Implementing 0.1 nm B₄C barriers in ultrashort period 1.0 nm W/Si multilayers for increased soft x-ray reflectance

Ultrashort period 1.0 nm W/Si multilayers have potential as dispersive Bragg reflectors in high-resolution x-ray fluorescence. However, formation of WSi_x leads to poor optical performance. To address this, we introduce ultrathin 0.1 nm B₄C diffusion barriers in sputter-deposited 1.0 nm W/Si, inhibiting W–Si interaction. We demonstrate that the peak reflectance at a wavelength of 0.834 nm increased with a factor of 3.4 compared to W/Si. Diffuse scattering measurements reveal no change in interfacial roughness when applying B₄C barriers compared to W/Si. X-ray reflectivity analysis shows a substantial increase in optical contrast between Si and W as well as sharper transitions between the layers. Chemical analysis suggests that the B₄C barrier reduces formation of WSix through partial substitution of W-silicide bonds with
W-carbide/boride bonds, leading to an increase in optical contrast. The resulting structure of W/Si with B₄C barriers offers a compelling alternative to the more established W/B₄C multilayer at the ultrashort scale due to its superior soft- and hard x-ray reflectance.