U.S. patent number RE38,273 [Application Number 09/599,608] was granted by the patent office on 2003-10-14 for baseband rf voltage-current probe.
This patent grant is currently assigned to ENI Technology, Inc.. Invention is credited to Kevin S. Gerrish, Daniel F. Vona, Jr..
United States Patent |
RE38,273 |
Gerrish , et al. |
October 14, 2003 |
**Please see images for:
( Certificate of Correction ) ** |
Baseband RF voltage-current probe
Abstract
An RF probe for a plasma chamber picks up current and voltage
samples of the RF power applied to an RF plasma chamber, and the RF
voltage and current waveforms are supplied to respective mixers. A
local oscillator supplies both mixers with a local oscillator
signal at the RF frequency plus or minus about 15 KHz, so that the
mixers provide respective voltage and current baseband signals that
are frequency shifted down to the audio range. The phase relation
to the applied current and voltage is preserved in the baseband
signals. These baseband signals are then applied to a stereo,
two-channel A/D converter, which provides a serial digital signal
to a digital signal processor or DSP. A local oscillator interface
brings a feedback signal from the DSP to the local oscillator. The
DSP can be suitably programmed to obtain complex Fast Fourier
Transforms of the voltage and current baseband samples. The
frequency-domain spectra are analyzed to obtain, with great
accuracy, magnitude of voltage and current and phase angle. Other
parameters are derived from these three.
Inventors: |
Gerrish; Kevin S. (Corona,
CA), Vona, Jr.; Daniel F. (Rochester, NY) |
Assignee: |
ENI Technology, Inc.
(Rochester, NY)
|
Family
ID: |
24749781 |
Appl.
No.: |
09/599,608 |
Filed: |
June 22, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
684833 |
Jul 22, 1996 |
05770922 |
Jun 23, 1998 |
|
|
Current U.S.
Class: |
315/111.21;
219/121.21; 219/121.36; 315/111.51; 315/224; 330/294 |
Current CPC
Class: |
H01J
37/32935 (20130101); H01J 37/32082 (20130101); H05H
1/0081 (20130101) |
Current International
Class: |
H01J
37/32 (20060101); H05H 1/00 (20060101); H01J
007/24 () |
Field of
Search: |
;315/111.21,111.51,111.41,111.71,224,39 ;333/32
;330/294,295,291,124R ;219/121.21,121.36,121.43,250 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Philogene; Haissa
Attorney, Agent or Firm: Harness, Dickey & Pierce,
P.L.C.
Claims
We claim:
1. A plasma arrangement in which an RF power generator supplies an
RF electrical wave in an RF frequency range to a power input of a
plasma chamber within which said RF electrical wave produces a
plasma, and in which a plasma probe picks up an RF voltage waveform
of said electrical wave and an RF current waveform of said
electrical wave; comprising the improvement wherein said plasma
probe comprises a controllable local oscillator providing a local
oscillator signal; a voltage signal mixer having inputs receiving
said RF voltage waveform and said local oscillator signal,
respectively, and an output providing a baseband voltage signal; a
current signal mixer having inputs receiving said RF current
waveform and said local oscillator signal, respectively, and having
an output providing a baseband current signal; an A/D converter
having a first channel input to which said baseband voltage signal
is applied, a second channel input to which said baseband current
signal is applied, and a serial output providing a time-synchronous
serial digital signal containing digital representations of said
baseband voltage waveform and said baseband current waveform; a
digital signal processor having an input coupled to the serial
output of said A/D converter, said digital signal processor being
suitably programmed to compute amplitude and relative phase of said
voltage baseband signal and said current baseband signal; external
interface means for providing an output determination based on said
amplitudes and relative phase; and local oscillator interface means
coupled to said digital signal processor and to said local
oscillator permitting said digital signal processor to control the
frequency of said local oscillator.
2. The arrangement of claim 1, wherein said local oscillator
provides said local oscillator frequency within approximately 0.2
KHz to 20 KHz of the frequency of the power input wave.
3. The arrangement of claim 1 wherein said A/D converter is a
high-fidelity audio-frequency stereo converter.
4. The arrangement of claim 1 wherein said local oscillator
provides said local oscillator signal at a local oscillator
frequency that differs from the frequency of said input power wave
by a difference of within 20 KHz.
5. The arrangement of claim 1 wherein said local oscillator
comprises a programmable oscillator.
6. The arrangement of claim 1 wherein said local oscillator also
comprises a divide-by-two frequency divider following said
programmable oscillator.
7. The arrangement of claim 1 wherein said A/D converter includes a
matched two-channel 20-bit stereo A/D converter.
8. The arrangement of claim 1 wherein said serial output provides
said time-synchronous serial digital signal containing digital
representations of said baseband voltage waveform and said baseband
current waveform so that said representation of current and voltage
appear alternately.
9. A method of deriving amplitude and relative phase information
for current and voltage of an RF power wave that is applied at a
radio frequency (RF) to a power input of a plasma changer within
which said RF power wave produces a plasma, and in which the plasma
probe picks up an RF voltage waveform and an RF current waveform of
said power wave; comprising the steps of generating a local
oscillator signal; mixing said local oscillator signal and said RF
voltage waveform to produce a voltage baseband signal at an audio
frequency; mixing said local oscillator signal and said RF current
waveform to produce a current baseband signal at said audio
frequency; .[.supplying a feedback signal from said digital signal
processor to control the frequency of said local oscillator
signal.]. converting said voltage baseband signal and said current
baseband signal to a serial digital signal; supplying said serial
digital signal to a suitably programmed digital signal processor;
supplying a feedback signal from said digital signal processor to
control the frequency of said local oscillator signal; and
computing the amplitudes and relative phase of said voltage and
current baseband signals.
10. The method of claim 9, wherein said step of providing said
local oscillator signal includes providing said local oscillator
signal at a frequency that is within between 0.20 KHz and 20 KHz of
the frequency of the applied RF power wave.
11. The method of claim 9, wherein said step of computing the
amplitudes and relative phase of said voltage and current baseband
signals includes calculating a Fast Fourier Transform of the
current and voltage baseband waveforms, and making phase and
magnitude measurements of the voltage and current baseband signals
by tracking the baseband frequency of said baseband signals.
12. The method of claim 11, wherein said step of making phase and
magnitude measurements includes, following said calculating of said
Fast Fourier Transform, extracting frequency spectra of said
voltage and current waveforms from said fast Fourier transform.
13. The method of claim 12, further comprising computing, from said
spectra of said voltage and current waveforms, a phase difference
as between said voltage and current waveforms.
14. The method of claim 12, wherein said computing the amplitudes
and relative phase of said voltage and current baseband signals
comprises transferring a plurality of samples of said serial
digital signal representing said baseband voltage waveform and said
baseband current waveform, respectively; multiplying said samples
by a predetermined window function to produce windowed current and
voltage signals; processing the respective voltage samples V and
associated current samples I as a complex waveform, W=V+j*I (where
j is the root of minus one), performing a complex Fast Fourier
Transform operation on said complex waveform FFT(W) to produce a
complex output, and extracting the current and voltage spectra from
said complex output.
15. The method of claim 14, wherein said computing the amplitudes
and relative phase of said voltage and current baseband signals
comprises extracting said relative phase angle by finding the
vector sums of said voltage and current spectra, and computing
arctangents of the resulting vector sums..Iadd.
16. A probe for monitoring an electrical signal transmitted between
a power generator and a plasma chamber input, the electrical signal
having an operating frequency, comprising: a pickup circuit coupled
to the plasma chamber input for providing a pickup signal that is
representative of a characteristic of the electrical signal, the
pickup signal having an operating frequency related to the
electrical signal operating frequency; a mixer circuit for
providing a baseband signal associated with the pickup signal, the
baseband signal being frequency shifted from the pickup signal
operating frequency to a lower frequency; and a detector circuit
coupled to the mixer circuit to process the baseband signal.
.Iaddend..Iadd.
17. The probe of claim 16 wherein the electrical signal
characteristic is selected from the group of: voltage, current, and
phase. .Iaddend..Iadd.
18. The probe of claim 16, wherein the mixer circuit comprises: a
local oscillator for providing a local oscillator signal; and a
signal mixer to mix the pickup signal and the local oscillator
signal to generate the baseband signal. .Iaddend..Iadd.
19. The probe of claim 18 wherein the detector has a sampling
conversion frequency, and wherein the local oscillator signal has
an operating frequency that is within approximately 0 kHz to Y kHz
of the electrical signal operating frequency, wherein Y kHz is
approximately equal to one-half of the detector sampling conversion
frequency. .Iaddend..Iadd.
20. The probe of claim 16 wherein the detector circuit comprises: a
converter coupled to the mixer circuit for converting the baseband
signal to a digital signal; and a digital signal processor coupled
to the converter for processing the digital signal.
.Iaddend..Iadd.
21. The probe of claim 20 wherein the converter has a sampling
conversion frequency, and wherein the converter includes an
anti-aliasing filter to band-limit the baseband signal to the range
of about 0 kHz to Y kHz, wherein Y kHz is approximately equal to
one-half of the converter sampling conversion frequency.
.Iaddend..Iadd.
22. The probe of claim 16 further including a local oscillator
interface coupled between the detector circuit and the mixer
circuit for controlling the frequency shifting of the pickup
signal. .Iaddend..Iadd.
23. The probe of claim 16 wherein the detector circuit comprises: a
signal processor coupled to the mixer circuit for processing the
baseband signal. .Iaddend..Iadd.
24. A probe for monitoring an electrical signal transmitted between
a power generator and a plasma chamber input, the electrical signal
having an operating frequency, comprising: a voltage pickup circuit
coupled to the plasma chamber input for providing a voltage pickup
signal that is representative of a voltage of the electrical
signal, the voltage pickup signal having an operating frequency
related to the electrical signal operating frequency; a current
pickup circuit coupled to the plasma chamber input for providing a
current pickup signal that is representative of a current of the
electrical signal, the current pickup signal having an operating
frequency related to the electrical signal operating frequency; a
mixer circuit for providing baseband signals associated with the
pickup signals; a voltage baseband signal associated with the
voltage pickup signal being frequency shifted from the voltage
pickup signal operating frequency to a lower frequency; and a
current baseband signal associated with the current pickup signal
being frequency shifted from the current pickup signal operating
frequency to a lower frequency; and a detector circuit coupled to
the mixer circuit to process the baseband signals.
.Iaddend..Iadd.
25. The probe of claim 24 wherein the detector circuit comprises: a
converter coupled to the mixer circuit for converting the baseband
signals to digital signals; and a digital signal processor coupled
to the converter for processing the digital signals.
.Iaddend..Iadd.
26. The probe of claim 25 wherein the mixer circuit comprises: a
local oscillator for providing a local oscillator signal; a first
signal mixer to mix the voltage pickup signal and the local
oscillator signal to generate the voltage baseband signal; and a
second signal mixer to mix the current pickup signal and the local
oscillator signal to generate the current baseband signal.
.Iaddend..Iadd.
27. The probe of claim 26 further including a local oscillator
interface coupled between the digital signal processor and the
local oscillator for controlling the local oscillator signal such
that the frequency shifting of the pickup signals is controllable.
.Iaddend.
Description
BACKGROUND OF THE INVENTION
This invention relates to plasma generation equipment, and is
particularly directed to probes for detecting the current, voltage,
and phase of radio frequency (RF) electrical power that is being
supplied to an RF plasma chamber.
In a typical RF plasma generator arrangement, a high power RF
source produces an RF wave at a preset frequency, i.e., 13.56 MHz,
and this is furnished along a power conduit to a plasma chamber.
Because there is typically a severe impedance mismatch between the
RF power source and the plasma chamber, an impedance matching
network is interposed between the two. There are non-linearities in
the plasma chamber, and because of these and because of losses in
the line and in the impedance matching network, not all of the
output power of the RF generator reaches the plasma chamber.
Therefore, it is conventional to employ a probe at the power input
to the plasma chamber to detect the voltage and current of the RF
wave as it enters the plasma chamber. By accurately measuring the
voltage and current as close to the chamber as possible, the user
of the plasma process can obtain a better indication of the quality
of the plasma. This in turn yields better control of the etching or
deposition characteristics for a silicon wafer or other workpiece
in the chamber.
At the present time, diode detection probes are employed to detect
the amplitude of the current and voltage waveforms. These probes
simply employ diode detector circuits to rectify the voltage and
current waveforms, and deliver simple DC metering outputs for
voltage and for current. These probes have at least two drawbacks
in this role. Diode detectors are inherently non-linear at low
signal levels, and are notoriously subject to temperature drift.
The diode detector circuits also are limited to detecting the
signal peaks for the fundamental frequency only, and cannot yield
any information about higher or lower frequencies present in the RF
power waveform. In addition to this, it is impossible to obtain
phase angle information between the current and voltage waveforms,
which renders the power measurement less accurate.
One proposal that has been considered to improve the detection of
RF power has been to obtain digital samples of the voltage and
current outputs of a probe, using flash conversion, and then to
process the samples on a high-speed buffer RAM. However, this
proposal does have problems with accuracy and precision. At the
present time, flash conversion has a low dynamic range, normally
being limited to eight bits of resolution. To gain reasonable phase
accuracy for plasma customer requirements, it is necessary to reach
a precision of at least twelve bits, so that a phase angle
precision of better than one degree can obtained at full power. In
addition, flash converters require an extremely fast RAM in order
to buffer a block of samples before they are processed in a digital
signal process (DSP), and fast RAM circuitry is both
space-consuming and expensive.
Voltage and current probes that are now in existence are limited in
their performance by the fact that they can only monitor the
voltage, current, and phase angle at one frequency, and even then
such probes have a poor dynamic range. Examining a different
frequency requires changing out the hardware, which can be costly
and time consuming. This means also that good performance will
ensue only if the load is linear, which is never the case with a
plasma chamber. Unlike capacitors, inductors, and resistors, plasma
chambers impose a highly non-linear load, which causes the
sinusoidal waveform of the input power to become distorted. This
distortion causes the resulting waveform to be a sum of sinusoids,
with the frequency of each additional sinusoid being some integer
multiple of the input sinusoidal frequency (i.e., harmonics).
Conventional probes can provide voltage, current and coarse phase
information, at best, for the fundamental voltage and current
waveforms. This severely limits the accuracy of the system, and
makes accurate and repeatable control impossible when there is a
significant amount of voltage or current appearing in the
harmonics.
OBJECTS AND SUMMARY OF THE INVENTION
It is an objective of this invention to provide a reliable and
accurate probe, at low cost, for detecting the current and voltage
of RF power being applied to a plasma chamber and for accurately
finding the phase angle between the applied voltage and applied
current.
It is a more specific object of this invention to provide a
frequency shifting arrangement that converts the voltage and
current to a lower frequency baseband signal to facilitate accurate
detection of RF current and voltage of the applied power, as well
as phase information.
According to an aspect of the invention, a plasma arrangement has
an RF power generator that supplies an RF electrical wave at a
predetermined frequency to a power input of a plasma chamber within
which the RF electrical wave produces a plasma. A plasma probe
picks up both an RF voltage waveform and an RF current waveform of
the electrical wave. The plasma probe sends the RF voltage and
current waveforms to an analysis board which converts the RF
waveforms to baseband voltage and current signals. A controllable
local oscillator provides a local oscillator signal which is a
square wave. A voltage signal mixer has inputs that receive the RF
voltage waveform and the local oscillator signal, respectively, and
an output that provides an audio frequency (AF) baseband voltage
signal. A current signal mixer has inputs that receive the RF
current waveform and the local oscillator signal, respectively, and
has an output that provides a baseband AF current signal. A stereo
A/D converter has a first channel input to which the baseband
voltage signal is applied, a second channel input to which the
baseband current signal is applied, and a serial output that
provides a time-synchronous serial digital signal containing
alternate digital representations of the baseband voltage waveform
and the baseband current waveform. A digital signal process has an
input coupled to the serial output of the stereo A/D converter. The
digital signal processor is suitably programmed to take the input
AF voltage and current signals, determine the amplitude and
relative phase of the voltage and current signals, and compute
relative RF parameters based on these signals. An external
interface provides an output determination based on the amplitudes
and relative phase. A local oscillator interface circuit couples
the digital signal processor to the local oscillator so that the
digital signal processor can control the frequency of the local
oscillator signal. In a preferred embodiment, the local oscillator
provides said local oscillator frequency within about 15 KHz of the
plasma RF frequency, so that the difference frequency, that is, the
baseband frequency of the baseband current and voltage signals, is
approximately 0.2 KHz to 15 KHz. Also, the stereo A/D converter is
preferably a high-fidelity audio-frequency stereo converter, and
can be of the type that is frequently used in high-fidelity audio
systems, such as a matched two-channel 20-bit A/D converter. The
A/D converters preferably incorporate anti-aliasing filters
band-limiting the input baseband signals to the range of 0.2 KHz to
20 KHz. The local oscillator preferably includes a programmable
oscillator, and can also include a divide-by-two frequency divider
following the programmable oscillator to maintain a constant duty
cycle. Information about harmonics can be derived by changing the
local oscillator signal to a multiple of the RF waveform frequency
plus or minus up to 20 KHz.
According to another aspect of this invention, amplitude and
relative phase information for current and voltage can be derived
for an RF power wave that is applied at a predetermined frequency
to a power input of a plasma chamber within which the RF power wave
produces a plasma. A plasma probe picks up an RF voltage waveform
and an RF current waveform of the applied power. The technique of
this invention involves generating a local oscillator signal and
mixing the local oscillator signal and the RF voltage and current
waveforms to produce the voltage baseband signal at an audio
frequency and the current baseband signal at an audio frequency. A
feedback signal is supplied from a digital signal processor to
control the frequency of said local oscillator signal. The voltage
baseband signal and said current baseband signal are converted to a
time-synchronous serial digital signal that is supplied to the
digital signal processor, which is suitably programmed to compute
the amplitudes and relative phase of the voltage and current
baseband signals. The local oscillator signal is produced at a
frequency that is within 0.20 KHz to 20 KHz of the predetermined
frequency of said RF power wave, so that the baseband signals will
have a frequency in the audio range of 200 Hz to 20 KHz. Preferably
this is about 10 KHz.
The digital signal processor computes the amplitudes and relative
phase of the voltage and current baseband signals, preferably by
means of a Fast Fourier Transform (FFT) of the current and voltage
baseband waveforms. Then, phase and magnitude measurements of the
voltage and current baseband signals are made by tracking the
baseband frequency of the current and voltage baseband signals. The
phase and magnitude measurements can be carried out after the Fast
Fourier Transform by extracting frequency spectra of the voltage
and current waveforms from the fast Fourier transform. The
extracted spectra are employed to compute the phase difference or
phase angle between the voltage and current waveforms.
Computing the amplitudes and relative phase of the voltage and
current baseband signals is carried out in the digital signal
processor. A predetermined number of samples of the serial digital
signal representing the baseband voltage waveform and the baseband
current waveform, respectively, are transferred into the DSP, and
these samples are multiplied by a predetermined window function to
produce windowed current and voltage signals. Then, windowed
voltage samples V and associated current samples I are processed as
a complex waveform, W=V+j *I (where j is the root of minus one),
and the digital signal processor performs a complex Fast Fourier
Transform operation FFT(W) on the complex waveform. This produces a
complex output, from which the digital signal processor can extract
current and voltage spectra. The amplitudes and relative phase of
said voltage and current baseband signals can be obtained from
vector summation of the voltage and current spectra, and from the
arctangent of the resulting vector sums.
From these data, other useful values can be calculated which can be
used in the accurate control of the RF plasma process, including
but not limited to RMS voltage, RMS current, delivered (dissipated)
power, forward power, reverse or reflected power, reactive power,
apparent power, magnitude of load impedance, phase of load
impedance, load resistance, load reactance, magnitude of reflection
coefficient, phase of reflectance coefficient, and voltage standing
wave ratio (VSWR).
The above and other objects, features, and advantages of this
invention will become apparent from the ensuing description of a
preferred embodiment, which should be read in conjunction with the
accompanying Drawing.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a block diagram of an RF plasma chamber, with associated
RF plasma generator, impedance match network, V-I pickup, and V-I
analysis board arrangement according to an embodiment of this
invention.
FIG. 2 is a simplified schematic diagram of the V-I pickup and the
signal analysis circuitry of the voltage probe arrangement.
FIG. 3 is a software logic flow diagram for explaining the
operation of the voltage-current probe arrangement of this
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
With reference to the Drawing, and initially to FIG. 1, a plasma
process arrangement 10, e.g., for etching a silicon wafer or other
workpiece, has an RF power generator 12, which produces RF power at
a prescribed frequency, e.g., 13.56 MHz at a predetermined power
level, such as one kilowatt. The generator 12 supplies RF power
along a conduit 14 to a matching network 16. The output of the
matching network 16 is coupled by a power conduit 18 to an input of
a plasma chamber 20. A probe voltage and current pickup device 22
samples the voltage V.sub.RF and the current I.sub.RF of the
applied RF power as it enters the input to the chamber 20. The
chamber 20 has a vacuum conduit associated with a not-shown vacuum
pump and a gas inlet through which a noble gas, e.g., argon, is
introduced into the chamber. The sampled voltage and current
V.sub.RF and I.sub.RF are fed to a voltage and current (V-I)
baseband probe arrangement 24 which measures the magnitudes or
amplitudes of the applied voltage and current, and also computes
the phase angle .PHI. between the applied voltage and current
waveforms. These three values can be computed with high accuracy,
and can in turn be used to calculate other parameters, as shall be
discussed below.
The baseband voltage-current probe permits accurate determination
of voltage amplitude .vertline.V.vertline., current amplitude
.vertline.I.vertline., and phase .PHI. between voltage and current
for an RF (radio frequency) signal. This can be in the range of
0.200 MHz to 67.8 MHz, permitting the user to analyze a plasma with
greater precision than has been possible with more conventional
analog techniques. The same concept can be applied beyond these
frequencies to other ranges. An end result of this improved
capability is improved process repeatability, improved process
endpoint determination, higher yields, and more consistent yields.
The voltage-current probe, when employed in connection with the RF
path in an RF plasma system, allows the user to achieve a higher
degree of control, and to achieve control using parameters beyond
simply peak voltage and current values of the RF wave. With the
baseband voltage-current probe arrangement 24 of this invention,
the user can control the plasma process based on power delivered to
the plasma, whether at the RF frequency of the generator or at any
other frequency, impedance of the plasma, either at the frequency
of the RF waveform or at any frequency within the bandwidth of the
arrangement 24. For example, harmonic analysis can be used for a
more accurately determination of completion for an etching step in
an integrated circuit (IC) wafer.
As shown in more detail in FIG. 2, the probe pickup 22 has a shield
or housing 23 that electrically seals the pickup. A voltage pickup
board 26 is coupled by a triax cable 28 to a super-high dynamic
range mixer 30 in the probe circuit arrangement 24. The triax cable
28 has an output braid coupled to the housing 23 and an inner braid
going to the chassis ground of the probe circuit arrangement 24. A
current pickup board 32 inside the pickup 22 is coupled by a
triaxial cable 34 to a super-high dynamic range mixer 36. The cable
34 has its outer and inner braids connected in a fashion similar to
that of the cable 28. A programmable local oscillator 38 generates
a local oscillator signal that is within twenty kilohertz of the
applied RF waveform, that is, at a frequency R.sub.RF.+-.0.20 KHz
to F.sub.RF.+-.20 KHz. The local oscillator has an associated
divide-by-two counter 40 to ensure a proper duty cycle. The local
oscillator 38 can favorably include a single chip phase lock loop
(PLL) frequency synthesizer, and this can have a design frequency
range of 0.320 to 120 MHz. The same local oscillator signal, at the
same frequency and phase, is fed to the local oscillator inputs of
both mixers 30 and 36. In the preferred embodiment the frequency of
the local oscillator 38 is chosen so that the local oscillator
(l.o.) signal, at the output of the divide-by-two counter 40, is
the applied waveform frequency plus or minus 15 KHz. The local
oscillator signal is fed to l.o. inputs of both mixers 30 and 36,
and the same produce a voltage baseband signal and a current
baseband signal, respectively. The baseband signals each have the
same baseband frequency, and as a result of the proper choice of
local oscillator frequency, the voltage and current baseband
signals are in the range of 0.20 to 20 KHz, that is, in the audio
frequency range. Proper matching of the baseband voltage and
current frequencies is assembled by the fact that the voltage and
current waveforms from the pickup 22 (which are, of course,
identical in frequency) are both mixed with the very same local
oscillator signal.
The baseband voltage signal and the baseband current signal are
then supplied to respective inputs L and R of a matched,
two-channel 20-bit stereo A/D converter 42. This is a low-cost,
available item that is frequently employed in high-fidelity audio
products. The preferred converter 42 incorporates two highly
matched, independent A/D converters, with a digital output that is
a simple synchronous serial digital signal that easily interfaces
with other digital components. The A/D converter incorporates
anti-aliasing filters which band-limit the input baseband signals
to the range of 0.20 to 20 KHz. The output is supplied over a
serial data interface 44, as alternate baseband voltage and current
samples, to a signal input of a digital signal processor or DSP 46.
There are well-known support hardware elements associated with the
DSP 46, and these are not shown in the Drawing.
The DSP 46 processes the digitized baseband voltage and current
signals, and calculates the magnitude .vertline.V.vertline. of the
voltage, the magnitude .vertline.I.vertline. of the current, and
the phase angles of the baseband voltage and current signals, from
which it derives the relative phase .PHI. of the applied voltage
and current. The DSP is coupled by means of a local oscillator
serial program interface 48 to a feedback input of the local
oscillator 38 to form a closed loop. The DSP 46 is also coupled to
an external serial interface, which in turn can be coupled to
controls for the plasma process arrangement, e.g., to control the
voltage or current supplied from the RF plasma generator 12 or to
control the impedance of the impedance match network 16.
The mixers 30 and 36, which both receive the same l.o. signal, make
it possible to reduce the frequency of the applied RF voltage and
applied RF current from the megahertz frequency range to the
kilohertz frequency range in a single mixing step. Once the two
signals are mixed to the baseband frequency range, the baseband
signals are filtered, via low-pass filters (not shown) to remove
the upper side band, thereby leaving only the lower sideband, or
baseband, signals. The phase relations in the applied voltage and
applied current waveforms are preserved in the two baseband
signals. There are fed to the A/D converter 42 where they are
converted, e.g., with a sampling conversion frequency of 48 KHz.
After a suitable number of baseband voltage and current samples are
taken, the DSP 46 carries out phase and magnitude measurements.
Once a predetermined (or selectable) number of baseband current and
voltage samples have been transferred from the converter 42 to the
DSP 46, the DSP carries out a series of complex signal processing
algorithms to process the data. This operation is carried out as
generally described now, with reference to FIG. 3. In this diagram
the solid lines between the routines or operations represent
processing of real numbers, while dash lines represent the
processing of complex (i.e., real plus imaginary) numbers.
Once a suitable number of samples of each of the voltage and
current baseband waveforms have been taken, as shown generally as
sampling subroutine 52, the sampled current data and sampled
voltage data are multiplied by a window function or window routine
54. The window function is chosen such that there is a minimum
amount of frequency peak spreading in the frequency domain. Here,
the default window function is the Harris-Blackman widow, but other
window functions could be employed and changed, via a window
generator subroutine.
In order to maintain processing efficiency, the orthogonal
characteristics of a single complex fast Fourier transform or FFT
are exploited to derive the spectrum of each of the voltage and
current waveforms. Due to the nature of the complex FFT, the phase
between the two spectra can be simply extracted using vector
summation and the arc tangent function. To achieve this, the
windowed current and voltage sample data are first combined
(complex sample data routine 56) into one complex waveform sample
W, that is W=V+j*I, where j is the base of imaginary numbers, i.e.
the square root of negative one, or j=(-1). The complex waveform W
is then subjected to a subroutine 58 that calculates the FFT of the
waveform W as a set of complex numbers, to wit, FFT(V+j*I). Once
the complex FFT is completed, the results are subjected to an
extraction routine 60, which extracts the current and voltage
spectra of the FFT output, using vector summation. From this stage,
the current baseband spectrum and the voltage baseband spectrum are
treated in a frequency domain maximum energy detection and tracking
algorithm 62. Here, the frequency of the maximum energy signal is
determined and tracked for both the voltage and current waveforms.
From these data, the magnitude .vertline.V.vertline. of the voltage
and the magnitude .vertline.I.vertline. of the current are
calculated, as in subroutines 64 and 66, and the phase angles of
voltage and current are calculated, as in routine 68. While the
instantaneous phase angle of voltage or current alone is not
particularly useful, the difference between these two produces the
relative phase angle .PHI., which represents the actual phase angle
of the plasma load. These values .vertline.V.vertline.,
.vertline.I.vertline., and .PHI. are employed in any of a set of
user-configurable calculation subroutines 70 to produce any of a
large number of parameters that can be employed in process control.
A short list of these values is provided as follows: a. RMS voltage
.vertline.V.vertline.=(avg(V.sup.2)).sup.0.5 =V.sub.rms b. RMS
current .vertline.I.vertline.=(avg(I.sup.2)).sup.0.5 =I.sub.rms c.
Phase angle .PHI.=.angle.I-.angle.V d. Delivered (dissipated) Power
P=.vertline.V.vertline.*.vertline.I.vertline.* Cos (.PHI.) e.
Forward (apparent) Power P.sub.F
=P.div.(1-.vertline..GAMMA..vertline..sup.2) f. Reverse Power
P.sub.R =P.sub.F -P g. Reactive Power P.sub.REACT
=.vertline.V.vertline.* .vertline.I.vertline.* Sin (.PHI.) h.
Magnitude of Load Impedance .vertline.Z.sub.
L.vertline.=.vertline.V.vertline..div..vertline.I.vertline. i.
Phase of Load Impedance .angle.Z.sub.L =.PHI. j. Load Resistance
Z.sub.LR =Real(Z.sub.L)=.vertline.Z.sub.L.vertline.* Cos (.PHI.) k.
Load Reactance Z.sub.LI =Imag(Z.sub.L)=.vertline.Z.sub.L.vertline.*
Sin (.PHI.) l. Magnitude of Reflection
The foregoing values, or others, are computed in near real time and
are conditioned in an output interface routine, where they are
supplied, e.g., through external serial interface 50, to control
the RF plasma generator 12 or the impedance match network 16.
It should be appreciated that with the probe arrangement of the
present invention, the above parameters are obtained with an
improvement in smaller size, lower cost, lower drift, higher
accuracy (especially at high phase angles) and with greater
flexibility of integration than with existing probe systems or
techniques. Moreover, unlike conventional, diode based systems, the
arrangement of this invention permits harmonic analysis and permits
plasma power and impedance measurements at user-selected
frequencies. Also, this invention permits the data to be easily
exported, and facilitates remote user operation and monitoring.
The phase measurement taken in this manner is highly accurate,
i.e., to within one-fifth degree, i.e. .+-.0.2.degree.. This cannot
be achieved with other techniques, such as zero-crossing
detectors.
Also, while the arrangement of the above-described embodiment has
been described in conjunction with an RF waveform frequency of
13.56 MHz, the invention can be used over a wide range of
frequencies, including other process RF frequencies such as 27.12
MHz, 40.68 MHz, etc.
While the invention has been described with reference to a
preferred embodiment, the invention is certainly not limited to
that precise embodiment. Rather, many modifications and variations
would present themselves to persons skilled in the art without
departing from the scope and spirit of the invention, as defined in
the appended claims.
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