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Posted: Aug 03, 2015

A facile process for the vertical orientation of block copolymer nanodomains

(Nanowerk Spotlight) Directed self-assembly (DSA) of block copolymers is one of the most promising techniques to enable the continued miniaturization of integrated circuits due to low cost, high speed, and simplicity of the process. The DSA process combines top-down photolithography for creation of guiding patterns with engineered new materials and processes to facilitate cost effective bottom-up techniques for pattern density multiplication and defect rectification. However, industrial-scale application of DSA still faces challenges and last year, for instance, a European consortium launched the 4.9 million CoLiSA.MMP project to develop new material and process models and a computational lithography framework for DSA.
"Although vertical orientation of lamellar and cylindrical nanodomains of block copolymers on a substrate is desirable, parallel orientation is usually obtained in a thin film because of different affinity between two block segments in a block copolymer toward the substrate and/or air," Jin Kon Kim, a professor in the Department of Chemical Engineering at Pohang University of Science and Technology (POSTECH), and Director of National Creativity Research Initiative Program for Smart Block Copolymers, tells Nanowerk. "In order to induce vertical orientation, researchers have developed diverse treatments to neutralize the preferential affinity."
He explains that, in order to induce vertical orientation of block copolymer nanodomains in the film state, diverse pre- or post-treatments to neutralize the preferential affinity between a substrate and each block of the block copolymer need to be introduced, for instance, solvent annealing, surface neutralization by random copolymer brush treatment, graphoepitaxy, temperature gradient, electric field or top coating.
However, such additional treatments make the fabrication process time-consuming, costly and complicated and thus impede commercialization of block copolymer DSA. Therefore, a simple and cost-effective DSA process that does not require these treatments is required for the fabrication of vertically oriented nanodomains to compete with conventional top-down lithography.
In new work, Kim and his team have now introduced a novel self-neutralization concept by designing molecular architecture of a block copolymer to develop vertically oriented lamellar or cylindrical nanodomains without pre- or post-treatments. By employing 18 arm star-shaped poly(methylmethacrylate)-block-polystyrene copolymers with two different polystyrene volume fractions (fPS of 0.60 and 0.78 corresponding to lamellar and PMMA cylindrical nanodomains), the researchers were able to develop vertically oriented lamellae and hexagonally packed cylinders on the substrate by spin-coating without using any pre- or post-treatment such as solvent annealing or neutralization process of the substrate.
vertical orientation of block copolymer nanodomains
The cylindrical (left) and lamellar (right) nanodomains are vertically aligned directly on the flexible substrate without additional treatment for neutralization of the preferential affinity between a substrate and each block of the block copolymer. This is attributed to the architecture of star-shaped block copolymer by forming different chain stacking from linear block copolymer. (Image: Prof. Jin Kon Kim, POSTECH)
They reported their findings in the July 30, 2015 online edition of Advanced Functional Materials ("Vertical Orientation of Nanodomains on Versatile Substrates through Self-Neutralization Induced by Star-Shaped Block Copolymers").
For their novel approach, the team formed vertically oriented nanodomains of the star-shaped block copolymer during spin coating. Although the as-spun film showed relatively poor spatial ordering of vertically aligned nanodomains, the spatial ordering was significantly improved by thermal annealing for a short time (less than 30 minutes).
The scientists attributed this to the significant entropy loss arising from junction points of star-shaped molecular architecture that overcomes enthalpy gain from the difference in the surface affinity between core and shell chains.
"Our facile and simple self-neutralization approach could greatly contribute toward commercialization of directed self-assembly (DSA) for microelectronics and data storage media," notes Kim.
"We used star-shaped block copolymer containing PS and etchable PMMA block, but sub 10 nm domain spacing is impossible due to the low χ," he points out. "It would greatly contribute toward commercialization of DSA for next-generation integrated circuit and data storage media if star-shaped block copolymer containing high χ blocks such as polystyrene-block-poly(dimethyl siloxane) that could provide sub 10 nm spacing could be aligned vertically by controlling molecular architecture – but this is also a future work."
Prior to this work, there were two main challenges that prevent the application of block copolymer DSA in industrial processes, though in principle it would provide a significantly more cost-effective bottom-up process. The first requirement is a perfect array over a large area (wafer scale) without defects. The second requirement is a simple and facile process for the vertical orientation of block copolymer nanodomains.
The POSTECH team overcame the second challenge; if researchers now would manage to solve the first one as well, block copolymer DSA could indeed replace conventional EUV photolithography.
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