I continue to be mystified by the universal, resounding success of minienvironments in reducing wafer contamination. Everyone who has examined and reported the impact of minienvironments on product contamination agrees that wafer contamination is reduced and reduced significantly when compared to similar processing operations carried out in the traditional ballroom type of cleanroom [Refs 1 -3].

Why should I be mystified? Clearly it's easier to clean up such small, tight enclosures as minienvironments than it is a huge ballroom with its multiple sources of potential leaks and obstructions to unidirectional gas flow. Particle concentration in an empty enclosure, sized to isolate a typical unit of process equipment, can be rapidly reduced to the background levels of the particle counter, even a condensation particle counter, by flushing it with ambient room air passed through a run-of-the-mill HEPA or ULPA filter. Particle concentration in a ballroom type cleanroom seldom reaches these low background levels. So it's easy to understand that the ambient air adjacent to the process equipment is made cleaner by enclosing the equipment in a minienvironment.

My problem, however, arises in reconciling the current consensus that the major source of particulate contamination in wafer production is no longer the ambient cleanroom environment or the personnel within the cleanroom but rather the production processes and the process materials themselves. It's not clear to me how a minienvironment reduces these equipment and process-related sources of particulate contamination. On the surface, a minienvironment reduces only a contamination source that is no longer thought to be dominant or as important as it once was. Any resulting improvement in wafer contamination during processing should thus be relatively minor, since processing within a minienvironment would seem not to reduce the major sources of wafer contamination in contemporary manufacturing.

It's hard to argue with the demonstrated success of minienvironments, but some of us still wonder why. Why do minienvironments perform so well?

References

1. Holzel, R., “Minienvironment Systems: Current Performance and Outlook”, 1993 Microcontamination Proceedings, pp. 278 – 297 (Canon Communications LLC, 11444 W. Olympic Blvd., Ste. 900, Los Angeles, CA 90064)
2. Rothman, L. B. et al, “SEMATECH Minienvironment Benchmarking Project”, 1994 Proceedings of the Institute of Environmental Sciences, pp. 387 – 395 (IEST, 940 East Northwest Highway, Mount Prospect, IL 60056)
3. Weiss, R. S., “SMIF Concept: Production Implementation of Micro-Environments”, 1990 Microcontamination Proceedings, pp. 391 – 400 (Canon Communications LLC, 11444 W. Olympic Blvd., Ste. 900, Los Angeles, CA 90064)

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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.