A subhalf-micron gap filling process for copper (Cu) films was developed by using an electron cyclotron resonance (ECR) plasma sputtering method. The achieved Cu film had a resistivity as low as that of bulk Cu and an excellent, void-free filling property in a 0.5-mu m-wide trench. This result is because the distance between the target and wafer was longer than conventional sputtering; the plasma could be generated at low pressure (<1 mTorr); and the ECR plasma sputtering method generates a highly anisotropic particle flux of Cu. Then, a reflow process of the Cu films deposited by using an ECR plasma sputtering method was tried. The reflow phenomenon was observed and the filling property was improved; void-free filling in a 0.4-mu m-wide trench was achieved. Moreover, a Cu interconnection of 0.5 mu m linewidth was fabricated by a chemical mechanical polishing method without dishing, scratching, and erosion.