Study Of Mechanical Properties Of Micro-Scale Materials By Micro-Tensile System And Method

The dimension of micro-electro-mechanical systems (MEMS) and micro-structure are in the range of millimeter or micrometer. The reduced dimensionality materials have obvious size effect when deformation length is in the range of micro or nano dimension. Moreover, preparation method has a very important effect on properties of micro-scale materials which has same dimension and ingredient. It is very necessary to obtain mechanical properties of micro-scale materials such as Young's modulus, tensile strength, yield limit and Poisson's ratio for improving reliability and evaluating service life of devices. However, these performance parameters can not be cited from bulk materials directly. Therefore, it is an important focal point to obtain accurate mechanical properties of micro-scale materials. Moreover, common methods are not suitable to measure properties of reduced dimensionality materials, and measurement of properties of micro-scale materials is attracted by researchers in MEMS field. However, many test methods only for measuring mechanical properties of single-crystal and multi-crystal Si in recent years. With the rapid development of micro-electro-mechanical systems technology, more thin film materials including metal and polymers are widely used in the MEMS field. Therefore, the development of MEMS is restricted to some extent because the mechanical properties of these films are lacked in recent years.There are many test methods for measuring properties of film materials including indentation, bulge, resonance, depiction and tension. Of all the existing methods proposed to measure properties of metal and silicon thin films, uniaxial tensile test is generally considered as the most reliable way, and it becomes an attractive task in MEMS. However, the difficulties of uniaxial tensile test are how to fabricate small and stress-free specimen, align and trip the specimen, generate small forces and measure strain. To solve previous difficulties, a new method for measuring mechanical properties of thin films is proposed in this paper. This method is suitable for thin films with large strain and can obtain in-situ images. The main work and the conclusions of this dissertation are as follows: (1) Design and fabrication of micro tensile systemTo solve the shortcoming of present tensile system, a new micro tensile system is designed which is suitable for thin films with large strain and can obtain in-situ images based on traditional tensile principle. The test chip is fabricated by wet-etching single crystal silicon and UV-LIGA technology which can hold good alignment and grip. The load is applied by high resolution piezoelectric motor and micro-spring force sensor to meet the micro tensile system. The strain and in-situ images can be obtained by laser sensor and high resolution CCD, respectively. To develop automatic micro tensile system, piezoelectric motor, laser sensor and CCD are integrated by VB software. The tensile system not only is suitable for measure mechanical properties of films with large strain, but also obtains in-situ images for analyzing failure mechanism.(2) Design and fabrication of test chipTo meet the requirement of films with large strain, the new test chip with S-shaped support spring is designed. The S-shaped support spring not only assures alignment of specimen, but also improves the force accuracy compared with U-shaped and straight support beams. Moreover, the proper S-shaped support spring is designed with appropriate parameter by ANSYS software simulation. The dumbbell shaped and linear film specimens are designed according to the ANSYS results. The measurement accuracy can be improved because both of ends of the film specimen are fixed to avoid damage and deformation. Two different test chips are fabricated by UV-LIGA technology to meet the requirement of different type films. One support frame of test chip is fabricated by wet-etching single crystal silicon and the other is fabricated by electronickelling.(3) Simulation, design and fabrication of force sensorIn this tensile system, a new micro spring force sensor is designed. The force sensor has two S-shaped springs, a pin and four alignment marks. The effect of line width, inner diameter, thickness, semicircles and length of straight beam of the spring on the elasticity coefficient are analyzed in detail by ANSYS software. Moreover, the elasticity coefficient is calculated by energy method. The micro spring force sensor with proper parameter is designed according to the simulated results. The front-end of the force sensor is connected with the movable table of test chip. The displacement of film specimen can be measured by the T-shaped displacement mark on the micro spring force sensor. The tensile force can be obtained by the displacement change of micro spring force sensor. To meet the requirement of different type films, different micro spring force sensors are fabricated by UV-LIGA technology and high resolution linear cutting technology.(4) Design and fabrication of fixed deviceTo assure the stability of test chip during tensile process, a new fixed device is designed in the micro tensile system. The fixing device is composed of fixed hole, locating pin, absorptive hole and vent pin. The test chip is fixed on the fixing device by vac-sorb pump. The test chip can be fixed easily by locating pin, and the alignment can be assured by double field microscope. The test chip can be fixed conveniently by this novel specimen-fixed stage.(5) Fabrication of device for measuring displacementTo measure the strain of the specimen directly, two gold line marks or indentation marks are electrodeposited on the surface of the specimen. The high resolution CCD can obtain images of the thin films during the tensile process. So the strain can be calculated from the difference of marks on images. Furthermore, the strain of film specimen can be measured by laser sensor during tensile process. Then the two methods for measuring strain of specimen can assure the accuracy of the measured results. The linear type film specimen is fabricated in order to integrate components of data acquisition.(6) Fabrication of actuating deviceIn this tensile system, loading process is realized by connecting the piezoelectric motor to the testing chip through the force sensor. The actuating speed of piezoelectric motor is controlled by pulse frequency. The maximum and minimum displacement of piezoelectric motor is 25 mm and 0.1μm, respectively. Moreover, the maximum and minimum speeds of piezoelectric motor are 70μm/S and 0.2μm/S, respectively. The actuating type includes manually operation, step operation and absolute value operation. The speed and step length of piezoelectric motor can be set by control panel or computer.(7) Measurement of mechanical properties of Ni filmElectrodeposited Ni components are widely used in MEMS for its high elasticity and flexibility. Consequently, they are chosen as the samples and tested in this paper. Specimens of Ni thin film, with the gauge section of 50μm wide, 100μm long and 5μm thick are tested. The measured Young's moduli of Ni thin film are about 95 GPa by photo method and 94.5 GPa by calculated method. The good consistency of Young's modulus by two methods verifies that the uniaxial tensile system has reliability for measuring mechanical properties of film specimen. Therefore, mechanical properties of micro-scale materials can be measured by this system to develop MEMS technology and application.