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First Printed in EOS/ESD Technology Dec 1991/Jan
1992
Clean Corona Ionization Can clean ionization be achieved by using air or nitrogen to minimize particle buildup on emitter points? Here's a look at IBM's research in this area. Kenneth D. Murray, Vaughn P. Gross- IBM General Technology Div., Essex Junction, VT 05452 Philip C.D. Hobbs- IBM Research Div., Thomas J. Watson Research Center, Yorktown Heights, NY 10598 In an attempt to achieve clean ionization and control particle generation, a standard bipolar DC ionizer was modified so it would supply filtered, dry compressed air or nitrogen to the emitter points via a perforated hose. This technique was designed to restrict the ammonium-nitrate buildup on the emitter points and also restrict the formation of ultra-fine aerosol particles (a product of high-voltage corona discharge) to measurable levels at or near background. Ionization and Particle Generation Corona-point air ionizers are widely used in clean-room environments to minimize ESD problems by neutralizing static charges and electrostatic fields (1-4). However, ionizers themselves can generate minute particles (5-8). Attempts have been made to identify the particles generated
by steady-state DC (SSDC) ionization. In 1989, the particles were tentatively
associated with ammonium nitrate If the particles are Test Setup and Procedures
The ionizers were placed 15 cm below the HEPA filters in a class 10, vertical laminar flow (approx. 100 cfm). Four condensation nucleus counters (CNCs) were used to monitor the particle concentrations. These were connected to a multiplexer processor and a computer. One meter of anti-static sample tubing was attached to each CNC. Air samples were taken 8 cm from the negative emitter points, and background counts were taken 3 cm below the HEPAs and above the influence of ionization. The effectiveness of the modified versus unmodified ionization was evaluated 1 m below the points with a charge plate monitor. The 0.05-micron filtered nitrogen had a water content o f0.1 parts per million (ppm). The 0.05-micron filtered compressed air water content was 1.0 PPM The percent relative humidity (%RH) inside the clean room lab was adjustable. As shown in Figure 1, the setup included particle monitoring under the control, the nitrogen-immersed, the room-air-immersed, and the compressed-air-immersed points. The monitors remained fixed in these positions for the duration of the experiment. The computer scanned every 10 seconds the average number of particles per cubic foot (diameter >0.01 micron) and recorded this information to the floppy disk every 10 minutes. The particle counts were recorded continuously in both high and low %RH for 30 days. After 30 days the emitter points were examined with a scanning electron microscope. Particle Concentrations
Tables 2 and 3 show typical particulate background
concentrations established at the beginning of the experiment. Of particular
interest is the average value of the compressed-air concentration over
time. The compressed air high-low range in Table 3 is noticeably similar
to typical concentrations recorded in Tables 4 and 5. If the control
range in Table 3 is subtracted from Tables 4 and 5, the remainder for
the "Comp. Air Point" would be at or near zero. For this case,
the results may be due to particles from the in-line filter. The same
may be true on the
Relative Humidity Tables 4 and 5 also reflect a 3X increase (approximately 10%) in range for relative humidity as shown by the "Air/RH Point." The typical data averaged for Tables 4 and 5 are plotted
in Figures 2 and 3. During the month-long data collection period, there
was a distinct behavior of the particle presence (See Figs. 2 and 3).
The particle presence under the room-air-immersed point appeared in
bursts with the lengths of time between low and high concentrations
varying, regardless of %RH. The particle concentration under the room-air-immersed
point consistently increased as predicted when the %RH increased (See
Figs. 2 and 3). Also shown, when moisture from the immediate vicinity
of the corona discharge only was eliminated, the humidity-dependent
particle production is restricted. This supports the theory that the
particles are associated with
Particle Buildup
Faceplate Collars A further reduction of the buildup of As indicated in Table 6, the use of faceplate collars
eliminated the ion/electron induced chemical reactions which lead to
the aerosol formation and underscored the significant influence that
The mechanisms of Material Loss Finally, the positive room-air-immersed point shows the typical tungsten material loss, which is consistent with our earlier findings (7). The gas-treated positive points show no such material loss. The loss of positive electrode material appears to be prevented by isolation of the corona points using the method described here. Conclusions The research and testing on a modified SSDC ionizer
described here shows that: Acknowledgments The authors appreciate the efforts by these individuals of IBM Essex Junction: S.J. Pierce (MFTIR, RAMP) and R.B. Dunn (SEMs). References 1. Dillenbeck, K., "Selection of Air Ionization
Within the Clean Room," Proceedings of the 32 Annual Technical
Meeting of the IES, 1986. |