Block copolymers exhibit rich self-assembly behavior which has been exploited to generate a wide range of periodic structures in bulk and in thin film form. Self-assembly of diblock copolymer thin films has been widely exploited to generate sub-10 nm microdomain patterns without the necessity for costly lithographic processing, and has yielded geometries based on parallel lines and close-packed arrays of dots. Three component triblock terpolymers extend the range of geometries which can be generated to include square arrays of dots, but thin films of triblock terpolymers suffer from the drawback that the patterns are usually broken up into small regions with only short-range order. Overcoming this problem, a team of researchers has now demonstrated that much longer range order can be imposed upon these triblock terpolymer structures through the selective application of surface topography and polymer brush coatings.
As reported in the July 13 issue of Nano Letters (DOI: 10.1021/nl201262f; p. 2849), C.A. Ross from the Massachusetts Institute of Technology, I. Manners from the University of Bristol, and their co-workers used a blend of polyisoprene-block-polystyrene-block-polyferrocenylsilane (PI-b-PS-b-PFS) and 15% polystyrene homopolymer to create cylinders of PI and PFS arranged in alternating square arrays within a PS matrix. Coating sufficiently thin films (~32 nm) of this material onto silicon wafers covered with short polymer chains (a polymer brush) caused the cylinders to orient themselves perpendicular to the surface. The extent of order in the resulting checkerboard pattern was honed by carefully adjusting the film thickness, the type of polymer brush, and solvent annealing. Swelling the film with the appropriate amount of chloroform allowed a relatively hydrophilic polyethylene oxide (PEO) brush to be used, whose similar interaction with all three components promoted large ordered domains of up to 1.75 μm in size. The films were then etched with an oxygen plasma, which preferentially removes the PS and PI regions to leave only a square pattern of PFS posts which etch more slowly due to their iron and silicon content.
The long-range order of the pattern was further improved by forming the block copolymer film on substrates which had been pre-patterned with walls and posts using electron beam lithography. The substrate pattern then acted as guiding surface features. While a more neutrally interacting flat surface promotes order, these vertical features are most beneficial when selectively interacting with the PFS block, and so were coated separately with a PFS brush. In this way, posts placed sparsely throughout the grid act as surrogate PFS cylinders and enforce order on the pattern over longer distances. Square arrays such as these are useful as etch masks through which a Cartesian grid can be transferred onto functional materials, providing a layout for minute circuitry or other nanoscale devices.
Thin films of the triblock terpolymer form a checkerboard pattern of polyisoprene (PI: yellow) and polyferrocenylsilane (PFS: orange) cylinders, of which only a square array of PFS remains after oxygen plasma treatment. Reproduced with permission from Nano Lett. 11 (7) 2011 (DOI: 10.1021/nl201262f; p. 2849). © 2011 American Chemical Society.
