A new plasma treatment method was proposed to improve silicon oxide films. The improvement of the films was attributed to the introduction of oxygen atoms, which were extracted from self-biased nitrous oxide plasma. The introduced oxygen atoms decreased the number of defect sites in the silicon oxide film by forming new silicon-oxygen bonds. After the proposed nitrous oxide plasma treatment, the nonstoichiometric silicon oxide film showed good quality similarly to the silicon dioxide film. However, when the self-bias was very high, extra nitrogen atoms were generated from the nitrous oxide plasma and were incorporated into the silicon oxide film. This generated many defect sites degrading the quality of the silicon oxide film.