论文部分内容阅读
从Scherzer1936年提出了关于旋转对称电磁透镜的球差不可能靠其本身场分布的设计而消除的理论以来,电子透镜的球差一直是限制电子显微镜分辨率的根本因素。虽然也有过一些采用非对称电子光学系统或其他方法矫正球差的工作,其中包括Scherzer本人建议的八极球差校正系统,但一直未能在高分辨率电镜上获得应用。近几年来,Crewe及其他一些人又提出了用六极透镜矫正球差的想法。在文献中Crewe证明了单一的六极透镜或两个六极透镜串连使用均不能形成矫正球差的条件。然而如果在两个六极透镜之间,装一弱的旋转对称透镜(图1)则可达到对矫正三级球差的条件。但三级以上像差的影响如何尚不清楚。本文详细推导了图1所示系统的三级像差及为矫正物镜固有三级像差的条件,并在Commodore—64微机上设计了计算该系统三极像差的程序“Sextupole—1”及计算包括三级像差在内直到五级以上像差的程序“Sextupole—2”。“Sextupole”—2程序还可用于同时计算矫正系统的各参数对实际矫正效果的影响。
Since Scherzer’s proposal of a patterned detector for global projection in 1936, the spherical aberration of the electron lens has been a fundamental factor limiting the resolution of the electron microscope. Although there have been some efforts to correct spherical aberration with asymmetric electro-optical systems or other methods, including an octupole spherical aberration correction system proposed by Scherzer himself, it has not been applied to high-resolution electron microscopy. In recent years, Crewe and others have proposed the idea of using a hexapole to correct spherical aberration. In the literature Crewe proved that a single hexapole or two hexapods can not be used to form spherical aberration. However, if a weak rotationally symmetric lens (Figure 1) is installed between two hexapoles, the condition for correcting the tertiary aberration can be achieved. However, the effect of aberrations above the third level is not yet clear. In this paper, the third-order aberrations of the system shown in Fig. 1 are deduced in detail and the conditions for correcting the inherent tertiary aberrations of the objective lens are derived. The program “Sextupole-1” for calculating the triplet aberration of the system and the “ The program ”Sextupole-2“ that includes aberrations up to the fifth level including the third-order aberrations is calculated. The ”Sextupole" -2 program can also be used to simultaneously calculate the effect of various parameters of the correction system on the actual correction.