For decades, pneumatic air tables have been the workhorse for the reduction of cleanroom vibrations for research and manufacturing, where essential microengineering equipment is utilized.
In the same way that pioneering technology has slowly developed within nano-applications in industrial laser or optical systems, biological research, and microelectronics fabrication, the requirement for enhanced precision has become more important in vibration isolation.
Pneumatic air tables are not meeting customer requirements as well as the more modern technology of negative-stiffness vibration isolation.
More than 20 years since its introduction, it has shown itself to be a benefit for thousands of applications in various fields of industry, government, and academia, including some of the most dynamic and difficult environments, for example cleanrooms.
A multitude of factors can cause vibration. All structures transmit noise. Inside a building, the ventilation and heating system, elevators, fans, and pumps are only some of the mechanical features that produce vibration.
How strongly the equipment will be influenced depends on how far the cleanroom equipment is located from these sources of vibration, and where the equipment is located within the structure, for example, on the third floor or in the basement.
The equipment can be affected by vibrations from nearby construction, loud noise from aircraft, adjacent road traffic, and even wind and different weather conditions that are external to the building but can create structural movements.
Sensitive equipment will be influenced by vibrations in the range of 2 Hertz (Hz) to 20000 Hz. These external and internal influences largely result in lower frequency vibrations.
These vibrations are transmitted through the structure, which produce powerful disruptions in the precision equipment employed in cleanrooms.
Vibration Isolation Equipment Within Cleanrooms
Fields that require sensitivity to environmental contaminants such as dust, airborne microbes, aerosol particles, and chemical vapors extensively use cleanrooms, for example, semiconductor manufacturing, biotechnology, the life sciences, and more.
Cleanrooms offer a restricted environment with a manageable degree of contamination that is specified by the number of particles per cubic meter at particular particle size.
The air externally entering a cleanroom is filtered to remove dust and the air inside is continually recirculated through high-efficiency particulate air (HEPA) and/or ultra-low particulate air (ULPA) filters to exclude contaminants that are internally generated.
The equipment utilized inside the cleanroom must be manufactured to create the least amount of air contamination.
This includes vibration isolation equipment, which can vary from basic breadboards, rubber blocks, and metal springs, to highly efficient active electronic systems, air systems and negative-stiffness systems - which is designed using more advanced materials and technologies for higher precision vibration isolation.
The equipment is designed using more advanced materials and technologies for higher precision vibration isolation.
Vibration Isolation Tables and Workstations
These are needed to achieve the same contamination and cleanroom requirements that the elements that they are shielding from vibration must meet.
Entirely enclosed isolation modules and vented exhaust systems are also available to ensure workstations are compliant with cleanroom standards.
Every surface of the isolation table should be accessible for cleaning and made so that they can easily be wiped to maintain cleanliness.
For compressed electrical supply and air tubing, isolator diaphragms should meet the established outgassing, non-volatile residue, and total mass loss specifications to ensure molecular contamination is kept at an absolute minimum.
Negative-Stiffness Vibration Isolation Versus Pneumatic Isolation Tables for Cleanroom Applications
Air tables have been employed since the 1960s for vibration isolation, and have the greatest installed base within cleanrooms.
With the increase in the sensitivity of instrumentation, specifically at the sub-atomic level, more accurate vibration isolation technology was required to deal with lower-hertz vibrations, which were negatively impacting results despite air tables being used.
Released over 20 years ago by Minus K Technology, negative-stiffness vibration isolation was designed specifically with the isolation of these low-frequency perturbations in mind.
The following are critical comparisons between negative-stiffness isolators and air tables that should be evaluated when looking at vibration isolation for cleanroom applications:
Vertical and Horizontal Isolation
Air tables can achieve isolation, but mostly in the vertical vector, with restricted horizontal isolation.
The horizontal vector is frequently overlooked because horizontal building vibrations are not as apparent, but they are still transmitted to the cleanroom instrumentation. Negative-stiffness isolators maintain a high degree of isolation in both horizontal and vertical directions.
Vibration Transmissibility of Low-Hertz Vibrations
Vibration transmissibility is an evaluation of the vibrations that are transmitted through the isolator in relation to the input vibrations. Each isolator will start isolating after amplifying at its resonant frequency.
Instead of reducing, air systems will amplify vibrations in a common range of 1.5 to 3 Hz because of the natural frequencies at which air tables resonate.
The low-cycle disruptions will directly contact the instrumentation. Air tables do not isolate to the extent that is fully required at very low resonance frequencies.
Negative-stiffness isolators resonate at 0.5 Hz and in some cases at lower frequencies both horizontally and vertically. At this frequency, there is practically no energy present.
It would be very rare to discover a significant vibration at 0.5 Hz. Vibrations with frequencies over 0.7 Hz are quickly attenuated with frequency increases.
When adjusted to 0.5 Hz, negative-stiffness isolators attain 93% isolation efficiency at 2 Hz; 99% at 5 Hz; and 99.7% at 10 Hz.
Some low-height negative-stiffness isolators offer natural frequencies of 1.5 Hz horizontal and 0.5 Hz vertically. Negative-stiffness isolators have the flexibility of customizing for higher resonant frequencies when the lower ones are not needed.
It should be noted that for an isolation system with a 0.5 Hz natural frequency, isolation starts at 0.7 Hz and becomes more efficient with the increase in the frequency of vibrations. The natural frequency is most often utilized to describe system performance.
Pneumatic isolation tables function on a supply of gaseous nitrogen or compressed air.
When employed in cleanroom environments (Class 10,000 and lower), the exhaust and supply gases need to be vented and piped out of the controlled areas, to make sure that the gases will not contaminate the cleanroom environment.
The air compressor, situated outside of the cleanroom, is also a source of low-hertz mechanical vibration.
Negative-stiffness isolators are based on an entirely mechanical concept where no electricity or air is required. No motors, pumps or chambers are used, and no maintenance is necessary as there is nothing to wear out. They operate completely in a passive mechanical mode.
Vertical-motion isolation is provided by a stiff spring that supports a weight load in combination with a negative-stiffness mechanism. The net vertical stiffness is very low without influencing the static load-supporting capability of the spring.
Beam-columns connected in series with the vertical-motion isolator offer horizontal-motion isolation. A beam-column performs as a spring along with a negative-stiffness mechanism.
The outcome is a compact passive isolator able to achieve very low horizontal and vertical natural frequencies, and very high internal structural frequencies.
If sensitive equipment in the cleanroom can be isolated from vibrations without having to work with compressed air or electricity, then it is a system that is easier to install, simpler to set-up, and more dependable to maintain and operate over the long-term.
Making them a more difficult fit for the stringent laboratory and production space requirements of cleanrooms, air tables are large and bulky.
Negative-stiffness systems, in contrast, are made to be compact and take up a smaller footprint. Instrumentation can be located or transported within a facility without needing to be concerned about feed-through for air hoses and electrical power. Large negative-stiffness workstations are also available for when more working space is needed.
Due to being highly efficient at isolation, negative-stiffness vibration isolation systems provide sensitive instruments, for example, scanning electron microscopes, optical profilers, laser-based interferometers, and scanning probe microscopes, the ability to be located wherever a cleanroom needs to be organized, whether that be in the basement or on the sixth floor of the building. For pneumatic isolation systems, extremely vibration-sensitive environments would not be suitable locations.
As vibration-handicapped environments become more common in relation to the location of cleanrooms, a more efficient vibration isolation solution will be necessary compared to what has been available for the past half-century with air tables. Negative-stiffness vibration isolation is meeting this requirement.
Produced from materials originally authored by Jim McMahon from Minus K.
Low Frequency Vibration Isolator for Heavy Loads