Electromigration failure mechanisms in bamboo-grained Al(Cu) interconnections

The electromigration in Al(Cu), Al(Pd, Nb) lines connected to either W studs or Si junction structures has been investigated. Mass transport as a function of temperature and current density was measured using both drift-velocity and resistance measurements. The dominant Al and Cu diffusion paths in fine Al(Cu) interconnects, besides grain boundaries, are along interfaces and subsequently on freshly formed free surfaces. An electromigration activation energy of 0.87 ± 0.03 eV has been obtained for both multigrained and bamboo-like structured lines. During electromigration stress, a sequential dissolution of the Al2Cu precipitates is observed, starting with the nearest precipitate to the cathode end. The observed edge displacement and resistance change in long Al(Cu) lines can be separated into three periods: incubation, slow Al motion, and steady state. A model is proposed between lifetime, τ, and current density, j, τ ∝ jm, where m changes from -2 to -1 during the process. © 1995.