Abstract
Abstract In the scanning transmission electron microscope, fast-scanning and frame-averaging are two widely used approaches for reducing electron-beam damage and increasing image signal noise ratio which require no additional specialized hardware. Unfortunately, for scans with short pixel dwell-times (less than 5 μs), line flyback time represents an increasingly wasteful overhead. Although beam exposure during flyback causes damage while yielding no useful information, scan coil hysteresis means that eliminating it entirely leads to unacceptably distorted images. In this work, we reduce this flyback to an absolute minimum by calibrating and correcting for this hysteresis in postprocessing. Substantial improvements in dose efficiency can be realized (up to 20%), while crystallographic and spatial fidelity is maintained for displacement/strain measurement.
Highlights
The scanning transmission electron microscope (STEM) is a powerful instrument for probing materials down to the atomic scale
More advanced tools for correcting for nonrigid distortions before this averaging have been shown to improve performance further (Berkels et al, 2014; Jones et al, 2015; Ophus et al, 2016). The algorithms in these previous studies work in slightly different ways, but all rely on one assumption; that the multiple observations provide a mathematically redundant set from which the time-varying distortions can be separated from the time-invariant ground-truth
Additional redundancy can be encoded into the dataset by varying the scan-orientation during the acquisition, either as perpendicular pairs (Ophus et al, 2016) or with images recorded with a 90° increment between successive frames (Sang & LeBeau, 2014; Jones et al, 2015)
Summary
The scanning transmission electron microscope (STEM) is a powerful instrument for probing materials down to the atomic scale. The most common approach to calculate sample exposure requires the user to know their beam current, and the spacing and dwell-time of each probe position. This leads to the expression in Equation (1): Dose = I ·C (dt ) 1 (dx) (1). For CS acquisitions, where only a fraction of pixels are illuminated, this number is multiplied by some fraction less than one From this equation we see the motivation to pursue approaches with both low beam currents and reduced dwell-times (Buban et al, 2010).
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