Surface investigation and aluminum oxide estimation on test filters for the ATHENA X-IFU and WFI detectors

The ATHENA mission provides the demanded capabilities to address the ESA science theme "Hot and Energetic Universe". Two complementary instruments are foreseen: the X-IFU (X-ray Integral Field Unit) and WFI (Wide Field Imager). Both the instruments require filters to avoid that the IR radiation heats the X-IFU cryogenic detector and to protect the WFI detector from UV photons. Previous experience on XMM filters recommends to employ bilayer membrane consisting of aluminum deposited on polyimide. In this work, we use the X-ray Photoelectron Spectroscopy (XPS) to quantify the native aluminum oxide thickness that affects the spectral properties of the filter. The estimation of the oxide thickness of the prototype filter for ATHENA is a considerable information for the conceptual design of the filters.