In SEM (secondary electron microscopy) observations, especially for electrically isolated structures, charging mechanisms originate from competition between the injection of primary electrons (PEs) into the specimen (contributing to negative charging) and the emission of secondary electrons (SEs) from the specimen (contributing to positive charging). During electron interaction with the specimen, self-regulation processes take place leading to changes in the SE yield δ from its initial value (δ0), to a steady-state value (δC∞). δ0 represents the SE yield obtained from short-pulse of electron beam to prevent charging and δC is the SE yield obtained under permanent irradiation, and the subscript ∞ represents the steady state under charged condition.
When the evacuation of charges to ground is negligible, the steady state for the SE yield corresponds to the ratio of number of electrons emitted into the vacuum and number of electrons injected into the specimen, meaning that the SE current minus the primary electron current determines the charging state of the specimen. Figure 4848 shows the schematic illustration of the SE yield d as a function of the
accelerating voltage of primary electrons (VPE). δmp represents the maximum yield at the most probable energy (MPE) of SE emission (Vmp). Vcr1 and Vcr2 are two critical voltages at which δ is equal to 1. Typically, Vcr1 is < 100 V and Vcr2 about 2 kV for insulators .
Figure 4848. Schematic illustration of the SE yield d as a function of the
accelerating voltage of primary electrons (VPE).
In Figure 4848, the two blue dotted lines show the two accelerating voltages of the primary electrons at which the electrically isolated structure is not charged at all during imaging. Therefore, the reason why the specimen needs to be grounded for high quality SEM imaging is that we cannot practically have such exact accelerating voltages for a “neutral” condition. The light yellow area shows the structure is positively charged at medium voltages, while the light red areas show that it is negatively charged at low and high voltages. Both LVSEM and CCVC pattern of e-beam testing work in the light yellow region with weak positive charging to avoid strong negative charging at high accelerating voltages.
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.
Furthermore, the total charge conservation leads to a steady state given by,
δC∞ + ηC∞ = 1
 Reimer, L. (1993) Specimen charging and damage. Image Formation in
Low Voltage Scanning Electron Microscopy, Chap. 5, pp. 71 - 135. SPIE