Non-fire propagating building materials and tools, along with adherence to fire safety standards, have made the cleanroom industry safer and more aware of the potential cost of cleanroom fires
By Vinnie DeGiorgio, FM Global
During the past 30 years, the semiconductor industry has experienced exponential growth that has significantly affected consumers’ daily lives. This growth, however, has come at the expense of semiconductor process-related fires that have caused devastating property damage, production interruption, and loss of market share.
Following the two large fires in Taiwan in 1996 and 1997, respectively, FM Global and other fire protection professionals stepped back and asked, “How can we improve fire safety and change the industry’s track record?”
Ten years later, with the collective efforts of many organizations and individual contributions, the journey toward the fire-safe cleanroom is almost complete.
Factors that have led to this tremendous improvement include the following:
- Widespread use of non-fire propagating construction materials
- Process liquids being heated remotely
- Third-party assessment of process equipment prior to installation
- Improved handling and disposal of silane gas
- Adherence to improved codes and standards
Current state
Released in 1997 by FM Approvals, a Nationally Recognized Testing Laboratory (NRTL), the Cleanroom Materials Flammability Test Protocol (Class 4910) has become the industry standard for the evaluation of construction materials used in cleanrooms.
FM4910 measures two crucial, fire-related elements of a product or material: the fire propagation index (FPI), an indicator of the tendency of a material to ignite and propagate fire, and the smoke damage index (SDI), an indicator of the amount of smoke generated.
For material to be considered non-fire propagating under FM4910, its FPI must be equal to or less than 6.0 and its SDI equal to or less than 0.4. Materials that meet the Cleanroom Materials Flammability Test Protocol do not require, in and of themselves, fixed fire protection when used according to the appropriate FM Global data sheets. Materials listed under FM4910 may burn locally in the ignition area but will not propagate a fire beyond the ignition zone. Additionally, such materials produce little, if any, smoke or corrosive byproducts, thus minimizing non-thermal damage.
Today, there are 17 manufacturers producing nearly 150 different types of FM4910-listed materials. A complete listing of FM4910 materials can be found at www.fmglobal.com/assets/pdf/4910Approved.doc.
This ever-expanding list has led semiconductor tool vendors to build the majority of products (e.g., wet benches) out of FM4910 materials. In fact, for some tool vendors, FM4910 material-constructed tools have become the standard. Tools made with less expensive (but highly combustible) polypropylene or polyvinyl chloride are, in many cases, now available only by special order.
While FM4910 materials are prevalent in semiconductor cleanrooms, they can easily be applied in other industries that utilize cleanrooms, such as the pharmaceutical, biotech, and food processing industries.
Although FM4910 fire-safe materials are helping prevent cleanroom fires, they can’t do it alone.
In the past, the typical fire scenario was a high-energy process liquid immersion heater or hot plate igniting combustible plastic associated with wet benches. Once ignited, the fire was drawn into the process exhaust ductwork, which many times was constructed of combustible plastic. Once the ductwork was ignited, the fire would spread inside the ductwork all the way to the scrubber.
Companies have eliminated the exposure created by combustible process exhaust ductwork by installing ductwork that meets FM Approvals’ Standard for Fume Exhaust Ducts or Fume and Smoke Exhaust Ducts (Class 4922). Even when subjected to a severe fire, FM4922-approved ductwork will not collapse or propagate fire and will release only minimal amounts of smoke.
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Increasingly, new and retrofitted cleanrooms are installing FM4922-approved ductwork; however, a considerable amount of combustible ductwork with no automatic sprinkler protection still remains in some facilities. Installing either FM4922 products or proper sprinkler protection in these cleanrooms is highly recommended.
Replacing combustible ductwork with FM4922-approved ductwork in existing, operating cleanrooms is not as daunting a challenge as it may seem. In fact, a major semiconductor manufacturer has successfully completed such a replacement. To that company, the benefits substantially outweigh the potentially astronomical loss.
Codes/standards
As mentioned previously, adherence to improved codes and standards (see Table 1) has been a significant contributing factor in improving cleanroom fire safety.
Many of these codes/standards recommend the use of fire-safe construction materials for cleanroom applications. In cases where such materials are not used, fixed fire detection and suppression systems are the recommended alternative. If neither measure is taken, the results can be catastrophic.
In 2005, FM Approvals issued an assessment standard for tools used in the semiconductor industry (Class 7701). This assessment standard evaluates the following aspects of semiconductor manufacturing equipment:
- Chemical
- Control and/or safety interlocks
- Electrical
- Ventilation
Currently, before new tools are installed, they often require on-site evaluation at either the manufacturer or client’s facility by a semiconductor specialist on a case-by-case basis. Due to the complexity and diversity of semiconductor manufacturing equipment, this can be a time-consuming and potentially costly endeavor.
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When a tool is evaluated under FM7701, it will only require a spot check after installation, saving tool vendors and semiconductor manufacturing companies significant amounts of time and money. Having fire-safe semiconductor equipment is critical in the event of a fire. Consider the fire damage consequences cited at the outset of this article.
A major semiconductor equipment manufacturer has recently completed the FM7701 assessment and will shortly become the first company to receive formal FM7701 recognition.
Silane gas usage
Silane, more so than other gases used in semiconductor manufacturing, can lead to severe explosions. It is a stable gas but is pyrophoric, which means that under certain conditions it can spontaneously ignite or have delayed ignition that could lead to an explosion.
Silane has been involved in a considerable number of fires. Common scenarios included untreated silane released into combustible fume exhaust ductwork and improper cylinder change-out procedures resulting in leaks at the cylinder connection points.
During the past couple of years, there have been several unfortunate events that have led to a renewed interest in the safe handling of silane. During the past 2 years, Air Products and Chemicals, Inc. organized a series of silane safety seminars held in Taiwan, Korea, China, and Singapore and most recently in Portland, OR. The goal of these highly successful seminars was to educate the audience on the behavior and best protection practices concerning silane.
Open issues
One significant cleanroom fire hazard still needs to be addressed: containers used to store in-process silicon wafers.
Wafer carriers or pods (200-mm wafers) and front-opening unified pods, or FOUPs (300-mm wafers), are currently made of highly combustible materials such as polycarbonate and polypropylene. These pods or FOUPs are typically placed inside vertical storage systems known as stockers. Fire burns for highly combustible materials placed in a vertical array.
FM Approvals issued the “Approval Standard for Wafer Carriers for use in Cleanrooms” (Class 4911). This standard provides testing criteria similar to FM4910 in order for a fire-safe wafer carrier to earn approval.
A supplier of wafer carriers and FOUPs is presently working with FM Approvals to develop a FOUP that will meet the requirements of FM4911 as well as the strict process requirements.
New fire safety products
The products in Table 2 have recently received FM Approvals certification and can improve overall cleanroom fire safety.
Future outlook
The outlook for further progress in cleanroom protection looks very promising. The use of FM4910 materials and FM4922 ductwork is well-established. Current codes and standards support the use of fire-safe materials. Attention needs to be focused toward bringing an FM4911-approved wafer carrier/FOUP to market. Recently released products have provided solutions to areas previously lacking fire-safe alternatives.
Vinnie DeGiorgio is FM Global’s principal engineer for the semiconductor industry. He has more than 26 years of property loss control and business impact risk assessment experience associated with the semiconductor and related high technology industries. DeGiorgio has a bachelor’s in engineering and a master’s in fire protection engineering. He maintains memberships in the NFPA and SFPE, is a board member of SESHA, and is the secretary of the NFPA 318 Technical Committee on Cleanrooms. More than one out of every three FORTUNE 1,000 companies work with FM Global (www.fmglobal.com) to develop robust property insurance and engineering solutions to protect their business operations from fire, natural disasters, and other types of property risk.