Research On Remote Plasma Source Characteristics And Polishing Process

Vacuum plasma etching and polishing technology is an emerging,efficient and promising optical element surface processing technology,which can obtain the processing effect that is difficult to be achieved by traditional mechanical polishing techniques,especially the ultra-smooth surface of medium-large aperture aspheric optical element processing and manufacturing,has been the concern of the relevant industries and has been widely used in semiconductor and optical components manufacturing,become a key technology in the field of microelectronics and micro-nano optics manufacturing.In order to meet the technical requirements for ultra-high precision,high reflectivity,and ultra-low damage of optical components in optoelectronic equipment and weapon systems,it is necessary to develop a highly efficient and precise optical component processing method to meet the urgent neegs of optoelectronic equipment for manufacturing batch and high-precision optical components.Because of its special polishing mechanism,free radical vacuum plasma polishing technology removes atomic-scale chemical reactions from materials without damaging the surface and sub-surfaces of the material,thereby obtaining an ultra-smooth surface.In this paper,the RPS(Remote Plasma Source)microwave vacuum plasma etching polishing system,ion source characteristics and the polishing process of silicon carbide and fused silica were studied.The research group independently designed polishing system experimental platform equipped with vacuum testing system,pneumatic system,workbench system,cooling system,constant pressure control system,automatic control system and other auxiliary systems to satisfy the working conditions of RPS microwave source,a large area of uniform and stable free radicals is produced,and the characteristics of the ion source and the etching process of optical elements are experimentally studied.Plasma-excited active free-radical particles react directly with the material to be processed,and their concentration,temperature and distribution have an important effect on the polishing process.In this paper,the emission spectroscopy diagnostic method was used to estimate the plasma electron density by Lorentz's fitting and Stark broadening method.The spectral intensity values were used to calculate the electron temperature of the plasma at different positions.The effects of working gas pressure,argon gas flow rate,ratio of etching gas flow rate,microwave power,substrate speed,bias power,and etching time on etch rate and surface roughness of fused silica substrates were investigated.The relationship between the plasma characteristics and the evaluation index of etching effect was studied.The etch process of silicon carbide material using microwave power,RF bias power,and etching gas flow ratio was studied using orthogonal experiments with three factors and three levels.The influence of technological factors on the etching mechanism was analyzed.Finally,the experimental research on the planarization of the optical element substrate by RF magnetron sputtering combined with the RPS and ICP etching and polishing techniques was carried out by the composite process experiment.The evolution of the surface roughness and reflectivity of the optical components was analyzed in the different processing before and after.The results show that:(1)The RPS-350 Microwave Plasma Etching and Polishing System was designed and built to provide the conditions for further study of plasma source characteristics and polishing process experiments;(2)The electron density of the microwave free radical plasma is about 1016,and the electron temperature varies with the power at different positions.The overall electron temperature range is 1300?8800K,and the free radical distribution in the chamber has a good uniformity;(3)The optimal etching parameters of fused silica were as follows:working pressure of 120Pa,etching gas flow ratio(CF4:02)of 50:30,microwave power of 1800W,substrate rotation speed of 20rpm,DC bias of 50V,etching time of 30min,the etching rate at this time reached of 2.4nm/min,but also verify the uniform distribution of free radical particles and etching uniformity;(4)Among the three factors in orthogonal experiment of silicon carbide material,the order of the primary and secondary factors influencing the etching rate is:microwave power>RF bias power>etching gas flow ratio,primary and secondary order of affecting surface roughness:microwave Power>etching gas flow ratio>RF bias power,the optimal etching process parameters are microwave power of 2000W,etching gas flow ratio of 50:20,RF bias power of 50W,working pressure of 120Pa,etching time of 5min;(5)In the composite process experiment,the optimal planarization film deposition parameters were obtained by orthogonal experiment on the silicon carbide surface:target distance of 4.4cm,Ar flow rate of 40sccm,working pressure of 1.2Pa and RF power of 120W.After RPS microwave plasma etching for 180s,the surface roughness of the original substrate decreased from 2.3nm to 1.8nm,the surface reflectance decreased by one percentage point from that of the original sample;(6)ICP optimum etching process parameters:RF power of 150W,RF bias power of 50W,working pressure of 1Pa,etching gas flow ratio(CF4:02)of 25:5,The silicon carbide surface roughness decreased from 1.8nm to 0.9nm after 210s etching under this parameter,while the surface reflectance increased by two percentage points.