The Scanning Electron Microscope (SEM)

And How It Works


The name 'Scanning Electron Microscope' comes from the fact that the beam of electrons scans over the surface of the sample. A beam of electrons from the illumination source is focused by electromagnetic condenser lenses to a very small spot on the sample. Another set of magnetic lenses, called scanning coils, deflects this spot in a rectangular pattern, called a raster, over the surface of the sample. At each spot, the number of electrons 'reflected' from the sample surface is counted by the signal detector. The number of electrons counted at each spot is displayed on a view CRT as an intensity--many electrons counted would give a bright spot, few electrons a dark spot. The individual spots, when viewed all together, form a detailed image of the sample. The image is formed in the same way a newspaper image is formed--seen up close, a newspaper image is just a collection of small spots. The resolution in the SEM is much better, however, because there are many more, small spots comprising a SEM image as compared to a newspaper image. Furthermore, you can only visualize objects that are larger than the spot itself--therefore, the smaller you can focus the illumination spot, the smaller the object you can observe. (Compare this to the limit of resolution in the TEM!)