Atomic-scale mechanisms for diffusion of impurities in transition-metal nitrides

We use results from first-principles calculations based on density-functional theory to evaluate the performance of ZrN, HfN, and TiN as diffusion barrier materials. We examine primarily migration of Cu impurities through the bulk or through inter-grain voids of nitride films and we elucidate the conditions that favor moderate diffusion in the former case and very rapid migration in the latter. Migration activation energies for Cu interstitials in the bulk of the materials lie in the range of 0.9-1.4 eV, while the corresponding values for surface diffusion of Cu adatoms vary between 0.1 and 0.55 eV. Based on the agreement between the calculated activation energies and available measured values we resolve previous conflicting suggestions that were used to interpret various experimental data. Overall, our findings highlight the role of native point defects, impurities, and film texture on the performance of nitride diffusion barrier materials.