U.S. patent application number 10/419511 was filed with the patent office on 2004-03-25 for materials and methods for control of stability and rheological behavior of particulate suspensions.
Invention is credited to Basim, Gul B., Brown, Scott C., Moudgil, Brij M., Vakarelski, Ivan U..
Application Number | 20040055993 10/419511 |
Document ID | / |
Family ID | 31998983 |
Filed Date | 2004-03-25 |
United States Patent
Application |
20040055993 |
Kind Code |
A1 |
Moudgil, Brij M. ; et
al. |
March 25, 2004 |
Materials and methods for control of stability and rheological
behavior of particulate suspensions
Abstract
The subject invention relates to a method and materials for the
control of the energy barrier to agglomeration with respect to
particles. Controlling the energy barrier to agglomeration can, for
example, cause the particles to enter suspension and/or depart from
suspension, as desired. By fine-tuning the energy barrier, a
variety of processes can be enhanced. The subject invention also
pertains to the control of the energy barrier between particles and
a surface. In a specific embodiment, the subject invention can be
utilized for chemical-mechanical polishing (CMP) processes. In a
preferred embodiment, the subject invention concerns a slurry
composition including abrasive particles, at least one dispersant,
and at least one competing agent, wherein the competing agent
competes with the dispersant for adsorption sites on the substrate,
thereby hindering the dispersant's influence on the
particle-substrate interaction within the composition, without
eliminating the dispersant's influence on the particle-particle
interaction (steric repulsion) within the composition, thereby
maintaining a stable solution.
Inventors: |
Moudgil, Brij M.;
(Gainesville, FL) ; Basim, Gul B.; (Dallas,
TX) ; Vakarelski, Ivan U.; (Gainesville, FL) ;
Brown, Scott C.; (Gainesville, FL) |
Correspondence
Address: |
SALIWANCHIK LLOYD & SALIWANCHIK
A PROFESSIONAL ASSOCIATION
2421 N.W. 41ST STREET
SUITE A-1
GAINESVILLE
FL
326066669
|
Family ID: |
31998983 |
Appl. No.: |
10/419511 |
Filed: |
April 21, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10419511 |
Apr 21, 2003 |
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09689537 |
Oct 12, 2000 |
|
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60159064 |
Oct 12, 1999 |
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60408405 |
Sep 4, 2002 |
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Current U.S.
Class: |
216/2 |
Current CPC
Class: |
C09G 1/02 20130101; B24B
37/044 20130101 |
Class at
Publication: |
216/002 |
International
Class: |
C23F 001/00 |
Goverment Interests
[0002] The subject invention was made with government support under
a research project supported by the National Science Foundation
Grant Nos. ECC-0121978, NSF-CPE 8005851 and EEC-94-02989.
Claims
We claim:
1. A composition for polishing a substrate, said composition
comprising particles, at least one dispersant, and at least one
competing agent.
2. The composition of claim 1, wherein said competing agent is
present within said composition in an amount effective to compete
with said dispersant for adsorption sites on said substrate,
thereby hindering said dispersant's influence on the
particle-substrate interaction within said composition.
3. The composition of claim 2, wherein said competing agent does
not eliminate said dispersant's influence on the particle-particle
interaction within said composition.
4. The composition of claim 1, wherein said competing agent
competes with said dispersant for the same adsorption sites on said
substrate as the dispersant, thereby weakening the affinity of said
dispersant for the substrate.
5. The composition of claim 1, wherein said dispersant is a
surfactant.
6. The composition of claim 1, wherein said competing agent
comprises a multivalent counterion.
7. The composition of claim 1, wherein said multivalent counterion
is selected from the group consisting of barium, calcium, aluminum,
and magnesium.
8. The composition of claim 1, wherein said competing agent
comprises a metal hydroxide.
9. The composition of claim 8, wherein said metal hydroxide is
selected from the group consisting of Ca(OH).sup.+ and
Ba(OH).sup.+.
10. The composition of claim 1, wherein said competing agent
comprises a polyelectrolyte.
11. The composition of claim 1, wherein said competing agent
comprises an organic amine.
12. The composition of claim 1, wherein said surfactant is a
cationic surfactant.
13. The composition of claim 12, wherein said cationic surfactant
is selected from the group consisting of long chain amines,
synthetic amines, diamines, polyamines, quaternary ammonium salts,
quaternized polyoxyalkylenated long chain amines, unquaternized
polyoxyalkylenated long chain amines, and amine oxides.
14. The composition of claim 12, wherein said cationic surfactant
comprises a long chain quaternary ammonium compound.
15. The composition of claim 12, wherein said cationic surfactant
has an alkyl chain 8 to 16 carbon atoms in length.
16. The composition of claim 12, wherein said cationic surfactant
comprises an alkyl trimethylammonium bromide.
17. The composition of claim 16, wherein said alkyl
trimethylammonium bromide has an alkyl chain 8 to 16 carbon atoms
in length.
18. The composition of claim 16, wherein said alkyl
trimethylammonium bromide is selected from the group consisting of
octyl trimethyl ammonium bromide, decyl trimethyl ammonium bromide,
and dodecyl trimethyl ammonium bromide.
19. The composition of claim 6, wherein said multivalent counterion
is a divalent cation.
20. The composition of claim 19, wherein said divalent cation is
selected from the group consisting of Ca.sup.++ and Ba.sup.++.
21. The composition of claim 1, wherein said surfactant and said
competing agent are the same substance.
22. The composite of claim 1, wherein said particles are abrasive
particles.
23. The composition of claim 22, wherein said abrasive particles
comprise silica.
24. The composition of claim 1, wherein the mean size of said
particles is within the range about 10 nm to 10 .mu.m.
25. The composition of claim 1, wherein the mean size of said
particles is within the range of about 10 nm and 300 nm.
26. The composition of claim 1, wherein said composition has a pH
within the range of about 9 to 12.
27. The composition of claim 1, wherein said composition has a pH
within the range of about 10 and about 11.
28. The composition of claim 1, wherein said dispersant is adsorbed
onto said substrate.
29. The composition of claim 24, wherein said substrate is
silica.
30. The composition of claim 1, wherein said dispersant is present
in said slurry composition at a concentration equal to or greater
than the critical micelle concentration.
31. The composition of claim 1, wherein said dispersant is present
in said slurry composition within a range of about 1 critical
micelle concentration to about 2 critical micelle
concentration.
32. The composition of claim 1, further comprising an anionic
surfactant.
33. The composition of claim 32, wherein said anionic surfactant
comprises sodium dodecyl sulfate.
34. The composition of claim 1, wherein said dispersant is a
cationic surfactant selected from the group consisting of octyl
trimethyl ammonium bromide, decyl trimethyl ammonium bromide, and
dodecyl trimethyl ammonium bromide, wherein said multivalent
counterion is a divalent cation selected from the group consisting
of Ca.sup.++ and Ba.sup.++, and wherein said cationic surfactant
present in said composition at a concentration equal to or greater
than the critical micelle concentration.
35. The composition of claim 1, wherein said particles are in
suspension.
36. The composition of claim 31, wherein said particles are in
stable suspension.
37. A method for polishing a substrate, said method comprising:
contacting a slurry composition to the substrate and polishing the
substrate, wherein the slurry composition comprises the composition
of claim 1.
38. The method according to claim 37, wherein said polishing
comprises applying a polishing pad to the substrate, wherein the
slurry composition is interposed between the substrate and the
polishing pad.
39. The method according to claim 37, further comprising placing
the substrate on a carrier, wherein said polishing comprises
applying a polishing pad to the substrate on the carrier, and
wherein the slurry composition is interposed between the substrate
and the polishing pad.
40. The method according to claim 37, wherein the substrate has a
surface, and wherein the dispersant is adsorbed onto the surface of
the substrate.
41. The method according to claim 37, wherein the substrate has a
surface, wherein the slurry composition is contacted to the surface
of the substrate, wherein said method further comprises removing
the abrasive particles from the surface of the substrate.
42. A method of making a slurry composition, said method comprising
admixing at least one dispersant with particles, and subsequently
adding at least one competing agent to the mixture.
43. The method of claim 42, wherein the competing agent is present
within the slurry composition in an amount effective to compete
with the dispersant for absorption sites on a substrate, thereby
weakening the affinity of the dispersant for the substrate.
44. A method for controlling the energy barrier to agglomeration of
particles in a composition containing at least one dispersant, said
method comprising modulating the interaction between the particles
and a substrate.
45. The method of claim 1, wherein said modulating comprises
weakening the affinity of the dispersant for the substrate, thereby
increasing the stability of the composition.
46. The method of claim 45, wherein said weakening comprises adding
at least one competing agent to the mixture, wherein the competing
agent competes with the dispersant for the same adsorption sites on
the substrate as the dispersant.
47. The method of claim 44, wherein said modulating comprises
strengthening the affinity of the dispersant for the substrate,
thereby decreasing the stability of the composition.
48. The method of claim 47, wherein the composition comprises at
least one competing agent, and wherein said stregthening comprises
inhibiting the competing agent's affinity for the substrate.
49. The method of claim 48, where said inhibiting comprises
removing the competing
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a continuation-in-part of U.S.
application Ser. No. 09/689,537, filed Oct. 12, 2000, which claims
priority to U.S. Provisional Application Serial No. 60/159,064,
filed Oct. 12, 1999; and claims priority to U.S. Provisional
Application Serial No. 60/408,405, filed Sep. 4, 2002; which are
hereby incorporated by reference herein in their entirety,
including any figures, tables, or drawings.
BACKGROUND OF THE INVENTION
[0003] The mechanical, electrical, and chemical properties of
nanocomposites, ceramics, coatings, and other nanofunctional
materials formed by consolidation of nanoparticulate slurries is
critically dependent on the stabilization of particles during
processing. Stabilization of nanoparticulate systems is not only
important in preventing agglomerate formulation that may induce
defects in the final body but also to ensure controlled Theological
behavior. As particle size becomes smaller, especially in
concentrated suspensions, small variations in the stability of the
suspensions may significantly impact the flowability and the
ability of the product to be successfully consolidated.
[0004] Precise control of dispersion in particulate suspensions is
of primary concern in colloidal systems. Environmental and safety
concerns have driven industrial processes towards aqueous media in
which polymeric dispersants have traditionally been used to
stabilize suspensions. Under conditions such as high electrolyte
concentration or extremes of pH, many of these traditional
polymeric dispersants may not perform adequately. The dispersant
may desorb from the surface, precipitate from solution, or change
conformation or charge, such that the barrier to agglomeration
between the particles may be overcome by van der Waals forces.
Additionally, many of these polymeric dispersants are large enough
to increase the effective diameter of a particle in suspension such
that, in highly concentrated or nanoparticulate suspensions, the
viscosity of the suspension is unduly increased, resulting in
processing difficulties.
[0005] Surfactant adsorption to surfaces is a phenomenon of
critical importance to various industrial processes ranging from
ore flotation, lubrication, and paint technology to enhanced oil
recovery. The process of and the factors affecting surfactant
aggregation or micellization in bulk solutions that may lead to
spherical or cylindrical micelles, bilayers, or bicontinuous phases
are relatively well understood. At interfaces, however, the
self-assembly process is influenced by additional
surfactant-surface and solvent-surface interactions, including the
free energy of adsorption, roughness, surface heterogeneity and
charge, and crystallinity.
[0006] Traditionally, adsorption isotherms combined with techniques
such as contact angle and surface potential have been used to
delineate the adsorption behavior of surfactants on substrates.
More recently these techniques have been joined by several methods
such as nuclear magnetic resonance (NMR), calorimetry,
ellipsometry, fluorescence decay, Raman spectroscopy, electronspin
resonance, infra-red adsorption (FT-IR), small angle neutron
scattering (SANS), neutron reflection, and surface force
measurement. Based on observations made via the above techniques,
many important insights into the aggregation process at surfaces
have been made including the critical hemi-micelle concentration,
and critical aggregation number. However, the structure and shape
of the adsorbed aggregates still remains controversial due to
limited direct experimental evidence.
[0007] In the last few years, the structure and shape of adsorbed
aggregates has been revealed by imaging coated surfaces with the
atomic force microscope (AFM). To produce such images, surfactant
aggregates must be securely adsorbed to a nearly atomically smooth
substrate. It has been found that there is a strong influence of
the surface in controlling the aggregate structure. For example, in
quaternary ammonium surfactant systems, mica tends to form full
cylindrical micelles that meander across the surface with changes
in direction that mirror the angles of the mica lattice. With
respect to amorphous silica, which lacks these atomic rows as such,
full spheres are observed.
[0008] In industry, there is strong motivation, based on
environmental concerns, to adopt aqueous processing schemes. Unlike
organic based processes, where methods have been established,
mostly based on carboxylate surfactants, to stabilize particulates
systems, best practices for aqueous suspension stabilization have
not been fully developed.
[0009] Based on standard application of DLVO theory, the maximum
energy barrier, responsible for the prevention of agglomeration
between particles, decreases as particle size decreases. Hence, a
smaller particle requires a higher surface charge to prevent
coagulation in the primary minimum than does a larger one. For
particles less than a micron in size the magnitude of surface
charge needed to stabilize the system may indeed become so large
that it cannot be practically achieved (.gtoreq.100 mV). As a
result, other methods such as the use of dispersants must be
applied.
[0010] For micron and larger sized particles, polymeric dispersants
with molecular weights on the order of 10,000-40,000 are often used
to enhance the stability of the suspension. However, as particle
size decreases, the radius of curvature of the particles approaches
that of the dispersant molecules. It has been shown that for these
very small particles higher molecular weight polymers result in a
significantly higher viscosity than very low molecular weight
dispersants (less than 6,000 MW). This behavior can be rationalized
if the high surface area of nanoparticulate materials is
considered. To produce a similar steric or electrosteric barrier, a
significantly higher concentration of polymer must be added to the
suspension. Hence, the effective volume fraction of the suspension
occupied by solid-like material dramatically increases.
Furthermore, for applications with higher volume fraction of
organic material in the consolidated body, organic burnout and the
density of consolidation become critical.
[0011] Chemical Mechanical Polishing (CMP) processes are widely
used in the microelectronics industry for planarization
(microscopically or macroscopically flattening) of metal and
dielectric layers on a substrate, such as a wafer, to achieve
multi-layer metallization. This process polishing the substrate
using a slurry that combines a chemical component, which oxidizes
the substrate surface, with a mechanical component, such as
abrasive particles, that abrades the oxides substrate, resulting in
a very flat surface. The major disadvantage of the process is the
large number of particles that can remain on the substrate surface
due to the large number in the slurry. Therefore, CMP is a source
of potentially problematic particle adhesion. Residual particles
often cause short-circuiting in the final integrated circuits. For
effective manufacturing of the microelectronic devices, it is
necessary to minimize the surface defects (such as scratches or
pitting) during the CMP process, while attaining a good planarity
with optimal material removal rate. One of the main sources of
defect formation in CMP is the presence of larger size particles in
the slurries.
[0012] In the case of magnetic disks, for example, particle
contamination can create topographic effects that are magnified as
additional layers are deposited. These topographic effects may
cause the read/write head of a disk drive assembly to crash.
Furthermore, the tolerance for topography caused by particulate
contamination on metal disk surfaces has decreased as the space
between the head and the disk is lessened. CMP uses an abrasive
slurry that may be very tenacious, particularly on a metal surface
that is negatively charged over the entire pH range of CMP slurry
solutions. This negatively charged surface electrostatically
attracts alumina slurry particles, for example, particularly in low
pH slurries such as ferric nitrate, where the alumina particles
acquire a positive surface charge. Therefore, robust dispersion of
the polishing slurries in extreme ionic strength and pH
environments of CMP is a must to achieve optimal performance.
[0013] In addition, to evenly planarize features across the whole
surface of the substrate, it is important to have uniform
pressure-sensitive material-specific removal rates across the
substrate. Higher features are exposed to higher download pressures
than their lower counterparts. To achieve a planar surface, the
material at these higher points must be removed more readily than
the lower features, which eventually should be equivalent
topographic heights. The material removed is the amount of material
abraded from the wafer surface during polishing. In the case of
Chemical Mechanical Polishing, the material removed primarily
consists of the chemically modified layer. In general, material
removal rates of 2000 to 6000 Angstroms per second are used in
practice to remove excess material from the wafer surface and
provide a planar surface of acceptable finish.
[0014] The use of uniform pressure-sensitive removal rates ensures
that higher features, which would be under the more pressure, are
reduced to the same level as lower features on the wafer giving a
planar surface. During CMP, the wafer is held on a rotating carrier
face down and is pressed against a polishing pad attached to a
rotating disk. For oxide or silicon polishing, an alkaline slurry
of colloidal silica (SiO.sub.2 particles in a KOH solution or
NH.sub.4OH) is continuously fed to the pad/wafer interface. In a
typical configuration for polishing the wafer, the rotating wafer
rests on a rotating pad system, which includes two pads (a top pad
and a sub-pad). A retaining ring surrounds the wafer and holds it
in place. With the exception of at the edges of the wafer, a
uniform load pressure distribution acts on the wafer. A uniform
pressure distribution of lower magnitude acts on the ring. The
primary inputs to the CMP process are (i) rotational speeds of the
pad and wafer (both constant), (ii) load pressure magnitude, (iii)
ring pressure, and (iv) pad conditioning (friction coefficient
between pad and wafer). The primary output of interest is the
uniformity of the wafer surface as measured by the within wafer
non-uniformity (WIWNU) and the average removal rate, as well as
it's relative defectivity. Defectivity refers to the overall
surface quality of the wafer at the nanoscale. Acceptable levels of
defectivity include RMS roughnesses of less than 2 nm and maximum
defect depths of less than 25 nm.
[0015] Accordingly, it would be advantageous to provide materials
and methods that impart stability to CMP slurries and other
suspensions in extreme chemical (e.g., pH, ionic strength, reactive
additives) and dynamic (e.g., shear and normal forces) environments
while attaining a controlled material removal rate and minimal
surface defects.
BRIEF SUMMARY OF THE INVENTION
[0016] The subject invention relates to a method and materials for
the control of the energy barrier to agglomeration with respect to
particles in solution. Controlling the energy barrier to
agglomeration can cause the particles to enter suspension and/or
depart from suspension, as desired. By fine-tuning the energy
barrier, a variety of processes can be enhanced. Thus, the subject
invention concerns methods for controlling the energy barrier to
agglomeration by modulating the interaction between particles and a
surface (also referred to herein as a substrate).
[0017] In one aspect, the present invention pertains to a method
for increasing the stability of a composition (such as a slurry
solution) containing particles and at least one dispersant, such as
a surfactant, by adding a at least one competing agent to the
composition, wherein the competing agent competes with the
dispersant for Coulombic (charged) and/or hydrogen bonding
adsorption sites on the substrate, thereby hindering the
dispersant's particle-substrate interaction within the composition.
However, the competing agent preferably does not eliminate the
dispersant's influence on the particle-particle interaction (steric
repulsion between particles) within the composition, thereby
achieving and maintaining a stable composition.
[0018] In another aspect, the present invention pertains to a
method for decreasing the stability (i.e., increasing
agglomeration) of a composition containing particles, at least one
dispersant, such as a surfactant, and at least one competing agent,
by enhancing the competitive activity of the competing agent. For
example, the ability of a competing agent to `compete` for an 5
adsorption site can be enhanced by adding an effective amount of
the competing agent to the composition to decrease the dispersant's
influence on the particle-substrate interaction by essentially
`out-numbering` the dispersant molecules.
[0019] In another aspect, the present invention pertains to a
stable composition comprising particles, such as abrasive
particles, at least one dispersant, such as a surfactant, and at
least one competing agent, wherein the competing agent competes
with the dispersant for adsorption sites (e.g., Coulombic or
hydrogen bonding sites) on the substrate, thereby hindering the
dispersant's influence on the particle-substrate interaction (e.g.,
lubrication) within the composition. However, the competing agent
preferably does not eliminate the dispersant's influence on the
particle-particle interaction (steric repulsion) within the
composition, thereby maintaining a stable solution.
[0020] In a specific embodiment, the subject invention can be
utilized to cause agglomeration of a first particle type while
maintaining suspension of a second particle type. In a preferred
embodiment, the barrier to agglomeration can be set at a specific
level, raised, and/or lowered, in order to meet the needs of a
particular situation. The subject method and materials are
particularly advantageous with respect to suspensions in extreme
environments. Such extreme environments can include, for example,
one or more of the following: high ionic strength (>0.01 Molar);
high temperature (>50.degree. C.); low temperature
(<10.degree. C.); high pH (>pH 13); low pH (<pH 2);
presence of chemical reactants such as oxidizers and/or reducing
agents; high forces (>10 mN/m); and high shear rate (>10,000
1/sec). For example, the subject methods and materials can be
utilized to provide high performing chemical-mechanical polishing
(CMP) slurries with excellent stability and material removal
characteristics, and which produce planarized substrates with
little or no surface defects during the polishing process.
[0021] Using the methods and materials of the subject invention,
mediation of the slurries with surfactants was shown to provide
stability to CMP slurries by introducing high enough repulsive
forces between the abrasive particles as a result of the
cohesiveness of the self-assembled surfactant structures. However,
it was also observed that, to design optimally performing CMP
slurries, the control of particle-particle interactions was not
sufficient as it may be in other applications. To enable an optimal
CMP performance, it was also necessary to have the abrasive
particles engage with the substrate so that material removal could
take place.
[0022] In one embodiment, the slurry composition of the subject
invention includes abrasive particles, a cationic surfactant, such
as alkyl trimethylammonium bromide, and a divalent cation, such as
Ca.sup.++ or Ba.sup.++. Preferably, the cationic surfactant is a
long chain surfactant having a nonpolar hydrocarbon tail of
approximately 8 to 16 carbon atoms in length. In a specific
embodiment, the cationic surfactant is a quaternary ammonium
compound.
[0023] In another aspect, the subject invention concerns a method
for polishing a substrate by applying a slurry composition of the
subject invention to the substrate and polishing the substrate. The
slurry composition can be contacted directly with the substrate
surface or applied to a polishing pad, which is contacted to the
substrate surface such that the slurry composition is interposed
between the pad and substrate surface.
[0024] The subject invention can utilize materials such as other
surfactants, polymers, mixtures of polymers, mixtures of
surfactants, mixtures of polymers and surfactants, biospecies,
microbes, bacteria, viruses, other particles, or combinations
thereof, which can affect the energy barrier. In a specific
embodiment, the subject invention can utilize materials that form
self-assembled systems. Examples of such materials include, but are
not limited to surfactants, which can form micelles; and block
copolymers comprising hydrophilic and hydrophobic subparts.
[0025] The subject invention further concerns abrasive articles,
such as polishing pads, and methods for making the slurry
compositions of the subject invention.
[0026] In a specific embodiment of the subject invention,
surfactant micelles or bilayers can be used as a dispersant for
nanoparticulate suspensions. Surfactant micelles or bilayers can
(i) be small in size relative to the particles, (ii) preferentially
partition to the solid liquid interface, (iii) have controlled
adsorption in multi-component systems, and (iv) provide adequate
steric as well as electrostatic repulsion between particles.
[0027] Surfactant molecules can be attractive dispersants for
nanoparticle suspensions because they are normally only a few
nanometers in length. Accordingly, surfactant molecules contribute
minimally to the effective volume fraction of a suspension.
Surfactant molecules are often used in the mining industry to
increase the hydrophobicity of surfaces such that the particles may
adhere to air bubbles. The specificity of this interaction has been
explored in detail such that for many different multi-component
systems only one mineral is hydrophobized and subsequently can be
separated.
[0028] At high concentrations, surfactants self-assemble into
bilayers or adsorbed aggregates of surfactant molecules. These
micelles have hydrophilic, possibly charged, surfaces, but still
may preferentially adsorb to particulate surfaces. Hence, as two
particles approach each other, they will first experience
electrostatic repulsion and then a repulsion due to the actual
compression of the micelle layers as they begin to overlap
(steric). However, for nanoparticle systems, the effect of the
electrostatic repulsion may be small. Accordingly, the stability of
suspensions stabilized in this manner can be critically dependent
on the strength of the adsorbed micelles (magnitude of the steric
repulsive barrier).
[0029] FIG. 16 demonstrates the effectiveness of the surfactant
dispersant concept in a model system. Plotted is the turbidity of a
suspension of 100 nm radius silica particles at the initially
unstable, as evidenced by low turbidity, condition of pH 4 with 100
mM NaCl sodium chloride. As the concentration of cationic
trimethylammonium bromide surfactant with a 12-carbon unit tail
(C.sub.12TAB) is increased, a sharp transition between the stable
and unstable region is observed (8 to 10 mM). This transition is
believed to correspond to the concentration where adsorbed micelles
begin to act as dispersants in this high electrolyte concentration
solution.
[0030] Controlling the onset of these repulsive forces and the
magnitude of the barrier to agglomeration can also be a critical
factor in nanoparticulate stability. One method of increasing the
strength of adsorbed micelles is to increase the length of the
hydrophobic tail. The greater the hydrophobic attraction between
the tails can lead to a more stable micelle and thus a greater
repulsive force. FIG. 4 depicts the height of the barrier to
agglomeration as a function of alkyl chain length as measured
between a mica surface and nanometer size silicon nitride probe
particle in atomic force microscopy. As is evident from the figure
the magnitude of the barrier is nearly linear with increasing chain
length.
[0031] Other parameters that can be manipulated by the subject
method to control the barrier to agglomeration in these systems
include, but are not limited to, salt concentration, addition of
cosurfactant, and addition of alcohol. Accordingly, the subject
invention can control the degree of stability in these systems such
that optimal processing properties can be achieved.
[0032] The subject method and apparatus pertain to the utilization
of adsorbed micelles to impart stability to suspensions, such as
CMP slurries. The use of micelles with respect to the subject
method and system is attractive due to their small size,
specificity of adsorption, and the ability to control the magnitude
of the barrier to agglomeration. Investigation of suspension
stabilization and interparticle forces indicate the advantages of
such a system.
[0033] The methods of the subject invention provide abrasive
solutions that are capable of good surface finishing when used to
polish substrates, with high material removal rate.
BRIEF DESCRIPTION OF DRAWINGS
[0034] FIGS. 1A-1C show schematic diagrams showing the possible
structures of surfactants adsorbed at the solid/liquid interface,
where FIG. 1A illustrates a bilayer formation, FIG. 1B illustrates
semicylindrical micelles or semispheres, and FIG. 1C illustrates
full cylinders or spheres.
[0035] FIG. 2 shows a force-distance curve for a 2 CMC C.sub.12TAB
solution showing the different stages as the tip approaches the
mica surface.
[0036] FIG. 3 shows a force-distance curves of C.sub.nTAB (for n
10, 12, 14 and 16) cylindrical micelles adsorbed on mica; a,
C.sub.10TAB; b, C.sub.12TAB; c, C.sub.14TAB; d, C.sub.16TAB.
[0037] FIG. 4 shows a plot of maximum compressive force as a
function of alkyl chain length for 2 CMC solutions of C.sub.nTAB
(for n=10, 12, 14 and 16).
[0038] FIG. 5 shows a plot of the change in electrical conductivity
due to micelle break-up in a pressure-jump experiment, where the
change in electrical conductivity is proportional to the signal
represented on the y-axis (.smallcircle. circles C.sub.10TAB;
.circle-solid. C.sub.16TAB).
[0039] FIG. 6 shows a plot of the maximum compressive force as
function of SDS concentration for a 2 CMC (32 mM) C.sub.12TAB
solution on mica.
[0040] FIG. 7 shows a plot of the turbidity of silica dispersions
(o) and the maximum compressive force on mica (.circle-solid.)
versus concentration of C.sub.12TAB at pH 4 after 60 seconds.
[0041] FIG. 8 shows a plot of the stability of silica dispersions
(o) and the maximum compressive force on mica (.circle-solid.)
versus concentration C.sub.12TAB in the presence of 100 mM NaCl at
pH 4.
[0042] FIG. 9 shows a plot of the stability of silica dispersions
(o) and the maximum compressive force on mica (.circle-solid.)
versus concentration of SDS in the presence of 100 mM NaCl and 5 mM
C.sub.12TAB at pH 4.
[0043] FIG. 10 shows a plot of relative turbidity versus
electrolyte concentration for 200 nm particles in the presence of
32 mM of C.sub.12TAB.
[0044] FIG. 11 shows a plot of maximum repulsive force versus
electrolyte (NaCl) concentration for an AFM tip and mica surface in
the presence of 32 mM C.sub.12TAB.
[0045] FIG. 12 shows a plot of maximum repulsive versus alcohol
(ethanol) concentration for an AFM tip and mica surface in the
presence of 32 mM C.sub.12TAB.
[0046] FIG. 13 shows a plot of maximum repulsive force versus Urea
concentration in the presence of 32 mM C.sub.12TAB.
[0047] FIG. 14 shows a plot of interaction force versus separation
distance for an AFM tip and mica surface in the presence of 32 mM
C.sub.12TAB and no urea, 1 M urea, or 5M urea.
[0048] FIG. 15 shows a plot of interaction force versus separation
distance for an AFM tip and mica surface in the presence of 100
mg/L Darvan C and NaCl or 0.1 M NaCl.
[0049] FIG. 16 shows a plot of maximum repulsive force versus
C.sub.12TAB concentration for an AFM tip and silica surface, and
for 200 nm silica particles, in the presence of 0.1 M NaCl.
[0050] FIG. 17 shows a plot of maximum repulsive force versus
cosurfactant (SDS) concentration for an AFM tip and silica surface,
and for 200 nm silica particles, in the presence of 0.1 M NaCl and
3 mM C.sub.12TAB.
[0051] FIG. 18 shows a plot of maximum repulsive force versus
C.sub.12TAB concentration for an AFM tip and mica surface in the
presence of no NaCl, or in the presence of 0.1 M NaCl.
[0052] FIG. 19 shows a plot of maximum repulsive force versus
C.sub.12TAB concentration for an AFM tip and a silica surface, and
for an AFM tip and a mica surface, in the presence of 0.1 M
NaCl.
[0053] FIG. 20 shows a plot of maximum repulsive force versus
temperature for an AFM tip and a silica surface in the presence of
11 mM C.sub.12TAB and 0.1 M NaCl.
[0054] FIG. 21 shows a plot of interaction force versus separation
distance for an AFM tip and silica surface in the presence of 11 mM
C.sub.12TAB and 0.1 M NaCl, at 25.degree. C., 32.degree. C., and
40.degree. C.
[0055] FIG. 22 shows a plot of viscosity versus temperature for 200
nm silica particles in the presence of 39 mg/g C.sub.12TAB.
[0056] FIG. 23 shows a plot of viscosity versus shear rate for 200
nm silica particles in the presence of 44 mg/g C.sub.12TAB, or in
the presence of 5 mg/g Darvan C.
[0057] FIG. 24 shows a plot of viscosity versus shear rate for 200
nm silica particles in the presence of 44 mg/g C.sub.12TAB and 0.1
M NaCl, or in the presence of 3 mg/g PEO 7500 MW.
[0058] FIG. 25 shows a plot of maximum repulsive force versus pH
for an AFM tip and alumina surface in the presence of 20 mM SDS and
0.001 M NaCl.
[0059] FIG. 26 shows a plot of interaction force versus separation
distance for an AFM tip and alumina surface in the presence of 16
mM SDS.
[0060] FIG. 27A shows a spectra of C.sub.12TAB at 4 mM bulk
concentration in the CH.sub.2 stretching region [asymmetric (2920
cm.sup.-1), symmetric (2850 cm.sup.-1)] with parallel (p) and
perpendicular (s) polarized infrared beam.
[0061] FIG. 27B shows a spectra of C.sub.12TAB at 8 mM bulk
concentration in the CH.sub.2 stretching region [asymmetric (2920
cm.sup.-1), symmetric (2850 cm.sup.-1)] with parallel (p) and
perpendicular (s) polarized infrared beam.
[0062] FIG. 28 shows the dependence of the order parameter S on the
dichroic ratio D.
[0063] FIG. 29 shows a schematic representation of surfactant
structures and orientation, and the order parameter values
associated with the structures.
[0064] FIG. 30 shows the adsorption isotherm (squares), zeta
potential (triangles), and contact angle (spheres) of silica
surfaces, in 0.1 M NaCl at pH 4.0 as a function of solution
C.sub.12TAB concentration.
[0065] FIG. 31 shows the contact angle of silicon surface
(triangle), measured maximum repulsive forces between an AFM tip
and silicon surface (squares), and order parameter (circles) of the
adsorbed surfactant structures on a silicon surface in 0.1 M NaCl
at pH 4.0 as a function of solution C.sub.12TAB concentration.
[0066] FIGS. 32A-32F show a schematic representation of the
proposed self-assembled surfactant films at concentrations
corresponding to regions A-F in FIGS. 30 and 31 where region A
represents individual surfactant adsorption, region B represents
low concentration of hemi-micelles on the surface, region C
represents higher concentration of hemi-micelles on the surface,
region D represents hemi-micelles and spherical surfactant
aggregates formed due to increased surfactant adsorption and
transition of some hemi-micelles to spherical aggregates, region E
represents randomly oriented spherical aggregates at onset of
steric repulsive forces, and region F represents surface fully
covered with randomly oriented spherical aggregates.
[0067] FIG. 33 shows a plot of maximum repulsive force versus
C.sub.12TAB concentration for an AFM tip and silica surface in the
presence of 0.1 M NaCl at a pH of 1, 2, and 4.
[0068] FIG. 34 shows a plot of maximum repulsive force versus
C.sub.12TAB concentration for an AFM tip and mica surface in the
presence of 0.1 M NaCl at a pH of 4 and 9.
[0069] FIG. 35 shows a plot of suspension turbidity versus SDS
concentration for 300 nm alumina particles and for an AFM tip and
saphire surface in the presence of 0.1 M NaCl and at a pH of 4.
[0070] FIG. 36 shows a schematic representation of the
particle-particle and particle-substrate interactions for the
silica-silica polishing systems in the presence of self assembled
surfactant aggregates.
[0071] FIG. 37 shows the maximum repulsive force response of
C.sub.12TAB, C.sub.10TAB and C.sub.8TAB surfactants at 32, 68 and
140 mM concentrations in the presence of 0.6 M NaCl at pH 10.5
obtained with AFM using 1.5 .mu.m particle attached to tip.
[0072] FIG. 38 shows the stability and material removal rate
responses for surfactant-mediated slurries.
[0073] FIG. 39 shows in situ friction force and material removal
rate responses of the baseline slurries (12 w%, 0.2 .mu.m primary
particle size) and the slurries containing C.sub.12TAB surfactants
at 32, 68 and 140 mM concentrations in the presence of 0.6 M NaCl
at pH 10.5.
[0074] FIGS. 40A and 40B show AFM friction force measurements on
silica wafer with 7 .mu.m size particle attached to the tip. FIG.
40A shows solutions containing C.sub.12TAB, C.sub.10TAB and
C.sub.8TAB surfactants at 32, 68 and 140 mM concentrations without
NaCl at pH 10.5. FIG. 40B shows the same surfactant solutions with
0.6 M salt in each solution.
[0075] FIG. 41 shows FTTR-ATR adsorption spectra of the solutions
in the presence of 32 mM C.sub.12TAB without salt and in the
presence of 0.6 M NaCl and 0.24 M CaCl.sub.2.
[0076] FIGS. 42A and 42B show AFM force measurements for
C.sub.12TAB mediated slurry in the presence of NaCl and CaCl.sub.2.
FIG. 42A shows repulsive force measurements. FIG. 42B shows
friction force measurements.
[0077] FIGS. 43A-43E show formulas for cationic surfactants that
can be used as dispersants in the present invention. FIG. 43A shows
an amine. FIG. 43B shows a diamine. FIG. 43C shows a quaternary
ammonium salt. FIG. 43D shows a polyoxyalkylenated amine. FIG. 43E
shows an amine oxide.
[0078] FIGS. 44A-44C show illustrations of individual surfactant
adsorption (FIG. 44A), Hemi-Micelle Formation (FIG. 44B), and
self-asembled surface aggregates (FIGS. 44C and 44D).
DETAILED DISCLOSURE OF THE INVENTION
[0079] The subject method and composition pertain to the control of
the energy barrier existing between particles, and/or particles and
a surface. The method of the present invention involves reducing
the strength of adhesion between particles in an abrasive slurry
composition and the substrate with which they are contacted,
without significantly compromising the strength of adhesion between
the particles themselves. For example, the selection of appropriate
dispersants, such as surfactants, and corresponding competing
agents for nanoparticulate suspensions can significantly impact
whether the particles remain separate in suspension or agglomerate.
The subject invention also relates to the use of an electro-steric
barrier arising from adsorbed micelles to control the stability of
a suspension.
[0080] The subject invention relates to methods and materials for
the control of the energy barrier to agglomeration with respect to
particles. Controlling such energy barrier can, for example, cause
the particles to enter suspension and/or depart from suspension, as
desired. By fine-tuning the energy barrier, a variety of processes
can be enhanced. The subject invention also pertains to the control
of the energy barrier between particles and a surface. In a
specific embodiment, the subject invention can be utilized to cause
agglomeration of a first particle type while maintaining suspension
of a second particle type. In a preferred embodiment, the barrier
to agglomeration can be set at a specific level, raised, and/or
lowered, in order to meet the needs of a particular situation.
[0081] The slurry compositions of the subject invention are
preferably aqueous solutions. Preferably, the aqueous component
comprises deionized water. In one embodiment, the slurry
composition of the subject invention includes a plurality of
abrasive particles, a cationic surfactant, such as alkyl
trimethylammonium bromide, and a divalent cation, such as Ca.sup.++
or Ba.sup.++. Preferably, the cationic surfactant is a long chain
surfactant having a nonpolar hydrocarbon tail of approximately 8 to
16 carbon atoms in length. In a specific embodiment, the cationic
surfactant is a quaternary ammonium compound.
[0082] The subject invention also concerns a method for polishing a
substrate by applying a slurry composition of the subject invention
to the substrate and polishing the substrate. The slurry
composition can be contacted directly with the substrate surface or
applied to a polishing pad, which is contacted to the substrate
surface such that the slurry composition is interposed between the
pad and substrate surface.
[0083] The subject method and materials are particularly
advantageous with respect to suspensions in extreme environments.
Such extreme environments can include, for example, one or more of
the following: high ionic strength (>0.01 Molar); high
temperature (>50.degree. C.); low temperature (<10.degree.
C.); high pH (>pH 13); low pH (<pH 2); presence of chemical
reactants such as oxidizers and/or reducing agents; high forces
(>10 mN/m); and high shear rate (>10000 1/sec). Particles to
which the subject techniques can be applied include, but are not
limited to, silica, alumina, ceria, cerium oxide, kaolin, titania,
hematite (iron oxide), and magnesia. Surfaces to which the subject
technique can be applied include, but are not limited to, tungsten,
copper, aluminum, tantalum nitride, zirconia, titanium, alumina,
silicon, and silicon nitride as well as any ceramic, metal or
polymer surface.
[0084] Preferably, dispersants for particulate suspensions should
(i) be small in size relative to the particles (ii) preferentially
partition to the solid liquid interface, (iii) have controlled
adsorption in multi-component systems, and (iv) provide adequate
repulsion between particles under given solution and mechanical
conditions. In a specific embodiment of the subject method and
system, self-assembled layers of surfactant molecules can be
utilized to disperse particulates. Advantageously, the subject
self-assembled layers of surfactant molecules can be useful under
extreme conditions.
[0085] Although much has been published on the structure of
surfactant films adsorbed on materials using AFM, very little has
been reported on the stability of these surfactant films and their
technological implications. In a specific embodiment, the subject
invention relates to the use of an adsorbed micelle layer in the
dispersion of a suspension. Atomic force microscopy can be used to
monitor onset of surface aggregation of surfactant molecules and
the magnitude of the barrier to agglomeration produced.
[0086] Other force measuring techniques can also be used in
accordance with the subject invention, including, but not limited
to, surface force apparatus (developed by Israel, Achvili, and
Adams) and interfacial gauge techniques (MASIF). When using AFM to
measure the magnitude of the barrier to agglomeration between
particles the AFM tip can be made of the same material as the
particles and the surface to which the tip is urged can also be the
same as the particles. For example, one or more particle of such
material can be glued onto the tip. Alternatively, the tip may be
composed of a material that is comparable to such material.
Likewise, when measuring the barrier between a particle and a
surface, the surface can be of the same material as the surface and
the tip can have a particle glued onto it or not, depending on the
situation.
[0087] FIG. 2 depicts the surface forces present between a silicon
nitride tip of an atomic force microscope (AFM) cantilever and a
mica substrate in the presence of a dodecyltrimethylammonium
bromide solution (C.sub.12TAB--where C.sub.12 represents the number
of carbon atoms in the alkyl chain) at twice the bulk critical
micelle concentration (32 mM). At this concentration and these
solution conditions mica is negatively charged and has been shown
to be coated with full cylindrical micelles. This was confirmed by
imaging the adsorbed micelle layer under these conditions.
[0088] The most striking aspect of the force interaction curve is
that at relatively short separation distances, 4 nm to 8 nm, a
strong repulsive force exists between the two surfaces. Then, at a
particular distance, the repulsion seems to disappear allowing the
tip to jump-in and contact the substrate. Although this interaction
may at first seem similar to the traditional electrostatic
repulsion followed by van der Waals attraction as described by DLVO
theory, it is important to note that because the radius of
curvature of the AFM tip is small, 10-20 nm, the interaction forces
measured in this case appear to be approximately two orders of
magnitude greater than electrostatic repulsion.
[0089] Based on images of the surface micelles and on the
assumption that they are cylindrical, it can be estimated that the
thickness of the adsorbed micelle layer is approximately 5.2 nm.
The very small, approximately 10-20 nm, radius of the AFM probe
allows forces to be accessed that are much greater in magnitude.
Since the ion-electrostatic repulsion is negligible at high
electrolyte concentrations, it is believed that the repulsive force
comes from the steric stabilization by an adsorbed film.
[0090] Different structures of surfactant aggregates have been
proposed as depicted in FIG. 1. For example, surfactant molecules
can form bilayers, semicylinders, full cylinders, semi spheres, and
full spheres adsorbed onto the surface. It is believed that the
maximum compressive strength of the adsorbed surfactant aggregates
is directly related to the stability of solid/liquid dispersions.
The strength of surfactant films adsorbed onto solid/liquid
interfaces can be controlled. In a specific embodiment, dispersion
stability can be controlled. Alkyl trimethylammonium bromides
adsorb as cylinders or spheres on surfaces, such as mica or silica,
even at relatively low bulk concentration (2 CMC). The stability of
the adsorbed layer, which was measured by the maximum compressive
force, increases linearly with alkyl chain length and can be
greatly enhanced by the addition of oppositely charged surfactant,
due to reduction of intramicellar repulsion. Dispersion stability
measurements have shown that unstable silica dispersions can be
stabilized by the addition of C.sub.12TAB. The addition of SDS
lowers the minimum concentration of C.sub.12TAB required to
stabilize the silica dispersion. Tuning the compressive barrier
required to stabilize solid/liquid dispersions can be a useful way
of stabilizing high ionic strength dispersions, such as high ionic
strength slurries used for Chemical Mechanical Polishing (CMP) of
wafers. Other substances involved in CMP should be taken into
account such that there is not interferences with the chemical
reactions between these other substances and the surface or each
other.
[0091] Referring to FIG. 2, the force interaction curve is shown
representing the surface forces present between a silicon nitride
tip of an atomic force microscope cantilever and a mica substrate
in the presence of a C.sub.12TAB solution at twice the bulk CMC.
The general shape of this curve is believed to be representative of
the force versus separation distance with respect to particles in
suspension. The force magnitude indicated in FIG. 2 as F.sub.max
represents the force above which a particle will adsorb to a
surface, or two particles will agglomerate. Advantageously, the
methods and materials of the subject invention can raise or lower
the value of F.sub.max, or energy barrier, in order to control the
adsorbtion and/or agglomeration of particles. This energy barrier
can be raised and/or lowered for a particular separation distance,
thus allowing the triggering of adsorption or agglomeration while
maintaining the same number of particles per unit volume. The
subject invention can allow precise tuning of this energy barrier.
Such control can enable many processes involving particles to be
optimized, saving materials, costs, and/or time. While prior
techniques have relied on electrostatic interaction to provide
sufficient particle-particle repulsion, the subject method can
utilize steric interaction between the particles, such as elastic
deformation of the dispersant layer on the particles, to provide
sufficient repulsion forces. Accordingly, various techniques to
reduce these steric forces can be utilized.
[0092] Control of this energy barrier can be accomplished, for
example, by one or more of the following: the addition of salt to
increase electrolytes; utilizing a co-surfactant (e.g., cationic
anionic, nonionic, amphoteric or zwitterionic surfactants) with a
longer, shorter or identical alkyl chain length; the addition of
molecules that compete for hydrogen bonding sites; the addition of
a structure breaking substance such as urea; the addition of a
structure making substance; the changing of the type of particle or
surface; the raising or lowering of the temperature; the dilution
of the suspension; the addition of alcohol; the addition of
anti-foaming agents; and the adjustment of the pH of the
suspension. Anti-foaming agents can include, for example, urea,
tributylphosphate, and alkyl tetra-ammonium halides.
[0093] In particular, the addition of salts can create a high ionic
strength slurry. As used herein, the term `high ionic strength
slurry` refers to a slurry with adequate electrolyte to
sufficiently lower the electrostatic repulsive energy between
particles to a level where it is not sufficient to overcome the
collision energy imparted to the particles due to Brownian motion.
Essentially, the surface charge is shielded by the presence of the
electrolytes, thereby lowering the electrostatic repulsion between
surfaces. Preferably, in accordance with the subject invention, the
ionic strength of the high ionic strength slurry is greater than
0.01 M.
[0094] Advantageously, the subject process and composition includes
adding an additional component to yield flexibility in competitive
absorption strategies. The addition of similarly charged ionic
moieties and/or molecules competing for hydrogen bonding sites
prevent excessive adsorption by the cationic surfactant onto the
substrate, thereby leaving a consistently smooth substrate surface.
Examples of similarly charged ionic moieties include, but are not
limited to, salts, anionic surfactants, polymers, sodium dodecyl
sulfate and proteins associated with a positively charged surface
in competition with other anionic ions. Examples of molecules that
compete for hydrogen bonding sites include polyether oxide (PEO),
polyvinyl acrylate (PVA), polyacrylic acid (PAA), polyacrylamide
(PAM) and poly(allylamine hydrochloride)(PAH). Optionally,
dispersants, although not competing for adsorption sites, are
useful as slurry stabilizers. Examples include, but are not limited
to, amphoteric surfactants, proteins or polymers useful as
dispersants.
[0095] Examples of processes which can benefit from the methods and
materials of the subject invention include: high speed coating of
surfaces, jet printing, polishing, grinding, homogenization,
particle synthesis, crystallization, cleaning, silk screening,
pesticide application, and fertilizer application. As a specific
example, when polishing the surface of a substrate, as during CMP,
it may be desired to have a low enough energy barrier to allow
polishing of a surface by the particles while the energy barrier is
high enough to keep the particles from agglomerating and
potentially scratching the surface. In another specific example, it
may be desirable to have a high enough energy barrier with respect
to a suspension of ceramic particles to allow pouring of the
ceramic particle suspension into a mold such that the mold voids
can be filled and then be able to overcome the energy barrier by
raising the temperature such that the ceramic particles
agglomerate. In this example, precise control of the energy barrier
can reduce the amount of temperature increase required to trigger
agglomeration and, therefore, reduce costs.
[0096] During CMP, as layers of materials are sequentially
deposited and removed, the uppermost surface of the substrate may
become non-planar across its surface and require planarization.
Planarizing a surface, or "polishing" a surface, is a process where
material is removed from the surface of the substrate to form a
generally even, planar surface. Planarization is useful to remove
excess deposited material and to provide an even surface for
subsequent levels of metallization and processing. Planarization
may also be used in removing undesired surface topography and
surface defects, such as rough surfaces, agglomerated materials,
crystal lattice damage, scratches, and contaminated layers or
materials. Chemical mechanical planarization, or chemical
mechanical polishing (CMP), is a common technique used to planarize
substrates. In conventional CMP techniques, a substrate (e.g.,
wafer) carrier or polishing head is mounted on a carrier assembly
and positioned in contact with a polishing pad in a CMP apparatus.
The carrier assembly provides a controllable pressure to the
substrate urging the substrate against the polishing pad. The pad
is typically moved relative to the substrate by an external driving
force, such as a rotating turntable or platen. Thus, the CMP
apparatus effects polishing or rubbing movement between the surface
of the substrate and the polishing pad while dispersing a polishing
composition (a slurry) to effect both chemical activity and
mechanical activity. As used herein, the terms "CMP",
"chemical-mechanical polishing", and "chemical-mechanical
planarization" are broadly defined as polishing a substrate by
chemical activity, mechanical activity, or a combination of both
chemical and mechanical activity. According to the methods of the
subject invention, the slurries of the invention can be applied to
(placed in contact with) the substrate. Optionally, the slurries of
the invention can be applied with a CMP apparatus as described,
wherein the slurry is placed in contact with, and interposed
between, the substrate surface and the polishing pad. Either the
substrate, the polishing pad, or both, can be rotated relative to
one another.
[0097] In another aspect, the subject invention concerns an
abrasive article, such as a polishing pad, that is at least
partially coated with or releases a slurry composition of the
subject invention. The abrasive article can include a backing sheet
with a plurality of abrasive elements adhered thereto. The slurry
composition can be applied to one side of the abrasive article,
which is then contacted with the substrate to be polished.
Alternatively, the slurry composition of the invention can be
contained within the abrasive article, such as in a reservoir,
which is then dispensed on the side of the abrasive article to be
placed in contact with the substrate surface to be polished.
[0098] The methods and materials of the subject invention can also
be applied to the mining of substances where the particles can be
retrieved and transported in suspension and then later agglomerate
the particles out of suspension in order to process and package
them. The methods and materials of the subject invention can also
be applied to gaseous fluid systems. For example, dry particles can
be coated in a vapor environment in order to alter the caking
properties of the particles. This can be advantageous in, for
example, the transport and processing of the particles.
[0099] The subject invention is particularly advantageous when
applied to nanoparticulates. Since nanoparticulates are very small
and, accordingly, it is very important to minimize the thickness of
the dispersant layer on the particles such that a larger number of
particles can be suspended per unit volume. The method and
materials of the subject invention allow fine-tuning of the energy
barrier such that the dispersant layer need not be any thicker than
necessary to achieve the desired processing objectives.
[0100] The subject invention provides consistently high performing
CMP slurries that can achieve high material removal rates while
reducing the formation of surface defects during the polishing
process. As the sizes of the microelectronic devices keep on
decreasing continuously, a very thin film of material needs to be
removed during CMP allowing lower tolerance for the slurry
performance for not only enabling effective CMP operations for the
current microelectronics device manufacturing requirements, but
also to enable the manufacturing of next generation devices. The
mechanisms of controlling the slurry stability and the material
removal by controlling the interaction forces described in this
invention are critical tools to develop consistently high
performing slurries for oxide polishing system. However, with the
developed fundamental understanding on the subject, slurries can be
generated for other CMP operations such as copper, tungsten of
aluminum. Furthermore, the method of modifying the surfactant
adsorption/desorption can also be applied to develop
material-selective slurries. The methods of the subject invention
provide CMP slurries that are stable with controlled material
removal rate response and which have minimal defects during
polishing as well as with high material selectivity.
[0101] The inconsistent performance of the currently available
commercial CMP slurries result in poor process control leading to
low productivity and economical losses. The proposed invention will
allow the consumer better control on the slurry material removal
rate, overall planarization and selectivity, as well as reduction
in the surface deformations. As the invention introduces the
guidelines to manipulate the material removal rates, it gives the
flexibility of preparing slurries meeting specific CMP
requirements. Therefore, this invention is expected to increase
process productivity with enhanced process control and minimization
of the defects. Furthermore, it will allow the adaptation of new
and more challenging CMP applications at a shorter time span as it
underlines the methods to control and modify the polishing and
planarization responses.
[0102] The CMP process needs to be improved to keep up with the
demands in the microelectronics industry such as the continuously
decreasing sizes of the microelectronic devices. Especially there
is a need for decreasing the defect formations during the polishing
process. The ability to predict and control material removal rates
for global planarization, while preventing defect formation,
considerably lowers the risks of adverse slurry performance while
providing a knowledgebase for tunable slurry design.
[0103] Although the currently used slurries may be effective on a
particular type of CMP process, they often exhibit inconsistent
performance from batch to batch even if they are claimed to have
the same properties. Slight variations in the slurry manufacturing
results in major variations in the CMP performance. In addition,
they cannot be modified to satisfy the minor or major changes in
the performance requirements. The subject invention, however,
introduces a method of manufacturing consistently stable slurries
with tunable material removal rate response. The slurry properties
can be tailored to meet the process requirements in terms of
material removal rate and material selectivity. Furthermore, the
inconsistencies in the slurry-performance can be minimized since
the interaction forces are closely controlled to prevent
destabilization and undesired mechanical interactions.
[0104] In accordance with subject method, the primary mechanism
resulting in the adsorption of the surfactants to the silica
surface is due to charge. The silica surface is highly negatively
charged under the process pH (.about.10.5) and the surfactants are
driven to the surface because they are cationic (oppositely
charged). The surfactants adsorb onto the surface initially as
individual molecules with the head closest to surface due to the
opposite charge attraction for the case of silica and most charged
hydrophilic substrates. It's important to note that the tails are
hydrophobic so that do their best to minimize their interaction
with the aqueous phase. This results in the self-assembly (i.e.,
the formation of monolayer patches and micelles as the solution
surfactant concentration is increased).
[0105] Advantageously, the slurry compositions of the subject
invention tolerate CMP polishing even when the abrasive particle
and the substrate are both highly negative charged as in, for
example, a silica particle and a silica wafer. Although micelles
are adsorbed onto the particles and the wafer surfaces, the
self-assembled aggregates and surfactant monomer effectively leave
the surfaces and do not intervene with the material removal
mechanism when the abrasive particle is pressed against the wafer
under CMP conditions.
[0106] Modifying the CMP slurry pH maximizes consistency of CMP
performance. In the case of silica CMP, adjusting the pH to a
regime where the silica is prone to dissolution forms the
chemically modified surface layer and allows CMP to occur. For the
polishing of silica, the pH should preferably lie between pH 9.0
and pH 12.00. More preferably, the pH should range lie from about
10 .0 to 11.0. For other systems, the pH will vary and will depend
on the particular process and substrate to be polished.
[0107] The slurry composition of the present invention comprises
abrasive particles, at least one surfactant, and at least one
competing agent. Examples of abrasive particles include, but are
not limited to, metal oxides, ceramic powders, polymer powders,
metal powders, alumina, titania, cerium, zirconia, silicon nitride
and silica. The selection of the appropriate abrasive particle is
dependent on application and the affinity the dispersant molecule
displays for the particle surface, thereby invoking slurry
stability. In particular, the abrasive particle, or the exterior
surface of a composite particle, should not adhere to the surface
of the substrate. In a specific embodiment, the modulus of the
particles should be sufficient enough to promote material removal
within CMP processing. Preferred abrasive particle and substrate
combinations include, but are not limited to, copper wafers and
alumina particles, tungsten wafers and alumina particles, silica
wafers and silica particles, and silicon nitride/silica patterned
wafers and cerium particles.
[0108] Preferred physical characteristics of the abrasive particles
includes size ranging from about 10 nm to 10 microns. In a specific
embodiment, the abrasive particle size ranges from about 10 nm to
300 nm when utilized in CMP processing. The preferred solids
loadings for the abrasive particles in solution depends on the size
and shape of the particles and the application. For example,
solids-loadings for CMP slurries range from 1 wt % to 40 wt %.
Preferably, solids loadings range from 10 wt % to about 15 wt
%.
[0109] The competing agents used in the subject invention are those
that can effectively compete for adsorption sites on the substrate
(e.g., wafer), under process conditions--thereby weakening the
affinity of the surfactant (or other dispersant moiety) to the
surface thereby allowing for enhanced material removal. For
example, any cation can compete with the cationic surfactant for
adsorption sites, and therefore may be suitable (depending on the
surfactant). Preferably, for the chosen surfactant, multi-valent
cations or a cationic moiety with additional hydrogen bonding
capabilities sufficiently compete with the dispersant molecule
(e.g., surfactant) for surface adsorption sites--thereby reducing
the lubrication effect between the abrasive slurry particles and
the surface. However, these molecules should not adversely alter
the substrate surface finish through `unwanted` reaction or slurry
destabilization.
[0110] The preferred divalent cation concentration is cation and
system specific. Effective cation concentrations range from about
0.0001M to the saturation level. Preferably, for CMP applications,
the concentration should lie in the range in which both acceptable
removal rates and surface defectivity are found.
[0111] In accordance with the method of the present invention
directed to production of stable compositions, the dispersant (such
as a cationic surfactant) should be added to the slurry
composition, before the addition of the competing agent (such as a
competing salt) for high ionic strength systems. Otherwise, strong
aggregates can form in the composition.
[0112] The materials and methods of the subject invention can be
used to manipulate the adsorption of dispersants to surfaces within
any particulate system. Therefore, the subject invention is of
critical importance to various industrial processes ranging from
ore flotation, lubrication, and paint technology, to enhanced oil
recovery.
[0113] In addition to one or more types of particles, at least one
dispersant (such as a cationic surfactant), and at least one
competing agent (such as a divalent cation), the slurry
compositions of the subject invention can optionally include one or
more acids, one or more oxidizing agents, one or more corrosion
inhibitors, one or more chelating agents, one or more amines, and
one or more salts.
[0114] Examples of suitable acids include, but are not limited to,
formic acid, acetic acid, propanoic acid, butanoic acid, pentanoic
acid, hexanoic acid, heptanoic acid, octanoic acid, nonanoic acid,
lactic acid, hydrochloric acid, nitric acid, phosphoric acid,
sulfuric acid, hydrofluoric acid, malic acid, tartaric acid,
gluconic acid, citric acid, phthalic acid, pyrocatechoic acid,
pyrogallol carboxylic acid, gallic acid, tannic acid, and mixtures
thereof. These acids can be present in the slurry composition of
the subject invention in a concentration of about 0.01% to about
10% of the total weight of the slurry.
[0115] Oxidizing agents are typically small organic molecules that
tend to form a thin oxide film to stop further etching once an
oxide film is formed. Examples of suitable oxidizing agents (also
referred to herein as oxidizers) include, but are not limited to,
hydrogen, peroxide, potassium ferracyanide, potassium dichromate,
potassium iodate, potassium bromate, vanadium trioxide,
hypochlorous acid, sodium hypochlorite, potassium hypochlorite,
calcium hypochlorite, magnesium hypochlorite, ferric nitrate,
ammonium persulfate, potassium permanganate, and mixtures thereof.
These oxidizers can be present in the slurry composition of the
subject invention in a concentration of about 0.01% to about 10% of
the total weight of the slurry.
[0116] Examples of suitable corrosion inhibitors include, but are
not limited to, benzotrizole, 6-tolylytrizole,
1-(2,3-dicarboxypropyl)benzotr- izole, and mixtures thereof. The
corrosion inhibitors may be present in the slurry in a
concentration of about 1 part per million (ppm) to about 300 ppm.
Carboxylic acids, if included, can also impart corrosion inhibition
properties to the slurry composition of the subject invention.
[0117] Examples of suitable chelating agents include, but are not
limited to, ethylenediaminetetracetic acid (EDTA),
N-hydroxyethylethylenediaminet- riacetic acid (NHEDTA),
nitrilotriacetic acid (NTA), diethylklenetriaminepentacetic acid
(DPTA), ethanoldiglycinate, and mixtures thereof. The chelating
agents may be present in the slurry in a concentration of about
0.01% to about 1% of the total weight of the slurry.
[0118] Examples of suitable amines include, but are not limited to,
hydroxylamine, monoethanolamine, diethanolamine, triethanolamine,
diethyleneglycolamine, N-hydroxylethylpiperazine, and mixtures
thereof. The amines can be present in the slurry in a concentration
of about 0.01% to about 1% of the total weight of the slurry.
[0119] Examples of suitable salts include, but are not limited to,
ammonium persulfate, potassium persulfate, potassium sulfite,
potassium carbonate, ammonium nitrate, potassium hydrogen
phthalate, hydroxylamine sulfate, and mixtures thereof. The salts
can be present in the slurry in a concentration of about 0.01% to
about 10% of the total weight of the slurry.
[0120] As used herein, the term "surfactant" refers to an
amphiphilic surface-active agent, the molecules of which have a
hydrophilic portion and a hydrophobic portion. These molecules can
include at least one long-chain hydrocarbon "tail" that is
hydrophobic and a polar (often ionic) hydrophilic "head" group.
Surfactants fall into four broad categories: anionic, cationic,
nonionic, and zwitterionic. In aqueous solutions, surfactant
molecules can arrange themselves into organized molecular
assemblies or clusters, referred to herein as micelles, if the
concentration of the surfactant exceeds a certain value, i.e., the
critical micelle concentration (CMC). On a logarithmic plot of
surface tension versus concentration, the break in the curve occurs
at the critical micellar concentration where there clusters form.
Depending upon the particular molecular architecture of the
surfactant molecule, a variety of microstructures can form in order
to minimize the free energy of the solution. Possible aggregate
structures are spherical micelles, worm-like micelles, spherical
vesicles, lamellar sheets, or a variety of other topologies.
[0121] As used herein, the term "cationic surfactant" refers to a
surfactant that ionizes in solution and has a positively charged
portion that is surface-active. Cationic surfactants are often
nitrogen-based, such as quaternary ammonium compounds, but cationic
surface-active sulphonium compounds are also known. Examples of
suitable cationic surfactants include, but are not limited to, long
chained amines derived from animal and/or vegetable acids, tall
oil, and synthetic amines, diamines and polyamines including ether
amines and imidazolines, quarternized and unquartemized
polyoxyalkylenated long chain amines, and amine oxides derived from
tertiary amines oxidized with hydrogen perioxide.
[0122] As used herein, the term "anionic surfactant" refers to a
surfactant that has a negatively charged portion. Preferably, the
anionic surfactant contributes greatly to the stability of the
system when the length of its tail is equivalent to the tail length
for the cationic surfactant. In accordance with the present
invention, controlling the stability of a slurry solution can be
manipulated by altering the degree of mismatch between the
hydrocarbon tails and the degree of interaction between the head
groups. All soaps, such as sodium oleate, sodium palmitate, sodium
myristate, and sodium stearate are examples of anionic surfactants.
Other anionic surfactants include alkyl sulfates (such as sodium
dodecyl sulfate (SDS)), alkyl sulfonates, alkyl benzenesulfonates,
alkyl phosphates, and sulfosuccinates, for example.
[0123] As used herein, the term "nonionic surfactant" refers to an
uncharged surfactant. The hydrophilic group is composed of some
other water-soluble moiety, such as a short, water-soluble polymer
chain, rather than a charged species. Nonionic surfactants can
include poly(ethylene) oxide chains as the hydrophilic group (the
water-soluble surfactant "head" group), for example. Examples of
nonionic surfactants include alcohol ethoxylate, alkylphenol
ethoxylate, alkyl polyglycosides, and sorbitan esters.
[0124] As used herein, the term "long chain quaternary ammonium
compound" refers to a cationic surfactant having four organic
substituents bonded to a nitrogen atom, wherein at least one of the
four substituents is a hydrocarbon chain (a hydrophobic "tail"
group) within the range of about 8 carbon atoms to about 24 carbon
atoms in length.
[0125] As used herein, the terms "adsorb", "adsorbed", and
"adsorption", and grammatical variations thereof, refer to the
taking up of a gas or liquid, such as a surfactant, at the surface
of another substance, such as a solid.
[0126] As used herein, the terms "stable" and "stabilized", and
grammatical variations thereof, in association with the slurry
compositions and micelles of the subject invention in the specific
embodiment for CMP, refer to slurries with sufficient repulsion
between primary particles such that during the CMP process the
formation of strongly bound larger clusters is prevented. As used
herein, the term "strongly bound larger clusters" and grammatical
variations thereof, refer to aggregates that adhere together with
enough strength such that they increase the extent of surface
defectivity (i.e., the degree of pitting, scratching, roughness,
and other blemishes on the wafer surface). In contrast, an unstable
slurry composition is one that forms strongly bound aggregates of
the primary particles. In the case of unstable slurry compositions
used in CMP, formation of these aggregates can occur before or
during the CMP process.
[0127] As used herein, the terms "polish" and "planarize", and
grammatical variations thereof, are used to refer to intentional
abrasive contact between a slurry composition of the subject
invention and a substrate, such as a wafer, chip, or other
microelectronic component, and the selective removal of topographic
features through material removal, respectively.
[0128] As used herein, the term "defectivity" refers to the overall
surface quality of the substrate (e.g., wafer) at the nanoscale.
Acceptable levels of defectivity include RMS roughness of less than
about 2 nm and maximum defect depths of less than about 25 nm.
[0129] As used herein, the term "material removal", and grammatical
variations thereof, refers to the amount of material abraded from a
wafer surface during polishing. Typically, the material that is
removed is from the chemically modified layer. In general, material
removal rates of 2000 to 6000 Angstroms per second are used in
practice to remove excess material from the wafer surface and
provide a planar surface of acceptable finish.
[0130] The terms "comprising", "consisting of", and "consisting
essentially of" are defined according to their standard meaning and
may be substituted for one another throughout the instant
application in order to attach the specific meaning associated with
each term.
Materials and Methods
[0131] FT-IR/ATR experiments. Fourier Transform Infrared/Attenuated
Total Reflectance (FT-IR/ATR) experiments were carried out on
silicon single-crystal parallelepiped internal reflection elements
(IRE) (55 mm.times.5 mm.times.2 mm, 45.degree. incident angle)
obtained from SPECTRA-TECH Inc. The interaction forces were
measured between a silicon wafer and the silicon nitride tip of an
atomic force microscope cantilever. The root mean square (RMS)
roughness of the silicon substrate, as measured by atomic force
microscopy is less than 0.5 nm. Triangular oxide sharpened contact
mode cantilevers of 0.12 N/m spring stiffness (200 mm long, thick
legged) were obtained from Digital Instruments. Water was produced
by a Millipore filtration system and had an internal specific
resistance of 18.2 M (and less than 7 ppb carbon.
Dodecyltrimethylammonium bromide surfactant (C.sub.12TAB) of 99%
purity was obtained from Aldrich Chemical Co. All other reagents
were at least ACS reagent grade. Surfaces were cleaned by rinsing
with acetone, methanol, and water in sequence. Surfaces were
cleaned immediately prior to experimentation.
[0132] Measurement. FT-IR experiments were conducted with a
nitrogen-purged Nicolet Magna 760 spectrometer equipped with a DTGS
detector. All the spectra were the results of 256 co-added scans at
a resolution of 4 cm.sup.-1. The background spectrum for all the
experiments was the single-beam spectra of the dry silicon crystal.
Spectrum of each sample was measured with 0.degree. and 90.degree.
plane-polarized light, obtained using a wire grid polarizer, in
order to calculate the dichroic ratio (dichroism). All the spectra
were obtained 5 times and the result presented is the average for
the measurements. Spectra of n-propanol was measured as a reference
for randomly oriented molecules, and spectra of C.sub.12TAB were
measured at various concentrations of C.sub.12TAB to investigate
average orientation of hydrocarbon chains in the surfactant films
adsorbed on the crystal. Asymmetrical CH.sub.2 stretching mode
peaks (near 2920 cm.sup.-1) were used for the calculation of
dichroism, although the symmetrical peaks (near 2850 cm.sup.-1),
have also been used in several cases, to verify the results.
Instrumental noise was measured using the same number of scans and
resolution and the same gain as used in the experiments. The
average peak to peak noise was approximately 2% of the peak
intensity, which translates to approximately 2% error in the
calculated values of the dichroism.
[0133] Calculations. The theory of attenuated total internal
reflection spectroscopy (ATR) was proposed by Harrick.sup.1 and
independently by Fahrenfort.sup.2 in the 1960s. This surface
sensitive technique involves the internal reflection of
electromagnetic wave (light) through an internal reflection element
(IRE). As the light is totally reflected at the IRE/sample
interface, an evanescent wave extends from the surface of the IRE
into the sample, with the intensity decaying exponentially. The
evanescent wave can be described by three orthogonal electric field
vectors, each of which interacts with the sample adsorbed at the
surface of the IRE. The electric field amplitudes have been derived
by Harrick,.sup.1 and in the final form by Haller and
Rice..sup.3
[0134] In order to determine molecular orientations within adsorbed
surfactant films, several researchers.sup.4-7 have used polarized
beam infrared spectroscopy. The key parameter in determining
molecular orientations is the dichroic ratio or dichroism, which is
defined as 1 D = A y A x + A z ( 1 )
[0135] where, A.sub.s and A.sub.p are the experimentally measured
absorption with perpendicular and parallel polarized beam
respectively. (Reciprocal value R=(1/D) has also been used by
several researchers for determining molecular orientation).
[0136] Deriving values of A.sub.s and A.sub.p from absorption due
to x, y, and z components of the electric field, dichroism can be
written as, 2 D = A s A p ( 2 )
[0137] where, A.sub.x, A.sub.y, and A.sub.z are the absorption due
to the x, y, and z components of the electric field. Axes x, y, and
z are directed along, across, and normal to the crystal (IRE)
surface. The absorption of electromagnetic beam, A, is proportional
to the square of the scalar product of the electric field vector of
evanescent wave, E, and transition dipole moment of the adsorbed
film, M.
A((E.multidot.M).sup.2 (3)
[0138] The values for A.sub.x, A.sub.y, and A.sub.z will depend on
the model of the adsorbed layer structure used. Haller and
Ulman.sup.8 have used a model with fixed angles of the alkyl chain
from the normal to the surface. The other model proposed by
Zbinden,.sup.9 which considers a uniaxial symmetric distribution of
the transition dipole moments M, about the alkyl chain (c axis)
with fixed angle .theta. between M and c, and a uniaxial symmetric
distribution of the c axis about the z axis (surface normal), with
fixed angle .gamma. between the c and z axes, appears to represent
the adsorbed film more realistically. Formulae for dichroism for
this model was first developed by Frey and Tamm.sup.4 and
Rabinovich et al. ,.sup.5 and used later in several papers..sup.6-7
Averaging A.sub.x, A.sub.y, and A.sub.z through rotation about the
c and z axes, the expression for dichroism was obtained.sup.4-5
from equations (2) and (3) as 3 = arcsin 2 1 + 2 E z 2 * D / ( E y
2 - E x 2 * D ) ( 4 )
[0139] Experimentally, the absorption of the symmetric and
asymmetric stretching modes of the methylene groups in the
surfactant was measured. The transition dipole moments of these
modes are perpendicular to the C-C axis, i.e., the angle .theta.
between M and the C-C axis is 90 degrees.
[0140] Solving equation 4 for .gamma. with .theta.=90.degree. gives
4 D = E y 2 E x 2 + 2 E z 2 .times. [ 2 2 sin 2 + 2 cos 2 * sin 2 -
sin 2 * sin 2 - 1 ] ( 5 )
[0141] Equations (4) and (5) were obtained by neglecting reflection
at the entrance and exit surface of the crystal..sup.8 The
absorption of the evanescent wave in the bulk solution of the
surfactant is negligible, due to the lower concentration of the
surfactants in the bulk, as compared to their concentration in the
adsorbed layer at the solid-liquid interface. This assumption was
confirmed by measuring absorption of internally reflected beam with
diluted solutions of hexane in water.
[0142] FIGS. 27A and 27B show the spectra obtained using parallel
and perpendicular plane polarized infrared beam, at two different
bulk concentrations of the surfactant. The value of D was
calculated from the experimental data using equation (1), and the
different components of the electric field E.sub.x, E.sub.y, and
E.sub.z were calculated from the formulae given by Haller and
Rice..sup.3 Refractive indices (n) used for these calculations were
n.sub.silicon=3.42, n.sub.water=1.42, and n.sub.surfactant
film=1.46. Index of refraction for water at wavenumber=2900
cm.sup.-1 was experimentally measured by Pinkley et al..sup.10 The
refractive index for the surfactant film, at the appropriate wave
numbers, was calculated, assuming that the film is composed of 50%
water (n=1.42) and 50% pure hydrocarbon (n=1.50). Using the
experimental values for D, and theoretical values of E.sub.x,
E.sub.y, and E.sub.z, the values of the angle .gamma. were
calculated from equation (5) for increasing concentrations of the
surfactant.
[0143] Following standard definition used for liquid crystals, an
order parameter S .sup.11 can be defined as
S=1.5 cos.sup.2.gamma.-0.5 (6)
[0144] FIG. 28 shows the dependence of the order parameter S on
dichroism D. The graph was plotted using equations (5) and (6) and
using the equations given by Haller and Rice.sup.3 to calculate the
values of E.sub.x, E.sub.y, and E.sub.z. (refractive index values
used are mentioned above). Knowing the experimental values of D,
the order parameter, S, can be estimated from FIG. 28. FIG. 29
schematically shows the surfactant structures and molecular
orientations at the interface for different values of the order
parameter S. For the alkyl chain axis normal to the surface
(homeotropic orientation), .gamma.=0.degree. and S=1, indicating
perfect order. For full disorder, i.e; random distributed
orientation of molecules, .gamma.=54.7.degree. and S=0. For perfect
planar orientation, i.e., the alkyl chain axis parallel to the
surface, .gamma.=90.degree. and S=-0.5.
[0145] Surface Force Measurement. Surface force was measured on a
Digital Instruments Nanoscope III in a fused silica liquid cell. In
a typical experiment, solutions of increasing surfactant
concentration were injected sequentially into the cell. It was
determined that the order of addition of surfactant does not affect
the measured interaction force and that rinsing with water quickly
produced a force profile equivalent to curves measured in absence
of surfactant. These observations indicate that equilibrium is
established quickly in this system. A new cantilever was used for
each experiment. Because, small variations in the spring constant
(stiffness), and the tip radius may occur between cantilevers, even
from the same batch, the interaction force between each tip and
silica surface in the presence of a standard solution of 32 mM
dodecyltrimethylammonium bromide at pH 6 and 0.0001 M NaCl was
measured. The characteristic maximum repulsive force (magnitude of
repulsion where the tip jumps into contact with the surface) under
these conditions was used to normalize the interactions between
different tip/surface combinations. The absolute values of the
maximum repulsion varied by as much as 20%. Because the radius of
the tip is not precisely known and the application of Derjaguin's
approximation with small radius probes is questionable, interaction
forces are presented without normalizing by the radius of the
probe.
[0146] Contact Angle. The angle formed between the solid-liquid
interface and the vapor-liquid interface is referred to as the
contact angle. A contact angle is a measure of wettability, which
is the ability of the liquid to wet the solid. A low angle
indicates high wettability, while a high angle indicates a low
wettability. Three basic causes for change in surface wettability
include surface roughness, physical interaction (e.g.,
electrostatic attraction between the liquid and the surface), and
chemical interaction of solid and water (e.g., reaction of the
surface with the liquid). Contact angle was measured between an air
bubble and a CVD silica substrate using a Rame-Hart goniometer
model A100. The angle of the substrate was adjusted to within
5.degree. of the angle at which an approximately 10 .mu.L air
bubble placed on the underside of the substrate would not leave the
surface. Six measurements of the advancing contact angle were
measured on each of at least three bubbles placed on the surface.
The results presented are the averages of these measurements.
[0147] Following examples illustrate procedures for practicing the
invention. These examples should not be construed as limiting. All
percentages are by weight and all solvent mixture proportions are
by volume unless otherwise noted.
Example 1
Adsorption of Alkyl Trimethylammonium Bromides
[0148] In these experiments, the adsorption of alkyl
trimethylammonium bromides (CNTAB for n=10, 12, 14 and 16) on
mica-was investigated by means of atomic force microscopy (AFM).
Images as well as force-distance curves have been obtained. The
results can be compared with dispersion stability studies of silica
particles in presence NaCl, SDS and C.sub.12TAB to delineate the
molecular mechanism by which surfactants influence dispersion
stability. The stability of the adsorbed micellar layer can then be
compared to the stability of micelles in bulk by means of the
pressure-jump technique with electrical conductively detection. The
stability of the adsorbed aggregates can then be compared with the
stability of silica dispersions.
[0149] An example of a force-distance curve for a 2 CMC C.sub.12TAB
solution on mica is given in FIG. 2. The mechanisms responsible for
initial repulsion at distances larger than about 8 nm is not yet
entirely clear. Electrical forces as predicted by the DLVO theory
(van der Waals and double layer forces) are approximately 10-100
times smaller than the forces measured. A possible explanation for
the forces experienced beyond 8 nm could be that of micelles
adsorbed onto the AFM tip. As the tip continues to approach the
surface, it is believed the micellar layer deforms due to
compression. The micellar compression results in a steep increase
in the interaction force up to approximately 4 nm where the micelle
breaks and the tip jumps into contact with the mica surface.
[0150] Force-distance curves for the entire series of C.sub.nTAB
(for n=10, 12, 14 and 16) are shown in FIG. 3 where curves a, b, c,
and d correspond to n=10, 12, 14, and 16, respectively. The maximum
compressive force as function of alkyl chain length is plotted in
FIG. 4. It appears that the maximum compressive force (F.sub.max in
FIG. 2) increases linearly as function of alkyl chain length.
Initially, octyltrimethylammonium bromide (C.sub.8TAB) was also
investigated in this study. However, images as well as
force-distance curves did not show adsorption of C.sub.8TAB on
mica, which is more or less confirmed by extrapolating the maximum
compressive force in FIG. 4 to shorter alkyl chain lengths. The
maximum compressive force approaches zero, indicating that
C.sub.8TAB does not adsorb in the same manner as the higher chain
length surfactants (n>8).
Example 2
Effect of Chain Length on the Stability of Adsorbed Micelles
[0151] The effect of chain length on the stability of adsorbed
micelles was compared with the stability of micelles in the bulk
solution. The stability of micelles in bulk solution is directly
related to the slow micellar relaxation time (.tau..sub.2), which
can be determined by the pressure-jump technique with electrical
conductivity detection. The change in electrical conductivity as
function of pressure was measured instead of .tau..sub.2. FIG. 5
shows the effect of pressure on the electrical conductivity of 2
CMC solutions of C.sub.nTAB (for n=10 and 16). Since the electrical
conductivity of surfactant solutions is directly related to the
surfactant monomer concentration (CMC) and since increasing
pressure increases the CMC, the slower increase in conductivity
with increasing pressure for C.sub.16TAB indicates that C.sub.16TAB
micelles are more stable than C.sub.10TAB micelles. For C.sub.16TAB
higher pressures are required to obtain the same change in
conductivity as compared to C.sub.10TAB, meaning that the micelles
are more stable. Others have measured the stability of a series of
sodium alkylsulfates and found an increase in micellar stability as
the chain length was increased from C.sub.10 to C.sub.16, which is
qualitatively consistent with the force-distance curves in FIG. 3.
The longer the hydrocarbon chain length, the stronger the
hydrophobic forces between the molecules, resulting in more stable
micelles. Accordingly, for applications with high pressures,
selection of a surfactant with an optimal chain length can allow
control of the energy barrier to enhance the processing of the
particles.
[0152] The stability of sodium dodecyl sulfate (SDS) micelles in
bulk solution (also measured by the pressure jump technique with
electrical conductivity detection) can be greatly enhanced by the
addition of C.sub.12TAB or long chain alcohols, due to the
introduction of intramicellar ion-ion or ion-dipole interactions.
In a specific embodiment of the subject invention, micellar
stability for adsorbed micelles can be tailored by, for example,
adding SDS to the C.sub.12TAB solutions. For example, the addition
of SDS can dramatically increase the magnitude of the repulsive
barrier.
[0153] In a specific example, an increasing amount of SDS was added
to the 2 CMC C.sub.12TAB (32 mM) solutions. FIG. 6 shows the
maximum compressive force for C.sub.12TAB cylindrical micelles as a
function of added SDS concentration. The C.sub.12TAB concentration
was kept constant at twice the CMC (32 mM). The chain lengths
between the two oppositely charged surfactants (C.sub.12TAB and
SDS) should preferably be kept the same in order to maximize the
lateral interactions, attributed to the chain length compatibility
effect. The maximum compressive force increases with SDS
concentration and levels off beyond approximately 20 mol % of SDS.
The addition of more than 20 mol % SDS to C.sub.12TAB solutions
results in precipitation and hence the maximum compressive force
was not measured in this range.
[0154] One might expect that the electrical force should decrease
as the net charge of the micelle decreases due to the addition of
SDS. Experimental results show an opposite effect on the maximum
compressive force, suggesting the steric origin of the repulsive
force. The maximum compressive force measured in these experiments
represents the stability of the micellar layer. Thus, micellar
stability for micelles adsorbed onto solid/liquid interfaces can be
tailored. In fact, the results of these experiments appear to
indicate that the stability of micelles adsorbed on a solid/liquid
interface is consistent with the stability of micelles in bulk.
Hence, the stability of the adsorbed layer, for micelles adsorbed
onto solid/liquid interfaces, can be controlled by, for example,
adjusting the chain length of the surfactant or by the addition of
additives, such as oppositely charged surfactants, alcohols or
electrolyte.
[0155] The strength of the adsorbed surfactant film on particles
(i.e., the stability of the adsorbed micellar layer) is an
important parameter in the stability of dispersions. At high ionic
strength, ion-electrostatic forces do not appear to play a
significant role, while steric repulsion of the adsorbed micelles
appears to be a predominate mechanism for the stability of the
dispersion. Therefore, dispersion stability experiments were
performed with 0.05 wt %, 200 nm, silica particles in solutions
containing C.sub.12TAB in presence and absence of (100 mM) NaCl or
a varying amount of SDS at pH 4. A pH of 4 (close to the IEP of
silica between 2 and 3) was chosen to assure an unstable dispersion
in the presence of NaCl. The pH was adjusted with HNO.sub.3.
[0156] FIG. 7 shows the turbidity of silica dispersions after 60
minutes as well as the maximum compressive force as a function of
C.sub.12TAB concentration on mica. A high turbidity indicates very
stable dispersions, whereas very low values indicate unstable
dispersions. Initially, the addition of C.sub.12TAB destabilizes
the dispersion due to the adsorption of C.sub.12TAB onto the silica
particles. At low concentrations of C.sub.12TAB (<2 mM), the
adsorption is driven by electrostatic, attractive forces between
the cationic surfactant and the oppositely charged silica surface.
This appears to result in monolayer coverage where the hydrophobic
groups point towards the water. The attractive forces between the
now hydrophobic silica surface results in a non-stable silica
dispersion. With an increase in surfactant concentration, surface
aggregation around the initial adsorption site occurs, which
results in bilayer formation. Further increase in surfactant
concentration results in the formation of micelles. Above 5 mM
concentration of C.sub.12TAB, the dispersion becomes stable. The
maximum compressive force (measured on mica) follows the same trend
as the stability of the dispersion and thus the strength of the
adsorbed micellar layers seems to be directly related to the
stability of the silica dispersion.
[0157] FIG. 8 shows the stability of the same systems as in FIG. 7,
in the presence of 100 mM NaCl. Adding 100 mM of electrolyte
compresses the electrical double layer and therefore the dispersion
becomes unstable. Interestingly, the presence of electrolyte
appears to enhance the formation of bilayers or cylinders, causing
the plateau to shift to lower C.sub.12TAB concentrations (from
approximately 20 to 10 mM C.sub.12TAB).
[0158] The influence of SDS on the stability of the dispersion in
the presence of 5 mM C.sub.12TAB and 100 mM NaCl (pH 4) is
presented in FIG. 9. The addition of SDS stabilizes the mixed
surfactant layer as shown in FIG. 6. It appears that even a very
small amount of SDS (0.007 mM) is enough to stabilize the silica
dispersion again. The ratio of C.sub.12TAB to SDS is approximately
4500:1 for achieving maximum stability. However, based upon these
adsorption studies, it is likely that SDS selectively partitions to
the solid/liquid interface making the actual ratio more like 1:10
to 1:2. It is remarkable that a very low concentration
(.lambda.0.001%) can strikingly enhance the stability of
dispersions. The stability the silica dispersions appears to be
directly related to the compressive strength of the adsorbed
micellar layer (measured against mica), which can be tuned the same
way as micellar stability in bulk solution, i.e. adjusting chain
length or by additives, such as oppositely charged surfactants,
electrolyte or long chain alcohols.
[0159] Referring to FIG. 10, the relative turbidity versus
electrolyte concentration is shown for 200 nm silica, at pH 4, and
32 mM C.sub.12TAB. Even at 5M NaCl, where electrostatic forces
between the particles are no longer responsible for the
particle-particle repulsion, the suspension is stable in the
presence of surfactant. It appears that steric forces are
responsible for the particle-particle repulsion.
[0160] Referring to FIG. 11, a plot of maximum repulsive force
between an AFM tip and a mica surface in the presence of 32 mM
C.sub.12TAB, at pH 6, versus electrolyte concentration (NaCl) is
shown. The plot shows that as electrolyte concentration increases,
micelle stability and barrier magnitude also increase. It is
believed that the addition of the electrolyte promotes screening of
the C.sub.12TAB head groups such that the individual surfactant
molecules can get closer together. This leads to more densely
packed micelles which provide higher steric forces. This effect is
contradictory to the general effect of electrolytes decreasing
forces in the electrostatic range. Accordingly, the addition of
electrolytes can be used to control the magnitude of the energy
barrier. As an example, electrolytes can be added when the
suspension undergoes other conditions which would tend to decrease
the energy barrier, in order to maintain the particles in
suspension.
[0161] Referring to FIG. 12, the maximum repulsive force between an
AFM tip and a mica surface in the presence of 32 mM C.sub.12TAB, at
pH 6, versus alcohol (ethanol) concentration is shown. The plot
shows that as alcohol concentration is increased the energy barrier
increases. Accordingly, alcohol can be utilized in accordance with
the subject invention to control the energy barrier between
particles and between particles and a surface.
[0162] Referring to FIGS. 13 and 14, the maximum repulsive force
between an AFM tip and a mica surface is the presence of 32 mM
C.sub.12TAB, at pH 6, versus Urea concentration is shown. As can be
seen, the energy barrier initially increases with increasing Urea
concentration and then decreases with further Urea concentration
increases. Accordingly, Urea can be utilized in the control of the
energy barrier in accordance with the subject invention. In a
particular embodiment, two different particle types can be in a
liquid medium and the concentration of Urea in the liquid can be
used to control the energy barriers between the two sets of like
particles. The two types of particles can have different energy
barriers. In this way, both particles can be in suspension for
transportation of the two types of particles and the concentration
of Urea can be increased until the particles of a first type having
the lowest energy barrier agglomerates. The particles of a second
type having the higher energy barrier can remain in suspension such
that the two types of particles can be separated.
[0163] FIG. 15 shows the interaction force versus separation
distance for an AFM tip and a mica surface in the presence of 100
mg/L Darvan C, pH 6, for no NaCl or 0.1 N NaCl. From the figure,
the addition of NaCl lowers the interaction force between the
particles thereby encouraging agglomeration. Indicating that common
polymeric dispersants such as Darvan C may not be suitable for use
in extreme conditions.
[0164] Referring to FIG. 16, the correlation between the onset
suspension stability and the force barrier is shown. The force
between a AFM tip/silica surface is shown as a function of
C.sub.12TAB concentration. Also shown is the suspension turbidity
for 200 nanometer silica particles as the concentration of
C.sub.12TAB is increased. This figure shows that having micells on
the surface of the silica appears to be critical to the increase of
the energy barrier.
[0165] Referring to FIG. 17, it is shown that a very low
concentration of cosurfactant can induce surface aggregation. The
maximum repulsive force between a AFM tip and a silica surface is
shown as a function of cosurfactant, SDS, concentration. It is
shown in the graph that very low concentrations of a cosurfactant,
SDS, added to a 3 mM C.sub.12TAB environment can induce surface
agglomeration. It appears that a substantial portion, if not
essentially all of the added cosurfactant goes to the surfaces.
Accordingly, the use of a cosurfactant can lower the amount of
surfactant needed to achieve the same result. In this case, the
addition of very low concentration SDS lowered the amount of
C.sub.12TAB from 10 mM to 3 mM. This can be a way to allow the
addition of a small volume of materials to create a large effect,
as well as to save costs on materials.
[0166] Referring to FIG. 18, the maximum repulsive force versus
C.sub.12TAB concentration with no NaCl present, or with 0.1 M NaCl
present, is shown. The presence of the electrolyte increases the
barrier magnitude and reduces the surfactant concentration needed
to produce the barrier. Stated differently, the electrolyte reduces
the onset of the barrier. Accordingly, electrolytes can be utilized
in accordance with the subject invention to control the magnitude
and onset of the barrier.
[0167] FIG. 19 illustrates the difference in barrier onset and
magnitude which can result from different surface, or particle,
material. These differences may be attributable to the impact the
surface microstructure has on self assembly of the surfactant and
can be important in the separation of dissimilar particles. For
example, the concentration of surfactant can be increased until the
particles of a first material agglomerate, with the particles of a
second type staying in suspension. In a specific embodiment, during
a mining operation two different particles can enter into
suspension for transportation, for example, through a pipeline or
in holding tanks. Upon arrival at their destination, the particles
surfactant can be added until the particles of a first material
agglomerate, leaving the particles of the second material in
suspension.
[0168] FIGS. 20 and 21 illustrate the effect of temperature on the
barrier, based on the interaction of an AFM tip and a silica
surface, with 11 mM C.sub.12TAB at pH 4 and 0.1 M NaCl. As shown,
as the temperature increases the barrier to agglomeration
decreases. Accordingly controlling the temperature can control the
barrier. In a specific example, particles such as alumina particles
can be maintained in a concentrated suspension and can have a
material such as SDS or C.sub.12TAB added to reduce viscosity while
maintaining high solids loading. This suspension can then be poured
into a mold. After the suspension has an opportunity to fill the
voids of the mold while a viscous form, the mold can be heated,
thereby lowering the barriers to agglomeration and causing the
particles to form a molded object. When the mold is removed and the
temperature lowered, the particles can form a solid object.
Accordingly, temperature can be utilized alone, or in conjunction
with one or more techniques disclosed herein, to control the
barrier and therefore affect the behavior of particle systems. By
controlling the barrier by selection of type and amount of the
material added, the amount of temperature increased can be kept low
to save costs.
Example 3
Control of Rheological Behavior
[0169] The methods and materials of the subject invention can also
be utilized to control the Theological behavior of particles. FIG.
22 illustrates the viscosity versus temperature of 200 nm silica
particles in 39 mg/g C.sub.12TAB. As shown, agglomeration above
55.degree. C. results in a sharp increase in viscosity. It appears
this increase in viscosity may be due to changes in the structure
of the surface layer.
[0170] FIG. 23 shows a plot of viscosity versus shear rate for 200
nm silicon particles in either 44 mg/g C.sub.12TAB or 5 mg/g Darvn
C, and a high salt content without the addition of C.sub.12TAB or
Darvan C. The 50 vol % 200 nm silica particles form a crumbly
material, wherein the addition of C.sub.12TAB or Darvan C produces
a honey-like consistency. It should be noted that 50 vol %
suspensions of 200 nm silica can not be made with electrostatic
stabilization or the non-ionic polymer polyethyleneoxid (PEO), even
under optimal conditions, e.g., without salt. Accordingly, the
surfactant appears to be a good dispersant even at this high salt
concentration. Referring to FIG. 23, it appears that the surface
layer formed on the particles by C.sub.12TAB is harder than the
surface layer formed by Darvan C. Accordingly, even at low shear
rates, C.sub.12TAB is effective in controlling viscosity.
[0171] FIG. 24 shows a plot of viscosity versus shear rate for 45
vol % 200 nm silica with 44 mg/g C.sub.12TAB or 3 mg/g PEO 7500 MW.
Over a wide range of shear rates, dispersion with the surfactant
yields a more Newtonian suspension. More Newtonian suspensions have
also been accomplished with, for example, kaolin and alumina
particles.
[0172] FIG. 25 illustrates the effect of surface potential on the
barrier. The barrier magnitude versus pH is shown for an AFM tip
and Alumina surface with 0.01 M NaCl and 20 mM SDS. FIG. 25 appears
to indicate that the barrier and corresponding particle stability
is independent of surface charge under these conditions. FIG. 26
shows the barrier versus separation distance for an AFM tip and
alumina surface with 16 mM SDS and no salt.
[0173] FIG. 33 illustrates the effect of surface change on the
barrier. The barrier magnitude versus C.sub.12TAB concentration is
shown for an AFM tip and silica surface in the presence of 0.1 M
NaCl at a pH of 1, 2, and 4. These results indicate a
non-electrostatic adsorption mechanism, as the barrier exists even
when the surface charge is nearly zero. Accordingly, the barrier
appears to be independent of surface charge.
[0174] FIG. 34 also illustrates the effect of surface charge on the
barrier. The barrier magnitude versus C.sub.12TAB concentration is
shown for an AFM tip and mica surface in the presence of 0.1 M NaCl
at a pH of 4 and 9. These results also indicate the barrier onset
is not strongly controlled by increasing surface charge.
[0175] FIG. 35 illustrates the effect of surface microstructure on
the barrier. The suspension turbidity versus SDS concentration is
shown for 300 nm alumnia particles and for an AFM tip and saphire
surface in the presence of 0.1 M NaCl at a pH of 4. These results
indicate the onset of the barrier occurs at a lower concentration
than the suspension stability. This indicates that the substrate
can have an effect and, in particular, the crystalline direction of
the substrate can have an effect on the barrier onset concentration
and the suspension stability concentration.
[0176] Accordingly, self-assembling surfactant aggregates can be
utilized to stabilize nanoparticulate suspensions. In particular,
the subject methods are useful in several environments. The
magnitude of the agglomeration barrier can be tailored to specific
application using one or more of the techniques described in the
subject disclosure.
Example 4
Effect of C.sub.12TAB Concentration on Adsorption, Zeta Potential,
and Contact Angle
[0177] Adsorption, zeta potential, and contact angle measurements
of silica surfaces, in 0.1 M NaCl at pH 4.0 as a function of
solution C.sub.12TAB concentration are shown in FIG. 30. Based on
the observations, possible self-assembled surfactant structures as
a function of the surfactant concentration are suggested. It
appears that initially (concentrations below 0.007 mM, Region A on
FIG. 30), individual surfactant adsorption takes place. In this
system, hemi-micelles seem to form at very low concentrations, as
evidenced by a linear (on the log-log plot) adsorption isotherm.
These hemi-micelles have a significant effect on both the zeta
potential and hydrophobicity of the surfaces. As the surfactant
concentration increases, at approximately 0.1 mM in region B, the
sign of the zeta potential reverses, but the contact angle
continues to increase, indicating that the reversal in zeta
potential is, either due to increased adsorption due to hydrophobic
association between the surfactant tails, or due to some kind of
specific adsorption, and more importantly that bi-layers are not
needed to reverse the sign of the zeta potential. In region B and
C, the concentration of hemi-micelles increases, but beyond a
certain concentration (approximately 2.3 mM), in region D the
hydrophobicity decreases. In concentration range D, the rate of
increase of the zeta potential also increases, indicating that an
increasing number of the polar head of the surfactant molecules are
oriented towards the solution. The sharp increase in zeta
potential, and a corresponding decrease in contact angle was
attributed either to the presence of mixtures of hemi-micelles and
bi-layers; or hemi-micelles and spherical aggregates; or
hemi-micelles and structures having semi-spheres on top of perfect
monolayers. At higher surfactant concentrations beyond the bulk CMC
(Region E and F), based on AFM imaging, either spherical
aggregates, or composite semi-spheres on top of perfect monolayers
are possible. Thus, based on the adsorption, zeta potential, and
contact angle, several plausible surfactant structures are proposed
at the interface at different concentrations of the surfactant. In
order to further investigate the structural transitions taking
place at the interface, FTIR/ATR technique, which can probe the
adsorbed structures directly, was employed.
[0178] Individual surfactant molecules, hemi-micelles, mono-layers,
bi-layers, and spherical/cylindrical aggregate at the interface
will have different average orientation of the alkyl chain with
respect to the surface normal. Different average orientation would
result in different dichroism of the IR beam, and can thus be used
to identify the surfactant structures at the interface.
[0179] FIG. 31 shows the variation of the contact angle
(triangles), repulsive steric forces (squares), and order parameter
(circle) as determined from FTIR/ATR, for a silicon substrate with
increasing bulk concentration of dodecyltrimethylammonium bromide
at pH 4.0 and 0.1 M NaCl concentration. As with the silica system,
initially in region A the contact angle remains similar to the bare
substrate (silicon contact angle=25.degree.), and then increases
beyond approximately 0.1 mM surfactant concentration and reaches a
maximum value between 0.3 mM and 4 mM in regions B and C,
indicating the presence of hemi-micelles at these concentrations.
As the surfactant amount is further increased (Region D), the
contact angle decreases and a sharp decrease in the contact angle
is observed, reaching a minimum value at 7 mM and higher surfactant
concentrations (Regions E and F).
[0180] In the case of interaction forces measured between silicon
substrate and the AFM tip, no repulsive forces are observed up to a
surfactant concentration of 6 mM, and a transition from no
repulsive forces to steric repulsive forces occurs between 6 and 8
mM. It is important to note that this is similar to the transition
observed on silica substrate, where the transition takes place
between 8 and 10 mM. Overall, changes in the contact angle and
forces on the silicon substrate follow the same trend as in the
case of silica. Hence, it is indicated that transitions in
surfactant structures, similar to those observed in case of silica,
are also responsible for observed changes in contact angle and
forces with silicon substrate. The sharp decrease in the contact
angle between 4 and 7 mM can thus be attributed to the transition
of hemi-micelles to bi-layers, spherical/cylindrical aggregates, or
semi-spherical/semi-cylindrical aggregates on top of perfect
mono-layers.
[0181] Correct choice of one of these structures can be done, by
calculating the order parameter of the adsorbed surfactant films
(procedure detailed earlier), based on FT-IR/ATR results (FIGS. 27A
and 27B). FIG. 31 also shows the variation in the order parameter
at the silicon surface as the concentration of
dodecyltrimethylammonium bromide is increased. The reproducibility
of the results for dichroism (D), obtained in different experiments
(under the same experimental conditions), as well as coinciding
values of dichroism for symmetrical and asymmetrical stretching
modes (with mutually orthogonal transitional dipole moments),
proves the validity of the adsorbed surfactant model suggested by
Zbinden,.sup.9 which was used in the present study to calculate the
tilt angle .gamma. and the order parameter S. At very low
surfactant concentrations (Region A), the value of the order
parameter is close to zero, indicating the presence of random
structures at the surface due to surfactant molecules adsorbing
individually. This observation is supported by the contact angle
data where no appreciable change in the contact angle is
determined, indicating that hemi-micellization has not yet started.
As the surfactant concentration increases beyond approximately 0.1
mM, the order parameter increases and becomes positive, indicating
the formation of hemi-micelles at the interface in regions B and C.
The sharp decrease in the order parameter in region D, corresponds
to the fall in the contact angle values between 4 and 7 mM of the
surfactant concentration. Increasing value of the order parameter
up to approximately 4 mM, indicates that increasing number of
hemi-micelles are being formed at the interface in regions B and
C.
[0182] Considering that a decrease in the order parameter is
observed beyond C, formation of bi-layers as the transition
structure may be ruled out, since the order parameter should have
increased or remained constant if hemi-micelles resulted in the
formation of bi-layers. The decrease in the order parameter could
also be explained if the interface structure consisted of a mixture
of decreasing amount of hemi-micelles and an increasing amount of
randomly oriented spherical or cylindrical aggregates. AFM imaging
has demonstrated the presence of spherical aggregates of
C.sub.12TAB on the surface of silica, which is similar in nature to
silicon. Hence, it was assumed that the randomly oriented
aggregates observed on silicon are spherical in shape. Beyond 7 mM
(Regions E and F), the observed values of the order parameter is
close to zero, indicating the presence of randomly oriented
spherical aggregates at the interface.
[0183] FIG. 31 also correlates the onset of repulsive force barrier
in the presence of surfactants to the structural transitions taking
place at the interface, as indicated by the order parameter
obtained from FTIR/ATR experiments. Spherical aggregates are
present at the interface at concentrations above 4 mM, along with
hemi-micelles, and complete transition to randomly oriented
spherical structures takes place between 6 mM and 8 mM. This is the
same transition region where steric repulsive forces become
dominant, indicating that the spherical aggregates provide the
steric barrier to agglomeration.
[0184] Based on the contact angle, order parameter, zeta potential,
adsorption, and the repulsive forces measured using AFM, the
structural transitions are summarized by the schematic shown in
FIG. 32, which indicates the structures present at the interface at
the concentration regions A-F in FIGS. 30 and 31.
Example 5
Stabilization of Slurries at High Ionic Strength
[0185] FIG. 36 illustrates the interparticle and particle-substrate
interactions for the slurries mediated with cationic surfactants at
1 to 2 CMC concentrations. The particle-particle repulsion
controlling the slurry stability is provided by the repulsive force
barrier created due to the cohesiveness of the self assembled
surfactant aggregates. FIG. 37 illustrates the repulsive force
barriers obtained with baseline (pH=10.5 with and without 0.6 M
NaCl) and C.sub.12TAB, C.sub.10TAB and C.sub.8TAB surfactants at
32, 68 and 140 mM concentrations (above CMC concentrations, at
pH=10.5 with 0.6 M NaCl), which is a direct measurement of the
cohesiveness of the self-assembled surfactant structures.
Initially, the baseline polishing slurry was stable due to the
presence of high negative charges around the silica particles at pH
10.5 creating electrostatic repulsion. However, CMP operations are
conducted in the presence of reactive additives such as oxidizers
and complexing agents, which results in slurry destabilization.
Therefore, to simulate the severe environments of the CMP slurries,
0.6 M NaCl was added to the silica slurry resulting in coagulation
of the slurry particle by screening the surface charge and
disappearance of the repulsive barrier. As a result, the mean
particle size of the slurry increased from 0.2 .mu.m to 4.3 .mu.m.
At 32 mM C.sub.12TAB and 68 mM C.sub.12TAB additions the polishing
slurry was completely stable (mean particles size 0.2 .mu.m) since
an adequate repulsive force barrier was reached under these
conditions. However, since the C.sub.8TAB surfactant did not
exhibit a repulsive force barrier, slurries mediated with the right
carbon chain surfactant were unstable.
Example 6
Polishing Performance with Surfactant Mediated Slurries
[0186] Polishing experiments were conducted to evaluate the
performance of the slurries mediated with C-TAB surfactants in
terms of the surface quality and the material removal rate
responses. The surface quality was quantified by measuring the
surface roughness and maximum depth of the surface defects (pits
and scratches) by imaging the polished wafer surfaces using AFM.
Slurries containing 0.6 M 35 nm obtained with the baseline slurry.
The poor surface quality in the presence of salt was due to the
coagulation of the baseline slurry with the salt addition. When
complete stabilization was reached with the addition of C.sub.10TAB
and C.sub.12TAB, surface roughness reduced to 0.5 nm and the
maximum surface depth of the pits was recorded to be only 20 nm.
Due to the lack of interparticle repulsion, slurry prepared using
C.sub.8TAB surfactant was unstable. Therefore, maximum surface
deformation values were higher than desired due to presence of
coagulates (50-60 nm). Material removal rate responses of the
slurries were also measured simultaneously with the surface quality
response. Addition of 0.6 M NaCl resulted in 7058 .ANG./min
material removal rate as compared to the 4300 .ANG./min with the
baseline slurry. Addition of 140 mM C.sub.2TAB resulted in 5170
.ANG./min material removal rate while this value decreased to 650
and 66 .ANG./min for the C.sub.10TAB and C.sub.12TAB mediated
slurries respectively. FIG. 38 summarizes the stability and
material removal rate responses of the surfactant mediated
slurries. The decrease in the material removal was attributed to
the lubrication of the surfaces with the surfactants resulting in
decreased frictional interactions except for the C.sub.8TAB
mediated slurries which were unstable and the mechanical material
removal created by the coagulates still gave high material removal
response.
Example 7
Surface Lubrication Effect on Particle-Substrate Interactions
[0187] To investigate the impact of surface lubrication on material
removal rate, frictional forces were measured at system level by
utilizing in-situ frictional force measurement and by AFM at the
single particle-wafer interaction level.
[0188] FIG. 39 compares the material rates obtained with the
baseline (with and without NaCl) and surfactant mediated slurries
(with 0.6 M NaCl) to the in-situ frictional force values. A direct
correlation was observed between the material removal rates and the
measured frictional forces. The baseline slurry without NaCl
addition resulted in 3.4 friction force, which increased to 7.2 N
in the presence of 0.6 M NaCl. The highly coagulated C.sub.8TAB
modified slurry (at 140 mM) also resulted in high frictional forces
(4.5 N), which polished at 5167 .ANG./min rate. For the C.sub.10TAB
(68 mM) and C.sub.12Tab (32 mM) mediated slurries, on the other
hand, friction force decreased to 2 N, which was -4 times lower
compared to the baseline value in the presence of 0.6 M NaCl.
Although this decrease agreed with the reduction in the material
removal rates, it was not as significant as the -10 times reduction
for the C.sub.10TAB, and 100 times reduction for the C.sub.12TAB
mediated slurries as compared to the polishing with 0.6 M NaCl
containing slurry. This inconsistency can be explained based on the
fact that this technique does not only represent the particle
substrate interactions, but the total frictional force in the
polishing system, which is composed of the combined
particle-particle, pad-particle, particle-substrate and
pad-substrate frictional interactions.
[0189] In order to understand the surfactant lubrication effect on
material removal rate at particle-wafer interaction level, AFM
friction force measurements were conducted. A silica particle
attached to the cantilever tip was made to raster on the silica
wafer surface in the presence of surfactants to simulate the single
particle-wafer interaction. FIG. 40A shows the lateral versus
normal force response of the baseline (pH-10.5) and surfactant
mediated solutions without salt addition (140, 68 and 32 mM,
C.sub.8TAB, C.sub.10TAB and C.sub.12TAB). It can be seen that for
baseline solution, frictional forces increased with the increasing
normal forces. On the other hand, in the absence of salt, all the
surfactant mediated slurries resulted in minimal friction. Indeed,
when polishing experiments were conducted at the given
concentrations of these surfactants in the absence of NaCl, the
material removal rate values were observed to be negligible for all
the chain lengths (61, 53 and 56 .ANG./min for C.sub.8TAB,
C.sub.10TAB, and C.sub.12TAB, respectively). This result indicated
that at any chain length, CTAB surfactant was able to form a
lubrication layer in the absence of salt. Consequently, minimal
particle-surface interactions occurred for-these systems resulting
in negligible material removal. It is also important to note that
all the slurries were stable without salt addition.
[0190] The polishing results in the presence of 0.6 M NaCl,
however, showed an increase in the material removal with decreasing
chain length of the surfactant (66,650 and 6167 .ANG./min for
C.sub.12TAB, C.sub.10TAB and C8TAB, respectively). When AFM
friction force measurements were conducted in the presence of salt,
an interesting behavior was observed. It can be seen in FIG. 5b
that, C.sub.8TAB and C.sub.10TAB mediated solutions started to
exhibit higher friction values above 750 nN. It appeared that the
lubricating surfactant layer is desorbed/destroyed beyond a certain
loading force. In the absence of salt, all the surfactants,
regardless of the chain length, form a compact adhesion layer due
to the electrostatic interaction between the negatively charged
silica surface and positively charged surfactant head group.
However, addition of salt resulted in competitive adsorption of the
salt molecules, weakening the adsorbed surfactant and hence
possibly desorption of surfactant lubrication layers beyond a
certain applied load. The impact of competitive adsorption of the
salt molecules was less effective on the longer chain length
surfactants, perhaps due to their ability to form more densely
packed and well-ordered layers. As the length of the hydrocarbon
chain increases, the lateral interactions between the hydrocarbon
chains become more pronounced resulting in formation of more
compact layers. In addition, the driving force leading the
surfactant to the substrate surface also increases with the
increased chain length, resulting in denser adsorption of the
surfactant. Thus, it is possible that NaCl addition does not affect
the lubrication layers created by C.sub.12TAB mediated slurries to
the same extend as surfactants with shorter chain lengths as seen
in FIG. 40B. Accordingly, C.sub.12TAB yielded negligible material
removal of 66 .ANG./min, whereas the shorter chain length
C.sub.10TAB surfactant, resulted in material removal of 650
.ANG./min indicating that the silica particle were able to engage
with the silica wafer surface up to some extend due to the removal
of the surfactant. Finally, C.sub.8TAB mediated slurries also
showed an increase in the frictional forces at single
particle-surface interaction level, suggesting that they as well
should polish the silica surface. However, the significantly high
removal rate for the C.sub.8TAB mediated slurries (5167 A/min)
should be attributed not only to easier removal of the loosely
packed 8-carbon chain surfactant layer but also to the coagulation
of the particulates in the absence of a repulsive force barrier for
C.sub.8TAB at 0.6 M salt concentration.
Example 8
Modification of the Interaction Forces by Calcium Addition
[0191] It is indicated in the previous section that ability of the
adsorbed surfactant to form a lubricating layer is adversely
affected by Na.sup.+ addition to the system. The Na.sup.+ ions tend
to approach the negatively charged silica surface at high pH values
due to electrostatic interactions. Furthermore, it has been shown
that in the case of silica, stability constants characterizing the
intensity of interaction between surface hydroxyl and the cation
increases with the increasing valency of the added salt. This
observation suggested that, by varying the valency of the added
salt. This observation suggested that, by varying the valency of
the added salt, the frictional forces could be manipulated.
[0192] To investigate this hypothesis, 0.24 M CaCl.sub.2 was added
into the polishing slurries (the ionic strength was kept equivalent
to 0.6 M NaCl). Table 1 summarizes the polishing performance in the
presence of CaCl.sub.2. It was observed that the slurry was stable
and hence the Table 1. Summary of polishing performance of the
baseline (with and Without CaCl.sub.2), And C.sub.12TAB mediated
slurries in the presence of 0.24 M CaCl.sub.2.
1TABLE 1 Summary of polishing performance of the baseline (with and
Without CaCl.sub.2), And C.sub.12TAB mediated slurries in the
presence of 0.24 M CaCl.sub.2. Mean Material Surface Particle
Removal Rough Maximum Defect Size (.sup..mu.m) Rate (.ANG./min) RMS
(nm) Depth R.sub.max (nm) W/O CaCl.sub.2 0.2 .+-. 0.01 3600 .+-.
428 0.08 .+-. 0.4 25 W CaC.sub.2 7.2 .+-. 2.4 6800 .+-. 916 3.10
.+-. 1.39 60 32 mM C.sub.12TAB 0.2 .+-. 0.06 4800 .+-. 714 0.47
.+-. 0.7 16
[0193] surface quality of the polished wafers was acceptable with
roughness value of 0.47 nm, and the maximum depth of the observed
defects less than 16 nm. Moreover, the material removal rate was
high even in the presence of 32 mM C.sub.12TAB surfactant (4800
.ANG./min). The speciation diagram of calcium at pH 10.5 shows that
C.sup.++ is the predominant specie in the solution with CaOH.sup.+.
The divalent Ca.sup.++ ions are more competitive in adsorbing on
the silica surface as their intensity of interaction on the
negatively charged silica surface is higher than Na+ions.
Specifically, it was shown that adsorption calcium species on
quartz extensively increase starting at pH 10. Therefore, the
adsorption of the cationic surfactant is expected to be much less
on the calcium activated silica surface resulting in weaker
lubrication.
[0194] FIG. 41 illustrates the FFR absorbance spectra for the
solutions containing 32 mM C.sub.12TAB at pH 10.5 without any salt
addition and in the presence of 0.6 M NaCl and 0.24 M CaCl.sub.2.
It can be seen that the most intense CH.sub.2 peak was obtained in
the absence of salt, indicating effective surfactant adsorption.
The frictional forces were recorded to be negligible for this
system (FIG. 40A). Addition of NaCl reduced the intensity of the
peak, which was attributed to the competitive adsorption of the
Na.sup.+ ions. CaCl.sub.2 addition resulted in the most significant
decrease, confirming that calcium species reduced the C.sub.12TAB
adsorption significantly. In agreement, the direct measurement of
the C.sub.12TAB (32 mM) adsorption on the silica surface in the
absence of salt gave 6.5.times.10.sup.-6 mol/m.sup.2 value, while
adsorption decreased to 5.2.times.10.sup.-6 and 1.4.times.10.sup.-6
mol/m.sup.2 with 0.6 M NaCl and 0.24 M NaCl.sub.2 addition,
respectively.
[0195] FIG. 42A shows the repulsive force barriers obtained with
the baseline and 32 mM C.sub.12TAB mediated slurries in the
presence of 0.6 M NaCl and 0.24 M CaCl.sub.2 (same ionic strength).
It can be seen that in all the cases there was a repulsive
interaction resulting in the stability of the slurries. The
repulsive force barrier observed with the calcium added slurries
was relatively smaller than in the presence of sodium. This may be
attributed to the reduced surfactant adsorption resulting in
smaller number of surfactant aggregate existence on the silica
surfaces for calcium.
[0196] The friction force measurements conducted in the presence of
0.24 M CaCl.sub.2 for C.sub.12TAB mediated slurries also
illustrated high frictional interactions, which was consistent with
the high material removal rates obtained with these systems. The
single particle-substrate level interactions showed in high
friction response as illustrated in FIG. 42B. Based on these
findings, it is suggested in this invention that, consistently high
performing slurries can be formulated by controlling the
interaction forces in CMP. It is also demonstrated that, control of
the interaction forces is possible by manipulating the slurry
-chemistry utilizing surfactants and varying the ionic
strength.
Example 9
Selection of Cationic Surfactant as a Dispersant
[0197] The preferred cationic surfactant preferably meets three
requirements. First, the cationic surfactant is preferably one in
which the surfactant associates with the surface of the slurry
particles in the form of assembled 3-dimensional structures, for
example, micelles or bilayers, in the absence of an added
electrolyte while still providing slurry stability. Second, the
preferred cationic surfactant preferably does not chemically react
with the surface under process conditions. This is determined by
surfactant head group chemistry. Third, the cationic surfactant
preferably has a suitable competing species (also referred to
herein as a competing agent) that allows for material removal and
sufficient slurry stability under processing conditions.
[0198] The degree of adsorption for a surfactant to a surface
depends on the specific interactions between the surface and
surfactant head group and the extent of hydrophobicity of the tail.
As the chain length increases, the tail's hydrophobicity increases,
creating stronger adsorptivity to the surface. Accordingly,
critical micelle concentration (CMC) decreases as the tail chain
length increases. Advantageously, less relatively expensive
surfactant reagents are required to maintain stability for systems
wherein micelles and bilayers are formed. However, the preferred
chain length is dependent on the headgroup. In one embodiment,
wherein the cationic surfactant is C.sub.nTAB, the chain length is
preferably above or equal to ten. Below this chain length,
C.sub.nTAB does not effectively stabilize at least some slurry
compositions, such as silica slurry solutions.
[0199] CMC is dependent on the surface and solution environment and
the stability contributions of any self-assembled structures formed
prior to CMC. Preferably, CMC is sufficient when self-assembled
structures are formed in the vicinity of the surface of the
abrasive slurry particles and mediate a suitable repulsion between
slurry particulates to maintain adequate slurry stability under the
given process conditions. The dispersion or stability of the slurry
is not reached until sufficient concentration of surfactant is
added to the slurries to form hydrophilic structures on the surface
of the particles (bilayers, micelles). Micelles form on the surface
of the particles near the CMC of the solution--the concentration at
which micelles are formed in solution. Therefore, less surfactant
is required to maintain stability of the system where micelles or
bilayers are formed.
[0200] Examples of suitable cationic surfactants include but are
not limited to long chained amines derived from animal and/or
vegetable acids, tall oil, and synthetic amines, diamines and
polyamines including ether amines and imidazolines, quartemized and
unquartemized polyoxyalkylenated long chain amines, and amine
oxides derived from tertiary amines oxidized with hydrogen
perioxide (as shown in FIGS. 43A-43E).
Example 10
Selection of Competing Agents
[0201] The competing agents used in the subject invention are those
that can effectively compete for adsorption sites on a substrate
(such as a wafer), under the particular conditions (such as under
CMP process conditions)--thereby weakening the affinity of the
surfactant (or other dispersant moiety) to the substrate, thereby
allowing for enhanced material removal. For example, where a cation
is used as the dispersant, any cation can compete with a cationic
surfactant for adsorption sites, and therefore may be suitable
(depending on the surfactant). Preferably, for the chosen
dispersant, multi-valent cations and/or monovalent hydroxides
(formed from the original cation under specific aqueous pH and
concentration conditions), sufficiently compete with the dispersant
molecule (e.g., surfactant) for surface adsorption sites--thereby
reducing the lubrication effect between the abrasive slurry
particles and the surface. However, these molecules should not
adversely alter the substrate surface finish through `unwanted`
reaction or slurry destabilization. The dispersant is preferably
added to the composition before the addition of the competing agent
(such as a competing salt) for high ionic strength systems, so as
to avoid formation of strong aggregates within the competition.
[0202] As used herein in the context of competing agents, the term
"an effective amount" represents an amount that partially or fully
interferes with the association of the dispersant with its binding
sites on the particular substrate. In the context of CMP, the
amount of competing agent added to the composition preferably does
not reach a concentration that adversely alters the substrate
finish through undesirable reaction or slurry destabilization. For
example, in screening for competitively adsorbing ions under the
conditions of pH 10.5 under 0.6 M ionic strength, it was observed
that a slurry containing 0.2M CaCl.sub.2 had acceptable polishing
properties (good removal rate with no macroscopic, multi-micron
sized scratches or pitting, or adverse reaction on the
surface).
[0203] In addition, in screening suitable multivalent cations at pH
10.5 and 0.6 M ionic strength, barium cations performed well for
silica CMP in competition with 32 mM C12TAB. Surface quality and
removal rates were similar to that produced by calcium. Other
potential competing agents include, for example, magnesium ions,
cadium cations, strontium cations, cerium cations, quartemary
amines, cationic polyelectrolytes, and metal hydroxide cations,
depending upon the concentration of the particular competing
agent.
Example 11
Competitive Adsorption Strategies
[0204] The stabilized compositions of the present invention are
based on the strategy of using a dispersant and a corresponding
competing agent. This strategy can be carried out using, for
example, charged ionic moieties (dispersant molecules versus
destabilizing molecules) competing for a surface of opposite
charge. An example for the case of anionic surfactants is given
below. The competing ion can be a salt molecule, for example.
[0205] Where the dispersant is an anionic surfactant, polymer, or
protein associated with a positively charged surface, other anionic
ions (particularly those that are multivalent) can be added as the
competing agent to compete with the anionic dispersant for
adsorption sites on the substrate. For example, where the
dispersant in the composition is the surfactant sodium dodecyl
sulfate (SDS), which can associate with alumina particles in a
composition at a low pH, phosphate ions can be added as the
competing agent.
[0206] In the compositions and methods of the subject invention,
the competing agents can compete with the dispersant for hydrogen
bonding sites. The polymers in Table 2 below are known to
specifically adsorb to either Bronsted acid or Lewis acid hydrogen
bonding sites. Similarly, surfactants or protiens with the similar
H-bonding head groups as the polymer molecules in Table 2 can be
used to adsorb to these sites as well. Organic molecules with
similar affinities for these sites may also be used for
competition. For example, the dispersant Triton-X 100 (having a PEO
type head group, as indicated in Table 2), which adsorbs to silica,
competes with methanol or ethanol.
[0207] Additionally, two or more of the above strategies can be
combined to compete with dispersant molecules for adsorption sites.
For instance, a cation metal hydroxide ion can compete with
cationic and/or hydrogen bonding surfactant ion for both negatively
charged and hydrogen bonding sites on a surface.
2TABLE 2 Active Polymer Repeating Unit Functionality Adsorbs To
Site PEO [CH.sub.2CH.sub.2O].sub.- n ether SiO.sub.2 Bronsted PVA
[CH.sub.2CH(OH)].sub.n hydroxyl SiO.sub.2 Bronsted PAA
[CH.sub.2CH(CO.sub.2H)].sub.n carboxylic Fe.sub.2O.sub.3 Lewis acid
Al.sub.2O.sub.3 PAM [CH.sub.2CH(CONH.sub.2)].sub.n amide
Fe.sub.2O.sub.3 Lewis Al.sub.2O.sub.3 PAH
[CH.sub.2CH(CH.sub.2NH.sub.2)].sub.n Amine SiO.sub.2 Bronsted
[0208] All patents, patent applications, provisional applications,
and publications referred to or cited herein are incorporated by
reference in their entirety, including all figures and tables, to
the extent they are not inconsistent with the explicit teachings of
this specification.
[0209] It should be understood that the examples and embodiments
described herein are for illustrative purposes only and that
various modifications or changes in light thereof will be suggested
to persons skilled in the art and are to be included within the
spirit and purview of this application.
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