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The thermal and chemical stabilities of amorphous-metal diffusion barrier films are of importance in high-temperature semiconductor device applications. The reaction characteristics of the barrier constituents with the surrounding elements as well as the crystallization temperature determine the thermal stability of an amorphous-alloy diffusion barrier. We report that suitable thin films of Ta-Cu have been prepared over a wide range of compositions, by cosputter deposition onto GaAs and fused-quartz substrates. The amorphous nature and crystallization behavior of the films have been monitored by x-ray diffraction and van der Pauw resistivity measurements. Films were found to be amorphous over the range of 55–95 at. % Ta. In addition, Auger electron spectroscopy surveys and depth profiles were used to investigate the various interdiffusion reactions between the amorphous diffusion barrier, polycrystalline Au overlayers, and GaAs substrates. Barriers of Ta93Cu7 are remarkably effective in preventing Au in-diffusion, a 3000-Å layer remaining unpenetrated after an annealing at 700 °C for 20 min. Diffusion of Ga and/or As into amorphous 93 at. % Ta is more rapid than that of Au. Interfacial reactions formed Ta3Au, CuAu, TaAs2, Ga3Cu7, Cu3As, and other unidentified compounds formed above 700 °C. Journal of Applied Physics is copyrighted by The American Institute of Physics.