The dependence of Si out-diffusion in HfO2 gate stack on the annealing pressure during rapid thermal anneal (RTA) at 950 degreesC is reported. We show enhanced Si diffusion from the substrate or interfacial layer into the HfO2 bulk, forming Hf-O-Si in a low-O-2 partial pressure annealing ambient. The formation of Hf-O-Si lowers the dielectric constant of the gate stack and changes the degree of crystallization. It is also found that dielectric constant after RTA depends on HfO2 film, thickness, and this dependence is attributed to the interfacial layer growth. The thermal stability of Hf-N bonds was investigated, and it is demonstrated that the Hf-N bond can be easily broken after RTA to form Si-N-x near the substrate, which further suppresses Si out-diffusion. The etching characteristic of HfO2 was shown to be dependent on the degree of crystallization, and this in turn depends on the amount of nitrogen near the substrate, the thickness of the film deposited as well as the composition in the bulk after RTA. (C) 2004 Elsevier B.V. All rights reserved.