Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf
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New instrumentation to enable novel imaging modalities using sub-50 keV transmitted helium ions
Helium ions offer an alternative imaging probe to electrons, with a smaller de Broglie wavelength at the same energy  . Furthermore, the ability for neutralisation means that images can be formed by collecting only post-sample neutrals or both neutrals and transmitted ions. A comparison between the two can map where ions are more easily neutralised and offers an alternative contrast mechanism not possible with electrons. Transmission helium ion imaging is quite an understudied field and more experiments are required to fully assess the possibilities and benefits with this new microscopy. With this aim in mind, a prototype transmission helium ion microscope (THIM) has been constructed at the Luxembourg -Institute of Science and Technology (LIST) (Figure 1). The ion source is a duoplasmatron operated at 10-20 keV with a minimum beam spot size of 100 µm and a beam current of 0.1-2 nA . A microchannel plate (MCP) located behind the sample converts the transmitted ion signal to an electron shower which then hits a phosphor screen for direct transmission imaging with a stationary beam . The detector is placed over 50 cm away from the sample. Imaging of crystalline powders showed unexpectedly large charging and deformation of the beam, producing collections of spots (Figure 2). Scanning transmission ion microscopy (STIM) can also be conducted if the phosphor screen is replaced with a metal anode plate. As the beam is scanned over the sample surface, the current from the plate is measured and gives the intensity at each pixel in the STHIM image. A secondary electron detector in front of the sample is used to record secondary electron images at the same time as STIM imaging (Figure 1). Post sample deflectors blank all ions in transmission, such that only neutral atoms are imaged and the fraction of neutralised ions can be estimated. Electrostatic blanking and using the anode plate current as a stop signal allows one to determine the energy of transmitted particles by measuring their time of flight (TOF). In addition, a position sensitive delay line detector has recently been installed, to add position sensitivity to the TOF measurements. This allows both the trajectory and energy of and ion to be measured at the same time, providing a more complete record of the transmission through the sample.
On a separate prototype machine, the ‘NPScope’ instrument, which combines a gas field ion source with a transmission delay line detector, STIM can be performed with nanometre spot size. This enables parallel bright and dark field imaging using the same detector (Figure 3).
Keywords: helium ion imaging; Scanning transmission ion microscopy
Virtual Early Career European Microscopy Congress 2020, 24.-26.11.2020, København, Danmark