The effect of the nitrogen content on the thermal stability and degradation mechanisms of Ta-Si-N diffusion barriers was studied using methods that prove Cu interdiffusion. On the one hand, glancing angle X-ray diffraction was applied to detect Cu3Si formation after annealing of Cu/Ta-Si-N/Si layer stacks. On the other band, a combined secondary ion mass spectroscopy and transmission electron microscopy analysis of Ta-Si-N/Cu/Ta-Si-N/SiO2/Si samples was performed. For a detailed investigation of the microstructure evolution, the crystallization behavior of both Cu-capped and uncapped Ta-Si-N/Si samples was analyzed using X-ray diffraction. In the case of an uncapped Ta73Si27 film, Si interdiffusion from the substrate precedes the layer crystallization. The substrate influence on the crystallization process decreases with increasing N content x(N) of the Ta-Si-N layer. Using Cu/Ta-Si-N/Si samples, a critical temperature for Cu silicide formation was determined. This temperature increases with increasing N content of the Ta-Si-N barrier. In the case of Ta-Si-N films with x(N)> 25 at.%, Cu interdiffusion into the substrate occurs before a significant barrier crystallization is observed. For Ta-Si-N layers with x(N <=)25 at.%, no indications for Cu diffusion before crystalline phase formation were detected. (c) 2005 Elsevier B.V. All rights reserved.