EP0261338B1 - Inductively excited ion source - Google Patents

Description

The invention relates to an inductively excited ion source with a vessel for receiving substances to be ionized, in particular gases, the substances to be ionized being surrounded by a waveguide which is connected to a high-frequency generator, and the two ends of the waveguide being on the same Potential. Such an ion source is known from DE-A-2 245 753.

With the help of ion sources, a beam of ions, i. H. of electrically charged atoms or molecules. The different types of ion sources adapted to the respective requirements mostly use a form of gas discharge for the ionization of neutral atoms or molecules.

The oldest, very simple ion source is the channel beam ion source or channel beam tube. Here, a gas discharge "burns" between two electrodes, which carry a voltage of a few 1000 volts, at a pressure of 10⁻¹ to 1 Pa, in which the ionization takes place by electron or ion impact. This ion source, in which the electrodes are immersed in the plasma, is also called an ion source with capacitive excitation.

Another type of ion generation is realized with the help of the high-frequency ion source. Here, the ions are generated by a high-frequency discharge in the MHz range at about 10⁻² Pa, between two specially shaped electrodes burns or is generated by an outer coil. The ions are extracted from the plasma and focused using a special extraction method (H. Oechsner: Electron cyclotron wave resonances and power absorption effects in electrodeless low pressure HF plasmas with superimposed static magnetic field, Plasma Physics, 1974, Volume 16, pp. 835 bis 841; J. Freisinger, S. Reineck, HW Loeb: the RF-Ion source RIG 10 for intense hydrogen ion beams, Journal de Physique, Colloque C7, Supplement au n ^o 7, Tome 40, July 1979, pp. C7-477 to C7-478; I. Ogawa: Electron cyclotron resonances in a radio-frequency ion source, Nuclear Instruments and Methods 16, 1962, pp. 227 to 232).

A disadvantage of many known ion sources with inductive excitation is, however, that they have a considerable RF power loss. This RF power loss occurs because the RF coil, which is wrapped around the vessel in which the plasma is located, must be adapted to the RF generator. For this purpose, a matching network is provided between the HF generator and the HF coil, which matches the generator power to the consumer power, ie. H. adapts to the coil power (see, for example, DE-A-25 31 812, reference number 40 in the figures). This adaptation consists in that the wave resistance of the coil loaded by the plasma is transformed into the wave resistance of the transmitter line. In the adapter circuit, a power loss of 20% to 50% of the total power delivered by the HF generator occurs.

Another disadvantage of the known ion source with inductive excitation is that the attachment of additional magnets in the vicinity of the vessel in which the plasma is located is difficult because the RF coil takes up a relatively large amount of space and because the magnets are in the magnetic field Heat up the RF coil. Such additional magnets are required in order to keep the plasma away from certain points on the vessel wall or to compress the plasma (cf. EP-A-0169744). In addition, the cooling of the coils is problematic because of the fact that these coils are flushed with cooling water on the one hand and on the other hand to HF potential lie, which requires space-consuming potential degradation lines to bring the potential from a high value to a low value. Since the potential is usually reduced by extending the coil, there is an increased power loss.

It is also known to form induction coils in a converter system as a waveguide and to cool them with a liquid (DE-A-25 44 275). Such liquid-cooled induction coils are, however, also used in high-frequency induction plasma torches (DE-B-21 12 888).

In addition, an inductively excited ion source is known which has a vessel for holding substances to be ionized (US Pat. No. 3,084,281). Here, a resonance circuit is provided, with λ / 4 lines being provided for the transmission of R.F. power.

It is also known to implement a high-quality resonance circuit with a cavity resonator (Fred. E. GARDIOL, "Introduction to Microwaves, 1984, Artech House, Inc., pages 109 to 129).

Finally, a device for carrying out a reaction between a gas and a material in an electromagnetic field is also known, which has a reaction chamber for receiving the gas and the material, a composite coil with two connected coil sections, the turns of which are wound in opposite directions, has a high-frequency source and a device for connecting the high-frequency source to the coil (DE-A-22 45 753). In this device, the two ends of the coil are interconnected so that they are at the same potential. In addition, one connection of the high-frequency source is connected to a point on the coil which is located between the two ends of the coil. However, the grounded connection of the radio frequency source is at a different potential than the ends of the coil. A disadvantage of this device is that an adaptation network is required.

The invention is therefore based on the object of providing an arrangement in an inductively excited ion source according to the preamble of claim 1, which dispenses with a special adapter network.

This object is achieved by an ion source according to claim 1.

The advantage achieved with the invention is in particular that the power losses of an inductively excited ion source can be considerably reduced. It is also possible to easily supply and remove the cooling water to earth potential.

Fig. 1

eine perspektivische Darstellung der äußeren mechanischen Form der erfindungsgemäßen Ionenquelle;

Fig. 2

eine Prinzipdarstellung der erfindungsgemäßen elektrischen Schaltungsanordnung;

Fig. 3

eine Schnittdarstellung durch die erfindungsgemäße Ionenquelle mit den zugehörigen elektrischen Anschlüssen;

Fig. 4

eine Schnittdarstellung durch eine Variante der erfindungsgemäßen Ionenquelle;

Fig. 5

eine besondere Anschaltung einer variablen Kapazität an eine Spule der erfindungsgemäßen Ionenquelle.

An embodiment of the invention is shown in the drawing and will be described in more detail below. Show it:

Fig. 1

a perspective view of the outer mechanical shape of the ion source according to the invention;

Fig. 2

a schematic diagram of the electrical circuit arrangement according to the invention;

Fig. 3

a sectional view through the ion source according to the invention with the associated electrical connections;

Fig. 4

a sectional view through a variant of the ion source according to the invention;

Fig. 5

a special connection of a variable capacitance to a coil of the ion source according to the invention.

In Fig. 1, an evacuated vessel 1 is shown, which is surrounded by an electrically conductive high-frequency coil 2 and is closed with an upper annular end plate 4. The ends 5, 6 of the high-frequency coil 2 are guided through corresponding openings in the lower end plate 4 to a cooling system, not shown. This cooling system has the effect that a cooling liquid is introduced through the end 5 of the high-frequency coil 2 designed as a hollow tube and is led out again through the end 6 of this coil 2. The high-frequency coil 2 consists, for example, of copper tube, which is arranged outside the vessel here, but can also be integrated into this or arranged within the vessel. The inflow and outflow of the Coolant is indicated by the arrows 7 and 8. Water is preferably used as the cooling liquid. In the exemplary embodiment, the high-frequency coil 2 has nine turns, a diameter of approximately 120 mm and a height of approximately 130 mm. Their length is λ / 2, where λ is related to the frequency of a high-frequency generator. Coil length is understood to mean the length of the drawn-out coil wire and not the length of the coil. It goes without saying that the high-frequency coil 2 can also have dimensions other than those specified here. In addition, it does not have to be wrapped around the vessel 1, but can, for example, also be located on the inner wall of the vessel 1 or be integrated into the vessel wall. On the underside of the vessel 1, a nozzle 9 is provided through which the gas to be ionized enters the vessel 1. The electrical coupling of the HF power takes place via a cable 10 connected to a high-frequency generator, which is connected to the coil 2 by means of a clamp 11.

2, apart from the end plates 3, 4, essentially the electrical circuit of the ion source according to the invention is shown. If the end plates 3, 4 are in turn connected to one another in a highly conductive manner, the coil ends 5, 6 can also be fastened to their own plate 3, 4 alone. 2 shows a high-frequency generator 12 which is grounded via a line 22 and which is connected to the high-frequency coil 2 via the cable 10. The electrical connection point of the generator 12 is designated 13. At another point in the coil 2 there is another electrical connection point 14 to which a capacitor 15 with variable capacitance is connected. However, this capacitor can also be omitted if the resonance frequency of the resonator consisting of the coil 2 and the enclosed plasma is precisely matched to the frequency of the high-frequency generator 12.

As a rule, however, this precise tuning is difficult to carry out, so that it is easier to resonate the resonant circuit by changing the capacitance of the capacitor 15.

The HF generator 12, the lower end plate 4 and the capacitor 15 are connected to earth or ground via the lines 21, 22, 23. The earthing is preferably carried out via a short, wide and well-conducting cable, which, for. B. consists of silver.

In terms of radio frequency, the coil has not only an inductance, but also an inherent capacitance. Inductance and capacitance together form the resonance frequency of the coil 2, the inductance and the capacitance being determined via the so-called induction coating and the capacitance coating. The coil 2 is therefore to be understood as a waveguide on which waves of the Lecher type propagate (see K. Simonyi: Theoretical Electrical Engineering, Berlin 1956, pp. 313 to 363, or H.-G. Unger: Electromagnetic waves on lines , Heidelberg, 1980). The winding of the coil 2 is to be regarded as a subordinate influencing variable in relation to its wire length.

The output frequency of the HF generator 12 is placed on the resonance frequency of the high-frequency coil 2, which can be influenced by the ions in the vessel 1. Thus, the total power consumed in the actual resonant circuit and not in an impedance matching is consumed, i. H. if the generator frequency is tuned to the resonance frequency, it is possible that practically no power loss occurs. The actual resonance circuit is understood to mean the combination of the excitation coil and plasma, that is to say the excitation coil loaded by the plasma. A high-frequency shielding housing may also be part of this actual resonance circuit. Such a shielding housing was not shown in the illustration in FIG. 2 because the appearance of these housings and their influence on the overall resonant circuit are known.

A power adjustment in the sense that the power of the high-frequency generator 12 is optimally given to the coil 2 is, however, not yet connected to the measures mentioned.

However, this power adjustment is possible by means of a suitable choice of the connection point 13 of the line 10 to the coil 2. The connection point 13 is selected so that the quotient of voltage and current at point 13 is equal to the characteristic impedance of line 10. If one continuously measures this quotient and compares it with the known characteristic impedance, an electric drive can be controlled with the aid of a control circuit so that it always brings point 13 into a position in which the above-mentioned condition applies. In this way it is possible to automate the performance adjustment.

In the illustration in FIG. 2, the high-frequency generator 12 is by no means short-circuited, as it might appear from a low-frequency view. Rather, the straight piece of the coil 2, which extends from the connection point 13 to the plate 4, has an inductance and a capacitance coating which prevents a high-frequency short circuit.

Instead of placing the frequency of the frequency generator 12 at the natural or resonant frequency of the coil 2, it is also possible to adapt the resonant frequency of the coil 2 to the predetermined frequency of the high-frequency generator 12. For this purpose, the capacitor 15 is provided, which is connected to the coil 2. By adjusting this capacitor 15, which is connected to the point of symmetry 14 of the coil 2, the resonance frequency of the system coil 2 / capacitor 15 is changed. By means of this change in the resonance frequency, the influence of the ions on the coil resonance frequency can be balanced.

If the coil 2 or the system coil 2 / capacitor 15 is acted upon by an alternating voltage whose frequency is equal to the resonance frequency of the coil 2 or the system coil 2 / capacitor 15 or a harmonic thereof, then the instantaneous currents and voltages are on the Coil 2 distributed as integer multiples of half wavelengths. Current bellies and voltage nodes always come on the coil ends 5, 6 to lie; ie the coil ends 5,6 are at ground potential. The cooling water can therefore be easily supplied and removed to earth potential. When there is resonance, there are always at least two points on the coil at which the ratio of voltage and current is equal to the characteristic impedance of line 10. If the line 10 is connected to such a point 13, the power of the high-frequency generator 12 is coupled in without loss. By shifting this coupling point 13, it is possible to compensate for changes in the natural frequency of the coil 2 which result from different plasma densities, ie different loads on the coil 2.

The arrangement according to the invention concentrates the entire magnetic field energy occurring in the coil 2, so that its magnetic field holds the plasma together very effectively and compresses it. Of course, the coil can also be different, e.g. meandering, designed to accommodate other field configurations, e.g. generate a "cusp" field or multipolar field as shown in Fig. 2 of EP-A-0169744.

The arrangement according to the invention is shown again in section in FIG. The vessel 1, which is cylindrical and made of a chemically inert material, is surrounded by the coil 2 and has an extraction grid system 16 at its upper end, which is connected to an extraction power supply 17. At the lower end of the vessel 1, the inlet port 9 is provided with its gas supply channel 18. If a pressure between about 2 x 10⁻² Pa and 50 Pa is set in the discharge space 19 of the vessel 1, a discharge can be ignited via the connection of the high-frequency generator 12. The ions formed in this process are sucked out by the extraction grid system 16 when a suitable voltage of the extraction power supply 17 is present at this grid system 16. The extraction grid system 16 is - in contrast to the annular end plates 3, 4 which are grounded via the lines 20, 21 or in contrast to the high-frequency generator 12 which is grounded via the line 22 - not at ground potential.
Although resonance phenomena play an important role in the invention, it differs considerably from other circuits for inductively coupled low-pressure plasma, which also work with resonances. In the known resonance inductor already mentioned above, an adaptation by means of capacitors and inductors must be carried out. But even when the coil or the inductor is fed via an asymmetrical line, for example a coaxial cable, this cable must be balanced and adapted to the inductor impedance. In the present invention, matching networks and impedance transformations are eliminated. Neither an impedance transformation using an RF transmitter, nor a π transformation or a T transformation is required.

FIG. 4 shows a variant of the ion source shown in FIG. 3. In this embodiment, the basic resonance frequency of the coil 2 is reduced from originally approximately 50 MHz by doubling its length to approximately half of its original value to approximately 25 MHz. The doubling of the coil length is achieved here by a second coil layer, which is designated by 25. The winding direction of the two coil layers 25, 26 can be in opposite directions, whereby particularly advantageous effects are achieved.

The efficiency of the ion source is improved by a small distance between the resonance and excitation frequency. In addition, the inductance increases with the number of turns of the coil, which leads to an improvement in the resonant circuit quality.

With the double-layer winding of the coil 2, ignition without pressure surge can be achieved, i. H. purely electrical ignition is possible.

FIG. 5 shows a variant of the connection of a capacitor 27 to the coil shown in FIG. 2. The capacitor 27 is connected to the coil 2 at two points 28, 29, while the oscillator 12 is at the "50-ohm point" 30 of the coil 2. This connection enables the HF ion source to be tuned to a low voltage level. The influence of the capacitor 27 on the tuning is less, and there is also a certain distortion of the current and Voltage distribution on, but the capacitor line 31 can be made longer because of the lower voltage. The advantage achieved in this way is, in particular, that the capacitor no longer has to sit directly on the ion source, but can be arranged at a certain distance from it, without substantial loss of power occurring due to stray capacitances which are at high voltage.

Claims (16)

1. Inductively excited ion source, having a vessel (1) in which substances to be ionized, in particular gases, or a plasma are located during operation, these substances or the plasma being surrounded by a waveguide (2) which is connected to a high-frequency generator (12), and the two ends of the waveguide (2) being at the same potential, characterized in that the length of the waveguide (2) is substantially $\text{n.c/(2f)}$

   

   , where n is a natural number greater than zero, c is the phase velocity of an electromagnetic wave guided by the waveguide (2), and f is the frequency of the high-frequency generator (12), the high-frequency generator (12) and the resonant frequency of the system comprising waveguide (2), substance or plasma to be ionized and where appropriate a high-frequency screening housing and where appropriate a device (15) for matching the resonant frequency or a harmonic of this resonant frequency being matched to one another, in that the high-frequency generator (12) is connected by way of a generator line (10) having a wave resistance directly to a point (13) on the waveguide (2) for the feeding in of the high-frequency power from the high-frequency generator (12), and in that at this point (13) of the waveguide (2) the quotient of voltage and current intensity is substantially equal to the wave resistance of the generator line (10).
2. Inductively excited ion source according to Claim 1, characterized in that a wound coil (25, 26) is provided as the waveguide (2).
3. Inductively excited ion source according to Claim 1, characterized in that the potential of the ends (5, 6) of the waveguide (2) and the one connection (22) of the high-frequency generator (2) is earth potential.
4. Inductively excited ion source according to Claim 2, characterized in that the wound coil (25, 26) has two winding layers, and in that the one winding layer (25) is wound in the opposite direction to the other winding layer.
5. Inductively excited ion source according to Claim 1, characterized in that a device is provided for matching the frequency of the high-frequency generator (12) to the natural frequency of the system or a harmonic of this natural frequency.
6. Inductively excited ion source according to Claim 1, characterized in that a device is provided for matching the natural frequency of the system or of a harmonic of this natural frequency to the frequency of the high-frequency generator (12).
7. Inductively excited ion source according to Claim 6, characterized in that the natural frequency of the system or of a harmonic of this natural frequency is matched with the aid of a variable capacitor (15).
8. Inductively excited ion source according to Claim 7, characterized in that the capacitor (15) is connected at the electrical point of symmetry (14) of the wave guide (2).
9. Inductively excited ion source according to Claim 7, characterized in that a coil is provided as the waveguide (2), and in that the one connection of the capacitor (15) is made to the coil and the other connection of this capacitor (15) is earthed.
10. Inductively excited ion source according to Claim 1, characterized in that the waveguide is a coil (2) constructed as a hollow tube through which a coolant flows during operation.
11. Inductively excited ion source according to Claim 10, characterized in that the coolant is water.
12. Inductively excited ion source according to Claim 1, characterized in that the point (13) for feeding the high-frequency power from the high-frequency generator (12) into the waveguide (2) is selectable such that at this point (13) the quotient of voltage and current intensity is equal to the wave resistance of the generator line (10) in the respective operating state of the ion source.
13. Inductively excited ion source according to Claim 12, characterized in that a device is provided for automatically adjusting the point (13) for feeding in the high-frequency power.
14. Inductively excited ion source according to Claim 1, characterized in that the vessel (1) has the shape of a hollow cylinder and is terminated by an upper and a lower closure plate (3 and 4 respectively), the upper closure plate (3) being provided with an extraction grid (16) and the lower closure plate (4) being provided with an opening conduit (9) for the gas supply, and the ends (5, 6) of the waveguide (2) being earthed by way of a closure plate (3 and 4 respectively).
15. Inductively excited ion source according to Claim 1, characterized in that during operation a direct current which generates a magnetic field guiding the ions additionally flows through the waveguide (2).
16. Inductively excited ion source according to Claim 1, characterized in that a variable capacitor (27) is provided which by means of its one connection is at earth potential and by means of its other connection is connected to two different points (28, 29) of a coil (2) serving as a waveguide.

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