By Robert P. Donovan
This month's column continues the discussion initiated last month concerning the somewhat subtle sources that can generate objectionable aerosol particles in cleanroom operations.
In November, the highlighted particle source was gas to particle conversions; this month, it's particle generation by electrical discharges. Most of the manifestations of these sources are now minor players in modern cleanrooms; however, both have been recommended at one time or another as a standard method for generating challenge aerosol particles as part of a test procedure. So, the potential for a particle problem can often be present where it is not wanted or even suspected.
Deliberately creating electrical discharges between closely spaced electrodes is the operating basis of air ionization systems widely used to minimize electrical charge accumulation on ungrounded surfaces in a cleanroom. A common design for air ionization systems is to mount a number of discharge sources near the ceiling of a cleanroom, just below the incoming air exiting the HEPA/ULPA filters.
In a DC air ionization system, half of the electrode pairs operate with a negative bias between the grounded electrode of the pair and its high-voltage partner; the other half operate with a positive bias between the grounded electrode and its high-voltage partner. Such an array of positively and negatively charged electrodes introduces large concentrations of positive and negative ions into the cleanroom that neutralize both incipient positive and negative charge build-ups on any surface or object downstream of the air ionizers.
Initial embodiments of equipment developed to implement this bipolar air ionization scheme did, in fact, generate significant concentrations of small aerosol particles—just as a recommended procedure had promised—along with the desired bipolar flood of ions.1
Happily, cleanroom air ionization system manufacturers recognized this objectionable feature early on and have long since switched to electrode materials that generate far fewer particles under the discharge conditions, while still producing the high concentrations of positive and negative air ions needed to neutralize incipient charge build-ups—for example, using emitter points made of silicon.2 In this case, the particle-generating problem associated with air ionizing systems seems to have been satisfactorily resolved, at least for the present.
But particle generation by electrical discharges still remains a potential problem in semiconductor manufacturing. Plasma processing, for example, is a widely used technique in semiconductor manufacturing and, sure enough, plasma processes have also been recognized as being useful methods for generating aerosol particles in applications where having significant, controllable concentrations of aerosol particles are desirable.3, 4, 5
The same, low-pressure, silane-based plasma processes now widely used throughout the industry can generate nanoparticles that could easily become killer particles for tomorrow's devices.6 Particle behavior in the complex electric fields, and gas flows associated with plasma systems, are not always clearly understood; so, shielding products from this contaminating source can require the development of new wafer protection technology as well.
It's not clear that the full force of this source of particulate contamination has yet been felt. Plasma-generated particles typically peak in the sub 50-nm size range—still a generation or so smaller than the critical dimensions of contemporary production lines.
Throughout the development of semiconductor technology, surprising phenomena have always been encountered. This pattern seems destined to repeat in the continuing development of new and exciting semiconductor technologies that often bring unanticipated, adverse side effects along with their benefits. 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]
- Liu, B. Y. H., D. Y. H. Pui, W. O. Kinstley and W. G. Fisher, “Aerosol Charging and Neutralization and Electrostatic Discharge in Clean Rooms,” J. Environmental Sci. 30(2), March/April 1987, pp. 42-46
- Ion Systems, “In-line Ionization Kit for SRDs” [http://www.ion.com/datasheets/4620_1.asp]
- Praburam, G. and J. Goree, “A New Plasma Method of Synthesizing Aerosol Particles,”
- Aerosol Sci. 27(8), 1996, pp. 1257-1268
- Borra, J. P., A. Goldman, M. Goldman and D. Bouland, “Electrical Discharge Regimes and Aerosol Production in Point-to-plane DC High-Pressure Cold Plasmas: Aerosol Production by Electrical Discharges,” J. Aerosol Sci. 29(5/6), 1998, pp. 661-674
- Kortshagen, U.R., U.V. Bhandarkar, M.T. Swihart, and S.L. Girshick, “Generation and Growth of Nanoparticles in Low-Pressure Plasmas,” Pure and Applied Chemistry, 71, 1999, pp. 1871-1877
- Selwyn, G. S., J. Singh and R. S. Bennett, “In situ laser diagnostic studies of plasma-generated particulate contamination”, J. Vac. Sci. Technol. A7(4), July/August 1989, pp. 2758-2765