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BACKGROUND AND SUMMARY OF THE INVENTION
This invention relates generally to integrated circuit processes and
fabrication, and more particularly, to a system and method of adhering
copper to a diffusion barrier surface.
The demand for progressively smaller, less expensive, and more powerful
electronic products, in turn, fuels the need for smaller geometry
integrated circuits (ICs), and large substrates. It also creates a demand
for a denser packaging of circuits onto IC substrates. The desire for
smaller geometry IC circuits requires that the interconnections between
components and dielectric layers be as small as possible. Therefore,
research continues into reducing the width of via interconnects and
connecting lines. The conductivity of the interconnects is reduced as the
surface area of the interconnect is reduced, and the resulting increase in
interconnect resistivity has become an obstacle in IC design. Conductors
having high resistivity create conduction paths with high impedance and
large propagation delays. These problems result in unreliable signal
timing, unreliable voltage levels, and lengthy signal delays between
components in the IC. Propagation discontinuities also result from
intersecting conduction surfaces that are poorly connected, or from the
joining of conductors having highly different resistivity characteristics.
There is a need for interconnects and vias to have both low resistivity,
and the ability to withstand volatile process environments. Aluminum and
tungsten metals are often used in the production of integrated circuits
for making interconnections or vias between electrically active areas.
These metals are popular because they are easy to use in a production
environment, unlike copper which requires special handling.
Copper is a natural choice to replace aluminum in the effort to reduce the
size of lines and vias in an electrical circuit. The conductivity of
copper is approximately twice that of aluminum and over three times that
of tungsten. As a result, the same current can be carried through a copper
line having half the width of an aluminum line.
The electromigration characteristics of copper are also much superior to
those of aluminum. Copper is approximately ten times better than aluminum
with respect to electromigration. As a result, a copper line, even one
having a much smaller cross-section than an aluminum line, is better able
to maintain electrical integrity.
There have been problems associated with the use of copper, however, in IC
processing. Copper pollutes many of the materials used in IC processes
and, therefore, care must be taken to keep copper from migrating. The
migration of copper into silicon semiconductor regions is especially
harmful. The conduction characteristics of the semiconductor regions are a
consideration in the design of a transistors. Typically, the fabrication
process is carefully controlled to produce semiconductor regions in
accordance with the design. Elements of copper migrating into these
semiconductor regions can dramatically alter the conduction
characteristics of associated transistors.
Various means have been suggested to deal with the problem of copper
diffusion into integrated circuit material. Several materials, especially
metallic ones, have been suggested for use as barriers to prevent the
copper diffusion process. For example, Cho et al., in the article entitled
"Copper Interconnection with Tungsten Cladding for ULSI," 1991 Symposium
on VLSI Technology, pg. 39, suggests the use of tungsten as a diffusion
barrier. Molybdenum and titanium nitride (TiN) have also been suggested
for use as copper diffusion barriers. Gardner, et al., in an article
entitled "Encapsulated Copper Interconnection Devices Using Sidewall
Barriers," in 1991 VMIC Conference, pg. 99, suggests the use of sidewall
structures to completely encapsulate the copper. However, the adhesion of
copper to these diffusion barrier materials has been, and continues to be,
an IC process problem.
Copper cannot be deposited onto substrates using the conventional processes
for the deposition of aluminum when the geometries of the selected IC
features are small. That is, new deposition processes have been developed
for use with copper in the lines and interconnects of an IC interlevel
dielectric. It is impractical to sputter metal, either aluminum or copper,
to fill small diameter vias, since the gap filling capability is poor. To
deposit copper, a chemical vapor deposition (CVD) technique has been
developed in the industry.
In a typical CVD process, copper is combined with a ligand, or organic
compound, to make the copper volatile. That is, copper becomes an element
in a compound that is vaporized into a gas. Selected surfaces of an
integrated circuit, such as diffusion barrier material, are exposed to the
copper gas in an elevated temperature environment. When the copper gas
compound decomposes, copper is left behind on the selected surface.
Several copper gas compounds are available for use with the CVD process.
It is generally accepted that the configuration of the copper gas
compound, at least partially, affects the ability the copper residue to
adhere itself to the selected surface.
Wang, et al. in the article "Chemical Mechanical Polishing of Copper
Metalized Multi-level Interconnection Devices," 1995 VMIC Conference, pg.
505, suggests the use of one particular copper gas compound, or precursor,
for improving the adhesion of copper to a TiN barrier surface. Although
certain precursors may improve the copper adhesion process, variations in
the diffusion barrier surfaces to which the copper is applied, and
variations in the copper precursors themselves, often result in the
unsatisfactory adhesion of copper to a selected surface.
It has become standard practice in the industry to apply CVD copper
immediately after the deposition of the diffusion barrier material to the
IC. Typically, the processes are performed in a single chamber or an
interconnected cluster chamber. It has generally been thought that the
copper layer has the best chance of adhering to the diffusion barrier
material when the diffusion barrier material surface is clean. Hence, the
diffusion barrier surface is often kept in a vacuum, or controlled
environment, and the copper is deposited on the diffusion barrier as
quickly as possible. However, even when copper is immediately applied to
the diffusion barrier surface, problems remain in keeping the copper
properly adhered. A complete understanding of why copper does not always
adhere directly to a diffusion barrier surface is lacking.
It would be advantageous to employ a method of improving the adhesion of
CVD copper to a diffusion barrier material surface.
It would also be advantageous if a method were employed for preparing a
diffusion barrier surface, in advance of CVD copper deposition, to improve
the adhesion of copper of the diffusion barrier surface.
Further, it would be advantageous if the adhesion improving process did not
degrade the electrical conductivity between the deposited copper and a
conductive diffusion barrier material. It would also be advantageous if
the process did not disrupt the silicon bonds and structures in adjoining
IC substrates.
Accordingly, a method of applying copper to selected integrated circuit
surfaces is provided. The selected copper-receiving surfaces are
predominately on diffusion barrier material applied to selected regions of
the IC. The method comprises the steps of: exposing each selected
copper-receiving surface to a reactive oxygen species; oxidizing a thin
layer of the diffusion barrier material surface in response to the oxygen
exposure; and stopping the exposure of the diffusion barrier material to
the oxygen before the oxide layer exceeds approximately 30 angstroms
(.ANG.), whereby the relatively thin oxide layer prepares the diffusion
barrier material receiving surface for adhesion to copper.
In a preferred embodiment of the invention, the method includes generating
the reactive oxygen species from a predominately oxygen plasma. A
preferred embodiment includes generating the reactive oxygen species from
an oxygen-contained plasma, with the oxygen-contained gas being selected
from the group consisting of CO, NO.sub.2, N.sub.2 O, and H.sub.2 O.
The method also provides a further step of depositing CVD copper on the
oxidized diffusion barrier material surface, whereby the copper is adhered
to a material which prevents the diffusion of copper into regions of the
IC underlying the diffusion barrier. A preferred embodiment of the
invention includes using a direct plasma source having a radio frequency
(RF) power level of less than approximately 200 watts to generate the
plasma, whereby the relatively low energy level of the plasma ions
minimizes the disruption of silicon crystalline structures. In its
preferred form, the Cu-receiving surface is exposed to reactive oxygen
species at a substrate temperature of less than approximately 200.degree.
C. to protect the silicon crystalline structure of the IC.
An integrated circuit is also provided comprising a first substrate layer
of diffusion barrier material having a surface. The integrated circuit
further comprises a layer of oxide having a thickness of less than
approximately 30 angstroms and a surface, the oxide layer overlying the
first substrate surface. The integrated circuit also comprises a layer of
copper overlying the oxide surface, whereby the oxide layer promotes
adhesion between the copper layer and the first substrate surface.
The integrated circuit further comprises a second substrate layer having a
surface underlying the diffusion barrier layer, whereby the diffusion
barrier prevents migration of the copper into the second substrate layer.
In a preferred embodiment of the invention the diffusion barrier material
is conductive and selected from the group consisting of TiN, TiON, TiSiN,
TaSiN, TaN, TiW, TiWN, Mo, and WN, whereby the copper layer is adhered to
a barrier material which permits electrical communication between the
copper layer and the second substrate surface.
A co-pending application Serial No. 08/717,315, filed Sep. 20, 1996,
entitled "Copper Adhered to a Diffusion Barrier Surface and Method for
Same", invented by Lawrence Charneski and Tue Nguyen, Docket No. SMT 243,
which is assigned to the same assignees as the instant patent, discloses a
method for using a variety of reactive gas species to improve copper
adhesion without forming an oxide layer over the diffusion barrier.
It has been standard practice in the industry to keep a diffusion barrier
surface, located on a selected surface of an IC, in a controlled
environment whenever possible, and to apply the copper as quickly as
possible. This practice is based on the belief that protecting the
diffusion barrier from uncontrolled gas environments, and keeping the
barrier clean, provide the best foundation for copper adhesion. However,
as demonstrated in the present invention, a thin layer of oxide promotes
chemical bonding between the copper layer and the diffusion barrier
surface. An oxide thickness of approximately 30 angstroms, or less, is
thick enough to promote chemical bonding, and thin enough to allow the
tunneling of electrons between the copper and the diffusion barrier so
that electrical conductivity is not degraded.
The above disclosed preparation of the diffusion barrier surface for a
deposition of CVD copper significantly improves the adhesion of deposited
copper to the diffusion barrier surface. The thin layer of oxide, formed
by exposure of the diffusion barrier surface to the reactive oxygen atoms,
improves the chemical bonding between the copper and diffusion barrier
material. Because of the relative thinness of the oxide layer, electrical
conductivity between the copper and the diffusion barrier surface is not
adversely affected.
The low power levels and temperatures required to perform this process
insure that minimum damage is done to the associated substrates in the
integrated circuit. Since the plasma exposure process is generally
completed in less than 60 seconds, a minimum of damage is done to the IC
crystalline structures and a speedy, commercially viable, process is
insured. The improved adhesion resulting from the oxide layer permits a
greater degree of variation in the uniformity of the diffusion barrier
surface and the precursor. Further, the oxidation process and the copper
deposition processes can be carried out in different chambers, and at
different times, because of the reduced concern over the cleanliness of
the processed diffusion barrier surface.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1 through 4 illustrate steps in the method of forming a completed
integrated circuit with a diffusion barrier surface prepared for copper
adhesion.
FIG. 5 is a flow chart illustrating the steps in the method of applying
copper to selected copper-receiving surfaces of an integrated circuit.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
FIGS. 1 through 4 illustrate steps in the method of forming a completed
integrated circuit with a diffusion barrier surface prepared for copper
adhesion. FIG. 1 illustrates an integrated circuit having a first
substrate layer of diffusion barrier material 10. Diffusion barrier 10 has
a surface 12 for receiving copper. The IC also comprises a second
substrate layer 14 having a surface 16 underlying diffusion barrier layer
10. Diffusion barrier material 10 is applied to selected areas of the IC
to prevent the migration of copper into second substrate layer 14.
FIG. 2 is the IC of FIG. 1 with copper-receiving surface 12 being exposed
to a reactive oxygen species. The oxygen exposure is represented by arrows
18. A thin layer of diffusion barrier material surface 12 is oxidized in
response to oxygen exposure 18.
FIG. 3 is the IC of FIG. 2 with a layer of oxide 20 having a thickness 22
of less than approximately 30 angstroms and a surface 24. Oxide layer 20
overlies first substrate surface 12. The exposure of diffusion barrier
material 10 to oxygen 18 is stopped before oxide layer 20 exceeds a
thickness of approximately 30 angstroms.
FIG. 4 is the integrated circuit of FIG. 3 with a layer of copper 26
overlying oxide surface 24. Oxide layer 24 promotes adhesion between
copper layer 26 and first substrate surface 10. In a preferred form of the
invention, diffusion barrier material 10 is conductive and selected from
the group consisting of TiN, TiON, TiSiN, TaSiN, TaN, TiW, TiWN, Mo, and
WN. When one of these materials is used as diffusion barrier material 10,
copper layer 26 is adhered to a barrier material which permits electrical
communication between copper layer 26 and regions of the IC underlying
diffusion barrier material 10, such as second substrate surface 16.
Typically, copper layer 26 is deposited to form an electrical interface to
second substrate layer 14 when substrate layer 14 is a transistor
semiconductor region. Depositing copper layer 26 directly upon substrate
surface 16 would allow copper to migrate into the semiconductor material,
altering its intended conduction characteristics. Diffusion material 10,
located between copper layer 26 and second substrate 14, prevents the
diffusion of copper into semiconductor substrate 14. When diffusion
material 10 is conductive, such as TiN, then electrical communication
between copper layer 26 and the semiconductor second substrate layer 14 is
maintained as semiconductor substrate 14 is protected from copper
diffusion. In this manner, safe and effective electrical interfaces are
made between copper lines and vias to silicon semiconductor material.
It is a feature of the invention that the exposure of oxide layer 20 is
stopped to produce oxide layer 20 being generally in the range between 30
angstroms and 50 angstroms. When oxide layer 20 has this increased
thickness, the bonding between copper layer 26 and diffusion barrier
material surface 12 is promoted when electrical conductivity between
copper layer 26 and diffusion barrier material surface 12 is not required.
The thicker oxide layer 20 enhances adhesion with the use of certain types
of diffusion barrier material 10.
In a preferred embodiment, oxide layer 20 has a thickness generally in the
range of 30 angstroms to 50 angstroms overlying a nonconductive diffusion
barrier material 10. That is, diffusion material 10 is also used with
copper layer 26 when second substrate 14 is a nonconductive material, such
as SiO, Si.sub.3 N.sub.6, or BN, buffering individual semiconductor
regions (not shown). Diffusion barrier 10 is used in these circumstances
when there is a fear that copper will migrate through the second substrate
buffer region 14 to a semiconductor region (not shown). When covering
non-conductive second substrate 14, electrical conductivity through oxide
layer 20 is not a concern, so that oxide thickness 22 can be thickened to
promote greater adhesion.
It is also a feature of the present invention that diffusion barrier
material is non-conductive and selected from the group consisting of BN,
Si.sub.3 N.sub.4, SiBN. When diffusion barrier material 10 is selected
from one of the above mentioned materials, the copper is adhered to in
electrically insulating material. Preferably, copper layer 26 is adhered
to a barrier material 10 that has a low dielectric constant to minimize
the capacitance between copper layer 26 and diffusion barrier 10. As is
well known in the art, the use of a low dielectric material under a
conductor allows for higher speed signal propagation.
Alternately, FIG. 4 may be described as an adherent copper conductor
interface on an integrated circuit comprising a semiconductor layer 14,
and a diffusion barrier 10 overlying semiconductor layer 14 for preventing
the diffusion of copper into semiconductor layer 14. An oxide layer 20,
having a thickness 22 of less than approximately 30 angstroms, overlies
diffusion barrier 10. Oxide layer 20 is formed by exposing diffusion
barrier 10 to a reactive oxygen species. A copper layer 26 overlies oxide
layer 20, whereby oxide layer 20 is formed to prepare diffusion barrier
material 10 for adhesion to copper.
FIG. 5 is a flow chart illustrating the steps in the method of applying
copper to selected IC surfaces. Step 50 provides selected integrated
circuit surfaces, the selected copper-receiving surfaces are predominately
on diffusion barrier material applied to selected regions of the IC. Step
52 exposes each selected copper-receiving surface to a reactive oxygen
species. A reactive oxygen species contains oxygen in a form that is
likely to combine with other atoms. One reactive oxygen form is the single
oxygen atom with its two unattached electrons. Atomic oxygen is produced
as a result of molecular disassociation caused by electron impacts. Atomic
oxygen readily combines with metal atoms of the diffusion barrier material
at relatively low temperatures. In this manner, stable oxide compounds are
formed on the diffusion barrier surface.
Step 54 oxidizes a thin layer of the diffusion barrier material surface in
response to the oxygen exposure in Step 52. As mentioned above, the
diffusion barrier material is typically metallic and, therefore, readily
forms a metal oxide when exposed to reactive oxygen species. Alternately,
oxygen compounds are formed with non-conductive diffusion barrier
materials.
Step 56 stops the exposure of the diffusion barrier material to the oxygen
in Step 52 before the oxide layer formed in Step 54 exceeds approximately
30 angstroms. This relatively thin oxide layer prepares the diffusion
barrier material receiving surface for adhesion to copper.
It is a feature of the invention to include a further Step 58, following
Step 56, of depositing CVD copper on the diffusion barrier material
surface oxidized in Step 54. Unattached Copper atoms, from the CVD
precursor, are able to bond to unattached oxygen atoms from the diffusion
material oxide to form copper oxide. In some circumstances the oxide layer
is a mixture of both copper oxide and the oxide of the diffusion material.
Step 60 is a product, an IC with copper adhered to a diffusion barrier
prepared with a thin oxide layer as an improved manner of adhering copper
to material which prevents the diffusion of copper into regions of the IC
underlying the diffusion barrier.
It is a feature of the invention that Step 52 includes generating the
reactive oxygen species from a predominately oxygen plasma so that the
copper-receiving surface is exposed to a predominately oxygen plasma in
Step 52. Plasma is used in many commercially prevalent forms of
anisotropic etching. Ashing, or plasma etching, is performed in a chamber
where an atmosphere of a relatively inert gas is introduced. The pressure
of the gas and the pumping rates are controlled. A voltage across the
chamber, at a predetermined frequency, is created to establish a flow of
ions in a known direction. In addition, the temperature of the substrate,
and the time of exposure to the ion flow, are controlled. As a consequence
of the radio frequency voltage in the chamber, the relatively inert gas is
transformed into a plasma consisting of unstable and, therefore, reactive
ions and radicals.
In addition to generating a reactive gas species to perform a chemical
process, plasma generation typically involves the physical effects of
bombarding the selected IC surface with ions. These bombarding atoms have
the potential of transferring their high energy to surface atoms to
influence chemical reaction rates. Ion bombardment can damage chemical
bonds, and damage single crystalline or polycrystalline structures.
Energetic ions can also cause electron or hole trapping in gate oxide, as
is well known in the art, that is only removable through annealing
processes.
As a plasma etchant, the reactive gas species are typically used to remove
materials, such as photoresist, from the surface of a substrate. The ions
and radicals react with the film layers on the IC wafer to form volatile
etch products which are then pumped away from the IC. Contrary to its
popular use, plasma is used in the present invention to generate a
reactive gas species which combines with the selected IC surface to add a
layer of oxide over the diffusion barrier surface.
In a preferred form of the invention, Step 52 includes generating the
reactive oxygen species from an oxygen-contained gas plasma. As used
herein, an oxygen-contained gas refers to a compound containing oxygen
atoms. It is a feature of the invention that the oxygen-contained gas is
selected from the group consisting of CO, NO.sub.2, N.sub.2 O, and H.sub.2
O. When these oxygen-contained gases become a plasma, oxygen atoms are
released from the compound to become a reactive oxygen species.
It is a feature of the invention that selected regions of the IC include
silicon, and that Step 52 includes using a direct plasma source having a
RF power level of less than approximately 200 watts to generate the
reactive oxygen species. The reactive oxygen species is generated from a
predominately oxygen plasma. Alternately, the reactive oxygen species is
generated from an oxygen-contained plasma. The relatively low energy of
the plasma ions created by the RF power source minimizes the disruption of
silicon crystalline structures. That is, energetic ions produced from such
a low level of power are unlikely to damage adjoining silicon IC
structures.
In a preferred embodiment of the invention, Step 52 is performed at a
substrate temperature of less than approximately 200.degree. C. Since the
oxygen species are especially reactive, the oxidation process can be
accomplished at a relatively low temperature. Low temperatures help insure
that a minimum of damage and stress is done to nearby IC silicon
crystalline structures or underlying IC substrates.
It is a feature of the invention that Step 56 stops the exposure of the
diffusion barrier material to oxygen generally within a time interval of
less than 60 seconds. Once again, because the oxygen species is so
reactive, a relatively short process time protects the silicon crystalline
structure of the IC.
In a preferred embodiment of the invention, Step 52 includes generating the
reactive oxygen species from a downstream plasma source. In this manner,
high energy ions are removed from the plasma flow to minimize damage to
the IC. A downstream plasma source ionizes gas at a site relatively remote
from the IC substrate. Baffles, or barriers, located between the plasma
source and the IC substrate, remove the high energy ions. Reactive oxygen
species resulting from the generation of plasma are typically moved to the
selected IC substrate surfaces through control of the relative pressures
between the plasma and IC chambers. The reactive gas species combine with
the diffusion barrier surface as a purely chemical process, without
energetic ions. Using a downstream plasma source allows the ions to be
generated at high energy without concern that energetic ions will harm the
selected IC substrate surface. Therefore, feed gases requiring a large
amount of power to generate the reactive oxygen species can be used in the
present invention when the oxygen species is generated from a downstream
plasma source.
A thin layer of oxide located between a copper layer and a diffusion
material surface improves the adhesion of the copper. Oxide bonds are
formed to both the copper layer and the diffusion barrier surface. These
relatively strong oxide bonds mitigate uncertainties introduced in
variations between copper precursor batches. The oxide bonds also mitigate
against variations in the condition and cleanliness of the diffusion
barrier surface. The present invention allows the deposition of copper to
be performed in a different chamber from where the diffusion barrier
material is deposited. The present invention reduces the need for
maintaining the diffusion barrier surface in a controlled environment. It
also allows the copper deposition process to be delayed to times more
convenient in the IC process.
The invention has been described above as comprising overlaying layers of
copper, oxide, diffusion material, and IC substrate. The present invention
is also applicable to the adhesion of copper to sidewall structures, and
other copper encapsulating structures. Other variations with the scope of
the present invention will occur to those skilled in the art.
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