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Determining the Young's modulus of SU-8 negative photoresist through tensile testing for MEMS applications

SU-8 is not only an epoxy-based negative-tone photoresist but also a potential building structural material which can be directly used as movable parts in micro structures for its unique UV curing characters and high-aspect ratio. It is particularly suitable for the fabrication of a wide range of low-temperature processes objects, such as micro valves, micro gear wheels and micro grippers. A standard mechanical property parameter database of SU-8 in microscale has not been established while its characters can vary significantly depending on the process conditions. The Young's modulus is one of the important parameters to evaluate the mechanical properties of this novel structural material, especially for the optimum design, performance realization and reliability analysis of micro devices. This paper presents the design of free-standing SU-8 thin film specimens with different sizes. The fabrication processing is demonstrated and the Young's modulus is achieved through tensile testing. The experiments are operated on a micro-tensile testing system. The force is measured by a load cell with accuracy of 0.25 mN and elongation of specimen is obtained from a displacement sensor with accuracy of 0.1 um. The measured Young's modulus of 60um wide SU-8 specimens from force-displacement curves is about 2.2±0.1 GPa. The behavior of plastic deformation is also detected before elastic deformation during the experimental process.