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Claims  |
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What is claimed is:
1. A device for generating plasma for use in semiconductor fabrication,
comprising:
a first radio frequency excitation source for outputting a first excitation
current having a first phase and a first amplitude;
a second radio frequency excitation source for outputting a second
excitation current having a second phase and a second amplitude;
a plasma generating element having a first end and a second end for
receiving respectively said first excitation current and said second
excitation current; and
a control circuit having a control input, said control input receiving a
user-variable signal indicative of a desired phase difference between said
first phase and said second phase, said control circuit, responsive to
said control input, outputs a control signal to one of said first radio
frequency excitation source and said second radio frequency excitation
source for controlling respectively one of said first phase and said
second phase, thereby causing an actual phase difference between said
first phase and said second phase to substantially approximate said
desired phase difference, whereby said device becomes essentially an
inductive coupling device when said first phase and said second phase are
opposite in phase, said device becomes essentially a capacitive coupling
device when said first phase and said second phase are in phase, said
device becomes a combination inductive and capacitive coupling device when
said first phase and said second phase differs by an angle that is between
in phase and opposite in phase.
2. The device of claim I wherein each of said first radio frequency
excitation source and said second radio frequency excitation source
includes:
a radio frequency generator; and
a matching circuit coupled to said radio-frequency generator for matching
an output impedance of said radio frequency generator with an input
impedance of said plasma generating element to optimize power delivery by
said radio frequency generator.
3. The device of claim 2, further comprising:
a first measurement device coupled to said first end and said control
circuit, said first measurement device outputting to said control circuit
a first measurement signal indicative of said first phase; and
a second measurement device coupled to said second end and said control
circuit, said second measurement device outputting to said control circuit
a second measurement signal indicative of said second phase, whereby said
first measurement signal and said second measurement signal are utilized
by said control circuit in deriving said control signal.
4. The device of claim 3 wherein said first phase is fixed and said second
phase is variable responsive to said control signal.
5. The device of claim 3 wherein said plasma generating element is a planar
coil.
6. The device of claim 3 wherein said plasma generating element is a
nonplanar coil.
7. The device of claim 3 wherein said first measurement device is a voltage
probe.
8. The device of claim 3 wherein said first measurement device is a current
probe.
9. The device of claim 3 wherein said control circuit comprises:
a phase comparison circuit coupled to said first measurement device and
said second measurement device for generating a phase differential signal;
and
a phase error amplifier coupled to said phase comparison circuit, said
control input and one of said first radio frequency excitation source and
said second radio frequency excitation source, said phase error amplifier
generating said control signal responsive to said phase differential
signal and said control input.
10. The device of claim 9 wherein said phase comparison circuit comprises:
a mixer circuit coupled to said first measurement signal and said second
measurement signal for generating a mixer output signal; and
a low pass filter coupled to said mixer circuit for generating, responsive
to said mixer output signal said phase differential signal.
11. The device of claim 9 wherein said phase comparison circuit employs
quadrature detection.
12. A method for generating plasma for use in plasma-enhanced semiconductor
processes, comprising:
generating a first excitation current using a first radio frequency
excitation source, said first excitation current having a first phase and
a first amplitude;
generating a second excitation current using a second radio frequency
excitation source, said second excitation current having a second phase
and a second amplitude;
providing said first excitation current and said second excitation current
to a plasma generating element, said plasma generating element having a
first end and a second end for receiving respectively said first
excitation current and said second excitation current; and
controlling one of said first excitation current and second excitation
current using a control circuit, said control circuit having a control
input for receiving a signal indicative of a desired phase difference
between said first phase and said second phase, said control circuit,
responsive to said control input, outputs a control signal to one of said
first radio frequency excitation source and said second radio frequency
excitation source for controlling respectively one of said first phase and
said second phase, thereby causing an actual phase difference between said
first phase and said second phase to substantially approximate said
desired phase difference, whereby said method generating essentially
inductively coupled plasma when said first phase and said second phase are
opposite in phase, said method generating essentially capacitively coupled
plasma when said first phase and said second phase are in phase, said
method generating essentially a combination of inductively coupled and
capacitively coupled plasma when said first phase and said second phase
differs by an angle that is between in phase and opposite in phase.
13. The method of claim 12 wherein said first radio frequency excitation
source comprises a first radio frequency generator and said second radio
frequency excitation source comprises a second radio frequency generator.
14. The method of claim 13 wherein said first radio frequency excitation
source further comprises a first matching circuit coupled to said first
radio frequency generator for matching an output impedance of said first
radio frequency generator with an input impedance of said plasma
generating element to optimize power delivery by said first radio
frequency generator, said second radio frequency excitation source further
comprises a second matching circuit coupled to said second radio frequency
generator for matching an output impedance of said second radio frequency
generator with an input impedance of said plasma generating element at
said first end to optimize power delivery by said second radio frequency
generator.
15. The method of claim 13, further comprising:
measuring an electrical parameter of said first excitation current, said
electrical parameter being a parameter from which said first phase can be
derived; and
providing said electrical parameter to said control circuit as a first
measurement signal, whereby said first measurement signal is used, in
combination with said control input, by said control circuit in deriving
said control signal.
16. The method of claim 15 wherein said measuring step is performed using a
voltage probe and said electrical parameter represents voltage.
17. The method of claim 13 wherein said first radio frequency generator is
a fixed phase radio frequency generator and said second radio frequency
generator is a variable phase radio frequency generator.
18. The method of claim 13 wherein said plasma generating element is a
planar coil.
19. The method of claim 13 wherein said plasma generating element is a
nonplanar coil.
20. An apparatus for generating plasma for use in plasma-enhanced
semiconductor processes, comprising:
first means for generating a first radio frequency excitation current, said
first radio frequency excitation current having a first phase and a first
amplitude;
second means for generating a second radio frequency excitation current,
said second radio frequency excitation current having a second phase and a
second amplitude;
means for generating said plasma, utilizing energy supplied by said first
radio frequency excitation current and said second radio frequency
excitation current; and
means for controlling one of said first radio frequency excitation current
and second radio frequency excitation current, said control means having a
control input for receiving a signal indicative of a desired phase
difference between said first phase and said second phase, said control
means, responsive to said control input, outputs a control signal to one
of said first means for generating said first radio frequency excitation
current and said second means for generating said second radio frequency
excitation current for controlling respectively one of said first phase
and said second phase, thereby causing an actual phase difference between
said first phase and said second phase to substantially approximate said
desired phase difference, whereby said apparatus generating essentially
inductively coupled plasma when said first phase and said second phase are
opposite in phase, said apparatus generating essentially capacitively
coupled plasma when said first phase and said second phase are in phase,
said apparatus generating essentially a combination of inductively coupled
and capacitively coupled plasma when said first phase and said second
phase differs by an angle that is between in phase and opposite in phase.
21. The apparatus of claim 20 wherein said first means for generating said
first radio frequency excitation current comprises a first radio frequency
generator and said second means for generating said second radio frequency
excitation current comprises a second radio frequency generator.
22. The apparatus of claim 21 wherein said first radio frequency excitation
source further comprises a first matching circuit coupled to said first
radio frequency generator for matching an output impedance of said first
radio frequency generator with an input impedance of said plasma
generating element to optimize power delivery by said first radio
frequency generator, said second radio frequency excitation source further
comprises a second matching circuit coupled to said second radio frequency
generator for matching an output impedance of said second radio frequency
generator with an input impedance of said plasma generating element at
said first end to optimize power delivery by said second radio frequency
generator.
23. The apparatus of claim 22, further comprising:
means for measuring an electrical parameter of said first excitation
current, said electrical parameter being a parameter from which said first
phase can be derived, whereby said first measurement signal is used, in
combination with said control input, by said control circuit in deriving
said control signal. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
The present invention relates to methods and apparatus for inducing plasma
in low pressure plasma systems, which are typically used in semiconductor
fabrication. More specifically, the invention relates to methods and
apparatus for variable control of the plasma generating element to achieve
combinations of inductive and/or capacitive coupling.
Plasma-enhanced semiconductor processes for etching, oxidation,
anodization, chemical vapor deposition (CVD), or the like are known.
For illustration purposes, FIG. 1 shows a chemical etch reactor 100,
representing a plasma generating system which utilizes an inductive coil
for plasma generation. Reactor 100 includes coil system 102 and chamber
124. Coil system 102 includes a coil element 106, which is biased by radio
frequency generator 110 to act as an electrode. Coil element 106 is
coupled to a matching circuit 108 for matching the impedance of coil
element 106 to that of radio frequency generator 110. The matching of the
impedances permits radio frequency generator 110 to efficiently deliver
power to coil element 106. To provide a path to ground, the chamber wall
of chamber 124 is typically grounded. Alternatively, the ground path may
be provided through the lower electrode, e.g., a chuck 128 of FIG. 1, when
the plasma is confined.
Within chamber 124, there typically exists a vacuum. A shower head 126 is
disposed above a chuck 128 and wafer 134, which is supported by chuck 128.
Chuck 128 acts as a second electrode and is preferably biased by its
independent radio frequency circuit 120 via a matching network 122. It
should be borne in mind that the components of FIG. 1, as well as of other
figures herein, are shown only representatively for ease of illustration
and to facilitate discussion. In actuality, coil element 106 and match 108
are typically disposed proximate to chamber 124 while RF generator 110 may
be placed in any reasonable location.
Shower head 126 represents the apparatus for dispensing deposition
materials onto wafer 134. Shower head 126 preferably includes a plurality
of holes for releasing gaseous source materials (typically around the
periphery edge of shower head 126) into the RF-induced plasma region
between itself and wafer 134 during operation. In one embodiment, shower
head 126 is made of quartz although it may also be made of other suitable
materials and may be left either electrically floating or grounded.
In the prior art, there exists capacitively coupled plasma systems. It has
been discovered, however, that inductively coupled plasma generates higher
plasma density, which is more suitable for certain low pressure processes.
In the prior art, the relative phases at first coil end 130 and second
coil end 132 of coil system 102 is a function of the electrical length of
the coil and the operating frequency and is consequently relatively fixed.
However, a plasma generating system that is preset to couple its plasma
either inductively or capacitively is inherently limiting. Modern
fabrication processes demand flexibility on the part of the equipment that
are used to fabricate semiconductor circuits. Consequently, there has been
efforts to provide for plasma generating systems that can be configured,
in a flexible manner, as either an inductive system, a capacitive system,
or one that provides for a combination of both inductive and capacitive
coupling.
By way of example, there exists in the prior art a control circuit which
utilizes four capacitors for producing either inductively coupled plasma
or capacitively coupled plasma. This prior art control circuit, which is
essentially analog in character, controls the coupling of the coil by
varying the capacitance of one or more capacitors.
Although the aforementioned control scheme has some advantages, it
nevertheless represents an electromechanical approach, which results in
many attendant disadvantages. For example, it is difficult to set up the
capacitors in the prior art analog control circuit because the setup
parameters depend on the specific measurements pertaining to a particular
reactor.
Further, the prior art electromechanical approach to providing the desired
combination of inductive/capacitive coupling is static and is therefore
difficult to change to accommodate, in a flexible and simple manner,
applications that demand different combinations of inductive and/or
capacitive coupling. Most significantly, it is difficult to vary, as a
function of time, combinations of inductive and capacitive coupling using
the prior art electromechanical approach.
In view of the foregoing, what is desired is new apparatus and methods for
achieving, in a flexible and simple manner, variable combinations of
inductive and/or capacitive coupling in a plasma generating system.
SUMMARY OF THE INVENTION
The invention relates, in one embodiment, to a device for generating plasma
for use in semiconductor fabrication, which includes a first radio
frequency excitation source for outputting a first excitation current
having a first phase and a first amplitude. The inventive device further
includes a second radio frequency excitation source for outputting a
second excitation current having a second phase and a second amplitude and
a plasma generating element having a first end and a second end for
receiving respectively the first excitation current and the second
excitation current.
Moreover, the inventive device includes a control circuit having a control
input for receiving a user-variable signal indicative of a desired phase
difference between the first phase and the second phase. The control
circuit, responsive to the control input, outputs a control signal to one
of the first radio frequency excitation source and the second radio
frequency excitation source for controlling respectively one of the first
phase and the second phase, thereby causing an actual phase difference
between the first phase and the second phase to substantially approximate
the desired phase difference. In so doing, the device becomes essentially
an inductive coupling device when the first phase and the second phase are
opposite in phase. When the first phase and the second phase are in phase,
the device becomes essentially a capacitive coupling device. Finally, when
the first phase and the second phase differs by an angle that is between
in phase and opposite in phase, the device becomes a combination inductive
and capacitive coupling device.
In another embodiment, the invention relates to a method for generating
plasma for use in plasma-enhanced semiconductor processes, which includes
the step of generating a first excitation current using a first radio
frequency excitation source. The first excitation current has a first
phase and a first amplitude. Further, the invention includes the step of
generating a second excitation current using a second radio frequency
excitation source, the second excitation current having a second phase and
a second amplitude.
Moreover, the inventive method includes the step of providing the first
excitation current and the second excitation current to a plasma
generating element, the plasma generating element having a first end and a
second end for receiving respectively the first excitation current and the
second excitation current. Additionally, there is provided the step of
controlling one of the first excitation current and second excitation
current using a control circuit. The control circuit has a control input
for receiving a signal indicative of a desired phase difference between
the first phase and the second phase.
The control circuit, responsive to the control input, outputs a control
signal to one of the first radio frequency excitation source and the
second radio frequency excitation source for controlling respectively one
of the first phase and the second phase, thereby causing an actual phase
difference between the first phase and the second phase to substantially
approximate the desired phase difference. In so doing, the method
generates essentially inductively coupled plasma when the first phase and
the second phase are opposite in phase. When the first phase and the
second phase are in phase, the method generates essentially capacitively
coupled plasma. Finally, when the first phase and the second phase differs
by an angle that is between in phase and opposite in phase, the method
generates essentially a combination of inductively coupled and
capacitively coupled plasma.
These and other advantages of the present invention will become apparent
upon reading the following detailed descriptions and studying the various
figures of the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows, for illustration purposes, a chemical etch reactor,
representing a plasma generating system which utilizes an inductive coil
for plasma generation;
FIG. 2 is a diagram of the plasma generating system in accordance with one
embodiment of the present invention;
FIG. 3 shows an example of one embodiment of a matching circuit, which
utilizes three variable capacitors in a T configuration;
FIG. 4 is a graph illustrating two signals, RF1 and RF2, including their
phases;
FIGS. 5A-5C shows the excitation currents output by the RF excitation
sources at various phase angles, along with the resulting couplings (in
symbolic form);
FIG. 6 shows one embodiment of a voltage probe for measuring the potential
level at one end of the plasma generating element;
FIG. 7 shows in one embodiment a current probe for measuring current
through a conductor;
FIG. 8 shows yet another embodiment of the inventive plasma generating
system;
FIG. 9 illustrates a phase detection scheme utilizing quadrature detection;
and
FIG. 10 illustrates a rotating phasor diagram in which two vectors are
shown for discussing the advantages associated with quadrature detection.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows, for illustration purposes, a chemical etch reactor 100,
representing a plasma generating system which utilizes an inductive coil
for plasma generation.
FIG. 2 is a diagram of the plasma generating system in accordance with one
embodiment of the present invention. Referring now to FIG. 2, there is
shown a plasma generating system 200, which includes a plasma generating
element 202. Plasma generating element 202 represents, in one embodiment,
an inductive coil, e.g., one known as a TCP.TM. (transformer coupled
plasma) coil, although the inventive plasma generating system of the
present invention may be extended to include any type of inductive plasma
generating element. The coil itself may have any number of configurations,
including the Archimedes spiral configuration. Further, plasma generation
element 202 is preferably planar although nonplanar plasma generation
elements are also suitable.
At a first end 204 of plasma generating element 202, there is coupled a
measurement device 206. Measurement device 206 preferably measures the
voltage at first end 204. In one embodiment, however, measurement device
206 may be configured to measure other electrical parameters such as
current, phase, vector impedance, and the like. Similarly, there is
coupled at second end 208 of plasma generating element 202 a second
measurement device 210. Like measurement device 206, measurement device
210 is preferably used to measure voltages although measurements of
current, phase, vector impedance, as well as any other desired electrical
perimeter at second end 208 may also be obtained if desired.
In one specific example, measurement devices 206 and 210 represent voltage
probes to measure the voltages at the ends of plasma generating element
202. These voltage measurements are preferably representations of the
currents being measured. (The term current is used herein to also denote
the flow of electrical energy, which occurs at measurable voltage levels).
Consequently, they preferably have the same waveform as those excitation
currents existing at the ends of plasma generating element 202. The
voltage measurements are utilized, in one embodiment, as feedback signals
by a control circuit 220 to, for example, ascertain the phase difference
between excitation currents at these two ends. Responsive to the
ascertained phase difference, control circuit 220 may then provide control
signals to one or both RF excitation sources 223 and 225 to modify the
phase of the current output thereby to achieve the desired phase
difference at the coil ends. In so doing, the desired combinations of
inductive and/or capacitive coupling are obtained.
Unlike the electromechanical approach of the prior art, control circuit 220
preferably implements a electronic controller, e.g., solid state. By
implementing control circuit 220 electronically, the inventive plasma
generating system has multiple advantages over the prior art. For example,
it is now possible to more finely derive the various combinations of
inductive and/or capacitive coupling provided by the plasma generating
element. Further, it is possible to make those combinations varying over
time, or to pulse between combinations or coupling modes if desired. As
discussed, these capabilities were lacking in the prior art
electromechanical control solutions.
Excitation currents are provided to plasma generating element 202 via radio
frequency (RF) excitation sources 223 and 225 respectively. In RF
excitation source 223, there is provided a RF generator 222 for providing
the sourcing current, which then traverses matching circuit 226 prior to
being input into one end of plasma generating element 202 as one of the
excitation currents. Similarly, RF excitation source 225 includes a RF
generator 224, representing the source for another excitation current. The
current output by RF generator 224 traverses matching circuit 228 to
provide another excitation current to the other end of plasma generating
element 202.
RF generators 222 and 224 may represent any number of commercially
available radio frequency generators, preferably one having a phase input
such as an AE model RFG 1250, which is available from Advanced Energy
Industries, Inc. of Fort Collins, Colo. As will be explained in detail
later, the inventive plasma generating system utilizes the RF generator
phase input to flexibly achieve combinations of capacitive and/or
inductive coupling for its plasma.
Note that unlike the prior art plasma generating systems, which uses only
one generator and one matching network for generating a fixed
capacitive/inductive coupling combination, the present invention utilizes
two radio frequency generators and two matching networks for the two ends
of the plasma generating element. Because of this, flexibility in the
control of the relative phases at those two ends is substantially
enhanced. Since the energy to the plasma generating element itself is
controlled so that different phases can be presented at the two ends of
the plasma generating element, there is practically no limit to the type
of waveforms that can be generated, even during a process run if desired,
across the plasma generating element. Advantageously, the ability to
derive various combinations capacitive/inductive coupling for the plasma
is substantially enhanced, with concomitant control over the plasma that
results therefrom.
As mentioned earlier, the function of a matching circuit is essentially to
match the output impedance of the RF generator to the input impedance of
the plasma generating element. As an example, plasma generating element
202 may have an input impedance of, say, about 2-3 .OMEGA.. However,
modern generators typically operate at about 50 .OMEGA.. By matching up
these impedances, a matching circuit enables the RF generator, e.g., RF
generator 222 or 224, to deliver power to its plasma generating element in
an efficient manner.
In one embodiment, matching circuits 226 and 228 are implemented by
transformer networks. In another embodiment, matching circuits 226 and 228
represent fixed non-tunable matching networks with the generators
providing the necessary current and voltage headroom required for any
plasma condition. However, it is contemplated that continuously tunable or
switch tuning elements may be employed in matching circuits 226 and 228,
if appropriate, without departing from the scope and spirit of the present
invention.
Control circuit 220 further includes a phase input terminal 232,
representing a control input whereby signals indicative of the desired
phase differences may be input. In one embodiment, the desired amplitude
is also input into control circuit 220 for generating amplitude control
signals to RF generator 222 and RF generator 224. When the amplitude is
changed, the center to edge distribution of power in the coil
correspondingly changes. Advantageously, the ability to dynamically
control the center-to-edge distribution of power in the coil provides
greater control over the plasma process since plasma parameters, such as
density or the like, may be flexibly and accurately controlled at run
time.
In operation, the electrical measurements obtained through measurement
devices 206 and 210 are used by control circuit 220 to ascertain the
difference in phases at first end 204 and second end 208. Control circuit
220, responsive to the ascertained phase difference at the ends of the
plasma generating element and the desired phase difference at phase input
terminal 232, then outputs a control signal to one or both of RF
generators 222 and 224. This control signal is input into the phase input
of the RF generator to modify the phase of the RF generator output.
In one embodiment, RF generator 222 provides a fixed phase reference
through matching network 226 and via the frequency at which plasma
generating system 200 operates. The other RF generator in the plasma
generating system 200, e.g., RF generator 224, also operates at the same
frequency. However, the phase and amplitude of RF generator 224 may be
varied through its phase input. In this embodiment, the control signal
from control circuit 220 needs to be input into only one of the RF
generators, e.g., RF generator 224, to modify the phase difference at the
two ends of plasma generating element 202.
In other embodiments, however, it is contemplated that both RF generators
222 and 224 may be configured to have, responsive to their phase inputs,
variable output phases and amplitudes. In this case, control circuit 220
preferably generates a plurality of control signals to individually
control the multiple RF generators in plasma generating system 200.
If RF generators 222 and RF generators 224 are controlled such that both
first end 204 and second end 208 are in phase, i.e., at the same phase,
there is no current through plasma generating element 202 and the coupling
is purely capacitive. On the other hand, if RF generator 222 and RF
generator 224 are controlled such that their respective phases are
opposite in phase, i.e., have a 180.degree. offset, the coupling becomes
purely inductive with almost no capacitive coupling. In between the in
phase and opposite in phase situations, a combination of
inductive/capacitive coupling may be obtained. The exact combination
depends on the input signal representing the desired phase difference,
which exists at phase input terminal 232 in one embodiment.
FIG. 3 shows an example of one embodiment of a matching circuit 300, which
utilizes three variable capacitors 302, 304, and 306 in a T configuration.
Matching circuit 300 may be used to implement, for example, matching
circuit 226 or matching circuit 228 of the plasma generating system 200 of
FIG. 2. As shown in FIG. 3, one end of the T-configured capacitor network
is coupled to the RF generator while another end is coupled to the plasma
generating element. In between these two capacitors, a third capacitor is
coupled to ground. The variable capacitors 302, 304, and 306 may be
individually adjusted to achieve the right match between the output
impedance of the RF generator and the input impedance of the plasma
generating element. It should be kept in mind that FIG. 3A is only
illustrative and other matching network designs, which are known in the
art, may also be employed in the present inventive plasma generating
system.
To further illustrate, FIG. 4 is a graph illustrating two signals, RF1 and
RF2, including their phases. Signal RF1 represents, for example, the phase
of the excitation current supplied to one end of plasma generating element
202 while signal RF2 may represent the phase of the excitation current
supplied to the other end of that plasma generating element. As is shown
in FIG. 4, signals RF1 and RF2 have substantially the same frequency but
differ in phases by an angle A. As mentioned earlier, it is this
difference in phases that determines whether the coupling provided by
plasma generating element 202 is purely inductive, purely capacitive, or a
combination of both.
It should be kept in mind that although the amplitudes of signals RF1 and
RF2 are shown to be substantially the same for ease of illustration, such
is not required. In fact, it is contemplated that user control over the
amplitudes of the signals output by the RF generators represents one
important aspect of the present invention. In one embodiment, the desired
amplitude of the RF generator output signal may be input into control
circuit 220 for use in generating an amplitude control signal. This
amplitude control signal may in turn be used to cause one or both RF
generators to output signals having the desired amplitudes.
The excitation currents output by the RF excitation sources, e.g.,
excitation sources 223 and 225 of FIG. 2, at various phases are shown in
FIGS. 5A-5C. Correspondingly, the resulting couplings caused by the output
phases are also shown (in symbolic form). In FIG. 5A, the excitation
currents into the ends of the plasma generating element are opposite in
phase, i.e., are 180.degree. out of phase. Therefore, the coupling is
purely inductive as shown symbolically in FIG. 5A.
In FIG. 5B, the excitation currents have their phases offset by an angle
that is between 0.degree. and 180.degree.. Consequently, the resulting
coupling is both inductive and capacitive, as shown symbolically in FIG.
5B. In FIG. 5C, the excitation currents are in phase, i.e., offset by
0.degree.. Therefore, the coupling is purely capacitive, as shown
symbolically in FIG. 5C.
FIG. 6 shows one embodiment of a voltage probe 390 for measuring the
potential level at a plasma generating element end. Voltage probe 390 may
represent, for example, measurement device 206 or measurement device 210
of FIG. 2. Referring now to FIG. 6, voltage probe 390 includes two
capacitors 400 and 402 in series between ground and point 392. Point 392
represents the point which has the potential to be measured, e.g., the end
of the plasma generating element. As is apparent, capacitors 400 and 402
act as a capacitor divider network for outputting a measurement signal,
which preferably has the same waveform as that of the signal being
measured.
In one embodiment, capacitors 400 and 402 are preferably selected to be
small capacitors, say, 10 pf for capacitor 400 and about 990 pf for
capacitor 402. These capacitors are preferably small to avoid unduly
affecting the electrical characteristics of the signal being measured. In
this example, the measurement signal on conductor 402 has the same
waveform as that at node 392, albeit having only 1% of the amplitude of
the latter. It should be kept in mind that FIG. 5 represents only one
scheme for measuring the voltage on a conductor and there are other known
schemes that may also be su | | |
