Block Copolymer Lamellae: Stand Up and Shear (Align)

1. Block Copolymer Lamellae: Stand Up and Shear (Align) NaiPhuanOng, Princeton University, DMR 0819860 IRG-C: Saswati Pujari, Michael Keaton,Paul Chaikin (NYU), and Rick Register Princeton Center for Complex Materials (PCCM) Block copolymer thin films are attractive templates for covering large areas with nanoscalefeatures: for example, grids of parallel metal nanowires which effectively polarize the short-wavelength ultraviolet light used in photolithography today. Lamellar block copolymers, with the lamellae standing perpendicular to the substrate, are attractive template candidates, but no process has existed to align such perpendicular lamellae in the in-plane direction on ordinary unpatterned substrates, over macroscopic distances. Recently, PCCM researchers have generated such aligned perpendicular lamellae, over centimeter-square areas, simply by shearing the film with a soft rubber pad in contact with the film surface. Instead of “lying down”, like a deck of cards set on edge and then tipped over, the lamellae remain perpendicular due to a neutralizing treatment of the supporting silicon wafer, combined with the thinness of the film. Shear alignment of a thin (18 nm) film of a polystyrene-polymethylmethacrylate (PS-PMMA) diblock copolymer, with lamellar period L0 = 30 nm, supported on a Si wafer substrate. As shown in the schematic at lower right, prior to depositing the block copolymer film, the substrate is grafted with a S-MMA-hydroxyethylmethacrylate random terpolymerto equalize the interaction energies of the different blocks with the substrate. Top left: atomic force microscopy (AFM) image of the top surface of the film prior to shearing, showing that the lamellae “stand up”, but are completely without in-plane direction. Top right: after shearing, the lamellae line up in the direction of shear. Alignment Reference: S. Pujari, M.A. Keaton, P.M. Chaikin, and R.A. Register, “Alignment of Perpendicular Lamellae in Block Copolymer Thin Films by Shearing”, Soft Matter, 8, 5358-5363 (2012).