Effects of thermal annealing on the interface between tungsten and CeO₂/La₂O₃ stack gate dielectrics

The impacts of thermal annealing on the interfacial reactions and bonding structures between the tungsten metal gate and CeO₂/La₂O₃ stacked dielectrics were investigated by x-ray photoelectron spectroscopy (XPS) measurements. We found that the amount of W oxidation increases with the depth closer to the CeO₂ layer. In addition, as the annealing temperature increases to 600 °C, out-diffusion of Ce and La atoms to the bulk W, leading to the formation of Ce-O-W or La-O-W phases at the W/CeO₂-La₂O₃ transition layer, were observed. A quantitative analysis on the oxidation states of tungsten (i. e. Wⁿ⁺, with n = 0, 2, 4, 5, and 6) were conducted by the Gaussian decomposition method. Results show that in the as-deposited sample the metallic W⁰ (about 60%) is the majority content state. Other oxidation states such as W²⁺, W⁴⁺, W⁶⁺, and W⁵⁺ are still noticeable. With 600 °C annealing, the amount of W⁶⁺ state dramatically increases to about 16% and it accompanies with the decrease of CeO₂ phase. These observations indicate that the high temperature annealing would not only lead to a significant oxidation of tungsten film, but also promote the material intermixing at W/CeO₂ interface. The mechanisms of interfacial reactions and their effect on EOT were discussed.