A Material Difference
As computer chips get smaller and faster, the manufacturing process required to create them gets more complex. Even the slightest delay in production can mean millions of dollars in lost revenue. To offset the potential for contamination, chips are manufactured in the highly filtered environment of a cleanroom.

However, because computer chips are so susceptible even to the tiniest speck of dust, cleanrooms, historically, have been difficult areas to protect from fires. Contamination from a fire, no matter how small, could potentially put a chip maker out of business for weeks—if not permanently.

In 1997, FM Approvals, a Nationally Recognized Testing Laboratory, introduced the Cleanroom Materials Flammability Test Protocol (Class 4910) [1].

In the past, cleanrooms and wet benches (plastic or stainless steel workstations upon which computer chips are manufactured) often needed to be protected by sprinklers or more expensive special fire-protection systems like carbon dioxide, fine-water spray, or halon. By the time a cleanroom fire propagated and triggered a sprinkler or special fire protection system, millions of dollars in property damage could already have occurred in the rest of the cleanroom. With FM4910, wet-bench manufacturers and users can now develop plastic materials and equipment capable of resisting fire and emitting little, if any, smoke.

Due to such factors as potential lost earnings, chip makers are requiring suppliers to use materials in wet-bench fabrication that are less flammable. This should reduce the need for additional—and costly—fire protection systems because the materials will be inherently safe when they arrive in the cleanroom. Consequently, interest in cleanroom plastics with low fire risk has soared among semiconductor manufacturers and tool providers since the turn of the millennium. During that time, an increasing amount of tools constructed of FM4910 fire-safe materials have been installed in semiconductor cleanrooms and the frequency of devastating cleanroom fires has plummeted by approximately 75 percent.

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 Development Index (SDI), an indicator of the amount of smoke generated.

For material to be considered fire-safe under FM4910, its FPI must be equal to or less than 6 and its SDI equal to or less than 0.4. Today, scores of FM4910-listed materials and products made with such materials are available from nearly two dozen manufacturers. 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 their 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 (pharmaceutical, biotech, food processing, etc.). FM4910 fire-safe materials are helping prevent cleanroom fires. But they can’t do it alone.

Codes and Standards
The following codes and standards have significantly improved cleanroom fire prevention:

• National Fire Protection Association (NFPA) 318 “Standard for the Protection of Semiconductor Fabrication Facilities”
• Semiconductor Equipment Materials International (SEMI) S2 “Environmental, Health, and Safety Guideline for Semiconductor Manufacturing Equipment”
• SEMI S14 “Safety Guidelines for Fire Risk Assessment and Mitigation for Semiconductor Manufacturing Equipment”
• FM Global Property Loss Prevention Data Sheet 7-7 “Semiconductor Fabrication Facilities”
  Each of these documents recommends the use of fire-safe construction materials for cleanroom applications. In cases where such materials are not used, fixed fire detection and suppression units are the recommended alternative. If neither measure is taken, the results can be catastrophic.

Proper Con-Duct
During the past ten years, a handful of semiconductor companies have suffered crippling fires in a frighteningly similar manner. A fire would start in a process tool, such as a wet bench, and then be drawn into the fume exhaust system. The fume exhaust system was typically made of combustible material and devoid of internal automatic sprinkler protection. As a result, the fire would spread rapidly throughout the ductwork system all the way to the scrubbers. In new fume exhaust ductwork installations, the preferred material is one which meets FM Approval Standard for Fume Exhaust Ducts or Fume and Smoke Exhaust Ducts (Class 4922) [1]. Even when subjected to a severe fire, FM4922-Approved ductwork will not collapse, will not propagate fire, and will release only minimal amounts of smoke. Increasingly, new and retrofitted cleanrooms are installing FM4922-Approved ductwork. However, a considerable amount of combustible ductwork with no automatic sprinkler protection remains in many 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 semiconductor cleanrooms is not as daunting a challenge as it may appear. In fact, a major semiconductor manufacturer is successfully conducting such a replacement right now. To that company, the benefits substantially outweigh the potentially astronomical costs of not having the proper protection. And more help is on the horizon.

Where We’re Headed

At least one significant cleanroom fire hazard still needs to be addressed: containers used to store in-process wafers.

Wafer carriers or pods (200mm wafers) and front opening unified pods, or FOUPs (300mm wafers), are currently made of highly combustible materials like polycarbonate and polypropylene. These pods or FOUPs are typically placed inside vertical storage systems known as stockers. Fire loves few things more than highly combustible materials placed in a vertical array.

In December 2002, FM Approvals issued the Approval Standard for Wafer Carriers for use in Cleanrooms (Class 4911) [1]. This standard provides testing criteria similar to FM4910 in order for a fire-safe wafer carrier to earn FM Approval. An FM4911-Approved wafer carrier is expected to be brought to market sometime this year.

In 2005, FM Approvals expects to issue an evaluation standard for Tools Used in the Semiconductor Industry (Class 7701). This new product test standard will address design and construction features of semiconductor production equipment including:

• Fire
• Chemical
• Materials
• Electrical
• Ventilation
• Control and/or safety interlocks

Currently, before new tools are installed, they often require on-site evaluation (either at the manufacturer or client’s facility) by a semiconductor manufacturing specialist on a case-by-case basis. Due to the complexity and diversity of semiconductor manufacturing equipment, this can be a time-consuming, potentially costly endeavor.

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.

While great progress has been made recently in cleanroom fire prevention, there’s still work to be done. We may not be there yet, but our destination is clearly in sight.

[1] Note: for more on FM4910 (including an up-to-date listing of manufacturers and materials), FM4922, and FM4911, visit www.fmglobal.com/approvals.

Vincent DeGiorgio is Industry Leader for Semiconductor & High Technology at FM Global, Inc., 1301 Atwood Avenue, P.O. Box 7500, Johnston, RI 02919. He can be reached at 401-275-3000 x 1994, or vincent.degiorgio@fmglobal.com.