A Brief Introduction to SEM (Scanning Electron Microscopy)
In SEM, a focused beam of electrons is scanned across the specimen surface in an evacuated chamber and the back scattered and secondary electrons produced are processed to provide a topographical image of the specimen surface.
The electron beam also generates characteristic X-rays of the specimen. With the use of energy-dispersive spectroscopy (EDS), it is possible to obtain an elemental map of the specimen surface layer.
In traditional SEM, the specimen must be coated with a conducting layer to overcome charging of the surface by the electron beam. This limits the surface sensitivity as all backs cattered, secondary electrons and emitted X-rays must penetrate the surface coating which means that high energy incident beams (say 30keV) need be used.
However, recent developments mean that a lower-energy electron beam (approximately 1-3keV) can be used which can eliminate the problem of surface charging and hence the necessity for coating the specimen surface.
If the electron beam is not scanned but held in one spot on the specimen, quantitative elemental analysis can be performed on the areas of interest under the electron beam.