In order to get high resolution images from any scanning beam microscope one must be able to produce a sufficiently small probe, have a small interaction volume in the substrate and have an abundance of information-rich particles to collect to create the image. A typical scanning electron microscope is able to meet all of these requirements to some degree. However, a helium ion microscope based on a Gas Field Ion Source (GFIS) has significant advantages over the SEM in all three categories.

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The scanning helium ion microscope has been used in transmission mode to investigate both the feasibility of this approach and the utility of the signal content and the image information available. Operating at 40 keV the penetration of the ion beam, and the imaging resolution achieved, in MgO crystals was found to be in good agreement with values predicted by Monte Carlo modeling. The bright-field and annular dark-field signals displayed the anticipated contrasts associated with beam absorption and scattering. In addition, the diffraction of the He ion beam within the sample gave rise to crystallographic contrast effects in the form of thickness fringes and dislocation images. Scanning transmission He ion microscopy thus achieves useful sample penetration and provides nanometer scale resolution, high contrast images of crystalline materials and crystal defects even at modest beam energies.

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