The effects of microwave plasma treatments on the physical and electrical characteristics of silicon dioxide films are discussed. Plasma treatments significantly improve the characteristics at low temperatures. Differences in the type of inert gas, O2 partial pressure, and total pressure cause differences in the plasma energy and active species concentrations, which affect reduction in the impurity concentrations, generation of dangling bonds, and effective working depth of the plasma. The changes in the electrical characteristics of the plasma-treated oxide films are consistent with those in the physical characteristics. The plasma conditions that result in the best improvements are determined.