Impact of B₄C buffer layer on interface diffusion in Cr/Sc multilayers: combined study by x-ray reflectivity, scattering and fluorescence

Abstract

In thin film multilayer based optical components of X-ray imaging system, diffusion of one material into the other degrades the reflectivity of the mirrors severely. Along with this thermodynamically driven diffusion, there are also growth generated interface roughness of different special frequencies and microstructures which can increase the diffused scattering from the multilayer and reduce the resolution of an image. Generally grazing incidence X-ray reflectivity in specular geometry (specular GIXR) and diffused X-ray scattering measurement in rocking scan geometry yield information regarding microstructure and overall diffusion at the interfaces of a multilayer. In this paper it is shown that grazing incidence X-ray fluorescence (GIXRF) measurement in standing wave condition alongwith the above measurements can give precise information regarding element-specific diffusion at the interfaces of a multilayer structure. Periodic multilayers made of 75 Cr/Sc bilayers with bilayer thickness ~ 4 nm with and without B4C barrier layer of 0.2 nm thickness at the interfaces have been prepared using ion beam sputtering system and characterized by GIXR, diffused X-ray scattering and GIXRF measurements using synchrotron X-ray radiation just above the Cr K-edge. From the above measurements, drastic reduction in interface diffusion of Cr and improvement of interface morphology after addition of B4C barrier layer at the interfaces of Cr/Sc multilayers have been observed which is also corroborated by cross-sectional transmission electron microscopy of the multilayers. Finally, in the water window soft X-ray region of 2.3 – 4.4 nm performance of these multilayers have been tested and the Cr/B4C/Sc multilayer with improved interface quality has been found to yield ~30.8% reflectivity at 3.11 nm wavelength which is comparable with the best reported reflectivities in the literature at this wavelength.