【作者】 张段芹；

【导师】 褚金奎；

【作者基本信息】 大连理工大学 ， 微机电工程， 2009， 博士

【摘要】 微机械电子系统(Microelectromechanical Sysetms,MEMS)具有很好的发展前景,而且应用范围特别广泛,如汽车用的微加速度器,办公用的喷墨打印机头、数字微镜阵列,通讯用的光开关,以及医疗用的微流控芯片等等。这些MEMS器件与IC芯片的主要不同之处是MEMS器件中包含有运动、接触、摩擦等微构件,使得微尺度下的力学性能研究成为MEMS的一个重要基础研究内容。然而,微尺度下的力学性能受尺度效应和微加工工艺等因素的影响,已不能用宏观下的理论来解释,需要重新对微尺度下构件的力学性能进行表征。微拉伸测试方法具有能够得到均匀的应力应变场,数据容易解释,通用性强等显著优点,是最常用的测试方法之一。本文研制了一种片外驱动微拉伸测试装置,目的是测量MEMS中常用薄膜材料的力学性能。此微拉伸测试装置采用压电陶瓷驱动,电感式测微仪(精度0.1μm)测量其驱动位移,微力传感器(分辨率0.25 mN)直接测量被测试样的拉力载荷,五维微动台用于调整动、静载物台使其轴向对准。主要解决两个技术难题:1)试样的轴向变形位移的检测。位移传感器测量的是总的驱动位移,它除了包含试样的轴向变形位移外,还包含微力传感器的输出变形和粘结胶的剪切变形,本文是通过在线测量测试装置的轴向刚度,从总的驱动位移中减去测试装置的轴向变形而获得拉伸试样的轴向变形。2)拉伸试样的轴向对准与夹持。由于被测试样的特征尺寸在微米量级,而测试装置的特征尺寸在毫米量级,试样的轴向对准问题是非常难解决的。本文提出了一种游标-凹槽载物片式对准机构,它由体硅微加工工艺制作。梳齿式游标用于检测非轴向对准时的角偏差,而凹槽和凹槽中的定位台用于实现试样的快速安装和定位。这种方法很大程度上提高了拉伸试样的轴向对准精度和重复对准精度。利用此微拉伸测试装置,测量了热氧化硅薄膜的力学性能。设计并制作了两种微拉伸试样:常规微拉伸试样和附加弹性梁的微拉伸试样。其中,后一种试样中的弹性梁可以有效减小因非轴向对准而引入的扭弯变形,保证微拉伸梁的单轴向拉伸。这两种试样采用相同的微加工工艺在同一硅片上制作,其关键步骤是双掩膜两次感应耦合等离子体(ICP)刻蚀形成阶梯状窗口结构,并最终在硅衬底表面形成释放了的热氧化硅薄膜梁。通过微拉伸测试,得到热氧化硅薄膜的弹性模量和断裂强度分别为65 GPa和350-489MPa。由于过大的残余压应力,氧化硅薄膜梁发生了过屈曲,通过对氧化硅薄膜梁的几个应力状态进行分析,提出了一种基于微拉伸屈曲梁法的测量残余应力的理论模型,氧化硅薄膜的初始残余压应力为354 MPa。另外,对电铸镍薄膜梁进行了微拉伸测试。电铸镍拉伸试样采用准LIGA技术制作,氨基磺酸镍为电铸液。测量结果表明:高电流密度下制备出的微结构致密度较差,具有较高的气孔率,并且杨氏模量与气孔率的关系遵循指数经验公式,其测量值与计算值比较吻合。在本文工艺条件下,当电流密度为20 mA/cm~2时,镍薄膜的杨氏模量为83±6GPa;当电流密度减小到10 mA/cm~2时,镍薄膜的杨氏模量为124±5 GPa。最后,探索了片内集成力敏单元的微拉伸测试。设计并制作了集成扩散硅压阻式力敏单元的微拉伸试样结构,使其能够进行片内拉力检测。利用片外驱动微拉伸测试装置进行压阻力敏单元的输出信号-输入拉力之间的关系曲线标定,测量得到力敏单元的灵敏度为0.017mV/mN。更多还原

【Abstract】 MEMS （Microelectromechanical systems） has huge development potential and is used in different fields, such as micro accelerometer used in motorcars, ink-jet printing head and digital microarray device used in office, optical switch used in communication, microfluidic chip in medical and so on. The marked difference between MEMS and IC （Integrated circuits） is that there are mechanical elements under moving, contact or friction in MEMS. So characteriazation of mechanical proporties in micro scale is a fundamental research in MEMS. However, the mechanical properties in micro scale could not be interpreted by theories in macro scale because the properties are influenced by size effect and micro-fabrication conditions of specimens. Thus, it is necessary to characterize the mechanical properties of materials in micro scale.Because the tensile test generates a uniform state of stress and strain, and provides readily interpretable data to extract valuable information, micro-tensile testing is one of the most favored methods. In this paper an off-chip actuated tensile test device has been developed for the mechanical properties characterization of materials in MEMS. In the tensile device, a piezoelectric actuator is used to drive a movable stage. And the displacement of the actuator is measured by an inductance type displacement sensor （with accuracy 0.1μm）. A load cell （with resolution 0.25 mN） is used to measure the tensile force in specimens directly, and a 5-axis translation stage is used for axial alignment between the fixed stage and the movable stage. Two technical matters are settled. The first one is the measurement of axial deformation of tensile specimen. The sensor measures the total displacement of the piezoelectric actuator, which includes deformation of the load cell and shearing deformation of glue as well as the axial deformation of the tensile specimen. In this paper the axial stiffness of the tensile system is measured in situ, and the axial deformation of tensile specimen can be obtained by substracting the axial deformation of tensile system from the total displacement of the actuator. The second matter is axial alignment and gripping of the tensile specimen. The size of the specimen is at micro-scale, whereas that of actuators and sensors is at macro-scale. So the alignment of the specimen in uniaxial tensile testing is an utterly intractable issue. In this paper, a vernier-groove plate, which is fabricated by bulk silicon micromachining technology, is presented. The vernier is used to detect the offset angle of non-alignment, and the groove and the locating boss is used to mount and locate the specimen rapidly. This method can largely improve the accuracy of alignment and repeatability.Using this micro-tensile testing, mechanical properties of thermal silicon oxide films are measured. Two kinds of specimens are designed and prepared: the traditional tensile specimens and the specimens with suspended spring beams. For the latter, the spring beams in the specimen could reduce effectively the torsion strain or bending strain of the specimen caused by non-axial force, and keep the specimen under uniaxial tension. Both kinds of specimens are fabricated in a wafer with the same process. The pivotal process in fabrication is double-masks-twice-ICP etching to form stair-opening structures and finally to obtain the free-standing thermal silicon oxide thin film specimens on the surface of the silicon wafer. Through tensile testing, the measured Young’s modulus and fracture strength of the SiO₂ film are 65 GPa and 350-489 MPa respectively. The measured thermal SiO₂ film beams are buckled due to the compressive residual stress. Through analysis of a series of stress states in the SiO₂ film, a model to calculate the residual stress is presented based on the method of micro-tensile buckled beams. The compressive residual stress of the SiO₂ film is 354 MPa.Also, the mechanical properties of electroplated Ni films are measured by micro-tensile testing. The specimens are fabricated using LIGA-like technology and sulphate electrolyte bath. The results of the testing show that the higher current density produced microstucture with low density and a higher volume fraction of pores. The measured values of Young’s modulus are consistent with that of calculated from the exponential empirical formula between Young’s modulus and porosity. Under the technological conditions in this paper, the obtained Young’s modulus is 83±6 GPa for nickel specimens electroplated at current density of 20 mA/cm² and it increases to 124±5 GPa as current density is decreased to 10 mA/cm².Finally, micro-tensile testing with a load cell in-chip is researched. The micro-tensile specimen integrated with a piezoresistive diffused-silicon load cell is designed and microfabricated. So the tensile force can be measured in-chip. The relational curve of the output signal and the input load curve for the piezoresistive load cell is calibrated by the off-chip actuated tensile test device, and the sensitivity coefficient of the load cell is 0.017 mV/mN.更多还原