SEM
Scanning
Electron Microscopy
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.
Operating Principles
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.
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.
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.
Limitations of the SEM
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.
Standardized
Methods
The standard practice
for the performance of a scanning electron microscope is covered in
ASTM E 968, "Scanning Electron Microscope Beam Size Characterization."
