- Fresnel Optics Investigated for Next-Generation Lithography
Daniel S. Burgess
Achromatic Fresnel optical sys-tems under development at Xradia Inc. of Concord, Calif., promise to compete with mirror-based solutions for next-generation extreme-ultraviolet lithography in the production of integrated circuits. The researchers at the company suggest that using such optics rather than the multilayer mirrors would reduce fabrication complexity and cost, and that they would be suitable for use with broadband radiation sources.
As they continue to stay apace with Moore's law, semiconductor chip manufacturers soon will require new tools to print the smaller feature sizes that enable higher circuit densities. Today's exposure systems are based on refractive lenses and therefore are constrained by the resolution limit to producing linewidths no smaller than about 65 nm, explained Yuxin Wang, director of research and development at Xradia.
Moreover, the promising illumination sources for next-generation lithography produce 13.4-nm radiation, which most materials readily absorb, making traditional refractive lens systems unusable.
As a result, the industry is investigating exposure tools based on reflective mirrors. These lithography cameras, which have demonstrated resolutions of approximately 50 nm, contain six or more large multilayer mirrors that require atomic-scale coating accuracies to produce the required surface shapes and layer thicknesses. "These mirror systems are truly technological marvels that rival the Hoover Dam and Hubble Space Telescope," he said. "Because they are such technological marvels, they are very difficult and expensive to produce."
An alternative technology is diffraction gratings, which are easier to fabricate and have demonstrated resolutions of 20 nm. Because these devices separate the incident radiation into different wavelengths, however, the systems that employ them cannot focus broadband radiation, and they suffer from chromatic aberration because the different wavelengths have different focal lengths, he said. As a result, the systems suffer from a lack of throughput when used with broadband illumination sources such as laser-produced plasmas and x-ray tubes.
Xradia's achromatic Fresnel optics would retain the benefits of diffractive elements but would solve the aberration problem by adding a thin refractive lens with chromatic aberration opposite to that of the other component, a diffractive Fresnel zone plate. The latter would feature concentric rings of diffractive material fabricated on a silicon substrate by lithographic techniques.
The refractive Fresnel lens also would be produced in silicon, employing focused ion-beam lithography, diamond turning or other techniques. Together, the optics would increase the usable bandwidth 100 to 1000 times, compared with a zone plate alone, depending on the resolution and on the size of the optics, Wang said.
The techniques for fabricating the zone plates are mature, he added, and such diffractive elements with a 50-nm resolution are fashioned routinely. The researchers are investigating approaches for the production of the refractive Fresnel lenses, and the company hopes to begin incorporating achromatic Fresnel optics into its products in 2004 or 2005.
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