Characterisation of iridium and low-density bilayer coatings for the Athena optics

The future Athena observatory will feature optics with unprecedented collecting area enabled by silicon pore optics technology. In order to achieve the telescope effective area requirements at 1 keV and 7 keV, thin film coatings of iridium with a low-density overcoat are deposited onto the mirror substrates. Assembling the coated silicon pore optics plates into mirror modules for the Athena optics requires wet chemical processing and thermal annealing. While iridium appears to be compatible with the post-coating processes, previous studies have shown degradation of the low-density material. The overcoat layer is particularly critical for the low-energy telescope performance, so several candidate materials (boron carbide, silicon carbide and carbon) have been studied to identify a compatible thin film design. We present the characterisation of x-ray mirror performance using x-ray reflectometry, as well as the measurements of residual film stress with stylus profilometry. Furthermore, we evaluate the effects of post-coating treatment in order to recommend the most suitable overcoat material for the telescope.