This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.
In the arrangement of the basic imaging systems of conventional transmission electron microscopy (CTEM), an electron beam emitted by a source radiates the TEM specimen, usually through an illuminating system of lenses as shown in blue in Figure 3766. The incident electrons interact with the specimen and thus are scattered. The electron beam after interaction is then focused by the objective lens, forming an image. The image is further magnified by projector lenses to produce an image at proper magnifications. Note that in modern TEM systems additional lenses, intermediate lenses are also placed between objective lens and projector lenses.
For scanning transmission electron microscopy (STEM), the same diagram can be used with a reversed direction of the electron beam also shown in Figure 3766. The electron source for STEM replaces the TEM detection system (image plane). In modern STEM systems, a field-emission gun can be used to produce an electron source as small as 10 nm in diameter. One or more projector lenses in TEM (acting as condenser lenses in STEM) with a short focal length demagnify the source to provide the flexibility in the beam parameter. The beam is then demagnified further by the main lens in the systems, which is objective lens to form a small probe of 0.2 – 1 nm on the specimen. Therefore, a STEM probe can be considered as a demagnified image of the source. Note that the focal length of the objective lens can be as small as 1 mm. The scanning coils are deﬂector coils, built into the bore of the objective lens, and serve to scan the electron probe over the specimen. The detector on the STEM image plane collects the electrons transmitted, or scattered, by the specimen.
There are two important factors determining the essential geometry of the imaging system: i) The wavelength of the electrons determines the angular range of the scattering from the specimen; ii) The aberrations of the electron lenses limit the angular range of scattered radiation.
Figure 3766. Schematic illustration of imaging geometries of TEM and STEM systems. The green lines show the common paths of the rays of CTEM and STEM illuminations. The blue arrows indicate the rays for the TEM illumination, while the red arrows indicate the directions of STEM rays. The STEM image is formed on the left-hand side, while the CTEM image is on the right-hand side. The yellow lenses are the common lenses for both STEM and CTEM systems.