Theory

Electron Beam/Specimen Interactions

Solid specimens subjected to electron beam excitation in an electron microscope exhibit complex interactions with primary beam electrons. These interactions result in a variety of signals that may be detected in the microscope. To analyze a specimen visually, one might choose to view an image of the specimen by collecting and displaying SE’s, BSE’s or transmitted electrons. To determine information about composition, one might choose to record x-ray or auger electrons.

1. Secondary Electrons

Secondary electrons are specimen electrons ejected by interactions with the beam electrons. They are used for imaging because of their high spatial resolution and topographical sensitivity. They carry very little information about elemental composition. They typically have energies less than 50eV.

2. Backscattered Electrons

Backscattered electrons (BSE’s) are beam electrons that interact with the nucleus of a sample atom and are elastically scattered with little loss of energy. BSE’s come from greater depths within the specimen so they have lower spatial resolution. They provide elemental information because the number of BSE’s produced is directly proportional to the atomic number of the elements within the specimen.

3. X-ray Continuum (Bremsstrahlung)

The beam electron may be scattered inelastically by the coulomb field of an atomic nucleus, thus giving up some of its energy. This energy is emitted in the form of x-ray radiation called bremsstrahlung (German for “braking radiation”). The beam electrons can give up any amount of its energy, so the energy distribution of the emitted x-rays is continuous up to the beam energy. This component of the x-ray signal is called continuum.

4. Characteristic X-rays

Characteristic x-rays are produced by the primary electron beam displacement of an electron in the valence shell of the specimen. As the electron is displaced, an electron from a higher valence shell must fill its orbit. The result is a small amount of energy loss in the form of an x-ray photon. The amount of energy lost is unique to the atom from which it is emitted. X-rays are produced deep within the specimen so they have very poor spatial resolution.

5. Auger Electrons

Sometimes a characteristic x-ray is produced and then reabsorbed within the same atom, ejecting a lower energy electron. This is an Auger electron. The Auger electron possesses an energy exactly equal to the difference between the energy of the original characteristic x-ray and the binding energy of the ejected electron. They carry specific chemical information about the atom from which they originated. They have very low energy (a few eV’s) and therefore carry information about the surface of the specimen (the first few atomic layers).

Nomenclature

The lines are usually named according to the shell in which the initial vacancy occurs and the shell from which an electron drops to fill that vacancy.

Example: If the initial vacancy occurs in the K shell and the vacancy-filling electron drops from the adjacent shell (the L shell), a Ka x-ray is emitted. We will be most concerned with K-, L- and M-series x-rays, so they are referred to as KLM lines.