Development of a differential deposition system based on wavefront metrology for ellipsoidal mirrors

Abstract

Ellipsoidal X-ray mirrors are ideal focusing optics, offering a high numerical aperture (NA), a long working distance, and high beam utilization efficiency. However, the figure accuracy of mirrors fabricated via the electroforming process is limited to approximately 100nm PV due to internal stress relaxation upon release from the mandrel. To achieve full-shell illumination and fully leverage their performance, a figure accuracy of around 10nm PV is required. In this study, we developed a high-precision correction system that integrates a wavefront measurement system with a differential deposition process using ion beam sputtering. For wavefront measurement, a Mach–Zehnder Point Diffraction Interferometer (PDI) with spatially separated optical paths was constructed to eliminate crosstalk in conventional PDIs, successfully recovering the focused wavefront phase without failure. For differential deposition, we evaluated the deposition characteristics within the confined inner cavity of the mirror, achieving a surface roughness of 0.23nm RMS— suitable for x-ray mirrors—and high processing stability (a deposition error of approximately 1.5% over repeated runs). Furthermore, we identified key challenges, including measurement repeatability (currently 1.4nm) and issues in film thickness profile measurement. These results demonstrate the effectiveness of our system toward establishing nano-accuracy figure correction for electroformed ellipsoidal mirrors.