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 ( φ Br = 3.9 ° grazing) and 8.4% at λ = 0.834 nm ( φ Br = 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.