| With the development of optical technology,large-aperture optical elements are used more and more widely in modern optical systems.The main application areas are:laser system devices,large-aperture astronomical telescope devices,solar devices,etc.In these high-performance large-scale optical system devices,every optical element is inseparable from the optical film,especially the high reflecting coating for large-diameter mirror which plays a reflective role.At present,with the increasing diameter of optical elements,the coating process of large-diameter mirror has been the focus of the field of thin film preparation.The gravity of the large-aperture reflector,the temperature change of the reflector produced during the coating process,and the film stress between the reflector and the film will all affect the surface shape of the mirror and affect the optical performance of the reflector.Therefore,to prepare a high-performance large-aperture high-reflective film,it is necessary to establish a finite element model of the surface shape change of the large-aperture mirror coating according to the structural parameters and material characteristics of the large-aperture mirror,and simulate and analyze the gravity,temperature rise,and film stress.Only by predicting the deformation size of the mirror surface in advance can the deformation control during the coating process be truly achieved.(1)This article first analyzes the surface deformation of the mirror.The working environment of the reflector is analyzed,and the design basis of the reflector is put forward.A three-dimensional model of a 650mm flat-back and open-back lightweight SiC mirror was established,and the model was imported into Ansys for finite element analysis.The reflection was analyzed in a coupled field with an inertial load of 9.8N/m2 and a temperature change of 80℃ on the mirror surface.For the performance of the mirror assembly,the RMS value of the mirror surface shape is 2.91 nm,and the PV value is 9.45 nm.On the basis of this finite element model,the film compressive stress of metal Al and the substrate is added to the reflector to-540Mpa,and the RMS value of the mirror surface shape is 51nm and the PV value is 111nm.(2)Secondly,the film thickness uniformity analysis was carried out on the Al film prepared by the thermal evaporation of the 1.1m coating machine.According to the geometric configuration of the vacuum chamber,the uniformity model of the film thickness of the large-diameter Al film was established.By controlling variables and changing the geometric configuration of the vacuum chamber,the uniformity of the Al film thickness is analyzed.When the evaporation characteristics of the Al evaporation source are constant,the factors that affect the distribution of the Al film thickness are the distance L from the evaporation source to the center of the vacuum chamber and the distance H from the substrate to the center of the vacuum chamber.When L=400 mm and H/L=1.10,the film thickness uniformity is the best,the unevenness is 9.614%,and the unevenness increases as the value of H/L increases.When H=500 mm and H/L=1.47,the film thickness uniformity is the best,the unevenness is 4.487%,and the unevenness increases as the value of H/L decreases.In order to further improve the uniformity of the Al film thickness,a correction function was introduced according to the film thickness distribution curve of the Al film under the rotating plane fixture,and a suitable correction baffle was designed to solve the problem of the uniformity of the Al film thickness.The film thickness uniformity of the Al film is improved from 17.8%when the correction baffle is not added to 3.9%,which further improves the film thickness uniformity.(3)Finally,the preparation process of the metal high-reflective film is explored.Vacuum thermal evaporation was used to prepare an Al film with a peak reflectivity of 90.3%,an average reflectivity of 88.87%,a thickness of 80nm and an Ag film with a peak reflectivity of 98.3%,an average reflectivity of 97.2%and a thickness of 110nm in the 380～780nm band.The influence of the thickness of the SiO2 protective film on the reflectivity of the Al film and the influence of the transition layer materials Cr and Al2O3 on the adhesion of the Ag film to the substrate are respectively explored.The results show that the thickness of the SiO2 protective film will affect the reflectivity of the Al film.When the thickness is about 100nm,the reflectivity of the Al film reaches the minimum value of 83.97%.When the thickness of SiO2 is about 200nm,the reflectivity of the Al film reaches the maximum value of 89.45%;the plating Cr and Al2O3 transition layer materials strengthen the adhesion of the Ag film to the substrate,and the effect of the Cr transition layer is better than that of Al2O3. |
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