The Experiment And Mechanism Research Of Magnetic Field Assisted Electrochemical Mechanic Polishing Of Single Crystal Silicon Carbide

Silicon carbide(SiC)crystal is the third generation of semiconductor materials,it has many advantages such as stability,thermal conductivity and high wear resistance,excellent electrical properties.It is widely used in aerospace,communications and other fields,but SiC is a typical hard and brittle material,unsesistant impact,sensitive to defects.The traditional grinding and polishing method has low material removal rate(MRR)and many surface defects.Magnetic field assisted electromechanical mechanical polishing(MECMP)technology,combined with the magnetorheological and electrochemical effects,can simultaneously meet the processing quality and processing efficiency requirements.In this paper,through the study of magnetic field,electric field,flow field simulation and polishing experimental research,the feasibility of this compound processing method is studied to realize the efficient removal of silicon carbide,and the influence of different process conditions on the grinding polishing characteristics is obtained,so as to realize the ultra precision machining of silicon carbide.First the MECMP testing machine has been independently developed.The auxiliary magnetic field has been simulated and optimized.Through the analysis of the force of the charged particle and abrasive force in magnetic field,the influence of magnetic field on electrochemical reaction and the effect on magnetic polishing pad and abrasive are discussed to further understand the effect mechanism of the magnetic field.Then,the three-dimensional CFD model considering multiphase flow and discrete phase is established.The velocity and pressure distribution of polishing liquid and the distribution of polishing abrasive grains under different working conditions are studied.It is found that the smaller the film thickness is,and the higher polishing disk and wafer speeds,the smaller distribution density of the abrasive grains is.The dynamic pressure process of the abrasive grains on the wafer surface during the polishing process is analyzed.Based on the theory of the energy conservation of fatigue fracture,a prediction model of the MRR under various working conditions is established,The MRRs of silicon carbide wafers under different working conditions are obtained.It is found that the MRR of SIC wafer decreases with the increase of the polishing process.And the smaller the film thickness is,the smaller MRR is.Next the electric field is simulated by using Maxwell software.The influence law of the polishing pad,polishing pad holes and excitation voltage is studied respectively on the electric field distribution.Results show that when the electric field travels down through the silicon carbide,current density concentrates upon the surface of the specimen.There is an obvious "skin effect".The current density changes at the interface between the polishing fluid and the pad.When polishing pad is added,the surface voltage of the wafer increases and the current density decreases,but the fluctuation of the electric field regularity increases.The distribution of the small hole of the polishing pad has little influence on the surface electric field distribution;with the increase of the excitation voltage,the current density increases,and the electrochemical anode loss mainly occurs at the small hole of the polishing pad.Finally,a data acquisition system is built to acheieve the collection and recording of real experimental data.The effects of different polishing fluid,external electric field,auxiliary magnetic field and magnetorheological fluid on the friction and wear properties of SIC wafer were studied.The results show that the MRR under dry friction condition is high,but the surface quality of the specimen is poor.The friction coefficient and the MRR are different under different conditions.Under the same conditions,The MRR of 5% K2MnO4 and 4% CeO2 was the highest and can be improved by applying positive voltage and external magnetic field to the specimen.The magnetic fluid mixed with K2MnO4 solution as the grinding liquid can produce better grinding effect.With the assistance of electric field and magnetic field,Magnetorheological fluid with 5% K2MnO4 and 4%CeO2 solution combined with composite polishing liquid can produce maximum material removal rate of 3.87mg/h.