Effect of Thermomigration–Electromigration Coupling on Mass Transport in Cu Thin Films

Abstract

Self-induced temperature gradients produced due to passage of electric current through thin film interconnects with bends can be very large, making thermomigration an important mass transport mechanism, in addition to electromigration. Here, we study effects of thermomigration–electromigration coupling on mass transport in Cu films deposited on SiO2/Si substrate, as per the Blech configuration, with a W or Ta interlayer. We observed a slowly growing depletion zone at the anode in addition to a rapidly expanding depleted zone at the cathode. Moreover, we also observed that the extent of the depletion zone at the cathode varied non-monotonically with the inverse of the length of the sample. These seemingly “anomalous” observations are attributed to the coupling between thermomigration and electromigration, where thermomigration becomes dominant as the current density is increased, the sample length is decreased and the affinity between interlayer and the Cu film is weakened. The findings in this work are augmented by finite element modeling of thermomigration–electromigration coupling in Cu film. An overview of impact of these findings on fabrication of thin film device-level interconnects is also presented.