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Magnetron plasma-enhanced chemical vapor deposition of diamond-like carbon thin films
Thin Solid Films 506 – 507 (2006) 63 – 67 www.elsevier.com/locate/tsf
Magnetron plasma-enhanced chemical vapor deposition of diamond-like carbon thin films V. Anita a,c,*, N. Saito b, O. Takai c b
a Department of Plasma Physics, ‘‘Al.I. Cuza’’ University, Iasi-6600, Romania Department of Molecular Design and Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan c EcoTopia Science Institute, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan
Available online 13 September 2005
Abstract An RF magnetron plasma-enhanced chemical vapor deposition system with variable magnetic field (MagPECVD) was utilized for high rate deposition of diamond-like carbon thin films. Deposition rate was nonuniform, with minimum value of 30 nm/min in the center of the cathode where the low ionic flux is determined by the absence of the E B electronic drift. The highest value of 1029 nm/min was registered in the position of the cathode with the highest ion flux determined by the presence of the E B electronic drift. Hardness, elastic modulus and RMS roughness of thin films were dependent on local values of magnetic field intensities and the values of input power. D 2005 Elsevier B.V. All rights reserved. Keywords: Diamond-like carbon; Methane; Magnetron discharge; Mechanical properties
1. Introduction In the last period, there has been a large interest on diamond-like carbon (DLC) on account of its useful properties such as high resistivity, hardness and density, wear and chemical resistance, smoothness, transparency in the infrared region and biocompatibility [1]. DLC has also the possibility to be doped with different elements in the view to improve these properties [2]. Many methods have been developed and studied to deposit DLC thin films: pulsed dc discharge [3], radio-frequency [4,5] or microwave plasma-enhanced chemical vapor deposition [6], arc discharge [7], liquid-based methods [8], ion beam-assisted systems [9], laser ablation [10] and sputtering [11]. All presented methods necessitate the presence of bias on the substrate in the view to control the energy of positive ions that play a very important role in establishing the amount of sp3 bonded carbon component in thin film [12]. One of the most utilized method and also simplest is the plasma* Corresponding author. EcoTopia Science Institute, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan. Tel.: +81 52 789 5163; fax: +81 52 789 3260. E-mail address: [email protected] (V. Anita). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.08.074
enhanced chemical vapor deposition with plane cathode and radio-frequency powered. This method is characterized by low deposition rate. To improve the deposition rate, there has been developed different variants like using magnetic fields with different configuration in the front of cathode surface or systems with high current density on the cathode and using precursors with low first ionizing energy [13]. Magnetron-enhanced sputtering of materials is one of the most utilized techniques in modern technologies for the fabrication of thin-film structures [14]. The presence of orthogonal E and B fields in the front of magnetron cathode surface generates an E B drift for electrons and the secondary electrons emitted by ion bombardment are confined to the near vicinity of the cathode and improve plasma density. The high plasma density reduces the discharge impedance and induces an increase in the discharge current density and a decrease of discharge voltage at low pressures. By adopting this configuration, mainly utilized for sputtering, we expected to obtain very high deposition rates of DLC thin films at reduced bias voltages. By adjusting the magnetic field value in front of the magnetron cathode, the self bias voltage responsible of ion energy could be controlled. We focused mainly on the effect of input power and values of parallel component of
64
V. Anita et al. / Thin Solid Films 506 – 507 (2006) 63 – 67
pumps was utilized. Basic vacuum pressure was 6 10 3 Pa and the pressure in the time of thin film deposition was kept at 20 Pa by adjusting the mass flow of the reactive gas. The surface morphology of DLC thin films was observed with an atomic force microscope (AFM; JSPM –5200 T, JEOL). The deposition rate was estimated by measuring the thickness of deposited thin films using a surface profilometer (Surfatest SV-600, Mitutoyo). The infrared spectra of the films were analyzed in order to characterize the bonding of hydrogen with carbon in thin films (FTS 175 C, BioRad). Raman spectra were recorded using a Jobin Yvon spectrometer with 514.5 nm laser excitation line and 10 mW output power. The spectra were fitted with a curve-fitting software (based on two Gaussian curve shapes) to identify the G and D peaks positions, to measure and compare the intensities (GRAMS/AIi from ThermoGalactic). The hardness and Young’s modulus of the DLC films were determined from loading– unloading curves on a nanoindentation instrument (TriboScope, Hysitron) using a Berkovich diamond tip. Ten indentations were made for each sample and the hardness and Young’s modulus were calculated by averaging the measurements.
magnetic field on the properties of deposited thin films in different radial positions on the magnetron cathode. The properties of DLC thin films and efficiency of MagPECVD were evaluated by different experimental techniques.
2. Experimental details A chemical vapor deposition system with plane cathode was modified by adding a variable magnetic field with closed lines in front of the cathode in the view to assure E B drift of electrons (Fig. 1). The diameter of obtained magnetron cathode was 100 mm and the parallel component of magnetic field could be varied between 0.02 and 0.065 T by using a movable magnetic circuit and permanent magnets positioned inside the cathode. Because DLC thin films are characterized by high resistivity, we have chosen a capacitive coupled RF power supply with 13.56 MHz frequency and 10 – 400 W variable output power. In this experimental set-up, the active area of cathode under magnetron plasma ring (30 cm2) was much smaller than the area of the anode (6000 cm2). Applied voltage distributed mainly between anode and cathode dark spaces and the larger voltage developed at the smaller electrode as a consequence of big differences between electrons and ion mobilities in the plasma [14]. In the time of deposition, the self bias voltage was monitored in order to obtain information about the average energy of ions involved in thin film growth. Methane (CH4) was utilized as precursor. Thin films were deposited on silicon and aluminum oxide substrates positioned directly on the surface of the magnetron cathode. To obtain vacuum in the process chamber, a system with rotary, mechanical booster and turbomolecular
3. Results and discussion Self bias voltage increased almost linearly with the input power and was dependent on the value of the parallel component of magnetic field at the surface of the cathode (Fig. 2). The highest value (765 V) was measured for normal PECVD (B = 0 T) at 400 W input RF power and 20 Pa pressure of methane precursor. By increasing the parallel component of the magnetic field, self bias voltage decreased
LOW DENSITY PLASMA
HIGH DENSITY PLASMA VACUUM PUMP SILICON SUBSTRATE
S
N
MAGNETRON CATHODE
S
C PLASMA IMPEDANCE MATCH RF POWER SUPPLY
L
V
Fig. 1. Experimental set-up.
SELF-BIAS MEASUREMENT
V. Anita et al. / Thin Solid Films 506 – 507 (2006) 63 – 67
160
800
PECVD
600 500 400
0.020 T
300
0.035 T 0.045 T
200
0.065 T
100
MagPECVD
Deposition rate (nm/min)
0T
700
Bias voltage (V)
65
140 120 100 80 60 40 50 W RF Power 20
0 0
50
100
100 150 200 250 300 350 400 450
200
300
400
500
600
700
Parallel magnetic field component (x 10-4T)
Power (W) Fig. 2. Relationship between self bias voltage and input power (the value of parallel magnetic field component as variable).
Fig. 4. Variation of deposition rate with the value of parallel magnetic field component (50 W input RF power).
to lower values (150 V for 0.065 T and 400 W input RF power). This high variation indicated a decrease in plasma impedance, an increase in discharge current and plasma density [14]. Deposited thin films were nonuniform; the deposition rate in the active area of the cathode was very high as compared with the normal cathode without magnetic field and increased with the input power for the same value of magnetic field. The minimum value (30 nm/min) was measured in the center of the cathode where the maximum value of perpendicular component of the magnetic field was highest (å 0.1 T). The maximum value of deposition rate was registered on the position of the cathode with the maximum value of the parallel component of magnetic field (0.065 T). The deposition rate was 282 nm/min for 80 W input RF power and increased to 1029 nm/min at 400 W input power, as presented in Fig. 3. In the case of normal discharge, without magnetic field, the maximum obtained deposition rate was 47 nm/min at 400 W input power. For a constant input power, the deposition rate increased with the increase in the value of the parallel magnetic field component (Fig. 4). Raman measurements were utilized to obtain the qualitative indication of the amount of hydrogen and
graphitization in DLC thin film. The spectra revealed differences among the properties of thin films obtained in different areas of the cathode. For a constant input power, the slopes of the background of the Raman spectra for thin films deposited in the regions with low deposition rate were high and this feature indicated high content of hydrogen in thin films [15]. The slopes of the background of the Raman spectra for thin films deposited in the regions with high deposition rates were lowest, indicating the decrease in the hydrogen content in thin films determined by high ion flux bombardment. By keeping a constant value of magnetic field and increasing the input power, the slopes of Raman spectra of thin film deposited in a fixed region of magnetron cathode decreased, indicating also the dehydrogenation in DLC thin films (Fig. 5). This feature is determined by highenergy ion bombardment which induces a local increase of temperature. By deconvolution the Raman intensities in graphite component (G band, I G – 1580 cm 1) and disordered graphite component (D band, I D –1370 cm 1) and calculating the integrated intensities ratio I D/I G, it was found that this ratio increased with the increase in the input power. From this evolution resulted that for low-input RF power, 50
1200 1000
40 400 W
Intensity (a.u)
Deposition rate (nm/min)
MagPECVD
800 600 80 W
400
30 W 30 20 200 W 10
200
PECVD 400 W 0
0 -60
-40
-20
0
20
40
Radial distance (mm) Fig. 3. Radial variation of deposition rate.
60
80
400 W 3500
3000
2500
2000
Raman shift
1500
1000
500
(cm-1)
Fig. 5. Raman spectra of thin films deposited at different input RF power.
V. Anita et al. / Thin Solid Films 506 – 507 (2006) 63 – 67
16
500
14
450
12
400
10
350
8
300 Hardness Magnetic field
6
250
4
Magnetic field (x 10-4 T)
the Raman peaks ratio I D/I G has had low values and this feature was associated with high sp3 bonded carbon fraction in deposited thin films [12]. The increase in input power led to a decrease in the sp3 fraction and an increase in the disordered sp2 fraction in deposited DLC thin films. Hardness and elastic modulus of thin films deposited with 0.046 T magnetic field intensity, in the region with the highest deposition rate, are presented in Fig. 6. Hardness increased from 11.4 GPa at 50 W input RF power to 16.2 GPa at 200 W input power and then decreased to 9.2 GPa at 400 W input power. The elastic modulus also increased from 89 GPa at 50 W input power to 121.6 GPa at 200 W and then decreased to 66.7 GPa at 400 W input power. The decrease of hardness and elastic modulus at higher values of input power can be associated with the increase in sp2 component in thin films, induced by high-energy ion bombardment. For a constant input power, the hardness and elastic modulus varied on the radial distance, in the point of the cathode with the highest value of parallel component of magnetic field and higher ion flux being registered a lower value. Fig. 7 presents the radial variation of hardness for 50 W input power and 0.046 T maximum value of parallel component of magnetic field and similarly variations were registered for the elastic modulus. The decrease in hardness and elastic modulus in the point of cathode surface with a high value of magnetic field can be associated with low values of bias voltages and high ionic flux on dielectric diamond-like carbon thin films deposited in these regions. The high ionic flux bombardment induces the increase of local surface temperature above 250 -C and causes a decline in hydrogen content and an increase in sp2 fraction [12]. AFM measurements revealed that the surface morphology of thin films was influenced by the value of utilized power: RMS roughness decreased from 8.41 nm at 50 W to 1.69 nm at 400 W for thin films obtained in the position of the cathode with the highest deposition rate. Sputtering by high energy ions contributed to the smoothing of thin films. For a constant input power of 400 W, RMS roughness
Hardness (GPa)
66
200 -30
-20
-10
0
10
20
30
Radial distance (mm) Fig. 7. Radial variations of hardness and parallel component of magnetic field.
varied on radial position of cathode, the lowest value being measured in the region of cathode with lowest values of parallel magnetic component: 1.69 nm in the position with parallel component of 0.065 T, 0.87 nm in the position with 0.030 T and 0.49 nm in the position with 0.010 T. This variation could be explained by the lower local values of bias voltage in the region of cathode with higher parallel magnetic component and indirectly by lower energy of ions hitting the growing film. DLC thin films were transparent in the IR range. The main peak of absorption spectra was centered approximately at about 2925 cm 1 and was associated with the majority presence of hydrogen in the form of CHx (x = 2,3) stretching. The spectra of deposited film showed mainly CH3 asymmetric mode at 2959 cm 1 and CH3 symmetric mode at 2867 cm 1. The intensity of the main absorption peak decreased with the increase in input power suggesting an increase of precursor dissociation. Electric measurements of DLC thin films deposited onto Al2O3 substrates showed that thin films obtained in methane plasma has very high resistivity, with values higher than 107 V cm.
4. Conclusion 130
20
Hardness (GPa)
110 16
100
14
90 80
12
70 10
Hardness Elastic modulus
8 0
60
Elastic modulus (GPa)
120
18
50 50 100 150 200 250 300 350 400 450
Power (W) Fig. 6. Variation of hardness and elastic modulus with input power for 0.046 T maximum value of parallel component of magnetic field.
By imposing a closed E B drift of electrons in the front of the cathode, the properties of normal PECVD were influenced. The bias voltage decreased to lower values indicating a decrease in plasma impedance and an increase in discharge current and plasma density. Very high deposition rate of DLC (1029 nm/min, about 20 times higher as compared with normally PECVD) was registered in the active area of the magnetron cathode. The values of hardness, elastic modulus and RMS roughness of thin films were dependent on the position of the substrate onto the cathode surface and on the input power. This behavior was connected with variable ion flux density on cathode surface, different local bias voltages developed on high-resistivity DLC films, the increase of ion energies and local temper-
V. Anita et al. / Thin Solid Films 506 – 507 (2006) 63 – 67
ature with input power. Raman measurements revealed that the increase in input power led to a decrease in the sp3 fraction and an increase of the disordered graphite sp2 fraction in deposited DLC thin films. To obtain uniform thin films by utilizing this geometry, it is necessary to move the substrate onto the surface of the cathode.
References [1] A. Grill, Diamond Relat. Mater. 8 (1999) 428. [2] H.-C. Tsai, D.B. Bogy, J. Vac. Sci. Technol., A 5 (1987) 3287. [3] J.L. Andular, M. Vives, C. Corbella, E. Bertran, Diamond Relat. Mater. 12 (2003) 98. [4] N.J. Ianno, R.O. Dillon, Abbas Ali, A. Ahmad, Thin Solid Films (1995) 275. [5] H. Yamada, O. Tsuji, P. Wood, Thin Solid Films (1995) 220.
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[6] D.S. Patil, K. Ramachandran, N. Venkatramani, M. Pandey, R. D’Cunha, Pramana—J. Phys. 55 (2000) 935. [7] H. Takikawa, K. Izumi, R. Miyano, T. Sakakibara, Surf. Coat. Technol. 163 – 164 (2003) 368. [8] Hao Wang, Ming-Rong Shen, Zhao-Yuan Ning, Chao Ye, Hai-Yan Dang, Chuan – Bao Cao, He-Sun Zhu, Thin Solid Films 293 (1997) 87. [9] D.J. Li, F.Z. Cui, H.Q. Gu, Appl. Surf. Sci. 137 (1999) 30. [10] I.H. Shin, T.D. Lee, J. Vac. Sci. Technol., B 18 (2) (2000) 1027. [11] Wan-yu Wu, Jyh-ming Ting, Thin Solid Films 420 – 421 (2002) 166. [12] J. Robertson, Mater. Sci. Eng., R 37 (2002) 129. [13] V. Anita, T. Butuda, T. Maeda, K. Takizawa, N. Saito, O. Takai, Proceedings of the 9th International Conference on New Diamond Science and Technology (ICNDST-9), Diamond and Related Materials 13 (11 – 12) (2004) 1993. [14] Brian Chapmann, Glow Discharge Processes, Sputtering and Plasma Etching, John Wiley and Sons, New York, 1980. [15] www.jyhoriba.com, A.N.07 (2000), Derivation of Physical Parameters from Raman Spectroscopy of Hard Carbon Films.
Magnetron plasma-enhanced chemical vapor deposition of diamond-like carbon thin films V. Anita a,c,*, N. Saito b, O. Takai c b
a Department of Plasma Physics, ‘‘Al.I. Cuza’’ University, Iasi-6600, Romania Department of Molecular Design and Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan c EcoTopia Science Institute, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan
Available online 13 September 2005
Abstract An RF magnetron plasma-enhanced chemical vapor deposition system with variable magnetic field (MagPECVD) was utilized for high rate deposition of diamond-like carbon thin films. Deposition rate was nonuniform, with minimum value of 30 nm/min in the center of the cathode where the low ionic flux is determined by the absence of the E B electronic drift. The highest value of 1029 nm/min was registered in the position of the cathode with the highest ion flux determined by the presence of the E B electronic drift. Hardness, elastic modulus and RMS roughness of thin films were dependent on local values of magnetic field intensities and the values of input power. D 2005 Elsevier B.V. All rights reserved. Keywords: Diamond-like carbon; Methane; Magnetron discharge; Mechanical properties
1. Introduction In the last period, there has been a large interest on diamond-like carbon (DLC) on account of its useful properties such as high resistivity, hardness and density, wear and chemical resistance, smoothness, transparency in the infrared region and biocompatibility [1]. DLC has also the possibility to be doped with different elements in the view to improve these properties [2]. Many methods have been developed and studied to deposit DLC thin films: pulsed dc discharge [3], radio-frequency [4,5] or microwave plasma-enhanced chemical vapor deposition [6], arc discharge [7], liquid-based methods [8], ion beam-assisted systems [9], laser ablation [10] and sputtering [11]. All presented methods necessitate the presence of bias on the substrate in the view to control the energy of positive ions that play a very important role in establishing the amount of sp3 bonded carbon component in thin film [12]. One of the most utilized method and also simplest is the plasma* Corresponding author. EcoTopia Science Institute, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan. Tel.: +81 52 789 5163; fax: +81 52 789 3260. E-mail address: [email protected] (V. Anita). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.08.074
enhanced chemical vapor deposition with plane cathode and radio-frequency powered. This method is characterized by low deposition rate. To improve the deposition rate, there has been developed different variants like using magnetic fields with different configuration in the front of cathode surface or systems with high current density on the cathode and using precursors with low first ionizing energy [13]. Magnetron-enhanced sputtering of materials is one of the most utilized techniques in modern technologies for the fabrication of thin-film structures [14]. The presence of orthogonal E and B fields in the front of magnetron cathode surface generates an E B drift for electrons and the secondary electrons emitted by ion bombardment are confined to the near vicinity of the cathode and improve plasma density. The high plasma density reduces the discharge impedance and induces an increase in the discharge current density and a decrease of discharge voltage at low pressures. By adopting this configuration, mainly utilized for sputtering, we expected to obtain very high deposition rates of DLC thin films at reduced bias voltages. By adjusting the magnetic field value in front of the magnetron cathode, the self bias voltage responsible of ion energy could be controlled. We focused mainly on the effect of input power and values of parallel component of
64
V. Anita et al. / Thin Solid Films 506 – 507 (2006) 63 – 67
pumps was utilized. Basic vacuum pressure was 6 10 3 Pa and the pressure in the time of thin film deposition was kept at 20 Pa by adjusting the mass flow of the reactive gas. The surface morphology of DLC thin films was observed with an atomic force microscope (AFM; JSPM –5200 T, JEOL). The deposition rate was estimated by measuring the thickness of deposited thin films using a surface profilometer (Surfatest SV-600, Mitutoyo). The infrared spectra of the films were analyzed in order to characterize the bonding of hydrogen with carbon in thin films (FTS 175 C, BioRad). Raman spectra were recorded using a Jobin Yvon spectrometer with 514.5 nm laser excitation line and 10 mW output power. The spectra were fitted with a curve-fitting software (based on two Gaussian curve shapes) to identify the G and D peaks positions, to measure and compare the intensities (GRAMS/AIi from ThermoGalactic). The hardness and Young’s modulus of the DLC films were determined from loading– unloading curves on a nanoindentation instrument (TriboScope, Hysitron) using a Berkovich diamond tip. Ten indentations were made for each sample and the hardness and Young’s modulus were calculated by averaging the measurements.
magnetic field on the properties of deposited thin films in different radial positions on the magnetron cathode. The properties of DLC thin films and efficiency of MagPECVD were evaluated by different experimental techniques.
2. Experimental details A chemical vapor deposition system with plane cathode was modified by adding a variable magnetic field with closed lines in front of the cathode in the view to assure E B drift of electrons (Fig. 1). The diameter of obtained magnetron cathode was 100 mm and the parallel component of magnetic field could be varied between 0.02 and 0.065 T by using a movable magnetic circuit and permanent magnets positioned inside the cathode. Because DLC thin films are characterized by high resistivity, we have chosen a capacitive coupled RF power supply with 13.56 MHz frequency and 10 – 400 W variable output power. In this experimental set-up, the active area of cathode under magnetron plasma ring (30 cm2) was much smaller than the area of the anode (6000 cm2). Applied voltage distributed mainly between anode and cathode dark spaces and the larger voltage developed at the smaller electrode as a consequence of big differences between electrons and ion mobilities in the plasma [14]. In the time of deposition, the self bias voltage was monitored in order to obtain information about the average energy of ions involved in thin film growth. Methane (CH4) was utilized as precursor. Thin films were deposited on silicon and aluminum oxide substrates positioned directly on the surface of the magnetron cathode. To obtain vacuum in the process chamber, a system with rotary, mechanical booster and turbomolecular
3. Results and discussion Self bias voltage increased almost linearly with the input power and was dependent on the value of the parallel component of magnetic field at the surface of the cathode (Fig. 2). The highest value (765 V) was measured for normal PECVD (B = 0 T) at 400 W input RF power and 20 Pa pressure of methane precursor. By increasing the parallel component of the magnetic field, self bias voltage decreased
LOW DENSITY PLASMA
HIGH DENSITY PLASMA VACUUM PUMP SILICON SUBSTRATE
S
N
MAGNETRON CATHODE
S
C PLASMA IMPEDANCE MATCH RF POWER SUPPLY
L
V
Fig. 1. Experimental set-up.
SELF-BIAS MEASUREMENT
V. Anita et al. / Thin Solid Films 506 – 507 (2006) 63 – 67
160
800
PECVD
600 500 400
0.020 T
300
0.035 T 0.045 T
200
0.065 T
100
MagPECVD
Deposition rate (nm/min)
0T
700
Bias voltage (V)
65
140 120 100 80 60 40 50 W RF Power 20
0 0
50
100
100 150 200 250 300 350 400 450
200
300
400
500
600
700
Parallel magnetic field component (x 10-4T)
Power (W) Fig. 2. Relationship between self bias voltage and input power (the value of parallel magnetic field component as variable).
Fig. 4. Variation of deposition rate with the value of parallel magnetic field component (50 W input RF power).
to lower values (150 V for 0.065 T and 400 W input RF power). This high variation indicated a decrease in plasma impedance, an increase in discharge current and plasma density [14]. Deposited thin films were nonuniform; the deposition rate in the active area of the cathode was very high as compared with the normal cathode without magnetic field and increased with the input power for the same value of magnetic field. The minimum value (30 nm/min) was measured in the center of the cathode where the maximum value of perpendicular component of the magnetic field was highest (å 0.1 T). The maximum value of deposition rate was registered on the position of the cathode with the maximum value of the parallel component of magnetic field (0.065 T). The deposition rate was 282 nm/min for 80 W input RF power and increased to 1029 nm/min at 400 W input power, as presented in Fig. 3. In the case of normal discharge, without magnetic field, the maximum obtained deposition rate was 47 nm/min at 400 W input power. For a constant input power, the deposition rate increased with the increase in the value of the parallel magnetic field component (Fig. 4). Raman measurements were utilized to obtain the qualitative indication of the amount of hydrogen and
graphitization in DLC thin film. The spectra revealed differences among the properties of thin films obtained in different areas of the cathode. For a constant input power, the slopes of the background of the Raman spectra for thin films deposited in the regions with low deposition rate were high and this feature indicated high content of hydrogen in thin films [15]. The slopes of the background of the Raman spectra for thin films deposited in the regions with high deposition rates were lowest, indicating the decrease in the hydrogen content in thin films determined by high ion flux bombardment. By keeping a constant value of magnetic field and increasing the input power, the slopes of Raman spectra of thin film deposited in a fixed region of magnetron cathode decreased, indicating also the dehydrogenation in DLC thin films (Fig. 5). This feature is determined by highenergy ion bombardment which induces a local increase of temperature. By deconvolution the Raman intensities in graphite component (G band, I G – 1580 cm 1) and disordered graphite component (D band, I D –1370 cm 1) and calculating the integrated intensities ratio I D/I G, it was found that this ratio increased with the increase in the input power. From this evolution resulted that for low-input RF power, 50
1200 1000
40 400 W
Intensity (a.u)
Deposition rate (nm/min)
MagPECVD
800 600 80 W
400
30 W 30 20 200 W 10
200
PECVD 400 W 0
0 -60
-40
-20
0
20
40
Radial distance (mm) Fig. 3. Radial variation of deposition rate.
60
80
400 W 3500
3000
2500
2000
Raman shift
1500
1000
500
(cm-1)
Fig. 5. Raman spectra of thin films deposited at different input RF power.
V. Anita et al. / Thin Solid Films 506 – 507 (2006) 63 – 67
16
500
14
450
12
400
10
350
8
300 Hardness Magnetic field
6
250
4
Magnetic field (x 10-4 T)
the Raman peaks ratio I D/I G has had low values and this feature was associated with high sp3 bonded carbon fraction in deposited thin films [12]. The increase in input power led to a decrease in the sp3 fraction and an increase in the disordered sp2 fraction in deposited DLC thin films. Hardness and elastic modulus of thin films deposited with 0.046 T magnetic field intensity, in the region with the highest deposition rate, are presented in Fig. 6. Hardness increased from 11.4 GPa at 50 W input RF power to 16.2 GPa at 200 W input power and then decreased to 9.2 GPa at 400 W input power. The elastic modulus also increased from 89 GPa at 50 W input power to 121.6 GPa at 200 W and then decreased to 66.7 GPa at 400 W input power. The decrease of hardness and elastic modulus at higher values of input power can be associated with the increase in sp2 component in thin films, induced by high-energy ion bombardment. For a constant input power, the hardness and elastic modulus varied on the radial distance, in the point of the cathode with the highest value of parallel component of magnetic field and higher ion flux being registered a lower value. Fig. 7 presents the radial variation of hardness for 50 W input power and 0.046 T maximum value of parallel component of magnetic field and similarly variations were registered for the elastic modulus. The decrease in hardness and elastic modulus in the point of cathode surface with a high value of magnetic field can be associated with low values of bias voltages and high ionic flux on dielectric diamond-like carbon thin films deposited in these regions. The high ionic flux bombardment induces the increase of local surface temperature above 250 -C and causes a decline in hydrogen content and an increase in sp2 fraction [12]. AFM measurements revealed that the surface morphology of thin films was influenced by the value of utilized power: RMS roughness decreased from 8.41 nm at 50 W to 1.69 nm at 400 W for thin films obtained in the position of the cathode with the highest deposition rate. Sputtering by high energy ions contributed to the smoothing of thin films. For a constant input power of 400 W, RMS roughness
Hardness (GPa)
66
200 -30
-20
-10
0
10
20
30
Radial distance (mm) Fig. 7. Radial variations of hardness and parallel component of magnetic field.
varied on radial position of cathode, the lowest value being measured in the region of cathode with lowest values of parallel magnetic component: 1.69 nm in the position with parallel component of 0.065 T, 0.87 nm in the position with 0.030 T and 0.49 nm in the position with 0.010 T. This variation could be explained by the lower local values of bias voltage in the region of cathode with higher parallel magnetic component and indirectly by lower energy of ions hitting the growing film. DLC thin films were transparent in the IR range. The main peak of absorption spectra was centered approximately at about 2925 cm 1 and was associated with the majority presence of hydrogen in the form of CHx (x = 2,3) stretching. The spectra of deposited film showed mainly CH3 asymmetric mode at 2959 cm 1 and CH3 symmetric mode at 2867 cm 1. The intensity of the main absorption peak decreased with the increase in input power suggesting an increase of precursor dissociation. Electric measurements of DLC thin films deposited onto Al2O3 substrates showed that thin films obtained in methane plasma has very high resistivity, with values higher than 107 V cm.
4. Conclusion 130
20
Hardness (GPa)
110 16
100
14
90 80
12
70 10
Hardness Elastic modulus
8 0
60
Elastic modulus (GPa)
120
18
50 50 100 150 200 250 300 350 400 450
Power (W) Fig. 6. Variation of hardness and elastic modulus with input power for 0.046 T maximum value of parallel component of magnetic field.
By imposing a closed E B drift of electrons in the front of the cathode, the properties of normal PECVD were influenced. The bias voltage decreased to lower values indicating a decrease in plasma impedance and an increase in discharge current and plasma density. Very high deposition rate of DLC (1029 nm/min, about 20 times higher as compared with normally PECVD) was registered in the active area of the magnetron cathode. The values of hardness, elastic modulus and RMS roughness of thin films were dependent on the position of the substrate onto the cathode surface and on the input power. This behavior was connected with variable ion flux density on cathode surface, different local bias voltages developed on high-resistivity DLC films, the increase of ion energies and local temper-
V. Anita et al. / Thin Solid Films 506 – 507 (2006) 63 – 67
ature with input power. Raman measurements revealed that the increase in input power led to a decrease in the sp3 fraction and an increase of the disordered graphite sp2 fraction in deposited DLC thin films. To obtain uniform thin films by utilizing this geometry, it is necessary to move the substrate onto the surface of the cathode.
References [1] A. Grill, Diamond Relat. Mater. 8 (1999) 428. [2] H.-C. Tsai, D.B. Bogy, J. Vac. Sci. Technol., A 5 (1987) 3287. [3] J.L. Andular, M. Vives, C. Corbella, E. Bertran, Diamond Relat. Mater. 12 (2003) 98. [4] N.J. Ianno, R.O. Dillon, Abbas Ali, A. Ahmad, Thin Solid Films (1995) 275. [5] H. Yamada, O. Tsuji, P. Wood, Thin Solid Films (1995) 220.
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[6] D.S. Patil, K. Ramachandran, N. Venkatramani, M. Pandey, R. D’Cunha, Pramana—J. Phys. 55 (2000) 935. [7] H. Takikawa, K. Izumi, R. Miyano, T. Sakakibara, Surf. Coat. Technol. 163 – 164 (2003) 368. [8] Hao Wang, Ming-Rong Shen, Zhao-Yuan Ning, Chao Ye, Hai-Yan Dang, Chuan – Bao Cao, He-Sun Zhu, Thin Solid Films 293 (1997) 87. [9] D.J. Li, F.Z. Cui, H.Q. Gu, Appl. Surf. Sci. 137 (1999) 30. [10] I.H. Shin, T.D. Lee, J. Vac. Sci. Technol., B 18 (2) (2000) 1027. [11] Wan-yu Wu, Jyh-ming Ting, Thin Solid Films 420 – 421 (2002) 166. [12] J. Robertson, Mater. Sci. Eng., R 37 (2002) 129. [13] V. Anita, T. Butuda, T. Maeda, K. Takizawa, N. Saito, O. Takai, Proceedings of the 9th International Conference on New Diamond Science and Technology (ICNDST-9), Diamond and Related Materials 13 (11 – 12) (2004) 1993. [14] Brian Chapmann, Glow Discharge Processes, Sputtering and Plasma Etching, John Wiley and Sons, New York, 1980. [15] www.jyhoriba.com, A.N.07 (2000), Derivation of Physical Parameters from Raman Spectroscopy of Hard Carbon Films.