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

Abstract: Proposed herein is an equipment for removing static electricity from charged articles existing in a clean space, particularly a clean room for the production of semiconductor devices, comprising 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; opposite conductors are also installed so that each may be positioned apart from each emitter by a predetermined distance; an DC voltage or voltages are applied to the opposite conductors; and by adjusting the DC voltage or voltages the densities of positive and negative ions generated by each emitter is controlled.

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. 17, 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. 18. DC type : As diagrammatically shown in Fig. 19, 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. 20, 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 positive 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. 19) form positive air ions, while the other emitters (emitters 103b on the bar 102b in the example shown in Fig. 19) 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. 19) 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. 21. 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. 22, 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. 22, the density of negative ions is markedly lower than that of positive ions. The measurement shown in Fig. 22 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. 22, 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 vertical 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 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; each opposite conductor of said discharge pairs is connected to a DC voltage source; and there is provided a means for adjusting a DC voltage out put from said DC voltage source. We have found that 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 also found that the basic problem of a great difference between densities of positive and negative ions associated with AC type ionizers can be solved by applying a predetermined DC voltage or voltages to opposite conductors. The discharge end of each emitter is preferably positioned upstream of the corresponding grid- or loop-like opposite conductor with respect to the flow of air by a predetermined distance. While it is essential in the equipment according to the invention to suitably select an intensity of the DC voltage or intensities of the voltages to be applied to the opposite conductors, there are roughly classified two systems of applying the DC voltage to each opposite conductor in order to realize a supply of ionized air well balance in positive and negative ion densities to charged articles. In the first system, a DC voltage adjusted at a predetermined intensity is applied to the opposite conductors of all the discharge pairs having substantially the same configuration and structure from a common DC source. According to the first system, positive and negative air ions are generated from each discharge pair substantially in the same density, alternately at a periodic interval corresponding to a frequency of the AC applied to the emitters. According to the second system, some discharge pairs continuously generate positive ions in a high density but do not substantially generate negative ions, while the other discharge pairs continuously generate negative ions in a high density but do not substantially generate positive ions. In the second system, a DC voltage of a certain intensity is applied to the discharge pairs which generate positive ions, while a DC voltage of a different intensity is applied to the discharge pairs which generate negative ions, and the positive ion generating discharge pairs and the negative ion generating discharge pairs are arranged in a two dimensional expanse at an appropriate distribution in a direction transversely of the flow of clean air, whereby ionized air well balance in positive and negative ion densities may be supplied to charged articles existing downstream of the air flow. We have further found that in addition to the application of a DC voltage or voltages to the opposite conductors, if an appropriate DC voltage biased to a positive or negative side is added to the AC to be applied to the emitters, positive and negative ions can be generated in higher densities. Thus, the invention provides 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; wherein 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 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, preferably perpendicular to, said flow of clean air; (a) wherein opposite conductors of said discharge pairs are connects to a common DC voltage source; and there is provided a means for adjusting a DC voltage put out from said DC voltage source so that each of said discharge pair may ionize air so as to provide a positive ion density and a negative ion density substantially balanced from each other; or (b). wherein opposite conductors of some of said discharge pairs are connected to a first DC voltage source, while opposite conductors of the other discharge pairs are connected to a second DC voltage source; and there is provided a means for independently adjusting DC voltage out puts from said first and second DC voltage sources so that the discharge pairs connected to said first DC voltage source may generate ions inclined to a positive or negative polarity, while the discharge pairs connected to said second DC voltage source may generate ions inclined to the opposite polarity; or (c) wherein each opposite conductor of said discharge pairs is connected to a DC voltage source; there is provided a means for adjusting a DC voltage out put from said DC voltage source; each emitter of said discharge pairs is connected to an AC source of a high voltage having added thereto a voltage biased to a positive or negative side; and there is provided a means for adjusting an intensity of the voltage out put from said AC source and an intensity and polarity of said bias voltage. 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 example of an air ionizer which may be 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 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 a perspective view showing a part of 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 an emitter and the corresponding opposite conductor used in the ionizer of Fig. 1; Fig. 8 is a side view showing another example of the relative position of an emitter and the corresponding opposite conductor 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 illustrates a testing apparatus used herein; Fig. 11 is a graph showing densities of positive and negative ions measured by an ion density meter plotted against the DC voltage applied to the opposite conductor obtained in the test of Fig. 10 under the indicated conditions; Fig. 12 is a graph showing densities of positive and negative ions measured by an ion density meter plotted against the DC voltage applied to the opposite conductor obtained in the test of Fig. 10 under the indicated conditions including addition of a DC bias voltage to the AC applied to the emitter ; Fig.13 is a schematic perspective view of another example of an air ionizer which may be used in the equipment according to the invention; Fig. 14 are wave diagrams of AC and DC applied to the equipment according to the invention; Fig. 15 is an explanatory diagram for showing the state of the electric field at the time an emitter is in a plus phase in a case wherein a minus DC voltage is applied to the opposite conductor; Fig. 16 is an explanatory diagram for showing the state of the electric field at the time an emitter is in a minus phase in a case wherein a minus DC voltage is applied to the opposite conductor; Fig. 17 is a schematic illustration of a conventional pulsed DC type ionizer; Fig. 18 is a wave diagram of a voltage applied to the ionizer of Fig. 17; Fig. 19 is a schematic illustration of a conventional DC type ionizer; Fig. 20 is a schematic illustration of a conventional AC type ionizer; Fig. 21 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. 22 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 which may be 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 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 position 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 located 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. All of the emitters 2 are communicated through a common insulated conductive line 6 with an out put terminal 7 of an AC voltage controlling device 5, which controls an AC voltage applied to the emitters 2. All of the opposite conductors 3 are communicated through a common insulated conductive line 8 with an out put terminal 10 of a DC voltage controlling device 9, which controls a DC voltage applied to the opposite conductors 3. A reference numeral 11 designates a voltage operating part for adjusting out put voltages from the AC voltage controlling device 5 and the DC voltage controlling device 9. 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 an 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 12 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 the ceramic cover spaced apart from the corresponding 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 21 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 pointed or may not be pointed. Fig. 6 is an enlarged perspective view showing a part of 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 8 having an insulating coating so that they may be installed substantially within a plane in a two dimensional expanse. The conductor 8 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. All of the ring-shaped opposite conductors 3 are communicated through the conductor 8 with the OUT PUT10 of the DC voltage controlling device 9. 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 the 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 coated with a ceramic material 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 coated with a ceramic material 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. Fig. 9 is a circuit diagram for the AC voltage controlling device 5 and its voltage operating part 11 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) and a transformer 32 attached to the in put terminal 31, and a rectification circuit 33, a constant voltage circuit 34, an inverter circuit 35 and a high voltage transformer 36 connected in series to the secondary side of the transformer 32. The AC from the transformer 32 undergoes all wave rectification in the rectification circuit 33, becoming a DC. The constant voltage circuit 34 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 33 varies accordingly, and in turn the in put voltage to the subsequent high voltage transformer 36 varies, and the eventual out put voltage can not be kept constant. Accordingly, the constant voltage circuit 34 is utilized. The inverter circuit 35 is incorporated with an oscillation circuit, and choppers the constant voltage DC from the constant voltage circuit 34 to a square wave, which is then transformed by the high voltage transformer 36 to a high voltage AC of a square wave and put out to the emitters 2 from the out put terminal 7 (see Fig. 1). The high voltage transformer 36 comprises an insulated transformer having incorporated with a slide rheostat, and thus, the intensity of the AC voltage put out to the emitters 2 can be controlled at will by operating the slide rheostat part of the high voltage transformer 36. Accordingly, this high voltage transformer 36 corresponds to the voltage operating part 11 of Fig. 1. 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. The DC voltage controlling device 9 of Fig. 1 may be a known one for converting a commercial AC to a DC. It is sufficient that it can convert a commercial AC of 100 V to a DC of a voltage e. g. within the range between - 1 kV and + 1 kV. In the equipment of Fig. 1, an AC high voltage is applied to to all the emitters 2 from the same AC voltage source, while a DC voltage is applied to all the opposite conductors 3 from the same DC voltage source, and all the discharge pairs 4 have substantially the same configuration and structure. Accordingly, when clean air flows uniformly through the discharge pairs 4, all the discharge pairs 4 show the same behavior to ionize air. Namely, each discharge pair 4 generates positive and negative air ions alternately at a periodic interval corresponding to a frequency of the AC applied to the emitters 2. If the DC voltage applied to the opposite conductors 3 is properly adjusted, it is possible to provide positive and negative ions substantially in the same density. The operation of the equipment of Fig. 1 will be specifically described by test examples. Fig. 10 illustrates an apparatus used in the tests. 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 is electrically communicated with the AC voltage controlling device 5 via the vertically extending glass tube 18 and the horizontally extending resin covered tube 22. An 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 opposite conductor 3 is supported in position by supporting its insulated conductive line 39 by acrylic bars 38 vertically suspended from the resin covered tube 22. A conductive line 8 connected with the insulated conductive line 39 is communicated with the DC voltage controlling device 9. A thickness of the stainless opposite conductor ring is 6 mm, and a diameter of the ring is 80 mm. A high voltage AC is applied to the emitter 2, while applying a DC voltage to the opposite conductor 3 to effect corona discharge, 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 40. An effective AC component of the AC applied to the emitter 2 and DC voltage applied to the opposite conductor 3 are represented by V and V e , respectively. Fig. 11 is a graph showing densities of positive and negative ions measured by the ion density meter 40 plotted against the DC voltage V e applied to the opposite conductor 3 under the test conditions including a distance of the discharge end 21 of the emitter 2 being 37 mm upstream of the opposite conductor 3 with respect to the air flow (G shown in Fig. 7 = + 37 mm), V = 13 kV and a frequency of the applied AC of 50 Hz. The result shown in Fig. 11 is very interesting in that in a case wherein no DC voltage is applied to the opposite conductor, the density of positive ions is extremely higher than the density of negative ions, providing ionized air extremely inclined to a positive side; whereas if a negative DC voltage is applied to the opposite conductors, as the absolute intensity of the applied negative DC voltage increases, the density of positive ions decreases, while the density of negative ions increases. Under the conditions employed, when the V e is approximately - 190 V, both the positive and negative ions are balanced, showing a density of about 48 × 10³ ions/cc). Accordingly, in a case wherein the same conditions as those of this test are applied to each discharge pair of Fig. 1, if a DC voltage of approximately - 190 V is applied to each opposite conductor, ionized air with substantially the same positive and negative ion densities may be continuously caused to flow downstream of the discharge pairs. In clean rooms an air flow is not significantly disturbed. Accordingly, it is possible to make ionized air with well balanced positive and negative ion densities to reach fairly downstream side. Fig. 12 is a graph showing densities of positive and negative ions measured by an ion density meter plotted against the DC voltage applied to the opposite conductor obtained in the test of Fig. 10 under the same conditions except that a positive DC bias voltage (V B ) is added to the AC applied to the emitter. While an intensity and polarity of the DC bias voltage added to the AC may be varied, Fig. 12 shows data of an example wherein the DC bias voltage added is 2.1 kV. In the equipment of Fig.1, the addition of a bias voltage to the AC can be made by connecting a DC transformer 41 to the AC voltage controlling device 5. Advantageous results of the addition of a DC bias voltage are apparent from the results of Fig. 12. Namely, in the case wherein a bias voltage of 2.1 kV is added as in Fig. 12, the density of negative ions become high as a whole, when compared with the case wherein no bias voltage is added as in Fig. 11. For example, in the case of Fig. 12, even if the V e is 0 V, the difference between the densities of positive and negative ions is smaller, and by application of a V e of only - 63 V to the opposite conductor positive and negative ions are well balanced at a density of about 63 × 10³ ions/cc, which is higher than about 48 × 10³ ions/cc in the case of Fig. 11. Accordingly it is preferable to add a DC transformer 41 to the AC voltage controlling device 5 of the equipment of Fig. 1, thereby adding a positive or negative DC bias voltage to the AC applied to the emitters. Fig.13 is a schematic perspective view of another example of an air ionizer which may be used in the equipment according to the invention. In this case, a DC voltage of a certain intensity is applied to opposite conductors of some discharge pairs, while a DC voltage of a different intensity is applied to opposite conductors of the other discharge pairs so that some discharge pairs may continuously generate positive ions in a high density, while the other discharge pairs may continuously generate negative ions in a high density. In the illustrated example, DC voltage controlling devices 9a and 9b are capable of putting out DC currents of different voltages from their respective OUT PUT 10a and 10b. Some opposite conductors 3a are communicated with the OUT PUT 10a via an insulated conductive line 8a while the other opposite conductors 3b are communicated with the OUT PUT 10b via an insulated conductive line 8b. More specifically, 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. Opposite conductors 3a in the first line of the figure and opposite conductors 3a in the third line of the figure are communicated through a common insulated conductive line 8a with the OUT PUT 10a of the DC voltage controlling device 9a, while the opposite conductors 3b in the second line of the figure and opposite conductors 3b in the fourth line of the figure are communicated through a common insulated conductive line 8b with the OUT PUT 10b of the DC voltage controlling device 9b. When a DC voltage of a negative side is put out from the OUTPUT 10a, while putting out a more positive DC voltage from the OUT PUT 10b, negative ion rich air is continuously generated from each opposite conductor 3a, while positive ion rich air is continuously generated from each opposite conductor 3b. For example, in a case wherein each discharge pair has the same structure as that used in the test of Fig. 11, and an AC voltage having a frequency of 50 Hz and a V of 13 kV is applied to the emitters, it will be possible to cause each opposite conductor 3a to generate ionized air high in the negative ion density and low in the positive ion density by putting out a DC voltage of e. g. more negative than - 300 V.from the OUT PUT 10a, and it will be possible to cause each opposite conductor 3b to generate ionized air high in the positive ion density and substantially free from negative ions by putting out a DC voltage of e. g. more positive than 0 V. Likewise, if a bias DC voltage of 2.1 kV is further added to the AC applied to emitters as in the test of Fig. 12 under the conditions of Fig. 12, negative ion rich air and positive ion rich air will be continuously and stably generated from each opposite conductor 3a and 3b, respectively, by putting out a DC voltage of e. g. - 400 V.from the OUT PUT 10a and a DC voltage of e. g. + 200 V.from the OUT PUT 10b. Accordingly, by appropriately arranging a plurality of the opposite conductors 3a generating negative ion rich air and the opposite conductors 3b generating positive ion rich air in a two dimensional expanse transversely of the air flow, for example, by alternately arranging a line of the opposite conductors 3a and a line of the opposite conductors 3b as shown in Fig. 13, or by arranging the individual opposite conductors 3a and 3b alternately or zigzag, or by arranging a small group of the opposite conductors 3a and a small group of the opposite conductors 3b alternately, it is possible to supply well balanced positive and negative ions to charged articles which are existing downstream of the ionizer. Figs. 14 to 16 are for illustrating effects of the DC voltage or voltages applied to the opposite conductors. AC type ionizers inevitably generate more positive ions than negative ions in cases where the V e is 0. However, under the conditions that a sufficient effective AC component for corona discharge as shown in Fig. 14 is being applied to the emitter, if a negative V e is applied to the opposite conductor in accordance with the invention, in either case wherein the emitter 2 is in a positive (Fig. 15) or negative (Fig. 16) phase, an electric field directing to the opposite conductor 3, as shown by broken arrows, is formed downstream of the opposite conductor 3 with respect to the air flow. Thus, by the electric field so formed, a Coulomb force for causing negative ions, which have gone through the opposite conductor 3, to move downwards always acts, irrespective of the polarity of the emitter, thereby increasing a density of negative ions arriving at charged articles existing downstream. If this reasoning is correct, the discharge end 21 of the emitter 2 should be preferably positioned upstream of the opposite conductor 3 with respect to the air flow, as shown in Fig. 7. If the discharge end 21 of the emitter 2 is positioned downstream of the opposite conductor 3 with respect to the air flow, as shown in Fig. 8, the intended effect of increasing a negative ion density will be reduced. We have experimentally found that although the structure of the discharge pair, as shown in Fig. 8, may be preferred in some cases wherein an AC high voltage having added thereto a certain bias is applied to the emitter side, in general the discharge end 21 of the emitter 2 should be preferably positioned upstream of the opposite conductor 3 with respect to the air flow, as shown in Fig. 7. We have repeated the tests while varying parameters G shown in Figs 7 and 8, D, V and V e . It has been found that optimum conditions for working the equipment according to the invention in a clean room with rates of air flow of from 0.15 to 0.6 m/sec include : - 80 mm ≦ G ≦ 80 mm, 50 mm ≦ D ≦ 150 mm, 8 kV ≦ V, and - 500V ≦ V e ≦ 500 V. In the test of Fig. 10 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 same tests wherein 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 applied to the emitter. The numbers of particles having a size of larger than 0.03 µm measured at a location 160 mm below the discharge end 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 by a predetermined distance from a 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; each opposite conductor of said discharge pairs is connected to a DC voltage source; and there is provided a means for adjusting a DC voltage out put from said DC voltage source. The equipment for removing static electricity from charged articles according to claim 1, characterised in that opposite conductors of said discharge pairs are connected to a common DC voltage source so that each of said discharge pair ionizes air so as to provide a positive ion density and a negative ion density substantially balanced from each other. The equipment for removing static electricity from charged articles according to claim 1, characterised in that opposite conductors of some of said discharge pairs are connected to a first DC voltage source, while opposite conductors of the other discharge pairs are connected to a second DC voltage source; and there is provided a means for independently adjusting DC voltage out puts from said first and second DC voltage sources so that the discharge pairs connected to said first DC voltage source generate ions inclined to a positive or negative polarity, while the discharge pairs connected to said second DC voltage source may generate ions inclined to the opposite polarity. The equipment for removing static electricity from charged articles according to claim 1, characterised in that each emitter of said discharge pairs is connected to an AC source of a high voltage having added thereto a voltage biased to a positive or negative side; and there is provided a means for adjusting the value of the voltage put out from said AC source and the value and polarity of said bias voltage. The equipment for removing static electricity from charged articles according to claim 1, 2, 3 or 4 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, 2, 3, 4 or 5 wherein said dielectric ceramic material is quartz. The equipment, for removing static electricity from charged articles according to claim 1, 2, 3, 4, 5 or 6 wherein the discharge end of each is positioned upstream of the corresponding 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 3 wherein the discharge pairs having the opposite conductors connected to the first DC voltage source and the discharge pairs having the opposite conductors connected to the second DC voltage source are discretely arranged alternately in at least one direction within said two dimensional expanse. The equipment for removing static electricity from charged articles according to claim 1, 2, 3, 4, 5, 6, 7 or 8 wherein the discharge pairs are arranged in a two dimensional expanse in a direction perpendicular to said flow of clean air.

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: H05F3, H01T23