Abstract: A process for forming a resist mask pattern includes the steps of forming a resist layer of organic material in a multilevel resist process on a layer to be etched, and selectively etching a planarizing lower layer used in the resist layer by using an etching gas of oxygen under a plasma condition, in which a compound gas of at least one element selected from the group consisting of B, Si, Ti, Al, Mo, W and S is added to the etching gas. For example, the compound gas comprises BCl.sub.3, BH.sub.3, TiCl.sub.4, S.sub.2 Cl.sub.2, SiCl.sub.4 or the like. During the etching, a compound oxide, e.g., B.sub.2 O.sub.3, SiO.sub.2 or the like, is deposited on sidewalls of the lower layer to form a protective layer which prevents undercutting.