Determination of thickness and defocus by quantitative comparison of experimental and simulated high-resolution images

Abstract

In this work, we present a refinement of the thickness, defocus, and beam tilt deduced from a defocus series of high-resolution electron microscope images using a computational method that is commonly employed in structure refinement by X-ray diffraction and in the analysis of γ-ray and X-ray spectra, namely unconstrained, non-linear, least-squares optimization. Defocus results obtained from the refinement are compared with analysis of the image contrast from amorphous material present in each image. Results indicate that refined values for defocus are consistent with those values determined using more conventional methods. The refinement method is more sensitive to defocus than the conventional method. In addition, the refinement method is sensitive to specimen thickness and beam tilt. Reasonable fits including thickness, defocus, and beam tilt are obtained for defoci near zero defocus, whereas the quality of the fit decreases with increasing defocus.