Copper films were prepared by evaporation of copper metal and simultaneous bombardment by nitrogen ions with ion energy in the range 0.2 keV to 2.0 keV. The adhesion of copper films was improved by increasing the ion energy of the nitrogen ions. The copper film prepared with 2.0 keV nitrogen ions had the strongest adhesion. The structure of the interlayer between the copper film and the polyimide substrate was evaluated by transmission electron microscopy. The copper atoms were diffused into the polyimide substrate by nitrogen ion bombardment. The chemical states of the polyimide film surface and the chemical binding states at the interlayer were analyzed by X-ray photoelectron spectrometry. Nitrogen ion bombardment caused carbonization of the polyimide surface, and the irradiated nitrogen ions combined with the carbon atoms of the polyimide film. At the interlayer, X-ray photoelectron spectrometry showed that copper compounds were formed by nitrogen ion bombardment. It was considered that the increase in adhesion due to nitrogen ion bombardment could be attributed to a combination of the anchor effect caused by the diffusion of copper atoms into the substrate and the formation of copper compounds at the interlayer.