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Wave analysis in microwave-excited plasma reactor using MSP antenna
Surface & Coatings Technology 202 (2008) 5306–5309
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
Wave analysis in microwave-excited plasma reactor using MSP antenna A. Tsuji ⁎, Y. Yasaka, H. Takeno Department of Electrical and Electronic Engineering, Graduate School of Engineering, Kobe University, 1-1, Rokkodai, Nada, Kobe 657-8501, Japan
A R T I C L E
I N F O
Available online 6 June 2008 PACS: 52.35.Hr 52.70.Ds 52.80.Pi
A B S T R A C T The structure of waves in a microwave-excited plasma reactor using the multi-slotted planar (MSP) antenna is investigated by experimental measurements and numerical analyses. As a result, it is shown that the radial mode number is fixed and equals to 5 in a wide range of the plasma density, and mode jumps of surface wave hardly appear. Mode jumps of surface wave cause the non-uniformity of the plasma density. Therefore, it is considered that the characteristic in the device is an advantage for controlling plasma parameters. © 2008 Elsevier B.V. All rights reserved.
Keywords: Microwave discharges Multi-slotted planar antenna Surface wave eigenmode Plasma resonance
1. Introduction Microwave plasmas sustained by the surface wave have a long history of investigation. The surface wave plasma (SWP) of those days was a long plasma column with small diameter [1], and it was shown that microwave plasmas can be used to etch materials in semiconductor processing [2,3]. In addition, the possibility of occurrence of resonant field enhancement at local plasma resonance was pointed and it contributed to understanding of heating mechanisms for microwave plasmas with high electron density [4]. Such plasma columns with small diameter, however, are not suitable for LSI manufacturing that demands large-area and high-speed processing, although they have accumulated knowledge in experimental and theoretical researches. Subsequently, developments of microwave plasma devices with a slot antenna enabled plasma generation with large diameter suitable for LSI manufacturing [5,6]. For the next generation plasma processing, recently, it is necessary to develop high-performance processing plasmas [6], and microwave plasmas are suitable for future processing plasma because they have a number of advantages over the RF plasmas, such as a high electron density (~1011 cm- 3), a low electron temperature (less than 3 eV) and discharges in a wide range of gas pressures [7]. In addition, SWPs with large diameter can be effectively generated by spatial resonance of surface wave. Moreover, characteristics of thin-firm formed by chemical vapor deposition (CVD) with microwave SWP are investigated [7–9], and the advantage as high-speed deposition is reported [9]. Microwave plasmas with high electron density have spatial resonance of surface wave and resonant absorption of body waves
⁎ Corresponding author. E-mail address: [email protected] (A. Tsuji). 0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.06.007
as heating mechanisms. In spatial resonance, surface wave depending on the device structure and the plasma density is excited at plasmaquartz interface and it has been reported that striped brightness pattern is observed in the experiment [6]. This phenomenon has been revealed by theoretical researches in the one interface model, and it is known that mode jumps appear when changing the electron density [10,11]. However, it is difficult to control plasmas maintained by spatial resonance because a specific mode pattern and mode jumps cause non-uniformity of plasmas. On the other hand, resonant absorption in local plasma resonance exists because plasmas have density gradient at the sheath, and it is shown by theoretical and experimental researches [12–14]. Therefore, it is key point which heating mechanism strongly influences. We have previously performed plasma generations, measurements of electric field distributions and simulations in a microwave-excited plasma device with multi-slotted planar (MSP) antenna developed for suppressing spatial resonance of surface wave [15–19]. In this study, first of all, we measured electric field distributions near the plasma-quartz interface and compared theoretical values with them. Next, we performed surface wave analyses by numerical calculation. As a result, it is shown that mode jumps of spatial resonance do not appear in the device. Finally, we generated Ar/N2 plasma and confirmed that the distribution of N radical has a good uniformity. 2. Experimental equipments and plasma characteristics Fig. 1 shows the experimental equipments schematically. A discharge chamber has the radius of R = 250 mm and a depth of 380 mm, and a quartz window with same diameter and a thickness of 30 mm is set on the top. The MSP antenna settled above the quartz
A. Tsuji et al. / Surface & Coatings Technology 202 (2008) 5306–5309
5307
using average Jis in the area of r b 15 cm in Ar plasmas with comparatively good uniformity and the electron temperature, Te, of 2.5 eV. It is shown that ne smoothly changes when changing external parameters and is an order of 1012 cm− 3. 3. Structure of waves It is important to know a structure of waves in the device because plasmas are maintained by fast electrons that are accelerated in electric fields. It is reported that the amplitude has a peak at some distance from plasma-quartz interface in z-direction, the azimuthal mode number, m, equals to 1 and the radial mode number, n, equals to 5 at a certain discharge condition [17]. The dispersion relation of the surface wave eigenmodes in a cylindrical plasma approximated by a simple 2-layer model consisting of a dielectric window and a plasma layer is calculated by
Fig. 1. Experimental equipments.
window is movable in the axial direction, and microwave driven at ω/ 2π = 2.45 GHz propagates as azimuthally rotating TE11 cylindrical mode in a cylindrical waveguide [15]. The radiation modes of microwave to the plasma, namely rotating and non-rotating excitation modes, are controllable by adjusting stab tuners suitably. Ar gas is fed from the side wall at a rate of 100 seem. To measure electric field distributions and plasma parameters in the radial direction, an electric field and a Langmuir probes are set, respectively, at z = 10 and 55 mm with z = 0 at the bottom surface of the quartz window. Two dimensional distributions of light emission from the plasma are observed by charge-coupled device (CCD) camera set in the substrate stage with a small circular window. The plasmas were generated in various conditions of gas pressures and incident powers, and the ion saturation current density, Jis, was measured by the Langmuir probe. Fig. 2 shows the relationship of electron density, ne, to external parameters, where ne is obtained
Fig. 2. Relationship of electron density to gas pressures and incident powers.
p1 p2 tanhðp1 hÞ þ ¼ 0 and ε1 εp
ð1Þ
q1 p2 tanðq1 hÞ− ¼ 0; ε1 εp
ð2Þ
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi where p1 ¼ λ2 −k20 ε1 ; p2 ¼ λ2 −k20 εp ; q1 ¼ k20 ε 1 −λ2 ; ε1 ¼ 4:0; εp ¼ 2 1− ωpe =ω ; k0 ¼ ω=c; λ ¼ jmn =R; h ¼ 0:03, ε1 is a dielectric constant of the quartz window, ωpe is plasma angular frequency, c is the light velocity, h is a thickness of the quartz window, and jmn is n-th root of the m-th-order Bessel function of the first kind, Jm [11]. Eqs. (1) and (2) represent dispersion equations of a pure and a hybrid modes, respectively. In Fig. 3, open symbols show their calculation results. It is shown that surface wave eigenmodes are excited at specific ne. We measured n of the electric field near the plasma-quartz interface by the electric field probe movable in the radial direction, where it is sensitive to the electric field in the z-direction. The radial mode number n is obtained by the number of zeros of Ez(r) in the radial range of 0 b r ≤ R. In the measurement circuit, a double-balanced mixer (DBM) to multiply a signal obtained from the electric field probe and a reference signal of microwave is used as shown in Fig. 1. The plasmas are generated by non-rotating TE11 excitation mode. Fig. 4 shows the output signals from DBM at gas pressures of 10– 50 mTorr and a fixed incident power of 2.0 kW, where a smoothing process is performed to the signals for the elimination of high frequency components. It is shown that n = 5 for different values of ne ranging 1–4 × 1012 cm− 3 as shown in Fig. 2.
Fig. 3. Surface wave eigenmodes and radical mode number calculated by the spectrum analysis of light emission intensities.
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A. Tsuji et al. / Surface & Coatings Technology 202 (2008) 5306–5309
Fig. 4. Output signals from DBM at gas pressures of 10–50 mTorr and a fixed incident power of 2.0 kW.
We observed light emission intensities of plasmas generated in various conditions by the CCD camera. If the plasmas predominantly absorb the microwave power by Joule heating, its intensity is proportional to E2, where E(r, θ) represents the electric field. The main component of E is the axial component, Ez, that has a radial dependence of Jm. Since it is reported that m = 1 in the device [17] , we assume that the light emission intensity S is represented by a linear combination of J21(j1nr/R) as S = a0 + ∑anJ21(j1nr/R). The spectrum analyses are performed by using the least square fitting so that S reproduces the measured radial variation of the light emission. As a result, radial mode number n is obtained by both the electric field distribution and the brightness pattern. The closed symbols in Fig. 3 show experimental results in the device. The radial mode number n is calculated by the spectrum analyses of the light emission intensities, and ne is estimated by Jis at r = 0, where we assumed Te = 2.5 eV. It is shown that n has a fixed value, 5, in a wide range of ne, 1–5 × 1012 cm− 3. Therefore, it is shown that n = 5 in a wide range of ne and surface wave eigenmodes are suppressed in the device. 4. Numerical analyses of surface wave The numerical analyses of surface wave in the SWP model and MSP model have been performed by the finite-difference time-domain (FDTD) method. The SWP model represents a cylindrical device with a few slots and has quartz and plasma layers, where a radius of all layers and a thickness of the quartz layer are 250 and 30 mm, respectively.
Fig. 6. Relative brightness distribution in azimuthal direction in Ar/N2 plasma.
Microwave is excited from two slots at the top surface of the quartz layer, where the slots are symmetrically set from the center and the distance is 170 mm. The broken line in Fig. 5 shows an amplitude ratio in the SWP model, where the amplitude ratio is a value in which a maximum value of at the plasma-quartz interface is divided by that at the excitation position. It is shown that the geometry of the SWP model has surface wave eigenmodes at specific ne because the calculation result indicates some sharp peaks. This means that the amplitude changes remarkably at a narrow range of ne. For instance, surface wave eigenmodes of (m, n) = (7,9) and (3,8) modes are consecutively excited in the SWP model. On the other hand, the MSP model represents the device in Fig. 1, and the MSP antenna is approximated by multi-rings. Microwave is excited by rotating TE11 excitation mode. The solid line in Fig. 5 shows an amplitude ratio in the approximate model of the device. It is shown that it is not easy to excite surface wave eigenmodes in the geometry of the MSP model because the calculation result has a few broad peak only. We presume that the key points are the axisymmetrical geometry and the excitation method. 5. Measurement of light emission of N radical in Ar/N2 plasma We measured light emission of N radical by CCD camera with a interference filter, where transparent wavelength and half bandwidth are 409.5 nm and 11.3 nm, respectively. Ar/N2 plasmas are generated by microwave of rotating TE11 excitation mode at the mixture gas pressure of 60 mTorr and incident power of 2.0 kW. Fig. 6 shows the relative brightness distribution in the azimuthal direction. It is shown that N radical distribution in the azimuthal direction is uniform and striped brightness pattern does not appear. In addition, we previously confirmed the density of N radical by using a quadrupole mass spectrometer (QMS) [19]. Therefore, it is confirmed that spatial resonance of surface wave is suppressed in Ar/N2 plasma and the plasmas are suitable for the nitriding process. 6. Summary
Fig. 5. Surface wave eigenmodes in the SWP and the MSP models.
In the experiments, it is shown that the structure of waves maintains n = 5 in a wide range of ne, 1–5 × 1012 cm− 3. In the numerical analyses, it is shown that it is not easy to excite surface wave eigenmodes in the geometry of the MSP model. In the measurement of the brightness distribution in Ar/N2 plasma, it is confirmed that N radical distribution is uniform in the azimuthal direction and striped brightness pattern does not appear. As a result, it is shown that spatial
A. Tsuji et al. / Surface & Coatings Technology 202 (2008) 5306–5309
resonance of surface wave is suppressed in the device. These characteristics of this device may be significantly important in controlling plasma parameters for plasma reactors in next generation. References [1] M. Moisan, C. Beandry, P. Leprince, IEEE Trans. Plasma Sci. PS-3 (1975) 55. [2] J. Hubert, M. Moisan, A. Ricard, Spectrochim. Acta, Part B: Atom. Spectrosc. 34 (1979) 1. [3] J. Paraszczak, J. Heidenreich, M. Hatzakis, M. Moisan, Microelectronic Engineering 3 (1985) 397. [4] M. Zethoff, U. Kortshagen, J. Phys. D, Appl. Phys. 25 (1992) 1574. [5] K. Komachi, S. Kobayashi, J. Microwave Power Electromagn. Enegy 24 (1989) 140. [6] H. Sugai, I. Ghanashev, M. Nagatsu, Plasma Sources Sci. Technol. 7 (1998) 192. [7] M. Umeno, S. Adhikary, Diamond and Related Materials 14 (2005) 1973. [8] H. Akasaka, N. Ohtake, Diamond and Related Materials 14 (2005) 1828. [9] Y. Hotta, H. Toyoda, H. Sugai, Thin solid films 515 (2007) 4983. [10] I. Ghanashev, M. Nagatsu, H. Sugai, Jpn. J. Appl. Phys. Vol. 36 (1997) 337.
5309
[11] H. Hojo, A. Shimamura, N. Uchida, Y. Yasaka, A. Mase, J. Plasma Fusion Res. Vol. 80 (No. 9) (2004) 719. [12] I. Ghanashev, S. Morita, N. Toyoda, M. Nagatsu, H. Sugai, Jpn. J. Appl. Phys. 38 (1999) 4313. [13] Y. Yasaka, D. Nozaki, K. Koga, M. Ando, T. Yamamoto, N. Goto, N. Ishii, T. Morimoto, Plasma Sources Sci. Technol. 8 (1999) 530. [14] Y. Yasaka, H. Hojo, Phys. Plasmas 7 (2000) 1601. [15] Y. Yasaka, K. Koga, N. Ishii, T. Yamamoto, M. Ando, M. Takahashi, Phys. Plasmas 9 (2002) 1029. [16] Y. Yasaka, N. Ishii, T. Yamamoto, M. Ando, M. Takahasi, IEEE Trans. Plasma Sci. 32 (2004) 101. [17] Y. Yasaka, A. Sakae, N. Sugimoto, H. Takeno, H. Hojo, Jpn. J. Appl. Phys. Vol. 45 (2006) 8059. [18] Y. Tamai, Y. Miyamoto, T. Hayashi, A. Tsuji, H. Takeno, Y. Yasaka, Thin Solid Films 506–507 (2006) 626. [19] Y. Miyamoto, Y. Tamai, K. Daimon, M. Esaki, H. Takeno, Y. Yasaka, Thin Solid Films 506–507 (2006) 622.
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
Wave analysis in microwave-excited plasma reactor using MSP antenna A. Tsuji ⁎, Y. Yasaka, H. Takeno Department of Electrical and Electronic Engineering, Graduate School of Engineering, Kobe University, 1-1, Rokkodai, Nada, Kobe 657-8501, Japan
A R T I C L E
I N F O
Available online 6 June 2008 PACS: 52.35.Hr 52.70.Ds 52.80.Pi
A B S T R A C T The structure of waves in a microwave-excited plasma reactor using the multi-slotted planar (MSP) antenna is investigated by experimental measurements and numerical analyses. As a result, it is shown that the radial mode number is fixed and equals to 5 in a wide range of the plasma density, and mode jumps of surface wave hardly appear. Mode jumps of surface wave cause the non-uniformity of the plasma density. Therefore, it is considered that the characteristic in the device is an advantage for controlling plasma parameters. © 2008 Elsevier B.V. All rights reserved.
Keywords: Microwave discharges Multi-slotted planar antenna Surface wave eigenmode Plasma resonance
1. Introduction Microwave plasmas sustained by the surface wave have a long history of investigation. The surface wave plasma (SWP) of those days was a long plasma column with small diameter [1], and it was shown that microwave plasmas can be used to etch materials in semiconductor processing [2,3]. In addition, the possibility of occurrence of resonant field enhancement at local plasma resonance was pointed and it contributed to understanding of heating mechanisms for microwave plasmas with high electron density [4]. Such plasma columns with small diameter, however, are not suitable for LSI manufacturing that demands large-area and high-speed processing, although they have accumulated knowledge in experimental and theoretical researches. Subsequently, developments of microwave plasma devices with a slot antenna enabled plasma generation with large diameter suitable for LSI manufacturing [5,6]. For the next generation plasma processing, recently, it is necessary to develop high-performance processing plasmas [6], and microwave plasmas are suitable for future processing plasma because they have a number of advantages over the RF plasmas, such as a high electron density (~1011 cm- 3), a low electron temperature (less than 3 eV) and discharges in a wide range of gas pressures [7]. In addition, SWPs with large diameter can be effectively generated by spatial resonance of surface wave. Moreover, characteristics of thin-firm formed by chemical vapor deposition (CVD) with microwave SWP are investigated [7–9], and the advantage as high-speed deposition is reported [9]. Microwave plasmas with high electron density have spatial resonance of surface wave and resonant absorption of body waves
⁎ Corresponding author. E-mail address: [email protected] (A. Tsuji). 0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.06.007
as heating mechanisms. In spatial resonance, surface wave depending on the device structure and the plasma density is excited at plasmaquartz interface and it has been reported that striped brightness pattern is observed in the experiment [6]. This phenomenon has been revealed by theoretical researches in the one interface model, and it is known that mode jumps appear when changing the electron density [10,11]. However, it is difficult to control plasmas maintained by spatial resonance because a specific mode pattern and mode jumps cause non-uniformity of plasmas. On the other hand, resonant absorption in local plasma resonance exists because plasmas have density gradient at the sheath, and it is shown by theoretical and experimental researches [12–14]. Therefore, it is key point which heating mechanism strongly influences. We have previously performed plasma generations, measurements of electric field distributions and simulations in a microwave-excited plasma device with multi-slotted planar (MSP) antenna developed for suppressing spatial resonance of surface wave [15–19]. In this study, first of all, we measured electric field distributions near the plasma-quartz interface and compared theoretical values with them. Next, we performed surface wave analyses by numerical calculation. As a result, it is shown that mode jumps of spatial resonance do not appear in the device. Finally, we generated Ar/N2 plasma and confirmed that the distribution of N radical has a good uniformity. 2. Experimental equipments and plasma characteristics Fig. 1 shows the experimental equipments schematically. A discharge chamber has the radius of R = 250 mm and a depth of 380 mm, and a quartz window with same diameter and a thickness of 30 mm is set on the top. The MSP antenna settled above the quartz
A. Tsuji et al. / Surface & Coatings Technology 202 (2008) 5306–5309
5307
using average Jis in the area of r b 15 cm in Ar plasmas with comparatively good uniformity and the electron temperature, Te, of 2.5 eV. It is shown that ne smoothly changes when changing external parameters and is an order of 1012 cm− 3. 3. Structure of waves It is important to know a structure of waves in the device because plasmas are maintained by fast electrons that are accelerated in electric fields. It is reported that the amplitude has a peak at some distance from plasma-quartz interface in z-direction, the azimuthal mode number, m, equals to 1 and the radial mode number, n, equals to 5 at a certain discharge condition [17]. The dispersion relation of the surface wave eigenmodes in a cylindrical plasma approximated by a simple 2-layer model consisting of a dielectric window and a plasma layer is calculated by
Fig. 1. Experimental equipments.
window is movable in the axial direction, and microwave driven at ω/ 2π = 2.45 GHz propagates as azimuthally rotating TE11 cylindrical mode in a cylindrical waveguide [15]. The radiation modes of microwave to the plasma, namely rotating and non-rotating excitation modes, are controllable by adjusting stab tuners suitably. Ar gas is fed from the side wall at a rate of 100 seem. To measure electric field distributions and plasma parameters in the radial direction, an electric field and a Langmuir probes are set, respectively, at z = 10 and 55 mm with z = 0 at the bottom surface of the quartz window. Two dimensional distributions of light emission from the plasma are observed by charge-coupled device (CCD) camera set in the substrate stage with a small circular window. The plasmas were generated in various conditions of gas pressures and incident powers, and the ion saturation current density, Jis, was measured by the Langmuir probe. Fig. 2 shows the relationship of electron density, ne, to external parameters, where ne is obtained
Fig. 2. Relationship of electron density to gas pressures and incident powers.
p1 p2 tanhðp1 hÞ þ ¼ 0 and ε1 εp
ð1Þ
q1 p2 tanðq1 hÞ− ¼ 0; ε1 εp
ð2Þ
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi where p1 ¼ λ2 −k20 ε1 ; p2 ¼ λ2 −k20 εp ; q1 ¼ k20 ε 1 −λ2 ; ε1 ¼ 4:0; εp ¼ 2 1− ωpe =ω ; k0 ¼ ω=c; λ ¼ jmn =R; h ¼ 0:03, ε1 is a dielectric constant of the quartz window, ωpe is plasma angular frequency, c is the light velocity, h is a thickness of the quartz window, and jmn is n-th root of the m-th-order Bessel function of the first kind, Jm [11]. Eqs. (1) and (2) represent dispersion equations of a pure and a hybrid modes, respectively. In Fig. 3, open symbols show their calculation results. It is shown that surface wave eigenmodes are excited at specific ne. We measured n of the electric field near the plasma-quartz interface by the electric field probe movable in the radial direction, where it is sensitive to the electric field in the z-direction. The radial mode number n is obtained by the number of zeros of Ez(r) in the radial range of 0 b r ≤ R. In the measurement circuit, a double-balanced mixer (DBM) to multiply a signal obtained from the electric field probe and a reference signal of microwave is used as shown in Fig. 1. The plasmas are generated by non-rotating TE11 excitation mode. Fig. 4 shows the output signals from DBM at gas pressures of 10– 50 mTorr and a fixed incident power of 2.0 kW, where a smoothing process is performed to the signals for the elimination of high frequency components. It is shown that n = 5 for different values of ne ranging 1–4 × 1012 cm− 3 as shown in Fig. 2.
Fig. 3. Surface wave eigenmodes and radical mode number calculated by the spectrum analysis of light emission intensities.
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A. Tsuji et al. / Surface & Coatings Technology 202 (2008) 5306–5309
Fig. 4. Output signals from DBM at gas pressures of 10–50 mTorr and a fixed incident power of 2.0 kW.
We observed light emission intensities of plasmas generated in various conditions by the CCD camera. If the plasmas predominantly absorb the microwave power by Joule heating, its intensity is proportional to E2, where E(r, θ) represents the electric field. The main component of E is the axial component, Ez, that has a radial dependence of Jm. Since it is reported that m = 1 in the device [17] , we assume that the light emission intensity S is represented by a linear combination of J21(j1nr/R) as S = a0 + ∑anJ21(j1nr/R). The spectrum analyses are performed by using the least square fitting so that S reproduces the measured radial variation of the light emission. As a result, radial mode number n is obtained by both the electric field distribution and the brightness pattern. The closed symbols in Fig. 3 show experimental results in the device. The radial mode number n is calculated by the spectrum analyses of the light emission intensities, and ne is estimated by Jis at r = 0, where we assumed Te = 2.5 eV. It is shown that n has a fixed value, 5, in a wide range of ne, 1–5 × 1012 cm− 3. Therefore, it is shown that n = 5 in a wide range of ne and surface wave eigenmodes are suppressed in the device. 4. Numerical analyses of surface wave The numerical analyses of surface wave in the SWP model and MSP model have been performed by the finite-difference time-domain (FDTD) method. The SWP model represents a cylindrical device with a few slots and has quartz and plasma layers, where a radius of all layers and a thickness of the quartz layer are 250 and 30 mm, respectively.
Fig. 6. Relative brightness distribution in azimuthal direction in Ar/N2 plasma.
Microwave is excited from two slots at the top surface of the quartz layer, where the slots are symmetrically set from the center and the distance is 170 mm. The broken line in Fig. 5 shows an amplitude ratio in the SWP model, where the amplitude ratio is a value in which a maximum value of at the plasma-quartz interface is divided by that at the excitation position. It is shown that the geometry of the SWP model has surface wave eigenmodes at specific ne because the calculation result indicates some sharp peaks. This means that the amplitude changes remarkably at a narrow range of ne. For instance, surface wave eigenmodes of (m, n) = (7,9) and (3,8) modes are consecutively excited in the SWP model. On the other hand, the MSP model represents the device in Fig. 1, and the MSP antenna is approximated by multi-rings. Microwave is excited by rotating TE11 excitation mode. The solid line in Fig. 5 shows an amplitude ratio in the approximate model of the device. It is shown that it is not easy to excite surface wave eigenmodes in the geometry of the MSP model because the calculation result has a few broad peak only. We presume that the key points are the axisymmetrical geometry and the excitation method. 5. Measurement of light emission of N radical in Ar/N2 plasma We measured light emission of N radical by CCD camera with a interference filter, where transparent wavelength and half bandwidth are 409.5 nm and 11.3 nm, respectively. Ar/N2 plasmas are generated by microwave of rotating TE11 excitation mode at the mixture gas pressure of 60 mTorr and incident power of 2.0 kW. Fig. 6 shows the relative brightness distribution in the azimuthal direction. It is shown that N radical distribution in the azimuthal direction is uniform and striped brightness pattern does not appear. In addition, we previously confirmed the density of N radical by using a quadrupole mass spectrometer (QMS) [19]. Therefore, it is confirmed that spatial resonance of surface wave is suppressed in Ar/N2 plasma and the plasmas are suitable for the nitriding process. 6. Summary
Fig. 5. Surface wave eigenmodes in the SWP and the MSP models.
In the experiments, it is shown that the structure of waves maintains n = 5 in a wide range of ne, 1–5 × 1012 cm− 3. In the numerical analyses, it is shown that it is not easy to excite surface wave eigenmodes in the geometry of the MSP model. In the measurement of the brightness distribution in Ar/N2 plasma, it is confirmed that N radical distribution is uniform in the azimuthal direction and striped brightness pattern does not appear. As a result, it is shown that spatial
A. Tsuji et al. / Surface & Coatings Technology 202 (2008) 5306–5309
resonance of surface wave is suppressed in the device. These characteristics of this device may be significantly important in controlling plasma parameters for plasma reactors in next generation. References [1] M. Moisan, C. Beandry, P. Leprince, IEEE Trans. Plasma Sci. PS-3 (1975) 55. [2] J. Hubert, M. Moisan, A. Ricard, Spectrochim. Acta, Part B: Atom. Spectrosc. 34 (1979) 1. [3] J. Paraszczak, J. Heidenreich, M. Hatzakis, M. Moisan, Microelectronic Engineering 3 (1985) 397. [4] M. Zethoff, U. Kortshagen, J. Phys. D, Appl. Phys. 25 (1992) 1574. [5] K. Komachi, S. Kobayashi, J. Microwave Power Electromagn. Enegy 24 (1989) 140. [6] H. Sugai, I. Ghanashev, M. Nagatsu, Plasma Sources Sci. Technol. 7 (1998) 192. [7] M. Umeno, S. Adhikary, Diamond and Related Materials 14 (2005) 1973. [8] H. Akasaka, N. Ohtake, Diamond and Related Materials 14 (2005) 1828. [9] Y. Hotta, H. Toyoda, H. Sugai, Thin solid films 515 (2007) 4983. [10] I. Ghanashev, M. Nagatsu, H. Sugai, Jpn. J. Appl. Phys. Vol. 36 (1997) 337.
5309
[11] H. Hojo, A. Shimamura, N. Uchida, Y. Yasaka, A. Mase, J. Plasma Fusion Res. Vol. 80 (No. 9) (2004) 719. [12] I. Ghanashev, S. Morita, N. Toyoda, M. Nagatsu, H. Sugai, Jpn. J. Appl. Phys. 38 (1999) 4313. [13] Y. Yasaka, D. Nozaki, K. Koga, M. Ando, T. Yamamoto, N. Goto, N. Ishii, T. Morimoto, Plasma Sources Sci. Technol. 8 (1999) 530. [14] Y. Yasaka, H. Hojo, Phys. Plasmas 7 (2000) 1601. [15] Y. Yasaka, K. Koga, N. Ishii, T. Yamamoto, M. Ando, M. Takahashi, Phys. Plasmas 9 (2002) 1029. [16] Y. Yasaka, N. Ishii, T. Yamamoto, M. Ando, M. Takahasi, IEEE Trans. Plasma Sci. 32 (2004) 101. [17] Y. Yasaka, A. Sakae, N. Sugimoto, H. Takeno, H. Hojo, Jpn. J. Appl. Phys. Vol. 45 (2006) 8059. [18] Y. Tamai, Y. Miyamoto, T. Hayashi, A. Tsuji, H. Takeno, Y. Yasaka, Thin Solid Films 506–507 (2006) 626. [19] Y. Miyamoto, Y. Tamai, K. Daimon, M. Esaki, H. Takeno, Y. Yasaka, Thin Solid Films 506–507 (2006) 622.