High-reflectance magnetron-sputtered scandium-based x-ray multilayer mirrors for the water window

Abstract : We present an experimental comparison of several Sc-based short period multilayer mirrors including Cr/Sc with B4C barrier layers and CrNx/Sc, and we propose a new material combination that provides high reflectance in the water window domain. Multilayer samples with period thickness in the range 1.5–1.7 nm have been deposited by magnetron sputtering and characterized by x-ray reflectometry with a Cu–Kalpha source and with synchrotron radiation near the Sc-L2,3 edge. Best results are achieved by combining the nitridation of Cr layers and the addition of B4C barrier layers. Near normal incidence reflectance as high as 23% has been measured at photon energy of 397 eV. A simulation model of the multilayer structure is proposed and it predicts that reflectance higher than 32% is achievable with CrNx/B4C/Sc mirrors.