Normal Incidence Reflectometry of Concave Multilayer Mirrors Using Synchrotron Radiation to Evaluate the Period Thickness Distribution

For the purpose of fabricating curved multilayer mirrors, we developed an ion beam sputtering deposition system with a programmable shutter for thickness distribution control. In this report we fabricated a Mo/Si multilayer concave mirror of a 100 mm diameter and a 300 mm radius of curvature to be used at 13.5 nm at an angle of incidence of 5°. At first a test multilayer was deposited without thickness distribution control and the lateral distribution of deposition rate was evaluated. We used the normal incidence EUV reflectometry to determine the multilayer period thickness while we usually use the small angle X-ray diffractometry when the substrate is plane. The measurements were performed at BL-12A of the Photon Factory, KEK. In an analysis of a spectral reflectance, the side band structure as well as the main peak was taken into account. The natural thickness at the outermost part was found to be 13% thinner than that at the center. Next a Mo/Si multilayer of uniform thickness over the substrate was deposited with the shutter programmed to compensate for the unevenness of deposition rate. By the normal incidence EUV reflectometry the thickness distribution was found to be successfully uniform within an error of P-V 0.3%.