Preparation of Ag–Fe₂O₃-Based black and electrically insulating coatings by magnetron sputtering from metal targets

Black and electrically insulating coatings provide touch panel displays with a classic appearance. In this study, black and electrically insulating Ag–Fe₂O₃-based coatings were prepared using reactive magnetron sputtering, stainless steel and silver foil. The thin films prepared with oxygen introduction and no Ag foil showed blue light absorption and high electrical resistance, which are comparable to those of Fe₂O₃ single crystals. The optimal Ag foil size for maintaining a high electrical resistivity of 10⁸ Ω·sq⁻¹ was determined to be 54 mol%Ag in the film samples. Black and electrically insulating thin films composed of metallic Ag, amorphous Ag–Fe–O, and slightly crystallized Fe₂O₃ were prepared using 3.4 or 3.9 %O₂. Metallic Ag segregated randomly in the high-refractive-index Fe₂O₃-based matrix, resulting in plasmonic absorption over a wide range of wavelengths centered in the red-wavelength region. At 3.9 %O₂, a high sheet resistance of 10⁸ Ω·sq⁻¹ and strong visible absorption of 35.2 μm⁻¹ (on average), slightly deviating by 2.4 μm⁻¹ were simultaneously achieved by clear phase separation of the Ag grains and Fe₂O₃-based matrix. A clear phase separation mechanism based on thermodynamic nonequilibrium oxidation of the metallic Ag phase is proposed.