Study On GaN Electrochemical Mechanical Polishing And Material Removal Mechanism

Gallium nitride(GaN),a third-generation semiconductor material,has many excellent properties,such as wide energy band gap,low dielectric constant,high thermal conductivity,and breakdown voltage,which has been widely applied in aerospace,detection and communication fields.However,GaN is considered as one of most difficult-to-machine material in ultra-precision polishing process owing to its high hardness,brittleness,and strong chemical inertness.Therefore,it is of great significance to obtaining the atomic-level surface quality of GaN crystal in its applications.In order to obtain GaN wafer with high material removal rate(MRR)and better surface quality,the ultra-precision polishing process of traditional chemical mechanical polishing(CMP)and CMP of GaN wafer under electrical induction conditions have been compared in this thesis.Effects of various polishing parameters on GaN waferswere explored and Cooperative etching mechanisms of H2O2 oxidation and PTA corrosion inhibition and the synergetic wear mechanism of H2O2 oxidation and PTA corrosion inhibition clarifies the GaN material removal mechanism under the synergistic effects of electric field and corrosion inhibitor have been studied.In addition,we present a physical model about the GaN electrical-induced CMP material removal mechanism.This work is important for promoting the development of integrated circuit and electronic information technology.The electrochemical etching mechanisms of PTA and H2O2 on GaN wafers were studied,and results show that flatten film can be formed on the surface of the GaN wafer after etching and the synergistic effects of H2O2 and PTA,indicating that oxidation and corrosion inhibition can be effectively controlled under the synergistic effects of H2O2 and PTA.Effects of various polishing parameters on the CMP polishing effect of GaN wafers under electrical induction conditions have been explored.Under the current of 40 mA and the corrosion time of 15 min,importance of each polishing parameter on the material removal rate of the GaN wafer as follows:polishing pressure,PTA concentration,the H2O2 concentration,the disc polishing speed.The removal rate of GaN had the highest value of?593.29 nm/h with the lowest roughness of?0.243 nm at the optimized parameters polishing pressure of 20.5 psi,the disc polishing speed of 125 rpm,the H2O2 concentration of 4 wt%and PTA concentration of 10 mmol/L.The surface of polished GaN is detected by cathode fluorescence(CL),and results show that GaN crystal surface without any the subsurface damage.The effects of current and hydrogen peroxide on the nano-hardness of the surface of GaN wafer was studied using an instrumental nano indenter.The results showed that the surface nanohardness of the original GaN is the largest,followed by wafer treated by chemical corrosion while the wafer treated by electrochemical corrosion has the lowest nano-hardness,which indicates that the electro-inducing assistance can accelerate the oxidation corrosion of GaN surface,and the hardness is 21450.06MPa,17458.32MPa and 15697.71 MPa.In the process of pressing in,there are multiple pop-in phenomena at different positions.This is because the sharper tip has plasticity at a shallower depth of indentation.X-ray photoelectron spectroscopy(XPS)was used to analyze the surface composition of GaN chip before and after chemical and electrochemical etching.The results showed that the content of Ga2O3 on the surface of GaN wafer after electrochemical etching is significantly higher than that after chemical etching,indicating that the current played a role of catalyzer in the solution.In addition,it can accelerate the rate of GaN oxidation corrosion reaction,which benefits the removal rate of GaN in CMP.The synergetic wear mechanism of H2O2 oxidation and PTA corrosion inhibition was revealed by instrumental micro-nano abrasion under the conditions of constant load and progressive load.The results showed that when PTA and H2O2 coexist,the friction coefficient of GaN wafers is lower,indicating that the synergistic effect of PTA and H2O2 is more likely to form a corrosion-inhibiting film on the surface of GaN,which presents elastic-plastic deformation and reduces the wear of furrow,it can not only avoid the excessive corrosion in the electrochemical etching process,but also alleviate the corrosion damage in the ultra-precision polishing of GaN wafer.Finally,the samples were tested by anodic polarization and Electrochemical Impedance Spectroscopy(EIS),the material removal mechanism of the ultra-precision polishing of GaN by the synergistic effect of electric field and corrosion inhibitor was clarified.The physical model of the material removal mechanism of GaN CMP assisted by electro-induction was established as well.