Safety Engineers Exchange Old Fab Concerns for New
Aaron Hand, Managing Editor -- Semiconductor International, 12/1/2000
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David Quadrini, who has been involved with the industry since the 1970s, saw how safety concerns developed during his years as fire marshal at Texas Instruments (TI, Dallas). "Wafer fabs have increased in cost tremendously, so that any fire, no matter how small, has much more of an impact than in years past. We're talking about hundreds of thousands of dollars, perhaps even millions," said Quadrini, who is now an independent consultant for TI. "So you have to worry about loss of production, cleanliness of the fab and getting the equipment back up."
Up in smoke
Fires are a considerable concern to fab safety. And for potential fire hazards, plastic is a major contributor.
"You'll find that virtually all the fires that you find in a fab end up being electrically related," Quadrini said. "If you have a short circuit in close proximity of plastic, you get a hot spot, it starts heating the plastic, and eventually the plastic will catch fire."
As a means to providing ultraclean process environments, Quadrini said, many of the tools are constructed of plastic. Although stainless steel certainly poses less of a fire threat, it just doesn't stand up to many of the corrosive chemicals used in such tools as wet benches or CMP systems. Wet benches, for example, are traditionally made of standard or fire-retardant polypropylene (PP) or polyvinyl chloride (PVC). Fire-retardant PP (FRPP) offers some protection against fire, but still requires that fire and smoke detection and suppression systems be incorporated into the processing tool.
Combustible tools constitute one of the most prevalent property hazards in semiconductor fabs, said Heron Peterkin, an engineering specialist for commercial and industrial property insurer FM Global (Johnston, R.I.). "When we see tools made out of polypropylene or PVC — if a fire were to occur, it would be a big fire," he said. "It's a real area of concern."
FRPP takes longer than standard PP to ignite, but the corrosive smoke and fumes that it emits when it does burn can be particularly damaging. In cooperation with International SEMATECH, Factory Mutual Research (Norwood, Mass.) developed the FM4910 Clean Room Materials Test Protocol about three years ago to make cleanrooms safer and less expensive to protect from fire. The plastics on its list now number more than 25 acceptable materials — including post-chlorinated polyvinyl chloride (CPVC), polyvinylidene fluoride (PVDF) and ethylenechlorotrifluoroethylene (ECTFE). The FM4910 materials still burn, but they do not propagate fire beyond the area of ignition or generate large quantities of smoke.
BFGoodrich (Cleveland) has developed Corzan 4910 CPVC as one alternative to standard cleanroom plastics. Corzan is FM4910-listed for use in wet benches, and is an important development for the industry, Quadrini says.
But for now, at least, Corzan and other materials like it come at a fairly high premium, he said. Because it's a new material, it's already likely to be a bit more expensive than standard materials. But where the cost really factors in is the tool manufacturers' learning curve in working with the new material. "Fabricators are still learning how to do these cost-efficiently," Quadrini said. "They all know how to use polypropylene — fire-retardant polypropylene — but they're still learning how to do things with the new materials."
What makes the materials so attractive to the industry — their fire safety — is also what makes them harder to work with. PP melts at a lower heat, making it easier to mold.
Although BFGoodrich, for example, may say that Corzan is priced competitively, the price is likely to be a considerable factor by the time it gets to the fab floor, Quadrini said. "Somewhere between the formulators of the plastic and those that manufacture the sheets, and the fabricators that take the sheets and build the tools, the price is coming up significantly," he said. However, he expects the price to come down once tool manufacturers get over the learning curve.
Leak hazards
Stainless steel is used more often, however, in tools that handle solvents, which are more likely than acids to cause fires, Quadrini said. But as process engineers continue to come up with new chemicals, he said, "all of the processes get to the point of being exotic," requiring a new look at fab safety.
Silane, a pyrophoric chemical, has certainly caused its share of accidents in the semiconductor industry. Used to put a very thin, uniform layer of silicon on wafers, it will immediately ignite if it comes in contact with atmosphere. If it accumulates, it will detonate, Quadrini said. So chipmakers must make sure that no silane is releasing where it can get trapped. If such trapped silane is disturbed or makes contact with air, it will explode.
| 1. Chemical accidents can occur when changing out cylinders in the chemical delivery system. (Source: BOC Edwards) |
However, space constraints become an issue when storing silane in bulk. It must be separated from the nearest building by about 200 feet, according to Quadrini. "That can be more of an issue in Asia, where space is at a premium," he said.
TI set up bulk silane programs at fabs in Texas and Italy. The silane is remotely located and piped into the fab. In the event of a leak, a deluge system goes off. Rather than put the fire out, the system keeps everything cool and stops the source of the silane. "You want it to burn immediately," Quadrini said. "If you've got silane burning, it's the better, accepted way to go as opposed to having silane release and accumulate, where it can detonate."
In fact, bulk chemical delivery systems almost completely eradicate the risks that the semiconductor industry faced some 20 years ago, when workers physically hand-poured the chemicals, noted Dennis Collins, senior safety engineer with Intel Corp. (Hillsboro, Ore.). In Intel's state-of-the-art facilities, he contends, visitors would be hard-pressed to find any evidence of chemicals because of the latest tool designs.
"In the old days, there were many open-face wet stations," he said. "Now they're totally enclosed, and the chemicals are delivered from other rooms."
Gas delivery has become safer as well, Collins said, moving from high-pressure cylinders to sub-atmospheric delivery, in which the gas is sucked out of the cylinders. "The only way the gas will leave the cylinders is if it's in a vacuum," he said. Although these systems are used only for selected materials and special applications, he said, their use will likely expand in the future.
Not to say that the whole industry has done away with gas and chemical hazards. "If you looked at smaller operations, you'd see the challenges that the industry had in the earlier days," Collins said. "It's the function of maturity that drives improvements."
But even the mini environments of today's state-of-the-art facilities are not foolproof. "The leak always manages to find its way outside the mini environment," said Peterkin of FM Global. "I can't say that I've really seen any instances where there's been a leak or fire, and the mini environment has saved us from a major loss."
Vinny DiGiorgio, president of Technical Risk Consulting (TRC, Attleboro, Mass.), agrees. "Leaking from valves and fittings — that just seems to happen no matter what," he said.
And any kind of liquid leakage can present problems for fab productivity. "Believe it or not, we've had more incidents involving liquid leakage than fires," Peterkin said. "And it always seems to leak over a critical piece of equipment. That's just the way Murphy operates."
| 2. Storing chemicals in the sub-floor and piping them up into the process tools helps create a safer fab environment. (Source: BOC Edwards) |
Future safety concerns
Looking at injury reports, Collins said, strains and sprains are the stuff of today's worker safety concerns. When it comes to repetitive handling of 200 mm wafers, he said, ergonomics has become the issue. But even those worries are fading as material handling becomes more automated. And the eventual move to 300 mm will make automated wafer handling a must.
But no chipmaking facility can be completely safe. For the very automation that eradicates what Collins sees as one of his few remaining safety concerns in fab design brings us back around to potential fire hazards. Automation brings robots — a potential source of electrical sparks — together with FOUPs (front opening unified pods), which are made of standard plastics that burn and smoke easily.
It does no good to fill a fab with fire-safe processing tools if you use wafer carriers that are made out of combustible materials. While wafers are being processed, FOUPs spend much of their time in stockers. "You can have hundreds of these carriers in a stocker. If a fire starts in a stocker, you can have a very large combustible fire," Peterkin said. "There's no question that — if you're looking from a probability point of view — a combustible wet bench is definitely a higher-probability event. But the consequences (of a stocker fire) are just as severe."
But FOUP manufacturers are not yet making the carriers out of FM4910 materials. "Several of the big guns like Asyst are trying to come up with fire-safe boxes," TRC's DiGiorgio said. Although they are making progress, the new plastic materials are presenting some problems related to wafer contact, he said.
The 300 mm process may also introduce as yet unknown obstacles to fab safety, simply because of the sheer size of the wafers and the need for automation, DiGiorgio said. For one, the larger wafers demand more combustible and flammable chemicals, he said.
Managing costs
No matter what new problems might arise, the fact is that chipmakers simply can't afford to be lax about fab safety. With fabs costing upwards of $1 billion to build, and every bit of downtime cutting into the productivity needed to stay in business, fab accidents can have serious consequences.
"A semiconductor cleanroom, especially one that's operating at 0.18 to 0.25 µm linewidths, can't afford to have any kind of contamination, whether from a fire or chemical leak," FM Global's Peterkin said.
And a fab that doesn't have key issues under control is likely to face very high insurance premiums, DiGiorgio noted. Taking care of the "key issues" — using fire-safe plastics, instituting redundancy of critical utilities, making sure chemical and gas distribution systems are properly arranged — reduces the "maximum foreseeable loss" number, which leads to a better insurance rate, he explained.
But sources maintain that the attitude has changed over the years for humanitarian as well as cost reasons. "Management is just more concerned about safety," Intel's Collins said. "It's just attitude; a maturing of the management group rather than cost."
Fab managers didn't concern themselves with fire protection so much in the past, Peterkin noted. "There's no fab manager around that wants to have a fire. But it's like you in your house; they just didn't think about it." Whereas protection used to mean installing a sprinkler system, chip companies are becoming more proactive, consciously looking at how to improve fab safety. •
