Evolution of interface and surface morphology of WC/SiC multilayers with increase in number of bilayers

Abstract

WC/SiC multilayer mirrors are promising elements for operation in the short wavelength range of hard X-rays and even gamma-rays. A set of WC/SiC multilayers with nominal bilayer thickness of 3.0 nm with 30, 50, 100, and 200 bilayers have been deposited by magnetron sputtering method. The evolution of interface and surface morphology with addition of the number of bilayers has been investigated. The methods of grazing incidence X-ray reflectivity, X-ray diffuse scattering, optical profiler, atomic force microscope, transmission electron microscopy and energy-dispersive X-ray spectroscopy have been used. It is found that the interface roughness plays a dominant role in interface imperfections of WC/SiC and it exhibits slight accumulation across the multilayers with the number of bilayers increasing. The surface roughness also increases in relatively high spatial frequency range as the number of bilayers has been added, due to the cumulative increase in interface roughness. The interface roughness accumulation ultimately contributes to the diminishment of optical performance of WC/SiC multilayers with a large number of bilayers. At the wavelength of 0.154 nm, a peak reflectivity of 69.2 % is achieved for a 100 bi-layer multilayer under the grazing incidence of approximately 1.5°, while lower reflectivity of 67.2 % for a 200 bi-layer multilayer.