Diamond Radiator Thinning Using an Excimer Laser

Laser Thinning of Diamond
Laser ablation will be used to thin the diamond chip to the precise thickness required for the radiator. An older excimer laser offered by a local AMO group will be converted from a XeCl 308 nm beam to the required ArF 193 nm beam. Although the system was last used 10 years ago, it was left in a fully functioning state and was properly flushed upon decomposition. There may also be another unused excimer laser that can be used for parts in case any repair is needed. I have included a link to my current Lab Journal which I plan to update bi weekly.

Work Schedule
The following is a list of tasks required to get the Ar-F excimer laser online. Long-term operating goals:
 * First, take the proper safety course required by the University of Connecticut Environmental Health and Safety department.
 * Prepare Dr. Well's laboratory for the transfer of the Excimer laser including setup of ventilation, three-phase (208V) power supply, cold water distribution and collection, and a sturdy table to put the laser on.
 * Replacement of Halogen Filters
 * Tend to the vacuum system: replace oil in diffusion pump, test for serious leaks at moderately low pressure (10−6 Torr)
 * Test the RF generator (most expensive/hard to replace!)
 * Procure a bottle of pure Ar gas
 * Check internal circulating fan bearings, as these are a possibly source of corrosion
 * Flush the tube, turn it on and test output power
 * Barring major repair: expected online by end of 2009 with $2-3k
 * Switch optics to 192nm
 * Get mixture of Ar-F, eventually procure a gas mixing system)
 * Ensure reliable pulse train and consistent power output (±5%)

Updates for November/December 2009

 * Original documentation of the Lambda Physik EMG 101 MSC Excimer laser has been scanned and posted on the following page,EMG 101 Documentation
 * Preparation for moving the Excimer laser has begun.
 * Initial diagnostics on the laser's state have also begun, however the real testing begins once we are able to take the laser head cover off.
 * After having a conversation with Jeff Edberg from Coherent Inc. it was recommended that we check a circulating fan inside the laser head. If the fan doesn't turn then it may lead to the end of the project as parts are extremely hard to find. Fortunately, after removing the laser head cover, the fan spun freely without restriction.
 * The following photos are of the inside of the Lambda Physik EMG 101 MSC Excimer Laser.
 * The next step is to transfer the laser down to Dr. Barry Wells' laboratory.
 * Once the power supply, main exhaust, water supply, and gases are installed properly, we will pressurize the system with helium and check for leaks as well as test the HV power supply.

Updates for January 2010

 * All safety classes and eye exams have been taken, the [[Media:S.O.P.pdf|S.O.P.]] is nearly ready for filing.
 * The laser has been transferred to Dr. Barry Wells' lab and placed on a reinforced table.
 * The main exhaust and vacuum pump exhaust are installed.
 * The HV power supply is connected to the laser head and hooked up to the mains (208V 3-phase)
 * Gas lines have been installed with new Swagelok fittings.
 * Gas solenoids have been repaired and the system has been charged with fresh Helium to 2600mbar.
 * Currently the system is leaking >25mbar per hour and we plan on replacing o-rings in main laser window to fix this.
 * Optics will be cleaned once we receive our new supplies and the laser must be realigned using a HeNe laser.
 * A Veeco Helium Leak Detector was used to check for leak sites. The front window has the largest source of Helium at the moment and the o-ring will be replaced shortly. Also, a number of the "spark plugs" that discharge into the laser had noticeable leak rates and I hope to service them shortly.

Updates for February 2010

 * New o-rings have been purchased to repair both the leaks in the window mounts and the "spark plugs".
 * The front window has been cracked and will need replacement.
 * A new window has been purchased from Lattice Electro Optics in California. The window has the following specs:
 * Material: CaF2 (calcium fluoride)
 * Dimensions: 36mm diameter, 5mm thickness
 * Scratch-Dig: 20-10
 * Surface Flatness: λ/10
 * Quantity: 2
 * Argon (99.995 % or better) and Fluorine/He (99.9 % or better, mixed in Helium) gas are going to be purchased from Air Gas.
 * I have dismantled the portion of the laser which blocks access to the spark plug array. After replacing all 45 o-rings I will contact Dr. Hines again and check my work with the helium leak detector at operating pressure (2200mbar). The pictures below show the laser with the thyratron, MSC, and capacitor array removed.
 * 44 of 45 spark plugs have been reinstalled with new o-rings. The brass nut on the last plug stripped due to corrosion while trying to extract it. Here is a picture of what it looks like
 * Performed secondary leak check using the Veeco helium leak detector. We were able to actively maintenance the plugs to ensure that each one was "leak free". After filling the system with 2200mbar (operating pressure) the leak rate was determined to be 10mbar/hour. Still two times greater than the excepted leak rate of 5mbar/hour.
 * The last plug is believed to be the main leak source as bubbles can be seen escaping from the site when Snoop leak detector is used. Plans are being made to extract this corroded/stripped/seized nut.
 * After much deliberation, it has been decided that the only way resolve the leak problem is to remove the spark plug array completely. This will allow us to remove the stripped nut.
 * The array has been removed from the lasing cavity and the nut was extricated using an "Easy Out" and a T- wrench. The photos below illustrate the process and capture the elusive inner cavity of the excimer laser.
 * The system has been pressurized to 2200mbar (operating pressure) and will be checked again within 24hrs to obtain the leak rate. If found to be less than 5mbar/hr the system will be reconstructed and tested for light output after the new window is received.

Updates for March 2010

 * After replacing all o-rings within the spark plug array, the leak rate was still at the unacceptable level of 26mbar/hour. Using the helium leak detector, three large leaks were found at the halogen and noble gas solenoid as well as the vacuum pump solenoid.
 * Once the gas solenoids were tied together and the vacuum solenoid was plugged, the leak rate dropped to 0.1mbar/hour.
 * A ballcock valve has been placed inline of the vacuum pump as a cost effective alternative to replacing the vacuum solenoid valve. The order form for said valve can be found here [[media:CCorderForm.pdf|Bürkert]]
 * We have received argon gas from Air Gas and have ordered F2 (5%in He)through Spectra gases.
 * A new halogen filter has been purchased from Coherent Inc. (p/n:261068; $280)
 * Lattice Electro Optics will be shipping new lens during the week of 4/5/2010
 * A fluorine resistant regulator will be purchased by the first week in April.

Updates for April 2010

 * 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.
 * Taking the X and Y projections of the focused beam and fitting them with a Gaussian distribution,we are able to attain $$\sigma_{X} = 0.63mm\,$$ and $$\sigma_{Y} = 0.23mm \,$$.


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


 * $$\mathrm{FWHM} = 2 \sqrt{2 \ln 2}\ \sigma. $$


 * The smallest values of $$\mathrm{FWHM_{X}}$$ and $$\mathrm{FWHM_{Y}}$$ were 1.49mm and 0.552mm respectively.
 * The Rayleigh Length, $$\mathrm{Z_{R}}$$ is defined as the distance from the beam waist along the axis of propagation to the point where its cross section is doubled ($$\mathrm\omega_{R}$$). This value represents the "play" we will have when trying to focus the beam onto the diamond target for ablation. Taking $$\mathrm\omega_{0}$$ as the beam waist, and using the $$\mathrm{FWHM}$$ as its value we are looking for the point where,
 * $$\omega_{R} = \sqrt{2}\ \omega_{0}. $$


 * Plotting $$\mathrm\omega_{R}$$ as a function of distance away from the beam waist center, we find an average Rayleigh Length,
 * $$\mathrm{Z_{RX}} =11.8mm$$ and $$\mathrm{Z_{RY}} =10.5mm$$


 * Knowing $$\mathrm\omega_{0}$$ 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 $$26J/cm^2.$$ Which is above the $$14J/cm^2$$ 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.

Updates for May 2010

 * An optics design has been made for focusing the 200mJ pulses from the excimer laser onto the diamond target. We are using a fused silica lens and a CaF2 entrance window. The entrance window will be adjusted at an angle to reduce the build-up of ablated material within the chamber as well as redirecting a small percentage of the beam for real-time power measurements. The photos below show the rail and the subsequent components.
 * These photos are taken from a TurboCAD rendering of the optics rail that I completed this week. I have also added the 0.5cm x 0.5cm x 100 micron thick diamond at the optimum focal length (17cm) and tilted at a 45 degree angle from the beam axis. The blue cone represents the ablation plume and its dimensions are taken from plume characteristics observed in local thin film ablation laboratories.
 * A beam enclosure has been constructed to shield observers from harmful UV light. It is made from Poly(methyl methacrylate)which filters 193nm light very well as shown by the following (graph).
 * Now that all components have arrived (Fluorine gas and regulator) I have begun testing the laser output. I first experienced no light except what was reflected from the pre-ionization pins. After the system was recharged with a fresh gas mixture, I began to see a light blue beam that was about 12"x6" in size (I believe this to be a fluorescence effect). It wasn't until about the 5th refill that I saw an actual laser beam. Unfortunately, the beam was very spread out, and only lasted a matter of minutes. Believing this to be an issue of passivation, I started refilling and running the laser while timing the duration of actual laser beam. The beam has hit a wall at around 5 minutes, after which it begins to mesh in with the background light until it is indistinguishable. Contacting Coherent Inc. contact Jeff Edberg lead to the proper passivation procedure. However, the red light has been only lasting a maximum of 7.5 minutes.
 * I decided to re-align the optics for the laser system and replace the cracked CaF2 lens with the new one purchased from Lattice Electro Optics. After which I saw an increase in beam intensity and an increase in geometrical sharpness of the beam, but no increase in beam duration.
 * I found an article which stresses that unclean gas handling lines can contain hydrocarbons which react with fluorine gas to produce $$CF_4$$, which can have a negative effect on pulse duration in ArF excimer lasers. Looking at the lines we currently use, it was clear that this may be an issue as they were from Dr. Eyler's original setup used for XeCl.
 * A new line has been installed and in order to remove and moisture from inside the line, I have "baked" it for over and hour at $$110\,^\circ C$$ while under vacuum. After the system cooled to $$60\,^{\circ}{\rm C}$$ I bled the lines and left fresh fluorine in the line for passivation. After 24 hours I will purge the system and give it a fresh fill, then test the laser output.

Updates for May 2010

 * After reinstalling the new 304L steel fluorine gas line, the red light seen in the passivation procedure lasted over 11.5 minutes. However, the laser beam is still on the 5 minute time scale.
 * Using a [[Media:coherent_sensor.pdf|Coherent J45LP-MUV]] pyro-electric energy sensor coupled to an oscilloscope, it is now possible to observe the laser beam quantitatively.
 * A maximum of 140mJ has been seen from the initial pulses reaching a fluence of $$17J/cm^2.$$.
 * It was thought that the gas was being contaminated by the act of recirculation within the laser cavity via the gas processor. To test this, the beam energy was first monitored every ten seconds with a standard fill (150mbar $$F_2$$, 350mbar Ar, 1700mbar He) and ran at 10Hz/26kV. After a new fill, the high voltage was increased from zero to 26kV (and held for 5 seconds) on every ten second mark and then on every one minute mark. If the circulation of the gas is to blame for the decrease in energy, then I expected to see the energy to drop as the original continuously running at 26kV) trial. This is what I saw after the three runs,

Updates for June
The plan is to take the laser completely apart and remove the gas cavity for cleaning. Depending on the amount of contamination present, we will be able to determine if this was a likely cause of poor passivation.
 * Based on the observed beam characteristics and advice from neighboring laser groups, it was decided that the main gas cavity may be dirty therefore preventing passivation of the system. After an exciplex molecule has emitted a photon it returns to its stable state of fluorine and argon, before it can be used for lasing again it requires a rest period. A flow of gas from a circulating fan supplies fresh gas to the lasing window to compensate for this lag time. As the gas is cycled it passes through a series of heat exchangers and a particle filtration system. Each of these are possible sites for corrosion build up which would contaminate the fluorine gas during the cycling process. The figure bellow illustrates the typical gas cavity setup within an excimer laser.
 * After removing the electronics and the lasing tube it was clear that there was a large amount of green/yellow corrosion within the gas cavity. Once we saw this we decided to remove the entire gas assembly and under go a complete cleaning of the gas recirculation system. Below are a series of pictures showing the pieces before and after the cleaning which was done with ethanol, sandpaper, and Kim wipes with a final propanol rinse.

Passivation for ArF
1) Bleed all lines prior to this process.

2) Turn on HV supply (turn key to on position) and allow the thyratron to warm up for at least ten minutes.

3) Evacuate the laser cavity until the fine gauge reads in the red (zero) by turning on the vacuum pump switch located on the laser head

4) Fill the system with 150mbar fluorine and 1850 He making the total pressure 2000mbar.

5) Set the Rep. Rate to 10Hz and turn on the HV supply (push button in)

6) Turn laser on (push button in) and set High Voltage to 20kV.

7) Laser will begin to pulse a red light, allow the laser to fire until the red light's intensity is reduced to about half. For the first few runs it may be necessary to re-fill a few times. The light should last AT LEAST 10 minutes.