Study Of Hafnium Dioxide Film As The Interlayer Of Anti-reflective Protection For Sapphire Infrared Window Application

Sapphire crystal is one of the common candidate materials for MWIR detectionwindow. However, its can not fully meet the future development trend ofhigh-precision IR imaging in the future. Besides, the anti-fog performance ofsapphire is poor. All of these will limit the application of sapphire IR window tosome extent. Silica(SiO2) is the preferred material as antireflective protective film ofsapphire in MWIR band, but there is serious thermal mismatch between sapphireand SiO2, which will lead to film delamination. In this study, in order to overcomethe film delamination, HfO2and SiO2were prepared for anti-reflective protectionbilayrer on sapphire, in which HfO2is as the intermediate layer for its high refractiveindex and SiO2is the outer layer for its low refractive index. Also, it was proposedfor the first time by using perfluoro dodecyl trichlorosilane (FDTS) to modify thesurface of sapphire with SiO2/HfO2bilayer to improve its hydrophobicity.Thetheoreticalcalculation results by Filmstar software showedthat the averagetransmittance of sapphire with SiO2/HfO2bilayer is93.2%in the range of3~5μm,and there is almost no influence on the transmittance with the variation of thethickness of HfO2in some range. The deviation analysis shows that, therefractiveindex of SiO2and HfO2filmshave significantimpactson the transmittanceofsapphire, while there are almost no influence on the transmittancewith thedeviationof film thickness.The sputtering experimental results of HfO2films showed that, with theincreasing RF power, the film deposition velocityis increased, and the O/Hf atomicconcerntration ratio of HfO2film is close to its standard stoichiometry, but the filmhas best crystallization properties and refractive index when RF power is150W.With increasing flow ratio of O2/Ar and sputtering pressure, the film deposition rateis decreased, and the O/Hf atomic concerntration ratio of HfO2film is close to itsstandard stoichiometry, but the refractive index is closest to the threotical valuewhen the flow ratio of O2/Ar is20:10and the sputtering pressure is5Pa. Theannealing in oxgen atmosphere can siginificantly improve the compositionstoichiometry, crystalline property, and its optical and mechanical characteristics.When the annealing temperature is800℃, the atomic ratio of O/Hf reaches1.9,which is closed to the standard stoichiometric ratio (2.0). The annealed film also hasa refractive index of2.09at632.5nm, which is closed to the theoretical value (2.08).Simultaneously,the film hardness and elastic modulus reach their largest valueof of77.5GPa and elastic modulus of235.4GPa, respectively. The performance chrarcterization of sapphire with SiO2/HfO2bilayer showedthat the antirefractive and protective performance is improved by introducing HfO2interlayer. The film adhesion of SiO2/HfO2bilayeris increased by70%comparedwithSiO2monolayer. The transmittance of sapphire with SiO2/HfO2bilayerdepositedon both sides is95.2%in the range of3~5μm, and transmittance at high temperatureis also significantly improved. The laser induced dymage threshold (LIDT) ofSiO2/HfO2bilayer is20.1J/cm2, which is increased by1.2J/cm2compared with SiO2monolayer.The surface modification experimental results show that the surface energy ofsapphire deposited with SiO2/HfO2bilayer is decreased by the fluorine element. Thecontact angle of surface with water is increased from58.7oup to120.1oaftermodifying by FDTS, and there is almost no influence on its transmittance. Therefore,the application fields of sapphire infrared windowsare broadened to a large extent.