SEM is a type of microscope that is used to obtain various information of a solid sample by an electron beam scanning. The signals generated from the electron beam scanning are analyzed for external morphology, chemical composition, crystalline structure and orientation of the surface. Among these signals, secondary electrons and backscattered electrons are displayed in the figure below. These two signals are commonly used for imaging samples with a magnification of up to 500,000 times.
The interactions of the electron beam with atoms near the surface of the sample yield secondary electron signals. This signal is responsible for showing morphology and topography on the sample. And, backscattered electrons are the signal resulted from the electron beam reflected from the sample. Because the intensity of the BSE signal is closely related to the atomic number of the specimen, the distribution of different elements in the sample becomes available.