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How Does a Scanning Electron Microscope (SEM) Work?
admin August 28th, 2009
Flip on the light in your kitchen and take a look around. In front of you most likely are your cabinets, refrigerator, and maybe the trusty coffee pot that you stumble downstairs to turn on every morning. We process the world through sight so naturally that rarely do we think about how it occurs, but the truth is that we do not actually observe the objects around us. Instead what we really see is the pattern of light that bounces off of them. The reason the coffee pot looks different from the fridge is that the light interacts with it differently, sending different patterns to your eye. If you find this difficult to believe, flip that light back off again. To your eye, your kitchen has vanished. The coffee pot still exists, but now there is no light to bounce off of it.
Electron microscopes work much the same way, but instead of bouncing light off of an object to figure out what it looks like, they use electrons. Why use electrons? Well, because they are smaller than photons (light “particles”) and like smaller pixels on a TV, they provide a more detailed picture of the thing you want to see. Imagine that you are blindfolded and given a small object (say a toy car) to draw and identify. At first, you are only allowed to feel the object with your arms. Later, you are allowed to hold and feel the toy with your fingers. Your fingers give you a much better description of the car because they are smaller, and can fit into all the small folds and crevices. Your two drawings will look very different. The same principle works for microscopy. The smaller the bits exploring the object, the more detail we can observe.
While light microscopes use glass lenses to focus light down on an object, electron microscopes use magnetic lenses to focus down the electrons. Electrons are scanned over the object, and information about how they interact is sent back to sensors. The sensors then use this information, just like your eye, to create an image of what the object looks like. As you can see above, SEMs can produce very highly detailed images of small objects, achieving up to 250 times the magnification of light microscopes.
Of course this is a very simple description of a complex technology. Yet strip away the details, and underneath it all is the simple idea that you’ll figure out more about an object by poking it with a pin than you will with a bowling ball.