Malcom Hicken and Dr. R. Steven Turley, Physics and Astronomy

Lithography is the process of focusing light to etch a micro-circuitry pattern on silicon chips. In order to get smaller patterns, and thus more condensed computer chips, bright sources of low-wavelength light are needed. The goal of this project was to produce a bright source of 13.5 nm extremeultraviolet radiation. A paper by J. J. Rocca alerted us to the possibility of using doubly-ionized lithium, whose 2s-1s transition gives off 13.5 nm radiation.1 T-butyl-lithium was selected as a compound that has a relatively high vapor pressure at low temperatures, thus avoiding the more elaborate high-temperature experimental set-up required for obtaining adequate vapor pressures from other lithium compounds.

A Marx generator was used to create a high-voltage electron discharge through a capillary. I designed a small stainless steel container to hold the t-butyl-lithium. This container was attached so that t-butyl-lithium vapor could flow from the container into the capillary. Then the Marx generator could be fired to produce a plasma, including the desired doubly-ionized lithium. This in turn would give off the 13.5 nm light.

T-butyl-lithium “sublimes readily at about 70 [degrees Celsius] and 0.1 mm pressure.”2 I spent quite a lot of time in trying to create a suitable vacuum in the capillary. I regreased all the o-rings (rubber rings that are placed between metal fittings so as to create a tight seal) and attached a roughing pump and a turbo pump to evacuate the capillary. Pressure in the capillary was measured by using a coldcathode gauge. I found several large leaks in the tubing connections by applying alcohol around the seal and noticing a sudden increase in pressure inside the capillary. By tightening these seals I was able to eliminate the large leaks. By leaving the roughing and turbo pumps running overnight I was able to achieve pressures as low as 6 x 10-5 T. However, there were still small leaks present. When I shut the valves that connected the pumps the pressure would then rise quickly.

I received the t-butyl-lithium dissolved in pentane. David Allred and Jim Thorne helped me to load it into the container and then pump of the pentane, leaving only t-butyl lithium. In order to avoid an explosion and toxic fumes this had to be down in a glove-bag in an inert nitrogen atmosphere. We measured the mass of t-butyl-lithium-pentane compound that we put in the container. We calculated the mass of t-butyl-lithium that was in the container so as to know by measuring the mass of the container and its contents when the pentane had been pumped off, leaving only t-butyl-lithium.

The light from the capillary passes through two narrow slits in order to create a “slit-source” of light which is then sent through a incident-grazing diffraction grating and recorded on special photographic film to determine the wavelengths and intensities of the spectrum. The plasma was found to corrode these slits, making the opening much wider. Following a suggestion by Shevelko, I attached some baffling to each slit at roughly a 45 degree angle so as to deflect incoming ions approaching the edge of the slits into the space in front of the opening and collide with ions traveling towards the opening itself. This would reduce the corrosion.3 The slits I made were roughly 25 micrometers apart. I presented a report at the Brigham Young University College of Physical and Mathematical Sciences 1998 Spring Research Conference on the design and predicted results of this experiment. The temperatures needed to doubly-ionize the lithium were within the range of the Marx generator.

I brought new de-ionized water to run through the Marx generator and cleaned off some rust. The water is used as a dielectric to build up higher voltages in the Marx generator. Ions in the water increase the water’s conductivity, thus negating the desired effect. I suggested that the rusty surface be painted in order to eliminate this source of ions–this was later done by Bob Bradford and Josh Arel.

I heated the t-butyl-lithium container to 80 degrees Celsius. The capillary was at a pressure of 7.5 x 10-5 T. I opened the valve to allow t-butyl-lithium vapor into the capillary. I fired 5-10 shots of the Marx generator at 4 different positions of film. When I developed the film I discovered that the opening over the film, that only allows a part of the film to be exposed, was too wide and I had overlapped the spectra.

The tubing was later taken apart and work was done on trying to improve the vacuum.

As soon as a suitable vacuum can be produced I can run my experiment and analyze the spectrum. Hopefully I will be able to accomplish this by December.

I would like to thank the Office of Research & Creative Activities for this scholarship. I would also like to thank all those who assisted me in my research.4

References

1. J. J. Rocca, M. A. Klosner, H. A. Bender, and W. T. Silfvast, Optics Letters 22, 34(1997).
2. Michael Weiner, George Vogel, and Robert West, Inorganic Chemistry 1, 655(1962).
3. Chris Crawford, “Spectroscopic Investigations of Soft X-Ray Amplification in a Capillary Discharge,” Master’s thesis (Brigham Young University, Provo, Utah, 1997).
4. R. Steven Turley, Bob Bradford, Joseph Young, David Allred, Justin Peatross, Wesley Lifferth, Department of Physics & Astronomy, Jim Thorn, Department of Chemistry.