Atomic layer deposited ultrathin HfO2 and Al2O3 films were studied as diffusion barriers between Cu and Si substrate. The thermal stability of 3 nm thick HfO2 and Al2O3 films was investigated after annealing at different temperatures for 5 min in N-2. X-ray diffraction analyses and sheet resistance measurements suggest that both barrier films were thermally stable and the formation of Cu3Si started to take place only after annealing at high temperatures. Specifically, our results show that 3 nm thick HfO2 and Al2O3 diffusion barriers break down after annealing in N-2 at 700 and 750 degrees C, respectively. (C) 2007 The Electrochemical Society.