Case 6:20-cv-00636-ADA Document 246-2 Filed 11/12/22 Page 1 of 13
`
`Exhibit E
`in support of the
`Declaration of Olivia
`Weber
`
`

`

`Case 6:20-cv-00636-ADA Document 246-2 Filed 11/12/22 Page 2 of 13
`
`USO09593411 B2
`
`(12) United States Patent
`Hoffman et al.
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 9,593.411 B2
`Mar. 14, 2017
`
`(54)
`
`(75)
`
`(73)
`
`(*)
`
`(21)
`(22)
`(65)
`
`(63)
`
`(51)
`
`(52)
`
`(58)
`
`PHYSICAL VAPOR DEPOSITION CHAMBER
`WITH CAPACTIVE TUNING AT WAFER
`SUPPORT
`
`Inventors: Daniel J. Hoffman, Fort Collins, CO
`(US); Karl M. Brown, San Jose, CA
`(US); Ying Rui, Santa Clara, CA (US);
`John Pipitone, Livermore, CA (US)
`Assignee: APPLIED MATERIALS, INC., Santa
`Clara, CA (US)
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 618 days.
`Appl. No.: 13/614,704
`Filed:
`Sep. 13, 2012
`
`Notice:
`
`Prior Publication Data
`US 2013 FOOO8778 A1
`Jan. 10, 2013
`
`Related U.S. Application Data
`Continuation of application No. 12/077,067, filed on
`Mar. 14, 2008.
`
`Int. Cl.
`C23C I4/34
`C23C I4/35
`
`(2006.01)
`(2006.01)
`(Continued)
`
`U.S. C.
`CPC .......... C23C 14/35 (2013.01); C23C 14/34.71
`(2013.01); C23C 14/50 (2013.01); H01.J
`37/3266 (2013.01); H01J 37/32091 (2013.01);
`H01J 37/32.174 (2013.01); H01J 37/32577
`(2013.01); H01J 37/32706 (2013.01);
`(Continued)
`Field of Classification Search
`USPC ............. 204/298.08, 298.12, 192.12, 192.15
`See application file for complete search history.
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`3,704,219 A * 1 1/1972 McDowell ............... 204,192.13
`3,892,650 A * 7/1975 Cuomo et al. ........... 204,192.15
`(Continued)
`
`FOREIGN PATENT DOCUMENTS
`
`CN
`JP
`KR
`
`1950922. A
`10-265952 A
`10-2000-00056O1 A
`
`4/2007
`6, 1998
`1, 2000
`
`OTHER PUBLICATIONS
`
`Rossnagel, S. M., “Directional and Ionized Physical Vapor Depo
`sition for Microelectronics'. Journal Vacuum Science Technology,
`Sep./Oct. 1998, vol. 16, No. 5, USA.
`(Continued)
`
`Primary Examiner — Jason M Berman
`(74) Attorney, Agent, or Firm — Moser Taboada; Alan
`Taboada
`
`ABSTRACT
`(57)
`In a plasma enhanced physical vapor deposition of a mate
`rial onto workpiece, a metal target faces the workpiece
`across a target-to-workpiece gap less than a diameter of the
`workpiece. A carrier gas is introduced into the chamber and
`gas pressure in the chamber is maintained above a threshold
`pressure at which mean free path is less than 5% of the gap.
`RF plasma source power from a VHF generator is applied to
`the target to generate a capacitively coupled plasma at the
`target, the VHF generator having a frequency exceeding 30
`MHz. The plasma is extended across the gap to the work
`piece by providing through the workpiece a first VHF
`ground return path at the frequency of the VHF generator.
`
`15 Claims, 5 Drawing Sheets
`
`
`
`122
`
`

`

`US 9,593.411 B2
`Page 2
`
`(2006.01)
`(2006.01)
`(2006.01)
`
`(51) Int. Cl.
`HOI. 37/32
`HOI. 37/34
`C23C I4/50
`(52) U.S. Cl.
`CPC ........ H01J 37/32715 (2013.01); H01J 37/34
`(2013.01); H01J 37/3444 (2013.01)
`References Cited
`
`(56)
`
`U.S. PATENT DOCUMENTS
`
`4,824,546 A * 4, 1989 Ohmi ....................... 204/298.08
`4,931,169 A * 6/1990 Scherer et al. .......... 204/298.11
`5.439,574 A
`8/1995 Kobayashi et al.
`6, 190,513 B1* 2/2001 Forster .................. HO1J 37,321
`204,192.12
`
`7,214,619 B2
`7,244,344 B2
`7,268,076 B2
`2006, OO73283 A1
`2006/0169578 A1
`2006,0169582 A1
`2006,0169584 A1
`2006/01725 17 A1
`2006/0172536 A1
`2006,019 1876 A1
`2007.0193982 A1
`2008, OO14747 A1
`
`5, 2007 Brown et al.
`7/2007 Brown et al.
`9, 2007 Brown et al.
`4/2006 Brown et al.
`8, 2006 Brown et al.
`8, 2006 Brown et al.
`8, 2006 Brown et al.
`8, 2006 Brown et al.
`8, 2006 Brown et al.
`8, 2006 Brown et al.
`8, 2007 Brown et al.
`1/2008 Brown et al.
`
`OTHER PUBLICATIONS
`
`Official Action dated Mar. 7, 2012 issued in corresponding Chinese
`Patent Application Serial No. 200980 1071444.
`
`* cited by examiner
`
`Case 6:20-cv-00636-ADA Document 246-2 Filed 11/12/22 Page 3 of 13
`
`

`

`U.S. Patent
`
`Mar. 14, 2017.
`
`Sheet 1 of 5
`
`US 9,593.411 B2
`
`124-Q
`
`122
`
`126
`
`Z Match
`
`112
`
`116-1
`
`116
`
`1 04
`
`20
`
`I
`
`
`x I 118
`
`DC Target
`Power Supply
`
`
`
`
`
`144
`
`155
`
`Z. Match
`
`134
`
`
`
`150 O
`
`136
`
`Case 6:20-cv-00636-ADA Document 246-2 Filed 11/12/22 Page 4 of 13
`
`FIG 1
`
`

`

`U.S. Patent
`
`Mar. 14, 2017.
`
`Sheet 2 of 5
`
`US 9,593.411 B2
`
`O
`
`Velocity
`Direction
`
`+90°
`
`FIG 2
`
`Ion
`Population
`
`-90'
`
`
`
`Case 6:20-cv-00636-ADA Document 246-2 Filed 11/12/22 Page 5 of 13
`
`

`

`U.S. Patent
`
`Mar. 14, 2017.
`
`Sheet 3 of 5
`
`US 9,593.411 B2
`
`Provide a Target Material Having a Low
`Bias Treshold for Deposition
`
`Introduce Process Gas Into Chamber
`
`Generate WHF Power Capacitively Coupled
`Plasma at the Target
`
`Extend VHF Plasma to Wafer by Providing VHF
`Ground Return Path Through the Wafer
`
`Maintain Narrow Wafer-to-Target Gap
`
`Maintain Sufficiently High Chamber Pressure
`for High Number of Collisions Per Transit
`
`Permit Collisions at Wafer Surface to Establish
`Broadened Ion Welocity Distribution
`
`
`
`Apportion Plasma Energy Between the Side Wall
`and Wafer by Apportioning the WHF Ground
`Return Impedance of the Wafer or Side Wall
`
`Case 6:20-cv-00636-ADA Document 246-2 Filed 11/12/22 Page 6 of 13
`
`Offset WHF Induced Wafer Bias
`with LF Bias Power
`
`FIG 4
`
`41 O
`
`412
`
`414
`
`416
`
`418
`
`420
`
`422
`
`424
`
`426
`
`

`

`U.S. Patent
`
`Mar. 14, 2017
`
`Sheet 4 of 5
`
`US 9,593.411 B2
`
`Z Match
`
`
`
`
`
`DC Chucking
`Power Supply
`
`Case 6:20-cv-00636-ADA Document 246-2 Filed 11/12/22 Page 7 of 13
`
`FIG 5
`
`

`

`U.S. Patent
`
`Mar. 14, 2017.
`
`Sheet 5 of 5
`
`US 9,593.411 B2
`
`
`
`134
`
`144
`
`Case 6:20-cv-00636-ADA Document 246-2 Filed 11/12/22 Page 8 of 13
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`

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`US 9,593411 B2
`
`1.
`PHYSICAL VAPOR DEPOSITION CHAMBER
`WITH CAPACTIVE TUNING AT WAFER
`SUPPORT
`
`2
`lowing plasma ignition, so that about 40% of the process is
`performed prior to stabilization of the impedance match and
`delivered power.
`
`CROSS-REFERENCE TO RELATED
`APPLICATIONS
`
`This application is a continuation of co-pending U.S.
`patent application Ser. No. 12/077,067, filed Mar. 14, 2008,
`which is herein incorporated by reference.
`
`10
`
`BACKGROUND
`
`15
`
`Plasma enhanced physical vapor deposition (PECVD)
`processes are used to deposit metal films such as copper onto
`semiconductor wafers to form electrical interconnections. A
`high level of D.C. power is applied to a copper target
`overlying the wafer in the presence of a carrier gas, such as
`argon. Plasma Source power may be applied via a coil
`antenna surrounding the chamber. PECVD processes typi
`cally rely upon a very narrow angular distribution of ion
`velocity to deposit metal onto sidewalls and floors of high
`aspect ratio openings. One problem is how to deposit
`sufficient material on the sidewalls relative to the amount
`deposited on the floors. Another problem is avoiding pinch
`off of the opening due to faster deposition near the top edge
`of the opening. As miniaturization of feature sizes has
`progressed, the aspect ratio (depth/width) of a typical open
`ing has increased, with microelectronic feature sizes having
`now been reduced to about 22 nanometers. With greater
`miniaturization, it has become more difficult to achieve
`minimum deposition thickness on the sidewall for a given
`deposition thickness on the floor or bottom of each opening.
`The increased aspect ratio of the typical opening has been
`addressed by further narrowing of the ion velocity angular
`distribution, through ever-increasing wafer-to-sputter target
`distance and ever lower chamber pressures, e.g., less than 1
`mT (to avoid velocity profile widening by collisions). This
`has given rise to a problem observed in thin film features
`near the edge of the wafer: At extremely small feature sizes,
`a portion of each high aspect ratio opening sidewall is
`shadowed from a major portion of the target because of the
`greater wafer-to-target gap required to meet the decreasing
`feature size. This shadowing effect, most pronounced near
`the wafer edge, makes it difficult if not impossible to reach
`a minimum deposition thickness on the shadowed portion of
`the side wall. With further miniaturization, it has seemed a
`further decrease and chamber pressure (e.g., below 1 mT)
`and a further increase in wafer-Sputter target gap would be
`required, which would exacerbate the foregoing problems.
`One technique employed to Supplement the side wall
`deposition thickness is to deposit an excess amount of the
`metal (e.g., Cu) on the floor of each opening and then
`re-sputter a portion of this excess on the opening side wall.
`This technique has not completely solved the shadowing
`problem and moreover represents an extra step in the
`process and a limitation on productivity.
`A related problem is that the Sputter target (e.g., copper)
`must be driven at a high level of D.C. power (e.g., in the
`range of kW) to ensure an adequate flow of ions to the wafer.
`Such a high level of D.C. power rapidly consumes the target
`(driving up costs) and produces an extremely high deposi
`tion rate so that the entire process is completed in less than
`five seconds. This time is about 40% of the time required for
`the RF source power impedance match to equilibrate fol
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`SUMMARY
`
`A method is provided for performing physical vapor
`deposition on a workpiece in a reactor chamber. The method
`includes providing a target comprising a metallic element
`and having a Surface facing the workpiece, and establishing
`a target-to-workpiece gap less than a diameter of the work
`piece. A carrier gas is introduced into the chamber and gas
`pressure in the chamber is maintained above a threshold
`pressure at which mean free path is less than 5% of the gap.
`RF plasma source power from a VHF generator is applied to
`the target to generate a capacitively coupled plasma at the
`target, the VHF generator having a frequency exceeding 30
`MHz. The method further includes extending the plasma
`across the gap to the workpiece by providing through the
`workpiece a first VHF ground return path at the frequency
`of the VHF generator.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`So that the manner in which the exemplary embodiments
`of the present invention are attained and can be understood
`in detail, a more particular description of the invention,
`briefly summarized above, may be had by reference to the
`embodiments thereof which are illustrated in the appended
`drawings. It is to be appreciated that certain well known
`processes are not discussed herein in order to not obscure the
`invention.
`FIG. 1 is a diagram of a plasma reactor in accordance with
`a first aspect.
`FIG. 2 is a graph of a random or near-isotropic Velocity
`distribution of ions at the wafer surface attained in the
`reactor of FIG. 1.
`FIG. 3 is a graph depicting angular Velocities in the
`distribution of FIG. 2.
`FIG. 4 is block diagram depicting method in accordance
`with one embodiment.
`FIG. 5 is a diagram of a plasma reactor in accordance with
`a second aspect.
`FIGS. 6A and 6B depict alternative embodiments of
`variable ground return impedance elements in the reactor of
`FIG. 1 or FIG. S.
`To facilitate understanding, identical reference numerals
`have been used, where possible, to designate identical
`elements that are common to the figures. It is contemplated
`that elements and features of one embodiment may be
`beneficially incorporated in other embodiments without fur
`ther recitation. It is to be noted, however, that the appended
`drawings illustrate only exemplary embodiments of this
`invention and are therefore not to be considered limiting of
`its scope, for the invention may admit to other equally
`effective embodiments.
`
`DETAILED DESCRIPTION
`
`In one embodiment, a PEPVD process provides complete
`uniform sidewall coverage at feature sizes of 25 nm and
`below (e.g., 18 nm) free of the non-uniformities caused by
`shadowing. The PEPVD process of this embodiment is
`carried out with VHF plasma source power on the sputter
`target to create a capacitively coupled RF plasma at the
`target. The process further employs a very low (or no) D.C.
`power on the sputter target. With the low D.C. power level
`
`Case 6:20-cv-00636-ADA Document 246-2 Filed 11/12/22 Page 9 of 13
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`US 9,593411 B2
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`3
`on the target, the process may be performed over a period of
`time that is longer than the settling time of the impedance
`match elements, and Sufficiently long for good process
`control and chamber matching. These advantages are real
`ized by using high chamber pressure to at least reduce or
`totally eradicate the neutral directionality from the metal
`target to the wafer. If, in fact the remaining deposition were
`to be accomplished from neutrals emanating from the
`vacuum gap of the mean free path above the wafer, the
`features would totally be pinched off. However, the RF
`power maintains a significant density of metal ions in the
`plasma, making them available for attraction to the wafer by
`electric field. The electric field can be derived by either a
`residual RF field from the VHF plasma source at the ceiling
`(target) or by a small RF bias power applied to the wafer
`directly. This then creates a predominantly vertical ion
`velocity distribution with some perpendicular (horizontal)
`velocity component for side wall coverage. The result is that
`the side wall and floor of each high aspect ratio opening is
`conformally covered with the sputtered material from the
`target. In Summary, the Source provides a nearly isotropic
`distribution of neutral velocity and a distribution of ion
`Velocity including a large vertical component and a rela
`tively smaller non-vertical or horizontal component. The
`isotropic neutral velocity distribution and somewhat broad
`ened but predominantly vertical ion velocity distribution at
`the wafer Surface is realized by maintaining the chamber at
`an extremely high pressure (e.g., 100 mT) to ensure an ion
`collision mean free path that is /20" of the wafer-to-sputter
`target gap. A high flux of Sputtered ions at the wafer is
`realized by: (1) minimizing the wafer-to-sputter target gap to
`a fraction of the wafer diameter, (2) generating a VHF
`capacitively coupled plasma at the Sputter target (as men
`tioned above) and (3) extending the capacitively coupled
`plasma down to the wafer. The plasma is extended down to
`the wafer by providing an attractive VHF ground return path
`through the wafer. The process window is expanded by
`reducing (or possibly eliminating) the D.C. power applied to
`the Sputter target, so that the target consumption rate is
`reduced and the process is less abrupt. This reduction in D.C.
`target power without loss of requisite sputtering is made
`possible by the high density plasma generated at the target
`by application of VHF power to the target and by the
`reduced wafer-to-sputter target gap.
`Undesirable ion bombardment by ions of the carrier gas
`(e.g., argon ions) is suppressed by selectively favoring the
`desired sputter target ions (e.g., copper ions) at the plasma
`sheath overlying the wafer surface. This selection is made by
`maintaining the wafer bias Voltage below an upper threshold
`Voltage (e.g., 300 volts) above which carrier gas (e.g., argon)
`ions interact with or damage thin film structures on the
`wafer. However, the wafer bias voltage is maintained above
`a lower threshold voltage (e.g., 10-50 volts) above which the
`sputter target material (e.g., copper) ions deposit on the
`wafer surface. Such a low wafer bias voltage is achieved by
`differential control of VHF ground return path impedances
`for the VHF source power through: (a) the wafer and (b) the
`chamber side wall, respectively. Decreasing the VHF ground
`return path impedance through the sidewall tends to
`decrease the wafer bias voltage. This control is provided by
`independent variable impedance elements governing ground
`return path impedances through the side wall and through
`the wafer, respectively.
`The wafer bias voltage is also minimized as follows: The
`VHF source power applied to the target creates a modest
`positive bias Voltage on the wafer, in the absence of any
`other applied RF power. This positive wafer bias may be
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`Case 6:20-cv-00636-ADA Document 246-2 Filed 11/12/22 Page 10 of 13
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`50
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`55
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`60
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`65
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`4
`offset by applying a small amount of optional low frequency
`(LF) RF bias power to the wafer. The LF bias power tends
`to contribute a negative bias Voltage on the wafer, so that its
`negative contribution may be adjusted or balanced against
`the positive contribution of the VHF source power to pro
`duce a net wafer bias Voltage that is close to Zero, if desired,
`or as Small as desired. Specifically, the wafer bias Voltage is
`reduced well below the carrier gas ion bombardment bias
`threshold referred to above.
`The foregoing process has been discussed above with
`reference to a PEPVD process for depositing copper. How
`ever, the process can be used to deposit a wide range of
`materials other than copper. For example, the Sputter target
`may be titanium, tungsten, tin (or other Suitable metallic
`materials or alloys) for PEPVD deposition of the metallic
`target material (e.g., titanium, tungsten or tin). Moreover, a
`metallic (e.g., titanium) target may be employed with a
`nitrogen process gas for deposition of titanium nitride or
`other metallic nitride, where a carrier gas (argon) is used for
`plasma ignition and is then replaced by nitrogen for the
`metallic (titanium) nitride deposition.
`Referring to FIG. 1, a PEPVD reactor in accordance with
`a first embodiment includes a vacuum chamber 100 defined
`by a cylindrical sidewall 102, a ceiling 104 and floor 106, the
`chamber containing a wafer support 108 for holding a wafer
`110 in facing relationship with the ceiling 104. A metal
`sputtering target 112 is Supported on the interior Surface of
`the ceiling 104. A vacuum pump 114 maintains pressure
`within the chamber 100 at a desired sub-atmospheric value.
`A conventional rotating magnet assembly or “magnetron'
`116 of the type well-known in the art overlies the ceiling 104
`directly above the sputtering target 112. A process gas
`Supply 118 furnishes a carrier gas such as argon into the
`chamber 100 through a gas injection apparatus 120, which
`may be a gas injection nozzle, an array of nozzles or a gas
`distribution ring.
`The wafer support 108 may embody an electrostatic
`chuck including a grounded conductive base 108-1, an
`overlying dielectric puck 108-3 having a wafer support
`surface 108-5, and an electrode or conductive mesh 108-7
`inside the puck 108-3 and separated from the wafer support
`surface by a thin layer 108-9 of the dielectric puck 108-3.
`A D.C. power supply 122 is connected to the center of the
`target 112 through a central aperture 116-1 in the magnetron
`116. An RF plasma Source power generator 124 having a
`VHF frequency is coupled through a VHF impedance match
`126 to the center of the target 112 through the center
`magnetron aperture 116-1. A D.C. chucking Voltage Supply
`128 is connected to the chuck electrode 108-7.
`In operation, the VHF power generator 124 provides
`about 4kW of plasma source power to Support a capacitively
`coupled plasma at the target 112 initially consisting of ions
`of the carrier gas. This plasma sputters the target 112 to
`generate free target (e.g., copper) atoms which become
`ionized in the plasma, the rotation of the magnetic fields of
`the magnetron 116 helping to distribute the consumption of
`the target 112 and promote ionization near the target 112.
`The reactor includes features that enable the plasma gener
`ated at the target 112 to reach the wafer 110. In accordance
`with one such feature, the plasma formed at the target 112 by
`the capacitively coupled VHF power from the generator 124
`is made to extend down to the wafer 110 by providing an
`attractive VHF ground return path through the wafer 110
`(i.e., through the wafer support 108). For this purpose, a
`variable impedance element 130 is coupled between the
`electrode 108-7 and ground. Other than this connection, the
`electrode 108-7 is insulated from ground, so that the imped
`
`

`

`US 9,593411 B2
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`ance element 130 provides the only connection between the
`electrode 108-7 and ground. The variable impedance ele
`ment 130 in one implementation consists of a series reac
`tance 132 Such as a capacitor, and variable parallel reac
`tances 134, 136, of which the reactance 134 may be a
`variable capacitor and the reactance 136 may be a variable
`inductor. The reactances 132, 134, 136 are selected to
`provide an impedance at the frequency of the VHF generator
`124 that allows current at that frequency to flow from the
`electrode 108-7 to ground.
`Another feature that enables the plasma to extend to from
`the target to the wafer is a reduction in the gap between the
`wafer 110 and the target 112. The wafer-target gap is reduced
`to a distance less than or a fraction of the wafer diameter. For
`example, the gap may be /$" of the wafer diameter. For a
`300 mm wafer diameter, the wafer-target gap may be 60
`.
`The pump 114 is set to provide a high chamber pressure
`(e.g., 50-200 mT). The chamber pressure is sufficiently high
`to set the mean free path to less than /20" of the length of
`the wafer-to-target gap. The space between the target 112
`and the wafer 110 is empty, i.e., free of other apparatus, to
`ensure maximum dispersion of angular velocity distribution
`of the neutrals by the large number of collisions in their
`transit from the target 112 to the wafer 110. The resulting
`angular distribution of Velocity of the neutrals in the plasma
`is very broad (nearly uniform) at the wafer surface within a
`hemispherical angular range from 0 (normal to the wafer
`surface) up to nearly 90° (parallel to the wafer surface). The
`ions have a less uniform angular distribution that peaks at
`the perpendicular direction, due to the attraction presented
`by the wafer bias voltage. FIG. 2 is a simplified diagram
`depicting the distribution of ion population as a function of
`direction in a cone of ion trajectories depicted in FIG. 3,
`showing a peak at the perpendicular direction. This peak has
`been broadened from a high variance (e.g., 0.8) to a variance
`(e.g., 0.2) by the large number of collisions, mainly with
`neutrals, that the ions experience within the wafer-target
`gap. These collisions compete with the electric field to
`reduce the sharp peak of ion velocity distribution about the
`vertical and provide a small component of non-vertical (e.g.,
`horizontal) ion velocity. This broadening of the ion angular
`trajectory distribution is combined with the nearly isotropic
`angular distribution of neutral velocities. This combination
`improves the uniformity or conformality of the deposited
`film. As a result of the broadened angular distribution of ion
`velocity at the wafer surface, coupled with the relatively
`Small wafer-to-target gap, metal is deposited on interior
`Surfaces of high aspect ratio openings in the Surface of the
`wafer with very high conformality and uniform thickness.
`The capacitively coupled plasma produced at the target
`112 contains ions from the carrier gas (e.g., argon ions) and
`ions from the target (e.g., copper ions). The copper ions
`require a relatively low plasma bias Voltage on the wafer to
`deposit on the wafer surface, typically around 50 volts or
`less. The argon ions are more volatile than the copper ions,
`and at the low sheath voltage tend to elastically collide with
`the features on the wafer surface and disperse, rather than
`imparting damage. At slightly greater bias Voltage levels
`(e.g., 300 volts), the argon ions collide inelastically with thin
`film features on the wafer and damage them. Therefore, ideal
`results can be achieved by limiting the wafer bias voltage to
`about 50 volts or less, for example. The problem is how to
`limit the wafer bias voltage to such a low level.
`A first feature for limiting wafer bias voltage is one that
`diverts a selected portion of the plasma ions away from the
`wafer 110 to the chamber sidewall 102 (which is formed of
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`a metal). This feature employs the variable VHF ground
`return impedance element 130 coupled to the chuck elec
`trode 180-7 and, in addition, a second variable VHF ground
`return impedance element 140 coupled to the side wall 102.
`The sidewall variable VHF impedance element 140 has a
`structure similar to that of the element 130, and (in one
`implementation) includes a series capacitor 142, a variable
`parallel capacitor 144 and a variable inductor 146. The
`sidewall variable impedance element 140 is connected
`between the sidewall 102 and ground, the sidewall 102 being
`insulated from ground with the exception of this connection.
`The impedances of the two elements 130, 140 are indepen
`dently adjustable, and determine the apportionment of the
`plasma current between the wafer 110 and the sidewall 102.
`Adjustment of the impedances of the elements 130, 140 is
`performed to reduce the wafer bias voltage down to a low
`level and to accurately select that level. For example, a
`reduction in wafer bias Voltage may be obtained by increas
`ing the resistance at the VHF source power generator
`frequency presented by the chuck electrode impedance
`element 130 (rendering the VHF ground return path through
`the wafer 110 less attractive) while reducing the resistance
`at the VHF source power generator frequency presented by
`the chamber sidewall impedance element 140 (rendering the
`VHF ground return path through the sidewall 102 more
`attractive). The relative impedances of the two impedance
`elements 130, 140 at the VHF source power frequency for a
`given apportionment of plasma current depends upon the
`relative areas of the wafer 110 and the conductive side wall
`102.
`In order to reduce the conductance through a selected one
`of two variable impedance elements 130, 140, the imped
`ance may be chosen to either behave as a very high resis
`tance or open circuit at the frequency of the VHF generator
`124, or to behave as a very low resistance or short circuit at
`harmonics of the frequency of the VHF generators (e.g. 2",
`3", 4" harmonics). Such behavior at the harmonic frequen
`cies may be implemented in either or both of the impedance
`elements 130, 140 with the addition of further adjustable
`tank circuits for each of the harmonics of interest. This may
`be accomplished by adding more variable reactances to the
`impedance elements 130, 140 to achieve the desired filter or
`pass band characteristics. For example, referring to FIG. 6A,
`the variable impedance element 130 may further include
`variable resonant circuits 130-1, 130-2, 130-3 that may be
`tuned to provide selected impedances at the second, third
`and fourth harmonics, respectively. Referring to FIG. 6B,
`the variable impedance element 140 may further include
`variable resonant circuits 140-1, 140-2, 140-3 that may be
`tuned to provide selected impedances at the second, third
`and fourth harmonics, respectively.
`A second feature for limiting wafer bias Voltage exploits
`the tendency of the VHF source power applied to the target
`112 to produce a modest positive bias voltage on the wafer
`110 (in the absence of any other RF power being applied).
`This second feature involves coupling an optional low
`frequency RF bias power generator 150 through an imped
`ance match 155 to the chuck electrode 108-7. As RF bias
`power from the generator 150 is increased from Zero, the
`wafer bias voltage, which is initially positive under the
`influence of the VHF source power from the VHF generator
`124, is shifted down and at Some point crosses Zero and
`becomes negative. By carefully adjusting the output power
`level of the bias power generator 150 to a small power level
`(e.g., about 0.1 kW), the wafer bias voltage can be set to a
`very small value (e.g., to less than 50 volts).
`
`

`

`7
`Since the VHF source power generator 124 provides
`plasma source power for the generation of plasma ions near
`the target 112, the D.C. power source 122 is not the sole
`Source of power for plasma generation. The demands on the
`D.C. power source 122 are further lessened because the
`reduced wafer-target gap reduces loss of plasma density
`between the target 112 and the wafer 110. Therefore, the
`D.C. power level of the D.C. supply 122 may be reduced
`from the conventional level (e.g., 35-40 kW) to as low as 2
`kW. This feature reduces the sputtering rate of the target 112
`and therefore reduces the consumption of the target, cost of
`operation and thermal load on the entire system. Moreover,
`it reduces the deposition rate on the wafer 110. At the higher
`D.C. power level (e.g., 38 kW) the deposition rate was
`extremely high, and the deposition time had to be limited to
`about 5 seconds for a typical copper film deposition thick
`ness, of which the first 2 seconds were spent by the imped
`ance match 126 reaching equilibrium or stability following
`plasma ignition. At the new (reduced) D.C. power level (of
`a few kW or less), the deposition time may be on the order
`of 30 seconds, so that the impedance match 126 is stable for
`a very high percentage of the process time.
`The increase in uniformity of the metal coating on interior
`Surfaces of high aspect ratio openings increases the process
`window over which the reactor may be operated. In the prior
`art, the metal deposition on the interior Surfaces of high
`aspect ratio openings was highly non-uniform, which
`allowed for only a very small wafer-to-wafer variation in
`performance and a very narrow process window within
`which adequate metal coating could be realized for all
`internal Surfaces of a high aspect ratio opening. Further
`more, the inadequate deposition thickness on sidewalls of
`high aspect ratio openings required the performance of a
`second step following deposition, namely a re-sputtering
`step in which excess material deposited on the floor or
`bottom of an opening is transferred to the sidewall. The
`re-sputtering step has typically required an excess thickness
`to be deposited on the floor of the high aspect ratio opening.
`With the present embodiment, the improved uniformity of
`the deposition on the floor and sidewall of the opening
`40
`eliminates the need for re-sputtering and the need for excess
`thickness on the bottom of the opening. This increases
`productivity and reduces the amount of material that must be
`removed when opening a via through the floor of the high
`aspect ratio opening.
`The total power applied to the chamber is reduced in the
`embodiment of FIG. 1 from a convention PEPVD process,
`as can be seen in the following table (Table I).
`
`10
`
`15
`
`25
`
`30
`
`35
`
`45
`
`US 9,593411 B2
`
`8
`sition in Such openings near the edge of a 300 mm wafer
`exhibited great non-uniformity due to shadowing effects.
`The ratio between the metal deposited on radially inner and
`outer sides of the opening sidewall was as great as 50:1,
`corresponding to a highly non-uniform sidewall deposition.
`With the process disclosed above, it is improved to nearly
`1:1, a uniform sidewall deposition.
`FIG. 4 depicts a method in accordance with one aspect. A
`target is provided having a material (such as copper) requir
`ing a low wafer bias voltage (e.g., 10-50 volts) for deposi
`tion (block 410 of FIG. 4). A carrier gas such as argon is
`introduced into the chamber that tends not to produce ion
`bombardment damage at a low wafer bias Voltage (block
`412). A plasma is generated at the target by capacitively
`coupling VHF source power using the target as an electrode
`(block 414). This plasma is extended down to the wafer by
`providing a VHF ground return path through the wafer
`(block 416), and by establishing a narrow wafer-to-target
`gap (block 418). The chamber pressure is maintained at a
`sufficiently high level (e.g., 100 mT) to ensure an ion mean
`free path length that is less than /20" of the wafer-target gap
`(block 420), to establish a random or nearly isotropic ion
`velocity distribution at the wafer surface (block 422). The
`wafer bias Voltage is minimized by apportioning the plasma
`between a ground return path through the wafer at a first
`impedance and a ground return path through the chamber
`sidewall at a second impedance (block 424). The wafer bias
`voltage is further minimized by offsetting a positive bias
`voltage induced by the VHF source power