Design and fabrication of X-ray non-periodic multilayer mirrors: Apodization and shaping of their spectral response

We have developed non-periodic Cr/Sc multilayer mirrors specifically designed to reflect energy photons between 2 and 4 keV with a pre-defined reflectivity profile. Furthermore, these mirrors have to work as filters in the 1–2 keV and 4–12 keV energy bandpass with a reflectivity as low as possible in these bandwidths.The mirrors were designed and optimized with the help of a commercial calculation code. Numerous combinations of layers have been investigated with two or three different materials in the multilayer. The interfacial effect, leading to thickness modifications, has been investigated. Layer thicknesses were accurately controlled, taking into account compaction effect at interfaces. The best results lead to non-periodic mirrors dedicated to work at 1.5° grazing incidence with a reflectivity above 15% in almost the entire energy range 2–4 keV and lower than 1% outside, except in the total reflection zone. The final choice of material was made from the experimental knowledge of corresponding layer deposited behavior. The multilayers have been deposited by magnetron sputtering. Grazing-incidence X-ray reflectance at 8.05 keV (0.154 nm) and at the Physikalisch-Technische Bundesanstalt laboratory (PTB) at the synchrotron radiation facility BESSY II in Berlin for X-ray reflectance over the whole range were used both in order to characterize the multilayers (thicknesses, complex indices, and roughnesses). The final results show good agreement with the requested reflectivity profile.