Thermally stable, low resistivity TaC diffusion barrier layers between Cu and Si were developed. The 5 nm thick Ta(53)3C(47) layer, which had the lowest electrical resistivity of 210 mu Omega cm, was found to prevent the Cu diffusion to Si after annealing at 600 degrees C for 30 min. Microstructural and diffusional analyses by X-ray diffraction and high-resolution electron microscopy indicated that the Ta53C47 layers had small grains with average diameter of similar to 10 nm, and that the grain boundaries would control the Cu diffusion through the barrier. The microstructure was found to depend on the carbon concentration in the TaC layer : Ta40C60 and Ta20C80 layers were believed to have microcrystalline structure with amorphous carbon phases, and the Cu diffusion was believed to be controlled by the amorphous carbon phase in the TaC layers.