DE102007039758B4 - Device and method for generating a plasma by low-frequency inductive excitation - Google Patents

Abstract

Device (10) for generating a plasma by inductive excitation, comprising a container (12) with a gas in which a plasma is to be generated, and an inductance (18) which is inductively coupled to the gas, and a power source which is suitable to generate an alternating current signal with an excitation frequency ν of less than 200 kHz in the inductance (18), the container (12) being spherical or approximately spherical and the inductance (18) being formed by a coil which the container (12 ), the container (12) further comprising an equatorial region (42) in which its cross-sectional area in a plane perpendicular to the induced magnetic field drops from a maximum value Amax to a value of Amax / 2, and comprises two pole regions (44) in which the cross-sectional area of Amax / 2 drops to zero, with at least one turn (18b, 18c) of the coil at least partially surrounding the container (12) in its equatorial region (42) and at least one turn each G...

Description

The present invention relates to a device for generating a plasma by inductive excitation according to the preamble of claim 1 and a method for generating a plasma by inductive excitation according to the preamble of claim 22.

Inductively coupled plasmas have been produced and studied for at least 100 years, as described, for example, in J. Hopwood, "Review of inductively coupled plasma for plasma processing", Plasma Sources Science and Technology, 1 (1992) 109-116. A particular advantage of generating a plasma by inductive coupling is that no electrodes must be in direct contact with the plasma. A device of the type mentioned comprises a container with a gas in which a plasma is to be generated, and an inductance, for. B. a coil which is inductively coupled to the gas. In the case of inductive coupling, the inductance can be regarded as the primary winding of a transformer which generates an alternating magnetic field in the gas. The time-varying magnetic flux induces in the gas an electromotive force which, with sufficient strength, can ignite and maintain a plasma. The discharge in the gas thereby constitutes an electrically conductive fluid, and the charge flow in the plasma can be regarded as a single secondary winding, which effectively forms a transformer with the inductance as the primary winding.

At present inductively coupled plasmas are usually excited in the high frequency spectrum. In this case, commercially available 13.56 MHz excitation sources are used in most cases.

The registration forms EP 0 849 766 A2 and WO 2003/030207 A1 disclose plasma generating devices for use primarily in semiconductor manufacturing in which a plasma is inductively AC-energized in a cylindrical coil-wound container. As excitation frequency calls the EP 0 849 766 A2 the range of 100 kHz to 100 MHz, preferably 300 kHz to 3 MHz, the application letter WO 2003/030207 A1 a spectrum of 10 kHz to 3 GHz, but preferably 400 kHz to 2 MHz.

The patent DE 198 25 056 C1 discloses a circuit arrangement for inductively feeding electrical energy into a plasma in the frequency range from 20 kHz to 100 kHz. The excitation occurs via an electronic control unit with an output stage driver and a subsequent stage consisting of two MOSFETs and one high-voltage IGBT. With suitable pulse patterns, energy is fed into the plasma source in the form of pulsed DC voltage or AC voltage.

The US 5,521,351 A shows a device and a method for treating the interior of a hollow body with plasma. The interior of the body to be treated is first evacuated and then filled with a plasma gas, so that the pressure inside the hollow body is higher than outside. The hollow body is surrounded by an induction coil, which is adapted to the outer shape of the body. In particular, the bodies can be spherical. By applying an AC signal to the induction coil, a plasma is generated inside the body. The excitation frequencies are 450 kHz to 27.1 MHz and in the preferred embodiment is 13.56 MHz.

The further progress in the generation of inductively excited low-pressure discharge plasmas will be dependent above all on the future achievable electron densities. Although the proportionate ionization is higher in conventional inductively coupled plasmas than in capacitively coupled plasmas, it is overall relatively low. At present, only a fraction of 0.01 to 0.1 of the gas atoms in the plasma can be ionized by inductive excitation. For example, if discharge plasmas are to be used for UV component imaging applications, a higher ion to neutral ratio would be of great benefit since the emission spectrum of the excited ions is higher than that of the excited neutral atoms at higher energies. In general, a higher degree of ionization is also advantageous because then arise at a given gas pressure higher electron densities.

The invention has for its object to provide an apparatus and a method for generating a plasma by inductive excitation, which can be achieved with higher electron densities in the plasma.

This object is achieved by a device having the features of claim 1 and a method having the features of claim 22. Advantageous developments are specified in the dependent claims.

According to the invention, a power source is used in the device for generating a plasma, which is suitable in the inductance, an AC signal with an excitation frequency ν of less than 200 kHz, preferably less than 100 kHz, particularly preferably from 1 kHz to 50 kHz and in particular from 10 kHz to 30 kHz. In this case, the term "AC signal" indicates that the excitation current in the inductance does not necessarily have to be a CW signal, but it may also be, for example, a damped oscillation with only a few zero crossings.

The essential difference of the invention over known devices and methods for producing inductively coupled plasmas consists in the comparatively very low excitation frequencies in the kHz range. The invention is based on the finding that higher electron densities than in previous devices and methods can be achieved by lowering the excitation frequency. The relationship between the excitation frequency and the achievable electron density is as follows.

wobei E die elektrische Feldstärke und σ_p die räumlich und zeitlich gemittelte Leitfähigkeit des Plasmas ist, für die gilt

In general, the power of the applied electric field is transmitted within a certain skin depth δ in an inductively coupled plasma discharge, see, for. BJT Gudmundsson and MA Lieberman: "Magnetic induction and plasma impedance in a planar inductive discharge", Plasma Sources Science and Technology, 7 (1998) 83-95. In a shock-dominated plasma, ie, a plasma in which the frequency ν _{c of} the collisions between electrons and neutral gas particles is much larger than the excitation frequency ν, it has been shown that a maximum efficiency of coupling energy at a skin depth of δ = 0.57r _p (1) occurs, where r _{p is} the radius of the plasma, which can be equated to a good approximation with the radius of the discharge vessel. Equation (1) above is described by MA Lieberman and AJ Lichtenberg: "Principles of Plasma Discharge and Materials Processing", Wiley & Sons, New Jersey, 2005 and J. Reece Roth: "Industrial Plasma Engineering Volume 1", IoP (Instit of Physics Publishing), 2003. This means that the skin depth is already essentially determined by the construtive structure. The density of the power absorbed by the plasma ẇ _abs is as follows:

where E is the electric field strength and σ _{p is} the spatially and temporally averaged conductivity of the plasma, for which applies

Substituting equations (1) and (3) into equation (2) gives the following relationship:

It can be seen from equation (4) that the power density absorbed by the plasma is inversely proportional to the excitation frequency ν. This means that higher power densities can be achieved under otherwise identical conditions, such as induced field strength E _eff and plasma radius r _p with low-frequency excited plasmas. Since excitation frequencies in the MHz range are used in conventional inductively supplied plasmas as mentioned above, the absorbed power density should be substantially increased when the excitation frequency is lowered according to the invention, and in particular an excitation frequency below 200 kHz is used. This was also confirmed in first experiments.

The result of equation (4) also allows an estimation of the achievable electron densities. Within the scope of equation (1), the electron density n _e scales linearly with the injected power, as described, for example, by Hopwood et al. was experimentally confirmed, see J. Hopwood et al .: J. Vac. Sci. Technol. All: 152, (1993). For the power dissipated in the plasma then: Ẇ _diss = n _e u _B A _eff W _T , (5) where u _B is Bohm's velocity, A _{eff is} the effective surface area of the discharge vessel and W _{T is} the total energy lost per Liebermann and Lichtenberg charge carrier pair (see above), which is composed of radiation losses and kinetic energy losses that occurs when the charge carriers reach the vessel wall. The "effective surface" A _eff corresponds to spherical containers the geometric surface, however, may be about 10% less than the geometric surface for other vessel shapes, such as cylindrical vessels.

The dissipated power Ẇ _diss according to equation (5) must correspond to the total power absorbed in the plasma due to the energy conservation. The total absorbed power Ẇ _abs corresponds to the volume integral over the power density of equation (4), which can be qualitatively approximated by multiplying the power density of equation (4) by the volume V _{p of} the plasma, yielding

By equating equations (5) and (6) (conserving energy) we obtain the following approximated expression for the electron density:

As can be seen from equation (7), the electron density n _{e is} indeed inversely proportional to the excitation frequency ν, which in turn means that higher electron densities n _e can be obtained at lower excitation frequencies. Therefore, the invention achieves the above object by using lower excitation frequencies than in the prior art.

Furthermore, it can be seen that the electron density n _{e is} proportional to the ratio between the volume V _p and the effective surface A _eff . This means, firstly, that higher electron densities can be achieved with larger containers. Second, this means that a spherical, ie spherical, vessel geometry in which the ratio of volume to surface is maximal, is also advantageous for achieving a high electron density n _e .

Since excitation frequencies ν are to be used in the context of the present invention, which are about three orders of magnitude lower than those used in conventional inductively coupled plasmas, completely new electronics are required for driving the inductance. One difficulty is that a certain induced electric field strength is necessary to ignite and maintain the plasma. In other words, the effective field strength E _eff from the above equation (4) must exceed a minimum value in order for ignition and maintenance of the plasma to occur at all. The induced field strength is in turn dependent on the magnitude of the time derivative of the magnetic flux in the plasma vessel, which in turn is proportional to the time derivative of the current in the inductor. It can therefore be assumed that certain current rise rates in the inductance must not be exceeded, so that it comes to the ignition and maintenance of a plasma. The lower the excitation frequency, the higher the current amplitude in the inductance must be selected in order to obtain a sufficient current increase rate. In order to be able to use the low excitation frequencies according to the invention, therefore, a power source must be provided which is suitable for generating a very high amplitude alternating current signal in the inductance.

In an advantageous embodiment of the invention, the power source comprises at least one capacitor which can be charged to an operating voltage, and at least one switching element, which is switchable to a conductive state and is connected so that the at least one capacitor in the conductive state of the switching element through the Can discharge inductance. As will be explained in more detail below on the basis of exemplary embodiments, sufficiently high current intensities can be generated with such a design using modern power switching elements, which lead to an ignition of the plasma even at the very low excitation frequencies of the invention.

The at least one capacitor and the inductance preferably form components of an undamped electrical resonant circuit whose natural frequency corresponds to said excitation frequency v. After this development, therefore, said AC signal is formed in an electrical resonant circuit, which contains the capacitor and the inductor. The inductance L ₀ and the capacitance C _{0 of} the capacitor can then be tuned so that the resonant circuit oscillates at the desired excitation frequency. In the oscillation of the resonant circuit is a damped oscillation, on the one hand due to the ohmic resistance of the inductance, but especially because of the inductive coupling with the plasma that is desired to transfer energy to the plasma. By these two damping sources results on the one hand compared to the undamped resonant circuit reduced natural frequency and on the other hand, the known decay of the damped oscillation. Such a decaying damped oscillation signal is an example of the above-mentioned "AC signal".

wobei ν wiederum die Anregungsfrequenz des Wechselstromsignals, I₀ die maximale Amplitude des Wechselstromsignals, L₀ die Induktivität und b den Umfang des Behälters in einer Ebene senkrecht zum von der Induktivität erzeugten Magnetfeld bezeichnet. Theoretische und experimentelle Untersuchungen des Erfinders haben ergeben, daß für diese Wahl der Parameter in der Tat eine Zündung und Aufrechterhaltung des Plasmas zu erwarten ist. Man beachte wiederum, daß gemäß der obigen Beziehung ein kleinerer Wert der Anregungsfrequenz ν eine um denselben Faktor höhere Stromstärke I₀ erforderlich macht. Dies deutet wiederum darauf hin, daß bei den angestrebten sehr kleinen Anregungsfrequenzen hohe Ströme in der Induktivität benötigt werden. Der Erfinder konnte jedoch bestätigen, daß sich ausreichend hohe Ströme mit modernen Leistungsschaltelementen schalten lassen, wie unten anhand mehrerer Ausführungsbeispiele näher erläutert wird.As mentioned above, the rate of current rise in the inductor must be sufficiently large to allow ignition and maintenance of the plasma. Preferably, the power source is constructed so that the following relationship applies:

where ν again denotes the excitation frequency of the alternating current signal, I ₀ the maximum amplitude of the alternating current signal, L ₀ the inductance and b the circumference of the container in a plane perpendicular to the magnetic field generated by the inductance. Theoretical and experimental investigations of the inventor have shown that ignition and maintenance of the plasma are indeed to be expected for this choice of parameters. Note again that, according to the above relationship, a smaller value of the excitation frequency ν requires a current factor I ₀ which is higher by the same factor. This in turn indicates that high currents are required in the inductance at the desired very small excitation frequencies. However, the inventor was able to confirm that sufficiently high currents can be switched with modern power switching elements, as will be explained in more detail below with reference to several exemplary embodiments.

Preferably, the switching element comprises at least one thyristor, at least one IGBT (insulated gate bipolar transistor, insulated gate bipolar transistor) or at least one gas discharge switch, such as an ignitron. The advantage of thyristors is that they can switch very high currents. IGBTs typically can not switch quite as high currents as thyristors, but have the advantage that they can be selectively switched off. The thyristor, however, is only offset by falling below a holding current in the blocking state.

In an advantageous embodiment, the switching element is arranged between the capacitor and the ground potential. In such an arrangement, the inductance is at ground potential prior to activation of the switching element, which brings advantages in terms of reliability with it. In an alternative advantageous embodiment, the switching element is located between the inductance and the ground potential. This arrangement is advantageous in so far as it contributes to the avoidance of parasitic inductances between the switching element and the resonant circuit.

In a particularly advantageous development, the switching element is formed by a full-wave bridge, which comprises four switches, in particular thyristors and / or IGBTs, which can be controlled in pairs alternately. Such a full-wave bridge is particularly suitable for "quasi-CW operation", especially in very high power applications, such as those required in the sterilization of large quantities of drinking water by plasma discharge irradiation. An example of such a full-wave bridge will be shown in the following description.

Preferably, the at least one capacitor or a plurality of capacitors connected in parallel have a total capacitance of 1 μF to 100 μF, and more preferably from 6 μF to 20 μF. Furthermore, in an advantageous development, the power source is designed to generate maximum current rise rates of 300 A / μs to 30 kλ / μs, preferably from 1 kM / μs to 10 kM / μs. Further, the inductance L _{0 is} preferably between 1 μH to 10 μH, and more preferably 1.5 μH to 2.5 μH.

As will be apparent from the above description, the power source must be designed to produce relatively high currents at relatively high current rise rates. However, the parameter ranges of the preferred embodiments show that this is entirely possible with modern power electronics components, and further concrete examples are shown in the following embodiments. At the same time, however, the route described here, in addition to the potentially higher achievable electron density, also has other practical advantages over conventional inductive plasma excitations in the MHz range. On the one hand, the power source described in the embodiments is much simpler in construction than HF sources, and the known EMC problems of high-frequency technology are almost completely eliminated. Also, the capacitive coupling between the inductor and the plasma is negligible compared to RF excitation.

Another advantage of the plasma excitation according to the invention is that the plasma is already trapped or localized due to the high currents used. This inclusion of the plasma is often called "confinement" in the literature. In RF plasmas, additional coils are needed instead to locate the plasma, which can be eliminated in the invention. Thus, although the original motivation of the invention was to produce higher electron densities, the invention also has a number of constructive advantages that are lacking in RF excitation.

The inductance is preferably formed by a coil surrounding the container. The coil preferably has a number of turns n ₀ from two to eight, and particularly preferably from four to six. Higher numbers of turns, which would bring about a higher inductance L ₀ , are disadvantageous because they would require an increase in the charging voltage of the capacitor, which in turn is undesirable from the point of view of simplicity and practicality. Namely, as mentioned above, the interaction between the inductance and the plasma resembles that between a primary coil and a secondary coil of a transformer, the "secondary coil" being formed by the charge current in the plasma and thus, to remain in the transformer image, only a "turn " having. Since the voltage applied to the inductance is stepped down according to the turns ratio, turns of less than eight are advantageous in terms of the voltage induced in the plasma, because then the primary voltage can remain in a practically manageable range, for example below 10 kV.

Preferably, the container, i. H. the discharge vessel spherical or approximately spherical. A spherical, d. H. Spherical container has the advantage that it has a large volume / surface ratio, which in turn allows higher electron density according to equation (7) above. As such, a spherical container is ideal for the purposes of the invention. An "approximately spherical" container in the present specification is a container whose shape is similar to that of a spherical container, at least insofar as it has a volume to surface ratio that differs by less than 20% from that of an exactly spherical container of equal volume.

A spherical and an "approximately spherical" container in the sense of the disclosure also agree insofar as the container comprises an equatorial region in which the cross-sectional area in a plane perpendicular to the induced magnetic field from a maximum value A _max to a value of A _max / 2 decreases, and has two Polbereiche in which the cross-sectional area of A _max / 2 drops to zero. As such, for example, a cylindrical container is not "approximately spherical" because it does not include an equatorial region and pole regions as defined herein.

At least one of the turns of the inductance preferably surrounds the container at least partially in its equatorial region, and at least one turn in each case surrounds the container at least partially in its pole regions. The term "at least partially" indicates that the winding can also lie on the boundary between the equatorial region and a pole region, which may well be due to the relatively large width of the conductor strips of the inductance. According to this development, there is thus at least one winding in the equatorial region, which has the effect of a conventional cylindrical coil, and at least one turn in each of the pole regions, which are similar in effect to a planar coil, which in the literature as "pancake coil" referred to as.

Planar coil have the advantage that they reduce leakage flux and thereby allow increased coupling efficiency. In the advantageous development described here, the coil arrangement can be understood as a combination of a conventional cylindrical coil and a conventional planar coil, which combines the advantages of both arrangements. In fact, the structure described here results in that a sufficient part of the leakage fluxes is contained in the plasma, whereby an increased magnetic coupling and thus an increased efficiency of the energy transfer between the inductance and the plasma is achieved. At the same time, despite the use of a planar coil-like geometry, the capacitive coupling between the coil and the plasma is negligible compared to RF operation.

Preferably, the device comprises a controller which is suitable for periodically activating the at least one switching element at a pulse frequency in such a way that it switches to its conductive state. This achieves pulsed operation of the device. The pulse frequency is at least 1 Hz, preferably at least 10 Hz and particularly preferably at least 50 Hz. If the alternating current signal drops, for example, within 150 to 200 μs, at such pulse frequencies, the excitation pulses are still relatively distant in time relative to their length, or in other words the duty cycle is relatively low. Adjusting the duty cycle can improve performance the institution can be specified. The power can be increased by increasing the pulse rate until, in extreme cases, a "quasicontinuous", ie a "quasi-CW" operation is achieved. A true CW operation is certainly out of the question for most applications because current strengths in the range of kA and voltages in the range of kV imply peak pulse power in the MW range. Only in very large discharge chambers, whose order of magnitude would be in the range of 1 m to 10 m, could a CW operation come into question.

The inductively coupled plasma generating device described herein may be used to advantage in a variety of applications. A particularly advantageous application of the invention consists in a device for sterilizing drinking water, in which the water to be sterilized is passed or passed through in the immediate vicinity of a plasma discharge source, wherein the plasma discharge source comprises a device for plasma generation according to one of the types described above. The plasma generation device according to the invention is particularly advantageous for the application of drinking water treatment, because it enables high power and high electron densities and at the same time is technologically simpler due to the operation in the kHz instead of in the MHz range and in particular circumvents the known EMC problems of high-frequency technology. In this regard, the device according to the invention is ideally suited for drinking water treatment.

Another very advantageous application of the invention is in the lithography of semiconductor structures. Due to the higher achievable electron density n _e , the emission spectrum shifts in the direction of shorter wavelengths, so that the invention is particularly suitable for the generation of short-wave UV light, so-called V-UV radiation, which is useful in semiconductor lithography. The device is also generally used as a light source for short-wave UV radiation.

Due to the high achievable electron density, the invention is also suitable for the production of industrial process plasma, as z. B. in the semiconductor industry in ion etching processes (so-called sputtering) can be used.

Due to the high plasma densities and temperatures, the device according to the invention is also suitable as a plasma engine. In this case, the enthalpy of a working gas is increased by electromagnetic induction electron-free and then made usable by gas expansion via a nozzle to generate a recoil.

For a better understanding of the present invention, reference will be made below to the preferred embodiments illustrated in the drawings, which are described in terms of specific terminology. It should be understood, however, that the scope of the invention should not be so limited since such changes and other modifications to the illustrated apparatus and method, as well as such other uses of the invention as set forth herein, are to be considered as current or future knowledge a competent person. The figures show embodiments of the invention namely:

1 a sectional side view of a plasma generating device;

2 a front view of a plasma generating device;

3 a rear view of a plasma generating device;

4 a rear view of an alternative plasma generating device, wherein the switching element is formed by thyristors;

5 a cross-sectional view of the plasma container and the surrounding coil;

6 an equivalent circuit diagram of the plasma generating device of 1 to 3 ;

7 an equivalent circuit diagram of the plasma generating device of 4 ;

8th an equivalent circuit diagram of an alternative embodiment of a plasma generating device;

9 an equivalent circuit diagram of an alternative embodiment of a plasma generating device using a full-wave bridge circuit with thyristors;

10 an equivalent circuit diagram of an alternative embodiment of a plasma generating device using an IGBT as a switching element; and

11 an equivalent circuit diagram of an alternative embodiment of a plasma generating device that uses a full-wave bridge circuit with four IGBTs.

1 shows a schematic sectional view of a first embodiment 10 a device for generating a plasma by inductive excitation according to a development of the invention. 2 shows a front view of the device 10 from 1 and 3 a rear view.

The device 10 from 1 to 3 comprises a spherical container 12 in which a noble gas, for example argon, is at a low pressure of, for example, 12 Pa. The container 12 , which is also referred to as a discharge vessel, has at its upper and its lower end a so-called CF-35 flange 14 , Further, the container 12 with a gas supply 16 connected, through which the noble gas into the container 12 is insertable.

The container 12 is of four turns 18a to 18d an induction coil 18 surrounded by a coil holder 20 made of an electrically insulating material (eg DURATEC). The turns 18a to 18d are interconnected by electrically conductive fasteners 22 connected in 1 in detail and in 3 are shown schematically. The four turns or segments 18a to 18d together form a coil with a total inductance of 2.1 μH.

As in particular in 3 It can be seen on the back of the device 10 two capacitors 24 provided, which are connected in parallel and together form a so-called capacitor bank. The capacitors 24 each have a first connection 26 that has a power supply 28 is connected to a power supply (not shown). The capacitors 24 each have a capacity of 6 μF, so together a total capacity of C ₀ = 12 μF. About the power supply 28 become the capacitors 24 charged with a precharge voltage of 4100V.

The capacitors 24 also each have a second connection 30 , both with a first end 32 the coil 18 ie in the representation of 1 to 3 upper end of the coil 18 , are connected. A second, in the presentation of 1 to 3 lower end 34 the coil 18 is with a switching element 36 connected, in which it is in the representation of 1 to 3 is a gas discharge switch, a so-called Ignitron.

In 5 is an equivalent circuit diagram of the first embodiment of 1 to 3 shown, wherein the reference numerals to the components of 1 to 3 Respectively. In the diagram of 6 is the coil 18 represented by a series connection of an inductance L ₀ of 2.1 μH and a resistance R ₀ of 8 mΩ.

In the following, the function of the plasma generating device will be described 10 from 1 to 3 and 5 explained. At a certain time t = 0 is the capacitor 24 charged with capacity Co with the charging voltage of 4100V. Furthermore, at the time t = 0, a control signal at the control input 38 of the Ignitron 36 (please refer 5 ), whereupon the Ignitron 36 is switched to a conductive state. The capacitor 24 discharges through the coil 18 through with a maximum current I ₀ of 9.6 kA and with a current increase rate of about 1 kA / μs (in the embodiment shown, specifically 1.8 kA / μs). Due to the rapid increase in current is in the gas inside the container 12 generates a temporally strong magnetic flux, which in turn induces an electric field, which is sufficient to ignite a plasma.

Since the plasma discharge can be considered as an electrically conductive fluid coming from the coil 18 is surrounded, it effectively forms a single secondary winding of an imaginary transformer with an inductance L _p and a resistive plasma resistance R _p . The capacitor 24 with capacity Co and the coil 18 with the resistor R ₀ and inductance L ₀ form a damped electrical series resonant circuit, so that the voltage across the capacitor 24 with a frequency ν oscillates and the current at the same frequency in the coil 18 runs back and forth. The frequency ν of the resonant circuit results from the parameters C ₀ , R ₀ and L ₀ , with the attenuation by the coupled plasma being added as a further and much more substantial damping component. In the embodiment described here results in a frequency of ν = 29 kHz, which is also the excitation frequency of the plasma. The oscillation of the resonant circuit lasts for around 150 to 200 μs, during which the plasma is ignited and maintained, then it goes out.

As a result of the construction described, a plasma can be produced by inductive coupling with an excitation frequency which is only about three orders of magnitude below the usual excitation frequencies at only 29 kHz. As mentioned at the beginning, a higher electron density in the plasma can be generated at such low excitation frequencies. First spectroscopic investigations of the plasma indicate indeed a significantly increased electron density.

After the plasma has gone out, the capacitor becomes 14 recharged until the Ignitron 36 is switched to the conductive state by another control signal for a re-discharge of the capacitor. This results in a pulsed operation with a discharge phase of about 150 to 200 microseconds and a dark phase until the next control signal. The control signal is output by a controller (not shown) at a periodic pulse rate, which in turn dictates the duty cycle of the device. The pulse frequency can be 50 Hz, for example. Since the power is during the pulse peaks in the MW range, the period of the pulse rate will generally be well below the duration of the discharge phase, however, in very large and powerful plants, such as those for drinking water treatment, the pulse rate can be so high that a quasi-continuous operation results.

4 shows a modified embodiment and 7 the associated circuit diagram. The modification consists only in that instead of the Ignitrons 36 a thyristor pair 40 is provided, which is also referred to as a "Press Pack".

In 5 is the container 12 with the surrounding induction coil 18 as used in the embodiments of 1 to 4 used, shown separately. The container 12 has a diameter of 20.3 cm and a spherical shape. The turns 18a to 18d the induction coil 18 are made of copper and have a rectangular cross-section of 2 mm × 20 mm.

The container 12 from 5 settles in an equatorial area 42 and two pole areas 44 divide. The equatorial area 42 is here defined as the area that is an equatorial plane 46 contains, ie a plane in which the cross-sectional area in a plane perpendicular to the direction of the induced magnetic field B → assumes its maximum value. Within the equatorial area 42 takes the cross-sectional area of the container 12 upwards and downwards continuously until the boundary to the respective adjacent pole area 44 is reached. The boundary to the pole region is defined in the present specification as the latitude at which the cross-sectional area has fallen to half, ie it is at a latitude of 45 ° or 135 °°.

Essential for the particularly good coupling between the induction coil 18 and the plasma in the embodiment shown here is that two coil segments 18b and 18c in the equatorial area 42 lie and two coil segments 18a . 18d at least partially in each pole area 44 lie. Although all turns 18a to 18d or segments contribute to the generation of the magnetic field, there are qualitative differences in their effect. The middle coil segments 18b and 18c act like a conventional cylindrical coil. The outer coil segments 18a and 18d at least partially in the pole areas 44 lie, have an effect similar to that of known flat coils, so-called "pancake coils". The advantage of such pancake coils is that the flux leakage is generally lower, but it is commonly believed that their increased capacitive coupling with the plasma compared to a solenoid is problematic. At the low excitation frequencies used here, however, the capacitive coupling plays a relatively minor role. With the in 5 The construction shown, the advantages of both coil arrangements, ie a conventional cylindrical coil on the one hand and a flat coil on the other hand agree. In fact, the leads in 5 geometry shown in the experiments carried out to a coupling efficiency of 0.8, that is, 80% of the energy of the capacitor is transferred to the plasma, and it can be achieved with a slightly modified structure even coupling efficiencies of 0.9.

Another reason for the very high coupling efficiency is that the coil segments 18a to 18d very close to the container 12 abut, with a maximum gap of 3 mm. Another advantage of the spherical geometry of the container 12 is because it has a maximum volume / surface ratio, which allows for increased electron density according to equation (7) above.

The embodiment specifically described here above all serves to demonstrate the principles of the invention, and many modifications are possible. Particularly promising are applications with larger containers and capacitors of higher capacities. In the following, some modifications regarding the power source and the circuit of the components will be discussed.

8th shows an alternative equivalent circuit diagram, which corresponds to that of 7 is basically similar. The main difference is that the thyristor 40 not here between the coil 18 and the ground potential, but is disposed between the capacitor and the ground potential. Note that in the illustration of 8th the thyristor with passage in the opposite direction has been omitted for the sake of simplicity. This arrangement has the advantage that the coil is at ground potential during the charging of the capacitor, and not, as in the arrangement of 7 , on high voltage. The arrangement of 7 However, has the advantage that the parasitic inductance between the switching element 14 and the resonant circuit is lower.

9 shows a further equivalent circuit diagram in which the switching element is formed by a full-wave bridge of four thyristors TH1 to TH4. In this bridge circuit, the thyristors TH1 and TH3 or TH2 and TH4 are alternately driven, so that the series resonant circuit is driven alternately with the charging voltage from a current source C _L. This thyristor full-wave bridge is particularly well suited for "quasi-CW operation".

The equivalent circuit diagrams of 10 and 11 correspond to those of 8th and 9 except that the thyristors are replaced by IGBTs.

Although preferred embodiments have been shown and described in detail in the drawings and the foregoing specification, this should be considered as illustrative and not restrictive of the invention. It should be understood that only the preferred embodiments are shown and described and all changes and modifications that are presently and in the future within the scope of the invention should be protected.

LIST OF REFERENCE NUMBERS

10

Device for plasma generation

12

container

14

flange

16

gas supply

18

induction coil

20

coil holder

22

Connecting elements between coil segments

24

capacitor

26

first connection of the capacitor 24

28

power

30

second connection of the capacitor 24

32

first end of the coil 18

34

second end of the coil 18

36

ignitron

38

Control input of the Ignitrons

40

thyristor

42

equatorial region

44

pole areas

46

equatorial plane

Claims (26)

Facility ( 10 ) for generating a plasma by inductive excitation, comprising a container ( 12 ) with a gas in which a plasma is to be generated, and an inductance ( 18 ), which is inductively coupled to the gas, and a power source which is suitable in the inductance ( 18 ) to generate an AC signal with an excitation frequency ν of less than 200 kHz, wherein the container ( 12 ) is spherical or approximately spherical and the inductance ( 18 ) is formed by a coil, the container ( 12 ), further comprising the container ( 12 ) an equatorial area ( 42 ) in which its cross-sectional area in a plane perpendicular to the induced magnetic field decreases from a maximum value A _max to a value of A _max / 2, and two pole regions ( 44 ) in which the cross-sectional area of A _max / 2 drops to zero, wherein at least one turn ( 18b . 18c ) of the coil Container ( 12 ) at least partially in its equatorial region ( 42 ) and at least one turn ( 18a . 18d ) of the bobbin the container ( 12 ) at least partially in its pole regions ( 44 ) and has a cross-sectional area that is less than A _max . Facility ( 10 ) according to claim 1, wherein the power source is suitable in the inductance ( 18 ) to generate an AC signal having an excitation frequency ν of less than 100 kHz, preferably from 1 kHz to 50 kHz and in particular from 10 kHz to 30 kHz. Facility ( 10 ) according to claim 1 or 2, wherein the power source comprises at least one capacitor ( 24 ), which is rechargeable to an operating voltage, and at least one switching element ( 36 . 40 ) which is switchable to a conductive state and is connected so that the at least one capacitor ( 24 ) in the conductive state of the switching element ( 36 . 40 ) by the inductance ( 18 ) can discharge through. Facility ( 10 ) according to claim 3, wherein the at least one capacitor ( 24 ) and the inductance ( 18 ) Form components of an undamped electrical resonant circuit whose natural frequency corresponds to said excitation frequency (ν). Facility ( 10 ) according to one of the preceding claims, wherein - the amplitude I _{0 of} the alternating current signal, - the excitation frequency ν of the alternating current signal, - the inductance L _{0 of} the inductance ( 18 ) and - a circumference b of the container ( 12 ) in a plane perpendicular to the inductance ( 18 ), the following relationship applies:

Facility ( 10 ) according to one of the preceding claims, in which the switching element comprises at least one thyristor ( 40 ), at least one IGBT or at least one gas discharge switch ( 36 ). Facility ( 10 ) according to one of claims 3 to 6, in which the switching element ( 36 . 40 ) between the capacitor ( 24 ) and the ground potential is arranged. Facility ( 10 ) according to one of claims 3 to 6, in which the switching element ( 36 . 40 ) between the inductance ( 18 ) and the ground potential is arranged. Facility ( 10 ) according to one of the preceding claims, in which the switching element ( 36 . 40 ) is formed by a full-wave bridge, which includes four switches, which are controlled in pairs alternately. Facility ( 10 ) according to claim 9, wherein the switches are formed by thyristors and / or IGBTs. Facility ( 10 ) according to one of claims 3 to 10, in which the at least one capacitor ( 24 ) or a plurality of parallel connected capacitors ( 24 ) has or have a total capacity of 1 μF to 100 μF, in particular of 6 μF to 20 μF. Facility ( 10 ) according to one of the preceding claims, in which the alternating current signal has maximum current rise rates of 300 A / μs to 30 kA / μs, in particular from 1 kA / μs to 10 kA / μs. Facility ( 10 ) according to one of the preceding claims, in which the inductance L _{0 of} the inductance ( 18 ) Is 1 μH to 10 μH, in particular 1.5 μH to 2.5 μH. Facility ( 10 ) according to one of the preceding claims, wherein the coil has a number of turns of two to eight, in particular from four to six. Device for sterilizing drinking water with a plasma discharge source, comprising a device ( 10 ) for generating plasma according to any one of the preceding claims and is passed in the water to be sterilized in the immediate vicinity of the plasma discharge source or passed through them. Device for lithography of semiconductor structures with a device for generating a plasma according to one of the preceding claims. Use of a device ( 10 ) for producing a plasma according to any one of the preceding claims for the sterilization of drinking water or for exposure in semiconductor lithography. Method for producing a plasma by inductive excitation, in which in an inductance ( 18 ) which is inductively coupled to a gas in which a plasma is to be generated, an AC signal having an excitation frequency v is generated which is below 200 kHz, wherein the inductance ( 18 ) is formed by a coil having a spherical or approximately spherical container ( 12 ), the container ( 12 ) an equatorial area ( 42 ) in which its cross-sectional area in a plane perpendicular to the induced magnetic field decreases from a maximum value A _max to a value of A _max / 2, and two pole regions ( 44 ) in which the cross-sectional area of A _max / 2 drops to zero, wherein at least one turn ( 18b . 18c ) of the bobbin the container ( 12 ) at least partially in its equatorial region ( 42 ) and at least one turn ( 18a . 18d ) of the bobbin the container ( 12 ) at least partially in its pole regions ( 44 ) and has a cross-sectional area that is less than A _max . Method according to Claim 18, in which the excitation frequency is below 100 kHz, preferably between 1 kHz and 50 kHz and in particular between 10 kHz and 30 kHz. The method of claim 19, wherein the AC signal is generated by applying a capacitor ( 24 ) is charged to an operating voltage and at least one switching element ( 36 . 40 ) is switched into a conductive state, so that the capacitor ( 24 ) discharges through the inductance. The method of claim 20, wherein the at least one capacitor ( 24 ) and the inductance ( 18 ) Form components of an undamped electrical resonant circuit whose natural frequency corresponds to said excitation frequency ν. Method according to one of Claims 19 to 21, in which - the amplitude I _{0 of} the alternating current signal, - the excitation frequency ν of the alternating current signal, - the inductance L _{0 of} the inductance ( 18 ) and - a circumference b of the container ( 12 ) in a plane perpendicular to the inductance ( 18 ), the following relationship applies:

Method according to one of Claims 19 to 22, in which the switching element comprises at least one thyristor ( 40 ), at least one IGBT or at least one gas discharge switch ( 36 ). Method according to one of claims 19 to 23, wherein the coil ( 18 ) has a number of turns of two to eight, preferably from four to six. Method according to one of claims 19 to 24, wherein the at least one switching element ( 36 . 40 ) is periodically driven at a pulse rate to switch to its conductive state. Method according to Claim 25, in which the pulse frequency is at least 1 Hz, preferably at least 10 Hz and particularly preferably at least 50 Hz.

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