U.S. patent application number 10/836161 was filed with the patent office on 2004-10-14 for ceric-ion slurry for use in chemical-mechanical polishing.
Invention is credited to Cadien, Kenneth C., Feller, A. Daniel, Miller, Anne E..
Application Number | 20040203245 10/836161 |
Document ID | / |
Family ID | 23072364 |
Filed Date | 2004-10-14 |
United States Patent
Application |
20040203245 |
Kind Code |
A1 |
Miller, Anne E. ; et
al. |
October 14, 2004 |
Ceric-ion slurry for use in chemical-mechanical polishing
Abstract
The invention provides a chemical-mechanical polishing slurry
comprising a liquid, cerium ions as an oxidizer, an abrasive, and a
pH increasing substance. The cerium ions are in the liquid in a
quantity equal to the inclusion of at least 0.02 molar ammonium
cerium nitrate in the liquid. The abrasive is also included in the
liquid. The liquid, the cerium ions and the abrasive jointly have a
first pH value. The pH increasing substance increases the first pH
value to a second pH value above 1.5.
Inventors: |
Miller, Anne E.; (Portland,
OR) ; Feller, A. Daniel; (Portland, OR) ;
Cadien, Kenneth C.; (Portland, OR) |
Correspondence
Address: |
Blakely, Sokoloff, Taylor & Zafman LLP
7th Floor
12400 Wilshire Boulevard
Los Angeles
CA
90025
US
|
Family ID: |
23072364 |
Appl. No.: |
10/836161 |
Filed: |
April 29, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10836161 |
Apr 29, 2004 |
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09280268 |
Mar 29, 1999 |
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6752844 |
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Current U.S.
Class: |
438/691 ;
257/E21.304 |
Current CPC
Class: |
H01L 21/3212 20130101;
C09K 3/1463 20130101; C09G 1/02 20130101; C23F 3/00 20130101 |
Class at
Publication: |
438/691 |
International
Class: |
H01L 021/302; H01L
021/461 |
Claims
What is claimed:
1. A chemical-mechanical polishing slurry comprising: a liquid;
cerium ions as an oxidizer in the liquid, the cerium ions being in
a quantity equal to the inclusion of at least 0.02 molar ammonium
cerium nitrate in the liquid; an abrasive in the liquid, the
liquid, the cerium ions and the abrasive together having a first pH
value; and a pH increasing substance in the liquid that increases
the first pH value to a second pH value above 1.5.
2. The slurry of claim 1 comprising cerium ions in quantity equal
to the inclusion of between 0.05 molar and 0.1 molar of ammonium
cerium nitrate.
3. The slurry of claim 1 comprising between 1 percent and 30
percent abrasive by weight.
4. The slurry of claim 1 wherein the abrasive is silica.
5. The slurry of claim 1 wherein the second pH value is at least
between 2.5 and 4.
6. The slurry of claim 1 wherein the substance is glycine.
7. The slurry of claim 1 which is environmentally green.
8. The slurry of claim 1 further comprising a complexing agent.
9. The slurry of claim 8 wherein the complexing agent is
glycine.
10. The slurry of claim 1 further comprising an anti-oxidizing
agent.
11. The slurry of claim 10 wherein the anti-oxidant is BTA.
12. The slurry of claim 11 comprising between 0.00200 molar and
0.00500 molar BTA.
13. The slurry of claim 1 comprising cerium ions in quantity equal
to between 0.02 molar and 0.1 molar ammonium cerium nitrate and BTA
comprising between 0.00200 and 0.00500 molar BTA.
14. The slurry of claim 13 wherein the second pH value is at least
2.5.
15. A method of preparing a chemical-mechanical polishing slurry,
comprising: adding together an abrasive and a complex cerium double
salt as a source of cerium ions.
16. The method of claim 15, wherein the double salt is selected
from the group consisting of ammonium cerium nitrate, and ammonium
cerium sulfate.
17. The method of claim 15 wherein the double salt is ammonium
cerium nitrate.
18. The method of claim 15 wherein the abrasive and the source of
cerium ions, in solution, has a first pH value, the method further
comprising adding a substance which increases the first pH value to
a second pH value above 1.5.
19. The method of claim 18 wherein the substance is glycine.
20. The method of claim 18 further comprising adding a complexing
agent.
21. A method of forming a metal line, comprising: forming a first
layer, with an opening therein, over a semiconductor substrate;
depositing a metal layer which fills the opening and covers the
first layer; applying a chemical-mechanical polishing slurry onto
the metal layer, the slurry comprising cerium ions as an oxidizer,
and an abrasive; contacting a polishing surface against the metal
layer; and moving the polishing surface over the metal layer.
22. The method of claim 21 wherein the slurry comprises cerium ions
in quantity sufficient to oxidize a portion of the metal layer, and
the abrasive in quantity sufficient to assist in removal of the
oxidized portion when the polishing surface is moved over the metal
layer.
23. The method of claim 21 wherein the metal layer is of a metal
selected from the group consisting of copper and tungsten.
24. The method of claim 21 further comprising: depositing a barrier
layer over the first layer and before depositing the metal layer,
the cerium ions selectively oxidizing the material of the metal
layer over the material of the barrier layer.
25. The method of claim 21 wherein the metal layer is removed at a
rate of at least 1000 angstroms per minute.
15. A method of preparing a chemical-mechanical polishing slurry,
comprising: adding together an abrasive and a complex cerium double
salt as a source of cerium ions.
16. The method of claim 15, wherein the double salt is selected
from the group consisting of ammonium cerium nitrate, and ammonium
cerium sulfate.
17. The method of claim 15 wherein the double salt is ammonium
cerium nitrate.
18. The method of claim 15 wherein the abrasive and the source of
cerium ions, in solution, has a first pH value, the method further
comprising adding a substance which increases the first pH value to
a second pH value above 1.5.
19. The method of claim 18 wherein the substance is glycine.
20. The method of claim 18 further comprising adding a complexing
agent.
Description
BACKGROUND OF THE INVENTION
[0001] 1). Field of the Invention
[0002] The present invention relates to a slurry for use in
chemical-mechanical polishing of a metal film on a semiconductor
substrate.
[0003] 2). Discussion of Related Art
[0004] The manufacture of integrated circuits involves the
fabrication of multiple electronic devices such as transistors,
diodes and capacitors in and on a silicon or other semiconductor
wafer, and then interconnecting the devices with metal lines, plugs
and vias.
[0005] During the manufacture of an integrated circuit, a number of
layers of different materials are alternately deposited on one
another and then partially removed. For example, during the
formation of metal lines, a metal layer may be blanket deposited
over the entire wafer so that metal of the metal layer covers
higher areas on the wafer and fills trenches between the higher
areas. The metal layer is then partially removed so that the higher
areas are exposed and metal lines are left behind in the
trenches.
[0006] One technique for removal of layers on a wafer is known in
the art as "chemical-mechanical polishing". In a
chemical-mechanical polishing operation, a chemical-mechanical
polishing slurry is applied over the metal layer which serves both
a chemical and a mechanical function.
[0007] Chemically, a slurry of the above kind usually includes an
oxidizer which oxidizes the metal layer by removal of electrons
therefrom. An easily removable oxidized film is so formed by an
upper portion of the metal film.
[0008] Mechanically, a slurry of the above kind also includes an
abrasive such as silica (SiO.sub.2) or ceria (CeO.sub.2). The
purpose of the abrasive is to abrade the oxidized film when a
polishing pad is pressed against and moved over the film, and so
remove the film.
[0009] Once the oxidized film is removed, the metal is again
oxidized to form another oxidized film which is again removed
utilizing the abrasive. The process is continued until the metal
layer is removed to a required depth.
[0010] Slurries containing oxidizers may be unstable due to
breakdown of the oxidizer over time resulting in unstable polish
processes or production of toxic components. Replenishing and
stabilizing these oxidizers are an added expense to be avoided. In
addition, the spent slurry may have disposal issues and slurry
oxidant requiring no waste treatment is desirable.
[0011] Another problem associated with the use of common
chemical-mechanical polishing slurries is that they usually have pH
values which are very low. Slurries having pH values which are very
low are corrosive and may be the cause of damage to polishing
equipment used in a chemical-mechanical polishing operation.
[0012] A further problem with conventional chemical-mechanical
polishing slurries is that they cause "erosion", "dishing" and
"recess" during polishing. Erosion is the preferential loss of a
layer due to high metal pattern density, and dishing is the
topography developed in a wide metal line due to the metal being
preferentially polished in the middle of the line with respect to
the edge of the line. Recess, is the uniform loss of metal within a
structure due to chemical activity and is typically dominant in
narrow metal lines. Erosion, dishing and recess are undesirable as
a planar surface is usually required.
SUMMARY OF THE INVENTION
[0013] The invention provides a chemical-mechanical polishing
slurry comprising a liquid, cerium ions as an oxidizer, an
abrasive, and a pH increasing substance. The cerium ions are in the
liquid in a quantity equal to the inclusion of at least 0.02 molar
ammonium cerium nitrate in the liquid. The abrasive is also
included in the liquid. The liquid, the cerium ions and the
abrasive jointly have a first pH value. The pH increasing substance
increases the first pH value to a second pH value above 1.5.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The invention is further described by way of example with
reference to the accompanying drawings wherein:
[0015] FIG. 1a is a side view illustrating a first layer, a barrier
layer, and a metal layer which are formed on a substrate;
[0016] FIG. 1b is a view similar to FIG. 1a illustrating oxidation
of a portion of the metal layer by a slurry;
[0017] FIG. 1c is a view similar to FIG. 1b illustrating removal of
the oxidized portion with a polishing pad; and
[0018] FIG. 1d is a view similar to FIG. 1c illustrating how the
barrier layer also acts as a polish stop layer.
DETAILED DESCRIPTION OF THE INVENTION
[0019] A chemical-mechanical polishing slurry, a method of
preparing a chemical-mechanical polishing slurry, and a method of
forming a metal line are described. In the following description,
for purposes of explanation, numerous specific details are set
forth in order to provide a thorough understanding of the present
invention. It will be evident, however, to one skilled in the art,
that the present invention may be practiced without these specific
details.
[0020] The invention provides a chemical-mechanical polishing
slurry, comprising cerium ions as an oxidizer, and, in addition to
the oxidizer, an abrasive. The slurry is environmentally green in
the sense that it does not require waste treatment when depleted.
Because the slurry contains very little cyanide, expensive
recirculation or destruction processes for depleted cyanide
containing slurries are avoided.
[0021] The slurry has also been found to be selective to barrier
materials such as tantalum, tantalum nitride, titanium, or titanium
nitride, which makes such a material a good choice for a polish
stop.
[0022] Cerium ions may be provided by adding and mixing a complex
double salt such as ammonium cerium nitrate
[(NH.sub.4).sub.2Ce(NO.sub.3).sub.6] together with the abrasive in
deionized liquid water. The ammonium cerium nitrate dissolves in
the water to provide Ce.sup.+4 ions in solution in the water. The
slurry preferably includes cerium ions in quantity equal to the use
of 0.01 molar to 0.1 molar of ammonium cerium nitrate. Other
sources of cerium ions may alternatively be used, such as a simple
salt like cerium tetrasulfate [Ce(SO.sub.4).sub.2.2H.sub.2SO.sub.4]
or a double salt like ammonium cerium sulfate
[(NH.sub.4).sub.4Ce(SO.sub.4).su- b.4.2H.sub.2O].
[0023] The slurry may be applied over a copper or other metal layer
which is formed on a semiconductor substrate such as a silicon
wafer. Cerium ions of the slurry may oxidize (remove electrons
from) copper of the copper layer according to one of the following
formulas:
2Ce.sup.+4+Cu.fwdarw.Cu.sup.+2+2Ce.sup.+3
Ce.sup.+4+Cu.fwdarw.Cu.sup.+1+Ce.sup.+3
[0024] The copper ions (Cu.sup.+2 or Cu.sup.+1) may then react with
the water within the slurry to form a copper oxide film such as
Cu.sub.2O or CuO which, with the assistance of the abrasive is
easily removable from the remainder of the copper layer during
polishing.
[0025] The abrasive in the slurry may be silica (SiO.sub.2), ceria
(CeO.sub.2), alumina (Al.sub.2O.sub.3) or any other suitable
abrasive. The slurry may include between 1 percent and 30 percent
of the abrasive by weight, more preferably between 1 percent and 5
percent of the abrasive by weight.
[0026] In all the examples that follow, a slurry is applied to a
copper layer on an 8 inch diameter wafer at a rate of 200
milliliter per minute, the wafer and a polishing pad are rotated
relative to one another at 280 revolutions per minute, and a
pressure of about 4 pounds per square inch is applied between the
wafer and the polish pad.
EXAMPLE 1
[0027] In one example a slurry was used which, in addition to
deionize water, consisted of 0.05 molar ammonium cerium nitrate and
2.5 percent silicate by weight. The slurry has a pH value of 0.90.
A polish rate of 5612 angstroms per minute (.ANG./min) was
obtained. Polish uniformity across the wafer had a 1 .sigma.
variance of 8.3 percent.
[0028] In general, in the absence of a pH increasing substance, the
slurry would have a pH below 1.5 if at least 0.02 molar ammonium
cerium nitrate is used. To prevent damage to polishing equipment,
the slurry preferably has a pH value of at least 1.5 and more
preferably at least 2. The slurry may include a substance which
increases its pH value. An amino acid such as glycine
(NH.sub.2--CH.sub.2--COOH) may be used to increase the pH value of
the slurry. Glycine also acts as a copper complexor, i.e. it is a
complexing agent which "grabs" onto oxidized copper thereby
increasing copper ion solubility and so assists removal of the
oxidized copper. The complexing is proposed to be similar to
complexing by ammonia (NH.sub.3) that occurs at a high pH such as
described in the references of J. M. Steigerwald, et al, "Chemical
Mechanical Planarization of Microelectronic Materials" by John
Wiley & Sons, Inc., and of V. Brusic, et al, "Electrochemical
Approach to Au and Cu CMP Process Development" by the
Electrochemical Society Proceeding Vol 96-22 and 176-185. Other
substances which may be used to increase a pH value of the slurry
include potassium hydroxide (KOH) and ammonium hydroxide (NH.sub.4
OH).
EXAMPLE 2
[0029] In another example a slurry was used comprising 0.05 molar
ammonium cerium nitrate, 2.5 percent silica by weight, and 1
percent glycine by weight. The slurry has a pH value of 1.65. A
polish rate of 4770 .ANG./min was obtained. Polish uniformity had a
1 .sigma. variance of 8.3 percent.
EXAMPLE 3
[0030] In another example a slurry was used comprising 0.05 molar
ammonium cerium nitrate, 2.5 percent silica by weight, and 2
percent glycine by weight. The slurry has a pH value of 2.26. A
polish rate of 4784 .ANG./min was obtained. Polish uniformity had a
1 .sigma. variance of 6.6 percent.
EXAMPLE 4
[0031] In another example a slurry was used comprising 0.05 molar
ammonium cerium nitrate, 2.5 percent silica by weight, and 3
percent glycine by weight. The slurry has a pH value of 2.43. A
polish rate of 5653 .ANG./min was obtained. Polish uniformity had a
1 .sigma. variance of 4.3 percent.
EXAMPLE 5
[0032] In another example a slurry was used comprising 0.025 molar
ammonium cerium nitrate, 1.25 percent silica by weight, and 2
percent glycine by weight. The slurry has a pH value of 3.2. A
polish rate of 2794 .ANG./min was obtained. Polish uniformity had a
1 .sigma. variance of 5.0 percent.
[0033] It can be seen by comparing Example 5 with Examples 2 to 4
that, although a reduction in cerium ion concentration
substantially increases the pH value of the slurry, polish rate
also decreases substantially. It is therefore preferred that the
slurry comprises cerium ions equal to at least 0.05 molar ammonium
cerium nitrate, while still having a sufficient amount of glycine,
or another pH increasing substance, so that the slurry has a pH
value of at least 2.
[0034] When certain metal layers, such as nickel, chromium,
tungsten or aluminum metal layers, are oxidized, a thin oxide film
forms on the metal layer which passivates the remainder of the
metal layer, i.e. the oxide film prevents further oxidation of
deeper laying metal of the metal layer. The oxide film is easier to
remove than the remainder of the metal layer so that only the oxide
film is removed during polishing with the remainder of the metal
layer resisting removal and thus forming a "polish stop". Copper,
however, does not form a stable, non-porous passivating oxide
layer, i.e. copper continues to oxidize until an oxidizing agent to
which the copper is exposed is removed, and does therefore not form
a polish stop.
[0035] It is believed that the fact that copper does not form a
stable, passivating oxide, and a polish stop, is the cause of more
local polish non-uniformity, or more dishing, when a copper layer
is planarized in a chemical-mechanical polishing operation, than
would be the case when a layer of another metal, which does form a
passivating layer, is planarized.
[0036] An anti-oxidizing agent or corrosion inhibitor may be
included in the slurry to control, or at least reduce, oxidation of
a metal such as copper, with corresponding less variation in polish
uniformity or less dishing. One anti-oxidizing agent which may be
used is benzotriazole (BTA, chemical formula:
C.sub.6H.sub.5N.sub.2)
[0037] Details of how BTA acts as an anti-oxidizing agent are
described in the reference of R. Walker "Benzotriazole as a
Corrosion Inhibitor for Antiques", Journal of Chemical Education,
volume 57, 1980, pp. 789-791.
EXAMPLE 6
[0038] Table 1 includes test results for a slurry containing 2.5
percent silicate by weight, 3 percent glycine by weight, 0.05 molar
ammonium cerium nitrate, and BTA in varying quantities.
1TABLE 1 Test results for varying amounts of BTA BTA Polish Rate 1
.sigma. variance in Polish (molar) pH (.ANG./min) Uniformity
(percent) 0.00100 2.43 5653 8.3 0.00200 2.78 5884 3.3 0.00268 2.81
5790 2.7 0.00300 2.82 6158 3.4 0.00400 2.88 6594 4.0 0.00500 2.82
5801 10.5 0.00514 2.78 347 6.8 0.00600 2.88 86 150
[0039] It can be seen from Table 1 that 1 .sigma. variance in
polish uniformity is between 2.7 percent and 4.0 percent for BTA
concentrations of between 0.00200 molar and 0.00400 molar. The 1
.sigma. variance in polish uniformity increases significantly for
BTA concentrations above 0.00400 molar. Polish rate also decreases
significantly for BTA concentrations above 0.00400 molar. The
slurry therefore preferably includes between 0.00200 molar and
0.00500 molar BTA in order to keep polish rate to at least 1000
.ANG./min, and more preferably includes between 0.00200 molar and
0.00500 molar BTA in order to obtain acceptable polish
uniformity.
[0040] Although the aforegoing description is primarily directed
towards a slurry and a method of preparing a slurry, it should be
understood, from the aforegoing description, that the invention
also extends to a method of forming metal lines. The method of
forming metal lines is now further described with reference to
FIGS. 1a to 1d.
[0041] FIG. 1a shows a structure which is formed over a
semiconductor substrate. A first layer 110, typically an interlayer
dielectric layer, is formed over the substrate. An opening 112 is
formed in the first layer 110. A barrier layer 114 is deposited
onto the first layer 110 and on side walls and within a base of the
opening 112. The barrier layer 110 is typically of tantalum,
tantalum nitride, titanium/titanium nitride and prevents
outdiffusing of metal from a metal layer which is eventually formed
in the opening 112.
[0042] A metal layer 116 is then deposited over the barrier layer
114. The metal layer 116 fills the opening 112 completely and
covers the barrier layer 114. The metal layer 116 may be deposited
in a conventional plating or vapor deposition process. The metal
layer is typically made of copper but may be made of another metal
such as tungsten.
[0043] As shown in FIG. 1b, a slurry 118, of the aforedescribed
kind, is then applied over the metal layer 116. The slurry
comprises cerium ions in quantity sufficient to oxidize a portion
120 of the metal layer 116.
[0044] When, as shown in FIG. 1C, a polishing surface of a
polishing pad 122 is contacted against and moved over the metal
layer, the oxidized portion of the metal layer 116 is removed. The
slurry comprises abrasive in quality sufficient to assist in
removal of the oxidized portion 120.
[0045] The process as shown in FIGS. 1a to 1c is repeated until, as
shown in FIG. 1d, the barrier layer 114 is exposed and a metal line
122 remains within the opening 112. The cerium ions in the slurry
are selective in that only the material of the metal layer 116 is
oxidized, and not the material of the barrier layer 114. The
barrier layer 114 is thus used as a polish stop layer which
prevents further removal of material.
[0046] Thus, a chemical-mechanical polishing slurry, a method of
preparing a chemical-mechanical polishing slurry, and a method of
forming a metal line are described. While certain exemplary
examples and embodiments have been described, it is to be
understood that such examples and embodiments are merely
illustrative and not restrictive of the current invention, and that
this invention is not restricted to the specific examples of
embodiments described, since modifications may occur to those
ordinarily skilled in the art.
* * * * *