Introduction

Scanning electron microscopes and electron microprobes have much in common, including a source of electrons (an ‘electron gun’), and electron lenses to focus the beam on the specimen, which together form the ‘column’. Column design is similar in principle for both types of instrument: descriptions in the following sections therefore apply equally to both for the most part (differences are explained where necessary). Beam-deflection coils and electron detectors enable scanning images to be produced. X-ray spectrometers (described in the next chapter) are a common SEM accessory and an essential component of the EMP. Other types of detector are available optionally. Features also described in the following sections include the vacuum system, specimen stage and, in the case of EMPs (but not usually SEMs), optical microscope.

The electron gun

The source of electrons in EMP and SEM instruments is held at a negative potential (typically 10–30 kV), which accelerates the electrons towards the sample. The commonest type of emitter is a tungsten filament (about 0.1 mm in diameter) bent into a ‘hairpin’ shape and attached to legs mounted on an insulator (Fig. 3.1). This is heated electrically to about 2700 K, giving electrons sufficient thermal energy to overcome the potential barrier at the surface. The grid or ‘wehnelt’ (Fig. 3.2) is held at a negative potential relative to the cathode and limits the effective emitting area of the filament to the region close to the tip.