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Nanocrystalline diamond growth on different substrates
Thin Solid Films 515 (2006) 1005 – 1010 www.elsevier.com/locate/tsf
Nanocrystalline diamond growth on different substrates W. Kulisch a , C. Popov a,⁎, V. Vorlicek b , P.N. Gibson c , G. Favaro d a
Institute of Nanostructure Technologies and Analytics (INA), University of Kassel, Germany Institute of Physics, Academy of Sciences of the Czech Republic, Prague, Czech Republic c Institute for Health and Consumer Protection, Joint Research Center, Ispra, Italy d CSM Instruments SA, Peseux, Switzerland
b
Available online 11 September 2006
Abstract Nanocomposite films consisting of diamond nanoparticles of 3–5 nm diameter embedded in an amorphous carbon matrix have been deposited by means of microwave plasma chemical vapour deposition (MWCVD) from CH4/N2 gas mixtures. Si wafers, Si coated with TiN, polycrystalline diamond (PCD) and cubic boron nitride films, and Ti–6Al–4V alloy have been used as substrates. Some of the substrates have been pretreated ultrasonically with diamond powder in order to enhance the nucleation density nnuc. It turned out that nnuc depends critically on the chemical nature of the substrate, its smoothness and the pretreatment applied. No differences to the nucleation behaviour of CVD PCD films were observed. On the other hand, the growth process seems to be not affected by the substrate material. The crystallinity (studied by X-ray diffraction) and the bonding environment (investigated by Raman spectroscopy) show no significant differences for the various substrates. The mechanical and tribological properties, finally, reflect again the influence of the substrate material: on TiN, a lower hardness was measured as compared to Si, PCD and c-BN, whereas the adhesion of c-BN/nanocrystalline diamond (NCD) system was determined by that of the c-BN film on the underlying Si substrate. © 2006 Elsevier B.V. All rights reserved. Keywords: Nanocrystalline diamond; Growth mechanisms; Nucleation mechanisms; Mechanical properties
1. Introduction Nanocrystalline diamond (NCD) films [1–3], either in pure form or as NCD/a-C nanocomposites, have recently attracted considerable interest as they combine (to a large extent) the extreme properties of diamond with very smooth surfaces, which makes them a candidate for many applications such as tribology, optics, microelectromechanical systems (MEMS) [4], and biomedicine [5,6] to name but a few. After more than a decade of research and development, suitable deposition and characterization methods, the dependencies of the most important properties on the deposition conditions, and first models of nucleation and growth have been established [1–3]. For obvious reasons (availability, smoothness, suitability for characterization purposes, etc.) most of the work described in the literature on NCD deposition has been performed with silicon substrates but there are notable exceptions. These concern especially application relevant substrates such as SiO2 [7], ⁎ Corresponding author. Tel.: +49 561 804 4205; fax: +49 561 804 4136. E-mail address: [email protected] (C. Popov). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.07.163
quartz [8,9], Si3N4 [10], Mo [11,12], Ti–6A1–4V alloy [12,13], and cemented carbide (WC-Co) [14,15]. However, systematic studies are missing, although from a theoretical point of view experiments with different substrate materials might be of great interest as they can provide further insight into the growth and especially nucleation mechanisms of nanocrystalline diamond films. In this paper we report on the deposition of NCD/a-C nanocomposite films on a variety of substrates such as Si, polycrystalline diamond, cubic boron nitride, TiN and Ti–6Al–4V alloy (the latter might also be of interest for biomedical applications of these films). 2. Experimental NCD/a-C nanocomposite films have been deposited by microwave plasma chemical vapour deposition (MWCVD) from CH4/N2 mixtures. Details of deposition set-up and procedure have been published earlier [16,17]. All experiments described in this paper have been prepared with constant parameters: 17% CH4 in N2 at a pressure of 3.8 kPa and a total
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flow of 100 sccm, a microwave power of 800 W and a substrate temperature TS of 600 °C. In one experiment (see below), TS = 770 °C was used. The deposition time was 390–420 min in all cases. The substrates employed in this investigation are listed in Table 1. Besides polished and unpolished (100) Si wafers, a polycrystalline diamond film (ca. 4 μm thick) deposited by hot filament chemical vapour deposition (HFCVD) from 0.7% CH4 in H2, a cubic boron nitride film gradient system (with a total thickness of about 700 nm), which is described in detail in Ref. [18], a sputtered TiN film (ca. 300 nm), and a piece of Ti–6Al– 4V alloy were used. Some of the substrates have been subjected to an ultrasonic pretreatment with diamond powder (average grain size 250 nm) suspended in n-pentane in order to enhance the nucleation density. All samples discussed in this paper have been characterized by scanning electron microscopy (SEM, morphology, structure and growth rates), X-ray diffraction (XRD, crystallinity), and Raman spectroscopy (bonding environment). Details of the measurements can be found elsewhere [19]. The mechanical properties of selected samples were investigated by nanoindentation and nano scratch tests (CSM Instruments) using the setups and conditions described in Ref. [20]. 3. Brief summary of film properties on Si substrates A set of NCD/a-C samples deposited under identical conditions but at a higher substrate temperature (770 instead of 600 °C) has been characterized comprehensively by a variety of methods besides SEM, XRD and Raman [19,21]. The results of these studies can be summarized as follows: The films consist of diamond nanocrystals of 3–5 nm diameter embedded in an amorphous carbon matrix with a width of 1–1.5 nm, the crystallite/matrix ratio being about unity. The matrix contains sp2 bonded carbon (20–30%) and ca. 20% hydrogen, mostly bonded to sp3 carbon. The nitrogen content is ≤ 1%. Lowering the deposition temperature from 770 to 600 °C has no influence on the crystallite size; however, the crystallite/matrix ratio decreases slightly. Furthermore, the total hydrogen content drops from 10 to ca. 6%, while Raman and Fourier transform infrared spectra indicate certain but still unidentified changes of the bonding environment of the matrix [22].
Table 1 Overview over the substrates used in the present investigation Substrate
PT
nnuc (cm− 2)
Polished Si Polished Si Unpolished Si PCD c-BN TiN TiN Ti–6Al–4V
No Yes Yes No No No Yes Yes
Extremely 2–4 × 108 Very high Extremely Extremely Extremely 1.5 × 109 Very high
dc (nm) H (GPa)
E (GPa) Lc (mN)
low 3–5 high 3.5 high 3.5 low 4
39.7 ± 2.2 387 ± 17 ≥250 33.7 ± 4.1 362 ± 49 37.2 ± 2.8 324 ± 15 135 ± 2 25.8 ± 2.4 209 ± 22 268 ± 5
Also given are some selected film properties. PT = pretreatment; nnuc = nucleation density; dc = crystallite size; H = nanoindentation hardness; E = indentation modulus; Lc = critical load for delamination of the film system.
4. Basic film properties 4.1. SEM Fig. 1a shows a SEM picture of a NCD/a-C film deposited on pretreated silicon at 770 °C. The film (4 μm thick) is continuous, homogeneous and rather flat. In contrast, coatings deposited at 600 °C on Si consist of individual nodules with an average diameter of 2.4 μm and a height of 1.5 μm (Fig. 1b). Further investigations [3,23] have shown, however, that also the homogeneous film in Fig. 1a has initially started to grow from such individual nodules. The difference can be found in the much higher growth rate at 770 °C (10 nm/min instead of 3–4 nm/min) resulting in much thicker films (the deposition times are identical). Thus, the nodules could coalesce to a thick, continuous film. The nodules themselves are a consequence of the rather low nucleation rate of 2–4 × 108 cm− 2 (Table 1) brought about by our pretreatment procedure [3]. On the other hand, on untreated Si wafers almost no nucleation at all was observed. The SEM image of the NCD film grown on a TiN surface in Fig. 1c shows that the film is continuous. But it can also be clearly seen that the growth started again from individual nodules. However, the nodules diameter is smaller (300–400 nm), indicating a higher nucleation density of 1.5 × 109 cm− 2. Thus, for a film thickness of ca. 1 μm, the nodules were able to coalesce to form a continuous coating. From a comparison of this sample and the film shown in Fig. 1b it must be concluded that the result of the substrate pretreatment process depends on the substrate material (on untreated TiN, again almost no nucleation was observed). Fig. 1d shows a NCD film deposited on a HFCVD PCD coating. It is continuous and homogeneous, following the contours of the very rough polycrystalline diamond. No voids can be seen at the interface, which indicates a very high nucleation density. The same holds for the c-BN/NCD system shown in Fig. 1e. Here, the interface (indicated by the mark) is hardly discernable. Fig. 1f, finally, shows a NCD film on a Ti–6Al–4V alloy substrate. Again the film is continuous, but it can also be seen that the Ti alloy substrate is extremely rough. From these results the following conclusions can be drawn concerning the nucleation of nanocrystalline diamond films [3] (see also Table 1): (i) without pretreatment, on flat, smooth surfaces, like polished Si and TiN, almost no nucleation is observed; (ii) substrate pretreatment leads to a drastic increase of the nucleation density although the value of some 108 cm− 2 on Si is rather low as compared to other work (1011 cm− 2 are possible [24]). Moreover the results of the pretreatment procedure depend on the substrate material; (iii) on (poly-) crystalline diamond and the closely related c-BN extremely high densities are observed. Obviously, every point of the surface can act as a nucleation site; iv) on rough surfaces (Si, Ti alloy) the nucleation density is very high. From all these it can be concluded that there are no differences between the nucleation mechanisms of poly- and nanocrystalline diamond: rapid nucleation requires either a pretreatment, very rough surfaces, or the template of a diamond (or related) surface. Thus
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Fig. 1. SEM pictures of NCD/a-C films deposited on (a) pretreated Si at 770 °C; (b) pretreated Si at 600 °C; (c) pretreated TiN; (d) polycrystalline diamond; (e) cubic boron nitride; and (f) pretreated Ti–6Al–4V alloy.
the differences between PCD and NCD deposition have to be found on the growth step as discussed in detail in Ref. [3]. 4.2. XRD XRD measurements have been carried out in grazing angle geometry with Cu Kα radiation (0.15418 nm). Selected results are presented in Fig. 2. From Fig. 2a showing patterns obtained at incident angle φ = 1° of a film on polished and pretreated silicon, together with the JCPDS standard for diamond (card 6675), it can be seen that both, the diamond (111) and the (220) peaks are present, indicating the presence of crystalline diamond. With respect to the powder standard, no deviations can be detected, which means that the films are free of stress and possess no texture. However, the peaks are very broad, indicating the presence of nanocrystalline material; from the FWHM of the (111) peak of this and similar patterns on silicon substrates, by means of the Debye–Scherrer formula a mean crystallite size of 3–5 nm could be derived.
Fig. 2b shows φ = 0.2° patterns of a NCD films deposited on polycrystalline diamond. Again, only the diamond (111) and (220) peaks are present, which are, however, much sharper than the peaks in Fig. 2a. A closer inspection (see the insert bi)) reveals that this peak can be deconvoluted into a narrow peak with a FWHM of 0.25° and a second, much broader one with FWHM = 2.35° at nearly identical positions (2θ = 43.95°). The former can be attributed to the PCD substrate (the FWHM is determined by the resolution of the scan), the latter to the NCD film. From its FWHM a crystallite size of 3.5 nm can be derived. This clear deconvolution of the (111) peak into two distinct contributions is a clear hint at a very sharp transition from the poly- to the nanocrystalline diamond, in agreement with the SEM picture in Fig. 1d. Fig. 2c shows a φ = 0.5° scan of a coating on a c-BN film system; also shown is the JCPDS standard 25–1033 for cubic boron nitride. Compared with diamond, the (111) and (222) peaks of c-BN are shifted to slightly lower 2θ values. However, as can be seen from Fig. 2c, the two peaks of this pattern are
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the TiN (111), (200) and (220) reflections are present as can be seen from the JCPDS standard for TiN also shown in Fig. 2d. Owing to the overlap of the Ti (200) and diamond (111) peaks it is not possible to determine the diamond crystallite size from this scan. However, as the insert (di) shows, for φ = 0.3° the TiN peaks have almost disappeared; from this scan a mean crystallite size of 4 nm can be determined from the FWHM of the D(111) peak. Summarizing, from the XRD measurements shown in Fig. 2 it can be concluded that the different substrates have no influence on the crystalline properties of the NCD films: the crystallite size is on the order of 3–5 nm; the peak positions close to the standard values indicate that no stresses are present, and the ratio I220/I111 also close to the standard value of 0.25 excludes the presence of textures (the deviation from this value for the patterns in Fig. 2b are due to a texture of the underlying HFCVD film). 4.3. Raman spectroscopy
Fig. 2. Normalized grazing angle XRD patterns of NCD films on various substrates: (a) Si, φ = 1°. Also given is the JCPDS standard for diamond (card 6675). (b) PCD substrate, φ = 0.2°. The inset (bi) shows the deconvolution of the (111) peak discussed in the text. (c) cubic boron nitride, φ = 0.5°. Also given is the JCPDS standard for c-BN (card 25-1033). (d) TiN substrate; φ = 0.5°. Also given is the JCPDS standard for TiN (card 38-1420). The insert (di) shows the patterns for φ = 0.3°.
located at the diamond positions, and no shoulders are observed at the positions of the c-BN peaks. The c-BN part of the underlayer is nanocrystalline also and about 150 nm thick [18]; moreover the scattering probability of boron nitride is very low. Thus the underlayer obviously does not contribute to the peaks in Fig. 2c. From the width of the (111) peak again a mean size of the diamond crystallites of 3.5 nm could be obtained. Fig. 2d, finally, shows φ = 0.5° patterns of a NCD film on a sputtered TiN film. Besides the diamond (111) and (220) peaks,
Raman spectra have been recorded with an excitation wavelength of 514 nm [19]. Fig. 3a shows a typical spectrum of a film on a TiN substrate after subtraction of the photoluminescence background. It can be seen that the broad band between 1100 and 1700 cm− 1 can be deconvoluted into four contributions: two of them (the graphite related D and G bands at 1360 and 1562 cm− 1, respectively), can be assigned to sp2 carbon. The other two at 1165 and 1475 cm− 1 are usually ascribed to the presence of nanocrystalline diamond. Although their origin is controversially discussed in the literature (see [19] and the references cited therein), to our knowledge at least the peak at 1165 cm− 1 has never been observed for amorphous carbon films nor for polycrystalline diamond. Thus it can be used as a fingerprint for the presence of NCD. Due to the differences in the sensitivity of sp2 and sp3 carbon towards 514 nm excitation, a quantitative evaluation of such spectra as in Fig. 3a is impossible [19]. Nevertheless, a careful investigation of the spectra of the samples investigated in this study revealed that there were no significant differences between them. In all spectra, the four peaks listed above were present; neither their positions nor their widths nor their relative intensities vary considerably. Thus it can be concluded that the bonding environment (as far as it is accessible by Raman spectroscopy) is not influenced by the substrate material. Fig. 3b shows a series of Raman measurements taken on a cross section of a PCD/NCD bilayer (total thickness ca. 5– 5.5 μm). At a distance x = 5.5 μm from the surface, only the principle diamond line at 1332 cm− 1 is present. The absence of graphite related peaks and of any photoluminescence background proves the good quality of the underlying HFCVD film. The intensity of the diamond line increases with decreasing distance x from the surface. This can be explained by the spot size of 1.2 μm of these measurements: for x = 5.5 μm, the spot is partly on the silicon substrate; the silicon peak at 520 cm− 1 is clearly present in the spectrum (not shown). For x ≤ 1.1 μm, the spot is more and more concentrated on the NCD film; as a consequence, the diamond line decreases, the PL background increases and the
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obtained at 600 °C were investigated. Nevertheless, with the exception of the TiN sample all hardness values fell within a close range; considering the error range, no significant deviation can be detected. The large error observed for the PCD/NCD system can be attributed to the rough surface of this coating (Fig. 1d). The absolute values are considerably lower than those for bulk diamond and NCD films (100 and 70–90 GPa, respectively, see [20] and the reference cited therein); as discussed in [20], this can be attributed to the relatively large volume fraction of the matrix of about 50%. For the TiN substrate, the hardness is considerably lower; this can be explained by the indentation depth of ca. 300 nm, which means that the substrate, which is softer in the case of TiN than the superhard materials diamond and c-BN, contributes to these values. On the other hand, for the measurements with the silicon substrate, the coating was thick enough to exclude a substrate contribution. All these arguments can also be applied to the values of the indentation modulus presented in Table 1. Concerning film adhesion, very good results were obtained for Si and TiN substrates (Table 1). Owing to the rough surface of the PCD/NCD system (Fig. 1d), scratch tests were difficult to perform. The low critical load for delamination of the c-BN/NCD system can be explained by the rather poor adhesion of c-BN films on silicon substrates [18,25]. 6. Summary
Fig. 3. (a) Raman spectrum of a NCD/a-C nanocomposite film on a TiN substrate after subtraction of the photoluminescence background. (b) Raman spectra performed on a cross-section of a PCD/NCD bilayer. The spectra are shown as measured but shifted along the y-axis for clarity. Parameter is the distance from the surface.
Nanocrystalline diamond/amorphous carbon nanocomposite films have been deposited on a variety of substrates which may be of technological importance such as Si, PCD, c-BN, TiN, and Ti–6Al–4V alloy. The chemical and morphological nature of the substrate has a strong influence on the nucleation process; to achieve high densities on smooth surfaces, either a pretreatment is required or the presence of diamond or diamond-related material. The growth process, on the other hand, is not affected by the substrate; no significant changes of the crystallinity and the bonding environment could be detected by XRD and Raman spectroscopy. The mechanical and tribological properties, finally, are determined by the entire system substrate/underlayer/ NCD film; failure is determined by the weakest link, e.g. the poor adhesion between Si substrate and c-BN layer. Acknowledgements
four peaks typical for NCD as discussed above become more and more pronounced (as a consequence of the roughness of the PCD underlayer (Fig. 1d), the diamond line at 1332 cm− 1 has not vanished completely for the x = 0 μm spectrum). 5. Mechanical properties For selected substrate materials, the mechanical and tribological properties of the NCD films have been investigated by nanoindentation and nano scratch test measurements (CSM Instruments), the results of which are summarized in Table 1. When comparing these results it must be kept in mind that the data for silicon stem from a 4 μm thick coating deposited at 770 °C while for the other substrates ca. 1 μm thick films
This research was supported by the Marie-Curie EIF within the 6th EC Framework Programme (MEIF-CT-2004-500038) and the Academy of Sciences of the Czech Republic (Research Project AVOZ10100520). The authors gratefully acknowledge the financial support of the NATO under the Collaborative Linkage Grant Program (CBP.EAP.CLG 981519). References [1] D.M. Gruen, Annu. Rev. Mater. Sci. 29 (1999) 211. [2] K. Okada, in: H.S. Nalwa (Ed.), Encyclopedia of Nanoscience and Nanotechnology, vol. 7, American Science Publishers, Stevenson Ranch, Cal., 2004, p. 691. [3] W. Kulisch, C. Popov, Phys. Status Solidi, A Appl. Res. 203 (2006) 203.
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[4] O. Auciello, J. Birrell, J.A. Carlisle, J.E. Gerbi, X. Xiao, B. Peng, H.D. Espinosa, J. Phys., Condens. Mater. 16 (2004) R539. [5] J.A. Carlisle, O. Auciello, Electrochem. Soc. Interface 12 (2003) 28. [6] W. Kulisch, in: R. Kassing, P. Petkov, W. Kulisch, C. Popov (Eds.), Functional Properties of Nanostructured Materials, Springer, Berlin, 2006, p. 461. [7] S. Bhattacharyya, O. Auciello, J. Birrell, J.A. Carlisle, L.A. Curtiss, A.N. Goyette, D.M. Gruen, A.R. Kraus, J. Schlueter, A. Sumant, P. Zapol, Appl. Phys. Lett. 79 (2001) 1441. [8] D.M. Bhusari, J.R. Yang, T.Y. Wang, K.H. Chen, S.T. Lin, L.C. Chen, Mater. Lett. 36 (1998) 279. [9] W.B. Yang, X.F. Lü, Z.X. Cao, J. Appl. Phys. 91 (2002) 10068. [10] M. Amaral, F.J. Oliveira, N. Belmonte, A.J.S. Fernandes, F.M. Costa, R.F. Silva, Diamond Relat. Mater. 13 (2004) 643. [11] K. Wu, E.G. Wang, J. Chen, N.S. Xu, J. Vac. Sci. Technol., B 17 (1999) 1059. [12] S.A. Catlegde, Y.K. Vohra, J. Appl. Phys. 84 (1998) 6469. [13] M.J. Papo, S.A. Catledge, Y.K. Vohra, C. Machado, J. Mater. Sci., Mater. Med. 15 (2004) 773. [14] S. Hogmark, P. Hollman, A. Alahelisten, P. Hedenquist, Wear 200 (1996) 225. [15] S. Takeuchi, S. Oda, M. Murakawa, Thin Solid Films 388/389 (2001) 238.
[16] C. Popov, W. Kulisch, in: M. Allendorf, F. Maury, F. Teyssandier (Eds.), Proceedings of the CVD XVI and EUROCVD-14 Conference, vol. 2003-08, The Electrochemical Society, Pennington, N.J., 2003, p. 1079. [17] C. Popov, M. Novotny, M. Jelinek, S. Boycheva, V. Vorlicek, M. Trchova, W. Kulisch, Thin Solid Films 506–507 (2006) 297. [18] R. Freudenstein, W. Kulisch, Thin Solid Films 410/421 (2002) 132. [19] C. Popov, W. Kulisch, P.N. Gibson, G. Ceccone, M. Jelinek, Diamond Relat. Mater. 13 (2004) 1371. [20] W. Kulisch, C. Popov, S. Boycheva, L. Buforn, G. Favaro, N. Conte, Diamond Relat. Mater. 13 (2004) 1997. [21] C. Popov, W. Kulisch, S. Boycheva, K. Yamamoto, G. Ceccone, Y. Koga, Diamond Relat. Mater. 13 (2004) 2071. [22] W. Kulisch, C. Popov, S. Boycheva, M. Jelinek, P.N. Gibson, V. Vorlicek, Surf. Coat. Technol. 200 (2006) 4731. [23] W. Kulisch, C. Popov, S. Boycheva, G. Beshkov, V. Vorlicek, P.N. Gibson, G. Georgiev, Thin Solid Films 469–470 (2004) 735. [24] R. Erz, W. Dötter, K. Jung, H. Ehrhardt, Diamond Relat. Mater. 2 (1993) 449. [25] W. Kulisch, R. Freudenstein, A. Klett, M.F. Plass, Thin Solid Films 377–378 (2000) 170.
Nanocrystalline diamond growth on different substrates W. Kulisch a , C. Popov a,⁎, V. Vorlicek b , P.N. Gibson c , G. Favaro d a
Institute of Nanostructure Technologies and Analytics (INA), University of Kassel, Germany Institute of Physics, Academy of Sciences of the Czech Republic, Prague, Czech Republic c Institute for Health and Consumer Protection, Joint Research Center, Ispra, Italy d CSM Instruments SA, Peseux, Switzerland
b
Available online 11 September 2006
Abstract Nanocomposite films consisting of diamond nanoparticles of 3–5 nm diameter embedded in an amorphous carbon matrix have been deposited by means of microwave plasma chemical vapour deposition (MWCVD) from CH4/N2 gas mixtures. Si wafers, Si coated with TiN, polycrystalline diamond (PCD) and cubic boron nitride films, and Ti–6Al–4V alloy have been used as substrates. Some of the substrates have been pretreated ultrasonically with diamond powder in order to enhance the nucleation density nnuc. It turned out that nnuc depends critically on the chemical nature of the substrate, its smoothness and the pretreatment applied. No differences to the nucleation behaviour of CVD PCD films were observed. On the other hand, the growth process seems to be not affected by the substrate material. The crystallinity (studied by X-ray diffraction) and the bonding environment (investigated by Raman spectroscopy) show no significant differences for the various substrates. The mechanical and tribological properties, finally, reflect again the influence of the substrate material: on TiN, a lower hardness was measured as compared to Si, PCD and c-BN, whereas the adhesion of c-BN/nanocrystalline diamond (NCD) system was determined by that of the c-BN film on the underlying Si substrate. © 2006 Elsevier B.V. All rights reserved. Keywords: Nanocrystalline diamond; Growth mechanisms; Nucleation mechanisms; Mechanical properties
1. Introduction Nanocrystalline diamond (NCD) films [1–3], either in pure form or as NCD/a-C nanocomposites, have recently attracted considerable interest as they combine (to a large extent) the extreme properties of diamond with very smooth surfaces, which makes them a candidate for many applications such as tribology, optics, microelectromechanical systems (MEMS) [4], and biomedicine [5,6] to name but a few. After more than a decade of research and development, suitable deposition and characterization methods, the dependencies of the most important properties on the deposition conditions, and first models of nucleation and growth have been established [1–3]. For obvious reasons (availability, smoothness, suitability for characterization purposes, etc.) most of the work described in the literature on NCD deposition has been performed with silicon substrates but there are notable exceptions. These concern especially application relevant substrates such as SiO2 [7], ⁎ Corresponding author. Tel.: +49 561 804 4205; fax: +49 561 804 4136. E-mail address: [email protected] (C. Popov). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.07.163
quartz [8,9], Si3N4 [10], Mo [11,12], Ti–6A1–4V alloy [12,13], and cemented carbide (WC-Co) [14,15]. However, systematic studies are missing, although from a theoretical point of view experiments with different substrate materials might be of great interest as they can provide further insight into the growth and especially nucleation mechanisms of nanocrystalline diamond films. In this paper we report on the deposition of NCD/a-C nanocomposite films on a variety of substrates such as Si, polycrystalline diamond, cubic boron nitride, TiN and Ti–6Al–4V alloy (the latter might also be of interest for biomedical applications of these films). 2. Experimental NCD/a-C nanocomposite films have been deposited by microwave plasma chemical vapour deposition (MWCVD) from CH4/N2 mixtures. Details of deposition set-up and procedure have been published earlier [16,17]. All experiments described in this paper have been prepared with constant parameters: 17% CH4 in N2 at a pressure of 3.8 kPa and a total
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flow of 100 sccm, a microwave power of 800 W and a substrate temperature TS of 600 °C. In one experiment (see below), TS = 770 °C was used. The deposition time was 390–420 min in all cases. The substrates employed in this investigation are listed in Table 1. Besides polished and unpolished (100) Si wafers, a polycrystalline diamond film (ca. 4 μm thick) deposited by hot filament chemical vapour deposition (HFCVD) from 0.7% CH4 in H2, a cubic boron nitride film gradient system (with a total thickness of about 700 nm), which is described in detail in Ref. [18], a sputtered TiN film (ca. 300 nm), and a piece of Ti–6Al– 4V alloy were used. Some of the substrates have been subjected to an ultrasonic pretreatment with diamond powder (average grain size 250 nm) suspended in n-pentane in order to enhance the nucleation density. All samples discussed in this paper have been characterized by scanning electron microscopy (SEM, morphology, structure and growth rates), X-ray diffraction (XRD, crystallinity), and Raman spectroscopy (bonding environment). Details of the measurements can be found elsewhere [19]. The mechanical properties of selected samples were investigated by nanoindentation and nano scratch tests (CSM Instruments) using the setups and conditions described in Ref. [20]. 3. Brief summary of film properties on Si substrates A set of NCD/a-C samples deposited under identical conditions but at a higher substrate temperature (770 instead of 600 °C) has been characterized comprehensively by a variety of methods besides SEM, XRD and Raman [19,21]. The results of these studies can be summarized as follows: The films consist of diamond nanocrystals of 3–5 nm diameter embedded in an amorphous carbon matrix with a width of 1–1.5 nm, the crystallite/matrix ratio being about unity. The matrix contains sp2 bonded carbon (20–30%) and ca. 20% hydrogen, mostly bonded to sp3 carbon. The nitrogen content is ≤ 1%. Lowering the deposition temperature from 770 to 600 °C has no influence on the crystallite size; however, the crystallite/matrix ratio decreases slightly. Furthermore, the total hydrogen content drops from 10 to ca. 6%, while Raman and Fourier transform infrared spectra indicate certain but still unidentified changes of the bonding environment of the matrix [22].
Table 1 Overview over the substrates used in the present investigation Substrate
PT
nnuc (cm− 2)
Polished Si Polished Si Unpolished Si PCD c-BN TiN TiN Ti–6Al–4V
No Yes Yes No No No Yes Yes
Extremely 2–4 × 108 Very high Extremely Extremely Extremely 1.5 × 109 Very high
dc (nm) H (GPa)
E (GPa) Lc (mN)
low 3–5 high 3.5 high 3.5 low 4
39.7 ± 2.2 387 ± 17 ≥250 33.7 ± 4.1 362 ± 49 37.2 ± 2.8 324 ± 15 135 ± 2 25.8 ± 2.4 209 ± 22 268 ± 5
Also given are some selected film properties. PT = pretreatment; nnuc = nucleation density; dc = crystallite size; H = nanoindentation hardness; E = indentation modulus; Lc = critical load for delamination of the film system.
4. Basic film properties 4.1. SEM Fig. 1a shows a SEM picture of a NCD/a-C film deposited on pretreated silicon at 770 °C. The film (4 μm thick) is continuous, homogeneous and rather flat. In contrast, coatings deposited at 600 °C on Si consist of individual nodules with an average diameter of 2.4 μm and a height of 1.5 μm (Fig. 1b). Further investigations [3,23] have shown, however, that also the homogeneous film in Fig. 1a has initially started to grow from such individual nodules. The difference can be found in the much higher growth rate at 770 °C (10 nm/min instead of 3–4 nm/min) resulting in much thicker films (the deposition times are identical). Thus, the nodules could coalesce to a thick, continuous film. The nodules themselves are a consequence of the rather low nucleation rate of 2–4 × 108 cm− 2 (Table 1) brought about by our pretreatment procedure [3]. On the other hand, on untreated Si wafers almost no nucleation at all was observed. The SEM image of the NCD film grown on a TiN surface in Fig. 1c shows that the film is continuous. But it can also be clearly seen that the growth started again from individual nodules. However, the nodules diameter is smaller (300–400 nm), indicating a higher nucleation density of 1.5 × 109 cm− 2. Thus, for a film thickness of ca. 1 μm, the nodules were able to coalesce to form a continuous coating. From a comparison of this sample and the film shown in Fig. 1b it must be concluded that the result of the substrate pretreatment process depends on the substrate material (on untreated TiN, again almost no nucleation was observed). Fig. 1d shows a NCD film deposited on a HFCVD PCD coating. It is continuous and homogeneous, following the contours of the very rough polycrystalline diamond. No voids can be seen at the interface, which indicates a very high nucleation density. The same holds for the c-BN/NCD system shown in Fig. 1e. Here, the interface (indicated by the mark) is hardly discernable. Fig. 1f, finally, shows a NCD film on a Ti–6Al–4V alloy substrate. Again the film is continuous, but it can also be seen that the Ti alloy substrate is extremely rough. From these results the following conclusions can be drawn concerning the nucleation of nanocrystalline diamond films [3] (see also Table 1): (i) without pretreatment, on flat, smooth surfaces, like polished Si and TiN, almost no nucleation is observed; (ii) substrate pretreatment leads to a drastic increase of the nucleation density although the value of some 108 cm− 2 on Si is rather low as compared to other work (1011 cm− 2 are possible [24]). Moreover the results of the pretreatment procedure depend on the substrate material; (iii) on (poly-) crystalline diamond and the closely related c-BN extremely high densities are observed. Obviously, every point of the surface can act as a nucleation site; iv) on rough surfaces (Si, Ti alloy) the nucleation density is very high. From all these it can be concluded that there are no differences between the nucleation mechanisms of poly- and nanocrystalline diamond: rapid nucleation requires either a pretreatment, very rough surfaces, or the template of a diamond (or related) surface. Thus
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Fig. 1. SEM pictures of NCD/a-C films deposited on (a) pretreated Si at 770 °C; (b) pretreated Si at 600 °C; (c) pretreated TiN; (d) polycrystalline diamond; (e) cubic boron nitride; and (f) pretreated Ti–6Al–4V alloy.
the differences between PCD and NCD deposition have to be found on the growth step as discussed in detail in Ref. [3]. 4.2. XRD XRD measurements have been carried out in grazing angle geometry with Cu Kα radiation (0.15418 nm). Selected results are presented in Fig. 2. From Fig. 2a showing patterns obtained at incident angle φ = 1° of a film on polished and pretreated silicon, together with the JCPDS standard for diamond (card 6675), it can be seen that both, the diamond (111) and the (220) peaks are present, indicating the presence of crystalline diamond. With respect to the powder standard, no deviations can be detected, which means that the films are free of stress and possess no texture. However, the peaks are very broad, indicating the presence of nanocrystalline material; from the FWHM of the (111) peak of this and similar patterns on silicon substrates, by means of the Debye–Scherrer formula a mean crystallite size of 3–5 nm could be derived.
Fig. 2b shows φ = 0.2° patterns of a NCD films deposited on polycrystalline diamond. Again, only the diamond (111) and (220) peaks are present, which are, however, much sharper than the peaks in Fig. 2a. A closer inspection (see the insert bi)) reveals that this peak can be deconvoluted into a narrow peak with a FWHM of 0.25° and a second, much broader one with FWHM = 2.35° at nearly identical positions (2θ = 43.95°). The former can be attributed to the PCD substrate (the FWHM is determined by the resolution of the scan), the latter to the NCD film. From its FWHM a crystallite size of 3.5 nm can be derived. This clear deconvolution of the (111) peak into two distinct contributions is a clear hint at a very sharp transition from the poly- to the nanocrystalline diamond, in agreement with the SEM picture in Fig. 1d. Fig. 2c shows a φ = 0.5° scan of a coating on a c-BN film system; also shown is the JCPDS standard 25–1033 for cubic boron nitride. Compared with diamond, the (111) and (222) peaks of c-BN are shifted to slightly lower 2θ values. However, as can be seen from Fig. 2c, the two peaks of this pattern are
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the TiN (111), (200) and (220) reflections are present as can be seen from the JCPDS standard for TiN also shown in Fig. 2d. Owing to the overlap of the Ti (200) and diamond (111) peaks it is not possible to determine the diamond crystallite size from this scan. However, as the insert (di) shows, for φ = 0.3° the TiN peaks have almost disappeared; from this scan a mean crystallite size of 4 nm can be determined from the FWHM of the D(111) peak. Summarizing, from the XRD measurements shown in Fig. 2 it can be concluded that the different substrates have no influence on the crystalline properties of the NCD films: the crystallite size is on the order of 3–5 nm; the peak positions close to the standard values indicate that no stresses are present, and the ratio I220/I111 also close to the standard value of 0.25 excludes the presence of textures (the deviation from this value for the patterns in Fig. 2b are due to a texture of the underlying HFCVD film). 4.3. Raman spectroscopy
Fig. 2. Normalized grazing angle XRD patterns of NCD films on various substrates: (a) Si, φ = 1°. Also given is the JCPDS standard for diamond (card 6675). (b) PCD substrate, φ = 0.2°. The inset (bi) shows the deconvolution of the (111) peak discussed in the text. (c) cubic boron nitride, φ = 0.5°. Also given is the JCPDS standard for c-BN (card 25-1033). (d) TiN substrate; φ = 0.5°. Also given is the JCPDS standard for TiN (card 38-1420). The insert (di) shows the patterns for φ = 0.3°.
located at the diamond positions, and no shoulders are observed at the positions of the c-BN peaks. The c-BN part of the underlayer is nanocrystalline also and about 150 nm thick [18]; moreover the scattering probability of boron nitride is very low. Thus the underlayer obviously does not contribute to the peaks in Fig. 2c. From the width of the (111) peak again a mean size of the diamond crystallites of 3.5 nm could be obtained. Fig. 2d, finally, shows φ = 0.5° patterns of a NCD film on a sputtered TiN film. Besides the diamond (111) and (220) peaks,
Raman spectra have been recorded with an excitation wavelength of 514 nm [19]. Fig. 3a shows a typical spectrum of a film on a TiN substrate after subtraction of the photoluminescence background. It can be seen that the broad band between 1100 and 1700 cm− 1 can be deconvoluted into four contributions: two of them (the graphite related D and G bands at 1360 and 1562 cm− 1, respectively), can be assigned to sp2 carbon. The other two at 1165 and 1475 cm− 1 are usually ascribed to the presence of nanocrystalline diamond. Although their origin is controversially discussed in the literature (see [19] and the references cited therein), to our knowledge at least the peak at 1165 cm− 1 has never been observed for amorphous carbon films nor for polycrystalline diamond. Thus it can be used as a fingerprint for the presence of NCD. Due to the differences in the sensitivity of sp2 and sp3 carbon towards 514 nm excitation, a quantitative evaluation of such spectra as in Fig. 3a is impossible [19]. Nevertheless, a careful investigation of the spectra of the samples investigated in this study revealed that there were no significant differences between them. In all spectra, the four peaks listed above were present; neither their positions nor their widths nor their relative intensities vary considerably. Thus it can be concluded that the bonding environment (as far as it is accessible by Raman spectroscopy) is not influenced by the substrate material. Fig. 3b shows a series of Raman measurements taken on a cross section of a PCD/NCD bilayer (total thickness ca. 5– 5.5 μm). At a distance x = 5.5 μm from the surface, only the principle diamond line at 1332 cm− 1 is present. The absence of graphite related peaks and of any photoluminescence background proves the good quality of the underlying HFCVD film. The intensity of the diamond line increases with decreasing distance x from the surface. This can be explained by the spot size of 1.2 μm of these measurements: for x = 5.5 μm, the spot is partly on the silicon substrate; the silicon peak at 520 cm− 1 is clearly present in the spectrum (not shown). For x ≤ 1.1 μm, the spot is more and more concentrated on the NCD film; as a consequence, the diamond line decreases, the PL background increases and the
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obtained at 600 °C were investigated. Nevertheless, with the exception of the TiN sample all hardness values fell within a close range; considering the error range, no significant deviation can be detected. The large error observed for the PCD/NCD system can be attributed to the rough surface of this coating (Fig. 1d). The absolute values are considerably lower than those for bulk diamond and NCD films (100 and 70–90 GPa, respectively, see [20] and the reference cited therein); as discussed in [20], this can be attributed to the relatively large volume fraction of the matrix of about 50%. For the TiN substrate, the hardness is considerably lower; this can be explained by the indentation depth of ca. 300 nm, which means that the substrate, which is softer in the case of TiN than the superhard materials diamond and c-BN, contributes to these values. On the other hand, for the measurements with the silicon substrate, the coating was thick enough to exclude a substrate contribution. All these arguments can also be applied to the values of the indentation modulus presented in Table 1. Concerning film adhesion, very good results were obtained for Si and TiN substrates (Table 1). Owing to the rough surface of the PCD/NCD system (Fig. 1d), scratch tests were difficult to perform. The low critical load for delamination of the c-BN/NCD system can be explained by the rather poor adhesion of c-BN films on silicon substrates [18,25]. 6. Summary
Fig. 3. (a) Raman spectrum of a NCD/a-C nanocomposite film on a TiN substrate after subtraction of the photoluminescence background. (b) Raman spectra performed on a cross-section of a PCD/NCD bilayer. The spectra are shown as measured but shifted along the y-axis for clarity. Parameter is the distance from the surface.
Nanocrystalline diamond/amorphous carbon nanocomposite films have been deposited on a variety of substrates which may be of technological importance such as Si, PCD, c-BN, TiN, and Ti–6Al–4V alloy. The chemical and morphological nature of the substrate has a strong influence on the nucleation process; to achieve high densities on smooth surfaces, either a pretreatment is required or the presence of diamond or diamond-related material. The growth process, on the other hand, is not affected by the substrate; no significant changes of the crystallinity and the bonding environment could be detected by XRD and Raman spectroscopy. The mechanical and tribological properties, finally, are determined by the entire system substrate/underlayer/ NCD film; failure is determined by the weakest link, e.g. the poor adhesion between Si substrate and c-BN layer. Acknowledgements
four peaks typical for NCD as discussed above become more and more pronounced (as a consequence of the roughness of the PCD underlayer (Fig. 1d), the diamond line at 1332 cm− 1 has not vanished completely for the x = 0 μm spectrum). 5. Mechanical properties For selected substrate materials, the mechanical and tribological properties of the NCD films have been investigated by nanoindentation and nano scratch test measurements (CSM Instruments), the results of which are summarized in Table 1. When comparing these results it must be kept in mind that the data for silicon stem from a 4 μm thick coating deposited at 770 °C while for the other substrates ca. 1 μm thick films
This research was supported by the Marie-Curie EIF within the 6th EC Framework Programme (MEIF-CT-2004-500038) and the Academy of Sciences of the Czech Republic (Research Project AVOZ10100520). The authors gratefully acknowledge the financial support of the NATO under the Collaborative Linkage Grant Program (CBP.EAP.CLG 981519). References [1] D.M. Gruen, Annu. Rev. Mater. Sci. 29 (1999) 211. [2] K. Okada, in: H.S. Nalwa (Ed.), Encyclopedia of Nanoscience and Nanotechnology, vol. 7, American Science Publishers, Stevenson Ranch, Cal., 2004, p. 691. [3] W. Kulisch, C. Popov, Phys. Status Solidi, A Appl. Res. 203 (2006) 203.
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