EP0258296B1 - Device for generating ions in gas streams - Google Patents

Abstract

A device for generating ions in gas streams includes an electrode arrangement exposed to the gas streams and pulse wave shaped high voltage supply which delivers a succession of alternating negative and positive pulses with steep sides. The electrode arrangement comprises at least a point discharge electrode (6) and at least a counter-electrode (4) in a fixed and defined mutual arrangement. The timing of the high voltage signal is determined so that the duration of each impulse corresponds to the time of flight of the ions between the electrodes and the interval between the pulses is adapted to the speed of the gas streams.

Description

The invention relates to a device for generating ions in gas streams for reducing electrostatic charges which are present on sensitive products such as e.g. Microchips, foils, magnetic disks, laser storage disks and printed circuit boards can lead to destruction in the event of uncontrolled discharge or result in increased particle deposition.

In the manufacture of highly integrated semiconductor components, in laser and magnetic storage disks and in other products with microstructures in the resolution range of one micrometer and less, both particle contamination and uncontrolled electrical discharges lead to considerable quality losses. Here, microstructures are also generally understood to mean sensitive plastic films or surfaces in which the deposition of microparticles leads to a loss in quality. The damage is caused by electrostatic charges. Such productions are found e.g. in clean rooms, the air of which is highly pre-filtered and flows through the clean room in a low-turbulence, piston-like displacement flow. The supply air of such clean rooms can be filtered to such an extent that almost no particles can get into the clean room via the air flow. Particle pollution during production essentially arises from the production process itself or from the operating personnel. However, the device according to the invention can also be operated at limited workplaces with a specially generated air flow.

The charges are generated by friction, influence or capacitive processes and are unavoidable when moving the product, especially on insulating surfaces. Charge densities can arise that lead to voltages of several thousand volts. On the one hand, these charged surfaces increasingly attract aerosols, in particular charged aerosols, by means of electrostatic forces.

One finds about 20-fold particle deposition on surfaces charged with 500 V compared to a neutral surface. On the other hand, such surface charges can be discharged in an uncontrolled manner via the microstructures. The microstructures can be destroyed either by an electrical breakdown or by high current densities. Sensitive metal oxide semiconductor structures on silicon wafers can be destroyed by discharging voltages around 50 V.

The charging of insulating surfaces on the product and the increased particle deposition can be prevented by the fact that the air flow contains the ions of positive and negative signs. As a result, charges are balanced both on airborne particles and on the product surfaces. Uncontrolled discharges through the microstructures cannot take place. Surface charges are broken down by controlled discharge via air ions. With electrostatically sensitive products, a uniform distribution of positive and negative ions is particularly important.

It is known to use the Townsend gas discharge in the inhomogeneous electric field of needle tips or wires to generate positive and negative air ions. A device for generating ions at tips is shown in US Pat. No. 1,356,211, while in DE-OS 28 09 054 a device for generating ions at wires is described. A discharge zone with an expansion of approximately 0.5 mm forms in the vicinity of the tips or the wire surface, in which the gas molecules are ionized. With increasing distance from the discharge zone, the speed slows down due to the weakening field. A condition for the ions to be carried away by the air flow is that their velocity in the inhomogeneous field decreases to a value which is less than the air velocity. A voltage of 6-7 kV is required to ignite a gas discharge on strongly curved surfaces. When operating such ionizers with a voltage of approx. 10 kV, the speed of the ions decreases within half to one meter to a value of less than one meter per second. The usual air flow speed at workplaces is around 0.5 m per second. From the foregoing it is clear that for the distribution of the ions in the air flow there is a close connection between the air speed on the one hand and the temporal course of the high voltage coupled with the charge electrode geometry on the other hand.

Conventional ionizers work with voltages between 10 and 20 kV. The time course of the voltage is either uniform (FIG. 1c) a sine voltage (FIG. 1a) from 50 to 60 Hz or a rectangular voltage course.

It is known that with the same field geometry of the discharge and the same voltage, more ions are generated at the negative emitter than at the positive. Since ionizers can only fulfill their task of neutralizing surface discharges if the same number of positive and negative ions are metered into the air flow, the sinusoidal alternating voltage for supplying emitters is disadvantageous, whereas in the case of a rectangular voltage curve and direct voltage supply, ions can also be set by setting the corresponding direct voltage level balanced polarity balance are generated.

FR-A-2 135 162 describes a device for generating ions, in which elongated electrodes are arranged in parallel above and below one another and in each case one discharge electrode lies between two counter electrodes. The discharge electrodes are supplied with a pulsed voltage which has a rectangular shape. This way Shaped voltage supply, like the rectangular voltage curve and the sinusoidal AC voltage, has the disadvantage that the polarity changeover takes place in periods which are short when the air flow velocity is considered. In this case, ions that have already been metered into the air are transported back to the discharge electrode by rapidly changing the polarity and are ineffective for the ionization of the air. In addition, the efficiency of ion dosing deteriorates. Efficiency is understood here to mean the ratio of the number of ions that enter the airflow to the total number of ions generated at the discharge electrode.

This disadvantage increases the current load on tip electrodes. With a high current load on the tip electrodes, there is increased material removal and thus an increase in the radius at the tips and an increased accumulation of particles at the tip. As the inhomogeneous field decreases, the generation of ions is reduced. Constant operating conditions are therefore called into question. In practice, these disadvantages are corrected by increasing the operating voltage, which leads to an acceleration of the disadvantages described.

The increased current load due to return transport is also not avoided in known systems by supplying two peak groups separately with DC voltage. In this case, the potential difference between the peaks is approximately 20 kV, and the distance between the peaks should be chosen to be approximately 30 cm. In this case, the average ion velocity remains so high that only a small proportion of ions from the peripheral zones of the electridic field are absorbed by the air flow. For this reason, the same disadvantages are to be expected as with ionizers operated with AC voltage. The formation of planar ionizers, such as can be installed, for example, over a large area under the ceiling of clean rooms, leads to locally discontinuous ion generation. In the border area of ionizers supplied in this way, there is excess ion polarity which, in contrast to the actual task of the ionizers, can lead to additional charges. Experience has proven even more disadvantageous that the constant field strengths parallel to the electrode plane, which arise between such electrodes which are supplied with direct voltage in the cross-sectional plane of the air flow and are supplied with DC voltage, lead to separation of negative and positive ions: With this separation, one can be caused by the excess of ions Polarity charges of several 100 V arise.

Operating experience with ionizers in clean rooms, for example class 10 according to US Federal Standard 209c with particularly high requirements, has shown disadvantages in operation in the three operating modes of the ionizers described in FIG. 1. These disadvantages relate to the removal of the tips, the introduction of metallic tip material into the clean room air and the deposition of impurities on the tips, as well as electrochemical conversion processes of gaseous products into solid particles. According to the latest studies by BY Liu et al, Tex. lnstr. Cor .: Characterization of Electronic lonizers for Clean Rooms; IES 1985, up to an additional 1.5 x 10 ⁶ particles per m ³ can be found in the clean room air. In contrast, in high-quality cleanrooms, particle concentrations of around 300 particles per m ³ and less are aimed for.

The invention has for its object to provide a device for generating ions in gas streams with an electrode arrangement exposed to the gas streams and a pulsed high voltage supply, which provides an alternating sequence of negative and positive pulses with steep flanks, which also have constant operating conditions over a longer period of time guaranteed with uniform ion distribution over the flow cross-section with good efficiency.

This object is achieved according to the invention by the characterizing features of the main claim.

The fact that tip-shaped discharge electrodes and associated counter electrodes are provided in a fixed and defined association with one another provides a defined electrical field and the time profile of the high voltage applied to the tip-shaped discharge electrodes can be correlated with the gas velocity and the ion flight time between the discharge electrodes and counter electrodes, so that the efficiency is increased. The duration of the pulses corresponds to the flight time of the ions, so that the entire space between the electrodes is filled with ions before switching off. The pulse pauses between the pulses are adapted to the speed of the gas in such a way that the ions generated during the pulses are largely carried out by the gas flow from the space exposed to the force of the electric field when the high voltage is switched on. Due to the geometrical arrangement of tip-shaped discharge electrodes and counter electrodes, a uniform ion diversification is produced over the flow cross section and the disruptive influence of other potentials in the room on the ion generation and distribution is prevented. The alternation of positive and negative high voltage at the same tip-shaped discharge electrode avoids constant DC fields perpendicular to the direction of flow of the gas, which lead to separation of the positive and negative ions.

Advantageous further developments and improvements are possible through the measures specified in the subclaims.

Characterized in that an erosion as the material for the tip-shaped discharge electrodes with niobium poor electrode material is selected, the removal behavior is improved and the tendency to sputter is reduced.

The device according to the invention can be used both in high-quality cleanrooms and outside of cleanrooms. In the non-high quality filtered air outside of clean rooms, contamination of the tip-shaped discharge electrodes can occur due to the accumulation of particulate air pollutants, which lead to impairment of ion generation. For cleaning purposes, the electrode carriers can therefore be removed from their spring-locked plug seat with a rotation lock and reinserted.

By providing high-voltage relays, the positive and negative high-voltage generators can be galvanically separated and the tip-shaped discharge electrodes can be supplied with positive and negative high-voltage via a single, single-core, shielded high-voltage cable. By switching the high-voltage relays without load, their lifespans are increased considerably.

By providing a high-voltage supply, which has a separate low-voltage control unit and a high-voltage module, the high-voltage module can be arranged in the vicinity of the electrode arrangement outside the gas flow, as a result of which no undesirable turbulence occurs in the gas flow. The low-voltage control unit, which controls the high-voltage module to regulate the positive and negative ion quantities, can be located in the immediate vicinity of the workplace. While the connection between the electrode arrangement and the high-voltage module is made via a shielded high-voltage cable, the high-voltage module is controlled by the low-voltage control unit with direct current, so that even long cable lengths can be used in the production area without the risk of disturbing sensitive electronic control and measuring devices due to radiated electromagnetic radiation.

Another advantage of the invention is that additional particle generation is significantly reduced. It was determined by measurements that, with a resolution of approximately 100 particles per m ^3, no additional particle generation could be recorded by the device according to the invention.

It is known that ionizers according to the prior art produce ozone by the gas discharge in a concentration which can be harmful to the health of the employed personnel. The measurements carried out during operation of the device according to the invention did not lead to an increase in the ozone concentration existing in the natural ambient air, since the current intensity in the discharges at the tip electrodes is extremely low with the aid of the voltage supply. The size of the high voltage is an important criterion for operational safety, but also for the loading of the tips of the discharge electrodes. With known ionizers, it can assume values of 30 kV. Because of the high efficiency of the ion dosing and because of the homogeneous distribution of the discrete ion sources in the flow cross section, the maximum operating voltage can be limited to values below 15 kV in the present invention. Despite the low operating voltage, discharge times are achieved that meet the high requirements, for example in chip production.

In known ionizers, the tip electrodes are directed towards the processing of sensitive products in order to achieve short discharge times. In this case, voltages above the sensitivity threshold of the products can be influenced. These disadvantages are largely avoided in the device according to the invention by horizontally aligning the changing fields in the cross-sectional plane of the air flow parallel to the working plane and by the dense, defined arrangement of the counterelectrode. The device according to the invention allows working in the immediate vicinity of the ionizer if a perforated metal sheet is attached to earth potential between the working level and the ionizer without reducing the effectiveness of the ionizer.

• Fig. 1 unterschiedliche zeitliche Verläufe von Hochspannungen zur Versorgung der Entladungselektroden,
• Fig. 2 der zeitliche Verlauf der Hochspannung zur Versorgung der Entladungselektroden gemäß der erfindungsgemäßen Vorrichtung,
• Fig. 3 einen Schnitt durch ein erstes Ausführungsbeispiel der erfindungsgemäßen Vorrichtung,
• Fig. 4 die schematische Darstellung der verschiedenen Bestandteile der erfindungsgemäßen Vorrichtung,
• Fig. 5a eine perspektivische schematische Darstellung der Elektrodenanordnung eines weiteren Ausführungsbeispiels,
• Fig. 5b und 5c schematische Schnittdarstellungen von weiteren Elektrodenanordnungen,
• Fig. 6 einen Teilschnitt durch einen Elektrodenträger gemäß Fig. 5a,
• Fig. 7a die schaltungsgemäße Ausgestaltung des Hochspannungsmoduls, und
• Fig. 7b ein Impulsdiagramm für das Hochspannungsmodul nach Fig. 7a.

The invention is illustrated in the drawing and is explained in more detail in the description below. Show it:

• 1 different time profiles of high voltages for supplying the discharge electrodes,
• 2 shows the time course of the high voltage for supplying the discharge electrodes according to the device according to the invention,
• 3 shows a section through a first exemplary embodiment of the device according to the invention,
• 4 shows the schematic representation of the various components of the device according to the invention,
• 5a shows a perspective schematic illustration of the electrode arrangement of a further exemplary embodiment,
• 5b and 5c are schematic sectional representations of further electrode arrangements,
• 6 shows a partial section through an electrode carrier according to FIG. 5a,
• 7a shows the circuit configuration of the high-voltage module, and
• Fig. 7b is a timing diagram for the high voltage module of Fig. 7a.

4 shows the device according to the invention, which has a low-voltage control unit 30, a high-voltage module 31 and the electrode arrangement 32. The electrode arrangement is arranged in the area of the air flow, for example in clean rooms in the ceiling area below the air outlets or the air filter. 5a schematically shows a grid-shaped electrode arrangement which is suitable for mounting under a clean room filter ceiling. The electrode arrangement 31 has cross members 1, 8 from metal semicircular profiles, which form a solid frame with tubular metal counter electrodes 4 lying on the ground. Electrode carriers 5, which carry needle-shaped or tip-shaped discharge electrodes 6, are fastened to the crossbeams 1, 8 via plug connections 3, 7. The counter electrodes 4 and the electrode carrier 5 are arranged parallel to one another in one plane, the tip-shaped discharge electrodes likewise preferably lying in a plane perpendicular to the counter electrodes 4. 5a, only three tip-shaped discharge electrodes 6 are provided per electrode carrier 5. Of course, more discharge electrodes can also be provided. The counter electrodes 4 or electrode carrier 5 have a diameter of about 3 to 15 mm and the distance is between 5 and 50 cm. The tip-shaped discharge electrodes 6 are arranged at regular intervals of approximately 5 to 30 cm.

The high voltage is supplied to the discharge electrodes 6 via protective resistors in the crossbar 1 and the plug connectors 3, the electrode carriers 5 being electrically connected in parallel. A terminal connection (not shown) for the electrical connection of the grounded shielding of the single-wire high-voltage cable 9 is also provided in or on the crossbar.

In Fig. 6 the section through an electrode carrier and in particular the connector 3, 7 is shown. The connector 3 has an acrylic tube 33 with a shoulder, inside which the high-voltage line 10 is guided. The approach is inserted into the electrode carrier 5, a socket 11 connected to the high-voltage line and a pin 12 provided in the electrode carrier 5 forming the electrical connection. The acrylic tube 33 provides a creepage distance between the high-voltage electrode carrier and the cross-beam 1 at ground potential. The connector 7 also has an insulating acrylic rod 34, the end of which is inserted into the electrode carrier and is fixed by means of a dowel pin 14. A compression spring 13 is supported on the end of the acrylic rod 34. The dowel pin 14 forms an anti-rotation device, so that the tip-shaped discharge electrodes cannot change their position with respect to the counter electrodes 4. The connectors 3 and 7 together form a spring-locked plug seat, so that the electrode carrier can be removed and cleaned without major difficulties.

The tip-shaped discharge electrodes are driven with a high voltage according to FIG. 2 alternately with positive and negative pulses with steep edges. For example, the high voltage is first applied over a period of time t ₁ , which is selected such that the space between the electrodes 4, 6 is filled with positive ions. During this time, due to the high ion velocity due to the high field strengths, hardly any ions are left in the perpendicular to the grid-shaped electrode arrangement. Fig. 5a metered flowing air flow. If the high voltage is switched off steeply after a time that corresponds to the ion flight time, the force of the electric field ceases and the ions can be removed from the area of greatest field strength between the electrodes 4, 5, 6 by the frictional force of the air flow. This takes place in the period t ₂ . Thereafter, the opposite pole, negative high voltage is applied to the same tip-shaped electrodes 6. The negative high voltage also remains switched on only until a negative ion cloud fills the space between the electrodes 4, 5, 6 (t ₃ ), in order then to be switched off on a steep side. The distance a according to FIG. 5a between the electrode carriers 5 with the discharge electrodes 6 and the counter electrodes 4 determines the switch-on time t and t _{3 of} the high voltage via the ion mobility. The switch-on times are, for example, between a few and a few 10 ms, in particular between 5 and 60 ms. With air flows between 0.1 and 1 m per second, the switch-off times, ie the interval between the pulses, are between 100 and 1000 ms. This results in duty cycles of 1: 5 to 1:20. Through this interaction of the fixed electrode arrangement and switching the high voltage on and off, the majority of the ions generated at the tips of the discharge electrodes are introduced into the air flow. This reduces the current load at the tips by orders of magnitude, which is responsible for the disadvantageous particle generation in the air flow.

A low-erosion electrode material is used for the discharge electrodes, stainless steel and tungsten being used in the prior art. Tungsten showed a low abrasion behavior. Investigations of further materials showed that with niobium and its alloys as the electrode material, significantly better results can be achieved, so that this material is used in the discharge electrodes 6. Table 1 shows the results of a test carried out over 1000 hours with 20 times, non-tactile current loading of the tip-shaped discharge electrodes. Column 2 shows a volume removed which is 6 times lower than that of tungsten. Tantalum also showed better results than tungsten.

The high-voltage module 31, which is preferably arranged in the vicinity of the electrode arrangement for reducing the length of the high-voltage cable 9 but outside the air flow, is shown in more detail in FIG. 7a. Two high-frequency oscillators 18 drive the primary winding of two high-voltage transformers 19 via low-voltage drivers (not shown), one transformer generating positive high voltage and the other negative high voltage, depending on the passage in the high-voltage diodes which are cast in each case. Two high-voltage relays 20 switch the respective one High voltage on the shielded high-voltage cable 9, which supplies the discharge electrodes 6. So that the high-voltage relays 20 switch load-free, the oscillators 18 and the relays 20 are driven in accordance with the pulse diagram according to FIG. 7b. From this it can be seen that the high-voltage relays 20 are switched on or off when the oscillators 18, which are driven in pulsed fashion, are not switched on.

The low-voltage control unit 30 can be located in the immediate vicinity of the workplace or can be accommodated in a central control cabinet. It outputs two direct currents with independently adjustable direct voltage values to the high-voltage module, whereby the positive and negative high-voltage values can be determined independently of one another. To regulate the DC voltage values generated by the low-voltage control unit 30 and thus to regulate the balance of the ion polarity are not shown in a. Control circuit, the currents used to generate the positive and negative ions are measured separately in the high-voltage module 31 and supplied to the low-voltage control unit 30 as a control variable.

Special counter electrodes 4 are provided in the electrode arrangement according to FIG. 5a. 5b and 5c, the counter electrodes are formed by system components surrounding the discharge electrodes 6. For example, according to FIG. 5b, a frame system 16, which is electrically connected to ground, is designed as a counter electrode. In Fig. 5c, a perforated plate 17 is provided as the counter electrode, which is on ground and which can serve as a screen or the like.

Another embodiment is shown in FIG. 3. In this exemplary embodiment, ions are not metered into a gas or air flow present in the room, but rather a closed device is provided which has a device for generating a rectified flow over a large cross section. This device has a blower 22 which supplies a pressure chamber 21 which is delimited on the outflow side by a uniformly air-permeable layer 23 designed as a baffle plate. The baffle forms the counter electrode to the tip-shaped discharge electrodes 6, which are arranged below the baffle 23 and are attached to electrode carriers 5 according to FIG. 5a. The rectified flow is stabilized in the surrounding space by a circumferential flow apron 24.

Claims (14)

1. Apparatus for producing ions in gas currents with an electrode system exposed to the gas currents comprising at least one discharge electrode and at least one counterelectrode at earth potential, which are in a fixed and defined association with one another, and with a pulse-like high voltage supply, which supplies an alternating sequence of negative and positive pulses with steep edges, characterized in that the at least one discharge electrode (6) is constructed in tip-shaped manner and that the duration of the positive or negative pulses is roughly of the same length as the transit time of the ions between the at least one discharge electrode and the at least one counterelectrode and that between the pulses are provided pulse intervals, whose length is adapted to the speed of the gas current in such a way that the ions produced during the pulses are largely removed by the gas current from the area exposed to the force action of the electric field.

2. Apparatus according to claim 1, characterized in that the pulse duration is approximately 5 to 60 ms and that the distance between the pulses at a gas current speed of approximately 0.1 to 1 m/s is approximately 100 to 1000 ms.

3. Apparatus according to claims 1 or 2, characterized in that the electrode system (31) has rod- like, parallel, alternating counterelectrodes (4) and electrode supports (5) supporting tip-shaped discharge electrodes (6), the latter being arranged in one plane preferably at right angles to the counterelectrodes (4).

4. Apparatus according to claim 3, characterized in that the counterelectrodes (4) and electrode supports (5) are preferably circular and have a diameter of approximately 3 to 15 mm and a reciprocal spacing between 5 and 50 cm and that the tip-shaped discharge electrodes have an identical reciprocal spacing of approximately 5 to 30 cm.

5. Apparatus according to claims 3 or 4, characterized in that the electrode supports (3) supporting the tip-shaped discharge electrode (6) are detachably fixed in a plug-in seat (3, 7).

6. Apparatus according to claim 6, characterized in that the electrode supports (5) are lockable in the plug-in seat (3,7).

7. Apparatus according to claims 1 or 2, characterized in that the at least one counterelectrode (16, 17, 23) is a component of other equipment parts, such as frame structures or safety plates formed from perforated plates, which are at a clearly defined distance from the tip-shaped discharge electrode (6) and are at a clearly defined potential.

8. Apparatus according to one of the claims 1 to 7, characterized in that the high voltage supply comprises a low voltage control unit (30), which supplies two direct currents with adjustable d.c. voltage values, and a high voltage module (31) spatially separated from the low voltage control unit (30) and connected thereto, the high voltage module (31) being positionable in the vicinity of the electrode system (32).

9. Apparatus according to claim 8, characterized in that the high voltage module (31) is connected across a single-wire high voltage line (9) to the electrode system (32).

10. Apparatus according to claims 8 or 9, characterized in that the high voltage module (31) has, for producing the positive and negative high voltage, in each case a voltage converter with an oscillator (18), a transformer (19) and a rectifier (25) and in each case one high voltage relay (20).

11. Apparatus according to claim 10, characterized in that the high voltage relay is switched load-free in the pulse intervals of the driver of the associated oscillators (18) with the same cycle as said driver.

12. Apparatus according to claim 8, characterized in that for controlling the balance of the ion polarity the currents necessary for producing the positive and negative ions are measured and are used as a controlled value for setting the d.c. voltage values.

13. Apparatus according to one of the claims 1 to 12, characterized in that the tip-shaped discharge electrodes (6) are made from niobium or its alloys.

14. Apparatus according to claim 1, characterized in that the gas flow is passed across a pressure chamber (21), which is closed by a deflector or perforated plate (23), etc., and that the discharge electrodes are arranged on the outflow side of the plate (23), the latter forming the defined counterelectrode.

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