A novel technique, focused ion beam biased sputtering, is used to repair opaque defects in both conventional binary intensity masks and phase shift masks without leaving post-repair residual defects caused by gallium ion staining. The biased repair involves sputter removal of a defect to an area larger than its original size. The reduction of light transmission due to gallium stain is then effectively compensated by the enlarged repair. Results are presented on the repair of both conventional and attenuated phase shift masks using the biased method in comparison with conventional (no bias) repair. Optimum bias is determined by computer simulation of optical imaging and resist development. Unlike other anti-staining techniques which require additional equipment and process steps, the biased repair method involves only a change of sputtering strategy and can be implemented in any existing focused ion beam mask repair tools.