A mechanism of effective work function change of the Ru oxide metal gate under various anneal conditions has been studied. High temperature annealing was found to result in changing the tetragonal phase of RuO2 to the hexagonal phase characteristic for Ru metal. By extracting the electrode-dielectric band offset magnitude, it was confirmed that such a change in the stoichiometry of Ru oxide leads to the lower effective work function values of the postanneal Ru oxide/high-k gate stacks. The low temperature oxygen anneal treatment of the high temperature annealed Ru oxide electrode stacks recovers the high work function value by supplying oxygen into the decomposed Ru oxide. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3290742] All rights reserved.