Ratchana Limary - Academia.edu (original) (raw)
Papers by Ratchana Limary
Langmuir, Jun 16, 1999
We examined the dynamics of dewetting of a thin symmetric diblock copolymer film on a substrate a... more We examined the dynamics of dewetting of a thin symmetric diblock copolymer film on a substrate above the bulk order-disorder transition temperature, TODT, of the copolymer using atomic force microscopy. The dewetting mechanism proceeded with the formation of discrete holes without their characteristic peripheral rims. During this early stage, the hole radii R increased exponentially with time. This stage was followed by a narrow intermediate regime where the rim develops and R ∼ t. When the rim was fully developed, R increased as t 2/3. The shape of the rim was highly asymmetrical and its width L increased as t 1/2. At the final stage of the process, droplets of the copolymer, a few microns in diameter and with heights on the order of tens of nanometers, existed on a dense copolymer "brush" of uniform thickness 7 nm anchored to the substrate. This clearly indicates that the process is autophobic, a phenomenon first documented in small molecule liquids.
Advances in Colloid and Interface Science, Nov 1, 2001
During the last decade, research on thin, sub-micron thick, block copolymer films was devoted tow... more During the last decade, research on thin, sub-micron thick, block copolymer films was devoted toward understanding and controlling microstructural and topographical features at temperatures T-T , where T is the order᎐disorder transition temperature below ODT ODT Ž. which thermodynamic interactions favor the formation of ordered phase separated microstructures. Symmetric diblock copolymers, the subject of this review, undergo an isotropic to lamellar transition when T-T. Topographical features, 'islands' or 'holes,' ODT of these films typically reflect the underlying phase separation; and the dimension of these features, normal to the substrate, is equal to the interlamellar spacing, L. Two aspects of block copolymer thin films that have not received much attention are discussed in this Ž. paper: 1 pattern formation in symmetric block copolymers under conditions of T) T ; ODT Ž. and 2 phase behavior of thin film symmetric diblock copolymerrhomopolymer mixtures, when T-T .
MRS Proceedings, 2000
Symmetric diblock copolymers undergo a disorder to order transition below a microphase separation... more Symmetric diblock copolymers undergo a disorder to order transition below a microphase separation transition temperature. In this temperature range the structure is characterized by alternating lamellae of thickness L. In thin film geometries, the lamellae are oriented normal to the substrate if there is a preferential interaction between either of the block constituents and the substrate/copolymer or copolymer/vacuum interfaces. Depending on the relation between the film thickness and L, the topography of the film might comprise of holes, islands or spinodal-like structures. We show that in a polystyrene-b-poly(methyl methacrylate) diblock copolymer of molecular weight 20, 000 g/mol, above the microphase separation transition temperature, the topography of the film depends on the thickness. A heirarchy of bicontinuous patterns and holes is observed with increasing film thickness for films thinner than 35 nm. FF1.5.1
European Physical Journal E, May 1, 2002
The substrate is shown to induce substantial ordering in diblock copolymer thin films above the b... more The substrate is shown to induce substantial ordering in diblock copolymer thin films above the bulk order-disorder transition (ODT) where, thermodynamically, a phase mixed state is favored. Initially, uniform films reorganize to form a hierarchy of transient surface patterns and stable film thicknesses that depend on the initial film thickness and on the substrate. Self-consistent field calculations of the free energy of the system for different situations, depending on the relative tendency for the different block components to be attracted to the substrate and/or free surface, provide an explanation of the formation of the stable film thicknesses. A continuum picture proposed earlier by Brochard et al. provides an explanation of the wetting characteristics of this system. In some cases the ordering destabilizes the film so that dewetting occurs (wetting autophobicity), whereas in other cases the surface ordering results in a kinetic stabilization of a film that would otherwise dewet.
American Chemical Society eBooks, Jul 23, 2001
Macromolecules, Jul 16, 2002
Advances in Patterning Materials and Processes XXXVI
Conventional doping of crystalline Si via ion implantation results in a stochastic distribution o... more Conventional doping of crystalline Si via ion implantation results in a stochastic distribution of doped regions in the x-y plane along with relatively poor control over penetration depth of dopant atoms. As the gate dimensions get to 10 nm, the related device parameters also need to be scaled down to maintain electrical activity. Thus highly doped abrupt, ultra-shallow junctions are imperative for source-drain contacts to realize sub-10 nm transistors. Uniform ultra-shallow junctions can be achieved via monolayer doping, wherein thermal diffusion of a self-limiting monolayer of dopant atomcontaining organic on Si surface yields sub-5 nm junctions. We have extended the use of organic dopant molecules in the monolayer doping technique to introduce a new class of spin-on polymer dopants. In effect, these new spin-on dopants offer a hybrid between the monolayer doping technique and traditional inorganic spin-on dopants. We have been able to uniformly introduce p- and n-type dopants with doping efficiencies comparable to the monolayer doping technique. Control over junction depth can be easily achieved via optimizing annealing temperature and time. Concurrently, sequestering the dopant precursors within the cores of block copolymer micelles allows us to achieve precise control over the spatial positions of dopant atoms in all three dimensions owing to the high periodicity of block copolymer domains on the 10-100 nm length scale.
Advances in Patterning Materials and Processes XXXVI, 2019
As devices become ever smaller and more sophisticated, there is also a general need for creating ... more As devices become ever smaller and more sophisticated, there is also a general need for creating high quality defect-free thin coatings of polymers on 3-dimensional wafer topography, for example, for shrinkage of the size of trench openings. To address this challenge, we developed a spin-on polymer brush material, which comprises of a dopant moiety with a universal adhesive dopamine end group. We demonstrate that the polymer coating is highly conformal and free of pinhole defects, even when only a few nm thick, or when coated over high aspect ratio over 200 nm deep trench topography. Our investigations demonstrate that the dopamine end group enables stable sub-10 nm thick conformal coatings on three-dimensional surfaces. Furthermore, on acute 3-dimensional semiconductor topography, the creation of highly doped abrupt, ultra-shallow junctions with three-dimensional control are essential for successful source-drain contacts. In consideration of this need, we extended the above polymer brush concept further by incorporating a suitable implant dopant atom, such as boron, into the monomer structure. After conformal coating and a subsequent rapid thermal annealing process, the dopant atom is driven into the semiconductor substrate underneath the polymer film. This is potentially very useful for uniform all-around doping of 3-dimensional topography such as FinFETs or Nanowire-FETs. A high dopant dosage on silicon substrate with appropriate shallow implant characteristics was demonstrated for the end-functionalized dopant polymer brush, highlighting one of the promising applications of such conformal coatings.
Bulletin of the American Physical Society, 2018
The stability of thin film symmetric diblock copolymers blended with layered silicate nanocomposi... more The stability of thin film symmetric diblock copolymers blended with layered silicate nanocomposites were examined using a combination of optical microscopy, atomic force microscopy (AFM), and X-ray diffraction (XRD). Two cases were examined PS-b-PMMA (polystyrene-b-polymethylacrylate) blended with montmorillonite stoichiometrically loaded with alkyl ammonium ions, OLS(S), and PS-b-PMMA blended with montmorillonite loaded with excess alkyl ammonium ions, OLS(E). XRD spectra show an increase in the gallery spacing of the OLSs, indicating that the copolymer chains have intercalated the layered silicates. AFM images reveal a distinct difference between the two nanocomposite thin films: regions in the vicinity of OLS(S) aggregates were depleted of material, while in the vicinity of OLS(E) aggregates, dewetting of the substrate occurred. We show that the stability of the copolymer/OLS nanocomposite films is determined by the enthalpic driving force associated with intercalation of the copolymer chains into the galleries of the modified OLS layers and by the substrate/organic modifier interactions.
Aps Meeting Abstracts, Mar 1, 2000
Thin, non-wetting, liquid films on substrates can become unstable and spontaneously rupture due t... more Thin, non-wetting, liquid films on substrates can become unstable and spontaneously rupture due to long and short-range intermolecular forces. Patterns ranging from discrete holes to bicontinous spinodal patterns, reflecting fluctuations in the local film thickness, are observed. Much of the research on this topic has been devoted to the behavior of simple polymeric fluids. We show that thin symmetric diblock copolymer films at temperatures above the bulk order-disorder transition (ODT) exhibit topographies that depend on the local film thickness. These topographies are transient and initially they range from holes to spinodal structures, depending on the film thickness, h. At long times the structures evolve into large droplets. The rate of evolution toward the droplet stage depends on the film thickness. The surface of films thicker than 35 nm remain smooth and stable indefinitely.
We examined the late stage structural evolution of droplets of a polystyrene-b-poly(methyl methac... more We examined the late stage structural evolution of droplets of a polystyrene-b-poly(methyl methacrylate) PS-b-PMMA diblock copolymer on a PS-b-PMMA brush. The initial droplet size distributions and shape distributions varied appreciably from one sample to another. Coarsening of the droplets occurred, wherein the average droplet size, S, increased with time, S ˜t^b, accompanied by a decrease in the number of droplets per unit area with time, N(t) ˜t^-b. The value of the power law exponent b was found to increase as the size distribution and shape distribution of droplets decreased, from 1/10 to approximately 2/5. The droplet probability distributions, F(s/S) vs. s/S, determined from our data were compared with theoretical probability distributions and the results strongly indicate that the mechanism of coarsening occurs via a, self-similar, dynamic coalescence process, not Ostwald ripening.
ACS Symposium Series, 2001
MRS Proceedings, 2000
ABSTRACTSymmetric diblock copolymers undergo a disorder to order transition below a microphase se... more ABSTRACTSymmetric diblock copolymers undergo a disorder to order transition below a microphase separation transition temperature. In this temperature range the structure is characterized by alternating lamellae of thickness L. In thin film geometries, the lamellae are oriented normal to the substrate if there is a preferential interaction between either of the block constituents and the substrate/copolymer or copolymer/vacuum interfaces. Depending on the relation between the film thickness and L, the topography of the film might comprise of holes, islands or spinodal-like structures. We show that in a polystyrene-b-poly(methyl methacrylate) diblock copolymer of molecular weight 20, 000 g/mol, above the microphase separation transition temperature, the topography of the film depends on the thickness. A heirarchy of bicontinuous patterns and holes is observed with increasing film thickness for films thinner than 35 nm.
Physical Review E, 2002
Thin films of poly͑styrene-b-methyl methacrylate͒ diblock copolymers above the bulk order-disorde... more Thin films of poly͑styrene-b-methyl methacrylate͒ diblock copolymers above the bulk order-disorder transition temperature, within a certain thickness range, are structurally unstable on SiO x /Si substrates. They dewet autophobically, forming droplets on a self-assembled brush. We investigated the late-stage evolution, coarsening, of the droplets on the brush. The average droplet cross-sectional area ͗S͘ increased with time, ͗S͘ϰt ␥. This was accompanied by a decrease in the number of droplets per unit area with time, N(t)ϰt Ϫ␥. We analyzed the droplet size distribution, F(S/͗S͘) vs S/͗S͘, and found that the shape of the curve was virtually identical at different stages throughout the process. This suggests that a structural self-similarity is associated with the process. A comparison of F(S/͗S͘) vs S/͗S͘ data with distributions based on Ostwald ripening and coalescence cluster coarsening ͑dynamic and static͒ mechanisms strongly indicates that the dominant coarsening mechanism involved motion of droplets across the brush and subsequent coalescence, i.e., dynamic coalescence coarsening, not Ostwald ripening.
Macromolecules, 2000
The stability of thin film nanocomposites on silicon substrates, formed from mixtures of a symmet... more The stability of thin film nanocomposites on silicon substrates, formed from mixtures of a symmetric diblock copolymer blended with layered silicate nanocomposites, was investigated using a combination of optical microscopy, atomic force microscopy (AFM), and X-ray diffraction (XRD). Two ...
Macromolecules, 1999
Thin symmetric poly(styrene-b-methyl methacrylate) (PS-b-PMMA) diblock copolymer films at tempera... more Thin symmetric poly(styrene-b-methyl methacrylate) (PS-b-PMMA) diblock copolymer films at temperatures higher than the bulk order-disorder transition temperature TODT are shown to dewet silicon substrates, forming topographical features that depend on the initial film thickness h. Films of thickness h < 3.5 nm dewet the substrate, forming bicontinuous spinodal-like patterns. When 3.5 nm < h < hL) 7 nm, discrete holes are observed randomly throughout the film. For films of thickness in the range hL < h < hH) 35 nm, the copolymer exhibited autophobic behavior, whereby the top layer of thickness (h-hL) dewets a dense "brush" of ordered copolymer of height hL anchored to the silicon substrate. The morphologies, which include a bicontinuous spinodal pattern for films of thickness in the range hL < h < 19 nm, and discrete holes, for films of thickness in the range 19 nm < h < hH, eventually evolve into droplets. Films of h > 35 nm remained stable, with smooth surfaces. The time-dependent evolution of the "spinodal" structures that evolve in the autophobic regime is discussed. In addition, the existence of surface-induced ordering of the copolymer at temperatures above the bulk TODT is also discussed.
Langmuir, 2003
We examined the late-stage structural evolution of droplets of a polystyrene-b-poly(methyl methac... more We examined the late-stage structural evolution of droplets of a polystyrene-b-poly(methyl methacrylate) PS-b-PMMA diblock copolymer on a PS-b-PMMA "substrate". The initial droplet size distributions and shape distributions varied appreciably from one sample to another. Coarsening of the droplets occurred, wherein the average droplet size, 〈S〉, increased with time, 〈S〉 ∝ t , accompanied by a decrease in the number of droplets per unit area with time, N(t) ∝ t-. The value of the power law exponent was found to increase as the size distribution and shape distribution of droplets decreased, from 1/10 to approximately 2/5. The droplet probability distributions, F(S/〈S〉) vs S/〈S〉, determined from our data were compared with theoretical probability distributions and the results strongly indicate that the mechanism of coarsening occurs via a self-similar, dynamic coalescence process, not Ostwald ripening. The differing exponents appear to be associated with quantitative differences between the details of the coarsening process as the droplets become smaller with a narrow shape distribution. We suggest evidence of a possible dimensional crossover from 3D to 2D dynamics.
Langmuir, 1999
We examined the dynamics of dewetting of a thin symmetric diblock copolymer film on a substrate a... more We examined the dynamics of dewetting of a thin symmetric diblock copolymer film on a substrate above the bulk order-disorder transition temperature, TODT, of the copolymer using atomic force microscopy. The dewetting mechanism proceeded with the formation of discrete holes without their characteristic peripheral rims. During this early stage, the hole radii R increased exponentially with time. This stage was followed by a narrow intermediate regime where the rim develops and R ∼ t. When the rim was fully developed, R increased as t 2/3. The shape of the rim was highly asymmetrical and its width L increased as t 1/2. At the final stage of the process, droplets of the copolymer, a few microns in diameter and with heights on the order of tens of nanometers, existed on a dense copolymer "brush" of uniform thickness 7 nm anchored to the substrate. This clearly indicates that the process is autophobic, a phenomenon first documented in small molecule liquids.
Langmuir, Jun 16, 1999
We examined the dynamics of dewetting of a thin symmetric diblock copolymer film on a substrate a... more We examined the dynamics of dewetting of a thin symmetric diblock copolymer film on a substrate above the bulk order-disorder transition temperature, TODT, of the copolymer using atomic force microscopy. The dewetting mechanism proceeded with the formation of discrete holes without their characteristic peripheral rims. During this early stage, the hole radii R increased exponentially with time. This stage was followed by a narrow intermediate regime where the rim develops and R ∼ t. When the rim was fully developed, R increased as t 2/3. The shape of the rim was highly asymmetrical and its width L increased as t 1/2. At the final stage of the process, droplets of the copolymer, a few microns in diameter and with heights on the order of tens of nanometers, existed on a dense copolymer "brush" of uniform thickness 7 nm anchored to the substrate. This clearly indicates that the process is autophobic, a phenomenon first documented in small molecule liquids.
Advances in Colloid and Interface Science, Nov 1, 2001
During the last decade, research on thin, sub-micron thick, block copolymer films was devoted tow... more During the last decade, research on thin, sub-micron thick, block copolymer films was devoted toward understanding and controlling microstructural and topographical features at temperatures T-T , where T is the order᎐disorder transition temperature below ODT ODT Ž. which thermodynamic interactions favor the formation of ordered phase separated microstructures. Symmetric diblock copolymers, the subject of this review, undergo an isotropic to lamellar transition when T-T. Topographical features, 'islands' or 'holes,' ODT of these films typically reflect the underlying phase separation; and the dimension of these features, normal to the substrate, is equal to the interlamellar spacing, L. Two aspects of block copolymer thin films that have not received much attention are discussed in this Ž. paper: 1 pattern formation in symmetric block copolymers under conditions of T) T ; ODT Ž. and 2 phase behavior of thin film symmetric diblock copolymerrhomopolymer mixtures, when T-T .
MRS Proceedings, 2000
Symmetric diblock copolymers undergo a disorder to order transition below a microphase separation... more Symmetric diblock copolymers undergo a disorder to order transition below a microphase separation transition temperature. In this temperature range the structure is characterized by alternating lamellae of thickness L. In thin film geometries, the lamellae are oriented normal to the substrate if there is a preferential interaction between either of the block constituents and the substrate/copolymer or copolymer/vacuum interfaces. Depending on the relation between the film thickness and L, the topography of the film might comprise of holes, islands or spinodal-like structures. We show that in a polystyrene-b-poly(methyl methacrylate) diblock copolymer of molecular weight 20, 000 g/mol, above the microphase separation transition temperature, the topography of the film depends on the thickness. A heirarchy of bicontinuous patterns and holes is observed with increasing film thickness for films thinner than 35 nm. FF1.5.1
European Physical Journal E, May 1, 2002
The substrate is shown to induce substantial ordering in diblock copolymer thin films above the b... more The substrate is shown to induce substantial ordering in diblock copolymer thin films above the bulk order-disorder transition (ODT) where, thermodynamically, a phase mixed state is favored. Initially, uniform films reorganize to form a hierarchy of transient surface patterns and stable film thicknesses that depend on the initial film thickness and on the substrate. Self-consistent field calculations of the free energy of the system for different situations, depending on the relative tendency for the different block components to be attracted to the substrate and/or free surface, provide an explanation of the formation of the stable film thicknesses. A continuum picture proposed earlier by Brochard et al. provides an explanation of the wetting characteristics of this system. In some cases the ordering destabilizes the film so that dewetting occurs (wetting autophobicity), whereas in other cases the surface ordering results in a kinetic stabilization of a film that would otherwise dewet.
American Chemical Society eBooks, Jul 23, 2001
Macromolecules, Jul 16, 2002
Advances in Patterning Materials and Processes XXXVI
Conventional doping of crystalline Si via ion implantation results in a stochastic distribution o... more Conventional doping of crystalline Si via ion implantation results in a stochastic distribution of doped regions in the x-y plane along with relatively poor control over penetration depth of dopant atoms. As the gate dimensions get to 10 nm, the related device parameters also need to be scaled down to maintain electrical activity. Thus highly doped abrupt, ultra-shallow junctions are imperative for source-drain contacts to realize sub-10 nm transistors. Uniform ultra-shallow junctions can be achieved via monolayer doping, wherein thermal diffusion of a self-limiting monolayer of dopant atomcontaining organic on Si surface yields sub-5 nm junctions. We have extended the use of organic dopant molecules in the monolayer doping technique to introduce a new class of spin-on polymer dopants. In effect, these new spin-on dopants offer a hybrid between the monolayer doping technique and traditional inorganic spin-on dopants. We have been able to uniformly introduce p- and n-type dopants with doping efficiencies comparable to the monolayer doping technique. Control over junction depth can be easily achieved via optimizing annealing temperature and time. Concurrently, sequestering the dopant precursors within the cores of block copolymer micelles allows us to achieve precise control over the spatial positions of dopant atoms in all three dimensions owing to the high periodicity of block copolymer domains on the 10-100 nm length scale.
Advances in Patterning Materials and Processes XXXVI, 2019
As devices become ever smaller and more sophisticated, there is also a general need for creating ... more As devices become ever smaller and more sophisticated, there is also a general need for creating high quality defect-free thin coatings of polymers on 3-dimensional wafer topography, for example, for shrinkage of the size of trench openings. To address this challenge, we developed a spin-on polymer brush material, which comprises of a dopant moiety with a universal adhesive dopamine end group. We demonstrate that the polymer coating is highly conformal and free of pinhole defects, even when only a few nm thick, or when coated over high aspect ratio over 200 nm deep trench topography. Our investigations demonstrate that the dopamine end group enables stable sub-10 nm thick conformal coatings on three-dimensional surfaces. Furthermore, on acute 3-dimensional semiconductor topography, the creation of highly doped abrupt, ultra-shallow junctions with three-dimensional control are essential for successful source-drain contacts. In consideration of this need, we extended the above polymer brush concept further by incorporating a suitable implant dopant atom, such as boron, into the monomer structure. After conformal coating and a subsequent rapid thermal annealing process, the dopant atom is driven into the semiconductor substrate underneath the polymer film. This is potentially very useful for uniform all-around doping of 3-dimensional topography such as FinFETs or Nanowire-FETs. A high dopant dosage on silicon substrate with appropriate shallow implant characteristics was demonstrated for the end-functionalized dopant polymer brush, highlighting one of the promising applications of such conformal coatings.
Bulletin of the American Physical Society, 2018
The stability of thin film symmetric diblock copolymers blended with layered silicate nanocomposi... more The stability of thin film symmetric diblock copolymers blended with layered silicate nanocomposites were examined using a combination of optical microscopy, atomic force microscopy (AFM), and X-ray diffraction (XRD). Two cases were examined PS-b-PMMA (polystyrene-b-polymethylacrylate) blended with montmorillonite stoichiometrically loaded with alkyl ammonium ions, OLS(S), and PS-b-PMMA blended with montmorillonite loaded with excess alkyl ammonium ions, OLS(E). XRD spectra show an increase in the gallery spacing of the OLSs, indicating that the copolymer chains have intercalated the layered silicates. AFM images reveal a distinct difference between the two nanocomposite thin films: regions in the vicinity of OLS(S) aggregates were depleted of material, while in the vicinity of OLS(E) aggregates, dewetting of the substrate occurred. We show that the stability of the copolymer/OLS nanocomposite films is determined by the enthalpic driving force associated with intercalation of the copolymer chains into the galleries of the modified OLS layers and by the substrate/organic modifier interactions.
Aps Meeting Abstracts, Mar 1, 2000
Thin, non-wetting, liquid films on substrates can become unstable and spontaneously rupture due t... more Thin, non-wetting, liquid films on substrates can become unstable and spontaneously rupture due to long and short-range intermolecular forces. Patterns ranging from discrete holes to bicontinous spinodal patterns, reflecting fluctuations in the local film thickness, are observed. Much of the research on this topic has been devoted to the behavior of simple polymeric fluids. We show that thin symmetric diblock copolymer films at temperatures above the bulk order-disorder transition (ODT) exhibit topographies that depend on the local film thickness. These topographies are transient and initially they range from holes to spinodal structures, depending on the film thickness, h. At long times the structures evolve into large droplets. The rate of evolution toward the droplet stage depends on the film thickness. The surface of films thicker than 35 nm remain smooth and stable indefinitely.
We examined the late stage structural evolution of droplets of a polystyrene-b-poly(methyl methac... more We examined the late stage structural evolution of droplets of a polystyrene-b-poly(methyl methacrylate) PS-b-PMMA diblock copolymer on a PS-b-PMMA brush. The initial droplet size distributions and shape distributions varied appreciably from one sample to another. Coarsening of the droplets occurred, wherein the average droplet size, S, increased with time, S ˜t^b, accompanied by a decrease in the number of droplets per unit area with time, N(t) ˜t^-b. The value of the power law exponent b was found to increase as the size distribution and shape distribution of droplets decreased, from 1/10 to approximately 2/5. The droplet probability distributions, F(s/S) vs. s/S, determined from our data were compared with theoretical probability distributions and the results strongly indicate that the mechanism of coarsening occurs via a, self-similar, dynamic coalescence process, not Ostwald ripening.
ACS Symposium Series, 2001
MRS Proceedings, 2000
ABSTRACTSymmetric diblock copolymers undergo a disorder to order transition below a microphase se... more ABSTRACTSymmetric diblock copolymers undergo a disorder to order transition below a microphase separation transition temperature. In this temperature range the structure is characterized by alternating lamellae of thickness L. In thin film geometries, the lamellae are oriented normal to the substrate if there is a preferential interaction between either of the block constituents and the substrate/copolymer or copolymer/vacuum interfaces. Depending on the relation between the film thickness and L, the topography of the film might comprise of holes, islands or spinodal-like structures. We show that in a polystyrene-b-poly(methyl methacrylate) diblock copolymer of molecular weight 20, 000 g/mol, above the microphase separation transition temperature, the topography of the film depends on the thickness. A heirarchy of bicontinuous patterns and holes is observed with increasing film thickness for films thinner than 35 nm.
Physical Review E, 2002
Thin films of poly͑styrene-b-methyl methacrylate͒ diblock copolymers above the bulk order-disorde... more Thin films of poly͑styrene-b-methyl methacrylate͒ diblock copolymers above the bulk order-disorder transition temperature, within a certain thickness range, are structurally unstable on SiO x /Si substrates. They dewet autophobically, forming droplets on a self-assembled brush. We investigated the late-stage evolution, coarsening, of the droplets on the brush. The average droplet cross-sectional area ͗S͘ increased with time, ͗S͘ϰt ␥. This was accompanied by a decrease in the number of droplets per unit area with time, N(t)ϰt Ϫ␥. We analyzed the droplet size distribution, F(S/͗S͘) vs S/͗S͘, and found that the shape of the curve was virtually identical at different stages throughout the process. This suggests that a structural self-similarity is associated with the process. A comparison of F(S/͗S͘) vs S/͗S͘ data with distributions based on Ostwald ripening and coalescence cluster coarsening ͑dynamic and static͒ mechanisms strongly indicates that the dominant coarsening mechanism involved motion of droplets across the brush and subsequent coalescence, i.e., dynamic coalescence coarsening, not Ostwald ripening.
Macromolecules, 2000
The stability of thin film nanocomposites on silicon substrates, formed from mixtures of a symmet... more The stability of thin film nanocomposites on silicon substrates, formed from mixtures of a symmetric diblock copolymer blended with layered silicate nanocomposites, was investigated using a combination of optical microscopy, atomic force microscopy (AFM), and X-ray diffraction (XRD). Two ...
Macromolecules, 1999
Thin symmetric poly(styrene-b-methyl methacrylate) (PS-b-PMMA) diblock copolymer films at tempera... more Thin symmetric poly(styrene-b-methyl methacrylate) (PS-b-PMMA) diblock copolymer films at temperatures higher than the bulk order-disorder transition temperature TODT are shown to dewet silicon substrates, forming topographical features that depend on the initial film thickness h. Films of thickness h < 3.5 nm dewet the substrate, forming bicontinuous spinodal-like patterns. When 3.5 nm < h < hL) 7 nm, discrete holes are observed randomly throughout the film. For films of thickness in the range hL < h < hH) 35 nm, the copolymer exhibited autophobic behavior, whereby the top layer of thickness (h-hL) dewets a dense "brush" of ordered copolymer of height hL anchored to the silicon substrate. The morphologies, which include a bicontinuous spinodal pattern for films of thickness in the range hL < h < 19 nm, and discrete holes, for films of thickness in the range 19 nm < h < hH, eventually evolve into droplets. Films of h > 35 nm remained stable, with smooth surfaces. The time-dependent evolution of the "spinodal" structures that evolve in the autophobic regime is discussed. In addition, the existence of surface-induced ordering of the copolymer at temperatures above the bulk TODT is also discussed.
Langmuir, 2003
We examined the late-stage structural evolution of droplets of a polystyrene-b-poly(methyl methac... more We examined the late-stage structural evolution of droplets of a polystyrene-b-poly(methyl methacrylate) PS-b-PMMA diblock copolymer on a PS-b-PMMA "substrate". The initial droplet size distributions and shape distributions varied appreciably from one sample to another. Coarsening of the droplets occurred, wherein the average droplet size, 〈S〉, increased with time, 〈S〉 ∝ t , accompanied by a decrease in the number of droplets per unit area with time, N(t) ∝ t-. The value of the power law exponent was found to increase as the size distribution and shape distribution of droplets decreased, from 1/10 to approximately 2/5. The droplet probability distributions, F(S/〈S〉) vs S/〈S〉, determined from our data were compared with theoretical probability distributions and the results strongly indicate that the mechanism of coarsening occurs via a self-similar, dynamic coalescence process, not Ostwald ripening. The differing exponents appear to be associated with quantitative differences between the details of the coarsening process as the droplets become smaller with a narrow shape distribution. We suggest evidence of a possible dimensional crossover from 3D to 2D dynamics.
Langmuir, 1999
We examined the dynamics of dewetting of a thin symmetric diblock copolymer film on a substrate a... more We examined the dynamics of dewetting of a thin symmetric diblock copolymer film on a substrate above the bulk order-disorder transition temperature, TODT, of the copolymer using atomic force microscopy. The dewetting mechanism proceeded with the formation of discrete holes without their characteristic peripheral rims. During this early stage, the hole radii R increased exponentially with time. This stage was followed by a narrow intermediate regime where the rim develops and R ∼ t. When the rim was fully developed, R increased as t 2/3. The shape of the rim was highly asymmetrical and its width L increased as t 1/2. At the final stage of the process, droplets of the copolymer, a few microns in diameter and with heights on the order of tens of nanometers, existed on a dense copolymer "brush" of uniform thickness 7 nm anchored to the substrate. This clearly indicates that the process is autophobic, a phenomenon first documented in small molecule liquids.