Mid-infrared optical coatings with improved transparency for 3 to 6μm spectral region using sputtered silicon and oxide films

Mid infrared optical coatings are commonly designed & manufactured using typically electron beam evaporated films of Silicon and Silicon oxide. However the transparency of these coatings is limited by optical absorption in the films when producing coatings for wavelengths beyond 4um approximately. This work reports improvements in mid infrared (3 to 6um) filter transparency achieved by exploiting recent advances in microwave plasma assisted pulsed DC magnetron sputtering technology. Sputtered silicon compound films have been used to demonstrate efficient room temperature deposited optical coatings for gas sensing applications at wavelengths between 3 to 6um. This process technology allows a new selection of film materials to be used in design of mid infrared filters, with transmission and thermal drift characteristics enhanced compared with conventional electron beam evaporated coatings. The spectral location of the optical coatings is controlled by a non-optical method, which avoids the complex optical monitoring configurations normally required. Durable filters are obtained at room temperature, which would otherwise be required in conventional evaporation processes. Importance of water partial pressure control during deposition for mid infrared is emphasised.