Electron microscopy
Coating to Avoid Charging in SEM Samples
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In SEM (secondary electron microscopy) observations, especially for electrically isolated structures, charging can occur and thus SEM imaging is degorated. In such cases, the SEM specimens are often coated with a grounded metallic layer:
          i) The fraction 1−η of incident electrons is trapped by the coated materials,
          ii) The secondary electrons are restricted to the coated layer and near surface.

This coating prevents from formation of positive or negative potentials at the surface and thus deflection of the incident electron beam is elliminated becasue the surface is electrically nuturalized.

However, this coating layer in fact forms a kind of Faraday cage only:
          i) this arrangment does not remove internal charging and electric field,
          iii) it does not prevent possible mechanical stress underneath the surface,
          iv) it does not prevent possible migration of mobile ions underneath the surface. Note that this ionic migration does not disturb the quality of SEM images except when the topography of the coating is modified by the accumulation of mobile species below the surface. [1] The authors established a simple model to explain the anion migration driven by the electrostatic field induced by SEM beam.










[1] O. Jbara, J. Cazaux, G. Remond and C. Gilles, Halogen ion electric field assisted diffusion in fluorite and polyvinyl chloride during electron irradiation, J. Appl. Phys. 79 (5), 1, 2309 (1996).