Study On Micro-tensile Test Method And Applications For MEMS Structure Materials

The typical size of microstructures in MEMS devices is smaller than micron; meanwhile, when the size of the material decreases to micro- or nano- scale, the properties of materials will be dramatically different from the bulk ones, due to the size effect. Moreover, sometimes the properties of materials with the same composite and size will also be different, because different fabrication process could lead to different microstructures, which will affect the properties. Therefore, familiar with the properties, for example, mechanical properties of materials is essential to the designing of the MEMS devices. Because more and more materials and a variety of fabrication processes have been used in MEMS, it is necessary to develop a high efficient, accurate and integrated test system for measuring mechanical properties of micro scale materials.This thesis is focusing on the design and fabrication of the specimen, data acquisition and analysis and applications of the micro-tensile test system to measure the mechanical properties of the MEMS structure materials.First of all, based on the theory of traditional unaxial tensile test and large deformation properties of non-silicon thin films, an integrated tensile test chip was proposed, which mainly consists of supporting springs, pines for connecting the force sensor and movable table, displacement mark for strain measurement by laser sensor, specimen fixing pines and alignment mark. The FEM (Finite Element Method) simulation results show that, 1) the spring constant of supporting springs is much smaller than the stiffness of the specimen in our design, which ensures the accuracy of the test system; 2) the straight with little arcs design of the specimen makes sure that the strain concentrates in straight section of the specimen, which prevent the fracture in the edge during the tensile process; at the same time, the straight specimen assures that the strain can be equivalent to the displacement of the strain mark on the movable table. The size of the specimen is 100μm long, 50μm wide and 10μm thick.According the above design, the integrated tensile test chips with nickel and silicon as the supporting structures have been fabricated by UV-LIGA process. The sacrificing process is the supplement for the back side silicon etching process. The sacrificing process is rather simple, the fabrication period is short, and the specimen does not have to be immerged in the high temperature concentrated alkali solution for a long time. Meanwhile, by adjusting the additives amount in the electrolyte, the residual stress can be almost eliminated, which could improve the accuracy during the test. In the fabrication process, in order to improve the uniformity of the specimens, two efficient methods were proposed: 1) improve the uniformity of the thickness of the specimen by adding the in-chip auxiliary electrode near it; 2) the"side effect"can be decreased by adding the isolated board and the out-chip auxiliary electrode in the electrolyte.A data acquisition and analysis system was programmed in the Visual Basic platform, and it realizes the controlling the step motor, acquire the displacement from laser sensor and real time tensile curves calculation during the test. The minimum tensile displacement is 0.1μm, the maximum tensile speed could be 70μm/s while the minimum speed could be 0.2μm/s; the resolution is 10nm.Finally, by using this tensile test system, the mechanical properties of two sulfamate electroplated nickel; Watts electroplated nickel and chloride electroplated nickel have been investigated. The results suggest that, in the specific conditions, the ultimate tensile strength (UTS) of the four sets of specimens is much higher than that of bulk nickel. Moreover, the difference in these specimens was observed. Watts specimens show the highest ultimate tensile strength (2178MPa), followed by chloride specimens (1525MPa), Specimens electroplated in Sulfamate added by saccharin only showed the lowest UTS (923Mpa). The Specimens electroplated in Sulfamate added by saccharin and butynediol show the UTS (1954Mpa) as twice as that of specimens in same electrolyte added by saccharin only (923Mpa).By analysing the bright field and dark field TEM images of four sets of specimens, their grain sizes were obtained. XRD test results show that the Watt specimens have no preferred orientation, while other three sets of specimen have the preferred orientation in (200) direction. Comparing the tensile test results of these three sets, their Ultimate tensile strength was determined by their grain size. The UTS increases with the decreasing of the grain size.