Self-Assembly of Triblock Terpolymer Films

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 and close-packed arrays of rings from and PS-PFS-P2VP. 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.