UConn PAN - User contributions [en]

Student Projects in Nuclear Physics

2017-10-24T11:11:01Z

Bpratt18: /* Theses */

<font size="-1"><i>This work is supported by the U.S. National Science Foundation under grant 1508238</i></font>

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== Active Projects ==
=== Tagger Microscope Installation and Commissioning ===
* [[Tagger Microscope Status]] - [[User: barnes|Alex Barnes]]
* [[Microscope Installation Pictures]] - [[User: barnes|Alex Barnes]], [[User: bpratt18|Brendan Pratt]], [[User: mcintyre|James McIntyre]]
* [[Fiber Installation]] - [[User: barnes|Alex Barnes]], [[User: bpratt18|Brendan Pratt]], [[User: mcintyre|James McIntyre]]
* [[Lab Journals|Inclusive listing of lab journals of students who worked on this project]]

=== Tagger Microscope Mechanical Construction ===
* [[Tagger Microscope Construction]] - [[User: mcintyre|James McIntyre]], [[User: barnes|Alex Barnes]]
* [[Construction of the Full-Scale Tagger Microscope]] - [[User: carrolla|Ann Marie Carroll]], [[User: mcintyre|James McIntyre]], [[User: jonathank| Jonathan Kulakofsky]], [[User: Liana|Liana Hotte]], [[User:brand| Kenny Brand]], [[User: Bartolotta| John Bartolotta]], [[User: Acarta|Aaron Carta]]
* [[Full Scale Tagger Microscope Drawings|Tagger Microscope Engineering Drawings]] - [[User: mcintyre|James McIntyre]]
* [[Replacement Fibers Construction and Assessment]] - [[User: micahwarren|Micah Warren]]

=== Tagger Microscope Electronics ===
* [[Microscope Electronics]] - [[User: Barnes|Alex Barnes]], [[User: Senderovich|Igor Senderovich]], [[User: jonathank| Jonathan Kulakofsky]]
* [[Euro Card Connectors]] - [[User:Barnes|Alex Barnes]], [[User: mcintyre|James Mcintyre]]
* [[Darkbox Fiber Testing Setup]] - [[User: Barnes|Alex Barnes]]
'''

=== Active Collimator Installation and Commissioning ===
* [[Assembly and bench tests with the active collimator]] - [[User: Bartolotta| John Bartolotta]]
* [[Active Collimator Installation Pictures]] - [[User: barnes|Alex Barnes]]

=== Diamond Radiator Fabrication and Mounting ===
* [[Diamond Radiator Thinning Using an Excimer Laser]] - [[User: bpratt18|Brendan Pratt]]
* [[Diamond Mounting]] - [[User: bpratt18|Brendan Pratt]]
* [[Analysis of Diamond Cantilever Vibration]] - [[User:jess|Jessica Hyde]]
* [[Surface Images and Thickness profile of Diamonds]] - [[User: mokaya|Fridah Mokaya]]

=== Detector Development ===
* [[Construction and testing of a GEM-based radon detector]] - [[User: andrewsampino|Andrew Sampino]], [[User: miravarma|Mira Varma]]

=== Hadron Spectroscopy ===
* [[Analysis of JETSET data: A Search for XYZ Meson Analogs|Summary of 2008 XYZ Meson Review]] - [[User:Senderovich|Igor Senderovich]]
* [[Exotic b1π Channel Simulation and Analysis]] - [[User:Senderovich|Igor Senderovich]]

=== Radphi Data Analysis ===
* [[Radphi_MonteCarlo|Omega photoproduction and backgrounds simulation]]

=== GPU Computing ===
*[[GPU/CPU Cooling System]] - [[User: mcintyre|James McIntyre]], [[User: Barnes|Alex Barnes]]
*[[Media:JBCooling_system1.pdf|Cooling System Specification]] - [[User: bartolotta|John Bartolotta]]
*[[GPU/CPU Cooling Loop Construction]] - [[User: andyw177|Andy Wang]]
*[[GPU/CPU Cooling Loop Controls]] - [[User: francesyu|Frances Yu]], [[User: jeremyprema|Jeremy Prema]], [[User: sabrinashen|Sabrina Shen]], [[User: amarsinha|Amar Sinha]], [[User: varunyetukuri|Varun Yetukuri]]

== Presentations and Posters ==
===Conferences and Workshops===
* [[media:AlexBarnes-JlabUG2016.pdf|Hall D & GlueX Update]], ''[[User: barnes| Alex Barnes]]'' - invited plenary talk, JLab Annual User's Group Meeting, Newport News, VA, June 20-22, 2016.
* [[media:FridahMokaya-DNP2016.pdf|Spin density matrix elements for radiative decays of the omega meson in photoproduction at 5 GeV]], ''[[User: mokaya| Fridah Mokaya]]'' - contributed talk, APS-DNP Annual April Meeting, Salt Lake City, NM, Apr. 16-19, 2016.
* [[media:AlexBarnes-DNP2015.pdf|Calibration of the Tagger Detectors with GlueX Commissioning Data]], ''[[User: barnes| Alex Barnes]]'' - contributed talk, APS-DNP Annual Fall Meeting, Santa Fe, NM, Oct. 28-31, 2015.
* [[media:FridahMokaya-HEreactions-6-2015.pdf|High-Statistics Analysis of All-Neutral Decays of Mesons with the Radphi Experiment]], ''[[User: mokaya|Fridah Mokaya]]'' - contributed talk, 2015 International Summer Workshop on Reaction Theory, Bloomington, IN, June 8-19, 2015.
* [[media:JamesMcIntyre-HEreactions-6-2015.pdf|High-Resolution Tagger Hodoscope for GlueX]], ''[[User: mcintyre|James McIntyre]]'' - contributed talk, 2015 International Summer Workshop on Reaction Theory, Bloomington, IN, June 8-19, 2015.
* [[media:AlexBarnes-HEreactions-6-2015.pdf|GlueX at JLab]], ''[[User: barnes| Alex Barnes]]'' - contributed talk, Hampton University Graduate Summer School 2015, Newport News, VA, May29 - June 17, 2015.
* [[media:BrendanPrattHUGS-2015.pdf|Diamond Radiator Development for the GlueX Experiment]], ''[[User: bpratt18|Brendan Pratt]]'' - contributed talk, 2015 International Summer Workshop on Reaction Theory, Bloomington, IN, June 8-19, 2015.
* [[media:CHESS-Users-Meeting-6-2014.pptx|Thin Diamond Radiator Characterization at CHESS for the GlueX Experiment]], ''[[User: bpratt18|Brendan Pratt]]'' - poster, CHESS Annual User's Meeting, Ithaca, NY, June 10-11, 2014.
* [[media:NDNC-2014.ppt|Thin Diamond Radiator Fabrication for the GlueX Experiment]] ''[[User: bpratt18|Brendan Pratt]]'' - contributed talk, New Diamond and Nano Carbons Conference, Chicago, IL, May 25-29, 2014.
* [[media:AlexBarnes-DNP2013.pdf|The Development and Construction of the Tagger Microscope for the GlueX Experiment]], ''[[User: Barnes| Alex Barnes]]'' - contributed talk, DNP-2013 Newport News, VA, October, 2013.
* [[media:ICDCM_2013.pdf|Thin Diamond Radiator Fabrication and Characterization for The GlueX Experiment]] - ''[[User: bpratt18|Brendan Pratt]]'' - poster, International Conference on Diamond and Carbon Materials, Riva del Garda, Italy, Sept. 2-5, 2013.
* [[media:CHESS-Users-Meeting-6-2013.pdf|CHESS Rocking Curve Measurements of Thin Diamonds for the GlueX Experiment]], ''[[User: barnes|Alex Barnes]]'' - poster, CHESS Annual Users Meeting, Ithaca, NY, June 4-5, 2013.
* [[media:Pratt_DNP2012.pdf|Thin Diamond Radiator Fabrication for the GlueX Experiment]], ''[[User: bpratt18|Brendan Pratt]]'' - contributed talk, APS-DNP Annual Fall Meeting, Newport Newport Beach, CA, Oct. 24-27, 2012.
* [http://zeus.phys.uconn.edu/halld/glueXtalks/jonesDNP-10-2012.ppt Collimation and Tagging Instrumentation for the GlueX Photon Beamline], ''[[User: senderovich|Igor Senderovich]]'' - contributed talk, APS-DNP Annual Fall Meeting, Newport Beach, CA, Oct. 24-27, 2012.
* [http://zeus.phys.uconn.edu/halld/glueXarticles/EmilyBriere_FinalReportREU2012.pdf Design and Fabrication of Calibration Device for Scintillating Fibers of Tagger Microscope: For use in GlueX’s QCD Experiment], ''Emily Briere'' - poster, Conference Experience for Undergraduates, Newport Beach, CA, Oct. 24-27, 2012.
* [http://zeus.phys.uconn.edu/halld/glueXarticles/hadron2011_GlueX_proc.pdf Search for Gluonic Excitations in Hadrons with GlueX], ''[[User: senderovich|Igor Senderovich]]'', [http://www.slac.stanford.edu/econf/C110613/ Proceedings of the XIV International Conference on Hadron Spectroscopy, eds. S. Paul and N. Barmbilla], Munich, Germany, July 13-17, 2011.

===Other Venues===
* [[media:mokaya-UConnGradPoster-4-2016.pdf|Spin density matrix elements for radiative decays of the omega meson in photoproduction at 5 GeV]], ''[[User: Mokaya|Fridah Mokaya]]'', poster, Annual Graduate Research Exhibition, Physics Department, University of Connecticut, March 11, 2016.
* [[media:pratt-UConnGradPoster-4-2016.pdf|Diamond Radiator Fabrication and Characterization for the GlueX Experiment]], ''[[User: Bpratt18|Brendan Pratt]]'', poster, Annual Graduate Research Exhibition, Physics Department, University of Connecticut, March 11, 2016.
*[[media:Ploen_Gluex_PGSA_Poster_Session_Mar_2016.pdf|Quantifying Light Loss in the Tagger Microscope for the GlueX Experiment ]], ''Christine Ploen'', poster, Annual Graduate Research Exhibition, Physics Department, University of Connecticut, March 11, 2016.
* [[media:Hotte-Frontiers2015.pdf|Construction and Testing of the Photon Tagger Microscope for the GlueX Experiment]], ''[[User: Liana|Liana Hotte]]'' - poster at [http://ugradresearch.uconn.edu/wp-content/uploads/sites/323/2015/06/2015-Frontiers-Program.pdf Frontiers in Undergraduate Research 2015], University of Connecticut, Storrs, CT, April 10-11, 2015.
* [[media:Barnes-Juniata-Talk.pdf|Understanding Confinement in Quantum Chromodynamics Through the GlueX Experiment]], ''[[User: barnes|Alex Barnes]]'' - invited seminar, Juniata College, Huntingdon, PA, Nov. 2 2012.
* [[Listing of past undergraduate research projects|Posters and talks from past years]]

== Completed Projects ==
=== Undergrad Honors Projects ===
* [[CP Fall 2015|Diagnostics of Light Yield from Scintillating Fibers]] - Christine Ploen, 5/2016.
* [[media:LianaHotte_Honors_thesis2015.pdf|Construction of the Photon Tagger Microscope for the GlueX Experiment]] - Liana Hotte, 12/2015.
* [http://zeus.phys.uconn.edu/halld/glueXarticles/EmilyBriere_FinalReportREU2012.pdf Design and Fabrication of Calibration Device for Scintillating Fibers of Tagger Microscope: For use in GlueX’s QCD Experiment], Emily Briere, REU project final report, August 1, 2012.
* [[Listing of past undergraduate research projects]]

=== High School Mentor Connection ===
* [[Mentor Connection 2014]] - ''Alden Richter, Omar Amer, and Suki Hyman'', 7/2014.
* [[Mentor Connection 2013]] - ''Michael Reisman and Kyle Lockwood'', 7/2013.
* [[all past years of Nuclear Physics Mentor Connection site|all past years]]

== Theses ==
*[[User: bpratt18|Brendan Pratt]], [http://opencommons.uconn.edu/dissertations/1578/ Diamond Radiator Fabrication, Characterization and Performance for the GlueX Experiment], PhD Thesis, University of Connecticut, August 24, 2017.
* [[User: Liana|Liana Hotte]], [[media:LianaHotte_Honors_thesis2015.pdf|Construction of the Photon Tagger Microscope for the GlueX Experiment]], Undergraduate Honors Thesis, University of Connecticut, December 18, 2015.
* [[User: senderovich|Igor Senderovich]], [http://zeus.phys.uconn.edu/halld/IgorThesis-9-2012/PhDthesis-final.pdf Search for Gluonic Excitations in Hadrons with GlueX], PhD thesis, University of Connecticut, August 24, 2012.
* Chris Pelletier,[[Vibration Analysis for Diamond Bremsstrahlung Targets]], Undergraduate Honors Thesis, University of Connecticut, May 11, 2011.
* Mitchell Underwood, [[Design of Electronics for a High-Energy Photon Tagger for the GlueX Experiment]], Undergraduate Honors Thesis, University of Connecticut, May 8, 2010.

== Help ==
* [[Administrative Guide for New Group Members]]
* [[How to start a Chrome Remote Desktop session on Linux]]
* [[Private:Poster printing directions|How to print a poster in P403]]
* [[Notes on distributed authoring software tools]]
* [[Example page]] - Richard Jones
* [[Test page]] - testing

== References ==
* [[Group Photos]]
* [[Physics References]]
* Jefferson Lab Accelerator Schedule [[media:JLabThree-YearSchedule.pdf|2014-2016]], [[media:JLabThree-YearSchedule2.pdf|2016-2018]]
* [https://coda.jlab.org/wiki/index.php/JLab_Module_Manuals JLab VME Module Manuals]

== Safety Resources ==

* [https://docs.google.com/spreadsheets/d/1QrP9avFvB7SAJM8cn0O1tRAdHM4H14bubLpcG-yVQlU/edit?usp=sharing Chemical Inventory P-403 and P-405]

Student Projects in Nuclear Physics

2017-04-12T15:35:54Z

Bpratt18: /* References */

<font size="-1"><i>This work is supported by the U.S. National Science Foundation under grant 1508238</i></font>

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== Active Projects ==
'''
=== Tagger Microscope Installation and Commissioning ===
* [[Tagger Microscope Status]] - [[User: barnes|Alex Barnes]]
* [[Microscope Installation Pictures]] - [[User: barnes|Alex Barnes]], [[User: bpratt18|Brendan Pratt]], [[User: mcintyre|James McIntyre]]
* [[Fiber Installation]] - [[User: barnes|Alex Barnes]], [[User: bpratt18|Brendan Pratt]], [[User: mcintyre|James McIntyre]]
* [[Lab Journals|Inclusive listing of lab journals of students who worked on this project]]

=== Tagger Microscope Mechanical Construction ===
* [[Tagger Microscope Construction]] - [[User: mcintyre|James McIntyre]], [[User: barnes|Alex Barnes]]
* [[Construction of the Full-Scale Tagger Microscope]] - [[User: carrolla|Ann Marie Carroll]], [[User: mcintyre|James McIntyre]], [[User: jonathank| Jonathan Kulakofsky]], [[User: Liana|Liana Hotte]], [[User:brand| Kenny Brand]], [[User: Bartolotta| John Bartolotta]], [[User: Acarta|Aaron Carta]]
* [[Full Scale Tagger Microscope Drawings|Tagger Microscope Engineering Drawings]] - [[User: mcintyre|James McIntyre]]

=== Tagger Microscope Electronics ===
* [[Microscope Electronics]] - [[User: Barnes|Alex Barnes]], [[User: Senderovich|Igor Senderovich]], [[User: jonathank| Jonathan Kulakofsky]]
* [[Euro Card Connectors]] - [[User:Barnes|Alex Barnes]], [[User: mcintyre|James Mcintyre]]
* [[Darkbox Fiber Testing Setup]] - [[User: Barnes|Alex Barnes]]
'''

=== Active Collimator Installation and Commissioning ===
* [[Assembly and bench tests with the active collimator]] - [[User: Bartolotta| John Bartolotta]]
* [[Active Collimator Installation Pictures]] - [[User: barnes|Alex Barnes]]

=== Diamond Radiator Fabrication and Mounting ===
* [[Diamond Radiator Thinning Using an Excimer Laser]] - [[User: bpratt18|Brendan Pratt]]
* [[Diamond Mounting]] - [[User: bpratt18|Brendan Pratt]]
* [[Analysis of Diamond Cantilever Vibration]] - [[User:jess|Jessica Hyde]]
* [[Surface Images and Thickness profile of Diamonds]] - [[User: mokaya|Fridah Mokaya]]

=== Detector Development ===
* [[Construction and testing of a GEM-based radon detector]] - Andrew Sampino

=== Hadron Spectroscopy ===
* [[Analysis of JETSET data: A Search for XYZ Meson Analogs|Summary of 2008 XYZ Meson Review]] - [[User:Senderovich|Igor Senderovich]]
* [[Exotic b1π Channel Simulation and Analysis]] - [[User:Senderovich|Igor Senderovich]]

=== Radphi Data Analysis ===
* [[Radphi_MonteCarlo|Omega photoproduction and backgrounds simulation]]

=== GPU Computing ===
*[[GPU/CPU Cooling System]] - [[User: mcintyre|James McIntyre]], [[User: Barnes|Alex Barnes]]
*[[Media:JBCooling_system1.pdf|Cooling System Specification]] - [[User: bartolotta|John Bartolotta]]

== Presentations and Posters ==
===Conferences and Workshops===
* [[media:AlexBarnes-JlabUG2016.pdf|Hall D & GlueX Update]], ''[[User: barnes| Alex Barnes]]'' - invited plenary talk, JLab Annual User's Group Meeting, Newport News, VA, June 20-22, 2016.
* [[media:FridahMokaya-DNP2016.pdf|Spin density matrix elements for radiative decays of the omega meson in photoproduction at 5 GeV]], ''[[User: mokaya| Fridah Mokaya]]'' - contributed talk, APS-DNP Annual April Meeting, Salt Lake City, NM, Apr. 16-19, 2016.
* [[media:AlexBarnes-DNP2015.pdf|Calibration of the Tagger Detectors with GlueX Commissioning Data]], ''[[User: barnes| Alex Barnes]]'' - contributed talk, APS-DNP Annual Fall Meeting, Santa Fe, NM, Oct. 28-31, 2015.
* [[media:FridahMokaya-HEreactions-6-2015.pdf|High-Statistics Analysis of All-Neutral Decays of Mesons with the Radphi Experiment]], ''[[User: mokaya|Fridah Mokaya]]'' - contributed talk, 2015 International Summer Workshop on Reaction Theory, Bloomington, IN, June 8-19, 2015.
* [[media:JamesMcIntyre-HEreactions-6-2015.pdf|High-Resolution Tagger Hodoscope for GlueX]], ''[[User: mcintyre|James McIntyre]]'' - contributed talk, 2015 International Summer Workshop on Reaction Theory, Bloomington, IN, June 8-19, 2015.
* [[media:AlexBarnes-HEreactions-6-2015.pdf|GlueX at JLab]], ''[[User: barnes| Alex Barnes]]'' - contributed talk, Hampton University Graduate Summer School 2015, Newport News, VA, May29 - June 17, 2015.
* [[media:BrendanPrattHUGS-2015.pdf|Diamond Radiator Development for the GlueX Experiment]], ''[[User: bpratt18|Brendan Pratt]]'' - contributed talk, 2015 International Summer Workshop on Reaction Theory, Bloomington, IN, June 8-19, 2015.
* [[media:CHESS-Users-Meeting-6-2014.pptx|Thin Diamond Radiator Characterization at CHESS for the GlueX Experiment]], ''[[User: bpratt18|Brendan Pratt]]'' - poster, CHESS Annual User's Meeting, Ithaca, NY, June 10-11, 2014.
* [[media:NDNC-2014.ppt|Thin Diamond Radiator Fabrication for the GlueX Experiment]] ''[[User: bpratt18|Brendan Pratt]]'' - contributed talk, New Diamond and Nano Carbons Conference, Chicago, IL, May 25-29, 2014.
* [[media:AlexBarnes-DNP2013.pdf|The Development and Construction of the Tagger Microscope for the GlueX Experiment]], ''[[User: Barnes| Alex Barnes]]'' - contributed talk, DNP-2013 Newport News, VA, October, 2013.
* [[media:ICDCM_2013.pdf|Thin Diamond Radiator Fabrication and Characterization for The GlueX Experiment]] - ''[[User: bpratt18|Brendan Pratt]]'' - poster, International Conference on Diamond and Carbon Materials, Riva del Garda, Italy, Sept. 2-5, 2013.
* [[media:CHESS-Users-Meeting-6-2013.pdf|CHESS Rocking Curve Measurements of Thin Diamonds for the GlueX Experiment]], ''[[User: barnes|Alex Barnes]]'' - poster, CHESS Annual Users Meeting, Ithaca, NY, June 4-5, 2013.
* [[media:Pratt_DNP2012.pdf|Thin Diamond Radiator Fabrication for the GlueX Experiment]], ''[[User: bpratt18|Brendan Pratt]]'' - contributed talk, APS-DNP Annual Fall Meeting, Newport Newport Beach, CA, Oct. 24-27, 2012.
* [http://zeus.phys.uconn.edu/halld/glueXtalks/jonesDNP-10-2012.ppt Collimation and Tagging Instrumentation for the GlueX Photon Beamline], ''[[User: senderovich|Igor Senderovich]]'' - contributed talk, APS-DNP Annual Fall Meeting, Newport Beach, CA, Oct. 24-27, 2012.
* [http://zeus.phys.uconn.edu/halld/glueXarticles/EmilyBriere_FinalReportREU2012.pdf Design and Fabrication of Calibration Device for Scintillating Fibers of Tagger Microscope: For use in GlueX’s QCD Experiment], ''Emily Briere'' - poster, Conference Experience for Undergraduates, Newport Beach, CA, Oct. 24-27, 2012.
* [http://zeus.phys.uconn.edu/halld/glueXarticles/hadron2011_GlueX_proc.pdf Search for Gluonic Excitations in Hadrons with GlueX], ''[[User: senderovich|Igor Senderovich]]'', [http://www.slac.stanford.edu/econf/C110613/ Proceedings of the XIV International Conference on Hadron Spectroscopy, eds. S. Paul and N. Barmbilla], Munich, Germany, July 13-17, 2011.

===Other Venues===
* [[media:mokaya-UConnGradPoster-4-2016.pdf|Spin density matrix elements for radiative decays of the omega meson in photoproduction at 5 GeV]], ''[[User: Mokaya|Fridah Mokaya]]'', poster, Annual Graduate Research Exhibition, Physics Department, University of Connecticut, March 11, 2016.
* [[media:pratt-UConnGradPoster-4-2016.pdf|Diamond Radiator Fabrication and Characterization for the GlueX Experiment]], ''[[User: Bpratt18|Brendan Pratt]]'', poster, Annual Graduate Research Exhibition, Physics Department, University of Connecticut, March 11, 2016.
*[[media:Ploen_Gluex_PGSA_Poster_Session_Mar_2016.pdf|Quantifying Light Loss in the Tagger Microscope for the GlueX Experiment ]], ''Christine Ploen'', poster, Annual Graduate Research Exhibition, Physics Department, University of Connecticut, March 11, 2016.
* [[media:Hotte-Frontiers2015.pdf|Construction and Testing of the Photon Tagger Microscope for the GlueX Experiment]], ''[[User: Liana|Liana Hotte]]'' - poster at [http://ugradresearch.uconn.edu/wp-content/uploads/sites/323/2015/06/2015-Frontiers-Program.pdf Frontiers in Undergraduate Research 2015], University of Connecticut, Storrs, CT, April 10-11, 2015.
* [[media:Barnes-Juniata-Talk.pdf|Understanding Confinement in Quantum Chromodynamics Through the GlueX Experiment]], ''[[User: barnes|Alex Barnes]]'' - invited seminar, Juniata College, Huntingdon, PA, Nov. 2 2012.
* [[Listing of past undergraduate research projects|Posters and talks from past years]]

== Completed Projects ==
=== Undergrad Honors Projects ===
* [[CP Fall 2015|Diagnostics of Light Yield from Scintillating Fibers]] - Christine Ploen, 5/2016.
* [[media:LianaHotte_Honors_thesis2015.pdf|Construction of the Photon Tagger Microscope for the GlueX Experiment]] - Liana Hotte, 12/2015.
* [http://zeus.phys.uconn.edu/halld/glueXarticles/EmilyBriere_FinalReportREU2012.pdf Design and Fabrication of Calibration Device for Scintillating Fibers of Tagger Microscope: For use in GlueX’s QCD Experiment], Emily Briere, REU project final report, August 1, 2012.
* [[Listing of past undergraduate research projects]]

=== High School Mentor Connection ===
* [[Mentor Connection 2014]] - ''Alden Richter, Omar Amer, and Suki Hyman'', 7/2014.
* [[Mentor Connection 2013]] - ''Michael Reisman and Kyle Lockwood'', 7/2013.
* [[all past years of Nuclear Physics Mentor Connection site|all past years]]

== Theses ==
* [[User: Liana|Liana Hotte]], [[media:LianaHotte_Honors_thesis2015.pdf|Construction of the Photon Tagger Microscope for the GlueX Experiment]], Undergraduate Honors Thesis, University of Connecticut, December 18, 2015.
* [[User: senderovich|Igor Senderovich]], [http://zeus.phys.uconn.edu/halld/IgorThesis-9-2012/PhDthesis-final.pdf Search for Gluonic Excitations in Hadrons with GlueX], PhD thesis, University of Connecticut, August 24, 2012.
* Chris Pelletier,[[Vibration Analysis for Diamond Bremsstrahlung Targets]], Undergraduate Honors Thesis, University of Connecticut, May 11, 2011.
* Mitchell Underwood, [[Design of Electronics for a High-Energy Photon Tagger for the GlueX Experiment]], Undergraduate Honors Thesis, University of Connecticut, May 8, 2010.

== Help ==
* [[Administrative Guide for New Group Members]]
* [[How to start a Chrome Remote Desktop session on Linux]]
* [[Private:Poster printing directions|How to print a poster in P403]]
* [[Notes on distributed authoring software tools]]
* [[Example page]] - Richard Jones
* [[Test page]] - testing

== References ==
* [[Group Photos]]
* [[Physics References]]
* Jefferson Lab Accelerator Schedule [[media:JLabThree-YearSchedule.pdf|2014-2016]], [[media:JLabThree-YearSchedule2.pdf|2016-2018]]
* [https://coda.jlab.org/wiki/index.php/JLab_Module_Manuals JLab VME Module Manuals]

== Safety Resources ==

* [https://docs.google.com/spreadsheets/d/1QrP9avFvB7SAJM8cn0O1tRAdHM4H14bubLpcG-yVQlU/edit?usp=sharing Chemical Inventory P-403 and P-405]

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:46:59Z

Bpratt18: /* Focal Spot Characterization */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|thumb|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser shown in Figure 2 was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 3: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure 4: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 3. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 4 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:spherabs.png|right|thumb|200px|Figure 6:Zygo image of pulses made on diamond after passing through a single focusing lens.]] [[Image:goodfocus.png|right|thumb|200px|Figure 7:Zygo image of pulses made on diamond after passing through the three lens system.]]
[[Image:ablation_full.png|center|thumb|300px|Figure 8:Rendering of ablation beamline]]
Figure 8 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process. Figure 6 shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.

The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 8. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 6 shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. Figure 7 shows the improvement in beam shape gained after using the three lens setup.
If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
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| [[Image:xscan_005_map.png|left|thumb|300px|Figure 9a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|Figure 9b]]
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{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
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| [[Image:xscan_005_fit.png|left|thumb|300px|Figure 9c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|Figure 9d]]
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Figures 9a and 9b are color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
Figures 9c and 9d are projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 9 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 9c,d. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 9b shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
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| [[Image:cutrate_raw.png|left|thumb|300px|Figure 10:Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Figure 11:Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
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The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
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| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Figure 12a:Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Figure 12b:Focused Beam Profile]]
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*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
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| [[Image:g_r1X.png|left|thumb|300px|Figure 13a:X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Figure 13b:Y-axis Projection of Focused Beam]]
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*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
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| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
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*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|thumb|400px|Figure 14: CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure 15: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure 16: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 16 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 17a.
The same study was conducted again, but the dial indicator was used to require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 17b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
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| [[Image:deltay_bad.png|left|thumb|300px|Figure 17a: The histogram shown in Figure 17a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure 17b: R The histogram in Figure 17b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:46:47Z

Bpratt18: /* Excimer Laser */

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:46:21Z

Bpratt18: /* Sub-Micron Precision Using Dial Indicators */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser shown in Figure 2 was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 3: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure 4: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 3. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 4 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:spherabs.png|right|thumb|200px|Figure 6:Zygo image of pulses made on diamond after passing through a single focusing lens.]] [[Image:goodfocus.png|right|thumb|200px|Figure 7:Zygo image of pulses made on diamond after passing through the three lens system.]]
[[Image:ablation_full.png|center|thumb|300px|Figure 8:Rendering of ablation beamline]]
Figure 8 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process. Figure 6 shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.

The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 8. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 6 shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. Figure 7 shows the improvement in beam shape gained after using the three lens setup.
If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_map.png|left|thumb|300px|Figure 9a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|Figure 9b]]
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|}
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
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| [[Image:xscan_005_fit.png|left|thumb|300px|Figure 9c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|Figure 9d]]
|-
|}
Figures 9a and 9b are color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
Figures 9c and 9d are projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 9 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 9c,d. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 9b shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:cutrate_raw.png|left|thumb|300px|Figure 10:Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Figure 11:Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
|-
|}

The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
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| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Figure 12a:Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Figure 12b:Focused Beam Profile]]
|-
|}
*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:g_r1X.png|left|thumb|300px|Figure 13a:X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Figure 13b:Y-axis Projection of Focused Beam]]
|-
|}

*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
|-
|}
*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|thumb|400px|Figure 14: CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure 15: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure 16: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 16 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 17a.
The same study was conducted again, but the dial indicator was used to require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 17b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:deltay_bad.png|left|thumb|300px|Figure 17a: The histogram shown in Figure 17a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure 17b: R The histogram in Figure 17b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:45:57Z

Bpratt18: /* Sub-Micron Precision Using Dial Indicators */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser shown in Figure 2 was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 3: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure 4: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 3. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 4 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:spherabs.png|right|thumb|200px|Figure 6:Zygo image of pulses made on diamond after passing through a single focusing lens.]] [[Image:goodfocus.png|right|thumb|200px|Figure 7:Zygo image of pulses made on diamond after passing through the three lens system.]]
[[Image:ablation_full.png|center|thumb|300px|Figure 8:Rendering of ablation beamline]]
Figure 8 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process. Figure 6 shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.

The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 8. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 6 shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. Figure 7 shows the improvement in beam shape gained after using the three lens setup.
If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_map.png|left|thumb|300px|Figure 9a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|Figure 9b]]
|-
|}
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_fit.png|left|thumb|300px|Figure 9c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|Figure 9d]]
|-
|}
Figures 9a and 9b are color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
Figures 9c and 9d are projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 9 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 9c,d. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 9b shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:cutrate_raw.png|left|thumb|300px|Figure 10:Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Figure 11:Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
|-
|}

The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Figure 12a:Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Figure 12b:Focused Beam Profile]]
|-
|}
*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:g_r1X.png|left|thumb|300px|Figure 13a:X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Figure 13b:Y-axis Projection of Focused Beam]]
|-
|}

*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
|-
|}
*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|thumb|400px|Figure 14: CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure 15: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure 16: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 16 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 17a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 17b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:deltay_bad.png|left|thumb|300px|Figure 17a: The histogram shown in Figure 17a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure 17b: R The histogram in Figure 17b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:44:38Z

Bpratt18: /* Ablation Chamber */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser shown in Figure 2 was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 3: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure 4: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 3. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 4 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:spherabs.png|right|thumb|200px|Figure 6:Zygo image of pulses made on diamond after passing through a single focusing lens.]] [[Image:goodfocus.png|right|thumb|200px|Figure 7:Zygo image of pulses made on diamond after passing through the three lens system.]]
[[Image:ablation_full.png|center|thumb|300px|Figure 8:Rendering of ablation beamline]]
Figure 8 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process. Figure 6 shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.

The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 8. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 6 shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. Figure 7 shows the improvement in beam shape gained after using the three lens setup.
If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_map.png|left|thumb|300px|Figure 9a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|Figure 9b]]
|-
|}
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_fit.png|left|thumb|300px|Figure 9c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|Figure 9d]]
|-
|}
Figures 9a and 9b are color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
Figures 9c and 9d are projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 9 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 9c,d. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 9b shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:cutrate_raw.png|left|thumb|300px|Figure 10:Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Figure 11:Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
|-
|}

The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Figure 12a:Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Figure 12b:Focused Beam Profile]]
|-
|}
*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:g_r1X.png|left|thumb|300px|Figure 13a:X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Figure 13b:Y-axis Projection of Focused Beam]]
|-
|}

*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
|-
|}
*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|thumb|400px|Figure 14: CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure 15: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 11 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 12a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 12b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:deltay_bad.png|left|thumb|300px|Figure: The histogram shown in Figure 11a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure: R The histogram in Figure 11b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:44:14Z

Bpratt18: /* Ablation Chamber */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser shown in Figure 2 was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 3: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure 4: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 3. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 4 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:spherabs.png|right|thumb|200px|Figure 6:Zygo image of pulses made on diamond after passing through a single focusing lens.]] [[Image:goodfocus.png|right|thumb|200px|Figure 7:Zygo image of pulses made on diamond after passing through the three lens system.]]
[[Image:ablation_full.png|center|thumb|300px|Figure 8:Rendering of ablation beamline]]
Figure 8 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process. Figure 6 shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.

The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 8. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 6 shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. Figure 7 shows the improvement in beam shape gained after using the three lens setup.
If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_map.png|left|thumb|300px|Figure 9a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|Figure 9b]]
|-
|}
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_fit.png|left|thumb|300px|Figure 9c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|Figure 9d]]
|-
|}
Figures 9a and 9b are color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
Figures 9c and 9d are projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 9 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 9c,d. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 9b shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:cutrate_raw.png|left|thumb|300px|Figure 10:Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Figure 11:Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
|-
|}

The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Figure 12a:Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Figure 12b:Focused Beam Profile]]
|-
|}
*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:g_r1X.png|left|thumb|300px|Figure 13a:X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Figure 13b:Y-axis Projection of Focused Beam]]
|-
|}

*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
|-
|}
*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|400px|Figure 14: CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure 15: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 11 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 12a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 12b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:deltay_bad.png|left|thumb|300px|Figure: The histogram shown in Figure 11a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure: R The histogram in Figure 11b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:43:42Z

Bpratt18: /* FORTRAN Simulations of Beam Spot */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser shown in Figure 2 was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 3: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure 4: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 3. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 4 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:spherabs.png|right|thumb|200px|Figure 6:Zygo image of pulses made on diamond after passing through a single focusing lens.]] [[Image:goodfocus.png|right|thumb|200px|Figure 7:Zygo image of pulses made on diamond after passing through the three lens system.]]
[[Image:ablation_full.png|center|thumb|300px|Figure 8:Rendering of ablation beamline]]
Figure 8 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process. Figure 6 shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.

The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 8. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 6 shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. Figure 7 shows the improvement in beam shape gained after using the three lens setup.
If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
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| [[Image:xscan_005_map.png|left|thumb|300px|Figure 9a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|Figure 9b]]
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{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
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| [[Image:xscan_005_fit.png|left|thumb|300px|Figure 9c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|Figure 9d]]
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Figures 9a and 9b are color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
Figures 9c and 9d are projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 9 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 9c,d. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 9b shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
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| [[Image:cutrate_raw.png|left|thumb|300px|Figure 10:Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Figure 11:Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
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The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
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| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Figure 12a:Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Figure 12b:Focused Beam Profile]]
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*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
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| [[Image:g_r1X.png|left|thumb|300px|Figure 13a:X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Figure 13b:Y-axis Projection of Focused Beam]]
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*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
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| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
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*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|400px| CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 11 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 12a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 12b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
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| [[Image:deltay_bad.png|left|thumb|300px|Figure: The histogram shown in Figure 11a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure: R The histogram in Figure 11b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
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==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:42:52Z

Bpratt18: /* Ablation Rate */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser shown in Figure 2 was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 3: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure 4: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 3. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 4 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:spherabs.png|right|thumb|200px|Figure 6:Zygo image of pulses made on diamond after passing through a single focusing lens.]] [[Image:goodfocus.png|right|thumb|200px|Figure 7:Zygo image of pulses made on diamond after passing through the three lens system.]]
[[Image:ablation_full.png|center|thumb|300px|Figure 8:Rendering of ablation beamline]]
Figure 8 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process. Figure 6 shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.

The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 8. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 6 shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. Figure 7 shows the improvement in beam shape gained after using the three lens setup.
If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
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| [[Image:xscan_005_map.png|left|thumb|300px|Figure 9a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|Figure 9b]]
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{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
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| [[Image:xscan_005_fit.png|left|thumb|300px|Figure 9c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|Figure 9d]]
|-
|}
Figures 9a and 9b are color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
Figures 9c and 9d are projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 9 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 9c,d. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 9b shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:cutrate_raw.png|left|thumb|300px|Figure 10:Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Figure 11:Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
|-
|}

The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Focused Beam Profile]]
|-
|}
*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:g_r1X.png|left|thumb|300px|X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Y-axis Projection of Focused Beam]]
|-
|}

*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
|-
|}
*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|400px| CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 11 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 12a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 12b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:deltay_bad.png|left|thumb|300px|Figure: The histogram shown in Figure 11a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure: R The histogram in Figure 11b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:42:14Z

Bpratt18: /* Focal Spot Characterization */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser shown in Figure 2 was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 3: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure 4: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 3. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 4 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:spherabs.png|right|thumb|200px|Figure 6:Zygo image of pulses made on diamond after passing through a single focusing lens.]] [[Image:goodfocus.png|right|thumb|200px|Figure 7:Zygo image of pulses made on diamond after passing through the three lens system.]]
[[Image:ablation_full.png|center|thumb|300px|Figure 8:Rendering of ablation beamline]]
Figure 8 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process. Figure 6 shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.

The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 8. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 6 shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. Figure 7 shows the improvement in beam shape gained after using the three lens setup.
If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_map.png|left|thumb|300px|Figure 9a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|Figure 9b]]
|-
|}
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_fit.png|left|thumb|300px|Figure 9c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|Figure 9d]]
|-
|}
Figures 9a and 9b are color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
Figures 9c and 9d are projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 9 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 9c,d. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 9b shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:cutrate_raw.png|left|thumb|300px|Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
|-
|}

The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Focused Beam Profile]]
|-
|}
*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:g_r1X.png|left|thumb|300px|X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Y-axis Projection of Focused Beam]]
|-
|}

*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
|-
|}
*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|400px| CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 11 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 12a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 12b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:deltay_bad.png|left|thumb|300px|Figure: The histogram shown in Figure 11a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure: R The histogram in Figure 11b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:40:11Z

Bpratt18: /* Laser Beamline */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser shown in Figure 2 was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 3: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure 4: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 3. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 4 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:spherabs.png|right|thumb|200px|Figure 6:Zygo image of pulses made on diamond after passing through a single focusing lens.]] [[Image:goodfocus.png|right|thumb|200px|Figure 7:Zygo image of pulses made on diamond after passing through the three lens system.]]
[[Image:ablation_full.png|center|thumb|300px|Figure 8:Rendering of ablation beamline]]
Figure 8 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process. Figure 6 shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.

The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 8. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 6 shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. Figure 7 shows the improvement in beam shape gained after using the three lens setup.
If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_map.png|left|thumb|300px|5a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|5b]]
|-
|}
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_fit.png|left|thumb|300px|5c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|5d]]
|-
|}
*Color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
*Projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 7 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 8. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 7a shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:cutrate_raw.png|left|thumb|300px|Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
|-
|}

The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Focused Beam Profile]]
|-
|}
*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:g_r1X.png|left|thumb|300px|X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Y-axis Projection of Focused Beam]]
|-
|}

*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
|-
|}
*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|400px| CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 11 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 12a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 12b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:deltay_bad.png|left|thumb|300px|Figure: The histogram shown in Figure 11a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure: R The histogram in Figure 11b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:36:22Z

Bpratt18: /* Gas Purification System */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser shown in Figure 2 was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 3: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure 4: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 3. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 4 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:spherabs.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through a single focusing lens.]] [[Image:goodfocus.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through the three lens system.]]
[[Image:ablation_full.png|center|thumb|300px|Rendering of ablation beamline]]
Figure 1 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process. Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.

The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 4. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_map.png|left|thumb|300px|5a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|5b]]
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|}
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
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| [[Image:xscan_005_fit.png|left|thumb|300px|5c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|5d]]
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*Color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
*Projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 7 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 8. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 7a shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
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| [[Image:cutrate_raw.png|left|thumb|300px|Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
|-
|}

The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
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| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Focused Beam Profile]]
|-
|}
*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:g_r1X.png|left|thumb|300px|X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Y-axis Projection of Focused Beam]]
|-
|}

*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
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| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
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*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|400px| CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 11 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 12a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 12b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:deltay_bad.png|left|thumb|300px|Figure: The histogram shown in Figure 11a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure: R The histogram in Figure 11b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:35:11Z

Bpratt18: /* Excimer Laser */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser shown in Figure 2 was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 2: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 1. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 3 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:spherabs.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through a single focusing lens.]] [[Image:goodfocus.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through the three lens system.]]
[[Image:ablation_full.png|center|thumb|300px|Rendering of ablation beamline]]
Figure 1 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process. Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.

The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 4. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
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| [[Image:xscan_005_map.png|left|thumb|300px|5a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|5b]]
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|}
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
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| [[Image:xscan_005_fit.png|left|thumb|300px|5c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|5d]]
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*Color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
*Projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 7 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 8. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 7a shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:cutrate_raw.png|left|thumb|300px|Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
|-
|}

The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
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| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Focused Beam Profile]]
|-
|}
*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:g_r1X.png|left|thumb|300px|X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Y-axis Projection of Focused Beam]]
|-
|}

*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
|-
|}
*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|400px| CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 11 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 12a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 12b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:deltay_bad.png|left|thumb|300px|Figure: The histogram shown in Figure 11a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure: R The histogram in Figure 11b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:34:02Z

Bpratt18: /* Ablation Chamber */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 2: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 1. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 3 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:spherabs.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through a single focusing lens.]] [[Image:goodfocus.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through the three lens system.]]
[[Image:ablation_full.png|center|thumb|300px|Rendering of ablation beamline]]
Figure 1 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process. Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.

The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 4. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_map.png|left|thumb|300px|5a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|5b]]
|-
|}
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_fit.png|left|thumb|300px|5c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|5d]]
|-
|}
*Color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
*Projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 7 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 8. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 7a shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:cutrate_raw.png|left|thumb|300px|Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
|-
|}

The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Focused Beam Profile]]
|-
|}
*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:g_r1X.png|left|thumb|300px|X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Y-axis Projection of Focused Beam]]
|-
|}

*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
|-
|}
*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|400px| CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 11 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 12a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 12b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:deltay_bad.png|left|thumb|300px|Figure: The histogram shown in Figure 11a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure: R The histogram in Figure 11b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:32:02Z

Bpratt18: /* Laser Beamline */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 2: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 1. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 3 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:spherabs.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through a single focusing lens.]] [[Image:goodfocus.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through the three lens system.]]
[[Image:ablation_full.png|center|thumb|300px|Rendering of ablation beamline]]
Figure 1 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process. Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.

The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 4. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_map.png|left|thumb|300px|5a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|5b]]
|-
|}
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_fit.png|left|thumb|300px|5c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|5d]]
|-
|}
*Color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
*Projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 7 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 8. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 7a shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:cutrate_raw.png|left|thumb|300px|Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
|-
|}

The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Focused Beam Profile]]
|-
|}
*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:g_r1X.png|left|thumb|300px|X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Y-axis Projection of Focused Beam]]
|-
|}

*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
|-
|}
*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|300px| CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 11 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 12a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 12b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:deltay_bad.png|left|thumb|300px|Figure: The histogram shown in Figure 11a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure: R The histogram in Figure 11b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:26:43Z

Bpratt18: /* Focal Spot Characterization */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 2: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 1. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 3 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:ablation_full.png|left|thumb|300px|Rendering of ablation beamline]]

Figure 1 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process.

[[Image:spherabs.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through a single focusing lens.]] [[Image:goodfocus.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through the three lens system.]]

Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.

The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 4. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
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| [[Image:xscan_005_map.png|left|thumb|300px|5a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|5b]]
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{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
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| [[Image:xscan_005_fit.png|left|thumb|300px|5c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|5d]]
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*Color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
*Projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 7 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 8. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 7a shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
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| [[Image:cutrate_raw.png|left|thumb|300px|Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
|-
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The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
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| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Focused Beam Profile]]
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*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
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| [[Image:g_r1X.png|left|thumb|300px|X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Y-axis Projection of Focused Beam]]
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*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
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| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
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*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|300px| CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 11 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 12a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 12b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:deltay_bad.png|left|thumb|300px|Figure: The histogram shown in Figure 11a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure: R The histogram in Figure 11b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:26:25Z

Bpratt18: /* Excimer Laser */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 2: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 1. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 3 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:ablation_full.png|left|thumb|300px|Rendering of ablation beamline]]

Figure 1 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process.

[[Image:spherabs.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through a single focusing lens.]] [[Image:goodfocus.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through the three lens system.]]

Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.

The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 4. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_map.png|left|thumb|300px|5a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|5b]]
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|}
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
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| [[Image:xscan_005_fit.png|left|thumb|300px|5c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|5d]]
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|}
*Color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
*Projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 7 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 8. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 7a shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:cutrate_raw.png|left|thumb|300px|Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
|-
|}

The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
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| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Focused Beam Profile]]
|-
|}
*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:g_r1X.png|left|thumb|300px|X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Y-axis Projection of Focused Beam]]
|-
|}

*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
|-
|}
*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|300px| CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 11 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 12a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 12b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:deltay_bad.png|left|thumb|300px|Figure: The histogram shown in Figure 11a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure: R The histogram in Figure 11b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:26:16Z

Bpratt18: /* Excimer Laser */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 2: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 1. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 3 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:ablation_full.png|left|thumb|300px|Rendering of ablation beamline]]

Figure 1 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process.

[[Image:spherabs.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through a single focusing lens.]] [[Image:goodfocus.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through the three lens system.]]

Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.

The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 4. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_map.png|left|thumb|300px|5a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|5b]]
|-
|}
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_fit.png|left|thumb|300px|5c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|5d]]
|-
|}
*Color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
*Projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 7 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 8. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 7a shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:cutrate_raw.png|left|thumb|300px|Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
|-
|}

The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Focused Beam Profile]]
|-
|}
*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:g_r1X.png|left|thumb|300px|X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Y-axis Projection of Focused Beam]]
|-
|}

*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
|-
|}
*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|300px| CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 11 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 12a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 12b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:deltay_bad.png|left|thumb|300px|Figure: The histogram shown in Figure 11a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure: R The histogram in Figure 11b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:25:58Z

Bpratt18: /* Focal Spot Characterization */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 2: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 1. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 3 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:ablation_full.png|left|thumb|300px|Rendering of ablation beamline]]

Figure 1 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process.

[[Image:spherabs.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through a single focusing lens.]] [[Image:goodfocus.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through the three lens system.]]

Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.

The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 4. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_map.png|left|thumb|300px|5a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|5b]]
|-
|}
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_fit.png|left|thumb|300px|5c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|5d]]
|-
|}
*Color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
*Projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 7 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 8. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 7a shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:cutrate_raw.png|left|thumb|300px|Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
|-
|}

The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Focused Beam Profile]]
|-
|}
*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:g_r1X.png|left|thumb|300px|X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Y-axis Projection of Focused Beam]]
|-
|}

*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
|-
|}
*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|300px| CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 11 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 12a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 12b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:deltay_bad.png|left|thumb|300px|Figure: The histogram shown in Figure 11a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure: R The histogram in Figure 11b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:25:46Z

Bpratt18: /* Laser Beamline */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 2: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 1. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 3 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:ablation_full.png|left|thumb|300px|Rendering of ablation beamline]]

Figure 1 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process.

[[Image:spherabs.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through a single focusing lens.]] [[Image:goodfocus.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through the three lens system.]]

Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.

The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 4. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_map.png|left|thumb|300px|5a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|5b]]
|-
|}
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_fit.png|left|thumb|300px|5c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|5d]]
|-
|}
*Color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
*Projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 7 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 8. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 7a shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:cutrate_raw.png|left|thumb|300px|Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
|-
|}

The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Focused Beam Profile]]
|-
|}
*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:g_r1X.png|left|thumb|300px|X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Y-axis Projection of Focused Beam]]
|-
|}

*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
|-
|}
*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|300px| CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 11 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 12a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 12b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:deltay_bad.png|left|thumb|300px|Figure: The histogram shown in Figure 11a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure: R The histogram in Figure 11b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:25:31Z

Bpratt18: /* Laser Beamline */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 2: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 1. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 3 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:ablation_full.png|left|thumb|300px|Rendering of ablation beamline]]

Figure 1 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process.

[[Image:spherabs.png|right|thumb|300px|Zygo image of pulses made on diamond after passing through a single focusing lens.]] [[Image:goodfocus.png|right|thumb|300px|Zygo image of pulses made on diamond after passing through the three lens system.]]

Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.

The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 4. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_map.png|left|thumb|300px|5a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|5b]]
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|}
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
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| [[Image:xscan_005_fit.png|left|thumb|300px|5c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|5d]]
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|}
*Color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
*Projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 7 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 8. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 7a shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:cutrate_raw.png|left|thumb|300px|Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
|-
|}

The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Focused Beam Profile]]
|-
|}
*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:g_r1X.png|left|thumb|300px|X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Y-axis Projection of Focused Beam]]
|-
|}

*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
|-
|}
*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|300px| CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 11 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 12a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 12b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:deltay_bad.png|left|thumb|300px|Figure: The histogram shown in Figure 11a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure: R The histogram in Figure 11b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:25:18Z

Bpratt18: /* Laser Beamline */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 2: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 1. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 3 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:ablation_full.png|left|thumb|300px|Rendering of ablation beamline]]

Figure 1 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process.

[[Image:spherabs.png|right|thumb|400px|Zygo image of pulses made on diamond after passing through a single focusing lens.]] [[Image:goodfocus.png|right|thumb|400px|Zygo image of pulses made on diamond after passing through the three lens system.]]

Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.

The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 4. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_map.png|left|thumb|300px|5a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|5b]]
|-
|}
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_fit.png|left|thumb|300px|5c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|5d]]
|-
|}
*Color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
*Projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 7 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 8. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 7a shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:cutrate_raw.png|left|thumb|300px|Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
|-
|}

The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Focused Beam Profile]]
|-
|}
*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:g_r1X.png|left|thumb|300px|X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Y-axis Projection of Focused Beam]]
|-
|}

*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
|-
|}
*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|300px| CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 11 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 12a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 12b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:deltay_bad.png|left|thumb|300px|Figure: The histogram shown in Figure 11a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure: R The histogram in Figure 11b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:25:03Z

Bpratt18: /* Focal Spot Characterization */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 2: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 1. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 3 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:ablation_full.png|left|thumb|300px|Rendering of ablation beamline]]

Figure 1 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process.

[[Image:spherabs.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through a single focusing lens.]] [[Image:goodfocus.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through the three lens system.]]

Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.

The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 4. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_map.png|left|thumb|300px|5a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|5b]]
|-
|}
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_fit.png|left|thumb|300px|5c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|5d]]
|-
|}
*Color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
*Projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 7 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 8. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 7a shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:cutrate_raw.png|left|thumb|300px|Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
|-
|}

The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Focused Beam Profile]]
|-
|}
*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:g_r1X.png|left|thumb|300px|X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Y-axis Projection of Focused Beam]]
|-
|}

*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
|-
|}
*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|300px| CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 11 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 12a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 12b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:deltay_bad.png|left|thumb|300px|Figure: The histogram shown in Figure 11a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure: R The histogram in Figure 11b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:24:39Z

Bpratt18: /* Focal Spot Characterization */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 2: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 1. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 3 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:ablation_full.png|left|thumb|300px|Rendering of ablation beamline]]

Figure 1 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process.

[[Image:spherabs.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through a single focusing lens.]] [[Image:goodfocus.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through the three lens system.]]

Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.

The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 4. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_map.png|left|thumb|300px|5a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|5b]]
|-
|}
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_fit.png|left|thumb|300px|5c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|5d]]
|-
|}
*Color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
*Projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 7 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 8. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 7a shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:cutrate_raw.png|left|thumb|300px|Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
|-
|}

The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Focused Beam Profile]]
|-
|}
*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:g_r1X.png|left|thumb|300px|X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Y-axis Projection of Focused Beam]]
|-
|}

*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
|-
|}
*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|300px| CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 11 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 12a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 12b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:deltay_bad.png|left|thumb|300px|Figure: The histogram shown in Figure 11a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure: R The histogram in Figure 11b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:24:05Z

Bpratt18: /* Laser Beamline */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 2: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 1. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 3 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:ablation_full.png|left|thumb|300px|Rendering of ablation beamline]]

Figure 1 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process.

[[Image:spherabs.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through a single focusing lens.]] [[Image:goodfocus.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through the three lens system.]]

Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.

The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 4. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_map.png|left|thumb|300px|5a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|5b]]
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|}
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
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| [[Image:xscan_005_fit.png|left|thumb|300px|5c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|5d]]
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*Color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
*Projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 7 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 8. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 7a shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:cutrate_raw.png|left|thumb|300px|Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
|-
|}

The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
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| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Focused Beam Profile]]
|-
|}
*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:g_r1X.png|left|thumb|300px|X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Y-axis Projection of Focused Beam]]
|-
|}

*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
|-
|}
*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|300px| CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 11 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 12a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 12b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:deltay_bad.png|left|thumb|300px|Figure: The histogram shown in Figure 11a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure: R The histogram in Figure 11b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:23:50Z

Bpratt18: /* Focal Spot Characterization */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 2: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 1. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 3 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:ablation_full.png|left|thumb|300px|Rendering of ablation beamline]]

Figure 1 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process.

[[Image:spherabs.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through a single focusing lens.]] [[Image:goodfocus.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through the three lens system.]]

Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.

The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 4. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_map.png|left|thumb|300px|5a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|5b]]
|-
|}
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_fit.png|left|thumb|300px|5c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|5d]]
|-
|}
*Color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
*Projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 7 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 8. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 7a shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:cutrate_raw.png|left|thumb|300px|Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
|-
|}

The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Focused Beam Profile]]
|-
|}
*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:g_r1X.png|left|thumb|300px|X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Y-axis Projection of Focused Beam]]
|-
|}

*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
|-
|}
*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|300px| CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 11 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 12a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 12b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:deltay_bad.png|left|thumb|300px|Figure: The histogram shown in Figure 11a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure: R The histogram in Figure 11b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:23:32Z

Bpratt18: /* Laser Beamline */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 2: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 1. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 3 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:ablation_full.png|left|thumb|300px|Rendering of ablation beamline]]

Figure 1 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process.

[[Image:spherabs.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through a single focusing lens.]] [[Image:goodfocus.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through the three lens system.]]

Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.

The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 4. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_map.png|left|thumb|300px|5a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|5b]]
|-
|}
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_fit.png|left|thumb|300px|5c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|5d]]
|-
|}
*Color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
*Projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 7 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 8. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 7a shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:cutrate_raw.png|left|thumb|300px|Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
|-
|}

The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Focused Beam Profile]]
|-
|}
*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:g_r1X.png|left|thumb|300px|X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Y-axis Projection of Focused Beam]]
|-
|}

*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
|-
|}
*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|300px| CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 11 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 12a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 12b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:deltay_bad.png|left|thumb|300px|Figure: The histogram shown in Figure 11a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure: R The histogram in Figure 11b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:23:18Z

Bpratt18: /* Gas Purification System */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 2: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 1. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 3 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:ablation_full.png|left|thumb|300px|Rendering of ablation beamline]]

Figure 1 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process.

[[Image:spherabs.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through a single focusing lens.]] [[Image:goodfocus.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through the three lens system.]]

Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.

The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 4. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_map.png|left|thumb|300px|5a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|5b]]
|-
|}
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_fit.png|left|thumb|300px|5c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|5d]]
|-
|}
*Color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
*Projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 7 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 8. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 7a shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:cutrate_raw.png|left|thumb|300px|Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
|-
|}

The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Focused Beam Profile]]
|-
|}
*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:g_r1X.png|left|thumb|300px|X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Y-axis Projection of Focused Beam]]
|-
|}

*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
|-
|}
*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|300px| CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 11 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 12a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 12b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:deltay_bad.png|left|thumb|300px|Figure: The histogram shown in Figure 11a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure: R The histogram in Figure 11b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:23:03Z

Bpratt18: /* Laser Beamline */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 2: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 1. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 3 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:ablation_full.png|left|thumb|300px|Rendering of ablation beamline]]

Figure 1 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process.

[[Image:spherabs.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through a single focusing lens.]] [[Image:goodfocus.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through the three lens system.]]

Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.

The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 4. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_map.png|left|thumb|300px|5a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|5b]]
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|}
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
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| [[Image:xscan_005_fit.png|left|thumb|300px|5c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|5d]]
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|}
*Color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
*Projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 7 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 8. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 7a shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:cutrate_raw.png|left|thumb|300px|Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
|-
|}

The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Focused Beam Profile]]
|-
|}
*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:g_r1X.png|left|thumb|300px|X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Y-axis Projection of Focused Beam]]
|-
|}

*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
|-
|}
*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|300px| CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 11 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 12a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 12b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:deltay_bad.png|left|thumb|300px|Figure: The histogram shown in Figure 11a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure: R The histogram in Figure 11b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:22:49Z

Bpratt18: /* Laser Beamline */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 2: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 1. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 3 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:ablation_full.png|left|thumb|300px|Rendering of ablation beamline]]

Figure 1 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process.

[[Image:spherabs.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through a single focusing lens.]] [[Image:goodfocus.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through the three lens system.]]

Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.

The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 4. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_map.png|left|thumb|300px|5a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|5b]]
|-
|}
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_fit.png|left|thumb|300px|5c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|5d]]
|-
|}
*Color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
*Projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 7 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 8. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 7a shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:cutrate_raw.png|left|thumb|300px|Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
|-
|}

The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Focused Beam Profile]]
|-
|}
*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:g_r1X.png|left|thumb|300px|X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Y-axis Projection of Focused Beam]]
|-
|}

*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
|-
|}
*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|300px| CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 11 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 12a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 12b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:deltay_bad.png|left|thumb|300px|Figure: The histogram shown in Figure 11a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure: R The histogram in Figure 11b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:22:32Z

Bpratt18: /* Focal Spot Characterization */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 2: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 1. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 3 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:ablation_full.png|left|thumb|300px|Rendering of ablation beamline]]

Figure 1 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process.

[[Image:spherabs.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through a single focusing lens.]] [[Image:goodfocus.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through the three lens system.]]

Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.

The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 4. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_map.png|left|thumb|300px|5a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|5b]]
|-
|}
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_fit.png|left|thumb|300px|5c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|5d]]
|-
|}
*Color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
*Projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 7 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 8. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 7a shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:cutrate_raw.png|left|thumb|300px|Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
|-
|}

The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Focused Beam Profile]]
|-
|}
*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:g_r1X.png|left|thumb|300px|X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Y-axis Projection of Focused Beam]]
|-
|}

*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
|-
|}
*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|300px| CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 11 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 12a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 12b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:deltay_bad.png|left|thumb|300px|Figure: The histogram shown in Figure 11a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure: R The histogram in Figure 11b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:22:16Z

Bpratt18: /* Laser Beamline */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 2: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 1. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 3 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:ablation_full.png|left|thumb|300px|Rendering of ablation beamline]]

Figure 1 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process.

[[Image:spherabs.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through a single focusing lens.]] [[Image:goodfocus.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through the three lens system.]]

Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.

The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 4. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_map.png|left|thumb|300px|5a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|5b]]
|-
|}
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_fit.png|left|thumb|300px|5c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|5d]]
|-
|}
*Color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
*Projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 7 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 8. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 7a shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:cutrate_raw.png|left|thumb|300px|Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
|-
|}

The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Focused Beam Profile]]
|-
|}
*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:g_r1X.png|left|thumb|300px|X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Y-axis Projection of Focused Beam]]
|-
|}

*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
|-
|}
*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|300px| CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 11 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 12a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 12b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:deltay_bad.png|left|thumb|300px|Figure: The histogram shown in Figure 11a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure: R The histogram in Figure 11b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:22:03Z

Bpratt18: /* Gas Purification System */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 2: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 1. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 3 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:ablation_full.png|left|thumb|300px|Rendering of ablation beamline]]

Figure 1 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process.

[[Image:spherabs.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through a single focusing lens.]] [[Image:goodfocus.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through the three lens system.]]

Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.

The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 4. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_map.png|left|thumb|300px|5a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|5b]]
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|}
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
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| [[Image:xscan_005_fit.png|left|thumb|300px|5c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|5d]]
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|}
*Color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
*Projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 7 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 8. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 7a shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:cutrate_raw.png|left|thumb|300px|Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
|-
|}

The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Focused Beam Profile]]
|-
|}
*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:g_r1X.png|left|thumb|300px|X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Y-axis Projection of Focused Beam]]
|-
|}

*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
|-
|}
*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|300px| CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 11 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 12a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 12b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:deltay_bad.png|left|thumb|300px|Figure: The histogram shown in Figure 11a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure: R The histogram in Figure 11b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:14:40Z

Bpratt18: /* Laser Beamline */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 2: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 1. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 3 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:ablation_full.png|left|thumb|300px|Rendering of ablation beamline]]

Figure 1 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process.

[[Image:spherabs.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through a single focusing lens.]] [[Image:goodfocus.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through the three lens system.]]

Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.

The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 4. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_map.png|left|thumb|300px|5a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|5b]]
|-
|}
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_fit.png|left|thumb|300px|5c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|5d]]
|-
|}
*Color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
*Projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 7 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 8. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 7a shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:cutrate_raw.png|left|thumb|300px|Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
|-
|}

The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Focused Beam Profile]]
|-
|}
*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:g_r1X.png|left|thumb|300px|X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Y-axis Projection of Focused Beam]]
|-
|}

*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
|-
|}
*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|300px| CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 11 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 12a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 12b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:deltay_bad.png|left|thumb|300px|Figure: The histogram shown in Figure 11a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure: R The histogram in Figure 11b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:14:20Z

Bpratt18: /* Laser Beamline */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 2: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 1. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 3 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:ablation_full.png|left|thumb|300px|Rendering of ablation beamline]]

Figure 1 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process.

[[Image:spherabs.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through a single focusing lens.]] [[Image:goodfocus.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through the three lens system.]]

Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.

The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 4. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_map.png|left|thumb|300px|5a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|5b]]
|-
|}
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_fit.png|left|thumb|300px|5c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|5d]]
|-
|}
*Color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
*Projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 7 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 8. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 7a shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:cutrate_raw.png|left|thumb|300px|Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
|-
|}

The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Focused Beam Profile]]
|-
|}
*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:g_r1X.png|left|thumb|300px|X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Y-axis Projection of Focused Beam]]
|-
|}

*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
|-
|}
*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|300px| CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 11 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 12a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 12b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:deltay_bad.png|left|thumb|300px|Figure: The histogram shown in Figure 11a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure: R The histogram in Figure 11b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:13:26Z

Bpratt18: /* Focal Spot Characterization */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 2: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 1. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 3 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:ablation_full.png|left|thumb|300px|Rendering of ablation beamline]]

Figure 1 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process.

[[Image:spherabs.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through a single focusing lens.]] [[Image:goodfocus.png|left|thumb|200px|Zygo image of pulses made on diamond after passing through the three lens system.]]

Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.

The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 4. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_map.png|left|thumb|300px|5a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|5b]]
|-
|}
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_fit.png|left|thumb|300px|5c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|5d]]
|-
|}
*Color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
*Projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 7 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 8. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 7a shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:cutrate_raw.png|left|thumb|300px|Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
|-
|}

The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Focused Beam Profile]]
|-
|}
*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:g_r1X.png|left|thumb|300px|X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Y-axis Projection of Focused Beam]]
|-
|}

*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
|-
|}
*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|300px| CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 11 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 12a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 12b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:deltay_bad.png|left|thumb|300px|Figure: The histogram shown in Figure 11a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure: R The histogram in Figure 11b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:13:15Z

Bpratt18: /* Laser Beamline */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 2: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 1. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 3 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:ablation_full.png|left|thumb|300px|Rendering of ablation beamline]]

Figure 1 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process.

[[Image:spherabs.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through a single focusing lens.]] [[Image:goodfocus.png|left|thumb|200px|Zygo image of pulses made on diamond after passing through the three lens system.]]

Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.

The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 4. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_map.png|left|thumb|300px|5a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|5b]]
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|}
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
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| [[Image:xscan_005_fit.png|left|thumb|300px|5c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|5d]]
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|}
*Color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
*Projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 7 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 8. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 7a shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:cutrate_raw.png|left|thumb|300px|Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
|-
|}

The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Focused Beam Profile]]
|-
|}
*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:g_r1X.png|left|thumb|300px|X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Y-axis Projection of Focused Beam]]
|-
|}

*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
|-
|}
*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|300px| CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 11 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 12a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 12b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:deltay_bad.png|left|thumb|300px|Figure: The histogram shown in Figure 11a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure: R The histogram in Figure 11b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:10:50Z

Bpratt18: /* Laser Beamline */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 2: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 1. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 3 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:ablation_full.png|left|thumb|300px|Rendering of ablation beamline]]

Figure 1 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process.

[[Image:spherabs.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through a single focusing lens.]] [[Image:goodfocus.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through the three lens system.]]

Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.

The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 4. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_map.png|left|thumb|300px|5a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|5b]]
|-
|}
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_fit.png|left|thumb|300px|5c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|5d]]
|-
|}
*Color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
*Projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 7 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 8. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 7a shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:cutrate_raw.png|left|thumb|300px|Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
|-
|}

The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Focused Beam Profile]]
|-
|}
*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:g_r1X.png|left|thumb|300px|X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Y-axis Projection of Focused Beam]]
|-
|}

*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
|-
|}
*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|300px| CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 11 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 12a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 12b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:deltay_bad.png|left|thumb|300px|Figure: The histogram shown in Figure 11a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure: R The histogram in Figure 11b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:10:28Z

Bpratt18: /* Laser Beamline */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 2: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 1. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 3 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:ablation_full.png|left|thumb|300px|Rendering of ablation beamline]]

Figure 1 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process.

[[Image:spherabs.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through a single focusing lens.]] [[Image:Image:goodfocus.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through the three lens system.]]

Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.

The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 4. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_map.png|left|thumb|300px|5a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|5b]]
|-
|}
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_fit.png|left|thumb|300px|5c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|5d]]
|-
|}
*Color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
*Projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 7 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 8. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 7a shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:cutrate_raw.png|left|thumb|300px|Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
|-
|}

The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Focused Beam Profile]]
|-
|}
*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:g_r1X.png|left|thumb|300px|X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Y-axis Projection of Focused Beam]]
|-
|}

*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
|-
|}
*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|300px| CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 11 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 12a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 12b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:deltay_bad.png|left|thumb|300px|Figure: The histogram shown in Figure 11a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure: R The histogram in Figure 11b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:07:42Z

Bpratt18: /* Laser Beamline */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 2: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 1. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 3 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:ablation_full.png|left|thumb|300px|Rendering of ablation beamline]]
Figure 1 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:spherabs.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through a single focusing lens.]]] ||   || [[Image:Image:goodfocus.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through the three lens system.]]
|-
|}
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.

The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 4. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_map.png|left|thumb|300px|5a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|5b]]
|-
|}
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_fit.png|left|thumb|300px|5c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|5d]]
|-
|}
*Color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
*Projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 7 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 8. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 7a shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:cutrate_raw.png|left|thumb|300px|Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
|-
|}

The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Focused Beam Profile]]
|-
|}
*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:g_r1X.png|left|thumb|300px|X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Y-axis Projection of Focused Beam]]
|-
|}

*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
|-
|}
*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|300px| CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 11 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 12a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 12b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:deltay_bad.png|left|thumb|300px|Figure: The histogram shown in Figure 11a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure: R The histogram in Figure 11b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:05:54Z

Bpratt18: /* Focal Spot Characterization */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 2: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 1. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 3 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:ablation_full.png|left|thumb|300px|Rendering of ablation beamline]]
Figure 1 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process.
[[Image:spherabs.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through a single focusing lens.]]
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.[[Image:goodfocus.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through the three lens system.]]
The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 4. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_map.png|left|thumb|300px|5a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|5b]]
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|}
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
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| [[Image:xscan_005_fit.png|left|thumb|300px|5c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|5d]]
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*Color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
*Projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 7 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 8. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 7a shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:cutrate_raw.png|left|thumb|300px|Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
|-
|}

The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
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| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Focused Beam Profile]]
|-
|}
*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:g_r1X.png|left|thumb|300px|X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Y-axis Projection of Focused Beam]]
|-
|}

*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
|-
|}
*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|300px| CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 11 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 12a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 12b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:deltay_bad.png|left|thumb|300px|Figure: The histogram shown in Figure 11a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure: R The histogram in Figure 11b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:05:38Z

Bpratt18: /* Laser Beamline */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 2: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 1. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 3 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:ablation_full.png|left|thumb|300px|Rendering of ablation beamline]]
Figure 1 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process.
[[Image:spherabs.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through a single focusing lens.]]
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.[[Image:goodfocus.png|right|thumb|200px|Zygo image of pulses made on diamond after passing through the three lens system.]]
The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 4. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_map.png|left|thumb|300px|5a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|5b]]
|-
|}
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_fit.png|left|thumb|300px|5c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|5d]]
|-
|}
*Color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
*Projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 7 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 8. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 7a shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:cutrate_raw.png|left|thumb|300px|Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
|-
|}

The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Focused Beam Profile]]
|-
|}
*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:g_r1X.png|left|thumb|300px|X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Y-axis Projection of Focused Beam]]
|-
|}

*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
|-
|}
*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|300px| CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 11 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 12a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 12b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:deltay_bad.png|left|thumb|300px|Figure: The histogram shown in Figure 11a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure: R The histogram in Figure 11b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:05:14Z

Bpratt18: /* Laser Beamline */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 2: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 1. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 3 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:ablation_full.png|right|thumb|300px|Rendering of ablation beamline]]
Figure 1 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process.
[[Image:spherabs.png|left|thumb|200px|Zygo image of pulses made on diamond after passing through a single focusing lens.]]
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.[[Image:goodfocus.png|left|thumb|200px|Zygo image of pulses made on diamond after passing through the three lens system.]]
The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 4. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_map.png|left|thumb|300px|5a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|5b]]
|-
|}
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_fit.png|left|thumb|300px|5c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|5d]]
|-
|}
*Color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
*Projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 7 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 8. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 7a shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:cutrate_raw.png|left|thumb|300px|Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
|-
|}

The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Focused Beam Profile]]
|-
|}
*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:g_r1X.png|left|thumb|300px|X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Y-axis Projection of Focused Beam]]
|-
|}

*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
|-
|}
*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|300px| CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 11 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 12a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 12b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:deltay_bad.png|left|thumb|300px|Figure: The histogram shown in Figure 11a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure: R The histogram in Figure 11b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:04:57Z

Bpratt18: /* Focal Spot Characterization */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 2: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 1. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 3 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:ablation_full.png|right|thumb|500px|Rendering of ablation beamline]]
Figure 1 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process.
[[Image:spherabs.png|left|thumb|200px|Zygo image of pulses made on diamond after passing through a single focusing lens.]]
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.[[Image:goodfocus.png|left|thumb|200px|Zygo image of pulses made on diamond after passing through the three lens system.]]
The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 4. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_map.png|left|thumb|300px|5a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|5b]]
|-
|}
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_fit.png|left|thumb|300px|5c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|5d]]
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|}
*Color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
*Projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 7 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 8. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 7a shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:cutrate_raw.png|left|thumb|300px|Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
|-
|}

The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Focused Beam Profile]]
|-
|}
*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:g_r1X.png|left|thumb|300px|X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Y-axis Projection of Focused Beam]]
|-
|}

*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
|-
|}
*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|300px| CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 11 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 12a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 12b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:deltay_bad.png|left|thumb|300px|Figure: The histogram shown in Figure 11a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure: R The histogram in Figure 11b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:04:41Z

Bpratt18: /* Focal Spot Characterization */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 2: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 1. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 3 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:ablation_full.png|right|thumb|500px|Rendering of ablation beamline]]
Figure 1 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process.
[[Image:spherabs.png|left|thumb|200px|Zygo image of pulses made on diamond after passing through a single focusing lens.]]
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.[[Image:goodfocus.png|left|thumb|200px|Zygo image of pulses made on diamond after passing through the three lens system.]]
The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 4. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_map.png|left|thumb|300px|5a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|5b]]
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|}
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
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| [[Image:xscan_005_fit.png|left|thumb|300px|5c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|5d]]
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*Color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
*Projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 7 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 8. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 7a shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:cutrate_raw.png|left|thumb|300px|Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
|-
|}

The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
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| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Focused Beam Profile]]
|-
|}
*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:g_r1X.png|left|thumb|300px|X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Y-axis Projection of Focused Beam]]
|-
|}

*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
|-
|}
*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|300px| CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 11 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 12a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 12b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:deltay_bad.png|left|thumb|300px|Figure: The histogram shown in Figure 11a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure: R The histogram in Figure 11b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:04:26Z

Bpratt18: /* Laser Beamline */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 2: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 1. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 3 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:ablation_full.png|right|thumb|500px|Rendering of ablation beamline]]
Figure 1 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process.
[[Image:spherabs.png|left|thumb|200px|Zygo image of pulses made on diamond after passing through a single focusing lens.]]
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.[[Image:goodfocus.png|left|thumb|200px|Zygo image of pulses made on diamond after passing through the three lens system.]]
The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 4. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_map.png|left|thumb|300px|5a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|5b]]
|-
|}
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_fit.png|left|thumb|300px|5c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|5d]]
|-
|}
*Color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
*Projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 7 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 8. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 7a shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:cutrate_raw.png|left|thumb|300px|Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
|-
|}

The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Focused Beam Profile]]
|-
|}
*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:g_r1X.png|left|thumb|300px|X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Y-axis Projection of Focused Beam]]
|-
|}

*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
|-
|}
*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|300px| CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 11 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 12a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 12b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:deltay_bad.png|left|thumb|300px|Figure: The histogram shown in Figure 11a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure: R The histogram in Figure 11b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:04:01Z

Bpratt18: /* Laser Beamline */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 2: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 1. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 3 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:ablation_full.png|right|thumb|500px|Rendering of ablation beamline]]
Figure 1 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process.
[[Image:spherabs.png|left|thumb|300px|Zygo image of pulses made on diamond after passing through a single focusing lens.]]
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.[[Image:goodfocus.png|left|thumb|300px|Zygo image of pulses made on diamond after passing through the three lens system.]]
The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 4. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_map.png|left|thumb|300px|5a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|5b]]
|-
|}
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_fit.png|left|thumb|300px|5c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|5d]]
|-
|}
*Color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
*Projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 7 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 8. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 7a shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:cutrate_raw.png|left|thumb|300px|Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
|-
|}

The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Focused Beam Profile]]
|-
|}
*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:g_r1X.png|left|thumb|300px|X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Y-axis Projection of Focused Beam]]
|-
|}

*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
|-
|}
*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|300px| CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 11 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 12a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 12b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:deltay_bad.png|left|thumb|300px|Figure: The histogram shown in Figure 11a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure: R The histogram in Figure 11b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:02:05Z

Bpratt18: /* Laser Beamline */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 2: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 1. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 3 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:ablation_full.png|center|thumb|500px|Rendering of ablation beamline]]
Figure 1 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process.
[[Image:spherabs.png|center|thumb|300px|Zygo image of pulses made on diamond after passing through a single focusing lens.]]
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.[[Image:goodfocus.png|center|thumb|300px|Zygo image of pulses made on diamond after passing through the three lens system.]]
The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 4. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_map.png|left|thumb|300px|5a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|5b]]
|-
|}
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:xscan_005_fit.png|left|thumb|300px|5c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|5d]]
|-
|}
*Color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
*Projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 7 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 8. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 7a shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:cutrate_raw.png|left|thumb|300px|Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
|-
|}

The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Focused Beam Profile]]
|-
|}
*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:g_r1X.png|left|thumb|300px|X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Y-axis Projection of Focused Beam]]
|-
|}

*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
|-
|}
*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|300px| CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 11 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 12a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 12b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
|-
| [[Image:deltay_bad.png|left|thumb|300px|Figure: The histogram shown in Figure 11a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure: R The histogram in Figure 11b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
|-
|}

==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']

Diamond Radiator Thinning Using an Excimer Laser

2016-12-15T22:01:37Z

Bpratt18: /* Gas Purification System */

<table align="right" cellpadding="3">
<tr><td><b>Important Documents</b></td></tr>
<tr><td bgcolor="#e0e0f0">[[media:LaserSafetyManual.pdf|UConn Laser Safety Manual]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:S.O.P.pdf|Excimer laser S.O.P.]]</td></tr>
<tr><td bgcolor="#e0e0f0">[[media:GP2000.pdf|Oxford GP 2000 User Manual]]</td></tr>
</table>

==Laser Thinning of Diamond==

[[Image:expsetup.png|right|thumb|400px|Figure 1: Illustration of the experimental setup used to ablate diamond using a UV excimer laser at the University of Connecticut.]]
A research group at Brookhaven National Laboratory ([https://zeus.phys.uconn.edu/halld/diamonds/BNLdev-1-2009/smedley-1-2009_2.pdf BNL]) published results using a focused high-power UV laser to mill diamond via a process known as ablation. Lasers with wavelengths above the band gap of diamond (213 nm) can excite electrons between the diamonds carbon atoms from a bound state to an ionized state. The top 100 nm of the diamond surface at the focal spot is instantly vaporized, emitting a plume of carbon-based plasma normal to the surface of the diamond crystal. By scanning the diamond across the focal spot, a sequence of overlapping spots is built up that results in uniformly milling the sample surface. The short duration (10 ns) and short absorption length (50 nm) of the laser pulse ensure that the pulse energy is absorbed in the upper 100 nm of the diamond and does not result in deep energy deposition in the bulk of the crystal. Most importantly, this technique allows the diamond to be thinned deferentially, creating a central window of 20 microns thickness while retaining the full thickness around the edges for stiffness and support. This would never be possible with an abrasive technique. The laser ablation technique on diamond was developed extensively here at UConn by the author, using a Lambda Physik EMG 101 excimer laser operating at a wavelength of 193 nm as the light source. An XYZ computer controlled stage was constructed to sweep the diamond (under a vacuum of 600 mtorr) across the laser focus. The bulk diamond crystal before machining is typically of size 7.2 x 7.2 x 0.250 mm^3 . A series of laser pulses was applied to a rectangular region in the center of the diamond, milling a thin window down to the required thickness and leaving untouched a zone around the perimeter which is called the frame. The components of the experimental setup shown in Figure 1 are discussed in detail in the sections below.

==Excimer Laser==
A Lambda Physik EMG 101 argon fluorine (ArF) excimer laser (shown in Figure 2) with an operating wavelength of 193 nm was used to ablate single-crystal CVD diamond. The laser is capable of delivering 120 mJ at a maximum repetition rate of 50 Hz and pulse duration of 20 nanoseconds. The pulse geometry is an asymmetrical flat top Gaussian with horizontal dimensions of 22 mm and vertical dimensions of 6 mm.
[[Image:Laser setup.jpg|center|300px|Figure 2: Image of Lambda Physik EMG 101 MSC excimer laser with cover removed.]]

This particular laser was over 30 years old when we first acquired it for the project. Much of my time (maybe too much :) ) has been spent restoring it so that it could maintain high energy levels for extended periods of time. All of my troubles are explicitly detailed in my logbooks which can be shared on request.

== Gas Purification System ==
[[Image:laser_cavity.png|right|thumb|300px| Figure 2: Illustration depicting the internals of a typical excimer laser.]]
Excimer lasers produce UV light via the spontaneous/stimulated emission of an excited complex comprised of a halogen, noble and buffer (fluorine, argon and helium respectively). These pseudo-molecules are created inside the laser cavity by large electric fields and live for only a short period of time before photo-emission occurs. Afterwards, the halogen and noble gases undergo a refractory period during which they cannot be re-excited. The internal mechanics of the Lambda Physik EMG 101 excimer laser provides a continuous flow of fresh lasing medium into the laser cavity via a circulating fan. Figure 3 depicts the cross section of a typical excimer laser and illustrates the path of the laser gas medium.
As shown, after the laser gas undergoes emission it passes over a series of heat exchangers. At repetition rates greater than 3 Hz a cooling unit must be run which passes 30◦ C de-ionized water through these heat exchangers. Once the hot gas is cooled it passes through particulate filters and is sent back into the laser tube to begin the cycle again. As this process continues the halogen component of the lasing medium reacts with the non-passivated metal released by the discharge of the laser cavity’s pre-ionization pins and other contaminants within the laser cavity. This contamination of the halogen gas reduces the average pulse energy of the laser until no lasing occurs and the 4 gas mixture must be completely replaced. For the purposes of laser ablating diamond, the laser gas medium is replaced when the average pulse energy is less than 50 mJ. Lambda Physik specifies this model laser can fire 400,000 pulses before the specified power reaches 50%. Assuming the laser starts at a maximum power of 120 mJ the laser would produce 480,000 pulses before it reached the minimum pulse energy of 50 mJ and had to be refilled. A 7.2 x 7.2 x 0.25 mm^3 diamond thinned with a central region of dimensions 6.7 x 6.7 x 0.02 mm^3 would require approximately 450,000 pulses per single complete pass over the diamond (assuming 0.5 mm s motor speed in x direction, 50 Hz laser repetition rate, and 0.01 mm motor step in y axis).
[[Image:GP2000b.png|left|thumb|250px| Figure: Average laser output as a function of total shots fired.]]
An average cut depth of 38µm per complete pass would estimate a total of over 2.6 million pulses to reach the final depth of 20 µm or roughly 7 complete laser medium fills. The ablation setup has methods to compensate for fluctuations in average laser energy so that the diamond surface remains smooth to within ± 0.5µm (these methods will be discussed in detail in a later section). However, even with these corrections, allowing the average laser energy to vary by 50% over the course of a single pass results in non-uniform ablation across the diamond which is too exaggerated to compensate for. It is then desirable to extend the lifetime of the laser gas medium so that average power remains constant over a single pass. Ideally, the laser would have a gas life time which exceeds the total number of pulses required to bring the diamond sample to 20µm. An Oxford GP-2000 cryogenic gas purification system and Millipore particulate filter were installed in a closed loop with the laser cavity as shown in Figure 1. The system pumps the laser gas medium through a liquid nitrogen cold trap removing contaminants generated during the lasing process, extending the laser gas life time by over an order of magnitude. The plot below shows the average pulse energy as a function of pulses completed. Figure 3 shows the comparison between running the laser with (blue) and without (red) the gas purification system. Using the gas purifier in line with the laser cavity resulted in an order of magnitude increase in number of total pulses fired. Also, the average output energy of the laser increased significantly due to filtration of halogen spoiling contaminants inside the laser cavity. In some cases only a single fill was required to ablate a diamond from start to finish-greatly reducing the surface variation on the diamond radiator and the cost of running the machine. It is conclusive to say that without the use of the gas purification system this laser would not be viable for use as a light source for diamond ablation purposes.

==Laser Beamline==
[[Image:ablation_full.png|center|thumb|500px|Rendering of ablation beamline]]
Figure 1 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process.
[[Image:spherabs.png|center|thumb|300px|Zygo image of pulses made on diamond after passing through a single focusing lens.]]
Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX.[[Image:goodfocus.png|center|thumb|300px|Zygo image of pulses made on diamond after passing through the three lens system.]]
The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.
The laser beam then passes through a series of lenses as shown in Figure 4. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.

==Focal Spot Characterization==

The focal spot was studied using a gold-tungsten harp scanner in both the x and y plane. Each harp scan was comprised of an aluminum mount machined at UConn and then anodized to insulate it from the 50 micron gold-tungsten wire that was stretched across it as can be seen in the CAD drawing below.
[[Image:2wire.png|center|thumb|300px|CAD drawing of harp scan mount]]

The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2mm and the z coordinate range is 12mm. Each pass has a single value of z, and sweeps over the full 2mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place

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| [[Image:xscan_005_map.png|left|thumb|300px|5a]] ||   || [[Image:yscan_003_map.png|right|thumb|300px|5b]]
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| [[Image:xscan_005_fit.png|left|thumb|300px|5c]] ||   || [[Image:yscan_003_fit.png|right|thumb|300px|5d]]
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*Color maps of the two orthoganol scans of beam focal region, where the color represents the charge per pulse seen on the wire in arbitrary units.
*Projections of the color maps shown in Figure 6 onto the transverse axis with Gaussian fits to central peak over a flat background.]]

Each pixel in the plots shown in Figures 7 represents one laser pulse, with the color representing the pulse height integral. The lower-most row should be ignored because the scanning program was not yet fully synchronized to the raster pattern. The widths of the focal spot in x and y are shown in the RMS values of the fits in Figure 8. The values in x and y are roughly the same, 65 µm vs 48 µm respectively. Figure 7a shows a maximum intensity at a z position of 4.8mm, however it is interesting to note that the shape of the focal spot does not appear to change drastically away from this point. The optical setup has a narrow focal spot with a wide depth of field which is ideal for the purpose of laser ablation. From this study it was also concluded that the use of a collimator at the focal point of L1 and L2 greatly reduced the background seen by the harp scan and should be used during the ablation process to protect the diamond surface away from the ablation point

==Ablation Rate==
The ablation rate of diamond using 193nm light was measured under a vacuum of 650 mtorr. The laser power was gradually decreased as each row was completed so that the complete operation range could be studied. The ablated surface was then measured using a Zygo white-light interferometer and the cut depth values for each row were extracted. These values were used in the model shown below to calculate the expected cut depth of a row as a function of the average laser energy. The figures below show the Zygo image of the ablated surface and the modeled cut depth.

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| [[Image:cutrate_raw.png|left|thumb|300px|Zygo image of 7mmx7mm diamond cut using laser operating 193nm with varying output energy]] ||   ||

[[Image:cutrate_fit.png|right|thumb|300px|Calculated ablation rate in diamond as a function of laser energy fit with a second order polynomial]]
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The histogram above was fitted with a second order polynomial and used to correct for fluctuations in the laser energy from row to row. Stacking laser pulses on top of each other increases the amount of diamond material removed within the region of overlap. This method was used to account for laser energy fluctuations while deferentially ablating diamond to within ±0.5µm surface variation.

==FORTRAN Simulations of Beam Spot==
*A FORTRAN program has been written which simulates rays exiting the laser aperture and then propagating through a fused silica plano-convex lens. Using this program we can now observe the geometry of the beam as it passes through the focusing lens onto a target. We have seen that the beam leaving the laser aperture has a flat top distribution in the X plane and a Gaussian distribution in the Y. As the beam is focused both the X and Y projections achieve Gaussian distributions.
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| [[Image:r0(2)r0(1).jpg|left|thumb|300px|Original Beam Profile]] ||   || [[Image:r2.png|right|thumb|300px|Focused Beam Profile]]
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*Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain <math>\sigma_{X} = 0.63mm\,</math> and <math>\sigma_{Y} = 0.23mm \,</math>.
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| [[Image:g_r1X.png|left|thumb|300px|X-axis Projection of Focused Beam]] ||   || [[Image:g_r1Y.png|right|thumb|300px|Y-axis Projection of Focused Beam]]
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*Assuming a Gaussian distribution at the waist of the beam, we now find the FWHM (full width at half maximum) by the following relation,

:<math>\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. </math>
*The smallest values of <math>\mathrm{FWHM_{X}}</math> and <math>\mathrm{FWHM_{Y}}</math> were 1.49mm and 0.552mm respectively.
*The Rayleigh Length, <math>\mathrm{Z_{R}}</math> is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled (<math>\mathrm\omega_{R}</math>). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking <math>\mathrm\omega_{0}</math> as the beam waist, and using the <math>\mathrm{FWHM}</math> as its value we are looking for the point where,
:<math>\omega_{R} = \sqrt{2}\ \omega_{0}. </math>
[[Image:rayleigh.png|center|thumb|400px|Describes the Rayleigh Length of a beam waist <math>\omega_{0}</math>.]]
*Plotting <math>\mathrm\omega_{R}</math> as a function of distance away from the beam waist center, we find an average Rayleigh Length,
:<math>\mathrm{Z_{RX}} =11.8mm</math> and <math>\mathrm{Z_{RY}} =10.5mm</math>
{| cellpadding="3" style="text-align:center; margin: 1em auto 1em auto"
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| [[Image:x_beam.png|left|thumb|300px|Waist of Beam through X-axis]] ||   ||

[[Image:y_beam.png|right|thumb|300px||Waist of Beam through Y-axis]]
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*Knowing <math>\mathrm\omega_{0}</math> also allows us to calculate the theoretical fluence of the beam. Assuming maximum power of 220mJ over a 1.49mm x 0.552mm area yields <math>26J/cm^2.</math> Which is above the <math>14J/cm^2</math> threshold value cited by Brookhaven National Laboratories who were conducting diamond ablation experiments with a 213nm Nd:YAG laser (213nm with the use of a 4 + 1 frequency mixing crystal). Our ArF excimer laser produces 193nm light that will be more readily absorbed by the surface of the diamond as diamond is opaque to wavelengths above the band gap. These calculations provide a level of confidence that we theoretically will be able to ablate diamond.

==Ablation Chamber==
An aluminum vacuum chamber was machined at UConn to house the diamond during the ablation process as shown in the figure below.
[[Image:ablationchamber.png|center|300px| CAD drawing of ablation chamber]]
A roughing pumps was attached to one of the ports on the ablation chamber while a second port was connected to a digital flow controller and needle valve. This enabled the user to maintain a constant pressure to within 1 mtorr over the course of an ablation run. Vacuum pressures of a few tens of mtorr were achievable using this setup, however were not typically desired due to the large amounts of amorphous carbon build up after ablation occurred.
[[Image:iso1.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]

The diamond sits at a 45◦ angle incident to the incoming laser pulse to prevent amorphous carbon from covering the entrance window. The setup is illustrated in the figure above.

==Sub-Micron Precision Using Dial Indicators==
[[Image:iso2.png|center|thumb|300px|Figure: Rendering of ablation setup showing position of dial indicator used to locate the x-translation stage.]]
As shown in the figure above the ablation chamber is mounted on two orthogonal translation stages, both with bidirectional repeatability of <1.5 µm and minimum achievable incremental movement of 0.05 µm/. The x-stage moves the diamond in the horizontal plane with respect to the lab floor. The y-stage increments at a 45◦ angle to the lab floor which is in the same plane as the diamond. Digital dial indicators with sub-micron resolution were installed to measure the positions of the x and y translation stage and were used in a study to measure the non-linearity of the y-translation stage motor. Non-linearity (movement in the lead-screw of the translation stage which does not place the stage at the requested position) of the y-stage is critical due to the row-by-row rastering sequence used to deferentially ablate the diamond sample. Each row has a unique sequence of laser pulses that correspond to an exact position on the diamond and the control of the ablation rate relies on the overlap between these rows. The dial indicator shown in Figure 11 is placed at a 45◦ angle and makes contact with an aluminum extension mounted to the y-stage. The dial indicator has a rolling bearing attached to its end so that it rides along the extension as the x-translation stage moves back and forth. The ablation chamber was moved to a series of y coordinates and the difference between the desired displacement and the displacement measured by the dial indicator was taken and is shown in Figure 12a.
The same study was conducted again, but the dial indicator was used to 17 require that the y-translation stage fall within 1 µm of the desired position. This was accomplished by creating an internal loop within the LabView software responsible for the movement of the translation stages. At the beginning of the sequence, the y-stage is homed and brought to an origin position. The reading of the dial indicator at this origin is recorded and all subsequent moves in the y-stage coordinate system are translated into the dial indicator coordinate system using this value. After the y-stage moves to the provided coordinate, the LabView software queries the dial indicator for a position. If the dial indicator value matches the expected value to within a micron, the sequence continues, if not the y-stage is moved by the dial indicator difference in position and the dial indicator is queried again until the 1 micron condition is satisfied. Figure 12b illustrates the improvement made to the y-stage which now has the accuracy of 1 micron. The study concluded that position of the diamond relative to the focal spot would be determined by the sub-micron dial indicators in both the x and y axis.
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| [[Image:deltay_bad.png|left|thumb|300px|Figure: The histogram shown in Figure 11a displays the difference between the position reported by the y-stage and the position measured by the dial indicator. The wide RMS suggests a large non-linearity in the motor.]] ||   ||

[[Image:deltay_good.png|right|thumb|300px|Figure: R The histogram in Figure 11b shows the same difference after the dial indicator was used to correct for the non-linearity in the y-stage.]]
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==Ablation Software==
Extensive software was written at UConn which converts surface measurements (taken with the Zygo) into a series of laser pulses and motor coordinates. The software uses a model of the beam spot and the Convolution Theorem to solve for the number and placement of laser pulses on the diamond so that it matches a end result specified by the user. The software used to create these "seq" files are listed below.

*[http://zeus.phys.uconn.edu/~pratt/software/ablator.C '''ablator.C: Core software used in creating ablation raster files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablator.h '''ablator.h: Header file for ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.cc '''Map2D.cc: Package required to run ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/Map2D.h '''Map2D.h: Header file for Map2D.''']
*[http://zeus.phys.uconn.edu/~pratt/software/ablate.py '''ablate.py: Python module with custom methods for processing Zygo images for use in ablator.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zygo2root.cc '''zygo2root.C: Software for converting hitched Zygo data files into root files.''']
*[http://zeus.phys.uconn.edu/~pratt/software/zfinder.cc '''zfinder.cc: Package needed for zygo2root''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.cc '''MetroProMap.cc: Package needed to run Zygo2root software.''']
*[http://zeus.phys.uconn.edu/~pratt/software/MetroProMap.h '''MetroProMap.h : Header file for MetroProMap.''']
*[http://zeus.phys.uconn.edu/~pratt/software/setenv.sh '''setenv.sh: sample of environment variables required to run above software.''']