The properties of high-kappa metal oxide gate stacks are often determined in the final processing steps following dielectric deposition. We report here results from medium energy ion scattering and x-ray photoelectron spectroscopy studies of oxygen and silicon diffusion and interfacial layer reactions in multilayer gate stacks. Our results show that Ti metallization of HfO2/SiO2/Si stacks reduces the SiO2 interlayer and (to a more,limited extent) the HfO2 layer. We find that Si atoms initially present in the interfacial SiO2 layer incorporate into the bottom of the high-kappa layer. Some evidence for Ti-Si interdiffusion through the high-kappa film in the presence of a Ti gate in the crystalline HfO2 films is also reported. This diffusion is likely to be related to defects in crystalline HfO2 films, such as grain boundaries. High-resolution transmission electron microscopy and corresponding electron energy loss spectroscopy scans show aggressive Ti-Si intermixing and oxygen diffusion to the outermost Ti layer, given high enough annealing temperature. Thermodynamic calculations show that the driving forces exist for some of the observed diffusion processes. (c) 2007 American Vacuum Society.