(12) INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT)
`
`(19) World Intellectual Property Organization
`International Boreao
`
`(43) International Publication Date
`21 March 2002 (21.03.2002)
`
`• I IIIII IIIIIII II IIIIII IIIII IIII I II Ill lllll lllll llll lllll llll 111111111111111111
`
`(JO) International Publication Number
`WO 02/23588 A2
`
`PCT
`
`(51) International Patent Classification 7:
`
`HOlJ 37/32
`
`{21) International Application Number: PCT/USOl/42 111
`
`(22) International Filing Date:
`1.2 September 2001 (12.09.2001)
`
`(25) Filing Language:
`
`(26) Publication Language:
`
`English
`
`English
`
`(30) Priority Data:
`60/231,878
`
`12 Sept.ember 2000 (12.09.2000) US
`
`(71) Applicant (for all designated States except US): TOKYO
`ELECTRON LIMITED [JP/JP); TBS Broadcast Center,
`3-6, Akasak 5-chome, Minato-ku, Tokyo 107 (JP).
`
`(72) Inventor; and
`(75) Inventor/Applicant (for US only): QUON, Bill, H.
`[US/US]; 1020 East Sunburst Lane, Tempe, AZ 85284
`(US).
`
`(74) Agents: LAZAR, Dale, S. et al.; Pi.llsbury Winthrop LLP,
`1600 Tysons Boulevard, McLean, VA 22102 (US).
`
`(81) Designatw States (national); AE, AG, AL, AM, AT, AU,
`AZ, BA, 8 8, BG, BR, B'Y, BZ, CA, CH, CN, CO, CR, CU,
`CZ, DE, DK, DM, DZ, EC. EE, ES, Fl, GB, GD, GE, GH,
`GM, HR, HU, ID, Il~, lN, TS, JP, KE, KG, KP, KR, KZ, LC,
`LK, LR, LS, LT, LU, LV, MA, MD, MG, MK, tv-IN, MW,
`MX, MZ, NO, NZ, PH, PL, PT, RO, RU, SD, SE, SG, SI,
`SK, SL, TJ, TM, TR, TI, Tl, UA, UG, US, UZ, VN, YU,
`ZA,ZW.
`
`(84) Designated St.ates (regional): ARil'O patent (GH, GM,
`KE, LS, MW, MZ, SD, SL, SZ, TZ, UG, ZW), Eurasian
`patent(AM, AZ, BY, KG, KZ, MD, RU, TJ, TM), European
`patent (AT, BE, CH, CY, DE, DK, ES, FI, FR, GB, GR, IB,
`IT, LU, MC, NL, PT, SE, TR), OAPI patent (BF, BJ, CF,
`CG, Cl, CM, GA, GN, GQ, GW, ML, MR, NE, SN, TD,
`TG).
`
`Published:
`without international search report and to be republished
`upon receipt of that report
`
`[Continued on next page}
`
`----iiiiiiiiii
`
`iiiiiiiiii
`
`-------------------------------------------
`
`-
`
`(54) Title: APPARATl JS AND tvfETIJOD TO CONTROL TI·IB lJNTFORN.UTY OF PLASlvfA BY REDUCING RADIAL LOSS
`
`!!!!!!!
`
`iiiiiiiiii -
`----
`
`70
`
`80
`
`90
`
`13
`
`BOUNDARY
`LAYER
`PLASMA
`
`DISK PLASMA
`
`: +.: t/'.·(7'~:!;'.? [ :: :",,'.
`
`100
`
`.... ,.
`-........ ·_ ·~ t.:.
`15
`14
`QO
`00 (57) Abstract: A capacitively coupled plasma reactor composed of: a reactor chamber enclosing a plasma region; upper and lower
`lt) main plasma generating e]ectrodes for generating a processing plasma in a central portion of the plasma region by transmitting
`~ e.lectrical power from a power source to t he centra.l portion while a gas is present in the pJasma region; and a magnetic mirror including
`g plasma. A capacitively coupled plasma reactor composed of: a reactor chamber enclosing a plasma region; upper and lower plasma
`-... at least one set of magnets for maintaining a boundary layer plasma in a bo11ndary pottion of lhe plasma region around the processing
`generating electrodes for generating a processing plasma in the plasma region by transmitting electrical power from a power source
`0 to the plasma region while a gas is present in the plasma region; and power supplies for applying a VHF drive voltage to the upper
`::-,.. plasma gener<1ting electrode and RF bias voltages at a lower frequency than the VHF d1ive voltage to the upper and lower plasma
`;;,,, generating electrodes.
`
`Page 1 of 26
`
`APPLIED MATERIALS EXHIBIT 1017
`
`

`

`WO 02/23588 A2
`
`I IIIII IIIIIIII II IIIIII IIIII IIII I II Ill lllll lllll lllll 1111111111111111111111111111
`
`For IH o-lcttcr codes cmd other abbrcl'iations, ru/i.-'r to t!,e "Guid(cid:173)
`ance Notes on Codes and Abbreviations" appearinsr at the b(gin(cid:173)
`ning o( each regular issue oftlze I'CT Gazette.
`
`Page 2 of 26
`
`

`

`WO 02/23588
`
`PCT/USOl/42111
`
`APPARATUS AND METHOD TO CONTROL THE UNIFORMITY
`
`OF PLASMA BY REDUCING RADIAL LOSS
`
`5
`
`This application is based on and derives priority from U.S. Provisional Patent
`Application No. 60/231,878, filed September 12, 2000, the contents of which are
`incorporated herein by reference.
`
`BACKGROUND OF THE INVENTION
`
`10
`
`The present invention relates to methods and apparatus for generating a
`plasma in a plasma chamber, the plasma being used for performing various industrial
`and scientific processes including etching and layer deposition on a semiconductor
`
`wafer.
`
`Plasma generating systems are currently widely used in a number of
`
`manufacturing procedures such as etching and layer deposition on wafers as part of
`integrated circuit manufacturing processes. The basic components of such a system
`are a plasma chamber enclosing a processing region in which a plasma will be
`formed, a plasma electrode, usually at the top of the chamber, for delivering RF
`
`electrical power into the chamber in order to initiate and sustain the plasma, and a
`wafer chuck, usually at the bottom of the chamber, to hold a wafer on which·
`
`integrated circuits will be formed. Such a system further necessarily includes
`associated devices for delivering plasma-forming gas and processing gas to the
`chamber and pumping gas out of the chan1ber in order to maintain both a desired gas
`pressure and a desired gas composition in the chan1ber. One of the key desiderata in
`plasma reactor design is to increase plasma density while maintaining plasma
`uniformity.
`
`15
`
`20
`
`25
`
`There are two major sources of plasma non-uniformity in parallel plate plasma
`reactors, or RF capacitively coupled plasma (CCP) systems, currently used in the
`industry: radial plasma losses; and highly localized harmonic contents.
`
`Page 3 of 26
`
`

`

`WO 02/23588
`
`PCT/USOl/42111
`
`In a CCP parallel plate plasma reactor, both the plasma electrode and the
`
`chuck, which can also be considered to be an electrode, are capacitively coupled to
`
`RF power sources, and self-bias potentials are developed on these electrodes. In
`
`existing systems, the plasma is typically associated with a halo plasma, which is a
`
`5
`
`scattered plasma surrounding the discharge gap existing everywhere inside the
`chamber. An electric field having a large gradient in the radial direction can be
`
`developed through the halo plasma in contact with the chamber wall. Since the
`
`plasma potential is time dependent in nature and the plasma always contacts the
`
`chamber wall in these CCP reactors, there is always a time dependent radial electric
`
`10
`
`field gradient in the plasma in these CCP reactors. This radial electric field gradient is
`
`associated with radial diffusion near the plasma edge. The diffusive loss generates a
`
`plasma density profile in which the plasma density is higher in the center and lower
`
`near the edge of the chamber. This diffusive radial plasma density profile is one
`
`major source of plasma non-uniformity due to radial plasma losses.
`
`15
`
`As concerns plasma non-uniformity caused by highly localized harmonic
`
`contents, if the driven frequency on the plasma electrode of a parallel plate reactor is
`
`increased, the energy contained at harmonic frequencies of the RF electric field
`
`increases rapidly. Interference among these harmonic contents always occurs inside
`
`the plasma chamber. The contribution to the total RF electric field due to the
`
`20
`
`harmonic interference causes the total RF electric field on the surface of the
`
`electrodes to become non-uniform. The non-uniformity in plasma density could be
`
`much greater than the total electric field non-uniformity because high frequency
`
`power is much more efficient in creating high plasma densities. The high harmonic
`
`frequencies create additional plasma density, but they contribute even more strongly
`
`25
`
`to the plasma non-uniformity. So the harmonic contents and their interference with
`
`each other is another major source of plasma non-uniformity.
`
`For the semiconductor industry, if a system with non-uniform plasma is used
`
`for semiconductor wafer processing, the non-uniform plasma discharge will produce
`
`non-uniform deposition or etching on the surface of the semiconductor wafer. Thus,
`
`30
`
`the control of the uniformity of the plasma directly affects the quality of the resulting
`
`integrated semiconductor chips.
`
`2
`
`Page 4 of 26
`
`

`

`WO 02/23588
`
`PCT/USOl/42111
`
`The trend in the semiconductor equipment industry is toward reactor sources
`for processing ever larger wafers, current efforts being devoted to progressing from
`plasma reactor sources capable of processing wafers with a diameter of 200 mm to
`those capable of processing wafers with a diameter of 300 mm. Since local field
`non-unifom1ity increases as a substantial function of the source dimension relative to
`wavelength, it is expected that greater non-uniformity will be found in 300-mm
`systems than in equivalent 200-mm systems. Thus, control of the uniformity of the
`plasma becomes critical for larger systems.
`
`In VHF CCP systems of the type currently used in the industry, both the upper
`electrode and the chuck are capacitively coupled to the RF power source or to
`respective power sources. The processing plasma in such systems makes contact with
`the chamber wall through the halo plasma existing in the chamber surrounding the
`
`discharge gap. Lack of control of the halo plasma makes it difficult to control the
`time dependent plasma potential. There is also a significant time-dependent radial
`electric field gradient existing near the outer edge of the processing plasma. This
`radial electric field gradient increases radial plasma loss, introduces charging damage
`near the wafer edge, and possibly causes sputtering on the chamber wall.
`
`5
`
`10
`
`15
`
`BRIEF SUMMARY OF THE INVENTION
`
`20
`
`The present invention provides improved plasma density uniformity in CCP
`systems.
`
`25
`
`The invention is implemented by a capacitively coupled plasma reactor
`comprising: a reactor chamber enclosing a plasma region; upper and lower main
`plasma generating electrodes for generating a processing plasma in a central portion
`of the plasma region by transmitting electrical power from a power source to the
`central portion while a gas is present in the plasma region; and means including at
`least one set of magnets for maintaining a boundary layer plasma in a boundary
`portion of the plasma region around the processing plasma.
`The invention is further implemented by a capacitively coupled plasma
`
`30
`
`reactor comprising: a reactor chamber enclosing a plasma region; upper and lower
`plasma generating electrodes for generating a processing plasma in a central portion
`
`3
`
`Page 5 of 26
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`

`

`WO 02/23588
`
`PCT/USOl/42111
`
`of the plasma region by transmitting electrical power from a power source to the
`
`central portion while a gas is present in the plasma region; and means for applying a
`
`VHF drive voltage to the upper plasma generating electrode and RF bias voltages at a
`
`lower frequency than the VHF drive voltage to the upper and lower plasma generating
`
`5
`
`electrodes.
`
`The invention is not limited to systems which employ VHF drive voltages, and
`
`at least some aspects of the invention apply to a wide range of RF frequencies used
`
`for semiconductor processing. However, the current industry trend is toward the use
`
`of VHF drive voltages for parallel plate, CCP process reactors. Although edge non-
`
`10
`
`uniformity due to radial losses can be observed in all such reactors, the plasma non(cid:173)
`
`uniformity associated with harmonics of the fundamental frequency can be greatly
`
`exacerbated when the fundamental drive voltage frequency is in the VHF range.
`
`BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
`
`15
`
`FIGs. 1, 2A, 2B and 2C are simplified cross-sectional pictorial views of four
`
`embodiments of apparatus according to the invention.
`
`FI Gs. 3A, and 3B are plasma potential waveform diagrams illustrating one
`
`aspect of the invention.
`
`FIG. 4 is a block circuit diagram illustrating another aspect of the invention.
`
`20
`
`FIGs. SA, SB, 6A and 6B are electrode voltage and plasma potential
`
`waveform diagrams illustrating a further aspect of the invention.
`
`FIG. 7 is a diagram of a RF power supply circuit for supplying power to
`
`electrodes of a reactor according to the invention.
`
`FIG. 8 is a pictorial illustration of the electron and ion gradient-B drifts in a
`
`25
`
`circular ring cusp magnetic field.
`
`DETAILED DESCRIPTION OF THE INVENTION
`
`This invention relates to an apparatus and a method for improving the radial
`
`uniformity of the plasma density profile in a plasma chan1ber by reducing the radial
`
`4
`
`Page 6 of 26
`
`

`

`WO 02/23588
`
`PCT/USOl/42111
`
`electric field gradient and radial losses. Apparatus according to this invention is a
`
`new type of capacitively coupled plasma reactor, and has been tem1ed a Capacitively
`
`Coupled Double Plasma (CCDP) reactor.
`
`A simplified cross-sectional pictorial view of one embodiment of an
`
`5
`
`apparatus according to the invention is shown in FIG. 1. The basic elements of the
`
`illustrated apparatus include: an upper disk electrode 10 within which a gas delivery
`element 12, commonly termed a showerhead, is disposed for injecting process gas
`
`into a plasma region where an etching or deposition operation is to be performed; a
`
`quartz shield ring 13; a wafer 14 to be processed; a chuck focus ring 15; a lower disk
`
`IO
`
`electrode 20 constituted by a chuck for supporting a wafer to be processed; an upper
`
`ring electrode 30; a lower ring electrode 40; a cylindrical electrode 50 surrounding
`
`electrodes 30 and 40; one or more rings of permanent magnets 60; an RF feed line 70;
`a ceramic washer 80; a top cover 90; and a vacuum chamber having a wall 110 of
`
`cylindrical shape provided with a pumping port I 00. The interior of the vacuum
`
`15
`
`chamber encloses the plasma region.
`
`In the embodiment shown in FIG. I, there are two vertically superposed rings
`
`of permanent magnets 60. Each permanent magnet has a radially extending
`
`polarization axis, with the north poles of the permanent magnets in one ring pointing
`
`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
`
`20
`
`extend generally vertically along arcuate paths between the two rings of magnets. In
`
`an alternate embodiment, any of the permanent magnets can be replaced with an
`
`electromagnet.
`
`Cylindrical electrode 50 is surrounded by magnets 60. Cylindrical electrode
`
`25
`
`50 and magnets 60 act together as a magnetic mirror wall for reflecting plasma away
`
`from wall I 10. The rings of permanent magnets 60 have a field strength and spacing
`
`to create a magnetic field that is sufficiently strong close to the surface of cylindrical
`
`electrode 50 to reflect the plasma in a manner to have a substantial confining effect
`
`and to keep the plasma density relatively uniform in the radial direction to a short
`
`30
`
`distance from electrode 50. By arranging magnets 60 in the manner described above,
`
`plasma near the magnetic mirror wall will circulate in a closed surface, and plasma
`
`loss and charge separation along the cusp axes will be reduced greatly.
`
`5
`
`Page 7 of 26
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`

`

`WO 02/23588
`
`PCT/USOl/42111
`
`The electrons and ions in the plasma near the magnetic mirror wall will be
`
`subject to several drift motions. First, there will be magnetic field gradient drift V v8
`
`and magnetic field line curvature drift V Re, viz.
`
`5
`
`VvB =± Yz V.L p (BXVB)/B2
`
`VRc = ±Yz v,1 p (RcXVB)IR/B2
`
`where the+ sign corresponds to electron and the- sign corresponds to ion, V.L and v11
`are the perpendicular and parallel thermal velocities, and p is the corresponding
`
`10
`
`gyroradius of the particle specie, respectively. In these drifts, electrons and ions are
`
`drifting in opposite directions, but are both drifting in directions perpendicular to the
`
`magnetic field lines and the direction of the magnetic field gradient or the direction of
`
`field line curvature. For the circular line cusp configuration used in embodiments of
`
`15
`
`the present invention, the electrons and ions are drifting azimuthally, but in directions
`
`opposite to each other, in closed orbits as illustrated in FIG 8.
`
`There is another drift due to the electric field in the plasma,
`
`20
`
`V mrn = (EXB)/B2
`
`which is always perpendicular to the magnetic field lines and the electric field E that
`
`is present in the plasma. In this case, the ions and electrons are drifting in the same
`
`direction.
`
`25
`
`Since the magnetic field near the magnet wall is decreasing as one moves
`
`away from the magnets, the gradient drift and the curvature drift are both atways
`
`present there. It is important to ensure that these drifts are in closed orbits so that no
`
`charge separation is present anywhere in the plasma. Otherwise, the particle drift
`
`30 motions can generate charge-separation, leading to a large-scale space-charge field E.
`
`The plasma can be moved collectively by the EXB drift, resulting in a large non(cid:173)
`
`uniformity in plasma density.
`
`6
`
`Page 8 of 26
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`

`

`WO 02/23588
`
`PCT/USOl/42111
`
`In the embodiment shown in FIG. 1, the processing plasma is generated
`
`between electrodes 10 and 20 and the boundary layer plasma is formed between ring
`
`electrodes 30 and 40 and is confined radially by magnetic mirror 50, 60.
`
`In the operation of the embodiment shown in FIG. 1, VHF RF power at 60
`
`5 MHz or 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 of2 MHz, is also
`
`applied to upper disk electrode 10 to create a DC self-bias on electrode 10. Lower
`
`frequency RF power at, for example, 2 MHz is also applied to lower disk electrode
`
`20, upper ring electrode 30 and lower ring electrode 40 to create DC self-biases on
`
`10
`
`these electrodes. A conventional power splitter with individual amplitude and phase
`
`control for each output (not shown) may be used to deliver individually controlled
`
`lower frequency RF power to each of the four electrodes. By applying the same low
`
`frequency bias voltages to the upper ring and disk electrodes and to the lower ring
`
`electrode and disk electrodes, controllability of the ion energy and the spatial potenti~l
`
`15
`
`uniformity of the processing plasma are improved. Cylindrical electrode 50 can be
`
`grounded, or biased with DC or low frequency RF voltage at 2 MHz for plasma
`
`potential control.
`
`During the plasma process, the CCDP process reactor creates two plasma
`
`discharges: one in the center between upper and lower disk electrodes 10 and 20 as a
`
`20
`
`processing plasma; and the other surrounding the center processing plasma between
`
`upper and lower ring electrodes 30 and 40 as a boundary layer ring plasma. The
`
`center processing plasma is mainly generated by the high :frequency power at 60 MHz
`
`or higher supplied to the upper disk electrode. The lower frequency RF power at 2
`
`MHz applied to the upper and lower disk electrodes generates the self-bias voltage on
`
`25
`
`these electrodes. The center processing plasma usually has a relatively high density,
`for example in the range of 1 to 3 x 1011 cm"3 and the boundary layer ring plasma can
`have a lower density, for example <1 x 1011 cm·3
`• The boundary layer plasma is
`predominantly generated by the low :frequency RF power at 2 MHz supplied to the
`
`upper and lower ring electrodes with confinement of that plasma by the magnetic
`
`30 mirror wall to maintain the desired boundary layer plasma density and profile. The
`
`magnetic mirror wall, consisting of cylindrical electrode 50 and one or more sets of
`
`7
`
`Page 9 of 26
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`

`

`WO 02/23588
`
`PCT/USOl/42111
`
`permanent magnets 60, reflects the plasma from cylindrical wall 110 and maintains
`
`the boundary layer ring plasma.
`
`In general, depending on the excitation frequency range, different physics
`
`phenomena occur in the plasma. At lower frequencies, secondary electrons generated
`
`5
`
`by ion bombardment are responsible for sustaining the plasma. Higher applied
`
`voltages are necessary for maintaining the plasma density as well as the etching or
`
`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 and little damage. The current trend is to apply a high
`
`10
`
`frequency, e.g., 60 MHz, to one electrode, typically the upper electrode, to create the
`
`processing plasma, and to apply a lower frequency, e.g., 2 MHz, bias voltage to the
`
`chuck to control ion energy thereabove. The low frequency bias voltages applied on
`
`the electrodes will strongly affect the time-dependent plasma potential.
`
`the boundary layer plasma is created essentially to influence the center
`
`15
`
`processing plasma. When the boundary layer plasma is biased by the same low
`
`frequency RF as the center processing plasma, the boundary layer plasma will be at
`
`about the same time-dependent plasma potential. As a result, radial, ambipolar
`
`diffusion from the center processing plasma will be minimized.
`
`Electrodes having a variety of shapes or other devices can be used to create a
`
`20
`
`boundary layer plasma having the desired shape. One preferred embodiment in the
`
`current structure is a set of ring electrodes, as described above with reference to FIG.
`
`1. The ring electrodes are used mainly to ensure that an axially symmetrical flat
`
`plasma potential profile is maintained in the entire center processing plasma.
`
`The apparatus shown in FIG. 1 can be operated in several modes. For
`
`25
`
`example, upper ring electrode 30 can be floated or RF biased and cylindrical electrode
`can be floating or grounded. An electrode is electrically floating when it is electrically
`
`isolated from both ground potential, 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 and electron current to the electrode is zero.
`
`8
`
`Page 10 of 26
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`

`

`WO 02/23588
`
`PCT/USOl/42111
`
`The boundary layer plasma can also be created by other means than the ring
`
`electrodes, as described below with reference to FIGs. 2A, 2B and 2C, in which
`
`components identical to those shown in FIG. 1 are given the same reference numerals.
`
`The second embodiment of apparatus according to the invention shown in
`
`5
`
`FIG. 2A differs from that of FIG. 1 in that ring electrodes 30 and 40 are replaced by
`
`an electrostatically shielded radio frequency (ESRF) loop antenna, or single turn coil,
`
`120 which is inductively coupled to the peripheral portion of the plasma region.to
`
`form the boundary layer plasma region. The magnetic mirror wall is made less lossy
`
`and more inclusive by adding two rings of permanent magnets 65 adjacent the lower
`
`10
`
`part of the peripheral portion of the plasma region, essentially in the position occupied
`
`by ring electrode 40 in the embodiment of PIG. 1. Magnets ·65 all have a vertically
`
`oriented polarization axis and are arranged in an inner ring of magnets whose north
`
`poles face downwardly and an outer ring of magnets whose north poles face
`
`upwardly. The inner and outer rings are centered on a common horizontal plane. In
`
`15
`
`this configuration, the cylindrical magnetic mirror wall is extended radially inwardly
`
`to cover the region immediately outside disk electrodes 10 and 20.
`
`The third embodiment shown in FIG. 2B differs from the embodiment of PIG.
`
`2A only by replacement of coil 120 with a slotted waveguide 130 connected to a
`
`microwave power source (not shown) to generate an electron cyclotron resonance
`
`20
`
`(ECR) plasma. The microwave power source can be a conventional device generating
`
`electrical power at a frequency of, for example, 2.45GHz.
`
`The fourth embodiment shown in FIG. 2C differs from that of PIG. 2B only in
`
`that slotted waveguide 130 and its connected microwave power source are replaced by
`
`two further rings of permanent magnets 140 disposed above the region containing the
`
`25
`
`boundary layer plasma. These magnets will be oriented in the same manner as
`
`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
`
`magnetic mirror.
`
`In all of the above~described embodiments, the magnetic mirror is used
`
`30
`
`distinctively for reflecting the plasma. In addition to confining the plasma in
`
`9
`
`Page 11 of 26
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`

`

`WO 02/23588
`
`PCT/USOl/42111
`
`cylindrical geometry and minimizing radial plasma loss, this mirror will further
`decouple the chamber from the plasma potential.
`
`The magnetic mirror wall can also be made in shapes other than those
`illustrated. For example, the mirror can be constituted by an array of magnets lying
`on a curved annular surface, like a portion of a torus.
`
`5
`
`In a CCDP process reactor according to the invention, only the center
`processing plasma is used for processing a workpiece, or wafer. The boundary layer
`ring plasma itself is not used for processing, but is provided mainly to make the center
`processing plasma more uniform and more controllable. The existence of the
`
`10
`
`boundary layer ring plasma minimizes any potential difference in the electric field
`between the center and the edge of the processing plasma, and helps maintain the
`center processing plasma more uniform.
`
`15
`
`Control of the time dependent plasma potential in the processing plasma is
`also of importance. In the configuration proposed in this invention, the center
`processing plasma is insulated completely from the system wall by the boundary layer
`ring plasma. In a capacitively coupled plasma discharge, electron current flows to
`any electrode that is biased at a potential more positive than the plasma potential, and
`ion current flows to any electrode that is biased at a potential more negative than the
`plasma potential. In a steady state, or repeated CW, operation, the time average
`
`20
`
`electron current must equal the time average ion current. There are two factors that
`determine the balance of these currents: (1) electrons have much higher mobility than
`
`ions; and (2) the electron current increases exponentially as the potential difference
`between the plasma potential and the electrode voltage increases. On a capacitively
`coupled electrode, a self-DC bias voltage is developed so that the most positive bias
`voltage on the electrode becomes about equal to the peak plasma potential. Thus in a
`multiple electrode system, the processing plasma potential will follow the most
`positive instantaneous potential of the upper or lower disk electrodes. This makes it
`possible to apply the top and bottom bias voltages in modes such that those voltages
`are in phase or out of phase with one another. In these modes of operation, the ion
`energy can be controlled to about -10 e V, determined by the accuracy of the
`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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`By applying the same low :frequency bias voltages to the upper and lower disk
`electrodes, the controllability of the ion energy is improved dramatically. The spatial
`potential uniformity of the center processing plasma will also be improved by
`
`applying the same bias voltage also to the upper and lower ring electrodes of the
`embodiment of PIG. 1.
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`FI Gs. 3A and 3B show the electrode voltage and the resulting time dependent plasma
`potentials in the center processing plasma and the boundary layer ring plasma,
`respectively, when a VHF drive voltage at a :frequency of, for example, 60 MHz, is
`applied to upper disk electrode 10 and a RF bias voltage at, for example, 2 MHz, is
`applied to all of the electrodes 10, 20, 30 and 40. The bias voltages applied to the
`electrodes can be in phase or out of phase with one another. When the bias voltages
`applied to associated upper and lower electrodes are in phase, plasma potential control
`can be improved. However, a phase difference of 180 degrees between electrodes 10
`and 20 or electrodes 3 0 and 40 can lead to a reduction of the transfer of power from
`the fundamental frequency to the harmonic frequencies. An optimal phase difference
`exists for each specific case.
`
`Because the same low :frequency bias voltage drives the lower disk and ring
`electrodes, the low frequency time dependent plasma potentials of the two plasmas
`are identical. This will greatly reduce radial ambipolar diffusion of the plasma, even
`though a high frequency drive voltage is being applied to upper disk electrode 10.
`The magnetic field acting on the boundary layer plasma must be strong enough to
`magnetize the electrons to reflect the plasma electrons magnetically. "Magnetized"
`electrons are electrons moving in a magnetic field that preferably move in helical
`motions about magnetic field lines and, in general, are constrained to move along
`field lines rather than across them. Typically, collisional processes are required to
`diffuse electrons across magnetic field lines. For the case presented herein, the
`desired field strength for magnetized electrons is approximately 200 Gauss, below
`which the degree of "magnetization" is lessened. There will be a surface layer rich in
`ions near the magnet mirror, which gives rise to a positive local potential to reflect the
`plasma ions electrostatically. The effective leak width on the ring cusp, for the case
`of ambipolar diffusion, is given by the so-called hybrid gyroradius: p= (PePi)112
`where Pe and p1 are the electron gyroradius and ion gyroradius, respectively.
`
`;
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`In accordance with a further feature of the present invention, cylindrical
`electrode 50 is maintained at a potential substantially equal to the plasma potential
`which varies at the low RF frequency. One embodiment of a 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
`respective voltage sensors 250, 252 and 254. The output voltages from sensors 250,
`252 and 254 are amplified to appropriate levels by respective amplifiers 260,262 and
`264. The output voltages from amplifiers 260, 262 and 264 are applied to a
`comparator circuit 266 composed of a differential amplifier, a buffer and an inverter,
`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
`the output of amplifier 264 is supplied to the input of a gate 274. The opening and
`closing of gates 272 and 274 is controlled by respective outputs of comparator 266 in
`such a manner that if the output from amplifier 262 is more positive, gate 272 is
`opened and if the output of amplifier 264 is more positive, gate 27 4 is opened. The
`outputs of gates 272 and 274 are connected to a combining element 280. Thus, the
`
`output signal from amplifier 260 is representative of the voltage on cylindrical
`electrode 50, while the output of combining circuit 280 is representative of the higher
`of the voltages on upper electrode 10 and lower electrode 20, which voltage
`corresponds to the potential of the processing plasma.
`
`The output voltages from amplifier 260 and combining circuit 280 are
`supplied to respective inputs of a differential amplifier 284 and the output, which is
`representative of the difference between the voltages of the output of amplifier 260
`and the output of combining unit 280, is supplied to the input of a power amplifier
`286 which drives electrode 50. Thus, with this circuit arrangement, the output voltage
`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.
`
`In the circuit of FIG. 4, use will be made of circuit components which have a
`sufficiently rapid response to allow the voltage on cylindrical electrode 50 to follow
`the low RF component of the potential of the processing plasma. As a result, low
`frequency current drawn into the surface of the chamber wall will be minimized.
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`This control of the voltage on cylindrical electrode 50, contributes
`significantly to suppression of the radial electric field gradient in the ring plasma,
`thereby further suppressing any radial electric field gradient in the processing plasma.
`
`The