Developemtn of High Resolution Scanning Transmission Electron Microscope through Spherical Aberration Correction by Means of a Foil Lens

将箔透镜应用于扫描透射电子显微镜（STEm），通过对探针形成透镜的球差进行校正，获得更高的分辨力。取得了新的进展。改进了STEM的疏散系统，减少了箔片对箔片的污染率，这决定了箔片的使用寿命。由此产生的干净的真空使箔透镜在没有明显污染的情况下使用约50小时。对箔透镜的特性进行了数值研究。当箔透镜位于成探针透镜主平面附近时，五阶球差可以忽略不计，而这是限制探针直径的一个因素。在此基础上设计了单箔透镜。为了去除部分散射在箔片上的电子，从而增加STEM图像的背景噪声，在试样平面的共轭处设置了不同直径的孔径膜片。当用小孔径光圈将视野限制在37 μ m时，40% od散射电子被去除。为了量化球面像差校正对图像的改善，构建了图像SN比测量系统。系统利用最大似然食物法对同一场的两幅数字化STEM图像计算SN比。用箔透镜拍摄细颗粒图像。测量到的SN比随着箔透镜电压的增大而增大，说明降低了球差的作用。研究了非旋转对称像差的分辨精度。结果表明，将箔透镜的加工精度提高10倍，可实现与传统透射电子显微镜分辨率相当的STEM。

A foil lens was applied to a scanning transmission electron microscope(STEm) to attain higher resolving power through correction of spherical aberration of the probe-forming lens. Following new developments were achieved.1. Evacuating system of the STEM was improved to reduce the contamination rate onto the foil fo the foil lens, which determines the life time of the foil lens. The resultant clean vacuum allowed the use of the foil lens without obvious contrmination for about fifty hours.2. Characteristics of the foil lens were investigated numerically. The fifth order spherical aberration, which had been a limiting factor of the probe diameter obtained, was found bto be negligibly small when the foil lens was located near the principal plane of the probe-forming lens. A one foil lens was designed based on this result.3. In order remove a part of electrons scatterd in the foil, which result in increase of the back ground noise in STEM images, aperture diaphragms of various diameters were located at the conjugate of the specimen plane. When visual field was limited to 37 um by an small sperture diaphragm, 40% od scatters electrons were removed.4. For quantification if improvement of the image due to spherical berration correction,a system for the measutemetn of SN ratio of the image was constructed. The system computes the SN ratio from two digiralized STEM images of the same field by means of maximum likelifood method.5. Images of fine particles were taken with the foil lens. The measured SN ratio increased with the foil lens voltage, indeicating the effect of reduced spherical aberration.6. The errect on resolution of non-rotationally symmetrical aberrations was investigated. The result showed that ten times higher machining accuracy of the foil lens would achieve the STEM with the resolution comparable to conventionl transmission electron micrscopes.