We investigate Vfb behavior of (TaC) 1-xAl x gated HfO 2 MOS capacitors as a function of post metal annealing (PMA) temperature. The positive Vfb shift appears in PMA temperature at above 700°C, while the negative Vfb shift occurs at below 600°C. At below 600°C, the effective work function (φ m,eff) of gate electrode becomes lower by increasing Al atoms with a low work function. We found that the positive Vfb shift occurs dominantly due to the φ m,eff change induced by Al diffusion from gate electrode at 700 and 800°C. At above 900°C, Al atoms reach to the HfO 2/SiO 2 interface and form the AlO x layers, and results in positive Vfb shift due to the bottom interface dipole. We can divide into two components of φ m,eff change of gate electrode and the bottom interface dipole due to AlO x layer as a function of PMA temperature.