Fundamental study on spherical aberration correction of the objective lens of an electron microscope using a foil lens

1.透射电子显微镜（TEM）物镜透镜用箔透镜的研制：为了设计具有最佳球差校正特性的箔透镜，计算了不同几何形状的箔透镜的球差系数。结果表明，将箔透镜置于靠近下极镜的位置，并使电极孔径小至0.3mm，即可实现校正。在此基础上研制了侧入式薄膜透镜。为了避免强透射电子束聚焦在箔上而在箔上产生污染物，采用了一种加热箔的装置。校正后的物镜透镜的球面像差的测量：由MgO颗粒的暗场TEM图像与相应的明场图像的偏差测量球面像差。球面像差随着箔透镜电压增加到600 V而减小，并且在600 V时变为负。 ...更多信息 然而，在最佳箔透镜电压附近引起的小球面像差不能被测量，因为MgO的暗场图像由于辐射损伤而在高放大率下快速地淡出。因此，在足够高的放大倍数下在最佳电压附近观察到对电子照射不敏感的金微粒，这也使我们能够减少由散焦引起的测量球面像差的误差，因为从碳支撑膜的相位对比度检测到散焦效应。在箔透镜电压为615 V时，暗场像的偏差消失，证明了球差的校正.光学傅里叶变换球差系数测量系统的构建。为了研究薄膜透镜在单衍射角和宽孔径角范围内的衍射特性，建立了由衍射仪和计算机组成的光学傅里叶变换系统。球面像差系数准确地测量从空间频率提供的最大值和最小值的功率谱的TEM图像的薄碳膜。少

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