Method and device for heating a medium using an RF signal

Description

[0001] The present invention relates to a method for heating a medium, in particular for generating a plasma, using an RF signal, comprising the steps Generating an RF feed signal with a defined first operating frequency and a defined first signal power, in particular by means of an RF generator, coupling the RF feed signal into the medium via a transmission path so that the medium is heated by the RF feed signal, determining a first RF signal reflection along the transmission path, in particular by means of a measuring and control device, and Changing the first operating frequency as a function of the first RF signal reflection in order to reduce subsequent RF signal reflections, in particular by means of the measuring and control device.

[0002] The invention also relates to a device for heating a medium, in particular for generating a plasma, using an RF signal with an RF generator which is configured to generate an RF feed signal with a defined first operating frequency and a defined first signal power, with a transmission path which is configured to couple the RF feed signal into the medium so that the medium can be heated by the RF feed signal, and with a measuring and control device which is configured to determine a first RF signal reflection along the transmission path and which is further configured to change the first operating frequency as a function of the first RF signal reflection in order to reduce subsequent RF signal reflections.

[0003] Such a method and a corresponding device are known, for example, from US 2014 / 0197761 A1.

[0004] The known device can be used, among other things, to generate a plasma which is used in etching and / or coating systems for the surface treatment of workpieces. Plasma is understood here to be a gas which is brought into an excited state by external energy absorption (heating) so that charge carriers in the gas are released from their respective atomic and / or molecular bonds and are present as free charge carriers. As a result, the plasma has an electrical conductivity which depends on the number of free charge carriers and thus on the respective energy absorption. A characteristic of a device for generating a plasma is that the impedance of the load, i.e. in this case the impedance of the plasma chamber to be supplied with the heating energy, can change very quickly and significantly.This places high demands on the RF generator, which generates the electrical RF signal to heat the gas and supply the plasma with electrical power, and on the transmission path, because the coupling of the electrical heating signal depends on how well the generator's output impedance is matched to the load's input impedance. Any mismatch leads to reflections, with the result that part or, in the worst case, even all of the electrical power does not reach the gas or plasma to be heated, but is reflected to the generator's output. Without appropriate countermeasures, this can lead to the destruction of the generator and / or other components of such a device. Furthermore, the plasma can extinguish if insufficient heating power is no longer reaching the gas.

[0005] US 2018 / 0151332 A1 discloses a plasma processing apparatus comprising a microwave output unit, a waveguide tube, a tuner, a demodulation unit, and a calculation unit. The microwave output unit outputs a microwave with a power corresponding to the tuned power while frequency-modulating the microwave within a tuned frequency range. The waveguide tube guides the microwave to an antenna of a chamber main body. The tuner is provided in the waveguide tube and adjusts the position of a movable plate. The demodulation unit is provided in the waveguide tube and detects traveling wave power and reflected wave power for each frequency. The calculation unit calculates a frequency at which a reflection coefficient calculated based on the traveling wave power and reflected wave power becomes a minimum point as an absorption frequency for each frequency.

[0006] US 2012 / 0152940 A1 discloses a microwave heating device. There, a control part performs an operation of alternately repeating a sweep operation for detecting reflected electric power with an electric power detection section while changing an oscillation frequency of an oscillation section and setting a phase value of a phase-variable section for detecting a reflected electric power characteristic with respect to the oscillation section, and a sweep operation for detecting reflected electric power with the electric power detection section while changing the phase value of the phase-variable section and setting the oscillation frequency of the oscillation section for detecting a reflected electric power characteristic with respect to the phase value, to determine an oscillation frequency and a phase value for use in heating an object to be heated.

[0007] US 2019 / 0080886 A1 discloses a microwave generation system. This system comprises a modular architecture that is configurable to deliver power from less than 1 kW to over 100 kW. The various power levels are achieved by combining the RF outputs of multiple RF power amplifiers in a single structure. Each system component contains a dedicated embedded microcontroller for powerful, real-time control response. The components are connected to a high-speed digital data bus and are controlled and monitored by a control program executing on a host computer.

[0008] The present invention is not limited to devices and methods for generating a plasma, although this is a preferred application. The described method and device can equally well be used for heating other media and / or for other purposes, such as heating liquid or solid media.

[0009] The known methods for adjusting the operating frequency of an RF signal generator used to heat a medium are comparatively complex to implement. Furthermore, in some implementations, the search for the optimal operating frequency is quite time-consuming. Therefore, it is an object of the present invention to provide a method and a device of the type mentioned above with which a suitable operating frequency of the RF signal generator can be set relatively easily and quickly.

[0010] According to one aspect of the present invention, this object is achieved by a method according to claim 1 and / or a device according to claim 14. Preferred embodiments of the method and / or the device are described in the subclaims and the description. Accordingly, this object is achieved by a method of the type mentioned at the outset, wherein the defined first operating frequency is changed, in particular increased, by a defined first frequency value during a first test interval in order to couple the first RF feed signal into the medium for a limited time with a changed, in particular increased, first operating frequency, and wherein the defined first operating frequency is changed, in particular reduced, in reverse by a defined second frequency value during a second test interval in order to couple the first RF feed signal into the medium for a limited time with a changed, in particular increased, first operating frequency.in particular reduced first operating frequency into the medium, wherein during the first test interval a second RF signal reflection is determined which correlates in time with the changed, in particular increased, first operating frequency, and wherein during the second test interval a third RF signal reflection is determined which correlates in time with the inversely changed, in particular reduced, first operating frequency, and wherein the RF feed signal is generated at a defined second operating frequency after the expiration of the second test interval and coupled into the medium, wherein the defined second operating frequency is selected as a function of the first, second and third RF signal reflections, in particular from the first operating frequency, the inversely changed, in particular reduced, first operating frequency and the changed, in particular increased, first operating frequency.

[0011] According to a further aspect, this object is achieved by a device of the type mentioned at the outset, wherein the measuring and control device is further configured to change, in particular increase, the defined first operating frequency during a first test interval by a defined first frequency value in order to couple the first RF feed signal into the medium for a limited period of time with a changed, in particular increased, first operating frequency, further to change, in particular reduce, the defined first operating frequency in the opposite direction during a second test interval by a defined second frequency value in order to couple the first RF feed signal into the medium for a limited period of time with a changed, in particular reduced, first operating frequency, further to determine a second RF signal reflection during the first test interval which is temporally related to the changed,in particular increased first operating frequency, further to determine a third RF signal reflection during the second test interval, which correlates in time with the inversely changed, in particular reduced, first operating frequency, and to generate the RF feed signal after the end of the second test interval with a defined second operating frequency and to couple it into the medium, wherein the defined second operating frequency is selected as a function of the first, second and third RF signal reflections, in particular from the first operating frequency, the changed, in particular increased first operating frequency and the inversely changed, in particular reduced first operating frequency.

[0012] The described device and the described method can be implemented very easily, in particular if the RF signal generator is a so-called solid-state generator with a voltage-controlled oscillator (VCO) and / or a phase-locked loop (PLL). Such an RF signal generator allows the instantaneous operating frequency to be changed in a simple and rapid manner. In some preferred embodiments, the RF signal generator generates the RF feed signal with an operating frequency that lies in the lower microwave range, in particular in the range from 2 GHz to 5 GHz. In one embodiment, the first operating frequency lies in the range from 2.4 GHz to 2.5 GHz, inclusive.

[0013] Starting from a current first operating frequency, which in some embodiments may be a preselected, nominal operating frequency, the described device and the described method test an increased first operating frequency and a reduced first operating frequency in two successive test intervals while the RF feed signal continues to be coupled into the medium. The temporal order of the first and second test intervals is, in principle, freely selectable. This means that in some embodiments, the defined first operating frequency can initially be increased and then reduced later. In other embodiments, however, the defined first operating frequency can initially be reduced and then increased later. Accordingly, the terms first test interval and second test interval do not imply a mandatory temporal order.In other words, the second test interval can be earlier than the first test interval.

[0014] In any case, the instantaneous operating frequency of the RF feed signal is increased at least once and reduced at least once in a test cycle. In preferred embodiments, the instantaneous operating frequency is increased exactly once in a test cycle and reduced exactly once before or after that. For each of the three operating frequencies thus obtained (instantaneous first operating frequency, increased first operating frequency, and reduced first operating frequency), associated or assignable RF signal reflections along the transmission path are determined. Thus, after such a test, at least three characteristic values ​​for RF signal reflections are available. Advantageously, the operating frequency from the (three) named operating frequencies is then very simply selected for further operation of the RF signal generator which correlates with the lowest RF signal reflection of the at least three detected RF signal reflections.

[0015] The determination and selection of the (future) second operating frequency is thus very simple and quick. In particular, the described method can be readily and very advantageously implemented in the firmware of an RF signal generator with a controllable operating frequency. Accordingly, in preferred embodiments of the described device, the increase and reduction of the respective operating frequency at time-limited test intervals is implemented in the firmware of a processor-controlled RF signal generator. Alternatively, in other embodiments, the increase and reduction of the first operating frequency could be performed "externally," i.e., initiated via a control signal supplied externally to the RF signal generator.

[0016] The described method and device have the advantage that, on the one hand, the search for a "better" operating frequency is limited to a small instantaneous search range. In the preferred embodiments, the search for a "better" operating frequency is limited to exactly two alternative values, namely one above and one below the instantaneous operating frequency. This search can therefore be carried out very quickly. On the other hand, the described method and device search "two-sidedly" for a possibly better operating frequency than the instantaneous first operating frequency. Therefore, the described method and device can follow dynamically changing impedance changes quickly and independently of direction, i.e., both toward higher impedances and toward lower impedances.

[0017] The described method and device offer a new way to very easily and quickly set a suitable operating frequency for the RF signal generator. The above-mentioned problem is therefore completely solved.

[0018] In a preferred embodiment of the invention, the increasing and reducing of the respective operating frequency is repeated cyclically with further test intervals, wherein the defined second operating frequency is used as the new first operating frequency after each cycle.

[0019] In this embodiment, the described method and device follow dynamically changing impedance changes of the load to be heated during operation of the RF signal generator. The current operating frequency is adaptively and dynamically adjusted to changing impedances. This achieves a long-term, highly efficient feed of heating power into the medium.

[0020] In a further embodiment, the respective operating frequency is increased and reduced cyclically with a cycle time T which is in a range from 1 ms to 500 ms, inclusive.

[0021] In preferred embodiments, these values ​​have proven to be very well suited to cope with dynamically changing impedances in a plasma load without having a lasting impact on the generation of the plasma by test-related increases and reductions of the respective operating frequency.

[0022] In a further embodiment, the first test interval has an interval length that is in a range from 50 µs to 500 µs, inclusive.

[0023] These values ​​have also proven to be very suitable in preferred embodiments for controlling dynamically changing impedances in a plasma load, while at the same time maintaining the plasma through constant power supply.

[0024] In a further embodiment, the defined first operating frequency is increased abruptly by the defined first frequency value at the beginning of the first test interval. Advantageously, the defined first operating frequency is decreased abruptly at the beginning of the second test interval.

[0025] A sudden increase or decrease in the first operating frequency enables very short test intervals and thus a fast and accurate assignment of the respective RF signal reflections. Alternatively, in other designs, the increase or decrease in the first operating frequency could be achieved with a non-sudden, thus more smooth transition, which in some scenarios may have the advantage of more stable plasma excitation. In many cases, however, the sudden increase or decrease appears more advantageous due to the minimal test time.

[0026] In a further embodiment, the defined first frequency value and the defined second frequency value are the same.

[0027] In this embodiment, the first operating frequency is midway between the increased first operating frequency and the reduced first operating frequency. This embodiment has the advantage that the instantaneous operating frequency of the RF signal generator remains largely constant on average over the test intervals, which also advantageously contributes to keeping the plasma excitation stable.

[0028] In a further embodiment, the defined first frequency value lies in a range from 0.0001% to 0.001% of the defined first operating frequency. In some embodiments, the defined first frequency value can, for example, be in the range from 5 kHz to 20 kHz, inclusive. In an advantageous embodiment, the first frequency value is 10 kHz at an operating frequency of the RF signal generator in the range of 2.45 GHz.

[0029] A first frequency value in the tenths of a percent range relative to the nominal operating frequency of the RF signal generator is very advantageous in order to ensure stable plasma generation on the one hand and to react specifically to impedance changes of the plasma load on the other.

[0030] In a further embodiment, the second test interval immediately follows the first test interval.

[0031] In this embodiment, the second test interval immediately precedes or immediately follows the first test interval. This results in the instantaneous operating frequency of the RF signal generator, after a temporary increase (reduction), being adjusted as quickly as possible to the opposite frequency value below (above) the original first operating frequency. In an embodiment in which the first frequency value and the second frequency value are equal, the operating frequency of the RF signal generator remains very close to the original first operating frequency on average over the two test intervals.In embodiments in which the defined first operating frequency is abruptly increased by the defined first frequency value at the beginning of the first test interval and abruptly reduced at the beginning of the second test interval, the instantaneous operating frequency thus jumps by twice the frequency value during the transition from the first test interval to the second test interval (or vice versa). This configuration advantageously contributes to keeping the plasma excitation very stable throughout the test intervals.

[0032] In a further embodiment, the defined first operating frequency is determined by transmitting an RF test signal with a current operating frequency over the transmission path, which passes through a defined frequency band, wherein RF signal reflections along the transmission path are determined.

[0033] In this embodiment, the first operating frequency, which essentially forms the starting value of the described method, is determined in a targeted scan across the defined frequency band. Alternatively, the RF signal generator could initially "start" with a nominal or otherwise selected operating frequency. In some embodiments, the defined frequency band includes the total bandwidth provided by the RF signal generator in a defined operating mode. Alternatively or additionally, the frequency band can be defined based on legal and / or regulatory requirements and, in particular, include all frequencies that are appropriate and / or approved for the desired operation and / or heating of the desired medium.In some embodiments, the defined frequency band covers a frequency range that is included in a range of 2% to 6%, inclusive, of the defined operating frequency, e.g., for a defined operating frequency of 2.45 GHz, in a range of approximately 50 MHz to 150 MHz, inclusive.

[0034] This embodiment complements the method described above with a "global scan," meaning a prior and / or interim search for an optimal operating frequency across the defined frequency band. This embodiment contributes significantly to finding the best operating frequency across the available frequency band in a simple and as fast a manner as possible.

[0035] In a further embodiment, the RF test signal is generated separately from the RF feed signal and transmitted over the transmission link. Advantageously, in some variants of this embodiment, the RF test signal can be transmitted over the transmission link in addition to, i.e., temporally parallel to, the RF feed signal. Alternatively, in other embodiments, the RF test signal can be transmitted over the transmission link at a temporally separate time from the RF feed signal.

[0036] This design makes it possible to test signal reflections independently of the RF feed signal and thus largely independent of the heating of the medium over a wide frequency range. This design therefore allows for great flexibility, which is particularly advantageous when very different media are to be heated with the RF signal generator. The design also includes the option of using the RF test signal as a modulation signal for the RF feed signal, thus combining it with the RF feed signal.

[0037] In a further embodiment, the RF feed signal forms the RF test signal.

[0038] In this configuration, the RF feed signal is used for a test scan across the defined frequency band. This enables a very cost-effective implementation.

[0039] In a further embodiment, the RF test signal is generated before the first test interval.

[0040] In this embodiment, the global scan is performed before or at the beginning of heating to obtain an initial optimal operating frequency as the starting point for the described process. This embodiment is quite simple to implement and allows for an optimal operating frequency right at the beginning of the described process.

[0041] In a further embodiment, the RF test signal is generated again after the second test interval has elapsed.

[0042] In this configuration, the "global scan" is repeated once or several times even after the start of heating, i.e., during heating. This configuration helps maintain an optimal operating frequency even with very significant changes in load impedance.

[0043] It is understood that the features mentioned above and those to be explained below can be used not only in the combination specified in each case, but also in other combinations or on their own, without departing from the scope of the present invention.

[0044] Embodiments of the invention are illustrated in the drawings and explained in more detail in the following description. They show: Fig. 1 is a simplified, schematic representation of a first embodiment of the described device, Fig. 2 is a simplified, schematic representation of a second embodiment of the described device, and Fig. 3 is a flow chart for explaining an embodiment of the described method.

[0045] The following descriptions of similar methods and / or devices cited below also serve to better classify the invention and the exemplary embodiments within the technical context: US 2014 / 0197761 A1, mentioned above, describes a device with a plurality of material processing assemblies connected in parallel to an RF signal generator. The material processing assemblies can, for example, be chambers for plasma-assisted coating of workpieces (PVD or PACVD chambers). US 2014 / 0197761 A1 discloses that each material processing assembly has a measuring device with which the respective reflected power is measured. Depending on the respective measured reflected power, the operating frequency of the RF feed signal is then to be varied until one or two differently defined threshold values ​​are reached for all material processing assemblies.In this context, US 2014 / 0197761 A1 discloses that the frequency of the RF feed signal influences the reflected power and describes a total of six different operating modes in which the frequency and / or power of the RF feed signal are varied until the defined threshold values ​​are reached.

[0046] US 2009 / 0237170 A1 discloses a method and apparatus for generating a plasma using an RF signal, wherein the operating frequency of the RF signal generator is varied depending on impedance changes of the plasma load and resulting reflections. The optimal operating frequency of the RF signal generator is to be found by dividing the total available bandwidth of the generator into sixteen subbands. During operation, the RF signal generator is to operate at its selected operating frequency 99% of the time. In the remaining 1% of the time, the sixteen subbands are to be checked sequentially for short periods of time to determine whether a more suitable operating frequency exists.

[0047] DE 10 2011 076 404 B4 describes a method for adapting the output impedance of a high-frequency power supply arrangement to the impedance of a plasma load. The operating frequency of the RF signal generator is monitored to determine whether it is within a specified frequency range. Adaptation is achieved by mechanical and / or electrical modification of a circuit downstream of the RF signal generator.

[0048] In a white paper from TRUMPF Hüttinger GmbH + Co. KG, 79111 Freiburg, Germany, entitled "A New Auto Frequency Tuning Algorithm" from July 2015, it is disclosed that the RF feed signal of an RF generator for a plasma load is constantly frequency modulated in order to find the best operating frequency during operation.

[0049] In Fig. 1 An embodiment of the described device is designated by reference numeral 10. The device 10 serves here to heat a gaseous medium 12 in a plasma chamber 14 (plasma chamber), in which, for example, workpieces (not shown here) are to be etched and / or coated with the aid of a plasma. In principle, however, the described method and the described device can also be used to heat solid and / or liquid media, as is well known in conventional household microwave cooking appliances.

[0050] The device 10 has an RF signal generator 16, which generates an RF feed signal 18 and couples it into the medium 12 via a transmission path 20. In the simplest case, electrodes (not shown here) are arranged in the plasma chamber 14, via which the RF feed signal 18 is coupled into the medium 12, for example in the form of an electromagnetic wave.

[0051] In this embodiment, the RF signal generator 16 has a voltage-controlled oscillator 22 (VCO), which is controlled by a microprocessor 24 (µC) and generates a radio-frequency signal 23 with an operating frequency that depends on the control voltage received from the microprocessor 24. Alternatively or in addition to the microprocessor 24, the RF signal generator 16 can comprise a microcontroller and / or other control circuitry, such as one or more ASICs, FPGAs, or the like. The operating frequency can be in a range of 2.4 GHz to 2.5 GHz in some embodiments.

[0052] The high-frequency signal 23 is fed to a splitter 26, which divides the high-frequency signal 23 into several parallel signal paths Nx, which results in Fig. 1 is indicated by vertically arranged dots. For reasons of clarity, only a single signal path is shown here. The parallel signal paths Nx are each constructed identically in the preferred embodiments and have the same elements. In particular, each of the parallel signal paths Nx contains a signal amplifier 28, which amplifies the corresponding signal component of the high-frequency signal 23 in the signal path Nx. Furthermore, each of the parallel signal paths Nx contains a circulator 30 and a directional coupler 32, which is connected upstream of the circulator 30 in the forward direction to the plasma chamber 14.

[0053] The signal components of the high-frequency signal 23 amplified in the parallel signal paths Nx are combined here using a coupler 34 (combiner). Preferably, the amplified signal components of the high-frequency signal 23 are combined in phase in the combiner 34. The coupler 34 (combiner) thus generates the powerful RF feed signal 18 from the individually amplified signal components of the high-frequency signal 23. The division of the high-frequency signal 23 into parallel signal paths Nx, each with at least one signal amplifier 28, and the subsequent combination in the combiner 34 facilitates a high power amplification of the original high-frequency signal 23 in preferred embodiments. In principle, however, the described method and the corresponding device can also be implemented with a single signal amplification path.

[0054] The directional coupler 32 in each signal path Nx is configured in a manner known per se to extract a small signal component of the RF signal 18 traveling to the plasma chamber 14 and to feed it to the microprocessor 24 via a rectifier 36a. The circulator 30 in each signal path Nx is configured to divert a signal reflection (typically in the form of a returning wave, not shown here). In the exemplary embodiment shown here, the circulator 30 in each signal path Nx directs a small portion of the returning signal reflection to the microprocessor 24 via a further rectifier 36b for measurement.

[0055] The circulator 30 in each signal path Nx serves, on the one hand, to keep the returning signal reflection away from the amplifier 27, the oscillator 22, and other sensitive components of the RF signal generator 16. Furthermore, the circulator 30 in each signal path Nx also acts similarly to the directional coupler 32 by feeding a small portion of the returning signal reflection to the microprocessor 24 for measurement.

[0056] In preferred embodiments, the microprocessor 24 may include one or more analog-to-digital converters (not shown here) or be combined with one or more upstream analog-to-digital converters (not shown here). The analog-to-digital converter(s) is / are advantageously configured to convert the signal components of the outgoing power coupled out via the directional coupler 32 and the returning power diverted by the circulator 30 into digital values ​​and thus make them available for digital signal processing in the microprocessor 24.

[0057] During operation of the device 10, the microprocessor 24 determines the respective RF signal reflection on the transmission path 18 with the aid of the powers received from the parallel circulators 30 and, depending thereon, varies the operating frequency of the voltage-controlled oscillator 22 in order to reduce the RF signal reflection on the transmission path 18. In this case, the microprocessor 24 can Fig. 3 implement simplified procedures.

[0058] First, in some embodiments, the optional step 50 can be executed. Here, the available frequency band is sequentially scanned using an RF test signal, which the microprocessor 24 can generate using the oscillator 22, to find an advantageous first operating frequency for heating the medium 12. Alternatively, a nominal first operating frequency can be selected based on theoretical considerations and / or practical experience.

[0059] According to step 52, the RF feed signal is then generated with the first operating frequency and coupled into the medium according to step 54. According to step 56, a first RF signal reflection on the transmission path 20 is determined. This can be done according to Fig. 1 This can advantageously be done using the circulator 30 in each signal path Nx and using the microprocessor 24 or alternatively with the aid of another, possibly analog power measuring device (not shown here). According to step 58, the operating frequency of the RF feed signal is then increased by a defined frequency value fstep during a first test interval, and according to step 60, a second RF signal reflection on the transmission path 20 is determined. The second RF signal reflection correlates in time with the operating frequency of the RF feed signal increased by fstep. According to step 62, the operating frequency of the RF feed signal is then reduced by twice the defined frequency value fstep during a second test interval, so that the operating frequency of the RF feed signal is reduced in the second test interval by a second frequency value, which here corresponds to the first frequency value fstep.According to step 64, a third RF signal reflection on the transmission link 20 is determined, which correlates temporally with the reduced operating frequency of the RF feed signal. According to step 66, it is now determined which of the three received signal reflections was the lowest, and the future operating frequency of the RF feed signal is set, according to step 68, to the (second) operating frequency from the triple "first operating frequency, increased first operating frequency, reduced first operating frequency" that correlates with the lowest signal reflection.

[0060] In the preferred embodiments, the now set second operating frequency according to loop 70 functions as the new first operating frequency, ie steps 54 to 68 are repeated cyclically while the RF feed signal is coupled into the medium.

[0061] In some embodiments, step 50 can also be repeated cyclically according to loop 72, wherein the cycle time of loop 72 is advantageously selected to be greater than the cycle time of loop 70. In particular, the global scan of the frequency band according to step 50 can be repeated with a cycle time that is 10 times, 100 times or even 1000 times the cycle time according to loop 70.

[0062] In Fig. 2 Another embodiment of a device 10' is shown, with which the described method can be advantageously implemented. The same reference numerals denote the same elements as before.

[0063] In the embodiment according to Fig. 2 A common directional coupler 32' for several or all parallel signal paths Nx is arranged at the output of the combiner 34. This allows the directional couplers 32 in the parallel signal paths Nx to be Fig. 1 The directional coupler 32' is advantageously configured to couple out both a portion of the forward power and a portion of the return power for determining the RF signal reflection. The embodiment according to Fig. 2 This therefore requires a smaller number of directional couplers. The circulators 30 in the parallel signal paths Nx serve only to divert any remaining signal reflection into a so-called dummy load (not shown here). The dummy load is a load whose primary purpose is to absorb the power of a signal reflection and convert it into heat ("dissipate").

Claims

1. A method for heating a medium (12), in particular for generating a plasma, using an HF signal, with the steps: - generating (52) an HF supply signal (18) with a defined first operating frequency and a defined first signal power, - coupling (54) the HF supply signal (18) into the medium (12) via a transmission path (20) so that the medium (12) is heated by the HF supply signal (18), - determining (56) a first HF signal reflection along the transmission path (18), and - changing (68) the first operating frequency depending on the first HF signal reflection in order to reduce subsequent HF signal reflections, characterized in that the defined first operating frequency is changed, in particular increased (58), by a defined first frequency value during a first test interval in order to couple the first HF supply signal into the medium (12) with a changed, in particular increased, first operating frequency for a limited time, and that the defined first operating frequency is changed in the reverse manner, in particular reduced (62), by a defined second frequency value during a second test interval in order to couple the first HF supply signal into the medium (12) for a limited time with a first operating frequency that has been changed in the reverse manner, in particular reduced, wherein a second HF signal reflection is determined (60) during the first test interval, which correlates in time with the changed, in particular increased, first operating frequency, and wherein a third HF signal reflection is determined (64) during the second test interval, which correlates in time with the first operating frequency, which has been changed in the reverse manner, in particular reduced, and wherein the HF supply signal is generated (68) at a defined second operating frequency after the second test interval has elapsed and is coupled into the medium (12), wherein the defined second operating frequency is selected (66) as a function of the first, second and third HF signal reflections.

2. The method according to claim 1, characterized in that increasing and reducing the respective operating frequency is cyclically repeated (70) with further respective test intervals, wherein the defined second operating frequency is used as the new first operating frequency after each cycle.

3. The method according to claim 1 or 2, characterized in that increasing and reducing the respective operating frequency is carried out cyclically with a cycle time (T) which lies in a range between and including 1 ms and 500 ms, respectively.

4. The method according to one of claims 1 to 3, characterized in that the first test interval has an interval length which lies in an area between and including 50 µs and 500 µs, respectively.

5. The method according to any one of claims 1 to 4, characterized in that the defined first operating frequency is suddenly increased by the defined first frequency value at the start of the first test interval.

6. The method according to any one of claims 1 to 5, characterized in that the defined first frequency value and the defined second frequency value are the same.

7. The method according to any one of claims 1 to 6, characterized in that the defined first frequency value is in a range between 0.0001% and 0.001% of the defined first operating frequency.

8. The method according to any one of claims 1 to 7, characterized in that the second test interval immediately follows the first test interval.

9. The method according to any one of claims 1 to 8, characterized in that the defined first operating frequency is determined by transmitting (50) an HF test signal over the transmission path at an instantaneous operating frequency which passes through a defined frequency band, wherein HF signal reflections along the transmission path (20) are determined.

10. The method according to claim 9, characterized in that the HF test signal is generated separately from the HF supply signal and is transmitted via the transmission path (20).

11. The method according to claim 9, characterized in that the HF supply signal (18) forms the HF test signal.

12. The method according to any one of claims 9 to 11, characterized in that the HF test signal is generated temporally before the first test interval.

13. The method according to any one of claims 9 to 12, characterized in that the HF test signal is generated again (72) after the second test interval has elapsed.

14. An apparatus for heating a medium (12), in particular for generating a plasma, using an HF signal, with - an HF generator (16, 16') configured to generate an HF supply signal 818) with a defined first operating frequency and a defined first signal power, - a transmission path (20) configured to couple the HF supply signal (18) into the medium (12) so that the medium (12) can be heated by the HF supply signal (18), and - a measuring and control device (24, 32, 36) configured to determine a first HF signal reflection along the transmission path (20) and further configured to change the first operating frequency as a function of the first HF signal reflection in order to reduce subsequent HF signal reflections, characterized in that the measuring and control device (24, 32, 36) is further configured to change the defined first operating frequency during a first test interval by a defined first frequency value, in particular increase (58) it, in order to couple the first HF supply signal into the medium (12) for a limited time with a changed, in particular increased, first operating frequency, change the defined first operating frequency during a second test interval by a defined second frequency value in the reverse manner, in particular reduce (62) it, in order to couple the first HF supply signal into the medium (12) for a limited time with a changed, in particular reduced, first operating frequency in the reverse manner, determine (60) a second HF signal reflection during the first test interval, which correlates in time with the changed, in particular increased, first operating frequency, determine (64) a third HF signal reflection during the second test interval, which correlates in time in the reverse manner with the changed, in particular reduced, first operating frequency, and generate (68) the HF supply signal after the second test interval has elapsed with a defined second operating frequency and couple it into the medium (12), wherein the defined second operating frequency is selected (66) as a function of the first, second and third HF signal reflections.