Helium Ion Lithography
Recent developments show that Scanning Helium Ion Beam Lithography (SHIBL) with a sub-nanometer beam diameter is a promising alternative fabrication technique for high-resolution nanostructures at high pattern densities. Key principles and critical condit
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Helium Ion Lithography Principles and Performance Emile van der Drift and Diederik J. Maas
Abstract
Recent developments show that Scanning Helium Ion Beam Lithography (SHIBL) with a sub-nanometer beam diameter is a promising alternative fabrication technique for high-resolution nanostructures at high pattern densities. Key principles and critical conditions of the technique are explained. From existing data, the fundamental factors underlying the sensitivity gain by 1–2 orders of magnitude and the prospects for high resolution at high pattern densities are analysed. State-of-the-art performance of the technique is illustrated with experimental achievements in HSQ and PMMA resists. Exploratory SHIBL work on aluminum oxide resist is presented as a novel approach to overcome potential shot noise effects in pattern definition and to improve masking capabilities in subsequent pattern transfer.
4.1
Introduction
The launch of the helium ion (He+) microscope by Zeiss/Alis [1] has opened novel perspectives in nanotechnology, both from an imaging and manufacturing point of view. The key point in this development is the sub-nanometer He+ beam technology. For imaging, the resolution of the novel helium ion microscope (HIM) progressed to about 0.35 nm [2]. In this, it nicely fills the gap between scanning electron and transmission electron microscopy. For manufacturing, thus far only a few studies with the sub-nanometer probe setup were reported, in ion milling [3], ion beam-induced growth [4] and ion beam lithography [5, 6]. Nanostructuring
E. van der Drift (*) Kavli NanoLab, Delft University of Technology, Delft, The Netherlands e-mail: [email protected] D.J. Maas TNO Science & Industry, Delft, The Netherlands M. Stepanova and S. Dew (eds.), Nanofabrication, DOI 10.1007/978-3-7091-0424-8_4, # Springer-Verlag/Wien 2012
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capabilities in both milling and beam-induced growth will be elaborated on in Chap. 11 on He+-induced processing. In the present chapter, we will focus on the promising potential of scanning He+ ion beam lithography (SHIBL). State-of-the art SHIBL performance is more or less equivalent to the best electron beam lithography (EBL) achievement [7], except for the 1–2 orders of magnitude higher sensitivity as compared to EBL. See Fig. 4.1. A great beneficial circumstance of SHIBL is the much more directional scattering profile of helium ions in matter with negligible backscattering, as compared to electrons. Figure 4.2 illustrates calculated trajectories of helium ions and electrons both at 30 keV for several depths (1,000, 100 and 20 nm) in silicon [8]. The top panels show that 30 keV electrons have substantial forward, lateral and backscattering over a range of about 6 mm, while most helium ions scatter merely forward in a 30 cone down to a depth of 400 nm. It illustrates two important beneficial features of SHIBL as compared to EBL. Firstly, He+ ions result in an energy deposit over trajectories being about a factor of about 15 shorter and so yield a c
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