U.S. patent application number 12/419625 was filed with the patent office on 2009-10-08 for free radical-forming activator attached to solid and used to enhance cmp formulations.
Invention is credited to Daniel Hernandez Castillo, Junaid Ahmed Siddiqui, Robert J. Small.
Application Number | 20090250656 12/419625 |
Document ID | / |
Family ID | 37890677 |
Filed Date | 2009-10-08 |
United States Patent
Application |
20090250656 |
Kind Code |
A1 |
Siddiqui; Junaid Ahmed ; et
al. |
October 8, 2009 |
Free Radical-Forming Activator Attached to Solid and Used to
Enhance CMP Formulations
Abstract
A chemical mechanical polishing composition having: a fluid
comprising water and at least one oxidizing compound that produces
free radicals when contacted with an activator; and a plurality of
particles having a surface and comprising at least one activator
selected from ions or compounds of Cu, Fe, Mn, Ti, or mixtures
thereof disposed on said surface, wherein at least a portion of
said surface comprises a stabilizer. Preferred activators are
selected from inorganic oxygen-containing compounds of B, W, Al,
and P, for example borate, tungstate, aluminate, and phosphate. The
activators are preferably ions of Cu or Fe. Advantageously, certain
organic acids, and especially dihydroxy enolic acids, are included
in an amount less than about 4000 ppm. Advantageously, activator is
coated onto abrasive particles after the particles have been coated
with stabilizer.
Inventors: |
Siddiqui; Junaid Ahmed;
(Richmond, VA) ; Small; Robert J.; (Tuscon,
AR) ; Castillo; Daniel Hernandez; (Laveen,
AR) |
Correspondence
Address: |
MORGAN LEWIS & BOCKIUS LLP
1111 PENNSYLVANIA AVENUE NW
WASHINGTON
DC
20004
US
|
Family ID: |
37890677 |
Appl. No.: |
12/419625 |
Filed: |
April 7, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11264027 |
Nov 2, 2005 |
7513920 |
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12419625 |
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10361822 |
Feb 11, 2003 |
7029508 |
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11264027 |
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10074757 |
Feb 11, 2002 |
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10361822 |
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Current U.S.
Class: |
252/79.1 |
Current CPC
Class: |
H01L 21/3212 20130101;
C09K 3/1463 20130101; C03C 19/00 20130101; C09G 1/02 20130101; C23F
3/04 20130101; G11B 5/3169 20130101; G11B 5/3163 20130101 |
Class at
Publication: |
252/79.1 |
International
Class: |
C09K 13/00 20060101
C09K013/00 |
Claims
1. A slurry composition for chemical-mechanical polishing a
substrate, said slurry composition comprising: a fluid comprising
water and at least one oxidizing compound that produces free
radicals when contacted with an activator, and wherein the fluid pH
is between about 2 to about 8; and a plurality of particles having
a surface and comprising at least one activator selected from ions
or compounds of Cu, Fe, Mn, Ti, or mixtures thereof disposed on
said surface, wherein at least a portion of said surface comprises
a stabilizer selected from inorganic compounds of B, W, and Al, and
wherein if the particle body is alumina, then the stabilizer
comprises B, W, or mixture thereof.
2. The slurry of claim 1 further comprising an
.alpha.,.beta.-dihydroxy enolic compound.
3. The slurry of claim 2 wherein the .alpha.,.beta.-dihydroxy
enolic compound is ascorbic acid, erythorbic acid, or derivatives
and/or mixtures thereof.
4. The slurry of claim 3, wherein the pH of the fluid is between
about 6 and about 7.
5. The slurry of claim 1 wherein: said particles comprise silica,
alumina, ceria, spinel, or combination thereof; said stabilizer
comprises B; and said activator is selected from the group
consisting of Cu, Fe, Mn, and mixture thereof said activator being
associated with the surface of the abrasive particle ad being
present in an amount sufficient to provide at least a 20% higher
substrate removal rate during chemical mechanical polishing of said
substrate, compared to the substrate removal rate during polishing
the substrate under the same conditions but wherein the abrasive
particle does not comprise activator.
6. The slurry of claim 5, wherein said stabilizer is associated
with the surface of the particle, said stabilizer forming a layer
between at least a portion of said activator and said particle
surface.
7. The slurry of claim 1, wherein said activator is present in an
amount between about 0.2 ppm and 12 ppm as metal, based on the
weight of the slurry.
8. The slurry composition of claim 7, wherein the activator is
present in an amount between about 3 ppm and 8 ppm as metal, based
on the weight of the slurry.
9. The slurry composition of claim 7 further comprising between
0.01% and 0.5% by weight total of ascorbic acid, alkyne diols,
citric acid, lactic acid, glycolic acid, and/or salicylic acid, or
combinations thereof.
10. An abrasive particle for use in chemical mechanical polishing,
said abrasive particle comprising: a body comprising silica,
alumina, ceria, spine, or combination thereof; a stabilizing
coating disposed on the exterior of the particle body, said
stabilizing coating comprising an inorganic compound comprising B,
W, Al, or mixtures thereof, wherein if the particle body is alumina
then the stabilizer comprises B, W, or mixture thereof; and an
activator selected from the group consisting of Cu, Fe, Mn, Ti, or
mixture thereof, said activator being associated with the surface
of the abrasive particle and being present in an amount sufficient
to provide at least a 20% higher substrate removal rate during
chemical mechanical polishing of said substrate when used with a
liquid comprising a per-type oxidizer capable of producing free
radicals, compared to polishing the substrate with the same liquid
and under the same conditions but wherein the abrasive panicle does
not comprise activator
11. The abrasive particle of claim 10, wherein the surface of the
particle body is modified by the stabilizer prior to the activator
becoming associated with the surface of the particle.
12. The abrasive particle of claim 11, wherein at least 80% of the
surface sites of the particle are coated with said stabilizer.
13. The abrasive particle of claim 11, wherein said stabilizer is
associated with the surface of the particle, said stabilizer
forming a layer between at least a portion of said activator and
said particle surface.
14. The abrasive particle of claim 11, wherein the particle body
comprises silica, the stabilizer comprises B, and the activator is
present in an amount between 0.01% to 3% by weight of the weight of
the particles.
15. The abrasive particle of claim 14, wherein said stabilizer is
associated with the surface of the particle, said stabilizer
forming a layer between said activator and said particle
surface.
16. The abrasive particle of claim 10, wherein at least 80% of the
surface sites of the particle are coated with said stabilizer.
17. The abrasive particle of claim 16, wherein the stabilizer
comprises B, and the activator is present in an amount between
0.01% to 3% by weight of the weight of the particles.
18. The abrasive particle of claim 17, wherein the activator
comprises iron.
19. The abrasive particle of claim 10, wherein the abrasive
particle is disposed on the surface of a polishing pad.
20. (canceled)
21. (canceled)
22. (canceled)
23. (canceled)
24. (canceled)
25. (canceled)
26. (canceled)
Description
RELATED APPLICATIONS
[0001] This application claims priority to and is a
continuation-in-part of pending U.S. application Ser. No.
10/361,822 filed Feb. 11, 2003, which is in turn a
continuation-in-part of U.S. application Ser. No. 10/074,757 filed
Feb. 11, 2002, the entire contents of which are each incorporated
herein for all purposes by express reference thereto. This
application also claims priority to pending U.S. application Ser.
No. 10/759,666 filed Jan. 16, 2004, which is incorporated herein
for all purposes by express reference thereto.
FIELD OF THE INVENTION
[0002] The invention relates generally to a system that is useful
in chemical mechanical polishing or planarization (hereafter CMP)
processes, and an associated method of polishing a substrate using
the system. More particularly, e invention relates to a composition
comprising a fluid, a oxidizing agent capable of producing
inorganic oxygen-containing free radical, and a free
radical-inducing activator ion or salt which is affixed to a solid
in contact with the composition, and which when contacted by the
oxidizing agent increases the rate at which the oxidizing agent
produces free radical. The composition is useful in the polishing
of various layers, such as metal layers, on substrates.
BACKGROUND OF THE INVENTION
[0003] A semiconductor wafer generally has a substrate surface on
which one or more integrated circuits is formed. The substrate
surface is desirably as flat, or planar, as possible before the
surface is processed to form the integrated circuits. A variety of
semiconductor processes such as for example photolithography are
used to form the integrated circuits on the flat surface, during
which the wafer takes on a defined topography. The topography is
subsequently planarized, because an irregular surface, because the
surface has an excess of material deposited thereon, or the surface
has imperfections which seriously impede subsequent fabrication
processes. Thus, it is necessary to polish the wafer surface to
render it as planar or uniform as possible and to remove surface
imperfections.
[0004] CMP is now widely known to those skilled in the art and has
been described in numerous patents and open literature
publications. In a typical CMP process, a substrate (e.g., a wafer)
is placed in contact with a rotating polishing pad attached to a
platen. One method has the wafer held in place on a mount using
negative pressure, such as vacuum, or hydrostatic or pneumatic
pressure, where the mount is situated near or over a polishing pad.
A CMP slurry, typically an abrasive and chemically reactive
mixture, is supplied to the pad during CMP processing of the
substrate. During the CMP process, the pad (fixed to the platen)
and substrate are rotated while a wafer carrier system or polishing
head applies pressure (downward force) against the substrate. The
slurry accomplishes the planarization (polishing) process by
chemically and mechanically interacting with the substrate film
being planarized due to the effect of the rotational movement of
the pad relative to the substrate. Polishing is continued in this
manner until the desired film on the substrate is removed with the
usual objective being to effectively planarize the substrate. For
such a semiconductor wafer, a typical CMP process involves
polishing the metal in a controlled manner to preferentially etch
certain conductors, insulators or both over the oxide beneath the
metal, such that the metal is substantially coplanar with the oxide
and remains in the grooves or stud vias of the oxide. After CMP,
the substantially coplanar surface is ready for further
processing.
[0005] Economic forces are requiring the use of faster processing.
One approach has involved increasing the downward pressure on the
wafer carrier in order to increase material removal rates. This
approach is generally disfavored as the requisite downward pressure
is considered too high and too likely to cause wafer damage.
Another approach has involved increasing the amount of oxidizing
agent used in the CMP slurry in an effort to increase chemical
removal of targeted material. This approach is largely disfavored
as the use of increased amounts of oxidizing agents increase
material costs and also detrimentally add to the handling issues
and environmental issues associated with many oxidizing agents and
also increase costs. Additional approaches have involved using
various protected combinations of oxidizers, chelators, corrosion
inhibitors, solvents, and other chemicals in the slurry, various
abrasives including for example a zirconium abrasive or mixed
abrasives, and/or using point-of-use mixing techniques. These
approaches are generally undesirable, as they typically complicate
CMP in terms of tooling and process control for example, consume
more process time, and/or increase costs.
[0006] It is generally known that oxidizers admixed in a solution
can provide synergistic etching rates. While ferric salts, cerium
salts, peroxides, persulfates, or hydroxylamines form the oxidizing
capacity of most commercially available CMP slurries, those of
ordinary skill in the art have long known that certain of these
oxidizers can be admixed with others in this group and also with
other oxidizers, and the resulting composition can show synergistic
results. For example, the compositions claimed in U.S. Pat. No.
6,117,783 to Small et al., which claims priority to a provisional
application filed Jul. 25, 1996, the contents of which is
incorporated herein by reference thereto, claims a CMP slurry
having a hydroxylamine compound and hydrogen peroxide, and teaches
in the specification that the two have a synergistic effect.
[0007] Many slurries use a metal ion, typically Fe ions or Ce ions,
as an oxidizer, alone or in combination with another oxidizer.
However, both iron and cerium, as well as other metal ions, causes
metallic contamination of substrates. U.S. Pat. No. 5,773,364
describes a CMP slurry where oxidizers include ferric nitrate or
cerium nitrate, and note the problem that metal ions are created as
a result of the oxidizing process. U.S. Pat. No. 5,958,288 suggests
limiting the amount of ferric nitrate to from about 0.001 to about
2.0 weight percent, where the slurry comprises another oxidizer The
ferric ion contamination may be exceedingly difficult to
subsequently remove. Raghunath et al showed in Mechanistic Aspects
Of Chemical Mechanical Polishing Of Tungsten Using Ferric Ion Based
Alumina Slurries, in the Proceedings of the First International
Symposium on Chemical Mechanical Planarization, 1997, that alumina
slurries containing ferric salts is conducive to the formation of
an insoluble layer of ferrous tungstate on tungsten. The industry
has developed methods to remove a portion of the metallic
contamination, for example by: physical desorption by solvents;
changing the surface charge with either acids or bases so that
Si--OH or M--OH group can be protonated (made positive) in acid or
made negative with bases by removing the proton; ion competition,
for example removing adsorbed metal ions by adding acid (i.e. ion
exchange); subsequent oxidation of metals to change the chemical
bonds between the impurities and substrate surface; and subsequent
etching the surface, wherein the impurity and a certain thickness
of the substrate surface is removed, as described in U.S. Pat. No.
6,313,039. There have been various post-CMP cleaners developed to
remove metallic contamination, but removal of all undesired metal
ions is substantially beyond the range of cleaners, and as the size
of the structures continues to decrease, even a very small number
of metallic atoms deposited on a surface will result in undesired
shorts or current leakage. Additionally, metal ion-containing
fluids and many post-CMP cleaners are environmentally undesirable
and expensive treatment may be needed prior to waste disposal of
used product.
[0008] Another problem with many soluble metal oxidizers is that
they react with and cause degradation of other oxidizers. When a
metal-containing oxidizer is admixed with a non-metal-containing
oxidizer, for example hydrogen peroxide in a solution, the two
often react in an undesirable fashion, and the oxidizing capacity
of the mixture declines rapidly, but without any rigorous
predictability, with time. For example, ferric nitrate reacts with
hydrogen peroxide in CMP formulations at essentially all usable
pHs, making the formulation oxidizing capacity fall with time,
which complicates polishing since there is a non-uniformity
problem, and also causing formation of undesired products. It is
known that if the pH is above about 5, iron precipitates as
Fe(OH).sub.3 which rapidly catalytically decomposes hydrogen
peroxide to oxygen, without forming hydroxyl radicals.
[0009] Therefore, despite the known advantages of having multiple
oxidizers, for example a metal-containing oxidizer admixed with
either another metal-containing oxidizer or with a
non-metal-containing oxidizer, there has been a tendency in the
industry to reduce the amount of metal ions in CMP slurries. For
example, Rodel, a large commercial manufacturer of CMP slurries
that at point of use are designed to be used with
non-metal-containing oxidizers such as peroxides and persulfates,
had about 30 ppm of metals, primarily iron, in the liquid portion
of an MSW1000.TM. slurry produced in 1995. While this iron would
have functioned as a promoter, it is likely the manufacturer did
not add the iron, but rater the iron was in the solution as a
result of impurities. Later generations of Rodel slurries, for
example the Rodel MSW1500.TM. slurry that was sold in 2002, has
only 4.2 ppm iron, according to their web-site. Even where soluble
iron ions, e.g., ferric nitrate, are added to increase rates, such
as described in U.S. Pat. No. 5,958,288, the preferred amount of
ferric nitrate added to a hydrogen peroxide solution is very small,
that is, 0.01 to about 0.05 weight percent, or about 100 ppm to
about 500 ppm. Another method of reducing metallic contamination is
to use sequential CMP polishing steps using sequential formulations
that have decreasing amounts of metal, so that metal deposited from
earlier formulations in a CMP process are removed by CMP with
subsequent formulations that are metal-free. For example, a Rodel
CMP slurry, the MSW2000.TM., has a first formulation having 12 ppm
Fe, and a second formulation that has less than 0.3 ppm Fe.
However, use of sequential formulations adds additional costs to
processing, as well as adding complexity to the required
equipment.
[0010] There is another mechanism for synergy that has been
described in co-owned U.S. published applications 20040029495,
20040006924, and 20030162398, the disclosures of which are
incorporated herein by disclosure thereto. In these applications,
various metals are absorbed onto abrasives in an ionic form. The
synergy is based on Fenton's reaction, where the relatively benign
oxidizers generate very strong, short-lived, non-organic,
oxygen-containing free radicals. The classic Fenton's reaction is
the production of free radicals as a byproduct of the oxidation of
soluble ferrous ions by hydrogen peroxide. The useful pH for
classical Fenton's reaction utilizing soluble iron ions is pH 3 and
pH 6, particularly 4 to 5.
[0011] It is clear that the industry is moving away from metals,
for example iron, in the fluids. Also, when iron or other
metal-containing formulation is admixed with non-metal-containing
oxidizers, the "pot-life" of the formulation is very short, so
mixing is generally point-of-use mixing, which complicates CMP
processes and equipment and can create start-up problems even after
a temporary interruption on the processing. Further developments in
the field of CMP technology are desired.
SUMMARY OF THE INVENTION
[0012] Selected objects of this invention are to provide a system
wherein 1) higher polishing rates of conductors, insulators,
barriers, and/or other surfaces are achieved from a combination of
chemicals and abrasives than were otherwise achievable, 2)
acceptable polishing rates of conductors, insulators, barriers,
and/or other surfaces are achieved from a combination of chemicals
and abrasives at lower concentrations than were achieved in the
prior art; 3) provide a system where CMP can be performed at
commercially acceptable removal rates with commercially acceptable
uniformity in the polished product; 4) provide a system where CMP
can be performed at commercially acceptable removal rates with
commercially acceptable uniformity in the polished product and with
substantially no metallic ion contamination of the substrate; 5)
provide a system where CMP can be performed at commercially
acceptable removal rates with commercially acceptable uniformity in
the polished product, wherein the chemicals used are
environmentally friendly, easily recoverable, or both; 6) provide a
system of increasing the effectiveness of oxidizers and/or
cleaners; 7) provide a method of recovering and re-using selected
components of the system which are otherwise considered consumable
components; and/or 8) provide a one-component system that exhibits
usable shelf life for a period of at least 24 hours; and/or 9)
provide an additive which increases the effectiveness of various
commercial CMP slurries, beneficially without introducing
additional compounds to the slurry fluid, and/or provide
formulations for use in polishing that have essentially no
dissolved metals, low amounts of abrasive, and low amounts of
oxidizer, wherein said abrasives have residue removal rates and
nonuniformity comparable or exceeding the prior art slurries which
used considerably more chemicals. These objects of the invention
are lot exhaustive, and it is realized that not all objects of the
invention will be reached by any one system.
[0013] Various specific embodiments for CMP of a substrate using
various systems of the current invention are disclosed below, and
the description will often include one or more preferred
components. It is recognized that the preferred components can be
substituted with one or more components of the same class, where
various acceptable substitutes are described in later. For example,
in most embodiments, the preferred transition metal is iron. Other
useful transition metals include ceria, copper, and silver, used
alone or in combination with one another and/or with iron. The
preferred abrasive is generally colloidal silica. Other useful
particles include alumina in various forms, ceria in various forms,
spinels, and the like, alone or in combination with silica. The
preferred oxidizer may be for example hydrogen peroxide,
persulfate, or periodic acid, but any of these, combination of
these, and/or other per type oxidizers that react with the
transition metals associated with particles can be used. It is also
recognized and is part of the invention that two or more
embodiments can be combined, that is, a single system is formed
having the criteria defining a plurality of below-defined
embodiments, so long as the criteria defining the embodiments do
not conflict. As used here, unless otherwise specified, the terms
"coated", "absorbed on", and "associated with" mean having a
transition metal of the current invention (an activator metal ion)
associated on a surface of the object that is coated, where the
activator metal ion is different than the substrate. The coating is
not derived from the substrate and may be placed upon the surface
via mechanical, physical, electrical, or chemical means. An
intermediate layer of material, in particular a layer (molecule or
molecules) of stabilizer, may be present between activator metal
and the surface of the substrate. Its not clear what form the iron
would be in if suspended from the substrate via an stabilizer
moiety (or "chain") comprising boron-oxygen, aluminum-oxygen,
tungsten-oxygen, phosphorus-oxygen, or titanium-oxygen, in
particular whether it would be of the form of an ion or as a single
molecule of activator-oxide. At this level, there is no easy
mechanism to tell. Although a coating may be of any thickness,
typically the coating has a thickness smaller than that of the
substrate, and is usually believed to be between one and only
several molecules or atoms in thickness. Unless otherwise
specified, (e.g., unless attached to the surface of the abrasive
via a stabilizer molecule or moiety, for example) the transition
metal activator is believed to be present as an ion or salt, for
example ferric or ferrous iron, where the ion or salt is a ached to
the surface of a particle. More specifically unless otherwise
specified, the transition metal activator is not an oxide, when
being used in the composition. If the coated abrasive is dried, we
realize the absorbed activator may at least in part be converted to
an oxide, but with sufficiently long immersion in water the
activator will again become an ion.
[0014] Generally, in many of the embodiments discussed below, the
emphasis is on minimizing components and/or concentrations in
slurries. Each embodiment, combinable with other embodiments, can
either comprise, consist essentially, or consist of the listed
components (where water is always present but may or may not be
listed). Unless specifically stated otherwise, all % are weight %,
any mention of transition metal coated or absorbed on a surface
means transition metal ions or salts (not oxides) adhering to or
absorbed on a abrasive, all particles sizes are number average
particle size, which may be determined utilizing known techniques
such as transmission electron microscopy (TEM). The mean particle
diameter refers to the average equivalent spherical diameter when
using TEM image analysis, i.e., based on the cross sectional area
of the particles
[0015] 1. Slurry CMP Method With Stabilizer-Modified
Activator-Coated Particles: Without being bound by theory, we
believe that the most effective mechanism to stabilize the
transition metal-coated particles is to have stabilizers be
attached to the particle, partially shielding the particle (through
steric and/or electrodynamic forces) from oxidizer in the fluid. As
used here, the term "stabilizer" means an agent effective to help
maintain the abrasive as a sol in an aqueous medium. (In subsequent
sections of this application, the term "stabilizer" will be used to
refer to chelating additives, which have a different function which
is to stabilize the oxidizer and to minimize metal contamination of
the substrate.)
[0016] Stabilizers have a stabilizing influence on for example a
coated silica slurry by retarding settling. Inorganic stabilizers
are highly preferred. Such coatings are advantageously resistant to
attack by free radicals. Suitable stabilizers include ions
containing boron, aluminum, tungsten, titanium, with boron being
most preferred. Some preferred absorbed stabilizer ions are borate,
titanate, tungstate, or aluminate.
[0017] Silanols which can be bound to the abrasive particles can
form a stabilizing coating on the particle, where the silanol bound
to the particle is stable and comprises a sterically effective
blocking group, making this material when bound to for example
silica resistant to attack by free radicals. Exemplary stabilizers
which can form bonds with the abrasive include the traditional
halogenated trialkylsilanes and halogenated trialkoxysilanes, for
example chlorotrimethylsilane and chlorotrimethoxysilane. The
silica-containing stabilizers can be added before the activator
ions are absorbed onto the abrasive, or after the activator ions
are absorbed onto the abrasive, so long as if added afterward the
silanes do not bond directly with the absorbed activator ions. In
one embodiment, the short chain stabilizers comprise a chelating
moiety to trap iron ions, copper ions, or the like. This will
promote stability, but the transition metals may be too loosely
held to the particle during polishing, and such chelating elements
are therefore not expected to reduce iron ion contamination of a
substrate during polishing.
[0018] The stabilizer may be a phosphorous-containing ion (e.g.,
from pyrophosphoric acid or from phosphoric acid) that is tightly
bound to the abrasive, e.g., the alumina or silica, preferably
silica. Preferably, however, the stabilizer comprises at least one
member selected from the group consisting of B, W and Al. In
addition to the stabilizer, the abrasive will also have absorbed on
its surface the at least one transition metal ion selected from the
group consisting of Ag, Ce, Cu, Fe, Mn, Ti, W and/or V, provided
that the at least one stabilizer and the at least one catalyst are
not simultaneously W. The preferred absorbed transition metal ions
or salts are iron and copper.
[0019] The stabilizer may comprise stannate ions absorbed onto the
abrasive.
[0020] In this particular case, in an alternate embodiment, wherein
the surface of the abrasive is modified by absorbed borate,
tungstate, and/or aluminate, the transition metal coated onto the
surface can be a molecular species, for example an oxide, and/or
the transition metal can be an absorbed ion or salt.
[0021] The most preferred stabilizer comprises B, which can be
provided from for example boric acid, and the most preferred
transition metal ion coated onto the abrasive comprises iron. It is
preferred that at least 1%, more preferably 40-95%, even more
preferably 80-99+% of available surface sites on the abrasive be
occupied by the stabilizer and the catalyst. It is more preferred
that 80-99+% of available surface sites on the abrasive be occupied
by the stabilizer, where the activator is added after the
stabilizer.
[0022] The surface coverage of the surface modified abrasive can be
characterized using zeta potential measurement. For example, the
amount of surface coverage of boric acid on the silica surface can
be measured using a Colloidal Dynamics instrument, manufactured by
Colloidal Dynamics Corporation, 1-Knight Street, Building E8,
Warwick, R.I., 02886. The Colloidal Dynamics instrument measures
the zeta potential (surface charge) of the surface modified silica
particles. During the preparation of boric acid modified silica,
boric acid is added to the deionized silica particles, which
changes the zeta potential of the silica particle surface. After
reaching the full surface coverage, there is no change in the zeta
potential of the surface modified silica. From this titration curve
of zeta potential as a function of grams of boric acid to a given
amount of deionized silica, it is possible to measure the percent
surface coverage of boric acid on the silica surface. After
completing the reaction with boric acid, the surface coverage
achieved by reacting the boron-modified sol with the selected
transition metal salts (activators), and completion of the reaction
can also be determined from the zeta potential.
[0023] The amount of stabilizer is typically between 0.1% to 5% of
the weight of the stabilizer-coated particles. The amount of
activator is typically between 0.01% to 3% by weight of the weight
of the activator(and optionally stabilizer)-coated particles. In
one embodiment, the stabilizer covers more than 95%, preferably 98%
or more of the surface of the activator-coated abrasive, and the
total amount of activator in the slurry is between 0.2 ppm and 12
ppm, for example between about 3 ppm and 8 ppm. In some
embodiments, more activator is added to the abrasive, and then some
activator is removed by for example contacting the activator-coated
abrasive with an acid that will remove a portion of the activator.
The acid can then be separated from the abrasive, leaving on the
abrasive only the more tenaciously bound activator. In preferred
embodiments there is between 0 and 3, preferably between 0 and 1.5
ppm soluble activator. Advantageously the amount of
activator-coated abrasive is between 0.2% to 2%, for example
between 0.25% and 1%, where the slurry can further comprise between
0 and 2% activator-free abrasive.
[0024] The invention includes a method of polishing by using a
slurry comprising a fluid portion comprising: water and a per-type
oxidizer capable of forming free radicals such as hydroxyl radicals
in the slurry; and particles comprising a transition metal coating
thereon, such coating being exposed to the oxidizer that in turn
reacts with the transition metal-coated particles to create free
radicals in an amount useful for accelerating a chemical mechanical
polishing process. The preferred the preferred transition metal is
iron, the preferred abrasive is colloidal silica, and preferred
oxidizers are hydrogen peroxide, peracetic acid, or periodic acid.
Not all abrasive need contain transition metal. One embodiment uses
transition metal coated-silica having a diameter of about 0.07 to
about 0.09 microns, and uncoated silica of size about 0.06 to 0.08
microns in diameter. Advantageously, however, even abrasive without
the absorbed transition metal ions should have stabilizer absorbed
thereon. The polishing composition is useful at all commercially
useful pH values, e.g., from about 1.5 to 9. The preferred pH is
between about 2 and about 8, for example between 2.5 and 7, but is
typically between 3 and 6.5.
[0025] It is surprising that iron can operate at high pH values,
i.e., pH greater than 5, as conventional wisdom would suggest that
the iron would form inactive (and highly detrimental) hydroxides.
Fe(OH).sub.3 start to precipitate at pH 2.5-3, and substantially
completely precipitate at pH 3.7-4 when the concentration of the
iron ions Fe.sup.3+ is 0.001M or about 60 ppm. When the slurry is
prepared without for example a boric acid stabilizer,
advantageously the pH of the silica in deionized water is adjusted
to about 2 with nitric acid, boric acid, phosphoric acid, or the
like in order to cover the surface with SiOH group before adding
ferric ions into the slurry. Surprisingly the silica surface then
adsorbs Fe.sup.3+ cations very strongly even though at this pH the
SiOH groups would not normally be ionized. The pH of the
composition in use is advantageously between 3.5 and 6.5. Without
being bound by theory, we believe that since Fe.sup.3+ is very
insoluble at this pH 3.5, the iron ions remain permanently capped
on silica surface. Equally surprising, despite the pH being equal
to or greater than 3.5 (which means we should have Fe.sub.xOH.sub.y
or Fe.sub.xO.sub.y) we still see Fenton's reagent activity).
[0026] Generally, the addition of between 20 and 2000 ppm,
typically 50 ppm to 400 ppm by weight of an
.alpha.,.beta.-dihydroxy enolic compound such as ascorbic acid or
derivative thereof, is beneficial to stabilize the slurry
(providing a slurry shelf life of days to even a week or so).
Exemplary .alpha.,.beta.-dihydroxy enolic five member ring
compounds include:
4-Dihydroxymethyl-2,3-dihydroxy-cyclopent-2-enone;
4-(1,2-Dihydroxy-ethyl)-2,3-dihydroxy-cyclopent-2-enone;
3,4-Dihydroxy-5-hydroxymethyl-5H-furan-2-one;
3,4-Dihydroxy-5-(1-hydroxy-propyl)-5H-furan-2-one;
3,4-Dihydroxy-5H-thiophen-2-one; 3,4-Dihydroxy-5H-furan-2-one;
2,3-Dihydroxy-cyclopent-2-enone; and
3,4-Dihydroxy-1,5-dihydro-pyrrol-2-one. Exemplary six member ring
dihydroxy enolic compounds include 2,3-dihydroxy-1,4-benzenedione;
2,3-Dihydroxy-1H-pyridin-4-one; 2,3-Dihydroxy-thiopyran-4-one;
Tetrahydroxy 1,4-benzoquinone, and in its most simple form
2,3-Dihydroxy-cyclohexa-2,5-dienone or
2,3-Dihydroxy-cyclohex-2-enone. Exemplary seven member ring
dihydroxy enolic compounds include
2,3-Dihydroxy-cyclohepta-2,6-dienone and
5,6-Dihydroxy-1,7-dihydro-azepin-4-one. The most preferred
.alpha.,.beta.-dihydroxy enolic compounds are ascorbic acid and the
similar erythorbic acid, or derivatives and/or mixtures thereof.
Representative derivatives of ascorbic acid include, but are not
limited to, ascorbic palmitate; dipalmitate L-ascorbate; or
ammonium L-ascorbate-2-sulfate. Advantageously, if ascorbic acid
derivatives are used, the molarity of the ascorbic acid derivatives
should be about the same as the molarity of the above-described
ascorbic acid in a slurry. Alternatively, or additionally the
composition can comprise between 20 and 1000 ppm of stable alkynes,
for example alkyne diols (Surfynol 104E) that complex with iron
and/or copper ions. In certain embodiments of the invention, the
polishing composition can also include an alkyne compound having at
least one hydroxyl substituent, as disclosed in pending U.S. patent
application Ser. No. 10/315,398, filed Dec. 9, 2002. Alternatively,
or additionally, the composition may comprise between 20 and 1000
parts of citric, lactic, glycolic, and/or salicylic acid per part
of dissolved iron and/or copper.
[0027] This is considered the basic slurry, and the other
embodiments can be considered to be variants of this slurry.
[0028] 2. Slurry CMP Method--Essentially Pure Slurry: The invention
includes a method of polishing by using a slurry consisting
essentially of a fluid portion comprising: water; a per-type
oxidizer capable of forming free radicals such as hydroxyl radicals
in the slurry, and optionally one or more pH-adjusting additives in
an amount less than 0.3%; and particles comprising a transition
metal coating thereon, such coating being exposed to the oxidizer
that in turn reacts with the transition metal-coated particles to
create free radicals in an amount useful for accelerating a
chemical mechanical polishing process. The preferred transition
metal is iron, the preferred abrasive is colloidal silica, and
preferred oxidizers are hydrogen peroxide or periodic acid. The
composition may optionally comprise abrasive that is free of a
transition metal coating thereon. This embodiment recognizes the
value of not having any (alternatively less than 1000 ppm) of
organic material containing 3 or more carbon atoms, including
organic corrosion inhibitors, chelators, and organic acids. If
organic compounds are present, they are preferably in an amount
less than 1000 ppm, and more preferably they are selected from
ascorbic acid, alkyne diols, citric acid, lactic acid, glycolic
acid, and/or salicylic acid, or combinations thereof. In addition
to quenching free radicals, too much chelators can adversely effect
some oxide structure. This embodiment recognizes the value of not
having any dissolved transition metals (alternatively less than 10
ppm, more preferably less than 3 ppm, for example about 2 ppm or
less, prior to polishing), such as dissolved ferric ions and the
like.
[0029] Generally, we have found that the fewer additives added, the
better the system performance. But a small amount of chelators,
preferably citric, lactic, glycolic, and/or salicylic acid, can be
necessary under certain conditions. For example, at pH 5 the
solubility of the various tungsten by-products are poor and in the
absence of chelators there can be a poor finish on the wafer due to
re-precipitation. W removal rate can progressively get higher as
the pH increases, up to .about.5500+ angstroms/minute in the
presence of sufficient chelators. Complexing agents are not needed
at control the decomposition rate of the hydrogen peroxide in the
storage tanks and distribution lines for periods of time in the
range of hours to days. A small amount of certain stabilizers can
extend tank life to a week or more.
[0030] 3. Slurry CMP Method With Low Oxidizer Concentrations: As
discussed above, we have beneficially found that the slurries and
fluids of the present invention can achieve commercially acceptable
substrate removal rates with very low oxidizer concentrations. This
low-oxidizer-concentration embodiments reduce the absolute amounts
of undesired hydrogen gas that can be produced, reduce chemical
cost, reduce problems of exposure of workers and equipment to high
concentrations of these somewhat hazardous compounds, facilitate
neutralization of the oxidizer prior to disposal or even allow
disposal without neutralization. By commercial rates it is meant
over 1000, for example over 2000, angstroms per minute for
components such as tungsten, aluminum, copper, tantalum, iron,
nickel, and the like, and over 300, for example over 600, angstroms
per minute for noble metals. If iron is the transition metal
forming the coating on the abrasive, then as little as 1 ppm iron
(based on the weight of the polishing slurry) coated on silica can
provide a 100% increase in polishing rates. More typically, between
2 and 4 ppm iron activator is present at normal concentrations
(about 2% to about 5%) of hydrogen peroxide. Polishing rates are
for wafers polished under normal parameters. The invention includes
a method of polishing by using a slurry comprising a fluid portion
comprising: water; a per-type oxidizer capable of forming free
radicals such as hydroxyl radicals in the slurry; and particles
comprising a transition metal coating thereon, such coating being
exposed to the oxidizer that in turn reacts with the transition
metal-coated particles to create free radicals in an amount useful
for accelerating a chemical mechanical polishing process. The
preferred the preferred transition metal is iron, the preferred
abrasive is colloidal silica, and preferred oxidizer is hydrogen
peroxide or periodic acid or mixture thereof. Slurries of this
embodiment contain from about 0.2% to about 3%, for example from
0.5% to 2%, e.g., about 0.5% to about 1.5%, by weight of hydrogen
peroxide, peracetic acid, periodic acid, a persulfate compound, a
periodate compound, or a combination thereof, compared to the
weight of the fluid. At such low concentrations of oxidizer, it is
preferable to have between about 4 and 20 ppm of iron coated on
silica abrasive.
[0031] 4. Solution and Activator-Coated-Abrasive Pad CMP Method:
The invention includes a method of polishing by using a slurry
comprising a fluid portion comprising: water; a per-type oxidizer
capable of forming free radicals such as hydroxyl radicals in the
slurry; and a polishing pad or roll having particles comprising a
transit on metal ionic coating thereon, such coating being exposed
to the oxidizer that in turn reacts with the transition
metal-coated particles to create free radicals in an amount useful
for accelerating a chemical mechanical polishing process.
Advantageously the particles, coated to or embedded on or in the
pad, comprise silica. The polishing pads may have an abrasive
character, such that the abrasion is primarily by action of the pad
rather than by the coated particles. In such an embodiment, the
coated particles may be very small, e.g., between 5 and 100
nanometers in diameter, or have a hardness such that the particles
are not abrasive, or both.
[0032] 5. Slurry and Abrasive Pad CMP Method: The invention
includes a method of polishing by using abrasive pads, where the
pad is used with a slurry comprising particles having transition
metal ions thereon, and at least one oxidizer that reacts with the
transition metal-coated particles to create free radicals in an
amount useful for accelerating a chemical mechanical polishing
process. Advantageously the particles comprise silica. In such an
embodiment, the coated particles may be very small, e.g., between 5
and 40 nanometers in diameter, or in low concentration, e.g., 0.05%
to 0.5% by weight of the slurry, or have a hardness such that the
particles are not abrasive, or any combination thereof.
[0033] 6. Slurry CMP Method With Stable Slurry: The invention
includes a method of polishing by using polishing slurries
comprising transition metal-coated particles in a slurry with at
least one oxidizer that reacts with the transition metal-coated
particles to create free radicals in an amount useful for
accelerating a chemical mechanical polishing process, wherein the
slurry can be on average premixed at least 12 hours, or at least 24
hours, for example at least 96 hours, prior to use. This is
advantageous because it allows the operator to make larger batches,
thereby reducing mixing time. A problem in the art has been that
slurries deteriorate by losing oxidizer and often having resultant
pH shifts over time, so the operator can not readily depend on a
constant slurry activity. The very pure embodiments and the low
transition metal embodiments, and particularly the combination
thereof, are preferred embodiments for stability. The slurry of
this invention loses less than 2% (preferably less than 1%) of the
oxidizer initially present per hour, or has a pH change of less
than 0.04 pH units preferably less than 0.01 pH units) per hour, or
both. The more preferred slurry of this invention loses
considerably less than 0.5% of the oxidizer initially present per
hour, and has a pH change of less than 0.02 pH units per hour. Such
a slurry can advantageously have between 20 and 500 ppm, for
example between 50 ppm and 200 ppm, of a free radical quencher
therein. The preferred fee radical quencher is ascorbic acid,
though some or all of the ascorbic acid can be replaced by
equimolar amounts of lactic acid or other chelators.
[0034] 7. Slurry CMP Method With Very Small Amounts of Coated
Particles: The invention includes a method of polishing by using
polishing slurries comprising very low levels of transition
metal-coated particles in a slurry with at least one oxidizer that
reacts with the transition metal-coated particles to create free
radicals in an amount useful for accelerating a chemical mechanical
polishing process. The preferred transition metal is iron, the
preferred transition-metal-coated-particle comprises silica.
Generally, there is between 1% and 6% by weight of silica having
transition metal (e.g., iron ions or salts) on the surface thereof.
In this embodiment, the total amount of
transition-metal-coated-particle abrasive is between 0.01% and 1%,
preferably 0.1% to 0.6%, for example about 0.2% to 0.5%, based on
the weight of the slurry. We have previously stated that typically
transition metal coated particles typically have between 0.01% to
3% of iron thereon by weight of the particles. The slurry may or
may not contain additional abrasives. The slurries of this
embodiment preferably have small particles, between about 0.005 and
0.1 microns, typically 0.04 to 0.07 microns, in diameter. The total
amount of abrasive can be very small. In a preferred embodiment,
the amount of abrasive is between about 0.2% and 2%, for example
between about 0.4% and about 1%, and in one preferred embodiment
the amount of abrasive is between about 0.5% and about 0.8% by
weight of the slurry, where the abrasive comprises fumed silica,
colloidal silica, or a mixture thereof with a particle size of
between about 0.01 and about 0.2 microns.
[0035] 8. Slurry CMP Method With Very Small Silica Abrasive: The
invention includes a method of polishing by using polishing
slurries comprising very low levels of transition metal-coated
silica particles in a slurry with at least one oxidizer that reacts
with the transition metal-coated particles to create free radicals
in an amount useful for accelerating a chemical mechanical
polishing process, wherein slurries of this embodiment have very
small particles, between about 0.003 and 0.07 microns in diameter,
preferably between about 0.007 and 0.03 microns in diameter. The
preferred transition metal is iron, the preferred particle
comprises silica. In one embodiment, the transition metal ion
coated silica has an average diameter of about 0.03 to about 0.05
microns in diameter. Another embodiment uses silica with an uses
coated-silica having a diameter of about 0.005 to about 0.025
microns in diameter, where at least a portion of the particles are
in the form of loosely bound aggregates, chains of individual
particles, or combinations thereof.
[0036] 9. Slurry CMP Method With Sub-Monolayer Amounts of Bound
Transition Metal: The invention includes a method of polishing by
using polishing slurries comprising transition metal-coated
particles in a slurry with at least one oxidizer that reacts with
the transition metal-coated particles to create free radicals in an
amount useful for accelerating a chemical mechanical polishing
process, where the transition metal(s) present are present on the
particles in an amount less than is needed to form a monolayer the
surface of the coated particles. In a preferred embodiment the
transition metal(s) present are present on the particles in an
amount needed to form a monolayer on between 10% and 90%, i.e.,
about 25% to about 75%, of the surface of the coated particles. In
another embodiment the transition metal(s) present are present on
the particles in an amount needed to form a monolayer on between
0.1 and 9%, i.e., about 1 and about 5%, of the surface of the
coated particles. In each of these embodiments, the surface is the
outer surface of the particles, which can be obtained by absorption
techniques using material that will not penetrate pores in the
particles, or by observation by for example a microscope, an
electron microscope, or other means of a statistically significant
number of particles to determine an average outer surface area. The
amount of transition metal present depends on the particle size of
the coated particles. For example, slurry having 1% by weight of
substantially spherical silica particles having a monolayer iron
coating on 25% of the outer surface would have about 10 ppm of
bound iron if particles are 0.14 microns in diameter, about 17 ppm
of bound iron if particles are 0.08 microns in diameter, and about
50 ppm of bound iron if particles are 0.025 microns in
diameter.
[0037] 10. Slurry CMP Method With Very Small Amounts of
Particle-Bound Transition-Metal: Relatively large amounts of
transition metals can be found even with sub-monolayer coatings of
transition metals. However, it has been shown that extremely low
levels of bound transition metal, especially iron and especially in
combination with hydrogen peroxide, are beneficial. The invention
includes a method of polishing by using polishing slurries
comprising very low levels of transition metal, where the
transition metal is coated to particles, in a slurry with at least
one oxidizer that reacts with the transition metal-coated particles
to create free radicals in an amount useful for accelerating a
chemical mechanical polishing process, wherein the total amount of
transition metal associated with the surface of particles is less
than about 40 ppm by weight of the slurry, preferably less than
about 25 ppm by weight of the slurry. The preferred transition
metal is iron, the preferred transition-metal-coated-particle
comprises silica. The total amount of transition metal (preferably
iron) of the current invention coated on abrasive particles is
between 0.5 ppm and about 40 ppm, for example between about 1 ppm
and about 25 ppm, for example between 1 ppm and 9 ppm. Very
surprisingly a composition having as little as 0.1% of iron (by
weight of silica) coated on the silica, where the silica is present
on an amount of 0.3% by weight in the slurry, will provide a slurry
with only 3 ppm of activator iron, but when used in a slurry with
the oxidizer capable of producing free radicals will provide well
over a 20% increase in polishing rate than if the silica is
uncoated and iron free (even if there is 3 ppm iron in solution in
the slurry. In preferred embodiments of the invention, the slurry
may contain about 3 ppm activator absorbed onto an abrasive, or
alternatively the slurry may contain about 4 ppm activator absorbed
onto an abrasive, or alternatively the slurry may contain about 5
ppm activator absorbed onto an abrasive, or alternatively about 10
ppm activator absorbed onto an abrasive. Again, we use "absorbed
on", "coated on", and "associated with" interchangably, and each
term encompasses embodiments where the activator is for example
bound to a stabilizer, which is in turn bound to a abrasive
particle. In fact, we have surprisingly found that, at total
activator levels between about 3 ppm and about 10 ppm, that iron
added to stabilized silica provided greater substrate removal rates
than did a similar amount of iron coated on un-stabilized silica.
The reason is unclear. Similar amounts can be useful for copper
coated on an abrasive, though silver, titanium, and/or tungsten may
require somewhat higher concentrations. If mixed with equal amounts
of say copper ions coated on silica, then the amount of iron can be
further reduced, by about a third, from the above low levels.
Unlike dissolved metal co-oxidizer formulations, where very small
amounts of transition metal have little effect, applicants have
found that very small levels of these particular transition metals,
when coated to abrasive particles, greatly increase polishing
rates. Very low embodiments, having for example 0.5 ppm to 9 ppm,
have a large effect on the rate of substrate removal but contribute
very little dissolved metal to the fluid portion of the slurry. The
slurry may or may not contain additional, i.e., non-coated,
abrasives, but preferred embodiments contain non-coated abrasives
as well as coated abrasives, and the transition metal-coated
abrasive have between 10% and 75% of the outer surface covered by a
monolayer of the transition metal(s). The slurries of this
embodiment advantageously have abrasive particles of average size
between about 0.005 and 0.2 microns in diameter. Generally, the
slurries of this embodiment also have very low amounts of abrasive,
i.e., less than about 3%, for example between about 0.5 and 2% by
weight of the slurry.
[0038] 11. Slurry CMP Method To Reduce Hydrogen Production: One
problem facing operators using certain slurries, for example
hydrogen peroxide and iron, is hydrogen generation. Hydrogen is
extremely explosive and is lighter than air, allowing hydrogen to
accumulate in areas where one would ordinarily not expect gas to
accumulate. Preferred polishing methods to minimize hydrogen
production are to use two component formulations, the first
containing a non-metal-containing oxidizer or oxidizers of the
current invention and the second containing the coated panic less
admixing them at point of use. After point of use, the coated
particles are advantageously immediately, i.e., within a few tens
of minutes, separated from the liquid portion of the slurry. The
invention includes a method of polishing by using a slurry
comprising: a first portion comprising water, a per-type oxidizer
capable of forming free radicals such as hydroxyl radicals in the
slurry, and optionally one or more pH-adjusting additives; and a
second portion comprising water and particles comprising a
transition metal coating thereon, such coating being exposed to the
oxidizer that in turn reacts with the transition metal-coated
particles to create free radicals in an amount useful for
accelerating a chemical mechanical polishing process, and
optionally one or more pH-adjusting additives, wherein the first
and second components are mixed within an hour of being used,
preferably within a minute of being used, and is typically mixed a
few seconds prior to time of use, use being the time when the
slurry contacts the substrate in a manner such that chemical
mechanical polishing occurs.
[0039] In one embodiment, the particles are substantially separated
from the fluid portion of the slurry within a few tens of minutes
of time of use. In some embodiments, fluids have less than 5 ppm,
for example less than 2 ppm, of dissolved transition metals (other
than those polished from the substrate, and other than tin, which
can be a stabilizer) in any fluid portion of the slurry. In some
embodiments, the oxidizer is one or more of persulfates, periodic
acid, peracetic acid, and the like. In some embodiments,
compositions have 2% or less by weight of oxidizer (which may
include or be exclusively hydrogen peroxide), thereby limiting the
absolute amount of hydrogen generation possible from a slurry. This
limited oxidizer slurry is an important embodiment, limiting
hydrogen gas generation, as well as minimizing purchasing costs and
disposal costs, and with the method of the current invention
commercially acceptable substrate removal rates can be maintained.
In some embodiments, a chelator can be added at point of use or
even after point of use, in very small amounts such as less that
0.1% by weight, to de-activate dissolved metals. In some
embodiments, a component that is readily oxidized is added to the
fluid or to the slurry after polishing to consume excess oxidizer,
where the added compound is environmentally benign.
[0040] 12. Slurry CMP Method with Silicates and/or Aluminates
without Abrasive:
[0041] The invention includes a method of polishing by using
polishing slurries comprising transition metal-containing silicates
or aluminates, which may be at least partially formed into discrete
particulates. The material may be in the form of a suspended
sol-gel, where in a slurry with at least one oxidizer; the
activator metal-containing sol-gel reacts with the oxidizer to
create tree radicals in an amount useful for accelerating a
chemical mechanical polishing process. The preferred transition
metal is iron, and the preferred sol-gel comprises silicon.
[0042] 13. Slurry CMP Method With Shaped Abrasive Material: The
invention includes a method of polishing by using polishing
slurries comprising transition metal-coated particles in a slurry
with at least one oxidizer that reacts with the transition
metal-coated particles to create free radicals in an amount useful
for accelerating a chemical mechanical polishing process, wherein
slurries of this embodiment comprise cocoon-shaped silica particles
are colloidal silica with a minor axis of 10 to 200 nm and ratio of
major/minor axis of 1.4 to about 2.2 as described in U.S. Pat. No.
6,544,307. The preferred transition metal is iron, the preferred
particle comprises silica. The coated particles of this embodiment
can be substantially spherical (i.e., ratio of major to minor axis
is less than 1.2, preferably less than 1.1), or the coated
particles can be the cocoon-shaped particles, or both substantially
spherical and cocoon-shaped particles can have one or more
transition metals of this invention associated with the outer
surface thereof.
[0043] 14. Slurry CMP Method With Aggregated or Chain-like Abrasive
Material The invention includes a method of polishing by using
polishing slurries comprising transition metal-coated particles in
a slurry with at least one oxidizer that reacts with the transition
metal-coated particles to create free radicals in an amount useful
for accelerating a chemical mechanical polishing process, wherein
slurries of this embodiment have particles that are loosely or
tightly aggregated into groups, or are loosely or tightly connected
in chain-like structures. The individual particles are very small,
for example between about 0.003 and 0.05 microns in diameter. In
one embodiment the particles form aggregates of a plurality, i.e.,
more than about 11, individual particles in a roughly spherical
shape. In another embodiment the particles form chain-like
structures, which can be simply paired particles, but preferably
the chains comprise three or more particles, i.e., more than 4
particles in length and one particle in width. The preferred
transition metal is iron, the preferred particle comprises silica.
The structure can comprise individual particles having transition
metal associated on the surface thereof and particles that do not
have transition metal associated on the surface thereof
[0044] 15. Slurry CMP Method With A BiModal Distribution Of
Abrasive Sizes.
[0045] The invention includes a method of polishing by using
polishing slurries comprising transition metal-coated particles in
a slurry with at least one oxidizer that reacts with the transition
metal-coated particles to create free radicals in an amount useful
for accelerating a chemical mechanical polishing process, wherein
slurries of this embodiment have particles with sizes resulting in
at least a bimodal distribution of particle size distribution, that
is, wherein the sizes and distribution of particles present in the
slurry are sufficient to give a bimodal (or trimodal) distribution
of particle sizes where a modal distribution comprises at least 1%,
for example at least 10%, by count of the total particles. The
diameter ratio of the smaller to the larger particles is between
about 1:10 and about 10:1. For example, one preferred embodiment
uses coated-silica having a diameter of about 0.08 microns, and
uncoated silica of size about 0.07 microns in diameter, where the
particle size is tightly controlled so that about 90% (y count) of
the particles fall within 5% of the stated size (i.e., 90% by
number of the coated silica has a diameter between 76 and 84
microns, and 90% by number of the un-coated silica has a diameter
between 66 and 74 microns). In another embodiment, the coated
particles have a diameter of about 0.06 to 0.09 microns in
diameter, and the uncoated abrasive has a diameter of about 0.10 to
0.28, for example between about 0.11 to 0.16, microns in diameter.
Of course, the particles that are transition metal (of this
invention) coated can be the larger or the smaller of the
particles, or both. The particles may be of one type, i.e., silica,
or the slurry used for the CMP method may comprise a plurality of
types of abrasives, for example comprising at least two of a
colloidal silica, a fumed silica, ceria, alpha-alumina, a spinel,
gamma alumina, a beta alumina, titania, germania, and/or silicon,
aluminum, cerium, titanium, germanium carbide and/or nitrides, or a
mixture thereof.
[0046] 16. Slum CMP Method With A Plurality Of Types Of Abrasive
Material: The invention includes a method of polishing by using
polishing slurries comprising transition metal-coated (ion or salt)
particles in a slurry with at least one oxidizer that reacts with
the transition metal-coated particles to create free radicals in an
amount useful for accelerating a chemical mechanical polishing
process, wherein slurries of this embodiment have a plurality of
types of abrasive material. This does not mean simply that some
material is coated and some material is not coated--those
embodiments are discussed elsewhere. This does not mean simply a
bi-modal distribution of abrasive--those embodiments are discussed
elsewhere. Here, there are a plurality of types of abrasives in the
slurry used for the CMP method, for example comprising at least two
of a colloidal silica, a fumed silica, ceria, alpha-alumina, a
spinel, gamma alumina, a beta alumina, titania, germania, and/or
silicon, aluminum, cerium, titanium, germanium carbide and/or
nitrides, or a mixture thereof or one preferred embodiment, the
slur comprises fumed silica and colloidal silica, where the
colloidal silica is coated with transition metal, and the fumed
silica optionally has no transition metal coated thereon. In
another embodiment, colloidal silica that is coated with transition
metal is admixed in a slurry with ceria. The silica is not the only
particle that can be coated with the transition metal(s) of this
invention. In another embodiment, the slurry comprises alumina, for
example alpha-alumina, having transition metal coated thereon, and
a beta or gamma alumina, or ceria, or silica (or mixture thereof.
In another embodiment, the slurry comprises a spinel, e.g., an iron
spinel, having transition metal coated thereon, and a different
abrasive. The different types of abrasives typically, but need not,
have different sizes, giving a bimodal (or trimodal) distribution
of particle sizes in the slurry.
[0047] 17. Slurry CMP Method With An Alumina Abrasive Material: The
invention includes a method of polishing by using polishing
slurries comprising transition metal-ion-coated particles in a
slurry with at least one oxidizer that reacts with the transition
metal-coated particles to create free radicals in an amount useful
for accelerating a chemical mechanical polishing process, wherein
slurries of this embodiment comprise transition metal coated
alumina abrasive material. In one embodiment, the slurry comprises
alumina, for example alpha-alumina, having the transition metal(s)
of this invention coated thereon. Alumina was surprisingly found to
hold transition metals, i.e., iron, tightly. Alumina is useful for
different substrates, and for different pHs, as is known in the
art. The coated alumina of this invention react with hydrogen
peroxide, persulfates, periodic acid, peracetic acid, or the like
to produce from the oxidizer free radicals, i.e., hydroxyl free
radicals, which accelerate the substrate removal rate.
[0048] 18. Slurry CMP Method With A Transition Metal Coated Spinel
Abrasive Material: The invention includes a method of polishing by
using polishing slurries comprising transition metal-coated
particles in a slurry with at least one oxidizer that reacts with
the transition metal-coated particles to create free radicals in an
amount useful for accelerating a chemical mechanical polishing
process, wherein slurries of this embodiment have transition metal
coated spinel abrasive material. In one embodiment, the slurry
comprises an iron spinel material, having the transition metal(s)
of this invention coated thereon. In another embodiment, the slurry
comprises an magnesium spinel material, having the transition
metal(s) of this invention coated thereon. In another embodiment,
the slurry comprises an zinc spinel material, having the transition
metal(s) of this invention coated thereon. Of course, in another
embodiment, the slurry comprises an spinel material comprising at
least two of iron, zinc, and magnesium, the spinel material having
the transition metal(s) of this invention coated thereon.
[0049] 19. Slurry CMP Method With An Iron Spinel Abrasive Material:
The invention includes a method of polishing by using polishing
slurries comprising iron spinel abrasive particles in a slurry with
at least one oxidizer that reacts with the iron spinel particles to
create free radicals in an amount useful for accelerating a
chemical mechanical polishing process. The iron spinel may be
treated to increase formation of free radicals to a commercially
acceptable amount. Advantageously, the slurry comprises at least
one additional abrasive material. The embodiments of this invention
beneficially also comprise transition-metal coated abrasive
material.
[0050] 20. Slurry CMP Method With An Iron Oxide And/Or Copper Oxide
Abrasive Material: The invention includes a method of polishing by
using polishing slurries comprising abrasive particles of iron
oxide, copper oxide, or both in a slurry with at least one oxidizer
that reacts with the iron oxide and/or copper oxide to create free
radicals in an amount useful for accelerating a chemical mechanical
polishing process. Highly advantageously the iron oxide and/or
copper oxide abrasives are substantially completely
surface-modified with a stabilizer, such as a stabilizer comprising
B, Al, W, or P, most preferably boric acid-stabilizer. The Fe/Cu
oxide may be treated, e.g., by adding absorbed iron ions and/or
absorbed copper ions to the surface of the oxide, to increase
formation of free radicals to a commercially acceptable or
desirable amount. Advantageously, the slurry comprises at least one
additional abrasive material. Advantageously, the pH of the
slurries of this embodiment are kept at levels between about 3 and
about 6, for example between about 4 and about 5. The embodiments
of this invention beneficially also comprise transition-metal
coated abrasive material. Advantageously the size of the iron oxide
particles is small, for example between 20 nm and 80 nm in
diameter, Advantageously, the slurry comprises a silica or alumina
abrasive of size 50 nm to about 200 nm in (average) diameter.
[0051] 21. Slurry CMP Method With A Titanium Dioxide Abrasive
Material: The invention includes a method of polishing by using
polishing slurries comprising or consisting essentially of an
oxidizer capable of generating free radicals and titanium dioxide
abrasive particles having one or more transition metal ions coated
thereon in an amount sufficient to cover between 1% and 100% of the
available surface area. Advantageously the titanium dioxide
comprises a stabilizer, for example a boric acid stabilizer, on the
surface thereof. Beneficially, the titanium dioxide is coated with
a transition metal that reacts oxidizer to create free radicals in
an amount useful for accelerating a chemical mechanical polishing
process. Even a TiO.sub.2/Ti.sub.2O.sub.3 abrasive such as is known
in the art can be improved with surface modification by stabilizers
described herein. In other embodiments, another abrasive type is
also present in the slurry, and this other abrasive is coated with
a transition metal that reacts oxidizer to create free radicals in
an amount useful for accelerating a chemical mechanical polishing
process.
[0052] 22. Slurry CMP Method With Silver-Coated Abrasive Material:
The invention includes a method of polishing by using polishing
slurries comprising transition metal-coated particles in a slurry
with at least one oxidizer that reacts with the transition
metal-coated particles to create free radicals in an amount useful
for accelerating a chemical mechanical polishing process, wherein
the transition metal comprises silver. In preferred embodiments,
the oxidizer is a persulfate or peracetic acid, more preferably a
persulfate. Silver is more expensive than either copper or iron,
and has a more limited effective window, and is expected to have a
lower long-term affinity to the surface of the abrasive (as the
metal may form atoms of Ag.sup.0 on the surface of the abrasives
during the reaction with persulfates to form free radicals.
[0053] 23. Slurry CMP Method With Copper-Coated Abrasive Material:
The invention includes a method of polishing by using polishing
slurries comprising transition metal-coated particles in a slurry
with at least one oxidizer that reacts with the transition
metal-coated particles to create free radicals in an amount useful
for accelerating a chemical mechanical polishing process, wherein
the transition metal comprises copper ions absorbed onto the
surface of an abrasive. In preferred embodiments, the abrasive is
silica or alumina, and the oxidizer is a peroxide or periodic
acid.
[0054] 24. Slurry CMP Method With Cerium-Coated Abrasive Material:
The invention includes a method of polishing by using polishing
slurries comprising cerium ion-coated particles in a slurry with at
least one oxidizer that reacts with activator-coated particles to
create free radicals in an amount useful for accelerating a
chemical mechanical polishing process. Cerium ions are not
efficient at increasing the production of free radicals, so in more
preferred embodiments the particles further comprise absorbed iron
ions, absorbed copper ions, or both. The preferred abrasive
material comprises silica and/or alumina. The cerium ions are
absorbed onto the surface of the abrasive, and are advantageously
salts, and not oxides.
[0055] 25. Slurry CMP Method With Titanium-Coated Abrasive
Material: The invention includes a method of polishing by using
polishing slurries comprising transition metal-coated particles in
a slurry with at least one oxidizer that reacts with the transition
metal-coated particles to create free radicals in an amount useful
for accelerating a chemical mechanical polishing process, wherein
the transition metal comprises titanium. Importantly, the metal is
an ion absorbed onto the silica or alumina surface, and is not an
oxide. The preferred abrasive material comprises silica and/or
alumina.
[0056] 26. Slurry CMP Method With Plurality Of Transition Metal
Ion-Coated Abrasive Material: The invention includes a method of
polishing by using polishing slurries comprising transition
metal-coated particles in a slurry with at least one oxidizer that
reacts with the transition metal-coated particles to create free
radicals in an amount useful for accelerating a chemical mechanical
polishing process, wherein the transition metal comprises at least
two of iron, copper, cerium, titanium, and silver. An individual
particle may have a plurality of transition metals coated thereon,
or there may be a plurality of particles with some particles having
a first metal coated thereon and other particles having a second
metal coated thereon, or any combination of these. The preferred
abrasive material comprises silica and/or alumina. The preferred
transition metal ion combinations are iron and cerium or iron and
copper for all oxidizers, and iron and silver if the slurry
comprises a persulfate.
[0057] 27. Slurry CMP Method With Zero-Valent
Transition-Metal-Coated Abrasive Material: The invention includes a
method of polishing by using polishing slurries comprising
transition metal-coated particles in a slurry with at least one
oxidizer that reacts with the transition metal-coated particles to
create free radicals in an amount useful for accelerating a
chemical mechanical polishing process, wherein one or more of the
transition metal(s) coating the particle is present (at least
partially, and/or at least initially) in the zero valent state,
e.g., the abrasive may be formed from iron or copper metal
sputtered onto an abrasive.
[0058] 28. Slurry CMP Method With Sulfate-Stabilized Slurry: We
have found that sulfate ions can have a stabilizing influence on
for example a coated silica slurry by retarding settling. Without
being bound by theory, we believe the sulfate forms a stable
double-layer about the bound iron or other transition metal.
Sulfate can be present for example in an amount between about 30
and about 500 ppm sulfate, preferably between about 50 and 300 ppm
sulfate, for example between about 100 and about 200 ppm sulfate. A
sulfate level of 170 ppm can extend the time before particle
settling becomes significant from about 2 days to about 5 days. The
slurry can contain between about 0.05% and 5% by weight of
transition metal coated particles, e.g., silica having iron ions or
salt absorbed thereon.
[0059] 29. CMP Method With Dissolved Transition-Metal Promoter: The
invention includes a method of polishing by using polishing
slurries comprising a transition metal coated to particles, in a
slurry with at least one oxidizer that reacts with the transition
metal-coated particles to create free radicals in an amount useful
for accelerating a chemical mechanical polishing process, and
between about 2 and 50 ppm of dissolved transition metal ions, for
example dissolved iron. If promoters, e.g., dissolved iron salts,
copper salts, and/or cerium salts, are present in the slurry, then
advantageously the composition comprises between 2 and 20 parts by
weight of a five- or six-member-enolic ring-structure alpha,
beta-dihydroxy compound such as ascorbic acid, erythorbic acid, or
a similar number of moles of derivatives and/or mixtures thereof,
per part by weight of dissolved promoter metal. Alternatively, or
additionally, the composition can comprise at least a molar
equivalent of one or more stable alkynes, for example alkyle diols
(Surfynol 104E) that complex with dissolved promoter metal.
Finally, the composition may comprise between 2 and 20 parts of
lactic acid, citric acid, glycolic acid, and/or salicylic acid per
part of dissolved promoter metal. We believe ascorbic acid
stabilizes a slurry and helps prevent metal ion contamination of
the slurry. We believe the stable alkynes strongly bind to the
dissolved iron or copper, and may even increase the effectiveness
of the dissolved metals on promoting an increase in the polishing
rate. We believe lactic acid, citric acid, glycolic acid, and/or
salicylic acid (lactic acid being particularly preferred) are good
chelators. Of course, these same three components would also be
very useful on prior art ferric nitrate/hydrogen peroxide
compositions that do not have transition metal coated abrasives
therein. The preferred absorbed transition metal is iron, the
preferred transition-metal-coated-particle comprises silica. The
total amount of transition-metal on coated-particle abrasive is
between 0.1 ppm and about 40 ppm, for example between about 0.5 ppm
and about 25 ppm. The slurry may or may not contain additional
abrasives. The slurries of this embodiment have small particles,
between about 0.02 and 0.2 microns in diameter, preferably between
0.04 and 0.08 microns in diameter.
[0060] The preferred fluid composition for a periodic acid/soluble
ferric nitrate solution, especially for use in polishing tungsten,
has 1.5% to 2.4%, for example 1.8% to 2.2% of periodic acid; the pt
is 1.5 to 4, for example 2.8 to 3,5, if modest loss of the
dielectric TEOS is acceptable, though a pH of about 4 to about 8,
preferably about 5.5 to about 7, is preferred if greater
selectivity between the tungsten and the dielectric is desired. In
the absence of transition metal ion coated abrasive, the ferric
nitrate concentration should be between 0.01 and 0.05 weight
percent. With transition metal ion coated abrasive, the ferric
nitrate concentration can be between 0.001% and 0.01% (if less than
0.001% ferric nitrate the soluble ferric ions contribute too little
activity). The preferred abrasive would be between about 2 to 4
weight percent of either alumina or a mixture of fumed and
colloidal silica with between 30% and 70% of the silica being
colloidal. Even with ascorbic acid, lactic acid, and/or stable
alkynes to combine with the soluble ferric ions, this composition
will result in greater contamination than using coated silica
abrasive alone.
[0061] 33. A CMP system, either a slurry comprising a liquid phase
and suspended abrasive particles, or a combination of a liquid
phase and abrasive particles bound to a polishing pad, wherein the
abrasive particles are previously surface-modified with one or more
inorganic stabilizers selected from borate, tungstate, aluminate,
stannate, or titanate, most preferably borate, in an amount greater
than 70%, for example between 80% and 99%, alternatively between
about 90% to about 98%, of the available surface area of the
abrasive particle, the abrasive particles further comprising one or
more activator metals added to the stabilizer-modified surface of
the abrasive, wherein the combination of stabilizers and activators
are advantageously sufficient to cover at least 90% of the
available surface area, for example between about 98% and about
120% of the amount sufficient to cover the surface area of the
abrasive particles. If the abrasive particles are suspended in a
CMP slurry, then advantageously the total amount of activator is
advantageously between about 1 ppm and about 100 ppm, preferably
between about 2 ppm and about 20 ppm, for example between about 3
ppm and about 10 ppm, based on the weight of the slurry.
Advantageously, if the abrasive particles are suspended in a CMP
slurry, then the amount of activator-coated abrasive is between
about 0.1% to about 10%, for example between about 0.2% to about
4%, and in some embodiments between about 0.3% to about 2%, based
on the weight of the slurry. The liquid phase of the CMP slurry
comprises the oxidizer that reacts with the activator to form
oxygen-containing free radicals, and further advantageously may
comprise between 1 ppm and about 1000 ppm, for example between
about 50 ppm and 400 ppm, of a chelator, wherein the preferred
chelators include lactic acid and the like, dihydroxy enolic
compounds and the like, or mixtures thereof. Advantageously the pH
of the liquid phase is between 3 and 7. In the presence of
dihydroxy enolic compounds, advantageously the pH is between about
6 and about 6.5.
[0062] Various combinations of the above embodiments are also part
of his invention, as the embodiments were simplified to show Me
particular advantages of only one or a few aspects of that
embodiment. The key to this invention is providing transition metal
coating onto particles, where the transition metal coating reacts
with a oxidizer in a Fenton-type reaction to provide a
super-oxidizer, i.e., a hydroxyl radical, where the hydroxyl
radical (or progeny of the hydroxyl radical) thereafter reacts with
the substrate, resulting in increased substrate removal rates.
DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0063] The invention involves CMP slurry systems that incorporate a
transition metal, preferably iron, associated with a particle and
contacting a fluid, wherein the fluid when contacting the
transition metal produces free radicals. In a preferred embodiment
the transition metal associated with a particle does not require
any form of energy exterior to the slurry, such as actininc energy,
to be effective. Preferably, the effectiveness of the transition
metal associated with a particle is not appreciably affected, i.e.,
less than about a 20% increase in rate, due to any form of energy
exterior to the slurry, such as actininc energy and heating above
normal temperatures of for example ambient to 45 C. The transition
metal is preferably not in the form of an oxide, but rater as
absorbed oxidized ion.
[0064] Substrates
[0065] The invention is a CMP slurry and a method of
chemically-mechanically polishing a substrate. The key requirements
of chemically-mechanically polishing a substrate are 1) to obtain a
commercially acceptable removal rate of components of the substrate
where removal is desired (and different removal rates of other
components of the substrate); 2) to obtain a commercially
acceptable finish, i.e., smoothness and non-uniformity; and 3) to
not damage or otherwise contaminate the substrate such that the
substrate is unfit for its desired purpose or is only made fit
after extensive additional processing.
[0066] The substrate can be a metal, a crystal, a semiconductor, an
insulator, a ceramic, a thin film transistor-liquid crystal
display, a glass substrate a fused silicon substrate, thin films,
memory storage devices including memory disks, optical instruments
including lens, nanotechnology machines, finely machined components
including microscopic components, close tolerance machine parts, or
a read/write head, for example. The present invention is
particularly suitable for polishing very high density semiconductor
substrate and memory devices. The composition or slurry of this
invention may be used to polish at least one feature or layer on a
substrate such as a semiconductor substrate or any other substrate
associated with integrated circuit. The present invention is
particularly suitable for polishing all structures found on
integrated circuit chips, including for example shallow trench
isolation structures, LAN structures, silicon on insulator, deep
gate structures, and the like.
[0067] The composition and associated methods of this invention are
effective for CMP of a wide variety of substrates, including
substrates having dielectric portions that comprise materials
having low dielectric constants (low-k materials, such as materials
having a dielectric constant less than 3.3). The polishing slurries
herein are particularly suitable for use with tungsten in silica,
PETEOS, or low-k substrates. Suitable low-k films in substrates
include, but are not limited to, organic polymers, carbon-doped
oxides, fluorinated silicon glass (FSG), inorganic porous
oxide-like materials, and hybrid organic inorganic materials,
Representative low-k materials and deposition methods include:
MesoElk.RTM. available from Air Products and Chemicals, a spin-on
hybrid organic-inorganic material; Black Diamond.TM. available from
Applied Materials, a chemical vapor deposition (CVD) Carbon-doped
oxide; SiLK.TM., Porous SiLK.TM.available from Dow Chemical, a
spin-on organic polymer; NANOGLASS.RTM. E available from Honeywell
Electronic Materials, a spin-on oxide-like inorganic; and
CORAL.RTM. available from Novellus Systems, a Plasma enhanced
chemical vapor deposition (PECVD) deposited carbon-doped oxide.
[0068] Similarly, the composition and associated methods of this
invention are effective for CMP of substrates comprised of various
metals, including, but not limited to, tungsten and copper. The
composition and associated methods of this invention are
particularly useful and preferred for tungsten CMP and afford very
high selectivities for removal of tungsten in relation to
dielectric. In certain embodiments, the selectivity for removal of
tungsten relative to removal of the dielectric from the substrate
is at least 5:1, more preferably at least 10:1, and even more
preferably at least 15:1.
[0069] The substrate can comprise, consist essentially of, or
consist of any suitable metal. The metals for which the invention
is useful, in the semiconductor industry, include but are not
limited to those containing at least one of tungsten, titanium,
aluminum, copper, and tantalum. The very strong hydroxyl and
superoxide radicals formed during the CMP processes of this
invention are strong enough to promote polishing of most noble
metals, including silver, gold, platinum, iridium, ruthenium,
germanium, rhodium, palladium, d osmium. The metal can be pure, be
an alloy, or be a compound. By way of example, the composition of
the present invention may K used in the CMP of a substrate having
one or more layers of aluminum, coppers copper-aluminum alloy,
tantalum titanium, tungsten, or tantalum-, titanium-, or
tungsten-containing alloys, such as tantalum nitride, titanium
nitride, titanium tungsten, or other combinations thereof.
[0070] The substrate can comprise, consist essentially of, or
consist of any suitable metal oxide. Typical insulative structures
include dielectrics such as silica, alumina, organic silicas,
polysilicon, gallium arsenide, and others known in the art. The
substrate can include, alumino-borosilicate, TEOS, borosilicate
glass, phosphosilicate glass (PSG), borophosphosilicate glass
(BPSG), SOS (silicon on sapphire), silicon-germanium alloys, and
silicon-germanium-carbon alloys. The compositions described herein
are suitable for use on substrates having strained silicon,
polysilicon, single-crystal silicon, poly-crystalline silicon,
amorphous silicon, silicon-on-insulator, and the like.
[0071] Slurry
[0072] A principal embodiment of the invention is a composition for
chemical-mechanical polishing a semiconductor or memory device
substrate, comprising: a fluid comprising at least one per-type
oxidizer that produces free radicals, preferably an Oxygen/Hydroxyl
Free Radical, when contacted with at least one activator, wherein
the activator is one or more transition metals that promote
production of the free radicals, and wherein the activator is
attached to a particle (a "coated particle" or "transition metal
coated particle"), preferably an abrasive particle, such coating
being exposed to the oxidizer that in turn reacts with the
transition metal-coated particles to create oxygen/hydroxyl free
radicals in an amount useful for accelerating a chemical mechanical
polishing process, and wherein the composition when used in a
chemical mechanical polishing process will remove desired metal but
will not create defects or nonuniformity such that the substrate
can not undergo further fabrication to become a finished operable
semiconductor or memory device.
[0073] The invention eliminates the need for adding soluble metal
catalysts to catalyze oxidation by hydrogen peroxide (but does not
necessarily exclude the use of soluble metal catalysts in certain
embodiments). The invention improves the CMP process as well as
minimizes contamination, as there is no need to add soluble salt
catalysts in the CMP formulation. Other advantages provided by at
least some embodiments of the invention include: (a) relatively low
CMP slurry costs; (b) no staining or yellowing of the polishing
pad; (c) low metal ions (or other counter-ions of metal salt
catalysts) as impurities in the CMP slurry, (d) employing commonly
available hydrogen peroxide or other popular oxidizers; and (e) low
static etch rate of metal substrates.
[0074] The first embodiment described in the summary of the
invention is a general depiction of a preferred slurry that is
useful in carrying out one or more embodiments of the invention. In
less preferred variants of the invention, the particles have little
or no stabilizer absorbed thereon. Each subsequent method,
material, and amount described herein are applicable to each of the
other embodiments to the extent the other embodiments do not
exclude that particular method, material, and/or amount.
[0075] The invention includes a method of polishing by using a
composition comprising movably contacting a substrate to be
polished with the fluid-that-produces-the-free-radicals and with
the coated particles, preferably by having these components be
urged against the substrate by a polishing pad that is movably
contacting the substrate, wherein the fluid and coated particles
are disposed between the substrate and the polishing pad.
[0076] The term "Oxygen/Hydroxyl Free Radical" means a free radical
that contains only oxygen and optionally hydrogen, and include the
hydroxyl radical, Atomic oxygen (singlet) radical, superoxide
radical, and the like. The useful free radicals of this invention
have an electrochemical potential of between about 1.6 V and 3 V,
and preferred free radicals have a potential greater than about 2.4
V. As used herein, the term "free radical" is used interchangably
with "Oxygen/Hydroxyl Free Radical." There are a plurality of
radicals formed of organic constituents, which may or may not
contain oxygen. These radicals formed of organic constituents do
not have sufficient electrochemical potential to oxidize the
substrates of interest. As used herein, the term "free radical" and
interchangably "Oxygen/Hydroxyl Free Radical" specifically does not
encompass free radicals containing organic components.
[0077] The oxygen/hydroxyl free radical is advantageously formed by
action of the transition metal coated onto a particle and the
oxidizer. The oxygen/hydroxyl free radical must be of sufficient
electrochemical potential to oxidize the substrate. The
oxygen/hydroxyl free radical must be formed in an amount sufficient
to substantially increase substrate removal rate, for example by at
least 10%, preferably at least 30%, and typically at least 50%,
compared to the same polishing system but without the transition
metal coated to the particle (but having an equivalent amount of
abrasives),
[0078] Transition Metal Activator
[0079] In every embodiment there is a selected transition metal
activator associated with a particle. As used herein, the phrase
"transition metal" only encompasses those metals which when
attached to or absorbed on a solid particle react with per-type
oxidizers to form oxygen/hydroxyl free radicals. The transition
metal must be one of the select few metals that will initiate a
Fenton-type free radical generation-type reaction with the selected
fluid, especially at 25.degree. to 45.degree. C. Note that the
Fenton's reaction is not a catalytic process where an oxidizer is
made to react faster with a substrate. Rather, the Fentons reaction
generates a different oxidizer, preferably a oxygen/hydroxyl free
radical, that has a greater oxidation potential than the oxidizers
in solution. Without being bound by theory or process, we believed
that the generated free radicals may react with the slurry as
follows (where free radicals are underlined):
Fe.sup.+2+H.sub.2O.sub.2.fwdarw.Fe.sup.+3+OH.sup.-+.degree.OH
.degree.OH+Fe.sup.+2.fwdarw.OH.sup.-+Fe.sup.+3
.degree.OH+H.sub.2O.sub.2.fwdarw.H.sub.2O+.degree.OOH
.degree.OOH+H.sub.2O.sub.2.fwdarw.O2+H.sub.2O+.degree.OH
Note that a hydroxyl radical can be quenched by ferrous ions, which
is yet another reason to minimize the concentration soluble iron
salts. A few reactions of Fenton's reagents coated on abrasives
with periodic acid are as follows:
H.sub.4IO.sub.6.sup.-+Fe.sup.+3.fwdarw.H.sub.3IO.sub.5.sup.-+.degree.OH+-
Fe.sup.+2
H.sub.3IO.sub.5.sup.-+Fe.sup.+2.fwdarw.IO.sub.3.sup.-+H.sub.2O+.degree.O-
H+Fe.sup.+3
Advantageously, hydroxyl radicals can be obtained both by the
conversion of Fe.sup.+2 to Fe.sup.+3 and by the conversion of
Fe.sup.+3 to Fe.sup.+2.
[0080] The transition metal (unless otherwise stated, the term
"transition metal" is limited to activators) is generally an ion,
and are beneficially multivalent ions. Unless otherwise specified,
the transition metal is present in an oxidized state, for example
ferric or ferrous iron. Unless otherwise specified, the oxidized
transition metal is not present as an oxide. Oxides have less
activity than absorbed activators and often require actinic energy
to be effective. Rather, the metal is present absorbed onto the
particle as a salt, likely with counterions such as sulfate near
the surface.
[0081] We have found that transition metal activator(s) of this
invention that are associated with solids, for example an abrasive,
a particle, or a pad, can initiate the creation of free radicals
without the undesirable side effects such transition metals may
have if they are in solution in the fluid contacting the substrate.
In particulars we have surprisingly found that transition metal
activators) of this invention associated with the surface of a
solid are effective at promoting the formation of free radicals,
but these transition metal-containing activators are not "in
solution" and therefore do not contaminate the substrate as much as
does soluble metal ion oxidizers. Further, we have surprisingly
found that the transition metal(s) of this invention so associated
with the surface of the solid do not cause significant degradation
of the hydrogen peroxide or of the oxide when admixed for a period
of at least several hours, often a day or more, which is a typical
storage time in semiconductor fabrication plants. A transition
metal activator that is "associated with", "absorbed" on, or
"coated" on (these phrases are used interchangably here) an
abrasive means the metal is not in solution in the slurry. Unless
otherwise stated, a transition metal associated with a surface is a
metal ion, wherein the metal is selected from the metals presented
herein, wherein the metal is in the form of an ions and not in the
form of an oxide, nitride, or carbide, or a metal. Metallic
(zero-valent) coatings of these metals can be used in limited
circumstances, though they are of limited utility and
effectiveness, and are often not specific enough for the desired
Fentons reaction promotion. The only metal oxide we found to be
effective was iron oxide--though we believe copper oxide may also
have some limited activity.
[0082] One method of expressing the concentration is that a
preferred transition metal-coated particle has between 2 and 100%
of the outer surface, i.e., that surface accessible to for example
mercury at 5 psi absolute pressure, covered with a monolayer or
bilayer of the transition metal(s). It is recognized that many
abrasives and particles have substantial porosity, and substantial
accompanying surface area. However, surface area deep within a pore
is of limited utility if free radicals generated therein do not
migrate out within a time span similar to the expected life of the
free radical-propagation chain. The most important surface area is
the outer surface area, that is, that surface readily visible to a
microscope or an SEM. The invention includes a method of polishing
by using polishing slurries comprising transition metal-coated
particles in a slurry with at least one oxidizer that reacts with
the transition metal-coated particles to create free radicals in an
amount useful for accelerating a chemical mechanical polishing
process, where the transition metal(s) present are present on the
particles in an amount less than is needed to form a monolayer the
surface of the coated particles. In a preferred embodiment the
transition metal(s) present are present on the particles in an
amount needed to form a monolayer on between 10% and 90%, i.e.,
about 25% to about 75%, of the surface of the coated particles. In
another embodiment the transition metal(s) present are present on
the particles in an amount needed to for a monolayer on between 0.1
and 9%, i.e., about 1 and about 5%, of the surface of the coated
particles. For example, slurry having 1% by weight of substantially
spherical silica particles having a monolayer iron coating on 50%
of the outer surface would have about 20 ppm of bound iron if
particles are 0.14 microns in diameter, about 35 ppm of bound iron
if particles are 0.08 microns in diameter, and about 100 ppm of
bound iron if particles are 0.025 microns in diameter.
[0083] Metal ions in solution, which we call "promoters" to
distinguish them from transition metals coated on an abrasive, will
cause degradation of a slurry, and will absorb onto and contaminate
a substrate. One added benefit of having activator on a surface of
an abrasive is if chemical reactions occur to cause the activator
to be reduced to a metallic state, the activator will still not
move from the surface of the abrasive, and therefore will not plate
out on the substrate. Additionally, activator associated with an
abrasive has a much lower tendency to spontaneously decompose
certain oxidants, for example hydrogen peroxide, even at higher pH
values where hydrogen decomposition by metal ions in solution is
known. While not being bound by theory, generally, an activator
associated with an abrasive is believed to only incidentally
contact the substrate.
[0084] Fenton's has a very limited number of metal activators.
Generally, ions of Cu, Fe, Mn, Ti, W and V are both physically
useful and not environmentally prohibitive. Silver is a useful
activator for many systems, and can be coated onto for example
silica, ceria, alumina, and other known abrasives. Silver is less
preferred, because silver is believed to be less tenacious to a
surface--as silver changes oxidation states, it may under some
conditions become un-associated from the solid material.
Additionally, the cost of silver is prohibitive unless
recovery/recycle systems are in place. Finally, silver ions can
complicate disposal of used slurry. Selected other metals promote
the generation of free radicals from the oxidizer capable of
forming the free radicals, but generally because they are
inefficient and also have one or more of high cost, toxicity,
and/or stringent environmental disposal regulations, that the use
of these other ions is not commercially advantageous.
[0085] The preferred transition metal species is iron. The iron can
be associated with the abrasive in the form of a salt, for example
a ferric salt, a ferrous salt, or both. Iron associated with an
abrasive is particularly useful and is the most preferred
activator. Iron associated with silica is the preferred system. The
silica, with its numerous OH groups, can multiply bind with the
iron, holding the iron firmly associated with the silica by a
number of covalent and/or ionic type bonds. Yet, the plurality of
bonds of iron onto the silica, be it absorbed, adsorbed, or coated,
allows easy transformation between oxidation states without the
iron having a tendency to dis-associate from the silica surface.
Surprisingly, iron associated with silica can be used at high pH
values, for example from pH 5 to pH 7 and in some cases up to pH 8.
It is known that soluble iron at these pH values forms undesirable
precipitates which contaminate substrate and which catalyze
degradation of hydrogen peroxide into oxygen and water, resulting
in unsafe explosive accumulations of gases. An additional advantage
of iron is that it is environmentally benign and does not pose
significant disposal problems. Absorbed iron ions associated with
alumina, ceria, and/or spinel are also useful abrasive/activators.
Still further provided is a composition comprising an abrasive
having a surface on which at least one stabilizer and iron ions are
bonded, wherein the abrasive is a member selected from the group
consisting of alumina, titania, zirconia, germania, silica, ceria
and mixtures thereof, the at least one stabilizer comprises at
least one member selected from the group consisting of B, W and
Al.
[0086] Iron ions associated with a surface of a metal oxide
abrasive, e.g., alumina, ceria, titania, or most preferably silica,
are useful embodiments. Many advantages of this system are
discussed in co-owned pending applications. A more preferred
embodiment uses a surface-modified abrasive modified with at least
one stabilizer and at least one activator metal differing from the
at least one stabilizer. Generally, inorganic stabilizers are
preferred over organic (carbon-containing) stabilizers. Iron
associated with stabilized silica, e.g., silica treated with a
boron-containing stabilizer which is absorbed or attached onto the
surface of the silica, is very useful. For silica, iron ions
absorbed onto a borate-stabilized silica is the preferred
system.
[0087] In preferred embodiments the quantity of activator iron,
that is associated with the surface of the abrasive, is about 1 to
about 200 ppm. These embodiments presume the transition metal is
primarily the most effective iron species. Higher concentrations,
for example two to ten times higher, may be needed for copper
and/or silver. Even with the efficient iron species, larger ppm
numbers can be used, for example up to 2000 ppm, but the higher
concentrations result in deterioration of a slurry comprising
certain oxidizers if stored over time, and also greater levels of
contamination of the substrate by activator ions, Any value below
500 ppm is preferred, though below 200 ppm is more preferred, and
below 100 ppm is most preferred. This ppm value is parts by weight
of the slurry, wherein the transition metal concentration is the
value obtained if the solids containing the transition metal are
separated out, leached of all surface-coated metal by for example
immersing the particles in excess heated aqueous ascorbic acid for
a time necessary to remove substantially all the bound transition
metal from the particle, and then analyzing the metals content of
the acid. Therefore, a low level of surface-bound iron is
preferred, providing the iron is present in an amount necessary to
create the desired amount of hydroxyl free radicals. Surprisingly,
however, even very low levels, for example between about 0.3 ppm
and about 8 ppm, alternately between about 0.5 ppm and about 4 ppm,
of particle-bound iron based on the weight of the slurry, provides
measurable and economically significant increased metal removal
rate that is believed due to the formation of free radicals. The
total amount of transition metal of the current invention coated on
abrasive particles is between 0.1 ppm and about 40 ppm, for example
between about 0.5 ppm and about 25 ppm, for example between 1 ppm
and 15 ppm. The amounts of iron in this embodiment can alternately
range from about 0.5 pp to 50 ppm, preferably from about 1 ppm to
about 30 ppm, for example between about 2 ppm and about 10 ppm, or
alternately from about 10 ppm to about 25 ppm. Similar amounts can
be useful for copper coated on an abrasive, though silver may
require somewhat higher concentrations.
[0088] Copper is a useful Fenton's agent, and therefore copper ions
associated with solids makes an excellent activator. As copper can
shift from a cuprous and cupric oxidation states. The copper can be
associated with the abrasive in the form of a salt, for example a
cupric salt, a cuprous salt or both. In alternate embodiments of
the invention, at least a portion of the copper can be a copper
oxide.
[0089] Absorbed titanium ions may also be useful in promoting the
generation of free radicals from the oxidizer. Absorbed tungsten
ions may also be useful in promoting the generation of free
radicals from the oxidizer.
[0090] Absorbed cerium ions also may be useful. Cerium may promote
the generation of hydroxyl radicals from hydrogen peroxide, but if
so it is not believed to be efficient. CeO.sub.2 does not
sufficiently promote a Fenton's reaction. Cerium is nevertheless a
preferred ion, especially in an absorbed state which will prevent
the cerium from forming cerium oxide. Cerium salts, be they
absorbed, adsorbed, or coated onto a solid, are useful. Like iron,
these ions can be strongly held by the active sites on the abrasive
and/or particle, and once absorbed, adsorbed or coated, do not tend
to become un-associated with the particle. Absorbed titanium ions
may promote the Fenton-like reaction to create free radicals,
though titanium oxides are not included herein as an effective
activator. If absorbed cerium and/or titanium ions are used, they
are beneficially included with a preferred activator metal such as
iron activator to achieve the desired substrate removal rates.
[0091] The invention includes a method of polishing by using a slum
comprising a fluid portion comprising: water; a per-type oxidizer
capable of forming free radicals such as hydroxyl radicals in the
slurry, and particles comprising an activator metal, e.g., iron,
coated thereon, such activator being exposed to the oxidizer that
in turn reacts with the activator-coated particles to create free
radicals in an amount useful for accelerating a chemical mechanical
polishing process. Simply admixing commercially available abrasive
with soluble activator metal ions will not result in
activator-coated abrasive. One method of forcing the activator ions
to absorb on the abrasive particle is using an ion exchange method.
An activated ion exchange material, preferably an acid-state ion
exchange material, is added to a slurry of the abrasive.
Advantageously the abrasive is in deionized water, but with
sufficient ions (often potassium ions) to provide a stable slurry.
The pH of the water is maintained such that the activator salt
would be soluble therein, and a soluble salt of the activator ion.
While an acetate salt is preferred, any soluble salt including a
nitrate salt can be added. Advantageously the composition is
heated, for example to >40.degree. C., and the slurry is
agitated to increase the kinetics of activator absorption on the
surface of the particle. The preferred the preferred transition
metal is iron, the preferred abrasive is colloidal silica, and
preferred oxidizer is hydrogen peroxide or periodic acid or mixture
thereof. A coated silica abrasive, for example, can be colloidal
silica, fumed silica, silica fumed, a silica admixed with one or
more adjuvants into a multicomponent particle such that the level
of silica in the particle surface, excluding absorbed iron and
other metals, is at least 20 percent, preferably at least 50%, or
mixtures thereof. We have found, however, that colloidal silica is
superior at absorbing the iron onto its surface and holding the
iron onto the surface during CMP processes.
[0092] An alternate method by which the activator metal can be
coated onto the particle is by growing the particle, i.e. by
precipitation, in the presence of the activator metal ions. A
preferred method of manufacturing colloidal silica particles having
the transition metal, e.g., iron, attached thereon is to grow
silica particles in the presence of iron ions. This method can
include dissolving silica, and then reprecipitating the silica. An
advantage is the transition metal can be incorporated deeper into
the silica structure, that is, for example, areas of the particle
exposed to the iron by dissolution under conditions where iron can
absorb onto the particle. Only the transition metal on the outer
surface is useful--transition metal coating must contact the
oxidizer to be effective.
[0093] Stabilizer Surface-Modified Abrasive
[0094] The preferred basic slurry comprises stabilized abrasive. As
used herein, the term "stabilizer" means an agent effective to help
maintain the abrasive as a sol in an aqueous medium. Suitable
stabilizers include metals and borderline metals, such as e.g.,
boron, aluminum, tungsten, and/or titanium, with boron being most
preferred Phosphorus is another useful stabilizer. Various
carbon-containing silanes and silanols can be used as a stabilizer.
Various organic chelating compounds can be used both as a
stabilizer and as a binding site for activator ions, if bound to
the surface of an abrasive particle. For example, polyvinylpyridine
polymers are useful for stabilizing the abrasives from coagulating
and also for immobilizing iron and copper as PVTY complexes on
silica surface, and the "coated" activator atoms had excellent
activity.
[0095] Generally, inorganic stabilizers are preferred over organic
(carbon-containing) stabilizers. One aspect of the invention is a
composition comprising an abrasive having a surface on which at
least one inorganic stabilizer and at least one activator are
bonded, wherein the abrasive is advantageously a member selected
from the group consisting of alumina, titania, zirconia, germania,
silica, ceria and mixtures thereof, the at least one stabilizer
comprises at least one member selected from the group consisting of
P, B, W, Al or mixture thereof, and the at least one activator
comprises at least one member selected from the group consisting of
Cu, Fe, Mn, Ti, W, V, and mixture thereof, provided that the at
least one stabilizer and the at least one catalyst are not
simultaneously W. A plurality of activator metals, preferably metal
ions, can be absorbed or coated on an abrasive, or a plurality of
abrasives in a slurry can have different activator metals absorbed
thereon, or a plurality of abrasives can have different amounts of
activator absorbed thereon. Similarly, a plurality of stabilizers
can be absorbed or coated on an abrasive, or a plurality of
abrasives in a slurry can have different stabilizers absorbed
thereon, or a plurality of abrasives can have different amounts of
stabilizer absorbed thereon.
[0096] Advantageously all the abrasive in a slurry, or
alternatively all the exposed abrasive on a polishing pad, or both,
comprise stabilizer. Some embodiments of the invention comprise
some stabilized abrasive and some abrasive that comprises less
stabilizer, or even no stabilizer. The surface coverage of the
surface modified abrasive can be characterized using zeta potential
measurement. It is possible to deduce a point of 100% surface area
coverage of an abrasive particle by a stabilizer, by suspending the
abrasive in a concentrated solution of excess stabilizer under
conditions where stabilizer is absorbed onto the particle, and
monitoring the zeta potential. Once the zeta potential is stable
and unchanging, the abrasive particles are assumed to have 100%
surface area coverage. For example, the amount of surface coverage
of boric acid on the silica surface can be measured using a
Colloidal Dynamics instrument, manufactured by Colloidal Dynamics
Corporation, 11-Knight Street, Building E8, Warwick, R.I., 02886.
The Colloidal Dynamics instrument measures the zeta potential
(surface charge) of the surface modified silica particles. During
the preparation of boric acid modified silica, boric acid is added
to the deionized silica particles, which changes the zeta potential
of the silica particle surface. After reaching the full surface
coverage, there is no change in the zeta potential of the surface
modified silica. From this titration curve of zeta potential as a
function of grams of boric acid to a given amount of silica, it is
possible to measure the percent surface coverage of boric acid on
the silica surface. After completing the reaction with boric acid,
the surface coverage achieved by reacting the boron-modified sol
with the second metal salt can also be determined from the zeta
potential.
[0097] Even with 100% surface coverage, it appears possible to add
activator ions onto the surface of the particle, especially in the
low concentrations (less than 100 ppm total activator absorbed on
0.3% to 4% activator-coated abrasive in a slurry) which are
preferred variants of the invention. Advantageously the abrasive
particles have sufficient stabilizer thereon to provide at least 1%
surface coverage, for example at least 10% surface coverage, more
preferably at least 40% surface, and for best stability of the
slurry at least 80% surface coverage. In an alternate embodiment,
it is preferred that at least 10%, more preferably 40% to 99%, even
more preferably 80% to more than 99% of available surface sites on
the abrasive be occupied by the stabilizer and the catalyst. The
percentage of surface sites covered on an abrasive in a composition
of this invention can range up to 100%.
[0098] While most of the useful absorbed or bound stabilizer and
activator are on the exterior surface of an abrasive particle, it
can be useful to have more of the interior surface area, present as
a result of porosity in the abrasive grain, to be coated with
stabilizer. It may be technically and commercially useful to use an
abrasive, such as an alumina abrasive, of sufficient porosity such
that the total of the activator and the non-aluminum-containing
stabilizer equal more than one percent by weight of the weight of
alumina. The molar ratio of activator to stabilizer can vary
depending upon the substrate, the nature and quantity of any
oxidizing agent being used, and the desired substrate removal rate.
For example, the molar ratio of activator to stabilizer can range
from 1:1 to 1:1000, more typically from 1:1 to 1:00, for example
from 1:10 to 1:100.
[0099] Colloidal abrasive particles that are smaller and which
consequently have less surface area generally require higher
relative amounts of stabilizer than do larger particles. As a
non-limiting illustrative example for boric acid surface-modified
colloidal silica, without any activator ions attached or absorbed
thereon, the various sizes of colloidal particles required the
approximate levels of boric acid modification as shown in Table 1
in order to achieve good stability towards gel formation in acidic
media, such as metal CMP polishing compositions.
TABLE-US-00001 TABLE 1 Boric Acid Particle Diameter (moles of boric
acid)/ % Surface (Nanometers, nm) (moles of silica) Modification*
12 0.08 92 23 0.06 95 50 0.043 98 100 0.02 99 *Based on 100%
surface modification with excess stabilizer.
[0100] Exemplary methods of obtaining stabilizer-modified abrasive
are now described. Boron surface-modified colloidal silica having
an average particle diameter of 50 to 60 nanometers, as measured by
Capillary Hydro-Dynamic Flow using a Matec Applied Sciences model
number CHDF 2000 instrument (a preferred method of measuring
particle size is the average particle size is >20 nanometers),
was prepared using the following procedure. An acid-state ion
exchange resin is activated by for example being washed with 20%
sulfuric acid solution and rinsed with deionized water. The
acid-state resin is then added to an aqueous slurry of silica, for
example alkali-stabilized silica, until the pH had dropped to
pH<3, for example a pH of .about.2. The slurry is then
advantageously allowed to react (preferably with agitation), and
then the resin is (and optionally the liquid) are advantageously
removed. The silica is then contacted with aqueous boric acid,
e.g., 0.2% to saturated, but preferable 1% to 5%, for a sufficient
amount of time and preferably with agitation to obtain the desired
level of surface modification. The modification process can be
shortened to five hours or less (often 10 minutes or less) by
heating the slurry to a temperature greater than 40.degree. C., for
example between 55.degree. C. to 60.degree. C. If there is excess
boric acid in the water, it may be advantageous to remove the
boric-acid-containing water and rinse the silica with deionized
water. The product is boron-surface-modified colloidal silica.
[0101] Potassium aluminate surface-modified colloidal silica having
an average particle diameter of 50 to 60 nanometers was prepared
using the following procedure. As described above, an activated
acid-state ion exchange resin is prepared, and added to silica, for
example potassium-stabilized silica until the pH had dropped to pH
of 2.5. The acid-state resin is then added to an aqueous slurry of
silica, for example alkali-stabilized silica, until the pH had
dropped to pH<3. The slurry is then advantageously allowed to
react (preferably with agitation), and then the resin is (and
optionally the liquid) are advantageously removed. The silica is
then contacted with aqueous potassium aluminate, e.g., 0.2% to
saturated, but preferable 1% to 3%. To promote absorption,
potassium hydroxide (at a concentration of 1%) can be then added,
and the slurry allowed to react (preferably with agitation) for a
sufficient amount of time to obtain the desired level of surface
modification. The modification process can be shortened to five
hours or less (often 10 minutes or less) by heating the slurry to a
temperature greater than 40.degree. C., for example between
55.degree. C. to 60.degree. C. If there is excess boric acid in the
water, it may be advantageous to remove the boric-acid-containing
water and rinse the silica with deionized water. The product is
aluminum-surface-modified colloidal silica.
[0102] The activator can be added to the abrasive particle before,
during, or after modifying the abrasive with stabilizer. This is
not to say that the end result will be equal. Recent data has shown
that, at least at activator levels in the slurry below about 10
ppm, that iron added to stabilized silica is significantly more
effective than iron added to unstabilized silica, so that to obtain
equal polishing rates between about 20% and about 50% or more iron
is needed in slurries where the iron was coated onto un-stabilized
silica. We are uncertain whether there is synergy between the
activator iron and the stabilized silica, or whether iron bound to
a boron stabilizer is more active than and/or more exposed than
iron bound to silica. To add activator to stabilizer-modified
abrasive, it is only necessary to contact the surface-modified
abrasive with an aqueous solution of the activator for a period of
time, and advantageously at elevated temperature (e.g., greater
than 40.degree. C.). To add activator at the same time the
stabilizer is being added, the following process can be followed.
As described above, an activated acid-state ion exchange resin in
water is prepared (pH<3), admixed with silica, and soluble
activator-metal salts are added to the composition in a
concentration between for example 0.01% to 0.5%. If the desire is
to place the activator on the abrasive first, this composition can
be allowed to react for a sufficient amount of time to add the
activator ions to the abrasive. An aqueous solution of sodium
tungstate is added under agitation and the pH was adjusted to pH 5.
The mixture is allowed to react (preferably with agitation), and
the result is tungsten-surface-modified activator-coated
abrasive.
[0103] There are significant advantages to using stabilized
abrasives as the carrier for the activator metal. First, as will be
described in the examples, the activator metal is relatively easy
to absorb onto stabilized metal oxide, particularly iron onto boric
acid-stabilized silica. Importantly, iron or other metal ions
attached to stabilized (e.g., boric acid) modified silica are much
more tightly bonded compared to metal ions adsorbed to silica. We
have surprisingly found that use of stabilizer-coated abrasive,
relative to activator absorbed directly onto abrasive without
stabilizers, results in less activator ions in solution in the
liquid phase. Additionally, especially at low activator
concentrations of below 20 ppm total absorbed activator in a
slurry, at much greater efficiency in increasing the rate of free
radical formation and of substrate removal. Each of these
properties offers unique advantages.
[0104] It is easy to get activator ions to absorb onto
stabilizer-coated abrasive, especially stabilizer-coated silica.
Generally, merely adding a soluble salt of the activator to the
deionized slurry of surface-modified abrasive, e.g., silica,
advantageously at 40.degree. C. or greater, will result in
activator ions being absorbed onto the surface-modified
abrasive.
[0105] Iron or other metal ions attached to stabilized
boron-surface-modified silica are much more tightly bonded compared
to metal ions adsorbed to silica. We have surprisingly found that
use of stabilizer-coated abrasive, relative to activator absorbed
directly onto abrasive without stabilizers, results in less
activator ions in solution in the liquid phase. Without being bound
to theory, we believe the greater affinity of activator to
stabilizer-modified abrasive is because the boric acid ligand is a
stronger base than the silicon-oxygen ligand (the greater
electronegativity difference between boron oxygen vs silicon-oxygen
bond), and/or because the Si--O--Si--O-- bond length is greater
than --O--B--O--B--O-- as boron atom has much smaller radius than
silicon, so iron is ionically much tightly attached on the
boron-oxygen surface as they are in closer proximity. An abrasive
that is less amenable to retaining activator ions attached thereto
can be made more useful by having the surface thereof be modified
by a stabilizer, particularly a boric acid stabilizer. Generally,
therefore, all other things being equal, polishing with a
stabilizer-surface-modified activator-coated abrasive, and most
particularly a boron-surface-modified iron-coated silica, will
typically result in fewer activator ions contaminating the surface
of the substrate being polished than would polishing with
activator-coated abrasive.
[0106] Chelators and Dihydroxy Enolic Compounds
[0107] Chelators include EDTA, DPTA, multivalent organic acids
including citric acid, polyhydroxyaromatics including catechols.
Generally, the presence of organic compounds in a slurry comprising
the activator and the oxidizer that reacts with activator to
produce the free radicals is not discouraged. While systems with
soluble iron need organic chelators to stabilize the slurry to give
modest shelf lisp no such stabilizers are needed in the coated
particles/oxidizer slurries of the current invention. Any organic
can quench a free radical, though ascorbic acid, thiamine, and
other "antioxidants" as they are known in diet parlance, as well as
alcohols, glycols, amino-alcohols, alkanolamines, and the like are
very effective free radical quenchers to be avoided. Preferably the
slurries have less than 0.5% of all of these.
[0108] Though chelators are free radical quenchers, we have found
that they are necessary under certain conditions. In certain
embodiments, some organics, particularly chelators, are useful, but
even so the quantity is preferably less than 0.4%, most preferably
less than about 0.2% by weight. For example, at pH 5 the solubility
of the various tungsten by-products are poor and at high removal
rates there is usually a poor finish on the wafer due to
re-precipitation. Therefore, polishing at pH 3.5 to 8 generally
involves the use of chelators, preferably a dihydroxy enolic acid
and/or one or more of citric, lactic, glycolic, and salicylic
acids. More preferred additives include ascorbic acid and/or lactic
acid. The use of a dihydroxy enolic compound such as ascorbic acid
is highly preferred in slurries of the present invention because it
is more effective at keeping the dielectric clean of activator
ions. The W removal rate can progressively get higher as the pH
increases, up to .about.5500+ angstroms per minute with a clean
surface in the presence of sufficient chelators. Complexing agents
are not needed to control the decomposition rate of the hydrogen
peroxide in the storage tanks and distribution lines, as it is when
soluble iron is present in a slurry, for pot life of hours
extending to for example 12 hours.
[0109] The greater affinity of activator to the modified abrasive
(as opposed to the affinity of iron on silica) is especially
important when a dihydroxy enolic compound, e.g., ascorbic acid, is
present in the slurry for extended periods of time. It is well
known that hydrogen peroxide is not very stable in the presence of
many metal ions without the use of stabilizers. Useful stabilizers
include phosphoric acid, organic acids (e.g., acetic, citric,
tartaric, orthophthalic, and ethylenediaminetetracetic acid), tin
oxides, phosphonate compounds and other ligands that bind to the
metal and reduce its reactivity toward hydrogen peroxide
decomposition. These additives can be used alone or in combination
and significantly decrease the rate at which hydrogen peroxide
decomposes, and may also effect metal polishing rates. While
hydrogen peroxide decomposition is surprisingly low when contacted
by activator-coated abrasive, to obtain a pot life of about a week
or more an additive that stabilizes the oxidizer is useful. For
example, ascorbic acid is advantageously present in a slurry at
concentrations between about 20 ppm and 1000 ppm, more typically
between about 100 ppm to 400 ppm. Its possible to use greater
amounts, but in slurries of the present invention having less than
100 ppm total activator absorbed onto abrasive, the additional
ascorbic acid will have little effect. However, both the abrasive
particle and the dielectric material on the substrate are typically
silica in one form or another, and we have found that even small
quantities of ascorbic acid can strip absorbed activator (iron)
ions off silica. While iron-ascorbic acid complexes in solution do
increase the polishing rate of a slurry containing a per-type
oxidizer, and while complexes of iron ions with ascorbic acid
(and/or with other enolic compounds substituted on both sides with
hydroxy groups, which we for simplicity call alpha, beta dihydroxy
enolic compounds, of which ascorbic acid is the most common
example) may be better than soluble ferric nitrate alone at
increasing substrate removal rates, nevertheless more (usually more
than twice as much) soluble iron is needed to provide the same
level of substrate removal as is provided by a certain amount of
activator absorbed onto an abrasive, e.g., silica. Fortunately,
ascorbic acid is much less potent in stripping absorbed activator
ions from boric acid-stabilized silica.
[0110] When using ascorbic acid with non-stabilizer-modified
activator-coated abrasive, the ascorbic acid is advantageously
added within 30 minutes or so of use, more preferable within a few
minutes of use, e.g., point of use mixing, so the ascorbic acid
does not strip activator from the abrasive. Point-of-use mixing can
also advantageously be done with stabilizer-surface-modified
activator-coated abrasive, but it is less important, as ascorbic
acid only strips a fraction of iron bound to stabilized silica.
[0111] As an aside, dihydroxy enolic compounds, and especially
ascorbic acid, are also very useful in preventing/removing metal
contamination resulting from conventional polishing slurries, for
example, for removing iron from the substrate resulting from use of
a conventional peroxide-ferric nitrate slurry, and removing cerium
from the substrate resulting from use of a conventional ceria
slurry or slurries having soluble cerium ions. Patents that
describe CMP slurries having a small amount of soluble rare earth
salts, especially cerium salts, which function as soluble metal
oxidizers or polishing accelerators for polishing dielectric
material include: U.S. Pat. No. 6,797,624, U.S. Pat. No. 6,399,492,
U.S. Pat. No. 6,752,844, and U.S. Pat. No. 5,759,917. Ascorbic
acid, erythorbic acid, or derivatives and/or mixtures thereof is
advantageously present in such prior art slurries at concentrations
between about 20 ppm and 4000 ppm, more typically between about 100
ppm to 1000 ppm. Ascorbic acid and/or erythorbic acid can also be
used in a post-CMP rinse.
[0112] Abrasive
[0113] The abrasive is typically a metal oxide abrasive preferably
selected from the group consisting of spinel, alumina, titania,
zirconia, germania, silica, ceria and mixtures thereof. Preferred
abrasives include, but are not limited to, alumina, spinel,
colloidal silica, colloidal ceria, and colloidal titania, with
colloidal silica being most preferred. The metal oxide abrasive may
be produced by any technique known to those skilled in the art. Due
to stringent purity requirements in the integrated circuit
industry, the preferred metal oxide should be of a high purity.
High purity means that the total impurity content, from sources
such as raw material impurities and trace processing contaminants
but not including the activator content and the stabilizer content,
is typically less than 0.1% and preferably is less than 0.01%
(i.e., 100 ppm).
[0114] The spinel or metal oxide abrasive consist of discrete,
individual particles, aggregates of particles, or both, having
diameters from 5 nanometers to 5 microns, preferably 10 nanometers
to 500 nanometers, more preferably from 20 nanometers to 200
nanometers. The percentage values used herein to describe the
nature of the abrasive particles in terms of particle size are
weight percentages, unless otherwise noted. The particle size of
the abrasive particles refers to the particle diameter. A spherical
or approximately spherical particle is preferred in this invention.
In some embodiments, and especially with particles smaller than
about 50 nanometers in diameter, cocoon- or peanut-shaped particles
can be useful. In preferred embodiments, the metal oxide abrasive
consists of metal oxide aggregates and particles having a size
distribution less than about 1.0 micron, a mean diameter less than
about 0.4 micron and a force sufficient to repel and overcome the
van der Waals forces between abrasive aggregates and particles
themselves. Such metal oxide abrasive has been found to be
effective in minimizing or avoiding scratching, pit marks, divots
and other surface imperfections during polishing. The particle size
distribution in the present invention may alternatively be
determined utilizing known techniques such as transmission electron
microscopy (TEM). The mean particle diameter refers to the average
equivalent spherical diameter when using TEM image analysis, i.e.,
based on the cross sectional area of the particles. By force is
meant that either the surface potential or the hydration force of
the metal oxide particles must be sufficient to repel and overcome
the van der Waals attractive forces between the particles.
[0115] Any particles can be useful for having the transition metals
of this invention coated thereon, provided they can hold the
transition metal activator to the surface thereof while allowing
the transition metal activator to be effective in promoting a
Fenton-type reaction, i.e., a reaction forming an oxygen and/or
hydroxyl free radical from oxidizer. The effectiveness of the
absorbed or bound activator ion is not expected to be the same when
the activator is on one abrasive as when it is attached to a
different abrasive. There is believed to be an interaction between
the abrasive and the activator ion, or between the stabilizer and
the activator ion, or both, that influences the effectiveness of
the ion in increasing the production of free radicals from the
oxidizer capable of producing free radicals. Generally, the
transition metal activator(s) of this invention are coated on at
least a part of the outer surface of one or more abrasives.
Examples of suitable abrasive particles, any of which may be at
least partially coated with a transition metal activator of the
current invention, include, but are not limited to, metal oxides
including particles comprising: alumina, silica, ceria (CeO.sub.2),
Ce.sub.2O.sub.3, both CeO.sub.2 and Ce.sub.2O.sub.3, titania
(TiO.sub.2), Ti.sub.2O.sub.3, both (TiO.sub.2) and Ti.sub.2O.sub.3,
zirconia, manganese dioxide, yttrium oxide (Y.sub.2O.sub.3),
Fe.sub.2O.sub.3, FeO, tin oxide, germania, copper oxide, nickel
oxide, manganese oxide, and tungsten oxide, as well as spinels
comprising one or more of Al, Mg, Zn, and Fe, and compounds other
than oxides, for example metal (of the metals listed in the
preceding oxides) nitrides such as zirconium nitride; metal (of the
metals listed in the preceding oxides) carbides, e.g., silicon
carbide, titanium carbide, or tungsten carbide; metal (of the
metals listed in the preceding oxides) silicides; or ceramics such
as metal (of the metals listed in the preceding oxides) titanate,
tantalate, zirconate, metal-germanium oxide, niobate, boride, or
combinations thereof; boron carbide; as well as polymeric particles
having a chelating capacity, polymer/metal oxide composite
particles, or even suspended agglomerations of silicates or
aluminate, or mixtures of any of the above, some or all of which
may optionally be coated with activator ions and/or be modified by
stabilizers. The plurality of particles having a surface and having
at least one activator associated with the surface can also
comprise a substantially spherical ceramic particle having an
average particle size from about 0.001 to about 1 micron and having
a particle size distribution such that: at least about 95% by
weight of the ceramic particles have a particle size within about
30% of the weight average particle size, wherein the ceramic
particle comprises at least one metallic oxide selected from the
group consisting of zinc oxide, bismuth oxide, cerium oxide,
germanium oxide, silica, aluminum oxide; and a metallic sulfide, a
metallic titanate, a metallic tantalate, a metallic zirconate, a
metallic silicate, a metallic germanium oxide, a metallic niobate,
a metallic borides, a metallic nitride, a metallic carbide, a
metallic telluride, a metallic arsenide, a metallic silicide,
metallic selenide, and mixtures or combinations thereof.
[0116] In selecting abrasive particles for particular CMP slurries,
the particle size, distribution of particle size, crystalline
phase, and uniformity of crystal-line phase are all properties that
affect the chemical mechanical polishing process. While the list
seems endless, those of skill in the art are aware that: only
certain abrasives are useful in selected pH ranges; only certain
abrasives are very useful for obtaining the desired selectivity of
one component over another, when both components are typically
found on a surface; certain abrasives are too hard or soft for
polishing particular substrates; certain abrasives result in
greater levels of ion contamination or defectivity; and there is a
substantial difference in cost of abrasives. The choice of abrasive
can depend on the particular nature of the substrate being polished
using prior art information, wherein the inclusion of coated
particles will generally accelerate the substrate removal rates.
Preferred abrasive particles include alumina (alpha-, beta-, and/or
gamma-alumina), silica (colloidal or fused), ceria, and spinels.
The more preferred abrasives for tungsten are colloidal silica and
alpha alumina, followed by fused silica and gamma alumina.
[0117] The physical properties of the abrasive particles and
particularly the crystalline form will also affect the surface
charge or Zeta potential of the polished surface. The Zeta
potential can have a great effect in the stability as well as on
the polishing performance of the slurry. In addition, an
undesirable Zeta potential can affect the residual particle surface
charge of the polished surface prior to post-CMP cleaning. The Zeta
potential is an electrostatic potential measurement of the
interaction of the electrostatic double layer ions (anions and
cations) that exist around each particle in a solution. The Zeta
potential can be positive, zero or negative and for the slurries of
the invention is preferably greater than or equal to +10 mV or less
than or equal to -10 mV. Another concern is that the Zeta potential
between the slurry and metal particles and the wafer will be such
that the particles will be attracted and adhere to the wafer
surface thereby possibly requiring additional post-CMP cleaning
steps to be performed to remove the adhering particles. If the Zeta
potential of the slurry composition does not have a desired value,
the slurry composition can be susceptible to settling of the slurry
particles, which can be detrimental to the performance of the
slurry during the CMP polishing process.
[0118] Various combinations of abrasive particles can be used if
desired. In addition, in a slurry, some abrasive can have a
transition metal activator of this invention coated thereon, and
other abrasive can be free of coated transition metal activator.
These coated and uncoated abrasives can be the same or different
composition, be the same or different type, or both.
[0119] When abrasive size is mentioned, the size is the average
particle diameter, which can be measured by for example microscopic
examination, or by other techniques generally used in the industry.
The surface area of the abrasives can vary widely, for example
between 1 and 2000 square meters per gram, as measured by BET. When
size ranges are specified, for example, from 10 to 80 nanometers,
this means that the average particle size falls within those
extremes, but preferably the individual particles have a size
distribution such that substantially all particles, i.e., at least
95% by count (for a single mode composition) have sizes within 30%,
preferably within 10%, of the average particle diameter. For the
example of a slurry with a particle size of between 10 to 80
nanometers, a preferred composition may have an average particle
size of 20 nanometers where at least 95% of particles have sizes
between 14 and 26 nanometers, more preferably between 18 and 22
nanometers.
[0120] In preferred preparations, the activator-coated particles
have an average particle size of less than 120 mm, for example
having an average particle size of about 50 to 80 nm. In some
embodiments of the invention, the particles are substantially
monodispersed. One preferred composition has abrasive particles
which are monodispersed and are of an average size which lies
between 30 and 100 nanometers, where the material is single-mode or
bimodal or trimodal. It is known to use cocoon-shaped abrasive,
which is generally defined in the industry as a particle where the
length component is a factor of two or three times the width
component. While it is possible to form cocoon-shaped particles of
a single matrix, it is preferred to form the cocoon-shaped
particles from 2 to 3 loosely bound smaller particles. One
composition has abrasive particles that are cocoon-shaped where 2
to 3 individual particles forming the cocoon are of an average size
of between about 10 and 40 nanometers.
[0121] The abrasive particles may form aggregates, which are
particles loosely or strongly held together in clumps, where the
number of particles in an aggregate depends largely on the
chemistry of the composition and on the particle size. Aggregates
typically have substantially the same dimensions, plus or minus
70%, measured in each direction, and have a plurality of particles
interconnected such that a plurality of particles contact at least
three other particles. Such aggregates can be desirable because
they have high polishing rates as found with bigger particles but
also have some resiliency, which reduces gouging. A useful
aggregate for very fine features are abrasives in aggregate form
with a particle size of 7 to 15 nanometers with an aggregate size
of 0.02 to 0.05 microns.
[0122] In some circumstances superior results are obtained with
chains formed of very small abrasive particles. As used herein a
chain is a structure with a length-to-width ratio of at least 4,
preferably at least 6, and in some embodiments at least 10. The
chain is not a single long crystal, but is a mass of small
individual, preferably substantially spherical silica particles
bound end to end. Such chains are believed to have an effective
size that is much larger than the average particle, but is much
more resilient than either an aggregate or a cocoon-shaped
structure.
[0123] Of course, it is realized that formulations may and usually
do contain a plurality of structures, be they monodispersed
particles, cocoon shaped particles, cocoon-shaped particles formed
of bound substantially spherical particles, chains, and aggregates.
As used herein, when a structure is specified, at least 50% of the
weight of abrasive should have that structure at least before
polishing. When two or more structures are specified, at least 70%
of the particles by weight should fall into the categories
mentions, at least before polishing.
[0124] The amount of particles can range across the ranges normally
used for solid abrasives or other solid particles in a CMP slurry,
for example from about 0.01% to 20%, for example between about 0.1%
to 4%, by weight based on the weight of the slurry. In one
important embodiment the amount of abrasive is kept very low.
Without being bound by theory, it is believed that the enhanced
chemical activity of the fluid surrounding the particles results in
less need for abrasive. At low abrasive concentrations, the current
invention still gives high substrate removal rates with good NU %.
Again without being bound by theory, the high amount of solids in
prior art formulations is believed to contribute to poorer wafer
uniformity as there are particle to particle interactions, as well
as solid migration due to centrifugal forces forming undesirable
accumulations. Such problems can be minimized by use of slurries
having less than 1% by weight abrasive.
[0125] The preferred concentration of coated particles is very
dependent on particle size, but is for sizes between about 0.005
and 0.4 microns is between about 0.05% to about 12% by weight, more
preferably between about 0.1% and 7%, for example between about 1%
and about 2.5% by weight.
[0126] Silica: The plurality of particles having a surface and
having at least one transition metal activator of this invention
associated with the surface in the principal embodiments can
comprise a silica, optionally wherein the particles have a BET
surface area between about 5 and 1500 m.sup.2/g, preferably with an
average particle size less than about 1 micron, and a particle size
distribution such that at least about 95% by weight of the silica
particles have a particle size within about 30% of the weight
average particle size, and additionally or alternatively, wherein
the particles have an average particle size form about 0.005 to
about 0.6 microns. By silica particle it is meant that a
substantial portion of the particle, for example at least 50% by
weight, preferably at least 95% by weight of the particle is
silica. Suitable abrasive particles are commercially available and
can be prepared by known methods, for example, by wet chemical
methods such as condensation-polymerization or colloidal
precipitation.
[0127] Advantageously the silica has a stabilizer, e.g., an
inorganic oxide coating of a stabilizer, of which boric acid is
preferred. Advantageously at least 90%, for example at least 95%,
of the available surface area of the particles comprise stabilizer.
Other stabilizers include aluminate, tungstate, and the like. One
un-tested yet potentially useful stabilizer may be a stannate.
[0128] The preferred silica for high removal rates is fumed silica.
Fumed silica is produced by a thermal (high temperature process)
and the material is known to have a "sharper edge" and a
corresponding increase in polishing rate as compared to colloidal
silica. For normal CMP, however, colloidal silica is preferred.
Colloidal silica polishes at a lower rate than does fumed silica,
but there are less defects and less abrasive-related contamination
when using colloidal silica as opposed to fumed silica. Colloidal
silica is preferred for transition metal activator coating. Very
small or even undefined particles colloidal-type particles, for
example a silica sol-gel, can also be used.
[0129] For colloidal silica, the preferred range of concentration
in the slurry is less than 4% by weight, preferably less than 3% by
weight, for example between about 0.05% and 2%, of which between
about one twentieth to about all of the colloidal silica has
transition metal activator attached thereto. Iron-coated colloidal
silica works best at 3% and lower concentrations, as at higher
levels the ILD (interlevel dielectric) oxide film is removed to
quickly.
[0130] Advantageously, in one embodiment, there is between about
0.3% and about 1% by weight of total abrasive, where the amount of
abrasive having transition metal activator associated therewith,
especially iron on colloidal silica, is between about 0.1% and
about 0.4%, where both percentages are based on the weight of the
particles compared to the weight of the slurry. This slurry is
especially superior for preventing the oxide film erosion outside
the metal structures. In one preferred silica: iron/hydrogen
peroxide system, there is only 0.6% total solids, all silica of
size between about 0.05 and 0.1 microns in diameter, of which 0.16%
is Fe coated and 0.44% is regular (uncoated) SiO.sub.2. While
colloidal silica is shown to be able to incorporate iron through
its matrix and to be able to retain absorbed iron more tenaciously
than does fumed silica, we have found that fumed silica, which is
more abrasive in character than colloidal silica, is beneficially
included because of its increased abrasive character relative to
colloidal silica. In one preferred embodiment, blends of fumed and
colloidal silica are used. Advantageously, the fumed silica has
little or no activator coated thereon. Additionally, it is known
that fumed silica, with its greater porosity, has a lower settling
rate in a slurry than does colloidal silica. The fumed silica can
be any of the high-surface-area silica particles used in the
industry, for example having a surface area (BET) of between about
3 and about 1000 m.sup.2/g. Very high porosity gives a more friable
particle, and very low surface area gives a slurry which settles
faster. A surface area of between about 20 and about 200 m.sup.2/g
gives good toughness and acceptable settling properties.
[0131] As an aside, we have surprisingly found that the use of
fumed plus colloidal silica also gave far larger removal rates
(with soluble ferric ions and a per-type oxidizer material present)
than either colloidal silica or fumed silica as the only particles.
Advantageously, the particles associated with the transition metal
activator are colloidal silica-based, and these can be admixed with
fumed silica that has a lower quantity of a transition metal
activator thereon, or no transition metal activator thereon.
However, the fumed silica can contain transition metal activator,
particularly iron, coated thereon.
[0132] Particle size of the silica will in most embodiments range
from 3 microns to 3 nanometers. As is known in the art, fine
control of particle size is needed to minimize defects. The silica
abrasives for very fine features are by necessity also very fine.
We have found that particles below 0.15 microns, preferably below
0.1 microns, are particularly useful. On the other extreme, unless
the zeta potential of the stabilized coated silica is controlled,
particles having a diameter of less than about 10-20 nanometers
will show an unfavorable tendency to stick to the substrate. The
particle size of the colloidal silica is beneficially between about
50 nanometer and about 200 nanometers, for example between about 60
nanometers and about 120 nanometers (alternately about 0.05 to 0.2
microns). In one preferred embodiment, the particle size of the
colloidal silica is between about 70 nanometers and about 100
nanometers, alternately between 0.07 and 0.1 microns
[0133] In some embodiments of the invention, the particles are
substantially monodispersed. One preferred composition has 0.5 to
3% silica where the particles are monodispersed and are of an
average size which lies between 30 and 100 nanometers, where the
material is single-mode or bimodal or trimodal. It is known to use
cocoon-shaped silica, which is generally defined in the industry as
clumps of between 2 and 3 individual particles, such that there is
a length component that is a factor of two or three times a width
component. While it is possible to form cocoon-shaped particles of
a single matrix, it is preferred to form the cocoon-shaped
particles from 2 to 3 loosely bound smaller particles. The
particles may form aggregates, which are silica particles loosely
or strongly held together in clumps, where the number of particles
in an aggregate depends largely on the chemistry of the composition
and on the particle size. For silica particles of average size of
15 nanometers, an aggregate may contain ten or more individual
particles such that the aggregate size is about 40 microns.
Aggregates typically have substantially the same dimensions, plus
or minus 70%, measured in each direction, and have a plurality of
particles interconnected such that a plurality of particles contact
at least three other particles. Such aggregates can be desirable
because they have high polishing rates as found with bigger
particles but also have some resiliency, which reduces gouging.
Particularly preferred for very fine features is silica in
aggregate form with a particle size of 15 nanometers (0.015
microns) with an aggregate size of 0.04 microns, or silica with a
particle size of 7 nanometers (0.007 microns) with an aggregate
size of 0.02 microns. In some circumstances acceptable results are
obtained with chains formed of very small silica particles. The
chain is not a single long crystal, but is a mass of small
individual, preferably substantially spherical silica particles
bound end to end. An exemplary chain is formed of 3 to 10 particles
each having an average diameter of between 3 and 20 nanometers.
Such chains are believed to have an effective size that is much
larger than the average particle, but is much more resilient than
either an aggregate or a cocoon-shaped structure
[0134] Smaller particles result in lower substrate removal rate.
For this reason, in many embodiments larger particles are
preferred. In another embodiment, the particle size of the
colloidal silica is beneficially between about 50 nanometer and
about 200 nanometers, for example between about 60 nanometers and
about 120 nanometers (alternately about 0.05 to 0.2 microns). In
one preferred embodiment, the particle size of the colloidal silica
is between about 70 nanometers and about 100 nanometers,
alternately between 0.07 and 0.1 microns. In another embodiment,
the particle size of the silica is beneficially between about 50
nanometer and about 250 nanometers, for example between about 60
nanometers and about 200 nanometers (alternately about 0.05 to
about 0.3 microns). In one preferred embodiment, the particle size
of the silica is between about 70 nanometers and about 150
nanometers.
[0135] In another embodiment, the transition metal activator-coated
particles comprise silica sol/gel having silicate agglomerations or
panicles particles in the range of about 10 to about 60 nanometers
in diameter. The particle size of the sol/gel having the transition
metal activator, i.e., iron, associated on the surface thereof is
beneficially between about 40 nanometer and about 60 nanometers.
The amount of the transition metal activator-containing silica
sol-gel can range from 0.05% to 6%, for example between 0.1% to 1%,
of the slurry by weight. This material is typically, but need not
be, mixed with an abrasive comprising particles greater than about
60 nanometers in diameter. The preferred concentration of abrasive
is between about 0.1% to about 6% by weight, more preferably
between about 0.3% and 3%, for example between about 0.5% and about
1% by weight, and the abrasive advantageously has an average
particle diameter of between about 70 and about 250 nanometers.
Alternatively, the pad can comprise abrasives.
[0136] Even more surprisingly, we have found that even at very low
quantities of abrasive, only a small fraction of the abrasive need
have transition metal activator associated therewith. While all of
the abrasive can have transition metal activator associated
therewith, we have found excellent results are obtained with only a
small amount of the abrasive having the transition metal activator
associated therewith admixed in a slurry with lower-cost normal
abrasive. The amount of abrasive that is coated with the transition
metal activator can range from about 0.01% to about 5% by weight of
the slurry is sufficient, based on the weight of the transition
metal activator-containing particles to the weight of the slurry.
In one preferred embodiment, the amount of abrasive (or other
particle) that is coated with the transition metal activator can
range from about 0.001% to about 2%, more preferably from about
0.01% to about 0.9%, for example from about 0.05% to about 0.3%,
based on the weight of the transition metal activator-containing
particles to the weight of the slurry. The total amount of abrasive
is advantageously less than 1% by weight of the slurry. The prior
art formulations recommend higher amounts of abrasive. For example,
EP 0844290 to Grumbine in sec. 0044 discloses a slurry with a fumed
silica concentration of 3 to 45% with 10-20% being best. We have
surprisingly found that lower abrasive content provides superior
results. In particular, we have found that less than 5% abrasive,
preferably less than 3% abrasive, for example between about 0.05%
and about 2.8% by weight of abrasive, provides superior results. In
a preferred embodiment, the amount of abrasive is between about
0.2% and 2%, for example between about 0.4% and about 1%, and in
one preferred embodiment the amount of abrasive is between about
0.5% and about 0.8% by weight. These are much lower concentrations
than are typically used in the industry, and represent an important
breakthrough in the art. The lower amount of abrasive allows
reduced cost as well as the ability to more closely engineer the
abrasive to find the particular and narrow properties, including
particle size, sharpness, and amount of transition metal activator
attached thereto.
[0137] Alternative Iron-Coated Silica Manufacture--In one
embodiment, fresh non-aged silica is advantageously utilized to
form the activator metal-containing particle. This involves
building the particle from a very small size and inserting the
metal onto the outer surface, advantageously by binding the
activator metal, i.e., the iron, with at least one and preferably
with a plurality of Si--O-- groups. First, finely divided silica
particles are precipitated, or alternately provided, beneficially
in finely divided particles, to an aqueous solution at an alkaline
pH, for example pH 9-10. Generally, very finely divided silica may
cause water to approach that high pH value. Colloidal silica is a
preferred particle to use with the activator metal, particularly
the more highly polymerized species or particles larger than about
50 .ANG.A. However, smaller particles are beneficial at higher pH,
and we have found down to 1-20 .ANG. particles useful starting
agents with pH>9 silica abrasives. Then, the pH is adjusted to a
very acidic value, for example between about 1 and 3 about 3, i.e.,
about 2. The low pH is believed to dissolve silicates, creating an
abundance of particles with a diameter that is believed to be
around 10 to 20 angstroms. At this point iron can be added,
generally to form a solution having between 0.01% and 0.5% by
weight, for example 0.05% to about 2% by weight. Monomeric silica
does not react with most metal ions in water at low pH where
Si(OH).sub.4 can exist. However, iron (and incidentally uranium)
are the only metal ions that form basic ions at the pH of 2, where
monomeric Si(OH).sub.4 is most stable. The other known reaction of
monomer with a metal cation is the case of ferric iron, reported by
Weber and Stumm and further examined by Porter and Weber in regard
to the effect of the degree of polymerization of silica. They
polymerized the silica at a concentration of 2280 ppm at pH 0-10
for various lengths of time, under conditions that are known to
give very small spherical particles. With increasing polymerization
of silica with formation of adjacent SiOH groups that can combine
with iron. At pH 2, the number of SiOH groups combined per iron ion
increases from one on the monomer to two or three as the particles
become larger. It is important that the pH of the solution
containing the iron and the silica is below about 3.5. At or
somewhat below pH 3.5 a soluble complex between Si(OH).sub.4 and
Fe.sup.3+ exists. The interaction of Si(OH).sub.4 with ferric iron
is evidenced by the fact that concentrations of 10.sup.-4-10.sup.-3
M SiO.sub.2 in water catalyze the oxidation of Fe.sup.2+ to
Fe.sup.3+. At pH 6-8, a ratio of 3 Si(OH).sub.4 to 1 Fe.sup.3+
prevents precipitation of Fe(OH).sub.3. However, in the case of
Al.sup.3+, a fivefold excess of Si--(OH).sub.4 is required to
prevent precipitation.
[0138] It is necessary to consider the polymerization of silica in
solution below pH 7 separately from that above pH 7 for the
following reasons. First, it is only at low pH that the
polymerization is sufficiently slow to allow the early
incorporation stages to be followed. Changes that occur in hours at
pH 2 are completed in minutes or seconds at pH 8-9. Second, the
polymer units or particles formed at low pH bear no charge and
unless the silica concentration is very low, aggregation begins to
occur soon after polymeric particles are formed, not only because
of the lack of charge on the particles, but also because the
particles are extremely small and cease to grow after they reach a
diameter of 2-3 nm. Since the rate of aggregation depends mainly on
the number of particles per unit volume and less on their size,
aggregation occurs even at very low silica concentrations. The
formation of "oligmers" generally means low molecular weight.
polysilicic acids which might include for example 3-8 groups. This
polymerization behavior appears to be explained by the following:
1). The monomer, Si(OH).sub.4, has a pKa of .about.9.9; 2. The
dimer pKa is reported higher but is likely lower than 9.9; and 3.
Higher polymerized species have a much lower pKa approaching 6.7
and hence are more highly ionized than the dimer or monomer. This
implies that the greater the number of siloxane linkages and the
fewer OH groups on a silicon atom, the stronger the acidity (the
pKa goes lower). Polymerization involves intermediate ionization to
iO.sup.- or to i.sup.+ below or above pH 2, respectively, but to
simplify discussion only iO.sup.- is used as the example. Only
recently was it shown by Harder and Flehmig that even at 20.degree.
C., quartz crystals were nucleated in a suspension of Fe(OH).sub.3
or AL(OH).sub.3 in only 14 days. The solutions were very dilute: 2
ppm Fe.sup.3+ or Al.sup.3+ and 0.4-5 ppm SiO.sub.2. In some manner,
amorphous hydroxides of Fe.sup.3+ (or Al, Mn, and Mg) can absorb
and hold up to 9 moles of SiO.sub.2 per mole of hydroxide.
Therefore, for this invention, the first step is to shift the pH of
a standard colloidal SiO.sub.2 from pH 10-11 down to a pH of
between about 1 and 4, preferably to a pH of about 2, and then
subsequently or concurrently contacting the particle with the iron
salt in a manner such that the iron salt becomes associated with
the metal, and then finally adjust back to pH 34. Silica,
especially colloidal silica, has a number of interactions with iron
species. The end result is iron that is incorporated not only on
the silica but within the silica matrix. This has certain
advantages, as the particles can be recycled and re-used more often
than for example silica having iron absorbed thereon.
[0139] Other mechanisms can be used to bind or associate a
transitional metal to a particle. For example, lower amines are
adsorbed as the substituted ammonium ions. Silica gel bearing these
adsorbed ions has use has useful adsorbent properties, especially
for weakly acidic substances. Also the ammonia- or amine-bearing
gels have selective affinity for certain metal ions, which are
strongly adsorbed. Selective adsorption controlled in this way has
been demonstrated by Neimark et al. We have surprisingly found
excellent results with our slurries that are ammonium-stabilized
commercial colloidal silica products that are neutralized with
nitric acid to pH 2 and then adjusted with NH4OH or TMAH
chemistries to rise the pH to .about.3. Potassium-stabilized
colloidal silica does not give similar results. Kolthoff and
Stenger found that when the ion Cu(NH.sub.3).sub.4.sup.2+ was
adsorbed on the silica surface, the NH.sub.3:Cu ratio was less than
4:1.
[0140] Another method of making transition metal-coated silica is
as follows. A master batch of SiO.sub.2 is made by pumping sodium
silicate into a reactor at a certain temperature to form small
silica particles (20-40 nm). Then add more silicate at a rate to
maintain a certain particle size (i.e., about 70-80 nm) while using
ion exchange resin to neutralize and remove much of the sodium in
the mixture. As the particles are growing, or alternately after the
particles have reached the desired size, add a transition metal
that is a salt of iron, copper, silver, or the less-preferred
metals, or combinations thereof. Advantageously the salts are such
that the anion can be readily removed, for example a sulfate. In a
preferred embodiment iron sulfate with HOAc is added. The slurry is
again exposed to ion exchange resin to remove a portion of the
anion, i.e., the sulfate, to allow the iron to adsorb onto the
silica. Once this operation is complete, the material can be
stabilized by adding salts. Non-metallic salts, for example a
buffer of sulfuric acid and ammonium sulfate or tetramethyl
ammonium sulfate, are preferred.
[0141] Alumina: The invention includes a method of polishing by
using polishing slurries comprising transition metal
activator-coated particles in a slurry with at least one oxidizer
that reacts with the transition metal activator-coated particles to
create free radicals in an amount useful for accelerating a
chemical mechanical polishing process, wherein slurries of this
embodiment have transition metal activator coated alumina abrasive
material. In one embodiment, the slurry comprises alumina,
preferably alpha-alumina, having the transition metal activator(s)
of this invention coated thereon. Alumina was surprisingly found to
hold transition metal activators, i.e., iron, tightly. Alumina is
useful for different substrates, and for different pHs, as is known
in the art. The coated alumina of this invention produces free
radicals, i.e., hydroxyl free radicals, which accelerate the
substrate removal rate. The plurality of particles having a surface
and having at least one transition metal activator associated with
the surface thereof can comprise a alumina, optionally wherein the
particles have a BET surface area between about 5 and 1000
m.sup.2/g and the weight average particle size is less than about
0.4 microns, and additionally or alternatively, wherein the
particles have an average particle size from about 0.001 to about
0.2 microns. By way of example, the alumina may be an
alpha-alumina, a gamma-alumina, colloidal alumina, fused alumina,
ceramic alumina, or other aluminas known in the art, or a
combination thereof. The alumina particles can comprise alpha
alumina, preferably wherein the particles have a BET surface area
between about 5 and 1000 m.sup.2/g and the average particle size is
less than about 0.8 microns, say between about 0.03 and about 0.4
microns.
[0142] Advantageously the alumina has a stabilizer, e.g., an
inorganic oxide coating of a stabilizer, of which boric acid is
preferred. Advantageously at least 90%, for example at least 95%,
of the available surface area of the particles comprise stabilizer.
Other stabilizers include tungstate, and the like. One un-tested
yet potentially useful stabilizer may be a stannate.
Advantageously, in some embodiments, the stabilizer and the
activator combine to equal at least 1% of the weight of the
alumina, for example between about 1% and 3% by weight of the
alumina, more preferably between about 1.1% and 2% by weight.
Clearly, such a level of coating will provide polishing and slurry
stability characteristics considerably different than for example
pure alumina, though the underlying morphology and hardness of the
alumina can be useful in polishing certain substrates.
[0143] Spinels: The invention includes a method of polishing by
using polishing slurries comprising transition metal
activator-coated particles in a slurry with at least one oxidizer
that reacts with the transition metal activator-coated particles to
create free radicals in an amount useful for accelerating a
chemical mechanical polishing process, wherein slurries of this
embodiment have transition metal activator coated spinel abrasive
material. In one embodiment, the slurry comprises an iron spinel
material, having the transition metal activator(s) of this
invention coated thereon. In another embodiment, the slurry
comprises an magnesium spinel material, having the transition metal
activator(s) of this invention coated thereon. In another
embodiment, the slurry comprises an zinc spinel material, having
the transition metal activator(s) of this invention coated thereon.
Of course, in another embodiment, the slurry comprises an spinel
material comprising at least two of iron, zinc, and magnesium, the
spinel material having the transition metal activator(s) of this
invention coated thereon. The particles having a transition metal
activator(s) of this invention coated thereon may comprise or
consist essentially of spinel particles having the formula
AO.xZ.sub.2O.sub.3 wherein A is at least one divalent cation, Z is
at least one trivalent cation, and x is between 0.01 to 100. In one
embodiment a slurry composition of the present invention is
preferably substantially free of particles having a Mohs hardness
of greater Man 8.5, e.g. alpha phase alumina particles, when used
in the final chemical mechanical polishing steps and thus provides
a scratch-free surface. In the spinel particles, it is also
preferred that x is between 1 and 12.5. Preferably, the A cations
in the slurry compositions of the invention are selected from the
group consisting of Mg, Fe, Zn, Mn, Ni, Ca and combinations
thereof. The Z cations are preferably selected from the group
consisting of Al, Fe, Cr, Ti, and mixtures thereof, wherein the A
cations and the Z cations are not both entirely iron. The spinet
particles can also include a small amount of at least one cation
selected from the group consisting of Zr, Si, Ga, Cu, Co, V, B, Li,
rare earth cations, and mixtures thereof. In one embodiment of the
invention, A is Mg, Z is Al, and the formula of the spinel
particles is MgO.xAl.sub.2O.sub.3. In another embodiment of the
invention, A is Zn and Z is Al such that the formula of the spinel
particles is ZnO.xAl.sub.2O.sub.3. In another embodiment of the
invention, A is Fe and Z is Al such that the formula of the spinet
particles is FeO.xAl.sub.2O.sub.3. Also preferred are spinet
particles include those having essentially a maghemite
(gamma-Fe.sub.2O.sub.3) or chromite structure. The spinet particles
are preferably present in an amount between about 0.05 and about 10
percent by weight. The slurry compositions can also include between
0.1 and 10 percent by weight of other abrasive particles.
[0144] In accordance with the invention, the spinel particles of
the invention are prepared to have certain desirable properties. In
particular, the mean crystallite diameter of the spinet particles
is preferably between 5 and 500 nm, more preferably between 5 and
100 nm. The mean crystallite diameter can be measured, e.g., using
x-ray diffraction. The spinet particles preferably have a specific
surface area of between 50 m.sup.2/g and 150 m.sup.2/g. The
preferred spinel particles of the invention also have a crystallite
phase distribution including greater than about 95%, preferably
greater than about 98% and more preferably greater than about 99%
gamma-like spinet phase as measured using x-ray diffraction. This
gamma-like phase has a spinel structure with a high level of
cationic vacancies. It has been discovered that this gamma-like
phase is particularly useful at polishing surfaces without
scratching. The spinet particles of the invention typically have
desirable Zeta potentials thus limiting the chance that the slurry
particles will settle out of the slurry or that the slurry
particles or metal ions will adhere to the wafer surface.
[0145] The invention includes a method of polishing by using
polishing slurries comprising iron oxide abrasive particles in a
slurry with at least one oxidizer that reacts with the iron oxide
particles to create free radicals in an amount useful for
accelerating a chemical mechanical polishing process. We have found
that certain forms of iron oxide, in crystalline form, exhibit some
increase in substrate removal rate suggesting increased oxidizer
activity. Iron oxides such as FeO and Fe.sub.2O.sub.3 can be
useful. The iron oxide may be treated to increase formation of free
radicals to a commercially acceptable amount. Advantageously, the
slurry comprises at least one additional abrasive material.
Advantageously, the pH of the slurries of this embodiment are kept
at levels between about 3 and about 7, for example between about 4
and about 5. Advantageously the particles are substantially
surface-modified with stabilizer, that is, greater than 80% of
available surface area covered by a stabilizer, for example from
boric acid or an aluminate.
[0146] PER-TYPE OXIDIZER Capable of Forming Oxygen/Hydroxyl Free
Radicals:
[0147] It is important to note that the bound activator metal does
not directly take part in the oxidation process. In preferred
embodiments, the iron levels are so low that it (the iron) is not
the direct reagent in the polishing process. The core of the
invention is the promotion of a Fenton-type reaction in which a
per-type oxidizer is induced to react with the activator to produce
free radicals in a sufficient amount and in sufficient proximity to
the substrate to be polished so that the generated free radicals
produce a substantially increased (e.g., at least greater than 10%,
preferably greater than 20%, and typically greater than 50%)
substrate removal rate. The preferred concentration of oxidizer is
between about 0.2% and 10% by weight, preferably between about 1%
and about 7% by weight, for example between about 2% and about 4%
by weight. The preferred oxidizer is hydrogen peroxide or periodic
acid. The per-type oxidizer that is induced to react with the
activator to produce free radicals oxidizing agent is per-type
oxidizer, which is generally defined as a compound containing an
element in its highest state of oxidation; or a compound containing
at least one peroxy group (--O--O--).
[0148] Suitable per-compounds containing at least one peroxy group
include peroxides. As used herein, the term "peroxides" encompasses
hydrogen peroxide and reaction products and complexes of hydrogen
peroxide and other compounds, including specifically peroxyformic
acid, peracetic acid, percarbonic acid, perboric acid,
propaneperoxoic acid, butaneperoxoic acid,
hydroperoxy-acetaldehyde, urea-hydrogen peroxide, and the like. Any
mention of an acid also includes any salts thereof. Generally, the
presence of organics, acids, and other anions is discouraged. The
preferred peroxide is hydrogen peroxide. The preferred
concentration is between about 0.2% and 10% by weight, preferably
between about 1% and about 7% by weight, for example between about
2% and about 4%, by weight of the fluid component of the slurry.
While lesser amounts are operative, the amount should of be so
little that the concentration of hydrogen peroxide decreases by a
significant percentage as the slurry travels over the substrate, as
this will result in undesirable variable spacial substrate removal
rates. In some embodiments, the hydrogen peroxide and the activator
metal coated particles are mixed at or just prior to point of use,
but the slurries are so stable this is not required.
[0149] Another preferred oxidizer that produces free radicals is
periodic acid and/or any periodiate salt. This is particularly
effective with iron-coated abrasives. The preferred concentration
of periodic acid is between about 0.2% and 10% by weight, for
example between about 0.5% and about 7% by weight, for example
between about 2% and about 4%, by weight of the fluid component of
the slurry.
[0150] Persulfate oxidizers can be the free-radical forming agent.
As used herein, the term "persulfates" encompasses monopersulfates,
di-persulfates, and acids and salts and adducts thereof. Included
for example is peroxydisulfates, peroxymonosulfuric acid and/or
peroxymonosulfates, Caro's acid, including for example a salt such
as potassium peroxymonosulfate, but preferably a non-metallic salt
such as ammonium peroxymonosulfate. Iron and copper are useful with
persulfates. Silver is active at forming oxygen/hydroxyl free
radicals only from persulfates. Thermal decomposition of
persulfates can also form highly reactive sulfate free radicals. At
temperatures above 40 C, persulfate begins decomposing to
SO.sub.4.sup.-, which has an electrochemical potential of about 2.6
volts. In some embodiments, slurries containing persulfate are
heated to about 40.degree. C. at or just prior to point of use.
[0151] Peroxydiphosphates are useful in this invention, and
preferred ranges are 0.5 to 10%, for example 2 to 6% by weight of
the slurry.
[0152] We have beneficially found that the slurries and fluids of
the present invention can achieve commercially acceptable substrate
removal rates with very low oxidizer concentrations. This
low-oxidizer-concentration embodiments reduce the absolute amounts
of undesired hydrogen gas that can be produced, reduce chemical
cost, reduce problems of exposure of workers and equipment to high
concentrations of these somewhat hazardous compounds, facilitate
neutralization of the oxidizer prior to disposal or even allow
disposal without neutralization. By commercial rates it is meant
over 1000, for example over 2000, angstroms per minute for
components such as tungsten, and over 300, for example over 600,
angstroms per minute for noble metals. Slurries of this embodiment
contain from about 0.1% to about 3%, for example from 0.2% to 2%,
i.e. about 0.5% to 1.5%, by weight of hydrogen peroxide or periodic
acid, compared to the weight of the fluid.
[0153] Additives
[0154] One preferred mechanism of manufacturing the iron-coated
silica useful in this invention is to place silica particles in a
solution, adding iron sulfate, and then quantitatively removing
sulfate with for example ion exchange resins. Iron sulfate is added
to a silica-containing slurry in an amount sufficient to add the
desired amount of iron to the silica, and the sulfate level of the
slurry is then beneficially brought to below detectable limits,
that is, less than 10 ppm sulfates. This has the effect of causing
the iron, or other metal which promotes a Fenton-type reaction, to
become bound to the surface of the silica. Advantageously, after
the iron is bound to the silica, sulfate ions can be re-introduced
to the slurry. We have found that sulfate ions can have a
stabilizing influence on the silica slurry by retarding settling.
Without being bound by theory, we believe the sulfate forms a
stable double-layer about the bound iron. This can also reduce the
activity of the iron. Sulfate can be present for example in an
amount between about 30 and about 500 ppm sulfate, preferably
between about 50 and 300 ppm sulfate, for example between about 100
and about 200 ppm sulfate. A sulfate level of 170 ppm can extend
the time before particle settling becomes significant from about 2
days to about 5 days.
[0155] Method of CMP
[0156] The present invention also includes a method of chemical
mechanical polishing a substrate using the slurry compositions
described above. The slurry composition is applied to a surface of
a substrate and the surface of the substrate is polished using the
slurry to a desired end-point or planarization to provide the
desired surface. In a typical chemical mechanical polishing
process, the substrate is placed in direct contact with a rotating
polishing pad. A carrier applies pressure against the backside of
the substrate. During the polishing process, the pad and table are
rotated while a downward force is maintained against the substrate
back. An abrasive and chemically reactive solution, commonly
referred to as a "slurry" is deposited onto the pad during
polishing. Polishing without an abrasive is also possible using
selected compositions of this invention. The slurry initiates the
polishing process by chemically reacting with the film being
polished. The polishing process is facilitated by the rotational
movement of the pad relative to the substrate as slurry is provided
to the wafer/pad interface. Polishing is continued in this manner
until the desired film on the insulator is removed. In its basic
components, a method for polishing a substrate including at least
one metal layer comprising the steps of:
[0157] (a) admixing the CMP fluid of this invention, the fluid
containing a compound that produces free radicals at least when
contacted by an activator;
[0158] (b) contacting the fluid with an activator to form free
radicals in the fluid;
[0159] (c) contacting the free radical-containing fluid to the
substrate; and
[0160] (d) mechanically abrading the substrate contacting the free
radical-containing fluid to the substrate, thereby removing at
least a portion of the metal layer from the substrate, wherein the
substrate (typically comprising a metal such as tungsten) removal
rate is at least 10% greater than the polishing rate of a similar
composition but without the activator.
[0161] A method of measuring activator is as follows: Provide a
slurry comprising an abrasive; separate and rinse the abrasive from
the fluid carrier and oxidizer (if present), digest 1 part by
weight of the abrasive in each of 2 parts, 4 parts, and 8 parts of
a solution of deionized water having 2% ascorbic acid at an
elevated temperature of between about 40.degree. C. to about
60.degree. C. for a period of 24 hours, with stirring, withdrawing
a portion of the digesting liquid, and analyzing the same for
activator metals using known techniques, for example ICP. The size
and variable composition of the slurry make separation of the
particles from the liquid a case by case issue. Typically,
particles can be separated from liquids by ultracentrifugation
[0162] One problem facing operators is that certain slurries is
hydrogen generation. Hydrogen is extremely explosive and is lighter
than air, allowing hydrogen to accumulate in areas where one would
ordinarily not expect gas to accumulate. One of the worst hydrogen
generating compositions are those that contain hydrogen peroxide
and a transition metal dissolved therein, such as iron, copper, or
the like, which can generate tens of cc of hydrogen per minute per
liter of slurry. Polishing methods using prior art slurries to
minimize hydrogen production use two component formulations,
admixing them at point of use. Though the slurries of the present
invention typically produce orders of magnitude less hydrogen than
would a typical iron nitrate hydrogen peroxide slurry, nevertheless
precautions can be taken. The invention includes a method of
polishing by using a slurry comprising: a first portion comprising
water, a per-type oxidizer capable of forming free radicals such as
hydroxyl radicals in the slurry, and optionally one or more
pH-adjusting additives; and a second portion comprising water and
particles comprising a transition metal coating thereon, and
optionally one or more pH-adjusting additives, wherein the first
and second components are mixed within an hour of being used,
preferably within a minute of being used, and is typically mixed a
few seconds prior to time of use, use being the time when the
slurry contacts the substrate in a manner such that chemical
mechanical polishing occurs. In a preferred embodiment, the
particles are substantially separated from the fluid portion of the
slurry within a few tens of minutes of time of use, for example,
within a minute after time of use. In some embodiments, operators
do not use oxidizers prone to generating excess hydrogen,
particularly hydrogen peroxide, but rather use persulfates, or
periodic acid.
[0163] Any or all of the following improvements can be incorporated
into the above method. In some embodiments, fluids or slurries have
less than 5 ppm, for example less than 2 ppm, of dissolved
transition metals (other than those polished from the substrate,
and other than tin, which can be a stabilizer) in any fluid portion
of the slurry. In some embodiments, compositions have 2% or less by
weight of oxidizer (which may include or be exclusively hydrogen
peroxide), thereby limiting the absolute amount of hydrogen
generation possible from a slurry. This limited oxidizer slurry is
an important embodiment, limiting hydrogen gas generation, as well
as minimizing purchasing costs and disposal costs, and with the
method of the current invention commercially acceptable substrate
removal rates can be maintained. In some embodiments, a chelator
can be added at point of use. The pH of the slurry is
advantageously between 2 and 7, say between about 2.6 and about
4.5, preferably between about 3 and about 3.5. The pH can be
adjusted as needed, preferably with mineral acids such as sulfuric
acid or nitric acid, and with bases such as ammonium hydroxide,
mono-, di-, tri-, or tetra-alkyl ammonium compound, or a mixture
thereof. The preferred base is tetramethyl ammonium hydroxide
(TMAH). Other additives, including chelators, stabilizers,
promotors, other oxidizers, other abrasives, corrosion inhibitors,
and the like can be included, though generally such additives are
not needed. Advantageously, the slurry has less than about 50 ppm
of soluble metal ions. Advantageously, the slurry comprises less
than about 10 ppm of a soluble form of the transition metal or
metals associated with the particle. Excluding for example soluble
tin, which may be used as a stabilizer, in preferred embodiments
the slurry has less than 20 ppm total soluble metal, for example
less than 5 ppm soluble metal.
[0164] In one embodiment of this invention particles having
associated activator are recovered from used CMP slurries, for
example by a simple expedient of separating particles having
activator from the slurry by for example filtration,
centrifugation, or the like, after polishing and are re-used. The
activator is not used up in the process. If activator coated
particles have different zeta potentials in the slurry than
non-activator coated abrasive, separation may be done by partially
destabilizing the slurry and recovering the particles having
activator associated thereon. Various additives such as salts can
be added to destabilize the slurry to enhance separation, but such
recovered material should subsequently be washed, for example with
a dilute mineral acid or ascorbic acid, prior to reuse. Such a
system would have an additional amount of activator-coated
particles added thereto to replace that lost to for example
grinding. A small fraction of the recycled activator-coated
particles may be disposed of to keep the amount of activator-coated
particles in the CMP slurry constant.
[0165] In some embodiments the temperature can be changed during a
CMP process, following a profile to give increased free radicals in
the initial polishing and less free radicals in the later stage of
polishing. Similarly, the amount of formation of free radicals can
be changed by altering the pH of the solution. Other combinations
will be devised by one of ordinary skill in the art with the
benefit of this disclosure.
[0166] As mentioned above, the abrasive material of the composition
is at least partially coated with the activator. As used herein,
"coating" and its various linguistic or grammatical forms or
counterparts generally refer to forming a physical connection
between the abrasive and the activator, such as by forming at least
a partial layer of activator material on at least a portion of the
abrasive, absorbing or adsorbing the activator material on at least
a portion of the abrasive, forming adhesion between the activator
material and at least a portion of the abrasive, and the like, by
any suitable means or method. By way of example, a method of
producing a silica sol coated with iron acetate is provided in U.S.
Pat. No. 4,478,742 of Payne, the entire contents of which are
incorporated herein by this reference. Similarly, U.S. Pat. Nos.
3,007,878, 3,139,406 and 3,252,917, which describe ways of putting
metals on a core of silica, are incorporated herein by this
reference. The activator may coat from about 0.001% to about 100%,
for example about 5 to about 100 percent of the surface of the
abrasive particle, such as from about 5 to about 80 percent of the
particle surface, or preferably, from about 25 to about 50 percent
of the particle surface.
[0167] In one embodiment, activator is put on to substantially all
the outer surface or all the connected surface, and then activator
is removed by for example washing in heated acids, oxidizers,
and/or chelators to obtain a desired coating, for example between
about 1% and about 25% of surface area coated. The remaining
activator will be very tenaciously bound to the surface, reducing
activator loss due to leaching to the solution.
[0168] The CMP composition or slurry of the present invention may
be prepared using conventional techniques. Typically, the water,
additives, and abrasive components are combined, activator-coated
abrasive is then added, oxidizer is then added, and the pH is
adjusted. Alternatively, according to one aspect of the present
invention, the activator-coated abrasive may be added to an
existing CMP composition, such as a commercially available CMP
composition that contains an oxidizing agent. For example, the
activator-coated abrasive may be added as a slurry to a previously
formulated peroxide composition to provide a CMP composition of
this invention. In some CMP processes, particularly some of the
advanced polishing processes, the composition is prepared by
adjusting the amount of each composition component in real time,
just prior to a re-mixing of the composition at the point of use.
For most CMP processes, the prepared composition is re-mixed at the
point of use, whereupon it is poured onto the polishing pad.
Typically, the composition is poured onto the pad as it is moved or
rotated. As the CMP process proceeds, additional slurry may be
added or excess slurry may be removed, as desired or necessary.
[0169] The compositions of the present invention are all of the
"coated activator" variety, comprising a activator-coated abrasive
rather than solely a free, soluble promoter such as iron nitrate.
As demonstrated above, this relatively stable, activator-coated
abrasive is an extremely effective component of the composition of
this invention.
[0170] The composition of the present invention is advantageously
used in conventional CMP processes, and more particularly, in CMP
processes that call for reduced carrier pressures. Generally,
carrier pressures of from about 0.5 to about 2 psi are considered
low carrier pressures, although this pressure range depends on the
particular CMP process under consideration. Low carrier pressures
are often desirable because they reduce the risk of wafer damage,
such as scratching, delaminating, or destroying of material layers,
particularly metal layers, on the wafer surface. When the
composition of the present invention is used in a
low-carrier-pressure process, desirable material removal rates are
obtainable even though the carrier pressure is low. Appropriate use
of the composition in CMP processes may reduce the risk of wafer
damage and improve wafer yield and performance.
[0171] Additionally, the composition of the present invention may
be advantageously used in the CMP of wafers layered with relatively
fragile films, such as porous films, that have low dielectric
constants. At the pressures used in typical CMP processes, these
films are particularly vulnerable to delamination, crushing, or
other damage. In advanced CMP processes used for these wafers,
carrier pressures of about 2 psi are desirable and carrier and
platen speeds are about the same as, or often greater than, those
used in typical CMP processes. For a wafer layered with a porous
material of relatively low dielectric constant, such as from about
1.5 or about 1.7 to about 2.3, and of about 0.1 micron in
thickness, a removal rate of greater than about 5000 A/mm is
desirable. As demonstrated herein, these removable rates are
obtainable when the composition of the present invention is used in
CMP, even when the carrier pressure is relatively low. The
compositions of the present invention are believed suitable for use
in CMP processes having even lower carrier pressures, such as the
low carrier pressures described above.
[0172] As demonstrated herein, the composition of the present
invention may be used in CMP processes to obtain desirable material
removal rates and within-wafer nonuniformity values. Merely by way
of example, the composition may be used in the CMP of a substrate
surface having a feature, layer or film thereon, such as a film of
aluminum, copper, titanium, tungsten, an alloy thereof, or any
combination thereof. Further by way of example, the composition may
be used in the CMP of such a substrate surface, where the film has
an adjacent or an underlying feature, layer or film, such as a film
of tantalum, tantalum nitride, titanium, titanium nitride, titanium
tungsten, tungsten, and any combination thereof.
[0173] Accordingly, the present invention includes a method of
polishing a substrate surface having at least one feature thereon
that comprises a metal, such as metal or metal alloy feature. The
substrate undergoing polishing may be any suitable substrate, such
as any of the substrates described herein. According to the method
of the invention, a composition of the invention is provided and
the feature on the substrate surface is polished. The polishing is
chemical mechanical polishing, such as that associated with any
conventional or known CMP process, any suitable later-developed CMP
process, or any CMP process described herein. The polishing process
parameters may be any suitable parameters, such as any of the
parameters described herein. For example, the carrier pressure
applied to the substrate surface, or the feature thereon, may be
from about 1 to about 6 psi.
[0174] Generally, the polishing of the substrate surface continues
until the targeted feature or layer is substantially coplanar with
surrounding material, such as an oxide material, on the substrate.
For example, the polishing of a metal-featured substrate may
continue until any metal excess is sufficiently removed to provide
a substantially uniform profile across the substrate surface. By
way of example, suitable surface uniformity (typically measured
using known wafer profiling techniques) is reflected by
within-wafer nonuniformity (WI WVNU) values of less than about 12%,
and preferably, from about 4% to about 6%, the lower values
typically reflecting better process control. Appropriate WIMNU
values may vary depending on the characterstics of the CMP process
and the substrates undergoing polishing.
[0175] The inventive method may be used to remove targeted
material, such as metal or metal alloy, from the substrate surface
at a rate of from about 100 to about 10,000 or to about 15,000
A/mm. The present method may be used to provide a polished
substrate surface of good uniformity, such as a substrate surface
having from about zero to about 40 percent, preferably, from about
zero to about 12 percent, or more preferably, from about zero to
about 10 percent, within-wafer nonuniformity. Further, the present
method may be used to provide a polished substrate surface wherein
any microscratch on the surface that is associated with the
polishing is less than about 20 A. The present invention further
encompasses a substrate produced by the inventive method, including
any of the substrates described herein, and any of the substrates
having any of the qualities, such as desirable uniformity values
and surface characteristics, described herein.
[0176] Generally, the CMP slurry compositions (or liquid
oxidizer/activator coated abrasive combinations) described herein
are useful with little or no modification in all CMP methods and
with all CMP equipment. Unlike certain systems, there is no need to
provide actinic energy to the slurry disposed between a polishing
pad and a substrate being polished.
[0177] Additionally, magnetism and electric field potentials, as
described for example in U.S. Pat. No. 6,030,425 may be useful, but
are not preferred as they unduly complicate the CMP equipment.
Similarly, the use of methods disclosed in U.S. Pat. No. 6,692,362
titled Methods of Producing Hydroxyl Radicals For Chemical
Reactions, including exposure of the slurry and/or substrate to
ultrasound/electrochemical energies to increase the rate of
formation of hydroxyl radicals can be useful to further increase
polishing rates. The use of an electric current and a membrane
comprising an ion exchange material, to quickly remove ions in the
polishing slurry formed by oxidizing the substrate, such as is
described in U.S. Pat. No. 6,722,950, may be useful. Various
aspects and features of the present invention have been explained
or described in relation to beliefs or theories, although it will
be understood that the invention is not bound to any particular
belief or theory. Further, although the various aspects and
features of the present invention have been described with respect
to preferred embodiments and specific examples herein, it will be
understood that the invention is entitled to protection within the
full scope of the appended claims.
EXAMPLES
[0178] The following examples are meant to illustrate, but in no
way limit the claimed invention. One of ordinary skill in the art
will see numerous obvious variations, which are intended to be
encompassed by the claims. Throughout this application, unless
otherwise specified, % is weight percent, temperature is in Degree
Centigrade, all slurries are water-based and have the named
components and the balance of the slurry is water. When expressed
as "ppm", the concentration is parts per million by weight based on
the total weight of the polishing slurry. Unless otherwise
specified, all tests are performed on blanket wafers having one
type of surface (typically CVD deposited tungsten, titanium
nitride, copper, or PETEOS) prepared for polishing experiments.
PETEOS means Plasma enhanced deposition of tetraethoxy silane.
Other abbreviations include "A" or more formally "A" for
angstrom(s); CMP for chemical mechanical planarization, or chemical
mechanical polishing; min for minute(s); ml for milliliter(s); mV
for millivolt(s), psi for pounds per square inch; and rpm for
revolutions per minute.
Comparative Example 1
[0179] The effect of soluble ferric nitrate on a slurry comprising
periodic acid ("PIA"), silica abrasive, and ammonia was monitored
over time as determined as follows. A prior art slurry having 2%
PIA, 3% Silica; 0.15% Ammonia; and an amount varying from 0 to
about 0.5% Fe(NO.sub.3).sub.3 were prepared and adjusted to pH 3.
The ferric nitrate concentrations were 0.00, 0.01, 0.05, 0.10, and
0.50 weight %. Tungsten polishing rates were determined with a
Strasbugh 6EC Polisher putting 6 psi down force pressure/0 psi back
pressure at 90 rpm table speed and 90 rpm carrier speed, and the
slurry flow was 175 ml over 60 sec polishing time, with 4 cycles of
the conditioning the IC1000 groove/Suba IV pad. There are two
components to polishing. The first is rate of substrate removal
rate, "RR", which unless otherwise specified is in Angstroms per
minute. The second is wafer non-uniformity, "NU %", and the
substrate may be also identified, for example WNU % is the Tungsten
wafer non-uniformity, TiNU % is titanium wafer nonuniformity, and
so on. If NU % is large then preferential areas of CMP action and
erosion have occurred, which lowers the process efficiency and the
wafer quality. The data is shown in Table 2.
TABLE-US-00002 TABLE 2 Comparative Examples With Soluble Ferric
Nitrate and Periodic Acid % Soluble Fe(NO.sub.3).sub.3 Tungsten RR
Tungsten NU % TEOS RR 0 2947 5.0 908 0.01 3135 4.7 909 0.05 3637
6.1 890 0.10 3627 6.6 869 0.50 3686 5.9 855
[0180] The removal rate of W increases by about 20% with 0.05% of
dissolved ferric nitrate in periodic acid, over the removal rate of
W in a slurry that is free of ferric nitrate. The increase was
accompanied by a small but significant increase in WNU %. Higher
quantities of dissolved ferric nitrate show little benefit. The
TEOS removal is not sensitive to the ferric nitrate level.
[0181] The effect of pH on the soluble iron/periodic acid system at
a ferric nitrate concentration of 0.01% was determined as follows.
A slurry was prepared by admixing 595 g deionized water, 200 g of
10 wt % periodic acid solution in water, 200 g of 30 wt % colloidal
silica, a trace, for example 5 g of 30 wt % ammonia was added to
the above slurry to adjust pH from 1.8 to 3 and to 7, and 0.22 g of
45% ferric nitrate solution was added, resulting in a slurry having
2% PIA, 3% Silica; and 0.01% Fe(NO.sub.3).sub.3. Polishing rates
were determined with a Strasbugh 6EC Polisher putting 6 psi down
force pressure/0 psi back pressure at 90 rpm table speed and 90 rpm
carrier speed, and the slurry flow was 175 ml over the 60 sec
polishing time, with 4 cycles of the conditioning the IC1000 K
groove/Suba IV pad. The results are shown in Table 3.
TABLE-US-00003 TABLE 3 Comparative Examples With 0.01% Soluble
Fe(NO.sub.3).sub.3 and Periodic Acid Slurry pH Tungsten RR Tungsten
NU (%) TEOS RR 1.8 3435 3.3 879 3.0 3494 4.1 893 7.0 3418 4.4
547
[0182] The pH had no significant effect on the tungsten polishing
rate. Ferric hydroxide (Fe(OH).sub.3) starts to precipitate at pH
2.7 and substantially completely precipitates at pH 4 when
[Fe.sup.3+]=0.01M (0.06% Fe). At pH 7 the Fe species should be in
the form of iron hydroxides which were expected to be inactive.
Without being bound by theory, it is believed that iron exists for
at least some time as some other species (not simply Fe.sup.+3) in
the slurries, and the most likely form is a pseudo-stable ferric
nitrate complex in or with water. Using the same slurry and
polishing system, the effect of the periodic acid concentration was
determined on these comparative example slurries, as shown in Table
4.
TABLE-US-00004 TABLE 4 Comparative Examples With 0.01% Soluble
Ferric Nitrate and Periodic Acid Slurry PIA, (wt %) Tungsten RR
Tungsten NU (%) TEOS RR 0.0 584 8.8 0.5 2226 8.8 886 1.0 3192 4.3
886 1.5 3433 3.8 905 2.0 3627 3.8 869
[0183] In the presence of constant iron activator concentration,
the tungsten removal rate increases with increasing concentration
of PIA, though the TEOS removal is not sensitive to the PTA
concentration. The amount of increase in tungsten removal rates is
greatest at low concentrations of PIA, and at concentrations
outside the range of 0.5% to 2% PLA further increases in the PTA
concentration have dubious value. Interestingly, the W-NU %
decreases sharply from 8.8% at 0.5% PTA to 4.3% at 1% PIA to 3.8%
at 1.5 PTA. Apparently having an excess of oxidizer, e.g., at least
1% of PIA, is necessary to achieve low NU %
[0184] Finally, the effect of various abrasives were determined on
these Comparative Examples, using the same 2% PTA, 0.01% ferric
nitrate, 3% abrasives slurries, again using the same polishing
parameters as discussed above. The data is in Table 5. The data
from the fumed & colloidal silica could be skewed because there
were a number of large silica particles in the slurry.
TABLE-US-00005 TABLE 5 Abrasive Tungsten RR Tungsten NU (%) TEOS RR
Colloidal Silica 3946 5.4 465 Alumina 3862 3.2 301 Fumed Silica
3797 11 -- Fumed and Colloidal 4843 5.6 528 Silica
[0185] Clearly, the combination of fumed silica and colloidal
silica gave the greatest removal rates. Unless used with colloidal
silica, however, fumed silica is less preferred.
[0186] The best system for a periodic acid/soluble ferric nitrate
sturry, especially for tungsten, has 1.5 to 2.4, for example 1.8 to
2.2, weight percent periodic acid; a pH of 1.5 to 4, for example
2.8 to 3.5, if modest loss of the dielectric TEOS is not a problem,
though a pH of about 4 to about 8, preferably about 6 to about 7,
is preferred if greater selectivity between the tungsten and the
dielectric is desired; a ferric nitrate concentration of between
0.01 and 0.05 weight percent; and between about 2 to 4 weight
percent of either alumina or silica, with alumina or a mixture of
fumed and colloidal silica with between 30% and 70% of the silica
being colloidal being preferred. These formulations, i.e.,
dissolved ferric ions and hydrogen peroxide, are unstable, and
ferric ions contaminate substrates.
Comparative example 2 and Example 2B
[0187] A series of tests were performed to determine whether
iron-coated silica performed better than a comparative example
where ferric nitrate was added to a pad. We performed additional
comparative experiments with iron nitrate solutions or iron nitrate
impregnated into a polishing pad and using a periodic
acid-containing slurry, and these experiments clearly showed a
lower tungsten polishing rate suggesting that free iron ions were
not very effective for W polishing. With iron, the best substrate
removal rate using soluble ions was about 60% of the absorbed iron
(Fenton's process) using periodic acid. Simply adding iron salts to
a silica does not cause the iron to become associated with the
silica. The presence or addition of silica to the a soluble iron
salt/oxidizer system, the tungsten removal rates do not show
appreciable increases. Further, the free iron catalyzes rapid
decomposition of oxidizers, in particular hydrogen peroxide, such
that commercial formulations of iron and oxidizer include chelator
to prevent rapid decomposition of the hydrogen peroxide.
[0188] For all tests, the polishing system was a Speedfam IPEC 472
Polisher with a Rodel IC1000 k grooved/IV polishing pad, with 6 psi
downforce, 0 psi backpressure, 110 rpm table speed and 70 rpm
carrier speed, and finally with 150 ml slurry flow to polish a
Sematech W blanket wafer. The slurry gross compositions were
identical, each having 2% PIA and 3% silica. Comparative Example 2A
(comparative) contained no ferric species, on a clean pad. In
Example 21 the silica was coated with about 25% of outer surface
area having a monolayer of absorbed iron (about 0.02% by weight of
iron based on the weight of the slurry). In Comparative Example 2C
the polishing pad soaked in 10% of ferric nitrate for 2 hours prior
to use, as described in U.S. Pat. No. 6,383,065 B1 (ferric nitrate
solution entered pad). In Comparative Example 2D, using the same
pad as in 2C, with the pad then soaked in 10% of ferric nitrate for
24 hours, and then dried, and then conditioned, prior to use. The
results are shown in Table 6, where polishing conditions were the
same.
TABLE-US-00006 TABLE 6 Tungsten RR NU % Example 2A (comparative)
2290 16.3 Example 2B 3430 7.9 Example 2C (comparative) 2270 15.3
Example 2D (comparative) 2070 15.3
[0189] The example of the current invention (example 2B) showed
significantly greater removal rates and much better uniformity than
any of the three comparative examples. The iron-coated-on-silica
slurry activates the Fenton's reaction to give tungsten removal
rate 3426 A and NU % of only 7.9%. Applying a fresh 10% ferric
nitrate solution on the pad that was already "conditioned" with the
very active iron-coated-on-silica slurry, and then polishing using
the PLA/uncoated silica slurry, only provided a tungsten polishing
rate of 2270 A with a high NU %. Apparently, at least for a system
using periodic acid as the oxidizer, soluble ferric nitrate on a
pad has little effect in terms of substrate removal rate, and
soluble ferric ions need to be on the substrate to have any effect.
These experiments show that the embodiments of U.S. Pat. No.
6,383,065 B1 were inoperable in PTA silica system.
Comparative Example 3
[0190] A known organic free radical initiator, Vazor.TM. 44 WSP
available from DuPont which is believed to be a hexanitrile
compound, was added to give a concentration of 1% Vazor in an
aqueous slurry containing 2% PIA and 0.16% iron-coated silica and
0.44% uncoated silica. Surprisingly, the addition of the organic
free radical initiator significantly decreased the removal rate of
tungsten, by as much as 75%, compared to the removal rate of
tungsten using the same slurry but without the organic free radical
initiator.
Example 4
[0191] We have surprisingly found that, unlike very low
concentrations of soluble iron salts which are not effective or are
very inefficient at concentrations below 20 ppm, very low
concentrations of iron-coated silica are effective to substantially
increase the removal rate of substrates. Two iron-coated silica
slurries were prepared. The silica was colloidal silica with a size
range of about 80 nm untreated, 100 nm iron-coated, and all
particles fell within the preferred range of 50 to 150 nm in
diameter. A trace amount of sulfuric acid was added to each slurry
to adjust the pH. The polishing conditions were similar to example
1.
[0192] The first slurry used to polish tungsten/TEOS wafers
contained 0.16% iron-coated silica, 0.44% uncoated silica, and 3%
hydrogen peroxide. The 0.16% iron-coated silica had approximately
11 ppm of surface-bound iron compared to the weight of the slurry.
The second slurry used to polish tungsten/TEOS wafers contained 1%
iron-coated silica and 3% hydrogen peroxide. The 1% of iron-coated
silica has about 66 ppm of surface-bound iron compared to the
weight of the slurry.
[0193] In this specification, when the activator is expressed in
parts per million, unless otherwise stated, this is the weight of
the activator metal compared to the total weight of the polishing
slurry. On the other hand, when the amount of activator-coated
abrasive is specified, unless otherwise stated, this is the
combined weight of the coated abrasive (which includes the weight
of activator), and the value is generally expressed as percent by
weight of the slurry.
[0194] Polishing rates on tungsten wafers were in the range of 3500
angstroms per minute to the first slurry and 5000 angstroms per
minute for the second slurry, Oxide polished at about 300 angstroms
per minute with the first slurry and 500 angstroms per minute with
the second slurry. TEOS polished at a rate of 50 angstroms per
minute with the first slurry and 400 angstroms per minute with the
second slurry. The % NU was excellent and within commercially
acceptable values. Additional tests were performed with slurries
containing 0.32% and 0.64% by weight of iron coated silica (which
corresponds to 21 to 43 ppm of surface-bound iron at point of use).
Substrate removal values fell as predicted between those of the
0.16% and 1% iron coated silica experiments.
Example 5
[0195] The performance of the iron-coated silica is reproducible
and is not overly wearing on equipment or disposables. A slurry
having 2% PIA, 0.6% acetic acid, 0.95% TMAH, and 3% iron-coated
silica (pH=3.5) was prepared. A number of wafers were tested,
sequentially, using a Speedfam IPEC472 Polisher, with 6 psi down
force pressure, 0 psi back pressure, 10 rpm table speed, 100 rpm
carrier speed, and an 150 ml slurry flow over a 60 sec polishing
time, with in-situ conditioning of a IC1000 K groove/Suba IV pad.
Tungsten removal rate were about 4900 angstroms per minute with
less than a 6% variation in the tungsten rate over a 26 wafer test,
and the nonuniformity (NrU %) was acceptable and showed only slight
changes over the 26 wafer test.
[0196] Additionally, again unlike soluble iron salts which reach a
maximum efficiency at 0.1% to 0.2% by weight, we have found that
the iron-coated silica, using "typical" concentrations of oxidizer
and abrasive, can increase rates to well beyond what is readily
controllable. At concentrations of above 6%-10% hydrogen peroxide
and 4%-6% iron-coated silica, tungsten removal rates .about.10000
angstroms per minute or greater were measured.
Example 6
[0197] This example describes the preparation of boron
surface-modified colloidal silica starting with colloidal silica
particles having an average particle diameter of 40 to 55
nanometers. The procedure to prepare activator-coated silica
advantageously starts with the preparation of de-ionized silica,
followed by addition of surface-modifying stabilizer and/or
activator salts, where some modifications of conditions may be
necessary to make the activators and inorganic stabilizers adhere
to the surface of the silica. Generally, when preparing the coated
abrasives, additions of material are done slowly to avoid very high
local concentrations of ingredients, as is taught by U.S. Pat. No.
3,922,393, the disclosure of which is incorporated herein by
reference thereto. U.S. Pat. No. 3,922,393 describes a process of
coating silica with alumina at high pH, while much of the coating
of particles of the present invention is done at low pH which has
the benefit of not requiring a large change in pH before adding
activator salts to the slurry to prevent precipitation of activator
salts as hydroxides
[0198] Preparation of De-ionized Silica: Approximately 1 kg of
AMBERLITE IR-120 ion exchange resin (available from Rohm and Haas
Company, Philadelphia, Pa.) was washed with 1 liter of 20% aqueous
sulfuric acid solution. The mixture was stirred and the resin was
allowed to settle. The aqueous layer was decanted and washed with
10 liters of deionized water. The mixture was again allowed to
settle and then the aqueous layer was decanted. This procedure was
repeated until the decanted water was colorless. This procedure
afforded an acidic form of resin. Then, 12 kg of SYTON.RTM. HT 50
(a potassium hydroxide-stabilized colloidal silica (available from
DuPont Air Products NanoMaterials L.L.C., Carlsbad, Calif.) was
placed in a five-gallon mix tank equipped with an agitator. 2.502
kg of deionized water were added to the tank and the solution was
allowed to mix a few minutes. The pH of the solution was measured
to be approximately 10.2. With continued pH monitoring, aliquots of
the previously-prepared acid-state resin were added, while allowing
the pH to stabilize n between additions until the stable pH had
dropped to pH 1.90-2.20. Once this pH limit had been reached and
was stable in this range, no further resin additions were made and
the mixture was stirred for 1-1.5 hours. At this time, it is
preferred to add stabilizers, activators, or both, in any order, to
the deionized silica.
[0199] Step 1: Adding Boron-Containing Stabilizer to Surface of
Silica: The above-prepared mixture was passed through a 500-mesh
screen to remove the resin and afforded deionized SYTON HT 50 at pH
2. A solution of 268 g of boric acid powder (Fisher Scientific,
2000 Park Lane, Pittsburgh, Pa., 15275) in 5.55 kg of deionized
water was prepared in a 10 gallon mixing tank equipped with an
agitator and a heater by slowly adding the boric acid powder until
all had been added to the water and then agitating the mixture for
15 hours and increasing the temperature of the mixture to
55-65.degree. C. The deionized and wetted SYTON HT 50 (12 kg silica
and 2.5 kg water at pH 2) was then added to the boric acid solution
slowly over about 1.2 hours by adding it at approximately 200
ml/minute and maintaining the temperature greater than 52.degree.
C. while agitating the mixture. After this addition was completed,
heating at 60.degree. C. and agitation of the mixture were
continued for 5.5 hours. While not done here, heating to higher
temperatures such as from 60.degree. C. to 100.degree. C., for
example from 85.degree. C. to 100.degree. C., may provide a
composition exhibiting even greater stability. The resulting
solution was subsequently filtered through a 1-micron filter to
afford boron surface-modified colloidal silica.
[0200] This boron surface-modified colloidal silica was
characterized for colloid stability over 15 days using a Colloidal
Dynamics instrument (11-Knight Street, Building E8, Warwick, R.I.,
02886), and was found to exhibit both constant pH (pH approximately
6.6) and zeta potential (zeta potential approximately -58
millivolts) over the 15-day test period. The percentage of surface
sites of this surface-modified colloidal silica occupied by
boron-containing compound(s) was approximately 98%.
[0201] Step 2: Reaction of Boron-modified SoI with Ferric Nitrate:
An aliquot (.about.1000 grams) of wet boron-modified silica from
Step 1 was transferred to a 4-liter beaker. Under agitation, 5.1
grams of ferric nitrate (1.2 grams iron) were added to the
boron-modified silica. The mixture was heated between 45.degree. to
50.degree. C. for 2 hours and 50 minutes. Again, heating to higher
temperatures may provide greater degree of absorption of iron onto
the silica, or shorten the time required for absorption, or both.
After heating the mixture, the dispersion was cooled. The
zeta-potential changed from 58 millivolts to +10.2 millivolts,
indicating addition of the iron to the surface of the particle. The
pH of the slurry was measured as 2.09.
[0202] The ready addition of the iron to the surface of the
particles was surprising. This provided a surface modified hybrid
bimetallic (B/Fe) sol having about 1.2 grams (0.02 moles) of ferric
ions, 22 grams (0.356 moles as H.sub.3BO.sub.3) of boric acid, and
1000 grams (16.64 moles) of silica. Since we had determined that
the first treatment with the boric acid stabilizer covered about
98% of the surface area, we would have expected less than 0.01
moles of ferric ions to be absorbable directly onto the remaining
silica (assuming parity of size of the boric acid stabilizer and
ferric ion activator). Since more than twice this much iron was
readily absorbed onto the surface-modified silica, we concluded
that at least a portion of the iron was bound to the
boron-containing stabilizer as opposed to being bound directly on
the silica. Significantly, the silica contained .about.2.4% by
weight of surface-modifying components, based on the weight of the
modified silica.
Example 7
[0203] It is not necessary to add the boric acid stabilizer before
adding the iron nitrate activator to de-ionized silica.
Additionally, the amounts of iron and boric acid can be varied over
a large range. In this procedure, deionized SYTON HT 50 at pH 2
(600 grams, supplied by DuPont Air Products NanoMaterials L.L.C.),
prepared as described in Example 3, was transferred to a 4-liter
beaker. Under agitation, 400 grams of deionized water were added,
followed by the addition of a mixture of boric acid (12 grams) and
ferric nitrate (10.1 grams). Recall that .about.2.25 grams of boric
acid will occupy substantially all of the surface sites on 100
grams of deionized SYTON HT 50, so the percentage of surface sites
estimated to be covered by stabilizer was .about.89%. After the
addition of boric acid and ferric nitrate, an additional 278 grams
of water were added. The mixture was heated between 45 and
50.degree. C. for 2.5 hours. After heating, the mixture was cooled,
the pH was 1.67, and the zeta potential was +16.4 millivolts. The
600 grams (10 moles) of silica had the surface thereof modified by
12 grams (0.2 moles) boric acid and 2.3 grams (0.04 moles) of the
Fe ion. The molar ratio of iron to silica was 0.04:10 and the molar
ratio of iron to boric acid was 0.04:0.2 or about 1:5.
Example 8
[0204] In this example, a slurry such as was described in Example 6
was prepared using boron-iron-modified silica using a molar ratio
of iron to silica of 1:4 (0.25) and a molar ratio of iron to boric
acid of 1:4.3 (0.23). The pH of the surface modified was 2.11, and
the zeta potential was +22.2 millivolts. This example shows that
the pH and zeta potential can be controlled by changing the
concentration of ferric nitrate or boric acid.
Example 9
[0205] Other stabilizers are also useful. Deionized SYTON HT 50 was
prepared in accordance with Example 1. 10.1 grams (.about.0.04
moles) of ferric nitrate were added to the deionized SYTON IT 50
(600 grams, pH=1.9 to 2.1). The mixture was heated for 1 hour at
50.degree. C. In a separate beaker, 300 grams of deionized water
were charged, and kept under agitation. To this water solution,
sodium tungstate (10.9 grams, .about.0.03 to 0.04) was added under
agitation during a period of 10 minutes. This solution had a pH of
7.14. After completing the addition of sodium tungstate to the
deionized water, 5 grams of 70% nitric acid were added to adjust
the pH to 5.02. The mixture was stirred at room temperature for an
additional 10 minutes. The tungstate solution was then added to the
iron-modified silica particles, and stirred for an additional 2
hours, Note that there are about the same number of nmoles of
tungstate as there are ferric ions on the silica, and that the
ferric ions were added before adding the tungstate, both of which
we believe may contribute to reduced activity of the composite. The
pH was 6.13.
Example 10
Polishing Experiments
[0206] In Table 4, polishing compositions are tabulated. The
polishing composition in comparative Example 10-A was prepared
using boron-modified silica (no activator) and the preparation
method of Example 6 (Step 1). The polishing composition in Example
10-B was prepared using bimetallic "boron-iron" modified silica and
the preparation method of Example 6 (Steps 1 and 2). The polishing
composition in Example 10-C was prepared using "iron-tungsten"
modified silica and the preparation method of Example 7. The
polishing composition in comparative Example 10-D was prepared
using boron-modified silica, the preparation method of Example 1
(Step 1), and soluble iron catalyst. A portion of the soluble iron
catalyst may have absorbed onto the boron stabilized silica. The
polishing composition in Example 10-E was prepared using
"boron-iron" modified silica of Example 8 and the preparation
method of Example 6 (Step 2). The polishing compositions were used
to polish CVD tungsten blanket wafers and PETEOS blanket wafers
(PETEOS, plasma enhanced tetraethoxy silane, dielectric oxide
layer) using a polishing tool. The blanket wafers were purchased
from Silicon Valley Microelectronics, 1150 Campbell Ave Calif.
95126. The PETEOS wafers had a film thickness specification of
15,000 .ANG. PETEOS. The CVD tungsten wafers had film stack
thickness specifications of 8000 .ANG. CVD tungsten/250 .ANG.
titanium/6300 .ANG. thermal oxide. A SpeedFam IPEC, model 472
(manufactured by SpeedFam IPEC, 305 North 54th street, Chandler. AZ
85226), polishing machine was used with conditions set as follows:
Down force 7 psi (pounds per square inch); Flow rate of polishing
composition=170 ml/min (milliliters per minute); Back pressure=0.5
psi; Carrier speed=35 psi; Platen speed=40 revolutions per minute
(RPM). The substrate was effectively planarized using the above
polishing composition under the stated conditions. The polishing
data is presented in Table 7.
TABLE-US-00007 TABLE 7 TABLE EX. 10-A EX. 10-D asd (comparative)
EX. 10-B EX. 10-C (comparative) EX 10-E Notes Boron modified SYTON
HT 50 SYTON HT 50 Boron modified SYTON HT SYTON HT 50 with modified
with modified SYTON HT 50 50 with (control) H.sub.3BO.sub.3--Fe--,
Fe-tungstate, with soluble modified bimetallic bimetallic ferric
nitrate as H.sub.3BO.sub.3--Fe--, surface surface soluble catalyst
bimetallic (35 ppm) surface pH 2.8 2.8 2.8 2.8 2.8 Silica 180 gram
180 grams 180 grams 180 grams 180 grams Activator -- 0.2 grams Fe
0.7 grams Fe -- 0.7 grams Fe (ppm) Stabilizer 4 grams BO.sub.3 4
grams BO.sub.3.sup.-3 2.6 grams WO.sub.4.sup.-2 4 grams BO.sub.3
3.5 gms BO.sub.3.sup.-3 (ppm) Water 3249 grams 3249 grams 3249
grams 3249 grams 3249 grams H.sub.2O.sub.2 400 grams 400 grams 400
grams 400 grams 400 grams (30%) Tungsten 427 .ANG./min 3880
.ANG./min 2655 .ANG./min 2049 .ANG./min 2751 .ANG./min removal rate
PETEOS 85 .ANG./min 217 .ANG./min 87 .ANG./min 83 .ANG./min 91
.ANG./min removal rate Selectivity 5 18 31 25 30 (W-RR/ PETEOS-
RR)
[0207] Clearly the method of preparation of the activator
iron-stabilizer modified silica had a resultant effect on the
polishing rates. The highest tungsten removal rate was observed on
the polishing system of Example 10-B (Example 6), wherein it is
believed that most or all of the iron activator was bonded to a
borate-based stabilizer which was in turn bonded to the silica
surface. This is the case despite example 10-B having less
activator than either 10-C or 10-E, suggesting that the use of
stabilizer-addition to near saturation of the available surface
sites before adding the activator, and/or the use of boron-based
activator prior to adding activator, seems to provide a synergistic
effect. While greater amounts of Fe activator were present in
Examples 10-C and 10-E, a portion or most of the iron is believed
to be bonded directly to the silica, and the stabilizer may in part
be shielding the activator. Additionally, it appears that the high
rates of Example 10-B are accompanied by a small decrease in the
tungsten: PETEOS selectivity, from about 30 to just under 18.
[0208] It is not known whether, or to what extent, the soluble
ferric nitrate added in Example 10-D subsequently was absorbed onto
the boric acid-surface-modified silica.
[0209] Tungstate-stabilized iron-coated silica also appears to be
less effective in promoting increased tungsten removal rate than
does the boric acid-stabilized iron coated silica.
Example 11
[0210] A slurry was prepared containing 4% peroxide, 1.25% 130 nm
silica, and 0.3% of 80 nm coated silica, and variable amounts of
lactic acid. This slurry was used to polish tungsten using the
following polishing parameters: down force 5 psi, back pressure 2.5
psi, Ring pressure 5.5 psi, Table rotation 110 RPM, Carrier
rotation 105 RPM, 150 ml/min slurry flow, using Strasbaugh 6EC
polisher, Rodel IC1000/SUBA IV pad, and Rodel DF200 Carrier film,
pH adjusted to 3.4 with ammonium hydroxide. The polishing results
of sequential tests using the same polishing pad are shown
below.
TABLE-US-00008 Lactic acid Angstroms/min 0 4060 0.1% 4300 0.3% 4380
0.5% 3710
[0211] It can be seen that a small amount of lactic acid, tat is,
from about 0.1% to about 0.3% by weight, can advantageously improve
the tungsten removal rate. Too much lactic acid, however, and the
tungsten removal rate drops. Therefore, it is beneficial to use a
chelator in an amount between about 0.01% to about 1%, but better
results are obtained if the chelator is present in an amount
between about 0.05% and 0.5%, for example between about 0.1% and
0.3%. Under the conditions of the above-described test, using an 8
inch wafer, a slurry flow rate of 150 ml/minute, and a polishing
rate of about 4300 angstroms per minute of tungsten, a lactic acid
concentration of 0.1% by weight will provide about 0.8 to 0.9 moles
lactic acid per mole of tungsten polished. For chelators that can
chelate one metal atom or complex, it appears advantageous to have
between 0.5 moles and 4 moles, for example from 0.8 moles to 3
mole, or alternatively from about 1 mole to about 2.7 moles, of
chelator per mole of metal being removed from the substrate. While
a small amount of chelator can be useful, after a certain point
addition of more lactic acid results in decreasing tungsten removal
rate, probably due to a combination of lactic acid forming a film
over the tungsten, free radical quenching effects, or both.
Example 12
[0212] We have experimentally found that tungsten wafers polished
with commercially available ferric nitrate/oxidizer/abrasive CMP
formulations leave iron residues on the wafer at amounts between
100.times.10.sup.+10 Atoms/cm.sup.2 to 200.times.10.sup.+10
Atoms/cm.sup.2 even after post-CMP cleaning and rinsing. Certain
preferred systems of the current invention, in which iron is bound
to silica, leave iron residues on the wafer at less than about
10.times.10.sup.+10 Atoms/cm.sup.2 or less after post-CMP cleaning
and rinsing. There is a strong desire to further minimize metallic
contamination of the substrate by metal ions, including but not
limited to the metal ion activators which become detached from the
surface of a particle, metal ions polished from the substrate
surface, as well as metal ion contamination from other sources.
[0213] The purpose of this Example was to show the efficiency for
reducing Fe-contamination on wafer surface after Tungsten CMP of
(i) CMP3600.TM. (CMP3600.TM. is a commercially available slurry
from DuPont Air Products NanoMaterials L.L.C., Tempe, Ariz.,
developed at DuPont-EKC Technologies) versus (ii) formulations
derived from the addition of organic additives to CMP3600.TM.. The
CMP3600.TM. is a "conventional" ferric ion-coated silica. The
formulation modifications of CMP3600.TM. are the inclusion of
organic additives to reduce the Fe contamination level on the
wafer; after W CMP and standard post CMP cleaning with dilute
ammonia solution. A previous screening effort had shown lactic acid
and ascorbic acid are more efficient at reducing trace Fe
contamination on the wafer surface post CMP than other chelators
tested.
[0214] The aqueous slurry contained about 0.5% CMP3600.TM. and
about 4% hydrogen peroxide. Polishing tests were performed using an
Ebara EPO222D.TM. Polish Tool, a Rigaku TXRF Measurement Tools (at
3 pts: 0.0; 0.50 and -50.0), a KLA Tencor SP1 (10 mm E), 4
Dimension 4 point probe: 49 pt line and KLA Tencor F5x: 49pt line.
The following table 8 compares the results. Note that the values in
the table below are measured prior to post-CMP cleaning and rinsing
with a dilute ammonia solution.
TABLE-US-00009 TABLE 8 Iron Contamination After CMP With Iron
Coated Silica followed by Rinsing, With No Post-CMP Cleaning Iron
On Wafer After CMP W Removal Rate Additive % Additive
(.times.10.sup.+10 Atoms/cm.sup.2) (.ANG./min) None 0 377 4010
Lactic Acid 0.14 62 4980 Lactic Acid 0.28 39 5370 Lactic Acid 0.42
30 5280 Ascorbic Acid 0.01 357 4580 Ascorbic Acid 0.04 94 4150
[0215] From the results given in the table above, it is obvious
that the addition of lactic and ascorbic acids to CMP3600.TM. would
result in the desired reduction in Fe contamination of the wafer
surface after performing W CMP and standard post CMP cleaning with
dilute ammonia solution.
[0216] Indeed, as shown in the previous example, addition of lactic
acid (at least to amounts less than 0.5%) showed substantial
increases in tungsten removal rate. The use of ascorbic acid gave
similar results, though there was less of an increase in the
tungsten removal rate and the rate of removal starts rapidly
declining with the addition of increasing amounts of ascorbic acid.
Ascorbic acid is, between the two, more effective both on a weight
percent basis and on a molar basis. The addition of lactic acid, in
amounts varying between 0.14% and 0.42%, significantly increased
the tungsten removal rate. The decline in tungsten removal rate
with the further addition of ascorbic acid is problematic, but the
reduction in Fe contamination did not decrease the W removal rate
to any rate below the control rate (with no organic acid
added).
[0217] The use of lower concentrations of activator in the slurry,
the presence of stabilizers on the abrasive, and the presence of
a,b-dihydroxy enolic compounds such as ascorbic acid in the fluid,
can greatly reduce this iron contamination. Additionally,
conditioning the activator-coated abrasive, for example by storage
in dilute (.about.200-2000 ppm) aqueous ascorbic acid for a time
between an hour and days, followed by separation and removal of the
aqueous composition, can provide a activator-coated abrasive that
is less likely to leave activator metal atoms on the surface of the
substrate. Additionally, exposing an aqueous slurry of
activator-coated abrasive to an elevated temperature, say between
about 70.degree. C. and 120.degree. C., more typically from
80.degree. C. and 100.degree. C., for a period of time can provide
a activator-coated abrasive that is less likely to leave activator
metal atoms on the surface of the substrate.
Example 13
[0218] We have found that a number of chelators, particularly
lactic acid, citric acid, and the like, are useful for minimizing
contamination from each of metal ion activators which become
detached from the surface of a particle, metal ions polished from
the substrate surface, as well as metal ion contamination from
other sources. On the other hand, alpha-, beta-dihydroxy enolic
compounds (enolic metal chelators with two hydroxyl attached to an
olefinic double bond, the most commercially available of which
include ascorbic acid, erythorbic acid, and derivatives and/or
mixtures thereof. Useful derivatives of ascorbic acid and
erythorbic acid are soluble (to the extent they are present in
solution, which is typically below 5000 ppm) and do not hinder the
action of the dihydroxy enolic functionality. Exemplary derivatives
include ascorbic palmitate and erythorbic palminate. These alpha-,
beta-dihydroxy enolic compounds have been found not only to prevent
deposition of metal ions on the surface of the substrate, but are
also believed to actively and efficiently strip metal ions absorbed
onto the surface of the substrate, which is typically silica or a
low-k silicon-containing material. This leads to a problem in that
most activators are metal ions absorbed on the abrasive, which is
typically silica, and dihydroxy enolic compounds can under certain
situations strip activator from the surface of an abrasive particle
and solubilize same. We have found that such complexed activator is
still useful in increasing substrate (e.g., tungsten) removal rates
during CMP, possibly to a greater extent than does "simple" metal
ion complexes such as provided by ferric nitrate, but the increase
in substrate removal rate is less than if the activator were
attached to the abrasive surface.
[0219] In the absence of alpha-, beta-dihydroxy enolic compounds,
most activator is absorbed on the surface of an abrasive. A boron
modified silica was prepared using a procedure as described
previously in Example 6. The boron modified silica was treated to
absorb or bond 7 ppm of ferric ion (based on the weight of the
slurry) on the boron modified silica and the pH was adjusted to at
pH=3.5. The "boron-ferric" ion modified silica dispersion was
centrifuged to separate the abrasive particles. Using ICPMS, "NO"
detectable iron (e.g., less than 0.1 ppm) was detected in the
solution phase. This suggests immobilization of ferric ion on the
boron coated silica at pH=3.5.
[0220] The ferric ion coated silica was mixed with 100 ppm of
ascorbic acid at pH=3,5. The addition of ascorbic acid eventually
stripped most of the 7 ppm ferric ion from the silica surface.
[0221] We found there was a strong effect of pH on the stripping
efficiency of ascorbic acid in the slurry formulation. At pH=2.5,
the water having 100 ppm ascorbic acid stripped all of 7 ppm ferric
ion from the silica surface. At pH 3.5, the water having 100 ppm
ascorbic acid stripped substantially all of 7 ppm ferric ion from
the silica surface.
[0222] A boric acid surface-modified silica was surface modified
with 7.6 ppm of iron, and the slurry was adjusted to pH 4.0 after
addition of 200 ppm of ascorbic acid. Subsequent analysis showed
5.7 ppm Fe in solution, implying about 1.9 ppm of the iron remained
absorbed on the surface of the abrasive and acts as an activator.
At pH about 6 or above, the ascorbic acid strips only a minor
amount of the iron activator from silica. At pH 6, a slurry having
100 ppm ascorbic acid stripped only 2.4 ppm ferric ion (about one
third) of the original 7.6 ppm of iron from the silica surface of a
slurry, leaving about 5 ppm (y weight of the slurry) of iron
disposed on the surface of the abrasive. A boric acid modified
silica was surface coated with 5.6 ppm of iron, and held at pH 6.5
in an aqueous slurry having 100 ppp ascorbic acid, and only 0.14
ppm iron was subsequently found in the aqueous phase. An
independent test at an outside testing agency found only 0.15 ppm
of the 5.6 ppm of iron originally present as activator disposed on
the surface of abrasive to be solubilized by water at pH 6.5 having
100 ppp ascorbic acid.
[0223] We have found that that alkyne diols, such as found for
example in Surfynol 104E (commercially available from Air Products)
forms a complex with soluble iron, and the "Fe-alkyne complexe"
increased tungsten removal rates in a slurry comprising hydrogen
peroxide. We believe that the "Fe-alkyne complexes" may be more
effective than soluble iron, for example as provided by dilute
ferric nitrate, when present in a polishing slurry having a
peroxide oxidizer, at increasing the substrate removal rates. That
is, the effectiveness at increasing the tungsten removal rate
during polishing may be between that of activator iron and that of
soluble iron nitrate promoters. A boric acid-modified silica was
surface coated with 7.6 ppm of iron, and held in an aqueous slurry
comprising 100 ppm ascorbic acid and 200 ppm of Surfynol 104E at a
pH of 6.5. Subsequent analysis showed that 1.5 ppm of the 7.6 ppm
of iron was stripped from the boric acid-surface-modified silica
and was solubilized by the ascorbic acid and/or Surfynol 104E.
[0224] Further, we found that the pH apparently had a effect on
polishing rates. Using a slurry to polish a tungsten-coated wafer,
under conditions similar to those described for Example 10, gave
the results in Table 9.
TABLE-US-00010 TABLE 9 Total Fe, ppm based on Tungsten Removal the
wt of the slurry pH (angstroms/min) 4.6 3.5 2900 6.6 3.5 2700 6.6 6
3120 6.6 2.5 3550
[0225] We had earlier shown that low pH, e.g., pH between 2.5 and
3.5, was preferred in a system using iron-activator-coated silica
and a per-type oxidizer, which we attributed to the higher
solubility of the tungsten by-products polished from the surface of
the substrate. We then found that high polishing rates could be
achieved at more neutral ph, e.g., between 4 and 7, preferably
between 4 and 6, in the presence of appropriate chelators. We now
found that, due to the propensity of ascorbic acid (and presumably
other di-hydroxy enol compounds) to strip iron activator from
silica, we have found that a preferred pH range for polishing
tungsten is about 6 to about 7, preferably from about 6 to about
6.5. Based on the above data, from the standpoint of tungsten
removal rates pH=6 appears to perform better than pH=3.5.
Interestingly 4.6 ppm seem to have about the same tungsten removal
rates as 6.6 ppm ferric ion at pH=3.5.
Example 14
[0226] A slurry was prepared containing 4% hydrogen peroxide, 1.25%
130 nm uncoated silica, and variable (X) amounts of 80 nm coated
silica. This slurry was used to polish tungsten using the following
polishing parameters: down force 5 psi, back pressure 2.5 psi, Ring
pressure 5.5 psi, Table rotation 110 RPM, Carrier rotation 105 RPM,
150 ml/min slurry flow, using Strasbaugh 6EC polisher, Rodel
IC1000/SUBA W pad, and Rodel DF200 Carrier film. Tungsten removal
rates are shown below.
TABLE-US-00011 Activator-coated Silica Angstroms/min Delta from 0%
(A/min/0.1%) 0 980 0.1 2910 1930 0.2 3740 1380 0.4 3800 705 0.8
7070 760
[0227] The coated silica was extremely effective at very low
concentrations. We believe that even as little as 0.01% iron-coated
silica would be economically significant in terms of increasing the
polishing rate of tungsten. Further, while the greatest gains in
substrate removal rate per unit quantity of activator-coated
abrasive added to a polishing composition are observed at the
lowest quantities of activator-coated abrasive, there is no
apparent "leveling off" of the substrate removal rates with
addition of greater amounts of activator-coated abrasive.
[0228] Another slurry was prepared containing 4% peroxide, 1.25%
130 nm silica, and variable amounts of 80 nm coated silica. This
slurry was used to polish tungsten using the following polishing
parameters: down force 5 psi, back pressure 2.5 psi, Ring pressure
5.5 psi, Table rotation 110 RPM, Carrier rotation 105 RPM, 150
ml/min slurry flow, using Strasbaugh 6EC polisher, Rodel
IC1000/SUBA IV pad, and Rodel DF200 Carrier film. The polishing
results of sequential tests using the same polishing pad are shown
below.
TABLE-US-00012 Activator-coated Silica Angstroms/min 0.3% 4500 0.8%
6550 0.6% 5980 0.3% 4060
Clearly, there is increasingly little benefit in adding greater
quantities of iron coated silica, but there are down sides in
having too much iron coated silica, particularly decreased shelf
life and cost. The amount of activator-coated abrasive rarely
should exceed 3%, and it should normally fall within a range
between 0.1% and 1% by weight of the slurry.
Example 15 and Comparative Examples
[0229] The following series of tests were performed to show that
iron coated onto a colloidal silica, in the presence of hydrogen
peroxide, will have a higher tungsten removal rate than either free
iron(Fe.sup.+3) or iron oxide(Fe.sub.2O.sub.3) in the presence of
colloidal silica and hydrogen peroxide. The following table 10
shows tungsten removal rates (in angstroms per minute) with wafers
run at 5 psi downforce, 0 psi backpressure, 90 rpm carrier speed,
90 rpm table speed, and 175 ml/min slurry flow. The slurry for all
the wafers run was 2.5% colloidal silica and 5% hydrogen peroxide.
Samples 15A, 15C, 15D, and 15E are comparative examples as they has
no iron-coated silica. The examples 15B, 15F, and 15G had only
small amounts of iron coated silica, sufficient to give only 3 to 4
ppm of iron to the slurry.
TABLE-US-00013 TABLE 10 Soluble Iron Coated onto Iron Oxide
Fe(NO.sub.3).sub.3 Tungsten Silica (as ppm Fe) (as ppm Fe) (as ppm
Fe) RR 15A (comp) 0 0 0 570 15B 3.3 0 0 2700 15C 0 20 0 2390 15D
(comp) 0 0 20 3930 15E (comp) 0 0 5 1460 15F 3.3 20 0 3090 15G 3.3
0 20 4320
[0230] Results show that as surface bound iron, 3.3 ppm gives a
2130 angstroms per minute increase over the removal rate of a
system with no iron. In contrast, 5 ppm soluble iron gives a 900
angstroms per minute increase over the removal rate of a system
with no iron, and 20 ppm soluble iron gives only a 3350 angstroms
per minute increase in tungsten removal rate (about the amount
expected from 6-7 ppm activator iron).
[0231] The Examples 15B, 15F, and 15G systems had very low amounts
(.about.3 ppm) of activator iron. It was not surprising that
addition of soluble ferric ions provided further increases in the
tungsten removal rate. While addition of soluble iron improves
removal rate, the effect tops out at a fairly low concentration of
iron, the iron ions contaminate the substrate, and the iron ions
degrade the oxidizer (giving short shelf life).
[0232] Surprisingly, 20 finely powdered iron oxide showed
substantial activity, giving an increase in removal rate
(.about.1800 angstroms per minute) that is within a factor of two
of the increase seen with comparable amounts of soluble iron. The
inclusion of iron oxide to a system, in amounts to provide between
about 5 ppm iron and 500 ppm, for example between about 11 ppm and
about 50 ppm as iron based on the weight of the slurry, are useful
to promote the tungsten removal rate. Iron oxide combined with
iron-coated silica gives a modest increase in rate over the
iron-coated silica itself, though greater increases can be achieved
by adding the extra iron as additional iron-coated particles, i.e.,
iron coated silica-containing particles. The addition of 3.3 ppm
iron coated silica to the slurry of 6C gave a 700 angstroms per
minute increase in rate.
[0233] In the previous examples we observed that adding either
soluble iron nitrate or iron oxide, in the range between about 5
ppm to about 20 ppm, to a slurry containing only 3 to 4 ppm iron
coated on silica provided significant increases in the tungsten
removal rate. At higher activator iron concentrations the effect of
soluble iron becomes insignificant. The following data shows the
effects of additional free iron on tungsten and TEOS removal rates
when it was added to an iron coated silica slurry having 49 ppm
activator iron coated on silica. The data in the table below were
all performed at the same process parameters: 5 psi and 0 psi
backpressure, at 90 rpm table and 90 rpm carrier rotation, with 175
ml/min slurry. All of the tests used 0.64% iron coated silica and
5% hydrogen peroxide and the pH about 2.5. The removal rate is in
Table 11, again, in angstroms per minute.
TABLE-US-00014 TABLE 11 Fe-coated silica Soluble Fe (as ppm Fe) (as
ppm Fe) Tungsten RR TEOS RR 15H 49 2.4 2900 274 15I 49 7.4 3000 283
15J 49 12.4 3100 256
[0234] Sample 15H had no added iron nitrate or iron sulfate, so the
2.4 ppm or soluble iron was believed to be residual from the
preparation of the iron-coated silica. Samples 15I and 15J had iron
sulfate added after the slurry comprising the iron-coated silica
was prepared. Results show that modest amounts of soluble iron (up
to 15 ppm, probably up to 20 ppm) have little effect on the
tungsten and TEOS removal rates.
[0235] Similar experiments were run where the iron was added as
Fe.sub.2O.sub.3. It is anticipated that at the low pH, the iron
oxides may partially or completely dissolve. The results in Table
12, again, show no significant increase in polishing rate.
TABLE-US-00015 TABLE 12 Fe-coated silica Fe.sub.2O.sub.3 (as ppm
Fe) (as ppm Fe) Tungsten RR TEOS RR 15K 49 2.4 2880 274 15L 49 7.4
2880 278 15M 49 12.4 2820 264
Example 16
[0236] A conventional iron-activator ion was placed on silica. The
abrasive used was Mirasol 3070.TM., hereafter "Mirasol", a
commercially available aqueous solution of abrasive silica
particles. Mirasol, commercially available from Precision Colloids,
LLC of Cartersville, Ga., contains approximately 30 weight percent
silica (Si0.sub.2) particles, which generally have an effective
diameter of approximately 70 nanometers. Mirasol 3070 coated with
activator contains the above-described Mirasol with for example
iron acetate activator coated/absorbed onto at least a portion of
the surface of the silica particles, i.e., on about 70 percent of
the surface area of each silica particle. Mirasol having as an
activator, i.e., cationic iron is hereafter Mirasol/Fe-Ac, or
copper which is hereafter Mirasol/Cu-Ac, provided the activator.
Generally, the compounds that form free radicals at an accelerated
rate when contacted by either the iron activator or the copper
activator (or both) include hydrogen peroxide (H.sub.2O.sub.2),
persulfate, periodic acid, and/or peracetic acid. Unless otherwise
specified, water formed the balance of the slurries.
[0237] A first example concerns two CMP compositions, Example 16A
with 3% H.sub.2O.sub.2 and Example 16B with 5% peracetic acid, both
at pH 2, which are particularly suited to CMP of a wafer, such as a
silicon wafer, having a tungsten layer or feature on its surface.
The components of the two compositions and the approximate amounts
thereof, as well as the approximate pH of the compositions, are set
forth in Table 13.
TABLE-US-00016 TABLE 13 CMP Slurry Compositions for Example 16A and
Example 16B H.sub.2O.sub.2 Peracetic Acid Mirasol Mirasol w/Fe--Ac
Example 16A 3 wt. % 0 wt. % 5 wt. % 0.5 wt. % Example 16B 0 wt. % 5
wt. % 5 wt. % 0.5 wt. %
[0238] Each of the Example 16A and 16B were used in a conventional
CMP process performed on a silicon substrate at least partially
layered with a tungsten film of about 8000 Angstroms (A) in
thickness. The process parameters for both included a carrier
pressure of about 6 pounds per square inch (psi), a carrier speed
of about 90 revolutions per minute (rpm), a platen speed of about
90 rpm, and a flow rate for the CMP composition used of about 175
milliliters per minute (ml/min). The processes differed only in
terms of which CMP composition was used. The results of each CMP
process in terms of the approximate material (tungsten) removal
rate in Angstroms per minute (A/mm) and the approximate
within-wafer nonuniformity percentage (WIWNU) are set forth in
Table 14.
TABLE-US-00017 TABLE 14 CMP Results on Tungsten Using Example 16A
or Example 16B Removal Rate (A/mm) Nonuniformity (% WIWNU) Example
16A 5040 10.9 Example 16B 5077 7.42
[0239] Both the sample with 3% hydrogen peroxide and the sample
with 5% peracetic acid had about the same tungsten removal rates,
which is not surprising because the moles of oxidizer per liter of
slurry was 20% within 20% of one another. As mentioned previously,
in CMP processes, and particularly modern or advanced CMP
processes, it is desirable to obtain acceptable or optimal such as
increased, material removal rates while using acceptable or
optimal, such as not unduly high, carrier pressures. In the CMP of
tungsten-layered wafers, a good carrier pressure is about 9 psi or
less, such as about 6 psi, and a good outcome at a pressure of
about 6 psi is a removal rate of greater than about 5000 A/mm.
Further, obtaining polished wafers with uniformity values of from
about 3 to about 12% WLWNIJ percent is considered a good result.
While the foregoing examples of process parameters, outcomes and
results are often desirable, other suitable outcomes and results
are contemplated herein.
[0240] In the CMP processes performed with Example 16A and Example
166B, desirable tungsten removal rates of about 5040 and 5077 A/mm,
respectively, were obtained. Additionally, the surfaces of the
polished wafers were substantially uniform, having 10.9 and 7.42%
WIWNU, respectively. Example 166B is generally preferred over
Example 16A, given its higher removal rate and better uniformity
value (lower % WIWNU). It should be noted that while there is a
general preference for compositions that provide high removal
rates, other factors, such as good uniformity values (for example,
low % WIWNU), efficient use of oxidizer, and good storage and
handling characteristics, are also important considerations in the
evaluation of a composition of the present invention.
[0241] A second example of the composition of the present invention
concerns two CMP compositions, Example 16C and Example 16D, which
were used in the CMP of a silicon wafer that had a copper layer or
feature on its surface. In this example, the copper layer had a
thickness of about 15,000 A. The oxidizer in Example 16C was 1.5%
peracetic acid and in Example 16D) was hydroxylamine (commercially
available in an aqueous composition as HAD.RTM., EKC Technology,
Inc.). Hydroxylamine is not generally considered a per-type
oxidizer, and Fenton-type reactions with hydroxylamine are not
generally known. The two compositions also differed in terms of pH,
Composition C having a pH of about 2 and Composition D having a pH
of about 6.7. The components of the two compositions and the
approximate amounts thereof, as well as the approximate pH of the
compositions, are set forth in Table 15.
TABLE-US-00018 TABLE 15 CMP Slurry Compositions for Example 16C and
Example 16D HDA .RTM. Peracetic Acid Mirasol Mirasol w/Fe--Ac pH
Example 0 wt. % 1.5 wt. % 5 wt. % 0.5 wt. % 2 16C Example 4 wt. % 0
wt. % 5 wt. % 0.5 wt. % 6.7 16D
[0242] Each of the Examples 16C and 16D were used in a conventional
CMP process performed on a silicon wafer at least partially layered
with copper. When Example 16C was polished, the process parameters
included a carrier pressure of about 4 psi, a carrier speed of
about 40 rpm, a platen speed of about 40 rpm, and a flow rate for
the Example 16C of about 100 m/min. When Example 166D was polished,
the process parameters included a carrier pressure of about 4 psi,
a carrier speed of about 75 rpm, a platen speed of about 75 rpm,
and a flow rate for the Example 16D of about 175 ml/mm. The
parameters of each CMP process are set forth in Table 16 and the
results thereof in terms of the approximate material (copper)
removal rate and the approximate within-wafer nonuniformity
percentage are set forth in Table 17.
TABLE-US-00019 TABLE 16 CMP Process Using Example 16C or Example
16D Carrier Pressure Carrier Speed Platen Speed Flow Rate (psi)
(rpm) (rpm) (ml/min) Example 16C 4 40 40 100 Example 16D 4 75 75
175
TABLE-US-00020 TABLE 17 CMP Results on Copper Using Example 16C or
Example 16D Removal Rate (A/mm) Nonuniformity (% WIWNU) Example 16C
~15,000 Not measurable Example 16D 7,800 8.87
[0243] As mentioned previously, in CMP processes, and particularly
modern or advanced CMI processes, it is desirable to obtain
acceptable or optimal, such as increased, material removal rates
while using acceptable or optimal, such as not unduly high, carrier
pressures. In the CMP of copper-layered wafers, a good carrier
pressure is about 6 psi or less, such as about 4 psi, and a good
outcome at a pressure of about 4 psi is a removal rate of greater
than about 7000 A.sup.1 mm. While the foregoing examples of process
parameters, outcomes and results are often desirable, other
suitable outcomes and results are contemplated herein
[0244] In the CMP process performed with Example 16C, an unusually
high copper removal rate was obtained, such that all of the copper
was removed. This result prevented measurement of a uniformity
value. Example C, with only 1.5% peracetic acid, is also a useful
composition of the present invention, although it may be a bit too
aggressive in terms of removal rate for some applications such as
the polishing of very thin layers of copper on a substrate. The
polishing compositions of this invention must be carefully
controlled to be used with copper, or too high a substrate removal
rate may result. Accordingly, for some applications, a CMP process
using Example C may be altered by diluting the composition,
diluting the activator-coated abrasive and/or oxidizing agent
components of the composition, changing the composition flow rate,
or the like.
[0245] In the CMP process performed with Example 16D, using the
activator-coated abrasive with a hydroxylamine activator, a
desirable copper removal rate was obtained. Additionally, the
surface of the wafer polished using Example 16D was substantially
uniform. Example D is thus a desirable composition of one
embodiment of the present invention.
[0246] Another example compares the CMP compositions Example 16B
and Example 16E to similar prior art compositions, each of which
were used in the CMP of a silicon wafer that had a tungsten layer
on its surface, the layer being of about 8000 A in thickness.
Example 16B was compared to a similar comparative example, Example
C1, and Example E was compared to a similar comparative example,
Example C-2. Neither of comparative examples C-1 and C-2 contained
activator-coated abrasive. Example E and comparative example 2
contained ethylene glycol, the purpose of which was to boost the
removal rate. The pH of all four compositions was about 2. The
components of the four compositions and the approximate amounts
thereof (balance water) are set forth in Table 18 below.
TABLE-US-00021 TABLE 18 CMP Examples 16 B and 16 E and Comparative
Examples C-1 and C-2 Weight %: H.sub.2O.sub.2 Peracetic Acid
Mirasol Mirasol/Fe--Ac Ethylene Glycol Example 16 B 0 5 5 0.5 0
Comp. Ex. C-1 0 5 5 0 0 Example 16 E 3 0 5 0.5 0.25 Comp. Ex. 2 3 0
5 0 0.25
[0247] Each of the four compositions were used in a conventional
CMP process having the same process parameters as previously
described in the first example and set forth in Table 3 above. Each
of Comparative Examples C-1 and C-2 were tested twice, in a Trail A
and a Trial B, respectively. The results of each CMP process in
terms of the approximate material (tungsten) removal rate in A/mm
and the approximate % WIWNU are set forth in Table 19.
TABLE-US-00022 TABLE 19 CMP Results Using Ex 16B or 16E or
Comparative Examples C-1 or C-2 Nonuniformity Removal Rate (A/mm)
(% WIWNU) Example 16B 5080 7.4 Comp. Ex. C-1, Trial A 2220 7.0
Comp. Ex. C-1, Trial B 2470 6.9 Example 16E 4480 4.6 Comp. Ex. 2,
Trial A 1560 3.4 Comp. Ex. 2, Trial B 1580 3.3
[0248] The utility of somewhat larger amounts (e.g., between 0.2%
and 1% by weight of the slurry) of activator-coated abrasive, in
this case iron ion activator bonded directly to silica with no
inorganic stabilizers, is clear from the above examples. In terms
of the tungsten removal rate, Examples 16B and 16 E each
outperformed Comparative Examples C-1 and C-2 by over 200 percent.
More importantly, the non-uniformity was very low, that is, between
4.6% and 7.4%. The CMP performances of Example 16B and Example 16E
are impressive, even when the moderate decreases in surface
uniformity are considered.
[0249] The above examples had 5% uncoated abrasive and 0.5% coated
abrasive. We found that the amount of non-activator-coated abrasive
could be reduced, provided there was sufficient activator-coated
abrasive, and the result was excellent polishing characteristics.
Example 16-F used a slurry having 5% peracetic acid, 2.5% Mirasol,
and 0.5% Mirasol with Fe-Acetate at pH 2. The etch rate through
tungsten was 4300 angstroms per minute, and the percent
nonuniformity was very low, between 2.7% and 5.6% for multiple
samples. This compares favorable with the 7.4% WIWNU observed when
a substrate was polished with example 16B, having a similar
polishing slurry composition but twice the uncoated silica. These
results demonstrate that the activator-coated abrasive is an
effective and potent component in the compositions of this
invention.
[0250] The minor increases in the Nonuniformity with the activator
coated abrasives may in part be due to using a mixture of a
relatively small quantity of highly activator-coated abrasive
(about 70% of outer surface coated with activator) and a greater
amount of abrasive without activator. It is believed that the free
radicals generated in the polishing composition have a relatively
short lifespan, and increases in polishing rate due to the higher
concentrations of free radicals is a fairly local phenomenon
restricted to the volume very near (e.g., perhaps within a few
microns or less) the activator-coated particle. It is believed that
a more uniform mixture comprising either only abrasive with
activator, where the activator is both present in a small
percentage of the surface area, or alternatively a mixture of
activator-coated abrasive and non-activator-coated abrasive where
the weight ratio thereof is between 0.2:1 to about 2:1, will reduce
non-uniformity.
Example 17
[0251] Example 17 I used a composition having 0.1% Mirasol with
Fe-acetate activator, 3% peracetic acid, and 5% Mirasol. This
composition exhibited superior CMP etch rate over a similar
composition without the activator for copper, ranging from 2200 to
4700 angstroms per minute at different processing conditions, but
the best nonuniformity observed in these tests was 13.7%. This is
somewhat higher than is desirable. While iron is a superior
activator, especially for tungsten polishing, other metals have
been found to work. Example 17F used a composition having 0.1%
Mirasol with copper activator, 5% peracetic acid, and 5% Mirasol.
This composition exhibited superior CMP etch rate over a similar
composition without the activator.
[0252] Example 17G used a composition having 0.2% Mirasol with Mn
(as the acetate salt) activator, 5% peracetic acid, and 5% Mirasol.
This composition exhibited superior CMP etch rate over a similar
composition without the activator. Example 17H used a composition
having 0.5% Mirasol with Mn-acetate activator, 3% hydrogen
peroxide, and 5% Mirasol. This composition exhibited superior CMP
etch rate over a similar composition without the activator, but the
rate for titanium was greatly enhanced while the rate for tungsten
improved only marginally: for tungsten the removal rate was 246
angstroms per minute; for TEOS the removal rate was 778 angstroms
per minute; and for titanium the removal rate was >2200
angstroms per minute. Manganese is a less effective activator than
either iron or copper as an activator, but this activator can be
useful.
[0253] One place where a less aggressive activator is useful is
when polishing copper substrates. For polishing copper metal
disposed on substrates, the activators copper and/or manganese are
very useful. Example 17 J used a composition having 0.5% Mirasol
with Mn-acetate activator, 5% hydrogen peroxide, and 5% Mirasol.
This composition exhibited superior CMP substrate removal rate
about 2380 angstroms per minute over a similar composition without
the activator which had etch rates of 270 to 380 angstroms per
minute for copper. Further, those wafers polished without activator
had about three times the nonuniformity as those wafers polished
with the slurry of this invention, which exhibited nonuniformity
between 8.8 and 11.9%.
Example 18
[0254] The next example shows slurry stability. This effective
activator-coated abrasive component functions optimally in
commercial settings when it is relatively, if not substantially,
stable. Slurry stability is a desirable characteristic in the
composition, as it facilitates control of the CMP process. Thus,
tests were conducted to determine the relative stability of the
activator-coated abrasive used in the composition of the present
invention, as compared with that of a soluble promoter of similar
chemical composition, in the presence of an oxidizing agent, in two
other compositions.
[0255] In these slurry stability tests, the activator-coated
abrasive was Mirasol/Fe-Ac, and an oxidizing agent in the form of
hydroxylamine and had a pH of about 7. The first "free promoter"
composition was composed of normal abrasive in the form of silica
particles, soluble promoter in the form of iron nitrate, and
oxidizing agent in the form of hydroxylamine, and had a pH of about
7. The second "free promoter" composition was composed of all of
the components of the first "free promoter" composition except for
the abrasive component. The three test compositions were prepared
as set forth below. A activator-coated abrasive preparation was
obtained by adding an appropriate amount of the activator-coated
abrasive to 50 milliliters of water, while a first "free promoter"
preparation was obtained by adding the silica particles to 50 ml of
water, and then adding an appropriate amount of the iron nitrate to
the water-abrasive mixture to give the same iron content in the
slurry. The amount of abrasive in the first "free promoter"
preparation was similar to the amount of activator-coated abrasive
used in the "coated activator" preparation. A second "free
promoter" preparation containing only iron nitrate dissolved in 50
ml of water (i.e., no abrasive) was also prepared.
[0256] The same designated amount of 50% hydroxylamine was added to
each of these preparations to obtain the three test compositions.
At a pH of over 6, hydroxylamine is a good reducing agent, the
stability of which is extremely sensitive to trace metals in
solution, hydroxylamine reacts easily with many soluble transition
metal ion promoters, such as cobalt, copper and iron ions,
resulting in the reduction of the metal ions by at least one
oxidation level and the formation of by-products including nitrogen
gas, ammonia (NH.sub.3), water, and possibly heat, depending on the
concentration of the hydroxylamine. A high level of reactivity, or
a very fast reaction rate, is a sign of relative instability.
[0257] When the hydroxylamine component was added to obtain the
"activator-coated abrasive" composition, little color change,
little or no outgassing, and little or no precipitation were
observed. When the first "free promoter" composition containing
silica abrasive was formed, an immediate color change (light orange
to brown), substantial outgassing, and precipitation were observed.
When the second "free promoter" composition containing no abrasive
was formed, an even more immediate color change (light orange to
very dark brown) and similar outgassing, as compared to the first
"free promoter" composition, were observed. The "activator-coated
abrasive" composition was clearly more stable than the two
relatively unstable "free promoter" compositions tested. The slurry
remained useable, that is, had a CMP rate on tungsten and TEOS of
at least about one half of the CMP rate for a freshly prepared
formulation, after 24 hours.
[0258] Another aspect of slurry stability is the stability of the
abrasive in terms of remaining stabilized in the liquid carrier. As
previously discussed, the stabilizers, and most particularly boric
acid-based stabilizers, can alter the zeta potential of particles
and can thereby greatly increase the suspension stability. In the
absence of stabilizers, or in addition to stabilizers, the presence
of certain ions can promote slurry stability. A series of tests
were performed to determine the pot life of slurries of this
invention. We have found through a series of experiments that
adding sulfate, either as an acid or as a salt, can extend the
colloidal stability of a slurry comprising 80 nm iron-coated silica
to up to about 5 days. The ferric coating, in the absence of salts
or stabilizers, results in colloids of 80 nm iron-coated silica
that settle out to a commercially unacceptable amount in only about
a day.
Example 19 and Comparative Example
[0259] This example illustrates use of boron-modified silica, in
particular a silica of mean diameter between 40 to 55 nanometers
manufactured as described in Example 6, following a procedure was
used for the preparation of boron-coated silica as described in
U.S. Pat. No. 6,743,267 (issued to DuPont AirProducts
Nanomaterials, inventors. P. Jemakoff and J. Siddiqui).
Approximately 1 kg of an acidic form of AMBERLITE.TM. IR-120 ion
exchange resin (Rohn and Haas Company, Philadelphia, Pa.) was
prepared. This ion exchange resin was then added incrementally to
12 kg of SYTON.TM. HT 50 (12 kg, approximately 2.27 gallons, DuPont
Air Products an Materas L.L.C., Tempe, Ariz.) in 2.50 kg of
deionized water, with agitation, until the slurry pH was about 2.
Subsequently, the mixture was passed through a 500-mesh screen to
remove the resin and afforded deionized SYTON.TM. HT 50.
[0260] The boric acid stabilizer was also added as previously
described. A solution of 268 g of boric acid powder (Fisher
Scientific, 2000 Park Lane, Pittsburgh, Pa., 15275) in 5.55 kg of
deionized water was prepared and heated to 55-65.degree. C.
Deionized and wetted SYTON.TM. HT 50 (14.5 kg) was then added to
the boric acid solution slowly over about 1.2 hours by adding it at
approximately 200 ml/minute and maintaining the temperature greater
than 52.degree. C. while agitating the mixture. After this addition
was completed, heating at 60.degree. C. and agitation of the
mixture were continued for 5.5 hours to afford boron
surface-modified colloidal silica. This boron surface-modified
colloidal silica was characterized for colloid stability over 15
days using a Colloidal Dynamics instrument (11-Knight Street,
Building ES, Warwick, R.I., 02886), and was found to exhibit both
constant pH (pH approximately 6.6) and zeta potential (zeta
potential approximately -58 millivolts) over the 15-day test
period. The percentage of surface sites of this surface-modified
colloidal silica occupied by boron-containing compound(s) was
approximately 98%.
[0261] Comparative Example 19-A, Step 2: After preparing the
boron-coated silica, in step 2, the boron-coated silica was used
for polishing tungsten wafers in comparative Example 19-A. In
comparative Example 19-A, the components of the slurry formulation
are: 1) 13.0 grams of boron-surface-modified Colloidal silica; 2)
30 ppm of 30% nitric acid in water; 3) 947 grams of De-ionized
water; and 4) 40 grams Hydrogen peroxide. The procedure for mixing
this 1.0 Kg batch size slurry was as follows. In a 2-liter beaker,
947 grams of de-ionized water were transferred. Under agitation,
13.0 grams of boron coated silica was added slowly during a period
of three minutes. After completing the addition of the boron coated
colloidal silica, 40 grams of hydrogen peroxide was added during a
period of 4 minutes to the silica sol mixture. The mixture was
agitated for additional 4 minutes. After 4 minutes of agitation,
under agitation, 30 ppm of 30% nitric acid was added. After
stirring the mixture for 4 minutes, pH was 4.7, and zeta-potential
was -141 mV.
[0262] Example 19-B, Step 2, used the previously described method
of adding ferric ions to boron-surface modified silica to
manufacture Iron-Boron coated silica. The slurry composition
consisted of: 13.0 grams Boron-modified colloidal silica described
above further modified with 16 ppm (based on the weight of the
slurry, providing 3.7 ppm of ferric ions) of Ferric nitrate; 10 ppm
of Nitric acid; 947 grams of De-ionized water; and 40 grams of
Hydrogen peroxide. The Step 2 procedure for the 1.0 Kg batch size
slurry was as follows. In a 2-liter beaker, 947 grams of de-ionized
water was transferred. After adding water to the beaker, it was
kept under agitation using a magnetic stirrer. Under agitation, 13
grams of boron modified colloidal silica was added slowly during a
period of 3 minutes. After completing the addition of the boron
modified colloidal silica, 3.7 ppm of ferric ions (in the form of
ferric nitrate) and the nitric acid were added to the dispersion.
After completing the addition of ferric nitrate, the dispersion was
stirred for additional 5 minutes, followed by the addition of 40.0
grams of hydrogen peroxide to the silica sol mixture. The mixture
was agitated for additional 4 minutes, the pH of the polishing
mixture was 4.7.
[0263] In Examples 19-C and 19-D, the polishing compositions were
prepared using the procedure described in Example 2, however, the
concentration of "iron" ion on the boron modified silica was
increased from 3.6 ppm to 5.7 ppm in Example 19-C, and 7.6 ppm of
"ferric" ion in Example 19-D. Example 19-E is same as Examples 19-C
and 19-D, except the amount of "ferric" ion concentration on the
boron modified silica surface was increased to the 57 ppm
level.
[0264] Evidence for iron coating on the boron modified silica was
obtained using two experimental methods, 1) zeta potential
measurements of the coated abrasive as measured by Colloidal
Dynamics, and 2) free ferric ion concentration in the dispersion
using ICP-MS (Inductive coupled Plasma mass spectroscopy.
Measurement of immobilized iron on the abrasive surface using Zeta
potential of iron coated silica is shown in the data in table TZBN,
zeta potential data of boric acid modified silica (example 19-A),
and different concentrations of ferric ions immobilized on the
boric acid coated silica (examples 19-B to 19-E) are summarized.
Clearly as the concentration of ferric ion increased form 3.6 ppm
to 57 ppm of ferric ions, zeta potential increased from (minus)
-141 mV (no ferric ions) to -39.3 mV for the particles having 7.6
ppm ferric ion on the surface. Interestingly, a charge reversal
occurred on the "iron-boron" coated silica as the concentration of
ferric ion increased to 57 PPM, the zeta potential increased to
+39.9 mV for the "iron boric" coated silica. Zeta potential data is
presented in table Tvbm. The increase in the zeta potential is
direct evidence that the iron was indeed being absorbed onto or
bound with the boron-stabilized silica. With lower amounts of iron,
the zeta potential increased about 13.5 mV per ppm Fe ions added,
but the increase "per ppm Fe ion added" was much lower at 57 ppm Fe
added Measurement of free ferric ions in Me solution phase of
"iron-boron" coated silica dispersion supports this conclusion.
Using ICPNS, the amount of free iron ion in known concentration of
fete nitrate solutions was measured using an appropriate
calibration curve. The solution part of the "iron-boric" coated
silica dispersion was separated from the abrasive part, and the
solution part was tested for ferric ion concentration. The measured
value was less than 0.1 ppm, which suggests iron coating on the
boric acid modified silica. The two experiments, as described
above, conclusively suggest that ferric ions readily coated boric
acid modified silica via acid-base reaction, where ferric ion is a
Lewis acid, and boric acid modified silica is a Lewis base.
[0265] The polishing formulations were used to polish tungsten and
PETEOS wafers, the results are summarized in table Tvbm. For all
polishing experiments, Mirra.RTM. polishing tool, manufactured by
Applied Materials, 3050 Bowers Avenue, Santa Clara, Calif., 95054,
was used. The polishing compositions were used to polish CVD
tungsten blanket wafers and PETEOS blanket wafers (PETEOS, plasma
enhanced tetraethoxy silane, dielectric oxide layer) using a
polishing tool. The blanket wafers were purchased from Silicon
Valley Microelectronics, 1150 Campbell Ave Calif. 95126. The PETEOS
wafers had a film thickness specification of 15,000 .ANG. PETEOS.
The CVD tungsten wafers had film stack thickness specifications of
8000 .ANG. CVD tungsten/250 .ANG. titanium/6300 .ANG. thermal
oxide. The polishing conditions on the Mirra.RTM. were set as
follows: Flow rate of polishing composition 120 ml/min (milliliters
per minute); Retaining ring (psi)=6.8 psi; Membrane pressure 3.5
psi; Inner tube pressure 6.3 psi; Platen speed=120 revolutions per
minute (RPM); and Head speed 130 RPM. The substrate was effectively
planarized using the above comparative polishing composition under
the stated conditions. The tungsten, PETEOS removal rates, and
tungsten to PETEOS selectivity is shown in example in table 20.
TABLE-US-00023 TABLE 20 Effect of Composite "Iron-Boron" coated
Silica on Removal Rates Comp Ex Ex 19-B Ex 19-C Ex 19-D, Ex 19-E:
19-A, B--Fe B--Fe B--Fe B--Fe Control, b- modified modified
modified modified modified silica (3.6 ppm silica (5.7 ppm silica
(7.6 ppm silica (57 ppm silica Fe) Fe) Fe) Fe) Colloidal silica 1.3
wt % 1.3 wt % 1.3 wt % 1.3 wt % 1.3 wt % Zeta potential -141 mV
-92.3 mV -64.2 mV -39.3 mV +39.9 mV H2O2 (wt %.) 4 4 4 4 4 Water
(wt. %) 94.7 94.7 94.7 94.7 94.65 pH 4.7 4.7 4.2 3.9 3.8 W RR
(A/min) 427 3800 5540 6230 7600 TEOS RR(A/min) 58 35 50 93 83 W:
TEOS Selectivity 7 109 111 67 92
[0266] In Table 20, example 19-A is the comparative example with
boron coated silica whereas examples 19-B to 19-E contain different
amounts of "ferric" ions coated on the boron coated silica. The
boron coated silica with no "ferric" ion on the silica surface gave
very low tungsten removal of 427 A/min, and "ferric" ion coated
silica in Example 19-B with 3.6 PPM of "ferric" ions gave tungsten
removal rate of 3804 A/min, a 9.times. fold increase in tungsten
removal rate. Even higher rates were observed with the higher
amounts of iron added to the boron-surface-modified silica
abrasive, Clearly data show that "ferric" ion coating on the boron
coated silica increased tungsten removal rates dramatically. A
dramatic increase in the tungsten removal rates with "ferric" ion
coated boron coated silica strongly suggests that abrasive
particles during chemical planarization of tungsten acts as a
powerful Fenton's reagent for the decomposition of hydrogen
peroxide, thus dramatically increasing tungsten removal rates and
excellent tungsten to PETEOS selectivity. Surprisingly, the rate of
polishing of PETEOS was about the same for the control sample 19-A
as for the B--Fe-modified silica abrasive.
Example 20
[0267] It has been found that CMP polishing compositions comprising
an iron-boron surface-modified silica, a peroxide-type oxidizing
agent, and ascorbic acid or a derivative thereof possess high
stability with regard to maintaining near constant levels of
components over long periods and consequently maintaining
propensity for affording high removal rates over long periods in
comparison to otherwise identical compositions without ascorbic
acid (or a derivative thereof) being present. The second benefit of
adding ascorbic acid or a derivative thereof is that it is
effective in reducing iron ion contamination on the polished
surface of wafers. As ascorbic acid (or a derivative thereof)
removes metal contamination during polishing, this eliminates
additional cleaning or buffing steps during the fabrication of
semiconductor devices.
[0268] Zeta potential measurements were made using a Colloidal
Dynamics instrument, manufactured by Colloidal Dynamics
Corporation, 11 Knight Street, Building E8, Warwick, R.I., 02886.
This instrument measures the zeta potential (surface charge) of
colloidal particles, such as surface-modified colloidal silica
particles. The CMP tool that was used is a Mirra.RTM., manufactured
by Applied Materials, 3050 Boweres Avenue, Santa Clara, Calif.,
95054. The polishing conditions on the Mirrat were set as follows:
Flow rate of polishing composition=120 ml/min (milliliters per
minute); Retaining ring=68 psi; Membrane pressure=3.5 psi; Inner
tube 6.3 psi; Platen speed 120 rpm; and Head speed=130 rm. A Rohm
and Haas Electronic Materials IC1010.TM. pad, supplied by Rohm and
Haas Electronic Material, 3804 East Watkins Street, Phoenix, Ariz.,
85034, seas used on the platen for the blanket wafer studies. The
blanket wafers used in this work were purchased from Silicon Valley
Microelectronics, 1150 Campbell Ave, Calif., 95126. PETEOS wafers
had 15,000 .ANG. on silicon, Tungsten wafers had 10,000 .ANG. CVD
over 5000 .ANG. thermal oxide on silicon; and Titanium nitride
wafers had 3000 .ANG. TiN over 3000 .ANG. thermal oxide. Rohm and
Haas Electronic Materials IC1010.TM. pads were used for polishing.
The IC1010.TM. pad consists of a rigid microporous polyurethane
with a radial grooving pattern top pad and a Suba.TM. TV
impregnated felt sub-pad. Rohm and Haas Electronic Materials is
based in Newark, Del. PETEOS thickness was measured with a
Nanometrics, model, #9200, manufactured by Nanometrics Inc, 1550
Buckeye, Milpitas, Calif. 95035-7418. The metal films were measured
with a ResMap CDE, model 168, manufactured by Creative Design
Engineering, Inc, 20565 Alves Dr, Cupertino, Calif., 95014. This
tool is a four-point probe sheet resistance tool. Twenty-five and
forty nine-point polar scans were taken with the respective tools
at 3-mm edge exclusion. Planarity measurements were conducted on a
P-15 Surface Profiler manufactured by KLA.RTM. Tencore, 160 Rio
Robles, San Jose, Calif. 95161-9055.
[0269] A batch of boric acid modified silica was prepared as
discussed in previous example 6, said boric acid modified silica
having 268 g boric acid added onto 12 kg of SYTON.RTM. HT 50. The
boric acid-stabilized silica was over a 15 day period found to
exhibit both constant pH (pH approximately 6.6) and zeta potential
(zeta potential approximately -58 millivolts). The percentage of
surface sites of this surface-modified colloidal silica occupied by
boron-containing compound(s) was approximately 98%.
[0270] Comparative Example 20-A had 43.5 grams Boron
surface-modified Colloidal silica in 823 grams of water, to which
was added 38 ppm of Ferric nitrate nonhydrate (7.6 ppm ferric ion),
as well as 30 ppm Nitric acid to control the pH, and 133.3 grams
aqueous 30% hydrogen peroxide. After stirring the mixture for 4
minutes, the pH was 4.7, and the zeta-potential -39.3 mV. In
Example 20-B, the formulation was same as Example 20-A, the only
difference being the addition of 200 ppm of ascorbic acid at the
last step during the preparation of the polishing formulation. The
polishing characteristics of these slurries were measured
immediately after preparing the slurry, again at 24 hours, and
finally again after six days. The data is presented in Table
21.
TABLE-US-00024 TABLE 21 Effect of Ascorbic acid on the
Stabilization of a CMP slurry Containing H.sub.2O.sub.2 in the
Presence of Iron-Boron surface-modified Silica Comp Ex Comp Ex Comp
Ex Ex 20-B: Ex 20-B: Ex 20- 20-A: 20-A: 24 20-A: Six Zero 24 B: Six
Zero time hours days time Hours days B/Fe (7.6 ppm) 43.5 g 43.5 g
43.5 g 43.5 g 43.5 g 43.5 g surface-modified colloidal silica
Ascorbic acid (ppm) 0 0 0 200 200 200 H.sub.2O.sub.2 (30 wt.) 133.3
g 133.3 g 133.3 g 133.3 g 133.3 g 133.3 g Water (wt.) 823 g 823 g
823 g 823 g 823 g 823 g pH 3.9 4.1 3.9 4.1 3.8 3.9 Tungsten RR 6590
5856 3832 6430 6236 5479 (.ANG./min) TEOS RR (.ANG./min) 189 168
135 249 210 184 TiN RR (.ANG./min) 1037 820 917 1458 1532 1519 W:
TiN Selectivity 6.3 7.1 4.2 4.4 4.0 3.6 W: TEOS Selectivity 35 35
28 26 29 30
[0271] It can be seen that in the absence of ascorbic acid
chelator, there is about a 11% degradation in substrate removal
rates after a CMP slurry of this preferred embodiment of the
invention. This modest amount of degradation can be commercially
acceptable, but after six days, the tungsten removal rate is so
degraded (>40% loss of tungsten removal rate) as to be
unacceptable. There is also a similar decline in the TEOS removal
rate, but the TiN removal rate showed an initial decline over 24
hours but little change (or even a rebound) thereafter.
[0272] In contrast, with only 200 ppm of ascorbic acid added to the
slurry, the decline in the tungsten removal rate was only 3% over
24 hours, which is commercially an excellent result, and the
decline in the tungsten removal rate even after 6 days was only
about 15%, which is commercially acceptable. Therefore,
manufacturing processes are made easier because in normal
operations a tank of slurry can be prepared and used without taking
into account slurry degradation, and even when production is
interrupted the slurry remains useful for several days. Another
advantage of ascorbic acid addition was in maintaining a consistent
titanium nitride removal rate from time zero to six days. Also,
addition of ascorbic acid increases the titanium nitride removal
rate. This is advantageous because the TiN is generally used as a
very thin barrier layer, and once reached the manufacturer will not
wish the have long polishing times to remove residual TiN, where
such long polishing times will increase tungsten erosion.
[0273] It is well known that tungsten and titanium nitride removal
rates are sensitive to the concentration of H.sub.2O.sub.2, so in a
separate set of experiments, the rate of decomposition of
H.sub.2O.sub.2 with and without ascorbic acid or/and ascorbic acid
derivates in the slurry formulation was investigated. The slurries
used for hydrogen peroxide stability nominally had the compositions
of Example 20-A, but started with about 4.3% hydrogen peroxide. To
one aliquot, 200 ppm ascorbic acid was added, providing a slurry
much like that described in Example 20-B. In a second aliquot, 200
ppm of ascorbic palminate was added to the slurry. Finally, as a
control, in a third aliquot, 200 ppm of acetic acid was added. Both
the aliquot having 200 ppm ascorbic acid and the aliquot having 200
ppm of ascorbic palminate showed substantially no decline (less
than 0.2% absolute concentration) over 8 days. In contrast, the
aliquot having 200 ppm acetic acid saw the hydrogen peroxide
concentration drop from 4.3% at time zero to 2.1% at day six, and
further fell to 1.6% by day eight. Clearly the data suggest that
slurry formulations with ascorbic acid or ascorbic palmitate
stabilized the slurry. This explains as to why the addition of
ascorbic acid (and ascorbic acid derivatives such as ascorbic
palmitate) in the slurry formulations stabilized the titanium
nitride and tungsten removal rates for several days.
[0274] The next set of experiments showed the effect of adding
ascorbic acid (to polishing formulations containing iron-boron
surface-modified colloidal silica) on iron ion contamination on the
polished PETEOS wafers after polishing. Example 20-C had boron
modified silica coated with 3.6 PPM of ferric ions, with no
ascorbic acid added. Two slurries used had the same compositions as
examples 20-A and 20-B, containing Boron modified silica coated
with 7.6 PPM of ferric ions with no ascorbic acid (20-A) or with
200 ppm ascorbic acid (20-B). Additionally, two more control
samples were prepared containing Boron modified silica coated with
7.6 PPM of ferric ions with no 200 ppm acetic acid (20-C) and with
800 ppm acetic acid (20-D). The slurries were used to polish PETEOS
wafers using method described above. After the polishing
experiment, iron ion concentration on the polished wafer surface
was measured using TXRW (X-ray fluorescence spectrometry) method.
First, we found that the amount of contamination of the polished
wafer is proportional to the amount of ferric ions (activator ions)
in the slurry. The slurry having 3.6 ppm total activator iron (and
no acid) left 81.4xE10 ferric atoms per square centimeter of
substrate, while the slurry having 7.3 ppm total activator iron
(and no acid) left 205xE10 ferric atoms per square centimeter of
substrate. The addition of only 200 ppm of ascorbic acid to the
slurry having 7.3 ppm total activator iron provided a remarkable
reduction in the iron contamination, leaving only 13.2xE10 ferric
atoms per square centimeter of substrate. This is only about 6% of
the iron contamination from a similar slurry without the ascorbic
acid. Acetic acid had little effect. The addition of 200 ppm of
acetic acid to the slurry having 7.3 ppm total activator iron left
only 212xE10 ferric atoms per square centimeter of substrate, about
the same as is left by a slurry having no acid. Increasing the
amount of acetic acid to 800 ppm reduced subsequent iron
contamination only slightly, to 188xE10 ferric atoms per square
centimeter of substrate.
Example 21
[0275] A similar set of experiments to determine the effectiveness
of ascorbic acid on reducing the iron contamination of substrate
surfaces were run on slurries comprising commercially available
iron-coated silica. In particular, this example provides data on
the efficiency for reducing iron ion contamination on polished
wafer surfaces after CMP using (i) MicroPlanar.RTM. CMP3600.TM.
alone versus (ii) formulations derived from the addition of
ascorbic acid to MicroPlanar.RTM. CMP3600.TM.. MicroPlanar.RTM.
CMP3600.TM. is a commercially available tungsten CMP slurry from
DuPont Air Products NanoMaterials L.L.C., Tempe, Ariz. The
formulation modification of CMP3600.TM. in these examples is the
inclusion of ascorbic acid to reduce the iron ion contamination
level on the PETEOS wafer; after CMP and standard post CMP cleaning
with dilute ammonia solution. Ascorbic acid was shown to be
efficient at reducing trace iron ion contamination on the wafer
surface post CMP.
[0276] Polishing of PETEOS blanket wafers was done under the
following conditions: Polish Tool used was an Ebara EPO222D, and
measurements were made using a Rigaku TARF-3 pt(0.0; 0.50 and
-50.0), a KLA Tencor SP1 (10 mm E), a 4 Dimension 4 point probe (49
pt line), and a KLA Tencor F5x (49pt line). The results that were
obtained are summarized below in Table 22. It is seen that the
addition of 400 ppm of ascorbic acid to MicroPlanar.RTM.
CMP3600.TM. resulted in a significant reduction in iron ion
contamination of the wafer surface after performing CMP and
standard post CMP cleaning with dilute ammonia solution. This
reduction in iron ion contamination did not decrease the tungsten
removal rate.
[0277] At the same time, the data clearly shows that use of
boron-stabilized ferric ion coated silica slurries having less than
10 ppm activator and 200 ppm ascorbic acid (such as were tested in
Example 20) provided a much lower level of iron contamination than
was achieved using the commercial slurry, even with the addition of
400 ppm ascorbic acid.
TABLE-US-00025 TABLE 22 Effect of Ascorbic Acid Added to a CMP3600
.TM. Iron-coated Silica- containing Slurry having 4% hydrogen
peroxide Iron Ion contamination on Tungsten Ascorbic Acid PETEOS
Wafer After Post Removal rate Example No at point-of-use CMP
Cleaning and Rinsing .ANG./min Comp. 0 ppm 376.7 .times. E10
atoms/cm.sup.2 4010 Ex. 21-A Ex. 21-B 100 ppm 356.6 .times. E10
atoms/cm.sup.2 4576 Ex. 21-C 400 ppm 93.5 .times. E10
atoms/cm.sup.2 4147
[0278] Addition of lactic acid instead of ascorbic acid into the
same slurry provided the results in Table 23.
TABLE-US-00026 TABLE 23 Effect of Lactic Acid Added to a CMP3600
.TM. Iron-coated Silica-containing Slurry having 4% hydrogen
peroxide Iron Ion contamination on Tungsten Lactic Acid PETEOS
Wafer After Post Removal rate Example No at point-of-use CMP
Cleaning and Rinsing .ANG./min Comp. 0% 376.7 .times. E10
atoms/cm.sup.2 4010 Ex. 21-A Ex. 21-D 0.14% 62.4 .times. E10
atoms/cm.sup.2 4981 Ex. 21-E 0.28% 38.5 .times. E10 atoms/cm.sup.2
5367 Ex. 21-E 0.42% 30.3 .times. E10 atoms/cm.sup.2 5282
[0279] It is seen that the addition of 0.1% to 0.5% of lactic acid
to MicroPlanar.RTM. CMP3600.TM. resulted in a significant reduction
in iron ion contamination of the wafer surface after performing CMP
and standard post CMP cleaning with dilute ammonia solution. The
relatively large, e.g., 0.1% to 0.5%, of a chelator like lactic
acid appears not only to effectively reduce the amount of iron
contamination of the substrate, but also provides a significant
increase in the tungsten removal rate.
Example 22
[0280] It is important that a slurry can retain activator on the
abrasive particles even at higher ascorbic acid concentrations.
Generally, slurries are sold as concentrates. In one embodiment, a
slurry concentrate may comprise for example 0.5% to 4%
activator-free abrasive, such as silica having an average diameter
between about 80 nm and 250 nm, for example between 150 nm and 200
nm in diameter; from 0.5% to 4% activator-coated abrasive such as
iron coated silica, where the total activator iron in the slurry
concentrate is between 30 and 300 ppm, for example between 150 and
250 ppm, and wherein the total ascorbic acid in the slurry
concentrate is between 600 ppm and 6000 ppm, for example between
1200 and 3000 ppm, where the slurry concentrate is to be diluted
and mixed with an oxidizer prior to use, where the total dilution
is between about 1 part slurry concentrate per about 2 part to
about 6 parts fluid (usually predominately water with some
oxidizer). Generally, such high concentrations of ascorbic acid
will over long periods of time (weeks to a few months, as are often
encountered in manufacturing, shipping, and using product) strip
much of the activator from a suspended abrasive. At low pH such as
pH 3 virtually all the activator can be stripped from the abrasive
by the more concentrated ascorbic acid solution. Increasing the pH
of the composition to between 6 and 7 will stabilize activator such
that a significant fraction of the activator remains on the surface
of the abrasive particles.
[0281] If the abrasive is first coated with a stabilizer, in
particular a boric acid-based stabilizer, prior to coating the
abrasive with activator, then the activator is more tenacious and a
greater fraction will remain attached onto the surface of the
abrasive, as compared to for example activator absorbed directly
onto the abrasive. Without being bound by theory, we believe the
boric acid ligand is a stronger base then the silica-oxygen ligand.
Also, the --Si--O--Si--O-- bond length is greater than the
--O--B--O--B--O-- bond length, so ionically the iron is more
tightly and tenaciously attached to a boron-oxygen surface that to
a silicon-oxygen surface. The most preferred slurry concentrate has
boron-iron-surface-modified abrasive material, where the pH of the
slurry concentrate is between 5 and 7, say between 6 and 6.5. The
addition of the boron also allows the zeta potential to be either
negative or positive, depending on the amount of activator iron
absorbed onto the abrasive. The amount of stabilizer and activator
can be varied to provide a desired zeta potential between about
-120 mV to about +30 mV.
[0282] The above examples are meant to illustrate certain aspects
of the invention, but are not intended to limit the invention in
any way.
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