Block Copolymer Thin Films

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 (body-centered-cubic packing in bulk) reconstruct to form a hexagonal packing of spheres, while cylinders typically lie down in the plane of the substrate.

Block Coploymer Films
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. We have developed a shear-alignment process to create long-range order in these films, using a soft silicone rubber (PDMS) pad, as schematized below:

Pad Shearing
Apparatus for the shear alignment of supported block copolymer thin films. Shear stress is applied through the PDMS pad, held in contact with the film by a simple dead load; temperature control is provided by a digital hot plate (Angelescu et al., Adv. Mater., 16, 1736 (2004)).

This simple approach is remarkably effective for the alignment of monolayers (or multilayers) of cylinder-forming block copolymers, as well as bilayers (or thicker stacks) of sphere-forming diblocks. Examples of successful alignment are shown in the tapping-mode atomic force microscope (TM-AFM) images below. We have also used such aligned films as nanofabrication templates to prepare ordered nanostructure arrays covering square-centimeter areas.

Shear-aligned Block Copolymer
TM-AFM images of shear-aligned block copolymer films. Shear direction indicated by red arrows; the alignment is coincident with the shear direction, over the entire centimeter-squared sheared area (set by the area of the PDMS pad). Both are polystyrene-poly(ethylene-alt-propylene) diblocks, PS-PEP, with a minority PS block; the rubbery PEP matrix and glassy PS inclusions provide excellent contrast for TM-AFM imaging at room temperature. Left: monolayer of cylinder-forming PS-PEP 5-13. Right: bilayer of sphere-forming PS-PEP 3-24; only the top layer of spheres is revealed by TM-AFM (cylinders:Angelescu et al., Adv. Mater., 16, 1736 (2004); spheres:Angelescu et al., Adv. Mater., 17, 1878 (2005)).

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