Studies of the multilayer mirror reflectivity in soft x-ray region

Abstract

Four factors (the interface roughness, the monotonous thickness drift, the interdiffusion between two layers and the change of extinction coefficients) which may affect the multilayer mirror reflectivity in soft x-ray region are investigated. Some conclusions are obtained by our theoretical analysis. In long wavelength region (λ > 12 nm), the change of extinction coefficients is the main cause reducing the multilayer mirror reflectivity and others (excluding the interface roughness) only shift the peak position. In short wavelength region (λ < 10 nm), all the four factors affect the reflectivity. In addition, the two factors, monotonous thickness drift and interdiffusion between two layers, shift the peak position. To check these conclusions, the measurement of a Nb/Si multilayer mirror fabricated by our magnet sputtering system is performed on an x-ray generator and a reflectometer installed on the Beijing Synchrotron Radiation Facility. The experimental results are in good agreement with our calculations, and the reflectivity up to 32% of the Nb/Si multilayer mirror (with 41 layers and at wavelength 17.59 nm) is attained.