Reflective coating for lightweight x-ray optics

X-ray reflective coating for next generation’s lightweight, high resolution optics for astronomy requires thin-film deposition that is precisely fine-tuned so that it will not distort the thin sub-mm substrates. Film of very low stress is required. Films with multi-layer or bi-layer can be deposited to give an effective low stress which cause negligible distortion. Alternatively, mirror distortion can be cancelled by precisely balancing the deformation by coating films on both sides of the substrates. We have been developing techniques to coat glass substrates that can provide good reflectivity in the soft x-ray band below 10 keV, and yet introduce negligible surface distortion for arc-second optics. These efforts include: low-stress deposition by magnetron sputtering and atomic layer deposition of the metals, balancing of gross deformation with two-layer depositions of opposite stresses and with depositions on both sides of the thin mirrors.