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Effect of deposition conditions on the chemical bonding in sputtered carbon nitride films
RE TD iJiv TE IALS
Diamond and Related Materials 5 (1996) 163-168
Effect of deposition conditions on the chemical bonding in sputtered carbon nitride films N. Ax&, G.A. Botton, R.E. Somekh, I.M. Hutchings Unit’ersity of Cambridge, Department of Materials Science and Metallurgy, Pembroke Street, Cambridge CB2 3QZ, UK Received 8 August 1995; accepted
in final form 21 September
1995
Abstract A balanced planar r.f. powered magnetron sputter source has been used to deposit carbon nitride films from a graphite target under various conditions. Sample temperature, bias voltage and nitrogen content in the gas mixture were varied. The effects of oxygen, methane and ammonia on the film growth were also studied. Special attention was paid to the effects of the deposition parameters on the structure of the films, in particular the hybridisation of the carbon and nitrogen bonding. The chemical bonding of the carbon and nitrogen atoms was studied by electron energy loss spectroscopy (EELS). The chemical composition was evaluated by Rutherford back-scattering. The intensity of transitions to n: antibonding orbitals, as revealed by EELS, was found to increase with the nitrogen content in the films. Ion bombardment of the films during growth and the addition of oxygen or hydrogen-rich gases further increased the proportion of 7c bonds of both the carbon and nitrogen atoms. It is suggested that the increase in the transitions to p antibond orbitals is to be explained by increased sp2 or possibly sp hybridisation of the carbon and nitrogen. Also, the effect of annealing on the bonding of nitrogen rich films after deposition was tested. The changes caused by nitrogen and deposition conditions are consistent with previous reports on the formation of paracyanogen structures. Keywords: Thin films; Carbon nitride; Sputter deposition; Electron energy loss spectroscopy
1. Introduction There is a continuous search for better tribological materials which combine high hardness with toughness, are chemically stable and exhibit a low friction against common counter-materials such as steel. There has been little theoretical work on the prediction of such properties. Liu and Cohen, however, developed a model to predict the bulk modulus of tetrahedrally bonded solids [1,2]. Their model has given results in good agreement with experimental data for several solids. They have also used the model to predict values for novel materials, such as C,N, with a structure analogous to /?-S&N,; this material is predicted to have a bulk modulus at least as high as that of diamond. Although the incorporation of nitrogen into solid carbon (both amorphous carbon and diamond) had already been tried by several investigators, often in small amounts to investigate the effect of nitrogen doping [3] but also in ratios up to about 1: 1 [4], the publications of Liu and Cohen initialised many attempts to specifically synthesise C3N,. The first attempts to form this potentially hard material 0925-9635/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved SSDI
0925-9635(95)00343-6
by a conventional sputter route were reported in 1992 [ 5,6]. By now a large number of the more readily available standard methods for the deposition of nitrides have been tested. The synthesis of covalently bonded and very hard C3N, does not seem to be easily achieved. However, some investigators have reported crystallites, with still uncertain structure, in nitrogen-rich carbon films [6,7] and the possibility of producing crystalline C,N4 by depositing carbon nitride as an interlayer between other nitrides with similar structures is currently being evaluated [S]. Despite the lack of progress in achieving superhard C3N4, other interesting properties have been found in the carbon nitride materials which have been synthesised. The very high hardness values reported in early works may be due to unusually high elastic recovery of the films after hardness indentation. There are now several reports confirming that the addition of 30-50 at.% nitrogen to amorphous carbon films changes their mechanical properties, and that the materials exhibit high elasticity in low load indentation tests [ 5,7,9,10]. For low indenta-
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tion loads (< 10 gf) the hardness indentations often cannot be found at all after unloading. Amorphous and partially crystalline carbon films have found application as tribological films, often showing very low friction values in combination with relatively high hardness. There are reports indicating that the addition of nitrogen to the amorphous structure might improve the friction properties further [11,12]. A low friction material with high elasticity could be the basis for a promising tribological material, even if it is not superhard. The bonding state of amorphous carbon has been extensively studied [ 12-151. Carbon is unique among the Group IV elements in its ability to form several different types of bonds. These correspond to different orbital configurations often denoted as sp, sp2 and sp3. Carbon also forms amorphous and poorly crystalline structures. It has been shown that the bonding hybridisation in amorphous carbon films can be either sp2 or sp3 dominated, depending on how the films are manufactured [ 131. However, the correlation between deposition parameters and bonding state in the films is only poorly understood. The addition of nitrogen results in an even more complex system. In this work carbon nitride films have been deposited from a graphitic target using an r.f. powered sputter apparatus with a balanced planar magnetron. The effects of a variety of sputter gas mixtures and heat treatments on the variations in hybridisation of the carbon and nitrogen have been evaluated by means of EELS and TEM.
2.
method
Carbon nitride films were deposited with an r.f. powered planar balanced magnetron sputtering system. A magnetron power of 100 W was used on a magnetron with dimensions of 35 x 55 mm (racetrack area% 300 mm2) placed 35 mm from the substrates to be coated. The magnetron was covered with a high purity graphite target. Before each deposition run the vacuum system was pumped to below lop8 mbar until the leakage rate fell below lo-’ mbar s-l, which was normally achieved after a few hours of pumping if the system had not been exposed to air for more than a few minutes. The deposition took place at a chamber pressure of 2.0 Pa, as measured with a Baratron gauge. The deposition temperature was measured with thermocouples on the substrate holder. Most films were deposited on sodium chloride crystals at temperatures below 300 “C. Higher temperatures were not used for deposition on NaCl since biased films showed traces of chloride contamination at this temperature. The salt substrates were subsequently dissolved in water and the films, being about 50 nm thick, collected on copper grids. For higher deposition or annealing
5 (1996) 163-168
temperatures (up to 1OOO’C)R-plane sapphire substrates were used. These substrates were ultrasonically cleaned in chloroform, acetone and subsequently iso-2-propanol before deposition of about 1 pm thick films (1 h deposition time). Films deposited at high temperatures were detached mechanically from the sapphire substrates and collected on lacey carbon-copper grids for transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) examination. The structure and chemical bonding in the films were investigated by means of TEM on a Philips CM30 at 100 kV, and EELS with a parallel Gatan detector (model 666). Under typical operation conditions, an energy resolution better than 1 eV (as determined by the full width of half maximum of the no-loss peak) was achieved. Spectra were collected in image mode without any objective aperture so as to integrate the energy loss angular distribution over all scattering angles. In order to assess any effect of possible film anisotropy in the spectra, angular collector conditions were varied (e.g. to diffraction mode with a small collection angle). These collection methods yielded comparable spectra for all films, except for some large grain graphitic films, which showed anisotropic behaviour in the energy loss. Spectra were corrected for detector gain changes by acquiring spectra shifted in energy [ 161. As the films varied in thickness, comparisons were carried out after normalising the spectra in an energy range about 10 eV above the threshold of the edges. This interval is less sensitive to effects due to multiple inelastic scattering caused by film thickness differences. Numerical deconvolution of the multiple inelastic effects using logarithmic techniques [ 171 was not attempted in this work, as quantitative assessment of the hybridisation fraction is complicated by the effects of film composition changes on the transition probability and consequently on the detailed band structure of the materials [ 181. In this work, therefore, only qualitative variations in the near edge structure features were compared. The sputter gas was an argon-nitrogen mixture, ranging from pure argon to pure nitrogen, pre-mixed before admission to the sputter system. The effects of oxygen, methane and ammonia in the sputter gas, as well as sample bias voltages of up to 300 V, were also evaluated. Oxygen and methane (or other hydrocarbons) are used as graphite etching gases in CVD deposition of diamond. Biasing the substrates during deposition leads to an increased ion bombardment of the growing films, which is known to facilitate the formation of other nitrides (such as TIN) at lower substrate temperatures compared to without bias. The chemical composition of the films was studied by Rutherford backscattering (RBS) and confirmed with EELS. With RBS the nitrogen content of the films was measured with an accuracy of better than 3% of the quoted value.
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165
3. Results 3.1. Nitrogen content of thejlms and its effect on the bonding The nitrogen content of the films increased with the amount of nitrogen in the sputter gas, saturating at around 5OYP70% nitrogen in a N,/Ar mixture. Films containing ca. 57 at.% nitrogen (corresponding to the C,N, stoichiometry), as shown by RBS measurements, can be achieved with the system [lo]. As the substrate temperature during deposition reaches ca. 800 “C, the nitrogen content decreases and at the same time the films start to crystallise into graphitic precipitates in an amorphous carbon nitride matrix. At 1000 “C the nitrogen content has decreased essentially to zero (Fig. 1). At high deposition temperatures (> 800 “C) platinum impurities from the heating stage were found in the films. Transmission electron microscopy revealed most of the films, whether nitrogen-containing or pure carbon, to be amorphous. Only the nitrogen-free films formed at temperatures above 900 “C were found to contain crystallites, < 10 nm in size. Electron diffraction and EELS showed these crystallites to be graphitic in structure. For the amorphous films, differences in the diffraction patterns as a function of nitrogen content could not be detected. The EELS spectra of amorphous pure carbon films show lower X* peaks at 284 eV than for the polycrystalline graphitic films. This is shown in Fig. 2(a), in which an EELS spectrum of a film deposited at 1000 “C with graphitic precipitates is compared with spectra for films with different nitrogen contents. The graphitic film showed strong rc* and (T*peaks typical of single crystals of graphite. As nitrogen was added to the sputter gas mixture the Z* peak at the carbon K edge increased in intensity. The height of the n* peak increased continuously with the nitrogen content. Whether this effect can
loo
L
.z
’ ~
z
I
0
0
200
.-,
400
Substrate
600 temperature,
800
,‘i loo0
“C
Fig. 1. Nitrogen content in the films as determined by RBS, as a function of the substrate holder temperature for film deposited with 100 W r.f. magnetron power at a gas pressure of 2.0 Pa.
Energy Loss (eV)
(4
F
390 (b)
400
410
420
430
440
450
Energy Loss (eV)
Fig. 2. EELS spectra from carbon and carbon nitride films deposited at 200 “C on NaCl substrates from a gas mixture containing different amounts of nitrogen. (a) Carbon K edges of films made with O%, 50% and 100% nitrogen in argon, together with a film deposited at 900 “C containing graphitic crystallites; (b) nitrogen K edges of films made with 25%, 50% and 100% nitrogen in argon.
be attributed to an increase of sp’ hybridisation or is caused by changes in the transition probability (significant changes in the chemical composition of the films and in the long range order would also change the scattering intensity) is not clear. For films deposited in pure nitrogen gas the n* peak is still lower than for the fully sp2 hybridised film with graphitic precipitates. The increased intensity of the Z* peak and the observation of a similar near edge structure in the N K edge (Fig. 2(b)) indicates that the introduction of nitrogen leads to the formation of C=N double bonds. The formation of double bonds between nitrogen atoms (N=N), as in molecular nitrogen (N2) trapped in the films, can be excluded, since they would lead to a nitrogen edge with a different fine structure from that seen in Fig. 2(b) [ 191. The difference in the energy loss
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distribution at energies lo-15 eV above the g* peaks are caused by film thickness differences influencing the electron energy loss, as a result of multiple inelastic scattering effects as discussed in Section 2. Annealing of nitrogen-rich samples after deposition was also carried out in order to promote crystallisation. The samples were held at temperatures between 800 and 1000 “C in the platinum heating stage in the sputter chamber for times ranging from 15 min to 2 h at a nitrogen pressure of 50 Pa. The nitrogen content was then estimated and the changes in bonding was studied with EELS. It was found that annealing under these conditions caused the nitrogen to leave the samples and was accompanied by graphitisation of the films. Annealing the films had a minimal effect on the carbon hybridisation before graphitisation, as deduced from the EELS spectra. 3.2. EfSect of bias and gas composition
Energy Loss
(4
(eV)
on structure and
bonding
Both the carbon and nitrogen K edges in the EELS spectra were very sensitive to the substrate bias voltage. A bias voltage of - 100 V had a slight effect, whereas -200 V substantially increased the height of the n* peaks for both carbon and nitrogen relative to the edges, as seen in Figs. 3(a) and 3(b). This indicates an increased sp’ hybridisation of both the carbon and nitrogen atoms, since no changes in the chemical composition or structure were observed. The X* peak for carbon is, however, still lower than for the graphitic film and increasing the substrate bias did not result in any crystallinity. Bias voltages reduced the deposition rate, being as low as about 50 nm h-’ for -300 V bias. Such films were polluted with sodium and chlorine from the substrates. The effects of oxygen, methane and ammonia on the formation of carbon nitride films were also investigated. All these gases were added to an argon-nitrogen (ratio 1:l) mixture and it was found that all reduced the deposition rate. In particular, O2 and NH, could not be added in greater proportions than about 5% without reducing the deposition rate to effectively zero. The original intention of incorporating large amounts of sp3 hybridised nitrogen in the form of ammonia could not be realised. The EELS n* peaks of both the carbon and nitrogen were very sensitive to the concentration of oxygen in the sputter gas. The peak intensity increased strongly with the oxygen content up to 5%, which was the highest amount at which analysis was possible due to the reduced deposition rate, see Figs. 4(a) and 4(b). The carbon and nitrogen rr* peaks for the oxygen films are surprisingly high and even higher than for the films with graphitic precipitates (this is seen from comparing with the intensity of the interval just after the rc* peak which should be insensitive to changes in film thickness),
390
(‘4
400
I
I
410
420
Energy
,
1
I
430
440
,
450
Loss (eV)
Fig. 3. EELS spectra of (a) the carbon K edges and (b) the nitrogen K edges from carbon nitride films r.f. deposited at 200°C from a sputter gas containing 50% nitrogen in argon with substrate bias voltages of 0, - 100 or - 200 V.
considering that significant changes in the ordering are not observed from diffraction patterns. It is possible that these high Z* peaks are caused by the formation of sp bonds in addition to sp’, since sp hybridisation would require two 71 bonding and antibonding orbitals. However, the x* peaks for the oxygen samples are at the same energy as for the other films analysed in this work and no significant change in the low loss spectra (in particular the intensity of the n plasmon at 6 eV) are observed. The fact that no oxygen K edge was detected in the EELS spectra, indicates that the larger X* peak cannot be attributed to significant presence of C-O or C=O bonds. Both ammonia and methane also increased the heights of the rt* peaks for carbon and nitrogen and thus had a similar effect, though less pronounced, to oxygen. Ammonia which could not be added in higher amounts
167
N. Ax& et al.lDiamond and Related Materials 5 (1996) 163-168
270
280
I
290
,
I
300
I
I
I
310
320
/
330
260
270
Energy Loss (eV) P
!
(b) N
380
K edge 390
(b)
280
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310
320
Energy Loss (eV)
“““““‘I
400
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I
(b) N K edge
440
Energy Loss (eV)
380
(W
390
R 400
I
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/
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I
430
,
I
440
j
Energy Loss (eV)
Fig. 4. EELS spectra of (a) the carbon K edges and (b) the nitrogen K edges from carbon nitride films r.f. deposited at 200°C on NaCl from a sputter gas containing equal amount of nitrogen and argon with addition of 5% oxygen
Fig. 5. EELS spectra of (a) the carbon K edges and (b) the nitrogen K edges from carbon nitride films r.f. deposited at 200°C on NaCl from a sputter gas containing equal amount of nitrogen and argon with additions of 4% ammonia or 5% methane.
than ca. 4%, had about the same effect as 5% of methane, see Figs. 5(a) and 5(b).
in Ref. [lo], that this is not the case. To the contrary, considering that the films have similar composition and structure, both these gases, as well as ammonia, increased the presence of rc bonds. The deposition rate was, however, strongly reduced by their presence in the sputter gas, suggesting that an etching process was indeed taking place, but leaving a material richer in sp’ bonding. These gases therefore have similar effects as hydrogen in amorphous C:H films prepared by plasma deposition [ 181. In early work on the sputtering of carbon in nitrogen [4] it was argued that the films formed had a paracyanogen structure. This possibility is also discussed in Refs. [7] and [lo]. The existence of a polymer-like paracyanogen solid and its properties have been described in the literature [4,21]. The molecular weight is indefinite and its formula is often denoted -(CN),-. Formation of
4. Discussion Methane and oxygen are commonly used in the CVD deposition of diamond to etch away graphitic carbon and promote the formation of diamond or other sp3 hybridised carbon films [ZO]. The original intention of using these gases was to establish whether they would also promote the sp3 hybridisation of the carbon in sputtered carbon nitride films. Although a quantitative analysis of the sp2 content is complex considering the possible changes in transition probability caused by the change in composition as nitrogen is added to the films, it has been clearly demonstrated in this work, as well as
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The addition of up to 5% of oxygen, methane or ammonia to the gas mixture increased the sp2 hybridisation of both the carbon and nitrogen. Higher amounts of these gases could not be used since they strongly reduced the effective deposition rate. It appears that they have an etching effect on the growing films, as in CVD, but in this case they promote the formation of sp2 or possibly sp bonds. The only films showing crystallinity were those deposited or annealed at temperatures above some 800 “C. These films contained graphitic carbon precipitates, < 10 nm in size.
Acknowledgements
Fig. 6. Molecular paracyanogen.
structure
of two different
suggested
structures
of
a paracyanogen structure would promote sp and sp2 and suppress sp3 hybridisations of the carbon, see Fig. 6. There is therefore no evidence in the EELS spectra obtained in this work which would be inconsistent with a paracyanogen structure. Further work with IR spectroscopy could confirm this hypothesis. The carbon nitrogen films formed within this work withstand surprisingly high temperatures. It was possible both to deposit and to anneal the films at 700 “C while maintaining high concentrations of nitrogen (ca. 50 at.%). Raising the temperature did, however, not lead to the formation of a crystalline carbon nitrogen phase.
5. Conclusions Amorphous carbon nitride films containing as much as 57 at.% nitrogen (corresponding to the stoichiometry of C3N4) have been formed by sputtering a carbon target with an r.f. powered balanced magnetron in argon/ nitrogen atmospheres containing more than 50% nitrogen. The intensity of the X* peak in EELS spectra, representing the presence of sp2 hybridised carbon, increased steadily with the nitrogen content of the sputter gas, with a maximum for films deposited in pure nitrogen. A direct relation to increased sp2 hybridisation cannot be easily conducted because of the changes in film composition. However, for changes in preparation conditions leading to similar nitrogen content in the films, we can infer that the sp2 hybridisation increased with substrate bias voltage, probably due to the formation of C=N double bonds. This can be possibly explained by the formation of paracyanogen structures. Further analysis with IR will have to be carried out to confirm this hypothesis.
N. Ax&n thanks the Human Capital and Mobility Programme of the EC and the Swedish Institute for their financial support. G.A. Botton acknowledges support from EPSRC (ROPA) and Darwin College in Cambridge for a research fellowship.
References Cl1 A.Y. Liu and M.L. Cohen, Science, 245 (1989) 841. Liu and M.L. Cohen, Phys. Rev. B, 41 (1990) 10727. c31 D.I. Jones and A.D. Stewart, Philos. Mag. 8, 46 (1982) 423. c41 J.J. Cuomo, P.A. Leary, D. Yu, W. Reuter and M. Frisch, J. Vat. Sci. Technol., 16 (1979) 299. c51 M.Y. Chen, X. Lin, V.P. Dravid, Y.W. Chung, MS. Wong and W.D. Sproul, Surf. Coat. Technol., 54/55 (1992) 360. 161 M.Y. Chen, X. Lin, V.P. Dravid, Y.W. Chung, M.S. Wong and W.D. Sproul, Tribal. Trans., 36 (1993) 491. I. Ivanov, M. Johansson, L. Hultman, J.-E. [71 H. SjastrBm, Sundgren, S.V. Hainsworth, T.F. Page and L.R. Wallenberg, Thin Solid Films, 246 (1994) 103. CSI D. Bradley, New Scientist, 3 (1995) 19. c91 D. Li, Y.-W. Chung, M.-S. Wong and W.D. Sproul, J. Appl. Phys, 74 (1993) 219. H.Q. Lou, R.E. Somekh and I.M. Cl01 N. Ax&n, G.A. Botton, Hutchings, Su$ Coat. Technol., (1995) in press. Y.-W. Chang, M.-S. Wong and W.D. Cl11 D. Li, E. Cutiongco, Sproul, Surf. Coat. Technol., 68169 (1994) 611. II121 T.-A. Yeh, C.-L. Lin, J.M. Sivertsen and J.H. Judy, J. Magn. Magn. Mater., 120 (1993) 314. [I31 D.C. Green, D.R. McKenzie and P.B. Lukins, Mater. Sci. Forum, 52/53 (1989) 103. and C.A. Davis, Diamond Related Mater., 4 Cl41 J. Robertson (1995) 441. Cl51 SD. Berger, D.R. McKenzie and P.J. Martin, Phil. Mag. Lett., 57 (1988) 285. K. Sato and K. Yamada, in L.D. Peachey and Cl61 C.B. Boothroyd, D.B. Williams (eds.), XII ICEM, San Fransisco Press, San Fransisco, CA, 1990, p. 80. Cl71 R.F. Egerton, EELS in the Electron Microscope, Plenum Press, New York, 1986. Cl81 J. Fink, in P.W. Hawkes (ed.), Advances in Electronics and Electron Physics, Vol. 75, p. 121, Academic Press, London, 1989. Cl91 C.C. Ahn, O.L. Krivanek, EELS Atlas, Gatan Inc., Warrendale, PA, USA. D. Leers and D.U. Wiechert, Ber. Bunsenges. [201 P.K. Bachmann, Phys. Chem., 95 (1991) 1390. The Chemistry of Organic Cyanogen c211 V. Mirgrdichian, Compounds, Reinhold USA, New York, 1947, p. 362.
CT1A.Y.
Diamond and Related Materials 5 (1996) 163-168
Effect of deposition conditions on the chemical bonding in sputtered carbon nitride films N. Ax&, G.A. Botton, R.E. Somekh, I.M. Hutchings Unit’ersity of Cambridge, Department of Materials Science and Metallurgy, Pembroke Street, Cambridge CB2 3QZ, UK Received 8 August 1995; accepted
in final form 21 September
1995
Abstract A balanced planar r.f. powered magnetron sputter source has been used to deposit carbon nitride films from a graphite target under various conditions. Sample temperature, bias voltage and nitrogen content in the gas mixture were varied. The effects of oxygen, methane and ammonia on the film growth were also studied. Special attention was paid to the effects of the deposition parameters on the structure of the films, in particular the hybridisation of the carbon and nitrogen bonding. The chemical bonding of the carbon and nitrogen atoms was studied by electron energy loss spectroscopy (EELS). The chemical composition was evaluated by Rutherford back-scattering. The intensity of transitions to n: antibonding orbitals, as revealed by EELS, was found to increase with the nitrogen content in the films. Ion bombardment of the films during growth and the addition of oxygen or hydrogen-rich gases further increased the proportion of 7c bonds of both the carbon and nitrogen atoms. It is suggested that the increase in the transitions to p antibond orbitals is to be explained by increased sp2 or possibly sp hybridisation of the carbon and nitrogen. Also, the effect of annealing on the bonding of nitrogen rich films after deposition was tested. The changes caused by nitrogen and deposition conditions are consistent with previous reports on the formation of paracyanogen structures. Keywords: Thin films; Carbon nitride; Sputter deposition; Electron energy loss spectroscopy
1. Introduction There is a continuous search for better tribological materials which combine high hardness with toughness, are chemically stable and exhibit a low friction against common counter-materials such as steel. There has been little theoretical work on the prediction of such properties. Liu and Cohen, however, developed a model to predict the bulk modulus of tetrahedrally bonded solids [1,2]. Their model has given results in good agreement with experimental data for several solids. They have also used the model to predict values for novel materials, such as C,N, with a structure analogous to /?-S&N,; this material is predicted to have a bulk modulus at least as high as that of diamond. Although the incorporation of nitrogen into solid carbon (both amorphous carbon and diamond) had already been tried by several investigators, often in small amounts to investigate the effect of nitrogen doping [3] but also in ratios up to about 1: 1 [4], the publications of Liu and Cohen initialised many attempts to specifically synthesise C3N,. The first attempts to form this potentially hard material 0925-9635/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved SSDI
0925-9635(95)00343-6
by a conventional sputter route were reported in 1992 [ 5,6]. By now a large number of the more readily available standard methods for the deposition of nitrides have been tested. The synthesis of covalently bonded and very hard C3N, does not seem to be easily achieved. However, some investigators have reported crystallites, with still uncertain structure, in nitrogen-rich carbon films [6,7] and the possibility of producing crystalline C,N4 by depositing carbon nitride as an interlayer between other nitrides with similar structures is currently being evaluated [S]. Despite the lack of progress in achieving superhard C3N4, other interesting properties have been found in the carbon nitride materials which have been synthesised. The very high hardness values reported in early works may be due to unusually high elastic recovery of the films after hardness indentation. There are now several reports confirming that the addition of 30-50 at.% nitrogen to amorphous carbon films changes their mechanical properties, and that the materials exhibit high elasticity in low load indentation tests [ 5,7,9,10]. For low indenta-
N. Ax& et ul./Diamondand
164
Related Materiuls
tion loads (< 10 gf) the hardness indentations often cannot be found at all after unloading. Amorphous and partially crystalline carbon films have found application as tribological films, often showing very low friction values in combination with relatively high hardness. There are reports indicating that the addition of nitrogen to the amorphous structure might improve the friction properties further [11,12]. A low friction material with high elasticity could be the basis for a promising tribological material, even if it is not superhard. The bonding state of amorphous carbon has been extensively studied [ 12-151. Carbon is unique among the Group IV elements in its ability to form several different types of bonds. These correspond to different orbital configurations often denoted as sp, sp2 and sp3. Carbon also forms amorphous and poorly crystalline structures. It has been shown that the bonding hybridisation in amorphous carbon films can be either sp2 or sp3 dominated, depending on how the films are manufactured [ 131. However, the correlation between deposition parameters and bonding state in the films is only poorly understood. The addition of nitrogen results in an even more complex system. In this work carbon nitride films have been deposited from a graphitic target using an r.f. powered sputter apparatus with a balanced planar magnetron. The effects of a variety of sputter gas mixtures and heat treatments on the variations in hybridisation of the carbon and nitrogen have been evaluated by means of EELS and TEM.
2.
method
Carbon nitride films were deposited with an r.f. powered planar balanced magnetron sputtering system. A magnetron power of 100 W was used on a magnetron with dimensions of 35 x 55 mm (racetrack area% 300 mm2) placed 35 mm from the substrates to be coated. The magnetron was covered with a high purity graphite target. Before each deposition run the vacuum system was pumped to below lop8 mbar until the leakage rate fell below lo-’ mbar s-l, which was normally achieved after a few hours of pumping if the system had not been exposed to air for more than a few minutes. The deposition took place at a chamber pressure of 2.0 Pa, as measured with a Baratron gauge. The deposition temperature was measured with thermocouples on the substrate holder. Most films were deposited on sodium chloride crystals at temperatures below 300 “C. Higher temperatures were not used for deposition on NaCl since biased films showed traces of chloride contamination at this temperature. The salt substrates were subsequently dissolved in water and the films, being about 50 nm thick, collected on copper grids. For higher deposition or annealing
5 (1996) 163-168
temperatures (up to 1OOO’C)R-plane sapphire substrates were used. These substrates were ultrasonically cleaned in chloroform, acetone and subsequently iso-2-propanol before deposition of about 1 pm thick films (1 h deposition time). Films deposited at high temperatures were detached mechanically from the sapphire substrates and collected on lacey carbon-copper grids for transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) examination. The structure and chemical bonding in the films were investigated by means of TEM on a Philips CM30 at 100 kV, and EELS with a parallel Gatan detector (model 666). Under typical operation conditions, an energy resolution better than 1 eV (as determined by the full width of half maximum of the no-loss peak) was achieved. Spectra were collected in image mode without any objective aperture so as to integrate the energy loss angular distribution over all scattering angles. In order to assess any effect of possible film anisotropy in the spectra, angular collector conditions were varied (e.g. to diffraction mode with a small collection angle). These collection methods yielded comparable spectra for all films, except for some large grain graphitic films, which showed anisotropic behaviour in the energy loss. Spectra were corrected for detector gain changes by acquiring spectra shifted in energy [ 161. As the films varied in thickness, comparisons were carried out after normalising the spectra in an energy range about 10 eV above the threshold of the edges. This interval is less sensitive to effects due to multiple inelastic scattering caused by film thickness differences. Numerical deconvolution of the multiple inelastic effects using logarithmic techniques [ 171 was not attempted in this work, as quantitative assessment of the hybridisation fraction is complicated by the effects of film composition changes on the transition probability and consequently on the detailed band structure of the materials [ 181. In this work, therefore, only qualitative variations in the near edge structure features were compared. The sputter gas was an argon-nitrogen mixture, ranging from pure argon to pure nitrogen, pre-mixed before admission to the sputter system. The effects of oxygen, methane and ammonia in the sputter gas, as well as sample bias voltages of up to 300 V, were also evaluated. Oxygen and methane (or other hydrocarbons) are used as graphite etching gases in CVD deposition of diamond. Biasing the substrates during deposition leads to an increased ion bombardment of the growing films, which is known to facilitate the formation of other nitrides (such as TIN) at lower substrate temperatures compared to without bias. The chemical composition of the films was studied by Rutherford backscattering (RBS) and confirmed with EELS. With RBS the nitrogen content of the films was measured with an accuracy of better than 3% of the quoted value.
N. Axtn et al.lDiamond and Related Materials 5 (1996) 163-168
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3. Results 3.1. Nitrogen content of thejlms and its effect on the bonding The nitrogen content of the films increased with the amount of nitrogen in the sputter gas, saturating at around 5OYP70% nitrogen in a N,/Ar mixture. Films containing ca. 57 at.% nitrogen (corresponding to the C,N, stoichiometry), as shown by RBS measurements, can be achieved with the system [lo]. As the substrate temperature during deposition reaches ca. 800 “C, the nitrogen content decreases and at the same time the films start to crystallise into graphitic precipitates in an amorphous carbon nitride matrix. At 1000 “C the nitrogen content has decreased essentially to zero (Fig. 1). At high deposition temperatures (> 800 “C) platinum impurities from the heating stage were found in the films. Transmission electron microscopy revealed most of the films, whether nitrogen-containing or pure carbon, to be amorphous. Only the nitrogen-free films formed at temperatures above 900 “C were found to contain crystallites, < 10 nm in size. Electron diffraction and EELS showed these crystallites to be graphitic in structure. For the amorphous films, differences in the diffraction patterns as a function of nitrogen content could not be detected. The EELS spectra of amorphous pure carbon films show lower X* peaks at 284 eV than for the polycrystalline graphitic films. This is shown in Fig. 2(a), in which an EELS spectrum of a film deposited at 1000 “C with graphitic precipitates is compared with spectra for films with different nitrogen contents. The graphitic film showed strong rc* and (T*peaks typical of single crystals of graphite. As nitrogen was added to the sputter gas mixture the Z* peak at the carbon K edge increased in intensity. The height of the n* peak increased continuously with the nitrogen content. Whether this effect can
loo
L
.z
’ ~
z
I
0
0
200
.-,
400
Substrate
600 temperature,
800
,‘i loo0
“C
Fig. 1. Nitrogen content in the films as determined by RBS, as a function of the substrate holder temperature for film deposited with 100 W r.f. magnetron power at a gas pressure of 2.0 Pa.
Energy Loss (eV)
(4
F
390 (b)
400
410
420
430
440
450
Energy Loss (eV)
Fig. 2. EELS spectra from carbon and carbon nitride films deposited at 200 “C on NaCl substrates from a gas mixture containing different amounts of nitrogen. (a) Carbon K edges of films made with O%, 50% and 100% nitrogen in argon, together with a film deposited at 900 “C containing graphitic crystallites; (b) nitrogen K edges of films made with 25%, 50% and 100% nitrogen in argon.
be attributed to an increase of sp’ hybridisation or is caused by changes in the transition probability (significant changes in the chemical composition of the films and in the long range order would also change the scattering intensity) is not clear. For films deposited in pure nitrogen gas the n* peak is still lower than for the fully sp2 hybridised film with graphitic precipitates. The increased intensity of the Z* peak and the observation of a similar near edge structure in the N K edge (Fig. 2(b)) indicates that the introduction of nitrogen leads to the formation of C=N double bonds. The formation of double bonds between nitrogen atoms (N=N), as in molecular nitrogen (N2) trapped in the films, can be excluded, since they would lead to a nitrogen edge with a different fine structure from that seen in Fig. 2(b) [ 191. The difference in the energy loss
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distribution at energies lo-15 eV above the g* peaks are caused by film thickness differences influencing the electron energy loss, as a result of multiple inelastic scattering effects as discussed in Section 2. Annealing of nitrogen-rich samples after deposition was also carried out in order to promote crystallisation. The samples were held at temperatures between 800 and 1000 “C in the platinum heating stage in the sputter chamber for times ranging from 15 min to 2 h at a nitrogen pressure of 50 Pa. The nitrogen content was then estimated and the changes in bonding was studied with EELS. It was found that annealing under these conditions caused the nitrogen to leave the samples and was accompanied by graphitisation of the films. Annealing the films had a minimal effect on the carbon hybridisation before graphitisation, as deduced from the EELS spectra. 3.2. EfSect of bias and gas composition
Energy Loss
(4
(eV)
on structure and
bonding
Both the carbon and nitrogen K edges in the EELS spectra were very sensitive to the substrate bias voltage. A bias voltage of - 100 V had a slight effect, whereas -200 V substantially increased the height of the n* peaks for both carbon and nitrogen relative to the edges, as seen in Figs. 3(a) and 3(b). This indicates an increased sp’ hybridisation of both the carbon and nitrogen atoms, since no changes in the chemical composition or structure were observed. The X* peak for carbon is, however, still lower than for the graphitic film and increasing the substrate bias did not result in any crystallinity. Bias voltages reduced the deposition rate, being as low as about 50 nm h-’ for -300 V bias. Such films were polluted with sodium and chlorine from the substrates. The effects of oxygen, methane and ammonia on the formation of carbon nitride films were also investigated. All these gases were added to an argon-nitrogen (ratio 1:l) mixture and it was found that all reduced the deposition rate. In particular, O2 and NH, could not be added in greater proportions than about 5% without reducing the deposition rate to effectively zero. The original intention of incorporating large amounts of sp3 hybridised nitrogen in the form of ammonia could not be realised. The EELS n* peaks of both the carbon and nitrogen were very sensitive to the concentration of oxygen in the sputter gas. The peak intensity increased strongly with the oxygen content up to 5%, which was the highest amount at which analysis was possible due to the reduced deposition rate, see Figs. 4(a) and 4(b). The carbon and nitrogen rr* peaks for the oxygen films are surprisingly high and even higher than for the films with graphitic precipitates (this is seen from comparing with the intensity of the interval just after the rc* peak which should be insensitive to changes in film thickness),
390
(‘4
400
I
I
410
420
Energy
,
1
I
430
440
,
450
Loss (eV)
Fig. 3. EELS spectra of (a) the carbon K edges and (b) the nitrogen K edges from carbon nitride films r.f. deposited at 200°C from a sputter gas containing 50% nitrogen in argon with substrate bias voltages of 0, - 100 or - 200 V.
considering that significant changes in the ordering are not observed from diffraction patterns. It is possible that these high Z* peaks are caused by the formation of sp bonds in addition to sp’, since sp hybridisation would require two 71 bonding and antibonding orbitals. However, the x* peaks for the oxygen samples are at the same energy as for the other films analysed in this work and no significant change in the low loss spectra (in particular the intensity of the n plasmon at 6 eV) are observed. The fact that no oxygen K edge was detected in the EELS spectra, indicates that the larger X* peak cannot be attributed to significant presence of C-O or C=O bonds. Both ammonia and methane also increased the heights of the rt* peaks for carbon and nitrogen and thus had a similar effect, though less pronounced, to oxygen. Ammonia which could not be added in higher amounts
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N. Ax& et al.lDiamond and Related Materials 5 (1996) 163-168
270
280
I
290
,
I
300
I
I
I
310
320
/
330
260
270
Energy Loss (eV) P
!
(b) N
380
K edge 390
(b)
280
290
300
310
320
Energy Loss (eV)
“““““‘I
400
I
410
I
420
I
I
430
I
(b) N K edge
440
Energy Loss (eV)
380
(W
390
R 400
I
410
/
420
I
430
,
I
440
j
Energy Loss (eV)
Fig. 4. EELS spectra of (a) the carbon K edges and (b) the nitrogen K edges from carbon nitride films r.f. deposited at 200°C on NaCl from a sputter gas containing equal amount of nitrogen and argon with addition of 5% oxygen
Fig. 5. EELS spectra of (a) the carbon K edges and (b) the nitrogen K edges from carbon nitride films r.f. deposited at 200°C on NaCl from a sputter gas containing equal amount of nitrogen and argon with additions of 4% ammonia or 5% methane.
than ca. 4%, had about the same effect as 5% of methane, see Figs. 5(a) and 5(b).
in Ref. [lo], that this is not the case. To the contrary, considering that the films have similar composition and structure, both these gases, as well as ammonia, increased the presence of rc bonds. The deposition rate was, however, strongly reduced by their presence in the sputter gas, suggesting that an etching process was indeed taking place, but leaving a material richer in sp’ bonding. These gases therefore have similar effects as hydrogen in amorphous C:H films prepared by plasma deposition [ 181. In early work on the sputtering of carbon in nitrogen [4] it was argued that the films formed had a paracyanogen structure. This possibility is also discussed in Refs. [7] and [lo]. The existence of a polymer-like paracyanogen solid and its properties have been described in the literature [4,21]. The molecular weight is indefinite and its formula is often denoted -(CN),-. Formation of
4. Discussion Methane and oxygen are commonly used in the CVD deposition of diamond to etch away graphitic carbon and promote the formation of diamond or other sp3 hybridised carbon films [ZO]. The original intention of using these gases was to establish whether they would also promote the sp3 hybridisation of the carbon in sputtered carbon nitride films. Although a quantitative analysis of the sp2 content is complex considering the possible changes in transition probability caused by the change in composition as nitrogen is added to the films, it has been clearly demonstrated in this work, as well as
N. Ax& et al.lDiamond
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The addition of up to 5% of oxygen, methane or ammonia to the gas mixture increased the sp2 hybridisation of both the carbon and nitrogen. Higher amounts of these gases could not be used since they strongly reduced the effective deposition rate. It appears that they have an etching effect on the growing films, as in CVD, but in this case they promote the formation of sp2 or possibly sp bonds. The only films showing crystallinity were those deposited or annealed at temperatures above some 800 “C. These films contained graphitic carbon precipitates, < 10 nm in size.
Acknowledgements
Fig. 6. Molecular paracyanogen.
structure
of two different
suggested
structures
of
a paracyanogen structure would promote sp and sp2 and suppress sp3 hybridisations of the carbon, see Fig. 6. There is therefore no evidence in the EELS spectra obtained in this work which would be inconsistent with a paracyanogen structure. Further work with IR spectroscopy could confirm this hypothesis. The carbon nitrogen films formed within this work withstand surprisingly high temperatures. It was possible both to deposit and to anneal the films at 700 “C while maintaining high concentrations of nitrogen (ca. 50 at.%). Raising the temperature did, however, not lead to the formation of a crystalline carbon nitrogen phase.
5. Conclusions Amorphous carbon nitride films containing as much as 57 at.% nitrogen (corresponding to the stoichiometry of C3N4) have been formed by sputtering a carbon target with an r.f. powered balanced magnetron in argon/ nitrogen atmospheres containing more than 50% nitrogen. The intensity of the X* peak in EELS spectra, representing the presence of sp2 hybridised carbon, increased steadily with the nitrogen content of the sputter gas, with a maximum for films deposited in pure nitrogen. A direct relation to increased sp2 hybridisation cannot be easily conducted because of the changes in film composition. However, for changes in preparation conditions leading to similar nitrogen content in the films, we can infer that the sp2 hybridisation increased with substrate bias voltage, probably due to the formation of C=N double bonds. This can be possibly explained by the formation of paracyanogen structures. Further analysis with IR will have to be carried out to confirm this hypothesis.
N. Ax&n thanks the Human Capital and Mobility Programme of the EC and the Swedish Institute for their financial support. G.A. Botton acknowledges support from EPSRC (ROPA) and Darwin College in Cambridge for a research fellowship.
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