A solid understanding of the user's manufacturing process requirements is essential to realizing economically efficient design and construction

BY RICHARD V. PAVLOTSKY, Ph.D, P.E.

Extremely sophisticated systems are built to ensure the purity of manufacturing processes in cleanrooms. Any combination of design criteria variables will produce a cleanroom of a specific quality. These same 15 variables are what determine the construction cost of a cleanroom. This cost (with or without tools, further fit-up and process equipment cost) can vary from $180/sq. ft. to $2,800/sq. ft.

The Eskimo have hundreds of words for snow, while our industry has only one word and price for cleanroom. Far too often, clients pay for design overkill, unnecessarily spending money for a cleanroom that exceeds their process needs.

Cleanroom design and construction are typically performed under very tight time constraints that limit opportunities for options, changes and other in-progress improvements. Thus, “cookie-cutter repeats” are encouraged in an attempt to avoid potential risks of new design approaches. These standardized cleanroom recipes may make the designer's task easier, but may not improve the project's economics or lower the construction cost. Since requirements of each cleanroom are different in subtle ways, it's inefficient if not impossible to address all cases with the same design/build template.

Unless the client has confidence in a particular design template, the engineer is often asked to design a cleanroom with more stringent specifications than are required in hopes that, if something goes wrong, they will at least get what they need. Design overkill wastes money, but is inevitable when a detailed assessment of real needs has not been developed and agreed upon by all parties. The cost of the cleanroom grows in reverse proportion to the mutual trust among the design and construction professionals, cleanroom developers, and client.

Take charge of the project

The optimum arrangement of the tools and utilities connections to the production equipment, location of service aisles, chases, corridors, mechanical equipment rooms and utilities rooms plays a major part in cleanroom economy decisions. At the onset of the project, these factors should be considered and simultaneously evaluated by the team of architects, engineers and contractors, with the client as an equal partner. This will ensure a thorough and shared understanding of the user's manufacturing process goals and requirements, which is the key to an economically successful design/build project.

Put simply, the selection of the right design features for the type of cleanroom required by the client has a monumental impact on the final cost. There is no “paint by numbers” solution. Client, engineer, architect and contractor must understand the cost impact of the selected combination of factors/variables that are not necessarily regulated by standards or ISOs, and that need be determined by evaluating each process' specifics and manufacturing requirements. The team must then work together to finalize the goals, needs and expectations before any design activity begins.

A case study

In analyzing cleanroom construction cost in relation to 15 key influential design factors, data was compiled for 20 cleanroom facilities in the United States:

• Ten microelectronics manufacturing facilities with cleanrooms ISO Class 5 and 6 built between 1983 and 2003;
• Four pharmaceutical and biopharmaceutical facilities with cleanrooms ISO Class 5 and 7 built between 1983 and 1996;
• Ten microelectronics manufacturing facilities with cleanrooms ISO Class 3, 5, 7 and 8 built between 1994 and 2003;
• Eight biopharmaceutical facilities ISO Class 4, 5, 7 and 8 built between 1994 and 2004.

Mechanical (HVAC, process piping) construction cost data was extrapolated and adjusted for site location, labor cost and inflation factors to obtain a unified “snapshot” picture of the mechanical construction cost breakdown per generic cleanroom ISO Class 3, 5, 7 and 8.

All facilities selected for analysis were approximately the same size—between 40,000 and 60,000 square feet—with 30 to 40 percent of the facility space occupied by the classified cleanroom, cleanroom corridors and airlocks. Similar arithmetic was applied to mechanical equipment operating costs to obtain the cost breakdown shown in Table 2. This approach allows unification of the mechanical construction cost without differentiation between microelectronics, pharmaceutical and biopharmaceutical cleanrooms. (The resulting data is shown in Tables 3, 4, 5, 6 and 7. Please click here to download a .pdf of the tables associated with this article.)

Click here to enlarge image

Based on these tables, the “Key value factor” chart (Figure 1, page 23) was constructed with Cleanroom ISO Class 5 mechanical construction cost as a baseline; cost factor variables shown in Table 8 (page 28) were obtained. These empirical variables may be applied as percent multipliers to the square foot construction cost for different types of cleanrooms, and may be helpful for the initial alternatives cost evaluation.

15 ways to success

Using the following 15 cost factor variables, we shall be able to reduce the facilities construction cost by maintaining the live database of design alternatives, staying close to facilities needs and engineering valuable improvements to the base cleanroom design and criteria. ATI Architects and Engineers (Danville, Calif.; www.ATIae.com) applies these methods for successful cleanroom design-and-build projects for pharmaceutical, biopharmaceutical, microelectronics and food processing industries.

1. Air Changes.

COST FACTOR VARIABLES: –0.08 to 0.19
Air distribution devices in cleanrooms (usually ceiling filters) are designed to provide a uniform “shower” of pure, filtered air. The quantity of air supplied to the room brings to mind the analogy of a fire hydrant as opposed to an ordinary showerhead.

American Federal Standard 209 E, Japanese JIS B9920, and ISO 14644-1 entitled “Cleanrooms and associated controlled environments—Part 1: Classification of air cleanliness,”provide guidelines for cleanroom parameters, classification and testing, but do not tell you how to get there economically. This is because each process and, therefore, cleanroom requirement are different; desired conditions may be reached utilizing different quantities of recirculating air.

There are always at least five different “possible” designs to achieve the desired results; however, there is only one design that “fits” best. Finding the optimal and most economical solution for the project is a fundamental goal of the project concept for cleanroom air changes.

As shown in Table 1 (page 24), the air changes and quantity of recirculation vary significantly even for the given room class. The cleanroom Class M2, for example, may be achieved with 300 air changes, or 540 air changes may not be enough.

The rate of room contamination and live particles generation is one of the major factors in cleanroom air quantity selection. The rate of particles removal from the room may be very important for one manufacturing process but may not have any effect on another. Other variables that may impact the quantity of recirculating air are: room configuration, equipment location, equipment surface temperature, convective flux, type of the airflow (unidirectional over sensitive areas only or over the entire room), room operations and protocol, materials and chemicals used, etc.

The room may need to certified at rest only, certified at working conditions, validated for the process, or cGMP-validated. Federal standard 209(e), set by the General Services Administration, suggests that air in a Class 100 cleanroom shall be at 90 cfm/sq. ft. or 90 fpm; however, it is possible to build a better than Class 100 room with lower air movement. In fact, it has been done with as low as 45 cfm/sq. ft.

2. Unidirectional, turbulent, vertical, or horizontal airflow.

COST FACTOR VARIABLES: –0.073 to 0.18
In most cases, unidirectional flow is only necessary over small, sensitive areas of the cleanroom and may be handled with minienvironments. Source of contamination may be localized with glove boxes and filtration modules, etc. The majority of cleanrooms designed for total unidirectional flow can achieve unidirectional flow only at rest without workers, equipment, and room exhausts. The choice of vertical or horizontal airflow depends on room configuration and equipment layout. In many facilities, turbulent airflow with properly engineered exhaust and return air locations works fine for contamination removal.

Raised floors were developed to help distribution of electrical wires, communication cables, utilities, piping, etc. between tools, equipment and utility sources. Subsequently, for economy purposes, they were utilized for returning air from the bays to the chases in bay-and-chase cleanroom configurations. In a pure unidirectional cleanroom, the particles are expected to flow from the working space toward the holes in the floor. We have successful installations of Class 10 and Class 100 ballrooms with low wall returns and without raised floors, and the cleanroom airflow is designed to be turbulent.

Another option is to isolate an ultra-clean area with a thin air jet moving at 500 to 800 fpm; it functions exactly the same way as the plastic curtain.

3. Air Filtration.

COST FACTOR VARIABLES: –0.032 to 0.10
Incoming air is filtered with terminal HEPA filters, providing efficiency of 99.99% at 0.3 to 0.5 µm and 99.999% at 0.12 µm, or ULPA filters providing efficiency of 99.99995% at 0.12 µm.

Other devices may also be required depending on cleanroom purpose, such as HEPA filtration in recirculating and make-up air handlers, VOC absorption with charcoal type or similar synthetic filters, electrostatic filtration chambers, etc.

Fan-filters are no longer new to the cleanroom market, and properly applied provide an excellent and economical sol-ution for many high-level cleanrooms—especially in buildings with ceiling height limitations. Applying higher-grade terminal filters should be economically justified and weighted against cost and lower pressure drop of more expensive filter media. (It is not always obvious that lower grade filters with higher pressure drops are more economical in the long run, through the life of the cleanroom.)

4. Air handlers.

COST FACTOR VARIABLES: –0.07 to 0.15
Make-up AHUs. Typically, the make-up air handling units (primary AHUs) provide the necessary make-up air for the recirculating air-handling units (secondary AHUs). The make-up air AHUs consist of draw-through centrifugal, vane axial or plug fans with filters, hot water coils for preheat and reheat, chilled water coils for cooling and dehumidification, and steam or adiabatic humidifiers. Add-ons and variations include static air mixers, steam preheat coils, ultrasonic humidifiers, brine, Dx or glycol subcooling coils for dehumidification, VOC absorption filters, sound attenuators, and VFD drives.

In the last 12 years, a performance success was achieved with reliable and very economical Whisper-Air and Compac Space fan systems, manufactured by M&I Heat Transfer Products (Mississaugua, ON; www.miair.com). These systems are very quiet and consume 20 percent less energy in comparison to conventional centrifugal or plug fans assigned for the same duty.

Make-up AHUs typically discharge into a common header with ductwork laterals balanced to supply the required make-up air to the recirculation AHUs. Air measuring stations are installed in the primary and secondary air supply main ducts to modulate the supply fans, VFDs, or inlet vane dampers to maintain constant air flow. Magnahelic gauges monitor the loading of HEPA filters, bag filters and prefilters located at each air-handling unit. In situations where ceiling fan filters are provided, the make-up air is to be evenly distributed in the space above. Air from the make-up AHUs enters draw-through recirculation AHUs.

Recirculation AHUs. Each recirculation AHU typically consists of an energy- efficient centrifugal fan with filters and sensible (dry) cooling coil. Add-ons and variations include reheat coils for zone temperature control, steam or ultrasonic humidifier for zone humidity control, and constant volume control boxes. Whisper-Air and Compac Space fan systems used for this application proved to be cost-saving and energy-conscientious selections.

Multiple recirculation AHUs discharge into supply ducts feeding the cleanroom through ULPA or HEPA filters, which typically cover 100 percent of the ceiling in the Class 10 and Class 1 areas. This vertical unidirectional flow passes down through the room, through the perforated raised floor tile into the return air space under the floor, and then up through vertical return air shafts, which are open to the ceiling return air plenum. The air then re-enters the recirculating AHUs and the cycle repeats. In the case of ceiling fan-filters, the sensible cooling may be provided by water or DX-cooled fan coil units located in the ceiling space above the fan filters.

5. Air pressure differential.

COST FACTOR VARIABLES: –0.041 to 0.08
Cleanroom pressurization is necessary to protect the cleanroom against contamination from adjacent areas, control the flow of unwanted contaminants, prevent cross-contamination between areas, and help maintain temperature and humidity at required levels.

Typical air pressure differential between cleanroom and reference corridor and other areas of the facility is maintained at 0.25 to 0.005 in. w.g. The higher number is usually more applicable to pharmaceutical facilities, with cascading air pressures between areas to avoid cross-contamination. Such areas normally require a series of cascading airlocks between them, plus pharmaceutical doors that allow air to escape at a high velocity, creating a pressure differential. A well-designed microelectronics cleanroom normally operates at 0.02 to 0.005 in. w.g. with a semi-hermetic air lock at the cleanroom entrance. The cloth change and gowning rooms often serve as airlocks. Mechanical air showers at the entrance are more a question of facility culture and cleanroom protocol than necessity.

Many microelectronics and photolithography cleanrooms operate successfully with passive air pressure control, and maintain only minimum air velocity of 50 to 100 fpm over the entrance with the door fully open. Some biopharmaceutical facilities require an active differential pressure control and supplement air escaping through the door (when the door is being opened) with make-up air automatically.

Differential pressure monitors, such as those by Henry G. Dietz Co., Inc. (Long Island City, N.Y.; www.lowpressure.com), may be used for this task. Monitors may be mounted outside the cleanroom with a small LED inside the cleanroom, having an audible alarm and two indicators to show when pressure is normal or abnormal. The device detects negative pressure in biocontainment areas common in biopharmaceuticals, as well as the positive pressure common to electronic facilities. It has a digital differential pressure display with resolution of 0.001-in. w.c. and pressure/ vacuum range of 0.5-in. w.c. Differential pressure is indicated by illuminated LEDs and an audio alarm. An internal adjustable time delay prevents activation of the audible alarm when the door is opened.

6. Temperature control.

COST FACTOR VARIABLES: –0.032 to 0.15
Temperature and humidity variations cause process equipment misalignment, impact the repeatability of the developed process, and eventually reduce the product's useful output and increase the quantity of waste. It is understandable that the goal is the most stringent cleanroom temperature requirements, but the cost often dictates otherwise.

Often, in an attempt to lower construction costs, the engineer is asked to design a precise temperature control area within a large space where the temperature is allowed to swing +/– 4 to 6 degrees Farenheit (i.e., the warehouse area with roll-up doors). But without hard walls and airlocks, it may be a very expensive alternative, and with virtually uncontrollable variations of humidity. Common sense says that cascading levels of cleanliness, temperature, humidity and pressure are easier to achieve and maintain. The allowable tolerance should be carefully evaluated.

Mechanical equipment and control systems for cleanrooms with stringent temperature control requirements (+/– 0.1 degress F) may cost 20 to 50 percent more than a cleanroom with typical requirements (68 to 72 degrees F) and setpoint (70 degrees F, +/– 2.0 degrees).

A zone thermostat typically controls the design temperature in each cleanroom zone. It actuates the duct-mounted zone reheat or recool coil to satisfy the room's sensible load conditions. In the case of fan coil units, the zone thermostat controls the temperature of air leaving the coils in the zone.

7. Humidity control.

COST FACTOR VARIABLES: –0.034 to 0.19
The relative humidity for each cleanroom is controlled by a zone humidistat. If there is a high relative humidity in the room, the humidistat lowers the cooling coil discharge air temperature to provide more dehumidification. At the same time, the reheat coil provides heat to maintain the room temperature. If the relative humidity of any cleanroom falls below the design limit, the zone humidistat actuates the duct-mounted zone humidifier. When precise humidity control is required, it can typically be achieved with adiabatic humidification of make-up air in the air handler, and by maintaining the cleanroom dew point.

Local variation in humidity levels may be handled with ultrasonic humidifiers located in the ductwork plenum before the filtration terminals. These work well, with RO/DI water quality and water resistivity near 3 to 5 mgm. If the relative humidity falls below the design limit, the humidistat actuates the humidifier to increase the supply air's moisture content. If there is a high relative humidity, the humidistat lowers the make-up air handlers' cooling coil discharge air dew point temperature to provide more dehumidification.

8. Exhaust systems.

COST FACTOR VARIABLES: –0.031 to 0.16
Process exhaust systems typically are: acid, solvents and VOC, toxic, heat, and general room exhaust. If ammonia is present in the exhaust air stream, some facilities prefer to run a separate ductwork and abatement for fumes. The most common abatement for acid exhausts is a horizontal or vertical scrubber. Solvents and VOC exhausts require absorption, concentration and removal of condensed solvents or on-site incineration. Toxic exhaust is usually abated by in-place or on-site high temperature destruction. Carefully determine the abatement types and the quantities of required exhaust air streams.

Good facility air management can help minimize quantities of exhaust air and, therefore, lower construction cost and energy waste during operation. It is important to maintain balance between potential needs for increase of exhaust quantity and expansion and economical value of installing such provisions. Far too often, oversized, expensive, lined stainless steel or FRP exhaust systems have been built in consideration for future expansion in cleanrooms while other utilities and mechanical systems (make-up air and cooling, for example) couldn't support such expansion. It is also common that acid exhaust systems have been built without consideration for future need, where velocity in the ductwork exceeds 4,000 fpm with added airflow and new ductwork branches. Such systems were difficult or impossible to balance and expensive to operate.

9. Vibration and noise control.

COST FACTOR VARIABLES: –0.027 to 0.10
Equipment size and weight affect vibration transfer and control. Concrete “waffle” slabs under the cleanroom floor worked well for keeping equipment vibration from transferring to other areas of production or to metrology tools.

Waffle slabs remain rigid even if holes are drilled in the floor for piping and conduit access to the cleanroom. Because the strength of the floor is in the grid system, rearrangement is feasible; additional holes can be punched in the floor without adversely affecting vibration considerations.

All mechanical equipment should be vibroisolated with springs, flexible connections, and isolated foundations to minimize the vibration effect. Quiet, energy-efficient fans and motors allow maintenance of the desired NC level in the cleanroom. The vibration and sound consultant should be a part of the team to verify mechanical and architectural concepts for vibration and noise control. It could be very expensive to fix it later. Many potentially costly measures can be eliminated, or replaced with more economical solutions if considered from the beginning and involving architects, mechanical engineers, client and consultants.

10. Magnetic and electromagnetic flux.

COST FACTOR VARIABLES: –0.037 to 0.15
Magnetic flux proposed to be the flow of the background oceanic particles of the galaxy. Galactic rotation is also proposed to be electromagnetic in nature. The word “flux” means flow, and we may think of magnetic field lines as lines of some type of fluid flow through an imaginary surface. The magnetic field magnitude is like a rate of flow, and its direction is the direction of flow. The magnetic flux is like total volume of flow through the surface. Better yet, we may think of magnetic flux as the number of field lines passing through the surface.

At a given speed, this force is greatest when the particle moves perpendicular to the magnetic field, and zero when the motion is parallel to the magnetic field.

At the most basic level, magnetic forces are exerted on moving charges by other moving charges, just as electrostatic forces are exerted by electric charges on other electric charges—whether or not they are moving.

It appears from comparative studies of planets that the Earth has a strong field because it rotates and has a molten metallic core. In theory, the field arises in internal electric currents that are induced by the Earth's rotation and by circulation in its fluid core. Magnetic flux density (instead of “Newton's per Amp-meter”) has its own unit—the Tesla. The typical value of the Earth's field near its surface is about half of a gauss, which is about one 200,000th of a Tesla. Occupational safety and health requirements limit magnetic field strength in areas open to the public to the 5 Gauss limit.

Magnetic shielding of a cleanroom can be very expensive. For example, 4 to 5-mm thick shielding on the envelope of the cleanroom with magnetic (Fe-Si) steel (M15 type) may lower the strength of magnetic flux from MRI equipment to 1.3 to 2.6 Gauss. But 14-mm of ordinary low-carbon steel shielding may not be sufficient. Current semiconductor, metrology and communication laboratories require the strength of magnetic flux in the area to be limited to 0.05 Gauss or less.

A very expensive cleanroom was once shut down shortly after it was built because it was constructed on a site with high magnetic flux. Another cleanroom process failure was attributed to a high-capacity, high-voltage cable buried many feet under the production floor. Again, these things should be assessed at the onset of the project so remedies may be found and time and costs saved. Indeed, the cost of magnetic shielding should also be considered.

11. Electrostatic charge of airand surfaces.

COST FACTOR VARIABLES: –0.037 to 0.12
“Static electricity” is present when surfaces in contact are separated. If the charge that arises from differences between the surfaces cannot escape to Earth quickly enough, then it is trapped and the charge will spread out over a material's surface—it is “static.”

Retained electrostatic charge creates risks and causes problems in many areas of the industry. It can cause ignition of flammable gases and even shock personnel. It can make thin films and light fabrics cling, attract airborne dust and debris, damage semiconductor devices and upset the operation of microelectronic equipment.

The hazard concerning flammable gases, vapors and powders relates both to the capacitively-stored energy in relation to minimum ignition energies, and to the breakdown voltage of the minimum gap from which an ignition will propagate.

Typically, the minimum ignition energies of common hydrocarbon gas/air mixtures are 0.2 millijoule (mJ) with a few kV minimum breakdown voltages. With powders, minimum ignition energies start at a few mJ.

Shocks from electrostatic discharges become discernible around 1 mJ and are likely to be uncomfortable in the 10 to 100-mJ range. They will cause major muscular contraction above 1J. Mechanical handling problems arise when electro-static forces become comparable to gravitational or other constraining forces. This relates to the strength of local electric fields and, hence, on insulators to surface-charge density.

In general, electrostatic forces are weak, but dust will be attracted to surfaces at charge densities less than a few mJ x 10^-7 per m^-2. Electrostatic charge will be generated on people by such normal activities as walking across carpets, getting up from chairs, rubbing clothing against surfaces, etc. The levels of charge will be higher in low humidity environments and where artificial fibers are extensively used. Body potentials up to 15 kV may be expected.

Electrostatic discharges will occur from charged fabrics, a charged body, and any metal objects held in the hand, etc. These electrostatic discharges may involve high potentials and so may be able to jump several millimeters of air through gaps in equipment casings directly to internal circuitry. The discharges can involve currents up to several amps and involve frequency spectra extending up to several hundred megahertz—particularly where a metal conductor acts as the source of the discharge.

Static damage to semiconductor devices is very dependent on the device type and design. CMOS devices and fine geometry structures are especially susceptible. The risks may be expressed in relation to the voltage involved with a “human body model” discharge, although damage is more likely related to discharge energy and voltage. Damage sensitivities down around 50 volts may be experienced. Problems with the upset of microelectronic systems are also usually expressed in relation to a human body model discharge. System immunities of several kilovolts (kV) are likely to be needed (preferably over 15 kV for uncontrolled environments) as high potentials can readily be generated on personnel in normal working environments by movements across flooring, etc.

To minimize risk and avoid potential problems, it is necessary to ensure thatstatic charge can dissipate more quickly than it is generated. For normal, manual-handling and body motion activities, this means the charge decay time needs to be .25 second or less. A new concept, relevant to risk control, is that if static charge experiences a high capacitance on a material, then only low surface voltages will be observed and potential problems and harm will be prevented.

Many firms have taken precautions against static by the installation of conductive floors and work surfaces, personnel earth bonding via wrist straps, and using anti-static or conductive bags for storing and transporting components and assemblies. The basic way to avoid electrostatic discharges affecting microelectronic systems is to mount the equipment within an enclosure, providing good electrostatic and magnetic shielding, and to suitably decouple all input and output connections.

12. Energy and operating cost.

COST FACTOR VARIABLES: –0.017 to 0.15
As can be seen in Table 2 (page 22), the operating costs of recirculating AHU fans and air-cooled DX units each contribute significantly—approximately 35 percent—to the total HVAC operating cost. An operating cost savings was achieved on newly designed facilities by using fan filters, VFD drives, energy efficient motors, a cooling system with cooling towers, hot water zone reheat, and adiabatic humidification.

The stringent cleanliness requirements for cleanrooms are coupled with their inherent high operating costs; however, you can cut costs, if you plan ahead:

• Precisely define the class of cleanroom desired, making sure it fits the process requirement.
• Precisely define the room operating air temperature and humidity, making sure it fits the process requirement.
• The amount of air exhausted from the cleanroom should be the minimum required by the process. Imbed the exhaust air management program from the beginning. Start with providing flow and differential pressure indicating instruments at all process exhaust air ductwork branches and tools.
• Limit ductwork and piping pressure drops by establishing maximum and minimum air velocities for the facility, and following these guidelines through construction.
• Use minienvironments, glove boxes, vacuum chambers and modular enclosures; reduce the need for large ballroom space and, therefore, reduce operating cost.
• Use energy-efficient motors with VFD drives.
• Loop supply and exhaust systems to reduce pressure drop and save space.

13. Form and function.

COST FACTOR VARIABLES: –0.038 to 0.16
Generally speaking, 20,000 to 30,000 sq. ft. of cleanroom space requires a 60,000 to 90,000 sq.ft. structure. The building height may be 21 to 26 ft. in order to accommodate an interstitial space of approximately 10 ft. in height. Another 12 to 16 ft. may be added for mechanical equipment above or below the cleanroom level.

For maximum flexibility, there should not be any columns inside the cleanroom. The roof structure should have a minimum depth and designed for loads of 100 to 200 pounds per sq./ft. to support the piping, ductwork and air-handling equipment that could be suspended from the overhead structure.

Frequently, the interstitial space must be used as a return air plenum, so the room and the building should be constructed of non-combustible material. The interstitial space, as well as the space below the ceilings, should be sprinklered.

Current NFPA codes should be followed to the letter, along with local safety codes. Obtaining permits for the construction (permitting process) may be long, expensive and frustrating if all building and fire safety codes are not followed. Also, all exempts and interpretations need to be documented and discussed with permitting officials ahead of time.

The cleanroom may be located against an interior wall with sufficient space provided outside the cleanroom to accommodate mechanical and process equipment, such as chillers, heat exchangers, pumps, DI water equipment, storage tanks, etc. Ideally, the floor slab in the cleanroom should not be poured until the designer and owner determine the requirements for process piping trenches and the location of vibration-sensitive equipment. A basement and or sub-basement space may be required for process and utilities piping, and cable distribution to tools and equipment, exhaust and return ductwork.

Extremely large electrical service with special redundancy features is often required, in which case it is necessary to confirm that adequate utilities are available.

The high water usage associated with semiconductor and pharmaceutical fabrication also requires waste treatment capacity and sewer lines large enough to handle the expected effluent. Minimizing wastewater discharge is always a goal. Today, zero-discharge facilities are commonplace—no longer a state-of-the-art feature.

An experienced architect's input into cleanroom facility design cannot be overstated or overlooked. It will have a tremendous impact on the quality and cost-efficiency of your facility. The high-tech facility lives and dies by a thorough understanding of its function. Like the need for speed determines the size and shape of a racecar, the cleanroom's specifics determine the good architectural design of the facility.

A cleanroom architect with solid knowledge of the facility's process and its present and future needs, teamed with a qualified mechanical cleanroom design engineer, structural and electrical experts, and client process group, form a critical element of every successful design.

14. Particulate.

COST FACTOR VARIABLES: –0.040 to 0.15

15. Process piping.

COST FACTOR VARIABLES: –0.021 to 0.20

These factors (14 and 15) should not be eliminated from the discussion of their effect on cost. At the same time, because these variables are particularly specific to each facility, it is best to not generalize; however, they are addressed within the included tables.

Only time will tell…

The process of cleanroom facility design is dynamic and creative. It is possible, dare I say very probable, that cleanrooms as we know them today will evolve into something completely different.

Mini-environments, barrier technology, glove boxes and automation have already changed cleanroom look and operation. But, just as today's cleanrooms were relatively unheard of only few decades ago, tomorrow's technical production facilities will undoubtedly reveal some interesting surprises… and challenges for us all.

RICHARD V. PAVLOTSKY, Ph.D., P.E. is a director of Advanced Technologies Division for Danville, Calif.-based ATI Architects and Engineers. He can be reached at: [email protected]

References

1. ASHRAE Handbook, Heating Ventilating and Air-conditioning Applications, 1999, Chapter 15, American Society of Heating, Refrigerating and Air–Conditioning Engineers, Inc., Atlanta, Ga.
2. Clean Room and Work Station Requirements, Federal Standard No.209E, The General Service Administration, Washington, D.C.,1993
3. J. N. Chubb – John Chubb Instrumentation papers for the IEEE-IAS meeting Oct, 1999 and for the ESA meeting at Niagara Falls, June 2000
4. Davies, D. K. “Electrostatic damage to semiconductor devices” International Symposium on Electrostatics- Application and Hazards
5. ISO 14644 Cleanrooms and associated controlled environments
6. ISO 14698 Cleanrooms and associated controlled environments-Biocontamination control

Taking action against static

J.N. Chubb of John Chubb Instrumentation (Cheltenham, U.K.; www.jci.co.uk) presented an excellent paper at the “Electrostatics Summer School '85,” held at the University College of North Wales, Bangor. His presentation describes approaches to controlling static electricity and the problems static can cause:

• Enhance system design to prevent influence of static; for example, ensure proper earth bonding of all metal parts of equipment and plant so that hazardous quantities of charge cannot be accumulated where they are available for easy and rapid discharge; use inert gas with flammable vapors to render atmosphere non-flammable; build microelectronic systems to have adequate discharge immunity; maintain room humidity level above 35 percent relative humidity.
• Enhance system design to accept the risk of direct consequences of static problems if the generation and discharge of static cannot be controlled; for example, provide explosion venting and/or explosion suppression facilities, and avoid manual loading of powders into reactors where flammable gases or clouds may be present.
• Change system design or operation to minimize static generation; for example, avoid sliding of insulating webs on stationary surfaces and on stalled rolls, minimize use and size of high resistivity plastic surfaces, limit speeds of pumping of high resistivity liquids.
• Change system design to enhance rate of static leakage so that high charge levels and high potentials cannot occur; for example, using antistatic additives in high resistivity liquids, using antistatic coatings on web surfaces, using earthling rods when loading powders into bins with insulating liners, using conductive flooring, footwear, clothing, work surfaces and wrist straps to prevent charges building up on personnel.
• Use only certified electrical equipment in areas where flammable gases may be present; earth-bond all instrumentation and portable equipment before introduction into hazardous areas.
• Charge neutralization; for example, using passive or active units or air ionization to limit charge levels on any highly insulating surfaces.

The above list is a simple outline of actions, which are generally applicable, and which should be taken wherever static is likely to be present in industrial plants and processes.—RP