Zeltmann Steven E, Müller Alexander, Bustillo Karen C, Savitzky Benjamin, Hughes Lauren, Minor Andrew M, Ophus Colin
Department of Materials Science and Engineering, University of California, Berkeley 94720, USA.
National Center for Electron Microscopy, Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley 94720, USA.
Ultramicroscopy. 2020 Feb;209:112890. doi: 10.1016/j.ultramic.2019.112890. Epub 2019 Nov 12.
Nanoscale strain mapping by four-dimensional scanning transmission electron microscopy (4D-STEM) relies on determining the precise locations of Bragg-scattered electrons in a sequence of diffraction patterns, a task which is complicated by dynamical scattering, inelastic scattering, and shot noise. These features hinder accurate automated computational detection and position measurement of the diffracted disks, limiting the precision of measurements of local deformation. Here, we investigate the use of patterned probes to improve the precision of strain mapping. We imprint a "bullseye" pattern onto the probe, by using a binary mask in the probe-forming aperture, to improve the robustness of the peak finding algorithm to intensity modulations inside the diffracted disks. We show that this imprinting leads to substantially improved strain-mapping precision at the expense of a slight decrease in spatial resolution. In experiments on an unstrained silicon reference sample, we observe an improvement in strain measurement precision from 2.7% of the reciprocal lattice vectors with standard probes to 0.3% using bullseye probes for a thin sample, and an improvement from 4.7% to 0.8% for a thick sample. We also use multislice simulations to explore how sample thickness and electron dose limit the attainable accuracy and precision for 4D-STEM strain measurements.
通过四维扫描透射电子显微镜（4D-STEM）进行纳米级应变映射依赖于确定一系列衍射图案中布拉格散射电子的精确位置，而动态散射、非弹性散射和散粒噪声使这项任务变得复杂。这些特性阻碍了对衍射盘进行准确的自动计算检测和位置测量，限制了局部变形测量的精度。在此，我们研究使用图案化探针来提高应变映射的精度。我们通过在探针形成孔径中使用二元掩模在探针上印上“靶心”图案，以提高峰值查找算法对衍射盘内强度调制的鲁棒性。我们表明，这种印记导致应变映射精度大幅提高，但空间分辨率略有下降。在对未应变的硅参考样品进行的实验中，我们观察到，对于薄样品，使用标准探针时应变测量精度为倒易晶格矢量的2.7%，而使用靶心探针时提高到了0.3%；对于厚样品，应变测量精度从4.7%提高到了0.8%。我们还使用多切片模拟来探索样品厚度和电子剂量如何限制4D-STEM应变测量可达到的准确度和精度。
