Fundamental study on spherical aberration correction of the objective lens of an electron microscope 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 with the optimum characteristics in spherical aberration correction, spherical aberration coefficients were calculated for variable geometries of the foil lens. It was found that the correction is achieved by locating the foil lens close to the lower polepiece and making the diameter of the electrode aperture as small as 0.3mm. A side-entry type foil lens was developed based on the calculation. A device for heating the foil was incorporated to avoid contamination build-up on the foil due to intense transmitted electron beam focused on it.2. Measurements of spherical aberration of the corrected objective lens : The spherical aberration was measured from the deviation of dark-field TEM images of MgO particles from the corresponding bright-field image. The spherical aberration decreased as the foil lens voltage was increased up to 600 V and became negative at … More 800 V.A small spherical aberration caused around the optimum foil lens voltage, however, could not be measured because the dark-field image of MgO quickly fades out at high magnifications due to radiation damage. Fine gold particles which is not sensitive to electron irradiation, therefore, were observed around the optimum voltage at sufficiently high magnifications, which also enabled us to reduce the error in measured spherical aberrations caused by defocus because the defocus effect was detected from the phase contrast of the carbon supporting film. At the foil lens voltage of 615 V the deviation of the dark-field image disappeared, which proved the correction of spherical aberration.3. Construction of an optical Fourier transform system for measurements of spherical aberration coefficients. : In order to investigate the performance of the foil lens not only at single diffraction angle but over a wide range of the aperture angle, an optical Fourier transform system, which consists of a diffractmeter and a computer for data treatments, was constructed. The spherical aberration coefficients were accurately measured from spatial frequencies providing maxima and minima in the power spectrum of TEM images of thin carbon films. Less