CROSS-REFERENCE TO RELATED APPLICATIONS
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
 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.
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.
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.
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.
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.
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.
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.
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 (≧100 mV). As a result, other methods such as the use of dispersants must be applied.
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.
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.
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.
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.
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 (SiO2 particles in a KOH solution or NH4OH) 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.
- BRIEF SUMMARY OF THE INVENTION
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.
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).
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.
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.
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.
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° C.); low temperature (<10° 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.
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.
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++ or Ba++. 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.
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.
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.
The subject invention further concerns abrasive articles, such as polishing pads, and methods for making the slurry compositions of the subject invention.
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.
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.
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).
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 (C12TAB) 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.
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.
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.
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.
BRIEF DESCRIPTION OF DRAWINGS
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.
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.
FIG. 2 shows a force-distance curve for a 2 CMC C12TAB solution showing the different stages as the tip approaches the mica surface.
FIG. 3 shows a force-distance curves of CnTAB (for n 10, 12, 14 and 16) cylindrical micelles adsorbed on mica; a, C10TAB; b, C12TAB; c, C14TAB; d, C16TAB.
FIG. 4 shows a plot of maximum compressive force as a function of alkyl chain length for 2 CMC solutions of CnTAB (for n=10, 12, 14 and 16).
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 (∘ circles C10TAB; C16TAB).
FIG. 6 shows a plot of the maximum compressive force as function of SDS concentration for a 2 CMC (32 mM) C12TAB solution on mica.
FIG. 7 shows a plot of the turbidity of silica dispersions (o) and the maximum compressive force on mica () versus concentration of C12TAB at pH 4 after 60 seconds.
FIG. 8 shows a plot of the stability of silica dispersions (o) and the maximum compressive force on mica () versus concentration C12TAB in the presence of 100 mM NaCl at pH 4.
FIG. 9 shows a plot of the stability of silica dispersions (o) and the maximum compressive force on mica () versus concentration of SDS in the presence of 100 mM NaCl and 5 mM C12TAB at pH 4.
FIG. 10 shows a plot of relative turbidity versus electrolyte concentration for 200 nm particles in the presence of 32 mM of C12TAB.
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 C12TAB.
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 C12TAB.
FIG. 13 shows a plot of maximum repulsive force versus Urea concentration in the presence of 32 mM C12TAB.
FIG. 14 shows a plot of interaction force versus separation distance for an AFM tip and mica surface in the presence of 32 mM C12TAB and no urea, 1 M urea, or 5M urea.
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.
FIG. 16 shows a plot of maximum repulsive force versus C12TAB concentration for an AFM tip and silica surface, and for 200 nm silica particles, in the presence of 0.1 M NaCl.
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 C12TAB.
FIG. 18 shows a plot of maximum repulsive force versus C12TAB concentration for an AFM tip and mica surface in the presence of no NaCl, or in the presence of 0.1 M NaCl.
FIG. 19 shows a plot of maximum repulsive force versus C12TAB 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.
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 C12TAB and 0.1 M NaCl.
FIG. 21 shows a plot of interaction force versus separation distance for an AFM tip and silica surface in the presence of 11 mM C12TAB and 0.1 M NaCl, at 25° C., 32° C., and 40° C.
FIG. 22 shows a plot of viscosity versus temperature for 200 nm silica particles in the presence of 39 mg/g C12TAB.
FIG. 23 shows a plot of viscosity versus shear rate for 200 nm silica particles in the presence of 44 mg/g C12TAB, or in the presence of 5 mg/g Darvan C.
FIG. 24 shows a plot of viscosity versus shear rate for 200 nm silica particles in the presence of 44 mg/g C12TAB and 0.1 M NaCl, or in the presence of 3 mg/g PEO 7500 MW.
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.
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.
FIG. 27A shows a spectra of C12TAB at 4 mM bulk concentration in the CH2 stretching region [asymmetric (2920 cm−1), symmetric (2850 cm−1)] with parallel (p) and perpendicular (s) polarized infrared beam.
FIG. 27B shows a spectra of C12TAB at 8 mM bulk concentration in the CH2 stretching region [asymmetric (2920 cm−1), symmetric (2850 cm−1)] with parallel (p) and perpendicular (s) polarized infrared beam.
FIG. 28 shows the dependence of the order parameter S on the dichroic ratio D.
FIG. 29 shows a schematic representation of surfactant structures and orientation, and the order parameter values associated with the structures.
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 C12TAB concentration.
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 C12TAB concentration.
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.
FIG. 33 shows a plot of maximum repulsive force versus C12TAB concentration for an AFM tip and silica surface in the presence of 0.1 M NaCl at a pH of 1, 2, and 4.
FIG. 34 shows a plot of maximum repulsive force versus C12TAB concentration for an AFM tip and mica surface in the presence of 0.1 M NaCl at a pH of 4 and 9.
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.
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.
FIG. 37 shows the maximum repulsive force response of C12TAB, C10TAB and C8TAB 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 μm particle attached to tip.
FIG. 38 shows the stability and material removal rate responses for surfactant-mediated slurries.
FIG. 39 shows in situ friction force and material removal rate responses of the baseline slurries (12 w%, 0.2 μm primary particle size) and the slurries containing C12TAB surfactants at 32, 68 and 140 mM concentrations in the presence of 0.6 M NaCl at pH 10.5.
FIGS. 40A and 40B show AFM friction force measurements on silica wafer with 7 μm size particle attached to the tip. FIG. 40A shows solutions containing C12TAB, C10TAB and C8TAB 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.
FIG. 41 shows FTTR-ATR adsorption spectra of the solutions in the presence of 32 mM C12TAB without salt and in the presence of 0.6 M NaCl and 0.24 M CaCl2.
FIGS. 42A and 42B show AFM force measurements for C12TAB mediated slurry in the presence of NaCl and CaCl2. FIG. 42A shows repulsive force measurements. FIG. 42B shows friction force measurements.
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.
DETAILED DISCLOSURE OF THE INVENTION
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).
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.
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.
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++ or Ba++. 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.
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.
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° C.); low temperature (<10° 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.
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.
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.
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.
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 (C12TAB—where C12 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.
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.
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.
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 C12TAB. The addition of SDS lowers the minimum concentration of C12TAB 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.
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 C12TAB 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 Fmax 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 Fmax, 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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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 (˜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).
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.
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.
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.
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 %.
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.
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.
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.
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.
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.
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.
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.
Examples of suitable corrosion inhibitors include, but are not limited to, benzotrizole, 6-tolylytrizole, 1-(2,3-dicarboxypropyl)benzotrizole, 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.
Examples of suitable chelating agents include, but are not limited to, ethylenediaminetetracetic acid (EDTA), N-hydroxyethylethylenediaminetriacetic 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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
- Materials and Methods
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.
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×5 mm×2 mm, 45° 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 (C12TAB) 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.
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−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° and 90° 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 C12TAB were measured at various concentrations of C12TAB to investigate average orientation of hydrocarbon chains in the surfactant films adsorbed on the crystal. Asymmetrical CH2 stretching mode peaks (near 2920 cm−1) were used for the calculation of dichroism, although the symmetrical peaks (near 2850 cm−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.
Calculations. The theory of attenuated total internal reflection spectroscopy (ATR) was proposed by Harrick1 and independently by Fahrenfort2 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,1 and in the final form by Haller and Rice.3
In order to determine molecular orientations within adsorbed surfactant films, several researchers4-7
have used polarized beam infrared spectroscopy. The key parameter in determining molecular orientations is the dichroic ratio or dichroism, which is defined as
where, As and Ap 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).
Deriving values of As
from absorption due to x, y, and z components of the electric field, dichroism can be written as,
where, Ax, Ay, and Az 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.
The values for Ax
, and Az
will depend on the model of the adsorbed layer structure used. Haller and Ulman8
have used a model with fixed angles of the alkyl chain from the normal to the surface. The other model proposed by Zbinden,9
which considers a uniaxial symmetric distribution of the transition dipole moments M, about the alkyl chain (c axis) with fixed angle θ between M and c, and a uniaxial symmetric distribution of the c axis about the z axis (surface normal), with fixed angle γ 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 Tamm4
and Rabinovich et al. ,5
and used later in several papers.6-7
, and Az
through rotation about the c and z axes, the expression for dichroism was obtained4-5
from equations (2) and (3) as
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 θ between M and the C-C axis is 90 degrees.
Solving equation 4 for γ with θ=90° gives
Equations (4) and (5) were obtained by neglecting reflection at the entrance and exit surface of the crystal.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.
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 Ex, Ey, and Ez were calculated from the formulae given by Haller and Rice.3 Refractive indices (n) used for these calculations were nsilicon=3.42, nwater=1.42, and nsurfactant film=1.46. Index of refraction for water at wavenumber=2900 cm−1 was experimentally measured by Pinkley et al.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 Ex, Ey, and Ez, the values of the angle γ were calculated from equation (5) for increasing concentrations of the surfactant.
Following standard definition used for liquid crystals, an order parameter S 11 can be defined as
S=1.5 cos2γ−0.5 (6)
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 Rice3 to calculate the values of Ex, Ey, and Ez. (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), γ=0° and S=1, indicating perfect order. For full disorder, i.e; random distributed orientation of molecules, γ=54.7° and S=0. For perfect planar orientation, i.e., the alkyl chain axis parallel to the surface, γ=90° and S=−0.5.
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.
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° of the angle at which an approximately 10 μL 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.
- Example 1
Adsorption of Alkyl Trimethylammonium Bromides
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.
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 C12TAB 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.
An example of a force-distance curve for a 2 CMC C12TAB 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.
- Example 2
Effect of Chain Length on the Stability of Adsorbed Micelles
Force-distance curves for the entire series of CnTAB (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 (Fmax in FIG. 2) increases linearly as function of alkyl chain length. Initially, octyltrimethylammonium bromide (C8TAB) was also investigated in this study. However, images as well as force-distance curves did not show adsorption of C8TAB 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 C8TAB does not adsorb in the same manner as the higher chain length surfactants (n>8).
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 (τ2), 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 τ2. FIG. 5 shows the effect of pressure on the electrical conductivity of 2 CMC solutions of CnTAB (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 C16TAB indicates that C16TAB micelles are more stable than C10TAB micelles. For C16TAB higher pressures are required to obtain the same change in conductivity as compared to C10TAB, 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 C10 to C16, 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.
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 C12TAB 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 C12TAB solutions. For example, the addition of SDS can dramatically increase the magnitude of the repulsive barrier.
In a specific example, an increasing amount of SDS was added to the 2 CMC C12TAB (32 mM) solutions. FIG. 6 shows the maximum compressive force for C12TAB cylindrical micelles as a function of added SDS concentration. The C12TAB concentration was kept constant at twice the CMC (32 mM). The chain lengths between the two oppositely charged surfactants (C12TAB 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 C12TAB solutions results in precipitation and hence the maximum compressive force was not measured in this range.
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.
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 C12TAB 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 HNO3.
FIG. 7 shows the turbidity of silica dispersions after 60 minutes as well as the maximum compressive force as a function of C12TAB concentration on mica. A high turbidity indicates very stable dispersions, whereas very low values indicate unstable dispersions. Initially, the addition of C12TAB destabilizes the dispersion due to the adsorption of C12TAB onto the silica particles. At low concentrations of C12TAB (<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 C12TAB, 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.
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 C12TAB concentrations (from approximately 20 to 10 mM C12TAB).
The influence of SDS on the stability of the dispersion in the presence of 5 mM C12TAB 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 C12TAB 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 (λ0.001%) 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.
Referring to FIG. 10, the relative turbidity versus electrolyte concentration is shown for 200 nm silica, at pH 4, and 32 mM C12TAB. 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.
Referring to FIG. 11, a plot of maximum repulsive force between an AFM tip and a mica surface in the presence of 32 mM C12TAB, 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 C12TAB 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.
Referring to FIG. 12, the maximum repulsive force between an AFM tip and a mica surface in the presence of 32 mM C12TAB, 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.
Referring to FIGS. 13 and 14, the maximum repulsive force between an AFM tip and a mica surface is the presence of 32 mM C12TAB, 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.
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.
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 C12TAB concentration. Also shown is the suspension turbidity for 200 nanometer silica particles as the concentration of C12TAB 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.
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 C12TAB 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 C12TAB 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.
Referring to FIG. 18, the maximum repulsive force versus C12TAB 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.
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.
- Example 3
Control of Rheological Behavior
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 C12TAB 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 C12TAB 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.
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 C12TAB. As shown, agglomeration above 55° 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.
FIG. 23 shows a plot of viscosity versus shear rate for 200 nm silicon particles in either 44 mg/g C12TAB or 5 mg/g Darvn C, and a high salt content without the addition of C12TAB or Darvan C. The 50 vol % 200 nm silica particles form a crumbly material, wherein the addition of C12TAB 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 C12TAB is harder than the surface layer formed by Darvan C. Accordingly, even at low shear rates, C12TAB is effective in controlling viscosity.
FIG. 24 shows a plot of viscosity versus shear rate for 45 vol % 200 nm silica with 44 mg/g C12TAB 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.
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.
FIG. 33 illustrates the effect of surface change on the barrier. The barrier magnitude versus C12TAB 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.
FIG. 34 also illustrates the effect of surface charge on the barrier. The barrier magnitude versus C12TAB 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.
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.
- Example 4
Effect of C12TAB Concentration on Adsorption, Zeta Potential, and Contact Angle
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.
Adsorption, zeta potential, and contact angle measurements of silica surfaces, in 0.1 M NaCl at pH 4.0 as a function of solution C12TAB 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.
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.
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°), 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).
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.
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,9 which was used in the present study to calculate the tilt angle γ 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.
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 C12TAB 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.
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.
- Example 5
Stabilization of Slurries at High Ionic Strength
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 6
Polishing Performance with Surfactant Mediated Slurries
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 C12TAB, C10TAB and C8TAB 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 μm to 4.3 μm. At 32 mM C12TAB and 68 mM C12TAB additions the polishing slurry was completely stable (mean particles size 0.2 μm) since an adequate repulsive force barrier was reached under these conditions. However, since the C8TAB surfactant did not exhibit a repulsive force barrier, slurries mediated with the right carbon chain surfactant were unstable.
- Example 7
Surface Lubrication Effect on Particle-Substrate Interactions
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 C10TAB and C12TAB, 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 C8TAB 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 Å/min material removal rate as compared to the 4300 Å/min with the baseline slurry. Addition of 140 mM C2TAB resulted in 5170 Å/min material removal rate while this value decreased to 650 and 66 Å/min for the C10TAB and C12TAB 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 C8TAB mediated slurries which were unstable and the mechanical material removal created by the coagulates still gave high material removal response.
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.
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 C8TAB modified slurry (at 140 mM) also resulted in high frictional forces (4.5 N), which polished at 5167 Å/min rate. For the C10TAB (68 mM) and C12Tab (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 C10TAB, and 100 times reduction for the C12TAB 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.
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, C8TAB, C10TAB and C12TAB). 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 Å/min for C8TAB, C10TAB, and C12TAB, 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.
- Example 8
Modification of the Interaction Forces by Calcium Addition
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 Å/min for C12TAB, C10TAB 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, C8TAB and C10TAB 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 C12TAB mediated slurries to the same extend as surfactants with shorter chain lengths as seen in FIG. 40B. Accordingly, C12TAB yielded negligible material removal of 66 Å/min, whereas the shorter chain length C10TAB surfactant, resulted in material removal of 650 Å/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, C8TAB 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 C8TAB 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 C8TAB at 0.6 M salt concentration.
It is indicated in the previous section that ability of the adsorbed surfactant to form a lubricating layer is adversely affected by Na+ addition to the system. The Na+ 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.
To investigate this hypothesis, 0.24 M CaCl2
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 CaCl2
. It was observed that the slurry was stable and hence the Table 1. Summary of polishing performance of the baseline (with and Without CaCl2
), And C12
TAB mediated slurries in the presence of 0.24 M CaCl2
|TABLE 1 |
|Summary of polishing performance of the baseline |
|(with and Without CaCl2), And C12TAB mediated |
|slurries in the presence of 0.24 M CaCl2. |
| ||Mean ||Material ||Surface || |
| ||Particle ||Removal ||Rough ||Maximum Defect |
| ||Size (μm) ||Rate (Å/min) ||RMS (nm) ||Depth Rmax (nm) |
| || |
|W/O CaCl2 ||0.2 ± 0.01 ||3600 ± 428 ||0.08 ± 0.4 ||25 |
|W CaC2 ||7.2 ± 2.4 ||6800 ± 916 ||3.10 ± 1.39 ||60 |
|32 mM |
|C12TAB ||0.2 ± 0.06 ||4800 ± 714 ||0.47 ± 0.7 ||16 |
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 C12TAB surfactant (4800 Å/min). The speciation diagram of calcium at pH 10.5 shows that C++ is the predominant specie in the solution with CaOH+. The divalent Ca++ 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.
FIG. 41 illustrates the FFR absorbance spectra for the solutions containing 32 mM C12TAB at pH 10.5 without any salt addition and in the presence of 0.6 M NaCl and 0.24 M CaCl2. It can be seen that the most intense CH2 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+ ions. CaCl2 addition resulted in the most significant decrease, confirming that calcium species reduced the C12TAB adsorption significantly. In agreement, the direct measurement of the C12TAB (32 mM) adsorption on the silica surface in the absence of salt gave 6.5×10−6 mol/m2 value, while adsorption decreased to 5.2×10−6 and 1.4×10−6 mol/m2 with 0.6 M NaCl and 0.24 M NaCl2 addition, respectively.
FIG. 42A shows the repulsive force barriers obtained with the baseline and 32 mM C12TAB mediated slurries in the presence of 0.6 M NaCl and 0.24 M CaCl2 (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.
- Example 9
Selection of Cationic Surfactant as a Dispersant
The friction force measurements conducted in the presence of 0.24 M CaCl2 for C12TAB 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.
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.
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 CnTAB, the chain length is preferably above or equal to ten. Below this chain length, CnTAB does not effectively stabilize at least some slurry compositions, such as silica slurry solutions.
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.
- Example 10
Selection of Competing Agents
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).
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.
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 CaCl2 had acceptable polishing properties (good removal rate with no macroscopic, multi-micron sized scratches or pitting, or adverse reaction on the surface).
- Example 11
Competitive Adsorption Strategies
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.
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.
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.
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.
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.
|TABLE 2 |
| || || || ||Active |
|Polymer ||Repeating Unit ||Functionality ||Adsorbs To ||Site |
|PEO ||[CH2CH2O]n ||ether ||SiO2 ||Bronsted |
|PVA ||[CH2CH(OH)]n ||hydroxyl ||SiO2 ||Bronsted |
|PAA ||[CH2CH(CO2H)]n ||carboxylic ||Fe2O3 ||Lewis |
| || ||acid ||Al2O3 |
|PAM ||[CH2CH(CONH2)]n ||amide ||Fe2O3 ||Lewis |
| || || ||Al2O3 |
|PAH ||[CH2CH(CH2NH2)]n ||Amine ||SiO2 ||Bronsted |
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.
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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