Session H45: Focus Session: Thin Film Block Copolymers - Phase Behavior

Controlling the Orientation of Block Copolymer Thin Films with Selective and Neutral Nanoparticles

The bottom up approach using self-assembly of block copolymers (BCP) have been considered as a powerful technique which can resolve the limitations of conventional nanolithograph. For practical applications, the perpendicular orientation of microdomains with respect to the substrate is a prerequisite. However in most cases, one of the domains has a preferential interaction with the substrate and this interaction induces parallel orientation of the microdomains. To overcome the preferential interaction and to obtain vertically orientated BCP microdomains, diverse approaches have been developed. Previously, we synthesized thermally stable core-shell gold nanoparticles using UV cross-linkable BCP and also precisely tuned the surface property of nanoparticles which are selective and neutral to PS and PMMA by adjusting the composition of polymeric ligand. Moreover, we demonstrated the effect of selective and neutral gold nanoparticles on the orientation of PS-PMMA thin films. Herein, we further investigated the effect of selective and neutral gold nanoparticles on the orientation of BCP thin films of lamellar and cylindrical BCP according to the film thickness. The thin film morphologies were characterized with AFM and SEM.   

Assembly and Photo-Induced Disorder in Block Copolymer-Additive Systems

Additives that hydrogen bond selectively to one block of a weakly ordered or disordered block copolymer can drive phase segregation to yield well ordered materials. Here we show that the addition of D- or L-tartaric acid to low molecular weight, weakly segregated poly(ethylene oxide-block-tert-butyl acrylate), PEO-b-PtBA, induces strong segregation and well ordered morphologies as evidenced by Small Angle X-ray Scattering. This strong interaction between enantiopure tartaric acid and the PEO block also suppresses PEO crystallinity at room temperature. While the addition of racemic tartaric acid does not strengthen segregation nor does it suppress PEO crystallization. The UV-exposure of well ordered films of PEO-b-PtBA/tartaric acid blends containing a photo acid generator followed by a post-exposure bake results in the deprotection of the tert-butyl acrylate block to yield poly(acrylic acid) (PAA). Since PAA and PEO are miscible and tartaric acid can interact strongly with either block, the system becomes disorder, resulting in a photo-induced disordering transition which can be exploited to pattern the surfaces. The kinetic behavior of the disordering transition upon deprotection of PtBA to PAA was studied using Grazing-Incidence Small-Angle X-ray Scattering.   

Diblock copolymer morphologies in ultra thin films under shear

We demonstrate that the shear alignment and the shear-induced transitions in sphere-forming diblock copolymer single layer and bilayer films observed experimentally can be explained by cell dynamics simulation, a simple model with a Ginzburg-Landau Hamiltonian. In two layer films the spheres align in various arrangements, like (100) or (110) bcc plains, or transform to cylinders depending on the shear rate and the temperature. We present a nontrivial alignment mechanism of a single layer of spherical domains in shear via slug-like movement of transient cylindrical micelles. In addition, we clarify the formation of the perpendicular cylinders, found in the recent particle based simulation. We also present results on lamellae shearing in ultra-thin films.   

Structure and dynamics of random block copolymers in the bulk and thin films

Using a soft, coarse-grained model and a Lennard-Jones bead-spring model, we study the morphology of random block copolymers in the bulk and in contact with a hard wall that preferentially attracts one component. We show that both coarse-grained models yield similar equilibrium morphologies at intermediate and long length scales, and identify a mapping between the parameters of the two models. For most parameters we observe a disordered, microemulsion-like morphology. We study the single-chain dynamics in the bulk and in contact with a preferential surface. The relaxation times of the soft, coarse-grained model is about two orders of magnitude faster than the Lennard-Jones bead-spring model. In both models the relaxation time increases with segregation but the Lennard-Jones bead-spring model is additionally slowed down by the densification of the local packing at low temperatures. We employ the soft, coarse-grained model to generate starting configurations for the bead-spring model. Then, the bead-spring model is quenched below its glass transition temperature, and we investigate the local mechanical properties of the disordered, yet structured morphology.   

Macrophase Separation of Block Copolymer Blends in Thin Films

The behavior of macrophase-separating blends of symmetric polystyrene-b-poly(methyl methacrylate) block copolymers in thin films on non-preferential surfaces was characterized with respect to the blends' molecular weights and composition. When the molecular weight ratio of block copolymers is at least five, the blends segregate into a domain of nearly pure small block copolymer with a small lamellar period and a domain of about equal volume fraction of each block copolymer with a larger lamellar period. The composition and structure of these blends was determined by analyzing the period and area fraction of each phase from plan-view scanning electron micrographs. The lamellae propagate through the thickness of the film. The use of block copolymer blends has potential in block copolymer lithography as a strategy to pattern different period structures in the same film.   

Phase segregation at the sub-5-nm scale using high $\chi $ Poly(ethylene oxide-b-dimethylsiloxane) copolymers

Silicon containing block copolymers, such as Poly(styrene-b-dimethylsiloxane) (PS-PDMS), have recently received significant attention for nanolithographic applications [Jung et al., Nano Letters 2010, 10, 1000]. PDMS provides indeed a robust and highly selective mask to oxygen reactive ion etching. In addition, the high Flory-Huggins $\chi _{PS-PDMS}$ parameter (about 0.3 at room temperature) favors the segregation of low molecular weight (16 kg/mol) block copolymers (BCPs) into well-organized structures with pitch as small as 17 nm. In an effort to downscale further the size of structures formed by BCP, we decided to turn to copolymers with even higher $\chi $ parameters. Copolymers of Poly(ethylene oxide) (PEO) and PDMS are known to have extremely high $\chi $ parameters (0.4 -1.1) [Galin et al., Macromolecules, 1981, 14, 677], but their bulk and thin film properties have not been investigated in detail. PEO-PDMS BCPs were synthesized by chain coupling \textit{via} a versatile copper-activated azide-alkyne click reaction. The unusually high $\chi $ parameter between EO and DMS allowed strong phase segregation to occur in copolymers with molecular weight as low as 5kg/mol. The full pitch were found to be less than 10 nm and we report on their bulk and thin film characteristics.   

Decoupling Bulk Thermodynamics and Wetting Characteristics of Block Copolymer Thin Films

The incorporation of a random copolymer molecular architecture has been known to induce notable changes in the physical properties, often with commercial implications. In this presentation, the consequences of controlled degrees of epoxidation on both bulk and thin-film properties of poly(isoprene) blocks in poly(styrene-$b$-isoprene) (PS-PI) diblock copolymers and poly(isoprene) (hPI) homopolymers have been studied, where the products after epoxidation are denoted as hPIxn and PS-PIxn. Small angle X-ray scattering and dynamic mechanical spectroscopy were conducted on PS-PIxn to calculate the effective interaction parameters $\chi _{eff}$ between the PS and PIxn blocks in bulk (3-D) while the surface energy of thin-film PIxn (2-D) was estimated based on contact angle measurements on hPIxn and lamellar orientations of thin-film PS-PIxn. A non-linear change with a minimum at the intermediate degrees of modification is observed for $\chi _{eff}$ in bulk whereas thin-film experiments suggest that the surface energy of PIxn increases linearly with epoxidation. This decoupling of bulk and thin-film thermodynamic behaviors is attributed to the different roles that a random copolymer architecture plays in establishing 3-D order versus wetting at a 2-D surface.   

Continuity and connectivity of lamellar-forming block copolymers in thin films

Thin films of block copolymers are emerging as a low-cost lithographic material. In order to effectively utilize this class of materials, the structure of the self-assembled morphologies in thin films must be well understood. In this work, the network structure formed by a lamellar diblock copolymer of polystyrene (PS) and poly(methyl methacrylate) (PMMA) was explored as the compositional symmetry was varied through homopolymer addition. The volume fraction of PMMA was varied from 0.45 to 0.55. The long-range connectivity of the PS and PMMA domains, as well as the branch and endpoint density, was characterized. Increasing the compositional asymmetry of the copolymer system leads to interconnected networks that span arbitrarily large areas, increased branch density, and decreased endpoint density. The network structure for each copolymer system also depends on annealing time, annealing temperature, and surface chemistry of the substrate. Improved understanding of the variability in lamellar morphologies will enable the selection of copolymers, annealing conditions, and surface chemistries to fabricate lithographic masks by self-assembly.   

Theoretical and Experimental Investigations of Contact Hole Shrink using PS-PMMA Block Copolymers

One possible application of block copolymer directed self-assembly (DSA) involves rectification or ``shrink'' of contact holes in a polarity switched photoresist (see, e.g., J. Cheng et al., ACS Nano 8, 4815 [2010]). The block copolymer (e.g., PS-PMMA) undergoes ordering inside the cylindrical hole; the central (PMMA) domain is then etched out so that the hole diameter is effectively reduced. We utilize strong segregation theory (SST) and numerical self-consistent field theory (SCFT) to calculate the phase behavior of the block copolymers as function of their molecular weight and composition, as well as the contact hole diameter and the surface chemistry of the walls. The model predictions were compared with experimental data, and a good agreement was found. The results illustrate how modeling can serve to guide block copolymer selection for DSA contact hole rectification application.   

Tailoring block copolymer morphology via control of topographical surface: A self consistent field theoretic study

It is well known that chemically patterned or topologically complex substrates can direct self-assembly of adsorbed layers or thin films of block copolymers. In this study we have examined the self-assembly of a lamella-forming diblock copolymer guided by topological complexity, namely, substrates composed of trenches with different heights and widths. In general, when the substrate is neutral to both blocks of the copolymer, the perpendicular lamella morphology is obtained. However, when the substrate has a preferred affinity to one of the blocks, a host of novel morphologies including different bi-continuous network structures can be created by judiciously manipulating the trench height and width. Overall, this study clearly demonstrates the impact of this class of simulations in rational design of morphologies in thin multi-component polymeric films with application to technologies such as filtration, and high-surface area membranes.   

Silicon patterning using self-assembled PS-b-PAA diblock copolymer masks for anti-reflective black silicon fabrication via plasma etching

The diblock copolymer of poly(styrene-b-acrylic acid) is a novel self-assembling mask material for pattern transfer applications. This material system has high dry etch selectivity and can produce a variety of feature types and size scales. Different vertical profiles were produced by altering the etch recipes and diblock copolymer or SiO2 mask processing. This patterning technique is used to fabricate antireflective silicon metamaterials that show broadband anti-reflection properties in the visible and infrared wavelength range ($<$5{\%} total reflection). These materials are potentially useful for solar cell and light sensing applications. Similar surface roughening by chemical etch, porous silicon, nanowires and other methods have been used previously to reduce reflectance from material interfaces for photovoltaics and antireflection applications. Unlike these methods, the BCP self-assembled pattern transfer via RIE produces robust patterns that are tunable in both the horizontal and vertical directions without harsh chemicals or expensive catalysts. This simple and rapid process can also be applied to semiconductors other than silicon.   

Assembly of block copolymer films between chemically patterned and chemically homogeneous surface

Many technologically useful block copolymer systems other that poly(styrene-block-methylmethacrylate) are currently not amenable for directed assembly because one of the blocks has a lower surface energy, segregates to the free surface of the film, and disrupts directed assembly of the film (at least with respect to realizing perpendicularly oriented through-film domains) on the underlying chemical pattern. Cross-linkable random copolymer mats were developed as well as methods to deposit them on the surfaces of block copolymer films. The chemistry of these ``top coats'' can be tuned to impart preferential and non-preferential wetting properties towards the blocks of the block copolymer films. The three-dimensional morphology of block copolymers assembled between lithographically-defined chemically patterned surfaces and top coats of varying wetting properties were characterized using specialized sample preparation techniques and cross-sectional scanning electron microscopy. The resulting structures compare favorably with molecular simulations. A primary technological objective of the top coat strategy is to direct the assembly of block copolymer systems that allow for sub-10 nm patterning and perpendicularly oriented domains.