DS 7-7R 17-12R Semiconductor Fabrication Facilities ... - FM Global

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7-7R REFERENCE DOCUMENT 17-12R SEMICONDUCTOR FABRICATION FACILITIES Page 22 Fig. 5. Clean bay service aisle. The air flow velocity in the cleanroom ranges from 40 to 100 ft/min (0.20 to 0.51 m/sec). Typical air flow volumes for new cleanrooms, whether recirculated at work stations, modules, or large global air systems, range from about 20 to 50 cfm/ft 2 (0.57 to 1.4 m 3 /min) of cleanroom. This assumes that 60 percent of the cleanroom is a service corridor with less stringent requirements. The method of returning the air from the cleanroom to the recirculation fans is accomplished by sidewall vents (Figs. 2 and 3 of Data Sheet 7-7/17-12), a perforated raised floor (Fig. 4 of Data Sheet 7-7/17-12), or a perforated raised floor opening into a basement plenum (Fig. 5 of Data Sheet 7-7/17-12). Sidewall return refers to the use of openings in the walls of the work area as the path for air return. A perforated raised floor is from 1 to 4 ft (0.3 to 1.2 m) above the structural floor and forms a plenum underneath the walking level for air return. Finally, perforations in the structural floor allows air flow directly to the basement which is used as an air plenum. ©2003 Factory Mutual Insurance Company. All rights reserved.
REFERENCE DOCUMENT 7-7R SEMICONDUCTOR FABRICATION FACILITIES 17-12R Page 23 Fig. 6. Tool service corridor. The air supplied to the cleanroom is usually a mixture of recirculated air and make-up air which compensates for leakage and exhaust losses. Since the recirculated air is cleaner and closer to the temperature and humidity requirements, a high ratio (80 to 95 percent) of recirculated-to-make-up air is provided. The concept of laminar air flow is used in nearly all semiconductor cleanrooms. Laminar flow occurs when air is made to flow in unidirectional layers when air flow velocities are maintained above 70 ft/min (0.36 m/sec). As the air flows from the supply side (usually the ceiling) to the return side (either a perforated floor or sidewall vent), particulate matter is ‘‘washed’’ away in a shower of air. The laminar flow cleanroom requires an air flow rate between 70 to 110 ft/min (0.36 to 0.56 m/sec) The average Class 100 room will operate at 90 ft/min (0.46 m/sec). If this room has a 9-ft high (2.7 m) ceiling, ten air changes per minute or 600 per hour would occur. The cleanroom is typically kept under a positive pressure in the range of about 0.15 in. W.G. (water gauge) (0.04 kPa). This is done because if there is any air leakage, or if a door or other passage is opened, the exchange of air will be from the inside to the outside. If outside air were to rush in, it would bring millions of airborne contaminates with it. In a vertical laminar flow (VLF) work station or hood (Fig. 7 and Figs. 1 through 4 of Data Sheet 7-7/17-12), the air enters from above and moves vertically downward over the work area. These stations are used in recirculating applications, or where fumes are generated, and must be removed and exhausted. The use of a VLF work station can reduce the size of the central air system and simultaneously provide a source of high velocity, filtered air to the work area. In the past, as more critical particulate control became necessary, the VLF hood approach had several drawbacks. But this problem was solved by dividing the fabrication area into separate tunnels or bays. Today, HEPA filters built into the ceilings serve the same purpose. ©2003 Factory Mutual Insurance Company. All rights reserved.
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DS 7-7R 17-12R Semiconductor Fabrication Facilities ... - FM Global

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