Ta thin films were grown on Si(001) and polycrystalline Si substrates by plasma-enhanced atomic-layer deposition (PE-ALD) using TaCl5 and atomic hydrogen as precursors. The grown films have resistivity of 150-180 mum cm with a small Cl concentration between 0.5 and 2 at.%. The diffusion barrier properties were investigated using bilayer structures consisting of 200 nm Cu deposited by sputtering on ALD Ta films with various thicknesses. Three in situ analysis techniques consisting of x-ray diffraction, elastic light scattering, and resistance analysis were used to determine the diffusion barrier failure temperature of Ta films. The barriers were annealed at. a temperature ramp rate of 3 degreesC/s from 100 to 1000 degreesC in forming gas. For this method using x-ray diffraction, the barrier failure temperatures were determined by monitoring the disappearance of the Cu(1 I I) x-ray diffraction peak and appearance of Cu silicide diffraction peaks. At the diffusion barrier failure temperature, elastic light scattering indicated a rapid increase. in surface roughness, and concurrent increases in sheet resistance of the blanket Cu/ALD Ta films on polycrystalline Si were monitored using a four-point-probe technique. The same analyses were performed on bilayer structures of Cu/physical-vapor-deposition Ta films with comparable thicknesses. The ALD Ta barrier films show significantly higher failure temperatures compared to PVD Ta; at least 70degreesC at comparable thicknesses, which is thought to be due to the uniform, smooth surface and amorphous or nanocrystalline microstructure of ALD Ta.