10 Introduction: The optical nature of a charged particle beam
Diffraction is familiar in light optics. For example, it imposes a
fundamental limit on the resolution of a microscope. Because of
diffraction, it is not possible in a conventional microscope to resolve any object which is appreciably smaller than the wavelength.
This turns out to be true for both a light microscope and an electron microscope. It is another example of the close analogy that
exists between charged particle optics and light optics. Since the
wavelength of a fast charged particle is much smaller than that of
visible light, it is expected that the resolving power of a charged
particle microscope should be much better than a light microscope.
This is indeed verified in practice. A modern electron microscope
can resolve a single atom, a feat which is in no way possible with
visible light.
Charged particles interact strongly with matter. This forms the
basis of many useful instruments. For example, a fast electron can
be scattered by an atomic nucleus of the target material, with
little energy transferred to the material. This is known as elastic
scattering, and forms the basis of contrast in a transmission electron microscope. Alternatively, the incident particle can transfer
energy to the sample material, giving rise to secondary processes.
For example, a secondary electron or ion can be ejected. By measuring the charge and mass of the ejected particle, useful chemical
and physical information about the sample is obtained.
Three generic types of electron microscopes exist. These are shown
schematically in Figure 1.5. A conventional transmission electron
microscope (TEM) is shown in (a). A transparent specimen S is
illuminated from above, where the illumination is omitted for simplicity. Some electrons are elastically scattered at the object point,
and some remain unscattered. The unscattered current passes
through an aperture A, and is imaged by a lens L onto the recording plane P, where P typically consists of an array of charged
coupled devices. Some fraction of the scattered current is stopped
by the aperture A. Areas of the specimen which scatter strongly
thus appear dark in the image, and areas which scatter weakly
appear bright. The object point is depicted as being off the cen
Diffraction is familiar in light optics. For example, it imposes a
fundamental limit on the resolution of a microscope. Because of
diffraction, it is not possible in a conventional microscope to resolve any object which is appreciably smaller than the wavelength.
This turns out to be true for both a light microscope and an electron microscope. It is another example of the close analogy that
exists between charged particle optics and light optics. Since the
wavelength of a fast charged particle is much smaller than that of
visible light, it is expected that the resolving power of a charged
particle microscope should be much better than a light microscope.
This is indeed verified in practice. A modern electron microscope
can resolve a single atom, a feat which is in no way possible with
visible light.
Charged particles interact strongly with matter. This forms the
basis of many useful instruments. For example, a fast electron can
be scattered by an atomic nucleus of the target material, with
little energy transferred to the material. This is known as elastic
scattering, and forms the basis of contrast in a transmission electron microscope. Alternatively, the incident particle can transfer
energy to the sample material, giving rise to secondary processes.
For example, a secondary electron or ion can be ejected. By measuring the charge and mass of the ejected particle, useful chemical
and physical information about the sample is obtained.
Three generic types of electron microscopes exist. These are shown
schematically in Figure 1.5. A conventional transmission electron
microscope (TEM) is shown in (a). A transparent specimen S is
illuminated from above, where the illumination is omitted for simplicity. Some electrons are elastically scattered at the object point,
and some remain unscattered. The unscattered current passes
through an aperture A, and is imaged by a lens L onto the recording plane P, where P typically consists of an array of charged
coupled devices. Some fraction of the scattered current is stopped
by the aperture A. Areas of the specimen which scatter strongly
thus appear dark in the image, and areas which scatter weakly
appear bright. The object point is depicted as being off the cen