To explore potential applications of tantalum and tantalum nitrides as diffusion barrier materials in integrated circuits with Cu interconnects, detailed first-principles simulations of Cu/Ta and Cu/TaN systems were carried out. Various interfacial structures between Cu- and Ta-based compounds were examined by considering different surface orientations, in-plane arrangements, surface terminations, and chemical compositions. The coexistence of strong Cu-N ionic bonding and Ta-Cu covalent/metallic bonding dictates the stable interfacial structures. Using the nudged elastic band method, calculations were made of the diffusion energy barriers of Cu to the pre-existing vacancies across Cu/Ta, Cu/TaN interfaces, and in bulk TaN compounds. As a comparison, Cu diffusion in Si was also studied. It was found that Cu could easily diffuse into Si either spontaneously or with small energy barriers. On the other hand, although the Cu/Ta interfacial diffusion barrier was low, the high vacancy formation energy in Ta renders Cu diffusion difficult. It was found that face-centered cubic TaN was an excellent candidate for diffusion barrier material owing to its extremely high interfacial diffusion energy barrier. The bulk diffusion barrier of Cu in face-centered cubic TaN was also very high.

First-Principles Simulations of Copper Diffusion in Tantalum and Tantalum Nitride. Y.Zhao, G.Lu: Physical Review B, 2009, 79[21], 214104