Block Coplymer Thin Films and Nanofabrication
The phase diagram of block copolymers in bulk is now well established, and the familiar sphere-cylinder-gyroid-lamellae progression has been verified in a number of material systems. When these same block copolymers are deposited as “thin” films—one or at most a few microdomain spacings in thickness—they can adopt structures mirroring those which they form in bulk, or reconstruct to form new structures which are stable only in thin films. Typically, sphere-forming block copolymers (bcc packing in bulk) reconstruct to form a hexagonal packing of spheres, while cylinders typically lie down in the plane of the substrate.
Structures typically adopted by asymmetric block copolymers in thin films, where the minority component is shown in blue. Left: hexagonal packing of spheres. Right: in-plane cylinders. When the minority component has an affinity for one or both film surfaces (either air or substrate), the polymer will form an extra brush-like “wetting layer”, typically half a microdomain spacing thick, at the relevant interface. In this example, wetting layers are present at both the substrate and air interfaces, which is the case for a polystyrene(red)-polydiene(blue) diblock deposited on an oxide-coated silicon wafer.
Block copolymers are easily deposited as thin films over large areas by spin coating. As in bulk, there is a strong thermodynamic preference for a particular microdomain size and periodicity, set by the block copolymer’s molecular weight. This makes such block copolymer thin films useful as templates for the fabrication of dense two-dimensional arrays of nanostructures on substrates. However, if left to themselves, the microdomains will organize into grains of very limited coherence length, typically a micron or so—a feature of “unguided” or “natural” self-assembly. As part of our current research efforts, we have developed a shear-alignment process to create long-range order in these films, and are using such aligned films as nanofabrication templates; these projects are carried out in collaboration with Professor Paul Chaikin (Physics, New York University) and Dr. Douglas Adamson (Princeton Institute for the Science and Technology of Materials).