Using a 50 keV, fine diameter electron beam lithography system, and a substrate consisting of a 10 nm‐thick gold plating base on a 1 μm‐thick, 2 cm‐diam SiNxx‐ray mask membrane, we have successfully exposed interdigitated electrode patterns for quantum‐effect devices having lines and spaces of 50 nm. The resist is a single layer of polymethyl methacrylate (PMMA), 496 K molecular weight, 250 nm thick. That such fine features are achievable in a single layer of thick resist is attributed to: (1) reduced backscattering from the very thin plating base (10 nm Au versus the standard 30 nm) and thin substrate (1 μm‐thick SiNx); (2) a well focused beam; (3) proximity‐effect correction; and (4) precise exposure and development control. Once developed and ‘‘de‐scummed,’’ 200 nm‐thick gold is electroplated into the PMMA mold, yielding high contrast (∼12 dB) x‐ray masks suitable for the CuLlines at 1.3 nm. To avoid problems of distortion and peeling of the 50 nm‐wide lines, the plating was done under current–density andpH conditions that produce zero stress. The x‐ray masks were replicated onto substrates and ‘‘daughter’’ x‐ray mask membranes and electroplated, yielding ‘‘polarity reversal.’’ These polarity reversed masks, with 50 nm line‐and‐space features, can then be exposed onto device substrates using either contact or proximity x‐ray lithography. This overall process takes advantage of the best aspects of electron‐beam and x‐ray nanolithographies, i.e., the capabilities of the former to create patterns of arbitrary geometry, and the robustness and high process latitude of the latter.