Abstract：
A quantitative four-dimensional scanning transmission electron microscopy (4D-STEM) imaging technique (q4STEM) for local thickness estimation across amorphous specimen such as obtained by focused ion beam (FIB)-milling of lamellae for (cryo-)TEM analysis is presented. This study is based on measuring spatially resolved diffraction patterns to obtain the angular distribution of electron scattering, or the ratio of integrated virtual dark and bright field STEM signals, and their quantitative evaluation using Monte Carlo simulations. The method is independent of signal intensity calibrations and only requires knowledge of the detector geometry, which is invariant for a given instrument. This study demonstrates that the method yields robust thickness estimates for sub-micrometer amorphous specimen using both direct detection and light conversion 2D-STEM detectors in a coincident FIB-SEM and a conventional SEM. Due to its facile implementation and minimal dose reauirements, it is anticipated that this method will find applications for in situ thickness monitoring during lamella fabrication of beam-sensitive materials.
摘要：
提出了一种定量的四维扫描透射电子显微镜（4D-STEM）成像技术（q4STEM），用于估算整个无定形样品的局部厚度，例如通过聚焦离子束（FIB）铣削薄片以进行（低温）TEM分析获得的。这项研究是基于测量空间分辨的衍射图，以获得电子散射的角度分布，或集成的虚拟暗场和亮场STEM信号的比率，并使用蒙特卡罗模拟对其进行定量评估。该方法与信号强度校准无关，只需要了解检测器的几何形状，对于给定的工具是不变的。这项研究表明，该方法在重合的FIB-SEM和常规SEM中使用直接检测和光转换2D-STEM检测器对亚微米非晶样品产生了稳健的厚度估计。由于其易于实施和最小剂量分配，预计该方法将在光束敏感材料的薄片制造过程中找到原位厚度监测的应用。
