Structure, energetics and dynamics of copper-tin interfaces

The Sn/Cu interface plays an important role in micro-electronic industry, in addition to its scientific and environmental importance. A better understanding of the basic processes involved in it, is a fundamental task. Using Molecular Dynamics, the evolution dynamics, energetics and morphology of the Sn/Cu(111) and Sn/Cu(100) interfaces has been investigated as a function of coverage and temperature. The atomic interactions are described by the modified embedded atom method (MEAM) potentials. The MEAM is a modification of the well known Embedded Atom Method (EAM). In this model, much of the energy is attributed to an embedding energy, the energy to embed each atom into the electronic density provided by the surrounding atoms. This embedding contribution is supplemented by a pair potential to account for core-core interactions. In the MEAM the electron density is given as a multipolar expansion to include angle-dependent terms that results in bond-bending forces; while in the EAM the electron density is spherically symmetric. I calculated some physical properties such as the segregation, vacancy migration and formation energies and the activation energies for the Sn-Cu. All of these, fundamental quantities to alloy formation. It was found that incorporation of Sn into the surface layer is favored by 0.546 eV for the Cu(100) surface and by 0.424 eV for the Cu(111) surface. This means that for these low-index surfaces, the energetics favor incorporation of deposited Sn into the surface layer. The calculated segregation energies of Sn in Cu and the diffusion activation energies of Cu in Sn and Sn in Cu agree well with the experimental values. I found that at low coverages Sn deposited on Cu(111) leads to the formation of a two-dimensional (2D) alloy phase with a p([special characters omitted] x [special characters omitted]) − R30° structure which is stable up to high temperatures. For deposition of Sn on Cu(100), a coverage of 1/4 of a monolayer (ML) results in the formation of a stable 2D alloy phase with a p(2 x 2) structure. These results are in agreement with ion-scattering experiments. It is found that on both Cu(100) and Cu(111) surfaces, the resulting alloy phases are rippled with the Sn atoms displaced outward from the surfaces. One notable result at large coverages of Sn on Cu(111) and Cu(100) was the formation of a 2D ordered structure by liquid Sn at the interface. It was found that this structure does not depend on the morphology of the substrate, seems that there is no correlation between the liquid Sn atoms and the Cu atoms of the substrate. This 2D structure of Sn atoms remains stable on both Cu surfaces (111) and (100), up to temperatures of the order of 800 K, and does not change with variations in the coverage. I also found that the Cu structure at the interface stays the same for both surfaces (111) and (100)