A titanium oxide film was deposited on a silicon substrate by RF-magnetron sputtering using a sintered oxide target in an argon-gas atmosphere. Phase transition and crystallization during the post-heat treatment were investigated. The thermodynamic stability of phase, oxygen diffusion through the film to form an interfacial SiO2 layer, and the reaction process in which the rutile-type TiO2 is crystallized were investigated. Rock-salt-type TiO with large deficiency of titanium ions is quenched in the as-deposited film in non-equilibrium. The following post-heat treatment in nitrogen gas transforms the film to Ti3O5, while post-oxidation transforms the film into rutile-type TiO2, even in an ultra-thin film of 5 nm, at the expense of the growth of interfacial SiO2 with a thickness of 2.5 nm. The growth of the interfacial oxide is explained in terms of three origins: oxygen defects incorporated into the rutile-type TiO2, fast oxygen diffusion via the defects, and the fact that non-stoichiometric TiO2 - x is less stable than SiO2. We also discuss the main criteria that must be satisfied in order to apply the high-permittivity oxide to a gate insulator. (C) 2002 Elsevier Science B.V. All rights reserved.