Controlled
Environments - January 2008
Vibration Isolation in Cleanrooms:
A System
for Virtually Every Application
By: Hamid Shaidani
Vibration isolation workstations and related equipment are
used in cleanrooms around the world to improve productivity
for research and other high-precision work such as manufacturing
semiconductors and optics. In general, the more precise the
work, the more vibration control is required beneath it. Because
such high-precision equipment must also match the appropriate
class of cleanroom specifications, today's market offers a
sometimes confusing array of vibration isolation products,
many of which did not exist 20 years ago. With so many choices,
it is important to match the equipment to the application.
Vibration isolation workstations for use in cleanrooms are
typically designed to minimize horizontal surfaces and facilitate
wipedown. Completely enclosed isolation modules and vented
exhaust systems are also available to keep these workstations
in compliance with cleanroom standards.
A simple vibration isolation system (Figure 1) can be represented
as a mass, spring, and dashpot. The mass (m) is infinitely
rigid. The spring is weightless, and its stiffness (k) can
be measured in pounds per inch. The damper, or dashpot, is
also weightless, and its damping coefficient (c) is typically
measured in pounds per inch per second.
Most vibration isolation equipment is Class 1000 compatible,
and Class 100 compatibility is also fairly common. Most manufacturers
of this equipment make sure that certain models can be configured
for Class 10 compatibility if the customer requests it. Equipment
with Class 1 compatibility is harder to find across the board,
but at least when you find it you know the pertinent attributes
have not been "tacked on" but rather designed with
only the specialized Class 1 market in mind.
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EVERYTHING VIBRATES
Because of traffic, machinery, HVAC systems, weather, and
natural frequency, everything vibrates. In an average building,
approximately 70-75% of the vibrations are vertical. In laboratories,
vibrations don't have to be obvious for performance to suffer.
Most vibrations are not even noticeable to a building's occupants,
but many cannot be tolerated by equipment used in research,
precision manufacturing, inspection, and quality control.
Figure 2 shows typical building vibrations. Day-to-day problems
may include excessive signal noise, low-frequency jitter,
and high-resolution image blur. At their worst, uncontrolled
vibrations can cause sensitive electromechanical and optical
equipment to undergo excessive wear and even structural damage.
If you think you might need vibration control, you probably
do. Most sensitive work benefits from vibration isolation
equipment, and installation costs are minimal compared with
the costs of discovering later, after your new operation or
facility is up and running, that your product is flawed or
production is stalled due to unwanted vibrations.
The two most important factors to consider in choosing vibration
isolation equipment are natural frequency and isolation efficiency.
The best systems achieve very low natural frequencies and
attenuate all potentially damaging vibration amplitudes in
the 8- to 200-Hz broadband, random-vibration spectrum. However,
there are many variables, so it is best to consult an expert
who can help you find the most appropriate equipment for your
location and requirements. Some manufacturers offer on-site
vibration surveys to ascertain those potentially damaging
vibration amplitudes and frequencies.
Where vibration is a problem, modern vibration-control solutions
are available. These range from relatively simple mounts and
breadboards to highly efficient air systems, active electronic
systems, and negative-stiffness systems constructed with technologies
and materials that take them far beyond conventional vibration
isolators, such as rubber blocks and metal springs. While
blocks and springs may still have their places, the increased
sensitivity of the latest laboratory equipment usually makes
more sophisticated vibration isolation a must.
TYPES OF VIBRATION EQUIPMENT AVAILABLE FOR CLEANROOMS
Breadboards
Many applications do not require a highly efficient vibration
isolation workstation or, for that matter, any true vibration
isolation at all. Certain prototyping, inspection, and assembly
tasks only require reliable flat, stable surfaces. For these
jobs, economical breadboards provide a solution.
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On the market today are breadboards
in larger variety than ever before. Many are constructed
with precision tooling and high-quality materials. For
instance, they may be bonded under pressure with vibration-damping,
hightech structural epoxies. Available in many sizes
and thicknesses with or without mounting holes, some
are general purpose while others stress a particular
feature such as a honeycomb-type core that lightens
the weight of the board without sacrificing rigidity.
Some breadboards offer nonmagnetic surfaces while others
are magnetic. One type has a top skin of stainless steel
and provides high static stiffness. Some can be ordered
with custom cutouts.
Air Systems
Many types of vibration-control equipment make use of
passive or active air systems. In addition to a variety
of workstations and benchtop platforms, these include
modular mounts, platforms, and "islands" (Figure
3) for large equipment.
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The Passive Air Systems
The first technology to advance vibration isolation beyond
springs and blocks was a "passive" air system, which
relies on manually adjustable air bladders ("air springs")
replenished with a hand pump or a tank of compressed air.
For a workstation or benchtop platform, passive air isolators
offer high-frequency vibration isolation for less critical
applications or wherever low-frequency vibrations below 10
Hz are not expected. These systems require manual leveling;
when unwanted vibrations occur, they only damp the vibrations
that are higher than the pre-set system frequency range.
Active Air Systems
"Active" air systems are self-leveling. Connected
to a compressor or other pressurized air source, they have
servo valves that automatically feed or bleed air from each
isolator as needed to maintain the tabletop at a pre-set zero-deflection
level as the load increases, decreases, or is moved. Vibration
isolation efficiencies can approach 99% for vertical and 95%
for horizontal.
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One type of active air system uses frictionless,
rolling-diaphragm air seals (Figure 4) in conjunction
with dual air chambers. With this design, stiffness
is a function of the combined air volume of the dual
chambers. Very low stiffness is needed to obtain the
desired low natural frequency and thus high vertical
isolation efficiency. Some systems even add pistons
that enhance horizontal isolation.
Active air workstations are available in a wide range
of payload capacities and tabletop sizes. Tabletops
are typically constructed of aluminum, steel, or composite
with plastic, anti-static, or stainless-steel laminate.
Corrosion-resistant metals and layered composites can
be combined to provide lightweight, rigid, highly damped
surfaces. Although tabletops need to be supported by
low-frequency support systems, the tops themselves should
have high internal natural frequencies. Tabletops can
be built with honeycomb-type cores of hexagonal cells
for lighter weight without sacrificing rigidity. In
fact, such cores can provide natural frequencies higher
than those attainable with either ribbed cast iron or
a solid granite block of the same size. Thick granite
tabletops are also available.
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Specialized Active Air Equipment
Small, personal workstations provide economical vibration
isolation in small labs and other tight spaces. Table-tops
may be constructed of steel with plastic laminate, anti-static
laminate, or magnetic stainless-steel laminate.
Workstations for Class 1 cleanrooms are often constructed
of welded, stainless-steel, tubular braces and may even use
stainless-steel valves. The overall finish may be electropolish
for Class 1 or clear passivate for Class 10. Tabletops may
be electropolished stainless-steel laminate, white epoxy powder
coat, or another impervious, easily cleaned surface. And so
it goes with the applicable attributes of any other type of
other clean-room-compatible vibration isolation equipment,
including optical tables (and their legstands), breadboards,
benchtop platforms, mounts, etc.
High-performance workstations can be easily customized with
accessories and typically have tabletops made of composite
with plastic laminate.
High-capacity workstations can handle heavy, tall, or moving
loads and can be ordered with a variety of table-tops, including
a granite slab that is 4" thick.
Variable-height workstations reduce user fatigue and back
stress. These are remotely activated with a quiet electrohydraulic
mechanism that allows the user to smoothly raise or lower
the tabletop in order to maintain a posture that is ergonomically
correct - a major benefit when accommodating different operators
on different shifts.
Active air benchtop platforms are available with a wide range
of efficiencies and a variety of platform surfaces (stainless
steel, plastic laminate, etc.), with or without mounting holes.
Some models offer horizontal as well as vertical vibration
control.
Active air workstations provide stability for balances; confocal,
tunneling, atomic-force, or optical microscopes; roundness
checkers; surface profilometers; and other equipment used
in tasks, such as cell injection, patch clamping, testing,
inspections, wafer probing, and mask aligning in industries,
such as medical research, semiconductor processing, telecommunications,
and aerospace engineering.
Active air benchtop platforms enhance the performance of certain
microscopes, microhardness testers, pro-filometers, balances,
audio components, and other devices for which vibrations can
be adequately controlled without a permanent table dedicated
to that function.
Also available are mounts and platforms for large equipment
weighing up to 20,000 pounds. These make use of passive and/or
active air technology, and the ones with active air provide
automatic leveling. For some large equipment, a raised-floor
"island" - an alternative to corner lift brackets
and poured-in-place isolation pads -can be built to provide
vibration isolation for a section of a room.
Active Electronic Equipment
Active electronic equipment provides vibration isolation different
from what is possible with air systems. Active electronic
systems have microprocessors that use "active feedback"
to sense vibrations and provide immediate responses to eliminate
them. Rapidly and automatically adjusting to different loads,
this type of system dynamically isolates all six translational
and rotational modes of vibration.
Active electronic benchtop platforms provide a portable vibration
isolation solution. Their tops can be ordered with or without
mounting holes and can be constructed of aluminum plate, ferromagnetic
stainless steel, plastic laminate, or anti-static laminate.
Up to 500 times stiffer than an air table, an active electronic
workstation can easily accommodate a high-center-of-gravity
or moving load. The best ones have ultra-low natural frequencies
(< 1 Hz) and produce no low-frequency resonance. Available
in a variety of load capacities and tabletops, they can actively
damp vibrations from 1.0 to 1000 Hz, whether the vibrations
originate from the building or from a piece of equipment on
the platform. Isolation efficiency is especially efficient
over 2 Hz, typically reaching 99% (40 dB) at 10 Hz. For frequencies
above 1000 Hz, these workstations provide passive vibration
control. For monitoring purposes, a multiplexed signal indicating
vibration levels with and without isolation can be displayed
on an oscilloscope.
Applications for active electronic benchtop platforms include
the support of interferometers, SPM, and imaging microscopes.
Active electronic workstations are for check-weighers, atomic
force microscopes, micro-hard ness testers, profilometers,
and other sensitive equipment
Negative-stiffness Equipment
Another type of vibration isolation workstation makes use
of a stiff spring and a "negative-stiffness" mechanism
to provide ultra-low natural frequencies, high internal structural
frequencies, and excellent vertical and horizontal isolation
efficiencies for static loads. Horizontal isolation is provided
by beam columns connected series with a vertical-motion isolator.
For supporting microscopes in semiconductor processing, aerospace
engineering, medical research, and other fields, workstations
can achieve superb vibration isolation: isolation efficiency
at 2 Hz, 99% at 5 Hz, and 99.7% 10 Hz.
Optical Tables
In the field of optics, as in other sciences, the increased
demand for precision has led to optical equipment that is
ultra-sensitive to vibrations. These devices require high
isolation capabilities at very low frequencies (2 and lower),
with quick response to micro disturbances This performance
can be achieved by vibration isolation support systems that
incorporate a combination mechanical passive and active isolation
concepts. A high quality optical table will offer quad-tuned
as well broadband vibration damping using individual absorbers
(tuned to the two lowest natural frequencies, bending and
torsion, of each table) embedded in all four corners of the
table. This is narrow-band selective damping, provided by
four frequency-tuned, mass-spring resonators dry-damped and
tuned to resonate 180° out of phase with the lowest natural
frequency of the table. The out-of-phase inertial forces induced
in each resonator mass act to cancel or absorb the motion
of the table at its natural frequency and lower the resonant
amplification.
CONCLUSION
Vibration isolation equipment performs at different efficiencies
depending on payload. For the best performance, the weight
of your typical load should be no more than 80% of the equipment's
rated load capacity. For a workstation or support system,
look for a manufacturer that offers equipment available in
more than one load range. Some manufacturers also offer modular
support systems that allow the retrofitting of workstations
or optical tables to increase or decrease load capacity in
the quest for greater accuracy.
Accuracy concerns are also related to ergonomics. There is
no point in eliminating blur and jitter from microscope work,
for example, if the operator cannot stay comfortable and alert.
Good design and appropriate construction materials can make
vibration isolation equipment user-friendly.
Many accessories are available to add convenience to workstations
or optical tables, including shelves, drawers, laser ports,
electrical outlets, monitor supports, lighting, guard rails,
padded armrests, and retractable casters. Faraday cages protect
sensitive operations from electromagnetic interference, while
other tabletop enclosures are available for creating a draft-free,
environmentally controlled atmosphere to protect against harsh
manufacturing environments, etc. A typical aluminum-frame
enclosure may have acrylic walls and a rigid composite roof
onto which a HEPA filter blower can be mounted.
Vibration-control scientists continue to integrate new ideas
into their products, but experience teaches that tomorrow's
cleanrooms are likely to pose new vibration isolation challenges.
Obviously, the right vibration isolation equipment for supporting
a particular device or operation depends not only on the performance
level and surface space required but also on the class of
the cleanroom in which it will be used. Ongoing research holds
the promise that we will be able to meet new challenges as
they arise.
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