U.S. patent application number 09/738796 was filed with the patent office on 2001-06-07 for manufacture and cleaning of a semiconductor.
Invention is credited to Chen, Gary, Hu, Yongjun Jeff, Li, Li.
Application Number | 20010003061 09/738796 |
Document ID | / |
Family ID | 23521213 |
Filed Date | 2001-06-07 |
United States Patent
Application |
20010003061 |
Kind Code |
A1 |
Chen, Gary ; et al. |
June 7, 2001 |
Manufacture and cleaning of a semiconductor
Abstract
Metal nitride and metal oxynitride extrusions often form on
metal suicides. These extrusions can cause short circuits and
degrade processing yields. The present invention discloses a method
of selectively removing such extrusions. In one embodiment, a novel
wet etch comprising an oxidizing agent and a chelating agent
selectively removes the extrusions from a wordline in a memory
array. In another embodiment, the wet etch includes a base that
adjusts the pH of the etch to selectively remove certain extrusions
relative to other substances in the wordline. Accordingly new metal
silicide structures can be used to form novel wordlines and other
types of integrated circuits.
Inventors: |
Chen, Gary; (Boise, ID)
; Li, Li; (Meridian, ID) ; Hu, Yongjun Jeff;
(Boise, ID) |
Correspondence
Address: |
KNOBBE MARTENS OLSON & BEAR LLP
620 NEWPORT CENTER DRIVE
SIXTEENTH FLOOR
NEWPORT BEACH
CA
92660
US
|
Family ID: |
23521213 |
Appl. No.: |
09/738796 |
Filed: |
December 15, 2000 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09738796 |
Dec 15, 2000 |
|
|
|
09385396 |
Aug 30, 1999 |
|
|
|
Current U.S.
Class: |
438/655 ;
252/79.1; 257/757; 257/E21.2; 257/E21.309; 257/E29.157; 438/656;
438/669 |
Current CPC
Class: |
H01L 21/28061 20130101;
H01L 29/4941 20130101; H01L 21/28247 20130101; H01L 21/32134
20130101 |
Class at
Publication: |
438/655 ;
438/656; 438/669; 252/79.1; 257/757 |
International
Class: |
H01L 021/44 |
Claims
What is claimed is:
1. A method of fabricating a wordline in a memory array, the method
comprising the acts of: depositing a tungsten silicide barrier
layer on a wordline stack; processing the wordline stack such that
tungsten nitride extrusions extend from an exposed surface of the
barrier layer; and selectively etching the tungsten nitride
extrusions with a solution comprising water, hydrogen peroxide and
EDTA.
2. The method of claim 1 wherein the amount of hydrogen peroxide in
the solution ranges from approximately 2% to approximately 50% of
the solution.
3. The method of claim 1 wherein the amount of hydrogen peroxide in
the solution is approximately 10% of the solution.
4. The method of claim 1 wherein the amount of EDTA in the solution
ranges from approximately 0.001% to approximately 5% of the
solution.
5. The method of claim 1 wherein the amount of EDTA in the solution
is approximately 0.03% of the solution.
6. The method of claim 1 wherein the solution further comprises
ammonium hydroxide.
7. The method of claim 6 wherein the amount of ammonium hydroxide
is ranges from approximately 0.5% to 30% of the solution.
8. A method of fabricating a wordline in a memory array, the method
comprising the acts of: depositing a low-resistance metal silicide
layer on a wordline stack; processing the wordline stack such that
metal nitride extrusions extend from the metal silicide layer; and
selectively etching the metal nitride extrusions--with a solution
comprising at least an oxidizing agent and a chelating agent.
9. The method of claim 1 wherein the oxidizing agent is a substance
selected from the group of hydrogen peroxide, ozonated water,
ozone, nitric acid, hypochlorous acid, chloric acid, ammonium
persulphate and sulfuric acid.
10. The method of claim 1 wherein the chelating agent is a
substance selected from the group of amines, carboxylic acid
compounds, oxalate, acetate, disodium ethylenediamine acid,
glycine, iminodiacetic acid, nitrolotriacetic acid, and tetrasodium
ethylenediaminetetraacetate.
11. The method of claim 1 wherein the solution further comprises a
base that adjusts the pH of the solution to about seven.
12. The method of claim 1 wherein the solution further comprises a
base that adjusts the pH of the solution to about eight.
13. The method of claim 1 wherein the base is ammonium
hydroxide.
14. A method of fabricating a wordline in a memory array, the
method comprising the acts of: depositing a tungsten silicide
barrier layer on a wordline stack; depositing a titanium silicide
conductive layer above the tungsten silicide barrier layer;
processing the wordline stack such that metal nitride extrusions
extend from the tungsten silicide barrier layer and the titanium
silicide conductive layer; and etching the metal nitride extrusions
at a faster rate than the conductive layer or the barrier
layer.
15. The method of claim 14 wherein the act of etching etches the
metal nitride extrusions at over 2000 .ANG./min.
16. The method of claim 14 wherein the metal nitride extrusions
comprise tungsten nitride and the act of etching etches the
tungsten nitride at over 2000 .ANG./min.
17. The method of claim 14 wherein the act of etching etches the
metal nitride extrusions at approximately 200 .ANG./min.
18. The method of claim 14 further comprising the act of processing
the wordline stack such that metal oxynitride extrusions extend
from the tungsten silicide barrier layer and the titanium silicide
conductive layer.
19. The method of claim 18 wherein the act of etching etches the
metal oxynitride extrusions at approximately 200 .ANG./min.
20. The method of claim 19 wherein the act of etching etches the
titanium silicide conductive layer at less than 1 .ANG./min.
21. The method of claim 19 wherein the act of etching etches the
tungsten silicide barrier layer at less than 1 .ANG./min.
22. A method of fabricating a semiconductor, the method comprising
the acts of: depositing polysilicon gate on a semiconductor
substrate to form a wordline stack; depositing a conductive layer
comprising metal silicide above the polysilicon gate; processing
the wordline stack in a manner that forms protrusions on the
conductive layer; and etching the protrusions at a faster rate than
the conductive layer.
23. The method of claim 22 wherein the act of etching etches the
protrusions at a faster rate than the polysilicon gate.
24. A method of forming a wordline gate comprising: forming metal
nitride extrusions on a wordline stack; and selectively removing
the metal nitride extrusions.
25. A method of forming a semiconductor structure comprising
selectively removing metal oxynitride extrusions from a
semiconductor structure in a manner that does not substantially
damage a conductive layer in the semiconductor structure.
26. A method of forming a semiconductor structure comprising
selectively removing metal oxynitride extrusions from a
semiconductor structure in a manner that does not substantially
damage a polysilicon layer in the semiconductor structure.
27. A method of forming a wordline gate comprising: forming metal
oxynitride extrusions on a wordline stack; and selectively removing
the metal oxynitride extrusions.
28. A wet etch for selectively removing tungsten nitride extrusions
comprising: at least 10% by weight hydrogen peroxide; and at least
0.03% by weight EDTA.
29. The wet etch of claim 28 further comprising at least 0.02% by
weight ammonium hydroxide.
30. The wet etch of claim 28 further comprising a buffer salt.
31. A wet etch for selectively removing metal nitride extrusions
comprising: at least 10% by weight an oxidizing agent; and at least
0.03% by weight a chelating agent.
32. The wet etch of claim 31 further comprising at least 0.02% by
weight a base.
33. The wet etch of claim 32 further comprising a buffer salt.
34. The wet etch of claim 33 wherein the buffer salt is potassium
phosphate.
35. A semiconductor structure comprising a tungsten silicide layer,
wherein the tungsten silicide layer has substantially etched
tungsten nitride extrusions formed thereon.
36. The semiconductor structure of claim 35 wherein the structure
is a transistor.
37. A semiconductor structure comprising a tungsten silicide layer,
wherein the tungsten silicide layer has substantially etched
tungsten oxynitride extrusions formed thereon.
38. A semiconductor structure comprising a titanium silicide layer,
wherein the titanium silicide layer has substantially etched
titanium nitride extrusions formed thereon.
39. A semiconductor structure comprising a titanium silicide layer,
wherein the titanium silicide layer, has substantially etched
titanium oxynitride extrusions formed thereon.
40. A semiconductor structure comprising: a barrier layer above a
semiconductor substrate, the barrier layer comprising a tungsten
silicide; a conductive layer above the wordline gate, the
conductive layer comprising titanium silicide; and a cap above the
conductive layer.
41. The semiconductor structure of claim 40 wherein the
semiconductor structure is a transistor.
42. The semiconductor structure of claim 40 wherein the
semiconductor structure is a synchronous dynamic random access
memory array.
43. The semiconductor structure of claim 40 wherein the
semiconductor structure is a static memory array.
44. The semiconductor structure of claim 40 wherein the
semiconductor structure is a dynamic memory array.
45. The semiconductor structure of claim 40 wherein the
semiconductor structure is an extended data out memory array.
46. The semiconductor structure of claim 40 wherein the
semiconductor structure is a wordline in a memory array.
47. The semiconductor structure of claim 40 wherein the barrier
layer is approximately 150 .ANG. thick.
48. The semiconductor structure of claim 40 wherein the barrier
layer has a resistivity of approximately a resistivity of
approximately 60 .mu..OMEGA./cm.
49. The semiconductor structure of claim 40 wherein the conductive
layer is approximately 150 .ANG. thick.
50. The semiconductor structure of claim 40 wherein the conductive
layer has a resistivity of approximately 15-20 .mu..OMEGA./cm.
51. The semiconductor structure of claim 40 wherein the
semiconductor substrate is silicon.
52. The semiconductor structure of claim 40 wherein the
semiconductor substrate is gallium arsenide.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The invention relates generally to the removal of extrusions
that form during the fabrication of integrated circuits. More
particularly, the invention relates to the removal of extrusions
that form on metal-oxide-semiconductor (MOS) structures.
[0003] 2. Description of the Related Art
[0004] In order to improve speed and performance in semiconductor
components, manufacturers typically attempt to decrease access time
of various semiconductor structures located in the components by
incorporating layers of materials which enhance electron flow into
semiconductor structures. For example, to increase read/write speed
in memory cells, such as dynamic random access memory (DRAM),
semiconductor manufacturers attempt to decrease wordline access
time by reducing the resistance or capacitance in a wordline.
Reduction of resistance is typically achieved by layering a
low-resistance conductive material over the gate in a semiconductor
structure.
[0005] Some manufacturers use materials such as titanium silicide
(TiSi.sub.x), known for its low resistance, to form the conductive
layer of a semiconductor structure. Titanium silicide, however, can
become unstable during subsequent high temperature processing steps
and can diffuse into the underlying gate layer (typically
polysilicon), also called titanium silicide agglomeration, thereby
damaging the semiconductor structure. To prevent titanium silicide
agglomeration, some manufacturers cover the gate layer with a
barrier layer, such as titanium nitride (TiN). Depending on the
thickness of the barrier layer, however, the titanium nitride may
possess a high resistance and thus detract from the low resistance
effect of the conductive layer. Furthermore, during subsequent
processing at high temperature and an oxygen atmosphere, titanium
nitride can oxidize to form titanium oxynitride
(TiO.sub.xN.sub.y).
[0006] Likewise, during subsequent processing, semiconductor
structures containing metal silicide layers that may be exposed to
nitrogen and oxygen rich environments at high temperature. These
gases react with metal rich portions of the semiconductor structure
to create metal oxynitrides. Such metal oxynitrides may expand to
create extrusions which protrude outwardly from the sides of the
semiconductor structure. If these extrusions contact other portions
of the semiconductor, short circuits result.
[0007] To reduce the probability of short circuits resulting from
extrusions, some manufacturers coat the semiconductor structure
with an insulator such as a nitride spacer. While these coatings
reduce the amount and extent of extrusions formed, application of
such coatings is a complex and expensive process. Furthermore,
these coatings may interfere with further processing steps thereby
increasing manufacturing costs and decreasing semiconductor
component yields.
SUMMARY OF THE INVENTION
[0008] A conductor, preferably composed of a tungsten silicide
(WSi.sub.x) barrier layer and a titanium silicide (TiSi.sub.x)
conductive layer, is disclosed for use in a semiconductor structure
such as a wordline, transistor or any other structure. A novel wet
etch, preferably composed of an oxidizing and chelating agent in
solution, is further disclosed for selectively removing extrusions
which may protrude from the conductor.
[0009] Exemplary methods of creating the conductor include
sputtering tungsten silicide onto a polysilicon gate to create a
tungsten silicide layer. Titanium silicide is then sputtered onto
the tungsten silicide layer. Exemplary formulations of the wet etch
include a combination of an oxidizing agent such as hydrogen
peroxide (H.sub.2O.sub.2) in water with a quantity of a chelating
agent such as ethylenediaminetetraacetic acid (EDTA). In other
embodiments, the wet etch comprises water, an oxidizing agent, a
chelating agent and a base such as ammonium hydroxide. In yet other
embodiments, the wet etch comprises water, an oxidizing agent, a
chelating agent, a base and a buffer salt such as ammonium
phosphate.
[0010] One aspect of the invention relates to a method of
fabricating a wordline in a memory array. The method comprises
depositing a tungsten silicide barrier layer on a wordline stack
and processing the wordline stack such that tungsten nitride
extrusions extend from an exposed surface of the barrier layer. The
method further comprises selectively etching the tungsten nitride
extrusions with a solution that comprises water, hydrogen peroxide
and EDTA.
[0011] Another aspect of the invention relates to a method of
fabricating a wordline in a memory array. The method comprises
depositing a metal silicide layer on a wordline stack and
processing the wordline stack such that metal nitride extrusions
extend from the metal silicide layer. The method further comprises
selectively etching the metal nitride extrusions with a solution
that comprises at least an oxidizing agent and a chelating
agent.
[0012] An additional aspect of the invention relates to a method of
fabricating a wordline in a memory array. The method comprises
depositing a tungsten silicide barrier layer on a wordline stack
and depositing a titanium silicide conductive layer above the
tungsten silicide barrier layer. The method further comprises
processing the wordline stack such that metal nitride extrusions
extend from the tungsten silicide barrier layer and the titanium
silicide conductive layer. The method also comprises etching the
metal nitride extrusions at a faster rate than the conductive layer
or the barrier layer.
[0013] One embodiment of the invention relates to a method of
fabricating a semiconductor structure. The method comprises
depositing a polysilicon gate on a semiconductor substrate to form
a wordline stack and depositing a conductive layer comprising metal
silicide above the polysilicon gate. The method further comprises
processing the wordline stack in a manner that forms protrusions on
the conductive layer and etching the protrusions at a faster rate
than the conductive layer.
[0014] Another embodiment of the invention relates to a method of
forming a wordline gate that comprises forming metal nitride
extrusions on a wordline stack and selectively removing the metal
nitride extrusions. An additional embodiment relates to a method of
forming a semiconductor structure. The method comprises selectively
removing metal oxynitride extrusions from a semiconductor structure
in a manner that does not substantially damage a conductive layer
in the semiconductor structure.
[0015] Yet another embodiment relates to a method of removing metal
oxynitride extrusions from a semiconductor structure in a manner
that does not substantially damage a polysilicon layer in the
semiconductor structure. A further embodiment relates to a method
of forming a wordline gate. The method comprises forming metal
oxynitride extrusions on a wordline stack and selectively removing
the metal oxynitride extrusions.
[0016] One aspect of the invention relates to a wet etch for
selectively removing tungsten nitride extrusions that comprises at
least 10% by weight hydrogen peroxide and at least 0.03% by weight
EDTA. Another aspect of the invention relates to a wet etch for
selectively removing metal nitride extrusions that comprises at
least 10% by weight an oxidizing agent, and at least 0.03% by
weight a chelating agent.
[0017] One embodiment of the invention relates to a semiconductor
structure that comprises a tungsten silicide layer, wherein the
tungsten silicide layer has substantially etched tungsten nitride
extrusions formed thereon. Another embodiment relates to a
semiconductor structure that comprises a tungsten suicide layer,
wherein the tungsten silicide layer has substantially etched
tungsten oxynitride extrusions thereon. An additional embodiment
relates to a semiconductor structure that comprises a titanium
silicide layer, wherein the titanium silicide layer has
substantially etched titanium nitride extrusions thereon. Yet
another embodiment relates to a semiconductor structure that
comprises a titanium silicide layer, wherein the titanium silicide
layer has substantially etched titanium oxynitride extrusions
thereon.
[0018] Another aspect of the invention relates to a semiconductor
structure that comprises a barrier layer above a semiconductor
substrate, the barrier layer comprising a tungsten silicide. The
semiconductor structure further comprises a conductive layer above
the wordline gate, the conductive layer comprising titanium
suicide. The semiconductor structure also comprises a cap above the
conductive layer.
[0019] For purposes of summarizing the invention, certain aspects,
advantages and novel features of the invention are described
herein. It is to be understood that not necessarily all such
advantages may be achieved in accordance with any particular
embodiment of the invention. Thus, for example, those skilled in
the art will recognize that the invention may be embodied or
carried out in a manner that achieves one advantage or group of
advantages as taught herein without necessarily achieving other
advantages as may be taught or suggested herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] These and other aspects, advantages, and novel features of
the invention will become apparent upon reading the following
detailed description and upon reference to the accompanying
drawings.
[0021] FIG. 1 is a diagram depicting communication between a
microprocessor and a memory device.
[0022] FIG. 2 shows a memory array containing multiple wordlines
and bitlines.
[0023] FIG. 3 is a cross-sectional view of a wordline in accordance
with one embodiment of the invention.
[0024] FIG. 4 shows a top view of wordline stacks in a memory
array.
[0025] FIG. 5 is a top angled view of a wordline stack after
source/drain reoxidation and before wet cleaning.
[0026] FIG. 6 is a top angled view of a wordline stack after wet
cleaning.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0027] A conductor, preferably composed of a tungsten silicide
(WSi.sub.x) barrier layer and a titanium silicide (TiSi.sub.x)
conductive layer, is disclosed for use in a semiconductor structure
such as a wordline, transistor or any other structure. A wet etch,
preferably composed of an oxidizing and chelating agent in
solution, is further disclosed for removing extrusions which may
protrude from these low-resistance segments.
[0028] As illustrated in FIG. 1, a novel memory array 10 is
disclosed herein that interfaces with other electronic circuitry 12
via conventional address signals 14 and data signals 16. The
address signals 14 select one or more memory cells in the memory
array 10. The data signals 16, on the other hand, carry data that
is stored in or retrieved from the memory array 10.
[0029] In one embodiment, the memory array 10 disclosed herein is a
synchronous dynamic random access memory (SDRAM). In other
embodiments the memory array 10 may comprise a wide variety of
memory devices such as static memory, dynamic memory, extended data
out memory or any other memory type known in the art.
[0030] The memory array 10 interfaces with different types of
electronic circuitry 12. By way of example, the electronic
circuitry 12 can include any device which accesses or relies on
memory including, but not limited to, processors, controllers, and
the like. In some embodiments, the memory array 10 and the
electronic circuitry 12 are implemented separately, in other
embodiments the memory array 10 and the electronic circuitry 12 are
integrated together. Furthermore, one of ordinary skill in the art
will recognize that the memory array 10 can be implemented in a
wide variety of devices, products and systems.
[0031] FIG. 2 illustrates a memory array 10 that comprises a
plurality of memory cells 20. These memory cells 20 are organized
into columns C.sub.N and rows R.sub.N. A column decoder 24 and a
row decoder 26 process the address signals to identify the column
C.sub.N and row R.sub.N of the targeted memory cells 20. The
columns are commonly known as bitlines and the rows are typically
known as wordlines to those skilled in the art.
[0032] FIG. 3 illustrates a cross-sectional view of a wordline
R.sub.N. In one embodiment, a semiconductor substrate 30 comprises
two field oxide regions, 32 and 34. The semiconductor substrate 30
may include semiconductor structures and/or other layers that have
been fabricated thereon or any doped silicon platform that is
commonly used in the art. While the illustrated semiconductor
substrate 30 comprises an intrinsically doped monocrystalline
silicon wafer, it will be understood by one of skill in the art of
semiconductor fabrication that the semiconductor substrate 30 in
other arrangements can comprise other forms of semiconductor layers
which include active or operable portions of semiconductor
devices.
[0033] The semiconductor substrate 30 is further processed to
create a gate comprising two n+ regions using methods common to
those of skill in the art. The first n+ region 40 acts as a source
and the second n+ region 42 acts as a drain. Hereinafter the first
n+ region 40 will be referred to as the source 40 and the second n+
region 42 will be referred to as the drain 42. Each wordline
R.sub.N further comprises a plurality of layers which will herein
be called a wordline stack 50. The layers in the wordline stack 50
comprises a dielectric layer 60, a polysilicon layer 70, a
conductive segment 80 and a cap 90.
[0034] In one embodiment, the dielectric layer 60 is a gate oxide
comprising silicon dioxide (SiO.sub.2). The dielectric layer 60 is
applied by wet or dry oxidation of the semiconductor substrate 30
followed by etching through a mask, or by other techniques common
to those of skill in the art. In one embodiment, the dielectric
layer 60 is approximately 60-90 angstroms (.ANG.) thick. The
polysilicon layer 70, in one embodiment, may be deposited onto the
dielectric layer 60 through chemical vapor in the art. In one
embodiment, the polysilicon layer 70 is approximately 1,000 .ANG.
thick.
[0035] The conductive segment 80 interconnects the source 40 and
drain 42. In one embodiment, the conductive segment 80 comprises a
conductive layer 100 and an optional barrier layer 102. The
conductive layer 100 is typically a metal layer that forms a
metal-poly gate. The conductive layer 100, when implemented over
the polysilicon layer 70, lowers the overall resistivity of the
wordlines R.sub.N while retaining the gate integrity provided by
the polysilicon layer 70. Typically, the conductive layer 100
comprises a layer of metal silicide. One of the functions of the
conductive layer 100 is to carry the charge to the memory cells 20
in communication with a wordline R.sub.N.
[0036] In one embodiment, the conductive layer 100 is comprised of
titanium silicide (TiSi.sub.x) which is deposited by physical vapor
deposition in an Argon environment. Suitable sputtering reactors
are commercially available from Applied Materials of Santa Clara,
Calif. In one embodiment, argon gas is introduced into the
sputtering chamber at a rate of between about 50 standard cubic
centimeters per minute (sccm) and 100 sccm for approximately 8 to
10 seconds. The DC magnetron power of the preferred sputtering
reactor is generally set at approximately 1.0 kilowatts (kW) to
approximately 2.0 kW, preferably at approximately 1.5 kW. In light
of the present disclosure, however, the skilled artisan can readily
determine the appropriate sputtering parameters through routine
optimization to achieve the desired titanium silicide composition
in a given reactor configuration.
[0037] In this embodiment the conductive layer 100 is approximately
1,000 .ANG. thick and has a resistivity of approximately 15-20
.mu..OMEGA./cm. The conductive layer 100 may also posses metal-rich
pockets. For example, if the conductive layer 100 comprises
titanium silicide, titanium-rich pockets may exist in the
conductive layer 100.
[0038] Those of ordinary skill in the art will recognize that the
conductive layer 100 may also be made from a variety of metals.
Additional examples of suitable metals for forming the conductive
layer 100 include, but are not limited to, copper, gold, aluminum,
silicon, cobalt silicide, nickel silicide, and the like. Mixtures
of metals are also suitable for forming the conductive layer 100.
Exemplary methods of depositing the conductive layer 100 include,
but are not limited to, Rapid Thermal Chemical Vapor Deposition
(RTCVD), Low Pressure Chemical Vapor Deposition (LPCVD), and
Physical Vapor Deposition (PVD).
[0039] When the memory array 10 is subjected to further processing,
portions of the conductive layer 100 may diffuse into the
polysilicon layer 70, thereby damaging the polysilicon layer 70.
For example, if the conductive layer 100 comprises titanium
silicide, the stability of the titanium silicide decreases when the
semiconductor structure is exposed to temperatures exceeding
850.degree. C. during further processing. Thus, absent a barrier
layer 102, the titanium silicide tends to diffuse into the
polysilicon layer 70 thereby degrading the polysilicon.
[0040] In one embodiment, the barrier layer 102 exists between the
conductive layer 100 and the polysilicon layer 70. In other
embodiments, the barrier layer 102 may not exist. While the barrier
layer 102 electrically connects the conductive layer 100 and the
polysilicon layer 70, it also inhibits diffusion of impurities
between those layers and protects the polysilicon layer 70 during
further processing as described below. In one embodiment, the
barrier layer 102 comprises a variety of metal silicides such as
tungsten silicide (WSi.sub.x) which is deposited by physical vapor
deposition in an argon environment.
[0041] Suitable sputtering reactors are commercially available from
Applied Materials of Santa Clara, Calif. In one embodiment, argon
gas is introduced into the sputtering chamber at a rate of between
about 50 standard cubic centimeters per minute (sccm) and 100 sccm
for approximately 5 to 10 seconds. The DC magnetron power of the
preferred sputtering reactor is generally set at approximately 0.5
kilowatts (kW) to approximately 2.0 kW, preferably at approximately
1.0 kW. In one embodiment, the applied tungsten silicon is
approximately WSi.sub.2.7. In addition, the tungsten silicide
remains stable up to temperatures of approximately 1050.degree. C.,
thereby protecting the polysilicon layer 70 during high temperature
anneals and other high-temperature processing.
[0042] In this embodiment, the barrier layer 102 is approximately
150 .ANG. thick and has a resistivity of approximately 60
.mu..OMEGA./cm. The thinness of the barrier layer 102 combined with
the low resistance of tungsten silicide improves gate access.
[0043] In light of the present disclosure, however, the skilled
artisan can readily determine the appropriate sputtering parameters
through routine optimization to achieve the desired barrier layer
102 composition in a given reactor configuration. Furthermore, one
of ordinary skill in the art will recognize that the thickness and
resistivity of the barrier layer 102 can vary over a range of
values. Those of ordinary skill in the art will also recognize that
the barrier layer 102 can comprise a variety of materials such as
titanium nitride, silicon nitride or other materials which protect
the polysilicon layer 70.
[0044] Local variations in the uniformity of the barrier layer 102
can create metal-rich areas. These metal-rich areas may be due in
part to stoichiometry variations due to the thinness of the barrier
layer 102. For example, if the barrier layer 102 comprises tungsten
silicide, tungsten-rich areas can exist within the tungsten
silicide composition. The wordline stack 50 further comprises the
cap 90 which is deposited onto the conductive segment 80 to protect
the underlying materials during further processing. In one
embodiment, the cap 90 comprises a layer of silicon oxide
(SiO.sub.2) which is deposited onto the conductive layer by
chemical vapor deposition, or by other techniques common to those
of skill in the art. In another embodiment, the cap 90 further
comprises a layer of silicon nitride (Si.sub.3N.sub.4) which is
deposited onto the silicon oxide layer by chemical vapor
deposition, or by other techniques common to those of skill in the
art. In yet another embodiment, the cap 90 comprises silicon
nitride as the primary passivating layer. Those of skill in the art
will recognize that the cap 90 may be made of various common
insulating materials or their combination.
[0045] The wordline stack 50 is further patterned through etching.
FIG. 4 illustrates a top view of two wordline stacks (50, 50') in
one embodiment of a memory array 10. Each wordline stack 50 in this
embodiment is "S" shaped. In the preferred embodiment, the etch
comprises a level 50 in-situ dry etch. The etch may also comprise
other dry or wet etches conunon to those of skill in the art. The
patterning stops at the gate oxide 60 or the substrate 30, and then
the source 40 and the drain 42 regions are formed. In one
embodiment, the source 40 and the drain 42 are doped with n-type
dopants using techniques common to those of skill in the art.
[0046] After patterning the wordline stack 50, or in other
processing steps, the wordline stack 50 is often exposed to
nitrogen rich environments. The nitrogen may react with metal rich
pockets in the conductive segment 80. For example, in an embodiment
where the barrier layer 102 comprises tungsten silicide with
tungsten-rich pockets, the tungsten rich pockets may react with the
nitrogen to form tungsten nitride (WN.sub.x). In other embodiments,
where the conductive layer 100 comprises titanium silicide with
titanium-rich pockets, the titanium-rich pockets may also react
with the nitrogen to form titanium nitride (TiN.sub.x).
[0047] In addition, a source/drain reoxidation is applied to partly
fix dry etch damage and to improve device reliability. This
reoxidation subjects the memory array 10 to an oxygen rich
environment. While the oxygen reoxidizes the areas existing near
the source and drain, the oxygen may also combine with the tungsten
nitride and titanium nitride areas existing in the conductive layer
100 and the barrier layer 102 in various embodiments. When the
oxygen reacts with these layers, metal oxynitrides, such as
tungsten oxynitride (WO.sub.xN.sub.y) and titanium oxynitride
(TiO.sub.xN.sub.y) are formed. Still further, titanium-rich and
tungsten-rich areas may combine with nitrogen and then oxygen to
form titanium tungsten oxynitrides (TiWO.sub.xN.sub.y). One of
ordinary skill in the art will also recognize that in other
embodiments, other metals in the barrier or conductive layers can
combine with oxygen to create different types of metal
oxynitrides.
[0048] As illustrated in FIG. 5, when the metal oxynitrides form,
they expand relative to the remainder of the other conductor
materials. This expansion can create extrusions 140 that extend
outwardly from the sides of the wordline stack 50. These extrusions
140 can contact other portions of the memory array 10, thereby
creating short circuits which can alter the operation of the memory
array 10 or even render the memory array 10 unusable.
[0049] For example, in one embodiment where the conductive layer
100 comprises titanium silicide and the barrier layer 102 comprises
tungsten silicide, the extrusions 140 may comprise tungsten
oxynitride, titanium oxynitride, and/or titanium tungsten
oxynitride. These extrusions 140 not only extend outward from the
exposed surface of the conductive layer 100, but also extend
outward from the exposed surface of the barrier layer 102. While
the dimensions of the extrusions 140 vary, the width is typically
less than 10 nm and the length is approximately 30 nm to
approximately 50 nm.
[0050] As explained above, these extrusions 140 can cause shorts in
the memory array 10. In one example, 360 memory arrays with a
titanium silicide conductive layer 100 and a tungsten silicide
barrier layer 102 were constructed on a single wafer. After the
wafer was processed, only 10 of the 360 memory arrays operated
properly.
[0051] Although the extrusions 140 have been described with respect
to the barrier layer 102 and conductive layer 100 of a wordline
stack 50, it should be understood that such extrusions 140 can
develop in a wide range of devices which use metal-based conductors
such as transistors and the like. For example, the extrusions 140
can develop on a wide variety of metal silicides which are then
exposed to nitridizing conditions. Furthermore, the extrusions 140
can develop metal nitrides which are exposed to oxidizing
conditions.
[0052] The extrusions 140 can be removed by a unique technique
which allows for selective dissolution of the extrusions 140
without significantly harming the materials of the wordline stack
50. In one embodiment, a memory array 10 is subjected to a novel
wet etch that removes the extrusions 140. The novel wet etch
comprises an oxidizing agent and a chelating agent in solution.
[0053] In one embodiment, the oxidizing agent is hydrogen peroxide
(H.sub.2O.sub.2). Preferably, the wet etch is approximately 10% by
weight hydrogen peroxide. In other embodiments, the amount of
hydrogen peroxide can vary from 2% to 50%. In further embodiments,
the oxidizing agent can include any of a variety of substances
including, but not limited to, ozonated water, ozone, nitric acid
(HNO.sub.3), hypochlorous acid (HClO), chloric acid (HClO.sub.3),
ammonium persulphate ((NH.sub.4).sub.2S.sub.2O- .sub.8), sulfuric
acid (H.sub.2SO.sub.4), and the like.
[0054] In one embodiment, the chelating agent is
ethylenediaminetetraaceti- c acid (EDTA) that is approximately
0.03% by weight EDTA. In other embodiments, the amount of EDTA in
the wet etch can range from approximately less than 0.001% to more
than 5% by weight EDTA. In further embodiments, the chelating agent
can be any of a variety of substances including, but not limited
to, ligands, the amine group, the carboxylic acid group compounds,
oxalate, acetate, disodium ethylenediamine acid
(H.sub.2NC.sub.2H.sub.4NH.sub.2), glycine (NH.sub.2CH.sub.2COOH),
iminodiacetic acid (NH(CH.sub.2COOH)O.sub.2), nitrolotriacetic acid
(N(CH.sub.2COOH).sub.3), tetraammonium ethylenediaminetetraacetate,
tetramethylammonium ethylenediaminetetraacetate, and the like.
[0055] In a further embodiment, the etch comprises a combination of
an oxidizing and chelating agent and further comprises a base. The
base may be used to adjust the pH of the etch solution to improve
cleaning and selectivity. In one embodiment, the base is ammonium
hydroxide (NH.sub.4OH). In another embodiment, the wet etch is
approximately 0.02% ammonium hydroxide, 0.042% EDTA and 10%
hydrogen peroxide and has a pH of about seven to eight. In other
embodiments, the amount of ammonium hydroxide can vary over a wide
range such as from approximately 0.5% to approximately 30% by
weight, depending on the desired pH. In further embodiments the
base can be any of a variety of bases including, but not limited
to, tetramethyl ammonium hydroxide (TMAH), trimethoxyamphetamine
(TMA), choline hydroxide, sodium hydroxide, potassium hydroxide,
and the like.
[0056] In still a further embodiment, the etch solution comprises a
combination of an oxidizing agent, a chelating agent, a base and a
buffer salt. The buffer salt may be added to help maintain a
constant pH level and to form a protective layer on the other
materials of the stack to assist selectivity. In one embodiment,
the buffer salt is potassium sulfate (K2SO4) that is approximately
2% potassium sulfate. In other embodiments, the amount of potassium
sulfate can vary over a wide range, depending on the pH and
concentration of other substances in the wet etch solution. In
further embodiments, the buffer salt can be any of a variety of
salts including, but not limited to, potassium phosphate,
diammonium phosphate, and the like.
[0057] In one embodiment, the wet etch is heated to 65.degree. C.
It has been found that a 65.degree. C. wet etch comprising 10%
hydrogen peroxide and 0.03% EDTA in water removes tungsten nitride
at approximately 2040 .ANG./min. and titanium nitride at
approximately 200 .ANG./min. This wet etch further removes tungsten
oxynitride at approximately 200 .ANG./min. Estimates indicate that
the wet etch also removes titanium oxynitride at approximately 20
.ANG./min. In contrast, the wet etch removes tungsten silicide,
titanium silicide, and polysilicon at lower rates such as, by way
of example, at approximately 1 .ANG./min. The rate of etching of
the tungsten oxynitride, the titanium oxynitride, the tungsten
silicide, the titanium silicide and the polysilicon however, can
vary.
[0058] While the wet etch is heated to 65.degree. C. in one
embodiment, the temperature of the wet etch may vary over a wide
range. For example, in certain embodiments, the temperature of the
wet etch varies from approximately room temperature to
approximately 85.degree. C. One of ordinary skill in the art will
recognize that the temperature of the wet etch can be adjusted
based on the composition of the etch solution and the composition
of the extrusions 140 to be dissolved.
[0059] In the preferred embodiment, ammonium hydroxide is added to
adjust the pH of the wet etch. For example, 0.02% by weight of
ammonium hydroxide adjusts the pH of the wet etch to approximately
7. The pH adjustment of the wet etch enhances the metal nitride
removal rates without significantly damaging the remainder of the
wordline stack 50.
[0060] In one embodiment, a memory array 10 is soaked in the
preferred wet etch for approximately 2 minutes to remove
approximately 50 nm of tungsten oxynitride extrusions 140. The
length of time of cleaning can vary over a wide range. For example,
in certain embodiments, the length of time of cleaning varies from
approximately 5 minutes to approximately 30 minutes. Those of skill
in the art will recognize that a memory array 10 can be soaked in a
wet etch solution for enough time to remove as much metal nitride
and/or metal oxynitride as required.
[0061] In the example where only 10 out of 360 memory arrays
constructed on a wafer operated properly after processing, a second
360 memory array wafer was constructed. With the novel wet etch,
200 of the 360 memory arrays operated properly after wet etch
cleaning.
[0062] While application of the wet etch has been described with
respect to removal of extrusions 140 in a wordline stack 50, it
should be understood that the wet etch can be applied to a wide
variety of semiconductor structures wherein extrusions 140 need to
be removed therefrom. Furthermore, although the foregoing invention
has been described in terms of certain preferred embodiments, other
embodiments will become apparent to those of ordinary skill in the
art in view of the disclosure herein. Accordingly, the present
invention is not intended to be limited by the recitation of
preferred embodiments, but is intended to be defined solely by
reference to the appended claims.
* * * * *