RESOLUTION
The following picture represents an electron beam, speeded up with a 20kV tension, filtering through a metallic material. The diameter of the probe is only a few nanometers long, but the beam widens in the material because of the impacts. It's possible to notice that the electrons progressively loose their energy while filtering through the material.
It ensures from this that the signal used can come from a much larger surface than the diameter of the probe. The resolution is determined by the size of the zone from which the signal (SE, BSE, RX), used to create the picture, is emitted. Indeed, it is impossible to see details smaller than this diameter.
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Secondary electron production area. |
Back scattering electron production area. |
X Rays production area. |
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Secondary electrons have little energy. They can therefore not go far into the material. This why they come from a small area (a few angstroms) which surrounded the incident beam. The SE are interactions which give the best resolution. |
Back scattering electrons have approximately the same energy as the incident beam (elastic impacts). The higher the accelerating tension used for the picture is the bigger the production area is. The smaller the tension is, the higher the resolution will be, but be careful, the lower the tension is, the less BSE is produced. |
X rays can travel important distances in materials. The resolution obtained with X rays will be always low. It will be function of the atomic weight of the material. The smaller Z is, the lower the resolution is. |
The diameter of the red area is function of the penetration depth which is principally function of three factors :
- The acceleration voltage
- The material's atomic number
- The angle of incident beam the with the surface
It seems that the higher the acceleration tension is, the bigger the electron penetration depth is. A big atomic number increase the number of impacts therefore reducing the electron penetration depth. The depth then varies from a few tenths of microns to several micrometers. This can be checked with the following example (with an accelerating tension of 2 to 16KV) on carbon) :
The electron penetration depth also depends on the atomic numbe. The bigger the atomic number of the target is, the more difficult it is for the electrons emitted by the beam to filter through. The penetration is greater when the incident beam hits the surface perpendicularly. In conclusion : The best resolution is obtained with secondary electrons and materials with a atomic weight.
electron/structure interactions
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