Self-assembly of block copolymers in thin films

Abstract

The self-assembly of block copolymers in thin films has been a subject of recent studies from both academic and industrial perspectives. One of its potential applications is nanolithography; block copolymers can function as novel mask materials intended for fabrication of small features, not easily realizable by current optical lithography. This dissertation addresses several fundamental issues associated with thin-film block copolymers. The bulk and interfacial wetting properties of partially epoxidized poly(styrene-b-isoprene) diblock copolymers, denoted as PS-PEI, were studied while varying the degree of the chemical modification. The incorporation of the random copolymer architecture induced decoupling between the bulk and the thin-film thermodynamics. The tunable surface wetting, a consequence of the partial modification, permitted control over the orientation of the domains in thin films. The morphologies of thin-film block copolymers were investigated using two different boundary conditions that involve one neutral interface and one preferential interface. The neutralities at the free surface and the underlying substrate were attained independently by the partial epoxidation in PS-PEI and the composition adjustment of random copolymer mats, respectively. For both boundary conditions, thin-film block copolymers formed an island/hole motif, characterized by 0.5 L₀ step heights (L₀: bulk lamellar periodicity). The thin-film behavior of PS-PEI block copolymers with random copolymer architecture was examined as the segregation strength (χN) was adjusted systematically across the order-disorder transition. Unlike in the bulk, the random copolymer architecture did not generate abnormal behavior in thin-film thermodynamics compared to plain linear architecture. With decreasing segregation strength, the thin-film system exhibited fluctuation-pervaded morphologies prior to reaching a disordered state. An agreement was found between the order-disorder transition temperatures in three dimensions (bulk) and in two dimensions (thin film). Lastly, the bulk properties and the thin-film structures of lamellae-forming poly(styrene-b-isoprene-b-methyl methacrylate) (SIM) triblock copolymers were studied. The thin-film morphology exhibited the dependence on the size of the poly(isoprene) (PI) middle block. While perpendicular lamellae were observed for the thin-film SIM block copolymer with a small PI volume fraction, complex behavior was observed for the sample with a large PI volume fraction.