Thorium free antireflection coating in MWIR region on Silicon optics

Most infrared transmitting optics have high refractive indices which in turn have high per surface reflection loss. So antireflection coating has very important role in increasing the transmission in the desired wavelength region. In this paper a study has been carried out on the design and fabrication of Thorium free antireflection coating effective for Silicon substrate in MWIR (3.6–4.9 μm) region. The wave band 3.6–4.9 μm is chosen for the reported work because the detected system used in MWIR region has a band selection filter effective in the same wavelength region. Comprehensive search method was used to design the multilayer stack on Silicon substrate. The coating materials used in the design were Germanium (Ge) and Silicon dioxide (SiO₂). The fabrication of coating was made in a coating plant fitted with Cryo pump system and Residual Gas Analyzer. The evaporation was carried out at high vacuum (2–6 × 10–6 mbar) using Electron Beam Gun and layer thicknesses were measured with crystal monitor. The result achieved for the antireflection coating was 96% average transmission in 3.6–4.9 μm band which withstood MIL-F-48616 environmental testing. This work provides an alternate antireflection coating on Silicon by replacing radioactive Thorium Fluoride, used as a coating material in most IR antireflection coating designs.