Study On High Efficiency And Precision Polishing Technology Of Sapphire Wafer With Core-Shell Composite Abrasives

Single crystal sapphire has become the most dominant substrate material for Ga N-based light-emitting diodes（LEDs）by virtue of its superior physicochemical properties.The substrate surface quality directly affects the Ga N film growth quality,which needs to achieve the quality requirement of ultra-smooth and no sub-surface damage after polishing process.However,due to the high hardness and brittleness of sapphire wafers,the single abrasive used in conventional polishing process faces the bottleneck of unable to combine ultra-smooth surface and high polishing efficiency at the same time due to its own limitation,which hinder the further improvement of the surface quality of sapphire wafers.In this paper,based on the micro-contact model and solid-state reaction principle,the research on low-damage,high-efficiency and pollution-free polishing technology for sapphire wafers was carried out by preparing different kinds of core-shell composite abrasives,relying on the mechanical properties of the coupled hard core and soft active shell to achieve synergistic cooperation between chemical corrosion and mechanical removal.Based on Gibbs free energy change and abrasive hardness,appropriate core and shell materials were selected.The core-shell Al₂O₃/Si O₂ and diamond/Si O₂ composite abrasives were prepared by sol-gel method using Al₂O₃ and diamond as the core respectively and ethyl orthosilicate as the silicon source.The synthesized composite abrasives were characterized by various characterization means,which demonstrated that the synthesized composite abrasives had good dispersion without agglomeration,and the amorphous Si O₂ shell was closely coated on the surface of the core particles.Based on the hydrolysis and polycondensation process of ethyl orthosilicate,the forming mechanism of the composite abrasive was analyzed.In addition,using an improved chemical precipitation method,core-shell diamond/Ce O₂ composite abrasives were successfully synthesized.The prepared diamond/Ce O₂ composite abrasives have uniform particle size and cubic fluorite structured Ce O₂ microcrystals were tightly coated on the diamond particle surface by Ce-O-C chemical bonding.Polishing experiments were designed to investigate the polishing performance of composite abrasives on sapphire wafers,and the combination of core and shell materials with the best synergistic cooperation and the most excellent polishing quality was selected.The results showed that,compared with single abrasives,composite abrasives all improve polishing efficiency while effectively reducing surface roughness.Among them,diamond/Si O₂ composite abrasives achieved the best polishing results,with a27.2%reduction in surface roughness and more than 8.8%increase in material removal rate.Through wear debris analysis and physical phase examination of the sapphire surface,the interfacial contact behavior with sapphire wafers during the polishing process of composite abrasives and its material removal mechanism were revealed.The improved polishing performance of the composite abrasives was attributed to the synergistic cooperation of chemical etching and mechanical removal.The soft active shell reacts with the sapphire in solid state to produce a reactive layer with low hardness and weak bonding with the sapphire matrix,which is more easily removed by the mechanical action of the abrasive grains,and this approach effectively avoids deep scratches and severe surface damage,thus improving the polishing quality.In order to further improve the polishing efficiency of composite abrasives,the planarization performance of composite abrasives for thermal-assisted polishing on sapphire wafer was evaluated on a home-made processing zone temperature control device,and the mechanism of thermal-assisted in promoting chemical corrosion effect on composite abrasives during the polishing process was revealed.Experimental results indicated that there is a 25.5%reduction in the surface roughness under 50℃,obtaining a scratch-free surface,as well as the MRR was improved by 115%.The significant improvement in surface quality and polishing efficiency was due to the thermal-assisted polishing that increased the solid-state reaction rate and accelerated the process of soft reaction layer generation and removal.