Investigation of high χ block copolymers for directed self-asssembly: Synthesis and characterization of PS-b-PHOST

Directed self assembly (DSA) of block copolymers (BCP) could enable high resolution secondary patterning via pitch multiplication from lower resolution primary lithographic patterns. For example, DSA could enable dense feature production at pitches less than 20 nm from patterns generated using 193 nm exposure tools. According to theory, microphase separation of block copolymers can only occur when the critical condition that χ^N>10.5 is met, where χ is the Flory Huggins interaction parameter and N is the total degree of polymerization for the block copolymer. In order to generate smaller DSA pattern pitches, the degree of polymerization of the block copolymer is reduced since this reduces the characteristic length scale for the polymer (e.g. radius of gyration). Thus, as N is reduced, the effect of this reduction on χ^N must be balanced by increasing χ to maintain a given level of phase separation. Currently, most DSA work has focused on the use of poly(styrene)-b-poly(methyl methacrylate) (PS-b-PMMA) copolymers whose low χ value (i.e. ∼0.04) limits the practical DSA pitch using such materials to approximately 20nm. The general goal of this work has been to explore new higher χ block copolymer systems, develop DSA patterning schemes based on such materials, and test their ultimate pitch resolution. This paper discusses the synthesis and characterization of poly(styrene)-b-poly(hydroxystyrene) (PS-b-PHOST) copolymers made via nitroxide mediated radical polymerization. The formation of lamellar fingerprint structures in PS-b-PHOST using solvent annealing is demonstrated. Using this fingerprint data, initial estimates of χ for PS-b-PHOST are made which show that it appears to be at least one order of magnitude larger than the χ for PS-b-PMMA. Finally, graphoepitaxy of self-assembled lamellar structures in PS-b-PHOST is demonstrated using SU-8 guiding patterns on cross-linked neutral underlayers.