Different Types of Electron Microscopes

Electron microscopy utilizes a focused beam of electrons to create high-resolution images of a target specimen. Whereas light microscopes are restricted in their magnification by the wavelength of photons, electron microscopes are limited by the much smaller wavelength of electrons, thus achieving magnification down to nearly 0.05 nanometers. There are four main types of electron microscopes, all of which can be roughly delimited by the type of reflected energy they record from the specimen.

History

• The first electron microscope, a transmission electron microscope, was constructed by the German engineers Max Knoll and Ernst Ruska in 1931. Although the original prototype achieved a lower magnification than that of current light microscopes, Knoll and Ruska successfully proved the design was possible and two years later surpassed the light microscope in magnification power. All subsequent iterations of the electron microscope are based on this original prototype.

Transmission Electron Microscope (TEM)

• Transmission electron microscopes produce images by recording the electron beam after it has passed through a thin slice of specimen. The specimen is placed on a copper wire grid and subjected to an electron beam, normally generated by running high voltage across a tungsten filament. The electron beam travels through a condenser lens, strikes the specimen and continues through objective and projective lenses before being collected onto a phosphor screen. As with all forms of electron microscopy, the target specimen must be dehydrated and isolated in a vacuum to avoid water vapor contamination, which can cause unwanted electron scattering. TEMs produce the highest magnification of all electron microscopes.

Scanning Electron Microscope (SEM)

• Scanning electron microscopes, along with transmission electron microscopes, are the most widely used. Unlike the TEMs, scanning electron microscopes produce images by collecting the secondary or inelastically scattered electrons that bounce off the surface of a specimen. The primary electron beam travels through several condenser lenses, scan coils and an objective lens before striking the surface of the specimen. The electron beam is scattered upon hitting the specimen and a secondary electron detector collects the scattered electrons. The electron data is then raster-scanned to produce surface images with considerable depth of field.

Reflection Electron Microscope (REM)

• Reflection electron microscopes operate very similar to SEMs in terms of structure. REMs, however, collect the backscattered or elastically scattered electrons after the primary electron beam strikes the specimen surface. Reflection electron microscopes are most commonly coupled with spin-polarized low-energy electron microscopy to image the magnetic domain signature of specimen surfaces in computer circuitry construction.

Scanning Transmission Electron Microscope (STEM)

• Scanning transmission electron microscopes, like traditional TEMs, pass an electron beam through a thin slice of specimen. Instead of focusing the electron beam after passing through the sample, a STEM focuses the beam beforehand and constructs the image through raster scanning. Scanning transmission electron microscopes are well suited for analytical mapping techniques such as electron energy loss spectroscopy and annular dark-field microscopy.