Groves T
Ultramicroscopy. 1975 Jul;1(1):15-31. doi: 10.1016/s0304-3991(75)80005-2.
A theory of resolution and image formation is presented for thick amorphous specimens in transmission electron microscopes. Eight modes of operation are considered, four in the scanning transmission electron microscope (STEM) and four in the conventional electron microscope (CEM). A thick specimen is defined here as one in which the resolution of detail is limited by plural scattering of the electron beam. In practice this includes films on the order of a micron in thickness. An analytic theory of plural incoherent scattering is developed which is general with respect to material and beam voltage. The theory gives the distribution of elastically scattered electrons as a function of transverse coordinate and angles, and is directly applicable to optical systems. The theory applies to all thicknesses normally encountered, and includes thin specimens as well as thick specimens. Criteria are proposed for evaluation of the quality of microscope images, and the modulation transfer function is applied to determine some practical estimates of picture quality. The STEM is found to have distinct advantages over the CEM for thick specimens. For a carbon specimen one micron thick a STEM operating in bright field at 90 keV produces an image which is roughly equivalent to that of a CEM operating in bright field at 1 MeV. Improvement can be obtained in the CEM by filtering out eneryg-loss electrons which degrade resolution due to chromatic aberration. This results in a reduction in signal intensity and usable thickness, however.
本文提出了一种用于透射电子显微镜中厚非晶试样的分辨率和图像形成理论。考虑了八种操作模式，其中四种在扫描透射电子显微镜（STEM）中，四种在传统电子显微镜（CEM）中。这里将厚试样定义为其中细节分辨率受电子束多次散射限制的试样。实际上，这包括厚度约为一微米的薄膜。发展了一种关于多次非相干散射的解析理论，该理论在材料和束电压方面具有通用性。该理论给出了弹性散射电子作为横向坐标和角度的函数的分布，并且直接适用于光学系统。该理论适用于通常遇到的所有厚度，包括薄试样和厚试样。提出了评估显微镜图像质量的标准，并应用调制传递函数来确定图像质量的一些实际估计值。发现对于厚试样，STEM相对于CEM具有明显优势。对于一微米厚的碳试样，在90keV下以明场操作的STEM产生的图像大致相当于在1MeV下以明场操作的CEM产生的图像。然而，通过滤除由于色差而降低分辨率的能量损失电子，可以在CEM中获得改进。不过，这会导致信号强度和可用厚度降低。
