TAIWAN The shift to 300mm silicon wafers, combined with the reduction in IC linewidths below a quarter micron, will have a significant impact on the design of next-generation cleanrooms. While the only 300mm wafer fabs in existence today are pilot lines and studies, the debate over 300mm requirements has been going on for several years. When the shift to volume production occurs, cleanroom contractors and equipment vendors say they will be ready.
With 300mm technology, the size, weight, and value of the wafers is so significant that such issues as robotics, minienvironments, contamination control, utilities consumption, and materials management are even more important.
When designing any system within the fab, one needs to understand the total costs associated with each design solution. When considering capital cost, energy consumption, and effect on building size, the FFU solution is more cost effective.
For the transition to 300mm wafers, semiconductor manufacturers are looking for a more cost-efficient cleanroom, according to Helmut Laub, president of M+W Zander Facility Engineering GmbH, a German-based cleanroom contractor. “Our clients are really looking at the cost of ownership. It is more a question that you do not over-specify, because the costs are so high.”
The general consensus in the industry is that 80 to 90 percent of the first-generation 300mm fabs will use such minienvironment approaches as standard mechanical interface (SMIF) and front-opening unified pod (FOUP). This amount dwarfs the estimated 30 percent use of SMIF for the first wave of 200mm fabs in the early 1990s.
The suppliers of equipment for these future fabs say they already have the technology solutions. “Frankly, SMIF and the minienvironment have been way ahead of its time,” says Mihir Parikh, chairman and CEO of Asyst Technologies Inc., a SMIF equipment vendor based in Fremont, CA. “We have capability today that is good for well below 0.1-micron manufacturing requirements.”
While vendors crow about the cleanliness of their minienvironments, smaller enclosures don't necessarily mean cleaner ones. Minienvironments control some environmental parameters, but there has been an assumption that airborne molecular contamination (AMC) is controlled. “Sometimes a small enclosure actually makes the [AMC] problem worse,” notes John K. Higley, vice president of sales and marketing at Extraction Systems Inc. (ESI), a supplier of AMC measurement and control products in Franklin, MA. “The solution to pollution is dilution. But in a small enclosure you don't get that dilution effect,” he says.
Laub of M+W Zander believes that while some “special solutions” will be required for AMC control, he foresees no technology barriers. One simple solution, offered by Higley, is to reduce the exposure time of the wafers to the gases in the air. “If you can reduce the wafer sit-time, you can reduce molecular problems in many cases,” he says. However, Higley concedes this is not always practical to do for all fab processes. For example in lithography this approach cannot be applied.
A significant difference in a 300mm cleanroom compared with earlier 200mm designs will be the height. With tools for 300mm wafers being larger and the requirement for more robotics to carry the heavier wafers, vertical layouts will be the norm. “The design of the cleanroom is becoming more vertical because of the space issue,” says Higley. For example, a furnace for 300mm production can be 3.5 meters high, according to M+W Zander. This will result in a revised cleanroom height recommendation of about 4.5 meters, taking into account transport systems, access, and hook-up requirements.
Also, for 300mm fabs, air filtration will be focused on the tool rather than the whole cleanroom. However, because 300mm process tools are larger, the accompanying filtration systems are also typically larger because they must filter more air. To counter this problem, ESI has developed small but powerful filtration systems targeted at the lithography area of the cleanroom. Says Higley, “300mm means the demand for highly powerful, reliable filtration is greater.” Not only that, but the monitoring technology for AMC is critical because financial losses from contamination are far greater with a large wafer.