High-end precision optics are generally manufactured through traditional processes, which produce unique shapes and smooth surfaces, and the modern figure correction process. Particularly for soft x-ray regions, the demand for accurate free-form optics with complex and steep shapes has been increasing recently. Wolter mirrors and ring-focusing mirrors are representative of these types of optics, which are expected to be the next-generation devices for soft x-ray microscopy. To fabricate these mirrors with sufficient accuracy to achieve ideal optical performances, both traditional and modern processes must be optimized ad hoc. In this study, we apply the high-precision lens fabrication process to a monolithic Wolter mirror and a ring-focusing mirror. The surface measurement results of the mirrors suggest that it is possible to fabricate single-nanometer accurate soft x-ray mirrors with steep shapes in complex geometries.
Ellipsoidal mirrors are ideal focusing optics for soft x-rays because of advantages that include high numerical aperture, high efficiency, and no chromatic aberrations. Shape accuracy of nanometer order is required on the internal surface of a mirror with a diameter of around 10 mm. Because of the difficulty of processing the internal surface, ellipsoidal mirrors are fabricated by replication of the shapes of master mandrels. In previous studies, a fabrication process was developed for x-ray ellipsoidal mirrors involving mandrel fabrication and nickel electroforming. 40-mm-long ellipsoidal mirrors were fabricated and a focused beam with full width at half maximum (FWHM) of 240 nm was obtained. For better focusing performance and expansion of the applicable energy range, we designed and fabricated a 120-mm-long ellipsoidal mirror from the master mandrel with a shape accuracy of 3.8 nm (root mean square). A focusing experiment was also performed at the synchrotron radiation facility, SPring-8 (BL25SU). A focused beam with FWHM of 1 μm was obtained.
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