University of Liège

How does it work?

The electron beam, which typically has an energy ranging from 2 to 30 keV, is focused by one or two condenser lenses into a beam with a very fine focal spot size. The beam passes through pairs of scanning coils or pairs of deflector plates in the electron optical column, typically in the objective lens, which deflect the beam horizontally and vertically so that it scans in a raster, fashion over a rectangular area of the sample surface.

The energy exchange between the electron beam and the sample results in the emission of electrons and electromagnetic radiation, which can be detected to produce an image, as described below.


Secondary electrons are emitted when the primary beam that has lost a part of its own energy, excites atoms of the sample. The secondary electrons have a little energy (about 50 eV) divided up on a large spectrum. Due to their low energy, these electrons originate within a few nanometers from the surface
Their low energy is a quality:
- It will be easily possible to deflect them (with a potential difference) and to collect a large amount of the electrons on the detector in order to have a picture with a good signal/noise ratio.
- They cannot cover a long distance, because they are soon stopped. They therefore come from an area closed to the beam and this position gives pictures with very good resolution. Using this technique, resolutions less than 1 nm are possible.
Pictures obtained with the detection of secondary electrons mostly represent the topography of the sample

Backscattered electrons consist of high energy electrons originating in the electron beam, that are reflected or backscattered out of the specimen interaction volume. Backscattered electrons may be used to detect contrast between areas with different chemical compositions, especially when the average atomic number of the various regions is different, since the brightness of the BSE image tends to increase with the atomic number.
High Vacuum Mode

The main advantage of the XL30 in high vacuum mode is its field-emission electron gun and advanced electron optics. So, a very high resolution/magnification is obtainable at any high voltage setting. 

High vacuum mode can be divided in two sub modes: 

  • Low voltage mode – for observing non coated samples. Very low voltage (0.2-1.0 kV) can be used for observation of beam sensitive samples.
  • Regular mode – for observing conductive and coated samples. Since useful resolution is very specimen dependent, this mode usually yields best results at highest magnifications.
Wet Mode

  • By utilizing cooling Peltier stage and high water vapor pressure in the specimen chamber it is possible to achieve high levels of humidity (up to 100%). In these conditions wet or hydrated specimens (cells, plant samples, tissue, etc.) will not dry and introduce any artifacts. Dynamic experiments are also possible; for example, drying or crystallization processes can be examined. 
  • Wet mode is very useful not only for observing hydrated specimens, but for observing non-conductive specimens as well. Paper, plastics, ceramics, fibers, fabrics, outgassing materials – everything goes! And  resolution in wet mode is as high as in high vacuum mode. 

The incident beam may excite an electron in an inner shell, prompting its ejection and resulting in the formation of an electron hole within the atom’s electronic structure. An electron from an outer, higher-energy shell then fills the hole, and the difference in energy between the higher-energy shell and the lower-energy shell is released in the form of an x-ray. The x-ray released by the electron is then detected and analyzed by the energy dispersive spectrometer. These x-rays are characteristic of the difference in energy between the two shells, and of the atomic structure of the element form which they were emitted.

Electron backscatter diffraction (EBSD), also known as backscatter Kikuchi diffraction (BKD), is a microstructural-crystallographic technique used to elucidate the crystallographic texture or preferred orientation of any crystalline or polycrystalline materials. EBSD can be used to index and identify the seven crystal systems, and as such it is applied to crystal orientation mapping, defect studies, phase identification, grain boundary and morphology studies, regional heterogeneity investigations, material discrimination, and using complimentary techniques, physico-chemical identification.