Morphology of Star-Shaped Block Copolymer in Thin Film

Abstract

Block copolymers have received great attention for their various nanostructures such as spheres, cylinders, gyroids, lamellae depending on the volume fraction of one block (f), degree of polymerization (N), Flory-Huggins segmental interaction parameter (χ), and molecular architecture of block copolymer. Nonlinear copolymers, for instance, grafted, miktoarm, comb-like, and star-shaped block copolymers showed unexpected morphology compared with linear block copolymers. Especially, the phase behavior of star-shaped block copolymers is different from that of linear block copolymers. Star-shaped block copolymer thin film showed that vertically oriented microdomains are formed without any surface modifications on various substrates. This is attributed to the entropic penalty arising from star-shaped molecular architecture which overcomes the favorable interaction between each block and air (or substrate). While the orientation of microdomains in a thin film greatly depends on various factors such as molecular weight, annealing condition, film thickness, and surface tension at the interface side, there are a few studies for investigation of the morphology and (or) orientation of star-shaped block copolymers in thin films. For the nanolithographic application of block copolymer self-assembled nanostructures, the orientation control of the microdomains in thin films on a substrate becomes very important. Therefore, controlling the morphology and (or) orientation of star-shaped block copolymers is essential to study in detail and fabricate well-ordered nanostructures on a substrate. In this thesis, I investigated how the molecular weight (M), film thickness, annealing conditions, and interface affinity affected the morphology and (or) orientation of star-shaped block copolymers consisting of polystyrene (PS) and poly(methylmethacrylate) (PMMA) blocks in thin films on various substrates. The choice of PMMA-block-PS copolymer as a model system is because this could be used for nanolithography due to easy removal of PMMA chains either by reactive ion etching or by UV irradiation followed by rinsing with acetic acid. In chapter 2, I investigated, via small-angle X-ray scattering (SAXS) and atomic force microscopy (AFM), the effect of M on the orientation of microdomains on various substrates. I successfully synthesized, by atom transfer radical polymerizations (ATRP), six-arm star-shaped PMMA-b-PS [(PMMA-b-PS)6] with two different microdomains (lamellae and PMMA cylinders). When the M is close to the critical molecular weight (Mcrit) above which block copolymers start to microphase separate, parallel orientation of lamellae (or PMMA cylinders) was obtained. This is due to the smaller entropic penalty for the formation of parallel orientation. However, even at a small increase in M (≳ 1.5 Mcrit), thin films showed vertically oriented microdomains regardless of substrates. Thus, star-shaped block copolymers are very effective to obtain vertically oriented microdomains on versatile substrates. In chapter 3, the thin film morphology of (PMMA-b-PS)6 with the volume fraction of PS block of ~0.50 was investigated by field-emission scanning electron microscopy (FE-SEM). The surface tension difference of PS and PMMA at the air surface is easily tuned depending on the annealing temperature. When the M is small, thin-film morphology greatly depends on the surface tension at the air side, when a substrate has preferential interaction with one block. For M ≲ 1.23 Mcrit, tube-like nanostructures, instead of vertically oriented lamellae (VL) were formed at the top of the film with a near-neutral air surface. However, the PMMA layer was formed on the bottom film contacting the silicon substrate with native oxide. This is because the combination of vertical and parallel lamellae generates a huge energy penalty at the T-junction connecting these two different lamellar orientations. Tube-like nanostructures were also formed on other substrates that are preferential to one block, for instance, gold or a substrate grafted by PS brush, when the film thickness does not meet the commensurability. On the other hand, when M is much higher than Mcrit, vertical lamellae were formed throughout the entire film thickness. In chapter 4, I investigated the nanostructures of (PMMA-b-PS)6 confined in a topologically pre-patterned trench to obtain long-range ordered microdomains. One of the most important parameters of block copolymers confined in the trench is the affinity between each block and the wall of the trench. The affinity between the block and the wall was controlled by using polymer brushes or selective gold (Au) deposition on one side of the walls. When the substrate and both walls are preferential to PMMA block, VL and nanotubes coexist between the walls. Because the PMMA block prefers strongly to locate at the walls with an Al2O3 layer, VL is formed near the walls with an Al2O3 layer. Also, the tube-like nanostructures are formed because the (PMMA-b-PS)6 is confined into a selective substrate to PMMA block near the wall, while the air-side is near-neutral. Thus, VL is formed near the walls and tube-like nanostructures are formed in the middle of the trench. However, for a trench grafted with PS brushes, tube-like structures are formed with a very thin PS layer. When selective Au was deposited only on one side of the walls, I observed dual nanopatterns with different numbers of lines on each side wall and the tube-like nanostructures in the middle of the trench. The formation of nanostructures of star-shaped block copolymer thin films was not only affected by the interface of air and a substrate but also the affinity between the block and both walls. By carefully controlling the affinity between the blocks and the walls, two or more different nanopatterns were realized on a single substrate.