- Email: [email protected]
High quality diamond like carbon thin film fabricated by ECR plasma CVD
applied
surface science ELSEVIER
Applied Surface Science
I 13, I 14 (I 997) 227-230
High quality diamond like carbon thin film fabricated by ECR plasma CVD K. Kuramoto
*, Y. Domoto, H. Hirano, S. Kiyama, S. Tsuda
New Materials R.C.. Sanyo Elecrric Co.. Ltd.. l-18-13 Hoshiridani. Hirakata, Osaka 57.3. Japan
Abstract Diamond like carbon (DLC) films have been fabricated by electron cyclotron resonance plasma enhanced chemical vapor deposition (ECR CVD>. Low temperature (less than 70°C) fabrication of the DLC films, with a hardness of more than 3000 Hv and a high deposition rate of more than 80 nm/min, was achieved by applying a bias voltage during deposition to accelerate the ion impinging energy to the substrate. Furthermore, an attempt was made to fabricate ultrathin DLC films for coating purposes. Ultrathin DLC films by this method with a thickness of 10 nm exhibited excellent wear characteristics and chemical inertness, indicating their usability as protective coating for electronic devices.
protective
1. Introduction Diamond like carbon (DLC) films exhibit a variety of useful properties such as a hardness approaching that of a diamond, transparency, a smooth surface morphology, wear resistance and chemical inertness [1,2]. A number of DLC film fabrication methods using sputtering [3], ion beam [4], and plasma deposition [5,6] have been investigated. However, a low deposition temperature and a high deposition rate could not be simultaneously achieved. Fabrication of DLC films by electron cyclotron resonance (ECR) plasma enhanced chemical vapor deposition (CVD) was expected to reduce the deposition temperature by separating the plasma generating region from the film deposition region. Additionally, independent control of the bias voltage was expected to control the energy of ions impinging upon the substrate better.
’ Corresponding author. Tel.: + 8 l-720-417869; 417842; e-mail: [email protected]. 0169.4332/97/$17.00 Copyright PII SO169-4332(96)00768-4
fax: + 8 I-720.
To apply DLC films to electronic devices as protective coating, two significant points should be considered. One is that low temperature fabrication of the hard DLC films is important to avoid thermal damage to devices, and the other is that the coating thickness should be reduced while maintaining film uniformity and surface smoothness to avoid configurative changes to devices. Until now, however, investigations on the properties of ultrathin DLC films have been scarce. In this paper, changes in the deposition rate, temperature and film properties with bias voltage are reported. Furthermore, the effect of ion impinging in the characterization and uniformity of films, and the fabrication of ultrathin DLC films by ECR CVD are presented.
2. Experimental
apparatus
and procedure
A schematic view of the experimental apparatus is shown in Fig. 1. Argon gas was decomposed in the
0 1997 Elsevier Science B.V. All rights reserved.
228
K. Kuramoto et al./Applied
Plasma flow
Substrate
\
Mi wa
I
Substrate
Sur$ace Science 1I3/
114 (1997) 227-230
holder
/
-_)
vacuum
0’
0
’
’ 20
’
’ 40
’ 60
’
Bias voltage Fig. 2. Dependence Fig. 1. Schematic view of experimental
’
’ 80
B
1 0
[-VI
of deposition rate on bias voltage.
apparatus.
ECR cavity (875 G, 2.45 GHz). The substrate holder was powered by an RF (13.56 MHz) generator to control the self-bias voltage to the substrate. Methane gas was introduced near the surface of the substrate. The argon and methane partial pressures were set at 7.6 X lo-* Pa and 1.3 X 10-l Pa, respectively. The microwave power was kept constant at 200 W, and the substrate bias voltage was varied from 0 to - 100 V by adjusting the RF power, for the purpose of controlling the ion impinging energy to the substrate.
3. Results and discussion
3. I. EfSect of bius aoltage on deposition temperature
rate and
The dependence of the deposition rate on the substrate bias voltage is shown in Fig. 2. As the figure shows, the deposition rate increases with bias voltage and then becomes almost constant (80 nm/min) for a substrate bias voltage above -50 V. This result indicates that the ions accelerated by the bias voltage play an important role in increasing the film deposition rate. Hardness and deposition temperature are important factors that need special consideration in practical applications. Fig. 3 shows the dependence of deposition temperature and film hardness on bias voltage. The deposition temperature was measured with a thermocouple in contact with the substrate
surface after deposition for 15 min. The hardness of the films drastically increased when a bias voltage of -20 V was applied, and then became almost constant with increasing bias voltage, but the deposition temperature was about 70°C or less at bias voltages of less than -50 V. The considerable reason for the increasing deposition temperature was ion impingement on the substrate throughout the process. In this case, the ion impinging energy was expectedly much smaller than the value of several hundred volts that have been commonly applied in RF CVD methods to obtain hard DLC films IS]. This low bias voltage conceivably contributed to the low temperature fabrication in this method. Accordingly, by applying a low bias voltage, low temperature fabrication of the films with a hardness of more than 3000 Hv and a high deposition rate of more than 80 nm/min could be achieved.
5000 Deposition:
0
20
40
1Smin
60
Bias voltage Fig. 3. Dependence of thin film hardness ture on bias voltage.
80
100
[-VI and substrate
tempera-
229
K. Kuramoto et al. /Applied Surface Science 113 / 114 I 1997) 227-230
Bias voltage:OV ThicknessfIOOnm
Kl
1800
1600
1400
1200
4. Properties of ultrathin DLC films I
1000
Wave number [cm-l] Fig. 4. Raman spectra of films without and with bias (- 50 V).
3.2. Structure of DLC film Fig. 4 shows the Raman spectra of films grown on a Si(100) substrate at bias voltages of 0 V and -50 V. As the figure shows, the Raman spectrum is broad when no bias is applied. On the other hand, the relative intensity of the Raman spectra increases as a bias voltage is applied, and a typical spectrum corresponding to DLC, which has a peak at around 1530 cm-’ with a shoulder at around 1400 cm-’ [6] can be observed. This result indicates that ion impingement promotes the formation of DLC films. Scanning electron micrographs of the surface and fractured cross section of films prepared on Si(100) substrates at bias voltages of 0 V and -50 V are shown in Fig. 5. By applying a substrate bias voltage, the film began to exhibit a minute structure and a smooth surface, which was conceivably caused by the peening effect of ion impingement.
In this method, a low deposition temperature of less than 70°C and an improvement in surface smoothness by the effect of ion impingement were achieved. Based on these results, fabrication of ultrathin DLC films was attempted. The scanning electron micrograph of a sample prepared by this method is shown in Fig. 6. The deposition thickness was set at 10 nm and the substrate bias voltage was set at -50 V. From Fig. 6, the formation of a uniform film with a smooth surface was confirmed. This film was also confirmed to be a typical DLC film with almost the same Raman spectrum as the lower spectrum in Fig. 4. RF plasma deposited hydrogenated amorphous silicon (a-Si:H) films having almost the same structure as DLC films, 10 nm scale clustery structures were observed, and they were caused by the formation and coalescence of nuclei [7]. However, the DLC film was formed by a uniform growth process at the initial stage of deposition. These results conceivably indicate that layer by layer like growth occurs in this method. Wear resistance and chemical inertness are important factors for coating materials on electronic devices. Wear characteristics were estimated by a friction test in which the ultrathin DLC film was reciprocally rubbed four hundred times by a loaded (50 gf> alumina (A1,OX> ball with a diameter of 10 mm. Fig. 7 shows wear tracks formed on Si substrates with and without the ultrathin DLC coating (thickness: IO nm>. An obvious wear track could be ob-
Surface
Surface
J-KY
Film
Si substrate Si substrate Bias voltage:OV
500nm
lOOnm Bias voltage:-50V
Fig. 5. Scanning electron micrographs of the surface and fractured cross section of films prepared on Si substrates.
Fig. 6. The scanning electron micrograph of the surface and fractured cross section of an ultrathin DLC film fabricated on a Si substrate.
K. Kuramoto et al. /Applied Surface Science I13 / I14 ( 1997) 227-230
230
resistance of the thin-film resistor without coating drastically increased at the beginning of the test. From these results, high quality ultrathin DLC films with excellent wear characteristics and chemical inertness were confirmed. Without DLC
With DLC (Thickness:lOnm)
5. Conclusion
Fig. 7. Wear tracks formed on Si substrates with and without ultrathin DLC coating (thickness: 10 nm) after a friction test.
served on the bare Si substrate, but it was quite faint on the ultrathin DLC film. This result indicates that ultrathin DLC films fabricated by this method have satisfactory wear characteristics to protect the surface of the Si substrate. The chemical inertness of the ultrathin DLC film coated on the surface of a thin-film resistor was estimated by a corrosion test in corrosive solution. Fig. 8 shows the variation of the relative resistance of the thin-film resistor with time in corrosive solution. The increase in the relative resistance of the thin-film resistor indicates that the thin-film resistor was etched by the corrosive solution. As Fig. 8 shows, the relative resistance to the initial value of the thin-film resistor coated with the ultrathin DLC film (10 nm) was stable. On the other hand, the e :’ AWith
2.5
7
DLC (Thickness:
Diamond like carbon (DLC) films have been fabricated by an ECR CVD process. With increasing bias voltage, the film hardness and deposition rate also increased, and the film began to exhibit a minute structure and smooth surface. Thus, ions in ECR plasma conceivably play an important role in the characterization and uniformity of DLC films. Consequently, low temperature (less than 70°C) fabrication of DLC films with a hardness of more than 3000 Hv and a high deposition rate of more than 80 nm/min at low bias voltage was confirmed. Furthermore, an ultrathin DLC film of 10 nm thickness was found to provide excellent protection for electronic device coating.
Acknowledgements The authors would like to thank Professor Dr. Y. Mori of Osaka University for his constant guidance and encouragement.
10nm)
References [I] A.
0.5
1 . 0
’
.
’
.
20 10 Duration of the corrosion
Fig. 8. Variation of the relative resistance with time in corrosive solution.
m 30 test
.
[2] [3]
’
40 [mm.]
of a thin-film
[4] [5] resistor
Bubcnzer, B. Dischler, G. Brandt and P. Koidl, J. Appl. Phys. 54 (1983) 4590. P. Koidl, Proc. Electrochem. Sot. 89 (1989) 237. X.-M. Tang, J. Weber, Y. Baer, C. Miiller, W. Hanni and H.E. Hintermann, Phys. Rev. B 48 (1993) 10124. S. Aisenberg and R. Chabot, J. Appl. Phys. 42 (1971) 2953. E.H.A. Dekempeneer, R. Jacobs, J. Smeets, J. Meneve, L. Eersels, B. Blanpain, J. Roos and D.J. Oostra, Thin Solid Films 217 (1992) 56.
surface science ELSEVIER
Applied Surface Science
I 13, I 14 (I 997) 227-230
High quality diamond like carbon thin film fabricated by ECR plasma CVD K. Kuramoto
*, Y. Domoto, H. Hirano, S. Kiyama, S. Tsuda
New Materials R.C.. Sanyo Elecrric Co.. Ltd.. l-18-13 Hoshiridani. Hirakata, Osaka 57.3. Japan
Abstract Diamond like carbon (DLC) films have been fabricated by electron cyclotron resonance plasma enhanced chemical vapor deposition (ECR CVD>. Low temperature (less than 70°C) fabrication of the DLC films, with a hardness of more than 3000 Hv and a high deposition rate of more than 80 nm/min, was achieved by applying a bias voltage during deposition to accelerate the ion impinging energy to the substrate. Furthermore, an attempt was made to fabricate ultrathin DLC films for coating purposes. Ultrathin DLC films by this method with a thickness of 10 nm exhibited excellent wear characteristics and chemical inertness, indicating their usability as protective coating for electronic devices.
protective
1. Introduction Diamond like carbon (DLC) films exhibit a variety of useful properties such as a hardness approaching that of a diamond, transparency, a smooth surface morphology, wear resistance and chemical inertness [1,2]. A number of DLC film fabrication methods using sputtering [3], ion beam [4], and plasma deposition [5,6] have been investigated. However, a low deposition temperature and a high deposition rate could not be simultaneously achieved. Fabrication of DLC films by electron cyclotron resonance (ECR) plasma enhanced chemical vapor deposition (CVD) was expected to reduce the deposition temperature by separating the plasma generating region from the film deposition region. Additionally, independent control of the bias voltage was expected to control the energy of ions impinging upon the substrate better.
’ Corresponding author. Tel.: + 8 l-720-417869; 417842; e-mail: [email protected]. 0169.4332/97/$17.00 Copyright PII SO169-4332(96)00768-4
fax: + 8 I-720.
To apply DLC films to electronic devices as protective coating, two significant points should be considered. One is that low temperature fabrication of the hard DLC films is important to avoid thermal damage to devices, and the other is that the coating thickness should be reduced while maintaining film uniformity and surface smoothness to avoid configurative changes to devices. Until now, however, investigations on the properties of ultrathin DLC films have been scarce. In this paper, changes in the deposition rate, temperature and film properties with bias voltage are reported. Furthermore, the effect of ion impinging in the characterization and uniformity of films, and the fabrication of ultrathin DLC films by ECR CVD are presented.
2. Experimental
apparatus
and procedure
A schematic view of the experimental apparatus is shown in Fig. 1. Argon gas was decomposed in the
0 1997 Elsevier Science B.V. All rights reserved.
228
K. Kuramoto et al./Applied
Plasma flow
Substrate
\
Mi wa
I
Substrate
Sur$ace Science 1I3/
114 (1997) 227-230
holder
/
-_)
vacuum
0’
0
’
’ 20
’
’ 40
’ 60
’
Bias voltage Fig. 2. Dependence Fig. 1. Schematic view of experimental
’
’ 80
B
1 0
[-VI
of deposition rate on bias voltage.
apparatus.
ECR cavity (875 G, 2.45 GHz). The substrate holder was powered by an RF (13.56 MHz) generator to control the self-bias voltage to the substrate. Methane gas was introduced near the surface of the substrate. The argon and methane partial pressures were set at 7.6 X lo-* Pa and 1.3 X 10-l Pa, respectively. The microwave power was kept constant at 200 W, and the substrate bias voltage was varied from 0 to - 100 V by adjusting the RF power, for the purpose of controlling the ion impinging energy to the substrate.
3. Results and discussion
3. I. EfSect of bius aoltage on deposition temperature
rate and
The dependence of the deposition rate on the substrate bias voltage is shown in Fig. 2. As the figure shows, the deposition rate increases with bias voltage and then becomes almost constant (80 nm/min) for a substrate bias voltage above -50 V. This result indicates that the ions accelerated by the bias voltage play an important role in increasing the film deposition rate. Hardness and deposition temperature are important factors that need special consideration in practical applications. Fig. 3 shows the dependence of deposition temperature and film hardness on bias voltage. The deposition temperature was measured with a thermocouple in contact with the substrate
surface after deposition for 15 min. The hardness of the films drastically increased when a bias voltage of -20 V was applied, and then became almost constant with increasing bias voltage, but the deposition temperature was about 70°C or less at bias voltages of less than -50 V. The considerable reason for the increasing deposition temperature was ion impingement on the substrate throughout the process. In this case, the ion impinging energy was expectedly much smaller than the value of several hundred volts that have been commonly applied in RF CVD methods to obtain hard DLC films IS]. This low bias voltage conceivably contributed to the low temperature fabrication in this method. Accordingly, by applying a low bias voltage, low temperature fabrication of the films with a hardness of more than 3000 Hv and a high deposition rate of more than 80 nm/min could be achieved.
5000 Deposition:
0
20
40
1Smin
60
Bias voltage Fig. 3. Dependence of thin film hardness ture on bias voltage.
80
100
[-VI and substrate
tempera-
229
K. Kuramoto et al. /Applied Surface Science 113 / 114 I 1997) 227-230
Bias voltage:OV ThicknessfIOOnm
Kl
1800
1600
1400
1200
4. Properties of ultrathin DLC films I
1000
Wave number [cm-l] Fig. 4. Raman spectra of films without and with bias (- 50 V).
3.2. Structure of DLC film Fig. 4 shows the Raman spectra of films grown on a Si(100) substrate at bias voltages of 0 V and -50 V. As the figure shows, the Raman spectrum is broad when no bias is applied. On the other hand, the relative intensity of the Raman spectra increases as a bias voltage is applied, and a typical spectrum corresponding to DLC, which has a peak at around 1530 cm-’ with a shoulder at around 1400 cm-’ [6] can be observed. This result indicates that ion impingement promotes the formation of DLC films. Scanning electron micrographs of the surface and fractured cross section of films prepared on Si(100) substrates at bias voltages of 0 V and -50 V are shown in Fig. 5. By applying a substrate bias voltage, the film began to exhibit a minute structure and a smooth surface, which was conceivably caused by the peening effect of ion impingement.
In this method, a low deposition temperature of less than 70°C and an improvement in surface smoothness by the effect of ion impingement were achieved. Based on these results, fabrication of ultrathin DLC films was attempted. The scanning electron micrograph of a sample prepared by this method is shown in Fig. 6. The deposition thickness was set at 10 nm and the substrate bias voltage was set at -50 V. From Fig. 6, the formation of a uniform film with a smooth surface was confirmed. This film was also confirmed to be a typical DLC film with almost the same Raman spectrum as the lower spectrum in Fig. 4. RF plasma deposited hydrogenated amorphous silicon (a-Si:H) films having almost the same structure as DLC films, 10 nm scale clustery structures were observed, and they were caused by the formation and coalescence of nuclei [7]. However, the DLC film was formed by a uniform growth process at the initial stage of deposition. These results conceivably indicate that layer by layer like growth occurs in this method. Wear resistance and chemical inertness are important factors for coating materials on electronic devices. Wear characteristics were estimated by a friction test in which the ultrathin DLC film was reciprocally rubbed four hundred times by a loaded (50 gf> alumina (A1,OX> ball with a diameter of 10 mm. Fig. 7 shows wear tracks formed on Si substrates with and without the ultrathin DLC coating (thickness: IO nm>. An obvious wear track could be ob-
Surface
Surface
J-KY
Film
Si substrate Si substrate Bias voltage:OV
500nm
lOOnm Bias voltage:-50V
Fig. 5. Scanning electron micrographs of the surface and fractured cross section of films prepared on Si substrates.
Fig. 6. The scanning electron micrograph of the surface and fractured cross section of an ultrathin DLC film fabricated on a Si substrate.
K. Kuramoto et al. /Applied Surface Science I13 / I14 ( 1997) 227-230
230
resistance of the thin-film resistor without coating drastically increased at the beginning of the test. From these results, high quality ultrathin DLC films with excellent wear characteristics and chemical inertness were confirmed. Without DLC
With DLC (Thickness:lOnm)
5. Conclusion
Fig. 7. Wear tracks formed on Si substrates with and without ultrathin DLC coating (thickness: 10 nm) after a friction test.
served on the bare Si substrate, but it was quite faint on the ultrathin DLC film. This result indicates that ultrathin DLC films fabricated by this method have satisfactory wear characteristics to protect the surface of the Si substrate. The chemical inertness of the ultrathin DLC film coated on the surface of a thin-film resistor was estimated by a corrosion test in corrosive solution. Fig. 8 shows the variation of the relative resistance of the thin-film resistor with time in corrosive solution. The increase in the relative resistance of the thin-film resistor indicates that the thin-film resistor was etched by the corrosive solution. As Fig. 8 shows, the relative resistance to the initial value of the thin-film resistor coated with the ultrathin DLC film (10 nm) was stable. On the other hand, the e :’ AWith
2.5
7
DLC (Thickness:
Diamond like carbon (DLC) films have been fabricated by an ECR CVD process. With increasing bias voltage, the film hardness and deposition rate also increased, and the film began to exhibit a minute structure and smooth surface. Thus, ions in ECR plasma conceivably play an important role in the characterization and uniformity of DLC films. Consequently, low temperature (less than 70°C) fabrication of DLC films with a hardness of more than 3000 Hv and a high deposition rate of more than 80 nm/min at low bias voltage was confirmed. Furthermore, an ultrathin DLC film of 10 nm thickness was found to provide excellent protection for electronic device coating.
Acknowledgements The authors would like to thank Professor Dr. Y. Mori of Osaka University for his constant guidance and encouragement.
10nm)
References [I] A.
0.5
1 . 0
’
.
’
.
20 10 Duration of the corrosion
Fig. 8. Variation of the relative resistance with time in corrosive solution.
m 30 test
.
[2] [3]
’
40 [mm.]
of a thin-film
[4] [5] resistor
Bubcnzer, B. Dischler, G. Brandt and P. Koidl, J. Appl. Phys. 54 (1983) 4590. P. Koidl, Proc. Electrochem. Sot. 89 (1989) 237. X.-M. Tang, J. Weber, Y. Baer, C. Miiller, W. Hanni and H.E. Hintermann, Phys. Rev. B 48 (1993) 10124. S. Aisenberg and R. Chabot, J. Appl. Phys. 42 (1971) 2953. E.H.A. Dekempeneer, R. Jacobs, J. Smeets, J. Meneve, L. Eersels, B. Blanpain, J. Roos and D.J. Oostra, Thin Solid Films 217 (1992) 56.