Stitching-angle measurable microscopic-interferometer: Surface-figure metrology tool for hard X-ray nanofocusing mirrors with large curvature

We propose an optical profilometer system for evaluating mirrors that are used for hard X-ray nanofocusing. A surface metrology system designed for these mirrors to focus to a size of 10 nm is required to ensure a precise measurement of a large curvature with a vertical accuracy of 1 nm and a lateral resolution less than 0.1 mm. The surface metrology system is based on a stitching interferometer that can determine stitching angles between adjacent local surface profiles. Local surface profiles and stitching angles are detected simultaneously by using two types of optical interferometers, a microscopic interferometer and a large-scale interferometer, respectively. We developed a nanofocusing mirror that has approximately the same surface curvature as a sub-10-nm focusing mirror, to examine the measurement accuracy of the developed surface metrology system. As a result of a performance assessment of the measurement instrument, we achieved measurement reproducibility and reliability of less than peak-to-valley height of 4 nm by measuring the focusing mirror. In addition, we achieved focusing performance of a full width at half maximum 27 nm at 7 keV using the mirror at SPring-8. We verified that the metrology system can be used to measure sub-10-nm focusing mirrors.