February, 2017
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Page 45 Precision Vibration Isolation in the Cleanroom By Jim McMahon F
or decades, pneumatic air tables have been the workhorse for reducing vibrations in cleanrooms for manufacturing and research,
where critical micro-engineering instrumentation is employed. But just as technology has steadily pushed the boundaries into nano-applications in microelectronics fabrication, industrial laser/opti- cal systems and biological research, so has the need increased for improved precision in vibration isolation. Increasingly, pneumatic air tables are taking
a back seat to the more recent technology of nega- tive-stiffness vibration isolation, which over the past 20 years since its introduction, has proven it- self in thousands of applications throughout indus- try, government and academia, including some of the most diverse and challenging environments, such as cleanrooms.
Vibration Sources Vibration can be caused by a
multitude of factors. Every structure is transmitting noise. Within a build- ing itself, the heating and ventilation system, fans, pumps and elevators are just some of the mechanical de- vices that create vibration. Depend- ing on how far away the cleanroom equipment is from these vibration sources, and where in the structure the equipment is located, whether on the third floor or in the basement, for example, will determine how strongly the equipment will be influenced. Ex- ternal to the building, the equipment can be influenced by vibrations from adjacent road traffic, nearby con- struction, loud noise from aircraft, and even wind and other weather conditions that can cause movement of the structure.
Sensitive equipment in cleanrooms can be
influenced by vibrations
from road traffic, nearby construction, aircraft
noise, and other factors that cause structural movement.
Vibrations in the range of 2 to
20,000 Hz will influence sensitive equipment. But these internal and external influences primarily cause lower frequency vibrations which are transmitted through the structure, creating strong disturbances in the precision equipment used in clean- rooms.
Vibration Isolation Equipment Used extensively in semiconduc-
tor manufacturing, biotechnology, the life sciences, and other fields that are very sensitive to environmental contaminants — such as dust, air- borne microbes, aerosol particles, and chemical vapors — cleanrooms pro- vide an enclosed environment with a controlled level of contamination that is specified by the number of particles per cubic meter at a certain particle size. The air entering a cleanroom from outside is filtered to exclude dust, and the air inside is constantly recirculated through high-efficiency particulate air (HEPA) and/or ultra- low particulate air (ULPA) filters to remove internally generated contam- inants. Equipment employed inside the cleanroom must be designed to generate minimal air contamination. This includes vibration isolation equipment, which can range from rel- atively simple rubber blocks, metal springs and breadboards, to highly
efficient air systems, active electronic systems, and negative-stiffness systems — constructed with more advanced technologies and materials for
higher precision vibration isolation. Vibration isolation tables and workstations
are required to meet the same cleanroom and con- tamination standards as the components that they protect. Completely enclosed isolation modules and vented exhaust systems are also available to keep these workstations in compliance with clean- room standards. All surfaces of the isolation table should be
Minus K benchtop negative-stiffness vibration isolator.
accessible for cleaning, and constructed so that they can be easily wiped to maintain cleanliness. Isolator diaphragms — for compressed air tubing and electrical supply — should meet the specified outgassing, nonvolatile residue and total mass loss specifications to keep molecular contamination to an absolute minimum.
Continued on next page
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