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The Probe
The Probe
No matter how well aligned the electron beam is, it will always have some measurable diameter at the sample surface. However, signal is generated from a larger volume. This Interaction Volume influences the resolution of an image and which surface features we can see.
Note in the diagram below how the teardrop-shaped interaction volume grows for signals generated deeper in the sample. Signal may come from the entire width of the interaction volume as signals escape through the surface.
Signals in SEM
Signals in SEM
When high energy electrons hit the surface of a material, most of the electrons keep going.
Some electrons inelastically scatter multiple times then escape through the surface. These are secondary electrons and show topographic detail. They are typically generated from the first few nm beneath a surface.
Some electrons elastically scatter once and return back out the surface. These are backscattered electrons and show material contrast. Heavier elements scatter electrons more strongly. They may be generated from up to 100nm beneath the surface.
Some electrons displace atomic electrons, whose vacancies must be filled with higher energy electrons. The energy released from these interactions is typically in the X-ray portion of the electromagnetic spectrum. Characteristic X-rays identify specific elements within a sample. These may be generated from up to 1um beneath the surface.
How can we control interactions?
How can we control interactions?
The penetration depth increases with higher beam energy (accelerating voltage). What happens to the interaction volume?
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