Abstract:
In one aspect, a method includes protecting passive components connected to a high-frequency generator. In another aspect, a system includes a high-frequency generator having an HF source generating a high-frequency power signal at a fundamental frequency, and having a first control circuit which is fed with a signal related to an HF power transmitted by a high-frequency cable between the high-frequency generator and a load.

Description:
TECHNICAL FIELD 
     The invention relates to a method for protecting passive components connected to a high-frequency generator. The invention also relates to a high-frequency generator having an HF source generating a high-frequency power signal at a fundamental frequency, and having a first control circuit which is fed with a signal related to an HF power transmitted by a high-frequency cable between the high-frequency generator and a load. 
     BACKGROUND 
     High-frequency cables, in particular coaxial cables, are used to transmit a high-frequency power generated at a fundamental frequency (also known as working frequency) from a high-frequency generator to a matching network (matchbox). The high-frequency power is transmitted from the matching network to a load, for example to a plasma chamber. If the matching network is unable to match the output resistance of the high-frequency generator with the load resistance, then some of the power is reflected on the load due to the mismatch. In this case, power is produced at harmonic frequencies of the fundamental frequency. Power at these frequencies passes through the matching network and the high-frequency cable and arrives at the high-frequency generator. 
     In the case of harmonic frequencies, the load (the plasma), an electrode, the matching network, the high-frequency cable and an output circuit of the high-frequency generator act as reactance elements with a high intrinsic quality. The excitation by the harmonics, caused by the reflection on the load (for example 100 W effective power of the harmonics with 10 kW power at the working frequency) can thus cause resonances with a high reactive power (for example 10 kVA). This reactive power represents an additional load on the high-frequency cable between high-frequency generator and matching network. This problem applies in particular to so-called dual frequency systems, since these systems have a particularly large number of reactance elements (two high-frequency generators, HF cables and impedance-matching networks with filters). The loading of the high-frequency cable with reactive power can result in failure of the high-frequency cable. 
     SUMMARY 
     The object of the present invention is therefore to provide a method and a high-frequency generator, by which it is possible to avoid the above-mentioned problems. 
     This object is achieved by a method for protecting passive components, in particular a high-frequency cable, connected to a high-frequency generator, the method comprising the following steps:
         a. generating a high-frequency power at a fundamental frequency by means of the high-frequency generator;   b. transmitting the high-frequency power to a load via a high-frequency cable;   c. determining a variable related to the high-frequency load, in particular at least at one frequency differing from the fundamental frequency, on at least one passive component, in particular on a high-frequency cable, connected to the high-frequency generator;   d. comparing the determined variable with a reference variable which describes the maximum load (maximum permissible load) on at least one connected passive component;   e. adjusting the high-frequency power generated by the high-frequency generator at the fundamental frequency on the basis of the comparison result.       

     By detecting the transmitted high-frequency power or a variable, which is related to the high-frequency load of a passive component, in particular of the high-frequency cable (HF cable), the generator power can be reduced to prevent an overload of the passive component(s), in particular of the high-frequency cable. It is thereby possible to control (open-loop or closed-loop loop control) the power generated by the high-frequency generator at the fundamental frequency based on the high-frequency load on the passive component, in particular on the HF cable which has been detected. It is possible to determine the transmitted high-frequency power or a variable related thereto at the fundamental frequency and/or at one or a plurality of frequencies differing therefrom. Passive components can be, for example, high-frequency cables, connection elements (plugs), measuring devices, electrodes, plasma chambers, etc. The method is preferably used when HF power is generated at fundamental frequencies within a range of 1 to 100 MHz and at wattages within a range of 1 kW to 100 kW and in plasma generation and laser excitation systems. 
     The high-frequency power generated by the high-frequency generator at the fundamental frequency can be controlled by an open-loop control or in particular by a closed-loop control on the basis of the comparison result. Thus, not only open-loop control, but also closed-loop control is possible. In particular, a limiting control can be performed. 
     Furthermore, a variable related to the high-frequency power transmitted by the high-frequency cable at the fundamental frequency can be determined, the determined variable can be compared with a set value of the high-frequency power, and the high-frequency power generated by the high-frequency generator at the fundamental frequency can be adjusted on the basis of the comparison result. Accordingly, the high-frequency power, generated by the high-frequency generator at the fundamental frequency can be adjusted according to two criteria. Firstly, care can be taken that the high-frequency load on the passive component(s), in particular on the high-frequency cable, does not become excessive. Secondly, the power which is provided for a load can be detected and the generated power can be adjusted accordingly to obtain as effectively as possible a set value for the power supplied to the load at the fundamental frequency. In particular, it is possible to regulate the power according to a nominal power. In this respect, for adjusting the high-frequency power generated by the high-frequency generator at the fundamental frequency, it is possible to use the comparison result which leads to a relatively greater reduction in the high-frequency power. Thereby, on the one hand an overload of the passive component(s), in particular of the high-frequency cable, is effectively counteracted and on the other, an excessive amount of power is prevented from being supplied to the load. 
     The power transmitted by the high-frequency cable at frequencies different from the fundamental frequency can be determined. When the power transmitted at frequencies different from the fundamental frequency or when a variable related thereto is determined, it is then particularly easy to determine the high-frequency load on the high-frequency cable or on the passive component(s). In particular, the load can be determined by harmonics. 
     It can also be provided that the entire power transmitted in one direction by the high-frequency cable is determined. In this respect, either the entire power, the power transmitted in the forward direction, i.e. from the high-frequency generator to the load, or the power reflected on the load can be determined. It is also possible to determine which power is transmitted for each direction. 
     Furthermore, the power transmitted in one direction by the high-frequency cable at the fundamental frequency can be determined. When the power transmitted in one direction at the fundamental frequency is determined and when the entire power transmitted in one direction is determined, it is possible, by forming the difference, to evaluate which high-frequency load is caused by frequencies differing from the fundamental frequency, for example by harmonics or frequencies below the fundamental frequency. 
     To monitor a high-frequency generator, it can be advantageous to display the entire power and/or the power at the frequencies different from the fundamental frequency transmitted in the direction of the load by the high-frequency cable. The displayed power or the displayed load on the high-frequency cable can use the fundamental frequency as a reference frequency. 
     A variant of the method can provide that a signal which is related to the power transmitted by the high-frequency cable is filtered using a broadband band-pass filter. As a result, it is possible in particular to determine the power in the case of harmonics. Filtering can be carried out within a range of 1 to 200 MHz. 
     In this respect, the band-pass filter can be modified with a correction frequency response. Subject to frequency, high-frequency cables have a differing power rating. If harmonics occur at different frequencies, higher frequencies for example can be evaluated more thoroughly by a correction frequency response to thus provide more accurate information about the load on the high-frequency cable. In particular it can thereby be considered that, in the case of mismatching, the cable rating is lower than the power rating specified in the data sheet for the high-frequency cable. 
     Furthermore, it can be provided that the high-frequency load on the high-frequency cable or on the passive component(s) is determined using the forward power supplied in the direction of the load by the high-frequency generator or using the reflected power. For example, if a high-frequency generator has a high reflection factor for all frequencies and if the load has a high reflection factor for all frequencies apart from the fundamental frequency, then it is possible for only the reflected power to be considered. This is because for each harmonic frequency, the forward power is the same as the reflected power. 
     However, if the high-frequency generator has a low reflection factor for all frequencies or for all frequencies apart from the fundamental frequency, and if the load has a high reflection factor for all frequencies apart from the fundamental frequency, all harmonics coming from the load are absorbed in the generator. The forward power and the reflected power have to be evaluated for measuring the harmonic power and for the cable load. 
     A variant of the method can provide that the high-frequency load on a plurality of high-frequency cables or passive components, by which a respective high-frequency generator is connected to the load, is determined and if the high-frequency load exceeds a predetermined value, the high-frequency power of all HF generators connected to the load, which is generated at the respective fundamental frequency, is reduced. 
     The reference variable describing the maximum load (maximum permissible load) of a passive component can be adjusted. As a result, it is possible to adjust to the passive component which is used in each case. 
     Furthermore, the reference variable describing the maximum load can be determined from a catalogue of predetermined characteristics of at least one passive component and/or of at least one active component connected to the plasma chamber. For example, a manufacturer can specify for a passive component one or more parameters, from which the reference variable is then determined, thus for example is selected or calculated. Active components connected to the plasma chamber can be, for example, matching networks (matchboxes) or high-frequency generators. 
     As an alternative or in addition, the reference variable describing the maximum load can be adjusted automatically. The reference variable can be automatically adjusted, for example, on the basis of the automatic detection of connected passive components, such as high-frequency cables, or of other high-frequency generators which can operate at other frequencies. 
     The scope of the invention also includes a high-frequency generator having an HF source generating a high-frequency power signal at a fundamental frequency, and having a first control circuit, which is fed with a signal related to an HF power transmitted by a high-frequency cable between the high-frequency generator and a load, the control circuit having a controller for controlling the HF source based on a set value (supplied to the controller) for the HF load, in particular for the HF cable load, in particular based on a maximum value as reference value, and the transmitted HF power. Accordingly, with a high-frequency generator of this type, the output power of the high-frequency generator can be controlled at the fundamental frequency on the basis of the (cable) load. It is thereby possible to not only regulate the output power at a set value for the output power. 
     The control circuit can have a broadband band-pass filter. A band-pass filter of this type makes it possible to determine the power which is transmitted at the harmonics. In particular, the filter can be adjusted to a frequency band within a range of 1 to 200 MHz. 
     The band-pass filter can have a correction frequency response. Consequently, it can be considered that the cable power rating is based on a 50Ω load case, while a reflection factor of 1 is present for harmonics. 
     Furthermore, a second control circuit can be provided, which is fed with a signal related to an HF power transmitted by a high-frequency cable between the high-frequency generator and a load. The control circuit has a controller for controlling the HF source based on a set value, supplied to the controller, for the high-frequency power to be output by the high-frequency generator at the fundamental frequency, and the transmitted HF power. It is thereby also possible to regulate to a set value for the high-frequency power. 
     The second control circuit can have a filter which is adjusted to the fundamental frequency. Consequently, it is possible to detect the power which is supplied to the load at the fundamental frequency or the power which is reflected to the fundamental frequency. 
     The controllers can be connected to the HF source by a priority circuit. In this respect, the priority circuit can be configured as a limiting controller. Alternatively, it is possible to configure the priority circuit as a digital controller. In particular, it is thereby possible for the control circuit, which produces a greater reduction in the HF power generated by the HF source at the fundamental frequency, to be prioritised for controlling the high-frequency source. 
     Furthermore, means can be provided for determining the harmonic power transmitted by the HF cable. It is thereby possible to determine the proportion of the (cable) load, which is made up by the harmonic power. 
     In addition, means can be provided for determining the high-frequency power transmitted by the HF cable. In particular, it is thereby possible to determine the entire power transmitted by the HF cable. 
     One or more display devices can be provided to display the harmonic power transmitted by the HF cable and/or to display the high-frequency power transmitted by the HF cable. Consequently, an operator can immediately receive information about the high-frequency load on the HF cable or on a further passive component or components. 
     The scope of the invention also includes a power supply arrangement comprising a high-frequency generator according to the invention, which is connected to a load by a high-frequency cable. Furthermore, at least one further high-frequency generator can be connected to the load by a high-frequency cable. This produces a so-called dual frequency system, this system being protected by the high-frequency generator according to the invention against the failure of high-frequency cables. 
     A directional coupler can be provided to determine the forward power or the reflected power. In this respect, it can be provided that the directional coupler has a fixed coupling factor. The directional coupler is preferably arranged at the high-frequency output of the high-frequency generator. The directional coupler can be provided externally or can be part of the high-frequency generator. As an alternative to using a directional coupler, it is possible to measure the current or voltage in order to determine the cable load. 
     The power supply arrangement can comprise a first and a second high-frequency generator. 
     In the power supply arrangement, the first high-frequency generator can generate a high-frequency power signal at a first fundamental frequency of in particular 3.39 MHz, and the second high-frequency generator can generate a high-frequency power signal at a second frequency of in particular 13.56 MHz. 
     The two high-frequency generators can each be connected by high-frequency cables to impedance matching elements which, in turn, are jointly connected to an electrode of a plasma chamber. 
     The high-frequency generators can each comprise a first control circuit, as described above. 
     The load can be configured as a plasma installation. 
     Further features and advantages of the invention are provided in the following detailed description of embodiments of the invention, with reference to the figures of the drawings which show details essential to the invention, and are also provided in the claims. The features shown there are presented such that the features according to the invention can be made clearly visible. The various features can be realised individually per se or in a plurality of combinations in variants of the invention. 
     Embodiments of the invention are illustrated in the schematic drawings and are described in more detail in the following description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a first embodiment of a power supply arrangement; 
         FIG. 2  shows a second embodiment of a power supply arrangement; 
         FIG. 3  is a schematic illustration of a dual frequency system. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a power supply arrangement  10 . The power supply arrangement  10  comprises a high-frequency generator  11  which comprises a high-frequency source  12 , an HF driver stage  12   a  and an HF output stage  12   b  as the output circuit. A measuring device  13 , configured as a directional coupler is connected to the output of the high-frequency generator  11 . From there, a high-frequency cable  14  extends to an impedance matching network  15  which, in turn, is connected to a load  16  configured as a plasma installation. 
     In the configuration of the power supply arrangement  10  according to  FIG. 1 , it is assumed that the high-frequency generator  11  has a high reflection factor for all frequencies. It is also assumed that the load  16  has a high reflection factor for all frequencies apart from the fundamental frequency. In this case, the forward power P 1  is the same as the reflected power P R  for every harmonic amplitude. This means that only the forward power, which is designated here by P 1 , has to be considered for the measurement of the harmonic power and of the cable load. The term “forward power” is understood as meaning the power which is conducted in the direction of the load  16  from the high-frequency generator  11 . 
     Thus, received at the output  17  of the measuring device  13  is a signal, for example within a range of between 0 and 1 W, which has both the fundamental frequency at which an HF power signal is generated by the high-frequency generator  11 , as well as other frequencies, for example harmonics. This signal is fed to a control circuit  18  and is there fed in particular to a broadband band-pass filter  19 . The band-pass filter  19  can filter frequencies within a range of 1 to 200 MHz. The band-pass filter  19  is influenced by a correction frequency response  20  of the cable power rating. 
     The filtered signal is fed to a detector  21  which generates from the filtered signal a direct voltage signal which shows a measurement of the actual value of the entire high-frequency power which is transmitted to the load  16  by the HF cable  14 . It is thereby possible for an actual value of the HF power to be displayed on the display device  22 . 
     The output signal from the detector  21  is inverted in the inverter  23  and the inverted signal is fed to a summing controller  24 . The set value of the HF cable power  25  is also fed to the controller  24  as a reference variable. Since the inverter  23  inverts the measurement of the actual power, the actual power is effectively subtracted in the controller  24  from the set value of the HF cable power  25 , and it is then determined whether the actual power exceeds the set value. 
     The controller  24  is connected to a mixer  27  of the HF source  12  via a limiting controller  26 . A high-frequency oscillator  28  is also connected to the mixer  27 . If the controller  24  establishes that the actual value of the HF cable power exceeds the set value of the HF cable power, the mixer  27  is actuated such that the high-frequency power output by the high-frequency generator  11  at the fundamental frequency is reduced. 
     Furthermore, the signal at the output  17  is fed to a band-pass filter  29  which is adjusted to the fundamental frequency, i.e. it is a narrow-band filter. The filtered signal, which is still only related to the power at the fundamental frequency without harmonics, is fed to a detector  30  which gives therefrom a direct-voltage signal which is a measurement of the actual value of the high-frequency power at the fundamental frequency without harmonics. This signal is inverted by the inverter  31  and is fed to a controller  32  configured as a summing controller. The set value of the high frequency power  33  at the fundamental frequency is also fed to the controller  32 . 
     The output signal from the controller  32  is fed to the mixer  27  via the limiting controller  36 . The limiting controllers  26 ,  36  cause the output value of that controller  24 ,  32 , which produces a greater reduction in the high-frequency power generated at the fundamental frequency, to influence the output power of the high-frequency generator  11 . For correct operation, a voltage signal is also provided which is connected via a power supply U and a current-limiting resistor. The controllers  24 ,  32  essentially make a comparison. Comparison elements could also be used instead of the controllers  24 ,  32 . In this case, the output signals from the detectors  21 ,  30  would not have to be inverted. Inverting is carried out so that a difference from a set value or reference value can be determined in the controllers  24 ,  32 . 
     In the summing element  34 , the inverted output signal from the detector  30  is added to the output signal from the detector  21 . This means that the measurement of the power at the fundamental frequency is subtracted from the measurement of the total power. The result is the actual value of the power or a variable related thereto at frequencies which differ from the fundamental frequency, in particular from the harmonic power, which power is then displayed on the display device  35 . 
       FIG. 2  shows an embodiment of a power supply arrangement  10 . 1 , in which the high-frequency generator  11 . 1  has a lower reflection factor for all frequencies or for all frequencies apart from the fundamental frequency. The load has a high reflection factor for all frequencies apart from the fundamental frequency. In this case, it is necessary to evaluate the forward power P 1  and the reflected power P R  at the HF output of the high-frequency generator  11 . 1  in order to measure the harmonic power and the cable load. Elements which correspond to those of  FIG. 1  have been provided with the same reference numerals in  FIG. 2 . 
     It can be seen that the control circuit  18 . 1  is now connected to the output  40  of the measuring device  13 . Also connected to the output  40  is a narrow-band band-pass filter  41  which has been adjusted to the fundamental frequency. Here, a variable is determined which is related to the reflected power at the fundamental frequency. The output signal from the band-pass filter  41  is fed to a detector  42 , the output signal of which is a direct-voltage signal which is inverted in the inverter  43 . Unlike the situation in  FIG. 1 , the output signal from the inverter  43  and not the output signal from the inverter  31  is fed to the summing element  34 . Here again, the output signal from the summing element  34  is a measurement of the actual value of the harmonic power which is displayed on the display device  35 . 
     The control circuit  18 . 1  no longer has an inverter  23 . For this purpose, a further summing element  44  is provided which is fed with the actual value of the reflected harmonic power (or with a variable related thereto) and with the actual value of the forward power at the fundamental frequency, i.e. with the output signal from the detector  30 . The result which is released at the output of the summing element  44  corresponds to the actual value of the high-frequency cable power. Furthermore, this signal is fed to the controller  24 . 
       FIG. 3  shows a power supply arrangement in the form of a so-called dual frequency system  50 . This system  50  comprises a first high-frequency generator  51  and a second high-frequency generator  52 . The high-frequency generator  51  generates, for example, a high-frequency power signal at a fundamental frequency of 3.39 MHz, whereas the high-frequency generator  52  generates, for example, a high-frequency power signal at a frequency of 13.56 MHz. Each high-frequency generator  51 ,  52  is connected by high-frequency cables  53 ,  54  to impedance matching elements  55 ,  56  which, in turn, are jointly connected to the electrode  57  of a plasma chamber  58 . Located in the plasma chamber  58  is a counter electrode  59  which is connected to earth. Plasma  60  is generated between electrode  57  and counter electrode  59 . 
     The high-frequency generators  51 ,  52  each have a control circuit  18 . 3 ,  18 . 4  of the type described in  FIG. 1  or in  FIG. 2 .