INTRODUCTION TO TEM AND ELECTRON DIFFRACTION IN THE TEM

摘要

As we heard in previous talks, both SEM and EPMA examine top-surface-near regions of bulk samples. In contrast to that, thin samples are examined in transmission electron microscopes (TEM). Further differences exist in both the principle of image formation and the applied accelerating voltage. As a consequence, the type of the obtained information is also different, as summarized in Fig. 1. The thin sample is illuminated by a plane-wave (wide parallel beam of electrons) in a TEM. The sample must be so thin that a significant fraction of the illuminating electrons is transmitted through the sample without a significant loss of energy. The objective lens below the sample forms an image with these transmitted electrons in analogy to the image formation of a concave lens in optics. The analytical electron microscope (AEM) / scanning transmission electron microscope (STEM) is a kind of combination of the two; thin samples are illuminated with high energy electrons that are focused and scanned for imaging and the focussed static beam is used for local analysis with EELS or EDS.