Suppression of reflectance drop of Ru-capped multilayer mirror by irradiation atmosphere

Relative reflectance drops of projection mirrors for EUV lithography arise from contaminations such as oxidation and/or carbon deposition. Mo/Si multilayer mirrors (MLMs) are used for the projection mirrors because of their high initial reflectance, but their reflectance drop rates were too fast to satisfy the specification. Ru is the most promising material among investigated materials as capping layers in order to keep the reflectance (Bajt et al., 2004). The reflectance dropping rate of the Ru-capped MLMs depended on the samples. The two different experimental systems were used for observation of the influence of hydrocarbon. The reflectance drop was almost suppressed under higher pressure of hydrocarbon. The hydrocarbon can help reducing the oxidation and keeping the reflectance while too much hydrocarbon makes carbon deposition on the mirror surface.