Improvement of Resolution of a Electron Microscope by Means of Correction of Spherical Aberration Using a Foil Lens

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1. Development of a foil lens for the objective lens of a transmission electron microscope (TEM).In order to design the foil lens which enables us to correct the spherical aberration at a lower electrostatic field than 120 kV/mm, that is the known limit of self-supporting of the foil electrode, optical characteristics of the foil lens were numerically studied. It was found that that the correction is achieved by locating the foil lens at the position 3.6 mm lower from the center of the gap of the objective polepieces. The spherical aberration is corrected at the foil lens voltage of 550 V when the distance of the aperture and foil electrodes. An improved side-entry type foil lens was developed based on the calculation. The foil lens holder has the thickness of 1.7 mm, 0.3 mm thinner than previous one.2. Observation of crystal lattice images with the improved foil lens.The foil lens was incorporated with a commercial TEM and the images of Au fine particles were observed at the accelerating voltage of 200 kV. At the foil lens voltage of 300 V, (111) lattice images were observed at nearly in-focus condition where the contrast of the Fresnel fringe was weak. Such reduction of aretefacts shows an effect of correction of spherical aberration.3. Measurements of spherical aberration of the corrected objective lens.The spherical aberration was measured with a newly developed method, in which the spherical aberration coefficient and the defocus are simultaneously determined from the deviation of the dark-field TEM images of Au particles from the corresponding bright-field image. It was shown that the improved foil lend yields smaller spherical aberration coefficients than the previous one at the same foil lens voltage.