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Laboratory
Product News - February 2007
Technical
Applications
New solutions needed for vibration control
in nanotechnology
By Jim McMahon
It wasn't long ago that making the decision where to locate
a scanning probe microscope was straightforward. Most labs put
it in the basement where ambient vibration was minimized. But
today, with nanotechnology applications growing exponentially,
scientists and engineers often need to put these sensitive instruments
in other locations, where vibration noise is significantly higher.
Scanning probe microscopes, interferometers and stylus profilers
are sometimes being sited in locations that pose a serious challenge
to vibration isolation.
"Vibration isolators are one of those necessities that
people are not really focused on when they go to purchase
an instrument like an AFM (atomic force microscope),"
says George McMurtry, CEO of NanoAndMore, a distributor of
AFM probes and nanotechnology peripherals.
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"But it is different with the bigger
scanning electron microscopes and transmission electron
microscopes, because you are dealing with a very expensive
piece of gear that technically needs all sorts of
isolation in order to work properly. They are more
apt to talk about it right up front.
"But with smaller instruments such as white light
interferometers, laser interferometers, stylus profilers,
and atomic force microscopes, the situation is different,
he says, and problems with site preparation are more
frequent. If, for example, the lab is on the 4th floor
of a building, without isolation, the users will end
up getting very poor images.
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"There are so many more people using AFMs in so many
different environments that isolators are needed more often,"
says Mark Flowers, president of Nanoscience Instruments, a
distributor of atomic force microscopy products. "In
the early days you could put your AFM in the basement of your
building, now people want to use their AFMs on the 3rd floor.
But in the basement you are going to have a much better environment,
and you could get by with an unsophisticated isolator."
The vibrations are subtle. Within the building itself many
things cause vibration, such as the heating and ventilation
system, fans, pumps that are not properly isolated, and elevators.
Outside the building, vibrations from adjacent traffic, wind,
construction, and other elements can also affect equipment.
The lower-frequency vibrations resulting from these forces
cause havoc for instruments trying to measure a very few angstroms
or nanometers of displacement.
Air tables with resonant frequencies at 2 to 2-1/2 Hz can
typically only handle vibrations down to about 8 to 10 Hz,
not quite low enough for optimum performance with modern nano-equipment.
Therefore, for purposes of clarity in scanning probe microscopes
and interferometers, air tables are an inefficient isolation
solution.
Another solution is active isolation, also known as electronic
force cancellation. Active isolation senses motion electronically,
and adds equal amounts of motion to compensate, effectively
cancelling out the motion. Isolation begins as low as 0.7
Hz, which is sufficient for isolating the lower frequencies
that are so damaging to image clarity with SPMs and interferometers.
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However, due to their dependence on
electricity, these isolators are vulnerable to electronic
dysfunction and power modulations, which can interrupt
scanning.
Another solution is a negative-stiffness
vibration isolation system, which has the added benefit
of lower operating costs than active systems.
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"This is a passive approach for achieving low vibration
environments and isolation against sub-Hertz vibrations,"
says Dr David Platus, inventor of the negative-stiffness technology,
and president and founder of Minus K Technology. "These
isolation systems enable vibration-sensitive instruments,
such as scanning probe microscopes, micro-hardness testers
and scanning electron microscopes to operate in severe vibration
environments, such as upper floors of buildings and clean
rooms. The images and data produced are many times better
than those achievable with pneumatic isolators."
Negative-stiffness isolators employ a mechanical concept
in low-frequency vibration isolation. Vertical-motion isolation
is provided by a stiff spring that supports a weight load,
combined with a negative-stiffness mechanism (NSM). The net
vertical stiffness is made very low without affecting the
static load-supporting capability of the spring. Beam-columns
connected in series with the vertical-motion isolator provide
horizontal-motion isolation. The horizontal stiffness of the
beam-columns is reduced by a beam-column effect. (A beam-column
behaves as a spring combined with an NSM.)
The result is a compact passive isolator capable of very
low vertical and horizontal natural frequencies and very high
internal structural frequencies. The isolators (adjusted to
1/2 Hz) achieve 93% isolation efficiency at 2 Hz; 99% at 5
Hz; and 99.7% at 10 Hz.
"Improved vibration isolation directly correlates to
improved instrument performance," says Patrick O'Hara,
president and CEO of Ambios Technology, a manufacturer of
SPMs, stylus profilers and optical interferometers used in
nanotechnology. "When you are trying to measure atomic
scale features, mechanically stable support structures are
critically important. Up until the advent of probe microscopes,
and some of the other very high-resolution imaging and data
acquisition techniques, air isolators were adequate for most
of applications. But not any longer."
He adds that although active isolation systems have fundamentally
no resonance, their transmissibility does not roll off as
fast as
negative-stiffness isolators. As a result, at building and
seismic frequencies, the transmissibility of active isolators
can be 10X greater than negative-stiffness isolators. This
causes substantial adverse measurement and imaging artifacts
in the data.
Further, he says that air isolators have the added disadvantage
that their 2 to 2-1/2 Hz resonance effects a significant loss
in isolation capability below about 5 Hz, and he feels that
negative-stiffness isolators are the most efficient choice
for probe microscopes.
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