The effect of modifying Cu/dielectric cap interface on electromigration (EM) in dual-damascene interconnect structures is presented. The Cu surface was treated with reducing (NH 3, H 2) and reactive (SiH 4) gases immediately after chemical mechanical polishing, but prior to SiN x dielectric cap deposition. Thus, compositions change at the interface capped by different layers including copper suicide and nitride. The Cu/SiN x, interf'acial reactions upon different treatments were detailed by X-ray photoelectron spectroscopy analysis, which correlated to the EM lifetimes very well. NHj treated samples showed similar failure times as control samples with no surface treatment. Significant improvement was observed for hydrogen plasma and silane treatment structures. It is found that, upon different surface treatments, formation of Cu suicide has a higher impact on EM lifetime improvement than formation of Cu nitride.