Temperature and grain size dependences of mechanical properties of nanocrystalline copper by molecular dynamics simulation

Nanocrystalline copper (Cu) is considered to be one of the best interconnected material in integration circuit (IC) industry, because of its ultra-low resistivity and high mechanical stability. Mechanical properties of nanocrystalline Cu are completely different from those of bulk monocrystalline Cu. These properties are of high importance in the assessment of the thermo-mechanical reliability of the interconnected IC structure. To investigate the effects of the grain sizes and temperature on the mechanical properties of nanocrystalline Cu, molecular dynamics simulations of uniaxial tensile test are performed in this study. The results show that the elastic modulus of nanocrystalline Cu with grain sizes of 4.65–12.41 nm gradually increases with the increase of the mean grain sizes, the corresponding flow stress concurrently increases, and the flow stress is proportional to the square-root of the grain size, which satisfies the inverse-Hall–Petch relation. Furthermore, the elastic modulus linearly decreases with the increase of temperature. The coupled effect of the flow stress, strain rate and temperature were elaborated by the Arrhenius hyperbolic sinusoidal model. Meanwhile, the deformation activation energy of nanocrystalline Cu for various grain sizes were obtained. All of the tensile simulation tests confirmed that the mechanism of plastic deformation for nanocrystalline Cu with 4.65–12.41 nm grain sizes is mainly specific to the grain boundary sliding and grain rotation. The dislocation nucleation and migration, which is usually the deformation mechanism of plasticity for macroscopic materials, is no longer the dominant factor for nanocrystalline Cu.