Decoupling interfacial neutrality and film stability in high-χ PS-b-PDMSB block copolymer thin films

The self-assembly of block copolymers (BCPs) provides a promising route toward high-resolution nanofabrication, yet the practical deployment of high-χ systems is hindered by film instabilities and the difficulty of controlling microdomain orientation, particularly in lamellar-forming materials. Here, we investigate the stability and microdomain organization of lamellar poly(styrene)-block-poly(1,1-dimethylsilacyclobutane) (PS-b-PDMSB) thin films deposited on silicon substrates modified either by HBr/O₂ plasma treatment or by grafted poly(2-ethylhexyl acrylate) (PEHA) homopolymers of varying molar mass. Atomic force microscopy (AFM), cross-sectional scanning transmission electron microscopy (STEM), and grazing-incidence small-angle X-ray (GISAXS) scattering demonstrate that both substrate treatments yield near-neutral wetting conditions, but lead to markedly different self-assembly behaviors. While plasma-treated substrates yield unstable films exhibiting predominantly in-plane lamellae, PEHA-grafted substrates stabilize smooth films with out-of-plane lamellar orientation immediately after spin-coating. This metastable perpendicular morphology persists for extended periods and is attributed to a soft-confinement mechanism, whereby grafted chains partially penetrate the BCP domains and reduce the effective interfacial energy between the blocks. The lifetime of this perpendicular orientation depends strongly on brush molar mass, with shorter PEHA chains providing enhanced stabilization during thermal annealing. Nevertheless, prolonged annealing ultimately induces reorientation toward the thermodynamically favored in-plane morphology as a consequence of the non-neutral air interface. These results demonstrate that grafted polymer underlayers can effectively modulate both the stability and orientation of high-χ BCP thin films, providing new insights into the interplay between confinement, interfacial energetics, and self-assembly kinetics while offering a practical strategy for the integration of sub-10 nm BCP lithographic materials.