Helium ion beam lithography

Abstract

As nanoelectronic device design pushes towards ever smaller feature sizes, there is an increasing need for new lithographic patterning techniques and resists. Helium ion beam lithography (HIBL), an emerging technique that uses a high-intensity, sub-nanometer focused beam of helium ions generated in the helium ion microscope (HIM) to expose resist, promises to drive nano-patterning beyond the capabilities of conventional electron beam lithography (EBL). In this work, an investigation on HIBL in direct comparison with EBL using both conventional EBL resist PMMA and a novel fullerene-based molecular resist is described. Analysis of large area exposures reveals a sensitivity of 2 μC/cm2 on PMMA and 40 μC/cm2 on fullerene derivative resist with 30-keV helium ion beam, which are 60 times and 500 times higher than those with an electron beam for the same resists respectively. This improved resist sensitivity in HIBL demonstrates the potential for faster pattern definition and therefore high throughput. The proximity effect, caused by the backscattered primary particles and known to limit the performance of EBL, was then quantitatively studied using a “doughnut” method on 20-nm-thick PMMA resist on silicon with 30 keV HIBL and EBL. The lithographic results were fitted to the proximity function and the backscattered electrons/ions range were found out to be ~3.26 μm and ~0.067 μm, revealing a 60 times proximity effect reduction in HIBL compared to EBL. This suggests that the negligible proximity effect in HIBL enables high-density pattern definition, whilst avoiding the inadvertent exposure of surrounding material. Finally, to demonstrate the benefit of the smaller spot size and the reduced proximity effect in HIBL, line dose optimisation was carried out with high-resolution single-pixel line arrays on both resists. Linewidth of ~11.7 nm was achieved in 20-nm PMMA with pitches down to 30 nm. In 10-nm fullerene resist, line widths of 7.2 nm were achieved in sparse line arrays. The fabrication of 8.5 nm half-pitched lines with good feature separation and 6 nm half-pitched lines with inferior but still resolvable separation is also shown in this resist. Thus, sub-10 nm patterning with high resist sensitivity and small proximity effect is demonstrated using HIBL with standard processing conditions, establishing its potential as an alternative to EBL for rapid prototyping of beyond CMOS devices.

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