Grazing-incidence reflectivity of Si-Au coatings for optics with high thermal load

Abstract

Measurements of the EUV, visible and near-infrared grazing-incidence reflectivity of Si-Au coatings are presented. Such coatings could be used for EUV optical components subjected to very high thermal load, as the optics for the EUV spectrometer of the Solar Orbiter mission. The mission consists in putting an orbiting spacecraft in close proximity (45 solar radii) to the Sun, then in a severe thermal environment (34 kW/m2). The thermal stresses are reduced if the optics looking at the disk are operated in grazing incidence. The common materials used as a grazing-incidence coatings with high EUV reflectivity have low reflectivity in the visible and near-infrared, on the contrary materials with high visible grazing-incidence reflectivity have poor EUV reflectivity. A suitable coating with high reflectivity both in the EUV and visible is a silicon layer (100-400 a) deposited on gold. The silicon has high EUV grazing-incidence reflectivity and is partially transparent in the visible and near-infrared, where the gold coating has high reflectivity. Measurements on Si-Au samples show both higher EUV reflectivity than gold samples and higher visible reflectivity than silicon samples.