The inhibition of Cu oxidation by alloying C into the Ru/TaN barrier stack is investigated. By using in-situ XRD measurement, severe copper oxide formation was observed for the Cu/Ru/barrier system during annealing process in He atmosphere. This phenomenon is explained using thermodynamics from the viewpoint of Gibbs free energy of oxide formation. The Cu oxidation can be inhibited by alloying C into the Ru layer, and the more C content in the RuC layer, the more evident the inhibition effect. The RuC/TaN stack also shows better diffusion barrier properties than the Ru/TaN bi-layer. Atomic layer deposition of Cu(2)O on the RuC substrate was carried out and reduction of the Cu(2)O to Cu was observed. The mechanism of inhibition of Cu oxidation on C alloyed Ru was investigated by a first principles calculation based on the density functional theory. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.041112jes] All rights reserved.