SAMPLING A COMPOSITE SIGNAL RELATING TO A PLASMA PROCESS

Abstract

Methods and devices for sampling a composite signal relating to a plasma process are provided. The composite signal has at least one plasma signal of interest and superimposed by at least one interfering signal. The methods include identifying the at least one interfering signal, digitalizing the composite signal by sampling the composite signal at a sampling frequency, and varying the sampling frequency during an operation of the plasma process according to at least one of a frequency of the at least one plasma signal of interest and a frequency of the at least one interfering signal.

Claims

1. A method of sampling a composite signal associated with a plasma process, wherein the composite signal comprises at least one plasma signal of interest on which at least one interfering signal is superimposed, the method comprising: identifying at least one interfering signal; digitizing the composite signal by sampling the composite signal at a sampling frequency; and varying the sampling frequency during operation of the plasma process in dependence on at least one of (i) a frequency of the at least one plasma signal of interest and (ii) a frequency of the at least one interfering signal.

2. The method of claim 1, further comprising: identifying a plurality of plasma signals of interest at the same sampling frequency based on a result of varying the sampling frequency.

3. The method of claim 1, further comprising: identifying at least one of (i) the at least one plasma signal of interest and (ii) the at least one interfering signal in a Nyquist zone that is higher than a frequency range up to a half of the sampling frequency.

4. The method of claim 1, further comprising: determining a digital representation of the frequency of the at least one plasma signal of interest and the frequency of the at least one interfering signal.

5. The method of claim 4, wherein the digital representation is determined by calculation.

6. The method of claim 1, further comprising: identifying respective digital representations of the frequencies of the at least one plasma signal of interest and of the at least one interfering signal by at least one frequency sweep of the sampling frequency at a start of operation of the plasma process.

7. The method of claim 1, further comprising: identifying respective digital representations of the frequencies of the at least one plasma signal of interest and of the at least one interfering signal by repeated frequency sweeps of the sampling frequency during operation of the plasma process.

8. The method of claim 1, further comprising: identifying respective digital representations of the frequencies of the at least one plasma signal of interest and of the at least one interfering signal by modulating the sampling frequency.

9. The method of claim 1, further comprising: identifying respective digital representations of the frequencies of the at least one plasma signal of interest and of the at least one interfering signal by frequency or amplitude modulation of a high-frequency power signal exciting the plasma process.

10. The method of claim 1, further comprising: tracking the sampling frequency uniformly with the frequency of the at least one interfering signal.

11. The method of claim 1, further comprising: tracking the sampling frequency uniformly with the same ratio with a frequency of a high-frequency power signal that excites the plasma process.

12. The method of claim 1, wherein digitizing the composite signal comprises using a single analogue/digital (A/D) convertor.

13. The method of claim 1, further comprising: filtering the composite signal prior to sampling the composite signal.

14. The method of claim 1, wherein varying the sampling frequency comprises changing the sampling frequency by a voltage-controlled oscillator (VCO) or a direct digital synthesizer (DDS).

15. The method of claim 1, further comprising: monitoring a frequency shift of the at least one interfering signal when the sampling frequency is varied; and identifying the at least one interfering signal based on a result of monitoring the frequency shift.

16. The method of claim 1, further comprising: identifying the at least one plasma signal of interest and the at least one interfering signal based on a result of varying the sampling frequency; and selecting a particular sampling frequency for sampling the composite signal at which the at least one plasma signal of interest can be detected without the superimposition of the at least one interfering signal.

17. The method of claim 1, wherein the at least one plasma signal of interest comprises a signal that occurs during the plasma process and is to be determined for analyzing or controlling the plasma process.

18. A device for sampling a composite signal associated with a plasma process, wherein the composite signal comprises at least one plasma signal of interest on which the at least one interfering signal is superimposed, the device comprising: a digital signal processor configured to identify and analyze at least one interfering signal; a sampling frequency generator connected to the digital signal processor and configured to: generate a sampling frequency for the composite signal based on a result of analyzing the at least one interfering signal, and vary the sampling frequency during an operation of the plasma process in dependence on at least one of a frequency of the at least one plasma signal of interest and a frequency of the at least one interfering signal; and an analogue/digital (A/D) converter connected to the digital signal processor and the sampling frequency generator and configured to: receive the sampling frequency, and digitize the composite signal by sampling the composite signal at the sampling frequency.

19. The device of claim 18, wherein the sampling frequency generator comprises an input for specifying a frequency of the plasma signal of interest.

20. The device of claim 18, wherein the sampling frequency generator comprises an input for specifying a frequency of the interfering signal.

Description

DETAILED DESCRIPTION

[0049] FIG. 1 shows a spectrum of a frequency range of 0 to 50 MHz for different sampling rates (fs) in a range between 0 and 100 MHz. In the embodiment shown, a plasma process was excited by a high-frequency power signal having a fundamental frequency of 13.56 MHz. The first plasma signal of interest is denoted by reference numeral 1. Since the fundamental frequency can be slightly varied around the nominal value thereof, the reference numeral 1 at the frequency of 13.56 MHz is not just a line, but rather a band.

[0050] Further signals relating to the plasma process are signals at harmonics of the high-frequency power signal having the fundamental frequency of 13.56 MHz, e.g., harmonic signals. The first harmonic is at approximately 27 MHz. This is denoted by reference numeral 2. The second harmonic is slightly above 40 MHz and is provided with reference numeral 3. The signals having reference numerals 1, 2, and 3 are, for example, plasma signals of interest. If a sampling frequency of between approximately 85 and 90 MHz is selected, the signals can be unambiguously identified. This is because the Nyquist criterion is fulfilled for these signals, e.g., the sampling rate is more than twice the highest frequency to be sampled. The Nyquist criterion corresponds to the line provided with reference numeral 4. The signal represented by reference numeral 3 is the alias of the signal represented by reference numeral 3 from the second Nyquist zone. The fifth harmonic of the fundamental frequency is at 81.36 MHz, and the signal shown, having reference numeral 8, is the alias thereof from the third Nyquist zone.

[0051] If a lower sampling frequency is to be selected, e.g., a frequency in the region of 60 MHz, it is possible to monitor the fundamental frequency at 13.56 MHz and the first harmonic at 27.12 MHz, but it would not be possible to monitor the second harmonic at 40.68 MHz, which in this case appears as the alias represented by reference numeral 3 from the second Nyquist zone at approximately 20 MHz, because the alias of the fifth harmonic (represented by reference numeral 8) is superimposed on the alias in the region shown by reference numeral 5. What is known as a distorted plasma signal of interest is formed here for the second harmonic of 40.68 MHz (alias 3). The Nyquist mirror image of the second harmonic 3 can thus no longer be unambiguously resolved.

[0052] In the following, the region 10, which is shown enlarged in FIG. 2A, will be explained in greater detail. It can be seen in FIG. 2A that the region 10 contains the plasma signal of interest 1 at the excitation frequency or fundamental frequency of 13.56 MHz. Moreover, the region contains a signal 11 that is the alias of the first harmonic 2 at the frequency of approximately 27 MHz, as well as the signal 3 that is the alias of the signal 3 at the frequency of slightly above 40 MHz. The signals 3, 11 occur because the sampling frequency is selected to be less than twice the frequencies of the signals 2, 3, and therefore mirroring occurs at half of the sampling frequency. Moreover, at the frequency of 13.56 MHz, an interfering signal 7 is superimposed on the signal 1, which interfering signal is the alias of the fifth harmonic of 81.36 MHz from the 4th Nyquist zone thereof. The signals 3, 11 are also interfering signals for the signal 1. The signal 1, and likewise the signals of the harmonics, have a particular width since, to control the plasma process, it is possible for the frequency of the high-frequency power signal used for exciting the plasma process to vary within certain limits.

[0053] The way in which an interfering signal can be determined on the basis of a spectrum produced in the digital representation and that is obtained at a particular sampling frequency, as shown in FIG. 2A, will be explained with reference to FIG. 2B, where As represents an amplitude of a signal. The signal 1 is free of interference at the sampling frequencies fs1, fs2. The interference components 7a, 7b are next to the plasma signal of interest 1. In the case of the sampling frequency fs3, however, the interfering signal 7c is superimposed on the plasma signal of interest 1. The original frequency or the original frequency range, and thus the Nyquist zone, of the interfering signal 7c, can thus be reconstructed by monitoring the frequency shift. It is thus possible to predict the behavior of the interfering signal 7 when the sampling rate fs is changed, and to prevent superimposition on the plasma signal of interest 1.

[0054] Alternatively, according to FIG. 3, a frequency sweep of the sampling frequency can be carried out. For example, in the detail 10 shown, the sampling frequency can be varied between 40 MHz and 55 MHz. In this method, instead of a spectrum, just a single frequency or a frequency band is monitored, which simplifies the digital signal processing. In this case, the interfering signals 20 are then detected by amplitude changes on the monitored frequency. This frequency sweep of the sampling frequency can take place at the start of the plasma process. Alternatively or in addition, the frequency sweep of the sampling frequency can be repeated.

[0055] FIG. 2A, 2B and 3 illustrate the identification of interfering signals.

[0056] FIG. 4 again shows the signal 1, i.e., the plasma signal of interest. If the sampling frequency is to be selected according to the dotted line 21 for example, the signal 1 could not be sampled free of interference. Interfering signals would always be superimposed on the signal 1. If, however, the interfering signals 22, 23, 24 are known, the sampling rate (or sampling frequency) can be varied according to line 25 for example. In some implementations, the rate or frequency can be varied uniformly with the frequency of the interfering signal 23. Alternatively, the sampling rate can be changed proportionally to the frequency variation of the signal 1, according to line 26. In this case, the sampling rate is changed in the same ratio as the frequency of the signal of interest 1, which signal remains, measured at the sampling rate fs, at the same relative position on the spectrum, and this simplifies the architecture of the digital signal processing. Thus, the signal 1 can be sampled in a region in which interfering signals are not superimposed thereon. To be able to appropriately select the sampling rate according to the lines 25, 26, the interfering signals or the frequencies of the interfering signals must be known. Therefore, according to the invention, the interfering signals are first identified. The manner in which the interfering signals are identified is shown in FIGS. 2A, 2B and 3 for example.

[0057] The block diagram in FIG. 5 shows a device 50 for carrying out the methods described herein. A composite signal to be sampled is fed to an A/D convertor 51. A sampling frequency is specified for the A/D convertor 51 by a sampling frequency generator 52, which may include a voltage-controlled oscillator (VCO) or a direct digital synthesizer (DDS). The sampling frequency generator 52 is, in turn, connected to a digital signal processor 53, which can identify interfering signals. On the basis of the interfering signal identified, the sampling frequency generator 52 can determine a corresponding sampling signal or a sampling frequency. Furthermore, the sampling frequency generator 52 includes an input 54 for specifying a frequency of a plasma signal of interest and an input 55 for specifying a frequency of an interfering signal.

[0058] Optionally, an analogue filter 56 can be provided for filtering a plasma signal of interest. In this case, the signal is filtered before it is fed to the A/D convertor 51.

OTHER EMBODIMENTS

[0059] A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.