Introduction

Abstract

Unlike transmission electron microscopy (TEM) described in the counterpart to this book [1.1], scanning electron microscopy (SEM) can image and analyse bulk specimens [1.2–28]. The principle of SEM is shown in Fig. 1.1. Electrons from a thermionic, Schottky or field-emission cathode are accelerated through a voltage difference between cathode and anode that may be as low as 0.1 keV or as high as 50 keV. The range between 0.1 and 5 keV is called low-voltage SEM (LVSEM). The smallest beam cross-section at the gun — the crossover — with a diameter of the order of 10–50 μm for thermionic emission or the virtual source, with a diameter of 10–100 nm for Schottky or field-envision guns, is demagnified by a two-or three-stage electron lens system, so that an electron probe of diameter 1–10 nm carrying an electron-probe current of 10⁻⁹−10⁻¹² A is formed at the specimen surface. For modes of operation that need a higher electron-probe current of ≃ 10⁻⁸ A, the probe diameter increases to ≃ 0.1 μm.