By Robert P. Donovan
Cleanrooms that are now used for full-scale production of precision products come in at least two varieties, minienvironments and ballrooms. A minienvironment, as many of you have become more familiar, is a protective enclosure customized for housing just one production tool or process—or perhaps a couple.
A ballroom, on the other hand, is a relatively large structure enclosing an entire production line or one of several large enclosures, each of which houses multiple tools and the personnel who operate and maintain those tools.
Compared to a ballroom cleanroom, a minienvironment is a relatively small structure enclosing only a relatively low volume, thus minimizing the air space that must be controlled to meet air quality requirements for a given industry such as semiconductor manufacturing.
Controlling contamination within a single, well-sealed small volume is both cheaper and more efficient than controlling a large-volume ballroom with imperfectly sealed walls. While the costs of building customized minienvironments for all the tools of a production line offset their single unit cost advantage, their efficiency advantage maintains throughout the production line.
All operators and production personnel are outside of the minienvironment enclosure and only access its interior to perform maintenance so that products are doubly isolated from anthropomorphic pollutants. Wafers enter and exit minienvironments through automated pass-throughs between the interior of the minienvironment and the environmentally isolated pods that deliver and retrieve cassettes of wafers and transport them between other process stations, which are also housed within minienvironments.
In a ballroom cleanroom, all operators and production personnel are within the environmentally controlled production space so that contaminant-sensitive products must be isolated not only from the emissions of other processes but also from those of the operating personnel. Ballroom cleanrooms feature bunny-suited personnel, often outfitted with helmets that capture exhaled breath and filter it before discharging it into the workspace environment.
Suiting up for entering the production environment of a ballroom cleanroom requires donning clothing and equipment that protect products from human contaminants. Worker comfort is only a minor consideration in specifying worker dress for duty in a ballroom cleanroom. Cosmetics are banned, beards must be covered.
Enclosing all wafer processing and transporting tools within minienvironments reduces the requirements on air quality in the space surrounding the tools. Minienvironment proponents like to allude to a future in which semiconductor manufacturing personnel will not need to wear bunny suits or other cumbersome equipment characteristic of the wafer-protecting gear required by all personnel in a ballroom cleanroom. IBM has announced that this worker-friendly feature is part of their new facility in East Fishkill, N.Y.
With such advantages, minienvironments threaten to make ballroom cleanrooms obsolete in semiconductor manufacturing or at least reduce the need for state-of-the-art ballroom cleanrooms. What does the future hold for the traditional ballroom cleanroom developed over the past 40 years? Will the fully automated, lights-out factory of the future utilizing minienvironments throughout render ballroom cleanrooms obsolete?
Most likely not. Even if semiconductor manufacturers, the major market for ballroom cleanrooms in the past, adopt minienvironments in all future facilities, other users are signing on. For example, a recent question the editors of PennWell's Laser Focus World posed to its readers was: Will cleanrooms be as important in the manufacture of photonic components as they are for semiconductor devices? The results showed that 81 percent responded “Yes,” 16 percent said “No” and 3 percent said “Don't Know.”
The rapidly expanding microelectromechanical systems (MEMS) activity requires clean manufacturing areas, but areas that are less demanding than those of today's microelectronics industry. The smallest dimensions of MEMS structures are typically larger than 1 micron, several generations behind that of the state-of-the-art in microelectronics and likely to remain so until the need for smaller mechanical structures becomes clear.
At present, MEMS products typically satisfy specialized needs in small quantities and are fabricated in small facilities compared with those of microelectronic manufacturers. Over the near term, it seems unlikely that production scales will justify customized minienvironments for MEMS.
The manufacture of flat panel displays (FPDs) actually bucks the trend toward minienvironments in that larger and larger ballroom cleanrooms are being built.1 This is not to deny the presence and importance of minienvironments for some phases of FPD production, but the size of the finished product and the equipment used to manufacture it still seems most compatible with ballroom cleanroom designs.
The criteria for choosing one cleanroom design over the other remain subjective and I suspect that both minienvironments and ballroom cleanrooms will remain part of the cleanroom industry for sometime to come. III
Robert P. Donovan is a process engineer assigned to the Sandia National Laboratories and a monthly columnist for CleanRooms magazine. He can be reached at [email protected].
References
1.O'Halloran, M., Grout, R. and Pettengill, N., “Oversized Cleanrooms Assume Huge Role in Emerging Flat-panel Display Production,” CleanRooms 16(8), August 2002, pp 24-26.