Reflective coatings for the future x-ray mirror substrates

We present the development of the reflective coating by magnetron sputtering deposition onto precisely-fabricated thin X-ray mirrors. Our goal is to remove distortion induced by the coating and then keep their surface profiles. We first addressed the uniform coating to minimize the distortion by introducing a mask to control the spatial distribution of the coating thickness. The uniformity was finally achieved within ±1%. We next tried a platinum single-layer coating on a glass substrate with a dimension of 200 mm × 125 mm. The distortion caused by the frontside coating with a thickness of 320 Å was found to be at most ∼ 1 μm, smaller than the previous results obtained from the non-uniform coating. We then carried out the platinum coating with the same amount of the thickness on the backside surface of the glass substrate. The surface profile of the glass substrate was fully recovered, indicating that the residual stress was successfully balanced by the backside coating. Furthermore, we tried to an iridium single-layer coating with a thickness of 150 Åon the silicon mirrors. The frontside coating caused the degradation of the imaging quality by 7.5 arcsec in half-power width. However, the backside coating with the same amount of the thickness reduced this degradation to be 3.4 arcsec. Finally, an additional backside coating with a thickness of 100 Å and the annealing to relax the residual stress were found to eliminate the distortion completely; the final degradation of the imaging quality was only 0.4 arcsec.