Session L18: Focus Session: Long Range Order in Polymeric Structure and Morphology II

Directed Self-assembly for Lithography Applications

Economics dictated that semiconductor devices need to be scaled approximately to 70 percent linearly in order to follow the pace of Moore's law and maintain cost effectiveness. Optical lithography has been the driving force for scaling; however, it approaches its physical limit to print patterns beyond 22nm node. Directed self-assembly (DSA), which combines ``bottom-up'' self-assembled polymers and ``top-down'' lithographically defined substrates, has been considered as a potential candidate to extend optical lithography. Benefit from nanometer-scale self-assembly features and the registration precision of advanced lithography, DSA provides precise and programmable nanopatterns beyond the resolution limit of conventional lithography. We have demonstrated DSA concepts including frequency multiplication and pattern rectification using guiding prepattern with proper chemical and topographical information generated by e-beam lithography. In addition, we seek to integrate DSA with 193 nm optical lithography in a straightforward manner in order to move DSA from the research stage to a viable manufacturing technology. Recently, we implemented various integration strategies using photolithography to produce guiding patterns for DSA. This new ability enables DSA to be applied to large areas with state-of-the-art lithography facilities.   

Directed Self-assembly of Block Copolymer Thin Films on 2D Chemical Patterns Made by Electro-oxidation Nanolithography

We have studied the wetting and self-assembly behavior of block copolymer thin films on chemical patterns. Carboxylic-terminated, mesh-like patterns were generated on OTS modified silicon wafers by AFM electro-oxidation lithography. The films were pinned on the carboxylic regions due to the strong interaction of the minor component block with the surface which was also found to suppress film dewetting over the unpatterned methyl regions. We have found that the cylindrical microdomains orient normal to the methyl-terminated patterns and remain laterally confined within them. Defect-free, hexagonally packed cylindrical microdomains could be obtained thanks to the ``corralling'' action of the patterns. Point defects arose when the dimensions or shapes of the patterns were not commensurate with the natural packing of the copolymers. Tetragonal packing of microdomains was observed when a square-shaped confinement geometry, with dimension comparable to 2L$_{0}$ (natural period), was used.   

Electric Field-Assisted Dip-Pen Nanolithography on P4VP Polymer Films

Dip-Pen Nanolithography (DPN) has attracted increased attention in recent years for its ability to generate nanometer-scale patterns on solid surface using an `ink'-coated atomic force microscope (AFM) tip. Herein we develop a modified DPN modality for creating nanostructures on Poly(4-vinylpyridine) (P4VP) polymer film, which exploits the mechanical swelling response of the substrate. The underlying working principle consists in delivering acidic ions onto polymer films to very locally trigger the protonation of the polymer film, causing the latter to swell. An AFM tip coated with phosphate buffer solution of pH 4 is used for the patterning process. More importantly, a reliable strategy results when applying an electric field between the AFM tip and polymer substrate to control the protonation process. We demonstrate the capability of the electric field-assisted DPN technique for reproducibly and reliably fabricating nanostructures originated from the swelling response of P4VP polymer. Our study includes a systematic pattern fabrication under different pattering parameters (applied bias and contact force), and provides evidence on the reversible character of the process.   

Orientation Control of Microphase-Separated Domains of Block Copolymer Thin Films Placed on Surfaces with Tunable Roughness

The orientation of microphase-separated domains of diblock copolymer (BCP) thin films deposited on surfaces with controlled roughness was investigated. To generate the controllable surface roughness, either ordered nanoparticle (NP) monolayers or hydrogen silsesquioxane (HSQ) patterns produced by the Atomic Image Projection E-beam Lithography (AIPEL) was realized on the substrate. The AIPEL employed in this study is the E-beam lithographic technique based on the lattice of crystalline materials used as a mask. We controlled the scale of ordered roughness by varying the size of NPs or the lattice image by adjusting the magnification in AIPEL. Furthermore, the shape of HSQ patterns could change from dot arrays to line/spacing patterns by AIPEL. On the surfaces with controlled roughness, we could obtain BCP films with perpendicularly orientated and long-ranged microdomains. The effect of size and shape of the substrate roughness on the orientation and long-range order of BCP microdomains will be discussed.   

Self-Assembly of Block Copolymer on Soft Textured Substrate

Soft textured (or sawtooth) surface was used to generate long-range lateral ordering of block copolymer microdomains (BCPs). The replication of the sawtooth pattern on the surface of reconstructed sapphire was accomplished with polymers, like poly(butylene terephthalate) or polyimide. Thin films of BCPs were prepared by spin-coating onto these soft textured substrates and solvent-annealed to develop long-range ordering of BCP microdomains. The surface facets were used to guide the self-assembly of block copolymers over macroscopic distances into a highly ordered array of densely packed cylindrical microdomains oriented normal to the film surface. We found that the amplitude and pitch of the sawtooth patterns could be adjustable by mechanical elongation of soft textured substrates. The lateral ordering of BCP microdomains on textured surface with different amplitude as well as different pitch distance was also investigated.   

Self-Assembled Polymer Nanostructures Formed from Homopolymers Induced by UV and Solvent Exposure

The fabrication of polymer nanostructures by self-assembly becomes an important lithography patterning technique, and much work has been focused on block copolymers. Here we report an alternative approach to self-assembled polymer nanostructures using homopolymers. It involves a simple process of illuminating a polymer thin film followed by solvent exposure. Periodic patterns were obtained on various substrates, the shapes and sizes of which depending on the molecular weight and solution concentration of the polymer, and the irradiation time. The photochemistry of polymers will be discussed and films characterized. The mechanism of the nanostructure formation will be elucidated.   

Templated Self-Assembly of Highly Ordered Square Arrays from an ABC Triblock Terpolymer

Square-symmetry patterns are of interest in nanolithography, but are not easily obtained from self-assembly of a diblock copolymer. Instead, we demonstrate 40 nm period square patterns formed in a thin film of 82 kg/mol polyisoprene-b-polystyrene-b-polyferrocenylsilane (PI-b-PS-b-PFS) triblock terpolymer with volume fraction of 25{\%}, 65{\%} and 10{\%}, respectively, blended with 15{\%} PS homopolymer. The square patterns consist of PFS pillars which remain after removal of the PI and PS with an oxygen plasma. On a smooth substrate, the correlation length of the square pattern is increased dramatically to several microns by the use of brush layers and specific solvent anneal conditions. The interaction between the square pattern and nanoscale topographical trenches and posts was also investigated, and controlled by the substrate functionalization.   

Highly Oriented and Aligned Line Patterns of Block Copolymer over Macroscopic Areas

We describe a novel process to prepare highly oriented and aligned line patterns over arbitrarily large surface areas without the use of photolithography, e-beam lithography, or other process used to chemically or topographically pattern a surface. Thin films of cylinder-forming PS-b-PEO were spin- coated onto the faceted surface of either hard (sapphire) or flexible (PBT) surfaces, and then exposed to THF and water. Scanning force microscopy demonstrated that highly aligned PEO cylindrical microdomains oriented parallel to the surface and normal to the surface facets were obtained over the entire surface. Grazing incidence small angle x-ray scattering (GI- SAXS) and 2D transmission SAXS were performed to characterize the ordering on the nanoscopic level over macroscopic length scales. X-ray results indicated that this film consisted of highly aligned and oriented cylindrical microdomains with no grains or misorientations over large areas owing to the ability of the faceted surfaces to direct the assembly of the block copolymer.   

Effect of chain extender on the phase behavior and morphology of high hard block content polyurethanes

Thermoplastic polyurethanes (TPUs) are linear block copolymers typically constructed of statistically alternating soft and hard segments, the hard segment itself being composed of an isocyanate and a short chain extender. In this project we focused on the effect that varying the chain extender used has on the phase behavior and morphology of high hard block content TPUs. Four different chain extenders were used. DSC, SAXS / WAXS, TEM / AFM, mechanical testing and FTIR were mainly used to characterize the morphology and properties of our materials. Through this work we were able to show that small changes in the chain extender chemical structure had dramatic effects on the properties of the TPUs. The use of 3-methyl-1,5-pentanediol resulted in a fully phase-mixed system with poor mechanical properties, while the use of 1,3-propanediol resulted in stiff materials with relatively high crystallinity and melting temperature. The use of 2-methyl-1,3-propanediol and 1,5-pentanediol resulted in similar materials, although 1,5-pentanediol was found to phase separate / crystallize on cooling while 2-methyl-1,3-propanediol was found to separate / crystallize on heating, suggesting a higher chain mobility in the latter materials.   

Reversible structuring of azobenzene polymer films by surface plasmons

It should be possible to move adsorbed nano-objects with relative ease, in large number and simultaneously. The essential idea is not to put more effort in fighting against the prevailing surface forces but rather to utilize them - in clear contrast to current techniques of nano-manipulation with atomic force microscopy [Santer, Adv Mat 2006]. For this, the topography should be reversible switching between different states by changing the morphology at the scale of objects to be moved. In this work, we choose light for changing the polymer topography. Here we present azo thin films [Seki, Chem Soc Jpn 2007] with integrated optically active elements supposed to support and steer the response of polymer films to external illumination by acting as nano-scale antennas. During irradiation surface plasmon (SP) waves are generated on a metallic mask. The interaction of the SP waves with azo polymers results in printing of near field intensity distributions into topography with the pattern size below the diffraction limit. We found that the topography can be driven reversible by changing polarization or wavelength. We also examine how the structuring process depends on the size of the metallic patterns. The results are confirmed by FTDT simulations and compared with imprints of photolithographic mask.