Study On Thermodynamic Matching Between Substrate And Films In Optical Thin Film Technology

Temperature change affects optical films throughout their entire life cycle from preparation to application.During the temperature change,the mismatch of the thermodynamic properties between films and substrates can easily lead to mechanical failure of the films,especially in the specific high and low-temperature environments application.For example,warhead and aircraft windows in flight must withstand the temperatures of 600?1000?,and infrared detection optical elements in outer space need to work under -200?.Therefore,matching the thermodynamic properties of film and substrate is critically important for thin-film technology.The principal objective of this study was to investigate the mechanism and methods of matching the thermodynamic properties of optical films and substrates.We investigated the thermodynamic matching of films with substrates from the following three perspectives:the thermodynamic properties of the materials,the distribution rules of thermal stress,and the improvement of the deposition methods.Based on these studies,we proposed several countermeasures and suggestions for improving the mechanical properties of optical thin films.Five parts are included in this work:The influence factors of the thermodynamic matching for the films and the substrates are introduced.The main methods of measuring thermodynamic parameters of thin films are also summarized.Then we discussed the researching significance and researching methods of matching the mechanical properties of thin films.At the same time,the relationship between film stress,adhesion and film failure mechanism are discussed.Based on the theoretical analysis of the factors affecting stress,adhesion and fracture energy,the thermodynamic matching suggestions between films and substrate are concluded.We analyzed the thermal stress distribution and stress concentration of Ta₂O₅ and SiO₂,high and low refractive index thin film materials commonly used in visible to near-infrared,deposited on four substrates?Si,Zn Se,CaF₂,fused silica?with a large difference in thermal expansion coefficients to consider the results of matching the same film system with different substrates.Theoretical and finite element method models were established to analyze the thermal stress distribution of multilayer films.The generations of ?-and ?-form cracks are discussed by analyzing the moment.The influence of Young's modulus and thickness on the failure of films are also discussed.Based on ellipsometry,we designed and constructed an experimental optical platform to measure the thermal expansion coefficient of optical films.Through measuring the variation of the ellipsoid parameters after heating,we fitted and calculated the thermal expansion coefficient of optical thin film materials.With this method,we measured the thermal expansion coefficients of five single-layer optical thin films,including Al₂O₃?Ti O₂?Ta₂O₅?Si O₂?Yb F₃ and Zn S.The results show that the thermal expansion coefficient of the films is almost one order of magnitude larger than that of the bulk materials,and the thermal expansion coefficient of the films decreases with the densification.At the same time,the temperature dependencies of the refractive index of the films were derived,and the stress states corresponding to the failure states of the films were discussed.We counted the adhesion and stress characteristics of several films with large difference of thermal expansion coefficient in visible and infrared waveband and proposed the relevant thermodynamic matching schemes between films and substrates.The failure of films is influenced by many factors,which includes the mechanical properties of the film itself,the stress accumulation of the film system,the thermal stress between the film and the substrate,the adhesion matching and the relative stiffness between the film and the substrate.In addition,we also discussed the thermodynamic matching of Ti O₂/Al₂O₃ infrared filter,Ta₂O₅/Si O₂ visible antireflection film,Ti O₂/Al₂O₃ three-wave segment objective antireflection film.Film matching of substrate materials with high coefficient of thermal expansion,such as Ga F₂?FK51?Zn Se?Ba F₂,were discussed.The spectral transmittance of the infrared filter decreases within 2.76?3.04?m wavelength band.We addressed the problem by material selection,improvement of deposition method and different membrane system collocation.We compared the matching of the three material combinations?Ti O₂/Al₂O₃?Zn Se/Yb F₃?Ge/Al₂O₃?with the Si substrate,and proposed a mixed deposition method.An infrared filter with an average transmittance of 92.2%was prepared.It breaks the mechanical properties restriction on the transmittance of infrared bandpass filters,and the spectral transmittance within 2.76?3.04?m has been increased by 12.2%.