Mechanism Reserch Of Electro-fenton Chemical Mechanical Polishing For Single Crystal GaN

As a third-generation semiconductor,single-crystal gallium nitride(GaN)is more excellent in band gap width,thermal conductivity,radiation resistance,breakdown electric field and electron saturation rate,and is widely used in 5G communication,high brightness LED,radar,unmanned driving,high frequency wireless technology and other fields.Good surface quality determines its application potential in the field of microelectronics.In order to meet the practical needs,it is required that the surface of GaN chip is super-smooth,without scratches,defects and sub-surface damage.GaN wafer is difficult to process because of its high hardness,brittleness and stable chemical properties.Therefore,the research of high efficiency and high quality GaN surface flattening processing method is the focus of attention at home and abroad.This thesis proposes a chemical mechanical polishing method based on electro-Fenton to control the chemical reaction rate for GaN wafer polishing,realizing the combination of chemical enhancement,reaction control,high efficiency and high quality.Firstly,we studied the friction and wear properties of single crystal GaN wafers in the electrofenton environment,and the optimal electrofenton system for the chemical action of GaN was selected.To master the chemical mechanism of electrofenton reaction on GaN,the effects of different electro-Fenton parameters(different media,H₂O₂,cathode material,cathode potential,solid phase catalyst and p H)on the GaN wear process were studied.The results show that:The variation of Coefficient of friction(COF)combined with wear section can well reflect the oxidation and corrosion characteristics of GaN,and GaN is easier to be removed in electro-Fenton medium with high oxidation capacity.5 wt%is the best initial mass fraction of H₂O₂ to realize electro-Fenton dielectric oxidation of single crystal GaN.Too low initial mass fraction of H₂O₂leads to low efficiency of electro-Fenton,and too high initial mass fraction of H₂O₂ leads to a large proportion of side reactions,which also reduces the efficiency of electro-Fenton.The regeneration rate of H₂O₂ during the electro-Fenton process is much lower than the consumption rate,so timing or quantitative supplementation of H₂O₂during the electro-Fenton reaction can effectively improve slurry oxidation.Graphite felt with porous fiber structure,large specific surface area and strong oxygen adsorption capacity is more suitable for the cathode material of electro-Fenton.Too low cathodic potential fails to exert the best efficiency of electro-Fenton,while too high cathodic potential will produce hydrogen evolution reaction,which will increase p H,accelerate the decomposition of H₂O₂,and reduce electro-Fenton efficiency.The cathode potential can control the electro-Fenton efficiency in real time,thereby controlling the strength of the chemical reaction.The iron-carbon catalyst with strong ionization ability is more suitable for the electro-Fenton system.The energization of the liquid can also control the rate of Fe²⁺ionization of the iron-carbon catalyst.The optimal mass fraction of the iron-carbon catalyst is 3 wt%.The p H rises,causing Fe³⁺precipitation.When the precipitate is wrapped on the surface of the catalyst,the catalytic activity will be reduced,which is not conducive to the continuous and efficient operation of the electro-Fenton,The medium oxidizing property of electro-Fenton under acidic conditions is much higher than that under weakly acidic conditions,because the low p H can not only effectively ionize Fe²⁺from the solid-phase catalyst,but also alleviate the formation of precipitation of Fe²⁺and Fe³⁺and reduce the attenuation of the Fenton reaction.possibility,while also preventing the rapid decomposition of H₂O₂.Moreover,more hydroxyl radicals(·OH)could be produced under the electrofenton conditions of 5 wt%initial H₂O₂,5 wt%H₂O₂ every 5 min,-0.5 V cathode potential,3 wt%Fe-C catalyst and p H 2,and the electro-Fenton chemical effect was the strongest.Then,the effect of polishing process parameters(abrasive type,abrasive particle size,polishing pressure,polishing disk speed and workpiece disk speed)on the electro-Fenton chemical mechanical polishing(CMP)of GaN wafers was investigated.Taking surface roughness Ra and surface topography as objective functions,different process parameters were optimized by single factor experiment.The results show that:the hardness and dispersion stability of abrasive directly affect the performance of polishing fluid,and improving the abrasive concentration can improve the surface quality.The polishing pressure is directly related to the removal efficiency,but too high polishing pressure will make the abrasive embedded too deep into the GaN surface,and GaN surface is very easy to produce scratches.Increasing polishing disc speed and workpiece disc speed can increase polishing efficiency and improve surface quality faster.The optimized process parameters are as follows:abrasive concentration is 10 wt%;polishing pressure is 40 k Pa;workpiece disk speed is 200 rpm;polishing disc speed is 120 rpm.The surface roughness Ra of GaN obtained under the optimum processing conditions is 1.3 nm.Finally,the removal mechanism of GaN electro-Fenton CMP was constructed by combining friction and wear experiments and electro-Fenton CMP experiments.The COF changes,wear conditions and energy spectrum analysis of friction and wear experiments(electrolyte group,H₂O₂ group,Fenton group and electro-Fenton group)were compared and analyzed.It was found that the degree of oxidation and corrosion of GaN affected the wear contact state during the wear process,and the electro-Fenton chemical mechanical coordination mechanism of single crystal GaN was explored.In the CMP experiment(no chemical group,Fenton group,electro-Fenton group),the results of three groups of polishing experiments were tracked,and the morphology and surface roughness of the wafer were analyzed.During the electro-Fenton CMP of single crystal GaN,the controllability and strong oxidation of electrofenton can not only help CMP to quickly remove the damaged layer on the wafer surface,but also make the polishing more efficient to obtain high surface quality.The material removal behavior model of electrofenton chemical mechanical polishing for single crystal GaN was established.Considering the influence of wafer material properties,polishing pad characteristics,abrasive types,abrasive concentration,polishing relative speed,polishing pressure and other factors on electro-Fenton CMP,the Preston equation was modified,and the material removal rate model of electro-Fenton CMP was proposed.