HOW THE SCANNING ELECTRON MICROSCOPE AND X-RAY ANALYZER WORK
Electrons are
much smaller than photons (the subatomic particles of light) and can be easily
manipulated, which is why the SEM uses electrons to imag
e
structures too small to be seen with an ordinary light microscope. A powerful
instrument, the SEM is the only type of advanced microscope that creates images
of ultratiny things in a way that that doesn't distort or destroy them in
the process. Unlike the more abstract information revealed by other types
of specialized microscopes, the forms and structures
in these pictures look natural, presenting the same three dimensional appearance
they would have if they were big enough to see with our eyes.
To create the
images, a filament inside an electron "gun" shoots a stream of electrons down
through a stack of electromagnetic lenses tha
t
focus them into a coherent beam. Landing as a fine point on the specimen below,
the beam is continually scanned across a chosen portion of the sample, which
responds by emitting electrons from its own surface that are collected by
a detector inside the sample chamber. This begins an electronic process that
results in a simultaneous recreation of the sample's surface on a viewing
screen. A larger area of the sample can be scanned to produce an overall view,
w
hile zooming
in on a small spot enlarges it on the screen to show details at higher magnification.
Graininess in the micrographs can be due to several causes; among them are
imaging in the higher magnification ranges and varying the current in atypical
ways.
The stage holding
the sample can be manipulated from the outside in order to move, tilt and
rotate the specimen so that it can be oriented in nearly any direction while
its image is being viewed on the screen. With the twist of a dial (in the
case of the SEM that took these pictures) or the wiggle of a mouse (in newer
models), the specimen's contours can be explored in any magnification ranging
from a low of about eight times actual size up to the hundreds of thousands
(Lamont's scope) or, lately, as much as a million times. Watching the image
on the screen is like looking out of a window of a plane - the landscape "below"
continually changing as your "plane" flies above or around it, zooming in
and out or hovering for a photo. Different types of images can be created
by different detectors optimized to "see" the several subatomic events that
take place simultaneously during the beam's impa
ct
on the sample. The one that recreates the three dimensional view of the sample
is called "secondary" imaging; this is the type of image shown in the gallery
section. Two shots of a feature, taken from two different angles, can be merged
to create a true stereo 3D effect.
The other
detectors inside the sample chamber can yield images evoking the specimen's
chemistry: a "cathodoluminescence" image, for instance, shows minerals
that fluoresce in visible light when bombarded by an electron beam; a
"backscattered" electron image is a picture in which differing grey levels
correspond to areas of different chemical composition. Several other research
instruments can be attached to the SEM to extend the range of information
a specimen can yield. One of these is called an energy-dispersive X-ray
analyzer (EDX, or EDS), which produces a spectrum of the elements present
in targeted areas of the sample that have been revealed in a backscatter
or cathodoluminescence image. These areas can then be chemically "mapped"
with different colors representing the distribution of different elements
or compounds in the sample. Using other programs, features can be analyzed
for size, shap
e,
and other physical characteristics; they can also be quantified in combination
with their chemistry.
Lamont's SEM is a manually-operated film-based instrument, but the modern models are software-controlled. Some of these newer machines still operate only in conventional high vacuum mode (for which the specimens must be dry and electrically conductive) but other varieties can vary the pressure in the sample chamber or even introduce water vapor, which not only makes hydrated samples like medical tissues available for imaging in their natural state, but introduces the dimension of time. Now, for example, crystals can be seen dissolving and resolidifying. Hot or cold sample stages can expand the SEM's capabilities even further. It seems that every month a new type of instrument is developed that can be added to an SEM to increase its capabilities.
Dee
Breger
Mgr. SEM/EDX Facility
Lamont-Doherty Earth Observatory
61 Route 9W Palisades, NY 10964 USA
T: 914/365-8640 F: 914/365-8155
micro@ldeo.columbia.edu
