Understanding of scanning-system distortions of atomic-scale scanning transmission electron microscopy images for accurate lattice parameter measurements

Atomic-scale scanning transmission electron microscopy (STEM) imaging has opened up the possibility of studying the local lattice parameters of crystalline materials. To ensure more accurate measurements, low-frequency distortions of the images should be properly calibrated, which requires a better understanding of their causes. Although the major possible causes are sample drift and the scanning systems of microscopes, their effects are intricate because the rates of sample drifts differ in respective measurements. In the present study, low-frequency distortions of STEM images and their dependence on scan rotations were evaluated by measuring the lattice parameters of a reference specimen, strontium titanate. The distortions due to sample drifts and the scanning system of a microscope were separately calculated and corrected using affine transformations. In the as-observed images, the length scales in the x and y directions were underestimated by 0.4–1.2% and 2.7–3.6%, respectively, with shear distortions of 0.6°–1.2°, and the magnitudes of the underestimation and shear distortions were dependent on the scan rotations. On the basis of these findings, a methodology was proposed for the correction of distortions for accurate measurement of the lattice parameters of materials.