Phase Imaging in STEM Allowing for Post-Acquisition Aberration Correction and 3D Optical Sectioning using Ptychography Wigner-Distribution Deconvolution

Abstract

Advances in fast pixelated detector technology have enabled coherent diffraction patterns in a scanning transmission electron microscope (STEM) to be recorded at every probe position to form a four dimensional (4D-) STEM dataset. The rich information contained in the 4D-STEM dataset has previously been poorly utilized by the STEM bright-field detector with a small collection angle, hindering the efficiency for imaging light elements. Furthermore, collecting the electron intensity using integrating detectors such as annular bright field and the annular dark field (ADF) detectors is not able to make full use of the diffraction information.Electron ptychography is a method for retrieving phase information using diffraction patterns acquired from a sequential array of overlapping illuminated areas, and has been first demonstrated on a 4D-STEM dataset more than 20 years ago for improving image resolution [1]. At that time, the image field of view was restricted by the limitations of the camera speed and computational processing power. Using a modern aberration-corrected STEM and a fast pixelated detector, quantitative phase images can be formed simultaneously with other STEM imaging modes such as ADF at atomic resolution [2]. This phase imaging mode has a monotonic phase contrast transfer function and provides an inherent filter of image noise without reducing the strength of the phase contrast [3].In this work we show that the large redundancy in phase information in the 4D-STEM data not only offers efficient phase imaging, but also allows lens aberrations to be detected and corrected through post-acquisition processing (Fig.1) by applying a ptychography method called Wigner-distribution deconvolution (WDD) [4]. The aberration measurement and correction method works on both amorphous and crystalline specimens. Moreover, three dimensional structure information is also available post-acquisition by reconstructing the phase image at a specific depth in the specimen along the e-beam direction, which can be performed even though the microscope may not have been focused at that depth (Fig.2). Theoretical calculations suggest that the three dimensional structure information using the WDD method is attributed to an optical sectioning effect, and our experimental results suggest that WDD optical sectioning gives a depth resolution that is equal to the depth of focus of the microscope.