U.S. patent number RE42,917 [Application Number 11/519,242] was granted by the patent office on 2011-11-15 for rf power control device for rf plasma applications.
This patent grant is currently assigned to Advanced Energy Industries, Inc.. Invention is credited to Theresa Beizer, Frederick Hauer, Anton Mavretic.
United States Patent |
RE42,917 |
Hauer , et al. |
November 15, 2011 |
RF power control device for RF plasma applications
Abstract
There is provided by this invention an improved rf power control
device for plasma applications for optimization of the feedback
control voltage in the presence of harmonic and non-harmonic
spurious frequencies. In this system, an oscillator and mixer,
similar to those normally used in radio receiver applications are
placed at the sampled output of the solid state rf signal source
used for plasma ignition. The sampled output is mixed to a low
frequency and filtered to remove the spurious frequencies that is
created in the non-linear plasma. In this way, the feedback power
control essentially ignores the spurious frequencies. In this
application, the oscillator and mixer do not interfere with other
desirable system characteristics and effectively isolate the
feedback control voltage from changes in plasma spurious content.
This allows rf power to be delivered to the plasma with greater
accuracy than would otherwise be possible with conventional power
control device and methods.
Inventors: |
Hauer; Frederick (Windsor,
CT), Beizer; Theresa (Cherry Hill, NJ), Mavretic;
Anton (Natick, MA) |
Assignee: |
Advanced Energy Industries,
Inc. (Fort Collins, CO)
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Family
ID: |
32927881 |
Appl.
No.: |
11/519,242 |
Filed: |
September 11, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
10620129 |
Jul 15, 2003 |
6791274 |
Sep 14, 2004 |
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Current U.S.
Class: |
315/111.21;
118/723MW; 315/111.51; 333/17.3 |
Current CPC
Class: |
H01J
37/32174 (20130101); H01J 37/32082 (20130101) |
Current International
Class: |
H01J
7/24 (20060101); H01P 5/08 (20060101); C23C
16/00 (20060101) |
Field of
Search: |
;315/111.21,111.51,111.71,111.01 ;333/17.3,99PL,17.1
;118/723R,723MW ;204/155,298.08 ;422/906 ;427/569,570 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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05266990 |
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Oct 1993 |
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JP |
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10041097 |
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Feb 1998 |
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JP |
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2003139804 |
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Feb 2001 |
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JP |
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2001044873 |
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Jan 2003 |
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JP |
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2003017296 |
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May 2003 |
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JP |
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Other References
Shuichi Miyamoto; Japanese Office Action; Application No.
2006-520335; Dec. 16, 2009; Japanese Patent Office. cited by other
.
Shuichi Miyamoto; Japanese Office Action; Application No.
2006-520335; Jun. 9, 2010; Japanese Patent Office. cited by other
.
Shuichi Miyamoto; Japanese Office Action; Application No.
2006-520335; Jan. 18, 2011; Japanese Patent Office. cited by other
.
Japanese Patent Office, Third Party Observation, Jun. 11, 2010.
cited by other .
Japanese Patent Office, Third Party Observation, Jan. 7, 2011.
cited by other.
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Primary Examiner: Vo; Tuyet Thi
Attorney, Agent or Firm: Neugeboren O'Dowd PC O'Dowd; Sean
R.
Claims
We claim:
1. A VHF generator for delivering rf power to a plasma, comprising,
a) a variable rf signal generator including a power amplifier
connected to a directional coupler; b) the directional coupler
having one output connected to a matching network wherein power is
delivered to plasma in a processing chamber; c) at least one output
of the directional coupler disposed to sample a forward power
signal of the rf signal generator and at least one output of the
directional coupler disposed to sample a reflected power signal of
the rf signal generator; d) each of the sampled forward and
reflected signals is connected to mixers for mixing with an
intermediate frequency of an oscillator; e) the mixed forward and
reflected signals are passed through low pass filters; f) the
filtered forward and reflected signals are connected to amplifiers
and detectors; and g) the detected forward and reflected signals
are fed back to a power control circuit wherein the power delivered
to the plasma is monitored without interference from spurious
frequency signals generated by the plasma.
2. A VHF generator for delivering rf power to a plasma as recited
in claim 1 wherein forward and reflected bandpass filters are
connected between the output of the directional coupler and the
mixers for removing harmonics and spurious low frequency
signals.
3. A VHF generator for delivering rf power to a plasma as recited
in claim 2 wherein the detectors are diode detectors.
4. A VHF generator for delivering rf power to a plasma as recited
in claim 2 wherein the detectors are RMS detectors.
5. A VHF generator for delivering rf power to a plasma, comprising,
a) an rf signal generator including a power amplifier connected to
a directional coupler; b) the directional coupler having one output
connected to a matching network wherein power is delivered to
plasma in a processing chamber; c) at least one output of the
directional coupler disposed to sample a forward power signal of
the rf signal generator and at least one output of the directional
coupler disposed to sample a reflected power signal of the rf
signal generator; d) each of the sampled forward and reflected
signals is connected to a first mixer; e) a first oscillator
connected to a second mixer for mixing a sampled output of the
variable rf signal generator with a first intermediate frequency;
f) the output of the second mixer is connected to a first band pass
filter and then connected to a third mixer for mixing with a second
intermediate frequency of a second oscillator; g) the output of the
third mixer is connected to a second band pass filter and connected
to the first mixer; h) the mixed forward and reflected signals are
passed through low pass filters; i) the filtered forward and
reflected signals are connected to amplifiers and detectors; and j)
the detected forward and reflected signals are fed back to a power
control circuit wherein the power delivered to the plasma is
monitored without interference from spurious frequency signals
generated by the plasma.
6. A VHF generator for delivering rf power to a plasma as recited
in claim 5 wherein forward and reflected bandpass filters are
connected between the output of the directional coupler and the
first mixer for removing harmonics and spurious low frequency
signals.
7. A VHF generator for delivering rf power to a plasma as recited
in claim 6 wherein the detectors are diode detectors.
8. A VHF generator for delivering rf power to a plasma as recited
in claim 6 wherein the detectors are RMS detectors.
.Iadd.9. An apparatus for delivering RF power to a plasma,
comprising: a. an RF power source that delivers power to a plasma
in a processing chamber; and b. an output sampler disposed between
the RF power source and the plasma, the output sampler disposed to
sample at least one output characteristic of the RF power source
and thereby generate at least one output signal; wherein the output
signal is mixed with an oscillator frequency to generate a mixed
signal, the mixed signal processed to generate a filtered mixed
signal, and the filtered mixed signal fed back to a control circuit
for controlling the RF power source..Iaddend.
.Iadd.10. The apparatus of claim 9 wherein the output sampler is a
directional coupler..Iaddend.
.Iadd.11. The apparatus of claim 9 wherein the output
characteristic is one of forward or reflected power..Iaddend.
.Iadd.12. The apparatus of claim 9, further comprising a matching
network disposed between the output sampler and the
plasma..Iaddend.
.Iadd.13. The apparatus of claim 9 wherein the RF power source is a
VHF generator..Iaddend.
.Iadd.14. The apparatus of claim 9 wherein the RF power source is a
variable frequency RF power source..Iaddend.
.Iadd.15. The apparatus of claim 10, further comprising forward and
reflected band pass filters disposed at the output of the
directional coupler for removing harmonics and spurious low
frequency signals..Iaddend.
.Iadd.16. The apparatus of claim 15 wherein the filters are
connected to detectors that monitor the power delivered to the
plasma..Iaddend.
.Iadd.17. The apparatus of claim 16 wherein the detectors are RMS
detectors..Iaddend.
.Iadd.18. A method of delivering RF power to a plasma, comprising,
a. providing an RF power source that delivers power to a plasma in
a processing chamber; b. providing an output sampler between the RF
power source and the plasma; c. sampling at least one output
characteristic of the RF power source using the output sampler,
thereby generating at least one output signal; d. mixing the output
signal with an oscillator frequency to generate a mixed signal; e.
processing the mixed signal to generate a filtered mixed signal;
and f. using the filtered mixed signal to control the RF power
source..Iaddend.
.Iadd.19. The method of claim 18 wherein the output sampler is a
directional coupler..Iaddend.
.Iadd.20. The method of claim 18 wherein the output characteristic
is one of forward or reflected power..Iaddend.
.Iadd.21. The method of claim 18 wherein the RF power source is a
VHF generator..Iaddend.
.Iadd.22. The method of claim 18 wherein the RF power source is a
variable frequency RF generator..Iaddend.
.Iadd.23. The method of claim 19, further comprising providing
forward and reflected band pass filters disposed at the output of
the directional coupler for removing harmonics and spurious low
frequency signals..Iaddend.
.Iadd.24. The method of claim 23 wherein the filters are connected
to detectors to monitor the power delivered to the
plasma..Iaddend.
.Iadd.25. The method of claim 24 wherein the detectors are RMS
detectors..Iaddend.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to plasma processing applications
utilizing RF power and more particularly to RF power generators
used in plasma processing applications having circuitry to improve
power delivery characteristics.
2. Brief Description of the Prior Art
Heretofore, RF power generators used in plasma applications have
relied on relatively simple diode peak detectors operating from the
output of a directional coupler to monitor the power delivered to
the plasma However, this method becomes unreliable in the presence
of plasma induced spurious frequencies because the diode detectors
cannot differentiate between the voltage of the generator output
frequency and that of the plasma induced spurious frequencies.
Synchronous detection methods have been used to eliminate the
effect of spurious frequencies, however the cost and complexity of
such a design is generally not acceptable for plasma generator
applications. Other power detectors relying on thermal response
have also been used. Bandpass filters of various types have been
used to eliminate spurious frequencies. However, insertion losses
in these filters are difficult to control, particularly when the
offending frequency is very close to the generator output
frequency.
FIG. 1 shows a typical VHF power generator 10 of the prior art. A
rf frequency source 12 supplies power to an amplifier 14 which is
fed to a directional coupler 16. The directional coupler 16 is
connected to diode detectors 18 and 20. In some designs filters
(not shown) are used between the directional coupler 16 and the
detectors 18 and 20 to remove spurious frequencies. However, design
of these filters is very difficult when the spurious frequencies
are very close to the generator output frequency. For example, if
the spurious frequency has an amplitude that is higher than the
generator output and this signal is close in phase to the output
frequency of the generator that is being fed back to monitor the
power delivered, the peak diode detectors cannot distinguish
between the true feedback signal and the signal of the spurious
frequency. Therefore, the detected signals connected to the
feedback power control circuitry 22 would be in error. A matching
network 24 is connected between the generator 10 and plasma chamber
26 in a typical system application. A second generator 30 coupled
through matching network 32 may supply power to the chamber at a
different operating frequency.
It would be desirable if there were provided an RF generator that
had improved power control stability by isolating the feedback
control voltage from the plasma induced spurious frequencies.
Accordingly, it is an object of this invention to provide an
improved RF power control method that is easily implemented to
replace more expensive and complex designs currently used in the
prior art.
It is a further object of this invention to replace the complex
narrowband filters as used in prior art with a relatively simple
low pass filter to remove the spurious signals.
It is yet another object of this invention to provide a means for
using methods other than voltage detection of the generator output
for monitoring its power.
It is yet another object of this invention to provide a means for
improved matching network tuning in the presence of spurious
frequencies.
SUMMARY OF THE INVENTION
There is provided by this invention an improved RF power control
method for plasma applications that optimizes the feedback control
voltage in the presence of harmonic and non-harmonic spurious
caused by interaction between multiple generators acting on the
non-linear plasma. In this system, an oscillator and mixer are
placed at the sampled output of the solid state RF source used for
plasma ignition. The sampled output is mixed to an intermediate
frequency and filtered to remove the spurious frequencies that are
created in the non-linear plasma. In this way, the feedback power
control essentially ignores the spurious frequencies. In this
application, the oscillator and mixer do not interfere with other
desirable system characteristics and effectively isolate the
feedback control voltage from changes in plasma spurious frequency
content. This allows RF power to be delivered to the plasma with
greater accuracy than would otherwise be possible with conventional
power control methods
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a block diagram of a prior art VHF power
generator;
FIG. 2 illustrates a block diagram of a VHF power generator a with
generalized heterodyne detector function incorporating the
principles of this invention;
FIG. 3 illustrates a block diagram of a VHF power generator with a
heterodyne detector function utilizing a rms detector; and
FIG. 4 illustrates a block diagram of a VHF power generator with a
variable frequency heterodyne detector.
BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT
There is shown in FIG. 2 a VHF generator 10' that incorporates the
principles of this invention that detects and monitors the power
delivered to plasma in a plasma processing system such as that
shown FIG. 1. In this preferred embodiment a 162 MHz rf source 12
is connected to the power amplifier 14 and sends power to the
directional coupler 16. Sampled 162 MHz forward and reflected
signals are taken from the directional coupler 16 and injected into
the detector circuit where these signals are mixed with a base
frequency. The detector circuit is comprised of band pass filters
20', 22', mixers 24', 26'; and a heterodyne oscillator 28. The
mixed frequencies are passed through low pass filters 30' and 32'
there they are amplified by amplifiers 34 and 36 and detected by
detectors 38 and 40. The detected signals are then fed back to a
generator control circuit 42. This heterodyne detection circuit
allows the output of the generator at 18 to be constantly adjusted
for changing plasma conditions. The detector circuit is capable of
taking several options for the oscillator frequency, with
appropriate change in filtering.
For this description, assume that the oscillator frequency is 162.2
MHz. A signal at the oscillator frequency is mixed (heterodyned)
with the sensed forward and reflected power signals. This
explanation describes only the forward power signal. The reflected
power signal is treated in exactly the same way. The forward power
signal has frequency components as follows,
P.sub.Fwd=V(f.sub.0,2f.sub.0,3f.sub.0, . . .
,f.sub.0+f.sub.1,f.sub.0-f.sub.1,f.sub.0+2f.sub.1,f.sub.0-2f.sub.1,
. . . ) where, f.sub.0=fundamental frequency (In our case this is
162 MHz). 2f.sub.0=2.sup.nd Harmonic of the fundamental frequency.
3f.sub.0=3.sup.rd Harmonic of the fundamental frequency.
f.sub.1=Second frequency (2 MHz in this case).
Thus the forward power signal may contain frequency components of
162 MHz, 324 MHz, 486 MHz, 648 MHz, 810 MHz, . . . etc.
When a signal with the above spectrum is mixed with the oscillator
frequency at 162.2 MHz, the mixed products are generated according
to the following trigonometric formula, cos .alpha. cos .beta.=1/2
cos(.alpha.-.beta.)+1/2 cos(.alpha.+.beta.)
Thus the mixed frequencies are at 200 kHz, 161.8 MHz, 164 MHz,
323.8 MHz, etc. This spectrum is then filtered through the low pass
filters 30',32' that only allows the 200 KHz signal to pass
through. Thus at the output of these filters there is a signal that
has the same amplitude information as the sensed 162 MHz forward
and reflected power signals. However, since it is filtered, it is
insensitive to any other mixed frequencies, including harmonics of
162 MHz, 2 MHz sidebands, or any other chamber induced frequency
that is sufficiently apart from the 162 MHz signal. The power
detector may be designed for either fixed frequency or variable
frequency generator operation.
FIG. 2 shows the heterodyne detector connected between the
directional coupler in place of the conventional diode detector.
The power control circuitry within the generator is effectively
isolated by the heterodyne detector from changes in the spurious
content of the waveform at the output of the generator and always
senses only the generator output power. This results in improved
plasma stability and more consistent process results. The power
control is optimized for operation at the output frequency without
compromises needed to maintain power control accuracy in the
presence of spurious. It is understood that the mixers and filters
shown may be of the active or passive type and the filters may use
either lumped element or distributed components.
The directional coupler forward and reflected power samples are
band pass filtered to remove harmonics and low frequency spurious
ahead of the mixers. It is understood that the output of the 162.2
MHz oscillator is filtered to reduce its harmonics. The outputs of
the mixers are passed through 200 kHz filters. Other oscillator
frequencies may be used to produce filter frequencies other than
200 kHz as long as the unwanted spurious frequencies appear outside
the filter band pass. The filter output is amplified, detected, and
used in the normal way for feedback power control of the
generator.
FIG. 3 illustrates another embodiment that utilizes true RMS
detectors 44 and 46 in place of the diode detectors. The block
diagram of FIG. 3 shows such detectors operating directly on the
200 kHz waveform.
Still another embodiment derives the oscillator frequency from the
generator variable frequency source, thus allowing the heterodyne
detector to track the variable output frequency of the generator.
The block diagram of this embodiment is shown in FIG. 4. A 57-63
MHz MHz application is shown. The output of the variable frequency
source 48 is mixed with an 80 MHz signal to give 137-143 MHz. This
is filtered and mixed with 79.8 MHz down to 57.2-63.2 MHz. This
signal is then mixed with the original 57-63 MHz to provide the
necessary 200 kHz for the low pass filter and the detector circuit.
It is understood that suitable amplification may be used along the
signal path to compensate for losses that occur during the
frequency conversion process. It is further understood that
although only the Forward power sample from the directional coupler
is shown, the reflected power sample may be detected in the same
way. It is still further understood that the variable frequency
injected into Mixer #1 may be derived by other means, such as an
additional output of the generator variable frequency source.
It will be further understood that, although the invention is
described for a VHF application, it may be used at any frequency
for the purpose of detecting and controlling generator output
power. It will be still further understood that, although power
control and metering is discussed as being internal to the
generator, it may be accomplished by any number of means and be
either internal or external to the generator as well as be
controlled by a system CPU. It will be still further understood
that detailed schematic diagrams have been eliminated for the
purpose of clarity. It will be still further understood that the
improved power detector may be used to accurately control power
from any or all generators operating on the plasma.
The invention has been described in detail with particular
reference to a preferred embodiment. It will be understood that
variations and modifications in addition to those described can be
effected within the spirit and scope of the invention. It will be
further understood that changes, alterations, modifications, or
substitutions can be made in the structure of the apparatus in
accordance with the invention without departing from the spirit and
scope of the invention.
* * * * *