Lithography Past Light's Limits

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Plasmonic lithography is "a technology that bears looking at because we need better solutions for sub-20-nanometer lithography than we have today," says John Hartley, director of the Advanced Lithography Center at the University of Albany's College of Nanoscale Science and Engineering. In optical lithography, light shines through a mask--a type of stencil--onto a substrate, such as a silicon wafer, that's coated with a light-sensitive chemical called a photoresist. The photoresist hardens where the light strikes it; elsewhere, it can be rinsed away, reproducing the pattern of the mask. It's possible to make finer features by using shorter-wavelength light, but this approach quickly becomes impractical, says Zhang. Shorter-wavelength light has higher energy, and producing it requires expensive lasers or, in the case of extreme ultraviolet light, a synchrotron. Other technologies, such as electron beams, can etch very fine features without masks, but they're slow. The Berkeley flying lens is much faster and will become faster still, says Zhang, when the number of plasmonic lenses in an array is increased from the current 16 to 100,000.

So far, the Berkeley researchers have demonstrated that they can use the technology to etch 80-nanometer lines. This is large compared with the best optical-lithography techniques currently in use. However, Zhang says, engineering the distance-control system was the hard part. Making the concentrators smaller, for example, will increase the technique's resolution.

But higher-resolution light beams won't do much good without a new generation of photoresists that can resolve features that are only five nanometers or so across; the photoresists on the market were designed to work with wider beams of light. Zhang says that he is currently collaborating with chemists to address this problem.

Zhang says that the air-bearing design could also enable other applications of plasmonics, particularly high-resolution imaging. "If we can print 50 nanometers, we can image 50 nanometers," he says. The flying lenses could be used as probes for evaluating the quality of computer chips or for biological imaging, allowing biologists to watch processes unfolding in living cells at the molecular level.

Zhang is in the process of spinning out a company to develop the technology and has been contacted by major semiconductor companies, he says.