Journal of Non-Crystalline Solids 354 (2008) 5504–5508
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Characteristics of sputtered amorphous carbon ﬁlms prepared by a closed-ﬁeld unbalanced magnetron sputtering method Yong Seob Park a, Byungyou Hong a,b,* a b
School of Information and Communication Engineering, Sungkyunkwan University, 300 Cheoncheon-dong, Jangan-gu, Suwon 440-746, Republic of Korea Center for Advanced Plasma Surface Technology (CAPST), Sungkyunkwan University, Republic of Korea
a r t i c l e
i n f o
Article history: Received 25 August 2007 Received in revised form 21 July 2008 Available online 18 September 2008 PACS: 87.64.Je 68.49.Uv 81.15.Cd 81.05.Uw 68.37.Og 62.20.Qp Keywords: Raman scattering UPS/XPS Sputtering Carbon STEM/TEM Hardness
a b s t r a c t We discuss the tribological performance of sputtered amorphous carbon (a-C) ﬁlms deposited by closedﬁeld unbalanced magnetron (CFUBM) sputtering with a graphite target using a mixture of helium (He) and argon (Ar) as sputtering gases. We investigated the effects of the graphite target power density on the micro-structural and physical properties. In the Raman spectra, the G-peak position moved to the higher wavenumbers. The ID/IG ratio increased with the increase of target power density in the ﬁxed DC bias voltage. This was the result of the structural change in the a-C ﬁlm that resulted with the increase in sp2 bonding fraction. Also, the maximum hardness of the a-C ﬁlm was 23 GPa, the friction coefﬁcient was 0.1, and the critical load was 25.9 N on the Si wafer. In addition, the compressive residual stress of the ﬁlm increased a little with increasing target power density. Consequently, the various properties of aC ﬁlms, with an increase of the target power density, were associated with the increase of cross-linked sp2 bonding fraction and the cluster size. The tribological properties of a-C ﬁlm showed clear dependence on the energy of ion bombardment with the increase of plasma density during ﬁlm growth. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Amorphous carbon (a-C) and hydrogenated amorphous carbon (a-C:H) ﬁlms are composed of a network of sp3 (diamond-like) and sp2 (graphite-like) co-ordinations. They possess excellent mechanical and tribological properties such as elevated hardness and excellent wear resistance . In addition, they have low friction coefﬁcients and provide protection for the counter parts . Therefore, these ﬁlms’ properties indicate that they have good prospects for use in a wide range of applications not only the typical mechanical applications such as in protective coating, wear resistant coating, corrosion resistant coating, and antireﬂective coating, but also in optical applications such as in photodiodes, light-emitting diodes, and electroluminescence devices [3,4]. We know that the properties of deposited a-C ﬁlms depend strongly on the deposition conditions and elaboration methods. We can obtain a-C ﬁlms with several techniques such as sputtering [2,3,11,14], ion beam deposition , CVD , and PLD. Sputtering methods are preferred for industrial applications because of * Corresponding author. Tel.: +82 31 290 7209; fax: +82 31 290 5669. E-mail address: [email protected]
(B. Hong). 0022-3093/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2008.08.007
these methods’ versatility, widespread use, and ease of scaling up. Also, these kinds of techniques allow for good control of the various deposition parameters such as the deposition rate, temperature, and gas composition during the deposition process. In this paper, we report on the effects of target power density on the tribological and structural properties of a-C ﬁlms synthesized by closed-ﬁeld unbalanced magnetron (CFUBM) sputtering. 2. Experimental The a-C ﬁlms were deposited onto 3 cm 3 cm p-type (1 0 0) silicon and glass substrates by using a closed-ﬁeld unbalanced magnetron (CFUBM) sputtering system consisting of two targets with 99.99% pure graphite and a diameter of 10.16 cm. The distance from target to substrate was 60 mm. Also, high purity argon (99.99%) and helium (99.99%) were used as the sputtering gases for the growth of the a-C ﬁlms. The silicon and glass substrates were cleaned successively for 5 min in acetone, methanol, and D.I. water. Then, the silicon substrates were etched in a hydroﬂuoric acid solution to strip off any native oxide. For the deposition process, the background pressure of the process chamber was evacuated to below 4.3 10 4 Pa using diffusion pumps and then the gases
Y.S. Park, B. Hong / Journal of Non-Crystalline Solids 354 (2008) 5504–5508 Table 1 Deposition conditions of a-C ﬁlms prepared by CFUBM sputtering Deposition parameters Base pressure (Pa) Conditions (value)
Ar/He ﬂow rate (sccm)
Ar/He pressure (PAr/PHe) (Pa)
Total pressure (Pa)
of Ar and He (14.2:2.5 sccm) were introduced into the chamber and were mixed. The total working pressure for the entire deposition run was 0.4 Pa. We obtained the a-C ﬁlms at various target power densities from 1.2 to 2.0 W/cm2 below the ﬁxed negative DC bias voltage. The thicknesses of the deposited all ﬁlms were approximately 200 nm. We summarize the experimental parameters in Table 1. The thickness of the a-C ﬁlms was measured using FE-SEM [Jeol, JSM6700F] and the internal structure of the ﬁlms was characterized by Raman spectrometry [Jasco, MRS-300], X-ray photoelectron spectroscopy [VG MICROTECH, ESCA-2000], and high resolution transmission electron microscopy (HRTEM) [JEOL, JEOL 300 kV]. The surface morphology of the ﬁlms was observed using atomic force microscopy [Seiko, SPA-400] and the value of the rootmean-square (RMS) roughness was obtained over an image area of 2 lm2. The residual stress of a-C ﬁlms was measured using stress tester [J&L Tech, JLCST022]. And also, the hardness and elastic modulus were measured using a commercial nano-indentation instrument [Nano-indenter XP] having a continuous stiffness method (CSM) option. Continuous loading–unloading indentations were applied up to a maximum load of 30 mN. Also, the friction
Substrate bias voltage (V) 200
Target power density (kW/ cm2) 1.2, 1.6, 2.0
coefﬁcients of the a-C ﬁlms with an increase of the target power density were analyzed using a ball-on disk (BOD) tribometer in normal dry ambient air against a polished AISI 52100 steel ball with a diameter of 4.72 mm. The sliding speed of the steel ball was maintained at a constant value of 60 rpm. Also the adhesion value of a-C ﬁlms was measured using a nanoscratch tester [J&L Tech. JLST022]. A diamond tip of nominal radius was used to scratch the ﬁlm surfaces and the normal load was increased to 35 N. Also, the scratch speed v and scratch distance were kept to values of 0.2 mm/s and 10 mm. 3. Results Raman spectroscopy is an effective method for the characterization of carbon materials [1,6]. Raman spectra of the a-C ﬁlms deposited at various target power densities are shown in Fig. 1(a). The ID/IG ratio and the G-peaks position, which were deconvoluted into two Gaussian curve ﬁts, in order to obtain quantitative information on the sp3 content in the ﬁlm, are shown in Fig. 1(b) for the various target power densities. From these ﬁgures, we observed that the G- and D-bands, corresponding to the gra-
a Intensity [arb. units]
Intensity [arb. units]
1.6 kW/cm 1375 cm
2.0 kW/cm 2 1.6 kW/cm
1.2 kW/cm 1562 cm 600
Target power density [kW/cm ] Fig. 1. Raman spectra (a) and the variation of the G-peak position and ID/IG ratio (b) of a-C ﬁlms prepared at various target power densities.
G peak position
0.54 0.53 0.52
sp /sp bonding ratio
ID / IG ratio
G peak position [cm-1]
Binding energy [eV]
Target power density [kW/cm ] Fig. 2. Carbon 1s XPS spectra (a) and the variation of sp3/sp2 bonding ratio of a-C ﬁlms prepared at various target power densities (b).
Y.S. Park, B. Hong / Journal of Non-Crystalline Solids 354 (2008) 5504–5508
phitic band and disordered band, were situated at approximately 1562 cm 1 and 1375 cm 1 [5,7], respectively. Also, we found that the positions of the G-peaks moved to lower wavenumbers and the ID/IG ratio gradually increased with the increase of target power density. Fig. 2 shows the carbon 1s XPS spectra and the variation of sp3/ 2 sp bonding ratio of a-C ﬁlms prepared at various target power densities. From the Gaussian ﬁtting results of the XPS spectra, which were deconvoluted into two components with binding energies of 284.4 eV and 286.8 eV corresponding to sp2 C–C and/or C–H and sp3 C–C and/or C–H [8,9], the position of the C 1s peak with the increase of target power density moved to a lower binding energy related to the polymerization of the ﬁlm [8,10]. Also, the sp3/sp2 bonding ratio with various target power densities showed low values below 0.55 and decreased with an increase of the target power density. We performed HRTEM analyses for a detailed investigation of ﬁlm structure. Fig. 3 shows the HRTEM micrographs of a-C ﬁlms. From the ﬁgures, the microstructure of a-C ﬁlm prepared at the target power density of 1.2 kW/cm2 was found to be a nanocrystalline graphite structure within an amorphous carbon matrix. And also, the a-C ﬁlm prepared at 2.0 kW/cm2 exhibited the dispersion of graphite sp2 bonding clusters of approximately 5 nm size and had an interplanar spacing of (1 1 1) and (0 1 0) planes. Also, we could see that with increasing target power density, the number of clusters in the a-C ﬁlm prepared at 2.0 kW/cm2 was higher than the number of clusters in the ﬁlm prepared at 1.2 kW/cm2.
Fig. 4 shows the growth rate, surface roughness, and friction coefﬁcient of a-C ﬁlms prepared with various target power densities at the ﬁxed negative DC bias voltage. From the ﬁgure, this result clearly shows that the growth rate increased signiﬁcantly and the surface roughness decreased with the increase of target power density. The increase of the growth rate can be explained by the enhancement of the reactive ions with the increase of sputtered carbon ﬂux as a result of the increase of target power density . Also, the decrease of the surface roughness is related to the increase of the energetic ions bombardment at the ﬁlm surface. And, the friction coefﬁcients of a-C ﬁlms prepared with various target power densities are shown in Fig. 4(c). It was clear that most of the a-C ﬁlms showed a low friction coefﬁcient below 0.13, which varied very little with target power density. The hardness, elastic modulus, residual stress, and critical load of the a-C ﬁlms prepared at various target power densities are shown in Fig. 5. We can observe in this ﬁgure that the hardness and elastic modulus increased with the increase of target power density and the maximum values exhibited about 23 GPa and
c Growth rate [nm/min.]
Linear fit 100 90 80 70 60
Coefficient of friction
Rms roughness [nm]
50 2.4 2.2 2.0 1.8 1.6 1.4 0.14 0.13 0.12 0.11 0.10 0.09 1.2
Target power density[ kW/cm ] Fig. 3. HRTEM micrographs of nano-structured a-C ﬁlms deposited at a target power density of 1.2 kW/cm2 (a) and 2.0 kW/cm2 (b).
Fig. 4. AFM topography images of a-C ﬁlms deposited at a target power density of 1.2 kW/cm2 (a) and 2.0 kW/cm2 (b) and the growth rate, rms roughness, and the friction coefﬁcient (c) of the a-C ﬁlms prepared at various target power densities.
Y.S. Park, B. Hong / Journal of Non-Crystalline Solids 354 (2008) 5504–5508
26 2.0 24
Critical load [N]
Residual stress [GPa]
Elastic modulus [GPa]
Target power density [kW/cm ] Fig. 5. The hardness, elastic modulus, residual stress, and critical scratch load of a-C ﬁlms deposited with various target power densities.
198 GPa. However, the residual stress of a-C ﬁlms increased a little. Also, we can see that the adhesion between a-C ﬁlm and substrate increased with the increase of target power density. 4. Discussion The variation of G-peak position and ID/IG ratio depends on the sp2 bonding fraction in the ﬁlm. From the Raman and XPS results, the increase of target power density is attributed to the increase of the sp2 bonding fraction in the carbon ﬁlm. These indicate that the increase of target power density leads to the enhancement of the sputtered carbon ﬂux from the graphite target in the plasma. Then, the increased carbon ﬂux attributed to the increase of ion bombardment at the surface by an applied negative DC bias voltage [9,11,12] and led to the rising of the surface temperature by the frequent bombardments with increasing target power density at the ﬁlm surface. Consequently, this behavior can be explained by the increase of the sp2 bonding fraction in the ﬁlm as a result of the increase of the growing temperature with increasing target power density, and also the structural variation of a-C ﬁlms associated closely with the density of the sputtered carbon ion ﬂux and the applied negative DC bias voltage. Also, from the results of HRTEM analysis, we conﬁrmed the existence of the clusters in the carbon. As seen in the pictures shown ﬁg. 3, the increase of target power density contributed to the increase of cross-linked sp2 clusters in the ﬁlm. These indicate that the enhancement of ion bombardment caused by energetic ions with increasing carbon ﬂux leads to the rising of the surface temperature at the ﬁlm surface and contributes to the increase of the number of cross-linked sp2 bonding clusters in the carbon networks [10–12]. In the result, the increase of sp2 bonding clusters in a-C ﬁlm can mainly be explained by the temperature rising at the ﬁlm surface caused by the increase of energetic carbon ﬂux. This is in agreement with previous observations. As seen in ﬁg. 4, the smooth surface of a-C ﬁlm at the high target power density is associated with the effect of ion bombardment as a result of the increase of sputtered carbon ﬂux. Generally, we
know that the coefﬁcient of friction depends on the variation of the surface morphology and the ﬁlm density caused by the chemical bonding of carbon and hydrogen . Indications are that the low value of the friction coefﬁcient results from the improvement of ion bombardments. This behavior is caused by the increase of the energetic ions at the surface as a result of the increase of the sputtered carbon ﬂux with the increase of target power. Consequently, the increase of target power density and the applied negative DC bias voltage can be attributed to the low friction coefﬁcient and the smooth surface due to the formation of graphite sp2 clusters [11,16]. We conﬁrmed the clusters in a-C ﬁlms generated with increasing target power density in ﬁg. 3. Also, we have reported the physical changes in ﬁg. 5. In the result, the increase of target power density leads to the increase of the hardness, elastic modulus, and critical load [13,17]. However, this caused the increase of the residual stress. These results indicate that the ion bombardment and the rising of surface temperature lead to the promotion of the formation of cross-liked sp2 carbon bond and the disorder degree of carbon network as a result of the improvement of carbon ﬂux with the increase of target power density and the addition of the energy to ions by supplying negative DC bias voltage during the ﬁlm deposition [11,15,16]. Also, these lead to the increase of the residual stress due to the increase of the dense degree of the ﬁlms. Consequently, the cross-linked sp2 bonding clusters generated by the target power density are attributed to the improvement of the physical properties of a-C ﬁlms. In this work, this ‘sp2 clusters’ can occur due to two main causes: the increase of ion bombardment by the sputtered carbon ﬂux and the relaxation process after the implantation of high energetic ions during the ﬁlm growth. 5. Conclusion We investigated the effect of target power density on tribological and structural properties the a-C ﬁlms deposited by a magnetron sputtering technique. From the structural results, we conﬁrmed the existence of the sp2 bonding clusters in carbon networks and knew the promotion of the formation of sp2 bonding clusters by the increase of target power density and the applied negative DC bias voltage. Consequently, the increase of the sputtered carbon ﬂux with the increase of target power density leads to the improvement of the tribological properties of a-C ﬁlm due to the increase of dispersed cross-linked sp2 clusters. Specially, our CFUBM sputtering method can prepare the a-C ﬁlms exhibited the excellent physical performance. Acknowledgment The authors are grateful for the ﬁnancial support provided by Grant No. R11-2000-086-0000-0 and No. R01-2008-000-10690-0 from the Center of Excellency program of the Korea Science and Engineering Foundation and by MOST through the Center for Advanced Plasma Surface Technology (CAPST) at Sungkyunkwan University. References  J. Robertson, Mater. Sci. Eng. R 37 (2002) 129.  V. Rigato, G. Maggioni, D. Boscarino, G. Mariotto, E. Bontempi, A.H.S. Jones, D. Camino, D. Tear, Surf. Coat. Technol. 116–119 (1999) 580.  M. Cremona, R. Reyes, C.A. Achete, R. Tarora Britto, S.S. Camargo, Thin Solid Films 447&448 (2004) 74–79.  E. Staryga, G.W. Bals, Diamond Relat. Mater. 14 (2005) 23.  D.J. Choi, W.H. Koo, S.M. Jeong, D.W. Han, D.Y. Lee, H.K. Baik, S.W. Jang, S.M. Lee, Vacuum 72 (2004) 445.  A.C. Ferrari, J. Robertson, Phys. Rev. B 61 (20) (2000) 14095.  S. Zhang, X.L. Bui, Y. Fu, Surf. Coat. Technol. 167 (2003) 137.
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