An experimental evaluation of bright and dark field imaging modes in high voltage electron microscopy

Abstract

An experimental evaluation in terms of resolution and contrast has been made of a number of useful imaging modes in high voltage electron microscopy. The results indicate that bright field imaging with a high order reflection in a systematic row is the most useful general purpose mode and so far appears to be the only technique which enables adequate intensity and contrast to be maintained in thicker regions of crystalline material. Although dark field imaging is only really useful in studying thin areas of a crystal it can, in certain cases, provide information which cannot be obtained by bright field, especially if defects or precipitates <∼10nm in size are of interest. In particular, first-order dark field imaging under weak beam conditions can give images with the highest resolution; second-order dark field imaging with s 2g≠ 0 can give images of high contrast and second-order dark field imaging at the critical voltage can give images with enhanced contrast and resolution of precipitates normally visible in bright field by structure factor contrast.