Radiation stability of EUV Mo/Si multilayer mirrors

The stability of Mo/Si multilayer mirrors under extreme ultraviolet (EUV) radiations has been investigated. Mirrors were deposited by DC magnetron sputtering and designed for maximal reflectivity at the wavelength of 13.5 nm. Investigating samples after irradiation by three different EUV sources (Xe-gas discharge, compact laser and X-ray tube), a layer was found on top of mirrors. These contamination layers mainly consisted of oxygen and carbon but with thicknesses depending on the kind of source which was used. A maximal carbon thickness of 45 nm after 210 min of irradiation existed with the compact laser source and a minimal one of 1 nm after 10 h of irradiation was found with the X-ray tube. Furthermore, this deposition process was demonstrated to have a two stages of contamination rates.