The properties of Ru(5 nm)/WCoCN(5 nm) stacked layers as a seedless Cu barrier system has been investigated. Its barrier properties compared to single 10 nm Ru film were investigated by sheet resistances, X-ray diffraction patterns, transmission electron microscopy, energy dispersive spectrometry spot analysis, line scans, and leakage currents. Thermal stability of the Ru(5 nm)/WCoCN(5 nm) improved by over 100 degrees C than that of Ru(10 nm) barrier. The results show that Ru(5 nm)/WCoCN(5 nm) can effectively block Cu diffusion up to 600 degrees C for 30 min. The Ru(5 nm)/WCoCN(5 nm) bilayer is a great Cu barrier candidate for seedless Cu interconnects. (c) 2009 Elsevier B.V. All rights reserved.