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Design
News - July 2008
TEST
& MEASUREMENT
Selecting vibration-isolation tables
for sensitive apps
As Nanotech
Applications Become More Diverse, the Need For Reliable Vibration
Control Has Become Increasingly Critical
By Jim McMahon
Minus K Technology, Inc., Inglewood, CA
http://www.minusk.com
For almost 40 years, pneumatic vibration isolators have
been the mainstay for stabilizing industrial and academia's
most critical micro-engineering instrumentation. However,
just as technology has been steadily migrating from micro
to nano, so has the need for more precise vibration isolation
in microelectronics fabrication, industrial laser/optical
systems, and biological research.
These so-called "passive system" air tables are
now being seriously challenged by the negative-stiffness vibration
isolators. Able to effectively isolate vibration in diverse
and challenging environments, negative-stiffness isolation
is rapidly gaining popularity in industrial and laboratory
environments.
Air table applications
An isolator is used to solve a problem, and how bad the problem
is determines the solution you need. Since the 1960's air
tables have been used for isolation. Basically cans of air,
they are still the most popular isolators used. But, air tables
with resonant frequencies at 2 to 2.5 Hz can typically only
handle vibrations down to about 8 to 10 Hz, not quite low
enough for optimum performance with modern nano-equipment.
Also, greater isolation efficiencies are needed in the frequency
ranges air isolators can handle.
For purposes of clarity in scanning probe microscopes and
interferometers, air tables are an inefficient isolation solution.
The air systems have been adequate up until a few years ago,
when better isolation was required for finer measurements.Because
of its very high isolation efficiencies, negative-stiffness
vibration isolation systems enable vibration-sensitive instruments
such as scanning probe microscopes, micro-hardness testers,
profilers and scanning electron microscopes to operate in
harsh conditions and severe vibration environments that would
not be practical with top-performance air tables and other
pneumatic isolation systems.
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How negative stiffness
works
Negative-stiffness isolators employ a unique - and completely
mechanical - concept in low-frequency vibration isolation
that was invented by Dr. David L. Platus, the founder
of Minus K Technology. With this technology, vertical-motion
isolation is provided by a stiff spring that supports
a weight load, combined with a negative-stiffness mechanism.
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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 the "beam-column"
effect. (A beam-column behaves as a spring combined with a negative-stiffness
mechanism.) 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 (see Fig. 1).
Negative-stiffness features
Since negative-stiffness isolators are a relatively new technology,
the features they offer can be understood in contrast to the
more familiar air tables. There are several significant areas
in which negative-stiffness systems offer new capabilities:
Low-frequency perturbations
All isolators will amplify at their resonant frequency,
and then they will start isolating. Air tables typically
amplify vibrations in a range of 2 to 7 Hz because of
that is where their natural resonance frequency occurs.
Any vibration in that range could not only fail to be
mitigated, it could be amplified. The low cycle perturbations
would come through to the instrument. Negative-stiffness
isolators resonate at 0.5 Hz, a frequency at which typically
there is almost no energy present to amplify - it would
be very unusual to find significant vibration at 0.5 Hz.
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Fig. 1.
Due to their lower resonant frequency, negative-stiffness
isolators provide better isolation at low frequencies
than air tables do. |
Image clarity
Compared with top-performance air tables, negative-stiffness
vibration isolation can reduce vibration noise levels in atomic-force
microscopes, for example, by a factor of 2 to 3 - particularly
significant in the sub-angstrom range and results in clearer
images and features not discernable with pneumatic isolation
systems.
Severe vibration
As use of nano-equipment becomes more prevalent, lab sites are
being set up in much more severe vibration-prone environments,
such as upper floors of buildings and clean rooms. Such severe
vibration locations are too extreme for current pneumatic isolators
to effectively do their job. Negative-stiffness isolators perform
well in such environments, producing better images and data.
Harsh environments
Air tables are not particularly compatible when it comes to
operating in vacuums, extreme high and low temperatures, and
radiation. Yet these harsh operating environments are often
necessary when conducting research and testing, such as with
cryogenic chambers in semiconductor research.All metal negative-stiffness
systems can be configured that are compatible with high vacuums
and other adverse environments, such as extreme high and low
temperatures, and radiation. With vacuums, for example, negative-stiffness
isolators can be used right inside the vacuum chambers. This
offers other advantages such as much lower payload weights,
more compact systems, and eliminates problems associated with
vacuum chamber feed-through.
Compressed air
Unlike air tables, negative-stiffness isolators do not require
a constant supply of compressed air. Compressed air requires
either a dedicated air line to be plumbed into a lab, a tank
of pressurized gas, or a small compressor.Even if a dedicated
compressed-air line is already available, an air table's location
is still limited by the length of the air line. As for large
tanks of compressed gas, they have to be mounted very securely
to minimize danger, and changing tanks can be difficult and
inconvenient. Compressors, on the other hand, are sources of
both mechanical and acoustic noise, making them very poor choices
from a vibration standpoint.
Equipment location choices
Air tables, especially high-performance units, are bulky structures
that can take up a lot of lab space. This can become a limiting
factor when laying out the equipment in your lab. Negative-stiffness
isolators are available in high-performance bench top configurations,
considerably more compact than air tables and easy to move around.
They are also available as workstations, tables, and floor platforms
where these configurations are required.
Load adjustment
Low-frequency passive vibration isolators are somewhat sensitive
to small changes in weight loads. Pneumatic systems utilize
leveling valves to mitigate the problem.
Negative-stiffness isolators provide a very simple manual adjustment
to accommodate variations in weight loads. For applications
where manual load adjustment is not practical they provide an
auto-adjust system that maintains the isolator in a precise
vertical equilibrium position.
Multiaxes isolation
Consider the case of scanning probe microscopes (SPMs), which
have vibration isolation requirements that are unparalleled
in the metrology world. The vertical axis is the most sensitive
for most SPMs, but they can be quite sensitive to vibrations
in the horizontal axes too.To achieve the lowest possible noise
floor, on the order of an angstrom, isolation is always used.
Benchtop air systems provide limited isolation vertically and
very little isolation horizontally. Negative-stiffness isolators
provide increased isolation performance and have the flexibility
of custom tailoring resonant frequencies vertically and horizontally.
Maintenance and expense
Because negative-stiffness isolators utilize simple elastic
structures and viscoelastic materials that deform, their isolation
performance does not degrade with micromotions typical of laboratory
floors and fabrication rooms, as do conventional pneumatic isolators.
Negative-stiffness isolators are comparably priced to air isolators
or lower for many applications.
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