Electron microscopy
 
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:
        i) if the specimen is very thin, then the absorption is negligible,
        ii) if the specimen is thick, then the absorption cannot be neglected.

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,
        absorption function ---------------------- [1184a]
where,
        µ -- absorption coefficient,
        Δz - specimen thickness.

Then, the transmission function can be given by,
        absorption function ---------------------- [1184a]
where,
          σ -- interaction constant,
          φ -- two-dimensional projected potential.

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.

 

 

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