A lithographic method using focused ion beam exposure and subsequent dry etching is examined. The lithographic mechanism is also investigated. Employing resist patterns formed by this method, an x‐ray mask is prepared. It is shown that Ga ion exposed regions of polymer resists act as aninsitumask during the O2reactive ion etching dry development of unexposed regions. Gallium distribution and its chemical shift in a resist after exposure or development are measured by XPS. The surface gallium atoms of resist exposed to oxygen plasma are changed to Ga2O3, which covers the resist surface and acts asinsitumasks against the oxygen plasma. The feasibility of the lithographic method is demonstrated by a preparation of submicron pattern x‐ray masks.