TiN, TaN and WxN films of 5, 10 or 20 nm thickness have been evaluated as diffusion barriers between Cu and SiO2. The main emphasis of this work is on electrical characterization of metal-oxide-semiconductor capacitors after bias temperature stress at different temperatures, using capacitance-vs-voltage, leakage current-vs-voltage, and triangular-voltage-sweep (TVS) measurements. The electrical tests are correlated with compositional information from X-ray photoelectron spectroscopy and secondary ion mass spectrometry. Our results indicate that TiN is a substantially inferior barrier compared to the other materials, and that WxN is at least equivalent to TaN. WxN may even be slightly better at the lowest barrier thickness. Moreover, electrical methods are generally more sensitive than structural methods in detecting failure of the diffusion barrier. Among the three electrical test methods, TVS is the earliest indicator of eventual barrier failure. However, the conditions for a consistent methodology in evaluating the reliability of diffusion barriers need to be more clearly defined. (C) 2003 Elsevier B.V. All nights reserved.