by Robert P. Donovan
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Cleanrooms provide ideal environments for chip and overall product health; but what about the health of the people inside? The concentrations of particle contaminants are far lower inside cleanrooms than in the outside ambient air. And the same is true for airborne molecular contaminants (AMCs) in certain protected areas of contemporary semiconductor fabs.
Who could ask for more, especially in these days of bioterrorism? The same HEPA/
ULPA filters that efficiently remove aerosol particles from the makeup air entering the distribution system of a cleanroom also remove anthrax spores and other potentially hazardous airborne bio-agents. A similar claim holds for chemical agents—filters or media that trap AMCs can be made to trap chemical warfare vapors.
Protected by this filration avante garde, cleanroom workers live in their own isolate cocoons, safe from the dangers of many of today's environmental threats. Or do they?
One problem is that cleanrooms are not round-the-clock living environments. While chambers and enclosures have been designed and built for 24/7 existence, the quality of life and the associated costs make this application unacceptable for widespread use.
There is also a sound scientific basis for thinking that cleanroom filters protect human health as well as the “health” of the chip. The same mechanisms that make particle filters so efficient in capturing aerosol particles (inertial impaction, interception and diffusion) also apply to aerosol particles in human respiratory systems. However, the differing dimensions of these two particle-capturing media result in significantly different filtration characteristics.
A ULPA fibrous filter typically has a maximum penetration particle size (MPPS) in the 0.1-µm to 0.2-µm range (and even within this range aerosol particle penetration is less than 0.001%). The passageways of the human respiratory system lead to an MPPS in the 1-µm to 2-µm range, an order of magnitude larger than that of a ULPA filter. Only a particle within that general size range reaches the alveoli of the lungs. Both smaller and larger particles get trapped in the upper airways of the respiratory system.
While those responsible for the anthrax terrorism evidently prepared spores to have and maintain the size needed to remain airborne indefinitely and to achieve maximum penetration in the respiratory system, aerosol particles in this size range are readily captured by ULPA filters. Thus, the series combination of a ULPA filter upstream of the human respiratory system should reduce lung penetration of anthrax to vanishingly small numbers.
Unfortunately, ULPA filters placed in the ceiling of a cleanroom do not offer immediate protection against hazardous species generated within the cleanroom by the cleanroom processes themselves or emitted from cleanroom building materials and furniture.
ULPA protection does not take effect until such emissions are recirculated out of the cleanroom and back through the ceiling filter system. Worker exposure in the cleanroom could have already occurred by this time. Cleanroom designers minimize this exposure by employing unidirectional airflow between the ceiling entry and the floor return and by locating known or suspected contaminant-generating equipment or processes as close as possible to the floor return vents.
For example, many semiconductor manufacturers do not allow pregnant women to work in a semiconductor cleanroom because of a suspected increase in spontaneous abortions [ref 1] and concern over potential teratogenic effects of some of the chemicals used in semiconductor processing operations. Male ion implantation workers exhibit increased restrictive lung abnormality.[ref 2]
The list of potential threats to human health found in a cleanroom can be lengthy, what with all the arsines, phosphines, solvents and ionizing radiation used through out the industry. While the cleanroom environment has properties that are desirable from a health viewpoint, it is far from being a cure-all.
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.
References
- LaDou, J. and T. Rohm, “The International Electronics Industry,” International Journal of Occupational and Environmental Health 4(1), Jan – Mar, 1998, pp 1 – 18.
- Luo, J. C. J., K. H. Hsu, L. L. Hsieh, C. J. Wong and M. J. W. Chang, “Lung Function and General Illness Symptoms in a Semiconductor Manufacturing Facility,” J. Occupational and Environmental Medicine 40 (10), Oct. 1998, pp 895 – 900.