Ultra-precision Grinding Technology Of Monocrystalline Silicon Substrates For Laser Reflector

Laser reflector is one of the core optical components in high-energy laser system.Monocrystalline silicon substrate is used for fabricating laser reflector and is required with a high surface quality and a high surface shape accuracy.At present,a process route of the monocrystalline silicon substrate is "lapping-polishing-surfacing”.In this process route,lapping is the key process because surface shape accuracy and surface damage after lapping greatly affect the accuracy and time of subsequent polishing.However,it is not only difficult to control the surface damage and surface shape accuracy,but also material removal rate is low in lapping process.In order to solve the problems,an ultra-precision grinding process based on the workpiece rotational grinding method is proposed to replace the existing lapping method for machining the monocrystalline silicon substrate in this thesis.According to processing characteristics of the workpiece rotary grinding method and machining requirements of the monocrystalline silicon substrate,the surface layer quality of the ground monocrystalline silicon substrates,the surface layer quality of the ground monocrystalline silicon substrates is evaluated by surface roughness and subsurface damage,the grinding wheel for low damage grinding is developed,the control method of surface shape accuracy of the monocrystalline silicon substrate considering effect of the clamping deformation are studied.Therefore,the high precision and low damage grinding process of the monocrystalline silicon substrate is finally developed.The main research contents and conclusions are as follows:(1)In order to control the surface quality of the monocrystalline silicon substrate ground by using the workpiece rotational grinding method,a model of grain cutting depth in ductile grinding is established.In this model,the minimum grain cutting depth,the radius of abrasive cutting edge and the elastic recovery of monocrystalline silicon are considered.The surface roughness and the subsurface damage depth are predicted.The influences of grinding speed on surface quality when grinding at the constant grain cutting depth are also analyzed.The prediction accuracy of grinding surface roughness and surface damage depth reach 78%and 73%,respectively.The results show that surface roughness and surface damage depth of the ground monocrystalline silicon substrate decrease with the increase of grinding speed.(2)In order to achieve ultra-precision and low-damage grinding of monocrystalline silicon substrate,the compound abrasive grinding wheel(CAGW)made of ultra-fine diamond and cerium oxide abrasive and the magnesium oxychloride bonded soft abrasive grinding wheel(SAGW)made of cerium oxide abrasive are developed.Through analyzing the surface roughness and the subsurface damage depth of the ground monocrystalline silicon substrate,the grinding performances of both grinding wheels are investigated.The results show that the surface roughness of the silicon substrate ground by using CAGW is about Ra 5.6 nm,the subsurface layer only present such damages as amorphous microstructure and dislocation,and the subsurface damage depth is about 100 nm,while the surface roughness by using SAGW is less than Ra 0.6 nm,and the depth of subsurface damage merely presenting as amorphous microstructure is less than 30 nm.(3)The effect of clamping deformation on surface shape accuracy of the monocrystalline silicon substrate is analyzed when it is ground under clanping with a vacuum chuck,the control method of surface shape accuracy of the ground monocrystalline silicon substrate is studied.A measuring device is designed and developed to measure the clamping deformation of the substrate,which is caused by clamp force due to incomplete fitting between the positioning surface of the substrate and the vacuum chuck surface.Through the actual measurement and finite element simulation,the effect of clamping deformation on surface shape accuracy of the ground silicon substrate is analyzed.Considering the clamping deformation of the substrate,the relationship between the ground surface shape of the substrate and the angle of grinding wheel spindle is established,and the method to control the ground surface shape accuracy by adjusting the angle of the grinding wheel spindle to compensate clamping deformation of the substrate is proposed.Experimental result shows that the ground surface shape accuracy of the substrate with a dimension of φ100 mm×6 mm is decreased from PV 4.68 μm before grinding to PV 1.32 μm after grinding.(4)The ultra-precision grinding process of the monocrystalline silicon substrate with low damage surface and high surface shape accuracy is developed.According to the material removal mechanisms and the processing characteristics of grinding monocrystalline silicon with UCGW and SAGW.the processing strategies for control feed grinding with the UCGW and control force grinding with the SAGW are proposed correspondingly,the processing parameters for grinding the monocrystalline silicon substrate by using the UCGW and the SAGW in sequence are determined.The surface shape accuracy,surface roughness and subsurface damage depth of the ground monocrystalline silicon substrate reach PV 1.41μm,Ra 0.7 nm and 30 nm respectively,the accuracy and surface quality of grinding meet the requirement of the subsequent chemical mechanical polishing(CMP).