Research On The Key Technology Of Optical Thin Film For SWIR/MWIR Detection System

With the rapid development of science and technology,infrared detection t echnology has gradually upgraded from single-band detection to multi-band fusi on detection.The short-wave infrared and medium-wave infrared?SWIR/MWIR?detection systems combine SWIR and MWIR detection technologies to acquire the unique advantages in low-light night vision,camouflage identification,and penetration of rain and fog,which has attracted the most attention in the field ofinfrared detection technology.The optical elements in a SWIR/MWIR detecti onsystem have a high refractive index.So,when light is incident on the surfac eof an optical element,the light will have a large energy loss,which will wea ken the energy entering the system,and reduce the accuracy and response spee dof the detection system.Therefore,it is necessary to deposit an optical film o nthe surface of the optical element,to improve the transmittance of the optical element,reduce the energy loss,thereby improving the performance of the det ection system.At the same time,SWIR/MWIR detection systems may work un der complex weather conditions such as rain and sand.Especially in military fi eld,such as reconnaissance,tracking,and air-to-air missile interception,the syst ems may face extreme environmental conditions represented by high and low t emperature,high humidity,and mold.Therefore,the optical films used in SWI R/MWIR detection systems not only need to have excellent optical properties with high transmittance,but also need to have good environmental resistance t o high and low temperature,mold,and corrosion.The optical films in SWIR/MWIR detection systems with good environmental adaptability is of great signi ficance to improve the performance of the systems.In this research,an optical film suitable for 1.5?5?m SWIR detection syst em is developed on the surface of a Si substrate.Specifically,Si_xGe_1-x,SiO,a nd MgF₂ are used to develop the anti-reflective film,and perhydropolysilazane?PHPS?cured Si O₂ is used to develop the protective film.The prepared film ha s the properties of high transmittance,friction resistance,and corrosion resistan ce.The specific research content is as follows.For the requirements of 1.5?5?m band and transmittance of T?96%,based on the calculation results of Willey's formula,Si O and MgF₂ were selected as the materials for preparing the thin films with medium and low refractive inde x,respectively.By contrast,the conventional high-refractive-index materials cou ld not meet the requirements,and there is a need to develop Si_xGe_1-x-x hybrid fi lmas the high-refractive-index thin film material.According to the requirements for the transparent region and refractive ind ex of the new high-refractive-index material,a vapor-phase mixed evaporation t echnique was used to study the preparation of the high-refractive-index Si_xGe_1-x-x film.Ar:H₂ ion beam was used to reduce absorption caused by amorphous def ects.According to OJL model and KKR relationship,the optical properties of Si_xGe_1-x film were fitted.To solve the problem that low-refractive-index MgF₂ film easily absorb m oisture,the preparation of low-water-absorption MgF₂ film was investigated.O₂w as used as the ion source reaction gas to increase the aggregation density whil e filling anion vacancies to suppress the inter-band absorption and chemical wa ter absorption caused by crystal structure defects,and reduce the adsorption of water vapor by MgF₂.For the requirements of the film's environmental resistance and spectral performance,the protective film with low absorption and high transmittance was researched.The protective film was prepared by spin coating curing with perhydropolysilazane,which filled the pores of the film layer,blocked water vapor,and improved the environmental resistance.Research was carried out on the uniform spin coating technology and low-temperature plasma curing technology ofperhydropolysilazane liquid film.Finally a SiO₂ protective film meeting the preset requirements was prepared.Using the above-mentioned thin film materials,the research on the structure design of film systems was carried out.Based on the linear programming model and the equivalent layer theory of a symmetric film system,the initial film system structure of wide-band high-transmittance thin film was designed.The genetic algorithm was used to optimize the structure of the film system,and its sensitivity to film thickness was analyzed.In order to reduce the film thickness sensitivity of the film system,a new evaluation function with optimization capabilities in both spectrum and film thickness was constructed.Besides,the film system with low film thickness sensitivity,wide spectrum and high transmittance was designed.For the problem of poor adhesion between the MgF₂ thin film and the Si_xGe_1-x-x film during the preparation of the film system,research was conducted to improve the adhesion of the MgF₂ thin film.According to the thermal stress moment model and adsorption theory,combined with the requirements for the material's refractive index by the film system design,the Al₂O₃ and SiO₂ by vapor-phase mixed evaporation were used as the transition layer which effectively improved the firmness of the film.The prepared film was tested for spectral performance and environmental r esistance,and the film properties were analyzed according to the test results.T he test results showed that in the range of 1.5?5?m,the average spectral trans mittance of the prepared film was 97.6%and the minimum spectral transmittan ce was 96.8%.At the same time,the prepared film showed excellent environm ental resistance to salt spray,mold,high and low temperature,which could me et the requirements for the optical elements used in SWIR/MWIR detection sys tems.