Advanced recording schemes for electron tomography
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Introduction Electron tomography allows three-dimensional (3D) imaging of the microstructure of materials on the nanometer scale. Projections are created by illuminating the specimen with an electron beam from different directions, and a 3D data set is reconstructed computationally from the projections. Scanning transmission electron microscopy (STEM) tomography has become one of the primary tools for analytical 3D characterization in materials science1–4 and finds increased application in biology5,6 with advantages for studying thick biological samples.7–9 The achievable image quality and resolution of electron tomography are limited by different factors that depend on the sample. Often, the limiting factor is the signal-to-noise ratio (SNR), resulting from the limited number of electrons per pixel. This is especially the case for dose-sensitive specimens, where imaging means finding a compromise between radiation damage and noise, thereby limiting the electron dose. In other situations, imaging is limited by the geometry of image acquisition, particularly for thick specimens that can only be tilted across a limited angular range. Several methods have been proposed to acquire the 3D data set containing the best representation of the original structure of a sample. The quality of the final tomogram generally
depends on three factors: (1) spatial and angular sampling (i.e., where spatial frequencies are acquired in all directions); (2) the SNR of the projections; and (3) the accuracy of the tomographic reconstruction algorithms. In this article, we give an overview of available recording schemes for STEM tomography and highlight some of the newer developments. We do not discuss other 3D electron microscopy techniques, which are mainly used in biology, including transmission electron microscopy (TEM) tomography,10,11 single-particle tomography,12 or 3D focused ion beam combined with scanning electron microscopy (FIB/SEM).13
Recording schemes for STEM tomography The most commonly used recording scheme for 3D STEM is single-tilt tomography. The specimen is rotated around a single axis perpendicular to the beam direction. Raster-scanned STEM images are then recorded at each tilt angle. The annular dark field (ADF) detector is mostly used to acquire the images, but bright field STEM can be also used. The latter has advantages when imaging thicker specimens because geometric broadening (i.e., the fact that objects at different heights are out of focus) can be reduced with this technique.8 The tilting takes place in either constant angular or slope increments.14 In some cases, samples can be processed to a cylindrical shape,
Tim Dahmen, German Research Center for Artificial Intelligence, Germany; [email protected] Patrick Trampert, German Research Center for Artificial Intelligence, Germany; [email protected] Niels de Jonge, Leibniz Institute for New Materials, Germany; [email protected] Philipp Slusallek, German Research Center for Artificial Intelligence, and Saarland University, Germany; [email protected] d
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