Emission characteristics of debris from CO_2 and Nd:YAG laser-produced tin plasmas for extreme ultraviolet lithography light source

摘要

We describe a comparative study of the emission characteristics of debris from CO_2 and Nd:YAG laser-produced tin plasmas for developing an extreme-ultraviolet (EUV) lithography light source. Tin (Sn) ions and droplets emitted from a Sn plasma produced by a CO_2 laser or an Nd: YAG laser were detected using Faraday cups and quartz crystal microbalance (QCM) detectors, respectively. The droplets were also monitored by using silicon substrates as witness plates. The results showed higher ion kinetic energy and lower particle emission for the CO_2 laser than the Nd:YAG laser for the same laser energy (50 mJ). The average ion energy was 2.2 keV for the CO_2 laser-produced plasma (LPP), and 0.6 keV for the Nd: YAG LPP. The debris accumulation of the CO_2 LPP detected by the QCM detectors, however, was less than one fourth of that of the Nd:YAG LPP for the same laser energy. Using ion energy data, the mirror lifetime is estimated for the CO_2 and Nd: YAG lasers. In both cases, the upper limit of the number of shots was of the order of 10~6.