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The thin film coating technology for the European Space Agency mission, Advanced Telescope for High-Energy Astrophysics (Athena) has been established. The X-ray optics of the Athena telescope is based on Silicon Pore Optics (SPO) technology which is enhanced by the thin film coatings deposited on the reflective surface of the SPO plates. In this work, we present a literature study of the coating process parameter space and provide an overview of the thin film properties with a focus on micro roughness, chemical composition and wear resistance when deposited under various process conditions. We determined, that the thin film density depends strongly on the mobility of the adatoms on the substrate surface. Some coating process parameters, which have a significant impact on the adatom mobility are the discharge voltage, the working gas pressure and the substrate temperature.

The Advanced Telescope for High ENergy Astrophysics (Athena) is a Cosmic Vision L-class mission of the European Space Agency selected for launch in the early 2030s1. The observatory features a single X-ray telescope with a 12 m focal length. The focusing Wolter-I optics is based on Silicon Pore Optics (SPO) technology2. The Athena science objectives require a high performance in the 0.3-12 keV range which is obtained with X-ray reflective thin films.

As part of the thin film development for the Athena X-ray telescope and X-ray optics in general, we investigated the residual stress in iridium and chromium thin films deposited using direct current magnetron sputtering. Residual stresses in thin films can affect the performance and adhesion properties of the fabricated thin film coated X-ray optics. We characterized the thin films using X-ray reflectometry to determine the thin film thicknesses and stylus profilometry to determine the residual film stresses. To counterbalance the compressive stress identified in the iridium thin films, we introduced a chromium thin film layer for which the residual stress is tensile beneath the iridium film. However, chromium thin films are known to exhibit a grainy growth resulting in a high surface roughness which was also observed in this work. In this paper, we evaluated the effect on the iridium surface roughness when introducing a chromium underlayer and discussed the effect on the X-ray optics performance.

The demand for X-ray optics increases and so do their performance requirements. Iridium (Ir) thin film coatings have proven to be very robust and exhibit excellent reflective properties within the X-ray regime. However, the residual stress in Ir thin films is high and studies show that thin film Ir coatings with a thickness of 10–30 nm exhibit residual compressive stress1 as illustrated in Figure 1.

Excellent X-ray reflective mirror coatings are key in order to meet the performance requirements of the ATHENA telescope. The baseline coating design of ATHENA was initially formed by Ir/B4C but extensive studies have identified critical issues with the stability of the B4C top layer which shows strong evolution over time and appears incompatible with the industrialization processes required for the production of mirror modules. Motivated by the need for a compatible top layer material to improve the telescope performance at low energies and based on simulated performance, a SiC top layer has been selected as the best substitute to B4C. We report the latest development of Ir/SiC bilayer coatings optimized for ATHENA and the characterization of coating performance and stability.

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.