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SCANNING ELECTRON MICROSCOPE – ENERGEY DISPERSIVE X-RAY
Electron Microscopy has many varied applications in life
sciences, earth sciences and in electrical and mechanical
engineering, using both transmission and scanning electron
microscope.
Electron microscope can be used for imaging, and with the
addition of an X-ray detector, acquisiton of quantitative
elemental analysis data is possible. With the rapid evolution
of computer based digital image processing, a wide range of
image analysis and enhancement techniques is available to the
microscopist.
Although both the SEM and TEM (Transmission Electron
Microscope) utilise a focused beam of high- energy electrons to
produce images, the method of image formation, and the final
images are very different. Whereas the TEM produces an image by
passing the electron beam through the specimen, the SEM provides
an image of the specimen surface. Consequently specimen
preparation is vey different but specimens for both SEM and TEM
must be dry, able to withstand the high vacuum, and remain
stable in the electron beam.
Specimen Preparation
Scanning electron microscopes produce images of specimen
surfaces, so thickness is unimportant. The sample size is only
limited by the size of the microscope specimen chamber. The
sample need to be free from any water or volatile component and
it must also be electrically conductive. However, new
developments in SEM design are the LOW VACUUM and ENVIRONMENTAL
SEM which maintain the specimen chamber at low vacuum, enabling
hydrated, uncoated samples to be imaged.
Parts of an SEM
The control console : contains circuitry to maintain stable
lens currents, evacuate the specimen chamber, produce a beam of
electrons, and so on.
The display output : produces the image in the form of a
television type picture on a cathode ray tube (CRT)
The column : contains the specimen and the means of
observing it.
Scanning electron microscopes produce images of specimen
surfaces, so thickness is unimportant. The sample size is only
limited by the size of the microscope specimen chamber. The
sample need to be free from any water or volatile component and
it must also be electrically conductive. However, new
developments in SEM design are the LOW VACUUM and ENVIRONMENTAL
SEM which maintain the specimen chamber at low vacuum, enabling
hydrated, uncoated samples to be imaged.
When a beam of electrons impinges on the sample, it knocks
other electrons loose from the object itself. These secondary
electrons scatter in directions determine by the angle of
incidence of the beam and the surface topography of the sample.
The SEM image display
The CRT beam is not static. It sweeps back and forth across
the face of the tube (exactly as in a TV set, which is what a
display CRT actually is) and its sweep is synchronized with that
of the electron beam in the column. When the column beam knocks
off large numbers of secondary electrons from some point on the
specimen, the CRT beam produces a bright spot on the viewing
screen. Conversely, small numbers of electrons produce low CRT
beam output, and a dark point on the viewing screen. Thus the
SEM image is really an analog of what is going on inside the
column; it's constructed, point-by-point and line-by-line, based
on the information supplied to the CRT by the detector
circuitry.
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