U.S. patent application number 09/918022 was filed with the patent office on 2001-12-13 for sputtering target having an annular vault.
Invention is credited to Fu, Jianming, Gopalraja, Praburam.
Application Number | 20010050223 09/918022 |
Document ID | / |
Family ID | 27413858 |
Filed Date | 2001-12-13 |
United States Patent
Application |
20010050223 |
Kind Code |
A1 |
Gopalraja, Praburam ; et
al. |
December 13, 2001 |
Sputtering target having an annular vault
Abstract
A target for a magnetron plasma sputter reactor. The target has
an annular vault facing the wafer to be sputter coated and has a
width of preferably at least 5 cm and an aspect ratio of at least
1:2, preferably 1:1. Various types of magnetic means positioned
around the walls of the vault, some of which may rotate along the
vault, create a magnetic field in the vault to support a plasma
extending over a large volume of the vault from its top to its
bottom. The large plasma volume within the vault increases the
probability that the sputtered metal atoms will become ionized and
be accelerated towards an electrically biased wafer support
electrode.
Inventors: |
Gopalraja, Praburam;
(Sunnyvale, CA) ; Fu, Jianming; (San Jose,
CA) |
Correspondence
Address: |
Applied Materials, Inc.
Patent/Legal Department
P.O. Box 450A
Santa Clara
CA
95052
US
|
Family ID: |
27413858 |
Appl. No.: |
09/918022 |
Filed: |
July 30, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09918022 |
Jul 30, 2001 |
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09703601 |
Nov 1, 2000 |
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09703601 |
Nov 1, 2000 |
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09518180 |
Mar 2, 2000 |
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6277249 |
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09518180 |
Mar 2, 2000 |
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09490026 |
Jan 21, 2000 |
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6251242 |
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Current U.S.
Class: |
204/298.12 ;
204/298.11; 204/298.13; 204/298.2; 257/E21.169 |
Current CPC
Class: |
H01L 21/76844 20130101;
H01L 21/76865 20130101; H01L 21/76805 20130101; C23C 14/165
20130101; C23C 14/35 20130101; H01J 37/3405 20130101; H01L 21/76862
20130101; H01L 21/76873 20130101; H01L 21/76843 20130101; H01J
37/3458 20130101; C23C 14/046 20130101; C23C 14/225 20130101; H01J
37/342 20130101; H01L 2221/1089 20130101; H01J 37/3452 20130101;
H01J 37/3423 20130101; C23C 14/185 20130101; H01J 37/3455 20130101;
H01L 21/2855 20130101 |
Class at
Publication: |
204/298.12 ;
204/298.13; 204/298.2; 204/298.11 |
International
Class: |
C23C 014/35 |
Claims
1. A vault-shaped sputtering target, comprising a continuous member
having a vault annular about a central axis, having a width of at
least 5 cm, and disposed on a first side of said target having an
annular inner sidewall, an opposed annular outer sidewall, a top
wall, and an annular flange extending radially outwardly from said
first side of said outer sidewall, an aspect ratio of a depth of
said vault to a width of said vault being at least 1:2, said target
being continuous within said vault and in an area radially inwardly
of said inner sidewall.
2. The target of claim 1, wherein said aspect ratio is at least
1:1.
3. The target of claim 1, wherein said continuous member
additionally comprises a planar face wall attached to said first
side of said inner sidewall.
4. The target of claim 3, wherein a well is formed by said inner
sidewall and said planar face wall able to accommodate a magnet
adjacent said inner sidewall.
5. The target of claim 1, wherein said continuous member
additionally comprises an annular knob extending from said first
side of said outer sidewall beyond said flange.
6. The target of claim 1, wherein said width is at least 7 cm.
7. The target of claim 6, wherein said width is at least 10 cm.
8. The target of claim 1, wherein said width is sized for a 200 mm
wafer.
9. The target of claim 1, wherein said width is at least 25 times a
dark space of a plasma.
10. The target of claim 1, wherein said target is a copper
target.
11. The target of claim 1, wherein said target is a titanium
target.
12. The target of claim 1, wherein said target is a tungsten
target.
13. The target of claim 1, wherein said sidewalls extend parallel
to each other and perpendicular to said top wall.
14. A vault-shaped sputtering target, comprising a continuous
member having a vault annular about a central axis and disposed on
a first side of said target having an annular inner sidewall, an
opposed annular outer sidewall, a top wall, an annular flange
extending radially outwardly from said first side of said outer
sidewall, and an annular knob extending toward said first side of
said outer sidewall away from said flange, an aspect ratio of a
depth of said vault to a width of said vault being at least 1:2,
said target being continuous within said vault and in an area
radially inwardly of said inner sidewall.
15. The target of claim 14, wherein said continuous member
additionally comprises a planar face wall attached to said first
side of said inner sidewall.
16. The target of claim 15, wherein a well is formed by said inner
sidewall and said planar face wall able to accommodate a magnet
adjacent said inner sidewall.
17. The target of claim 14, wherein said vault has a width of at
least 5 cm.
18. The target of claim 17, wherein said vault has a width of at
least 7 cm.
19. The target of claim 14, which is a copper target.
20. A target for use in a sputter reactor having a magnetron being
at least partially rotatable about a central axis, comprising a
vault-shaped continuous member having a vault annular about said
central axis with a width of at least 7 cm and disposed on a first
side of said target facing an interior of said reactor and having
an annular inner sidewall, an opposed annular outer sidewall, a top
wall, and an annular flange extending radially outwardly from said
first side of said outer sidewall configured to be supported on
said reactor, an aspect ratio of a depth of said vault to a width
of said vault being at least 1:2, said target being continuous
within said vault and in an area radially inwardly of said inner
sidewall.
21. The target of claim 20, wherein said continuous member
additionally comprises a planar face wall attached to said first
side of said inner sidewall.
22. The target of claim 21, wherein a well is formed by said inner
sidewall and said planar face wall able to accommodate a portion of
said magnetron adjacent said inner sidewall.
23. The target of claim 20, wherein said aspect ratio is at least
1:1.
24. The target of claim 20, wherein said continuous member is
formed of copper.
25. A sputtering target adapted for use in a magnetron sputter
reactor designed to sputter material of said target onto a wafer
having a maximum diameter and comprising a continuous member having
a vault annular about a central axis and disposed on a first side
of said target having an annular inner sidewall, an opposed annular
outer sidewall of a diameter greater than said maximum diameter of
said wafer, a top wall, and an annular flange extending radially
outwardly from said first side of said outer sidewall, an aspect
ratio of a depth of said vault to a width of said vault being at
least 1:2, said target being continuous within said vault and in an
area radially inwardly of said inner sidewall.
26. The sputtering target of claim 25, wherein said aspect ratio is
at least 1:1.
27. The sputtering target of claim 25, wherein a width of said
vault is at least 5 cm.
28. The sputtering target of claim 25, which is a copper
target.
29. A plasma sputter reactor, comprising: a chamber wall; a
pedestal for supporting a substrate to be sputter coated; an
isolator supported on said chamber wall; and a metal shield
protecting said isolator and said chamber wall from being sputter
coated; and a target supported on said isolator and comprising a
continuous vault-shaped member having a vault annular about a
central axis and disposed on a first side of said target having an
annular inner sidewall, an opposed annular outer sidewall, a top
wall, an annular flange extending radially outwardly from said
first side of said outer sidewall, and an annular knob extending
toward said first side of said outer sidewall away from said flange
and forming a plasma dark space in opposition to said shield, an
aspect ratio of a depth of said vault to a width of said vault
being at least 1:2.
30. The reactor of claim 29, wherein said aspect ratio is at least
1:1.
31. The reactor of claim 29, wherein a width of said vault is at
least 5 cm.
32. The reactor of claim 29, wherein said target is a copper
target.
33. The reactor of claim 29, wherein a diameter of said outer
sidewall is greater than a diameter of said pedestal.
Description
RELATED APPLICATIONS
[0001] This application is a division of Ser. No. 09/703,601, filed
Nov. 1, 2000, which is a continuation in part of Ser. No.
09/518,180, filed Mar. 2, 2000, issue fee paid, which is a
continuation in part of Ser. No. 09/490,026, filed Jan. 21, 2000,
now issued as U.S. Pat. No. 6,251,242.
FIELD OF THE INVENTION
[0002] The invention relates generally to plasma sputtering. In
particular, the invention relates to the sputter target and
associated magnetron used in a sputter reactor and to an integrated
via filling process using sputtering.
BACKGROUND ART
[0003] A semiconductor integrated circuit contains many layers of
different materials usually classified according to whether the
layer is a semiconductor, a dielectric (electrical insulator) or
metal. However, some materials such as barrier materials, for
example, TiN, are not so easily classified. The two principal
current means of depositing metals and barrier materials are
sputtering, also referred to as physical vapor deposition (PVD),
and chemical vapor deposition (CVD). Of the two, sputtering has the
inherent advantages of low cost source material and high deposition
rates. However, sputtering has an inherent disadvantage when a
material needs to filled into a deep narrow hole, that is, one
having a high aspect ratio. The same disadvantage obtains when a
thin layer of the material needs be coated onto the sides of the
hole, which is often required for barrier materials. Aspect ratios
of 3:1 present challenges, 5:1 becomes difficult, 8:1 is becoming a
requirement, and 10:1 and greater are expected in the future.
Sputtering itself is fundamentally a nearly isotropic process
producing ballistic sputter particles which do not easily reach the
bottom of deep narrow holes. On the other hand, CVD tends to be a
conformal process equally effective at the bottom of holes and on
exposed top planar surfaces.
[0004] Up until the recent past, aluminum has been the metal of
choice for the metallization used in horizontal interconnects and
in the vias connecting two levels of metallization. In more recent
technology, copper vias extend between two levels of horizontal
copper interconnects. Contacts to the underlying silicon present a
larger problem, but may still be accomplished with either aluminum
or copper. Copper interconnects are used to reduce signal delay in
advanced ULSI circuits. It is understood that copper may be pure
copper or a copper alloy containing up to 10% alloying with other
elements such as magnesium and aluminum. Due to continued downward
scaling of the critical dimensions of microcircuits, critical
electrical parameters of integrated circuits, such as contact and
via resistances, have become more difficult to achieve. In
addition, due to the smaller dimensions, the aspect ratios of
inter-metal features such as contacts and vias are also increasing.
An advantage of copper is that it may be quickly and inexpensively
deposited by electrochemical processes, such as electroplating.
However, sputtering or possibly CVD of thin copper layers onto the
walls of via holes is still considered necessary to act as an
electrode for electroplating or as a seed layer for the
electroplated copper. The discussion of copper processes will be
delayed until later.
[0005] The conventional sputter reactor has a planar target in
parallel opposition to the wafer being sputter deposited. A
negative DC voltage is applied to the target of magnitude
sufficient to ionize the argon working gas into a plasma. The
positive argon ions are attracted to the negatively charged target
with sufficient energy to sputter atoms of the target material.
Some of the sputtered atoms strike the wafer and form a sputter
coating thereon. Most usually, a magnetron is positioned in back of
the target to create a larger magnetic field adjacent to the
target. The magnetic field traps electrons, and, to maintain charge
neutrality in the plasma, the ion density also increases. As a
result, the plasma density and sputter rate are increased. The
conventional magnetron generates a magnetic field lying principally
parallel to the target.
[0006] Much effort has been expended to allow sputtering to
effectively coat metals and barrier materials deep into narrow
holes. High-density plasma (HDP) sputtering has been developed in
which the argon working gas is excited into a high-density plasma,
which is defined as a plasma having an ionization density of at
least 10.sup.11 cm.sup.-3 across the entire space the plasma fills
except the plasma sheath. Typically, an HDP sputter reactor uses an
RF power source connected to an inductive coil adjacent to the
plasma region to generate the high-density plasma. The high argon
ion density causes a significant fraction of sputtered atoms to be
ionized. If the pedestal electrode supporting the wafer being
sputter coated is negatively electrically biased, the ionized
sputter particles (metal ions) are accelerated toward the wafer to
form a directional column that reaches deeply into narrow
holes.
[0007] HDP sputter reactors, however, have disadvantages. They
involve a somewhat new technology and are relatively expensive.
Furthermore, the quality of the sputtered films they produce is
often not the best, typically having an undulatory surface. Also,
high-energy ions, particularly the argon ions which are also
attracted to the wafer, tend to damage the material already
deposited.
[0008] Another sputtering technology, referred to as self-ionized
plasma (SIP) sputtering, has been developed to fill deep holes.
See, for example, U.S. patent application Ser. No. 09/373,097 filed
Aug. 12, 1999 by Fu and U.S. Patent Application filed Oct. 8, 1999
by Chiang et al. Both of these patent applications are incorporated
by reference in their entireties. In its original implementations,
SIP relies upon a somewhat standard capacitively coupled plasma
sputter reactor having a planar target in parallel opposition to
the wafer being sputter coated and a magnetron positioned in back
of the target to increase the plasma density and hence the
sputtering rate. The SIP technology, however, is characterized by a
high target power density, a small magnetron, and a magnetron
having an outer magnetic pole piece enclosing an inner magnetic
pole piece with the outer pole piece having a significantly higher
total magnetic flux than the inner pole piece. In some
implementations, the target is separated from the wafer by a large
distance to effect long-throw sputtering, which enhances collimated
sputtering. The asymmetric magnetic pole pieces causes the magnetic
field to have a significant vertical component extending far
towards the wafer, thus enhancing and extending the high-density
plasma volume and promoting transport of ionized sputter
particles.
[0009] The SIP technology was originally developed for sustained
self-sputtering (SSS) in which a sufficiently high number of
sputter particles are ionized that they may be used to further
sputter the target and no argon working gas is required. Of the
metals commonly used in semiconductor fabrication, only copper has
a sufficiently high self-sputtering yield to allow sustained
self-sputtering.
[0010] The extremely low pressures and relatively high ionization
fractions associated with SSS are advantageous for filling deep
holes with copper. However, it was quickly realized that the SIP
technology could be advantageously applied to the sputtering of
aluminum and other metals and even to copper sputtering at moderate
pressures. SIP sputtering produces high quality films exhibiting
high hole filling factors regardless of the material being
sputtered.
[0011] Nonetheless, SIP has some disadvantages. The small area of
the magnetron may require circumferential scanning of the magnetron
in a rotary motion at the back of the target to achieve even a
minimal level of uniformity, and even with rotary scanning, radial
uniformity is difficult to achieve. Furthermore, very high target
powers have been required in the previously known versions of SIP.
High-capacity power supplies are expensive and necessitate
complicated target cooling. Lastly, known versions of SIP tend to
produce a relatively low ionization fraction of sputter particles,
for example, 20%. The remaining non-ionized fraction of sputtered
particles has a relatively isotropic distribution rather than
forming a forward directed column which results from metal ions
being accelerated toward a biased wafer. Also, the target diameter
in a typical commercial sputter reactor is only slightly greater
than the wafer diameter. As a result, those holes being coated
located at the edge of the wafer have radially outer sidewalls
which see a larger fraction of the target and are more heavily
coated than the radially inner sidewalls. Therefore, the sidewalls
of the edge holes are asymmetrically coated.
[0012] Other sputter geometries have been developed which increase
the ionization density. One example is a multi-pole hollow cathode
target, several variants of which are disclosed by Barnes et al. in
U.S. Pat. No. 5,178,739. Its target has a hollow cylindrical shape,
usually closed with a circular back wall, and is electrically
biased. Typically, a series of magnets, positioned on the sides of
the cylindrical cathode of alternating magnetic polarization,
create a magnetic field extending generally parallel to the
cylindrical sidewall.
[0013] Another approach uses a pair of facing targets facing the
lateral sides of the plasma space above the wafer. Such systems are
described, for example, by Kitamoto et al. in "Compact sputtering
apparatus for depositing Co--Cr alloy thin films in magnetic
disks," Proceedings: The Fourth International Symposium on
Sputtering & Plasma Processes, Kanazawa, Japan, Jun. 4-6, 1997,
pp. 519-522, by Yamazato et al. in "Preparation of TiN thin films
by facing targets magnetron sputtering, ibid., pp. 635-638, and by
Musil et al. in "Unbalanced magnetrons and new sputtering systems
with enhanced plasma ionization," Journal of Vacuum Science and
Technology A, vol. 9, no. 3, May 1991, pp. 1171-1177. The facing
pair geometry has the disadvantage that the magnets are stationary
and create a horizontally extending field that is inherently
non-uniform with respect to the wafer.
[0014] Musil et al., ibid., pp. 1174, 1175 describe a coil-driven
magnetic mirror magnetron having a central post of one magnetic
polarization and surrounding rim of another polarization. An
annular vault-shaped target is placed between the post and rim.
This structure has the disadvantage that the soft magnetic material
forming the two poles, particularly the central spindle, are
exposed to the plasma during sputtering and inevitably contaminate
the sputtered layer. Furthermore, the coil drive provides a
substantially cylindrical geometry, which may not be desired in
some situations. Also, the disclosure illustrates a relatively
shallow geometry for the target vault, which does not take
advantage of some possible beneficial effects for a concavely
shaped target.
[0015] Helmer et al. in U.S. Pat. No. 5,482,611 describe a target
having an annular groove or vault facing the substrate. Stationary
magnets are arranged on the outside of the vault sidewalls with
parallel magnetic polarities so as to create a magnetic field
generally parallel to the vault walls within the vault and having a
magnetic cusp or null spot near the opening of the vault. The
magnetic cusp directs the metal sputter ions in a beam towards the
wafer. However, Helmer et al. admit that uniformity of deposition
with this magnetic configuration is not good. Lantsman in U.S. Pat.
No. 5,589,041 discloses an plasma etch chamber having a dielectric
roof that is formed with a vault so as to shape the plasma.
[0016] It is thus desired to combine many of the good benefits of
the different plasma sputter reactors described above while
avoiding their separate disadvantages.
[0017] Returning now to copper processing and the structures that
need to be formed for copper vias, as is well known to those in the
art, in a typical copper interconnect process flow, a thin barrier
layer is first deposited onto the walls of the via hole prior to
the copper deposition. The barrier layer prevents copper from
diffusing into the insulating dielectric layer separating the two
copper levels and also to prevent intra metal and inter metal
electrical shorts. A typical barrier for copper over silicon oxide
includes Ta or TaN or a combination thereof, but other materials
have been proposed, such as W/WN and Ti/TiN among others. In a
typical barrier deposition process, the barrier layer is deposited
using PVD or other method to form a continuous layer between the
underlying and overlying copper layers including the contact area
at the bottom of the via hole. Thin layers of these barrier
materials have a small but finite transverse resistance. A
structure resulting from this copper interconnect process produces
a contact having a finite characteristic resistance (known in the
art as a contact or via resistance) that depends on the geometry.
Conventionally, the barrier layer at the bottom of the contact or
via hole contributes about 30% of the total contact or via
resistance. Geffken et al. disclose in U.S. Pat. No. 5,985,762 a
separate directional etching step to remove the barrier layer from
the bottom of the via hole over an underlying copper feature but
not from the via sidewalls so that, during the sputter removal of
the copper oxide at the via bottom, the dielectric is not poisoned
by the sputtered copper. This process requires presumably a
separate etching chamber. Furthermore, the process deleteriously
also removes the barrier at the bottom of the trench in a
dual-damascene structure. They accordingly deposit another
conformal barrier layer, which remains under the metallized
via.
[0018] As a result, there is a need in the art for a method and
apparatus to form a low-resistance contact between underlying and
overlying copper layers and having a low contact resistance without
unduly complicating the process.
[0019] A copper layer used to form an interconnect is conveniently
deposited by electrochemical deposition, for example,
electroplating. As is well known, an adhesion or seed layer of
copper is usually required to nucleate an ensuing electrochemical
deposition on the dielectric sidewalls as well as to provide a
current path for the electroplating. In a typical deposition
process, the copper seed layer is deposited using PVD or CVD
methods, and the seed layer is typically deposited on top of the
barrier layer. A typical barrier/seed layer deposition sequence
also requires a pre-clean step to remove native oxide and other
contaminants that reside on the underlying metal that has been
previously exposed in etching the via hole. The pre-clean step, for
example, a sputter etch clean step using an argon plasma, is
typically performed in a process chamber that is separate from the
PVD chamber used to deposit the barrier and seed layers. With
shrinking dimension of the integrated circuits, the efficacy of the
pre-clean step, as well as sidewall coverage of the seed layer
within the contact/via feature, become more problematical.
[0020] As a result, the art needs a method and apparatus that
improves the pre-clean and deposition of the seed layer. Further,
the seed layer needs to be conformally deposited in all portions of
the via hole even if the barrier layer is removed in portions of
the hole.
SUMMARY OF THE INVENTION
[0021] The invention includes a magnetron producing a large volume
or thickness of a plasma, preferably a high-density plasma. The
long travel path through the plasma volume allows a large fraction
of the sputtered atoms to be ionized so that their energy and
directionality can be controlled by substrate biasing.
[0022] The target may be formed with more than one annular vault on
the side facing the substrate. Each vault should have a width of at
least 2.5 cm, preferably at least 5 cm, and more preferably at
least 7 cm and should have an aspect ratio of at least 1:2,
preferably at least 1:1. The width is thus at least 10 times and
preferably at least 25 times the dark space, thereby allowing the
plasma sheath to conform to the vault outline.
[0023] In one embodiment of the invention, the target includes at
least one annular vault on the front side of the target. The
backside of the target includes a central well enclosed by the
vault and accommodating an inner magnetic pole of one polarity. The
backside of the target also includes an outer annular space
surrounding the vault and accommodating an outer magnetic pole of a
second polarity. The outer magnetic pole may be annular or be a
circular segment which is rotated about the inner magnetic
pole.
[0024] In one embodiment, the magnetization of the two poles may be
accomplished with soft pole pieces projecting into the central well
and the outer annular space and magnetically coupled to magnets
disposed generally behind the well and outer annular space. In a
second embodiment, the two poles may be radially directed magnetic
directions. In a third embodiment, a magnetic coil drives a yoke
having a spindle and rim shape.
[0025] In one advantageous aspect of the invention, the target
covers both the spindle and the rim of the yoke as well as forming
the vault, thereby eliminating any yoke sputtering.
[0026] According to another aspect of the invention, the relative
amount of sputtering of the top wall of the inverted vault relative
to the sidewalls may be controlled by increasing the magnetic flux
in the area of the top wall. An increase of magnetic flux at the
sidewalls may result in a predominantly radial distribution of
magnetic field between the two sidewalls, resulting in large
sputtering of the sidewalls.
[0027] One approach for increasing the sputtering of the top wall
places additional magnets above the top wall with magnetic
polarities aligned with the magnets just outside of the vault
sidewalls. Another approach uses only the top wall magnets to the
exclusion of the sidewall magnets. In this approach, the back of
the target can be planar with no indentations for the central well
or the exterior of the vault sidewalls. In yet another approach,
vertical magnets are positioned near the bottom of the vault
sidewalls with vertical magnetic polarities opposed to those the
corresponding magnets near the top of the vault sidewalls, thereby
creating semi-toroidal fields near the bottom sidewalls. Such
fields can be adjusted either for sputtering or for primarily
extending the top wall plasma toward the bottom of the vault and
repelling its electrons from the sidewalls. A yet further approach
scans over top wall a small, closed magnetron having a central
magnetic pole of one polarity and a surrounding magnetic pole of
the other polarity.
[0028] Various magnetron configurations are possible for use with
the vaulted target. A particularly advantageous design includes an
annular inner sidewall magnet of one polarity, an outer sidewall
magnet of the other polarity, and a roof magnet that rotates about
the central axis. The roof magnet may be composed of an annular
outer magnet of the second polarity surrounding an inner magnet of
the first polarity. The inner sidewall magnet is preferably divided
into two axial portions separated by a non-magnetic spacer, thereby
smoothing the erosion pattern on the inner target sidewall because
the magnetic field is curved towards the non-magnetic; however,
although the non-magnetic spacer is not required for all aspects of
the invention.
[0029] The invention also includes a two-step sputtering process,
the first producing high-energy ionized copper sputter ions, the
second producing a more neutral, lower-energy sputter flux. The
two-step process can be combined with an integrated copper fill
process in which the first step provides high sidewall coverage and
may break through the bottom barrier layer and clean the copper.
The second step completes the seed layer. Thereafter, copper is
electrochemically deposited in the hole. For sputtering into a
dual-damascene structure, the conditions are preferably set so that
the first step sputters the barrier from the bottom of the via hole
but not from the more accessible trench floor.
[0030] After forming a first level of metal on a wafer and pattern
etching a single or dual damascene structure for a second level of
metal on the wafer, the wafer is processed in a PVD cluster tool to
deposit a barrier layer and a seed layer for the second metal
level.
[0031] Instead of using a pre-clean step (for example, a sputter
etch clean step), in accordance with one aspect of the present
invention, a simultaneous clean-deposition step (i.e., a self-clean
deposition step) is carried out. The inventive self clean
deposition is carried out using a PVD deposition chamber that is
capable of producing high-energy ionized target material. In
accordance with one embodiment of the present invention, the
high-energy ions physically remove material on flat areas of a
wafer. In addition, the high-energy ions can dislodge material from
a barrier layer disposed at the bottom of a contact/via feature.
Further, in accordance with one embodiment of the present
invention, wherein an initial thickness of the barrier layer is
small, the high-energy ions can removed enough material from the
barrier layer to provide direct contact between a seed layer and
the underlying metal (for example, between a copper underlying
layer and a copper seed layer). In addition to providing direct
contact between the two copper layers, the inventive sputtering
process also causes redeposition of copper over sidewalls of the
contact/via to reinforce the thickness of the copper seed layer on
the sidewall. This provides an improved path for current
conduction, and advantageously improves the conformality of a layer
subsequently deposited by electroplating.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] FIG. 1 is a schematic cross-sectional view of a first
embodiment of a magnetron sputter reactor of the invention using a
stationary, circularly symmetric magnetron.
[0033] FIG. 2 is a schematic cross-sectional diagram illustrating
the collimating function of the target of the invention.
[0034] FIG. 3 is a schematic cross-sectional view of a second
embodiment of a magnetron sputter reactor of the invention using a
rotating, segmented magnetron with vertically magnetized
magnets.
[0035] FIG. 4 is a schematic cross-sectional view of a third
embodiment of a magnetron sputter reactor of the invention using a
rotating, segmented magnetron with radially magnetized magnets.
[0036] FIG. 5 is a schematic cross-sectional view of a fourth
embodiment of a magnetron sputter reactor of the invention using an
electromagnetic coil.
[0037] FIG. 6 is a cross-sectional view of a fifth embodiment of a
magnetron of the invention using additional magnets at the roof of
the vault to increase the roof sputtering.
[0038] FIG. 7 is a cross-sectional view of a sixth embodiment of a
magnetron of the invention using only the vault magnets.
[0039] FIG. 8 is a cross-sectional view of a seventh embodiment of
a magnetron of the invention using additional confinement magnets
at the bottom sidewall of the vault.
[0040] FIG. 9 is a cross-sectional view of an eighth embodiment of
a magnetron of the invention using a closed magnetron over the
vault roof and separate magnets for the vault sidewalls.
[0041] FIGS. 10-12 are cross-sectional views of ninth through
eleventh embodiments of magnetrons of the invention.
[0042] FIGS. 13 and 14 are respectively a cross-sectional view and
a schematic plan view of a twelfth embodiment of the invention
using stationary outer sidewall magnets and rotating inner sidewall
magnets.
[0043] FIG. 15 is a schematic plan view of a variant of the twelfth
embodiment.
[0044] FIG. 16 is a cross-sectional view of the target and
magnetron of the twelfth embodiment illustrating the resultant
magnetic field.
[0045] FIG. 17 is a graph of sputtering yield as a function of
copper ion energy.
[0046] FIGS. 18 and 19 are cross-sectional views illustrating the
effects of high-energy ionized sputter deposition, particularly the
effect of a high-energy copper PVD deposition removing the barrier
layer at the bottom of the via.
[0047] FIG. 20 is a cross-sectional view illustrating how one
copper PVD reactor can be used to both remove the barrier at the
via bottom and to deposit a copper layer in its place.
[0048] FIG. 21 is a sectioned orthographic view of a desired
barrier layer in a dual-damascene interconnect.
[0049] FIGS. 22 and 23 are cross-sectional view of a desirable
structure for a barrier layer and copper seed layer in a
dual-damascene interconnect.
[0050] FIG. 24 is a flow diagram of a process usable for achieving
the desired interconnects of FIGS. 20 and 23 including a
electroplating via fill.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0051] The invention uses a complexly shaped sputter target and a
specially shaped magnetron which have the combined effect of
impressing a magnetic field producing a thick region of relatively
high plasma density. As a result, a large fraction of the metal
atoms sputtered from the target can be ionized as they pass through
the plasma region. Sputtered metal ions can be advantageously
controlled by substrate biasing to coat the walls of a deep, narrow
hole and to selectively interact with the already deposited barrier
layer dependent upon the local geometry.
[0052] The inventive apparatus has been used to achieve several
novel processes involving selective removal of layers at the bottom
of high aspect-ratio holes and the selective sputter deposition on
areas dependent upon their geometries. Multi-step processes
involving both removal and deposition can be performed in the same
sputter reactor, for example, the inventive reactor with a novel
target and associated magnetron.
Apparatus
[0053] A magnetron sputter reactor 10 of a first embodiment of the
invention is illustrated in the schematic cross-sectional view of
FIG. 1. It includes specially shaped sputter target 12 and
magnetron 14 symmetrically arranged about a central axis 16 in a
reactor otherwise described for the most part by Chiang et al. in
the above referenced patent application. This reactor and
associated processes will be referred to as SIP.sup.+ sputtering in
contrast to the SIP sputter reactor of Chiang et al., which uses a
planar target. The shaped target 12 or at least its interior
surface is composed of the material to be sputter deposited. The
invention is particularly useful for sputtering copper, but it may
be applied to other sputtering materials as well. It is understood
that target may be composed of alloys, typically to less than 10%
of alloying. For example, copper is often alloyed with silicon,
aluminum, or magnesium. As is known, reactive sputtering of
materials like TiN and TaN can be accomplished by using a Ti or Ta
target and including gaseous nitrogen in the plasma. Other
combinations of metal targets and reactive gases are possible.
[0054] The target 12 includes an annularly shaped downwardly facing
vault 18 opposed to a wafer 20 being sputter coated. The vault 18
could alternatively be characterized as an inverted annular trough
or moat. The vault 18 has an aspect ratio of its depth to radial
width of at least 0.5:1 and preferably at least 1:1. The tested
embodiment had a vault width of 7.5 cm and an aspect ratio of
1.4:1. The vault 18 has an cylindrical outer sidewall 22 outside of
the periphery of the wafer 20, a cylindrical inner sidewall 24
overlying the wafer 20, and a generally flat, annular vault top
wall or roof 25 (which extends across the space between the annular
sidewalls 22, 24 and closes the bottom of the downwardly facing
vault 18). The sidewalls 20, 22 in this embodiment extend generally
parallel to the central axis 16, and the roof 25 extends generally
perpendicularly. The target 12 also includes a central portion
forming a spindle 26 including the inner sidewall 24 and a
generally planar face 28 spanning the space formed at the bottom
terminus of the inner sidewall 24 in parallel opposition to the
wafer 20. The target 12 is continuous across its parts 22, 25, 24,
28 with no structure intervening between these parts and the
process space between the target 12 and the wafer 20. The target 12
also includes a flange 29 which extends radially outwardly from
near the terminus of the outer sidewall 22 and which is vacuum
sealed to the lower chamber body of the sputter reactor 10.
[0055] The magnetron 14 of the embodiment illustrated in FIG. 1
includes one or more central magnets 30 having a first vertical
magnetic polarity and one or more outer magnets 32 of a second
vertical magnetic polarity opposite the first polarity and arranged
in an annular pattern. That is, one is N-S; the other, S-N. In this
embodiment the magnets 30, 32 are permanent magnets, that is,
composed of strongly ferromagnetic material and are stationary. The
inner magnets 30 are radially disposed within or axially upward of
a cylindrical central well 36 formed in the spindle 26 and between
the opposed portions of the inner target sidewall 24 while the
outer magnets 32 are disposed generally radially outside of the
outer target sidewall 22. A circular magnetic yoke 34 magnetically
couples tops of the inner and outer magnets 30, 32. The yoke is
composed of a magnetically soft material, for example, a
paramagnetic material, such as SS410 stainless steel, that can be
magnetized to thereby form a magnetic circuit for the magnetism
produced by the permanent magnets 30, 32. Permanently magnetized
yokes are possible but are difficult to obtain in a circular
geometry.
[0056] A cylindrical inner pole piece 40 of a similarly
magnetically soft material abuts the lower ends of the inner
magnets 30 and extends deep within the target well 36 adjacent to
the inner target sidewall 24 to produce, in this case, a N pole
within the well 36. (It is of course appreciated that the selection
of an N or S pole is for the most part arbitrary because almost all
practical magnetic effects depend only upon the relative polarities
of different poles.) If the magnetron 14 is generally circularly
symmetric, it is not necessary to rotate it for uniformity of
sputter deposition. A tubular outer pole piece 42 of a magnetically
soft material abuts the lower end of the outer magnets 32 and
extends downwardly outside of the outer target sidewall 22. The
magnetic pole pieces 40, 42 of FIG. 1 differ from the usual pole
faces in that they and the magnets 30, 32 are configured and sized
to emit a magnetic field B in the target vault 18 that is largely
perpendicular to the magnetic field of the corresponding associated
magnets 30, 32. In particular, the magnetic field B is generally
perpendicular to the target vault sidewalls 22, 24.
[0057] This configuration has several advantages. First, the
electrons trapped by the magnetic field B, although gyrating about
the field lines, otherwise travel generally horizontally and
radially with respect to the target central axis 16. Plasma sheaths
are formed on both vault sidewalls 22, 24 which reflect electrons
traveling along the magnetic field lines toward the vault
sidewalls. As a result, the electrons are substantially bound
within the vault 18, and electron loss is minimized, thus
increasing the plasma density. Secondly, the vertical depth of the
magnetic field B intensifying the plasma density is determined by
the height of the target sidewalls 22, 24. This depth can be
considerably greater than that of a high-density plasma region
created by magnets in back of a planar target. As a result,
sputtered atoms traverse a larger region of a high-density plasma
and are accordingly more likely to become ionized. The support
structure for the magnetron 14 and its parts is not illustrated but
can be easily designed by the ordinary mechanic. The support
structure usually includes an overlying cover shielding and
supporting the magnetron.
[0058] The remainder of the sputter reactor 10 is similar to that
described by Chiang et al. in the above referenced patent
application although a short-throw rather than a long-throw
configuration may be used. Long throw is defined by Chiang et al.
as the separation between the target and wafer as being at least
80% and preferably at least 140% of the wafer diameter. The target
12 is vacuum sealed to a grounded vacuum chamber body 50 through a
dielectric target isolator 52. The wafer 20 is clamped to a heater
pedestal electrode 54 by, for example, a clamp ring 56 although
electrostatic chucking is possible. An electrically grounded shield
58 acts as an anode with respect to the cathode target 12, which is
negatively biased by a variable DC power supply 60. DC magnetron
sputtering is conventional in commercial applications, but RF
sputtering can enjoy the advantages of the target and magnetron of
the invention and is especially advantageous for sputtering
non-metallic targets. An electrically floating shield 62 is
supported on the electrically grounded shield 58 or chamber 50
through a dielectric shield isolator 64. An annular cylindrical
knob 66 extending downwardly from the outer target sidewall 22 and
positioned inwardly of the uppermost part of the floating shield 62
protects the upper portion of the floating shield 62 and the target
isolator 52 from being sputter deposited from the strong plasma
disposed within and slightly vertically outwardly of the target
vault 18. The gap between the upper portion of the floating shield
62 and the target knob 66 and flange 29 is small enough to act as a
dark space preventing the plasma from propagating into the gap.
[0059] A working gas such as argon is supplied into the chamber
from a gas source 68 through a mass flow controller 70. A vacuum
pumping system 72 maintains the chamber at a reduced pressure,
typically a base pressure in the neighborhood of 10.sup.-8 Torr.
Although a floating pedestal electrode 54 can develop a desired
negative self-bias, it is typical in high plasma-density sputtering
for an RF power supply 74 to RF bias the pedestal electrode 54
through an isolation capacitor, which results in a controlled
negative DC self-bias. A controller 76 regulates the power supplies
60, 74, mass flow controller 70, and vacuum system 72 according to
a sputtering recipe prerecorded in it with recordable magnetic or
optical media.
[0060] The structures of the target and magnetron have several
advantages. As mentioned previously, secondary electrons are
largely trapped within the vault 18 with little loss even upon
collision with the target sidewalls 22, 24, more specifically
reflected from the plasma sheaths adjacent the sidewalls. Also, the
plasma thickness is relatively large, determined by the sidewall
heights, thereby increasing the ionization fraction of the
sputtered target atoms. The separation of the inner and outer poles
40, 42 is relatively small, thereby increasing the magnetic field
intensity within the vault 18. The target 12 is continuous across
the pole pieces 40, 42, thus preventing the magnetic material of
the poles from being sputtered and deposited on the semiconductor
wafer 20.
[0061] The relatively high ionization fraction allows this fraction
of the sputtered target atoms to have their trajectories toward the
wafer be controlled both by the magnetic field looping from the
target toward the wafer and by the electric field induced by the DC
self-bias applied to the pedestal. Increasing the DC self-bias
draws the ions into high aspect-ratio holes, thereby allowing high
bottom and sidewall coverage of such high aspect-ratio holes. On
the other hand, the ionization fraction is less than 100%, and the
remaining sputtered atoms are neutral. In some situations a finite
neutral component is useful, and the ratio of neutrals to ions can
be controlled by adjusting power levels and chamber pressures.
[0062] The high aspect ratio of the vault 18 also improves the
symmetric filling of holes located near the edge of the wafer,
particularly in configurations having a shorter throw than that
illustrated in FIG. 1. As schematically illustrated in FIG. 2, a
hole 78 located at the right edge of the wafer 20 is to have a
conformal layer sputter deposited on its sides. The size of the
hole 78 and the thickness of the wafer 20 are greatly exaggerated,
but the geometry remains approximately valid. If a planar target
were being used, the right side of the wafer hole 78 would see a
much larger fraction of the target than the left side and would
thus be coated with a commensurately thicker layer. However, with
the vault-shaped target 12, the hole 78 sees neither the inner
sidewall 24 of the left side of the vault 18 nor the left vault top
wall 25. Even the upper portion of the outer sidewall 22 of the
left side of the vault 18 is shielded from the wafer hole 78 by the
inner sidewall 24 of the left side of the vault 18. As a result,
the two sidewalls of the hole 78 to be coated see areas of the
vault-shaped target that are much closer in size than for a planar
target, and the sidewall coating symmetry is thereby greatly
increased.
[0063] The target structure, as a result, can provide sputtered
particles having trajectories preferentially aligned
perpendicularly to the wafer surface, but without an apertured
collimator, which tends to become clogged with sputtered material.
The effect is increased by a high aspect ratio for the vault,
preferably at least 1:2, and more preferably at least 1:1. The
tested target had a vault with an aspect ratio of 1.4:1.
[0064] A sputter reactor 80 of second embodiment of the invention
is illustrated in the schematic cross-sectional view of FIG. 3. A
magnetron 82 includes the previously described inner magnets 30 and
inner pole piece 40. However, one or more outer magnets 84 and an
outer pole piece 86 extend around only a segment of the
circumference of the target, for example between 15.degree. and
90.degree.. An asymmetric magnetic yoke 88 shaped as a sector
magnetically couples the inner and outer magnets 30, 84 but only on
the side of target well 36 toward the outer magnets 84. In fact, a
circular yoke 88, although larger, would not affect the operative
magnetic field. As a result, a high-density plasma is generated in
only a small circumferential portion of the target vault 18. For
self-ionized plating (SIP) and particularly sustained
self-sputtering (SSS), a high plasma density is desired. In view of
the limited capacity of realistic power supplies 60, the high
plasma density can be achieved by reducing the volume of the
magnetron 82.
[0065] To achieve uniform sputtering, a motor 90 is supported on
the chamber body 50 through a cylindrical sidewall 92 and roof 94
preferably electrically isolated from the biased target flange 29.
The motor 90 has a motor shaft connected to the yoke 88 at the
target axis 16 and rotates the magnetron 82 about that axis 16 at a
few hundred rpm. Mechanical counterbalancing may be provided to
reduce vibration in the rotation of the axially offset magnetron
82. The mechanical details are not accurately represented in FIG. 3
but will be described more completely below.
[0066] Other magnet configurations are possible to produce similar
magnetic field distributions. A sputter reactor 100 of a third
embodiment of the invention is illustrated in the schematic
cross-sectional view of FIG. 4 A magnetron 102 includes an inner
magnet 104 having a magnetization direction generally aligned with
a radius of the target 12 about the target axis 16. One or more
outer magnets 106 are similarly radially magnetized but
anti-parallel to the magnetization of the inner magnet 104 with
respect to the center of the vault 18. A C-shaped magnetic yoke has
two arms 110, 112 in back of and supporting the respective magnets
104, 106 and a connector 114 supported on and rotated by the shaft
of the motor 90.
[0067] The magnets 104, 106 may be advantageously positioned only
on reduced circumferential portions of the sidewalls 24, 22 of the
target vault 18 so as to concentrate the magnetic field there.
Furthermore, in this configuration extending along only a small
segment of the target periphery, the magnets 104, 106 may be
conveniently formed of plate magnets.
[0068] Electromagnetic coils may replace the permanent magnets of
the previously described embodiments. A sputter reactor 120 of a
fourth embodiment of the invention is illustrated in the schematic
cross-sectional view of FIG. 5. A magnetron 122 includes a magnetic
yoke including a central spindle 124 fit into the well 36 of the
target 12 and a tubular rim 126 surrounding the outer sidewall 24
of the target vault 18. The magnetic yoke also includes a generally
circular back piece 128 magnetically coupling the spindle 124 and
the rim 126. An electromagnetic coil 130 is wound around the
spindle 124 below the back piece 128 and inside of the rim 126. The
coil 130 is preferably powered by a DC electrical source but a
low-frequency AC source can be used. The coil 130 in conjunction
with the magnetic yoke creates a generally radial magnetic field in
the target vault 18.
[0069] The previously described embodiments have emphasized
sputtering the vault sidewalls 22, 24 preferentially to sputtering
the vault top wall or roof 25 (see FIG. 1) since relatively few of
the magnetic field lines terminate on the vault roof 25. The metal
ionization fraction can be increased if sputtering is increased in
the vault roof 25 since the plasma thickness experienced by the
average sputtered atom is increased. Also, the directionality of
sputtered material leaving the vault 18 is increased.
[0070] The increased roof sputtering can be achieved in a number of
ways. In a fifth embodiment of a magnetron 140 illustrated in
cross-section in FIG. 6 with the remainder of the sputtering
chamber being similar to the parts illustrated in FIG. 3. A target
142 is similar to the previously described target 12 except for a
thinner roof portion 144. Similarly to the magnetron 82 of FIG. 3,
it includes the rotatable yoke 88 supporting the inner magnets 30
of a first vertical polarity magnetically coupled to the inner pole
piece 40 and the outer magnets 84 of a second vertical polarity
magnetically coupled to the outer pole piece 86. These magnets 30,
84 and pole pieces 40, 86 produce a generally radial magnetic field
B extending between the sidewalls 22, 24 of the vault 18. The
magnetron 82 additionally supports on the magnetic yoke 88 an inner
roof magnet 146 of the first vertical polarization aligned with the
inner magnets 30 and an outer roof magnet 148 of the second
vertical polarization aligned with the outer magnets 86. The
opposed roof magnets 146, 148 magnetically coupled by the yoke 88
produce a semi-toroidal magnetic field B penetrating the vault roof
144 at two locations. Thereby, electrons are trapped along the
semi-toroidal magnetic field and increase the plasma density near
the vault roof 144, thereby increasing the sputtering of the vault
roof 144.
[0071] In the illustrated embodiment, the outer magnets 84 and
outer pole piece 86 occupy only a segment of the periphery of the
target 142 but are rotated along that periphery by the motor 90.
Similarly, inner and outer roof magnets 146, 148 extend only along
a corresponding segment angle. However, a corresponding
non-rotating magnetron can be created by making the roof magnets
146, 148, outer magnet 84, and outer pole piece 86 in annular
shapes. The same circularly symmetric modification may be made to
the embodiments described below.
[0072] The roof sputtering can be further emphasized by a sixth
embodiment of a magnetron 150, illustrated in FIG. 7, which
includes the inner and outer roof magnets 146, 148 but which in the
illustrated embodiment includes neither the inner magnets within
the well 36 nor the outer magnets outside of the outer sidewall 22.
This configuration produces a relatively strong semi-toroidal
magnetic field B adjacent to the vault roof 144 and a weaker
magnetic field B in the body of the vault 18 adjacent to the
sidewalls 22, 24. Therefore, there will be much more sputtering of
the roof 144 than of the sidewalls 22, 24. Nonetheless, magnetic
field lines in the vault body terminate at the sidewalls 22, 24,
thereby decreasing electron loss out of the plasma. Hence, the
magnetic field intensity may be low in the vault, but the plasma
density is still kept relatively high there so that the target
atoms sputtered from the roof 144 still traverse a thick plasma
region and are accordingly efficiently ionized.
[0073] Since no magnets or pole pieces are placed in the target
well 36 or outside of the outer target sidewall 22 and assuming the
target material is non-magnetic, the inner and outer sidewalls 24,
22 may be increased in thickness even to the point that there is no
well and no appreciable volume between the outer sidewall 22 and
the chamber wall. That is, the back of the target 142 may have a
substantially planar face 152, 154, 156. However, the inventive
design of this embodiment still differs from a target having a
circularly corrugated surface in that the spacing of the opposed
roof magnets 146, 148 is at least half of the radial vault
dimension and preferably closer to unity. This is in contrast to
the embodiments of FIGS. 1, 3, and 4 in which the two sets of
magnets are separated preferably by between about 100% and 150% of
the vault width. Alternatively stated, the width of the vault 18 in
the radial direction should be at least 2.5 cm, preferably at least
5 cm, and most preferably at least 10 cm. These dimensions,
combined with the vault aspect ratio being at least 1:2 assures
that the vault width is at least 10 times and preferably at least
25 times the plasma dark space, thus guaranteeing that the plasma
conforms to the shape of the vault 18. These vault widths are
easily accommodated in a sputter reactor sized for a 200 mm wafer.
For larger wafers, more complex target shapes become even easier to
implement.
[0074] A seventh embodiment of a magnetron 160 illustrated in the
cross-sectional view of FIG. 8 includes the inner and outer main
magnets 30, 84, although they are preferably somewhat shorter and
do not extend below the vault roof 144. The magnetron also includes
the inner and outer roof magnets 146, 148. However, neither the
inner pole piece nor the outer pole piece needs to be used to
couple the magnetic field from the main magnets 30, 84 into the
vault 18. Instead, all these magnets produce a horizontally
oriented semi-toroidal field B adjacent the vault roof 144. Some of
these magnets may be eliminated as long as there are opposed
magnets associated with the inner and outer target sidewalls 22,
24. Instead of ferromagnetic or paramagnetic pole pieces,
non-magnetic (e.g. aluminum or hard stainless steel) or even
diamagnetic spacers 162, 164 are supported below the inner and
outer main magnets 30, 84 respectively. Henceforth, nonmagnetic
materials will be assumed to include diamagnetic materials unless
specifically stated otherwise. The inner spacer 162 supports on its
lower end an inner sidewall magnet 166 of the second magnetic
polarity, that is, opposite that of its associated main inner
magnet 30. Similarly, the outer spacer 164 supports on its lower
end an outer sidewall magnet 168 of the first magnetic polarity,
that is, opposite that of its associated main outer magnet 84. Both
the sidewall magnets 166, 168 are located near the bottom of the
respective vault sidewalls 24, 22. Because, they have polarities
opposed to those of their associated main magnets 30, 84 they
create two generally vertically extending semi-toroidal magnetic
fields B' and B" near the bottom of the vault sidewalls 24, 22.
Because of their opposed magnetic orientations, the sidewall
magnets 166, 168 create two anti-parallel components of radial
magnetic field across the vault 18. However, because of the
relative spacings of the poles, the semi-toroidal magnetic fields
B' and B" dominate.
[0075] In one sub-embodiment, the horizontally extending magnetic
field B near the vault roof 144 is much stronger than the
vertically extending magnetic fields B' and B" near the vault
sidewalls 24, 22. As a result, sputtering of the roof 144
predominates. Alternatively, increased sidewall fields B' and B"
can increase the amount of sidewall sputtering in a controlled way.
In any case, the vertically extending sidewall fields B' and B" are
sufficient to support a plasma throughout much of the body of the
vault 18. Also, the sidewall fields B' and B" are oriented to repel
electrons in the plasma flux from the roof 144, thereby decreasing
the electron loss of that plasma.
[0076] All of the previous embodiments have used magnets that
extend generally along either the entire circumference or a segment
of the circumference of various radii of the target. However, an
eighth embodiment of a magnetron 170 illustrated in the
cross-section view of FIG. 9 treats the planar vault roof 144
distinctly differently than the band-shaped vault sidewalls 22, 24.
The sidewall magnetic assembly is similar to that of FIG. 6 and
includes the rotatable yoke 88 supporting the inner magnets 30 of a
first vertical polarization magnetically coupled to the inner pole
piece 40 and the segmented outer magnets 84 of an opposed second
vertical polarization magnetically coupled to the outer pole piece.
These produce a generally radially directed magnetic field B across
the vault 18. The rotating magnetic yoke 88 also supports a closed
magnetron over the vault roof 144 including an inner magnet 172 of
one vertical magnetic polarization and a surrounding outer magnet
174 of the other vertical magnetic polarization producing between
them a cusp-shaped magnetic field B' adjacent the vault roof 144.
In the simplest sub-embodiment, the inner magnet 172 is
cylindrical, and the outer magnet 174 is annular or tubular,
surrounds the inner magnet 172, thereby producing a circularly
symmetric cusp field B'. However, other shapes are possible, such
as a radially or circumferentially aligned racetrack or a pair of
nested segment-shaped magnets. The roof magnetron of FIG. 9 is the
general type of magnetron described by Fu and by Chiang et al. in
the previously referenced patent applications for SIP sputtering of
a planar target, and those references provide guidance on the
design of such a closed unbalanced magnetron having a strong outer
pole surrounding a weaker inner pole of the opposite polarity.
[0077] The figure does not adequately illustrate the magnetic yoke
88 which in the conceptually simplest implementation would
magnetically isolate the roof magnets 172, 174 from the sidewall
magnets 30, 84 while still magnetically coupling together the roof
magnets 172, 174 and separately coupling together the sidewall
magnets 30, 84. However, in view of the large number of magnets, a
more complex magnetic circuit can be envisioned.
[0078] As has been shown in the cited patent applications, such a
small closed roof magnetron will be very effective in highly
ionized sputtering of the target roof 144. The sidewall magnets 30,
84 on the other hand will extend the plasma region down the height
of the sidewalls 22, 24 as well as cause a degree of sidewall
sputtering depending on the relative magnetic intensities.
[0079] The relative magnetic polarizations of roof magnets 172, 174
relative to those of the sidewall magnets 30, 84 may be varied.
Also, the sidewall magnets 30, 84 and particularly the outer
sidewall magnet 84 may be made fully annular so as to close on
themselves so that optionally they do not need to be rotated and
may be coupled by their own stationary yoke while the roof magnets
172, 174 do rotate about the circular planar area on the back of
the vault roof 144 and are coupled by their rotating own yoke.
[0080] Other combinations of the closed roof magnetron and the
sidewall magnets of other embodiments are possible.
[0081] A ninth embodiment of a magnetron 180 of the invention is
illustrated in the cross-sectional view of FIG. 10 and includes the
inner and outer magnets 172, 174 overlying the vault roof 144. Side
magnets 180, 182 disposed outside of the vault sidewalls 142 have
opposed vertical magnetic polarities but they are largely decoupled
from the roof magnets 172, 174 because they are supported on the
magnetic yoke 88 by non-magnetic supports 186, 188. As a result,
the side magnets 182, 184 create a magnetic field B in the vault 18
that has two generally anti-parallel components extending radially
across the vault 18 as well as two components extending generally
parallel to the vault sidewalls. Thus, the magnetic field B extends
over a substantial depth of the vault 18 and further repels
electrons from the sidewalls. In the illustrated embodiment, all
the side magnets 182, 184 are segmented and rotate with the roof
magnets 172, 174. However, a mechanically simpler design forms the
side magnets 182, 184 in annular shapes and leaves one or both of
them stationary. As illustrated, the polarities are such that the
top pole of the inner side magnet 182 has the same polarity as the
bottom pole of the adjacent annular top magnet 174 while the outer
side magnet 180 has the opposite relationship with the annular top
magnet 174. However, these polarities may be varied.
[0082] A tenth embodiment 190 illustrated in the cross-sectional
view of FIG. 11 is similar to the magnetron 180 of FIG. 10 except
that an inner side magnet 192 is smaller than the outer side magnet
184, thereby allowing tailoring of the magnetic fields on the two
vault sidewalls. The opposite size relationship is also
possible.
[0083] An eleventh embodiment 200 illustrated in the
cross-sectional view of FIG. 12 dispenses with the top magnets and
uses only the two side magnets 182, 184 which may be of the same
size or of unequal size. In this case, the yoke 88 need not be
magnetic.
[0084] A twelfth embodiment 210 illustrated in the cross-sectional
side view of FIG. 13 is the subject of U.S. patent application Ser.
No. 09/703,738, filed on Nov. 11, 2000 by A. Subramani et al.,
incorporated herein by reference in its entirety. This embodiment
has similar functionality to that of FIG. 10 but has further
capabilities.
[0085] The illustrated upper part of the sputtering chamber
includes a cylindrical wall composed of a bottom frame 212 and a
top frame 214, on which is supported a chamber roof 216. The
SIP.sup.+ vault-shaped target 12 is fixed to the bottom frame 212.
All these parts are sealed together to allow cooling water to
circulate in a space 218 in back of the target 12. The vault-shaped
target 12 includes the annular vault 18 having the outer sidewall
22, the inner sidewall 24, and the vault roof 25, all generally
circularly symmetric with respect to the vertical chamber axis 16.
The inner and outer vault sidewalls 22, 24 extend generally
parallel to the chamber axis 16 while the vault roof 25 extends
generally perpendicularly thereto. That is, the vault 18 is
annularly shaped with a generally rectangular cross section.
[0086] A magnetron 220 is placed in back of the vaulted target 12
in close association with the vault 18. The magnetron 220 includes
a stationary ring-shaped outer sidewall magnet assembly 222 placed
outside the outer vault sidewall 22 and having a first vertical
magnetic polarity. The preferred structure for the outer sidewall
magnets 224 is more complicated than that illustrated, as is
described in the patent application to Subramani et al., but the
functions remain much the same. A rotatable inner sidewall magnet
assembly 224 includes an upper tubular magnet 226 and a lower
tubular magnet 228 separated by a non-magnetic tubular spacer 230
having an axial length at least half the respective lengths of the
two tubular magnets 226, 228. The two tubular magnets 226, 228 have
a same second vertical magnetic polarity opposite that of the outer
sidewall magnet 222. However, the non-magnetic spacer 230 is not
required, and other magnet configurations may be selected to
achieve a desired erosion pattern. The bottom of the lower tubular
magnet 228 is separated from the back of a central planar portion
232 of the vaulted target 12 by a small gap 234 having an axial
extent of between 0.5 to 1.5 mm.
[0087] The magnetron also includes a rotatable roof magnet assembly
236 in a nested arrangement of an outer ring magnet 238, generally
circularly shaped, having the first magnetic polarity surrounding
an inner rod magnet 240 having the second magnetic polarity and a
magnetic yoke 242 supporting and magnetically coupling the magnets
238, 240. It is preferred that the total magnetic flux of the outer
ring magnet 238 be substantially greater than that of the inner
magnet 240, for example, having a ratio of at least 1.5. It is
preferred, although not required, that the magnetic polarity of the
outer ring magnet 238 be anti-parallel to that of the inner
sidewall magnet 224 so as to avoid strong magnetic fields adjacent
to the inner upper corner of the target vault 18 and instead to
intensify the magnetic field at the outer upper corner, which is
being more quickly scanned. The metal ions produced in the very
high-density plasma adjacent the roof are focused by the sidewall
magnets 222, 224 into a column directed to the wafer.
[0088] Both the inner sidewall magnet 224 and the roof magnet
assembly 236 are rotatable about the chamber axis 16. The inner
sidewall magnet 224 is connected to and supported by a shaft 244
rotated about the chamber axis 16 by a motor 246. The magnetic yoke
242 supporting the roof magnets 238, 240 is also fixed to the
rotating shaft 244.
[0089] The motor shaft 244 and the inner sidewall magnet 224
includes an inner passageway 250 configured for passage of cooling
fluid, usually water, supplied from a chiller 252 through an inlet
port 254 to a rotary union 256 connected to the motor shaft 244.
The cooling water flows from the bottom of the inner sidewall
magnet 224 through the gap 232 at the bottom of the inner sidewall
magnet 224. It then flows upwardly between the inner vault sidewall
24 and the inner sidewall magnet 224. The rotating roof magnet
assembly 236 stirs up the cooling water in the back of the target
12, thereby increasing its turbulence and cooling efficiency. The
cooling water then flows down next to the outer vault sidewall 22.
As explained by Subramani et al., the tubular outer sidewall magnet
assembly 222 is composed of a large number of rod magnets, and they
are separated from the actual walls of the target 12. As a result,
the cooling water can flow both through and below the outer
sidewall magnet 222 to one of several outlets 258 in the bottom
frame 212 and then through several risers 260 in the frames 212,
214 to an outlet port 262 in the upper frame 214, whence the warmed
cooling water is returned to the chiller 252. This cooling design
has the advantage of supplying the coldest water to the hottest,
central portions of the target 12.
[0090] Even though the inner sidewall magnet 224 is rotating, its
circular symmetry causes it to produce the same magnetic field as
that produced by a stationary cylindrical magnet. A schematic plan
view of the magnets is shown in FIG. 14. This figure is intended to
represent the effective magnetic poles rather than actual magnets.
The labeled polarities correspond to the uppermost poles and do not
necessarily reflect the effective polarities within the vault 18.
The inner sidewall magnet 224 is included within the inner vault
sidewall 24 and is essentially circularly symmetric even though it
may be rotating. Similarly the outer sidewall magnet 222 is
positioned on the radial exterior of the outer vault sidewall 22,
and it also is substantially circularly symmetric. The roof magnet
assembly 236 including the outer and inner roof magnets 238, 240 is
positioned over the vault roof between the outer and inner
sidewalls 22, 24, and it rotates about the center of the target. It
is apparent that the roof magnet assembly 236 occupies less than
20% of the area of the vault 18, and its effective magnetic fields
occupy less than 10%. These factors provide corresponding increases
in effective target power densities. Nonetheless, circumferential
scanning provides uniform sputtering of the target.
[0091] It is possible to increase the number of roof magnet
assemblies. For example, as illustrated in the schematic plan view
of FIG. 15, a second roof magnet assembly 260 has outer and inner
magnets 262, 262 of sizes and polarities matched to those of the
first roof magnet assembly 236. The second roof magnet assembly 260
is disposed over the vault 18 diametrically opposite the first roof
magnet assembly 236 and is rotated with it. Additional roof magnet
assemblies may be added. While the multiple roof magnet assemblies
increase the sputtering rate, particularly of metal ions, they
require additional power to achieve the same peak plasma
density.
[0092] The asymmetry between the roof magnet assembly 236 and the
sidewall magnets 222, 234 for the embodiment of FIGS. 13 and 14
produces distinctly different magnetic field strengths and
distributions in different parts of the vault 18, as is
schematically illustrated in FIG. 16. In the portion of the vault
18 with the roof magnet assembly 236, there is a strong magnetic
field B adjacent the vault roof 25. With the illustrated magnetic
polarities, the magnetic field is relatively weaker at the upper
corner of the inner sidewall 24 but much stronger at the upper
corner of the outer sidewall 22. The inclusion of the non-magnetic
spacer 230 between the two tubular magnets 226, 228 of the inner
sidewall magnet assembly 224 produces a magnetic field distribution
that is more parallel to the inner vault sidewall 24, thereby
evening the erosion pattern there. In contrast, as illustrated on
the left side, in portions of the vault 18 distant from the roof
magnet assembly 236, the magnetic field B' has a reduced intensity,
particularly near the vault roof 25. As a result, there is
relatively more sputtering of the vault roof 25 in areas of the
roof magnet assembly 236 than elsewhere, and the metal ionization
fraction in that portion is substantially higher. In contrast,
distant from the roof magnet assembly 236, there is relatively
substantial sputtering of the vault sidewalls 22, 24 with an
increased fraction of neutral metal atoms being produced.
[0093] It is known that low-pressure sputtering requires a
relatively high magnetic field. It is thus possible to select
chamber pressure and target power such that the plasma is supported
only adjacent the roof magnet assembly 236 or to select another
combination of chamber pressure and target power such that the
plasma is supported throughout the vault 18.
[0094] It is thus seen that the complex geometry of the magnetron
and target of the various embodiments of the invention provides
additional controls on the intensity, directionality, and
uniformity of sputtering.
[0095] It is possible to include multiple concentric vaults and to
associate magnetic means with each of them.
[0096] It is also possible to additionally include an RF inductive
coil to increase the plasma density in the processing space between
the target and wafer. However, the unique configurations of the
target and magnetron of the invention in large part eliminate the
need for expensive coils.
[0097] Although the described embodiments have included a magnetron
with a vault having vertical sidewalls and producing a
substantially horizontal magnetic field in the vault. However, it
is appreciated that the magnetic field cannot be completely
controlled, and inclinations of the magnetic field may extend up to
about 25.degree.. Furthermore, the sidewalls may form more of a
V-shaped vault with sidewall slope angles of up to 25.degree., but
a maximum of 10.degree. is preferred.
[0098] Although the invention has been described with respect to
sputtering a coating substantially consisting of the material of
the target, it can be advantageously used as well for reactor
sputtering in which a gas such as nitrogen or oxygen is supplied
into the chamber and reacts with the target material on the wafer
surface to form a nitride or an oxide.
Processes and Structures
[0099] The magnetron 180 of FIG. 12 using stationary annular side
magnets has been used in a number of experiments with sputtering
copper and has shown unusual capabilities. We believe that the
unusual results arise from the enhanced ionization fraction of the
sputtered copper as it passes through the extended magnetic field
in the vault and the restriction of the high-density plasma to only
a portion of the vault. The copper ions can be attracted to the
wafer by the inherent DC self-bias of a floating pedestal and the
attraction can be increased by RF biasing the pedestal. The
controlled attraction controls the energy and directionality of the
copper ions incident on the wafer and deep into the via hole and
allows controlled fractions of ionized and neutral sputtered
atoms.
[0100] The sputtering yield for copper ions as a function of the
ion energy is plotted in FIG. 17. Thus, the higher sputter
particles energies possible with the inventive magnetron and other
magnetrons can produce a high copper yield in the case that the
underlying copper is exposed during sputter processing by
high-energy copper ions. Furthermore, the ratio of sputtering yield
of tantalum relative to copper is 1:4 and further lower for TaN,
thereby providing selectivity over copper. The believed effect of
high-energy sputtered copper is schematically illustrated in the
cross-sectional view of FIG. 18. A substrate 270 is formed with a
lower copper metal feature 272. An inter-level dielectric layer 274
is deposited thereover and photolithographically etched to form a
via hole 276. After pre-cleaning, a thin barrier layer 278 is
substantially conformally coated in the via 276 hole and over the
top of the dielectric 274. The subsequent high-energy copper ion
sputter deposition and resultant resputtering of the copper already
deposited on the wafer reduces the deposition on the field area on
the planar top of the oxide 274 and at the bottom 280 of the via
hole 276. However, the copper atoms resputtered from the bottom of
the via hole 276 is of lower energy than the incident copper ions
and are emitted generally isotropically. As a result, they tend to
coat the via sidewalls 282 even more than the via bottom 220
because the sidewalls 282 are not exposed to the anisotropic
high-energy copper ion column. The bottom sputtering further is
likely to etch through the barrier layer 218 at the via bottom 220,
thus exposing the underlying copper 272. Furthermore, the top layer
of the copper 272 is cleaned in what is generally a PVD process. As
a result, as illustrated in FIG. 19, the barrier layer 278 is
removed at the bottom of the via hole 276 and a recess 284,
experimentally observed to be concave, is formed in the underlying
copper 272. Further, relatively thick copper sidewalls 286 of
thickness d.sub.S are deposited while a copper field layer 288 of
thickness d.sub.B is formed over the planar top of the dielectric
274. Because of the high resputtering, overhangs do not form on the
lip of the via 276. The sidewall coverage d.sub.S/d.sub.B is
observed to be in the neighborhood of 50 to 60% for high target
power and low chamber pressure. The result may be described as
selective PVD.
[0101] The removal of the lower barrier layer has two implications.
The contact resistance is reduced because the barrier material is
removed in the direct current path, specifically at the interface
of the via metal being deposited with the underlying metal feature,
and the copper of the upper-level metallization is in direct
contact with the copper of the lower-level metallization, namely,
the copper feature 272. Furthermore, in prior systems, a high
resistivity oxide of the underlying metal needed to be removed by a
pre-metallization cleaning step. With the invention, the pre-clean
that was necessary for that function to clean the oxide or residue
at the top of the underlying copper 272 prior to depositing the
barrier or seed layer is no longer required to assure direct
contact between the two copper levels because the since the PVD
step is itself removing the barrier and cleaning the underlying
copper or other metal layer. Pre-cleaning on the sidewalls and top
of the dielectric is much less of a requirement and may in some
circumstances be eliminated.
[0102] It is noted that the structure illustrated in FIG. 19 shows
the removal of the barrier layer 278 on the horizontal bottom of
the via hole 276 but its barrier field portion 290 remaining in the
field area on the planar top of the dielectric layer 274. This is
possible if the metal ionization fraction is less than 100% so that
a substantial number of unaccelerated metal neutrals are sputtered
onto the field area. The neutrals, however, are shielded from
reaching the via bottom. As a result, the high-energy metal ions
can sputter the barrier layer 278 at the bottom of the via hole
276, but they are overcome by the lower-energy metal neutrals at
the top, and there is a net deposition of copper and no barrier
removal in the field area on top of the dielectric layer 274. This
result differs from the process disclosed by Geffken et al. in the
above patent in which all horizontally extending barrier layers are
removed.
[0103] The sidewall coverage afforded by the high-energy ionized
sputter deposition is sufficient for use as a seed layer. It is
believed that about 5 nm sidewall coverage is required in 3
.mu.m-deep vias having an 11:1 aspect ratio. However, the copper
field coverage is reduced over the conventional sputtering process
and does not provide a sufficient electrical path for the
electroplating current. Therefore, a short, more conventional
copper sputter process may be used to complete the copper seed
layer and eliminate any voids in it and to thicken the field
coverage. The more conventional sputtering produces not only
lower-energy copper ions but a larger fraction of neutral copper
sputter particles, which do not respond to wafer biasing. The two
steps can be balanced to provide a balance between bottom coverage,
sidewall coverage, and blanket thickness. That is, the conformality
can be tailored. The more conventional copper sputter could be
performed in a separate sputter reactor. However, in view of the
small quantity of copper needed to complete the seed layer, the
same reactor used for the high-energy sputtering can be adjusted to
effect lower-energy sputtering. To accomplish this second step, for
example, the target power can be reduced to reduce the plasma
density and metal ionization fraction, the chamber pressure can be
raised above 1 milliTorr, preferably about 1.5 milliTorr or higher,
to reduce the wafer self-bias, thus reducing the ion energy, and to
decrease the metal ionization fraction, the RF pedestal bias power
can be reduced to decrease the acceleration of ions toward the
wafer, or a combination of the three changes can be made between
the two steps.
[0104] The structure of FIG. 19 is accomplished by producing a
relatively high but not very high copper ionization fraction. It is
possible to perform within a single sputter reactor a two-step
copper PVD process in which the first step produces the structure
of FIG. 19 and the second step is performed with chamber parameters
adjusted to reduce the copper ion energy so that, as illustrated in
the cross-sectional view of FIG. 20, the via bottom is coated with
a second copper layer 292 covering all areas including a via bottom
portion 294. Further, it is noted that the two-step copper PVD
process can be advantageously used even in the case where the first
step does not leave a barrier field layer 290 and a first copper
field layer 288 is not deposited. For single-level damascene, the
field region is subjected to CMP, and these extra layers in the
field region are not crucial.
[0105] Following the formation of the second copper layer 292, the
via hole is filled and overfilled by electro chemical plating of
copper using the second copper layer 292 both as a seed layer and
an electrode. Thereafter, the copper and typically the barrier
layers exposed over the field area are removed by chemical
mechanical polishing.
[0106] Although the description above is directed to removing the
barrier layer at the bottom of the via hole, a similar two-step
process may be used to produce a more conformal seed layer coating
even if the bottom barrier layer is not removed. The chamber
parameters for the first step are adjusted to emphasize middle
sidewall coverage with little or no bottom and/or field coverage.
The chamber parameters are then changed for the second step to
emphasize bottom, upper sidewall, and field coverage. In most
cases, this means that there is a substantial fraction of energetic
metal ions in the first step and a larger fraction of neutrals
relative to energetic ions in the second step. The two-step process
is superior to a one-step process with intermediate chamber
parameters because the latter tend to immediately begin producing
an overhanging lip at the top of the via hole which would interfere
with bottom and middle sidewall coverage.
[0107] The invention can be advantageously applied to more complex
and demanding structures desired in advanced integrated circuits. A
dual damascene structure is illustrated in the sectioned
orthographic view of FIG. 21, which allows inter-level vias and
horizontal interconnects to be metallized in a single metallization
process. A generally dielectric underlayer 300 includes a copper
feature 302 in its surface that needs to be electrically contacted
through an overlying inter-level dielectric layer 304. A
horizontally extending trench 306 is formed at the top of
inter-level dielectric layer 304, and one or more vias 308 (only
one of which is illustrated) are formed between the bottom of the
trench 306 and the corresponding ones of the copper features 302. A
single sequence of metallization steps are used to simultaneously
metallize the trench 306 (providing the horizontal interconnects)
and the vias 308 to the lower-level metallization 302. However, a
barrier layer 310 is required between the metal and any neighboring
dielectric materials, for example, a TaN barrier for copper
metallization. The barrier layer 310 is divided into a field
portion 312 on top of the upper dielectric layer 304, a trench
sidewall portion 314, a trench floor portion 316, and a via
sidewall portion 318. All these portions 312, 314, 316, and 318 are
desired for a reliable integrated circuit. However, it is desired
that the barrier layer 310 not extend over the bottom of the via
hole 308 in order to reduce the contact resistance to the metal
feature. Accordingly, it is greatly desired that a sputtering
process be available which has high bottom coverage in the trench
306 and no bottom coverage in the via hole 308. The dual-damascene
process disclosed by Geffken et al. in the above cited patent lacks
this selectivity. It is noted that the trench 306 has a very low
aspect ratio along its axis but may have a relatively high aspect
ratio transverse to its axis. Chen et al. describe a somewhat
similar selective removal and deposition in U.S. patent application
Ser. No. 09/704,161, filed on Nov. 1, 2000 by L. Chen et al.
incorporated herein by reference in its entirety. The grandparent
application Ser. No. 09/518,180 discloses a similar process to that
discussed here with respect to FIGS. 14 and 15.
[0108] Such a selective removal of the barrier layer and selective
deposition of copper is possible by adjusting the copper PVD
process parameters to assure a balance between energetic copper
ions and low-energy copper neutrals to produce the structure
illustrated in cross section in FIG. 22. A first copper seed layer
320 is deposited with relatively high copper ion energy but a
substantial neutral fraction so that the barrier layer 310 at the
bottom of the via hole 308 is removed and the underlying copper
feature 302 is cleaned. However, the copper layer 320 is deposited
over and thus protects the barrier layer 310 on the via sidewall
322, the trench floor 324, the trench sidewall 326, and the field
area 328 because these are either less exposed to the high-energy
copper ions or more exposed to the lower-energy copper
neutrals.
[0109] It is advantageous to perform the second copper seed
deposition to produce a conformal second copper seed layer 330,
illustrated in the cross-sectional view of FIG. 23, to assure a
thick sidewall and via bottom coverage as well as thick field
coverage. The second copper seed layer 330 is in direct contact
with the cleaned upper surface of the underlying copper feature
302, thus assuring a good electrical contact.
[0110] Following the deposition of the second copper seed layer
330, the via hole 308 and trench 306 are filled with copper by
electrochemical plating using the second copper layer 330 as both a
seed layer and a plating electrode. Thereafter, chemical mechanical
polishing removes any copper exposed above the field area 328
outside of the trench 306 and typically also the barrier layer 310
in the area.
[0111] For a given PVD chamber, particularly one of the SIP+chamber
described above, the metal ionization fraction is increased by
operating at a lower pressure or a higher target power. The metal
ion energy can be increased by these same two techniques or by
increasing the pedestal self-bias by any technique.
[0112] It has been observed that the DC self-bias on a floating
pedestal depends on the chamber pressure. For example, at 0.85
milliTorr, a self bias of about -20 VDC develops; and at 0.64
milliTorr, about -100 VDC. Thus, the chamber pressure can be used
to control the copper ion energy. Similarly, increases of the
target power from 20 kW to 40 kW show about the same sequence of
floating self-bias voltages, providing yet another tool for copper
ion energy.
[0113] An alternative approach to differentiate between the bottom
and top of the via hole is to use an auxiliary electromagnetic coil
wrapped around the outside of the central axial portion of the
chamber about its central axis to selectively generate an axial
magnetic field between the target and wafer. When the field is
turned on in the first step, the metal ions are preferentially
guided toward the wafer compared to when the field is turned off or
reduced in the second step. Wei discloses such an auxiliary
electromagnet in U.S. patent application Ser. No. 09/612,861, filed
Jul. 10, 2000, incorporated herein by reference in its
entirety.
[0114] We believe that a sputter reactor such as those of FIGS. 11,
13, and 14 having vaulted target and one or more nested top magnet
assemblies and continuous inner and outer sidewall magnet can be
operated in two distinct modes determined by a combination of
target power and chamber pressure. At higher power and lower
pressure, the self-bias on the pedestal is between -100 to -150 VDC
while at lower power and higher pressure, the self-bias assumes the
more normal value of -30 VDC. A related difference is that, below a
certain argon pressure, the target voltage is between about -450
and -700 VDC while above that pressure the target voltage drops to
about -400 VDC. Although we are not bound by our understanding of
the invention, we believe that at lower pressure and higher power
the plasma is maintained in the vault only in the area of the top
nested magnet assembly. Elsewhere, the plasma is extinguished. The
magnetic fields in the area of the localized plasma may be
sufficient to funnel an ionized copper flux towards the wafer. The
copper ionization fraction in this mode may be quite high, near
50%, and the high wafer self-bias draws highly energetic ions to
the wafer and deep within high aspect-ratio holes. We believe that
at higher pressure the sidewall magnets are sufficient to maintain
a plasma throughout the entire length of the vault. The lower
plasma densities and increased scattering produce a more neutral
flux of copper atoms.
[0115] Applying RF bias to the pedestal through a coupling
capacitor will also increase the DC self-bias. Some of the more
pronounced high-energy sputtering results were obtained with a
chamber pressure of 0.5 milliTorr, 40 kW of target power, and 300 W
of RF bias applied to the pedestal.
[0116] A process for accomplishing a copper via is illustrated in
the flow diagram of FIG. 24. In step 340, a inter-metal dielectric
layer of, for example, TEOS silicon dioxide or a low-k dielectric
whether carbon-based or silicon-based, is deposited, usually by a
CVD process and photolithographically patterned with via holes
using a plasma etching process. The dielectric patterning may be
dual damascene, which includes both the vias and interconnect
trenches in a common connecting structure. These steps are not
directly part of the invention, and may be practiced in any number
of ways. It is assumed that the material underlying the via holes
is copper. Contact holes to underlying silicon require a somewhat
more complex process.
[0117] Thereafter, the wafer is placed in a multi-chamber
integrated processing system. In some circumstances, no plasma
preclean need be performed. Instead, one PVD system is used in step
342 to deposit the barrier layer into the via hole and on top of
the dielectric. Chemical vapor deposition (CVD) can instead be used
for the barrier layer, or a combination of CVD and PVD can be used.
In step 344, the high-energy ionized copper deposition both cleans
the bottom of the via hole and coats its sidewalls, as has been
described. This step also cleans the interface of the underlying
copper exposed beneath the barrier layer. Even in this mode, a
substantial neutral flux is present that cannot penetrate to the
bottom of the via hole but does deposit on the planar field portion
above the dielectric. As a result, the barrier layer on the field
portion is not sputtered away by the energetic copper ions but is
protected by some deposition of neutral copper.
[0118] In step 346, a lower-energy, more neutral copper sputter
deposition is performed to complete the seed layer, also used as
the electroplating electrode. Whatever copper ions are present are
accelerated by a lesser self-bias voltage and thus do not
significantly sputter. Therefore, the lower energy copper ions the
bottom of the via to provide bottom coverage and the neutral copper
effectively coats the exposed planar field portion.
[0119] The two steps 344, 348 can be at least partially separated
by requiring the first step 344 to be performed at a pressure of
less than 1.0 milliTorr, more preferably 0.7 milliTorr or less, and
most preferably 0.5 milliTorr or less, while the second step 236 is
performed at 1.5 milliTorr or above.
[0120] By proper timing of the two steps 344, 346 and their
associated target powers and chamber pressure, not only is the
bottom barrier layer removed but the conformality of the copper
deposition at the via bottom, via sidewall, and field portion can
be adjusted.
[0121] In step 348, the copper metallization is completed with an
electroplating or other electrochemical process.
[0122] Although this process has been described with reference to
the inventive vault magnetron, similar high-energy ionized copper
sputtering can be achieved in other ways. Achieving the desired
selective PVD is believed to be eased by creating an energy
distribution of the copper ions in the plasma with a peak energy of
between 50 and 300 eV and/or by maintaining the ratio of argon ions
to copper ions Ar.sup.+/Cu.sup.+ in the plasma at 0.2 or less. Of
course, the ultimate low fraction is obtained with sustained
self-sputtering. The low fraction of argon ions reduces the
problems commonly experienced with HDP sputtering.
[0123] Further, it has been shown the inventive SIP.sup.+ reactors
can be used for the sputter deposition of Ta, TaN, Al, Ti, and TiN
and should be usable with W, especially for the effects of
selective removal, selective deposition, and a multi-step
process.
[0124] The inventive process need not completely remove the barrier
layer at the bottom of the via to reduce the contact resistance.
The outer portion, for example, of TiN while providing the barrier
function has the highest resistivity. Hence, removing just the
nitride portion would be advantageous.
[0125] Of course, the invention can be used with copper alloyed
with a five percent of an alloying element such as silicon,
aluminum, or magnesium. Further, many aspects of the invention are
applicable as well to sputtering other materials.
* * * * *