Precision Vibration Isolation in the Cleanroom

Pneumatic air tables have historically been the dependable option for decreasing vibrations in cleanrooms for research and manufacturing, where important micro-engineering instrumentation is used.

The demand has increased for more precision in vibration isolation, just as technology has slowly pushed the boundaries into nano applications in microelectronics fabrication, industrial laser or optical systems, and biological research.

Pneumatic air tables are increasingly taking a back seat to the new technology of negative-stiffness vibration isolation.

Since its introduction over 20 years ago, it has assisted in thousands of applications in a range of government, industry, and academic fields, including some of the most flexible and complicated environments, for example, cleanrooms.

Vibration Sources

Vibration can be a result of several factors. All structures generate noise. The heating and ventilation system, pumps, elevators, and fans within a building are just a few of the mechanical devices that generate vibration.

Considering how far away the cleanroom equipment is located from these vibration sources, and where the equipment is located in the structure, such as in the basement or on the third floor, will establish the strength of its influence on the equipment.

Factors that are external to the building but can influence the equipment are vibrations from nearby road traffic, construction occurring close to the building, loud noise from aircraft, and even wind and alternative weather conditions that can result in movement of the structure.

Vibrations ranging from 2 to 20000 Hz will impact sensitive equipment. Lower frequency vibrations that are transmitted through the structure are mostly caused by internal and external factors that create strong disturbances in the precision equipment utilized in cleanrooms.

Vibration Isolation Equipment

Cleanrooms are used widely in biotechnology, life sciences, semiconductor manufacturing, and different fields that are highly sensitive to environmental contaminants such as chemical vapors, aerosol particles, airborne microbes, and dust.

They provide a limited environment with a managed contamination level that is determined by the number of particles per cubic meter at specific particle sizes.

The outside air entering a cleanroom is filtered to take the dust away, and the air inside is always recirculated through high-efficiency particulate air (HEPA) and/or ultra-low particulate air (ULPA) filters to eliminate internally generated contaminants.

Laser Interferometer Vibration Isolation

Equipment used inside the cleanroom must be created to produce the least air contamination.

This entails vibration isolation equipment, which begins with comparably simple metal springs, breadboards, and rubber blocks, to highly effective air systems, active electronic systems, and negative-stiffness systems, all created with increasingly advanced materials and technologies for a greater degree of precision in vibration isolation.

Vibration isolation tables and workstations are necessary to meet the same contamination and cleanroom standards as the components they protect from vibration.

It is possible to acquire completely enclosed isolation modules and vented exhaust systems to keep these workstations in adherence to cleanroom standards.

All surfaces of the isolation table should be designed so that cleaning and wiping are easily achieved to ensure cleanliness.

Isolator diaphragms should meet total mass loss, compressed outgassing, and non-volatile residue specifications to keep molecular contamination to a complete minimum.

Negative-Stiffness Versus Pneumatic Isolation

Since the 1960s, air tables have been employed for vibration isolation, and have the largest installed base within cleanrooms.

More precise vibration isolation technology is required to resolve low-hertz vibrations, which negatively impact results despite the use of air tables. This is particularly important with the increase in the sensitivity of instrumentation, especially at the subatomic level.  

Negative-stiffness vibration isolation was introduced over 20 years ago by Minus K Technology and is purpose-built to isolate these low-frequency perturbations.

The following are essential comparisons between negative-stiffness isolators and air tables which should be assessed when considering vibration isolation for cleanroom applications:

Vertical and Horizontal Isolation

Air tables achieve isolation, but mainly in the vertical vector with reduced horizontal isolation.

The horizontal vector is commonly not considered due to the building vibrations being less apparent, but they still transmit to cleanroom equipment. Negative-stiffness isolators enable a high level of isolation in both horizontal and vertical directions.

Transmissibility of Low-Hertz Vibrations

Vibration transmissibility is an evaluation of the vibrations that are transmitted through the isolator depending on the input vibrations. Each isolator will amplify at its resonant frequency and then begin isolating.

Air systems enhance rather than reduce vibrations, in a normal range of 1.5 to 3 Hz, because of their natural frequencies in which air tables resonate.

The low-cycle perturbations will reach directly through to the instrumentation. Air tables do not isolate to the degree that is required at very low resonance frequencies.

Negative-stiffness isolators resonate at 0.5 Hz. In some cases, they resonate at lower frequencies both horizontally and vertically. There is almost no energy present at this frequency.

To find a significant vibration at 0.5 Hz would be very uncommon. Vibrations with frequencies of more than 0.7 Hz are rapidly attenuated with increases in frequency.

It should be noted that for an isolation system with a 0.5 Hz natural frequency, isolation starts at 0.7 Hz and becomes better with an increase in the vibration frequency.

The natural frequency is used more often to describe the performance of the system. When adjusted to a natural frequency of 0.5 Hz, negative-stiffness isolators have approximately 93% isolation efficiency at 2 Hz vibration, 99% at 5 Hz, and 99.7% at 10 Hz.

Certain low height negative-stiffness isolators give natural frequencies of 1.5 Hz horizontal and 0.5 Hz vertical. Negative-stiffness isolators have the flexibility of customizing for higher resonant frequencies when lower ones are not needed.

Mechanical Simplicity

Pneumatic isolation tables are powered by a supply of gaseous nitrogen or compressed air. When utilized in cleanroom environments (Class 10,000 and lower), the exhaust and supply gases must be vented and piped out of the controlled areas, to guarantee that the gases will not contaminate the cleanroom environment. Located outside of the cleanroom, the air compressor is itself a source of low-hertz mechanical vibration.

Negative-stiffness isolators utilize an entirely mechanical concept, with no electricity or air required.There are no pumps, chambers, or motors, and no maintenance is required because there is nothing to wear out. They function completely in a passive mechanical mode.

A stiff spring supporting a weight load, combined with a negative-stiffness mechanism, provides vertical motion isolation. The overall vertical stiffness is very low without influencing the ability of the spring to support a static load.

Beam-columns connected in series with the vertical motion isolator offer horizontal-motion isolation. A beam-column functions as a spring along with a negative-stiffness mechanism.

If it is possible to isolate sensitive equipment in the cleanroom from vibrations, without the requirement of compressed air or electricity, a system is created that is easier to install, simpler to set up, and more reliable to maintain and operate over the long-term.

Location Flexibility

Air tables are large and difficult to maneuver, making them an unsuitable solution for the limited production and laboratory space requirements of cleanrooms.

Negative-stiffness systems can be made compact where they take up minimum space.Instrumentation can be fixed or transported around a facility with no need for concern about feed-through for air hoses and electrical power. If larger working spaces are needed, large negative-stiffness workstations are also available.

Negative-stiffness vibration isolation systems enable sensitive instruments due to their very high isolation efficiencies.

These sensitive instruments include scanning electron microscopes, optical profilers, laser-based interferometers, and scanning probe microscopes. These can be located wherever a cleanroom must be arranged, whether that be in the sixth floor or the basement.

Extreme vibration-sensitive environments would not be adequate locations for pneumatic isolation systems.

As vibration-handicapped environments become more frequent in the placement of cleanrooms, an optimized vibration isolation solution will be needed compared to what has been offered in the past 50 years with air tables. Negative-stiffness vibration isolation is solving this challenge.

This information has been sourced, reviewed and adapted from materials provided by Minus K Technology.

For more information on this source, please visit Minus K Technology.

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