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When we use an SEM, the beam rasters, or scans, across the sample generating signal one pixel at a time.
The longer the beam dwells on a single pixel, the more signal is generated.
When the beam scans across edges, more electrons can escape through the increased surface area. This is called the Edge Effect.
Electrons may accumulate at certain locations - especially edges - if the beam scans too slowly or the sample is not conductive. This is called charging.
The animation below illustrates how the intensity (brightness) of each pixel varies with the surface topography according to the number of electrons escaping the surface.
See Beam-Sample Interactions for more information on signal generation.
Notice how the tip of the electron probe does not track the surface. How does this affect the focus?
How does the scanning speed affect images?
How does the scanning speed affect images?
If we slow down the probe, the beam dwells on a single point for a longer time, allowing the beam to generate more signal (and less noise,) but be careful! If we scan too slowly, the sample may begin charging.
How do I reduce noise when my sample charges at slow speeds?
How do I reduce noise when my sample charges at slow speeds?
Most SEMs have an averaging or integration mode which combines the signal from multiple frames into a single image. Since noise is random, the signal to noise ratio improves as more frames are added.
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