Credit: ©2008 AAAS

Consumer demand for smaller and faster microelectronics is driving improvements in the photolithographic techniques used to create integrated circuits of metal oxide semiconductors, but this is expected to reach a lower size limit of 30 nm. Improvements beyond this could be reached by block-copolymer lithography, with feature sizes of 10–30 nm that are controlled by molecular weight. Unfortunately, although hexagonal arrays are relatively easy to produce, standard integrated circuit designs demand square arrays, which are still a challenge.

Now, a team at the University of California, Santa Barbara, have used a hierarchical self-assembly strategy to generate1 nanoscale square patterns of diblock copolymers. This was achieved by combining the supramolecular assembly of hydrogen-bonding units with controlled phase separation. The polymers, made from blocks of polystyrene with additional hydrogen-bonding units and either poly(ethylene oxide) or poly(methyl methacrylate), were blended and spun-coated onto silica wafers. The hydrogen bonding suppresses macrophase separation in favour of separation on a smaller scale. As a result, the diblock copolymers separated into cylindrical domains, creating a square pattern with centre-to-centre spacing of 51 nm.

The team, led by Craig Hawker, Edward Kramer and Glenn Fredrickson, used the polymers as lithographic masks by successively etching away different diblock domains to leave a pattern of 22-nm holes on a silica substrate. In order to confirm the critical role of these supramolecular interactions, Hawker and colleagues performed control experiments using similar polymers but without the hydrogen-bonding units. These did not show the same square ordering, indicating that the interactions were responsible for producing both the long-range order and the nanoscale features.