2.1. Effects of Electron Bombardment

Electron bombardment of a sample is unique to microprobe analysis and produces a large number of effects from the target material (Figure 2.1a.). The incident electrons interact with specimen atoms and are significantly scattered by them (rather than penetrating the sample in a linear fashion). Most of the energy of an electron beam will eventually end up heating the sample (phonon excitation of the atomic lattice); however, before the electrons come to rest, they undergo two types of scattering: elastic and inelastic.

In elastic scattering, the electron trajectory changes, but its kinetic energy and velocity remain essentially constant (due to large differences between the mass of the electron and nucleus). This process is known as electron backscattering (although later we will confine the term "backscattered electrons" to those scatter out of the sample).

In inelastic scattering, the trajectory of the incident electron is only slightly perturbed, but energy is lost through interactions with the orbital electrons of the atoms in the specimen. Inelastic interactions produce diverse effect including:

• phonon excitation (heating)

• cathodoluminescence (visible light fluorescence)

• continuum radiation (bremsstrahlung or “braking” radiation)

• characteristic x-ray radiation

• plasmon production (secondary electrons)

• Auger electron production (ejection of outer shell electrons)

 

Figure 2.1a. Effects produced by electron bombardment of a material.

Two major factors control which effects can be detected from the interaction volume. First, some effects are not produced from certain parts of the interaction volume (Figure 2.1b). Beam electrons lose energy as they traverse the sample due to interactions with it and if too much energy is required to produce an effect, it will not be possible to produce it from deeper portions of the volume. Second, the degree to which an effect, once produced, can be observed is controlled by how strongly it is diminished by absorption and scattering in the sample.

For example, although secondary and Auger electrons are produced throughout the interaction volume, they have very low energies and can only escape from a thin layer near the sample's surface. Similarly, soft X-rays, which are absorbed more easily than hard X-rays, will escape more readily from the upper portions of the interaction volume. Absorption is an important phenomenon and is discussed in more detail below.

Figure 2.1b. Generalized illustration of interaction volumes for various electron-specimen interactions. Auger electrons (not shown) emerge from a very thin region of the sample surface (maximum depth about 50 Å) than do secondary electrons (50-500 Å).

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