Fabrication of soft x-ray monolithic Wolter mirror based on surface scanning measurement using touch probe

Abstract

The monolithic Wolter mirror is an ideal optical device for focusing soft x rays to a submicron-sized spot, with the advantages of high efficiency, large acceptance, achromaticity, and robustness to alignment error. The fabrication process for this type of mirror has not been established because of the difficulty in highly accurate figure measurement of free-form surfaces with small radii of curvature and steep profiles. In this study, we employed tactile scanning measurement for surface characterization to fabricate a high-precision Wolter mirror. First, it was demonstrated that the touch probe measurement did not leave scratches on the raw surface of the mirror substrate. Next, the measurement capability of the surface profiler was assessed, and the data analysis conditions were determined. Finally, the Wolter mirror was fabricated through repeated figure correction based on the tactile measurement, and the figure error of the final surface was evaluated. Wave-optical simulations that used this error as reference suggested that the size of the beam focused by the mirror was equivalent to the theoretical value at 1000 eV. The reflected image with uniform intensity distribution obtained at SPring-8 also revealed the effectiveness of the present fabrication approach based on tactile measurement.

Surface evaluation of high-precision monolithic mirror for soft x-ray focusing

“…The angle between the ellipsoidal and hyperboloidal surfaces is approximately 165°. The deviation from a fitted plane in the ellipsoidal and hyperboloidal parts was 0.12 mm and 0.29 mm, respectively, which was approximately 10 times larger than a previously fabricated mirror [7]. The average radius of curvature in the sagittal direction was 32 mm.…”

“…Mirrors with such steep shapes make it difficult to apply stitchinginterferometry-based surface measurements represented by MSI and RADSI [5,6], which prevents their fabrication with high accuracy. Recently, we developed a mirror fabrication process that involves surface measurements using a surface profiler with a touch probe [7]. Tactile measurements generally have the advantage of a wide measurable range of depths and angles, which is suitable for steep mirrors.…”

“…Tactile measurements generally have the advantage of a wide measurable range of depths and angles, which is suitable for steep mirrors. In previous studies, the effectiveness of the process was reported through the fabrication of a focusing mirror for soft X-rays at photon energies of 300 and 1000 eV [1,7]. To further investigate the measurement performance of a surface profiler with a contact probe, we herein compared the measurement performance of the profiler and a stitching interferometer for a flat substrate with a waviness of 4 mm-period.…”

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

“…In the intermediate phase, the 5-mm-period error component was significantly eliminated, and all spatial frequency components below 1 mm -1 were reduced. The figure error of the final surface was evaluated from a series of measurements performed and analyzed under two conditions [7] . First, the surface was raster scanned at low speed and processed using a band-pass filter (λc = 0.8-5 mm).…”

“…Scanning measurement with a contact probe is one of the most suitable methods for evaluating the figure error of soft X-ray mirrors with steep shapes and complex geometries owing to its wide range of measurable depths and angles and its robustness to measurement errors dependent on sample curvature. Its effectiveness has been reported for the fabrication of monolithic Wolter mirrors for extreme ultraviolet and soft X-ray focusing [7,8] .…”