Electromigration-induced Cu motion and precipitation in bamboo Al–Cu interconnects

Abstract The kinetics of θ phase (Al2Cu) precipitation in passivated, bamboo Al-0.2 at.% Cu segments was studied in-situ in an SEM during electromigration. The electrical current was used to reversibly transport Cu back and forth along the segment and allowed for a systematic study of Cu diffusion and precipitate nucleation, growth, and dissolution. The observations allowed an independent determination of diffusion with and without an electric current and showed that both interface and lattice diffusion are important in Cu precipitation in bamboo segments. It was also found that a substantial supersaturation of Cu is required to nucleate the θ phase. A simple model for electromigration-induced motion of Cu can be used to explain the data and predicts that an interconnect with Cu both in solution and in periodically spaced precipitates will have optimal reliability.