Nanometer-scale period Sc/Cr multilayer mirrors and their thermal stability

Results of comprehensive characterization of Sc/Cr multilayers for soft X-ray mirrors working in the water window range (2.4–4.4 nm) are presented. Multilayer samples were prepared by ion beam sputtering with up to 250 periods in a range of 1.3–1.75 nm. They were analyzed by transmission electron microscopy (TEM), high resolution TEM, X-ray diffractometry, specular X-ray scattering and diffuse X-ray scattering. The TEM inspection showed good periodicity of the multilayer structure. From simulation studies of the specular reflectivity and a reciprocal space map of the diffuse scattering, it follows that the effective roughness of interfaces is 0.25–0.28 nm, being equal to the geometrical roughness data. Lateral and vertical correlation lengths of the roughness are 7 and 35 nm, respectively. Heat treatment study of the Sc/Cr multilayers revealed a reasonable thermal stability. An increase of the multilayer period of 2.4% was observed after 33 h annealing at 280 °C and a considerable decrease of reflectivity followed above 300 °C annealing for 3 h, which corresponds to the low mutual miscibility between Sc and Cr.