Scanning Electron Microscopy (SEM)

Image Formation in the SEM

At any given moment, the specimen is bombarded with electrons over a very small area. Several things may happen to these electrons. They may be elastically reflected from the specimen, with no loss of energy. They may be absorbed by the specimen and give rise to secondary electrons of very low energy, together with X- rays. They may be absorbed and give rise to the emission of visible light (an effect known as cathodoluminescence). And they may give rise to electric currents within the specimen. All these effects can be used to produce an image. By far the most common, however, is image formation by means of the low-energy secondary electrons.

Figure 1. Schematic drawing of a scanning electron microscope with secondary electrons forming the images on the TV screen.

The secondary electrons are selectively attracted to a grid held at a low (50 volt) positive potential with respect to the specimen. Behind the grid is a disc held at about 10 kilovolts positive with respect to the specimen. The disc consists of a layer of scintillant coated with a thin layer of aluminum. The secondary electrons pass through the grid and strike the disc, causing the emission of light from the scintillant. The light is led down a light pipe to a photomultiplier tube which converts the photons of light into a voltage. The strength of this voltage depends on the number of secondary electrons that are striking the disc. Thus the secondary electrons produced from a small area of the specimen give rise to a voltage signal of a particular strength. The voltage is led out of the microscope column to an electronic console, where it is processed and amplified to generate a point of brightness on a cathode ray tube (or television) screen. An image is built up simply by scanning the electron beam across the specimen in exact synchrony with the scan of the electron beam in the cathode ray tube.

The SEM does not contain objective, intermediate and projector lenses to magnify the image as in the optical microscope. Instead magnification results from the ratio of the area scanned on the specimen to the area of the television screen. Increasing the magnification in an SEM is therefore achieved quite simply by scanning the electron beam over a smaller area of the specimen.

This description of image formation in the SEM is equally applicable to elastically scattered electrons, X-rays, or photons of visible light - except that the detection systems are different in each case. Secondary electron imaging is the most common because it can be used with almost any specimen.

A modern trend in electron microscopy is to fit X-ray analysis equipment as a bolt-on accessory. Bombarding a specimen with electrons causes X-rays of characteristic wavelengths and energies to be emitted from the spot where the beam strikes the specimen. Computer analysis of the wavelength or energy spectra makes it possible to measure accurately the nature and quantity of different elements in the material. The technique is of little use to biologists because light elements such as carbon produce too weak an X-ray signal. But it is of great value in materials science, particularly because an area as small as I square micrometer can be analyzed with precision.

Figure 2 shows an SEM view of salt crystals (NaCl) and the energy spectrum of the characteristic X-rays emitted from the salt. You can see peaks from sodium (Na) and chlorine (Cl).

Figure 2.(top) Scanning electron microscope image of salt crystals. (below) Spectrum of the energy of characteristic X-rays emitted from the salt (NaCl) crystals shown above. In the energy spectrum the X-ray peaks from sodium (Na) and chlorine (Cl) are identified. (Images by John Wheatley, ASU)