INTERACTION OF INTENSE LASER PULSES WITH DROPLET AND CLUSTER SOURCES: APPLICATION TO EXTREME ULTRAVIOLET LITHOGRAPHY AND PLASMA WAVEGUIDE GENERATION
Milchberg, Howard M
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Several topics were studied in the interaction of intense laser pulses with droplet and cluster sources. Laser pulse-formatting that can enhance laser-to-EUV conversion efficiency in the 13.5nm band for next geration lithography was first explored, using droplet sources as the laser target. Xenon droplets size distribution was measured, and the droplet plasma spectrum irradiated by various laser energies was scanned. A 2-pulse heater setup and a 4-pulse stacker scheme were built and studied. Results suggest that, unlike droplets of argon (Ar) and krypton (Kr), the ionizationstate distribution in xenon may be much more transient. The decay timescales for 13.5nm emissions are approximately 0.5-1.5ns, a timescale intermediate to the results from excitation and recombination emission in Ar and Kr droplet targets. Comparing xenons results with argons, the argon droplets generated a more robust ionizationstage distribution, while the xenon droplets generated a more transient ionizationstage distribution. The second topic studied was an investigation of plasma waveguides generated in clustered gases, using 100ps long pump pulses axicon-generated Bessel beam. The plasma waveguide space and time evolution was measured, using picosecond interferometry. The resulting waveguides have both central densities as low as ~1018cm-3 and small diameters, a desirable but hard to achieve combination for either hydrodynamic shock waveguides using conventional gases or for other techniques, such as discharge capillaries. Extremely efficient absorption of laser pulses by cluster targets was shown to extend to pulses significantly longer than the timescale for cluster explosive disassembly. These long pulse absorption efficiencies can be more than a factor of 10 greater than those in unclustered gas targets of the same volume average atomic density. The maximum long pulse absorption observed in cluster jets under our range of conditions was 35%. A third topic explored was resonant pulse-shortening of Bessel beams in the under-dense (Ne is ~10^(-2) Ncr) plasma channels formed by 100ps Nd:YAG laser pulses. Pulse shortening was seen at two pressures under our range of conditions: 340torr and 460torr.