Carbon accumulation and mitigation processes, and secondary electron yields of ruthenium surfaces

摘要

Metallic ruthenium capping layers ～2 nm thick protect and extend the lifetimes of Mo/Si multilayer mirrors used in extreme ultraviolet lithography (EUVL) applications. However, Ru-capped mirrors experience a loss of reflectivity after prolonged exposure to EUV radiation. In the present work, we use ultrahigh vacuum surface science methods to address several aspects of Ru surface chemistry that may impact on Ru capping layer stability and mitigation processes. (1) We characterize the composition and stability of Ru surfaces that simulate surfaces of Ru-capped multilayer mirrors, under exposure to different background gases (water, methyl methacrylate (MMA)) and to electron irradiation. Evidence for some mitigation of carbon accumulation during electron bombardment in MMA + water vapor is found. (2) We report the photon-energy dependence of secondary electron yield (SEY) measurements for clean Ru, O-dosed and C-dosed Ru, and Ru-capped multilayer mirrors using synchrotron radiation near 13.5 nm at Brookhaven National Synchrotron Light Source (NSLS). Much of the radiation-induced chemistry on the surfaces of capping layers is induced by low-energy secondary electrons rather than direct photoexcitation, so the SEY is an important parameter affecting mirror lifetimes in EUVL.