The electromigration resistance of Cu lines is significantly improved by depositing a low-k capping layer thereon, e.g., a silicon carbide capping layer having a dielectric constant of about 4.5 to about 5.5. Embodiments include sequentially treating the exposed planarized surface of inlaid Cu with a plasma containing NH₃ diluted with N₂, discontinuing the plasma and flow of NH₃ and N₂, pumping out the chamber; introducing trimethylsilane, NH₃ and He, initiating PECVD to deposit the silicon carbide capping layer, as at a thickness of about 200 to about 800 . Embodiments also include Cu dual damascene structures formed in dielectric material having a dielectric constant (k) less than about 3.9.