Characterization of CaF2 substrates for VUV fluoride coatings

CaF2 has become the most important substrate for vacuum ultraviolet (VUV) optical coatings. The popularity of CaF2 relies on its transparency range, which extends down to 120 nm. A major drawback of the material is, however, its mechanical softness. Hence careful assessment of the surface quality is necessary in order to obtain low loss components. By combining roughness data from Nomarski Microscopy and Atomic Force Microscopy (AFM) measurements with total and angle resolved scattering measurements at 157 nm scatter losses due to interface roughness as well as bulk inhomogeneities are being studied. Measured losses are furthermore compared with theoretically predicted results. Results are also presented on VUV coatings. The question of the coupling between the substrate surface finishing quality and the optical properties of the coatings is specially addressed.