by Robert P. Donovan
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Many precision products, while still requiring some degree of environmental contaminant control, can settle for less than the state of-the-art cleanroom used in semiconductor manufacturing. One definition of a precision product is a product that must be manufactured in a cleanroom.
Cleanrooms now come in various sizes, shapes and designs. Ballroom cleanrooms refer to building layouts, generally covering large areas, in which a large number of operations are carried out with a common air supply and recirculation loop.
Process stations within a ballroom cleanroom are often compartmentalized and thus partially isolated from one another. But the unique feature of a ballroom design is that multiple process stations share a common air inlet supply and return.
Many of today's 300 mm fabs continue to use (and plan to continue to use) such design construction.
Over the last decade, however, the use of minienvironments for semiconductor manufacturing has begun to catch on. Minienvironments represent the opposite end of the design size spectrum from ballrooms.
A minienvironment is an enclosure surrounding just one production apparatus. More and more manufacturers are opting for minienvironments in new fab construction as the preferred design to achieve the reduced environmental contamination required by new products. Entrances and exits for product must be provided for each minienvironment, a number of which are generally located within a larger single ballroom having somewhat lower air quality than a stand-alone ballroom design. A major advantage of a minienvironment is the lower concentrations of aerosol particles that can be achieved inside a small enclosure compared to that achievable in even the latest ballroom-type cleanrooms.
While both these designs achieve very high quality air—ISO Class 3 (Class 1) or better—both are costly and represent overkill for many applications that are less demanding in contamination control than semiconductor manufacturing.
Other cleanroom options exist. A ballroom can be designed with less than 100 percent of the ceiling covered with ultra-low penetration air (ULPA) or high-efficiency particulate air (HEPA) filters, achieving somewhat lower quality air than provided by the 100 percent ceiling filter coverage typical of today's semiconductor ballroom structures.
Alternatively, small benchtop enclosures, similar in structure and size to a minienvironment, can provide high quality clean space for specific tasks in which a manually performed process or procedure is carried out inside the enclosure by an operator reaching inside through access ports. Such benchtop enclosures can be set up and operated in rooms having no more than standard heating and air conditioning service.
There is at least a third option, falling somewhere between these two extremes in size. This class of cleanroom design is often called the modular cleanroom. It is larger than a benchtop enclosure but is not room size. It usually comes equipped with its own filters and blowers. A typical design is about 8 feet in height and has walls that extend from the ceiling to six or so inches above the floor.
The area covered can vary widely and can be easily increased by joining additional modules together. Makeup air enters the ceiling filters from the surrounding room, flows downward through the enclosure and exits at the floor level.
Its walls are frequently made of flexible plastic members that are easily separated, permitting personnel to enter and exit the cleanroom without having special doors built into the walls.
The performance of these modular cleanrooms is surprisingly good. Air quality within the modular cleanroom often qualifies as ISO Class 5 (Class 100). This level of air cleanliness is adequate for manufacturing many precision products and conducting particle-sensitive research. But alas, it is not quite adequate for state-of-the-art semiconductor manufacturing. Nonetheless, modular cleanrooms often represent the most cost-effective solution for carrying out a host of second-tier, contamination-sensitive tasks.
Robert P. Donovan is a process engineer assigned to the Sandia National Laboratories as a contract employee by L&M Technologies Inc., Albuquerque, NM. His Sandia project work is developing technology for recycling spent rinse waters from semiconductor wet benches.