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
(); a material found to
precipitate onto the emitter points over time (4,7). Micro Fourier
Transform Infrared (MFTIR) and Raman microprobe (RAMP) showed the
material to have major spectral features and Raman shifts indicative
of , respectively (4).
There were not enough aerosol particles collected to determine conclusively,
by MFTIR or RAMP, the particles' primary chemical compound. However,
energy dispersive x-ray results showed similar elemental associations
between the particles captured and the collected
from the points (7).
If the particles are ,
then their formation would require a source of hydrogen, with the
most plausible one being atmospheric water vapor. According to test
results, particle production can be consistently restricted with
dry, compressed air or nitrogen-immersed corona points. This technique
can also restrict buildup
of the points. Particle generation has also been shown to be a function
Test Setup and Procedures
SSDC ionizers were modified as shown in Figure 1. A plastic covering,
or faceplate, was attached to half the length of each unit. Behind
the faceplates, gas bleeder lines were used to flood the inside
cavities of each ionizer with reasonably dry, filtered gas. The
holes cut in the faceplates, in front of each point, were 1 cm in
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.
In Table 1, voltage decay rates are shown in seconds under the modified
vs. unmodified ionizations. The gas-treated points took about 3
seconds longer to decay 5000V to zero (averaging 14.4 seconds).
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 nitrogen
side. To address this concern, an improved filter system or an extended
breaking-in period would be needed.
Tables 4 and 5 also reflect a 3X increase
(approximately 10%) in range for relative humidity as shown by the
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 .
30 days of operation, the precipitate
could easily be seen on the room-air-immersed emitter points. Scanning
electron micrographs (SEMs) of the points were taken at 100 power
(Photos 1-9). The SEMs show that all of the positive points had
buildup to some degree. For the positive points the buildup was
dendritic. The room-air-immersed negative points had deposits which
were nodular in appearance. In comparison, the overall buildup was
much less pronounced on the dry gas-treated points.
A further reduction of the buildup
of may be achieved by
the attachment of collars
(3 mm in length) around the faceplate openings cut in front of each
point (Figure 4). These collars or sleeves would prevent room air
(with its moisture content) from being entrained by turbulence to
the vicinity of the corona. The differences in the dendrite growth,
for example, on the nitrogen versus compressed air points in additions
to the different particle counts found under the two gasses are
largely due to this room air entrainment.
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 plays in
the creation of these ultrafine particles.
The mechanisms of aerosol
particle formation under corona discharge treatment of combustion
flue gasses and the important role that plays
on the formation was recently discussed in detail by Chang (9).
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
The research and testing on a modified
SSDC ionizer described here shows that:
* The production of aerosols by high-voltage corona discharge can
be reduced or eliminated;
* The buildup on the corona
points can be avoided or significantly restricted;
* The erosion of the positive electrodes can be prevented;
* The potential problems of metallic and organic contamination to
clean room environments can be prevented;
* The ammonium nitrate buildup on the corona points and the aerosol
particle presence are highly humidity dependent;
* The data presented demonstrates for the first time the effectiveness
of a new method for truly clean corona ionization.
The authors appreciate the efforts
by these individuals of IBM Essex Junction: S.J. Pierce (MFTIR,
RAMP) and R.B. Dunn (SEMs).
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