Scanning electron microscope

The scanning electron microscope (SEM) is a type of electron microscope capable of producing high resolution images of a sample surface. Due to the manner in which the image is created, SEM images have a characteristic 3-dimensional quality and are useful for judging the surface structure of the sample.

SEM image of an ant head ()

Table of contents

1 Scanning process 2 Detection of secondary electrons 3 Detection of back-scattered electrons

Scanning process

In a typical SEM configuration, electrons are thermionically emitted from a tungsten or LaB₆ cathode filiment towards an anode. The electron beam, which typically has an energy ranging from a few keV to 50 keV, is focused by two successive condenser lenses into a beam with a very fine spot size (~ 5nm). The beam then passes through the objective lens, where pairs of scanning coils deflect the beam either linearly or in a raster fashion over a rectangular area of the sample surface. As the primary electrons strike the surface they are inelastically scattered by atoms in the sample. Through these scattering events, the primary beam effectively spreads and fills a teardrop-shaped volume extending about 1 μm into the surface. Interactions in this region lead to the subsequent emission of electrons and x-rays, which are then detected to produce an image.

Detection of secondary electrons

The most common imaging mode monitors low energy (<50 eV) secondary electrons. Due to their low energy, these electrons must originate within a few tenths of a nanometer from the surface. The electrons are detected by a scintillator-photomultiplier device and the resulting signal used to modulate the intensity of a CRT that is rastered in conjunction with the raster-scanned primary beam. Because the secondary electrons come from the near surface region, the brightness of the signal depends on the surface area that is exposed to the primary beam. This surface area is relatively small for a flat surface, but increases for steep surfaces. Thus steep surface and edges (cliffs) tend to be brighter than flat surfaces resulting in images with good three-dimensional contrast. Using this technique, resolutions of the order of 5 nm are possible.

Detection of back-scattered electrons

In addition to the secondary electrons, backscattered electrons (essentially elastically scattered primary electrons) can also be detected. Due to their much higher energy (approximately the same as the primary beam), these electrons may be scattered from fairly deep within the sample, resulting in less topological contrast than for the case of secondary electrons. However, the probability of backscattering is a weak function of atomic number, thus some contrast between areas with different chemical compositions can be observed.
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