Session B45: Focus Session: Thin Film Block Copolymers - Swelling and Ordering

Effect of Solvent Removal Rate on the Morphology of Solvent Vapor Annealed ABA Triblock Copolymer Thin Films

Solvent vapor annealing (SVA) treatments can be used to kinetically trap unique self-assembled nanostructures in block copolymer thin films that are not achievable by traditional thermal annealing methods. In this work, we kinetically trapped the thin film morphologies of a cylinder-forming ABA triblock copolymer at key points during the solvent removal process following SVA in order to gain insight into the re-ordering mechanisms associated with solvent removal. Specifically, we identified morphology transformations as a function of solvent removal rate, showed that the mechanism for cylinder reorientation involved the propagation of changes at the free surface through the film as a front, and validated a film etching scheme to image the through-film morphology using successive ultra-violet ozone (UVO) etching steps followed by atomic force microscopy (AFM). This facile real-space analysis of the thin film internal structure is more easily implemented in comparison to cross-sectional imaging. The results and methodology of our work are significant not only for improving our understanding of block copolymer thin film self-assembly, but also for tailoring solution processing methods to fabricate nanostructured materials (e.g., for nanotemplate and membrane applications) in general.   

Obtaining Perpendicular Block Copolymer Morphologies with Solvent Annealing

Being able to control block copolymer (BCP) thin film morphology and orientation is of interest for lithographic applications where creation of feature sizes ranging from 10-100nm is desirable. Perpendicular oriented cylinders and lamellae are especially valuable due to their high aspect ratios but are difficult to achieve in BCP systems with a large Flory-Huggins interaction parameter ($\chi )$. We explore the morphological phase behavior that films (30-200nm) of poly(styrene-b-dimethylsiloxane) (PS-PDMS, 45kg/mol, $\chi $=0.26) exhibit under different solvent conditions with focus on conditions that produce perpendicular microdomains. The microdomains are revealed by selectively etching the PS with an oxygen plasma (50W CF4). Variation in the solvent vapor conditions results in selective swelling of the different blocks of the copolymer depending on the relative Hildebrand solubility parameters (e.g. PS- 18.5, toluene-18.3 MPa$^{1/2}$), affecting the microdomain morphologies, and the solvent evaporation and deswelling process influences the orientation of the microdomains. Two different strategies are presented involving solvent vapor annealing that result in perpendicular morphologies in films of PS-PDMS and the results are compared with self-consistent field theory modeling of solvent-polymer systems.   

Mesoscale Dynamics of Solvent Evaporation in Block Copolymer Thin Films

Block copolymer thin films are being investigated for a wide variety of applications ranging from separation membranes, organic photovoltaics, and lithographic masks. In order to accelerate defect annihilation in the periodic structures that develop within these films, solvent annealing techniques are often employed that exploit control over solvent atmosphere to modify the free surface thermodynamics and evaporation rate in an attempt to influence the alignment of ordered domains. The inherently non-equilibrium nature of this problem complicates standard theoretical treatments based on relative free energy calculations, so we have employed a dynamical extension of Self-Consistent Field Theory coupled with a solvent evaporation mechanism to gain insights into the interplay between component-surface interactions, evaporation rate, and observed film morphology. The effects of these factors on the micro-phase separation trajectory of the film will be discussed.   

Optimizing the Combination of Solvent Annealing and Directed Self-Assembly in Thin Film Block Copolymer Systems Using Self-Consistent Field Theory

We show how self-consistent field theory simulations can be used to optimize the combination of solvent annealing and directed self-assembly using topographical and chemical templating in order to achieve an ultimate goal of arbitrary pattern generation in thin film block copolymer systems. The simulations use a combination of field boundary conditions to model topographical features such as posts and chemically distinct surfaces along with a variety of compositions to model a range of solvent, homopolymer, and block copolymer multi-component blends [Macromolecules 2010, 43, 8290--8295]. Computational results are compared with experimental systems that use polydimethylsiloxane, polystyrene, and polyferrocenylsilane polymers, heptane, toluene, and chloroform solvents, and electron beam lithography created hydrogen silsesquioxane posts. By varying the different polymers used and thus their $\chi $ interaction parameters, the relative fractions of solvent and different polymer blocks, and surface feature affinity and shape, the optimal requirements to create arbitrary complex features for nanolithography applications will be demonstrated.   

Sub-10 nm block copolymer patterns with mixed morphology and period using electron irradiation and solvent annealing

High resolution patterns with controllable period and feature geometry are of intense interest for nanolithography applications, but to date this has been challenging to accomplish from a single block copolymer, which produces patterns of a fixed period and morphology. Here we show how patterns consisting of coexisting sub-10 nm spheres and cylinders and sphere patterns with a range of periods can be created using a combination of serial solvent anneal processes and electron-beam irradiation of selected areas of a film of poly(styrene-block-dimethylsiloxane). We also addtionally combined with topographical templates consisting of either removable polymeric regions to make patterns with highly aligned lines, dots and featureless regions, or post arrays which can simultaneously align and register the line and dot arrays. These techniques offer the possibility of forming a wide range of aperiodic pattern geometries including single lines or ordered line segments, and significantly extend the ability of block copolymer lithography to produce patterns essential for nanoscale device fabrication.   

Raster solvent vapor annealing of block copolymer thin films

Nanoscale phase separation in block copolymer (BCP) thin films makes them attractive for a variety of applications such as membranes, organic electronics, and nanoscale templating. Annealing BCP thin films (thermal or solvent) promotes ordering of their microphase-separated structures into useful patterns. Solvent vapor annealing (SVA) is an attractive approach as it avoids thermal degradation and provides greater flexibly in morphology control compared to thermal annealing through overall film swelling and preferential swelling of one or more blocks. SVA is typically conducted in an annealing chamber that permits exposure of the film to a controlled vapor environment. However, control over the vapor environment locally may be more desirable for nano-patterning applications. In this work we introduce raster solvent vapor annealing, a method that gives us precise control over the annealed region and excellent point control of the resulting morphology allowing for specific mixed morphologies on a single sample. The simplest setup, needle directed solvent vapor with controlled solvent flow rate and rastering speed, allows ``drawn'' ordered parallel and perpendicular cylinders in a cylinder forming BCP thin film. The efficacy of this method will be discussed in the oral presentation.   

Confined drying of copolymer solutions

We developed a simple tool for the rapid screening of phase diagrams of polymer and surfactant solutions. Our technique is based on the controlled drying of a droplet solution in a confined geometry. A $\mu$L-sized droplet of an aqueous solution is confined between two wafers (diameter 3 cm), separated by a controlled thickness ($\approx$ 150 $\mu$m). The confinement casts a well-defined timescale to the drying kinetics, mainly governed by the wafer area. Indeed, water removal only occurs through a diffusive process from the edge of droplet to the edge of the wafer. Confinement also permits a simple 2D description, and allows simple observations of the drying. We studied the drying of an aqueous solution of a tribloc copolymer (Pluronics, P104) thanks to three different techniques: polarized microscopy, fluorescent microscopy, and Raman imaging. With our tool and techniques, we not only build an accurate phase diagram of the solution (with one microliter only) but also measure both the mutual diffusion coefficient and the activity of the solution as a function of its concentration, including the Flory-Huggins parameter.   

Rapid Self- Assembly and Perpendicular Alignment in lamellar PS-b-PEO System for Fabrication of Sub 20 nm Nanolithography Templates

Creating perpendicular alignment in lamellar block copolymer systems has considerable industrial and commercial significance. In general, these lamellar systems require careful interface engineering to obtain vertical orientation of the blocks. To avoid the strong preferential adsorption of one block to either the substrate or the polymer/air interface, the surface must be `neutralised' by chemical brushes or external forces e.g. solvent fields. Reported here is a stepwise thermo/solvent annealing process called ``combinatorial annealing'' allowing the formation of perpendicular domains of polystyrene-b-polyethylene oxide (PS-b-PEO) lamellar structures while avoiding brush or other surface modification. The Thermo/solvent annealing is done in a commercial microwave reactor and perpendicular alignment is observed within a few minutes. This BCP has a very This BCP has a small minimum feature size, relevant to the fabrication of nano-features in electronic devices and results are presented here.   

Controlled Deposition of Ordered Block Copolymer Thin Films by Electrospray

Electrospray offers a potentially useful platform for the controlled delivery of a variety of materials, but surprisingly, its application to block copolymer thin film deposition remains unexplored. Here we show that under appropriate conditions, well ordered films of PEO cylinder-forming poly(styrene-b-ethylene oxide) may be continuously deposited by electrospray. Ordered film formation is predicated on fast thermal equilibration relative to rate of deposition. We conduct time-resolved observations and investigate the effects of process parameters that underpin film morphology including solvent selectivity, substrate temperature and flow rate of the electrospray feed solution. For the particular system studied, we uncover a wide temperature window from 90$^{o}$C to 160$^{o}$C and an ideal flow rate (2$\mu$L/min) for ordered film growth, but no strong influence of solvent selectivity was observed. PEO cylinders were observed to align with their long axes perpendicular to the substrate at optimal spray conditions.   

Two distinct timescales in the ordering of symmetric diblock copolymer films

At equilibrium, an ordered symmetric diblock copolymer film forms lamellae parallel to the substrate interface. Furthermore, unless a film's thickness is exactly commensurate with the intrinsic height of the layered structure, the free surface will break up into holes or islands with a thickness corresponding to one lamellae. This ensures that the preferred lamellar spacing of the film is achieved while volume is conserved. We study the internal dynamics of ordering as a lamellar forming thin film transitions from the disordered state to its equilibrium morphology. In particular, before the free surface nucleates to form islands or holes, a film must begin by ordering internally. Using ellipsometry, we measure small changes in the internal film structure as the diblock orders prior to any change of the free surface topology. We probe two distinct timescales along the pathway to an equilibrium state: 1) An initial ordering time where molecules begin to align and form lamellae; and 2) an incubation time where the structure remains constant before nucleation of holes or islands. We will show the effect that film height and commensurability has on these two timescales, allowing us to better understand the effect of confinement on the ordering dynamics of lamellar forming thin films.   

The hydration and ordering of lamellar block copolymer films prior to the formation of polymer vesicles

Polymersomes -- vesicles based on self-assembled bilayers in turn composed of amphiphilic copolymers -- are good candidates for molecular delivery systems; hydrophilic molecules can be enclosed within the aqueous core, to be released by a trigger, which disrupts the vesicle's wall. The key to the use of these polymer vesicles as effective molecular delivery system is in the ability to efficiently encapsulate a molecular payload within the vesicle. To understand the formation mechanism of polymer vesicles via the thin film rehydration method, we have evaluated the hydration and ordering of PEO-PBO diblock copolymer thin films in a controlled water vapor atmosphere. We have performed Neutron Reflectivity, Ellipsometry and Atomic Force Microscopy measurements during the hydration process. These results show that the film swells slowly in the initial stage. It then swells rapidly at a certain critical point and makes ordered structure at the same time. The lamellae are gradually oriented parallel to the substrate with increasing water absorption.   

Effect of thickness on microdomain orientation in shear-aligned, cylinder-forming PS-PHMA thin films

Previous work has shown that application of a shear stress can impart well-defined orientational order to the microdomains of cylinder-forming poly(styrene)-b-poly(n-hexylmethacrylate) (PS-PHMA) thin films. Unlike many block copolymer thin films, PS-PHMAs tend not to form terraces (islands/holes). As a result, film thickness plays an important role in determining the orientation of the cylindrical microdomains. Here we study the effect of film thickness on the morphology of aligned and unaligned PS-PHMA films. Films with a gradient in thickness were generated, via flow coating, and then imaged using atomic force microscopy to examine the microdomain morphology before and after shear alignment. As a function of thickness the unsheared film morphology oscillated between dots (either spheres or cylinders oriented perpendicularly to the substrate) and lines (cylinders oriented parallel to the substrate), with the highest fraction of lines occurring at film thicknesses corresponding to an integral number of cylinder domain spacings. For thicknesses larger than three domain spacings, the oscillation ceased and complete coverage by line patterns was observed. Once shear-aligned, the thicknesses that exhibited the largest fraction of lines pre-shear, showed the highest quality of alignment post-shear.   

GISAXS simulation and analysis on GPU clusters

We have implemented a flexible Grazing Incidence Small-Angle Scattering (GISAXS) simulation code based on the Distorted Wave Born Approximation (DWBA) theory that effectively utilizes the parallel processing power provided by the GPUs. This constitutes a handy tool for experimentalists facing a massive flux of data, allowing them to accurately simulate the GISAXS process and analyze the produced data. The software computes the diffraction image for any given superposition of custom shapes or morphologies (e.g. obtained graphically via a discretization scheme) in a user-defined region of k-space (or region of the area detector) for all possible grazing incidence angles and in-plane sample rotations. This flexibility then allows to easily tackle a wide range of possible sample geometries such as nanostructures on top of or embedded in a substrate or a multilayered structure. In cases where the sample displays regions of significant refractive index contrast, an algorithm has been implemented to perform an optimal slicing of the sample along the vertical direction and compute the averaged refractive index profile to be used as the reference geometry of the unperturbed system. Preliminary tests on a single GPU show a speedup of over 200 times compared to the sequential code.   

Modeling Line Edge Roughness in Lamellar Block Copolymer Systems

Block copolymers offer an appealing alternative to current lithographic techniques with regard to fabrication of the next generation microprocessors. However, if copolymers are to be useful on an industrial manufacturing scale, they must meet or exceed lithography specifications for placement and line edge roughness (LER) of resist features. Here we discuss a field theoretic approach to modeling the LER in the lamellar phase of a strongly segregated block copolymer system. Our model is based on the Leibler-Ohta-Kawasaki free energy functional, which takes the Flory-Huggins parameter and index of polymerization as inputs. We consider a domain with a finite number of phase separated microdomains; at the system boundary, we apply conditions akin to a chemical pinning field. Using a path integral formalism, we determine how fluctuations of the microdomain boundaries depend on distance from the system boundary, number of microdomains, the Flory-Huggins parameter, and index of polymerization.