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Facilities
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Scanning Electron Microscope (SEM)
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Scanning electron
microscopes have been commercially available since the late 1960's. The
instrument has evolved from a highly specialized research laboratory instrument
into an accepted part of analytical laboratories and production facilities. With
the capacity of magnifying features from 10 to 100,000X, it can be found in the
businesses of semiconductor and nylon fiber quality assurance, pollution
particle characterization, and equipment failure analysis. The SEM also serves
as a platform for micro-analytical techniques, such as Energy Dispersive X-ray
Spectroscopy (EDS). In combination with this technique, the SEM becomes a
powerful tool for ferreting out and characterizing evidence for root cause
failure analysis. Both mechanical and corrosion related failures often have
features that can be best discerned at high magnification and identified by
x-ray microanalysis. |
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Operating Principles
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In the SEM, a very finely
focuses beam of electrons is scanned over the surface of the specimen. As the
electron beam scans the specimen, it not only provides topographical
information, but also, as it penetrates the surface, interacts with the sample
to cause effects such as electron backscattering, x-ray emission, secondary
electron emission, and cathode luminescence. |
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| Cracking on surface |
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The imaging process of the
SEM takes place when a cathode ray tube (CRT) is scanned simultaneously with the
electron beam. The most common method of imaging utilizes secondary electrons.
As the electron beam is scanned over the specimen, surface features in line of
sight of the secondary electron detector will generate proportionally more
electrons. The detector generates a signal that is proportional to the number of
the electrons received as various surface features come under the electron beam.
The intensity of the CRT beam is modulated proportionally to represent the
magnitude of the signal arriving from the secondary electron detector. A picture
is built up that represents the surface topographical features and are discerned
by the effect that, for a "hill" on the surface, the side facing the detector
will generate more secondary electrons that are likely to arrive at the
detector, and consequently, that side will appear brighter than the side that is
not in the line of sight of the detector.
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Another commonly used
image is produced by the detection of backscattered electrons. The intensity of
these electrons is influenced by differences in atomic number. Microstructural
phase with different average atomic numbers and grain boundaries in unetched
specimens can be imaged with these detectors. |
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Limitations of the SEM
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Feature resolution is limited to 70 to 100
Angstroms for most microscopes. Specimens must be resistant to vacuum;
liquids with vapor pressures less than 10-3 torr cannot be analyzed. Since
the scattering takes place in vacuum, the maximum size of the specimens to
be examined is limited to the size of the chamber at the base of the
microscope. Consequently, typical specimen dimensions are limited to less
than 3 cm on a side. |
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Standardized Methods
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The
standard practice for the performance of a scanning electron microscope is
covered in ASTM E 968, "Scanning Electron Microscope Beam Size
Characterization."
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