Aberration measurement by electron ptychography and consistency among different algorithms

Abstract

Control over and knowledge of the electron probe is important in all scanning transmission electron microscopy (STEM) techniques. This is emphasized especially in electron ptychography, where the accurate probe wave function is required to deconvolve illumination from the specimen. The majority of ptychographic algorithms, such as the extended ptychographic iterative engine, reconstruct the electron probe on a pixelated grid numerically self-consistently. Solutions are thus not necessarily bound to wave functions physically realizable by the optical system. A method is presented to characterize reconstructed probes by conventional lens aberrations. The fitted aberrations are then used to investigate the quality of the retrieved probes, and their consistency is examined in a systematic study using a 4D-STEM focal series recorded for a thin SnS2 2D flake. Additionally, the influences of partial coherence and limited electron dose on the retrieved probes are analyzed, and the usefulness of the retrieved probes for different ptychographic methods, such as single sideband, Wigner distribution deconvolution ptychography, and gradient descent-based schemes, is elucidated. Finally, applications for ptychography-driven alignments of aberration-correcting electron optics are outlined.