Self-assembly goes around bends

Many researchers are working on the idea of self-assembly -- coaxing materials to assemble at the molecular level. Self-assembly could someday be used to build nanoscale structures and to build materials molecule-by-molecule.

Researchers from the University of Wisconsin at Madison, the Korea Advanced Institute of Science and Technology, and the Paul Scherrer Institute in Switzerland have taken the idea to step forward with a way to make polymer chains automatically assemble in non-regular patterns, including sharp angles. The method could eventually be used to build precise features as small as ten nanometers -- the length of a row of 100 hydrogen atoms.

Existing polymer self-assembly methods are able to form patterns, but are limited to making arrays of dots or straight lines. The new method makes for more precise patterns, includes the ability to make less regular features, and is very precise, according to the researchers. They produced patterns with 50- to 80-nanometer-wide parallel lines that contained bends of 45, 90 and 135 degrees.

These patterns can be oriented precisely on chips, and could be used to make computer circuits that are smaller than those made using today's silicon technology, according to the researchers.

The key to the method is a mix of polymers and a nanoscale template. To make the patterns, the researchers mixed block copolymers, or pairs of polymer chains connected at one end, and homopolymers, or single polymer chains so that they self-assembled into structures on chemically nano-patterned templates.

They made the patterns by coating a silicon wafer with a light-sensitive plastic and using light and chemicals to etch an initial pattern of lines. They then etched away the lines to a flat surface whose surface chemistry still followed the initial pattern. The last step was to apply the blend of polymers to this surface; the block copolymer covers the areas where the lines had been and the homopolymer covered the remaining areas.

The method could be used practically in five to ten years, according to the researchers. The work appeared in the June 2, 2005 issue of Science (Directed Assembly of Block Copolymer Blends into Nonregular Device-Oriented Structures).