Research On Fabrication Key Technology Of Polyimide Membrane Diffractive Lens

The next generation of super large aperture space optical system needs new principle and technology,which breaks through the limitations of traditional technology.Microstructure imaging technology is one of the representatives.Using micro-nano structures to modulate the amplitude and phase of light waves,and preparing micro-nano structures on large-diameter optical membrane substrate material can greatly reduce the weight of main mirror,and then combining multiple main mirrors by splicing,can realize larger aperture and foldable function.This technology becomes the trend of future space imaging optical system.However,it is not easy to prepare large-diameter optical membrane substrate materials and micro nano structures on its surface,which meet the requirements.This paper will focus on these problems and carry out research on the key technology of polyimide membrane lens preparation.In this paper,the application requirements of large aperture diffractive lens and main technical bottlenecks in the preparation of polyimide membrane lens are studied,and a new technology route for the preparation of polyimide membrane lens is proposed.Based on the optical design and tolerance distribution of polyimide membrane lens,the synthesis of optical grade polyimide material and film forming process are studied.The micro-structure fabrication and testing of large aperture polyimide membrane lens are overcomed.The influencing process factors of wavefront error of polyimide membrane lens are studied,and the imaging quality of polyimide membrane lens is improved.The main research work includes:(1)The research background and significance of polyimide membrane diffractive lens are introduced.Through the analysis of research status at home and abroad,this paper summarizes the processing technology difficulties and existing main processing methods of polyimide membrane lens,and points out the advantages and technical bottlenecks of the mainstream process.(2)The basic theory of diffractive optical system is introduced.Based on the theory of wave optics,schupmann achromatic model is used to eliminate the chromatic aberration of diffractive primary mirror with diffractive secondary mirror group.The system aperture is 1.5m,the spectral range is 0.55?m~0.65?m,the field of view is greater than 0.12°,the modulation transfer function(MTF)is greater than 0.5 at 62.5lp/mm.The diffractive optical system is designed with ZEMAX optical simulation software with wavefront error better than ?/5(?=0.6?m).The design and tolerance distribution of off-axis edge sub-lens are studied.The diameter of diffractive pattern is 352 mm,the lens material is polyimide,the number of levels is 4,the minimum linewidth of microstructure is 2.1?m,the linewidth error and the depth error are less than 10%,the nesting error is less than 0.5? m,the position error is less than 1.0?m,and the average diffractive efficiency is hiher than 65% and wavefront error is less than 20 nm.(3)The synthesis and molding of optical grade polyimide membrane materials were studied.The polyimide membrane was prepared by polycondensation reaction with MTOL as diamine,PMDA and BPDA as dianhydride,NMP as solvent,followed by spin coating.The film was cured at 60 ? and imidized at 380 ?.By adjusting the ratio of PMDA and bpda,the performance of polyimide was controlled.Through the optimization of the glass transition temperature(TG),coefficient of thermal expansion(CTE),mechanical properties,UV spectrum and other properties,the polyimide membrane which can basically meet the technical specifications was obtained.On this basis,the influence of process parameters such as rotational speed and glue concentration on film thickness and uniformity was studied.After optimization of the process parameters,good optical uniformity was achieved on the large diameter polyimide membrane.(4)We studies the processing method of diffractive lens based on polyimide membrane substrate,analyzes the difficulties in the processing of film lens,puts forward a new technological route,separates the film forming and the preparation of diffractive pattern,overcomes the coupling problem of transmission wave front,places the film support in the front of diffractive pattern processing,and solves the pattern distortion problem caused by the sudden change of stress.The experimental results show that the error of the microstructure of the diffractive lens is less than 10%,the diffraction efficiency is higher than 60%,and the RMS of the wavefront error is less than 30 nm.The test results are close to the theoretical analysis,which shows that the technical route proposed in this paper is feasible.(5)The influence factors of wavefront error of polyimide membrane lens were studied from three aspects.Firstly,based on the optical uniformity of the thin film substrate,the correction technology based on the reactive ion etching method to improve the membrane optical uniformity is proposed.The RMS of the transmission wave front of the membrane substrate is better than 20 nm.Secondly,the influence of polyimide material's hygroscopic swelling property on the phase difference of lens wave and the method of improving the dimension stability of the material are studied.Finally,the photoelastic properties of the thin film materials are studied,and the photoelastic coefficient of the thin film is about 400nm/MPa.cm The mathematical relationship between the optical delay angle and the ratio of the principal stress is analyzed and summarized.The physical relationship between the stress distribution,the optical delay amount and the delay angle is established,which is helpful to improve the stress uniformity in film support and fixing.