This work concentrates on the diffusion barrier stability of very thin (10 or 20 nm) alpha- or beta-Ta, TaN, Ta(O) and Ta(N,O) films in the Cu/barrier/Si system. Based on the classical theory of the thin film growth and know how of material transport in thin films, the various Ta-based films were classified according to their density of free short-circuit paths. Using scanning electron microscopy, transmission electron microscopy, glow discharge optical emission spectroscopy and secondary ion mass spectrometry, the 20 nm thin polycrystalline columnar-structured P-Ta films were found to be stable up to 500 degrees C for 1 h. After 1 h at 600 degrees C Cu3Si was formed due to short-circuit diffusion of Cu throughout the P-Ta films. The 20 nm thin giant-grained cr-Ta films show equivalent performance to the P-Ta films. On the other hand, the 10 nm thin stuffed nanocrystalline face-centered-cubic (fcc) TaN films were able to protect the Si from Cu diffusion up to at least 600 degrees C/1 h. Ten nm thin amorphous-like Ta(N,O) and Ta(O) films also show barrier stability that is comparable to fee TaN. While Ta(N,O) tends to recrystallize mainly into hexagonal-close-packed Ta2N above 500 degrees C, the Ta(O) remains amorphous even at 600 degrees C. Besides the amorphous-like microstructure, the high recrystallization temperature of Ta(O) is the reason why the introduction of 5 nm thin Ta(O) film into the Cu/5 nm Ta(O)/5 nm beta-Ta/Si structure leads to a stability increase up to at least 600 degrees C for 1 h. O 1999 American Vacuum Society. [S0734-2101 (99)04803-4].