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process. “For an SEM to work it has to
be in a vacuum,” said Phenom-World’s
Schipperen. “Every time you load and
unload samples, you have to evacuate, or
pump-down, the vacuum chamber, which
can take 5 or 6 minutes.”
A tabletop SEM vacuum system typically
provides rapid pump-down, from 20
seconds to 1 minute, allowing analysis to
begin sooner. “Some of this is predicated
on tabletop units having smaller chambers,”
said Greg Ott, president of ASPEX
Corp., Delmont, Pa., manufacturer of the
PSEM eXpress tabletop unit.
Phenom’s unit incorporates a sample
cup that attaches to the machine instead
of using a stage in the vacuum chamber.
Only the cup has to be evacuated—not
the entire chamber. Pump-down time is
less than 30 seconds.
However, using the sample cup instead
of a stage limits the sample size.
“That is the tradeoff with the Phenom,”
said Schipperen. “The maximum sample
size is 1" in diameter. That is still pretty
big for a sample size, though. If you are
going to enlarge something up 10,000×,
you don’t need a lot of it.” The manufacturer
does offer larger, customized sample
containers.
The maximum sample size for most
tabletop units is around 70mm in diameter
× 50mm high.
Also making things faster and easier,
tabletop SEMs eliminate the range of accelerating
voltages from which the user
must choose. Conventional SEMs feature
accelerating voltages ranging from
approximately 0.1kV to 30kV. The higher
the voltage, the deeper the penetration of
the beam into the sample. However, the
higher voltage can damage some beamsensitive
samples and decrease the image
surface detail. Higher accelerating voltages
are also required for energy-dispersive
X-ray analysis systems, used for
elemental identification.
Most tabletop models offer users just
a few accelerating voltages, such as 5kV,
10kV, 15kV and 20 kV. “The lower voltages
give you better surface imaging,”
said Robert J. Gordon, vice president of
the Nanotechnology and Life Science
Division of Hitachi High Technologies
The tabletop SEM bridges the gap between optical
microscopy and full-blown electron microscopy.
America Inc., the Pleasanton, Calif., manufacturer
of the TM-1000 and TM-3000
tabletop SEMs. “At lower voltages, the
beam is not as energetic and you can use
beam-sensitive materials.”
Another feature that makes tabletop
SEMs easy to use is they are not
as affected by noise and vibration as
traditional SEMs. Imaging at high magnification
in a conventional SEM requires
an environment free from vibration, ambient
noise and electromagnetic interference.
“Because tabletop SEMs have lower
magnifications, vibration is not always a
major factor,” said Wolfe. Most of the tabletop
units feature vibration-dampening
systems as well.
One dramatic difference between conventional
and tabletop SEMs is cost. Traditional
SEMs start around $120,000 and
go up to $700,000. Tabletop models start
Nikon Instruments
Nikon’s NeoScope
JCM-5000 can
inspect electronic
components, such as
these printed circuit
board connections.
around $65,000. Leasing is an option, but
most customers purchase them outright.
Another difference is maintenance. “I
used to shudder when I heard there was
a problem with our [conventional] SEM,”
said Courtney Martin, manufacturing
engineer at Adaptive Materials Inc., an
Ann Arbor, Mich.-based fuel cell manufacturer
that uses the Hitachi TM-1000.
“Conventional SEMs require constant
care and preventative maintenance for
their multiple parts, and ample training
to get great images.”
Maintenance issues are typically minimal
with tabletop SEMs. One consumable
is the filament, the source for
electron generation. Most tabletop SEMs
feature a tungsten filament, which is simple
for the user to replace. However, they
typically have just a 200-hour life.
The Phenom unit’s lanthanum-hexaboride
source provides 1,500 hours of operation.
“It lasts longer and gives a brighter
image than the tungsten, but is quite a
bit more expensive,” said Schipperen. The
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