Minienvironments cost significantly less to construct than full-scale cleanroom facilities.
In 1970, the initial cost of building and equipping a high-volume, state-of-the-art production cleanroom was $30 million. By 1990, this cost had increased tenfold to $300 million. The use of the minienvironment can offer savings in reduced initial construction costs for environmental conditioning systems, commensurate reduced annual operating costs, a highly flexible equipment layout, and reduced costs for personnel and operator gowning procedures. [McIlvaine, 1992]
The volume of clean air required to maintain acceptable contamination levels in the cleanroom is dramatically reduced. According to Cleanrooms - 1992-2000, Rooms and Components Vol. Three (1992) to maintain
... a conventional cleanroom of 40,000 square feet uniformly clean to Class 1 standards requires the recirculation, conditioning, and filtration of over four million cubic feet of clean air per minute. Operating the required air handling equipment consumes approximately ten million kWh per year of electricity. This can cost as much as $1 million per year at an assumed cost of $0.10/kWh. A [small] fraction of the ... clean air volume is required in an equivalent minienvironment cleanroom because, while the local environment around the product and the processing equipment is kept ultra-clean, the rest of the room is kept at a much lower level of cleanliness. [The McIlvaine Co., Illinois, Correspondence, October 1995]
Different cleanroom design concepts and their energy consumption have been compared by Lynn (1991). All four of the concepts studied are currently being used in industry to obtain Class 1 Performance.
1.Clean Bay Return Chase (This concept is a common configuration in Class 10 or better cleanrooms.),
2.Clean Bay/Supply Air to Chase,
3.Ballroom, and
4.Minienvironments.
To compare these concepts on equal terms, the process equipment load and building exhaust flow were held constant; the only variation among the four concepts came from varying air-flow rates for the cleanroom recirculation systems.
Total energy consumption varied by site location, but energy consumption for the minienvironment was the lowest for both locations modeled: New England and San Jose, CA. While the percent differentials among concepts varied by location, the minienvironment concept ranked the lowest in each location.
The San Jose location yielded a variation of 29% between the annual maximum and minimum total energy consumption. [Lynn, 1991]
Fan energy consumes a greater percentage of the total energy in San Jose than in New England, resulting in a wider variation in energy use between these two geographic locations. Colder climates require more heating energy; therefore, the total energy impact of fan energy savings is reduced. Cost savings for reducing fan energy can be significant in milder climates. Minimizing total cleanroom air flow directly affects the operating costs of a facility. [Lynn, 1991]
In another case study, a 50,000 sq. ft. cleanroom was designed to produce 30,000 wafers per month. According to O'Reilly and Rhine (1995), "the design used Class 1000 open ballroom, with minienvironments to provide Class 0.1 where wafers were exposed." [O'Reilly and Rhine, 1995]
In this case study, savings were found in both construction and operating costs. The final design reduced the number of air handling units by 52 percent, reduced the number of ULPA filters required by 54 percent, and reduced the clean air volume flow-rate by 49 percent. "Savings for operating costs were calculated at $2 per wafer,." according to O'Reilly and Rhine (1995) and an additional benefit that was not foreseen "was the increase in ramp-up of the process, which gave them a 4 month advance on getting product to the market."
In another case study noted by O'Reilly and Rhine (1995), "the requirement was for a Class 100 cleanroom for development of optical devices." However, because of a limited budget, "[a] general Class 10,000 cleanroom [was designed instead] with minienvironments. The air handling volume was reduced by 75% with a better than Class 10 level for the equipment was maintained."
