United States Patent r191
`Dible
`
`I lllll 1111111111111111111111111111111 11111111111111111111111111 111111111111
`US005573595A
`[llJ Patent Number:
`[45] Date of Patent:
`
`5,573,595
`Nov. 12, 1996
`
`[54] METHODS AND APPARATUS FOR
`GENERATING PLASMA
`
`Assistant Examiner-Joni Y. Chang
`Attorney, Agent, or Finn-Hickman Beyer & Weaver
`
`[75]
`
`Inventor: Robert D. Dible, Fremont, Calif.
`
`[57]
`
`ABSTRACT
`
`[73] Assignee: Lam Research Corporation, Fremont,
`Calif.
`
`[21] Appl. No.: 536,574
`
`Sep. 29, 1995
`
`[22] Filed:
`Int. Cl.6
`..................................................... C23C 16/00
`[51]
`[52] U.S. Cl. ............................... 118/723 MP; 118/723 E;
`118/723 I; 156/643.1; 427/569
`[58] Field of Search ......................... 1181723 E, 723 ER,
`118/723 MP, 723 I, 723 IR; 156/643.1,
`345, 627.1; 216/68, 71; 204/298.34, 298.08,
`298.06, 298.12; 427/569
`
`[56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`4,539,068
`5,116,482
`5,147,493
`5,228,939
`5,273,610
`5,316,645
`5,332,880
`5,414,324
`5,433,813
`5,436,424
`
`9/1985 Takagi et al ............................ 156/614
`5/1992 Setoyama et al .................. 204/298.08
`9/1992 Nishimura et al ...................... 156/345
`7/1993 Chu ......................................... 156/345
`12/1993 Thomas, III et al. .................. 156/345
`5/1994 Yamagami et al ................. 204/298.06
`7/1994 Kubota et al ...................... 219/121.52
`5/1994 Roth et al .......................... 315/111.21
`7/1995 Kuwahara ............................... 156/345
`7/1995 Nakayama et al. ................ 219/121.43
`
`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 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. More(cid:173)
`over, 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 induc(cid:173)
`tive and capacitive coupling device.
`
`Primary Examiner-R. Bruce Breneman
`
`23 Claims, 7 Drawing Sheets
`
`FIXED
`RF
`GENERATOR
`
`226
`
`206
`
`210
`
`228
`
`VARIABLE
`RF
`GENERATOR
`
`MATCH
`CIRCUIT
`
`MEASURE(cid:173)
`MENT
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`MEASURE(cid:173)
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`MATCH
`CIRCUIT
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`614
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`c s
`0
`I
`N G
`T N
`R A
`0 L
`L
`
`Ex.1007 p.1
`
`

`
`U.S. Patent
`
`Nov. 12, 1996
`
`Sheet 1of7
`
`5,573,595
`
`100 \
`
`110
`
`108
`
`RF
`GENERATOR
`
`MATCH
`
`130
`
`132
`
`102
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`26
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`120
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`122
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`RF
`GENERATORl---1 MATCH t - - - - - - - - -1
`
`FIG.1
`
`124
`
`Ex.1007 p.2
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`GENERATOR
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`222
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`
`Ex.1007 p.3
`
`

`
`U.S. Patent
`
`Nov. 12, 1996
`
`Sheet 3 of 7
`
`5,573,595
`
`TO
`GENERATOR
`
`/
`
`300
`
`A
`M
`p
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`T
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`304
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`TO PLASMA
`GENERATING
`ELEMENT
`
`306
`
`FIG.3
`
`RF2
`
`TIME
`
`I
`-+-A_.I
`
`FIG.4
`
`Ex.1007 p.4
`
`

`
`U.S. Patent
`
`Nov. 12, 1996
`
`Sheet 4 of 7
`
`5,573,595
`
`I
`~
`
`FIG. 5( a)
`
`INDJCTIVE
`
`+1
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`INDJCTIVE I CAPACITIVE
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`FIG. 5(b)
`
`1
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`
`CAPAOTIVE
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`
`FIG. 5( c)
`
`Ex.1007 p.5
`
`

`
`U.S. Patent
`
`Nov. 12, 1996
`
`Sheet 5 of 7
`
`5,573,595
`
`To Matching
`Circuit
`
`To Plasma
`t------F--------1 Generating
`Element
`
`400
`
`402
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`~390
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`FIG.6
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`FIG.7
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`504
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`Ex.1007 p.6
`
`

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`FIG. 8
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`FILTER
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`
`Ex.1007 p.7
`
`

`
`U.S. Patent
`
`Nov. 12, 1996
`
`Sheet 7 of 7
`
`5,573,595
`
`606
`
`'\
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`902
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`920
`
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`
`FROM MEASUREMENT
`DEVICE
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`912
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`SIGNAL
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`SIGNAL
`PROCESSING
`CIRCUIT
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`CONTROL
`INPUT
`
`FIG. 9
`
`614
`
`FIG. 10
`
`Ex.1007 p.8
`
`

`
`1
`METHODS AND APPARATUS FOR
`GENERATING PLASMA
`
`BACKGROUND OF THE INVENTION
`
`5,573,595
`
`5
`
`2
`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.
`10 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 par(cid:173)
`ticular reactor.
`Further, the prior art electromechanical approach to pro-
`15 viding the desired combination of inductive/capacitive cou(cid:173)
`pling is static and is therefore difficult to change to accom(cid:173)
`modate, in a flexible and simple manner, applications that
`demand different combinations of inductive and/or capaci(cid:173)
`tive coupling. Most significantly, it is difficult to vary, as a
`20 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 cou(cid:173)
`pling in a plasma generating system.
`SUMMARY 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 spe(cid:173)
`cifically, 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 25
`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 representa(cid:173)
`tively 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 45
`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 con(cid:173)
`sequently relatively fixed.
`However, a plasma generating system that is preset to
`couple its plasma either inductively or capacitively is inher(cid:173)
`ently limiting. Modem fabrication processes demand flex(cid:173)
`ibility 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 con(cid:173)
`figured, 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.
`
`40
`
`The invention relates, in one embodiment, to a device for
`generating plasma for use in semiconductor fabrication,
`30 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
`35 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 essen(cid:173)
`tially 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 semicon(cid:173)
`ductor processes, which includes the step of generating a
`60 first excitation current using a first radio frequency excita(cid:173)
`tion 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
`65 having a second phase and a second amplitude.
`Moreover, the inventive method includes the step of
`providing the first excitation current and the second excita-
`
`50
`
`55
`
`Ex.1007 p.9
`
`

`
`5,573,595
`
`10
`
`3
`tion 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 5
`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, out(cid:173)
`puts 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 15
`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 20
`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 draw-
`ings.
`
`4
`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™
`(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 configu(cid:173)
`rations, including the Archimedes spiral configuration. Fur(cid:173)
`ther, 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 mea(cid:173)
`surement device 206, measurement device 210 is preferably
`used to measure voltages although measurements of current,
`phase, vector impedance, as well as any other desired
`25 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 mea(cid:173)
`surable 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
`40 ends. Responsive to the ascertained phase difference, control
`circuit 220 may then provide control signals to one or both
`RF excitation sources 223 and 22S to modify the phase of
`the current output thereby to achieve the desired phase
`difference at the coil ends. In so doing, the desired combi-
`45 nations of inductive and/or capacitive ·coupling are obtained.
`Unlike the electromechanical approach of the prior art,
`control circuit 220 preferably implements a electronic con(cid:173)
`troller, e.g., solid state. By implementing control circuit 220
`electronically, the inventive plasma generating system has
`50 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 com-
`55 binations 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
`60 and 22S 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 22S 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
`
`30
`
`35
`
`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, RFl and RF2,
`including their phases;
`FIGS. SA-SC 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 65
`reactor 100, representing a plasma generating system which
`utilizes an inductive coil for plasma generation.
`
`Ex.1007 p.10
`
`

`
`5,573,595
`
`5
`another excitation current to the other end of plasma gen(cid:173)
`erating element 202.
`RF generators 222 and 224 may represent any number of
`commercially available radio frequency generators, prefer(cid:173)
`ably one having a phase input such as an AE model RFG 5
`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 combi(cid:173)
`nation, the present invention utilizes two radio frequency
`generators and two matching networks for the two ends of 15
`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 20
`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. Advan(cid:173)
`tageously, the ability to derive various combinations capaci(cid:173)
`tive/inductive coupling for the plasma is substantially 25
`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 gen(cid:173)
`erator 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 Q. How(cid:173)
`ever, modem generators typically operate at about 50 Q. 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 embodi(cid:173)
`ment, matching circuits 226 and 228 represent fixed non(cid:173)
`tunable matching networks with the generators providing the
`necessary current and voltage headroom required for any
`plasma condition. However, it is contemplated that continu(cid:173)
`ously 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 differ(cid:173)
`ence at phase input terminal 232, then outputs a control
`signal to one or both of RF generators 222 and 224. This
`
`6
`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
`10 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 con(cid:173)
`trol 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 capaci(cid:173)
`tive. 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° offset, the
`coupling becomes purely inductive with almost no capaci(cid:173)
`tive coupling. In between the in phase and opposite in phase
`30 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 capaci(cid:173)
`tors 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
`40 system 200 of FIG. 2. As shown in FIG. 3, one end of the
`T-configured capacitor network is coupled to the RF gen(cid:173)
`erator 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
`45 306 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
`50 also be employed in the present inventive plasma generating
`system.
`To further illustrate, FIG. 4 is a graph illustrating two
`signals, RFl and RF2, including their phases. Signal RFl
`represents, for example, the phase of the excitation current
`55 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 RFl and RF2 have substan(cid:173)
`tially the same frequency but differ in phases by an angle A.
`60 As mentioned earlier, it is this difference in phases that
`determines whether the coupling provided by plasma gen(cid:173)
`erating element 202 is purely inductive, purely capacitive, or
`a combination of both.
`It should be kept in mind that although the amplitudes of
`65 signals RFl 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
`
`35
`
`Ex.1007 p.11
`
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`5,573,595
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`5
`
`15
`
`7
`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 22S of FIG. 2, at
`various phases are shown in FIGS. SA-SC. Correspond- 10
`ingly, the resulting couplings caused by the output phases
`are also shown (in symbolic form). In FIG. SA, the excita(cid:173)
`tion currents into the ends of the plasma generating element
`are opposite in phase, i.e., are 180° out of phase. Therefore,
`the coupling is purely inductive as shown symbolically in
`FIG. SA.
`In FIG. SB, the excitation currents have their phases offset
`by an angle that is between 0° and 180°. Consequently, the
`resulting coupling is both inductive and capacitive, as shown
`symbolically in FIG. SB. In FIG. SC, the excitation currents
`are in phase, i.e., offset by 0°. Therefore, the coupling is
`purely capacitive, as shown symbolically in FIG. SC.
`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, mea(cid:173)
`surement 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, I 0 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 40
`waveform as that at node 392, albeit having only 1 % of the
`amplitude of the latter. It should be kept in mind that FIG.
`S represents only one scheme for measuring the voltage on
`a conductor and there are other known schemes that may
`also be suitable for the purposes described herein.
`FIG. 7 shows in one embodiment a current probe 490 for
`measuring current through a conductor. Current probe 490
`may be used to, for example, implement measurement
`device 206 or measurement device 210. For best accuracy,
`a measurement device, such as current probe 490, is pref(cid:173)
`erably placed close to the end of the coils although this is not
`an absolute requirement. Referring now to FIG. 7, there is
`shown a conductor SOO for carrying a current to be mea(cid:173)
`sured. The current to be measured may represent, for
`example, the excitation current provided to one end of the
`plasma generating element. Surrounding conductor SOO is a
`toroid S04, which is preferably made of ferrite. On toroid
`S04, a conductor S02 is wound for a specified number of
`windings. For ease of illustration, 50 windings are arbitrarily
`chosen. The current in conductor S02 is then the ratio of the 60
`current through conductor SOO divided by the number of
`turns, or windings, that conductor S02 makes on toroid S04,
`or \!So of the current through conductor SOO. This current in
`conductor S02 further has substantially the same waveform
`as that of the current in conductor SOO.
`The current in conductor S02 may be used for control
`purposes by, for example, control circuit 220 of FIG. 2, to
`
`8
`generate the appropriate amplitude and phase control signals
`for output to one or both of the RF generators. It should be
`borne in mind that the current probe of FIG. 7 is meant to
`be illustrative and there exists other known schemes for
`taking a measurement of the current of a signal which may
`also be suitable.
`FIG. 8 shows yet another embodiment of the inventive
`plasma generating system 200. In FIG. 8, control circuit 606
`is implemented by a phase