A reactive ion etching (RIE) system with a magnetic field, mainly parallel to the dark space electric field on the etch electrode, is described. This axial-field REE system has been used with a molecular bromine (Br-2) plasma to fabricate Si stencil masks for ion beam lithography. Silicon dioxide (SiO2) films were used as masking layers, and the etching process was optimized for smallest silicon wall angle at a constant Si/SiO2 selectivity of 50. The optimized process was used to fabricate 0.75-1.0-mu m-thick Si masks with sub-50 nm resolution. Over a thickness of 0.75 mu m, change in linewidth was found to be less than 10 nm, corresponding to a wall angle of less than 0.2 degrees or 3 mrad.