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Abstract
We present a quantitative four-dimensional scanning transmission electron microscopy (4D-STEM)imaging technique (q4STEM) for local thickness estimation across amorphous specimen such asobtained by focused ion beam (FIB)-milling of lamellae for (cryo-)TEM analysis. Our method isbased on measuring spatially resolved diffraction patterns to obtain the angular distribution of electronscattering, or the ratio of integrated virtual dark and bright field STEM signals, and their quantitativeevaluation using Monte Carlo simulations. The method is independent of signal intensity calibrationsand only requires knowledge of the detector geometry, which is invariant for a given instrument.We demonstrate that the method yields robust thickness estimates for sub-micrometer amorphousspecimen using both direct detection and light conversion 2D-STEM detectors in a coincident FIBSEMand a conventional SEM. Due to its facile implementation and minimal dose requirements,we anticipate that this method will find applications for in-situ thickness monitoring during lamellafabrication of beam-sensitive materials.
