Increasing soft x-ray reflectance of short-period W/Si multilayers using B4C diffusion barriers.

Short-period multilayer mirrors are used in wavelength-dispersive x-ray fluorescence to extend the wavelength range available with naturally occurring Bragg-crystals. W/Si multilayer mirrors with a period of 2.5 nm are used to reflect and disperse elements in the O-Kα–Al-Kα range. However, the reflectance is far from theoretical due to nanoscale W-Si intermixing and formation of WSi_x. In this work, B₄C diffusion barriers were applied in sputter deposited 2.5 nm W/Si multilayers to inhibit W–Si interaction. A peak reflectance of 45% at 9.7° grazing was measured at a wavelength of 0.834 nm—the highest reported in the literature so far. Diffuse scattering measurements revealed no change in interfacial roughness when applying B₄C barriers compared to W/Si. A hybrid grazing incidence x-ray reflectivity and x-ray standing wave fluorescence analysis revealed an increase in W concentration of the absorber layer after the application of B₄C barriers. Chemical analysis suggests a partial replacement of W silicide bonds with W carbide/boride bonds from the B₄C barrier. The formed W_xB_y and W_xC_y instead of W_xSi_y is hypothesized to increase reflectance at 0.834 nm due to its higher W atomic density. [ABSTRACT FROM AUTHOR]