Producing ultra-dense, orderly arrays of nanometre-sized elements for next-generation electronic devices is no easy task, but researchers in the US have come up with a new way of tackling the problem.

Their technique involves placing a thin film of block copolymers onto the surface of a commercially available sapphire wafer. The process produces large films of highly patterned and densely packed nanostructures that are almost defect free (Science 323 1030).

The technique could lead to dramatic improvements in the data-storage capacity of electronic media, say Tom Russell at the University of Massachusetts Amherst, Ting Xu at the University of California Berkeley and colleagues.

The new work relies on the fact that molecules in thin films of block copolymers — two or more chemically dissimilar polymer chains linked together — will self-assemble into ordered patterns when spread out on a surface.

The problem in the past however, was that the order broke down as the area increased. This lack of order means that you can no longer write to or read individual domains, which means that such patterns cannot be used in data storage devices.

Saw-toothed ridges

Russell, Xu and colleagues have overcome this problem by layering the film of copolymers onto the surface of a commercially available "miscut" sapphire crystal — a crystal cut at a small angle to an atomic plane. When this crystal is heated to between 1300 and 1500 °C for 24 hours, its surface reorganizes into a highly ordered patterned of saw-toothed ridges that can then be used to guide the self-assembly of the block copolymers.

Using their technique, the researchers succeed in making almost defect-free arrays, measuring more than 3 × 3 cm2, that contain individual elements with feature sizes as small as 3 nm. This translates into a potential data-storage density of about 10 terabits per square inch. By contrast, the best commercial hard drives can store information at densities of about 200 gigabits per square inch, which is about one fiftieth the density achieved by the new technique.

Although sapphire was used in this work, Russell and Xu explain that other single-crystalline off-the-shelf materials, like silicon, can be employed to guide self-assembly too.

The technique is also better than using "top down" methods of nanopatterning, such as nano-photolithography, which is expensive, environmentally unfriendly (it uses harsh chemicals) and rapidly approaching the resolution limits of light.

The researchers have filed a joint patent on the technology.