Samsung Electronics Co., Ltd. v. Demaray LLC
`Samsung Electronic's Exhibit 1017
`Exhibit 1017, Page 1
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`

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`WO 02/23588 A2—__MINITTITANIATMi
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`bor two-letter codes and other abbreviations, refer to the "Guid-
`ance Notes on Codes andAbbreviations" appearing at the begin-
`ning ofeach regularissue ofthe PCT Gazette.
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`Ex. 1017, Page 2
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`Ex. 1017, Page 2
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`WO 02/23588
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`PCT/US01/42111
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`APPARATUS AND METHOD TO CONTROL THE UNIFORMITY
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`OF PLASMA BY REDUCING RADIAL LOSS
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`5
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`This application is based on and derives priority from U.S. Provisional Patent
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`Application No. 60/231,878,filed September 12, 2000, the contents of which are
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`incorporated herein by reference.
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`BACKGROUND OF THE INVENTION
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`The present invention relates to methods and apparatus for generating a
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`10
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`plasmain a plasma chamber, the plasma being used for performing various industrial
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`and scientific processes including etching and layer deposition on a semiconductor
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`wafer.
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`Plasma generating systems are currently widely used in a number of
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`manufacturing procedures such as etching and layer deposition on wafers as part of
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`15
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`integrated circuit manufacturing processes. The basic components of such a system
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`are a plasma chamberenclosing a processing region in which a plasmawill be
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`formed, a plasma electrode, usually at the top of the chamber, for delivering RF
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`electrical power into the chamberin orderto initiate and sustain the plasma, and a
`wafer chuck, usually at the bottom of the chamber, to hold a wafer on which
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`20
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`integrated circuits will be formed. Such a system further necessarily includes
`associated devices for delivering plasma-forming gas and processing gas to the
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`chamber and pumping gas out of the chamberin order to maintain both a desired gas
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`pressure and a desired gas composition in the chamber. One ofthe key desiderata in
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`plasmareactor design is to increase plasma density while maintaining plasma
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`25
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`uniformity.
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`There are two major sources of plasma non-uniformity in parallel plate plasma
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`reactors, or RF capacitively coupled plasma (CCP) systems, currently used in the
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`industry: radial plasmalosses; and highly localized harmonic contents.
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`Ex. 1017, Page 3
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`Ex. 1017, Page 3
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`Ina CCPparallel plate plasma reactor, both the plasmaelectrode and the
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`chuck, which can also be considered to be an electrode, are capacitively coupled to
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`RF powersources, and self-bias potentials are developed onthese electrodes. In
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`existing systems, the plasmais typically associated with a halo plasma, whichis a
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`scattered plasma surrounding the discharge gap existing everywhereinside the
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`chamber. An electric field having a large gradient in the radial direction can be
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`developed throughthe halo plasma in contact with the chamberwall. Since the
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`plasmapotential is time dependent in nature and the plasma always contacts the
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`chamber wall in these CCP reactors, there is always a time dependentradial electric
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`10
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`field gradient in the plasmain these CCP reactors. This radial electric field gradientis
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`associated with radial diffusion near the plasma edge. The diffusive loss generates a
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`plasmadensity profile in which the plasma density is higher in the center and lower
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`near the edge of the chamber. This diffusive radial plasma density profile is one
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`major source of plasma non-uniformity due to radial plasmalosses.
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`15
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`Asconcerns plasma non-uniformity caused by highly localized harmonic
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`contents, if the driven frequency on the plasmaelectrode of a parallel plate reactoris
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`increased, the energy contained at harmonic frequencies of the RF electric field
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`increases rapidly. Interference among these harmonic contents always occurs inside
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`the plasma chamber. The contribution to the total RF electric field due to the
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`20
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`harmonic interference causes thetotal RF electric field on the surface of the
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`electrodes to become non-uniform. The non-uniformity in plasma density could be
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`’ muchgreater than the total electric field non-uniformity because high frequency
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`power is much moreefficient in creating high plasma densities. The high harmonic
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`frequencies create additional plasma density, but they contribute even more strongly
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`25
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`to the plasma non-uniformity. So the harmonic contents and their interference with
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`each other is another major source of plasma non-uniformity.
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`For the semiconductorindustry, if a system with non-uniform plasmais used
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`for semiconductor wafer processing, the non-uniform plasma discharge will produce
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`non-uniform deposition or etching on the surface of the semiconductor wafer. Thus,
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`30
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`the control of the uniformity of the plasmadirectly affects the quality of the resulting
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`integrated semiconductorchips.
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`Ex. 1017, Page 4
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`Ex. 1017, Page 4
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`PCT/US01/42111
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`The trend in the semiconductor equipmentindustry is toward reactor sources
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`for processing ever larger wafers, current efforts being devoted to progressing from
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`plasmareactor sources capable ofprocessing wafers with a diameter of 200 mm to
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`those capable of processing wafers with a diameter of 300 mm.Sincelocalfield
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`non-uniformity increases as a substantial function of the source dimensionrelative to
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`wavelength, it is expected that greater non-uniformity will be found in 300-mm
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`systems than in equivalent 200-mm systems. Thus, control of the uniformity of the
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`plasma becomescritical for larger systems.
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`In VHF CCPsystemsofthe type currently used in the industry, both the upper
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`10
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`electrode and the chuck are capacitively coupled to the RF power source or to
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`respective power sources. The processing plasma in such systems makes contact with
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`the chamber wall through the halo plasma existing in the chamber surrounding the
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`discharge gap. Lack of control of the halo plasma makesit difficult to control the
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`time dependent plasma potential. There is also a significant time-dependentradial
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`15
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`electric field gradient existing near the outer edge ofthe processing plasma. This
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`radial electric field gradient increases radial plasma loss,
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`introduces charging damage
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`near the wafer edge, and possibly causes sputtering on the chamber wall.
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`BRIEF SUMMARY OF THE INVENTION |
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`20
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`The present invention provides improved plasma density uniformity in CCP
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`systems.
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`The invention is implemented by a capacitively coupled plasma reactor
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`comprising: a reactor chamber enclosing a plasma region; upper and lower main
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`plasma generating electrodes for generating a processing plasmain a central portion
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`25
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`of the plasmaregion by transmitting electrical power from a powersourceto the
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`central portion while a gas is present in the plasma region; and meansincluding at
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`least one set of magnets for maintaining a boundary layer plasma in a boundary
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`portion ofthe plasma region aroundthe processing plasma.
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`The invention is further implemented by a capacitively coupled plasma
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`30
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`reactor comprising: a reactor chamberenclosing a plasma region; upper and lower
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`plasma generating electrodes for generating a processing plasmain a central portion
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`Ex. 1017, Page 5
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`Ex. 1017, Page 5
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`ofthe plasma region by transmitting electrical power from a powersourceto the
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`central portion while a gas is present in the plasma region; and meansfor applying a
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`VHFdrive voltage to the upper plasma generating electrode and RF bias voltages at a
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`lower frequency than the VHF drive voltage to the upper and lower plasma generating
`electrodes.
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`The invention is not limited to systems which employ VHFdrive voltages, and
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`at least some aspects of the invention apply to a wide range of RF frequencies used
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`for semiconductor processing. However, the current industry trend is toward the use
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`of VHFdrive voltages for parallel plate, CCP process reactors. Although edge non-
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`10
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`uniformity dueto radial losses can be observed in all such reactors, the plasma non-
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`uniformity associated with harmonics of the fundamental frequency can be greatly
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`exacerbated when the fundamental drive voltage frequencyis in the VHF range.
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`BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
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`15
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`FIGs. 1, 2A, 2B and 2C are simplified cross-sectional pictorial views of four
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`embodiments of apparatus according to the invention.
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`FIGs. 3A, and 3B are plasmapotential waveform diagramsillustrating one
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`aspect of the invention.
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`FIG.4 is a block circuit diagram illustrating another aspect of the invention.
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`20
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`FIGs. 5A, 5B, 6A and 6B are electrode voltage and plasma potential
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`waveform diagramsillustrating a further aspect of the invention.
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`FIG. 7 is a diagram of a RF power supply circuit for supplying power to
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`electrodes of a reactor according to the invention.
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`FIG.8 is a pictorial illustration of the electron and ion gradient-B drifts in a
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`25
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`circular ring cusp magnetic field.
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`DETAILED DESCRIPTION OF THE INVENTION
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`This invention relates to an apparatus and a method for improving the radial
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`uniformity of the plasmadensity profile in a plasma chamberby reducing theradial
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`Ex. 1017, Page 6
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`Ex. 1017, Page 6
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`electric field gradient and radial losses. Apparatus accordingto this invention is a
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`new type of capacitively coupled plasma reactor, and has been termed a Capacitively
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`Coupled Double Plasma (CCDP)reactor.
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`A simplified cross-sectional pictorial view of one embodiment of an
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`apparatus according to the invention is shown in FIG. 1. The basic elements ofthe
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`illustrated apparatus include: an upper disk electrode 10 within which a gas delivery
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`element 12, commonly termed a showerhead, is disposed for injecting process gas
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`into a plasma region where an etching or deposition operation is to be performed; a
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`quartz shield ring 13; a wafer 14 to be processed; a chuck focus ring 15; a lower disk
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`10
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`electrode 20 constituted by a chuck for supporting a wafer to be processed; an upper
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`ring electrode 30; a lowerring electrode 40; a cylindrical electrode 50 surrounding
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`electrodes 30 and 40; one or more rings of permanent magnets 60; an RF feed line 70;
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`a ceramic washer 80; a top cover 90; and a vacuum chamberhaving a wall 110 of
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`cylindrical shape provided with a pumping port 100. The interior of the vacuum
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`15
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`chamberencloses the plasmaregion.
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`In the embodiment shown in FIG.1, there are two vertically superposed rings
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`of permanent magnets 60. Each permanent magnethasa radially extending
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`polarization axis, with the north poles of the permanent magnets in one ring pointing
`inwardly and those ofthe other ring pointing outwardly. Thus,in this embodiment,
`permanent magnets 60 form an annular magnetic field having magnetic field lines that
`extend generally vertically along arcuate paths between the two rings of magnets. In
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`20
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`an alternate embodiment, any of the permanent magnets can be replaced with an
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`electromagnet.
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`Cylindrical electrode 50 is surrounded by magnets 60. Cylindrical electrode
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`25
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`50 and magnets 60 act together as a magnetic mirror wall for reflecting plasma away
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`from wall 110. The rings of permanent magnets 60 haveafield strength and spacing
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`to create a magnetic field that is sufficiently strong close to the surface of cylindrical
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`electrode 50 to reflect the plasma in a manner to have a substantial confining effect
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`and to keep the plasmadensity relatively uniform in the radial direction to a short
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`30
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`distance from electrode 50. By arranging magnets 60 in the manner described above,
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`plasma near the magnetic mirror wall will circulate in a closed surface, and plasma
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`loss and charge separation along the cusp axes will be reduced greatly.
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`Ex. 1017, Page 7
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`Theelectrons and ions in the plasma near the magnetic mirror wall will be
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`subject to several drift motions. First, there will be magnetic field gradient drift Vvp
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`and magneticfield line curvature drift Vac, viz.
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`Vvs =+%v. p (BXVB)B’
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`Vro = +% vi p (ReXVBYReB’
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`where the + sign correspondsto electron and the — sign correspondsto ion, v, and vy
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`10
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`are the perpendicular and parallel thermal velocities, and p is the corresponding
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`gyroradius of the particle specie, respectively. In these drifts, electrons and ions are
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`drifting in opposite directions, but are both drifting in directions perpendicular to the
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`magnetic field lines and the direction of the magnetic field gradient or the direction of
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`field line curvature. For the circular line cusp configuration used in embodiments of
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`15
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`the present invention, the electrons and ionsare drifting azimuthally, but in directions
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`opposite to each other, in closed orbits as illustrated in FIG 8.
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`There is another drift due to the electric field in the plasma,
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`20
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`Vix = (EXB)/B*
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`which is always perpendicular to the magnetic field lines and the electric field E that
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`is present in the plasma. In this case, the ions and electronsare drifting in the same
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`direction.
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`25
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`Since the magnetic field near the magnet wall is decreasing as one moves
`away from the magnets,the gradientdrift and the curvature drift are both always
`present there. It is important to ensure that these drifts are in closed orbits so that no
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`charge separation is present anywhere in the plasma. Otherwise, the particle drift
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`30
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`motions can generate charge-separation, leading to a large-scale space-charge field E.
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`The plasma can be movedcollectively by the EXB drift, resulting in a large non-
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`uniformity in plasma density.
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`Ex. 1017, Page 8
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`Ex. 1017, Page 8
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`In the embodiment shown in FIG. 1, the processing plasmais generated
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`between electrodes 10 and 20 and the boundarylayer plasma is formed between ring
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`electrodes 30 and 40 and is confined radially by magnetic mirror 50, 60.
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`In the operation of the embodiment shown in FIG. 1, VHF RF power at 60
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`MHzor higher may be applied to upper disk electrode 10 via a DC blocking capacitor
`while RF power at a lower frequency, for example of the order of 2 MHz,is also
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`applied to upperdisk electrode 10 to create a DC self-bias on electrode 10. Lower
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`frequency RF powerat, for example, 2 MHz isalso applied to lower disk electrode
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`20, upper ring electrode 30 and lowerring electrode 40 to create DC self-biases on
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`10
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`these electrodes. A conventional powersplitter with individual amplitude and phase
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`control for each output (not shown) may be usedto deliver individually controlled
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`lower frequency RF powerto eachof the four electrodes. By applying the same low
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`frequency bias voltages to the upper ring and disk electrodes and to the lower ring
`electrode and disk electrodes, controllability ofthe ion energy andthe spatial potential
`uniformity of the processing plasma are improved. Cylindrical electrode 50 can be
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`15
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`grounded, or biased with DC or low frequency RF voltage at 2 MHz for plasma
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`potential control.
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`During the plasma process, the CCDPprocessreactor creates two plasma
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`discharges: one in the center between upper and lowerdisk electrodes 10 and 20 as a
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`20
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`processing plasma; and the other surrounding the center processing plasma between
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`upper and lowerring electrodes 30 and 40 as a boundary layer ring plasma. The
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`center processing plasma is mainly generated by the high frequency power at 60 MHz
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`or higher supplied to the upper disk electrode. The lower frequency RF powerat 2
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`MHzapplied to the upper and lowerdisk electrodes generates the self-bias voltage on
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`25
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`these electrodes. The center processing plasmausually has a relatively high density,
`for example in the range of 1 to 3 x 10'’ cm® andthe boundary layerring plasma can
`have a lowerdensity, for example <1 x 10’! cm®. The boundary layerplasmais
`predominantly generated by the low frequency RF power at 2 MHz suppliedto the
`upper and lowerring electrodes with confinementof that plasma by the magnetic
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`30
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`mirror wall to maintain the desired boundary layer plasma density and profile. The
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`magnetic mirror wall, consisting of cylindrical electrode 50 and one or moresets of
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`Ex. 1017, Page 9
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`Ex. 1017, Page 9
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`permanent magnets 60,reflects the plasma from cylindrical wall 110 and maintains
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`the boundary layer ring plasma.
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`In general, depending on the excitation frequency range, different physics
`phenomenaoccur in the plasma. At lower frequencies, secondary electrons generated
`by ion bombardmentare responsible for sustaining the plasma. Higher applied
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`voltages are necessary for maintaining the plasma density as well as the etching or
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`deposition rate. At higher frequencies, e.g. higher than .13.56 MHz, high plasma
`density can be generated with lower applied voltages so that high processing rates can
`be realized with low bias andlittle damage. The current trend is to apply a high
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`10
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`frequency, e.g., 60 MHz, to one electrode, typically the upper electrode, to create the
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`processing plasma, and to apply a lower frequency,e.g., 2 MHz, bias voltage to the
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`chuck to control ion energy thereabove. The low frequency bias voltages applied on
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`the electrodes will strongly affect the time-dependent plasmapotential.
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`The boundary layer plasma is created essentially to influence the center
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`15
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`processing plasma. When the boundary layer plasmais biased by the same low
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`frequency RF as the center processing plasma, the boundary layer plasma will be at
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`about the same time-dependent plasmapotential. As a result, radial, ambipolar
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`diffusion from the center processing plasma will be minimized.
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`Electrodes having a variety of shapes or other devices can be used to create a
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`20
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`boundary layer plasma having the desired shape. One preferred embodimentin the
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`current structure is a set of ring electrodes, as described above with reference to FIG.
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`1. The ring electrodes are used mainly to ensure that an axially symmetrical flat
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`plasmapotential profile is maintained in the entire center processing plasma.
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`The apparatus shown in FIG. 1 can be operated in several modes. For
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`25
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`example, upperring electrode 30 can be floated or RF biased and cylindrical electrode
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`can be floating or grounded. An electrodeis electrically floating whenit is electrically
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`isolated from both groundpotential, as by using capacitive coupling, and a bias
`potential. The electrode then achieves a potential, commonly referred to as the
`floating potential, such that the net ion andelectron currentto the electrode is zero.
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`Ex. 1017, Page 10
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`The boundary layer plasma can also be created by other meansthan thering
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`electrodes, as described below with reference to FIGs. 2A, 2B and 2C, in which
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`components identical to those shown in FIG. | are given the same reference numerals.
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`The second embodiment of apparatus according to the invention shown in
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`FIG. 2A differs from that of FIG. 1 in that ring electrodes 30 and 40 are replaced by
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`an electrostatically shielded radiofrequency (ESRF) loop antenna,or single turn coil,
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`120 whichis inductively coupled to the peripheral portion of the plasma region.to
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`form the boundary layer plasma region. The magnetic mirror wall is madeless lossy
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`and more inclusive by adding two rings of permanent magnets 65 adjacent the lower
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`10
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`part of the peripheral portion of the plasmaregion,essentially in the position occupied
`by ring electrode 40 in the embodimentof FIG. 1. Magnets 65 all have a vertically
`oriented polarization axis and are arranged in an inner ring of magnets whose north
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`poles face downwardly and an outer ring of magnets whose north poles face
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`upwardly. The inner and outer rings are centered on a commonhorizontal plane. In
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`15
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`this configuration, the cylindrical magnetic mirror wall is extended radially inwardly
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`to cover the region immediately outside disk electrodes 10 and 20.
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`The third embodiment shown in FIG. 2B differs from the embodiment of FIG.
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`2A only by replacementof coil 120 with a slotted waveguide 130 connected to a
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`microwave powersource (not shown) to generate an electron cyclotron resonance
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`20
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`(ECR) plasma. The microwave powersource can be a conventional device generating
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`electrical power at a frequency of, for example, 2.45GHz.
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`The fourth embodiment shown in FIG. 2C differs from that of FIG. 2B only in
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`that slotted waveguide 130 and its connected microwave powersource are replaced by
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`two further rings ofpermanent magnets 140 disposed above the region containing the
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`25
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`boundary layer plasma. These magnets will be oriented in the same manneras
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`magnets 65. Thus, in this embodiment, the boundary layer plasma is enclosed on three
`sides by permanent magnets which cooperate with cylindrical electrode 50 to form the
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`magnetic mirror.
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`In all of the above-described embodiments, the magnetic mirroris used
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`30
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`distinctively for reflecting the plasma. In addition to confining the plasma in
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`Ex. 1017, Page 11
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`cylindrical geometry and minimizing radial plasmaloss, this mirror will further
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`decouple the chamber from the plasmapotential.
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`The magnetic mirror wall can also be made in shapes other than those
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`illustrated. For example, the mirror can be constituted by an array of magnets lying
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`on a curved annular surface, like a portion of a torus.
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`In a CCDPprocess reactor according to the invention, only the center
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`processing plasmais used for processing a workpiece, or wafer. The boundary layer
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`ring plasmaitself is not used for processing, but is provided mainly to make the center
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`processing plasma more uniform and more controllable. The existence of the
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`10
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`boundary layer ring plasma minimizes any potential difference in theelectric field
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`between the center and the edge of the processing plasma, and helps maintain the
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`center processing plasma more uniform.
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`Control of the time dependent plasmapotential in the processing plasmais
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`also of importance. In the configuration proposedin this invention, the center
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`15
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`processing plasmais insulated completely from the system wall by the boundary layer
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`ring plasma. In a capacitively coupled plasma discharge, electron current flows to
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`any electrode that is biased at a potential more positive than the plasmapotential, and
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`ion current flows to any electrodethat is biased at a potential more negative than the
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`plasma potential. In a steady state, or repeated CW, operation, the time average
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`20
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`electron current must equal the time average ion current. There are two factors that
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`determine the balance of these currents: (1) electrons have much higher mobility than
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`ions; and(2) the electron current increases exponentially as the potential difference
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`between the plasma potential and the electrode voltage increases. On a capacitively
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`coupled electrode, a self-DC bias voltage is developed so that the most positive bias
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`25
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`voltage on the electrode becomes about equalto the peak plasma potential. Thus in a
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`multiple electrode system, the processing plasmapotential will follow the most
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`positive instantaneous potential of the upper or lower disk electrodes. This makesit
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`possible to apply the top and bottom bias voltages in modes such that those voltages
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`are in phase or out of phase with one another. In these modesofoperation, the ion
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`30
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`energy can be controlled to about ~10 eV, determined by the accuracy of the
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`amplitudes and phasesof the top and bottom bias voltages, if such low ion energyis
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`desirable for the process applications.
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`10
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`Ex. 1017, Page 12
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`By applying the same low frequency bias voltages to the upper and lowerdisk
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`electrodes, the controllability of the ion energy is improved dramatically. The spatial
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`potential uniformity of the center processing plasmawill also be improved by
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`applying the samebias voltage also to the upper and lowerring electrodes of the
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`embodimentof FIG.1.
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`FIGs. 3A and 3B showthe electrode voltage and the resulting time dependent plasma
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`potentials in the center processing plasma and the boundary layer ring plasma,
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`respectively, when a VHFdrive voltage at a frequency of, for example, 60 MHz,is
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`applied to upper disk electrode 10 and a RF bias voltageat, for example, 2 MHz,is
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`10
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`appliedto all of the electrodes 10, 20, 30 and 40. The bias voltages applied to the
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`electrodes can be in phase or out of phase with one another. Whenthe bias voltages
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`applied to associated upper and lowerelectrodes are in phase, plasmapotential control
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`can be improved. However, a phase difference of 180 degrees between electrodes 10
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`and 20 or electrodes 30 and 40 can lead to a reduction ofthe transfer of power from
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`15
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`the fundamental frequency to the harmonic frequencies. An optimal phase difference
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`exists for each specific case.
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`Because the same low frequencybias voltage drives the lowerdisk and ring
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`electrodes, the low frequency time dependent plasmapotentials of the two plasmas
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`are identical. This will greatly reduce radial ambipolar diffusion of the plasma, even
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`20
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`though a high frequency drive voltage is being applied to upper disk electrode 10.
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`The magnetic field acting on the boundary layer plasma must be strong enough to
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`magnetize the electronsto reflect the plasma electrons magnetically. “Magnetized”
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`electrons are electrons moving in a magnetic field that preferably movein helical
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`motions about magnetic field lines and, in general, are constrained to move along
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`25
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`field lines rather than across them. Typically, collisional processes are required to
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`diffuse electrons across magnetic field lines. For the case presented herein, the
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`desired field strength for magnetized electrons is approximately 200 Gauss, below
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`which the degree of “magnetization” is lessened. There will be a surface layer rich in
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`ions near the magnet mirror, which givesrise to a positive local potential to reflect the
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`plasmaions electrostatically. The effective leak width on the ring cusp,for the case
`1/2 ,
`of ambipolar diffusion, is given by the so-called hybrid gyroradius: p= (pep)~;
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`30
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`where p, and p, are the electron gyroradius and ion gyroradius, respectively.
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`11
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`Ex. 1017, Page 13
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`In accordance with a further feature ofthe present invention, cylindrical
`electrode 50 is maintained at a potential substantially equal to the plasma potential
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`which varies at the low RF frequency. One embodimentofa circuit for achieving
`such control is shown in FIG.4 in which voltages at the low RF frequency on
`cylindrical electrode 50, upper electrode 10 and lower electrode 20 are monitored by
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`respective voltage sensors 250, 252 and 254. The output voltages from sensors 250,
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`252 and 254 are amplified to appropriate levels by respective amplifiers 260, 262 and
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`264. The output voltages from amplifiers 260, 262 and 264 are applied to a
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`comparator circuit 266 composedofa differential amplifier, a buffer and an inverter,
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`10
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`the function of which will be described below.
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`The output of amplifier 262 is further supplied to the input of a gate 272, while
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`the output of amplifier 264 is supplied to the input of a gate 274. The opening and
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`closing of gates 272 and 274 is controlled by respective outputs of comparator 266 in
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`such a mannerthat if the output from amplifier 262 is more positive, gate 272 is
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`15
`
`opened andif the output of amplifier 264 is more positive, gate 274 is opened. The
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`outputs of gates 272 and 274 are connected to a combining element 280. Thus, the
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`output signal from amplifier 260 is representative ofthe voltage on cylindrical
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`electrode 50, while the output of combining circuit 280 is representative of the higher
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`ofthe voltages on upperelectrode 10 and lower electrode 20, which voltage
`
`20
`
`correspondsto the potential of the processing plasma.
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`The output voltages from amplifier 260 and combining circuit 280 are
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`supplied to respective inputs ofa differential amplifier 284 and the output, which is
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`representative of the difference between the voltages of the output of amplifier 260
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`and the output of combining unit 280, is supplied to the input of a power amplifier
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`25
`
`286 which drives electrode 50. Thus, with this circuit arrangement, the output voltage
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`from power amplifier 286 will act to maintain the voltage on cylindrical electrode 50
`
`equal to the higher value of the voltages on electrodes 10 and 20.
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`In the circuit of FIG. 4, use will be made of circuit components which have a
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`sufficiently rapid response to allow the voltage on cylindrical electrode 50 to follow
`
`30
`
`the low RF componentofthe potential of the processing plasma. Asa result, low
`
`frequency current drawn into the surface of the chamber wall will be minimized.
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`12
`
`Ex. 1017, Page 14
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`Ex. 1017, Page 14
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`WO 02/23588
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`PCT/US01/42111
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`This control of the voltage on cylindrical electrode 50, contributes
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`significantly to suppression ofthe radial electric field gradient in the ring plasma,
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`thereby further suppressing any radial electric field gradient in the processing plasma.
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`The combination of a magnetic mirror wall with a cylindrical electrode and
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`feedback circuit according to the present invention offers the advantageof reflecting
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`electrons as well as ions. The reflection of plasma from the magnetic mirror wall
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`controls the radial profile of the plasma and reducesradial losses of the plasma. The
`result is a greater processing plasma uniformity and increased plasmadensity. The
`magnetic mirror wall accordingto the invention also helps to insulate the outer
`chamber wall from the plasma. The outer chamber wall does not draw any current
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`and is no longer subjected to sputtering damage.
`
`As described earlier herein, a reactor according to the invention can be
`operated with a VHFdrive voltage applied to the upper disk electrode and a low RF
`bias voltage applied to both the upper and lower disk electrodes. According to a
`further feature of the invention, such an operating scheme can be used to reduce the
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`harmonic content of the electric field generated in the plasma. This feature will be
`
`described with reference to FIGs. 5A, 5B, 6A and 6B.
`
`Whena high frequencyvoltageis applied to upperdisk electrode 10 and a low
`frequency voltage is applied to lower disk electrode, or chuck, 20, as is done in prior
`art systems, these voltagesare rectified in the plasma because the plasmapotentialis
`always greater than or equalto zero volts (ground potential) and equal to the more
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`positive one of the potentials on the twoelectrodes.
`
`FIG. 5A showsthe voltages applied to both electrodes, where, according to the
`
`prior art, only a high frequency (VHF)voltage is applied to upper disk electrode 10
`and only a low frequency RF voltageis applied to chuck 20. FIG. 5B showsthe
`resultant plasma potential, which is always the morepositive of the two electrode
`voltages. Here, high frequency harmonics are generated continuously, i.e., throughout
`the entire cycle of the low frequency wave. Therefore, the electric field in the plasma
`
`has a substantial harmonic content
`
`10
`
`15
`
`20
`
`25
`
`30
`
`In the contrast, FIGs. 6A and 6B showelectrode voltages and plasma
`potential, respectively, when both a low RF frequency voltage and a high frequency
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`13
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`Ex. 1017, Page 15
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`Ex. 1017, Page 15
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`WO 02/23588
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`PCT/US01/42111
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`voltage are applied to upper disk electrode 10, and only the low RF frequency
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`modulation voltage is applied to chuck 20. The low frequency RF voltages applied to
`
`upper disk electrode 10 and chuck 20 are equal in magnitude but out ofphase by 180°.
`
`FIG. 6B showsthe resultant plasma potential, where the high frequency harmonics are
`
`generated only over one-half of each cycle of the low frequency RF wave, thereby
`
`reducing the harmonic content of the electric field in the plasma. In general, it has
`
`been found that application of a RF bias to upper disk electrode 10 which is 180° out
`
`of phase from the RF bias applied to chuck 20 producesdesirableeffects, i.e. reduced
`
`harmonics and improved uniformity. However, there may be a different phase
`
`10
`
`difference which is optimal, as described above. The level of generation of harmonics
`
`or transfer ofpower to the harmonic frequencies can dramatically affect the process
`
`uniformity to a greater extent than the mean processrate, or plasma density.
`
`The feature illustrated in FIGs. 6A and 6B can beutilized advantageously in a
`
`15
`
`CCPreactor of conventional construction, i.e., one not equipped to produce or sustain
`a ring plasma and with or withouta cylindrical electrode 50 and control circuit of the
`type shown in FIG.4.
`
`FIG. 7 showsa diagram ofa circuit that may be operated to produce the
`
`electrode voltages and plasmapotential illustrated in FIGs. 6A and 6B. The
`
`associated reactor may be identical to that shown in FIG. 1 with electrodes 30 and 40
`
`20
`
`removed. Thecircuit includes a RF power source 302 producing an output voltage of,
`
`for example, 2 MHz, whic