Microroughness analysis of thin film components for VUV applications

For the roughness characterization of optical surfaces a new procedure based on the analysis of their power spectral density (PSD) functions has been developed. The method consists of the fitting of the PSD obtained from Atomic Force Microscopy measurements at different scan sizes to mathematical models. With this procedure the micro-structural properties of optical surfaces and coatings can be represented by a reduced set of numbers that correspond to the characteristic parameters of the mathematical models. For optical coatings this method allows a separate study of the influence of the substrate and layers on the overall sample roughness. As an example, the method is applied to MgF2 and LaF3 films for VUV applications. We investigated a set of single layers deposited onto superpolished CaF2, fused silica and Si substrates. The samples were deposited by ion beam sputtering, boat and e-beam evaporation. A comparison of the influence of the substr ate on the development of the roughnesses and lateral structures has been performed, as well as a study of the dependence of the roughness properties of the coatings on the deposition process. Complementary investigations of roughness-related scattering consisting of measurements of Total Scatter at 193nm and 633nm and calculation of expected scattering based on the theory are presented.