PHASE RETRIEVAL WITH 4D-STEM: LIMITS ON SENSITIVITY, RESOLUTION, AND SPEED
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Moore’s Law predicts that the number of transistors in an integrated circuit (IC) doubles approximately every two years, leading to a continuous reduction in the size of individual circuits. As features on ICs shrink to nanometer or even Ångstrom scales, conducting microscopic local studies poses significant challenges, demanding improved resolution of imaging methods. Scanning transmission electron microscopy (STEM) emerges as a standout solution for its unparalleled resolution in material characterization. Recent advances in segmented and pixelated detectors have driven the development of 4D-STEM. This approach captures a full diffraction pattern at each scanning position, offering a significant enhancement in phase retrieval applications such as magnetic imaging and super-resolution ptychography. This thesis explores how to optimize sensitivity, resolution, and/or speed as a function of the number of detector pixels in 4D-STEM. One application is successfully disentangling nanoscale magnetic contrast from grain contrast in a polycrystalline chiral magnetic thin film using a pixelated detector. Another outcome is a high-throughput super-resolution imaging method for 2D materials. By implementing upsampled electron ptychography with just a 4-pixel segmented detector, we achieved a 40% improvement in resolution compared to dark field and integrated differential phase contrast imaging on the same detectors. This extends the super-resolution imaging technique to segmented detectors and reveals opportunities to implement ptychography for in-situ temporal imaging with acquisition times down to the millisecond scale.