Characterizing CaF2 for VUV optical components: Roughness, surface scatter, and bulk scatter

Methods for evaluating the quality of CaF2 substrates for vacuum ultraviolet (VUV) low loss optical components are presented. Today superpolished CaF2 is available. However, major differences might still occur between batches and careful control is therefore necessary. By using roughness data from AFM measurements combined with total scattering measurements at 193 nm and 157 nm surface roughness as well as inhomogeneities in the bulk of the material can be studied. Results are also presented of anti-reflective (AR) and highly reflective (HR) multilayer coatings on CaF2 where reduced total backscatter scattering was found for the AR -coating as compared to the substrate.