In SEM (secondary electron microscopy) observations, especially for thick insulators, charging mechanisms originate from competition between the trapping of primary electrons (PEs) injected into the specimen or generated secondary electrons (SEs) (contributing to negative charging) and the emission of secondary electrons (SEs) from the specimen (contributing to positive charging).
Based on the dependence of interaction volume and maximum escape depth on PE (primary electron) beam energy, there are three different cases regarding SE emissions: i) RL ≤ 5λ, ii) RM > 5λ, and iii) RH >> 5λ as shown in Figure 4567 (R is maximum interaction volume and λ is maximum escape depth). For the case of R ≤ 5λ, the two positive and negative charge areas are very close. For the cases of R > 5λ and R >> 5λ, there are two distinct regions inside the interaction volume: a thin near-surface layer with thickness T ~ 5λ in positive charge Q+, and a negatively charged bulk (Q-) with a thickness of about (R–T). As a result, the implanted charge is ΔQ = Q+ + Q-, and the built-in electric field (Ebi) governed by ΔQ is established.
Figure 4567. Schematics of the charge distribution and built-in electric field inside the interaction volume depending on maximum PE range (R).
The actual charging process of an electrically isolated structure is very complicated and is determined by many factors:
i) The incident electrons interact with the specimen in the manner of many complex and dynamic processes, including scattering, diffusion, mobility, trapping, combination, etc;
ii) The charging process is time-dependent;
iii) The irradiation condition and the internal property of the specimen can affect the charging process;
iv) Adjacent structure can affect the charging process of a location.