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

1.扫描电子显微镜(STEM)用箔片透镜的研制：为了设计出性能最佳的箔片透镜，计算了其三阶和五阶球差系数。结果表明，在实际的箔透镜电压下，通过使电极孔径减小到0.4 mm即可实现校正。在此基础上研制了侧入口式箔片透镜。信号电子探测系统的构建与优化：采用明场/暗场探测器对信号电子探测系统进行了优化。当探测到离开样品的电子角度达到26mrad时，被夹角大于6.3mrad的箔透镜所作用的电子被探测孔径光圈和高信噪比的图像屏蔽。箔片透镜矫正特性的测量与评价：新研制的箔片透镜在S…进行手术用阴影成像法测量球差系数和透射电子显微镜。球差系数随着箔片透镜电压的增加而减小，在350V左右变为负值，当考虑五级球差时，测量的球差系数与计算值吻合较好，证明了球差的修正。使用箔片透镜的扫描电子显微镜图像观察：在200kV的加速电压下，使用箔片透镜和不使用箔片透镜观察到细小的金粒子，并对图像进行比较。使用箔片透镜可以分辨距离较小的细小颗粒，因此，使用箔片透镜第一次显示出分辨率的提高。根据计算的电子探头的电流密度分布进行评估：为了评估校正实验的结果，采用波光学方法计算了探头的轮廓。350V的箔片透镜的轮廓几乎与理想的轮廓相同，没有任何像差。最优电压与实验电压吻合较好，具有最高的分辨率，得到了近乎完美的校正。较少

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