How are backscattered electrons formed?

A backscattered electron forms through the interaction of a beam electron with the Coulombic field associated with the nucleus of an atom. If the beam electron interacts in an elastic manner, its direction will change, but there will be no loss in energy. If this beam electron undergoes many of these elastic interactions, it is possible that the electron will eventually exit the sample altogether. These electrons that exit from the sample with nearly the same energy with which they entered are called backscattered electrons.

What do backscattered electrons tell us about the sample?

Backscattered electrons have high energy, and their abundance is determined by the average atomic number of the spot that the beam is hitting. The higher the average atomic number, the larger the fields associated with the nuclei of the atoms. The larger the Coulombic fields, the more likely a beam electron will interact with it and produce a backscattered electron. Therefore, mapping the abundance of backscattered electrons will provide us with information about the composition of the sample. In the backscattered electron image to the right, one can see three phases, each with a very different backscattered electron signal. The round or oval shaped dark grains have the lowest average atomic number, and the white matrix has the highest average atomic number. From this image we can determine the area occupied by each phase without doing any chemical analyses.