(12) Unlted States Patent
`(16) Patent N0.:
`US 6,306,265 B1
`
`Fu et al.
`(45) Date of Patent:
`Oct. 23, 2001
`
`USOO6306265B1
`
`(54) HIGH-DENSITY PLASMA FOR IONIZED
`METAL DEPOSITION CAPABLE OF
`EXCITING A PLASMA WAVE
`
`(75)
`
`Inventors: Jianming Fu, San Jose; Praburam
`Gopalraja, Sunnyvale; Fusen Chen,
`Saratoga; John Forster, San Francisco,
`all of CA (us)
`
`(73) Assignee: Applied Materials, Inc., Santa Clara,
`CA (US)
`
`(*) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(21) Appl. No.: 09/546,798
`
`(22)
`
`Filed:
`
`Apr- 11, 2000
`
`(
`
`63
`
`Related US. Application Data
`.
`.
`.
`.
`.
`Cont1nuat10n—1n— art of a
`lication No. 09 373 097 filed on
`) Aug. 12, 1999, nIdw Pat. IITO. 6,183,614, w/hichjis ajcontinu—
`ation—in—part of application No. 09/249,468, filed on Feb. 12,
`1999'
`Int. Cl.7 ..................................................... C23C 14/34
`(51)
`(52) us. Cl.
`.................................... 204/19212; 204/2982
`(58) Field of Search ............................ 204/298.19, 298.2,
`204/298.22, 192.12
`
`(56)
`
`References Cited
`
`U~S- PATENT DOCUMENTS
`
`204/2982
`............
`4,444,643 *
`4/1984 Garrett
`204/2982
`5,252,758
`10/1993 Demaray et a1.
`204/2982
`5,770,025
`6/1998 Kiyota .............
`204/2982
`5,897,752 *
`4/1999 Hong et a1.
`.
`5,966,607 * 10/1999 Chee et al.
`........................... 438/305
`FOREIGN PATENT DOCUMENTS
`
`
`
`2 241 710 *
`62—89864
`63—282263
`11—074225
`
`9/1991 (GB)
`4/1987 (JP) ~
`11/1988 (JP) .
`3/1999 (JP) .
`
`............................... 204/298.19
`
`OTHER PUBLICATIONS
`
`B. Window et al. “Charged particle fluxes from planar
`magnetron sources”, J. Vac. Sci. Technol. A4(2), Mar/Apr.
`1986, pp. 196—202.*
`J. Musil, et al. “Unbalanced magnetrons and new sputtering
`systems with enhanced plasma ionization”, J. Vac. Sci.
`Technol. A 9(3), May/Jun. 1991, pp. 1171—1177.*
`W. Munz “The unbalanced magnetron: current status of
`development”, Surface and Coatings Technology, 48 (1991),
`pp. 81—94.*
`Matsuoka et al., “Dense plasma production and film depo-
`sition by new high—rate sputtering using an electric mirror,”
`Journal of Vacuum Science and TechnologyA, vol. 7, No. 4,
`Jul/Aug. 1989, pp. 2652—2657.
`
`* Cited by examiner
`Primary Examiner—Nam Nguyen
`.
`.
`ASSlsmm Examiner—Gregg Cantelmo
`(74) Attorney) Agent, 07‘ FWm—Charles 5- Guenzer, Esq
`(57)
`ABSTRACT
`
`low-pressure
`A magnetron especially advantageous for
`Plasma Wittering 0r “Stained self'spmtering haVing
`reduced area but full
`target coverage. The magnetron
`includes an outer pole face surrounding an inner pole face
`with a gap therebetween. The outer pole of the magnetron of
`the invention is smaller than that of a circular magnetron
`similarly extending from the center to the periphery of the
`target A Preferred triangular Shape haVing a small apex
`angle of 20 to 30° may be formed from outer bar magnets of
`one magnetic polarity enclosing an inner magnet of the other
`magnetic polarity. The magnetron allows the generation of
`plasma waves in the neighborhood of 22 MHZ which
`interact with the 1 to 20 eV electrons of the plasma to
`thereby increase the plasma den51ty.
`
`06—620583A
`0 691 419 A1 *
`
`10/1994 (EP).
`1/1996 (EP)
`
`................................ 204/298.19
`
`22 Claims, 11 Drawing Sheets
`
`40
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`46
`
`GILLETTE 1014
`
`GILLETTE 1014
`
`

`

`US. Patent
`
`Oct. 23, 2001
`
`Sheet 1 0f 11
`
`US 6,306,265 B1
`
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`(PRIOR ART)
`
`FIG.
`
`1
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`

`

`US. Patent
`
`Oct. 23, 2001
`
`Sheet 2 0f 11
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`US 6,306,265 B1
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`(PRIOR ART)
`
`FIG. 3
`
`56 57
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`
`I? 54
`_)
`
`

`

`US. Patent
`
`Oct. 23, 2001
`
`Sheet 3 0f 11
`
`US 6,306,265 B1
`
`72
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`68
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`US. Patent
`
`Oct. 23, 2001
`
`Sheet 4 0f 11
`
`US 6,306,265 B1
`
`
`
`1 1 O
`
`1 08
`
`122
`
`106
`
`1 14
`
`78
`
`112
`
`FIG. 8
`
`

`

`US. Patent
`
`Oct. 23, 2001
`
`Sheet 5 0f 11
`
`US 6,306,265 B1
`
`A////"—
`
`126
`
`152
`
`136
`144
`
`148
`
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`
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`150
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`
`152
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`

`

`US. Patent
`
`Oct. 23, 2001
`
`Sheet 6 0f 11
`
`US 6,306,265 B1
`
`150
`
`164
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`FIG.
`
`11
`
`

`

`US. Patent
`
`Oct. 23, 2001
`
`Sheet 7 0f 11
`
`US 6,306,265 B1
`
`5&2
`T
`
`174
`
`178
`
`FIG. 13
`
`90°
`
`8
`
`180°
`
`P/RT
`
`7T+2
`
`7T
`
`2
`
`178
`
`180
`
`90°
`
`180°
`
`9
`
`

`

`US. Patent
`
`Oct. 23, 2001
`
`Sheet 8 0f 11
`
`US 6,306,265 B1
`
`
`
`

`

`US. Patent
`
`Oct. 23, 2001
`
`Sheet 9 0f 11
`
`US 6,306,265 B1
`
`200
`
`206 202
`
`
`
`

`

`US. Patent
`
`Oct. 23, 2001
`
`Sheet 10 0f 11
`
`US 6,306,265 B1
`
`
`
`80
`
`60
`
`BOTTOM
`
`COVERAGE
`m 40
`
`——o—— NO Bms
`20 —-{}-100w Bms
`
`---A—-- 250w BIAS
`
`ASPECT RATIO
`
`

`

`US. Patent
`
`Oct. 23, 2001
`
`Sheet 11 0f 11
`
`US 6,306,265 B1
`
`PRESSURE
`(mT)
`
`
`
`
`1
`
`20
`
`4O
`
`60
`
`80
`
`100
`
`FIG. 22
`
`N2 FLOW (sccm)
`
`50
`
`3O
`STEP
`COVERAGE
`(%)
`20
`
`4o
`
`1O
`
`FIG 23
`
`ASPECT RATIO
`
`

`

`US 6,306,265 B1
`
`1
`HIGH-DENSITY PLASMA FOR IONIZED
`METAL DEPOSITION CAPABLE OF
`EXCITING A PLASMA WAVE
`
`RELATED APPLICATION
`
`This application is a continuation in part of Ser. No.
`09/373,097, filed Aug. 12, 1999, now US. Pat. No. 6,183,
`614 Feb. 6, 2001 which is a continuation in part of Ser. No.
`09/249,468, filed Feb. 12, 1999.
`
`FIELD OF THE INVENTION
`
`The invention relates generally to sputtering of materials.
`In particular, the invention relates to the magnetron creating
`a magnetic field to enhance sputtering.
`
`BACKGROUND ART
`
`10
`
`15
`
`Sputtering, alternatively called physical vapor deposition
`(PVD), is the most prevalent method of depositing layers of
`metals and related materials in the fabrication of semicon-
`
`20
`
`ductor integrated circuits. A conventional PVD reactor 10 is
`illustrated schematically in cross section in FIG. 1, and the
`illustration is based upon the Endura PVD Reactor available
`from Applied Materials, Inc. of Santa Clara, California. The
`reactor 10 includes a vacuum chamber 12 sealed to a PVD
`
`target 14 composed of the material, usually a metal, to be
`sputter deposited on a wafer 16 held on a heater pedestal 18.
`A shield 20 held within the chamber protects the chamber
`wall 12 from the sputtered material and provides the anode
`grounding plane. A selectable DC power supply 22 nega-
`tively biases the target 14 to about —600VDC with respect to
`the shield 20. Conventionally, the pedestal 18 and hence the
`wafer 16 are left electrically floating.
`A gas source 24 supplies a sputtering working gas,
`typically the chemically inactive gas argon, to the chamber
`12 through a mass flow controller 26. In reactive metallic
`nitride sputtering, for example, of titanium nitride, nitrogen
`is supplied from another gas source 27 through its own mass
`flow controller 26. Oxygen can also be supplied to produce
`oxides such as A1203. The gases can be admitted to the top
`of the chamber, as illustrated, or at its bottom, either with
`one or more inlet pipes penetrating the bottom of the shield
`or through the gap between the shield 20 and the pedestal 18.
`A vacuum system 28 maintains the chamber at a low
`pressure. Although the base pressure can be held to about
`10'7 Torr or even lower, the pressure of the working gas is
`typically maintained at between about 1 and 1000 mTorr. A
`computer-based controller 30 controls the reactor including
`the DC power supply 22 and the mass flow controllers 26.
`When the argon is admitted into the chamber, the DC
`voltage between the target 14 and the shield 20 ignites the
`argon into a plasma, and the positively charged argon ions
`are attracted to the negatively charged target 14. The ions
`strike the target 14 at a substantial energy and cause target
`atoms or atomic clusters to be sputtered from the target 14.
`Some of the target particles strike the wafer 16 and are
`thereby deposited on it, thereby forming a film of the target
`material. In reactive sputtering of a metallic nitride, nitrogen
`is additionally admitted into the chamber 12, and it reacts
`with the sputtered metallic atoms to form a metallic nitride
`on the wafer 16.
`
`To provide efficient sputtering, a magnetron 32 is posi-
`tioned in back of the target 14. It has opposed magnets 34,
`36 creating a magnetic field within the chamber in the
`neighborhood of the magnets 34, 36. The magnetic field
`traps electrons and, for charge neutrality, the ion density also
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`2
`increases to form a high-density plasma region 38 within the
`chamber adjacent to the magnetron 32. The magnetron 32 is
`usually rotated about the center of the target 14 to achieve
`full coverage in sputtering of the target 14. The form of the
`magnetron is a subject of this patent application, and the
`illustrated form is intended to be only suggestive.
`The advancing level of integration in semiconductor
`integrated circuits has placed increasing demands upon
`sputtering equipment and processes. Many of the problems
`are associated with contact and via holes. As illustrated in
`the cross-sectional view of FIG. 2, via or contact holes 40
`are etched through an interlevel dielectric layer 42 to reach
`a conductive feature 44 in the underlying layer or substrate
`46. Sputtering is then used to fill metal into the hole 40 to
`provide inter-level electrical connections. If the underlying
`layer 46 is the semiconductor substrate, the filled hole 40 is
`called a contact;
`if the underlying layer is a lower-level
`metallization level, the filled hole 40 is called a via. For
`simplicity, we will refer hereafter only to vias. The widths of
`inter-level vias have decreased to the neighborhood of 0.25
`pm and below while the thickness of the inter-level dielectric
`has remained nearly constant at around 0.7 pm. As a result,
`the via holes in advanced integrated circuits have increased
`aspect ratios of three and greater. For some technologies
`under development, aspect ratios of six and even greater are
`required.
`Such high aspect ratios present a problem for sputtering
`because most
`forms of sputtering are not strongly
`anisotropic, a cosine dependence off the vertical being
`typical, so that the initially sputtered material preferentially
`deposits at
`the top of the hole and may bridge it,
`thus
`preventing the filling of the bottom of the hole and creating
`a void in the via metal.
`
`It has become known, however, that deep hole filling can
`be facilitated by causing a significant fraction of the sput-
`tered particles to be ionized in the plasma between the target
`14 and the pedestal 18. The pedestal 18 of FIG. 1, even if it
`is left electrically floating, develops a DC self-bias, which
`attracts ionized sputtered particles from the plasma across
`the plasma sheath adjacent to the pedestal 18 and deep into
`the hole 40 in the dielectric layer 42. The effect can be
`accentuated with additional DC or RF biasing of the pedestal
`electrode 18 to additionally accelerate the ionized particles
`extracted across the plasma sheath towards the wafer 16,
`thereby controlling the directionality of sputter deposition.
`The process of sputtering with a significant fraction of
`ionized sputtered atoms is called ionized metal deposition or
`ionized metal plating (IMP). Two related quantitative mea-
`sures of the effectiveness of hole filling are bottom coverage
`and side coverage. As illustrated schematically in FIG. 2, the
`initial phase of sputtering deposits a layer 50, which has a
`surface or blanket thickness of s1, a bottom thickness of s2,
`and a sidewall thickness of s3. The bottom coverage is equal
`to S2/S1, and the sidewall coverage is equal to s3/s1. The
`model
`is overly simplified but
`in many situations is
`adequate.
`One method of increasing the ionization fraction is to
`create a high-density plasma (HDP), such as by adding an
`RF coil around the sides of the chamber 12 of FIG. 1. An
`
`HDP reactor not only creates a high-density argon plasma
`but also increases the ionization fraction of the sputtered
`atoms. However, HDP PVD reactors are new and relatively
`expensive, and the quality of the deposited films is not
`always the best. It is desired to continue using the principally
`DC sputtering of the PVD reactor of FIG. 1.
`Another method for increasing the ionization ratio is to
`use a hollow-cathode magnetron in which the target has the
`
`

`

`US 6,306,265 B1
`
`3
`shape of a top hat. This type of reactor, though, runs very hot
`and the complexly shaped targets are very expensive.
`It has been observed that copper sputtered with either an
`inductively coupled HDP sputter
`reactor or a hollow-
`cathode reactor tends to form an undulatory copper film on
`the via sidewall, and further the deposited metal tends to
`dewet. The variable thickness is particularly serious when
`the sputtered copper layer is being used as a seed layer of a
`predetermined minimum thickness for a subsequent depo-
`sition process such as electroplating to complete the copper
`hole filling.
`A further problem in the prior art is that the sidewall
`coverage tends to be asymmetric with the side facing the
`center of the target being more heavily coated than the more
`shielded side facing a larger solid angle outside the target.
`Not only does the asymmetry require excessive deposition to
`achieve a seed layer of predetermined minimum thickness,
`it causes cross-shaped trenches used as alignment indicia in
`the photolithography to appear to move as the trenches are
`asymmetrically narrowed.
`that promotes deep hole
`Another operational control
`is generally believed that
`filling is chamber pressure. It
`lower chamber pressures promote hole filling. At higher
`pressures,
`there is a higher probability that sputtered
`particles, whether neutral or ionized, will collide with atoms
`of the argon carrier gas. Collisions tend to neutralize ions
`and to randomize velocities, both effects degrading hole
`filling. However, as described before, the sputtering relies
`upon the existence of a plasma at least adj acent to the target.
`If the pressure is reduced too much, the plasma collapses,
`although the minimum pressure is dependent upon several
`factors.
`
`The extreme of low-pressure plasma sputtering is sus-
`tained self-sputtering (SSS), as disclosed by Fu et al. in US.
`patent application, Ser. No. 08/854,008, filed May 8, 1997.
`In SSS, the density of positively ionized sputtered atoms is
`so high that a sufficient number are attracted back to the
`negatively biased target to resputter more ionized atoms.
`Under the right conditions for a limited number of target
`metals, the self-sputtering sustains the plasma, and no argon
`working gas is required. Copper is the metal most prone to
`SSS, but only under conditions of high power and high
`magnetic field. Copper sputtering is being seriously devel-
`oped because of copper’s low resistivity and low suscepti-
`bility to electromigration. However, for copper SSS to
`become commercially feasible, a full-coverage, high-field
`magnetron needs to be developed.
`Increased power applied to the target allows reduced
`pressure, perhaps to the point of sustained self-sputtering.
`The increased power also increases the ionization density.
`However, excessive power requires expensive power sup-
`plies and increased cooling. Power levels in excess of 30 kW
`are expensive and should be avoided if possible. In fact, the
`pertinent factor is not power but the power density in the
`area below the magnetron since that
`is the area of the
`high-density plasma promoting effective sputtering. Hence,
`a small, high-field magnet would most easily produce a high
`ionization density. For this reason, some prior art discloses
`a small circularly shaped magnet. However, such a magne-
`tron requires not only rotation about the center of the target
`to provide uniformity, but it also requires radial scanning to
`assure full and fairly uniform coverage of the target. If full
`magnetron coverage is not achieved, not only is the target
`not efficiently used, but more importantly the uniformity of
`sputter deposition is degraded, and some of the sputtered
`material redeposits on the target in areas that are not being
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`4
`sputtered. Furthermore, the material redeposited on unsput-
`tered areas may build up to such a thickness that it is prone
`to flake off, producing severe particle problems. While radial
`scanning can potentially avoid these problems, the required
`scanning mechanisms are complex and generally considered
`infeasible in a production environment.
`One type of commercially available magnetron is kidney-
`shaped, as exemplified by Tepman in US. Pat. No. 5,320,
`728. Parker discloses more exaggerated forms of this shape
`in US. Pat. No. 5,242,566. As illustrated in plan view in
`FIG. 3, the Tepman magnetron 52 is based on a kidney shape
`for the magnetically opposed pole faces 54, 56 separated by
`a circuitous gap 57 of nearly constant width. The pole faces
`54, 56 are magnetically coupled by unillustrated horseshoe
`magnets bridging the gap 57. The magnetron rotates about a
`rotational axis 58 at the center of the target 14 and near the
`concave edge of the kidney-shaped inner pole face 54. The
`convexly curved outer periphery of the outer pole face 56,
`which is generally parallel to the gap 57 in that area, is close
`to the outer periphery of the usable portion if the target 14.
`This shape has been optimized for high field and for uniform
`sputtering but has an area that is nearly half that of the target.
`It is noted that the magnetic field is relatively weak in areas
`separated from the pole gap 57.
`For these reasons,
`it
`is desirable to develop a small,
`high-field magnetron providing full coverage so as to pro-
`mote deep hole filling and sustained copper self-sputtering.
`
`SUMMARY OF THE INVENTION
`
`The invention includes a sputtering magnetron having an
`oval or related shape of smaller area than a circle of equal
`diameter where the two diameters extend along the target
`radius with respect to the typical rotation axis of the mag-
`netron. The shapes include racetracks, ellipses, egg shapes,
`triangles, and arced triangles asymmetrically positioned
`about the target center.
`The magnetron is rotated on the backside of the target
`about a point preferably near the magnetron’s thin end, and
`the thicker end is positioned more closely to the target
`periphery. Preferably,
`the total magnetic flux is greater
`outside than inside the half radius of the target.
`The magnetic intensity away from the target can be
`increased for a triangular magnetron having a relatively
`small apex angle by using bar magnets.
`The small area allows an electrical power density of at
`least 600 W/cm2 to be applied from an 18 kW power supply
`to a fully covered sputtering target used to sputter deposit a
`200 mm wafer.
`
`The high power density and the magnetic field extending
`far away from the target are two means possible to produce
`a plasma wave which can further drive the plasma to a
`higher density and ionization. Advantageously, a primary
`plasma wave is generated at a higher frequency in the range
`of hundreds of megahertz, which is parametrically converted
`to another wave at a much lower frequency, for example, 5
`to 75 MHz, corresponding to the thermal velocity of elec-
`trons in the plasma produced by capacitively coupling DC
`power to the target.
`The magnetron is configured to produce less magnetic
`flux in its inner pole than in its surrounding outer pole.
`Thereby, the magnetic field reaches further into the sputter-
`ing chamber to promote low-pressure sputtering and sus-
`tained self-sputtering.
`The invention also includes sputtering methods achiev-
`able with such a magnetron. The high magnetic field extend-
`
`

`

`US 6,306,265 B1
`
`5
`ing over a small closed area facilitates sustained self-
`sputtering. Many metals not subject
`to sustained self-
`sputtering can be sputtered at chamber pressures of less than
`0.5 milliTorr, often less than 0.2 milliTorr, and even at 0.1
`milliTorr. The bottom coverage can be further improved by
`applying an RF bias of less than 250 W to a pedestal
`electrode sized to support a 200 mm wafer. Copper can be
`sputtered with 18 kW of DC power for a 330 mm target and
`200 mm wafer either in a fully self-sustained mode or with
`a minimal chamber pressure of 0.3 milliTorr or less.
`The invention provides for high-power density sputtering
`with power supplies of reduced capacity.
`The invention also includes sputtering with condition,
`such as a sufficiently target power and high magnetic field
`away from the target, that a non-linear wave-beam interac-
`tion occurs that pumps energy into plasma electrons, thereby
`increasing the plasma density.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a schematic diagram of a DC plasma sputtering
`reactor.
`FIG. 2 is a cross-sectional view of a inter-level via in a
`
`semiconductor integrated circuit.
`FIG. 3 is a plan view of a conventional magnetron.
`FIG. 4 is a plan view of the pole pieces of an embodiment
`of the magnetron of the invention taken along the view line
`4—4 of FIG. 7.
`
`FIG. 5 is a plan view of the magnets used in the magne-
`tron of FIG. 4.
`
`FIG. 6 is a cross-sectional view of one of the magnets
`used in conjunction with the embodiments of the invention.
`FIG. 7 is a cross-sectional view of the magnetron of FIG.
`
`4.
`
`FIG. 8 is a plan view of an egg-shaped magnetron.
`FIG. 9 is a plan view of a triangularly shaped magnetron.
`FIG. 10 is a plan view of a modification of the triangularly
`shaped magnetron of FIG. 9, referred to as an arced trian-
`gular magnetron.
`FIG. 11 is a plan view of the magnets used in the arced
`triangular magnetron of FIG. 10.
`FIG. 12 is a plan view of two model magnetrons used to
`calculate areas and peripheral lengths.
`FIG. 13 is a graph of the angular dependences of the areas
`of a triangular and of a circular magnetron.
`FIG. 14 is a graph of the angular dependences of the
`peripheral lengths of the two types of magnetrons of FIG.
`12.
`
`FIG. 15 is a bottom plan view of a magnetron of the
`invention using bar magnets.
`FIG. 16 is a bottom plan view of an alternative to the
`magnetron of FIG. 15.
`FIG. 17 is a side view of an idealization of the magnetic
`field produced with the described embodiments of the inven-
`tion.
`
`FIGS. 18 and 19 are a top plan view and a schematic side
`view of a chamber and magnetron arranged for measuring
`plasma wave generated by a magnetron of the invention.
`FIG. 20 is a graph of a typical energy distribution of
`plasma electrons.
`FIG. 21 is a graph showing the effect of RF wafer bias in
`bottom coverage in titanium sputtering.
`FIG. 22 is a graph of the dependence of chamber pressure
`upon nitrogen flow illustrating the two modes of deposition
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`obtained in reactive sputtering of titanium nitride with a
`magnetron of the invention.
`FIG. 23 is a graph of the step coverage obtained in the two
`sputtering modes for reactive sputtering of titanium nitride
`with a magnetron of the invention.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`
`One embodiment of the invention is a racetrack magne-
`tron 60, illustrated in plan view in FIG. 4. The racetrack
`magnetron 60 has a central bar-shaped pole face 62 of one
`magnetic polarity having opposed parallel middle straight
`sides 64 connected by two rounded ends 66. The central,
`bar-shaped pole face 62 is surrounded by an outer elongated
`ring-shaped pole face 68 of the other polarity with a gap 70
`of nearly constant width separating the bar-shaped and
`ring-shaped pole faces 62, 68. The outer pole face 68 of the
`other magnetic polarity includes opposed parallel middle
`straight sections 72 connected by two rounded ends 74 in
`general central symmetry with the inner pole face 62. The
`middle sections 72 and rounded ends 74 are bands having
`nearly equal widths. Magnets, to be described shortly, cause
`the pole faces 62, 68 to have opposed magnetic polarities. A
`backing plate, also to be described shortly, provides both a
`magnetic yoke between the magnetically opposed pole faces
`62, 68 and support for the magnetron structure.
`Although the two pole faces 62, 68 are illustrated with
`specific magnetic polarities producing magnetic fields
`extending generally perpendicularly to the plane of
`illustration, it is of course appreciated that the opposite set
`of magnetic polarities will produce the same general mag-
`netic effects as far as the invention is concerned. The
`
`illustrated assembly produces a generally semi-toroidal
`magnetic field having parallel arcs extending perpendicu-
`larly to a closed path with a minimal field-free region in the
`center. There results a closed tunnel of magnetic field
`forming struts of the tunnel.
`The pole assembly of FIG. 4 is intended to be continu-
`ously rotated during sputter deposition at a fairly high
`rotation rate about a rotation axis 78 approximately coinci-
`dent with the center of the target 14 of uniform composition.
`The rotation axis 78 is located at or near one prolate end 80
`of the outer pole face 68 and with its other prolate end 82
`located approximately at the outer radial usable extent of the
`target 14. The asymmetric placement of the rotating mag-
`netron 60 with respect to the target center provides a small
`magnetron nonetheless achieving full target coverage. The
`outer usable periphery of the target is not easily defined
`because different magnetron designs use different portions
`of the same target. However, it is bounded by the flat area of
`the target and almost always extends to significantly beyond
`the diameter of the wafer being sputter deposited and is
`somewhat less than the area of the target face. For 200 mm
`wafers, target faces of 325 mm are typical. A 15% unused
`target radius may be considered as an upper practical limit.
`Racetrack magnetrons are well known in the prior art, but
`they are generally positioned symmetrically about the center
`of the target. In the described invention, the racetrack is
`asymmetrically positioned with its inner end either overly-
`ing the target center or terminating at a radial position
`preferably within 20% and more preferably within 10% of
`the target radius from the target center. The illustrated
`racetrack extends along a diameter of the target.
`As illustrated in the plan view of FIG. 5, two sets of
`magnets 90, 92 are disposed in back of the pole faces 62, 68
`to produce the two magnetic polarities. The combination of
`
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`US 6,306,265 B1
`
`7
`the pole faces 62, 68, the magnets 90, 92, and possibly a
`back magnetic yoke produces two opposite magnetic poles
`having areas defined by the pole faces 62, 68. Other means
`may be used to achieved such poles.
`The two types of magnets 90, 92 may be of similar
`construction and composition producing an axially extend-
`ing magnetic flux on each vertically facing end. If they are
`of different, magnetic composition, diameter, or length, the
`flux produced by different magnets may be different. A
`cross-sectional view of a magnet 90, 92 is shown in FIG. 6.
`A cylindrical magnetic core 93 extending along an axis is
`composed of a strongly magnetic material, such as neody-
`mium boron iron (NdBFe). Because such a material is easily
`oxidized, the core 93 is encapsulated in a case made of a
`tubular sidewall 94 and two generally circular caps 96
`welded together to form an air-tight canister. The caps 96 are
`composed of a soft magnetic material, preferably SS410
`stainless steel, and the tubular sidewall 96 is composed of a
`non-magnetic material, preferably SS304 stainless steel.
`Each cap 96 includes an axially extending pin 97, which
`engages a corresponding capture hole in one of the pole
`faces 62, 68 or in a magnetic yoke to be shortly described.
`Thereby, the magnets 90, 92 are fixed in the magnetron. The
`magnetic core 93 is magnetized along its axial direction, but
`the two different types of magnets 90, 92 are oriented in the
`magnetron 60, as illustrated in the cross-sectional view of
`FIG. 7, so that the magnets 90 of the inner pole 62 are
`aligned to have their magnetic field extending vertically in
`one direction, and the magnets 92 of the outer pole 68 are
`aligned to have their magnetic field extending vertically in
`the other direction. That is, they have opposed magnetic
`polarities.
`As illustrated in the cross-sectional view of FIG. 7, the
`magnets 90, 92 are arranged closely above (using the
`orientation of FIG. 1) the pole faces 62, 68 located just
`above the back of the target 14. A magnetic yoke 98 having
`a generally closed shape generally conforming to the outer
`periphery of the outer pole face 68 is closely positioned in
`back of the magnets 90, 92 to magnetically couple the two
`poles 62, 68. As mentioned previously, holes in the pole
`faces 62, 68 and in the yoke 98 fix the magnets 90, 92, and
`unillustrated hardware fix the pole faces 62, 68 to the yoke
`98.
`
`The inner magnets 90 and inner pole face 62 constitute an
`inner pole of one magnetic polarity while the outer magnets
`92 and the outer pole face 68 constitute a surrounding outer
`pole of the other magnetic polarity. The magnetic yoke 98
`magnetically couples the inner and outer poles and substan-
`tially confines the magnetic field on the back or top side of
`the magnetron to the yoke 98. Asemi-toroidal magnetic field
`100 is thereby produced, which extends through the non-
`magnetic target 14 into the vacuum chamber 12 to define the
`high-density plasma region 38. The field 100 extends
`through the non-magnetic target 14 into the vacuum cham-
`ber 12 to define the extent of the high-density plasma region
`38. The magnets 90, 92 may be of different magnetic
`strength. However, it is desired for reasons to be explained
`later that the total magnetic flux produced by the outer
`magnets 92 be substantially greater than that produced by
`the inner magnets 90. As illustrated,
`the magnetron 60
`extends horizontally from approximately the center of the
`target 14 to the edge of the usable area of the target 14. The
`magnetic yoke 90 and the two pole faces 62, 68 are
`preferably plates formed of a soft magnetic material such as
`$8416 stainless steel.
`
`The inner prolate end 80 of the magnetron 60 is connected
`to a shaft 104 extending along the rotation axis 78 and
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`rotated by a motor 106. As illustrated, the magnetron 60
`extends horizontally from approximately the center of the
`target 14 to the right hand side of the usable area of the target
`14. Demaray et al.
`in US. Pat. No. 5,252,194 disclose
`exemplary details of the connections between the motor 106,
`the magnetron 60, and the vacuum chamber 12. The mag-
`netron assembly 60 should include counter-weighting to
`avoid flexing of the shaft 104. Although the center of
`rotation 78 is preferably disposed within the inner prolate
`end 74 of the outer pole face 72,
`its position may be
`optimized to a slightly different position, but one preferably
`not deviating more than 20%, more preferably 10%, from
`the inner prolate end 80 as normalized to the prolate length
`of the magnetron 60. Most preferably, the inner end of the
`outer pole face 68 near the prolate end 80 overlies the
`rotation center 78.
`
`The racetrack configuration of FIG. 4 has the advantage
`of simplicity and a very small area while still providing full
`target coverage. As will be discussed later, the asymmetric
`magnetic flux of the two poles is advantageous for low-
`pressure sputtering and sustained self-sputtering.
`The racetrack configuration of FIG. 4 can be alternatively
`characterized as an extremely flattened oval. Other oval
`shapes are also included within the invention, for example,
`continuously curved shapes of continuously changing diam-
`eter such as elliptical shapes with the major axis of the
`ellipse extending along the radius of the target and with the
`minor axis preferably parallel to a rotational circumference.
`Tabuchi illustrates a symmetric oval magnetron in Laid-
`open Japanese Patent Application 63-282263. This shape
`however has the disadvantage of a complex shape, espe-
`cially for packing the magnets in the inner pole.
`Another oval shape is represented by an egg-shaped
`magnetron 106, illustrated in plan view in FIG. 8. It has an
`outer pole face 108 of one magnetic polarity surrounding an
`inner pole face 110 of the other polarity with a nearly
`constant gap 122 between them. Both pole faces 108, 110 are
`shaped like the outline of an egg with a major axis extending
`along the radius of the target. However, an inner end 112 of
`the outer pole face 108 near the rotation axis 78 is sharper
`than an outer end 114 near the periphery of the target. The
`egg shape is related to an elliptical shape but is asymmetric
`with respect to the target radius. Specifically, the minor axis
`is pushed closer to the target periphery than its center. The
`inner pole face 110 and the gap 122 are similarly shaped.
`Such an egg shape places more of the magnetic flux closer
`to the target periphery so as to impro