- Email: [email protected]
Graphitization of thin films formed by focused-ion-beam chemical-vapor-deposition
Diamond & Related Materials 18 (2009) 490–492
Contents lists available at ScienceDirect
Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d
Graphitization of thin films formed by focused-ion-beam chemical-vapor-deposition Kazuhiro Kanda a,⁎, Noriko Yamada a, Makoto Okada a, Jun-ya Igaki a, Reo Kometani b, Shinji Matsui a a b
Laboratory of Advanced Science and Technology for Industry, University of Hyogo, Kamigori, Hyogo 678-1205, Japan Graduate School of Engineering, The University of Tokyo, Bunkyo-ku, Tokyo 113-8658, Japan
a r t i c l e
i n f o
Available online 30 October 2008 Keywords: Ion-beam deposition Diamond-like carbon Thermal properties Surface structure
a b s t r a c t The effect of annealing on the structure of diamond-like carbon (DLC) thin film fabricated by focused-ionbeam chemical-vapor deposition (FIB–CVD) was investigated. The near-edge x-ray absorption fine-structure spectrum of the carbon K-edge over the excitation energy range of 275–320 eV was measured on the FIB– CVD DLC thin film by annealing for 1 h in a temperature region from room temperature (RT) to 1273 K. The sp2/(sp2 + sp3) ratio maintained ≈0.4 in an annealing temperature region from room temperature to 523 K and steeply increased from ≈0.4 to ≈0.7 with annealing temperature from 523 K to 773 K. The I(D)/I(G) ratio, which was obtained from the Raman spectrum, also indicated a similar dependence on annealing temperature. The graphitization of thin film fabricated by focused-ion-beam chemical-vapor deposition (FIB– CVD) by annealing was confirmed to start at ≈600 K. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Focused-ion-beam chemical-vapor deposition (FIB–CVD) has been developed as an effective technique of producing nanostructures. This enables three-dimensional (3D) nanostructures to be fabricated, that form complex shapes with overhanging, hollow, and bridging structures [1]. The Young's modulus of a FIB–CVD pillar was reported to exceed 600 GPa [2], which is sufficiently large so that nanodevices fabricated using FIB–CVD offer enormous possibilities for various applications, such as those in biochemical and electromechanical fields. With this method, scanning with a focused Ga ion beam under a phenanthrene gas atmosphere used as a source gas leads to carbon material being deposited in arbitrary nanoscale areas. The fundamental structure of carbon material formed by FIB–CVD was found to be diamond-like carbon (DLC) from measurements of the Raman spectrum [1], and residual Ga atoms used as an ion source were contained in DLC [3]. The atomic fraction of as-deposited FIB–CVD DLC film was determined to be C:Ga:H= 87.4:3.6:9.0 at.% from Rutherford backscattering spectrometry (RBS) and elastic recoil detection analysis (ERDA) [4]. It is known that the material properties of nanodevices fabricated with FIB–CVD are affected by annealing. Residual Ga atoms in the FIB– CVD DLC pillar exited from the pillar and a DLC pillar was graphitized by annealing at 873 K, which was established from transmission electron microscopy (TEM) observations [3]. However, elementary analysis of the FIB–CVD DLC films indicates that residual Ga atoms left the films at higher than 573 K and H atoms left the films at 823 K [5]. Structural analysis by using near-edge x-ray absorption fine structure (NEXAFS) spectroscopy indicated the residual Ga atoms, populated ⁎ Corresponding author. E-mail address: [email protected] (K. Kanda). 0925-9635/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2008.10.013
around the as-deposited film bottom, moved to the neighboring surface of the DLC film and left from the thin film surface by annealing [6]. This movement of Ga in the FIB–CVD DLC film was supported by the x-ray photoelectron spectroscopy observations [7]. The graphitization of FIB–CVD DLC film, on the other hand, was not observed. In the present study, we analysed the structure of FIB–CVD DLC films at various annealing temperatures. The carbon K-edge NEXAFS spectrum of the FIB–CVD DLC films after 1 h of annealing in a temperature range from room temperature (RT) to 1273 K was observed over the excitation energy range 275–320 eV. In addition, the Raman spectra of these samples were observed to confirm their fundamental structures. 2. Experimental FIB–CVD DLC films were formed on a Si substrate surface using a commercially available FIB–CVD system with a Ga+ ion beam operating at 30 keV. The details of synthesis to fabricate the FIB– CVD DLC film have been described in a previous paper [1]. In the present experiment, we prepared nine samples using two kinds of FIB–CVD systems (SII Nano Technology, SMI9200 and SMI2050MS), to investigate what effect annealing had by varying the annealing temperatures. The current of the ion beam was 20 nA for the SMI9200 and 28 nA for the SMI2050MS. The Ga content and H content in the film were expected to be slightly different in each series, because the deposition conditions were not completely identical [5]. The film thicknesses of the samples ranged from 80 to 270 nm. The produced samples were held for 1 h at all annealing temperatures and kept in a dry box after annealing. Annealing temperatures were varied in a region from RT to 1273 K. The samples used in this work were identical to those used for measuring the RBS/ERDA and nanoindentation reported in ref. [5].
K. Kanda et al. / Diamond & Related Materials 18 (2009) 490–492
491
Fig. 2. Dependence of sp2/(sp2 + sp3) ratio on annealing temperature. Open circles and open blocks correspond to sp2/(sp2 + sp3) ratio of FIB–CVD DLC films fabricated by SMI9200 and SMI2050MS.
Fig. 1. C K-edge NEXAFS spectra of FIB–CVD DLC films after a 1-h of annealing at various temperatures.
The NEXAFS measurements were carried out using the BL8B1 stage at the Ultraviolet Synchrotron Orbital Radiation Facility (UVSOR) of the Institute for Molecular Science [8]. The experimental apparatus and procedures employed in this study were almost identical to those in our previous study [9]. Typical slit widths of the monochromator were 20 μm, and the resolution of exciting light energy was about 0.5 eV Full Width Half Maximum (FWHM) for these slit widths. The uncertainty in calibrated wavelength was estimated to be ±0.2 eV. The electrons leaving the sample were detected in the total electron yield (TEY) mode. The carbon K-edge NEXAFS spectra were measured in at energy range of 275\320 eV. Raman spectroscopy was conducted ex situ, at RT with back scattering geometry (NRS-2100; JASCO). The spectra were excited with a 514.5-nm line of an Ar ion laser, analyzed with a triple monochromator, and detected with a liquid-nitrogencooled CCD array detector. The estimated spectral resolution was about 1 cm− 1. The linearity of the working curves of DLC films obtained by NEXAFS has been reported to be excellent up to a film thickness of 4.0 nm [10]. The I(D)/I(G) ratio determined by Raman spectroscopy can be regarded as the bulk structure of DLC film. A more thorough understanding of the local structure of DLC films can be obtained from the complementary measurements.
increased with annealing temperature. As a result, the spectrum of the FIB–CVD DLC film after 1-h of annealing at 1273 K closely resembled the spectrum of graphite. It is known that the sp2 content obtained by NEXAFS study is a good index for DLC properties and NEXAFS spectroscopy is a useable method of measurement to evaluate DLC films. A method of determining the sp2 content from NEXAFS spectrum was described in ref. [12]. In the present study, the value of the sp2/(sp2 + sp3) ratio was estimated from a comparison with that of graphite. The error range of the determined sp2/(sp2 + sp3) ratio was estimated to be less than 10%. The dependence of the sp2/(sp2 + sp3) ratio on annealing temperature is plotted in Fig. 2. The sp2/(sp2 + sp3) ratio of as-deposited FIB–CVD DLC film was ≈0.4. After 1-h of annealing at 523 K, the sp2/(sp2 + sp3) ratio was almost the same. However, it steeply increased from ≈0.4 to ≈0.7 with annealing temperatures from 523 K to 773 K and increased to ≈0.9 with increasing annealing temperature. This indicates that the graphitization of FIB–CVD DLC film by annealing starts at ≈600 K. Raman spectroscopy gave complementary information on the film structure to NEXAFS. Raman spectroscopy is sensitive to structural changes in DLC films. Fig. 3 shows the Raman spectra of FIB–CVD DLC films after 1-h of annealing at various temperatures. The Raman spectra of amorphous carbon materials are characterized by a peak at
3. Results and discussion Fig. 1 shows the NEXAFS carbon K-edge spectra of FIB–CVD DLC films after 1-h annealing at various temperatures. The pre-edge resonance located at 285.4 eV is ascribed to the 1 s→π⁎ resonance transition originating from carbon double bonding. The broad peak located in energy ranges higher than 293 eV is ascribed to 1 s→σ⁎ resonance transition. The peak at 289.0 eV in the NEXAFS spectrum of FIB–CVD DLC was attributed to the 1 s→σ⁎ resonance transition originating from carbon–gallium bonding in the previous work [9]. The spectral features of FIB–CVD DLC film after 1-h of annealing at 523 K resemble those of as-deposited film. However, the intensity of the peak originating from the C–Ga site at 289.0 eV decreased with increasing annealing temperatures from 523 K to 773 K. However, the intensity of the peak originating from the C = C site at 285.4 eV increased from 523 K to 773 K. In addition, the peak height at ≈307 eV, which is a characteristic of the NEXAFS spectrum of graphite [11],
Fig. 3. Raman spectra of FIB–CVD DLC films after 1-h of annealing at various temperatures.
492
K. Kanda et al. / Diamond & Related Materials 18 (2009) 490–492
FIB–CVD DLC films that were 9-nm thick, while we used ≈100-nm thick samples in the present work. The effect of Ga as a catalyst can be considered to be proportional to the density of Ga in the film in a unit area, because Ga atoms moved by annealing from the bottom region populated as-deposited to the surface. Therefore, the effect of graphitization was estimated to be small in the thin film used in the previous work. 4. Conclusion
Fig. 4. Dependence of I(D)/I(G) ratio on annealing temperature. Open circles and open blocks correspond to I(D)/I(G) ratio of FIB–CVD DLC films fabricated by SMI9200 and SMI2050MS.
about 1550\1600 cm− 1 (the G band of the crystalline graphite arising from the zero-center E2g photon) with a shoulder at ≈1350 cm− 1 (due to the so-called D band, assigned to an A1g zone-edge mode activated because of the finite size of the graphic domains) [13–16]. The D peak was hardly observed in the Raman spectrum of as-deposited FIB–CVD DLC film. Its spectral shape resembles that of amorphous carbon film including a high content of sp3 hybridized carbon atoms [9]. After 1-h of annealing at 523 K, the spectral features of FIB–CVD DLC film were almost the same as those of the as-deposited film. However, a D peak appeared after the 1-h of annealing at 523 K and its peak intensity increased with annealing temperature. To compare the peak intensity on a more quantitative basis, the ratio of the intensity of the D peak to that of the G peak, I(D)/I(G), was estimated by fitting the spectra with a Gaussian function. The dependence of the I(D)/I(G) ratio on the annealing temperature is plotted in Fig. 4. The I(D)/I(G) ratio of asdeposited FIB–CVD DLC film was ≈0.5. After 1-h of annealing at 523 K, it did not vary. However, it steeply increased from ≈0.5 to ≈1.2 with annealing temperatures from 523 K to 773 K and increased to ≈1.5 with increased annealing temperature. The increase of D band by 1-h of annealing can be regarded as the increase of finite size of the graphic domains. This dependence of I(D)/I(G) on annealing temperature is in good agreement with that of the sp2/(sp2 + sp3) ratio estimated from the NEXAFS study. This supports the finding that graphitization in the surface and bulk regions of FIB–CVD DLC film by annealing starts at ≈600 K. Our elementary analysis indicates that the exodus of Ga atoms from FIB–CVD DLC films by 1-h of annealing started at ≈500 K and that of H atoms began at ≈700 K [5]. By taking these results into account, graphitization was enhanced by the movement of Ga in the FIB–CVD DLC film during annealing. This conclusion was supported by TEM observation where the FIB–CVD DLC pillar was graphitized by the movement of a metal particle in the pillar [17]. In other words, the metal atoms or metal particles work as a catalyst in FIB–CVD DLC in the graphitization of amorphous carbon. In our previous study, graphitization was not found by measuring the NEXAFS spectra as described in the introduction [6]. This discrepancy was considered to be due to variations in film thickness. In our previous study, we used
The dependence of the local structure of DLC thin film fabricated by FIB–CVD on annealing temperature was investigated by measuring the near-edge x-ray absorption fine-structure spectra and Raman spectra. The sp2/(sp2 + sp3) ratio remained at ≈0.4 in the annealingtemperature region from RT to 523 K and steeply increased from ≈0.4 to ≈0.7 with annealing temperature from 523 K to 773 K. The graphitization of a FIB–CVD DLC thin film by annealing can be regarded as starting at ≈600 K. These results provide fundamental information on the development and use of nanodevices fabricated utilizing FIB–CVD technology. Acknowledgements Part of this work was supported by the New Energy and Industrial Technology Development Organization (NEDO) under an "Advanced Fundamental Research Project on Hydrogen Storage Materials" and a Joint Studies Program by the Institute for Molecular Science. The authors wish to thank UVSOR staff, especially Dr. Nakamura, for the NEXAFS measurements at BL8B1. References [1] S. Matsui, T. Kaito, J. Fujita, M. Komuro, K. Kanda, Y. Haruyama, J. Vac. Sci. Technol. B 18 (2000) 3181. [2] J. Fujita, M. Ishida, T. Ichihashi, T. Sakamoto, Y. Ochiai, T. Kaito, S. Matsui, Jpn. J. Appl. Phys. 41 (2002) 4423. [3] J. Fujita, M. Ishida, T. Ichihashi, Y. Ochiai, T. Kaito, S. Matsui, Nucl. Instrum. Methods Phys. Res., Sect. B 206 (2003) 472. [4] J. Igaki, A. Saikubo, R. Kometani, K. Kanda, T. Suzuki, K. Niihara, S. Matsui, Jpn. J. Appl. Phys. 46 (2007) 8003. [5] K. Kanda, J. Igaki, A. Saikubo, R. Kometani, T. Suzuki, K. Niihara, H. Saitoh, S. Matsui, Jpn. J. Appl. Phys. 47 (2008) 7464. [6] A. Saikubo, K. Kanda, Y. Kato, J. Igaki, R. Kometani, S. Matsui, Jpn. J. Appl. Phys. 46 (2007) 7512. [7] R. Kometani, Y. Haruyama, K. Kanda, T. Kaito, S. Matsui, Jpn. J. Appl. 46 (2007) 7987. [8] A. Hiraya, E. Nakamura, M. Hasumoto, T. Kinoshita, K. Sakai, Rev. Sci. Instrum. 66 (1995) 2104. [9] K. Kanda, J. Igaki, Y. Kato, R. Kometani, S. Matsui, Radiat. Phys. Chem. 75 (2006) 1850. [10] M. Niibe, Y. Kakutani, K. Koida, S. Matsunari, T. Aoki, S. Terashima, H. Takase, K. Murakami, Y. Fukuda, J. Vac. Sci. Technol. B 25 (2007) 2118. [11] P.E. Batson, Phys. Rev. B 48 (1995) 2608. [12] K. Kanda, T. Kitagawa, Y. Shimizugawa, Y. Haruyama, S. Matsui, M. Terasawa, H. Tsubakino, I. Yamada, T. Gejo, M. Kamada, Jpn. J. Appl. Phys. 41 (2002) 4295. [13] F. Tuinstra, J.L. Koenig, J. Chem. Phys. 53 (1970) 1126. [14] R.J. Nemanich, S.A. Solin, Phys. Res. B 20 (1979) 392. [15] R.O. Dillon, J.A. Woollam, V. Katkanant, Phys. Res. B 29 (1984) 3482. [16] D.S. Knight, W.B. White, J. Mater. Res. 4 (1989) 385. [17] J. Fujita, T. Ichihashi, S. Nakazawa, S. Okada, M. Ishida, Y. Ochiai, T. Kaito, S. Matsui, Appl. Phys. Lett. 88 (2006) 83109.
Contents lists available at ScienceDirect
Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d
Graphitization of thin films formed by focused-ion-beam chemical-vapor-deposition Kazuhiro Kanda a,⁎, Noriko Yamada a, Makoto Okada a, Jun-ya Igaki a, Reo Kometani b, Shinji Matsui a a b
Laboratory of Advanced Science and Technology for Industry, University of Hyogo, Kamigori, Hyogo 678-1205, Japan Graduate School of Engineering, The University of Tokyo, Bunkyo-ku, Tokyo 113-8658, Japan
a r t i c l e
i n f o
Available online 30 October 2008 Keywords: Ion-beam deposition Diamond-like carbon Thermal properties Surface structure
a b s t r a c t The effect of annealing on the structure of diamond-like carbon (DLC) thin film fabricated by focused-ionbeam chemical-vapor deposition (FIB–CVD) was investigated. The near-edge x-ray absorption fine-structure spectrum of the carbon K-edge over the excitation energy range of 275–320 eV was measured on the FIB– CVD DLC thin film by annealing for 1 h in a temperature region from room temperature (RT) to 1273 K. The sp2/(sp2 + sp3) ratio maintained ≈0.4 in an annealing temperature region from room temperature to 523 K and steeply increased from ≈0.4 to ≈0.7 with annealing temperature from 523 K to 773 K. The I(D)/I(G) ratio, which was obtained from the Raman spectrum, also indicated a similar dependence on annealing temperature. The graphitization of thin film fabricated by focused-ion-beam chemical-vapor deposition (FIB– CVD) by annealing was confirmed to start at ≈600 K. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Focused-ion-beam chemical-vapor deposition (FIB–CVD) has been developed as an effective technique of producing nanostructures. This enables three-dimensional (3D) nanostructures to be fabricated, that form complex shapes with overhanging, hollow, and bridging structures [1]. The Young's modulus of a FIB–CVD pillar was reported to exceed 600 GPa [2], which is sufficiently large so that nanodevices fabricated using FIB–CVD offer enormous possibilities for various applications, such as those in biochemical and electromechanical fields. With this method, scanning with a focused Ga ion beam under a phenanthrene gas atmosphere used as a source gas leads to carbon material being deposited in arbitrary nanoscale areas. The fundamental structure of carbon material formed by FIB–CVD was found to be diamond-like carbon (DLC) from measurements of the Raman spectrum [1], and residual Ga atoms used as an ion source were contained in DLC [3]. The atomic fraction of as-deposited FIB–CVD DLC film was determined to be C:Ga:H= 87.4:3.6:9.0 at.% from Rutherford backscattering spectrometry (RBS) and elastic recoil detection analysis (ERDA) [4]. It is known that the material properties of nanodevices fabricated with FIB–CVD are affected by annealing. Residual Ga atoms in the FIB– CVD DLC pillar exited from the pillar and a DLC pillar was graphitized by annealing at 873 K, which was established from transmission electron microscopy (TEM) observations [3]. However, elementary analysis of the FIB–CVD DLC films indicates that residual Ga atoms left the films at higher than 573 K and H atoms left the films at 823 K [5]. Structural analysis by using near-edge x-ray absorption fine structure (NEXAFS) spectroscopy indicated the residual Ga atoms, populated ⁎ Corresponding author. E-mail address: [email protected] (K. Kanda). 0925-9635/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2008.10.013
around the as-deposited film bottom, moved to the neighboring surface of the DLC film and left from the thin film surface by annealing [6]. This movement of Ga in the FIB–CVD DLC film was supported by the x-ray photoelectron spectroscopy observations [7]. The graphitization of FIB–CVD DLC film, on the other hand, was not observed. In the present study, we analysed the structure of FIB–CVD DLC films at various annealing temperatures. The carbon K-edge NEXAFS spectrum of the FIB–CVD DLC films after 1 h of annealing in a temperature range from room temperature (RT) to 1273 K was observed over the excitation energy range 275–320 eV. In addition, the Raman spectra of these samples were observed to confirm their fundamental structures. 2. Experimental FIB–CVD DLC films were formed on a Si substrate surface using a commercially available FIB–CVD system with a Ga+ ion beam operating at 30 keV. The details of synthesis to fabricate the FIB– CVD DLC film have been described in a previous paper [1]. In the present experiment, we prepared nine samples using two kinds of FIB–CVD systems (SII Nano Technology, SMI9200 and SMI2050MS), to investigate what effect annealing had by varying the annealing temperatures. The current of the ion beam was 20 nA for the SMI9200 and 28 nA for the SMI2050MS. The Ga content and H content in the film were expected to be slightly different in each series, because the deposition conditions were not completely identical [5]. The film thicknesses of the samples ranged from 80 to 270 nm. The produced samples were held for 1 h at all annealing temperatures and kept in a dry box after annealing. Annealing temperatures were varied in a region from RT to 1273 K. The samples used in this work were identical to those used for measuring the RBS/ERDA and nanoindentation reported in ref. [5].
K. Kanda et al. / Diamond & Related Materials 18 (2009) 490–492
491
Fig. 2. Dependence of sp2/(sp2 + sp3) ratio on annealing temperature. Open circles and open blocks correspond to sp2/(sp2 + sp3) ratio of FIB–CVD DLC films fabricated by SMI9200 and SMI2050MS.
Fig. 1. C K-edge NEXAFS spectra of FIB–CVD DLC films after a 1-h of annealing at various temperatures.
The NEXAFS measurements were carried out using the BL8B1 stage at the Ultraviolet Synchrotron Orbital Radiation Facility (UVSOR) of the Institute for Molecular Science [8]. The experimental apparatus and procedures employed in this study were almost identical to those in our previous study [9]. Typical slit widths of the monochromator were 20 μm, and the resolution of exciting light energy was about 0.5 eV Full Width Half Maximum (FWHM) for these slit widths. The uncertainty in calibrated wavelength was estimated to be ±0.2 eV. The electrons leaving the sample were detected in the total electron yield (TEY) mode. The carbon K-edge NEXAFS spectra were measured in at energy range of 275\320 eV. Raman spectroscopy was conducted ex situ, at RT with back scattering geometry (NRS-2100; JASCO). The spectra were excited with a 514.5-nm line of an Ar ion laser, analyzed with a triple monochromator, and detected with a liquid-nitrogencooled CCD array detector. The estimated spectral resolution was about 1 cm− 1. The linearity of the working curves of DLC films obtained by NEXAFS has been reported to be excellent up to a film thickness of 4.0 nm [10]. The I(D)/I(G) ratio determined by Raman spectroscopy can be regarded as the bulk structure of DLC film. A more thorough understanding of the local structure of DLC films can be obtained from the complementary measurements.
increased with annealing temperature. As a result, the spectrum of the FIB–CVD DLC film after 1-h of annealing at 1273 K closely resembled the spectrum of graphite. It is known that the sp2 content obtained by NEXAFS study is a good index for DLC properties and NEXAFS spectroscopy is a useable method of measurement to evaluate DLC films. A method of determining the sp2 content from NEXAFS spectrum was described in ref. [12]. In the present study, the value of the sp2/(sp2 + sp3) ratio was estimated from a comparison with that of graphite. The error range of the determined sp2/(sp2 + sp3) ratio was estimated to be less than 10%. The dependence of the sp2/(sp2 + sp3) ratio on annealing temperature is plotted in Fig. 2. The sp2/(sp2 + sp3) ratio of as-deposited FIB–CVD DLC film was ≈0.4. After 1-h of annealing at 523 K, the sp2/(sp2 + sp3) ratio was almost the same. However, it steeply increased from ≈0.4 to ≈0.7 with annealing temperatures from 523 K to 773 K and increased to ≈0.9 with increasing annealing temperature. This indicates that the graphitization of FIB–CVD DLC film by annealing starts at ≈600 K. Raman spectroscopy gave complementary information on the film structure to NEXAFS. Raman spectroscopy is sensitive to structural changes in DLC films. Fig. 3 shows the Raman spectra of FIB–CVD DLC films after 1-h of annealing at various temperatures. The Raman spectra of amorphous carbon materials are characterized by a peak at
3. Results and discussion Fig. 1 shows the NEXAFS carbon K-edge spectra of FIB–CVD DLC films after 1-h annealing at various temperatures. The pre-edge resonance located at 285.4 eV is ascribed to the 1 s→π⁎ resonance transition originating from carbon double bonding. The broad peak located in energy ranges higher than 293 eV is ascribed to 1 s→σ⁎ resonance transition. The peak at 289.0 eV in the NEXAFS spectrum of FIB–CVD DLC was attributed to the 1 s→σ⁎ resonance transition originating from carbon–gallium bonding in the previous work [9]. The spectral features of FIB–CVD DLC film after 1-h of annealing at 523 K resemble those of as-deposited film. However, the intensity of the peak originating from the C–Ga site at 289.0 eV decreased with increasing annealing temperatures from 523 K to 773 K. However, the intensity of the peak originating from the C = C site at 285.4 eV increased from 523 K to 773 K. In addition, the peak height at ≈307 eV, which is a characteristic of the NEXAFS spectrum of graphite [11],
Fig. 3. Raman spectra of FIB–CVD DLC films after 1-h of annealing at various temperatures.
492
K. Kanda et al. / Diamond & Related Materials 18 (2009) 490–492
FIB–CVD DLC films that were 9-nm thick, while we used ≈100-nm thick samples in the present work. The effect of Ga as a catalyst can be considered to be proportional to the density of Ga in the film in a unit area, because Ga atoms moved by annealing from the bottom region populated as-deposited to the surface. Therefore, the effect of graphitization was estimated to be small in the thin film used in the previous work. 4. Conclusion
Fig. 4. Dependence of I(D)/I(G) ratio on annealing temperature. Open circles and open blocks correspond to I(D)/I(G) ratio of FIB–CVD DLC films fabricated by SMI9200 and SMI2050MS.
about 1550\1600 cm− 1 (the G band of the crystalline graphite arising from the zero-center E2g photon) with a shoulder at ≈1350 cm− 1 (due to the so-called D band, assigned to an A1g zone-edge mode activated because of the finite size of the graphic domains) [13–16]. The D peak was hardly observed in the Raman spectrum of as-deposited FIB–CVD DLC film. Its spectral shape resembles that of amorphous carbon film including a high content of sp3 hybridized carbon atoms [9]. After 1-h of annealing at 523 K, the spectral features of FIB–CVD DLC film were almost the same as those of the as-deposited film. However, a D peak appeared after the 1-h of annealing at 523 K and its peak intensity increased with annealing temperature. To compare the peak intensity on a more quantitative basis, the ratio of the intensity of the D peak to that of the G peak, I(D)/I(G), was estimated by fitting the spectra with a Gaussian function. The dependence of the I(D)/I(G) ratio on the annealing temperature is plotted in Fig. 4. The I(D)/I(G) ratio of asdeposited FIB–CVD DLC film was ≈0.5. After 1-h of annealing at 523 K, it did not vary. However, it steeply increased from ≈0.5 to ≈1.2 with annealing temperatures from 523 K to 773 K and increased to ≈1.5 with increased annealing temperature. The increase of D band by 1-h of annealing can be regarded as the increase of finite size of the graphic domains. This dependence of I(D)/I(G) on annealing temperature is in good agreement with that of the sp2/(sp2 + sp3) ratio estimated from the NEXAFS study. This supports the finding that graphitization in the surface and bulk regions of FIB–CVD DLC film by annealing starts at ≈600 K. Our elementary analysis indicates that the exodus of Ga atoms from FIB–CVD DLC films by 1-h of annealing started at ≈500 K and that of H atoms began at ≈700 K [5]. By taking these results into account, graphitization was enhanced by the movement of Ga in the FIB–CVD DLC film during annealing. This conclusion was supported by TEM observation where the FIB–CVD DLC pillar was graphitized by the movement of a metal particle in the pillar [17]. In other words, the metal atoms or metal particles work as a catalyst in FIB–CVD DLC in the graphitization of amorphous carbon. In our previous study, graphitization was not found by measuring the NEXAFS spectra as described in the introduction [6]. This discrepancy was considered to be due to variations in film thickness. In our previous study, we used
The dependence of the local structure of DLC thin film fabricated by FIB–CVD on annealing temperature was investigated by measuring the near-edge x-ray absorption fine-structure spectra and Raman spectra. The sp2/(sp2 + sp3) ratio remained at ≈0.4 in the annealingtemperature region from RT to 523 K and steeply increased from ≈0.4 to ≈0.7 with annealing temperature from 523 K to 773 K. The graphitization of a FIB–CVD DLC thin film by annealing can be regarded as starting at ≈600 K. These results provide fundamental information on the development and use of nanodevices fabricated utilizing FIB–CVD technology. Acknowledgements Part of this work was supported by the New Energy and Industrial Technology Development Organization (NEDO) under an "Advanced Fundamental Research Project on Hydrogen Storage Materials" and a Joint Studies Program by the Institute for Molecular Science. The authors wish to thank UVSOR staff, especially Dr. Nakamura, for the NEXAFS measurements at BL8B1. References [1] S. Matsui, T. Kaito, J. Fujita, M. Komuro, K. Kanda, Y. Haruyama, J. Vac. Sci. Technol. B 18 (2000) 3181. [2] J. Fujita, M. Ishida, T. Ichihashi, T. Sakamoto, Y. Ochiai, T. Kaito, S. Matsui, Jpn. J. Appl. Phys. 41 (2002) 4423. [3] J. Fujita, M. Ishida, T. Ichihashi, Y. Ochiai, T. Kaito, S. Matsui, Nucl. Instrum. Methods Phys. Res., Sect. B 206 (2003) 472. [4] J. Igaki, A. Saikubo, R. Kometani, K. Kanda, T. Suzuki, K. Niihara, S. Matsui, Jpn. J. Appl. Phys. 46 (2007) 8003. [5] K. Kanda, J. Igaki, A. Saikubo, R. Kometani, T. Suzuki, K. Niihara, H. Saitoh, S. Matsui, Jpn. J. Appl. Phys. 47 (2008) 7464. [6] A. Saikubo, K. Kanda, Y. Kato, J. Igaki, R. Kometani, S. Matsui, Jpn. J. Appl. Phys. 46 (2007) 7512. [7] R. Kometani, Y. Haruyama, K. Kanda, T. Kaito, S. Matsui, Jpn. J. Appl. 46 (2007) 7987. [8] A. Hiraya, E. Nakamura, M. Hasumoto, T. Kinoshita, K. Sakai, Rev. Sci. Instrum. 66 (1995) 2104. [9] K. Kanda, J. Igaki, Y. Kato, R. Kometani, S. Matsui, Radiat. Phys. Chem. 75 (2006) 1850. [10] M. Niibe, Y. Kakutani, K. Koida, S. Matsunari, T. Aoki, S. Terashima, H. Takase, K. Murakami, Y. Fukuda, J. Vac. Sci. Technol. B 25 (2007) 2118. [11] P.E. Batson, Phys. Rev. B 48 (1995) 2608. [12] K. Kanda, T. Kitagawa, Y. Shimizugawa, Y. Haruyama, S. Matsui, M. Terasawa, H. Tsubakino, I. Yamada, T. Gejo, M. Kamada, Jpn. J. Appl. Phys. 41 (2002) 4295. [13] F. Tuinstra, J.L. Koenig, J. Chem. Phys. 53 (1970) 1126. [14] R.J. Nemanich, S.A. Solin, Phys. Res. B 20 (1979) 392. [15] R.O. Dillon, J.A. Woollam, V. Katkanant, Phys. Res. B 29 (1984) 3482. [16] D.S. Knight, W.B. White, J. Mater. Res. 4 (1989) 385. [17] J. Fujita, T. Ichihashi, S. Nakazawa, S. Okada, M. Ishida, Y. Ochiai, T. Kaito, S. Matsui, Appl. Phys. Lett. 88 (2006) 83109.