Development of EUV light source by CO₂ laser-produced plasma with nano-structured SnO₂ targets

EUV lithography is scheduled on the international technology roadmap for semiconductors (ITRS). In order to introduce the EUV light source into the production lines, EUV light sources having an average power of more than 100 W at 13.5 nm are needed. Such EUV light sources have been under development by using the laser-produced plasma (LPP) or the discharge-produced plasma (DPP). In the case of Sn LPP, the efficiency is several times higher than that of Xe LPP, but the problem of debris generation that limits the lifetime of the optics in the lithographic system is serious. The mechanism of debris generation is considered to be splashes of a melted surface by rapid heating of the sub-surface within a depth of about 100 nm. So we have suspected that the nano-particles with a diameter of less than 100 nm produce no debris, because the sub-surface can not be produced within such a small particle. In this study, we developed nano-structured SnO₂ targets and investigated the emission characteristics of EUV light from CO₂ laser produced plasma with those targets.