DETERMINATION OF THICKNESS AND DEFOCUS BY QUANTITATIVE COMPARISON OF EXPERIMENTAL AND SIMULATED HIGH-RESOLUTION IMAGES

In this work, we present a refinement of the thickness, defocus, and b eam tilt deduced from a defocus series of high-resolution electron mic roscope images using a computational method that is commonly employed in structure refinement by X-ray diffraction and in the analysis of ga mma-ray and X-ray spectra, namely unconstrained, non-linear, least-squ ares optimization. Defocus results obtained from the refinement are co mpared with analysis of the image contrast from amorphous material pre sent in each image. Results indicate that refined values for defocus a re consistent with those values determined using more conventional met hods. The refinement method is more sensitive to defocus than the conv entional method. In addition, the refinement method is sensitive to sp ecimen thickness and beam tilt. Reasonable fits including thickness, d efocus, and beam tilt are obtained for defoci near zero defocus, where as the quality of the fit decreases with increasing defocus.