Electron beam broadening in electron‐transparent samples at low electron energies
Electron beam broadening in electron‐transparent samples at low electron energies
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低电子能量下电子透明样品中的电子束展宽
DOI：
10.1111/jmi.12793
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发表时间：
2019
影响因子：
2
通讯作者：
D. Gerthsen
中科院分区：
文献类型：
--
作者：
M. Hugenschmidt;E. Müller;D. Gerthsen
近年来，低初级电子能量下的扫描透射电子显微镜（STEM）受到越来越多的关注，因为可以避免撞击损伤，并且可以获得弱散射材料的高对比度。然而，样品中的电子束的加宽在低电子能量下是显著的，这降低了分辨率并限制了最大样品厚度。在这项工作中，我们已经研究了电子束展宽的材料与原子序数Z之间的10和32（MgO，Si，SrTiO 3，Ge）和厚度高达900 nm。使用安装在扫描电子显微镜中的多段STEM检测器在15和30千电子伏之间的电子能量下直接测量光束展宽。由于实验的原因，电子束直径被定义为只包含总束流强度的68%，而不是通常使用的总束流强度的90%。利用反常扩散的概念，引入了Hurst指数H_s，它随样品厚度和弹性平均自由程长度的变化而在0.5和1之间变化。计算还取决于定义电子束直径的束强度的分数。Hurst指数H为1是t/Λel→ 0的弹道散射状态的特征，并且可以排除我们研究的实验条件为6 μ t/Λel ≤ 30。我们从测量的光束直径推导出H = 0.75，它大于在扩散条件下预期的H = 0.5。偏向largerH值可以合理化，我们的定义，电子直径仅包含68%的总束强度，因此需要更大的样品厚度之前的扩散制度是达到。我们的结果明显偏离了以前描述光束展宽的分析方法（Goldsteinet等人，Reed，Williamset al.，Kohl和Reimer）。测量的光束直径进行了比较与模拟的，这是通过求解电子输运方程。与常用的蒙特卡罗模拟相比，这种方法具有优势，因为它是电子输运方程的精确解，并且需要更少的计算机时间。模拟的电子束直径与实验数据吻合得很好，产率H = 0.80。Lay描述在扫描透射电子显微镜（STEM）中，聚焦电子束在电子透明样品上扫描，通过STEM检测器检测透射电子的强度来形成图像。STEM分辨率最终受到电子束直径的限制，对于最好的显微镜，分辨率可以优于0.1 nm。然而，电子束直径随着样品厚度的增加而增加，因为电子被样品材料和电子的相互作用散射。电子散射导致电子传播方向的改变并降低电子束的聚焦。相关的电子束展宽降低了STEM的横向分辨率，并且通常限制了可以以良好分辨率成像的最大样品厚度。到目前为止，STEM主要在80 keV及以上的高电子能量下进行。较低的电子能量有利于弱散射和辐射敏感材料的研究，但随着电子能量的降低，电子束展宽变得更加明显。因此，光束展宽的知识对于解释…
Scanning transmission electron microscopy (STEM) at low primary electron energies has received increasing attention in recent years because knock‐on damage can be avoided and high contrast for weakly scattering materials is obtained. However, the broadening of the electron beam in the sample is pronounced at low electron energies, which degrades resolution and limits the maximum specimen thickness. In this work, we have studied electron beam broadening in materials with atomic numbersZbetween 10 and 32 (MgO, Si, SrTiO3, Ge) and thicknesses up to 900 nm. Beam broadening is directly measured using a multisegmented STEM detector installed in a scanning electron microscope at electron energies between 15 and 30 keV. For experimental reasons, the electron beam diameter is defined to contain only 68% of the total intensity instead of the commonly used 90% of the total beam intensity.The measured beam diameters can be well described with calculated ones based on a recently published model by Gauvin and Rudinsky. Using the concept of anomalous diffusion the Hurst exponentHis introduced that varies between 0.5 and 1 for different scattering regimes depending ont/Λelwith the specimen thicknesstand the elastic mean free path lengthΛel. The calculations also depend on the fraction of the beam intensity that defines the electron beam diameter. A Hurst exponentHof 1 is characteristic for the ballistic scattering regime witht/Λel→ 0 and can be excluded for the experimental conditions of our study with 6 ≦t/Λel≦ 30. We deducedH= 0.75 from measured beam diameters which is larger thanH= 0.5 that is expected under diffusion conditions. The deviation towards largerHvalues can be rationalised by our definition of electron diameter that contains only 68% of the total beam intensity and requires therefore larger sample thicknesses before the diffusion regime is reached. Our results clearly deviate from previous analytical approaches to describe beam broadening (Goldsteinet al., Reed, Williamset al., Kohl and Reimer). Measured beam diameters are compared with simulated ones, which are obtained by solving the electron transport equation. This approach is advantageous compared to the commonly used Monte Carlo simulations because it is an exact solution of the electron transport equation and requires less computer time. Simulated beam diameter agree well with the experimental data and yieldH= 0.80.Lay DescriptionIn scanning transmission electron microscopy (STEM), a focused electron beam is scanned over an electron‐transparent sample and an image is formed by detecting the intensity of the transmitted electrons by a STEM detector. STEM resolution is ultimately limited by the electron beam diameter and can be better than 0.1 nm for the best microscopes. However, the electron‐beam diameter increases with increasing specimen thickness because electrons are scattered by the interaction of the specimen material and electrons. Electron scattering leads to a change of the electron propagation direction and reduces focusing of the electron beam. The associated electron‐beam broadening degrades the lateral resolution of STEM and generally limits the maximum specimen thickness that can be imaged with good resolution. STEM is up to now mainly performed at high electron energies of 80 keV and above. Lower electron energies are beneficial for the study of weakly scattering and radiation‐sensitive materials but electron beam broadening becomes more pronounced with decreasing electron energies. Knowledge of beam broadening is therefore particularly important for the interpretation of …
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影响因子：
2.2
作者：
Bell, David C.;Russo, Christopher J.;Kolmykov, Dmitry V.
通讯作者：
Kolmykov, Dmitry V.
用于计算薄膜中入射电子束展宽的通用方程。
影响因子：
2.2
作者：
R. Gauvin;S. Rudinsky
通讯作者：
S. Rudinsky
分析电子显微镜中探针尺寸和空间分辨率的一致定义
影响因子：
2
作者：
J. Michael;D. Williams
通讯作者：
D. Williams
低能扫描透射电子显微镜中非晶碳薄膜的电子束展宽
影响因子：
2.2
作者：
H. Drees;E. Müller;M. Dries;D. Gerthsen
通讯作者：
D. Gerthsen
扫描电子显微镜中扫描透射电子显微镜（STEM）成像的进展
影响因子：
2.8
作者：
C. Sun;E. Müller;M. Meffert;D. Gerthsen
通讯作者：
D. Gerthsen