The team, led by Caroline Ross and Karl Berggren, has shown that a block copolymer, polystyrene-polydimethylsiloxane, can be "forced" to form a complex set of 3D patterns on a substrate surface. The technique, detailed in Advanced Materials, involves using a simple template comprising an array of small pillars made of silica, explains team member Amir Tavakkoli. "We have found that we can create a rich variety of microdomain morphologies on a single substrate by modifying the layout of the pillars," he said. "One such morphology is a high-resolution square lattice of dots, but cylinders, spheres, ellipsoids and double cylinders can easily be produced too."

The spaces between features in the final patterns can be smaller than the original periodicity in the block copolymer, he adds. This means that the number of components that can be packed onto the sample substrate is increased. Indeed, the feature sizes that can be made in this way are very small, at around 10–20 nm. In contrast, those produced by conventional photolithography are at least 10 nm bigger. "Being able to fabricate such small structures will be important for the future because feature sizes are continuing to shrink, in accordance with Moore's Law," Tavakkoli reminds us.

Another big advantage of the new technique is that it can produce square and rectangular structures. These shapes form the basis of most microchip layouts but are quite difficult to produce through conventional self-assembly processes, says Ross. "When molecules self assemble, they have a natural tendency to create hexagonal shapes – as in a honeycomb or an array of soap bubbles between sheets of glass. They do not naturally form squares or rectangles."

The MIT researchers' fabrication technique starts with the construction of a precisely controlled pattern of nanopillars on a silicon substrate surface using high-resolution electron beam lithography. Next, the pillars are chemically coated with a thin polystyrene "brush" layer that subsequently interacts with the block copolymer when it is applied to the substrate surface. The copolymer then self assembles into a pattern that is guided by the pillars.

Polymer strain

The process works because the template coating is arranged in such a way as to repel one of the components in the polymer. This produces a significant amount of strain in the polymer, forcing it to twist and turn, explains Berggren. "In doing so, the polymer rearranges itself on the substrate surface into more interesting patterns."

The team says that it now plans to investigate how to remove the physical post-template from the final pattern, and then transfer the pattern to the substrate. "We would then like to make some functional devices and also want to understand and model the self-assembly process so that we can generalize this work to other block copolymers and feature geometries," Tavakkoli told nanotechweb.org.