Direct observation, by means of in situ scanning electron microscopy, of void heterogeneous nucleation and migration controlled electromigration failure mechanism in Cu dual damascene interconnect structures has been recently reported [A. V. Vairagar, S. G. Mhaisalkar, A. Krishnamoorthy, K. N. Tu, A. M. Gusak, M. A. Meyer, and E. Zschech, Appl. Phys. Lett. 85, 2502 (2004)] In the present study, a dual damascene structure with an additional 25 nm Ta diffusion barrier embedded into the upper Cu layer was fabricated. This thin layer of diffusion barrier blocked voids from propagating into the via, thus eliminating the previously reported failure mechanism. With this structure, a lifetime improvement of at least 40 times was achieved. Analysis on failed samples suggested that failures in samples with the embedded Ta barrier layer occurred at the bottom of the via, which were caused by void migration along the bottom of the Cu lines.