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Synthesis of carbon nitride thin films by vacuum arcs
ELSEVIER
Materials Science and Engineering A209 (1996) 10-15
Synthesis of carbon nitride thin films by vacuum arcs Imad F. Huseina, YuanZhong Zhoua, Fan Lia, Ryne C. Allen'\ Chung Chana, Jacob I. Kleiman b , Yu Gudimenko b , Clark V. Cooperc a Plasma
Science Laboratory, Department oj' Electrical and Computer Engineering, Northeastern University, Boston, M A 02115, USA bIntegrity Testing Lahoratory/University 0/ Toronto Institute/or Aermpace Studies, Do1t'l7sviell', Ont, M3H 5T6, Canada C United Technologies Research Center, East Hartford, CT 06108, USA
Abstract Carbon nitride (CN) thin films were synthesized by combining vacuum arcs and plasma ion implantation techniques. Three methods were investigated: plasma ion implantation into carbon films deposited by anodic vacuum arcs (AAPII), continuous cathodic vacuum arc with plasma ion implantation (CAPII) and pulsed cathodic vacuum arc (PCA), The films were found to be amorphous by X-ray diffraction (XRD). X-Ray photoelectron spectroscopy (XPS) and Raman spectroscopy analysis indicated the formation ofC-N, C=N and C=N bonds. Calculations of the surface tension components (dispersion and polar) of the films using the contact angle measurement technique suggested the formation of covalent carbon-nitrogen bonds. The CN films exhibited improved adhesion relative to the pure carbon films as indicated by adhesion calculations and the reduction in interfacial tension between the films and the substrate. A hardness of 18.9 GPa was obtained by nanoindentation measurements for CN films with an N/C ratio of 0.135. Keywords: Carbon nitride thin films; Plasma ion implantation; Vacuum arcs
1. Introduction The possibility of forming solids with covalent CN bonds, such as fJ -C 3N 4 , as new hard materials was proposed by Liu and Cohen in 1989 based on theoretical calculations [1,2]. These calculations predicted that carbon nitrides with the composition C 3N 4 in the /f phase (fJ -C 3N 4 ) would have a high bulk modulus comparbale with that of diamond and a moderately large cohesive energy. More recently, Liu and Wentzcovitch [3] identified zinc-blende-like cubic and graphite-like metastable carbon nitrides with the composition C 3N 4 . Cohesive energy calculations showed that fJ and graphite-like structures have very similar cohesive energies, which suggests that in forming the fJ phase we must overcome the competition from the energetically favorable Sp2 bond found in the graphite-like phase [3]. Many experimental methods have been used to synthesize fJ -C 3N 4 employing different film deposition techniques: ion beam deposition [4], r.f. diode sputtering [5- 7] , pulsed laser ablation [8], ion and vapor deposition [9], d.c. magnetron sputtering [4,10] and plasma
decomposition of CH 4 and N 2 [II]. In this paper, we present a preliminary study of carbon nitride films deposited on silicon wafers by three methods utilizing vacuum arcs and plasma ion implantation techniques: (I) plasma immersion ion implantation [12] into carbon films deposited by anodic vacuum arcs (AAPII) [13IS]; (2) continuous cathodic vacuum arcs with plasma ion implantation (CAPII); (3) pulsed cathodic vacuum arcs (PCA) [16]. The films were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), measurement of the contact angles of water, ethylene glycol and diiodomethylene on the deposited surfaces, nanoindentation measurements and Raman spectroscopy. The use of cathodic and anodic vacuum arcs with plasma immersion ion implantation (PIlI) is a new technique which has the potential to succeed in forming carbon nitride films. This technique combines the advantages of vacuum arcs in synthesizing diamond-like films with high hardness (95 GPa) and a high percentage of Sp3 bonds [17,18] with the advantages of the PIlI method in improving adhesion and reducing stress in thin films [19]. 0921-5093/96/$15,00 © 1996 -
Elsevier Science S.A, All rights reserved SSDI0921-5093(95)10096-2
/.F. Husein el 11/.
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2. Experimental details
2.4. Film c1wructeri:ation
2.1. CNfilms prepared by the AAPIlmethod
The crystal structure of the films was investigated by XRD. A 0-5000 XRD Siemens equipment using the Cu K:x line was employed. The composition and CN bonding nature of the films were investigated by XPS. This analysis was performed on a Leybold MAX 200 X-ray photoelectron spectrometer using monochromatized Mg K:x exciting radiation. Survey scans and high resolution scans were performed on all samples deposited by the three methods. To characterize the adhesion and surface tension properties of the CN films, measurements of the advancing contact angle for three droplets of testing liquid on carbon and CN films were made. Deionized water with a surface tension energy of 72.8 dyn em I, ethylene glycol with a surface tension energy of 48.2 dyn cm I and diiodomethylene with a surface tension energy of 50.8 dyn cm I were used. A custom-designed set-up was used and the geometric parameters of the droplets were measured by an optical system and a photo-camera. The sessile drop technique [20,21] was employed with droplet volumes of 2- 3 pI. The contact angle was measured as a tangent angle formed between the liquid and the surface. The film hardness was measured by a Nanoindenterl equipment (Nano Instruments Inc.); it is equipped with a three-sided diamond Berkovick indenter. The rate of loading and unloading was 400 p N s I
In this method, carbon films were deposited on silicon wafers utilizing the anodic vacuum arc technique [13-15]. An arc was ignited by an electric trigger between two graphite electrodes of cylindrical geometry and sustained by a consumable anode. The anodic vacuum arc produced a partially ionized carbon vapor plasma (less than 20% ionized) [13]. Films with thicknesses around 0.8-1.7 ,um (measured by a Dektak3 surface profilometer) were deposited with a deposition rate of 0.5 ,um min - 1 using currents between 50 and 60 A. Higher deposition rates can be achieved depending on factors such as the geometry of the electrodes, arc current and distance between the arc and the substrate. These carbon films were later immersed in a nitrogen plasma with a working gas pressure maintained at about 1.3 x 10 2 Pa (l x 10 4 Torr). Negative voltage pulses were applied to the substrate in order to extract the nitrogen ions from the plasma. Nitrogen ions also diffused into the films between the high voltage pulses via thermal diffusion [12]. Voltages of 12kV were used with a pulse length of 4-8 ps and a repetition frequency of 6-14 kHz. 2.2. CN films deposited by the CAPII method
In this method, the pulsed cathodic arc source in Ref. [16] was modified to run in a continuous mode, i.e. continuous cathodic are, providing a flux of fully ionized carbon plasma, unlike the partially ionized plasma of the anodic arc. A baffled magnetic duct was attached to the source to filter the macroparticles and neutrals from the carbon ions [16]. Nitrogen was fed into the vacuum chamber with a base pressure of 1.3 x 10 4 Pa (l x 10- 6 Torr) and the working pressure was maintained at about 1.3 x 10 2 Pa (l x 10 4 Torr). Thermionic emission was used to form a nitrogen plasma background surrounding the silicon substrate. Negative voltage pulses of 2 kV with a pulse duration of 10 ps and a frequency of 1 kHz were applied to the substrate. CN films of 0.8 pm thickness were deposited with a deposition rate of 0.1 pm min I. 2.3. CN films deposited by the PCA method
A magnetically filtered pulsed cathodic arc [16] was used as the plasma source of highly ionized carbon ions. The operating parameters were as follows: arc current, 200-250 A; pulse duration, 1.5 ms; pulse repetition rate, I-lOps. Nitrogen ions were supplied by a nitrogen plasma similar to the CAPII method. A negative d.c. voltage (- 50 V) was used to bias the substrate. CN films of 1.0 pm thickness were deposited using this method.
3. Results and discussion 3.1. XRD analysis
All films deposited by the AAPII and CAPII methods were found to be amorphous. 3.2. XPS analysis
The surface elemental composition was obtained from the survey spectra. The highest N/C ratio of 0.286 was found in the films deposited by the PCA method. N /C ratios between 0.135 and 0.25 were found in the films prepared by the AAPII and CAPII methods. A small amount of oxygen contamination was detected in the films. The N/C ratios are considerably lower than the optimal ratio predicted for carbon nitride. The XPS results for the films deposited by the AAPII and CAPII methods are similar and will be discussed first. A typical high resolution CIs spectrum of the CN films prepared by the AAPII and CAPII methods is shown in Fig. 1. The peak deconvolution results for pure C and CN films with variable N /C ratios are summarized in Table I. Four carbon peaks at binding energies of 284.5 eV, 285.6 eV, 287.1 eV and 289.3 eV
[.F. Husein et al. / Materials Science and Engineering A209 (1996) 10- 15
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Binding Energy. eV
Fig. 1. A typical XPS CIs spectrum. CN film with N/C = 0.15 synthesized by the CAPII method.
are identified in all the films. Carbon bound to itself, regardless of hybridization, has a characteristic peak at 285.0 eV which is usually used as the binding energy reference [22]. Thus the peak at 284.5 eV in the high resolution CIs spectrum indicates the formation of -C-C-C-C- groups. The main graphitic C Is peak from a carbon film exhibits an asymmetric tail towards high binding energy. The tail is attributed to the interaction of the positive core hole which is formed as a result of the primary photoemission process with the conduction electrons (conduction band interaction, CBI). Thus the peaks at 285.6 eV, 287.1 eV and 289.3 eV can be associated with three different processes, i.e. surface oxidation, formation of carbon-nitrogen bonds and the CBI mechanism. Table 1 compares the intensities of these peaks. The ratio of the peak intensity at 285.6 eV to the main peak at 284.5 eV is consistent for both the C film and the CN films with a range of 0.44-0.49. This observation suggests that the peak at 285.6 eV is predominantly due to the graphitic structure and can be associated with the CBI mechanism which is intrinsic to the photoemission process. The presence of
Fig. 2. A typical XPS N Is spectrum. CN film with N/C=0.15 synthesized by the CAPII method.
the peak at 289.3 eV indicates the formation of some C(O)-type groups on the surface. Analysis of the ratio changes for the peak at 287.1 eV shows that the CN films have ratios which increase with increasing nitrogen content. These ratios are between 0.4 and 0.6, i.e. much higher than the ratio of the pure carbon film (0.23). These observations suggest that the peak at 287.1 eV can be associated with the formation of carbon-nitrogen bonds. Fig. 2 shows a typical high resolution N 1s spectrum of CN films prepared by the AAPII and CAPII methods and Table 1 summarizes the peak deconvolution results. Four nitrogen peaks were identified at binding energies of 398.4 eV, 399.7 eV, 402.9 eV and 405.2 eV. The peaks at 402.9 eV and 405.2 eV can be attributed to N(O) bondings. The two major nitrogen peaks at 398.4 eV and 399.7 eV originate from C-N and C=N bonds respectively [4]. The intensity of the peaks indicate the dominance of the C=N bonds over the C- N bonds. The percentage of the C- N bonds increases with increasing N/C ratio. Similar XPS results were obtained for the AAPII and CAPII films with similar
Table 1 XPS C Is and N Is peak deconvolution of C and CN films with different N/C ratios deposited by AAPII and CAPII methods Peak
Binding energy (eV)
Peak deconvolution Carbon film
C Is
N Is
284.5 285.6 287.1 289.3 398.4 399.7 402.9 405.2
N/C=0.135 (AAPII)
N/C = 0.25 (AAPII)
N/C = 0.15 (CAPII)
at. Ii'l,
Ratio
at.%
Ratio
at.%
Ratio
at.%
Ratio
55.1 27.2 12.9 4.7
1.0 0.49 0.23 0.09
51.9 24.2 20.9 3.0 25.1 71.4 2.7 0.9
1.0 0.46 0.40 0.06
47.9 22.1 29.0 1.0 30.9 65.2 3.9
1.0 0.46 0.6 0.02
50.4 22.2 24.5 2.8 25.3 70.9 1.5 2.3
1.0 0.44 0.49 0.06
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406.0
402.0
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Fig. 3. XPS N Is spectrum of a CN mm with N/C sized by the PCA method.
13
Marerials Science und Engineering A209 (1996) 10 15
=
0.286 synthe-
NjC ratios. Table 1 shows the similarity between an AAPII (NjC = 0.135) and a CAPII (NjC = 0.15) film. Initial XPS studies were conducted on the PCA films. Fig. 3 shows the N Is spectrum of a CN film deposited by PCA with an NjC ratio of 0.268. The C-N bonds (at a binding energy of 398.6 eV) dominate the C=N bonds (at 399.9 eV) in this film.
3.3. Contact angle measurements and surface tension calculations
The measurement of the contact angle is a highly sensitive technique for the determination of the composition and properties of surfaces [21]. From contact angle measurements, accurate calculations of the surface tension (and surface free energy) of a solid and interfacial tension between the deposited film and the substrate can be obtained. Many methods have been proposed to calculate the surface tension of a solid, such as the geometric mean method, eq uation of state method and harmonic mean method. The latter method is more accurate and preferred to the other methods [21]. Knowledge of the surface tension and components of the CN films may be helpful in studying the deposited film structures and adhesion properties.
The results obtained for the contact angles of deionized water, ethylene glycol and diiodomethylene on carbon and CN films with variable NjC ratios (prepared by the AAPII and CAPII methods on polished and rough surfaces of Si wafers) are presented in Table 2. As can be seen from the table, the contact angles for the selected liquids reflect the changes in the compositions of the investigated film surfaces. For water and ethylene glycol, the contact angles decrease with increasing N/C ratio, and for diiodomethylene, they increase. As in the XPS analysis, a strong similarity is observed between the AAPII and CAPII films with similar N IC ratios. The surface tension (y s) of the films and its dispersion (y~)) and polar (yf) components were calculated using the harmonic mean method (21]. Given the surface tension of the testing liquids and the contact angles, the dispersion and polar components of a solid surface tension can be calculated. Table 3 summarizes these calculations and the polarity (X P = yfjys) calculations. rncreasing the N/C ratio increases the polar component (and the polarity) of the surface tension. This suggests the formation of new covalent CN bonds, which confirms the results obtained by XPS analysis. The interfacial tension and work of adhesion (WJ between the films and the Si wafer (films were deposited on both polished and rough surfaces of the wafer) are calculated from the dispersion and polar components of the deposited films and the Si substrate. These calculations are summarized in Table 2. Comparison between the carbon and CN films shows that the latter have a higher work of adhesion and a lower interfacial tension. This indicates that increasing the nitrogen content of the films improves their adhesion to the substrate surface (both polished and rough). 3.4. Nanoindentation analysis
Nanoindentation measurements were conducted on a carbon film, an AAPII film (N jC = 0.135) and a CAPII
Table 2 Measurements of the advancing contact angle for three droplets of testing liquid. Calculated interfacial tension and work of adhesion between C or CN films and Si wafer surfaces (polished and rough) deposited by the AAPII and CAPII methods __ ..
. ~ ~ . _ .
Film composition (deposition method)
C film (anodic arc) N/C = 0.135 (AAPII) N/C = O.IS (CAPII) C film (anodic arc) N/C = O.2S (AAPII)
Advancing contact angles
~ ~ _
..•..
~ ~ - - _ . ~ - ~ ~ - _ .
n
Substrate surface (Si)
Surface tension between films and Si wafer (dyn cm- 1)
Work of adhesion (erg cm- C)
Water
Ethylene glycol
Diiodomethylene
77 68
37 33
32 34
Polished Polished
10.4 5.0
69.0 73.2
69 69 63
33 35 29
34 36 41
Polished Rough Rough
5.3 3.9 1.5
no
-~~--
71.3 75.5
14
I.F. Husein et al. j Materials Science and Engineering A209 (1996) 10-15
Table 3 Surface tension components and polarity of the C and CN films of Table 2 calculated by the harmonic mean method at 20°C Film composition (deposition method)
Substrate surface (Si)
Cfilm (anodic arc) NjC = 0.135 (AAPII) NjC=0.15 (CAPII) Cfilm (anodic arc) NjC = 0.25 (AAPII) Si wafer Si wafer
Polished Polished Polished Rough Rough Polished Rough
Surface tension components and polarity (dyn cm -1) Dispersion
Polar
Total C',)
Polarity (X P)
(i?)
(;,r)
;\ = ~'~)+;';
x, p = "PI" Is/Is
38.4 33.6 34.0 32.5 25.0 20.3 18.5
5.4 9.0 8.7 10.0 18.8 15.3 14.7
43.8 42.6 42.7 42.5 43.8 32.6 33.2
0.12 0.21 0.20 0.24 0.43 0.43 0.44
film (N/C = 0.15). The highest hardness of 18.9 ± 0.6 GPa was obtained for the AAPII film; Fig. 4 shows the load-displacement curve of this film. This is higher than the value obtained for the unimplanted carbon film which has a hardness of 16.9 ± 0.6 GPa. A value of 16.4 ± 0.3 GPa was found for the CAPII film.
3.5. Raman spectroscopy Preliminary studies using this technique were performed on a carbon film and an AAPII film (N/C = 0.25). The Raman spectrum of the AAPII film shows a peak around 2330 cm - I, which is indicative of the formation of the CooN bond [23]. The pure carbon film shows a small peak at around the same wavelength with a much lower intensity. This can be attributed to the presence of a C=C bond.
4. Conclusions Amorphous carbon nitride thin films were synthesised by three methods (AAPII, CAPII and PCA) utiliz8r---------------~
7
ing anodic and cathodic vacuum arcs as sources of partially and fully ionized carbon plasmas respectively and plasma ion implantation as the primary nitrogen source. Although the N/C ratios of our CN films were considerably lower than the optimal ratio, interesting preliminary results were obtained from the use of the above three techniques. (1) XPS analysis of the films prepared by the three methods showed that increasing the nitrogen content of the films increased the intensity of the Cis peak at 287.1 eV and the N Is peak at 398.4 eV, which are associated with C- N bond formation. (2) The increase in the polar component of the surface tension as the N/C ratio increases suggests the formation of covalent carbon-nitrogen bonds. (3) The CN films exhibited an improvement in adhesion to the Si substrate relative to the pure carbon films as indicated by the interfacial tension and work of adhesion calculations from the contact angle measurements.
Acknowledgements Special thanks are due to Mr. Joe A. Genevich for helping to construct much of the apparatus used in the experiments.
6
z
References
5
E 4
".3 o
3 2
Displacement (nm)
Fig. 4. Load-displacement curve of a CN film with NjC = 0.135 prepared by the AAPII method. A hardness of 18.9 GPa was calculated for this film.
[I] A.Y. Liu and M.L. Cohen, Science, 245 (1989) 841. [2] A.Y. Liu and M.L. Cohen, Phys. Rev. B, 32 (1985) 7988. [3] A.Y. Liu and R.M. Wentzcovitch, Phys. Rev. B, 50 (1994) 10362. [4] D. Marton, K.J. Boyd, A.H. AI-Bayati, S.S. Todorov and J.W. Rabalais, Phys. Rev. Lell., 73 (1994) 118. [5] T.A. Yeh, e.L. Lin, J.M. Sivertsen and J.H. Judy, IEEE Trans. Magn., 27 (1991) 5163. [6] K.M. Yu, M.L. Cohen, E.E. Haller, W.L. Hansen, A.Y. Liu and I.e. Wu, Phys. Rev. B, 49 (1994) 5034. [7] C.J. Torng, J.M. Sivertsen, J.H. Judy and e. Chang, J. Mater. Res., 5 (II) (1990) 2490. [8] e. Niu, Y.Z. Lu and e.M. Lieber, Science, 261 (1993) 334.
J.F. Husein et 01.
Materials Science and Engineering A209 (1996) 10 15
[9] K. Ogata, 1. Chubaci and F. Fujimoto, 1. Appl. PhI's .. 76 (6) (1994) 3791. [10] D. Li. E. Cutiongco, Y. Chung, M. Wong and W. Sproul, Surf Coat. Technol. 68/69 (1994) 611. [11] H. Han and BJ. Feldman. Solid State COl11mun., 65 (9) (1988) 921. [12] Z. Xia and C. Chan, 1. Appl. Phys .. 73 (1993) 3651. [13] S. Meassick, C. Chan and R. Allen, SUfI Coot. Tee/mol., 54 (1992) 343. [14] S. Meassick, J. Kumpf, R. Allen, C. Chan and T. Sroda, Mater. Lett., 14 (1992) 63. [IS] V. Buck, J.B. Orde and M. Mausbach, Mater. Sci. Eng. A, 140 (1991) 770. [16] T. Sroda, S. Meassick and C. Chan, Appl. Phys. Lett., 60 (1992) 1075.
15
[17] D.M. Sanders. D.B. Boercker and S. Falabella, IEEE Trans. Plasma Sci., 18 (6) (1990) 883. [18] R. Lossy, D.L. Pappas, R.A. Roy and J.J. Cuomo, Appl. Phys. Lett., 61 (2) (1992) 171. [19] A. Andres, S. Andres, l.G. Brown, M.R. Dickinson and R.A. MacGilL 1. Vac. Sci. Technol. B, 12 (12) (1994) 815. [20] W. Gutowski. in L.H. Lee (ed.), Fundal11entals of Adhesion, Plenum, New York. 1991, p. 126. [21] S. Wu. Polymer Interface and Adhesion, Marcel Dekker, New York, 1982. p. 178. [22] C.D. Wagner. Handhook of X-Ray Photoelectron Spectroscopy. Perkin-Elmer, Physical Electronics Division. Eden Prairie. MN, 1978. [23] J.J. Cuomo. P.A. Leary, D. Yu, W. Reuter and M. Frisch, 1. Vac. Sci. Technol., 16 (2) (1979) 299.
Materials Science and Engineering A209 (1996) 10-15
Synthesis of carbon nitride thin films by vacuum arcs Imad F. Huseina, YuanZhong Zhoua, Fan Lia, Ryne C. Allen'\ Chung Chana, Jacob I. Kleiman b , Yu Gudimenko b , Clark V. Cooperc a Plasma
Science Laboratory, Department oj' Electrical and Computer Engineering, Northeastern University, Boston, M A 02115, USA bIntegrity Testing Lahoratory/University 0/ Toronto Institute/or Aermpace Studies, Do1t'l7sviell', Ont, M3H 5T6, Canada C United Technologies Research Center, East Hartford, CT 06108, USA
Abstract Carbon nitride (CN) thin films were synthesized by combining vacuum arcs and plasma ion implantation techniques. Three methods were investigated: plasma ion implantation into carbon films deposited by anodic vacuum arcs (AAPII), continuous cathodic vacuum arc with plasma ion implantation (CAPII) and pulsed cathodic vacuum arc (PCA), The films were found to be amorphous by X-ray diffraction (XRD). X-Ray photoelectron spectroscopy (XPS) and Raman spectroscopy analysis indicated the formation ofC-N, C=N and C=N bonds. Calculations of the surface tension components (dispersion and polar) of the films using the contact angle measurement technique suggested the formation of covalent carbon-nitrogen bonds. The CN films exhibited improved adhesion relative to the pure carbon films as indicated by adhesion calculations and the reduction in interfacial tension between the films and the substrate. A hardness of 18.9 GPa was obtained by nanoindentation measurements for CN films with an N/C ratio of 0.135. Keywords: Carbon nitride thin films; Plasma ion implantation; Vacuum arcs
1. Introduction The possibility of forming solids with covalent CN bonds, such as fJ -C 3N 4 , as new hard materials was proposed by Liu and Cohen in 1989 based on theoretical calculations [1,2]. These calculations predicted that carbon nitrides with the composition C 3N 4 in the /f phase (fJ -C 3N 4 ) would have a high bulk modulus comparbale with that of diamond and a moderately large cohesive energy. More recently, Liu and Wentzcovitch [3] identified zinc-blende-like cubic and graphite-like metastable carbon nitrides with the composition C 3N 4 . Cohesive energy calculations showed that fJ and graphite-like structures have very similar cohesive energies, which suggests that in forming the fJ phase we must overcome the competition from the energetically favorable Sp2 bond found in the graphite-like phase [3]. Many experimental methods have been used to synthesize fJ -C 3N 4 employing different film deposition techniques: ion beam deposition [4], r.f. diode sputtering [5- 7] , pulsed laser ablation [8], ion and vapor deposition [9], d.c. magnetron sputtering [4,10] and plasma
decomposition of CH 4 and N 2 [II]. In this paper, we present a preliminary study of carbon nitride films deposited on silicon wafers by three methods utilizing vacuum arcs and plasma ion implantation techniques: (I) plasma immersion ion implantation [12] into carbon films deposited by anodic vacuum arcs (AAPII) [13IS]; (2) continuous cathodic vacuum arcs with plasma ion implantation (CAPII); (3) pulsed cathodic vacuum arcs (PCA) [16]. The films were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), measurement of the contact angles of water, ethylene glycol and diiodomethylene on the deposited surfaces, nanoindentation measurements and Raman spectroscopy. The use of cathodic and anodic vacuum arcs with plasma immersion ion implantation (PIlI) is a new technique which has the potential to succeed in forming carbon nitride films. This technique combines the advantages of vacuum arcs in synthesizing diamond-like films with high hardness (95 GPa) and a high percentage of Sp3 bonds [17,18] with the advantages of the PIlI method in improving adhesion and reducing stress in thin films [19]. 0921-5093/96/$15,00 © 1996 -
Elsevier Science S.A, All rights reserved SSDI0921-5093(95)10096-2
/.F. Husein el 11/.
Jfarerill/s Science ilnd Engineering·L:lN (/996) /0 /5
11
2. Experimental details
2.4. Film c1wructeri:ation
2.1. CNfilms prepared by the AAPIlmethod
The crystal structure of the films was investigated by XRD. A 0-5000 XRD Siemens equipment using the Cu K:x line was employed. The composition and CN bonding nature of the films were investigated by XPS. This analysis was performed on a Leybold MAX 200 X-ray photoelectron spectrometer using monochromatized Mg K:x exciting radiation. Survey scans and high resolution scans were performed on all samples deposited by the three methods. To characterize the adhesion and surface tension properties of the CN films, measurements of the advancing contact angle for three droplets of testing liquid on carbon and CN films were made. Deionized water with a surface tension energy of 72.8 dyn em I, ethylene glycol with a surface tension energy of 48.2 dyn cm I and diiodomethylene with a surface tension energy of 50.8 dyn cm I were used. A custom-designed set-up was used and the geometric parameters of the droplets were measured by an optical system and a photo-camera. The sessile drop technique [20,21] was employed with droplet volumes of 2- 3 pI. The contact angle was measured as a tangent angle formed between the liquid and the surface. The film hardness was measured by a Nanoindenterl equipment (Nano Instruments Inc.); it is equipped with a three-sided diamond Berkovick indenter. The rate of loading and unloading was 400 p N s I
In this method, carbon films were deposited on silicon wafers utilizing the anodic vacuum arc technique [13-15]. An arc was ignited by an electric trigger between two graphite electrodes of cylindrical geometry and sustained by a consumable anode. The anodic vacuum arc produced a partially ionized carbon vapor plasma (less than 20% ionized) [13]. Films with thicknesses around 0.8-1.7 ,um (measured by a Dektak3 surface profilometer) were deposited with a deposition rate of 0.5 ,um min - 1 using currents between 50 and 60 A. Higher deposition rates can be achieved depending on factors such as the geometry of the electrodes, arc current and distance between the arc and the substrate. These carbon films were later immersed in a nitrogen plasma with a working gas pressure maintained at about 1.3 x 10 2 Pa (l x 10 4 Torr). Negative voltage pulses were applied to the substrate in order to extract the nitrogen ions from the plasma. Nitrogen ions also diffused into the films between the high voltage pulses via thermal diffusion [12]. Voltages of 12kV were used with a pulse length of 4-8 ps and a repetition frequency of 6-14 kHz. 2.2. CN films deposited by the CAPII method
In this method, the pulsed cathodic arc source in Ref. [16] was modified to run in a continuous mode, i.e. continuous cathodic are, providing a flux of fully ionized carbon plasma, unlike the partially ionized plasma of the anodic arc. A baffled magnetic duct was attached to the source to filter the macroparticles and neutrals from the carbon ions [16]. Nitrogen was fed into the vacuum chamber with a base pressure of 1.3 x 10 4 Pa (l x 10- 6 Torr) and the working pressure was maintained at about 1.3 x 10 2 Pa (l x 10 4 Torr). Thermionic emission was used to form a nitrogen plasma background surrounding the silicon substrate. Negative voltage pulses of 2 kV with a pulse duration of 10 ps and a frequency of 1 kHz were applied to the substrate. CN films of 0.8 pm thickness were deposited with a deposition rate of 0.1 pm min I. 2.3. CN films deposited by the PCA method
A magnetically filtered pulsed cathodic arc [16] was used as the plasma source of highly ionized carbon ions. The operating parameters were as follows: arc current, 200-250 A; pulse duration, 1.5 ms; pulse repetition rate, I-lOps. Nitrogen ions were supplied by a nitrogen plasma similar to the CAPII method. A negative d.c. voltage (- 50 V) was used to bias the substrate. CN films of 1.0 pm thickness were deposited using this method.
3. Results and discussion 3.1. XRD analysis
All films deposited by the AAPII and CAPII methods were found to be amorphous. 3.2. XPS analysis
The surface elemental composition was obtained from the survey spectra. The highest N/C ratio of 0.286 was found in the films deposited by the PCA method. N /C ratios between 0.135 and 0.25 were found in the films prepared by the AAPII and CAPII methods. A small amount of oxygen contamination was detected in the films. The N/C ratios are considerably lower than the optimal ratio predicted for carbon nitride. The XPS results for the films deposited by the AAPII and CAPII methods are similar and will be discussed first. A typical high resolution CIs spectrum of the CN films prepared by the AAPII and CAPII methods is shown in Fig. 1. The peak deconvolution results for pure C and CN films with variable N /C ratios are summarized in Table I. Four carbon peaks at binding energies of 284.5 eV, 285.6 eV, 287.1 eV and 289.3 eV
[.F. Husein et al. / Materials Science and Engineering A209 (1996) 10- 15
12
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55000
c
II
"'~ 0
60000
II
C 50000
80
;; Q
60
.~
40
45000 40000
20 296
292
408
280
406
404
402
400
398
396
Binding Energy. eV
Binding Energy. eV
Fig. 1. A typical XPS CIs spectrum. CN film with N/C = 0.15 synthesized by the CAPII method.
are identified in all the films. Carbon bound to itself, regardless of hybridization, has a characteristic peak at 285.0 eV which is usually used as the binding energy reference [22]. Thus the peak at 284.5 eV in the high resolution CIs spectrum indicates the formation of -C-C-C-C- groups. The main graphitic C Is peak from a carbon film exhibits an asymmetric tail towards high binding energy. The tail is attributed to the interaction of the positive core hole which is formed as a result of the primary photoemission process with the conduction electrons (conduction band interaction, CBI). Thus the peaks at 285.6 eV, 287.1 eV and 289.3 eV can be associated with three different processes, i.e. surface oxidation, formation of carbon-nitrogen bonds and the CBI mechanism. Table 1 compares the intensities of these peaks. The ratio of the peak intensity at 285.6 eV to the main peak at 284.5 eV is consistent for both the C film and the CN films with a range of 0.44-0.49. This observation suggests that the peak at 285.6 eV is predominantly due to the graphitic structure and can be associated with the CBI mechanism which is intrinsic to the photoemission process. The presence of
Fig. 2. A typical XPS N Is spectrum. CN film with N/C=0.15 synthesized by the CAPII method.
the peak at 289.3 eV indicates the formation of some C(O)-type groups on the surface. Analysis of the ratio changes for the peak at 287.1 eV shows that the CN films have ratios which increase with increasing nitrogen content. These ratios are between 0.4 and 0.6, i.e. much higher than the ratio of the pure carbon film (0.23). These observations suggest that the peak at 287.1 eV can be associated with the formation of carbon-nitrogen bonds. Fig. 2 shows a typical high resolution N 1s spectrum of CN films prepared by the AAPII and CAPII methods and Table 1 summarizes the peak deconvolution results. Four nitrogen peaks were identified at binding energies of 398.4 eV, 399.7 eV, 402.9 eV and 405.2 eV. The peaks at 402.9 eV and 405.2 eV can be attributed to N(O) bondings. The two major nitrogen peaks at 398.4 eV and 399.7 eV originate from C-N and C=N bonds respectively [4]. The intensity of the peaks indicate the dominance of the C=N bonds over the C- N bonds. The percentage of the C- N bonds increases with increasing N/C ratio. Similar XPS results were obtained for the AAPII and CAPII films with similar
Table 1 XPS C Is and N Is peak deconvolution of C and CN films with different N/C ratios deposited by AAPII and CAPII methods Peak
Binding energy (eV)
Peak deconvolution Carbon film
C Is
N Is
284.5 285.6 287.1 289.3 398.4 399.7 402.9 405.2
N/C=0.135 (AAPII)
N/C = 0.25 (AAPII)
N/C = 0.15 (CAPII)
at. Ii'l,
Ratio
at.%
Ratio
at.%
Ratio
at.%
Ratio
55.1 27.2 12.9 4.7
1.0 0.49 0.23 0.09
51.9 24.2 20.9 3.0 25.1 71.4 2.7 0.9
1.0 0.46 0.40 0.06
47.9 22.1 29.0 1.0 30.9 65.2 3.9
1.0 0.46 0.6 0.02
50.4 22.2 24.5 2.8 25.3 70.9 1.5 2.3
1.0 0.44 0.49 0.06
I.F. Husein
fI
al.
-eo
410.0
406.0
402.0
3980
8inding Energy (eV)
Fig. 3. XPS N Is spectrum of a CN mm with N/C sized by the PCA method.
13
Marerials Science und Engineering A209 (1996) 10 15
=
0.286 synthe-
NjC ratios. Table 1 shows the similarity between an AAPII (NjC = 0.135) and a CAPII (NjC = 0.15) film. Initial XPS studies were conducted on the PCA films. Fig. 3 shows the N Is spectrum of a CN film deposited by PCA with an NjC ratio of 0.268. The C-N bonds (at a binding energy of 398.6 eV) dominate the C=N bonds (at 399.9 eV) in this film.
3.3. Contact angle measurements and surface tension calculations
The measurement of the contact angle is a highly sensitive technique for the determination of the composition and properties of surfaces [21]. From contact angle measurements, accurate calculations of the surface tension (and surface free energy) of a solid and interfacial tension between the deposited film and the substrate can be obtained. Many methods have been proposed to calculate the surface tension of a solid, such as the geometric mean method, eq uation of state method and harmonic mean method. The latter method is more accurate and preferred to the other methods [21]. Knowledge of the surface tension and components of the CN films may be helpful in studying the deposited film structures and adhesion properties.
The results obtained for the contact angles of deionized water, ethylene glycol and diiodomethylene on carbon and CN films with variable NjC ratios (prepared by the AAPII and CAPII methods on polished and rough surfaces of Si wafers) are presented in Table 2. As can be seen from the table, the contact angles for the selected liquids reflect the changes in the compositions of the investigated film surfaces. For water and ethylene glycol, the contact angles decrease with increasing N/C ratio, and for diiodomethylene, they increase. As in the XPS analysis, a strong similarity is observed between the AAPII and CAPII films with similar N IC ratios. The surface tension (y s) of the films and its dispersion (y~)) and polar (yf) components were calculated using the harmonic mean method (21]. Given the surface tension of the testing liquids and the contact angles, the dispersion and polar components of a solid surface tension can be calculated. Table 3 summarizes these calculations and the polarity (X P = yfjys) calculations. rncreasing the N/C ratio increases the polar component (and the polarity) of the surface tension. This suggests the formation of new covalent CN bonds, which confirms the results obtained by XPS analysis. The interfacial tension and work of adhesion (WJ between the films and the Si wafer (films were deposited on both polished and rough surfaces of the wafer) are calculated from the dispersion and polar components of the deposited films and the Si substrate. These calculations are summarized in Table 2. Comparison between the carbon and CN films shows that the latter have a higher work of adhesion and a lower interfacial tension. This indicates that increasing the nitrogen content of the films improves their adhesion to the substrate surface (both polished and rough). 3.4. Nanoindentation analysis
Nanoindentation measurements were conducted on a carbon film, an AAPII film (N jC = 0.135) and a CAPII
Table 2 Measurements of the advancing contact angle for three droplets of testing liquid. Calculated interfacial tension and work of adhesion between C or CN films and Si wafer surfaces (polished and rough) deposited by the AAPII and CAPII methods __ ..
. ~ ~ . _ .
Film composition (deposition method)
C film (anodic arc) N/C = 0.135 (AAPII) N/C = O.IS (CAPII) C film (anodic arc) N/C = O.2S (AAPII)
Advancing contact angles
~ ~ _
..•..
~ ~ - - _ . ~ - ~ ~ - _ .
n
Substrate surface (Si)
Surface tension between films and Si wafer (dyn cm- 1)
Work of adhesion (erg cm- C)
Water
Ethylene glycol
Diiodomethylene
77 68
37 33
32 34
Polished Polished
10.4 5.0
69.0 73.2
69 69 63
33 35 29
34 36 41
Polished Rough Rough
5.3 3.9 1.5
no
-~~--
71.3 75.5
14
I.F. Husein et al. j Materials Science and Engineering A209 (1996) 10-15
Table 3 Surface tension components and polarity of the C and CN films of Table 2 calculated by the harmonic mean method at 20°C Film composition (deposition method)
Substrate surface (Si)
Cfilm (anodic arc) NjC = 0.135 (AAPII) NjC=0.15 (CAPII) Cfilm (anodic arc) NjC = 0.25 (AAPII) Si wafer Si wafer
Polished Polished Polished Rough Rough Polished Rough
Surface tension components and polarity (dyn cm -1) Dispersion
Polar
Total C',)
Polarity (X P)
(i?)
(;,r)
;\ = ~'~)+;';
x, p = "PI" Is/Is
38.4 33.6 34.0 32.5 25.0 20.3 18.5
5.4 9.0 8.7 10.0 18.8 15.3 14.7
43.8 42.6 42.7 42.5 43.8 32.6 33.2
0.12 0.21 0.20 0.24 0.43 0.43 0.44
film (N/C = 0.15). The highest hardness of 18.9 ± 0.6 GPa was obtained for the AAPII film; Fig. 4 shows the load-displacement curve of this film. This is higher than the value obtained for the unimplanted carbon film which has a hardness of 16.9 ± 0.6 GPa. A value of 16.4 ± 0.3 GPa was found for the CAPII film.
3.5. Raman spectroscopy Preliminary studies using this technique were performed on a carbon film and an AAPII film (N/C = 0.25). The Raman spectrum of the AAPII film shows a peak around 2330 cm - I, which is indicative of the formation of the CooN bond [23]. The pure carbon film shows a small peak at around the same wavelength with a much lower intensity. This can be attributed to the presence of a C=C bond.
4. Conclusions Amorphous carbon nitride thin films were synthesised by three methods (AAPII, CAPII and PCA) utiliz8r---------------~
7
ing anodic and cathodic vacuum arcs as sources of partially and fully ionized carbon plasmas respectively and plasma ion implantation as the primary nitrogen source. Although the N/C ratios of our CN films were considerably lower than the optimal ratio, interesting preliminary results were obtained from the use of the above three techniques. (1) XPS analysis of the films prepared by the three methods showed that increasing the nitrogen content of the films increased the intensity of the Cis peak at 287.1 eV and the N Is peak at 398.4 eV, which are associated with C- N bond formation. (2) The increase in the polar component of the surface tension as the N/C ratio increases suggests the formation of covalent carbon-nitrogen bonds. (3) The CN films exhibited an improvement in adhesion to the Si substrate relative to the pure carbon films as indicated by the interfacial tension and work of adhesion calculations from the contact angle measurements.
Acknowledgements Special thanks are due to Mr. Joe A. Genevich for helping to construct much of the apparatus used in the experiments.
6
z
References
5
E 4
".3 o
3 2
Displacement (nm)
Fig. 4. Load-displacement curve of a CN film with NjC = 0.135 prepared by the AAPII method. A hardness of 18.9 GPa was calculated for this film.
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