| The development of the semiconductor industry is closely related to our national defense,military,aerospace,energy and other important areas of science and technology.As the representative of the third generation semiconductor material,silicon carbide(SiC)single crystal is an important new wide band gap semiconductor material,which can be used as a substrate material for the growth of gallium nitride(GaN)and graphene.At the same time,it has a high Young's modulus,high hardness,high temperature resistance,corrosion resistance and so on,it can be widely used in the production of high temperature,high frequency,high power devices.Especially,in the military industry,it is the core of the new generation of radar,satellite communications,and it has important application value and broad prospects for development,therefore,it has become the focus of international attention today.As the development of silicon(Si)electronic components has become the limit,it is more important and urgent to study the third generation wide band gap semiconductor materials.At the same time,it will lead the revolution of the semiconductor industry of the third generation.The processing quality and precision of SiC single crystal substrate have a direct impact on the performance of the device,so it is required that the machined surface is smooth,free of defects and no damage.Ultra precision polishing technology is the last step of the whole process,which is divided into two processes:mechanical polishing and chemical mechanical polishing.Mechanical polishing plays a decisive role in the material removal rate and flatness,while chemical mechanical polishing is the key to the realization of atomic surface roughness.It is one of the key technologies to ensure the high precision,high efficiency and low cost of SiC single crystal substrate.In this paper,the process and mechanism of mechanical polishing and chemical mechanical polishing were studied based on the surface roughness,flatness and material removal rate of 3 inch SiC single crystal substrate.The relationship between the surface roughness and the material removal rate was evaluated by the comprehensive scoring method.The optimum process parameters are obtained under different conditions according to different requirements.Based on the micro scale and macro scale,the effects of chemical action,mechanical action and chemical mechanical coupling on the removal mechanism of SiC single crystal substrate on atomic level were analyzed.The main contents of this paper include the following aspects:(1)The geometric model of the double-sided mechanical polishing without planet carrier is established,the relative trajectory equation of SiC single crystal substrate and the abrasive grains on the polishing pad is derived.The influence of the distribution radius of the abrasive particle,the distribution radius of the SiC substrate,the speed ratio of gear ring and sun gear,the speed ratio of polishing disk and sun gear on the polishing trajectory and curvature is analyzed.The effects of three factors named the particle interval radius,the speed ratio of gear ring and sun gear,the speed ratio of polishing disk and sun gear on the uniform polishing and abrasive wear of SiC single crystal substrate were studied by constructing the polishing uniformity function and the statistical method to calculate the coefficient of variation.(2)The three-dimensional physical model of the double-sided mechanical polishing without planet carrier is established,the coincidence of the displacement,velocity and acceleration of 5 point symmetry of the surface of 3 inch SiC single crystal substrate with time is analyzed,and the correctness of the theoretical model and the feasibility of the planetary differential gear train parameters are verified.Orthogonal experiment of 3 inch SiC single crystal substrate mechanical polishing was designed.The effects of three factors named polishing pressure,the speed of lower polishing disk,diamond abrasive diameter on the material removal rate,surface roughness and flatness were studied through single factor analysis and comprehensive analysis method.Thus,high quality,high efficiency and low cost SiC single crystal substrate and optimized process parameters were obtained.(3)From the microscopic scale,the 6H-SiC single crystal cell model is established.The molecular dynamics characteristics of the cell model are calculated by quantum mechanics.Based on the density functional theory and the first principle,the CASTEP module is used to simulate the band structure,total density,electron density and charge density.The use of the Forcite module to(100),(010),three(001)of the atomic density,the relative concentration distribution structure disordering degree of radial distribution function,the temperature distribution and velocity distribution changes with position were studied.The changes of potential energy,kinetic energy,non-bond energy,total energy and total enthalpy are analyzed.(4)Chemical mechanical polishing fluid solid coupling model of 3 inch SiC single crystal substrate was established,the effect of the polishing fluid on the unidirectional fluid solid coupling of SiC substrate was analyzed based on ANSYS.From the macroscopic scale,the effects of four parameters named pressure,stress,strain and strain energy on the fluid flow and the bottom surface of the polishing fluid are analyzed,and the mechanism of chemical mechanical polishing is expounded.(5)The effects of chemical mechanical polishing parameters on the material removal rate and surface roughness of the Si and C surfaces of SiC substrate are studied.Firstly,the orthogonal test with six factors and five levels was designed,the six factors include pressure,the speed of the polishing disk,the diameter of the abrasive grains,the pH value,the concentration of the polishing fluid and the concentration of the oxidant.The influence order of the factors of variance and variance is analyzed,and the optimal combination of process parameters is obtained.Secondly,the weight coefficient of the material removal rate and the surface roughness is measured by the comprehensive score analysis.The optimal process parameters of different targets are obtained.Then,the three most influential factors named pressure,polishing speed and polishing fluid concentration are analyzed.The quadratic regression orthogonal experiment design of three factors is designed and the mathematical model is established.The influence of the coupling between the factors on the target is analyzed by the surface response diagram to provide the theoretical guidance and reference for the selection of process parameters in the actual machining process and to provide a guarantee for the next step of precision digital control. |
PDF Full Text Request