| Micro-electronic and photoelectron manufacturing which are represented by integrated circuit(IC) and optical electronics photoelectronic device are the core of the electronic information industry, also the most competitive and most rapid industry all over the world. Semiconductor materials are the foundation of integrated circuit, the innovation of semiconductor technology leads the development of information industry and optoelectronic industry. Monocrystalline SiC as the third generation semiconductor materials has the advanatge of wide band gap, high breakdown field, high thermal conductivity, high electronic saturation velocity rate, higher anti-radiation ability, good chemical stability and so on. Hence, monocrystalline SiC are widely used as substrates materials of power electronics equipment and photoelectronic device with characteristic of high temperature resistance, high pressure resistant, radiation resistance, high power, high density integrated. These substrates demand for ultra-flattened and ultra-smooth surface with defect free and damage free, the machined quality directly determines the applied value and the device performance. However, the processing efficiency of monocrystalline SiC substrates is difficult to improve by the traditional mechanical method for its high hardness and brittleness characteristic, and it is difficult to achieve ultra-smooth and damage free surface by the traditional CMP method for the chemical stability characteristic. The ultra-precision machining theory of monocrystalline SiC substrates is need to be perfected.In this paper, the deformational behavior of monocrystalline SiC was researched, and then the efficiency and ultra-precision processing technology were researched by used lapping, grinding and cluster magnetorheological (MR) polishing method. Material removal mechanism and ultra-smooth surface formation rule were studied by the surface damage detection, and the efficiency thinning and ultra-smooth polishing technology were achieved at last. The main contents and results of this paper are included below:1) The deformational behavior of monocrystalline SiC was researched by the nano-indentation, nanoscratch and Vickers indentation method. The results show that, the hardness (H) and elastic modulus (E) of C face is higher than Si face, the hardness of monocrystalline SiC is much bigger than the mohs hardness 9, and the ratio of E/H is less than 14, so the SiC belongs to the typical brittle material. The deformational behavior of (0001) crystal plane effected by the abrasive grains can be divided into five stages:elastic deformation, plastic deformation, plastic removal, brittle-ductile transition, brittle fracture, and the critical load of above stages change are 3.5mN,16mN,63mN and 385mN.2) A rapid detection method was proposed to accurately judge the subsurface of monocrystalline SiC whether exist microcracks damage and measure the microcracks deepth. Then the subsurface microcracks characteristic of monocrystalline 6H-SiC substrates were observed, and the subsurface microcracks distribution rules of lapping, grinding and cluter MR polishing were analysised through this method.3) The material removal models of single sided lapping was built up, and single factor experiment of abrasives kind, abrasives size, abrasives dispersion, lapping pressure, lapping speed, slurry concentration, slurry flow, lapping plate material and so on were carried out by the KD15BX precision plane grinding machine. The relations of material removal rates, surface roughness and material removal manner effected by above factors were detailedly analyzed, and the process conditions of two-body lapping and three-body lapping were obtained. Micro cutting is the main material removal pattern of two-body lapping, smooth and sheen surface with little shadow scratchs can be obtained, the scratchs include plastic removal mode, brittle removal mode and brittle-ductile removal mode. But the material removal method of three-body lapping is roll broken, which can get uniform rough surface with roll broken hole. In the coarse lapping stage, higher than 20μm/min of material removal rate can be got by using cast iron plate and process conditions:-slurry concentration of W20 diamond is 4wt%, lapping pressure is 4.408Psi, lapping plate rotation speed is 80r/min and slurry flow is 20ml/min. And in the fine lapping stage, even and no scratches surface with roughness Ra 0.024 μm can be realized by the condition of process variables which consist of lapping plate mateial, abrasive size, slurry concentration, lapping pressure, lapping plate rotation speed, slurry flow are copper, W3,4wt%,4.408psi,80r/min and 15ml/min, respectively.4) The grinding experiments of monocrystalline 6H-SiC substrates was carried out on DMG-6011V ultra-precision horizontal surface grinder by using #325 metal-bonded diamond wheel and #8000 vitrified bonded diamond wheel. Surface roughness, surface topography and subsurface damage microcracks affected by processing parameters were analysised. The surface quality is better and the surface roughness is reduced near to the grinded substrates centre for the abrasive particle trajectory density become denser and the grinding marks are smaller. With the decrease of the grinding wheel feed, the surface quality become well and the surface roughness is reduced. The maximum depth of subsurface damage is kept linear relationship to surface roughness Rz. The relation from grinding wheel wear to the material removal amount, the wheel surface topography, the maximum grinding force and surface quality were observed by using the same process parameter grinding continuously. With the grinding wheel wearing, the maximum grinding force become bigger, also the brittle fracture becomes more and more obvious. Finaly, a flat substrate with surface roughness Ra 0.012pm, subsurface damage depth (SSD) less than 4pm, TTV less than 3μm can be obtained by using the #325 optimal processing parameters.5) The magnetic field characteristics of pole arrangement, the pole tip shape and different magnetic pole size were analysed by static magnetic field finite element software. It is found that soft polishing pad is more easily composed of multiple independent micro-grinding head by select cylindrical flat bottom magnetic pole and arrange in the same direction, then the contact area of workpiece and micro grinding head maximization can be realized. The polishing pressure of an individual MR micro-grinding head on workpieces was investigated and the mathematical model of cluster MR polishing was established based on the Preston equation and the properties of the workpiece material. Kinematic model of abrasive particles for certain eccentric and linear interpolation swing uncertain eccentric, uncertain eccentricity of X direction and Y direction in plane polishing based on cluster MR effect were built and simulated by computer to analysis the influence rule of each factors.6) By using cylindrical flat bottom magnetic pole arrange in the same direction and ring magnet to build up cluster MR polishing pad, K9 glass, single crysta silicon wafers and single crystal 6H-SiC wafers were polished for testing and verifying the cluster characteristics of micro-grinding head. Otherwise, it’s found that the cluster MR polishing gives a priority of material removal for the bumps on the workpiece and thus accelerates the flattening of the workpiece. The profile of 6H-SiC wafer arc polishing region’s cross section and the surface 3D morphology were observed and compared with the simulation curves to verify the obtained model. Ultra-smooth and damage free surface with roughness Ra 0.293nm can be obtained by optimizing 6H-SiC wafer MR fluid component such as PH value, abrasive concentration, propotion of abrasive and sol, polishing slurry temperature, oxidizer and so on. |
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