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Standards Development
This document defines the procedure for determining the point resolution, called Scherzer resolution, of high-resolution transmission electron microscopes (HREM), which have the ability to visualize the sample structure with sub-nanometer fineness. The measurement of real spherical aberration coefficient of the objective lens is also included in the measurement procedure. This document does not treat the information limits, lattice resolution, and STEM resolution. In addition, Cs-corrected TEM is not included the target instrument.
The definition of point resolution in high-resolution electron microscopy utilizes the resolution theory proposed by Scherzer. That is, point resolution is given by the structure size corresponding to the spatial frequency at which the phase contrast transfer function under certain specific focus conditions indicates first zero contrast. The specific focus is called Scherzer focus and can be determined theoretically by the spherical aberration coefficient of the objective lens and the electron wavelength. However, it is known that the coefficient value included in the theoretical equation representing the Scherzer focus is not a fixed value, but have different values within a certain range depending on the purpose and target of observation. Since this makes to lead some difference values in the theoretical point resolution values, it is not desirable for evaluating the performance of the instrument. To solve this problem, this new proposal item adopted the optimal coefficient for the purpose of evaluating the performance of the HREM. In addition, to obtain the correct Scherzer focus, it is necessary to use the real spherical aberration coefficient, so this proposal, which shows an experimental procedure for measuring the point resolution correctly, includes a procedure for determining the real spherical aberration coefficient. Furthermore, this new proposal was also planned on the basis of the results of a questionnaire survey on the requirements of TEM-related standards conducted in 2005.
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