Structural properties of thin Ta and Ta-N films acting as diffusion barriers were investigated. Blanket Ta-based films of 10 nm thickness were deposited by conventional sputtering techniques onto (100)-Si and covered with a Cu cap layer. X-ray diffraction, depth profile analysis and electron microscopy were used to correlate results of microstructure and phase characterization with diffusion phenomena. Different barrier failure mechanisms were observed after annealing at temperatures between 450 and 800 degreesC. Th and Cu silicides were formed suddenly in layer stacks with pure Ta and Ta-20 at.% N barrier films at 550 degreesC. The application of the stoichiometric TaN as a diffusion barrier prevents the formation of Ta silicides and does not lead to significant Cu silicide formation up to 800 degreesC. However, trace Cu diffusion into the substrate was also detected at lower temperatures. The barrier stability against Cu diffusion is improving with increasing N content.