U.S. patent number RE39,126 [Application Number 08/527,954] was granted by the patent office on 2006-06-13 for two-step chemical mechanical polishing process for producing flush and protruding tungsten plugs.
This patent grant is currently assigned to Micron Technology, Inc.. Invention is credited to Trung T. Doan, Chris C. Yu.
United States Patent |
RE39,126 |
Yu , et al. |
June 13, 2006 |
Two-step chemical mechanical polishing process for producing flush
and protruding tungsten plugs
Abstract
A method for forming conductive plugs within an insulation
material is described. The inventive process results in a plug of a
material such as tungsten which is more even with the insulation
layer surface than conventional plug formation techniques.
Conventional processes result in recessed plugs which are not
easily or reliably coupled with subsequent layers of sputtered
aluminum or other conductors. The inventive process uses a two-step
chemical mechanical planarization technique. An insulation layer
with contact holes is formed, and a metal layer is formed
thereover. A polishing pad rotates against the wafer surface while
a slurry selective to the metal removes the metal overlying the
wafer surface, and also recesses the metal within the contact holes
due to the chemical nature and fibrous element of the polishing
pad. A second CMP step uses a slurry having an acid or base
selective to the insulation material to remove the insulator from
around the metal. The slurry also contains abrasive materials which
polish the metal surface so as to make the metal level with the
insulation layer surface. Removal of the insulation material can
continue, thereby producing a slightly protruding plug which
results in a more reliable contact from the substrate to subsequent
conductive layers.
Inventors: |
Yu; Chris C. (Aurora, IL),
Doan; Trung T. (Boise, ID) |
Assignee: |
Micron Technology, Inc. (Boise,
ID)
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Family
ID: |
25242798 |
Appl.
No.: |
08/527,954 |
Filed: |
September 14, 1995 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
07824980 |
Jan 24, 1992 |
05244534 |
Sep 14, 1993 |
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Current U.S.
Class: |
438/633; 438/648;
438/672 |
Current CPC
Class: |
C23F
3/00 (20130101); H01L 21/3212 (20130101); H01L
21/76819 (20130101); H01L 21/7684 (20130101) |
Current International
Class: |
H01L
21/461 (20060101) |
Field of
Search: |
;438/597,622,631,633,637,645,648,672,693,750,692 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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239927 |
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Oct 1986 |
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DD |
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0 343 698 |
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Nov 1989 |
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EP |
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63-90838 |
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Apr 1988 |
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JP |
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01017879 |
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Jan 1989 |
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JP |
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2-42728 |
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Feb 1990 |
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JP |
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3-244130 |
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Oct 1991 |
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JP |
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Other References
J J. Estabil et al., "Electromigration Improvements with Titanium
Underlay and Overlay in Al(Cu) Metallurgy", VMIC Conference, pp.
242-248, Jun. 11-12, 1991. cited by other .
Ronald R. Uttecht et al., "A Four-Level-Metal Fully Planarized
Interconnect Technology for Dense High Performance Logic and SRAM
Applications", VMIC Conference, pp. 20-26, Jun. 11-12, 1991. cited
by other .
"Advanced Metallization for ULSI Applications", Proceedings of the
Conference held Oct. 8-10, 1991. cited by other .
S. Wolf, "4.4.11 Chemical-Mechanical Polishing," Silicon Processing
for the VLSI Era, vol. 2--Process Integration, pp. 238-239, 1990.
cited by other.
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Primary Examiner: Pert; Evan
Attorney, Agent or Firm: TraskBritt
Claims
What is claimed is:
1. A method of producing a conductive plug in an insulation layer,
comprising the steps of: a) removing a portion of the insulation
layer to form a contact hole within the insulation layer; b)
applying a layer of conductive material to a surface of the
insulation layer, thereby filling said contact hole with said
conductive material and resulting in a layer of said conductive
material over said insulation layer surface; c) removing at least a
portion of said conductive material from said surface of the
insulation layer and leaving said contact hole substantially filled
with said conductive material, said removing being performed by
chemical mechanical planarization with a slurry comprising an
abrasive material and an oxidizing component, said oxidizing
component comprising hydrogen peroxide and water wherein a ratio by
volume of hydrogen peroxide to water is in the range of 1:0 to 1:1;
d) removing some of the insulation layer to lower said insulation
layer surface with respect to an upper surface of said conductive
material.
2. The method of claim 1, wherein said contact hole is formed by
etching.
3. The method of claim 1, wherein said .Iadd.layer of
.Iaddend.conductive .[.layer.]. .Iadd.material .Iaddend.is formed
by chemical vapor deposition.
4. The method of claim 1, wherein said abrasive material comprises
aluminum oxide.
5. The method of claim 1, wherein step d) is done by chemical
mechanical planarization.
6. The method of claim 5, wherein step d) comprises the use of a
slurry having .[.an.]. .Iadd.a second .Iaddend.abrasive material
and a compound which selectively removes a portion of the
insulation layer.
7. The method of claim 6, wherein said .Iadd.second
.Iaddend.abrasive material comprises silica.
8. The method of claim 6, wherein said compound comprises potassium
hydroxide.
9. The method of claim 1, wherein between 0.5K.ANG. and 3K.ANG. of
the .[.substrate material.]. .Iadd.insulation layer .Iaddend.is
removed during step d).
10. The method of claim 1, wherein step d) continues until said
insulation layer surface is substantially even with said upper
surface of said conductive material.
11. The method of claim 1, wherein step d) continues until said
insulation layer surface is lower than said upper surface of said
conductive material, thereby resulting in said conductive material
protruding from said insulation layer surface.
12. The method of claim 1, wherein the plug formed comprises
tungsten.
13. The method of claim 12, wherein during step c) a portion of
said tungsten reacts with said hydrogen peroxide to form tungsten
oxide.
14. The method of claim 1, wherein the insulation layer comprises
.[.an insulating.]. .Iadd.a .Iaddend.dielectric .[.layer.].
.Iadd.material.Iaddend..
15. The method of claim 14, wherein between 0.5 K.ANG. and 3K.ANG.
of the .[.substrate material.]. .Iadd.insulation layer .Iaddend.is
removed in step d).
16. The method of claim 1, wherein the .[.substrate material.].
.Iadd.insulation layer .Iaddend.comprises polyimide.
17. A method of chemical mechanical planarization of an oxide
material, comprising the steps of: a) etching a portion of the
oxide material to form a contact hole within said oxide material;
b) applying a layer of tungsten to a surface of the oxide material,
thereby filling said contact hole with said tungsten and resulting
in a layer of said tungsten material over said oxide material; c)
chemically and mechanically removing at least a portion of said
tungsten from said surface of said oxide material with a first
solution comprising H.sub.2O.sub.2 water, and an abrasive material,
and leaving said contact hole substantially filed with said
tungsten, wherein a ratio by a volume of said H.sub.2O.sub.2 to
said water is in the range of 1:0 to 1:1; d) chemically and
mechanically removing a portion of the oxide material with a second
solution comprising KOH and an abrasive material to lower said
surface of said oxide material with respect to an upper surface of
said tungsten; whereby a plug of tungsten is formed in said contact
hole.
18. The method of claim 17, wherein between 0.5K.ANG. and 3K.ANG.
of said oxide material is removed during step d).
19. The method of claim 17, wherein step d) continues until said
surface of said oxide material is substantially even with said
upper surface of said tungsten.
20. The method of claim 17, wherein step d) continues until said
surface of said oxide material is lower than said upper surface of
said conductive material, thereby resulting in said tungsten
protruding from the oxide material.
21. A method of producing a conductive plug in an insulation layer,
comprising the steps of: a) removing a portion of the insulation
layer to form a contact hole within the insulation layer; b)
applying a layer of conductive material to a surface of the
insulation layer, thereby filling said contact hole with said
conductive material and resulting in a layer of said conductive
material over said insulation layer surface; c) removing at least a
portion of said conductive material from said surface of the
insulation layer and leaving said contact hole substantially filled
with said conductive material; d) removing some of the insulation
layer by chemical mechanical planarization using a slurry, said
slurry having an abrasive material and a compound which selectively
removes a portion of said insulation layer to lower said insulation
layer surface with respect to an upper surface of said conductive
material; e) removing at least a portion of said conductive
material from said surface of the insulation layer and leaving said
contact hole substantially filled with said conductive material,
said removing being performed by chemical mechanical planarization
with a slurry comprising an abrasive material and an oxidizing
component and a carrier, wherein a ratio by volume of the oxidizing
component to the carrier is in the range of 1:0 to 1:1; f) removing
some of the insulation layer to lower said insulation layer surface
with respect to an upper surface of said conductive material.
22. The method of claim 21, wherein said abrasive material
comprises silica.
23. The method of claim 21, wherein said compound comprises
potassium hydroxide.
.Iadd.24. A method of producing a conductive plug in an insulation
layer, comprising the steps of: a) removing a portion of the
insulation layer to form a contact hole within the insulation
layer; b) applying a layer of conductive material to a surface of
the insulation layer, thereby filling said contact hole with said
conductive material and resulting in a layer of said conductive
material over said insulation layer surface; c) removing at least a
portion of said conductive material from said surface of the
insulation layer and leaving said contact hole substantially filled
with said conductive material, said removing being performed by
chemical mechanical planarization with a slurry comprising an
abrasive material and an oxidizing component, said oxidizing
component consisting essentially of hydrogen peroxide and,
optionally, water in a ratio by volume of water to hydrogen
peroxide of up to 1:1; and d) removing some of the insulation layer
to lower said insulation layer surface with respect to an upper
surface of said conductive material..Iaddend.
.Iadd.25. The method of claim 24, wherein said contact hole is
formed by etching..Iaddend.
.Iadd.26. The method of claim 24, wherein said conductive layer is
formed by chemical vapor deposition..Iaddend.
.Iadd.27. The method of claim 24, wherein said abrasive material
comprises aluminum oxide..Iaddend.
.Iadd.28. The method of claim 24, wherein step d) is done by
chemical mechanical planarization..Iaddend.
.Iadd.29. The method of claim 28, wherein step d) comprises the use
of a slurry having a second abrasive material and a compound which
selectively removes a portion of the insulation layer..Iaddend.
.Iadd.30. The method of claim 29, wherein said second abrasive
material comprises silica..Iaddend.
.Iadd.31. The method of claim 29, wherein said compound comprises
potassium hydroxide..Iaddend.
.Iadd.32. The method of claim 24, wherein between 0.5K.ANG. AND
3K.ANG. of the insulation layer is removed during step
d)..Iaddend.
.Iadd.33. The method of claim 24, wherein step d) continued until
said insulation layer surface is substantially even with said upper
surface of said conductive material..Iaddend.
.Iadd.34. The method of claim 24, wherein step d) continues until
said insulation layer surface is lower than said upper surface of
said conductive material, thereby resulting in said conductive
material protruding from said insulation layer
surface..Iaddend.
.Iadd.35. The method of claim 24, wherein the plug formed comprises
tungsten..Iaddend.
.Iadd.36. The method of claim 35, wherein during step c) a portion
of said tungsten reacts with said hydrogen peroxide to form
tungsten oxide..Iaddend.
.Iadd.37. The method of claim 24, wherein the insulation layer
comprises a dielectric material..Iaddend.
.Iadd.38. The method of claim 37, wherein between 0.5K.ANG. and
3K.ANG. of the insulation layer is removed in step d)..Iaddend.
.Iadd.39. The method of claim 24, wherein the insulation layer
comprises polyimide..Iaddend.
.Iadd.40. A method of chemical mechanical planarization of an oxide
material, comprising the steps of: a) etching a portion of the
oxide material to form a contact hole within said oxide material;
b) applying a layer of tungsten to a surface of the oxide material,
thereby filling said contact hole with said tungsten and resulting
in a layer of said tungsten material over said oxide material; c)
chemically and mechanically removing at least a portion of said
tungsten from said surface of said oxide material with a first
solution and an abrasive material, and leaving said contact hole
substantially filled with said tungsten, said first solution
consisting essentially of hydrogen peroxide and, optionally, water
in a ratio by volume of water to hydrogen peroxide of up to 1:1; d)
chemically and mechanically removing a portion of the oxide
material with a second solution comprising KOH and an abrasive
material to lower said surface of said oxide material with respect
to an upper surface of said tungsten; and whereby a plug of
tungsten is formed in said contact hole..Iaddend.
.Iadd.41. The method of claim 40, wherein between 0.5K.ANG. and
3K.ANG. and said oxide material is removing during step
d)..Iaddend.
.Iadd.42. The method of claim 40, wherein step d) continues until
said surface of said oxide material is substantially even with said
upper surface of said tungsten..Iaddend.
.Iadd.43. The method of claim 40, wherein step d) continues until
said surface of said oxide material is lower than said upper
surface of said conductive material, thereby resulting in said
tungsten protruding from the oxide material..Iaddend.
.Iadd.44. A method of producing a conductive plug in an insulation
layer, comprising the steps of: a) removing a portion of the
insulation layer to form a contact hole within the insulation
layer; b) applying a layer of conductive material to a surface of
the insulation layer, thereby filling said contact hole with said
conductive material and resulting in a layer of said conductive
material over said insulation layer surface; c) removing at least a
portion of said conductive material from said surface of the
insulation layer and leaving said contact hole substantially filled
with said conductive material, said removing being performed by
chemical mechanical planarization with a slurry comprising an
abrasive material and a solution, said solution consisting
essentially of an oxidizing component and, optionally, a carrier in
a ratio by volume of carrier to oxidizing component of up to 1:1;
and d) removing some of the insulation layer to lower said
insulation layer by chemical mechanical planarization using a
slurry, said slurry having an abrasive material and a compound
which selectively removes a portion of said insulation layer
surface with respect to an upper surface of said conductive
material..Iaddend.
.Iadd.45. The method of claim 44, wherein said abrasive material
comprises silica..Iaddend.
.Iadd.46. The method of claim 44, wherein said compound comprises
potassium hydroxide..Iaddend.
Description
FIELD OF THE INVENTION
The disclosed invention relates to the field of semiconductor
manufacture. More specifically, a chemical mechanical wafer
polishing process is described which produces improved flush and
protruding tungsten plugs rather than the recessed plugs produced
by conventional tungsten plug etchback techniques. Coupling with
subsequent layers of conductive material such as sputtered aluminum
is therefore more easily accomplished.
BACKGROUND OF THE INVENTION
Integrated circuits are chemically and physically integrated into a
substrate, such as a silicon or gallium arsenide wafer, by
patterning regions in the substrate, and by patterning layers on
the substrate. These regions and layers can be conductive, for
conductor and resistor fabrication. They can also be of different
conductivity types, which is essential for transistor and diode
fabrication. Up to a thousand or more devices are formed
simultaneously on the surface of a single wafer of semiconductor
material.
It is essential for high device yields to start with a flat
semiconductor wafer. If the process steps of device fabrication are
performed on a wafer surface that is not uniform, various problems
can occur which may result in a large number of inoperable
devices.
Previous methods used to ensure the wafer surface planarity
included forming an oxide such as borophosphosilicate glass (BPSG)
layer on the wafer surface, then heating the wafer to reflow and
planarize the oxide layer. This "reflow" method of planarizing the
wafer surface was sufficient with fairly large device geometries,
but as the technology allowed for smaller device feature sizes,
this method produced unsatisfactory results.
Another method which has been used to produce a planar wafer
surface is to use the oxide reflow method described above, then
spin coat the wafer with photoresist. The spin coating of the
material on the wafer surface fills the low points and produces a
planar surface from which to start. Next, a dry etch, which removes
photoresist and oxide at a rate sufficiently close to 1:1, removes
the photoresist and the high points of the wafer, thereby producing
a planar oxide layer on the wafer surface.
Most recently, chemical mechanical planarization (CMP) processes
have been used to planarize the surface of wafers in preparation
for device fabrication. The CMP process involves holding a thin
flat wafer of semiconductor material against a rotating wetted
polishing pad surface under a controlled downward pressure. A
polishing slurry such as a mixture of either a basic or acidic
solution used as a chemical etch component in combination with
alumina or silica particles used as an abrasive etch component may
be used. A rotating polishing head or wafer carrier is typically
used to hold the wafer under controlled pressure against a rotating
polishing platen. The polishing platen is typically covered with a
relatively soft wetted pad material such as blown polyurethane.
Such apparatus for polishing thin flat semiconductor wafers are
well known in the art. U.S. Pat. Nos. 4,193,226 and 4,811,522 to
Gill, Jr. and U.S. Pat. No. 3,841,031 to Walsh, for instance,
disclose such apparatus.
Deposited conductors are an integral part of every integrated
circuit, and provide the role of surface wiring for conducting
current. Specifically, the deposited conductors are used to wire
together the various components that are formed in the surface of
the wafer. Electronic devices formed within the wafer have active
areas which must be contacted with conductive runners, such as
metal. Typically, a layer of insulating material is applied stop
the wafer and selectively masked to provide contact opening
patterns. The layer is subsequently etched, for instance with a
reactive ion etch (RIE), to provide contact openings from the upper
surface of the insulating layer down into the wafer to provide
electrical contact with selected active areas.
Certain metals and allows deposited by vacuum evaporation and
sputtering techniques do not provide the most desired coverage
within the contact openings when applied to the surface of a wafer.
An example of a metal which typically provides such poor coverage
is sputtered aluminum, or alloys of aluminum with silicon and/or
copper. One metallization scheme which does provide good coverage
within contact vias is tungsten deposited by the chemical vapor
deposition (CVD) technique. Tungsten is not, however, as conductive
as aluminum. Accordingly, a tungsten layer is typically etched or
polished back to provide a plug within the insulation layer, the
plug having a flat upper surface which is flush with the surface of
the insulator. A layer of aluminum would subsequently be applied
atop the wafer surface to contact the plug. The aluminum layer is
then selectively etched to provide the desired interconnecting
runners coupling the tungsten with other circuitry.
FIG. 1 shows a desirable outcome of a process to produce a tungsten
plug. In accordance with wafer fabrication techniques, a material
such as an oxide layer 10 covers the material of the wafer surface
12. The tungsten 14 which fills the contact hole 16 in the oxide
material 10 is level with the surface of the oxide layer. FIG. 2
illustrates one problem with present methods of tungsten etch
backs, an over etching within the contacts which recesses the
tungsten 14 within the contact hole 16 in the wafer surface 10.
This can provide for poor contact between the tungsten plug 14 with
the aluminum or aluminum alloy layer (not shown) which would be
subsequently deposited by sputtering. It is difficult to provide
reliable contacts between the aluminum and the recessed tungsten
plugs which result from conventional tungsten etchback techniques
such as reactive ion etching (RIE).
In addition to RIE, another conventional tungsten etch back means
includes a single-step CMP etchback using a polishing slurry and
polishing pad. A layer of tungsten is formed by CVD or other means
onto the wafer surface, thereby filling the contact holes in the
insulation layer with tungsten. The surface of the wafer is
polished to remove the tungsten overlying the surface of the wafer,
which leaves the contact holes filled with tungsten. Due to the
chemical nature of the slurry and compressible nature of the
polishing pad, a certain amount of the tungsten material is removed
from the contact holes, leaving the recessed tungsten structure 14
of FIG. 2.
U.S. Pat. No. 4,992,135 describes a method of etching back tungsten
layers, which is incorporated herein by reference.
A need remains for improved methods of etching back tungsten layers
on semiconductor wafers to allow for good contact with layers of
metal or other conductive material which are subsequently
deposited.
SUMMARY OF THE INVENTION
An object of the invention is to provide a process for forming
contacts (plugs) of tungsten or other conductive materials that
results in a more uniform, nonrecessed plug.
Another object of the invention is to provide a process for forming
a plug of tungsten or other conductive material which results in a
better surface to connect with another material such as a layer of
aluminum by virtue of the more uniform, nonrecessed characteristics
of the plug.
Yet another object of the invention is to provide a process for
forming a plug of tungsten of other conductive material which can
produce uniform, protruding plugs which allow for easier coupling
with subsequent layers of conductive material than recessed plugs
produced by conventional methods.
These objects of the invention are realized with an inventive
two-step process of plug formation which uses chemical mechanical
planarization (CMP) technology. A substrate of a material such as
silicon having a layer of oxide (BPSG) is manufactured with contact
holes therein, and a layer of metal such as tungsten is formed upon
the substrate to fill the contact holes. A first CMP step, which is
selective to the plug material, removes the upper layer of tungsten
from the oxide surface while removing very little or no oxide from
the wafer surface. During the last phase of the step which
completely removes metal residue including barriers such as
titanium nitride and titanium layers over the surface of the wafer,
a portion of the tungsten below the level of the oxide surface is
also removed, thereby recessing the tungsten plugs. This recessed
plug, which is typical of conventional plug formation, is difficult
to couple with a subsequent layer of metal or other material.
Therefore, a second CMP step which is selective to oxide material
of the wafer surface, removes a portion of the insulation material
to a level even with, or slightly below, the level of the tungsten
plugs. To shape the tungsten extending above the surface so as to
remove the concave shape resulting from the plug recess, the slurry
of the oxide CMP can be formulated so as to remove a desired amount
of tungsten. This can be accomplished by increasing the amount of
etchant that is selective to the material of the plug.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-section of a desirable plug;
FIG. 2 is a cross-section of a recessed plug typically produced by
a conventional CMP process;
FIG. 3 is a cross-section of a first step in the inventive process
showing a layer of conductive material (such as tungsten) formed
over the substrate; and
FIG. 4 is a cross-section of a protruding plug embodiment which can
be produced by the inventive two-step process.
DETAILED DESCRIPTION OF THE INVENTION
The inventive process formed plugs from a conductive material (in
the instant case tungsten) which were even with, and in a second
embodiment slightly protruding from, .[.a.]. .Iadd.an
.Iaddend.insulation layer .Iadd.comprising an insulating material
or a dielectric material, .Iaddend.such as .Iadd.an .Iaddend.oxide
.Iadd.material .Iaddend.(BPSG in the instant case, or other
materials such as SiO.sub.2) .Iadd.or a nonoxide material (such as
polyimide).Iaddend.. The shape of the protruding plugs was
controllably convex and allowed for an improved surface with which
to couple a subsequent layer of conductive material such as
aluminum.
The inventive process began with a wafer as shown in FIG. 3
fabricated by means known in the art having a layer of insulation
material 10 such as an oxide (BPSG) which is approximately 2-3
.mu.m thick. Contact holes 16 were formed into material 10 by any
conventional process. A layer of metal 30, tungsten in the instant
case, filled the contact holes 16 and extended over the insulation
10 surface. The tungsten layer 30 was formed by chemical vapor
deposition (CVD) to most efficiently fill the contact holes 16, but
workable methods known in the art are also possible. The layer of
tungsten 30 over the oxide surface 10 in the instant case was
approximately 10K.ANG. thick, but other thicknesses are possible as
the layer is removed in subsequent wafer processing steps.
Next, the wafer was subjected to a chemical mechanical polishing
(CMP) process which was selective to tungsten. The process employed
a polishing pad mounted on a rotating platen. A slurry, containing
abrasive particles such as Al.sub.2O.sub.3 and etchants such as
H.sub.2O.sub.2 and either KOH or NH.sub.4OH, or other acids or
bases, removed the tungsten at a predictable rate, while removing
very little of the insulation. This process is described in U.S.
Pat. No. 4,992,135. The polishing pad was held in contact with the
wafer surface at a pressure of 7-9 psi for approximately 5-10
minutes. This process resulted in the structure of FIG. 2, a
tungsten plug 14 within the contact holes 16 in the oxide 10. The
tungsten 14 was slightly recessed at this stage in the process as
shown resulting from the mechanical erosion of the tungsten from
the fibers of the polishing pad. The magnitude of the recess
typically varied from approximately 0.5K.ANG. to 3K.ANG. below the
surface of the oxide 10. To selectively remove the tungsten, the
chemical component of the slurry oxidized the tungsten, and the
tungsten oxide was removed mechanically with the abrasive material
in the slurry. Additionally, a small portion of the tungsten was
removed by the abrasive. In any case, the CMP process used is
selective to tungsten and leaves the insulation layer relatively
unaffected.
The second step involved a CMP process which was selective to the
material of the insulation layer, although it may be desirable to
remove a small amount of the tungsten as well to either to polish
the tungsten or to provide a convex protruding plug. If tungsten is
removed at this step, it is done at a much slower rate than the
removal of the insulation material. A slurry containing etchants
selective to the oxide was added between a rotating polish pad and
the wafer surface. The colloidal silica slurry used in the instant
case contained abrasives as described above, and also etchants
selective to the oxide, such as a basic mixture of H.sub.2O and
KOH. In most cases, if other nonoxide insulators are used other
chemical etchants would be required. As shown in FIG. 1, the
insulation material 10 was removed from around the tungsten plugs
14, resulting in a plug 14 which was even with the surface of the
insulation material 10. The action of the pad abraded the surface
of the tungsten and the oxide material sufficient to polish out
surface irregularities. The tungsten was polished at a slow rate,
less than 50.ANG./minute, but the oxide underlayer was polished at
a high rate, greater than 2500.ANG./minute. Typically, a layer of
0.5K.ANG.-3K.ANG. of the insulation material is removed at the
second CMP step, as this is the usual extent to which the tungsten
is recessed within the contact hole.
A second embodiment of the first step was also used to successfully
form the tungsten plugs. This process used a novel polishing slurry
comprising aluminum oxide (Al.sub.2O.sub.3) abrasive particles and
a basic mixture of H.sub.2O and H.sub.2O.sub.2. It was found that
the second base of the mixture as described above, KOH or
NH.sub.4OH, had little effect on the speed or quality of the etch.
In this novel slurry, H.sub.2O.sub.2 is used to oxidize the
tungsten surface, forming tungsten oxide. The formed tungsten oxide
is subsequently removed by the polishing process, creating a fresh
tungsten surface for continued surface reaction between
H.sub.2O.sub.2 and the tungsten surface. In contrast, the first
embodiment of the first step describes the use of H.sub.2O.sub.2
and a second chemical component such as KOH or NH.sub.4OH which
served to remove tungsten oxide chemically. It has been found that
the tungsten oxide is sufficiently removed by the mechanical
polishing effect of the abrasive within the slurry. With this new
slurry, a polishing rate of 1K.ANG./minute to 3K.ANG./minute was
found, depending on the H.sub.2O.sub.2 to H.sub.2O ratio. A 100%
solution of H.sub.2O.sub.2 removed the tungsten oxide at about
3K.ANG./minute while a 1:1 ratio by volume of H.sub.2O.sub.2 to
H.sub.2O removed the tungsten oxide at around 0.5K.ANG./minute.
Using the inventive slurry, a good tungsten to insulation (i.e.
BPSG) polishing selectivity was obtained, and was determined to be
approximately 20:1.
In other embodiment of the invention, the second wafer polishing
step which removed the oxide 10 was continued to remove additional
insulation material 10 and to produce a convexly rounded protruding
tungsten plug 40 as shown in FIG. 4, although this is not a
requirement of the inventive process. The rounded surfaces of the
tungsten plugs 40 provided surfaces which were easily coupled to
layers of aluminum (not shown) formed by sputtering or other means
during subsequent wafer processing steps. Tungsten plugs with a
diameter of less than 1 micron were produced
In addition to producing uniform plugs which were not recessed
within the insulation layer, the inventive two-step process
resulted in more planarized wafer surface due to the oxide
polishing in the second step.
What have been described are specific configurations of the
invention, as applied to particular embodiments. Clearly,
variations can be made to the original methods and materials
described in this document for adapting the invention to other
embodiments. For example, insulators other than those comprising
oxide could be used, for example Si.sub.3N.sub.4. For these
nonoxide insulators, however, a chemical etchant other than the KOH
and water solution would most likely be required. Also, various
acids, bases, and abrasive materials can be used in the CMP slurry
to maintain the scope and spirit of the invention. Therefore, the
invention should be read as limited only by the appended
claims.
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