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Annealing effect on the structural, mechanical and electrical properties of titanium-doped diamond-like carbon films
Thin Solid Films 518 (2009) 1503–1507
Contents lists available at ScienceDirect
Thin Solid Films 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 / t s f
Annealing effect on the structural, mechanical and electrical properties of titanium-doped diamond-like carbon films Yu-Hung Lin a, Hong-Da Lin a, Chun-Kuo Liu a, Meng-Wen Huang b, Ya-Chi Chen b, Jiann-Ruey Chen a, Han C. Shih a,c,⁎ a b c
Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan Department of Materials Science and Engineering, National Chung Hsing University, Taichung 402, Taiwan Institute of Materials Science and Nanotechnology, Chinese Culture University, Taipei 111, Taiwan
a r t i c l e
i n f o
Available online 3 October 2009 Keywords: Diamond-like carbon (DLC) Ti-doped DLC Metal vapor vacuum arc (MeVVA) Thermal annealing
a b s t r a c t Titanium-doped diamond-like carbon (Ti-doped DLC) films with a Ti content of 1.1 at.% were synthesized on a Si substrate by a process that involves filtered cathodic vacuum arc (FCVA) and metal vapor vacuum arc (MeVVA) systems. The effect of annealing temperature on the microstructure, surface roughness, hardness and electrical resistivity of the resulting films was evaluated in this study. The Raman spectra revealed that the degree of graphitization of the Ti-doped DLC thin films was increased from 25 to 600 °C and the microstructure of the films is converted to a nano-crystalline graphite structure. The resulting films maintain a smooth surface after the annealing process. The hardness of the Ti-doped DLC films increases as the annealing temperature increases up to 400 °C because the induced defects and the inter-atomic bonds are repaired after the annealing process. But the hardness decreases at the higher temperature due to the increase of number and size of the nano-crystalline graphitic domains. Since the degree of graphitization of the thin films increases, the electrical resistivity of the Ti-doped DLC thin films decreases from 0.038 to 0.006 Ω cm. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Diamond-like carbon (DLC) films are a form of amorphous carbon that contains a significant fraction of sp3 bonds. They are well known for their superior physical and chemical properties of high hardness, chemical inertness, optical transparency, thermal conductivity and biocompatibility [1–6]. To enhance their electrical, optical and mechanical properties, DLC films are frequently doped with nitrogen, fluorine, boron and metal contents [7–17]. Recently, metal-doped DLC films have stimulated considerable interests and have been widely investigated owing to their extraordinary microstructure of the nanocrystalline metal carbide precipitated in an amorphous carbon matrix. Various methods have been developed for depositing metal-doped DLC films, such as plasma-enhanced chemical vapor deposition (PECVD) [11–13], filtered cathodic vacuum arc (FCVA) [14,15], sputtering deposition [16], plasma source ion implantation (PSII) [17], pulsed laser deposition [18] and metal vapor vacuum arc (MeVVA) [19–21]. Of these methods, MeVVA has been recently adopted to modify the surface properties and control the concentration of dopants, proving its efficiency in the synthesis of metal-doped DLC thin films at low cost and with industrial applications [22]. However, the microstructure and inter-atomic bonds of ion-implanted DLC thin films are destroyed ⁎ Corresponding author. Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan. Fax: +886 3 5710290. E-mail address: [email protected] (H.C. Shih). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.09.096
following the bombardment with highly energetic positive ions, which reduces the hardness of the resulted films [21]. In this work, DLC films were synthesized on a Si substrate using a FCVA system, and Ti+ ions were subsequently implanted using a MeVVA ion source. To improve the mechanical property and electrical conductivity of the ion-implanted Ti-doped DLC films, annealing treatments ~200–600 °C are appropriate to eliminate the defects which have been formed by the metal ion bombardments. In a series of experiments, it is found that the hardness of the Ti-doped DLC films steadily increases as the annealing temperature increases from 25 to 400 °C but decreases as the temperature further increases further from 400 to 600 °C, and the electrical resistivity of the resulting films increases from 0.038 to 0.006 Ω cm as the annealing temperature increases from 200 to 600 °C. The chemical composition, microstructure, surface roughness and hardness were also investigated in this study. 2. Experimental details 2.1. Film synthesis and annealing treatments DLC films were deposited to a thickness of 150 nm onto a Si substrate (2 × 2 cm2; p-type) under a dc pulsed negative bias of −300 V at 25 kHz with a duty cycle of 50% at a fixed flow rate of 5 sccm Ar in a 90°-bend magnetic FCVA system. The main arc was initiated using a trigger to start pulse, and maintained by the main arc pulse until it
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was shut off. All of the neutral atoms and macroparticles were filtered out by a 90°-bend magnetic filter with a curved axial field [23]. During the deposition of the DLC films, the working pressure of the vacuum chamber was maintained at 9.3 × 10− 2 Pa at 300 K for a deposition time of 3 min. After the DLC films were deposited, Ti-doped DLC films were obtained by Ti ion implantation into the as-prepared DLC films using a MeVVA ion source. The implantation dose was fixed at 1 × 1017 ions/cm2 at the accelerating voltage of 30 kV. Thermal annealing treatment was performed at 200 °C, 300 °C, 400 °C, 500 °C and 600 °C in a quartz tube furnace at 1 × 10− 2 Pa for 3 h with a heating rate of 20 °C/min. It was demonstrated that the process result has been directly related to the film microstructure, and can be controlled by the annealing temperature. 2.2. Characterization, hardness and electrical resistivity of Ti-doped DLC films The chemical bonding and elemental compositions of DLC and Tidoped DLC films were investigated by X-ray photoelectron spectroscopy (XPS Perkin-Elmer Model PHI1600 system) using a single MgKα (1253.6 eV) X-ray source, operated at 250 W. The microstructure of the film was also analyzed by micro-Raman spectroscopy, with an excitation achieved by means of the 632.8 nm (1.96 eV) line of an He– Ne laser. The surface morphology and roughness of the deposited films were observed over 1 μm × 1 μm by an atomic force microscopy (AFM, Digital Instrument NS3a Controller with D3100 Stage) in tapping mode. The hardness of the Ti-doped DLC films after annealing at various temperatures was measured by a nano-indentation approach to a maximum force of 1 mN, using a Berkovich diamond indenter. The indentation depths were maintained at one tenth of the thickness of the film to prevent any substrate effect. Finally, the electrical resistivity of the Ti-doped DLC films was evaluated using a four-point probe. 3. Results and discussion Fig. 1 shows the C 1s and Ti 2p core-level spectra of the DLC and ion-implanted Ti-doped DLC films. In Fig. 1 (a), the C 1s core-level spectra are composed of four peaks of C O at 288 eV, C–O at 286.5 eV, C–C at 284.6 eV and C–Ti at 282 eV [24]. The formation of C–Ti bonds clearly reveals that the Ti ions were successfully doped into the DLC films using MeVVA ion implantation. As shown in Fig. 1 (b), the appearance of Ti 2p 1/2 peak at 462.4 eV and Ti 2p 3/2 peak at 456.8 eV also indicates that the Ti ions have been doped into the films. Additionally, the atomic percentage of the Ti in Ti-doped DLC films was around 1.08 at.%, determined by the ratio of the area under the C 1s and Ti 2p peaks in the XPS core-level spectra, corrected by the atomic sensitivity factors (C: 0.314 and Ti: 2.077). The microstructure of the DLC films was determined using microRaman spectroscopy, which is frequently applied to measure the microstructure of carbon-based materials. Fig. 2 (a)–(f) presents the evolved Raman spectra of the Ti-doped DLC films that were annealed at various temperatures, i.e., from room temperature up to 600 °C. The peaks in the spectrum were fitted as the D-band (~1350 cm− 1) and the G-band (~1550 cm− 1), characteristic of several DLC films: the D-band and G-band were attributed to the bond-angle disorder of the sp3 bond and the E2g symmetric vibrational mode of the sp2 bond, respectively [25]. The D- and G-bands were significant at 500 °C and the separation increased with the temperature, indicating a pronounced transformation of sp3-bonded carbon to sp2-bonded carbon. Fig. 3 (a) reveals that the full width at half maximum (FWHM) of the G-band decreases and the peak shifts to a higher wave number side from 1525 to 1584 cm− 1 as the annealing temperature increases, indicating that the grain size and the degree of graphitization increase with the annealing temperature [26]. Fig. 3 (b) shows that the ratio of intensities of the D-band to the G-band (ID/IG) is also a function of the
Fig. 1. The XPS core-level spectra of the Ti-doped and un-doped DLC films: (a) C 1s; and (b) Ti 2p.
annealing temperature, and increases from 25 to 600 °C. The behavior of the ID/IG ratio can be consistent with sp2 structural integrity [27] of the graphitization because of the increase in the number of graphite micro domains and sp2-bonded clusters in the films as the ID/IG increases. This observation is similar to those in previous investigations of the thermal stability of DLC and Cr-doped DLC films [28,29]. Furthermore, the significant increase of the ID/IG ratio as a result of the observable separation of the D- and G-band is caused by the conversion of the carbon structure of the Ti-doped DLC films to the nano-crystalline graphite and carbide as the annealing temperature increases. The conversion of nano-crystalline graphite also results in a considerable transfer of the sp3 bonds to sp2 bonds. The morphology and the root mean square roughness (Rrms) of the Ti-doped DLC films following annealing at various temperatures were investigated by AFM. Fig. 4 shows three-dimensional AFM morphological images of the films before annealing and annealed at 600 °C. Fig. 5 presents detailed data on the variation of Rrms of the Tidoped DLC films at various annealing temperatures. In all cases, the roughness of the films remained low between 0.19 nm and 0.21 nm. However, the Ti-doped DLC films maintain their smooth surface and low friction coefficients after annealing. DLC films have been applied as hard coatings and their mechanical properties are therefore very important. In this study, the hardness of the Ti-doped DLC films was investigated by nano-indentation. Fig. 6 shows that the hardness of the Ti-doped DLC films steadily increases as the annealing temperature increases from 25 to 400 °C but decreases as the temperature further increases further from 400 to 600 °C. Highly energetic Ti+ ion bombardment can destroy the microstructure of the DLC films and produce numerous defects in the
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Fig. 2. The Raman spectra of the Ti-doped DLC films evolved with the annealing temperature at (a) 25 °C, (b) 200 °C, (c) 300 °C, (d) 400 °C, (e) 500 °C and, (f) 600 °C.
process of the MeVVA ion implantation, which reduces the hardness of the film [21]. The increase in hardness from 29.2 to 32.4 GPa with a lower annealing temperature from 25 to 400 °C occurs because the induced defects and the inter-atomic bonds are repaired. However, the hardness decreases from 32.4 to 28.8 GPa at a higher annealing temperature, which is consistent with the observation in Raman spectra of the Ti-doped DLC films. As the annealing temperature further increases to 500 °C, the D- and G-bands start to split as shown in Fig. 2 (e) and (f), indicating that the carbon atoms are converted to a nano-crystalline graphite structure. Such conversion to nanocrystalline graphite must significantly increase the sp2-bonded carbon fraction and ID/IG ratio. Therefore, the hardness of the Ti-doped DLC films decreases as the annealing temperature increases because of the increase of the number and size of nano-crystalline graphitic domains.
However, the variation of the decreasing hardness of the Ti-doped DLC films is slight, compared with that of the pure DLC films after the annealing from 400 to 600 °C [29]. The comparatively slight decrease in hardness is a result of the nano-crystalline TiC carbide-existence in the amorphous carbon matrix. This metal carbide is stable even at the temperature of 600 °C and therefore maintains the hardness of the Tidoped DLC films at high annealing temperatures, which is consistent with the reported study of Zhang et al. [30]. Fig. 7 plots the electrical resistivity of the Ti-doped DLC films measured using the four-point probe technique, which demonstrated that the electrical resistivity decreases from 0.038 to 0.006 Ω cm as the annealing temperature increases from 25 to 600 °C. The decrease in electrical resistivity is attributed to the formation of a large amount of the bonded conductive sp2 carbons, which is consistent with the Raman spectra.
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Fig. 4. AFM 3-D images of Ti-doped DLC films (a) before annealing and (b) annealed at 600 °C. Fig. 3. The annealing temperature evolved characters of the Ti-doped DLC films: (a) G-band position and full width at half maximum (FWHM), and (b) ID/IG ratio.
A decrease in electrical resistivity notably occurs at 300 °C, but a significant increase in the ID/IG ratio appears at higher annealing temperature of 400 °C. It is apparent that the new sp2 sites measured by the resistivity are more sensitive to that measured by the Raman spectrum, which is consistent with the study of Ferrari et al. [31]. 4. Conclusions
other hand, the electrical conductivity of the Ti-doped DLC films can be improved by the process of thermal annealing. Moreover, this study demonstrates that the new sp2 sites measured by the electrical resistivity are more sensitive to that measured by the Raman spectrum. Acknowledgement The authors would like to thank the National Science Council of the Republic of China for the financial support of this research under the contract number: NSC 96-2221-E-034-006-MY2.
In this work, Ti-doped DLC films with Ti concentration of 1.1 at.% were synthesized on Si substrate by a process that combines a FCVA system with a MeVVA system. The most important results are summarized as follows. 1. The microstructure of the films is converted to a nano-crystalline graphite structure and as a result the ratio of ID/IG increases with the increasing annealing temperature. 2. The surface roughness (Rrms) of the Ti-doped DLC films retains a value of 0.20 ± 0.0 after annealing, which indicates that the resulting films maintain a smooth surface. 3. The hardness of the Ti-doped DLC films increases as the annealing temperature increases from 25 to 400 °C but decreases as the annealing temperature still increases further. The increase in the hardness of the resulting films is due to the induced defects and the inter-atomic bonds are repaired after the annealing process. However, the film hardness decreases at further higher annealing temperature owing to the increase of number and size of the nanocrystalline graphitic domains. 4. The electrical resistivity decreases as the annealing temperature increases because the sp2 content in the volume increases. On the
Fig. 5. Slight variation of the measured Rrms around 0.2 for the Ti-doped DLC films annealed at various temperatures.
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References [1] [2] [3] [4] [5]
[6] [7] [8] [9] [10] [11] [12] [13] Fig. 6. Variation of the measured hardness of the Ti-doped DLC films annealed at various temperatures, showing that the film hardness increases from 25 to 400 °C but decreases thereafter.
[14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31]
Fig. 7. Variation of the measured electrical resistivity of the Ti-doped DLC films annealed from 25 to 600 °C.
A. Grill, Diamond Relat. Mater. 8 (1999) 428. S. Sattel, J. Robertson, H. Ehrhardt, J. Appl. Phys. 82 (1997) 4566. J. Robertson, Surf. Coat. Technol. 50 (1992) 185. F. Ruiz, W.D. Sun, F.H. Pollak, C. Venkatraman, Appl. Phys. Lett. 73 (1998) 1802. N. Boutroy, Y. Pernel, J.M. Rius, F. Auger, H.J. von Bardeleben, J.L. Cantin, F. Abel, Andreas Zeinert, C. Casiraghi, A.C. Ferrari, J. Robertson, Diamond Relat. Mater. 15 (2006) 921. D.P. Dowling, P.V. Kola, K. Donnelly, T.C. Kelly, K. Brumitt, L. Lloyd, R. Eloy, M. Therin, N. Weill, Diamond Relat. Mater. 6 (1997) 390. S.R.P. Silva, J. Robertson, G.A.J. Amaratunga, J. Appl. Phys. 81 (1997) 2626. J. Li, W. Zheng, C. Gu, Z. Jin, Y. Zhao, X. Mei, Z. Mu, C. Dong, C. Sun, Carbon 42 (2004) 2309. X.W. Liu, L.H. Chan, W.J. Hsieh, J.H. Lin, H.C. Shih, Carbon 41 (2003) 1143. W.J. Hsieh, S.H. Lai, L.H. Chan, K.L. Chang, H.C. Shih, Carbon 43 (2005) 820. W.J. Wu, M.H. Hon, Surf. Coat. Technol. 111 (1999) 134. W.J. Meng, T.J. Curtis, L.E. Rehn, P.M. Baldo, J. Appl. Phys. 83 (1998) 6076. Y.K. Cho, W.S. Jang, S. Yoo, S.G. Kim, S.W. Kim, Surf. Coat. Technol. 202 (2008) 5390. J.S. Chen, S.P. Lau, Z. Sun, G.Y. Chen, Y.J. Li, B.K. Tay, J.W. Chai, Thin Solid Films 398 (2001) 110. K.W. Weng, Y.C. Chen, T.N. Lin, D.Y. Wang, Thin Solid Films 515 (2006) 1053. W.Y. Wu, J.M. Ting, Carbon 44 (2006) 1210. K. Baba, R. Hatada, Surf. Coat. Technol. 169 (2003) 287. R.J. Narayan, Diamond Relat. Mater. 14 (2005) 1319. L. Cui, L. Guoqing, C. Wenwu, M. Zongxin, Z. Chengwu, W. Liang, Thin Solid Films 475 (2005) 279. E. Byon, J.K. Kim, J.J. Rha, S.C. Kwon, Z. Mu, C. Liu, G. Li, Surf. Coat. Technol. 201 (2007) 6670. K.W. Weng, C.L. Chang, D.Y. Wang, Diamond Relat. Mater. 11 (2002) 1447. A.D. Liu, H.X. Zhang, T.H. Zhang, Surf. Coat. Technol. 193 (2005) 65. C.C. Lin, W.J. Hsieh, J.H. Lin, U.S. Chen, X.J. Guo, H.C. Shih, Surf. Coat. Technol. 200 (2006) 5052. D. Rats, D. Rats, L. Vandenbulcke, R. Herbin, R. Benoit, R. Erre, V. Serin, J. Sevely, Thin Solid Films 270 (1995) 177. A. Richter, H.J. Scheibe, W. Pompe, K.W. Brezeinka, I. Muhling, J. Non-Cryst. Solids 88 (1986) 131. J. Robertson, Mater. Sci. Eng. 37 (2002) 129. M.A. Tamor, W.C. Vassell, J. Appl. Phys. 76 (1994) 3823. M.C. Chiu, W.P. Hsieh, W.Y. Ho, D.Y. Wang, F.S. Shieu, Thin Solid Films 476 (2005) 258. W.J. Yang, Y.H. Choa, T. Sekino, K.B. Shim, K. Niihara, K.H. Auh, Thin Solid Films 434 (2003) 49. S. Zhang, X.L. Bui, X. Li, Diamond Relat. Mater. 15 (2006) 972. A.C. Ferrari, B. Kleinsorge, N.A. Morrison, A. Hart, V. Stolojan, J. Robertson, J. Appl. Phys. 85 (1999) 7191.
Contents lists available at ScienceDirect
Thin Solid Films 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 / t s f
Annealing effect on the structural, mechanical and electrical properties of titanium-doped diamond-like carbon films Yu-Hung Lin a, Hong-Da Lin a, Chun-Kuo Liu a, Meng-Wen Huang b, Ya-Chi Chen b, Jiann-Ruey Chen a, Han C. Shih a,c,⁎ a b c
Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan Department of Materials Science and Engineering, National Chung Hsing University, Taichung 402, Taiwan Institute of Materials Science and Nanotechnology, Chinese Culture University, Taipei 111, Taiwan
a r t i c l e
i n f o
Available online 3 October 2009 Keywords: Diamond-like carbon (DLC) Ti-doped DLC Metal vapor vacuum arc (MeVVA) Thermal annealing
a b s t r a c t Titanium-doped diamond-like carbon (Ti-doped DLC) films with a Ti content of 1.1 at.% were synthesized on a Si substrate by a process that involves filtered cathodic vacuum arc (FCVA) and metal vapor vacuum arc (MeVVA) systems. The effect of annealing temperature on the microstructure, surface roughness, hardness and electrical resistivity of the resulting films was evaluated in this study. The Raman spectra revealed that the degree of graphitization of the Ti-doped DLC thin films was increased from 25 to 600 °C and the microstructure of the films is converted to a nano-crystalline graphite structure. The resulting films maintain a smooth surface after the annealing process. The hardness of the Ti-doped DLC films increases as the annealing temperature increases up to 400 °C because the induced defects and the inter-atomic bonds are repaired after the annealing process. But the hardness decreases at the higher temperature due to the increase of number and size of the nano-crystalline graphitic domains. Since the degree of graphitization of the thin films increases, the electrical resistivity of the Ti-doped DLC thin films decreases from 0.038 to 0.006 Ω cm. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Diamond-like carbon (DLC) films are a form of amorphous carbon that contains a significant fraction of sp3 bonds. They are well known for their superior physical and chemical properties of high hardness, chemical inertness, optical transparency, thermal conductivity and biocompatibility [1–6]. To enhance their electrical, optical and mechanical properties, DLC films are frequently doped with nitrogen, fluorine, boron and metal contents [7–17]. Recently, metal-doped DLC films have stimulated considerable interests and have been widely investigated owing to their extraordinary microstructure of the nanocrystalline metal carbide precipitated in an amorphous carbon matrix. Various methods have been developed for depositing metal-doped DLC films, such as plasma-enhanced chemical vapor deposition (PECVD) [11–13], filtered cathodic vacuum arc (FCVA) [14,15], sputtering deposition [16], plasma source ion implantation (PSII) [17], pulsed laser deposition [18] and metal vapor vacuum arc (MeVVA) [19–21]. Of these methods, MeVVA has been recently adopted to modify the surface properties and control the concentration of dopants, proving its efficiency in the synthesis of metal-doped DLC thin films at low cost and with industrial applications [22]. However, the microstructure and inter-atomic bonds of ion-implanted DLC thin films are destroyed ⁎ Corresponding author. Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan. Fax: +886 3 5710290. E-mail address: [email protected] (H.C. Shih). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.09.096
following the bombardment with highly energetic positive ions, which reduces the hardness of the resulted films [21]. In this work, DLC films were synthesized on a Si substrate using a FCVA system, and Ti+ ions were subsequently implanted using a MeVVA ion source. To improve the mechanical property and electrical conductivity of the ion-implanted Ti-doped DLC films, annealing treatments ~200–600 °C are appropriate to eliminate the defects which have been formed by the metal ion bombardments. In a series of experiments, it is found that the hardness of the Ti-doped DLC films steadily increases as the annealing temperature increases from 25 to 400 °C but decreases as the temperature further increases further from 400 to 600 °C, and the electrical resistivity of the resulting films increases from 0.038 to 0.006 Ω cm as the annealing temperature increases from 200 to 600 °C. The chemical composition, microstructure, surface roughness and hardness were also investigated in this study. 2. Experimental details 2.1. Film synthesis and annealing treatments DLC films were deposited to a thickness of 150 nm onto a Si substrate (2 × 2 cm2; p-type) under a dc pulsed negative bias of −300 V at 25 kHz with a duty cycle of 50% at a fixed flow rate of 5 sccm Ar in a 90°-bend magnetic FCVA system. The main arc was initiated using a trigger to start pulse, and maintained by the main arc pulse until it
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was shut off. All of the neutral atoms and macroparticles were filtered out by a 90°-bend magnetic filter with a curved axial field [23]. During the deposition of the DLC films, the working pressure of the vacuum chamber was maintained at 9.3 × 10− 2 Pa at 300 K for a deposition time of 3 min. After the DLC films were deposited, Ti-doped DLC films were obtained by Ti ion implantation into the as-prepared DLC films using a MeVVA ion source. The implantation dose was fixed at 1 × 1017 ions/cm2 at the accelerating voltage of 30 kV. Thermal annealing treatment was performed at 200 °C, 300 °C, 400 °C, 500 °C and 600 °C in a quartz tube furnace at 1 × 10− 2 Pa for 3 h with a heating rate of 20 °C/min. It was demonstrated that the process result has been directly related to the film microstructure, and can be controlled by the annealing temperature. 2.2. Characterization, hardness and electrical resistivity of Ti-doped DLC films The chemical bonding and elemental compositions of DLC and Tidoped DLC films were investigated by X-ray photoelectron spectroscopy (XPS Perkin-Elmer Model PHI1600 system) using a single MgKα (1253.6 eV) X-ray source, operated at 250 W. The microstructure of the film was also analyzed by micro-Raman spectroscopy, with an excitation achieved by means of the 632.8 nm (1.96 eV) line of an He– Ne laser. The surface morphology and roughness of the deposited films were observed over 1 μm × 1 μm by an atomic force microscopy (AFM, Digital Instrument NS3a Controller with D3100 Stage) in tapping mode. The hardness of the Ti-doped DLC films after annealing at various temperatures was measured by a nano-indentation approach to a maximum force of 1 mN, using a Berkovich diamond indenter. The indentation depths were maintained at one tenth of the thickness of the film to prevent any substrate effect. Finally, the electrical resistivity of the Ti-doped DLC films was evaluated using a four-point probe. 3. Results and discussion Fig. 1 shows the C 1s and Ti 2p core-level spectra of the DLC and ion-implanted Ti-doped DLC films. In Fig. 1 (a), the C 1s core-level spectra are composed of four peaks of C O at 288 eV, C–O at 286.5 eV, C–C at 284.6 eV and C–Ti at 282 eV [24]. The formation of C–Ti bonds clearly reveals that the Ti ions were successfully doped into the DLC films using MeVVA ion implantation. As shown in Fig. 1 (b), the appearance of Ti 2p 1/2 peak at 462.4 eV and Ti 2p 3/2 peak at 456.8 eV also indicates that the Ti ions have been doped into the films. Additionally, the atomic percentage of the Ti in Ti-doped DLC films was around 1.08 at.%, determined by the ratio of the area under the C 1s and Ti 2p peaks in the XPS core-level spectra, corrected by the atomic sensitivity factors (C: 0.314 and Ti: 2.077). The microstructure of the DLC films was determined using microRaman spectroscopy, which is frequently applied to measure the microstructure of carbon-based materials. Fig. 2 (a)–(f) presents the evolved Raman spectra of the Ti-doped DLC films that were annealed at various temperatures, i.e., from room temperature up to 600 °C. The peaks in the spectrum were fitted as the D-band (~1350 cm− 1) and the G-band (~1550 cm− 1), characteristic of several DLC films: the D-band and G-band were attributed to the bond-angle disorder of the sp3 bond and the E2g symmetric vibrational mode of the sp2 bond, respectively [25]. The D- and G-bands were significant at 500 °C and the separation increased with the temperature, indicating a pronounced transformation of sp3-bonded carbon to sp2-bonded carbon. Fig. 3 (a) reveals that the full width at half maximum (FWHM) of the G-band decreases and the peak shifts to a higher wave number side from 1525 to 1584 cm− 1 as the annealing temperature increases, indicating that the grain size and the degree of graphitization increase with the annealing temperature [26]. Fig. 3 (b) shows that the ratio of intensities of the D-band to the G-band (ID/IG) is also a function of the
Fig. 1. The XPS core-level spectra of the Ti-doped and un-doped DLC films: (a) C 1s; and (b) Ti 2p.
annealing temperature, and increases from 25 to 600 °C. The behavior of the ID/IG ratio can be consistent with sp2 structural integrity [27] of the graphitization because of the increase in the number of graphite micro domains and sp2-bonded clusters in the films as the ID/IG increases. This observation is similar to those in previous investigations of the thermal stability of DLC and Cr-doped DLC films [28,29]. Furthermore, the significant increase of the ID/IG ratio as a result of the observable separation of the D- and G-band is caused by the conversion of the carbon structure of the Ti-doped DLC films to the nano-crystalline graphite and carbide as the annealing temperature increases. The conversion of nano-crystalline graphite also results in a considerable transfer of the sp3 bonds to sp2 bonds. The morphology and the root mean square roughness (Rrms) of the Ti-doped DLC films following annealing at various temperatures were investigated by AFM. Fig. 4 shows three-dimensional AFM morphological images of the films before annealing and annealed at 600 °C. Fig. 5 presents detailed data on the variation of Rrms of the Tidoped DLC films at various annealing temperatures. In all cases, the roughness of the films remained low between 0.19 nm and 0.21 nm. However, the Ti-doped DLC films maintain their smooth surface and low friction coefficients after annealing. DLC films have been applied as hard coatings and their mechanical properties are therefore very important. In this study, the hardness of the Ti-doped DLC films was investigated by nano-indentation. Fig. 6 shows that the hardness of the Ti-doped DLC films steadily increases as the annealing temperature increases from 25 to 400 °C but decreases as the temperature further increases further from 400 to 600 °C. Highly energetic Ti+ ion bombardment can destroy the microstructure of the DLC films and produce numerous defects in the
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Fig. 2. The Raman spectra of the Ti-doped DLC films evolved with the annealing temperature at (a) 25 °C, (b) 200 °C, (c) 300 °C, (d) 400 °C, (e) 500 °C and, (f) 600 °C.
process of the MeVVA ion implantation, which reduces the hardness of the film [21]. The increase in hardness from 29.2 to 32.4 GPa with a lower annealing temperature from 25 to 400 °C occurs because the induced defects and the inter-atomic bonds are repaired. However, the hardness decreases from 32.4 to 28.8 GPa at a higher annealing temperature, which is consistent with the observation in Raman spectra of the Ti-doped DLC films. As the annealing temperature further increases to 500 °C, the D- and G-bands start to split as shown in Fig. 2 (e) and (f), indicating that the carbon atoms are converted to a nano-crystalline graphite structure. Such conversion to nanocrystalline graphite must significantly increase the sp2-bonded carbon fraction and ID/IG ratio. Therefore, the hardness of the Ti-doped DLC films decreases as the annealing temperature increases because of the increase of the number and size of nano-crystalline graphitic domains.
However, the variation of the decreasing hardness of the Ti-doped DLC films is slight, compared with that of the pure DLC films after the annealing from 400 to 600 °C [29]. The comparatively slight decrease in hardness is a result of the nano-crystalline TiC carbide-existence in the amorphous carbon matrix. This metal carbide is stable even at the temperature of 600 °C and therefore maintains the hardness of the Tidoped DLC films at high annealing temperatures, which is consistent with the reported study of Zhang et al. [30]. Fig. 7 plots the electrical resistivity of the Ti-doped DLC films measured using the four-point probe technique, which demonstrated that the electrical resistivity decreases from 0.038 to 0.006 Ω cm as the annealing temperature increases from 25 to 600 °C. The decrease in electrical resistivity is attributed to the formation of a large amount of the bonded conductive sp2 carbons, which is consistent with the Raman spectra.
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Fig. 4. AFM 3-D images of Ti-doped DLC films (a) before annealing and (b) annealed at 600 °C. Fig. 3. The annealing temperature evolved characters of the Ti-doped DLC films: (a) G-band position and full width at half maximum (FWHM), and (b) ID/IG ratio.
A decrease in electrical resistivity notably occurs at 300 °C, but a significant increase in the ID/IG ratio appears at higher annealing temperature of 400 °C. It is apparent that the new sp2 sites measured by the resistivity are more sensitive to that measured by the Raman spectrum, which is consistent with the study of Ferrari et al. [31]. 4. Conclusions
other hand, the electrical conductivity of the Ti-doped DLC films can be improved by the process of thermal annealing. Moreover, this study demonstrates that the new sp2 sites measured by the electrical resistivity are more sensitive to that measured by the Raman spectrum. Acknowledgement The authors would like to thank the National Science Council of the Republic of China for the financial support of this research under the contract number: NSC 96-2221-E-034-006-MY2.
In this work, Ti-doped DLC films with Ti concentration of 1.1 at.% were synthesized on Si substrate by a process that combines a FCVA system with a MeVVA system. The most important results are summarized as follows. 1. The microstructure of the films is converted to a nano-crystalline graphite structure and as a result the ratio of ID/IG increases with the increasing annealing temperature. 2. The surface roughness (Rrms) of the Ti-doped DLC films retains a value of 0.20 ± 0.0 after annealing, which indicates that the resulting films maintain a smooth surface. 3. The hardness of the Ti-doped DLC films increases as the annealing temperature increases from 25 to 400 °C but decreases as the annealing temperature still increases further. The increase in the hardness of the resulting films is due to the induced defects and the inter-atomic bonds are repaired after the annealing process. However, the film hardness decreases at further higher annealing temperature owing to the increase of number and size of the nanocrystalline graphitic domains. 4. The electrical resistivity decreases as the annealing temperature increases because the sp2 content in the volume increases. On the
Fig. 5. Slight variation of the measured Rrms around 0.2 for the Ti-doped DLC films annealed at various temperatures.
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References [1] [2] [3] [4] [5]
[6] [7] [8] [9] [10] [11] [12] [13] Fig. 6. Variation of the measured hardness of the Ti-doped DLC films annealed at various temperatures, showing that the film hardness increases from 25 to 400 °C but decreases thereafter.
[14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31]
Fig. 7. Variation of the measured electrical resistivity of the Ti-doped DLC films annealed from 25 to 600 °C.
A. Grill, Diamond Relat. Mater. 8 (1999) 428. S. Sattel, J. Robertson, H. Ehrhardt, J. Appl. Phys. 82 (1997) 4566. J. Robertson, Surf. Coat. Technol. 50 (1992) 185. F. Ruiz, W.D. Sun, F.H. Pollak, C. Venkatraman, Appl. Phys. Lett. 73 (1998) 1802. N. Boutroy, Y. Pernel, J.M. Rius, F. Auger, H.J. von Bardeleben, J.L. Cantin, F. Abel, Andreas Zeinert, C. Casiraghi, A.C. Ferrari, J. Robertson, Diamond Relat. Mater. 15 (2006) 921. D.P. Dowling, P.V. Kola, K. Donnelly, T.C. Kelly, K. Brumitt, L. Lloyd, R. Eloy, M. Therin, N. Weill, Diamond Relat. Mater. 6 (1997) 390. S.R.P. Silva, J. Robertson, G.A.J. Amaratunga, J. Appl. Phys. 81 (1997) 2626. J. Li, W. Zheng, C. Gu, Z. Jin, Y. Zhao, X. Mei, Z. Mu, C. Dong, C. Sun, Carbon 42 (2004) 2309. X.W. Liu, L.H. Chan, W.J. Hsieh, J.H. Lin, H.C. Shih, Carbon 41 (2003) 1143. W.J. Hsieh, S.H. Lai, L.H. Chan, K.L. Chang, H.C. Shih, Carbon 43 (2005) 820. W.J. Wu, M.H. Hon, Surf. Coat. Technol. 111 (1999) 134. W.J. Meng, T.J. Curtis, L.E. Rehn, P.M. Baldo, J. Appl. Phys. 83 (1998) 6076. Y.K. Cho, W.S. Jang, S. Yoo, S.G. Kim, S.W. Kim, Surf. Coat. Technol. 202 (2008) 5390. J.S. Chen, S.P. Lau, Z. Sun, G.Y. Chen, Y.J. Li, B.K. Tay, J.W. Chai, Thin Solid Films 398 (2001) 110. K.W. Weng, Y.C. Chen, T.N. Lin, D.Y. Wang, Thin Solid Films 515 (2006) 1053. W.Y. Wu, J.M. Ting, Carbon 44 (2006) 1210. K. Baba, R. Hatada, Surf. Coat. Technol. 169 (2003) 287. R.J. Narayan, Diamond Relat. Mater. 14 (2005) 1319. L. Cui, L. Guoqing, C. Wenwu, M. Zongxin, Z. Chengwu, W. Liang, Thin Solid Films 475 (2005) 279. E. Byon, J.K. Kim, J.J. Rha, S.C. Kwon, Z. Mu, C. Liu, G. Li, Surf. Coat. Technol. 201 (2007) 6670. K.W. Weng, C.L. Chang, D.Y. Wang, Diamond Relat. Mater. 11 (2002) 1447. A.D. Liu, H.X. Zhang, T.H. Zhang, Surf. Coat. Technol. 193 (2005) 65. C.C. Lin, W.J. Hsieh, J.H. Lin, U.S. Chen, X.J. Guo, H.C. Shih, Surf. Coat. Technol. 200 (2006) 5052. D. Rats, D. Rats, L. Vandenbulcke, R. Herbin, R. Benoit, R. Erre, V. Serin, J. Sevely, Thin Solid Films 270 (1995) 177. A. Richter, H.J. Scheibe, W. Pompe, K.W. Brezeinka, I. Muhling, J. Non-Cryst. Solids 88 (1986) 131. J. Robertson, Mater. Sci. Eng. 37 (2002) 129. M.A. Tamor, W.C. Vassell, J. Appl. Phys. 76 (1994) 3823. M.C. Chiu, W.P. Hsieh, W.Y. Ho, D.Y. Wang, F.S. Shieu, Thin Solid Films 476 (2005) 258. W.J. Yang, Y.H. Choa, T. Sekino, K.B. Shim, K. Niihara, K.H. Auh, Thin Solid Films 434 (2003) 49. S. Zhang, X.L. Bui, X. Li, Diamond Relat. Mater. 15 (2006) 972. A.C. Ferrari, B. Kleinsorge, N.A. Morrison, A. Hart, V. Stolojan, J. Robertson, J. Appl. Phys. 85 (1999) 7191.