First-principles study of thermodynamical and mechanical stabilities of thin copper film on tantalum

The adhesion, stability, and wetting behavior at interfaces between thin Cu films and clean Ta (110) substrates are investigated by first-principles calculations using density functional theory (DFT) in the local-density approximation. Interfaces between pseudomorphic body-centered-tetragonal thin films of Cu, strained face-centered-cubic thin films of Cu, and a single pseudomorphic monolayer of Cu on body-centered-cubic Ta (110) surfaces are studied. Various high-symmetry interface configurations are considered for each case. The mechanical stability of the interfaces is studied by the ideal work of separation, while the thermodynamic stability is investigated by Gibbs' excess interface energy. All three interfaces are found to be thermodynamically unstable. An energy-weighting scheme extends the use of the DFT calculations to the case of an incoherent misfitting interface. The incoherent monolayer of Cu on Ta is thereby found to be thermodynamically stable. For coverages by more than a monolayer, the Cu atoms are expected to form three-dimensional islands on top of the Cu monolayer. With respect to interface separation, the monolayer is found to be bound more strongly to the Ta substrate than the thin film. Hence, failure is expected to occur not at the $\mathrm{Cu}∕\mathrm{Ta}$ interface but inside the Cu.