Triblock terpolymers are interesting because they can form a much wider diversity of 3D structures than diblock copolymers, including rings and square-symmetry patterns, which may be useful in nanolithography. Two examples of pattern formation in triblock terpolymers have been investigated, showing square arrays of dots from PI-PS-PFS ^[1] and close-packed arrays of rings from and PS-PFS-P2VP ^[2] . Square patterns are of particular interest for structures such as arrays of vias. In the PI-PS-PFS triblock terpolymer, the minority blocks (PI and PFS) form cylinders alternating with square symmetry. Oxygen etching removes the PI and PS, leaving oxidized PFS arrays with a period of 44 nm. On a smooth substrate, the correlation length of the square pattern is increased dramatically to several microns by the use of brush layers and specific solvent annealing conditions. The interaction between the square pattern and nanoscale topographical trenches and posts can be controlled by substrate functionalization, templating the structure.

For the PS-PFS-P2VP, the bulk structure consists of P2VP core-PFS shell cylinders in a PS matrix. Removing the P2VP and PS leaves ring-shaped PFS features. The cylinders can be oriented perpendicular to the top surface of the film by controlling the film thickness and annealing conditions. However, the cylinders typically lie in plane at the substrate-film interface, and the ring pattern therefore cannot be transferred directly by etching an underlying film. Pattern transfer was achieved instead by imprinting using the PFS rings as an imprint stamp. These examples show some of the diversity of geometries that can be achieved using triblock terpolymers. A wide range of geometries remains to be investigated, including lines and spaces of unequal widths, lines with width modulation, or tiling patterns, and self-assembly of these patterns may facilitate the fabrication of new types of devices.

1. V. P. Chuang, J. Gwyther, R. A. Mickiewicz, I. Manners, and C. A. Ross,  “Templated self-assembly of square symmetry arrays from an ABC triblock terpolymer,” Nano Lett., vol. 9, pp. 4364-9, 2009.
2. V. P. Chuang, C. A. Ross, J. Gwyther, and I. Manners, “Self-assembled nanoscale ring arrays from a polystyrene-b-polyferrocenylsilane-b-poly(2-vinylpyridine) triblock terpolymer thin film,” Adv.Mater., vol,  21, pp. 3789-93, 2009.

Block copolymers can be used to make a variety of functional electronic or magnetic devices. We have developed pattern transfer processes to produce metal, silicon, oxide, or polymer patterns using a mask made from a PS-PDMS (polystyrene-polydimethylsiloxane) block copolymer.

Metal films are patterned using a “damacene” process in which a metal film is deposited over a pattern made from the oxidized PDMS microdomains. Reactive ion etching planarizes and slowly removes the metal until the oxidized PDMS is exposed. The oxidized PDMS etches much faster than the metal, and termination of the etch process at this point leaves a metal pattern that is the reverse contrast of the block copolymer pattern. This process has been used to make magnetic nanostructures such as dot, line, and antidot arrays, as well as narrow metallization lines.

We have also deposited films of block copolymers on top of materials such as conductive polymers or graphene, and then used the block copolymer pattern as an etch mask to form structures such as arrays of conducting polymer nanowires or graphene nanoribbons. Other pattern transfer includes the formation of high aspect silicon nanowires by catalytic etching of a Si wafer using a BCP-patterned gold catalyst and the formation of inverted pyramid arrays by anisotropic etching of silicon through a block copolymer mask.

1. Y. S. Jung and C. A. Ross, “Fabrication of diverse metallic nanowire arrays based on block copolymer self-assembly,”Nano Lett.  vol. 10, pp. 3722-3726, 2010.
2. S.-W. Chang, V. P. Chuang, S. T. Boles, C. A. Ross, and C. V. Thompson, “Densely-packed arrays of ultrahigh-aspect-ratio silicon nanowire fabricated using block copolymer lithography and metal-assisted etching,” Adv. Functional Materials vol. 19, pp. 2495-2500, 2009.
3. Y. Sik Jung and C. A. Ross, “Well-ordered thin film nanopore arrays formed using a block copolymer template,” Small, vol. 5, pp. 1654-1659, 2009.
4. Y. S. Jung, W. Jung, H. L. Tuller, and C. A. Ross, “Nanowire conductive polymer gas sensor patterned using self-assembled block copolymer lithography,” Nano Lett., vol. 8, pp. 3776-3780, 2008.