Orienting Block Copolymer Thin Films via Entropy

Abstract

Controlling the orientation of nanostructured thin films of block copolymers (BCPs) is essential for next-generation lithography using BCPs. According to conventional wisdom, the orientation of BCP thin films is mainly determined by molecular interactions (enthalpy-driven orientation). Here, we show that the entropic effect can be used to control the orientation of BCP thin films. Specifically, we used the architecture of star-block copolymers consisting of polystyrene (PS) and poly­(dimethylsiloxane) (PDMS) blocks to regulate the entropic contribution to the self-assembled nanostructures. The study unequivocally demonstrate that for star-block copolymers with the same volume fractions of PS and PDMS, perpendicularly oriented BCP nanostructures could be induced via an entropic effect regulated by the number of arms. Also, the feasibility of using the star-block copolymer thin films for practical applications is demonstrated by using the thin film as a mask for nanolithography or as a template for the fabrication of nanoporous monolith.

Directed Self‐Assembly of Star‐Block Copolymers by Topographic Nanopatterns through Nucleation and Growth Mechanism

“…To examine the morphological textures of the self‐assembled star‐block PS–PDMS within a line pattern from the top, a reactive ion etching (RIE) using CF 4 /O 2 mixed gas was carried out to simultaneously remove the PDMS thin wetting layer resulting from the low surface energy and to degrade the PS matrix followed by O 2 plasma to oxide PDMS into SiO 2 and simultaneously to remove the PS, giving the required height contrast for SEM observation of the forming perpendicular SiO 2 cylinders. All the top‐view SEM images shown in this article were prepared in a similar fashion following our previous works . Figure a shows numerous arrays of hexagonally packed dots, resulting from the perpendicular SiO 2 cylinders within the unmodified (native) line patterns; this morphological observation suggests the formation of perpendicularly oriented PDMS cylindrical domains of the microphase‐separated 3‐arm star‐block PS–PDMS from the bottom of the substrate driven by entropy (Figure S1, Supporting Information).…”

“…BCP Synthesis : The synthesis of the 3‐arm star‐block PS–PDMS samples was described in the previous studies . First, the PS‐ b ‐PDMS sample was prepared by sequential anionic polymerization of styrene (Acros Organics, 99%) and hexamethylcyclotrisiloxane (D3, Acros Organics, 98%) by using high‐vacuum techniques.…”

“…Therefore, the use of silicon‐containing BCP thin films for nanopatterning becomes a main stream for BCP lithography by using such a self‐assembled pattern with nanoscale texture as a mask for lithographic works. In our previous study, we have demonstrated the entropy‐driven perpendicular orientation of 3‐arm star‐block polystyrene (PS)–poly(dimethylsiloxane) (PDMS) in the thin‐film state due to the architecture effect on BCP self‐assembled thin film, and the potential of using the nanostructured thin film as a mask for nanolithography . In this study, we aim to examine the self‐assembling mechanism and kinetics of 3‐arm star‐block PS–PDMS as nanostructured thin films with perpendicular orientation and controlled lateral ordering by DSA using topographically patterned substrates fabricated from a specific lithographic approach.…”

Thermal Approaches to Perpendicular Block Copolymer Microdomains in Thin Films: A Review and Appraisal

“…In another work, Lo et al . used a high‐χ 3‐arm PS‐ b ‐PDMS star BCP to produce perpendicular microdomains that propagate from the substrate but ultimately cap off parallel at the film surface due to the low‐γ PDMS block.…”

Precise Synthesis of Fluorine-containing Block Copolymers via RAFT

“…Recently, silicon-containing BCPs have received much attention due to their high etching contrast and small feature dimensions derived from their component incompatibility [2,3]. However, controlling the orientation of such materials is challenging because silicon-containing BCPs with strong segment-segment interaction generally exhibit disparate polarities and a large difference in the surface free energies between the two blocks.…”