A new technique, ‘‘reactively masked sputter etching’’ of Al, is described here. This process can pattern fine lines in Al or in Al‐Cu‐Si alloy but does not have many of the problems associated with presently available reactive sputter etching methods. The technique combines deposition and etching in the one process; Al2O3, Al, and Al‐Cu‐Si alloy are etched, while all other materials are coated with a layer of SiOx. This results in essentially infinite (Al/mask) and (Al/substrate) etch rate ratios. In addition, the etch gas contains no Cl, which is a common cause of corrosion and undercut. In effect, the etch combines the advantages of both reactive and nonreactive sputter etching. The gas described here is a SiF4/O2 mixture, and it is shown that additions of most impurity gases have very little effect on the etch. However, water or H2 addition significantly improves the performance of the etch, and typical profiles are shown for a SiF4/O2/H2 mixture with a photoresist masking layer.