Study On Removal Characteristics Of Variable Aperture Ion Beam Based On Aperture Method

With the development of modern optical technology,the level of optical processing has also improved significantly.On the one hand,larger optics enable higher levels of processing accuracy,on the other hand,The accuracy of shape error of optical elements with different spatial frequencies is improved.The most representative modern optical processing technology is the ion beam polishing technology based on the CCOS(Computer Controlled Optical Surface)principle.The ion beam processing is used replace of the traditional polishing.The non-contact polishing method can effectively avoid the wear of the polishing head,Edge damage and other issues,and the definitive polishing effect is good.By optimizing the ion beam diameter,it effectively enhances the ion beam's ability to modify the shape.In this paper,the diameter of the ion beam emitted from the ion source is reduced by clamping the diaphragm,to optimize the ion beam polishing tool,and to obtain the removal characteristic of fused quartz,K9 and glass-ceramic under different parameters of the diaphragm assembly.The main tasks completed are as follows:(1)Analysis of material characteristics of optical elements.By analyzing the three common optical glass components,the characteristics of different materials are obtained.And SRIM software to analyze the sputtering yield of different materials is used.With the same incident ion energy,It is found that fused quartz has the largest yield and the smallest crystallites,and K9 is somewhere in between,when the ion beam is incident vertically.(2)Diaphragm structure design.According to the optimal beam beam waist position and beam spot size obtained earlier,a simulation analysis based on the aperture method of variable aperture ion beam is performed.The target distance and the size of the diaphragm are used as the analysis objects.The larger the diameter,the higher the peak intensity of the ion beam,and vice versa.The structure of the diaphragm assembly was designed by using graphite as the diaphragm material,and 5mm,8mm,12 mm,and 16 mm aperture diaphragms and 10 mm,15mm,and 20 mm target distance diaphragm assemblies were designed.(3)Stability analysis of the ion source.The Faraday cup scanning and line scanning etching methods were used to analyze the ion beam stability.The etching morphology of different positions on the line scanning grooves were analyzed,The fluctuation range of the corresponding peak removal rate and the full width at half maximum at different positions is within 5%,and the ion source stability is good.(4)Ion beam etching experiment using diaphragm method.The ion source was clamped with different parameter apertures,and a Faraday cup scan was performed to obtain the ionbeam intensity distribution.Different optical materials is etched to obtain the corresponding removal function distribution.The change trend of the ion beam removal function under different target distances and aperture diameters is analyzed,and the removal characteristics between different materials are analyzed,compared with the intensity distribution of the ion beam current,and explained by the material-related constants.According to the obtained fused silica removal characteristics of different beam diameters,the realization of the equal removal of the ion beam is analyzed,and the optimal superposition distance for equal removal is obtained.By polishing a 140 mm × 140 mm fused silica planar optical element(the initial surface RMS value is 76.284nm),With a target distance of 10 mm and an aperture stop of 16 mm,the RMS value of the workpiece to be processed was reduced to 18.015 nm.And then a 5mm aperture is used,the RMS value of the workpiece to be processed was reduced to 12.037 nm.