Ta/TaN bi-layer structure is currently used in the ultra-large scale integrated circuits (ULSI) interconnect as barrier for copper because of its good adhesion to both SiO2 and Cu wire. In this work Cu, Ta and TaN layers were prepared by sputtering technology. X-ray diffraction, Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) as well as transmission electron microscopy (TEM) were applied to characterize the thin film thermal stability and microstructure evolution. The results show that the Ta/TaN bi-layer structure has much better diffusion barrier properties than pure Ta or pure TaN film. A mechanism was proposed to explain the better thermal stability of the Ta/TaN bi-layer structure based on the correlation between TaN layer thickness and TaN crystallization kinetics. The microstructure evolution for Ta/TaN bi-layer structure during annealing was described. (c) 2006 Elsevier B.V. All rights reserved.