Unveiling the effect of adding B₄C at the W-on-Si interface

W/Si thin-film structures are used in various applications, including X-ray optics. While many studies reported that adding a sub-nm-thick B₄C film at the W/Si interfaces is beneficial for the overall reflectivity of the structures, the physical mechanisms involved were not yet fully understood. In this study, we investigate the W-on-Si interface and the effect of adding a B₄C interlayer at such interface, using low-energy ion scattering (LEIS) spectroscopy, X-ray reflectivity, X-ray diffraction, and transmission electron microscopy with energy dispersive X-ray spectroscopy. We extract the effective width of the interface in three different structures having: no-B₄C, 0.24 nm, and 1.2 nm of B₄C deposited between the W and the Si films. The analysis reveals that the W distribution does not get significantly sharper when B₄C atoms are deposited at the W-on-Si interface, showing that B₄C does not act as a physical barrier against the diffusion of atoms during the deposition of these structures. This study highlights the importance of systematically extracting the values of the effective interface width for the analysis and understanding of thin-film growth, which is highly valuable for the design of multilayer optics in soft X-ray applications.