A method for forming a composite barrier layer that also functions as an etch stop in a damascene process is disclosed. A SiC layer is deposited on a substrate in a CVD process chamber followed by deposition of a silicon nitride layer to complete the composite barrier layer. The SiC layer exhibits excellent adhesion to a copper layer in the substrate and is formed by a method that avoids reactive Si+4 species and thereby prevents CuSiX formation. The silicon nitride layer thickness is sufficient to provide superior barrier capability to metal ions but is kept as thin as possible to minimize the dielectric constant of the composite barrier layer. The composite barrier layer provides excellent resistance to copper oxidation during oxygen ashing steps and enables a copper layer to be fabricated with a lower leakage current than when a conventional silicon nitride barrier layer is employed.