U.S. patent application number 11/222245 was filed with the patent office on 2006-08-03 for physical vapor deposition plasma reactor with rf source power applied to the target.
This patent application is currently assigned to APPLIED MATERIALS INC.. Invention is credited to Karl M. Brown, Vineet Mehta, John Pipitone.
Application Number | 20060169584 11/222245 |
Document ID | / |
Family ID | 36755338 |
Filed Date | 2006-08-03 |
United States Patent
Application |
20060169584 |
Kind Code |
A1 |
Brown; Karl M. ; et
al. |
August 3, 2006 |
Physical vapor deposition plasma reactor with RF source power
applied to the target
Abstract
A physical vapor deposition reactor includes a vacuum chamber
including a sidewall, a ceiling and a wafer support pedestal near a
floor of the chamber, a vacuum pump coupled to the chamber, a
process gas inlet coupled to the chamber and a process gas source
coupled to the process gas inlet. A metal sputter target is located
at the ceiling and a high voltage D.C. source coupled to the
sputter target. An RF plasma source power generator is coupled to
the metal sputter target and has a frequency suitable for exciting
kinetic electrons. Preferably, the wafer support pedestal comprises
an electrostatic chuck and an RF plasma bias power generator is
coupled to the wafer support pedestal having a frequency suitable
for coupling energy to plasma ions. Preferably, a solid metal RF
feed rod having a diameter in excess of about 0.5 inches engages
the metal sputter target, the RF feed rod extending axially above
the target through the ceiling and being coupled to the RF plasma
source power generator.
Inventors: |
Brown; Karl M.; (Mountain
View, CA) ; Pipitone; John; (Livermore, CA) ;
Mehta; Vineet; (Sunnyvale, CA) |
Correspondence
Address: |
PATENT COUNSEL;APPLIED MATERIALS, INC.
Legal Affairs Dept., M/S 2061
P.O. Box 450A
Santa Clara
CA
95052
US
|
Assignee: |
APPLIED MATERIALS INC.
|
Family ID: |
36755338 |
Appl. No.: |
11/222245 |
Filed: |
September 7, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11052011 |
Feb 3, 2005 |
|
|
|
11222245 |
Sep 7, 2005 |
|
|
|
Current U.S.
Class: |
204/298.02 ;
204/298.12; 257/E21.169 |
Current CPC
Class: |
H01J 37/32082 20130101;
H01L 21/76844 20130101; H01J 37/32706 20130101; C23C 14/358
20130101; H01J 37/3408 20130101; H01L 21/76843 20130101; H01L
21/2855 20130101; C23C 14/345 20130101; C23C 14/046 20130101 |
Class at
Publication: |
204/298.02 ;
204/298.12 |
International
Class: |
C23C 14/00 20060101
C23C014/00 |
Claims
1. A physical vapor deposition reactor, comprising: a vacuum
chamber including a sidewall, a ceiling and a wafer support
pedestal near a floor of the chamber, and a vacuum pump coupled to
the chamber; a process gas inlet coupled to said chamber and a
process gas source coupled to said process gas inlet; a metal
sputter target at said ceiling; a high voltage D.C. source coupled
to said sputter target; and an RF plasma source power generator
coupled to said metal sputter target and having a frequency
suitable for exciting kinetic electrons.
2. The reactor of claim 1 wherein said wafer support pedestal
comprises an electrostatic chuck.
3. The reactor of claim 1 further comprising an RF plasma bias
power generator coupled to said wafer support pedestal and having a
frequency suitable for coupling energy to plasma ions.
4. The reactor of claim 1 further comprising a solid metal RF feed
rod having a diameter in excess of about 0.5 inches engaging said
metal sputter target, said RF feed rod extending axially above said
target through said ceiling and being coupled to said RF plasma
source power generator.
5. The reactor of claim 4 further comprising an RF match circuit
coupled between said RF plasma source power generator and said RF
feed rod, said RF match circuit being mounted on said RF feed
rod.
6. The reactor of claim 4 further comprising a magnet array
overlying said ceiling and a center axle about which said magnet
array is rotatable, said center axle having an axially cylindrical
hollow passageway therethrough, said metal rod extending through
said passageway.
7. The reactor of claim 6 further comprising planetary motion
apparatus coupled between said magnet array and said center axle,
said magnet array comprising an array of magnetic poles arranged to
produce a generally toroidal-shaped confinement pattern of ions
near said target.
8. The reactor of claim 1 wherein said wafer support and said
target are separated by a distance which does not exceed about one
fourth of the diameter of said wafer support.
9. The reactor of claim 1 further comprising a removable shield
surrounding a processing zone encompassing the wafer support
pedestal and separating said processing zone from the sidewall of
the chamber.
10. The reactor of claim 9 further comprising a conductive tab
coupled between said shield and said wafer support pedestal.
11. The reactor of claim 1 wherein said metal sputter target
comprises copper.
12. The reactor of claim 1 wherein said metal sputter target
comprises tantalum.
13. The reactor of claim 1 wherein said RF plasma source power
generator is a VHF generator.
14. The reactor of claim 13 wherein said VHF generator has a
frequency of about 81 MHz.
15. The reactor of claim 1 wherein said metal target has an
inverted dome shape opening toward said wafer support pedestal.
16. The reactor of claim 1 wherein said metal target has an annular
truncated conical shape.
17. For use in a physical vapor deposition reactor in which VHF
source power is coupled from a VHF generator through a solid metal
elongate RF feed rod: a sputter target comprising a solid metal
disk having a receptacle for firmly engaging said RF feed rod.
18. The sputter target of claim 17 wherein said receptacle of said
sputter target is threaded for threadably engaging said RF feed
rod.
19. The sputter target of claim 17 wherein said sputter target is a
copper disk.
20. The sputter target of claim 17 wherein said sputter target is a
tantalum disk.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of U.S.
application Ser. No. 11/052,011 filed Feb. 3, 2005, entitled
"APPARATUS FOR METAL PLASMA VAPOR DEPOSITION AND RE-SPUTTER WITH
SOURCE AND BIAS POWER FREQUENCIES APPLIED THROUGH THE WORKPIECE",
by Karl M. Brown, et al. and assigned to the present assignee.
BACKGROUND OF THE INVENTION
[0002] With recent technological advances in integrated circuit
design, there are now as many as six to ten interconnect layers of
a multilayer interconnect structure overlying the semiconductor
transistors. Insulator layers separate the successive conductor
layers. The conductor interconnect layers can have completely
different conductor patterns and are connected to one another and
to the transistor layer at different locations through vias
extending vertically through the insulator layers. It is the
formation of the vias with which the present invention is
concerned.
[0003] To reduce resistive power losses in the integrated circuit,
the interconnect layers and the vias typically employ aluminum and,
more recently, copper as the principal conductor. The insulator
layers are silicon dioxide, although more recently dielectric
materials other than silicon dioxide having a lower dielectric
constant are increasingly being employed. Because copper tends to
diffuse over time through the insulator layer to cause catastrophic
short circuiting, a barrier layer that blocks copper diffusion is
placed between the copper material and the dielectric material
wherever the two materials interface in the integrated circuit. The
barrier layer is typically formed of an underlying tantalum nitride
or titanium nitride layer contacting the insulator layer, an
overlying pure (or nearly pure) tantalum (or titanium) layer and,
finally, a copper seed layer over the pure tantalum (or titanium)
layer. If the conductor is to be copper, then tantalum is
preferred. The copper conductor is deposited on the copper seed
layer. Such a barrier layer prevents migration or diffusion of
copper atoms into the dielectric material. The tantalum and
tantalum nitride (or titanium and titanium nitride) layers are poor
conductors compared to copper. The formation of the barrier layer
and of the copper conductors is carried out by physical vapor
deposition. Other deposition processes may be employed for the
metal fill step (copper deposition) such as chemical vapor
deposition, plasma enhanced chemical vapor deposition or
electroplating.
[0004] A problem arises in forming the vias extending vertically
between the horizontal interconnect layers, as follows. Each
vertical via opening is formed by etching a hole through an
overlying horizontal insulator (silicon dioxide) layer so as to
expose a portion of the copper conductor in the underlying
horizontal interconnect layer. It is this exposed portion to which
connection is to be made through the via. A barrier layer must be
deposited on the interior surfaces of the via before the copper
conductor can be formed in the via, to prevent copper migration as
explained above. This barrier layer, in covering all interior
surfaces of the via, covers the exposed portion of the underlying
copper conductor. Since the barrier layer is an inferior conductor,
it must be selectively removed from the underlying copper conductor
(in an etch process) without removing the remainder of the barrier
layer from the other interior surfaces of the via. This removal
step has required interruption of the physical vapor deposition
process in order to place the wafer in an etch chamber where the
step of selectively removing the barrier layer from the underlying
copper surface is carried out. The wafer is then returned to a
physical vapor deposition reactor for formation of the copper
conductor(s).
[0005] The interruption represented by the selective removal of the
barrier layer entails a higher production cost and consumes
production time. In recent years, a dual purpose reactor was
developed capable of both physical vapor deposition of the barrier
layer and selective removal of the barrier layer after the barrier
layer formation step, without removing the wafer from the reactor.
As a result, great savings in production cost and production time
have been realized. This was accomplished by providing in the
physical vapor deposition chamber a separate coil near the wafer.
After barrier layer formation, the coil is used to form an
inductively coupled plasma which selectively sputters the barrier
layer from horizontal surfaces (i.e., the floor formed by the
underlying copper conductor). Such selective sputtering
(hereinafter referred to as "re-sputtering") is achieved by
applying RF bias power to the wafer to achieve an ion velocity
distribution that is predominantly vertical. While this
dual-purpose reactor works extremely well, it does entail some
additional expense. For example, since the barrier layer deposition
step involves sputtering a metal target and therefore deposits
metal over all interior surfaces of the reactor chamber, the
re-sputtering coil must be located inside the chamber so that no
metallized surfaces shield the coil or otherwise prevent inductive
coupling of RF power from the re-sputtering coil to the plasma. In
order to avoid process contamination, the re-sputtering coil is
formed of pure tantalum, adding cost. The coil is subject to very
large temperature fluctuations during its lifetime, and must be
changed periodically. RF power must be coupled to the re-sputtering
coil through the vacuum seal of the reactor chamber and through an
environment that periodically is completely filled with metal
vapor. Therefore, an RF feedthrough must be employed that can
tolerate the metal deposition, and whose exterior surfaces are
textured to avoid excessive accumulation of deposited materials and
flaking, and that can tolerate wide temperature excursions over its
lifetime.
[0006] Another well-known dual-purpose reactor employs an external
inductive coil overlying a portion of the ceiling not blocked by
the metal sputter target. One problem is that the metal vapor
deposition process can coat the ceiling with metal and thereby
block inductive coupling from the coil antenna. A more significant
problem is that the RF plasma produced by the coil produces a high
proportion of metal ions from the target, so that the wafer bias
cannot be optimized for etch selectivity to horizontal surfaces
without impacting (de-optimizing) the flux of metal ions/vapor from
the target. Therefore, the metal deposition process and the
re-sputter process must be performed at separate times.
[0007] It should be noted that although such dual purpose reactors
are capable of performing both the Ta/TaN barrier layer deposition
step and the re-sputtering step, a different reactor is typically
employed to perform the subsequent copper deposition step. This is
because a high flux of copper ions on the wafer is desired, and the
PVD reactor must be specially configured in order for the sputtered
copper atoms to self-ionize in a very dense plasma at the copper
target. Specifically, a very high D.C. power level (40-56 kWatts)
is applied to the copper target and a specially configured
magnetron is employed for a more concentrated plasma at the target.
Because of the high density of copper ions near the target, it is
placed very high above the wafer (390 mm), which limits the copper
deposition rate to an acceptable threshold (as well as providing
some beneficial collimation of copper neutrals). Typically,
however, are large share of the copper ions are deposited on
shields in the chamber and otherwise lost while traveling over this
long distance.
[0008] In addition to the requirement for a copper PVD chamber and
a barrier PVD chamber, a third chamber, an etch chamber, must be
employed to carry out a pre-deposition cleaning process, since the
copper PVD chamber and the barrier PVD/re-sputter chamber are not
suitable for clean/etch processes.
[0009] Another problem is the tendency of the tantalum and/or
tantalum nitride material deposited during formation of the barrier
layer to deposit with non-uniform thickness along the via walls,
and in particular to accumulate faster near the top corners of the
vertical walls and thereby exhibit some tendency toward pinch-off.
This makes it necessary to restrict the process window in order to
ameliorate such problems. This problem is solved to some extent
when, upon completion of the barrier layer deposition process, the
re-sputtering process is performed, because the re-sputtering
process tends to remove tantalum or tantalum nitride from the tops
and corners of the via walls faster than elsewhere, while
transferring tantalum (or tantalum nitride) material removed from
the horizontal surfaces (floors) of the vias onto the lower
portions of the via sidewalls. Nevertheless, it would be beneficial
to avoid altogether the initial non-uniform tantalum or tantalum
nitride deposition problem, to eliminate any risk of pinch-off,
permitting some liberalization of the process window.
[0010] It would also be beneficial to avoid the necessity of the
internal re-sputtering coil provided at least some of its benefits
could be realized in a simpler manner.
[0011] The present invention provides benefits at least approaching
those afforded by the internal re-sputtering coil without the need
for such a coil. The present invention furthermore provides a way
of ameliorating or avoiding non-uniform deposition of the barrier
layer, and a way of avoiding or minimizing deposition of the
barrier layer on the exposed copper conductor surface forming the
floor of the via during formation of the barrier layer.
[0012] Another problem that is to be solved is that, as
technological advances dictate smaller geometries and higher aspect
ratios for vias and other features, the degree of ionization of the
sputtered material (e.g., copper) must be increased to achieve the
requisite conformality of the deposited film. Such an increase in
ionization requires greater VHF power applied to the wafer support
pedestal. Increased demands on process performance require that the
temperature of the workpiece be more precisely controlled,
dictating the use of an electrostatic chuck to clamp the wafer to a
temperature-controlled surface. Use of an electrostatic chuck
limits the amount of VHF power that can be applied to the wafer.
This is, in large part, because the electrode to which RF power is
applied in the electrostatic chuck typically consists of a small
molybdenum mesh within an aluminum nitride puck. The small gauge of
the mesh (e.g., 100 microns) greatly limits the efficiency of the
mesh as an RF radiator and limits the amount of RF power that can
be applied to the mesh to as low as 1.5 kW maximum. The requisite
degree of ionization for conformal deposition on high aspect ratio
openings of small feature sizes (e.g., 45 nm) can only be achieved
with much higher VHF power, e.g., 3.5 kW or higher.
SUMMARY OF THE INVENTION
[0013] A physical vapor deposition reactor includes a vacuum
chamber including a sidewall, a ceiling and a wafer support
pedestal near a floor of the chamber, a vacuum pump coupled to the
chamber, a process gas inlet coupled to the chamber and a process
gas source coupled to the process gas inlet. A metal sputter target
is located at the ceiling and a high voltage D.C. source coupled to
the sputter target. An RF plasma source power generator is coupled
to the metal sputter target and has a frequency suitable for
exciting kinetic electrons. Preferably, the wafer support pedestal
comprises an electrostatic chuck and an RF plasma bias power
generator is coupled to the wafer support pedestal having a
frequency suitable for coupling energy to plasma ions. Preferably,
a solid metal RF feed rod having a diameter in excess of about 0.5
inches engages the metal sputter target, the RF feed rod extending
axially above the target through the ceiling and being coupled to
the RF plasma source power generator.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a cut-away side view of a plasma reactor embodying
the invention.
[0015] FIG. 2 is an enlarged cut-away view of a wafer support
pedestal in accordance with one aspect.
[0016] FIG. 3 is an enlarged cut-away view of a wafer support
pedestal in accordance with another aspect.
[0017] FIG. 4 illustrates the use of two different plasma bias
power frequencies in the reactor of FIG. 1.
[0018] FIG. 5 is a graph illustrating the combination of the
different ion energy distributions of the different bias power
frequencies in the embodiment of FIG. 4.
[0019] FIGS. 6A through 6E are sequential cross-sectional views of
a portion of an integrated circuit, in accordance with a process of
the invention.
[0020] FIG. 7 is an enlarged cross-sectional view of a barrier
layer formed in the process of FIGS. 6A through 6E.
[0021] FIGS. 8A through 8C illustrate the formation of a barrier
layer in one embodiment of a process of the invention.
[0022] FIGS. 9A and 9B illustrate the formation of a barrier layer
in the preferred embodiment of a process of the invention.
[0023] FIG. 10 is a block flow diagram of a preferred process of
the invention.
[0024] FIGS. 11A, 11B and 11C are cross-sectional side views of an
narrow opening through a dielectric layer, and depict deposition
results in three respective modes of the reactor of FIG. 1, namely
a conformal mode, a non-conformal mode and a punch-through mode,
respectively.
[0025] FIG. 12 illustrates a plasma reactor in accordance with a
first alternative embodiment.
[0026] FIG. 13 illustrates a plasma reactor in accordance with a
second alternative embodiment.
[0027] FIG. 14 illustrates a plasma reactor in accordance with a
third alternative embodiment.
[0028] FIG. 15 illustrates a plasma reactor in accordance with a
fourth alternative embodiment.
[0029] FIG. 16 illustrates a plasma reactor in accordance with a
fifth alternative embodiment.
[0030] FIG. 17 depicts a process in which the reactor of FIG. 1
performs a pre-deposition cleaning process, a dielectric barrier
layer deposition process and a metal barrier layer deposition
process on the same wafer.
[0031] FIGS. 18A and 18B illustrate one embodiment of an RF
grounding conductive tab of FIG. 2.
[0032] FIG. 19 is a diagram of a first preferred embodiment of a
PVD reactor having an electrostatic chuck and a highly efficient
VHF radiator and employing a rotating magnet array on the
sputtering target.
[0033] FIG. 20 is a top view corresponding to FIG. 19.
[0034] FIG. 21 depicts the toroidal magnetic confinement pattern of
the magnet array of FIGS. 19 and 20.
[0035] FIG. 22 is a diagram of a second preferred embodiment of a
PVD reactor having an electrostatic chuck and a highly efficient
VHF radiator and employing a rotating magnet array on the
sputtering target.
[0036] FIG. 23 depicts a first pattern of RF feed columns on the
sputter target of the reactor of FIG. 22.
[0037] FIG. 24 depicts a second pattern of RF feed columns on the
sputter target of the reactor of FIG. 22.
[0038] FIGS. 25, 26 and 27 depict first, second and third
alternative embodiments, respectively.
[0039] FIG. 28 depicts a process for carrying out physical vapor
deposition in accordance with one aspect of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0040] A plasma reactor forms barrier layers (such as a
tantalum/tantalum nitride film or titanium/titanium nitride film)
for conductors (such as copper, for which the barrier should be
tantalum/tantalum nitride) in trenches or through vias between
successive interconnection layers of an integrated circuit. The
plasma reactor is capable of both physical vapor deposition and of
highly selective re-sputtering to remove barrier material from the
exposed horizontal surfaces of the underlying conductor
constituting the floor of the via. Significantly, the reactor
accomplishes all this without an internal coil that had previously
been required for a fully and precisely controllable re-sputtering
step. Instead, a plasma is formed near the wafer to perform the
re-sputtering step. For this purpose a process gas such as argon
may be introduced and source power is applied to the wafer at an RF
frequency effective for capacitively coupling energy to kinetic
electrons to excite argon plasma ions near the wafer. The term
"source power" as employed in this specification refers to RF power
suitable for maintain an RF-coupled plasma by coupling power to
kinetic electrons in the plasma. This is to be distinguished from
use of the term "source" when referring to the D.C. excited metal
sputtering target, which is the "source" of the metal atoms or ions
in a metal deposition process. Typically, the source power
frequency is a VHF frequency because of the low mass-to-charge
ratio of electrons. The ions of the VHF-coupled plasma formed near
the wafer are employed in the re-sputtering step. The selectivity
of the re-sputtering step for horizontal surfaces is established by
applying bias power to the wafer at an RF frequency effective for
coupling energy to the ions (e.g., argon ions), which is typically
an HF or LF frequency because of the high mass-to-charge ratio of
the ions. This constricts the ion velocity distribution across the
plasma sheath at the wafer surface to a small directional range
about the chamber axis, typically a vertical direction, making the
re-sputtering step highly selective for surfaces perpendicular to
the chamber axis, typically horizontal surfaces. A significant
feature is that the bias power controls the selectivity of the ion
re-sputter or etch step without affecting the flux of metal atoms
from the target toward the wafer. This is facilitated by a low
power (2-5 kW) D.C. discharge plasma at the target surface which
produces primarily neutral metal particles or atoms from the target
that are unaffected by the bias power applied to the wafer. Thus,
two different plasmas are employed, one being a D.C. discharge
plasma at the target and the other being an RF (VHF) plasma at the
wafer. Therefore, the target sputtering may be optimized without
affecting the re-sputtering plasma at the wafer, while the bias
voltage may be optimized without affecting the target sputtering.
This feature is not possible in conventional ion physical vapor
deposition reactors that employ RF coupled plasmas near the target
to produce metal ions for the physical vapor deposition
process.
[0041] An advantageous mode is provided that is unique to the
present invention in which the physical vapor deposition and the
re-sputtering can be performed simultaneously, and the selectivity
of the re-sputter or etch process is adjusted without affecting the
flux of metal atoms from the target.
[0042] The re-sputtering step tends to compensate for non-uniform
deposition of the barrier layer material. Therefore, in one
embodiment of the invention, the re-sputter step is performed
simultaneously with the barrier deposition step. This is possible
because a preferred embodiment of the invention produces two
somewhat independent plasmas, namely a D.C. discharge plasma near
the ceiling or target that is confined by a magnetron above the
target and an RF plasma near the wafer surface to perform the
re-sputtering. Thus, the plasma near the ceiling is optimized for
sputtering the target while (simultaneously) the plasma at the
wafer is optimized for re-sputtering and selectively etching the
floor of each via. One advantage is that prominent non-uniformities
in the deposited barrier layer film are reduced or are never
formed, thereby reducing the risk of pinch-off or other problems in
the vias. Another advantage of this embodiment is that throughout
the entire barrier deposition/re-sputter process, accumulation of
barrier material can be entirely avoided on the exposed horizontal
surface of the underlying conductor forming the floor of the via.
This is accomplished by adjusting the barrier material deposition
rate (controlled largely by the tantalum target D.C. sputter
voltage) relative to the re-sputter rate (controlled largely by the
VHF plasma source power applied to the wafer.)
[0043] The invention can afford certain advantages depending upon
it application. For example, a low power (2-5 kW) D.C. power level
is employed to sputter the metal target for all deposition process,
including copper, tantalum and titanium, because metal neutrals are
desired. Therefore, the target height above the wafer and the
magnetron design are the same in all these processes, so that the
same chamber can perform any or all of them. The target height
above the wafer can be relatively low, or between about 225 mm and
290 mm, for example. In addition, the VHF source power applicator
and the HF bias power applicator can be used without a target to
excite a plasma (such as argon plasma) to perform a pre-deposition
clean process prior to each deposition process. Such a pre-clean
process can be repeated prior to each and all of the deposition
steps, including the barrier metal (tantalum) deposition step, the
barrier dielectric (tantalum nitride) barrier deposition step, the
copper seed layer deposition step and the copper conductor
deposition step.
Reactor Apparatus:
[0044] Referring to FIG. 1, a reactor of a first embodiment of the
invention includes a vacuum chamber defined by a cylindrical side
wall 10, a disk-shaped ceiling 12, and a wafer support pedestal 14
for supporting a semiconductor wafer 16 to be processed. A target
18 of a metal (e.g., tantalum) to be deposited on the wafer 16 is
mounted on the ceiling 12. A process kit consisting of a
cylindrical shield 20 surrounds the wafer 16 and the target 18. A
magnetron 22 of the conventional type overlies the target 18 on the
external side of the ceiling 12. A high voltage D.C. source 24 is
coupled to the target 18. A process gas injector 26 furnished
process gas from a supply 28 into the interior of the chamber. A
vacuum pump 30 maintains a desired sub-atmospheric pressure in the
vacuum chamber.
[0045] The wafer pedestal 14 is coupled through an impedance match
network 34 to a VHF plasma source power generator 36 and to an HF
or LF plasma bias power generator 38. The high voltage D.C. source
maintains an upper plasma 40 near the target 18. The VHF plasma
source power generator 36 maintains a lower plasma 42 at or near
the surface of the wafer 16. The two plasmas 40, 42 may be
maintained simultaneously or may be produced at different times.
The upper plasma 40 is a D.C. discharge plasma that enables
sputtering of the target 18 to produce mainly neutral metal atoms
from the target 18 that deposit on the wafer, with some metal ions
from the target 18. The lower plasma 42 is a capacitively coupled
RF plasma that promotes selective etching of horizontal surface on
the wafer 16. The two plasmas 40, 42 may be controlled
independently, for separate control of the metal deposition process
and the re-sputter process. The LF bias power applied to the wafer
determines the selectivity of the re-sputter/etch process for
horizontal surfaces. Plasma uniformity, particularly uniformity of
the plasma 42 nearest the wafer, is controlled by an
electromagnetic coil 43 wrapped around the cylindrical sidewall of
the reactor chamber and supplied with D.C. current by a current
source controller 45.
[0046] Coupling of the VHF source power and HF or LF bias power to
the wafer is illustrated in FIG. 2. The wafer support pedestal 14
can be an electrostatic chuck (ESC) for electrostatically holding
the wafer 16 in place. In this case, the ESC or pedestal consists
of an insulating layer 50 mounted on a conductive base 51 and an
electrode 52 such as a conductive mesh buried inside the insulating
layer 50. The pedestal structure can extend downwardly in a coaxial
arrangement consisting of a center conductor 54 connected to the
electrode 52, an insulating intermediate layer 56 and an outer
conductor 58 connected to the conductive base 51. The conductive
base can be coupled to the bottom of the cylindrical shield 20 by
conductive tabs 60 to provide a more continuous ground reference.
The center conductor 54 is coupled to the RF power sources while
the outer conductor 58 is grounded. The impedance match network 34
consists of a conventional impedance match circuit 64 coupled to
the RF (VHF) plasma source power generator 36 and a conventional
impedance match circuit 68 coupled to the RF (HF or LF) plasma bias
power generator 38. The output of the impedance match circuit 64 is
connected through a high pass filter 65 to the wafer pedestal
center conductor 54, while the output of the impedance match
circuit 68 is connected through a low pass filter 69 to the wafer
pedestal center conductor 54. In addition, a D.C. chuck voltage
source 74 is connected to the wafer pedestal center conductor 54
and is isolated from RF power by an isolation capacitor 76. The
high pass filter 65 has a sufficiently high cut-off frequency to
prevent HF or LF current from the RF generator 38 from reaching the
VHF match circuit 64, while the low pass filter has a sufficiently
low cut-off frequency to prevent VHF current from the RF generator
36 from reaching the HF (or LF) match circuit 68.
[0047] FIG. 3 illustrates another embodiment of the wafer support
pedestal 14 in which the electrode 52 contacts the wafer, and there
is no electrostatic chucking of the wafer. In this case, since the
electrode 52 is potentially exposed to the plasma, the electrode 52
can be formed of the material to be deposited on the wafer, such as
tantalum.
[0048] FIG. 4 illustrates an embodiment in which the bias power
consists of both HF and LF frequencies (e.g., 13.56 MHz and 2 MHz,
respectively, for example). For this purpose, there are two bias
power RF generators, namely an HF bias power generator 38a and an
LF bias power generator 38b, the generators 38a, 38b being coupled
through respective match circuits 68a, 68b and filters 69, 69a to
the wafer pedestal center conductor 54. The VHF source power
generator 36 is coupled through its impedance match circuit 64 and
through the high pass filter 65 to the wafer pedestal center
conductor 54. One advantage of this embodiment is that the lower
ion energy distribution of the HF bias and the higher ion energy
distribution of the LF bias (both shown in FIG. 5) can be combined
to produce an ion energy distribution that peaks between the peaks
of the LF and HF ion energy distributions. This peak can be shifted
up or down in energy by adjusting the relative power levels of the
LF and HF power generators 38a, 38b.
[0049] The deposition rate of the target material (e.g., tantalum)
is mainly determined by the power delivered by the D.C. voltage
source to the target. The selectivity of the etch/re-sputter
process (for horizontal surfaces) is determined by the bias power,
while the rate of the etch/re-sputter process is mainly determined
by the source power level. Therefore, there are three parameters
that can be controlled independently of one another, namely metal
deposition rate, etch selectivity for horizontal surfaces and etch
rate. Since all these can be controlled independently of one
another, the metal deposition and etch/re-sputter process may be
performed simultaneously, if desired.
PVD/Re-Sputter Method:
[0050] The reactor of FIG. 1 is particularly useful in the
formation of metal contacts between successive interconnection
layers of an integrated circuit. Typically, an integrated circuit
includes an active semiconductor layer having thousands of
transistors and many insulated interconnection layers stacked over
the active semiconductor layer and providing complex
interconnection circuits among the transistors. The connections
between interconnection layers are formed through vias or vertical
holes between the interconnection layers by filling the holes with
a metal such as copper. In order to prevent failure by short
circuiting due to diffusion of copper through insulating material,
a barrier layer of tantalum and tantalum nitride is placed between
the copper and the insulating material. It is in the deposition of
the barrier layer within the via that the reactor of FIG. 1
provides great advantage.
[0051] FIG. 6A is an enlarged cross-sectional side view of a
partially fabricated portion of an interconnection layer 100 in a
stack of many interconnection layers overlying an active
semiconductor layer (now shown). FIG. 6B is the corresponding top
view. The interconnection layer 100 includes, among many
conductors, a pair of parallel copper conductors 102, 104 embedded
in an insulator (silicon dioxide) layer 106. Each copper conductor
102, 104 is separated from the dielectric layer 106 by a barrier
layer 108 that prevents diffusion of copper atoms into the
dielectric layer 106. Preferably, the barrier layer 108 includes a
tantalum nitride layer contacting the insulator layer 106 and
covered by a tantalum layer and a copper seed layer. The structure
is best shown in the enlarged view of FIG. 7, which shows a
tantalum nitride layer 110 covering the insulator layer 106, a
tantalum metal layer 112 overlying the tantalum nitride layer 110
and a copper seed layer 114 overlying the tantalum metal layer 112.
A copper conductor, such as the copper conductor 102, overlies the
copper seed layer 114. The tantalum metal layer 112 establishes a
high quality bond with the tantalum nitride layer 110, while the
copper seed layer establishes a high quality bond with the tantalum
metal layer 112 on one side and the copper conductor 102 on the
opposite side.
[0052] Before the next interconnection layer is formed on the top
surface of the insulator layer 106, a vertical opening or via 120
is opened through the insulator layer 106, typically by etch
processes (FIGS. 6A and 6B). The via 120 consists of a large
opening 122 extending only partly through the insulator layer 106
and a pair of smaller openings 124 extending down to the respective
copper conductors 102, 104. Typically, the etching process that
forms the two smaller openings 124 is carried out sufficiently to
remove the exposed portion of the barrier layer 108 overlying each
conductor 102, 104 (FIG. 6A).
[0053] The via 120 forms a vertical conductor when later filled
with copper. However, before copper is deposited into the via 120,
a barrier layer 130 is deposited onto all surfaces in the via 120
and onto the top surface of the insulator layer 106 as shown in
FIG. 6C. The barrier layer 130 has the same structure as that
described above with reference to FIG. 7, including a tantalum
nitride layer 110, a metal tantalum layer 112 and a copper seed
layer 114. In one embodiment of the invention, each sub-layer 110,
112, 114 of the barrier layer 120 is deposited in a separate step
by providing the appropriate material for the metal target 18
(tantalum for the sub-layers 110, 112 and copper for the sub-layer
114). The target 18 is sputtered by introducing a process gas which
is ionized in the vicinity of the target by the large D.C.
sputtering voltage from the sputter voltage source 24. In order to
deposit the tantalum nitride sub-layer 110, nitrogen is employed as
the process gas, and the tantalum atoms combine with nitrogen atoms
as they impact the wafer to form a tantalum nitride film. When
depositing the metal tantalum layer 112 and, later, when depositing
the copper seed layer 114, the process gas is an inert or
non-reactive gas, such as Argon. Thus, three deposition steps are
carried out. First, using a tantalum sputtering target and nitrogen
process gas, tantalum nitride is deposited. Second, using a
tantalum target and argon process gas, metallic tantalum is
deposited. Third, using a copper target and argon gas, a copper
seed layer is deposited.
[0054] In one embodiment of the invention, no plasma RF (VHF)
source power is applied to the wafer support pedestal 14, although
a modest level of plasma RF (HF or LF) bias power may be applied.
In this way, metal is sputtered from the target 18 and deposited
onto the wafer 16. As a result, the barrier layer 130 is formed by
carrying out the three deposition steps for the successive
sub-layers 110, 112, 114. The barrier layer 130 covers all exposed
surfaces including the portions of the copper conductors 102, 104
exposed by the small openings 124, as shown in FIG. 6C.
[0055] After each of the three deposition steps have been
completed, an etch/re-sputter step is carried out (FIG. 6D) in
which the VHF plasma source power generator 36 and the HF (or LF)
plasma bias power generator 38 deliver power to the wafer support
pedestal 14. This produces a plasma near the wafer that furnishes
ions guided to the horizontal surfaces by the applied plasma bias
power. These ions sputter the horizontal surfaces to remove films
deposited thereon, such as the portion of the barrier layer 130 at
the bottom of the small openings 124. Within the small openings
124, the close proximity of the vertical walls (or small diameter
of the openings 124) promotes the re-deposition of atoms sputtered
from the floor 124a of each opening 124 onto the vertical side
wall. This uncovers the copper conductors, as desired, as
illustrated in FIG. 6D. In other areas where there is no vertical
wall, such as the vast area of the top surface of the insulator
layer 106, the sputtered atoms re-deposit onto the horizontal
surface, so that there is no net loss.
[0056] In a preferred embodiment, each of the three deposition
process steps (corresponding to the three layers 110, 112, 114) is
performed simultaneously with the etch and re-sputter process step.
In this preferred embodiment, the sputter voltage source 24
delivers power to the target 18, while simultaneously the VHF
plasma source power generator 36 and the HF (or LF) plasma bias
power generator 38 deliver power to the wafer support pedestal 14.
This produces a plasma near the wafer that furnishes ions guided to
the horizontal surfaces by the applied plasma bias power while
atoms from the target 18 are being deposited. Deposition of atoms
from the target 18 on the horizontal surfaces such as the exposed
portions of the copper conductors 102, 104 is avoided provided the
etch rate on the horizontal surfaces rival the deposition rate of
atoms from the target 18. This condition is realized by an
appropriate adjustment of the voltage of the D.C. sputter voltage
source 24 (to adjust the deposition rate) and of the power level of
the VHF source power generator 36. The power level of the HF (or
LF) bias power generator 38 is adjusted to achieve the desired
degree of etch selectivity for horizontal surfaces. The result is
that the barrier layer 130 is not formed over the horizontal floor
of the via 120, so that the process sequence skips the state
illustrated in FIG. 6C.
[0057] The next step (FIG. 6E) is to deposit a thick copper layer
to form the next interconnect layer 200 and vertical conductors
extending through the via 120 between the lower interconnect
conductors 102, 104 and the upper interconnect layer 200, as shown
in FIG. 6E.
[0058] The sequence illustrated in FIGS. 8A through 8C shows one
advantage of the re-sputtering step of FIG. 6D. FIG. 8A illustrates
one of the small openings 124 prior to deposition of the barrier
layer 130. FIG. 8B shows how the barrier layer 130 is formed in
cases where the re-sputtering step is not carried out
simultaneously with the deposition step. Specifically, the barrier
layer 130 tends to accumulate with greater thickness near the top
edge of the small opening 124 and with smaller thickness near the
bottom of the opening 124. The re-sputtering step removes the
excess material from the top edge and removes the material from the
floor of the small opening and re-deposits it on the vertical side
wall, so that the thickness distribution along the vertical side
wall becomes more uniform, as indicated in FIG. 8C. The problem is
that the thicker accumulation of barrier material near the top edge
of the small opening 124 may lead to pinch-off, so that the
re-sputter step could have no beneficial effect, the device would
fail.
[0059] This risk is avoided in the preferred method in which the
re-sputter step (FIG. 6D) and the deposition step (FIG. 6C) are
carried out simultaneously. In this preferred mode, the process
begins with a newly formed small opening 124 in FIG. 9A and
transitions directly to a uniform barrier layer 130 in FIG. 9B. The
simultaneous deposition/re-sputtering prevents the deposition
process from forming significant non-uniformities in the barrier
layer 130. This eliminates the risk of the pinch-off effect
illustrated in FIG. 8B.
[0060] FIG. 10 is a block diagram illustrating the preferred
process. In block 310 of FIG. 10, a deposition D.C. discharge
plasma is generated around the target 18 (near the ceiling) to
deposit atoms from the target onto the wafer. In block 312, a
re-sputter capacitively coupled RF plasma is generated near the
wafer to produce ions to bombard the wafer to re-sputter the
deposited atoms from the target 18. In the step of block 314,
plasma RF bias power is applied to the wafer. The bias power is
sufficient to render the sputtering highly selective for horizontal
surfaces. In block 320, the plasma source power applied to the
wafer and the D.C. sputter voltage applied to the target are
adjusted relative to one another so that the re-sputter rate is at
least nearly as great as the sputter deposition rate.
[0061] One embodiment of the PVD/re-sputter reactor of the
invention is capable of depositing three different type of layers
by operating in three different modes, specifically, in mode (A), a
highly conformal layer with uniform sidewall and horizontal surface
coverage, in mode (B), a non-conformal layer with little or no
sidewall coverage, and in mode (C), a "punch through" layer with
good sidewall coverage and good horizontal surface coverage in a
large field, but no coverage of bottom surfaces of high aspect
ratio openings. The conformal layer of mode (A), illustrated in
FIG. 11A, is obtained by applying a relatively low level of D.C.
power to the target (e.g., 5 kW), high VHF source power to the
wafer (1 kW at 60 MHz) and a low level of HF bias power to the
wafer (about 100 W at 13.56 MHz). The non-conformal layer of mode
(B), illustrated in FIG. 11B, is obtained under the same conditions
except that the HF bias power level is reduced to zero. The
"punch-through" layer of mode (C), illustrated in FIG. 11C, is
obtained by increasing the bias power to a high level (500 W at
13.56 MHz). The conformal mode is particularly useful for
depositing the copper conductor layer. The non-conformal mode is
particularly useful for covering the bottom or floor of a via with
a low-resistance metal such as tantalum or titanium. The
punch-through mode is the preferred mode for depositing the barrier
layer (Ta and TaN) in a via.
[0062] In some cases, the plasma density distribution may differ
between the three modes described above. In order to maintain more
uniform plasma density distribution, the current source controller
45 may cause different levels of D.C. current to flow through the
electromagnet coil 43 in different ones of the three modes. In any
case, the current level furnished by the current controller 45 is
preferably optimized to improve the radial plasma ion density
uniformity in the process zone.
[0063] Each of the three modes described above with reference to
FIGS. 11A, 11B and 11C may be implemented by a process controller
210 shown in FIG. 1 whose outputs control the power level of the
target high voltage D.C. supply 24, the power level of the VHF
plasma source power generator 36 and the power level of the HF or
LF bias power generator 38. The process controller 210 may be
controlled by a user through a user interface 212, allowing the
user to program the controller to have the reactor of FIG. 1
automatically transition between any of the operating states
defined above for the conformal mode, the non-conformal mode and
the punch through re-sputter mode. The process controller (or
processor) 210 therefore has three states to which the user may set
or program the processor 210 to enter into. One state is the
conformal deposition mode in which the processor 210 sets the D.C.
power level of the supply 24 to a low level, sets the power level
of the VHF generator 36 to a high level and the HF/LF bias
generator 36 to a low level. The another state is the non-conformal
mode in which the processor 210 sets the D.C. power level of the
supply 24 to a low level, sets the power level of the VHF generator
36 to a high level and the HF/LF bias generator 38 to zero (or
nearly zero) level. The remaining state is the punch through state
in which the processor 210 sets the D.C. power level of the supply
24 to a low level, sets the power level of the VHF generator 36 to
a high level and the HF/LF bias generator 38 to a high level.
[0064] The processor 210 may also govern the electromagnet current
source 45, so that in each of the three modes (FIGS. 11A, 11B,
11C), the current level is optimized for a more uniform radial
distribution of plasma ion density distribution.
[0065] The metal target 18 may assume a shape different from the
disk shape illustrated in FIG. 1. For example, as shown in FIG. 12,
a modified target 18' having an annular shape may be employed,
leaving a center portion 12a of the ceiling 12 exposed and an
annular portion 12' blocked by the target 18'. The overlying
magnetron 22' has a corresponding annular configuration.
Optionally, VHF plasma source power may be applied to the ceiling
center portion 12a by an additional VHF source power generator 36'
(dashed line in FIG. 12). This may be in addition to or in lieu of
the VHF source power generator 36 coupled to the wafer support
pedestal 14. However, it is preferred to couple VHF source power to
the pedestal 14 rather than to the ceiling 12.
[0066] FIG. 13 depicts another option, in which a coil antenna 400
may be placed over the ceiling center portion 12a and coupled to an
RF source power generator 410 through an impedance match circuit
415 to produce an inductively coupled plasma. A louvered shield 420
may cover the ceiling center portion 12a during metal deposition to
avoid metal coverage of the ceiling center portion 12a, so that the
coil antenna 400 will not be shielded from the plasma.
[0067] FIG. 14 illustrates how the configuration of the coil
antenna 400 and target 18' may be reversed from that illustrated in
FIG. 13. In FIG. 14, the coil antenna 400 is in an annular shape
while the disk-shaped target 18 is located at the ceiling center
portion 12a. The reactor of FIG. 14 may be modified as follows: The
annular coil antenna 400 may be removed and the peripheral portion
12' of the ceiling 12 may be conductive and coupled to the VHF
plasma source power generator 410 through the impedance match
circuit 415, as indicated in dashed line in FIG. 14.
[0068] FIG. 15 illustrates another alternative embodiment in which
the target 18 is configured in an inverted cup shape. As shown in
FIG. 16, the cup shape may be of sufficient height to accommodate
an array of magnets 450 along its side to enhance ion
distribution.
[0069] FIG. 17 is a flow diagram of a process carried out in the
PVD/re-sputter chamber of FIG. 1. In the step of block 1710, a
pre-clean process is performed by applying no power to the metal
target, introducing an etchant precursor gas such as a
fluorine-contain gas or a neutral gas such as argon, applying
sufficient VHF plasma source power to the wafer from the VHF
generator 36 and applying a small amount of HF bias power to the
wafer from the HF generator 38. In the following step of block
1720, the dielectric film (e.g., TaN) of a barrier layer is
deposited by introducing nitrogen gas and sputtering the metal
target (e.g., tantalum) while re-sputtering by maintaining the
VHF-driven argon plasma near the wafer and applying bias power to
the wafer. In the next step (block 1730), the metal film of the
barrier layer is deposited by stopping the supply of nitrogen gas,
so that pure metal is deposited while the VHF-driven argon plasma
performs re-sputtering. Thus, three successive processes may be
performed in the PVD re-sputter reactor of FIG. 1 without removing
the wafer from the reactor.
[0070] Use of the shield 20 in the reactor of FIG. 1 with the VHF
source power supply 36 can be problematic. This is because process
control can be lost due to uncontrolled plasma generation on the
outside of the shield. This can occur because the RF return path
for plasma near the wafer periphery is along the inside (wafer
side) surface of the shield 20 and up to the top of the shield 20,
and then down the inside surface of the chamber side wall 10
(traveling the entire height of the side wall 10) and along the
bottom of the chamber body to the wafer support pedestal 14. At a
VHF frequency such as 60 MHz, this RF return path exceeds more than
one wavelength, so that along the path there will be several
standing wave peaks of the VHF source power at which plasma is
likely to be generated. To the extent such a peak falls on the
outside of the shield 20, plasma will be generated on the outside
of the shield 20, outside of the wafer processing zone enclosed by
the shield 20. Such uncontrolled plasma generation will divert RF
source power from the wafer process, causing a loss of process
control.
[0071] This problem is solved by providing the RF connection bridge
or conductive tab 60 referred to above with reference to FIG. 2.
The RF connection bridge provides an extremely short RF return path
between the bottom edge of the shield 20 and the conductive base 51
of the wafer support pedestal 14. The RF return path for plasma
over the wafer 16 near the bottom of the shield 20 is therefore
much shorter than the VHF source power wavelength, so that there
are no standing wave peaks along the shield, and therefore no
undesirable plasma generation along surface of the shield 20.
[0072] FIGS. 18A and 18B illustrate a preferred embodiment of the
connection bridge or tab 60 as an elastically deformable
cylindrical spring 224 formed of a conductive material (e.g.,
copper). The cylindrical spring 224 is supported on a conductive
table 226 that is connected to the conductive base 51 of the wafer
support pedestal 14. As shown in FIG. 18A, when the pedestal 14 is
raised to the top position (where it is held during processing),
the cylindrical spring 224 is pressed against the bottom edge of
the shield 20 so as to be elastically deformed to at least
partially conform with the shape of the bottom corner of the shield
20, thereby providing a very low resistance connection to the
shield 20. This path extends from the shield 20 through the tab 60
and radially along the base 51 to the coaxial outer conductor 58 of
the coaxial RF feed to the pedestal. In FIG. 18B, the pedestal 14
has been withdrawn to a lower position, allowing the cylindrical
spring 224 to return to its undeformed cylindrical shape. The
cylindrical axis of the spring 224 lies generally parallel with the
plane of the wafer 16.
Copper Deposition Using a Single VHF Frequency:
[0073] The reactor of FIG. 1 may be used to deposit copper, such as
the copper seed layer 114 of FIG. 7, for example. This is
accomplished by using copper instead of tantalum as the metal
target 18 in the reactor of FIG. 1. However, using the reactor of
FIG. 1 for copper deposition involves special problems. One problem
involves undesirable formation (during copper deposition) of an
overhanging copper protrusion or "neck" on the sidewall of each
narrow aspect ratio opening or via (of the type illustrated in FIG.
8B). Specifically, it is more difficult in a copper deposition
process to prevent formation of a metal neck or protrusion on the
sidewall. The neck is formed from copper sputtered from the top
corner edge of the opening onto the opposing surface of the
sidewall. Such an overhanging metal formation neck near the top of
the opening (FIG. 8B) tends to protrude into the narrow opening or
via, eventually enclosing an unacceptable void at the bottom of the
opening by closing off the opening. Sputtering is mainly caused by
the carrier gas (Argon) ions. Sputtering of the top corner edge of
the via or narrow opening is very high relative to other features
because the sputtering rate is maximum for ion incidence angles
between 30 and 50 degrees. Using a carrier gas such as Argon to
promote the formation of kinetic electrons (for VHF plasma
generation near the wafer), we have found it difficult to find a
process window of power levels of the VHF and HF power sources 36,
38 in which overhang formation or close-off of the opening is
avoided. It seems that the HF bias power required to direct a
sufficient flux of copper atoms to the bottom of the via opening
(for good bottom coverage) has the undesired effect of creating
intense re-sputtering of copper from the top corner edge of the
narrow opening onto the opposing sidewall surface of the opening to
create the problematic copper protrusion on the sidewall (resulting
in a deposition profile similar to that illustrated in FIG. 8A). As
the sidewall protrusion grows, it can pinch-off the opening,
thereby preventing deposition of copper on the floor of the
opening. This problem likely is caused by two factors:
[0074] First, such harmful re-sputtering of the copper from the top
corner edge is worse at higher ion energy levels. This is a serious
problem because the HF and LF bias power frequencies of FIG. 1
produce a wide ion energy distribution having significant
components at very high ion energies (e.g., an ion energy component
corresponding to the peak-to-peak bias voltage).
[0075] Secondly, selection of a bias power level sufficient to
accelerate a sufficient amount of copper ions to the via floor
necessarily accelerates many more times that number of argon atoms
to the wafer, which sputter away the copper atoms from the top
corner edge of the narrow opening. This happens because copper and
argon have (for present purposes) fairly similar atomic masses, so
that the effect of the bias voltage in accelerating ions across the
plasma sheath at the wafer will be about the same for both copper
and argon. This is problematic because, due to the limited
efficiency of the target 18 of FIG. 1 as a source of copper, there
can be one (or more) orders of magnitude more Argon ions present
than Copper ions, so that Argon sputtering of Copper overwhelms the
deposition process. In some cases, sputtering by Argon ions can
also reach the via floor and compete with copper deposition on the
via floor.
[0076] We have solved this problem (at least for 65 nm feature size
design rules) by applying only VHF source power from the VHF source
36 while applying no HF or LF bias power from the HF/LF source 38.
In the preceding description of this specification, a similar mode
of operation of the reactor of FIG. 1 was mentioned in which no HF
or LF bias power is applied. At nominal VHF power levels (e.g., 100
Watts to 300 Watts), such a mode produces a non-conformal metal
deposition (characterized by a relatively thin sidewall coverage)
due to lack of re-sputtering without HF or LF bias power.
[0077] However, this mode is rendered highly conformal by
compensating for the lack of HF or LF bias power by increasing the
VHF source power level to a medium level or a high level, until
achieving a requisite flux of ions perpendicular to the wafer
surface (i.e., parallel to the axis of each high aspect ratio
opening) to obtain the desired re-sputtering effects. We have found
this achieves the highly conformal effects of the now-missing HF or
LF bias power. We have discovered that the lack of HF or LF bias
power may be completely compensated by increasing the VHF source
power level in this manner. This is believed to be because the VHF
source power level from the VHF source 36 of FIG. 1 may be
increased to the point at which good via floor coverage from direct
copper deposition and good sidewall coverage by copper
re-sputtering is obtained. (The re-sputtering effect is discussed
at length in previous portions of this specification.) This is
possible because increasing the VHF source power level does not
significantly increase ion energy but mainly increases the flux of
copper ions. This is particularly true if the VHF source power
frequency is particularly high, e.g., 60 MHz. Increasing source
power at such a VHF frequency does not appreciably increase ion
energy at the wafer surface because the frequency is too high for
ions to follow the oscillations. Instead, the principal portion (or
all) of the source power is consumed in generating kinetic
electrons from the carrier gas (which are sufficiently light to
follow the source power oscillations). This principal portion is
preferably greater than at least 50%. This principal portion may be
around 80% at a source power frequency of 60 MHz. As a result, the
undesirable effects of HF or LF bias power are avoided while
conformal copper deposition is achieved.
[0078] The increase in VHF power required to achieve the foregoing
effect may be significant. The nominal power range of VHF power at
which deposition is non-conformal lies in a range of about 50-300
Watts, depending upon chamber design and other process parameters
(pressure, gas composition, gas flow rate, etc.). The desired
degree of deposition conformality may be achieved in this novel
single frequency conformal mode by increasing the VHF power to a
medium level, between approximately 300 to 800 Watts. In some
cases, it may be necessary to increase the VHF power to a high
level (800-1500 Watts) before realizing a desired degree of
conformality. The increase in VHF power required to carry out this
single frequency conformal mode is readily ascertained empirically
by measuring deposition conformality at increasing VHF power
levels.
[0079] In summary, a new copper deposition mode of the reactor of
FIG. 1 is one in which the metal target 18 is copper, and the only
RF power source employed is the VHF plasma source power supply 36.
No plasma bias power is applied (i.e., RF power having a
sufficiently low frequency to be followed by ions such as copper).
In this single frequency mode, the VHF source power level is
increased to reach a high degree of conformality in the deposited
copper layer. Specifically, the VHF source power is increased until
a sufficient flux of copper ions at the wafer is achieved to
provide good copper coverage on the floors of high aspect ratio
openings with sufficient re-sputtering to provide adequate sidewall
copper coverage. As in the other modes disclosed previously in this
specification, the High Voltage D.C. supply 24 and the magnetron 22
need only provide a medium amount of power, and the copper plasma
generated near the target 18 may or may not be self-ionizing.
Dual Frequency Copper Deposition Modes:
[0080] While the single frequency copper deposition mode described
immediately above is effective at feature sizes as small as 65 nm,
it is felt that copper deposition at smaller feature sizes (e.g.,
45 nm, 35 nm) are best accomplished using an HF or LF bias in
combination with VHF source power. As discussed above, the VHF
power may be increased to increase the flux of copper ions without
a proportionate increase in ion energy. This advantage is enhanced
by increasing the VHF source power frequency from 60 MHz to 81 MHz.
With such an elevated frequency, the principal portion of the
source power dedicated to kinetic electron generation (to generate
plasma ions) is as high as 90% or 95%, almost none of it increasing
ion energy. Thus, the VHF source power level may be increased to an
even greater degree (without a significant harmful increase in ion
energy) in order to increase copper ion flux at the wafer to
increase copper deposition at the via floor and enhance uniform
copper re-sputtering for uniform deposition on the via side walls.
This feature reduces the required HF bias power level for a
conformal copper deposition. For example, the required HF power
level may be reduced from 5 kWatts to 3 kWatts, and in some cases
to even lower levels (e.g., 1 k Watt). By reducing the HF bias
power level, undesirable copper sputtering by ions of the carrier
gas (e.g., argon) is reduced accordingly. It should be noted that
the HF bias power is of a sufficiently low frequency so that ions
near the plasma sheath readily follow its oscillations so that
nearly all of its power is consumed in accelerating ions toward the
wafer and thereby increasing ion energy.
[0081] In order to regulate or prevent re-sputtering of the copper
from the top corner via edge to the via sidewall by ions of the
carrier gas (e.g., argon ions), a carrier gas is employed having a
much lower atomic mass than copper. Specifically, a light inert gas
(helium) is employed as the carrier gas. Although helium has a low
atomic mass, it nevertheless suffices nearly as well as argon to
produce, under the stimulation of the VHF source power, sufficient
kinetic electrons to produce an adequate copper plasma near the
wafer surface. If the carrier gas ions are much lighter than the
copper ions, then they do not sputter copper atoms from the via top
corner edge onto the via sidewall at such a fast rate. This feature
therefore achieves enhanced control of the re-sputtering process.
In some embodiments employing a higher bias power level, there is
an additional problem of Argon ions sputtering deposited copper
atoms from the via floor faster than the copper deposition rate.
This additional problem is solved by using the lighter (Helium)
carrier gas, because the lighter Helium atoms do not produce such a
high copper sputter rate. As a result, the re-sputtering of copper
from the top corner via edge to the via sidewall is better
regulated or controlled. Furthermore, the additional problem
(encountered in some cases) of removing copper from the via floor
by the more populous carrier gas ions is eliminated or at least
reduced. An inert gas such as helium is selected because it
introduces no additional chemical reactions or effects and does not
chemically react with the deposited copper.
[0082] Another problem unique to copper deposition is that the
surface of the deposited copper has a relatively high surface
energy, and is active (like a fluid) during plasma enhanced
deposition, enabling it to reduce its surface energy by
accumulating into small globs or balls of copper on the surface.
The result is formation of a low-quality copper surface. This
problem is solved by adding to the carrier gas a species that
combines with copper bonds on the deposited copper surface (on the
wafer) to reduce the copper surface energy. The preferred choice
for such a surface energy-reducing agent is hydrogen. In the helium
plasma, diatomic hydrogen molecules dissociate into monatomic
hydrogen, which adheres to open copper atomic bonds in the surface
of the deposited copper layer. This surface reaction reduces the
copper surface energy, allowing the deposited copper atoms to flow
during deposition in thin uniformly flat layers, thereby forming a
uniformly smooth high quality copper surface.
[0083] This embodiment is also effective for removing copper oxide
from the deposited copper, or for preventing formation of copper
oxide during processing. In removing copper oxide, the hydrogen
reduces the copper oxide, capturing oxygen atoms to form water
molecules that are released into the chamber.
[0084] The hydrogen gas may form as much as 10% of the process gas.
If the reactor is specially constructed to handle pure hydrogen
gas, then hydrogen gas may constitute all or nearly all of the
process gas.
[0085] The bias power window or range within which the copper
deposition rate at the 65 nm via floor exceeds the neck growth rate
on the sidewall is a "65 nm window". The bias power window or range
within which the copper deposition rate at the 45 nm via floor
exceeds the neck growth rate on the sidewall is a "45 nm window",
which is significantly narrower than the 65 nm window because of
the greater aspect ratio of the opening. Within each of these
windows, the copper floor deposition rate exceeds the sidewall neck
growth rate sufficiently to allow copper deposition on the via
floor to reach a target thickness (e.g., 50-500 Angstroms) before
the sidewall copper neck pinches off the opening. The problem was
that these windows either did not exist or were too narrow for
practical operation.
[0086] One way in which the process window is widened in the
present invention is by increasing the VHF source power level,
which increases the flux of copper ions without a concomitant
increase in sputtering or neck growth rate. Another way of widening
the process window is to employ a low-atomic mass carrier gas
species (Helium). This tends to decrease the neck growth rate by
reducing sputtering. Using a low atomic mass carrier gas also tends
to increase the copper deposition rate at the via floor, by
preventing the removal of deposited copper from the floor surface
by sputtering from the carrier gas.
[0087] Another advantage of the VHF source power is that it further
widens the process window by at least partially masking the
nonuniformity in radial distribution of the HF (or LF) bias power.
In some cases, bias power radial distribution is non-uniform across
the wafer surface, which restricts the allowable RF power range (to
avoid exceeding process limits in any radial location on the
wafer), thereby narrowing the bias power process windows. At least
partially masking this nonuniformity by applying VHF source power
reduces this effect, thereby preserving a wider process window.
[0088] In summary, a highly conformal layer of copper is deposited
on the floor and sidewall of high aspect ratio openings in the dual
frequency reactor of FIG. 1. For this process, the metal target 18
of FIG. 1 is copper. The process gas is about 80% to 95% helium and
about 5% to 10% hydrogen. The bias power is either an LF or HF
frequency (or a combination of both) at a preferred level of about
20-40 Watts, or under 100 Watts. The source power is a VHF
frequency of about 60 MHz, although superior performance may be
achieved using a VHF source power frequency of 81 MHz. Similarly,
in all of the embodiments described above in this specification,
the VHF source power frequency may be increased to 81 MHz to
improve process performance. This may allow the VHF source power
level to be further increased in order to enhance process
performance without increasing the ion energy.
PVD Reactor with ESC and Efficient VHF Radiator:
[0089] In the reactor of FIG. 19, the low efficiency and low power
capacity of the electrostatic chuck as a VHF radiator is
circumvented by integrating the sputter target and rotating magnet
in a highly efficient VHF radiator having high maximum VHF power
capability.
[0090] Referring to FIG. 19 now, the reactor includes a vacuum
chamber defined by a cylindrical side wall 10, a disk-shaped
ceiling 12, and a wafer support pedestal 14 for supporting a
semiconductor wafer 16 to be processed. A target 18 of a metal
(e.g., copper) to be deposited on the wafer 16 is mounted on the
ceiling 12. A process kit consisting of a cylindrical shield 20
surrounds the wafer 16 and the target 18. A rotating magnet
("magnetron") 22 of the conventional type overlies the target 18 on
the external side of the ceiling 12. A high voltage D.C. source 24
is coupled to the target 18 through a low-pass filter 25 that
includes a series inductor 25a and a shunt capacitor 25b. A process
gas injector 26 furnishes process gas from a supply 28 into the
interior of the chamber. A vacuum pump 30 maintains a desired
sub-atmospheric pressure in the vacuum chamber.
[0091] The wafer pedestal 14 is coupled through an impedance match
network 34 to an HF or LF plasma bias power generator 38. The LF
bias power applied to the wafer determines the selectivity of the
re-sputter/etch process for horizontal surfaces. Plasma uniformity
is enhanced by an electromagnetic coil 43 wrapped around the
cylindrical sidewall of the reactor chamber and supplied with D.C.
current by a current source controller 45.
[0092] The wafer support pedestal 14 is an electrostatic chuck
(ESC) of the type illustrated in FIG. 2. Referring to FIG. 2, the
wafer support pedestal 14 of FIG. 19 consists of an insulating
layer 50 mounted on a conductive base 51 and an electrode 52 such
as a conductive mesh buried inside the insulating layer 50. The
pedestal structure can extend downwardly in a coaxial arrangement
consisting of a center conductor 54 connected to the electrode 52,
an insulating intermediate layer 56 and an outer conductor 58
connected to the conductive base 51. The conductive base 51 can be
coupled to the bottom of the cylindrical shield 20 by conductive
tabs 60 to provide a more continuous ground reference. The center
conductor 54 is coupled through an isolation capacitor 76 to the RF
match 34 while the outer conductor 58 is grounded. The impedance
match network 34 is coupled to the RF (HF or LF) plasma bias power
generator 38. In addition, a D.C. chuck voltage source 74 is
connected through a low pass filter or inductor 73 to the wafer
pedestal center conductor 54.
[0093] In FIG. 19, the magnetron 22 has a center axle 80 coupled to
a center arm 81 that is coupled to a magnet array 85. An electric
motor (not shown) produces orbital motion by rotating the center
axle 80.
[0094] FIG. 20 depicts an alternative embodiment, in which the
rotating magnet or "magnetron" 22 has its center axle 80 coupled to
the proximal end of the center arm 81, a dual planetary axle 82
connected to the distal end of the center arm 81 and to the
proximal end of a planetary arm 83. A dual spin axle 84 is
connected to the distal end of the planetary arm 83 and a magnet
array 85 that is spun around the axle 84. Planetary motion of the
magnet array can be attained through a planetary gear arrangement
(not shown).
[0095] In the preferred embodiment of FIG. 19, the magnets 85a
within the magnet array 85 may be permanent magnets arranged
symmetrically in the manner of FIG. 21 so as to produce a toroidal
confinement pattern of the ions near the target 18, in accordance
with the magnetic confinement pattern 85b outlined in dashed line
in FIG. 21. In one implementation, the target 18 was nineteen
inches in diameter and the magnet array 85 was five inches in
diameter.
[0096] Referring again to FIG. 19, the metal (copper) sputter
target 18 and the magnetron 22 are integrated in a highly efficient
high power VHF radiator or applicator by providing a relatively
thick (0.75 inch diameter) RF rod 86 embedded in the metal target
18 and extending axially through a cylindrical hollow in the center
axle 80 and through the ceiling 12. The RF rod 86 may be
electrically insulated from the center axle 80. A VHF impedance
match circuit 87 is coupled to (or mounted on) the external (top)
end of the rod 86 and a VHF source power generator 88 is coupled to
the match circuit 87. An output capacitor (not shown) of the RF
impedance match 87 isolates the match 87 from D.C. current from the
D.C. supply 24. Preferably, the VHF generator 88 applies RF power
at a frequency of about 81 MHz at a power level of about 3.5 kW (or
more). In one implementation, the target 18 is copper and the RF
rod 86 is a 0.75 inch diameter copper rod that is threaded into the
target 18. In any case, the copper rod 86 is relatively thick,
being at least 0.50 inch in diameter or thicker (0.70 inch or more
in diameter).
[0097] FIG. 22 illustrates another embodiment in which plural RF
rods 90 extend radially from the VHF impedance match circuit 87 to
axial RF rods 91 that are coupled to the edge of the target 18. In
the illustrated embodiment, the RF match circuit 87 is centered
relative to the disk-shaped metal target 18, the rods 90 are of a
uniform length and the rods 91 are of a uniform length. The RF rods
90, 91 are sufficiently thick (e.g., 0.70 inch in diameter) to
provide highly efficient coupling of VHF power and withstand high
VHF power levels (e.g., 3.5 kW and above). In the embodiment of
FIG. 22, there may be three sets of symmetrically arranged RF rods
90, 91 (as shown in the top view of FIG. 23) or four sets of
symmetrically arranged RF rods 90, 91 (as shown in the top view of
FIG. 24) or more.
[0098] In the embodiments of FIGS. 1 and 19, the D.C. power from
the source 24 applied to the metal (copper) target 18 initially
ionizes the carrier gas (e.g., helium), which starts the sputtering
of the metal target 18. A relatively small fraction of the metal
atoms sputtered from the target 18 are ionized in this process.
Ions of the carrier gas (e.g., helium) produced by the D.C. power
and some copper ions are confined at the surface of the target 18
by the field of the magnetron 22 in the toroidal confinement
pattern discussed above. The ionization fraction is insufficient
for deposition on floors of high aspect ratio openings on the
wafer. In order to provide a very high ionization fraction (e.g.,
in excess of 80%) that is capable of such deposition, VHF power is
employed to ionize the neutral metal atoms sputtered from the
target 18. For this purpose, in the embodiment of FIG. 1, VHF power
was coupled to the wafer to generate a VHF plasma near the wafer
surface, which was possibly separate from the plasma generated at
the target 18 by the D.C. power applied to the target. In FIG. 1,
the maximum VHF power level was severely limited by the fine
geometry of the mesh electrode of the ESC, thus limiting the
maximum ionization fraction at the wafer to as low as 20% in some
cases. We feel that an optimum ionization fraction for deposition
in high aspect ratio openings of a 65 nm or 45 nm feature size is
closer to or in excess of 80%. Therefore, in order to overcome the
limitation of the ESC, in the embodiment of FIG. 19 the VHF power
is applied to the target 18 through the thick RF rod 86, as
described above. The thickness of the metal target 18 and the
thickness of the RF rod 86 is such that very high levels of VHF
power may be applied to the target 18 and, moreover, the target 18
is a relatively efficient VHF radiator. As a result, VHF power
delivered to the plasma may be nearly tripled, in some cases. The
wafer-to-target spacing can now be reduced, as the invention
achieves a good level of ionization at a controllable deposition
rate, without the need for a large spacing.
[0099] In operation, the ionization fraction is no longer limited
(e.g., below 20% in some cases) by the low power threshold and
efficiency of the ESC 14, and instead very high levels of VHF power
may be applied by the VHF generator 88. The resulting high
ionization fraction (e.g., greater than 80% in some cases)
eliminates the need for very high D.C. power from the supply 24
while enabling, for the first time, deposition of highly conformal
coatings in very high aspect ratio openings or vias. Preferably,
the D.C. target power level is in a range of about 1000 Watts to
2500 Watts. However, the deposition rate can be tightly controlled
and set at very low levels by reducing the D.C. target power from
the supply 24 to very low levels (for example, 500 Watts or less),
while using the VHF power boost the ionization fraction to the
desired level. This reduces the metal deposition rate sufficiently
to eliminate the need for a large wafer-to-target spacing.
Heretofore, without the use of VHF power, the high level of D.C.
power required to attain the desired ionization fraction produced
uncontrollably high deposition rates unless the wafer-to-ceiling
distance was increased to about 400 mm (to increase the deposition
time to a mere 7 seconds). With VHF power and D.C. power driving
the target 18, the wafer-to-target spacing may be reduced to 50-70
mm or less (for example, for processing a 300 mm diameter wafer).
The result is that there is very little (or no) decrease in
ionization fraction from the VHF-driven metal target 18 to the
wafer. In this way, the VHF plasma, generated by driving the
(copper) target 18 with the VHF generator 88, is very close to the
wafer, so that there is no need to apply VHF to the ESC 14 for
sufficient ionization at the wafer. The D.C. power may be reduced
as desired without appreciably reducing ionization fraction,
provided the VHF power is not reduced. Therefore, it is preferable
to reduce or limit the D.C. power to obtain a highly controllable
deposition rate at the wafer, e.g., a deposition process that
requires on the order of a minute (in contrast to the earlier
techniques that afforded a deposition process time on the order of
only seconds and which was therefore difficult to control).
[0100] Application of HF power to the ESC 14 to generate an
adequate rate of re-sputtering of copper from horizontal surfaces
or corners onto the high aspect ratio opening sidewalls has been
discussed previously in this specification. The HF power level
required for this purpose is sufficiently low so that the
limitations of the ESC 14 (e.g., the fine mesh electrode) do not
limit the re-sputtering process. Therefore, the HF power for
controlling re-sputtering is applied through the ESC 14 in the
reactor of FIG. 19.
[0101] An advantage of reducing the wafer-to-ceiling spacing is
that the surface area of the shield 20 is reduced, which reduces
the amount of sputtered metal (e.g., copper) that is wasted by
depositing on the shield instead of the wafer. As a result, the
metal target 18 and the shield 20 can be used to process a greater
number of wafers (e.g., 20,000 wafer) before being replaced, thus
reducing the per-wafer cost of operating the reactor. This
represents an improvement of about an order of magnitude.
[0102] Applying D.C. power and VHF power to the target 18
simultaneously while applying HF or LF power to the ESC 14
facilitates the independent simultaneous control of three key
parameters: deposition rate, ionization fraction and re-sputtering
(re-flow) rate. The deposition rate is controlled by the level of
D.C. power applied to the target 18 by the D.C. supply 24. The
ionization fraction is controlled by the level of VHF power applied
to the target 18 by the VHF generator 88. The re-sputter rate is
controlled by the level of HF (or LF) power applied to the ESC 14
by the HF (or LF) generator 38. In a preferred operating mode, very
low D.C. power (e.g., less than 500 Watts) is applied to the
sputter target for a very low deposition rate, very high VHF power
(e.g., over 3.5 kwatts) is applied to the sputter target for a high
ionization fraction sufficient for conformal coating in very high
aspect ratio openings, and a moderate level of HF power is applied
to the ESC 14 to provide an adequate rate of re-sputtering of
deposited metal for re-deposition of metal on the sidewalls of high
aspect ratio openings. The deposition rate may be increased if
desired (by increasing the target D.C. power) up to a limit at
which the flux of metal atoms through the VHF-generated plasma
exceeds the ionization rate of which the plasma is capable, at
which point the ionization fraction decreases significantly.
Conversely, the VHF power may be decreased while applying a given
level of D.C. power to the target 18, until the same limit is
reached at which the VHF-plasma density is overwhelmed by the flux
of metal atoms from the target.
[0103] In one working example, the VHF power level was 3.5 kW. This
induced a relatively small or negligible D.C. voltage on the target
18, so that the deposition rate is controlled almost entirely by
the D.C. target power. The D.C. power applied to the target 18 in
this example was 500 Watts, the D.C. supply furnishing about 300
Volts and about 1.5 Amperes.
[0104] In one embodiment, the reactor of FIGS. 19 and 20 was
employed for plasma enhanced physical vapor deposition of copper,
the target 18 being copper. Helium gas was employed as the carrier
gas to moderate the re-sputter rate at the wafer, and hydrogen was
included in the process gas (up to about 10% hydrogen) to prevent
de-wetting of the deposited copper on the sidewalls of high aspect
ratio openings. The reactor of FIGS. 19 and 20 may be used to
deposit barrier layers (such as a tantalum nitride barrier layer
and/or a tantalum barrier layer). In such a case, the target 18 is
tantalum (or titanium or other suitable barrier metal). For a
deposition of a tantalum nitride barrier layer, nitrogen gas is
introduced into the chamber.
[0105] As illustrated in FIG. 25, the metal sputtering target may
be shaped as a truncated cone 18' surrounding a flat circular
ceiling 12a. The RF rod 86 engages the target 18' in a manner
similar to that of FIG. 19, but is necessarily off-center because
of the target shape. An alternative symmetrical or centered
arrangement is suggested in dashed line, in which the VHF source
87, 88 is axially centered and feeds the target through multiple RF
rods 86 that may be evenly spaced.
[0106] As illustrated in FIG. 26, the target 18 may be in
registration with the centered circular ceiling 12a and surrounded
by the truncated conical ceiling section 12'. One option (not
illustrated in FIG. 26) is to provide an inductive antenna over the
truncated conical ceiling section 12' in the manner of FIG. 14.
[0107] As illustrated in FIG. 27, the metal sputtering target 18
may be shaped as an inverted bell or "U" shape, in the manner of
FIG. 16. In one implementation, the RF rod 86 extends upwardly from
the center of the target, as illustrated in solid line in FIG. 27.
However, a preferred way may be to feed the VHF power at plural
evenly spaced feed points 18a, 18b along the periphery of the
U-shaped target 18 of FIG. 27, through radial and axial RF rods
90', 91' as indicated in dashed line.
[0108] The flow diagram of FIG. 28 illustrates a method of
performing physical vapor deposition of copper onto an integrated
circuit in a vacuum chamber of a plasma reactor, as follows:
providing a copper target near a ceiling of the chamber (block 92
of FIG. 28), placing an integrated circuit wafer on a wafer support
pedestal facing the target near a floor of the chamber (block 93),
introducing a carrier gas into the vacuum chamber (block 94),
establishing a deposition rate on the wafer by applying D.C. power
to the copper target (block 95), establishing a desired plasma
ionization fraction near the wafer by applying VHF power to the
copper target (block 96), promoting re-sputtering of copper on
vertical surfaces on the wafer by coupling HF or LF power to the
wafer (block 97), and maintaining a sufficiently small distance
between the wafer and the target so that the VHF power control
plasma ionization fraction at the surface of the wafer (block
98).
[0109] A similar process may be employed using the reactor of FIG.
19 to deposit a barrier layer, such as tantalum and tantalum
nitride barrier layers, prior to copper deposition. For this
purpose the target 18 is tantalum. In order to deposit a tantalum
nitride barrier layer, nitrogen is added to the process gas.
[0110] While the invention has been described in detail with
reference to preferred embodiments, it is understood that
variations and modifications thereof may be made without departing
from the true spirit and scope of the invention.
* * * * *