Design and Fabrication of Thin Film a-Si/Al2O3 Infrared Filter

ABSTRACTLong-wavelength infrared filter operated at various temperature are critical for imaging applications. In this paper anew type of infrared filter is investigated, this infrared filter uses a crystalline silicon substrate coated with multi layers of aluminum oxide (AL2O3) and amorphous silicon (a-Si) to produce the multi layers thin film infrared filter a-Si/AL2O3 operating in the range (8-14)um. Amorphous silicon is used in this paper due to its high refractive index, while AL2O3 due to its low refractive index material. The a-Si/ AL2O3 thin film structure were designed using the Thin Film Design software (TF Calc 3.5.6 version). The simulated results obtained shows that the transmittance of the a-Si/ AL2O3 infrared filter with 19 layers is about 90% for (8-12.5) m wavelength, while it is about 95% for 47 layers for (8.25-13.25) m wavelength. The investigated filter was fabricated using vacuum evaporation process and the results obtained were comparable with the simulated one. The fabricated a-Si/ AL2O3 filter is compared with the Ge/ZnS infrared filter and it is found that the results is comparable but the cost of the fabricated filter is small compared to the Ge/ZnS filter.Keyword: Infrared Filter, Silicon, Aluminum Oxide.