(12) United States Patent
`US 6,319,355 B1
`(10) Patent N0.:
`Holland
`(45) Date of Patent:
`Nov. 20, 2001
`
`USOO6319355B1
`
`(54) PLASMA PROCESSOR WITH COIL
`RESPONSIVE TO VARIABLE AMPLITUDE
`RF ENVELOPE
`
`(75)
`
`Inventor:
`
`John Patrick Holland, San Jose, CA
`(US)
`
`(73)
`
`Assignee: Lam Research Corporation, Fremont,
`CA (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.
`
`(21)
`
`(22)
`
`(51)
`
`(52)
`
`(58)
`
`(56)
`
`Appl. No.: 09/343,246
`
`Filed:
`
`Jun. 30, 1999
`
`Int. Cl.7 ....................................................... H05H 1/00
`
`US. Cl.
`
`.......................................... 156/345; 118/723 I
`
`Field of Search ........................... 156/345; 118/723 I
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`.
`
`.
`
`1/1990 Ooiwa et al.
`4,891,118
`8/1990 Ogle.
`4,948,458
`7/1993 Chen et al.
`5,226,967
`1/1994 Ogle.
`5,277,751
`4/1994 Coultas et al.
`5,304,279
`5/1994 Dokietal..
`5,310,452
`11/1994 Chen et al.
`.
`5,368,710
`.
`3/1995 Patrick et al.
`5,401,350
`9/1996 Okumura et al.
`5,558,722
`3/1998 Gates .
`5,731,565 *
`6/1998 Holland et al.
`5,759,280
`8/1998 Ishii et al.
`.
`5,795,429
`9/1998 Holland et al.
`5,800,619
`5,827,435 * 10/1998 Samukawa .
`.
`5,897,713 *
`4/1999 Tomioka et al.
`FOREIGN PATENT DOCUMENTS
`
`.
`
`.
`
`.
`
`.
`
`08 13 227 A2 * 12/1997 (EP) .
`3—323326 *
`6/1991 (JP) .
`99/07913
`2/1999 (W0) .
`
`OTHER PUBLICATIONS
`
`H. Sugai et al., “Diagnostics and control of radicals in an
`inductively coupled etching reactor,” XP—002148446, J.
`Vac. Sci. Technol. A 13(3), May/Jun 1995, pp. 887—893.*
`
`Seiji Samukawa, et al., “Pulsed—time—modulated Electron
`Cyclotron Resonance Plasma Discharge for Highly Selec-
`tive, Highly Anistoropic, and Charge—free Etching,” J. VAC.
`Sci. Technol. A 14(6), Nov/Dec 1996, 1996 American
`Vacuum Society, pp. 3049—3058.
`
`Sumio Ashida, et al., “Time Modulated Operation of High
`Density Plasma Sources,” 1995 Dry Process Symposium,
`pp. 21—26.
`
`A. Yokozawa, et al., “Simulation for Afterglow Plasma in
`Time—modulated Cl 2Plasma,” 1995 Dry Process Sympo-
`sium, pp. 27—32.
`
`Sumio Ashida, et al., “Measurements of Pulsed—power
`Modulated Argon Plasmas in an Inductively Coupled
`Plasma Source,”J. Vac. Sci. Technol. A 14(2), Mar/Apr
`1996, 1996 American Vacuum Society, pp. 391—397.
`
`H. Sugai, et al., “Diagnostics and Control of Radicals in an
`Inductivity Coupled Etching Reactor,” J. Vac. Sci. Technol.
`A 13(3), May/Jun 1995, 1995 American Vacuum Society,
`pp. 887—893.
`
`(List continued on next page.)
`
`Primary Examiner—Th1 Dang
`(74) Attorney, Agent, or Firm—Lowe Hauptman Gilman &
`Berner, LLP
`
`(57)
`
`ABSTRACT
`
`A vacuum plasma processor includes a coil for reactively
`exciting a plasma so plasma incident on a workpiece has
`substantially uniformity. The coil and a Window which
`reactively couples fields from the coil to the plasma have
`approximately the same diameter. An r.f. source supplies a
`pulse amplitude modulated envelope including an r.f. carrier
`to the coil.
`
`39 Claims, 2 Drawing Sheets
`
`28
`.
`DANCE
`
`
`
`
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`
`
`smLRScEE
`' 305805
`ImECH'NG
`27
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`7
`NETWORK
`29
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`30
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`MR
`56/
`V—ol
`DUTY CYCLE
`CONTROLLER
`CONTROL
`7
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`l
`l
`34
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`INPUT
`331
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`
`
`1o
`/
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`32
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`WORKPIECE
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`2°
`PLATEN
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`a1
`33
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`
`K
`K
`K
`IMPEDANCE
`14
`22
`RF
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`
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`_4 MATCHING
`NETWORK 1
`SOURCE
`L
`T
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`TSMC et al. v. Zond, Inc.
`GILLETTE-1015
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`Page 1 of 12
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`TSMC et al. v. Zond, Inc.
`GILLETTE-1015
`Page 1 of 12
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`

`

`US 6,319,355 B1
`Page 2
`
`OTHER PUBLICATIONS
`
`Seiji Samukawa, et al., “Pulsed—time Modulated Electron
`Cyclotron Resonance Plasma Etching for Highly Selective,
`Highly Anisotropic, and Less—charging Polycrystalline Sili-
`con Patterning,” J. Vac. Sci. Technol. B 12(6), Nov/Dec
`1994, American Vacuum Society, pp. 3300—3305.
`Seiji Samukawa, “Time—modulated Electron Cyclotron
`Resonance Plasma Discharge for Controlling the Polymer-
`ization in SiO 2Etching”, Jpn. J. Appl. Phys. vol. 32 (1993)
`pp. 6080—6087, Part 1, No. 12B, Dec. 1993.
`Nobuo Fuj iware, et al., “Pulse Plasma Processing for Reduc-
`tion of Profile Distortion Induced by Charge Build—up in
`ECR Plasma,” 1995 Dry Process Symposium, pp. 51—56.
`
`Seiji Samukawa, “Highly Selective and Highly Anisotropic
`SiOzEtching in Pulse—time Modulated Electron Cyclotron
`Resonance Plasma,” Jpn. J. Appl. Phys. vol. 33 (1994) pp.
`2133—2138, Part 1, No. 4B, Apr. 1994.
`Shigenori Sakamori, et al., “Reduction of Electron Shading
`Damage With Pulse—modulated ECR Plasma,” 1997 2nd
`International Symposium on Plasma Process—induced Dam-
`age, May 13—14, Monterey, CA, Copyright 1997 American
`Vacuum Society, pp. 55—58.
`K. Hashimoto et al., “Reduction of Electron Shading Dam-
`age by Using Synchronous bias in Pulsed Plasma,” 1995 Dry
`Process Symposium, pp. 33—37.
`
`* cited by examiner
`
`GILLETTE-1015 / Page 2 of 12
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`GILLETTE-1015 / Page 2 of 12
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`

`

`US. Patent
`
`Nov. 20, 2001
`
`Sheet 1 012
`
`US 6,319,355 B1
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`29 I SOURCE r
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`F
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`—: 25
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`28
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`NETWORK
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`8'!
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`55
`24
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`58
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`AMP
`
`.
`CONTROL 18
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`DUTY CYCLE
`CONTROLLER
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`31
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`RF
`SOURCE
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`IA'|
`\A
`IIsmmmmmm ’r
`”I
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`
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`
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`PLATEN
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`20
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`A I
`33 WWI“
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`IMPEDANCE
`NETWORK
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`MATCHING
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`k22
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`14
`Fig_ 1
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`GILLETTE-1015 / Page 3 of 12
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`GILLETTE-1015 / Page 3 of 12
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`GILLETTE-1015 / Page 4 of 12
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`US 6,319,355 B1
`
`1
`PLASMA PROCESSOR WITH COIL
`RESPONSIVE TO VARIABLE AMPLITUDE
`RF ENVELOPE
`
`FIELD OF INVENTION
`
`The present invention relates generally to plasma proces-
`sors including radio frequency (r.f.) responsive coils for
`exciting gases in vacuum chambers to plasmas that process
`workpieces in the chamber and more particularly to such a
`processor and to a processing method wherein plasma
`density on the workpiece is controlled by varying the
`amplitude of the envelope of the r.f. applied to the coil.
`BACKGROUND ART
`
`One type of processor for treating workpieces with an r.f.
`plasma in a vacuum chamber includes a coil responsive to an
`r.f. source. The coil responds to the r.f. source to produce
`magnetic and electric fields that excite ionizable gas in the
`chamber to a plasma. Usually the coil is on or adjacent to a
`dielectric window that extends in a direction generally
`parallel to a planar horizontally extending surface of the
`processed workpiece. The excited plasma interacts with the
`workpiece in the chamber to etch the workpiece or to deposit
`material on it. The workpiece is typically a semiconductor
`wafer having a planar circular surface or a solid dielectric
`plate, e.g., a rectangular glass substrate used in flat panel
`displays, or a metal plate.
`Ogle, US. Pat. No. 4,948,458 discloses a multi-turn spiral
`coil for achieving the above results. The spiral, which is
`generally of the Archimedes type, extends radially and
`circumferentially between its interior and exterior terminals
`connected to the r.f. source via an impedance matching
`network. Coils of this general type produce oscillating r.f.
`fields having magnetic and capacitive field components that
`propagate through the dielectric window to heat electrons in
`the gas in a portion of the plasma in the chamber close to the
`window. The oscillating r.f. fields induce in the plasma
`currents that heat electrons in the plasma. The spatial
`distribution of the magnetic field in the plasma portion close
`to the window is a function of the sum of individual
`
`magnetic field components produced by each turn of the
`coil. The magnetic field component produced by each of the
`turns is a function of the magnitude of r.f. current in each
`turn which differs for different turns because of transmission
`
`line effects of the coil at the frequency of the r.f. source.
`For spiral designs as disclosed by and based on the Ogle
`’458 patent, the r.f. currents in the spiral coil are distributed
`to produce a torroidal shaped magnetic field region in the
`portion of the plasma close to the window, which is where
`power is absorbed by the gas to excite the gas to a plasma.
`At low pressures, in the 1.0 to 10 mTorr range, diffusion of
`the plasma from the ring shaped region produces plasma
`density peaks just above the workpiece in central and
`peripheral portions of the chamber, so the peak densities of
`the ions and electrons which process the workpiece are in
`proximity to the workpiece center line and workpiece
`periphery. At intermediate pressure ranges, in the 10 to 100
`mTorr range, gas phase collisions of electrons, ions, and
`neutrons in the plasma prevent substantial diffusion of the
`plasma charged particles outside of the torroidal region. As
`a result, there is a relatively high plasma flux in a ring like
`region of the workpiece but low plasma fluxes in the center
`and peripheral workpiece portions.
`These differing operating conditions result in substan-
`tially large plasma flux (i.e., plasma density) variations
`between the ring and the volumes inside and outside of the
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`2
`ring, resulting in a substantial standard deviation, i.e., in
`excess of three, of the plasma flux incident on the workpiece.
`The substantial standard deviation of the plasma flux inci-
`dent on the workpiece has a tendency to cause non-uniform
`workpiece processing, i.e, different portions of the work-
`piece are etched to different extents and/or have different
`amounts of molecules deposited on them.
`Many coils have been designed to improve the uniformity
`of the plasma. The commonly assigned US. Pat. No. 5,759,
`280, Holland et al., issued Jun. 2, 1998, discloses a coil
`which, in the commercial embodiment, has a diameter of 12
`inches and is operated in conjunction with a vacuum cham-
`ber having a 14.0 inch inner wall circular diameter. The coil
`applies magnetic and electric fields to the chamber interior
`via a quartz window having a 14.7 inch diameter and 0.8
`inch uniform thickness. Circular semiconductor wafer work-
`
`pieces are positioned on a workpiece holder about 4.7 inches
`below a bottom face of the window so the center of each
`
`workpiece is coincident with a center line of the coil.
`The coil of the ’280 patent produces considerably smaller
`plasma flux variations across the workpiece than the coil of
`the ’458 patent. The standard deviation of the plasma flux
`produced by the coil of the ’280 patent on a 200 mm wafer
`in such a chamber operating at 5 milliTorr is about 2.0, a
`considerable improvement over the standard deviation of
`approximately 3.0 for a coil of the ’458 patent operating
`under the same conditions. The coil of the ’280 patent causes
`the magnetic field to be such that the plasma density in the
`center of the workpiece is greater than in an intermediate
`part of the workpiece, which in turn exceeds the plasma
`density in the periphery of the workpiece. The plasma
`density variations in the different portions of the chamber for
`the coil of the ’280 patent are much smaller than those of the
`coil of the ’458 patent for the same operating conditions as
`produce the lower standard deviation.
`Other arrangements directed to improving the uniformity
`of the plasma density incident on a workpiece have also
`concentrated on geometric principles, usually concerning
`coil geometry. See, e.g., US. Pat. Nos. 5,304,279, 5,277,
`751, 5,226,967, 5,368,710, 5,800,619, 5,401,350, and 5,847,
`704.
`
`It is accordingly an object of the present invention to
`provide a new and improved vacuum plasma processor and
`method of operating same wherein the plasma density
`incident on the workpiece can be controlled at will.
`An additional object of the present invention to provide a
`new and improved vacuum plasma processor and method of
`operating same wherein the plasma density incident on the
`workpiece has relatively high uniformity.
`Another object of the invention is to provide a new and
`improved vacuum plasma processor having the same geom-
`etry as the prior art but which is operated to have controlled
`plasma density characteristics.
`An added object of the invention is to provide a new and
`improved vacuum plasma processor having the same geom-
`etry as the prior art but which is operated to have greater
`plasma density uniformity characteristics than the prior art.
`A further object of the invention is to provide a new and
`improved vacuum plasma processor including an r.f. exci-
`tation coil that is operated so the plasma density incident on
`a workpiece is substantially less than 2.0.
`SUMMARY OF THE INVENTION
`
`I have discovered that the foregoing objects are attained
`by varying the amplitude of the envelope of r.f. applied to a
`plasma excitation coil, such as the coil disclosed in the ’280
`patent.
`
`GILLETTE-1015 / Page 5 of 12
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`GILLETTE-1015 / Page 5 of 12
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`

`

`US 6,319,355 B1
`
`3
`In one embodiment, the invention is directed to a vacuum
`plasma processor for processing workpieces that includes
`(1) a vacuum chamber for processing the workpieces with a
`plasma, (2) a holder in the chamber for the workpieces, (3)
`a coil for exciting gas in the chamber into the plasma, and
`(4) an r.f. source for supplying an r.f. carrier to the coil
`having a variable amplitude envelope wherein the (a) vari-
`able envelope amplitude, (b) pressure within the chamber,
`and (c) arrangement of the (i) the coil, (ii) chamber and (iii)
`holder are such as to cause the plasma to have a substantially
`constant flux across the workpiece while the workpiece is on
`the holder.
`
`A further aspect of the invention relates to a method of
`plasma processing a workpiece on a holder in a vacuum
`plasma processor chamber by exciting gas in the chamber to
`a plasma by applying an r.f. carrier having envelope ampli-
`tude variations to a coil that responds to the r.f. carrier to
`couple magnetic and electric plasma excitation fields to the
`plasma. The variations of the envelope amplitude and the
`coil geometry, as well as the arrangement of the chamber,
`workpiece and holder, are such that the density of the plasma
`across the workpiece has substantially lower standard devia-
`tion than when the plasma envelope has a constant ampli-
`tude. Hence controlling the variations of the envelope ampli-
`tude maintains the density of the plasma across the
`workpiece substantially constant.
`Another aspect of the invention relates to a vacuum
`plasma processor including a workpiece holder in a vacuum
`chamber, and a coil for exciting gas in the chamber into the
`plasma driven by an r.f. source having a variable amplitude
`envelope wherein the variable amplitude, pressure within
`the chamber, and the arrangement of (a) the coil, (b) the
`chamber and (c) the holder cause the plasma to have a higher
`flux on a portion of the workpiece aligned with the center of
`the coil than on a portion of the workpiece aligned with a
`portion of the coil removed from the center of the coil when
`a constant amplitude r.f. envelope is applied to the coil, and
`the variable amplitude envelope causes the plasma flux
`across the workpiece to be such as to cause the plasma flux
`in a portion of the workpiece removed from the portion of
`the workpiece aligned with the center of the coil to be at least
`equal to the plasma flux in the portion of the workpiece
`aligned with the center of the coil.
`An added aspect of the invention concerns such a vacuum
`plasma processor wherein the source driving the coil is
`arranged so that the amplitude of the high amplitude seg-
`ments increases with increases in the spacing of the high
`amplitude segments from each other.
`A further aspect of the invention concerns such a plasma
`processor wherein the variable amplitude envelope of an r.f.
`source driving the coil is controlled so that the plasma flux
`on a portion of the workpiece removed from a portion of the
`workpiece aligned with the center of the coil is at least equal
`to the plasma flux at a portion of the workpiece aligned with
`the center of the coil.
`
`The variable amplitude envelope preferably has high and
`low amplitude portions. The low amplitude portions have a
`low enough magnitude to prevent excitation of charge
`particles in the gas to a plasma. The sufficient duration and
`magnitude of each low amplitude portion are such as to
`enable charge particles in the plasma to diffuse to a much
`greater extent than charge particle diffusion which occurs
`when the envelope has a high value to excite the plasma to
`a state which causes processing of the workpiece.
`Preferably the variable amplitude envelope is in the form
`of pulses having zero amplitude periods spaced between
`
`4
`finite amplitude periods. The zero amplitude durations are,
`in one embodiment, short enough that the plasma is not
`extinguished during them (e.g., the period is approximately
`ten microseconds) and, in a second embodiment, are long
`enough that the plasma is extinguished during them (e.g., the
`period of the zero amplitude is about ten milliseconds).
`I am aware that there are disclosures in the prior art of
`pulsing a plasma excitation source. In particular, I am aware
`of the following: Sumio Ashida, M. R. Shim and M. A.
`Lieberman, “Measurements of Pulsed-power Modulated
`Argon Plasmas in an Inductively Coupled Plasma Source,”
`J. Vac. Sci. Technol. A 14(2), March/April 1996, 1996
`American Vacuum Society, pages 391—397; H. Sugai and K.
`Nakamura, “Diagnostics and Control of Radicals in an
`Inductively Coupled Etching Reactor,” J. Vac. Sci. Technol.
`A 13(3), May/June 1995, 1995 American Vacuum Society,
`pages 887—893; Seiji Samukawa and Kazuo Terada, “Pulse-
`time Modulated Electron Cyclotron Resonance Plasma
`Etching for Highly Selective, Highly Anisotropic, and Less-
`charging Polycrystalline Silicon Patterning,” J. Vac. Sci.
`Technol. B 12(6), November/December 1994, American
`Vacuum Society, PAGES 3300—3305; Seiji Samukawa,
`“Time-modulated Electron Cyclotron Resonance Plasma
`Discharge for Controlling the Polymerization in SiO2
`Etching,” Jpn. J. Appl. Phys. Vol. 32 (1993), pages
`6080—6087, Part 1, No. 12B, December 1993; Nobuo
`Fujiwara, Takahiro Maruyama and Masahiro Yoneda, “Pulse
`Plasma Processing for Reduction of Profile Distortion
`Induced by Charge Build-up in ECR Plasma,” 1995 Dry
`Process Symposium, pages 51—56; Seiji Samukawa,
`“Highly Selective and Highly Anisotropic SiO2 Etching in
`Pulse-time Modulated Electron Cyclotron Resonance
`Plasma,” Jpn.
`J. Appl. Phys. Vol. 33 (1994) pages
`2133—2138, Part 1, No. 4B,April 1994; Shigenori Sakamori,
`Takahiro Maruyama, Nobuo Fujiwara, Hiroshi Miyatake
`and Masahiro Yoneda, “Reduction of Electron Shading
`Damage with Pulse-modulated ECR Plasma,” 1997 2nd
`International Symposium on Plasma Process-Induced
`Damage, May 13—14, Monterey, Calif., 1997 American
`Vacuum Society, pages 55—58; K. Hashimoto, Y. Hikosaka,
`A. Hasegawa and M. Nakamura, “Reduction of Electron
`Shading Damage by Using Synchronous Bias in Pulsed
`Plasma,” 1995 Dry Process Symposium, pages 33—37;
`Samukawa, “Plasma Processing Method and Equipment
`used Therefor” US. Pat. No. 5,827,435, issued Oct. 27,
`1998; Ooiwa et al., “Plasma Processing Apparatus” US. Pat.
`No. 4,891,118, issued Jan. 2, 1990; Doki et al. “Plasma
`Process Apparatus and Plasma Processing Method” US.
`Pat. No. 5,310,452, issued May 10, 1994.
`While several of these references deal with plasmas
`excited by coils, none of the references disclosing such
`excitation sources indicates the pulsed excitation of the coil
`has an effect on plasma density uniformity, and in particular
`improved plasma density uniformity. This is not surprising
`because the coils mentioned in the references are not dis-
`
`closed as having geometry conducive to providing plasmas
`with substantial plasma flux uniformity. In one case, the coil
`is a solenoid while in other cases the coil appears to be
`conventional, relatively small conventional fiat spiral coil.
`Such coils do not have a diameter and geometry conducive
`to providing substantially uniform plasma flux during con-
`tinuous operation.
`In contrast, the coil of the preferred embodiment of the
`present invention is specifically designed to provide a sub-
`stantially uniform plasma flux across the entire workpiece.
`The specific coil is designed to be “centerfast.” Pulsing the
`r.f. field the coil derives enables the plasma to diffuse during
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`US 6,319,355 B1
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`5
`the interval between excitation or when low amplitude
`excitation occurs to promote an “edge-fast” plasma flux and
`thereby provide substantial plasma flux uniformity across
`the entire workpiece.
`While Samukawa et al. disclose pulsing a microwave
`electron cyclotron resonance field in their November/
`December, 1996, Journal of Vacuum Science Technology
`publication, to attain improved uniformity, the mechanism
`involved in microwave electron cyclotron resonance opera-
`tion is entirely different from the mechanisms involved in
`coil excited plasmas. In an electron cyclotron resonance
`plasma, a very substantial DC magnetic field must be
`applied, usually by a solenoid coil, to the plasma to obtain
`the required swirling action of the charged particles. The DC
`magnetic field interacts with the microwave energy to pro-
`vide the plasma excitation. An r.f. plasma does not require
`a large DC magnetic field source or a microwave generator.
`An r.f. plasma excited by a coil requires a relatively simple,
`inexpensive r.f. (non-microwave) source, typically having a
`frequency of 13.56 mHz, and a relatively inexpensive coil.
`A further advantage of r.f. coil plasma excitation over
`microwave excitation is that greater diffusion occurs with a
`pulsed plasma excited by a coil than can occur with an
`electron cyclotron resonance process because a plasma
`excited by a coil is not coupled with a strong DC magnetic
`field.
`
`The above and still further objects, features and advan-
`tages of the present invention will become apparent upon
`consideration of the following detailed descriptions of sev-
`eral specific embodiments thereof, especially when taken in
`conjunction with the accompanying drawings.
`
`BRIEF DESCRIPTION OF THE DRAWING
`
`FIG. 1 is a schematic diagram of a preferred embodiment
`of a vacuum plasma processor using a pulsed r.f. source
`exciting a coil in accordance with a preferred embodiment of
`the present invention;
`FIG. 2 is a top view of a preferred coil in the processor of
`FIG. 1; and
`FIG. 3 is a perspective view of the coil illustrated in FIG.
`
`2.
`
`DETAILED DESCRIPTION OF THE DRAWING
`
`The vacuum plasma workpiece processor of FIG. 1 of the
`drawing includes vacuum chamber 10, shaped as a cylinder
`having grounded metal wall 12, metal bottom end plate 14,
`and circular top plate structure 18, consisting of a dielectric
`window structure 19, having the same thickness from its
`center to its periphery. Sealing of vacuum chamber 10 is
`provided by conventional gaskets (not shown). The proces-
`sor of FIG. 1 can be used for etching a semiconductor,
`dielectric or metal substrate or for depositing molecules on
`such substrates.
`
`A suitable gas that can be excited to a plasma state is
`supplied to the interior of chamber 10 from a gas source (not
`shown) via port 20 in side wall 12. The interior of the
`chamber is maintained in a vacuum condition, at a pressure
`that can vary in the range of 1—100 milliTorr, by a vacuum
`pump (not shown), connected to port 22 in end plate 14.
`The gas in the chamber is excited to a plasma having a
`spatially substantially uniform density by a suitable electric
`source. The electric source includes a substantially planar
`coil 24, mounted immediately above window 19 and excited
`by r.f. power source 26 (typically having a fixed frequency
`of 13.56 mHz) having a variable amplitude envelope pro-
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`vided by periodic pulse source 27. Source 27 turns the r.f.
`output of source 26 on and off from time to time, preferably
`periodically. When pulse source 27 turns r.f. source 26 on,
`the amplitude of the r.f. voltage applied to coil 24 is
`sufficiently high to produce sufficient current in the coil to
`generate a high magnetic field. The current
`in coil 24
`generates a large enough magnetic field flux in chamber 10
`in proximity to window 19 to excite ionizable gas in the
`chamber to a plasma. When source 27 turns source 26 off or
`reduces the voltage of the envelope of the source r.f. output
`to a relatively low value the magnetic field coil 24 applies to
`the gas in chamber 10 is insufficient to excite the gas to a
`plasma state. As a result there is much greater charge particle
`diffusion than the charge particle diffusion which occurs
`when the r.f. envelope applied to coil 24 has a high value.
`Impedance matching network 28, connected between
`output terminals of r.f. source 26 and excitation terminals of
`coil 24, couples the variable amplitude envelope the r.f.
`source derives to the coil. Impedance matching network 28
`includes variable reactances (not shown) which a controller
`varies in a known manner to achieve impedance matching
`between source 26 and a load including coil 24 and the
`plasma load the coil drives.
`Duty cycle controller 29 responds to input device 31 to
`control the duration of the pulses source 27 derives. Input
`device 31 can be a manual device, such as a potentiometer
`or numeric duty cycle per cent representing keys of a key
`pad, or a microprocessor responsive to signals stored in a
`computer memory for different processing recipes of work-
`piece 32. Variables of the recipes include (1) species of gases
`flowing through port 22 into chamber 10, (2) pressure in
`chamber 10 controlled by the vacuum pump connected to
`port 20, (3) duty cycle of the r.f. envelope source 26 derives
`in response to the pulses source 27 derives, and (4) average
`power r.f. source 26 supplies to coil 24.
`I have found that etch rate of material from workpiece 32
`by the plasma in chamber 10 depends on average power of
`the plasma. Usually etch rate is desirably maintained con-
`stant. Consequently, peak power and duty cycle are desir-
`ably inverse functions of each other so that as the duty cycle
`of r.f. source 26 increases the envelope of the r.f. voltage
`source 26 derives decreases in a reciprocal manner and vice
`versa for decreases in the duty cycle of source 26.
`To these ends, duty cycle controller 29 supplies amplitude
`controller 35 with a signal having an amplitude that
`is
`inversely proportional to the duty cycle of the control signal
`controller 29 supplies to pulse source 27. Amplitude con-
`troller 35 responds to the signal from controller 29 to vary
`the output voltage and power of the r.f. envelope source 26
`supplies to coil 24. The recipe controls the initial, continuous
`wave (100% duty cycle) amplitude of the envelope source
`26 derives. The recipe also causes the signal from controller
`35 to increase the envelope amplitude as duty cycle
`decreases. It is to be understood that the amplitude control
`of the envelope r.f. source derives could also be directly
`controlled by a separate output of input device 31 in a
`coordinated manner with the duty cycle of source 27.
`Workpiece 32 is fixedly mounted in chamber 10 to a
`surface of workpiece holder (i.e., chuck) 30; the surface of
`holder 30 carrying workpiece 32 is parallel to the surface of
`window 19. Workpiece 32 is usually electrostatically
`clamped to the surface of holder 30 by a DC potential of a
`DC power supply (not shown). R.f. source 31 supplies a
`constant amplitude r.f. voltage to impedance matching net-
`work 33,
`that
`includes variable reactances (not shown)
`Matching network 33 couples the output of source 31 to
`
`GILLETTE-1015 / Page 7 of 12
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`GILLETTE-1015 / Page 7 of 12
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`US 6,319,355 B1
`
`7
`
`holder 30. A controller (not shown) controls the variable
`reactances of matching network 33 to match the impedance
`of source 31 to the impedance of an electrode (not shown)
`of holder 30. The load coupled to the electrode is primarily
`the plasma in chamber 10. As is well known the r.f. voltage
`source 31 applies to the electrode of holder 30 interacts with
`charge particles in the plasma to produce a DC bias on
`workpiece 32.
`Surrounding planar coil 24 and extending above top end
`plate 18 is a metal tube or can-like shield 34 having an inner
`diameter somewhat greater than the inner diameter of wall
`12. Shield 34 decouples electromagnetic fields originating in
`coil 24 from the surrounding environment. The distance
`between shield 34 and the peripheral regions of coil 24 is
`large enough to prevent significant absorption by shield 34
`of the magnetic fields generated by the peripheral regions of
`coil 24.
`
`The diameter of cylindrically shaped chamber 10 is large
`enough to prevent absorption by chamber walls 12 of the
`magnetic fields generated by the peripheral regions of coil
`24. The diameter of dielectric window structure 19 is greater
`than the diameter of chamber 10 to such an extent that the
`
`entire upper surface of chamber 10 is comprised of dielectric
`window structure 10. The distance between the treated
`
`surface of workpiece 32 and the bottom surface of dielectric
`window structure 19 is chosen to provide the most uniform
`plasma flux on the exposed, processed surface of the work-
`piece. For a preferred embodiment of the invention,
`the
`distance between the workpiece processed surface and the
`bottom of the dielectric window is approximately 0.3 to 0.4
`times the diameter of chamber 10;
`the inner diameter of
`chamber 12 is 14 inches, the diameter of coil 24 is 12 inches,
`the inner diameter of cylindrical shield 34 is 14.7 inches, and
`the distance between the workpiece processed surface and
`the bottom of the dielectric window is 4.7 inches.
`
`Planar coil 24 functions as a transmission line to produce
`a standing wave pattern along the length of the coil. The
`standing wave pattern results in variations in the magnitude
`of the r.f. voltages and currents along the length of the coil.
`The dependence of the magnetic flux generated by the coil
`on the magnitude of these r.f. currents results in differing
`amounts of plasma being produced in different portions of
`chamber 10 beneath different portions of the coil.
`The variations in the r.f. current magnitude flowing in
`different parts of the coil are spatially averaged to assist in
`deriving a uniform plasma. Spatially averaging these differ-
`ent current values in the different parts of the coil substan-
`tially prevents non-radial asymmetries in the plasma density,
`particularly at regions of high r.f. current in the coil seg-
`ments near the coil periphery. The transmission line behav-
`ior of the r.f. current in planar coil 24 increases the amount
`of magnetic flux generated by the peripheral coil segments
`relative to the center coil segments. This result is achieved
`by exciting coil 24 with r.f. so the regions of maximum r.f.
`current are on the peripheral coil segments.
`As illustrated in FIGS. 2 and 3, planar coil 24 includes
`interior substantially semicircular loops 40, 42 and periph-
`eral substantially circular segments 46 and 48 and an inter-
`mediate substantially circular segment 44. Loops 40 and 42
`form half turns of coil 24 while each of loops 44, 46 and 48
`forms almost a complete full turn; the full and half turns are
`connected in series with each other. All of segments 40, 42,
`44, 46 and 48 are coaxial with central coil axis 50, coinci-
`dent with the center axis of chamber 10. Opposite excitation
`terminals 52 and 54, in the center portion of coil 24, are
`respectively coupled by leads 48 and 56 to opposite termi-
`
`8
`nals of r.f. source 26 via matching network 28 and one
`electrode of capacitor 80, the other electrode of which is
`grounded. Terminal 60, at the end of loop 40 opposite from
`terminal 52, is connected to end terminal 66 of outer loop
`segment 48 by conductive strap 64 which is located in a
`region slightly above the plane of coil 24 and does not touch
`any of the coil segments which run beneath it so the strap is
`electrically insulated from coil 24, except at terminals 60
`and 66.
`
`Segment 48 has a second terminal 68 slightly less than
`360° from terminal 66; terminal 68 is connected to terminal
`70 of loop segment 46 via strap 72. Loop 46, having an
`angular extent of almost 360°, has a second end terminal 74
`connected to terminal 76 of loop 44 via strap 78. Loop 44,
`having an angular extent of almost 360°, has a second end
`terminal 80 which is connected by strap 82 to terminal 62 at
`the end of segment 42 opposite from terminal 54.
`
`Capacitor 80, having a capacitive impedance anp=1/
`(jZJ'ch), where j=\/:1, f is the frequency of r.f. source 26, and
`C is the capacitance of capacitor 30, shifts the phase and
`therefore location of the voltage and current distribution
`across the entire length of coil 24. The voltage and current
`distribution are shifted in coil 24 so the coil produces r.f.
`electric and magnetic fields which provide substantially
`uniform plasma flux on the processed surface of workpiece
`32.
`
`For the preferred embodiment, the voltage and current of
`coil 24 are distributed by selecting the value of capacitor 80
`so the peak-to-peak r.f. current at coil
`terminal 54 is a
`minimum and equals the peak-to-peak r