The ability of random copolymer brushes and cross-linked mats to induce the vertical orientation of domains in overlying films of lamellae-and cylinder-forming block copolymers was investigated as a function of the composition. The substrate-modifying layers consisted of styrene and methyl methacrylate random copolymers and contained either a terminal hydroxyl group or a third polar comonomer of 2-hydroxyethyl methacrylate (HEMA) for grafting brushes to silicon oxide surfaces or glycidyl methacrylate (GMA) for crosslinking the random copolymer into a mat. Polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) lamellaeand cylinder-forming block copolymers (both PS and PMMA minority block copolymers) were deposited and annealed on the modified surfaces. In all cases the vertical orientation of domains was observed for a range of random copolymer composition, but the ranges of composition were different for each combination of surface layer and block copolymer. The cylindrical domains of PS exhibited vertical structures for a very narrow range of compositions compared to cylindrical domains of PMMA or lamellae. As expected, the incorporation of polar HEMA or GMA monomers in the surface layers shifted the composition range for the perpendicular orientation of domains to higher fractions of styrene. The results are discussed in terms of the equilibration of the films in the presence of the chemically modified surfaces.

We report a method to fabricate high-quality patterned magnetic dot arrays using block copolymer lithography, metal deposition, and a dry lift-off technique. Long-range order of cylindrical domains oriented perpendicular to the substrate and in hexagonal arrays was induced in the block copolymer films by prepatterning the substrate with topographic features and chemically modifying the surface to exhibit neutral wetting behaviour towards the blocks of the copolymer. The uniformity of the domain size and row spacing of block copolymer templates created in this way was improved compared to those reported in previous studies that used graphoepitaxy of sphere-forming block copolymers. The pattern of block copolymer domains was transferred to a pattern of magnetic metal dots, demonstrating the potential of this technology for the fabrication of patterned magnetic recording media.

Random copolymers of P(S-r-MMA-r-HEMA)s with a distribution of surface reactive hydroxyl groups were synthesized to formulate neutral surface layers on a SiO2 substrate. The layers were designed to drive vertical orientation of lamellar microdomains in a top P(S-b-MMA) thin film. Copolymers with a styrene weight fraction (f(St)) of 0.58 and a HEMA fraction (f(HEMA)) ranging from 0.01 to 0.03, with a corresponding MMA fraction (f(MMA)) ranging from 0.41 to 0.39, in the P(S-r-MMA-r-HEMA) copolymer showed neutral surface characteristics. The morphology of block copolymer thin films was studied by scanning electron microscopy (SEM). P(S-r-MMA-r-HEMA) copolymers prepared by both living and classical free-radical polymerizations were equally effective in demonstrating the neutrality of the surface. These side-chain-grafted random copolymer brushes showed faster grafting kinetics than the end-chain-grafted P(S-r-MMA) because of multipoint attachment to the surface. The modified surfaces had a very thin layer of random copolymer brush (5-7 nm), which is desirable for effective pattern transfer. Furthermore, neutral surfaces could be obtained even when the grafting time was reduced to 3 h. These results indicate that the composition of the random copolymer brush, rather than its PDI or molecular weights, is the most important factor in controlling the neutrality of the surface. These results also demonstrate the feasibility of using a third comonomer (C) in the random copolymer brush P(A-r-B-r-C) to alter the interfacial and surface energies of a diblock copolymer (A-b-B).

Block copolymer (BCP) lithography has emerged as a promising strategy to create highly regular and dense dot or line arrays at the sub-50 nm length scale. [1][2][3][4][5][6] Block copolymer lithography refers to the use of self-assembled domain structures, typically spheres, cylinders, and lamellas in thin-film form as a template for the addition and subtraction nanofabrication processes. [5] Mainly two pattern geometries have been studied extensively: dense arrays of dots and dense arrays of lines and spaces. The former can be generated from sphereforming BCP or from cylinder-forming BCP with domains oriented perpendicular to the substrate, and the latter from cylinder-forming BCP with domains oriented parallel to the substrate or lamella forming BCP with domains oriented vertically to the substrate. These periodic arrays based on self-assembly in BCP thin films have been examined for fabrication of magnetic storage media, quantum dot arrays, photonic crystals, and nanowire transistors. [4,[7][8][9][10] Vertically oriented domain structures have distinct advantages in terms of pattern transfer to the underlying substrate and fabrication of high-aspect ratio features.[11] In the case of perpendicular cylinder or perpendicular lamellae, the orientation of the BCP microdomains can be controlled by solvent evaporation, electric fields, directional crystallization, [12][13][14][15] physical constraints (topography), or chemical patterns. [16][17][18][19] Perhaps the most common means to induce perpendicular domain orientation, however, is to chemically modify and control the interaction between the BCP and the substrate. [20,21] For example, the substrate can be chemically modified by using self-assembled monolayers (SAMs) or random copolymer brush. SAMs of alkylthiols (on gold) or alkylsiloxanes (on Si/SiO 2 ) with polar or non-polar terminal groups have been utilized to modify the substrate. [22][23][24] Exposure to different doses of X-ray in the presence of oxygen can further alter the polarity and hence the wetting behavior of the SAM resulting in a symmetric, neutral or asymmetric BCP morphology. [25] Mansky and coworkers demonstrated that the interfacial energies between the PS and PMMA blocks of P(S-b-MMA) and the substrate can be carefully balanced by grafting an end-hydroxy functionalized P(S-r-MMA) polymer on the SiO x /Si substrate. [20,26,27] Covalent grafting of P(S-r-MMA)having f St (where f st is the styrene fraction) of 0.58 resulted in the neutral wetting behavior and the vertical orientation of lamellar or cylindrical domains. In this original report covalent grafting is usually achieved through the dehydration reaction between the end-hydroxy functionalized P(S-r-MMA) and the native oxide layer of silicon substrate. Ryu et. al recently reported an elegant chemistry to create substrate independent neutral surface through the self thermal crosslinking reaction of benzocyclobutene (BCB)-containing random copolymers at temperatures above 200°C.[28] The high temperature required for efficient cross-lin...

A morphological transition from cylinders to spheres was induced in an asymmetric diblock copolymer, poly(styrene)-block-poly(tert-butyl acrylate) (PS-b-PtBA). The periodic arrays of the poly(tert-butyl acrylate) (PtBA) domains were transformed to the ordered poly(acrylic anhydride) (PAA) spheres via the thermal deprotection of tert-butyl acrylate linkages and the subsequent volume change of the minority block. Coupled with techniques to direct the assembly of cylinder-forming block copolymers, this finding provides new routes to fabricate ordered geometries of nanodot arrays.