Analysis of microroughness evolution in X-ray astronomical multilayer mirrors by surface topography with the MPES program and by X-ray scattering

摘要

Future hard X-ray telescopes (e.g. SIMBOL-X and Constellation-X) will make use of hard X-ray optics with multilayer coatings, with angular resolutions comparable to the achieved ones in the soft X-rays. One of the crucial points in X-ray optics, indeed, is multilayer interfacial microroughness that causes effective area reduction and X-Ray Scattering (XRS). The latter, in particular, is responsible for image quality degradation. Interfacial smoothness deterioration in multilayer deposition processes is commonly observed as a result of substrate profile replication and intrinsic random deposition noise. For this reason, roughness growth should be carefully investigated by surface topographic analysis, X-ray reflectivity and XRS measurements. It is convenient to express the roughness evolution in terms of interface Power Spectral Densities (PSD), that are directly related to XRS and, in turn, in affecting the optic HEW (Half Energy Width). In order to interpret roughness amplification and to help us to predict the imaging performance of hard X-ray optics, we have implemented a well known kinetic continuum equation model in a IDL language program (MPES, Multilayer PSDs Evolution Simulator), allowing us the determination of characteristic growth parameters in multilayer coatings. In this paper we present some results from analysis we performed on several samples coated with hard X-ray multilayers (W/Si, Pt/C, Mo/Si) using different deposition techniques. We show also the XRS predictions resulting from the obtained modelizations, in comparison to the experimental XRS measurements performed at the energy of 8.05 keV.