Clean rooms for semiconductor manufacturing are getting purer with less-combustible plastics in tools, wet benches, ducts, and wall and floor coverings.
A single semiconductor immersion and cleaning wet bench can cost $2 million, and the usual room, or fab, making a product line 200-300 millimeters in diameter may have six to 10 wet-bench ensembles.
The choice of materials is growing, and fab operators more readily are accepting the expensive, mostly high-temperature thermoplastics listed under the FM4910 flammability test protocol.
A research arm of FM Global tests materials. Criteria were stiffened in early 1997. Commercial, industry and property insurer FM Global of Norwood, Mass., takes a serious approach to fab risks.
Fire-retardant polypropylene remains in wide use, but other entrants include polyvinylidene fluoride, Halar polyethylene chlorotrifluoroethylene and chlorinated PVC.
FM lists nearly 40 sheet materials and 16 resins vs. 16 sheet materials and one resin a couple of years ago, said Heron Peterkin, a semiconductor engineering specialist with FM Global.
He estimates that at least 60 percent of domestic producers have adopted the FM4910 materials, though “some people have made a conscious decision not to,” Peterkin said. Some semiconductor processors at the low end of the spectrum use the old flame-retardant PP, said Ted Hutton, business development engineer in Wetmore, Colo., with Atofina Chemicals Inc.'s technical polymers group. Those processors “pay the extra insurance.”
Making a tool totally 4910-compliant can pose difficulties.
Forming a shell is easy, but it's hard to get 4910 materials into internal processing areas, small valves, pumps, housings, knobs and buttons.
Even small amounts of combustible polymers present product-liability issues, particularly if a semiconductor maker demands total compliance.
Chemical compatibility issues in fabricating the equipment “keep bringing us back to using polymers in exotic fashion for complex applications,” said Al Fleming, director of product engineering for SCP Global Technologies, a major tool and wet-bench manufacturer in Boise, Idaho.
SCP suppliers make injection or rotational molded tanks, exhaust transmissions and large plenums and vacuum formed enclosure doors and plenum liners.
“Many applications require material selection for chemical compatibility or structural performance for which no FM4910-approved material exists,” Fleming said. “In most cases, tools are composed of a hybrid of FM4910-approved and nonapproved materials and augmented with fire detection and suppression to reduce fire-risk potential.”
Resins face stiff challenges, sometimes because employees at firms such as SCP still are learning about material compatibility after relying on flame-retardant PP for decades.
“Chemical process capabilities all play roles in selection of wet-bench materials,” Fleming said. Problems include chemical and gas resistance, polymer performance at varying temperatures, manufacturing-related distortion, leaching of ions, welding and bonding characteristics, outgassing and shedding of particles.
Certain chemicals can strip fluorine ions off the end of polymer chains causing decomposition and emitting ions that can attack circuitry in semiconductors, Fleming said.
“That can be a real detriment,” he said.
Often, SCP's resin and material suppliers use mold-release and stabilizing agents and additives in forming molded assemblies or bar stock. Agents can become adulterants and contaminate a product, Fleming said.
SCP uses the fluoropolymer Halar to coat aluminum or steel facing a corrosive environment.
“[Halar] is a beautiful polymer but difficult from a manufacturing standpoint,” Fleming said. “We can get pipe, sheet and fittings [in other polymers] from many sources, but Halar is not widely available.”
Montedison Group's Ausimont USA Inc. unit in Thorofare, N.J., makes Halar ECTFE resin in pellet, powder and micropellet forms.
Fire-safe materials have higher purity than a flame-retardant PP, Atofina's Hutton said in a telephone interview. Hutton has North American market responsibility for the semiconductor and chemical industries for Atofina, a unit of TotalFinaElf SA.
He recalled a major user getting contamination using flame-retardant PP.
“They couldn't figure it out,” he said. “Some wet chemistry was dripping back into process and contaminating [components].”
PVDF sheet, which Atofina markets, can cost five to seven times more than flame-retardant PP sheet, he said. The trade-off: no need to install or maintain a suppression system, if the PVDF parts are properly constructed and the site receives FM's approval.
Suppression systems, which supplement sprinklers, remain activated during normal maintenance. Hutton noted that Murphy's Law about things going wrong could cause a simple maintenance procedure to activate a suppression device. Cleanups and delayed production are costly.
Hutton said PVDF's higher cost is relatively small as a portion of a fully outfitted wet bench.
PVDF may cost $40,000-$50,000 vs. $10,000 for flame-retardant PP, he said.
Hutton chairs an American Welding Society subcommittee upgrading the use of thermoplastics in the welding process through standards and certifications.
In addition to Atofina and Ausimont, domestic clean room material sources include Solvay SA's advanced polymers unit in Houston; DuPont's fluoroproducts business unit in Wilmington, Del.; and Noveon Inc.'s Polymer Solutions Group in Cleveland.