A Method to Measure Source Size in Aberration Corrected Electron Microscopes

Abstract

The ability to correct spherical aberrations in electron optics [1,2], along with improvements in instrumental stability [3], have enabled sub-Ångström imaging to be achieved in both TEM and STEM. This has introduced the possibility of obtaining new and local information about a specimen via an exciting range of new experiments. However, extracting the maximum amount of information from such experiments requires experimental data to be interpreted quantitatively by means of comparisons with simulations. This, in turn, requires that the microscope’s performance is well-characterized. In particular, experimental data is strongly influenced by the degree of coherence of the electron beam. Currently, however, there is no established method for measuring this. In the present work, we will describe a method for measuring the spatial coherence of the electron beam arising from finite source size and instrumental instabilities. This method involves the analysis of a Ronchigram obtained from a crystalline material.