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Materials Science on CD-ROM User Guide

Using the Scanning Electron Microscope

Version 2.1

Peter Goodhew, University of Liverpool
Ian Jones, MATTER

October 1997

Assumed Pre-knowledge

This module is one of several devoted to aspects of microscopy. It would be helpful to be familiar with the material covered in the companion modules "Introduction to Electron Microscopes", "Beam-Specimen Interactions" and "Image Fundamentals".

Module Structure

The module is designed to be used by students with a background in any area of science. In order to make the illustrations relevant to the principle interests of the user, the module opens by offering the choice of images from materials sciences or life sciences. Selection at this stage ensures that each future section is illustrated with an appropriate set of images. However, this selection can be changed at any stage by using the "Click here to change the image type" selector which is at the bottom left of every active screen (see screen shots below).

Introduction

The first few pages allow the user to check that they are familiar with the arrangement of the key components of an SEM. They include a drag-and-drop exercise (see screen shot below) which tests whether seven critical components can be recognised and located in a typical SEM

The Specimen

This section deals with the specimen and its location in the chamber of the microscope. A simple simulation of the chamber itself is designed to indicate that specimen movement is limited by the presence of the objective lens and the electron detectors. The problem of charging is dealt with both in terms of its origin (secondary electron yields) and the ways in which it can be overcome (coating or low voltage operation).

Image Modes

The section opens with a review of the most useful secondary effects (secondary electrons, backscattered electrons and X-rays). Students might find it useful to review the module "Beam-Specimen Interactions" if these points are still unfamiliar.

The main emphasis of this section is on the emission and detection of secondary and backscattered electrons, and the appearance of images formed in these modes. The effect of coating is mentioned and the differences between carbon- and gold-coated specimens are explored. Finally, the beam scanning modes appropriate to imaging and channelling are simulated so that the distinction between scanning and rocking the beam can be appreciated.

Improving the Image

The key concepts in this section are the effect of noise and objective lens settings on the image. The section contains images which can be examined in and out of focus, and with different signal-to-noise ratios and hence different noise levels. The statistical nature of noise is explored via a simulation involving the counting of electrons per pixel. The terms under and over-focus are defined and the important concept of depth of field is introduced through a simulation involving a variable-diameter objective aperture and variable working distance.

Improving the Resolution

The emphasis in this section is on beam current and electron probe diameter. The real meaning of the term resolution is examined through a set of questions. The resolution is in practice limited by both beam diameter and beam spreading in the specimen. The broadening of the ideal beam diameter because of lens aberrations and diffraction at apertures is plotted interactively.

Bibliography

The student is referred to the following resources in this module: