Stereo Scan 440 - DCU

The zone ofp~mary excitation increases in depth and diameter with a high
accelerating voltage. Backscattered electrons which are generated mainly
beneath the specimen surface possess enough energy to ionise one or more
specimen atoms before escaping from the specimen surface. Each of these
ionisations produces more secondary electrons which are added to the true
secondary electron signal and result in a loss of surface detail. The use of too
high a voltage also results in parts of the sample exhibiting very bright edges.
The 'edge effect' is caused by the electron beam completely penetrating the edge
of the feature, so that secondary electrons are produced from a larger effective
surface area, see Figures 3.2(a) and 3.2(b) below.
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Figure 3.2(a) Sample of fractured aluminium at
high voltage showing edge effect
Stereoscan 440 Operator Manual
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