Ion Assisted Sputtering of Tungsten Oxide Solar-Control Films

Spectrally selective solar control films can greatly reduce solar heat gain from the infrared while admitting a high fraction of visible daylight. Single-crystal tungsten bronzes of general formula M_xWO₃, where M is a metal, can have sharp spectral selectivity, but this property is difficult to obtain in thin films. Reversible electrochromic coloration caused by insertion of alkali metal or hydrogen ions has been extensively studied in WO₃. Polycrystalline films were made by reactive magnetron sputtering of WO₃ together with substrate heating and/or ion bombardment. Bronze forming elements were introduced after deposition either by electrochemical injection of H⁺ or Li⁺ or by ion implantation of Ag and Cu. During deposition, H_xWO₃ or substoichiometric WOx could be formed by controlling the sputtering atmosphere. Films treated by ion bombardment had optical properties approaching those of single crystals.