Superfilling of submicrometer trenches by direct copper electrodeposition onto physical vapor deposited and atomic layer deposited Ru barriers is demonstrated. The Cu nucleation and growth mode is found to be sensitive tothe oxidation state of the Ru surface as well as the copper deposition parameters. Depending on the processing conditions, Cu deposition may or may not occur competitively with oxide reduction. Failure to remove the air-formed 3D oxide film results in Volmer-Weber (island) growth and consequently poor trench filling, as well as poor adhesion between Cu and Ru. In the case of thin resistive oxide-covered Ru seed layers, the "terminal effect" further exacerbates the difficulties in obtaining a compact, fully coalesced Cu film because the rate of Ru oxide reduction is decreased along with the density of Cu nuclei. In contrast, Cu deposition on a reduced "oxide-free" Ru surface results in more rapid coalescence involving the formation of a wetting Cu underpotential deposition layer. Electrochemical reduction of the oxidized Ru seed layer in a deaerated sulfuric acid solution, followed by rapid wet transfer to a Cu plating bath, enables robust superfilling of trenches and improved adhesion between Cu and Ru. Early film coalescence is favored by deposition at high (η -0.25 V) overpotentials.