Electron Scattering within TEM Specimen
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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 4103a lists the very common and important classification, sorted by elastic and inelastic scattering.

Table 4103a. Electron scattering from a specimen in TEM measurements (Incoherent does not imply inelastic scattering; however, inelastic scattering is necessarily incoherent in EM measurements).
Scattering type
Energy loss
Wave (phase) property
Scattering direction
Scattering angle
Electron property
Full name
Elastic No energy change of the wave after scattering Usually coherent (when the specimen is thin and crystalline) Forward scattering 1° ~ 10° Wave Coherent elastic scattering
Incoherent > 10° Particle Incoherent elastic scattering
Back scattering  
Inelastic There is energy change of the wave after scattering Almost always incoherent Forward scattering < 1° Incoherent inelastic scattering
Coherent Does not exist in EM measurement, but it shows in neutron scattering     Coherent inelastic scattering
*   Incoherence does not imply inelastic scattering, while inelastic electron scattering is incoherent in electron microscopy.
** Inelastic scattering is not necessarily incoherent in neutron scattering.

 

Table 4103b. 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 4103 shows the coherence of various scattered electron rays from TEM and SEM specimens.

Coherence of various scattered electron rays Coherence of various scattered electron rays
Figure 4103. 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.

 

 

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