Underlayer type dependence of EM threshold in Al-Cu interconnect

The underlayer type dependence of the electromigration (EM) threshold has been investigated in the Ti (bottom)/Al-Cu and the Ti (bottom)/TiN (top)/Al-Cu structures. From the activation energies of Cu-drift and Al-drift, the beneficial effects of Ti as an Al underlayer is shown to be associated not only with higher residual tensile stress but also with higher compressive Al-Cu yield stress. The implications of underlayer optimization are also discussed.