Development of a high resolution scanning transmission electron microscope with a foil lens for correction of spherical aberration

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1. Development of a foil lens for the scanning transmission electron microscope (STEM) : In order to design the foil lens with the optimum characteristics, the third and fifth order spherical aberration coefficients were calculated. It was found that the correction is achieved at a practical foil lens voltage by making the diameter of the electrode aperture as small as 0.4mm. A side-entry type foil lens was developed based on the calculation.2. Construction and optimization of the system for detection of signal electrons : A detection system of signal electrons was optimized with a bright field/dark field detector. When electrons leaving the specimen at angles up to 26 mrad were detected, electrons acattered by the foil of the foil lens with angles of larger than 6.3 mrad were screened by a detection aperture diaphragm and images of high signal-to noise ratios.3. Measurements and evaluation of correction characteristics of the foil lens : The newly developed foil lens was operated in S … More TEM and the spherical aberration coefficient was measured with shadow image method. The spherical aberration coefficient decreased as the foil lens voltage was increased and turned negative at around 350 V.The measured spherical aberration coefficients were in good agreement with the calculation when the fifth order spherical aberration was taken into account, which proved the correction of spherical aberration.4. Observation of STEM images with the foil lens : Fine gold particles were observed with and without the foil lens at the accelerating voltage of 200 kV,and the images were compared. Fine particles of a smaller distance were resolved with the foil lens and, therefore, an improvement of resolution using the foil lens was shown for the first time.5. Evaluation from calculated current density distributions of the electron probe : For evaluation of the result of the correction experiment, the probe profile was calculated wave-optically. The profile for the foil lens voltage of 350 V was nearly the same as the ideal profile without any aberrations. The optimum voltage was in good agreement with the experimental one giving the highest resolution, which suggests that almost perfect correction was obtained. Less