Transmission helium ion imaging and time-of-flight spectroscopy


Transmission helium ion imaging and time-of-flight spectroscopy

Mousley, M.; Bouton, O.; Klingner, N.; Serralta Hurtado De Menezes, E.; Hlawacek, G.; Eswara, S.; Wirtz, T.

Helium ions are alternative imaging probes to electrons, offering lower de Broglie wavelengths at the same energies and the possibility for different contrast mechanisms [1] [2][3]. A prototype Transmission Helium Ion Microscope (THIM) has been constructed at the Luxembourg Institute of Science and Technology (LIST) [4]. The use of post sample deflection allows the detection of the transmitted ions and neutrals or neutrals only (Figure 1 A). The source is a duo plasmatron with a spot size on the sample of approximately 100 µm and a beam current between 0.1-2nA. There are 2 Einzel lenses and 3 XY deflectors along the column to guide the beam. A MCP detector behind the sample can be used in one of 4 different output mechanisms. Firstly a phosphor screen can be used to produce a transmission helium ion image (THIM) directly which can be captured by an external CCD. Secondly, an anode plate can be used to collect the current directly whilst the beam is scanned, the current recorded at each pixel can then form a scanning image (STHIM). Thirdly, fast electronics are used to blank the beam and provide the start signal for time of flight (TOF) measurements, whilst the anode signal can be used as the stop signal [5]. This allows the generation of TOF-STHIM data. Finally, a delay line detector (DLD) can be placed behind the MCP, from which location and time of flight, of individual particles, can be recorded simultaneously, producing energy spectra and images at the same time. The prototype can image in different modes, THIM , STHIM (scanning THIM), THIM TOF and STHIM TOF. When scanning the beam a secondary electron image can be recorded at the same time (Figure 1 B). In THIM mode the formation of spot patterns due to sample charging was seen when imaging insulating inorganic crystal samples with a stationary broad beam. This was found to be due to unexpectedly large sample charging. We will present preliminary TOF spectra for the transmitted helium ion signal recorded with an anode plate detector and a position sensitive delay line detector. Images formed from different time windows from the TOF spectra show different contrast (Figure 2B) and the spectra for a single layer graphene sample showed increased counts after the main peak (Figure 3), indicative of processes causing energy loss.

[1] Scipioni,L.;, Sanford,C. A.;, Notte,J.;, Thompson,B.;, McVey,S.;, J. Vac. Sci. Technol. B Microelectron. Nanom. Struct., 2009, vol. 27, no. 6, p. 3250, 10.1116/1.3258634.
[2] Kavanagh,K. L.;, Herrmann,C.;, Notte,J. A.;, J. Vac. Sci. Technol. B, Nanotechnol. Microelectron. Mater. Process. Meas. Phenom., 2017, vol. 35, no. 6, p. 06G902, 10.1116/1.4991898.
[3] Wirtz,T.;, De Castro,O.;, Audinot,J.-N.;, Philipp,P.;, Annu. Rev. Anal. Chem., 2019, vol. 12, no. 1, 10.1146/annurev-anchem-061318-115457.
[4] Mousley,M.;, Eswara,S.;, De Castro,O.;, Bouton,O.;, Klinger,N.;, Koch,C. T.;, Hlawacek,G.;, Wirtz,T.;, Submitt. to MRS Commun., vol. 1, pp. 1–10.
[5] Klingner,N.;, Heller,R.;, Hlawacek,G.;, Borany,J. von;, Notte,J.;, Huang,J.;, Facsko,S.;, Ultramicroscopy, 2016, vol. 162, pp. 91–97, 10.1016/j.ultramic.2015.12.005.

Figure 1: A) transmission images formed with ions and neutrals of a copper grid with a single layer graphene membrane pitch 85µm (31µm bar 54µm hole). B) Secondary electron and transmission ion images recorded concurrently in scanning mode.

Figure 2: A) The effect of offsetting the beam aperture on zero loss peak width B) STHIM images, of a 200 mesh copper grid, formed from two different peaks in the TOF spectrum.

Figure 3: The TOF spectrum for a single layer graphene sample on a 300 mesh copper grid, shows extra peaks compared to a background spectrum without a sample.

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  • Lecture (Conference) (Online presentation)
    European Microscopy Congress 2020, 23.-28.08.2020, Copenhagen, Denmark

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