SEM Signal types, Contrast Mechanisms, and Imaging Schemes

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Signal types


Secondary electrons (SEs)

Secondary electrons (SEs) are low energy electrons emitted from the sample due to the energetic passage of beam electrons. Typical energies of SEs are on the order of a few eV, although a somewhat arbitrary convention regards anything less that 50eV emitted from a sample as a SE. Indeed, as the energy ranges of the different emitted electron types overlap, and electron energy is the easiest way of distinguishing types, this is a fairly practical rule of thumb. SEs are emitted throughout the interaction volume, but due to their low energy they don't travel far in the sample before being re-captured. It is for this reason that SEs are thought of as a surface-sensitive signal. The emission of SEs occurs throughout the intersection of the surface with the interaction volume, but as the beam electron flux density is the highest where the beam first enters the sample, the highest concentration of SEs also comes from this region. This fraction of the SE signal is what allows most high resolution SEM imaging.

Backscattered electrons (BSEs)
Back Scattered Electrons (BSEs) are primary beam electrons that have have undergone one or more elastic (billiard ball-type) collisions within the sample and eventually have bounced around enough to re-emerge from the surface they came in. Typically BSEs will also undergo several inelastic collisions during their time inside the solid as well, and so will typically have an energy that is lower than the beam energy upon their escape. The energy spread of BSEs goes from E0, the beam energy, all the way down to zero; typically this energy distribution is peaked somewhere between 0.4E0 and 0.95E0 or so, with the peak being sharper and more pronounced at higher atomic number.

The total BSE yield (total fraction of beam electrons that backscatter) from a randomly oriented polycrystal tends to increase monotonically with the atomic number, Z. Although there are a few exceptions, Mn(25)-Fe(26), Co(27)-Ni(28), Sn(50)versus its neighbors, the general trend is for the BSE yield to increase with increasing Z, and flattening out at higher Z. A general expression approximating the BSE yield for a flat polished sample is:

BSE yield = -0.0254 + 0.016Z - 1.86*10-4 Z2 + 8.3*10-7Z3

This works best for the higher beam energies ( > about 5kV). Other approximations exist for lower beam energies.

In addition to the BSE dependence on Z, surface orientation also affects the total BSE yield, as well as the preferred direction of the BSEs. For a flat sample surface that is normal to the beam, the BSE distribution is peaked in the normal direction, such that the highest BSE yield comes straight back up. For surfaces at small angles away from the normal, this distribution still more or less holds. At larger tilt angles, the beam and the peak in the BSE distribution can be thought of as occupying mirror positions with respect to the sample surface, although this is a very crude approximation at best.

For crystalline samples, the wave nature of the incident electrons interacts with the periodic structure of the sample to subtly affect the scattering probability of the beam electrons dependent on their path relative to the crystal lattice. This effect is known as electron channeling. The most notable features of electron channeling are the typically abrupt changes in BSE yield (as much as 10% or more) at the various Bragg conditions with the crystal.

X-Rays
Two types of X-Rays are produced from a sample during bombardment by an energetic electron beam. The continuum, or Bremsstrahlung, X-rays are formed from direct energy loss of the beam electrons as they are slowed in the sample, and form a continuous energy spectrum from 0eV up to E0. The intensity of the Bremsstrahlung spectrum is directly proportional to the atomic number of the sample. Typically of more interest are the characteristic X-rays, which are formed by energy level transitions within the inner electron shells of the target atoms. Characteristic X-rays are produced when a beam electron knocks out an inner shell electron , producing a 'hole'. This hole is quickly filled by any other nearby electron, typically from some other energy level within the same atom. Because there is always some energy difference between the electrons in the different shells, this filling of the inner shell hole will free up some energy, which is sometimes emitted from the atom as a electromagnetic packet of energy, an X-ray. Thus, the energies of the characteristic X-rays emitted are dependent on the energy level differences seen between the electron shells in an atom, which are unique to each atomic species.

Cathodoluminescence
Cathodoluminescence is the condition where some material emits visible photons under the influence of the electron beam. The operative mechanism is the same as for characteristic X-ray production, except the energies under consideration are very low, corresponding to (typically) energy level transitions of a few eV (conduction-valence band transitions and the like). This is mostly seen in translucent and transparent semiconductors.