Determination of Partially Coherent Parameters by Lattice Image Contrast in TEM

Abstract

A new method is proposed to measure a beam divergence angle and chromatic defocus spread in a transmission electron microscope. In the proposed method, these two parameters are determined by the defocus dependence of lattice image contrast, instead of contrast transfer functions obtained from high resolution images of weak phase objects such as amorphous carbon thin films. As a result of comparison of the lattice image contrast with an image formation theory under partially coherent illumination, the beam divergence angle and the chromatic defocus spread were estimated to be 0.4 ± 0.1 mrad and 12 ± 2 nm, respectively. Since strongly diffracted beams from a crystal are used in the proposed method, stronger contrast than that of an amorphous carbon image can be obtained, which is advantageous for determining the parameters precisely