Soft x-ray photoelectron spectroscopy using synchrotron radiation has been employed to study the interface between Y2O3 films and Si(100). Y2O3 films Of similar to8, similar to15, and 65 A were formed by plasma assisted chemical vapor deposition on HF-last Si(100). With this deposition technique, SiO2 forms at the interface and a kinetically limited silicate layer forms between the resulting SiO2 deposited Y2O3. For 65 Angstrom films, the Y3d(5/2) binding energy was between 158.8 and 159.0 eV, 2.2-2.4 eV higher than the reported value of 156.6 eV for Y2O3. For 8 and 15 Angstrom films, the Y3d(5/2) binding energies were 159.6 and 158.9 eV, respectively. The relatively high binding energies are attributed to hydroxide incorporation in the film. For the ultrathin films, similar to10 Angstrom of SiO2, was formed at the interface during or after the deposition. For the 8 Angstrom film, no silicate is detectable whereas for the 15 Angstrom film, an estimated 4 Angstrom of silicate is present between the interfacial SiO2 and Y2O3 overlayer. Because this transition layer does not form in the 8 Angstrom film, it is concluded that the mixing is kinetically limited. For the 8 Angstrom film, the Si 2p(3/2) [SiO2] binding energy was 3.65 eV relative to the substrate peak. For the 15 Angstrom deposition, the Si 2P(3/2) [SiO2] binding energy was 3.44 eV and the Si 2P(3/2) [silicate] binding energy was 2.65 eV relative to the substrate peak. (C) 2003 American Vacuum Society.