The droplet laser plasma source for EUV lithography

Summary form only given.Extreme ultraviolet lithography (EUVL) is now considered the leading technology to replace optical lithography at the 100-nm node for the production of advanced computer chips later this decade. EUVL is based on the use of all-reflective astigmatic multilayer-coated optics at 13 or 11 nm and a reflective 5:1 reduction mask illuminated by a bright point source. Currently the source of choice is a high repetition-rate (>1 kHz) laser-plasma, that must provide at least 7 W of radiation within a 3% spectral bandwidth, and be sufficiently free of particulate debris to ensure the long-lime life of multilayer collimating optics. Two laser-plasma source approaches are presently being pursued, one based on a high-pressure xenon gas jet target, and the other on plasmas produced from microscopic water droplets. The water droplet laser plasma was introduced in 1993, as a bright source of 13-nm and 11.6-nm line emission from lithium-like oxygen, generated by 10-ns duration, Nd:YAG laser pulses at /spl sim/10/sup 12/ W/cm/sup 2/. These mass-limited targets are expected to be totally ionized, avoiding completely the particulate target debris that plagued earlier sources. Our previous work with a 100-kHz droplet system and a 10-Hz laser demonstrated the attractive features of this approach.