Friday, May 8, 2009

Safeguarding the Semiconductor Fabrication Facility

Safeguarding the Semiconductor Fabrication Facility

In spite of all the precautions taken to protect the highly sensitive equipment and instruments within a semiconductor fabrication facility, it doesn’t take much to severely damage them. Typical semiconductor fabs are full of highly flammable and easily ignited combustible plastics. These include wet benches, plating benches, spray tools, and cables. In addition, there are numerous highly flammable liquids and gases and many hot surfaces. It is no wonder why this type of manufacturing is regarded as a high-risk operation. Because of these hazards, there is the potential for severe property loss from thermal and non-thermal damage, as well as substantial interruption costs caused by even a small fire incident. A little smoke, fire, water and/or vapor can halt business and shut down a facility.

According to Engineering Specialist Heron Peterkin, “A single interruption to a semiconductor fab process, even shutting down for one day, could be considered a major loss, one that could cost upwards of a $1 million or more in lost production.”

Fab Facts

Hazards associated with the semiconductor industry are related to the materials used in the construction of cleanrooms or buildings and the process tools, as well as the chemicals used in the manufacturing process.

Most semiconductor fabs and tools contain a variety of plastics including panels made of corrugated fiberglass reinforced plastic (FRP), polyvinyl chloride (PVC), and polypropylene (PP).

Many of these plastics are easily ignited; some burn fast and some give off dense smoke. Plastics will melt and become dripping, burning pieces. Additives can reduce the fire hazard, but may result in an even greater generation of smoke. Keep in mind that smoke damage can exceed fire damage, and dense smoke can hamper fire fighting efforts.

Ducts also play an essential role as part of semiconductor fab fume and smoke exhaust systems, and makeup and recirculating air systems. The wide variety of corrosive products and by-products used during the manufacturing process requires corrosion-resistant ducts, hoods, and scrubbers. Producing a state-of-the-art semiconductor device, is truly an extraordinarily technical process, and one that involves the use of a wide variety of flammable liquids and gases including silane which, when mixed with air, can self-ignite at room temperature.

Commonly used ductwork is fabricated from various plastics, including FRP, PVC, and PP. Scrubbers are usually made of FRP and PVC. The combustibility of these materials presents a multimillion-dollar damage potential from fire and products of combustion to the building (especially semiconductor fab areas), equipment, in-process products, and finished products.

The appropriate design, installation, and maintenance of the various cleanroom air handling and smoke/containment control system ducts have been part of an extensive research program at FM Global Research. The findings of this program have resulted in the updating of FM Global Data Sheet 7-7, “Semiconductor Fabrication Facilities,” which includes numerous recommendations for air handling, fume exhaust systems, and smoke containment systems. Scrubbers and other system components should be noncombustible or FM-Approved, and meet the specifications of the new “Cleanroom Materials Flammability Test Protocol.” Plastic as well as coated-metal ductwork, acceptable for use in cleanrooms, must be FM-Approved and also meet the specifications of the Test Protocol.

Prevention Necessary

Combustible plastics are one of the major concerns in a semiconductor fab. There have been many losses involving the ignition of plastic wet benches by immersion heaters, hot plates, and other electrical sources. Once the plastic wet bench is ignited, the fire is usually drawn into the fume exhaust ductwork system.

Loss example: An overheated electric heater caused a fire in a PP wet bench located in a 5,000 sq. foot (465 sq. meter) Class 100 to 1000 bay-and-chase style cleanroom. One ceiling sprinkler operated and controlled the fire. The public fire service provided final extinguishment. The PP wet bench burned and melted. Metal surfaces within the cleanroom and 2,300 in-process silicon wafers were subject to chloride contamination. The interior of the 5,000 sq. foot (465 sq. meter) cleanroom was contaminated by smoke, soot, and humidity. It took 26 days to reach 66% of pre-incident production levels and 74 days to reach 100% of pre-incident production levels. This loss shows that even a quickly controlled ceiling sprinkler fire still caused a large loss, one estimated at more than $12 million.

Combustible wet benches are difficult to protect. They usually need sprinkler protection but, generally, that’s not enough. By the time a fire in a plastic tool is large enough to operate a ceiling sprinkler, millions of dollars of smoke damage usually has occurred. Therefore, a fast-acting supplemental fire protection system, such as fine-water spray, CO2 or FM-200 is needed within the tool itself. While very effective, this approach can be very expensive and requires ongoing, vigilant maintenance. But even that combination is not risk free.

To add to the problem, products manufactured in semiconductor fabs are expensive and easily destroyed. In fact, they are so vulnerable that the tiniest bit of contamination from any size fire could destroy millions of dollars of work in process. Specialty computer chips can cost as much as $1,000 each, meaning one major loss could cost a company as much as a billion dollars. In fact, it’s not unusual that a typical maximum foreseeable loss for a semiconductor fab contamination could be in excess of $100 million or as high as $1 billion. Small incidents in semiconductor fabs average about $2.5 million compared to a cost of $250,000 for small incidents at other occupancies.

According to Peterkin, a company that manufactures computer chips cannot afford to lose even one day of production. “A semiconductor fabrication facility is a delicate operation,” said Peterkin. “Equipment is finely calibrated to repeatedly produce the same device. If there is any equipment damage, it could take months to repair or qualify new equipment.”

How do you prevent business interruption? “FM Global recommends that its customers do not make semiconductor fab tools out of combustible plastics,” said Peterkin. “The best way to build a fab is to use a noncombustible material like stainless steel. If that’s not an option, then use non-propagating material that has been tested to the 4910 Cleanroom Protocol. In the interim, make sure you have a good fire and smoke suppression system in place within each tool.”

Other precautions include storing and dispensing flammable and corrosive liquids according to Data Sheet 7-29, “Flammable Liquid Storage in Portable Containers” and Data Sheet 7-32, “Flammable Liquid Operations,” as applicable. If at all possible, these liquids shouldn’t be stored in the semiconductor fab area. If it’s absolutely necessary to store them there, flammable liquids should be stored in FM-approved safety cabinets and corrosive liquids should be stored inside metal cabinets or inside cabinets made with plastic materials which are listed to meet the 4910 Protocol.

In addition, semiconductor fabs should be protected from exposures from other occupancies located within your facility and from natural hazard-related occurrences such as flood, windstorm, and freezing weather. They also should be separated from adjacent occupancies and should contain noncombustible interior subdivision walls to limit the spread of smoke and other contaminants in case of an accident.

Semiconductor fabs are very dependent on electrical power supplies. If you’re building a new semiconductor fab facility, a complete reliability study of the local electric utility should be conducted by a qualified engineering consulting firm, prior to the final selection of a new site. It should focus on the number, duration, and causes of major system outages to the proposed site location over a 10-year period because electric service interruption is one of the major causes of semiconductor fab losses.

1 comment:

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