I've watched a few videos like this one and know the basic structure. We have an e-source, an anode which pulls electrons out, then a condenser which starts to focus all beams, scanning coils and finally the stage with our specimen.

Now I know the basics of how a magnetic lens works and how electrons behave when they enter one. Which is what I believe happens in the condenser in the video I linked.

But even though I have looked I have a hard time finding any info about the 'scanning coils' in particular. Could someone explain and/or link me something basic to understand what their purpose is?

In addition the more info I get about the working principle of a SEM the better. But I am mostly interested in the lenses and coils since they seem to be the least discussed.

Thanks in advance!


The scanning coils work in principle just like the deflection coils of a CRT ("fat TV" or analog oscilloscopes). They deflect the electron beam, making the focus move in a raster pattern over the sample.

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    $\begingroup$ And then the detector output is tied to the scanning signals to generate the image. (Side rant on modern digital oscilloscopes not having an XYZ mode deleted). $\endgroup$ – Jon Custer Oct 1 '19 at 16:28
  • $\begingroup$ Yes ok, but how they do it? Does a current flow through them such that a magnetic field is generated which then deflect the electron beam? Also since these scanning coils seem to be connected, it seems to me that we have no control over in which direction we deflect the electron beam? We just do something by turning the coil on. But I don't know exactly what. $\endgroup$ – Caito Oct 1 '19 at 16:58
  • $\begingroup$ @Caito Yes, a current generates a magnetic field the deflects the electron beam. There are two pairs of coils for orthogonal directions. The currents are varying, slow in one direction (frequency of the frame rate), much faster in the other direction (ratio given by the number of lines), sawtooth or triangular waveforms. And as Jon says, directly coupled to the display. (This used to be common knowledge in the age of fat TV.) $\endgroup$ – Pieter Oct 1 '19 at 17:21

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