We have studied the effects of step coverage of TaN diffusion barrier layer on Cu electromigration (EM) in detail and found that the EM lifetime strongly depends on the step coverage of via sidewall thickness. We conclude that the EM failures are caused by mechanisms such as unbalanced thermal stress, thermal expansion between Cu and TaN, and surface morphology of TaN including surface roughness and grain sizes. These factors were investigated and evaluated systematically through transmission electron microscopy, field emission scanning electron microscopy, and atomic force microscopy. Furthermore, we also address the enhancement of step coverage to examine Cu EM performance by performing a lifetime experiment, and suggest that the step coverage of TaN barrier becomes an important process integration subject for sub-0.13 mu m and beyond technologies. (c) 2006 The Electrochemical Society.