Developments of a Large-Convergence-Angle Scanning Transmission Electron Microscope Operable Over a Wide Range of Accelerating Voltages

It is widely recognized that the microstructure of a material significantly influences its properties. Scanning/transmission electron microscopy (S/TEM) is an excellent tool that enables atomic-scale charactarization of such microstructures. In recent years, as the diversity of materials has expanded significantly-from hard materials such as metal, ceramics and semi-conductor to soft materials composed of light elements-there is increasing demand for electron microscopes capable of handling this broad range of materials. Hard materials are generally more resistant to electron irradiation than soft materials and are therefore often observed at high accelerating voltages (e.g., 300 kV or 200 kV). In STEM mode at an accelerating voltage of 300 kV, the JEM-ARM300F2 (JEOL Ltd.) has achieved a spatial resolution of 40.5 pm [1]. In TEM mode, the same instrument provides a spatial resolution of 50 pm . Furthermore, atomic-resolution elemental analysis using EDS and EELS is feasible under these conditions. However, achieving such performance requires high accelerating voltages on the order of 300 kV or 200 kV. Soft materials, in contrast, are easily damaged by electron irradiation at high accelerating voltages, making it difficult to obtain reliable data. Although lowering the accelerating voltage is an effective approach to mitigate beam induced damage, the spatial resolution decreaces due to enhanced chromatic and diffraction aberrations. Moreover, lower voltages result in reduced beam brightness for a given probe size, necessitating the use of larger probe sizes to obtain sufficient signal for analytical measurements. To improve resolution at low accelerating voltages, it is essential to increase the convergence semi-angle that contributes to probe formation, which requires phase alignment over higher angles in the diffraction plane. Consequently, electron microscopes capable of correcting higher-order aberrations and supporting large convergence angles are required. Access to large convergence angles enables the use of larger condenser apertures, allowing higher beam currents and providing advantageous optical conditions for analytical applications. Meeting these low-voltage requirements using electron microscopes equipped with high maximum accelerating voltages is crucial for enabling high-resolution imaging and analysis of diverse materials across a wide range of accelerating voltages. To address these challenges, we collaborated with CEOS GmbH to integrate the CEOS LASCOR (Large Aperture STEM CORrector), a three-stage hexapole spherical aberration corrector [2,3], into the JEM-ARM300F2 (maximum accelerating voltage 300 kV). This integration enabled us to expand the aberration-free (flat) region of the Ronchigram beyond conventional limits, thereby increasing the usable angular range for probe formation to more than 100 mrad (semi-angle). Fig. 1 shows the Ronchigram obtained in STEM mode at 300 kV, where the flat region extends up to a convergence semi-angle of 100 mrad. Fig. 2(a) and 2(b) present the STEM image and corresponding FFT pattern of GaN [212], respectively, a spatial frequency corresponding to 40.5 pm is observed. Fig. 3 shows the Ronchigram obtained in STEM mode at 80 kV, where the flat region extends to a convergence semi-angle of 100 mrad. Fig. 4(a) and 4(b) show the STEM image of Ge [112] and its FFT pattern at 80 kV, with observable spots corresponding to 76 pm. These results demonstrate that the newly developed large-convergence-angle STEM Corrector is capable of correcting higher-order aberrations beyond conventional limits and achieving high spatial resolution across a wide accelerating voltage range from at least 300 kV down to 80 kV. Ronchigram image taken at 300 kV. (a) STEM image of GaN [212] and (b) FFT pattern taken at 300 kV. Ronchigram image taken at 80 kV. (a) STEM image of Ge [112] and (b) FFT pattern taken at 80 kV.