Sub-50 keV helium ions as an imaging probe in transmission microscopy and secondary ion mass spectrometry

Transmission imaging with helium ions is expected to offer contrast not possible with electron beams. Whilst MeV helium ion transmission imaging is possible1, sub-50 keV helium ions are more widely available, for example, in helium ion microscope (HIM) machines. These HIM systems can perform high-resolution secondary electron imaging as well as secondary ion mass spectrometry (SIMS) elemental analysis2. Despite recent interest in the transmission imaging capabilities of low energy helium3, this method still requires detailed evaluation. Time of flight (TOF) spectroscopy for backscattered helium in a HIM has also been reported4 but transmission TOF energy spectroscopy with sub-50 keV energy helium remains unexplored. This technique should obtain information not accessible using solely transmission images.
This work focuses on the contrasts available using sub-50 keV He+ in a transmission-HIM (THIM). Our experiments used an in-house developed THIM utilising a duoplasmatron ion source operated below 50 kV. Powdered crystals of BN, NaCl and MgO were investigated in the THIM, using 10 keV He+ stationary beam illumination. A large scattering effect was visible (fig. 1B) and has been explained by sample charging, which will require consideration in all future THIM imaging experiments. Scanning THIM (STHIM) results will also be discussed, SE and STHIM images were recorded in parallel for a Cu mesh (fig. 1C ). As an additional mode, deflector plates can pulse the primary beam, allowing our THIM to record STHIM-TOF spectra (see fig. 1D) .We will also present selected recent results obtained using our in-house developed SIMS system, attached to a Zeiss Nanofab HIM2. Fig. 2 shows images from a CIGS photovoltaic solar cell active layer, containing Na, In and Cu. HIM-SIMS can efficiently analyse the nanometre scale distribution of Na, the presence of which can reduce the efficiency of the solar cell.
Funding from Luxembourg National Research Fund (FNR) project STHIM (C16/MS/11354626) and EU Horizon 2020 Grant No. 720964.

Figure 1 A) THIM image of MgO sample on Cu grid, hole width 108 µm bar width 19 µm (scale bar approx. 2.5 mrad), B) a spot pattern from MgO. C) SE and STHIM images of a Cu grid recorded in parallel, hole width approx. 90 µm. D) TOF spectrum of source emission for 10 keV He+.

Figure 2A) HIM-SE image and HIM-SIMS images showing the distributions of Na, In and Cu for an active layer of a CIGS solar cell. SE image recorded using 20 keV He+, SIMS images recorded, using 20 keV Ne+ ions, in a Zeiss Nanofab equipped with an in-house developed SIMS system. Field of view is 10 µm.

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