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Discharge profiles of internal-antenna-driven inductively-coupled plasmas
Surface & Coatings Technology 202 (2008) 5234–5237
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 e v i e r. c o m / l o c a t e / s u r f c o a t
Discharge profiles of internal-antenna-driven inductively-coupled plasmas Yuichi Setsuhara ⁎, Takashi Sera, Kosuke Takenaka Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Osaka 567-0047, Japan
A R T I C L E
I N F O
Available online 5 June 2008 PACS: 52.25.-b; 52.50.Qt; 52.50.Dg Keywords: Inductively coupled plasmas Plasma treatment Internal antenna Plasma source
A B S T R A C T In this work radio frequency inductively-coupled plasmas (ICPs) sustained with a single low-inductance antenna (LIA) have been characterized in an attempt to clarify profiles of plasma parameters in the nearantenna region. The present investigations have been carried out for the purpose of understanding the local discharge features with a single LIA unit, which are regarded as one of the elements for sustaining large-area plasma source with multiple LIA configurations. The ion-saturation current profiles had a peak at a distance of 10–20 mm from the outer surface of the antenna insulator and then decreased with increasing distance from the antenna, while the electron temperature peaked in the vicinity of the outer surface of the antenna insulator. These results suggest that the discharge sustained with a single LIA is mainly excited via the induction electric field, which is inherent in near-field nature of the induction electric fields peaking in the vicinity of the antenna and attenuating with a depth of penetration (skin depth) in the plasma. Distribution of the RF fluctuation of the floating potential was also measured to evaluate the electrostatic coupling of the antenna to the plasma. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Radio frequency (RF) inductively inductively-coupled plasmas (ICPs) are widely used for fabrications of semiconductor devices and flat-panel-displays (FPDs) because of ability to produce dense plasmas [1–4]. Currently, the ICP sources applied to semiconductor and FPD processing are required to attain dense uniform plasma generation over large area and lowering of plasma damage to materials surface under processing. However the scale-up of the conventional ICPs with spiral antenna coils leads to problems including large antenna impedance, which causes huge RF voltages (1–10 kV order) at the powered end of the antenna, thick dielectric window between the plasma and the antenna and the standing wave effects [4–9]. To solve these problems, we have developed ICP sources with multiple low-inductance antenna (LIA) units [10]. In our previous studies [10–16], we have demonstrated that this unique antenna for ICP generation allows high-density (1011–1012 cm− 3) plasma production with low plasma damage (as low as 10 V) via low-voltage operation of ICPs. Furthermore the internal-antenna configuration with the LIA units has the feature of flexibility to meet various configurations of plasma sources. Deposition of microcrystalline silicon (μc-Si) films for thin-film transistors was also demonstrated using a rectangular reactor with multiple LIA units and highly crystallized μc-Si films with a crystalline fraction of over 90% were
⁎ Corresponding author. Tel./fax: +81 6 6879 8641. E-mail address: [email protected] (Y. Setsuhara). 0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.06.003
successfully deposited on glass substrates at a substrate temperature of 300 °C with a deposition rate of as high as 70 nm/min [17]. This work is motivated to perform detailed investigations on profiles of plasma parameters in the near-antenna region of the ICPs sustained with a single LIA unit, which is regarded as one of the elements for the formation of large-area plasma reactors with multiple LIA configurations. Detailed understanding of the local discharge feature is significant in designing and control of large-area processes with multiple LIA configurations due to the following reasons; a) plasma parameters (ion density and electron temperature) and their uniformity in the downstream process region are closely correlated with the discharge features in the source region, b) plasma potential and its fluctuations in the downstream process region are considerably affected by the static coupling to the antenna RF voltage, and c) radicals and/or ions with higher excitation and/or ionization energy via electron impact are expected to be produced preferentially in the vicinity of the antenna, through which the RF discharge power is deposited locally. For the purpose of designing uniform large-area plasma reactors with the multiple LIA configurations, in which the process uniformity can be attained via overlapping of the plasmas sustained with each of the LIA units, therefore, it is of key importance to take into account the local discharge features, especially for the uniformity control of the reactive species (radicals and ions). Furthermore, the antenna RF voltage and hence the power transfer efficiency are also expected to inherently depend on the local discharge features of the ICPs sustained with LIA, because the enhancement of the electron density in the vicinity of the antenna directly raise the RF power absorption
Y. Setsuhara et al. / Surface & Coatings Technology 202 (2008) 5234–5237
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to the electrons in the plasma, which increases the power powerabsorption efficiency and thus decreases the antenna RF current and the antenna RF voltage, as commonly modeled in the transformer formalism of ICPs [2,18,19]. 2. Experimental Fig. 1(a) shows schematic diagram of cylindrical chamber for investigation of ICPs sustained with a single internal LIA. A U-shaped antenna conductor with a 70 mm width and a 160 mm height was mounted on a vacuum flange as shown in Fig. 1(a) and was coupled to a 3-kW RF power generator at 13.56 MHz via a matching network. The antenna was made of copper hollow conductor, which was fully covered with dielectric tubing with 20 mm outer diameter for complete isolation from plasmas [10]. The stainless-steel cylindrical discharge chamber was 948 mm in length and 750 mm in internal diameter. The base pressure of the discharge system evacuated with a turbo molecular pump was 10− 4 Pa. The argon pressure was set at 1.3 and 6.7 Pa for the present experiments. Profiles of plasma parameters were measured using five cylindrical Langmuir probes. In this work, x-, y- and z-axis are taken to be along each axis of the discharge system as illustrated in Fig. 1(a) and (b). The z = 0 position lies in the inner surface of the antenna-mounted flange and the positive z values are taken in bottom-to-top direction. The probe tips were inserted axially from the flange opposite to the
Fig. 2. Variation of ion-saturation current as a function of distance from antennainsulator surface for the plasma sustained at argon pressures of 1.3 and 6.7 Pa at RF power of 200 W.
antenna-mounted flange at positions of x = 0 and y = −125, −65, 0, 65 and 125 mm, as schematically illustrated in Fig. 1(b). 3. Results and discussion As a first step to evaluate the local features of plasma parameters in the near-antenna region of the ICPs sustained with a single LIA unit, one-dimensional profiles of plasma parameters were measured using a Langmuir probe. Figs. 2 and 3 show the profiles of the ion ionsaturation current and the electron temperature measured along a line axis located at x = 0 mm and y = 0 mm as a function of distance from the outer surface of the antenna insulator (z = 180 mm), in which the argon plasmas were produced at 1.3 Pa and 6.7 Pa at RF power of 200 W. In the downstream region, as commonly observed, the electron temperature decreases with increasing pressure from ~2.5 eV at 1.3 Pa to ~ 1.8 eV at 6.7 Pa. Here it is noted that the plasma density in the near-antenna region estimated from the ion-saturation current and the electron temperature is on the order of 1011 cm− 3, which is
Fig. 1. Schematic diagrams of (a) RF-plasma production chamber and (b) Langmuir probes measurement system.
Fig. 3. Variation of electron temperature as a function of distance from antennainsulator surface for the plasma sustained at argon pressures of 1.3 and 6.7 Pa at RF power of 200 W.
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comparable to that reported for the ICP with the planar-coil type at a comparable distance from the quartz window and at the same RF power of 200 W [20]. The local features of plasma parameters in the near-antenna region show the following. First the plasma density increased with increasing distance from the outer surface of the insulator up to 10– 20 mm and then decreased with increasing distance from the antenna as shown in Fig. 2. On the other hand the electron temperature had a peak in the vicinity of the outer surface of the antenna insulator showing a relatively steep decay in the region within ~ 20 mm and a slight decay in the downstream region as shown in Fig. 3. Recalling that the RF induction electric fields induced by the RF antenna are primarily tangential to the direction of the RF current and inherent in near-field nature with a cutoff where the electron plasma frequency exceeds the driving RF frequency, the penetration depth of the induction electric fields may be estimated by the skin depth as a measure of the attenuation length of these RF electric fields in the plasma [21]. For the typical density of approximately 1011 cm− 3 in the present investigation, the penetration depth of the RF induction field in the plasma is estimated to be 17 mm from the outer surface of the antenna insulator. Thus the electron-temperature increase in the region within approximately 20 mm while approaching the outer surface of the antenna insulator (Fig. 3) is considered to be due to significant power absorption and/or heating of electrons via the penetrated induction electric fields. On the other hand, the steep decay feature of the ion ion-saturation current and/or the plasma density in the region within approximately 20 mm (Fig. 2) is considered to be determined by the balance between the ionization enhancement via the accelerated electrons in this power-absorption region and the significant chargedparticle loss (wall loss) at the surface of the insulator; i.e., due to the ionization enhancement via the significant power absorption in this penetration region of the induction fields the plasma density is considered to increase with increasing distance from the insulator even though the charged particles generated in this region are significantly lost as a wall loss at the surface of the insulator. Furthermore, the decay feature of the ion-saturation current and/or the plasma density in the down stream region is considered to be typical of diffusion-dominated profiles, in which relatively slight decay of the electron temperature is observed. The antenna RF voltage and hence the power transfer efficiency are expected to inherently depend on the local discharge features of the ICPs, since the electron density profiles in the vicinity of the antenna can directly affect the characteristics of the inductive coupling, as commonly modeled in the transformer formalism of ICPs [2]. In order to clarify these features the antenna RF voltage was measured at 1.3 and 6.5 Pa at RF power of 200 W. With increasing pressure, the peak-to-peak value of the antenna RF voltage decreased from 640 V at 1.3 Pa to 590 V at 6.5 Pa. These results demonstrate that the local density enhancement in the near-antenna region leads directly to enhancement of the RF power absorption to electrons in the plasma, which results in enhancement of the power transfer efficiency to the plasma and thus leads to lowering of the antenna RF current and voltage, as commonly expected from the transformer formalism of ICPs [2]. From the viewpoint of designing large-area processes with multiple LIA configurations, the local discharge features as discussed so far in the present investigation are significant not only as a basic information to predict density distribution in the downstream region but also as localized features for production of radicals and/or ions with higher excitation and/or ionization energy via electron impact. As shown in the present investigation, both of the plasma density and the electron temperature are generally much higher in the nearantenna region than in the downstream region. Therefore, this local feature involved in the present regime with the multiple LIA
configurations implies that the near-antenna region may serve as major production source of radicals and/or ions with higher excitation and/or ionization energy via electron impact. In the case of chemical vapor deposition of microcrystalline silicon films in hydrogen-diluted silane gas, for example, the near-antenna region can serve as a major source of atomic hydrogen radicals, which are significant for crystallization of the silicon films. Thus in designing and control of the largearea processes with the multiple LIA configurations, it is of key importance to consider not only the uniformity of the plasma density but also the uniformity control of the reactive species (radicals and ions) through the diffusion from the near-antenna region to the substrate. Finally, in order to evaluate the potential fluctuation in the vicinity of the antenna, two-dimensional distribution of the floating-potential fluctuation was measured using 5 movable Langmuir probes as schematically shown in Fig. 1. Fig. 4 is a typical example for the measured distribution of the floating-potential fluctuation (peak-topeak value) in the plasma sustained at argon pressure of 6.7 Pa and RF power of 200 W. The fluctuation of the floating potential is observed to be concentrated in the vicinity of the antenna and is found to increase considerably while approaching the bottom end of the antenna or the chamber wall, whereas the fluctuation in the downstream region is found to be negligibly small (less than 0.4 V). In addition, the maximum value of the fluctuation at the powered side of the antenna was measured to be approximately 2.5 V, which was higher than that observed at the grounded side (1.8 V). The increase in the fluctuation at the powered side of the antenna is considered to be due to the capacitive coupling generated by the antenna-terminal RF voltage, which is applied at the powered side of the antenna. Furthermore, the electron loss at the chamber wall due to the electron acceleration via the oscillated induction electric field, which is tangential to the antenna conductor, may also be considered to contribute to the potential fluctuation, since both of the potential fluctuations at the powered and grounded sides were found to increase while approaching the bottom end of the antenna or the chamber wall. From the viewpoint of the plasma process, the potential fluctuation in the vicinity of the antenna may cause impurity generation via the sputtering of the insulator and/or the chamber wall with the ions accelerated by the electrostatic potential formation due to the fluctuation in the plasma, whereas the potential fluctuation in the downstream process region may cause unwanted ion damages to the substrate, which hinder high-quality processing, by the ions accelerated by the electrostatic potential formation at the substrate due to the potential fluctuation. The results observed in the present investigation suggest that the plasma generation with the LIA is attractive since the potential fluctuation in the near-antenna region (less than 2.5 V) is sufficiently small enough to avoid sputtering and that in the downstream region (less than 0.4 V) is negligibly small for
Fig. 4. Two-dimensional distribution of floating-potential fluctuation for the plasma sustained at argon pressure of 6.7 Pa and RF power of 200 W.
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damage formation in materials processing. This feature of suppressed potential fluctuation may be attributed primarily to the generation of ICPs at low terminal RF voltages (low-voltage operation of ICPs), which is mainly due to reduction of the antenna impedance in the present regime using LIAs.
This work was supported partly by the Grant-in-Aid for Cooperative Research Project of Nationwide Joint-Use Research Institutes on Development Base of Joining Technology for New Metallic Glasses and Inorganic Materials from The Ministry of Education, Culture, Sports, Science and Technology, Japan.
4. Summary
References
Detailed investigations on profiles of plasma parameters in the near-antenna region of the ICPs sustained with a single LIA unit at argon pressure of 1.3 and 6.5 Pa and RF power of 200 W have shown that the ion-saturation current profiles had a peak at a distance of 10– 20 mm from the outer surface of the antenna insulator, while the electron temperature peaked in the vicinity of the outer surface of the antenna insulator. These local features related to the plasma density and the electron-temperature profiles have been evaluated via the penetration of the induction electric fields. Significance of these local features have been discussed from the viewpoint of designing and control of large-area processes with multiple LIA configurations. Studies on potential fluctuations in the plasma sustained with a single LIA unit at argon pressure of 6.5 Pa and RF power of 200 W have shown that the potential fluctuation in the near-antenna region (less than 2.5 V) is sufficiently small enough to avoid sputtering and that in the downstream region (less than 0.4 V) is negligibly small for damage formation in materials processing.
[1] J. Hopwood, Plasma Sources Sci. Technol. 1 (1992) 109. [2] R.B. Pjejak, V.A. Godyak, B.M. Alexandrovich, Plasma Sources Sci. Technol. 1 (1992) 179. [3] W.Z. Collison, T.Q. Ni, M.S. Barnes, J. Vac. Sci. Technol., A, Vac. Surf. Films 16 (1998) 100. [4] S.S. Kim, H.Y. Chang, C.S. Chang, N.S. Yoon, Appl. Phys. Lett. 77 (2000) 492. [5] Y.J. Lee, K.N. Kim, G.Y. Yeom, M.A. Lieberman, Appl. Phys. Lett. 85 (2004) 1677. [6] J. Perrin, J. Schmitt, C. Hollenstein, A. Howling, L. Sansonnens, Plasma Phys. Control. Fusion 42 (2000) B353. [7] Y. Wu, M.A. Lieberman, Plasma Sources Sci. Technol. 9 (2000) 210. [8] M.H. Khater, L.J. Overzet, Plasma Sources Sci. Technol. 9 (2000) 545. [9] Y. Setsuhara, J. Plasma Fusion Res. 81 (2005) 85. [10] Y. Setsuhara, T. Shoji, A. Ebe, S. Baba, N. Yamamoto, K. Takahashi, K. Ono, S. Miyake, Surf. Coat. Technol. 174–175 (2003) 33. [11] Y. Setsuhara, K. Takenaka, A. Ebe, K. Nishisaka, Plasma Process. Polym. 4 (2007) S628. [12] Y. Setsuhara, K. Takenaka, A. Ebe, K. Nishizaka, Solid State Phenom. 127 (2007) 239. [13] H. Deguchi, H. Yoneda, K. Kato, K. Kubota, T. Hayashi, K. Ogata, A. Ebe, K. Takenaka, Y. Setsuhara, Jpn. J. Appl. Phys. 45 (2006) 8042. [14] K. Takenaka, Y. Setsuhara, K. Nishisaka, A. Ebe, Jpn. J. Appl. Phys. 45 (2006) 8046. [15] K. Takenaka, Y. Setsuhara, K. Nishisaka, A. Ebe, Plasma Process. Polym. 4 (2007) S1009. [16] K. Takenaka, T. Sera, A. Ebe, Y. Setsuhara, Plasma Process. Polym. 4 (2007) S1013. [17] E. Takahashi, Y. Nishigami, A. Tomyo, M. Fujiwara, H. Kaki, K. Kubota, T. Hayashi, K. Ogata, A. Ebe, Y. Setsuhara, Jpn. J. Appl. Phys. 46 (2007) 1280. [18] T. Shoji, Y. Sakawa, S. Nakazawa, K. Kadota, T. Sato, Plasma Sources Sci. Technol. 2 (1993) 5. [19] J.Q. Zhang, Y. Setsuhara, T. Ariyasu, S. Miyake, J. Vac. Sci. Technol., A, Vac. Surf. Films 14 (1996) 2163. [20] J. Hopwood, C.R. Guarnieri, S.J. Whitehair, J.J. Cuomo, J. Vac. Sci. Technol., A, Vac. Surf. Films 11 (1993) 152. [21] M.A. Lieberman, A.J. Lichtenberg, Principles of Plasma Discharges and Materials Processing, 2nd editionWiley, NewYork, 2005 pp. 464.
Acknowledgments One of the authors (Y. Setsuhara) would like to acknowledge the partial support to formulate the basis of this study under the auspices of Priority Assistance for the Formation of Worldwide Renowned Centers of Research — The Global COE Program (Project: Center of Excellence for Advanced Structural and Functional Materials Design) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.
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 e v i e r. c o m / l o c a t e / s u r f c o a t
Discharge profiles of internal-antenna-driven inductively-coupled plasmas Yuichi Setsuhara ⁎, Takashi Sera, Kosuke Takenaka Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Osaka 567-0047, Japan
A R T I C L E
I N F O
Available online 5 June 2008 PACS: 52.25.-b; 52.50.Qt; 52.50.Dg Keywords: Inductively coupled plasmas Plasma treatment Internal antenna Plasma source
A B S T R A C T In this work radio frequency inductively-coupled plasmas (ICPs) sustained with a single low-inductance antenna (LIA) have been characterized in an attempt to clarify profiles of plasma parameters in the nearantenna region. The present investigations have been carried out for the purpose of understanding the local discharge features with a single LIA unit, which are regarded as one of the elements for sustaining large-area plasma source with multiple LIA configurations. The ion-saturation current profiles had a peak at a distance of 10–20 mm from the outer surface of the antenna insulator and then decreased with increasing distance from the antenna, while the electron temperature peaked in the vicinity of the outer surface of the antenna insulator. These results suggest that the discharge sustained with a single LIA is mainly excited via the induction electric field, which is inherent in near-field nature of the induction electric fields peaking in the vicinity of the antenna and attenuating with a depth of penetration (skin depth) in the plasma. Distribution of the RF fluctuation of the floating potential was also measured to evaluate the electrostatic coupling of the antenna to the plasma. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Radio frequency (RF) inductively inductively-coupled plasmas (ICPs) are widely used for fabrications of semiconductor devices and flat-panel-displays (FPDs) because of ability to produce dense plasmas [1–4]. Currently, the ICP sources applied to semiconductor and FPD processing are required to attain dense uniform plasma generation over large area and lowering of plasma damage to materials surface under processing. However the scale-up of the conventional ICPs with spiral antenna coils leads to problems including large antenna impedance, which causes huge RF voltages (1–10 kV order) at the powered end of the antenna, thick dielectric window between the plasma and the antenna and the standing wave effects [4–9]. To solve these problems, we have developed ICP sources with multiple low-inductance antenna (LIA) units [10]. In our previous studies [10–16], we have demonstrated that this unique antenna for ICP generation allows high-density (1011–1012 cm− 3) plasma production with low plasma damage (as low as 10 V) via low-voltage operation of ICPs. Furthermore the internal-antenna configuration with the LIA units has the feature of flexibility to meet various configurations of plasma sources. Deposition of microcrystalline silicon (μc-Si) films for thin-film transistors was also demonstrated using a rectangular reactor with multiple LIA units and highly crystallized μc-Si films with a crystalline fraction of over 90% were
⁎ Corresponding author. Tel./fax: +81 6 6879 8641. E-mail address: [email protected] (Y. Setsuhara). 0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.06.003
successfully deposited on glass substrates at a substrate temperature of 300 °C with a deposition rate of as high as 70 nm/min [17]. This work is motivated to perform detailed investigations on profiles of plasma parameters in the near-antenna region of the ICPs sustained with a single LIA unit, which is regarded as one of the elements for the formation of large-area plasma reactors with multiple LIA configurations. Detailed understanding of the local discharge feature is significant in designing and control of large-area processes with multiple LIA configurations due to the following reasons; a) plasma parameters (ion density and electron temperature) and their uniformity in the downstream process region are closely correlated with the discharge features in the source region, b) plasma potential and its fluctuations in the downstream process region are considerably affected by the static coupling to the antenna RF voltage, and c) radicals and/or ions with higher excitation and/or ionization energy via electron impact are expected to be produced preferentially in the vicinity of the antenna, through which the RF discharge power is deposited locally. For the purpose of designing uniform large-area plasma reactors with the multiple LIA configurations, in which the process uniformity can be attained via overlapping of the plasmas sustained with each of the LIA units, therefore, it is of key importance to take into account the local discharge features, especially for the uniformity control of the reactive species (radicals and ions). Furthermore, the antenna RF voltage and hence the power transfer efficiency are also expected to inherently depend on the local discharge features of the ICPs sustained with LIA, because the enhancement of the electron density in the vicinity of the antenna directly raise the RF power absorption
Y. Setsuhara et al. / Surface & Coatings Technology 202 (2008) 5234–5237
5235
to the electrons in the plasma, which increases the power powerabsorption efficiency and thus decreases the antenna RF current and the antenna RF voltage, as commonly modeled in the transformer formalism of ICPs [2,18,19]. 2. Experimental Fig. 1(a) shows schematic diagram of cylindrical chamber for investigation of ICPs sustained with a single internal LIA. A U-shaped antenna conductor with a 70 mm width and a 160 mm height was mounted on a vacuum flange as shown in Fig. 1(a) and was coupled to a 3-kW RF power generator at 13.56 MHz via a matching network. The antenna was made of copper hollow conductor, which was fully covered with dielectric tubing with 20 mm outer diameter for complete isolation from plasmas [10]. The stainless-steel cylindrical discharge chamber was 948 mm in length and 750 mm in internal diameter. The base pressure of the discharge system evacuated with a turbo molecular pump was 10− 4 Pa. The argon pressure was set at 1.3 and 6.7 Pa for the present experiments. Profiles of plasma parameters were measured using five cylindrical Langmuir probes. In this work, x-, y- and z-axis are taken to be along each axis of the discharge system as illustrated in Fig. 1(a) and (b). The z = 0 position lies in the inner surface of the antenna-mounted flange and the positive z values are taken in bottom-to-top direction. The probe tips were inserted axially from the flange opposite to the
Fig. 2. Variation of ion-saturation current as a function of distance from antennainsulator surface for the plasma sustained at argon pressures of 1.3 and 6.7 Pa at RF power of 200 W.
antenna-mounted flange at positions of x = 0 and y = −125, −65, 0, 65 and 125 mm, as schematically illustrated in Fig. 1(b). 3. Results and discussion As a first step to evaluate the local features of plasma parameters in the near-antenna region of the ICPs sustained with a single LIA unit, one-dimensional profiles of plasma parameters were measured using a Langmuir probe. Figs. 2 and 3 show the profiles of the ion ionsaturation current and the electron temperature measured along a line axis located at x = 0 mm and y = 0 mm as a function of distance from the outer surface of the antenna insulator (z = 180 mm), in which the argon plasmas were produced at 1.3 Pa and 6.7 Pa at RF power of 200 W. In the downstream region, as commonly observed, the electron temperature decreases with increasing pressure from ~2.5 eV at 1.3 Pa to ~ 1.8 eV at 6.7 Pa. Here it is noted that the plasma density in the near-antenna region estimated from the ion-saturation current and the electron temperature is on the order of 1011 cm− 3, which is
Fig. 1. Schematic diagrams of (a) RF-plasma production chamber and (b) Langmuir probes measurement system.
Fig. 3. Variation of electron temperature as a function of distance from antennainsulator surface for the plasma sustained at argon pressures of 1.3 and 6.7 Pa at RF power of 200 W.
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comparable to that reported for the ICP with the planar-coil type at a comparable distance from the quartz window and at the same RF power of 200 W [20]. The local features of plasma parameters in the near-antenna region show the following. First the plasma density increased with increasing distance from the outer surface of the insulator up to 10– 20 mm and then decreased with increasing distance from the antenna as shown in Fig. 2. On the other hand the electron temperature had a peak in the vicinity of the outer surface of the antenna insulator showing a relatively steep decay in the region within ~ 20 mm and a slight decay in the downstream region as shown in Fig. 3. Recalling that the RF induction electric fields induced by the RF antenna are primarily tangential to the direction of the RF current and inherent in near-field nature with a cutoff where the electron plasma frequency exceeds the driving RF frequency, the penetration depth of the induction electric fields may be estimated by the skin depth as a measure of the attenuation length of these RF electric fields in the plasma [21]. For the typical density of approximately 1011 cm− 3 in the present investigation, the penetration depth of the RF induction field in the plasma is estimated to be 17 mm from the outer surface of the antenna insulator. Thus the electron-temperature increase in the region within approximately 20 mm while approaching the outer surface of the antenna insulator (Fig. 3) is considered to be due to significant power absorption and/or heating of electrons via the penetrated induction electric fields. On the other hand, the steep decay feature of the ion ion-saturation current and/or the plasma density in the region within approximately 20 mm (Fig. 2) is considered to be determined by the balance between the ionization enhancement via the accelerated electrons in this power-absorption region and the significant chargedparticle loss (wall loss) at the surface of the insulator; i.e., due to the ionization enhancement via the significant power absorption in this penetration region of the induction fields the plasma density is considered to increase with increasing distance from the insulator even though the charged particles generated in this region are significantly lost as a wall loss at the surface of the insulator. Furthermore, the decay feature of the ion-saturation current and/or the plasma density in the down stream region is considered to be typical of diffusion-dominated profiles, in which relatively slight decay of the electron temperature is observed. The antenna RF voltage and hence the power transfer efficiency are expected to inherently depend on the local discharge features of the ICPs, since the electron density profiles in the vicinity of the antenna can directly affect the characteristics of the inductive coupling, as commonly modeled in the transformer formalism of ICPs [2]. In order to clarify these features the antenna RF voltage was measured at 1.3 and 6.5 Pa at RF power of 200 W. With increasing pressure, the peak-to-peak value of the antenna RF voltage decreased from 640 V at 1.3 Pa to 590 V at 6.5 Pa. These results demonstrate that the local density enhancement in the near-antenna region leads directly to enhancement of the RF power absorption to electrons in the plasma, which results in enhancement of the power transfer efficiency to the plasma and thus leads to lowering of the antenna RF current and voltage, as commonly expected from the transformer formalism of ICPs [2]. From the viewpoint of designing large-area processes with multiple LIA configurations, the local discharge features as discussed so far in the present investigation are significant not only as a basic information to predict density distribution in the downstream region but also as localized features for production of radicals and/or ions with higher excitation and/or ionization energy via electron impact. As shown in the present investigation, both of the plasma density and the electron temperature are generally much higher in the nearantenna region than in the downstream region. Therefore, this local feature involved in the present regime with the multiple LIA
configurations implies that the near-antenna region may serve as major production source of radicals and/or ions with higher excitation and/or ionization energy via electron impact. In the case of chemical vapor deposition of microcrystalline silicon films in hydrogen-diluted silane gas, for example, the near-antenna region can serve as a major source of atomic hydrogen radicals, which are significant for crystallization of the silicon films. Thus in designing and control of the largearea processes with the multiple LIA configurations, it is of key importance to consider not only the uniformity of the plasma density but also the uniformity control of the reactive species (radicals and ions) through the diffusion from the near-antenna region to the substrate. Finally, in order to evaluate the potential fluctuation in the vicinity of the antenna, two-dimensional distribution of the floating-potential fluctuation was measured using 5 movable Langmuir probes as schematically shown in Fig. 1. Fig. 4 is a typical example for the measured distribution of the floating-potential fluctuation (peak-topeak value) in the plasma sustained at argon pressure of 6.7 Pa and RF power of 200 W. The fluctuation of the floating potential is observed to be concentrated in the vicinity of the antenna and is found to increase considerably while approaching the bottom end of the antenna or the chamber wall, whereas the fluctuation in the downstream region is found to be negligibly small (less than 0.4 V). In addition, the maximum value of the fluctuation at the powered side of the antenna was measured to be approximately 2.5 V, which was higher than that observed at the grounded side (1.8 V). The increase in the fluctuation at the powered side of the antenna is considered to be due to the capacitive coupling generated by the antenna-terminal RF voltage, which is applied at the powered side of the antenna. Furthermore, the electron loss at the chamber wall due to the electron acceleration via the oscillated induction electric field, which is tangential to the antenna conductor, may also be considered to contribute to the potential fluctuation, since both of the potential fluctuations at the powered and grounded sides were found to increase while approaching the bottom end of the antenna or the chamber wall. From the viewpoint of the plasma process, the potential fluctuation in the vicinity of the antenna may cause impurity generation via the sputtering of the insulator and/or the chamber wall with the ions accelerated by the electrostatic potential formation due to the fluctuation in the plasma, whereas the potential fluctuation in the downstream process region may cause unwanted ion damages to the substrate, which hinder high-quality processing, by the ions accelerated by the electrostatic potential formation at the substrate due to the potential fluctuation. The results observed in the present investigation suggest that the plasma generation with the LIA is attractive since the potential fluctuation in the near-antenna region (less than 2.5 V) is sufficiently small enough to avoid sputtering and that in the downstream region (less than 0.4 V) is negligibly small for
Fig. 4. Two-dimensional distribution of floating-potential fluctuation for the plasma sustained at argon pressure of 6.7 Pa and RF power of 200 W.
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damage formation in materials processing. This feature of suppressed potential fluctuation may be attributed primarily to the generation of ICPs at low terminal RF voltages (low-voltage operation of ICPs), which is mainly due to reduction of the antenna impedance in the present regime using LIAs.
This work was supported partly by the Grant-in-Aid for Cooperative Research Project of Nationwide Joint-Use Research Institutes on Development Base of Joining Technology for New Metallic Glasses and Inorganic Materials from The Ministry of Education, Culture, Sports, Science and Technology, Japan.
4. Summary
References
Detailed investigations on profiles of plasma parameters in the near-antenna region of the ICPs sustained with a single LIA unit at argon pressure of 1.3 and 6.5 Pa and RF power of 200 W have shown that the ion-saturation current profiles had a peak at a distance of 10– 20 mm from the outer surface of the antenna insulator, while the electron temperature peaked in the vicinity of the outer surface of the antenna insulator. These local features related to the plasma density and the electron-temperature profiles have been evaluated via the penetration of the induction electric fields. Significance of these local features have been discussed from the viewpoint of designing and control of large-area processes with multiple LIA configurations. Studies on potential fluctuations in the plasma sustained with a single LIA unit at argon pressure of 6.5 Pa and RF power of 200 W have shown that the potential fluctuation in the near-antenna region (less than 2.5 V) is sufficiently small enough to avoid sputtering and that in the downstream region (less than 0.4 V) is negligibly small for damage formation in materials processing.
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Acknowledgments One of the authors (Y. Setsuhara) would like to acknowledge the partial support to formulate the basis of this study under the auspices of Priority Assistance for the Formation of Worldwide Renowned Centers of Research — The Global COE Program (Project: Center of Excellence for Advanced Structural and Functional Materials Design) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.