Particle Contamination from Air Ionizing Devices
Particle contamination from air ionizing devices is produced by three mechanisms: attraction, erosion and generation.
By William J. Larkin
As cleanrooms give way to smaller and much cleaner minienvironments, so too does the use of room ionization give way to localized, focused ionization for very specific neutralization. The use of more static dissipative materials throughout cleanrooms and the inefficiency and
high maintenance of room ionization has made full room ionization obsolete in many applications. In addition, the accumulation and generation of particles in high voltage corona ionization (HVCI) ceiling or clean booth units becomes a major concern when the same technology is localized and captured in an ultraclean minienvironment.
The generation of particles cannot be stemmed by changing materials or dry purging alone, but can be solved in some applications by using an ionizing source which does not generate or attract particles. However, using ionizing guns and in-line air ionizers in ultraclean environments present some unique and unexpected problems, which need to be examined and evaluated. These problems include: particle generation, high voltage exposure and close proximity to product of air ionizing guns.
Addressing ionizing problems
High voltage corona ionization utilizes emitter points to ionize the air. The accumulation of particles on and around these points has been a problem since HVCI`s inception. Particles effect the ionizers` efficiency in terms of total ionization current production and the plus/minus polarity balance of the ionization produced. To control particles, the largest manufacturer of HCVI devices has obtained a patent for a brush device to sweep across the emitter points from its ionizers.
HVCI emitter points use high voltage–4-10 kV and greater. High voltage is present at or near the output end, or tips, of ionizing nozzles and air guns because an ionized gas cannot travel long distances, or pass through filters, or small diameter openings without recombination of the ions occurring. HVCI must occur at the exit point of the compressed gas in order to be efficient. HVCI creates considerable voltage exposure for Electro Static Discharge Sensitive (ESDS) devices, particularly if a device is being grounded by a grounded operator or if the device is sitting on a grounded surface. In addition, because ionizing airguns are hand held, they could be dropped and handled roughly, therefore, weekly HCVI calibration may be necessary.
Often, hand-held ionizing air guns are brought very near the surface to be neutralized. This means that the ion balance or offset voltage is critical when using HVCI. Calibrating HVCI ion balance is performed at six inches from its target, per EOS/ESD Association Standard 3.1-1991. In actual use, a hand-held air gun is often brought to within 1-inch of its target. Any imbalance or offset voltage can result in residual voltage on the target. The residual voltage can cause further particle attraction and an electrostatic discharge from isolated conductors within the target device.
Phillipe J. Poidras reported that “due to the thinness of insulting layers within an electronic component a relatively low offset voltage can result in a very high field inside the dielectric. For example, 20 volts on a surface of a 0.1-micron thick insulator will create a field of 200 MV/m, which can cause the breakdown of the insulator.” [1]
Ultraclean minienvironments require that an ionizer be brought very close to the product. If the product in a minienvironment becomes charged during storage and handling, it will need to be neutralized before being transferred out of its ultraclean space. Also, the movement of the product as it exits its minienvironment can cause it to charge and attract particles, so the transfer may require continuous ionization.
Particle contamination by HVCI points
Mark Blitshteyn and Scott Shelton of the Simco Company reported that “HVCI points attract particles because the high voltage behaves as an electrostatic precipitator.” They described typical formations of particles on emitter points showing a distinct pattern following the lines of the electrical field around the point. They analyzed this formation of particles common to HVCI points in cleanrooms and found it to be predominantly silicon. Silicon occurs chiefly as oxide and silicates with particle sizes normally greater than one micron. After testing various materials as emitter points, they concluded that the point materials could not be the source of the silicon. [2]
They also showed that Steady State Direct Current (SSDC) Systems` positive HCVI points eroded to a degree significantly greater than the negative points. In some of the testing, the positive points were virtually destroyed over time.
At the same time Blitshteyn and Shelton were reporting their HVCI test results, Byh Lui et al [3] recorded production of 15,000 to 40,000 particles per cubic foot from tungsten emitters and up to 70,000 particles per cubic foot from stainless steel emitters. The majority of particles were smaller than 0.03 microns. He also reported a plume of these particles about four feet wide under the emitter points. He reported 10 particles per cubic foot less than 0.03 microns using a laser optical particle counter.
In 1988, IBM studied particulate contamination by emitter points, confirming an earlier study of particle presence. [4] Analysis of the material buildup on SSDC points indicated ammonium nitrate and major silicon (Si) and sulfur (S). In 1989, Murray and Gross [5] collected air samples which were taken just above a work surface one meter below the emitter points. They used a Condensation Nucleus Counter (CNC) sample tube with only a 3/8-inch I.D. The clean hood was rated at Class 1 or better. Particle presence was noted with the ionizer on and not detected with the ionizer off. Hood airflow was 90 cfm. “These continuous counts under SSDC at one time approached 47 k/cubic foot average.”
In this evaluation, a test was set up in a 90-100 cfm of better than Class 1 clean air hood. In fact, background particle count readings were near zero for particles larger than 0.01 microns. When the SSDC emitters are activated, a large reading of small particles greater than 0.01 microns are discovered, however few particles are greater than 0.6 microns or greater than 5 microns. Placing the CNC sample tubing 39 inches away from the emitter points might explain the finding of so few larger particles. If HVCI is used in ultraclean minienvironments or as an in-line ionizing purge for such an environment, sampling the air for particles should be done closer to the emitter.
This can also be seen in IBM`s study of the emitter points themselves. In the study, substantial material loss to the positive emitter point is suggested as a function of its operation. This was also shown by Lui and Blitshteyn.
IBM found “approximately 4.30䁾E6 cubic microns of tungsten” was released into the air during this one month time frame (assuming particle spheres). Where did this mass of material go? If, as Lui postulates, the material was vaporized by the heat and electro chemical affect on the surface of the positive emitter, where did it collect? Even though there was clear evidence of large amounts of tungsten being released, little evidence of it was detected by the sampling technique employed. Although a Vacuum Filter Collection analysis was performed, again one meter under the emitters, only one particle of tungsten out of 226 particles collected was found.
Are the particles being propelled away from the emitter in many directions and not following the flow of clean air as the smaller particles do? To answer this question, further studying needs to be performed of close-to-the-emitter-point particles. Obviously, there is a lot of action occurring at a HVCI positive emitter with a large loss of mass.
In 1990, IBM published “Clean Corona Ionization” [6] at the EOS/ESD Symposium. It concluded that particulate generation by HVCI points is a function of relative humidity. Its report seems to show small particles increase rapidly as humidity increases near the HVCI points. By keeping the amount of moisture at or near the HVCI points very low, there was less small particle generation. “This lends support to the theory that the particles are associated with ammonium nitrate (NH4NO4).”
IBM`s study also confirmed the dendritic buildup on positive emitters which seemed to be slowed by continuous, very low humidity purging of the points. In terms of cleanroom ionizing air guns, continuous purging of the ionizing point(s) is not practical. It would require having the guns purging a very clean and very dry gas continuously to try to keep the HVCI points from seeing any moisture. As soon as the gun stopped purging, the HVCI points would see ambient moisture because they must be located close to the tip of the gun to avoid recombining the ions. Also keeping the RH at 5 percent or less in a more localized work area or minienvironment would cause an exponential increase in static electricity accumulation on surfaces and materials, which could cause more problems than it prevented.
In 1992, Intel set up a similar study in a Class 1 area with the CNC sensor 18-inches away from emitter points that were non-metallic. [7] They found, “no particles of 0.014 micron or greater were generated by ionizers using the non-metallic ionizing points.” Although there was buildup of material on the non-metallic emitter tips after only one month of operation, the testing did not detect any particles.
This testing was set up as if the tests were for ionization balance or charge decay. The small collector or sensor of the CNC was placed 18 inches from the emitter points. The tests averaged an air sample traveling in a 100 cfm, Class 1 air flow. The obvious particulate accumulating on the emitter tip was never detected. Murray [4], found few metallic particles, but did detect many small particles as did Lui [3].
Murray [4] clearly demonstrated a relationship between particle generation and relative humidity. Others were claiming that the particulate generation could be reduced by changing to non-metallic HVCI points. Intel [7] has shown that even non-metallic points accumulated the classic particulate.
Both studies rely on the flow of air and particles over relatively long distances from the emitter point to the CNC sensor. Intel`s sampling in a 100 cfm, Class 1 air flow may not have detected the plume away from the CNC sensor.
All the studies [2-6] including the Intel study [7] of non-metallic emitter points, showed the classic particulate accumulation on the emitter points.
A portion of a study presented at the EOS/ESD Association Symposium in 1987 titled “A Novel Nuclear Ionization Device Employing A Pulsed Electric Field” [8] by Robert Wilson is very relevant to the problem of particulate generation. This study is relevant not because Nuclear Air Ionizers employ a pulsed voltage, but because of Wilson`s testing of the alpha ionizers in terms of particle generation.(See Table 1 and Table 2)
In the study of the alpha source in-line cartridge type, fewer particles were found because the air flow was carefully controlled, and the alpha ionizing source produced virtually no small particles from its ionization source.
In order to confirm the findings of Wilson, I submitted an in-line alpha ionized to IBM Analyzer services. The unit uses a gold foil system to encase the Polonium-210. Polonium-210 is a pure alpha emitter which produces a balanced ionized gas. (See Table 3)
This test showed there was less than 0.4 particles larger than 0.01 microns over a period of three hours.
This test was performed in-line and thus, no particles could be undetected.
Conclusions
Alpha ionizers using a gold foil encapsulation process are virtually particle free. For ultraclean minienviroments, gold foil ionizers are free of the continuing particle generation problems of HVCI. Testing of HVCI and alpha ionizers for particulate should be performed in-line so that all particulate being generated can be counted. High voltage and balance testing for ionizing air guns should be performed as close as 1 inch from the nozzle to reflect real life usage. n
References
1. Poidras, Phillipe, J., Piret, John J., Descriptions of Various Techniques Developed to Evaluate the Effectiveness of Air Ionization of a New Design of Electrostatic Neutralizer, EOS/ESD Symposium, 1989.
2. Blitshteyn, Mark; Shelton, Scott, Simco Company, “Contamination and Erosion of Cleanroom Ionizer Emitters,” Microcontamination, Aug. 1985, pp. 28-32.
3. Lui, BYH, Pui, DYH, Kinstley, WO, et al. Characterization of Electronic Ionizers for Cleanrooms. Institute of Environmental Sciences, Las Vegas, April-May 1985.
4. Murray, K.D., Amsworth G.F., and Gross, V.P. “Hood Ionization in Semiconductor Manufacturing and Evaluation,” EOS/ESD Symposium, Sept. 1988.
5. Murray, K.D., and Gross, V.P., “Ozone and Small Particle Production by Steady State DC Hood Ionization: An Evaluation,” Proceedings of the 11th EOS/ESD Symposium, 1989.
6. Murray, K.D., and Gross, V.P. “Clean Corona Ionization,” EOS/ESD Symposium Proceedings, 1990.
7. Fehrenbach, D.M., and Tsao R.R. “An Evaluation of Air Ionizers for Static Charge Reduction and Particle Emission,” EOS/ESD Symposium Proceedings, 1992.
8. Wilson, Robert, 3M Company, “A Novel Nuclear Ionization Device Employing a Pulsed Electric Field,” EOS/ESD Symposium Proceedings, 1987.
William J. Larkin is president of Alphastat Company (Salem, MA). He has more than 20 years experience in cleanroom static control. His experience centers around ionization and particle control but also includes electrostatic discharge materials, testing, consulting and training. He has 15 years experience with the 3M Company Static Control Systems Division. In 1984, he formed Alphastat, an ESD consulting and training company.
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