Optical losses of dielectric VUV-mirrors deposited by conventional evaporation, IAD, and IBS

Abstract

The deposition of high-reflecting dielectric coatings for the VUV spectral range requires the use of fluoride materials because oxide layers show high absorption losses at wavelengths below 200 nm. As shown in a previous paper, the reflectances of fluoride mirrors deposited conventionally are limited by volume and interface scattering due to the columnar microstructure of the layers; reduced scattering was observed in layers deposited by IAD or IBS. In these processes, oxygen was used as a reactive gas to compensate for the fluorine deficiency caused by preferential sputtering during deposition. The resulting oxide content in the layers, however, leads to high absorption losses at wavelengths below 200 nm. To obtain layers with improved stoichiometry by using IAD or IBS, these processes were performed with fluorine as a reactive gas. Single layers and high-reflecting quarterwave stacks were deposited and investigated. Their optical properties are compared to data obtained for conventionally deposited coatings and for coatings deposited by IAD or IBS using oxygen as a reactive gas. The results indicate that the ion processes are promising tools for the deposition of low-loss dielectric VUV mirrors.