Perpendicular Domain Orientation in Thin Films of Polystyrene−Polylactide Diblock Copolymers

Block copolymer thin films are ideal templates for a wide range of technologies where large area patterns of nanoscale features are desired. One of the main challenges in using lamellae- and cylinder-forming block copolymers for this purpose is to induce the block copolymer domains to orient perpendicular to the film surface. We here show that perpendicular domain orientation can be easily achieved with polystyrene-b-polylactide (PS−PLA) thin films. Cylinder-forming PS−PLA films were prepared by spin-coating on a variety of substrates followed by thermal annealing. The molecular weight, film thickness, annealing temperature, and annealing time were varied. When the film thickness was larger than the repeat spacing of the bulk morphology, the domains oriented perpendicular to the surface independent of the substrate/film interface. The films were then used to prepare nanoporous templates by a combination of hydrolytic PLA degradation and oxygen reactive ion etching (O2-RIE). The template pattern was then transferred to the substrate using CF4-RIE to form an array of nanoscale pits.