- Email: [email protected]
Effects of pulse parameters on macro-particle production in pulsed cathodic vacuum arc deposition
Surface & Coatings Technology 201 (2007) 6542 – 6544 www.elsevier.com/locate/surfcoat
Effects of pulse parameters on macro-particle production in pulsed cathodic vacuum arc deposition Yawei Hu a,c , Liuhe Li a,b,⁎, Hua Dai a , Xiaoling Li a , Xun Cai a , Paul K. Chu c a
b
School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China School of Mechanical Engineering and Automation, Beijing University of Aeronautics and Astronautics, Beijing, 100083, China c Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong Available online 16 November 2006
Abstract The effects of the pulse parameters on the production of macro-particles in vacuum arc deposition are studied. A power supply that can provide either direct current or pulsed power is used and the influence of the current and duty cycle are independently investigated. Copper is used as the cathode and glass is used as the substrate. Optical microscopy and scanning electron microscopy with image processing software are used to analyze the macro-particles deposited on the substrate. Our results show the general trend that the density of macro-particles increases with the direct current but there is no obvious correlation with the duty cycles. The lowest degree of macro-particle contamination is observed at a duty cycle of about 40.5%. © 2006 Elsevier B.V. All rights reserved. PACS: 52.77.Dq; 52.25.Vy; 68.55.Ln; 81.15.Ef Keywords: Macro-particles; Cathodic vacuum arc; Deposition
1. Introduction Cathodic vacuum arc deposition offers high ionization efficiency and deposition rates, and so it is widely used in deposition of protective and decorative coatings such as TiN [1,2], TiC [3], DLC [4,5], etc. However, the presence of macroparticles (MPs) limits wider applications in the optics and semiconductor industry. MPs originate from the plasma solid interaction at the cathode spot. The cathode spot is necessary to provide sufficient energy for plasma formation, electron emission, and current transport between the cathode and anode [6]. The heat generated by the high current and ion impact induces melting of the cathode materials, and the violent evaporation events causes the emission of MPs [7]. The size of these macro-particles is generally in the range of 0.1–10 μm, and there also exist smaller particles called nano-particles (NP) [8]. A filter is usually used to separate the particles from the plasma, but plasma loss is evitable [6,9,10]. The cathode materials, arc ⁎ Corresponding author. School of Mechanical Engineering and Automation, Beijing University of Aeronautics and Astronautics. Beijing, 100083, China. Tel./fax: +86 10 82338135. E-mail address: [email protected] (L. Li). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.09.071
current, distance, and angle can all influence MP production or distribution [11,12], and the appropriate magnetic field and bias can reduce the quantity of MPs [13,14]. The style and parameters of arc also has important influence. The number of large droplets is significantly reduced in HCA (high-current pulsed arc) deposition [15], and the density of the particles increases with increasing arc pulse duration time [12]. The modified arc deposition process combining the DC (direct current) and pulses arcs can increase the arc stability, improve the target consumption, increase the spot velocity, and decrease the number of particles [16,17], but the effects of pulse parameters such as duty cycle and frequency have not been investigated in details. In this work, an arc source combining DC and pulsed arcs is used to study the influence of the duty cycle, current (DC), and frequency on the production and distribution of macro-particles and possible reasons are discussed. 2. Experimental details The experiments were performed in a cylindrical vacuum chamber 80 cm in diameter and 120 cm in height. The source and position of the substrate are schematically shown in Fig. 1. The angle between the axis of the target and substrate was about 25°. The cathode was made of 99.99% pure copper and had a diameter
Y. Hu et al. / Surface & Coatings Technology 201 (2007) 6542–6544
Fig. 1. Schematic diagram of cathodic vacuum arc deposition system.
of 84 mm and length of 60 mm. The substrates were 2 cm× 2 cm glass slides placed about 45 cm from the cathode. The base pressure was below 2 × 10− 3 Pa. The working gas was argon and the partial pressure was about 1.7 × 10− 1 Pa. The film deposition time was 600 s. A power supply which could generate independent DC and pulsed current was used. Deposition was conducted with different currents (DC), duty cycles, and frequencies. After deposition, the films were characterized by optical microscopy and scanning electron microscopy (SEM, JSM6460). The number of MPs deposited onto the substrate was measured from six different locations using image analysis software. Particles that are smaller than 0.2 μm and larger than 10 μm were not counted. The density of the particles was evaluated by taking the average number of particles measured in six locations.
6543
Fig. 3. Density of particle as a function of duty cycles with different currents (DC) (peak values of pulse current = 40 A).
DC currents were respectively kept at about 40 and 70 A, whereas the duty cycle was varied from 23.5% to 91.5%. Fig. 2 shows the distribution of the densities of particles at different
3. Results 3.1. Effects of duty cycles and frequencies In the experiments, 4 different frequencies, 5 Hz, 35 Hz, 75 Hz, and 200 Hz, were used. The peak values of the pulsed and
Fig. 2. Density of particles as a function of duty cycles (peak value of pulsed current = 40 A, current (DC) = 70 A).
Fig. 4. Particle size distribution for estimated heat flux values (pulsing frequency = 75 Hz, duty cycle = 40.5%, pulse current = 40 A): (a): DC: 70 A, (b) DC: 45 A ((○): original data; (□): Gaussian fitted distribution).
6544
Y. Hu et al. / Surface & Coatings Technology 201 (2007) 6542–6544
duty cycles. It can be shown that the density of particles does not linearly change with the duty cycle, and the minimum density of particles appears at the same duty cycle (40.5%) for different frequencies. The density of particles first increases and subsequently decreases with increasing pulsing frequencies. 3.2. Effects of DC With the peak values and frequency of the pulsed current being constant, the DC current is respectively adjusted to 45 or 70 A. For duty cycles increasing from 20.5% to 91.5%, the density of MPs as a function of the duty cycle is shown in Fig. 3. The change in the density of particles with duty cycle is different with varying current (DC). When it is kept at 45 A, the density of particles quickly increases with increasing duty cycles from 23.5% to 74.5% and then slowly increases upon further increase in the duty cycles. 4. Discussion The power input is important to the cathode affecting the temperature in the cathode spot and volume of melted materials. Therefore, particle emission can be correlated with the heat load in this zone. When the heat load rises, the volume of molten materials formed at the cathode spot increases, and the number of particles emitted from the cathode spot increases as well. Increasing the arc current may lead to a greater number of spots or a larger spot size [18], and the density of particle increases with higher currents [12]. Fig. 3 shows that raising the current (DC) gives rise to a higher density of particles. The size distribution of the MPs shows that smaller particles are dominant. Some studies have indicated the number of particles has an exponential relationship with the particles size [11,19–21]. Fig. 4 depicts the distribution observed as well as the mean particle size Φp. The average particle size increases from 1.34 μm to 1.42 μm when the heat load is varied. The results show that an increase in the heat load in the cathode spot area increases the density of the emitted particles as well as the width of the size distribution. The cathode spot speed also has an effect on the density of particles. The power input increases the size of the cathode spot and number of spots, and the speed of the cathode spot as well, thereby resulting in the reduction of the resident time of the cathode spot. As shown in Fig. 3, with a low current (DC), the increase in the duty cycle and pulse current has great effect on the heat load and the production of particles. However, at a high current (DC), the effect of the duty cycle on the heat load diminishes whereas that on the cathode speed is enhanced. These two factors have alternating effects on the spot and so the density of the particles does not linearly increase with the duty cycles and shows a minimum at a duty cycle of 40.5% at a high current (DC) (Fig. 2). The variation of the particle density with
frequency may be attributed to the change of heat load and the resident time of the cathode spot. 5. Conclusion The effects of the pulse parameters on the production of macro-particles in vacuum arc deposition are studied. A power supply that can provide either DC (direct current) or pulsed power is used. The density of particles increases with higher current (DC) when other parameters are kept constant. This trend can be attributed to the increased heat input. The density of particles does not increase linearly with the duty cycle when other parameters are kept constant. It is due to the influence of the heat load and cathode spot speed. The lowest density of particles is achieved at a duty cycle of about 40.5%. Acknowledgments The financial support of this work was jointly provided by the National Natural Science Foundation of China. No.50271004 and City University of Hong Kong Direct Allocation Grant No. 9360110. References [1] H. Ichimura, F.M. Rodriguez, A. Rodrigo, Surf. Coat. Technol. 127 (2000) 138. [2] P.J. Martin, A. Bendivid, T.J. Kinder, IEEE Trans. Plasma Sci. 25 (1997) 675. [3] X. Ding, B.K. Tay, H.S. Tan, S.P. Lau, W.Y. Cheung, S.P. Wong, Surf. Coat. Technol. 138 (2001) 301. [4] L.H. Li, J.Z. Tian, X. Cai, Q.L. Chen, M. Xu, Y.Q. Wu, Ricky K.Y. Fu, Paul K. Chu, Surf. Coat. Technol. 196 (2005) 241. [5] M.S. Leu, S.Y. Chen, J.J. Chang, I.G. Chao, W. Lin, Surf. Coat. Technol. 177/178 (2004) 566. [6] A. Anders, Surf. Coat. Technol. 120–121 (1999) 319. [7] B. Jutter, J. Phys. D: Appl. Phys. 14 (1981) 1265. [8] O.R. Monteiro, A. Anders, IEEE Trans. Plasma Sci. 27 (1999) 1030. [9] J. Storer, J.E. Gaivin, I.G. Brown, J. Appl. Phys. 66 (1989) 5245. [10] S. Anders, A. Anders, M.R. Dickinson, R.A. Macgill, I.G. Brown, IEEE Trans. Plasma Sci. 25 (1997) 670. [11] S. Anders, A. Anders, K.M. Xu, X.Y. Yao, I.G. Brown, IEEE Trans. Plasma Sci. 21 (1993) 440. [12] M. Kandah, J.L. Meunier, J. Vac. Sci. Technol. A 13 (1995) 2444. [13] P.D. Swift, J. Phys. D: Appl. Phys. 29 (1996) 2025. [14] R.R. Aharonov, M. Chhowalla, S. Dhar, R.P. Fontana, Surf. Coat. Technol. 82 (1996) 334. [15] P. Siemroth, T. Schukke, T. Witke, Surf. Coat. Technol. 68/69 (1994) 314. [16] K. Keutel, H. Fuchs, H. Mecke and Ch. Edelmann, IEEE. 18th. Symp. on. Discharge and Electrical Insulation in Vacuum-Eindhoven-1998, August 17–21:p562. [17] M. Ellrodt, H. Meche, Surf. Coat. Technol. 74/75 (1995) 241. [18] B.E. Djakov, R. Holmes, J. Phys. D: Appl. Phys. 4 (1971) 504. [19] T. Utsumi, J.H. English, J. Appl. Phys. 46 (1975) 126. [20] D.T. Tuma, C.L. Chen, D.K. Davies, J. Appl. Phys. 49 (1978) 3821. [21] J.E. Daddler, J. Phys. D: Appl. Phys. 9 (1976) 2379.
Effects of pulse parameters on macro-particle production in pulsed cathodic vacuum arc deposition Yawei Hu a,c , Liuhe Li a,b,⁎, Hua Dai a , Xiaoling Li a , Xun Cai a , Paul K. Chu c a
b
School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China School of Mechanical Engineering and Automation, Beijing University of Aeronautics and Astronautics, Beijing, 100083, China c Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong Available online 16 November 2006
Abstract The effects of the pulse parameters on the production of macro-particles in vacuum arc deposition are studied. A power supply that can provide either direct current or pulsed power is used and the influence of the current and duty cycle are independently investigated. Copper is used as the cathode and glass is used as the substrate. Optical microscopy and scanning electron microscopy with image processing software are used to analyze the macro-particles deposited on the substrate. Our results show the general trend that the density of macro-particles increases with the direct current but there is no obvious correlation with the duty cycles. The lowest degree of macro-particle contamination is observed at a duty cycle of about 40.5%. © 2006 Elsevier B.V. All rights reserved. PACS: 52.77.Dq; 52.25.Vy; 68.55.Ln; 81.15.Ef Keywords: Macro-particles; Cathodic vacuum arc; Deposition
1. Introduction Cathodic vacuum arc deposition offers high ionization efficiency and deposition rates, and so it is widely used in deposition of protective and decorative coatings such as TiN [1,2], TiC [3], DLC [4,5], etc. However, the presence of macroparticles (MPs) limits wider applications in the optics and semiconductor industry. MPs originate from the plasma solid interaction at the cathode spot. The cathode spot is necessary to provide sufficient energy for plasma formation, electron emission, and current transport between the cathode and anode [6]. The heat generated by the high current and ion impact induces melting of the cathode materials, and the violent evaporation events causes the emission of MPs [7]. The size of these macro-particles is generally in the range of 0.1–10 μm, and there also exist smaller particles called nano-particles (NP) [8]. A filter is usually used to separate the particles from the plasma, but plasma loss is evitable [6,9,10]. The cathode materials, arc ⁎ Corresponding author. School of Mechanical Engineering and Automation, Beijing University of Aeronautics and Astronautics. Beijing, 100083, China. Tel./fax: +86 10 82338135. E-mail address: [email protected] (L. Li). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.09.071
current, distance, and angle can all influence MP production or distribution [11,12], and the appropriate magnetic field and bias can reduce the quantity of MPs [13,14]. The style and parameters of arc also has important influence. The number of large droplets is significantly reduced in HCA (high-current pulsed arc) deposition [15], and the density of the particles increases with increasing arc pulse duration time [12]. The modified arc deposition process combining the DC (direct current) and pulses arcs can increase the arc stability, improve the target consumption, increase the spot velocity, and decrease the number of particles [16,17], but the effects of pulse parameters such as duty cycle and frequency have not been investigated in details. In this work, an arc source combining DC and pulsed arcs is used to study the influence of the duty cycle, current (DC), and frequency on the production and distribution of macro-particles and possible reasons are discussed. 2. Experimental details The experiments were performed in a cylindrical vacuum chamber 80 cm in diameter and 120 cm in height. The source and position of the substrate are schematically shown in Fig. 1. The angle between the axis of the target and substrate was about 25°. The cathode was made of 99.99% pure copper and had a diameter
Y. Hu et al. / Surface & Coatings Technology 201 (2007) 6542–6544
Fig. 1. Schematic diagram of cathodic vacuum arc deposition system.
of 84 mm and length of 60 mm. The substrates were 2 cm× 2 cm glass slides placed about 45 cm from the cathode. The base pressure was below 2 × 10− 3 Pa. The working gas was argon and the partial pressure was about 1.7 × 10− 1 Pa. The film deposition time was 600 s. A power supply which could generate independent DC and pulsed current was used. Deposition was conducted with different currents (DC), duty cycles, and frequencies. After deposition, the films were characterized by optical microscopy and scanning electron microscopy (SEM, JSM6460). The number of MPs deposited onto the substrate was measured from six different locations using image analysis software. Particles that are smaller than 0.2 μm and larger than 10 μm were not counted. The density of the particles was evaluated by taking the average number of particles measured in six locations.
6543
Fig. 3. Density of particle as a function of duty cycles with different currents (DC) (peak values of pulse current = 40 A).
DC currents were respectively kept at about 40 and 70 A, whereas the duty cycle was varied from 23.5% to 91.5%. Fig. 2 shows the distribution of the densities of particles at different
3. Results 3.1. Effects of duty cycles and frequencies In the experiments, 4 different frequencies, 5 Hz, 35 Hz, 75 Hz, and 200 Hz, were used. The peak values of the pulsed and
Fig. 2. Density of particles as a function of duty cycles (peak value of pulsed current = 40 A, current (DC) = 70 A).
Fig. 4. Particle size distribution for estimated heat flux values (pulsing frequency = 75 Hz, duty cycle = 40.5%, pulse current = 40 A): (a): DC: 70 A, (b) DC: 45 A ((○): original data; (□): Gaussian fitted distribution).
6544
Y. Hu et al. / Surface & Coatings Technology 201 (2007) 6542–6544
duty cycles. It can be shown that the density of particles does not linearly change with the duty cycle, and the minimum density of particles appears at the same duty cycle (40.5%) for different frequencies. The density of particles first increases and subsequently decreases with increasing pulsing frequencies. 3.2. Effects of DC With the peak values and frequency of the pulsed current being constant, the DC current is respectively adjusted to 45 or 70 A. For duty cycles increasing from 20.5% to 91.5%, the density of MPs as a function of the duty cycle is shown in Fig. 3. The change in the density of particles with duty cycle is different with varying current (DC). When it is kept at 45 A, the density of particles quickly increases with increasing duty cycles from 23.5% to 74.5% and then slowly increases upon further increase in the duty cycles. 4. Discussion The power input is important to the cathode affecting the temperature in the cathode spot and volume of melted materials. Therefore, particle emission can be correlated with the heat load in this zone. When the heat load rises, the volume of molten materials formed at the cathode spot increases, and the number of particles emitted from the cathode spot increases as well. Increasing the arc current may lead to a greater number of spots or a larger spot size [18], and the density of particle increases with higher currents [12]. Fig. 3 shows that raising the current (DC) gives rise to a higher density of particles. The size distribution of the MPs shows that smaller particles are dominant. Some studies have indicated the number of particles has an exponential relationship with the particles size [11,19–21]. Fig. 4 depicts the distribution observed as well as the mean particle size Φp. The average particle size increases from 1.34 μm to 1.42 μm when the heat load is varied. The results show that an increase in the heat load in the cathode spot area increases the density of the emitted particles as well as the width of the size distribution. The cathode spot speed also has an effect on the density of particles. The power input increases the size of the cathode spot and number of spots, and the speed of the cathode spot as well, thereby resulting in the reduction of the resident time of the cathode spot. As shown in Fig. 3, with a low current (DC), the increase in the duty cycle and pulse current has great effect on the heat load and the production of particles. However, at a high current (DC), the effect of the duty cycle on the heat load diminishes whereas that on the cathode speed is enhanced. These two factors have alternating effects on the spot and so the density of the particles does not linearly increase with the duty cycles and shows a minimum at a duty cycle of 40.5% at a high current (DC) (Fig. 2). The variation of the particle density with
frequency may be attributed to the change of heat load and the resident time of the cathode spot. 5. Conclusion The effects of the pulse parameters on the production of macro-particles in vacuum arc deposition are studied. A power supply that can provide either DC (direct current) or pulsed power is used. The density of particles increases with higher current (DC) when other parameters are kept constant. This trend can be attributed to the increased heat input. The density of particles does not increase linearly with the duty cycle when other parameters are kept constant. It is due to the influence of the heat load and cathode spot speed. The lowest density of particles is achieved at a duty cycle of about 40.5%. Acknowledgments The financial support of this work was jointly provided by the National Natural Science Foundation of China. No.50271004 and City University of Hong Kong Direct Allocation Grant No. 9360110. References [1] H. Ichimura, F.M. Rodriguez, A. Rodrigo, Surf. Coat. Technol. 127 (2000) 138. [2] P.J. Martin, A. Bendivid, T.J. Kinder, IEEE Trans. Plasma Sci. 25 (1997) 675. [3] X. Ding, B.K. Tay, H.S. Tan, S.P. Lau, W.Y. Cheung, S.P. Wong, Surf. Coat. Technol. 138 (2001) 301. [4] L.H. Li, J.Z. Tian, X. Cai, Q.L. Chen, M. Xu, Y.Q. Wu, Ricky K.Y. Fu, Paul K. Chu, Surf. Coat. Technol. 196 (2005) 241. [5] M.S. Leu, S.Y. Chen, J.J. Chang, I.G. Chao, W. Lin, Surf. Coat. Technol. 177/178 (2004) 566. [6] A. Anders, Surf. Coat. Technol. 120–121 (1999) 319. [7] B. Jutter, J. Phys. D: Appl. Phys. 14 (1981) 1265. [8] O.R. Monteiro, A. Anders, IEEE Trans. Plasma Sci. 27 (1999) 1030. [9] J. Storer, J.E. Gaivin, I.G. Brown, J. Appl. Phys. 66 (1989) 5245. [10] S. Anders, A. Anders, M.R. Dickinson, R.A. Macgill, I.G. Brown, IEEE Trans. Plasma Sci. 25 (1997) 670. [11] S. Anders, A. Anders, K.M. Xu, X.Y. Yao, I.G. Brown, IEEE Trans. Plasma Sci. 21 (1993) 440. [12] M. Kandah, J.L. Meunier, J. Vac. Sci. Technol. A 13 (1995) 2444. [13] P.D. Swift, J. Phys. D: Appl. Phys. 29 (1996) 2025. [14] R.R. Aharonov, M. Chhowalla, S. Dhar, R.P. Fontana, Surf. Coat. Technol. 82 (1996) 334. [15] P. Siemroth, T. Schukke, T. Witke, Surf. Coat. Technol. 68/69 (1994) 314. [16] K. Keutel, H. Fuchs, H. Mecke and Ch. Edelmann, IEEE. 18th. Symp. on. Discharge and Electrical Insulation in Vacuum-Eindhoven-1998, August 17–21:p562. [17] M. Ellrodt, H. Meche, Surf. Coat. Technol. 74/75 (1995) 241. [18] B.E. Djakov, R. Holmes, J. Phys. D: Appl. Phys. 4 (1971) 504. [19] T. Utsumi, J.H. English, J. Appl. Phys. 46 (1975) 126. [20] D.T. Tuma, C.L. Chen, D.K. Davies, J. Appl. Phys. 49 (1978) 3821. [21] J.E. Daddler, J. Phys. D: Appl. Phys. 9 (1976) 2379.