Electron beam evaporated rigid optical solar reflectors for space applications

Abstract

Rigid optical solar reflectors (OSRs) for space applications were developed by using the electron beam evaporation technique. The component layers of the full stack were optimised for their electrical and optical properties. Thin films of Ta₂O₅ show absorption in the ultraviolet–visible range at high substrate temperatures, which can be eliminated by deposition at an oxygen partial pressure of >4 × 10⁻⁵ torr. Under optimised deposition conditions, indium–tin oxide (ITO) films have a sheet resistance R_sh of <0·95 kΩ/□ and a transmittance of <95% in the visible region. The electrical and optical properties of the OSR stacks were investigated as a function of various deposition parameters, such as substrate temperature, thickness and deposition rate. It was found that the functional properties of the OSRs developed with the electron beam technique are equivalent to those of the rigid OSRs deposited by the sputtering technique. A solar absorptance α_s value of <0·047 and a sheet resistance R_sh value of <3 kΩ/□ were obtained for the OSR deposited under the optimised conditions. The samples were found to be environmentally durable.