Temperature resistant anti-reflective coating on Si-wafer for long-wave infra-red imaging
Résumé
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, antireflective 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 YF 3 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 8-12µm wavelength range as compared to an uncoated wafer.
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