Modelling Cu diffusion into a Ta barrier

Ta has been used as a diffusion barrier for Cu interconnect technologies. The microstructure of Cu seed layer on a Ta barrier deposited by sputtering is fine-grained due to the nature of nucleation process. Grain boundaries in the Cu seed layer may have become one of the sources of vacancies for Cu to diffuse cross the Cu/Ta interface into Ta. Another potential source for vacancies is sputtering itself. The kinetic energy of Cu ions for ionized Cu PVD process is typically capable of creating vacancies at Ta surface. In this work, we calculated defect formation and migration energies in Cu and Ta, including Cu migration energy in Ta and Ta migration energy in Cu. The results indicate that with vacancies readily available at the Cu/Ta interface, Cu diffusion into Ta is more rapid compared to Ta diffusion into Cu. The calculated results support previous experimental observations. A brief discussion on how a dopant can be added to the Cu seed layer or the barrier layer to slow down Cu diffusion will also be given.