Development of a thermal-oxide patterning method for mitigating coating stress-induced distortion in silicon mirrors for the Lynx X-Ray Telescope mission concept (Conference Presentation)

Abstract

The thermal oxide patterning method has been proven effective for compensating coating stress on flat silicon wafers. In this paper we report progress of applying this method on thin X-ray telescope silicon mirrors, produced by GSFC, which are 100 mm long, 0.5 mm thick, 30 degree in azimuthal span and 312 mm in radius of curvature (Wolter-I geometry). The mirrors’ front sides are sputter coated by 20 nm iridium layers which have ~-70 N/m compressive stress. Due to the difference in geometries, we developed a new FEA model to calculate the duty cycle in thermal oxide hexagon patterns to compensate the coating stress. In addition, a new fabrication process with customized photo lithographic tools has been developed to achieve optimum patterning-precision on curved surfaces. Measurement results show that the developed method can mitigate coating-induced distortion by a factor of ~4, corresponding to ~0.5 arc-second in RMS slope error. The residual surface error is dominated by mid-frequency ripples produced by the annealing process, which will be resolved in the future. The developed thermal oxide patterning method could be inexpensive and precise, which could be a candidate to resolve coating stress issues for the Lynx telescope in the future.