Description: “Wet Benches” are stations for wet etching and cleaning of wafers and devices. (“Litho Benches” in contrast, are used for resist processing.) The various wet benches differ in the specific process modules available and the materials allowed at each station. For information about some of the various wet processes available, click <here>. The available Wet Benches are:
Wet Bench Fire Safety: Update
By: Lise Laurin and Marcia Tate
Where there’s smoke there’s fire…but where there’s fire there doesn’t have to be smoke, corrosive fumes, or catastrophic damage, because cooperation between semiconductor and plastics manufacturers, industrial insurance providers, and wet bench manufacturers has resulted in safer, more efficient wet benches.
Today’s wet benches, made from specially engineered plastics, have come a long way from their beginnings in the 1970s, when they were simply modified work benches made of painted steel. By the 1980s, specialized manufacturers were making wet benches from plastics such as polypropylene. In the late 1980s, attempts to produce safer wet benches led to the development of fire retardant polypropylene (FRPP); subsequent research, however, has shown that although the FRPP formula lengthens ignition time, the corrosive smoke from burning FRPP actually causes more damage than was caused by the earlier generations of polypropylene. All plastics burn, but not all burn in the same way.
Readily available, easily fabricated, and able to resist corrosive chemicals without deteriorating or leaching out into the bath, specialized plastics are the material of choice for wet bench construction. Yet in the semiconductor industry, the very nature of the wet bench process—an electronically controlled process usually containing corrosive chemicals and sometimes incorporating electrically heated baths—means there will always be a potential source of ignition nearby. While fire isn’t the only safety issue associated with wet benches, it’s certainly the one that attracts the most attention because of the astronomical amount of damage one fire and its resulting smoke and corrosive fumes can cause.
With loss control as motivation, industrial insurance pro-viders joined plastics manufacturers and wet bench manufacturers to find plastics that don’t propagate fire, release little smoke, and don’t produce corrosive fumes as they burn.
Factory Mutual Research, a division of FM Global, began the move toward safer wet benches by establishing an index against which to measure cleanroom materials: Test Standard FM4910, Cleanroom Materials Flammability Test Protocol (1997). The first and most important feature of the FM4910 test standard is the Fire Propagation Index (FPI), which indicates tendency of a material to propagate fire. The test also indicates the comparative amounts of smoke generated according to the Smoke Development Index (SDI). Initially, the test also measured corrosive fire byproducts (CDI), but that measurement was later dropped.
In order for a material to be considered “fire-safe,” it must have an FPI equal to or less than the established acceptable level of 6. Regular polypropylene, FRPP, and polyvinyl chloride (PVC)—the conventional wet bench materials and the first materials tested—do not meet this minimum requirement.
With this measure as a guide, plastics manufacturers began formulating plastics to meet the specifications and submitting them for testing. Semiconductor manufacturers became concerned that while these new materials might be fire-safe, they might not be compatible with their processes. To address these fears, International SEMATECH, a research consortium of semiconductor manufacturers, established testing procedures to determine the process compatibility of materials destined for use in wet bench construction. The aim was to establish a set of parameters that would allow anyone to test the compatibility of any plastic material.
The Price of Progress
International SEMATECH’s tests of new plastics show that many concerns about the need to compromise purity to gain fire safety are unwarranted. The new standards have not only given plastics manufacturers an incentive to develop new materials, but also provided guidelines as to what criteria the new products must meet. As a result, many of the materials submitted for testing are proving to be superior to conventional wet bench materials.
While most new plastics are more expensive than conventional ones, the materials usually represent only a small percentage of the cost of a wet bench; in most cases the added cost of new materials is less than the cost of the built-in fire suppression system.
Complicating the conversion to new materials, however, is the lack of “off-the-shelf” parts made of fire-safe materials. To maximize safety, wet bench manufacturers must custom-build parts, and this adds to costs. Todd Thomas, president of Amerimade Technology, a wet bench manufacturer, states that at present his company is choosing its new plastics from the poly-propylene family. Since many of the smaller parts, such as fittings and latches, are not yet available in the new materials, keeping materials’ properties as similar as possible allows for easier construction and welding. Although new materials may meet criteria for construction of the wet bench structure, no test results are yet available to qualify these materials for use in process baths. To achieve a totally fire-safe system, the process baths must be upgraded to Teflon or PVDF, materials that are inherently fire-safe.
As more plastics measure up to the FM4910 and SEMA-TECH standards, and as wet bench manufacturers become more comfortable with the characteristics of the new plastics, the costs associated with their use are decreasing. Customers, too, are becoming aware of and educated about the new materials. At first, semiconductor manufacturers showed some reluctance to switch from the materials with which they were familiar; there was uncertainty about whether the change was necessary and concern about potential adverse effects on production. Increased research results, several catastrophic fab fires, and the potential of reduced insurance coverage, however, helped overcome these obstacles. According to Thomas, as recently as one year ago, wet benches constructed of the new fire-safe plastics accounted for only 10-15% of his company’s sales; that figure is now up to 50%. Informed customers are now requesting specific materials.
As plastics manufacturers rushed to submit materials for testing, it became apparent that the limited availability of the Factory Mutual Research test equipment and subsequent high cost and long lead time of the tests were hindering the process. Acceptable alternative materials were not becoming available soon enough to meet demand. Many materials that passed the fire propagation and smoke tests (20% of those submitted for testing) failed the corrosion tests. Specifically, most of the semitransparent plastics suitable for windows in minienvironments were found to produce corrosive fumes as they burned.
Concerned engineers proposed eliminating the corrosion index, allowing the use of materials that do not comply to FM4910 as long as they are not close to an ignition source, and moving to qualify more labs to run the 4910 tests. In response to industry pressure and after a review board concluded that the CDI results were not adequately repeatable, Factory Mutual Research removed the corrosion damage index from the FM4910 test standard. Factory Research Mutual researchers and others involved felt that if a material passes the stringent fire propagation and smoke development tests, it would likely cause minimal corrosion damage.
Still, there was pressure to speed up the qualification process and to make it more economical. HSB Industrial Risk Insurers chartered Underwriters Laboratory (UL) to formulate a less expensive test that could produce results as reliable as those of FM4910. While Factory Mutual Research’s tests require equipment with limited availability, the UL test uses a standard cone calorimeter. No one involved disputed Factory Mutual Research’s contributions, but input from another source was welcome. This “second opinion” validated Factory Mutual Research’s effort and research.
Working with samples donated by plastics distributors, UL and Factory Mutual Research have completed a round-robin of testing to verify the equivalency and repeatability of their tests. Representatives from UL, FM Global, HSB, and several plastics manufacturers met on November 17, 1999, to discuss the results of UL’s test. UL disclosed the results of the tests using the standard cone calorimeter. Valid test results from this less costly, more readily available equipment (installed in more than 100 laboratories worldwide), would enable quicker, less expensive testing and result in more options and lower prices. It would also allow testing in intermediate steps during R&D.
The goal of the round-robin was to formulate a single set of “fire-safety” testing standards to present to the National Fire Protection Association (NFPA) at its annual spring meeting. This would align all parties involved (insurers, materials manufacturers, semiconductor manufacturers, and process equipment manufacturers) striving for the same result, which would ultimately result in better products and safer cleanrooms.
Factory Mutual Research retested eight of the nine plastics used in the UL tests; the results of this round of testing were consistent with Factory Mutual Research’s previous findings but not identical to UL’s findings.
Two types of tests include a “full-scale” test (Photo 1) and a “small-scale” test (Photo 2). Results of the full-scale (“Parallel Panel”) test are obtained by setting up two 2 x 8 ft. panels of the same plastic and starting a fire between the panels. Researchers monitor the panels to discover how quickly they ignite and what happens to them as they burn.
In the small-scale test, small samples of material are burned in a piece of laboratory equipment, where they are monitored to measure the time it takes the material to ignite, how much heat it releases, and the amount of smoke it generates. Since a much smaller sample of plastics is used in the small-scale test, results must be compared to the Parallel Panel test results. UL correlated the cone calorimeter tests (ASTM E1354) to the Parallel Panel tests.
UL and Factory Mutual Research achieved very similar results from the panel tests, but slightly less clear-cut results from the small-scale testing. Factory Mutual Research’s apparatus and the standard cone calorimeter yielded slightly different data. Factory Mutual Research felt that further testing on a wider selection of plastics was advisable; only nine plastics were thoroughly tested using the UL procedure as opposed to the hundreds that have been tested using FM4910. While there was total agreement on the properties of the plastics at the extremes of the test parameters, there were gray areas in the middle ranges.
After reviewing the data, the NFPA committee responsible for presenting recommendations for updating the NFPA 318, the standard of protection of cleanrooms, to the NFPA annual spring meeting discussed adopting FM4910 and UL2360, the new UL subject test standard, as reference standards. A note was included indicating that the subject test standard UL2360 was under development, since UL had not completed its internal standards review process. The recommendation, which will be submitted for final approval at the international annual conference in May 2000, is that in order for a material to be considered fire-safe or noncombustible, it must pass a reference standard test conducted by a recognized testing laboratory and be “listed” by that laboratory. Robert Pearce from HSB stated that both UL and Factory Mutual Research are acceptable testing laboratories.
FM Global and its insureds are already working under a version of the fire-safe standard. Since most wet benches are custom fabricated, Factory Mutual Research must inspect each of its clients’ wet benches, both at the manufacturer’s site and on-site in its final location, to determine whether or not the bench can be considered fire-safe without the addition of a fire suppression system. Once manufacturers have a full complement of fire-safe materials, along with a set of clear-cut standards to follow, this process will flow more smoothly.
Factory Mutual Research has submitted the FM4910 to the NFPA Fire Test Committee, ASTM, and ANSI, which effectively puts it in the public domain. The test would become NFPA Standard 287 upon final review and acceptance in 2000. The test is undergoing final review and comment from ASTM, and adoption is also expected in 2000.
Looking beyond wet benches, other major applications for fire-safe materials in semiconductor applications include wafer carriers, parts cleaners, wall panels, and insulation materials that can represent significant combustible loads. FM Global’s Paul Higgins states that his company is planning to partner with several wafer transport equipment manufacturers to move away from ordinary polypropylene and polycarbonate carriers. New 300 mm wafer carrier storage systems, known as stockers, could present fire protection challenges. Sprinkler systems would not prevent smoke and water damage to their contents, and the larger enclosures required could be very difficult and expensive to protect with fire suppression systems.
So far, the push for fire safety has been a win-win process for all involved. The reduced risk of fire will simplify business for the risk managers and insurers. Semiconductor manufacturers will benefit from more stable insurance coverage—from the knowledge that their facilities and workers are more secure and from knowing that wet benches made from new materials are more process-compatible than their predecessors. Plastics manufacturers have created new markets for superior materials, tailored specifically to their clients’ needs.
From the very beginning of the research to the presentation of the joint findings to the NFPA, there has been a spirit of cooperation among all parties involved. Everyone stands to gain from a clear definition of “fire-safe” and the adoption of standards to ensure uniform requirements throughout the industry.
Thanks to Todd Thomas, Amerimade Technology (Pleasanton, CA); Paul Higgins, FM Global (Johnston, RI): Robert Pearce, HSB Industrial Risk Insurers, (San Francisco, CA).