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.