Assessing substrate quality and contamination on thin film coatings for X-ray optics

This work investigates the importance of substrate quality and contamination of thin film coatings, essential for high-energy reflective mirrors used in astronomical missions and other applications, focusing on the manufacturing process from substrate selection to final assembly of the optics. Characterization of thin film coatings is crucial for evaluating the performance of X-ray mirrors. Multifaceted and synergized methodologies utilizing Xray reflectometry (XRR), X-ray photoelectron spectroscopy (XPS) and optical microscopy (OM) are presented. Herein, we discuss coating performance, encompassing substrate preparation, coating deposition, and storage conditions. XRR scans provide detailed insights into thin film properties, however, the dependence on accurate a priori knowledge necessitates a robust model for solving the inverse problem. Addressing this limitation, XPS proves invaluable in revealing the chemical composition of thin films, improving the accuracy of the XRR model. Combined characterization through OM and XRR is very useful to find visual insights into surface contamination-induced changes when mirrors are stored for long periods in a clean room environment, as might be the case for some astronomical missions. The synergy among these techniques is pivotal for evaluating coating quality for high-energy astronomical telescopes, with a specific focus on NewAthena and upcoming missions. This research not only advances methodologies in this field but also highlights the collaborative power of XRR, XPS, and OM in providing a comprehensive understanding of thin film coatings, emphasizing the importance of pre-coating mirror quality and mitigating contamination effects throughout the optics production process to ensure optimal performance.