U.S. patent application number 09/755522 was filed with the patent office on 2002-03-07 for method of etching tungsten or tungsten nitride electrode gates in semiconductor structures.
This patent application is currently assigned to Applied Materials, Inc.. Invention is credited to Nallan, Padmapani, Oluseyi, Hakeem.
Application Number | 20020028582 09/755522 |
Document ID | / |
Family ID | 25039504 |
Filed Date | 2002-03-07 |
United States Patent
Application |
20020028582 |
Kind Code |
A1 |
Nallan, Padmapani ; et
al. |
March 7, 2002 |
Method of etching tungsten or tungsten nitride electrode gates in
semiconductor structures
Abstract
The present invention relates to a method of etching tungsten or
tungsten nitride in semiconductor structures, and particularly to
the etching of gate electrodes which require precise control over
the etching process. We have discovered a method of etching
tungsten or tungsten nitride which permits precise etch profile
control while providing excellent selectivity, of at least 175:1,
for example, in favor of etching tungsten or tungsten nitride
rather than an adjacent oxide layer. Typically, the oxide is
selected from silicon oxide, silicon oxynitride, tantalum
pentoxide, zirconium oxide, and combinations thereof. The method
appears to be applicable to tungsten or tungsten nitride, whether
deposited by physical vapor deposition (PVD) or chemical vapor
deposition (CVD). In particular, an initial etch chemistry, used
during the majority of the tungsten or tungsten nitride etching
process (the main etch), employs the use of a plasma source gas
where the chemically functional etchant species are generated from
a combination of sulfur hexafluoride (SF.sub.6) and nitrogen
(N.sub.2), or in the alternative, from a combination of nitrogen
trifluoride (NF.sub.3), chlorine (Cl.sub.2), and carbon
tetrafluoride (CF.sub.4). Toward the end of the main etching
process, a second chemistry is used in which the chemically
functional etchant species are generated from Cl.sub.2 and O.sub.2.
This final portion of the etch process may be referred to as an
"overetch" process, since etching is carried out to at least the
surface underlying the tungsten or tungsten nitride. However, this
second etch chemistry may optionally be divided into two steps,
where the plasma source gas oxygen content and plasma source power
are increased in the second step.
Inventors: |
Nallan, Padmapani; (San
Jose, CA) ; Oluseyi, Hakeem; (Stanford, CA) |
Correspondence
Address: |
PATENT COUNSEL
APPLIED MATERIALS, INC.
Legal Affairs Department
P.O.BOX 450A
Santa Clara
CA
95052
US
|
Assignee: |
Applied Materials, Inc.
|
Family ID: |
25039504 |
Appl. No.: |
09/755522 |
Filed: |
January 5, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09755522 |
Jan 5, 2001 |
|
|
|
09614396 |
Jul 12, 2000 |
|
|
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Current U.S.
Class: |
438/710 ;
257/E21.206; 257/E21.311 |
Current CPC
Class: |
H01L 21/28088 20130101;
C23F 4/00 20130101; H01L 21/28123 20130101; H01L 21/28079 20130101;
H01L 21/32136 20130101 |
Class at
Publication: |
438/710 |
International
Class: |
H01L 021/302; H01L
021/461 |
Claims
We claim:
1. A method of plasma etching tungsten or tungsten nitride, wherein
an etch selectivity of greater than about 75:1 is obtained relative
to an adjacent silicon oxide layer by using a plasma source gas
comprising chemically functional etchant species which are
generated from Cl.sub.2 and O.sub.2, wherein the volumetric
percentage of O.sub.2 in said plasma source gas is at least 35%,
and wherein a plasma density is at least 1.6.times.10.sup.11
e.sup.-/cm.sup.3.
2. The method of claim 1, wherein said plasma density is obtained
by applying a source power of at least 1600 W.
3. The method of claim 1, wherein said etch selectivity is greater
than about 100:1, said volumetric percentage of O.sub.2 is at least
40%,and said plasma density is at least 1.65.times.10.sup.11
e.sup.-/cm.sup.3.
4. The method of claim 3, wherein said plasma density is obtained
by applying a source power of at least 1650 W.
5. The method of claim 1, wherein said etch selectivity is greater
than about 175:1, said volumetric percentage of O.sub.2 is at least
45%, and said plasma density is at least 1.8.times.10.sup.11
e.sup.-/cm.sup.3.
6. The method of claim 5, wherein said plasma density is obtained
by applying a source power of at least 1800 W.
7. A method of selectively etching tungsten or tungsten nitride
relative to an underlying oxide layer in semiconductor structures,
comprising: plasma etching a first majority portion of said
tungsten or tungsten nitride using a first plasma source gas
comprising at least one halogen-based chemically functional etchant
species; and plasma etching a remaining portion of said tungsten or
tungsten nitride using a second plasma source gas comprising
chemically functional etchant species generated from Cl.sub.2
and
8. The method of claim 7, wherein said underlying oxide is selected
from the group consisting of silicon oxide, silicon oxynitride,
tantalum pentoxide, zirconium oxide, and combinations thereof
9. The method of claim 8, wherein said halogen is selected from the
group consisting of fluorine, chlorine, and combinations
thereof.
10. The method of claim 9, wherein said first majority portion of
said plasma etching is carried out using a first plasma source gas
selected from the group consisting of a combination of SF.sub.6 and
N.sub.2, and a combination of NF.sub.3, Cl.sub.2, and CF.sub.4.
11. The method of claim 9, wherein said underlying oxide comprises
silicon oxide, and wherein a volumetric percentage of O.sub.2 in
said second plasma source gas is greater than 20% by volume.
12. The method of claim 11, wherein said O.sub.2 volumetric
percentage ranges from greater than 20% to about 45%.
13. The method of claim 9, wherein said underlying oxide comprises
silicon oxide, and wherein a plasma density of said etchant plasma
is at least 8.0.times.10.sup.10 e.sup.-/cm.sup.3.
14. The method of claim 13, wherein said plasma density ranges from
about 8.0.times.10.sup.10 e.sup.-/cm.sup.3 to about
3.0.times.10.sup.11 e.sup.-/cm.sup.3.
15. The method of claim 14, wherein said plasma density ranges from
about 8.0.times.10.sup.10 e.sup.-/cm.sup.3 to about
2.0.times.10.sup.11 e.sup.-/cm.sup.3.
16. The method of claim 13, wherein a plasma source power ranges
from about 800 W to about 3000 W.
17. The method of claim 16, wherein said plasma source power ranges
from about 800 W to about 2000 W.
18. The method of claim 9, wherein said underlying oxide comprises
silicon oxide, and wherein said etching which is carried out using
said plasma source gas comprising said chemically functional
etchant species generated from Cl.sub.2 and O.sub.2 is carried out
in two steps, wherein a volumetric concentration of O.sub.2 in a
first step ranges from greater than 20% by volume to about 35% by
volume, and wherein a volumetric concentration of O.sub.2 in a
second step ranges from greater than about 35% by volume to about
45% by volume.
19. The method of claim 9, wherein said underlying oxide comprises
tantalum pentoxide, and wherein a volumetric percentage of O.sub.2
in said second plasma source gas ranges from about 20% to about
50%.
20. The method of claim 19, wherein said underlying oxide comprises
tantalum pentoxide, and wherein a plasma density of said etchant
plasma ranges from about 2.0.times.10.sup.10 e.sup.-/cm.sup.3 to
about 1.8.times.10.sup.11 e.sup.-/cm.sup.3.
21. The method of claim 20, wherein a plasma source power ranges
from about 200 W to about 1800 W.
22. The method of claim 19, wherein said underlying oxide comprises
tantalum pentoxide, and a substrate bias power ranges from about 60
W to about 300 W.
23. A method of selectively etching tungsten or tungsten nitride
relative to an underlying oxide layer in semiconductor structures,
comprising: plasma etching a first majority portion of said
tungsten or tungsten nitride using a first plasma source gas
comprising fluorine species; and plasma etching a remaining portion
of said tungsten or tungsten nitride using a second plasma source
gas comprising chemically functional etchant species generated from
Cl.sub.2 and O.sub.2.
24. The method of claim 23, wherein said underlying oxide is
selected from the group consisting of silicon oxide, silicon
oxynitride, tantalum pentoxide, zirconium oxide, and combinations
thereof.
25. The method of claim 24, wherein said first majority portion of
said plasma etching is carried out using a first plasma source gas
selected from the group consisting of a combination of SF.sub.6 and
N.sub.2, and a combination of NF.sub.3, Cl.sub.2, and CF.sub.4.
26. The method of claim 25, wherein said first plasma source gas
comprises SF.sub.6 and N.sub.2, and wherein a volumetric ratio of
SF.sub.6:N.sub.2 in said first plasma source gas ranges from about
2:5 to about 6:1.
27. The method of claim 24, or claim 25, or claim 26, wherein said
underlying oxide comprises silicon oxide, and wherein a volumetric
percentage of O.sub.2 in said second plasma source gas is greater
than 20% by volume.
28. The method of claim 27, wherein said O.sub.2 volumetric
percentage ranges from greater than 20% to about 45%.
29. The method of claim 27, wherein a plasma density of said
etchant plasma is at least 8.0.times.10.sup.10
e.sup.-/cm.sup.3.
30. The method of claim 29, wherein said plasma density ranges from
about 8.0.times.10.sup.10 e.sup.-/cm.sup.3 to about
3.0.times.10.sup.11 e.sup.-/cm.sup.3.
31. The method of claim 30, wherein said plasma density ranges from
about 8.0.times.10.sup.10 e.sup.-/cm.sup.3 to about
2.0.times.10.sup.11 e.sup.-/cm.sup.3.
32. The method of claim 27, wherein a plasma source power ranges
from about 800 W to about 3000 W.
33. The method of claim 32, wherein said plasma source power ranges
from about 800 W to about 2000 W.
34. The method of claim 28, wherein a plasma density of said
etchant plasma is at least 8.0.times.10.sup.10
e.sup.-/cm.sup.3.
35. The method of claim 34, wherein said plasma density ranges from
about 8.0.times.10.sup.10 e.sup.-/cm.sup.3 to about
3.0.times.10.sup.11 e.sup.-/cm.sup.3.
36. The method of claim 34, wherein a plasma source power ranges
from about 800 W to about 3000 W.
37. The method of claim 24, wherein said underlying oxide comprises
silicon oxide, and wherein said etching which is carried out using
said plasma source gas comprising chemically functional etchant
species generated from Cl.sub.2 and O.sub.2 is carried out in two
steps, wherein a volumetric concentration of O.sub.2 in a first
step ranges from greater than 20% by volume to about 35% by volume,
and wherein the volumetric concentration of O.sub.2 in a second
step ranges from greater than about 35% by volume to about 45% by
volume.
38. The method of claim 37, wherein a plasma density during said
first step ranges from about 8.times.10.sup.10 e.sup.-/cm.sup.3 to
about 1.6.times.10.sup.11 e.sup.-/cm.sup.3, and wherein a plasma
density during said second step ranges from about
1.6.times.10.sup.11 e.sup.-/cm.sup.3 to about 3.0.times.10.sup.11
e.sup.-/cm.sup.3.
39. The method of claim 37, wherein a plasma source power during
said first step ranges from about 800 W to about 1600 W, and
wherein a plasma source power during said second step ranges from
about 1600 W to about 3000 W.
40. The method of claim 24, wherein said underlying oxide comprises
tantalum pentoxide, and wherein a volumetric percentage of O.sub.2
in said second plasma source gas ranges from about 20% to about
50%.
41. The method of claim 40, wherein said underlying oxide comprises
tantalum pentoxide, and wherein a plasma density of said etchant
plasma ranges from about 2.0.times.10.sup.11 e.sup.-/cm.sup.3 to
about 1.8.times.10.sup.11 e.sup.-/cm.sup.3.
42. The method of claim 41, wherein a plasma source power ranges
from about 200 W to about 1800 W.
43. The method of claim 40, wherein said underlying oxide comprises
tantalum pentoxide, and a substrate bias power ranges from about 60
W to about 300 W.
44. A method of selectively etching tungsten nitride relative to
silicon oxide during the etching of semiconductor structures,
comprising: plasma etching a majority portion of said tungsten
nitride using a plasma source gas comprising fluorine species; and
plasma etching a remaining portion of said tungsten nitride using a
second plasma source gas comprising chemically functional etchant
species generated from Cl.sub.2 and O.sub.2, wherein said O.sub.2
concentration is about 35% by volume or greater, and wherein a
plasma density of said etchant plasma is about 1.6.times.10.sup.11
e.sup.-/cm.sup.3 or greater.
45. The method of claim 44, wherein said first majority portion of
said plasma etching is carried out using a first plasma source gas
selected from the group consisting of a combination of SF.sub.6 and
N.sub.2, and a combination of NF.sub.3, Cl.sub.2, and CF.sub.4.
46. The method of claim 44, wherein said plasma density is created
by application of a plasma source power of 1600 W or greater.
47. The method of claim 44, wherein said O.sub.2 concentration is
about 40% by volume or greater, and wherein a plasma density of
said etchant plasma is about 1.8.times.10.sup.11 e.sup.-/cm.sup.3
or greater.
48. The method of claim 47, wherein said plasma density is created
by application of a plasma source power of 1800 W or greater.
49. A method of plasma etching tungsten or tungsten nitride,
wherein an etch selectivity of greater than about 75:1 is obtained
relative to an adjacent tantalum pentoxide layer by using a plasma
source gas comprising chemically functional etchant species which
are generated from Cl.sub.2 and O.sub.2, wherein the volumetric
percentage of O.sub.2 in said plasma source gas ranges from about
20% to about 50%, and wherein a plasma density ranges from about
2.0.times.10.sup.10 e.sup.-/cm.sup.3 to about 1.8.times.10.sup.11
e.sup.-/cm.sup.3.
50. The method of claim 49, wherein said plasma density is obtained
by applying a source power within the range of about 200 W to about
1800 W.
51. The method of claim 49, wherein a substrate bias power ranges
from about 60 W to about 300 W.
52. The method of claim 5 1, wherein a substrate bias power ranges
from about 60 W to about 200 W.
53. The method of claim 52, wherein a substrate bias power ranges
from about 100 W to about 200 W.
54. A method of selectively etching tungsten nitride relative to
tantalum pentoxide during the etching of semiconductor structures,
comprising: plasma etching a majority portion of said tungsten
nitride using a plasma source gas comprising fluorine species; and
plasma etching a remaining portion of said tungsten nitride using a
second plasma source gas comprising chemically functional etchant
species generated from Cl.sub.2 and O.sub.2, wherein said O.sub.2
concentration ranges from about 20% to about 50% by volume, and
wherein a plasma density of said etchant plasma ranges from about
2.0.times.10.sup.10 e.sup.-/cm.sup.3 to about 1.8.times.10.sup.11
e.sup.-/cm.sup.3.
55. The method of claim 54, wherein said first majority portion of
said plasma etching is carried out using a first plasma source gas
selected from the group consisting of a combination of SF.sub.6 and
N.sub.2, and a combination of NF.sub.3, Cl.sub.2, and CF.sub.4.
56. The method of claim 54, wherein said plasma density is obtained
by applying a source power within the range of about 200 W to about
1800 W.
57. The method of claim 54, wherein a substrate bias power ranges
from about 60 W to about 200 W.
58. The method of claim 51, wherein a substrate bias power ranges
from about 60 W to about 200 W.
59. The method of claim 52, wherein a substrate bias power ranges
from about 100 W to about 200 W.
60. A semiconductor gate structure comprising a layer of tungsten
and an underlying layer of tantalum pentoxide, deposited on a
silicon substrate, wherein said tantalum pentoxide layer has
thickness of less than about 100 .ANG..
61. The semiconductor gate structure of claim 60, wherein said
tantalum pentoxide layer has a thickness within the range of about
30 .ANG. to about 50 .ANG..
Description
RELATED APPLICATIONS
[0001] This application is a continuation-in-part of U.S.
application Ser. No. 09/614,396, filed Jul. 12, 2000.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention pertains to a method of etching
tungsten or tungsten nitride electrode gates in semiconductor
structures. In particular, one etch chemistry is used during the
majority of the etching process and a second etch chemistry is used
toward the end of the etching process.
[0004] 2. Brief Description of the Background Art
[0005] Semiconductor devices as a whole typically include
self-aligned contact structures and gate electrodes which are
fabricated from multiple film layers of differing compositions.
Tungsten nitride films have previously been used as barrier layers,
and tungsten has been used as a conductor in various semiconductor
device structures. Recently, both tungsten and tungsten nitride
have been developing as gate materials, as a result of smaller
device geometries.
[0006] In many instances, the tungsten or tungsten nitride film
(layer) is deposited over a thin (less than about 50 .ANG. thick)
silicon oxide inorganic dielectric layer. During patterned etching
of the multiple film layer structure, it is desired to plasma dry
etch through the tungsten or tungsten nitride layer and to stop
etching at the surface of the silicon oxide layer. This makes it
important that the etch selectivity for etching of tungsten or
tungsten nitride (in preference over silicon oxide) be high. (The
term "selectivity" is typically used to refer to a ratio of etch
rates of two materials.) Further, as the device geometries become
smaller, etching of layers of material must be more precise,
providing a profile which permits placement of more devices over a
given surface area. In the case of a tungsten gate, for example,
the gate may be in the form of a thin line or pad, and the
cross-sectional profile of the etched gate feature is preferably
one where the sidewalls of the etched feature are essentially
perpendicular to an underlying silicon oxide substrate layer, for
example. This means the tungsten must be completely etched to the
surface of the silicon oxide substrate layer (no residual "feet" at
the bottom of the etched tungsten sidewall). Control of the etch
process is critical in providing proper etched tungsten feature
profile while avoiding etching away critical thickness of the
underlying silicon oxide film substrate.
[0007] One reference describes a dry etching method wherein a
multilayer film including one selected from tungsten, molybdenum,
and a silicide thereof, is etched as the first layer. Underlying
the "first layer" is a second layer of polycrystalline silicon,
which overlies a silicon oxide insulation film. The etching step
for the first layer uses a plasma etchant source gas made up of a
first gas selected from fluorine, sulfur hexafluoride, and nitrogen
trifluoride, or a mixture gas containing the first gas and a second
gas selected from hydrogen chloride, hydrogen bromide, chlorine,
bromine, and carbon tetrachloride. Etching of the second layer of
polycrystalline silicon is carried out using a plasma etchant
source gas made up of the second gas and a third gas selected from
an inert gas, nitrogen gas, oxygen gas, silicon tetrachloride gas,
and carbon monoxide gas. In the second etch step, the amount of the
third gas added to the second gas should preferably be in the range
between 0 and 10 volume % of the total etching gas mixture.
[0008] Another reference discloses a method for fabricating a
silicon-based MOS transistor having an inverse-T refractory metal
gate structure. The gate fabricated comprises a main CVD tungsten
portion and a lower sputtered tungsten portion outwardly extending
from the bottom of the CVD portion. A Cl.sub.2/O.sub.2 plasma etch
is used to etch the CVD tungsten layer and a chemical etch
(KH.sub.2PO.sub.4/KOH/K.sub.3Fe(CN).su- b.6) is used to etch the
sputtered tungsten portion. The sputtered tungsten layer is said to
act as a shield to protect the underlying gate oxide layer from ion
damage throughout the fabrication process. In particular, the
sputtered tungsten is said to be more resistant to Cl.sub.2/O.sub.2
reactive ion etch than is CVD tungsten.
[0009] Another reference describes a method of fabricating sidewall
spacers for a self-aligned contact hole. A metal, such as tungsten,
is RIE etched using a conventional etchback procedure, without the
use of a photoresist masking, using a
Cl.sub.2--SF.sub.6--BCl.sub.3-- Ar etchant gas mixture for plasma
generation.
[0010] For further background information, the reader is directed
to U.S. Pat. No. 5,295,923, to Hori et al.; U.S. Pat. No.
5,599,725, to Dorleans et al.; and U.S. Pat. No. 6,033,962, to Jeng
et al.
SUMMARY OF THE INVENTION
[0011] The present invention relates to a method of etching
tungsten or tungsten nitride in semiconductor structures, and
particularly to the etching of gate electrodes which require
precise control over the etching process. We have discovered a
method of etching tungsten or tungsten nitride which permits
precise etch profile control while providing excellent selectivity
in favor of etching tungsten (or tungsten nitride) rather than a
thin underlying oxide layer. Typically, the oxide is selected from
silicon oxide, silicon oxynitride, tantalum pentoxide, zirconium
oxide, and combinations thereof. The method appears to be
applicable to tungsten or tungsten nitride, whether deposited by
physical vapor deposition (PVD) or chemical vapor deposition
(CVD).
[0012] In particular, an initial etch chemistry, used during the
majority of the tungsten or tungsten nitride etching process (the
main etch), employs the use of a plasma source gas where the
chemically functional etchant species are typically generated from
a combination of sulfur hexafluoride (SF.sub.6) and nitrogen
(N.sub.2), or in the alternative, from a combination of nitrogen
trifluoride (NF.sub.3), chlorine (Cl.sub.2), and carbon
tetrafluoride (CF.sub.4). Toward the end of the main etching
process, a second chemistry is used in which the chemically
functional etchant species are generated from Cl.sub.2 and O.sub.2.
This final portion of the etch process may be referred to as an
"overetch" process, since etching is carried out to at least the
surface underlying the tungsten or tungsten nitride. However, this
second etch chemistry may optionally be divided into two steps,
where the plasma source gas oxygen content and plasma source power
are increased in the second step.
[0013] We have discovered that an unexpectedly high etch
selectivity for tungsten in preference over an underlying oxide
layer (in the range of 175:1, for tungsten:silicon oxide, for
example) may be obtained when a sufficiently high concentration of
O.sub.2 is used in combination with a sufficiently high plasma
density. In particular, when the O.sub.2 concentration is greater
than about 20% by volume in the plasma source gas, further
increases in O.sub.2 content have a limited effect at plasma
densities below about 8.times.10.sup.10 e.sup.-/cm.sup.3, because
there is insufficient power input to energize the active oxygen
species. To obtain selectivity in favor of etching tungsten or
tungsten nitride relative to an underlying oxide layer, it is
necessary to increase both the oxygen content of the plasma source
gas and the source power applied to create and maintain the plasma.
For example, at a plasma density of about 1.6.times.10.sup.11
e.sup.-/cm.sup.3, and a substrate bias voltage of about -90 V
(about 90 W applied bias), an increase in plasma source gas oxygen
content from about 30 volume percent to about 40 volume percent
produces an increase in selectivity for etching tungsten relative
to silicon oxide from about 40:1 to about 75:1. However, at 40
volume percent O.sub.2, if the plasma density is increased to about
1.8.times.10.sup.11 e.sup.-/cm.sup.3, the selectivity for etching
tungsten relative to silicon oxide unexpectedly increases from
about 75:1 to about 160:1.
[0014] Although carrying out the "overetch" step at the conditions
which produce the highest selectivity protects the underlying oxide
layer, the etched tungsten or tungsten nitride feature profile may
be affected by the rapid tungsten etch rates obtained (about 1800
.ANG. per minute at the conditions which produce 175:1
tungsten:silicon oxide selectivity). To enable maintenance of
feature profile while removing residual tungsten "feet" at the base
of a feature, it may be advantageous to etch under conditions which
provide a lower selectivity, of about 30:1 tungsten: silicon oxide,
for example, and a tungsten etch rate of about 1000 .ANG. per
minute, and then change process conditions to those which provide a
selectivity of 175:1 tungsten: silicon oxide and a tungsten etch
rate of about 10-15 .ANG. per minute for a limited time at the end
of the etch, to clean residue off the oxide flat surface
surrounding the etched feature in general.
[0015] When the underlying dielectric layer comprises tantalum
pentoxide, we have discovered that the source gas composition and
process conditions described above provide an almost infinite
selectivity for etching tungsten relative to the underlying
tantalum pentoxide layer. This extraordinarily high tungsten:
tantalum pentoxide etch selectivity allows a thinner tantalum
pentoxide dielectric layer to be used (relative to a corresponding
silicon oxide layer). The tantalum pentoxide layer typically may
have a thickness of less than about 100 .ANG., preferably within
the range of about 30 .ANG. to about 50 .ANG.. The thinness of the
oxide layer can be important in terms of increasing a semiconductor
device speed.
[0016] We have discovered that tantalum pentoxide is more resistant
to ion bombardment than silicon oxide; this permits a higher
substrate bias power to be used during the tungsten overetch step
when tantalum pentoxide is used as the underlying oxide layer. The
use of a higher bias power (from about 100 W up to about 300 W)
during the overetch step provides improved profile control (i.e., a
more vertical profile is obtained), while maintaining a high
selectivity for etching tungsten relative to the underlying
tantalum pentoxide. When a higher bias power is employed, a plasma
source gas having a lower oxygen content can be used.
[0017] When the underlying oxide layer comprises tantalum
pentoxide, a tungsten etch rate of about 2000 .ANG. per minute can
be achieved while maintaining a tungsten: tantalum pentoxide etch
selectivity of about 75:1 or better. In particular, one etch
process includes using a plasma source gas having a volumetric
percentage of O.sub.2 of about 20%, with a plasma density within
the range of about 4.0.times.10.sup.10 e.sup.-/cm.sup.3 to about
6.0.times.10.sup.10 e.sup.-/cm.sup.3, a plasma source power within
the range of about 400 W to about 600 W, a substrate bias power of
about 175 W, and a process chamber pressure of about 4 mTorr, for
example. To enable maintenance of feature profile while removing
residual tungsten "feet" at the base of a feature, it may be
advantageous to adjust the etch conditions toward the end of the
etch. For example, the final etch conditions may provide a
selectivity of at least 175:1 tungsten: tantalum pentoxide, where
the tungsten etch rate is less than about 1000 .ANG. per minute.
This etch selectivity and etch rate can be obtained by using a
plasma source gas having a volumetric percentage of O.sub.2 of
about 20%, with a plasma density of about 2.0.times.10.sup.10
e.sup.-/cm.sup.3, a plasma source power of about 200 W, a substrate
bias power of about 175 W, and a process chamber pressure of about
6 mTorr, for example.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 shows a schematic of the plasma processing apparatus
which was used to carry out the etching processes described
herein.
[0019] FIGS. 2A and 2B show schematics of photomicrographs of
etched tungsten when no overetch step is used, i.e., there is no
change in the etch chemistry toward the end of the tungsten
etching, and the etch is stopped in sufficient time to avoid
etching an underlying silicon oxide substrate.
[0020] FIG. 3 shows a schematic of a photomicrograph of etched
tungsten where there is a change in the etch chemistry toward the
end of the tungsten etching, and etching is permitted to continue
to the surface of the silicon oxide substrate.
[0021] FIG. 4 is a graph showing that an increase in oxygen content
of a plasma source gas has a diminishing effect on the etch rate of
a silicon oxide substrate, when all other process variables are
held constant.
[0022] FIG. 5A is a graph showing the effect of increasing the
plasma source power on the etch rate of tungsten; the etch rate of
silicon oxide; and on the selectivity (in terms of an increase in
etch rate of tungsten relative to the etch rate of silicon oxide)
during an overetch step, when the oxygen concentration is about 20%
by volume.
[0023] FIG. 5B is a three-dimensional graph showing tungsten etch
rate in an overetch step, as a function of plasma source power and
oxygen flow rate, all other variables held constant.
[0024] FIG. 5C is a three-dimensional graph showing silicon oxide
etch rate in an overetch step, as a function of plasma source power
and oxygen flow rate, all other variables held constant.
[0025] FIG. 5D is a three-dimensional graph showing selectivity
(etch rate ratio of W:SiO.sub.x) as a function of plasma source
power and oxygen flow rate, all other variables held constant.
[0026] FIG. 6A shows a beginning semiconductor structure 600 for
performing an embodiment of the method of the invention for etching
tungsten, when the underlying dielectric layer is tantalum
pentoxide. The semiconductor structure 600 includes, from top to
bottom, a patterned silicon nitride hard mask layer 612, a
patterned silicon oxynitride ARC layer 610, a tungsten layer 608, a
tungsten nitride wetting layer 606, and a tantalum I pentoxide
layer 604, all deposited on a silicon substrate 602. The relative
thicknesses of the film stack layers are not shown to scale.
[0027] FIG. 6B shows the semiconductor structure 600 after etching
through a major portion of the tungsten 608/tungsten nitride 606
during the main etch step.
[0028] FIG. 6C shows the semiconductor structure 600 after
performance of the overetch step. The entire thickness of the
tungsten 608/tungsten nitride 606 has been etched through, exposing
the upper surface of the tantalum pentoxide layer 604.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0029] The present disclosure relates to a method of etching
tungsten or tungsten nitride in semiconductor structures. In
particular, the method pertains to the etching of feature sizes of
less than 0.5 .mu.m, where control of etch selectivity of tungsten
or tungsten nitride relative to a thin layer (typically less than
about 50 .ANG.) of an underlying oxide is of primary importance.
The term "feature" typically refers to metal lines, trenches and
openings in a dielectric layer, as well as other structures which
make up the topography of the substrate surface.
[0030] In particular, an initial etch chemistry, used during the
majority of the tungsten or tungsten nitride etching process, is
typically one employing a plasma source gas comprising at least one
halogen-based chemically functional etchant species. The halogen is
typically fluorine, chlorine, or combinations thereof. A source gas
comprising fluorine species is preferably used, because the
fluorine is capable of removing oxides and other residues which may
form on the surface of the tungsten if it is exposed to air during
processing. If the plasma source gas used during the main tungsten
etch step does not include fluorine species (for example, if
Cl.sub.2 is used), a breakthrough etch step employing a plasma
generated from a fluorine-containing gas may be needed in order to
remove the tungsten oxide from the substrate surface prior to
etching of the tungsten layer. Typical process conditions for the
breakthrough etch are as follows: 50-200 sccm CF.sub.4; 4-10 mTorr
process chamber pressure; 300-1000 W source power; 40-200 W
substrate bias power; and 50.degree. C. substrate temperature.
Other fluorine-containing gases, such as SF.sub.6 or NF.sub.3, for
example, can be used in place of, or in combination with, CF.sub.4.
A non-reactive diluent gas, such as, for example, argon, may be
used in combination with the fluorine-containing gas. A typical
duration for the breakthrough etch is approximately 10 seconds.
[0031] The tungsten main etch is preferably performed using a
plasma source gas where the chemically functional etchant species
are generated from sulfur hexafluoride (SF.sub.6) and nitrogen
(N.sub.2), or from a combination of NF.sub.3, Cl.sub.2, and
CF.sub.4. These etch chemistries provide a rapid tungsten etch rate
with excellent etch profile characteristics, and do not require the
performance of a breakthrough etch prior to the main tungsten
etch.
[0032] For example, in one embodiment, the main etch is carried out
using a source gas consisting of SF.sub.6 and N.sub.2. The
volumetric flow rates for SF.sub.6 typically range from about 30
sccm to about 100 sccm in a CENTURA.RTM. DPST processing chamber.
The volumetric flow rates for N.sub.2 typically range from about 30
sccm to about 100 sccm as well. The preferred volumetric ratio of
SF.sub.6 : N.sub.2 generally ranges from about 20:50 to about
60:10. The process chamber pressure ranges from about 2 mTorr to
about 20 mTorr, and is preferably maintained at a pressure within a
range of about 2 mTorr to about 10 mTorr. The substrate temperature
ranges from about 20.degree. C. to about 100.degree. C., with lower
temperatures being preferred, since apparatus costs are lower and
the possibility of damage to a gate oxide is reduced. The applied
plasma source power ranges from about 200 W to about 2000 W, and
the applied substrate biasing power ranges from about 40 W to about
200 W. ( The term "plasma source power" typically refers to the
power that is responsible for sustaining the plasma by providing a
major portion of the energy to ionize the neutral species in the
chamber, while the term "substrate bias power" typically refers to
the power applied to the substrate to attract high energy plasma
species toward the substrate.) Using conditions within the ranges
just described, adjusted for apparatus variables, a tungsten or
tungsten nitride etch rate ranging from about 1500 to about 4000
.ANG. per minute is obtained. During the etching of fine (about
0.15 .mu.m in width) lines, the profile of the line sidewall
relative to the underlying substrate surface is vertical at about
88.degree. to 90.degree. (discounting "feet" which may be formed
near the base of the sidewall).
[0033] Toward the end of the etching process, a second chemistry is
used in which the chemically functional etchant species are
generated from Cl.sub.2 and O.sub.2. The process during etch of the
remaining portion of the tungsten may be referred to as an overetch
process, since the etch is carried out to the surface of an
underlying film surface and may etch for a limited distance into
the underlying film surface. However, this second etch chemistry
may optionally be divided into two steps, where the oxygen content
and plasma source power are increased during the second step.
[0034] To obtain a satisfactory etch rate for the tungsten or
tungsten nitride and the desired selectivity in favor of etching
tungsten relative to an underlying silicon oxide gate layer, the
volumetric percentage of the O.sub.2 in a Cl.sub.2 /O.sub.2 source
gas mixture is controlled to range from greater than 20% up to
about 45%. Preferably, the volumetric percentage of O.sub.2 in a
Cl.sub.2/O.sub.2 source gas mixture ranges from about 35% to about
45%. However, to obtain the benefit of the O.sub.2 content
specified above, the plasma density in the etch process chamber
must be sufficiently high. We have determined that a plasma density
of at least about 8.times.10.sup.10 e.sup.-/cm.sup.3 (a plasma
source power of about 800 W in a CENTURA.RTM. DPSTM processing
chamber available from Applied Materials, Inc., of Santa Clara,
Calif.) is required to obtain benefit from increasing the
volumetric percentage of oxygen above 20%. (The term "decoupled
plasma source" or "DPS" as used herein refers to a plasma etch
apparatus with separate controls for the inductively coupled RF
source power used to generate and maintain a plasma and the bias
power applied to a semiconductor substrate to direct high energy
species toward the substrate).
[0035] An alternative to using a single overetch step is to use a
two-step overetch process. For example, after the main etch step, a
first overetch step may be carried out in which the plasma source
gas composition is a combination of Cl.sub.2 and O.sub.2, where the
volumetric content of O.sub.2 ranges from greater than 20% to about
35%. The plasma density typically ranges from about
8.0.times.10.sup.10 e.sup.-/cm.sup.3 (800 W applied source power
and a process chamber pressure of 4-6 mTorr) to about
1.6.times.10.sup.11 e.sup.-/cm.sup.3 (1600 W applied source power
and a process chamber pressure of 4-6 mTorr). The process chamber
pressure may range from about 2 mTorr to about 10 mTorr, and
preferably between about 2 mTorr and 6 mTorr. The substrate
temperature ranges from about 20.degree. C. to about 100.degree. C.
The applied substrate biasing power ranges from about 40 W to about
200 W. Using conditions within these ranges, adjusted for apparatus
variables, a "foot" which forms at the bottom of the fine line can
be removed, while maintaining the line profile at the vertical
88.degree. to 90.degree. profile.
[0036] Subsequently, a second overetch or finishing step is carried
out, to clean etch residue from the surface of an underlying thin
oxide film, without significant etching into the film (less than
10% of the thin oxide film thickness is etched). In the second
overetch step, the selectivity in favor of etching tungsten or
tungsten nitride relative to an underlying oxide film is critical.
It is desired to remove tungsten or tungsten nitride residue from
the flat surface of the thin oxide layer surrounding the etched
feature without etching through the thin oxide layer. Again, the
chemical etchants in the plasma source gas are Cl.sub.2 and
O.sub.2, where the volumetric percentage of O.sub.2 in the mixture
typically ranges from about 36% to about 41%. The plasma density
typically ranges from about 1.6.times.10.sup.11 e.sup.-/cm.sup.3 to
about 2.0.times.10.sup.11 e.sup.-/cm.sup.3, with the other process
conditions being the same as those specified above for the first
overetch step. A tungsten etch rate under these process conditions
is about 1750 .ANG. per minute, while the selectivity (W:SiO.sub.x)
is about 175:1. Increased plasma source gas O.sub.2 content and
increased plasma densities are expected to be useful as well
[0037] The selectivity of 175:1 obtained using the process
conditions just described may be compared with the selectivity
obtained using other process conditions which, at first glance, do
not appear to be significantly different, but which provide
surprisingly different results. For example, a change in the
O.sub.2 flow rate to provide a volumetric concentration of O.sub.2
of 36%, in combination with a plasma density of 1.6.times.10.sup.11
e.sup.-/cm.sup.3 (1600 W), provides a W:SiO.sub.x selectivity of
about 75:1; and a volumetric concentration of O.sub.2 of 20%, in
combination with a plasma density of 1.5.times.10.sup.11
e.sup.-/cm.sup.3 (1500 W), provides a W:SiO.sub.x selectivity of
about 28:1. As these data indicate, there is an unexpected increase
in selectivity in favor of etching tungsten or tungsten nitride
relative to silicon oxide which occurs as a result of increasing
the oxygen flow rate and the plasma density above particular ranges
simultaneously. This is further illustrated in the Examples
provided below
[0038] The oxide layer underlying the tungsten or tungsten nitride
layer may alternatively comprise tantalum pentoxide, zirconium
oxide, or silicon oxynitride, instead of silicon oxide. We have
discovered that, when the underlying oxide layer comprises tantalum
pentoxide, that a higher substrate bias power can be employed
during the tungsten overetch step. The use of a higher bias power
(about 100 W up to about 300 W) during the overetch step provides
improved profile control (i.e., a more vertical profile is
obtained), while maintaining a high selectivity (at least 75:1) for
etching tungsten relative to the underlying tantalum pentoxide.
Typically, a plasma source gas having a volumetric percentage of
O.sub.2 within the range of about 20% to about 50% is employed,
with a plasma density within the range of about 2.0
.times.10.sup.10 e.sup.-/cm.sup.3 to about 1.8.times.10.sup.11
e.sup.-/cm.sup.3. In the exemplary DPS system, this can be achieved
using a plasma source power within the range of about 200 W to
about 1800 W, a substrate bias power within the range of about 100
W to about 200 W, and a process chamber pressure within the range
of about 2 mTorr to about 10 mTorr. Tungsten etch rates of greater
than 1000 .ANG. per minute are typically obtained using this
process regime to etch tungsten over an underlying tantalum
pentoxide layer.
[0039] As a preface to the detailed description of the Examples, it
should be noted that, as used in this specification and the
appended claims, the singular forms "a", "an", and "the" include
plural referents, unless the context clearly dictates otherwise.
Thus, for example, the term "a semiconductor" includes a variety of
different materials which are known to have the behavioral
characteristics of a semiconductor.
[0040] I. AN APPARATUS FOR PRACTICING THE INVENTION
[0041] The embodiment etch processes described herein were carried
out in a CENTURA.RTM. Integrated Processing System, available from
Applied Materials, Inc., of Santa Clara, Calif. Although the etch
process chamber used in the EXAMPLES presented herein is shown in
schematic in FIG. 1, any of the etch processors available in the
industry should be able to take advantage of the etch chemistry
described herein, with some adjustment to other process parameters.
The equipment shown in schematic in FIG. 1 includes a Decoupled
Plasma Source (DPS) of the kind described by Yan Ye et al. at the
Proceedings of the Eleventh International Symposium of Plasma
Processing, May 7, 1996, and as published in the Electrochemical
Society Proceedings, Volume 96-12, pp. 222-233 (1996). The plasma
processing chamber enables the processing of an 8 inch (200 mm)
diameter wafer.
[0042] FIG. 1 shows a schematic of a side view of an individual
CENTURA.RTM. DPS.TM. polysilicon etch chamber 100. The etch chamber
100 consists of an upper chamber 104 having a ceramic dome 106, and
a lower chamber 108. The lower chamber 108 includes a monopolar
electrostatic chuck (ESC) cathode 110. Gas is introduced into the
chamber via gas injection nozzles 114 for uniform gas distribution.
Chamber pressure is controlled by a closed-loop pressure control
system (not shown) using a throttle valve 118. During processing, a
substrate 120 is introduced into the lower chamber 108 through
inlet 122. The substrate 120 is held in place by means of a static
charge generated on the surface of electrostatic chuck (ESC)
cathode 110 by applying a DC voltage to a conductive layer (not
shown) located under a dielectric film (not shown) on the chuck
surface. The cathode 110 and substrate 120 are then raised by means
of a wafer lift 124 and sealed against the upper chamber 104 in
position for processing. Etch gases are introduced into the upper
chamber 104 via gas injection nozzles 114. The etch chamber 100
uses an inductively coupled plasma source power 126 and matching
network 128 operating at 12.56 MHz for generating and sustaining a
medium to high density plasma. The wafer is biased with an RF
source 130 and matching network 132 operating at 13.56 MHz. Plasma
source power 126 and substrate biasing means 130 are controlled by
separate controllers (not shown).
[0043] II. EXAMPLES OF EMBODIMENTS OF THE INVENTION
[0044] FIG. 2A shows a cross-sectional side view schematic of an
etched tungsten structure 200, where the etched pattern is lines
203 and spaces 205. The line width is approximately 0.165 .mu.m,
and the space width is approximately 0.21 .mu.m. The structure
includes a thin (.apprxeq.45 .ANG.) silicon oxide layer 213 on a
silicon substrate 202, overlying silicon oxide layer 213 is a 1650
.ANG. thick layer of tungsten 204, a 400 .ANG. thick image-focusing
antireflective coating (ARC) layer 206, and the residue 208 of a
photoresist layer which was used to pattern the etched structure
200. The tungsten layer 204 includes a thin (.about.100 .ANG.)
wetting layer of tungsten nitride (not shown) which aids in
adhesion of the tungsten to the silicon oxide layer 213. The
tungsten nitride is etched during the same etch process as the
tungsten. Therefore, as used herein, the term "tungsten layer"
refers to a layer of tungsten, optionally in combination with a
thin underlying layer of tungsten nitride.
[0045] FIG. 2B shows a more three-dimensional view of the same
etched structure 200, showing the surface finish 222 of the etched
tungsten in the bottom of a trench (space 205), and the smoother
upper surface 224 of the overlying photoresist layer residue 208.
Both Figures show a remaining unetched tungsten thickness 214
overlying upper surface 210 of silicon oxide layer 213. The
unetched tungsten thickness 214 is about 0.023 .mu.m.
[0046] The tungsten 204 etch was carried out using only a single
etch chemistry, in which the etchant species in the plasma were
SF.sub.6 at 30 sccm and N.sub.2 at 50 sccm. (There was no second
etch chemistry used to etch the final portion of the tungsten layer
204 to the surface 210 of oxide layer 213, i.e., there was no
overetch carried out.) Other etch process conditions were as
follows: The etch process chamber pressure was about 4 mTorr; the
substrate temperature was about 50.degree. C.; the plasma source
power was about 500 W; the substrate bias power was about 80 W. The
etch profile obtained was good, with the sidewall angle of the
etched lines 203 from silicon oxide layer 213 surface 210 being
about 89.degree.; however, there was some tapering of the profile
at the base of the line, since the tungsten was not etched all the
way to the surface 210 of silicon oxide layer 203. The tungsten
etch rate was about 1500 .ANG. per minute. The term "tapered"
profile, with reference to an etched pattern of lines and spaces,
refers to a cross-sectional profile where the width of the line is
wider at the base of the line than at the top surface of the line.
A "vertical profile" is one where the side walls of the lines are
perpendicular to the surface of the silicon oxide substrate. An
"undercut" profile is one where the width of the line is more
narrow at the base of the line than at the top surface of the line.
A vertical profile is typically preferred, because it enables
closer placement of device structures on a given surface area.
[0047] As shown in FIG. 2B, the etched tungsten surface 222 at the
bottom of spaces 205 exhibited a rougher finish than the line 203
upper photoresist residue 208 surface 224.
[0048] FIG. 3 shows a schematic of a photomicrograph of etched
tungsten 300 where there is a change in the etch chemistry toward
the end of the tungsten 304 etching, and etching is permitted to
continue to the surface 310 of the thin silicon oxide layer 313.
Again, the etched tungsten structure was a pattern of lines 303 and
spaces 305. The line width is approximately 0.120 .mu.m, and the
space width is approximately 0.165 .mu.m. The structure included a
thin (.apprxeq.45 .ANG.) silicon oxide layer 313 on a silicon
substrate 302, a 1650 .ANG. thick overlying layer of tungsten 304
(i.e., tungsten/tungsten nitride, as described above with reference
to FIG. 2), a 400 .ANG. thick image-focusing antireflective coating
(ARC) layer 306, and a photoresist layer residue 308.
[0049] The tungsten 304 etch was carried out using two etch
chemistries. During the first portion of the etching, the
chemically reactive etchant species in the plasma were SF.sub.6 at
30 sccm and N.sub.2 at 50 sccm (i.e., 37.5 volume % SF.sub.6, 62.5
volume % N.sub.2). Other etch process conditions were as follows:
The etch process chamber pressure was about 4 mTorr; the substrate
temperature was about 50.degree. C.; the plasma source power was
500 W; the substrate bias power was about 80 W. The etch profile
obtained was good, with the sidewall angle of the etched lines 303
from silicon oxide substrate 302 surface 310 being about
89.degree.. The tungsten etch rate was about 1650 .ANG. per minute.
Approximately 1500 .ANG. of the initial 1650 A thickness of the
tungsten layer 304 was etched using this first etch chemistry.
[0050] Etching of the remaining 150 .ANG. of tungsten layer 304 was
carried out using a plasma in which the chemically reactive etchant
species were produced from a source gas of Cl.sub.2 at a flow rate
of about 70 sccm and O.sub.2 at a flow rate of about 40 sccm. Other
etch process conditions were as follows: The etch process chamber
pressure was about 6 mTorr; the substrate temperature was about
50.degree. C.; the plasma source power was 600 W; the substrate
bias power was about 80 W. The etch profile obtained was excellent,
with the sidewall angle of the etched lines 303 from silicon oxide
substrate 302 surface 310 being about 89.degree. to 90.degree.. The
tungsten etch rate was about 1500 .ANG. per minute. The etching was
allowed to continue until the upper surface of the silicon oxide
substrate 310 was slightly etched. It was determined that the
silicon oxide etch rate was about 19 .ANG. per minute. The etch
rate selectivity of tungsten: silicon oxide was about 79:1.
Tungsten residue was cleared off the space 305 open areas, but
there was a slight tungsten "foot" (not shown) at the bottom of
tungsten lines 303 in the areas where the spacing between lines was
less than that shown in FIG. 3 (in the more dense areas).
Subsequent experimentation demonstrated that adjustment of the
O.sub.2 flow rate to 45 sccm, the plasma source power to 1800 W,
and the process chamber pressure to 3 mTorr results in removal of
the "foot".
[0051] FIG. 4 is a graph 400 showing the etch rate (shown on axis
404) of silicon oxide as a function of the plasma source gas oxygen
flow rate (shown on axis 402). The data shown in graph 400 were
obtained by etching a layer of silicon oxide only, which was
deposited on a silicon wafer. The total flow of Cl.sub.2 and
O.sub.2 used was 110 sccm, and the volume % of oxygen in the plasma
source gas may be calculated by dividing the sccm shown on axis 402
by the total gas flow 100 sccm and multiplying by 100. The other
etch process conditions used during the silicon oxide etching were
as follows: The etch process chamber pressure was about 6 mTorr;
the substrate temperature was about 50 .degree. C.; the plasma
source power was about 1600 W; the substrate bias power was about
80 W. Graph 400 indicates that there is a diminishing effect
obtained by increasing the O.sub.2 flow rate, with the etch rate
leveling out at about 19 .ANG. per minute between about 35 sccm and
40 sccm of O.sub.2. We later discovered that it was necessary to
increase the plasma density (plasma source power) to obtain the
full benefit of an increase in the O.sub.2 flow rate over about 35
sccm.
[0052] FIG. 5A is a graph 500 showing the effect of increasing the
plasma source power (shown in Watts on axis 502), on the etch rate
of tungsten (W) (shown by curve 508) and on the etch rate of
SiO.sub.x (shown by curve 510), at a constant O.sub.2 flow rate of
about 20 sccm. The etch rate units in each case are shown on axis
504. Graph 500 also shows the selectivity for W:SiO.sub.x as a
function of the plasma source power, illustrated by curve 512, at
the constant O.sub.2 flow rate of 20 sccm. The nominal selectivity
is shown on axis 506. As can be observed from graph 500, an
increase in plasma source power results in an increase in tungsten
etch rate, a decrease in SiO.sub.x etch rate, and an increase in
selectivity. However, at an O.sub.2 flow rate of 20 sccm, up to a
plasma source power of about 1600 W, the selectivity of W:SiO.sub.x
is only about 40:1. The process chamber pressure was about 6 mTorr,
and the substrate temperature was about 50 .degree. C.
[0053] FIG. 5B is a three-dimensional graph 530 showing tungsten
etch rate in an overetch step, as a function of plasma source power
and oxygen flow rate, all other variables held constant. In
particular, the wafer etched was a sputtered tungsten layer
overlying a silicon wafer. The process chamber pressure was 6
mTorr, and the substrate temperature was about 50.degree. C. The
tungsten etch rate units (.ANG./min.) are shown on axis 534, the
plasma source power units (W) are shown on axis 532, and the
O.sub.2 flow rate units (sccm) are shown on axis 536. Curve 538
clearly shows that an increase in O.sub.2 flow rate alone from
about 30 sccm to about 40 sccm, with plasma source power held
constant at about 1,600 W, increased the tungsten etch rate from
about 1170 .ANG./min. to about 1500 .ANG./min. A simultaneous
increase in O.sub.2 flow rate from about 30 sccm to about 45 sccm,
and increase in plasma source power from about 1600 W to about 1800
W, increased the tungsten etch rate from about 1170 .ANG./mm, to
about 1750 .ANG./min.
[0054] FIG. 5C is a three-dimensional graph 540 showing silicon
oxide etch rate in an overetch step, as a function of plasma source
power and oxygen flow rate, all other variables held constant. In
particular, the wafer etched was a silicon substrate having a layer
of thermal silicon oxide on its surface. The process chamber
pressure was 6 mTorr, and the substrate temperature was about
50.degree. C. The silicon oxide etch rate units (.ANG./min.) are
shown on axis 544, the plasma source power units (W) are shown on
axis 542, and the O.sub.2 flow rate units (sccm) are shown on axis
546. Curve 538 shows that an increase in O.sub.2 flow rate alone
from about 20 sccm to about 40 sccm, with plasma source power held
constant at about 1600 W, decreased the silicon oxide etch rate
from about 37 .ANG./min. to about 19 .ANG./min. A simultaneous
increase in O.sub.2 flow rate from about 20 sccm to about 45 sccm,
and increase in plasma source power from about 1600 W to about 1800
W, decreased the silicon oxide etch rate from about 37 .ANG./min.
to about 10 .ANG./min.
[0055] FIG. 5D is a three-dimensional graph 550 showing selectivity
(etch rate ratio of W:SiO.sub.x) as a function of plasma source
power and oxygen flow rate, all other variables held constant. FIG.
5D is derived from FIGS. 5B and 5C, and makes apparent the striking
increase in selectivity which can be achieved by increasing both
the oxygen flow rate and the plasma source power simultaneously.
The selectivity (W:SiO.sub.x) nominal units are shown on axis 554,
the plasma source power units (W) are shown on axis 552, and the
O.sub.2 flow rate units (sccm) are shown on axis 556. Curve 558
shows that an increase in O.sub.2 flow rate alone from about 30
sccm to about 40 sccm, with plasma source power held constant at
about 1600 W, increased the selectivity from about 50:1 to about
80:1. A simultaneous increase in oxygen flow rate from about 30
sccm to about 45 sccm, and increase in plasma source power from
about 1600 W to about 1800 W, increased the selectivity from about
50:1 to about 175:1. For example, a selectivity of about 100:1 is
obtained when the O.sub.2 content is about 40%, and the plasma
source power is about 1670 W. A selectivity of about 175:1 is
obtained when the O.sub.2 content is about 45% and the plasma
source power is about 1800 W. An increase in the selectivity to
175:1 was unexpected in view of the much smaller increase in
selectivity obtained when only the oxygen flow rate was
increased.
[0056] To better appreciate this surprising increase in
selectivity, it is helpful to compare FIG. 5D with FIG. 5A. FIG. 5A
showed that when the oxygen flow rate was held constant at about 20
sccm, and the plasma source power was increased from about 800 W up
to about 1600 W, the selectivity of W: SiO.sub.x increased from
about 19:1 to about 40:1. Further, as illustrated in FIG. 5D, an
increase in oxygen flow rate from about 30 sccm to about 40 sccm,
with the plasma source power held constant at 1600 W, provided a
selectivity increase from about 50:1 to about 80:1. It was only
with the synergistic combination of increased oxygen flow rate and
increased plasma source power that a selectivity of 175:1 was
achieved.
[0057] Table I, below, provides a summary of various process
conditions and the tungsten (or tungsten nitride) etch rate which
is expected to be obtained at those process conditions. Table I
also shows the selectivity relative to silicon oxide which is
expected to be obtained, and the etched sidewall profile angle
which is expected to be obtained.
1TABLE I Typical Process Conditions During Final Portion Etching
(Overetch) of Tungsten or Tungsten Nitride, With Underlying Silicon
Oxide Layer Process Condition Process Condition Process Condition
Broad Range Preferred Range and Result and Results and Results
Total Gas Flow 50-200 75-125 (sccm) O.sub.2 Flow Rate >20-60
30-50 (sccm) Vol. % O.sub.2 in mixt. of >20-50 30-40 O.sub.2 and
Cl.sub.2 Substrate 20-100 40-60 Temperature (.degree. C.) Process
Chamber 2-20 3-6 Pressure (mTorr) Source Power (W) 800-3000
1400-1800 Substrate Bias Power (W) 40-200 80-100 Etch Rate W or
WN.sub.2 1000-3000 1500-2000 (.ANG./min) Selectivity 10:1 to 200:1
100:1 to 175:1 W:SiO.sub.x Etched Profile, Vertical or
88-90.degree. vertical profile Tapered or Undercut (.degree.)
89-90.degree.
[0058] The dielectric layer underlying the tungsten or tungsten
nitride layer may alternatively comprise tantalum pentoxide,
zirconium oxide, or silicon oxynitride, instead of silicon oxide.
FIG. 6 illustrates an embodiment of the method of the invention for
etching tungsten, when the underlying dielectric layer is tantalum
pentoxide. FIG. 6A shows a beginning semiconductor structure 600
prior to performance of the method of the invention. The structure
600 includes a thin (50 .ANG.) tantalum pentoxide layer 604 on a
silicon substrate 602, overlying tantalum pentoxide layer 604 is a
300 .ANG. thick wetting layer of tungsten nitride 606, a 700 .ANG.
thick layer of tungsten 608, a 300 .ANG. thick image-focusing
antireflective coating (ARC) layer 610 (in this case, silicon
oxynitride), and a 1500 .ANG. thick silicon nitride hard mask layer
612, which is used to pattern etch the structure 600. The silicon
nitride hard mask layer 612 and silicon oxynitride ARC layer 610
have been previously patterned using techniques known in the
art.
[0059] Etching of the tungsten 608/tungsten nitride 606 was carried
out using two etch chemistries. During the first portion of the
etching, the chemically reactive etchant species in the plasma were
SF.sub.6 at 20 sccm and N.sub.2 at 45 sccm (i.e., 31 volume %
SF.sub.6, 69 volume % N.sub.2). Other etch process conditions were
as follows: The etch process chamber pressure was about 4 mTorr;
the substrate temperature was about 50.degree. C.; the plasma
source power was 200 W; the substrate bias power was about 175 W.
The tungsten/tungsten nitride etch rate was about 2000 .ANG. per
minute. As shown in FIG. 6B, almost all (at least 950 .ANG.) of the
initial 1000 .ANG. combined thickness of the tungsten 608/tungsten
nitride 606 was etched using this first etch chemistry, with a
small amount of tungsten/tungsten nitride residue remaining at the
bottom of the feature.
[0060] Etching of the remaining tungsten 608/tungsten nitride 606
was carried out using a plasma in which the chemically reactive
etchant species were produced from a source gas of Cl.sub.2 at a
flow rate of about 72 sccm and O.sub.2 at a flow rate of about 18
sccm. Other etch process conditions were as follows: The etch
process chamber pressure was about 4 mTorr; the substrate
temperature was about 50.degree. C.; the plasma source power was
400 W; the substrate bias power was about 175 W. The
tungsten/tungsten nitride etch rate was about 1871 .ANG. per
minute. The etching was allowed to continue until the upper surface
of the tantalum pentoxide layer 604 was slightly etched, as shown
in FIG. 6C. It was determined that the tantalum pentoxide etch rate
was about 20 .ANG. per minute. The etch rate selectivity of
tungsten:tantalum pentoxide was about 94:1. The etch profile
obtained was about 89.degree. to 90.degree..
[0061] When the underlying oxide layer comprises tantalum
pentoxide, we have discovered that we can use the same source gas
composition and process conditions during the tungsten overetch
step as those which were used when silicon oxide was used as the
underlying layer. Since the tantalum pentoxide is resistant to ion
bombardment which occurs during anisotropic etching, we can achieve
an almost infinite selectivity for etching tungsten relative to an
underlying tantalum pentoxide layer. This extraordinarily high
tungsten : tantalum pentoxide etch selectivity allows a thinner
tantalum pentoxide layer to be used (relative to a corresponding
silicon oxide layer). The tantalum pentoxide layer typically has
thickness of less than about 100 .ANG., preferably within the range
of about 30 .ANG. to about 50 .ANG.. The thinness of the oxide
layer can be important in terms of increasing a semiconductor
device speed.
[0062] Our discovery that tantalum pentoxide is more resistant to
ion bombardment than silicon oxide permits a higher substrate bias
power to be used during the tungsten overetch step over an
underlying tantalum pentoxide layer. The use of a high bias power
(from about 100 W up to about 300 W) during the overetch step
provides improved profile control (i.e., a more vertical profile is
obtained), while maintaining a high selectivity for etching
tungsten relative to the underlying tantalum pentoxide. When a
higher bias power is employed, a plasma source gas having a lower
oxygen content can be used.
[0063] When the underlying oxide layer comprises tantalum
pentoxide, a tungsten etch rate of about 2000 .ANG. per minute can
be achieved while maintaining a tungsten:tantalum pentoxide etch
selectivity of about 75:1 or better using a plasma source gas
having a volumetric percentage of O.sub.2 of about 20%, with a
plasma density within the range of about 4.0.times.10.sup.10
e.sup.-/cm.sup.3 to about 6.0.times.10.sup.10 e.sup.-/cm.sup.3,
which may be achieved in the exemplary DPS chamber using a plasma
source power within the range of about 400 W to about 600 W, a
substrate bias power of about 175 W, and a process chamber pressure
within the range of about 4 mTorr. To enable maintenance of feature
profile while removing residual tungsten "feet" at the base of a
feature, it may be advantageous to adjust the etch conditions
toward the end of the etch. For example, the final etch conditions
may provide a selectivity of at least 175:1 tungsten : tantalum
pentoxide, where the tungsten etch rate is less than about 1000
.ANG. per minute. This etch selectivity and etch rate can be
obtained by using a plasma source gas having a volumetric
percentage of O.sub.2 of about 20%, with a plasma density of about
2.0.times.10.sup.10 e.sup.-/cm.sup.3, which can be achieved in the
exemplary DPS chamber using a plasma source power of about 200 W, a
substrate bias power of about 175 W, and a process chamber pressure
of about 6 mTorr.
[0064] Experimental results obtained by varying particular process
parameters (O.sub.2 flow rate, process chamber pressure, plasma
source power, substrate bias power) during tungsten overetching are
presented in Table II, below.
2TABLE II Tungsten Overetch with Underlying Tantalum Pentoxide
Layer, Development Data Run # 1 2 3 4 5 6 7 Cl.sub.2 Flow Rate 72
72 72 72 72 72 72 (sccm) O.sub.2 Flow Rate 18 18 18 22 26 18 18
(sccm) Vol. % O.sub.2 20 20 20 23.4 26.5 20 20 Process Chamber 4 4
6 4 4 4 4 Pressure (mTorr) Plasma Source 200 200 200 200 200 400
600 Power (W) Substrate Bias 175 100 175 175 175 175 175 Power (W)
Substrate 50 50 50 50 50 50 50 Temperature (.degree. C.) W Etch
Rate 634 344 1014 481 42 1871 2198 (.ANG./min.) Ta.sub.2O.sub.5
Etch Rate 22 12 5.7 5.3 4.7 20 28 (.ANG./min.) W:Ta.sub.2O.sub.5
Etch 29:1 29:1 178:1 90:1 9:1 94:1 79:1 Selectivity
[0065] Table III, below, provides a summary of various process
conditions and the tungsten (or tungsten nitride) overetch rate
which is expected to be obtained at those process conditions. Table
III also shows the selectivity for etching tungsten/tungsten
nitride relative to tantalum pentoxide which is expected to be
obtained, and the etched sidewall profile angle which is expected
to be obtained.
3TABLE III Typical Process Conditions During Final Portion Etching
(Overetch) of Tungsten or Tungsten Nitride, With Underlying
Tantalum Pentoxide Layer Process Condition Process Condition
Process Condition Broad Range Preferred Range and Result and
Results and Results Total Gas Flow 30-150 90-120 (sccm) O.sub.2
Flow Rate 15-50 20-50 (sccm) Vol % O.sub.2 in mixt. of 15-50 20-30
O.sub.2 and Cl.sub.2 Substrate 40-60 40-60 Temperature (.degree.
C.) Process Chamber 2-10 4-8 Pressure (mTorr) Source Power (W)
200-1800 400-1800 Substrate Bias Power (W) 60-300 60-200 Etch Rate
W or WN.sub.2 300-2200 1000-2000 (.ANG./min) Selectivity 10:1 to
1000:1 30:1 to 600:1 W:Ta.sub.2O.sub.5 Etched Profile, Vertical or
Vertical Vertical Tapered or Undercut (.degree.)
[0066] The above described embodiments are not intended to limit
the scope of the present invention, as one skilled in the art can,
in view of the present disclosure, expand such embodiments to
correspond with the subject matter of the invention claimed
below.
* * * * *