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Protection from surface oxidation of Ru capping layers for EUVL projection optics mirrors by introducing hydrocarbon gas

We investigated the protection of a Ru-capped Mo/Si multilayer from surface oxidation under exposure to EUV radiation in the presence of water vapor and isopropyl alcohol (IPA). Degradation of the reflectance of the Ru-capped Mo/Si multilayer by EUV irradiation was controlled by introducing IPA gas. We also investigated the reduction effect of the oxide layer in a multilayer mirror by introducing ethanol and exposed EUV. The Ru-capped multilayer sample was exposed to EUV radiation in the presence of only water vapor to oxidize its surface. The reflectance decreased by about 1.5%. Then the sample was exposed to EUV radiation in the presence of only ethanol vapor. The reflectivity of the sample was recovered to +0.5%, and the atomic concentration of oxygen in the irradiated area was decreased by EUV irradiation in the presence of ethanol.