ANALYSING RF SIGNALS FROM A PLASMA SYSTEM

• US 20150276833A1
• Filed: 07/24/2013
• Published: 10/01/2015
• Est. Priority Date: 07/25/2012
• Status: Abandoned Application

• Current Assignee: Impedans Ltd.
• Original Assignee: Impedans Ltd.
• Inventors: Hopkins, Michael, Scullin, Paul
• Application Number:
  US14/416,778
• Publication Number:
  US 20150276833A1
• Time in Patent Office: Days
• US Class Current: 1/1
• CPC Class Codes: G01R 23/20 Measurement of non-linear distortion
• CPC Class Codes: H01J 37/32082 Radio frequency generated discharge H01J37/32357, H01J37/32366, H01J37/32394 and H01J37/32403 take precedence
• CPC Class Codes: H01J 37/32972 Spectral analysis

Abstract

Samples representing signals and noise from a plasma system across a frequency range are collected. A first complex frequency-domain signal component is identified from a sample corresponding to a frequency value F at which a local maximum signal is found. This first component is phase-adjusted by a variable angle θ to a predetermined phase angle φ, and stored. A further complex component is identified corresponding to a frequency F(N) representing an Nth order harmonic of F. This further component is phase-adjusted by an angle N×θ, and stored. The procedure is repeated to build up sets of phase-adjusted first and further components, with θ chosen in each iteration for the first component to give a constant phase angle φ, and within any iteration the value of θ used for the first component is employed in the adjustment of the further component. The aggregated, phase-adjusted components exhibit increased signal-to-noise.

Claims

1. A method of analysing RF signals from a plasma system, comprising the steps of:

(a) receiving from the plasma system one or more signal channels, wherein the or each signal channel comprises a data source representing signals and noise from the plasma system across a frequency range;

(b) identifying, in a first RF signal sample from one of said signal channel(s), a first frequency value, F, at which a local maximum is found in the frequency domain;

(c) determining a first complex frequency-domain signal component at said first frequency value, F in said first signal sample;

(d) identifying, in said first RF signal sample or in a further RF signal sample from one of said signal channel(s), a further frequency value F(N), where N is an integer representing an Nth order harmonic of said first frequency F, and wherein N=1 denotes a 1st order harmonic at the first frequency F;

(e) determining a further complex frequency-domain component at said further frequency value in the RF signal sample in which said further frequency value is identified;

(f) transforming the first complex frequency-domain component by adjusting its phase by an angle θ

to a predetermined phase angle φ

, to provide a phase-adjusted first complex signal component;

(g) transforming the further complex frequency-domain component by adjusting its phase by an angle equal to N×

θ

, to provide a phase-adjusted further complex signal component;

(h) iteratively repeating steps (b) to (g) in respect of a plurality of signal samples from the same plasma system, wherein in each iteration the value of θ

is chosen to give a constant phase angle φ

for the phase-adjusted first complex signal component across different iterations, and wherein within any iteration the value of θ

chosen in step (f) is employed in the adjustment of step (g);

(i) aggregating or averaging the phase adjusted first complex signal components obtained in each iteration;

(j) aggregating or averaging the phase-adjusted further complex signal components obtained in each iteration.

2. The method of claim 1 wherein the signal channel in which the first frequency value is identified is not the same as the signal channel in which the further frequency value is identified.

3. The method of claim 1, in which the signal channel in which the first frequency value is selected is one of a voltage signal channel, a current signal channel, and an optical signal channel, and the signal channel in which the further frequency value is identified is a different one of a of a voltage signal channel, a current signal channel, and an optical signal channel.

4. The method of claim 1, wherein on each iteration, steps (d), (e) and (g) are carried out more than once such that a plurality of further frequency values are identified in step (d) each representing a harmonic of said first frequency F found within a signal channel from the plasma system, and for each such harmonic a respective further complex frequency-domain component is determined in step (e) which is transformed in step (g) to provide a respective phase-adjusted further complex signal component, and wherein step (i) is repeated separately for each such phase adjusted further complex signal component.

5. The method of claim 4, wherein on each iteration, steps (d), (e) and (g) are carried out at least once for a further frequency value identified in said first RF signal sample, and at least once for a further frequency value identified in a further RF signal sample from a different signal channel.

6. The method of claim 5, wherein the first frequency value is identified as a fundamental frequency in a voltage signal channel and wherein at least one further frequency value is a harmonic of said fundamental frequency in said voltage signal channel of order N>

1, and further wherein another of said further frequency values is identified as a harmonic (N>

=1) of the same fundamental frequency within a signal sample from a current signal channel or an optical signal channel.

7. The method of claim 5, wherein the first frequency value is identified as a fundamental frequency in a current signal channel and wherein at least one further frequency value is a harmonic of said fundamental frequency in said current signal channel of order N>

1, and further wherein another of said further frequency values is identified as a harmonic (N>

=1) of the same fundamental frequency within a signal sample from a voltage signal channel or an optical signal channel.

8. The method of claim 6, wherein the first frequency value and plurality of further frequency values include at least the first and second order harmonics from a current signal channel and the first and second order harmonics from a voltage signal channel.

9. The method of claim 1, wherein:

the step (b) of identifying a first frequency value comprises identifying a bin number b1 of a discrete Fourier transform in which a local maximum of signal magnitude is located or expected, and

the step (d) of identifying a further frequency value comprises determining the identity of a bin number b2=N×

b1, subject to modulo arithmetic where the modulus is the total number of bins, and selecting a bin which lies within N bins adjacent to bin b2.

10. The method of claim 9, wherein the selection of a bin which lies within N bins adjacent to bin b2 comprises identifying a bin within said range where a local maximum of signal magnitude is found.

11. The method of claim 9, wherein the selection of a bin which lies within N bins adjacent to bin b2 comprises selecting a bin in which a frequency multiple F(N) will be found where the frequency F is known with a precision greater than the bin size.

12. The method of claim 1, wherein the value of the first frequency F drifts between successive iterations and wherein on each iteration the first frequency value F is identified as a local maximum signal within an expected frequency range.

13. The method of claim 1, further comprising the initial steps of receiving at least one RF signal sample from a signal channel and performing a transform of said at least one RF signal sample to the frequency domain.

14. The method of claim 13, wherein said transform is a discrete Fourier transform, and preferably a fast Fourier transform.

15. The method of claim 1, wherein steps (b) to (j) are repeated based on a different first frequency F′

, at which a local maximum is found in the frequency domain, where F′ and

F are not harmonics of one another.

16. A computer program product comprising machine-readable instructions which, when executed in a processor provided with data representative of one or more RF signals of a plasma system, are effective to carry out the method of claim 1.

17. A system for analysing RF signals from a plasma system, comprising one or more processing circuits programmed to execute the method of claim 1.

18. The system of claim 16, wherein said one or more processing circuits are implemented as a field programmable gate array and wherein said programming of said circuits comprises configuring said field programmable gate array with a logic function implementing said method.