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Stress evolution of tetrahedral amorphous carbon upon boron incorporation
Scripta Materialia 57 (2007) 141–144 www.elsevier.com/locate/scriptamat
Stress evolution of tetrahedral amorphous carbon upon boron incorporation Manlin Tan,* Jiaqi Zhu, Jiecai Han, Xiao Han, Li Niu and Wangshou Chen Center for Composite Materials, Harbin Institute of Technology, No. 2 Yi-kuang Street, Nan-gang District, Harbin 150001, China Received 26 January 2007; revised 10 March 2007; accepted 11 March 2007 Available online 23 April 2007
This paper reports the evolutions of stress and mechanical properties of tetrahedral amorphous carbon films upon boron incorporation. A sharp stress relief and slight density reduction combined with moderate hardness and Young’s modulus were observed for the films with boron concentration up to 15% in the cathodes. Raman spectroscopy shows that a clustering of sp2 sites without a notable decrease in sp3 content occurs in the films, which is responsible for the mechanical behavior after boron incorporation. Ó 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Thin films; Amorphous materials; Raman spectroscopy; Residual stress; Mechanical properties
In the past two decades, tetrahedral amorphous carbon (ta-C) films have drawn widespread attention due to their excellent properties for optical and mechanical coatings [1,2]. However, for films deposited under optimum conditions with respect to hardness, intrinsic high compressive stress occurs which prevents the production of micrometer-thick films due to adhesion failure from the substrates [3,4]. Therefore, the preparation of lowstress a-C films which retain their diamond-like properties is extremely important for technological applications. Thermal annealing has been the general way to reduce the stress after films deposition, and a complete stress relief of ta-C at 600 °C was successfully reported by Sullivan et al. [5,6] and subsequent researchers [7,8]. This makes it possible to prepare a stress-free thick film upon cyclic deposition and annealing. However, the annealing method is limited by the substrate materials due to the high annealing temperature [8]. One alternative solution to deal with the stress is the incorporation of another element into the film, such as boron [9], silicon [10] or metals [11]. Preliminary reports have shown that a low boron content can act to reduce the stress to 1–3 GPa in ta-C without markedly lowering the sp3 hybridizations [9]. However, few studies have been performed on the correlation between boron incorporation and internal stress, especially at higher concentrations. In this work, we prepared boron-incorporated a-C (a-C:B) films by filtered cathodic vacuum arc (FCVA) * Corresponding author. E-mail: [email protected]
deposition, in which the boron content was controlled by varying the B weight percentage in the mixed graphite cathodes. Compared with the other similar deposition method [9], the effects on the stress, density and mechanical properties of a-C:B films were investigated in films with a much wider range of boron compositions. The deposition of a-C:B films using the FCVA method, which employed a double-bend off-plane filter to remove the macroparticles and neutrals [12], was carried out on Si(1 1 1) substrates. A series of films with thickness 120 nm determined by spectroscopic ellipsometry were grown at a constant negative bias of 80 V while the magnitude of B in the cathodes ranged from 0 to 15%. As expected, the atomic content of B in the as-deposited a-C:B films increases from 0 to 6.04 at.%, as determined by X-ray photoelectron spectroscopy. The structure of the films was characterized by visible Raman spectroscopy (458 nm), which was carried out on a Jobin Ybon HR800 spectrometer. Determination of the compressive stress was conducted by measuring the substrate curvatures both before and after film deposition, using the well-known Stoney’s equation [13]. The film density was acquired from X-ray reflectivity (XRR) curves with a Bruker AXS GIXR reflectometer. The values of hardness and Young’s modulus were obtained through nanoindentation measurements. In order to produce the necessary resolution, the hardness and elastic modulus measurements were conducted by the continuous stiffness measurements technique on a nanoindenter XP system (MTS Systems Corp.) with a Berkovich three-sided pyramid diamond indenter. The average
1359-6462/$ - see front matter Ó 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2007.03.036
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M. Tan et al. / Scripta Materialia 57 (2007) 141–144
pressure under the indenter is obtained from the applied load and the projected area of contact between the indenter and the sample. Figure 1 shows the compressive stress of the films as a function of boron content in the cathodes. The stress in the intrinsic ta-C film after growth is 7.5 GPa, which is comparable to that reported by other researchers [14,15]. It can be seen that the stress decreases gradually to 4.9 GPa as the boron concentration exceeds 6%. For a further increase in boron addition, the stress reduces sharply to 2.3 GPa at 10% B and then down to 1.4 GPa at 15% B. This result shows that, for a composition range of 10–15%, the stress of the films can be greatly reduced through boron incorporation. Figure 2 shows the mass density derived from the XRR measurements. A typical XRR curve is also demonstrated in the inset. As the incident angle hi increases above a critical angle hc, X-rays start to penetrate into the film. According to Snell’s law, at the air/film interface [16] the critical angle of a material with elements j can be described as sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi N A r0 X C ðZ C þ fC0 Þ þ X B ðZ B þ fB0 Þ ð1Þ q hc ¼ k X CM C þ X BM B p where r0 is the classical electron radius, k is the X-ray wavelength, NA is Avogadro’s number, q is the overall mass density, Zj is the atomic number, Mj is the atomic mass, Xj is the atomic fraction and fj0 0 for the disper˚ with XB = 1 XC, the sion corrections. At k = 1.5418 A density q is obtained as q¼
ph2c ðX C þ 11Þ N A r0 k2 ðX C þ 5Þ
ð2Þ
For the pure ta-C film, the density has a very high value of 3.35 g cm3. As the boron content in the cathode increases, a gradual decrease in the mass density for the a-C:B films can be observed. With a boron composition ranging from 0% to 15%, the mass density first dramatically decreases from 3.35 to 3.12 g cm3 at 1% B, and then reduces slightly to 2.98 g cm3 at the maximum boron incorporation of 15%. Considering the roughly linear relation between density and sp3 content provided by Ferrari et al. [16], a considerable fraction of sp3 bonding could be expected in the a-C:B films.
Figure 1. Compressive stress of a-C:B films as a function of boron concentration in the cathodes.
Figure 2. Mass density of a-C:B films as a function of boron concentration determined from XRR measurements. A typical XRR curve is also presented in the inset.
The variation in stress and density with boron content can be understood based upon changes in the film structure. A deep insight of the bonding transformations is possible from Raman spectroscopy. Figure 3 shows the Raman spectra of ta-C and a-C:B films deposited using targets with different boron concentrations. As has been confirmed, the origin of the visible Raman features of ta-C in the region of 1100–1900 cm1 is associated with the sp2 bonding in the films, while a small peak located at around 950 cm1 arises from the second-order scattering of silicon substrate [17]. The line shapes of the Raman spectra are similar to each other for the a-C:B films with increased boron content, except for a small downshift of peak position to lower frequencies. In order to quantify the spectra for analysis, a simple fit in terms of two Gaussians, corresponding to D and G peaks, is carried out. The G peak is formed by olefinic sp2 bonds as well as sp2 stretch vibrations in benzene rings, while the D peak relates to the breathing mode of the sp2 sites only in rings [18]. The intensity ratio of D and G peaks, I(D)/I(G), is a measure of sp2 clustering, and can be used to reflect the relative fractions of sp3 bonding in the films [19]. As the boron
Figure 3. Raman spectra of a-C:B films prepared using the cathodes with boron content of (a) 0%, (b) 1%, (c) 3%, (d) 6%, (e) 10%, (f) 15%.
M. Tan et al. / Scripta Materialia 57 (2007) 141–144
concentration increases from 0 to 10%, I(D)/I(G) increases gradually from 0.14 to 0.19, and subsequently increases appreciably to the higher value of 0.27 at 15%. In order to evaluate the relaxation process of a-C:B films, we plot the corresponding stress vs. I(D)/I(G) of the films in Figure 4. As seen in the figure, a transformed ‘‘Z’’-shaped stress variation with I(D)/I(G) is observed, i.e. when the added boron does not exceed 6%, minor structural changes take place in the films which result in a slight relief of stress, while with boron incorporation of up to 10% or higher, remarkable bonding transformations combined with notable stress relaxation can be estimated in the films. The explanation for the stress behavior in the a-C:B has been suggested from the atomic level of local atoms distribution [9]. Sullivan et al. [5] proposed that sp2 sites are larger than sp3 sites in atomic volume but are smaller in in-plane size due to shorter bond length. Thus the formation of sp2 sites with theirrplane aligned in the plane of compression will relieve a biaxial compressive stress [20]. In the carbon network, B atoms are predominantly sp2 hybridized. If the sp2 bonded B sites also induce the nearest C atoms to be sp2 bonded, the total stress will be relaxed in the surrounding sp3 C–C bonded amorphous network. The changes in stress and strain in a thin film under biaxial stress can be described as E De ð3Þ 1t where r is the stress, E is the Young’s modulus, t is the Poission’s ratio and e is the strain [20]. By ignoring any possible variations in the elastic constants, Ferrari et al. [7] reported that a small strain De 1.2% was needed if the change in the film stress was taken as Dr 10 GPa and E/(1 t) 870 GPa. Thus only a minor structure modification, like a variation in bonding angle and length, is required to relieve the stress of the film. The increase in the sp2 phase with the incorporation of boron in a-C:B films, indicated by the Raman spectra, is the primary reason for the reduction of internal stress. This can be accounted for by the slight decrease in the mass density and Raman characteristics for the cases of our a-C:B films. The structural modification induced by boron incorporation can therefore be depicted as follows. When a carbon atom is replaced by B and forms sp2 sites with Dr ¼
Figure 4. Compressive stress vs. I(D)/I(G) ratio for the a-C:B films prepared using cathodes with different boron concentrations.
143
C, only a small fraction of new sp2 carbon is formed and inside a completely sp3 phase without any appreciable increase of sp2 clustering with the boron concentration increasing from 0% to 6%. This leads to an almost constant I(D)/I(G) ratio with only a slight increase in magnitude, being determined by the clustering of sp2 sites. Thereafter, as the overall sp2 content increases up to a certain high level, the isolated or olefinic sp2bonded carbon in chains starts to cluster into ordered rings. Upon the further addition of boron, these rings tend to align in parallel planes, going on to form a larger carbon ring. The increased size of the carbon ring will substantially enhance the D peak intensity in the Raman spectra, and cause the I(D)/I(G) ratio to increase quickly from 0.19 to 0.27 as the boron content in the cathode exceeds 10%. In the formation of the carbon ring, the transformed p bonds in the dispersive sp2 sites will align in the vertical orientation of the plane, and the atomic volume will become larger in the local carbon network. This allows for the internal stress to be seriously relieved at the high level of boron incorporation. Figure 5 shows the results of nanoindention measurements, where the hardness and Young’s modulus as determined from the loading–unloading curves using the Pharr–Oliver method are plotted as a function of boron concentration. Considering the influence of the silicon substrate on the hardness values of the films, the maximum applied load varied from 50 to 800 lN. A general result is then acquired from the average of six separate indents for each sample, and the error bars are calculated from the standard deviation of these six measurements. As seen in the figure, the hardness of the prepared ta-C film without boron incorporation is
Figure 5. Evolutions of (a) hardness and (b) Young’s modulus of aC:B films with boron concentration in the cathodes.
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about 55 GPa, which is a little higher than that reported by Chhowalla et al. [9] at the optimum ion energy condition. After the addition of boron, the hardness decreases quickly from an initial 55 GPa to 36 GPa at a boron content of 3%. For a further increase in boron concentration, the hardness descends linearly to a moderate value of 17 GPa at 15% with a slower rate. This may be because there is still a high content of sp3 carbon in the films in spite of the boron incorporation. The Young’s modulus has a similar change with boron addition, i.e. a gradual reduction of elastic modulus for the a-C:B films was also found in the same range of boron concentration. At a boron content of 15%, a still fairly large value of 230 GPa was obtained comparable to the hydrogenated ta-C of 300 GPa [21]. This is due solely to the effect of boron, like hydrogen, on the lowering of the carbon network coordination, as the elasticity of a random network is related to its coordination according to the constraint-counting model of Phillips and Thorpe [22,23]. In summary, boron-incorporated a-C films were prepared by a filtered cathodic vacuum arc deposition system on silicon substrates. The incorporation of B atoms in the films leads the stress first slowly and then quickly to reduce from 7.5 GPa to 1.4 GPa with the weight concentration varying from 0% to 15%. The mass density, hardness and Young’s modulus of the films were also found to decrease gradually as the boron content in the cathode increases. Raman spectra show that the incorporation of B results in a steady increase in the I(D)/I(G) ratio, indicating that a clustering of sp2 sites occurs in the films. The clustering of sp2 sites with minimal sp3 bonding reduction is responsible for the slight decrease in the mass density of the films, which still possess a moderate hardness value of 17 GPa at a boron concentration of 15%. This work was supported by the National Natural Science Foundation of China (Grant No. 50602012).
[1] Y. Lifshitz, Diamond Relat. Mater. 8 (1999) 1659. [2] J. Robertson, Mater. Sci. Eng. R 37 (2002) 129. [3] D.R. Mckenzie, D. Muller, B.A. Pailthorpe, Phys. Rev. Lett. 67 (1991) 773. [4] P.J. Fallon, V.S. Veerasamy, C.A. Davis, J. Robertson, G.A.J. Amaratunga, W.I. Milne, J. Koskinen, Phys. Rev. B 48 (1993) 4777. [5] J.P. Sullivan, T.A. Friedmann, A.G. Baca, J. Electron. Mater. 26 (1997) 1021. [6] J.P. Sullivan, T.A. Friedmann, R.G. Dunn, et al., Mater. Res. Soc. Symp. Proc. 498 (1998) 97. [7] A.C. Ferrari, B. Kleinsorge, N.A. Morrison, A. Hart, V. Stolojan, J. Robertson. J. Appl. Phys. 85 (1999) 7191. [8] B.K. Tay, X. Shi, E. Liu, S.P. Lau, L.K. Cheah, Z. Sun, J. Shi, Surf. Coating Technol. 120–121 (1999) 448. [9] M. Chhowalla, Y. Yin, G.A.J. Amaratunga, D.R. McKenzie, Th. Frauenheim, Appl. Phys. Lett. 69 (1996) 2344. [10] K. Okuri, T. Arai, Surf. Coating Technol. 47 (1991) 710. [11] B.K. Tay, Y.H. Cheng, X.Z. Ding, S.P. Lau, X. Shi, G.F. You, D. Sheeja, Diamond Relat. Mater. 10 (2001) 1082. [12] J.Q. Zhu, J.C. Han, X. Han, S.H. Meng, Z.C. Jia, J. Yan, K. Chen, Scripta Mater. 55 (2006) 1167. [13] B.K. Gupta, B. Bhushan, Thin Solid Films 270 (1995) 391. [14] Y.B. Zhang, S.P. Lau, D. Sheeja, B.K. Tay, Surf. Coating Technol. 195 (2005) 338. [15] D. Sheeja, B.K. Tay, L.J. Yu, S.P. Lau, J.Y. Sze, C.K. Cheong, Thin Solid Films 420–421 (2002) 62. [16] A.C. Ferrari, A. Libassi, B.K. Tanner, V. Stolojan, J. Yuan, L.M. Brown, S.E. Rodil, B. Kleinsorge, J. Robertson, Phys. Rev. B 62 (2000) 16. [17] J.Q. Zhu, J.C. Han, S.H. Meng, J.H. Wang, W.T. Zheng, Vacuum 72 (2004) 285. [18] A.C. Ferrari, J. Roberson, Phys. Rev. B 61 (2000) 14095. [19] M.A. Tamor, W.C. Vassell, J. Appl. Phys. 76 (1994) 3823. [20] A.C. Ferrari, S.E. Rodil, J. Robertson, W.I. Milne, Diamond Relat. Mater. 11 (2002) 994. [21] S. Sattel, J. Robertson, H. Ehrhardt, J. Appl. Phys. 82 (1997) 4566. [22] J.C. Philips, J. Non Cryst. Solids 34 (1979) 153. [23] M.F. Thorpe, J. Non-Cryst. Solids 57 (1983) 355.
Stress evolution of tetrahedral amorphous carbon upon boron incorporation Manlin Tan,* Jiaqi Zhu, Jiecai Han, Xiao Han, Li Niu and Wangshou Chen Center for Composite Materials, Harbin Institute of Technology, No. 2 Yi-kuang Street, Nan-gang District, Harbin 150001, China Received 26 January 2007; revised 10 March 2007; accepted 11 March 2007 Available online 23 April 2007
This paper reports the evolutions of stress and mechanical properties of tetrahedral amorphous carbon films upon boron incorporation. A sharp stress relief and slight density reduction combined with moderate hardness and Young’s modulus were observed for the films with boron concentration up to 15% in the cathodes. Raman spectroscopy shows that a clustering of sp2 sites without a notable decrease in sp3 content occurs in the films, which is responsible for the mechanical behavior after boron incorporation. Ó 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Thin films; Amorphous materials; Raman spectroscopy; Residual stress; Mechanical properties
In the past two decades, tetrahedral amorphous carbon (ta-C) films have drawn widespread attention due to their excellent properties for optical and mechanical coatings [1,2]. However, for films deposited under optimum conditions with respect to hardness, intrinsic high compressive stress occurs which prevents the production of micrometer-thick films due to adhesion failure from the substrates [3,4]. Therefore, the preparation of lowstress a-C films which retain their diamond-like properties is extremely important for technological applications. Thermal annealing has been the general way to reduce the stress after films deposition, and a complete stress relief of ta-C at 600 °C was successfully reported by Sullivan et al. [5,6] and subsequent researchers [7,8]. This makes it possible to prepare a stress-free thick film upon cyclic deposition and annealing. However, the annealing method is limited by the substrate materials due to the high annealing temperature [8]. One alternative solution to deal with the stress is the incorporation of another element into the film, such as boron [9], silicon [10] or metals [11]. Preliminary reports have shown that a low boron content can act to reduce the stress to 1–3 GPa in ta-C without markedly lowering the sp3 hybridizations [9]. However, few studies have been performed on the correlation between boron incorporation and internal stress, especially at higher concentrations. In this work, we prepared boron-incorporated a-C (a-C:B) films by filtered cathodic vacuum arc (FCVA) * Corresponding author. E-mail: [email protected]
deposition, in which the boron content was controlled by varying the B weight percentage in the mixed graphite cathodes. Compared with the other similar deposition method [9], the effects on the stress, density and mechanical properties of a-C:B films were investigated in films with a much wider range of boron compositions. The deposition of a-C:B films using the FCVA method, which employed a double-bend off-plane filter to remove the macroparticles and neutrals [12], was carried out on Si(1 1 1) substrates. A series of films with thickness 120 nm determined by spectroscopic ellipsometry were grown at a constant negative bias of 80 V while the magnitude of B in the cathodes ranged from 0 to 15%. As expected, the atomic content of B in the as-deposited a-C:B films increases from 0 to 6.04 at.%, as determined by X-ray photoelectron spectroscopy. The structure of the films was characterized by visible Raman spectroscopy (458 nm), which was carried out on a Jobin Ybon HR800 spectrometer. Determination of the compressive stress was conducted by measuring the substrate curvatures both before and after film deposition, using the well-known Stoney’s equation [13]. The film density was acquired from X-ray reflectivity (XRR) curves with a Bruker AXS GIXR reflectometer. The values of hardness and Young’s modulus were obtained through nanoindentation measurements. In order to produce the necessary resolution, the hardness and elastic modulus measurements were conducted by the continuous stiffness measurements technique on a nanoindenter XP system (MTS Systems Corp.) with a Berkovich three-sided pyramid diamond indenter. The average
1359-6462/$ - see front matter Ó 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2007.03.036
142
M. Tan et al. / Scripta Materialia 57 (2007) 141–144
pressure under the indenter is obtained from the applied load and the projected area of contact between the indenter and the sample. Figure 1 shows the compressive stress of the films as a function of boron content in the cathodes. The stress in the intrinsic ta-C film after growth is 7.5 GPa, which is comparable to that reported by other researchers [14,15]. It can be seen that the stress decreases gradually to 4.9 GPa as the boron concentration exceeds 6%. For a further increase in boron addition, the stress reduces sharply to 2.3 GPa at 10% B and then down to 1.4 GPa at 15% B. This result shows that, for a composition range of 10–15%, the stress of the films can be greatly reduced through boron incorporation. Figure 2 shows the mass density derived from the XRR measurements. A typical XRR curve is also demonstrated in the inset. As the incident angle hi increases above a critical angle hc, X-rays start to penetrate into the film. According to Snell’s law, at the air/film interface [16] the critical angle of a material with elements j can be described as sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi N A r0 X C ðZ C þ fC0 Þ þ X B ðZ B þ fB0 Þ ð1Þ q hc ¼ k X CM C þ X BM B p where r0 is the classical electron radius, k is the X-ray wavelength, NA is Avogadro’s number, q is the overall mass density, Zj is the atomic number, Mj is the atomic mass, Xj is the atomic fraction and fj0 0 for the disper˚ with XB = 1 XC, the sion corrections. At k = 1.5418 A density q is obtained as q¼
ph2c ðX C þ 11Þ N A r0 k2 ðX C þ 5Þ
ð2Þ
For the pure ta-C film, the density has a very high value of 3.35 g cm3. As the boron content in the cathode increases, a gradual decrease in the mass density for the a-C:B films can be observed. With a boron composition ranging from 0% to 15%, the mass density first dramatically decreases from 3.35 to 3.12 g cm3 at 1% B, and then reduces slightly to 2.98 g cm3 at the maximum boron incorporation of 15%. Considering the roughly linear relation between density and sp3 content provided by Ferrari et al. [16], a considerable fraction of sp3 bonding could be expected in the a-C:B films.
Figure 1. Compressive stress of a-C:B films as a function of boron concentration in the cathodes.
Figure 2. Mass density of a-C:B films as a function of boron concentration determined from XRR measurements. A typical XRR curve is also presented in the inset.
The variation in stress and density with boron content can be understood based upon changes in the film structure. A deep insight of the bonding transformations is possible from Raman spectroscopy. Figure 3 shows the Raman spectra of ta-C and a-C:B films deposited using targets with different boron concentrations. As has been confirmed, the origin of the visible Raman features of ta-C in the region of 1100–1900 cm1 is associated with the sp2 bonding in the films, while a small peak located at around 950 cm1 arises from the second-order scattering of silicon substrate [17]. The line shapes of the Raman spectra are similar to each other for the a-C:B films with increased boron content, except for a small downshift of peak position to lower frequencies. In order to quantify the spectra for analysis, a simple fit in terms of two Gaussians, corresponding to D and G peaks, is carried out. The G peak is formed by olefinic sp2 bonds as well as sp2 stretch vibrations in benzene rings, while the D peak relates to the breathing mode of the sp2 sites only in rings [18]. The intensity ratio of D and G peaks, I(D)/I(G), is a measure of sp2 clustering, and can be used to reflect the relative fractions of sp3 bonding in the films [19]. As the boron
Figure 3. Raman spectra of a-C:B films prepared using the cathodes with boron content of (a) 0%, (b) 1%, (c) 3%, (d) 6%, (e) 10%, (f) 15%.
M. Tan et al. / Scripta Materialia 57 (2007) 141–144
concentration increases from 0 to 10%, I(D)/I(G) increases gradually from 0.14 to 0.19, and subsequently increases appreciably to the higher value of 0.27 at 15%. In order to evaluate the relaxation process of a-C:B films, we plot the corresponding stress vs. I(D)/I(G) of the films in Figure 4. As seen in the figure, a transformed ‘‘Z’’-shaped stress variation with I(D)/I(G) is observed, i.e. when the added boron does not exceed 6%, minor structural changes take place in the films which result in a slight relief of stress, while with boron incorporation of up to 10% or higher, remarkable bonding transformations combined with notable stress relaxation can be estimated in the films. The explanation for the stress behavior in the a-C:B has been suggested from the atomic level of local atoms distribution [9]. Sullivan et al. [5] proposed that sp2 sites are larger than sp3 sites in atomic volume but are smaller in in-plane size due to shorter bond length. Thus the formation of sp2 sites with theirrplane aligned in the plane of compression will relieve a biaxial compressive stress [20]. In the carbon network, B atoms are predominantly sp2 hybridized. If the sp2 bonded B sites also induce the nearest C atoms to be sp2 bonded, the total stress will be relaxed in the surrounding sp3 C–C bonded amorphous network. The changes in stress and strain in a thin film under biaxial stress can be described as E De ð3Þ 1t where r is the stress, E is the Young’s modulus, t is the Poission’s ratio and e is the strain [20]. By ignoring any possible variations in the elastic constants, Ferrari et al. [7] reported that a small strain De 1.2% was needed if the change in the film stress was taken as Dr 10 GPa and E/(1 t) 870 GPa. Thus only a minor structure modification, like a variation in bonding angle and length, is required to relieve the stress of the film. The increase in the sp2 phase with the incorporation of boron in a-C:B films, indicated by the Raman spectra, is the primary reason for the reduction of internal stress. This can be accounted for by the slight decrease in the mass density and Raman characteristics for the cases of our a-C:B films. The structural modification induced by boron incorporation can therefore be depicted as follows. When a carbon atom is replaced by B and forms sp2 sites with Dr ¼
Figure 4. Compressive stress vs. I(D)/I(G) ratio for the a-C:B films prepared using cathodes with different boron concentrations.
143
C, only a small fraction of new sp2 carbon is formed and inside a completely sp3 phase without any appreciable increase of sp2 clustering with the boron concentration increasing from 0% to 6%. This leads to an almost constant I(D)/I(G) ratio with only a slight increase in magnitude, being determined by the clustering of sp2 sites. Thereafter, as the overall sp2 content increases up to a certain high level, the isolated or olefinic sp2bonded carbon in chains starts to cluster into ordered rings. Upon the further addition of boron, these rings tend to align in parallel planes, going on to form a larger carbon ring. The increased size of the carbon ring will substantially enhance the D peak intensity in the Raman spectra, and cause the I(D)/I(G) ratio to increase quickly from 0.19 to 0.27 as the boron content in the cathode exceeds 10%. In the formation of the carbon ring, the transformed p bonds in the dispersive sp2 sites will align in the vertical orientation of the plane, and the atomic volume will become larger in the local carbon network. This allows for the internal stress to be seriously relieved at the high level of boron incorporation. Figure 5 shows the results of nanoindention measurements, where the hardness and Young’s modulus as determined from the loading–unloading curves using the Pharr–Oliver method are plotted as a function of boron concentration. Considering the influence of the silicon substrate on the hardness values of the films, the maximum applied load varied from 50 to 800 lN. A general result is then acquired from the average of six separate indents for each sample, and the error bars are calculated from the standard deviation of these six measurements. As seen in the figure, the hardness of the prepared ta-C film without boron incorporation is
Figure 5. Evolutions of (a) hardness and (b) Young’s modulus of aC:B films with boron concentration in the cathodes.
144
M. Tan et al. / Scripta Materialia 57 (2007) 141–144
about 55 GPa, which is a little higher than that reported by Chhowalla et al. [9] at the optimum ion energy condition. After the addition of boron, the hardness decreases quickly from an initial 55 GPa to 36 GPa at a boron content of 3%. For a further increase in boron concentration, the hardness descends linearly to a moderate value of 17 GPa at 15% with a slower rate. This may be because there is still a high content of sp3 carbon in the films in spite of the boron incorporation. The Young’s modulus has a similar change with boron addition, i.e. a gradual reduction of elastic modulus for the a-C:B films was also found in the same range of boron concentration. At a boron content of 15%, a still fairly large value of 230 GPa was obtained comparable to the hydrogenated ta-C of 300 GPa [21]. This is due solely to the effect of boron, like hydrogen, on the lowering of the carbon network coordination, as the elasticity of a random network is related to its coordination according to the constraint-counting model of Phillips and Thorpe [22,23]. In summary, boron-incorporated a-C films were prepared by a filtered cathodic vacuum arc deposition system on silicon substrates. The incorporation of B atoms in the films leads the stress first slowly and then quickly to reduce from 7.5 GPa to 1.4 GPa with the weight concentration varying from 0% to 15%. The mass density, hardness and Young’s modulus of the films were also found to decrease gradually as the boron content in the cathode increases. Raman spectra show that the incorporation of B results in a steady increase in the I(D)/I(G) ratio, indicating that a clustering of sp2 sites occurs in the films. The clustering of sp2 sites with minimal sp3 bonding reduction is responsible for the slight decrease in the mass density of the films, which still possess a moderate hardness value of 17 GPa at a boron concentration of 15%. This work was supported by the National Natural Science Foundation of China (Grant No. 50602012).
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