64 - General Introduction

Abstract

This chapter gives an introduction to the process of image formation in various high-resolution transmission electron microscopes. It defines working of latest generation microscopes like: CTEM (conventional transmission electron microscope), STEM (scanning transmission electron microscope), TEM (transmission electron microscope), and SEMs (Scanning electron Microscopes). Earlier, transmission electron microscopes had electron guns, condenser lenses, and objective lenses to provide the magnification while the latest technology like the STEM is a microscope in which a small probe explores the specimen and information collected by the beam electrons as they traverse each small picture-element (pixel) of the specimen is extracted by various detectors downstream from the latter. It produces the information that is used to generate its numerous images sequentially. The modern day microscopes apply, the general rule that in elastically scattered electrons are deflected, on average, through smaller angles than elastically scattered electrons, forming a crude but very effective separation by the annular dark-field detector in the imaging mode. In the standard imaging mode, the source is de-magnified by the condenser and objective lenses, so that crossover, a selected-area aperture and specimen are conjugates.