Tabletop system generates extreme-UV laser light

Tabletop system generates extreme-UV laser light
BOULDER, Colo. — 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 13-nanometer EUV beam in Murnane's lab might help the VLSI community break the 100-nm feature size barrier. Unfortunately, Murnane's very compact EUV laser only produces about a milliwatt today, whereas direct VLSI lithography requires about 100 watts. 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.

The breakthrough has come from a fast-developing field called "high-harmonic generation" (HHG), where researchers fire visible or ultraviolet laser light into a gas, ionizing it, which then transfers energy from the fundamental frequency into very high harmonic orders. This type of research has required extremely high-powered lasers, and while the converted light is phase-coherent, it is not tightly focused into a spatially collimated beam.

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 claim 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 region — you would use it for morphology studying structures, to check the photomask, to check the integrity of the optics, to check anything morphological," Murnane said.

Direct observations of nanoscale structures have been hampered by the relatively long wavelength of light. Researchers have had to deduce many aspects of nanostructures indirectly using scanning-tunneling or atomic-force microscopy. Electron microscopes, in addition to being large and expensive, require specially prepared specimens to be housed in vacuum chambers. However, with a 13-nm-wavelength laser that can be pulsed in 5 femtoseconds, the whole world of nanoscale phenomena can be directly observed.

By making a microscope that incorporates such a light source, engineers not only could "see" nanoscale features, but also could freeze-frame processes at the molecular level as atoms bond to one another — a sort of slow motion for observing nanoscale processes.

The ability to do true holography with soft X-rays would be an exciting new capability in itself. EUV radiation has stretched conventional refractive and reflective optics to their physical limits, since the photons tend to penetrate most surfaces. EUV-lithographic systems have moved entirely to reflective optics using highly specialized surfaces to get around the problem.

"Even though our average power is only a milliwatt, our peak power is very high, and the wavelength is completely tunable, so there are lots of applications for such a laser in photo inspection, in holography, in microscopy — we could make an EUV microscope with this laser, no problem," Murnane said.

In 1998 the research team showed that the process by which its current laser is made was very efficient, but the team's recent announcement follows a four-year period of careful characterization of the beam. Murnane claims the group has proved that all the good properties of visible-wavelength lasers are preserved in the new EUV source.

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 version — taking blue or ultraviolet light into the soft-X-ray region — much higher harmonics are accessed and the energy output can be far higher. Up to now, the downside has been the lack of spatial coherence in the beam.

For the future, Murnane plans to demonstrate laserlike EUV beams that extend the "good" qualities of visible-light lasers down to the EUV region. "We are trying to make smart structures from these waveguides, so we are modulating them to extend the same kind of phase matching that has been demonstrated at visible wavelengths to much shorter wavelengths," Murnane said.