Practical Electron Microscopy and Database

Chapter/Index: Introduction | A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | Appendix

Electron Scattering within TEM Specimen

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. 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.