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Influence of a modulated magnetic field on the behavior of particulates in silane plasma CVD
Surface and Coatings Technology 97 (1997) 366–371
Influence of a modulated magnetic field on the behavior of particulates in silane plasma CVD Sung-Chae Yang *, Yoko Maemura, Kazuhiko Tazoe, Yoshinobu Matsuda, Hiroshi Fujiyama Department of Electrical Engineering and Computer Science, Nagasaki University, Nagasaki 852, Japan
Abstract The spatio-temporal evolution of silicon particles has been investigated by using a laser light scattering method in a static and/or modulated magnetic field. If the crossed static magnetic field is applied at the same time as the discharge starts, then the appearance times of the Mie scattering intensity by silicon particles become later with increasing applied magnetic flux density and the density of silicon particles decreases with increasing applied magnetic flux density. If the modulated magnetic field is applied, the particle density decreases more than in the case of static applied magnetic field; however, the appearance time of silicon particles has an optimum frequency for the discharge condition. Finally, it is considered that the fluctuation of discharge current in the presence of a modulated magnetic field is caused by the effects of both generation of silicon particles in the discharge space and the deposition of a-Si:H thin film on the cathode surface. © 1997 Elsevier Science S.A. Keywords: Fluctuation of discharge current; Modulated magnetic field; Silicon particle
1. Introduction During the last 10 years the problems of particulate (dusty particle) generation and their behavior in processing plasmas has been extensively studied by many authors [1–15]. These studies of dusty plasmas have mainly focused on the contaminative point of view, in the plasma-enhanced chemical vapor deposition (PECVD) process for the deposition of hydrogenated amorphous silicon (a-Si:H ) thin films. Consequently, it is found that dusty particles are typically a few nm to tens of mm in size and occur at >106 cm−3 density in low-pressure plasmas, inducing serious problems in the fabrication of microelectronics for integrated circuits, solar cells and other devices [2–8]. It has also been found that various forces affect the generation and spatial distribution of the particulates, such as electrostatic force, ion drag force, neutral drag force, gravitation force, thermophoretic force and polarization force [9–12]. However, the primary forces acting on the spatiotemporal distribution of particulates are different at each articles except for the electrostatic force. This means that the dynamics of dusty plasmas are not * Corresponding author. 0257-8972/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 02 5 7 -8 9 7 2 ( 9 7 ) 0 0 16 4 - 3
sufficiently understood. Moreover, at the present stage of study of dusty particles in plasmas, little is known about the effects of a magnetic field perpendicular to the electric field. We have investigated the dynamics of silicon particles in silane plasmas in the presence of a static and/or modulated magnetic field B, perpendicular to discharge electric field E, in DC silane plasmas [13–15]. From these studies, it is found that negatively charged silicon particles are transported in the direction opposite to the E×B drift of plasma, and that the density of the silicon particles decreases with increasing applied magnetic flux density. Therefore, it is considered that use of a crossed magnetic field is very useful to exclude particulate contamination. The goal of our study is to prepare largearea, uniform, a-Si:H thin films under particle-free conditions by control of the spatio-temporal distribution and the elimination of particles from the discharge space. In this paper, we report experimental results on the spatio-temporal evolution of silicon particles in a DC silane plasma by using a laser light scattering method (Mie scattering). Variation of the silicon particle density and their appearance time are also discussed by observing the spatially integrated Mie scattering intensity (SIMSI ) for the condition of applied static and/or modulated magnetic field at the same time as the
S.-C. Yang et al. / Surface and Coatings Technology 97 (1997) 366–371
discharge starts. We also report on the influence of the modulated magnetic field on the discharge current in dusty plasmas. Finally, the correlation between the generation of silicon particles and fluctuation of the discharge current in modulated magnetic DC silane plasmas is investigated.
2. Experimental apparatus and methods 2.1. Experimental apparatus A schematic diagram of the experimental apparatus is shown in Fig. 1. In this study, a conventional cylindrical vacuum chamber 32 cm in diameter and 230 cm long was used. Two parallel approximate Rogowsky electrodes made of stainless steel (SUS304), with dimensions of 90 mm diameter and 30 mm gap length, were used. In the experiments, 100% Ar and 10% SiH gas 4 diluted with Ar gas were used. Four solenoid coils were attached outside the vacuum chamber to produce homogeneous magnetic fields of 0–76 Gauss in the axial direction of the chamber. The experimental apparatus described in detail in [14,15]. 2.2. Particle detection In order to observe the spatio-temporal evolution (i.e., the generation and behavior) of dusty particles in the discharge space, the Mie scattering method was used. An He–Ne laser beam (5 mW at 632.8 nm) and/or a pulsed-dye laser pumped by an Nd:YAG laser (40 mW at 488 nm) were used alternatively as the laser source for Mie scattering. Both laser beams were incident to the discharge space from a horizontal view port. The expanded laser beam was shaped like a sheet 28 mm in height and 1 mm width by using a beam expander system and an aperture. An image-intensified charge-
Fig. 1. Experimental apparatus.
367
coupled device (ICCD) camera with a metallic interference filter (632.8 nm and/or 488 nm center wavelength, 1 nm full-width at half-maximum) was used to detect Mie scattering light by silicon particles. The data obtained by the ICCD camera were digitized on a computer and processed, giving a two-dimensional profile (contours) and the SIMSI. Since the Mie scattering intensity (MSI ) is a complicated function of size, number density and refractive index of the particles, the SIMSI represents only the overall particle features. In this study, the experiments were mainly performed for the following conditions: gas pressure, P=0.3–0.4 torr and discharge voltage, V =400–600 V. d 3. Experimental results and discussion 3.1. Spatio-temporal evolution of MSI Fig. 2 shows the conventional spatio-temporal evolution of two-dimensional MSI by silicon particles for the conditions P=0.3 Torr, V =500 V and magnetic flux d density B=0 Gauss in DC silane plasmas. As can be seen from Fig. 2, the MSI of silicon particles can be detected at about 10 min after the discharge starts, and it increases with increasing discharge time. If the magnetic field B perpendicular to discharge electric field E was applied at this discharge space inclusive of silicon particles, then silicon particles were transported in the opposite direction to the E×B drift, the particle density decreased with increasing applied magnetic flux density, and finally silicon particles were expelled from the discharge space. This result was verified experimentally over a wide range of discharge conditions as described in detail in [13–15].
Fig. 2. Spatio-temporal evolution of Mie scattering intensity.
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Fig. 3 shows the temporal variation of the SIMSI and the discharge current. In this figure, the solid line (a), the dashed line (b) and the dotted line (c) indicate results for the following discharge conditions: (a) applied magnetic flux density B=0 Gauss, frequency of modulated magnetic field f =0 Hz, (b) B=38 Gauss, B f =0 Hz and (c) B=38 Gauss, f =0.5 Hz, respectively. B B Here, the crossed magnetic field was applied at the same time as the discharge was started. In this figure, the SIMSI is normalized by the maximum value at the discharge time T =60 min. d The case (a) shows the temporal variation of the SIMSI obtained from the result of Fig. 2. This result shows that it takes about 10 min to observe the MSI by silicon particles, resulting from the slow growth rate of particulates under this discharge condition. The SIMSI is increased dramatically because of the change from the initial nucleation to the coagulation stage. From case (b) it is found that the MSI of silicon particles appears at about 20 min after the discharge starts, and it increases with discharge time. This appearance time is slower than the result from case (a). From this, it is considered that growth of the silicon particles is restrained by the applied magnetic field because the nuclei of silicon particles are expelled from the discharge space by the E×B drift velocity generated by the crossed magnetic field B perpendicular to the electric field E. Therefore, the particle growth and coagulating reaction of the nuclei of silicon particles are restrained. This experimental result agrees with theoretical calculation results that the E×B drift velocity increases with decreasing particle radius [15]. From case (c) it is found that the MSI of silicon particles appears at about 40 min after discharge start,
Fig. 3. Temporal variation of the SIMSI and discharge current.
and it increases with discharge time. This appearance time is slower than the result of case (b) as well as that of case (a). The particle density (SIMSI ) is reduced more than others. From this result, it is considered that growth of the silicon particles is restrained more by the modulated magnetic field than by the static magnetic field because the number of excluded nuclei of silicon particles increases with the modulated magnetic field. Case (d ) shows that the discharge current increases to a maximum value at an early stage in the discharge time and decreases dramatically after the peak value, finally saturating with increasing discharge time. From the above results, it is considered that the temporal variation of discharge current is caused by the effects of both the generation of silicon particles in the discharge space and the deposition of a-Si:H thin films on the cathode surface. In the early stage, the discharge current has very small value caused by a decrease in electron density by the active nucleation process owing to electron attachment. Afterwards, it increases by the saturation of electron attachment because of the change from the nucleation stage to the coagulation stage. Finally, it decreases and tends to saturate because of the deposition of a-Si:H thin films on the cathode surface [16 ]. The temporal variation of the discharge current agrees qualitatively with those of emission intensities from SiH* (414.2 nm) and H (656.3 nm). These results a are related to the temporal variation of the plasma resistance [17]. 3.2. Appearance time of silicon particles Fig. 4a shows the variation of the appearance time with applied magnetic flux density under the discharge conditions V =500 V and 600 V at P=0.3 Torr. In this d experiment, the MSI by silicon particles was not detected for applied magnetic flux densities of B=57 and 76 Gauss at V =500 V and B=76 Gauss at V =600 V. d d From this figure, it is found that the appearance time is delayed with increasing applied magnetic flux density because the amount of silicon particle nuclei excluded from the discharge space increases with the applied magnetic flux density. Fig. 4b shows the variation of appearance time with modulation frequency of the applied magnetic flux density under V =500 V and 600 V, for P=0.3 Torr and d B=38 Gauss. From this figure, it is found that the appearance time is most delayed at the modulation frequency f =0.5 Hz for both discharge voltages B (V =500 V and 600 V ). These results can be explained d as follows. At a modulation frequency lower than f =0.5 Hz, the excluded nuclei of silicon particles B increase with the modulated magnetic field because the movements of these nuclei increase with the modulation frequency of the magnetic field. On the other hand, at modulation frequency higher than f =0.5 Hz, the B
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of frequency lower than f =0.5 Hz is useful to control B the particle behavior. 3.3. Variation of discharge current fluctuation From the results of our studies, it is considered that using the crossed magnetic field is very useful to exclude particle contamination. However, in order to prepare large-area a-Si:H thin films under particle-free conditions, the influence of the modulated magnetic field on dusty plasmas has to be investigated, i.e., the silicon particle generation, growth and behavior, etc. Fig. 5 shows the temporal variation of the discharge current in the presence of the modulated magnetic field. As can be seen in Fig. 5, there is a fluctuation of the discharge current owing to modulation of the applied magnetic field. It is found from Fig. 5 that the variation of the overall discharge current is roughly same as that of Fig. 3d. However, there is a complicated variation of the discharge current amplitude (fluctuation) with discharge time. Fig. 6 shows the temporal variation of the discharge current fluctuation in the case of Ar (100%) (6) and SiH (10%)/Ar (%) plasma and the SIMSI (1) of 4 silicon particles for the process conditions P=0.3 Torr, V =500 V, B=38 Gauss and f =0.5 Hz. Here, the flucd B tuation of the discharge current for Ar and SiH plasmas 4 is normalized by the maximum value at the discharge times T =0 and T =45 min, respectively. The SIMSI d d is also normalized by the maximum value at the discharge time T =60 min. d As can be seen in Fig. 6, there is a complex variation of the discharge current fluctuation in the SiH plasma 4 compared with the simple variation in the Ar plasma. In the case of SiH plasma, the fluctuation of discharge 4 current decreases to a minimum value at an early stage in the discharge time [stage (a)] and increases dramatically after attaining the minimum [stage (b)], finally saturating with increasing discharge time [stage (c)]. On
Fig. 4. Variation of the appearance time with (a) applied magnetic flux density and (b) modulation frequency of applied magnetic field.
movements of excluded nuclei decrease with increasing modulation frequency of the magnetic field because these mainly large nuclei cannot adapt to the variation of the modulated frequency. However, the movements of the nuclei in the discharge space increase and, as a result, the coagulation reaction becomes active. From the above results it is considered that, under this experimental condition, a modulation magnetic field
Fig. 5. Temporal variation of discharge current in the presence of the modulated magnetic field.
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on the cathode surface as a result electron density is also saturated. From the above discussion, it is considered that measurement of the variation of discharge current fluctuation is able to provide a measure of the silicon particles in the presence of the modulated magnetic field because it includes information about silicon particle generation and growth that cannot be detected by the Mie scattering method. However, since there is more complicated variation in stage (a), and relatively low power laser (5 mW ) was used to detect the MSI, the correlation between MSI and the fluctuation of discharge current could not be understood sufficiently. However, results from our recent study, in which we used a high-power He–Ne laser (80 mW at 632.8 nm), are roughly the same as those obtained here by using the low-power laser even though the appearance time was somewhat faster. These experimental results will be reported shortly. Fig. 6. Temporal variation of the discharge current fluctuation and SIMSI in the presence of the modulated magnetic field.
the other hand, the SIMSI of silicon particles appears after about 40 min, and the fluctuation of discharge current is beginning to saturate. Therefore, it is considered that the variation of discharge current fluctuation depends on the silicon particle generation and growth and the a-Si:H thin-film deposition on the cathode surface. Generally, in magnetized plasmas, the fluctuation of discharge current means the relative electron density at that time. It indicates that the increment in discharge current as a result of the applied magnetic field is larger for the condition of high electron density than for low electron density. On the other hand, several works have shown that the mechanism of silicon particle generation and growth consists of three phases: nucleation, coagulation and growth saturation [18–20]. According to this mechanism, our experimental results in the presence of the modulated magnetic field perpendicular to the discharge electric field may be explained as follows. In the early discharge, stage (a), the nucleation of particles mainly occurs. The particles are very small and easily expelled from the discharge space by E×B drift before they coagulate. Consequently, the nucleation and expulsion of particles is repeated in this stage and as a result the electron density begin to decrease, because the majority of the electrons are spent on the attachment reaction. In stage (b), the coagulation of particles is the main occurrence, the particles grow rapidly and the SIMSI increases drastically. In this stage, electron density is regained because the main reaction is coagulation and electron attachment is a minor reaction. In stage (c), particle growth is saturated and the decrement of the discharge current by the effect of thin-film deposition
4. Conclusion The spatio-temporal evolution of silicon particle was investigated as two-dimensional spatial profiles. The growth of silicon particles is restrained by the applied static and/or modulated magnetic field because the nuclei of these silicon particles are expelled from the discharge space by the E×B drift velocity. In order to control particle contamination, a modulated magnetic field with the frequency lower than f =0.5 Hz is useful. The B variation of discharge current fluctuation depends on the effects of both particle generation and growth in the discharge space and the deposition of a-Si:H thin films on the cathode surface. Measurement of the variation of discharge current fluctuation is useful method to detect silicon particles because it includes some information about the particles. However, much more work is necessary to improve our understanding of this process and to solve the dynamics of the silicon particles.
Acknowledgement This work was supported partly by a Grant-in-Aid for Scientific Research on Free Radical Science in Priority Areas by the Ministry of Education, Science and Culture. It is a pleasure to thank M. Morita of Nagasaki University for his technical assistance.
References [1] K.G. Spear, T.J. Robinson, R.M. Roth, IEEE Trans. Plasma Sci. 14 (1986) 179.
S.-C. Yang et al. / Surface and Coatings Technology 97 (1997) 366–371 [2] G.S. Selwyn, J. Singh, R.S. Bennett, J. Vac. Sci. Technol. A7 (1989) 2758. [3] Y. Watanabe, M. Shiratani, M. Yamashita, Appl. Phys. Lett. 61 (1992) 1520. [4] J.P. Boeuf, Ph. Belengurer, J. Appl. Phys. 71 (1992) 4751. [5] L. Boufendi, A. Bouchoule, Plasma Sources Sci. Technol. 3 (1994) 262. [6 ] L. Dubost, A. Rhallabi, J. Perrin, J. Schmitt, J. Appl. Phys. 78 (1995) 3784. [7] C. Courteille, J.-L. Dorier, Ch. Hollenstein, L. Sansonnens, A.A. Howling, Plasma Sources Sci. Technol. 5 (1996) 210. [8] W.W. Stoffels, E. Stoffels, G.M.W. Koesen, F.J. de Hoog, J. Appl. Phys. 78 (1995) 4867. [9] M.S. Barnes, J.H. Keller, J.C. Forster, J.A. O’Neill, D.K. Coultas, Phys. Rev. Lett. 68 (1992) 313. [10] D. Winske, M.E. Jones, IEEE Trans. Plasma Sci. 22 (1994) 454. [11] S. Hamaguchi, R.T. Farouki, Phys. Rev. E 49 (1994) 4430. [12] J.E. Daugherty, D.B. Graves, J. Appl. Phys. 78 (1995) 2279.
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[13] H. Fujiyama, T. Yamashita, T. Takahashi, H. Matsuo, Appl. Phys. Lett. 50 (1987) 1322. [14] H. Fujiyama, H. Kawasaki, S.-C. Yang, Y. Matsuda, Jpn J. Appl. Phys. 33 (1994) 4216. [15] S.-C. Yang, Y. Nakajima, Y. Maemura, Y. Matsuda, H. Fujiyama, Plasma Sources Sci. Technol. 5 (1996) 333. [16 ] S.-C. Yang, Y. Matsuda, H. Fujiyama, in: Proceedings of 6th Symposium on Plasma Science for Materials, ( Tokyo, Japan) 1993, p. 21. [17] S.-C. Yang, Y. Matsuda, H. Fujiyama, in: Proceedings of 2nd Asia–Pacific Conference on Plasma Science and Technology, (Daejon, Korea) 1994, p. 204. [18] L. Boufendi, J. Hermann, A. Bouchoule, B. Dubreuil, E. Stoffels, W.W. Stoffels, M.L. de Giorgi, J. Appl. Phys. 76 (1994) 148. [19] A.A. Fridman, L. Boufendi, T. Hbid, B.V. Potapkin, A. Bouchoule, J. Appl. Phys. 79 (1996) 1303. [20] M. Shiratani, H. Kawasaki, T. Fukuzawa, T. Yoshioka, Y. Ueda, S. Singh, Y. Watanabe, J. Appl. Phys. 79 (1996) 104.
Influence of a modulated magnetic field on the behavior of particulates in silane plasma CVD Sung-Chae Yang *, Yoko Maemura, Kazuhiko Tazoe, Yoshinobu Matsuda, Hiroshi Fujiyama Department of Electrical Engineering and Computer Science, Nagasaki University, Nagasaki 852, Japan
Abstract The spatio-temporal evolution of silicon particles has been investigated by using a laser light scattering method in a static and/or modulated magnetic field. If the crossed static magnetic field is applied at the same time as the discharge starts, then the appearance times of the Mie scattering intensity by silicon particles become later with increasing applied magnetic flux density and the density of silicon particles decreases with increasing applied magnetic flux density. If the modulated magnetic field is applied, the particle density decreases more than in the case of static applied magnetic field; however, the appearance time of silicon particles has an optimum frequency for the discharge condition. Finally, it is considered that the fluctuation of discharge current in the presence of a modulated magnetic field is caused by the effects of both generation of silicon particles in the discharge space and the deposition of a-Si:H thin film on the cathode surface. © 1997 Elsevier Science S.A. Keywords: Fluctuation of discharge current; Modulated magnetic field; Silicon particle
1. Introduction During the last 10 years the problems of particulate (dusty particle) generation and their behavior in processing plasmas has been extensively studied by many authors [1–15]. These studies of dusty plasmas have mainly focused on the contaminative point of view, in the plasma-enhanced chemical vapor deposition (PECVD) process for the deposition of hydrogenated amorphous silicon (a-Si:H ) thin films. Consequently, it is found that dusty particles are typically a few nm to tens of mm in size and occur at >106 cm−3 density in low-pressure plasmas, inducing serious problems in the fabrication of microelectronics for integrated circuits, solar cells and other devices [2–8]. It has also been found that various forces affect the generation and spatial distribution of the particulates, such as electrostatic force, ion drag force, neutral drag force, gravitation force, thermophoretic force and polarization force [9–12]. However, the primary forces acting on the spatiotemporal distribution of particulates are different at each articles except for the electrostatic force. This means that the dynamics of dusty plasmas are not * Corresponding author. 0257-8972/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 02 5 7 -8 9 7 2 ( 9 7 ) 0 0 16 4 - 3
sufficiently understood. Moreover, at the present stage of study of dusty particles in plasmas, little is known about the effects of a magnetic field perpendicular to the electric field. We have investigated the dynamics of silicon particles in silane plasmas in the presence of a static and/or modulated magnetic field B, perpendicular to discharge electric field E, in DC silane plasmas [13–15]. From these studies, it is found that negatively charged silicon particles are transported in the direction opposite to the E×B drift of plasma, and that the density of the silicon particles decreases with increasing applied magnetic flux density. Therefore, it is considered that use of a crossed magnetic field is very useful to exclude particulate contamination. The goal of our study is to prepare largearea, uniform, a-Si:H thin films under particle-free conditions by control of the spatio-temporal distribution and the elimination of particles from the discharge space. In this paper, we report experimental results on the spatio-temporal evolution of silicon particles in a DC silane plasma by using a laser light scattering method (Mie scattering). Variation of the silicon particle density and their appearance time are also discussed by observing the spatially integrated Mie scattering intensity (SIMSI ) for the condition of applied static and/or modulated magnetic field at the same time as the
S.-C. Yang et al. / Surface and Coatings Technology 97 (1997) 366–371
discharge starts. We also report on the influence of the modulated magnetic field on the discharge current in dusty plasmas. Finally, the correlation between the generation of silicon particles and fluctuation of the discharge current in modulated magnetic DC silane plasmas is investigated.
2. Experimental apparatus and methods 2.1. Experimental apparatus A schematic diagram of the experimental apparatus is shown in Fig. 1. In this study, a conventional cylindrical vacuum chamber 32 cm in diameter and 230 cm long was used. Two parallel approximate Rogowsky electrodes made of stainless steel (SUS304), with dimensions of 90 mm diameter and 30 mm gap length, were used. In the experiments, 100% Ar and 10% SiH gas 4 diluted with Ar gas were used. Four solenoid coils were attached outside the vacuum chamber to produce homogeneous magnetic fields of 0–76 Gauss in the axial direction of the chamber. The experimental apparatus described in detail in [14,15]. 2.2. Particle detection In order to observe the spatio-temporal evolution (i.e., the generation and behavior) of dusty particles in the discharge space, the Mie scattering method was used. An He–Ne laser beam (5 mW at 632.8 nm) and/or a pulsed-dye laser pumped by an Nd:YAG laser (40 mW at 488 nm) were used alternatively as the laser source for Mie scattering. Both laser beams were incident to the discharge space from a horizontal view port. The expanded laser beam was shaped like a sheet 28 mm in height and 1 mm width by using a beam expander system and an aperture. An image-intensified charge-
Fig. 1. Experimental apparatus.
367
coupled device (ICCD) camera with a metallic interference filter (632.8 nm and/or 488 nm center wavelength, 1 nm full-width at half-maximum) was used to detect Mie scattering light by silicon particles. The data obtained by the ICCD camera were digitized on a computer and processed, giving a two-dimensional profile (contours) and the SIMSI. Since the Mie scattering intensity (MSI ) is a complicated function of size, number density and refractive index of the particles, the SIMSI represents only the overall particle features. In this study, the experiments were mainly performed for the following conditions: gas pressure, P=0.3–0.4 torr and discharge voltage, V =400–600 V. d 3. Experimental results and discussion 3.1. Spatio-temporal evolution of MSI Fig. 2 shows the conventional spatio-temporal evolution of two-dimensional MSI by silicon particles for the conditions P=0.3 Torr, V =500 V and magnetic flux d density B=0 Gauss in DC silane plasmas. As can be seen from Fig. 2, the MSI of silicon particles can be detected at about 10 min after the discharge starts, and it increases with increasing discharge time. If the magnetic field B perpendicular to discharge electric field E was applied at this discharge space inclusive of silicon particles, then silicon particles were transported in the opposite direction to the E×B drift, the particle density decreased with increasing applied magnetic flux density, and finally silicon particles were expelled from the discharge space. This result was verified experimentally over a wide range of discharge conditions as described in detail in [13–15].
Fig. 2. Spatio-temporal evolution of Mie scattering intensity.
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Fig. 3 shows the temporal variation of the SIMSI and the discharge current. In this figure, the solid line (a), the dashed line (b) and the dotted line (c) indicate results for the following discharge conditions: (a) applied magnetic flux density B=0 Gauss, frequency of modulated magnetic field f =0 Hz, (b) B=38 Gauss, B f =0 Hz and (c) B=38 Gauss, f =0.5 Hz, respectively. B B Here, the crossed magnetic field was applied at the same time as the discharge was started. In this figure, the SIMSI is normalized by the maximum value at the discharge time T =60 min. d The case (a) shows the temporal variation of the SIMSI obtained from the result of Fig. 2. This result shows that it takes about 10 min to observe the MSI by silicon particles, resulting from the slow growth rate of particulates under this discharge condition. The SIMSI is increased dramatically because of the change from the initial nucleation to the coagulation stage. From case (b) it is found that the MSI of silicon particles appears at about 20 min after the discharge starts, and it increases with discharge time. This appearance time is slower than the result from case (a). From this, it is considered that growth of the silicon particles is restrained by the applied magnetic field because the nuclei of silicon particles are expelled from the discharge space by the E×B drift velocity generated by the crossed magnetic field B perpendicular to the electric field E. Therefore, the particle growth and coagulating reaction of the nuclei of silicon particles are restrained. This experimental result agrees with theoretical calculation results that the E×B drift velocity increases with decreasing particle radius [15]. From case (c) it is found that the MSI of silicon particles appears at about 40 min after discharge start,
Fig. 3. Temporal variation of the SIMSI and discharge current.
and it increases with discharge time. This appearance time is slower than the result of case (b) as well as that of case (a). The particle density (SIMSI ) is reduced more than others. From this result, it is considered that growth of the silicon particles is restrained more by the modulated magnetic field than by the static magnetic field because the number of excluded nuclei of silicon particles increases with the modulated magnetic field. Case (d ) shows that the discharge current increases to a maximum value at an early stage in the discharge time and decreases dramatically after the peak value, finally saturating with increasing discharge time. From the above results, it is considered that the temporal variation of discharge current is caused by the effects of both the generation of silicon particles in the discharge space and the deposition of a-Si:H thin films on the cathode surface. In the early stage, the discharge current has very small value caused by a decrease in electron density by the active nucleation process owing to electron attachment. Afterwards, it increases by the saturation of electron attachment because of the change from the nucleation stage to the coagulation stage. Finally, it decreases and tends to saturate because of the deposition of a-Si:H thin films on the cathode surface [16 ]. The temporal variation of the discharge current agrees qualitatively with those of emission intensities from SiH* (414.2 nm) and H (656.3 nm). These results a are related to the temporal variation of the plasma resistance [17]. 3.2. Appearance time of silicon particles Fig. 4a shows the variation of the appearance time with applied magnetic flux density under the discharge conditions V =500 V and 600 V at P=0.3 Torr. In this d experiment, the MSI by silicon particles was not detected for applied magnetic flux densities of B=57 and 76 Gauss at V =500 V and B=76 Gauss at V =600 V. d d From this figure, it is found that the appearance time is delayed with increasing applied magnetic flux density because the amount of silicon particle nuclei excluded from the discharge space increases with the applied magnetic flux density. Fig. 4b shows the variation of appearance time with modulation frequency of the applied magnetic flux density under V =500 V and 600 V, for P=0.3 Torr and d B=38 Gauss. From this figure, it is found that the appearance time is most delayed at the modulation frequency f =0.5 Hz for both discharge voltages B (V =500 V and 600 V ). These results can be explained d as follows. At a modulation frequency lower than f =0.5 Hz, the excluded nuclei of silicon particles B increase with the modulated magnetic field because the movements of these nuclei increase with the modulation frequency of the magnetic field. On the other hand, at modulation frequency higher than f =0.5 Hz, the B
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of frequency lower than f =0.5 Hz is useful to control B the particle behavior. 3.3. Variation of discharge current fluctuation From the results of our studies, it is considered that using the crossed magnetic field is very useful to exclude particle contamination. However, in order to prepare large-area a-Si:H thin films under particle-free conditions, the influence of the modulated magnetic field on dusty plasmas has to be investigated, i.e., the silicon particle generation, growth and behavior, etc. Fig. 5 shows the temporal variation of the discharge current in the presence of the modulated magnetic field. As can be seen in Fig. 5, there is a fluctuation of the discharge current owing to modulation of the applied magnetic field. It is found from Fig. 5 that the variation of the overall discharge current is roughly same as that of Fig. 3d. However, there is a complicated variation of the discharge current amplitude (fluctuation) with discharge time. Fig. 6 shows the temporal variation of the discharge current fluctuation in the case of Ar (100%) (6) and SiH (10%)/Ar (%) plasma and the SIMSI (1) of 4 silicon particles for the process conditions P=0.3 Torr, V =500 V, B=38 Gauss and f =0.5 Hz. Here, the flucd B tuation of the discharge current for Ar and SiH plasmas 4 is normalized by the maximum value at the discharge times T =0 and T =45 min, respectively. The SIMSI d d is also normalized by the maximum value at the discharge time T =60 min. d As can be seen in Fig. 6, there is a complex variation of the discharge current fluctuation in the SiH plasma 4 compared with the simple variation in the Ar plasma. In the case of SiH plasma, the fluctuation of discharge 4 current decreases to a minimum value at an early stage in the discharge time [stage (a)] and increases dramatically after attaining the minimum [stage (b)], finally saturating with increasing discharge time [stage (c)]. On
Fig. 4. Variation of the appearance time with (a) applied magnetic flux density and (b) modulation frequency of applied magnetic field.
movements of excluded nuclei decrease with increasing modulation frequency of the magnetic field because these mainly large nuclei cannot adapt to the variation of the modulated frequency. However, the movements of the nuclei in the discharge space increase and, as a result, the coagulation reaction becomes active. From the above results it is considered that, under this experimental condition, a modulation magnetic field
Fig. 5. Temporal variation of discharge current in the presence of the modulated magnetic field.
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on the cathode surface as a result electron density is also saturated. From the above discussion, it is considered that measurement of the variation of discharge current fluctuation is able to provide a measure of the silicon particles in the presence of the modulated magnetic field because it includes information about silicon particle generation and growth that cannot be detected by the Mie scattering method. However, since there is more complicated variation in stage (a), and relatively low power laser (5 mW ) was used to detect the MSI, the correlation between MSI and the fluctuation of discharge current could not be understood sufficiently. However, results from our recent study, in which we used a high-power He–Ne laser (80 mW at 632.8 nm), are roughly the same as those obtained here by using the low-power laser even though the appearance time was somewhat faster. These experimental results will be reported shortly. Fig. 6. Temporal variation of the discharge current fluctuation and SIMSI in the presence of the modulated magnetic field.
the other hand, the SIMSI of silicon particles appears after about 40 min, and the fluctuation of discharge current is beginning to saturate. Therefore, it is considered that the variation of discharge current fluctuation depends on the silicon particle generation and growth and the a-Si:H thin-film deposition on the cathode surface. Generally, in magnetized plasmas, the fluctuation of discharge current means the relative electron density at that time. It indicates that the increment in discharge current as a result of the applied magnetic field is larger for the condition of high electron density than for low electron density. On the other hand, several works have shown that the mechanism of silicon particle generation and growth consists of three phases: nucleation, coagulation and growth saturation [18–20]. According to this mechanism, our experimental results in the presence of the modulated magnetic field perpendicular to the discharge electric field may be explained as follows. In the early discharge, stage (a), the nucleation of particles mainly occurs. The particles are very small and easily expelled from the discharge space by E×B drift before they coagulate. Consequently, the nucleation and expulsion of particles is repeated in this stage and as a result the electron density begin to decrease, because the majority of the electrons are spent on the attachment reaction. In stage (b), the coagulation of particles is the main occurrence, the particles grow rapidly and the SIMSI increases drastically. In this stage, electron density is regained because the main reaction is coagulation and electron attachment is a minor reaction. In stage (c), particle growth is saturated and the decrement of the discharge current by the effect of thin-film deposition
4. Conclusion The spatio-temporal evolution of silicon particle was investigated as two-dimensional spatial profiles. The growth of silicon particles is restrained by the applied static and/or modulated magnetic field because the nuclei of these silicon particles are expelled from the discharge space by the E×B drift velocity. In order to control particle contamination, a modulated magnetic field with the frequency lower than f =0.5 Hz is useful. The B variation of discharge current fluctuation depends on the effects of both particle generation and growth in the discharge space and the deposition of a-Si:H thin films on the cathode surface. Measurement of the variation of discharge current fluctuation is useful method to detect silicon particles because it includes some information about the particles. However, much more work is necessary to improve our understanding of this process and to solve the dynamics of the silicon particles.
Acknowledgement This work was supported partly by a Grant-in-Aid for Scientific Research on Free Radical Science in Priority Areas by the Ministry of Education, Science and Culture. It is a pleasure to thank M. Morita of Nagasaki University for his technical assistance.
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