The emerging field of helium ion microscopy (HIM) is rapidly establishing itself as a valuable surface imaging technique, capable of generating images exhibiting sub-nm resolution and a high depth of field, surpassing that possible with scanning electron microscopy (SEM). This is enabled by the atomically sharp helium ion source and the larger momentum (and so shorter de Broglie wavelength) of helium ions compared to electrons, which together result in a sub-nm probe focused on the sample and a low beam divergence angle. The combination of a small probe size and the low energy of the secondary electrons (SE) generated through the interaction of the helium beam with the sample leads to a small interaction volume and hence enables high resolution imaging of the sample surface. Our Orion at Southampton is capable of an edge resolution of 0.35 nm (a good field emission gun SEM achieves approximately 1 nm). A microchannel plate can be inserted to collect the back-scattered helium ions, forming images that compliment those created by the SE detector, and provide more materials contrast. The system is also fitted with an integrated electron flood gun for charge neutralization which allows insulating samples to be imaged without the application of a conductive coating. Furthermore, the large depth of field can be exploited with stereo imaging techniques to extract 3D information from a sample.
In addition to its imaging capabilities, the focused beam of helium ions generated by the HIM can also used be used for nanofabrication through the direct modification and patterning of material, analogous to the way in which the gallium ion beam is used in focused ion beam (FIB) systems. The smaller probe size in the HIM enables the definition of finer patterns and more controlled milling than with a Ga FIB, with less damage to the surrounding material. A gas injection system also provides the capability for beam induced deposition of metals in well-defined patterns.
Here at Southampton, we are developing both imaging and nanofabrication applications for the helium ion microscope. Examples of these include:
- The imaging of biological micro and nano structures such as those found on the wing scales of lepidoptera (butterflies and moths), which are responsible for the vivid colouration and remarkable optical effects observed in these creatures. Charge neutralization with the flood-gun together with the high resolution and large depth of field provided by the HIM is allowing the fine details on these structures to be imaged clearly for the first time .
- Ion induced luminescence spectroscopy with the Gatan MonoCL system, including tests on materials known to exhibit cathodoluminescence in the visible- near IR range, e.g. quantum dots, fluorescent dyes, and rare-earth doped nanocrystals. The aim is to image biological samples tagged with luminenscent species to a resolution beyond that which is possible with cathodoluminescence in an SEM .
- The fabrication by direct milling of nanoelectronic devices in materials such as extremely thin silicon-on-insulator and graphene. The technique enables the rapid prototyping of structures such as quantum point contacts, nanowires, side-gated transistors and quantum dot devices in novel thin materials for next-generation computing .
- The characterization of the nanoscale chemical variations in polymeric semiconductor thin-film blends being developed for organic solar cells .
 S. A. Boden, A. Asadollahbaik, H. N. Rutt, and D. M. Bagnall, âHelium ion microscopy of Lepidoptera scales.,â Scanning, vol. 33, pp. 1-14, Jul. 2011.
 S. A. Boden, T. M. W. Franklin, L. Scipioni, D. M. Bagnall, and H. N. Rutt, âIonoluminescence in the helium ion microscope,â Microscopy and Microanalysis 18, 1253-1262 (2012).
 S. A. Boden, Z. Moktadir, D. M. Bagnall, H. Mizuta, and H. N. Rutt, âFocused helium ion beam milling and deposition,â Microelectronic Engineering, vol. 88, pp. 2452-2455, Nov. 2011.
 A. J. Pearson, S. A. Boden, D. M. Bagnall, D. G. Lidzey, and C. Rodenburg, âImaging the bulk nanoscale morphology of organic solar cell blends using helium ion microscopy.,â Nano letters, vol. 11, no. 10, pp. 4275-81, Oct. 2011.