An amorphous Ta-Zr alloy film was studied as a diffusion barrier in the Cu metallization. On the experimental part, a Cu/Ta(50)Zr(50)/Si stack with thickness of 50 nm amorphous film was first prepared by sputtering and found effectively to suppress the penetration of Cu atoms into the substrate under rapid thermal annealing up to 650 degrees C. However, by examining the thermal stability of the barrier it revealed that these amorphous Ta(50)Zr(50) films crystallized at 800 degrees C, much higher than its failure temperature. Moreover, three metal silicides, TaSi(2), ZrSi(2) and Cu(3)Si were found almost simultaneously when samples annealed at 650 degrees C. This result indicates that the existence of Cu layer not only promotes the diffusion of Ta and Zr to form metal silicides but also the diffusion of itself to pass through the barrier film to react with Si. A failure mechanism of the diffusion barrier is therefore proposed and verified quantitatively based on the relation between the thermal stress and the activation energy of diffusion. (C) 2009 Published by Elsevier B.V.