Inhibition of Contamination of Ru-Capped Multilayer Mirrors for Extreme Ultraviolet Lithography Projection Optics by Ethanol

Abstract

The inhibition of the contamination of Ru-capped Mo/Si multilayer mirrors was systematically investigated by introducing ethanol into a controlled vacuum that mainly consisted of water vapor. Water vapor was introduced at a pressure of up to 1.3×10-5 Pa, which is a typical pressure in extreme ultraviolet (EUV) lithography production tools. Additionally, ethanol was introduced at several partial pressures ranging from 1.0×10-7 to 3.8×10-5 Pa. At the lowest ethanol pressure, the same degree of reflectance degradation as in the water-only case was observed. However, reflectance degradation was suppressed at ethanol pressures higher than 2.0×10-6 Pa. As a result of surface analyses using X-ray photoelectron spectroscopy, we determined that the reflectance degradation was caused by oxidation. The inhibition of contamination may be caused by a mechanism of a self-limiting carbon layer from ethanol adsorption on a hydroxylated mirror surface. It was found that oxides at the mirror surface were deoxidized by introduced ethanol. Therefore, deoxidization of the Ru layer may be one of the mechanisms of the inhibition of contamination. The suppression of reflectance degradation was independent of the photon intensity and measured for up to 1200 J/mm2. This dose corresponds to about 3 months of operation for projection optics mirrors at the maximum power expected. Therefore, the introduction of ethanol was found to be a promising method for inhibiting the contamination of projection optics mirrors, which are irradiated by a wide range of EUV power.