We have investigated the effectiveness and failure mechanism of thin tantalum layers as diffusion barriers to copper. 50 nm tantalum films were sputtered onto unpatterned single-crystal 〈100〉 Si wafers and overlaid with 100 nm Cu. Material reactions in these films were followed as a function of annealing temperature by in situ resistance measurements, and characterized by Rutherford backscattering spectroscopy and cross-section transmission electron microscopy. While pure Cu on Si reacts at 200°C, the Ta film prevents Cu silicon interaction up to 600°C. At higher temperatures, reaction of the Si substrate with Ta forms a planar layer of hexagonal TaSi 2. Cu rapidly penetrates to the Si substrate, forming η]-Cu 3Si precipitates at the Ta-Si2-Si interface.