- Email: [email protected]
Spatial distribution of high-density microwave plasma for fast deposition of microcrystalline silicon film
Solar Energy Materials & Solar Cells 66 (2001) 137}145
Spatial distribution of high-density microwave plasma for fast deposition of microcrystalline silicon "lm Hajime Shirai *, Yoshikazu Sakuma , Koichi Yoshino , Hiroyuki Ueyama Department of Functional Materials Science, Faculty of Engineering, Saitama University, 255 Shimo-Okubo, Urawa, Saitama 338-8570, Japan Nihon Koshuha Co., Ltd., 1119 Nakayama, Midori-ku, Yokohama, Kanagawa 226-0011, Japan
Abstract The high-density microwave plasma utilizing a spokewise antenna was successfully applied to fast deposition of highly crystallized and photoconductive microcrystalline silicon (lc-Si:H) "lm at low temperatures. Among various deposition parameters, spatial distribution of ion energy (IDF) mainly determines "lm crystallinity. The best crystallinity was obtained at the axial distance, Z from the quartz glass plate, where the spread of mean ion energy is minimum. By optimizing the axial distance, Z and total pressure, highly crystallized and photoconductive lc-Si:H "lm could be fabricated with a high deposition rate of 47 As /s at &50 mTorr in SiH and Ar plasma. 2001 Elsevier Science B.V. All rights reserved. Keywords: Microcrystalline silicon; High density; Low temperature; Microwave plasma; Spokewise antenna; Fast deposition; Ion energy distribution
1. Introduction Hydrogenated microcrystalline silicon (lc-Si:H) has recently been given much attention because it is one of the most promising materials for thin-"lm solar cells of lower cost, higher conversion e$ciency and better stability than amorphous-siliconbased solar cells. In reactive plasmas widely used for silicon thin-"lm growth, generally, there are e!ects of electron density, n and temperature, ¹ on the products * Corresponding author. Fax: #81-48-858-3676. E-mail address: [email protected] (H. Shirai). 0927-0248/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 4 8 ( 0 0 ) 0 0 1 6 6 - 5
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obtained from such processings. Higher n promotes the dissociation of mother molecule and it is desirable for enhancing the deposition rate. So far, to this aim, electron-cyclotron-resonance (ECR) and helicon wave plasma sources were applied for fast deposition of a-Si:H, a-SiGe :H and lc-SiC alloy "lms [1,2]. However, higher V V ¹ promotes excess secondary reaction and ionization in the plasma, which causes damage to the growing surface and interface. Therefore, a novel high-density and low-temperature plasma source is strongly required for precise control of gas-phase reaction at lower pressure condition. This is because higher generation e$ciency of neutral radicals such as SiH is expected in SiH plasma without excess secondary reaction. Several high-density and low-temperature plasmas without use of magnetic "eld have been applied for ULSI technologies, i.e., inductive coupling plasma (ICP), surface wave plasma (SWP) and ultra-high-frequency (UHF: 500 MHz) plasma [3}5]. We have applied a novel high-density and low-temperature microwave plasma (2.45 GHz) utilizing a spokewise antenna for fast deposition of lc-Si:H "lm. The plasma maintains a uniform state with high n '10 cm\ and a low ¹ of 2 eV in Ar plasma [6]. In addition, we reported that a highly crystallized and photoconductive lc-Si:H(Cl) "lm can be obtained from dichlorosilane, SiH Cl at a high depos ition rate of 20 As /s [7]. In this study, we demonstrate the fast deposition of highly crystallized and photoconductive lc-Si:H "lm at '40 As /s in SiH and Ar plasma. In particular, the spatial distribution of microwave plasma for fast deposition of highly crystallized lc-Si:H "lm is documented.
2. Experimental The lc-Si:H "lms have been deposited using a high-density microwave discharge utilizing a spokewise antenna with an outer diameter of 22 cm. The details of the deposition system are described elsewhere [5]. The microwave power is supplied to the spokewise antennas. The antenna's spoke length is "xed at 4 cm, that is, of the microwave length. A microwave power is supplied to the antenna assembly and radiated into the discharge chamber through a 15 mm thick quartz plate. Optical emission spectroscopy (OES) was employed using a periscope, which collects photons through a 3 mm diameter optical "ber leading to a monochromator. The spatial distribution of Ar I 750.4 nm emission intensity, corresponding to a threshold energy of 13.5 eV. The n and ¹ were determined in Ar plasma by means of a standard Langmuir probe technique as well as saturation ion current. The spatial distribution of ion energy was also examined using an electrostatic analyzer (Faraday cup) (10 mm in diameter) which is composed of four electrodes at di!erent Z positions and pressures. The distance between the grids was 0.5 mm. The deposition conditions of "lm included the #ow rates of SiH , Fr(SiH ): 5}30 sccm, Fr(H ): 0}50 sccm and Fr(Ar): 5 sccm and substrate temperature, ¹ : 200}2503C, total pressure: 30}200 mTorr and microwave power, P : 600 W. The "lm
structure was characterized by X-ray di!raction (XRD), spectroscopic ellipsometry (SE), Raman spectroscopy and atomic force microscopy (AFM). The residual hydrogen content in the "lms was measured by Fourier transform infrared spectroscopy
H. Shirai et al. / Solar Energy Materials & Solar Cells 66 (2001) 137}145
139
Table 1 Fr(SiH ) Fr(H ) Fr(Ar) ¹ P
Pressure
10 sccm 10 sccm 5 sccm 2003C 600 W &50 mTorr
(FTIR). Dark and photoconductivity measurements were also performed with a 100 lm gap coplanar Al electrode.
3. Results and discussion Figs. 1(a) and (b) show the deposition rate and the Raman spectra of lc-Si:H "lms fabricated at di!erent Z positions, respectively. The deposition conditions used in this study are summarized in Table 1. The deposition rate slightly increases with a shortening of the axial distance, Z from 15 to 4 cm, accompanied with an improvement of "lm crystallinity as shown in Fig. 1(b). In addition, the degree of surface roughness decreases with decreasing the axial distance, Z, which is con"rmed by SE and AFM observations. FTIR study also revealed that residual hydrogen content in the "lm monotonically decreases with shortening the axial distance, Z, though no change of the SiH (n"1, 2) bonds con"guration was observed. Thus, we found that L highly crystallized lc-Si:H "lm was fabricated with less surface roughness and hydrogen content at Z"6 cm at a high deposition rate of 16 As /s. To better understand the e!ect of the axial distance, Z on the spatial distribution of the plasma, the OES intensity was measured in Ar plasma by a periscope, which collects photons along a 3 mm diameter beam line and leads to a spectrometer. We monitored the OES emission intensity of Ar I (750.4 nm, 4s P4p ), corresponding to a threshold energy of 13.5 eV. Figs. 2(a) and (b) show the axial distributions of the OES Ar I intensity and probe saturation ion current, respectively, at di!erent pressure conditions. The OES intensity exponentially decreases with an increase in the axial distance, Z, at each pressure condition. Another feature is that the OES intensity decreases with decreasing pressure near the quartz glass plate, whereas it increases with decreasing the pressure in the Z'6 cm position. Thus, the pressure dependence of the OES emission intensity depends on the Z position. The integrated intensity is also shown in the inset. The integrated intensity has a maximum at &50 mTorr, at which the highest deposition rate is obtained [8]. On the other hand, saturation ion current slightly increases with increasing the axial distance, Z and it shows a maximum at the Z&6 cm position at a pressure of 50 mTorr, though its pressure dependence is not so systematic compared with that of OES. In addition, to study the axial distribution of plasma more precisely, a H and Ar mixture plasma exposure experiment was conducted on c-Si wafer at di!erent Z
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Fig. 1. (a) Deposition rate and (b) Raman spectra of lc-Si:H "lms deposited at di!erent axial distances, Z from the quartz glass plate under the same plasma condition.
positions. Fig. 3 shows the imaginary part of pseudodielectric function, 1e 2 spectra before and after 10 min plasma exposure at di!erent Z positions. The magnitude of 1e 2 drastically decreases at Z"4 and 15 cm compared with that at Z"6}10 cm. Generally, the magnitude of 1e 2 is tightly correlated with the surface roughness and bulk void fraction. Therefore, the damage of the c-Si wafer by Ar plasma is lower at Z&6}10 cm than that at Z(4 cm and Z'10 cm positions. These results originate from the axial distribution of plasma parameters, i.e., n and ¹ . However, it is still controversial why the highest deposition rate and highest crystallinity are obtained at the Z"&6 cm position (Fig. 1).
H. Shirai et al. / Solar Energy Materials & Solar Cells 66 (2001) 137}145
141
Fig. 2. Axial distributions of (a) OES Ar I signal intensity (750.4 nm), corresponding to a threshold energy of 13.5 eV and (b) probe saturation ion current measured at di!erent pressure conditions.
Further detailed study was made by measuring the axial distribution of ion energy (IED) in Ar plasma using an electrostatic analyzer (Faraday cup). Figs. 4(a) and (b) show "rst derivative, dI /d<, curves of Faraday cup characteristics measured at di!erent Z positions and pressure conditions, respectively [9]. The measurements were made under the conditions of (a) &50 mTorr and (b) Z"6 cm position. It is seen that IED provides an almost Maxwellian EDF with the narrowest half-width at (50 mTorr and Z"6 cm, while IED spreads toward the lower energy region, forming two peaks at Z(8 cm and higher pressure conditions, although the mean ion energy is almost independent of the Z position. The conditions at which the narrowest IED is observed almost agree with those for the highest "lm crystallinity obtained (Fig. 1(b)) Thus, lowered ion energy and its narrower distribution are essential for obtaining higher crystallinity. We found that the highest "lm crystallinity was
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Fig. 3. The imaginary part of pseudodielectric function, 1e 2 spectra of c-Si wafer before and after 10 min exposure of H (10 sccm) and Ar (5 sccm) mixture plasma at di!erent Z positions.
obtained at the Z"6 cm position at &50 mTorr with higher deposition rate, at which the spread of ion energy distribution (IED) is minimum in this plasma. Similar concepts have been reported in the growth of a-Si:H "lm with high hole mobility. A high hole mobility of &0.3 cm/V s, which is increased by two orders of magnitude compared to the conventional value, was attained by an intentional control of the combination of RF potential #uctuation and electron and ion temperatures by using a biased mesh grid [10]. At this stage, we have an idea that "lm crystallinity is also improved by using a similar concept in the growth of not only a-Si:H but also lc-Si:H. However, further detailed study is needed on why the spread of IED becomes narrower at Z"&6 cm and pressure of &50 mTorr in this high-density microwave plasma. To improve the deposition rate and "lm crystallinity, deposition study was carried out by increasing Fr(SiH ) at a pressure of &50 mTorr. As reported previously, hydrogen addition deteriorates "lm crystallinity in this plasma [8]. Therefore, in the following, "lm deposition was carried out using SiH and Ar mixture plasma. Fig. 5(a) shows the "lm deposition rate and OES intensity, SiH plotted as a function of Fr(SiH ) under a steady #ow of 5 sccm Ar plasma. Results are shown at Z"10 and 6 cm positions as symbols, E and 夹, respectively. The deposition rate increases linearly with increasing Fr(SiH ) and no saturation can be observed up to a 30 sccm supply of SiH . Thus, we can expect further increase of the deposition rate. The Raman spectra show high crystallinity, though the XRD crystal orientation is random as shown in Fig. 5(b). Film crystallinity can be improved by increasing the ¹ of 2503C, maintain ing a high deposition rate of 40 As /s at Z"10 cm. As a consequence, a high deposition rate is achieved up to 40 and 47 As /s at Z"10 and 6 cm positions, respectively, under the Fr(SiH )"30 sccm and pressure: &50 mTorr condition. The optical absorption
H. Shirai et al. / Solar Energy Materials & Solar Cells 66 (2001) 137}145
143
Fig. 4. First derivative of Faraday cup characteristics in 10 sccm pure Ar plasma at (a) di!erent Z positions (50 mTorr) and (b) pressure (Z"6 cm) conditions.
spectra in these "lms are almost identical to those of c-Si, though it shifts to near-infrared region, which is due to the surface roughness and/or bulk inhomogeneities. Dark and photoconductivities are in the range of 10\ and 10}}10\ S/cm under 100 mW/cm illumination, respectively. In addition, this high-density microwave plasma source is available for further scale up of the chamber size by designing the antenna's con"guration. This is because the concept is based on the interdigital "lter, though the antenna's spoke length is "xed at 4 cm. Recently, a novel UHF (500 MHz) plasma source has been applied for ULSI technologies and it is more suitable than microwave (2.45 GHz) for enlargement of the chamber size, because each antenna's length can be designed at longer than that of microwave [11]. However, it is still very expensive to use the 500 MHz system, because the magnetron for UHF (500 MHz) plasma is not generally commercialized except the application to telecommunication system. Additionally, lower cost and
144
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Fig. 5. (a) Deposition rate and the OES signal intensity, SiH plotted as a function of Fr(SiH ) at a total pressure of &50 mTorr. (b) Raman spectra of lc-Si:H "lms deposited at each corresponding Z position. The symbols, E and 夹 denote the results at Z"10 and 6 cm, respectively. The microwave power and Fr(Ar) are constant at 600 W and 5 sccm, respectively.
a simpli"ed deposition system are strongly required and available for practical production of a more than 1-m-sized solar cell. Therefore, the high-density and low-temperature microwave plasma utilizing a spokewise antenna has a high potential for large-area electronic processings in the next generation such as solar cell devices.
4. Conclusions Highly crystallized and photoconductive lc-Si:H "lm was fabricated at high deposition rates of up to 47 As /s in SiH and Ar mixture plasma. Higher "lm crystallinity
H. Shirai et al. / Solar Energy Materials & Solar Cells 66 (2001) 137}145
145
was obtained at the Z"6 cm position with higher deposition rate under the condition at which the spread of ion energy distribution (IDF) is minimum. The high-density and low-temperature microwave plasma demonstrated here has a high potential for practical production of solar cell devices in the next generation.
Acknowledgements The authors would like to thank Miss. Wasai, Mrs. N. Nabatova-Gabin and Mr. K. Kainuma in Atago Co., Ltd. for allowing the use of Raman spectrometer and spectroscopic ellipsometer. This work is in part supported by Suzuki Foundation and Nippon Sheet Glass Foundation for Materials Science and Engineering.
References [1] T. Watanabe, M. Tanaka, K. Azuma, M. Nakatani, T. Sonobe, T. Shimada, Jpn. J. Appl. Phys. 26 (1987) L288. [2] Y. Matsumoto, G. Hirata, H. Takakura, H. Okamoto, H. Hamakawa, J. Appl. Phys. 67 (1990) 6538. [3] M. Goto, H. Toyoda, T. Kojima, M. Nishitani, M. Kitagawa, H. Yamazoe, H. Sugai, Jpn. J. Appl. Phys. 38 (1999) 3655. [4] M. Goto, M. Toyoda, M. Kitagawa, T. Hirao, H. Sugai, Jpn. J. Appl. Phys. 34 (1997) 3714. [5] R. Yoshida, S. Sumiya, B. Mebarki, M. Ito, M. Hori, T. Goto, S. Samukawa, T. Tsukada, Proceedings of Dry process Symposium (IEEE of Japan), 1998, p. 73. [6] H. Shirai, T. Arai, H. Ueyama, Jpn. J. Appl. Phys. 37 (1998) 1078. [7] H. Shirai, Y. Sakuma, H. Ueyama, Thin Film Solids 345 (1999) 7. [8] H. Shirai, Y. Sakuma, Y. Moriya, C. Fukai, H. Ueyama, Jpn. J. Appl. Phys. 38 (1999) 6629. [9] Y. Ohtsu, K. Mori, H. Fujita, Jpn. J. Appl. Phys. 38 (1999) 4393. [10] K. Kato, S. Iizuka, G. Ganguly, T. Ikeda, A. Matsuda, N. Sato, Jpn. J. Appl. Phys. 36 (1997) 4547. [11] S. Samukawa, Y. Nakagawa, T. Tsukada, H. Ueyama, K. Shinohara, Jpn. J. Appl. Phys. 34 (1995) 6805.
Spatial distribution of high-density microwave plasma for fast deposition of microcrystalline silicon "lm Hajime Shirai *, Yoshikazu Sakuma , Koichi Yoshino , Hiroyuki Ueyama Department of Functional Materials Science, Faculty of Engineering, Saitama University, 255 Shimo-Okubo, Urawa, Saitama 338-8570, Japan Nihon Koshuha Co., Ltd., 1119 Nakayama, Midori-ku, Yokohama, Kanagawa 226-0011, Japan
Abstract The high-density microwave plasma utilizing a spokewise antenna was successfully applied to fast deposition of highly crystallized and photoconductive microcrystalline silicon (lc-Si:H) "lm at low temperatures. Among various deposition parameters, spatial distribution of ion energy (IDF) mainly determines "lm crystallinity. The best crystallinity was obtained at the axial distance, Z from the quartz glass plate, where the spread of mean ion energy is minimum. By optimizing the axial distance, Z and total pressure, highly crystallized and photoconductive lc-Si:H "lm could be fabricated with a high deposition rate of 47 As /s at &50 mTorr in SiH and Ar plasma. 2001 Elsevier Science B.V. All rights reserved. Keywords: Microcrystalline silicon; High density; Low temperature; Microwave plasma; Spokewise antenna; Fast deposition; Ion energy distribution
1. Introduction Hydrogenated microcrystalline silicon (lc-Si:H) has recently been given much attention because it is one of the most promising materials for thin-"lm solar cells of lower cost, higher conversion e$ciency and better stability than amorphous-siliconbased solar cells. In reactive plasmas widely used for silicon thin-"lm growth, generally, there are e!ects of electron density, n and temperature, ¹ on the products * Corresponding author. Fax: #81-48-858-3676. E-mail address: [email protected] (H. Shirai). 0927-0248/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 4 8 ( 0 0 ) 0 0 1 6 6 - 5
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obtained from such processings. Higher n promotes the dissociation of mother molecule and it is desirable for enhancing the deposition rate. So far, to this aim, electron-cyclotron-resonance (ECR) and helicon wave plasma sources were applied for fast deposition of a-Si:H, a-SiGe :H and lc-SiC alloy "lms [1,2]. However, higher V V ¹ promotes excess secondary reaction and ionization in the plasma, which causes damage to the growing surface and interface. Therefore, a novel high-density and low-temperature plasma source is strongly required for precise control of gas-phase reaction at lower pressure condition. This is because higher generation e$ciency of neutral radicals such as SiH is expected in SiH plasma without excess secondary reaction. Several high-density and low-temperature plasmas without use of magnetic "eld have been applied for ULSI technologies, i.e., inductive coupling plasma (ICP), surface wave plasma (SWP) and ultra-high-frequency (UHF: 500 MHz) plasma [3}5]. We have applied a novel high-density and low-temperature microwave plasma (2.45 GHz) utilizing a spokewise antenna for fast deposition of lc-Si:H "lm. The plasma maintains a uniform state with high n '10 cm\ and a low ¹ of 2 eV in Ar plasma [6]. In addition, we reported that a highly crystallized and photoconductive lc-Si:H(Cl) "lm can be obtained from dichlorosilane, SiH Cl at a high depos ition rate of 20 As /s [7]. In this study, we demonstrate the fast deposition of highly crystallized and photoconductive lc-Si:H "lm at '40 As /s in SiH and Ar plasma. In particular, the spatial distribution of microwave plasma for fast deposition of highly crystallized lc-Si:H "lm is documented.
2. Experimental The lc-Si:H "lms have been deposited using a high-density microwave discharge utilizing a spokewise antenna with an outer diameter of 22 cm. The details of the deposition system are described elsewhere [5]. The microwave power is supplied to the spokewise antennas. The antenna's spoke length is "xed at 4 cm, that is, of the microwave length. A microwave power is supplied to the antenna assembly and radiated into the discharge chamber through a 15 mm thick quartz plate. Optical emission spectroscopy (OES) was employed using a periscope, which collects photons through a 3 mm diameter optical "ber leading to a monochromator. The spatial distribution of Ar I 750.4 nm emission intensity, corresponding to a threshold energy of 13.5 eV. The n and ¹ were determined in Ar plasma by means of a standard Langmuir probe technique as well as saturation ion current. The spatial distribution of ion energy was also examined using an electrostatic analyzer (Faraday cup) (10 mm in diameter) which is composed of four electrodes at di!erent Z positions and pressures. The distance between the grids was 0.5 mm. The deposition conditions of "lm included the #ow rates of SiH , Fr(SiH ): 5}30 sccm, Fr(H ): 0}50 sccm and Fr(Ar): 5 sccm and substrate temperature, ¹ : 200}2503C, total pressure: 30}200 mTorr and microwave power, P : 600 W. The "lm
structure was characterized by X-ray di!raction (XRD), spectroscopic ellipsometry (SE), Raman spectroscopy and atomic force microscopy (AFM). The residual hydrogen content in the "lms was measured by Fourier transform infrared spectroscopy
H. Shirai et al. / Solar Energy Materials & Solar Cells 66 (2001) 137}145
139
Table 1 Fr(SiH ) Fr(H ) Fr(Ar) ¹ P
Pressure
10 sccm 10 sccm 5 sccm 2003C 600 W &50 mTorr
(FTIR). Dark and photoconductivity measurements were also performed with a 100 lm gap coplanar Al electrode.
3. Results and discussion Figs. 1(a) and (b) show the deposition rate and the Raman spectra of lc-Si:H "lms fabricated at di!erent Z positions, respectively. The deposition conditions used in this study are summarized in Table 1. The deposition rate slightly increases with a shortening of the axial distance, Z from 15 to 4 cm, accompanied with an improvement of "lm crystallinity as shown in Fig. 1(b). In addition, the degree of surface roughness decreases with decreasing the axial distance, Z, which is con"rmed by SE and AFM observations. FTIR study also revealed that residual hydrogen content in the "lm monotonically decreases with shortening the axial distance, Z, though no change of the SiH (n"1, 2) bonds con"guration was observed. Thus, we found that L highly crystallized lc-Si:H "lm was fabricated with less surface roughness and hydrogen content at Z"6 cm at a high deposition rate of 16 As /s. To better understand the e!ect of the axial distance, Z on the spatial distribution of the plasma, the OES intensity was measured in Ar plasma by a periscope, which collects photons along a 3 mm diameter beam line and leads to a spectrometer. We monitored the OES emission intensity of Ar I (750.4 nm, 4s P4p ), corresponding to a threshold energy of 13.5 eV. Figs. 2(a) and (b) show the axial distributions of the OES Ar I intensity and probe saturation ion current, respectively, at di!erent pressure conditions. The OES intensity exponentially decreases with an increase in the axial distance, Z, at each pressure condition. Another feature is that the OES intensity decreases with decreasing pressure near the quartz glass plate, whereas it increases with decreasing the pressure in the Z'6 cm position. Thus, the pressure dependence of the OES emission intensity depends on the Z position. The integrated intensity is also shown in the inset. The integrated intensity has a maximum at &50 mTorr, at which the highest deposition rate is obtained [8]. On the other hand, saturation ion current slightly increases with increasing the axial distance, Z and it shows a maximum at the Z&6 cm position at a pressure of 50 mTorr, though its pressure dependence is not so systematic compared with that of OES. In addition, to study the axial distribution of plasma more precisely, a H and Ar mixture plasma exposure experiment was conducted on c-Si wafer at di!erent Z
140
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Fig. 1. (a) Deposition rate and (b) Raman spectra of lc-Si:H "lms deposited at di!erent axial distances, Z from the quartz glass plate under the same plasma condition.
positions. Fig. 3 shows the imaginary part of pseudodielectric function, 1e 2 spectra before and after 10 min plasma exposure at di!erent Z positions. The magnitude of 1e 2 drastically decreases at Z"4 and 15 cm compared with that at Z"6}10 cm. Generally, the magnitude of 1e 2 is tightly correlated with the surface roughness and bulk void fraction. Therefore, the damage of the c-Si wafer by Ar plasma is lower at Z&6}10 cm than that at Z(4 cm and Z'10 cm positions. These results originate from the axial distribution of plasma parameters, i.e., n and ¹ . However, it is still controversial why the highest deposition rate and highest crystallinity are obtained at the Z"&6 cm position (Fig. 1).
H. Shirai et al. / Solar Energy Materials & Solar Cells 66 (2001) 137}145
141
Fig. 2. Axial distributions of (a) OES Ar I signal intensity (750.4 nm), corresponding to a threshold energy of 13.5 eV and (b) probe saturation ion current measured at di!erent pressure conditions.
Further detailed study was made by measuring the axial distribution of ion energy (IED) in Ar plasma using an electrostatic analyzer (Faraday cup). Figs. 4(a) and (b) show "rst derivative, dI /d<, curves of Faraday cup characteristics measured at di!erent Z positions and pressure conditions, respectively [9]. The measurements were made under the conditions of (a) &50 mTorr and (b) Z"6 cm position. It is seen that IED provides an almost Maxwellian EDF with the narrowest half-width at (50 mTorr and Z"6 cm, while IED spreads toward the lower energy region, forming two peaks at Z(8 cm and higher pressure conditions, although the mean ion energy is almost independent of the Z position. The conditions at which the narrowest IED is observed almost agree with those for the highest "lm crystallinity obtained (Fig. 1(b)) Thus, lowered ion energy and its narrower distribution are essential for obtaining higher crystallinity. We found that the highest "lm crystallinity was
142
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Fig. 3. The imaginary part of pseudodielectric function, 1e 2 spectra of c-Si wafer before and after 10 min exposure of H (10 sccm) and Ar (5 sccm) mixture plasma at di!erent Z positions.
obtained at the Z"6 cm position at &50 mTorr with higher deposition rate, at which the spread of ion energy distribution (IED) is minimum in this plasma. Similar concepts have been reported in the growth of a-Si:H "lm with high hole mobility. A high hole mobility of &0.3 cm/V s, which is increased by two orders of magnitude compared to the conventional value, was attained by an intentional control of the combination of RF potential #uctuation and electron and ion temperatures by using a biased mesh grid [10]. At this stage, we have an idea that "lm crystallinity is also improved by using a similar concept in the growth of not only a-Si:H but also lc-Si:H. However, further detailed study is needed on why the spread of IED becomes narrower at Z"&6 cm and pressure of &50 mTorr in this high-density microwave plasma. To improve the deposition rate and "lm crystallinity, deposition study was carried out by increasing Fr(SiH ) at a pressure of &50 mTorr. As reported previously, hydrogen addition deteriorates "lm crystallinity in this plasma [8]. Therefore, in the following, "lm deposition was carried out using SiH and Ar mixture plasma. Fig. 5(a) shows the "lm deposition rate and OES intensity, SiH plotted as a function of Fr(SiH ) under a steady #ow of 5 sccm Ar plasma. Results are shown at Z"10 and 6 cm positions as symbols, E and 夹, respectively. The deposition rate increases linearly with increasing Fr(SiH ) and no saturation can be observed up to a 30 sccm supply of SiH . Thus, we can expect further increase of the deposition rate. The Raman spectra show high crystallinity, though the XRD crystal orientation is random as shown in Fig. 5(b). Film crystallinity can be improved by increasing the ¹ of 2503C, maintain ing a high deposition rate of 40 As /s at Z"10 cm. As a consequence, a high deposition rate is achieved up to 40 and 47 As /s at Z"10 and 6 cm positions, respectively, under the Fr(SiH )"30 sccm and pressure: &50 mTorr condition. The optical absorption
H. Shirai et al. / Solar Energy Materials & Solar Cells 66 (2001) 137}145
143
Fig. 4. First derivative of Faraday cup characteristics in 10 sccm pure Ar plasma at (a) di!erent Z positions (50 mTorr) and (b) pressure (Z"6 cm) conditions.
spectra in these "lms are almost identical to those of c-Si, though it shifts to near-infrared region, which is due to the surface roughness and/or bulk inhomogeneities. Dark and photoconductivities are in the range of 10\ and 10}}10\ S/cm under 100 mW/cm illumination, respectively. In addition, this high-density microwave plasma source is available for further scale up of the chamber size by designing the antenna's con"guration. This is because the concept is based on the interdigital "lter, though the antenna's spoke length is "xed at 4 cm. Recently, a novel UHF (500 MHz) plasma source has been applied for ULSI technologies and it is more suitable than microwave (2.45 GHz) for enlargement of the chamber size, because each antenna's length can be designed at longer than that of microwave [11]. However, it is still very expensive to use the 500 MHz system, because the magnetron for UHF (500 MHz) plasma is not generally commercialized except the application to telecommunication system. Additionally, lower cost and
144
H. Shirai et al. / Solar Energy Materials & Solar Cells 66 (2001) 137}145
Fig. 5. (a) Deposition rate and the OES signal intensity, SiH plotted as a function of Fr(SiH ) at a total pressure of &50 mTorr. (b) Raman spectra of lc-Si:H "lms deposited at each corresponding Z position. The symbols, E and 夹 denote the results at Z"10 and 6 cm, respectively. The microwave power and Fr(Ar) are constant at 600 W and 5 sccm, respectively.
a simpli"ed deposition system are strongly required and available for practical production of a more than 1-m-sized solar cell. Therefore, the high-density and low-temperature microwave plasma utilizing a spokewise antenna has a high potential for large-area electronic processings in the next generation such as solar cell devices.
4. Conclusions Highly crystallized and photoconductive lc-Si:H "lm was fabricated at high deposition rates of up to 47 As /s in SiH and Ar mixture plasma. Higher "lm crystallinity
H. Shirai et al. / Solar Energy Materials & Solar Cells 66 (2001) 137}145
145
was obtained at the Z"6 cm position with higher deposition rate under the condition at which the spread of ion energy distribution (IDF) is minimum. The high-density and low-temperature microwave plasma demonstrated here has a high potential for practical production of solar cell devices in the next generation.
Acknowledgements The authors would like to thank Miss. Wasai, Mrs. N. Nabatova-Gabin and Mr. K. Kainuma in Atago Co., Ltd. for allowing the use of Raman spectrometer and spectroscopic ellipsometer. This work is in part supported by Suzuki Foundation and Nippon Sheet Glass Foundation for Materials Science and Engineering.
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