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Surface Plasmons Enable Subwavelength Lithography

Photonics Spectra
Aug 2005
Hank Hogan

A pair of researchers at the University of Texas at Austin have found that the way to subwavelength nanoscale photolithography is only skin -- or surface -- deep. They used surface-plasmon-assisted photolithography with polarized 355-nm laser radiation to image 130-nm features. The technique could have applications in the fabrication of integrated circuits.

Importantly, said Shaochen Chen, an associate professor of mechanical engineering who worked with Dongbing Shao on the project, the approach is compatible with existing lithography machines.

Surface Plasmons Enable Subwavelength Lithography
Exploiting the effects of surface plasmons, researchers created features as small as 130 nm with 355-nm polarized laser radiation using masks with 100-nm-wide slits spaced 500 (above) or 1000 nm apart (right). Courtesy of Shaochen Chen.

Semiconductor lithography is an optical affair, but diffraction places limits on how small a feature can be imaged for an exposure source of a given wavelength. Recent discoveries have revealed that perforated metal films transmit radiation exceptionally well, as a result of the workings of surface plasmons, groups of excited electrons that arise at the boundary between a conductor and insulator and that can alter the impact of local electromagnetic effects.

Chen and Shao thus extended the surface plasmon effect of a perforated film to a planar film for photolithography applications. In their demonstration, they constructed a metal mask by creating a repeating pattern of lines and spaces in a 70-nm-thick layer of gold or titanium atop a quartz plate; the slits in the metal were 100 nm wide and spaced 500 or 1000 nm apart. They then coated a silicon substrate with an 80-nm-thick titanium shield and covered that with a negative-tone photoresist.

They placed the metal mask in contact with the resist and illuminated the back with a 355-nm polarized beam from a Continuum Nd:YAG laser, exposing the resist. The thin, perforated metal enhanced the amount of radiation traveling through the subwavelength apertures, while the surface plasmons in the shield helped to confine it. As a result, the threshold illumination of the photoresist was reached, and 130-nm features, well below the 355-nm source wavelength, were imaged. The results matched what the researchers had predicted based on electromagnetic theory.

Chen noted that the method can image even smaller features. He also said that ongoing research will make the technique more like that used today in semiconductor manufacturing.

"We are working on another system that has a metal mask, photoresist, oxide and silicon -- just like traditional lithography, but with a sub-80-nm resolution," he said.


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