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Detectors attract and measure the signal generated by beam-sample interactions at each point.
Why do we see shadows?
Why do we see shadows?
Not all SEM detectors are positioned directly above the sample. If they are offset to one side of the chamber, then any topography of the sample between a given point and the detector will block the detection of generated electrons, in addition to the edge effect.
Sample rotation will change the arrangement of shadows, but image rotation will not.
How does the working distance affect signal collection?
How does the working distance affect signal collection?
Most SEMs have an optimal working distance, or range of working distances, for the detector.
For samples that generate little signal, it may help to shorten the working distance (CAREFULLY) so the electrons do not need to travel as far to reach the detector.
How do we collect different signals?
How do we collect different signals?
Secondary electrons are relatively low energy, so a bias is applied to the detector to attract these electrons.
Backscattered electrons are relatively high energy, so may be collected by reducing the detector bias.
Some systems include a dedicated backscatter detector that can be positioned below the pole piece to take advantage of the difference in takeoff angle between secondary and backscattered electrons.
Characteristic x-rays are very high energy, but are collected by a dedicated x-ray detector that is often offset to the side and calibrated for a specific working distance.
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