Optical performance of W/B4C multilayer mirror in the soft x-ray region

W/B4C x-ray multilayers (MLs) with 300 layer pairs and a period in the range of d = 2–1.6 nm are fabricated and investigated for the x-ray optical element in the soft x-ray regime. The structural analyses of the MLs are carried out by using hard x-ray reflectivity (HXR) measurements at 8.047 keV. Well-defined successive higher order Bragg peaks (up to 3rd order) in HXR data collected up to glancing incidence angles of ∼9° reveal a good quality of the periodic structure. The ML mirrors have an average interface width of ∼0.35 nm and have a compressive residual stress of ∼0.183 GPa and 0. 827 GPa for d = 1.62 nm and d = 1.98 nm, respectively. MLs maintain structural stability over a long time, with a slight increase in interface widths of the W layers by 0.1 nm due to self-diffusion. Soft x-ray reflectivity (SXR) performances are evaluated in the energy range of 650 to 1500 eV. At energy ∼ 1489 eV, measured reflectivities (energy resolution, ΔE) are ∼ 10% (19 eV) and 4.5% (13 eV) at glancing incident angles...