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Impurity control of CsLiB6O10 for improving UV-induced damage tolerance
|Kei Takachiho 1,2, Masashi Yoshimura 1,2, Kazuki Masuda 1,2, Yoshinori Takahashi 1,2, Mamoru Imade 1, Takatomo Sasaki 1,2, Yusuke Mori 1,2|
1. Osaka UNIV., Yamadaoka 2-1, Suita 565-0871, Japan
|CsLiB6O10 (CLBO) is an excellent nonlinear optical crystal for generating high-power UV output with wavelengths below 300 nm. The deep-UV sources employed on CLBO have been prevalent for high-resolution inspection in the semiconductor manufacturing process. Laser-induced damage of crystals is one of critical issues for scaling the UV power. We confirmed that UV output degradation and beam distortion in CLBO occurred at a lower peak power density than the bulk laser-induced damage threshold (LIDT) . In this study, we report on impurity control of CLBO for improving UV-induced degradation resistance.
Al-doped CLBO crystals were grown from 0.5 mol% Al2O3- and 10 mol% LiF-added stoichiometric melt by the solution-stirring top-seeded solution growth method. The addition of a fluoride component was used to decrease the solution viscosity. High-quality single crystal with the size of 79×36×22 mm3 was obtained by the LiF flux solution. The concentration of Al incorporated into the crystal was about 10 wt ppm. On the other hand, the water impurities in a CLBO sample significantly degrade its UV optical properties and the bulk LIDT . Therefore, prior to experiment, the water impurities in Al-doped and undoped CLBO samples were reduced by post-growth heat treatment at 150 oC in ambient and dry atmosphere.
We evaluated UV-induced degradation in CLBO. Figure 1 shows a schematic of the experimental setup for measuring degradation induced by UV pulses. UV beam (wavelength: 266 nm; peak power density at the focal point: 56 MW/cm2) was tightly focused at the center of CLBO sample to observe the degradation behavior in a short time. The transmitted power through the aperture after the sample was measured to detect the UV-induced degradation that occurred near the focal point in CLBO. Figure 2 shows a typical result obtained for undoped and Al-doped CLBO samples. Pattern distortion occurred after UV illumination for a while, and the transmitted power then decreased gradually. As is the case with the bulk LIDT , the UV-induced degradation resistance was significantly improved by eliminating water impurities from the crystal. The more important result is that Al-doping of CLBO can slow down UV-induced degradation. We will discuss about properties of Al-doped crystals and the degradation phenomena in detail.
Fig. 1. Schematic of the experimental setup for measuring degradation induced by UV pulses.
Fig. 2. Top: 266 nm transmitted power degradation through CLBO sample at 150 oC and aperture. (a) Undoped CLBO before dehydration, (b) undoped CLBO after dehydration, and (c) Al-doped CLBO after dehydration. Bottom: (d) Initial and (e) distorted transmitted UV beam through CLBO. The images were observed on fluorescent paper in the far-field.
 K. Takachiho, M. Yoshimura, Y. Fukushima, Y. Takahashi, M. Imade, T. Sasaki, and Y. Mori, Appl. Phys. Express 6 (2013) 022701.
 T. Kawamura, M. Yoshimura, Y. Honda, M. Nishioka, Y. Shimizu, Y. Kitaoka, Y. Mori, and T. Sasaki, Appl. Opt. 48 (2009) 1658.
Presentation: Oral at 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17, Topical Session 6, by Kei Takachiho
See On-line Journal of 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17
Submitted: 2013-03-28 08:59 Revised: 2013-07-25 12:31