Study On Stress Properties And Stress Deformation Control Of Ta₂O₅/SiO₂ Optical Films Deposited By Ion Beam Sputtering

High-precision optical systems are increasingly demanding optical films with better optical and mechanical properties.The optical components should not only meet the requirement from high spectral characteristics,ultra-low absorption and scattering losses,but also high environmental stability and high surface accuracy.Ion beam sputtering(IBS)technology has been widely used in high precision optical film fabrication,due to its stable performance,excellent optical and mechanical properties.However,the films deposited by IBS technology usually suffer from high compressive stress,which would result in surface distortion,coating cracking,shedding and other failure phenomena.It is of great theoretical significance and engineering application value to systematically study the stress properties and stress deformation control technology of optical film deposited by IBS.In this paper,the effects of oxygen flows and sputtering methods on stress and optical properties of Ta2O5 and SiO2 films were investigated.The regularities of stress evolution of Ta2O5 and SiO2 films at different annealing temperatures were systematically investigated;based on which,a method of large curvature radius accuracy control by using the stress properties of SiO2 film was proposed.A stress control model for IBS deposited Ta2O5/SiO2 multilayer was established;a film thickness monitoring strategy based on a broadband complex film and an optimization model for thickness uniformity correction were proposed;the stress deformation control of Ta2O5/SiO2 multilayers on large-aperture optical elements is successfully realized.The main conclusions are drawn as follows:1.In the process of preparing Ta2O5 film,the absorption of Ta2O5 film can be effectively reduced by adjusting the oxygen flows.Ta2O5 film has high environmental stability.The stress state of Ta2O5 films can be obviously controlled by subsequent annealing treatment.The reduction of oxygen flow results in a higher deposition rate and film surface roughness.The wavefront deformation of Ta2O5 films increases linearly with film thickness(within 20 ?m).After 60 days of aging test at room temperature,the results show that the stress of Ta2O5 film was stable and no release was observed.Annealing treatment can affect the stress state,optical properties and structure of Ta2O5 films.With the increase of annealing temperature,the compressive stress of Ta2O5 films decreases.When the annealing temperature rises up to 591 K,the stress of the films begins to change from compressive stress to tensile stress,and the tensile stress increases with the increase of annealing temperature.With the increase of annealing temperature,the optical thickness and the surface roughness of the film increase.and the stoichiometric ratio of surface elements tends to an ideal stoichiometric ratio.When annealing temperature rises up to 933 K,the amorphous Ta2O5 film crystallizes to hexagonal phase.2.In the process of preparing SiO2 film,the sputtering method can obviously change its microstructure,optical and mechanical properties.SiO2 film has high environmental stability.Subsequent annealing treatment can significantly reduce the stress of SiO2 film.The SiO2 film obtained by directly sputtering SiO2 target has lower roughness and less compressive stress.The wavefront deformation of SiO2 films increases linearly with film thickness(within 30?m).After 60 days of aging test at room temperature,the results show that the stress of SiO2 film is stable and with no releasing.Annealing treatment can affect the stress state,optical properties and structure of SiO2 films.With the increase of annealing temperature,the compressive stress of SiO2 film decreased linearly.However,stress of SiO2 film still maintains compressive during annealing treatment.Based on the regularity of stress evolution of SiO2 films at different annealing temperatures,the method of accurately controlling the radius of curvature of quartz optics with large curvature is proposed.With this method,the radius of curvature can be controlled within 0.2%,and the quartz optics can also maintain good optical properties because of the closeness of the index of SiO2 film to bulk material.3.Based on the regularity of stress evolution of Ta2O5 and SiO2 films at different annealing temperatures,the stress control model of Ta2O5/SiO2 multilayer was established.The stress deformation of Ta2O5/SiO2 multilayer can be effectively reduced by using this model.Considering the effect of Ta2O5/SiO2 period number on the wavefront deformation of optics,the Ta2O5/SiO2 multilayer stress control model was then modified.The wavefront deformation control of no-quarter-wave(QW)high reflection coating was realized by adjusting the thickness ratio of Ta2O5to SiO2 film.In order to maintain the spectral characteristics at center wavelength of single wavelength coating,the "absentee layer" was introduced to optimize the multilayer stress control model,and the wavefront deformation control of the quarter-wave high reflection coating is realized.In order to maintain the broadband spectral characteristics of the complex coating,the "buffer layer" was proposed to optimize the multilayer stress control model,and the wavefront deformation control of the anti-reflection coating is realized without spectral characteristics changes in the broadband range.4.For complex coatings thickness monitoring,the film thickness error can be effectively reduced and the yield of the coating can be improved by using the broadband optical monitoring combined with time monitoring and wavelength-indirect monitoring strategies.A monitoring strategy combining broadband optical monitoring and time monitoring is proposed to realize thickness control of sensitive layer,ultra-thin layer and index matching layer;a wavelength-indirect monitoring strategy with small random film thickness errors is proposed to realize indirect monitoring of wavelengths in broadband complex coatings.Current thickness uniformity correction model is optimized by introducing the actual shielding arc length correction factor into it,and the correction efficiency and precision of the film thickness uniformity are improved.A film thickness uniformity better than±0.1%in the 360 mm aperture is achieved by using the optimized model.