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Multiple scattering normally does not occur within a thin TEM specimen but is significant within a thick SEM specimen where the primary electrons lose most or all of their energy. Table 4102 lists the thickness dependence of electron scattering from a specimen in EM (electron microscope) measurements.
| Table 4102. Thickness dependence of electron scattering from a specimen in EM (electron microscope) measurements. |
Scattering direction |
EM method |
Scattering event/angle |
Scattering direction |
Electrons |
Wave (phase) property |
Electron property |
Contrast interpretation |
| Thin specimen |
TEM |
Less scattering events and then smaller scattering angle
|
Forward
scattering |
More electrons |
Strong coherent signal |
Strong wave property |
Easy |
|
Back scattering |
Fewer electrons |
Weak incoherent signal |
Weak particle property |
|
| Thick specimen |
TEM/SEM |
|
Forward
scattering |
Fewer electrons |
Weak coherent signal |
Weak wave property |
Middle |
| Electron is scattered more than once (plural scattering), and then scattering angle is greater |
Back scattering |
More electrons |
Strong incoherent signal |
Strong particle property |
|
| Very thick or bulk specimen |
SEM |
More scattering events and then greater scattering angle |
Forward
scattering |
Fewest electrons |
Weakest coherent signal |
Weakest wave property |
Difficult |
| Electron is scattered >20 times
(multiple scattering), and then scattering angle is greatest |
Back scattering |
Most electrons |
Strongest incoherent signal |
Strongest particle property |
|
Figure 4102 shows the coherence of various scattered electron rays from TEM and SEM specimens.
| Figure 4102. Coherence of various scattered electron rays: (a) Thin film specimen in TEM and/or STEM modes, and (b) Bulk specimen in SEM mode. The electron rays in red are incoherent, while the rays in green are coherent. |
|
