Protected silver coatings for reflectors

Abstract

For ground- and spaced based applications, Ag coated reflectors are indispensable because of their high reflectivity. The transport, assembling and storage of these reflectors takes a long time, before they are finally commissioned for the actual applications. To endure this period without a decrease of reflectivity, protective coatings with a final layer, which offers a high resistance to aqueous solutions and a low mechanical stress should be used. These criteria were taken into account for the selection of a final layer for a protected Ag-coating, which was applied for reflectors utilized in the CRIRES+- instrument (an IR spectrograph used at the VLT). Reactively sputtered Al₂O₃, SiO₂ and Si₃N₄ layers were investigated with regard to these criteria. In aqueous (basic) solutions, the investigated Si₃N₄ layers are more stable than the SiO₂ layers, and the SiO₂ layers are more stable than the Al₂O₃ layers. This shows the influence of the intrinsic material properties. The mechanical stress of the sputtered layers depends on the deposition conditions and thus on the selected parameters. A Si₃N₄ layer with a high resistance to aqueous solutions also offers a low and stable mechanical stress. Therefore, the deposition-parameters which have been used for this layer were applied for sputtering the final layer of the protected Ag-coating for the reflectors.