- Email: [email protected]
Hollow electrode enhanced RF glow plasma for the fast deposition of microcrystalline silicon
Surface and Coatings Technology 173 (2003) 243–248
Hollow electrode enhanced RF glow plasma for the fast deposition of microcrystalline silicon Toshihiro Tabuchi*, Masayuki Takashiri, Hiroyuki Mizukami Research Division, Electronic Material Research Department, Komatsu Ltd, 1200 Manda, Hiratsuka, Kanagawa 254-8567, Japan Received 6 December 2002; accepted in revised form 24 March 2003
Abstract A hollow electrode enhanced RF glow plasma excitation technique has been newly developed. In this technique, the reactor is divided into a capacitively-coupled RF glow discharge space and a processing space by counter electrode, which has a hollow structure and is placed between RF electrode and substrate. Hollow electrode discharge is induced inside this hollow structure. The application of this discharge type for semiconductor processing is studied in the case of plasma enhanced chemical vapor deposition of hydrogenated microcrystalline silicon (mc-Si) thin films. It is found that high crystallinity, photo sensitivity and maximum deposition rate of 4.9 nmys can be achieved at the plasma excitation frequency of 13.56 MHz. Properties of this plasma are investigated by observation of the plasma emission pattern viewed through the window, optical emission spectral analysis and the plasma potential. Plasmas generated inside the new apparatus is compared to those of normal capacitivelycoupled RF plasmas. In our apparatus, high intensity plasma emission is observed near and inside the hollow structure attached to the counter electrode. The spectral and plasma potential analysis showed that both the intensity of SiH* and the plasma potential of our plasma are higher than that of normal capacitively-coupled plasma. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Hollow electrode; Plasma CVD; RF glow discharge; High-density plasma; mc-Si thin film; Fast deposition; Low temperature
1. Introduction High-density plasma and its application to semiconductor fabrication processing are still under development all over the world. In order to achieve both an improvement of the processing speed and a reduction of the plasma damage to films or substrates, many novel plasma apparatuses w1,2x, such as the very high frequency plasma technique w3–6x or hotwire chemical vapor deposition (CVD) w7,8x, have been proposed. Hollow discharge w9,10x can generate much higher density plasma and has been applied for surface coating w11–13x. On the other hand, hollow discharge has been considered to be too harmful for semiconductor processing such as amorphous silicon thin film deposition for solar cells or thin film transistors, because it may cause serious damage to films or substrates. This is the reason why *Corresponding author. Tel.: q81-463-35-9323; fax: q81-463-359287. E-mail addresses: [email protected] (T. Tabuchi), [email protected] (M. Takashiri), [email protected] (H. Mizukami).
plasma equipment for semiconductor manufacturing is designed not to generate hollow discharge w14,15x. We, however, have developed a hollow electrode high-density plasma generation technique with less plasma damage by modifying a well-known capacitively-coupled RF plasma reactor w16–18x. Using SiH4 and H2 gases, it is found that mc-Si thin films with good photosensitivity can be deposited rapidly. Maximum deposition rate of 4.9 nmys has been achieved. This shows that properly designed hollow discharge is well suited for semiconductor processing, which requires both low plasma damage and fast surface treatment. There are two distinctive features on our apparatus. One is chamber separation into a discharge space and a processing space. The other is the hollow structure prepared at the center of the counter electrode, which is placed between the RF electrode and the substrate. First, RF plasma is excited at the ‘normal’ capacitively-coupled RF plasma discharge space (discharge chamber). Next, this RF plasma is strongly enhanced inside the hollow structure by the hollow electrode discharge effect. Then plasma processing is carried out at the space (deposition cham-
0257-8972/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0257-8972(03)00664-9
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T. Tabuchi et al. / Surface and Coatings Technology 173 (2003) 243–248
ber) remote from the RF glow discharge space. The authors, therefore, think that this phenomenon is a sort of remote plasma enhanced by a hollow effect, and call this technique the hollow electrode enhanced plasma transportation (HEEPT). In this paper, the plasma characteristics and the properties of deposited films are both described.
Table 1 Growth conditions RF frequency Input power Pressure SiH4 H2 Tsub
13.56 MHz 20–100 W 0.4 Torr 7 sccm 100 sccm 300 8C
2. Apparatus A schematic diagram of the HEEPT reactor is shown in Fig. 1. The reactor is made of stainless steel, and its inner diameter is 210 mm. Both counter electrode and RF electrode are made of aluminum. The RF electrode has a showerhead structure for uniform distribution of gases and also has a cave structure of 5 mm in height. Its diameter is 180 mm and its thickness is 7 mm. The counter electrode has a diameter of 208 mm, thickness of 7 mm and it surrounds the RF electrode. The substrate holder is set up under the counter electrode and has a diameter of 160 mm, and it is grounded with both counter electrode and reactor wall. Substrates, which are placed on the substrate holder, can be heated during experiments. An orifice is prepared at the center of the counter electrode, and a straight aluminum tube (nozzle) is attached to it. The total length of this nozzle including the thickness of the counter electrode is 19 mm. Internal diameter of this nozzle (dA) is 13 mm. Processing gases flow into the discharge chamber through gas inlet holes opened at the RF electrode, each of which is 1 mm in diameter. When RF power (fs13.56 MHz) is applied to the RF electrode, a plasma is generated at the discharge chamber. This plasma is enhanced inside of the nozzle and reaches the substrate, which is located in the deposition chamber. The distance between counter electrode and RF electrode is 13 mm, and the distance from the bottom end of the nozzle to substrate holder is 15 mm. In order to observe the plasma emission shape of the discharge chamber, a small rectangular quartz
window is prepared in a small portion of the guard ring. This guard ring is attached directly to the counter electrode. 3. Experimental First, the relation between inner diameter of the nozzle (dA) and its plasma pattern was investigated with a flow rate of H2s100 sccm under a pressure of 0.4 Torr. The range of values of dA we studied was from 5 to 50 mm. Next, comparative studies of plasma characteristics and silicon thin film deposition were carried out under the conditions of Table 1 between our reactor (dAs13 mm) and a ‘normal’ capacitively-coupled (conventional) plasma CVD reactor. The reactor shown in Fig. 1 can be easily converted to the conventional plasma CVD by removing the counter electrode and guard ring. The distance between RF electrode and substrate holder, which plays as the grounded electrode in this conventional plasma CVD, is 47 mm. Concerning the equipment for plasma measurement, there is no change on its arrangement. After the observation of hydrogen plasma emission patterns, silicon thin film deposition and its plasma diagnostics were carried out. Corning 7059 glasses, thickness 1.1 mm, were used as substrates, which were heated up to 300 8C during deposition. The deposition rate, conductivity and crystallinity of silicon films were evaluated. The deposition rate was calculated from the thickness of the area just below the
Fig. 1. Schematic diagram of the HEEPT reactor.
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Fig. 2. H2 plasma emission near the nozzle on various dA, (a) 5 mm, (b) 13 mm, and (c) 50 mm under the conditions of 0.4 Torr and 30 W.
nozzle. Conductivity was measured by a two-terminal method. In order to make contact with films, aluminum electrodes were deposited in parallel at intervals of 1 mm using an electron beam evaporation technique. Photo-conductivity measurements were carried out under the condition of AM1.5, 100 mWycm2. Crystallinity of films was investigated with Raman spectroscopy and Xray diffraction. The Raman crystallinity, Ic yIa, was obtained from the intensity ratio for crystalline peak at 520 cmy1 (Ic) and amorphous peak at 480 cmy1 (Ia). The plasma emissions passed through the quartz window to an optical fiber for optical emission spectroscopy (OES). The optical axis of this fiber is tuned to become parallel to the substrate and also is adjusted to the position 10 mm above from the substrate. The intensities of OES were, therefore, obtained by integrating the emission from the processing space. OES from the plasma was measured for SiH* (412.7 nm) and Ha (656.3 nm). The RF voltage and self-bias of the RF electrode were measured by an electrical circuit w19x, which was inserted between the RF matching box and the RF electrode. 4. Results and discussion 4.1. Observation of plasma patterns Fig. 2 shows the relation between dA and plasma emission near the nozzle. When dA is 5 mm, no plasma can be seen either inside the nozzle or deposition chamber. On the other hand, in the case of dAs50 mm, there is a weak plasma in the deposition chamber, but no bright plasma excitation can be observed either near the orifice or inside the nozzle. However, when dA is 13 mm, a mushroom-shaped high intensity plasma,
which extends over the orifice and leads to the nozzle, is clearly observed. This high intensity plasma is located near the counter electrode and orifice of the discharge chamber and its diameter is approximately 50 mm. Another high intensity plasma also can be seen in the deposition chamber, especially near the exit of the nozzle, and it looks like diffusion from nozzle to chamber wall. Identical plasma emission was observed when hydrogen gas was simply shut off, confined in the reactor by a closed gate-valve, under a pressure of 0.4 Torr. In this study, the high intensity plasma with a mushroom shape was observed in the range of 10OdAO15 mm, and the brightest plasma was generated at dAs13 mm. With increasing dA up to 30 mm, emission intensity became weak gradually but the mushroom shape still remained. No enhanced plasma was seen either near or inside the nozzle for dA)40 mm. Concerning the intensity distribution of the plasma, no bright spot and no irregular discharge have been observed in the plasma. In addition, authors have also observed inside of the RF electrode under all process conditions through very small quartz window, which has been prepared at the side of RF electrode (not shown in Fig. 1). It has been confirmed that no plasma is induced inside the RF electrode through this work. Furthermore, it has also been confirmed that the same plasmas are induced when a flat aluminum plate, which has no hole for gas distribution, is used as the RF electrode. Hydrogen is supplied through the side of the reactor in this study. It is therefore obvious that the intense plasma of this system is caused by the hollow electrode effect of the orifice with the nozzle in the counter electrode. 4.2. Deposition of silicon thin films The silicon thin films are prepared from gas mixtures of SiH4 and H2. Fig. 3 shows the deposition rate, the
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Fig. 4. Raman profile of the mc-Si film of 1.5 mm in thickness deposited at 4.9 nmys on glass.
Fig. 3. The deposition rate and the Raman crystallinity (top), and the conductivity (bottom) as a function of the input RF power.
Raman crystallinity, Ic yIa, and the conductivity as a function of input RF power. In the case of HEEPT technique, the deposition rate increases rapidly up to 35 W and exceeds 4.0 nmys, and then almost saturates. The deposition rate attains a maximum of 4.9 nmys at 75 W. This speed is approximately ten times as faster than that observed in conventional plasma CVD. The film crystallinity, however, remains amorphous up to 50 W, and is improved drastically to Ic yIa of over 3 at 75 W. Similarly, the photo- and dark-conductivity indicates that the film structure suddenly changes from amorphous to microcrystal at 75 W. To the contrary, we can never obtain mc-Si films by the conventional plasma CVD in this study. Figs. 4 and 5 show the Raman and X-ray diffraction patterns, respectively, of mc-Si film of 1.5 mm in thickness deposited by our apparatus at 75 W. These results mean that it is possible for us to deposit photosensitive device grade mc-Si films at a high speed by utilizing hollow electrode discharge enhanced plasma.
techniques. One is the intensity itself of SiH*, and the other is the behavior of Ha intensity in the high power region of over 50 W. The Intensity of SiH* for the HEEPT technique is more than two times larger than that for conventional CVD, even though the intensity of Ha is nearly equal for both method. The only intensity of Ha for conventional CVD increases linearly with RF power, while the SiH* for conventional CVD saturates and two OES curves of HEEPT technique show a tendency to saturate over 50 W. The characteristics of the Ha ySiH* intensity ratio of both techniques is quite different in region where RF power is greater than 50 W. Concerning the HEEPT technique, not only deposition rate but also the Ha ySiH* ratio almost saturate above 50 W in spite of the fact that both Ha and SiH* intensities in the high power region continue increasing with RF power. These results indicate that silicon film is being etched by hydrogen radicals nearly as fast as it is being deposited in the high power region. The RF power dependence of the plasma potential is shown in Fig. 7. The plasma potential is calculated by
4.3. Diagnostic study of the plasma Fig. 6 shows the RF power dependence of Ha, SiH* intensities and the Ha ySiH* intensity ratio obtained from both the HEEPT and conventional plasma technique. There are two striking differences between the
Fig. 5. X-ray diffraction pattern of the mc-Si film of 1.5 mm in thickness deposited at 4.9 nmys on glass.
T. Tabuchi et al. / Surface and Coatings Technology 173 (2003) 243–248
247
ratio are almost same. This power region corresponds to the region where both deposition rate and Ha ySiH* ratio almost saturate. Furthermore, we can obtain crystallized silicon thin films under higher plasma potential using the HEEPT technique. It is thought, therefore, that not only high-density radical generation for deposition but also high-speed etching by hydrogen radicals accompanied with some degrees of ion attacking from the plasma contribute to the crystallization of films at a deposition speed of 4.9 nmys. 5. Conclusions It was demonstrated that a hollow electrode enhanced high intensity plasma can be induced with the proper size of nozzle diameter attached to the counter electrode, which divides the reactor into a discharge space and a processing space. Highly crystallized and photo-conductive mc-Si films have been fabricated by SiH4 and H2 at a high deposition rate of 4.9 nmys, using this hollow electrode plasma at 13.56 MHz. Comparative studies of OES and plasma potential between the newly developed apparatus and conventional capacitively-coupled plasma CVD were carried out. The results suggest that a hollow electrode enhanced plasma has high intensity SiH* emission, low Ha ySiH* ratio and high plasma potential for the fast deposition of device grade mc-Si. Fig. 6. Optical emission intensities of Ha and SiH* (top), and the intensity ratio of HaySiH* (bottom) as a function of the input RF Power.
Acknowledgments
the following equation:
The authors wish to thank K. Ishida for his technical assistance, Y. Toyoshima for the valuable discussion, S. Sano and K. Ogaki for their continuous encouragement.
VssŽVrfqVdc.y2
(1)
where Vs is the plasma potential, Vrf is the RF voltage (which is equal to half the value of the RF peak to peak voltage, that is Vp–p) and Vdc is the self-bias voltage. The plasma potential of the HEEPT technique is approximately 50 V higher than that of the conventional plasma CVD, and in both methods increase slowly when the RF power is increased. As SiH* intensity is very strong in the HEEPT technique, it is thought that the density of radicals, which contribute to deposition, is higher in this plasma. We believe that this is the reason why we can achieve high speed deposition. Especially, in the HEEPT technique, compared to the conventional plasma CVD, SiH* radicals are generated more effectively than hydrogen radicals by increasing the RF power. We speculate that this is due to the difference of electron temperature between the HEEPT technique and the conventional one. On the other hand, the film structure remains amorphous at 50 W and turns to microcrystalline at 75 W even though both the deposition rate and the Ha ySiH*
References w1x C. Smit, E.A.G. Hamers, B.A. Korevaar, R.A.C.M.M. van Swaaij, M.C.M. van de Sanden, J. Non-Cryst. Solids 299– 302 (2002) 98.
Fig. 7. Plasma potential as a function of the input RF power.
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w2x H. Shirai, Y. Sakuma, H. Ueyama, Thin Solid Films 345 (1999) 7. w3x J. Meier, R. Fluckiger, H. Keppner, S. Shah, Appl. Phys. Lett. 65 (1994) 860. w4x F. Finger, P. Hapke, M. Luysberg, R. Carius, H. Wagner, M. Scheib, Appl. Phys. Lett. 65 (1994) 2588. w5x S. Oda, J. Noda, M. Matsumura, Jpn. J. Appl. Phys. 29 (1990) 1889. w6x K. Ro, K. Nakahata, T. Kamiya, C.M. Fortmann, I. Shimizu, J. Non-Cryst. Solids 266–269 (2000) 1088. w7x H. Matsumura, Jpn. J. Appl. Phys. 30 (1991) L1522. w8x T. Oshima, K. Abe, A. Yamada, M. Konagai, Jpn. J. Appl. Phys. 34 (1995) L1425. w9x V.I. Miljevic, Appl. Opt. 23 (1984) 1598. w10x A. Anders, S. Anders, Plasma Sources Sci. Technol. 4 (1995) 571.
w11x L. Bardos, Surf. Coat. Technol. 86–87 (1996) 648. w12x A. Hellmich, T. Jung, A. Kielhorn, M. Ribland, Surf. Coat. Technol. 98 (1998) 1541. w13x H. Barankova, L. Bardos, Surf. Coat. Technol. 120–121 (1999) 704. w14x K. Takeda, H. Siraku, H. Nishiwaki, Japan Patent No. 2895768, March 1999. w15x S. Handa, H. Fujinaka, in: M. Sugawara (Ed.), Practical Dry Etching Technology, Realize Inc, Tokyo, Japan, 1992, pp. 125–126, in Japanese. w16x T. Tabuchi, M. Takashiri, K. Ishida, H. Mizukami, Y. Toyoshima, S. Fujimoto, Japan Patent P2000-66106, March 2000. w17x T. Tabuchi, K. Ishida, H. Mizukami, M. Takashiri, Japan Patent P2000-83838, March 2000. w18x T. Tabuchi, K. Ishida, H. Mizukami, M. Takashiri, US Patent 09y730,813 December 2000. w19x J.W. Coburn, E. Kay, J. Appl. Phys. 43 (1972) 4965.
Hollow electrode enhanced RF glow plasma for the fast deposition of microcrystalline silicon Toshihiro Tabuchi*, Masayuki Takashiri, Hiroyuki Mizukami Research Division, Electronic Material Research Department, Komatsu Ltd, 1200 Manda, Hiratsuka, Kanagawa 254-8567, Japan Received 6 December 2002; accepted in revised form 24 March 2003
Abstract A hollow electrode enhanced RF glow plasma excitation technique has been newly developed. In this technique, the reactor is divided into a capacitively-coupled RF glow discharge space and a processing space by counter electrode, which has a hollow structure and is placed between RF electrode and substrate. Hollow electrode discharge is induced inside this hollow structure. The application of this discharge type for semiconductor processing is studied in the case of plasma enhanced chemical vapor deposition of hydrogenated microcrystalline silicon (mc-Si) thin films. It is found that high crystallinity, photo sensitivity and maximum deposition rate of 4.9 nmys can be achieved at the plasma excitation frequency of 13.56 MHz. Properties of this plasma are investigated by observation of the plasma emission pattern viewed through the window, optical emission spectral analysis and the plasma potential. Plasmas generated inside the new apparatus is compared to those of normal capacitivelycoupled RF plasmas. In our apparatus, high intensity plasma emission is observed near and inside the hollow structure attached to the counter electrode. The spectral and plasma potential analysis showed that both the intensity of SiH* and the plasma potential of our plasma are higher than that of normal capacitively-coupled plasma. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Hollow electrode; Plasma CVD; RF glow discharge; High-density plasma; mc-Si thin film; Fast deposition; Low temperature
1. Introduction High-density plasma and its application to semiconductor fabrication processing are still under development all over the world. In order to achieve both an improvement of the processing speed and a reduction of the plasma damage to films or substrates, many novel plasma apparatuses w1,2x, such as the very high frequency plasma technique w3–6x or hotwire chemical vapor deposition (CVD) w7,8x, have been proposed. Hollow discharge w9,10x can generate much higher density plasma and has been applied for surface coating w11–13x. On the other hand, hollow discharge has been considered to be too harmful for semiconductor processing such as amorphous silicon thin film deposition for solar cells or thin film transistors, because it may cause serious damage to films or substrates. This is the reason why *Corresponding author. Tel.: q81-463-35-9323; fax: q81-463-359287. E-mail addresses: [email protected] (T. Tabuchi), [email protected] (M. Takashiri), [email protected] (H. Mizukami).
plasma equipment for semiconductor manufacturing is designed not to generate hollow discharge w14,15x. We, however, have developed a hollow electrode high-density plasma generation technique with less plasma damage by modifying a well-known capacitively-coupled RF plasma reactor w16–18x. Using SiH4 and H2 gases, it is found that mc-Si thin films with good photosensitivity can be deposited rapidly. Maximum deposition rate of 4.9 nmys has been achieved. This shows that properly designed hollow discharge is well suited for semiconductor processing, which requires both low plasma damage and fast surface treatment. There are two distinctive features on our apparatus. One is chamber separation into a discharge space and a processing space. The other is the hollow structure prepared at the center of the counter electrode, which is placed between the RF electrode and the substrate. First, RF plasma is excited at the ‘normal’ capacitively-coupled RF plasma discharge space (discharge chamber). Next, this RF plasma is strongly enhanced inside the hollow structure by the hollow electrode discharge effect. Then plasma processing is carried out at the space (deposition cham-
0257-8972/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0257-8972(03)00664-9
244
T. Tabuchi et al. / Surface and Coatings Technology 173 (2003) 243–248
ber) remote from the RF glow discharge space. The authors, therefore, think that this phenomenon is a sort of remote plasma enhanced by a hollow effect, and call this technique the hollow electrode enhanced plasma transportation (HEEPT). In this paper, the plasma characteristics and the properties of deposited films are both described.
Table 1 Growth conditions RF frequency Input power Pressure SiH4 H2 Tsub
13.56 MHz 20–100 W 0.4 Torr 7 sccm 100 sccm 300 8C
2. Apparatus A schematic diagram of the HEEPT reactor is shown in Fig. 1. The reactor is made of stainless steel, and its inner diameter is 210 mm. Both counter electrode and RF electrode are made of aluminum. The RF electrode has a showerhead structure for uniform distribution of gases and also has a cave structure of 5 mm in height. Its diameter is 180 mm and its thickness is 7 mm. The counter electrode has a diameter of 208 mm, thickness of 7 mm and it surrounds the RF electrode. The substrate holder is set up under the counter electrode and has a diameter of 160 mm, and it is grounded with both counter electrode and reactor wall. Substrates, which are placed on the substrate holder, can be heated during experiments. An orifice is prepared at the center of the counter electrode, and a straight aluminum tube (nozzle) is attached to it. The total length of this nozzle including the thickness of the counter electrode is 19 mm. Internal diameter of this nozzle (dA) is 13 mm. Processing gases flow into the discharge chamber through gas inlet holes opened at the RF electrode, each of which is 1 mm in diameter. When RF power (fs13.56 MHz) is applied to the RF electrode, a plasma is generated at the discharge chamber. This plasma is enhanced inside of the nozzle and reaches the substrate, which is located in the deposition chamber. The distance between counter electrode and RF electrode is 13 mm, and the distance from the bottom end of the nozzle to substrate holder is 15 mm. In order to observe the plasma emission shape of the discharge chamber, a small rectangular quartz
window is prepared in a small portion of the guard ring. This guard ring is attached directly to the counter electrode. 3. Experimental First, the relation between inner diameter of the nozzle (dA) and its plasma pattern was investigated with a flow rate of H2s100 sccm under a pressure of 0.4 Torr. The range of values of dA we studied was from 5 to 50 mm. Next, comparative studies of plasma characteristics and silicon thin film deposition were carried out under the conditions of Table 1 between our reactor (dAs13 mm) and a ‘normal’ capacitively-coupled (conventional) plasma CVD reactor. The reactor shown in Fig. 1 can be easily converted to the conventional plasma CVD by removing the counter electrode and guard ring. The distance between RF electrode and substrate holder, which plays as the grounded electrode in this conventional plasma CVD, is 47 mm. Concerning the equipment for plasma measurement, there is no change on its arrangement. After the observation of hydrogen plasma emission patterns, silicon thin film deposition and its plasma diagnostics were carried out. Corning 7059 glasses, thickness 1.1 mm, were used as substrates, which were heated up to 300 8C during deposition. The deposition rate, conductivity and crystallinity of silicon films were evaluated. The deposition rate was calculated from the thickness of the area just below the
Fig. 1. Schematic diagram of the HEEPT reactor.
T. Tabuchi et al. / Surface and Coatings Technology 173 (2003) 243–248
245
Fig. 2. H2 plasma emission near the nozzle on various dA, (a) 5 mm, (b) 13 mm, and (c) 50 mm under the conditions of 0.4 Torr and 30 W.
nozzle. Conductivity was measured by a two-terminal method. In order to make contact with films, aluminum electrodes were deposited in parallel at intervals of 1 mm using an electron beam evaporation technique. Photo-conductivity measurements were carried out under the condition of AM1.5, 100 mWycm2. Crystallinity of films was investigated with Raman spectroscopy and Xray diffraction. The Raman crystallinity, Ic yIa, was obtained from the intensity ratio for crystalline peak at 520 cmy1 (Ic) and amorphous peak at 480 cmy1 (Ia). The plasma emissions passed through the quartz window to an optical fiber for optical emission spectroscopy (OES). The optical axis of this fiber is tuned to become parallel to the substrate and also is adjusted to the position 10 mm above from the substrate. The intensities of OES were, therefore, obtained by integrating the emission from the processing space. OES from the plasma was measured for SiH* (412.7 nm) and Ha (656.3 nm). The RF voltage and self-bias of the RF electrode were measured by an electrical circuit w19x, which was inserted between the RF matching box and the RF electrode. 4. Results and discussion 4.1. Observation of plasma patterns Fig. 2 shows the relation between dA and plasma emission near the nozzle. When dA is 5 mm, no plasma can be seen either inside the nozzle or deposition chamber. On the other hand, in the case of dAs50 mm, there is a weak plasma in the deposition chamber, but no bright plasma excitation can be observed either near the orifice or inside the nozzle. However, when dA is 13 mm, a mushroom-shaped high intensity plasma,
which extends over the orifice and leads to the nozzle, is clearly observed. This high intensity plasma is located near the counter electrode and orifice of the discharge chamber and its diameter is approximately 50 mm. Another high intensity plasma also can be seen in the deposition chamber, especially near the exit of the nozzle, and it looks like diffusion from nozzle to chamber wall. Identical plasma emission was observed when hydrogen gas was simply shut off, confined in the reactor by a closed gate-valve, under a pressure of 0.4 Torr. In this study, the high intensity plasma with a mushroom shape was observed in the range of 10OdAO15 mm, and the brightest plasma was generated at dAs13 mm. With increasing dA up to 30 mm, emission intensity became weak gradually but the mushroom shape still remained. No enhanced plasma was seen either near or inside the nozzle for dA)40 mm. Concerning the intensity distribution of the plasma, no bright spot and no irregular discharge have been observed in the plasma. In addition, authors have also observed inside of the RF electrode under all process conditions through very small quartz window, which has been prepared at the side of RF electrode (not shown in Fig. 1). It has been confirmed that no plasma is induced inside the RF electrode through this work. Furthermore, it has also been confirmed that the same plasmas are induced when a flat aluminum plate, which has no hole for gas distribution, is used as the RF electrode. Hydrogen is supplied through the side of the reactor in this study. It is therefore obvious that the intense plasma of this system is caused by the hollow electrode effect of the orifice with the nozzle in the counter electrode. 4.2. Deposition of silicon thin films The silicon thin films are prepared from gas mixtures of SiH4 and H2. Fig. 3 shows the deposition rate, the
246
T. Tabuchi et al. / Surface and Coatings Technology 173 (2003) 243–248
Fig. 4. Raman profile of the mc-Si film of 1.5 mm in thickness deposited at 4.9 nmys on glass.
Fig. 3. The deposition rate and the Raman crystallinity (top), and the conductivity (bottom) as a function of the input RF power.
Raman crystallinity, Ic yIa, and the conductivity as a function of input RF power. In the case of HEEPT technique, the deposition rate increases rapidly up to 35 W and exceeds 4.0 nmys, and then almost saturates. The deposition rate attains a maximum of 4.9 nmys at 75 W. This speed is approximately ten times as faster than that observed in conventional plasma CVD. The film crystallinity, however, remains amorphous up to 50 W, and is improved drastically to Ic yIa of over 3 at 75 W. Similarly, the photo- and dark-conductivity indicates that the film structure suddenly changes from amorphous to microcrystal at 75 W. To the contrary, we can never obtain mc-Si films by the conventional plasma CVD in this study. Figs. 4 and 5 show the Raman and X-ray diffraction patterns, respectively, of mc-Si film of 1.5 mm in thickness deposited by our apparatus at 75 W. These results mean that it is possible for us to deposit photosensitive device grade mc-Si films at a high speed by utilizing hollow electrode discharge enhanced plasma.
techniques. One is the intensity itself of SiH*, and the other is the behavior of Ha intensity in the high power region of over 50 W. The Intensity of SiH* for the HEEPT technique is more than two times larger than that for conventional CVD, even though the intensity of Ha is nearly equal for both method. The only intensity of Ha for conventional CVD increases linearly with RF power, while the SiH* for conventional CVD saturates and two OES curves of HEEPT technique show a tendency to saturate over 50 W. The characteristics of the Ha ySiH* intensity ratio of both techniques is quite different in region where RF power is greater than 50 W. Concerning the HEEPT technique, not only deposition rate but also the Ha ySiH* ratio almost saturate above 50 W in spite of the fact that both Ha and SiH* intensities in the high power region continue increasing with RF power. These results indicate that silicon film is being etched by hydrogen radicals nearly as fast as it is being deposited in the high power region. The RF power dependence of the plasma potential is shown in Fig. 7. The plasma potential is calculated by
4.3. Diagnostic study of the plasma Fig. 6 shows the RF power dependence of Ha, SiH* intensities and the Ha ySiH* intensity ratio obtained from both the HEEPT and conventional plasma technique. There are two striking differences between the
Fig. 5. X-ray diffraction pattern of the mc-Si film of 1.5 mm in thickness deposited at 4.9 nmys on glass.
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ratio are almost same. This power region corresponds to the region where both deposition rate and Ha ySiH* ratio almost saturate. Furthermore, we can obtain crystallized silicon thin films under higher plasma potential using the HEEPT technique. It is thought, therefore, that not only high-density radical generation for deposition but also high-speed etching by hydrogen radicals accompanied with some degrees of ion attacking from the plasma contribute to the crystallization of films at a deposition speed of 4.9 nmys. 5. Conclusions It was demonstrated that a hollow electrode enhanced high intensity plasma can be induced with the proper size of nozzle diameter attached to the counter electrode, which divides the reactor into a discharge space and a processing space. Highly crystallized and photo-conductive mc-Si films have been fabricated by SiH4 and H2 at a high deposition rate of 4.9 nmys, using this hollow electrode plasma at 13.56 MHz. Comparative studies of OES and plasma potential between the newly developed apparatus and conventional capacitively-coupled plasma CVD were carried out. The results suggest that a hollow electrode enhanced plasma has high intensity SiH* emission, low Ha ySiH* ratio and high plasma potential for the fast deposition of device grade mc-Si. Fig. 6. Optical emission intensities of Ha and SiH* (top), and the intensity ratio of HaySiH* (bottom) as a function of the input RF Power.
Acknowledgments
the following equation:
The authors wish to thank K. Ishida for his technical assistance, Y. Toyoshima for the valuable discussion, S. Sano and K. Ogaki for their continuous encouragement.
VssŽVrfqVdc.y2
(1)
where Vs is the plasma potential, Vrf is the RF voltage (which is equal to half the value of the RF peak to peak voltage, that is Vp–p) and Vdc is the self-bias voltage. The plasma potential of the HEEPT technique is approximately 50 V higher than that of the conventional plasma CVD, and in both methods increase slowly when the RF power is increased. As SiH* intensity is very strong in the HEEPT technique, it is thought that the density of radicals, which contribute to deposition, is higher in this plasma. We believe that this is the reason why we can achieve high speed deposition. Especially, in the HEEPT technique, compared to the conventional plasma CVD, SiH* radicals are generated more effectively than hydrogen radicals by increasing the RF power. We speculate that this is due to the difference of electron temperature between the HEEPT technique and the conventional one. On the other hand, the film structure remains amorphous at 50 W and turns to microcrystalline at 75 W even though both the deposition rate and the Ha ySiH*
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Fig. 7. Plasma potential as a function of the input RF power.
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