Fundamentals of Imaging in Scanning Electron Microscope (SEM) 

Scanning electron microscopy (SEM) is possibly the most commonly used microscopy technique across research domains; viz., ranging from physical to engineering to biological sciences. Usually the surface topographical features of the concerned samples (which include looking at particles and grains) and contrast between different regions of a sample (by virtue of them having different average atomic numbers), are imaged with SEM. While the former is typically done using the secondary electron (SE) mode, the latter is imaged using the back-scattered electron (BSE) mode. The possible magnifications in the present generation SEMs can range from a few thousands to a few lakhs, with resolution down to even ~1 nm; which, of course depend on the electron source (viz., field-emission gun vs. W-filament), signal-type (viz., SE vs. BSE), imaging conditions (viz., accelerating voltage, working distance, aperture size etc.) and quality/type of the sample. Beyond imaging, SEM can also be used for compositional analysis, via either energy-dispersive spectroscopy (EDS) or wavelength dispersive spectroscopy (WDS). In the above context, the talk will touch upon some of the basic aspects of functioning of SEM, types of signals generated/used, imaging conditions and spectroscopic characterizations. The talk will highlight the need to use the right signal(s) and condition(s) for obtaining the best possible image quality and associated information.