United States Patent c191
`Qian et al.
`
`[54]
`
`INDUCTIVELY COUPLED RF PLASMA
`REACTOR WITH FLOATING COIL
`ANTENNA FOR REDUCED CAPACITIVE
`COUPLING
`
`[75]
`
`Inventors: Xue-Yu Qian, Milpitas; Arthur H.
`Sato, Santa Clara, both of Calif.
`
`[73] Assignee: Applied Materials, Inc., Santa Clara,
`Calif.
`
`[21] Appl. No.: 480,174
`Jun. 7, 1995
`
`[22] Filed:
`Int. CL6
`........................................................ C23F 1/02
`[51]
`[52] U.S. Cl ......................... 1561345; 216/68; 1181723 IR
`[58) Field of Search ......................... 1181723 E, 723 ER,
`118n23 I, 723 IR, 723 MP; 150/345, 627.1,
`626.1; 216/68; 204/298.34, 298.08
`
`[56)
`
`References Oted
`
`U.S. PATENf DOCUMENTS
`
`11111~111111111111
`US005683539A
`5,683,539
`c111 Patent Number:
`[451 Date of Patent:
`Nov. 4, 1997
`
`5,458,732 10/1995 Buder et al ............................... 216161
`611996 Hama et al ........................... 1181723 I
`5,525,159
`5,540,800
`7/1996 Qian ................................. , ...... 1561345
`
`Primary Examiner-John Niebling
`Assistant Examiner-Joni Y. Chang
`Attorney, Agent, or Firm-Michaelson & Wallace
`
`(57]
`
`ABSTRACT
`
`In an inductively coupled RF plasma reactor having an
`inductive coil antenna connected through an RF impedance
`match network to an RF power source, capacitive coupling
`from the antenna to the plasma is reduced by isolating the
`coil antenna from the RF power source by an isolation
`transformer, so that the potential of the coil antenna is
`floating. The output of the RF impedance match network is
`connected across the primary winding of the isolation trans(cid:173)
`former while the floating coil antenna is connected across
`the secondary winding of the isolation transformer.
`
`4,320,716
`
`3/1982 Boulanger et al ...................... 1181723
`
`28 Claims, 2 Drawing Sheets
`
`90
`
`/80
`
`84
`
`14
`
`70
`
`60
`
`MATCH
`
`82
`
`RF
`GENERATOR
`
`110
`
`00
`
`Ex.1009 p.1
`
`

`
`U.S. Patent
`
`Nov. 4, 1997
`
`Sheet 1of2
`
`5,683,539
`
`0 co \ "'l:f"
`
`CXl
`
`0
`CJ)
`
`N '°
`
`:I:
`(.)
`~
`~
`
`,.....
`0
`
`0:::
`0
`LL~
`a::: w
`z w
`"
`
`0
`U)
`
`0
`
`T"""
`
`l 0
`
`0
`0
`
`0
`T"""
`T"""
`
`11
`
`•
`..__.
`~
`~
`
`o /
`co
`
`Ex.1009 p.2
`
`

`
`'-=
`°' ~
`~ ...
`="'
`...
`
`(It
`
`N
`
`r N
`
`s,
`
`~
`~
`
`~
`
`~ .
`:z
`
`[
`~
`0 • rJ1 •
`
`1000 1500 2000 2500 3000
`
`RF POWER INPUT (WATIS)
`750
`
`.
`
`I
`
`------
`
`~.
`
`84b
`
`B2a
`
`I SECONDARY
`
`'
`
`I
`
`---
`
`\"" --
`
`I
`
`PRIMARY \
`
`FIG. 3
`
`84
`
`84a
`
`500
`
`'
`
`300
`
`FIG. 4
`
`--
`
`·
`
`. -!------.....-
`
`(mA)
`
`0.00
`0.05
`.
`0.10
`0.15
`.
`0.20
`RF CURRENT 0 13MHz
`CAPACITIVELY COUPLED O 25
`0.30
`0.35
`0.40
`0.45
`
`Ex.1009 p.3
`
`

`
`5,683,539
`
`2
`FIG. 4 is a graph illustrating capacitively coupled RF
`cWTent to the wafer from the inductive coil antenna with and
`without the isolation transformer of the embodiment of FIG.
`1.
`
`DETAil..ED DESCRJPTION OF THE
`PREFERRED EMBODIMENTS
`1. Technical Field
`Referring to FIG. 1, an inductively coupled RF plasma
`The invention is related to improvements in inductively
`reactor includes a grounded reactor chamber 10 having a
`coupled radio frequency (RF) plasma reactors for reducing 10 grounded side wall 12 and ceiling 14 enclosing a wafer
`capacitive coupling from the coil antenna to the semicon-
`pedestal 20 for supporting a semiconductor wafer 30 to be
`processed. A gas inlet 40 introduces a processing gas into the
`doctor wafer.
`chamber 10. The gas is ionized to produce a plasma over the
`2. Background Art
`wafer 30 by RF power inductively coupled to the plasma
`An inductively coupled RF plasma reactor typically
`includes a reactor chamber with a wafer pedestal for sup- 15 from an inductive coil antenna 50 wound over the ceiling of
`porting a semiconductor wafer inside the chamber and a coil
`the chamber 10. The coil antenna 50 is coupled to an RF
`inductor or antenna over the chamber ceiling connected
`generator 60 through an RF impedance match network 70.
`through an RF impedance match network to an RF power
`Plasma ion energy at the wafer surface can be controlled by
`source. Gas introduced into the reactor chamber is ionized
`connecting a bias RF power generator 75 between the wafer
`by the RF power coupled into the reactor chamber from the 20 pedestal 20 and ground.
`In order to isolate the inductive coil antenna 50 from the
`coil antenna to produce a plasma over the wafer. For various
`types of plasma processes carried out on the wafer, it is
`RF power generator 60, an isolation transform.er 80 is
`desirable that the plasma ion energy be distributed over a
`interposed between the match network 70 and the inductive
`narrow range in order to optimize certain process param-
`coil antenna 50. Specifically, the isolation transfonner 80
`eters. For example, in an RF plasma etch process for etching 25 has a primary winding 82 and a secondary winding 84. The
`a polysilicon layer formed over a thin oxide layer (e.g., a
`match network 70 and RF generator 60 are connected across
`gate oxide layer) on the wafer, the etch process must have
`the primary winding 82 in the manner shown in the drawing,
`both high selectivity of polysilicon and high anisotropy.
`while the inductive coil antenna 50 is connected across the
`These goals can be met if the plasma ion energy is distrib-
`secondary winding 84.
`uted over a narrow range.
`The isolation transformer 80 reduces or virtually elimi-
`The plasma ion energy is controlled by a bias RF power
`nates any D.C. potential between the generator 60 and the
`generator connected to the wafer pedestal. The bias RF
`inductive coil antenna 50, so that the electric potential of the
`power applied to the wafer pedestal is capacitively coupled
`coil antenna 50 is floating with respect to the wafer pedestal
`to the plasma and can provide the desired narrow distribu-
`20. The advantage is that capacitive coupling between the
`ti.on of plasma ion energy, in the absence of other capacitive 35 coil antenna 50 and the pedestal 20/wafer 30 is reduced as
`well. The result is that the coil antenna 50 has less effect
`coupling. The problem is that stray capacitive coupling from
`the coil antenna to the plasma broadens the distribution of
`upon plasma ion energy at the wafer surface, namely less
`plasma ion energy and thus reduces the performance of the
`broadening of the plasma ion energy distnbution. A narrow
`RF plasma process.
`plasma ion energy distribution is required in a plasma etch
`It is therefore a principal object of the invention to reduce 40 process, for example, to obtain high etch selectivity and high
`etch anisotropy.
`any stray capacitive coupling from the coil antenna to the
`The isolation transformer 80 may have an air gap between
`plasma.
`the primary and secondary windings 82, 84. Alternatively,
`the isolation transformer 80 may include a ferrite core 90
`around which the primary and secondary windings 82, 84
`are wound
`Referring to FIG. 2, the ferrite core 90 may be circular and
`have a diameter of between about three and five inches. The
`50 primary and secondary windings 82, 84 may each have
`between about 5 and 10 turns.
`Referring to FIG. 3, if an air gap is employed in lieu of the
`ferrite core 90, the primary and secondary windings 82, 84
`may be in close proximity to one another. Specifically, as
`ss illustrated in FIG. 3, the primary winding 82 may consist of
`a single turn 82a disposed between a pair of successive turns
`84a, 84b of the secondary winding 84. In accordance with
`one embodiment, the primary and secondary windings 82,
`84 of FIG. 3 may all have a uniform diameter of about two
`60 inches and the primary winding turn 82a is separated from
`the secondary winding turn 84a by 0.5 inch and from the
`other secondary winding 84b by 0.5 inch.
`The reduction in capacitive coupling achieved by the
`present invention has been quantitatively measured.
`65 Specifically, it has been found that the RF cWTent from the
`plasma to the wafer pedestal induced by capacitive coupling
`is reduced by more than a factor of two.
`
`SUMMARY OF THE INVENTION
`In an inductively coupled RF plasma reactor having an
`inductive coil antenna connected through an RF impedance
`match network to an RF power source, capacitive coupling
`from the antenna to the plasma is reduced by isolating the
`coil antenna from the RF power source by an isolation
`transformer, so that the coil antenna has a floating potential.
`The output of the RF impedance match network is connected
`across the primary winding of the isolation transformer
`while 1he floating coil antenna is connected across the
`secondary winding of the isolation transformer. The reduc(cid:173)
`tion in capacitive coupling has been quantitatively measured
`to be more than a factor of two, a significant advantage.
`
`1
`INDUCTIVELY COUPLED RF PLASMA
`REACTOR WITH FLOATING con.
`ANTENNA FOR REDUCED CAPACITIVE
`COUPLING
`
`BACKGROUND OF THE INVENTION
`
`s
`
`30
`
`45
`
`BRJEF DESCRIPilON OF THE DRAWINGS
`FIG. 1 is a schematic diagram illustrating a plasma reactor
`apparatus embodying the present invention.
`FIG. 2 is a schematic diagram of a prefeITed embodiment
`of the isolation transformer employed in carrying out the
`invention.
`FIG. 3 is a schematic diagram of an alternative embodi(cid:173)
`ment of the isolation transformer employed in carrying out
`the invention.
`
`Ex.1009 p.4
`
`

`
`5,683,539
`
`35
`
`3
`4
`coreless and having an air gap between said primary
`A method of quantitatively measuring the RF current
`and secondary windings.
`induced by capacitive coupling alone (as distinguished from
`2. The reactor of claim 1 wherein said isolation trans-
`that induced by inductive coupling alone) is described in
`former comprises a primary winding coupled to said RF
`co-pending U.S. application Ser. No. 08/475,878, filed on
`date even herewith by the inventors herein and entitled s generator and a secondary winding coupled to said inductive
`"METHOD OF MEASURING THE AMOUNT OF
`antenna.
`CAPACITIVE COUPLING OF RF POWER IN AN
`3. The reactor of claim 1 further comprising an RF
`INDUCl'IVELY COUPLED PLASMA'', the disclosure of
`impedance match network connected between said genera-
`which is ineotporated herein by reference. Essentially, that
`tor and said isolation transformer.
`method involves separating the measured RF current from 10
`4. The reactor of claim 3 wherein said isolation trans-
`the plasma to the wafer pedestal into different frequency
`former comprises a primary winding connected to said RF
`components, including a component at the fundamental
`impedance match network and a secondary winding con-
`frequency (F) of the RF generator 68 and the second
`nected to said inductive antenna.
`hannonic (2F) thereof. The method further involves mea-
`suring the amount of capacitive coupling by measuring the
`5. The reactor of claim 4 wherein said inductive antenna
`15 comprises a coiled conductor having a pair of ends, said
`current of the fundamental component at frequency F.
`coiled conductor being wound over a portion of said cham(cid:173)
`In testing the present invention, the quantitative measure-
`her and wherein said secondary winding is connected across
`ment method of the referenced co-pending application
`said pair of ends of said roiled conductor.
`employed a current probe lM monitoring the current from
`6. The reactor of claim 1 wherein said secondary winding
`the wafer pedestal to ground (with the bias RF power
`generator 75 being bypassed) and an oscilloscope 110 con- 20 comprises a pair of successive turns and said primary
`nected to the oulput of the current probe lit. The measure-
`winding comprises a turn disposed between said pair of
`.
`.
`successive turns of said secondary winding.
`ment was earned ?ut with the plasma source RF power
`7. The reactor of claim 1 further comprising instrumen-
`i
`tali
`generator 68 operating at a fundamental frequency of 13.56
`.
`"ti el
`led
`titati 1
`ve Y meas~g a capact .v Y coup
`MHz. The inductively coupled component of the current
`on or quan
`measured by the current probe lM produced a 27.12 MHz 25 component of current ~m sud p~ ~~aid pedestal:
`·
`tr
`th
`cill
`·
`8. The reactor of claim 7 wherem said mstrumentation
`
`118 hil th ca e
`w e
`sme wave ace on
`e os
`oscope
`pact-
`tively induced component of the current measured by the
`comprises:
`current probe lM produced a 13.56 MHz sine wave trace on
`a conductor connected between said pedestal and ground;
`the oscilloscope 110. The amplitude of the latter component
`a current probe adjacent said conductor; and
`indicates the quantity of capacitive coupling and was 30 means for observing a component of CUITent through said
`observed over a large range of RF power of the generator 68.
`conductor at a frequency equal to said fundamental
`The results are plotted in the graph of FIG. 4, in which the
`frequency.
`capacitively coupled component of the RF current through
`9. The reactor of claim 1 further comprising a bias RF
`the probe lM is plotted along the ordinate while the RF
`generator connected between said pedestal and ground.
`power is plotted along the abscissa. The curve with white
`10. An inductively coupled plasma reactor for use with a
`squares illustrates the results obtained using the present
`plasma source power RF generator having an RF frequency
`invention. In a second test carried out in order to obtain
`F, said reactor comprising:
`comparative results, the isolation transformer 80 was
`a reactor chamber with a gas inlet for introducing a
`processing gas into said chamber and a pedestal for
`bypassed and the RF power varied over the same range
`while the capacitively coupled component of the current
`supporting a semiconductor substrate inside said cham-
`through the probe lM was monitored at the oscilloscope
`ber;
`118, and the results are plotted in FIG. 4 as the curve with
`an inductively coupled antenna adjacent said chamber for
`producing a plasma from said gas by inductive cou-
`black squares. Comparing the two curves, it is clear that the
`capacitively coupled component of the RF current to the
`pling of RF power, said inductively coupled antenna
`wafer pedestal is reduced, at 300 Watts RF power far
`comprising a coiled conductor wound over a portion of
`said chambt.r, said coil conductor being terminated at a
`example, from 0.4 mA to 0.16 mA, which is an improvement
`by more than a factor of two.
`pair of ends;
`a plasma source power RF generator coupled to said
`While the invention has been described in detail by
`specific reference to preferred embodiments, it is understood 50
`antenna having a fundamental RF frequency F;
`that variations and modifications thereof may be made
`an RF impedance match network having an input con-
`nected to an output of said RF generator; and
`without departing from the true spirit and scope of the
`invention.
`an isolation transformer coupling said RF generator to
`said inductive antenna and having a primary winding
`What is claimed is:
`1. An inductively coupled plasma reactor for use with a ss
`connected to an output of said RF impedance match
`plasma source power RF generator having an RF frequency
`network and a secondary winding, said pair of ends
`F. said reactor comprising:
`being connected across said secondary winding, and
`a reactor chamber adapted to admit a processing gas into
`wherein said isolation transformer is coreless and has
`an air gap between said primary and secondary wind-
`said chamber and a pedestal for supporting a semicon-
`ductor substrate inside said chamber;
`ings.
`an inductively coupled antenna adjacent said chamber for
`11. The reactor of claim 10 wherein said secondary
`producing a plasma from said gas by inductive cou-
`winding comprises a pair of successive turns and said
`primary winding comprises a turn disposed between said
`pling of RF power;
`a plasma source power RF generator coupled to said
`pair of successive turns of said secondary winding.
`12. The reactor of claim 10 further comprising a measurer
`antenna having a fundamental RF frequency F; and
`an isolation transformer coupling said RF generator to
`for quantitatively measuring a capacitively coupled compo-
`said inductive antenna. said isolation transformer being
`nent of current from said plasma to said pedestal.
`
`40
`
`4
`5
`
`60
`
`65
`
`Ex.1009 p.5
`
`

`
`5,683,539
`
`5
`13. The reactor of claim 12 wherein said measurer com(cid:173)
`prises:
`a conductor connected between said pedestal and ground;
`a current probe adjacent said conductor; and
`means for observing a component of current through said
`conductor at a frequency equal to said fundamental
`frequency.
`14. The reactor of claim 10 further comprising a bias RF
`generator connected between said pedestal and ground.
`15. An inductively coupled plasma reactor for use with a
`plasma source power RF generator having an RF frequency
`F, said reactor comprising:
`a reactor chamber adapted to admit a processing gas into
`said chamber and a pedestal for supporting a semicon- 15
`ductor substrate inside said chamber;
`an inductively coupled antenna adjacent said chamber for
`producing a plasma from said gas by inductive cou(cid:173)
`pling of RF power;
`a plasma source power RF generator coupled to said 20
`antenna having a fundamental RF frequency F; and
`an isolation transformer coupling said RF generator to
`said inductive antenna, said isolation transformer hav(cid:173)
`ing a primary winding and a secondary winding
`wherein said primary winding is wound around a first 25
`portion of a ferrite core and the secondary winding is
`wound around a separate second portion of the ferrite
`core.
`16. The reactor of claim 15 wherein the primary winding
`is coupled to said RF generator and the secondary winding 30
`coupled to said inductive antenna.
`17. The reactor of claim 15 further comprising an RF
`impedance match network connected between said genera(cid:173)
`tor and said isolation transformer.
`18. The reactor of claim 17 wherein the primary winding
`is connected to said RF impedance match network and the
`secondary winding connected to said inductive antenna.
`
`6
`19. The reactor of claim 18 wherein said inductive
`antenna comprises a coiled conductor having a pair of ends,
`said coiled conductor being wound over a portion of said
`chamber and wherein said secondary winding is connected
`5 across said pair of ends of said coiled conductor.
`20. The reactor of claim 15 wherein said ferrite core has
`a quadrangular shape.
`21. The reactor of claim 15 wherein said ferrite core has
`10 a circular shape.
`22. The reactor of claim 21 wherein said ferrite core has
`a diameter of between about three and five inches.
`23. The reactor of claim 15 wherein the primary winding
`and secondary winding each have between about five to ten
`turns.
`24. The reactor of claim 15 further comprising instrumen(cid:173)
`tation for quantitatively measuring a capacitively coupled
`component of current from said plasma to said pedestal.
`25. The reactor of claim 24 wherein said instrumentation
`comprises:
`a conductor connected between said pedestal and ground;
`a current probe adjacent said conductor; and
`means for observing a component of current through said
`conductor at a frequency equal to said fundamental
`frequency.
`26. The reactor of claim 15 further comprising a bias RF
`generator connected between said pedestal and ground.
`27. The reactor of claim 6 wherein each turn of the
`secondary winding and the turn of the primary winding are
`circular and have a diameter of about two inches.
`28. The reactor of claim 6 wherein each turn of the
`secondary winding is separated from the turn of the primary
`35 winding by 0.5 inches.
`
`* * * * *
`
`Ex.1009 p.6