The present invention relates to a bilayer cap structure for interconnect structures that comprise copper metallization or other conductive metallization. Such bilayer cap structure includes a first cap layer formed by an unbiased high density plasma (HDP) chemical vapor deposition process, and a second cap layer over the first cap layer, where the second cap layer is formed by a biased high density plasma (bHDP) chemical vapor deposition process. During the bHDP chemical vapor deposition process, a low AC bias power is applied to the substrate to increase the ion bombardment on the substrate surface and to induce resputtering of the capping material, thereby forming a seamless second cap layer with excellent reactive ion etching (RIE) selectivity.