The degradation mechanisms of 10 nm thick Ta and Ta73Si27 diffusion barriers deposited between Cu and SiO2 are compared by means of X-ray reflectometry, glancing angle X-ray diffraction, glow discharge optical emission spectroscopy depth profiling and transmission electron microscopy analysis. Annealing the samples at T-an = 600 degreesC for t(an) = 1 h results in a diffusion of Ta atoms through the Cu capping layer to the sample surface, particularly in the case of the pure Ta barrier. During longer heat treatment (t(an) > 1 h), microstructural changes occur within both barrier types. Whereas the amorphous Ta-Si layer starts to crystallize into Ta5Si3, the initially grown metastable beta-Ta of the pure Ta film transforms into the equilibrium alpha-Ta phase. Analyzing the diffusion rate of Cu atoms into the substrate, the Ta73Si27 barrier shows an enhanced thermal stability compared to the pure Ta film. (C) 2003 Elsevier B.V. All rights reserved.