PARTICLE-EMISSION DEBRIS FROM A KRF LASER-PLASMA X-RAY SOURCE

The production of plasma debris ejected from various solid targets hea ted and ionized by intense KrF laser pulses was studied. The plasma is produced by focusing a train of 50 ps KrF laser pulses to an 8-mu m-d iam spot on the surface of solid tape targets. The plasma conditions a re those of interest for the generation of a point source of the keV a nd sub-keV x rays for microlithography (Cu and Fe targets) and x-ray m icroscopy (plastic targets). Experimental measurements of the amounts of debris produced, the statistical distribution of the debris size, a nd angular distribution in vacuum are presented. The results of simila r measurements in a background gas of helium are also presented togeth er with measurements of the stopping power of the gas. A large flux of micron and submicron size particles is generated in vacuum even when employing thin tape targets. The introduction of a background gas redu ces the damaging influence of such particles dramatically but only if enough path length exists to stop the particles. Theoretical calculati ons have been carried out of the stopping power of the background gas based on Stokes law and the Newtonian theory of deceleration of partic les in gas. (C) 1996 American Vacuum Society.