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Lasers & Sources

Going to extremes in ultraviolet

Eye on Technology - ultraviolet sources

From oemagazine April 2001
31 April 2001, SPIE Newsroom. DOI: 10.1117/2.5200104.0001

At universities and research institutes across Japan, researchers are working to develop new approaches to the generation of extreme ultraviolet (EUV) light, providing short wavelength sources for commercial and scientific applications.

It's a bird. It's a plane. No, it's Super SOR, and when it's completed in 2005, this synchrotron at the University of Tokyo (Tokyo, Japan) will be the most brilliant light source in the world, topping the output of such installations as the Advanced Light Source (Berkeley, CA) and the Elettra Synchrotron Light Source (Trieste, Italy). According to Tadashi Koseki, head of the university's synchrotron project, Super SOR's output will range from vacuum ultraviolet (VUV) to hard x-rays, which will be accomplished by varying the electron energy between 1.0 and 1.6 GeV. Beam emittance will be 0.7 nm/rad at 1 GeV and 6 nm/rad at 1.6 GeV.

All the design and R&D work has been done, Koseki says. "We have already overcome most of the technical difficulties. We are only waiting for approval from the government." Koseki expects approval with the FY '02 budget, hence his 2005 completion estimate. The new facility will be located on the University of Tokyo's Kashiwa Campus in Chiba prefecture.

lithography

Far south and west of Tokyo, Shoichi Kubodera heads a research team working on EUV lasers at Miyazaki University (Miyazaki, Japan). While the lithography industry is concentrating on fluorine (F2) excimer lasers as the source of choice for 157-nm lithography, Kubodera may have found an alternative.

Rare gas excimers radiate in the EUV region, and electron beams have long been used to achieve high-power laser radiation with rare gas molecules. According to Kubodera, there have been promising conclusions about the possibility of discharge-pumped rare gas excimer lasers, but as far as he is aware, no groups have reported success thus far.

The Miyazaki University researchers demonstrated EUV krypton (Kr2) rare gas excimer-laser oscillation using a compact discharge device. They observed glow discharge in up to 10 atm of pure Kr2, with an EUV emission intensity centered at 147.8 nm. When the charging voltage exceeded a threshold value, the intensity suddenly increased. "We got self-sustained discharge at 10 atm Kr2 at charging voltages of between 27 and 31 kV," Kubodera explains. "But when voltage topped 29 kV, the Kr2 emission abruptly increased. The peak intensity at 31 kV was almost one order of magnitude higher than it was at 29 kV." The spectral emission characteristics changed dramatically as well. The spectral width was initially 4 nm FWHM at 27 kV, but narrowed to 0.4 nm at 31 kV. "This spectral narrowing is the direct evidence of the stimulated emission of Kr2 at 148 nm," Kubodera says.

William Arnold of ASM Lithography (Veldhoven, Netherlands) says that EUV lithography and electron-projection lithography are the two most promising next-generation lithography technologies.

Kubodera feels his team's effort demonstrates an alternative to the F2 lasers currently targeted by the optical-lithography industry. "More importantly, a similar endeavor could lead to an argon (Ar2) laser at 126 nm, and that would make a real impact," he says.

putting EUV to work

Professor Kaoru Yamanouchi of the department of chemistry at the University of Tokyo uses EUV light to study molecule dynamics. Many researchers use fixed-wavelength excimer lasers in this region—usually either argon-fluoride (193 nm) or F2. According to Yamanouchi, only a handful worldwide use tunable EUV laser-light sources generated by four-wave difference, or sum, frequency mixing for spectroscopic and dynamic studies on molecules. "As far as I know, my group is the only one in Japan that uses such tunable VUV laser light on a daily basis," Yamanouchi says.

The group has been studying selective photodissociation of carbonyl sulfide (OCS), using the output beams of two dye lasers simultaneously pumped by a xenon-chloride (XeCl) excimer laser. They tuned the frequency-doubled output of one dye laser to the two-photon resonant frequency of the Xe, and tuned the output of the other dye laser to generate EUV frequencies corresponding to the three lowest resonant peaks in the photofragment-excitation (PHOFEX) spectrum of the OCS.

Only the EUV light beam was introduced into the main chamber, where it crossed the free jet of sample gas 12 mm downstream from the nozzle orifice at an angle of 45°.

The group used a second tunable EUV laser as a probe to determine the rotational and vibrational states of the carbon monoxide (CO) fragments. "We investigated the photodissociation of OCS by measuring the rotational and vibrational state distributions of the CO fragments produced through the three lowest vibrational resonances," Yamanouchi says. He adds that their tunable system afforded an ideal opportunity to study the pure nuclear dynamics that occur in the course of molecular dissociation.