Surface profile measurement of hard X-ray nanofocusing mirrors

Abstract

Metrology plays an important role in surface figuring with subnanometer accuracy. We have developed RADSI (relative angle determinable stitching interferometry) for the surface figuring of elliptical mirror for hard x–ray nanofocusing. On the mirror surfaces figure accuracy of a few nanometers is necessary. In the measurement using the Fizeau interferometer, the density of the interference fringes on the X–ray mirror surface is so high that the entire area of surface profile cannot be acquired at once. The conventional stitching method using a common area between neighboring shots generates stitching errors larger than the figure accuracy of nanofocusing mirror. In the RADSI, stitching angles are determined not by the conventional method, but by the new method using the mirror′s tilt angles measured at times when profile data are acquired. Using the RADSI, the high measurement accuracy of approximately 3 nm peak–to–valley was achieved in the measurement of the elliptical mirror. We have carried out line focusing tests of the fabricated elliptical mirror at the 1–km–beamline in SPring–8. The line focus profile with a 40 nm full width at half maximum (FWHM) was realized.