Plasma Science and Technology, Vol.10, No.5, Oct. 2008 Uniformity of Plasma Density and Film Thickness of Coatings Deposited Inside a Cylindrical Tube by Radio Frequency Sputtering CUI Jiangtao (wô7) 1,TIANXiubo(X?Å) 1,2, YANG Shiqin (fl ) 1,2, HU Tao ( 7) 1,RickyK.Y.FU 3,PaulK.CHU 3 1 School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China 2 Shenzhen Key Lab of Composite Materials, Shenzhen-Tech-Innovation International, Shenzhen 518057, China 3 Department of Physics and Material Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China Abstract Plasma surface modification of the inner wall of a slender tube is quite difficult to achieve using conventional means. In the work described here, an inner coaxial radio frequency (RF) copper electrode is utilized to produce the plasma and also acts as the sputtered target to deposit copper films in a tube. The influence of RF power, gas pressure, and bias voltage on the distribution of plasma density and the uniformity of film thickness is investigated. The experimental results show that the plasma density is higher at the two ends and lower in the middle of the tube. A higher RF power and pressure as well as larger tube bias lead to a higher plasma density. Changes in the discharge parameter only affect the plasma density uniformity slightly. The variation in the film thickness is consistent with that of the plasma density along the tube axis for different RF power and pressure. Although the plasma density increases with higher tube biases, there is an optimal bias to obtain the highest deposition rate. It can be attributed to the reduction in self-sputtering of the copper electrode and re-sputtering effects of the deposited film at higher tube biases. Keywords: plasma distribution, inner surface, radio-frequency plasma PACS: 52.77, 52.80 1 Introduction Hollow tube structures are often used in water pipes, gas pipes, chemical pipes, medical implants, and so on in the industry [1]. These tubes are often required to have good corrosion and wear resistance. Therefore, it is sometimes necessary to enhance the inner surface by depositing a protective film. However, coating the inner wall of a hollow tube using conventional physical vapor deposition (PVD) techniques is difficult, particularly the tubes with small diameters. The technical challenge here is to generate a stable and uniform plasma throughout the length of the interior of the entire tube in the axial direction [2]. A number of plasma-based techniques have been attempted to improve the properties of the inner surface of hollow tubes. ENSINGER et al applied a highenergy ion beam to sputter conical-shaped rod inside the tube [3]. MINATO et al [4] and SATO et al [5] deposited a layer of TiN film by using a movable hollow cathode discharge gun inside the tube. FUJIYAMA et al reported the application of coaxial magnetron pulsed plasma or ECR plasma [6]. MATSON obtained a 100 μm thick tantalum film by using inner triode sputtering inside the tube [7]. In this study, a coaxial radio frequency (RF) copper electrode is utilized to deposit copper films inside a thin and long hollow tube. A thin copper rod is inserted into the tube as not only the RF antenna to trigger the argon plasma but also the sputtered target to deposit the required film. The distribution of the plasma density and the uniformity of film thickness along the axis of tube are investigated. 2 Experimental details The vacuum chamber with size of φ 400 mm 400 mm was evacuated by a mechanical pump and an oil diffusion pump. The base pressure in the chamber was about 9.0 10 3 Pa. The sputtering apparatus is schematically shown in Fig. 1. A stainless steel tube with a lengh of 200 mm and an inner diameter of 15 mm was mounted vertically on the holder. The coaxial copper rod of 240 mm in length and 2 mm in diameter was placed at the center of the tube. The copper rod was connected to a 13.56 MHz capacitively-coupled RF supported jointly by Natural Science Foundation of China (Nos. 10575025, 10775036) and City University of Hong Kong Applied Research Grants (Nos. 9667002, 9667011)
CUI Jiangtao et al.: Uniformity of Plasma Density and Film Thickness of Coatings power supply to act as the antenna to generate and sustain plasma. Meanwhile, it also served as the sputtered target source to deposit the copper film on the inner wall of the hollow tube. Fig.1 Schematic diagram of the experimental setup for inner coatings Prior to deposition of the copper film, the hollow cathode discharge was ignited to sputter and clean the inner wall of the tube. Sputter cleaning was carried out for 20 minutes using a RF power of 200 W and an argon pressure of 15 Pa. Afterwards, RF power was applied to the coaxial copper electrode and a pulsed negative voltage was applied to the tube. The argon pressure ranged from 28 Pa to 42 Pa and the RF power was varied between 400 W and 600 W. The negative pulsed bias on the tube was changed from 0 V to 100 V at a repetition rate of 14 khz and a pulse duration of 20 μs. The treating time was 60 min. The distribution of plasma density in the tube was monitored using measurement circuit illustrated schematically in Fig. 2. Five probes insulated by ceramic tubes were inserted into the tube. The probes were positively biased with a direct current (DC) voltage. The voltage across the resistors was recorded by a digital oscilloscope (Tektronix TDS340A) in order to determine the distribution of the electron density. After film deposition, the treated tubes were cut into several parts and the cross-sections with the copper film were observed by optical microscopy (Olympus BX60M) to measure the film thickness. 3 Results and discussion 3.1 Plasma generation The radio frequency discharge is ignited to generate the high density plasma inside the slender tube with a diameter of 15 mm. The surface of the RF electrode is much smaller than that of the chamber wall and the negative self bias voltage appears on the RF electrode [8]. Thus, the atoms on the central rod are sputtered off and deposited onto the inner surface of the tube. Compared to a DC discharge, the RF discharge is easier to achieve inside the slender tube due to the lower breakdown potential. According to the Paschen formula [9], the breakdown potential (U) depends on the p d value in the DC discharge mode: Bpd U = ln(pd)+lna ln ln( 1+γ, (1) γ ) where p is the gas pressure, d is the electrode gap width, γ is the coefficient of secondary electron emission, and A and B are constants determined by gas and electron temperature. In order to generate the discharge in the thin tube, a very high breakdown potential is necessary because of the small electrode gap (for example, 7.5 mm). However, the B value is smaller in the RF discharge [10], leading to a lower breakdown potential and a discharge that is easier to ignite. In our experiments, when the pressure is 14 Pa, the DC breakdown voltage in the tube is as high as 1250 V and an RF power of about 300 W may be sufficient to sustain a stable glow discharge. 3.2 Plasma density measurement The distribution of the electron density corresponds to that of the ion density in the plasma. The electron density is monitored by a positively biased probe. Fig. 3 displays the I-V characteristics of the probe at a pressure of 28 Pa. The I-V curve obtained at an RF power of 300 W shows that the probe current is proportional to the positive potential of the probe below Fig.2 Schematic diagram of the setup to monitor the plasma density (a) 300 W; (b) 600 W Fig.3 I-V characteristics measured by the probe in the tube with a length of 200 mm and inner diameter of 15 mm at different RF power 561
Plasma Science and Technology, Vol.10, No.5, Oct. 2008 +50 V. When the probe potential exceeds +50 V, the probe current goes up sharply. This sudden change is probably due to abnormal glow discharge on the tip of the probe. Similar phenomena are also observed for a RF power of 600 W but at a reduced potential of +40V.Thatistosay,ifahigherpotentialisapplied to the probe, e.g., +100 V, abnormal discharge induced by the probe potential may occur and there are larger measurement errors [11]. Thus a small probe potential such as +10 V is chosen here to monitor the distribution of the electron density in the tube. atoms. In fact, the hollow cathode effect is observed in conjunction with RF discharge. 3.3 Plasma density distribution The distribution of the plasma density for different RF powers, gas pressures, and tube bias voltages is presented in Fig. 4. In all cases, the plasma density is higher at the two ends of the tube and much lower in the middle section. This is consistent with Paschen law. At the ends of the tube, the larger discharge distance from the RF electrode to the chamber wall (ground potential) leads to a larger p d value. Consequently, the discharge can be easily ignited near the two ends of the tube due to the free discharge path and consequently higher plasma density may be achieved. In contrast, it is more difficult to form the discharge in the middle section as the electrode is further away from the grounded chamber wall and the discharge path is blocked by the tube. Hence, it is not easy to ignite the discharge leading to a lower plasma density within the tube. Fig. 4(a) illustrates that a higher RF power may enhance the plasma density. It has been reported that a higher RF power produces a higher self-bias voltage in the form of V self Wrf 0.5, which gives rise to a larger kinetic energy of the incident ions [12]. Hence, more secondary electrons are emitted and ionization of the argon atoms is enhanced in the tube. The influence of the gas pressure on the plasma density is displayed in Fig. 4(b). The plasma density increases with higher gas pressure. A higher gas pressure, such as 42 Pa, may produce a higher plasma density in the middle of the tube. From this viewpoint, better plasma density uniformity may be achieved at a higher pressure. Thus, it is easier to form the discharge in the central region at a higher pressure. On the other hand, the higher plasma density at the two ends may also lead to more particles diffusing from the ends to the center. Consequently, a higher plasma density can be accomplished in the middle of tube. Fig. 4(c) demonstrates the dependence of the plasma density on the tube bias. A higher bias voltage generates a higher plasma density. As the tube is at a zero potential, the glow discharge inside the entire tube looks very faint and the average plasma density is low. In contrast, when the tube is biased negatively, the glow discharge becomes more intense and brighter. The increased plasma density may be attributed to that the repelling effect of the negative bias enhances the electron oscillation and increases the probability of electron impact ionization on the argon (a) Different RF power at a pressure of 36 Pa and tube bias of 100 V, (b) Different pressure at a tube bias of 100 V and RF power of 600 W, (c) Different tube bias at RF power of 600 W and pressure of 36 Pa Fig.4 Influence of the RF power, pressure, and tube bias on the plasma density distribution along the tube axis 3.4 Axial distribution of film thickness The typical optical images of the cross sections with the deposited copper films are exhibited in Fig. 5. As expected from the plasma density distribution discussed above, the maximum film thickness appears near the two ends of the tube. However the distribution of film thickness is asymmetric, which is not well consistent with that of plasma density. It may be attributed to the unsteady RF discharge in the thin tube. It is occasionally observed that the inner discharge may abruptly extinguish and maintains outside arbitrarily at different ends due to harsh discharge condition. This leads to the non-uniformity of average plasma density near the two ends. Consequently the film thickness is not symmetric at the two ends. In contrast the plasma density is monitored at steady discharge and symmetrically distributed. 562
CUI Jiangtao et al.: Uniformity of Plasma Density and Film Thickness of Coatings bias to sputter the copper electrode to increase the film thickness. However, further increase in the bias voltage may not produce such effects because the bombarding energy of the argon ions diminishes due to the competitive absorption effect of the negative tube bias. Consequently, the sputtering rate decreases leading to a reduced deposition rate of the copper film. On the other hand, the higher re-sputtering effect on account of the higher tube bias also decreases the thickness of the film [16,17]. Therefore, it is necessary to optimize the tube bias during coaxial RF sputtering deposition. (a) 20 mm; (b) 80 mm Fig.5 Optical images of the cross sections of copper film prepared using RF power of 600 W, pressure of 36 Pa, and tube bias of 100 V at different distances from the end Fig. 6 depicts the distribution of the film thickness along the tube axis for different RF power values. A higher RF power may give rise to a thicker film but contribute little to the improvement of film thickness uniformity. A higher RF power results in a larger deposition rate, which is consistent with the results of HEGEMANN et al [13]. With a larger RF power, the ionization of argon is enhanced, leading to more ions bombarding onto the copper electrode. Meanwhile, a higher negative self-bias voltage is also induced and increases the kinetic energy of the ions resulting in a higher sputtering rate [14]. Fig. 7 shows the influence of the pressure on the film thickness. The film thickness is observed to increase with the increase in pressure. A higher pressure leads to a reduced mean free path and consequently increased the collisions in the negative glow region [15]. Therefore, a higher pressure enhances the plasma density and the total sputtering yield. In contrast to the variation of plasma density affected by gas pressure, the film thickness changes more significantly, for example, from 36 Pa to 42 Pa. This may be attributed to the different discharge intensity at different pressures. At higher pressure (42 Pa) the discharge with green color was observed, indicating the occurance of more ionizing events. This leads to a higher sputtering and deposition rate. The copper films become evidently thicker and more non-uniform in thickness. As shown in Fig. 8, there is an optimal tube bias voltage for a thickest film although the plasma density increases with the increase in tube bias. As the negative bias is varied from 0 V to 50 V, the film thickness increases due to the higher ion density in the tube as shown in Fig. 4(c). More ions are generated by the higher tube Fig.6 Influence of the RF power on the film thickness along the tube with a length of 200 mm and an inner diameter of 15 mm Fig.7 Influence of the pressure on the film thickness along the tube with a length of 200 mm and an inner diameter of 15 mm Fig.8 Influence of the tube bias on the film thickness along the tube with a length of 200 mm and an inner diameter of 15 mm 563
4 Conclusion A coaxial RF electrode is used both as the plasma source and the sputtered target to deposit a copper film onto the inner wall of a slender tube. The plasma density and the thickness of the film are not uniform due to Paschen law. The discharge near the tube ends is more intense and so the local deposition rate is higher. The plasma density increases with the RF power and the pressure, and the variation in the copper film thickness is consistent with the plasma density distribution in the tube. The tube bias effectively intensifies the discharge in the tube and increases the plasma density due to the hollow cathode effect. However, the tube bias must be optimized in order to achieve a high deposition rate since a too high bias may adversely affect the sputtering effect on the copper electrode and enhance the re-sputtering effect on the inner wall of the hollow tube. References 1 Masamune S, Yukimura K. 2003, Nucl. Instr. and Meth., B206: 682 2 Bardos L, Berg S, Barankova H, et al. 1993, J. Vac. Sci. Technol., A11: 1486 3 Ensinger W. 1996, Surf. Coat. Technol., 84: 434 4 Minato M, Itoh Y. 1997, Nucl. Instr. and Meth., B121: 187 Plasma Science and Technology, Vol.10, No.5, Oct. 2008 5 Sato M, Nishiura M, Oishi M, et al. 1996, Vacuum, 47: 753 6 Fujiyama H. 2000, Surf. Coat. Technol., 131: 278 7 Matson D W, McClananhan E D, Rice J P. 2000, Surf. Coat. Technol., 133 134: 411 8 Yonemura S, Nanbu K. 2006, Thin Solid Films, 506 507: 517 9 Druyvesteyn M, Penning F M. 1940, Reviews of Modern Physics, 12: 87 10 Raizer Y P. 1991, Gas Discharge Physics. Berlin: Springer-Verlag 11 Cui J T, Tian X B, Yang S Q, et al. 2007, Surf. Coat. Technol., 201: 6651 12 Grill A. 1994, Cold Plasma in Materials Fabrication: From Fundamentals to Applications. New York: IEEE Press 13 Brunner H, Hegemann D, Oehr C. 2001, Surf. Coat. Technol., 142 144: 849 14 Lim J M, Lee C M. 2006, Materials Chemistry and Physics, 95: 164 15 Bulbul F, Efeoglu I, Arslan E. 2007, Applied Surface Science, 253: 4415 16 Li J J, Zheng W T, Jin Z, et al. 2002, Applied Surface Science, 191: 273 17 Choi S R, Park I W, Kim S H. 2004, Thin Solid Films, 447 448: 371 (Manuscript received 28 September 2007) E-mail address of TIAN Xiubo: xiubotian@163.com 564