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Increasing the Usable Frame-rate of your Existing STEM

Increasing the Usable Frame-rate of your Existing STEM

Session

Stream 2: EMAG - Instrumentation Development (incl Detector technology)

Authors

Dr Jonathan Peters (2, 1), Tiarnan Mullarkey (2, 1), Clive Downing (1), Dr Lewys Jones (2, 1)

Affiliations

1. Advanced Microscopy Laboratory, CRANN
2. School of Physics, Trinity College Dublin

Keywords

STEM, Frame rate, Real time, Dose rate, Precision

Abstract text

Scanning transmission electron microscopy has increasingly become the preferred imaging mode in TEM due to its intuitive interpretation and ability to collect multiple signals simultaneously. However, the serial nature of STEM gives rise to several limitations that must be considered in any experiment. STEM is vulnerable to sample drift effects, presenting as non-rigid distortions in an image [1], and the high local beam current density and the resulting electron dose rate is capable of damaging many technologically interesting specimens. Recent developments in in-situ microscopy have led to the desire to capture dynamic events at high frame rates. Current scanning speeds attainable in typical STEMs are on the order of seconds for a 512x512 image, leading to a high probability of distorted images of beam damaged samples with low temporal resolution.

All of these problems can be mitigated by increasing the scan speed when imaging. The acquisition time of a single STEM image results from several variables: the pixel count, the pixel dwell time and the line flyback time. Decreasing the pixel count is an easy way to increase STEM frame rates, but often comes with an unacceptable penalty to resolution and/or field of view. Minimum usable pixel dwell times are typically limited by detector afterglow and the control electronics, with values on the order of 1 μs typically achievable [2]. Flyback waiting time is to account for the hysteresis in the scan coils arising from induction effects in the scan coils [3]. The Flyback time is typically on the order of 500 μs, and can account for a significant proportion of the image acquisition time. While is it possible to use improved hardware to reduce flyback hysteresis or dwell times, this presents a price hurdle and potentially disrupts integration into existing hardware and/or software [4].

Here we present a methodology to increase the achievable framerates of a Gatan Digiscan II controlling a Nion UltraSTEM. Through a careful, one-time calibration of the hysteresis behaviour of the scan system, we are able to correct for this in real time at the microscope. We show that our high framerate acquisition does not present any loss of precision through distortion analyses. Finally, we discuss the remaining limitations on STEM frame rates and strategies for future improvement.

References

[1] L. Jones, H. Yang, T. J. Pennycook, M. S. J. Marshall, S. Van Aert, N. D. Browning, M. R. Castell, and P. D. Nellist, Adv. Struct. Chem. Imaging 1, 8 (2015).
[2] T. Mullarkey, C. Downing, and L. Jones, Microsc. Microanal. 27, 99 (2020).
[3] J. P. Buban, Q. Ramasse, B. Gipson, N. D. Browning, and H. Stahlberg, J. Electron Microsc. (Tokyo). 59, 103 (2010).
[4] R. Ishikawa, Y. Jimbo, M. Terao, M. Nishikawa, Y. Ueno, S. Morishita, M. Mukai, N. Shibata, and Y. Ikuhara, Microscopy 69, 240 (2020).