Solvent Swelling Induced Self-assembly of Silicon-containing Block Copolymers

Abstract

Silicon-containing diblock copolymer, such as polystyrene-b-polydimethylsiloxane (PS-PDMS), possessing high density of Si in the backbone of PDMS provides extremely high etch contrast between constituted blocks under oxygen plasma treatment, which is advantageous for pattern transfer applications. Also, the strong segregation of PS-PDMS enables the formation of ordered structures with a smaller size. Block copolymers (BCPs) can self-assemble into a variety of ordered nanostructures through microphase separation for different volume fraction. To acquire a variety of nanostructure would require to synthesis a series of BCPs with different volume fraction. In this study, a simple method to create a variety of nanostructures resulting from the self-assembly of one-composition block copolymer (BCP) was developed. By using selective solvents for PS-PDMS self-assembly, the phase behavior of intrinsic BCP system can be enriched due to the variation in constituted fraction through preferential swelling the microdomain by selective solvent. Most interestingly, the equilibrium phase of PS-PDMS/solvent mixtures can be successfully preserved after solvent evaporation. Namely, the microphase-separated morphologies of effective constituted volume fractions can be preserved. We speculate that the preservation is attributed to the high segregation strength of PS-PDMS. Consequently, by controlling the solvent selectivity, a variety of nanostructures from microphase separation can be obtained from a single-composition of BCP. In contrast to the intrinsic phase of BCP (that is lamellae phase), these kinetically trapped phases are classified as metastable phases. Also, stable lamellae phase can be reformed by thermal annealing those metastable phases, further demonstrating the feasibility to control the metastability of the microphase-separated morphologies. Meanwhile, following the annealing process, phase transitions from gyroid to stable lamellae phase were well examined by using time-resolved SAXS profile combining with TEM results. As observed, we suggested a non-epitaxial phase transition behavior between gyroid phase and lamellae phase. Most interestingly, on the basis of electron tomographic results, a mesh-like structure between the gyroid and lamellae during phase transition can be found. As a result, new insights for the phase transition mechanism might be direcetly visualized.