Degradation of electromigration lifetime of Cu/Low-k interconnects by postannealing

The total thermal budget of the wafer fabrication process for structures with multilevel Cu/low-k interconnects has been increasing, and itseffect on the electromigration (EM) reliability of the lower-level interconnects has become a concern. The annealing of packaged samples including two-level interconnects for EM tests was shown here to be an effective method of evaluating the effect of the thermal budget on interconnects. EM lifetime was reduced by postannealing at the maximum process temperature (350 °C), and its failure mode was a slitlike void generated at the interface between the via and the Cu line. It was shown that the degradation mechanism was related to the contact between the via and the Cu line and that postannealing may affect the stress at this contact by changing the stress in the dielectric interlayer film.