Study On The Technique And Mechanism Of Photocatalysis Assisted Polishing Silicon Carbide Wafer

Single-crystal silicon carbide?4H-SiC?has excellent thermal and electronic properties,such as wide band gap,high breakdown field,high thermal conductivity,high temperature stability and low dielectric constant.Moreover,silicon carbide has superior characteristics such as lattice constant and thermal expansion coefficient similar to gallium nitride.Single-crystal silicon carbide can be widely used in the fields of high temperature,high frequency,high power and radiation resistant electronic devices.As the production cost of single-crystal silicon carbide decreasing,silicon chip may be replaced by silicon carbide,to solve the bottleneck problem of the chip material self performance,which may be revolutionary change in electronics industry.If silicon carbides are to be used in these application,it should be precisely machined and fine polished due to the poor surface roughness and accuracy.Therefore,surface planarization become one of key required technologies for silicon carbide applying in high-tech fields.However,the high hardness and good chemical inertness of silicon carbide bring many challenges to the traditional techniques.The mechanical polishing method is easy to introduce scratches and subsurface damages to the surface of silicon carbide due to its mechanical removal mechanism.The polishing techniques based on chemical and mechanical synergy,such as chemical mechanical polishing,catalyst etching,electrochemical mechanical polishing and plasma-assisted polishing have been proposed,have the limitation of low material removal rate,reagents easy to decomposet,and complex polishing system,etc.Therefore,it is necessary to investigate the material removal mechanism and develop a new chemical and mechanical synergistic polishing technique for silicon carbide.This study focues on the theories of chemical thermodynamics,chemical kinetics and photocatalytic oxidation on the basis of chemical and mechanical synergy.And it proposed a new photocatalytic assisted polishing method for single crystal silicon carbide to realize high efficiency,ultra precision and low damage polishing.Main research works and results are as following:?1?The chemical thermodynamics and kinetics theory of the SiC oxidation reaction were analyzed.Materials Studio and LAMMPS molecular dynamics simulation are be employed to demonstrate the feasibility and oxidation rate of silicon carbide oxidized by common oxidants and hydroxyl radicals·OH.It reveals that selectting reasonable oxidants,increasing temperature,increasing surface energy,introducing mechanical action are effective measures to accelerate the oxidation rate of silicon carbide.The lattice distortion introduced by mechanical action and the strong oxidizing property of hydroxyl radical·OH mainly drives the oxidation of silicon carbide,and the abrasive mechanical friction benefits to the removal of the oxide layer material in photocatalytic assisted polishing process.?2?Efficient and stable slurry for photocatalytic assisted polishing silicon carbide was prepared.Theoretical analysis and experimental research show that the preferable slurry should includes P25?0.75g/L?,H₂O₂?0.66mol/L?,voltage?15V?,pH?3?and silica sol?10wt%?.Furthermore,the hydroxyl radical·OH in the slurry was captured by dihydroxybenzoic acid,and the intermediate product was detected by an ultraviolet spectrophotometer.The analysis of static corrosiveness and polishing effect of the slurry showed that the etched silicon carbide surface contains a large number of protrusions and pits.And the oxygen O can be detected on the polished surface by spectrum analysis.The analysis with Olympus 3D laser microscopy,atomic force microscopy and X-ray electron spectrometer showed that the polished surface was flat and smooth with a surface roughness Ra of 0.348nm in 0.353×0.265mm area and functional groups such as Si-C,Si-C-O,Si-O,and C-O,etc.,which verified the oxidization of silicon carbide in polishing.?3?A photocatalytic assisted polishing test device for polishing silicon carbide wafers and a friction measuring device were prepared and the polishing technique is studied.A force model of a single abrasive grain is first established.The theoretical analysis and experimental results show that the experimental value of the material removal rate is similar to the calculated value of the force model of a single abrasive grain,and the achieved surface roughness is about Ra 8nm in the lapping process.By studing the effects of polishing technique parameters on frictional force,material removal rate and surface quality,the novel polishing parameters are achieved as an ideal polishing pressure of 0.025MPa,polishing plate speed of 60r/min,silica of 5wt%,polishing pad of synthetic fiber polymer.With the optimized polishing technique,the surface roughness of silicon carbide can reach to about Ra 0.218nm with material removalrate of about 1.18?m/h.?4?The material removal mechanism of photocatalytic assisted polishing silicon carbide have been studied base on mechanical and chemical interactions.The atomic indentation test showed that the depth of the surface layer with lower hardness was about 0?25nm.Silicon carbide is removed under the mode of chemical and mechanical interaction.X-ray photoelectron spectroscopy and transmission electron microscopy were used to study the surface composition and subsurface structure of the polished silicon carbide.The results show that there are various forms of functional groups such as Si-C,Si-C-O,Si-O,Si₄C₄O₄,C-O,and C=O on the polished surface.Little damage and cracks can be found on the surface and subsurface layer of silicon carbide.The amorphous layer thickness produced by mechanical action is about several nanometers.The thickness of the oxide layer is about 4?6nm.Theoretical analysis and experimental studies show that the chemical reaction is promoted by the mechanical action of abrasive.The oxide layer formed on the surface of silicon carbide is beneficial to the mechanical removal.Chemical and mechanical synergy results the material removal in polishing process.The study illustrates material removal mechanism of silicon carbide as:the binding energy between silicon carbide atoms and reaction activation energy are reduced by mechanical activation,and then the surface of silicon carbide wafer was oxidized by hydroxyl radical·OH in the slurry.Following,the oxidation layer on the surface of silicon carbide is removed by mechanical scratching,exposing a new surface to promote the chemical reaction of silicon carbide atoms.