Industrialization of the mirror plate coatings for the ATHENA mission

The Advanced Telescope for High-ENergy Astrophysics (ATHENA) was selected by the European Space Agency (ESA) as their future flagship to study X-rays generated in the hot and energetic Universe. ATHENA’s foreseen throughput and spectral resolution will provide ground-breaking science about active galactic nucleus, black holes, galaxy formation etc. ESA is continuing its efforts to mature the optics technology and the associated mass production techniques for ATHENA. The optics technology is based on the state-of-the-art Silicon Pore Optics mirror technology employing a bi-layer coating composed of iridium and boron carbide (top-layer). The boron carbide is of great significance to achieve the science objectives of ATHENA. The low density material increases the effective area of the optics at 1 keV. For the mission to be formally adopted (planned for late 2021) efforts are driven by the programmatic and technical requirement of reaching technology readiness level 6, including industrializing the mirror plate coatings.
The main purpose of this thesis was to establish a new coating facility dedicated for the ATHENA mission to demonstrate the capability of processing 300 mirror plates per day (∼100,000 mirror plates in a time-frame of two years), while at the same time reproducing stabile coatings. This work included the design, install, test, and commission of a new coating machine, to which the well-established coating process and techniques developed at the Technical University of Denmark (DTU) Space were successfully transferred. A direct current magnetron sputtering drum coater (BS1500S) was purchased and set to work in the Netherlands in-line with the stacking facility. Experiments were established to investigate the film quality and the coating uniformity. The iridium thin film quality deposited in the BS1500S fulfilled the requirements listed by ESA, having excellent X-ray reflecting properties and chemical process compatibility. The boron carbide thin films indicated good time stability with a dense structure. A preliminary result implies that the boron carbide thin film is fairly resistant to the chemical exposure.
In parallel with the establishment of the new coating facility, a continuation of the iridium and boron carbide thin film deposition technology was carried out. Efforts to increase the effective area at 6 keV led to the development of an optimized linear graded multilayer composed of iridium and boron carbide that was deposited on mirror plates, after which they were exposed to a chemical process to enable atomic bonding of the mirror plates. The thickness, density and roughness of the iridium and boron carbide layers were derived by modeling the X-ray reflectance in an energy range from 3.4 – 10.0 keV. The composition and the time-stability of non-reactively and reactively sputtered iridium and boron carbide single layers were systematically studied. The films were characterized by means of X-ray photoelectron spectroscopy and 8.047 keV X-ray reflectometry. The iridium films were stabile with time; this was shown from the reflectivity curves. Another result was observed for the boron carbide films, for which the measured reflectances indicated a significant reduction in layer thickness within a week. Indications of boron oxide formation on the surface of the boron carbide thin films propagating into the film interior with time were shown in the composition study. The boron carbide composition was changing rapidly when stored in atmospheric environment.
The stability of the boron carbide thin films indicated a strong chamber and target material dependency, showing the complexity of employing the material for X-ray optics and raising the awareness of potential degradation with time.