Particle emission debris from a KrF laser-plasma x-ray source

The production of plasma debris ejected from various solid targets heated 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-diam spot on the surface of solid tape targets. The plasma conditions are those of interest for the generation of a point source of the keV and sub-keV x rays for microlithography (Cu and Fe targets) and x-ray microscopy (plastic targets). Experimental measurements of the amounts of debris produced, the statistical distribution of the debris size, and angular distribution in vacuum are presented. The results of similar measurements in a background gas of helium are also presented together 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 reduces the damaging influence of such particles dramatically but only if enough path length exists to stop the particles. Theoretical calculations have been carried out of the stopping power of the background gas based on Stokes law and the Newtonian theory of deceleration of particles in gas. (C) 1996 American Vacuum Society.