U.S. patent application number 10/940582 was filed with the patent office on 2005-03-17 for polishing pad for chemical mechanical polishing.
This patent application is currently assigned to PsiloQuest. Invention is credited to Obeng, Yaw S..
Application Number | 20050055885 10/940582 |
Document ID | / |
Family ID | 34375319 |
Filed Date | 2005-03-17 |
United States Patent
Application |
20050055885 |
Kind Code |
A1 |
Obeng, Yaw S. |
March 17, 2005 |
Polishing pad for chemical mechanical polishing
Abstract
The present invention provides in one embodiment, a polishing
pad 100 for chemical mechanical polishing. The polishing pad
comprises a polishing body 110. The polishing body comprises a
thermoplastic foam substrate 115 having a surface 120 comprising
concave cells 125. A polishing agent 130 coats an interior surface
135 of the concave cells. The polishing agent comprises an
inorganic metal oxide that includes carbides or nitrides. Yet
another embodiment of the present invention is a method for
preparing a polishing pad 200.
Inventors: |
Obeng, Yaw S.; (Orlando,
FL) |
Correspondence
Address: |
HITT GAINES P.C.
P.O. BOX 832570
RICHARDSON
TX
75083
US
|
Assignee: |
PsiloQuest
Orlando
FL
|
Family ID: |
34375319 |
Appl. No.: |
10/940582 |
Filed: |
September 14, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60503152 |
Sep 15, 2003 |
|
|
|
Current U.S.
Class: |
51/293 ; 427/569;
51/298; 51/307 |
Current CPC
Class: |
B24D 3/348 20130101;
B24D 3/26 20130101; B24D 13/147 20130101 |
Class at
Publication: |
051/293 ;
051/307; 051/298; 427/569 |
International
Class: |
B24D 003/00 |
Claims
What is claimed is:
1. A polishing pad for chemical mechanical polishing comprising: a
polishing body comprising a thermoplastic foam substrate having a
surface comprising concave cells; and a polishing agent coating an
interior surface of said concave cells, wherein said polishing
agent comprises an inorganic metal oxide that includes carbides or
nitrides.
2. The polishing pad as recited in claim 1, wherein said carbides
or nitrides are incorporated into a lattice of said inorganic metal
oxides.
3. The polishing pad as recited in claim 2, wherein said nitrides
comprise silicon nitrides or titanium nitrides.
4. The polishing pad as recited in claim 2, wherein said carbides
include silicon carbides or titanium carbides.
5. The polishing pad as recited in claim 4, wherein said lattice of
said inorganic metal oxides comprise silicon oxide or titanium
oxide.
6. The polishing pad as recited in claim 1, wherein said inorganic
metal oxide comprise silicon carbides and silicon nitrides.
7. The polishing pad as recited in claim 1, wherein said nitrides
comprise about 10 mol percent of said polishing agent.
8. The polishing pad as recited in claim 1, wherein said carbides
comprise about 10 mol percent of said polishing agent.
9. The polishing pad as recited in claim 1, wherein said polishing
agent has a oxygen to silicon ratio of at least about 8:1.
10. The polishing pad as recited in claim 1, wherein said polishing
pad has a hardness of about 60 KPa or greater.
11. The polishing pad as recited in claim 1, wherein said polishing
pad has an elastic modulus of greater than about 3 MPa.
12. The polishing pad as recited in claim 1, wherein said polishing
pad has an loss modulus of about 0.4 MPa or greater.
13. A method for preparing a polishing pad for chemical mechanical
polishing, comprising: exposing closed cells within a thermoplastic
foam substrate to provide a substrate surface comprising concave
cells; and coating an interior surface of said concave cells with a
polishing agent comprising an inorganic metal oxide, wherein
carbides and nitrides are incorporated into said inorganic metal
oxide during said coating.
14. The method as recited in claim 13, wherein coating comprises:
exposing said substrate surface to an initial plasma reactant in a
plasma enhanced chemical vapor deposition (PECVD) process to
produce a modified surface thereon; and exposing said modified
surface to a secondary plasma reactant in said PECVD process to
form said polishing agent.
15. The method as recited in claim 14, wherein said initial plasma
reactant comprises argon or helium, and exposure to said initial
plasma proceeds for about 30 seconds.
16. The method as recited in claim 14, wherein exposure to a
secondary plasma reactant proceeds for at least about 30
minutes.
17. The method as recited in claim 14, wherein exposure to a
secondary plasma reactant proceeds for between about 30 minutes and
about 60 minutes.
18. The method as recited in claim 14, wherein said secondary
plasma reactant comprises tetraethoxy silane or titanium
alkoxide.
19. The method as recited in claim 14, wherein an interior of said
closed cells comprise nitrogen gas and said nitrogen gas reacts
with said secondary plasma reactant to form said nitride.
20. The method as recited in claim 14, wherein said thermoplastic
foam substrate reacts with said secondary plasma reactant to form
said carbide.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This Application claims the benefit of U.S. Provisional
Application Ser. No. 60/503,152 filed on Sep. 15, 2003, entitled
"CORROSION RETARDING POLISHING SLURRY FOR THE CHEMICAL MECHANICAL
POLISHING OF COPPER SURFACES," commonly assigned with the present
invention and incorporated herein by reference which is a
continuation-in-part of U.S. application Ser. No. 10/241,074,
entitled, "A POLISHING PAD SUPPORT THAT IMPROVES POLISHING
PERFORMANCE AND LONGEVITY," to Yaw S. Obeng and Peter Thomas, filed
on Sep. 11, 2002, which in turn, is a continuation in part of U.S.
Pat. No. 6,579,604 entitled, "A METHOD OF ALTERING AND PRESERVING
THE SURFACE PROPERTIES OF A POLISHING PAD AND SPECIFIC APPLICATIONS
THEREFOR," to Yaw S. Obeng and Edward M. Yokley, filed on Nov. 27,
2001, and a continuation-in-part of and of U.S. patent application
Ser. No. 10/241,985 entitled, MEASURING THE SURFACE PROPERTIES OF
POLISHING PADS USING ULTRASONIC REFLECTANCE, to Yaw S. Obeng, filed
on Sep. 12, 2002 and incorporated by reference as if reproduced
herein in their entirety.
TECHNICAL FIELD OF THE INVENTION
[0002] The present invention is directed to chemical mechanical
polishing for creating a smooth, ultra-flat surface on such items
as glass, semiconductors, dielectrics, metals and composites
thereof, magnetic mass storage media and integrated circuits.
BACKGROUND OF THE INVENTION
[0003] Chemical mechanical polishing (CMP) has been successfully
used for planarizing both metal and dielectric films. In one
plausible mechanism of planarizing, the polishing process is
thought to involve intimate contact between high points on the
wafer surface and the pad material, in the presence of slurry. In
this scenario, corroded materials, produced from reactions between
the slurry and wafer surface being polished, are removed by
shearing at the pad-wafer interface. The elastic properties of pad
material significantly influence the final planarity and polishing
rate. In turn, the elastic properties are a function of both the
intrinsic polymer and its foamed structure.
[0004] Historically, polyurethane-based pads have been used for CMP
because of their high strength, hardness, modulus and high
elongation at break. While such pads can achieve both good
uniformity and efficient topography reduction, their ability to
rapidly and uniformly remove surface materials drops off rapidly as
a function of use. The drop off in material removal rates as a
function of time observed for polyurethane-based pads has been
attributed to changes in the mechanical response of such polishing
pads under conditions of critical shear. It is generally believed
that the loss in functionality of polyurethane-based CMP pads is
due to pad decomposition from the interaction between the pad and
the slurries used in the polishing.
[0005] Moreover, decomposition produces a surface modification in
and of itself in the case of the polyurethane pads which can be
detrimental to uniform polishing. Alternatively, in some instances,
the surface modification of materials used for CMP polishing pads
may improve the application performance. Such modifications,
however may be temporary, thus requiring frequency replacement or
retreatment of the CMP pad. Polyurethane pads also generally
require a break-in period before polishing, in addition to the
reconditioning and retreatment after a period of use. It is often
also necessary to keep traditional pads wet in while polishing
equipment is in idle mode. These characteristics undesirably reduce
the overall efficiency of CMP when using polyurethane or similar
conventional pads.
[0006] Accordingly, what is needed is an improved CMP pad capable
of providing a highly planar surface during CMP and having improved
longevity, while not experiencing the above-mentioned problems.
SUMMARY OF THE INVENTION
[0007] To address the above-discussed deficiencies of the prior
art, the present invention provides in one embodiment, a polishing
pad for chemical mechanical polishing. The polishing pad comprises
a polishing body comprising a thermoplastic foam substrate. The
thermoplastic foam substrate has a surface comprising concave
cells. A polishing agent coating an interior surface of the concave
cells comprises an inorganic metal oxide that includes carbides or
nitrides.
[0008] Another embodiment of the present invention is directed to a
method for preparing a polishing pad. The method comprises exposing
closed cells within a thermoplastic foam substrate to provide a
substrate surface comprising concave cells. The method further
includes coating an interior surface of the concave cells with a
polishing agent comprising an inorganic metal oxide, wherein
carbides and nitrides are incorporated into the inorganic metal
oxide during the coating.
[0009] The foregoing has outlined preferred and alternative
features of the present invention so that those skilled in the art
may better understand the detailed description of the invention
that follows. Additional features of the invention will be
described hereinafter that form the subject of the claims of the
invention. Those skilled in the art should appreciate that they can
readily use the disclosed conception and specific embodiment as a
basis for designing or modifying other structures for carrying out
the same purposes of the present invention. Those skilled in the
art should also realize that such equivalent constructions do not
depart from the scope of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] For a more complete understanding of the present invention,
reference is now made to the following descriptions taken in
conjunction with the accompanying drawings, in which:
[0011] FIG. 1 illustrates a cross sectional view of a polishing pad
of the present invention;
[0012] FIGS. 2-4 illustrate cross sectional views of selected step
in a method of the present invention for preparing a polishing
pad;
[0013] FIG. 5 presents representative near infrared spectra of
samples of thermoplastic foam polishing pads after variable periods
of coating with a polishing agent precursor comprising tetraethoxy
silane (TEOS);
[0014] FIG. 6 illustrates changes in the near infrared signal for
representative thermoplastic foam polishing pads exposed to
different coating periods with TEOS;
[0015] FIG. 7 illustrates exemplary indentation curves for a
thermoplastic foam polishing pad after being coated with TEOS;
[0016] FIG. 8 illustrates the representative change in (Xa),
`pop-in` for thermoplastic foam polishing pads as a function of
coating time with TEOS;
[0017] FIG. 9 illustrates the representative change in hardness for
thermoplastic foam polishing pads as a function of coating time
with TEOS;
[0018] FIG. 10 illustrates the representative change in elastic
modulus for thermoplastic foam polishing pads as a function of
coating time with TEOS;
[0019] FIG. 11 illustrates the representative change in storage and
loss modulus for thermoplastic foam polishing pads as a function of
coating time with TEOS;
[0020] FIG. 12 presents representative XPS spectra of polishing
pads after various periods of coating time with TEOS;
[0021] FIG. 13 presents the change in the Oxygen to Si intensity
ratio values calculated from the XPS spectra of polishing pads
after various periods of coating times with TEOS; and
[0022] FIG. 14 presents the change in relative Blanket Tungsten
Removal Rate (WRR) and the Static Coefficient of Friction (COF) for
thermoplastic foam polishing pads as a function of coating time
with TEOS.
DETAILED DESCRIPTION
[0023] The present invention benefits from the previously
unrecognized advantages of using a thermoplastic polymer as the
substrate for depositing a uniform coating of a polishing agent on
concave cells. The interior surface of the concave cells was
discovered to form excellent receptacles for receiving a uniform
coating of the polishing agent. It is hypothesized that the center
of the concave cell serves as an excellent nucleating point for
coating because the surface energy of the cell at the center is
lowest. It is believed that the initiation of coating at this
location facilitates the uniform coverage of the interior surface
of the concave cell with the polishing agent, thereby facilitating
the polishing performance of a pad having such a surface.
[0024] The polishing agent of the present invention comprises an
inorganic metal oxide that includes nitrides or carbides. Using
such metal-oxides to coat the pad surface also advantageously makes
the polishing pad surface permanently hydrophilic. Preferably, the
inorganic metal oxide has a lattice of atoms that incorporate the
nitrides or carbides into the lattice. The use of such polishing
agent surface coatings enhances polishing by modifying the surface
mechanical properties of polishing pads.
[0025] For instance, altering the nitride or carbide content of
particular inorganic metal oxides allows the mechanical properties
of the polishing pad surface to be fine tuned so as to match the
mechanical properties the surface being polished. In turn, matching
the mechanical properties of the polishing pad surface to the
surface being polished improves polishing rate selectivity and
reduces process-induced defects, such as scratching. Tuning the
surface mechanical properties is accomplished by taking advantage
of thermoplastic polymer substrate alterations produced by the
plasma enhanced chemical vapor deposition (PECVD) surface coating
and the secondary thermal induced reactions in the bulk of the
thermoplastic substrate.
[0026] One embodiment of the present invention is a polishing pad
for chemical mechanical polishing semiconductor devices. FIG. 1
presents an exemplary polishing pad 100 of the present invention.
The polishing pad 100 comprises a polishing body 110. The polishing
body 110 comprises a thermoplastic foam substrate 115 having a
surface 120 comprising concave cells 125. A polishing agent 130
coating an interior surface 135 of the concave cells 125 comprises
an inorganic metal oxide that includes carbides or nitrides.
[0027] The polishing agent 130 comprises ceramic compounds composed
of one or more inorganic metal oxides formed by the grafting of
secondary reactants on the thermoplastic foam substrate 115 surface
120 in a plasma-enhanced chemical vapor deposition (PECVD) process.
As further explained below, in the present invention, the PECVD
process can be altered to promote the inclusion of carbides or
nitride in the inorganic metal oxide.
[0028] It is preferable for one or both of the carbides or nitrides
to be incorporated into a lattice of the inorganic metal oxides.
For instance, when the inorganic metal oxide comprises a silicon
oxide, then the lattice can comprise silicates in with polymeric
Si--O--Si structures having tetrahedral and distorted tetrahedral
configurations. The nitrides can comprise silicon nitrides that are
incorporated into these lattices. Alternatively, when the inorganic
metal oxide comprises a titanium oxide, then the nitrides can
comprise titanium nitrides incorporated into titanium oxide
lattices. Similarly, silicon carbides and titanium carbides can be
incorporated into a polishing agent 130 whose inorganic metal
oxides comprise silicon oxide and titanium oxide, respectively. In
some preferred embodiments, nitrides, such as silicon nitride,
comprise about 10 mol percent of the polishing agent 130, while in
other embodiments, carbides, such as silicon carbide, comprise
about 10 mol percent of the polishing agent. In some preferred
embodiments the polishing agent 130 can comprise both nitride and
carbides at these concentrations.
[0029] As the PECVD process is extended to longer periods, the
silanol concentration in the polishing agent 130 decreases,
resulting in a decrease in the ratio of oxygen to silicon. In some
advantageous embodiments of the polishing pad 100 where the
polishing agent 130 comprises silicon oxide, the O:Si ratio is at
least about 8:1, and in some cases, at least about 9.9:1.
[0030] Including nitrides and carbides in the polishing agent 130
provides an additional, heretofore unrecognized, means to alter the
mechanical properties of the polishing pad 100, and thereby alter
the pad's polishing properties. In some preferred embodiments, the
polishing pad 100 has a hardness of greater than 60 KPa, and more
preferably greater than 70 KPa. In other preferred embodiments, the
polishing pad 100 has a hardness between about 62 KPa and about 70
KPa. In certain preferred embodiments, the polishing pad 100 has an
elastic modulus of about greater than about 3 MPa, and more
preferably, 4 MPa or greater. In other preferred embodiments, the
polishing pad has an elastic modulus of between about 4 MPa and 8.2
MPa, respectively. In yet other embodiments, the polishing pad 100
has a loss modulus of about 0.4 MPa or greater, and more preferably
at least about 0.6 MPa.
[0031] As disclosed in U.S. Pat. No. 6,579,604 and U.S. application
Ser. No. 10/241,074, incorporated herein by reference, the
inorganic metal oxide of the polishing agent 130 can be produced
from a variety of oxygen-containing organometallic compound used as
the secondary reactant in a PECVD process. For example, the
secondary plasma mixture may include a transition metal such as
titanium, manganese, or tantalum. However, any metal element
capable of forming a volatile organometallic compound, such as
metal ester contain one or more oxygen atoms, and capable of being
grafted to the polymer surface is suitable. Silicon may also be
employed as the metal portion of the organometallic secondary
plasma mixture. In these embodiments, the organic portion of the
organometallic reagent may be an ester, acetate, or alkoxy
fragment. In preferred embodiments, the inorganic metal oxide of
the polishing agent 130 comprises silicon oxides or titanium
oxides, such as silicon dioxide or titanium dioxide, respectively;
tetraethoxy silane polymer; or titanium alkoxide polymer.
[0032] Other secondary plasma reactants include ozone, alkoxy
silanes, water, ammonia, alcohols, mineral sprits or hydrogen
peroxide. In some preferred embodiments, the secondary plasma
reactant comprises titanium esters; tantalum alkoxides, including
tantalum alkoxides wherein the alkoxide portion has 1-5 carbon
atoms; manganese acetate solution in water; manganese alkoxide
dissolved in mineral spirits; manganese acetate; manganese
acetylacetonate; aluminum alkoxides; alkoxy aluminates; aluminum
oxides; zirconium alkoxides, wherein the alkoxide has 1-5 carbon
atoms; alkoxy zirconates; magnesium acetate; and magnesium
acetylacetonate. Other embodiments are also contemplated for the
secondary plasma reactant, for example, alkoxy silanes and ozone,
alkoxy silanes and ammonia, titanium esters and water, titanium
esters and alcohols, or titanium esters and ozone.
[0033] Some preferred embodiments of the thermoplastic foam
substrate 115 comprise cross-linked polyolefins, such as
polyethylene, polypropylene, and combinations thereof. In certain
preferred embodiments, the thermoplastic foam substrate 115
comprises a closed-cell foam of crosslinked homopolymer or
copolymers. Examples of closed-cell foam crosslinked homopolymers
comprising polyethylene (PE) include: Volara.TM. and Volextra.TM.
from Voltek (Lawrence, Mass.); Aliplast.TM., from JMS Plastics
Supply, Inc. (Neptune, N.J.); or Senflex T-Cell.TM. (Rogers Corp.,
Rogers, Conn.). Examples of closed-cell foams of crosslinked
copolymers comprising polyethylene and ethylene vinyl acetate (EVA)
include: Volara.TM. and Volextra.TM. (from Voltek Corp.); Senflex
EVA.TM. (from Rogers Corp.); and J-foam.TM. (from JMS Plastics JMS
Plastics Supply, Inc.)
[0034] In other preferred embodiments, the closed-cell
thermoplastic foam substrate 115 comprises a blend of crosslinked
ethylene vinyl acetate copolymer and a low density polyethylene
copolymer (i.e., preferably between about 0.1 and about 0.3 gm/cc).
In yet other advantageous embodiments, the blend has a ethylene
vinyl acetate:polyethylene weight ratio between about 1:9 and about
9:1. In certain preferred embodiments, the blend comprises EVA
ranging from about 5 to about 45 wt %, preferably about 6 to about
25 wt % and more preferably about 12 to about 24 wt %. Such blends
are thought to be conducive to the desirable production of closed
cells 140 of the thermoplastic foam substrate 115, having a small
size (e.g., diameters between about 10 and about 500 microns, and
more preferably between about 50 to 150 microns). In still more
preferred embodiments, the blend has an ethylene vinyl
acetate:polyethylene weight ratio between about 0.6:9.4 and about
1.8:8.2. In even more preferred embodiments, the blend has an
ethylene vinyl acetate:polyethylene weight ratio between about
0.6:9.4 and about 1.2:8.8.
[0035] As further illustrated in FIG. 1, the thermoplastic foam
substrate 115 comprises closed cells 140. The term closed cell 140
as used herein, refers to any volume defined by a membrane within
the substrate 115 occupied by air, or other gases used as blowing
agents, such as nitrogen or helium. The closed cells 140 form a
substantially concave cell 135 formed upon skiving of the substrate
115. The concave cells 135 need not have smooth or curved walls,
however. Rather, the concave cells 135 may have irregular shapes
and sizes. Several factors, such as the composition of the
thermoplastic foam substrate 115 and the procedure used to prepare
the thermoplastic foam substrate 115, may affect the shape and size
of the closed cells 140 and the concave cells 135.
[0036] As further illustrated in FIG. 1, the thermoplastic foam
substrate 115 can be coupled to an optional backing material 145.
In some preferred embodiments the backing material 145 is stiff. A
stiff backing advantageously limits the compressibility and
elongation of the foam during polishing, which in turn, reduce
erosion and dishing effects during metal polishing via CMP. In some
cases, the backing material 145 comprise a high density
polyethylene (i.e., greater than about 0.98 gm/cc), and more
preferably a condensed high density polyethylene. In certain cases,
coupling to the thermoplastic foam substrate 115 is achieved via
chemical bonding using a conventional adhesive 150, such as epoxy
or other materials well known to those skilled in the art. In some
preferred embodiments, coupling is achieved via extrusion coating
of the molten backing material 145 onto the thermoplastic foam
substrate 115. In still other preferred embodiments, the backing
material 145 is thermally welded to the thermoplastic foam
substrate 115.
[0037] Another aspect of the present invention is a method for
preparing a polishing pad for chemical mechanical polishing. FIGS.
2 to 4 present selected steps in an exemplary method of preparing a
polishing pad 200. Any of the embodiments of the polishing pad and
its component parts, including the above described primary and
secondary plasma reactants, can be incorporated into the method of
preparing the polishing pad 200.
[0038] Turning now to FIG. 2, shown is the partially constructed
polishing pad 200 after exposing closed cells 210 within a
thermoplastic foam substrate 220 of the polishing pad 200 to
provide a substrate surface 230 comprising concave cells 240. The
concave cells 240 are formed on the substrate's surface 230 by
skiving. The term skiving as used herein means any process to cut
away a thin layer of the surface of the substrate 220 so as to
expose concave cells 240 within the thermoplastic foam substrate
220. Skiving may be achieved using any conventional technique
well-know to one of ordinary skill in the art.
[0039] FIGS. 3 and 4 illustrate selected stages in a surface
coating process. Turning first to FIG. 3, illustrated is the
partially completed polishing pad 200 after exposing the substrate
surface 230 to an initial plasma reactant, followed by exposure to
a secondary plasma reactant in a PECVD process.
[0040] Exposure to the initial plasma reactant forms a modified
surface 310 of the thermoplastic foam substrate 220. It is
important to carefully control the conditions and duration of the
plasma treatment to avoid excessive damage to the thermoplastic
foam substrate 220. For example, an excessively high or
uncontrolled radio flow discharge electrode temperature can cause
the thermoplastic foam substrate 220 to melt, warp or crack. In
some preferred embodiments of the method, the radio flow discharge
electrode temperature is maintained between about 20.degree. C. and
100.degree. C. and more preferably between about 30.degree. C. and
50.degree. C. In some cases an RF operating power between about 250
and about 1000 Watts, and more preferably about between about 300
Watts and 400 Watts is used. In certain preferred embodiments, the
initial plasma reactant comprises an inert gas such as neon, and
more preferably, argon or helium. In some cases, exposure to the
initial plasma proceeds for between about 1 second and 60 seconds,
and more preferably about 30 seconds. In some embodiments, the
PECVD reaction chamber is maintained at between about 300 mTorr and
about 400 mTorr, and more preferably about 350 mTorr.
[0041] FIG. 3 also shows the partially completed polishing pad 200
after exposing the modified surface 310 to a secondary plasma
reactant. In some preferred embodiments the secondary plasma
reactant comprises tetraethoxy silane (TEOS) or titanium alkoxide
(TYZOR). In some cases, the secondary plasma reactant also includes
the first plasma reactant, for example, TEOS or TYZOR vapor mixed
with helium or argon gas. Exposure to the secondary plasma reactant
results in the grafting of the secondary plasma reactant to the
modified surface 310 to form a polishing agent 320 comprising
inorganic metal oxides. The polishing agent 320 coats an interior
surface 330 of the concave cells 240.
[0042] Again, the conditions and duration of exposure to the
secondary plasma reactant is carefully controlled to avoid damaging
the thermoplastic foam substrate 220, or the polishing agent 320,
and to achieve long-lasting coatings of polishing agent 320. In
some embodiments, the PECVD reaction chamber is maintained at
between about 300 mTorr and about 400 mTorr, and more preferably
about 350 mTorr. In some instances, the radio flow discharge
electrode temperature is maintained at between about 20.degree. C.
and 100.degree. C., and more preferably between about 30.degree. C.
and 50.degree. C. In some cases, an RF operating power between
about 50 and about 500 Watts, and more preferably, about 250 to
about 350 Watts, is used.
[0043] Turning now to FIG. 4, illustrated is the partially
completed polishing pad 200 after a period of exposure to the
secondary plasma reactant of at least about 30 minutes. In some
preferred embodiments, exposure to the secondary plasma reactant is
for between about 30 minutes and about 60 minutes. In other
preferred embodiments, exposure to the secondary plasma reactant is
for between about 30 minutes and about 45 minutes. Such periods of
exposure advantageously enhance the incorporation of nitrides or
carbides, or both, into the inorganic metal oxide of the polishing
agent 320. In some embodiments of the method, an interior of closed
cells 410 of the thermoplastic foam substrate 220 comprise nitrogen
gas. The nitrogen gas can react with the secondary plasma reactant
to form nitrides. In other embodiments of the method, at least a
portion of the thermoplastic foam substrate 220 reacts with the
secondary plasma reactant to form carbides. For example, in some
embodiments, carbon radical species within in an about 1 micron
depth of the thermoplastic foam substrate 220 from the modified
surface 310 can react with the secondary plasma reactant.
[0044] As discussed above and further illustrated in the example
section to follow, the deposition of the polishing agent 320 via
PECVD modifies the surface properties of certain thermoplastic
foams 230, such as polyolefin foams. Surface coating of the
thermoplastic foam surface 310 with a polishing agent 320 for up to
about 30 minutes occurs by one mechanism. After this period,
however, surface coating occurs by a different mechanism. This, in
turn, results in the production of a polishing pad surface 410
having distinct differences in surface micromechanics and
chemistry, depending on the coating time.
[0045] In some cases, as the coating time increases, the
temperature thermoplastic foam substrate 220 temperature increases.
This, in turn, causes out-gassing of the nitrogen gas used in
foaming the substrate 220 and located in the closed cells 410 of
the substrate 240. In some embodiments, for instance where the
polishing agent 320 comprises silicon oxides, the out-gassed
nitrogen reacts with the silicon species on the pad surface, which
causes formation of Si.sub.3N.sub.4 species in stoichiometric
conversion from SiO.sub.4 to Si.sub.3N.sub.4. Of course, analogous
reactions can occur in embodiments where the polishing agent
comprises other inorganic metal oxides such as titanium oxides.
[0046] Similarly, at long coating times, the ion bombardment of the
thermoplastic foam substrate 240 surface 310 generates appreciable
amounts of carbon radicals on the surface 310 of the pad 200. In
some embodiments, for instance where the polishing agent 320
comprises silicon oxides, these radicals react with the silicon
species to form silicon carbide (SiC), which are subsequently
incorporated into the polishing agent 320 coating the pad 200.
[0047] The incorporation of species such as Si.sub.3N.sub.4 and SiC
into the polishing agent 320 modifies the polishing pad's 200
properties, such as enhancing its stiffness, hardness, and altering
its modulus of elasticity, as compared to the starting
thermoplastic foam substrate or substrates subject to brief coating
periods.
[0048] Having described the present invention, it is believed that
the same will become even more apparent by reference to the
following experiments. It will be appreciated that the experiments
are presented solely for the purpose of illustration and should not
be construed as limiting the invention. For example, although the
experiments described below may be carried out in a laboratory
setting, one skilled in the art could adjust specific numbers,
dimensions and quantities up to appropriate values for a full-scale
plant setting.
Experiments
[0049] Experiments were conducted to: 1) characterize the chemical
composition of thermoplastic foam substrates coated with polishing
agents as a function of coating time; 2) characterize the
mechanical properties of the foam substrate coated with polishing
agents; and 3) measure the polishing properties of the polishing
pads coated with polishing agents as a function of coating
time.
[0050] A thermoplastic foam substrate was formed into circular
polishing pads of approximately 120 cm diameter of about 0.3 cm
thickness. The commercially obtained thermoplastic foam substrate
(J-foam from JMS Plastics, Neptune N.J.), designated as "J-60SE,"
comprised a blend of about 18% EVA, about 16 to about 20% talc, and
balance polyethylene and other additives, such as silicates,
present in the commercially provided substrate. The J-60 sheets
were skived with a commercial cutting blade (Model number D5100 K1,
from Fecken-Kirfel, Aachen, Germany). The sheets were then manually
cleaned with an aqueous/isopropyl alcohol solution.
[0051] The J-60SE substrate was then coated with a polishing agent
comprising Tetraethoxy Silane (TEOS), by placing the skived
substrate into a reaction chamber of a conventional commercial
Radio Frequency Glow Discharge (RFGD) plasma reactor having a
temperature controlled electrode configuration (Model PE-2;
Advanced Energy Systems, Medford, N.Y.). The plasma treatment of
the substrate was commenced by introducing the primary plasma
reactant, Argon, for 30 seconds within the reaction chamber
maintained at 350 mTorr. The electrode temperature was maintained
at 30.degree. C., and a RF operating power of 300 Watts was used.
Subsequently, the secondary reactant was introduced, for periods
ranging from about 0 to about 45 minutes at 0.10 SLM, and
comprising TEOS mixed with He or Ar gas. The amount of secondary
reactant in the gas stream was governed by the vapor back pressure
(BP) of the secondary reactant monomer at the monomer reservoir
temperature (MRT; 50.+-.10.degree. C.).
[0052] The polishing properties of the J60SE polishing pads were
examined by polishing wafers having an about 4000 Angstrom thick
tungsten surface and an underlying about 250 Angstrom thick
tantalum barrier layer. Tungsten polishing properties were assessed
using a commercial polisher (Product No. EP0222 from Ebara
Technologies, Sacramento, Calif.). Unless otherwise noted, the
removal rate of tungsten polishing was assessed using a down force
of about 25 kPa of substrate, table speed of about 100 to about 250
rpm (Product Number MSW2000, from Rodel, Newark Del.). A
conventional slurry (Product Number MSW2000, from Rodel, Newark
Del.) adjusted to a pH of about 2 was used.
[0053] FIG. 5 illustrates FTIR spectra of the substrate's surface
after different periods of coating with TEOS. Spectra were obtained
on a FTIR spectrometer (FTIR 1727, Perkin-Elmer System detector,
equipped with a Series-I FTIR Microscope (MCT detector) and having
a spectral range from 10,000 to 370 cm-1. Signals at about 1010 and
about 950 cm-1 were assigned to the asymmetric Si--O--Si stretch of
silica and the Si--O--X (where X refers to polymeric
--(Si--O--Si)n-- structures not in the tetrahedral configuration),
stretch of silicates, respectively. A signal at 850 cm-1 is due to
free and associated silanols (Si--O--H). The silanols associate
through hydrogen bonding with the extent of association increases
with increasing surface concentration of silanols.
[0054] As illustrated in FIG. 6, as coating time increases up to
about 30 minutes, both of these signals montonically decreased, due
to a net decrease in the surface concentration of Si--O Thereafter,
there was a change in deposition kinetics and mechanism, indicating
an increased surface Si--O concentration. The latter observation is
inconsistent with generally accepted notions of TEOS deposition
mechanisms and kinetics, and promoted further investigation of the
coating process, especially in the post-30 minutes coating
period.
[0055] Nanoindentation testing was used to assess the mechanical
properties of the surface coating coatings, and more specifically
to measure the elastic modulus and the hardness. Indentations were
carried out on the thermoplastic foam substrates coated with
polishing agent for different periods. A NANOTEST 600.RTM.,
Nanoindenter located at Advanced Material and Characterization
Facility (AMPAC, Orlando, Fla.) was used for all measurements. The
machine rests upon a vibration isolation table and is enclosed in a
temperature-controlled cabinet. Two separate heaters placed on
either side in front of the cabinet provide a thermal barrier. The
temperature controller was set to a value about 2 or 3.degree. C.
above the room temperature, with expected stability at
.+-.0.1.degree. C. The indenter was allowed to settle for at least
half an hour to attain thermal stability before starting the
experiment.
[0056] Indentation parameters such as the type of indenter, the
maximum depth and loading/unloading rate, were established by
performing preliminary tests of the coated polishing pads. The
polishing pad surfaces were found to contain asperities and pores
of varying sizes of the order of few microns. Based on these
observations, a spherical indenter with tip diameter of .about.1 mm
was chosen, so that the indenter would sample enough pad material.
For the same reason, a spatial resolution of greater than 500
microns was chosen. Indentations were performed under ultra low
load range, with an initial load of 0.1 mN, which is a machine
parameter. The control parameter was set to depth controlled and
each pad was indented for varying depths with a maximum depth of
10,000 nanometers. The results were typically examined as an
average of 10 indentations. Both Oliver-Pharr method and Hertz
method were used to evaluate and validate the results. Load-depth
(P-h) curves obtained from the nanoindenter were analyzed using the
Oliver-Pharr method.
[0057] The polishing agent-coated polishing pads exhibited
non-uniform penetration during the indentation experiments. Visual
examination of the indentation (P-h) curves shows some unique
characteristics. As shown in FIG. 7, distinct events, labeled as
`pop-in` (Xb), `pop-out` or `kink-back` (Xc), are discernable from
the curves. For example, `pop-in` occurs during the compression
cycle when there is a sudden penetration of the indenter tip into
the sample. These events correlate with several experimental
parameters, such as coating time, loading/unloading rate and depth
of indentation. The mixed response exhibited by the coated
polishing pads revealed that `pop-in` events occurred more often
for indenter penetration depth around 1000 nanometers, `pop-out`
events seemed not to be affected by indentation depths, and rates
of loading/unloading were affected both of these events.
[0058] Further analysis of the P-h curve for various depths and
different coating times, revealed that the loading curve increase
steeply and decreases, this X coordinates, or the depth of this
transition point, is designated as Xa. Similarly, Xb and Xc are the
corresponding X coordinates or depth in nanometers. Such
non-uniform penetration of the indenter tip into the coatings
probably results from the onset of plastic deformation. It is
thought that plastic deformation is a critical attribute of CMP
pads, affecting the efficiency of the CMP process. Thus, the
above-described events in the load-depth curves was hypothesized to
be predictors of pad performance. The initial surface penetration
events (Xa) were found to be a function of PECVD coating time,
maximum penetration depth and load rate. For instance, FIG. 8 shows
a representative correlation of Xa with the TEOS coating time. The
data suggests that Xa is related to the thickness of surface foam
that has been modified by the dielectric coating.
[0059] FIGS. 9 and 10, respectively illustrate the hardness and
elastic modulus of polishing pads for different TEOS coating times,
calculated using the Oliver-Pharr method. The effective surface
modulus and hardness were found to increase with increasing coating
time. For coating times of 30 minutes, 40 minutes and 45 minutes,
the polishing pads had hardness values of about 65 KPa, 62 KPa and
70 KPa, respectively. For shorter coating times, the hardness was
60 KPa or less. For coating times of 30 minutes, 40 minutes and 45
minutes, the polishing pads had elastic modulus values of about 4
MPa, 5.5 MPa and 8.2 MPa, respectively. For shorter coating times,
the elastic modulus values was 3 MPa or less.
[0060] The changes in the mechanical properties of the pad surface
are attributed to the effect of the coatings deposited on the foam
substrate. These data indicate that the previously noted
discontinuities in the FTIR data are indicative of changes in the
pad surface chemistry, rather than net removal of TEOS-derived
coatings.
[0061] Dynamic mechanical analysis (DMA) was carried out on samples
of coated polishing pads using commercial equipment operating in
the tension mode from -125 to 200.degree. C. at a frequency of 1
Hz, at 10 micron amplitude with a programmed heating rate of
5.degree. C./min. Liquid nitrogen was used to achieve the
sub-ambient temperature. Samples were equilibrated at a predefined
initial temperature for 10 minutes before measurements were made.
All of the polishing pad samples were prepared to have the same
dimensions of 15 cm.times.5 cm, and were vacuum dried (30.degree.
C. at .about.1.times.10-2 Torr) for 24 hours prior to DMA
measurements, so as to avoid having to consider moisture
effects.
[0062] As illustrated in FIG. 11, the DMA studies indicate an
abrupt change in the loss modulus at long PECVD coating times. At a
coating time of 45 minutes the loss modulus increase to about 0.6
MPa, as compares to values ranging from 0.37 to 0.23 for coating
times of 10 minutes to 40 minutes.
[0063] This is contrary to the generally accepted view that PECVD
coatings only modify the surface of substrates. This surprising
result suggests that other processes occur that alter the bulk
mechanical properties of the foam substrate during the surface
coating. It was hypothesized that residual reactants in the
thermoplastic foam substrate were reacting during PECVD coatings in
a time dependent fashion.
[0064] The surface modification of polishing pads, subjected to
different coating times, was further characterized using X-ray
photoelectron spectroscopy (XPS). A commercial X-ray photoelectron
spectrometer was operated at a base pressure of 10.sup.-10 Torr and
the spectrometer was calibrated using a metallic gold standard (Au
(4.sub.f7/2): 84.0.+-.0.1 eV). A non-monochromatic Mg K .varies.
X-ray source with an energy of 1253 eV at a power of 250 W, was
used for the analysis. Charging shift produced by the polishing pad
samples were removed by using binding energy scale referenced with
respect to the binding energy of the hydrogen part of adventitious
carbon line at 285.0 eV. Peak deconvolution was carried out using
commercial software.
[0065] The XPS analysis provides several insights about the
chemical nature of the topography resulting from TEOS adsorption a
n d dissociation. The Carbon (1s) signal was resolved into three
major peaks: two peak at .about.285.0 eV, corresponding to C--C and
C--H bonds and peak observed at .about.286.5 eV corresponding to
C--O bonds. A peak centered at .about.289 to .about.289.3 eV was
attributed to carbamide [--O--C(NH.sub.2).dbd.O] functional group
from residual blowing agents used in the thermoplastic foam
substrate manufacturing process. For the specimens coated for 40
and 45 minutes respectively, another peak near .about.283.6 eV was
observed, and was tentatively assigned to C--Si bonds.
[0066] FIG. 12 presents exemplary peak fitted XPS signals of the Si
(2p) envelop obtained from pads after TEOS coating times of: (a) 10
min, (b) 20 min, (c) 30 min, (d) 40 min, and (e) 45 min. Peaks were
identified as: (1) Si--O, (2) Silicate, (3) Si--N, and (4) Si--C
bonds. Each spectrum was deconvoluted into two major peaks at
.about.102.3 and .about.103.4 eV, corresponding to bonds in
silicate and Si--O species, respectively.
[0067] These data indicate that for short coating times (e.g., less
than .about.30 minutes) the pad surface is rich in silanol,
consistent with TEOS films deposited at low process temperatures.
As further illustrated in FIG. 13 the Oxygen to Si intensity ratio,
calculated from the XPS data, is high early during coating
indicative of a high the concentration of the silanol in the
deposited coatings. The silanol concentration decreases with
coating times up 30 minutes, then starts increasing. For example,
as shown in FIG. 9, the O:Si ratio equals about 7.4, 4.8, 3.6, 7.1
and 9.9, after coating times of about 10 min, 20 min, 30 min, 40
min and 45 min, respectively.
[0068] Turning again to FIG. 12, for coating times of 30, 40 and 45
min, a small peak is observed at .about.102.1 eV, corresponding to
Si--N bonds. Over this same period, there is an abrupt reduction in
Si to N ratio, which indicates an increase in nitrogen species on
the surface.
[0069] These observations suggest that PECVD-based coating
involving competition between several processes. The PECVD-based
coating produces both silica and silicates on the (SiO.sub.x and
SiO.sub.2) on the foam surface. Moreover, surface chemistry of the
substrates changes as a function the coating time. For coating
times below 30 minutes, it there is net etching of the deposits
from the Ar-ion bombardment of the surface. The sample also heats
up from the plasma, so thermal processes also occur. As the coating
time increases, the silicate content on the substrate surface
starts to decrease and the pad becomes denser, so as to increase
the hardness of the pad.
[0070] For coating times of 30 minutes of longer, the substrate
temperature is high enough to cause out-gassing of the nitrogen gas
used in foaming the substrate or decomposition of any residual
blowing agent left in the foam to produce nitrogen gas. The
nitrogen reacts with Si-containing intermediates in the gas phase
or Si-species on the pad surface, to form nitrides such as
Si.sub.3N.sub.4 at the expense of SiO.sub.2. Such nitrides are
incorporated into the polishing agent in concentrations up to 10
mol %. Furthermore, for such coating times, the ion bombardment of
the foam surface generates appreciable amounts of Carbon radicals
on the pad surface. These radicals react with the silicon species
to form carbides, such as silicon carbide (SiC), which is
incorporated into the polishing agent in concentrations up to 10
mol %.
[0071] FIG. 14 compares the Relative Blanket Tungsten Removal Rate
(W-RR) and the Static Coefficient of Friction (COF) for
thermoplastic foam substrate subjected to different periods of
coating times with TEOS. Both the W-RR and COF both increase with
increased coating times up to 30 minutes, signifying an increase in
the thickness of the polishing agent. For coating times between 30
and 60 minutes, the W-RR and COF both decrease, and then increase.
These results suggest that the pad appears polishes by one
mechanism for surfaces coated for up to 30 minutes, and by a
different mechanism for surfaces coated for more than 30 minutes,
due to differences in surface micromechanics and chemistry.
[0072] Although the present invention has been described in detail,
those skilled in the art should understand that they can make
various changes, substitutions and alterations herein without
departing from the scope of the invention.
* * * * *