Young’s Modulus, Yield Strength and Fracture Strength of Microelements Determined by Tensile Testing

Abstract

Some mechanical properties of thin film microelements, e.g. fracture strength, depend on the manufacturing process, the load application as well as on size and shape of the microelements. Hence, the test structures that are used to determine mechanical properties should have dimensions of the same order of magnitude as an application structure, i.e. microelements must be used to accurately characterise MEMS. The fabrication of test structures must be realised in the same process as an intended application in order to give accurate results. Microelements are easily viewed in an SEM, but to be handled and tested in situ a micromanipulator was developed. Test structures were designed as released beams fixed to the substrate at one end, with a ring at the other. A high-precision testing unit was mounted on the micromanipulator next to the test structures. In the SEM, the testing unit was manoeuvred to grip the ring of the test structure beam and a tensile test of the beam was then executed. From the test data Young’s modulus and fracture strength of polysilicon and single crystalline silicon were evaluated. Relative measurement of test structures with different beam lengths enabled Young’s modulus to be evaluated with an accuracy of ±5%. Young’s modulus was determined to 172±7 GPa for polysilicon and 142±9 GPa for single crystalline silicon in the <100> direction. The fracture surfaces were examined and compared. Young’s modulus, yield strength and fracture strength of microelements made from electroplated nickel and nickel-iron alloy were also measured. Young’s modulus was evaluated to 231±12 GPa for nickel and 155±8 GPa for nickel-iron alloy composed of 72 at% nickel and 28 at% iron.