In 1993, Charles Smithart was convicted of the murder of an 11-year-old girl in the town of Glennallen, Alaska. Prosecutors suspected Smithart after he was spotted at the scene of the crime, but they had no evidence directly linking him to the murder. That's where a scanning electron microscope (SEM) came in.
Using the X-ray spectroscopy detector of an SEM, a forensic scientist analyzed bits of iron found at the scene of the crime. He found that they had a globular shape that only welding or grinding produces. As it turned out, Smithart had a welding rig in his shop and would sometimes repair bicycles for the local children. Thanks to the tremendous capabilities of scanning electron microscopes, prosecutors had the evidence they needed to link Smithart to the crime.
Why was an SEM, rather than a regular light, or optical, microscope from the local high school, necessary to examine the evidence for Smithart's trial? For one thing, SEMs can magnify objects at upward of 300,000 times the size of the object studied. Scientists refer to this number as the magnification power and denote it, for example, as 300,000x. In contrast, run-of-the-mill optical microscopes tend to have a magnification power of a few hundred times. SEMs also have tremendous depth of field compared to traditional microscopes, providing an almost 3-D image for researchers to analyze, as compared to the flatter image an optical microscope produces. Lastly, these advanced microscopes can look past the surface of an object, telling researchers information about its composition. All of these attributes proved essential in examining evidence from the Smithart case.
Of course, SEMs have their share of drawbacks as well, like cost. Even the cheapest among them cost tens of thousands of dollars. They're also bulky and complex instruments, requiring considerable expertise to operate. As a result, their use is typically limited to research and industrial applications, though recent breakthroughs have made SEMS more accessible in other applications.
In this article, we'll learn how SEMs are able to produce such detailed and striking images. In the process, we'll explore what goes into operating one, as well as some of the most recent breakthroughs in SEM technology. But before we learn about where the technology is headed, let's look at where it all began.
The History of Scanning Electron Microscopes
The development of SEMs started with more of a whimper than a bang. When the technology was first unveiled in 1935, a group of marketing professionals was asked to evaluate the new instrument's potential in the marketplace. After polling the scientific community, the marketing experts weren't too optimistic. They estimated a need for, at most, 10 of the devices worldwide. As it turns out, the experts vastly underestimated the potential of SEMs, and thankfully, their dour outlook failed to deter further development of the technology. As a result, more than 50,000 SEMs fill laboratories and businesses across the globe [source: Breton]. So how did SEMs go from near obsolescence to the essential research tools that they are today?
For one thing, scientists had pushed optical microscopes to their limits. Optical microscopes had been around for centuries, and while you can still find them in classrooms across the country, their dependence on light had become a problem. Light's tendency to diffract, or bend around the edges of optical lenses, limits the magnification capability and resolution of optical microscopes. As a result, scientists began to develop new ways to examine the microscopic world around them and, in 1932, produced the world's first transmission electron microscope (TEM). This instrument directs a beam of electrons through the sample under observation and then projects the resulting image on a fluorescent screen. TEMs, as you might guess, share a lot in common with SEMs, and it was only a matter of a few years before SEMs were developed.
Since development of TEMs was well under way by the time SEMs came along, the latter were initially considered unnecessary. It took the unwavering resolution of C.W. Oatley, a professor of engineering at Cambridge University, to move the newer microscope forward. Working closely with several of his colleagues and graduate students, Oatley was able to demonstrate both the SEM's magnification potential and the astonishing 3-D quality of images it produced. Today, SEMs are routinely used in tasks like inspecting semiconductors for defects or exploring how insects work.
The Key Components of a Scanning Electron Microscope
We've begun to get an idea of what SEMs are capable of. Now we're ready to take a look at the various components of one and how they work together to form an image. While the variations from one model to the next are seemingly endless, all SEMs share the same basic parts.
Electron gun: Electron guns aren't some futuristic weapon used in the newest Vin Diesel movie. Instead, they produce the steady stream of electrons necessary for SEMs to operate. Electron guns are typically one of two types. Thermionic guns, which are the most common type, apply thermal energy to a filament (usually made of tungsten, which has a high melting point) to coax electrons away from the gun and toward the specimen under examination. Field emission guns, on the other hand, create a strong electrical field to pull electrons away from the atoms they're associated with. Electron guns are located either at the very top or at the very bottom of an SEM and fire a beam of electrons at the object under examination. These electrons don't naturally go where they need to, however, which gets us to the next component of SEMs.
Lenses: Just like optical microscopes, SEMs use lenses to produce clear and detailed images. The lenses in these devices, however, work differently. For one thing, they aren't made of glass. Instead, the lenses are made of magnets capable of bending the path of electrons. By doing so, the lenses focus and control the electron beam, ensuring that the electrons end up precisely where they need to go.
Sample chamber: The sample chamber of an SEM is where researchers place the specimen that they are examining. Because the specimen must be kept extremely still for the microscope to produce clear images, the sample chamber must be very sturdy and insulated from vibration. In fact, SEMs are so sensitive to vibrations that they're often installed on the ground floor of a building. The sample chambers of an SEM do more than keep a specimen still. They also manipulate the specimen, placing it at different angles and moving it so that researchers don't have to constantly remount the object to take different images.
Detectors: You might think of an SEM's various types of detectors as the eyes of the microscope. These devices detect the various ways that the electron beam interacts with the sample object. For instance, Everhart-Thornley detectors register secondary electrons, which are electrons dislodged from the outer surface of a specimen. These detectors are capable of producing the most detailed images of an object's surface. Other detectors, such as backscattered electron detectors and X-ray detectors, can tell researchers about the composition of a substance.
Vacuum chamber: SEMs require a vacuum to operate. Without a vacuum, the electron beam generated by the electron gun would encounter constant interference from air particles in the atmosphere. Not only would these particles block the path of the electron beam, they would also be knocked out of the air and onto the specimen, which would distort the surface of the specimen.
As with many things, an SEM is more than the sum of its parts. Read on to see how all of these components work together to create astounding images of very, very tiny things.
How Does a Scanning Electron Microscopes Work Its Magic?
In some ways, SEMs work in the same way key copying machines work. When you get a key copied at your local hardware store, a machine traces over the indentations of the original key while cutting an exact replica into a blank key. The copy isn't made all at once, but rather traced out from one end to the other. You might think of the specimen under examination as the original key. The SEM's job is to use an electron beam to trace over the object, creating an exact replica of the original object on a monitor. So rather than just tracing out a flat one-dimensional outline of the key, the SEM gives the viewer more of a living, breathing 3-D image, complete with grooves and engraving.
As the electron beam traces over the object, it interacts with the surface of the object, dislodging secondary electrons from the surface of the specimen in unique patterns. A secondary electron detector attracts those scattered electrons and, depending on the number of electrons that reach the detector, registers different levels of brightness on a monitor. Additional sensors detect backscattered electrons (electrons that reflect off the specimen's surface) and X-rays (emitted from beneath the specimen's surface). Dot by dot, row by row, an image of the original object is scanned onto a monitor for viewing (hence the "scanning" part of the machine's name).
Of course, this entire process wouldn't be possible if the microscope couldn't control the movement of an electron beam. SEMs use scanning coils, which create a magnetic field using fluctuating voltage, to manipulate the electron beam. The scanning coils are able to move the beam precisely back and forth over a defined section of an object. If a researcher wants to increase the magnification of an image, he or she simply sets the electron beam to scan a smaller area of the sample.
While it's nice to know how an SEM works in theory, operating one is even better.
Operating a Scanning Electron Microscope
Before researchers can take their first SEM image of, say, a mosquito, they have to prepare the specimen. Because SEMs, unlike optical microscopes, operate in a vacuum and rely on electric fields to work, sample preparation can be a complicated process. Researchers start by cleaning it of any dust or debris. Once clean, it's ready to be mounted in the SEM if the specimen is fairly conductive. Otherwise, it's coated in a conductive material like gold or platinum through a process called sputter coating before it's ready for viewing. Sputter coating allows a sample to be grounded, preventing it from being damaged by the electron beam.
Since specimens placed in the microscopes also are subject to a vacuum, they sometimes undergo additional preparation to ensure that they hold up under such extreme conditions. Biological samples, for instance, are typically dehydrated before being placed in an SEM. Otherwise, the low atmospheric pressure of a vacuum would cause the water in biological samples to evaporate quickly, destroying the sample in the process. Other specimens are frozen before they are examined, and still others are chemically treated so that they survive the magnification process.
Researchers, like photographers, have a variety of controls over the images they produce. The magnification, focus, contrast and brightness of an image are all at the fingertips of the operator of an SEM. While some models have dedicated hardware for these settings, the more recent integration of computerized controls has both lowered the cost of SEMs and simplified their operation.
Finally, make sure to observe some safety precautions when operating the instrument. In the process of scanning specimens, SEMs generate small levels of radiation in the form of X-rays as electrons beneath the surface of a specimen are dislodged and replaced by other electrons. While X-rays are inherently dangerous to humans, you shouldn't be too worried about operating an SEM. Most of the instruments have a highly isolated specimen chamber, designed to keep out electrical and magnetic interference, so any X-rays generated in the magnification process shouldn't pose a threat to the operator. Still, researchers should make sure to observe any safety precautions concerning the operation of the SEMs at their institution.
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More Great Links
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