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The mechanical and physical properties of silicon nitride thin films have been characterized, particularly for their application in load-bearing MEMS applications. Both stoichiometric (high-stress) and silicon-rich (low-stress) films deposited by LPCVD have been studied. Young's modulus, E, has been determined using conventional lateral resonators and by bulge testing of membranes, and tensile strength has been determined using a specially designed microtensile specimen. All microdevices have been fabricated using standard micromachining. We have also measured the thermal expansion coefficient of stoichiometric silicon nitride. Our best estimate of E is 325±30 GPa for stoichiometric and 295±30 GPa for silicon-rich silicon nitride. The average tensile strength for the stoichiometric material is 6.4±0.6 GPa, while that for the silicon-rich material is 5.5±0.8 GPa; the burst strength of membranes of the stoichiometric material is 7.1±0.2 GPa.