What is a Scanning Electron Microscope (SEM)?

Unlike optical microscopes, SEM magnification is not about the power of the objective lens. The SEM does have an objective lense - but theoretically it could work entirely without one.

And that's why:

Scanning Electron Microscope

Martin Pueschel © Martin Pueschel

The SEM's function is not to "image" the specimen - but to focus the beam to a spot: An electron gun generates a beam with sufficiently small diameter. The magnification results from the ratio of the dimensions of the raster on the specimen and the raster on the display device. Magnification is controlled by the current (supplied to scan coils), or the voltage (supplied to deflector plates).

Specimens tend to charge when scanned by the electron beam. For conventional SEM imaging specimens must be electrically grounded to prevent scanning faults caused by electrostatic charge at the surface - therefore specimens get coated with an ultrathin coating, usually gold. This process is called sputter coating. This coating also increases the signal and surface resolution.

Our expert for the tiniest structures on Earth is Sue Lindsay. She is manager of the Microscopy & Microanalysis Unit, and operates the SEM.

Here she explains how it's done.






Mr Martin Pueschel , Scientific Illustrator
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