Diamond Radiator Thinning Using an Excimer Laser

Important Documents

UConn Laser Safety Manual

Excimer laser S.O.P.

Oxford GP 2000 User Manual

Laser Thinning of Diamond

Current Excimer Laser to be brought online

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.

Excimer Laser

Specs on EMG 101 MSC excimer laser.

Gas Purification System

specs on GP2000

Laser Beamline

laser optics

Ablation Rate

ablation rate

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.

Original Beam Profile

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 $\sigma _{X}=0.63mm\,$ and $\sigma _{Y}=0.23mm\,$.

X-axis Projection of Focused Beam

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,

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

• 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,

$\omega _{R}={\sqrt {2}}\ \omega _{0}.$

Describes the Rayleigh Length of a beam waist $\omega _{0}$.

• 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 $\mathrm {Z_{RY}} =10.5mm$

Waist of Beam through X-axis

Waist of Beam through Y-axis

• 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

• 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 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,

Beam energy w.r.t. time

Beam energy w.r.t. the number of pulses

Updates for June

• 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.

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.

• 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.

Bpratt18 17:33, 28 June 2010 (UTC)