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Optical lithography can cover 12nm, says MIT

Posted: 24 Jul 2008 ?? ?Print Version ?Bookmark and Share

Keywords:optical? lithography? interference?

Optical lithography can be extended to 12nm, according to Massachusetts Institute of Technology (MIT) researchers who have so far demonstrated 25nm lines using a new technique called scanning beam interference lithography.

"We are demonstrating 25nm features, but using our present technique we believe we can achieve even finer resolutions by another factor of two [12nm] at least," said Mark Schattenburg, director of the Space Nanotecnology Laboratory, MIT. "Using our scanning beam interference lithography technique, optical lithography is mainly limited by the roughness of materials and our ability to see such fine features," he added.

Interference lithography uses two lasers of different frequencies that interfere with each other to create a grating image with much higher resolution than is possible with either laser alone. Nevertheless, non-optical limits to interference lithography have limited its use to testing photoresist processes rather than actually imaging circuit patterns on wafers.

By adding scanning to the mix, the MIT researchers believe interference lithography can become commercially viable at the 25nm node and beyond.

"In traditional interference lithography the wafer is stationary, but in scanning beam interference lithography the wafer is constantly moving," said Schattenburg.

Unfortunately, moving the wafer relative to the lasers Doppler shifts their frequencies. This causes irregularities in the grating image, a pitfall that had to be overcome to make scanning beam interference lithography feasible. The researchers claimed to have solved the problem with a novel method that uses sound.

"We synchronize the grating image with the movement of the wafer using 100MHz sound waves," said Schattenburg. The sound waves vibrate the laser's crystals, slightly shifting their frequency up and down as they recede from and approach toward, respectively, the desired feature being imaged. This compensation produces a stable, consistent grating image across the patterns being transferred to the wafer, according to the researchers.

Schattenburg has founded a lithography company called Plymouth Grating Laboratory, which is currently considering the commercialization of the new lithography technique.

Research funding was provided by the Space Nanotechnology Laboratory, the Kavli Institute of Astrophysics and Space Research, National Aeronautics and Space Administration and the National Science Foundation.

- R. Colin Johnson
EE Times

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