Transmission ion microscopy and time-of-flight spectroscopy

A Transmission Ion Microscope (TIM), the Galileo prototype, has been built at the Luxembourg Institute of Science and Technology ( LIST) [1]. This is part of a new interest in the imaging properties of transmitted helium ions [2] [3] [4]. This allows the combination of both helium ions and neutrals to be detected after passing through a sample , or, if post sample deflection is used, then only the signal from the neutrals ( Figure 1 A). The helium ions have an energy between 10 keV and 20 keV and are produced in a Duoplasmatron ion source.
The prototype instrument is very flexible and uses a microchannel plate (MCP) which can be configured in multiple ways to enable analysis modalities, producing datasets of varying dimensionality. 2D images can be obtained either with a phosphor screen and a defocussed beam in direct TIM mode (analogous to transmission electron microscopy) or with an anode plate to collect the total detector signal in scanning mode providing scanning-TIM (STIM) images. By using fast blanking electronics (similar to [5] ), pulses of ions can be used to add time-of-flight (TOF) information (Figure 3), allowing a TOF-STIM mode to collect 3D datasets (x, y, t). Alternatively, a delay line detector (DLD) can be used to provide detector plane images with corresponding TOF values at each detector pixel, to collect TOF-TIM (3D datasets (, x’, y’, t). Finally, detector imaged DLD TOF-STIM can be used, in this mode, for each beam position on the sample, the arrival time and position on the detector is recorded for each count (5D datasets, x, y, x’, y’, t). The prototype TIM is also equipped with a secondary electron (SE) detector providing additional SE intensity for each pixel position in STIM modes (Figure 1 B) .
Example TOF datasets from materials science related samples (e.g. Au on Si) will be presented. In addition to microscopy, the effects of 20 keV helium ion irradiation on Au-Silica core-shell nanoparticles have been evaluated. Using bright field Transmission Electron Microscopy (TEM) imaging of irradiated particles, the effects of irradiation were tracked for increasing fluences [6]. It was seen that satellite particles are formed around the main Au core (Figure 2 , A and B) and neighbouring silica shells fuse together (Figure 2, C and Figure 2D). These effects will determine the suitable doses when imaging nanoparticles with 20 keV helium ions.

[1] M. Mousley et al., “Stationary beam full-field transmission helium ion microscopy using sub-50 keV He+: Projected images and intensity patterns,” Beilstein J. Nanotechnol., vol. 10, pp. 1648–1657, Aug. 2019, doi: 10.3762/bjnano.10.160. [Online]. Available: https://www.beilstein-journals.org/bjnano/articles/10/160
[2] K. L. Kavanagh C. Herrmann and J. A. Notte, “Camera for transmission He + ion microscopy,” J. Vac. Sci. Technol. B, Nanotechnol. Microelectron. Mater. Process. Meas. Phenom., vol. 35, no. 6, p. 06G902, 2017, doi: 10.1116/1.4991898. [Online]. Available: http://avs.scitation.org/doi/10.1116/1.4991898
[3] T. Wirtz O. De Castro J.-N. Audinot and P. Philipp, “Imaging and Analytics on the Helium Ion Microscope,” Annu. Rev. Anal. Chem., vol. 12, no. 1, 2019, doi: 10.1146/annurev-anchem-061318-115457.
[4] E. Serralta et al., “Scanning transmission imaging in the helium ion microscope using a microchannel plate with a delay line detector,” Beilstein J. Nanotechnol., vol. 11, pp. 1854–1864, 2020, doi: 10.3762/BJNANO.11.167.
[5] N. Klingner R. Heller G. Hlawacek S. Facsko and J. von Borany, “Time-of-flight secondary ion mass spectrometry in the helium ion microscope,” Ultramicroscopy, vol. 198, no. March 2019, pp. 10–17, 2019, doi: 10.1016/j.ultramic.2018.12.014. [Online]. Available: https://doi.org/10.1016/j.ultramic.2018.12.014
[6] M. Mousley et al., “Structural and chemical evolution of Au-silica core–shell nanoparticles during 20 keV helium ion irradiation: a comparison between experiment and simulation,” Sci. Rep., vol. 10, no. 1, pp. 1–13, 2020, doi: 10.1038/s41598-020-68955-7. [Online]. Available: https://doi.org/10.1038/s41598-020-68955-7