reflected electron, backscattered electron

"Backscattered electron(s)" mean the electrons which are scattered backward against the incident electrons (primary electrons) in the electron-scattering process. These electrons are sometimes called "reflected electrons". Fig. (a) shows an energy spectrum of electrons emitted from a specimen for an incident electron probe of 20 keV. The energy of the backscattered electron ranges from about 50 eV up to 20 keV. Small peaks around the center of the spectrum are attributed to Auger electrons.
A wide energy spread of the backscattered electron is due to the fact that the incident electrons repeat not only elastic scattering but also inelastic scattering along the incident electron path, and the electrons suffered by energy loss are emitted from the specimen.
Since the energy of the backscattered electron is much larger than that of the secondary electron, the escape depth of backscattered electrons from the specimen is about two orders of magnitudes larger than that of secondary electron. As an example, when the incident electron energy is 15 keV, the escape depth of backscattered electrons for Al and Fe is about 0.7 μm and 0.2 μm, respectively. The backscattered electrons emitted from the deep region degrade the resolution.
Fig. (b) shows the dependence of the backscattered electron coefficient on the atomic number. As the atomic number increases, the backscattered electron coefficient increases. Owing to this characteristic feature, the backscattered electron image reveals the compositional difference in the specimen. It should be noted that, since the secondary electron emission coefficient has no relation with the atomic number, the secondary electron image cannot distinguish the compositional difference.
Fig. (c) shows the change of the backscattered electron emission with the angle of the specimen surface against the incident electron probe. The backscattered electron emission is large in the direction of the specular reflection of the incident electrons with respect to the specimen surface. Thus, when the detector is fixed at a certain position, the topographic image of the specimen is obtained. Since the backscattered electron intensity is very sensitive to the topography of the specimen, the image can reveal even gentle undulations of the specimen without sharp edges. It is, however, noted that the backscattered electron image does not provide the edge effect which appears in the secondary electron image. Thus, the backscattered electron image is less sharp than the secondary electron image for a specimen with very fine and sharp edges.
It is noted that backscattered electrons can excite secondary electrons (SE2) near the specimen surface, and also hit the SEM components (objective lens, etc.) and excite secondary electrons (SE3). Those electrons degrade the quality of the secondary electron image.

Fig.(a) Energy spectrum of electrons emitted from a specimen for a primary electron probe of 20 keV.
Backscattered electrons are the electrons emitted from the specimen with an energy ranging from about 50 eV up to 20 keV. Small peaks around the center of the spectrum are attributed to Auger electrons.

Fig.(b) Dependence of backscattered electron coefficient on the atomic number.
As the atomic number increases, the backscattered electron coefficient also increases.

Fig.(c) Dependence of backscattered electron emission on the angles of the specimen surface against the incident electron probe.
The backscattered electron emission is large in the direction of specular reflection of the incident electron probe with respect to the specimen surface. When the detector is fixed at a certain position, the detector detects only the backscattered electrons traveling straight into the detector and thus, an image as if the specimen illuminated from the detector is obtained. Owing to this characteristic feature, the topographic image of the specimen is obtained.

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