Mechanical Properties Characterization Of Micro-scale Materials Through Micro-tensile Testing Method

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.