Equipment for removing static electricity from charged articles existing in clean space

Abstract: Proposed herein is an equipment for removing static elec­ tricity from charged articles existing in a clean space, particularly a clean room for the production of semiconductor devices, com­ prising an AC ionizer having a plurality of needle-like emitters disposed in a flow of clean air wherein a discharge end of each emitter is coated with a dielectric ceramic material; some emitters are connected to a high voltage AC source having added thereto a minus bias voltage thereby acting as pseudo negative pole emit­ ters capable of forming negative ion rich air, while the other emitters are connected to a high voltage AC source having added thereto a bias voltage biased to a more positive side than the above-mentioned minus bias voltage thereby acting as pseudo positive pole emitters capable of forming positive ion rich air.

Description: Background of the Invention In the production of semiconductor elements in clean rooms attention has been drawn to various difficulties caused by static electrification. Such difficulties include breakdown and performance deterioration of semiconductor devices, surface contamination of products due to absorption of fine particles and fault functions of electronic instruments in the rooms. As high integration, high speed calculation and saving energy are promoted in the semiconductor devices, oxide insulation films of semiconductor elements have become thinner and circuits and metal electrodes of the elements have been miniaturized, and thus, static discharge frequently causes pit formation in the elements and/or fusion or evaporation of metallic parts of the elements, leading to breakdown and performance deterioration of the semiconductor devices produced. For example, some MOS-FET and GaAs can not withstand a voltage as low as 100 to 200 volts, and thus, it is frequently required to keep the surface voltage of elements of such semiconductor materials at about 20 volts or lower. When semiconductor elements have been completely broken down, they may be detected upon delivery examination. It is, however, very difficult to find out performance deterioration of elements. In order to reduce static difficulties, it is, therefore, essential to reduce chances as far as possible for semiconductors to be encountered by static electricity, that is, to prevent charged articles as far as possible from approaching to semiconductor elements and substrates having incorporated with semiconductor elements, and to destaticize all in all charged articles. However, by prior art technology it has been impossible to completely do so. An example of surface voltage measurements on various articles involved in the production of semiconductor devices reported that surface voltages were 5 kV for wafer, 35 kV for wafer carrier, 8 kV for acrylic cover, 10 kV for surface of table, 30 kV for storage cabinet, 10 kV for working clothes and 1.5 kV for quartz palette. On the other hand, with recent clean rooms it has become possible to realize such a super cleanliness that a flow of clean air supplied contains no particles having a size of 0.03 µm or more. Fine particles are, however, inevitably generated from operators, robots and various manufacturing apparatus existing in the clean rooms. Such internally generated particles may have a size of from 0.1 µm to several tens µm, and when deposited on wafers of recent LSI and VLSI having the minimum line distance as small as 1 µm, result in fault products reducing the yield. It has been recently established that the deposition of fine particles on wafers is primarily attributed to electrostatic attraction and is substantially irrelevant to particular patterns of air flow in the vicinity of the wafers. Accordingly, prevention of such surface contamination of products due to deposition of fine particles may only be achieved by development of a technology for removing static electricity which does not directly relate to a technology for enhancing the cleanliness of clean rooms, including a technology for improving performances of filters. Furthermore, in cases wherein electronic equipments are existing in the clean room, discharge currents created by discharge of charged articles, for example charged human bodies and sheets of paper of a printer, may become static noise causing fault functions of the electronic equipments. To avoid such fault functions it is also desired to remove static electricity from charged articles existing in the clean room. To eliminate the above-discussed various difficulties due to static electrification in the clean room, it is effective to destaticize, that is to remove static electricity from charged articles existing in the clean room. In cases wherein charged articles are electrically conductive, destaticizing can be carried out simply by grounding the articles whereby static charges can be rapidly removed. However, it is practically impossible to ground all the charged articles in the clean room, and in cases wherein charged articles are insulators, they can not be destaticized by grounding. As to wafers, although they themselves are conductive, they are transported and handled in the condition that they are contained in casette cases or palettes which are insulating. Accordingly, it is difficult to destaticize wafers by grounding. For these reasons, there have been proposed destaticizing systems by means of ionizers. The underlying principle is as follows. In a clean room air cleaned by passing through filters is flowing substantially in one direction. An ionizer for ionizing air by corona discharge ( ion generator) is disposed upstream of the flow of clean air (normally in the vicinity of air exhaling surfaces of the filters) to provide a flow of ionized air, which comes in contact with charged articles to neutralize static electricity on the charged articles. Thus, positively and negatively charged articles are destaticized by negatively and positively ionized air, respectively. As corona discharge ionizers there are known pulsed DC type, DC type and AC type ionizers. In such an air ionizer, emitters are disposed in air and a high DC or AC voltage is applied to each emitter so that an electric field of an intensity higher than that of insulation failure of air may be created in the vicinity of the emitter, thereby effecting corona discharge. The known types of air ionizers will now be described in some detail. Pulsed DC type : As diagrammatically shown in Fig. 19, in this type of ionizer, direct currents, for example, having voltages of from + 13 kV to + 20 kV and from - 13 kV to - 20 kV, respectively, are alternately applied with a time interval (pulse) of e. g. from 1 to 11 seconds to a pair of needle-like emitters (tungsten electrodes) 100a and 100b disposed opposite from each other with a predetermined distance (for example several tens cm) therebetween, thereby alternately generating positive and negative air ions from each of the emitters 100a and 100b. The air ions so generated are carried by the air flow to a charged article 101 and neutralize static charges of opposite polarity on the articles. An example of the pulse is shown in Fig. 20. DC type : As diagrammatically shown in Fig. 21, in this type of ionizer, a pair of electrically conductive bars 102a and 102b with insulating coatings respectively having a plurality of emitters 103a and 103b buried therein at intervals of from 1 to 2 cm, are disposed opposite from each other with their bar axes in parallel and a predetermined distance (for example several tens cm) therebetween. A DC voltage of e. g. from + 12 to + 30 kV is applied to the emitters 103a of the bar 102a, while applying a DC voltage of e. g. from - 12 to - 30 kV to the emitters 103b of the bar 102b, thereby ionizing air. AC type : In this type of ionizer, an AC high voltage of a commercial frequency of 50/60 Hz is applied to needle like emitters. As diagrammatically shown in Fig. 22, a plurality of emitters 104 are arranged in a two dimensional expanse and connected to a high voltage AC source 105 via a frame work of conductive bars 106 having insulating coatings. For each emitter, a grounded grid 107 is disposed as an opposite conductor so that the grid 107 may surround the discharge end of the emitter 104 with a space therebetween. When the AC of a high voltage is applied to the emitter 104, there is formed an electric field between the emitter 104 and the grounded grid 107, which field inverts its polarity in accordance with a cycle of the applied AC, whereby positve and negative air ions are generated from the emitter 104. All such known types of ionizers pose various problems as noted below, when they are employed in destaticizing of charged articles in a clean room. First of all, the emitters in themselves contaminate the clean room. It is said that tungsten is the most preferred material for the emitter. When a high voltage is applied to the tungsten emitter to effect corona discharge, a great deal of fine particles (almost all of them having a size of 0.1 µm or less) are sputtered from the discharge end of the emitter upon generation of positive air ions, carried by the flow of clean air and contaminate the clean room. Furthermore, since the discharge end of the emitter is damaged by the sputtering, the emitter should be frequently renewed. Secondly, when an ionizer is caused to work for a prolonged period of time in a clean room, white particulate dust primarily comprised of SiO₂ deposits and accumulates on the discharge end of the emitter to a visible extent. While a cause of such white particulate dust is believed to be a material constituting filters for cleaning air, the deposition and accumulation of the particulate dust on the discharge end of the emitter poses a problem of reduction in ion generation and a problem of contamination due to scattering of the dust. Accordingly, the emitter must be frequently cleaned. Thirdly, a plurality of emitters disposed on the ceiling of the clean room may increase the concentration of ozone in the clean room. Although the increased ozone concentration is not very harmful to human bodies, ozone is reactive and undesirable in the production of semiconductor devices. In addition to the above-discussed common problems, individual types of known ionizers involve the following individual problems. With DC type ionizers, in which some emitters (emitters 103a on the bar 102a in the example shown in Fig. 21) form positive air ions, while the other emitters (emitters 103b on the bar 102b in the example shown in Fig. 21) form negative air ions, and these ions are carried by the air flow, there is frequently a case wherein air ions unduly inclined to a positive or negative side arrive at a charged article. The charged article often receives only air ions having the same polarity as that of the static charge thereon. In this case the charged article is not destaticized. On the contrary there can be a case wherein an article uncharged or slightly charged may be staticized by air ions carried thereto. While such phenomena are likely to occur in cases wherein the distance between the electrodes ( the distance between the rods 102a and 102b in the example shown in Fig. 21) is fairly large, if the distance is made short, a problem of sparking is posed. With pulsed DC type ionizers in which the polarity of air ions is inverted at a predetermined period, positive and negative air ions are alternately supplied to a charged article in accordance with the periodic generation of respective ions. Accordingly, the condition that positive or negative ions are continuously supplied to the charged article, as is the case with the DC type ionizers, is avoided. However, if the period is short, increased are chances for the positive and negative ions to be admixed in the air flow and to disappear before they reach the charged article. To the contrary, if the period is long, although chances for the ions to disappear are decreased, large masses of positive and negative ions will alternately arrive at the charged article. It is reported by Blitshteyn et al. in Assessing The Effectiveness of Cleanroom Ionization Systems, Microcontamination, March 1985, pages 46 - 52, 76 that with pulsed DC type ionizers, a potential of a charged surface decays zigzag, for example, as shown in Fig. 23. According to this report, static electricity on a charged surface does not disappear, rather static loads of about + 500 volts and about - 500 volts alternately appear on the charged surface. Such a surface potential as large as 500 volts may reduce the yield of products, since recent super LSI may be damaged even by a surface potential on the order of several tens volts. AC type ionizers involve a basic problem in that the amount of generated positive ions and the amount of generated negative ions are greatly different. It is frequently experienced that positive ions are generated in an amount of more than ten times the amount of negative ions generated. M. Suzuki et al. have reported an example of measurement of densities of positive and negative ions generated by an AC type ionizer as shown in Fig. 24, in a Japanese language literature, Proceedings of The 6th. Annual Meeting For Study of Air Cleaning and Contamination Control, (1987) pages 269 - 276, and in the corresponding English language literature, M. Suzuki et al., Effectiveness of Air Ionization Systems in Clean Rooms, 1988 Proceedings of The IES Annual Technical Meeting, Institute of Environmental Sciences, Mt. Prospect, Illinois, pages 405 to 412. As seen from Fig. 24, the density of negative ions is markedly lower than that of positive ions. The measurement shown in Fig. 24 was made with an AC type ionizer installed in a space wherein clean air is caused to flow vertically downwards from horizontally disposed HEPA filters. In Fig. 24, a reference symbol "d" designates a vertical distance of the point where the measurement was carried out from the emitter points, a reference symbol "l" designates a horizontal distance of the point where the measurement was carried out from a vertocal line passing through a central point of the ionizer, and BACKGROUND indicates positive and negative ion densities of the air flow when the ionizer is OFF. With the conventional AC type ionizers supplying positive ion rich air, the charged surface is not destaticized, rather it may remain positively charged at a potential of the order of from + several tens volts to about + 200 volts. Object of the Invention Accordingly, an object of the invention is to provide an equipment for removing static electricity from charged articles existing in a clean space, particularly a clean room for the production of semiconductor devices, thereby overcoming difficulties caused by static electrification. Particularly, the invention aims to solve the above-discussed problem of ion imbalance associated with known AC type ionizers as well as the above-discussed problems common to known ionizers, that is, contamination of clean rooms due to emitter sputtering, deposition and accumulation of particulate dust on emitters and generation of ozone, thereby achieving effective prevention of static electrification in an environment for the production of semiconductor devices. Summary of the Invention The object is achieved by an equipment for removing static electricity from charged articles existing in a clean space according to the invention, which equipment comprises an AC ionizer having a plurality of needle-like emitters disposed in a flow of clean air which has passed through filters wherein an AC high voltage is applied to said emitters to effect corona discharge for ionizing air whereby a flow of ionized air is supplied onto said charged articles to neutralize static electricity thereon, and is characterized in that: a discharge end of each of said needle-like emitters is coated with a dielectric ceramic material; each of said emitters is disposed with its discharge end spaced apart by a predetermined distance from a grounded grid- or loop-like opposite conductor to form a discharge pair; a plurality of such discharge pairs being arranged in a two dimensional expanse in a direction transversely of said flow of clean air; and emitters of some of said discharge pairs are connected to a high voltage AC source having added thereto a minus bias voltage thereby forming pseudo negative pole emitters, while emitters of the other discharge pairs are connected to a high voltage AC source having added thereto a bias voltage biased to a more positive side than said minus bias voltage thereby forming pseudo positive pole emitters, said pseudo negative pole emitters and pseudo positive pole emitters being discretely arranged in said two dimensional expanse. We have found that by coating a discharge end of a needle-like emitter with a thin film of dielectric ceramic material, dust generation from the discharge end upon corona discharge by application of an AC high voltage can be minimized without substantially lowering an ionizing ability of the emitter, and that when such an emitter having the discharge end coated with a ceramic material is used in a clean room, not only deposition of particulate dust on the discharge end can be avoided, but also ozone generation in the clean room can be minimized. Suitable dielectric ceramic materials which can be used herein include, for example, quartz, alumina, alumina-silica and heat resistant glass. Of these, quartz, in particular transparent quartz is preferred. The thickness of the ceramic coating on the discharge end of the emitter is suitably 2 mm or less. In the case of transparent quartz, the thickness is preferably from 0.05 to 0.5 mm. Incidentally, if a DC high voltage is applied to such an emitter having the discharge end coated with a ceramic material, air can be ionized by an electric field generated at the discharge end of the emitter for a moment of application of the DC high voltage. However, after the lapse of a particular time (for example 0.1 second in an air flow of 0.3 m/sec), air ions of a polarity opposite to that of the applied voltage surround the emitter to weaken the electric field at the discharge end of the emitter, whereby generation of ions is no longer continued. Accordingly, it is necessary to use an AC high voltage. We have further found that the basic problem of a great difference between densities of positive and negative ions associated with AC type ionizers as well as the problem of neutralization of generated ions in the air flow due to change of the polarity with time in accordance with the frequency of the applied AC can be almost completely solved by adding predetermined bias voltages to the applied AC high voltage so that some emitters (pseudo positive pole emitters) may continuously form positive ion rich air, while the other emitters ( pseudo negative pole emitters) may continuously form negative ion rich air in spite of the fact that an AC high voltage is applied. Thus, by suitably locating such pseudo positive pole emitters and pseudo negative pole emitters in a flow of clean air, it is possible to supply air with balanced amounts of positive and negative ions to charged articles to be destaticized. The discharge end of each pseudo negative pole emitter is preferably positioned downstream of the corresponding grounded grid- or loop-like opposite conductor with respect to the flow of air by a predetermined distance. It is advantageous that emitters of some discharge pairs are connected to a common high voltage AC source having added thereto a minus bias voltage thereby forming pseudo negative pole emitters, while emitters of the other discharge pairs are connected to a common high voltage AC source having added thereto a plus bias voltage thereby forming pseudo positive pole emitters. Both the high voltage AC sources may be conveniently provided by a voltage controlling device equipped with a means for transforming a commercially available AC to an AC of a predetermined high voltage, means for adding respective predetermined positively and negatively biased DC voltages to the transformed AC and a voltage operating part for adjusting the AC high voltage and the biased DC voltages. Detailed Description of the Invention The invention will now be described in detail with reference to the attached drawings in which: Fig. 1 is a schematic perspective view of an air ionizer used in the equipment according to the invention; Fig. 2 is a cross-sectional view of an example of an emitter which may be used in the ionizer of Fig. 1; Fig. 3 is an enlarged side view showing a pair of emitter and opposite conductor which may be used in the ionizer of Fig. 1; Fig. 4 is a cross-sectional view of another example of an emitter which may be used in the ionizer of Fig. 1; Fig. 5 is a cross-sectional view of a further example of an emitter which may be used in the ionizer of Fig. 1; Fig. 6 is an enlarged perspective view showing a part of grounded loop-shaped opposite conductors which may be used in the ionizer of Fig. 1; Fig. 7 is a side view showing an example of the relative position of the emitter and the corresponding opposite conductor which may be used in the ionizer of Fig. 1; Fig. 8 is a side view showing another example of the relative position of the emitter and the corresponding opposite conductor which may be used in the ionizer of Fig. 1; Fig. 9 is a diagram showing an example of a circuit for a voltage controlling device and its voltage operating part which may be used in the ionizer of Fig. 1; Fig. 10 is a diagram showing an example of a preferred assembly of circuits for a voltage controlling device and its voltage operating part which may be used in the ionizer of Fig. 1; Fig. 11 shows examples of square wave obtained by the circuit assembly of Fig. 10; Fig. 12 illustrates a testing method and apparatus used herein; Fig. 13 is a wave diagram for illustrating an effective AC component of a high voltage AC applied in the test of Fig. 12; Fig. 14 is a wave diagram for illustrating a bias voltage used in the test of Fig. 12; Fig. 15 is a graph showing densities of positive and negative ions measured by an ion density meter plotted against the added bias voltage V B obtained in the test of Fig. 12 under the indicated conditions; Fig. 16 is an AC wave diagram for illustrating effects of a bias voltage; Fig. 17 is an explanatory diagram for showing the state of the discharge part at the time a positive voltage (a) of Fig. 16 is being applied; Fig. 18 is an explanatory diagram for showing the state of the discharge part at the time a negative voltage (b) of Fig. 16 is being applied; Fig. 19 is a schematic illustration of a conventional pulsed DC type ionizer; Fig. 20 is a wave diagram of a voltage applied to the ionizer of Fig. 19; Fig. 21 is a schematic illustration of a conventional DC type ionizer; Fig. 22 is a schematic illustration of a conventional AC type ionizer; Fig. 23 shows an example of a change of a surface potential of a charged article with time when a conventional pulsed DC type ionizer is used; and Fig. 24 shows an example of densities of positive and negative ions generated by a conventional AC type ionizer. Fig. 1 schematically shows an example of an air ionizer used in the equipment according to the invention. The ionizer comprises a plurality of discharge pairs 4, each comprising a needle-like emitter 2 and a grounded loop-shaped opposite conductor 3. The discharge pairs 4 are arranged in a two dimensional expanse in a direction transversely of a flow of clean air shown by an arrow 1. HEPA or ULPA filters (not shown) are disposed upstream of the positions of the discharge pairs 4, and air cleaned by the filters passes through the discharge pairs 4. A unidirectional air flow which has passed through the discharge pairs 4 is directed to charged articles. In the illustrated example, each needle-like emitter 2 is disposed with its end toward a downstream direction of the air flow, and each ring-shaped opposite conductor 3 is arranged transversely of the air flow. The end of the emitter 2 is positioned on about an imaginary vertical line passing through the center of the ring of the opposite conductor 3. Further, in the illustrated example, six discharge pairs 4, each comprising the emitter 2 and the opposite conductor 3, are arranged in a line at substantially the same interval, and four such lines are arranged substantially in parallel and substantially within a plane. Emitters 2a in the first line of the figure and emitters 2a in the third line of the figure are communicated through a common insulated conductive line 6a with an out put terminal 7a of a voltage controlling device 5, while emitters 2b in the second line of the figure and emitters 2b in the fourth line of the figure are communicated through a common insulated conductive lines 6b with an out put terminal 7b of the voltage controlling device 5. As described later in more detail, the out put terminal 7b supplies a high AC voltage having added thereto a predetermined voltage biased to a minus side, whereas the out put teminal 7a supplies a high AC voltage having added thereto a predetermined voltage biased to a minus side to a less extent than the out put terminal 7b, or optionally biased to a plus side. A reference numeral 8 designates a voltage operating part of the voltage controlling device 5. All of the ring-like opposite conductors 3 are grounded by a common insulated conductive line 9 to the earth 10. Fig. 2 is a cross-sectional view of an example of the emitter 2. The emitter used herein is characterized in that its discharge end is coated with a dielectric ceramic material. The emitter illustrated in Fig. 2 comprises a tungsten rod 12 having a tapered needle portion 13 at one end and a tube 14 of a ceramic material concentrically containing the tungsten rod 12. The ceramic tube 14 also has a sealed tapered end portion 15, and the tungsten rod 12 is placed so that the end of its tapered needle portion 13 may come in contact with inside surface of the tapered end portion 15 of the ceramic tube 14 whereby the tapered needle portion 13 of the tungsten rod 12 may be coated with the thin ceramic tube 14. In the example shown in Fig. 2, the outer diameter of the tungsten rod 12 is slightly smaller than the inner diameter of the ceramic tube 14, and the tapered needle portion 13 of the tungsten rod 12 has an angle more acute than that of the tapered end portion 15 of the ceramic tube 14. Thus, by coating the tungsten rod 12 with the ceramic tube 14 so that the tapered needle portion 13 of the former may contact the tapered end portion 15 of the latter, the center of the end of the tapered needle portion 13 of the tungsten rod 12 may be naturally fitted to the center of the inside surface of the tapered end portion 15 of the ceramic tube 14. The other end 16 of the tungsten rod is jointed to a metallic conductor 17. This joint is made by intimately and concentrically inserting a predetermined depth of the tungsten rod 12 at its end 16 into an end of a metallic rod 17 having a diameter larger than that of the tungsten rod 12. The metallic rod 17 is received in a tube 18 of an insulating material such as glass, to which the other end 19 of the ceramic tube 14 is also connected via a seal member 20. As shown in Fig. 3, the emitter 2 is positioned with its discharge end 21 having a ceramic cover spaced apart from the corresponding grounded ring-shaped opposite conductor 3 by a predetermined distance and substantially on an imaginary vertical central line of the opposite conductor ring 3. This positioning is made by suspendedly supporting the emitters 2 on an insulated conductor 6 strong enough to support the emitters 2, thus in itself serving as a frame member for supporting the emitters. The insulated conductor 6 may comprise a relatively thick metallic conductor 17 coated with an insulating resin 22 (for example, fluorine resins such as "Teflon"), and also serves as a frame member for supporting opposite conductors 3 via insulating supporting members. By connecting the emitters 2 to the insulated conductor 6 via respective joint members 23 at intended positions, the emitters 2 can be arranged in the air flow without significantly disturbing the air flow. The emitter 2 used herein should have its discharge end coated with a dielectric ceramic material. Examples of such a dielectric ceramic material include, for example, quartz, alumina, alumina-silica and heat resistant glass. Of these, quartz, in particular, transparent quartz is preferred. The thickness of the ceramic coating on the needle portion 13 of the tungsten rod 12 is suitably 2 mm or less, preferably from 0.05 to 0.5 mm. The ceramic coating should also has a tapered end portion (an acute end 15 as shown in Fig. 2). Portions of the tungsten rod 12 other than its needle portion, which do not normally act as the discharge end, such as a body portion of the tungsten rod 12, is not necessarily coated with a ceramic material. Such examples are shown in Figs. 4 and 5. Fig. 4 depicts a tungsten rod 12 with its tapered end coated with a ceramic tube 14. Namely, the needle portion 13 of the tungsten rod 12 is tightly coated with the tapered end portion 15 of the ceramic tube 14, and the body portion of the tungsten rod 12 is coated with another insulating material (e. g. an insulating resin) 25. The ceramic tube 14 is bonded to the tungsten rod 12 by means of a adhesive (e. g. an epoxy resin based adhesive) 26, and the bond portion is covered with a sealing agent (e. g. a silicone sealing agent) 27 so that the tungsten may not be exposed. In this example, there is no opening between the outside surface of the tapered needle portion 13 of the tungsten rod 12 and the inside surface of the tapered end portion 15 of the ceramic tube 14. Fig. 5 depicts an example in which a conductive adhesive 29 is filled between an end 28 of the tungsten rod 12 and the tapered end portion 15 of the ceramic tube 14. Namely, the end 28 of the tungsten rod 12 extending beyond the insulating coat 25 is covered by the ceramic tube 14 having the tapered end portion 15 with an opening therebetween, and the opening is filled with the conductive adhesive 29. A reference numeral 27 designates a sealing agent, as is the case with Fig. 4. Examples of the conductive adhesive which can be used herein include, for example, a dispersion of particulate silver in an epoxy adhesive and a colloidal dispersion of graphite in an adhesive. In the example shown in Fig. 5, the end 28 of the tungsten rod may be pointed or may not be pointed. Fig. 6 is an enlarged perspective view showing a part of grounded loop-shaped opposite conductors 3 of Fig. 1. In this example, each opposite conductor 3 comprises a metal ring, and required numbers of such rings are connected together at a predetermined interval by a conductor 9 having an insulating coating so that they may be installed substantially within a plane in a two dimensional expanse. The conductor 9 used is strong enough to support the ring-shaped opposite conductors 3 in position, and thus serves as a frame for supporting the opposite conductors in position. The opposite conductors 3 are grounded to the earth 10 by means of the conductor 9. Since the conductor 9 serves as a frame for supporting the opposite conductors 3, a separate member for supporting the opposite conductors 3 is not required, and thus, a flow of clean air passing through the assembly of the opposite conductors 3 will not be significantly disturbed. The opposite conductors 3 are preferably of a shape of a perfect circle as illustrated herein. But they may be of a shape of an ellipse or a polygon. Alternatively, they may be grids as in conventional AC type ionizers formed by perpendicularly intersecting a plurality of straight lines within a plane. In any event, the opposite conductor 3 is not coated with a ceramic material, and is used with the metal surface exposed. Figs 7 and 8 show examples of the relative position of the emitter 2 and the corresponding opposite conductor 3, which constitute a discharge pair 4. In both the examples, the emitter 2 and the opposite conductor 3 are installed along the direction of and transversely of the air flow shown by an arrow, respectively so that the emitter may be positioned about on an imaginary vertical line passing through the center of the opposite conductor 3. In the example of Fig. 7 the emitter 2 is installed with its discharge end 21 positioned upstream of the opposite conductor 3 with respect to the air flow by a distance of G. Whereas in the example of Fig. 8 the emitter 2 is installed with its discharge end 21 positioned downstream of the opposite conductor 3 with respect to the air flow by a distance of G. Namely, the emitter 2 goes through the ring of the opposite conductor 3 in the example of Fig. 8, whereas it does not in the example of Fig. 7. Which embodiment should be adapted is determined depending upon the conditions of applying voltage, as described hereinafter. As already described, the first characteristic feature of the invention resides in the use of emitters with their discharge ends coated with a dielectric ceramic material in an AC type ionizer. The second characteristic feature of the invention resides in the manner of applying an AC high voltage to the emitters. We have found that upon application of an AC high voltage to the emitters with their discharge ends coated with a dielectric ceramic material, by adding appropriate bias voltages to the AC high voltage it is possible to cause some emitters to continuously form positive ion rich air, while causing the other emitters to continuously form negative ion rich air in spite of the fact that an AC high voltage is applied. Conventional AC type ionizer were to alternately generate positive and negative ions in accordance with the frequency of the AC utilized, although there was a great difference between the densities of the generated positive and negative ions. On the other hand, as already described, when a DC high voltage is applied to an emitter having the discharge end coated with a ceramic material, although air can be ionized for a moment of application of the DC high voltage, air ions of a polarity opposite to that of the applied AC voltage immediately surround the emitter to weaken the electric field at the discharge end of the emitter, and thus, generation of ions is no longer continued. In accordance with one aspect of the invention there is provided an improved AC type ionizer capable of continuously generating positive ions from some emitters while continuously generating negative ions from the other emitters. The ionizer described herein generates substantially only positive ions from some of its emitters while generating substantially only negative ions from its remaining emitters in spite of the fact that an AC high voltage is applied to the emitters, instead of alternately generating positive and negative ions in accordance with the frequency of the AC applied. Most typically, an AC high voltage having added thereto a minus bias voltage is applied to some emitters, while an AC high voltage having added thereto a plus bias voltage is applied to the other emitters. Now coming back to Fig. 1, an AC high voltage having added thereto a minus bias voltage is applied to a group of emitters denoted by 2b, thereby causing them to continuously form negative ion rich air, and an AC high voltage having added thereto a voltage biased to a more positive side is applied to a group of emitters denoted by 2a, thereby causing them to continuously form positive ion rich air. Strictly speaking, every emitter may become either positive or negative pole, since an AC voltage is applied thereto. For explanation purposes, an emitter to which an AC high voltage having added thereto a minus bias voltage is applied and which is capable of continuously forming negative ion rich air is referred to herein as "a pseudo negative pole emitter" and an emitter to which an AC high voltage having added thereto a voltage biased to a more positive side is applied and which is capable of continuously forming positive ion rich air is referred to herein as "a pseudo positive pole emitter". In Fig. 1, the emitters 2a are pseudo positive pole emitters, while the emitters 2b are pseudo negative pole emitters. All of the pseudo positive pole emitters 2a are communicated with the OUT PUT 7a of the voltage controlling device 5 by the insulated conductive wire 6a, while all the pseudo negative pole emitters 2b are communicated with the OUT PUT 7b of the the voltage controlling device 5 by the insulated conductive wire 6b. The OUT PUT 7a and the OUT PUT 7b put out an AC high voltage having added thereto bias voltages different from each other in the polarity and intensity, respectively. A reference numeral 8 in Fig. 1 designates a voltage operating part for operating or controlling nature of the AC voltages put out from the OUT PUT 7a and 7b. Fig. 9 is a diagram showing a circuit for a voltage controlling device 5 and its voltage operating part 8 which may be used in the ionizer of Fig. 1. The illustrated circuit comprises a common IN PUT 31 of a commercial AC ( AC of 100 V in the illustrated example) and 4 transformers 32, 33, 34 and 35 arranged in parallel. Variable resistances (slide rheostats) T₁, T₂, T₃ and T₄ are provided in the in put side of the transformers 32, 33, 34 and 35, respectively. These slide rheostats constitute the voltage operating part 8 of Fig. 1. The transformer 32 transforms the commercial AC (100 V) to a voltage of e. g. 8 kV or higher and put out the transformed AC to the OUT PUT 7a communicating with the pseudo positive pole emitters 2a, while the transformer 33 transforms the commercial AC (100 V) to a voltage of e. g. 8 kV or higher and put out the transformed AC to the OUT PUT 7b communicating with the pseudo negative pole emitters 2b. Accordingly, the transformers 32 and 33 are ordinary AC transformers which transform the commercial AC to a higher voltage without altering the frequency. Whereas the transformers 34 and 35 include a respective rectifier and serve to rectify the commercial AC to a DC and thereafter transform the DC to a higher voltage. Accordingly, the transformers 34 and 35 will be referred to herein as DC transformers. The DC transformer 34 puts out a DC of an elevated minus voltage, and is connected to one side of a secondary coil of the transformer 33. Thus, from the OUT PUT 7b there is applied a combined voltage of the AC component of a voltage elevated by the transformer 33 combined with the DC voltage biased to a minus side by the predetermined extent. On the other hand, the DC transformer 35 puts out a DC of an elevated plus voltage, and is connected to one side of a secondary coil of the transformer 32. Thus, from the OUT PUT 7a there is applied a combined voltage of the AC component of a voltage elevated by the transformer 32 combined with the DC voltage biased to a plus side by the predetermined extent. In Fig. 9, a reference symbol F designates a fuse, SW a switch for the electric source, and Z₁ and Z₂ spark killers for absorbing noise at the time of switching-on thereby reducing supply of a pulse component. According to the circuit of this construction, intensities of the AC voltage and DC voltage biased to a plus side which are to be put out from the OUT PUT 7a to the pseudo positive pole emitters 2a can be controlled at will by operating the slide rheostats T₁ and T₄. Likewise, intensities of the AC voltage and DC voltage biased to a minus side which are to be put out from the OUT PUT 7b to the pseudo negative pole emitters 2b can be controlled at will by operating the slide rheostats T₂ and T₃. Fig. 10 is a diagram showing a preferred assembly of circuits for a voltage controlling device 5 and its voltage operating part 8 which may be used in the ionizer of Fig. 1. The illustrated circuit assembly comprises an in put terminal 31 for a commercial AC (AC of 100 V), a transformer 37 attached to the in put terminal 31, and a rectification circuit 38, a constant voltage circuit 39, an inverter circuit 40, a high voltage transformer 41 and a high voltage block connected in series to the secondary side of the transformer 37. The AC from the transformer 37 undergoes all wave rectification in the rectification circuit 38, becoming a DC. The constant voltage circuit 39 is to provide an out put of a constant voltage. When the voltage of the commercial AC employed varies for some reasons, the voltage of the DC from the rectification circuit 38 varies accordingly, and in turn the in put voltage to the subsequent high voltage transformer 41 varies, and the eventual out put voltage can not be kept constant. Accordingly, the constant voltage circuit 39 is utilized. The inverter circuit 40 is incorporated with an oscillation circuit, and choppers the constant voltage DC from the constant voltage circuit 39 to a square wave, which is then transformed by the high voltage transformer 41 to an AC of a square wave as shown in Fig. 11 (a) by a reference numeral 43. The high voltage transformer 41 comprises an insulated transformer having incorporated with a slide rheostat, and can vary the out put AC voltage. The AC voltage from the high voltage transformer 41 is passed through the high voltage block 42, in which high voltage rectifiers (diodes D1 and D2 and high voltage resistances R1 to R6 are incorporated, and put out to the OUT PUT 7a and 7b. In the high voltage block 42, a secondary coil of the transformer 41 is branched so that it is communicated with a grounded line 44 at one side and with out put lines 45 and 46 respectively leading to the OUT PUT 7a and 7b at the other side. Between the out put line 45 leading to the OUT PUT 7a and the grounded line 44 there is inserted a diode D1 which does not cause a current of a plus side to flow and allows only a current of a minus side to flow. Between the out put line 46 leading to the OUT PUT 7b and the grounded line 44 there is inserted a diode D2 which does not cause a current of a minus side to flow and allows only a current of a plus side to flow. Further, resistances R1 to R6 are incorporated in the high voltage block 42 in the manner as shown in Fig. 10. Thus, to the OUT PUT 7a, a voltage of a plus side from the transformer 41 is applied as it is, but a voltage of a minus side applied to the OUT PUT 7a approaches 0 by an amount which has flow to the earth through the diode D1. The amount of the minus current allowed to flow to the earth can be adjusted by the resistances R1 and R5. As a result, a voltage biased to a plus side, e. g. having a wave 47 shown in Fig. 11 (b) is applied to the OUT PUT 7a. In this case, it can be said that a plus side bias voltage V B has been added. Likewise, a voltage biased to a minus side, e. g. having a wave 48 shown in Fig. 11 (c) is applied to the OUT PUT 7b. In this case, it can be said that a minus side bias voltage V B has been added. In the case of the circuit assembly shown in Fig. 10, the intensity of the AC voltage which is put out to the pseudo positive pole emitters 2a and to the pseudo negative pole emitters 2b can be controlled at will by the slide rheostat part of the high voltage transformer 41. Further, the intensity of the plus side bias voltage V B which is put out from the OUT PUT 7a to the pseudo positive pole emitters 2a can be controlled at will by adjusting a ratio of the resistances R1 and R5, more precisely by adjusting the ratio R5/(R1 + R5) . Likewise, the intensity of the minus side bias voltage V B which is out put from the OUT PUT 7b to the pseudo negative pole emitters 2b can be controlled at will by adjusting a ratio of the resistances R2 and R6, more precisely by adjusting the ratio R6/(R2 + R6) . The electric circuit or circuits for the voltage controlling device 5 and its voltage operating part 8 shown in Figs. 9 and 10 are preferred ones. What is required is that the OUT PUT 7b can provide a high voltage AC which is obtained by transformation of a commercial AC to a high voltage of e. g. 8 kV or more followed by addition thereto of a voltage biased to a minus side, the increase in the voltage by the transformation and the bias amount being adjustable, and that the OUT PUT 7a can provide a high voltage AC which is obtained by transformation of a commercial AC to a high voltage of e. g. 8 kV or more followed by addition thereto of a voltage biased to a less negative side than the above-mentioned bias voltage, optionally to a plus side, the increase in the voltage by the transformation and the bias amount being adjustable. So far as these requirements are met, any circuit or circuits can be used herein. During the operation of the equipment according to the invention, the pseudo negative pole emitters 2b, in spite of the fact that an AC high voltage is being applied, may continuously form ionized air having a high negative ion density and a positive ion density of approximately 0, and the so formed negative ion rich air is carried by the flow of clean air to charged articles. On the other hand, the pseudo positive pole emitters 2a, in spite of the fact that an AC high voltage is being applied, may continuously form ionized air having a high positive ion density and a low negative ion density, and the so formed positive ion rich air is carried by the flow of clean air to charged articles. Accordingly, by appropriately arranging a plurality of the pseudo negative pole emitters 2b and pseudo positive pole emitters 2a in a two dimensional expanse transversely of the air flow, for example, by alternately arranging a line of the emitters 2b and a line of the emitters 2a as shown in Fig. 1, or by arranging the individual emitters 2b and 2a alternately or zigzag, or by arranging a small group of the emitters 2b and a small group of the emitters 2a alternately, it is possible to supply well balanced positive and negative ions to charged articles which are existing downstream of the ionizer. The invention will be further described by test examples. Fig. 12 illustrates a testing method and apparatus used herein. A single emitter 2 covered with quartz having the construction shown in Fig. 2 is disposed with its axis held vertical in a flow of clean air flowing downwards at a rate of 0.3 m/sec in a vertical laminar flow clean room. The tungsten rod 12 of the emitter 2 has a diameter of 1.5 mm. The quartz tube 14 of the emitter 2 has an outer diameter of 3.0 mm and an inner diameter of 2.0 mm, and the length of the tapered end portion 15 of the quartz tube is 5 mm. The glass tube 18 of the emitter 2 has an outer diameter of 8 mm and an inner diameter of 6 mm, and contains the metallic conductor 17 of a diameter of 3 mm passing therethrough. The emitter 2 is electrically communicated with the voltage controlling device 5 via the vertically extending glass tube 18 and the horizontally extending resin covered tube 22. A grounded opposite conductor 3 comprising a ring of stainless steel is disposed so that its imaginary vertical center line may substantially coincide the axis of the emitter 2. The distance G between the the discharge end 21 of the emitter 2 and the center of the opposite conductor ring 3 is controlled by vertically sliding the opposite conductor 3. In cases wherein the discharge end 21 is positioned upstream of the opposite conductor 3 with respect to the air flow (in cases shown in Fig. 7), the distance G is positive. Whereas, in cases wherein the discharge end 21 goes through the opposite conductor ring 3 and is positioned downstream of the opposite conductor 3 with respect to the air flow (in cases shown in Fig. 8), the distance G is negative. A diameter of the opposite conductor ring 3 is represented by D. A high voltage AC having added thereto a bias voltage is applied to the emitter 2, and densities of positive and negative ions (in × 10³ ions/cc) are measured at a location 1200 mm below the discharge end 21 of the emitter 2 by means of an air ion density meter 50. An effective AC component of the AC applied to the emitter 2 and the bias voltage added to the AC are represented by V and V B , respectively. The effective AC component is 1/√2 times the peak voltage, as shown in Fig. 13. The bias voltage V B is a DC component added to an AC wave, as shown in Fig. 14. The V B is positive when the added bias is in a plus side, and is negative when the added bias is in a minus side. Fig. 15 is a graph showing densities of positive and negative ions measured by the air ion density meter 50 plotted against the added bias voltage V B under the conditions including D = 80 mm, G = - 25 mm, V = 11 kV and a frequency of the applied AC of 50 Hz. The result shown in Fig. 15 is very interesting in that in spite of the fact that an AC is applied to the emitter, ionized air extremely inclined to positive or negative ions is formed by controlling the V B . The positive ion density is maximum where the V B is about + 2 kV, and drastically decreases as the V B decreases to 0 through - 2 kV. On the other hand, the negative ion density is maximum where the V B is about - 4 kV, and drastically decreases as the V B increases to - 2 through 0 kV. Under the conditions employed, it is possible to generate substantially only either positive or negative ions by appropriately controlling the V B . For example, if the V B more positive than 0 is added, positive ions are generated in a high density without substantial generation of negative ions. If the V B more negative than - 3 kV, preferably more negative than - 4 kV, is added, negative ions are generated in a high density without substantial generation of positive ions. Under the conditions employed, both positive and negative ions are generated where the V B is within the range between - 3 kV and 0 kV. Thus, it is possible to generate both positive and negative ions from one and the same emitter. In this case, positive and negative ions are generated alternately in accordance with the frequency of the AC applied. Such a system in which positive and negative ions are generated from one and the same emitter alternately at a high frequency is, however, not necessarily advantageous partly because the generated positive and negative ions are likely to be mutually neutralized before they reach charged articles, resulting in reduction of effective ions for the purpose of destaticizing and partly because since a slight change of V B within the above-mentioned range invites a great change of ion densities, it is not easy to control the V B . Under the conditions employed, if the V B more positive than 0 is added to a certain emitter, it becomes an emitter capable of generating only positive ions (that is a pseudo positive pole emitter 2a). If the V B more negative than - 3 kV is added to a certain emitter, it becomes an emitter capable of generating substantially only negative ions (that is a pseudo negative pole emitter 2b). Accordingly, by appropriately discretely arranging both the pseudo emitters 2a and 2b in a two dimensional expanse transversely of the air flow, it is possible to supply well balanced positive and negative ions to charged articles. Figs. 16 to 18 are for illustrating effects of the bias voltage. With an AC having added thereto a minus bias voltage, the intensity of a positive voltage, shown by (a) in Fig. 16, is (V - |V B |), which is lower than the effective AC component V by |V B |. Whereas, the intensity of a negative voltage, shown by (b) in Fig. 16, is (V + |V B |), which is higher than the effective AC component V by |V B |. Accordingly, when this AC voltage is applied to the emitter, the intensity of electric field in the vicinity of the discharge end of the emitter is stronger in the case of (b) than in the case of (a), whereby a Coulomb force for causing negative ions to move downwards is much larger than a Coulomb force for causing positive ions to move downwards. Fig. 17 is an explanatory diagram for showing the state of the discharge end at the time a positive voltage (a) of Fig. 16 is being applied, and Fig. 18 is an explanatory diagram for showing the state of the discharge end at the time a negative voltage (b) of Fig. 16 is being applied. In these figures, arrows attached to ions indicate the strength of the Coulomb force exerting the respective ions. Thus, in this case, while positive and negative voltages are applied to the emitter, more negative ions reach the air ion density meter 50 than positive ions. We have repeated the tests using rates of air flow from 0.15 to 0.6 m/sec and varying parameters V, G, D and V B . It has been found that optimum conditions for a pseudo positive pole emitter 2a include : 8 kV ≦ V, - 80 mm ≦ G ≦ 80 mm, 50 mm ≦ D ≦ 150 mm, and - 8 kV ≦ V B ≦ 8 kV and that optimum conditions for a pseudo negative pole emitter 2b include : 8 kV ≦ V, - 80 mm ≦ G ≦ 0 mm, 50 mm ≦ D ≦ 150 mm, and - 8 kV ≦ V B ≦ 0 kV. Thus, in the case of the pseudo negative pole emitter 2b, the G is preferably negative, that is, the discharge end 21 of the emitter 2 preferably goes through the opposite conductor ring 3 so that the discharge end 21 may be positioned downstream of the opposite conductor 3 with respect to the air flow, as shown in Fig. 8, and the V B is preferably negative. In the case of the pseudo positive emitter 2a, the G may either positive or negative, that is, the discharge end 21 of the emitter 2 may be positioned upstream of the opposite conductor 3 with respect the air flow, as shown in Fig. 7, or it may go through the opposite conductor ring 3 so that it may be positioned downstream of the opposite conductor 3 with respect to the air flow, as shown in Fig. 8, and the V B may be negative or positive. In the test of Fig. 12 wherein an AC high voltage of 20 kV was applied to the emitter, no generation of dust from the discharge end 21 was detected. In contrast the tests of Fig. 12 except that an emitter with the tungsten rod 12 exposed was used with other conditions remaining the same, indicated remarkable generation of dust from the discharge end 21 when an AC high voltage in excess of 6 kV was used. The numbers of particles having a size of larger than 0.03 µm measured at a location 160 mm below the discharge end 21 were 7.4 × 10² pieces/ft³ with 6 kV, 2.5 × 10⁴ pieces/ft³ with 10 kV, and 2.9 × 10⁴ pieces/ft³ with 20 kV. An emitter having a quartz tube 14 recommended herein was caused to work for a continued period of 1050 hours. At the end of the period the discharge end of the emitter was examined by a microscope. It could not be distinguished from a new one, and no deposition of particulate dust and no damage were observed. Furthermore, an AC of 11.5 kV was applied to an emitter recommended herein and an ozone concentration was examined at a location 12.5 cm below the discharge end of the emitter. Ozone in excess of 1 ppb was not detected. By the equipment according to the invention almost all problems associated with the prior art can be solved and difficulties caused by static electrification in the production of semiconductor devices can be overcome.

Claims: An equipment for removing static electricity from charged articles existing in a clean space comprising an AC ionizer having a plurality of needle-like emitters disposed in a flow of clean air which has passed through filters wherein an AC high voltage is applied to said emitters to effect corona discharge for ionizing air whereby a flow of ionized air is supplied onto said charged articles to neutralize static electricity thereon characterized in that: a discharge end of each of said needle-like emitters is coated with a dielectric ceramic material; each of said emitters is disposed with its discharge end spaced apart at a predetermined distance from a grounded grid-or loop-like opposite conductor to form a discharge pair; a plurality of such discharge pairs being arranged in a two dimensional expanse in a direction transversely of said flow of clean air; and emitters of some of said discharge pairs are connected to a high voltage AC source having added thereto a minus bias voltage thereby forming pseudo negative pole emitters, while emitters of the other discharge pairs are connected to a high voltage AC source having added thereto a bias voltage biased to a more positive side than said minus bias voltage thereby forming pseudo positive pole emitters, said pseudo negative pole emitters and pseudo positive pole emitters being discretely arranged in said two dimensional expanse. The equipment for removing static electricity from charged articles according to claim 1 wherein said clean space is one for the production of semiconductor devices. The equipment for removing static electricity from charged articles according to claim 1 or 2 wherein said dielectric ceramic material is quartz. The equipment for removing static electricity from charged articles according to claim 1, 2 or 3 wherein the discharge end of each pseudo negative pole emitter is positioned downstream of the corresponding grounded grid- or loop-like opposite conductor with respect to the flow of air. The equipment for removing static electricity from charged articles according to claim 1, 2 or 3 wherein emitters of some discharge pairs are connected to a common high voltage AC source having added thereto a minus bias voltage thereby forming pseudo negative pole emitters, while emitters of the other discharge pairs are connected to a common high voltage AC source having added thereto a plus bias voltage thereby forming pseudo positive pole emitters. The equipment for removing static electricity from charged articles according to claim 1, 2, 3, 4 or 5 wherein the high voltage AC sources are provided by a voltage controlling device equipped with means for transforming a commercially available AC to an AC of a predetermined high voltage, means for adding respective predetermined positively and negatively biased DC voltages to the transformed AC and a voltage operating part for adjusting the AC high voltage and the biased DC voltages. The equipment for removing static electricity from charged articles according to claim 1, 2, 3, 4, 5 or 6 wherein the pseudo negative pole emitters and pseudo positive pole emitters are discretely arranged alternately in at least one direction within said two dimensional expanse.

Inventors: OKADA TAKAO, SAKATA SOICHIRO, YOSHIDA TAKANORI, OKADA, TAKAO, SAKATA, SOICHIRO, YOSHIDA, TAKANORI
Applicants: TAKASAGO THERMAL ENGINEERING, TAKASAGO THERMAL ENGINEERING CO. LTD.
Application year: 1989
Main classification: H05F3
IPC codes: H02M9, H05F3, H01T23