Dynamic Thermal Field-Induced Gradient Soft-Shear for Highly Oriented Block Copolymer Thin Films

• Published in: ACS nano 2012-11, Vol.6 (11), p.10335-10342
• Main Authors: Singh, Gurpreet, Yager, Kevin G, Berry, Brian, Kim, Ho-Cheol, Karim, Alamgir
• Format: Article
• Language: English
• Subjects: Alignment; Block copolymers; Crystallization - methods; Dimethylpolysiloxanes - chemical synthesis; Dynamics; Hardness; Materials Testing; Membranes, Artificial; Nanostructure; Nylons - chemical synthesis; Self assembly; Shear Strength; Silicone resins; Spreads; Temperature; Thermal Conductivity; Thermal expansion; Thin films
• ISSN: 1936-0851; 1936-086X
• DOI: 10.1021/nn304266f

As demand for smaller, more powerful, and energy-efficient devices continues, conventional patterning technologies are pushing up against fundamental limits. Block copolymers (BCPs) are considered prime candidates for a potential solution via directed self-assembly of nanostructures. We introduce here a facile directed self-assembly method to rapidly fabricate unidirectionally aligned BCP nanopatterns at large scale, on rigid or flexible template-free substrates via a thermally induced dynamic gradient soft-shear field. A localized differential thermal expansion at the interface between a BCP film and a confining polydimethylsiloxane (PDMS) layer due to a dynamic thermal field imposes the gradient soft-shear field. PDMS undergoes directional expansion (along the annealing direction) in the heating zone and contracts back in the cooling zone, thus setting up a single cycle of oscillatory shear (maximum lateral shear stress ∼12 × 104 Pa) in the system. We successfully apply this process to create unidirectional alignment of BCP thin films over a wide range of thicknesses (nm to μm) and processing speeds (μm/s to mm/s) using both a flat and patterned PDMS layer. Grazing incidence small-angle X-ray scattering measurements show absolutely no sign of isotropic population and reveal ≥99% aligned orientational order with an angular spread Δθfwhm ≤ 5° (full width at half-maximum). This method may pave the way to practical industrial use of hierarchically patterned BCP nanostructures.