Molecular dynamics simulations of grain boundary diffusion in Al using embedded atom method potentials
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Molecular dynamics (MD) simulations of diffusion in a 25(310) [001] Al tilt grain boundary were performed using for the first time three different potentials based on the embedded atom method (EAM). The EAM potentials that produce more accurate melting temperatures also yield activation energies in better agreement with experimental data. Compared to pair potentials, the EAM potentials also give more accurate results.
Grain boundary (GB) dynamics is of great interest both in materials science and metallurgical engineering because many important processes are dominated by the unique properties of grain boundaries. One import GB phenomenon is grain boundary diffusion. An example of its macroscopic effect is in electromigration in thin-film Al metallizations where tilt boundaries act as fast-diffusing paths for Al atoms. Electromigration is a growing concern to the semiconductor industry as a frequent cause of interconnect failure. It is generally believed that as with bulk diffusion, GB diffusion is mediated by point defects.1-2 Molecular dynamics (MD) simulations with various pair potentials have been performed to study diffusion in Al boundaries. 34 These simulations gave low activation energies for grain boundary diffusion.4 To our knowledge, MD simulations of GB diffusion have not been attempted using more accurate embedded atom method (EAM) potentials for Al. The EAM was developed by Daw and Baskes5 as a semiempirical means of computing the total energy of an arbitrary arrangement of atoms in metals. In the EAM formalism, the energy of an atom consists of two components: a sum of electrostatic pairwise interactions with each of its near neighbors and an energy to embed the atom in the local electron density created by its near neighbors. The latter component provides a "manybody" effect sensitive to the atom's local environment, making possible the accurate treatment of low-symmetry systems such as free surfaces, interfaces, and grain boundaries.5 Recent EAM calculations using molecular statics (MS) techniques to compute diffusion energetics in various [001] tilt grain boundaries in Ag 6 have shown good agreement with experiments by Ma and Balluffl7 of low temperature diffusion in the C-kinetics regime where diffusion across the boundaries is negligible. Several EAM potentials for Al have been developed. The first of these was by Foiles and Daw8 (we refer to J. Mater. Res., Vol. 10, No. 7, Jul 1995 http://journals.cambridge.org
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it as EAM-FD here) for study of Ni3Al alloy properties. The free parameters in the potential were obtained by fitting to several experimentally determined quantities,9 namely, the equilibrium lattice constant, sublimination energy, bulk modulus, elastic constants, and vacancy formation energy. Voter and Chen10 also derived an EAM potential (EAM-VC) by performing additional fitting to diatomic parameters. Recently, Erocolessi and Adams11 have developed an EAM potential for Al (EAM-EA) using the results from first-principle calculations as input parameters. Specif
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