Improvement of imaging performance of silicon micropore X-ray optics by ultra long-term annealing

We have been developing a light-weight X-ray telescope using micro electro mechanical systems technologies for future space missions. Micropores of 20 μm width are formed in a 4-inch Si wafer with deep reactive ion etching, and their sidewalls are used as X-ray reflection mirrors. The flatness of the sidewall is an important factor to determine the imaging performance, angular resolution. It is known that hydrogen annealing is effective to smooth a Si surface. We tested 150 hour annealing to achieve the ultimately smooth sidewalls. After 50 hour, 100 hour, and 150 hour annealing, the angular resolution improved 10.3, 4.0, and 2.6 arcmin in full width at half maximum (FWHM) and 17.0, 14.5, and 10.8 arcmin in half-power width (HPW). In spite of this improvement, the edge shapes of the sidewall were rounded. Therefore, both edges of the sidewall were ground and polished, and then the angular resolution was improved further to 3.2 arcmin (FWHM) and 5.4 arcmin (HPW).