Thin film parylene-aluminium filters (ParAlF) for silicon x-ray detectors in high energy astrophysics missions

The next generation of high energy astrophysics missions, driven by rapid, accurate, and highly sensitive silicon-based detectors, call for a new class of optical light filters. Minimal optical light contamination is required for detectors to maximize sensitivity in harsh space conditions. While externally mounted filter designs currently meet these functional requirements, they simultaneously introduce dead layers and are susceptible to stray optical background radiation through gaps between detector and filter surface, affecting the low energy x-ray detector response. This work presents the ongoing Parylene Aluminium Filters (ParAlF) project, that investigates the design, testing, and performance characterization of thin film optical filters coated directly on the surface of silicon x-ray detectors. Funded by the INAF Fundamental Research Program 2023, ParAlF aims to maximize x-ray transparency and minimize optical light entry for silicon-based detectors, utilizing a combination of Parylene and Aluminium thin films. Initially conceived to meet the requirements of Silicon Drift Detectors (SDDs) of the X and Gamma Imaging Spectrometer (XGIS) instrument onboard the Transient High Energy Sky and Early Universe Surveyor (THESEUS) mission, these filters offer potential for versatile applications in spaceborne as well as terrestrial high energy silicon-based detection systems. We present here the on-chip filter deposition concept and progress status of the ParAlF. Furthermore, the approaches employed for thin film depositions and sample characterizations, along with the next steps based on performance results from previous stages, will be highlighted.