Thermal Stability of Sputtered Tungsten Carbide as Diffusion Barrier for Copper Metallization

This work investigated the thermal stability of tungsten carbide (WC_x) films deposited by a sputtering process with a WC target as diffusion barrier layer between Cu and Si. The as-deposited WC_x film has a nanocrystalline structure and a low electrical resistivity of around 227 μΩ cm. Film characterization reveals that the WC_x film was able to preserve the integrity of the Cu (2000 Å)/WC_x (500 Å)/n-Si structure without formation of Cu₃Si, up to a 600°C annealing for 30 min. In addition, diode leakage current measurements on the same contact structure, but with a p⁺ n-Si substrate did not show deterioration of electrical characteristics up to a 550°C annealing. As the thickness of the WC_x barrier was reduced to 150 Å, the WC_x film retained the integrity of diodes up to 500°C without increasing the diode leakage current. The failure of WC_x film after high temperature annealing is attributed to the Cu diffusion into the Si substrate through grain boundaries or local detects of the WC_x barrier layer, in which some local defects may arise from the formation of W₅Si₃.