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Physical properties of a-C:H films prepared by electron cyclotron resonance microwave plasma chemical vapor deposition
Materials Science and Engineering B77 (2000) 229 – 234 www.elsevier.com/locate/mseb
Physical properties of a-C:H films prepared by electron cyclotron resonance microwave plasma chemical vapor deposition X.T. Zhou, S.T. Lee *, I. Bello, A.C. Cheung, D.S. Chiu, Y.W. Lam, C.S. Lee, K.M. Leung, X.M. He Center of Super-Diamond & Ad6anced Films and Department of Physics and Materials Science, City Uni6ersity of Hong Kong, Kowloon, Hong Kong Received 14 May 1999; accepted 16 May 2000
Abstract a-C:H films have been deposited on silicon and glass using electron cyclotron resonance microwave plasma decomposition of CH4 diluted with Ar gas at 0–160 V rf negative bias and 65 – 250 W microwave power. The deposition rates, absorption coefficients, optical bandgaps, refractive indices and internal stresses of the a-C:H films grown under varying preparation parameters have been measured. The microstructure of the carbon films has been evaluated by Raman spectroscopy. It has been found that the properties and structure of the carbon films are strongly dependent upon the rf substrate bias voltage. At lower rf biases (0 and 40 V), the films are transparent polymer-like carbon films with lower refractive indices, lower internal stresses, and higher optical bandgaps; at higher rf biases (80, 120 and 160 V), they are semi-transparent diamond-like carbon films with higher refractive indices, higher internal stresses and lower optical bandgaps. Microwave power has little influence on the properties and structure of the a-C:H films. © 2000 Elsevier Science S.A. All rights reserved. Keywords: a-C:H films; Physical properties; ECRMPCVD
1. Introduction Diamond-like carbon (DLC) films are formed by the relaxation of molecules from highly non-equilibrium states into more or less stable phases. DLC films possess a number of outstanding properties including high hardness, high electrical resistance and high optical transparency [1]. These properties allow diverse applications of this material such as wear-resistant coatings [2], integrated circuit passivation coatings [3], infrared optical coatings [4,5] and biocompatible coatings [6]. The basic mechanism of diamond-like carbon deposition involves the bombardment of a substrate by
* Corresponding author. Tel.: +852-27889606; fax: 27844696. E-mail address: [email protected] (S.T. Lee).
+ 852-
energetic ions during deposition. Several ion-assisted deposition methods, including ion-plating, magnetronsputtering, plasma enhanced chemical vapor deposition, and catholic arc deposition have been developed [7,8]. However, electron cyclotron resonance microwave plasma chemical vapor deposition (ECR-MPCVD) is preferred for the steady state growth of a-C:H films. This technique allows high production rate of plasma species, low substrate temperature, and independent control of ion energy during depositions [9–11]. Currently, some studies on DLC deposition by ECR-MPCVD method are performed. These works indicate that the properties of a-C:H films are strongly dependent upon experimental conditions. In this work, we studied the properties (deposition rate, absorption coefficient, optical gap, refractive index and internal stress) and evaluated the structure of a-C:H films prepared by the ECR-MPCVD method systematically.
0921-5107/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 0 ) 0 0 4 8 7 - 6
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X.T. Zhou et al. / Materials Science and Engineering B77 (2000) 229–234
2.2. Properties measurement The thicknesses of the prepared a-C:H films were measured with an Alpha-Step 500 profilometer. The optical transmittance of the films was measured with a UV/VIS spectrometer (Perkin Elmer-Lambda 2s), and the wavelengths ranged from 190 to 1100 nm. The absorption coefficients a of a-C:H films on glass were systematically calculated from the obtained absorbency and thickness data. The optical bandgaps E0 were calculated on the basis of the Tauc relation [1]: (aE)1/2 = B(E− E0)
Fig. 1. Plot of deposition rate vs. rf bias voltage (WM = 200 W).
where E is the photon energy and B the Tauc slope. The refractive indices of the films deposited on silicon were determined by an automatic ellipsometer (AutoELR-2). Raman spectroscopic measurements were performed at room temperature with a Renishaw Raman System 200 imaging microscope using a 25-mW He-Ne laser (632.8 nm). The internal stresses of the carbon films were measured by determining the bending deflection of the substrates before and after the film deposition. Internal stress was calculated from the following formula [12]: s= 4Es ts 2d/(3(1− n)L 2 tf)
Table 1 Growth conditions for a-C:H films Substrate Gas Pressure
Si, glass Ar, CH4 0.1–0.2 Pa
Flow rate Ar CH4
70 sccm 7 sccm
Deposition time rf bias Microwave power
10 min to 3 h 0–160 V 65–250 W
(1)
(2)
where Es is the Young’s modulus of the substrate, n is the Poisson’s ratio of the substrate, tf and ts are the thicknesses of the film and substrate, respectively, and L is the length of the scanned substrate. The samples used in these measurements were deposited on glass substrates (24× 12× 0.18 mm3) and had a thickness of about 0.5 mm. The deflection d was determined by an Alpha-step 100 profilometer.
3. Results
3.1. Deposition rate
2. Experimental methods
2.1. Film deposition The technology applied here has been described in detail previously [11]. Briefly, the films were prepared by plasma, decomposition of a mixture of argon and methane in an ASTeX AX2040 ECR-MPCVD system. A 13.56-MHz rf bias was coupled to bias the substrate holder. Prior to deposition, the chamber was first evacuated to 10 − 4 Pa and then Ar was introduced for sputter cleaning of the substrate (200 W microwave power, 100 V rf bias, 10 min). Finally, CH4 was introduced for the deposition of a-C:H films. The growth conditions of the films are shown in Table 1.
Fig. 1 shows the relationship between the deposition rates of a-C:H films on silicon and rf bias voltages, at a microwave power of 200 W. Even without any rf bias (VB = 0), an a-C:H film can still be deposited on the substrate. The deposition rate increases with increasing rf bias and reaches a maximum at 120 V. At higher rf voltages, the experimental deposition rate decreases after passing through a maximum. This reduction may be attributed to the film resputtering by energetic ions, especially Ar+. The energy of the ions in the ECR plasma increases with increasing rf bias, hindering the growth of films. Fig. 2 shows the deposition rates of a-C:H films on silicon versus microwave power, at a bias of 80 V. With increasing microwave power, the deposition rate increases and reaches a maximum at 220 W. However, at microwave powers greater than 250 W, no films were
X.T. Zhou et al. / Materials Science and Engineering B77 (2000) 229–234
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deposited, because increase in microwave power directly raise the amount of ions, especially Ar+. The Ar+ ion sputtering etches the films so rapidly that no films can be formed when the microwave power is greater than the critical value.
3.2. Absorption coefficient Figs. 3 and 4 show the plots of the absorption coefficients of the a-C:H films prepared on glass at various biases and microwave powers, respectively. The absorption coefficients of all the samples decreased with increasing wavelengths of light. This indicates that
Fig. 4. Plot of absorption coefficient vs. wavelength (VB = − 80 V).
Fig. 2. Plot of deposition rate vs. microwave power (VB = −80 V).
longer wavelengths of light penetrate the a-C:H films than shorter wavelengths of light. In addition, the absorption coefficients are strongly dependent upon the rf biases applied during the preparations of the films. At higher rf biases (80, 120 and 160 V), the absorption coefficients of the a-C:H films are higher. At lower rf biases (0 and 40 V), the absorption coefficients are approximately one order of magnitude lower than those of the a-C:H films prepared at higher rf biases. Raman spectroscopy shows that the a-C:H films prepared at higher rf biases are diamond-like carbon (DLC) films while the films prepared at lower rf biases are transparent polymer-like carbon films. However, absorption coefficients of the a-C:H films prepared at various microwave powers, were about the same at shorter wavelengths of light but differed slightly at longer wavelengths of light.
3.3. Optical bandgap Fig. 5 shows the optical bandgaps of a-C:H films prepared at various rf biases. The bandgap decreases from 2.0 to 1.0 eV with increasing rf bias. This implies transformation of a-C:H films from transparent polymer-like carbon to semi-transparent DLC films. The optical bandgaps of the a-C:H films prepared at various microwave powers are about the same.
3.4. Refracti6e index
Fig. 3. Plot of absorption coefficient vs. wavelength (WM =200 W).
Plots of refractive indices a-C:H films of light (wavelength of 632.8 nm) for as functions of rf bias and microwave power are shown in Figs. 6 and 7. The refractive index increases rapidly from 1.54 at rf bias=
232
X.T. Zhou et al. / Materials Science and Engineering B77 (2000) 229–234
0 V, to 2.06 at bias = 160 V. At a bias of 80 V, the values of refractive indices at various microwave powers are greater than 1.7 and increase slowly with increasing microwave power.
3.5. Internal stress Figs. 8 and 9 show the dependence of the internal stress of the films upon the rf bias and microwave power. All specimens showed compressive stress. At lower rf biases (0 and 40 V), the stress is very low
Fig. 7. Plot of refractive index vs. microwave power (VB = − 80 V, l = 632.8 nm).
Fig. 5. Plot of optical bandgap vs. rf bias voltage (WM = 200 W).
Fig. 8. Plot of internal stress vs. rf bias voltage (WM = 200 W).
(1×108 Pa). Then, the stress increases rapidly with increasing rf bias. At higher rf biases (120 and 160 V), the stress maintains a high value (9 × 108 Pa). At rf bias of 80 V, the stresses of the films prepared at various microwave powers are generally high (5–9×108 Pa).
3.6. Raman spectra
Fig. 6. Plot of refractive index vs. rf bias voltage (WM =200 W, l= 632.8 nm).
Raman spectra of carbon films deposited at various rf bias voltages and a microwave power of 200 W are shown in Fig. 10. The films deposited at rf bias voltages
X.T. Zhou et al. / Materials Science and Engineering B77 (2000) 229–234
233
under 40 V display intense fluorescence, which indicates the formation of polymerized organic structures with high hydrogen content. This is similar to the result obtained by Nagai et al. [10]. The film deposited at 80 V indicates a relatively sharp Raman band at 1550 cm − 1 (G band) with a shoulder at 1360 cm − 1 (D band). This suggests that the microstructure of the films changes from a polymer-like structure to a DLC structure as the rf bias voltage increases from 40 to 80 V. However, from the two peaks (1360 and 1550 cm − 1) obtained at higher rf bias voltages (120 and 160 V), it
Fig. 11. Raman spectra of carbon films deposited at the bias voltage of − 80 V with a variety of microwave powers: 65, 150, 200, 220 W.
Fig. 9. Plot of internal stress vs. microwave power (VB = −80 V).
implies that glassy carbon was formed due to higher substrate temperatures at higher rf biases. In addition, the ratio of sp3 to sp2 configurations decreases when the rf bias increases from 80 to 160 V. This is because the ratio of the carbon films in the amorphous phase to the graphitic phase decreases as the ratio of the intensity of the 1360-cm − 1 band to the intensity of the 1580-cm − 1 band increases. This is contrary to the result obtained by using a dc bias where the above band intensity ratio decreased with increasing dc bias from 150 to 250 V [10]. Fig. 11 shows the Raman spectra of carbon films deposited at various microwave powers and a rf bias voltage of 80 V. All the curves show a Raman band at 1550 cm − 1 and a shoulder at 1360 cm − 1, implying that the films contain DLC structure.
4. Discussions
Fig. 10. Raman spectra of carbon films deposited at the microwave power of 200 W with a variety of rf bias voltages: 0, 40, 80, 120, 160 V.
In this study, the physical characteristics and structure of a-C:H films are found to be highly dependent upon the substrate rf bias, the effect of ion bombardment is crucial for depositing DLC films, regardless of method used. For example, in a rf plasma deposition system, VB \ 100 V is generally needed to produce hard DLC films. In our experiment, the ratio of CH4:Ar is 1/10, the total pressure is 0.1–0.2 Pa and the mean path is approximately several centimeters. However, there is a high concentration of Ar+ ions in ECR plasma, and the ions are easily accelerated by the electric field induced by the substrate bias. The energy at which the ions bombard the substrate is strongly dependent upon
234
X.T. Zhou et al. / Materials Science and Engineering B77 (2000) 229–234
the substrate bias. The higher the substrate bias, the higher the energy of the ions in the plasma. Obviously, the energy of the ions plays an important role in the growth of carbon films using the ECRMPCVD method. At lower rf biases (0 and 40 V), the films are transparent polymer-like carbon films with a lower refractive index (1.6), higher optical bandgap (2.0), and lower internal stress (1×108 Pa). This is attributed to the lower energy of ion bombardment on the growing films, which is similar to the polymer-like carbon films prepared by rf plasma deposition at VB B 100 V [13]. At higher rf biases (80, 120 and 160 V), the films are semi-transparent DLC films with higher refractive indices (\1.8), lower optical bandgaps (B1.4 eV) and higher internal stresses (\ 5 ×108 Pa). This is due to the high-energy bombardment of ions on the growing films. Microwave power has little influence on the properties and microstructure of a-C:H films, except for a minor effect on the deposition rate and refractive index. Microwave power effects only the amount, not the energy, of the ions and neutral radicals, which bombard the growing film.
5. Conclusion a-C:H films have been deposited on silicon and glass as through electron cyclotron resonance microwave
.
plasma decomposition of mixtures of argon and methane. The properties and structure of the carbon films are strongly dependent upon the rf bias voltage. At lower rf biases (0 and 40 V), the films are transparent polymer-like carbon films with lower refractive indices, lower internal stresses and higher optical bandgaps; and at higher rf biases (80, 120 and 160 V), they are semi-transparent diamond-like carbon films with higher refractive indices, higher internal stresses and lower optical bandgaps. Microwave power has little influence on the structure and properties of a-C:H films.
References [1] J.C. Angus, P. Koidl, S. Domity, Plasma Deposited Thin Films, CRC Press, Boca Raton, FL, 1986, p. 90. [2] H. Tai, D. Bogy, J. Vac. Technol. A5 (1987) 3287. [3] A. Ono, T. Baba, H. Funamato, A. Nishikawa, Jpn. J. Appl. Phys. 25 (1986) L808. [4] B. Dischler, A. Bubenzer, P. Koidl, Appl. Phys. Lett. 42 (1983) 636. [5] F.M. Kimock, D.J. Knapp, Surf. Coat. Technol. 56 (1993) 273. [6] A.H. Lettington, D.J. Knapp, Diamond Relat. Mater. 1 (1992) 805. [7] F. Richter, et al., Thin Solid Films 212 (1992) 245. [8] W. Scharff, et al., Thin Solid Films 171 (1989) 157. [9] P.S. Audry, P.W. Pastel, W.J. Varhue, J. Mater. Res. 11 (1) (1996) 221. [10] I. Nagai, et al., J. Appl. Phys. 67 (6) (1990) 2890. [11] X.M. He et al. J. Mater. Res., in press. [12] J.W. Zou, et al., J. Appl. Phys. 67 (1) (1989) 487.
Physical properties of a-C:H films prepared by electron cyclotron resonance microwave plasma chemical vapor deposition X.T. Zhou, S.T. Lee *, I. Bello, A.C. Cheung, D.S. Chiu, Y.W. Lam, C.S. Lee, K.M. Leung, X.M. He Center of Super-Diamond & Ad6anced Films and Department of Physics and Materials Science, City Uni6ersity of Hong Kong, Kowloon, Hong Kong Received 14 May 1999; accepted 16 May 2000
Abstract a-C:H films have been deposited on silicon and glass using electron cyclotron resonance microwave plasma decomposition of CH4 diluted with Ar gas at 0–160 V rf negative bias and 65 – 250 W microwave power. The deposition rates, absorption coefficients, optical bandgaps, refractive indices and internal stresses of the a-C:H films grown under varying preparation parameters have been measured. The microstructure of the carbon films has been evaluated by Raman spectroscopy. It has been found that the properties and structure of the carbon films are strongly dependent upon the rf substrate bias voltage. At lower rf biases (0 and 40 V), the films are transparent polymer-like carbon films with lower refractive indices, lower internal stresses, and higher optical bandgaps; at higher rf biases (80, 120 and 160 V), they are semi-transparent diamond-like carbon films with higher refractive indices, higher internal stresses and lower optical bandgaps. Microwave power has little influence on the properties and structure of the a-C:H films. © 2000 Elsevier Science S.A. All rights reserved. Keywords: a-C:H films; Physical properties; ECRMPCVD
1. Introduction Diamond-like carbon (DLC) films are formed by the relaxation of molecules from highly non-equilibrium states into more or less stable phases. DLC films possess a number of outstanding properties including high hardness, high electrical resistance and high optical transparency [1]. These properties allow diverse applications of this material such as wear-resistant coatings [2], integrated circuit passivation coatings [3], infrared optical coatings [4,5] and biocompatible coatings [6]. The basic mechanism of diamond-like carbon deposition involves the bombardment of a substrate by
* Corresponding author. Tel.: +852-27889606; fax: 27844696. E-mail address: [email protected] (S.T. Lee).
+ 852-
energetic ions during deposition. Several ion-assisted deposition methods, including ion-plating, magnetronsputtering, plasma enhanced chemical vapor deposition, and catholic arc deposition have been developed [7,8]. However, electron cyclotron resonance microwave plasma chemical vapor deposition (ECR-MPCVD) is preferred for the steady state growth of a-C:H films. This technique allows high production rate of plasma species, low substrate temperature, and independent control of ion energy during depositions [9–11]. Currently, some studies on DLC deposition by ECR-MPCVD method are performed. These works indicate that the properties of a-C:H films are strongly dependent upon experimental conditions. In this work, we studied the properties (deposition rate, absorption coefficient, optical gap, refractive index and internal stress) and evaluated the structure of a-C:H films prepared by the ECR-MPCVD method systematically.
0921-5107/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 0 ) 0 0 4 8 7 - 6
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X.T. Zhou et al. / Materials Science and Engineering B77 (2000) 229–234
2.2. Properties measurement The thicknesses of the prepared a-C:H films were measured with an Alpha-Step 500 profilometer. The optical transmittance of the films was measured with a UV/VIS spectrometer (Perkin Elmer-Lambda 2s), and the wavelengths ranged from 190 to 1100 nm. The absorption coefficients a of a-C:H films on glass were systematically calculated from the obtained absorbency and thickness data. The optical bandgaps E0 were calculated on the basis of the Tauc relation [1]: (aE)1/2 = B(E− E0)
Fig. 1. Plot of deposition rate vs. rf bias voltage (WM = 200 W).
where E is the photon energy and B the Tauc slope. The refractive indices of the films deposited on silicon were determined by an automatic ellipsometer (AutoELR-2). Raman spectroscopic measurements were performed at room temperature with a Renishaw Raman System 200 imaging microscope using a 25-mW He-Ne laser (632.8 nm). The internal stresses of the carbon films were measured by determining the bending deflection of the substrates before and after the film deposition. Internal stress was calculated from the following formula [12]: s= 4Es ts 2d/(3(1− n)L 2 tf)
Table 1 Growth conditions for a-C:H films Substrate Gas Pressure
Si, glass Ar, CH4 0.1–0.2 Pa
Flow rate Ar CH4
70 sccm 7 sccm
Deposition time rf bias Microwave power
10 min to 3 h 0–160 V 65–250 W
(1)
(2)
where Es is the Young’s modulus of the substrate, n is the Poisson’s ratio of the substrate, tf and ts are the thicknesses of the film and substrate, respectively, and L is the length of the scanned substrate. The samples used in these measurements were deposited on glass substrates (24× 12× 0.18 mm3) and had a thickness of about 0.5 mm. The deflection d was determined by an Alpha-step 100 profilometer.
3. Results
3.1. Deposition rate
2. Experimental methods
2.1. Film deposition The technology applied here has been described in detail previously [11]. Briefly, the films were prepared by plasma, decomposition of a mixture of argon and methane in an ASTeX AX2040 ECR-MPCVD system. A 13.56-MHz rf bias was coupled to bias the substrate holder. Prior to deposition, the chamber was first evacuated to 10 − 4 Pa and then Ar was introduced for sputter cleaning of the substrate (200 W microwave power, 100 V rf bias, 10 min). Finally, CH4 was introduced for the deposition of a-C:H films. The growth conditions of the films are shown in Table 1.
Fig. 1 shows the relationship between the deposition rates of a-C:H films on silicon and rf bias voltages, at a microwave power of 200 W. Even without any rf bias (VB = 0), an a-C:H film can still be deposited on the substrate. The deposition rate increases with increasing rf bias and reaches a maximum at 120 V. At higher rf voltages, the experimental deposition rate decreases after passing through a maximum. This reduction may be attributed to the film resputtering by energetic ions, especially Ar+. The energy of the ions in the ECR plasma increases with increasing rf bias, hindering the growth of films. Fig. 2 shows the deposition rates of a-C:H films on silicon versus microwave power, at a bias of 80 V. With increasing microwave power, the deposition rate increases and reaches a maximum at 220 W. However, at microwave powers greater than 250 W, no films were
X.T. Zhou et al. / Materials Science and Engineering B77 (2000) 229–234
231
deposited, because increase in microwave power directly raise the amount of ions, especially Ar+. The Ar+ ion sputtering etches the films so rapidly that no films can be formed when the microwave power is greater than the critical value.
3.2. Absorption coefficient Figs. 3 and 4 show the plots of the absorption coefficients of the a-C:H films prepared on glass at various biases and microwave powers, respectively. The absorption coefficients of all the samples decreased with increasing wavelengths of light. This indicates that
Fig. 4. Plot of absorption coefficient vs. wavelength (VB = − 80 V).
Fig. 2. Plot of deposition rate vs. microwave power (VB = −80 V).
longer wavelengths of light penetrate the a-C:H films than shorter wavelengths of light. In addition, the absorption coefficients are strongly dependent upon the rf biases applied during the preparations of the films. At higher rf biases (80, 120 and 160 V), the absorption coefficients of the a-C:H films are higher. At lower rf biases (0 and 40 V), the absorption coefficients are approximately one order of magnitude lower than those of the a-C:H films prepared at higher rf biases. Raman spectroscopy shows that the a-C:H films prepared at higher rf biases are diamond-like carbon (DLC) films while the films prepared at lower rf biases are transparent polymer-like carbon films. However, absorption coefficients of the a-C:H films prepared at various microwave powers, were about the same at shorter wavelengths of light but differed slightly at longer wavelengths of light.
3.3. Optical bandgap Fig. 5 shows the optical bandgaps of a-C:H films prepared at various rf biases. The bandgap decreases from 2.0 to 1.0 eV with increasing rf bias. This implies transformation of a-C:H films from transparent polymer-like carbon to semi-transparent DLC films. The optical bandgaps of the a-C:H films prepared at various microwave powers are about the same.
3.4. Refracti6e index
Fig. 3. Plot of absorption coefficient vs. wavelength (WM =200 W).
Plots of refractive indices a-C:H films of light (wavelength of 632.8 nm) for as functions of rf bias and microwave power are shown in Figs. 6 and 7. The refractive index increases rapidly from 1.54 at rf bias=
232
X.T. Zhou et al. / Materials Science and Engineering B77 (2000) 229–234
0 V, to 2.06 at bias = 160 V. At a bias of 80 V, the values of refractive indices at various microwave powers are greater than 1.7 and increase slowly with increasing microwave power.
3.5. Internal stress Figs. 8 and 9 show the dependence of the internal stress of the films upon the rf bias and microwave power. All specimens showed compressive stress. At lower rf biases (0 and 40 V), the stress is very low
Fig. 7. Plot of refractive index vs. microwave power (VB = − 80 V, l = 632.8 nm).
Fig. 5. Plot of optical bandgap vs. rf bias voltage (WM = 200 W).
Fig. 8. Plot of internal stress vs. rf bias voltage (WM = 200 W).
(1×108 Pa). Then, the stress increases rapidly with increasing rf bias. At higher rf biases (120 and 160 V), the stress maintains a high value (9 × 108 Pa). At rf bias of 80 V, the stresses of the films prepared at various microwave powers are generally high (5–9×108 Pa).
3.6. Raman spectra
Fig. 6. Plot of refractive index vs. rf bias voltage (WM =200 W, l= 632.8 nm).
Raman spectra of carbon films deposited at various rf bias voltages and a microwave power of 200 W are shown in Fig. 10. The films deposited at rf bias voltages
X.T. Zhou et al. / Materials Science and Engineering B77 (2000) 229–234
233
under 40 V display intense fluorescence, which indicates the formation of polymerized organic structures with high hydrogen content. This is similar to the result obtained by Nagai et al. [10]. The film deposited at 80 V indicates a relatively sharp Raman band at 1550 cm − 1 (G band) with a shoulder at 1360 cm − 1 (D band). This suggests that the microstructure of the films changes from a polymer-like structure to a DLC structure as the rf bias voltage increases from 40 to 80 V. However, from the two peaks (1360 and 1550 cm − 1) obtained at higher rf bias voltages (120 and 160 V), it
Fig. 11. Raman spectra of carbon films deposited at the bias voltage of − 80 V with a variety of microwave powers: 65, 150, 200, 220 W.
Fig. 9. Plot of internal stress vs. microwave power (VB = −80 V).
implies that glassy carbon was formed due to higher substrate temperatures at higher rf biases. In addition, the ratio of sp3 to sp2 configurations decreases when the rf bias increases from 80 to 160 V. This is because the ratio of the carbon films in the amorphous phase to the graphitic phase decreases as the ratio of the intensity of the 1360-cm − 1 band to the intensity of the 1580-cm − 1 band increases. This is contrary to the result obtained by using a dc bias where the above band intensity ratio decreased with increasing dc bias from 150 to 250 V [10]. Fig. 11 shows the Raman spectra of carbon films deposited at various microwave powers and a rf bias voltage of 80 V. All the curves show a Raman band at 1550 cm − 1 and a shoulder at 1360 cm − 1, implying that the films contain DLC structure.
4. Discussions
Fig. 10. Raman spectra of carbon films deposited at the microwave power of 200 W with a variety of rf bias voltages: 0, 40, 80, 120, 160 V.
In this study, the physical characteristics and structure of a-C:H films are found to be highly dependent upon the substrate rf bias, the effect of ion bombardment is crucial for depositing DLC films, regardless of method used. For example, in a rf plasma deposition system, VB \ 100 V is generally needed to produce hard DLC films. In our experiment, the ratio of CH4:Ar is 1/10, the total pressure is 0.1–0.2 Pa and the mean path is approximately several centimeters. However, there is a high concentration of Ar+ ions in ECR plasma, and the ions are easily accelerated by the electric field induced by the substrate bias. The energy at which the ions bombard the substrate is strongly dependent upon
234
X.T. Zhou et al. / Materials Science and Engineering B77 (2000) 229–234
the substrate bias. The higher the substrate bias, the higher the energy of the ions in the plasma. Obviously, the energy of the ions plays an important role in the growth of carbon films using the ECRMPCVD method. At lower rf biases (0 and 40 V), the films are transparent polymer-like carbon films with a lower refractive index (1.6), higher optical bandgap (2.0), and lower internal stress (1×108 Pa). This is attributed to the lower energy of ion bombardment on the growing films, which is similar to the polymer-like carbon films prepared by rf plasma deposition at VB B 100 V [13]. At higher rf biases (80, 120 and 160 V), the films are semi-transparent DLC films with higher refractive indices (\1.8), lower optical bandgaps (B1.4 eV) and higher internal stresses (\ 5 ×108 Pa). This is due to the high-energy bombardment of ions on the growing films. Microwave power has little influence on the properties and microstructure of a-C:H films, except for a minor effect on the deposition rate and refractive index. Microwave power effects only the amount, not the energy, of the ions and neutral radicals, which bombard the growing film.
5. Conclusion a-C:H films have been deposited on silicon and glass as through electron cyclotron resonance microwave
.
plasma decomposition of mixtures of argon and methane. The properties and structure of the carbon films are strongly dependent upon the rf bias voltage. At lower rf biases (0 and 40 V), the films are transparent polymer-like carbon films with lower refractive indices, lower internal stresses and higher optical bandgaps; and at higher rf biases (80, 120 and 160 V), they are semi-transparent diamond-like carbon films with higher refractive indices, higher internal stresses and lower optical bandgaps. Microwave power has little influence on the structure and properties of a-C:H films.
References [1] J.C. Angus, P. Koidl, S. Domity, Plasma Deposited Thin Films, CRC Press, Boca Raton, FL, 1986, p. 90. [2] H. Tai, D. Bogy, J. Vac. Technol. A5 (1987) 3287. [3] A. Ono, T. Baba, H. Funamato, A. Nishikawa, Jpn. J. Appl. Phys. 25 (1986) L808. [4] B. Dischler, A. Bubenzer, P. Koidl, Appl. Phys. Lett. 42 (1983) 636. [5] F.M. Kimock, D.J. Knapp, Surf. Coat. Technol. 56 (1993) 273. [6] A.H. Lettington, D.J. Knapp, Diamond Relat. Mater. 1 (1992) 805. [7] F. Richter, et al., Thin Solid Films 212 (1992) 245. [8] W. Scharff, et al., Thin Solid Films 171 (1989) 157. [9] P.S. Audry, P.W. Pastel, W.J. Varhue, J. Mater. Res. 11 (1) (1996) 221. [10] I. Nagai, et al., J. Appl. Phys. 67 (6) (1990) 2890. [11] X.M. He et al. J. Mater. Res., in press. [12] J.W. Zou, et al., J. Appl. Phys. 67 (1) (1989) 487.