Infrared transparent conductive coatings deposited by activated reactive evaporation

摘要

Abstract: Infrared transparent conductive coatings were developed for ZnS substrates. The conductive layer was an indium oxide film deposited in a reactive atmosphere consisting of low energy oxygen ions. Activation of the gas molecules was performed by an ion source of the hollow cathode type. The deposited films were characterized by their transparency, conductivity and environmental durability. It was found that the positioning of the ion source relative to the substrates was critical for the film properties. High transmission and low sheet resistance of the film were obtained by optimization of the evaporation parameters which included oxygen pressure, cathode current, deposition rate, film thickness and substrate temperature. Application of the conductive film for both 3 - 5 $mu@m and 8 - 12 $mu@m infrared atmospheric windows was possible. The transmission of the indium oxide film depend on the infrared free carriers absorption and the reflections at the substrate and air interfaces. Improvement of the optical performance was accomplished by depositing antireflective and induced transmission dielectric stacks. Several dielectric stacks were designed and matched to indium oxide layers of 50 approximately ega@/sq and 30 approximately ega@/sq sheet resistance for maximum transmission, either at a single wavelength or for a wide band in the 3 - 5 $mu@m and 8 - 11.5 $mu@m spectral regions. Typical average transmittance values of ZnS substrates coated with conductive stacks were 82% and 74% in the 3 - 5 $mu@m region and 77% and 63% in the 8 - 12 $mu@m region for sheet resistances of 50 approximately ega@/sq and 30 approximately ega@/ sq, respectively. At 10.5 $mu@m peak transmittance values higher than 80% were obtained. The coatings were durable and passed adhesion, humidity and moderate abrasion tests in accordance with MIL-C-675C. !