Investigation of ALD coatings for next generation x-ray telescope mirrors

Future x-ray telescopes require reflective coatings on extremely thin mirror shells to achieve both high optical throughput and good angular resolution. Traditional coating methods such as DC magnetron sputtering create coatings with high density and low roughness, which have delivered quality coatings for NuSTAR and Chandra. However, these sputtered coatings have intrinsic compressive stress, which distorts the figure of the thin mirror shells, degrading angular resolution or necessitating complex correction process on fabrication. In this paper, we investigate Atomic Layer Deposition (ALD) for coating x-ray telescope mirrors. ALD has many potential advantages, including 1) Simultaneous, net distortion-free coating of both sides of thin mirrors, and 2) Uniform thickness control across multiple mirrors, essential for mass production. We study these capabilities through the ALD coating of both curved silicon mirrors and flat silicon wafers with 30 nm iridium layers. We investigate ALD coating using optical metrology to measure its figure distortion, x-ray reflectometer (XRR) to measure its surface roughness and density, and monitor its long-term stability.