Principle And Method Of Photoelectrochemical Mechanical Polishing For GaN Wafer

With the increasing demand for the performance of semiconductor devices,the semiconductor material manufacturing technology has made great progress.Gallium nitride(GaN)has shown excellent application value and prospect in the fields of advanced 5G communication RF chips,high power devices,and military radar due to its characteristics.GaN is a representative third-generation semiconductor material with high electron mobility,radiation and high temperature resistance.When GaN wafer is used as a substrate for homogeneous epitaxy or device growth to obtain better growth quality and give full play to device performance,the wafer is required to have ultra-smooth and near damage-free surface quality.At present,the chemical mechanical polishing(CMP)is mainly used for the final processing of GaN wafers.Although good wafer surface quality can be obtained,CMP has inefficient and time-consuming processing due to the extreme mechanical hardness and strong chemical inertness of GaN.As a result,it has become one of the bottlenecks that seriously restrict the development and application of GaN-based semiconductor devicesTo solve the problem of low material removal rate(MRR)in CMP of GaN wafer,this thesis proposed the new photoelectrochemical mechanical polishing(PECMP)method considering the semiconductor properties of GaN,the principle of photoelectrochemical oxidation,and multiple fields effects of light,electricity,chemistry and mechanical.Through systematic theoretical and experimental research,the photoelectrochemical oxidation reaction conditions of GaN wafer were determined;the photoelectrochemical oxidation reaction system was constructed;the PECMP material removal mechanism was revealed;the PECMP apparatus was developed;the PECMP experimental study was carried out,and proved to successfully polishing the surface of GaN wafer to become ultra-smooth and near damage-free.The main research contents and conclusions are as follows.(1)According to the band theory of GaN,the oxidation process of the wafer surface in CMP processing was analyzed.The generation,recombination and separation process ofelectron(e-)-hole(h+)pairs in GaN wafer under direct UV irradiation were studied.It was found that the low surface oxidation reaction rate was the key to restrict the improvement of CMP efficiency of GaN wafer.Due to the fact that the photoexcited e--h+pairs in wafer were easy to recombine and disappear,voltage was applied to enable efficient separation of e--h+pairs.On this basis,a new method of PECMP that under the action of the applied voltage was proposed.In principle,the whole wafer surface could be photoelectrochemically oxidized and mechanically polished alternatively.(2)According to the principle of PECMP,an experimental apparatus was developed.The apparatus can achieve direct UV irradiation,promote photoelectrochemical oxidation,perform mechanical polishing,load polishing pressure,apply voltage,detect current and voltage signal.Therefore,the synergistic effects of photoelectrochemical oxidation and mechanical polishing in the PECMP process can be realized on the wafer surface.(3)According to GaN material characteristics and PECMP reaction requirements,PECMP slurry system was developed using SiO2 as abrasives,H2SO4 and KOH as pH regulator,and K2SO4 as supporting electrolyte under weak acid,alkaline or neutral conditions without the need of strong oxidant.Further,the morphology of SiO2 abrasives in the slurry was tested and characterized,and the influence of different SiO2 abrasives sizes and mass concentrations on the light transmittance of the slurry was investigated.Considering the material removal capacity of SiO2 abrasive with different sizes and the processed surface quality,the slurry with SiO2 abrasive size of 25 nm and mass concentration of 5 wt%was finally selected.(4)The effect of applied voltage on photocurrent in PECMP of GaN wafer was studied by using linear sweep voltammograms(LSV)method.Also,the main chemical reactions and the separation of e--h+ by the applied voltage were analyzed.The applied voltage was found out to be the root cause of separation of photoexcited e--h+ pairs,revealing that the formation of photocurrent increased with greater applied voltage,but such increasing trend gradually flattened out when the applied voltage exceeded 1.5 V.The reasonable potential range for effective separation of photoexcited e--h+pairs was determined,and PECMP mechanism was revealed as follows.When GaN wafer surface was exposed to the direct UV irradiation,e--h+pairs would be generated and then separated by the applied voltage.The separated h+would accumulate at wafer surface under the action of electric field forces,oxidizing the surface to form an oxide layer.The oxide layer was subsequently mechanical polished by SiO2 abrasive to achieve a high-efficiency and high-quality polishing of GaN wafer surface.(5)The effects of applied voltage and pH of slurry on MRR were examined by potentiostatic PECMP tests on GaN wafer.The experimental results showed that when the slurry pH was 1 and the applied voltage was 2.5 V,GaN wafer MRR in PECMP was about 1.2μm/h,which was about one order of magnitude higher than that of CMP(0.159 μm/h).Furthermore,after 135 minutes of PECMP,the damaged layer caused by the previous grinding process was removed and the wafer surface roughness reduced from Sa 1 run(as-received)to about Sa 0.122 nm.A GaN wafer with high-efficiency and near damage-free PECMP surface of was acquired.