Scattering of EUV optics - substrate, coating, and degradation effects
Scattering resulting from interface imperfections crucially affects the throughput and image contrast of EUV optics. Since both the substrate finish and the intrinsic thin film roughness influence the scattering, thorough investigations are needed to separate the different effects and to identify the most promising starting points for further optimizations. Mo/Si multilayer coatings deposited onto different substrates are investigated by utilizing an instrument for EUV reflectance and scattering measurements at 13.5 nm recently developed at the Fraunhofer IOF. The influences of the substrate finish and the deposition process onto the scattering are separated. Furthermore, the instrument allowed the EUV-induced degradation of Mo/Si mirrors to be investigated at the wavelength of application. In particular the impact of top-layer oxidation and roughening on the scattering properties is discussed.