Amorphous Ta-Si-N thin film was deposited by dc sputterring of Ta5Si3 target in (Ar+N2) atmosphere. The crystal structure and the thermal stability of Ta-Si-N thin films were investigated by XRD, RBS, AES, Nomarski microscope, and TEM. When the concentration of nitrogen in Ta-Si-N thin film was higher than 40 at.%, the Ta-Si-N thin film remained the amorphous state after the annealing at 1100°C for 60 min. In this case, the Cu diffusion was prevented by the amorphous Ta-Si-N thin film even if the annealing temperature increased up to 900°C for 30 min. Whereas, the Ta-Si-N thin film with nitrogen concentration less than 40 at. % was transformed from the amorphous to the polycrystalline TaSi2 phases after the annealing at 900°C for 60 min and failed to prevent the Cu diffusion after the annealing at 700°C for 30 min.