Numerical Simulation And Analysis Of Surface Quality In Ultra-fine Grinding Of Monocrystalline Silicon Wafer

Back thinning process is a critical part of semiconductor chip production,and unnecessary carrier material at the back side of IC is removed in this process.The rotational grinding is widely used in the back thinning process of wafer.Grinding process causes subsurface damage in work material inevitable and the extension of subsurface damage will cause the failure of chip.To guarantee the quality of chip,the subsurface damage of wafer will be removed in the following polishing process.But the material removing rate of polishing is ten times lower than grinding.In this way,the surface quality and subsurface damage need to be controlled in back thinning process to reduce the workload of polishing process and improve the efficiency of the whole production line.Therefor the prediction of surface quality and subsurface damage depth in grinding process has significant meaning to research of grinding mechanism and optimizing of grinding parameters.With the development of computing technology,the grinding process can be simulated using numerical method.Simulation models now available can output some feature of grinding marks according to grinding parameters.The work material is assumed as rigid plastic material in most available rotational grinding simulation models for hard and brittle material.But according to the research of nanoscale cutting process of hard and brittle material,the contact status between grains and work material is elastic-plastic in ultra-precision grinding of hard and brittle material,which is different with the fundamental assumption of simulation models now available.What is more,most subsurface depth prediction model takes depth of crack as the evaluation index of subsurface damage.But in ultra-precision grinding,subsurface damage exists in forms of dislocation,thus the subsurface damage depth prediction model now available still cannot be used in simulation of ultra-precision grinding.Taking the kinematic characters of rotational grinding and material removing feature in ultra-precision grinding into account,a new numerical simulation model which can predict the surface topography and subsurface topography of square simulation zone is proposed based on Matlab platform.Different roughness parameters and maximum depth of subsurface damage which can reflect the surface and subsurface feature of simulation zone are analyzed and evaluated in this article.The results of experiments proved that the simulation model proposed in this article can reflect the surface and subsurface feature of ultra-fine grinding.The main contents of this paper are as follows:(1)Taking the kinematic characters of grains and simulation zone in rotational grinding into account,a new simulation model that can predict the surface topography of square simulation zone is developed based on Matlab platform.This model can output the topography of simulation zone and different roughness parameters useful for the evaluating of topography in simulation zone.(2)Taking the elastic plastic effect of work material,cutting edge radius of grains,critical cutting depth of chip forming and amount of effective grains into account,a new material removing model applicable to ultra-precision grinding is developed.This material removing model is integrated into the surface topography prediction model and the effect of new factors in material removing model on roughness parameters is analyzed.The accuracy of new surface quality prediction model is verified by the comparing of simulation results and rotational grinding results in different grinding parameters.(3)A new subsurface damage depth prediction model that takes dislocation depth as the evaluation parameter of subsurface damage depth is developed by build the relationship between cutting depth of grain,scratching force of grain and depth of subsurface damage in work material.By integrating this subsurface damage topography prediction model into the surface topography prediction model,a simulation model that can predict the subsurface topography of simulation zone is developed.The accuracy of this model is verified by comparing of prediction results and rotational grinding results in different grinding parameters.