charge balance

Charge balance means that there is no electric charging on a specimen.
When a non-conductive specimen is observed without conductive coating, the specimen can be electrically charged partially due to imbalance between the electrons emitted from the specimen and the incident (primary) electrons. Positive charging occurs when the amount of the electrons emitted from the specimen is greater than that of the incident electrons. Negative charging occurs for the opposite case. In theory, as shown in the figure below, if the number of the incident electrons to the specimen is the same as that of the electrons emitted from the specimen, the charges that enter and exit are balanced and the specimen is not charged. For the specimens in which conductive coating is not possible, it is requested to find the condition for no charging or charge balance by lowering the accelerating voltage or increasing the specimen tilt angle because these two measures can increase the efficiency of the secondary electron emission. For example, by lowering the accelerating voltage to about 1 kV or by increasing the tilt angle to 60 degrees or more, the charging phenomenon in the secondary electron image may almost disappear in some cases. However, the secondary electron emission ratio is not the same for the entire specimen surface because of its local compositional difference and/or local unevenness. Thus, it is difficult to accomplish charge balance in the whole specimen surface except for a special specimen with uniform composition and flat surface.

Fig. Charge balance

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