These are high energy electrons. All the electrons with energy greater than 50 eV to right up to the primary or probe beam energy are termed as Backscattered Electrons” . The high energy backscattered electrons result from elastic scattering of the primary beam electrons with atoms of the specimen being probed. The yield of BSE is much smaller than that of SE. Also, a yield of BSE has a direct correlation with an atomic number or Z of the matter being probed. Therefore, BSE imaging provides Z-contrast; however, it can also be manipulated to provide topographic contrast.
Angular spread of the BSE around the original beam and therefore, a detector for the BSE has to be arranged around the primary beam to be able to collect most of the BSE signal generated by the material. X-raysWhenever a matter is exposed by electron beam it produces X-rays. Depending upon the energy of the electron beam and the material being exposed to, two kinds of X-rays are produced – Bremsstrahlung X-rays or continuous X-rays and Fluorescent or Characteristic X-rays. Bremsstrahlung X-rays are produced due to deceleration of electrons in the electric field.
Characteristic or fluorescent X-rays, on the other hand, are produced by electronic transitions within the atoms being exposed by the primary electron beam. When high energy electrons strike the sample, it knocks out core-shell electrons and a vacancy is thus created. This vacancy is filled by the electronic transition from a higher energy level shell. The difference in the energy is released in the form of an X-ray. This X-ray is characteristic of the atom it is coming from and therefore is used for microchemistry determination by using wavelength dispersive spectroscopy (WDS) or energy dispersive spectroscopy (EDS).
Now that the signals relevant to SEM generated by electron – matter interaction are discussed it is relevant to understand configuration and functioning of a typical scanning electron microscope.
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