Event-driven MHz 4D STEM for low dose and in situ electron microscopy

Abstract text (incl. figure legends and references)

Four-dimensional scanning transmission electron microscopy (4D STEM) measures 2D reciprocal space diffraction from a focused electron probe scattered at each pixel across a 2D real space area. This high-dimensional dataset allows much richer information to be retrieved than conventional STEM [1]. Particularly the past decade has witnessed the advance of this technique because of the introduction of fast electron-counting detectors [2], which runs at 1-10 kHz continuous readout, while the conventional CCD only works at 1-10 Hz level. However, this "fast" frame rate is still 2-3 orders of magnitude slower than the conventional STEM with single-pixel or segmented detectors. This would prevent at least two kinds of applications which should have been benefited from the versatile 4D STEM measurements. The first is atomic-resolution low-dose phase contrast imaging of sensitive specimens. With a limited amount of dose budget and a fine scanning pixel size (for resolving atomic resolution, unless compressed sensing or ptychography is used), the electron probe current would be impractically low when the diffraction frame rate is only at kHz. As an example, for a total dose of 100 e-/A2 and a pixel size of 0.5 A, the probe current has to reduce to 0.004 pA for 1 kHz detector but can be 4 pA when the detector runs at 1 MHz. Thousandths of pA is either inaccessible in some microscopes or in practice hardly usable in most cases. The second is in situ study of dynamic behavior upon external stimuli. Pushing 4D STEM towards conventional STEM speed or beyond would permit time-resolved investigation of dynamic functionalities of the devices from the rich crystallographic and/or phase contrast information.

In contrast to the conventional frame-based detectors, event-driven readout architecture has very recently been applied to electron detection in cryo-EM [3,4], 4D STEM [5,6], as well as coincidence [7,8] and time-resolved [9] spectroscopy. We installed an event-based Timepix3 detector from Amsterdam Scientific Instruments with 2x2 chips (in total 512x512 pixels) on a monochromated and probe-corrected Thermo Fisher Scientific Titan STEM, and tested its performance in 4D STEM experimental setup and applications. Benefiting from its 1.56 ns Time of Arrival (TOA) binning resolution, we can now run 4D STEM measurements at MHz frame rate with reasonable probe current. Digital blanking of detector pixel/area was sometimes used to gain higher probe current within the maximum event rate. We will present three applications that have been tested so far: i) low-dose atomic-resolution phase contrast imaging, ii) large field-of-view crystallography phase mapping, and iii) in situ centre of mass analysis of e-beam-induced charging. Further prospects on programmable scanning path to manipulate the e-beam-induced charging or damage as well as near-real-time analysis of event-driven data based on sparse matrix computation will also be discussed during the presentation.

References

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[3] J. P van Schayck et al., Ultramic. 218, 113091 (2020)
[4] H. Guo et al., IUCrJ 7, 860 (2020)
[5] D. Jannis et al., Ultramic. 233, 113423 (2022)
[6] P. M. Pelz et al., IEEE Signal Process. Mag. 39, 25 (2022)
[7] D. Jannis et al., Appl. Phys. Lett. 114, 143101 (2019)
[8] N. Varkentina et al., Sci. Adv. 8, eabq4947 (2022)
[9] Y. Auad et al., Ultramic. 239, 113539 (2022)