charging phenomenon, charging

Charging is a phenomenon which gives rise to anomalous-contrast in SEM images in observing a non-conductive specimen. When the non-conductive specimen is observed without conductive coating, the specimen can be electrically charged due to the fact that the amount of the electrons emitted from the specimen is larger (or smaller) than that of the incident (primary) electrons in some locations of the specimen surface. This charging causes the anomalous contrast. Various charging phenomena occur depending on small or large charging. The secondary electron image is easily affected by charging. This is because the energy of the secondary electrons is low, so that it is sensitive to the potential of the specimen surface. Typical charging phenomena are described below. These can occur individually or in duplicate.

• Image contrast becomes low, resulting in an image showing less topographic contrast.
• Anomalously bright or dark areas appear locally in the specimen. Sometimes, these anomalously bright or dark areas accompany tails in the scanning direction of the electron probe.
• Image distortion or image drift arise because the incident electron beam is deflected.
• The specimen can be broken, for example, powder particles are blown away and disappear.

Figures below show anomalous contrast generated in a secondary electron image of an aluminum-based ceramic specimen (left) and a secondary electron image of the same specimen where charging is removed by a metal coating (right).

To recognize the presence of charging during observation, it is effective to lower the accelerating voltage or increase the scan speed of the electron probe and examine the change in the secondary electron image. If anomalous phenomena due to charging are suppressed by taking these two measures, the presence of charging is confirmed.

Figs. Comparison of secondary electron images of an aluminum-based ceramic specimen with charging (left) and without charging (right), taken at an accelerating voltage of 10 kV.
Left image (with charging) shows an anomalously bright area at the upper left. Bright and dark lines are seen from left to right (scan direction of the electron probe). Furthermore, the topographic contrast is weak in the whole image.
Right image (without charging) exhibits the removal of charging owing to the application of gold-palladium coating. Anomalous contrast is not seen unlike the left image, and a clear topographic contrast is seen.

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