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Internal stress of a-C:H(N) films deposited by radio frequency plasma enhanced chemical vapor deposition
Diamond and Related Materials 8 (1999) 1214–1219
Internal stress of a-C:H(N ) films deposited by radio frequency plasma enhanced chemical vapor deposition Y.H. Cheng *, Y.P. Wu, J.G. Chen, X.L. Qiao, C.S. Xie Institute of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, Hubei, People’s Republic of China Received 21 September 1998; accepted 5 January 1999
Abstract Amorphous hydrogenated carbon nitride [a-C:H(N )] films were deposited from the mixture of C H and N using the radio 2 2 2 frequency plasma enhanced chemical vapor deposition technique. The films were characterized by X-ray photon spectroscopy, infrared, and positron annihilation spectroscopy. The internal stress was measured by substrate bending method. Up to 9.09 at% N was incorporated in the films as the N content in the feed gas was increased from 0 to 75%. N atoms are chemically bonded 2 to C as C–N, CNN and CON bond. Positron annihilation spectra shows that density of voids increases with the incorporation of nitrogen in the films. With rising nitrogen content the internal stress in the a-C:H(N ) films decrease monotonically, and the rate of decrease in internal stress increase rapidly. The reduction of the average coordination number and the relax of films structure due to the decrease of H content and sp3/sp2 ratio in the films, the incorporation of nitrogen atoms, and the increases of void density in a-C:H(N ) films are the main factors that induce the reduction of internal stress. © 1999 Elsevier Science S.A. All rights reserved. Keywords: a-CH(N ) films; Internal stress; IR spectroscopy; Positron annihilation spectroscopy
1. Introduction There has been considerable interest in the last two decades in hydrogenated amorphous carbon films (a-C:H ) due to a large extent to their outstanding properties, such as high hardness, high transparency, high resistivity, chemical inertness, biocompatibility, which make them suitable to use in microelectronics, optics, wear resistance and biomedicine [1,2]. For a-C:H films used as prospective coatings in various tribological applications, their mechanical behavior and stability, which determine the lifetime of a coated tool or wear part, are of primary importance. Up to now, one of the main problem that affect the use of a-C:H films is the large compressive stress developed during their deposition. High compressive stresses have been found by many research workers in a-C:H films deposited by plasma vapor deposition (PVD), chemical vapor deposition (CVD) and plasma enhanced chemical vapor deposition (PECVD) methods [3–6 ]. In many cases these stress are expected to affect a-C:H films adhesion * Corresponding author. Fax: +86 027 87545438. E-mail address: [email protected] ( Y.H. Cheng)
to the substrate and causes delimitation and buckling of the films. When the adhesion is excellent and the compressive stress in the film exceeds the tensile stress in the substrate failures will occur in the buck material [7]. Over the last few years a-C:H(N ) films have been intensively studied. These films can be as hard as a-C:H films and the nitrogen incorporation cause a reduction of the internal stress without a noticeable change in their hardness [8–10]. It is very attractive in terms of the application of a-C:H(N ) as mechanical protective films. In this work a-C:H(N ) films were deposited by radio frequency (RF ) PECVD method, the internal stress and, composition and structure of a-C:H(N ) films were studied. The emphasis is placed on the factor that affect the internal stress of a-C:H(N ) films.
2. Experimental 2.1. Specimen preparation a-C:H(N ) films were prepared by the RF PECVD technique. The detail experimental apparatus used in
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Y.H. Cheng et al. / Diamond and Related Materials 8 (1999) 1214–1219
this work was published elsewhere [11]. The chamber of the equipment consists of stainless steel. A water cooled copper electrode acted as substrate holder and was capacitatively coupled to the 13.56 MHz RF generator via an impedance matching network. The other electrode was held at ground potential. The RF power was measured by wattmeter and was varied from 0 to 500 W. The negative DC bias can be continuously adjusted in the range of 0–1200 V. The mixture of high purity Nitrogen and acetylene were used as the reactant gas. The different N content which was determent 2 according to the partial pressure of different gas, was controlled by mixture container. The substrates for deposition were put directly on the cathode which was cooled by water. All samples were thoroughly cleaned ultrasonically in acetone. Before deposition the chamber was evacuated to 5×10−3 Pa, then Ar was introduced for sputter cleaning of the substrates, the condition is: −600 V bias, 0.1–1 Pa pressure, 10 min. Subsequently the chamber was evacuated to 5×10−3 Pa again. The reactant gas was introduced to deposit films. All depositions were performed at a total pressure of 10 Pa, a self bias voltage of −650 V and nitrogen partial pressures ranging from 0 to 75%. Substrate were held at ambient temperatures during deposition.
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where d is the diameter of the mth dark interference m fringe, l is the light wavelength and Na light (l= 589.3 nm) was used as light source. 2.3. Characterization of a-C:H(N) films The films were analyzed by X-ray photon spectroscopy ( XPS) using a XSAM-800 with Mg Ka X-rays. The infrared (IR) transmittance in the range 4000– 400 cm−1 was measured by employing a Nicolet-170sx type Fourier transform IR (FTIR) spectrometer with the KBr pellet technique. The positron annihilation measurements were carried out in a modified bent solenoid apparatus. In this case crystalline Si was chosen as the substrate. The energy of the monoenergetic positrons varied from 0 to 12 keV. The annihilation symbol c−ray was detected by an HPGe (ORTEC GGEM-10175) detector with a resolution of 1.12 keV (full-width half maximum, FWHM ) for a 514 keV c-ray of 85Sr. The date were recorded by collecting ca 1×105 counts for each positron energy. The shape parameter S is used to characterize the annihilation symbol c−ray, it is defined as the ratio of the counts in the central region of the annihilation line (1 keV ) to the total number of the counts in the line.
2.2. Measurement of internal stress 3. Results To measure the internal stress of a-C:H(N ) films, slide glass substrate (18×18×0.17 mm3) were used. Due to the mechanical internal stress in the film, the substrate will be curvated. The internal stress was evaluated by measuring the curvature of the wafer after deposition on only one side, using Stoney’s equation[12]: E t2 s s , 6(1−n )t R s c where E and n are the substrate Young’s modules and s s Position’s ratio, t and t are the thickness of the s c substrate and coating, R is the spherical radius of curvature of the substrate steel disk. The values adopted for the glass substrate constants (E =1.03×1010 Nm−2, s n =0.22) were taken from Zou et al. [13]. s In order to measure the geometrical thickness (t ) of c films, a glass substrate (20×20×3 mm3) which was partially covered with slide glass was coated with a-C:H(N ) films to produced a step between the portions of covered and uncovered glass surface. Then the optical interference method was used to measured the films thickness. The radius of curvate (R) was measured using the Newton’s rings method. The radius of the spherical substrate curvature is given by:
s=
R=
d2 m 4ml
3.1. Composition of a-C:H(N) films Fig. 1 shows a typical XPS spectrum of a-C:H(N ) films deposited from the mixture of 75% N and 25% 2 C H . Except for a small oxygen contamination, the 2 2 films are mainly compose of C and N. The nitrogen content incorporated in the a-C:H(N ) films can be obtained from the ratio of integrated net intensities of the N 1s to C 1s lines by computer, corrected by the corresponding photoionization cross sections. Fig. 2 shows the nitrogen concentration in the films as a function of the relative nitrogen concentration,
(m=1, 2, 3 …) Fig. 1. Typical XPS spectra of a-C:H(N ) films containing 9.09 at% N.
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Fig. 2. Nitrogen content in a-C:H(N ) films as a function of N content 2 in the feed gas.
N /(N +C H ), in the gas phase. It is clear from Fig. 2 2 2 2 2 that N is incorporated into the a-C:H network, and increases as a function of the N concentration in the 2 gas. Up to 9.09 at% N can be incorporated in the films. 3.2. Internal stress The internal stresses in a-C:H(N ) films are compressive stress. Fig. 3 show the dependence of mechanical internal stress of the films on the nitrogen content. It can be observed that the internal stress in the a-C:H(N ) films decrease monotonically from 1.6 to 0.15 GPa with rising nitrogen content in the films from 0 to 9.09 at%, and the rate of decrease in internal stress increase rapidly. 3.3. IR spectrum Fig. 4 shows the IR spectrum of a series of a-C:H(N ) films with various nitrogen content (0, 4.07, 9.09 at%) in the range of 3400–340 cm−1. It could be observed from spectrum a, which contains 0% nitrogen, that there is a strong broad band at ca 2900 cm−1 due to C–H stretch vibrations. The strong absorption centered around 3300, 1600 and 1000 cm−1 are due to the contaminant of the O–H and Si–O bonds. The weak absorption at ca 1384 cm−1 correspond to out of phase bending
Fig. 3. Internal stress as a function of the amount of nitrogen incorporated in the films.
Fig. 4. FTIR transparent spectrum of a-C:H(N ) films containing (a) 0, (b) 4.07 and (c) 9.09 at% N.
vibrations of sp3CH [14,15]. The intense absorption at 3 ca 750–950 cm-1 indicates large sp2 domains [16 ]. By comparing the IR spectrum of samples a, b, and c in the range of 1000–1700 cm−1, it can be obviously observed that the IR transmission between 1000 and 1600 cm−1 decrease strongly especially with the highest nitrogen incorporation in a-C:H(N ) films. Kaufman et al. [17] explained this effect by an incorporation of nitrogen into the sixfold carbon rings structure in a-C:H films, which breaks the symmetry in the sp2 domains causing the Raman-active G and D bands to become IR active. Schwan et al. [18] found that IR active C–N and CNN bonds each also contributing to the IR active in that wave region. But due to the overlap, the separation of the peaks corresponding to C–N, C–C and C– H bonds in this region is difficult. The additional weak peaks at ca 1271, 1622 and 1320 cm−1, which are assigned to the CNN, C–N and sp3 C–C stretching mode [19], can also be seen in the IR spectrum of sample b and c. The additional strong broad peak in sample c at ca 1429 cm−1, which correspond to C–N and >CNC< bonds, is also be observed. By comparing the three spectrum in the range of 1700–2700 cm−1, an additional band at 2200 cm−1 in the 9.09% N sample can been found which is due to the stretching vibration of a CON bond [20]. This indicates that the incorporated nitrogen atoms do not exist merely as occluded gaseous species but are chemically bonded to carbon. It can be found in Fig. 4 that there are dramatically difference among the three spectrum in the range of 2700–3000 cm−1. In order to studied the influence of
Y.H. Cheng et al. / Diamond and Related Materials 8 (1999) 1214–1219
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which indicates that rising nitrogen content in a-C:H(N ) attributes to the decrease of the sp3/sp2 bonds ratio. 3.4. Positron annihilate spectrum
Fig. 5. FTIR absorbance spectrum between 3300 and 2300 cm−1 of a-C:H(N ) films containing (a) 0, (b) 4.07 and (c) 9.09 at% N.
nitrogen content on the C–H stretch vibrations spectrum, the absorb spectrum of three samples in the range of 2300–3300 cm−1 is shown in Fig. 5. A strong broad band was observed in all three spectra at ca 2900 cm−1 due to C–H stretch vibrations, indicating relatively high hydrogen content in all films. With increasing the nitrogen content, a clear reduction in the intensity of the CH stretching band can be seen. It was shown by Dischler [21] that the area of C–H stretching bands was proportion to the total amount of bonded hydrogen. This indicates that the increase of nitrogen content causes the decrease of the total hydrogen content in a-C:H(N ) films. It can also be observed from Fig. 5 that the IR spectrum changes continuously from a double peaked spectrum at 2920 and 2850 cm−1 to a single broad line at ca 2920 cm−1. The band at 2850 cm−1 is assigned to the sp3 CH symmetric vibra2 tion mode, and the band at 2920 cm−1 is assigned to the sp3 CH asymmetric vibration mode or the sp3 CH 2 stretching band. According to the deconvolution results of the C–H stretching absorption band from a-C:H films, Palshin [22] found the existence of bands at 2870 cm−1, which is assigned to the sp3 CH stretching 3 mode, between the 2850 and 2920 cm−1 bands. The increase in intensity for 2870 cm−1 bands with an increase in the nitrogen content leads to the appearance of single broad peak for higher nitrogen content samples, which suggest that the film structure changes toward a more polymer-like one [23]. A weak shoulder peak at 2955 cm−1, which is assigned to the sp2 CH olefinic 2 stretching vibration mode, can also be observed in the curve, and the relative intensity for the peak decrease with the increase of nitrogen content in a-C:H(N ) films,
Positron annihilate measurements have been conducted with a variable energy positron bean to obtain a depth resolved open volume defect signal [24–26 ]. This technique can detect open volume defects at a sensitivity level of 5×1015 cm−3. In order to study the effect of void density on the internal stress, a-C:H(N ) films were studied using a Positron annihilate spectrum. Fig. 6 shows the shape parameter S as a function of the incident positron energy for different nitrogen content films on Si substrate. The parameter S was normalized to the vale obtained at the Si substrate (positron energies >12 keV ). The S–E data of all the films show three distinct regions, corresponding to the film, interface and substrate. In a-C:H(N ) films, the S values remain constant through the film, starts to increase near the interface and finally reaches the value of the Si substrate. It is known that the Doppler broadened annihilation c lines carries information on the momentum of the electron-positron annihilation pair. The Doppler broadening of the annihilation line was analyzed in terms of the standard S parameter which is defined as the ratio of the integral over a fixed, central portion of peak A (510.3–511.7 keV ) and the total counts B (504.5– 517.5 keV ) [27]. The sensitivity of this parameter to open volume defects arises from the annihilation characteristics of the thermalized positron with the electrons in the investigated material. When a positron annihilates with an electron, the center of mass of the electron positron pair will have a certain nonzero momentum that is essentially determined by the momentum of the electron since the positron is thermalized. This effect causes a Doppler broadening of the annihilation line, the width of which is essentially determined by the momentum of the electron. At open volume type defects, the positrons show attractive potential because of the missing ion cores and therefore can get trapped by these defects. Since the overlap of the positrons with core electrons is reduced in such a configuration, the width of the Doppler spectrum is significantly narrower than for the defect free material. So, the annihilation parameters S at the defect can be used as fingerprints of the open volume of the defect. In the same material, higher values of S correspond to a higher density of open structure (void and/or micropores). The constant of the S value in the films region observed from Fig. 6 indicates a constant distribution of voids throughout the films. It can also be observed from Fig.6 that at very low incident energy the S parameter is higher, which is consistent with impurities in the surface region. In order to remove the influence of surface defect on the S parameter of a-C:H(N ) films, the S parameter measured
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Fig. 6. Shape parameter S for a-C:H(N ) films as a function of positron energy.
The intrinsic stress in a-C:H films, which was the main fact that embarrass the application of a-C:H films, have been widely studied. It is believed now that a-C:H films are overconstrained in the sense that the number of mechanical constraints actually established by chemical bonding is greater than the ideal value which is predicted by the fully constrained network model (FCN ) proposed by Angus and Jansen [29,30], considering the observed hydrogen content. The consequent enhanced crosslinking leads to higher values of internal stress, due to bond angle and bond length distortions. Thorpe [31] defined a mean coordination number (r) for a skeleton network in terms of the fraction of threefold, fourfold coordinated, and H sites in a network: 4C +3C −C sp3 sp2 H, 1−C H where C is the carbon sp3 content, C is the carbon sp3 sp2 sp2 content and C the hydrogen content in the H a-C:H(N ) films. The result of IR spectrum (Figs. 4 and 5) indicates that the incorporation of nitrogen in a-C:H films leads to the decrease of hydrogen content and sp3/sp2 ratio. This will lead to the decrease of the coordination number for a-C:H(N ) films, which can be easily deduced from the above formula. Silva et al. [32] studied quantitatively the effect of nitrogen content on the coordination number for a-C:H(N ) films deposited in a magnetically confined RF PECVD system. The results also show that coordination of a-C:H(N ) films decreases gradually with the N content. The decrease of the coordination number will decrease the degree of overconstraining in a-C:H(N ) films and therefore the internal stress. The decrease of H content and sp3/sp2 ratio will be the first reason for the decrease of internal stress by the incorporation of N in a-C:H films. These are in good agreement with previous results which show that the hydrogen loss and structure change towards a more graphite-like one caused by increasing the self bias[13], substrate temperature [33] or by high energy implantation [34] will lead to the stress reduction in a-C:H films. But Schwan et al. [18] found a different results which indicated the loss of hydrogen at ion energies ca 100 eV is accompanied by a maximum in internal stress of a-C:H films deposited by a plasma beam source. The reason for this may be that in our case the internal stress is too lower than the threshold at 10–12 GPa to cause the phase transition for the reduction of hydrogen. The reduction of hydrogen are due to the incorporation of N atoms in a-C:H(N ) films. In addition, the date of Fig. 4 indicate that N atoms are chemically bonded to carbon as C–N, CNN, and CON bond in a-C:H(N ) films. In the case of C–N and CNN bonds, carbon atoms in a benzene ring are replaced by nitrogen atoms. Concerning the effects of nitrogen incorporation on the chemical bonds in a-C:H(N ) films, nitrogen atoms show a coordination (r)=
at a positron of 2 keV is presented in Fig. 7 as a function of the amount of nitrogen incorporated in the films. It can be found that the S value increases with nitrogen content, which indicates an increase in the density of voids with increasing the nitrogen content in the films. This is in good agreement with Freire et al.’s results [9]. The FTIR spectrum of a-C:H(N ) films with different N content indicates that the H content in a-C:H(N ) films decrease as the N content in the films increases. Asoka-Kumar et al.’s [26,28] studies indicated that hydrogen can fill open-volume regions and can passivate dangling bond sites. The decrease of H will create more open volume defects and dangling bonds, which corresponds to the increase in the open-volume content in a-C:H(N ) films sensed by positrons.
4. Discussion The total internal stresses in the films are the sum of the thermal and the intrinsic stresses. The thermal stress component is negligible since the film deposition and stress measurement were carried out at room temperature.
Fig. 7. Shape parameter S for a-C:H(N ) films as a function of the amount of nitrogen in the films.
Y.H. Cheng et al. / Diamond and Related Materials 8 (1999) 1214–1219
number equal to 3 at most (sp3 hybridized nitrogen) [10]. The replacement of carbon by nitrogen in a-C:H films necessarily implies a reduction of the average coordination number and hence of the degree of overconstraining. This change in coordination number could also be responsible for the internal stress reduction. When N atoms incorporate with carbon atoms as triple bonds, all valence electrons of nitrogen are saturated, so that this nitrogen forms terminated sites in the amorphous network, leading to a weaker network between the sp2 clusters in a-C:H(N ) films compared with the network in a-C:H films [32,35]. These will also correspond to the reduction of internal stress in a-C:H(N ) films. The effect of void in a-C:H films on the stress reduction were also studied by many researchers. Puchert et al. [12] studied the thickness dependent stress in sputtered carbon films and propose a structure model in which the voids are the most likely stress relief mechanism in the carbon films. Davis [36 ] found that the compressive stress in the chromium films produced by injection of the particles into the films is reduced and even changed to tensile stress by formation of the voids in the film. Freire et al. [9] also found that the voids in a-C:H(N ) films permit film relaxation which lead to the decrease of internal stress. By comparing Fig. 3 with Fig. 7, it can be seen that the dependence of internal stress on N content is in agreement with the dependence 2 of the void density on N content. This indicates that 2 the increase in the density of voids will be the third reason that causes internal stress reduction in a-C:H(N ) films. 5. Conclusion In this work a-C:H(N ) films were prepared from the mixture of C H and N using the RF PECVD tech2 2 2 nique. Up to 9.09 at% N atoms can be incorporated in the films, N atoms are chemically bonded to C as C-N, CNN and CON bond. The incorporation of nitrogen lead to the decrease of H content and sp3/sp2 ratio and the increase of the density of voids in a-C:H(N ) films. The internal stress in the films decrease with the increase of nitrogen content gradually. The reduction of the average coordination number and the relax of films structure due to the decrease in H content and sp3/sp2 ratio in the films, the incorporation of nitrogen atoms, and the increases of void density in a-C:H(N ) films are the main factors that induce a reduction in internal stress. Acknowledgements The authors would like to express their gratitude for the support of this work by the State Key Laboratory
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of Plastic Forming Simulation and Die & Mould Technology of China.
References [1] J. Robertson, Surf. Coat. Technol. 50 (1992) 185. [2] A. Matthews, S.S. Eskildsen, Diamond Relat. Mater. 3 (1994) 902. [3] D. Nir, Thin Solid Films 112 (1984) 41. [4] T. Zehnder, J. Balmer, W. Luthy, H.P. Weber, Thin Solid Films 263 (1995) 198. [5] S.B. IIyer, K.S. Harshavardhan, V. Kumar, Thin solid Films 256 (1995) 94. [6 ] Y.H. Cheng, Y.P. Wu, J.G. Chen, Vacuum Sci. Technol. 17 (6) (1998) 415. [7] H. Freller, A. Hempel, J. Lilge, H.P. Lorenz, Diamond Relat. Mater. 1 (1992) 563. [8] P. Wood, T. Wydeven, O. Ysuji, Thin Solid Films 258 (1995) 151. [9] F.L. Freire Jr,, G. Mariotto, C.A. Achete, Surf. Coat. Technol. 74 (1995) 75–382. [10] D.F. Frabceschini, C.A. Achete, F.L. Freire Jr,, Appl. Phys. Lett. 60 (26) (1992) 3229. [11] Y.H. Cheng, Y.P. Wu, J.G. Chen, J. Functional Materials (Chinese) 29 (1) (1998) 54. [12] M.K. Puchert, P.Y. Timbrell, R.N. Lamb, J. Vac. Sci. Technol. A 12 (3) (1994) 727. [13] J.W. Zou, K. Reichelt, K. Schmidt, B. Dischler, J. Appl. Phys. 65 (10) (1989) 3914. [14] Y. Bounouh, L. Chahed, A. Sadki, Diamond Relat. Mater. 4 (1995) 492. [15] Y.H. Cheng, Y.P. Wu, J.G. Chen, Surf. Coat. Technol. 111 (1999) 141. [16 ] S. Bhatacharya, C. Cardinaud, G. Turban, J. Appl. Phys. 83 (8) (1998) 4491. [17] J.H. Kaufman, S. Metin, D.O. Saperstein, Phys. Rev. B 39 (1989) 13053. [18] J. Schwan, V. Batori, S. Uirich, J. Appl. Phys. 84 (4) (1998) 2071. [19] R. Hauert, A. Glisenti, S. Metin, Thin Solid Films 268 (1995) 22. [20] M.Y. Chen, X. Lin, V.P. Dravid, Y.W. Chung, Surf. Coat. Technol. 54/55 (1992) 360. [21] B. Dischler, G. Brandt, P. Koidl, Appl. Phys. Lett. 42 (1983) 636. [22] V. Palshin, E.I. Meletis, S. Ves, S. Loggothhetidis, Thin Solid Films 270 (1995) 165. [23] L.H. Chou, H.W. Wang, J. Appl. Phys. 74 (7) (1993) 4673. [24] L. Liszkay, C. Corbel, L. Baroux, Appl. Phys. Lett. 64 (11) (1994) 1380. [25] M. Misheva, N. Djourelov, Tzv. Kotlarova, Thin Solid Films 283 (1996) 26. [26 ] P. Asoka-Kumar, L.G. Llynn, D.O. Welch, J. Appl. Phys. 76 (9) (1994) 4935. [27] T. Friessnegg, M. Boudreau, P. Mascher, J. Appl. Phys. 84 (2) (1998) 786. [28] P. Asoka-kumar, B.F. Dorfman, M.G. Abraizov, J. Vac. Sci. Technol. A 13 (3) (1995) 1044. [29] J.C. Angus, Diamond Relat. Mater. 1 (1991) 61. [30] J.C. Angus, F. Jansen, J. Vac. Sci. Technol. A 6 (1988) 1778. [31] M.F. Thorpe, J. Non-Cryst. Solids 182 (1995) 135. [32] S.R.P. Silva, J. Robertson, G.A.J. Amaratunga, J. Appl. Phys. 81 (6) (1998) 2626. [33] J.A. Thornton, D.W. Hoffman, Thin Solid Films 171 (1989) 5. [34] J.W. Zou, K. Schmidt, K. Reichelt, J. Vac. Sci. Technol. A 6 (1988) 3103. [35] J. Schwan, W. Dworschak, K. Jung, Diamond Relat. Mater. 3 (1994) 1034. [36 ] C.A. Davis, Thin Solid Films 226 (1993) 30.
Internal stress of a-C:H(N ) films deposited by radio frequency plasma enhanced chemical vapor deposition Y.H. Cheng *, Y.P. Wu, J.G. Chen, X.L. Qiao, C.S. Xie Institute of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, Hubei, People’s Republic of China Received 21 September 1998; accepted 5 January 1999
Abstract Amorphous hydrogenated carbon nitride [a-C:H(N )] films were deposited from the mixture of C H and N using the radio 2 2 2 frequency plasma enhanced chemical vapor deposition technique. The films were characterized by X-ray photon spectroscopy, infrared, and positron annihilation spectroscopy. The internal stress was measured by substrate bending method. Up to 9.09 at% N was incorporated in the films as the N content in the feed gas was increased from 0 to 75%. N atoms are chemically bonded 2 to C as C–N, CNN and CON bond. Positron annihilation spectra shows that density of voids increases with the incorporation of nitrogen in the films. With rising nitrogen content the internal stress in the a-C:H(N ) films decrease monotonically, and the rate of decrease in internal stress increase rapidly. The reduction of the average coordination number and the relax of films structure due to the decrease of H content and sp3/sp2 ratio in the films, the incorporation of nitrogen atoms, and the increases of void density in a-C:H(N ) films are the main factors that induce the reduction of internal stress. © 1999 Elsevier Science S.A. All rights reserved. Keywords: a-CH(N ) films; Internal stress; IR spectroscopy; Positron annihilation spectroscopy
1. Introduction There has been considerable interest in the last two decades in hydrogenated amorphous carbon films (a-C:H ) due to a large extent to their outstanding properties, such as high hardness, high transparency, high resistivity, chemical inertness, biocompatibility, which make them suitable to use in microelectronics, optics, wear resistance and biomedicine [1,2]. For a-C:H films used as prospective coatings in various tribological applications, their mechanical behavior and stability, which determine the lifetime of a coated tool or wear part, are of primary importance. Up to now, one of the main problem that affect the use of a-C:H films is the large compressive stress developed during their deposition. High compressive stresses have been found by many research workers in a-C:H films deposited by plasma vapor deposition (PVD), chemical vapor deposition (CVD) and plasma enhanced chemical vapor deposition (PECVD) methods [3–6 ]. In many cases these stress are expected to affect a-C:H films adhesion * Corresponding author. Fax: +86 027 87545438. E-mail address: [email protected] ( Y.H. Cheng)
to the substrate and causes delimitation and buckling of the films. When the adhesion is excellent and the compressive stress in the film exceeds the tensile stress in the substrate failures will occur in the buck material [7]. Over the last few years a-C:H(N ) films have been intensively studied. These films can be as hard as a-C:H films and the nitrogen incorporation cause a reduction of the internal stress without a noticeable change in their hardness [8–10]. It is very attractive in terms of the application of a-C:H(N ) as mechanical protective films. In this work a-C:H(N ) films were deposited by radio frequency (RF ) PECVD method, the internal stress and, composition and structure of a-C:H(N ) films were studied. The emphasis is placed on the factor that affect the internal stress of a-C:H(N ) films.
2. Experimental 2.1. Specimen preparation a-C:H(N ) films were prepared by the RF PECVD technique. The detail experimental apparatus used in
0925-9635/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0 9 2 5- 9 6 3 5 ( 9 9 ) 0 0 09 9 - 0
Y.H. Cheng et al. / Diamond and Related Materials 8 (1999) 1214–1219
this work was published elsewhere [11]. The chamber of the equipment consists of stainless steel. A water cooled copper electrode acted as substrate holder and was capacitatively coupled to the 13.56 MHz RF generator via an impedance matching network. The other electrode was held at ground potential. The RF power was measured by wattmeter and was varied from 0 to 500 W. The negative DC bias can be continuously adjusted in the range of 0–1200 V. The mixture of high purity Nitrogen and acetylene were used as the reactant gas. The different N content which was determent 2 according to the partial pressure of different gas, was controlled by mixture container. The substrates for deposition were put directly on the cathode which was cooled by water. All samples were thoroughly cleaned ultrasonically in acetone. Before deposition the chamber was evacuated to 5×10−3 Pa, then Ar was introduced for sputter cleaning of the substrates, the condition is: −600 V bias, 0.1–1 Pa pressure, 10 min. Subsequently the chamber was evacuated to 5×10−3 Pa again. The reactant gas was introduced to deposit films. All depositions were performed at a total pressure of 10 Pa, a self bias voltage of −650 V and nitrogen partial pressures ranging from 0 to 75%. Substrate were held at ambient temperatures during deposition.
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where d is the diameter of the mth dark interference m fringe, l is the light wavelength and Na light (l= 589.3 nm) was used as light source. 2.3. Characterization of a-C:H(N) films The films were analyzed by X-ray photon spectroscopy ( XPS) using a XSAM-800 with Mg Ka X-rays. The infrared (IR) transmittance in the range 4000– 400 cm−1 was measured by employing a Nicolet-170sx type Fourier transform IR (FTIR) spectrometer with the KBr pellet technique. The positron annihilation measurements were carried out in a modified bent solenoid apparatus. In this case crystalline Si was chosen as the substrate. The energy of the monoenergetic positrons varied from 0 to 12 keV. The annihilation symbol c−ray was detected by an HPGe (ORTEC GGEM-10175) detector with a resolution of 1.12 keV (full-width half maximum, FWHM ) for a 514 keV c-ray of 85Sr. The date were recorded by collecting ca 1×105 counts for each positron energy. The shape parameter S is used to characterize the annihilation symbol c−ray, it is defined as the ratio of the counts in the central region of the annihilation line (1 keV ) to the total number of the counts in the line.
2.2. Measurement of internal stress 3. Results To measure the internal stress of a-C:H(N ) films, slide glass substrate (18×18×0.17 mm3) were used. Due to the mechanical internal stress in the film, the substrate will be curvated. The internal stress was evaluated by measuring the curvature of the wafer after deposition on only one side, using Stoney’s equation[12]: E t2 s s , 6(1−n )t R s c where E and n are the substrate Young’s modules and s s Position’s ratio, t and t are the thickness of the s c substrate and coating, R is the spherical radius of curvature of the substrate steel disk. The values adopted for the glass substrate constants (E =1.03×1010 Nm−2, s n =0.22) were taken from Zou et al. [13]. s In order to measure the geometrical thickness (t ) of c films, a glass substrate (20×20×3 mm3) which was partially covered with slide glass was coated with a-C:H(N ) films to produced a step between the portions of covered and uncovered glass surface. Then the optical interference method was used to measured the films thickness. The radius of curvate (R) was measured using the Newton’s rings method. The radius of the spherical substrate curvature is given by:
s=
R=
d2 m 4ml
3.1. Composition of a-C:H(N) films Fig. 1 shows a typical XPS spectrum of a-C:H(N ) films deposited from the mixture of 75% N and 25% 2 C H . Except for a small oxygen contamination, the 2 2 films are mainly compose of C and N. The nitrogen content incorporated in the a-C:H(N ) films can be obtained from the ratio of integrated net intensities of the N 1s to C 1s lines by computer, corrected by the corresponding photoionization cross sections. Fig. 2 shows the nitrogen concentration in the films as a function of the relative nitrogen concentration,
(m=1, 2, 3 …) Fig. 1. Typical XPS spectra of a-C:H(N ) films containing 9.09 at% N.
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Fig. 2. Nitrogen content in a-C:H(N ) films as a function of N content 2 in the feed gas.
N /(N +C H ), in the gas phase. It is clear from Fig. 2 2 2 2 2 that N is incorporated into the a-C:H network, and increases as a function of the N concentration in the 2 gas. Up to 9.09 at% N can be incorporated in the films. 3.2. Internal stress The internal stresses in a-C:H(N ) films are compressive stress. Fig. 3 show the dependence of mechanical internal stress of the films on the nitrogen content. It can be observed that the internal stress in the a-C:H(N ) films decrease monotonically from 1.6 to 0.15 GPa with rising nitrogen content in the films from 0 to 9.09 at%, and the rate of decrease in internal stress increase rapidly. 3.3. IR spectrum Fig. 4 shows the IR spectrum of a series of a-C:H(N ) films with various nitrogen content (0, 4.07, 9.09 at%) in the range of 3400–340 cm−1. It could be observed from spectrum a, which contains 0% nitrogen, that there is a strong broad band at ca 2900 cm−1 due to C–H stretch vibrations. The strong absorption centered around 3300, 1600 and 1000 cm−1 are due to the contaminant of the O–H and Si–O bonds. The weak absorption at ca 1384 cm−1 correspond to out of phase bending
Fig. 3. Internal stress as a function of the amount of nitrogen incorporated in the films.
Fig. 4. FTIR transparent spectrum of a-C:H(N ) films containing (a) 0, (b) 4.07 and (c) 9.09 at% N.
vibrations of sp3CH [14,15]. The intense absorption at 3 ca 750–950 cm-1 indicates large sp2 domains [16 ]. By comparing the IR spectrum of samples a, b, and c in the range of 1000–1700 cm−1, it can be obviously observed that the IR transmission between 1000 and 1600 cm−1 decrease strongly especially with the highest nitrogen incorporation in a-C:H(N ) films. Kaufman et al. [17] explained this effect by an incorporation of nitrogen into the sixfold carbon rings structure in a-C:H films, which breaks the symmetry in the sp2 domains causing the Raman-active G and D bands to become IR active. Schwan et al. [18] found that IR active C–N and CNN bonds each also contributing to the IR active in that wave region. But due to the overlap, the separation of the peaks corresponding to C–N, C–C and C– H bonds in this region is difficult. The additional weak peaks at ca 1271, 1622 and 1320 cm−1, which are assigned to the CNN, C–N and sp3 C–C stretching mode [19], can also be seen in the IR spectrum of sample b and c. The additional strong broad peak in sample c at ca 1429 cm−1, which correspond to C–N and >CNC< bonds, is also be observed. By comparing the three spectrum in the range of 1700–2700 cm−1, an additional band at 2200 cm−1 in the 9.09% N sample can been found which is due to the stretching vibration of a CON bond [20]. This indicates that the incorporated nitrogen atoms do not exist merely as occluded gaseous species but are chemically bonded to carbon. It can be found in Fig. 4 that there are dramatically difference among the three spectrum in the range of 2700–3000 cm−1. In order to studied the influence of
Y.H. Cheng et al. / Diamond and Related Materials 8 (1999) 1214–1219
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which indicates that rising nitrogen content in a-C:H(N ) attributes to the decrease of the sp3/sp2 bonds ratio. 3.4. Positron annihilate spectrum
Fig. 5. FTIR absorbance spectrum between 3300 and 2300 cm−1 of a-C:H(N ) films containing (a) 0, (b) 4.07 and (c) 9.09 at% N.
nitrogen content on the C–H stretch vibrations spectrum, the absorb spectrum of three samples in the range of 2300–3300 cm−1 is shown in Fig. 5. A strong broad band was observed in all three spectra at ca 2900 cm−1 due to C–H stretch vibrations, indicating relatively high hydrogen content in all films. With increasing the nitrogen content, a clear reduction in the intensity of the CH stretching band can be seen. It was shown by Dischler [21] that the area of C–H stretching bands was proportion to the total amount of bonded hydrogen. This indicates that the increase of nitrogen content causes the decrease of the total hydrogen content in a-C:H(N ) films. It can also be observed from Fig. 5 that the IR spectrum changes continuously from a double peaked spectrum at 2920 and 2850 cm−1 to a single broad line at ca 2920 cm−1. The band at 2850 cm−1 is assigned to the sp3 CH symmetric vibra2 tion mode, and the band at 2920 cm−1 is assigned to the sp3 CH asymmetric vibration mode or the sp3 CH 2 stretching band. According to the deconvolution results of the C–H stretching absorption band from a-C:H films, Palshin [22] found the existence of bands at 2870 cm−1, which is assigned to the sp3 CH stretching 3 mode, between the 2850 and 2920 cm−1 bands. The increase in intensity for 2870 cm−1 bands with an increase in the nitrogen content leads to the appearance of single broad peak for higher nitrogen content samples, which suggest that the film structure changes toward a more polymer-like one [23]. A weak shoulder peak at 2955 cm−1, which is assigned to the sp2 CH olefinic 2 stretching vibration mode, can also be observed in the curve, and the relative intensity for the peak decrease with the increase of nitrogen content in a-C:H(N ) films,
Positron annihilate measurements have been conducted with a variable energy positron bean to obtain a depth resolved open volume defect signal [24–26 ]. This technique can detect open volume defects at a sensitivity level of 5×1015 cm−3. In order to study the effect of void density on the internal stress, a-C:H(N ) films were studied using a Positron annihilate spectrum. Fig. 6 shows the shape parameter S as a function of the incident positron energy for different nitrogen content films on Si substrate. The parameter S was normalized to the vale obtained at the Si substrate (positron energies >12 keV ). The S–E data of all the films show three distinct regions, corresponding to the film, interface and substrate. In a-C:H(N ) films, the S values remain constant through the film, starts to increase near the interface and finally reaches the value of the Si substrate. It is known that the Doppler broadened annihilation c lines carries information on the momentum of the electron-positron annihilation pair. The Doppler broadening of the annihilation line was analyzed in terms of the standard S parameter which is defined as the ratio of the integral over a fixed, central portion of peak A (510.3–511.7 keV ) and the total counts B (504.5– 517.5 keV ) [27]. The sensitivity of this parameter to open volume defects arises from the annihilation characteristics of the thermalized positron with the electrons in the investigated material. When a positron annihilates with an electron, the center of mass of the electron positron pair will have a certain nonzero momentum that is essentially determined by the momentum of the electron since the positron is thermalized. This effect causes a Doppler broadening of the annihilation line, the width of which is essentially determined by the momentum of the electron. At open volume type defects, the positrons show attractive potential because of the missing ion cores and therefore can get trapped by these defects. Since the overlap of the positrons with core electrons is reduced in such a configuration, the width of the Doppler spectrum is significantly narrower than for the defect free material. So, the annihilation parameters S at the defect can be used as fingerprints of the open volume of the defect. In the same material, higher values of S correspond to a higher density of open structure (void and/or micropores). The constant of the S value in the films region observed from Fig. 6 indicates a constant distribution of voids throughout the films. It can also be observed from Fig.6 that at very low incident energy the S parameter is higher, which is consistent with impurities in the surface region. In order to remove the influence of surface defect on the S parameter of a-C:H(N ) films, the S parameter measured
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Fig. 6. Shape parameter S for a-C:H(N ) films as a function of positron energy.
The intrinsic stress in a-C:H films, which was the main fact that embarrass the application of a-C:H films, have been widely studied. It is believed now that a-C:H films are overconstrained in the sense that the number of mechanical constraints actually established by chemical bonding is greater than the ideal value which is predicted by the fully constrained network model (FCN ) proposed by Angus and Jansen [29,30], considering the observed hydrogen content. The consequent enhanced crosslinking leads to higher values of internal stress, due to bond angle and bond length distortions. Thorpe [31] defined a mean coordination number (r) for a skeleton network in terms of the fraction of threefold, fourfold coordinated, and H sites in a network: 4C +3C −C sp3 sp2 H, 1−C H where C is the carbon sp3 content, C is the carbon sp3 sp2 sp2 content and C the hydrogen content in the H a-C:H(N ) films. The result of IR spectrum (Figs. 4 and 5) indicates that the incorporation of nitrogen in a-C:H films leads to the decrease of hydrogen content and sp3/sp2 ratio. This will lead to the decrease of the coordination number for a-C:H(N ) films, which can be easily deduced from the above formula. Silva et al. [32] studied quantitatively the effect of nitrogen content on the coordination number for a-C:H(N ) films deposited in a magnetically confined RF PECVD system. The results also show that coordination of a-C:H(N ) films decreases gradually with the N content. The decrease of the coordination number will decrease the degree of overconstraining in a-C:H(N ) films and therefore the internal stress. The decrease of H content and sp3/sp2 ratio will be the first reason for the decrease of internal stress by the incorporation of N in a-C:H films. These are in good agreement with previous results which show that the hydrogen loss and structure change towards a more graphite-like one caused by increasing the self bias[13], substrate temperature [33] or by high energy implantation [34] will lead to the stress reduction in a-C:H films. But Schwan et al. [18] found a different results which indicated the loss of hydrogen at ion energies ca 100 eV is accompanied by a maximum in internal stress of a-C:H films deposited by a plasma beam source. The reason for this may be that in our case the internal stress is too lower than the threshold at 10–12 GPa to cause the phase transition for the reduction of hydrogen. The reduction of hydrogen are due to the incorporation of N atoms in a-C:H(N ) films. In addition, the date of Fig. 4 indicate that N atoms are chemically bonded to carbon as C–N, CNN, and CON bond in a-C:H(N ) films. In the case of C–N and CNN bonds, carbon atoms in a benzene ring are replaced by nitrogen atoms. Concerning the effects of nitrogen incorporation on the chemical bonds in a-C:H(N ) films, nitrogen atoms show a coordination (r)=
at a positron of 2 keV is presented in Fig. 7 as a function of the amount of nitrogen incorporated in the films. It can be found that the S value increases with nitrogen content, which indicates an increase in the density of voids with increasing the nitrogen content in the films. This is in good agreement with Freire et al.’s results [9]. The FTIR spectrum of a-C:H(N ) films with different N content indicates that the H content in a-C:H(N ) films decrease as the N content in the films increases. Asoka-Kumar et al.’s [26,28] studies indicated that hydrogen can fill open-volume regions and can passivate dangling bond sites. The decrease of H will create more open volume defects and dangling bonds, which corresponds to the increase in the open-volume content in a-C:H(N ) films sensed by positrons.
4. Discussion The total internal stresses in the films are the sum of the thermal and the intrinsic stresses. The thermal stress component is negligible since the film deposition and stress measurement were carried out at room temperature.
Fig. 7. Shape parameter S for a-C:H(N ) films as a function of the amount of nitrogen in the films.
Y.H. Cheng et al. / Diamond and Related Materials 8 (1999) 1214–1219
number equal to 3 at most (sp3 hybridized nitrogen) [10]. The replacement of carbon by nitrogen in a-C:H films necessarily implies a reduction of the average coordination number and hence of the degree of overconstraining. This change in coordination number could also be responsible for the internal stress reduction. When N atoms incorporate with carbon atoms as triple bonds, all valence electrons of nitrogen are saturated, so that this nitrogen forms terminated sites in the amorphous network, leading to a weaker network between the sp2 clusters in a-C:H(N ) films compared with the network in a-C:H films [32,35]. These will also correspond to the reduction of internal stress in a-C:H(N ) films. The effect of void in a-C:H films on the stress reduction were also studied by many researchers. Puchert et al. [12] studied the thickness dependent stress in sputtered carbon films and propose a structure model in which the voids are the most likely stress relief mechanism in the carbon films. Davis [36 ] found that the compressive stress in the chromium films produced by injection of the particles into the films is reduced and even changed to tensile stress by formation of the voids in the film. Freire et al. [9] also found that the voids in a-C:H(N ) films permit film relaxation which lead to the decrease of internal stress. By comparing Fig. 3 with Fig. 7, it can be seen that the dependence of internal stress on N content is in agreement with the dependence 2 of the void density on N content. This indicates that 2 the increase in the density of voids will be the third reason that causes internal stress reduction in a-C:H(N ) films. 5. Conclusion In this work a-C:H(N ) films were prepared from the mixture of C H and N using the RF PECVD tech2 2 2 nique. Up to 9.09 at% N atoms can be incorporated in the films, N atoms are chemically bonded to C as C-N, CNN and CON bond. The incorporation of nitrogen lead to the decrease of H content and sp3/sp2 ratio and the increase of the density of voids in a-C:H(N ) films. The internal stress in the films decrease with the increase of nitrogen content gradually. The reduction of the average coordination number and the relax of films structure due to the decrease in H content and sp3/sp2 ratio in the films, the incorporation of nitrogen atoms, and the increases of void density in a-C:H(N ) films are the main factors that induce a reduction in internal stress. Acknowledgements The authors would like to express their gratitude for the support of this work by the State Key Laboratory
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of Plastic Forming Simulation and Die & Mould Technology of China.
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