SEM (Scanning Electron Microscope) Notes
I’ve been looking at buying an SEM, possibly for shared use and mostly because I’ve seen a couple going very cheaply and having a microscope capable of 100000x magnification intrigues me.
This post will serve a a location for me to keep my notes on SEMs.
Sample/Operation issues
In general you want to minimize the acceleration voltage, to avoid damaging your sample. However this also results in a poorer resolution image. So there are trade offs to be made in acceleration voltage. The JOEL SEM Guide has extensive information on the issues involved. The figure below sums up the issues:
You can see how the applied voltage has effected image resolution in the image below:
You also have similar issues with probe current, minimizing probe current yields sharper images (higher resolution) but you lose sharpness (there’s more noise). High currents can also damage the sample.
The sample angle can also have a significant effect, causing edge effects at certain tilt angles:
Another issue that comes up is sample “charge-up”. If your sample is an insulator a charge can build up on the surface, which will result in poor image quality:
This can be countered against by using a lower voltage, or by coating the sample with a conductive layer (e.g. with a sputtering machine).
EDS/EDAX
Some SEMs come equipped with an EDS system (the most popular appears to be the EDAX). This is a X-ray spectroscopy method mostly used for biological samples. It allows the user to chemically characterize their sample.
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