Electromigration tests for critical stress and failure mechanism evaluation in Cu/W via/Al hybrid interconnect

Electromigration in a hybrid interconnect which consists of copper metallization in via below, aluminum metallization in via above, and tungsten via in between has been investigated. Fatal failures are found to occur in copper segments of the hybrid structures we tested. Two distinct failure mechanisms in copper segments are observed. One type of failure occurs due to void nucleation at the interface between barrier metal of tungsten via and copper. Time to failure is highly dependent on types of barrier metals applied. Critical stresses for void nucleation at the interface for 3 types of barrier metals are obtained using a simulation tool, and the average stress ranges from 61MPa to 246MPa. Second type of failure, which occurs less frequently than the first type, is by void growth and spanning through width and thickness of the line. Failures by void growth occur at a specific time range and failures are independent of barrier metal variation, which suggests that the failure is initiated by a pre-existing void or a defect. Thus, in order to effectively enhance the EM resistance in this hybrid interconnect structure, one should not only optimize the barrier metal, but also minimize pre-existing voids or defects in the line.