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Experiment and modeling of plasma and neutral transports in slot-excited microwave discharges
Thin Solid Films 506 – 507 (2006) 622 – 625 www.elsevier.com/locate/tsf
Experiment and modeling of plasma and neutral transports in slot-excited microwave discharges Y. Miyamoto a,*, Y. Tamai a, K. Daimon b, M. Esaki b, H. Takeno a, Y. Yasaka a a b
Department of Electrical and Electronics Engineering, Kobe University, 1-1, Rokkodai, Nada-ku, Kobe 657-8501, Japan Advanced Engineering Faculty, Maizuru National College of Technology, 234 Shiroya, Maizuru, Kyoto 625-8511, Japan
Abstract A planar microwave discharge device has been developed, where microwave power is radiated uniformly from an antenna of multiple slots, and overdense argon plasmas with uniformities better than 3 % for 30 cm diameter has been obtained [Y. Yasaka, D. Nozaki, K. Koga, M. Ando, T. Yamamoto, N. Goto, N. Ishii, T. Morimoto, Plasma Sources Sci. Technol. 8 (1999) 530.]. In order to apply this device to material processing, an operation mixing nitrogen gas was preformed and density of nitrogen radical was measured by using a mass spectroscopy. The result was that the density of nitrogen radical had better uniformity when the base argon plasma is uniform. Numerical simulation including gas phase reactions also gave better uniformity of nitrogen radical on uniform base plasma. D 2005 Published by Elsevier B.V. Keywords: Plasma processing and deposition; Multi-slotted planar antenna; Nitrogen radical; Mass spectroscopy
1. Introduction We have been investigating a planar microwave discharge device for a large diameter plasma of high-density for high speed processing, where microwave power is radiated uniformly from an antenna of multiple slots and is strongly damped in the plasma, via resonant absorption within a short distance, without forming any standing waves. Furthermore, the radiated field is rotating in the azimuthal direction smoothing out azimuthal nonuniformity, if any. Using this multi-slotted planar (MSP) antenna device, we have obtained overdense argon plasmas with uniformities better than 3% for 30-cm diameter [1]. In order to apply this MSP device to material processing, we investigate transports of plasma and neutral particles associated with convection, diffusion, and chemical reactions. In this study, we treat nitrogen radicals produced in argon plasma. Relative permittivity of a silicon nitride is high and it can make a physical film thicker than a silicon oxidization film. So it is considered to be one of the most important thin film materials. When silicon and nitrogen atoms have a dangling-bond and impurities such as OH or H, a nitriding * Corresponding author. 0040-6090/$ - see front matter D 2005 Published by Elsevier B.V. doi:10.1016/j.tsf.2005.08.065
film has trap levels into its band gap. If trap of an electron hole or an electron takes place to the portion, the characteristic of the semiconductor device will be changed. Therefore, the process in low-pressure is needed. In order to use an MSP device for the process such as silicon nitride thin film formation, uniformity is necessary for not only argon plasma but also nitrogen radicals. In this paper, the uniformity of nitrogen radicals is investigated by experiments and numerical simulations. Using a quadrupole mass spectrometer (QMS), distribution of nitrogen radicals was measured. As for the simulation, we employed 2-D and 3-D codes [2] using several schemes such as FEM and FDTD to calculate the wave excitation, wave propagation, power absorption, plasma transport, and gas phase reaction.
2. Experimental equipments The experimental equipments used in this study are shown in Fig. 1. A discharge chamber has a diameter of 50 cm and a depth of 38 cm, and has a quartz glass window of the same diameter on the top. The MSP antenna is installed on the glass window and is driven by a microwave source at
Y. Miyamoto et al. / Thin Solid Films 506 – 507 (2006) 622 – 625
623
Fig. 3. Relative density of uniform and nonuniform plasma.
At first, only argon is fed in, and a plasma is generated by the microwave field of rotating (circularly polarized) TE11
mode. In Fig. 2, radial distribution of plasma density measured by the probe is shown. This was measured under the conditions that gas pressure p = 20 mTorr (low pressure), incident microwave power P A = 1.11 kW, gas flow rate is 100 sccm and antenna type is EC5-n3. The uniformity in this case is less than T 3.0 % in the radius of 15 cm. The electron density n e is 4.7 1011 cm 3 and temperature Te is 2.8 eV at the centre of plasma. When p = 500 mTorr (high pressure), P A = 2.30 kW, an antenna type is EC-21 and gas flow rate is 120 sccm, the uniformity is able to be made less than T 2.0% in the radius of 13 cm. The n e is 2.5 1012 cm 3 and Te is 1.6 eV at the centre of plasma. The uniformity strongly depends on the polarization of the microwave field in the feeder. When we purposely change the polarization from circular to elliptic, the plasma uniformity changes from T few % to worse than T 10%. We illustrate two distinct cases with uniformity of + 3/ 1% for p = 30 mTorr and P A = 2.5 kW and of + 15.5/ 12.8%for P A = 1.0 kW in Fig. 3, where along the azimuthal direction the relative density of plasma at r = 6cm is evaluated by taking a ratio to the density on the axis and is plotted as a radar chart. Solid and dashed curves are for uniform and nonuniform plasmas, respectively. Furthermore in the case of rotating and non-rotating TE11 mode, the distributions of light emission from the plasma are measured by the CCD camera in low pressure condition. High uniformities are confirmed in both the radial and
Fig. 2. Radial profile of plasma density.
Fig. 4. An example of the signals from QMS.
Fig. 1. Experimental equipments.
a frequency of 2.45 GHz. The MSP antenna consists of a feeder connected to the microwave source, a slotted plate with concentric array of slots distributed over the plate, and a back plate. Slots are 23 –30 mm in length, 2 mm in width, and 4 mm in slot interval in the radial direction, and are distributed all over about 1000 pieces except the central part. Argon and nitrogen gas are fed from the side wall. Flow rates are controlled independently at around 50 –200 sccm, and those values are used for the evaluation of mixing rate. Plasma parameters are measured by a radially fast-scanning Langmuir probe. Two-dimensional distribution of light emission from the plasma is monitored using a charge coupled device (CCD) camera located beneath a substrate stage. The QMS can be installed in place of the CCD camera and is movable by 5 cm in the plane perpendicular to the axis.
3. Results 3.1. Uniformity of Ar plasma
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Y. Miyamoto et al. / Thin Solid Films 506 – 507 (2006) 622 – 625
Fig. 5. Relative density of N radical.
AA and dwell time was 500 As. Argon and nitrogen were supplied into the discharge chamber at 50/50% of a rate. An example of the signals of the QMS is shown in Fig. 4. Solid curve shows the case of with plasma and dotted curve shows the case of without plasma. A great difference between both signals is found in low energy (15 – 25 eV) region. This energy corresponds to an ionization energy of nitrogen radicals which are surely created by the plasma. The density of nitrogen radical is ¨ 1013 cm 3, and the ratio of the density of nitrogen radical to that of nitrogen molecule is about 2%. The relative density of nitrogen radical is evaluated by taking a ratio to the density on the axis. In Fig. 5, the results are indicated with a radar chart. Four directions (up, down, right, and left) correspond to four measured points by relative azimuthal angles, and an ellipse passing through the measured values expresses an azimuthal distribution of relative density. Solid and dotted curves are for uniform and nonuniform plasmas, respectively. As shown in the figure, uniformities of relative density of nitrogen radical change in accordance with the uniformity of plasma. In the uniform plasma case, relative density of nitrogen radical is uniform and it is nonuniform for the nonuniform plasma case. 3.3. Study by numerical simulation
Fig. 6. Illustration of simulation model.
azimuthal directions in rotating TE11 mode. Dual peaks of high intensity are observed around the center in non-rotating TE11 mode, which means the plasma is nonuniform. 3.2. Measurement of nitrogen radicals The QMS was used to measure the density of nitrogen radical, and the signal of nitrogen radical (mass number 14) was measured on the axis and the points with a radial position of 50 mm [3]. The incident microwave power P A = 2 kW, pressure p = 50 mTorr, emission current was 120
We consider a system in which plasma is located in the bottom-half of a cylindrical conducting chamber with no magnetic field as shown in Fig. 6. Microwave power at x/ 2p = 2.45 GHz is introduced into the cavity chamber (tophalf) by an antenna which is specified by current segments aligned in the azimuthal direction. There is a dielectric window between the cavity and the plasma chamber. Reactive gases may be injected from gas nozzles located in the plasma chamber. The plasma density, neutral gas densities, and electron temperature can be inhomogeneous in the two (r –z)- or three (r – h – z)-dimensional directions. The global electric field E inside the system is obtained by solving the wave equation based on the Maxwell equations. The local power density p e absorbed by electrons from the wave is calculated from the complex amplitude of E and the
Fig. 7. Distribution of electron density n e and electron temperature Te for argon plasma obtained by 2-D code.
Y. Miyamoto et al. / Thin Solid Films 506 – 507 (2006) 622 – 625
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results of the relative density of nitrogen radical at a radius of 50 mm as those of experiments. Fig. 8(a) and (b) are for the case of rotating TE11 mode of uniform plasma and non-rotating TE11 mode of non-uniform plasma, respectively. Although the absolute value of relative density is different, the radical distribution is similar to the experimental result.
4. Conclusion Fig. 8. Distribution of relative density of nitrogen radical by simulation.
anti-Hermitian part of the dielectric tensor. Then, the electron density and temperature are calculated by using transport equations including p e. The reactive neutral densities are solved by the rate equations corresponding to the reactions of argon and nitrogen [4,5]. The plasma reactor size is similar to that used in the experiment but is somewhat simplified. The detailed procedures of the calculation are described in Ref. [2]. Fig. 7 displays birds-eye-view of n e and Te obtained in 2D calculation assuming argon plasma with p Ar = 50 mTorr and P A = 1500 W. In each frame, r- and z-axis are indicated, and values are plotted in vertical direction. The rear plane in Fig. 7 corresponds to the bottom surface of the glass window in Fig. 6. Both n e and Te have maximum values close to this surface, which are 2.8 1012 cm 3 and 2.6 eV, respectively. Then we use 3-D code to calculate gas phase reaction of added nitrogen, and take a close look on the azimuthal distribution of radicals. The plasma chamber is filled with argon gas at a pressure of 25 mTorr, and nitrogen gas is injected from nozzles placed at the sidewall at a rate of 100 sccm. The antenna current is given to excite primarily TE11 mode and its magnitude is adjusted in every time step so that the total absorbed microwave power is equal to 1000 W. In this simulation, we consider the following chemical reaction besides ionization, dissociation and excitation of nitrogen molecule, N2+ + eY2N [6,7]. Two cases of the antenna modes, rotating and nonrotating TE11 modes, are calculated to simulate the azimuthally uniform and non-uniform plasmas observed in the experiment. In Fig. 8, we indicate the calculation
In this study uniform argon plasma can be generated by an experimental method which we have proposed. By mixing nitrogen gas, density of nitrogen radical is investigated. The spatial distribution of relative density of nitrogen radical was measured for the first time by using a QMS. The results of both experiments and simulations show that the uniformity of relative density of nitrogen radical depends on the uniformity of plasma strongly. Therefore, it is necessary that the distribution of plasma density is uniform to obtain the uniformity of nitrogen radical.
Acknowledgements The authors would like to thank T. Hayashi and H. Tsuji for valuable discussion. This work was supported by Tokyo Electron Limited.
References [1] Y. Yasaka, D. Nozaki, K. Koga, M. Ando, T. Yamamoto, N. Goto, N. Ishii, T. Morimoto, Plasma Sources Sci. Technol. 8 (1999) 530. [2] Y. Yasaka, N. Uda, J. Appl. Phys. 89 (2001) 3594. [3] P. Kae-Nune, J. Perrin, J. Guillon, J. Jolly, Jpn. J. Appl. Phys. 81 (1994) 4303. [4] Traiq, Majeed, Douglas J. Strickland, J. Phys. Chem. Ref. Data 26 (1997) 335. [5] Electron-Impact Cross Section Database (National Institute of Standards and Technology; NIST) http://physics.nist.gov/PhysRefData/Ionization/molTable.html. [6] N. Itagaki, S. Iwata, K. Muta, A. Yonesu, S. Kawakami, N. Ishii, Y. Kawai, Thin Solid Films 435 (2003) 259. [7] E.C. Zipf, Geophys. Res. Lett. 7 (1980) 9.
Experiment and modeling of plasma and neutral transports in slot-excited microwave discharges Y. Miyamoto a,*, Y. Tamai a, K. Daimon b, M. Esaki b, H. Takeno a, Y. Yasaka a a b
Department of Electrical and Electronics Engineering, Kobe University, 1-1, Rokkodai, Nada-ku, Kobe 657-8501, Japan Advanced Engineering Faculty, Maizuru National College of Technology, 234 Shiroya, Maizuru, Kyoto 625-8511, Japan
Abstract A planar microwave discharge device has been developed, where microwave power is radiated uniformly from an antenna of multiple slots, and overdense argon plasmas with uniformities better than 3 % for 30 cm diameter has been obtained [Y. Yasaka, D. Nozaki, K. Koga, M. Ando, T. Yamamoto, N. Goto, N. Ishii, T. Morimoto, Plasma Sources Sci. Technol. 8 (1999) 530.]. In order to apply this device to material processing, an operation mixing nitrogen gas was preformed and density of nitrogen radical was measured by using a mass spectroscopy. The result was that the density of nitrogen radical had better uniformity when the base argon plasma is uniform. Numerical simulation including gas phase reactions also gave better uniformity of nitrogen radical on uniform base plasma. D 2005 Published by Elsevier B.V. Keywords: Plasma processing and deposition; Multi-slotted planar antenna; Nitrogen radical; Mass spectroscopy
1. Introduction We have been investigating a planar microwave discharge device for a large diameter plasma of high-density for high speed processing, where microwave power is radiated uniformly from an antenna of multiple slots and is strongly damped in the plasma, via resonant absorption within a short distance, without forming any standing waves. Furthermore, the radiated field is rotating in the azimuthal direction smoothing out azimuthal nonuniformity, if any. Using this multi-slotted planar (MSP) antenna device, we have obtained overdense argon plasmas with uniformities better than 3% for 30-cm diameter [1]. In order to apply this MSP device to material processing, we investigate transports of plasma and neutral particles associated with convection, diffusion, and chemical reactions. In this study, we treat nitrogen radicals produced in argon plasma. Relative permittivity of a silicon nitride is high and it can make a physical film thicker than a silicon oxidization film. So it is considered to be one of the most important thin film materials. When silicon and nitrogen atoms have a dangling-bond and impurities such as OH or H, a nitriding * Corresponding author. 0040-6090/$ - see front matter D 2005 Published by Elsevier B.V. doi:10.1016/j.tsf.2005.08.065
film has trap levels into its band gap. If trap of an electron hole or an electron takes place to the portion, the characteristic of the semiconductor device will be changed. Therefore, the process in low-pressure is needed. In order to use an MSP device for the process such as silicon nitride thin film formation, uniformity is necessary for not only argon plasma but also nitrogen radicals. In this paper, the uniformity of nitrogen radicals is investigated by experiments and numerical simulations. Using a quadrupole mass spectrometer (QMS), distribution of nitrogen radicals was measured. As for the simulation, we employed 2-D and 3-D codes [2] using several schemes such as FEM and FDTD to calculate the wave excitation, wave propagation, power absorption, plasma transport, and gas phase reaction.
2. Experimental equipments The experimental equipments used in this study are shown in Fig. 1. A discharge chamber has a diameter of 50 cm and a depth of 38 cm, and has a quartz glass window of the same diameter on the top. The MSP antenna is installed on the glass window and is driven by a microwave source at
Y. Miyamoto et al. / Thin Solid Films 506 – 507 (2006) 622 – 625
623
Fig. 3. Relative density of uniform and nonuniform plasma.
At first, only argon is fed in, and a plasma is generated by the microwave field of rotating (circularly polarized) TE11
mode. In Fig. 2, radial distribution of plasma density measured by the probe is shown. This was measured under the conditions that gas pressure p = 20 mTorr (low pressure), incident microwave power P A = 1.11 kW, gas flow rate is 100 sccm and antenna type is EC5-n3. The uniformity in this case is less than T 3.0 % in the radius of 15 cm. The electron density n e is 4.7 1011 cm 3 and temperature Te is 2.8 eV at the centre of plasma. When p = 500 mTorr (high pressure), P A = 2.30 kW, an antenna type is EC-21 and gas flow rate is 120 sccm, the uniformity is able to be made less than T 2.0% in the radius of 13 cm. The n e is 2.5 1012 cm 3 and Te is 1.6 eV at the centre of plasma. The uniformity strongly depends on the polarization of the microwave field in the feeder. When we purposely change the polarization from circular to elliptic, the plasma uniformity changes from T few % to worse than T 10%. We illustrate two distinct cases with uniformity of + 3/ 1% for p = 30 mTorr and P A = 2.5 kW and of + 15.5/ 12.8%for P A = 1.0 kW in Fig. 3, where along the azimuthal direction the relative density of plasma at r = 6cm is evaluated by taking a ratio to the density on the axis and is plotted as a radar chart. Solid and dashed curves are for uniform and nonuniform plasmas, respectively. Furthermore in the case of rotating and non-rotating TE11 mode, the distributions of light emission from the plasma are measured by the CCD camera in low pressure condition. High uniformities are confirmed in both the radial and
Fig. 2. Radial profile of plasma density.
Fig. 4. An example of the signals from QMS.
Fig. 1. Experimental equipments.
a frequency of 2.45 GHz. The MSP antenna consists of a feeder connected to the microwave source, a slotted plate with concentric array of slots distributed over the plate, and a back plate. Slots are 23 –30 mm in length, 2 mm in width, and 4 mm in slot interval in the radial direction, and are distributed all over about 1000 pieces except the central part. Argon and nitrogen gas are fed from the side wall. Flow rates are controlled independently at around 50 –200 sccm, and those values are used for the evaluation of mixing rate. Plasma parameters are measured by a radially fast-scanning Langmuir probe. Two-dimensional distribution of light emission from the plasma is monitored using a charge coupled device (CCD) camera located beneath a substrate stage. The QMS can be installed in place of the CCD camera and is movable by 5 cm in the plane perpendicular to the axis.
3. Results 3.1. Uniformity of Ar plasma
624
Y. Miyamoto et al. / Thin Solid Films 506 – 507 (2006) 622 – 625
Fig. 5. Relative density of N radical.
AA and dwell time was 500 As. Argon and nitrogen were supplied into the discharge chamber at 50/50% of a rate. An example of the signals of the QMS is shown in Fig. 4. Solid curve shows the case of with plasma and dotted curve shows the case of without plasma. A great difference between both signals is found in low energy (15 – 25 eV) region. This energy corresponds to an ionization energy of nitrogen radicals which are surely created by the plasma. The density of nitrogen radical is ¨ 1013 cm 3, and the ratio of the density of nitrogen radical to that of nitrogen molecule is about 2%. The relative density of nitrogen radical is evaluated by taking a ratio to the density on the axis. In Fig. 5, the results are indicated with a radar chart. Four directions (up, down, right, and left) correspond to four measured points by relative azimuthal angles, and an ellipse passing through the measured values expresses an azimuthal distribution of relative density. Solid and dotted curves are for uniform and nonuniform plasmas, respectively. As shown in the figure, uniformities of relative density of nitrogen radical change in accordance with the uniformity of plasma. In the uniform plasma case, relative density of nitrogen radical is uniform and it is nonuniform for the nonuniform plasma case. 3.3. Study by numerical simulation
Fig. 6. Illustration of simulation model.
azimuthal directions in rotating TE11 mode. Dual peaks of high intensity are observed around the center in non-rotating TE11 mode, which means the plasma is nonuniform. 3.2. Measurement of nitrogen radicals The QMS was used to measure the density of nitrogen radical, and the signal of nitrogen radical (mass number 14) was measured on the axis and the points with a radial position of 50 mm [3]. The incident microwave power P A = 2 kW, pressure p = 50 mTorr, emission current was 120
We consider a system in which plasma is located in the bottom-half of a cylindrical conducting chamber with no magnetic field as shown in Fig. 6. Microwave power at x/ 2p = 2.45 GHz is introduced into the cavity chamber (tophalf) by an antenna which is specified by current segments aligned in the azimuthal direction. There is a dielectric window between the cavity and the plasma chamber. Reactive gases may be injected from gas nozzles located in the plasma chamber. The plasma density, neutral gas densities, and electron temperature can be inhomogeneous in the two (r –z)- or three (r – h – z)-dimensional directions. The global electric field E inside the system is obtained by solving the wave equation based on the Maxwell equations. The local power density p e absorbed by electrons from the wave is calculated from the complex amplitude of E and the
Fig. 7. Distribution of electron density n e and electron temperature Te for argon plasma obtained by 2-D code.
Y. Miyamoto et al. / Thin Solid Films 506 – 507 (2006) 622 – 625
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results of the relative density of nitrogen radical at a radius of 50 mm as those of experiments. Fig. 8(a) and (b) are for the case of rotating TE11 mode of uniform plasma and non-rotating TE11 mode of non-uniform plasma, respectively. Although the absolute value of relative density is different, the radical distribution is similar to the experimental result.
4. Conclusion Fig. 8. Distribution of relative density of nitrogen radical by simulation.
anti-Hermitian part of the dielectric tensor. Then, the electron density and temperature are calculated by using transport equations including p e. The reactive neutral densities are solved by the rate equations corresponding to the reactions of argon and nitrogen [4,5]. The plasma reactor size is similar to that used in the experiment but is somewhat simplified. The detailed procedures of the calculation are described in Ref. [2]. Fig. 7 displays birds-eye-view of n e and Te obtained in 2D calculation assuming argon plasma with p Ar = 50 mTorr and P A = 1500 W. In each frame, r- and z-axis are indicated, and values are plotted in vertical direction. The rear plane in Fig. 7 corresponds to the bottom surface of the glass window in Fig. 6. Both n e and Te have maximum values close to this surface, which are 2.8 1012 cm 3 and 2.6 eV, respectively. Then we use 3-D code to calculate gas phase reaction of added nitrogen, and take a close look on the azimuthal distribution of radicals. The plasma chamber is filled with argon gas at a pressure of 25 mTorr, and nitrogen gas is injected from nozzles placed at the sidewall at a rate of 100 sccm. The antenna current is given to excite primarily TE11 mode and its magnitude is adjusted in every time step so that the total absorbed microwave power is equal to 1000 W. In this simulation, we consider the following chemical reaction besides ionization, dissociation and excitation of nitrogen molecule, N2+ + eY2N [6,7]. Two cases of the antenna modes, rotating and nonrotating TE11 modes, are calculated to simulate the azimuthally uniform and non-uniform plasmas observed in the experiment. In Fig. 8, we indicate the calculation
In this study uniform argon plasma can be generated by an experimental method which we have proposed. By mixing nitrogen gas, density of nitrogen radical is investigated. The spatial distribution of relative density of nitrogen radical was measured for the first time by using a QMS. The results of both experiments and simulations show that the uniformity of relative density of nitrogen radical depends on the uniformity of plasma strongly. Therefore, it is necessary that the distribution of plasma density is uniform to obtain the uniformity of nitrogen radical.
Acknowledgements The authors would like to thank T. Hayashi and H. Tsuji for valuable discussion. This work was supported by Tokyo Electron Limited.
References [1] Y. Yasaka, D. Nozaki, K. Koga, M. Ando, T. Yamamoto, N. Goto, N. Ishii, T. Morimoto, Plasma Sources Sci. Technol. 8 (1999) 530. [2] Y. Yasaka, N. Uda, J. Appl. Phys. 89 (2001) 3594. [3] P. Kae-Nune, J. Perrin, J. Guillon, J. Jolly, Jpn. J. Appl. Phys. 81 (1994) 4303. [4] Traiq, Majeed, Douglas J. Strickland, J. Phys. Chem. Ref. Data 26 (1997) 335. [5] Electron-Impact Cross Section Database (National Institute of Standards and Technology; NIST) http://physics.nist.gov/PhysRefData/Ionization/molTable.html. [6] N. Itagaki, S. Iwata, K. Muta, A. Yonesu, S. Kawakami, N. Ishii, Y. Kawai, Thin Solid Films 435 (2003) 259. [7] E.C. Zipf, Geophys. Res. Lett. 7 (1980) 9.