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Not just that, electron microscopy was further augmented with suitable detectors to even extract quantitative micro-chemistry and micro-crystallographic information not only from the surfaces (in case of SEM) but also from within the bulk of the material in case of TEM. Enhancement in resolution could be possible because of much smaller value of the wavelength of the accelerated electrons (which are the probe in electron microscopy) as compared to that of the visible light (which is the probe in case of optical microscopy). Why electron microscopy could be expanded to provide much more information is because interaction of electrons with matter leads to generation of a variety of signals like different kinds of electrons and X-rays and these signals contain valuable information about not only the topography of the surface but also about chemistry of the material and orientation of the grains. Therefore, it becomes relevant to present a brief introduction to electron – matter interaction for better understanding of electron microscopy in general and SEM in particular.
When an electron beam strikes with matters it interacts with the electrons – outer shell as well as core electrons and nucleus of the matter. Depending on the energy and intensity of the incident electron beam and thickness of the material being exposed to the electron beam different kind of signals are generated. A schematic diagram showing a typical electron – matter interaction is presented in Figure 1.
Electron beam can penetrate through the specimen only if its thickness is less than 100 nm and only then the transmitted signals are produced. Transmitted signals are used in Transmission Electron Microscope (TEM) and related analytical equipment like STEM (Scanning TEM), ATEM (Analytical TEM), HRTEM (High Resolution TEM) etc with attachments like EELS, HAADF etc. These signals are not