(6ae) Ordering of Spherical-Domain Block Copolymers in Monolayers and Multilayers

Thin films of block copolymers are a simple, inexpensive route for patterning nanoscale features, and are widely investigated in the semiconductor industry as an alternative (or complement) to established lithographic techniques. While the physics of bulk systems are well understood, the ordering of domains confined to a thin film can differ due to surface interactions and geometric frustration. We have identified some remarkably complex behavior in monolayers and multilayers of a model spherical-domain diblock copolymer using grazing-incidence small-angle x-ray scattering (GISAXS), which provides a quantitative measure of the structure over macroscopic areas. Below the melting transition, the hexagonally-packed monolayer has long-range orientational order and quasi-long-range translational order (algebraic decay), consistent with theoretical predictions for two-dimensional crystals. As the number of layers of spheres is increased from one to 23, competition between the packing preferred in the bulk with that at the interfaces drives a transition from a hexagonal to a body-centered cubic lattice. The hexagonal symmetry of the monolayer persists through four layers, and at five layers the symmetry breaks to form a face-centered orthorhombic lattice. As the number of layers is further increased from five to 23, the face-centered orthorhombic unit cell deforms to approach asymptotically a structure similar to the body-centered cubic lattice with the closest-packed (110) plane oriented parallel to the substrate. GISAXS results are supported by real-space imaging of the lattice with scanning force microscopy (both high resolution and under-sampled) and transmission electron microscopy.

Publications:

7. J. Bang, B.J. Kim, G.E. Stein, T.P. Russell, X. Li, J. Wang, E.J. Kramer, C.J. Hawker, ''Effect of humidity on the ordering of PEO-based copolymer thin films,'' submitted for publication 2007.

6. G.E. Stein, E.J. Kramer, X. Li, J. Wang, ''Thickness dependent ordering in laterally confined monolayers of spherical-domain block copolymers,'' Macromolecules, In Press, 2007.

5. G.E. Stein, E.W. Cochran, K. Katsov, G.H. Fredrickson, E.J. Kramer, X. Li, J. Wang, ''Symmetry Breaking of In-Plane Order in Confined Copolymer Mesophases,'' Physical Review Letters, vol. 98, no. 158302, 2007.

4. G.E. Stein, E.J. Kramer, X. Li, J. Wang, ''Layering transitions in thin films of spherical-domain block copolymers,'' Macromolecules, vol. 40, pp. 2453-2460, 2007.

3. G.E. Stein, E.J. Kramer, X. Li, J. Wang, ''Single-crystal diffraction from two-dimensional block copolymer arrays,'' Physical Review Letters, vol. 98, no. 086101, 2007.

2. V. Khanna, E.W. Cochran, A. Hexemer, G.E. Stein, G.H. Fredrickson, E.J. Kramer, X. Li, J. Wang, S.F. Hahn, ''Effect of chain architecture and surface energies on the ordering behavior of lamellar and cylinder forming block copolymers,'' Macromolecules, vol. 39, pp. 9346-9356, 2006.

1. A. Hexemer, G.E. Stein, E.J. Kramer, S. Magonov, ''Block copolymer monolayer structure measured with scanning force microscopy moiré patterns,'' Macromolecules, vol. 38, pp. 7083-7089, 2005.