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Investigation of photolithography process on SPOs for the Athena mission

As part of the ongoing effort to optimize the throughput of the Athena optics we have produced mirrors with a state-of-the-art cleaning process. We report on the studies related to the importance of the photolithographic process. Pre-coating characterization of the mirrors has shown and still shows photoresist remnants on the SiO2- rib bonding zones, which influences the quality of the metallic coating and ultimately the mirror performance. The size of the photoresist remnants is on the order of 10 nm which is about half the thickness of final metallic coating. An improved photoresist process has been developed including cleaning with O2 plasma in order to remove the remaining photoresist remnants prior to coating. Surface roughness results indicate that the SiO2-rib bonding zones are as clean as before the photolithography process is performed.