| As a third-generation semiconductor material,monocrystalline GaN has an urgent need in the fields of electronic information,integrated circuits and national defense and military industry due to its characteristics of direct band gap width,stable chemical properties,high thermal conductivity,high hardness and strength.Chemical mechanical polishing（CMP）is a common method of GaN wafer polishing,but due to the high hardness and chemical stability of GaN materials,the polishing efficiency is low and the processing cost is high,and it is of great research significance to seek a chemical mechanical polishing slurry suitable for singlecrystal GaN wafers and a new high-efficiency polishing method.Chemical mechanical polishing（CMP）is a common method for polishing GaN wafers.However,due to the high hardness and chemical stability of GaN materials,the polishing efficiency is low and the processing cost is high.Therefore,it is of great significance to find a suitable chemical mechanical polishing slurry and a new efficient polishing method for single crystal GaN wafers.Therefore,in this thesis,an electrochemical-assisted chemical mechanical polishing method（ECMP）combining electrochemical-mechanical polishing is proposed for singlecrystal GaN,which optimizes the electrochemical-assisted chemical-mechanical polishing slurry components and polishing process by electrochemically controlling and enhancing the chemical corrosion rate in chemical mechanical polishing,and provides a theoretical basis for the processing of GaN wafers.First,the oxidant type,p H value and chemical potential in the GaN polishing slurry were optimized by electrochemical experiments.The effects of different p H and concentration of KMn O4,Na Cl O and Na OH on the electrochemical properties of GaN in solution were studied by the dynamic potential polarization curve method,and the experimental results showed that the slurry corrosion current was the largest and the corrosion potential was the most negative under the conditions of p H 2.5 and 5 wt%KMn O4,indicating that the corrosive oxidation of this component on GaN was the strongest,which was conducive to the removal of CMP process materials,followed by Na OH,and Na Cl O was the weakest.By performing cyclic voltammetry on GaN,the results show that the electrochemical reaction on the surface of GaN is weak at low potential,which is conducive to electrochemical polishing when the potential is greater than 2V,and the polarization curve experimental results are verified by evaluating the size of the corrosion current.The potential range suitable for GaN electrochemical polishing should be between 2V and 6V.Then,the mechanical effect is increased by electrochemical auxiliary friction and wear experiment,and the influence of slurry components on the surface state and material removal rate of the wafer is analyzed by detecting the friction coefficient（COF）and wear area,and the chemical and mechanical effects are comprehensively measured,and the slurry components are further optimized.The results show that compared with the non-application electrochemical test,the oxygen content on the surface of GaN is significantly increased,the corrosion oxidation effect becomes better,the COF becomes larger,and the Wear Rate of GaN increases.Changing the slurry p H and concentration,the friction test showed that the material removal effect was best under the conditions of p H 2.5 and KMn O4 concentration of 5 wt%.When only the abrasive in the slurry is mechanically removed and there is no electrochemical action,the COF is large and the GaN surface has obvious scratches,but the GaN wear rate is low;when the basic slurry adds oxidant and applies electrochemical action,the COF curve fluctuates significantly due to the formation of a viscoelastic oxide layer on the GaN surface;when the abrasive and oxidant are added to the slurry and the electrochemical action is applied,the mechanical and chemical combination makes the GaN wear rate reach the maximum,and then the same slurry component is used for polishing test.The material removal results can basically correspond to the friction and wear test.Finally,the electrochemical assisted CMP polishing experiment is carried out on GaN,and the influence of abrasives and process parameters in actual polishing on the processing effect is considered.Through univariate experiments and orthogonal experiments,the influence of different parameters on GaN surface roughness and material removal rate was analyzed,and the GaN polishing process parameters were optimized and the polishing slurry components were further optimized.Experimental results show that the degree of influence on the material removal rate of GaN is as follows: abrasive concentration > voltage >workpiece disc speed > polishing pressure.When the material removal rate is evaluated,the best process conditions are polishing pressure of 10 psi,workpiece disc speed of 240 rpm,abrasive concentration of 7 wt%,voltage of 7V,polishing time of 0.5h,and the material removal rate of 2436.3nm/h.The degree of influence on the surface roughness after GaN electrochemical mechanical polishing is as follows: the workpiece disc speed > the polishing pressure > voltage > the abrasive concentration,when the surface roughness is the evaluation index,the best process conditions are the polishing pressure 10 psi,the workpiece disc speed is 240 rpm,the abrasive concentration is 4.5 wt%,the voltage is 4.5V,the polishing time is0.5h,and the surface roughness after polishing is Ra 1.16 nm.According to the process experiment,a single crystal GaN polishing process combination is proposed: when rough polishing,in order to improve the material removal rate,the particle size is 0.5μm,the concentration is 7 wt% diamond as the abrasive,the polishing pressure is 10 psi,the polishing time is 0.5h,the workpiece disc speed is 240 rpm,the spinner disc speed is 60 rpm,the voltage is 7V;when semi-fine polishing,the particle size is 0.5μm,and the concentration is 4.5 wt%diamond as the abrasive.Polishing pressure 10 psi,polishing time 0.5h,workpiece disc speed300 rpm,polishing disc speed 60 rpm,voltage 4.5V;finally fine polishing,to improve the surface quality for the purpose,the use of particle size 0.5μm,concentration of 4.5 wt%diamond for the abrasive,polishing pressure 10 psi,polishing time 0.5h,workpiece disc speed240 rpm,polishing disc speed 60 rpm,voltage 4.5V. |
