Center for Functional Nanomaterials Seminar

Controlled diblock copolymer self-assembly offers a method of sub-50 nm patterning for device applications. Here we describe the development of a combined block copolymer patterning system, poly(alpha-methylstyrene)-block-poly(4-hydroxystyrene), or PaMS-b-PHOST. A particular advantage to this cylinder-forming diblock is that the majority component of the cylinder-forming block copolymer, PHOST, is a well-known negative-tone photoresist, allowing combined top-down/bottom-up patterning techniques. The minority component, PaMS, can also be selectively removed. Demonstration of the use of solvent annealing to gain sufficient polymer mobility for long range order is a critical step toward the use of this polymer for patterning applications. Solvent annealing can be combined with other ordering techniques, in this case demonstrated with graphoepitaxy, to gain self-aligned patterns with a predictable number of repeat units in trenches up to 900nm wide. Furthermore, choice of solvent has an affect on the morphology of the dried film, which has been studied both at the surface (AFM) and within the film interior (Grazing Incidence SAXS). Annealing the cylinder-forming polymer in tetrahydrofuran, a good solvent for both blocks, leads to parallel cylinder orientation. Alternatively, annealing in acetone, a preferential solvent for the PHOST majority component, leads to a spherical morphology upon drying. In the swollen state, the selective swelling of the PHOST block leads to an order-order transition to spherical morphology, which is kinetically trapped upon evaporation of the solvent. This work is not only useful for understanding self assembly for this polymer system, but has broader application toward understanding solvent annealing in other block copolymers, several of which have been investigated as well.