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

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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.