Origin of Flatband Voltage Shift in Poly-Si/Hf-Based High-k Gate Dielectrics and Flatband Voltage Dependence on Gate Stack Structure

Abstract

Hafnium silicate films (HfSiO) were studied to assess the origin of the flatband voltage (Vfb) shift relative to SiO2. We systematically investigated its dependence on the structure of the film stack and the gate electrodes. Our experimental results indicate that the surface composition of Hf is the most critical factor in controlling the Vfb shift. We also found that the poly-Si deposition process or impurity boron in the poly-Si gate electrode did not give rise to an additional fixed charge that can account for the huge Vfb shift. In addition, we found that the shift occurred even before activation annealing of the ion-implanted poly-Si gate electrode. The results clearly demonstrate that the Vfb shift results from the Hf-originated defects at the poly-Si/HfSiO interface. We also discuss the suitable structure for reducing the huge Vfb shift by decreasing the number of Hf atoms near the interface of the poly-Si, for example, by reducing the Hf content and/or by adding a thin cap layer. These are both effective methods of reducing the Vfb shift.