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Effect of H2 and O2 plasma etching treatment on the surface of diamond-like carbon thin film
Applied Surface Science 254 (2008) 7925–7928
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Effect of H2 and O2 plasma etching treatment on the surface of diamond-like carbon thin film Deok Yong Yun a, Won Seok Choi b, Yong Seob Park a, Byungyou Hong a,c,* a
School of Information and Communication Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea Department of Electrical Engineering, Hanbat National University, Daejeon 305-719, Republic of Korea c Center for Advanced Plasma Surface Technology, Sungkyunkwan University, Suwon 440-746, Republic of Korea b
A R T I C L E I N F O
A B S T R A C T
Article history:
In this study, we investigated the surface properties of diamond-like carbon (DLC) films for biomedical applications through plasma etching treatment using oxygen (O2) and hydrogen (H2) gas. The synthesis and post-plasma etching treatment of DLC films were carried out by 13.56 MHz RF plasma enhanced chemical vapor deposition (PECVD) system. In order to characterize the surface of DLC films, they were etched to a thickness of approximately 100 nm and were compared with an as-deposited DLC film. We obtained the optimum condition through power variation, at which the etching rate by H2 and O2 was 30 and 80 nm/min, respectively. The structural and chemical properties of these thin films after the plasma etching treatment were evaluated by Raman and Fourier transform infrared (FT-IR) spectroscopy. In the case of as-deposited and H2 plasma etching-treated DLC film, the contact angle was 86.48 and 83.78, respectively, whereas it was reduced to 35.58 in the etching-treated DLC film in O2 plasma. The surface roughness of plasma etching-treated DLC with H2 or O2 was maintained smooth at 0.1 nm. These results indicated that the surface of the etching-treated DLC film in O2 plasma was hydrophilic as well as smooth. ß 2008 Elsevier B.V. All rights reserved.
Available online 4 April 2008 PACS: 78.55.Qr 78.30. j 71.55.Jv 77.84.Bw Keywords: Diamond-like carbon Plasma etching Surface treatment Hydrophilic
1. Introduction Diamond-like carbon (DLC) films have been intensively studied for over two decades, owing to their excellent characteristics such as extremely high hardness, low friction coefficients, chemical inertness, wear resistance, optical transparency, and biocompatibility [1,2]. The studies on this material have recently been conducted at a hectic pace since its unique characteristics make it a promising candidate as a hard mask [3,4] and, in particular, in biomedical applications [5–7]. Recent studies have reported that surface-modified DLC thin films improved biocompatibility, lubricity, stability and cell adhesion [8–11]. These characteristics are related to structural bonds [12,13], surface roughness [6,14] and whether the film is hydrophobic or hydrophilic [15,16]. The surface modification of DLC films have been performed by doping with suitable elements [9–11,17] and plasma treatment [18,19]. Because of chemically active species in plasma and the ease of processing, the plasma treatment is the most widely used method for surface modification. In this study, DLC thin films were synthesized by RF plasma enhanced chemical vapor deposition (PECVD) method with
* Corresponding author. Tel.: +82 31 290 7208; fax: +82 31 290 5669. E-mail address: [email protected] (B. Hong). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.03.170
methane (CH4) and hydrogen (H2) gas, and the films were etched to approximately 100 nm by the plasma etching treatment. The surface treatment of DLC films by plasma has been performed using H2 and O2 gas, respectively, at various plasma powers. In order to characterize the surface properties of DLC films after the etching treatment, as-deposited DLC was compared with etchingtreated DLC films in the H2 and O2 plasma. The molecular structures, morphology and surface profiles of the films were investigated by Fourier transform infrared (FT-IR), Raman spectroscopy, the contact angle measurement, and atomic force microscopy (AFM).
2. Experiment The synthesis of DLC films and plasma etching treatment were carried out by using 13.56 MHz RF-PECVD. Prior to the DLC film deposition, Si substrate was pretreated in H2 plasma at 150 W for 5 min in order to remove the contaminants on the surface and to improve the strength of the film adhesion to the substrate. The DLC films were deposited on p-type (1 0 0) silicon with a size of 2 cm 2 cm at a deposition rate of approximately 15 nm/min with RF power of 150 W. A mixture of CH4 (20 sccm) and H2 (80 sccm) gas was used to produce the hydrogenated DLC films. For the plasma etching of deposited films, the films were treated in O2 and
D.Y. Yun et al. / Applied Surface Science 254 (2008) 7925–7928
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Fig. 1. Etching rate of O2 (flow = 10 sccm) and H2 (flow = 80 sccm) as a function of RF power.
H2 plasma. Both the synthesis and the etching of DLC films were performed at a working pressure of 1 Torr, and the substrate temperature was maintained at room temperature. The plasma etching was performed by varying RF power from 50 to 250 W. All samples were etched to a thickness of approximately 100 nm and the thickness was measured using Alpha-step. The samples for the characterization measurement were etched based on an etching rate for the 150 W plasma power, and the etching rate was 30 nm/ min in H2 plasma and 80 nm/min in O2 plasma. The structural property of the samples before and after the plasma etching treatment was analyzed by Raman spectroscopy (Renishaw, 2000) excited with He–Ne laser at a wavelength of 633 nm. Their bonding structure was characterized by FT-IR (BRUKER IFS 66v/S) in transmission mode and all the spectra were detected in the range of 400–4000 cm 1. The surface morphology and roughness value of DLC films were characterized by AFM (SII Nanotechnology, SPA400). The hydrophilicity of the surface was evaluated by measuring the wetting contact angles by the sessile drop method at three different spots in each sample. 3. Results and discussions Fig. 1 shows the behavior of each etching rate in O2 and H2 plasma as a function of RF power. In the case of O2, the etching rate increases rapidly with increasing RF power, but the etching rate of
Fig. 2. Raman spectra of the (a) as-deposited DLC film (150 nm) and etching-treated DLC films (100 nm) in (b) H2 and (c) O2 plasma.
H2 increases rather slowly. According to Komatsu et al. [20], the etching rate by O2 plasma on the film surface was much more affected by the change in gas pressure and was five times higher than H2 plasma. These are considered that ion-bombardment energy of oxygen is bigger and generates more quantity of bonds such as CO2, CO, H2O, etc. than hydrogen [21]. The O2 plasma produces energetic oxygen species, which can be reacted easily to the surface of DLC films. This can improve the etching rate by the increase in carbon–oxygen bonds on the surface. In contrast, hydrogen atoms are decomposed into the small amount of water vapor and free hydrogen. The Raman spectra for the DLC films were deconvoluted with two Gaussian curve lines shown in Fig. 2. The peak at around 1365 cm 1 indicates disordered graphite (D-peak), and the peak at around 1540 cm 1 is attributed to crystalline graphite (G-peak) [15]. Based on the fitting parameters, the peak position and the ratio of integrated areas under the D and G peaks (ID/IG) are summarized in Table 1. The results show that the plasma etching treatment of DLC films leads to the shift of the two peaks towards lower wave numbers and a decrease in the ID/IG ratio. In the result, we can deduce that the sp3/sp2 ratio increased after the etching in H2 and O2 plasma. This means that H ions generated by H2 plasma leads to the increase in sp3 fraction consisted of sp3 C–H group on the surface and the
Table 1 Gaussian fitting results of Raman spectra Sample
D-peak (cm Position (cm
As-deposited DLC (150 nm) H2 plasma-treated DLC (100 nm) O2 plasma-treated DLC (100 nm)
1368.2 1363.2 1363.4
1
)
1
G-peak (cm )
Intensity (a.u.)
Position (cm
18,220 13,060 9822
1543.1 1535.9 1534.3
1
)
ID/IG
1
)
Intensity (a.u.) 19,671 15,479 11,849
Fig. 3. Photos of the wetting contact angle of the (a) as-deposited DLC film (100 nm) and DLC films (100 nm) after (b) H2 and (c) O2 plasma treatments.
1.76 1.53 1.49
D.Y. Yun et al. / Applied Surface Science 254 (2008) 7925–7928
7927
Fig. 4. AFM images and RMS roughness of the (a) as-deposited DLC film (100 nm) and etching-treated DLC films (100 nm) in (b) H2 plasma and (c) O2 plasma.
Table 2 Contact angle of the water drop on the prepared samples Sample
Contact angle, u (8)
As-deposited DLC (100 nm) H2 plasma-treated DLC (100 nm) O2 plasma-treated DLC (100 nm)
86.4 1.6 83.7 1.6 35.5 0.1
inside film; O ions by O2 plasma is related to the increase in dangling bonds (sp3 phase) desorbed at the carbon network of the surface. Consequently, H and O reactions on the surface by the etching attributes to the increase in sp3/sp2 ratio on DLC films. The photos of the wetting contact angle on the samples are shown in Fig. 3. The measurement of the contact angle was carried out by an image analysis system, which calculated both the left and the right contact angles from the shape of the droplet, and the results are summarized in Table 2. The contact angle of distilled water on the as-deposited DLC film is 86.48, and the contact angles on the etching-treated DLC films in H2 and O2 plasma are 83.78 and 35.58, respectively. The hydrophobic property of DLC films was nearly maintained in H2 plasma; however, it was possible to obtain the hydrophilic property for DLC films by the O2 plasma etching treatment. This means that the high concentration of carbon– oxygen sites on the surface of DLC films by O2 plasma etching contributed to the adsorption hysteresis related the high surface energy [22]. Fig. 4 shows the AFM images of the as-deposited DLC film and two plasma-treated DLC films. The surface roughness was measured over an area of 2 mm 2 mm. We initially anticipated that the surface of DLC films treated by O2 plasma will be rougher due to the larger ion-bombardment energy of C than H [23]. However, the surfaces of three different types of DLC film were still considerably smooth as the RMS roughness of approximately 0.1 nm after etching treatment. This indicates that the roughness of the DLC films remained almost unaffected under H2 or O2 plasma etching. In other words, we confirmed that the low surface roughness of all DLC films can be controlled by the bonding fraction, not bonded atoms on the surface.
FTIR studies were carried out to characterize the bonding structure of the DLC films. Fig. 5 shows the FT-IR spectra for the asdeposited DLC films with two different thickness and the etchingtreated DLC films in H2 and O2 plasma. The IR absorption peak at 2350 cm 1, which is weakly observed in two plasma etchingtreated DLC films, is associated with the CO2 asymmetrical stretching mode and is considered due to O2 injected as impurity during deposition [24]. The broad band at around 3250 cm 1 can be attributed to the O–H stretching vibrations [25], and the O–H broad band is only observed for the samples etched in O2 plasma. It is believed that this is affected mainly as the oxygen atoms are bonded to hydrogen atoms existing on the surface during O2 plasma treatment. And the O–H bonds (hydroxyl group) revealed by O2 plasma treatment made the surface hydrophilic [19]. A broad band is observed at around 2900 cm 1 due to the C–H stretching vibration mode. According to literature [26], the band at 2860 cm 1 is assigned to the C–H2 symmetric vibration mode, whereas the band at 2925 cm 1 is attributed to the C–H2
Fig. 5. FT-IR spectra of the (a) as-deposited DLC film (150 nm), (b) (100 nm) and etching-treated DLC films (100 nm) in (c) H2 and (d) O2 plasma.
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asymmetric vibration mode or to the C–H stretching mode. C–H (b) and C–H (s) peak intensity of the DLC films were decreased by plasma etching treatment, and the DLC films (samples b, c and d) with thickness of 100 nm show nearly same intensity for C–H (b) and C–H (s). While the C C peak was presented at around 1600 cm 1 for the as-deposited DLC films [25], the plasma etching treatment of DLC film leads to the shift of the peak towards lower wave number. This is because of an increase of the weak carbon double bonds related to the reduction of sp2 bonding fraction as the results of Raman measurement [27]. These results indicate that the etching of DLC film by O2 plasma leads to the reduction of hydrogen contents related to C–H groups, and also contributes the reduction in the weak double bonds generated by high energetic plasma ions in carbon networks [24]. The etching by H2 plasma causes the increase in H incorporation by the hydrogen diffusion and the reduction of dangling bonds. 4. Conclusion Synthesis and plasma etching treatment of DLC films were carried out by using 13.56 MHz RF-PECVD. The properties of the DLC films were investigated through Raman, FT-IR, AFM and contact angle measurements. The sp3/sp2 ratio increased after H2 and O2 plasma etching treatment because of the increase in the generation of the unstable C bonds and the hydrogen incorporation caused by the energetic ions. Especially, broad O–H group on the DLC film was formed through O2 plasma etching treatment. This means that the surface becomes more hydrophilic, and it is consistent with the result of contact angle, whereas the plasma-treated DLC film in H2 plasma was hardly changed in the hydrophobic property. The surface roughness was kept ultra smooth as RMS roughness of about 0.12 nm after H2 and O2 plasma etching treatment. Consequently, the etching technique of DLC film can be applied for biomedicine as well as we can control the surface properties of DLC films by using the plasma treatment. Further studies are necessary to determine the effect of biomaterials such as protein and deoxyribonucleic acid (DNA) on DLC film. Acknowledgments This work is outcome of the fostering project of the Specialized Graduate School supported financially by the Ministry of
Commerce, Industry and Energy (MOCIE). And the authors are grateful for the financial support provided by Grant No. R11-2000086-0000-0 from the Center of Excellency Program of the Korea Science and Engineering Foundation and most through the Center for Advanced Plasma Surface Technology (CAPST) at Sungkyunkwan University.
References [1] S.M. Mominuzzaman, K.M. Krishna, T. Soga, T. Jimbo, M. Umeno, Jpn. J. Appl. Phys. 38 (1999) 658. [2] P. Yang, N. Huang, Y.X. Leng, J.Y. Chen, R.K.Y. Fu, S.C.H. Kwok, Y. Leng, P.K. Chu, Biomaterials 24 (2003) 2821. [3] K.A. Pears, Microelectron. Eng. 77 (2005) 255. [4] K.A. Pears, J. Stolze, Microelectron. Eng. 81 (2005) 7. [5] V.-M. Tiainen, Diamond Relat. Mater. 12 (2003) 583. [6] W.J. Ma, A.J. Ruys, R.S. Mason, P.J. Martin, A. Bendavid, Z. Liu, M. Ionescu, H. Zreiqat, Biomaterials 28 (2007) 1620. [7] R.K. Roy, K.R. Lee, Journal of Biomedical Materials Research Part B: Applied Biomaterials 83B (2007) 72. [8] E.T. Uzumaki, C.S. Lambert, A.R. Santos Jr., C.A.C. Zavaglia, Thin Solid Films 515 (2006) 293. [9] S. Zhang, H. Du, S.-E. Ong, K.-N. Aung, H.-C. Too, X. Miao, Thin Solid Films 515 (2006) 66. [10] R. Hauert, Diamond Relat. Mater. 12 (2003) 583. [11] T. Nakamura, T. Ohana, M. Suzuki, M. Ishihara, A. Tanaka, Y. Koga, Surf. Sci. 580 (2005) 101. [12] J.Y. Chen, L.P. Wang, K.Y. Fu, N. Huang, Y. Leng, Y.X. Leng, P. Yang, J. Wang, G.J. Wan, H. Sun, X.B. Tian, P.K. Chu, Surf. Coat. Technol. 156 (2002) 289. [13] Q. Zhao, Y. Liu, C. Wang, S. Wang, Diamond Relat. Mater. 16 (2007) 1682. [14] J.A. McLaughlin, B. Meenan, P. Maguire, N. Jamieson, Diamond Relat. Mater. 5 (1996) 486. [15] T. Yokota, T. Terai, T. Kobayashi, T. Meguro, M. Iwaki, Surf. Coat. Technol. 201 (2007) 8048. [16] A. Rich, A.K. Harris, J. Cell Set. 50 (1981) 1. [17] T.I.T. Okpalugo, A.A. Ogwu, P.D. Maguire, J.A.D. McLaughlin, Biomaterials 25 (2004) 239. [18] S.-W. Ha, R. Hauert, K.-H. Ernst, E. Wintermantel, Surf. Coat. Technol. 96 (1997) 293. [19] R.K. Roy, H.-W. Choi, S.-J. Park, K.-R. Lee, Diamond Relat. Mater. 16 (2007) 1732. [20] Y. Komatsu, A. Alanazi, K.K. Hirakuri, Diamond Relat. Mater. 8 (1999) 2018. [21] S. Zhang, X.T. Zeng, H. Xie, P. Hing, Surf. Coat. Technol. 123 (2000) 256. [22] S. Meskinis, S. Tamulevicius, V. Kopustinskas, M. Andrulevicius, A. Guobiene, R. Gudaitis, I. Liutviniene, Thin Solid Films 515 (2007) 7615. [23] X.L. Peng, Z.H. Barber, T.W. Clyne, Surf. Coat. Technol. 138 (2001) 23. [24] S.P. Louh, I.C. Leu, M.H. Hon, Diamond Relat. Mater. 14 (2005) 1005. [25] J. Coates, Interpretation of Infrared Spectra, a Practical Approach, Wiley, 2000. [26] T. Heitz, B. Dre’villon, C. Godet, J.E. Bourre´e, Phys. Rev. B 58 (1998) 13957. [27] R.M. Silverstein, F.X. Webster, Spectrometric Identification of Organic Compounds Sixth Edition, John Wiley & Sons Inc., Canada, 1998.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Effect of H2 and O2 plasma etching treatment on the surface of diamond-like carbon thin film Deok Yong Yun a, Won Seok Choi b, Yong Seob Park a, Byungyou Hong a,c,* a
School of Information and Communication Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea Department of Electrical Engineering, Hanbat National University, Daejeon 305-719, Republic of Korea c Center for Advanced Plasma Surface Technology, Sungkyunkwan University, Suwon 440-746, Republic of Korea b
A R T I C L E I N F O
A B S T R A C T
Article history:
In this study, we investigated the surface properties of diamond-like carbon (DLC) films for biomedical applications through plasma etching treatment using oxygen (O2) and hydrogen (H2) gas. The synthesis and post-plasma etching treatment of DLC films were carried out by 13.56 MHz RF plasma enhanced chemical vapor deposition (PECVD) system. In order to characterize the surface of DLC films, they were etched to a thickness of approximately 100 nm and were compared with an as-deposited DLC film. We obtained the optimum condition through power variation, at which the etching rate by H2 and O2 was 30 and 80 nm/min, respectively. The structural and chemical properties of these thin films after the plasma etching treatment were evaluated by Raman and Fourier transform infrared (FT-IR) spectroscopy. In the case of as-deposited and H2 plasma etching-treated DLC film, the contact angle was 86.48 and 83.78, respectively, whereas it was reduced to 35.58 in the etching-treated DLC film in O2 plasma. The surface roughness of plasma etching-treated DLC with H2 or O2 was maintained smooth at 0.1 nm. These results indicated that the surface of the etching-treated DLC film in O2 plasma was hydrophilic as well as smooth. ß 2008 Elsevier B.V. All rights reserved.
Available online 4 April 2008 PACS: 78.55.Qr 78.30. j 71.55.Jv 77.84.Bw Keywords: Diamond-like carbon Plasma etching Surface treatment Hydrophilic
1. Introduction Diamond-like carbon (DLC) films have been intensively studied for over two decades, owing to their excellent characteristics such as extremely high hardness, low friction coefficients, chemical inertness, wear resistance, optical transparency, and biocompatibility [1,2]. The studies on this material have recently been conducted at a hectic pace since its unique characteristics make it a promising candidate as a hard mask [3,4] and, in particular, in biomedical applications [5–7]. Recent studies have reported that surface-modified DLC thin films improved biocompatibility, lubricity, stability and cell adhesion [8–11]. These characteristics are related to structural bonds [12,13], surface roughness [6,14] and whether the film is hydrophobic or hydrophilic [15,16]. The surface modification of DLC films have been performed by doping with suitable elements [9–11,17] and plasma treatment [18,19]. Because of chemically active species in plasma and the ease of processing, the plasma treatment is the most widely used method for surface modification. In this study, DLC thin films were synthesized by RF plasma enhanced chemical vapor deposition (PECVD) method with
* Corresponding author. Tel.: +82 31 290 7208; fax: +82 31 290 5669. E-mail address: [email protected] (B. Hong). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.03.170
methane (CH4) and hydrogen (H2) gas, and the films were etched to approximately 100 nm by the plasma etching treatment. The surface treatment of DLC films by plasma has been performed using H2 and O2 gas, respectively, at various plasma powers. In order to characterize the surface properties of DLC films after the etching treatment, as-deposited DLC was compared with etchingtreated DLC films in the H2 and O2 plasma. The molecular structures, morphology and surface profiles of the films were investigated by Fourier transform infrared (FT-IR), Raman spectroscopy, the contact angle measurement, and atomic force microscopy (AFM).
2. Experiment The synthesis of DLC films and plasma etching treatment were carried out by using 13.56 MHz RF-PECVD. Prior to the DLC film deposition, Si substrate was pretreated in H2 plasma at 150 W for 5 min in order to remove the contaminants on the surface and to improve the strength of the film adhesion to the substrate. The DLC films were deposited on p-type (1 0 0) silicon with a size of 2 cm 2 cm at a deposition rate of approximately 15 nm/min with RF power of 150 W. A mixture of CH4 (20 sccm) and H2 (80 sccm) gas was used to produce the hydrogenated DLC films. For the plasma etching of deposited films, the films were treated in O2 and
D.Y. Yun et al. / Applied Surface Science 254 (2008) 7925–7928
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Fig. 1. Etching rate of O2 (flow = 10 sccm) and H2 (flow = 80 sccm) as a function of RF power.
H2 plasma. Both the synthesis and the etching of DLC films were performed at a working pressure of 1 Torr, and the substrate temperature was maintained at room temperature. The plasma etching was performed by varying RF power from 50 to 250 W. All samples were etched to a thickness of approximately 100 nm and the thickness was measured using Alpha-step. The samples for the characterization measurement were etched based on an etching rate for the 150 W plasma power, and the etching rate was 30 nm/ min in H2 plasma and 80 nm/min in O2 plasma. The structural property of the samples before and after the plasma etching treatment was analyzed by Raman spectroscopy (Renishaw, 2000) excited with He–Ne laser at a wavelength of 633 nm. Their bonding structure was characterized by FT-IR (BRUKER IFS 66v/S) in transmission mode and all the spectra were detected in the range of 400–4000 cm 1. The surface morphology and roughness value of DLC films were characterized by AFM (SII Nanotechnology, SPA400). The hydrophilicity of the surface was evaluated by measuring the wetting contact angles by the sessile drop method at three different spots in each sample. 3. Results and discussions Fig. 1 shows the behavior of each etching rate in O2 and H2 plasma as a function of RF power. In the case of O2, the etching rate increases rapidly with increasing RF power, but the etching rate of
Fig. 2. Raman spectra of the (a) as-deposited DLC film (150 nm) and etching-treated DLC films (100 nm) in (b) H2 and (c) O2 plasma.
H2 increases rather slowly. According to Komatsu et al. [20], the etching rate by O2 plasma on the film surface was much more affected by the change in gas pressure and was five times higher than H2 plasma. These are considered that ion-bombardment energy of oxygen is bigger and generates more quantity of bonds such as CO2, CO, H2O, etc. than hydrogen [21]. The O2 plasma produces energetic oxygen species, which can be reacted easily to the surface of DLC films. This can improve the etching rate by the increase in carbon–oxygen bonds on the surface. In contrast, hydrogen atoms are decomposed into the small amount of water vapor and free hydrogen. The Raman spectra for the DLC films were deconvoluted with two Gaussian curve lines shown in Fig. 2. The peak at around 1365 cm 1 indicates disordered graphite (D-peak), and the peak at around 1540 cm 1 is attributed to crystalline graphite (G-peak) [15]. Based on the fitting parameters, the peak position and the ratio of integrated areas under the D and G peaks (ID/IG) are summarized in Table 1. The results show that the plasma etching treatment of DLC films leads to the shift of the two peaks towards lower wave numbers and a decrease in the ID/IG ratio. In the result, we can deduce that the sp3/sp2 ratio increased after the etching in H2 and O2 plasma. This means that H ions generated by H2 plasma leads to the increase in sp3 fraction consisted of sp3 C–H group on the surface and the
Table 1 Gaussian fitting results of Raman spectra Sample
D-peak (cm Position (cm
As-deposited DLC (150 nm) H2 plasma-treated DLC (100 nm) O2 plasma-treated DLC (100 nm)
1368.2 1363.2 1363.4
1
)
1
G-peak (cm )
Intensity (a.u.)
Position (cm
18,220 13,060 9822
1543.1 1535.9 1534.3
1
)
ID/IG
1
)
Intensity (a.u.) 19,671 15,479 11,849
Fig. 3. Photos of the wetting contact angle of the (a) as-deposited DLC film (100 nm) and DLC films (100 nm) after (b) H2 and (c) O2 plasma treatments.
1.76 1.53 1.49
D.Y. Yun et al. / Applied Surface Science 254 (2008) 7925–7928
7927
Fig. 4. AFM images and RMS roughness of the (a) as-deposited DLC film (100 nm) and etching-treated DLC films (100 nm) in (b) H2 plasma and (c) O2 plasma.
Table 2 Contact angle of the water drop on the prepared samples Sample
Contact angle, u (8)
As-deposited DLC (100 nm) H2 plasma-treated DLC (100 nm) O2 plasma-treated DLC (100 nm)
86.4 1.6 83.7 1.6 35.5 0.1
inside film; O ions by O2 plasma is related to the increase in dangling bonds (sp3 phase) desorbed at the carbon network of the surface. Consequently, H and O reactions on the surface by the etching attributes to the increase in sp3/sp2 ratio on DLC films. The photos of the wetting contact angle on the samples are shown in Fig. 3. The measurement of the contact angle was carried out by an image analysis system, which calculated both the left and the right contact angles from the shape of the droplet, and the results are summarized in Table 2. The contact angle of distilled water on the as-deposited DLC film is 86.48, and the contact angles on the etching-treated DLC films in H2 and O2 plasma are 83.78 and 35.58, respectively. The hydrophobic property of DLC films was nearly maintained in H2 plasma; however, it was possible to obtain the hydrophilic property for DLC films by the O2 plasma etching treatment. This means that the high concentration of carbon– oxygen sites on the surface of DLC films by O2 plasma etching contributed to the adsorption hysteresis related the high surface energy [22]. Fig. 4 shows the AFM images of the as-deposited DLC film and two plasma-treated DLC films. The surface roughness was measured over an area of 2 mm 2 mm. We initially anticipated that the surface of DLC films treated by O2 plasma will be rougher due to the larger ion-bombardment energy of C than H [23]. However, the surfaces of three different types of DLC film were still considerably smooth as the RMS roughness of approximately 0.1 nm after etching treatment. This indicates that the roughness of the DLC films remained almost unaffected under H2 or O2 plasma etching. In other words, we confirmed that the low surface roughness of all DLC films can be controlled by the bonding fraction, not bonded atoms on the surface.
FTIR studies were carried out to characterize the bonding structure of the DLC films. Fig. 5 shows the FT-IR spectra for the asdeposited DLC films with two different thickness and the etchingtreated DLC films in H2 and O2 plasma. The IR absorption peak at 2350 cm 1, which is weakly observed in two plasma etchingtreated DLC films, is associated with the CO2 asymmetrical stretching mode and is considered due to O2 injected as impurity during deposition [24]. The broad band at around 3250 cm 1 can be attributed to the O–H stretching vibrations [25], and the O–H broad band is only observed for the samples etched in O2 plasma. It is believed that this is affected mainly as the oxygen atoms are bonded to hydrogen atoms existing on the surface during O2 plasma treatment. And the O–H bonds (hydroxyl group) revealed by O2 plasma treatment made the surface hydrophilic [19]. A broad band is observed at around 2900 cm 1 due to the C–H stretching vibration mode. According to literature [26], the band at 2860 cm 1 is assigned to the C–H2 symmetric vibration mode, whereas the band at 2925 cm 1 is attributed to the C–H2
Fig. 5. FT-IR spectra of the (a) as-deposited DLC film (150 nm), (b) (100 nm) and etching-treated DLC films (100 nm) in (c) H2 and (d) O2 plasma.
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asymmetric vibration mode or to the C–H stretching mode. C–H (b) and C–H (s) peak intensity of the DLC films were decreased by plasma etching treatment, and the DLC films (samples b, c and d) with thickness of 100 nm show nearly same intensity for C–H (b) and C–H (s). While the C C peak was presented at around 1600 cm 1 for the as-deposited DLC films [25], the plasma etching treatment of DLC film leads to the shift of the peak towards lower wave number. This is because of an increase of the weak carbon double bonds related to the reduction of sp2 bonding fraction as the results of Raman measurement [27]. These results indicate that the etching of DLC film by O2 plasma leads to the reduction of hydrogen contents related to C–H groups, and also contributes the reduction in the weak double bonds generated by high energetic plasma ions in carbon networks [24]. The etching by H2 plasma causes the increase in H incorporation by the hydrogen diffusion and the reduction of dangling bonds. 4. Conclusion Synthesis and plasma etching treatment of DLC films were carried out by using 13.56 MHz RF-PECVD. The properties of the DLC films were investigated through Raman, FT-IR, AFM and contact angle measurements. The sp3/sp2 ratio increased after H2 and O2 plasma etching treatment because of the increase in the generation of the unstable C bonds and the hydrogen incorporation caused by the energetic ions. Especially, broad O–H group on the DLC film was formed through O2 plasma etching treatment. This means that the surface becomes more hydrophilic, and it is consistent with the result of contact angle, whereas the plasma-treated DLC film in H2 plasma was hardly changed in the hydrophobic property. The surface roughness was kept ultra smooth as RMS roughness of about 0.12 nm after H2 and O2 plasma etching treatment. Consequently, the etching technique of DLC film can be applied for biomedicine as well as we can control the surface properties of DLC films by using the plasma treatment. Further studies are necessary to determine the effect of biomaterials such as protein and deoxyribonucleic acid (DNA) on DLC film. Acknowledgments This work is outcome of the fostering project of the Specialized Graduate School supported financially by the Ministry of
Commerce, Industry and Energy (MOCIE). And the authors are grateful for the financial support provided by Grant No. R11-2000086-0000-0 from the Center of Excellency Program of the Korea Science and Engineering Foundation and most through the Center for Advanced Plasma Surface Technology (CAPST) at Sungkyunkwan University.
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