edge effect

The edge effect means a phenomenon appearing in a secondary electron image, where the tip of a protrusion and the edge of a step on a specimen surface become extremely bright.
The effect arises because secondary electrons are emitted much more from the tip of a protrusion and the edge of a step than those from flat regions when the incident electrons scatter in the specimen.
Fig. (a) schematically shows the difference in signal intensities of secondary electrons with respect to the specimen shape. Blue arrows indicate the magnitude of emission of secondary electrons at flat-, inclined-, and edge- regions.
The edge effect depends on the accelerating voltage of the primary electrons. When the accelerating voltage increases, the bright region in the image increases. This is because the scattering volume of the incident electrons is large for a high accelerating voltage, and then the volume to generate secondary electrons becomes large. Fig. (b) (left and right) show secondary electron images of the folds on acicular hairs growing in the compound eyes of a Drosophila, taken at accelerating voltages of 2 kV and 10 kV, respectively. In the image of 2 kV, the folds of the hairs are clearly seen due to a strong edge effect. To the contrary in the image of 10 kV, it is difficult to see the folds because the edge effect is weak and the contrast of the folds is low. It is noted that the specimen is plasma coated with osmium tetroxide (OsO₄) to prevent electric charging.

Fig. (a) Intensities of emitted secondary electrons for different specimen shapes
Red arrows: Incident electrons, Blue arrows: Emitted secondary electrons

Fig. (b) Secondary electron images of the folds of acicular hairs on the compound eyes of a Drosophila (with plasma coating of osmium tetroxide (OsO₄))
Accelerating voltage (Left) 2 kV, (Right) 10 kV. The two bottom figures are the enlarged images of the frame-enclosed parts.

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