Methods for Displaying Defocus and Astigmatism of High Resolution Stem Images

Abstract

Focussing and stigmation of the objective lens for high resolution STEM imaging is usually achieved by repeated reduced area scans of a given specimen area under manual control of the instrument. This method is particularly laborious since the scan times for images can be upwards of several seconds which subjects a specimen to unwanted electron beam exposure. Methods have been proposed to optimize STEM images such as the use of shadow images1 where a fixed beam pattern for a highly defocussed objective lens is observed. This method again requires a high beam dose in a given specimen area. Alternately, if a white noise object such as a carbon film is used, granular image features without any directionality are desired. In this case the eye must integrate over a large number of small image features. A better alternative is the use computer control to digitize images in real time and compute the power spectrum of an image could be employed. This would require a dedicated high speed computer system for a 1000 × 1000 display, but this latter method would have the advantage that only a single scan of image is required.