【作者】 张鹏；

【导师】 冯显英；

【作者基本信息】 山东大学 ， 机械电子工程， 2017， 博士

【摘要】 半导体产业的发展密切关系到我国国防、军事、航空航天、能源等重要科技领域。以碳化硅(SiC)单晶为代表的第三代半导体材料是一种重要的新型宽禁带半导体材料,通过外延可以作为生长氮化镓(GaN)、石墨烯的衬底材料。同时,它具有高杨氏模量、高硬度、耐高温、耐腐蚀等性质,可广泛用于制作高温、高频的大功率器件。尤其在军工方面,是新一代雷达、卫星通讯的核心,具有重要的应用价值和广阔的发展前景,已然经成为当今国际关注的焦点。由于硅(Si)电子元器件的开发已趋于极限,因此,研究第三代宽带隙半导体材料显得更加重要和尤为迫切;同时,它将引领第三次半导体产业革命。SiC单晶衬底的加工质量和精度直接影响器件的性能,故要求被加工表面超光滑、无缺陷、无损伤。超精密抛光技术是整个加工工艺的最后一步,分为机械抛光和化学机械抛光两道工艺。机械抛光对材料去除率和平坦度起到决定性作用,化学机械抛光是实现原子级表面粗糙度的核心。因此,超精密抛光技术是保证SiC单晶衬底高精度、高效率、低成本的关键。本文以3英寸SiC单晶衬底的表面粗糙度、平坦度和材料去除率为目标,对机械抛光和化学机械抛光的工艺和机理进行了研究。使用综合评分法来权衡表面粗糙度和材料去除率之间的关系,根据不同要求得到不同条件下的最佳工艺参数。从微观、宏观两个尺度出发,分析化学作用、机械作用以及化学机械耦合作用对SiC单晶衬底原子级去除机理的影响。具体的研究内容主要包括以下几个方面:(1)建立了无架行星式双面机械抛光二维几何模型,推导了 SiC单晶衬底和抛光垫上磨粒的相对运动轨迹方程。分析了磨粒分布半径、SiC衬底分布半径、齿圈与太阳轮转速比、抛光盘与太阳轮转速比对抛光轨迹和曲率的影响情况。构建抛光均匀性函数,使用统计学方法计算变异系数,研究了磨粒间隔半径、齿圈与太阳轮转速比、抛光盘与太阳轮转速比三个因素对SiC单晶衬底均匀抛光和抛光垫上磨粒磨损的影响规律。(2)建立了基于无架行星差动轮系的双面机械抛光机构的三维物理模型,分析了 3英寸SiC单晶衬底表面对称5点的位移、速度、加速度随时间变化曲线的重合情况,验证了理论模型的正确性和行星差动轮系参数的可行性。基于该模型设计了 3英寸SiC单晶衬底机械抛光正交试验,通过单因素分析法和综合分析法,研究了抛光压力、下抛光盘转速以及金刚石微粉直径三个因素对材料去除率、表面粗糙度和平坦度的影响规律,获得了最优工艺参数。(3)从微观尺度出发,建立了6H-SiC单晶晶胞模型,进行量子力学的计算,分析了晶胞模型分子动力学特性。根据密度泛函理论和第一性原理,使用CASTEP模块对能带结构、总态密度、电子密度以及电荷密度进行了分子动力学仿真。使用Forcite模块对(1 0 0)、(0 1 0)、(0 0 1)三个面的原子密度的相对浓度分布、结构无序化程度的径向分布函数、温度分布以及速度分布随位置的变化进行了研究,分析了晶胞势能、动能,非键合能,总能,总焓的变化规律。(4)建立了3英寸SiC单晶衬底化学机械抛光流固耦合模型,基于ANSYS分析了抛光液对SiC衬底被加工表面的单向流固耦合作用。分析了抛光液流量和抛光液底面转速对流固耦合界面的压力、应力、应变和应变能四个指标的影响情况,为化学机械抛光机理的研究提供理论指导。(5)研究了SiC衬底的Si面和C面的化学机械抛光的工艺参数对材料去除率和表面粗糙度的影响。首先,设计了化学机械抛光六因素五水平正交试验(抛光压力、抛光盘转速、磨粒直径、PH值、抛光液浓度和氧化剂浓度),通过极差、方差分析法确定了单因素的主次顺序,分别得到了材料去除率和表面粗糙度单目标的最优工艺参数。其次,通过综合评分法确定了材料去除率和表面粗糙度之间的权重系数,得到了多目标的最优工艺参数。然后,针对其中影响最大的三个因素(抛光压力、抛光盘转速和抛光液浓度),设计了三因子二次回归正交旋转试验,建立了回归模型的数学方程。通过曲面响应图分析了多因素之间的耦合作用对目标的影响规律,为实际加工过程中工艺参数的选择提供理论指导和借鉴,为下一步精密数字化控制提供保障。更多还原

【Abstract】 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.更多还原