Techniques formation of high purity copper (Cu)-filled lines and vias are provided. In one aspect, a method of fabricating lines and vias filled with high purity copper with is provided. The method includes the following steps. A via is etched in a dielectric. The via is lined with a diffusion barrier. A thin ruthenium (Ru) layer is conformally deposited onto the diffusion barrier. A Cu layer is deposited on the Ru layer by a sputtering process. A reflow anneal is performed to eliminate voids in the lines and vias.