Characterization of Tungsten Carbide as Diffusion Barrier for Cu Metallization

In this paper, the physical and electrical properties as well as thermal stability of the sputter-deposited tungsten carbide (WC_x) film used as a diffusion barrier layer for copper metallization were investigated for the first time. It is found that the as-deposited WC_x film has a nanocrystalline structure and a low electrical resistivity of approximately 227 µΩ·cm. According to the four-point probe technique, X-ray diffraction (XRD) and secondary ion mass spectroscopy (SIMS) measurements, it is found that WC_x film can preserve the integrity of the Cu(2000 Å)/WC_x(600 Å)/n-Si structure without the formation of Cu₃Si up to 600°C annealing in N₂ for 30 min. A more sensitive diode leakage current measurement of the Cu(2000 Å)/WC_x(600 Å)/p⁺n-Si structure shows that WC_x film is effective against Cu diffusion into Si substrate up to 550°C. The failure of WC_x film in preventing Cu diffusion is attributed to the Cu diffusion into the Si substrate through local defects of the WC_x barrier layer. It is found that Cu diffusion is strongly enhanced by the formation of W₅Si₃ at the WC_x/Si interface after high-temperature annealing.