Sputter deposited Ta and TaN single layers of 10 nm thickness as well as graded TaN/Ta and Ta/TaN/Ta layer stacks that act as diffusion barriers for Cu metallization were investigated after annealing at temperatures between T-an = 300 and 700 degreesC. By means of glancing angle X-ray diffraction, glow discharge optical emission spectroscopy and transmission electron microscopy, results of microstructure and phase characterization were correlated with diffusion phenomena. For the pure Ta barrier, Ta diffusion through the Cu cap layer to the sample surface is observed at T-an = 500 degreesC, and the transformation of initially grown metastable beta-Ta into the equilibrium alpha-Ta phase occurs at T-an = 600 degreesC. In contrast, a fee TaN layer remains stable at least up to T-an =700 degreesC. In the case of the graded layer stacks, first signs of N diffusion out of the TaN film into the adjacent Ta layers are observed after annealing at T-an = 300 degreesC, and formation of hexagonal Ta2N starts at T-an = 500 degreesC. Whereas in the course of thermal treatments for the threefold graded Ta/TaN/Ta barrier all TaN reacts with Ta to form TaN, some fee TaN remains in the twofold graded TaN/Ta barrier. (C) 2003 Elsevier Science B.V. All rights reserved.