Degradation in Euv Reflectance of Ion-Sputtered SiC Films

Abstract

Silicon Carbide (SiC) formed by chemical vapor deposition (CVD) has the highest reflectivity in the extreme ultraviolet (EUV) of any currently used optical material. The high temperature required for the CVD process, however, limits its suitability for coating optical components. To address this problem thin films have been sputtered onto optical surfaces from CVD β-SiC targets. These films, while having reflectivity lower than that of CVD SiC, are nonetheless the best coatings available for reflectance in the spectral region below 1000Å. While the initial properties are good, the EUV reflectivity degrades with time after deposition. A relative decrease of about 25% is evident in the reflectivity at 920Å after 2.5 years, and about 85% of this change occurs in the first three months. In fact, a decrease is observed in the minutes following deposition. In this study the degradation is characterized and a mechanism is proposed. Efforts underway to reduce or eliminate the degradation are discussed.