How Scanning Electron Microscopes Work

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 largely under-rated the market potential of SEMs, and gracefully, their dull appearance did not prevent further advancement in the technology.More than 50,000 SEMs occupy laboratories and businesses around the world.How did SEMs come out from near obsolescence to becoming the most sort after research tool of today.

For most part,researchers have greatly improved optical microscopes to their limits.Optical microscopes had been used continuously for centuries, and you can sight them in classrooms across the globe,their reliance on light is big drawback. Light's ability to bend round edges of optical lenses,reduces the magnification ability and resolution of optical microscopes.

So,scientists started developing new means to view the microscopic things round them and, in 1932, came the world's foremost transmission electron microscope (TEM). This device funnels a beam of electrons through a sample under observation and projects the produced image on a fluorescent screen. TEMs, as being called, have a lot in common with SEMs, and it was just only a few years before SEMs were unveiled.

As the days of development of TEMs was well under way,so SEMs came in-view, the latter considered not needed.It took the unquenchable resolution of C.W. Oatley, a professor of engineering at Cambridge University, to proper the recent microscope further.Working together with numerous colleagues and graduate students, Oatley was able to show case both the SEM's magnification ability and the amazing 3-D quality of images it took. Recently,SEMs are constantly employed in jobs such as checking semiconductors for flaws or exploring how insects move.

Color in a Black and White World

Scanning electron microscopes has numerous advantages over optical microscopes, but can not produce color images.Recently,scientists have developed an electron microscope with ability to fish out energy signatures emitted during the magnification.This data lets the microscope to give colors to different elements, such as titanium and manganese, depending on the energy signatures they emit. With this technology, researchers are able to know accurately where one material ends and another starts,all in color.

The Main Parts of a Scanning Electron Microscope

We now have an idea of what SEMs are able to do.Lets have a look at the various parts and how they work in unison to produce an image.The slight variations from one version to another are ending, all SEMs have the same functional basic parts.

Electron gun: Electron guns have been around for sometime.They produce constant streams of electrons needed for SEMs to operate.Electron guns are usually of two types.Thermionic guns are the most common type,employ thermal energy on a filament (typically made of tungsten,with has a high melting point) to force electrons from the gun and to move towards the specimen under observation.Field emission guns,on the other hand, create a strong electrical field to force electrons from the atoms they are located within. Electron guns are usually placed either at the top or the base of an SEM and fire a targeted beam of electrons at the object under observation.These electrons do not normally travel where they need to.

Lenses: Like any optical microscope, SEMs employ lenses to produce highly clear and precision images.The lenses in used these devices,operate differently. For most part,they are not made from glass, the lenses are made from magnets able to bend the path of electrons.By taking this action,the lenses focus and controls the electron beam,making sure the electrons move exactly where they are required to go.

Sample chamber: The sample chamber of an SEM is where all researchers put the sample being examined.Since the specimen must be kept totally still for the microscope to make clear images, the sample chamber must be highly sturdy and protected from vibration.In actual fact,SEMs are highly sensitive to vibrations that they usually installed on the ground floor any building.The sample chambers of an SEM not just to keep a specimen steady.They have ability to re-position the specimen, placing it at different angles and moving it so that researchers need not have to continuously remount the object to obtain different images.

Detectors: You can compare the various type of an SEM's detector to eyes of the microscope.These part figures out the various ways,the electron beam interacts with the sample viewed.For example,Everhart-Thornley detectors locate secondary electrons, which are electrons deflected from the outer surface of a specimen.These detectors are able to produce the highly resolute images of an object's surface. Other detectors, such as backscattered electron detectors and X-ray detectors, can reveal to researchers about the components of a substance.

Vacuum chamber: SEMs need a vacuum to function.Without a vacuum, the electron beam produced by the electron gun will overcome 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.

Just like other machines,an SEM is more than the sum of its parts.

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 underneath the specimen's surface). Dot after dot, row after row, an image of the original object is scanned onto a monitor for viewing (hence the "scanning" part of the machine's name).

This whole process would not be possible if the microscope can not control the travelling of an electron beam. SEMs employ use scanning coils,that 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

For a researcher to take a SEM image of, say, an ant, he has to get the specimen prepared. SEMs, unlike optical microscopes,work in a vacuum and depend on electric fields to operate,sample preparation is a highly complex process. Researchers begin by removing any dirt.After cleaning,it is alright to be mounted in the SEM if the specimen is a little bit conductive.Or else,it is coated with a conductive material such as gold or platinum in a process known as sputter coating before it can be viewed.Sputter coating makes a sample to be grounded,thus preventing it from damage by the electron beam.

Since specimens placed in the microscopes also are placed in a vacuum, they often need to undergo extra preparation to make sure,they bind up under such an extreme condition.Biological samples,are usually dried before they are placed in an SEM. Or else,the lowered atmospheric pressure in a vacuum will make the water in biological samples evaporate easily,destroying the sample in the process.Some other specimens are frozen before they are examined, and others are treated chemically so that they survive the magnification process.

Researchers just like photographers, have ability to control the quality of images they produce. The magnification, focus, contrast and brightness of an image are all within the reach of the person operating an SEM. Certain versions have specific hardware for these settings, the recent introduction of computerized controls has totally reduced the cost of SEMs and demystify their operation.

Some safety precautions need to be taken when using the equipment. During the process of scanning specimens, SEMs usually generate small amounts of radiation in the form of X-rays as electrons below the surface of a specimen are swept away and replaced by other electrons. X-rays are obviously inimical to humans,you should not worry about using a SEM. Nearly all of the equipment have a totally isolated specimen chamber,custom made to screen out electrical and magnetic interference, so all the X-rays produced during the magnification process would not pose a risk to the user. Yet,researchers must ensure to take all safety measures relating to the operation of the SEMs at their workplace.

Why SEM Is All About Wet

SEMs operate in a vacuum,so researchers often think that samples observed using a SEM usually require to be  free from moisture. This requirement prevented them from viewing biological specimens such as living cells. Luckily, the latest versions of SEMs has overcome this drawback. Recently some SEMs need only a moderately strong vacuum for use. These microscopes forgo high resolution in the process,they usually highly versatile for types of the samples they can scan. Some organizations have invented a means of viewing samples in solution. By separating the samples from the vacuum chamber using a very strong film, these SEMs can view things which have not being subjected to higher levels of magnification earlier.