Electron interactions and detected signals
When the electron beam is focussed onto a surface it can interact with the atoms in the sample in two ways:
- Elastically - the electron impacts and recoils from the sample with no loss or transfer of energy
- Inelastically - the electron loses some of its energy during the interaction, initiating secondary or even tertiary processes
These two interactions give rise to the many signals that can be detected in SEM, and all the information that they carry. Secondary electrons and X-rays are examples of inelastic scattering processes, while back-scattered electrons are generated by elastic scattering.
Secondary electrons are created through inelastic scattering of the electron beam with outer-shell electrons in the atoms of the surface. They typically have theses properties:
- Low kinetic energy, <50 eV
- Surface sensitive, only the top-most few nanometres
- Small interaction area, good for high resolution
- Best for topographic contrast
Back-scattered electrons are the initial electrons from the beam reflecting from the surface. They occur due to elastic scattering, where the electron trajectory is deflected due to interaction with the nucleus of the atom. Because it is elastic, they lose very little energy.
- High kinetic energy, approximately the beam energy (e.g. 15 keV)
- Sub-surface information from microns deep
- Larger interaction area, poorer resolution
- Gives material/compositional contrast
X-rays are created when the electron beam inelastically scatters from core-shell electrons. The atom will replace this scattered electron with one from a higher shell, and relax by releasing the excess energy as an X-ray photon. This X-ray has energy equal to the different in orbital energies, and therefore is characteristic of the atom.
- Fingerprint for the elemental composition
- Identifies elements and relative abundance
- Largest interaction volume
- Generally lower resolution signal
Information depth
Because of the different processes involved in generating these signals, they also have different interaction volumes, i.e. regions where the signals can come from.
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Secondary electrons (SE) are very low energy, which means they cannot escape from deep within the sample, or away from where the beam is focussed. This means the SE signal will give you the best resolution, the best surface sensitivity, and is ideal for topographic contrast.
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Back-scattered electrons (BSE) are high energy, and are effectively the electron beam bouncing elastically from the sample with no change in kinetic energy. This signal can carry information from microns deep into the sample, and can originate further away from the beam position. It gives you compositional contrast, but the resolution is not as good as the SE signal.
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X-rays will easily travel through many microns of material, and thus this signal can originate from the deepest within the sample for a given electron beam energy. It gives the most rich information about elemental composition, but at much poorer resolution compared to both the SE and BSE signals.