The etching characteristics of ion implanted chromium films in gas plasmas have been studied. It was found that in the case of Sb+ and As+ the etching rate of implanted chromium films decreased with increasing ion dose. It was also found that the etching rate depends on the surface impurity concentration, not on the total implanted dose of ions in the film. Using chromium film samples implanted with 20 keV ions at a dose of 1×1016 cm−2, reversely etched LSI photomask patterns have been obtained with good edge profiles. It is speculated that oxides of impurities are responsible for the reversal gas plasma etching.