| With the rapid development of mobile communications and semiconductor technology industries,new generation optoelectronic crystal materials such as sapphire(Al2O3),gallium nitride(GaN),and silicon carbide(SiC)are increasingly used in IC and optoelectronic devices.As new semiconductor materials,single crystal SiC,GaN,Ga2O3 and sapphire have become the necessary materials for semiconductor devices because of their outstanding electrical,thermal and optical properties.They are widely used in mobile communications,high-speed rail transit,aerospace,optoelectronic devices and other industries.With the development of optoelectronic wafers in the direction of large size and ultra-thinness,the grinding and polishing requirements of optoelectronic wafers have also increased,requiring low roughness(industrial requirements Ra?0.1 nm),low subsurface damage,high surface accuracy,and ultra-thinning.Therefore,efficient planarization machining of ultra-smooth surface with low machining damage and residual stress has become a challenge in the application of high-performance devices.Magnetorheological finishing technology is a flexible and stable ultra-precision processing method with low roughness and low damage.Aiming at the problems of high hardness,difficult interface reaction,low processing efficiency and high cost of optoelectronic wafer materials,a global dynamic pressure planarization processing method of high efficiency photoelectric wafer cluster magnetorheological finishing is proposed,which combines the controllability,low damage and high pressure and high efficiency of magnetorheological finishing technology with high pressure and high efficiency of hydrodynamic pressure.To realize the ultra-smooth and low-damage planarization processing of optoelectronic wafers,the main research contents are as follows:(1)Based on the tribological principle and hydrodynamic pressure theory,the shear force and positive force model of the cluster magnetorheological global dynamic pressure flattening process were deduced,and the polishing shear force and positive force were detected by a rotary dynamometer.The correctness of the two mechanical models was verified.The corresponding material removal model was established according to the mechanical model and speed model.Two-inch sapphire was used for polishing experiments to verify that the theoretical calculation results of the material removal rate were in good agreement with the actual measurement results.From the theoretical point of view,it was proved that the cluster magnetorheological global dynamic pressure planarization processing was feasible(2)The rheological properties of magnetorheological polishing fluid were measured by rotating magnetorheometer,and the Bingham constitutive model of the polishing fluid under the condition of the processing magnetic field strength was constructed.The fluid simulation model of magnetorheological dynamic pressure ultra-smooth planarization processing based on 3D microstructure was established.The mechanism of 3D microstructure strengthening polishing force is revealed.Simulation results show that the 3D microstructure on the disk surface can enhance the magnetic flux density mode,reduce the fluid flow rate,strengthen the fluid pressure and the polishing pad acting on the surface polishing force of the workpiece.Two-inch single crystal sapphire specimens were used for polishing experiments of smooth polishing disk and group-holes polishing disk.The experimental results were in good agreement with the simulation results.(3)By optimizing the design of the 3D microstructure of the polishing disk surface,a rectangular array of micro-holes was made on the surface of the polishing disk to increase the fixation force of the polishing disk to the polishing pad immediately generated in place.The material removal rate of the group-holes polishing diss was higher than that of the smooth polishing disk,the material removal rate is as high as 107%,the surface roughness Ra is reduced by 34.6%,and the PV value is reduced by 75%.The process parameters of magnetorheological dynamic pressure ultra-smooth planarization processing based on 3D microstructure of group holes were optimized by single factor experiment,and the original surface roughness of sapphire can be reduced from Ra 5.14 nm to Ra 0.24 nm.The material removal rate of the group-holes polishing disk using diamond,SiO2,Cr2O3 and silica sol is higher than that of the smooth polishing disk,but the surface roughness of the silica sol processing is reduced while the roughness of other abrasive processing is increased.The chemical action produced by the hydration reaction of silica sol and sapphire has a synergistic effect with the mechanical removal process of abrasive.The effect of polishing sapphire with silica sol abrasive and group-holes polishing disk is obviously higher than that of smooth polishing disk.(4)A flow field simulation model based on gap-varying magnetorheological ultra-smooth planarization processing under dynamic pressure was established.The results show that the flow field of variable-gap magnetorheological finishing has periodic dynamic changes and turbulence formation.The pressure and velocity on the workpiece surface show periodic changes.The positive polishing pressure and material removal rate obtained through polishing experiments verify the feasibility of the simulation model.Combined with the CFD simulation results,a gap-varying magnetorheological ultra-smooth planarization processing under dynamic pressure is proposed to achieve active control and enhancement of the flow field in the polishing zone,further control the fluid characteristics,achieve the enhancement of mechanical removal,and appropriately increase the frequency.Properly increasing the frequency and amplitude of variable gap is helpful to obtain better polishing effect.Secondly,the variable clearance movement of the workpiece promotes the radial movement of the abrasive along the surface of the workpiece.The impact angle between the abrasive and the surface of the workpiece is generally larger,which enhances the shearing force of the abrasive particles on the surface of the workpiece,and the surface roughness decreases rapidly.Finally,the surface roughness of sapphire can be reduced from Ra 6.44 nm to Ra 0.081 nm after 5 h polishing.(5)The systematic process parameters of single crystal sapphire wafer are optimized by cluster magnetorheological global dynamic pressure planarization machining method,and the optimized process is used to polish the sapphire.The workpiece surface can obtain a low roughness and low damage processing surface,and the local surface roughness is less than Ra 0.4 nm,but the large surface roughness in other areas leads to uneven distribution of the surface roughness of the workpiece.In order to solve the problem of poor polishing uniformity,a mathematical model of the abrasive particle motion trajectory of the cluster magnetorheological global dynamic pressure flattening process was established,and the uniformity of the abrasive particle motion trajectory distribution was evaluated by the variation coefficient of standard deviation.The deflection method,deflection amplitude and optimal process parameters of the wafer are tested.The polishing experiment results are in good agreement with the numerical simulation results.Using the sapphire wafer to carry out the process polishing experiment after uniformity optimization,the average surface roughness after polishing can be reduced from Ra 5.61 nm to Ra 0.33 nm,and the surface roughness difference is reduced from ?Ra 0.67 nm to ?Ra 0.21 nm,realizing a global photoelectric wafer ultra-smooth,low-damage flattening processing.The results of this thesis's research work on cluster magnetorheological global dynamic pressure planarization processing theory and implementation methods have not only been successfully used in the ultra-smooth,low-damage flattening processing of optoelectronic wafers,but also provide a new technical approach for the finishing of the new generation of optoelectronic wafers. |
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