Physical Principles of Electron Microscopy: An Introduction to TEM ...

134 Chapter 5
Figure 5-7. (a) Secondary-electron image of a small crystal; the side-mounted Everhart-
Thornley detector is located toward the top of the image. (b) Backscattered-electron image
recorded by the same side-mounted detector, which also shows topographical contrast and
shadowing effects. Such contrast is much weaker for a BSE detector mounted directly above
the specimen. From Reimer (1998), courtesy of Springer-Verlag.
Taking advantage of the orientation dependence of �, the whole sample is
often tilted away from a horizontal plane and toward the detector, as shown
in Fig. 5-1. This increases the overall SE signal, averaged over all regions of
the sample, while preserving the topographic contrast due to differences in
surface orientation.
To further increase the SE signal, a positively-biased electrode is used to
attract the secondary electrons away from the specimen. This electrode could
be a simple metal plate that absorbs the electrons, generating a small current
that could be amplified and used to generate a SE image. However, the
resulting SE signal would be weak and noisy. The amount of electronic noise
could be reduced by limiting the frequency response (bandwidth) of the
amplifier, but the amplified signal might not follow the fast changes in input
signal that occur when the electron probe is scanned rapidly over a nonuniform
specimen.
A stronger signal is obtained from an Everhart-Thornley detector, named
after the scientists who first applied this design to the SEM. The secondaries
are first attracted toward a wire-mesh electrode biased positively by a few
hundred volts; see Fig. 5-8. Most of the electrons pass through the grid and
are accelerated further toward a scintillator that is biased positive Vs by
several thousand volts. The scintillator can be a layer of phosphor (similar to
the coating on a TEM screen) on the end of a glass rod, or a light-emitting
plastic or a garnet (oxide) material, each made conducting by a thin metallic
surface coating. The scintillator has the property (cathodoluminescence) of