The improvement of the electrical properties of silicon oxynitride layers formed by nitridation of silicon dioxide layers using the electron impact plasma method has been achieved by a platinum (Pt) method which includes the Pt deposition on the oxynitride layers followed by heat treatment at 300 degreesC in an oxygen atmosphere. A leakage current density and the density of oxide fixed positive charges induced by the plasma treatment are markedly decreased by the Pt treatment. Although the thickness of the oxynitride layers is not changed and the nitrogen concentration is decreased by the Pt treatment, the dielectric constant increases after the Pt treatment probably due to the elimination of defect states. It is concluded that the improvement of the electrical characteristics results from the injection of oxygen ions (O-), produced by the catalytic activities of Pt, into the oxynitride layers, and the elimination of defect states by a reaction with the O- ions. (C) 2002 Elsevier Science B.V. All rights reserved.