FABRICATION OF 50 NM LINE-AND-SPACE X-RAY MASKS IN THICK AU USING A 50 KEV ELECTRON-BEAM SYSTEM

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-mu-m-thick, 2 cm-diam SiNx x-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-mu-m-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 (approximately 12 dB) x-ray masks suitable for the Cu(L) lines at 1.3 nm. To avoid problems of distortion and peeling of the 50 nm-wide lines, the plating was done under current-density and pH 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.