Nano-engineering with a focused helium ion beam
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Nano-engineering with a focused helium ion beam Diederik J. Maas1, Emile W. van der Drift2, Emile van Veldhoven1, Jeroen Meessen3, Maria Rudneva2, and Paul F. A. Alkemade2 1 TNO - van Leeuwenhoek Laboratory, Stieltjesweg 1, 2826 CK Delft, The Netherlands 2 Kavli Institute of Nanoscience, Delft University of Technology, Lorentzweg 1, 2628 CJ Delft, The Netherlands 3 ASML Netherlands B.V., de Run 6501, 5500 AH Veldhoven, The Netherlands ABSTRACT Although Helium Ion Microscopy (HIM) was introduced only a few years ago, many new application fields are budding. The connecting factor between these novel applications is the unique interaction of the primary helium ion beam with the sample material at and just below its surface. In particular, the HIM secondary electron (SE) signal stems from an area that is very well localized around the point of incidence of the primary beam. This makes the HIM wellsuited for both high-resolution imaging as well as high resolution nanofabrication. Another advantage in nanofabrication is the low ion backscattering fraction, leading to a weak proximity effect. The lack of a quantitative materials analysis mode (like EDX in Scanning Electron Microscopy, SEM) and a relatively low beam current as compared to the SEM and the Gallium Focused Ion Beam are the present drawbacks of the HIM. INTRODUCTION The HIM scans sample surfaces with a sub-nanometer sized probe of fast helium ions[1]. Similar to the primary electron beam in SEM, at the sample the helium ions collide with surface and bulk atoms and thereby create secondary electrons (SEs). By recording the intensity of the SE signal while scanning the ion probe, the HIM creates an image of the sample surface with sub-nanometer resolving power[2, 3]. Figure 1 illustrates the main differences in interaction between SEM and HIM. The shorter wavelength of the heavier helium ion (as compared to electrons) enables one to focus to approximately the same spot size at a typically 5 times smaller numerical aperture. Hence, the depth-of-focus is much larger for HIM. The velocity of a 30 keV helium ion is comparable to that of a 4 eV electron. As a consequence, helium ions collide mainly with the valence electrons of the target atoms. These weak collisions generate hardly any X-rays or Auger electrons, and only a minor fraction of the helium is back-scattered, thereby restraining the analytical capabilities of the HIM[4]. The yellow areas in Figure 1 represent schematically the interaction volume of each primary beam within the sample. Livengood et al have simulated the shape and range of the interaction volume as a function of the primary particle energy for electrons, gallium and helium ions in silicon [5, 6]. As in Figure 1, Livengood’s trajectory simulations show that primary electrons are (back-)scattered in a wider region than helium ions. Along their trajectory, the helium ions generate Secondary Electrons (iSE) with energy below 20 eV [7, 8], while primary electrons generate eSEs with energy up to a large fraction of the beam energy. In most materia
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