Non-Newtonian Fluid Polishing Of Single Crystal Silicon

Single crystal silicon is a kind of semiconductor material with excellent performance,which is widely used in large-scale integrated circuits and optical fields.Its application puts forward higher and higher requirements for the surface shape accuracy and surface roughness of single crystal silicon.As an anisotropic hard and brittle material,single crystal silicon is prone to surface defects caused by brittle removal during processing,and it needs to be polished to improve the surface quality.It is of great significance to research and develop the high-efficiency precision polishing technology of single crystal silicon to solve the problems of low processing efficiency,complex equipment and high cost of the existing polishing methods.In this study,non-Newtonian fluid is proposed to polish single crystal silicon.Based on the study of the rheological properties of different non-Newtonian fluids,that is,the shear thickening characteristics and viscoelasticity,combined with the numerical simulation of fluid polishing,the composition ratio of polishing fluid and the polishing parameters are optimized.The precise polishing of the single crystal silicon surface is realized finally.In this paper,shear thickening viscous polishing fluid and shear thinning viscoelastic polishing fluid are selected to study the rheological properties.The effects of different components on the rheological properties of shear thickening viscous polishing fluid were studied through orthogonal experiments.The results show that the effect of each component on the shear thickening characteristics are in descending order of the dispersion phase mass fraction,the type of dispersion medium,the particle size of abrasive,and the mass fraction of abrasive.The higher the mass fraction of dispersed phase is,the stronger the thickening effect is,and the compounding of dispersed media is beneficial to enhance the effect of shear thickening.The effect of xanthan gum mass fraction on the viscosity-temperature characteristics and dynamic viscoelasticity of shear thinning viscoelastic polishing fluid was studied by single factor experiment.The addition of xanthan gum is beneficial to improve the viscosity and dynamic modulus of the fluid.When the mass fraction is 0.5%,the comprehensive performance of the fluid is the optimal.The constitutive equations of two types of non-Newtonian fluids were obtained by fitting power law model and PTT model respectively,and the hydrodynamic simulation was conducted using the Fluent and Polyflow integrated in Ansys.The flow field pressure,velocity and shear rate which affect material removal were analyzed.The characteristics of the flow field distribution at different fluid composition ratios of shear thickening viscous polishing fluid,and the effects of different xanthan gum content of shear thinning viscoelastic polishing fluid and polishing disc channel corner on the flow field distribution in the polishing area were studied and obtained.The optimized non-Newtonian polishing fluid was used to polish the single crystal silicon wafers.The surface roughness Ra of silicon wafer is reduced from 68 nm to 11 nm,by using shear thickening viscous polishing fluid.The amount of polishing fluid is only 5 ml/min,which shows that it can achieve precision polishing of single crystal silicon wafer with low loss.The silicon wafer with initial surface roughness Ra of about 10 nm was polished with shear thinning viscoelastic polishing fluid.The effects of polishing time,abrasive particle size and channel corner on the processing quality and material removal rate were studied.When the channel corner is 0°,the abrasive particle size is 3 μm and the polishing time is 10 min,the wafer surface roughness Ra is reduced from 11.05 nm to 4.21 nm.