Method and apparatus for heating a medium using an HF signal - Patents.com

Abstract

The HF signal is used to generate an HF supply signal (18) having a prescribed first operating frequency and a prescribed first signal power for heating the medium (12), in particular for generating a plasma. The HF supply signal (18) is coupled to the medium (12) via a transmission path (20). A first HF signal reflection along the transmission path (20) is determined, and the first operating frequency is varied in dependence on the first HF signal reflection to reduce subsequent HF signal reflections over time. The first operating frequency is increased by a prescribed first frequency value during a first test period to couple the first HF supply signal to the medium (12) at the increased first operating frequency for a limited time. During a second test period, the first operating frequency is decreased by a prescribed second frequency value to couple the first HF supply signal to the medium (12) at the reduced first operating frequency for a limited time. A second HF signal reflection is determined during the first test period, and a third HF signal reflection is determined during the second test period. After the second test period has elapsed, an HF feed signal is generated at a second operating frequency selected in dependence on the first, second and third HF signal reflections.

Description

[Technical field]

[0001] The present invention relates to a method for heating a medium, in particular for generating a plasma, using an HF signal, comprising the following steps: In particular, the method includes the steps of: generating, using an HF generator, an HF supply signal having a defined first operating frequency and a defined first signal power; coupling an HF supply signal to a medium via a transmission path such that the medium is heated by the HF supply signal; In particular, determining a first HF signal reflection along a transmission path using a measurement and control device; In particular, using the measurement and control device, varying the first operating frequency in dependence on the first HF signal reflection to reduce subsequent HF signal reflections in time.

[0002] The invention further relates to a device for heating a medium, in particular for generating a plasma, using an HF signal, which device comprises: an HF generator configured to generate an HF feed signal having a defined first operating frequency and a defined first signal power; a transmission path configured to couple the HF supply signal to the medium such that the medium can be heated by the HF supply signal; configured to determine a first HF signal reflection along a transmission path; and and a measurement and control device configured to vary the first operating frequency in dependence on the first HF signal reflection to reduce subsequent HF signal reflections in time. [Background technology]

[0003] Such a method and a corresponding device are known, for example, from US Pat. No. 5,399,323.

[0004] The known device can be used in particular to generate plasmas used in etching and / or coating devices for the surface treatment of workpieces. In this document, a plasma is understood as a gas that is brought into an excited state by external energy absorption (heating), as a result of which the charge carriers of the gas are released from their respective atomic and / or molecular bonds and are present as free charge carriers. The plasma therefore has an electrical conductivity that depends on the number of free charge carriers and thus on the respective energy absorption. Devices for generating plasma are characterized by the fact that the impedance of the load, i.e. in this case the impedance of the plasma chamber to which the heating energy is supplied, can change very quickly and significantly. This places high demands on the HF generator, which generates the HF signal for heating the gas and supplying power to the plasma, and on the transmission path, since the coupling of the electrical heating signal depends on how well the output impedance of the generator is matched to the input impedance of the load. Mismatches cause reflections, with the result that part of the power, in the worst case all of it, does not reach the gas or plasma to be heated and is instead reflected back to the output of the generator. This can lead to damage to the generator and / or to other components of such devices, if there are no appropriate measures. Furthermore, if sufficient heating power does not reach the gas, the plasma may be extinguished.

[0005] The present invention is not limited to the apparatus and method for generating plasma, although this is a preferred application, as the above methods and apparatus may be used for heating other media and / or for other purposes as well, such as heating liquid or solid media. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] US Patent Application Publication No. 2014 / 0197761A1 [Patent Document 2] US Patent Application Publication No. 2009 / 0237170A1 [Patent Document 3] German Patent Application Publication No. 102011076404B4 [Non-patent literature]

[0007] [Non-Patent Document 1] A white paper entitled “A New Auto Frequency Tuning Algorithm” (July 2015) by TRUMPF Huettinger GmbH+Co.KG, Freiburg, Germany Summary of the Invention [Problem to be solved by the invention]

[0008] The known methods for adjusting the operating frequency of an HF signal generator used for heating a medium are relatively complex to implement. Moreover, the search for the optimal operating frequency in each case takes a rather long time in some implementations. Therefore, it is an object of the present invention to provide a method and a device of the type mentioned at the beginning, by which a suitable operating frequency of an HF signal generator can be set relatively easily and quickly. [Means for solving the problem]

[0009] According to an aspect of the invention, this problem is solved by a method according to claim 1 and / or an apparatus according to claim 14. Preferred embodiments of the method and / or the apparatus are set out in the dependent claims and the description. Thus, this problem is solved by a method of the type mentioned in the introduction, in which a defined first operating frequency is changed, in particular increased, by a defined first frequency value during a first test period in order to couple the first HF supply signal into the medium in a time-limited manner with a changed, in particular increased, first operating frequency, the defined first operating frequency is reversely changed, in particular lowered, by a defined second frequency value during a second test period in order to couple the first HF supply signal into the medium in a time-limited manner with a reversely changed, in particular lowered, first operating frequency, and a second HF signal reflection is determined during the first test period, the second HF signal reflection being time-correlated with the altered, in particular increased, first operating frequency, and during the second test period determining a third HF signal reflection, the third HF signal reflection being time-correlated with the inversely altered, in particular lowered, first operating frequency, and generating an HF feed signal at a defined second operating frequency after the second test period and coupling it to the medium, wherein the defined second operating frequency is selected in dependence on the first, second and third HF signal reflections, in particular from the first operating frequency, the inversely altered, in particular lowered, altered first operating frequency and the altered, in particular increased, first operating frequency.

[0010] According to a further aspect, this problem is solved by an apparatus of the type mentioned in the introduction, in which the measurement and control device furthermore varies, in particular increases, the defined first operating frequency by a defined first frequency value during a first test period in order to couple the first HF supply signal into the medium in a time-limited manner with a varied, in particular increased, first operating frequency, and furthermore varies, inversely, varies, in particular decreases, the defined first operating frequency by a defined second frequency value during a second test period in order to couple the first HF supply signal into the medium in a time-limited manner with a reversely varied, in particular decreased, first operating frequency, and furthermore determines a second HF signal reflection during the first test period. the second HF signal reflection being time-correlated with the altered, in particular increased, first operating frequency, and further configured to determine a third HF signal reflection during a second test period, the third HF signal reflection being time-correlated with the conversely altered, in particular lowered, first operating frequency, and further configured to generate and couple an HF feed signal to the medium at a predefined second operating frequency after the second test period has elapsed, the predefined second operating frequency being selected in particular from the first operating frequency, the altered, in particular increased, first operating frequency, and the conversely altered, in particular lowered, first operating frequency, depending on the first, second and third HF signal reflections.

[0011] In particular, the above apparatus and method can be implemented very easily when the HF signal generator is a so-called solid-state generator with a voltage controlled oscillator (VCO) and / or a phase-locked loop (PLL). Such an HF signal generator can quickly and easily change its current operating frequency. In some preferred embodiments, the HF signal generator generates an HF supply signal with an operating frequency in the low microwave range, in particular in the range of 2 GHz to 5 GHz. In one embodiment, the first operating frequency is in the range of 2.4 GHz to 2.5 GHz.

[0012] Starting from a current first operating frequency, which in some embodiments may be a preselected nominal operating frequency, the described apparatus and method tests the increased and reduced first operating frequency in two successive test periods in time while the HF supply signal continues to be coupled to the medium. The time sequence of the first and second test periods is in principle freely selectable. This means that in some embodiments the defined first operating frequency can be increased first and reduced later in time. Whereas in other embodiments the defined first operating frequency can be reduced first and increased later in time. The references first test period and second test period thus do not imply any mandatory time sequence in this specification. In other words, the second test period may precede the first test period in time.

[0013] In each case, the current operating frequency of the HF supply signal is increased at least once in the test period and decreased at least once. In a preferred embodiment, the current operating frequency is increased exactly once in the test period and decreased exactly once in time before or after. For each of the three operating frequencies thus obtained (current first operating frequency, increased first operating frequency, decreased first operating frequency), an associated or correlable HF signal reflection along the transmission path is determined. Thereby, after such a test, there are at least three characteristic values ​​of the HF signal reflection. Advantageously, the operating frequency that correlates to the smallest HF signal reflection of the at least three detected HF signal reflections is very simply selected from the (three) operating frequencies for further operation of the HF signal generator.

[0014] By this method, the determination and selection of the (future) second operating frequency is possible very simply and quickly. In particular, the described method can be easily and very advantageously implemented in the firmware of an HF signal generator having a controllable operating frequency. Accordingly, the increase and decrease of the respective operating frequency during the time-limited test period is implemented in the firmware of a processor-controlled HF signal generator in a preferred embodiment of the described device. Alternatively, in other embodiments, the increase and decrease of the first operating frequency can be initiated "externally", i.e. via a control signal supplied from outside to the HF signal generator.

[0015] 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 a preferred embodiment, the search for a "better" operating frequency is limited to exactly two alternative values, i.e. one above and one below the current operating frequency. This search can therefore be performed very quickly. On the other hand, the described method and device searches "on both sides" for an operating frequency that is better than the current first operating frequency. The method and device can therefore follow dynamically changing impedance changes quickly and independently of direction, i.e. both in the direction of higher impedance and in the direction of lower impedance.

[0016] The described method and device provide new possibilities for very simple and fast setting of a suitable operating frequency of an HF signal generator, thus completely solving the above-mentioned problems.

[0017] In a preferred embodiment of the invention, each increase and decrease in operating frequency is repeated cyclically with a respective further test period, and the defined second operating frequency is used as the new first operating frequency after each period.

[0018] In this embodiment, the described method and device tracks the dynamically changing impedance of the load to be heated during operation of the HF signal generator, and the current operating frequency is dynamically adapted to the changing impedance, respectively, so that the heating power is delivered to the medium very efficiently in the long term.

[0019] In a further embodiment, each operating frequency is increased and decreased periodically with a periodic time (T) in the range of 1 ms to 500 ms.

[0020] In the preferred embodiment, these values ​​have been proven highly suitable for controlling the dynamically changing impedance in the plasma load by test increasing and decreasing the respective operating frequencies without permanently affecting the generation of the plasma.

[0021] In a further embodiment, the first test period has a period length in the range of 50 μs to 500 μs.

[0022] In the preferred embodiment, these values ​​have also proven to be highly suitable for controlling the dynamically changing impedance in the plasma load while at the same time maintaining the plasma with a constant power supply.

[0023] In a further embodiment, the defined first operating frequency is abruptly increased by a defined first frequency value at the start of the first test period, and advantageously, the defined first operating frequency is abruptly decreased at the start of the second test period.

[0024] Each abrupt increase or decrease in the first operating frequency allows for a very short test period, thereby allowing for a fast and accurate classification of each HF signal reflection occurring in parallel. Alternatively, in other embodiments, the increase or decrease in the first operating frequency can be made with a non-abrupt, and thus rather smooth, transition, which may have the advantage in some scenarios that the plasma excitation is more stable. However, in many cases, abrupt increases or decreases are considered more advantageous for minimal test times.

[0025] In a further embodiment, the prescribed first frequency value and the prescribed second frequency value are identical.

[0026] In this embodiment, the first operating frequency is intermediate between the increased first operating frequency and the decreased first operating frequency. An advantage of this embodiment is that the current operating frequency of the HF signal generator remains approximately constant on a time average over the test period, which advantageously helps to keep the plasma excitation stable.

[0027] In a further embodiment, the prescribed first frequency value is in the range of 0.0001% to 0.001% of the prescribed first operating frequency. In some embodiments, the prescribed first frequency value may be in the range of 5 kHz to 20 kHz, for example. In an advantageous embodiment, the first frequency value is 10 kHz for an operating frequency of the HF signal generator in the range of 2.45 GHz.

[0028] A first frequency value in the region of tenths of a second relative to the nominal operating frequency of the HF signal generator is highly advantageous, on the one hand, for ensuring a stable plasma generation and, on the other hand, for an adequate reaction to impedance changes of the plasma load.

[0029] In a further embodiment, the second test period directly follows the first test period.

[0030] In this embodiment, the second test period immediately follows the first test period or immediately follows the first test period. This results in the current operating frequency of the HF signal generator being set as quickly as possible to the opposite frequency value below (above) the original first operating frequency after a time-limited increase (decrease). In an embodiment where the first and second frequency values ​​are identical, the operating frequency of the HF signal generator remains very close to the original first operating frequency on average over the two test periods. In an embodiment where the defined first operating frequency is abruptly increased by the defined first frequency value at the start of the first test period and abruptly decreased at the start of the second test period, the current operating frequency is abruptly changed by twice its frequency value at the transition from the first test period to the second test period (or vice versa). This embodiment advantageously contributes to keeping the plasma excitation very stable over the test periods.

[0031] In a further embodiment, the defined first operating frequency is determined by transmitting an HF test signal having the current operating frequency across a defined frequency band through a transmission path, and determining HF signal reflections along the transmission path.

[0032] In this embodiment, the first operating frequency, which in a sense forms a starting value for the described method, is determined in a targeted search over a defined frequency band. Alternatively, the HF signal generator may initially "start" with a nominal or other selected operating frequency. In some embodiments, the defined frequency band includes the entire bandwidth provided by the HF signal generator in the defined operating mode. Alternatively or additionally, the frequency band may be defined based on legal and / or regulatory requirements and may in particular include the overall frequencies that are useful and / or permitted for the desired operation and / or heating of the desired medium. In some embodiments, the defined frequency band covers a frequency range that is comprised between 2% and 6% of the defined operating frequency, so that for example, at a defined operating frequency of 2.45 GHz, it is comprised between about 50 MHz and 150 MHz.

[0033] This embodiment complements the above method to some extent by a "global scan", i.e. an up-front and / or intermediate time search for the optimum operating frequency over a defined frequency band, which contributes very advantageously to finding the optimum operating frequency over the available frequency band simply and as quickly as possible.

[0034] In a further embodiment, the HF test signal is generated separately from the HF supply signal and transmitted over the transmission path. Advantageously, the HF test signal can be transmitted in some variants of this embodiment additionally, i.e. in parallel in time, with the HF supply signal over the transmission path. Alternatively, the HF test signal can be transmitted in other examples separately in time from the HF supply signal over the transmission path.

[0035] This embodiment allows testing the signal reflections independently of the HF supply signal, and therefore independently of heating of the medium, over a wide frequency range. This embodiment therefore allows great flexibility, which is particularly advantageous when heating very different media with the HF signal generator. This embodiment also includes the possibility of using the HF test signal as a modulation signal for the HF supply signal, and of combining it with the HF supply signal in this way.

[0036] In a further embodiment, the HF supply signal forms an HF test signal.

[0037] In this embodiment, the HF feed signal is used for test scans over a defined frequency band, which allows for a very cost-effective implementation.

[0038] In a further embodiment, the HF test signal is generated temporally before the first test period.

[0039] In this embodiment, a "global scan" is performed before or at the start of heating in order to obtain a starting optimal operating frequency as a starting point for the described method. This embodiment is very simple to implement and allows an optimal operating frequency already at the start of the described method.

[0040] In a further embodiment, the HF test signal is generated again after a second test period has elapsed.

[0041] In this embodiment, the "global scan" is repeatedly performed once or several times after the start of heating, i.e., during heating. This embodiment contributes to maintaining the optimal operating frequency even when the load impedance changes significantly.

[0042] It is understood that the features mentioned above and those to be described below can be used not only in the combinations shown in each case, but also in other combinations or alone, without going beyond the scope of the invention.

[0043] Exemplary embodiments of the invention are illustrated in the drawings and are explained in more detail in the following description. [Brief description of the drawings]

[0044] [Figure 1] FIG. 1 is a simplified schematic diagram of a first embodiment of the device described. [Diagram 2] FIG. 2 is a simplified schematic diagram of a second embodiment of the device described. [Diagram 3] 4 is a flow chart illustrating one embodiment of the described method. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0045] For a better integration of the invention and embodiments into a technical environment, the following cited descriptions of similar methods and / or apparatuses are also helpful.

[0046] The above-mentioned US Pat. No. 6,299,366 describes an apparatus comprising a number of material processing assemblies connected in parallel to an HF signal generator. The material processing assemblies can be, for example, chambers for plasma-assisted coating of workpieces (PVD or PACVD chambers). The US Pat. No. 6,299,366 discloses that each material processing assembly has a measurement device for measuring the respective reflected power. Depending on the measured reflected power, the operating frequency of the HF supply signal is varied until a defined threshold value is reached, which is one or two different for all material processing assemblies. In this connection, the US Pat. No. 6,299,366 discloses that the frequency of the HF supply signal influences the reflected power, and describes a total of six different operating modes in which the frequency and / or power of the HF supply signal is varied until a defined threshold value is reached.

[0047] US Patent No. 5,399,633 discloses a method and apparatus for generating plasma using an HF signal, in which the operating frequency of the HF signal generator is varied depending on the impedance change of the plasma load and the resulting reflections. In this document, the optimum operating frequency of the HF signal generator is determined by dividing the total available bandwidth of the generator into 16 sub-bands. During operation, the HF signal generator needs to operate at the selected operating frequency 99% of the time. In the remaining 1% of the time, it needs to check each of the 16 sub-bands in turn for a short period of time to see if there is a more suitable operating frequency.

[0048] Patent document 3 describes a method for adapting the output impedance of a high frequency power supply assembly to the impedance of a plasma load. The operating frequency of an HF signal generator is monitored to see if it is within a predetermined frequency range. Adjustment is achieved by mechanically and / or electrically modifying the circuitry downstream of the HF signal generator.

[0049] In Non-Patent Document 1, it is disclosed that the HF supply signal of the HF generator for the plasma load is constantly frequency modulated in order to find the optimum operating frequency during operation.

[0050] In figure 1 an embodiment of the described apparatus is given the reference number 10. This apparatus 10 is used for heating a gaseous medium 12 in a plasma chamber 14 in which, for example, a workpiece (not shown here) is etched and / or coated with the plasma. In principle, however, the described method and the described apparatus can also be used for heating solid and / or liquid media, as is well known in domestic microwave ovens.

[0051] The apparatus 10 comprises an HF signal generator 16 which generates an HF feed signal 18 which is coupled to the medium 12 via a transmission path 20. In the simplest case, an electrode (not shown here) is arranged in the plasma chamber 14, via which the HF feed signal 18, for example in the form of an electromagnetic wave, is coupled to the medium 12.

[0052] In this embodiment, the HF signal generator 16 comprises a voltage controlled oscillator 22 (VCO) controlled via a microprocessor 24 (μC) to generate a radio frequency signal 23 having an operating frequency that depends on a control voltage received from the microprocessor 24. Alternatively, or in addition to the microprocessor 24, the HF signal generator 16 may comprise a microcontroller and / or other control circuitry such as one or more ASICs, FPGAs, etc. In some embodiments, the operating frequency may be in the range of 2.4 GHz to 2.5 GHz.

[0053] The high frequency signal 23 is fed to a splitter 26, which in this embodiment splits the high frequency signal 23 into a number of parallel signal paths Nx, which are shown as vertically arranged dots in FIG. 1. For clarity, only a single signal path is shown here. In a preferred embodiment, each parallel signal path Nx has the same structure and the same elements. In particular, each parallel signal path Nx includes a signal amplifier 28, which amplifies a corresponding signal component of the high frequency signal 23 in the signal path Nx. Furthermore, each parallel signal path Nx includes a circulator 30 and, in this embodiment, a directional coupler 32 connected upstream of the circulator 30 in the forward direction toward the plasma chamber 14.

[0054] The amplified signal parts of the high frequency signal 23 in the parallel signal paths Nx are combined in this embodiment using a combiner 34. Preferably, the amplified signal parts of the high frequency signal 23 are combined in the correct phase in the combiner 34. The combiner 34 thereby generates a high power HF feed signal 18 from the individually amplified signal parts of the high frequency signal 23. The division of the high frequency signal 23 into the 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 the preferred embodiment. In principle, however, the described method and the corresponding device can also be realized with a single signal amplification path.

[0055] The directional coupler 32 in each signal path Nx is configured in this embodiment in a manner known per se to split off a small signal portion of the HF feed signal 18 going to the plasma chamber 14 and feed it to the microprocessor 24 via a rectifier 36a. The circulator 30 in each signal path Nx is configured in this embodiment to direct the signal reflection (typically in the form of a return wave, not shown here). In this embodiment, the circulator 30 in each signal path Nx further directs the small portion of the return signal reflection to the microprocessor 24 via a rectifier 36b.

[0056] The circulator 30 of each signal path Nx, in this embodiment, serves, on the one hand, to redirect return signal reflections away from the amplifier 28, the oscillator 22, and other sensitive components of the HF signal generator 16. Additionally, the circulator 30 of each signal path Nx acts similarly to a directional coupler 32 by providing a small portion of the return signal reflections to the microprocessor 24 for measurement.

[0057] The microprocessor 24 may in a preferred embodiment include one or more analog / digital converters (not shown here) or may be combined with one or more upstream analog / digital converters (not shown here). The analog / digital converter(s) are advantageously configured to convert the signal portion of the outgoing power separated via the directional coupler 32 and the return power derived by the circulator 30 into digital values, thereby making them available for digital signal processing in the microprocessor 24.

[0058] During operation of the device 10, the microprocessor 24 uses the power received from the parallel circulator 30 to determine respective HF signal reflections on the transmission path 20 and varies the operating frequency of the voltage controlled oscillator 22 in dependence on the HF signal reflections in order to reduce the HF signal reflections on the transmission path 20. The microprocessor 24 may implement a method as shown in simplified form in FIG.

[0059] In some embodiments, an optional step 50 may be performed first, in which an available frequency band is sequentially scanned using an HF test signal, which may be generated by microprocessor 24 using oscillator 22, to find a first operating frequency that is advantageous for heating medium 12. Alternatively, a nominal first operating frequency may be selected based on theoretical considerations and / or based on practical experience.

[0060] Then, according to step 52, an HF supply signal is generated according to a first operating frequency and, according to step 54, is coupled to the medium. According to step 56, a first HF signal reflection on the transmission path 20 is determined. This determination can advantageously be performed according to FIG. 1 by using the circulators 30 in each signal path Nx and by using the microprocessor 24 or by using another, if necessary, analog power measurement device (not shown here). According to step 58, the operating frequency of the HF supply signal is increased during a first test period by a defined frequency value fstep, and according to step 60, a second HF signal reflection on the transmission path 20 is determined. The second HF signal reflection correlates in time with the operating frequency of the HF supply signal increased by fstep. According to step 62, the operating frequency of the HF supply signal is then lowered during a second test period, here by twice the defined frequency value fstep, thereby lowering the operating frequency of the HF supply signal in the second test period by a second frequency value, here corresponding to the first frequency value fstep. A step 64 determines a third HF signal reflection on the transmission path 20, which third HF signal reflection correlates in time with the reduced operating frequency of the HF supply signal. A step 66 determines which of the three resulting signal reflections was the smallest, and a step 68 sets the future operating frequency of the HF supply signal to that (second) operating frequency that correlates with the smallest signal reflection from the three operating frequencies: "first operating frequency, increased first operating frequency, reduced first operating frequency."

[0061] In a preferred embodiment, the set second operating frequency serves as the new first operating frequency by loop 70, i.e., steps 54 through 68 are repeated periodically while the HF feed signal is coupled to the medium.

[0062] In some embodiments, step 50 can furthermore be repeated periodically by a loop 72, the periodic time of which is advantageously selected to be greater than the periodic time of loop 70. In particular, the global scan of the frequency band by step 50 can be repeated with a periodic time which is 10, 100 or even 1000 times the periodic time of loop 70.

[0063] 2 shows a further embodiment of a device 10' in which the method described can be advantageously implemented, the same reference numbers denote the same elements as above.

[0064] In the embodiment shown in Fig. 2, a directional coupler 32' is arranged at the output of the combiner 34, which is common to several or all of the parallel signal paths Nx. This allows the directional couplers 32 in the parallel signal paths Nx from Fig. 1 to be omitted. The directional couplers 32' are advantageously arranged to separate a part of the outgoing power and a part of the returning power for the determination of the HF signal reflection. The embodiment according to Fig. 2 therefore makes do with a smaller number of directional couplers. The circulators 30 of the parallel signal paths Nx are used in this embodiment only to divert the remaining signal reflections to so-called dummy loads (not shown here). Dummy loads are loads that are mainly used to absorb the power of the signal reflections and convert them into heat ("dispose" of them). [Explanation of symbols]

[0065] 10, 10' device 12 Medium 14 Plasma chamber 16, 16' HF Generator 18 HF supply signal 20 Transmission Path 22 Voltage Controlled Oscillator 23 High Frequency Signals (HF Signals) 24 Microprocessors 26 Splitter 28 Signal Amplifier 30 Circulator 32, 32' directional coupler 34 Combiner 36a, 36b rectifier 50. Determination of Nominal First Operating Frequency 52 Method Steps 54 Method Steps 56 Method Steps 58 Method Steps 60 Method Steps 62 Method Steps 64 Method Steps 66 Method Steps 68 Method Steps 70 Loops 72 Loop Nx parallel signal paths T period time

Claims

1. 1. A method for heating a medium (12), in particular for generating a plasma, using an HF signal, comprising the steps of: Generating (52) an HF supply signal (18) having a defined first operating frequency and a defined first signal power; coupling (54) the HF supply signal (18) to the medium (12) via a transmission path (20) such that the medium (12) is heated by the HF supply signal (18); determining (56) a first HF signal reflection along said transmission path (20); and varying (68) the first operating frequency in dependence on the first HF signal reflection to reduce a subsequent HF signal reflection in time, modifying (58) said defined first operating frequency by a defined first frequency value during a first test period in order to couple a first HF supply signal into said medium (12) in a time-limited manner with said modified, in particular increased, first operating frequency, inversely varying, in particular decreasing (62), said defined first operating frequency by a defined second frequency value during a second test period in order to couple said first HF supply signal into said medium (12) in a time-limited manner with said inversely varied, in particular decreasing, first operating frequency, determining (60) a second HF signal reflection during the first test period, the second HF signal reflection being correlated in time with the altered, in particular increased, first operating frequency; determining (64) a third HF signal reflection during a second test period, the third HF signal reflection being correlated in time with the inversely altered, in particular the lowered, first operating frequency; generating (68) the HF supply signal at a second prescribed operating frequency after the second test period and coupling to the medium (12); selecting (66) said defined second operating frequency in dependence on said first, second and third HF signal reflections.

2. cyclically repeating (70) each increase and decrease in operating frequency with respective further test periods; 2. The method of claim 1, wherein said defined second operating frequency is used as a new first operating frequency after each period.

3. 3. A method according to claim 1 or 2, characterized in that each increase and decrease in the operating frequency is performed periodically with a periodic time (T) lying in the range of 1 ms to 500 ms.

4. The method according to any one of claims 1 to 3, characterized in that the first test period has a duration in the range of 50 μs to 500 μs.

5. 5. The method according to claim 1, characterized in that the defined first operating frequency is abruptly increased by the defined first frequency value at the start of the first test period.

6. Method according to any one of the preceding claims, characterized in that the defined first frequency value and the defined second frequency value are identical.

7. A method according to any one of the preceding claims, characterized in that the defined first frequency value is in the range 0.0001% to 0.001% of the defined first operating frequency.

8. A method according to any one of claims 1 to 7, characterised in that the second test period immediately follows the first test period.

9. determining the first predefined operating frequency by transmitting (50) an HF test signal having a current operating frequency over a predefined frequency band through the transmission path; A method according to any one of the preceding claims, characterised in that the HF signal reflections along the transmission path (20) are determined.

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

11. 10. The method of claim 9, wherein the HF supply signal (18) forms the HF test signal.

12. Method according to any one of claims 9 to 11, characterized in that the HF test signal is generated in time before the first test period.

13. 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 period has elapsed.

14. A device for heating a medium (12), in particular for generating a plasma, using an HF signal, comprising: an HF generator (16, 16') configured to generate an HF supply signal (18) having a defined first operating frequency and a defined first signal power; a transmission path (20) configured to couple the HF supply signal (18) to the medium (12) such that the medium (12) can be heated by the HF supply signal (18); a measurement and control device (24, 32, 36) configured to determine a first HF signal reflection along the transmission path (20) and to vary the first operating frequency in dependence on the first HF signal reflection in order to reduce subsequent HF signal reflections in time, The measurement and control device (24, 32, 36) further comprises: said first operating frequency being varied, in particular increased (58), by a defined first frequency value during a first test period in order to couple a first HF supply signal into said medium (12) in a time-limited manner with said varied, in particular increased, first operating frequency, inversely varying, in particular decreasing (62) said defined first operating frequency by a defined second frequency value during a second test period in order to couple said first HF supply signal to said medium (12) in a time-limited manner with said inversely varied, in particular decreased, first operating frequency, determining (60) a second HF signal reflection during said first test period, said second HF signal reflection being correlated in time with said altered, in particular increased, first operating frequency; determining (64) a third HF signal reflection during a second test period, this third HF signal reflection being correlated in time with the inversely altered, in particular lowered, first operating frequency; configured to generate (68) the HF supply signal at a second defined operating frequency and couple to the medium (12) after the second test period has elapsed; wherein the defined second operating frequency is selected (66) dependent on the first, second and third HF signal reflections.

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