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).

Grinding and chemical mechanical polishing process for micropore x-ray optics fabricated with deep reactive ion etching.

Silicon micropore optics using deep reactive ion etching of silicon wafers has been being developed for future x-ray astronomy missions. Sidewalls of the micropores through a thin wafer with a typical thickness of hundreds of micrometers and a pore width of ∼20 µm are used for x-ray mirrors. However, burr structures observed after etching with a typical height of a few micrometers at the micropore edges are known to significantly reduce x-ray reflectivity. A new grinding and chemical mechanical polishing process is introduced to remove the burr structures. Both sides of the silicon wafer were ground and precisely polished after etching. X-ray reflectivity measurements confirmed an increase of reflectivity by 2-15 times at incident angles of 0.8-0.2 deg. The surface microroughness worsened from 2.0±0.2 nm rms to 7.8-0.8+0.6 nm rms; however, an additional annealing recovered the smooth surface and the estimated surface microroughness was <1.4 nm rms. This new process enables not only removing the burr structures but also choosing a flat part of the sidewalls for better angular resolution.