In this work, we investigate the crystalline structure and the chemical properties of a "metal/high k" gate stack with TaN as the gate electrode and HfO2 as the dielectric. Both TaN and HfO2 were deposited using atomic vapor deposition. We show that a 3 nm thick layer of HfO2 is crystallized when it is integrated, whereas it is amorphous as deposited. Increasing the TaN thickness increases the amount of the TaN crystalline face-centered cubic phase. Concerning the gate-stack chemistry, oxygen and nitrogen diffusion between the layers is shown. This occurs during the gate electrode deposition for all TaN thicknesses. We show that the gate stacks, including thick TaN layers, are not chemically stable under the spike annealing used for dopant activation because oxygen and nitrogen diffusion is increased by this thermal treatment. Gate stacks with a thinner TaN layer are much more stable under annealing because they are already partially oxidized before annealing. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3125723] All rights reserved.