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Evanescent wave litho to surface at SPIE

Posted: 14 Feb 2006 ?? ?Print Version ?Bookmark and Share

Keywords:EWL? evanescent wave lithography? contact lithography? Rochester Institute of Technology? 26nm?

Is a technology called evanescent wave lithography (EWL) the next big thing in semiconductor manufacturing?

For years, researchers at the California Institute of Technology, the University of California at Berkeley and other universities have been exploring EWL. Some know this near-field technology as "contact lithography."

In this approach, which is still in the R&D stage but talked about for some time, an evanescent optical field is set up directly below a contact mask, enabling sub-micron resolutions, according to researchers.

At the upcoming SPIE Microlithography conference, which is slated from Feb. 19-24, the Rochester Institute of Technology (RIT) is expected to present a paper that claims it has produced a 26nm image based on EWL. This, in turn, opens EWL as an extension to conventional projection lithography as a means for sub-32nm chip production, according to RIT.

"This is very different from the evanescent wave imaging, also called near-field or contact imaging, at Berkeley and others," said Bruce Smith, RIT's professor of microelectronic engineering and director of the Center for Nanolithography Research in the Kate Gleason College of Engineering.

"We do not place a mask in contact with an image plane, which would be difficult to implement into manufacturing. Our evanescent wave lithography (EWL vs. evanescent wave imaging) is a more logical extension to optical lithography," Smith said.

"We are using projection imaging to direct images from the bottom element of the optical system through media with refractive indices lower than the numerical aperture of the imaging system," he said. "We have been able to achieve numerical apertures up to 1.85NA, well beyond the refractive index of any immersion fluid or photoresist currently available for 193nm exposure."

The technology has potential in the future. "Immersion lithography has pushed the limits of optical imaging," he added. "Evanescent wave lithography continues to extend this reach well into the future."

Immersion lithography is limited by Snell's Law to numerical apertures (NA) less than the lowest refractive index material in a lithography imaging system. Currently, with fused silica optics, high-index immersion fluids, and ArF photoresists, the largest theorectical numerical aperture is about 1.65.

In EWL, RIT "has made use of sapphire as an optical element, with a refractive index of 1.92 at 193nm," Smith said. "The approach that we've used to eliminate the media (i.e. fluid) and the photoresist limitations is to make use of the evanescent field that exists near the interfaces between the sapphire, fluid, and photoresist by propagating the field distances less than 100nm and recording it directly into the photoresist."

RIT, in effect, believes the technology is a next-generation lithography candidate. "Though this 'frustrated total internal reflection' produces an image only tens of nanometers deep, this can correspond to the image depth requirements for sub-32nm lithography," according to Smith.

"We have achieved, for example, an NA of 1.85 that propagates into a photoresist with a refractive index of only 1.70 to produces images 26nm in size," he added.

Over the years, others are taking different approaches. According to a paper from the California Institute of Technology, which was issued some time ago, researchers have been exploring newand exotic near-field lithographic technologiesas alternatives to conventional optical scanners.

Among those near-field technologies include ion-beam lithography and atomic force microscopes, among others. Two parallel approaches that do not require short-wavelength light are evanescent near-field lithography and evanescent interferometric lithography, according to the paper.

In one approach outlined some time ago by the University of Alabama, an intensity mask with sub-wavelength gaps is placed in close proximity to the resist. The evanescent wave emanating from the gap exposes the resist. The field penetrates only a few tens of nanometers from the resist, enabling sub-micron resolutions.

- Mark LaPedus
EE Times

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