Scanning Electron Microscopy

• Type of physical science: Condensed matter physics
• Field of study: Surfaces

A scanning electron microscope uses an electron beam to scan and magnify surfaces. The close-up images are known for their detail, depth of focus, and three-dimensional appearance.

Overview

The scanning electron microscope (SEM) is a powerful magnifying device that uses an electron beam to form an image of the surface of an object. The image is characterized by great depth of field and shadowing, which gives the viewer clarity and perspective. Like the transmission electron microscope (TEM), the SEM uses an electron gun and a system of condenser lenses to focus the electrons on the specimen. SEMs use much lower voltages and thinner beams than TEMs. The electron gun and lens system are very similar to the ones that produce a beam of electrons to form an image in the picture tube of a CRT (cathode-ray tube) television. The electron gun, which is operated at between one thousand and thirty thousand volts in an SEM, is a heated filament that provides a source of electrons for the probing beam.

As the electrons leave the gun through its opening, they are focused and directed by electromagnetic fields acting as lenses. No glass lenses are used, since they would interfere with the beam. Because moving electrons generate their own magnetic fields, electromagnets can be used to control the pathway of the beam. Electrically charged plates can also be used to produce fields that direct the beam. These magnetic and electrical fields function as adjustable lenses. The system produces a very narrow beam, which is critical to a good electron microscope. In the SEM, the size of the beam and the scattering of electrons limit the resolving power. The high charge-to-mass ratio of the electron allows relatively easy steering with electric and magnetic fields, however; an SEM beam can be converged to a spot of about five nanometers.

Several important effects occur when the beam is focused on the surface of the specimen. First, some of the beam electrons are reflected, or backscattered, by the specimen. Second, the bombarding electrons cause secondary electrons to be released from the surface of the material. These secondary electrons have gained enough energy to escape from the atoms of the specimen. Both types of electron emission can be detected as a signal and thus can be amplified and recorded. These emissions are used to form a picture of the specimen's surface.

The intensity of the returning signal caused by secondary and reflected electrons depends on the angle at which the beam hits the irregularities in the surface. The fewest electrons are emitted from the surface when the beam impacts directly at a ninety-degree angle, whereas more emissions occur when the angle is more acute. Thus, the SEM creates an image of the surface by producing a composite display of the number of emissions resulting from each spot that the beam probes.

Collecting the emissions is technologically crucial. Different types of detectors are used to collect the different types of emissions—mainly backscattered electrons and secondary electrons, but other emissions are produced as well, including photons and diffracted backscattered electrons. SEMs almost always have a secondary electron detector, and most have a backscattered electron detector as well. Additional detectors may also be used.

The SEM differs from the TEM in its use of the electron beam. In a TEM, the whole picture is recorded at once, and electrons pass through the specimen. By contrast, the SEM moves a very narrow beam of electrons across the surface of a specimen in a back-and-forth pattern called a raster. As the needle-thin electron beam rapidly scans over the surface of the specimen, the emission signal from each spot is amplified and displayed on a cathode-ray tube, which has its raster synchronized with that of the scanning beam. Thus, the reactions of each spot to the electron beam are recorded sequentially. The selected ratio between the size of the scanned area and the size of the display tube determines the magnification of the image. A typical SEM can use this ratio to magnify the image anywhere between 10 and 200,000 times. The high quality of the SEM surface image is tied to its creation of shadowing and depth.

While the number of emission electrons varies with the angle of the surface, the position of the electron detector will also affect the number of emissions collected from any one location. If a beam is focused on a microscopic hill with the same angle of incline on all sides, it will cause an equal number of electrons to be released on all sides, but emission electrons from the side of the hill facing the detector are more likely to be collected, while the far side of the hill will have a shadow. In an SEM picture, the side that appears to have a light source, usually at the top of the photograph, is actually the location of the detector. Tilting the specimen's surface relative to the direction of the electron beam also affects the image. This option often helps to add perspective. The observer always has the viewpoint of being in the electron gun. Tilting makes closer objects appear larger and allows the observer to see over them to the far end of the specimen. The tilting, shadowing, and large depth of field combine to produce a fascinating three-dimensional appearance that allows easy interpretation of specimen surfaces.

Applications

The scanning electron microscope has numerous applications in metallurgy, geology, electronics, medicine, and biology. SEM images are of value in industry for both quality control and research. SEM allows one to look at a surface rather than through it. In its basic emission modes, SEM can be used to study particles more than ten micrometers in diameter. Crystalline and amorphous structures can be studied. The SEM can be used on metallic, organic, and inorganic surfaces that are either smooth or rough. Measurements can be made once the machine is calibrated. Stereo pair pictures of a surface can be made at different tilts and used with a stereoviewer to achieve a true three-dimensional effect. One can even locate voltage potentials in images of semiconductor devices and integrated circuits. In such pictures, the negative potentials are brighter, whereas positive areas appear darker.

The SEM has some inherent limitations. First, in most SEMs, a high vacuum is needed to prevent scattering of the primary electron beam. Because of this, any samples that might produce vapor, such as biological samples with significant water content, must be dried or frozen first. Second, as a result of the specimen's natural electrical properties, it could become charged while it is being scanned and act as a capacitor, which would compromise the image. To prevent the buildup of charge by the electron beam, the specimen is often coated with metal or carbon and grounded to its mounting with a colloidal silver paste. Such preparations and the need for a high vacuum have generally ruled out the study of wet and living materials by the standard SEM, although low-vacuum and environmental SEMs are available to circumvent these limitations.

As long as only the surface of a specimen is being examined, its thickness is not important, which contrasts with the TEM. The only limitation is that the investigator must be able to fit the mounted object into the specimen chamber. An item with the approximate volume of a tennis ball is generally the upper limit, though larger SEMs are able to accommodate greater volumes, but smaller is better if the operator plans to move and tilt the specimen for better results. The range of the SEM's available magnification (ten to five hundred thousand times) places it between the light microscope and the high-resolution TEM. All of these instruments have qualities that do not allow one to replace the other. The SEM can be combined with the TEM or the light microscope.

An SEM can be operated in modes that use different detectors, provided there is enough room to place them in the specimen chamber and sufficient funds to pay for the added features. For example, the SEM can be used to analyze the chemical composition of a material and the distribution of those chemicals in a sample. Images of a specimen are formed by collecting only the reflected electrons and ignoring the secondary electrons. The probability of electron reflection increases as the square of the atom's atomic number. Besides the secondary and reflected electrons, low-energy Auger electrons, named for French physicist Pierre Victor Auger, may be released from the surface by the electron beam. These electrons come from the inner shell of the atoms in the specimen and can be used to determine the chemical composition of the material.

Another possible operating mode is to measure the x-rays that are generated by the electron beam. This mode is one of the SEM's most valuable adaptations, since it permits nondestructive chemical analysis. Such analysis requires the addition of a crystal spectrometer, which reveals both the amounts of various chemicals and their locations. Some scientists have used the SEM with an x-ray analyzer to date coatings on rocks. Scientists have also used the SEM to take advantage of the fact that many specimens emit light when bombarded by electrons; this phenomenon, called cathodoluminescence, permits the study of structural details that may not be viewable by other methods. Among other things, cathodoluminescence can be used in conjunction with a SEM to study the localized surface plasmon resonance, or oscillation of electrons when excited by photons, of metal nanoparticles.

Various advances in SEM technology, including the capacity to detect backscattered electrons and perform x-ray analysis of various elements, have made SEMs ideal for analysis of nanomaterials. They can also be incorporated into other instruments for use in the creation of nanotechnology, via techniques such as electron-beam nanolithography, in which the electron beam is used to expose the surface of the nanomaterial in a particular pattern so that specific areas can be removed.

The need for applications led to the development of the environmental scanning electron microscope (ESEM) by Gerasimos D. Danilatos in Australia in the 1970s and 1980s. The ESEM works without a continuous high vacuum and allows wet specimens to be studied. The key to this modification involves allowing the specimen to be under pressure while keeping most of the beam path at high vacuum. Danilatos's innovation was to provide a pressure gradient from the electron gun to the specimen. By the time the beam reaches the specimen, the pressure is great enough to allow the specimen to remain wet without degrading the beam. Another innovation was a new detector that measures ionization of the gas surrounding the specimen. Some applications of the ESEM have included dynamic studies of biological material, crystal growth, absorption, drying, melting, corrosion, and other chemical reactions. Notable uses include nondestructive examinations of moisture-laden historical artifacts, such as monitoring restoration of the Sistine Chapel and preservation of the Dead Sea Scrolls.

Another variation on the SEM is the focused ion beam (FIB), which operates much like the SEM, except with a beam of ions instead of electrons. While an FIB can be used on its own, it can also be incorporated into an SEM, allowing the microscope to deploy a beam of either electrons or ions. The ion beam can be used for imaging purposes, a practice known as scanning ion microscopy (SIM); it is particularly good for showing grain contrast in polycrystalline materials, making combined FIB/SEMs ideal for searching for shale gas in sedimentary rock.

The FIB, unlike the SEM, destroys samples as it scans them. While this feature may be problematic in some cases, it also means that the ion beam, like the electron beam, can be used for nanomachining, as well as to slice nanometer-scale cross-sections from a sample for further study.

Context

The light microscope is limited in magnification by the wave properties of light. Light bends, or diffracts, as it moves past the edges of openings in materials. Diffraction is not a problem if an opening is large and most of the light is not near an edge; if the opening is small, however, it becomes a major problem, because it makes the edge appear fuzzy. Two or more spreading wave fronts may cross each other and cause interference patterns. The waves combine their energies as they overlap, sometimes augmenting and sometimes canceling each other, thus causing the ability to view small details with light to be lost. The resolution of light microscopes is limited to about 200 nanometers, or as little as 97 nanometers with a specially designed scattering lens. In other words, a good light microscope with a scattering lens will allow the observer to distinguish two objects until they are closer than 97 nanometers.

In 1924, Louis de Broglie theorized that since moving electrons must obey the laws of quantum physics, they must have the properties of frequency and wavelength. The wavelength of an electron, according to de Broglie's calculation, is one hundred thousand times shorter than that of visible light. Because the wavelength of light was recognized as the barrier to further resolving power in the light microscope, scientists investigated the possibility of microscopes that would use electrons and developed a new family of magnifying instruments. Experimental electron microscopes were soon built. The first experimental SEMs appeared in the late 1930s and early 1940s. By 1964, the first SEM had become commercially available as a research tool, mainly because of improvements in the collection and amplification of the emission signals. Since then, many models have been manufactured that range from very basic setups to higher-priced versions with many added features.

The SEM is a member of an evolving family of instruments. The history of science is linked closely to the history of instrumentation. The need for information drives the invention of new technologies, and new instruments in turn tend to raise more basic questions and promote more science.

Principal terms

DIFFRACTION: the bending of waves around the edge of an opening, causing a fuzzy image

ELECTRON: a subatomic particle with a negative charge, normally found around the outside of an atom's nucleus

PRIMARY BEAM: a stream of high-energy electrons, produced by the electron gun of the scanning electron microscope, that is focused and aimed at the surface of the specimen

REFLECTED ELECTRONS: electrons from the beam that have bounced off the specimen, identified by having energies above those of the secondary electrons; also referred to as backscattered electrons

RESOLVING POWER: the minimum distance between two objects at which a magnifying instrument is still able to identify them as separate objects; also referred to as resolution

SECONDARY ELECTRONS: electrons that gain enough energy to escape from the atoms of the specimen being bombarded by the primary electron beam; identifiable by their lower energy levels

TRANSMISSION ELECTRON MICROSCOPE (TEM): a magnifying tool that uses a beam of electrons to penetrate thin specimens; an image is formed by the electrons that go through the object

Bibliography

Butcher, Alan R., and Herman J. Lemmens. "Advanced SEM Technology Clarifies Nanoscale Properties of Gas Accumulations in Shales." American Oil & Gas Reporter, July 2011, pp. 118+.

García de Abajo, F. J. "Optical Excitations in Electron Microscopy." Reviews of Modern Physics, vol. 82, no. 1, 2010, pp. 209–75.

Grillone, Lisa, and Joseph Gennaro. Small Worlds Close Up. Crown, 1978.

Hoffmann, Banesh. The Strange Story of the Quantum. Dover, 1959.

Kelley, Michael J. "Electron Microscopy: A Tutorial." Chemtech, vol. 17, 1987, pp. 232–40.

Kessel, Richard G., and Randy H. Kardon. Tissues and Organs: A Text-Atlas of Scanning Electron Microscopy. Freeman, 1979.

Kessel, Richard G., and C. Y. Shih. Scanning Electron Microscopy in Biology. Springer, 1976.

Lott, John N. A. A Scanning Electron Microscope Study of Green Plants. Mosby, 1976.

Schatten, Heide, editor. Scanning Electron Microscopy for the Life Sciences. Cambridge UP, 2013.

Shimizu, Kenichi, and Tomoaki Mitani, editors. New Horizons of Applied Scanning Electron Microscopy. Springer, 2010.

Watt, Ian M. The Principles and Practice of Electron Microscopy. Cambridge UP, 1985.

Zhou, Weilie, and Zhong Lin Wang, editors. Scanning Microscopy for Nanotechnology: Techniques and Applications. Springer, 2007.