High power excimer laser-generated plasma source for x-ray microlithography

This paper describes a high-intensity, high pulse-repetition-rate picosecond-pulse excimer laser system and plasma x-ray source, which generates up to 3 W of average x-ray power, into 2π steradians, in a spectral band from 10--16 angstrom. The XeCl excimer laser system output, at 308 nm, consists of a train of 16 pulses, each ∼45 ps in duration and spaced by 2 ns. The energy of each pulse in the train is ∼25 mJ, and the pulse-train repetition rate is 60 Hz. Each pulse in the train is focused to a spot of <10 microm diameter on a metal tape target, resulting in an intensity of 1 x 1015 W cm-2. Spectral and spatial characteristics of the x-ray emission have been studied, and the laser energy to x-ray dose conversion efficiency has been measured in an experiment which simulates the x-ray lithography process. Lithographic efficiencies of 5.9% and 10.9% have been measured for copper and stainless steel targets, respectively