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Optics Express

Optics Express

  • Editor: Michael Duncan
  • Vol. 13, Iss. 17 — Aug. 22, 2005
  • pp: 6445–6453
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Microstructure of femtosecond laser-induced grating in amorphous silicon

Geon Joon Lee, Jisun Park, Eun Kyu Kim, YoungPak Lee, Kyung Moon Kim, Hyeonsik Cheong, Chong Seung Yoon, Yong-Duck Son, and Jin Jang  »View Author Affiliations


Optics Express, Vol. 13, Issue 17, pp. 6445-6453 (2005)
http://dx.doi.org/10.1364/OPEX.13.006445


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Abstract

The femtosecond laser-induced grating (FLIG) formation and crystallization were investigated in amorphous silicon (a-Si) films, prepared on glass by plasma-enhanced chemical-vapor deposition. Probe-beam diffraction, micro-Raman spectroscopy, atomic force microscopy, scanning electron microscopy, and transmission electron microscopy were employed to characterize the diffraction properties and the microstructures of FLIGs. It was found that i) the FLIG can be regarded as a pattern of alternating a-Si and microcrystalline-silicon (μc-Si) lines with a period of about 2 μm, and ii) efficient grating formation and crystallization were achieved by high-intensity recording with a short writing period.

© 2005 Optical Society of America

1. Introduction

Amorphous semiconductors have attracted considerable interests due to their various applications to solar cell, xerography, and flat-panel display. Much of this attention has focused on amorphous silicon (a-Si) and its crystallization for the use in active-matrix-based flat-panel displays such as active-matrix liquid-crystal displays and active-matrix organic-light-emitting diodes [1–3

1. D. E. Carlson and C.R. Wronski, “Amorphous silicon solar cell,” Appl. Phys. Lett. 28, 671–673 (1976). [CrossRef]

]. In comparison with a-Si, crystallized silicon reveals larger carrier mobility, faster switching, and higher stability, all of which are key for the design of thin-film transistors. The crystallization of a-Si is typically achieved by excimer-laser annealing [4

4. J. S. Im and H. J. Kim, “Phase transformation mechanisms involved in excimer laser crystallization of amorphous sillicon films,” Appl. Phys. Lett. 63, 1969–1971 (1993). [CrossRef]

, 5

5. M. Miyasaka and J. Stoemenos, “Excimer laser annealing of amorphous and solid-phase-crystallized sillicon films,” J. Appl. Phys. 86, 5556–5565 (1999). [CrossRef]

] or solid-phase crystallization (SPC) [6

6. S. Y. Yoon, J. Y. Oh, C. O. Kim, and J. Jang, “Low temperature solid-phase crystallization of amorphous sillicon at 380 °C,” J. Appl. Phys. 84, 6463–6465 (1998). [CrossRef]

, 7

7. A. Mimura, N. Konishi, K. Ono, J. Ohwada, Y. Hosokawa, Y. Ono, T. Suzuki, K. Miyata, and H. Kawakami, “High performance low-temperature poly-Si n-channel TFT’s for LCD,” IEEE Trans. Electron Devices 36, 351–359 (1989). [CrossRef]

]. The excimer-laser crystallization has been improved through lateral laser-induced crystallization [8

8. J. S. Im and H. J. Kim, “On the super lateral growth phenomenon observed in excimer laser-induced crystallization of thin Si films,” Appl. Phys. Lett. 64, 2303–2305 (1994). [CrossRef]

, 9

9. A. T. Voutsas, A. Limanov, and J. S. Im, “Effect of process parameters on the structural characteristics of laterally grown, laser-annealed polycrystalline silicon films,” J. Appl. Phys. 94, 7445–7452 (2003). [CrossRef]

], and SPC has been enhanced by performing the crystallization process in the presence of a metal catalyst [10

10. C. Hayzelden and J. L. Batstone, “Silicide formation and silicide-mediated crystallization of nickel-implanted amorphous silicon thin films,” J. Appl. Phys. 73, 8279–8289 (1993). [CrossRef]

, 11

11. J. Jang, J. Y. Oh, S. K. Kim, K. J. Cho, S. Y. Yoon, and C. O. Kim, “Electric-field-enhanced crystallization of amorphous silicon,” Nature (London) 395, 481–483 (1998). [CrossRef]

]. Although several studies have been conducted on the excimer-laser-based crystallization of a-Si [8

8. J. S. Im and H. J. Kim, “On the super lateral growth phenomenon observed in excimer laser-induced crystallization of thin Si films,” Appl. Phys. Lett. 64, 2303–2305 (1994). [CrossRef]

, 9

9. A. T. Voutsas, A. Limanov, and J. S. Im, “Effect of process parameters on the structural characteristics of laterally grown, laser-annealed polycrystalline silicon films,” J. Appl. Phys. 94, 7445–7452 (2003). [CrossRef]

], only a few experiments have been reported on the femtosecond-laser crystallization [12

12. J.-M. Sieh, Z.-H. Chen, B.-T. Dai, Y.-C. Wang, A. Zaitsev, and C.-L. Pan, “Near-infrared femtosecond laser-induced crystallization of amorphous silicon,” Appl. Phys. Lett. 85, 1232–1234 (2004). [CrossRef]

]. Femtosecond laser can provide an extremely high heating rate in a particular material region, thus causing a rapid accumulation of energy and making it possible to induce a phase change in the amorphous or the crystalline structure of material. Femtosecond laser also allows the precise structuring, since the electron-lattice coupling by the transfer of the absorbed laser energy is ignored [13

13. B. N. Chichkov, C. Momma, S. Nolte, F. von Alvensleben, and A. Tunnermann, “Femtosecond, picosecond and nanosecond laser ablation of solids,” Appl. Phys. A 63, 109–115 (1996). [CrossRef]

]. Recently, the use of femtosecond laser in fabricating particular patterns inside a bulk or on a surface has been increased because applications such as gratings, waveguides, splitters, directional couplers, optical memory and photonic crystals can be accelerated utilizing such a patterning [14–16

14. Y. Kuroiwa, N. Takeshima, Y. Narita, and S. Tanaka, “Arbitrary micropatterning method in femtosecond laser microprocessing using diffractive optical elements,” Opt. Express 12, 1908–1915 (2004), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-12-9-1908. [CrossRef] [PubMed]

]. By combining the laser crystallization with the laser patterning, crystallization can be spatially controlled. Laser interference pattern is employed to obtain a spatially-selected crystallization in a-Si. Therefore, it is worthwhile to investigate the femtosecond laser-induced grating (FLIG) formation and crystallization in a-Si film.

In this work, the FLIG formation and crystallization were studied by femtosecond holography, in which the gratings were recorded in the a-Si film using a two-beam interference of near-infrared femtosecond laser pulses. The diffraction behaviors of the probe beam from the FLIGs were monitored to find the grating formation and relaxation profiles. The laser-induced surface deformations were also investigated by optical microscopy and atomic force microscopy (AFM). The structural characterization of laser-crystallized silicon was performed by micro-Raman spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM).

2. Experiment

Hydrogenated amorphous silicon films were deposited on Corning 1737 glass by plasma-enhanced chemical-vapor deposition (PECVD) using a gas mixture of silane (SiH4) and hydrogen (H2) at a substrate temperature of 270°C. The other deposition parameters were fixed as follows: Silane and hydrogen gas flow rates were fixed at 300 and 100 sccm (standard cubic centimeters), respectively. Chamber working pressure and RF power were 2 torr and 30 W, and the resulting film was 100 nm thick.

The femtosecond laser system used in this experiment was a regeneratively amplified Ti:Sapphire laser (Spectra-Physics, Hurricane) with an 800-nm output wavelength, 130-fs pulse duration, 1.0-mJ maximum pulse energy, and 1-kHz repetition rate. In order to record the gratings in a-Si film, we used the two-beam interference of femtosecond laser pulses. The pulse energies of the two interfering beams were equal to each other, and their intersection angle was 24°. Synchronization of the two writing pulses was achieved by adjusting the optical delay of the two pulses. The grating formation and relaxation profiles were recorded using a photomultiplier tube and a digitizing oscilloscope, where the probe beam was from a He-Ne laser. The surface profiles of the FLIGs were investigated with an optical microscope and an atomic force microscope (PSIA, XE-100). In order to study the crystallization of the femtosecond-laser-modified region, we measured the Raman spectra of all the samples at room temperature using the 514.5 nm line of a stabilized argon ion laser (Coherent, Innova 307) as the excitation source. At the sample, the laser power was measured at 3 mW. In order to obtain the micro-Raman spectra for the microcrystalline silicon (μc-Si) film formed by femtosecond holography, the laser was focused onto the sample using a microscope objective (x60). The scattered light was collected by the same objective lens, dispersed by a 55 cm monochromator and then detected by a liquid-nitrogen-cooled back-thinned charge-coupled-device array detector. TEM (JEOL, JEM-2010) was used to characterize the laser-treated sample. TEM samples were prepared by mechanical grinding and an ion mill. SEM (JEOL, JSM-6630F) was also employed to characterize the surface morphology.

3. Results and discussion

Fig. 1. (a), (b) Diffraction behaviors and (c) diffraction pattern of the probe beam from the FLIGs recorded in a-Si film using two interfering femtosecond-laser pulses: (a) 68 μJ and 68 μJ; (b), (c) 365 μJ and 365 μJ. In the grating formation dynamics [(a) and (b)], the horizontal bars represent the writing periods of two interfering beams for recording the grating. Here and all the following figures, the abbreviation “a. u.” stands for arbitrary units.
Fig. 2. AFM images for the FLIGs formed in a-Si film by femtosecond holography. The gratings were fabricated using (a) 51720 shots of two 68-μJ laser beams and (b) 200 shots of two 365-μJ beams. In the line profile average at the bottom of each figure, the surface profile is vertically averaged in order to show the grating structure along the horizontal direction.
Fig. 3. Typical micro-Raman spectra for the femtosecond-laser-modified and the unexposed regions. Solid circles and squares represent the micro-Raman spectra of μc-Si and a-Si regions, respectively.

Fig. 4. SEM and FESEM images for the μc-Si film formed by femtosecond holography. (a), (b) the μc-Si film, and (c) the a-Si film. (a) ×8000, (b) ×50000, and (c) ×8000. The high-resolution image in (b) was obtained by FESEM. The μc-Si sample was fabricated using 200 shots of two 365-μJ laser beams.

In order to obtain direct evidence for laser crystallization and to estimate the grain size in the crystallized silicon film, we measured SEM and TEM images for the femtosecond-laser-treated samples. Figure 4 shows the plan-view SEM images for the femtosecond-laser-modified region. The SEM micrograph in Fig. 4(a) shows regularly spaced bright regions which are thought to be the microcrystallized regions with irradiation of the femtosecond laser. The crystallites can be clearly seen in the field-emission scanning electron microscopy (FESEM) image [Fig. 4(b)] for the laser-crystallized region. Meanwhile, Fig. 4(c) indicates that the surface morphology of the amorphous film in the unexposed region has a uniform and featureless topology, and surface is dark unlike Fig. 4(a). Figure 5 shows the bright-field TEM images and the corresponding indexed electron diffraction (ED) patterns for the μc-Si film formed by femtosecond holography. TEM micrographs [Fig. 5(a) and 5(b)] clearly reveal that the FLIG can be regarded as a pattern of alternating a-Si and μc-Si lines with a period of about 2 μm. The selected area ED patterns [Fig. 5(c) and 5(d)] indicate that polycrystalline silicon (Poly-Si) is formed by femtosecond holography and that the crystals have a diamond cubic structure, as can be seen from the indexed diffraction pattern.

Fig. 5 (a), (b) Plan-view, bright-field TEM images and (c), (d) the corresponding indexed ED patterns for μc-Si films formed by femtosecond holography. For comparison, the ED pattern of an unexposed a-Si film is shown in (e). The μc-Si samples were fabricated using 51720 shots of two 68-μJ laser beams [see (a) and (c)] and 200 shots of two 365-μJ laser beams [(b) and (d)].
Fig. 6 (a) Average grain size as a function of laser-shot number. The average grain sizes were determined from the TEM analysis. (b) Diffraction intensity and crystalline Raman peak height as a function of laser-shot number. The diffraction intensity vs. laser-shot number plot was obtained by measuring the time evolution of diffraction signals during the irradiation of 90000 laser shots, and the Raman peak height vs. laser-shot number plot by measuring the micro-Raman spectra of seven different μc-Si samples crystallized with various laser-shot numbers.

4. Conclusion

By femtosecond holography, the FLIG formation and crystallization were studied in a-Si films prepared on a glass plate by the PECVD method. Probe-beam diffraction, micro-Raman spectroscopy, AFM, SEM, and TEM analyses reveal that i) the FLIG can be regarded as a pattern of alternating a-Si and μc-Si lines with a period of about 2 μm, and ii) efficient laser crystallization is achieved by high-intensity recording with a short writing period. The TEM analysis indicated that the average grain size of 170 nm was obtained through laser-interference crystallization using 200 shots of two 365-μJ laser beams.

Acknowledgments

This work was supported by the KOSEF through Quantum Photonic Science Research Center at Hanyang University, Seoul, Korea.

References and links

1.

D. E. Carlson and C.R. Wronski, “Amorphous silicon solar cell,” Appl. Phys. Lett. 28, 671–673 (1976). [CrossRef]

2.

B. K. Nayak, B. Eaton, J. A. A. Selvan, J. Mcleskey, M. C. Gupta, R. Romero, and G. Ganguly, “Semiconductor laser crystallization of a-Si:H on conducting tin-oxide-coated glass for solar cell and display applications,” Appl. Phys. A 80, 1077–1080 (2005). [CrossRef]

3.

T. Suzuki and S. Adachi, “Optical properties of amorphous Si partially crystallized by thermal annealing,” Jpn. J. Appl. Phys. 32, 4900–4906 (1993). [CrossRef]

4.

J. S. Im and H. J. Kim, “Phase transformation mechanisms involved in excimer laser crystallization of amorphous sillicon films,” Appl. Phys. Lett. 63, 1969–1971 (1993). [CrossRef]

5.

M. Miyasaka and J. Stoemenos, “Excimer laser annealing of amorphous and solid-phase-crystallized sillicon films,” J. Appl. Phys. 86, 5556–5565 (1999). [CrossRef]

6.

S. Y. Yoon, J. Y. Oh, C. O. Kim, and J. Jang, “Low temperature solid-phase crystallization of amorphous sillicon at 380 °C,” J. Appl. Phys. 84, 6463–6465 (1998). [CrossRef]

7.

A. Mimura, N. Konishi, K. Ono, J. Ohwada, Y. Hosokawa, Y. Ono, T. Suzuki, K. Miyata, and H. Kawakami, “High performance low-temperature poly-Si n-channel TFT’s for LCD,” IEEE Trans. Electron Devices 36, 351–359 (1989). [CrossRef]

8.

J. S. Im and H. J. Kim, “On the super lateral growth phenomenon observed in excimer laser-induced crystallization of thin Si films,” Appl. Phys. Lett. 64, 2303–2305 (1994). [CrossRef]

9.

A. T. Voutsas, A. Limanov, and J. S. Im, “Effect of process parameters on the structural characteristics of laterally grown, laser-annealed polycrystalline silicon films,” J. Appl. Phys. 94, 7445–7452 (2003). [CrossRef]

10.

C. Hayzelden and J. L. Batstone, “Silicide formation and silicide-mediated crystallization of nickel-implanted amorphous silicon thin films,” J. Appl. Phys. 73, 8279–8289 (1993). [CrossRef]

11.

J. Jang, J. Y. Oh, S. K. Kim, K. J. Cho, S. Y. Yoon, and C. O. Kim, “Electric-field-enhanced crystallization of amorphous silicon,” Nature (London) 395, 481–483 (1998). [CrossRef]

12.

J.-M. Sieh, Z.-H. Chen, B.-T. Dai, Y.-C. Wang, A. Zaitsev, and C.-L. Pan, “Near-infrared femtosecond laser-induced crystallization of amorphous silicon,” Appl. Phys. Lett. 85, 1232–1234 (2004). [CrossRef]

13.

B. N. Chichkov, C. Momma, S. Nolte, F. von Alvensleben, and A. Tunnermann, “Femtosecond, picosecond and nanosecond laser ablation of solids,” Appl. Phys. A 63, 109–115 (1996). [CrossRef]

14.

Y. Kuroiwa, N. Takeshima, Y. Narita, and S. Tanaka, “Arbitrary micropatterning method in femtosecond laser microprocessing using diffractive optical elements,” Opt. Express 12, 1908–1915 (2004), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-12-9-1908. [CrossRef] [PubMed]

15.

K. M. Davis, K. Miura, N. Sugimoto, and K. Hirao, “Writing waveguides in glass with a femtosecond,” Opt. Lett. 21, 1729–1731 (1996). [CrossRef] [PubMed]

16.

S. Kawata, H.-B. Sun, T. Tanaka, and K. Takada, “Finer features for functional microdevices: Micromachines can be created with higher resolution using two-photon absorption,” Nature (London) 412, 697–698 (2001). [CrossRef]

17.

Z. Iqbal and S. Veprek, “Raman scattering from hydrogenated microcrystalline and amorphous silicon,” J. Phys. C: Solid State Phys. 15, 377–392 (1982). [CrossRef]

18.

L. Houben, M. Luysberg, P. Hapke, R. Carius, F. Finger, and H. Wagner, “Structural properties of microcrystalline silicon in the transition from highly crystalline to amorphous growth,” Philos. Mag. A 77, 1447–1460 (1998). [CrossRef]

19.

T. Sameshima, “Laser processing for thin film transistor applications,“ Mater. Sci. Eng. B 45, 186–193 (1997). [CrossRef]

20.

G. Aichmayr, D. Toet, M. Mulato, P. V. Santos, A. Spangenberg, S. Christiansen, M. Albrecht, and H. P. Strunk, “Dynamics of lateral grain growth during the laser interference crystallization of a-Si,“ J. Appl. Phys. 85, 4010–4023 (1999). [CrossRef]

21.

C.-H. Oh, M. Ozawa, and M. Matsumura, “A novel phase-modulated excimer-laser crystallization method of silicon thin films,“ Jpn. J. Appl. Phys. 37, L492–L495 (1998). [CrossRef]

OCIS Codes
(050.0050) Diffraction and gratings : Diffraction and gratings
(160.2750) Materials : Glass and other amorphous materials
(300.6450) Spectroscopy : Spectroscopy, Raman
(320.7090) Ultrafast optics : Ultrafast lasers

ToC Category:
Research Papers

History
Original Manuscript: June 15, 2005
Revised Manuscript: August 5, 2005
Published: August 22, 2005

Citation
Geon Joon Lee, Jisun Park, Eun Kim, YoungPak Lee, Kyung Kim, Hyeonsik Cheong, Chong Yoon, Yong-Duck Son, and Jin Jang, "Microstructure of femtosecond laser-induced grating in amorphous silicon," Opt. Express 13, 6445-6453 (2005)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-13-17-6445


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References

  1. D. E. Carlson, and C.R. Wronski, "Amorphous silicon solar cell," Appl. Phys. Lett. 28, 671-673 (1976). [CrossRef]
  2. B. K. Nayak, B. Eaton, J. A. A. Selvan, J. Mcleskey, M. C. Gupta, R. Romero, and G. Ganguly, "Semiconductor laser crystallization of a-Si:H on conducting tin-oxide-coated glass for solar cell and display applications," Appl. Phys. A 80, 1077-1080 (2005). [CrossRef]
  3. T. Suzuki, and S. Adachi, "Optical properties of amorphous Si partially crystallized by thermal annealing," Jpn. J. Appl. Phys. 32, 4900-4906 (1993). [CrossRef]
  4. J. S. Im, and H. J. Kim, "Phase transformation mechanisms involved in excimer laser crystallization of amorphous sillicon films," Appl. Phys. Lett. 63, 1969-1971 (1993). [CrossRef]
  5. M. Miyasaka, and J. Stoemenos, "Excimer laser annealing of amorphous and solid-phase-crystallized sillicon films," J. Appl. Phys. 86, 5556-5565 (1999). [CrossRef]
  6. S. Y. Yoon, J. Y. Oh, C. O. Kim, and J. Jang, "Low temperature solid-phase crystallization of amorphous sillicon at 380 °C," J. Appl. Phys. 84, 6463-6465 (1998). [CrossRef]
  7. A. Mimura, N. Konishi, K. Ono, J. Ohwada, Y. Hosokawa, Y. Ono. T. Suzuki, K. Miyata, and H. Kawakami, "High performance low-temperature poly-Si n-channel TFT's for LCD," IEEE Trans. Electron Devices 36, 351-359 (1989). [CrossRef]
  8. J. S. Im, and H. J. Kim, "On the super lateral growth phenomenon observed in excimer laser-induced crystallization of thin Si films," Appl. Phys. Lett. 64, 2303-2305 (1994). [CrossRef]
  9. A. T. Voutsas, A. Limanov, and J. S. Im, "Effect of process parameters on the structural characteristics of laterally grown, laser-annealed polycrystalline silicon films," J. Appl. Phys. 94, 7445-7452 (2003). [CrossRef]
  10. C. Hayzelden, and J. L. Batstone, "Silicide formation and silicide-mediated crystallization of nickel-implanted amorphous silicon thin films," J. Appl. Phys. 73, 8279-8289 (1993). [CrossRef]
  11. J. Jang, J. Y. Oh, S. K. Kim, K. J. Cho, S. Y. Yoon, and C. O. Kim, "Electric-field-enhanced crystallization of amorphous silicon," Nature (London) 395, 481-483 (1998). [CrossRef]
  12. J.-M. Sieh, Z.-H. Chen, B.-T. Dai, Y.-C. Wang, A. Zaitsev, and C.-L. Pan, "Near-infrared femtosecond laser-induced crystallization of amorphous silicon," Appl. Phys. Lett. 85, 1232-1234 (2004). [CrossRef]
  13. B. N. Chichkov, C. Momma, S. Nolte, F. von Alvensleben, and A. Tunnermann, "Femtosecond, picosecond and nanosecond laser ablation of solids," Appl. Phys. A 63, 109-115 (1996). [CrossRef]
  14. Y. Kuroiwa, N. Takeshima, Y. Narita, and S. Tanaka, "Arbitrary micropatterning method in femtosecond laser microprocessing using diffractive optical elements," Opt. Express 12, 1908-1915 (2004), <a href= "http://www.opticsexpress.org/abstract.cfm?URL=OPEX-12-9-1908">http://www.opticsexpress.org/abstract.cfm?URL=OPEX-12-9-1908</a>. [CrossRef] [PubMed]
  15. K. M. Davis, K. Miura, N. Sugimoto, and K. Hirao, "Writing waveguides in glass with a femtosecond," Opt. Lett. 21, 1729-1731 (1996). [CrossRef] [PubMed]
  16. S. Kawata, H.-B. Sun, T. Tanaka, and K. Takada, "Finer features for functional microdevices: Micromachines can be created with higher resolution using two-photon absorption," Nature (London) 412, 697-698 (2001). [CrossRef]
  17. Z. Iqbal, and S. Veprek, "Raman scattering from hydrogenated microcrystalline and amorphous silicon," J. Phys. C: Solid State Phys. 15, 377-392 (1982). [CrossRef]
  18. L. Houben, M. Luysberg, P. Hapke, R. Carius, F. Finger, and H. Wagner, "Structural properties of microcrystalline silicon in the transition from highly crystalline to amorphous growth," Philos. Mag. A 77, 1447-1460 (1998). [CrossRef]
  19. T. Sameshima, "Laser processing for thin film transistor applications," Mater. Sci. Eng. B 45, 186-193 (1997). [CrossRef]
  20. G. Aichmayr, D. Toet, M. Mulato, P. V. Santos, A. Spangenberg, S. Christiansen, M. Albrecht, and H. P. Strunk, "Dynamics of lateral grain growth during the laser interference crystallization of a-Si," J. Appl. Phys. 85, 4010-4023 (1999) . [CrossRef]
  21. C.-H. Oh, M. Ozawa, and M. Matsumura, "A novel phase-modulated excimer-laser crystallization method of silicon thin films," Jpn. J. Appl. Phys. 37, L492-L495 (1998). [CrossRef]

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