Method and apparatus for heating a medium using an HF signal

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, generating an HF supply signal using an HF generator 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 the 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 the transmission path; and and a measurement and control device configured to change 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. 5,649,999.

[0004] The known device can be used to generate plasmas, particularly those used in etching and / or coating devices for the surface treatment of workpieces. In this context, plasma is understood to mean a gas that is excited by external energy absorption (heating), resulting in the gas's charge carriers being released from their respective atomic and / or molecular bonds and existing as free charge carriers. Plasma therefore has an electrical conductivity that depends on the number of free charge carriers and, therefore, their 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 rapidly 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 generator's output impedance matches the load's input impedance. Mismatches cause reflections, resulting in some, or in the worst case, all of the power not reaching the gas or plasma to be heated but being reflected back to the generator's output. Without appropriate countermeasures, this can lead to damage to the generator and / or other components of such devices. 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-described method 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] Known methods for adjusting the operating frequency of an HF signal generator used to heat a medium are relatively complex to implement. Furthermore, the search for the optimal operating frequency in each case takes a considerable amount of time in some implementations. Therefore, it is an object of the present invention to provide a method and device of the type mentioned at the beginning, which allows a suitable operating frequency of an HF signal generator to be set relatively easily and quickly. [Means for solving the problem]

[0009] According to one 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 apparatus are set out in the dependent claims and the description. Therefore, 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 a first HF supply signal into the medium in a time-limited manner with the changed, in particular increased, first operating frequency, and 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 the 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 is time-correlated with the altered, particularly increased, first operating frequency, and during the second test period, a third HF signal reflection is determined, the third HF signal reflection being time-correlated with the reversely altered, particularly lowered, first operating frequency; and an HF feed signal is generated at the specified second operating frequency after the second test period has elapsed and coupled to the medium, wherein the specified second operating frequency is selected in dependence on the first, second and third HF signal reflections, particularly from the first operating frequency, the reversely altered, particularly lowered, altered first operating frequency, and the altered, particularly 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 further modifies, 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 the modified, in particular increased, first operating frequency, and further modifies, inversely, modifies, 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 the reversely modified, in particular decreased, first operating frequency, and further determines a second HF signal reflection during the first test period. the second HF signal reflection is time-correlated with the altered, particularly increased, first operating frequency; and during a second test period, a third HF signal reflection is determined, the third HF signal reflection being time-correlated with the reversely altered, particularly lowered, first operating frequency; and after the second test period, an HF feed signal is generated and coupled to the medium at a specified second operating frequency, the specified second operating frequency being selected, in particular, from the first operating frequency, the altered, particularly increased, first operating frequency, and the reversely altered, particularly lowered, first operating frequency, depending on the first, second, and third HF signal reflections.

[0011] The above apparatus and method can be implemented very easily, especially when the HF signal generator is a so-called solid-state generator having 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, particularly 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 an increased first operating frequency and a decreased first operating frequency in two successive test periods while the HF supply signal continues to be coupled to the medium. The temporal order of the first and second test periods is, in principle, freely selectable. This means that in some embodiments, the specified first operating frequency can be increased first and then decreased in time. In contrast, in other embodiments, the specified first operating frequency can be decreased first and then increased in time. Therefore, the references to a first test period and a second test period do not imply a required temporal order herein. In other words, the second test period can 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 and decreased at least once during the test period. In a preferred embodiment, the current operating frequency is increased exactly once during the test period and decreased exactly once in time, either before or after. For each of the three operating frequencies thus obtained (current first operating frequency, increased first operating frequency, and decreased first operating frequency), an associated or correlable HF signal reflection along the transmission path is determined. Thereby, after such testing, there are at least three characteristic values ​​of HF signal reflection. Advantageously, the operating frequency correlated with the smallest HF signal reflection of the at least three detected HF signal reflections can be 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 apparatus 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, 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 apparatus search "on both sides" for an operating frequency that is better than the current first operating frequency. This allows the method and apparatus to follow dynamically changing impedance changes quickly and regardless of direction, i.e., in the direction of higher impedances and in the direction of lower impedances.

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

[0017] In a preferred embodiment of the present 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 apparatus 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 accommodate the changing impedance, respectively, thereby providing heating power to the medium in a highly efficient manner over the long term.

[0019] In a further embodiment, each operating frequency is increased and decreased periodically with a period (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 ​​also prove 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 enabling rapid 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 performed with a smooth transition rather than an abrupt one, which may have the advantage in some scenarios of more stable plasma excitation. However, in many cases, abrupt increases or decreases are considered more advantageous for minimal test times.

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

[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 one 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 1 / 10 of a mille relative to the nominal operating frequency of the HF signal generator is highly advantageous, on the one hand, to ensure stable plasma generation and, on the other hand, to respond appropriately to impedance changes of the plasma load.

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

[0030] In this embodiment, the second test period immediately precedes or follows the first test period. This results in the current operating frequency of the HF signal generator being set as quickly as possible to a frequency value below (above) the original first operating frequency after a time-limited increase (decrease). In an embodiment in which 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 in which the specified first operating frequency increases abruptly by the specified first frequency value at the start of the first test period and decreases abruptly at the start of the second test period, the current operating frequency abruptly fluctuates by twice that frequency value when transitioning from the first test period to the second test period (or vice versa). This embodiment advantageously contributes to keeping the plasma excitation very stable throughout the test periods.

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

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

[0033] This embodiment complements the above method to some extent by a "global scan", i.e., a preliminary and / or intermediate search for the optimum operating frequency across a defined frequency band, which contributes very advantageously to finding the optimum operating frequency across 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 feed signal and transmitted over the transmission path. Advantageously, the HF test signal can be transmitted over the transmission path in addition to, i.e., parallel in time to, the HF feed signal in some variations of this embodiment. Alternatively, the HF test signal can be transmitted over the transmission path separately in time from the HF feed signal in other embodiments.

[0035] This embodiment allows for testing signal reflections independently of the HF feed signal, and thereby independent of heating of the medium over a wide frequency range. This embodiment therefore allows for 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 feed signal and, in this way, combining it with the HF feed signal.

[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, as a starting point for the described method, a "global scan" is performed before heating or at the start of heating to obtain the optimum starting operating frequency. This embodiment is very simple to implement and allows for an optimum 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 optimum 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] An embodiment of the invention is illustrated in the drawings and explained in more detail in the following description. [Brief explanation of the drawings]

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

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

[0046] The aforementioned Patent Document 1 describes an apparatus comprising multiple material processing assemblies connected in parallel to an HF signal generator. The material processing assemblies may be, for example, chambers for plasma-assisted coating of workpieces (PVD or PACVD chambers). Patent Document 1 discloses that each material processing assembly has a measurement device that measures its respective reflected power. Depending on the measured reflected power, the operating frequency of the HF supply signal is varied until it reaches one or two different, defined thresholds for all material processing assemblies. In this regard, Patent Document 1 discloses that the frequency of the HF supply signal affects 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 it reaches a defined threshold.

[0047] Patent Document 2 discloses a method and apparatus for generating plasma using an HF signal, in which the operating frequency of the HF signal generator varies depending on the impedance change of the plasma load and the resulting reflection. In this document, the optimal operating frequency of the HF signal generator is determined by dividing the generator's total available bandwidth into 16 subbands. During operation, the HF signal generator must operate at the selected operating frequency 99% of the time. For the remaining 1% of the time, it must check each of the 16 subbands in turn for a short period of time to see if a more suitable operating frequency is available.

[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 ensure that 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] Non-Patent Document 1 discloses 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, one embodiment of the described apparatus is designated by the reference number 10. This apparatus 10 is used to heat 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 through which the HF feed signal 18 is coupled to the medium 12, for example in the form of an electromagnetic wave.

[0052] In this embodiment, the HF signal generator 16 includes a voltage controlled oscillator 22 (VCO) controlled via a microprocessor 24 (μC) to generate a high 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 include 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] In this embodiment, the RF signal 23 is fed to a splitter 26, which splits the RF signal 23 into multiple parallel signal paths Nx, as shown by vertically arranged dots in FIG. 1 . For clarity, only a single signal path is shown. In a preferred embodiment, each parallel signal path Nx has the same structure and components. In particular, each parallel signal path Nx includes a signal amplifier 28 that amplifies a corresponding signal component of the RF signal 23 in the signal path Nx. Additionally, 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 portions 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 portions of the high frequency signal 23 are combined in phase in the combiner 34. The combiner 34 thereby generates a high power HF feed signal 18 from the individually amplified signal portions of the high frequency signal 23. The division of the high frequency signal 23 into the parallel signal paths Nx, each having at least one signal amplifier 28 and subsequent combination in the combiner 34, facilitates high power amplification of the original high frequency signal 23 in the preferred embodiment. However, in principle, the described method and corresponding apparatus can also be implemented 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 destined for the plasma chamber 14 and feed it via a rectifier 36a to the microprocessor 24. 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 also 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 direct return signal reflections away from the amplifier 28, 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] Microprocessor 24 may, in a preferred embodiment, include one or more analog-to-digital converters (not shown here) or may be combined with one or more upstream analog-to-digital converters (not shown here), which are advantageously configured to convert the signal portions of the outgoing power decoupled via directional coupler 32 and the return power derived by circulator 30 into digital values, thereby making them available for digital signal processing in microprocessor 24.

[0058] During operation of device 10, microprocessor 24 uses the power received from parallel circulator 30 to determine each HF signal reflection on transmission path 20 and varies the operating frequency of voltage controlled oscillator 22 depending on the HF signal reflection in order to reduce the HF signal reflection on transmission path 20. Microprocessor 24 can implement the method shown in simplified form in FIG.

[0059] In some embodiments, optional step 50 may be performed first, in which an available frequency band is sequentially scanned using an HF test signal that 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 practical experience.

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

[0061] In the preferred embodiment, the established 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 further be repeated periodically by a loop 72, the period of which is advantageously selected to be greater than the period of loop 70. In particular, the global scan of the frequency band by step 50 can be repeated with a period that is 10, 100, or even 1000 times the period of loop 70.

[0063] 2 shows a further embodiment of an apparatus 10' in which the described method can be advantageously implemented, with the same reference numerals indicating the same elements as above.

[0064] In the embodiment shown in FIG. 2, a directional coupler 32′ common to several or all of the parallel signal paths Nx is arranged at the output of the combiner 34. This allows the directional coupler 32 in the parallel signal paths Nx from FIG. 1 to be omitted. The directional coupler 32′ is advantageously configured to separate a portion of the outgoing power from a portion of the returning power for determining HF signal reflections. Therefore, the embodiment according to FIG. 2 requires a smaller number of directional couplers. The circulators 30 in the parallel signal paths Nx are used in this embodiment only to divert remaining signal reflections to so-called dummy loads (not shown here). Dummy loads are loads that are primarily used to absorb the power of 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 Nominal First Operating Frequency Determination Process 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 Loops Nx parallel signal paths T period time

Claims

1. 1. A method for generating a plasma using an HF signal, comprising: 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 a medium (12) via a transmission path (20) such that the medium (12) for generating plasma is heated by the HF supply signal (18); Deriving (56) a first HF signal reflection along the transmission path (20); and varying (68) the first operating frequency based on the first HF signal reflection to reduce a temporally subsequent HF signal reflection, increasing (58) the predetermined first operating frequency by a predetermined first frequency value during a first test period to couple a first HF supply signal into the medium (12) in a time-limited manner at the increased first operating frequency; reversely reducing (62) the prescribed first operating frequency by a prescribed second frequency value during a second test period to couple the first HF supply signal to the medium (12) in a time-limited manner at the reversely reduced first operating frequency; deriving (60) a second HF signal reflection during the first test period, the second HF signal reflection being correlated in time with the increased first operating frequency; deriving (64) a third HF signal reflection during a second test period, the third HF signal reflection being inversely correlated in time with the reduced first operating frequency; generating (68) the HF supply signal at a predetermined second operating frequency after the second test period has elapsed and coupling it to the medium (12); selecting (66) the defined second operating frequency based on the first, second, and third HF signal reflections.

2. cyclically repeating (70) each increase and decrease in operating frequency with a respective further test period; 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. 3. The method according to claim 1, wherein the first test period has a duration in the range of 50 μs to 500 μs.

5. 3. The method of claim 1, wherein the first predetermined operating frequency is increased abruptly by the first predetermined frequency value at the start of the first test period.

6. 3. The method according to claim 1, wherein the first predetermined frequency value and the second predetermined frequency value are identical.

7. 3. The method of claim 1, wherein the predetermined first frequency value is in the range of 0.0001% to 0.001% of the predetermined first operating frequency.

8. 3. The method of claim 1, wherein the second test period immediately follows the first test period.

9. determining the first predetermined operating frequency by transmitting (50) an HF test signal having a current operating frequency over a predetermined frequency band through the transmission path; 3. A method according to claim 1 or 2, characterized in that the HF signal reflection along the transmission path (20) is derived.

10. 10. The method of claim 9, wherein 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. 10. The method of claim 9, wherein the HF test signal is generated temporally before the first test period.

13. 10. The method of claim 9, further comprising generating (72) the HF test signal again after the second test period has elapsed.

14. 1. An apparatus 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) so that the HF supply signal (18) can heat the medium (12) to generate a plasma; a measurement and control device (24, 32, 36) configured to derive a first HF signal reflection along the transmission path (20) and to modify the first operating frequency based on the first HF signal reflection to reduce subsequent HF signal reflections in time; The measurement and control device (24, 32, 36) further comprises: increasing (58) the predetermined first operating frequency by a predetermined first frequency value during a first test period to couple a first HF supply signal to the medium (12) in a time-limited manner at the increased first operating frequency; and decreasing (62) the predetermined first operating frequency by a predetermined second frequency value during a second test period to couple the first HF supply signal to the medium (12) in a time-limited manner with the decreased first operating frequency. deriving (60) a second HF signal reflection during the first test period, wherein the second HF signal reflection is correlated in time with the increased first operating frequency; deriving (64) a third HF signal reflection during a second test period, wherein the third HF signal reflection is inversely correlated in time with the reduced first operating frequency; the HF supply signal is generated (68) at a predetermined second operating frequency and coupled to the medium (12) after the second test period has elapsed; The apparatus of claim 1, wherein the predetermined second operating frequency is selected (66) based on the first, second, and third HF signal reflections.

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