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s) using VHF hollow electrode enhanced glow plasma
Surface & Coatings Technology 204 (2010) 3525–3529
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Development of microcrystalline silicon thin films with high deposition rate (over 10 nm/s) using VHF hollow electrode enhanced glow plasma M. Takashiri a,⁎, T. Tabuchi b a b
Research Division, Komatsu Ltd., 1200 Manda, Hiratsuka, Kanagawa, Japan Department of Enviromental Management, Komatsu Ltd., 2-3-5 Akasaka, Minato-ku, Tokyo, Japan
a r t i c l e
i n f o
Article history: Received 21 January 2010 Accepted in revised form 6 April 2010 Available online 10 April 2010 Keywords: Microcrystalline Silicon thin films VHF–HEEPT High deposition rate
a b s t r a c t Microcrystalline silicon (μc-Si) thin films with high deposition rate have been fabricated by VHF hollow electrode enhanced glow plasma transportation (VHF–HEEPT). Plasma observation is performed in the discharge chamber and the deposition chamber, and plasma emission spectroscopy is performed in the deposition chamber. The results of the plasma emission spectroscopy indicate that VHF–HEEPT yields a plasma with lower electron temperature compared to RF–HEEPT and conventional capacitively-coupled VHF plasmas. The structural and electrical properties of μc-Si thin films are estimated as a function of the SiH4/H2 ratio. The thin films made by VHF–HEEPT are microcrystalline and have a high deposition rate (12.5 nm/s), while thin films made by RF–HEEPT become amorphous for deposition rates over 10 nm/s. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Microcrystalline silicon (μc-Si) thin films have attracted much attention owing to their promise for applications as high-efficiency and stable solar cells [1–3]. In order to expand the use of μc-Si thin film solar cells into the commercial scene, it is necessary to reduce their fabrication cost while maintaining their film quality. The primary candidate to reduce the fabrication cost is to increase the deposition rate of the films using plasma-enhanced chemical vapor deposition (PECVD). There have been several plasma apparatuses proposed for generating the target plasma, such as the hollow plasma [4,5], the very high frequency (VHF) plasma [6–8], or the other high density plasma [9,10]. We have previously proposed a plasma generation system called radio frequency (13.56 MHz) hollow electrode enhanced glow plasma transportation (RF–HEEPT) [11] and achieved μc-Si thin films with the maximum deposition rate of 6.0 nm/s [12] and also achieved 6.9 nm/s by applying a magnetic field to the HEEPT system [13]. It has also been demonstrated that the HEEPT system can deposit films of uniform thickness over a large area [14]. In this work, to further increase the deposition rate of the μc-Si thin films we applied a higher plasma excitation frequency of 105 MHz to the HEEPT system (VHF–HEEPT). Compared to a RF plasma, a VHF plasma is known to have a higher plasma density and a lower electron temperature, thus resulting in a large amount of atomic hydrogen and long lifetime radicals such as SiH3 radicals which are expected to
⁎ Corresponding author. E-mail address: [email protected] (M. Takashiri). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.04.013
fabricate high quality thin films with high deposition rates [15–18]. The plasma characteristics were investigated by visual observation and emission spectroscopy. Several silicon thin films were deposited with different values of the gas flow rate ratio (SiH4/H2) with a view to optimizing deposition conditions, and the film properties were characterized by deposition rate, Raman spectroscopy, and electrical conductivity. 2. Apparatus A schematic diagram of the VHF–HEEPT reactor is shown in Fig. 1. The structure of the VHF–HEEPT reactor is similar to the RF–HEEPT reactor, and the detailed setup is described elsewhere [11,12]. In brief, the VHF–HEEPT reactor basically consists of two spaces, having a VHF electrode (cathode electrode), a counter electrode (anode electrode), and a substrate holder with heater. The space between the VHF electrode and counter electrode is called the discharge chamber. The space between counter electrode and substrate holder is called the deposition chamber. The VHF electrode has a showerhead structure with holes of 3 mm diameter for uniform distribution of gases, and also has a cave structure (the sub-discharge chamber) of 5 mm in height. The holes and cave structure operate as a hollow VHF electrode discharge space. An orifice is prepared at the center of the counter electrode, and a straight aluminum tube (nozzle) is attached to the orifice. The total length of this nozzle including the thickness of the counter electrode is 19 mm, and the internal diameter is 13 mm. The distance between counter electrode and VHF electrode is 8 mm, while the distance from the bottom end of the nozzle to the substrate holder is 15 mm. In order to observe the plasma patterns and emissions, small rectangular quartz windows are inserted in small
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M. Takashiri, T. Tabuchi / Surface & Coatings Technology 204 (2010) 3525–3529
Fig. 1. Schematic of the VHF–HEEPT reactor.
portions of the grounded guard ring, the side of the VHF electrode, and the reactor wall. When process gases flow into the reactor and VHF power is applied to the VHF electrode, a plasma is generated both in the discharge chamber and in the sub-discharge chamber. The plasma density is subsequently enhanced inside the nozzle, and then reaches the substrate located in the deposition chamber. 3. Experimental The plasma patterns in the discharge chamber, the sub-discharge chamber, and the deposition chamber were observed though the quartz windows with a hydrogen (H2) gas flow rate of 100 sccm and a VHF input power of 250 W. We also measured the H2 plasma emission spectra in the deposition chamber. The plasma emissions passed through the quartz windows to an optical fiber, and were detected by a monochromator. The optical axis of this fiber was parallel to the substrate and offset by 10 mm above the substrate. The intensity of plasma emission detected by the fiber was effectively an integral of the emission throughout the chamber. In this observation, the emission intensities of Hα (656 nm) and Hβ (486 nm) were measured, and their intensity ratio was evaluated because the ratio approximately corresponds to the electron temperature [19,20]. Then, the results were compared to those of RF–HEEPT and the conventional capacitively-coupled VHF plasma. Silicon films with a thickness of approximately 1 μm were deposited on glass (Corning 7059) substrates at a temperature of 300 °C. The excitation frequency was kept constant at 105 MHz. The VHF power was fixed at 250 W which was the maximum output power of our apparatus. The gas pressure was varied from 27 to 133 Pa while the plasma observations were performed. The gas flow rate ratio (SiH4/H2) was varied from 0.04 to 0.27, holding the H2 flow rate constant at 100 sccm. The deposition rate was calculated from the thickness of the area just below the nozzle. The structural properties of the thin films were investigated by Raman spectroscopy with the 514.5 nm line of an Ar+ ion laser (Renishaw System 2000). The electrical conductivity was measured at room temperature by a two-terminal method. In order to make contact with the films, aluminum electrodes of 10 mm in length were deposited in parallel at intervals of 1 mm using an electron beam evaporation technique. We measured both the photo-conductivity
and dark-conductivity. The photo-conductivity measurement was carried out under the condition of AM1.5, 100 mW/cm2. 4. Results and discussion 4.1. Diagnostic study of the plasma We employed only hydrogen plasmas for the observation of plasma patterns and the measurement of emission intensities, because mixtures of SiH4 gas in hydrogen gas caused silicon films to be deposited on the reactor window which made it difficult to observe the plasma patterns and the plasma emission. It is known that the electron temperature of a plasma changes by adding SiH4 gas into hydrogen gas. However, even in the case of a purely hydrogen plasma, it is possible to compare the relative magnitude of electron temperature among plasmas generated by different systems. The hydrogen plasma patterns in the discharge chamber including the sub-discharge chamber and the deposition chamber are shown in Fig. 2 for various gas pressures. In the deposition chamber, the plasma patterns were almost the same shape for all gas pressures. Many bright funnel-shaped irregular discharges were generated in the discharge chamber near and inside some of the holes in the showerhead of the VHF electrode (cathode electrode). Similar strong inhomogeneous plasma was also observed inside the sub-discharge chamber. Every irregular spot converged at the holes in the showerhead. This indicated that the hollow cathode discharge was induced at the holes in the VHF electrode for all gas pressures. High intensity plasma was also localized near the center of the counter electrode (anode electrode). In the deposition chamber, mushroomshaped plasma was generated near and inside the nozzle in the counter electrode. Though the plasma became smaller as the gas pressure increased, the hollow anode discharge was induced in the nozzle of the counter electrode for all gas pressures. As a result, the double hollow plasma, which means a plasma at both hollow cathode and hollow anode, was generated for all gas pressures in the VHF– HEEPT system. In our previous study of a RF–HEEPT system, we found that μc-Si thin films could be deposited at a higher rate using a double hollow plasma as compared to only a hollow anode plasma [12]. Therefore we also expect that the VHF–HEEPT with double hollow plasma has a high potential to deposit μc-Si thin films with a higher deposition rate than a single hollow anode plasma.
M. Takashiri, T. Tabuchi / Surface & Coatings Technology 204 (2010) 3525–3529
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Fig. 3. Optical emission intensities of Hα and Hβ (a) and intensity ratio (b) as a function of the gas pressure operated by VHF–HEEPT, RF–HEEPT and VHF-CCP for comparison. In both VHF–HEEPT and RF–HEEPT, the error of emission intensities of Hα, Hβ and the intensity ratio of Hβ/Hα of are ±2%, ±4% and ±6%, respectively.
HEEPT. As a result, the emission intensity ratio (Hβ/Hα) of VHF–HEEPT was 41% smaller than in RF–HEEPT. We also confirmed that the Hβ/Hα emission intensity ratio of VHF–HEEPT was lower than that of the VHF conventional capacitively-coupled plasma (VHF-CCP) at the same gas pressure [20]. Thus VHF–HEEPT yielded a plasma with lower electron temperature than RF–HEEPT and VHF-CCP, while the plasma density was kept nearly as high as RF–HEEPT. For the fabrication of μc-Si thin films with high deposition rate and high film quality, a large amount of atomic hydrogen and SiH3 radicals, which are effectively yielded in plasmas with low electron temperature, are necessary [21]. Therefore, we expect that the plasma properties of VHF–HEEPT are suitable for the deposition of μc-Si thin films with high deposition rate and high film quality. 4.2. Deposition of the silicon thin films
Fig. 2. H2 plasma emission by VHF–HEEPT in the discharge chamber, and in the deposition chamber for various gas pressures.
Optical emission spectroscopy (OES) plasma diagnostics were performed in the hydrogen plasma as a function of gas pressure with a view to optimizing deposition conditions. The hydrogen plasma emission intensities of Hα and Hβ, which approximately corresponds to the plasma density, decreased approximately linearly with increasing gas pressure as shown in Fig. 3 (a). The emission intensity ratio of Hβ/Hα also decreased with increasing gas pressure, as shown in Fig. 3 (b). We compared the plasma properties of VHF–HEEPT with those of RF–HEEPT, which was applied to the same apparatus by using a different power supply. At the gas pressure of 53 Pa, the Hα emission intensity of VHF–HEEPT was 21% smaller than in RF–HEEPT, and the Hβ emission intensity of VHF–HEEPT was 54% smaller than in RF–
For the deposition of μc-Si thin films, we employed a gas pressure of 80 Pa as a compromise to obtain both low electron temperature and high plasma density. The deposition rate of the thin films is shown in Fig. 4 as a function of the SiH4/H2 ratio. The deposition rate at the SiH4/ H2 ratio of 0.04 was 0.7 nm/s, and then it increased linearly up to 12.5 nm/s at the SiH4/H2 ratio of 0.27. Fig. 5 shows the Raman spectra of the μc-Si thin films as a function of the SiH4/H2 ratio. The Raman spectra of the thin films broadened with increasing the SiH4/H2 ratio. The Raman peak position at the lowest SiH4/H2 ratio was close to 520 cm−1, which indicated the formation of a crystalline silicon thin film. Then the slight shift of the Raman peak to a shorter wave number with increasing SiH4/H2 ratio revealed the increase of residual stress in the thin films. For detailed analysis of the structural properties of μc-Si thin films, the crystalline volume fraction, the full width at half maximum (FWHM), and the residual stress were estimated from the Raman
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Fig. 4. Deposition rate of μc-Si thin films by VHF–HEEPT as a function of the SiH4/H2 ratio.
spectra presented in Fig. 5. Employing the Si–TO phonon Raman spectra, the crystalline volume fraction, XC, was roughly estimated as Ic/(Ic + Ia), where Ic and Ia denoted the integrated intensities of spectral components around 520 and 480 cm− 1, respectively. As shown in Fig. 6 (a), increasing the SiH4/H2 ratio caused the estimated crystalline volume fraction to gradually decrease, from 88% at the SiH4/H2 ratio of 0.04 to 64% at the ratio of 0.27. This indicated that the crystallinity of the thin films at higher SiH4/H2 ratio was low but the phase still remained microcrystalline even for deposition rates over 10 nm/s. The FWHM of the Raman spectra for the thin films as a function of the SiH4/H2 ratio is presented in Fig. 6 (b). The asymmetry and broadening of the Raman spectra are caused by small crystal grains and high defect densities in thin films due to phonon scattering from the grain boundaries [22–24]. In Fig. 6 (b), the increase of the SiH4/H2 ratio corresponds to a broadening of the FWHM of the Raman spectra, from 10.5 cm− 1 at the SiH4/H2 ratio of 0.04 to 21.3 cm− 1 at the ratio of 0.27, indicating that the crystal grains became smaller or the defect density increased with increasing deposition rate. The residual stress of the thin films as a function of the SiH4/H2 ratio is also presented in Fig. 6 (b). It is known that the residual stress in thin films
Fig. 5. Raman spectra of μc-Si thin films by VHF–HEEPT as a function of Raman shift at various SiH4/H2 ratios.
Fig. 6. Crystalline volume fraction (a), FWHM and residual stress (b) of μc-Si thin films by VHF–HEEPT as a function of the SiH4/H2 ratio.
correlates with the crystalline defects, and can be estimated from the Raman spectra by the following equation [25]. −1
σðMPaÞ = −250Δωðcm
Þ:
ð1Þ
In Eq. (1), σ is the in-plane stress and Δω = ωs − ω0, where ω0 is the optical phonon wave number of the stress-free single crystal and ωs is the wave number of the stressed sample. Here, the positive and negative values of the residual stress show the tensile stress and the compressive stress, respectively. As shown in Fig. 6 (b), the SiH4/H2 ratio dependence of the residual stress of the thin films follows a similar trend as the FWHM. The increase of the SiH4/H2 ratio increased the tensile stress from 250 MPa at the SiH4/H2 ratio of 0.04 to
Fig. 7. Photo- and dark-conductivity of μc-Si thin films by VHF–HEEPT as a function of the SiH4/H2 ratio.
M. Takashiri, T. Tabuchi / Surface & Coatings Technology 204 (2010) 3525–3529
5. Conclusion
Table 1 Properties of silicon thin films deposited by VHF–HEEPT and RF–HEEPT. Plasma type
dr (nm/s)
Xc (%)
Phase
VHF–HEEPT
4.8 12.5 6.0 12.9
86 64 78 29
µc-Si µc-Si µc-Si a-Si
RF HEEPT a
Electrical conductivity σdark (S/cm)
σphoto (S/cm)
3.4 × 10− 6 4.2 × 10− 8 4.9 × 10− 6 4.5 × 10− 10
3.3 × 10− 5 2.5 × 10− 7 6.6 × 10− 5 3.2 × 10− 7
3529
Reference
This work [12]a
Our experimental results.
1200 MPa at the ratio of 0.27, indicating that the thin films with higher deposition rate are expected to have a higher defect density. Fig. 7 shows the electrical conductivity of the μc-Si thin films as a function of the SiH4/H2 ratio. The photo- and dark-conductivity of the thin films at the SiH4/H2 ratio of 0.04 were 1.5 × 10− 4 and 8.9 × 10− 6 S/ cm, respectively, and the photo-conductivity was an order of magnitude higher than that of high-performance thin films at the deposition rate of 5.5 nm/s [26]. One possible explanation of this effect could be that the photo-conductivity was increased by unintentional doping by oxygen due to changes in the charge state of recombination centers. We have not measured the oxygen concentration of the thin films in this study, but thin films deposited with similar deposition rate and conditions in an RF–HEEPT system have an oxygen concentration estimated as 1.0 × 1019 cm− 3 by using secondary ion mass spectrometry (SIMS), so we expect that the oxygen concentration of the thin films in this study is also approximately 1.0 × 1019 cm− 3. The photo- and dark-conductivity of the thin films gradually decreased with increasing SiH4/H2 ratio. At the SiH4/H2 ratio of 0.27, the photo- and dark-conductivity were 2.5 × 10− 7 and 4.2 × 10− 8 S/cm, respectively. This might be because the dark-conductivity was decreased by the increase of the amorphous phase in the thin films, and then the crystalline defects induced by the residual stress in the thin films could also cause the corresponding low photo-conductivity. We compared the properties of the μc-Si thin films in this work with those of thin films deposited by RF–HEEPT, as listed in Table 1. At the deposition rate of around 5 nm/s, the thin films deposited by both VHF–HEEPT and RF–HEEPT exhibited approximately the same crystallinity and photo-conductivity. On the other hand, at the deposition rate over 10 nm/s, the thin films deposited by VHF– HEEPT still maintained a microcrystalline phase, while the thin films deposited by RF–HEEPT turned into an amorphous phase. The deposition mechanism of μc-Si thin films with higher deposition rate by VHF–HEEPT is not yet clearly understood. However, the main models which are currently proposed have in common that the presence of a large amount of atomic hydrogen and SiH3 radicals is a key factor for forming the μc-Si with high quality [15–18]. The VHF–HEEPT system yielded a large amount of atomic hydrogen and SiH3 radicals because the electron temperature of VHF–HEEPT was lower than its RF–HEEPT counterpart while both systems obtained a high plasma density. Therefore an important accomplishment of this work is that the VHF–HEEPT system was able to maintain high crystallinity even for a deposition rate of over 10 nm/s. However, the photo-conductivity of the thin films is not sufficient to fabricate high quality solar cells so that the further optimization of the crystal structure and electrical properties, such as the reduction of the residual stress and the oxygen contamination in the thin films, is necessary.
We developed the VHF–HEEPT system to fabricate μc-Si thin films with a high deposition rate (12.5 nm/s). At first, plasma observation and plasma emission spectroscopy were performed. The results of the plasma emission spectroscopy indicated that VHF–HEEPT yielded a plasma with lower electron temperature compared to RF–HEEPT and VHF-CCP. Next, thin films were deposited for several different values of the SiH4/H2 ratio, and then the structural and electrical properties of the μc-Si thin films were estimated. The thin films deposited by VHF–HEEPT had a high deposition rate (12.5 nm/s), while RF–HEEPT yielded amorphous thin films for deposition rates over 10 nm/s. Though the deposition mechanism of the μc-Si thin film deposited by the VHF– HEEPT system has not yet been clearly understood, the key factors are considered to be the presence of a large amount of atomic hydrogen and SiH3 radicals. The VHF–HEEPT system yielded a large amount of atomic hydrogen and SiH3 radicals because the electron temperature of VHF– HEEPT was lower than its RF–HEEPT counterpart while both systems obtained a high plasma density. Therefore the VHF–HEEPT system succeeded in depositing μc-Si thin films with high deposition rate over 10 nm/s, while thin films made by RF–HEEPT became amorphous at almost the same deposition rate. However, the photo-conductivity of the VHF–HEEPT thin films is not sufficient to fabricate high quality solar cells and thus further optimization of the crystal structure and electrical properties is necessary. Acknowledgements The authors wish to thank K. Ishida for experimental support during this investigation, and Professor Dames at the University of California, Riverside for valuable comments. References [1] Y. Nasuno, M. Kondo, A. Matsuda, Sol. Energy Mater. Sol. Cells 74 (2002) 497. [2] S. Klein, T. Repmann, T. Brammer, Sol. Energy 77 (2004) 893. [3] Y. Mai, S. Klein, R. Carius, J. Wolff, A. Lambertz, F. Finger, X. Geng, J. Appl. Phys. 97 (2005) 114913. [4] V.I. Miljevic, Appl. Opt. 23 (1984) 1598. [5] A. Anders, S. Anders, Plasma Sources Sci. Technol. 4 (1995) 571. [6] J. Meier, R. Fluckiger, H. Keppner, S. Shah, Appl. Phys. Lett. 65 (1994) 860. [7] M. Ryo, Y. Sakurai, T. Kobayashi, H. Shirai, Jpn. J. Appl. Phys. 45 (2006) 8484. [8] Y. Sobajima, M. Nishino, T. Fukumori, M. Kurihara, T. Higuchi, S. Nakano, T. Toyama, H. Okamoto, Sol. Energy Mater. Sol. Cells 93 (2009) 980. [9] Y. Hotta, H. Toyoda, H. Sugai, Thin Solid Films 515 (2007) 4983. [10] C. Smit, A. Klaver, B.A. Korevaar, A.M.H.N. Petit, D.L. Williamson, R.A.C.M.M. van Swaaij, M.C.M. van de Sanden, Thin Solid Films 491 (2005) 280. [11] T. Tabuchi, M. Takashiri, H. Mizukami, Surf. Coat. Technol. 173 (2003) 243. [12] T. Tabuchi, H. Mizukami, M. Takashiri, J. Vac. Sci. Technol. A 22 (2004) 2139. [13] T. Tabuchi, M. Takashiri, K. Ishida, Surf. Coat. Technol. 202 (2007) 114. [14] M. Takashiri, K. Ishida, T. Tabuchi, Japan Patent. No.P2001-230208 (filed 2001). [15] C.C. Tsai, G.B. Anderson, R. Thompson, B. Wacker, J. Non-Cryst. Solids 114 (1989) 151. [16] K. Nakamura, K. Yoshida, S. Takeoka, I. Shimizu, Jpn. J. Appl. Phys. 34 (1995) 442. [17] A. Matsuda, J. Non-Cryst. Solids 59/60 (1983) 767. [18] M. Takai, T. Takagi, T. Nishimoto, M. Kondo, A. Matsuda, Surf. Coat. Technol. 131 (2000) 50. [19] H. Jia, J.K. Saha, N. Ohse, H. Shirai, J. Phys. D 39 (2006) 3844. [20] M. Takai, T. Nishimoto, M. Kondo, A. Matsuda, Appl. Phys. Lett. 77 (2000) 2828. [21] M. Tsuda, S. Oikawa, K. Sato, J. Chem. Phys. 91 (1989) 6822. [22] Z. Iabal, S. Veprek, A.P. Webb, P. Capezzuto, Solid State Commun. 37 (1981) 993. [23] H. Richter, Z.P. Wang, L. Ley, Solid State Commun. 39 (1981) 625. [24] I.H. Campbell, P.M. Fauchet, Solid State Commun. 58 (1986) 739. [25] I.D. Wolf, Semicond. Sci. Technol. 11 (1996) 139. [26] M. Kondo, M. Fukawa, L. Guo, A. Matsuda, J. Non-Cryst. Solids 266–269 (2000) 84.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Development of microcrystalline silicon thin films with high deposition rate (over 10 nm/s) using VHF hollow electrode enhanced glow plasma M. Takashiri a,⁎, T. Tabuchi b a b
Research Division, Komatsu Ltd., 1200 Manda, Hiratsuka, Kanagawa, Japan Department of Enviromental Management, Komatsu Ltd., 2-3-5 Akasaka, Minato-ku, Tokyo, Japan
a r t i c l e
i n f o
Article history: Received 21 January 2010 Accepted in revised form 6 April 2010 Available online 10 April 2010 Keywords: Microcrystalline Silicon thin films VHF–HEEPT High deposition rate
a b s t r a c t Microcrystalline silicon (μc-Si) thin films with high deposition rate have been fabricated by VHF hollow electrode enhanced glow plasma transportation (VHF–HEEPT). Plasma observation is performed in the discharge chamber and the deposition chamber, and plasma emission spectroscopy is performed in the deposition chamber. The results of the plasma emission spectroscopy indicate that VHF–HEEPT yields a plasma with lower electron temperature compared to RF–HEEPT and conventional capacitively-coupled VHF plasmas. The structural and electrical properties of μc-Si thin films are estimated as a function of the SiH4/H2 ratio. The thin films made by VHF–HEEPT are microcrystalline and have a high deposition rate (12.5 nm/s), while thin films made by RF–HEEPT become amorphous for deposition rates over 10 nm/s. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Microcrystalline silicon (μc-Si) thin films have attracted much attention owing to their promise for applications as high-efficiency and stable solar cells [1–3]. In order to expand the use of μc-Si thin film solar cells into the commercial scene, it is necessary to reduce their fabrication cost while maintaining their film quality. The primary candidate to reduce the fabrication cost is to increase the deposition rate of the films using plasma-enhanced chemical vapor deposition (PECVD). There have been several plasma apparatuses proposed for generating the target plasma, such as the hollow plasma [4,5], the very high frequency (VHF) plasma [6–8], or the other high density plasma [9,10]. We have previously proposed a plasma generation system called radio frequency (13.56 MHz) hollow electrode enhanced glow plasma transportation (RF–HEEPT) [11] and achieved μc-Si thin films with the maximum deposition rate of 6.0 nm/s [12] and also achieved 6.9 nm/s by applying a magnetic field to the HEEPT system [13]. It has also been demonstrated that the HEEPT system can deposit films of uniform thickness over a large area [14]. In this work, to further increase the deposition rate of the μc-Si thin films we applied a higher plasma excitation frequency of 105 MHz to the HEEPT system (VHF–HEEPT). Compared to a RF plasma, a VHF plasma is known to have a higher plasma density and a lower electron temperature, thus resulting in a large amount of atomic hydrogen and long lifetime radicals such as SiH3 radicals which are expected to
⁎ Corresponding author. E-mail address: [email protected] (M. Takashiri). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.04.013
fabricate high quality thin films with high deposition rates [15–18]. The plasma characteristics were investigated by visual observation and emission spectroscopy. Several silicon thin films were deposited with different values of the gas flow rate ratio (SiH4/H2) with a view to optimizing deposition conditions, and the film properties were characterized by deposition rate, Raman spectroscopy, and electrical conductivity. 2. Apparatus A schematic diagram of the VHF–HEEPT reactor is shown in Fig. 1. The structure of the VHF–HEEPT reactor is similar to the RF–HEEPT reactor, and the detailed setup is described elsewhere [11,12]. In brief, the VHF–HEEPT reactor basically consists of two spaces, having a VHF electrode (cathode electrode), a counter electrode (anode electrode), and a substrate holder with heater. The space between the VHF electrode and counter electrode is called the discharge chamber. The space between counter electrode and substrate holder is called the deposition chamber. The VHF electrode has a showerhead structure with holes of 3 mm diameter for uniform distribution of gases, and also has a cave structure (the sub-discharge chamber) of 5 mm in height. The holes and cave structure operate as a hollow VHF electrode discharge space. An orifice is prepared at the center of the counter electrode, and a straight aluminum tube (nozzle) is attached to the orifice. The total length of this nozzle including the thickness of the counter electrode is 19 mm, and the internal diameter is 13 mm. The distance between counter electrode and VHF electrode is 8 mm, while the distance from the bottom end of the nozzle to the substrate holder is 15 mm. In order to observe the plasma patterns and emissions, small rectangular quartz windows are inserted in small
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M. Takashiri, T. Tabuchi / Surface & Coatings Technology 204 (2010) 3525–3529
Fig. 1. Schematic of the VHF–HEEPT reactor.
portions of the grounded guard ring, the side of the VHF electrode, and the reactor wall. When process gases flow into the reactor and VHF power is applied to the VHF electrode, a plasma is generated both in the discharge chamber and in the sub-discharge chamber. The plasma density is subsequently enhanced inside the nozzle, and then reaches the substrate located in the deposition chamber. 3. Experimental The plasma patterns in the discharge chamber, the sub-discharge chamber, and the deposition chamber were observed though the quartz windows with a hydrogen (H2) gas flow rate of 100 sccm and a VHF input power of 250 W. We also measured the H2 plasma emission spectra in the deposition chamber. The plasma emissions passed through the quartz windows to an optical fiber, and were detected by a monochromator. The optical axis of this fiber was parallel to the substrate and offset by 10 mm above the substrate. The intensity of plasma emission detected by the fiber was effectively an integral of the emission throughout the chamber. In this observation, the emission intensities of Hα (656 nm) and Hβ (486 nm) were measured, and their intensity ratio was evaluated because the ratio approximately corresponds to the electron temperature [19,20]. Then, the results were compared to those of RF–HEEPT and the conventional capacitively-coupled VHF plasma. Silicon films with a thickness of approximately 1 μm were deposited on glass (Corning 7059) substrates at a temperature of 300 °C. The excitation frequency was kept constant at 105 MHz. The VHF power was fixed at 250 W which was the maximum output power of our apparatus. The gas pressure was varied from 27 to 133 Pa while the plasma observations were performed. The gas flow rate ratio (SiH4/H2) was varied from 0.04 to 0.27, holding the H2 flow rate constant at 100 sccm. The deposition rate was calculated from the thickness of the area just below the nozzle. The structural properties of the thin films were investigated by Raman spectroscopy with the 514.5 nm line of an Ar+ ion laser (Renishaw System 2000). The electrical conductivity was measured at room temperature by a two-terminal method. In order to make contact with the films, aluminum electrodes of 10 mm in length were deposited in parallel at intervals of 1 mm using an electron beam evaporation technique. We measured both the photo-conductivity
and dark-conductivity. The photo-conductivity measurement was carried out under the condition of AM1.5, 100 mW/cm2. 4. Results and discussion 4.1. Diagnostic study of the plasma We employed only hydrogen plasmas for the observation of plasma patterns and the measurement of emission intensities, because mixtures of SiH4 gas in hydrogen gas caused silicon films to be deposited on the reactor window which made it difficult to observe the plasma patterns and the plasma emission. It is known that the electron temperature of a plasma changes by adding SiH4 gas into hydrogen gas. However, even in the case of a purely hydrogen plasma, it is possible to compare the relative magnitude of electron temperature among plasmas generated by different systems. The hydrogen plasma patterns in the discharge chamber including the sub-discharge chamber and the deposition chamber are shown in Fig. 2 for various gas pressures. In the deposition chamber, the plasma patterns were almost the same shape for all gas pressures. Many bright funnel-shaped irregular discharges were generated in the discharge chamber near and inside some of the holes in the showerhead of the VHF electrode (cathode electrode). Similar strong inhomogeneous plasma was also observed inside the sub-discharge chamber. Every irregular spot converged at the holes in the showerhead. This indicated that the hollow cathode discharge was induced at the holes in the VHF electrode for all gas pressures. High intensity plasma was also localized near the center of the counter electrode (anode electrode). In the deposition chamber, mushroomshaped plasma was generated near and inside the nozzle in the counter electrode. Though the plasma became smaller as the gas pressure increased, the hollow anode discharge was induced in the nozzle of the counter electrode for all gas pressures. As a result, the double hollow plasma, which means a plasma at both hollow cathode and hollow anode, was generated for all gas pressures in the VHF– HEEPT system. In our previous study of a RF–HEEPT system, we found that μc-Si thin films could be deposited at a higher rate using a double hollow plasma as compared to only a hollow anode plasma [12]. Therefore we also expect that the VHF–HEEPT with double hollow plasma has a high potential to deposit μc-Si thin films with a higher deposition rate than a single hollow anode plasma.
M. Takashiri, T. Tabuchi / Surface & Coatings Technology 204 (2010) 3525–3529
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Fig. 3. Optical emission intensities of Hα and Hβ (a) and intensity ratio (b) as a function of the gas pressure operated by VHF–HEEPT, RF–HEEPT and VHF-CCP for comparison. In both VHF–HEEPT and RF–HEEPT, the error of emission intensities of Hα, Hβ and the intensity ratio of Hβ/Hα of are ±2%, ±4% and ±6%, respectively.
HEEPT. As a result, the emission intensity ratio (Hβ/Hα) of VHF–HEEPT was 41% smaller than in RF–HEEPT. We also confirmed that the Hβ/Hα emission intensity ratio of VHF–HEEPT was lower than that of the VHF conventional capacitively-coupled plasma (VHF-CCP) at the same gas pressure [20]. Thus VHF–HEEPT yielded a plasma with lower electron temperature than RF–HEEPT and VHF-CCP, while the plasma density was kept nearly as high as RF–HEEPT. For the fabrication of μc-Si thin films with high deposition rate and high film quality, a large amount of atomic hydrogen and SiH3 radicals, which are effectively yielded in plasmas with low electron temperature, are necessary [21]. Therefore, we expect that the plasma properties of VHF–HEEPT are suitable for the deposition of μc-Si thin films with high deposition rate and high film quality. 4.2. Deposition of the silicon thin films
Fig. 2. H2 plasma emission by VHF–HEEPT in the discharge chamber, and in the deposition chamber for various gas pressures.
Optical emission spectroscopy (OES) plasma diagnostics were performed in the hydrogen plasma as a function of gas pressure with a view to optimizing deposition conditions. The hydrogen plasma emission intensities of Hα and Hβ, which approximately corresponds to the plasma density, decreased approximately linearly with increasing gas pressure as shown in Fig. 3 (a). The emission intensity ratio of Hβ/Hα also decreased with increasing gas pressure, as shown in Fig. 3 (b). We compared the plasma properties of VHF–HEEPT with those of RF–HEEPT, which was applied to the same apparatus by using a different power supply. At the gas pressure of 53 Pa, the Hα emission intensity of VHF–HEEPT was 21% smaller than in RF–HEEPT, and the Hβ emission intensity of VHF–HEEPT was 54% smaller than in RF–
For the deposition of μc-Si thin films, we employed a gas pressure of 80 Pa as a compromise to obtain both low electron temperature and high plasma density. The deposition rate of the thin films is shown in Fig. 4 as a function of the SiH4/H2 ratio. The deposition rate at the SiH4/ H2 ratio of 0.04 was 0.7 nm/s, and then it increased linearly up to 12.5 nm/s at the SiH4/H2 ratio of 0.27. Fig. 5 shows the Raman spectra of the μc-Si thin films as a function of the SiH4/H2 ratio. The Raman spectra of the thin films broadened with increasing the SiH4/H2 ratio. The Raman peak position at the lowest SiH4/H2 ratio was close to 520 cm−1, which indicated the formation of a crystalline silicon thin film. Then the slight shift of the Raman peak to a shorter wave number with increasing SiH4/H2 ratio revealed the increase of residual stress in the thin films. For detailed analysis of the structural properties of μc-Si thin films, the crystalline volume fraction, the full width at half maximum (FWHM), and the residual stress were estimated from the Raman
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Fig. 4. Deposition rate of μc-Si thin films by VHF–HEEPT as a function of the SiH4/H2 ratio.
spectra presented in Fig. 5. Employing the Si–TO phonon Raman spectra, the crystalline volume fraction, XC, was roughly estimated as Ic/(Ic + Ia), where Ic and Ia denoted the integrated intensities of spectral components around 520 and 480 cm− 1, respectively. As shown in Fig. 6 (a), increasing the SiH4/H2 ratio caused the estimated crystalline volume fraction to gradually decrease, from 88% at the SiH4/H2 ratio of 0.04 to 64% at the ratio of 0.27. This indicated that the crystallinity of the thin films at higher SiH4/H2 ratio was low but the phase still remained microcrystalline even for deposition rates over 10 nm/s. The FWHM of the Raman spectra for the thin films as a function of the SiH4/H2 ratio is presented in Fig. 6 (b). The asymmetry and broadening of the Raman spectra are caused by small crystal grains and high defect densities in thin films due to phonon scattering from the grain boundaries [22–24]. In Fig. 6 (b), the increase of the SiH4/H2 ratio corresponds to a broadening of the FWHM of the Raman spectra, from 10.5 cm− 1 at the SiH4/H2 ratio of 0.04 to 21.3 cm− 1 at the ratio of 0.27, indicating that the crystal grains became smaller or the defect density increased with increasing deposition rate. The residual stress of the thin films as a function of the SiH4/H2 ratio is also presented in Fig. 6 (b). It is known that the residual stress in thin films
Fig. 5. Raman spectra of μc-Si thin films by VHF–HEEPT as a function of Raman shift at various SiH4/H2 ratios.
Fig. 6. Crystalline volume fraction (a), FWHM and residual stress (b) of μc-Si thin films by VHF–HEEPT as a function of the SiH4/H2 ratio.
correlates with the crystalline defects, and can be estimated from the Raman spectra by the following equation [25]. −1
σðMPaÞ = −250Δωðcm
Þ:
ð1Þ
In Eq. (1), σ is the in-plane stress and Δω = ωs − ω0, where ω0 is the optical phonon wave number of the stress-free single crystal and ωs is the wave number of the stressed sample. Here, the positive and negative values of the residual stress show the tensile stress and the compressive stress, respectively. As shown in Fig. 6 (b), the SiH4/H2 ratio dependence of the residual stress of the thin films follows a similar trend as the FWHM. The increase of the SiH4/H2 ratio increased the tensile stress from 250 MPa at the SiH4/H2 ratio of 0.04 to
Fig. 7. Photo- and dark-conductivity of μc-Si thin films by VHF–HEEPT as a function of the SiH4/H2 ratio.
M. Takashiri, T. Tabuchi / Surface & Coatings Technology 204 (2010) 3525–3529
5. Conclusion
Table 1 Properties of silicon thin films deposited by VHF–HEEPT and RF–HEEPT. Plasma type
dr (nm/s)
Xc (%)
Phase
VHF–HEEPT
4.8 12.5 6.0 12.9
86 64 78 29
µc-Si µc-Si µc-Si a-Si
RF HEEPT a
Electrical conductivity σdark (S/cm)
σphoto (S/cm)
3.4 × 10− 6 4.2 × 10− 8 4.9 × 10− 6 4.5 × 10− 10
3.3 × 10− 5 2.5 × 10− 7 6.6 × 10− 5 3.2 × 10− 7
3529
Reference
This work [12]a
Our experimental results.
1200 MPa at the ratio of 0.27, indicating that the thin films with higher deposition rate are expected to have a higher defect density. Fig. 7 shows the electrical conductivity of the μc-Si thin films as a function of the SiH4/H2 ratio. The photo- and dark-conductivity of the thin films at the SiH4/H2 ratio of 0.04 were 1.5 × 10− 4 and 8.9 × 10− 6 S/ cm, respectively, and the photo-conductivity was an order of magnitude higher than that of high-performance thin films at the deposition rate of 5.5 nm/s [26]. One possible explanation of this effect could be that the photo-conductivity was increased by unintentional doping by oxygen due to changes in the charge state of recombination centers. We have not measured the oxygen concentration of the thin films in this study, but thin films deposited with similar deposition rate and conditions in an RF–HEEPT system have an oxygen concentration estimated as 1.0 × 1019 cm− 3 by using secondary ion mass spectrometry (SIMS), so we expect that the oxygen concentration of the thin films in this study is also approximately 1.0 × 1019 cm− 3. The photo- and dark-conductivity of the thin films gradually decreased with increasing SiH4/H2 ratio. At the SiH4/H2 ratio of 0.27, the photo- and dark-conductivity were 2.5 × 10− 7 and 4.2 × 10− 8 S/cm, respectively. This might be because the dark-conductivity was decreased by the increase of the amorphous phase in the thin films, and then the crystalline defects induced by the residual stress in the thin films could also cause the corresponding low photo-conductivity. We compared the properties of the μc-Si thin films in this work with those of thin films deposited by RF–HEEPT, as listed in Table 1. At the deposition rate of around 5 nm/s, the thin films deposited by both VHF–HEEPT and RF–HEEPT exhibited approximately the same crystallinity and photo-conductivity. On the other hand, at the deposition rate over 10 nm/s, the thin films deposited by VHF– HEEPT still maintained a microcrystalline phase, while the thin films deposited by RF–HEEPT turned into an amorphous phase. The deposition mechanism of μc-Si thin films with higher deposition rate by VHF–HEEPT is not yet clearly understood. However, the main models which are currently proposed have in common that the presence of a large amount of atomic hydrogen and SiH3 radicals is a key factor for forming the μc-Si with high quality [15–18]. The VHF–HEEPT system yielded a large amount of atomic hydrogen and SiH3 radicals because the electron temperature of VHF–HEEPT was lower than its RF–HEEPT counterpart while both systems obtained a high plasma density. Therefore an important accomplishment of this work is that the VHF–HEEPT system was able to maintain high crystallinity even for a deposition rate of over 10 nm/s. However, the photo-conductivity of the thin films is not sufficient to fabricate high quality solar cells so that the further optimization of the crystal structure and electrical properties, such as the reduction of the residual stress and the oxygen contamination in the thin films, is necessary.
We developed the VHF–HEEPT system to fabricate μc-Si thin films with a high deposition rate (12.5 nm/s). At first, plasma observation and plasma emission spectroscopy were performed. The results of the plasma emission spectroscopy indicated that VHF–HEEPT yielded a plasma with lower electron temperature compared to RF–HEEPT and VHF-CCP. Next, thin films were deposited for several different values of the SiH4/H2 ratio, and then the structural and electrical properties of the μc-Si thin films were estimated. The thin films deposited by VHF–HEEPT had a high deposition rate (12.5 nm/s), while RF–HEEPT yielded amorphous thin films for deposition rates over 10 nm/s. Though the deposition mechanism of the μc-Si thin film deposited by the VHF– HEEPT system has not yet been clearly understood, the key factors are considered to be the presence of a large amount of atomic hydrogen and SiH3 radicals. The VHF–HEEPT system yielded a large amount of atomic hydrogen and SiH3 radicals because the electron temperature of VHF– HEEPT was lower than its RF–HEEPT counterpart while both systems obtained a high plasma density. Therefore the VHF–HEEPT system succeeded in depositing μc-Si thin films with high deposition rate over 10 nm/s, while thin films made by RF–HEEPT became amorphous at almost the same deposition rate. However, the photo-conductivity of the VHF–HEEPT thin films is not sufficient to fabricate high quality solar cells and thus further optimization of the crystal structure and electrical properties is necessary. Acknowledgements The authors wish to thank K. Ishida for experimental support during this investigation, and Professor Dames at the University of California, Riverside for valuable comments. References [1] Y. Nasuno, M. Kondo, A. Matsuda, Sol. Energy Mater. Sol. Cells 74 (2002) 497. [2] S. Klein, T. Repmann, T. Brammer, Sol. Energy 77 (2004) 893. [3] Y. Mai, S. Klein, R. Carius, J. Wolff, A. Lambertz, F. Finger, X. Geng, J. Appl. Phys. 97 (2005) 114913. [4] V.I. Miljevic, Appl. Opt. 23 (1984) 1598. [5] A. Anders, S. Anders, Plasma Sources Sci. Technol. 4 (1995) 571. [6] J. Meier, R. Fluckiger, H. Keppner, S. Shah, Appl. Phys. Lett. 65 (1994) 860. [7] M. Ryo, Y. Sakurai, T. Kobayashi, H. Shirai, Jpn. J. Appl. Phys. 45 (2006) 8484. [8] Y. Sobajima, M. Nishino, T. Fukumori, M. Kurihara, T. Higuchi, S. Nakano, T. Toyama, H. Okamoto, Sol. Energy Mater. Sol. Cells 93 (2009) 980. [9] Y. Hotta, H. Toyoda, H. Sugai, Thin Solid Films 515 (2007) 4983. [10] C. Smit, A. Klaver, B.A. Korevaar, A.M.H.N. Petit, D.L. Williamson, R.A.C.M.M. van Swaaij, M.C.M. van de Sanden, Thin Solid Films 491 (2005) 280. [11] T. Tabuchi, M. Takashiri, H. Mizukami, Surf. Coat. Technol. 173 (2003) 243. [12] T. Tabuchi, H. Mizukami, M. Takashiri, J. Vac. Sci. Technol. A 22 (2004) 2139. [13] T. Tabuchi, M. Takashiri, K. Ishida, Surf. Coat. Technol. 202 (2007) 114. [14] M. Takashiri, K. Ishida, T. Tabuchi, Japan Patent. No.P2001-230208 (filed 2001). [15] C.C. Tsai, G.B. Anderson, R. Thompson, B. Wacker, J. Non-Cryst. Solids 114 (1989) 151. [16] K. Nakamura, K. Yoshida, S. Takeoka, I. Shimizu, Jpn. J. Appl. Phys. 34 (1995) 442. [17] A. Matsuda, J. Non-Cryst. Solids 59/60 (1983) 767. [18] M. Takai, T. Takagi, T. Nishimoto, M. Kondo, A. Matsuda, Surf. Coat. Technol. 131 (2000) 50. [19] H. Jia, J.K. Saha, N. Ohse, H. Shirai, J. Phys. D 39 (2006) 3844. [20] M. Takai, T. Nishimoto, M. Kondo, A. Matsuda, Appl. Phys. Lett. 77 (2000) 2828. [21] M. Tsuda, S. Oikawa, K. Sato, J. Chem. Phys. 91 (1989) 6822. [22] Z. Iabal, S. Veprek, A.P. Webb, P. Capezzuto, Solid State Commun. 37 (1981) 993. [23] H. Richter, Z.P. Wang, L. Ley, Solid State Commun. 39 (1981) 625. [24] I.H. Campbell, P.M. Fauchet, Solid State Commun. 58 (1986) 739. [25] I.D. Wolf, Semicond. Sci. Technol. 11 (1996) 139. [26] M. Kondo, M. Fukawa, L. Guo, A. Matsuda, J. Non-Cryst. Solids 266–269 (2000) 84.