We demonstrate that exposure to a plasma stream produced by electron-cyclotron resonance (ECR) of a gas mixture of Ar and O-2 improves the metal-oxide-semiconductor (MOS) diode characteristics of ultrathin films of Al2O3. Al2O3 films were formed by ECR sputtering deposition under "metal-mode" conditions. The films were then left as is or exposed to the above plasma stream or an Ar plasma stream in order to test the relative improvements in their electrical properties. The MOS-diode characteristics of films after plasma exposure and annealing in a high vacuum (around 10(-4) Pa) were studied in detail. While exposure to the Ar-O-2 plasma stream for 10-120 s had little effect on the flatband voltage shift, a very small width of capacitance-voltage hysteresis (+3 mV), a small equivalent-oxide thickness (1.0 nm), and a low value for leakage current (1.5 X 10(-4) A /cm(2)) were obtained for films after 20-30 s of exposure. Exposure to the Ar plasma stream, however, did not improve the MOS-diode characteristics of the films. Through x-ray photoelectron spectroscopy analysis, we found energy shifts indicating that exposure to the Ar-O-2 plasma stream oxidized the metal bonds in the metal-mode-deposited Al2O3 films. (c) 2005 American Vacuum Society.