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 depositionparameters which have been used for this layer were applied for sputtering the final layer of the protected Ag-coating for the reflectors.

Large-scale collecting mirrors for ITER optical diagnostic

“…Properties of the protecting films depend on layer thickness, composition and surface morphology, which in their turn are function of deposition parameters. Reactively sputtered Al 2 O 3 , SiO 2 and Si 3 N 4 layers were investigated with regard to protect silver to aqueous solutions [38]. It was shown that the influence of the intrinsic material properties is higher than the influence of the deposition conditions.…”

PECVD SiNx Thin Films for Protecting Highly Reflective Silver Mirrors: Are They Better Than ALD AlOx Films?

“…Currently, a number of different materials are used for the thin film protective coatings, including Al 2 O 3 , TiO 2 , HfO 2 , Y 2 O 3 , Nb 2 O 5 , Ta 2 O 5 , SiO 2 , SiON x and Si 3 N 4 , as single films or in stacks [3][4][5][6][7][8][9][10][11][12][13][14][15][16]. Most of them are magnetron sputtered, but also ion-beam sputtered, evaporated with ion assistance and deposited with atomic layer deposition (ALD).…”