The work opens the possibility of circuit elements only a few molecules across, roughly ten times smaller than the features in current-generation chips such as the Pentium 4 with features 100-150 nanometers (microns) in size. With the new self-assembling nanotech method, “achieving dimensions of tens of nanometers is inexpensive and routine,” says team leader Richard Nealey. His team’s work with these substances is detailed in the July 24 issue of Nature.
The new technology is called “templated,” or “directed” self-assembly, and uses standard photolithographic techniques to chemically alter the surface of a standard silicon wafer. However, also employed are clever optical manipulations with extreme ultraviolet light, which has a much shorter and industrially impractical wavelength compared to the light used in conventional microchip lithography. With this special light, the team was able to lay down an alternating pattern of straight, parallel, chemically activated stripes only 20 to 30 nanometers wide with no inherent electrical properties, just tiny trenches in silicon.
Next, the team washed the patterned silicon surface with a solution containing block copolymers, plastic-like compounds composed of two or more long polymer chains connected at the ends. Without the etched template, these polymers recombined in curved, twisting parallel patterns like a circular maze. However, with the etched base to connect to, the polymers recombined in perfectly straight, perfectly formed lines of great length, right on top of the nanoscale stripes showing no evidence of swirling or other undirected behavior. “The final result is completely free of defects, which is very hard to do at these small sizes,” said Nealey. “Our technique delivers registration and overlay — two critical requirements that have been missing from other attempts at nanoscale lithography. This kind of hybrid technology can integrate self-assembling materials, such as block co-polymers, into existing manufacturing processes, such as lithography, and deliver molecular-level control.”
In the future, two important additions are expected to convert this technique into actual nanoscale electronic devices. First, different formulations of the polymers used can add various metal ions to the basic carbon chains and so impart various required electrical properties to the plastic lines. Second, new types of integrated circuit designs tailored to this material will be needed to turn the resulting plastic lines into arrays of diodes and transistors. “The industry has been using the same integrated circuit designs for years,” says Nealey, “just shrinking them as the chips are packed more densely. But here we can’t do that; with our technique we can only make very simple shapes like lines and circles. On the other hand, if we can make these simple designs very inexpensively, then the question for the chip designers becomes, ‘What can we do with this?'”