Session H18: Block Copolymer Thin Films I

Analysis of block-copolymer thin film ordering through a moving thermal zone

Block-copolymer thin films self-assemble into well-defined structures at the nanometer lengthscale. It has been shown that the morphology, orientation, and degree of order resulting from assembly is sensitive to a variety of preparation parameters, including annealing time and temperature, solvent exposure, substrate surface energy, application of electric fields, etc. A moving thermal zone can also strongly affect the ordering. We have shown that relatively ``cold'' zone annealing (CZA) conditions (above the glass-transition but well below the disordering temperature) can induce a preferential orientation of the microdomains in thin films. We further analyze this effect by measuring the ordering through the thermal front, using atomic force microscopy and scattering techniques (reflectivity and GI-SAXS), which are combined to quantify the order and 3D orientational distribution. Zone annealing leads to increased grain sizes and substantially faster coarsening kinetics, as compared to oven annealing. Moreover, the evolution of order through the thermal front constrains models which aim to explain the CZA's ability to induce orientational bias.   

Neutral Parameter Window for Perpendicularly Oriented Block Copolymer Resists Deposited on Organosilicate Substrates with Tunable Surface Energy

Balancing the interfacial interactions of a block copolymer (BCP) with a substrate as well as the free surface can induce the perpendicular orientation of microdomains, allowing the BCP films to serve as templates for nanofabrication. However, it is known that such orientation of microdomains is quite sensitive to the film thickness. In this presentation, we investigated the effect of film thickness on the orientation of microdomains in lamellae-forming P(S-$b$-MMA) thin films placed on thermally cured organosilicate (OS) substrates. For the film thickness of a P(S-$b$-MMA) ranging from 1 L$_{0}$ up to 2.5 L$_{0}$, we varied the surface energy of the OS substrate around the value close to the enthalpically neutral condition by controlling the substrate cure temperature. We will demonstrate the origin of the observed thickness effect by taking into account the increase in surface area at the free surface when P(S-$b$-MMA) films make holes or islands depending on the incommensurable conditions of the BCP film. This analysis allows us to define a more accurate neutral window for the P(S-$b$-MMA) in terms of both substrate surface energy and BCP film thickness.   

Effect of Surface Energetics on Block Copolymer Thin Film Phase Behavior

The development of block copolymer materials for future nanotechnologies requires an understanding of how surface energetics affect block copolymer thin film phase behavior. In this work, we use combinatorial methods to study these effects and to identify transitions in thin film phase behavior and microstructure orientation. Surface energy gradients were created using a vapor deposition technique developed by our group in which cross-diffusion of functionalized chlorosilanes under dynamic vacuum results in a linear gradient in surface energy on a silicon substrate. These gradients were characterized using x-ray photoelectron spectroscopy (XPS) and contact angle measurements. We then cast a thin film of block copolymer on the modified substrates using a flow coating technique. Finally, we used thermal and solvent annealing conditions to affect the surface energy at the free surface. The surface morphology of the films was examined with atomic force microscopy (AFM), and morphological changes across the gradient were found.   

Systematic tunability of self-assembled block copolymer patterns

The morphology and length scale of diblock copolymers (BCPs) are determined by the chain lengths, and therefore to obtain different geometries and feature sizes, polymers with different chain lengths or BCP/homopolymer blends have been employed. Here, we report on the solvent vapor induced tunability of pattern dimension and morphology of thin films of polystyrene-polydimethylsiloxane (PS-$b$-PDMS) BCPs, which provide robust patterns with exceptionally good ordering due to their large interaction parameter.$^{[}$\footnote{ Y. S. Jung \textit{et al.}, Nano Letters 7, pp.~2046-2050 (2007); Science 321, pp. 939 - 943 (2008); Nano Letters 8, pp. 2975-2981 (2008)\par }$^{]}$ Vapor pressure can control the interfacial interaction between the two blocks, and a mixed solvent can manipulate the effective volume fraction of each block. We show both coupled and independent control of the microdomain size and the periodicity by changing the vapor pressure and the mixing ratio of a selective (heptane) and a partially selective (toluene) solvent. We also demonstrate the transformations from spheres to cylinders and from cylinders to perforated lamellar structures by increasing the portion of selective solvent in the vapor. These results are supported by a theoretical model.   

Solvent annealing of Micropatterned PS-b-PEO copolymer films

Solvent annealing of block copolymer thin films have been known as an effective way to control both orientation of microdomains with respect to the surface and their registration into a well ordered periodic lattice structure. We have recently demonstrated hierarchically ordered microdomains in a thin poly(styrene-b-ethylene oxide)(PS-b-PEO) film combined with microcontact printing. The solvent annealing gave rise to well ordered spherical PEO microdomains in large area by the confined dewetting of thin PS-b-PEO films which had been micropatterned on chemically modified surface during solvent annealing. In this presentation, we intentionally prepare a micropatterned dewet film of PS-b-PEO by spincoating a block copolymer solution on a topographic PDMS pre-pattern. Convex lens shaped spherical caps of PS-b-PEO individually located on each PDMS mesa were successfully transferred to a Si substrate by a conventional transfer printing technique. We investigate the effect of solvent on not only film wettability but also formation of hierarchical nanostructures.   

Transition Behavior of Block Copolymer Thin Films

The phase transitions in block copolymers (BCPs), like the order-to-disorder transition, occur when the enthalpic term of free energy of mixing is equal to the entropic term. In thin films, interactions at the substrate/polymer and polymer/air interfaces influence this free energy balance, resulting in a change in the transition behavior. Here, we report on the transition behavior of BCP thin films. The thickness dependence of the transition temperature shows that interfacial interactions enhance the orientation of the lamellar microdomain parallel to the film surface even in 40L0 in thickness, where L0 is the equilibrium period of the BCP in the bulk. In thin film geometry, this phenomenon can be attributed to the fact that a preferential interaction of one component with the substrate leads to an amplification of a periodic variation in the composition and a shift of transition temperature.   

Thin Film Morphology of Diblock and Triblock Copolymers with Bulk Order-Order Transitions (OOT)

The thin film morphology of SEB and SEBS block copolymers that have an OOT in the bulk from cylinders to spheres as the annealing temperature is increased was studied as a function increasing film thickness using AFM and GISAXS. For both SEB and SEBS, the morphology is the same no matter if the film is annealed above or below the bulk OOT. The SEB morphology is governed by the free energy penalty due to chain stretching, showing spheres when the film thickness is less than that of a monolayer of cylinders. The cylindrical morphology dominates when the film thickness is larger than that of a monolayer of cylinders. On the other hand, the SEBS morphology is governed by the free energy penalty due to looping of the midblock at the surface. Spheres require a lower fraction of midblocks to loop at the surface than cylinders, therefore spheres pay a lower free energy penalty due to chain looping and were found for all film thicknesses studied (up to $\sim$100nm).   

UV-Induced Order-to-Order Transition (OOT) in Thin Films of Supramolecular Diblock Copolymer Assemblies Containing 2-(4'-Hydroxyphenylazo)benzoic Acid

Long-range lateral ordering and orientation in block copolymer thin films, which are highly desired for the applications requiring addressability, as in magnetic storage, may be obtained in a controlled way via an order-to-order transition (OOT), i.e. a morphological transition in a microphase-separated system. The photoisomerization of azobenzene results in volume changes that, when integrated into copolymers, can bring about phase transitions that, in turn, by sweeping the light across a surface, will promote long-range lateral ordering, similar to zone-refinement process used to produce large single crystals. We investigated UV-induced OOT in the supramolecule-assembled thin films of 2-(4-hydroxyphenylazo)benzoic acid and polystyrene-\textit{block}-poly(2-vinylpyridine) diblock copolymer. Grazing incidence small angle X-ray scattering demonstrated that phase transition from lamellae to hexagonally packed cylinders occurred at 150 $^{\circ}$C after UV radiation for 1 hour due to a significantly enhanced interfacial fluctuations induced by photoisomerization as evidenced by X-ray Reflectivity. This suggested that UV light can be utilized to control OOT in the supramolecule-assembled thin films and, hence, to fabricate long-range ordered nanostructures, and even smart responsive surfaces.   

Dilute Micelle Arrays in Block Copolymer Thin Films

Thin films of sphere-forming block copolymers are attractive templates for surface patterning and nanofabrication. While the areal density of spheres (micelles) can be adjusted through the diblock's molecular weight, sparse micelle arrays are quite difficult to achieve. Instead, we blend the diblock with matrix homopolymer in the ``dry brush'' regime, which eliminates the ``terracing'' (island/hole formation) present in films of the neat diblock. Furthermore, by choosing a system where the sphere-forming block wets the substrate and/or free surface, we can achieve very sparse micelle arrays without correspondingly extensive homopolymer dilution, by using the film thickness as the control parameter rather than the blend ratio. Specifically, we employ a polystyrene-polyisoprene diblock (PS/PI blocks of 68/12 kg/mol), blended with PS homopolymer; the PI block wets both the free surface and the SiOx substrate. For sufficiently thin films ($<$60 nm for 50 wt{\%} homoPS), all the block copolymer goes to form brush-like layers at the two surfaces, yielding no micelles. For thicker films, sufficient block copolymer remains to form spherical microdomains between the brushes; the areal density of micelles can be continuously tuned via the film thickness. We evaluate this approach by preparing a film with a thickness gradient, and apply a simple model to the measured areal densities of micelles.   

Surface Dynamics of Segregation Layer in Blockcopolymer Films

We have investigated the surface dynamics of supported block copolymer films of poly(styrene)-b-poly(dimethylsiloxane) (PS-b- PDMS) in the spherical phase, i.e., PDMS cores surrounded by PS shells by x-ray photon correlation spectroscopy (XPCS) in grazing angle geometry. The experiment was performed at the beamline 8ID-I in Advanced Photon Source. We found that the PDMS-rich layer near the surface appears at the temperature higher than the glass transition temperature. We applied the modified theory for bilayer model with surface capillary waves on simple viscoelastic liquid films. The viscosity obtained in this study is compared with that from the rheology measurement for bulk.   

Block Copolymer Brushes

Using self-consistent field theory (SCFT), we examine dry brushes of AB diblock copolymer, where the B ends are uniformly grafted to a planar substrate. Four different morphologies are predicted, which are conveniently described as the uniform, stripe, hexagonal, and inverted hexagonal phases on the basis of the patterned formed at the air surface by the A-rich domain. Phase diagrams are calculated for different grafting densities and for different A-segment surface affinities. In contrast to unanchored diblock-copolymer films, the brush system has a much greater tendency to form chemically-patterned surfaces.