We report a novel dry process suitable for submicron lithography using focused ion beam writing and plasma development of SiO2. Robust masks are fashioned in layers of SiO2 with a focused Ga ion beam, followed by development using CF4/O2 reactive ion etching. We find the development selectivity (etch rate ratio) between unimplanted and implanted SiO2 increases with increasing Ga dose. Using an implant dose of 1.5×1016 cm-2 we form SiO2 line and circular dot masks with dimensions as small as 300 nm. To demonstrate the utility of these durable masks, the patterns are transferred into GaAs by reactive ion etching, producing structures with aspect ratios (depth/width) greater than 10. Since masks formed of SiO2 are appropriate for ion bombardment processes such as found in plasma etching, as well as high‐temperature regrowth steps, this dry lithographic technique is promising for in situ vacuum integrated fabrication.