Temperature resistant anti-reflective coating on Si-wafer for long-wave infra-red imaging

A micromachined Silicon lid, sealed by CuSn solid liquid interdiffusion bonding is a promising approach for hermetic sealing of microbolometers for use in low-cost thermal cameras. However, since ∼30% of long-wave infrared light is reflected at an uncoated single Si-air interface, anti-reflective treatments are required. Traditional anti-reflective coatings are inapplicable since CuSn solid liquid interdiffusion bonding requires heating to about 270 °C and these multi-layer coatings fail due to differing coefficients of thermal expansion for the different layers and the substrate. For this purpose, an anti-reflective coating that maintains its anti-reflective properties after being heat-cycled to 300 °C has been developed. This coating was developed using a simple 2-layer structure composed of ZnS and YF₃ and deposited at 100 °C. The development process that led to the successful coating has also been described in this paper. The final sample shows a 30% average increase in transmission in the 8-12 μm wavelength range as compared to an uncoated wafer.