Enhancing Contrast and Resolution for Electron-beam Lithography on Insulating Substrates

We report on the effect of ambient gas on the contrast and the resolution of electron beam lithography (EBL) in gaseous environments on insulating substrates. Poly(methyl methacrylate) (PMMA) films were exposed in an environmental scanning electron microscope using a 30 keV electron-beam under 1 mbar pressure of helium, water, nitrogen and argon. We found that the choice of ambient gas results in significant variations in contrast, and the clearing dose increases with the gases molecular weight and proton number, consistent with the increase in scattering cross-section. Significantly higher contrast values are obtained for exposure under helium and are accompanied by improved sensitivity. Despite higher sensitivity, helium exhibited the best resolution with 20-nm half-pitch dense lines and spaces. However, water vapor offered a larger process window, particularly on fused silica substrates. We also demonstrate that higher sensitivity results from effective charge dissipation. Thus, for EBL on insulating substrates, helium and water vapor may be desirable choices for charge dissipation depending on the substrate and process conditions.