Imaging by Transmission Ion Microscopy and Secondary Ion Mass Spectrometry using sub-50 keV He+ ion beams

The recent availability of high-brightness helium ion sources has enabled exciting new possibilities in the fields of microscopy and nanofabrication. When compared to electron beams of same energy, He+ ions have a smaller interaction volume and thus offer higher lateral resolution (< 0.5 nm) in the secondary electron (SE) imaging mode1. While the majority of the applications of the commercial Helium Ion Microscope - HIM (Zeiss Nanofab) have been in SE imaging and nanofabrication, the complete range of imaging possibilities is still not fully explored. In this context, transmission ion microscopy is expected to offer new contrast mechanisms (e.g. charge neutralization) which are not possible in a Transmission Electron Microscope (TEM). Transmission microscopy using MeV He+ ions has already been demonstrated2, but, they are not very widely available. With the increasing availability of HIM which operate at primary energies below 50 keV, the potential to use it for transmission ion microscopy and ion energy-loss spectroscopy still need to be fully explored. To address this, we developed a prototype Transmission Helium Ion Microscope (THIM) that can operate on both stationary full-field THIM mode as well as Scanning THIM (STHIM) mode with simultaneous SE imaging possibility. This prototype has a duoplasmatron ion source and offers full flexibility in terms of instrumental configurations. This is a significant advantage in comparison to using the commercial instrument in which space below the specimen plane is very limited and thus restrict the possible experimental configurations. We imaged BN, NaCl and MgO crystalline powders in the stationary full-field THIM imaging using 10 keV He+ and investigated the distribution of transmitted ion intensities. The scattered intensity form unexpected spot patterns that may be explained by sample charging and morphology. Furthermore, we have added electronics to pulse the primary ion beam in the prototype instrument. This allows us to perform Time-of-Flight (TOF) multispectral imaging in both THIM and STHIM modes in addition to the standard Bright-Field, Dark-Field and SE imaging modes. Our presentation will focus mainly on the transmission ion configuration. We will also briefly discuss the recent developments in the Secondary Ion Mass Spectrometers (SIMS) that we developed for Zeiss Nanofab instruments (HIM-SIMS) which allow direct chemical mapping at nanoscale.
1 G. Hlawacek and A. Gölzhäuser, editors , Helium Ion Microscopy, 1st ed. (Springer, 2016).
2 F. Watt et al, Nucl. Instruments Methods Phys. Res. Sect. B Beam Interact. with Mater. Atoms 306, 6 (2013).