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Coating process parameter influence on thin films for the ATHENA x-ray optics

The thin film deposition technology for fabrication of the mirror optics for the Advanced Telescope for HighEnergy Astrophysics (ATHENA) has been established. Numerous coating process parameters impact the quality of the thin films. Defining a margin within the coating process parameter space, where the deposited thin film performs similar in X-ray reflectivity is key to avoid unforeseen risks within the coating process for the ATHENA flight optics production. In this work, we investigate the coating process parameter influence on the thin film properties with a focus on micro roughness, deposition rate and residual film stress when deposited under various process conditions. The thin films were produced by varying the following three coating process parameters: discharge power, discharge voltage and working gas pressure. The thin films were characterized using X–ray reflectometry at 3.4–10.0 keV. A main result of this work is that the residual stress of single layer iridium and boron carbide films can be reduced by a factor of approximately two, by increasing the working gas pressure while maintaining a high film quality.