Relation between electron- and photon-caused oxidation in EUVL optics

Abstract

Extreme ultraviolet (EUV)-induced oxidation of silicon-capped, [Mo/Si] multilayer mirrors in the presence of background levels of water vapor is recognized as one of the most serious threats to multilayer lifetime since oxidation of the top silicon layer is an irreversible process. The current work directly compares the oxidation on a silicon-capped, [Mo/Si] multilayers caused by EUV photons with the oxidation caused by 1 keV electrons in the presence of the same water vapor environment (2 x 10^-6 Torr). Similar, 4 nm, silicon-capped, [Mo/Si] multilayer mirror samples were exposed to photons (95.3 eV) + water vapor at the ALS, LBNL, and also to a 1 keV electron beam + water vapor in separate experimental systems. The results of this work showed that the oxidation produced by ~1 µA of e-beam current was found to be equivalent to that produced by ~1 mW of EUV exposure. These results will help allow the use of 1 keV electrons beams, instead of EUV photons, to perform environmental testing of multilayers in a low-pressure water environment and to more accurately determine projected mirror lifetimes based on the electron beam exposures.