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A self-DLC coated cathode plasma source
Surface & Coatings Technology 203 (2009) 2743–2746
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
A self-DLC coated cathode plasma source A. Deachana a,b,⁎, D. Boonyawan a, B. Yodsombat a a b
Fast Neutron Research Facility, Department of Physics, Faculty of Science, Chiang Mai University, Chiang Mai, 50200, Thailand Department of Physics and General Science, Faculty of Science and Technology, Songkhla Rajaphat University, Songkhla, 90000, Thailand
a r t i c l e
i n f o
Available online 11 March 2009 Keywords: DLC Aluminum cathode Microwave plasma source OES Raman spectroscopy RBS
a b s t r a c t A novel self-microwave plasma source modified by diamond-like carbon (DLC) coating self-cathode for density enhancement was devised. This simple method had been once routed for enhanced efficient microwave plasma with very low-cost. The coating parameters in fabrication of the self-coated DLC cathode for plasma density enhancement were investigated. DLC-Al cathodes coating by self cathode of microwave plasma source were fabricated to control and yielded strong non-activated emission. The novel cathodes showed enhancement of current density up to 240%. In addition, the contamination in the plasma source could be avoided with the new cathodes. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved.
1. Introduction The diamond-like carbon (DLC) as a cathode coating material of source component was of interest because of its excellent properties, low and negative surface electron affinity, chemically inert, high thermal conductivity, toughness [1] and low threshold field emission from diamond even with flat surface [2–4]. Several groups have investigated the field emission behavior of DLC films containing different amounts of bonded carbon ratio [5–9]. While the film containing only sp3 bonded carbon had to be activated in order to show field emission, the film containing conductive inclusions of sp2 shows strong non-activated emission [5]. Karabutov et al. [6] concluded that the positive influence on the field emission was produced by the following factors: (1) high content of the sp3 carbon in DLC film. (2) nitrogen or sulphur doping. (3) nanometre-scale thickness in case of high resistivity films, and (4) post-growth annealing resulting in a partial graphitization. Different types of the surface treatment and coatings have been carried out to improve emission behavior of the DLC films. The emission parameters were found to be governed by an intrinsic characteristic of the DLC films. Probably electron emission processes are that electron generating from graphitic clusters to the sp3 medium of DLC films, electron tunneling through the forbidden gap of sp 3 medium and then being injected into the vacuum (anode) [7]. In the nitrogen doped DLCcoated silicon emitters, the turn-on field decreased and the emission current increased with the elevation of nitrogen doping concentration. The work function decreased from 4.30 eV for the non-coated silicon emitters to 0.47 eV for nitrogen doped DLC-coated silicon
⁎ Corresponding author. Fast Neutron Research Facility, Department of Physics, Faculty of Science, Chiang Mai University, Chiang Mai, 50200, Thailand. Tel.: +66 53 943367; fax: +66 53 222776. E-mail address: [email protected] (A. Deachana).
emitters. As nitrogen doping concentrations increased, the work function of the nitrogen doped DLC-coated silicon emitters decreased [8]. In DLC films doped with both boron and nitrogen, the electron field emission properties of the as-deposited samples at fixed nitrogen doping vary significantly with boron content. The results infer that only when proper dopants are incorporated, the electron conduction in the DLC films is improved and the electron emission is enhanced [9]. There are many coating methods used to fabricate DLC films, including direct ion beam deposition [10], pulsed laser deposition (PLD) [11], ion beam conversion coatings [12], filtered cathodic arc deposition, plasma immersion ion implantation and deposition (PIII&D) [13,14], magnetrons sputter coating [15], and rf plasmaenhanced chemical vapor deposition (RF-PECVD) [16,17]. In these methods, DLC films are fabricated from a variety of carbonaceous precursor materials, and in most of these methods the characteristics of DLC depended on deposition conditions. For example, the flow rate of CH4, synthesis pressure, rf-power, bias voltage and substrate temperature are used as deposition conditions to reduce internal stress in the RF-PECVD method [18]. Additionally, in a simplified arc discharge system, DLC with a smooth surface and low friction coefficient could be formed at low negative-voltage pulses [19,20], and high adhesion of DLC coatings resulted from rise rates of the short arc discharge pulse [21]. These clearly show that the deposition conditions play a key role in controlling the DLC characteristics. One of the newer methods used to fabricate DLC films is to use microwave plasma. However, in this method the plasma obtained has limitations, e.g. cut-off density and skin depth [22,23]. Two main approaches to enhance the density are (1) variation of geometry and configuration of microwave plasma source, and (2) selection of suitable materials of source component. The dc-bias microwave plasma hybrid technique is shown to increase the cut-off density. This self-source technique provides the surface treatment and coating
0257-8972/$ – see front matter. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.02.132
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A. Deachana et al. / Surface & Coatings Technology 203 (2009) 2743–2746
to modify cathode's surface to enhance the plasma density. The simple method has been once routed for enhanced efficient microwave plasma with very low-cost. As a self-modification technique, there is no need to incorporate other coating systems, which is the main advantage of such method. Therefore, the major aims of this work are to devise the microwave plasma source coating system, as described in details in Section 2, and to study coating parameters in fabrication of the self-coated DLC cathode for plasma density enhancement. As will be seen in the results in Section 3, the self-coated DLC coating system developed here is shown to improve the plasma density significantly. 2. Experimental procedures 2.1. Microwave power setup The experimental setup consists of a homemade rectangular waveguide WR 340 (8.64 cm × 4.32 cm) with magnetron (commercial cooking microwave, Sharp Model: RV-MZ 4080, 2 M241) coupled at one end and a plasma source tube at the other, as shown in Fig. 1. The three-position tuner is placed between the magnetron and plasma source tube. Two cooking microwave transformers are employed as a power supply of magnetron, first for stepping up high voltage and second for varying the power of magnetron filament by using the variable transformer. The microwave power has already been adjusted to calibrated power with the water as the power load and calculated microwave power from the water heating. The conventional fan cooled magnetron has a maximum forward power of 100 W at 2.45 GHz with 200 V AC input. The vacuum system consists of a rotary-backing pump (Edward, No.2) and a turbomolecular pump (Edwards, EXT250/ISO 100), which allows a background pressure of plasma source tube and main chamber of 3.0 and 5.3 × 10− 4 Pa, respectively. Gas flow is measured by a mass flow controllers/meter (MKS, Type 246) and controlled via a precise leak valve of mass flow control (MKS, 1179A21CR1BV). The pressure is a function of the gas flow rate. The pressure difference between a plasma source and the main chamber is about 1.3 × 10− 2 Pa. 2.2. Plasma source setup The plasma source consists of a plasma source tube with adjustable electrode distance, as shown in Fig. 1(b), and a tapered WR340 wave guide with integrated fan cooling for anode at the upper part and high pressure air feeder for cathode at lower part with the source tube
inserted through wave guide and plugged into the main chamber, as schematically described in Fig. 1(a). The plasma is ignited inside the quartz tube of 7.5 mm diameter inside the waveguide since the internal pressure (p) and adjusted electrode distance (d) are close to the Paschen minimum. For optimization and better comparison, d = 10 mm is fixed in all experiments. The source tube can be operated in wide range of argon gas pressure from 1.3 to 1.0 × 103 Pa. The quartz tubes is confined in two electrode housings with stainless steel electrodes with aluminum tips inserted inside both housings as a cathode and an anode in the quartz tube. The source tube is electrically isolated from the waveguide. Gas is fed through the upper electrode, where the pressure gauge is connected to monitor the incoming gas pressure, whereas the lower electrode serves as the plasma outlet. In the source body, both aluminum tips can be biased independently to control plasma reactor. The electrons can diffuse through a pin hole with diameter of 300 µm to the main chamber. The plasma produced is then diagnostic with; (1) optical emission spectroscopy (OES, Ocean Optics S2000) via the fiber optic inserted through an insulator between upper housing and wave-guide, (2) the current–voltage (I–V) by biasing voltage between electrodes, and current density (J) obtained from I is correlated with the plasma density (n) for a stationary sheath [24]. 2.3. Cathode coating setup All substrate cathodes with diameter of 7.5 mm and a pin hole at center of the cathode with diameter of 300 µm are machined by highprecision CNC Machine from conventional aluminum plates of thickness 3 mm. The substrates are mechanically polished by AP-A Suspension (Struers) of 1–0.3 µm. The substrates are successively cleaned with water and followed with acetone, ethanol at 60 °C in the ultrasonic water bath (TRANSSONIC T 460/H) and finally dried with air prior to coating preparation. Fabrication of DLC films is carried out by a self-cathode of source tube, which is negatively biased with direct current (DC) from power supply (BERTAN, 205B-20R). This coating system is similar to microwave plasma enhanced chemical vapor deposition (MECVD) in the plasma source tube. The experimental coating conditions are listed in Table 1. Note that all of coating conditions, argon gas is used as active gas with argon gas pressure of 19.9 Pa as vacuum base pressure. The source tube pressure is varied from 1.5 × 102 to 5.8 × 102 Pa and before each experiment all substrates are sputtered for surface cleaning with a condition of 20 min, 19.9 Pa from 1.44 sccm of Ar gas, and cathode-bias of 400 V DC.
Fig. 1. Schematic diagram of (a) plasma source to the main chamber, and (b) zoom-in of plasma source tube.
A. Deachana et al. / Surface & Coatings Technology 203 (2009) 2743–2746
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Table 1 Conditions of films coating. Samples
Microwave power (W)
Time (min.)
Pressure (×102 Pa)
Flow rate (sccm)
Bias (V)
Gas
DLC210506 DLC260506 DLC070606 DLC100606
40 40 40 40
9 16 6 20
1.5 2.9 3.8 5.8
1.44 1.65 1.85 6.45
400 400 400 400
C2H2 C2H2 C2H2 C2H2
For plasma diagnosis, acetylene (C2H2) is used to monitor ion species during coating operation. Raman measurements are performed at room temperature using a spectrometer of the type Jobin Yvon T64000, France, in the quasi-back scattering geometry of an Ar laser operating with a power of 200 mW. The thickness of DLC films are roughly evaluated using Rutherford backscattering spectrometry (RBS). Field emission is measured for each sample with Ar-plasma by using a power supply, low voltage high current (LTRONIX, B60-15R) with negative bias to cathode, while multimeter (KEITHLEY 197) is used to measure bias-current. 3. Results and discussion 3.1. DLC film properties From 40 W microwave plasma, Fig. 2 (solid line) shows the spectra of time integrated optical emission, argon activated acetylene plasma with negative dc-bias voltages of 60 V at a gas pressure of 1.5 × 102 Pa. The main emission spectra are C at 818.2 nm, C at 711.2 nm and H at 841.3 nm, but CH at 431.4 nm [25] cannot be observed. The emitted lines result either from dissociative excitation of the acetylene or argon or from electron impact excitation of radicals and atoms. Though the thickness of DLC films cannot be analyzed by RBS because the films obtained are very thin, but some information of DLC films are derived. All of the films show dominant peak of oxygen contamination, which could be induced during the preparation of Al-substrates since Al-substrates are very sensitive and could easily form aluminum oxide. Fig. 3 shows typical Raman spectra of the DLC films at the bias voltage of 400 V, which are fitted by Gaussian distribution. All the spectra of the DLC films show similar broad peak at around 1590 cm− 1 and another obvious lower frequency shoulder at approximately 1360 cm− 1, commonly referred to as the G band and D band, respectively [26]. It should be noted that the D peaks observed in Fig. 3
Fig. 3. Raman spectra of DLC films prepared at various pressures.
are attributed to the carbon particles. Based on the fitting parameters, the peak position and ID/IG (the ratio of the intensity of the D and G peaks) are summarized in Table 2. It has been reported that the position of G band is related to the bond-angle disorder or sp3 bonding content, while the ID/IG ratio is correlated with the ratio of sp3/sp2 bonds [27]. These two factors play the most important role in determining the Raman spectra. The sp3/sp2 ratio in the DLC films cannot be derived directly from the Raman spectra, but some qualitative information can be extracted. The results from Table 2 show the correspondence between increasing pressures and the sp3/sp2 ratio, (2.9 × 102 Pa, 0.788), (3.8 × 102 Pa, 0.826), (5.8 × 102 Pa, 0.919). Clearly, the pressure has strong influence on the sp3/sp2 ratio. However, (1.5× 102 Pa, 0.873) does not follow the same trend probably because a greater fraction of sp3 bonds can be obtained about 100 eV, when the ion energy is high enough for displacement of low energy sp2 bonds, but still not enough for displacement of high-energy sp3 bonds. At low pressure, the ions take a long time (collision time) to gain energy from the bias voltage. When the ion energy is too high, the ions may damage the films surface and cause sp3 bonds to transform into sp2 bonds [28,29]. 3.2. Ion current density Even at low microwave power of 40 W and low negative dc-bias of 20 V at an argon gas pressure of 19.9 Pa, the OES spectra dominantly show, as seen in Fig. 2 (dash line) sputtering Al atoms at 517.3 nm, 669.8 nm, and 684.8 nm, but the Al spectra decrease significantly in DLC coating process (Fig. 2 (solid line)). This is because the microwave plasma contaminates Al-cathode, while the DLC provides contamination-free Al-cathode. Therefore, dc-bias of 30 V is set as condition for I–V characteristics of plasma, and each I–V curve is repeated 5 times for data collection. Fig. 4 shows the comparison of current density of between Al-cathode and DLC-coated Al-cathodes. All of DLC-Alcathodes show very strong enhancement in the current density. The improved current density could imply the enhanced plasma density through stationary sheath, as described earlier [24]. As listed in Table 2, the IG factor plays dominant role in the enhancement, probably as a result of the surface micro-irregularities in the DLC-Al cathodes. To understand the improved current density, the field
Table 2 Fitting results of Raman spectra of DLC films prepared at various pressures.
Fig. 2. Typical OES spectra of 40 W microwave plasma at various conditions during coating process: negative dc bias cathode 20 V with Ar plasma (dash line) and 60 V with mixed Ar and C2H2 plasma (solid line).
Samples
D band (cm− 1)
G band (cm− 1)
ID/IG
IG (a.u.)
DLC210506 DLC260506 DLC070606 DLC100606
1329.5 1368.0 1360.7 1367.9
1607.1 1590.9 1597.6 1582.6
0.873 0.788 0.826 0.919
1513.379 1218.038 505.788 549.867
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Acknowledgment The project was supported by Fast Neutron Research Facility, Department of Physics, Faculty of Science and the Graduate School, Chiang Mai University, Chiang Mai, 50200, Thailand.
References
Fig. 4. I–V curves of Al-cathode and DLCs coated Al-cathode.
emission of DLC films can be explained as follows. When the cathode is negatively biased, the electrons are driven back from the cathode and ions with the space-charge limit flux according to the Child– Langmuir's Law. However, the sheath edge will induce high electric field at interface layer between sheath edge and the DLC surface, which in turn causes the local field to drive electrons out through a potential barrier following the Fowler–Nordheim [30,31]. In the DLC films, conducting channels (corresponding to IG) are formed in insulating matrix (corresponding to ID), which is already present in the deposition process. These conducting channels act as tip-like structure leading to field emission enhancement [5]. This enhancement plays an important role in controlling of the plasma density, and the plasma density enhancement by DLC-coated cathode, which also depends on the film morphology, distribution and size of conducting channels, the thickness and optimization of sp2 content. 4. Conclusions This work presents a novel microwave plasma source coating system used to enhance the plasma density through self-coated DLC cathode. The coating parameters in fabrication of the self-coated DLC cathode for plasma density enhancement are studied. It is found that the plasma current density is enhanced with increasing IG factor of the DLC films. The threshold voltage of enhancement is approximately 5– 12 V and the density enhancement between 1.6–2.4 times at 30 V is obtained.
[1] T. Yamada, P.R. Vinod, D.-S. Hwang, H. Yoshikawa, S.-I. Shikata, N. Fujimori, Diam. Relat. Mater. 14 (2005) 2047. [2] T. Yamada, A. Sawabe, S. Koizumi, T. Kamio, K. Okano, Diam. Relat. Mater. 11 (2002) 257. [3] M.W. Geis, J.C. Twichell, J. Macaulay, K. Okano, Appl. Phys. Lett. 67 (1995) 1328. [4] T. Sugino, C. Kimura, K. Kuriyama, Y. Yokota, S. Koizumi, M. Kamo, Phys. Status Solidi (a) 174 (1999) 145. [5] O. Gröning, O.M. Küttel, P. Gröning, L. Schlapbach, Appl. Surf. Sci. 111 (1997) 135. [6] A.V. Karabutov, V.I. Konov, V.G. Ralchenko, E.D. Obraztsova, V.D. Frolov, S.A. Uglov, H.-J. Scheibe, V.E. Strelnitskij, V.I. Polyakov, Diamond Relat. Mater. 7 (1998) 802. [7] V.G. Litovchenko, A.A. Evtukh, R.I. Marchenko, N.I. Klyui, V.A. Semenovich, Appl. Surf. Sci. 111 (1997) 213. [8] M.-S. Choi, J.-H. Kim, Y.-S. Kim, J. Non-Cryst. Solids 324 (2003) 187. [9] Y.-H. Wu, C.-M. Hsu, C.-T. Chia, I.-N. Lin, C.-L. Cheng, Diam. Relat. Mater. 11 (2002) 804. [10] Y. Liu, A. Erdemir, E.I. Meletis, Surf. Coat. Technol. 82 (1996) 48. [11] A.A. Voevodin, M.S. Donley, J.S. Zabinski, Surf. Coat. Technol. 92 (1997) 42. [12] C.G. Fountzoulas, T.Z. Kattanis, J.D. Demaree, J.K. Hirvonen, in: A. Feldman, Y. Tzeng, W.A. Yarbrough, M. Yoshikawa, M. Murakawa (Eds.), Applications of Diamond Films and Related Materials, National Institute of Standards and Technology, Washington., DC, 1995, p. 907. [13] S. Anders, A. Anders, I.G. Brown, B. Wei, K. Komvopoulos, J.W. Ager, K.M. Yu, Surf. Coat. Technol. 68 (1994) 388. [14] J.H. Liang, M.H. Chen, W.F. Tsai, S.C. Lee, C.F. Ai, Nucl. Instrum. Methods B 257 (2007) 696. [15] X.L. Peng, Z.H. Barber, T.W. Clyne, Surf. Coat. Technol. 138 (2001) 23. [16] H. Miki, T. Takeno, T. Takagi, Thin Solid Films 516 (2008) 5414. [17] A. Erdemir, I.B. Nilufer, O.L. Eryilmaz, M. Beschliesser, G.R. Fenske, Surf. Coat. Technol. 120 (1999) 589. [18] S. Takeuchi, A. Tanji, H. Miyazawa, M. Murakawa, Thin Solid Films 447 (2004) 208. [19] T. Watanabe, K. Yamamoto, O. Tsuda, A. Tanaka, Y. Koga, Surf. Coat. Technol. 156 (2002) 317. [20] T. Watanabe, K. Yamamoto, O. Tsuda, A. Tanaka, Y. Koga, O. Takai, Diam. Relat. Mater. 12 (2003) 2083. [21] E. Alakoski, M. Kiuru, V.-M. Tiainen, Diam. Relat. Mater. 15 (2006) 34. [22] J Reece Roth, Industrial Plasma Engineering, vol. 1, Institute of Physics Publishing, Philadelphia, PA 19106, USA, 1995, p. 502. [23] I. Ganachev, H. Sugai, Surf. Coat. Technol. 174 (2003) 15. [24] J. Brutscher, R. Günzel, W. Möller, Plasma Sources Sci. Technol. 5 (1996) 54. [25] M. Kirinuki, M. Onoi, K. Nishikawa, Y. Oka, K. Azuma, E. Fujiwara, M. Yatsuzuka, Thin Solid Films 506 (2006) 68. [26] F. Balon, V. Stolojan, S.R.P. Silva, M. Michalka, A. Kromka, Vacuum 80 (2005) 163. [27] J. Sui, W. Cai, L. Zhao, Nucl. Instrum. Methods B 248 (2006) 67. [28] J. Robertson, Diam. Relat. Mater. 3 (1994) 361. [29] J. Robertson, Diam. Relat. Mater. 14 (2005) 942. [30] O.S. Panwar, S. Kumar, S.S. Rajput, R. Sharma, R. Bhattacharyya, Vacuum 72 (2004) 183. [31] N.S. Xu, S. Ejaz Huq, Mater. Sci. Eng., R 48 (2005) 47.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
A self-DLC coated cathode plasma source A. Deachana a,b,⁎, D. Boonyawan a, B. Yodsombat a a b
Fast Neutron Research Facility, Department of Physics, Faculty of Science, Chiang Mai University, Chiang Mai, 50200, Thailand Department of Physics and General Science, Faculty of Science and Technology, Songkhla Rajaphat University, Songkhla, 90000, Thailand
a r t i c l e
i n f o
Available online 11 March 2009 Keywords: DLC Aluminum cathode Microwave plasma source OES Raman spectroscopy RBS
a b s t r a c t A novel self-microwave plasma source modified by diamond-like carbon (DLC) coating self-cathode for density enhancement was devised. This simple method had been once routed for enhanced efficient microwave plasma with very low-cost. The coating parameters in fabrication of the self-coated DLC cathode for plasma density enhancement were investigated. DLC-Al cathodes coating by self cathode of microwave plasma source were fabricated to control and yielded strong non-activated emission. The novel cathodes showed enhancement of current density up to 240%. In addition, the contamination in the plasma source could be avoided with the new cathodes. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved.
1. Introduction The diamond-like carbon (DLC) as a cathode coating material of source component was of interest because of its excellent properties, low and negative surface electron affinity, chemically inert, high thermal conductivity, toughness [1] and low threshold field emission from diamond even with flat surface [2–4]. Several groups have investigated the field emission behavior of DLC films containing different amounts of bonded carbon ratio [5–9]. While the film containing only sp3 bonded carbon had to be activated in order to show field emission, the film containing conductive inclusions of sp2 shows strong non-activated emission [5]. Karabutov et al. [6] concluded that the positive influence on the field emission was produced by the following factors: (1) high content of the sp3 carbon in DLC film. (2) nitrogen or sulphur doping. (3) nanometre-scale thickness in case of high resistivity films, and (4) post-growth annealing resulting in a partial graphitization. Different types of the surface treatment and coatings have been carried out to improve emission behavior of the DLC films. The emission parameters were found to be governed by an intrinsic characteristic of the DLC films. Probably electron emission processes are that electron generating from graphitic clusters to the sp3 medium of DLC films, electron tunneling through the forbidden gap of sp 3 medium and then being injected into the vacuum (anode) [7]. In the nitrogen doped DLCcoated silicon emitters, the turn-on field decreased and the emission current increased with the elevation of nitrogen doping concentration. The work function decreased from 4.30 eV for the non-coated silicon emitters to 0.47 eV for nitrogen doped DLC-coated silicon
⁎ Corresponding author. Fast Neutron Research Facility, Department of Physics, Faculty of Science, Chiang Mai University, Chiang Mai, 50200, Thailand. Tel.: +66 53 943367; fax: +66 53 222776. E-mail address: [email protected] (A. Deachana).
emitters. As nitrogen doping concentrations increased, the work function of the nitrogen doped DLC-coated silicon emitters decreased [8]. In DLC films doped with both boron and nitrogen, the electron field emission properties of the as-deposited samples at fixed nitrogen doping vary significantly with boron content. The results infer that only when proper dopants are incorporated, the electron conduction in the DLC films is improved and the electron emission is enhanced [9]. There are many coating methods used to fabricate DLC films, including direct ion beam deposition [10], pulsed laser deposition (PLD) [11], ion beam conversion coatings [12], filtered cathodic arc deposition, plasma immersion ion implantation and deposition (PIII&D) [13,14], magnetrons sputter coating [15], and rf plasmaenhanced chemical vapor deposition (RF-PECVD) [16,17]. In these methods, DLC films are fabricated from a variety of carbonaceous precursor materials, and in most of these methods the characteristics of DLC depended on deposition conditions. For example, the flow rate of CH4, synthesis pressure, rf-power, bias voltage and substrate temperature are used as deposition conditions to reduce internal stress in the RF-PECVD method [18]. Additionally, in a simplified arc discharge system, DLC with a smooth surface and low friction coefficient could be formed at low negative-voltage pulses [19,20], and high adhesion of DLC coatings resulted from rise rates of the short arc discharge pulse [21]. These clearly show that the deposition conditions play a key role in controlling the DLC characteristics. One of the newer methods used to fabricate DLC films is to use microwave plasma. However, in this method the plasma obtained has limitations, e.g. cut-off density and skin depth [22,23]. Two main approaches to enhance the density are (1) variation of geometry and configuration of microwave plasma source, and (2) selection of suitable materials of source component. The dc-bias microwave plasma hybrid technique is shown to increase the cut-off density. This self-source technique provides the surface treatment and coating
0257-8972/$ – see front matter. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.02.132
2744
A. Deachana et al. / Surface & Coatings Technology 203 (2009) 2743–2746
to modify cathode's surface to enhance the plasma density. The simple method has been once routed for enhanced efficient microwave plasma with very low-cost. As a self-modification technique, there is no need to incorporate other coating systems, which is the main advantage of such method. Therefore, the major aims of this work are to devise the microwave plasma source coating system, as described in details in Section 2, and to study coating parameters in fabrication of the self-coated DLC cathode for plasma density enhancement. As will be seen in the results in Section 3, the self-coated DLC coating system developed here is shown to improve the plasma density significantly. 2. Experimental procedures 2.1. Microwave power setup The experimental setup consists of a homemade rectangular waveguide WR 340 (8.64 cm × 4.32 cm) with magnetron (commercial cooking microwave, Sharp Model: RV-MZ 4080, 2 M241) coupled at one end and a plasma source tube at the other, as shown in Fig. 1. The three-position tuner is placed between the magnetron and plasma source tube. Two cooking microwave transformers are employed as a power supply of magnetron, first for stepping up high voltage and second for varying the power of magnetron filament by using the variable transformer. The microwave power has already been adjusted to calibrated power with the water as the power load and calculated microwave power from the water heating. The conventional fan cooled magnetron has a maximum forward power of 100 W at 2.45 GHz with 200 V AC input. The vacuum system consists of a rotary-backing pump (Edward, No.2) and a turbomolecular pump (Edwards, EXT250/ISO 100), which allows a background pressure of plasma source tube and main chamber of 3.0 and 5.3 × 10− 4 Pa, respectively. Gas flow is measured by a mass flow controllers/meter (MKS, Type 246) and controlled via a precise leak valve of mass flow control (MKS, 1179A21CR1BV). The pressure is a function of the gas flow rate. The pressure difference between a plasma source and the main chamber is about 1.3 × 10− 2 Pa. 2.2. Plasma source setup The plasma source consists of a plasma source tube with adjustable electrode distance, as shown in Fig. 1(b), and a tapered WR340 wave guide with integrated fan cooling for anode at the upper part and high pressure air feeder for cathode at lower part with the source tube
inserted through wave guide and plugged into the main chamber, as schematically described in Fig. 1(a). The plasma is ignited inside the quartz tube of 7.5 mm diameter inside the waveguide since the internal pressure (p) and adjusted electrode distance (d) are close to the Paschen minimum. For optimization and better comparison, d = 10 mm is fixed in all experiments. The source tube can be operated in wide range of argon gas pressure from 1.3 to 1.0 × 103 Pa. The quartz tubes is confined in two electrode housings with stainless steel electrodes with aluminum tips inserted inside both housings as a cathode and an anode in the quartz tube. The source tube is electrically isolated from the waveguide. Gas is fed through the upper electrode, where the pressure gauge is connected to monitor the incoming gas pressure, whereas the lower electrode serves as the plasma outlet. In the source body, both aluminum tips can be biased independently to control plasma reactor. The electrons can diffuse through a pin hole with diameter of 300 µm to the main chamber. The plasma produced is then diagnostic with; (1) optical emission spectroscopy (OES, Ocean Optics S2000) via the fiber optic inserted through an insulator between upper housing and wave-guide, (2) the current–voltage (I–V) by biasing voltage between electrodes, and current density (J) obtained from I is correlated with the plasma density (n) for a stationary sheath [24]. 2.3. Cathode coating setup All substrate cathodes with diameter of 7.5 mm and a pin hole at center of the cathode with diameter of 300 µm are machined by highprecision CNC Machine from conventional aluminum plates of thickness 3 mm. The substrates are mechanically polished by AP-A Suspension (Struers) of 1–0.3 µm. The substrates are successively cleaned with water and followed with acetone, ethanol at 60 °C in the ultrasonic water bath (TRANSSONIC T 460/H) and finally dried with air prior to coating preparation. Fabrication of DLC films is carried out by a self-cathode of source tube, which is negatively biased with direct current (DC) from power supply (BERTAN, 205B-20R). This coating system is similar to microwave plasma enhanced chemical vapor deposition (MECVD) in the plasma source tube. The experimental coating conditions are listed in Table 1. Note that all of coating conditions, argon gas is used as active gas with argon gas pressure of 19.9 Pa as vacuum base pressure. The source tube pressure is varied from 1.5 × 102 to 5.8 × 102 Pa and before each experiment all substrates are sputtered for surface cleaning with a condition of 20 min, 19.9 Pa from 1.44 sccm of Ar gas, and cathode-bias of 400 V DC.
Fig. 1. Schematic diagram of (a) plasma source to the main chamber, and (b) zoom-in of plasma source tube.
A. Deachana et al. / Surface & Coatings Technology 203 (2009) 2743–2746
2745
Table 1 Conditions of films coating. Samples
Microwave power (W)
Time (min.)
Pressure (×102 Pa)
Flow rate (sccm)
Bias (V)
Gas
DLC210506 DLC260506 DLC070606 DLC100606
40 40 40 40
9 16 6 20
1.5 2.9 3.8 5.8
1.44 1.65 1.85 6.45
400 400 400 400
C2H2 C2H2 C2H2 C2H2
For plasma diagnosis, acetylene (C2H2) is used to monitor ion species during coating operation. Raman measurements are performed at room temperature using a spectrometer of the type Jobin Yvon T64000, France, in the quasi-back scattering geometry of an Ar laser operating with a power of 200 mW. The thickness of DLC films are roughly evaluated using Rutherford backscattering spectrometry (RBS). Field emission is measured for each sample with Ar-plasma by using a power supply, low voltage high current (LTRONIX, B60-15R) with negative bias to cathode, while multimeter (KEITHLEY 197) is used to measure bias-current. 3. Results and discussion 3.1. DLC film properties From 40 W microwave plasma, Fig. 2 (solid line) shows the spectra of time integrated optical emission, argon activated acetylene plasma with negative dc-bias voltages of 60 V at a gas pressure of 1.5 × 102 Pa. The main emission spectra are C at 818.2 nm, C at 711.2 nm and H at 841.3 nm, but CH at 431.4 nm [25] cannot be observed. The emitted lines result either from dissociative excitation of the acetylene or argon or from electron impact excitation of radicals and atoms. Though the thickness of DLC films cannot be analyzed by RBS because the films obtained are very thin, but some information of DLC films are derived. All of the films show dominant peak of oxygen contamination, which could be induced during the preparation of Al-substrates since Al-substrates are very sensitive and could easily form aluminum oxide. Fig. 3 shows typical Raman spectra of the DLC films at the bias voltage of 400 V, which are fitted by Gaussian distribution. All the spectra of the DLC films show similar broad peak at around 1590 cm− 1 and another obvious lower frequency shoulder at approximately 1360 cm− 1, commonly referred to as the G band and D band, respectively [26]. It should be noted that the D peaks observed in Fig. 3
Fig. 3. Raman spectra of DLC films prepared at various pressures.
are attributed to the carbon particles. Based on the fitting parameters, the peak position and ID/IG (the ratio of the intensity of the D and G peaks) are summarized in Table 2. It has been reported that the position of G band is related to the bond-angle disorder or sp3 bonding content, while the ID/IG ratio is correlated with the ratio of sp3/sp2 bonds [27]. These two factors play the most important role in determining the Raman spectra. The sp3/sp2 ratio in the DLC films cannot be derived directly from the Raman spectra, but some qualitative information can be extracted. The results from Table 2 show the correspondence between increasing pressures and the sp3/sp2 ratio, (2.9 × 102 Pa, 0.788), (3.8 × 102 Pa, 0.826), (5.8 × 102 Pa, 0.919). Clearly, the pressure has strong influence on the sp3/sp2 ratio. However, (1.5× 102 Pa, 0.873) does not follow the same trend probably because a greater fraction of sp3 bonds can be obtained about 100 eV, when the ion energy is high enough for displacement of low energy sp2 bonds, but still not enough for displacement of high-energy sp3 bonds. At low pressure, the ions take a long time (collision time) to gain energy from the bias voltage. When the ion energy is too high, the ions may damage the films surface and cause sp3 bonds to transform into sp2 bonds [28,29]. 3.2. Ion current density Even at low microwave power of 40 W and low negative dc-bias of 20 V at an argon gas pressure of 19.9 Pa, the OES spectra dominantly show, as seen in Fig. 2 (dash line) sputtering Al atoms at 517.3 nm, 669.8 nm, and 684.8 nm, but the Al spectra decrease significantly in DLC coating process (Fig. 2 (solid line)). This is because the microwave plasma contaminates Al-cathode, while the DLC provides contamination-free Al-cathode. Therefore, dc-bias of 30 V is set as condition for I–V characteristics of plasma, and each I–V curve is repeated 5 times for data collection. Fig. 4 shows the comparison of current density of between Al-cathode and DLC-coated Al-cathodes. All of DLC-Alcathodes show very strong enhancement in the current density. The improved current density could imply the enhanced plasma density through stationary sheath, as described earlier [24]. As listed in Table 2, the IG factor plays dominant role in the enhancement, probably as a result of the surface micro-irregularities in the DLC-Al cathodes. To understand the improved current density, the field
Table 2 Fitting results of Raman spectra of DLC films prepared at various pressures.
Fig. 2. Typical OES spectra of 40 W microwave plasma at various conditions during coating process: negative dc bias cathode 20 V with Ar plasma (dash line) and 60 V with mixed Ar and C2H2 plasma (solid line).
Samples
D band (cm− 1)
G band (cm− 1)
ID/IG
IG (a.u.)
DLC210506 DLC260506 DLC070606 DLC100606
1329.5 1368.0 1360.7 1367.9
1607.1 1590.9 1597.6 1582.6
0.873 0.788 0.826 0.919
1513.379 1218.038 505.788 549.867
2746
A. Deachana et al. / Surface & Coatings Technology 203 (2009) 2743–2746
Acknowledgment The project was supported by Fast Neutron Research Facility, Department of Physics, Faculty of Science and the Graduate School, Chiang Mai University, Chiang Mai, 50200, Thailand.
References
Fig. 4. I–V curves of Al-cathode and DLCs coated Al-cathode.
emission of DLC films can be explained as follows. When the cathode is negatively biased, the electrons are driven back from the cathode and ions with the space-charge limit flux according to the Child– Langmuir's Law. However, the sheath edge will induce high electric field at interface layer between sheath edge and the DLC surface, which in turn causes the local field to drive electrons out through a potential barrier following the Fowler–Nordheim [30,31]. In the DLC films, conducting channels (corresponding to IG) are formed in insulating matrix (corresponding to ID), which is already present in the deposition process. These conducting channels act as tip-like structure leading to field emission enhancement [5]. This enhancement plays an important role in controlling of the plasma density, and the plasma density enhancement by DLC-coated cathode, which also depends on the film morphology, distribution and size of conducting channels, the thickness and optimization of sp2 content. 4. Conclusions This work presents a novel microwave plasma source coating system used to enhance the plasma density through self-coated DLC cathode. The coating parameters in fabrication of the self-coated DLC cathode for plasma density enhancement are studied. It is found that the plasma current density is enhanced with increasing IG factor of the DLC films. The threshold voltage of enhancement is approximately 5– 12 V and the density enhancement between 1.6–2.4 times at 30 V is obtained.
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