In this study, oxygen penetration through two different SiCxNy cap layers was con-elated with in situ electromigration-induced void growth results. One of the SiCxNy caps was shown to be permeable to oxygen when blanket test structures were annealed in air at elevated temperatures. Electromigration results for the corresponding interconnect structures subject to the same anneals showed an increase in void growth rate as well as a decrease in the measured activation energy for diffusion and electromigration. The second SiCxNy cap, deposited using an alternate recipe, was shown to be significantly less penneable to oxygen and electromigration results showed essentially no change in void growth rate or activation energy. This suggests that oxygen concentration at the Cu/SiCxNy interface is critical in determining the mass transport along thee top surface of the Cu interconnect lines. (c) 2006 Elsevier B.V. All rights reserved.