`Samsung Electronic's Exhibit 1017
`Exhibit 1017, Page 1
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`WO 02/23588 A2
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`For two-letter codes and other abbreviations, refer to the ”Guid-
`ance Notes on Codes andAbbreviations " appearing at the begin-
`ning ofeach regular issue 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/USOl/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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`10
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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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`plasma in 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 chamber enclosing a processing region in which a plasma will 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 chamber in order to initiate and sustain the plasma, and a
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`wafer chuck, usually at the bottom of the chamber, to hold a wafer on which
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`integrated circuits will be formed. Such a system firrther necessarily includes
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`associated devices for delivering plasma-forming gas and processing gas to the
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`chamber and pumping gas out of the chamber in order to maintain both a desired gas
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`pressure and a desired gas composition in the chamber. One of the key desiderata in
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`plasma reactor design is to increase plasma density while maintaining plasma
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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 plasma losses; 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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`In a CCP parallel plate plasma reactor, both the plasma electrode 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 power sources, and self-bias potentials are developed on these electrodes. In
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`existing systems, the plasma is typically associated with a halo plasma, which is a
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`scattered plasma surrounding the discharge gap existing everywhere inside the
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`chamber. An electric field having a large gradient in the radial direction can be
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`developed through the halo plasma in contact with the chamber wall. Since the
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`plasma potential 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 dependent radial electric
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`field gradient in the plasma in these CCP reactors. This radial electric field gradient is
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`associated with radial diffusion near the plasma edge. The diffusive loss generates a
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`plasma density 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 plasma losses.
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`As concerns plasma non-uniformity caused by highly localized harmonic
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`contents, if the driven frequency on the plasma electrode of a parallel plate reactor is
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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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`harmonic interference causes the total 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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`‘ much greater than the total electric field non-uniformity because high frequency
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`power is much more efficient 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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`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 semiconductor industry, if a system with non-uniform plasma is 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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`the control of the uniformity of the plasma directly affects the quality of the resulting
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`integrated semiconductor chips.
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`Ex. 1017, Page 4
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`Ex. 1017, Page 4
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`PCT/USOl/42111
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`The trend in the semiconductor equipment industry is toward reactor sources
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`for processing ever larger wafers, current efforts being devoted to progressing from
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`plasma reactor sources capable of processing wafers With a diameter of 200 mm to
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`those capable of processing wafers with a diameter of 300 mm. Since local field
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`non-uniformity increases as a substantial function of the source dimension relative 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 ZOO-mm systems. Thus, control of the uniformity of the
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`plasma becomes critical for larger systems.
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`In VHF CCP systems of the type currently used in the industry, both the upper
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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 makes it difficult to control the
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`time dependent plasma potential. There is also a significant time-dependent radial
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`electric field gradient existing near the outer edge of the 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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`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 plasma in a central portion
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`of the plasma region by transmitting electrical power from a power source to the
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`central portion while a gas is present in the plasma region; and means including at
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`least one set of magnets for maintaining a boundary layer plasma in a boundary
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`portion of the plasma region around the processing plasma.
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`The invention is further implemented by a capacitively coupled plasma
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`reactor comprising: a reactor chamber enclosing a plasma region; upper and lower
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`plasma generating electrodes for generating a processing plasma in a central portion
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`Ex. 1017, Page 5
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`Ex. 1017, Page 5
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`of the plasma region by transmitting electrical power from a power source to the
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`central portion while a gas is present in the plasma region; and means for applying a
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`VHF drive 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
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`electrodes.
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`‘
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`The invention is not limited to systems which employ VHF drive 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 VHF drive voltages for parallel plate, CCP process reactors. Although edge non-
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`uniformity due to 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 frequency is in the VHF range.
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`BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
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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 plasma potential waveform diagrams illustrating 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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`FIGS. 5A, 5B, 6A and 6B are electrode voltage and plasma potential
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`waveform diagrams illustrating 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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`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 plasma density profile in a plasma chamber by reducing the radial
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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 according to 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 of the
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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 lower ring 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 chamber having 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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`chamber encloses the plasma region.
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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 magnet has a radially extending
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`polarization axis, with the north poles of the permanent magnets in one ring pointing
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`inwardly and those of the other ring pointing outwardly. Thus, in this embodiment,
`permanent magnets 60 form an annular magnetic field having magnetic field lines that
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`20
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`extend generally vertically along arcuate paths between the two rings of magnets. In
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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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`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 have a field 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 plasma density 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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`Ex. 1017, Page 7
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`The electrons 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 VVB
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`and magnetic field line curvature drift VRc, viz.
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`VVB =i 1/2 vi p (BXVB)/B2
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`ya = M v” p (REXVBnyB2
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`where the + sign corresponds to electron and the — sign corresponds to ion, vi and v//
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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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`the present invention, the electrons and ions are 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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`VEXB = (143)03sz
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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 electrons are drifting in the same
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`direction.
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`Since the magnetic field near the magnet wall is decreasing as one moves
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`away from the magnets, the gradient drift and the curvature drift are both always
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`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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`motions can generate charge-separation, leading to a large-scale space-charge field E.
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`The plasma can be moved collectively 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 plasma is generated
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`between electrodes 10 and 20 and the boundary layer 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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`MHz or higher may be applied to upper disk electrode 10 via a DC blocking capacitor
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`while RF power at a lower frequency, for example of the order of 2 MHZ, is also
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`applied to upper disk electrode 10 to create a DC self-bias on electrode 10. Lower
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`frequency RF power at, for example, 2 MHz is also applied to lower disk electrode
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`20, upper ring electrode 30 and lower ring electrode 40 to create DC self-biases on
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`these electrodes. A conventional power splitter with individual amplitude and phase
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`control for each output (not shown) may be used to deliver individually controlled
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`lower frequency RF power to each of 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
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`electrode and disk electrodes, controllability of the ion energy and the spatial potential
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`uniformity of the processing plasma are improved. Cylindrical electrode 50 can be
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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 CCDP process reactor creates two plasma
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`discharges: one in the center between upper and lower disk electrodes 10 and 20 as a
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`processing plasma; and the other surrounding the center processing plasma between
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`upper and lower ring 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 power at 2
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`MHz applied to the upper and lower disk electrodes generates the self-bias voltage on
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`these electrodes. The center processing plasma usually has a relatively high density,
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`for example in the range of 1 to 3 x 1011 cm'3 and the boundary layer ring plasma can
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`have a lower density, for example <1 x 1011 cm'3. The boundary layer plasma is
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`predominantly generated by the low frequency RF power at 2 MHz supplied to the
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`upper and lower ring electrodes with confinement of 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 more sets 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
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`phenomena occur in the plasma. At lower frequencies, secondary electrons generated
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`by ion bombardment are 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
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`density can be generated with lower applied voltages so that high processing rates can
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`be realized with low bias and little damage. The current trend is to apply a high
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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 plasma potential.
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`The boundary layer plasma is created essentially to influence the center
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`processing plasma. When the boundary layer plasma is 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 plasma potential. As a result, radial, ambipolar
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`diffiision 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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`boundary layer plasma having the desired shape. One preferred embodiment in 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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`plasma potential 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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`example, upper ring electrode 30 can be floated or RF biased and cylindrical electrode
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`can be floating or grounded. An electrode is electrically floating when it is electrically
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`isolated from both ground potential, as by using capacitive coupling, and a bias
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`potential. The electrode then achieves a potential, commonly referred to as the
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`floating potential, such that the net ion and electron current to the electrode is zero.
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`Ex. 1017, Page 10
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`Ex. 1017, Page 10
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`The boundary layer plasma can also be created by other means than the ring
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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. 1 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 radio frequency (ESRF) loop antenna, or single turn coil,
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`120 which is inductively coupled to the peripheral portion of the plasma regionto
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`form the boundary layer plasma region. The magnetic mirror wall is made less 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 plasma region, essentially in the position occupied
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`by ring electrode 40 in the embodiment of FIG. 1. Magnets '65 all have a vertically
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`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 common horizontal plane. In
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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 replacement of coil 120 with a slotted waveguide 130 connected to a
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`microwave power source (not shown) to generate an electron cyclotron resonance
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`(ECR) plasma. The microwave power source 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 power source are replaced by
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`two further rings of permanent magnets 140 disposed above the region containing the
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`boundary layer plasma. These magnets will be oriented in the same manner as
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`magnets 65. Thus, in this embodiment, the boundary layer plasma is enclosed on three
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`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 mirror is used
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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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`Ex. 1017, Page 11
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`cylindrical geometry and minimizing radial plasma loss, this mirror will further
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`decouple the chamber from the plasma potential.
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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 CCDP process reactor according to the invention, only the center
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`processing plasma is used for processing a workpiece, or wafer. The boundary layer
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`ring plasma itself 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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`boundary layer ring plasma minimizes any potential difference in the electric 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 plasma potential in the processing plasma is
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`also of importance. In the configuration proposed in this invention, the center
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`processing plasma is 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 plasma potential, and
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`ion current flows to any electrode that 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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`electron current must equal the time average ion current. There are two factors that
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`determine the balance of these currents: (l) 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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`voltage on the electrode becomes about equal to the peak plasma potential. Thus in a
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`multiple electrode system, the processing plasma potential will follow the most
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`positive instantaneous potential of the upper or lower disk electrodes. This makes it
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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 modes of operation, the ion
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`energy can be controlled to about ~10 eV, determined by the accuracy of the
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`amplitudes and phases of the top and bottom bias voltages, if such low ion energy is
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`desirable for the process applications.
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`Ex. 1017, Page 12
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`By applying the same low frequency bias voltages to the upper and lower disk
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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 plasma will also be improved by
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`applying the same bias voltage also to the upper and lower ring electrodes of the
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`embodiment of FIG. 1.
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`FIGS. 3A and 3B show the 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 VHF drive voltage at a frequency of, for example, 60 MHz, is
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`applied to upper disk electrode 10 and a RF bias voltage at, for example, 2 MHz, is
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`applied to 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. When the bias voltages
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`applied to associated upper and lower electrodes are in phase, plasma potential 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 of the transfer of power from
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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 frequency bias voltage drives the lower disk and ring
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`electrodes, the low frequency time dependent plasma potentials 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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`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 electrons to reflect the plasma electrons magnetically. “Magnetized”
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`electrons are electrons moving in a magnetic field that preferably move in helical
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`motions about magnetic field lines and, in general, are constrained to move along
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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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`10
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`20
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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 gives rise to a positive local potential to reflect the
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`30
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`plasma ions electrostatically. The effective leak width on the ring cusp, for the case
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`of ambipolar diffusion, is given by the so-called hybrid gyroradius: p= (pepi)1/2 ;
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`where p6 and p1 are the electron gyroradius and ion gyroradius, respectively.
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`Ex. 1017, Page 13
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`In accordance with a further feature ofthe present invention, cylindrical
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`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 embodiment of a circuit for achieving
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`such control is shown in FIG. 4 in which voltages at the low RF frequency on
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`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 composed of a 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 manner that if the output from amplifier 262 is more positive, gate 272 is
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`15
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`opened and if 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 fiom amplifier 260 is representative of the 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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`of the voltages on upper electrode 10 and lower electrode 20, which voltage
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`20
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`corresponds to 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 of a 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
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`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
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`the low RF component of the potential of the processing plasma. As a result, low
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`frequency current drawn into the surface of the chamber wall will be minimized.
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`12
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`Ex. 1017, Page 14
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`Ex. 1017, Page 14
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`This control of the voltage on cylindrical electrode 50, contributes
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`significantly to suppression of the 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 advantage of 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 reduces radial losses of the plasma. The
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`result is a greater processing plasma uniformity and increased plasma density. The
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`magnetic mirror wall according to the invention also helps to insulate the outer
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`10
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`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.
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`As described earlier herein, a reactor according to the invention can be
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`operated with a VHF drive voltage applied to the upper disk electrode and a low RF
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`bias voltage applied to both the upper and lower disk electrodes. According to a
`
`15
`
`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
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`described with reference to FIGS. 5A, 5B, 6A and 6B.
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`When a high frequency voltage is applied to upper disk electrode 10 and a low
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`frequency voltage is applied to lower disk electrode, or chuck, 20, as is done in prior
`
`20
`
`art systems, these voltages are rectified in the plasma because the plasma potential is
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`always greater than or equal to zero volts (ground potential) and equal to the more
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`positive one of the potentials on the two electrodes.
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`FIG. 5A shows the voltages applied to both electrodes, where, according to the
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`prior art, only a high frequency (VHF) voltage is applied to upper disk electrode 10
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`25
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`and only a low fiequency RF voltage is applied to chuck 20. FIG. 5B shows the
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`resultant plasma potential, which is always the more positive of the two electrode
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`voltages. Here, high frequency harmonics are generated continuously, i. e., throughout
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`the entire cycle of the low frequency wave. Therefore, the electric field in the plasma
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`has a substantial harmonic content
`
`30
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`In the contrast, FIGS. 6A and 6B Show electrode voltages and plasma
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`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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`PCT/USOl/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
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`upper disk electrode 10 and chuck 20 are equal in magnitude but out of phase by 180°.
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`FIG. 6B shows the resultant plasma potential, where the high frequency harmonics are
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`generated only over one-half of each cycle of the low frequency RF wave, thereby
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`reducing the harmonic content of the electric field in the plasma. In general, it has
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`been found that application of a RF bias to upper disk electrode 10 which is 180° out
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`of phase from the RF bias applied to chuck 20 produces desirable‘effects, i.e. reduced
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`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
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`or transfer of power to the harmonic frequencies can dramatically affect the process
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`uniformity to a greater extent than the mean process rate, or plasma density.
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`The feature illustrated in FIGS. 6A and 6B can be utilized advantageously in a
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`CCP reactor of conventional construction, 1'. e., one not equipped to produce or sustain
`a ring plasma and with or without a cylindrical electrode 50 and control circuit of