In very large scale integrated circuit technology, Al–Si–Cu alloys have been used for interconnections to improve electromigration resistance and suppress stress‐induced voids. Copper addition, however, makes plasma etching difficult because of the formation of residues. We found that micromasks of Al2Cu remain on the etched surface that could cause conical residues of the aluminum alloy. In this article, the formation mechanism of the residues is presented based on structural analyses of the conical residues by transmission electron microscope/energy dispersive x‐ray analysis and x‐ray photoelectron spectroscopy. A multistep, sequential etching process is proposed that effectively eliminates the residues.