`Tunnel` Approach May Offer a Cost-Effective Alternative to Class 100 Ballrooms
Unit Instruments estimates that its tunnel cleanroom approach reduced the cost of its basic cleanroom approximately 25 percent from that of a raised floor. Here`s what they did.
By Eric Redemann
Class 10 (M 2.5) to Class 100 (M 3.5) vertical laminar flow cleanrooms can be successfully built using two very different approaches. And, although the raised-floor “ballroom” design is more typical, a “tunnel” approach may prove to
be a cost-effective solution, especially for cleanrooms where small mechanical products requiring hand assembly and testing at bench-top workstations are manufactured.
A Class 100 cleanroom facility, built for Unit Instruments Inc. (Yorba Linda, CA) by Pacific Environmental Technologies Inc. (PETI, also in Yorba Linda) to manufacture its mass flow controllers (MFCs), is an example of a successful implementation of the tunnel approach.
Among Unit`s requirements was the need to meet strict building standards for earthquake damage prevention, and the fact that the construction would take place inside an existing building. A unique piping system also had to be designed to feed nine calibration gases into every workstation in the cleanroom. The tunnel approach facilitated the construction of the piping system, which was configured for optimum purging and minimum dead space inside the piping. The tunnel approach also allowed servicing without disrupting the manufacturing process, whereas a ballroom design would entail lifting up panels in the floor to access calibration gas piping and process plumbing, as well as electrical circuits and equipment.
The parameters of the project included a $2 million budget and a building with a 35-ft ceiling. The building is a typical Southern California tilt-up with a built-up wood-frame roof with minimal live-load capability. All building materials used inside the chases and inside the tunnels had to be compatible with a Class 100 operation, as well as the peculiar needs of certain customers. For example, no silicone caulking could be used to seal the tunnels, and the HEPA filters also could use neither silicones nor DOP integrity testing. The HEPA media had to be made hydrophobic to prevent it from becoming a breeding ground for bacteria. This is traditionally done with a silicone treatment. However, in the hard-disk-media industry, these materials have been found to spontaneously create silica particles which damage the product. Unit also requested a direct-to-air chilled water system, because the energy benefits of a cooling tower would not offset the additional operating and maintenance complexity.
Initial proposals for this work contained four different approaches to the new cleanrooms:
1. Locate large air handlers outside the building, with ducts through the wall. The ducts would be big enough to drive a semi-trailer truck through. A “strut” grid suspended from the building roof would support ceiling T-bar and HEPA filters.
2. Place air handlers on top of the cleanrooms themselves, so the walls of the cleanrooms would be load-bearing, with double-side walls for air return into the plenum under the air handlers.
3. Mount large air conditioners on the building roof, with ducts through the ceiling. This would require an increase in roof load, with corresponding roof reinforcement.
4. Construct a freestanding equipment platform to hold the air handlers and leave the actual cleanroom walls non-load bearing. This approach was chosen.
PETI`s free-standing-platform approach simultaneously solved a number of problems (see Fig. 1). For example, it allowed the heavy air handlers to be put up and out of the way, utilizing the 35-ft ceiling, as well as the use of the equipment platform structure to form a return-air plenum around the tunnels. With side-wall-return cleanroom designs, the vertical laminar airflow must turn horizontally toward the return air intakes. However, since this natural “tenting” effect can be held to acceptable levels if the maximum lateral travel is limited to between 6 and 7 feet, many of the expenses and operating difficulties of a raised-floor design can be avoided by configuring the manufacturing processes into long 14-ft wide tunnels. Additionally, the tunnel approach offers advantages in terms of installation and maintenance of complex connections to multiple workstations.
The platform configuration also met the seismic Zone 4 building code while providing support for the extensive calibration gas distribution system, equipment exhaust, and high-vacuum manifold. Thus, the tunnel walls are non-load bearing, which also minimizes the effect of shift or settling and in turn makes it easier to seal the tunnels. If they do leak, the air pressure inside the tunnels is positive, and the pressure in the air-return plenum is negative (with respect to the tunnel, yet positive overall) so the air will carry particles out of the tunnels and not in. (The negative plenum is Class 1,000, while the tunnels are Class 100.)
The negative-air-pressure plenum surrounds the tunnels on three sides and also forms the service chase. Most construction takes place inside the existing building (or “rain tent”) which avoids the problems associated with modifying an existing structure.
Three moderately sized air handlers, each weighing 1,850 lbs and moving 12,000 ft3 of air per minute, provide a total of 36,000 cfm for each 14 ¥ 80-ft tunnel. The use of three air handlers per tunnel distributes the weight over the equipment platform and minimizes the size of ducting necessary to carry conditioned air to the individual HEPA filters in the cleanroom ceilings. It also provides redundancy and ease of access as well as the potential for three different temperature control zones to accommodate different heat loads.
The chiller is a Trane #RTAA200 packaged system that offers 195 tons of chilled water capacity, with dual compressors with continuous turn-down to 10 percent each, and drawing 400 amps from a 480V3 electrical service. The 11,100 lb chiller is located outside on an equipment pad with the two chilled water circulating pumps, each of which delivers 185 gpm.
The cleanroom ceilings include 50 percent HEPA filters over the workstations and 35 percent HEPA filters over the entire ceiling of the cleanrooms (see Fig. 2). This provides for optimum laminar flow over the workstations and adequate clean air flow over the central aisle.
Because the production line assemblers use flammable gases in the calibration of MFC products, electronic ionization methods of static control could present an extreme safety hazard. Instead, static is controlled by ensuring a constant relative humidity of 55 䔮 percent, static-dissipative welded-seam PVC flooring, and aluminum wall-panel skins coated with baked polyester paint. Special ceiling panels, for the grid spaces between the HEPA filters and lights, are also made from static-dissipative material.
Since plastic “cracked-ice”-style light diffusers will retain a static charge as well as provide a hiding place for particles, open metallized egg-crate diffusers were used instead. All 20 workstations in each cleanroom have static-dissipative tops and wrist-strap connections which are tied to tunnel “common-earth” ground lines.
Distribution of calibration gases
Precise calibration of MFCs is vital to the semiconductor industry, and the greatest satisfaction occurs when this calibration is performed in conditions which exactly mimic the customer`s operating conditions. The calibration gas distribution system was not part of the general contractor`s scope of work, and was done by a company called Industrial Mechanical (Phoenix, AZ).
At each of the 20 workstations, in each of five tunnels, the assembly technician has access to nine surrogate gases that mimic 145 gases used in customer wafer fabs. The gas delivery network includes over 6,000-ft of tubing, more than 800 fitting specialties and over 800 valves. The internally electropolished 316L tubing is orbitally welded; major subassemblies are prefabricated in a cleanroom; and all field welds are purged with high purity Argon, then helium leak-checked after assembly.
Each gas originates on the outdoor gas pad where it has a source selection panel with redundant filters (retention of particles down to 0.003 microns delivering M1.5 quality gas) and provisions to switch between empty and full sources without interrupting service. Additionally, point-of-use filtration is provided in the test equipment at every workstation. Cryogenic sources are used whenever practical because of their intrinsically lower levels of particulate and condensable contaminants compared with cylinder gases.
Diaphragm valves used
Instead of using valves such as ball, plunger and poppet valves, that twist internally and produce friction, only non-friction diaphragm valves were used. These work by clamping a movable (usually metal) diaphragm across the gas passage; they produce virtually no particles.
A unique manifold of pneumatically-actuated diaphragm valves for surrogate gas selection is located in the equipment chase remote from, but adjacent to, each workstation. This allows valves to be worked on without technicians having to crawl underneath workstations. The piping is designed to minimize the length of pipe from source to workstation and configured to minimize dead volumes. Nitrogen is always the last gas at the last place on the gas stick, to allow complete nitrogen (N2) purging. The individual diaphragm valves are activated through a matching set of 24-Vdc pilot valves allowing for either manual or computer-driven gas sequencing. An emergency shutdown mechanism dumps the pneumatic source of the diaphragm valves –all diaphragm valves in the entire gas system will automatically return to a normally closed condition, regardless of the pilot valves state, because there will no longer be pneumatic power available.
Each workstation uses a vacuum connection that is capable of pumping 100+ SLM (standard liters per minute), while maintaining a pressure of a few torr (one atmosphere = 760 torr and 28.3 SLM = 1 SCFM, so this is a few hundred cfm displacement), to mimic the environment of an MFC operating in a modern semiconductor tool. Multiple oil-free dry pumps are manifolded together to provide both the necessary pumping capacity and redundancy for uninterrupted operation.
Although opinions vary as to which type of cleanroom is better, with each having its own pluses and minuses, Unit Instruments estimates that the tunnel design reduced the cost of its basic cleanroom approximately 25 percent from that of a raised floor. Also, although the tunnels were designed to Class 100, they measure better than that in actual operation. The tunnel architecture also made it possible to add to Unit`s original cleanroom area while continuing production six days a week, 20 hours a day. n
Eric Redemann is technical director for gas systems at Unit Instruments (Yorba Linda, CA). He has over 20 years experience in sophisticated technical environments and specializes in building and managing teams in interdisciplinary technologies. His research experience includes Los Alamos National Laboratory.
Figure 1. Cross-section shows Class 100 tunnels under free-standing equipment platform, which forms Class 1,000 negative return air plenum and holds the weight of the air handlers. These air handlers move the pre-filter air before moving it through HEPA filters in ceilings of tunnels. This design proved to be an advantage over the raised-floor “ballroom” approach.
Figure 2. A drawing of a gas distribution system shows how nine remotely controlled pneumatic diaphragm valves are used to choose among nine gases for MFC calibration. This original Unit design is less costly for materials, eliminates dead space where contamination can gather, provides total access to the gas piping outside the cleanroom and allows quick purge of gas lines between gas. N2 is used for purge gas and to control pneumatic valves.
Gas distribution system was designed to provide easy access for service, an advantage over the raised-floor “ballroom” approach. Nine surrogate gases mimic 145 customer gases; each workstation has access to all gases, plus exhaust and vacuum systems to remove spent gas.
Unique line-up of pneumatically controlled individual diaphragm valves provides single delivery line from manifold to point of use. Piping size was chosen to keep uniform gas flow and constant PSI throughout the system.
Class 100 cleanroom workers under HEPA filters in ceiling. Laminar air flows down over workers and workstations, then out through side wall return vents. All building materials meet Class 100 specifications.
Equipment chase doubles as a Class 1,000 negative return-air plenum, with a lower air pressure than cleanrooms, to remove particles. The air pressure in the chase is positive, but less than that inside tunnels. Return air pre-filters are visible below windows into tunnels; primary filtration is in the air handlers.