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Thin Solid Films 516 (2008) 4452 – 4455 www.elsevier.com/locate/tsf
Investigation of plasma parameters in 915 MHz ECR plasma with SiH4/H2 mixtures Doan Ha Thang a,⁎, Hiroshi Muta b , Yoshinobu Kawai a a
b
Research Institute for Applied Mechanics, Kyushu University, Japan Department of Advanced Energy Engineering Science, Kyushu University, Japan Available online 14 January 2008
Abstract The plasma parameters in 915 MHz ECR plasma with SiH4/H2 mixtures were investigated using a heated Langmuir probe where extremely high dilution ratio of H2 to SiH4 was used for preparing microcrystalline silicon thin films. As the incident microwave power was increased, the electron temperature (Te) decreased from 7 eV to 2–3 eV and the electron density (ne) increased from 0.5 × 1011 cm− 3 to 1.3 × 1011 cm− 3, that is, low Te ECR plasma with high ne was realized using 915 MHz microwaves. As a result of the film deposition, it was found that there is a correlation between the Te and crystallinity of the microcrystalline silicon. Furthermore, it was shown that high deposition rate can be realized by increasing the gas flow rate. © 2007 Elsevier B.V. All rights reserved. Keywords: Microcrystalline silicon; ECR plasma; Electron temperature; High deposition rate
1. Introduction Microcrystalline silicon (μc–Si) thin film has important device applications such as solar cell and thin film transistor for PDP [1,2]. As a plasma source, VHF plasma has been often used because of the high deposition rate due to the high plasma density. However, it has a problem that the standing wave and skin effect deteriorate uniformity and reproducibility of the plasma [3,4]. On the other hand, electron cyclotron resonance (ECR) plasmas have advantageous characteristics such as high density (ne∼1012 cm− 3) under low gas pressure (∼10− 3 Torr) operation. Although their electron temperature (Te) is relatively high, the use of 915 MHz excitation for ECR plasma leads to lower both Te and sheath potential compared with those of conventional 2.45 GHz excitation. Accordingly, 915 MHz ECR plasma can meet high deposition rate and high quality film with less dust particles. As is well known, almost reactions in the low pressure plasma depend on Te, so that the control of Te is essential to find out the best conditions necessary for qualified material processing. In addition, the Te control is now crucially required ⁎ Corresponding author. E-mail address: [email protected] (D.H. Thang). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.10.099
from industry because high Te plasma sources cause serious problems such as substrate damage due to ion bombardment [5]. Therefore, it is the most important subject to measure Te and ne in a 915 MHz ECR plasma. In this paper, the parameters of a 915 MHz ECR plasma were measured with a heated Langmuir probe. At the same time, the prepared films were evaluated by Raman spectroscopy and spectrophotometer. On the basis of the experimental data obtained under various experimental conditions, the correlation between Te and crystallinity of microcrystalline silicon and the film deposition rate were discussed. 2. Experimental apparatus and operating conditions A schematic diagram of the experimental apparatus is shown in Fig. 1. The cylindrical vacuum chamber was made of stainless steel with an inner diameter of 290 mm and a length of 1200 mm. Before the introduction of gases, the chamber was pumped to below 2 × 10− 6 Torr with a turbo molecular pump. H2 gas and SiH4/H2 gas were introduced into the chamber at a flow rate controlled to 50–200 sccm by mass flow meters from a periphery of the substrate. The total pressure was kept at 5–10 mTorr. The substrate was located at the axial position of z = 550 mm from the quarts window, and isolated electrically and heated at a constant
D.H. Thang et al. / Thin Solid Films 516 (2008) 4452–4455
4453
Fig. 1. Schematic diagram of the experimental apparatus.
of 250 °C. The 915 MHz microwave with TM01 mode was injected into the chamber at the input power of 0.6–2.2 kW. Matching between the microwave circuit and the plasma was regulated with an automatic stub tuner in such a way that the reflected microwave powers were as low as possible. In the previous research using nitrogen and argon, the Te of 915 MHz ECR plasma was considerably controlled by a specific magnetic field distribution [6]. Accordingly, we used similar magnetic configuration in expectation of high controllability of the Te. Fig. 2 shows the axial magnetic field distribution. The ECR position corresponding to 0.0327 T was set over a relatively wide area at the center of chamber (r = 0 cm).
Te, we separately measured Te as the component perpendicular (Te⊥) and parallel (Te//) to the magnetic field. As a result, the increase of Te above 1.5 kW was entirely attributed to the Te⊥ rather than Te//. It suggests that the increase of Te⊥ is due to the heating by the electromagnetic waves. Therefore, it is considered that the microwave power was spent on the electron heating bellow 1.5 kW and the ionization above 1.5 kW. Next, we show the results for SiH4/H2 mixture plasma. Fig. 4 (a) and (b) shows the dependence of the Te and the ne on the microwave power, respectively. The ne was calculated assuming
3. Experimental results and discussion First, the Te and ne of H2 ECR plasma were measured as a function of the microwave power and gas flow rate, prior to SiH4/H2 plasma. Fig. 3(a) and (b) shows the dependences of the Te and ne on the microwave power, respectively. As seen in Fig. 3 (a), as the microwave power was increased, the Te decreased and began to increase above 1.5 kW. On the other hand, the ne increased monotonically and was almost saturated above 1.7 kW. In order to understand this unusual tendency of
Fig. 2. Axial distribution of magnetic field.
Fig. 3. The microwave power dependences of (a) Te and (b) np in the H2 plasma at 10 mTorr and the gas flow rate from 100 to 200 sccm.
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D.H. Thang et al. / Thin Solid Films 516 (2008) 4452–4455
1.5 kW. In general, the ion bombard energy is evaluated by the following expression. rffiffiffiffiffiffiffiffiffiffiffi kTe mi ln Vs Vf ¼ e 2pme Here e is electron charge and k is Boltzman constant. Vs and Vf are plasma potential and floating potential, me and mi are electron and ion mass, respectively. Equation (1) means that the ion bombardment energy is proportional to Te. Namely, the best microcrystalline silicon was realized when Te was minimum and as a result the ion bombardment energy (Vs − Vf) was minimum. Therefore, it is considered that one of the reasons for the maximum crystallinity at the minimum Te was attributed to the minimum ion bombardment damage to the film. Regarding the decrease of the deposition rate at the minimum Te, it is beyond our knowledge at the present stage. The change of the film property from the amorphous state to the microcrystalline state may be associated with the film growth.
Fig. 4. The microwave power dependences of (a) Te and (b) np in SiH4/H2 mixture plasma at 10 mTorr, D = 0.1.
that the positive ions are composed of both H+ and SiH3+. It was observed that Te had a local minimum value at 1.5 kW. On the other hand, ne was observed to be slightly monotonously increasing for the gas flow rate of 100, 150 and 200 sccm. This tendency was same as that of pure H2 plasma. Furthermore, silicon thin films were actually prepared on the glass substrate (Corning 7059) at a constant temperature of 250 °C in accordance with the experimental conditions described above. They were evaluated by Raman scattering spectroscopy as shown in Fig. 5. The production of μc-Si was clearly observed only at the microwave power between 1.25 kW and 1.75 kW (Fig. 5 (a)). The best crystallinity was obtained at 1.5 kW as shown in Fig. 5(b). Then, the volume fraction Ic/Ia was 2.1 and μc–Si was prepared homogeneously in the radial direction of r = ± 2 cm from the center of substrate. Fig. 6 shows the dependence of the film property on the microwave power. The volume fraction of μc-Si was evaluated using Ic/Ia obtained from the Raman scattering intensity. Here Ic and Ia indicate the Raman intensity at a wave number of 520 cm− 1 and 480 cm− 1, respectively. The deposition rate was evaluated from the film thickness measured using a spectrophotometer. From the comparison between Figs. 4 (a) and 6 (a), we notice that the volume fraction is maximum and the deposition rate is minimum when the Te has a minimum value at
Fig. 5. Raman scattering spectrum (a) and at 1.5 kW where the best microcrystalline was obtained (b).
D.H. Thang et al. / Thin Solid Films 516 (2008) 4452–4455
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Fig. 7. The effect of gas flow rate on the deposition rate.
flow for high deposition rate, that is, the higher the gas flow rate is, the higher the deposition rate becomes. 4. Conclusions
Fig. 6. The dependence of μc Si film on the microwave power. (a) Volume fraction (b) Deposition rate.
In the fabrication of microcrystalline silicon films, high deposition rate is one of the most important factors for cost reduction because of the thick absorber layer needed. Therefore, we investigated the deposition rate as a function of gas flow rate. In general, ECR plasma is essentially low pressure (typically order of mTorr) discharge so that it is disadvantageous for high speed deposition in spite of high plasma density. However, increasing the gas flow rate tends to increase the deposition rate because it makes the flux of SiH3 radicals increase. As shown in Fig. 7, the deposition rate linearly increases between 100 sccm and 200 sccm. This result demonstrates the importance of gas
The plasma parameters in a 915 MHz ECR plasma with H2 and SiH4/H2 mixtures were measured successfully using a heated Langmuir probe. In addition, the microcrystalline silicon thin film was fabricated. The increase of gas flow rate was tried because high gas flow causes the high fluxes of radicals (especially SiH3 here) to the substrate. The experimental results revealed that the deposition rate linearly increased between 100 sccm and 200 sccm with Ic/Ia of 2.1 and deposition rate of 1.12 nm/s at 200 sccm gas flow rate. According to the recent VHF plasma research, typical deposition rate is 2 nm/s at 600 sccm. Therefore, these results suggest that ECR plasma can meet the high deposition rate by increasing the gas flow rate. It was found that a close correlation among the electron temperature, crystallinity and deposition rate of the μc–Si film. References [1] [2] [3] [4]
W.E. Spear, P.G. LeComber, Solid State Commun. 17 (1975) 1193. C.R. Wronski, et al., Appl. Phys. Lett. 29 (1976) 602. W. Tsai, J. Vac. Sci. Technol. B14 (1996) 3276. Y.P. Raizer, M.N. Schneider, N.A. Yatsenko, Radio-Frequency Capacitive Discharges, CRC Press, Boca Raton, FL, 1993 sect. 1.3.6. [5] G. Ganguly, A. Matsuda, Mater. Res. Soc. Proc. 336 (1994) 7. [6] Y. Kawai, et al., Surf. Coat. Technol. 193 (2005) 11.
Thin Solid Films 516 (2008) 4452 – 4455 www.elsevier.com/locate/tsf
Investigation of plasma parameters in 915 MHz ECR plasma with SiH4/H2 mixtures Doan Ha Thang a,⁎, Hiroshi Muta b , Yoshinobu Kawai a a
b
Research Institute for Applied Mechanics, Kyushu University, Japan Department of Advanced Energy Engineering Science, Kyushu University, Japan Available online 14 January 2008
Abstract The plasma parameters in 915 MHz ECR plasma with SiH4/H2 mixtures were investigated using a heated Langmuir probe where extremely high dilution ratio of H2 to SiH4 was used for preparing microcrystalline silicon thin films. As the incident microwave power was increased, the electron temperature (Te) decreased from 7 eV to 2–3 eV and the electron density (ne) increased from 0.5 × 1011 cm− 3 to 1.3 × 1011 cm− 3, that is, low Te ECR plasma with high ne was realized using 915 MHz microwaves. As a result of the film deposition, it was found that there is a correlation between the Te and crystallinity of the microcrystalline silicon. Furthermore, it was shown that high deposition rate can be realized by increasing the gas flow rate. © 2007 Elsevier B.V. All rights reserved. Keywords: Microcrystalline silicon; ECR plasma; Electron temperature; High deposition rate
1. Introduction Microcrystalline silicon (μc–Si) thin film has important device applications such as solar cell and thin film transistor for PDP [1,2]. As a plasma source, VHF plasma has been often used because of the high deposition rate due to the high plasma density. However, it has a problem that the standing wave and skin effect deteriorate uniformity and reproducibility of the plasma [3,4]. On the other hand, electron cyclotron resonance (ECR) plasmas have advantageous characteristics such as high density (ne∼1012 cm− 3) under low gas pressure (∼10− 3 Torr) operation. Although their electron temperature (Te) is relatively high, the use of 915 MHz excitation for ECR plasma leads to lower both Te and sheath potential compared with those of conventional 2.45 GHz excitation. Accordingly, 915 MHz ECR plasma can meet high deposition rate and high quality film with less dust particles. As is well known, almost reactions in the low pressure plasma depend on Te, so that the control of Te is essential to find out the best conditions necessary for qualified material processing. In addition, the Te control is now crucially required ⁎ Corresponding author. E-mail address: [email protected] (D.H. Thang). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.10.099
from industry because high Te plasma sources cause serious problems such as substrate damage due to ion bombardment [5]. Therefore, it is the most important subject to measure Te and ne in a 915 MHz ECR plasma. In this paper, the parameters of a 915 MHz ECR plasma were measured with a heated Langmuir probe. At the same time, the prepared films were evaluated by Raman spectroscopy and spectrophotometer. On the basis of the experimental data obtained under various experimental conditions, the correlation between Te and crystallinity of microcrystalline silicon and the film deposition rate were discussed. 2. Experimental apparatus and operating conditions A schematic diagram of the experimental apparatus is shown in Fig. 1. The cylindrical vacuum chamber was made of stainless steel with an inner diameter of 290 mm and a length of 1200 mm. Before the introduction of gases, the chamber was pumped to below 2 × 10− 6 Torr with a turbo molecular pump. H2 gas and SiH4/H2 gas were introduced into the chamber at a flow rate controlled to 50–200 sccm by mass flow meters from a periphery of the substrate. The total pressure was kept at 5–10 mTorr. The substrate was located at the axial position of z = 550 mm from the quarts window, and isolated electrically and heated at a constant
D.H. Thang et al. / Thin Solid Films 516 (2008) 4452–4455
4453
Fig. 1. Schematic diagram of the experimental apparatus.
of 250 °C. The 915 MHz microwave with TM01 mode was injected into the chamber at the input power of 0.6–2.2 kW. Matching between the microwave circuit and the plasma was regulated with an automatic stub tuner in such a way that the reflected microwave powers were as low as possible. In the previous research using nitrogen and argon, the Te of 915 MHz ECR plasma was considerably controlled by a specific magnetic field distribution [6]. Accordingly, we used similar magnetic configuration in expectation of high controllability of the Te. Fig. 2 shows the axial magnetic field distribution. The ECR position corresponding to 0.0327 T was set over a relatively wide area at the center of chamber (r = 0 cm).
Te, we separately measured Te as the component perpendicular (Te⊥) and parallel (Te//) to the magnetic field. As a result, the increase of Te above 1.5 kW was entirely attributed to the Te⊥ rather than Te//. It suggests that the increase of Te⊥ is due to the heating by the electromagnetic waves. Therefore, it is considered that the microwave power was spent on the electron heating bellow 1.5 kW and the ionization above 1.5 kW. Next, we show the results for SiH4/H2 mixture plasma. Fig. 4 (a) and (b) shows the dependence of the Te and the ne on the microwave power, respectively. The ne was calculated assuming
3. Experimental results and discussion First, the Te and ne of H2 ECR plasma were measured as a function of the microwave power and gas flow rate, prior to SiH4/H2 plasma. Fig. 3(a) and (b) shows the dependences of the Te and ne on the microwave power, respectively. As seen in Fig. 3 (a), as the microwave power was increased, the Te decreased and began to increase above 1.5 kW. On the other hand, the ne increased monotonically and was almost saturated above 1.7 kW. In order to understand this unusual tendency of
Fig. 2. Axial distribution of magnetic field.
Fig. 3. The microwave power dependences of (a) Te and (b) np in the H2 plasma at 10 mTorr and the gas flow rate from 100 to 200 sccm.
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D.H. Thang et al. / Thin Solid Films 516 (2008) 4452–4455
1.5 kW. In general, the ion bombard energy is evaluated by the following expression. rffiffiffiffiffiffiffiffiffiffiffi kTe mi ln Vs Vf ¼ e 2pme Here e is electron charge and k is Boltzman constant. Vs and Vf are plasma potential and floating potential, me and mi are electron and ion mass, respectively. Equation (1) means that the ion bombardment energy is proportional to Te. Namely, the best microcrystalline silicon was realized when Te was minimum and as a result the ion bombardment energy (Vs − Vf) was minimum. Therefore, it is considered that one of the reasons for the maximum crystallinity at the minimum Te was attributed to the minimum ion bombardment damage to the film. Regarding the decrease of the deposition rate at the minimum Te, it is beyond our knowledge at the present stage. The change of the film property from the amorphous state to the microcrystalline state may be associated with the film growth.
Fig. 4. The microwave power dependences of (a) Te and (b) np in SiH4/H2 mixture plasma at 10 mTorr, D = 0.1.
that the positive ions are composed of both H+ and SiH3+. It was observed that Te had a local minimum value at 1.5 kW. On the other hand, ne was observed to be slightly monotonously increasing for the gas flow rate of 100, 150 and 200 sccm. This tendency was same as that of pure H2 plasma. Furthermore, silicon thin films were actually prepared on the glass substrate (Corning 7059) at a constant temperature of 250 °C in accordance with the experimental conditions described above. They were evaluated by Raman scattering spectroscopy as shown in Fig. 5. The production of μc-Si was clearly observed only at the microwave power between 1.25 kW and 1.75 kW (Fig. 5 (a)). The best crystallinity was obtained at 1.5 kW as shown in Fig. 5(b). Then, the volume fraction Ic/Ia was 2.1 and μc–Si was prepared homogeneously in the radial direction of r = ± 2 cm from the center of substrate. Fig. 6 shows the dependence of the film property on the microwave power. The volume fraction of μc-Si was evaluated using Ic/Ia obtained from the Raman scattering intensity. Here Ic and Ia indicate the Raman intensity at a wave number of 520 cm− 1 and 480 cm− 1, respectively. The deposition rate was evaluated from the film thickness measured using a spectrophotometer. From the comparison between Figs. 4 (a) and 6 (a), we notice that the volume fraction is maximum and the deposition rate is minimum when the Te has a minimum value at
Fig. 5. Raman scattering spectrum (a) and at 1.5 kW where the best microcrystalline was obtained (b).
D.H. Thang et al. / Thin Solid Films 516 (2008) 4452–4455
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Fig. 7. The effect of gas flow rate on the deposition rate.
flow for high deposition rate, that is, the higher the gas flow rate is, the higher the deposition rate becomes. 4. Conclusions
Fig. 6. The dependence of μc Si film on the microwave power. (a) Volume fraction (b) Deposition rate.
In the fabrication of microcrystalline silicon films, high deposition rate is one of the most important factors for cost reduction because of the thick absorber layer needed. Therefore, we investigated the deposition rate as a function of gas flow rate. In general, ECR plasma is essentially low pressure (typically order of mTorr) discharge so that it is disadvantageous for high speed deposition in spite of high plasma density. However, increasing the gas flow rate tends to increase the deposition rate because it makes the flux of SiH3 radicals increase. As shown in Fig. 7, the deposition rate linearly increases between 100 sccm and 200 sccm. This result demonstrates the importance of gas
The plasma parameters in a 915 MHz ECR plasma with H2 and SiH4/H2 mixtures were measured successfully using a heated Langmuir probe. In addition, the microcrystalline silicon thin film was fabricated. The increase of gas flow rate was tried because high gas flow causes the high fluxes of radicals (especially SiH3 here) to the substrate. The experimental results revealed that the deposition rate linearly increased between 100 sccm and 200 sccm with Ic/Ia of 2.1 and deposition rate of 1.12 nm/s at 200 sccm gas flow rate. According to the recent VHF plasma research, typical deposition rate is 2 nm/s at 600 sccm. Therefore, these results suggest that ECR plasma can meet the high deposition rate by increasing the gas flow rate. It was found that a close correlation among the electron temperature, crystallinity and deposition rate of the μc–Si film. References [1] [2] [3] [4]
W.E. Spear, P.G. LeComber, Solid State Commun. 17 (1975) 1193. C.R. Wronski, et al., Appl. Phys. Lett. 29 (1976) 602. W. Tsai, J. Vac. Sci. Technol. B14 (1996) 3276. Y.P. Raizer, M.N. Schneider, N.A. Yatsenko, Radio-Frequency Capacitive Discharges, CRC Press, Boca Raton, FL, 1993 sect. 1.3.6. [5] G. Ganguly, A. Matsuda, Mater. Res. Soc. Proc. 336 (1994) 7. [6] Y. Kawai, et al., Surf. Coat. Technol. 193 (2005) 11.