A scanning electron microscope (SEM) scans a focused electron beam over a surface to create an image. The electrons in the beam interact with the sample, producing various signals that can be used to obtain information about the surface topography and composition.
Given sufficient light, the human eye can distinguish two points 0.2 mm apart, without the aid of any additional lenses. This distance is called the resolving power or resolution of the eye. A lens or an assembly of lenses (a microscope) can be used to magnify this distance and enable the eye to see points even closer together than 0.2 mm.
A modern light microscope has a maximum magnification of about 1000x. The resolving power of the microscope was not only limited by the number and quality of the lenses but also by the wavelength of the light used for illumination. White light has wavelengths from 400 to 700 nanometers (nm). The average wavelength is 550 nm which results in a theoretical limit of resolution (not visibility) of the light microscope in white light of about 200 – 250 nm. The figure below shows two points at the limits of detection and the two individual spots can still be distinguished. The right image shows the two points so close together that the central spots overlap.
Two points showing the limits of detection
The electron microscope was developed when the wavelength became the limiting factor in light microscopes. Electrons have much shorter wavelengths, enabling better resolution.
As dimensions are shrinking for materials and devices, many structures can no longer be characterized by light microscopy.
Optical microscope image of nanofibers
For example, to determine the integrity of a nanofiber layer for filtration, as shown here, electron microscopy is required to characterize the sample.
Scanning electron microscope image at 4000x magnification of same nanofibers
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Basic principles of scanning electron microscopy
Types of electrons generated and detected in an SEM
Electron source, lenses, detectors and more for an SEM