Helium Ion Microscopy

In 2006, the Atomic Level Imaging Systems Corporation (ALIS), now part of Carl Zeiss NTS, introduced a revolutionary new ion source that reliably extracts high ion currents from a source with a virtual size of only a single atom. Application of this source in the illumination column of a microscope enables a new range of contrast mechanisms that will give new insights in the structure of materials at superior resolution [1].

The Physics of Interfaces and Nanomaterials group operates a custom-built, unique Helium Ion Microscope (HIM) that was developed in collaboration with Carl Zeiss NTS and financed by the Foundation for Technical Sciences (STW). It includes features like a much-improved vacuum, as well as enhanced detection capabilities. A HIM has all the capabilities of a standard SEM, but easily extends its capabilities into the subnanometer regime through the differences in interaction volume.

It also touts an increased depth of field and gives increased chemical contrast when using secondary electrons. In addition, and most importantly, Rutherford Backscattered Ions can be used to form an image from which detailed compositional information can be obtained, effectively constituting a spatially resolved version of the Rutherford Backscattering Spectroscopy (RBS) technique. Crystallographic contrast in polycrystalline samples can be achieved through ion-scattering mechanisms like channeling and the use of positively charged He ions means that charge neutralization can be included for non-conducting samples. An image example of the HIM is shown in Fig. 1 below.

Figure 1: Secondary electron (left) and backscattered ion (right) images that illustrate the different information that is obtained from SE and RBI images. In the left image topographic information dominates and little materials contrast is observed. In the right hand side image, Pb (bright) and Sn (dark) areas can be easily distinguished.

[1] J. Morgan, J.A. Notte, R. Hill, and E.W. Ward, Microscopy Today 14 (2006), 24.