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Low-temperature formation of silicon nitride films using pulsed-plasma CVD under near atmospheric pressure
Applied Surface Science 254 (2008) 6208–6210
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Low-temperature formation of silicon nitride films using pulsed-plasma CVD under near atmospheric pressure M. Matsumoto a,*, Y. Inayoshi a, M. Suemitsu a, E. Miyamoto b, T. Yara b, S. Nakajima b, T. Uehara b, Y. Toyoshima c a b c
Center for Interdisciplinary Research, Tohoku University, 6-3 Aramaki Aza-Aoba, Aoba-ku, Sendai 980-8578, Japan Sekisui Chemicals Co. Ltd., 2-3-17 Toranomon, Minato-ku, Tokyo 105-8450, Japan Energy Technology Research Institute, AIST, 1-1-1 Umezono, Tsukuba 305-8568, Japan
A R T I C L E I N F O
A B S T R A C T
Article history:
Silicon nitride (SiNX) film fabrication on polyethylene terephthalate (PET) substrates has been achieved at a low temperature (100 8C) by plasma enhanced chemical vapor deposition operated at near atmospheric pressures. A short-pulse based power system was employed to maintain a stable discharge of SiH4, H2 and N2 in near atmospheric pressures without the use of any inert gases such as He. The deposited films were characterized by X-ray photoelectron spectroscopy. Cross sections of the films were observed by scanning electron microscope (SEM). Despite the use of N2 in place of NH3, a high deposition rate (290 nm/min) was obtained by this near-atmospheric-pressure plasma. ß 2008 Elsevier B.V. All rights reserved.
Available online 20 March 2008 PACS: 81.15.Gh Keywords: Silicon nitride PECVD Polyethylene terephthalate Atmospheric pressure
1. Introduction Silicon nitride (SiNX) is a useful gate insulator in amorphous silicon-based thin film transistors (TFTs) and as protection/ passivation layers in solar cells [1–3]. There have been many reports on the fabrication of SiNX films including plasma enhanced chemical vapor deposition (PECVD) with a gas mixture of silane (SiH4) and ammonia (NH3) [4–6]. Use of NH3, however, is somewhat problematic because of its toxicity and causticity. If NH3 is replaced with N2, safety and low cost is promised. Several works [7,8] have been reported on formation of SiNX using N2 instead of NH3. Their growth rates at around 10 nm/min, however, are impractically lower than those obtained in conventional PECVD using NH3. We have been working on pulsed-plasma enhanced CVD under near atmospheric pressure for growth of Si-related materials on glass [9] and polyethylene terephthalate (PET) [10] substrates. When PECVD is operated under near atmospheric pressure, it provides an essential advantage in film growth rate because the number density of the reactive species in the gas phase can be significantly increased as compared to those in reduced pressure
* Corresponding author. Tel.: +81 22 217 5484; fax: + 81-22-217-5484. E-mail address: [email protected] (M. Matsumoto). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.02.186
operations. In addition, we have achieved incubation-layer-free growth of polycrystalline Si at a high rate (60 nm/min) on glass [9] as well as polyethylene terephthalate (PET) [10] substrates. Judging from the generally accepted concept that a large excess of atomic hydrogen supplied to the growing surface favors the crystallite nucleation, we have interpreted this incubation-free growth in terms of enhanced dissociation of H2 molecules into atoms in our pulsed discharge system [9,10]. We expect similar effect to the N2 dissociation and subsequent increase in the growth rate. In this paper, we report a high-rate growth (290 nm/min) of SiNX on PET substrate at a low-temperature (100 8C) by using our pulsed PECVD without employing NH3 gas under near atmospheric pressure. 2. Experiment Fig. 1 shows the schematic diagram of the PECVD apparatus. The discharge plasma is generated by applying a pulsed electric bias on the hot electrode pair (20 mm 20 mm each) located opposite to the substrate on the grounded electrode. The H2–SiH4– N2 mixture gas flows through the 1 mm gap between the hot electrodes and the substrate. Pulsed discharge is operated by applying 30 kHz bipolar pulses of 14.8 kV height, with a singlepulse discharge duration of about 5 ms. A flexible PET substrate of 0.2 mm 50 mm 50 mm was used in each experiment. The
M. Matsumoto et al. / Applied Surface Science 254 (2008) 6208–6210
6209
Fig. 1. A schematic diagram of the PECVD apparatus.
operating pressure was fixed at 500 Torr, the substrate temperature at 100 8C, the SiH4 flow at 1.5 ml/min, and the H2 flow rate at 1500 ml/min. The N2 flow rate was varied for 0–3000 ml/min with the growth time being fixed at 5 min. The deposited films were characterized by X-ray photoemission spectroscopy (XPS). Cross sections of the films were observed by scanning electron microscope (X-SEM). 3. Results and discussion Fig. 2 shows the N2 flow rate dependence of the N/Si ratio obtained from the XPS spectral intensity ratio between Si2p and N1s with calibration. The N/Si ratios are 0, 0.7, 0.9, and 1.0 for the N2 flow rates of 0, 500, 1500, 3000 ml/min, respectively. As shown, the N/Si ratio increases with the N2 flow rate. Fig. 3 shows Si2p and N1s core level spectra from films grown by using N2 flow rates of 500, 1500 and 3000 ml/min. Si2p peak position is calibrated to C1s (284.5 eV) originating from casual surface contaminant. The Si2p spectra are peak-separated into four Gaussian components: the Si bonding feature at 99.6 eV, the Si3N2 at 100.8 eV, the Si3N4 at 101.7 eV and the SiO2 at 103 eV. With increasing the N2 flow rate, both the Si and Si3N2 peaks decrease their intensities, leaving the Si3N4 peak dominant. This behavior is consistent with the trend observed in Fig. 2. The increase of the SiO2 peak intensity with the N2 flow rate (Fig. 3(a)) has not been
Fig. 2. N2 flow rate dependence of the N/Si ratio.
Fig. 3. (a) Si2p and (b) N1s spectra of a-SiNX films growth at 100 8C and 500 Torr with different N2 flow rate, as indicated.
understood yet. One possibility is increase of defects in the film, which act as an oxidation site during exposure to air after depositing. This is suggested because the plasma discharge becomes unstable with the increase of the N2 flow rate as a result
Fig. 4. XPS depth profile of atomic concentrations in the SiNX film growth at 100 8C and 500 Torr with N2 flow rate of 3000 ml/min.
6210
M. Matsumoto et al. / Applied Surface Science 254 (2008) 6208–6210
4. Summary Low-temperature deposition of SiNX films on PET substrates without NH3 gas has been achieved by employing a pulsed discharge based PECVD operated at near atmospheric pressures. It should be emphasized that the deposition rate of 290 nm/min is generally higher than other techniques using N2 as the nitrogen source. The authors believe that the present result may suffice to indicate the high potentiality of this pulsed-discharge near-atmospheric-pressure PECVD in flexible electronic devices such as flexible display and flexible solar cells. We are currently planning on the electrical evaluation of our SiN films in device structures. Acknowledgement Fig. 5. Cross section of the film grown at 100 8C and 500 Torr with N2 flow rate of 3000 ml/min observed by scanning electron microscope (SEM).
of the higher dissociation energy of N2 molecule than that of H2 molecule. The N1s spectra consist of a single band, indicating that all N atoms are in a same chemical environment (highly likely to be fully coordinated to three Si atoms) in agreement with Refs. [11] and [12]. Fig. 4 shows the XPS depth profile of the atomic concentration in the SiNX film grown at a N2 flow rates of 3000 ml/min. It is shown that the atomic concentration stays almost constant (N/Si = 1) within the film. It is also observed in Fig. 4 that oxygen is found to exist into a considerable depth of the film. Fig. 5 shows the typical X-SEM image of the film grown at 100 8C with a N2 flow rate of 3000 ml/min. From the image, the film thickness is estimated to be about 1450 nm, which corresponds to a growth rate of 290 nm/min. This growth rate is generally higher than those obtained in conventional PECVD [7,8].
This research has been supported by the Japan Science and Technology Agency and the Tohoku University Global COE program ‘‘Center of Education and Research for Information Electronics Systems’’. References [1] K. Cherenack, A. Kattamis, K. Long, I.-C. Cheng, S. Wagner, J.-C. Sturm, Mater. Res. Soc. Symp. Proc. 936 (2006) L01–L05. [2] J.K. Holt, D.G. Goodwin, A.M. Gabor, F. Jiang, M. Stavola, H.A. Atwater, Thin Solid Films 430 (2003) 37. [3] A. Masuda, H. Umemoto, H. Matsumura, Thin Solid Films 501 (2006) 149. [4] A.J. Flewitt, A.P. Dyson, J. Robertson, W.I. Milne, Thin Solid Films 383 (2001) 172. [5] J. Sancho-Parramon, S. Bosch, A. Canillas, Appl. Surf. Sci. 253 (2006) 65. [6] B. Karunagaran, S.J. Chung, S. Velumani, E.-K. Suh, Mater. Chem. Phys. 106 (2007) 130. [7] K.M. Chang, C.C. Cheng, C.C. Lang, Solid-State Electron. 46 (2002) 1399. [8] Y.T. Kim, D.S. Kim, D.H. Yoon, Mater. Sci. Eng. B 118 (2005) 242. [9] H. Kitabatake, M. Suemitsu, H. Kitahata, S. Nakajima, T. Uehara, Y. Toyoshima, Jpn. J. Appl. Phys. 44 (2005) L683. [10] M. Matsumoto, M. Suemitsu, T. Yara, S. Nakajima, T. Uehara, Y. Toyoshima, S. Itou, ECS Trans. 3 (2006) 119. [11] C.H.F. Peden, J.W. Rogers, N.D. Shinn, K.B. Kidd, K.L. Tsang, Phys. Rev. B 47 (1993) 15622. [12] L.G. Jacobsohn, R.K. Schulze, L.L. Daemen, I.V. Afanasyev-Charkin, M. Nastasi, Thin Solid Films 494 (2006) 219.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Low-temperature formation of silicon nitride films using pulsed-plasma CVD under near atmospheric pressure M. Matsumoto a,*, Y. Inayoshi a, M. Suemitsu a, E. Miyamoto b, T. Yara b, S. Nakajima b, T. Uehara b, Y. Toyoshima c a b c
Center for Interdisciplinary Research, Tohoku University, 6-3 Aramaki Aza-Aoba, Aoba-ku, Sendai 980-8578, Japan Sekisui Chemicals Co. Ltd., 2-3-17 Toranomon, Minato-ku, Tokyo 105-8450, Japan Energy Technology Research Institute, AIST, 1-1-1 Umezono, Tsukuba 305-8568, Japan
A R T I C L E I N F O
A B S T R A C T
Article history:
Silicon nitride (SiNX) film fabrication on polyethylene terephthalate (PET) substrates has been achieved at a low temperature (100 8C) by plasma enhanced chemical vapor deposition operated at near atmospheric pressures. A short-pulse based power system was employed to maintain a stable discharge of SiH4, H2 and N2 in near atmospheric pressures without the use of any inert gases such as He. The deposited films were characterized by X-ray photoelectron spectroscopy. Cross sections of the films were observed by scanning electron microscope (SEM). Despite the use of N2 in place of NH3, a high deposition rate (290 nm/min) was obtained by this near-atmospheric-pressure plasma. ß 2008 Elsevier B.V. All rights reserved.
Available online 20 March 2008 PACS: 81.15.Gh Keywords: Silicon nitride PECVD Polyethylene terephthalate Atmospheric pressure
1. Introduction Silicon nitride (SiNX) is a useful gate insulator in amorphous silicon-based thin film transistors (TFTs) and as protection/ passivation layers in solar cells [1–3]. There have been many reports on the fabrication of SiNX films including plasma enhanced chemical vapor deposition (PECVD) with a gas mixture of silane (SiH4) and ammonia (NH3) [4–6]. Use of NH3, however, is somewhat problematic because of its toxicity and causticity. If NH3 is replaced with N2, safety and low cost is promised. Several works [7,8] have been reported on formation of SiNX using N2 instead of NH3. Their growth rates at around 10 nm/min, however, are impractically lower than those obtained in conventional PECVD using NH3. We have been working on pulsed-plasma enhanced CVD under near atmospheric pressure for growth of Si-related materials on glass [9] and polyethylene terephthalate (PET) [10] substrates. When PECVD is operated under near atmospheric pressure, it provides an essential advantage in film growth rate because the number density of the reactive species in the gas phase can be significantly increased as compared to those in reduced pressure
* Corresponding author. Tel.: +81 22 217 5484; fax: + 81-22-217-5484. E-mail address: [email protected] (M. Matsumoto). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.02.186
operations. In addition, we have achieved incubation-layer-free growth of polycrystalline Si at a high rate (60 nm/min) on glass [9] as well as polyethylene terephthalate (PET) [10] substrates. Judging from the generally accepted concept that a large excess of atomic hydrogen supplied to the growing surface favors the crystallite nucleation, we have interpreted this incubation-free growth in terms of enhanced dissociation of H2 molecules into atoms in our pulsed discharge system [9,10]. We expect similar effect to the N2 dissociation and subsequent increase in the growth rate. In this paper, we report a high-rate growth (290 nm/min) of SiNX on PET substrate at a low-temperature (100 8C) by using our pulsed PECVD without employing NH3 gas under near atmospheric pressure. 2. Experiment Fig. 1 shows the schematic diagram of the PECVD apparatus. The discharge plasma is generated by applying a pulsed electric bias on the hot electrode pair (20 mm 20 mm each) located opposite to the substrate on the grounded electrode. The H2–SiH4– N2 mixture gas flows through the 1 mm gap between the hot electrodes and the substrate. Pulsed discharge is operated by applying 30 kHz bipolar pulses of 14.8 kV height, with a singlepulse discharge duration of about 5 ms. A flexible PET substrate of 0.2 mm 50 mm 50 mm was used in each experiment. The
M. Matsumoto et al. / Applied Surface Science 254 (2008) 6208–6210
6209
Fig. 1. A schematic diagram of the PECVD apparatus.
operating pressure was fixed at 500 Torr, the substrate temperature at 100 8C, the SiH4 flow at 1.5 ml/min, and the H2 flow rate at 1500 ml/min. The N2 flow rate was varied for 0–3000 ml/min with the growth time being fixed at 5 min. The deposited films were characterized by X-ray photoemission spectroscopy (XPS). Cross sections of the films were observed by scanning electron microscope (X-SEM). 3. Results and discussion Fig. 2 shows the N2 flow rate dependence of the N/Si ratio obtained from the XPS spectral intensity ratio between Si2p and N1s with calibration. The N/Si ratios are 0, 0.7, 0.9, and 1.0 for the N2 flow rates of 0, 500, 1500, 3000 ml/min, respectively. As shown, the N/Si ratio increases with the N2 flow rate. Fig. 3 shows Si2p and N1s core level spectra from films grown by using N2 flow rates of 500, 1500 and 3000 ml/min. Si2p peak position is calibrated to C1s (284.5 eV) originating from casual surface contaminant. The Si2p spectra are peak-separated into four Gaussian components: the Si bonding feature at 99.6 eV, the Si3N2 at 100.8 eV, the Si3N4 at 101.7 eV and the SiO2 at 103 eV. With increasing the N2 flow rate, both the Si and Si3N2 peaks decrease their intensities, leaving the Si3N4 peak dominant. This behavior is consistent with the trend observed in Fig. 2. The increase of the SiO2 peak intensity with the N2 flow rate (Fig. 3(a)) has not been
Fig. 2. N2 flow rate dependence of the N/Si ratio.
Fig. 3. (a) Si2p and (b) N1s spectra of a-SiNX films growth at 100 8C and 500 Torr with different N2 flow rate, as indicated.
understood yet. One possibility is increase of defects in the film, which act as an oxidation site during exposure to air after depositing. This is suggested because the plasma discharge becomes unstable with the increase of the N2 flow rate as a result
Fig. 4. XPS depth profile of atomic concentrations in the SiNX film growth at 100 8C and 500 Torr with N2 flow rate of 3000 ml/min.
6210
M. Matsumoto et al. / Applied Surface Science 254 (2008) 6208–6210
4. Summary Low-temperature deposition of SiNX films on PET substrates without NH3 gas has been achieved by employing a pulsed discharge based PECVD operated at near atmospheric pressures. It should be emphasized that the deposition rate of 290 nm/min is generally higher than other techniques using N2 as the nitrogen source. The authors believe that the present result may suffice to indicate the high potentiality of this pulsed-discharge near-atmospheric-pressure PECVD in flexible electronic devices such as flexible display and flexible solar cells. We are currently planning on the electrical evaluation of our SiN films in device structures. Acknowledgement Fig. 5. Cross section of the film grown at 100 8C and 500 Torr with N2 flow rate of 3000 ml/min observed by scanning electron microscope (SEM).
of the higher dissociation energy of N2 molecule than that of H2 molecule. The N1s spectra consist of a single band, indicating that all N atoms are in a same chemical environment (highly likely to be fully coordinated to three Si atoms) in agreement with Refs. [11] and [12]. Fig. 4 shows the XPS depth profile of the atomic concentration in the SiNX film grown at a N2 flow rates of 3000 ml/min. It is shown that the atomic concentration stays almost constant (N/Si = 1) within the film. It is also observed in Fig. 4 that oxygen is found to exist into a considerable depth of the film. Fig. 5 shows the typical X-SEM image of the film grown at 100 8C with a N2 flow rate of 3000 ml/min. From the image, the film thickness is estimated to be about 1450 nm, which corresponds to a growth rate of 290 nm/min. This growth rate is generally higher than those obtained in conventional PECVD [7,8].
This research has been supported by the Japan Science and Technology Agency and the Tohoku University Global COE program ‘‘Center of Education and Research for Information Electronics Systems’’. References [1] K. Cherenack, A. Kattamis, K. Long, I.-C. Cheng, S. Wagner, J.-C. Sturm, Mater. Res. Soc. Symp. Proc. 936 (2006) L01–L05. [2] J.K. Holt, D.G. Goodwin, A.M. Gabor, F. Jiang, M. Stavola, H.A. Atwater, Thin Solid Films 430 (2003) 37. [3] A. Masuda, H. Umemoto, H. Matsumura, Thin Solid Films 501 (2006) 149. [4] A.J. Flewitt, A.P. Dyson, J. Robertson, W.I. Milne, Thin Solid Films 383 (2001) 172. [5] J. Sancho-Parramon, S. Bosch, A. Canillas, Appl. Surf. Sci. 253 (2006) 65. [6] B. Karunagaran, S.J. Chung, S. Velumani, E.-K. Suh, Mater. Chem. Phys. 106 (2007) 130. [7] K.M. Chang, C.C. Cheng, C.C. Lang, Solid-State Electron. 46 (2002) 1399. [8] Y.T. Kim, D.S. Kim, D.H. Yoon, Mater. Sci. Eng. B 118 (2005) 242. [9] H. Kitabatake, M. Suemitsu, H. Kitahata, S. Nakajima, T. Uehara, Y. Toyoshima, Jpn. J. Appl. Phys. 44 (2005) L683. [10] M. Matsumoto, M. Suemitsu, T. Yara, S. Nakajima, T. Uehara, Y. Toyoshima, S. Itou, ECS Trans. 3 (2006) 119. [11] C.H.F. Peden, J.W. Rogers, N.D. Shinn, K.B. Kidd, K.L. Tsang, Phys. Rev. B 47 (1993) 15622. [12] L.G. Jacobsohn, R.K. Schulze, L.L. Daemen, I.V. Afanasyev-Charkin, M. Nastasi, Thin Solid Films 494 (2006) 219.