“We show that it is possible to produce self-assembling block copolymers (BCPs) that are completely defect-free over large areas,” explains team leader Juan de Pablo. “Having defect-free material is important to the semiconductor industry for commercially fabricating efficient electronic devices.”

BCPs could be used to make well-defined periodic nanopatterns with feature sizes ranging from nanometres to hundreds of nanometres and directed self-assembly of BCPs by such surface patterns could be used to fabricate next-generation nanocircuits by lithography. It has recently come to light that kinetics, and not only thermodynamics, plays a key role for polymeric materials to self-assemble into perfect, defect-free ordered states. “Perfect” in this context implies not more than one defect (around 5 nm in size) over a sample area as large as 100 cm2 (a “needle in a haystack”, say the researchers).

Annealing at the order−disorder transition temperature

De Pablo and colleagues obtained their results using molecular simulations of self-assembling block polystyrene-polymethyl methacrylate (PS-b-PMMA) copolymers on patterned substrates. Along with sophisticated molecular modelling algorithms, they determined the molecular pathways through which a material “escapes” defective states, and the amount of energy required to do so. The team found that an extraordinarily large thermodynamic driving force is not necessarily sufficient for removing defects but that they can be removed by annealing at temperatures near the so-called order−disorder transition of the material.

“Anyone using lithography to fabricate ultra-small devices will benefit from this research”, de Pablo tells nanotechweb.org. “Computer chip manufacturers or data storage media companies, for example, have been looking into the possibility of using block copolymer assembly to reach the small critical dimensions needed for devices that calculate faster or store larger amounts of data.”

The team, which includes researchers from Chonnam National University in Korea, Cornell University in the US, Georg-August-Universität in Germany and Fudan University in China, are now planning to study materials other than just PS-b-PMMA, as well as more complex patterns. “We need to characterise these materials in greater detail – for example, their roughness and resistance to etch,” says de Pablo. “We also envisage developing methods based on self assembly for fabricating 3D structures.”

The work is detailed in PNAS doi: 10.1073/pnas.1508225112.