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Development of a User Adjustable Pole-piece Gap Objective-lens

Session

PST.3 - New Instrumentation

Authors

Mr Patrick McBean (1, 2), Mr David O’Mahony (1), Dr Lewys Jones (1, 2, 3)

Affiliations

1. School of Physics, Trinity College Dublin
2. Advanced Microscopy Laboratory (CRANN)
3. AMBER Research Centre, Trinity College Dublin

Keywords

transmission electron microscopy (TEM), objective-lens (OL) design, instrumentation development

Abstract text

The modern transmission electron microscope (TEM) is one of the most powerful characterisation tools available today to both physical and life scientists. A flagship TEM instrument may well include some combination of; advanced guns and/or monochromator, aberration correctors, fast cameras, one or more sensitive spectrometers and/or a range of in-situ holders. These features combined, as well as the room the instrument sits in, may represent an infrastructure investment well in excess of €5M. But, along with all these diverse options available at the time of commissioning there is one more choice to be made; the choice of pole-piece.

As the choice of pole-piece drastically impacts the future capabilities of the instrument throughout what might be a 15 year (or more) service life, pole-piece development remains an active area of research [1]. Smaller pole-gaps result in lower spherical and chromatic aberration and are typically preferred by the high-magnification physical science community, whilst larger pole-gaps offer wider opportunities for sample-tilting, tomography, in-situ experiments, or energy-dispersive x-ray spectroscopy (EDX) collection efficiency and may be preferred by the analytical or life-science communities [2]. The result then (unless you buy a second column) is that every laboratory’s experience of TEM procurement has begun with performance compromise before installation has even begun.

As an alternative approach, we propose the design and construction of a User Adjustable Pole-piece (UAP) with a gap that may be adjusted by the microscopist to suit a variety of experiments. To be of practical use, any such design must be; operable without the need to vent the microscope to air (for both time and contamination reasons), must be adjustable without the need to disassemble/remove adjacent lenses/detectors/goniometers, must be mechanically stable with ultra-precise alignment about the optic axis, and must be realignable to a good tuning without a specialist engineer present.

Our proposed adjustable pole-piece design, which is based around the current JEOL 200kV geometry, is shown in Figure 1.

Figure 1.  Simplified drawing of the proposed User Adjustable Pole-piece (UAP) showing a) a small ultra-high-resolution gap, b) an intermediate gap, and c) an expanded experimental gap for analytical or in-situ work.

Using designs constructed in Solidworks™ based on actual pole-piece dimensions, we construct multi-physics modelling simulations in the COMSOL™ environment. These are used to evaluate candidate designs for the UAP to aid in its construction (Figure 2).

Figure 2.  Magnetic flux density calculations of a) the entire objective-lens of a JEOL2100, and b) an enlargement of the detail within the pole-gap itself with the flux evaluated along the optic-axis.

We use these 3D computer models geometrically for the prediction of EDX spectrometer collection solid-angles as well as attainable tilt-ranges using a variety of holders, as well as electron-optically to determine the relative aberration performance for different pole-gaps.

In this presentation, we will elaborate on our progress in the design of this UAP. We will present our preliminary results which are in agreement with the prediction that the larger gaps yield better tilt/access for experiments, while the smaller gaps yield reduced aberrations. We will present calculated CTF’s for the range of gaps, and discuss the potential applications in low-Cc low-voltage imaging [3].

References

[1]    N. Shibata et al., “Atomic resolution electron microscopy in a magnetic field free environment,” Nat. Commun., vol. 10, no. 1, p. 2308, Dec. 2019.

[2]    https://www.jeol.co.jp/en/products/detail/JEM-2200FS.html

[3]    The authors would like to acknowledge financial support from Science Foundation Ireland (SFI), The Royal Society, and the AMBER Centre. We also acknowledge JEOL Tokyo for fruitful discussion and support.