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How Scanning Electron Microscopes Work

        Science | Optics

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.