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/SiO2 was observed at a bottom layer thinner than 7 nm with the tunable range of nearly 1.3 eV. 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 1 nm and the other is the metal diffusion of the top layer for a thicker (< 7nm) bottom layer. (c) 2006 The Electrochemical Society.