Fundamental Aspects of Effective Work Function Instability of Metal/Hf-based High-k Gate Stacks

Abstract

Instability of the effective work function (WF) of metal/Hf-based high-k gate stacks was investigated by means of the flat-band voltage (Vfb) shift in capacitance-voltage curves and interface dipole characterized by x-ray photoelectron spectroscopy. We observed a negative Vfb shift and corresponding interface dipole, which suggest formation of oxygen vacancy (Vo) in the Hf-based oxides. In contrast, we observed an opposite (positive) Vfb shift and interface dipole when Au electrodes were formed on cleaned Hf-based dielectrics. This indicates that Au-Hf bond hybridization at the Au/HfSiON interface also causes effective WF modulation, as theoretically predicted by Shiraishi et al. Moreover, the interface dipole causing positive Vfb shift was found to be stable under vacuum and dry ambient, but it was gradually released when the gate stacks were exposed to the air and wet ambient.