Some of you probably know this already, but there’s actually more than one kind of electron microscope. In electronics work, the scanning electron microscope (SEM) is the most common. You hit something with electrons and watch for secondary electron emissions. However, biologists more often use a TEM — a transmissive electron microscope — which passes electrons through a sample to image it. [Breaking Taps] built a small device to convert his SEM into a TEM.
One key idea is that in a SEM, the beam’s position on the target is the only thing that matters. Any secondary electron detected is a result of that spot’s composition, no matter where you collect them. Common detectors pick up back-scattered electrons bouncing back toward the electron source. There are also low-energy electrons bouncing off in random directions, depending on the topology of the target.
The slow electrons can be attracted by a single detector that has a strong positive charge. TEM doesn’t detect secondary electron emissions. Instead, it passes electrons through a target and collects the ones that pass through a very thin sample using a screen that glows when electrons hit it.
The idea, then, is to create a STEM-SEM device. There’s a sample holder and an angled reflector that shoots electrons passing toward the SEM’s detector. The back-scatter detector is not used, and a shield prevents the detector from seeing secondary emissions from the target itself.
You can buy these, but they are well over $1,000, so in true hacker fashion, [Breaking Taps] made his own. You could, too, but you’d need a pretty good machine shop and — oh yeah — a scanning electron microscope.
While we have seen some home labs with electron microscopes, you need some high-tech vacuum and high-voltage gear, so it isn’t too common. Armed with a STEM, you can even see the shadows of atoms.
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