A new E-beam excited plasma reactor was developed for sub-100-nm etching, which had an interface film between an E-beam source and a reaction chamber. The plasma was excited by an E-beam that was transmitted through a 1-mu m polyimide interface film. This ultra thin polyimide film was fabricated by a new method of spin coating on a copper plate with evaporated aluminium film. The maximum transmitted E-beam current was 3 mA for a 30-mm phi interface window, which was enough to induce plasma. A fine pattern of 50-nm lines and spaces was successfully fabricated by CF4 plasma etching on a Si wafer through a resist mask.