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

We present an experimental comparison of several Sc-based short period multilayer mirrors including Cr/Sc with B₄C barrier layers and CrN_x/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-Kα source and with synchrotron radiation near the Sc-L_2,3 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 CrN_x/B₄C/Sc mirrors.