Research On Ultrasonic-photoelectric Assisted Chemical Mechanical Polishing Of Single Crystal Silicon Carbide

Silicon carbide（SiC）has superior chemical and mechanical properties,such as wider energy gap,higher thermal conductivity,lower thermal expansion coefficient,and higher breakdown electric field,compared with first-and second-generation semiconductor materials,and its application prospects are very promising.In this paper,based on the analysis of domestic and international SiC polishing technologies,the ultrasonic-photovoltaic-assisted chemical-mechanical polishing of SiC is investigated by combining simulation and experiment with the aim of obtaining polishing process parameters with high material removal rate（MRR）and high surface quality.Firstly,the electric field under photoelectric assisted action was modeled and simulated.The influence of polishing fluid conductivity on the current density on the lower surface of the specimen and the distribution of the current density on the lower surface of the specimen were derived from the simulation analysis,and the influence of the conductive ring on the current density distribution and the characteristics of the electric field distribution at the defects on the surface of the specimen were analyzed.The results show that:the current density increases with the increase of polishing fluid conductivity,and the current density distribution on the lower surface of the specimen is very inhomogeneous;the conductive ring can improve this inhomogeneity while increasing the current density on the lower surface of the specimen;the current density and electric field density at the defects on the surface of the specimen will increase abruptly,and the MRR at these locations is higher.Then the mechanical removal and temperature fields under ultrasonic vibration were modeled and simulated.For the mechanical removal effect,the larger the impact angle,speed and abrasive size and density,the more obvious the impact effect is;the smaller the contact area when abrasive impact is made,the more obvious the impact effect is;the impact effect with oxide layer is obviously better than without oxide layer,and the thickness of oxide layer should not be too low or too high.For the temperature field,the higher the frequency of longitudinal ultrasonic vibration,the faster the temperature of the bottom surface of the specimen rises,but also the faster it reaches thermal equilibrium,and the lower the temperature when it reaches thermal equilibrium;the higher the ultrasonic amplitude,polishing pressure,friction coefficient and polishing speed,the faster the temperature of the bottom surface of the specimen rises,and the higher the temperature when it reaches thermal equilibrium,these factors do not affect the speed of reaching thermal equilibrium.Finally,the polishing experiments of SiC were carried out under different auxiliary conditions,and the following conclusions were drawn:the MRR under photoelectric assistance is 10%-20%higher than that of photocatalytic polishing,and the ultrasound-photoelectric assistance can improve it by about 15%,but the ultrasonic vibration will increase the risk of SiC specimen fragmentation;the best combination of photocatalyst and abrasive grains is P25+Ce O₂,and the best concentration of abrasive grains is 1%by mass The best concentration of abrasive grains is 1%of mass fraction;the higher the UV power,the stronger the photocatalytic reaction;the best current density of electric field is 1m A/cm²;finally,the polishing experiments were carried out under the preferred conditions,and the MRR was1.013μm/h and the surface roughness Ra was 2.36nm when polyurethane polishing pad was used,and the MRR was 0.881μm/h and the surface roughness Ra was 1.515 nm.