Equipment - July 2010
Environmental & Field Testing
Vibration Isolation Faceoff
The use of air
tables for vibration isolation is now being challenged by
compact negative-stiffness vibration isolators.
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 migrating from micro to nano, so has the
need for more precise vibration isolation in microelectronics
fabrication, industrial laser/optical systems and biological
These so-called "passive
system" air tables are now being challenged by
the newer negative-stiffness vibration isolators because
of their ability to effectively isolate vibration in
diverse and challenging environments.
Air tables have been used since the 1960s. Basically
cans of air, they are Still the most popular isolators
used; but, with resonant frequencies at 2 to 2.5 Hz,
they can only handle vibrations down to about 8 to 10
Hz -not quite low enough for optimum performance with
modern nano-equipment. systems.
Figure 1. Negative-stiffness vibration
Also, greater isolation efficiencies are needed in the frequency
ranges air isolators can handle. Negative-stiffness vibration
isolation systems, on the other hand, enable vibration-sensitive
instruments - 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
How it works
Negative-stiffness isolators employ a unique, completely mechanical
concept in low-frequency vibration isolation. A stiff spring
provides vertical-motion isolation and 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 a series with the
vertical-motion isolator provide horizontal-motion isolation.
The "beam-column" effect reduces the horizontal
stiffness of the beam-columns, which behave as springs combined
with a negative-stiffness mechanism.
The resulting compact, passive isolator is
capable of low vertical and horizontal natural frequencies
and high internal structural frequencies. The isolators (adjusted
to 0.5 Hz) achieve 93% isolation efficiency at 2 Hz, 99% at
5 Hz, and 99.7% at 10 Hz.
The following ten key points demonstrate the benefits of negative-stiffness
isolators compared to air isolation systems:
1. Low hertz perturbations: An air
table amplifies vibrations in a typical range of 2 to 7 Hz
because of the natural frequencies at which air tables resonate.
All isolators amplify at their resonant frequency and then
start isolating. Thus, with an air table, any vibration in
that range not only fails to be mitigated but also amplified.
The low cycle perturbations come straight through to the instrument.
Negative-stiffness isolators resonate at 0.5
H7 where almost no energy is present.
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 and
results in clearer images and features not discernible with
pneumatic isolation systems.
3. Severe vibration
environments: As nano-equipment use becomes more
prevalent, labs are being set up in more severe vibration-prone
environments, such as upper floors of buildings and
cleanrooms. Such locations are too extreme for pneumatic
isolators to do their job effectively. Negative-stiffness
isolators, however, perform well in such environments,
producing better images and data.
4. Harsh environments:
Air tables are not particularly compatible when
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.
|Figure 2. Transmissibility
- Negative-stiffness isolators versus air tables.
All-metal negative-stiffness systems 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, offering other advantages such as lower payload
weights and more compact systems. It also eliminates problems
associated with vacuum chamber feed-through.
5. Compressed air: Air tables require
a constant supply of compressed air from a dedicated compressed
air line plumbed in to the lab, a tank of pressurized gas,
or a small compressor.
Even if a lab has a dedicated compressed air
line, the length of the line limits the table's location.
Also, large tanks of compressed gas must be mounted securely
to minimize their danger, and changing them can be difficult
and inconvenient. As for compressors, they are sources of
both mechanical and acoustic noise and are poor choices from
a vibration standpoint.
Negative-stiffness isolators do not require
compressed air. They operate in a mechanical mode, eliminating
one more factor scientists have to worry about when setting
up and working in the lab.
6. Location selection: Bulky air tables
take up a lot of space, and high-performance air tables are
even bigger. This can become limiting when laying out equipment
in the lab.
Compact and easy to move around, negative-stiffness
isolators are offered in high-performance benchtop configurations.
They are also available as workstations, tables and floor
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 simple manual adjustment to accommodate variations
in weight loads. For applications where manual adjustment
is not practical, they provide an auto-adjust system
that maintains the isolator in a precise vertical
8. Scanning probe microscopes:
SPMs have vibration isolation requirements that are
unparalleled in the metrology world. The vertical
axis is the most sensitive for nearly all SPMs, and
they can be 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
Figure 3. Schematic of negative-stiffness
Benchtop air systems provide limited isolation vertically
and little isolation horizontally. Negative-stiffness isolators,
however, offer 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 industrial environment,
are susceptible to vibrations from the environment and thus
usually require vibration isolation. Negative-stiffness isolators
provide 10 to l00x the performance of air tables, depending
on the vibration frequency.
Laser-based interferometers are capable of
resolving nanometer-scale motions and features, and they often
have long mechanical paths that make them extremely sensitive
to vibrations. The sophisticated, modern ellipsometry techniques
that allow this high performance rely on low noise to detect
fringe movement. Properly isolating an interferometer allows
it to provide the highest possible resolution.
Optical profilers have similar sensitivity
to vibrations. Optical component systems are often quite complex,
and the long optical paths can lead to angular magnification
of vibrations. Optical air tables can make the problem worse
because their resonant frequency often matches that of floor
vibrations. Negative-stiffness 0.5-Hz isolators provide isolation
in these environments where air tables simply cannot.
10. Maintenance and expense: Negative-stiffness
isolators utilize simple elastic structures and viscoelastic
materials that deform. Thus, their isolation performance does
not degrade with the micromotions typical of laboratory floors
and fabrication rooms, as do conventional pneumatic isolators.
Regarding cost, negative-stiffness isolators
are comparably priced to air isolators or lower priced for
For more information, contact
Steve Varma, Minus K Technology, Inc., at 310-348-9656 or
firstname.lastname@example.org. Minus K Technology, Inc. develops and manufactures
advanced vibration isolation products based on the company's
patented negative-stiffness-mechanism technology.
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