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Tabletop system generates extreme-UV laser light

Posted: 01 Aug 2002 ?? ?Print Version ?Bookmark and Share

Keywords:table laser system? optical fiber? optical waveguide? lithography system?

A small, inexpensive tabletop laser system that can produce coherent extreme-ultraviolet laser light has been built by a research team at the University of Colorado. The system uses a hollow optical fiber filled with argon gas as a nonlinear optical waveguide that upconverts visible laser light into the extreme-ultraviolet range a critical part of the spectrum for advanced lithography systems.

The group began with a $100,000 commercial laser and redesigned it. The result is an EUV source that can be built using only $5,000 of off-the-shelf parts. But more important is the laser quality of the radiation produced by the new EUV laser, said Margaret Murnane, a professor of physics at the University of Colorado at Boulder.

"What this technology allows us to do is to take the same high-quality laser beam in the visible [spectrum], and translate that into much shorter wavelengths, which is normally very difficult to do," Murnane said.

"It is a very unique light source. The pulse duration is only 5 femtoseconds, which pretty much means you can watch element-specific reactions happen on surfaces," she said.

The coherent 13nm EUV beam in Murnane's lab might help the VLSI community break the 100nm feature size barrier. Unfortunately, Murnane's very compact EUV laser only produces about a milliwatt today, whereas direct VLSI lithography requires about 100W. The waveguide used in the experiment, however, has produced the most efficient upconversion of laser light into soft x-rays that has so far been observed in this type of experiment.

Development breakthrough

The breakthrough has come from a fast-developing field called "high-harmonic generation" (HHG), where researchers fire visible or UV laser light into a gas, ionizing it, which then transfers energy from the fundamental frequency into very high harmonic orders. This type of research required extremely high-powered lasers.

By using a long fiber to collimate the original light, the new approach has achieved the highest-quality EUV laser light of any HHG project, the researchers claimed in a report in the journal Science. "We can make a very high-quality laser light beam, in other words one that can make interference patterns, holograms and VLSI photolithography," Murnane said.

Although the system may not be ready for a high-end industrial process like VLSI lithography, it could become a significant tool for directly imaging small-scale features on silicon chips. "Think of our laser as the helium neon [highest quality/lowest wattage] of the UV regionyou would use it for morphology [studying structures], to check the photomask, to check the integrity of the optics, to check anything morphological," Murnane said.

The new approach closely resembles the established technique of frequency doubling used with infrared-laser diodes. An optically nonlinear medium such as a lithium niobate crystal transfers energy from the fundamental frequency to the first harmonic in the blue region of the spectrum. The shorter wavelengths can be used to double the capacity of optical disks. In the higher-energy versiontaking blue or UV light into the soft-X-ray regionmuch higher harmonics are accessed and the energy output can be far higher.

R. Colin Johnson

EE Times

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