The barrier properties and failure mechanism of co-sputtered Ta-Si amorphous thin films of various concentrations were studied for the application as a Cu diffusion barrier in Cu metallization schemes. After annealing the Si/Ta-Si(30 nm)/Cu(100 nm)/Ta(100 nm) structures at temperatures from 500 to 600 degrees C in an Ar-H-2(10%) ambient for 1 h, the failure temperatures of Ta-Si barriers were determined using X-ray diffraction (XRD), Auger electron spectroscopy (AES) and cross-sectional transmission electron microscopy (XTEM). Ta74Si26 and Ta63Si37 amorphous barriers containing less Si than Ta5Si3 undergo an abrupt failure between Si and Cu at 575 and 600 degrees C, respectively, with the crystallization to tantalum silicides. However, Ta60S40 and Ta56Si44 amorphous barriers containing more Si than Ta5Si3 react with the Cu overlayer before the crystallization and form the Cu3Si precipitates at the local Ta-Si/Cu interface at 575 and 550 degrees C, respectively. Among all compositions presented in this work, Ta63Si37 was the most stable barrier between Si and Cu.