Coating AXIS mirrors with near zero net-stress

Future x-ray telescopes require reflective coatings on extremely thin mirror shells to achieve both high optical throughput and good angular resolution. DC magnetron sputtering, the standard coating method for missions like Chandra and NuSTAR, produces coatings with high density and low roughness, but in the meantime introduces intrinsic compressive stress that distorts the figure of the thin mirror shells and degrading the angular resolution. To address this challenge, we are developing a stress compensation method using silicon oxide patterned onto the backside of thin silicon mirror with UV-direct mask-printing and acid etching. This approach is scalable, cost effective and ready for mass production, allowing precise local stress control without complex photo-lithographic processing. Initial results on silicon mirror shells demonstrate a successful stress compensation and satisfied repeatability, making this technique promising for future x-ray telescope missions. This paper will report on the status and results of this work.