A scanning electron microscope (SEM) projects and scans a focused stream of electrons over a surface to create an image. The electrons in the beam interact with the sample, thereby producing various signals that can be used to obtain information about the surface’s topography and composition.
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Why use electrons instead of light in a microscope?
Given sufficient light, the human eye can distinguish two points in space 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 much closer together than 0.2 mm.
A modern light microscope has a maximum magnification of about 1000x. The resolving power of the microscope is not only limited by the number and quality of lenses but also by the wavelength of the light used for illumination. White light specifically ranges from 400 to 700 nanometers (nm) in wavelength, whereas the average wavelength of that range is 550 nm. This 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.
The electron microscope was developed when the wavelength became the limiting factor in light microscopes. Electrons have much shorter wavelengths, enabling better resolution.
Compare an Optical Microscope vs. a Scanning Electron Microscope
As dimensions are shrinking for materials and devices, many structures can no longer be characterized by light microscopy. For example, to determine the integrity of a nanofiber layer for filtration — as shown here — electron microscopy is required to characterize the sample.
How a Scanning Electron Microscope Works
The main SEM components include:
- Source of electrons
- Column down which electrons travel with electromagnetic lenses
- Electron detector
- Sample chamber
- Computer and display to view the images
Electrons are produced at the top of the column, accelerated down, and passed through a combination of lenses and apertures to produce a focused beam of electrons which then strikes the surface of the sample. The sample itself is mounted on a stage in the chamber area and (unless the microscope is designed to operate at low vacuums) both the column and the chamber are evacuated by a combination of pumps. The level of the vacuum will depend on the design of the microscope.
The position of the electron beam on the sample is controlled by scan coils situated above the objective lens. These coils allow the beam to be scanned over the surface of the sample. This beam rastering or scanning enables information about a defined area on the sample to be collected. As a result of the electron-sample interaction, a number of signals are produced. These signals are then detected by appropriate detectors.
Sample-Electron Interaction
The scanning electron microscope (SEM) produces images by scanning the sample with a high-energy beam of electrons. As the electrons interact with the sample, they produce secondary electrons, backscattered electrons, and characteristic X-rays. These signals are collected by one or more detectors to form images which are then displayed on the computer screen. When the electron beam hits the surface of the sample, it penetrates the sample to a depth of a few microns, depending on the accelerating voltage and the density of the sample. Many signals like secondary electrons and X-rays are produced as a result of this interaction inside the sample.
The maximum resolution obtained in an SEM depends on multiple factors like the electron spot size and interaction volume of the electron beam with the sample. While it cannot provide atomic resolution, some SEMs can achieve resolution below 1 nm. Typically, modern full-sized SEMs provide resolution between 1-20 nm, whereas desktop systems can provide a resolution of 20 nm or more.