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Several large semiconductor cleanroom fires in Asia and the potential for a large number of fire losses annually in wafer fab production have led to an entirely new philosophy and to guidelines for the selection of cleanroom materials of construction.

Factory Mutual (now FM Global) has estimated that 1 in 10 fab locations suffer a fire loss annually. In comparison, this figure was estimated to be as low as 1 in 100 in other industrial locations. In addition, the average fab loss equaled $8,000,000 as opposed to a $250,000 average for other industrial accidents. The leading loss factor by a considerable percentage (47%) was deemed to be caused by fire.1 This large loss equation led to sweeping changes in industry norms and perceptions when specifying or constructing materials for wet bench efficiency and safety.Cost, Purity. Due to the purity-conscious nature of semiconductor wafer fab production (low leachables), a large quantity of published extraction data is available.

 

 

PVDF has played a dominant role within the microelectronics industry since the 1970s for this reason.3 One of the advantages of PVDF is its availability in multiple component forms (pipe, pump, valves, stock shapes, filtration devices).

ECTFE is also considered to have desirable characteristics for semiconductor applications. ECTFE has primarily been used as a coatings material for pipes, tanks and ductwork.Welding. The welding performance of a particular material of construction is essential due to the extensive quantity and quality of welds required in wet benches (chemical distribution cabinets, spin rinse dryers, acid tools, parts washers, piping, valves). Standard DVS recommended welding conditions for polymer materials are shown in Table 2.

CPVC, due to its additives and stabilizers for processing, has been most prominently used in industrial applications for low-temperature chemical applications and its smoke and flame characteristics. Although more expensive than FRPP or polyvinyl chloride (PVC), CPVC costs less than fluoropolymer alternatives (Table 1). The cost of plastic sheet has been said to represent from 15 to 30% of the total tool price.4 This estimation is determined by its size, complexity, and degree of automation. 

 

 1 FM Global, “Understanding The Hazard, Semiconductor Clean Rooms,” 2001 Semicon West Show Forum, pp. 4-14.2 Tewarson, Archibald. “Flammability of Clean Room Materials,” Factory Mutual Research Corporation, November 1996, pp. 1-6.3 Henley, Mike. “PVDF Remains Favorite For Piping In Semiconductor Plants,” Ultrapure Water, December, 1997, pp. 16-22.4 Roman, Diana. “Making Informed Choices in Wet Bench Fire Safety,” A2C2, February 2002, pp. 13-15, 2002).

The National Fire Protection Association (NFPA) 318 standard for the protection of semiconductor facilities now includes welding requirements for thermoplastic materials. This will serve to create a standard for welding quality enforcement. In this work, the use of FM4910-compliant materials at the workstation allows for a greater usage of chemicals. This factor may play a more significant role as the industry moves to larger 300 mm chip technology and the subsequent volume of chemicals is expected to increase.

Cooperation between semiconductor industry manufacturers, code bodies, and material of construction producers has created an environment for improved safety in cleanrooms. This article was written to trace the evolution of these new standards and factors for the design engineer to consider when selecting a material of construction for their particular fabrication.

References

 

 

 

 

 

Back To Its Roots

The cleanroom wet bench fire safety issue was revolutionized in 1996 when Factory Mutual Research Corporation prepared a report titled, “Flammability of Clean Room Materials.”2 The original purpose of the testing and document was threefold: first, to determine the fire properties of common cleanroom materials; second, to identify materials which would provide low non-thermal damage; third, and likely its most important legacy, to develop guidelines for the use of cleanroom materials to reduce or eliminate both thermal and non-thermal damage. This quickly led to a flurry of committee work by semiconductor industry associations who debated and examined the industry standards in place at the time. These groups proceeded to scientifically create testing procedures to determine the best obtainable solutions. In essence, the issues boiled down to cleanrooms are associated with very high loss expectancies in fire accidents. This takes into account both equipment damage and the high down time costs incurred with loss of production. It thus was deemed essential that any fire be contained within the ignition zone with a very limited release of smoke and corrosive products. The dilemma was that this needed to be accomplished without compromising industry standards such as purity and yield rates.

The smoke and flame specification enacted, Factory Mutual 4910 (FM 4910), determines an index criteria for fire propagation behavior (FPI) and smoke contamination (SDI).

FPI indicates the rate at which the surface of a material is involved on fire. SDI indicates the smoke contamination of the environment during a fire. FPI values were correlated using information from other common non-fire propagation tests, such as the plenum wire specification called Underwriter’s Laboratories 910 (UL 910). A pass value was then determined. The SDI criteria are based on limiting smoke concentration. A similar standard, UL 2360, was later developed to provide several recognized sources for testing.

Materials of Construction

Materials which were once utilized in wet bench constructions, most notably flame retardant polypropylene (FRPP), began to fall out of favor as the industry increasingly began to accept the new FM4910 and UL2360 fire safety standards. Non-FM4910 materials such as flame retardant polypropylene (FRPP) can still be used but require internal fire extinguishing and suppression systems. One of the drawbacks cited with this approach remains potential accidental discharge creating clean up and down time costs. This led to a host of new wet bench fire safe polymer alternatives including CPVC, PVDF, PVDF Flex 2850, and ECTFE.

Fluoropolymers (PVDF, PVDF Flex, ECTFE, ETFE) offer an advantage over other polymers in that they are inherently both fire and chemically resistant (no additives required). This is due to their overall molecular stability attributed to the carbon-fluorine bond. The flame and smoke resistance of the fluoropolymer family of resins can be traced back to plenum wire and cable applications since 1981. This plenum wire standard (UL 910) had created the single largest market for fluoropolymer materials in the 1980s. The concept, similar to FM4910 and UL2360, was to eliminate or restrict fire damage.

Selection Criteria

Factors beyond fire and chemical compatibility to consider when selecting an appropriate wet bench material of construction include cost, purity, mechanical properties, and weldability.

 

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