单轴应力下热氧化SiO_2的介电电致伸缩系数研究

微机电系统和集成电路中常用的热氧化SiO2是各向同性材料,研究了其在单轴应力场中介电常数的变化规律。依据介质在自由和束缚两种边界条件下受到单轴应力作用产生的应变不同,从电动力学基本关系出发推导了各向同性电介质两种边界下的介电电致伸缩系数计算公式,表明介电电致伸缩系数是与电介质的初始介电常数、杨氏模量和泊松比有关的常数。计算得到热氧化SiO2薄膜在自由和束缚条件下的介电电致伸缩系数M12分别为-0.143×10-21和-0.269×10-21 m2/V2。搭建了基于三维微动台的微位移加载系统,测量了在单轴应力下微悬臂梁SiO2薄膜电容的变化,测量得到热氧化SiO2薄膜的M12为(-0.19±0.01)×10-21 m2/V2,表明实际SiO2薄膜介质层的边界条件处于自由和束缚之间。

Study of the Dielectrostriction Coefficient of Thermal SiO2 Subjected to the Uniaxial Stress

The thermal SiO2 widely used in MEMS and integrated circuits is an isotropic dielectric material,its dielectric variation in the uniaxial stress filed was studied.According to the fact that dielectric material strains are different with the uniaxial stress under nonconstrained and constrained boundary conditions,and based on the basic relations of dynamics,the formulas of the dielectrostriction coefficients of isotropic dielectric in two cases were derived.It shows that the dielectrostriction coefficient is a constant relating with the initial dielectric constant,Young’s modulus and Poisson’s ratio.The computed dielectrostriction coefficient M12 of the thermal SiO2 thin film under nonconstrained and constrained conditions is-0.143×10-21 and-0.269×10-21 m2/V2,respectively.The micro displacement loading system based on 3D micromovement sets was constructed,the capacitance change of micro-cantilever SiO2 thin film was measured under the uniaxial stress.The measured M12 of thermal SiO2 thin film is(-0.19±0.01)×10-21 m2/V2.It shows that the boundary condition of the dielectric layer of SiO2 thin films is between the nonconstrained and constrained conditions.