Work Function Shift Mechanism of Metal-Gate Electrode with Ru ∕ Ti Bilayer

The work function shift mechanism of the bilayer metal-gate electrode system has been investigated. An abrupt variation of the metal work function of Ru∕Ti∕SiO2was observed at a bottom layer thinner than 7nmwith the tunable range of nearly 1.3eV. Two plausible explanations are suggested for the work function shift. One is the deposition coverage ratio of two metal layers on gate dielectric due to the sporadic and fast islandlike growth of an ultrathin bottom layer roughly below 1nmand the other is the metal diffusion of the top layer for a thicker (<7nm)bottom layer.