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methane ratios
Carbon 45 (2007) 2004–2010 www.elsevier.com/locate/carbon
Characteristics of carbon coatings on optical fibers prepared by plasma enhanced chemical vapor deposition using different argon/methane ratios Hung-Chien Lin a, Sham-Tsong Shiue a,*, Yu-Hang Cheng a, Tsong-Jen Yang b, Tung-Chuan Wu c, Hung-Yi Lin c a
Department of Materials Science and Engineering, National Chung Hsing University, 250 Kuo Kuang Road, Taichung, 402 Taiwan, ROC b Department of Materials Science and Engineering, Feng Chia University, 100 Wen Hwa Road, Taichung, 407 Taiwan, ROC c Mechanical and Systems Research Laboratories, Industrial Technology Research Institute, Hsinchu, 310 Taiwan, ROC Received 23 January 2007; accepted 2 June 2007 Available online 13 June 2007
Abstract The effect of argon/methane ratios on the characteristics of carbon coatings on optical fibers prepared by plasma enhanced chemical vapor deposition is investigated. The argon/methane ratios are selected at 0, 1, 2, 4, 6, and 8. Results show that if the argon/methane ratio increases from 0 to 4, the deposition process is mainly sustained by the Penning effect. The Penning effect enhances the dissociation of the hydrocarbon gas molecules. The carbon coating shifts to the graphite-like carbon and the surface of the carbon coating becomes smoother. As the argon/methane ratio increases from 4 to 8, etching becomes more important. The sp2 C@C bonding in the carbon coating transforms from order into disorder. Because the disordered sp2 C@C bonding increases, the surface of the carbon coating becomes rougher. When the argon/methane ratio is 4, the surface roughness of the carbon coating is a minimum, while the water contact angle of carbon coating is a maximum. At this argon/methane ratio, the carbon coating has the best ability to minimize low temperature induced delamination or microcracks, so is the best for the production of a hermetically sealed optical fiber coating. 2007 Elsevier Ltd. All rights reserved.
1. Introduction Recently, various properties of carbon films have been found, such as wide bandgap, high hardness, inert chemical attack, infrared transparency, and high water resistance [1,2]. Many researches successfully applied carbon films to the commercial applications such as hard disk, frictional resistance films, protective optical materials, biomedical products, and so on. Meanwhile, the excellent properties of carbon films have been also applied to become hermetical coatings of optical fibers and the carbon-coated optical fiber is expected to be a key technology for optical transmission lines [3–10]. *
Corresponding author. Fax: +886 4 22857017. E-mail address: [email protected] (S.-T. Shiue).
0008-6223/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2007.06.003
Preparation of carbon films can be carried out at a low substrate temperature and high deposition rate using various methods, such as radio-frequency-plasma enhanced chemical vapor deposition (rf-PECVD) [7,10–12], plasma enhanced chemical vapor deposition with electron cyclotron wave resonance source [13], microwave plasma chemical vapor deposition [14], sputtering deposition [15], filtered cathodic vacuum arc deposition [16], and ion beam deposition [17]. The carbon film prepared by rf-PECVD using methane (CH4) as the precursor gases contains a percentage of hydrogen, so it is usually classified as the hydrogenated amorphous carbon (a-C:H) film in the literature [2]. Recently, a-C:H films for hermetical optical fiber coatings prepared by rf-PECVD using methane (CH4) and hydrogen (H2) as the precursor gases were studied [7,10]. When the carbon films were prepared by rf-PECVD
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method, the characteristics of carbon films are affected by many factors such as the mixture ratio of reactive gases, self-bias voltage, and so on [10,18–20]. However, no work was found to study the effect of argon/methane (Ar/CH4) ratios on the properties of carbon coatings on optical fibers prepared by rf-PECVD method. Hence, the purpose of this article is to investigate the influence of Ar/CH4 ratios on the characteristics of carbon coatings on optical fibers. Additionally, the optimal Ar/CH4 ratio to fabricate the carbon-coated optical fibers will be proposed.
2. Experimental The experimental details proceeded as follows. First, silica glass fibers and silica glass plates were cleaned in ultrasonic bath of ethanol, acetone, and de-ionized (DI) water in sequence. These pre-treatments were typically applied to improve the adhesion of carbon coatings onto these substrates. Second, the carbon coatings were simultaneously deposited on silica glass fibers and silica glass plates using a 13.56 MHz capacitively coupled rfPECVD system in which a cylindrical stainless steel reaction chamber with two parallel planar electrodes was employed. The distance between two parallel planar electrodes was 3 cm. Before deposition, the reaction chamber was pumped so that the base pressure was less than 7.5 · 106 Torr. The carbon coatings were deposited with working pressure of 6 · 101 Torr (about 80 Pa) and radio-frequency power of 250 W. The substrate temperature was controlled by water cooling system and the substrate temperature was set at 293 K (20 C). Six kinds of carbon coatings were prepared with the Ar/CH4 ratios being 0, 1, 2, 4, 6, and 8, where the flow rate of CH4 gas was fixed at 10 sccm. The self-bias voltage was recorded at different deposition conditions. The deposition time was controlled such that carbon coatings of identical thickness (about 400 nm) were produced for all the Ar/CH4 ratios. The thickness of carbon coatings was obtained by examining the cross-section of carbon-coated optical fiber using the field emission scanning electron microscope (FESEM, JEOL JSM-6700F). Each thickness was estimated from the average value of five data with the same deposition condition. Thirdly, the structure of carbon coatings was examined by the Raman scattering spectroscopy (RSS, JOBIN YVON Triax 550) and the Fourier transform infrared (FTIR) spectroscopy (Thermo-Nicolet NEXUS 470). The RSS was measured in the back-scattering geometry with the 633 nm line of a He–Ne laser at room temperature in the spectral range from 800 to 2000 cm1. The laser power was 25 mW and irradiation time of the laser on the coatings was 20 s/point. The FTIR can identify the nature of bonding (C–Hx and C– C stretching modes) by absorbance spectrum at 298 K. All spectra were detected in the range of 1250–3600 cm1 with 128 scans at resolution of 1 cm1. Fourthly, the mechanical property of carbon coatings was measured by the nanoindenter (NI, UMIS Nanoindentation, Based Model, CRISO, Australia). In each of indentation experiment, the indenter was loaded and unloaded three times at the maximum penetration depth less than 10% of the film thickness. Fifthly, the roughness of carbon coatings was examined by the atomic force microscopy (AFM, Digital Instrument NS4/D3100CL/MultiMode). The value of surface roughness, Ra was estimated by tapping mode method and was evaluated in the area of 5 · 5 lm. Each datum was obtained from the average value of three different positions on the same specimen surface. Meanwhile, the water-repellency of carbon coatings was measured by contact angle (CA) meter with sessiledrop technique. A DI water droplet with a volume of 2 ll was released onto the surface of the sample from a syringe needle. Each measurement was repeated 10 times and reproducibility was never worse than ±1. Finally, the carbon-coated optical fibers were immersed in liquid nitrogen for one day to verify the low temperature performance of carbon coatings. To examine whether the carbon coatings on the optical fibers were broken or delaminated, the outer surfaces of the carbon-coated optical fibers before and after immersion in liquid nitrogen for one day were observed using optical microscope (OM). Notably, the thickness and the outer sur-
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face of carbon films were measured using the samples with silica glass fiber substrates. Nevertheless, the other properties of carbon films were measured using the samples with silica glass plate substrates.
3. Results 3.1. Self-bias voltage and deposition rate When the radio-frequency power is fixed at 250 W, the self-bias voltage (Vb) and deposition rate of carbon coatings as a function of Ar/CH4 ratio are shown in Fig. 1. Fig. 1 indicates that the self-bias voltage decreases from 283 to 143 V as the Ar/CH4 ratio increases from 0 to 8. On the other hand, the deposition rate also decreases with increasing the Ar/CH4 ratio. When the inert gas Ar is added into the methane plasma, the concentration of methane plasma and self-bias voltage are decreasing simultaneously. As the self-bias voltage decreases, the possibility of the hydrocarbon radicals with low ion energy in the plasma coming to rest on the film surface also decreases. Consequently, the deposition rate of carbon coatings decreases with increasing the Ar/CH4 ratio. 3.2. Microstructure analysis The Raman spectra of carbon coatings are mainly used to identify the carbon bonding types in the structure. For quantitative analysis, Raman spectra are decomposed into two Gaussian peaks which are denoted as D (disorder) band and G (graphite) band, respectively, located around 1330 and 1580 cm1. The D band is attributed to the breathing mode of the sp2 sites only in aromatic rings which makes Raman active by disorder effect [21–23]. The G band is resulting from stretching vibrations at all sp2 sites [24,25]. Fig. 2 shows the fitting results of Raman spectra for all Ar/CH4 ratios, where symbols x, FWHM, and I represent the Raman shift, full width half maximum, and integrated intensity, respectively; subscripts D and G indicate the D band and G band, respectively. The Raman signal sometimes is low due to the high film luminescence [26]. The determination of the Raman shift of G band
Fig. 1. The self-bias voltage (Vb) and deposition rate of carbon coatings at different Ar/CH4 ratios.
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Fig. 3. FTIR spectra of carbon coatings prepared with different Ar/CH4 ratios.
Fig. 2. The xD, FWHMD, and ID/IG of carbon coatings as a function of the Ar/CH4 ratio.
(xG) and full width half maximum of G band (FWHMG) contains the uncertainty, so xG and FWHMG are not shown here. Fig. 2 reveals that if the Ar/CH4 ratio is below 4, the Raman shift of D band (xD), full width half maximum of D band (FWHMD), and integrated intensity ratio of D band and G band (ID/IG) decrease with increasing the Ar/CH4 ratio. On the other hand, if the Ar/CH4 ratio is over 4, the xD, FWHMD, and ID/IG increase as the Ar/ CH4 ratio increases from 4 to 8. According to the analytic results of Raman spectra, if the Ar/CH4 ratio changes from 0 to 4, the number of ordered sp2 sites in the carbon coating structure increases with the decrease of xD, FWHMD, and ID/IG [11,25,27]. On the other hand, if the Ar/CH4 ratio increases from 4 to 8, the number of disordered sp2 sites in the carbon coating structure increases with the increase of xD, FWHMD, and ID/IG [11,25,27]. The FTIR spectra can be also used to identify the nature of bonding (C–Hx and C–C stretching modes) in the carbon coatings. Fig. 3 illustrates FTIR spectra of carbon coatings prepared at different Ar/CH4 ratios. In the C–H stretching bonds ranging from 2800 to 3100 cm1, two significant peaks at 2855 and 2920 cm1 are found in all of spectra, which represent the sp3-CH2 symmetric and sp3CH2 asymmetric bonds, respectively [2,28]. Fig. 3 shows that the peak intensity at 2855 and 2920 cm1 and the integrated peak area in the region of 2800 and 3100 cm1 decrease with increasing the Ar/CH4 ratio. In the other FTIR absorbance band ranging from 1300 to 2000 cm1, four significant peaks at 1415, 1450, 1580, and 1640 cm1 appeared. The two peaks at 1415 and 1450 cm1 represent
the olefinic sp2-CH2 and sp3-CH2 asymmetric stretching modes, respectively; the other two peaks at 1580 and 1640 cm1 indicate the aromatic and olefinic sp2 C@C stretching modes, respectively [2,29,30]. Notably, the peak at 1640 cm1 can also indicate the C@O stretching modes [31]. However, the base pressure in the reaction chamber was less than 7.5 · 106 Torr, so it is believed that the peak at 1640 cm1 is not attributed to the C@O stretching modes. When the Ar/CH4 ratio increases, the CHx modes, located at 1415 and 1450 cm1, decrease gradually. However, the sp2 C@C bonds, appearing at 1580 and 1640 cm1, are more obvious with increasing the Ar/CH4 ratio. The above results indicate that if the inert gas Ar is added into the methane plasma, the content of hydrocarbon bonds in the carbon coating decreases gradually with the increase of the sp2 carbon bonds which should present as olefinic and aromatic sp2 C@C bonding structure. 3.3. Mechanical property Fig. 4 shows the measured hardness and Young’s modulus of carbon coatings at different Ar/CH4 ratios. If the Ar/CH4 ratio is below 4, the hardness of carbon coatings rapidly decreases from 9.4 to 7.5 GPa and Young’s modulus of carbon coatings also strictly decreases from 82.1 to
Fig. 4. The hardness and Young’s modulus of carbon coatings as a function of the Ar/CH4 ratio.
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70.0 GPa. Alternatively, if the Ar/CH4 ratio is over 4, the hardness of carbon coatings slightly increases from 7.5 to 8.2 GPa and Young’s modulus of carbon coatings also slightly increases from 70.0 to 73.9 GPa. Notably, the measured Young’s modulus Er is usually not the exact Young’s modulus Es of coating materials. The measured Young’s modulus Er can be expressed as 1=Er ¼ ½ð1 m2s Þ=Es þ ½ð1 m2i Þ=Ei ;
ð1Þ
where Ei is Young’s modulus of the indenter, and ms and mi are Possion’s ratios of the sample and indenter, respectively. Because Ei is much larger than Es, the last term ð1 m2i Þ=Ei on the right hand side of Eq. (1) can be omitted. On the other hand, ms is commonly smaller than 0.3 for most of the coating materials [32]. Therefore, the measured Young’s modulus is close to Young’s modulus of the sample in this work.
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where hrough is the apparent contact angle of the rough surface, htrue is the contact angle of a flat surface with the identical surface chemistry, and r is the ratio of actual to project surface area. When the surface roughness increases, the ratio of actual to project surface area r should increase. Hence, Eq. (2) indicates that the water CA decreases with increasing the surface roughness as the water CA is smaller than 90. On the other hand, the water CA can be also dependent on the surface chemistry. From the FTIR analysis, it is known that the C–H bond in the carbon coating changes to the C@C bond as the Ar/CH4 ratio increases. The water CA on a C@C bonded surface is larger than that on a C–H bonded surface [34]. Hence, the water CA would be also increased with the Ar/CH4 ratio due to the surface chemistry. Consequently, it is believed that the change of the water CA for all Ar/CH4 is attributed to the surface roughness and surface chemistry of carbon coatings. 3.5. Surface morphology
3.4. Surface roughness and water-repellency Fig. 5 shows the relationship between the average surface roughness Ra of carbon coatings and Ar/CH4 ratio. Fig. 5 depicts that Ra decreases from 0.43 to 0.34 nm with increasing the Ar/CH4 ratio from 0 to 4. Notably, when the Ar/CH4 ratio is between 0 and 1, Ra of carbon coatings is almost the same and the value of Ra varies only within 0.02 nm. On the other hand, as the Ar/CH4 ratio increases from 4 to 8, Ra increases rapidly from 0.34 to 0.55 nm. Fig. 5 also indicates the relationship between the water CA of carbon coatings and Ar/CH4 ratio. Fig. 5 shows that if the Ar/CH4 ratio increases from 0 to 4, the water CA increases from 78.0 to 81.5. Alternatively, the water CA decreases from 81.5 to 79.1 with increasing the Ar/CH4 ratio from 4 to 8. It is worthy to mention that the water CA of carbon coatings for all Ar/CH4 ratios is larger than the water CA of silica glass plate, which is about 58. The relationship between the surface roughness and contact angle had been developed by Wenzel [33], and it is given as: cos hrough ¼ r cos htrue ;
Figs. 6 and 7 illustrate the examined results of OM before and after immersion in liquid nitrogen for one day, respectively. Before immersion in liquid nitrogen, Fig. 6a shows that obvious wrinkles and buckles in the carbon coating, prepared with the Ar/CH4 ratio being 0, are revealed. Fig. 6b indicates that few microcracks in the carbon coating surface are observed as the Ar/CH4 ratio is 1. Fig. 6c and d illustrate that a perfect surface of the carbon coating without any microcrack is found as the Ar/CH4
ð2Þ
Fig. 5. The average roughness (Ra) and water contact angle of carbon coatings at different Ar/CH4 ratios.
Fig. 6. Morphology of outer surfaces of the carbon-coated optical fibers before immersion in liquid nitrogen for one day examined by OM. The Ar/ CH4 ratios are (a) 0, (b) 1, (c) 2, (d) 4, (e) 6, and (f) 8.
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Fig. 7. Morphology of outer surfaces of the carbon-coated optical fibers after immersion in liquid nitrogen for one day examined by OM. The Ar/ CH4 ratios are (a) 0, (b) 1, (c) 2, (d) 4, (e) 6, and (f) 8.
ratio is between 2 and 4. If the Ar/CH4 ratio is 6, few microcracks in the carbon coating surface are observed again as shown in Fig. 6e. When the Ar/CH4 ratio is 8, the carbon coating is mostly delaminated from the surface of optical fiber as depicted in Fig. 6f. The above results indicate that the hermetical property of as-deposited carbon coatings on the glass fiber is poor as the Ar/CH4 ratio is less than 1 or larger than 6. After immersion in liquid nitrogen for one day, Fig. 7a–c reveal that the low temperature induced coating delamination on the outer surfaces of carbon-coated optical fibers becomes more conspicuous as the Ar/CH4 ratio changes from 0 to 2. If the Ar/CH4 ratio is 4, the carbon coating without any microcracks is observed as depicted in Fig. 7d. With increasing the Ar/ CH4 ratio from 6 to 8, the strictly damaged carbon coatings are revealed as illustrated in Fig. 7e and f. Based on the above results, it is found that if the Ar/CH4 ratio is 4, the carbon coating has the best property to sustain the thermal loading. The maximum thermal stresses in hermetically coated optical fibers had been derived in previous works [7,35]. Because the radius r1 of the carbon coating is close to the radius r0 of the glass fiber, the thermal stresses in the carbon coatings can be simplified as [7,35]
where rhmax is the maximum tangential stress in the carbon coating, and smax is the maximum shear stress at the interface between the glass fiber and carbon coating. Symbols E, a, and t indicate the Young’s modulus, thermal expansion coefficient, and Poisson’s ratio, respectively; subscripts 0 and 1 represent the glass fiber and carbon coating, respectively. DT(=298 K 77 K=221 K) is the temperature change. The parameters of the carbon-coated optical fiber are as following: r0 = 62.5 lm, r1 = 62.9 lm, E0 = 72.5 GPa, a0 = 0.56 · 106/C, a1 = 2.24 · 106/C, t0 = 0.17, and t1 = 0.3 [7,36]. Eqs. (3a) and (3b) show that the thermal stresses increase with increasing the Young’s modulus of carbon coatings, so the carbon coating deposited at the Ar/CH4 ratio of 4 possesses the lowest thermal stress. On the other hand, the tensile strength of the carbon coating is inversely related to the square root of surface roughness [10], so the carbon coating deposited using an Ar/CH4 ratio of 4 has the highest tensile strength. Furthermore, if the amorphous carbon coating is deposited on the amorphous substrate such as the silica fiber, the interface between the carbon coating and silica fiber can be found the number of SiC bonds [8]. It is predicted that the ordered sp2 sites in the carbon coating would promote the SiC bonding, so the carbon coating deposited using an Ar/CH4 ratio of 4 may has the highest interfacial shear strength. Consequently, the carbon coating deposited at the Ar/CH4 ratio of 4 owns the lowest thermal stress and highest mechanical strength, and is the best to sustain the thermal loading.
4. Discussion In this study, the carbon coatings are adopted to become hermetical coatings of optical fibers. In order to understand why the carbon-coated optical fibers have the best hermetical properties, the characteristics of carbon coatings prepared using different Ar/CH4 ratios are addressed in detail by the measurement of Raman, FTIR, hardness, Young’s modulus, roughness, and water CA. According to the above investigation, it is known that if the Ar/CH4 ratio increases from 0 to 4, the xD, FWHMD, ID/IG, hardness, Young’s modulus, and surface roughness decrease, while the water CA increases. On the other hand, if the Ar/CH4 ratio increases from 4 to 8, the xD, FWHMD, ID/IG, hardness, Young’s modulus, and surface roughness increase, while the water CA decreases. Consequently, the results of Raman spectra, hardness, Young’s modulus, Ra, and water CA have a turning point at the Ar/CH4 ratio equaling 4. Alternatively, based on the results of FTIR spectra, the number of hydrocarbon bonds in the carbon
rhmax ¼ E1 ½a1 ð1 þ t1 Þ a0 ð1 þ t0 ÞDT =ð1 t21 Þ; smax ¼ ð1 þ t1 Þða1
a0 ÞDT ½E1 ðr21
1=2 r20 Þ=r20 =f½3=ð2E0 Þ
ð3aÞ þ ½ð2 þ t1 Þ=E1 g
1=2
;
ð3bÞ
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coating reduces with increasing the Ar/CH4 ratio from 0 to 8, while the olefinic and aromatic sp2 C@C bonds increase. In order to know the coating structure in detail, the deposition mechanism at different Ar/CH4 ratios must be described first. The deposition process of carbon coatings at different Ar/CH4 ratios is actually controlled by the Penning effect and etching effect [18,37]. Both Penning effect and etching effect increase with increasing the Ar/CH4 ratio. When the Ar/CH4 ratio is between 0 and 4, the deposition process is mainly sustained by the Penning effect. The Penning effect enhances the dissociation of the hydrocarbon gas molecules; that is, the metastable argon species with large cross-section are easily to collide with CH4 gas molecules [37]. During collisions of metastable argon species with CH4 gas molecules, the methyl radicals (CH3) are quickly removed from the CH4 gas phase and the successive collision process finally results in the production of carbon–carbon dimer (C2) in the plasma [38,39]. This fact results in the decrease of hydrocarbon bonds in the carbon coating, and the sp2 sites should present as the olefinic and aromatic sp2 C@C bonding structure. The sp2 C@C bonding structure gradually changes into an order phase as the xD, FWHMD, and ID/IG decrease [11,25,27]. As a result, when the Ar/CH4 ratio increases from 0 to 4, the coating structure shifts to the graphite-like carbon with the decrease of hardness and Young’s modulus [18]. Meanwhile, the ordered sp2 C@C bonding structure is more obvious, so the surface of the carbon coating becomes smoother with the decrease of Ra and increase of water CA. The wrinkles, buckles, and microcracks on the outer surface of carbon-coated optical fibers before and after immersion in liquid nitrogen also reduce. On the other hand, when the Ar/CH4 ratio increases from 4 to 8, the etching effect becomes more important. Etching effect makes the weak bonding, such as hydrocarbon bonds, to be broken from the coating structure. This fact results in the rapid decrease of hydrocarbon bonds. The sp2 site, which presented as the olefinic and aromatic sp2 C@C bonding structure, is still conspicuous, but the sp2 C@C bonding structure shifts from order phase to disorder phase as the xD, FWHMD, and ID/IG increase [11,25,27]. The disordered sp2 C@C bonding structure increases, so the hardness and Young’s modulus of the carbon coating increase. Meanwhile, the surface of the carbon coating becomes rougher with the increase of Ra and decrease of water CA. Additionally, the damaged carbon coatings on the outer surface of optical fibers are gradually appeared as the Ar/CH4 ratio changes from 4 to 8. From the above discussion, it is found that if the Ar/CH4 ratio is 4, the carbon coating has the lowest Ra, the highest water CA, and the best ability to sustain the thermal loading. Hence, it is the best for the production of a hermetical optical fiber coating. In our unpublished work, the carbon-coated optical fiber was prepared by rf-PECVD with different hydrogen/ methane ratio. When the radio-frequency power, work
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pressure, and substrate temperature are set at 250 W, 6.5 · 101 Torr, and 20 C, respectively, it is shown that the carbon coating has the maximum water CA of 78 as the H2/CH4 ratio is 4. In this work, the maximum water CA of the carbon coating is 81.5, so the result of this work is better than that of our pervious work. Notably, it is difficult to measure the RSS, FTIR, NI, Ra and water CA of carbon coatings using the samples with silica glass fiber substrates, so these properties of carbon coatings are measured using the samples with silica glass plate substrates. The difference of the measured results between these two kinds of substrates is not clear. However, it is believed that the qualitative variation of these properties with the Ar/CH4 ratio for these two kinds of substrates would be the same. 5. Conclusions The influence of Ar/CH4 ratios on the characteristics of carbon coatings on optical fibers prepared by rf-PECVD is investigated. The deposition process of carbon coatings by changing the Ar/CH4 ratio is actually controlled by the Penning effect and etching effect. When the Ar/CH4 ratio increases from 0 to 4, the deposition process is mainly sustained by the Penning effect. The Penning effect enhances the dissociation of the hydrocarbon gas molecules. As a result, the hydrocarbon bonds in the carbon coating decrease, and the sp2 sites present as the olefinic and aromatic sp2 C@C bonding structure. Meanwhile, the coating structure shifts to the graphite-like carbon and the surface of the carbon coating becomes smoother. On the other hand, when the Ar/CH4 ratio increases from 4 to 8, the Penning effect in plasma is still sustained in the mixture of hydrocarbon gas, but the etching effect becomes more important. Etching effect makes the weak bonding, such as hydrocarbon bonds, to be broken from the coating structure, so the hydrocarbon bonds still decrease. The sp2 site, which presented as the olefinic and aromatic sp2 C@C bonding structure, is still conspicuous, but the disordered sp2 C@C bonding structure is gradually formed. Because the disordered sp2 C@C bonding structure is obvious, the surface of the carbon coating becomes rougher. Consequently, the results of Raman spectra, hardness, Young’s modulus, Ra and water CA have a turning point at the Ar/CH4 ratio equaling 4. When the Ar/CH4 is 4, the carbon coating has the lowest Ra, the highest water CA, and the best ability to sustain the thermal loading. Hence, it is the best for the production of a hermetical optical fiber coating. Acknowledgments This work was supported by the National Science Council, Taiwan, under the Grant Nos.: NSC 94-2215-E-005-002 and NSC 95-2221-E005-115.
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[21] Kawashima Y, Katagiri G. Fundamentals, overtones, and combinations in the Raman spectrum of graphite. Phys Rev B 1995;52(14): 10053–9. [22] Thomsen C, Reich S. Double resonant raman scattering in graphite. Phys Rev Lett 2000;85(24):5214–7. [23] Cancado LG, Pimenta MA, Neves BRA, Dantas MSS, Jorio A. Influence of the atomic structure on the raman spectra of graphite edges. Phys Rev Lett 2004;93(24):247401-1–4. [24] Abrasonis G, Gago R, Vinnichenko M, Kreissig U, Kolitsch A, Moller W. Sixfold ring clustering in sp2-dominated carbon and carbon nitride thin films: A Raman spectroscopy study. Phys Rev B 2006;73(12):125427-1–13. [25] Ferrari AC, Robertson J. Interpretation of Raman spectra of disordered and amorphous carbon. Phys Rev B 2000;61(20): 14095–107. [26] Ray SC, Fanchini G, Tagliaferro A, Bose B, Dasgupta D. Amorphous carbon films prepared by the ‘‘dip’’ technique: Deposition and film characterization. J Appl Phys 2003;94(2):870–8. [27] Lejeune M, Durand-Drouhin O, Henocque J, Bouzerar R, Zeinert A, Benlahsen M. Optical investigations and Raman scattering characterization of carbon bonding in hard amorphous hydrogenated carbon films. Thin Solid Films 2001;389(1–2):233–8. [28] Chakrabarti K, Basu M, Chaudhuri S, Pal AK. Mechanical, electrical and optical properties of a-C:H:N films deposited by plasma CVD technique. Vacuum 1999;53(3–4):405–13. [29] Mihailescu IN, Gyorgy E, Alexandrescu R, Luches A, Perrone A, Ghica C, et al. Optical studies of carbon nitride thin films deposited by reactive pulsed laser ablation of a graphite target in low pressure ammonia. Thin Solid Films 1998;323(1–2):72–8. [30] Szo¨re´nyi T, Fuchs C, Fogarassy E, Hommet J, Normand FLe. Chemical analysis of pulsed laser deposited a-CNx films by comparative infrared and X-ray photoelectron spectroscopies. Surf Coat Technol 2000;125(1–3):308–12. [31] Smidt E, Eckhardt KU, Lechner P, Schulten HR, Leinweber P. Characterization of different decomposition stages of biowaste using FT-IR spectroscopy and pyrolysis-field ionization mass spectrometry. Biodegradation 2005;16(1):67–79. [32] Jiang X, Reichelt K, Stritzker B. The hardness and Young’s modulus of amorphous hydrogenated carbon and silicon films measured with an ultralow load indenter. J Appl Phys 1989;66(12):5805–8. [33] Wenzel RN. Resistance of solid surfaces to wetting by water. Ind Eng Chem 1936;28(8):988–94. [34] Ostrovskaya L, Perevertailo V, Ralchenko V, Dementjev A, Loginova O. Wettability and surface energy of oxidized and hydrogen plasma-treated diamond films. Diam Relat Mater 2002;11(3–6): 845–50. [35] Shiue ST, Lin YS. Thermal stresses in metal-coated optical fibers. J Appl Phys 1998;83(11):5719–23. [36] Shiue ST, Lin HC, Shen TY, Ouyang H. Residual stress measurement in carbon coatings of optical fibers from the fiber bending curvature and coating thickness difference. Appl Phys Lett 2005;86(25):1–3 [251910]. [37] Grill A. Cold plasma in materials fabrication, vol. 56. New York: IEEE Press; 1993. [38] Han YS, Kim YK, Lee JY. Effect of argon and oxygen addition to the CH4-H2 feed gas on diamond synthesis by microwave plasma enhanced chemical vapor deposition. Thin Solid Films 1997; 310(1–2):39–46. [39] Riccardi C, Barni R, Sindoni E, Fontanesi M, Tosi P. Gaseous precursors of diamond-like composition of CH4/Ar plasmas. Vacuum 2001;61(2–4):211–5.
Characteristics of carbon coatings on optical fibers prepared by plasma enhanced chemical vapor deposition using different argon/methane ratios Hung-Chien Lin a, Sham-Tsong Shiue a,*, Yu-Hang Cheng a, Tsong-Jen Yang b, Tung-Chuan Wu c, Hung-Yi Lin c a
Department of Materials Science and Engineering, National Chung Hsing University, 250 Kuo Kuang Road, Taichung, 402 Taiwan, ROC b Department of Materials Science and Engineering, Feng Chia University, 100 Wen Hwa Road, Taichung, 407 Taiwan, ROC c Mechanical and Systems Research Laboratories, Industrial Technology Research Institute, Hsinchu, 310 Taiwan, ROC Received 23 January 2007; accepted 2 June 2007 Available online 13 June 2007
Abstract The effect of argon/methane ratios on the characteristics of carbon coatings on optical fibers prepared by plasma enhanced chemical vapor deposition is investigated. The argon/methane ratios are selected at 0, 1, 2, 4, 6, and 8. Results show that if the argon/methane ratio increases from 0 to 4, the deposition process is mainly sustained by the Penning effect. The Penning effect enhances the dissociation of the hydrocarbon gas molecules. The carbon coating shifts to the graphite-like carbon and the surface of the carbon coating becomes smoother. As the argon/methane ratio increases from 4 to 8, etching becomes more important. The sp2 C@C bonding in the carbon coating transforms from order into disorder. Because the disordered sp2 C@C bonding increases, the surface of the carbon coating becomes rougher. When the argon/methane ratio is 4, the surface roughness of the carbon coating is a minimum, while the water contact angle of carbon coating is a maximum. At this argon/methane ratio, the carbon coating has the best ability to minimize low temperature induced delamination or microcracks, so is the best for the production of a hermetically sealed optical fiber coating. 2007 Elsevier Ltd. All rights reserved.
1. Introduction Recently, various properties of carbon films have been found, such as wide bandgap, high hardness, inert chemical attack, infrared transparency, and high water resistance [1,2]. Many researches successfully applied carbon films to the commercial applications such as hard disk, frictional resistance films, protective optical materials, biomedical products, and so on. Meanwhile, the excellent properties of carbon films have been also applied to become hermetical coatings of optical fibers and the carbon-coated optical fiber is expected to be a key technology for optical transmission lines [3–10]. *
Corresponding author. Fax: +886 4 22857017. E-mail address: [email protected] (S.-T. Shiue).
0008-6223/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2007.06.003
Preparation of carbon films can be carried out at a low substrate temperature and high deposition rate using various methods, such as radio-frequency-plasma enhanced chemical vapor deposition (rf-PECVD) [7,10–12], plasma enhanced chemical vapor deposition with electron cyclotron wave resonance source [13], microwave plasma chemical vapor deposition [14], sputtering deposition [15], filtered cathodic vacuum arc deposition [16], and ion beam deposition [17]. The carbon film prepared by rf-PECVD using methane (CH4) as the precursor gases contains a percentage of hydrogen, so it is usually classified as the hydrogenated amorphous carbon (a-C:H) film in the literature [2]. Recently, a-C:H films for hermetical optical fiber coatings prepared by rf-PECVD using methane (CH4) and hydrogen (H2) as the precursor gases were studied [7,10]. When the carbon films were prepared by rf-PECVD
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method, the characteristics of carbon films are affected by many factors such as the mixture ratio of reactive gases, self-bias voltage, and so on [10,18–20]. However, no work was found to study the effect of argon/methane (Ar/CH4) ratios on the properties of carbon coatings on optical fibers prepared by rf-PECVD method. Hence, the purpose of this article is to investigate the influence of Ar/CH4 ratios on the characteristics of carbon coatings on optical fibers. Additionally, the optimal Ar/CH4 ratio to fabricate the carbon-coated optical fibers will be proposed.
2. Experimental The experimental details proceeded as follows. First, silica glass fibers and silica glass plates were cleaned in ultrasonic bath of ethanol, acetone, and de-ionized (DI) water in sequence. These pre-treatments were typically applied to improve the adhesion of carbon coatings onto these substrates. Second, the carbon coatings were simultaneously deposited on silica glass fibers and silica glass plates using a 13.56 MHz capacitively coupled rfPECVD system in which a cylindrical stainless steel reaction chamber with two parallel planar electrodes was employed. The distance between two parallel planar electrodes was 3 cm. Before deposition, the reaction chamber was pumped so that the base pressure was less than 7.5 · 106 Torr. The carbon coatings were deposited with working pressure of 6 · 101 Torr (about 80 Pa) and radio-frequency power of 250 W. The substrate temperature was controlled by water cooling system and the substrate temperature was set at 293 K (20 C). Six kinds of carbon coatings were prepared with the Ar/CH4 ratios being 0, 1, 2, 4, 6, and 8, where the flow rate of CH4 gas was fixed at 10 sccm. The self-bias voltage was recorded at different deposition conditions. The deposition time was controlled such that carbon coatings of identical thickness (about 400 nm) were produced for all the Ar/CH4 ratios. The thickness of carbon coatings was obtained by examining the cross-section of carbon-coated optical fiber using the field emission scanning electron microscope (FESEM, JEOL JSM-6700F). Each thickness was estimated from the average value of five data with the same deposition condition. Thirdly, the structure of carbon coatings was examined by the Raman scattering spectroscopy (RSS, JOBIN YVON Triax 550) and the Fourier transform infrared (FTIR) spectroscopy (Thermo-Nicolet NEXUS 470). The RSS was measured in the back-scattering geometry with the 633 nm line of a He–Ne laser at room temperature in the spectral range from 800 to 2000 cm1. The laser power was 25 mW and irradiation time of the laser on the coatings was 20 s/point. The FTIR can identify the nature of bonding (C–Hx and C– C stretching modes) by absorbance spectrum at 298 K. All spectra were detected in the range of 1250–3600 cm1 with 128 scans at resolution of 1 cm1. Fourthly, the mechanical property of carbon coatings was measured by the nanoindenter (NI, UMIS Nanoindentation, Based Model, CRISO, Australia). In each of indentation experiment, the indenter was loaded and unloaded three times at the maximum penetration depth less than 10% of the film thickness. Fifthly, the roughness of carbon coatings was examined by the atomic force microscopy (AFM, Digital Instrument NS4/D3100CL/MultiMode). The value of surface roughness, Ra was estimated by tapping mode method and was evaluated in the area of 5 · 5 lm. Each datum was obtained from the average value of three different positions on the same specimen surface. Meanwhile, the water-repellency of carbon coatings was measured by contact angle (CA) meter with sessiledrop technique. A DI water droplet with a volume of 2 ll was released onto the surface of the sample from a syringe needle. Each measurement was repeated 10 times and reproducibility was never worse than ±1. Finally, the carbon-coated optical fibers were immersed in liquid nitrogen for one day to verify the low temperature performance of carbon coatings. To examine whether the carbon coatings on the optical fibers were broken or delaminated, the outer surfaces of the carbon-coated optical fibers before and after immersion in liquid nitrogen for one day were observed using optical microscope (OM). Notably, the thickness and the outer sur-
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face of carbon films were measured using the samples with silica glass fiber substrates. Nevertheless, the other properties of carbon films were measured using the samples with silica glass plate substrates.
3. Results 3.1. Self-bias voltage and deposition rate When the radio-frequency power is fixed at 250 W, the self-bias voltage (Vb) and deposition rate of carbon coatings as a function of Ar/CH4 ratio are shown in Fig. 1. Fig. 1 indicates that the self-bias voltage decreases from 283 to 143 V as the Ar/CH4 ratio increases from 0 to 8. On the other hand, the deposition rate also decreases with increasing the Ar/CH4 ratio. When the inert gas Ar is added into the methane plasma, the concentration of methane plasma and self-bias voltage are decreasing simultaneously. As the self-bias voltage decreases, the possibility of the hydrocarbon radicals with low ion energy in the plasma coming to rest on the film surface also decreases. Consequently, the deposition rate of carbon coatings decreases with increasing the Ar/CH4 ratio. 3.2. Microstructure analysis The Raman spectra of carbon coatings are mainly used to identify the carbon bonding types in the structure. For quantitative analysis, Raman spectra are decomposed into two Gaussian peaks which are denoted as D (disorder) band and G (graphite) band, respectively, located around 1330 and 1580 cm1. The D band is attributed to the breathing mode of the sp2 sites only in aromatic rings which makes Raman active by disorder effect [21–23]. The G band is resulting from stretching vibrations at all sp2 sites [24,25]. Fig. 2 shows the fitting results of Raman spectra for all Ar/CH4 ratios, where symbols x, FWHM, and I represent the Raman shift, full width half maximum, and integrated intensity, respectively; subscripts D and G indicate the D band and G band, respectively. The Raman signal sometimes is low due to the high film luminescence [26]. The determination of the Raman shift of G band
Fig. 1. The self-bias voltage (Vb) and deposition rate of carbon coatings at different Ar/CH4 ratios.
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Fig. 3. FTIR spectra of carbon coatings prepared with different Ar/CH4 ratios.
Fig. 2. The xD, FWHMD, and ID/IG of carbon coatings as a function of the Ar/CH4 ratio.
(xG) and full width half maximum of G band (FWHMG) contains the uncertainty, so xG and FWHMG are not shown here. Fig. 2 reveals that if the Ar/CH4 ratio is below 4, the Raman shift of D band (xD), full width half maximum of D band (FWHMD), and integrated intensity ratio of D band and G band (ID/IG) decrease with increasing the Ar/CH4 ratio. On the other hand, if the Ar/CH4 ratio is over 4, the xD, FWHMD, and ID/IG increase as the Ar/ CH4 ratio increases from 4 to 8. According to the analytic results of Raman spectra, if the Ar/CH4 ratio changes from 0 to 4, the number of ordered sp2 sites in the carbon coating structure increases with the decrease of xD, FWHMD, and ID/IG [11,25,27]. On the other hand, if the Ar/CH4 ratio increases from 4 to 8, the number of disordered sp2 sites in the carbon coating structure increases with the increase of xD, FWHMD, and ID/IG [11,25,27]. The FTIR spectra can be also used to identify the nature of bonding (C–Hx and C–C stretching modes) in the carbon coatings. Fig. 3 illustrates FTIR spectra of carbon coatings prepared at different Ar/CH4 ratios. In the C–H stretching bonds ranging from 2800 to 3100 cm1, two significant peaks at 2855 and 2920 cm1 are found in all of spectra, which represent the sp3-CH2 symmetric and sp3CH2 asymmetric bonds, respectively [2,28]. Fig. 3 shows that the peak intensity at 2855 and 2920 cm1 and the integrated peak area in the region of 2800 and 3100 cm1 decrease with increasing the Ar/CH4 ratio. In the other FTIR absorbance band ranging from 1300 to 2000 cm1, four significant peaks at 1415, 1450, 1580, and 1640 cm1 appeared. The two peaks at 1415 and 1450 cm1 represent
the olefinic sp2-CH2 and sp3-CH2 asymmetric stretching modes, respectively; the other two peaks at 1580 and 1640 cm1 indicate the aromatic and olefinic sp2 C@C stretching modes, respectively [2,29,30]. Notably, the peak at 1640 cm1 can also indicate the C@O stretching modes [31]. However, the base pressure in the reaction chamber was less than 7.5 · 106 Torr, so it is believed that the peak at 1640 cm1 is not attributed to the C@O stretching modes. When the Ar/CH4 ratio increases, the CHx modes, located at 1415 and 1450 cm1, decrease gradually. However, the sp2 C@C bonds, appearing at 1580 and 1640 cm1, are more obvious with increasing the Ar/CH4 ratio. The above results indicate that if the inert gas Ar is added into the methane plasma, the content of hydrocarbon bonds in the carbon coating decreases gradually with the increase of the sp2 carbon bonds which should present as olefinic and aromatic sp2 C@C bonding structure. 3.3. Mechanical property Fig. 4 shows the measured hardness and Young’s modulus of carbon coatings at different Ar/CH4 ratios. If the Ar/CH4 ratio is below 4, the hardness of carbon coatings rapidly decreases from 9.4 to 7.5 GPa and Young’s modulus of carbon coatings also strictly decreases from 82.1 to
Fig. 4. The hardness and Young’s modulus of carbon coatings as a function of the Ar/CH4 ratio.
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70.0 GPa. Alternatively, if the Ar/CH4 ratio is over 4, the hardness of carbon coatings slightly increases from 7.5 to 8.2 GPa and Young’s modulus of carbon coatings also slightly increases from 70.0 to 73.9 GPa. Notably, the measured Young’s modulus Er is usually not the exact Young’s modulus Es of coating materials. The measured Young’s modulus Er can be expressed as 1=Er ¼ ½ð1 m2s Þ=Es þ ½ð1 m2i Þ=Ei ;
ð1Þ
where Ei is Young’s modulus of the indenter, and ms and mi are Possion’s ratios of the sample and indenter, respectively. Because Ei is much larger than Es, the last term ð1 m2i Þ=Ei on the right hand side of Eq. (1) can be omitted. On the other hand, ms is commonly smaller than 0.3 for most of the coating materials [32]. Therefore, the measured Young’s modulus is close to Young’s modulus of the sample in this work.
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where hrough is the apparent contact angle of the rough surface, htrue is the contact angle of a flat surface with the identical surface chemistry, and r is the ratio of actual to project surface area. When the surface roughness increases, the ratio of actual to project surface area r should increase. Hence, Eq. (2) indicates that the water CA decreases with increasing the surface roughness as the water CA is smaller than 90. On the other hand, the water CA can be also dependent on the surface chemistry. From the FTIR analysis, it is known that the C–H bond in the carbon coating changes to the C@C bond as the Ar/CH4 ratio increases. The water CA on a C@C bonded surface is larger than that on a C–H bonded surface [34]. Hence, the water CA would be also increased with the Ar/CH4 ratio due to the surface chemistry. Consequently, it is believed that the change of the water CA for all Ar/CH4 is attributed to the surface roughness and surface chemistry of carbon coatings. 3.5. Surface morphology
3.4. Surface roughness and water-repellency Fig. 5 shows the relationship between the average surface roughness Ra of carbon coatings and Ar/CH4 ratio. Fig. 5 depicts that Ra decreases from 0.43 to 0.34 nm with increasing the Ar/CH4 ratio from 0 to 4. Notably, when the Ar/CH4 ratio is between 0 and 1, Ra of carbon coatings is almost the same and the value of Ra varies only within 0.02 nm. On the other hand, as the Ar/CH4 ratio increases from 4 to 8, Ra increases rapidly from 0.34 to 0.55 nm. Fig. 5 also indicates the relationship between the water CA of carbon coatings and Ar/CH4 ratio. Fig. 5 shows that if the Ar/CH4 ratio increases from 0 to 4, the water CA increases from 78.0 to 81.5. Alternatively, the water CA decreases from 81.5 to 79.1 with increasing the Ar/CH4 ratio from 4 to 8. It is worthy to mention that the water CA of carbon coatings for all Ar/CH4 ratios is larger than the water CA of silica glass plate, which is about 58. The relationship between the surface roughness and contact angle had been developed by Wenzel [33], and it is given as: cos hrough ¼ r cos htrue ;
Figs. 6 and 7 illustrate the examined results of OM before and after immersion in liquid nitrogen for one day, respectively. Before immersion in liquid nitrogen, Fig. 6a shows that obvious wrinkles and buckles in the carbon coating, prepared with the Ar/CH4 ratio being 0, are revealed. Fig. 6b indicates that few microcracks in the carbon coating surface are observed as the Ar/CH4 ratio is 1. Fig. 6c and d illustrate that a perfect surface of the carbon coating without any microcrack is found as the Ar/CH4
ð2Þ
Fig. 5. The average roughness (Ra) and water contact angle of carbon coatings at different Ar/CH4 ratios.
Fig. 6. Morphology of outer surfaces of the carbon-coated optical fibers before immersion in liquid nitrogen for one day examined by OM. The Ar/ CH4 ratios are (a) 0, (b) 1, (c) 2, (d) 4, (e) 6, and (f) 8.
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Fig. 7. Morphology of outer surfaces of the carbon-coated optical fibers after immersion in liquid nitrogen for one day examined by OM. The Ar/ CH4 ratios are (a) 0, (b) 1, (c) 2, (d) 4, (e) 6, and (f) 8.
ratio is between 2 and 4. If the Ar/CH4 ratio is 6, few microcracks in the carbon coating surface are observed again as shown in Fig. 6e. When the Ar/CH4 ratio is 8, the carbon coating is mostly delaminated from the surface of optical fiber as depicted in Fig. 6f. The above results indicate that the hermetical property of as-deposited carbon coatings on the glass fiber is poor as the Ar/CH4 ratio is less than 1 or larger than 6. After immersion in liquid nitrogen for one day, Fig. 7a–c reveal that the low temperature induced coating delamination on the outer surfaces of carbon-coated optical fibers becomes more conspicuous as the Ar/CH4 ratio changes from 0 to 2. If the Ar/CH4 ratio is 4, the carbon coating without any microcracks is observed as depicted in Fig. 7d. With increasing the Ar/ CH4 ratio from 6 to 8, the strictly damaged carbon coatings are revealed as illustrated in Fig. 7e and f. Based on the above results, it is found that if the Ar/CH4 ratio is 4, the carbon coating has the best property to sustain the thermal loading. The maximum thermal stresses in hermetically coated optical fibers had been derived in previous works [7,35]. Because the radius r1 of the carbon coating is close to the radius r0 of the glass fiber, the thermal stresses in the carbon coatings can be simplified as [7,35]
where rhmax is the maximum tangential stress in the carbon coating, and smax is the maximum shear stress at the interface between the glass fiber and carbon coating. Symbols E, a, and t indicate the Young’s modulus, thermal expansion coefficient, and Poisson’s ratio, respectively; subscripts 0 and 1 represent the glass fiber and carbon coating, respectively. DT(=298 K 77 K=221 K) is the temperature change. The parameters of the carbon-coated optical fiber are as following: r0 = 62.5 lm, r1 = 62.9 lm, E0 = 72.5 GPa, a0 = 0.56 · 106/C, a1 = 2.24 · 106/C, t0 = 0.17, and t1 = 0.3 [7,36]. Eqs. (3a) and (3b) show that the thermal stresses increase with increasing the Young’s modulus of carbon coatings, so the carbon coating deposited at the Ar/CH4 ratio of 4 possesses the lowest thermal stress. On the other hand, the tensile strength of the carbon coating is inversely related to the square root of surface roughness [10], so the carbon coating deposited using an Ar/CH4 ratio of 4 has the highest tensile strength. Furthermore, if the amorphous carbon coating is deposited on the amorphous substrate such as the silica fiber, the interface between the carbon coating and silica fiber can be found the number of SiC bonds [8]. It is predicted that the ordered sp2 sites in the carbon coating would promote the SiC bonding, so the carbon coating deposited using an Ar/CH4 ratio of 4 may has the highest interfacial shear strength. Consequently, the carbon coating deposited at the Ar/CH4 ratio of 4 owns the lowest thermal stress and highest mechanical strength, and is the best to sustain the thermal loading.
4. Discussion In this study, the carbon coatings are adopted to become hermetical coatings of optical fibers. In order to understand why the carbon-coated optical fibers have the best hermetical properties, the characteristics of carbon coatings prepared using different Ar/CH4 ratios are addressed in detail by the measurement of Raman, FTIR, hardness, Young’s modulus, roughness, and water CA. According to the above investigation, it is known that if the Ar/CH4 ratio increases from 0 to 4, the xD, FWHMD, ID/IG, hardness, Young’s modulus, and surface roughness decrease, while the water CA increases. On the other hand, if the Ar/CH4 ratio increases from 4 to 8, the xD, FWHMD, ID/IG, hardness, Young’s modulus, and surface roughness increase, while the water CA decreases. Consequently, the results of Raman spectra, hardness, Young’s modulus, Ra, and water CA have a turning point at the Ar/CH4 ratio equaling 4. Alternatively, based on the results of FTIR spectra, the number of hydrocarbon bonds in the carbon
rhmax ¼ E1 ½a1 ð1 þ t1 Þ a0 ð1 þ t0 ÞDT =ð1 t21 Þ; smax ¼ ð1 þ t1 Þða1
a0 ÞDT ½E1 ðr21
1=2 r20 Þ=r20 =f½3=ð2E0 Þ
ð3aÞ þ ½ð2 þ t1 Þ=E1 g
1=2
;
ð3bÞ
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coating reduces with increasing the Ar/CH4 ratio from 0 to 8, while the olefinic and aromatic sp2 C@C bonds increase. In order to know the coating structure in detail, the deposition mechanism at different Ar/CH4 ratios must be described first. The deposition process of carbon coatings at different Ar/CH4 ratios is actually controlled by the Penning effect and etching effect [18,37]. Both Penning effect and etching effect increase with increasing the Ar/CH4 ratio. When the Ar/CH4 ratio is between 0 and 4, the deposition process is mainly sustained by the Penning effect. The Penning effect enhances the dissociation of the hydrocarbon gas molecules; that is, the metastable argon species with large cross-section are easily to collide with CH4 gas molecules [37]. During collisions of metastable argon species with CH4 gas molecules, the methyl radicals (CH3) are quickly removed from the CH4 gas phase and the successive collision process finally results in the production of carbon–carbon dimer (C2) in the plasma [38,39]. This fact results in the decrease of hydrocarbon bonds in the carbon coating, and the sp2 sites should present as the olefinic and aromatic sp2 C@C bonding structure. The sp2 C@C bonding structure gradually changes into an order phase as the xD, FWHMD, and ID/IG decrease [11,25,27]. As a result, when the Ar/CH4 ratio increases from 0 to 4, the coating structure shifts to the graphite-like carbon with the decrease of hardness and Young’s modulus [18]. Meanwhile, the ordered sp2 C@C bonding structure is more obvious, so the surface of the carbon coating becomes smoother with the decrease of Ra and increase of water CA. The wrinkles, buckles, and microcracks on the outer surface of carbon-coated optical fibers before and after immersion in liquid nitrogen also reduce. On the other hand, when the Ar/CH4 ratio increases from 4 to 8, the etching effect becomes more important. Etching effect makes the weak bonding, such as hydrocarbon bonds, to be broken from the coating structure. This fact results in the rapid decrease of hydrocarbon bonds. The sp2 site, which presented as the olefinic and aromatic sp2 C@C bonding structure, is still conspicuous, but the sp2 C@C bonding structure shifts from order phase to disorder phase as the xD, FWHMD, and ID/IG increase [11,25,27]. The disordered sp2 C@C bonding structure increases, so the hardness and Young’s modulus of the carbon coating increase. Meanwhile, the surface of the carbon coating becomes rougher with the increase of Ra and decrease of water CA. Additionally, the damaged carbon coatings on the outer surface of optical fibers are gradually appeared as the Ar/CH4 ratio changes from 4 to 8. From the above discussion, it is found that if the Ar/CH4 ratio is 4, the carbon coating has the lowest Ra, the highest water CA, and the best ability to sustain the thermal loading. Hence, it is the best for the production of a hermetical optical fiber coating. In our unpublished work, the carbon-coated optical fiber was prepared by rf-PECVD with different hydrogen/ methane ratio. When the radio-frequency power, work
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pressure, and substrate temperature are set at 250 W, 6.5 · 101 Torr, and 20 C, respectively, it is shown that the carbon coating has the maximum water CA of 78 as the H2/CH4 ratio is 4. In this work, the maximum water CA of the carbon coating is 81.5, so the result of this work is better than that of our pervious work. Notably, it is difficult to measure the RSS, FTIR, NI, Ra and water CA of carbon coatings using the samples with silica glass fiber substrates, so these properties of carbon coatings are measured using the samples with silica glass plate substrates. The difference of the measured results between these two kinds of substrates is not clear. However, it is believed that the qualitative variation of these properties with the Ar/CH4 ratio for these two kinds of substrates would be the same. 5. Conclusions The influence of Ar/CH4 ratios on the characteristics of carbon coatings on optical fibers prepared by rf-PECVD is investigated. The deposition process of carbon coatings by changing the Ar/CH4 ratio is actually controlled by the Penning effect and etching effect. When the Ar/CH4 ratio increases from 0 to 4, the deposition process is mainly sustained by the Penning effect. The Penning effect enhances the dissociation of the hydrocarbon gas molecules. As a result, the hydrocarbon bonds in the carbon coating decrease, and the sp2 sites present as the olefinic and aromatic sp2 C@C bonding structure. Meanwhile, the coating structure shifts to the graphite-like carbon and the surface of the carbon coating becomes smoother. On the other hand, when the Ar/CH4 ratio increases from 4 to 8, the Penning effect in plasma is still sustained in the mixture of hydrocarbon gas, but the etching effect becomes more important. Etching effect makes the weak bonding, such as hydrocarbon bonds, to be broken from the coating structure, so the hydrocarbon bonds still decrease. The sp2 site, which presented as the olefinic and aromatic sp2 C@C bonding structure, is still conspicuous, but the disordered sp2 C@C bonding structure is gradually formed. Because the disordered sp2 C@C bonding structure is obvious, the surface of the carbon coating becomes rougher. Consequently, the results of Raman spectra, hardness, Young’s modulus, Ra and water CA have a turning point at the Ar/CH4 ratio equaling 4. When the Ar/CH4 is 4, the carbon coating has the lowest Ra, the highest water CA, and the best ability to sustain the thermal loading. Hence, it is the best for the production of a hermetical optical fiber coating. Acknowledgments This work was supported by the National Science Council, Taiwan, under the Grant Nos.: NSC 94-2215-E-005-002 and NSC 95-2221-E005-115.
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