Through-Thickness Vertically Ordered Lamellar Block Copolymer Thin Films on Unmodified Quartz with Cold Zone Annealing

Abstract

Template-free directed self-assembly of ultrathin (approximately tens of nanometers) lamellar block copolymer (l-BCP) films into vertically oriented nanodomains holds much technological relevance for the fabrication of next-generation devices from nanoelectronics to nanomembranes due to domain interconnectivity and high interfacial area. We report for the first time the formation of full through-thickness vertically oriented lamellar domains in 100 nm thin polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) films on quartz substrate, achieved without any PMMA-block wetting layer formation, quartz surface modification (templating chemical, topographical) or system modifications (added surfactant, top-layer coat). Vertical ordering of l-BCPs results from the coupling between a molecular and a macroscopic phenomenon. A molecular relaxation induced vertical l-BCP ordering occurs under a transient macroscopic vertical strain field, imposed by a high film thermal expansion rate under sharp thermal gradient cold zone annealing (CZA-S). The parametric window for vertical ordering is quantified via a coupling constant, C (= v∇T), whose range is established in terms of a thermal gradient (∇T) above a threshold value, and an optimal dynamic sample sweep rate (v ∼ d/τ), where τ is the l-BCP's longest molecular relaxation time and d is the T - T distance. Real-time CZA-S morphology evolution of vertically oriented l-BCP tracked along ∇T using in situ grazing incidence small angle X-ray scattering (GISAXS) exhibited an initial formation phase of vertical lamellae, a polygrain structure formation stage, and a grain coarsening phase to fully vertically ordered l-BCP morphology development. CZA-S is a roll-to-roll manufacturing method, rendering this template-free through-thickness vertical ordering of l-BCP films highly attractive and industrially relevant.

Photothermally Directed Assembly of Block Copolymers

“…For a variety of interesting BCP materials, the ODT may not be readily accessible (e.g., the material may degrade before disordering); as a result, the CZA regime was investigated . Annealing during CZA occurs on both the heating and cooling fronts; annealing occurs below T ODT but above the material's glass transition temperature, increasing molecular mobility to induce ordering without passing through a disordered state . In‐plane thermal gradients (on the order of 10–70 °C mm −1 ) generated during CZA were shown to induce weakly aligned BCP morphologies with control over orientation through processing parameters, and provide accelerated kinetics relative to traditional oven thermal annealing at comparable temperatures (Figure b) .…”

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

“…Such zonal heating introduces another parameter to the film, a sharp temperature gradient which induces a thermal‐expansion‐induced strain. Basutkar et al . achieved perpendicular PS‐ b ‐PMMA lamellae on unmodified quartz substrate by using a zone annealing technique coined cold zone annealing (CZA).…”

“…d,e) GISAXS images of (b) and (c), respectively. Reproduced with permission . Copyright 2017, American Chemical Society.…”

“…[159,160] Such zonal heating introduces another parameter to the film, a sharp temperature gradient which induces a thermal-expansioninduced strain. Basutkar et al [76] achieved perpendicular PSb-PMMA lamellae on unmodified quartz substrate by using a zone annealing technique coined cold zone annealing (CZA). The term cold stems from the fact that the BCP is locally heated above its T g but below its ODT temperature (T ODT ), [161] in contrast to hot zone annealing in which the BCP is heated above the T ODT .…”

“…These methods include substrate modification, [29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46] use of top coats, [24,[47][48][49][50][51][52][53][54][55][56][57] incorporation of additives, [58][59][60][61][62][63][64] exploitation of the architecture effect, [65][66][67][68] use of modified [42,43,[69][70][71] or newly synthesized BCPs, [72][73][74][75] and use of cold zone annealing techniques. [76]…”