(12) United States Patent
`Jewett et al.
`
`USOO629 1938B1
`US 6,291,938 B1
`(10) Patent No.:
`Sep. 18, 2001
`(45) Date of Patent:
`
`METHODS AND APPARATUS FOR
`IGNITING AND SUSTAINING INDUCTIVELY
`COUPLED PLASMA
`
`Inventors: Russell F. Jewett, Charlotte, NC (US);
`Curtis C. Camus, Fort Collins, CO
`(US)
`Assignee: Litmas, Inc., Matthews, NC (US)
`
`Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`Appl. No.: 09/490,696
`Filed:
`Jan. 24, 2000
`Related U.S. Application Data
`Provisional application No. 60/174,110, filed on Dec. 31,
`1999.
`Int. Cl. ...................................................... H01J 7/24
`U.S. Cl. ................................. 315/111.51; 315/111.21;
`315/111.41; 219/121 R; 156/345
`Field of Search .......................... 315/111.21, 111.41,
`315/111.51; 219/121 R; 156/345
`
`(54)
`
`(75)
`
`(73)
`(*)
`
`(21)
`(22)
`
`(60)
`
`(51)
`(52)
`
`(58)
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`1/1975 Dundas et al. ................... 219/121 R
`
`3,862,393
`
`9/1975 Huchital et al. ...................... 330/4.3
`3.909,736
`6/1993 Mintz et al. .
`5,223,457
`1/1995 Farrell et al. ........................ 356/316
`5,383,019
`3/1995 Moslehi ......
`315/111.51
`5,397.962
`9/1995 Hanawa ............................... 156/343
`5.449,432
`10/1995 Wellerdieck ...
`... 204/298.08
`5,460,707
`11/1995 Patricket al. ...
`5,468,296
`... 118/723 MP
`5,506,475 * 4/1996 Alton ..........
`... 315/111.41
`5,578,165
`11/1996 Patrick et al. .................... 156/643.1
`5,639,519
`6/1997 Patrick et al. ....................... 427/569
`5,685,941
`11/1997 Forster et al. ....................... 156/345
`5,815,047
`9/1998 Sorensen et al.
`... 333/17.3
`5,849,136
`11/1998 Mintz et al. ......................... 156/345
`6,097,157 * 8/2000 Overzet et al. ..
`... 315/111.21
`6,156,667
`12/2000 Jewett .................................. 438/715
`* cited by examiner
`
`
`
`Primary Examiner Don Wong
`Assistant Examiner Jimmy Vu
`(74) Attorney, Agent, or Firm-Larry Williams
`(57)
`ABSTRACT
`
`Plasma processing is carried out in an apparatus having
`improved stability and reliability for plasma ignition. The
`improved plasma ignition characteristics result from a modi
`fied RF induction coil. One or more nonresonant Sections
`have been added to the RF power induction coil. The
`nonresonant Sections generate enhanced electric fields for
`igniting the plasma.
`
`31 Claims, 8 Drawing Sheets
`
`230
`
`A
`
`220
`
`RF POWER
`SOURCE
`
`20
`
`250 GAS OUT
`SN
`
`N N N NNN SN N
`
`a
`
`
`
`21 O
`
`240
`
`-O
`260
`NN
`N
`
`N
`
`245
`
`GASN
`
`RENO EXHIBIT 2024
`Advanced Energy v. Reno, IPR2021-01397
`
`

`

`U.S. Patent
`
`Sep. 18, 2001
`
`Sheet 1 of 8
`
`US 6,291,938 B1
`
`200
`
`
`
`250 GAS OUT
`
`O O O O O O O O O O O O O NNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNZ Ø % | Ø % % NNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNZ
`oooooºoooooº
`
`
`
`C)) 26
`
`FIG. 1
`
`220
`
`
`
`————
`
`210
`
`240
`
`

`

`U.S. Patent
`
`Sep. 18, 2001
`
`O CN
`
`<!=}Ø
`
`230
`
`
`
`or)CN CDO CDO
`)
`
`C/DCDO O O O OO O O O O Q O O O O O O
`
`
`
`FIG. 2
`
`US 6,291,938 B1
`
`Ñ
`
`O O O O O O O O O O O O O O O O O O
`
`OOO
`§ÑR
`
`Ë4SSSSSSSSSSSSSSSSØCN
`2O|ZZ Š§ 2á ºg
`-¿t||----
`5 %SSSSSSSSSSSSSSSSSSSSSSSSSSSS)2 § %Â Â ?ØØ?
`
`

`

`U.S. Patent
`
`Sep. 18, 2001
`
`Sheet 3 of 8
`
`US 6,291,938 B1
`
`240
`
`210
`
`230
`
`RF
`
`FIG. 3A
`
`240
`
`21 O
`
`3OO
`
`290 - 230
`
`RF
`
`FIG. 3B
`
`

`

`U.S. Patent
`U.S. Patent
`
`Sep. 18, 2001
`Sep. 18, 2001
`
`Sheet 4 of 8
`Sheet 4 of 8
`
`US 6,291,938 B1
`US 6,291,938 B1
`
`27O
`270
`
`21 O
`210
`
`280
`280
`
`In I
`LL
`
`230
`
`RF
`RF
`
`FIG. 4A
`
`270
`270
`
`21 O
`210
`
`280
`280
`
`|
`|
`|
`ft
`
`

`

`U.S. Patent
`U.S. Patent
`
`Sep. 18, 2001
`Sep. 18, 2001
`
`Sheet 5 of 8
`Sheet 5 of 8
`
`US 6,291,938 B1
`US 6,291,938 B1
`
`310
`305
`315
`310
`305
`315
`-- (-1^ N- N
`——_ aay
`
`eoweeye
`
`RF
`
`=
`
`FIG. 5A
`
`330
`330
`
`325
`325
`
`320
`320
`
`RF
`RF
`
`---
`=
`
`FIG.SB
`FIG. 5B
`
`360
`360
`
`350
`350
`
`355
`355
`
`() ()
`66 s () ()
`J) S555656 OD
`
`RF
`RF
`
`=
`-
`-
`
`FIG.SD
`FIG. 5D
`
`

`

`U.S. Patent
`U.S. Patent
`
`Sep. 18, 2001
`Sep. 18, 2001
`
`Sheet 6 of 8
`Sheet 6 of 8
`
`US 6,291,938 B1
`US 6,291,938 B1
`
`335
`335
`
`aF
`
`340
`340
`
`FIG.SC
`FIG. 5C
`
`375
`365
`eeee
`
`
`
`370
`
`RF
`
`=
`
`FIG.SE
`
`

`

`U.S. Patent
`U.S. Patent
`
`Sep. 18, 2001
`Sep. 18, 2001
`
`Sheet 7 of 8
`Sheet 7 of 8
`
`US 6,291,938 B1
`US 6,291,938 B1
`
`385
`
`385
`
`380
`380
`
`390
`
`390
`
`O
`[1
`
`C C
`
`d
`e
`C d-RF
`st d
`C d
`
`RF
`
`FIG.SF
`FIG. 5F
`
`405
`405
`—~—
`--
`
`400
`400
`——_r~
`--
`
`395
`395
`
`FIG. SG
`FIG. 5G
`
`

`

`U.S. Patent
`
`Sep. 18, 2001
`
`Sheet 8 of 8
`
`US 6,291,938 B1
`
`
`
`O
`
`28O
`
`2 3.
`Zo
`%O 3O
`Zo
`2O
`2O
`
`%O 2 2O 2 3o
`
`N. N.
`
`210
`
`270
`
`245
`
`GAS IN
`
`FIG. 6
`
`

`

`US 6,291,938 B1
`
`1
`METHODS AND APPARATUS FOR
`IGNITING AND SUSTAINING INDUCTIVELY
`COUPLED PLASMA
`
`CROSS-REFERENCES
`The present application claims the benefit of U.S. Provi
`sional Patent Application No. 60/174,110, filed on Dec. 31,
`1999. The present application is related to U.S. patent
`application Ser. No. 09/476020, filed on Dec. 31, 1999 U.S.
`Provisional Patent Application No. 60/174,110, entitled
`“Methods and Apparatus for Process Operations with RF
`Power” by Russell F. Jewett and Curtis C. Camus filed on
`Dec. 31, 1999, U.S. Patent Application No. 09/490,128,
`entitled “Methods and Apparatus for RF Power Process
`Operations with Automatic Input Power Control” filed on
`Jan. 24, 2000, and U.S. Patent Application No. 09/490,496
`entitled “Methods and Apparatus for Plasma Processing” by
`Russell F. Jewett, filed on Jan. 24, 2000 now U.S. Pat. No.
`6,156,667. All of these applications are incorporated herein
`by this reference.
`
`15
`
`2
`pounds that have been decomposed or abated with plasmas
`include chlorofluorocarbons (CFC) and perfluorocom
`pounds (PFC).
`One frequently encountered problem with Standard induc
`tively coupled RF plasma Systems is the difficulty of igniting
`and Sustaining the plasma. Plasma ignition is unreliable
`because coupling an ignition Voltage high enough to gener
`ate the energetic Species needed to produce the plasma is
`difficult. The Voltage required to generate the energetic
`Species is frequently referred to as the breakdown Voltage.
`The breakdown voltage for a gas depends upon a variety of
`factors. Two major factors are the pressure of the gas and the
`electronic properties of the gas Such as the electronegativity
`of the gas and its plasma products. The absolute value of the
`magnitude of the breakdown Voltage undergoes a minimum
`with respect to the pressure of the gas. Specifically, the
`magnitude of the breakdown voltage increases for plasma
`ignition at preSSures higher or lower than the pressures at
`which the minimum breakdown Voltage occurs.
`Consequently, igniting plasmas at very low pressures and at
`high preSSures is difficult. The electronegativity of the gas
`affects the magnitude of the breakdown Voltage So that the
`gas with higher electronegativity requires higher breakdown
`Voltages at every pressure.
`Unfortunately for Standard inductively coupled plasma
`technology, the absence of Strong electric fields and the
`absence of Strong capacitive coupling make it difficult to
`overcome the plasma ignition problems resulting from gas
`preSSure and gas electronegativity. At pressures that are too
`high or too low or for gases with high electronegativities, the
`required breakdown voltage may equal or exceed the capac
`ity of the RF power Source, making plasma ignition unre
`liable. As a result, it may be necessary to make Several
`attempts to ignite the plasma, greatly reducing the produc
`tivity and efficiency of the plasma process. The unreliable
`plasma ignition can waste valuable proceSS gases, can
`increase pollution problems, and can ruin Valuable product
`In addition to the problem of igniting the plasma, there is
`also the problem of poor plasma Stability. After the plasma
`has been ignited it is possible for the plasma to go out, i.e.
`become extinguished, because of changes in RF power
`delivery conditions. For instance, the plasma can go out
`while performing a proceSS and cause the same unfortunate
`results that occur for unreliable plasma ignition.
`Clearly, there are numerous applications requiring reliable
`and efficient methods and apparatus for igniting and Sus
`taining inductively coupled RF plasmas. Unfortunately,
`typical methods and apparatus for old-style inductively
`coupled RF plasma Systems have characteristics that are
`undesirable for Some applications. There is a need for
`methods and apparatus for igniting and Sustaining induc
`tively coupled RF plasmas that are simple to use, operate
`automatically, and provide high reliability.
`SUMMARY
`This invention Seeks to provide methods and apparatus
`that can overcome deficiencies in known RF power induc
`tively coupled plasma technology. One aspect of the present
`invention includes methods and apparatus for igniting and
`Sustaining an inductively coupled RF powered plasma. The
`methods and apparatus make it easier to ignite and Sustain
`the RF plasma and can provide greater reliability and greater
`Stability than is typical for Standard RF plasma technology.
`The apparatus has features built in that automatically facili
`tate igniting and Sustaining the plasma. The methods and
`apparatus are simple to use, operate automatically, and
`provide high reliability.
`
`BACKGROUND
`This invention relates to improved methods and apparatus
`for igniting and Sustaining inductively coupled plasmas
`produced from radio frequency (RF) power for process
`operations.
`RF plasma is extensively used in a wide variety of
`applications for carrying out process operations. For
`example, thermal plasmas can be used to promote chemical
`reactions because of the high temperatures of the thermal
`plasma. Alternatively, thermal plasmas are able to promote
`chemical reactions because of the ability of the energetic
`electrons to break chemical bonds and allow chemical
`reactions to occur that would proceed with difficulty under
`non-plasma conditions.
`In other applications, RF power is used to produce non
`thermal plasmas, also referred to as non-equilibrium plas
`mas. The manufacture of Semiconductor devices is one area
`in which non-thermal plasmas are extensively used. During
`the manufacture of Semiconductor devices, etch processes
`involving RF plasmas and deposition processes involving
`RF plasmas are used repeatedly during the fabrication
`process. One of the main benefits of using the non-thermal
`plasma is the ability of the non-thermal plasma to Stimulate
`chemical reactions that would otherwise require tempera
`tures that are too high for use in the fabrication of Semicon
`ductor devices.
`RF non-thermal plasmas are also used in cleaning pro
`ceSSes in manufacture of Semiconductor devices. The non
`thermal plasmas are commonly used to Strip photoresist
`materials from Semiconductor wafers as part of post etch
`wafer clean procedures. Resist material is Stripped from the
`Surface of the wafers by creating a non-thermal plasma in a
`gas containing oxidizing Species Such as Oxygen and possi
`bly halogen Species that are capable of reacting with and
`Volatilizing the resist material. In Some applications, the
`non-thermal plasma is maintained at a position upstream of
`the wafer. Reactive Species generated in the non-thermal
`plasma flow downstream and react with the wafer Surface
`for the Stripping process. Another cleaning process that uses
`non-thermal plasmas is the cleaning of reaction chambers
`used in Semiconductor manufacturing.
`RF plasmas have also been used for decomposition of
`chemical compounds that are hazardous or otherwise unde
`Sirable. Some of the compounds are highly refractory in
`nature and are difficult to decompose. Examples of com
`
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`

`3
`In one embodiment, the apparatus comprises a load Such
`as a plasma chamber capable of receiving gases for gener
`ating a plasma, an RF power Source, and an RF power
`antenna Such as a RF power induction coil. The RF power
`induction coil is positioned near the plasma chamber So the
`induction coil can couple RF power to the plasma. The
`resonant Section is connected with the RF power Source to
`receive RF power from the RF power source. At least one
`non-resonant Section is electrically attached to a location on
`the resonant Section. Application of a Substantially Steady
`State magnitude of RF power to the resonant Section of the
`RF coil produces a high current in the resonant Section in the
`absence of the plasma. In addition, the at least one nonreso
`nant Section maintains high Voltages that produce an
`enhanced electric field in the plasma chamber. The enhanced
`electric field in the plasma chamber facilitates igniting the
`plasma. After the plasma has been ignited, the plasma is
`sustained by inductive coupling of the RF power to the
`plasma. Inductive coupling of RF power to the plasma
`causes the current in the resonant Section to decrease. The
`decrease in the current in the resonant Section causes the
`Voltages in the at least one nonresonant Sections to decrease
`and maintain lower Voltages. Consequently, after the plasma
`is ignited, the plasma is Sustained by inductive coupling of
`RF power from the resonant Section, and the nonresonant
`Sections do not contribute significant amounts of power to
`the plasma.
`A further embodiment of the present invention includes a
`control System responsive to the presence or absence of the
`plasma when RF power is applied to the resonant Section of
`the coil. The control System sends a signal to the RF power
`Source if the plasma is absent while RF power is been
`applied to the resonant Section. The signal commands the RF
`power Source to provide an output RF power pulse to the
`resonant Section. The magnitude of the output RF power
`pulse is Substantially greater, preferably at least five percent
`greater, than the Steady State output RF power. More
`preferably, the magnitude of the RF power pulse is five times
`greater than the rated Steady-state RF power output for the
`RF power source. The steady-state output RF power refers
`to the RF power magnitude Selected for a particular proceSS
`as a Selected process parameter. Whereas, the rated Steady
`state RF output is a characteristic of the RF power source
`specified by the manufacturer of the RF power source.
`In alternative embodiments of the present invention, the
`resonant Section may have any shape Suitable for inductively
`coupling RF power to the plasma. Examples of Suitable
`shapes for the resonant Section are cylindrical coils, planar
`coils, cylindrical coils having varying diameters for the coil
`turns, and coils shaped like domes, partial spheres, or cones.
`In addition, the nonresonant Sections may include shapes
`other than coil turns. The nonresonant Sections can comprise
`any shape Suitable for an electrical conductor Such as sheets,
`plates, Strips, hollow cylinders, cylindrical coils, planar
`coils, grids, applied film, and deposited film.
`Another aspect of the present invention includes a method
`of igniting and Sustaining an inductively coupled RF power
`plasma. In one embodiment, the method includes multiple
`Steps. One Step includes providing an ionizable gas. Another
`Step involves providing an RF power induction coil having
`a resonant Section and providing at least one nonresonant
`Section, electrically connected with the resonant Section. Yet
`another Step includes providing a steady-state magnitude of
`RF power to the resonant section of the coil so the at least
`one nonresonant Section of the coil maintains a high Voltage
`in the absence of the plasma. The at least one nonresonant
`Section of the coil produces an enhanced electric field in the
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`US 6,291,938 B1
`
`4
`gas when the plasma is absent. The enhanced electric field
`facilitates igniting the plasma. The method further includes
`continuing to apply the Steady-state magnitude of RF power
`to the resonant Section So the at least one nonresonant
`Section maintains a reduced Voltage when the plasma is
`present, and, inductively coupling RF power from the reso
`nant Section to the plasma at about the Steady-state magni
`tude of RF power output.
`A further aspect of the method includes an additional Step
`to facilitate igniting the plasma. The additional Step includes
`providing a pulse of output RF power if the plasma is absent
`while the Steady-State RF power is applied to the resonant
`Section. The pulse of output RF power has a magnitude
`Substantially greater than the magnitude of the Steady-state
`RF power, preferably at least five percent greater than the
`Steady-state RF power output.
`In various Separate embodiments of the present invention,
`the load that receives the RF power may use the RF power
`for different applications. Exemplary functions of the loads
`for various applications are as follows. The load may absorb
`the RF power to produce a thermal plasma Such as those
`used for chemical processing, materials processing, analyti
`cal chemistry, or driving optical devices. The load may
`absorb the RF power to produce a non-thermal plasma Such
`as those used for chemical processing or materials proceSS
`ing. The load may absorb RF power to produce non-thermal
`plasmas Such as plasmas used for Semiconductor device
`fabrication processes like etching, deposition, cleaning,
`doping, oxidation, drying, photoresist Stripping, parts
`cleaning, reaction chamber cleaning, and annealing. The
`load may absorb RF power to produce a plasma for Stimu
`lating chemical reactions that cannot proceed or proceed
`slowly under non-plasma conditions. The load may absorb
`RF power to produce a plasma for decomposing chemical
`compounds. The load may absorb RF power to produce a
`plasma for Synthesizing chemical compounds.
`In another aspect of the present invention, the delivered
`RF power is used for abatement of gaseous halogenated
`organic compounds, other refractory organic compounds,
`perfluorocompounds, and refractory inorganic compounds.
`The apparatus uses a non-thermal plasma, generated by RF
`power, for creating free radicals in a dielectric reaction
`chamber. In a further aspect of the present invention, the
`treatment of gases is enhanced by the addition of Suitable
`ancillary reaction gases including water, methane, hydrogen,
`ammonia, hydrogen peroxide, Oxygen, or mixtures thereof.
`Embodiments of the present invention provide methods
`and apparatus for generating plasmas.
`Embodiments of the present invention provide methods
`and apparatus for generating thermal plasmas.
`Embodiments of the present invention provide methods
`and apparatus for generating non-thermal plasmas.
`Embodiments of the present invention provide methods
`and apparatus for RF power delivery for promoting chemical
`reactions.
`Embodiments of the present invention provide methods
`and apparatus for generating plasmas for Semiconductor
`device fabrication StepS Such as etching, deposition,
`cleaning, doping, oxidation, drying, photoresist Stripping,
`parts cleaning, reaction chamber cleaning, and annealing. In
`one embodiment, the plasma generation and the Semicon
`ductor device fabrication Step occur in the same chamber. In
`an alternative embodiment, the plasma generation occurs at
`the plasma location and reactive species from the plasma are
`transported to another chamber for the Semiconductor device
`fabrication Step. In one embodiment, the plasma chamber is
`
`

`

`US 6,291,938 B1
`
`S
`connected with a Semiconductor process tool So that the
`process tool receives reactive species from the plasma.
`Embodiments of the present invention provide methods
`and apparatus for removal of refractory compounds from
`waste Streams. Refractory compounds include compounds
`that show a high degree of Stability with respect to tempera
`ture and reactivity and are difficult to decompose.
`Embodiments of the present invention provide new and
`useful methods and apparatus for the destruction of refrac
`tory compounds Such as perfluorocompounds, Such as car
`bon fluorides, carbon tetrafluoride, nitrogen triflouride, and
`Sulfur hexafluoride by reactions facilitated by a plasma.
`Embodiments of the present invention provide methods
`and apparatus for gas waste treatment using a non-thermal
`plasma generated by RF power.
`Embodiments of the present invention provide methods
`and apparatus that are Suitable for processing waste Streams
`emanating from one or more individual Semiconductor pro
`ceSS tools and the apparatus can become an integral part of
`the Semiconductor device fabrication process. In one
`embodiment, the apparatus is connected with Semiconductor
`process tools Such as chemical vapor deposition tools,
`plasma etching tools, plasma cleaning tools, doping tools,
`photoresist Stripping tools, and plasma deposition tools.
`An advantage of embodiments of the present invention is
`the ability to provide an economical apparatus and method
`for the destruction of refractory compounds contained in
`gaseous waste Streams.
`Another advantage of embodiments of the present inven
`tion is the ability to provide waste treatment of undiluted off
`gases from individual Semiconductor device fabrication
`tools. Embodiments of the present invention can be made
`compact enough to be integrated into and attached directly
`to one or more than one wafer processing tools.
`The above and still further features and advantages of the
`present invention will become apparent upon consideration
`of the following detailed descriptions of Specific embodi
`ments thereof, especially when taken in conjunction with the
`accompanying drawings.
`
`DESCRIPTION OF THE DRAWINGS
`FIG. 1 is a schematic block diagram of a first embodiment
`of the present invention.
`FIG. 2 is a Schematic block diagram of a Second embodi
`ment of the present invention.
`FIG. 3 is an equivalent circuit diagram of embodiments of
`the present invention.
`FIG. 4 is another equivalent circuit diagram of embodi
`ments of the present invention.
`FIG. 5 is a diagram of Several example configurations
`suitable for use with embodiments of the present invention.
`FIG. 6 is a schematic block diagram of an embodiment of
`the present invention including RF pulse capability.
`
`DESCRIPTION
`Reference is now made to FIG. 1 wherein there is a
`plasma chamber 200 shown in cross-section. An RF power
`induction coil having a resonant Section 210 is shown wound
`around plasma chamber 200. Resonant section 210 is shown
`in cross-section. An RF power source 220 is connected with
`resonant section 210. An electrical ground connection 230 is
`connected with resonant Section 210. A nonresonant Section
`240 is shown in cross-section as a Section of coil electrically
`connected with resonant section 210. Nonresonant section
`
`6
`240 makes electrical contact with resonant Section 210 So
`that charge flow, or rather current, to and from nonresonant
`section 240 only occurs via the electrical contact with
`resonant section 210. Specifically, nonresonant section 240
`does not have a ground or other type of electrical connection
`between the connection with resonant section 210 and the
`end 260 of the nonresonant section 240. The end 260 of
`nonresonant Section 240 is ungrounded.
`Plasma chamber 200 has an opening 245 capable of
`allowing gas into the plasma chamber. Plasma chamber 200
`also has an opening 250 capable of allowing gas removal
`from plasma chamber 200. Plasma chamber 200 includes
`materials that are Substantially transparent to RF power So
`that RF power can be coupled to the interior of the plasma
`chamber So as to ignite and Sustain a plasma.
`RF power source 220 may be connected with resonant
`Section 210 via a standard RF match network, not shown, for
`matching the impedance of RF power source 220 so that RF
`power can be coupled to gas in plasma chamber 200 to
`generate the plasma.
`The design of resonant coils for coupling RF power to a
`load is well known in the art. Typical resonant coils have at
`least a tenth of a coil turn, i.e. a 36-degree Segment, more
`typically, at least one coil turn, i.e. a 360-degree Segment,
`and frequently multiple coil turns. The resonant coil is
`designed to have an inductance that allows resonant cou
`pling of RF power to the plasma. In a preferred embodiment,
`resonant Section 210 has multiple coil turns.
`Nonresonant section 240 is electrically conductive. In a
`preferred embodiment, nonresonant section 240 is made of
`the same material as resonant Section 210. Furthermore,
`nonresonant Section 240 may have a variety of Structural
`designs. In one embodiment, nonresonant Section 240 has at
`least one coil turn for cylindrical coils, but more preferably,
`nonresonant Section 240 has multiple coil turns for cylin
`drical coils.
`The embodiment shown in FIG. 1 can facilitate igniting
`and Sustaining an inductively coupled RF power plasma.
`Application of a steady-state magnitude of output RF power
`to resonant Section 210 produces a high current in resonant
`section 210 in the absence of the plasma. The high current
`causes nonresonant Section 240 to maintain high Voltages
`that produce an enhanced electric field in the plasma cham
`ber. The enhanced electric field in the plasma chamber
`facilitates igniting the plasma. Inductive coupling of RF
`power to the plasma causes the current in resonant Section
`210 to decrease. The decrease in the current in resonant
`Section 210 causes the Voltages in nonresonant Section 240
`to decrease. After the plasma is ignited, the plasma is
`Sustained by inductive coupling of RF power from resonant
`Section 210. Nonresonant section 240 does not contribute
`Significant amounts of RF power to the plasma; Substantially
`all of the RF power to the plasma is inductively coupled.
`Reference is now made to FIG. 2 wherein there is a
`plasma chamber 200 shown in cross-section. An RF power
`induction coil having a resonant Section 210 is shown wound
`around plasma chamber 200. Resonant section 210 is shown
`in cross-section. An RF power Source 220 and an electrical
`ground connection 230 are connected with resonant Section
`210 via a parallel capacitor 290 and a series capacitor 300 so
`as to allow application of RF power to resonant section 210.
`A first nonresonant section 270 is shown in cross-section
`as a section of coil. First nonresonant section 270 is elec
`trically connected with resonant section 210 at a first loca
`tion. A Second nonresonant Section 280, also shown in
`croSS-Section as a Section of coil, is electrically connected
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`US 6,291,938 B1
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`with resonant section 210 at a second location so that first
`nonresonant Section 270 and Second nonresonant Section
`280 will be at different voltages when RF power is applied
`to resonant section 210. For the embodiment shown in FIG.
`2, the first location and the Second location are at the two
`ends of resonant Section 210 So that resonant section 210,
`first nonresonant Section 270, and Second nonresonant Sec
`tion 280 form a continuous cylindrical coil wound around
`plasma chamber 200.
`First nonresonant section 270 and second nonresonant
`Section 280 are electrically connected to resonant Section
`210 so that charge flow, or rather current, to and from first
`nonresonant Section 270 and Second nonresonant Section
`280 only occurs via the connection with resonant section
`210. Specifically, nonresonant section 270 and nonresonant
`section 280 do not have an electrical ground or other type of
`electrical connection other than the connection with resonant
`Section 210.
`The embodiment shown in FIG. 2 can facilitate igniting
`and Sustaining an inductively coupled RF power plasma.
`Application of a steady-state magnitude of output RF power
`to resonant Section 210 produces a high current in resonant
`section 210 in the absence of the plasma. The high current
`causes first nonresonant Section 270 and Second nonresonant
`section 280 to maintain a high voltage difference that
`produces an enhanced electric field in the plasma chamber.
`The enhanced electric field in the plasma chamber facilitates
`igniting the plasma. Inductive coupling of RF power to the
`plasma causes the current in resonant Section 210 to
`decrease. The decrease in the current in resonant Section 210
`causes the Voltage difference between first nonresonant
`Section 270 and second nonresonant section 280 to decrease.
`After the plasma is ignited, the plasma is Sustained by
`inductive coupling of RF power from resonant section 210.
`First nonresonant Section 270 and Second nonresonant Sec
`tion 280 do not contribute significant amounts of RF power
`to the plasma; Substantially all of the RF power to the plasma
`is inductively coupled.
`An advantage that results from having two nonresonant
`Sections instead of a Single nonresonant Section is that two
`nonresonant Sections produce a more enhanced electric field
`for igniting the plasma.
`The enhanced electric field generated by nonresonant
`sections such as nonresonant section 240 (FIG. 1) and
`nonresonant sections 270 and 280 (FIG. 2) allow plasma
`ignition to occur under conditions that would make plasma
`ignition extremely difficult without the benefit of the
`enhanced electric field. For example, the embodiments
`shown in FIG. 1 and FIG. 2 make it easier to ignite and
`50
`Sustain a plasma over a large range of preSSures. Low
`preSSure plasmas Such as those operating at below 100
`milliTorr can be ignited more easily using the embodiments
`shown in FIG. 1 and FIG. 2. In addition, higher-pressure
`plasmas Such as those operating in the range of one hundred
`milliTorr to about one atmosphere can be ignited more easily
`using embodiments of the present invention. A Suitable
`range of preSSures for using embodiments of the present
`invention is from about five milliTorr to about one
`atmosphere, including all preSSures and ranges of pressures
`Subsumed therein. A preferred range of pressures for using
`embodiments of the present invention is the range from
`about 100 milli Torr to about 10 Torr.
`In addition to facilitating plasma ignition under extreme
`pressure conditions, the embodiments shown in FIG. 1 and
`in FIG. 2 are also useful in igniting plasmas when the plasma
`chamber is Substantially incapable of Supporting capaci
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`tively coupled RF power. Capacitively coupling RF power
`to a plasma chamber is extremely difficult if the plasma
`chamber is Substantially all dielectric. Such a situation can
`occur when there is insufficient electrically grounded Surface
`in the plasma chamber that can Support capacitive coupling
`of RF power. In addition, capacitively coupling RF power to
`a plasma chamber is extremely difficult if the plasma cham
`ber is designed So that grounded Surfaces that may be
`exposed to the plasma are far away from the resonant Section
`of the RF induction coil; the plasma ignition will be very
`difficult because of the very long discharge length. Dis
`charge length is interpreted to mean the distance between the
`resonant Section of the RF coil and the electrically grounded
`Surface inside the chamber. The enhanced electric fields, as
`described for FIG. 1 and FIG. 2, can facilitate plasma
`ignition for plasma chambers having poor capacitive cou
`pling characteristics. Using two nonresonant Sections as
`described for FIG. 2 is especially beneficial because of the
`more enhanced electric field.
`Reference is now made to FIG. 3 wherein there are shown
`equivalent circuits for embodiments of the present invention
`having one nonresonant Section. FIG. 3A shows the equiva
`lent circuits for resonant Section 210 and nonresonant Sec
`tion 240. Non-resonant section 240 is a coil as described
`earlier. Note that the nonresonant section 240 is electrically
`connected only with resonant Section 210. Resonant Section
`210 is connected acroSS the RF power Source and ground
`connection 230.
`FIG. 3B shows resonant section 210 connected with the
`RF power source and electrical ground connection 230 via
`parallel capacitor 290 and series capacitor 300. Non
`resonant section 240 is connected with resonant section 210.
`Reference is now made to FIG. 4 wherein there are shown
`equivalent circuits for embodiments of the present invention
`having two nonresonant Sections. FIG. 4A shows resonant
`Section 210 connected across an RF power input and elec
`trical ground connection 230. First nonresonant section 270
`is connected with resonant Section 210. Second nonresonant
`Section 280 is connected with resonant section 210.
`FIG. 4B shows resonant section 210 connected with the
`RF power source and electrical ground connection