AbstractâDeep ultraviolet optical fibers are fabricated using modified SiO2 glasses containing 2000-ppm fluorine for the clad and 200 ppm for the core.
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 13, NO. 9, SEPTEMBER 2001
Optical Fiber for Deep Ultraviolet Light M. Oto, S. Kikugawa, N. Sarukura, M. Hirano, and H. Hosono
Abstract—Deep ultraviolet optical fibers are fabricated using modified 2 glasses containing 2000-ppm fluorine for the clad and 200 ppm for the core. The transmission at 193 nm is improved to more than 60%/m by optimizing the fiber drawing condition. The 2 -impregnation into the fiber suppresses the degradation of the transmission by irradiation of ArF excimer laser (50 mJ cm2 pulse). Further improvement may be expected by reducing oxygen-deficient center (I) defect generation in the drawing process.
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Index Terms—Optical fibers, optical fiber losses, optical fiber materials, optical propagation, ultraviolet spectroscopy.
I. INTRODUCTION
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XPANSION of deep UV (DUV) light applications stimulates the need for optical fibers to transmit DUV light energy. The flexibility of optical fibers, in terms of supplying light power, provides compact and uniform irradiation systems in such applications as excimer laser lithography for semiconductors and excimer lamp assisted photochemical surface cleaning of semiconductor wafers or glass substrates for flat-panel display. However, no optical fiber with both high transparency and high resistivity to DUV light has yet been obtained. In this letter, we have fabricated optical fibers using modiglasses developed for optical materials for vacuum fied , characterized by UV (VUV) lithography. The modified F-doped and OH-free, exhibits a high optical transmittance in VUV region, as well as an excellent resistance to color center formation by the irradiation [1], [2]. The fluorine doping is effective to remove strained Si–O–Si bonds, which govern optical transparency in VUV region and radiation sensitivity [1], [2]. The suppression of defect formation during the fiber drawing process is one of major concerns in the present study. II. FABRICATION Preforms were prepared by a rod-in-tube technique, using the modified silica glasses having different fluorine concentrations both for core and clad parts. The core glass contains 200- ppm fluorine and from 5- to 10-ppm OH, while the concentration of fluorine in the clad-glass is 2000 ppm. The OH and fluorine contents in the glasses were measured by IR absorption around 3673 cm , and electron probe microanalyzer (EPMA), respectively. The refractive index difference, measured in infrared region, was 0.01. Thus, the relative refractive index difference Manuscript received April 30, 2001; revised June 12, 2001. M. Oto is with Showa Electric Wire and Cable Company, Ltd., 229-1133 Sagamihara, Japan. S. Kikugawa is with Asahi Glass Company, Ltd., 221-8755 Yokohama, Japan. N. Sarukura, M. Hirano, and H. Hosono are with the Transparent ElectroActive Material Project, ERATO, JST, 213-0012 Kawasaki, Japan. Publisher Item Identifier S 1041-1135(01)07544-9.
Fig. 1. A typical loss spectrum of the modified silica fiber. Inset shows the structure of the fiber.
as defined by the equation is caland culated to be 0.7% (see inset of Fig. 1), where represent refractive indexes of core and clad, respectively. The preforms were drawn to form a step index type multimode fiber with diameters of 750 m for the clad and 600 m for the core. Drawing temperatures were changed between 1780 C–1860 C, and drawing speed from 0.5 to 2.0 m/min. after drawing, fibers were immersed in a To impregnate hydrogen atmosphere of 10 MPa for two days. III. RESULTS Transmission loss spectra in UV–VUV region were measured by a cutback method using a Seya–Namioka-type VUV spectrophotometer (Bunkoh–Keiki Co., Ltd.), and invisible infrared region by a conventional Czerny–Turner-type monochromator system (JASCO). Cutback length of each fiber was 1 m from m. A typical loss spectrum of the fabrithe total length of cated fiber is shown in Fig. 1. Absorption of the fiber abruptly increases around 200 nm, and two peaks are seen in infrared region (1240, 1380 nm), both of which are assigned to higher orders harmonics of OH stretching vibration [3]. The transmission loss spectra in the DUV–VUV region depend on the drawing conditions. The lowest loss in the DUV nm , was obtained in the fiber fabricated region under the condition of a drawing temperature of 1780 C and a drawing speed of 0.5 m/min, demonstrating that the transmittance at 193 nm (wavelength of ArF excimer laser) is 60 /1 m (open circles in Fig. 2). With increases in both the drawing temperature and speed, the absorption band around 220 nm increases noticeably presumably due to the formation
1041–1135/01$10.00 ©2001 IEEE
OTO et al.: OPTICAL FIBER FOR DEEP ULTRAVIOLET LIGHT
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Fig. 2. Optical loss spectra of the fibers drawn with different conditions. ( 1780 C and 0.5 m/min, : 1860 C and 2.0 m/min).
Fig. 3. Optical loss spectra of a hydrogen-loaded fiber before ( ) and after ( ) ArF laser irradiation. Inset shows the change of the relative transmission at 193 nm during the irradiation.
of center [4], as shown fiber prepared by a drawing temperature of 1860 C and a drawing speed of 2.0 m/min (solid circles in Fig. 2). To examine laser damage in the fiber, we subjected the fiber to an ArF excimer laser radiation with a repetition rate of 20 Hz and a total irradiation of 10 doses. The power density at the entrance of the fiber was 50 mJ/cm pulse. During the irradiation, changes in the transmittance of the fiber were measured in situ by detecting the output energy using biplaner detectors, normalized by the input laser intensity. The inset of Fig. 3 shows the change in the transmission of the hydrogen-loaded fiber at 193 nm during the ArF laser irradiation, exhibiting slight decrease in the initial stage, followed by a quick saturation about 90% till the end of the irradiation. In addition, restoration of the transmittance was observed in about ten min. after the termination of the irradiation. Fig. 3 shows the absorption spectra
Fig. 4. Optical loss spectra of an As-drawn fiber before ( ) and after ( ) ArF laser irradiation. Inset shows the change of the relative transmission at 193 nm during the irradiation.
before (open circles) and after (solid circles) the irradiation, demonstrating no defect formation took place by the irradiation. Hydrogen impregnation has dual roles in suppression of color centers, i.e., elimination of laser-induced color centers by the and reduction of strained Si–O–Si bonds, which reaction of are the precursor of laser-induced color centers [4]–[6]. Similar absorption spectrum and time dependent change of an as-drawn fiber are shown in Fig. 4. A strong absorption band center in silica [4], was inpeaked at 220 nm, attributed to duced by the irradiation, and correspondingly to the induction, the transmittances at 193 nm decreased abruptly with the radiation (inset of Fig. 4). Comparison between Figs. 3 and 4 clearly into the fiber plays a critical indicates the impregnation of center genrole in the suppression and/or restoration of the eration. For the purpose of realizing what restricts the optical transmission of the fiber in DUV range, we prepared a piece of 10 mm-length fiber with both sides polished to measure VUV transmission spectrum. For comparison, a sliced disc of the preform rod and a piece of all-core fiber that was drawn from a core rod without jacketing clad glass were also measured. The transmittance in the measurements includes loss of surface reflection, because the fibers are too short to measure by cutback method. The transmission spectra of the normal fiber (open circles in Fig. 5), and the all-core fiber (solid circles) exhibit strong absorption bands at 163 nm (7.6 eV) assigned as oxygen-deficient center [ODC(I)] [4], [7], together with weak absorptions center. On the other hand, the spectrum of the due to the preform rod disc does not exhibit such absorption bands (solid line). It is therefore obvious, that ODC(I) is created during the fiber drawing processes. The ODC(I) center seems to limit the transmission loss of the present fiber in the VUV region. No significant change was observed in the transmission spectrum of the fiber when lowering the temperature from room temperature (solid circles in Fig. 6) to 77 K (open circles). This observation rules out the possibility
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 13, NO. 9, SEPTEMBER 2001
[8]. Thus, the improvement of the transmission of the fiber is still possible to suppress the generation of the defects by optimizing the drawing conditions.
IV. CONCLUSION Optical fibers for DUV were fabricated, composed of 200- ppm F-doped silica core and 2000- ppm F-doped clad. The fibers fabricated under the optimal drawing conditions, low drawing temperature and speed, followed by the hydrogen treatment exhibit transparency of 60%/1-m long at 193 nm with a high stability against UV irradiation. The optical transmission loss seems to be limited by ODC(I) defects generated in the drawing process.
Fig. 5. Transmission spectra of the pieces of the normal fiber ( ), the all core fiber ( ) and a disc of the preform rod (solid line). The length or the thickness of each specimen was 10 mm.
ACKNOWLEDGMENT The authors would like to thank A. Naruse, T. Yamamoto, and M. Ohyagi for their technical assistance.
REFERENCES
Fig. 6. Temperature dependence of the transmittance of the fiber. ( Spectrum measured at 77 K, : Spectrum at 293 K).
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that the loss is attributed to Urbach tail of the intrinsic absorption, which should exhibit a large blue shift with temperature
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