Length-Scale Dependence of Elasticity in Nanocrystalline Materials for MEMS Applications

Length-scale dependence of the elastic properties of thin film materials bears significance in the design of Microelectronic and Micro-electro-mechanical (MEMS) systems, which are usually desired to operate in the elastic range under different operational loading conditions. In this study, we investigate elastic properties of freestanding ultra-high purity Aluminum and Gold thin films with thickness varying from 30 to 350 nanometers. Uniaxial tension test results indicate that for truly polycrystalline films, Young’s modulus can be as low as 85% and 60% of the bulk value for Aluminum and Gold respectively with average grain size of 20 nanometers. We present, for the first time, the evidence of non-linear elasticity with total strain up to 1.0% in nanocrystalline thin films and attempt to provide fundamental understanding of the length-scale dependence of elasticity in thin films with a simple model based on inter-atomic force-distance relationships.