In a scanning electron microscope (SEM), a beam of electrons is scanned across the surface of a sample. The electrons interact with the atoms in the sample, and their interactions produce signals that are used to create an image of the sample.
The energy of the electrons in the SEM beam can range from 0.1 keV to 30 keV. The type of interactions that occur between the electrons and atoms in the sample depend on the energy of the electron beam. At low energies (< 1 keV), there are mostly elastic collisions between electrons and atoms. These collisions do not cause any damage to either party, and they only result in a small deflection of each particle's trajectory. However, at higher energies (> 10 keV), there are more inelastic collisions between electrons and atoms. These types of collisions can cause damage to both parties, but they also result in much bigger deflections compared to elastic collisions.
The vast majority of SEMs use an electron beam with an energy range from 2-20 keV, which is optimal for imaging most materials without causing too much damage. In some cases, lower-energy beams (< 1keV) can be used for imaging delicate samples that can not withstand higher-energy beams without being damaged; however.
Backscattered electrons (BSE)
The backscattered electron (BSE) signal is an intensity measurement of the number of electrons that are scattered from the specimen surface back toward the electron gun. The BSE signal is used to detect surface features and to create topographic images. Secondary electrons (SE): The secondary electrons (SE) emitted from a specimen under high-energy electron bombardment are used to detect composition and morphology of surfaces. In addition, these can be used in combination with BSE for imaging non conducting specimens.
Backscattered electrons originate from within the sample being analyzed and are ejected backwards, away from the direction of the primary incident beam. They provide information about both surface topography and chemical composition. Backscattered electron microscopy (BSEM) uses a specialized detector to image these signals, providing high-resolution images of both structures on the surface as well as those buried beneath it.
Secondary electrons are emitted preferentially from regions with lower atomic numbers; thus they tend to come predominantly from surfaces or near-surface regions where there has been less material build-up over time. This makes them useful for characterizing thinner films or measuring interface roughness between different materials. In addition, because they interact more weakly with materials than do backscattered electrons, they can be used to image non conducting specimens without damage caused by charging effects.
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Secondary electrons (SE)
Secondary electrons are emitted from the surface of a specimen and typically have low energies (a few eV). They travel in straight lines away from the specimen surface, but may be scattered by electric fields. In an SEM, SEs originate primarily due to backscattered electrons that emerge when the incident beam is scattered by imperfections at or near …
Secondary electrons (SE): Secondary electrons are emitted from the surface of a specimen and typically have low energies (a few eV). They travel in straight lines away from the specimen surface, but may be scattered by electric fields. In an SEM, SEs originate primarily due to backscattered electrons that emerge when the incident beam is scattered by imperfections at or near the surface of the sample. Because they are emitted from close to the sample surface, SEs tend to be very low in energy and therefore provide high-resolution imaging. However, their small mass also means that they interact very strongly with matter, making them difficult to control and limiting their usefulness for some applications.
Backscattered electrons (BSE): Backscattered electrons are high-energy secondary electrons that are ejected from the sample after being elastically scattered by atoms within the material. Unlike SEs, BSEs do not originate from bremsstrahlung radiation; instead, they arise due exclusively to elastic scattering processes. As a result, BSEs tend to have much higher energies than SEs-on the order of tens of keV rather than just a few eV-making them ideal for certain imaging applications such as detecting impurities or defects within materials. BSEs also interact much more weakly with matter than SEs do, meaning that they can be used to create images with better contrast and lower noise levels.
