Block copolymers have attracted intensive interest in the worldwide since their ability to form self-organized nanostructures.1,2 When the strength of the repulsive interaction between blocks is sufficient, block copolymer, which consists of two or more chemically different chains covalently linked together at one end, leads to microphase separation of dissimilar polymer chains into periodic domains.1 The typical domain periodicity range from 10 to 50 nm, which were considered as a span in which the requirement of next generation fabricating materials could be met.3
1.1.1 Block copolymer self-assembly
The phase separation behavior and self-assembled nanostructure of diblock copolymers has been comprehensively studied in theoretical perspectives.4,5 In brief, the domain periodicity of microphase separation (d) scales as d ~ aN2/3 ??1/6.3 ?? refers to Flory-Huggins interaction parameter and N is the degree of polymerization. Reducing N is the most plausible method to decrease the value of d because d is more strongly function of N than ??. However, the possibility of block copolymers to phase separates into periodic structure is determined by the repulsive interactions of each blocks which is characterized by the product ?? N. 3,6 Microphase separation can occur when the value of ?? N larger than the critical value for order-disorder transition (10.5 for symmetric diblocks).1,3,7 Thus, it is crucial to pursue block copolymers that have small N and high ?? values to minimize self-assembled feature sizes. In diblock copolymers, the morphology of nanostructure ranges from spheres to lamellar (shown in Figure 1-1) depending on the volume fraction of one block.
Figure 1-1. Schematics of diblock copolymer self-assembly structure in which the A-B type diblock copolymer is depicted as a two-color chain. The morphology is determined by the volume fraction of the polymer block (fA). Prog. Polym. Sci. 2007, 32, 1152-1204.
Traditional organic-organic BCPs such as Polystyrene-b-Poly(methyl methacrylate) (PS-b-PMMA) could readily form periodic structure with d-spacing at sub-45 nm scale.8 By replacing the component with strong phase-segregation interactions is a well-known approach to fabricate nanostructure with even smaller feature size.3,6,7 In addition, changing molecular architecture provides an alternative way to further shrink the domain periodicity. Hawker et al. have previously found by changing the architecture from linear to cyclic PS-b-PEO can yield cylindrical morphologies with d-spacing 19.5 nm.9
Furthermore, previous research has shown that incorporating a sufficient amount of silicon or metal into one block creates periodic structure with feature size below 20 nm. In addition, organic-inorganic BCPs such as poly(lactide-b-dimethylsiloxane-b-lactide) (PLA-b-PDMS-b-PLA),10 poly(styrene-b-dimethylsiloxnae) (PS-b-PDMS)11 and poly(styrene-b-ferrocenylsilane) (PS-b-PFS)12 exhibit high etching contrast due to their incorporated inorganic components.
1.1.2 Block copolymer lithography
In the last five decades, Moore's Law has successfully provided a roadmap for integrated circuits industry. Photolithography, which is widely used in semiconductor industry, can reach the scale as small as 45 nanometers with acceptable precision. However, according to the Rayleigh equation, the feature structure fabricated by photoresist is limited by the wavelength. To further decrease the scale to meet the requirement of industry to get sub-22 nm silicon structure, traditional photolithography reached physical limits. 8
Electron beam lithography and other top-down alternative methods are both costly and time-consuming.3,8 For example, creating a bit-patterned template with a 1T bit/in2 dot array by e-beam lithography is estimated to require a cost exceeding 1 million dollars and more than a month of continuous writing.3 Thus, the development of bottom-up methods, in which molecules self-assembled into well-defined structures, is being pursued intensively.
In recent several decades, block copolymers are emerging as new candidates for sub-22 nm lithography to meet the requirement of microelectronic industry. The self-assembly of BCPs provides an economic and versatile avenue to fabricate nanopatterns at large area.1-4
Figure 1-2. (A) Schematic of a nanolithography template consisting of a monolayer of PB spherical microdomains on silicon nitride (Si3N4) substrate. (B) The processing line when an ozonated copolymer film is used, which produces holes in the substrate. (C) Schematic of the processing flows when an osmium-stained copolymer film is utilized, which produces dots in Si3N4. Science 1997, 276, 1401-1404.
However, the essential of lithography lies on the pattern transfer. The pattern transfer from block copolymer to underlying substrates was first demonstrated by Park et al. 13 Spin-coating was utilized to prepare poly(styrene-b-butadiene) and poly(styrene-b-isoprene) thin films in which well-organized spherical or cylindrical nanodomains were formed and used as templates. The nanopatterns in thin films were transferred directly to the underlying substrates by two different techniques that resulted in holes and dots patterns. Dense arrays of holes and dots with hexagonally ordered periodic structure have been successfully fabricated in the underlying substrates. This process opens a versatile route for nanometers scale surface patterning by utilizing synthetic materials, which exhibit promising application in semiconductor lithography.
Furthermore, the orientation control of the microphase nanostructure in block copolymer thin films is essential to their utilization in BCP lithography. In contrast to the self-assembly in the bulk samples, the morphology in thin films strongly depends on the surface and interfacial interaction as well as the commensrability between the thin film thickness h and the period of the microdomain L0. Strong preferential interactions of one block with the substrate or a lower surface of one component cause segregation of the block to either the interface at substrate or the surface of the thin film, respectively.
Netural layer has been developed by Hawker atnd coworkers in order to solve this problem. A random copolymer, end-functionalized random copolymers of P(S-r-MMA), was synthesized and employed to prepare thin films with thickness about 20 nm.14 Neutral layers have been proved to be a effective way to tune polymer-surface interaction therefore to control the orientation. Also, solvent annealing is another effective method to manipulate the orientation of nanostructure in block copolymer thin films. Solvent evaporation could be used to induce the ordering and orientation of nanodomain. In the solvent annealing system, solvent acts as plasticizer, which enhances the formation of well-organized nanodomain at ambient temperature.
Recently, polarity-switching top coats were developed by Willson's group to mitigate interfacial forces of high block copolymers which is quite challenging to prepare well-defined thin films due to the disparate interfacial energy of each block.15 Top coats were applied to the lamellar-forming block copolymers poly(styrene-b-trimethylsilystyrene-b-styrene) and poly(trimethylsilystyrne-b-lactide) which were thermally annealed to produce orientation-directed features with line width 15 and 9 nm, respectively.
Figure 1-3. (A) AB diblock copolymer forming lamellar pattern with three different orientations. (B) The molecular structures of the two top coats. Science 2012, 338, 775-779.
1.1.3 Directed self-assembly of block copolymer thin films
Fabrication of nanopattern with long-rang order, regular domain size, as well as placement accuracy is vital to the application of block copolymers in semiconductor industry. Defects may still exist in block copolymer thin films, even after long-time thermal/solvent annealing. Thus, directed self-assembly (DSA) which refers to the integration of traditional manufacturing process of photolithography with the self-assembling materials was developed to meet this grand challenge. 16,17 The key techniques of DSA is to take advantage of the self-assembling properties of materials to reach nanoscale dimensions and, at the same time, provides a method to generate nanostructures with high spatial resolution and excellent placement accuracy. Chemical registration16 and graphoepitaxy17 are major methods that are widely investigated both in academic and industrial fields.
Figure 1-4. The strategy of the chemical registration to create precisely aligned lamellar pattern at large area. (a) A SAM of PETS was deposited on a silicon wafer. (b) Photoresist was spin-coated on the SAM, and (c) The alternating lines with spaces of period Ls were patterned by EUV-IL. (d) The nanopattern in the photoresist was converted to a chemical pattern on the surface of the SAM by irradiating the sample with soft X-rays in the presence of oxygen. (e) The photoresist was then removed with repeated solvent washes. (f) A symmetrics, lamella-forming PS-b-PMMA copolymer of period L0 was spin-coated onto the patterned SAM surface and (g) annealed, resulting in surface-directed block copolymer morphologies. Chemically modified regions of the surface presented polar groups containing oxygen and were preferentially wetted by the PMMA block, and unmodified regions exhibited neutral wetting behavior by the block. Nature 2003, 424, 411-414.
Paul Nealey and coworkers16 demonstrated the chemical registration methods to achieve well-registered nanostructure with exact placement accuracy in flat substrate with alternating hydrophobic and hydrophilic stripes (shown in Figure 1-4). A thin film of PS-b-PMMA was casted on the chemical different surface and then annealing was conducted to direct the self-assembly of block copolymers to fabricate well-registered perpendicularly oriented lamellar structure over large area. While, graphoepitaxy provide another approach to solve this problem. Templates with various length scale topographical features were proved to be a good method to control the self-assembly behavior of block copolymers. Moreover, the width of the topographical pattern (Ls) may be tens or hundreds time of the period of block copolymers. Segalman et al.17 reported the fabrication of long-range order sphere arrays by using topographically patterned substrates. A monolayer of poly(2-vinylpyridine) was grafted on the underlying patterned substrates which was obtained by photolithography. Next, PS-b-P2VP was casted on the substrates and then was annealed to generate well-ordered arrays at several micrometer scales. In brief, the orientation control as well as directed self-assembly techniques provide a versatile avenue to regulate the self-assembled nanostructure which is essential to the application of BCP lithography in semiconductor industry. Currently, the first full-scale commercial semiconductor production line using DSA technology is being developed by the industry.
Figure 1-5. (a) SFM of PS-b-PVP film on top of a mesa in which a single crystal is formed. b) The FFT pattern indicates that the grain is ordered in a single crystal with hexagonal symmetry. c) Schematic illustration of the molecular arrangement of PS-b-P2VP in trenched pattern. Adv. Mater. 2001, 13, 1152-1155.
1.1.4 POSS-containing block copolymers
As we mentioned above, organic-inorganic block copolymers exhibit high Flory-Huggins parameter (??) and thus eventually leads to smaller feature sizes could be obtained by decreasing the degree of the polymerization (N). 3 Among of organic-inorganic BCPs, polyhedral oligomeric silsesquioxnae (POSS) -containing block copolymers have drawn attentions from world wide since their extraordinary etching contrast and compatibility with the underlying Si wafer.18,19 POSS exhibits a well-defined molecular structure with the formula (RSiO3/2)n, in which organic substitutes R are attached to a silicon-oxygen cage.18 This intramolecular organic-inorganic hybrid structure endows POSS with extraordinary oxygen reactive ion etching (O2-RIE) resistance. 18
Figure 1-6. Schematic illustration for the fabrication of nanopattern by POSS-containing block copolymers. PS-b-PMAPOSS and PMMA-b-PMAPOSS were spin coated onto silicon substrate and exposed to solvent vapors to induce vertical orientation of the lamellar and cylinder domains. After oxygen plasma etching, silicon oxide lines and pore patterns were formed. Adv. Mater. 2009, 21, 4334-4338.
In the previous research, PS-b-PMAPOSS and PMMA-b-PMAPOSS were synthesized and their self-assembled structures were investigated. Results show that POSS-containing BCPs could form various periodic structures such as lamellar, cylinder and cubic structure at sub-20 nm scale.18-19 By utilizing spin-coating and thermal/solvent annealing techniques, POSS-containing BCP thin films were obtained. Hexagonal packed dots (from cylindrical or spherical domains) and periodic line patterns (from lamellar or cylindrical domains) at 10 nm scale could be fabricated in these BCP thin films. To meet the requirement of semiconductor industry, lithography materials with periodicity less than 10 nm should be developed in the near future. By reducing the molecular weight of PMMA-b-PMAPOSS combined with chemical registration and thermal annealing, the feature size of nanostructures could be decreased as small as 9.7 nm still with highly ordered periodic structure. 21,22
However, POSS-containing polymer meets its physical limits at sub-10 nm scale since the repulsive forces between each segment cannot provide strong driving force to form well-defined periodic structure. Thus, a new approach needs to be explored to meet this grand challenge.
1.2 Giant molecules
It is well known that a variety of amphiphilic low-molecular-weight molecules or oligomers can form a variety of long-range ordered supramolecular self-assembled structures on the nanometer scales, especially in less than 10 nm.23 Furthermore, phase-segregated structures of crystals or liquid crystals such as layered, 24 columnar, 25 cubic 26 and other types of complex structures, which two or more incompatible segments in the molecules must undergo microphase segregation, are similar in morphology to those of the self-assembled structures of BCPs at a single nanometer scale. The only difference is that, in the former case, the self-aggregated length in domains is larger. However, in general, they usually lack the required interdomain dry etching contrast because the molecular structure has not designed for further lithographically fabrication as seen in BCP lithography materials. Therefore, new materials must be developed to form well-defined sub-10 nm scale nanostructures and simultaneously to adjust for lithographically fabrication process.
Giant molecules are emerging as alternatives of block copolymers to fabricate nanostructure with smaller feature in recent years.27-30 Herein, giant molecules refer to oligomers with precisely defined chemical structures that serves as building elements for supramolecular self-assembly. In contrast to block copolymers, giant molecules were synthesized via organic chemical reaction instead of living/controlled polymerization.30 Giant molecules with precisely defined chemical structures have been proposed to present new approaches to fabricate engineering hierarchical structures with sub-10 nm features sizes and sharp boundaries which is difficult for traditional diblock copolymers.27 This class of materials is designed to bridge the gap between the two traditional self-assembling materials and possesses advantages of both with a domain periodicity ~ 10 nm.
1.2.1 Nano-building blocks for giant molecules
Molecular nanoparticles (MNPs) refer to shape- and volume-persistent nano-objects with exact molecular structures and specific symmetries, which have been utilized as nano-building blocks for the precisely synthesis of giant molecules. 30 Fullerenes, 29, 31-33 polyhedral oligomeric silsesquioxane (POSS) 27, 28, 30 and polyoxometalates (POMs) 34,35 are typical MNPs.
Figure 1-7. Chemical structures of (a) fullerene, (b) polyoxometalates, and (c) polyhedral oligomeric silsesquioxane.
Among the MNPs, POSS has attracted intensive interest over the last several decades due to its extraordinary structure and properties. Cage silsesquioxane exhibits a well-defined molecular structure with the formula (RSiO3/2)n, 20 in which organic substitutes R are attached to a silicon-oxygen cage. This intramolecular organic-inorganic hybrid structure endows POSS with extraordinary properties such as excellent thermal stability,36 an ultra low dielectric constant,37 and extraordinary oxygen plasma etching resistance.19 In addition, the surface groups can easily be chemically modified, which makes POSS a nearly perfect nanobuilding block for the fabrication of precisely defined giant molecules.30 On the other side, double-decker silsesquioxane (DDSQ), as a novel type of silsesquioxnae, exhibits extraordinary incomplete cage structure, which makes it is possible to prepare mutifunctionlaized silsesquioxnes by corner capping method. 36-38 Double-decker silsesquioxne was utilized as monomers to prepare silsesquioxane-containing main chain polymers. 41,42
It is necessary to carefully control of surface functional groups to prepare functional groups to prepare functional MNPs as building blocks to the construction of giant molecules 30 Site-selective mono-functionalization, regio-functionalization and simultaneous multisite functionalization are among the most common and important functionalization methods for MNPs.20 POSS is usually prepared from the condensation of a silane or silanol precursor. 20 Great efforts were taken to synthesis mono-functionalized and multi-functionalized POSS derivatives by many groups in the worldwide. Laine, Feher and other researchers successfully found novel methods to synthesis and plenty of POSS compounds were obtained. Furthermore, a category of mono-substituted POSS compounds could be purchased from Hybrid Plastics. These commercial available products provide a molecular platform to get various functionalized POSS for the further research.
1.2.2 POSS-containing giant molecules
Stephen Cheng and coworkers proposed the concept of giant molecules and a library of POSS-containing oligomers were synthesized. 27-30 These POSS-containing giant molecules include, but not limited to, giant surfactants, giant shape amphiphiles, and giant polyhedral. Giant surfactants are polymer or oligomer tail tethered to MNPs where two compontents exhibit chemical difference and thus leads to amphiphilicity that resembles low-molecular-weight amphilphiles. 30
A series of POSS-containing giant surfactants were synthesized and their self-assembled structures were reported.27 Polystyrene with narrow molecular weight dispersity was synthesized and incorporated into hydrophilic POSS via click chemistry. The volume fraction could be tuned by the repeat unit of styrene or the numbers of cage silsesquioxane. Various self-assembled nanostructure could be fabricated by these giant surfactants. As it claimed, giant surfactants bridge the gap between small-molecule surfactants and block copolymers and demonstrate a duality of both materials in terms of their self-assembly behaviors. The controlled structural variations of these giant molecules through precision synthesis reveals their self-assemblies are sensitive to primary chemical structures, leading to well-defined self-assembled structure with feature sizes around 10 nm in the bulk and solution state. These findings are thought to provide a versatile platform for engineering nanostructures with sub-10 nm feature sizes.
However, in fact, these POSS-containing giant surfactants exhibit narrow dispersed molecular weight since anionic polymerization is utilized for the synthesis. While, the primary structure of giant molecular is considered to vital to the self-assembled structure, especially when the feature size shrinks to sub-10 nm scale. Thus, precisely synthetic methods should be taken into consideration to replace the living polymerization. On the other side, the feature sizes obtained by the previous work is around 10 nm; the smallest obtained feature size is 8.1 nm, which may be able to be further decreased.
Figure 1-8. Self-assembled structure of POSS-containing giant surfactants. (A) The chemical structure of giant surfactants DPOSS-PS, APOSS-PS, and FPOSS-PS. (B) Phase diagram of DPOSS-PS observed from SAXS and TEM results. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, 10078'10083.
1.3 Brief overview of this research
In this study, we examined a new approach for fabricating and shrinking the feature size of self-assembled nanostructures. The basis of these new materials is the combination of the phase-segregation behavior of a small molecule, and the use of the different property in dry etching resistance to reactive ions. Incorporating the silsesquioxane into the self-assembling small molecules could favor the formation of periodic structures for a nanopatternable material.
Polystyrene, poly(ethylene glycol) and branched long alkyl chains were incorporated into double-decker silsesquioxane and cage silesquioxnae. Various experimental techniques have also been utilized to comprehensively investigate their self-assembled structure in great detail. Also, the influence of tethered chains to the microphase separation, thermal behavior, morphology of self-assembled nanostructure were also demonstrated.
The synthesis and self-assembled behavior of double-decker silsesquioxane (DDSQ)-containing oligomers, DDSQ-polystyrene (DDSQ-PS), DDSQ-poly(ethylene glycol) (DDSQ-PEG) and alkylated DDSQs, are reported in chapter 2.
In chapter 3, a wedge-shaped building block, 3,4,5-tris(octadecyloxy)benzoyl acid, was incorporated into amine-terminated cage silsesquioxane via amidation reaction. The thermal behavior and self-assembled nanostructure of this alkylated POSS were investigated. Results show that the intermolecular interaction of the long alkyl chains of this alkylated cage silsesquioxane could be manipulated to form long-range straight order hierarchical structure with periodicity at 5.3 nm. Moreover, the transmission electron microscopy (TEM) images clearly indicate the cage silsesquioxane molecules are arranged in highly ordered fashion with a 'head-to-head' type bilayer structure.
In chapter 4, a series of alkylated cage silsesquioxanes, were synthesized accords to the method we mentioned in chapter 3. Thermal behaviors and self-assembled structure of these alkylated silsesquioxane were comprehensively investigated. This work demonstrates that by carefully tuning parameters of molecular design such as alkyl chain length and branching number, well-defined lamellar structure with various periodicities can be obtained. Furthermore, the long-range straight ordered lamellar structure with sharp boundaries could be reliably formed in the samples of alkylated POSS derivatives by thermal annealing.
Chapter 5 provides a general conclusion of this research work. In brief, a set of silsesquioxane-containing oligomers and giant molecules were synthesized and their self-assembled structures were investigated. Results indicate that giant molecules provide a versatile approach to fabricate long-range straight order hierarchical lamellar structure with sharp boundaries and sub-10 nm scale periodicities. The feature sizes of self-assembled nanostructure could be precisely controlled by carefully tuning the parameter of the molecular design. As far as we know, the formation of such a long-range ordered lamellar structure with sharp interfacial boundaries is difficult to be achieved by diblock copolymers. In our mind, these findings are not only scientifically intriguing in understanding the principles of self-assembly but also technologically relevant.
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