Ultrathin random copolymer-grafted layers for block copolymer self-assembly.

Hydroxyl-terminated P(S-r-MMA) random copolymers (RCPs) with molecular weights (Mn) from 1700 to 69000 and a styrene unit fraction of approximately 61% were grafted onto a silicon oxide surface and subsequently used to study the orientation of nanodomains with respect to the substrate, in cylinder-forming PS-b-PMMA block copolymer (BCP) thin films. When the thickness (H) of the grafted layer is greater than 5-6 nm, a perpendicular orientation is always observed because of the efficient decoupling of the BCP film from the polar SiO2 surface. Conversely, if H is less than 5 nm, the critical thickness of the grafted layer, which allows the neutralization of the substrate and promotion of the perpendicular orientation of the nanodomains in the BCP film, is found to depend on the Mn of the RCP. In particular, when Mn = 1700, a 2.0 nm thick grafted layer is sufficient to promote the perpendicular orientation of the PMMA cylinders in the PS-b-PMMA BCP film. A proximity shielding mechanism of the BCP molecules from the polar substrate surface, driven by chain stretching of the grafted RCP molecules, is proposed.

Thermal stability of functional P(S-r-MMA) random copolymers for nanolithographic applications.

Two strategies are envisioned to improve the thermal stability of the grafted layer and to allow the processing of the random copolymer/block copolymer (RCP/BCP) system at high temperature. From one side, a high-temperature thermal treatment of a commercial α-hydroxyl ω-2,2,6,6-tetramethylpiperidinyloxy functional RCP, namely, TR58, leads to the formation of a stabilized layer able to induce the perpendicular orientation of a symmetric BCP to temperatures higher than 310 °C. On the other side, an α-hydroxyl ω-Br functional RCP, namely, BrR58, with the same molar mass and composition of TR58, was prepared by activator regenerated by electron transfer atom transfer radical polymerization. The resulting brush layer can sustain the self-assembly of the symmetric BCP for processing temperatures as high as 330 °C. In both systems, the disruption of the BCP film, deposited on the grafted RCP layer, occurs because of the formation of bubbles, due to a low-temperature evolution of monomers from the RCP layer. The extent of the low-temperature monomer evolution is higher for TR58 than it is for BrR58 and starts at lower temperatures. For both copolymers, the thermal treatment offsets the low-temperature monomer evolution while still maintaining surface characteristics suitable to induce the perpendicular orientation of the BCPs, thus ultimately extending the range of processing temperatures of the BCP film and consequently speeding the self-organization process.