Keeping on Track with Cleanroom Floors, Walls and Ceilings
Seamless “microbiological” flooring that is antistatic or static-dissipative and “total systems solutions”– cleanrooms that are ready to be “plugged in”– are in demand by major segments of the industry.
By Susan English
The market for cleanroom floors, walls and ceilings is expected to keep pace with the projected rise in worldwide orders for cleanrooms and cleanroom components from the semiconductor, disk drive, flat panel and pharmaceutical in-
dustries. For example, 1995 orders for floors were $34.8 million and expected to increase by over 50 percent to $77.5 million by the year 2000, according to a report by The McIlvaine Company (Northbrook, IL).
Floors
To maintain the highest level of clean air, Class 1 and sub-Class 1 cleanrooms generally use vertical laminar air flow systems with perforated, raised access floors. Space between the raised floor and concrete-slab on grade is used as a return air duct and also as a service area for wiring, cable and piping. Perforated panels provide a way to achieve vertical laminar flow conditions while controlling ESD, air flow, particulate contamination and vibration. Floor panels vary according to the application and may include aluminum, fabricated steel, aluminum casting, woodcore, gypsum and cement.
Walter Harriman, vice president of Maxcess Technologies, Inc. (Summerville, SC) says one concern voiced by semiconductor manufacturers is that standard cleanroom access floors can`t adequately handle the weight of today`s processing tools, especially during initial move-in. The average floor load of a cleanroom is 200 lbs./ft2, but it can reach 350 lbs./ft2 in isolated spaces, depending on equipment configuration and weight, according to industry statistics. Manufacturers generally must certify their system performance under rolling loads.
Harriman says, “The challenge for aluminum floor producers has always been how to increase the strength of our product without adding so much material that the cost becomes prohibitive. The rolling-load issue will become even more important as IC suppliers move toward 300-mm wafer production.” To meet the demand, Maxcess Technologies has developed a new line of heavy duty die-cast aluminum panels and grates that adds only slightly more than one pound of aluminum to achieve additional strength.
With more and heavier process equipment being installed in cleanrooms and the increased use of waffle-slab construction, Pace Co. (Portland, OR) sees a need for better space utilization under the floor. Its Pace Access Floor is made from die-cast aluminum panels that drop onto a system of pedestals. Unlike conventional 2-ft on-center pedestal layouts, Pace features a 4-ft on-center modular design. By reducing the number of pedestals required to support the floor, the grid understructure reduces installation labor, says the company. It also reduces pedestal crowding while increasing usable subfloor area for piping, spill containment, electrical conduit and process equipment.
In pharmaceutical applications, seamless flooring–epoxy coatings that create an impervious, seamless surface–is preferred, because bacteria or particulate could become trapped in the crevices of seamed tile flooring. Michael Kramer, president of International Coatings, Inc. (Franklin Park, IL), a manufacturer of epoxy coating, flooring and grouting materials, says he`d “like to think that the industry is going towards seamless flooring, which is very resistant to chemical attack and moisture penetration.” The company`s IcoGuard flooring can be applied as a high-build coating, self-leveling floor, or heavy duty quarter-inch flooring system.
Epoxy or other protective coatings are often used to coat the structural concrete beneath the raised floors of semiconductor fabs. “With many companies converting more and more of their old production into new cleanrooms,” says Kramer, “we have to fill all the cracks and turn an old concrete floor–which might have had tile on it at one time–into a very smooth, seamless surface.” The company also manufactures IcoGlaze wall coating, a solid seamless epoxy wall and ceiling coating system for cleanroom and food environments.
Most vendors of cleanroom flooring offer antistatic coatings as either standard or optional features. Access floor panels and pedestals should provide a positive contact for continuous electrical grounding of the entire floor system with an electrical resistance of less than 10 ohms, according to figures cited in an industry report. Garland Floor Co. (Cleveland, OH), a manufacturer of epoxy flooring systems, has introduced Chemi-Cote ESD Ep 1000, a high solids, low-odor epoxy floor coating that offers high impact resistance and controls static electricity in cleanrooms. The system can be applied to floors made of a variety of substrates, including existing non-conductive coatings, resurfacers and concrete. Independent tests have shown that the performance of its ESD coatings is not dependent on relative humidity. Body voltage generation is a minimum (<15 V); measured body voltage decay is 5,000 to 0 V in less than 0.1 second.
According to Tom Mulligan, Dycem`s sales and marketing manager, 80 percent of the contamination entering a cleanroom arrives on the feet of personnel or wheels of carts. Adhesive mats, says Mulligan, remove slightly more than half the contamination on the shoe at one step. Flooring by Dycem (Warwick, RI) is designed to trap and keep contaminants–in the gowning room, in airlocks, air showers and entranceways outside the cleanroom. A tacky finish added to polyvinyl chloride floors produces a high-friction surface that collects and holds dust, grit, fibers and microscopic hairs. The polymeric compound and processing technology produce a material with an exceptionally smooth surface–comparable to that of a liquid–which creates a high degree of surface energy to attract particulate contamination. The soft surface allows a dense packing of particles, enabling the mat or flooring to remain effective even after continuous contaminating overstrikes in the same area. For inside the cleanroom, Dycem offers CleanZone antistatic or static-dissipative flooring, as well as its line of Protecta-Mats.
“Microbiological” flooring, which contains antimicrobial chemistry that inhibits the growth of bacteria, has also been developed. While Dycem has largely targeted the pharmaceutical and medical markets with its products, Mulligan says the semiconductor industry is also beginning to be concerned about microbial contamination.
Oakridge Industries Inc.`s (Pomona, CA), Silikal fast-curing concrete protection systems are acrylic reactive resins based on methyl methacrylate polymer chemistry. For pharmaceutical applications, Silikal Resin Systems contain Microban B, an FDA- and EPA-registered additive engineered to combat microbial growth on flooring and concrete. A cell-wall penetrant rather than a biocide, Microban additive B is incorporated into the Silikal resins at the point of manufacture. According to the company, it migrates to the surface on demand, acting like microscopic tacks striking the thin cell walls of microorganisms, disrupting their ability to function, grow and reproduce. The company has also introduced the Universal Guardian antimicrobial topcoat system, with special bonding properties that make it possible to convert existing flooring to an antimicrobial floor.
The Dex-O-Tex Division of Crossfield Products Corp. (Rancho Dominguez, CA) makes Electro-Flor epoxy flooring to reduce static buildup and sparking. Its Dex-O-Tex Cheminert and Chem-Rez flooring, a troweled epoxy resin composition flooring, provides chemical resistance. Its Dexcide line of epoxy floors contains a biocide that prevents microbial growth by eliminating reproductive capacity. The Dexcide antimicrobial additive can also be applied to wall surfaces.
Dex-O-Tex also manufactures wall-coating systems that can be integrated into seamless environments suitable for cleanrooms. Its Wall-Glas sanitary wall-coating system, consists of multiple layers of epoxy with woven glass fabric reinforcement embedded in the first coat. It can also be applied to ceilings.
Walls
Soft wall cleanrooms are often used for manufacturing, filling, packaging or assembly operation, where temperature and humidity control is not critical. Soft walls permit easy upgrading for size requirements, equipment needs and degrees of cleanliness. Since they may be ceiling suspended, floor mounted or mounted on a heavy-duty steel frame with optional casters for portability, they are also flexible and economical.
“Hard wall” cleanrooms are typically enclosed by a demountable wall system that allows the removal of individual panels without affecting adjacent wall panels. Hard walls come in porcelain-enameled steel, aluminum sandwiched around a polyethylene core, mica, polyester, vinyl, urethane and porcelainized metal on inner cores of Styrofoam, paper honeycomb, gypsum, plywood or pressed chipwood. Porcelain-enamel finishes are effective in environments where there is exposure to strong acid or harsh cleaning regimens. Baked-enamel finishes provide smooth, durable, color-coordinated walls that are functional and economical. Stainless steel panels are designed for aseptic environments in which harsh cleaning agents are used.
Utilizing its own Build Clean program procedures, Hodess Building Co. (Rumford, RI) developed the Biowall wall and ceiling system. Biowall is non-outgassing and non-particulating, and is composed of 12 layers of resin-saturated paper with a white melamine surface, compressed into a 4-mm thick, high pressure laminate panel. It is designed to resist moisture, chemicals and heavy impact. It can be used as a wall or as ceiling panels in a number of applications, including high abuse, high moisture applications such as chemical storage or packing rooms, chemical hoods and spray booth enclosures. Biowall also meets USDA/FDA sanitary standards for use in food-processing areas.
USG Interiors, Inc. (Chicago, IL) makes wall systems for cleanrooms. The rooms can be operated with negative or positive air pressure, and any panel can be removed and replaced without disturbing other panels. The company`s conductive coating finish helps control ESD. For semiconductor applications, the Clean Room Wall System interfaces with HEPA ceiling systems and access floors and with structural framing systems. It can serve as a liner for other walls or as a one- or two-sided wall. All models allow airflow to be configured as needed and feature grilles with or without dampers that snap into place like the wall panels. They can form an air chase, or the wall cavity itself can be used as a return air duct.
Clean Rooms West, Inc. (Tustin, CA) manufactures and installs its own cleanroom wall and ceiling systems. The 4-in. wall system is used to support overhead mechanical equipment. When combined with the heavy duty “Power T” ceiling system, clear spans up to 50 feet can be constructed. The Power T powder-coat painted aluminum ceiling system is suspended from the overhead deck using standard mounting hardware and accommodates a variety of lighting and HEPA filter sizes.
Ceilings
Cleanroom ceilings, whether open-plenum or flush-grid, come prefabricated or in standard, lay-in panels. They should be of hard non-shedding, non-porous material and free from holes, crevices and cracks. Coves should be used to round off wall junctions. Cleanroom ceiling surfaces are generally faced in vinyl or metal and should also be cleanable, non-porous, non-shedding and easily maintained, as well as water impermeable and resistant to cleaning agents and disinfectants. Metal may be finished with an epoxy enamel and come prefabricated or in standard 2-ft ¥ 4-ft lay-in panels, which are flame retardant. Flush-grid ceilings for semiconductor wafer fabs can be used with filter modules, plenum modules or as a stock-built (T-grid) system.
The 2-in. extruded aluminum flush-grid ceiling system patented by Huntair (Tigard, OR), provides an architectural match to cleanroom wall systems. Greg Lanz, a designer at Huntair, says the 2-in. ceiling provides greater filter area. Overcoming the difficulties of getting a sprinkler system and high enough light levels into the 2-in. space were major obstacles, according to the company. If a typical cleanroom ceiling air filter measures 2 ft. ¥ 4 ft. and has 0.3 ft2 of filter face area, in a 10,000-ft2 cleanroom, the 2-in. ceiling system could support the equivalent of 50 additional filters.
USG Corp. offers Class 10-100,000 cleanroom ceiling panels in vinyl, with a vinyl latex seal and back coating to control airborne particles. The 0.002-in.-thick panel face-coating has been Gardner-Scrubbability-tested to 5,000 cycles. The panels are made to withstand temperatures up to 90°F and relative humidity of 90 percent without becoming delaminated. Class 100 panels feature an impervious surface of vinyl-coated white aluminum foil. Class 10,000-100,000 panels have a perforated, vinyl-coated white aluminum foil surface. All three come in standard 2-ft ¥ 2-ft. ¥ 4-ft sizes as well as metric dimensions.
All maintenance can be done from above with Clestra Cleanroom`s (N. Syracuse, NY) CP-3 and Sylab ceiling systems. Monoblock and non-progressive, they allow overhead access without disturbing the cleanroom envelope. Clestra`s custom aseptic Sylab Ceiling System is capable of supporting the weight of maintenance workers. The company`s recently UL-certified flush-grid ceiling is specially designed for Class 1 or sub-Class 1 semiconductor wafer fabs. “Flush-grid has really taken off,” says Scott Mackler. “Our system is caulk free and requires less gel, and therefore less time to pour, which results in cost savings. We also use it for Class 100 or 10.” Clestra offers three load-bearing grid systems: the slotted-grid for minienvironment and material handling requirements; the gasket (T-grid); and the gel-grid.
Pace Co.`s Clean-Pak Division (Clackamas, OR) manufactures Clean-Trak modular flush-surface ceiling systems with flush lighting, built-in fire protection ports and diffusers. It also offers stick-built ceiling systems with the non-progressive construction that some fabs require. In addition, the stick-built system can be used where vertical flexibility is needed (a 14-ft ceiling section in a 12-ft fab) or where assembled modules may be impractical and costly, especially in off-shore applications. Eight-foot sticks with pre-wired lighting are fastened together with a coupler to 2-ft cross members, available with either built-in sprinkler ports or electrical ports. The Pace Stick-Built design requires fewer hangers and struts than other stick-built systems, according to the company. Ducted walkable filters and return grilles can be used in bay-and-chase designs, making the entire ceiling walkable for access to equipment and ceiling modules. Pace Systems Co. does turnkey cleanroom installations using Clean-Pak airhandlers, Clean-Trak 2.6 modular flush-surface ceiling systems, floors, and walls.
Integrated systems and “total systems solutions”
Complex modular systems are turnkey-tested and ready to be plugged in. They usually come complete with monitoring systems and everything from air showers to ionizers. Until recently, Daw Technologies (Salt Lake City, UT) saw itself as one of the only companies to provide complete systems solutions. Now, other companies are ramping up to turn out full systems. Says Ryan Young, Daw`s marketing director, “the trend is reflected more and more in a higher percentage of systems sales vs. sales of bare components.” Young cites reduced costs, economies of scale, and better scheduling. Daw`s floor, wall and ceiling components are all available on an individual basis, but “our primary stress is the system,” says Young. “Users want to deal less and less with vendors directly.” The systems approach also provides considerable latitude in tailoring installations to customer requirements. Daw supplied its systems to Advanced Micro Devices for its $1.4 billion Fab 25 in Austin, TX. Young says some of the technologies used in the ceiling, flooring and even the walls “met some needs we hadn`t foreseen, unique applications that custom fit exactly what they were looking for. ” (See the profile of Fab 25: “Megafab plans 0.25-micron processing in 1998,” CleanRooms, June 1996, p. 6.)
Young says semiconductor wafer fab designers and operators are always looking for better ways to manage their utilities, such as piping and cableways, which are typically located between the raised access floor and the concrete sub-floor or waffle slab. Daw considers its VariSpan2 access flooring structure to be “an emerging technology” because it allows supporting pedestals to be installed at 4 feet instead of 2 feet on center like traditional raised access flooring. Pedestals can impede access to services and require significant additional expense in secondary structures, such as tube steel, if the waffle slab is designed as a 4-feet on-center structure. Installing supporting pedestals at 4 feet on center would save over 65 percent of the number of pedestals, he says, providing large open areas for the installation of services and allowing a system of interconnecting beams just below finished floor height from which to hang pipework and cableways.
Clestra Cleanroom designs and builds prefabricated and custom modular cleanrooms ranging from Class 1 to Class 100,000. It utilizes flush-grid ceilings, 2-in. load-bearing grids, gasket grids, gel grids and slotted grids, and helps design, supervise and install the ceilings. American Cleanroom Systems (Rancho Santa Margarita, CA), a manufacturer of a variety of different floor, wall and air systems, offers modular and conventional construction, HVAC, electrical, plumbing and sprinkler systems, standard and access flooring, plus cleanroom testing and certification. Wall panels are available in vinyl-faced gypsum board, prepainted steel, aluminum, fiberglass-reinforced plastic (FRP), high pressure laminate (HPL); melamine and stainless steel. Panel cores are comprised of structural honeycomb in paper, plastic, aluminum, expanded polystyrene (EPS) and polyurethane. Glazed window panels are factory assembled with tempered glass. Glazing is held in place by extruded aluminum frame and gasket, aluminum framing 6063-T6 aluminum with a clear anodized finish or white-painted baked-enamel finish. Ceilings include T-bar suspension systems–aluminum and steel; aluminum snap-grid; perforated grid; vinyl tiles; FRP tiles. Flooring: vinyl composition tile (VCT); sheet vinyl; heat/chem weld seamless roll flooring; cove/top set base; epoxy paint/sealer; a return air/access floor.
Both floor and wall suppliers are closely watching the activities of the ISO/TC 209 Technical Committee, specifically, the activities of Working Group 4, concerned with the design and construction of cleanrooms, says Michael Campion of Stonhard, Inc., one of the largest manufacturers and installers of polymer flooring systems. “The document emanating from this Group will provide end-users and vendors with selection criteria for cleanroom materials, requiring vendors to provide finished products that will meet or exceed the performance criteria established by the Committee.” n
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Freudenberg Building Systems, Inc. (Lawrence, MA) Noraplan Plus AL and Noraplan Plus AL are smooth-surfaced ESD control rubber floor coverings designed to eliminate the problem of carbon contamination.
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USG Interiors offers wall systems for every kind of controlled environment.
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An installation by Clean Rooms West, Inc. includes a 4-in. wall system combined with the “Power T” ceiling system made of powder-coat painted aluminum. Spans up to 50 feet can be constructed.
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Clestra Cleanroom, Inc.`s CP-3 and Sylab ceiling systems allow overhead access without disturbing the cleanroom envelope.
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Tate Access Floors` new chamfered perforated access floor panel made of aluminum and designed for Class M1 applications. According to the company, the chamfered hole design provides increased air flow performance as compared to standard perforated hole design.