Finishing procedure for high-performance synchrotron optics

Abstract

Modern synchrotron radiation sources of the 3rd generation like BESSY II, Spring-8 and others with their high brilliance beam characteristics need very high quality optics to exploit the full power of this radiation. For the grazing incidence reflecting type of that optics (flat, spherical or aspherical) besides roughness the slope deviation error is the most important spec, which has to be improved to meet the present and future performance requirements. Together with partners from industry we investigate and develop on the one hand surface figuring and polishing techniques for final finishing by using mainly ion beam milling technology and on the other hand we improve and make use of the combination of the surface shape measurements by means of interferometry, long trace and auto-collimation profilometry. We aim to achieve the following slope deviation errors on silicon optical elements: flat surface 310 mm long 0.03 arcsec rms, flat surface 100 mm long 0.02 arcsec rms and elliptical cylinder surface 210 mm long 0.1 arcsec rms. This is a five to ten-fold improvement compared to the present state of the art in production. To achieve the demanding specification it is necessary to measure and to deterministically machine the surface over a wide range of spatial wavelength down to the sub-millimeter range. In depth scale the sub-nanometer shape error level has to be achieved. The roughness of about 0.2 nm rms has not to be increased during the shape finishing.