Optical constants of beryllium thin layers determined from Mo Be multilayers in spectral range 90 to 134 eV

Mo Be multilayers are promising optical elements for extreme ultraviolet EUV lithography and space optics. Experimentally derived optical constants are necessary for accurate and reliable design of beryllium containing optical coatings. We report optical constants of beryllium derived from synchrotron radiation based reflectivity data of Mo Be multilayers. Results are in good agreement with available data in the literature obtained from the well known absorption measurements of beryllium thin films or foils. We demonstrate synchrotron based at wavelength reflectometry as an accurate and non destructive technique for deriving EUV optical constants for materials that are difficult or unstable to make thin foils for absorption measurement