Real-time CZA-S morphology evolution of vertically oriented l-BCP tracked along ∇ T using in-situ Grazing Incidence Small Angle X-ray Scattering 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. 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 g,heat- T g,cool distance. 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). Vertical ordering of l-BCPs results from the coupling between a molecular and a more » macroscopic phenomenon.
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We report for the first time, the formation of full thru-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). Here, template-free directed self-assembly of ultrathin (~10’s nm) lamellar block copolymer (l-BCP) films of high-interfacial area into vertically oriented nanodomains holds much technological relevance for fabrication of next-generation devices from nanoelectronics to nanomembranes due to domain interconnectivity and high interfacial area.
In conclusion, such insights into the development of highly ordered BCP nanostructures under template-free thermal gradient fields can potentially have important ramifications in the field of BCP-directed self-assembly and self-assembling polymer systems more broadly.
A novel aspect of this study is the identification of a grain-coarsening regime on the cooling edge of the gradient, previously obscure in zone annealing studies of BCPs. The ordering pathway in the different regimes is further described within the framework of an energy landscape. Four distinct regimes of BCP ordering were observed within the gradient that include microphase separation from an “as cast” unordered state (Regime I), evolution of vertical cylinders under a thermally imposed strain gradient (Regime II), reorientation of a fraction of cylinders due to preferential substrate interactions (Regime III), and finally grain-coarsening on the cooling edge (Regime IV). To that end, we developed a customized CZA-S assembly to probe the dynamic structure evolution and ordering of the PS- b-PMMA cBCP film in situ as it undergoes the CZA-S process using the grazing incidence small-angle X-ray scattering (GISAXS) technique. A thorough understanding of the molecular level mechanisms and pathways of the BCP ordering that occur during this CZA-S process is presented, useful to fully more » exploit the potential of CZA-S for large-scale BCP-based device fabrication. As a demonstration, hexagonally packed predominantly vertical cylindrical morphology, technologically relevant for applications such as membranes and lithography, was achieved in 1 μm thick cylinder-forming PS- b-PMMA (cBCP) films by applying sharp thermal gradients (CZA-Sharp) at optimum sample sweep rates. = ,ĭynamic thermal gradient-based processes for directed self-assembly of block copolymer (BCP) thin films such as cold zone annealing (CZA) have demonstrated much potential for rapidly fabricating highly ordered patterns of BCP domains with facile orientation control.
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