High reflectance ultrashort period W/B4C x-ray multilayers via intermittent ion polishing

Ultrashort period W/B4C multilayers (MLs) are essential for high-resolution x-ray optics, but their performance is limited by inherent interface roughness. In this study, we introduced the technique of intermittent ion beam polishing (I-IBP) to sputter-deposited W/B4C MLs with 1.0 and 1.1 nm periods. This novel approach differs from traditional ion polishing by using polished B4C interlayers to disrupt the accumulation of roughness throughout the multilayer. The 1st Bragg peak reflectance doubled compared to non-polished W/B4C, reaching 20% at λ = 0.154 nm (φBr=3.9° grazing) and 8.4% at λ = 0.834 nm (φBr=22° grazing) for 1.1 nm W/B4C. The average interface width decreased from 0.27 to 0.22 nm, which is attributed to interface smoothing. X-ray diffuse scattering and STEM images showed reduced interface roughness and improved lateral and vertical correlation lengths. This study demonstrates that I-IBP significantly enhances the structural and optical properties of ultrashort period MLs, advancing ML-based optics for both soft and hard x rays.