Abstract: Techniques for forming a ruthenium (Ru) capping layer on a copper (Cu) wire are provided. In one aspect, a method of forming a Ru capping layer on at least one exposed surface of a Cu wire embedded in a dielectric structure includes the following steps. A first Ru layer is selectively deposited onto the Cu wire and the dielectric structure by chemical vapor deposition (CVD) for a period of time during which selective nucleation of the Ru occurs on the surface of the Cu wire. Any nucleated Ru present on the dielectric structure is oxidized. The oxidized Ru and an aqueous acid are contacted to remove the oxidized Ru from the dielectric structure based on a selectivity of the aqueous acid in dissolving the oxidized Ru. A second Ru layer is selectively deposited onto the first Ru layer by CVD to produce a thicker Ru layer.