Measurement of the lithographic point-spread function of a focused helium ion beam in negative-tone PMMA and fullerene resists on ultrathin membranes

Abstract

Helium-ion-beam lithography has many advantages relevant to the fabrication of dense arrays of nanostructures such as a small probe size, large depth of field, and reduced proximity effect. Here, we calculate and measure the lithographic point-spread functions (PSFs) of 30 keV He${}^{+}$ ions in negative-tone polymethyl methacrylate (PMMA) and a fullerene-derivative resist on ultrathin silicon nitride membranes and compare the results to similar work by Manfrinato et al. (2017) measuring the PSF of 200 keV electrons in PMMA using an aberration-corrected scanning transmission electron microscope (STEM). PSFs were calculated using the method reported previously by Winston et al. (2012). Our results show that both the He${}^{+}$ ion/PMMA and He${}^{+}$ ion/fullerene-derivative resist PSFs decay more rapidly with distance, r, from the point of incidence of the beam than the corresponding aberration-corrected electron beam/PMMA PSF. In fact, the He${}^{+}$ ion PSFs decay approximately with r⁻⁴ while the aberration-corrected EBL PSF decays approximately with r⁻². This result implies that the lateral area exposed by the focused beam increases more rapidly with dose for e${}^{-}$ beams than He${}^{+}$ beams. Effectively, this should result in reduced proximity effect in helium-ion-beam lithography. This work provides further evidence that HIBL offers distinct advantages over EBL for high-resolution and high-density patterning as well as highlighting some benefits of the fullerene-derivative resist over PMMA.