WO2009024347A1

WO2009024347A1 - Device and method for generating plasma by low-frequency inductive excitation

Info

Publication number: WO2009024347A1
Application number: PCT/EP2008/006894
Authority: WO
Inventors: Christian Teske; Joachim Jacoby
Original assignee: Johann Wolfgang Goethe-Universität
Priority date: 2007-08-22
Filing date: 2008-08-21
Publication date: 2009-02-26
Also published as: DE102007039758A1; DE102007039758B4

Abstract

The invention relates to a device and a method for generating plasma by inductive excitation. The device comprises a container (12) with a gas, wherein plasma is to be generated, and an induction coil (18), that is inductively coupled to the gas. Furthermore, a power source is provided, being able to generate an alternating current signal in the inductivity (18) with an excitation frequency v of below 200 kHz, preferably under 100 kHz, particularly preferably of 1 kHz to 50 kHz and especially of 10 kHz to 30 kHz.

Description

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

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 10 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 proccessing", Plasma Sources Science and Technology, I (1992) 109-116 advantage of generating a plasma by inductive coupling is that no i ₅ electrodes must be in direct contact with the plasma. a device of the type mentioned above comprises a container with a gas, in which a plasma is to be generated, and an inductance, For example, a coil that is inductively coupled to the gas In inductive coupling, the inductance can be considered as the primary winding of a transformer that generates a magnetic alternating field in the gas

Magnetic flux induces an electromotive force in the gas 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.

Currently, 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 further advance in the generation of inductively excited low-pressure discharge plasmas will be above all the future achievable electron densities be dependent. 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. If, for example, discharge plasmas are to be used for illumination applications with one component in the UV spectrum, a higher ratio of ions to neutral atoms would be of great advantage, since the emission spectrum of the excited ions is higher at higher energies compared to that of the excited neutral atoms , In general, a higher degree of ionization is also advantageous because then arise at a given gas pressure higher electron densities.

The object I ₀ underlying the invention is to provide an apparatus and method for generating a plasma by inductive stimulus, which can achieve 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 given in the i ₅ 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 alternating current signal with an excitation frequency v of less than 200 kHz, preferably less than 100 kHz, more preferably from 1 kHz to 50 kHz and in particular from 10 kHz to 30 kHz. The 0 term "alternating current 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 is 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. In general, in an inductively coupled plasma discharge, the power of the applied electric field is transmitted within a certain skin depth δ, see, for example, JT Gudmundsson and MA Lieberman: "Magnetic induction and plasma impedance in a planar inductive discharge", Plasma Sources Science and Technology, 7 (FIG. 1998) 83-95. In a collision-dominated plasma, ie in a plasma in which the frequency v _{c of} the collisions between electrons and neutral gas particles is much larger than the excitation frequency v, 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 w _abs absorbed by the plasma 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

σ, = - -J-. (3) μ _Q vδ ²

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

6,16 ₂

^W abs * ... -.2 ^e ff ^' (4)

/ W ₁ P From equation (4) it can be seen that the power density absorbed by the plasma is inversely proportional to the excitation frequency v. This means that under otherwise identical conditions, such as induced field strength E _ej f and plasma radius r _{p can be achieved} with low-frequency excited plasmas higher power densities. 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 if the excitation frequency is lowered according to the invention, and in particular an excitation frequency of less than 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:

W _diss = n _e u _B A _eff W _τ , (5)

where Ü _{B is} the Böhm speed,

effective surface of the discharge vessel and W _{T is} the total energy loss per carrier pair produced by Liebermann and Lichtenberg (see above), which is composed of radiation losses and losses of kinetic energy, which occurs when the charge carriers reach the vessel wall. The "effective surface" A _e ff corresponds to the geometric surface in spherical containers, but can be about 10% less than the geometric surface in other vessel shapes, such as cylindrical vessels.

The dissipated power W _diss according to equation (5) must correspond to the total power absorbed in the plasma due to the energy conservation. The total absorbed power W _abs corresponds to the volume integral over the power density of equation (4), which can be approximated in a qualitative view by multiplying the power density of equation (4) by the volume V _{p of} the plasma, yielding: 6.16

W _abs «E] _ff

/ W ₁ _* v _P - (6)

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

₅ As shown in equation (7) can be seen, the electron density n _e in fact inversely proportional to the excitation frequency v, which in turn means that higher electron density 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 I ₀ volume V _p and the effective surface A _e ff. This means, firstly, that higher electron densities can be achieved with larger containers. Secondly, this means that a spherical, ie spherical, vessel geometry in which the ratio of volume to surface area is maximum, is also advantageous for achieving a high electron density n _e .

Since excitation frequencies v, which are about three orders of magnitude lower than those used in conventional inductively coupled plasmas, are to be used within the scope of the present invention, 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. With others

2Q words, the effective field strength E _ej f from the above equation (4) must exceed a minimum value, so that it comes to the ignition and maintenance of the plasma 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 increase rates in the inductance must not be fallen short of, so that it comes to the ignition and maintenance of a plasma. The lower the excitation frequency, the higher it must be the current amplitude in the inductance can be chosen to obtain a sufficient current slew 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 can discharge the inductance. As explained below in more detail with reference to embodiments, Las I ₀ sen generate sufficiently high currents, which lead, even at very low excitation frequencies of the invention to an ignition of the plasma with such a construction using modern power switching elements.

The at least one capacitor and the inductance preferably form components of an undamped electrical resonant circuit, the natural frequency of which corresponds to the mentioned i ₅ 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 Lo and the capacitance Co of the capacitor can then be tuned so that the resonant circuit oscillates at the desired excitation frequency. The oscillation of the resonant circuit is a damped oscillation,

20 on the one hand due to the ohmic resistance of the inductance, but above all because of the inductive coupling with the plasma, which is indeed 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 attenuated oscillation signal 5 is an example of the above-mentioned "AC signal".

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: ^^ - ≥ lkV / m,

where v denotes again the excitation frequency of the AC signal, Io the maximum amplitude of the AC signal, Lo 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 v necessitates a higher current magnitude Io 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, for example, 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 embodiment, the switching element is formed by a full-wave bridge, which comprises four switches, in particular thyristors and / or IGBTs, the in pairs can be controlled alternately. Such a full-path bridge is particularly suitable for "quasi-CW operation", particularly in very high power applications, such as those required in the sterilization of large quantities of drinking water by plasma discharge irradiation Description shown.

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 kA / μs, preferably from 1 μA / μs to 10 kA / μs. Further, the inductance Lo 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

I ₅ electronic components is quite possible, 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 exemplary embodiments is much simpler in its 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 the 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 plasma entrapment is often referred to in the literature as "confinement." RF plasmas, instead, require additional coils to locate the plasma that can be eliminated in the invention, and thus, although the original motivation of the invention was higher electron densities independently, 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 no from two to eight, and particularly preferably from four to six. Higher number 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 practicability. 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. the discharge vessel spherical or approximately spherical. A spherical, i. 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 decreases from a maximum value A _max to a value of A _max l2 having two pole pieces and in which the - l2 drops cross-sectional area of a _max to zero this respect, a cylindrical container, for example, not "approximately spherical", because it does not include the equatorial region and no pole portions according to this definition.. 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 the case due to the relatively large width of the inductance conductor strips a winding in the equatorial region which has the effect of a conventional cylindrical coil, and at least one winding in each of the pole regions, which are similar in effect to a planar coil, which is referred to in the literature as "pancake coil".

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 within 150 to 200 .mu.s, for example, at such pulse frequencies the excitation pulses are still relatively long in time relative to their length - removed, or in other words, the duty ratio is relatively low. By adjusting the duty cycle, the performance of the device 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 separates for most applications certainly because currents 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, which would range in size from 1 m to 10 m, could a CW operation be considered.

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 in the immediate vicinity of a plasma discharge source is passed or passed therethrough, 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 use of drinking water treatment, because it enables high performance 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 the known EMC problems of high-frequency technology bypasses. 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 electron density that can be achieved, the invention is also suitable for the production of industrial process plasmas, e.g. used in the semiconductor industry in Ionenätzpro- processes (so-called sputtering).

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 by way 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 heretofore indicated, will be deemed to be conventional or may be made future expertise of a person skilled in the art. The figures show embodiments of the invention namely:

Fig. 1 is a sectional side view of a plasma generating device;

Fig. 2 is a front view of a plasma generating device;

Fig. 3 is a rear view of a plasma generating device;

Fig. 4 is a rear view of an alternative plasma generating device in which the switching element is formed by thyristors;

Fig. 5 is a cross-sectional view of the plasma container and the coil surrounding it;

Fig. 6 is an equivalent circuit diagram of the plasma generating device of Figs. 1 to 3;

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

Fig. 8 is an equivalent circuit diagram of an alternative embodiment of a plasma generating device;

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

10 is an equivalent circuit diagram of an alternative embodiment of a plasma generating device using an IGBT as a switching element; and 11 is an equivalent circuit diagram of an alternative embodiment of a plasma generator employing a full-lane circuit with four IGBTs.

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

The device 10 of FIGS. 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, at its upper and its lower end each have a so-called CF-35 flange 14. Further, the container 12 is connected to a gas supply 16, through which the inert gas is introduced into the container 12 ,

The container 12 is surrounded by four turns 18a to 18d of an induction coil 18 consisting of a coil support 20 made of an electrically insulating material (e.g., DURATEC). The windings 18a to 18d are interconnected by electrically conductive connecting elements 22, which are shown schematically in FIG. 1 and in FIG. The four turns or segments 18a to 18d together form a coil with a total inductance of 2.1 μH.

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

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

FIG. 5 shows an equivalent circuit diagram of the first embodiment of FIGS. 1 to 3, wherein the reference numbers refer to the components of FIGS. 1 to 3. In the circuit diagram 5 of FIG. 6, the coil 18 is represented by a series connection of an inductance Lo of 2.1 μH and a resistance Ro of 8 mΩ.

In the following, the function of the plasma generating device 10 of Figs. 1 to 3 and 5 will be explained. At a certain time ^ = 0, the capacitor 24 is charged with capacitance Co at the charging voltage of 4100V. Further, at time t - 0, a control signal on

I ₀ control input 38 of the Ignitrons 36 (see Fig. 5) is applied, after which the Ignitron 36 is switched to a conductive state. The capacitor 24 discharges through the coil 18 with a maximum current Io of 9.6 kA and with a current increase rate of more than 1 kA / μs (in the illustrated embodiment specifically 1.8 kA / μs). By the rapid increase in current, a strongly varying magnetic temporally i is in the gas within the container 12 ₅ generates flux in turn induces an electric field which is sufficient for igniting a plasma.

Since the plasma discharge can be considered as an electrically conductive fluid, which is surrounded by the coil 18, it effectively forms a single secondary winding of an imaginary transformer with an inductance L _p and an ohmic plasma resistance

20 R _p . The capacitor 24 with capacitance Co and the coil 18 with the resistor Ro and inductance Lo form a damped electrical series resonant circuit, so that the voltage on the capacitor 24 oscillates at a frequency v and the current at the same frequency in the coil 18 reciprocates. The frequency v of the resonant circuit results from the parameters Co, Ro and Lo, where as further and much more substantial damping component nor the

2 ₅ attenuation added by the coupled plasma. In the embodiment described here results in a frequency of v = 29 kHz, which is simultaneously 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 in the introduction, a higher electron density in the plasma can be generated at such low excitation frequencies. First spectroscopic investigations of the plasma do indeed indicate a significantly increased electron density.

After the plasma has been extinguished, the capacitor 14 is recharged until the ignitor 36 is switched to the conductive state by another control signal for recharging the capacitor. This results in a pulsed operation with a discharge phase of about 150 to 200 μs 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 during the pulse peaks is in the MW range, the period of the pulse frequency is in the

I ₅ generally 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 chosen so high that there is a quasi-continuous operation.

Fig. 4 shows a modified embodiment and Fig. 7, the associated circuit diagram. The modification consists merely in that a thyristor pair 40 is provided instead of the ignitrone 36, which is also referred to as a "press pack".

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

The container 12 of FIG. 5 can be divided into an equatorial region 42 and two pole regions 44. The equatorial region 42 is defined herein as the region containing an equatorial plane 46, 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 Reichs 42 decreases the cross-sectional area of the container 12 upwards and downwards continuously until the limit to the respective adjacent pole region 44 is reached. The boundary to the pole region is defined in the present description as the latitude at which the cross-sectional area has fallen to half, ie it is at a latitude of s 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 are located in the equatorial region 42 and two coil segments 18a, 18d at least partially each in a pole region 44. Although all windings 18a to 18d or segments for generating

I ₀ contribute contribution 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, which lie at least partially in the pole regions 44, have an effect similar to that of known flat coils, so-called "pancake coils." The advantage of such flat coils is that the magnetic flux leakage is generally lower however, it is commonly believed that their increased capacitive coupling with the plasma is problematic compared to a solenoid, but capacitive coupling plays a relatively minor role at the low excitation frequencies used herein On the other hand, the geometry shown in Figure 5 results in a coupling efficiency of 0.8 in the experiments carried out, ie 80% of the energy of the capacitor is transferred to the plasma , and it's with a lei Depending on the modified structure, coupling efficiencies of 0.9 can even be achieved.

Another reason for the very high coupling efficiency is that the coil segments ₅ 18a to 18d bear very close to the container 12, with a maximum gap of 3 mm. Another advantage of the spherical geometry of the container 12 is that it has a maximum volume / surface ratio, which allows for increased electron density according to equation (7) above.

The specific embodiment described here serves above all to demonstrate the principles of the invention, and many modifications are possible. Promising are special 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.

FIG. 8 shows an alternative equivalent circuit diagram, which is fundamentally similar to that of FIG. 7. The main difference is that the thyristor 40 is not arranged between the coil 18 and the ground potential, but between the capacitor and the ground potential. Note that in the illustration of Fig. 8, the reverse direction thyristor has been omitted for the sake of simplicity. This arrangement has the advantage that the coil is at ground potential during the charging process of the capacitor and not at high voltage, as in the arrangement of FIG. However, the arrangement of Fig. 7 has the advantage that the parasitic inductance between the switching element 14 and the resonant circuit is lower.

FIG. 9 shows a further equivalent circuit diagram in which the switching element is formed by a full-path bridge comprising four thyristors TH1 to TH4. In this bridge circuit, the thyristors THl 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 Figs. 10 and 11 are the same as those of Figs. 8 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 containers

14 flange

16 gas supply

18 induction coil

20 coil holder

22 connecting elements between coil segments

24 capacitor

26 first terminal of the capacitor 24th

28 power supply

30 second terminal of the capacitor 24th

32 first end of the coil 18th

34 second end of the coil 18th

36 Ignitron

38 Control input of the Ignitron

40 thyristor pair

42 equatorial area

44 pole areas

46 equatorial plane

Claims

Patentansprüche5

A 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, characterized by a Power source that is suitable in inductance

I ₀ an AC signal with an excitation frequency v of less than 200 kHz, preferably below 100 kHz, more preferably from 1 kHz to 5O kHz and in particular from 10 kHz to 30 kHz to produce.

2. Device (10) according to claim 1, wherein the power source comprises at least one Kondenis capacitor (24) which is chargeable to an operating voltage, and at least one

Switching element (36, 40) which is switchable into a conductive state and is connected so that the at least one capacitor (24) in the conductive state of the switching element (36, 40) can discharge through the inductance (18) therethrough. 0 3. Device (10) according to claim 2, wherein the at least one capacitor (24) and the inductance (18) form components of an undamped electrical resonant circuit whose natural frequency of said excitation frequency (v) corresponds.

4. Device (10) according to one of the preceding claims, wherein for 5 - the amplitude Io of the AC signal, the excitation frequency v of the AC signal, the inductance LQ of the inductor and a circumference b of the container (12) in a plane perpendicular to the of Inductance generated magnetic field following relationship applies: 0 ^^ ≥ lkVIm. b

5. Device (10) according to one of the preceding claims, wherein the switching element comprises at least one thyristor (40), at least one IGBT or at least one gas discharge switch (36).

6. Device (10) according to one of claims 2 to 5, wherein the switching element (36, 40) between the capacitor (24) and the ground potential is arranged.

7. Device (10) according to one of claims 2 to 5, wherein the switching element (36, 40) between the inductance (18) and the ground potential is arranged.

8. Device (10) according to one of the preceding claims, wherein the switching element is formed by a full-wave bridge comprising four switches, in particular thyristors and / or IGBTs, which are controlled in pairs alternately.

9. Device (10) according to one of claims 2 to 8, wherein the at least one capacitor (24) or a plurality of parallel-connected capacitors (24) has a total capacity of 1 μF to 100 μF, preferably from 6 μF to 20 μF exhibit.

10. Device (10) according to one of the preceding claims, wherein the AC signal maximum current rise rates of 300 A / μs to 30 kA / μs, preferably from

1 kA / μs to 10 kA / μs.

11. Device (10) according to one of the preceding claims, in which the inductance Lo of the inductance is 1 μH to 10 μH, preferably 1.5 μH to 2.5 μH.

12. Device (10) according to one of the preceding claims, in which the inductance is formed by a coil (18) which surrounds the container (12).

13. Device (10) according to claim 12, wherein the coil has a number of turns of two to eight, preferably from four to six.

14. A device (10) according to any one of the preceding claims, wherein the container (12) is spherical or approximately spherical.

The apparatus (10) of claim 14, wherein the spherical or approximately spherical container (12) includes an equatorial region (42) having a cross-sectional area in a plane perpendicular to the induced magnetic field from a maximum value A _ma χ to a value of A _max / 2 and includes two pole regions in which the cross-sectional area decreases from A _max / 2 to zero.

16. Device (10) according to claim 12 or 13 and claim 15, in which at least one of the turns (18b, 18c) at least partially surrounds the container (12) in its equatorial region (42) and at least one turn (18a, 18d). the container (12) at least partly in its pole regions (44) surrounds.

17. Device (10) according to one of the preceding claims, comprising a controller which is suitable for periodically activating the at least one switching element (36, 40) with a pulse frequency in such a way that it switches into its conductive state. 5

18. Device (10) according to claim 17, wherein the pulse frequency is at least 1 Hz, preferably at least 10 Hz and more preferably at least 50 Hz.

19. A device for sterilizing drinking water in which water to be sterilized is passed or passed through in the vicinity of a plasma discharge source, characterized in that the plasma discharge source has a plasma generating device (10) according to one of the preceding claims.

20. A device for lithography of semiconductor structures, comprising a device for generating a plasma according to any one of the preceding claims.

21. Use of a device (10) for generating a plasma according to one of the preceding claims for the sterilization of drinking water or for exposure in HaIb- Leiterlithographie.

22. A method for generating a plasma by inductive excitation, characterized in that in an inductance (18) which is inductively coupled with a gas in which a plasma is to be generated, an alternating current signal with an excitation frequency ω is generated, which is below 20O kHz, preferably below 100 kHz, more preferably between 1 kHz and 50 kHz and in particular between 10 kHz and 30 kHz.

₅ 23. The method of claim 22, wherein the AC signal is generated by a capacitor (24) is charged to an operating voltage and at least one switching element (36, 40) is switched to a conducting state so that the capacitor (24) discharged by the inductor.

I ₀ 24. The method of claim 23, wherein the at least one capacitor (24) and the inductance (18) components of a not overdamped electrical oscillating circuit form, corresponds v whose natural frequency of said excitation frequency.

25. The method according to any one of claims 22 to 24, wherein for i ₅ - the amplitude / o of the alternating current signal, the excitation frequency v of the alternating current signal, the inductance Lo of the inductance and a circumference b of the container (12) in a plane perpendicular to The magnetic field produced by the inductance has the following relation: 0 ^^ ≥ lkV / m.

26. The method of claim 22, wherein the switching element comprises at least one thyristor, at least one IGBT or at least one gas discharge switch. 5

27. The method according to any one of claims 22 to 26, wherein the inductance is formed by a coil (18) surrounding a container (12).

A method according to claim 27, wherein the coil (18) has a number of turns of two to eight, preferably four to six.

29. The method according to any one of claims 22 to 28, wherein the container (12) is spherical or approximately spherical.

30. The method of claim 29, wherein the spherical or approximately spherical container (12) comprises an equatorial region (42) in which the cross-sectional area in a plane perpendicular to the induced magnetic field from a maximum value A _ma χ to a

5 value of A _ma χ / 2 decreases, and includes two Polbereiche in which the cross-sectional area of A _ma χ / 2 drops to zero.

A method according to claim 27 or 28 and claim 30, wherein at least one of the turns (18b, 18c) at least partially surrounds the container (12) in its equatorial region

10 (42) surrounds and at least one turn (18a, 18d) at least partially surrounds the container (12) in its pole regions (44).

32. The method according to any one of claims 22 to 31, wherein the at least one switching element (36, 40) is driven periodically at a pulse frequency so that it in its Leit-

I ₅ enabled state switches.

33. The method of claim 32, wherein the pulse rate is at least 1 Hz, preferably at least 10 Hz and more preferably at least 50 Hz.