Fabrication and surface evaluation of a high-precision monolithic mirror with complex geometry for soft x-ray focusing

Abstract

High-precision monolithic mirrors are used in the soft x-ray beamlines of synchrotron radiation facilities. A figure measurement technique is essential for the fabrication of high-precision mirrors. In recent years, mirrors of various shapes have been proposed, and a versatile figure measurement technique is required. Tactile measurements are one of the most suitable methods for evaluating the figure errors of x-ray mirrors with steep and complex geometries. Because a tactile measurement probe has a wide range of measurable depths and angles and can eliminate measurement errors depending on the sample curvature, tactile measurements have been applied to the fabrication of monolithic mirrors for soft x-ray focusing. In this study, a soft x-ray focusing mirror with concave and convex freeform surfaces on its monolithic substrate was fabricated based on tactile measurements for figure correction. The high- and mid-spatial-frequency roughness were 0.14-0.17 nm and 0.8 nm in RMS, respectively. Furthermore, the figure error evaluated using the tactile measurement was 2.64 nm in RMS.