GB2478509A

GB2478509A - A method or system for identifying the resonance frequency of a probe resonating in plasma

Info

Publication number: GB2478509A
Application number: GB0822072A
Authority: GB
Inventors: Shantanu Kumar Karkari; Cezar Gaman; Albert Rogers Ellingboe
Original assignee: Dublin City University
Current assignee: Dublin City University
Priority date: 2008-12-03
Filing date: 2008-12-03
Publication date: 2011-09-14
Anticipated expiration: 2028-12-03
Also published as: GB2478509A8; GB0822072D0; GB2478509B

Abstract

A method of identifying the resonance frequency of a probe resonating in a plasma. The method comprising obtaining at a first time, a first response signal representative of a probe resonating in a plasma. Obtaining at a second later time, a second response signal representative of the probe resonating in a plasma, wherein a perturbation is applied while the second response signal is being obtained. The first and second response signals are then combined to eliminate common background noise. The perturbation applied can be to the plasma itself i.e. power, flow rate or pressure thereof, or to the probe e.g. variation in the voltage applied thereto.

Description

A method and system for identifying the resonance frequency of a probe resonating in plasma

Field of the Invention

The present invention relates to a method of identifying the resonance frequency of a probe resonating in plasma. The present invention more particularly relates to combining a pair of time differentiated response signals of the probe resonating in plasma for eliminating common background noise.

Background

Probes for measuring the electron density of plasma are known in the art.

One such prior art probe 100 is illustrated in Figure 1. The probe 100 is provided as a u-shaped transmission line shorted at one end and open at the opposite end thereof. The probe 100 as a result of its u-shaped configuration is commonly known in the art as a hairpin probe. The probe 100 has a characteristic resonance at a particular frequency which depends on the length of the probe 100 and the dielectric constant of the medium in which the probe is placed. Typically, use of the probe 100 is a twofold step; firstly the probe 100 is made to resonate in a vacuum and then secondly in the plasma of interest. It will be observed that the resonance frequency of the probe shifts when resonating in the plasma as compared to when resonating in the vacuum due to differences in the dielectric constants of the two mediums. Figure 2 shows a first signal fpiasma representative of the probes' response to a frequency signal applied to the probe while the probe is placed in plasma, and a second signal fvacuum representative of the probes' response to a frequency signal applied to the probe while the probe is placed in a vacuum. A difference signal is generated by combining the signals fpiasma and fvacuum for identifying the resonance frequency of the probe. The difference signal 115 has a positive peak 120 which corresponds to the resonance frequency of the probe in plasma and a negative peak 122 which corresponds to the resonance frequency of the probe in a vacuum. Figure 3 illustrates a difference signal obtained by subtracting fvacuum from fpiasma resulting in a single positive dominant peak so that the resonance frequency of the probe in plasma is readily identifiable.

However, the probe 100 interacts with other elements in its proximity such as metallic objects resulting in the generation of background noise. The background noise may provide additional peaks which may be dominant over the resonance peak of the probe which makes it difficult to identify the resonance frequency of the probe from the background noise. The graph of figure 4 includes background noise which is significantly higher than the resonance signal from the probe 100 due to the probes' 100 interaction with proximate metallic objects. Figure 4 shows the resonance frequency fvacuum to be -2.5 GHz and the resonance frequency fpiasma to be -3.75 GHz. The background noise provides a peak at -4.75 GHz which is dominant over fpiasma.

As can be seen in figure 5 if the plasma density is changed slightly the resonance peak of the probe 100 also shifts slightly to reflect the new plasma density. The background noise signal does not remain static over time and may drift. As a consequence, the difference signal in Figure 5 has a poor signal to noise ratio (s/n) which makes it near impossible to identify the resonance peak of fpiasma from the noise peaks.

The previous technique used by the applicants of the present invention for identifying the resonance peak associated with plasma relied on the subtraction of a signal representative of the probe 100 resonating in a vacuum from a signal representative of the probe 100 resonating in plasma. If the background is constant, then subtraction eliminates the common background noise while retaining the resonance peaks, as illustrated Figure 3. This technique is not reliable when the background signal drifts with time, and/or when the plasma density varies slightly.

There is therefore a need for a system and method of identifying the resonance frequency of a probe resonating in plasma.

Sum mary These and other problems are addressed by combining first and second time differentiated response signals of a probe resonating in plasma where a perturbation is applied during the second response signal.

Accordingly, a first embodiment of the invention provides a method as detailed in claim 1. The invention also provides a system as detailed in claim 28. Advantageous embodiments are provided in the dependent claims.

These and other features will be better understood with reference to the followings Figures which are provided to assist in an understanding of the teaching of the invention.

Brief Description Of The Drawings

The present invention will now be described with reference to the accompanying drawings in which: Figure 1 is a diagrammatic view of a prior art probe.

Figure 2 is a graph illustrating response signals of the probe of Figure 1 to microwave signals while the probe is in a vacuum and in plasma.

Figure 3 is a difference signal obtained by combing the response signals ofFigure2.

Figure 4 is a graph illustrating response signals of the probe of Figure 1 to microwave signals while the probe is in a vacuum and in plasma in the

presence of dominant background noise.

Figure 5 is a graph illustrating time differentiated response signals of a probe while the probe is in plasma according to the present invention, and a difference signal generated by combining these response signals.

Figure 6 is a diagrammatic view of a experimental set up for carrying out the method of identifying the resonance frequency of a probe resonating in plasma in accordance the teaching of the present invention.

Figure 7 is a diagrammatic view of the probe of Figure 6 immersed in plasma with different external biases applied to it.

Figure 8(a) is the response of the probe to a microwave signal at two different bias conditions.

Figure 8(b) is a difference signal obtained by combining the signals of Figure 8(a).

Detailed Description Of The Drawings

The invention will now be described with reference to some exemplary systems which are provided to assist in an understanding of the teaching of the invention.

Referring to Figures 6 to 8 of the drawings and initially to Figure 6 there is provided a system 200 for implementing a method of identifying the resonance peak associated with plasma. The system 200 comprises a probe 205 which is substantially similar to the prior art probe 100 illustrated in Figure 1. In accordance with the present teaching, the probe 205 may be excited on separate occasions in plasma; with each excitation time effecting a generation of a corresponding response signals fpiasmai and fplasma2. The present inventors have realised that by combining fpiasmai which is generated in the absence of perturbation from a controlled perturbation source with fpIasma2WhCh is generated in the presence of perturbation from a controlled perturbation source significantly reduces the dominance of background noise in a resulting difference signal. A perturbation is a secondary influence on the system 200 that causes it to deviate slightly.

The probe 205 may be provided in any one of a number of different configurations. In this exemplary arrangement, the probe is provided as a u-shaped transmission line comprising a pair of spaced apart transmission elements 207 shorted together at one end and open at the opposite end. A frequency generator 210 applies a stimulus in the form of a microwave signal to the probe 207 via a 500 co-axial transmission line 220. A loop-antenna 223 couples the inner and outer conductors of the 500 co-axial transmission line 220 together thereby providing a return path along which the microwave signal is reflected. The loop antenna 223 is electrically isolated from the probe 205 and from the plasma. The microwave signal from the frequency generator 210 is injected into the loop-antennae 223 via the 50 0 coaxial transmission line 220. A directional coupler 227 is located intermediate the frequency generator 210 and the co-axial transmission line 220 which directs the reflected microwave signal to be measured. A Schottky diode 230 receives the reflected microwave signal from the directional coupler 227 and converts the reflected microwave signal to a (negative) DC signal which is displayed on a measuring means, in this case, an oscilloscope 235. In this way the measuring means uses a frequency domain reflectrometry technique.

As the microwave signal sweeps through the frequency range the probe 205 will resonate at its resonance frequency, which is dependent on a number of factors including the nature of medium where the probe is located and the length of the probe 207. When the quarter wavelength of the drive frequency 2.5 -5.0 GHz from the frequency generator 210 equals the length of the probe 205, the probe 205 is driven into resonance. At this frequency, strong inductive coupling of the microwave signal from the loop antenna 223 into the probe 205 takes place. Therefore when the scanning frequency approaches the resonance frequency of the probe 205, significant microwave power is absorbed by the probe 205 in driving oscillatory current through the finite resistances of the wire of the probe 205 resulting in substantial reduction of the reflected microwave power at the resonant frequency. This frequency dependent loading of the loop antenna 205 results in a drop in reflected microwave power at the resonant frequency. The reflected microwave power may be measured on the oscilloscope 235, or some other means for capturing the resonant frequency.

In operation, the probe 205 is placed in plasma. During a first time period the frequency generator 210 conducts a first sweep through a predetermined frequency range, such as for example the frequency range 1.0-18.0GHz. This microwave signal is injected into the loop-antennae 223 and then effects a response in the probe 205 in the form of a response signal fpiasmai. It will be appreciated that fpiasmai is generated in the absence of perturbation from a controlled perturbation source. The probe 205 is electrically floating. During the first sweep when the scanning frequency of the microwave signal approaches the resonance frequency of the probe 205, significant power is coupled to the probe 205 by driving a large ac current through the probe 205. The reflected microwave signal during the first sweep is converted by the Schottky diode 230 into a first DC signal which is displayed on the oscilloscope 235.

During a second time period the frequency generator 210 conducts a second sweep through the frequency range 1.0-18.0GHz while the probe 205 is still in the plasma. The second microwave signal is injected into the loop-antennae 223 in a similar fashion to that described with reference to the first sweep. This injected microwave signal into the loop-antennae 223 effects a response in the probe 205 in the form of a response signal fplasma2 which is generated in the presence of perturbation from a controlled perturbation source.

The perturbation source, namely, a voltage source applies a negative DC voltage in the range of --20 to -30 Volts to the probe 205 during the second sweep. Preferably the voltage source applies a negative DC voltage in the range of --15 to -20 Volts to the probe 205 during the second sweep. The measuring means combines fpiasmai and fplasma2by subtracting fplasma2frOm fpiasmai which results in a difference signal. The response signal fpiasmai is illustrated by the continuous line in Figure 8a, and the response signal fplasma2 is illustrated by the broken line in Figure 8a. Figure 8b represents the difference signal as a result of subtracting fplasma2frOmfplasmal which substantially eliminates common back ground noise present in both fpiasmai and fplasma2 which results in the resonance peaks of the probe being highly pronounced. Thus, the difference signal has a good signal to noise ratio.

When the probe 205 is immersed in plasma, the positive (ions) and negative charges (electrons or negative ions) re-organizes themselves around the probe 205 in such a manner that the probe 205 repels the negative charge and attracts the positive charge to its surface. A region of positive space charge called a "sheath" is formed around the transmission elements 207 of the probe as illustrated in Figure 7. The dimensions of the sheath is negligible as compared with the separation between the transmission elements 207. The sheath has a dielectric constant which is the same as that of a vacuum Evacuum.

When an external negative bias is applied to the probe 205, the electron free sheath region grows in volume and therefore influencing the effective dielectric constant between the transmission elements 207. This results in shifting the resonance frequency towards a lower frequency. The plasma region between the transmission elements 207 is reduced as the sheath thickness increases due to the exclusion of plasma electrons with the increase in negative bias. At a certain negative potential the sheath around the transmission elements 207 will overlap, hence the effective dielectric constant equals to that of vacuum. At this condition the probe measures the vacuum resonance frequency given by f=cI4L, where: f is the resonance frequency, c is the speed of light (3 x 108 m/s) L is the length of the transmission elements.

Therefore it is possible to emulate a response signal of a probe resonating in a vacuum while the probe is located in plasma by controlling the sheath through a negative voltage. Thus, fplasma2 in the present invention is equivalent to fvacuum of the prior art technique. As the probe is maintained in the plasma while fpiasmai and fplasma2 are being generated it will experience common background noise. This is not the case in the prior art when the probe is moved from a vacuum to a plasma, the background noise will also change. When fplasma2 is subtracted from fpiasmai the common background noise is substantially eliminated resulting in two dominant peaks one positive and one negative. The dominant positive peak represents the resonance frequency of the probe in plasma while the dominant negative peak represents the resonance frequency of the probe in a vacuum.

In an alternative arrangement the electron density of the plasma in which the probe is located is varied as a function of time. In the previous arrangement the electron density of the plasma was in a steady state, in this arrangement, the electron density of the plasma may be pulsed resulting in a time varying state. In operation, initially the plasma is in a steady state condition and during a first time period the frequency generator 210 applies a microwave signal to the probe 205, generating a first response signal fpiasmai. Also, during the steady state condition for a second time period, the frequency of the microwave signal is scanned generating the response signal fplasma2at a second instant. The perturbation is applied to the plasma during the second time period by changing the power coupled to the plasma or by changing the pressure, flow rates, or by introducing a trace gas to the plasma which causes the electron density of the plasma to vary which contributes to a shift in the resonance peak.

The measuring means subtracts fplasma2 from fpiasmai which results in a difference signal with two dominant resonance peaks representative of the plasma during the first and second time periods, respectively. This technique substantially eliminates the background noise by relative subtraction of the reflected signal at two states of the plasma having slightly different electron densities. The advantage is that no perturbation is applied to the probe is required.

In addition an experimental setup with repetitive pulsed plasma may be used, where the plasma is produced by a train of rectangular DC or radio-frequency power which is ON and OFF for specific times during the pulse. This set-up can be simply applied for finding the resonance peak against the background noise. In this set-up the microwave frequency generator applies a constant frequency to the probe, while the density of the plasma is in a time varying state which urges the probe to resonate at a certain time of the discharge pulse generating a first response signal. By tuning the frequency of the first response signal slightly above or below, the probe will resonate at a different time of the discharge pulse generating a second response signal.

By repeating the procedure over small steps in frequency, a first response signal at a specific time of the discharge pulse can be generated from the spectra of waveforms corresponding to discrete frequency versus time.

Similarly second response signals can be generated. The constant background noises can be eliminated by subtracting one of the second response signals from the first response signal. Therefore the above technique does not necessarily require the probe to be externally biased in the plasma. Hence the process becomes faster, simple and cost-effective.

It will be understood that what has been described herein are some exemplary systems for identifying the resonance peak associated with plasma.

While the present invention has been described with reference to some exemplary arrangements it will be understood that it is not intended to limit the teaching of the present invention to such arrangements as modifications can be made without departing from the spirit and scope of the present invention. In this way it will be understood that the invention is to be limited only insofar as is deemed necessary in the light of the appended claims.

Similarly the words comprises/comprising when used in the specification are used to specify the presence of stated features, integers, steps or components but do not preclude the presence or addition of one or more additional features, integers, steps, components or groups thereof.

Claims

Claims 1. A method of identifying the resonance frequency of a probe in plasma, the method comprising: Obtaining at a first time, a first response signal representative of a probe resonating in a plasma, Obtaining at a second later time, a second response signal representative of the probe resonating in a plasma, Providing a perturbation while the second response signal is being obtained,and Combining the first and second response signals to eliminate commonbackground noise.
2. A method as claimed in claim 1, wherein the perturbation is applied to the probe.
3. A method as claimed in claim 1, wherein the perturbation is applied to the plasma.
4. A method as claimed in claim 2, wherein the perturbation is provided as a voltage.
5. A method as claimed in claim 4, wherein the voltage is a DC voltage.
6. A method as claimed in claim 4 or 5, wherein the voltage is a negative voltage.
7. A method as claimed in claim 6, wherein the voltage is in the range of -20 to -30 volts.
8. A method as claimed in claim 6, wherein the voltage is in the range of -15 to -20 volts.
9. A method as claimed in any preceding claim, wherein a time varying signal is applied to the probe for generating the first and second response signals.
10. A method as claimed in claim 9, wherein a frequency generator applies the time vary signal to the probe.
11. A method as claimed in any preceding claim, wherein a radio frequency signal is applied to the probe for generating the first and second response signals.
12. A method as claimed in any of claims 1 to 11, wherein a microwave signal is applied to the probe for generating the first and second response signals.
13. A method as claim in claimed 3, wherein the perturbation applied to the plasma changes a characteristic of the plasma.
14. A method as claimed in claim 13, wherein the perturbation is applied by changing the power coupled into the plasma.
15. A method as claimed in claim 13, wherein the perturbation is applied by changing the pressure of the plasma.
16. A method as claimed in claim 13, wherein the perturbation is applied by changing the flow rates of the plasma.
17. A method as claimed in claim 13, wherein the perturbation is applied by introducing a trace gas into the plasma.
18. A method as claimed in claim 13, wherein the perturbation changes the properties of the plasma.
19. A method as claimed in any preceding claim, wherein the plasma is in a steady state.
20. A method as claimed in any of claimed 1 to 18, wherein the plasma is in a varying state.
21. A method as claimed in any preceding claim, wherein the probe is provided as a transmission line.
22. A method as claimed in claim 21, wherein the transmission line comprises first and second transmission spaced apart elements.
23. A method as claimed in claim 22, wherein the first and second transmission elements are shorted together at one end of the transmission line and open ended at the opposite end thereof
24. A method as claimed in any preceding claim, wherein the probe is u-shaped.
25. A method as claimed in any preceding claim, wherein the combination of the first and second response signals results in a difference signal.
26. A method as claimed in claim 25, wherein the difference signal comprises a pair of resonance peaks representative of the probe resonating in plasma.
27. A method as claimed in claim 26, wherein one of the resonance peaks is shifted by the perturbation to emulate a resonance peak in a vacuum.
28. A plasma system for identifying the resonance frequency of a probe resonating in plasma, the system comprising: a means for generating time differentiated first and second response signals representative of a probe resonating in plasma respectively, a perturbation source for applying a perturbation while the second response signal is being generated, and a means for combining the first and second response signals to eliminateat least some background noise.
29. A plasma system as claimed in claim 28, wherein the system comprises a frequency generator for applying time differentiated stimulus signals to the probe.
30. A plasma system as claimed in claim 29, wherein the system comprises a measuring means for measuring the response of the probe to the stimulus signals.
31. A plasma system as claimed in claim 30, wherein the measurement means generates a difference signal by combining the first and second response signals.
32. A plasma system as claimed in claim 31, wherein the measuring means comprises a means using a frequency domain reflectrometry technique to generate the difference signal over a defined frequency range.
33. A plasma system as claimed in any of claims 28 to 32, wherein the perturbation source is applied to the probe.
34. A plasma system as claimed in any of claims 28 to 32, wherein the perturbation source is applied to the plasma.
35. A plasma system as claimed in claim 33, wherein the perturbation source comprises an electrical source.
36. A plasma system as claimed in claim 35, wherein the perturbation source applies a biasing voltage to the probe.
37. A plasma system as claimed in claim 36, wherein the voltage is a DC voltage.
38. A plasma system as claimed in claim 36 or 37, wherein the voltage is a negative voltage.
39. A plasma system as claimed in claim 38, wherein the voltage is in the range of -20 to -30 volts.
40. A plasma system as claimed in claim 38, wherein the voltage is in the range of -15 to -20 volts.
41. A plasma system as claimed in claim 34, wherein the perturbation is applied by changing the power coupled into the plasma.
42. A plasma system as claimed in claim 34, wherein the perturbation is applied by changing the pressure of the plasma.
43. A plasma system as claimed in claim 34, wherein the perturbation is applied by changing the flow rates of the plasma.
44. A plasma system as claimed in claim 34, wherein the perturbation is applied by introducing a trace gas into the plasma.
45. A plasma system as claimed in claim 34, wherein the perturbation changes the properties of the plasma.
46. A plasma system as claimed in any of claims 28 to 45, wherein the plasma is in a steady state.
47. A plasma system as claimed in any of claims 28 to 45, wherein the plasma is in a varying state.
48. A plasma system as claimed in any of claimed 28 to 47, wherein the probe comprises a transmission line.
49. A plasma system as claimed in claim 48, wherein the transmission line comprises first and second spaced apart transmission elements.
50. A plasma system as claimed in claim 49, wherein the first and second transmission elements are shorted together at one end of the transmission line and open ended at the opposite end thereof.
51. A plasma system as claimed in any of claims 28 to 50, wherein the probe is u-shaped.
52. A plasma system substantially as described hereinbefore with reference to the drawings.
53. A method of identifying the resonance frequency of a probe in plasma substantially as described hereinbefore.