Dynamic scan control in STEM: spiral scans

Abstract

Scanning transmission electron microscopy (STEM) has emerged as one of the foremost techniques to analyze materials at atomic resolution. However, two practical difficulties inherent to STEM imaging are: radiation damage imparted by the electron beam, which can potentially damage or otherwise modify the specimen and slow-scan image acquisition, which limits the ability to capture dynamic changes at high temporal resolution. Furthermore, due in part to scan flyback corrections, typical raster scan methods result in an uneven distribution of dose across the scanned area. A method to allow extremely fast scanning with a uniform residence time would enable imaging at low electron doses, ameliorating radiation damage and at the same time permitting image acquisition at higher frame-rates while maintaining atomic resolution. The practical complication is that rastering the STEM probe at higher speeds causes significant image distortions. Non-square scan patterns provide a solution to this dilemma and can be tailored for low dose imaging conditions. Here, we develop a method for imaging with alternative scan patterns and investigate their performance at very high scan speeds. A general analysis for spiral scanning is presented here for the following spiral scan functions: Archimedean, Fermat, and constant linear velocity spirals, which were tested for STEM imaging. The quality of spiral scan STEM images is generally comparable with STEM images from conventional raster scans, and the dose uniformity can be improved.

Fast reconstruction of atomic-scale STEM-EELS images from sparse sampling

“…To achieve better performance with reduced acquisition time, several predefined scan patterns were proposed such as regular scan [32], random horizontal lines [24,17], mixed regular-random scan [18,32], spiral scans [33,34,17] or square-shape scan [17]. These results tend to show that the best performance is achieved by semi-random scan patterns, which introduce randomness and avoid large holes.…”

Two-level structural sparsity regularization for identifying lattices and defects in noisy images

“…The B5 image in Figure 12 was used to demonstrate the capability of our proposed approach to identify the global lattice grid of atoms in a sample Mo-V-M-O catalyst under local image distortions. Although a precision of a scanning electron microscope has been significantly advanced, the precise control of a electron probe at a sub-angstrom level is still challenging, so the probe location assigned for imaging may deviate from the actual probe location, which causes mild image distortions (Sang et al, 2016a). When such image distortions occurred during imaging a crystal material with single Figure 11: bias of the estimates of lattice basis p g and q g for different simulation designs.…”

Spatial and spectral dynamics in STEM hyperspectral imaging using random scan patterns

“…Alternative spiral scans have also been recently proposed for reducing fly-back image distortions arising at high scanning rates. [2][3][4] In the last years, subsampling has been extensively discussed as a very effective strategy for dose-reduced image acquisition. Indeed, STEM images are often associated with a certain degree of over-redundancy, and a good approximation of the full image can already be obtained from an appropriate subset of pixels.…”