Tungsten-rhenium alloy as a high-density absorber in ultra-short-period multilayer for hard x-ray mirrors

Ultra-short-period multilayer mirrors (period d below 5 nm) are key components for hard X-ray optics in the 20 keV to 40 keV range, but their performance rapidly degrades when the period approaches a few nanometers due to density loss and/or interfacial defects (roughness or intermixing). In this work, we investigate a tungsten–rhenium alloy absorber (WRe, 50 at.% Re) in WRe/SiC multilayers and compare it with a pure W absorber. We also assess the impact of the magnetron sputtering mode, conventional direct-current magnetron sputtering (dcMS) versus high-power impulse magnetron sputtering (HiPIMS), on the optical contrast and reflectivity of WRe/SiC multilayers with d ≈ 3 nm. Four periodic stacks Abs/SiC40 (Abs = W or WRe) were deposited and characterized by multi-energy X-ray reflectometry from a laboratory diffractometer and synchrotron beamlines (BAMline-BESSY-II and BM05–ESRF). Simultaneous fitting of reflectivity curves at multiple photon energies yields estimates of period, densities, and interfacial roughness. This specific fitting process is based on a sequential approach, providing an accurate structure model. These results are complemented by high-resolution TEM analysis of the microstructure. Compared with W/SiC, WRe/SiC exhibits a higher absorber density and a lower spacer density (SiC). This significant increase in the density contrast between the absorber and the spacer results in higher peak reflectivity at comparable Bragg angles. Under these conditions, dense WRe layers are obtained by dcMS, and HiPIMS provides similar densification within experimental uncertainties, while preserving the interfacial quality. These results demonstrate that WRe/SiC multilayers are a promising alternative to conventional W/SiC systems for high-energy X-ray mirrors at nanometer-scale periods.