Quantitative Assessment of Lower-Voltage TEM Performance Using 3D Fourier Transform of Through-Focus Series

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The spherical aberration corrector was a quantum leap in high-resolution transmission electron microscopy (TEM) [1]. It brought about a breakthrough in high-resolution lower-voltage TEM, and a variety of novel nanostructured materials have been explored utilizing its high spatial resolution, high image contrast and low knock-on damage [2,3]. The performance of an aberration-corrected TEM is determined by the information limit that is often evaluated using Young’s fringe method, in which the diffractogram of a double-exposure image with a given image shift, is analyzed. It was already reported, however, that Young’s fringe method shows unexpected high frequency information [4] due to the nonlinear terms, which correspond to interference between diffracted waves. The presence of the non-linear terms is a major difficulty in the existing procedures for evaluating TEM performance as pointed out by a few researchers [5,6]. The three-dimensional (3D) Fourier transform (FT) of through-focus TEM images allows us to discriminate between the linear imaging terms and the non-linear imaging terms [7– 9]. The linear imaging terms are observed on twin Ewald spheres in the 3D FT using an amorphous specimen. Here, we use the 3D FT of through-focus TEM images for the assessment of two TEM systems. Two spherical-aberration-corrected microscopes were assessed and compared at a relatively lower acceleration voltage. One transmission electron microscope was a Titan 3 (FEI) equipped with a monochromator and a spherical aberration corrector for image forming (CEOS, CETCOR) operated at an acceleration voltage of 80 kV. The energy spread of the electron source was 0.1 eV under monochromated condition. The other microscope, the TripleC microscope, was equipped with a cold field-emission gun (CFEG) and the spherical aberration corrector developed for the TripleC project. This microscope was operated at 60 and 30 kV [10], and the energy spread was 0.3-0.4eV.