Characterization of carbon thin films as an overcoating candidate material for the optics of NewATHENA

The NewATHENA mission has been chosen by ESA to cover the topic of “The hot and energetic Universe”. This will be carried out by studying the x-ray Universe with a space-based telescope to be launched in the second half of 2030s. Several materials have been considered as overcoating candidates for the Silicon-Pore Optics (SPO) mirror plates developed for NewATHENA. Previously, carbon did not qualify as an overcoating material since it was not compatible with the manufacturing processes. However, new coatings made in the dedicated chamber for NewATHENA located at cosine Research BV, together with an improvement on the selected cleaning procedure, have shown that carbon is now compatible with the process. This work covers the characterization of the mirror coatings for NewATHENA following a complete and representative wet chemistry process. The primary focus is on the overcoating carbon layer and the hybridisation of the atomic orbitals within its constituent atoms. For the characterization we used X-Ray Reflectometry (XRR) and X-ray Photoelectron Spectroscopy (XPS) techniques. We performed θ−2θ reflectivity measurements at fixed energies of 1.487 keV and 8.048 keV, and at a fixed incident angle of 0.6° from 3.4 to 10.0 keV. We performed Angle Resolved XPS (ARXPS) measurements with an energy of 1.487 keV. We found that both techniques are in agreement showing an sp3 -content on the samples around 20%, indicating a certain level of diamond-likeness. Although it remains unclear if this is the reason for the compatibility of carbon with the manufacturing process, our work shows that the species of carbon grown in the film are able to ensure a good stability of the mirror.