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Polycrystalline Si thin film growth on glass using pulsed d.c. magnetron sputtering
Thin Solid Films 420 – 421 (2002) 429–432
Polycrystalline Si thin film growth on glass using pulsed d.c. magnetron sputtering Min J. Junga,*, Yun M. Junga, Leonid R. Shaginyanb, Jeon G. Hana a
Center for Advanced Plasma Surface Technology, Sungkyunkwan University, 300 Chunchun-dong, Jangan-gu, Suwon 440-746, South Korea b Institute for Problems of Materials Science of Academy of Sciences of Ukraine, Kiev, Ukraine
Abstract Crystalline poly-Si thin films were deposited on Corning glass substrates at a temperature of 500 8C using a pulsed d.c. magnetron sputtering source. The increased bias voltage causes an enhancement of the potential difference between the plasma and substrate. In case of the bipolar pulsed d.c. magnetron sputtering, Vp shows higher value than that in the unipolar pulsed d.c. as the bias voltage increases. Therefore, the mobility of the sputtered adatoms increases by the surface heat accumulation of energetic sputtered particles with the increased bias voltage. Consequently, more higher mobility (41 cm2 Vsy1 ) and crystallinity were obtained for the Si film grown with the bipolar-pulsed d.c. magnetron sputtering due to the energy incorporation by enhanced bombardment of Ar ions. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Magnetron sputtering; Polysilicon; Substrate bias; Hall mobility; Raman spectra
1. Introduction Polycrystalline Si thin film is widely used as an industrial material for thin film transistors for flat panel display (FPD) and photovoltaic applications because it shows high mobility, electrical conductivity and highenergy conversion efficiency compared with a-Si w1,2x. Over the past few years, there have been a variety of techniques on the thin film growth of poly-Si. Among them, solid phase crystallization (SPC) and excimer laser annealing (ELA) have been considered as the most frequently used methods. However, the SPC method has a too high crystallization temperature (f650 8C) for the glass substrate. Although the ELA method is suitable at low temperature for glass substrates, there are some problems such as non-uniformity of grain growth on large area glass substrates and expensive processing costs w3x. Recently, a metal-induced crystallization (MIC) method of amorphous silicon has been studied to prepare poly-Si thin films on glass at low temperature w4 x . A magnetron sputtering method by a pulsed d.c. source was focused to improve the crystallinity and *Corresponding author. Tel.: q82-31-299-6646; fax: q82-31-2905669. E-mail address: [email protected] (M.J. Jung).
grain growth of poly-Si thin film at low temperature w5x. It is also well known that thin film growth can be substantially modified by ion-bombardment during sputtering w6x. This is particularly important in the case of thin-film deposition at low temperatures where the film growth occurs under highly non-equilibrium conditions. An attractive way to promote the crystalline growth at low temperature is the creation of additional energy in to the surface of the growing film by the bombardment of hyperthermal particles during sputtering processing. Therefore, to investigate the relationships between the film microstructure and growth mechanism of poly-Si thin film, poly-Si was deposited on the glass substrate with a bias voltage for the bombardment of hyperthermal particles by magnetron sputtering. In addition, the epitaxial orientation, microstructual characteristics and surface properties of the films were analyzed by transmission electron microscopy (TEM) and atomic force microscopy (AFM). For the electrical characterization of these films, their properties were obtained from the Hall effect measurement by the van der Pauw measurement. 2. Experimental procedure Poly-Si films were prepared by a pulsed d.c. unbalanced magnetron sputtering system from an undoped
0040-6090/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 2 . 0 0 8 0 7 - 6
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M.J. Jung et al. / Thin Solid Films 420 – 421 (2002) 429–432
Fig. 1. Raman spectra of poly-Si thin films on glass substrate with different bias and power. (a) Bipolar pulsed d.c., (b) bipolar and unipolar pulsed d.c (fixed bias: y100 V).
silicon target (2 inch) in pure Ar atmosphere at a pressure of 0.3 Pa and the substrate temperature of 500 8C. The base pressure of system was maintained at 0.03 Pa. A symmetrical bipolar and unipolar pulsed power generator was operated in a constant power mode (20 W cmy2) at the same pulse frequency (20 kHz) and duty ratio (50%). Films were deposited on Corning 1737 glass with a SiO2 layer (thickness 50 nm) coated by plasma-enhanced chemical vapor deposition (PECVD). Thickness of all samples was fixed at 200 nm. Since the glass substrate is an insulator, it was Cucoated partially to realize equipotential surfaces at the substrates by induced negative bias voltage to enhance ion bombardment. Prior to deposition, substrates were cleaned by immersing into acetone with the aid of ultrasonic cleaning. The distance between the target and substrate was kept at 65 mm. During sputtering, the characteristics of plasma were analyzed by the in-situ Doubling Langmuir probe method. The thickness of films was measured using a stylus-type surface tracer. TEM was also used to characterize the microstructure of silicon thin films. The microcrystallinity and surface profile of these films were investigated by Raman spectroscopy and AFM. For the electrical characterization of these films, their properties were obtained from the Hall effect measurement by the van der Pauw measurement. The van der Pauw measurements were carried out at a sample temperature of 300 K in a magnetic field of 3000 G, using a Keithley 181 nanovoltagemeter and Keithley 236 current source. Metal contact to the sample for the Hall effect measurements was realized with the evaporation of Indium. 3. Results and discussion Fig. 1 shows Raman scattering spectra of silicon thin films prepared with different pulsed d.c. power and bias
voltage. The spectra indicate that crystallization occurred in all films because there are no signal peaks from the amorphous phase at approximately 480 cmy1. It is clear from the spectra that the Si–Si TO mode is located at 517–521 cmy1 w7x. The band width broadens with the decreased bias voltage, which means a decrease in grain size. This result is attributed to an increased probability of grain growth as the ion energy, or equivalently the bias voltage is increased w8x. The AFM microstructure images of Si films deposited on the glass substrate as different pulsed d.c. power and bias voltage are presented in Fig. 2. As it is clear from the pictures, the grain growth increases as a function of bias voltage. Basically, the mobility of the sputtered adatoms increases by the surface heat accumulation of energetic sputtered particles as an increased bias voltage w9x. From these results, it is clear that the tendency of the grain growth with the bias voltage is the same for both unipolar and bipolar pulsed d.c. Therefore, we conclude that the grain growth of poly-Si film is highly influenced by the substrate bias voltage due to the different growth probability by ion bombardment. Fig. 3. shows the cross sectional TEM for the crystalline Si thin film deposited by bipolar pulsed d.c. magnetron sputtering with increased substrate bias voltage. TEM observation with bright-field image and electron diffraction pattern provide clear evidence of the formation of Si crystallites. As shown in the diffraction pattern of Fig. 3, the crystallinity in the film is enhanced as the bias voltage increases. The same result of enhanced crystallinity was obtained by Raman spectra in Fig. 1. In the previous study, we have found that substrate bias voltage has a strong influence on the crystallinity and grain growth. In the case of the increased negative substrate bias, this behavior may arise from the potential difference between plasma and substrate, which enhanc-
M.J. Jung et al. / Thin Solid Films 420 – 421 (2002) 429–432
431
Fig. 2. of poly-Si films deposited on glass as different power and bias voltage. (a) Unipolar pulsed d.c.: bias 0 V, (b) unipolar pulsed d.c.: bias y100 V, (c) bipolar pulsed d.c.: bias 0 V, (d) bipolar pulsed d.c.: bias y100 V.
Fig. 3. TEM micrographs for poly-Si deposited by bipolar pulsed d.c as different bias voltage. (a) Bias 0 V, (b) bias y100 V.
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magnetron sputtering, Vp shows higher value than that in the unipolar pulsed d.c. as the bias voltage increases. Therefore, the substrate is subjected to be a enhanced ion bombardment condition. This result suggests that previous referred modification of the films arise from the energy incorporation into the growing surface of the films by an enhanced bombardment of Ar ions having high energy. The Hall mobility of Si films deposited on the glass substrate with different pulsed d.c. power and bias voltage are presented in Fig. 5. For both unipolar and bipolar pulsed d.c. magnetron sputtering, the Hall mobility increases with the bias voltage. Higher mobility (41 cm2 Vsy1) was obtained for the Si film grown with the bipolar pulsed d.c. magnetron sputtering due to the energy incorporation by enhanced bombardment of Ar ions. 4. Conclusions
Fig. 4. Bias dependence of plasma potentials as different pulsed d.c. power.
es the energy transfer into the growing film by the increasing ion bombardment w6x. In order to get more direct insight about the sputtering process, the associated potentials were studied using Langmuir-probe measurement. The probe was immersed into the plasma between the target and substrate. The plasma potential (Vp) can be detected by probe measurement. The results are explained in Fig. 4. In case of the bipolar pulsed d.c.
The crystalline Si thin films were deposited on Corning glass substrates at the temperature of 500 8C using pulsed d.c. magnetron sputtering source. The substrate bias voltage has a significant effect on the properties of the films. The increased bias voltage causes an enhancement of the potential difference between the plasma and substrate. This difference is responsible for enhanced energy transfer in the film by Ar ions. Therefore, more higher mobility (41 cm2 Vsy1) and crystallinity were obtained from the Si film grown with the bipolar pulsed d.c. magnetron sputtering because of the energy incorporation by enhanced bombardment of Ar ions. Acknowledgments The authors are grateful for the financial support provided by the Korea Science and Engineering Foundation through the Center for Advanced Plasma Surface Technology at Sungkyunkwan University. References
Fig. 5. Bias dependence of the Hall mobility of poly-Si thin films as different pulsed d.c. power.
w1x R.B. Bergmann, J. Kohler, R. Dassow, C. Zaczek, J.H. Werner, Phys. Stat. Sol. 166 (1998) 587. w2x M.K. Hatalis, D.N. Kouvatsos, J.H. Kung, A.T. Voutsas, J. Kanicki, Thin Solid Films 338 (1999) 281. w3x S.Y. Yoon, S.J. Park, K.H. Kim, J. Jang, Thin Solid Films 383 (2001) 34. w4x J. Jang, J.I. Soo, S.Y. Yoon, K.H. Kim, Vacuum 51 (1998) 774. w5x J.H. Boo, H.K. Park, K.H. Nam, J.G. Han, Surf. Coat. Technol. 131 (2000) 211. w6x P. Reinig, V. Alex, F. Fenske, W. Fuhs, B. Selle, Thin Solid Films 403y404 (2002) 86. w7x Y. Feng, M. Zhu, F. Liu, J. Liu, H. Han, Y. Han, Thin Solid Films 395 (2001) 213. w8x M. Oden, C. Ericsson, G. Hakansson, H. Ljungcrantz, Surf. Coat. Technol. 114 (1999) 39–51. w9x C. Gautier, J. Machet, Thin Solid Films 295 (1997) 43–52.
Polycrystalline Si thin film growth on glass using pulsed d.c. magnetron sputtering Min J. Junga,*, Yun M. Junga, Leonid R. Shaginyanb, Jeon G. Hana a
Center for Advanced Plasma Surface Technology, Sungkyunkwan University, 300 Chunchun-dong, Jangan-gu, Suwon 440-746, South Korea b Institute for Problems of Materials Science of Academy of Sciences of Ukraine, Kiev, Ukraine
Abstract Crystalline poly-Si thin films were deposited on Corning glass substrates at a temperature of 500 8C using a pulsed d.c. magnetron sputtering source. The increased bias voltage causes an enhancement of the potential difference between the plasma and substrate. In case of the bipolar pulsed d.c. magnetron sputtering, Vp shows higher value than that in the unipolar pulsed d.c. as the bias voltage increases. Therefore, the mobility of the sputtered adatoms increases by the surface heat accumulation of energetic sputtered particles with the increased bias voltage. Consequently, more higher mobility (41 cm2 Vsy1 ) and crystallinity were obtained for the Si film grown with the bipolar-pulsed d.c. magnetron sputtering due to the energy incorporation by enhanced bombardment of Ar ions. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Magnetron sputtering; Polysilicon; Substrate bias; Hall mobility; Raman spectra
1. Introduction Polycrystalline Si thin film is widely used as an industrial material for thin film transistors for flat panel display (FPD) and photovoltaic applications because it shows high mobility, electrical conductivity and highenergy conversion efficiency compared with a-Si w1,2x. Over the past few years, there have been a variety of techniques on the thin film growth of poly-Si. Among them, solid phase crystallization (SPC) and excimer laser annealing (ELA) have been considered as the most frequently used methods. However, the SPC method has a too high crystallization temperature (f650 8C) for the glass substrate. Although the ELA method is suitable at low temperature for glass substrates, there are some problems such as non-uniformity of grain growth on large area glass substrates and expensive processing costs w3x. Recently, a metal-induced crystallization (MIC) method of amorphous silicon has been studied to prepare poly-Si thin films on glass at low temperature w4 x . A magnetron sputtering method by a pulsed d.c. source was focused to improve the crystallinity and *Corresponding author. Tel.: q82-31-299-6646; fax: q82-31-2905669. E-mail address: [email protected] (M.J. Jung).
grain growth of poly-Si thin film at low temperature w5x. It is also well known that thin film growth can be substantially modified by ion-bombardment during sputtering w6x. This is particularly important in the case of thin-film deposition at low temperatures where the film growth occurs under highly non-equilibrium conditions. An attractive way to promote the crystalline growth at low temperature is the creation of additional energy in to the surface of the growing film by the bombardment of hyperthermal particles during sputtering processing. Therefore, to investigate the relationships between the film microstructure and growth mechanism of poly-Si thin film, poly-Si was deposited on the glass substrate with a bias voltage for the bombardment of hyperthermal particles by magnetron sputtering. In addition, the epitaxial orientation, microstructual characteristics and surface properties of the films were analyzed by transmission electron microscopy (TEM) and atomic force microscopy (AFM). For the electrical characterization of these films, their properties were obtained from the Hall effect measurement by the van der Pauw measurement. 2. Experimental procedure Poly-Si films were prepared by a pulsed d.c. unbalanced magnetron sputtering system from an undoped
0040-6090/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 2 . 0 0 8 0 7 - 6
430
M.J. Jung et al. / Thin Solid Films 420 – 421 (2002) 429–432
Fig. 1. Raman spectra of poly-Si thin films on glass substrate with different bias and power. (a) Bipolar pulsed d.c., (b) bipolar and unipolar pulsed d.c (fixed bias: y100 V).
silicon target (2 inch) in pure Ar atmosphere at a pressure of 0.3 Pa and the substrate temperature of 500 8C. The base pressure of system was maintained at 0.03 Pa. A symmetrical bipolar and unipolar pulsed power generator was operated in a constant power mode (20 W cmy2) at the same pulse frequency (20 kHz) and duty ratio (50%). Films were deposited on Corning 1737 glass with a SiO2 layer (thickness 50 nm) coated by plasma-enhanced chemical vapor deposition (PECVD). Thickness of all samples was fixed at 200 nm. Since the glass substrate is an insulator, it was Cucoated partially to realize equipotential surfaces at the substrates by induced negative bias voltage to enhance ion bombardment. Prior to deposition, substrates were cleaned by immersing into acetone with the aid of ultrasonic cleaning. The distance between the target and substrate was kept at 65 mm. During sputtering, the characteristics of plasma were analyzed by the in-situ Doubling Langmuir probe method. The thickness of films was measured using a stylus-type surface tracer. TEM was also used to characterize the microstructure of silicon thin films. The microcrystallinity and surface profile of these films were investigated by Raman spectroscopy and AFM. For the electrical characterization of these films, their properties were obtained from the Hall effect measurement by the van der Pauw measurement. The van der Pauw measurements were carried out at a sample temperature of 300 K in a magnetic field of 3000 G, using a Keithley 181 nanovoltagemeter and Keithley 236 current source. Metal contact to the sample for the Hall effect measurements was realized with the evaporation of Indium. 3. Results and discussion Fig. 1 shows Raman scattering spectra of silicon thin films prepared with different pulsed d.c. power and bias
voltage. The spectra indicate that crystallization occurred in all films because there are no signal peaks from the amorphous phase at approximately 480 cmy1. It is clear from the spectra that the Si–Si TO mode is located at 517–521 cmy1 w7x. The band width broadens with the decreased bias voltage, which means a decrease in grain size. This result is attributed to an increased probability of grain growth as the ion energy, or equivalently the bias voltage is increased w8x. The AFM microstructure images of Si films deposited on the glass substrate as different pulsed d.c. power and bias voltage are presented in Fig. 2. As it is clear from the pictures, the grain growth increases as a function of bias voltage. Basically, the mobility of the sputtered adatoms increases by the surface heat accumulation of energetic sputtered particles as an increased bias voltage w9x. From these results, it is clear that the tendency of the grain growth with the bias voltage is the same for both unipolar and bipolar pulsed d.c. Therefore, we conclude that the grain growth of poly-Si film is highly influenced by the substrate bias voltage due to the different growth probability by ion bombardment. Fig. 3. shows the cross sectional TEM for the crystalline Si thin film deposited by bipolar pulsed d.c. magnetron sputtering with increased substrate bias voltage. TEM observation with bright-field image and electron diffraction pattern provide clear evidence of the formation of Si crystallites. As shown in the diffraction pattern of Fig. 3, the crystallinity in the film is enhanced as the bias voltage increases. The same result of enhanced crystallinity was obtained by Raman spectra in Fig. 1. In the previous study, we have found that substrate bias voltage has a strong influence on the crystallinity and grain growth. In the case of the increased negative substrate bias, this behavior may arise from the potential difference between plasma and substrate, which enhanc-
M.J. Jung et al. / Thin Solid Films 420 – 421 (2002) 429–432
431
Fig. 2. of poly-Si films deposited on glass as different power and bias voltage. (a) Unipolar pulsed d.c.: bias 0 V, (b) unipolar pulsed d.c.: bias y100 V, (c) bipolar pulsed d.c.: bias 0 V, (d) bipolar pulsed d.c.: bias y100 V.
Fig. 3. TEM micrographs for poly-Si deposited by bipolar pulsed d.c as different bias voltage. (a) Bias 0 V, (b) bias y100 V.
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M.J. Jung et al. / Thin Solid Films 420 – 421 (2002) 429–432
magnetron sputtering, Vp shows higher value than that in the unipolar pulsed d.c. as the bias voltage increases. Therefore, the substrate is subjected to be a enhanced ion bombardment condition. This result suggests that previous referred modification of the films arise from the energy incorporation into the growing surface of the films by an enhanced bombardment of Ar ions having high energy. The Hall mobility of Si films deposited on the glass substrate with different pulsed d.c. power and bias voltage are presented in Fig. 5. For both unipolar and bipolar pulsed d.c. magnetron sputtering, the Hall mobility increases with the bias voltage. Higher mobility (41 cm2 Vsy1) was obtained for the Si film grown with the bipolar pulsed d.c. magnetron sputtering due to the energy incorporation by enhanced bombardment of Ar ions. 4. Conclusions
Fig. 4. Bias dependence of plasma potentials as different pulsed d.c. power.
es the energy transfer into the growing film by the increasing ion bombardment w6x. In order to get more direct insight about the sputtering process, the associated potentials were studied using Langmuir-probe measurement. The probe was immersed into the plasma between the target and substrate. The plasma potential (Vp) can be detected by probe measurement. The results are explained in Fig. 4. In case of the bipolar pulsed d.c.
The crystalline Si thin films were deposited on Corning glass substrates at the temperature of 500 8C using pulsed d.c. magnetron sputtering source. The substrate bias voltage has a significant effect on the properties of the films. The increased bias voltage causes an enhancement of the potential difference between the plasma and substrate. This difference is responsible for enhanced energy transfer in the film by Ar ions. Therefore, more higher mobility (41 cm2 Vsy1) and crystallinity were obtained from the Si film grown with the bipolar pulsed d.c. magnetron sputtering because of the energy incorporation by enhanced bombardment of Ar ions. Acknowledgments The authors are grateful for the financial support provided by the Korea Science and Engineering Foundation through the Center for Advanced Plasma Surface Technology at Sungkyunkwan University. References
Fig. 5. Bias dependence of the Hall mobility of poly-Si thin films as different pulsed d.c. power.
w1x R.B. Bergmann, J. Kohler, R. Dassow, C. Zaczek, J.H. Werner, Phys. Stat. Sol. 166 (1998) 587. w2x M.K. Hatalis, D.N. Kouvatsos, J.H. Kung, A.T. Voutsas, J. Kanicki, Thin Solid Films 338 (1999) 281. w3x S.Y. Yoon, S.J. Park, K.H. Kim, J. Jang, Thin Solid Films 383 (2001) 34. w4x J. Jang, J.I. Soo, S.Y. Yoon, K.H. Kim, Vacuum 51 (1998) 774. w5x J.H. Boo, H.K. Park, K.H. Nam, J.G. Han, Surf. Coat. Technol. 131 (2000) 211. w6x P. Reinig, V. Alex, F. Fenske, W. Fuhs, B. Selle, Thin Solid Films 403y404 (2002) 86. w7x Y. Feng, M. Zhu, F. Liu, J. Liu, H. Han, Y. Han, Thin Solid Films 395 (2001) 213. w8x M. Oden, C. Ericsson, G. Hakansson, H. Ljungcrantz, Surf. Coat. Technol. 114 (1999) 39–51. w9x C. Gautier, J. Machet, Thin Solid Films 295 (1997) 43–52.