2.3.2. Secondary Electrons
When a sample is bombarded with electrons, the strongest region of the electron energy spectrum is due to secondary electrons. The secondary electron yield depends on many factors, and is generally higher for high atomic number targets, and at higher angles of incidence. Secondary electrons are produced when an incident electron excites an electron in the sample and loses some of its energy in the process. The excited electron moves towards the surface of the sample undergoing elastic and inelastic collisions until it reaches the surface, where it can escape if it still has sufficient energy. Their energies are a function of Eo and the surface work function, Ew, which defines the amount of energy needed to remove electrons from the surface of a material. One of the major reasons for coating a non-conductive specimen with a conductive materials is to increase the number of secondary electrons that will be emitted from the sample (decrease Ew).
Secondary electrons, by convention, are those emitted with energies less than 50 eV. This is only a small fraction of the electrons emitted from the sample (Figure 2.3.2a.).
|
|
|
![[Distribution of Secondary Electrons]](img/SEDistrib.gif)
The mean free path length of secondary electrons in many materials is approximately 10 Å. Thus, although electrons are generated throughout the region excited by the incident beam, only those electrons that originate less than 10 Å deep in the sample escape to be detected as secondary. This volume of production is very small compared with BSE and X-rays. Therefore, the resolution using SE is better than either of these and is effectively the same as the electron beam size. The shallow depth of production of detected secondary electrons makes them very sensitive to topography and they are used for scanning electron microscopy (SEM).
Back: 2.3.1. Backscattered Electrons | Next: 2.3.3. Auger Electrons | Home: Course Overview
Copyright 1997-2003, James H. Wittke
Last update: 01/18/2006 01:47 PM.