Annihilating nanoscale defects
Researchers at the University of Chicago and the US Department of Energy's Argonne National Laboratory claim to have found a way for the semiconductor industry to hit miniaturisation targets on time and without defects.
The researchers’ technique includes creating patterns on semiconductor surfaces that allow block copolymer molecules to self-assemble into specific shapes, but thinner and at much higher densities than those of the original pattern. The researchers can then use a lithography technique to create nano-trenches where conducting wire materials can be deposited.
This is a contrast to the industry practice of using homo-polymers in complex ‘photoresist’ formulations, where researchers have become unable to make the material smaller.
Before they could develop their fabrication method, however, the scientists needed to understand exactly how block copolymers self-assemble when coated onto a patterned surface - their concern being that certain constraints cause copolymer nanostructures to assemble into undesired metastable states. To reach the level to fabricate high-precision nanocircuitry, the team had to eliminate some of these metastable states.
"Molecules in these metastable states are comfortable, and they can remain in that state for extraordinarily long periods of time," explained Juan de Pablo of the University of Chicago's and Argonne's Institute for Molecular Engineering. "In order to escape such states and attain a perfect arrangement, they need to start rearranging themselves in a manner that allows the system to climb over local energy barriers, before reaching a lower energy minimum.
“What we have done in this work is predict the path these molecules must follow to find defect-free states and designed a process that delivers industry-standard nanocircuitry that can be scaled down to smaller densities without defects," he added.
De Pablo and his team used the Mira and Fusion supercomputers at the Argonne Leadership Computing Facility to generated molecular simulations of self-assembling block polymers along with sophisticated sampling algorithms to calculate where barriers to structural rearrangement would arise in the material.
After all the calculations were done, the researchers could predict the pathways of molecular rearrangement that block copolymers must take to move from a metastable to stable state. They could also experiment with temperatures, solvents and applied fields to further manipulate and decrease the barriers between these states.
"Manufacturers have long been exploring the feasibility of using block copolymer assembly to reach the small critical dimensions that are demanded by modern computing and higher data storage densities," de Pablo said. "Their biggest challenge involved evaluating defects; by following the strategies we have outlined, that challenge is greatly diminished."
The research team says it will continue their investigations with a wider class of materials, increasing the complexity of patterns and characterising materials in greater detail while also developing methods based on self-assembly for fabrication of three-dimensional structures.
Their long-term goal is to arrive at an understanding of directed self-assembly of polymeric molecules that will enable creation of wide classes of materials with exquisite control over their nanostructure and functionality for applications in energy harvesting, storage and transport.
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