Investigation of Chemical Pattern Design and Block Copolymer Formulation on Directed Self-assembly

Abstract

Directed self-assembly (DSA) of block copolymer (BCP) films is in development to augment advanced lithography capabilities. By combining state-of-the-art lithography techniques to guide the thermodynamically driven self-assembly, well-ordered arrays of nanoscale structures can be achieved over macroscopic dimensions. Both the design of the block copolymer and the substrate have tremendous impact on the properties of the self-assembled film. In chapters 2-4, chemoepitaxial surface patterns are used to direct the BCP; they are composed of guide stripe and background regions that are designed so that the minimum free energy of the system is achieved when the lamellae are aligned to the stripes. Chapter 2 characterizes a novel architecture for chemical patterns, where three separate regions of the pattern contain distinct wetting behavior; the guide stripe formed a trapezoidal cross-section and the sidewalls have distinct wetting behavior compared to the guide stripe and the background. Chapter 3 examines the role of the chemistry of the guide stripe material, which influences the interfacial energy between the guide stripe and the BCP domain in contact with it. The guide stripe ideally makes the largest difference in interfacial energy with respect to the BCP chemistries. Less selective guide stripes have impose a weaker driving force for DSA and therefore have slower assembly kinetics. The properties of block copolymers can be tuned by blending homopolymers (HP) or other block copolymers. Binary blends of BCP and HP are used in Chapter 4 as a proxy for modifying the overall volume fraction of the BCP during DSA. Whereas unpatterned BCP behavior is symmetric with volume fraction, the asymmetry stabilizes different defect mechanisms in DSA depending on whether the majority or minority block is guided by the chemical pattern. PMMA-rich formulations stabilized disclination-style defects due to weaker pattern registration whereas PS-rich formulations formed microbridges that interfered with pattern transfer. Chapter 5 characterizes the macrophase separation behavior of BCP-BCP blends in thin films; Macrophase separation occurs in symmetric BCP of matching polymer chemistries when the ratio of the molecular weights is at least five and the volume fraction of short BCP is high enough to saturate the interface of the long BCP. This critical composition of macrophase separation is different in thin film compared to the bulk, which influences not only the amount of each phase as determined by the lever rule, but also the period of the large period phase as determined by the miscible blend scaling equation.