The diffusion barrier properties of 100 nm thick films, both as deposited and annealed, were investigated in the metallization system using sheet resistance measurements, etch pit observation, x‐ray diffractometry, and cross‐sectional transmission electron microscopy (XTEM). No reaction of the Cu with layer was observed up to 650°C for 1 h. However, by Secco etching of the Si surface, etch pits were first observed after annealing at 550°C for 1 h. XTEM analysis shows that the in‐diffusion of Cu results in the formation of dislocations in the Si substrate along the projection of Si {111} plane and precipitates (presumably Cu silicides) around the dislocation. Considerable densification of film was achieved by annealing at 450°C for 30 min in ambient. In system, it is shown that barrier properties are enhanced by annealing the prior to Al deposition. However, in Cu/annealed system, the postannealing temperature at which etch pits are first observed is still 550°C, and the size and density of etch pits are similar to the case of as‐deposited . Thus, we conclude that annealing of film does not enhance the diffusion barrier property of in system as it does in system.