R&D
Magazine - June 2010
Test, Measurement & Analysis - Tools of the Trade
For handling vibration, negative-stiffness
isolators can offer big improvement over air tables
By Jim McMahon
For almost forty years pneumatic vibration isolators have been
the mainstay for stabilizing industrial and academia’s
most critical micro-engineering instrumentation. But, 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 newer negative-stiffness
vibration isolators. Negative-stiffness isolation is rapidly
gaining popularity in industrial and laboratory environments,
and to no small degree because of its ability to effectively
isolate vibration in diverse and challenging environments.
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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 2-1/2 Hz can typically only handle vibrations
down to about 8 to 10 Hz, not quite low enough for optimum
performance with modern nano-equipment. 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.
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 vibration isolation works
Negative-stiffness isolators employ a unique—and completely
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. 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.
Negative-stiffness versus air isolation
Following are ten key points which demonstrate the benefits
of negative-stiffness isolators compared to air isolation
systems:
#1: Low Hertz Perturbations
An air table will amplify vibrations in a typical range of
2 to 7 Hz, this is because of the natural frequencies where
air tables resonate. All isolators will amplify at their resonant
frequency, and then they will start isolating. With an air
table, any vibration in that range could not only fail to
be mitigated, it could be amplified. The low cycle perturbations
would just come straight through to the instrument.
Negative-stiffness isolators resonate at 0.5 Hz. At this frequency
there is almost no energy present. It would be very unusual
to find a significant vibration at 0.5 Hz.
#2: Image Clarity
Negative-Stiffness vibration isolation can reduce vibration
noise levels in Atomic Force Microscopes, for example, by
a factor of 2 to 3 when compared with top-performance air
tables. This is particularly significant for noise levels
in the sub-Angstrom range. This results in clearer images
and features not discernable with pneumatic isolation systems.
#3: Severe Vibration Environments
As nano-equipment use 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 pneumatic isolators
to effectively do their job. But negative-stiffness isolators
perform well in such environments, producing much better images
and data than can be obtained with even the best high-performance
air tables.
#4: Harsh Environments - Vacuums, High/Low
Temperature Extremes, Radiation
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
which 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.
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#5: Compressed Air
Air tables require a constant supply of compressed air. This
requires either a dedicated compressed air line to be plumbed
in to your lab, a tank of pressurized gas or a small compressor.
Even if you are lucky enough to have a dedicated compressed
air line your table’s location is still limited by the
length of air line you have. Large tanks of compressed gas have
to be mounted very securely to minimize their danger. Changing
the tanks can be quite difficult and inconvenient as well. Compressors
are sources of both mechanical and acoustic noise and are very
poor choices from a vibration standpoint.
If you can get your nano-environment mechanically isolated without
having to deal with compressed air to run your vibration isolator,
then you will be better off. The nice thing about negative-stiffness
isolators is they do not require compressed air. They operate
purely in a mechanical mode. One less thing you have to worry
about when you are setting up your lab and working in it.
#6: Location Selection for Vibration-Sensitive Equipment
Let’s face it, air tables are big, bulky structures, they
take up a lot of lab space. The high-performance air tables
are even bigger. 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.
#7: Load Adjustment
Low-frequency passive vibration isolators are somewhat sensitive
to small changes in weight loads, as well as to large displacements.
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.
#8: Scanning Probe Microscopes
Scanning Probe Microscopes (SPMs) have vibration isolation requirements
that are unparalleled in the metrology world. The vertical axis
is the most sensitive for most SPMs. They can also be quite
sensitive to vibrations in the horizontal axes. In order to
achieve the lowest possible noise floor, on the order of an
Angstrom, isolation is always used.
Bench top air systems provide limited isolation vertically and
very little isolation horizontally. Negative-stiffness isolators
provide increased isolation performance for SPMs over air tables,
while offering better ease-of-use and no facility requirements.
Negative-stiffness isolators have the flexibility of custom
tailoring resonant frequencies vertically and horizontally.
#9: Laser/Optical Equipment
Laser and optical systems, whether used in an academic lab or
in an industrial environment, are very susceptible to vibrations
from the environment. These instruments almost always need vibration
isolation. Traditionally, large air tables have been the isolators
preferred for optical systems, but Negative-stiffness isolators
are becoming a popular choice. Negative-stiffness isolators
provide 10 to100 times the performance of air tables, depending
on the vibration frequency.
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Laser based interferometers are extremely sensitive
devices that are capable of resolving nanometer scale
motions and features. They often have very long mechanical
paths which makes them even more sensitive to vibrations.
The sophisticated modern ellipsometry techniques that
allow this high performance rely on low noise to be
able to detect fringe movement. Properly isolating an
interferometer will allow it to provide the highest
possible resolution.
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Optical profilers have similar sensitivity to vibrations. Optical
component systems are often quite complex. The long optical
paths can lead to angular magnification of vibrations. Optical
air tables can make the problems worse since they have a resonant
frequency that often matches that of floor vibrations. Negative-stiffness
0.5 Hz isolators provide isolation in these environments when
air tables simply cannot.
#10: 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.
Cost-wise, negative-stiffness isolators are comparably priced
to air isolators or lower priced for many applications.
The need for a better vibration isolation solution
The need for vibration isolation will continue to increase in
importance as the precision of research and test applications
embraces smaller and smaller magnitudes of scale.
As industrial researchers and universities continue to broaden
their nano-tech work, necessitating more sensitive equipment
and expanded lab facilities, vibration-handicapped environments
will become more prevalent, and a better vibration isolation
solution will be required than what has been available for the
past almost half-century with air tables. It appears negative-stiffness
vibration isolation will fill that void.
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