Electron Absorption EM Specimen (& Thickness Dependence)
-- Fraction of Absorbed Electrons--
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Besides other contrast factors, the contrast of TEM images can also depend on the change of the amplitude of the transmitted beam or the diffracted beam due to absorption of incident electrons in the specimens:
Under electron bombardment, the absorbed electrons are the excess electrons which present in the specimen and are lead to ground and measured as a specimen current. The number of absorbed electrons per unit time (or so-called the specimen current), equals the number of primary electrons minus the number of scattered, secondary and transmitted electrons per unit time. Therefore, the fraction of electrons being absorbed depends on many parameters, including the composition, thickness and roughness of the specimen, the primary electron energy, the electron incidence angle and local electrostatic fields when present.
In the case of thick specimen, the effect of electron absorption can be given by the absorption function below,
Then, the transmission function can be given by,
Higher energy electrons are less readily absorbed by the specimen than lower energy electrons. For a very thick specimen, especially in SEM, the transmitted electrons are absorbed in the specimen and give rise to a specimen current (see page3768).
In EM measurement, especially in SEM measurement of a bulk material, when an electron beam hits a dielectric, its absorbed electrons accumulate on its surface due to the lack of a positive charge flowing from the ground.
Figure 1184 shows Monte Carlo simulation of backscattered and absorbed electrons in Al (aluminium) with 7 keV and 4 keV incident electron beam. Both the interaction volumes of the backscattered and absorbed electrons for 4 keV are smaller than those for 7 keV. The depths of backscattered electrons are ~140 nm and ~ 50 nm, respectively.
 Jorg Nissen and Dirk Berger, Lateral resolution of quantitative element analysis of low-Z elements, https://doi.org/10.1002/9783527808465.EMC2016.6229.