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Charge-carrier extraction from air and nitrogen gas streams that entrain charge from dc corona ionizers
Journal of Electrostatics 54 (2002) 271–282
Charge-carrier extraction from air and nitrogen gas streams that entrain charge from dc corona ionizers Charles G. Noll* ITW Static Control and Air Products, 2257 North Penn Road, Hatfield, PA 19440-1998, USA Received 5 July 2000; received in revised form 18 April 2001; accepted 30 April 2001
Abstract Charge decay upon a pre-charged target was studied in a gas flow channel that contained a pair of point-to-plane corona-discharge emitters. Charge decay was primarily determined by charge carriers entrained from the negative corona discharge and gas flow. The positive glow corona removed excess negative carriers and added positive carriers to achieve balanced residual charge on targets. Discharge modes with extended structures, such as streamers in air and nitrogen and sparking, yielded positive and negative charge carriers that were most likely to be entrained. The magnitudes of corona currents in ionizers were of lesser importance. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Static elimination; Charge decay; Mobility; Air flow; Nitrogen corona
1. Introduction The elimination of static electricity from insulating materials and electrically floating conductors is important to the success of industrial processes such as the handling of paper and plastic films and sheets, the loading and unloading of powder and bulk solids, and the manufacture and assembly of electrostatic-discharge (ESD) sensitive electronic products. Static elimination is used to improve handling and processing, safeguard components and assemblies, and eliminate shocks and incendive events. The environment for static elimination is usually room air, but
*Tel.: +1-215-822-2171; fax: +1-215-822-3795. E-mail address: [email protected] (C.G. Noll). 0304-3886/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 8 8 6 ( 0 1 ) 0 0 1 7 0 - X
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at times processes demand static elimination at elevated temperature, or in reduced oxygen, typically nitrogen environments. The charge carriers that are responsible for static elimination in many applications are blown from electrical corona to charged targets. Only a small fraction, typically on the order of 0.1%, of corona-discharge current is entrained from an ionizer by a gas stream, yet it is the control of the entrained charge-carriers that leads to a chargebalanced target and a short charge-decay time to reach the charge-balanced condition. Recently, work was reported on static elimination in variable ion mobility environments [1]. Such environments have significant changes in charge-carrier mobility over time during the normal process of static elimination. The tests were done in a semiconductor-device handler with the temperature in the range of 213– 300 K in nitrogen-diluted air and 300–433 K in air. The results showed that balanced static elimination can be achieved in the nitrogen environment, and that the previously-reported large difference between the charge-decay rates for positive- and negative-charged targets [2,3] can be significantly reduced by allowing large freeelectron dominated currents at the negative emitters. For setups of the ionizer that yielded a steady-state balanced-charge condition at the target, the charge-decay times remained relatively independent of the air/nitrogen composition of the environment, suggesting that charge decay arises from a common process in these gases. Charge decay from targets was weakly dependent on corona currents and more strongly determined by gas flow between the ionizer and target. Independent control of the positive and negative point-to-plane corona-emitter assemblies through physical isolation was found helpful towards achieving balanced static elimination. By setting the negative-emitter potential and a current-limit for the power supply, the ionizer could be balanced in both air and nitrogen by adjustment of the positive corona current. The earlier work [1] was done in a commercial semiconductor-device handler. Although temperature control was good, the interpretation of data was complicated by relatively poor control of gas composition and flow, and by grounded-structural interferences along the path of the conveyed charge carriers from the ionizer to the target for static elimination. Work was then done with an electrically insulating flow channel [4]. The gas-flow channel gave improved control of the entrained chargecarrier stream, emitter geometry, gas composition and flow, and the target for static elimination. The temperatures for this work were extended to 213–493 K for purenitrogen environments and 283–493 K for air. Gas flows were controlled in the 0–5 m/s range. The thrust of the work described in Ref. [4] was to explore charge-carrier entrainment from positive- and negative-polarity corona in air and nitrogen streams over a range of industrially important temperatures. Attention was focused on the current–voltage (I2V) characteristics, and net charge-carrier currents Is that are blown downstream from single emitters and pairs of positive- and negative-polarity point-to-plane corona emitters. The residual potential Vr on an electrically-floating target in the gas stream was also explored. It was found that the mode of the corona discharge had a significant influence on charge-carrier entrainment from the
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discharge. In particular, charge-carriers were more easily entrained from spatiallyextended discharges, such as the negative/positive streamer-type corona, than from the tightly-bound positive glow discharge. In fact, charge balance Vr ¼ 0 was achieved on targets in a nitrogen environment with a negative-polarity, currentlimited discharge and a positive dc corona-free electrode to remove excess negative carriers (free electrons). Although the previous work [4] revealed evidence of charge balance at targets and control of entrained charge-carrier currents, it yielded little information on the extraction of charge-carriers from the gas stream that is fundamental to static elimination. It was clear, however, that gas flow determined both Is and Vr ; and that electric field adjustments at the source determined what charge-carriers were available for entrainment to the target region. The present work is focused on the extraction of charge carriers from flowing gas streams that simultaneously entrain and convey positive- and negative-polarity charge carriers from corona discharges. Charge extraction selectively removes charge carriers from these gas streams to pre-charged targets, yielding the possibility to gain information on charge-carrier density and mobility effects that act during normal conditions for static elimination. In particular, the present work explores chargedecay processes that occur in gas streams with bipolar gas-entrained charge carriers, where Is measurements only reveal information on net charge-carrier entrainment. Air and nitrogen were chosen for these studies as representative of those gases which attach free electrons and those which do not, respectively. The selection of gases provides conditions with large differences in mobility between free-electron and negative-ion charge carriers.
2. Experimental arrangement 2.1. Equipment The experimental arrangement (Fig. 1) was used in earlier work where additional detail can be found [4]. The central element of the experimental arrangement was a gas flow channel with point-to-plane corona-emitter assemblies. This flow channel was positioned within a thermally controlled environmental chamber. A gas handling system, heat exchanger, and instrumentation were then used to control the gaseous environment in the flow channel and gain information on the corona and charge-carrier entrainment processes. The target, used for measuring charge decay and initial charge extraction currents Ice ; was a 9.5 mm diameter brass sphere on a 2–56 threaded rod. The rod supported the target and served as an electrical contact from which a PTFE insulated wire made connection to the plate of a charged-plate monitor (Trek Model 156A CPM). The support for the target was covered with PTFE insulation and sealed with silicone rubber so that the measured charge was collected only on the sphere. The target was positioned on the axis of the flow channel, 17.6 cm downstream from the emitter electrodes. The combined capacitance C of the target, insulated wire, and
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Fig. 1. Flow channel for charge extractions studies.
plate of the charged-plate monitor was 213 pF. The average field between the emitter and target was approximately one-tenth that between the emitter and its counterelectrode. The collector plate area was approximately ten times the target’s surface area. Under floating conditions in a gas stream with entrained carriers, the target assumes the residual potential Vr : Downstream from the target three layers of 1.6 1.6 mm2 wire mesh were positioned across the section of the duct as a final current collector. The current to the screen Is is the net current of charge carriers entrained from the corona discharge and the streaming current of those carriers in the flow channel. 2.2. Measurements of charge extraction and charge decay Charge extraction from the gas stream was studied by the conventional method outlined in EOS/ESD Std. 3.1 [5], suitably modified for the channel-flow study. In the standard method a target with known capacitance C with respect to its environment is charged to an initial potential V0 so that it is known to contain an initial charge Q0 ¼ CV0 : The target is then exposed to a gas flow containing charge carriers and its potential is observed to decay with time. This potential decay is then reported as charge decay. The charge decay is typically exponential towards the balanced condition and a charge-decay time CDT is defined in EOS/ESD Std. 3.1 as the time for the potential to decay from its initial value to 10% of this value. The charge-decay time, CDT ¼ t ln 10, where 1=t is the charge extraction rate. Charge decays are typically reported for positive and negative initial charges on the target with corresponding charge-decay times CDT+ and CDT, respectively. Although the method was built around standardization of capacitance and target arrangement, the concept is transferable to the present work. Here the target was a 9.5 mm brass sphere that was directly connected by wire to the charged-plate of the CPM. The charge stored on the spherical target, plate, and wire is at equipotential
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and this charge is reduced over time by exposure of the sphere to the charge-carriers in the gas stream in the flow channel. More generally, unbalanced concentrations of charge carriers in the gas stream can produce an unbalanced residual charge on a target. In this case the charge-decay data can be represented by potential decay to a steady-state residual charge Qr ðtÞ as indicated in (1). QðtÞ ¼ ðQ0 Qr ðtÞÞet=t þ Qr ðtÞ:
ð1Þ
Note that the steady-state charge on the target can be time varying, but takes on average, steady-state values for particular setups. In experiments the initial potential is +1000 V and charge decay continues to the steady-state residual potential Vr ¼ Qr =C: The charge-extraction rate, 1=t; can then be estimated from (2) using the initial slope of the target’s potential decay in the gas stream. 1 DV=Dt D : t ðV0 Vr Þ
ð2Þ
The initial charge-extraction current Ice is determined from the capacitance of the target/measurement system C and the initial plate potential V0 : Ice ¼ CV0 =t: When Qr ðtÞ ¼ 0; the quantity t is simply related to the CDT as noted above. The charge decay depends upon the capacitance of the measurement system and exposure of the target to the gas stream. The results in the present case thus reveal information on mechanisms of the static elimination process, yet do not assess the performance of a particular ionizer configuration against the EOS/ESD standard.
3. Experimental results 3.1. Charge extraction in air In air at ambient temperature (Fig. 2) Ice was about 20% higher for the positive target, and remained so relatively independent of Vr as determined through adjustment of positive-emitter current. In this figure vs ¼ 2:1 m/s, and based on earlier work in Ref. [1], the larger value for Ice to a positive target is attributed to the higher mobility of the negative charge carriers [6] at this gas speed. The Ice for both positive- and negative-polarity charge-decay processes increased as the positive emitter current was increased from low levels. The Ice were determined by entrainment of positive- and negative-polarity charge carriers from the negative corona, a finding that is consistent with the observed neutralization of wall charge when gas flow was passed over the negative corona. There was some additional charge decay that followed from the positive corona as it injected additional charge to neutralize the wall of the insulating channel and contributed to a reduction in Is at higher positive emitter currents. The ten-fold enhancement in Is that occurred with the application of a low voltage on the positive emitter [4] established the streaming current from ionizer, and the positive and negative charge-carrier components of Is did not rise by more than 50% as the
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Fig. 2. Charge extraction in room temperature air at vs ¼ 2:1 m/s.
carrier stream was balancedFeven though the positive-emitter current increased by a factor of one hundred to achieve balance. In other words, Vr and Ice were determined by relatively independent processes. Measured CDTs depend upon Vr with maximum values near Vr ¼ 0: At that point the CDTs were equal to those that were estimated from Ice measurements. Also, the CDTs estimated from the Ice measurements appeared to reach a minimum near the balance condition. The weak dependence of the CDTs on emitter current observed in Ref. [1] is now not surprising if streaming currents resulted from charge-carrier pair production in the negative corona, and were further developed by the unipolar positive corona. Fig. 3 shows the general increase in Ice with increasing vs : Note that both positive and negative targets were neutralized by charge carriers that were entrained from the negative corona. When the positive emitter was energized at normal levels with the negative emitter grounded, only the negative target was neutralized. This was again an indication that both positive and negative charge carriers were entrained from electric-discharge streamers and not the localized glow of a positive corona discharge. The positive corona, however, was occasionally observed in two modes, a glow corona and a pre-breakdown streamer mode. When the positive corona is transitioned from the glow corona to the pre-breakdown streamer mode, and the negative emitter was operated with near zero current, Vr fell from +1140 to +780 V with no significant change in the negative target’s Ice : This indicated that balance shifted by negative charge-carrier injection from the positive emitter in the prebreakdown streamer mode, while there was little change in the conveyed current of positive carriers. At the same time the positive-polarity emitter current nearly doubled by the change from glow to pre-breakdown streamer mode. As with the
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Fig. 3. Carrier transfer in flowing air.
negative corona, gas flow raised the pre-breakdown streamer current of positive corona and this was attributed to neutralization of wall charge by negative carriers from the corona. The positive glow corona (and wall neutralization) was not influenced by gas flow. Fig. 4 shows Ice as a function of temperature at vs ¼ 3:0 m/s. The currents to the ionizer were adjusted to give Vr ¼ 0 before each test. Under balanced conditions Is E2 pA. The higher Ice measurements for positive targets were attributed to higher negative carrier mobility, since the downstream screen was at zero potential and showed balanced carrier number density at balance. This result is consistent with findings in Ref. [1]. The Ice measurements were of the same order of magnitude as the saturation values of Is under unbalanced conditions [4]. The carriers of each polarity that were entrained from the corona discharges appeared relatively independent of corona currents in the ionizer. The electric fields in the electrode region then removed excess carriers or carriers of higher mobility to bring about balance. At balance, carrier pairs seemed to be produced, so that Is E0; yet differences in ion mobility affected charge decay at targets. The decrease in Ice with temperature with vs ¼ 3:0 m/s was of particular interest. It was shown in Fig. 3 that higher vs yielded increased streaming currents from the ionizer, at least at room temperature. The work in the semiconductor-device handler [1] indicated that carrier mobility plays a significant role in determining charge extraction at this vs : It is, therefore, believed that the observed reduction in Ice with temperature was related to the lower mobility of both polarity charge carriers at lower temperature [5]. The effect of charge-carrier mobility may have arisen from influences of the electric field in the ionizer, or from the electric fields that determine
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Fig. 4. Charge extraction at a spherical target in air at vs ¼ 3:0 m/s.
charge-carrier extraction at the target. Since the saturation values for Vr and Is under unbalanced conditions were independent of temperature and vs is in a range where mobility effects are active, it was believed that a major contributor to the temperature dependence in Fig. 4 was a change in carrier mobility near the target. The increasing currents [4] in the ionizer with temperature were, therefore, separately another manifestation of higher charge-carrier mobility. At low superficial velocities (vs o1 m/s) a minimum vs was necessary to entrain charge carriers from the corona ionizer. This disappeared with increasing temperature, and at 433 K the entrainment was higher at vs ¼ 1:0 m/s than it was at 2.0 m/s. There was possibly an interaction of charge carrier removal and entrainment mechanisms acting at higher temperatures that favored entrainment at lower vs : The mechanism remains unknown at this time. 3.2. Charge extraction in nitrogen In nitrogen Ice was controlled by free electrons and free-electron processes. Table 1 summarizes data at room temperature and 213 K. Data Points 1–2 and 9–12 show that Ice increased with increasing vs past the emitter/target system. The Ice were also lower at lower temperatures, as expectedFcompare Data Points 1 and 9. A reduction in the potential on the positive emitter (Data Points 2–4) resulted in a small increase in Ice to a negative target and a larger increase in Ice to positive targets. A small increase in the current to the negative emitter’s collector plate was also seenFthe current from this emitter was constant and indicated that the corona current-density distribution on the negative-emitter side became more localized at lower positive-emitter potentials. After reducing the potential on the positive emitter
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C.G. Noll / Journal of Electrostatics 54 (2002) 271–282 Table 1 Charge extraction and decay from charge-carrier streams in flowing nitrogen gas T ¼ 300 K Data point 1 2 3 4 5 6 7 8 T ¼ 213 K 9 10 11 12 13 14 15
Vþ (kV)
Iþ (mA)
V (kV)
I (mA)
vs (m/s)
Iceþ (nA)
Ice (nA)
VR (V)
4.4 4.4 3.8 3.3 0.2 0.2 3.7 0
+1.56 +1.65 +1.13 +0.85 3.0 2.1 +31.7 3.3
2.9 2.9 2.9 2.9 2.9 2.9 0.0 2.8
655 625 640 647 707 688 0.1 711
0.382 0.873 0.873 0.873 0.803 1.71 0.450 0.450
7.42 24.4 39.5 49.2 93.3 204 0 F
+1.92 +4.11 +4.35 +5.07 +4.62 +7.39 +0.53 +1.46
+80 +80 +20 6 268 302 >+1100 190
3.8 3.8 3.8 3.8 off 10.0 10.0
+0.66 +0.77 +0.81 +0.87 2.5 +10.2 +12.9
2.9 2.9 2.9 2.9 3.2 0.0 3.3
695 685 630 610 657 +.02 420
0.379 0.687 1.41 2.30 2.30 2.30 2.31
3.34 13.1 54.4 232 325 0 0
+1.08 +2.41 +3.65 +8.96 +7.90 F +10.9
30 18 +23 +80 309 F >+1100
to 200 V, there was little change in the Ice for negative targetsFsee Data Points 4–5, and 12–13. The charge on positive targets then decayed faster. Grounding the positive emitter stopped charge decay on positive targets, and reduced Ice on negative targetsFsee Data Points 1 and 8. On the other hand, if the positive emitter current was maintained and the negative current was turned off (Data Points 7 and 14), very low Ice were observed for both the positive and negative targets. The negative carriers produced were much fewer in number, but some may also have an ionic character and were entrained to give the low Ice results. The Ice for the negative target was similar to that found in air for a negative target. This is shown by comparing Figs. 4 and 5 for vs ¼ 3 m/s. At low superficial velocities (vs o1 m/s) with Vr ¼ 0 the Ice for a positive target was similar to that found in air. However, there was some anomalous behavior around 380 K as shown in Fig. 6 for the case with nitrogen. Upon increasing vs to approximately 4 m/s, the region of the anomalous behavior increased. Fig. 7 shows the Ice to a positive target with vs ¼ 4 m/s and Fig. 8 shows typical charge-decay data. Above 300 K there was initially a very rapid charge decay followed by a slower decay. These two charge decays were summarized as Ice in Fig. 7. It is concluded that the mobility of negative charge carriers increased with temperature and above 300 K the negative charge carriers were drawn downstream from the corona by the electric field of the target. At even higher temperatures, the negative charge carriers were more confined within the ionizer and the high Ice for negative-polarity carriers became lower. The negative current in the ionizer was fixed since it was determined by the current limit of the power supply.
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Fig. 5. Extraction current to a negative target in nitrogen, vs ¼ 3 m/s.
Fig. 6. Extraction current to positive target in nitrogen, vs ¼ 0:8 m/s.
The view that the high current was drawn by an electric field from the ionizer is supported by the fact that the fast charge decay in Fig. 8 was followed in potential by the lower charge-decay rate. There was a threshold potential for obtaining the fast charge-decay rate. The switching between the fast charge-decay rate and lower charge-decay rate was as abrupt as the transition that occurred when carriers began to by-pass the counterelectrodes and pass between the positive and negative polarity emitters at high temperature. Further, the lower Ice (Fig. 7) also decreased with increasing temperature, suggesting its decrease was determined by charge-carrier removal in the ionizer. However, the lower Ice in Fig. 7 was significantly higher than the currents in air and the Ice to positive targets in nitrogen. It is probably the free electron current that
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Fig. 7. Charge extraction currents to a positive target in nitrogen, vs ¼ 4 m/s.
Fig. 8. Typical charge decay curves for positive and negative targets in nitrogen T ¼ 380 K, vs ¼ 4 m/s.
was normally blown from the ionizer when the electric field between ionizer and target was too low to directly draw carriers from the ionizer. This high current was not seen in earlier work [1], but in that case the target was further downstream and somewhat sheltered by metal bars of the semiconductor-device handler. Finally, it should be noted that the high Ice effect did not show up in Is measurements. Since the screen was at ground potential, there was no reason to draw charge carriers to it.
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Finally, currents drawn from ionizers by direct electric field effects are known to yield much higher Ice than those where charge carriers are entrained by gas flow to targets. The typical Ice for static bars placed in close proximity to highly charged webs is on the order of 1 mA per needle, whereas the gas-entrained charge-carrier currents in the present work were on the order of 10 nA. The higher Ice ; observed around 370 K in the present work, were approximately 0.5 mA.
4. Conclusions Charge decays at targets in flowing gas streams, containing entrained carriers from corona discharges, were determined by the presence of extended corona structures rather than the corona current itself. Much static elimination occurred from negative corona streamers, and the positive glow corona served to bias the electrode system and introduce additional carriers to balance the ionization. Charge balance and charge extraction processes were then somewhat independent processes. The prebreakdown streamer mode of positive corona acted similarly to the negative streamer mode, yet did not significantly enhance the positive- and negative-polarity streaming currents. Gas flow provided the entrainment mechanism and conveyance of charge carriers from the ionizer to the target in typical applications of static elimination. In nitrogen, the ratio of initial charge-extraction currents was much smaller than the ratio of emitter currents. The small ratio of initial extraction currents taken together with the findings [4] for charge carrier entrainment from positive and negative corona, indicated that streamers from the negative emitter provide both positive and negative carriers that are responsible for static elimination. The negative charge carriers that were entrained from these streamers may arise from impurities in the gas or necessarily follow the positive space charge as charge-carrier pairs blown from the ionizer. Additional work is in progress towards understanding charge extraction at elevated temperature in air.
References [1] C.G. Noll, Balanced static elimination in variable ion mobility environments, J. Electrostat. 49 (2000) 169–194. [2] H. Inaba, T. Ohmi, M. Morita, M. Nakamura, Antistatic protection in wafer drying process by spin– drying, IEEE Trans. Semicond. Manuf. 5 (1992) 359–367. [3] J.-S. Chang, K.G. Harasym, P.C. Looy, A.A. Berezin, C.G. Noll, Neutralization of static surface charges by an ac ionizer in a nitrogen environment, Proceedings of Electrostatics 1999, IOP Conference Series 163, IOP, London, pp. 289–294. [4] C.G. Noll, Temperature dependence of dc corona and charge-carrier entrainment in a gas flow channel, J. Electrostat. 54 (2002) 245–270, this issue. [5] Protection of electrostatic discharge susceptible itemsFionization, ANSI-EOS/ESD-S3.1-1991, ESD Association, Rome, NY. ANSI approved 20 January 1994. [6] P.A. Lawless, L.E. Sparks, Measurement of ion mobilities in air and sulfur dioxide-air mixtures as a function of temperature, Atmos. Environ. 14 (1980) 481–483.
Charge-carrier extraction from air and nitrogen gas streams that entrain charge from dc corona ionizers Charles G. Noll* ITW Static Control and Air Products, 2257 North Penn Road, Hatfield, PA 19440-1998, USA Received 5 July 2000; received in revised form 18 April 2001; accepted 30 April 2001
Abstract Charge decay upon a pre-charged target was studied in a gas flow channel that contained a pair of point-to-plane corona-discharge emitters. Charge decay was primarily determined by charge carriers entrained from the negative corona discharge and gas flow. The positive glow corona removed excess negative carriers and added positive carriers to achieve balanced residual charge on targets. Discharge modes with extended structures, such as streamers in air and nitrogen and sparking, yielded positive and negative charge carriers that were most likely to be entrained. The magnitudes of corona currents in ionizers were of lesser importance. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Static elimination; Charge decay; Mobility; Air flow; Nitrogen corona
1. Introduction The elimination of static electricity from insulating materials and electrically floating conductors is important to the success of industrial processes such as the handling of paper and plastic films and sheets, the loading and unloading of powder and bulk solids, and the manufacture and assembly of electrostatic-discharge (ESD) sensitive electronic products. Static elimination is used to improve handling and processing, safeguard components and assemblies, and eliminate shocks and incendive events. The environment for static elimination is usually room air, but
*Tel.: +1-215-822-2171; fax: +1-215-822-3795. E-mail address: [email protected] (C.G. Noll). 0304-3886/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 8 8 6 ( 0 1 ) 0 0 1 7 0 - X
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at times processes demand static elimination at elevated temperature, or in reduced oxygen, typically nitrogen environments. The charge carriers that are responsible for static elimination in many applications are blown from electrical corona to charged targets. Only a small fraction, typically on the order of 0.1%, of corona-discharge current is entrained from an ionizer by a gas stream, yet it is the control of the entrained charge-carriers that leads to a chargebalanced target and a short charge-decay time to reach the charge-balanced condition. Recently, work was reported on static elimination in variable ion mobility environments [1]. Such environments have significant changes in charge-carrier mobility over time during the normal process of static elimination. The tests were done in a semiconductor-device handler with the temperature in the range of 213– 300 K in nitrogen-diluted air and 300–433 K in air. The results showed that balanced static elimination can be achieved in the nitrogen environment, and that the previously-reported large difference between the charge-decay rates for positive- and negative-charged targets [2,3] can be significantly reduced by allowing large freeelectron dominated currents at the negative emitters. For setups of the ionizer that yielded a steady-state balanced-charge condition at the target, the charge-decay times remained relatively independent of the air/nitrogen composition of the environment, suggesting that charge decay arises from a common process in these gases. Charge decay from targets was weakly dependent on corona currents and more strongly determined by gas flow between the ionizer and target. Independent control of the positive and negative point-to-plane corona-emitter assemblies through physical isolation was found helpful towards achieving balanced static elimination. By setting the negative-emitter potential and a current-limit for the power supply, the ionizer could be balanced in both air and nitrogen by adjustment of the positive corona current. The earlier work [1] was done in a commercial semiconductor-device handler. Although temperature control was good, the interpretation of data was complicated by relatively poor control of gas composition and flow, and by grounded-structural interferences along the path of the conveyed charge carriers from the ionizer to the target for static elimination. Work was then done with an electrically insulating flow channel [4]. The gas-flow channel gave improved control of the entrained chargecarrier stream, emitter geometry, gas composition and flow, and the target for static elimination. The temperatures for this work were extended to 213–493 K for purenitrogen environments and 283–493 K for air. Gas flows were controlled in the 0–5 m/s range. The thrust of the work described in Ref. [4] was to explore charge-carrier entrainment from positive- and negative-polarity corona in air and nitrogen streams over a range of industrially important temperatures. Attention was focused on the current–voltage (I2V) characteristics, and net charge-carrier currents Is that are blown downstream from single emitters and pairs of positive- and negative-polarity point-to-plane corona emitters. The residual potential Vr on an electrically-floating target in the gas stream was also explored. It was found that the mode of the corona discharge had a significant influence on charge-carrier entrainment from the
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discharge. In particular, charge-carriers were more easily entrained from spatiallyextended discharges, such as the negative/positive streamer-type corona, than from the tightly-bound positive glow discharge. In fact, charge balance Vr ¼ 0 was achieved on targets in a nitrogen environment with a negative-polarity, currentlimited discharge and a positive dc corona-free electrode to remove excess negative carriers (free electrons). Although the previous work [4] revealed evidence of charge balance at targets and control of entrained charge-carrier currents, it yielded little information on the extraction of charge-carriers from the gas stream that is fundamental to static elimination. It was clear, however, that gas flow determined both Is and Vr ; and that electric field adjustments at the source determined what charge-carriers were available for entrainment to the target region. The present work is focused on the extraction of charge carriers from flowing gas streams that simultaneously entrain and convey positive- and negative-polarity charge carriers from corona discharges. Charge extraction selectively removes charge carriers from these gas streams to pre-charged targets, yielding the possibility to gain information on charge-carrier density and mobility effects that act during normal conditions for static elimination. In particular, the present work explores chargedecay processes that occur in gas streams with bipolar gas-entrained charge carriers, where Is measurements only reveal information on net charge-carrier entrainment. Air and nitrogen were chosen for these studies as representative of those gases which attach free electrons and those which do not, respectively. The selection of gases provides conditions with large differences in mobility between free-electron and negative-ion charge carriers.
2. Experimental arrangement 2.1. Equipment The experimental arrangement (Fig. 1) was used in earlier work where additional detail can be found [4]. The central element of the experimental arrangement was a gas flow channel with point-to-plane corona-emitter assemblies. This flow channel was positioned within a thermally controlled environmental chamber. A gas handling system, heat exchanger, and instrumentation were then used to control the gaseous environment in the flow channel and gain information on the corona and charge-carrier entrainment processes. The target, used for measuring charge decay and initial charge extraction currents Ice ; was a 9.5 mm diameter brass sphere on a 2–56 threaded rod. The rod supported the target and served as an electrical contact from which a PTFE insulated wire made connection to the plate of a charged-plate monitor (Trek Model 156A CPM). The support for the target was covered with PTFE insulation and sealed with silicone rubber so that the measured charge was collected only on the sphere. The target was positioned on the axis of the flow channel, 17.6 cm downstream from the emitter electrodes. The combined capacitance C of the target, insulated wire, and
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Fig. 1. Flow channel for charge extractions studies.
plate of the charged-plate monitor was 213 pF. The average field between the emitter and target was approximately one-tenth that between the emitter and its counterelectrode. The collector plate area was approximately ten times the target’s surface area. Under floating conditions in a gas stream with entrained carriers, the target assumes the residual potential Vr : Downstream from the target three layers of 1.6 1.6 mm2 wire mesh were positioned across the section of the duct as a final current collector. The current to the screen Is is the net current of charge carriers entrained from the corona discharge and the streaming current of those carriers in the flow channel. 2.2. Measurements of charge extraction and charge decay Charge extraction from the gas stream was studied by the conventional method outlined in EOS/ESD Std. 3.1 [5], suitably modified for the channel-flow study. In the standard method a target with known capacitance C with respect to its environment is charged to an initial potential V0 so that it is known to contain an initial charge Q0 ¼ CV0 : The target is then exposed to a gas flow containing charge carriers and its potential is observed to decay with time. This potential decay is then reported as charge decay. The charge decay is typically exponential towards the balanced condition and a charge-decay time CDT is defined in EOS/ESD Std. 3.1 as the time for the potential to decay from its initial value to 10% of this value. The charge-decay time, CDT ¼ t ln 10, where 1=t is the charge extraction rate. Charge decays are typically reported for positive and negative initial charges on the target with corresponding charge-decay times CDT+ and CDT, respectively. Although the method was built around standardization of capacitance and target arrangement, the concept is transferable to the present work. Here the target was a 9.5 mm brass sphere that was directly connected by wire to the charged-plate of the CPM. The charge stored on the spherical target, plate, and wire is at equipotential
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and this charge is reduced over time by exposure of the sphere to the charge-carriers in the gas stream in the flow channel. More generally, unbalanced concentrations of charge carriers in the gas stream can produce an unbalanced residual charge on a target. In this case the charge-decay data can be represented by potential decay to a steady-state residual charge Qr ðtÞ as indicated in (1). QðtÞ ¼ ðQ0 Qr ðtÞÞet=t þ Qr ðtÞ:
ð1Þ
Note that the steady-state charge on the target can be time varying, but takes on average, steady-state values for particular setups. In experiments the initial potential is +1000 V and charge decay continues to the steady-state residual potential Vr ¼ Qr =C: The charge-extraction rate, 1=t; can then be estimated from (2) using the initial slope of the target’s potential decay in the gas stream. 1 DV=Dt D : t ðV0 Vr Þ
ð2Þ
The initial charge-extraction current Ice is determined from the capacitance of the target/measurement system C and the initial plate potential V0 : Ice ¼ CV0 =t: When Qr ðtÞ ¼ 0; the quantity t is simply related to the CDT as noted above. The charge decay depends upon the capacitance of the measurement system and exposure of the target to the gas stream. The results in the present case thus reveal information on mechanisms of the static elimination process, yet do not assess the performance of a particular ionizer configuration against the EOS/ESD standard.
3. Experimental results 3.1. Charge extraction in air In air at ambient temperature (Fig. 2) Ice was about 20% higher for the positive target, and remained so relatively independent of Vr as determined through adjustment of positive-emitter current. In this figure vs ¼ 2:1 m/s, and based on earlier work in Ref. [1], the larger value for Ice to a positive target is attributed to the higher mobility of the negative charge carriers [6] at this gas speed. The Ice for both positive- and negative-polarity charge-decay processes increased as the positive emitter current was increased from low levels. The Ice were determined by entrainment of positive- and negative-polarity charge carriers from the negative corona, a finding that is consistent with the observed neutralization of wall charge when gas flow was passed over the negative corona. There was some additional charge decay that followed from the positive corona as it injected additional charge to neutralize the wall of the insulating channel and contributed to a reduction in Is at higher positive emitter currents. The ten-fold enhancement in Is that occurred with the application of a low voltage on the positive emitter [4] established the streaming current from ionizer, and the positive and negative charge-carrier components of Is did not rise by more than 50% as the
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Fig. 2. Charge extraction in room temperature air at vs ¼ 2:1 m/s.
carrier stream was balancedFeven though the positive-emitter current increased by a factor of one hundred to achieve balance. In other words, Vr and Ice were determined by relatively independent processes. Measured CDTs depend upon Vr with maximum values near Vr ¼ 0: At that point the CDTs were equal to those that were estimated from Ice measurements. Also, the CDTs estimated from the Ice measurements appeared to reach a minimum near the balance condition. The weak dependence of the CDTs on emitter current observed in Ref. [1] is now not surprising if streaming currents resulted from charge-carrier pair production in the negative corona, and were further developed by the unipolar positive corona. Fig. 3 shows the general increase in Ice with increasing vs : Note that both positive and negative targets were neutralized by charge carriers that were entrained from the negative corona. When the positive emitter was energized at normal levels with the negative emitter grounded, only the negative target was neutralized. This was again an indication that both positive and negative charge carriers were entrained from electric-discharge streamers and not the localized glow of a positive corona discharge. The positive corona, however, was occasionally observed in two modes, a glow corona and a pre-breakdown streamer mode. When the positive corona is transitioned from the glow corona to the pre-breakdown streamer mode, and the negative emitter was operated with near zero current, Vr fell from +1140 to +780 V with no significant change in the negative target’s Ice : This indicated that balance shifted by negative charge-carrier injection from the positive emitter in the prebreakdown streamer mode, while there was little change in the conveyed current of positive carriers. At the same time the positive-polarity emitter current nearly doubled by the change from glow to pre-breakdown streamer mode. As with the
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Fig. 3. Carrier transfer in flowing air.
negative corona, gas flow raised the pre-breakdown streamer current of positive corona and this was attributed to neutralization of wall charge by negative carriers from the corona. The positive glow corona (and wall neutralization) was not influenced by gas flow. Fig. 4 shows Ice as a function of temperature at vs ¼ 3:0 m/s. The currents to the ionizer were adjusted to give Vr ¼ 0 before each test. Under balanced conditions Is E2 pA. The higher Ice measurements for positive targets were attributed to higher negative carrier mobility, since the downstream screen was at zero potential and showed balanced carrier number density at balance. This result is consistent with findings in Ref. [1]. The Ice measurements were of the same order of magnitude as the saturation values of Is under unbalanced conditions [4]. The carriers of each polarity that were entrained from the corona discharges appeared relatively independent of corona currents in the ionizer. The electric fields in the electrode region then removed excess carriers or carriers of higher mobility to bring about balance. At balance, carrier pairs seemed to be produced, so that Is E0; yet differences in ion mobility affected charge decay at targets. The decrease in Ice with temperature with vs ¼ 3:0 m/s was of particular interest. It was shown in Fig. 3 that higher vs yielded increased streaming currents from the ionizer, at least at room temperature. The work in the semiconductor-device handler [1] indicated that carrier mobility plays a significant role in determining charge extraction at this vs : It is, therefore, believed that the observed reduction in Ice with temperature was related to the lower mobility of both polarity charge carriers at lower temperature [5]. The effect of charge-carrier mobility may have arisen from influences of the electric field in the ionizer, or from the electric fields that determine
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Fig. 4. Charge extraction at a spherical target in air at vs ¼ 3:0 m/s.
charge-carrier extraction at the target. Since the saturation values for Vr and Is under unbalanced conditions were independent of temperature and vs is in a range where mobility effects are active, it was believed that a major contributor to the temperature dependence in Fig. 4 was a change in carrier mobility near the target. The increasing currents [4] in the ionizer with temperature were, therefore, separately another manifestation of higher charge-carrier mobility. At low superficial velocities (vs o1 m/s) a minimum vs was necessary to entrain charge carriers from the corona ionizer. This disappeared with increasing temperature, and at 433 K the entrainment was higher at vs ¼ 1:0 m/s than it was at 2.0 m/s. There was possibly an interaction of charge carrier removal and entrainment mechanisms acting at higher temperatures that favored entrainment at lower vs : The mechanism remains unknown at this time. 3.2. Charge extraction in nitrogen In nitrogen Ice was controlled by free electrons and free-electron processes. Table 1 summarizes data at room temperature and 213 K. Data Points 1–2 and 9–12 show that Ice increased with increasing vs past the emitter/target system. The Ice were also lower at lower temperatures, as expectedFcompare Data Points 1 and 9. A reduction in the potential on the positive emitter (Data Points 2–4) resulted in a small increase in Ice to a negative target and a larger increase in Ice to positive targets. A small increase in the current to the negative emitter’s collector plate was also seenFthe current from this emitter was constant and indicated that the corona current-density distribution on the negative-emitter side became more localized at lower positive-emitter potentials. After reducing the potential on the positive emitter
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C.G. Noll / Journal of Electrostatics 54 (2002) 271–282 Table 1 Charge extraction and decay from charge-carrier streams in flowing nitrogen gas T ¼ 300 K Data point 1 2 3 4 5 6 7 8 T ¼ 213 K 9 10 11 12 13 14 15
Vþ (kV)
Iþ (mA)
V (kV)
I (mA)
vs (m/s)
Iceþ (nA)
Ice (nA)
VR (V)
4.4 4.4 3.8 3.3 0.2 0.2 3.7 0
+1.56 +1.65 +1.13 +0.85 3.0 2.1 +31.7 3.3
2.9 2.9 2.9 2.9 2.9 2.9 0.0 2.8
655 625 640 647 707 688 0.1 711
0.382 0.873 0.873 0.873 0.803 1.71 0.450 0.450
7.42 24.4 39.5 49.2 93.3 204 0 F
+1.92 +4.11 +4.35 +5.07 +4.62 +7.39 +0.53 +1.46
+80 +80 +20 6 268 302 >+1100 190
3.8 3.8 3.8 3.8 off 10.0 10.0
+0.66 +0.77 +0.81 +0.87 2.5 +10.2 +12.9
2.9 2.9 2.9 2.9 3.2 0.0 3.3
695 685 630 610 657 +.02 420
0.379 0.687 1.41 2.30 2.30 2.30 2.31
3.34 13.1 54.4 232 325 0 0
+1.08 +2.41 +3.65 +8.96 +7.90 F +10.9
30 18 +23 +80 309 F >+1100
to 200 V, there was little change in the Ice for negative targetsFsee Data Points 4–5, and 12–13. The charge on positive targets then decayed faster. Grounding the positive emitter stopped charge decay on positive targets, and reduced Ice on negative targetsFsee Data Points 1 and 8. On the other hand, if the positive emitter current was maintained and the negative current was turned off (Data Points 7 and 14), very low Ice were observed for both the positive and negative targets. The negative carriers produced were much fewer in number, but some may also have an ionic character and were entrained to give the low Ice results. The Ice for the negative target was similar to that found in air for a negative target. This is shown by comparing Figs. 4 and 5 for vs ¼ 3 m/s. At low superficial velocities (vs o1 m/s) with Vr ¼ 0 the Ice for a positive target was similar to that found in air. However, there was some anomalous behavior around 380 K as shown in Fig. 6 for the case with nitrogen. Upon increasing vs to approximately 4 m/s, the region of the anomalous behavior increased. Fig. 7 shows the Ice to a positive target with vs ¼ 4 m/s and Fig. 8 shows typical charge-decay data. Above 300 K there was initially a very rapid charge decay followed by a slower decay. These two charge decays were summarized as Ice in Fig. 7. It is concluded that the mobility of negative charge carriers increased with temperature and above 300 K the negative charge carriers were drawn downstream from the corona by the electric field of the target. At even higher temperatures, the negative charge carriers were more confined within the ionizer and the high Ice for negative-polarity carriers became lower. The negative current in the ionizer was fixed since it was determined by the current limit of the power supply.
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Fig. 5. Extraction current to a negative target in nitrogen, vs ¼ 3 m/s.
Fig. 6. Extraction current to positive target in nitrogen, vs ¼ 0:8 m/s.
The view that the high current was drawn by an electric field from the ionizer is supported by the fact that the fast charge decay in Fig. 8 was followed in potential by the lower charge-decay rate. There was a threshold potential for obtaining the fast charge-decay rate. The switching between the fast charge-decay rate and lower charge-decay rate was as abrupt as the transition that occurred when carriers began to by-pass the counterelectrodes and pass between the positive and negative polarity emitters at high temperature. Further, the lower Ice (Fig. 7) also decreased with increasing temperature, suggesting its decrease was determined by charge-carrier removal in the ionizer. However, the lower Ice in Fig. 7 was significantly higher than the currents in air and the Ice to positive targets in nitrogen. It is probably the free electron current that
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Fig. 7. Charge extraction currents to a positive target in nitrogen, vs ¼ 4 m/s.
Fig. 8. Typical charge decay curves for positive and negative targets in nitrogen T ¼ 380 K, vs ¼ 4 m/s.
was normally blown from the ionizer when the electric field between ionizer and target was too low to directly draw carriers from the ionizer. This high current was not seen in earlier work [1], but in that case the target was further downstream and somewhat sheltered by metal bars of the semiconductor-device handler. Finally, it should be noted that the high Ice effect did not show up in Is measurements. Since the screen was at ground potential, there was no reason to draw charge carriers to it.
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Finally, currents drawn from ionizers by direct electric field effects are known to yield much higher Ice than those where charge carriers are entrained by gas flow to targets. The typical Ice for static bars placed in close proximity to highly charged webs is on the order of 1 mA per needle, whereas the gas-entrained charge-carrier currents in the present work were on the order of 10 nA. The higher Ice ; observed around 370 K in the present work, were approximately 0.5 mA.
4. Conclusions Charge decays at targets in flowing gas streams, containing entrained carriers from corona discharges, were determined by the presence of extended corona structures rather than the corona current itself. Much static elimination occurred from negative corona streamers, and the positive glow corona served to bias the electrode system and introduce additional carriers to balance the ionization. Charge balance and charge extraction processes were then somewhat independent processes. The prebreakdown streamer mode of positive corona acted similarly to the negative streamer mode, yet did not significantly enhance the positive- and negative-polarity streaming currents. Gas flow provided the entrainment mechanism and conveyance of charge carriers from the ionizer to the target in typical applications of static elimination. In nitrogen, the ratio of initial charge-extraction currents was much smaller than the ratio of emitter currents. The small ratio of initial extraction currents taken together with the findings [4] for charge carrier entrainment from positive and negative corona, indicated that streamers from the negative emitter provide both positive and negative carriers that are responsible for static elimination. The negative charge carriers that were entrained from these streamers may arise from impurities in the gas or necessarily follow the positive space charge as charge-carrier pairs blown from the ionizer. Additional work is in progress towards understanding charge extraction at elevated temperature in air.
References [1] C.G. Noll, Balanced static elimination in variable ion mobility environments, J. Electrostat. 49 (2000) 169–194. [2] H. Inaba, T. Ohmi, M. Morita, M. Nakamura, Antistatic protection in wafer drying process by spin– drying, IEEE Trans. Semicond. Manuf. 5 (1992) 359–367. [3] J.-S. Chang, K.G. Harasym, P.C. Looy, A.A. Berezin, C.G. Noll, Neutralization of static surface charges by an ac ionizer in a nitrogen environment, Proceedings of Electrostatics 1999, IOP Conference Series 163, IOP, London, pp. 289–294. [4] C.G. Noll, Temperature dependence of dc corona and charge-carrier entrainment in a gas flow channel, J. Electrostat. 54 (2002) 245–270, this issue. [5] Protection of electrostatic discharge susceptible itemsFionization, ANSI-EOS/ESD-S3.1-1991, ESD Association, Rome, NY. ANSI approved 20 January 1994. [6] P.A. Lawless, L.E. Sparks, Measurement of ion mobilities in air and sulfur dioxide-air mixtures as a function of temperature, Atmos. Environ. 14 (1980) 481–483.