Scanning Electron Microscopy (SEM)
In the SEM, an electron beam is focused into a fine probe and rastered over a small rectangular area. As the electrons interact with the sample, various signals, including those from secondary electrons, are created and detected. These highly localized signals are used to modulate the brightness of a CRT display rastered synchronously with the electron beam. The image formed on the CRT is a highly magnified image of the sample. The technique is usually nondestructive, although instrumental sample requirements often mandate alteration of the sample before analysis. Magnification is from 10X to 300,000X and lateral resolution is 10-50nm, allowing features as small as 100 Å to be seen. The analysis depth is 20-50 Å. SEM Analysis provides excellent topographical data, and in backscatter mode atomic number information (compositional) also. Images can be e-mailed or delivered on various storage media for client manipulation.
Back-Scattered Electron Images
Back-scattered electron (BSE) images are useful for locating phase differences and finding specific features on a sample. When a high energy electron interacts with an atom, it undergoes either inelastic scattering with the nucleus. BSEs result from elastic scattering and leave the sample with energies comparable to the primary beam. The likelihood of backscattering increases with the atomic number (Z) of the material. High-Z materials give a stronger signal (brightness) than low-Z materials, thereby giving image contrast from elemental differences.