マルチパス4D-STEMのドリフト補正方法

Phase contrast imaging in (scanning) transmission electron microscopy ((S)TEM) is among the most effective approach for investigating local structures, at near atomic resolution in beam-sensitive weakly scattering materials including biological samples. However, they often show poor contrast and low signal-to-noise ratio (SNR) at low electron fluence. Multi-pass data acquisition can be used to improve the SNR, however sample drift between data acquisition often complicates the multi-pass approach, particularly at high magnifications. Although numerous drift correction methods have been developed for conventional phase contrast imaging in the TEM, effective drift correction for multi-pass 4D-STEM data acquisition for low fluence electron ptychography has not been extensively explored. In this paper, we report on two approaches for calculating drift vectors at each probe position between passes; one based on the reconstructed ptychographic phase in real space and the other using diffraction patterns. We demonstrate that both methods are effective in calculating and correcting drift when a defocused probe is used for 4D-STEM data acquisition and improve the contrast of low SNR ptychographic phase reconstructions.