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Low-temperature processing of sol–gel-derived lead–zirconate–titanate thin films by oxygen-plasma treatment
Current Applied Physics 2 (2002) 407–409 www.elsevier.com/locate/cap
Low-temperature processing of sol–gel-derived lead–zirconate–titanate thin films by oxygen-plasma treatment Eung Kil Kang a, Hyuk Kyoo Jang a, Sung Kyun Lee a, Eung Ryul Park a, Cheol Eui Lee a,*, Kyung Min Kim b, Seung Jeong Noh b, Seung-Jin Yeom c a
b
Department of Physics, Korea University, Seoul 136-701, South Korea Department of Applied Physics, Dankook University, Seoul 140-714, South Korea c Hynix Semiconductor Inc., Kyongki 467-701, South Korea Received 23 January 2002; accepted 30 August 2002
Abstract Ferroelectric thin films of sol–gel-derived Pb(Zrx , Ti1x )O3 (lead–zirconate–titanate, PZT) were obtained by the low-temperature processing employing oxygen-plasma treatment. The as-coated PZT films were annealed in oxygen ambience at 450 °C, followed by oxygen-plasma treatment at 200 °C, which gave rise to the ferroelectric hysteresis. Annealing of the as-coated PZT films followed by oxygen-plasma teratment at 200 °C gave rise to the ferroelectric hysteresis. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 77.80.)e; 81.65.)b; 52.77.)j; 81.10.Jt Keywords: Low-temperature processing; PZT; Thin films; Oxygen-plasma treatment
1. Introduction Lead–zirconate–titanate (PZT) thin films have been extensively studied for applications in dynamic random access memories and nonvolatile memories for their remarkable ferroelectric and dielectric properties. The PZT family is generally regarded as the most promising candidate for ferroelectric memory applications [1,2]. High-temperature annealing at about 650 °C is generally required to obtain the perovskite PZT phase for practical applications. However, integration of PZT thin films with memory cells in ferroelectric random access memories requires a low-temperature processing in order to minimize the oxidation of the TiN electrodes, which are generally used for fabrication of FRAM capacitors. Oxide layers are formed between the Pt and TiN layers during the high-temperature processing employed for the formation of the PZT perovskite phase. In the integration of the PZT thin films with semiconductor devices, it is also necessary to avoid high
*
Corresponding author. Tel.: +82-2-3290-3098; fax: +82-2-9273292. E-mail address: [email protected] (C.E. Lee).
processing temperatures, which degrade the Al/Si interconnect, and damage the CMOS read-out circuitry. Thus, processing temperatures not significantly higher than 450 °C are highly desired [3–5]. We have recently shown that room temperature oxygen-plasma treatment gives rise to much improved electrical characteristics of the PZT thin films [6]. It is the purpose of this work to explore a low-temperature processing of PZT thin films by introducing an oxygen-plasma treatment process after a low-temperature annealing in oxygen ambience.
2. Experiment The Pb(Zrx , Ti1x )O3 (x ¼ 0:3) specimens used in this study were fabricated by depositing 250-nm-thick PZT films on the Pt(1 1 1)/Ti/SiO2 /Si(1 0 0) substrates by a sol–gel method. The as-coated films were annealed for 30 min in oxygen ambience at 450 °C, which would mostly form the pyrochlore PZT phase. The PZT crystallization is known to undergo the formation of an amorphous phase, the pyrochlore phase, and finally, the perovskite phase [7–9]. The plasma treatment was done by exposing the PZT films to 0.5 Torr oxygen-plasma in
1567-1739/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 1 5 6 7 - 1 7 3 9 ( 0 2 ) 0 0 1 5 0 - 5
408
E.K. Kang et al. / Current Applied Physics 2 (2002) 407–409
a radio-frequency plasma reactor at 200 °C, which was the temperature of the substrate holder. In order to characterize the electrical properties of the gate capacitors, the platinum (Pt) top electrode with an area of 1:11 103 cm2 was deposited by direct-current sputtering using a shadow mask at room temperature after the plasma treatment. The measurements of ferroelectric hysteresis and the leakage current were taken using a RT66A tester and a HP4155A semiconductor parameter analyzer, respectively.
3. Results and discussion No ferroelectric hysteresis was observed in the 450 °C-annealed as-coated PZT sample before the plasmatreatment. However, the oxygen-plasma treatment at 200 °C gave rise to the P –E ferroelectric hysteresis loops, from which the remanent polarization values were obtained. Fig. 1 shows the P –E hysteresis loops for a sample plasma treated for 20 min. It is worth noting the ferroelectricity induced by the all low-temperature processing of oxygen-plasma treatment at 200 °C, followed by annealing of the as-coated PZT films at 450 °C. The average remanent polarization as a function of plasma treatment time is shown in Fig. 2. Fig. 3 shows the leakage current density as a function of the applied electric field for the PZT films. It is seen that the leakage current density decreases with the oxygen-plasma treatment time. As the leakage current density is generally attributed to the defect densities, oxygen-plasma treatment is shown to reduce the defect densities perhaps through the oxygen defect passivation [6]. In PZT films, oxygen vacancies appear as natural point defects or arise from switching when electric field is applied, giving rise to the leakage currents.
Fig. 2. The average remanent polarization as a function of the plasma treatment time.
Fig. 3. The leakage current density for the 450 °C-annealed PZT samples with various oxygen-plasma treatment times.
Fig. 1. The P –E hysteresis loop for a 450 °C-annealed PZT samples after the oxygen-plasma treatment for 20 min.
We now discuss a possible mechanism concerning the oxygen-plasma treatment effects leading to crystallization of the PZT film into the perovskite phase. While annealing the as-coated film at 450 °C can give rise to the perovskite nucleation, the growth energy of the perovskite phase, which is about four times the nucleation energy, cannot be supplied by the low-temperature annealing at 450 °C. Then, our results indicate that the oxygen-plasma treatment supplies the growth energy necessary for the perovskite phase growth, which appears to start from the top layers of the film by the oxygen-plasma treatment at 200 °C. In conclusion, by employing oxygen-plasma treatment we have developed a novel method of lowtemperature processing of the PZT ferroelectric thin
E.K. Kang et al. / Current Applied Physics 2 (2002) 407–409
films. Oxygen-plasma treatment at 200 °C, after a 450 °C-annealing of the as-coated films, gave rise to ferroelectric hysteresis loops. The occurrence of the ferroelectricity was explained by the perovskite phase formation starting from the surface layers. In effect, we have demonstrated that the oxygen-plasma treatment can be used as a simple and effective method of lowtemperature processing of the PZT films for practical applications. Higher-temperature plasma treatment is currently being explored for optimal ferroelectric performances. Acknowledgements This work was supported by the Korea Ministry of Science and Technology (National Research Laboratory) and the Korea Research Foundation. A grant from Hynix Semiconductor and measurements at the Korea Basic Science Institute (KBSI)-Seoul Branch are acknowledged.
409
References [1] J.F. Scott, C.A. Arujo, B.M. Melnick, L.D. McMillan, R. Zuleeg, J. Appl. Phys. 70 (1991) 382; H. Tabata, T. Kawai, O. Murata, J. Fujioka, Appl. Phys. Lett. 59 (1991) 2354. [2] T. Mihara, H. Watanabe, C.A. Arujo, Jpn. J. Appl. Phys. 33 (1992) 5281; C.H. Peng, S.B. Desu, Appl. Phys. Lett. 61 (1992) 16. [3] M. Sayer, K. Sreenivas, Science 247 (1990) 1056. [4] Z. Huang, Q. Zhang, R. Whatmore, J. Mater. Sci. Lett. 17 (1998) 1157. [5] Z. Huang, Q. Zhang, R. Whatmore, J. Appl. Phys. 85 (1999) 7355. [6] H.K. Jang, S.K. Lee, C.E. Lee, S.J. Noh, W.I. Lee, Appl. Phys. Lett. 76 (2000) 882. [7] M. Chen, J. Wu, T. Wu, Jpn. J. Appl. Phys. 34 (1995) 4870; K.G. Brooks, I.M. Reaney, R. Klissurska, Y. Huang, L. Bursill, N. Setter, J. Mater. Res. 9 (1994) 2540. [8] M. Klee, R. Eusemann, R. Waser, W. Brandt, H. van Hal, J. Appl. Phys. 72 (1992) 1566; Y. Liu, P. Phule, J. Am. Ceram. Soc. 79 (1996) 495. [9] N. Floquet, J. Hector, P. Gaucher, J. Appl. Phys. 84 (1998) 3815; B.A. Tuttle et al., J. Mater. Res. 7 (1992) 1876.
Low-temperature processing of sol–gel-derived lead–zirconate–titanate thin films by oxygen-plasma treatment Eung Kil Kang a, Hyuk Kyoo Jang a, Sung Kyun Lee a, Eung Ryul Park a, Cheol Eui Lee a,*, Kyung Min Kim b, Seung Jeong Noh b, Seung-Jin Yeom c a
b
Department of Physics, Korea University, Seoul 136-701, South Korea Department of Applied Physics, Dankook University, Seoul 140-714, South Korea c Hynix Semiconductor Inc., Kyongki 467-701, South Korea Received 23 January 2002; accepted 30 August 2002
Abstract Ferroelectric thin films of sol–gel-derived Pb(Zrx , Ti1x )O3 (lead–zirconate–titanate, PZT) were obtained by the low-temperature processing employing oxygen-plasma treatment. The as-coated PZT films were annealed in oxygen ambience at 450 °C, followed by oxygen-plasma treatment at 200 °C, which gave rise to the ferroelectric hysteresis. Annealing of the as-coated PZT films followed by oxygen-plasma teratment at 200 °C gave rise to the ferroelectric hysteresis. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 77.80.)e; 81.65.)b; 52.77.)j; 81.10.Jt Keywords: Low-temperature processing; PZT; Thin films; Oxygen-plasma treatment
1. Introduction Lead–zirconate–titanate (PZT) thin films have been extensively studied for applications in dynamic random access memories and nonvolatile memories for their remarkable ferroelectric and dielectric properties. The PZT family is generally regarded as the most promising candidate for ferroelectric memory applications [1,2]. High-temperature annealing at about 650 °C is generally required to obtain the perovskite PZT phase for practical applications. However, integration of PZT thin films with memory cells in ferroelectric random access memories requires a low-temperature processing in order to minimize the oxidation of the TiN electrodes, which are generally used for fabrication of FRAM capacitors. Oxide layers are formed between the Pt and TiN layers during the high-temperature processing employed for the formation of the PZT perovskite phase. In the integration of the PZT thin films with semiconductor devices, it is also necessary to avoid high
*
Corresponding author. Tel.: +82-2-3290-3098; fax: +82-2-9273292. E-mail address: [email protected] (C.E. Lee).
processing temperatures, which degrade the Al/Si interconnect, and damage the CMOS read-out circuitry. Thus, processing temperatures not significantly higher than 450 °C are highly desired [3–5]. We have recently shown that room temperature oxygen-plasma treatment gives rise to much improved electrical characteristics of the PZT thin films [6]. It is the purpose of this work to explore a low-temperature processing of PZT thin films by introducing an oxygen-plasma treatment process after a low-temperature annealing in oxygen ambience.
2. Experiment The Pb(Zrx , Ti1x )O3 (x ¼ 0:3) specimens used in this study were fabricated by depositing 250-nm-thick PZT films on the Pt(1 1 1)/Ti/SiO2 /Si(1 0 0) substrates by a sol–gel method. The as-coated films were annealed for 30 min in oxygen ambience at 450 °C, which would mostly form the pyrochlore PZT phase. The PZT crystallization is known to undergo the formation of an amorphous phase, the pyrochlore phase, and finally, the perovskite phase [7–9]. The plasma treatment was done by exposing the PZT films to 0.5 Torr oxygen-plasma in
1567-1739/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 1 5 6 7 - 1 7 3 9 ( 0 2 ) 0 0 1 5 0 - 5
408
E.K. Kang et al. / Current Applied Physics 2 (2002) 407–409
a radio-frequency plasma reactor at 200 °C, which was the temperature of the substrate holder. In order to characterize the electrical properties of the gate capacitors, the platinum (Pt) top electrode with an area of 1:11 103 cm2 was deposited by direct-current sputtering using a shadow mask at room temperature after the plasma treatment. The measurements of ferroelectric hysteresis and the leakage current were taken using a RT66A tester and a HP4155A semiconductor parameter analyzer, respectively.
3. Results and discussion No ferroelectric hysteresis was observed in the 450 °C-annealed as-coated PZT sample before the plasmatreatment. However, the oxygen-plasma treatment at 200 °C gave rise to the P –E ferroelectric hysteresis loops, from which the remanent polarization values were obtained. Fig. 1 shows the P –E hysteresis loops for a sample plasma treated for 20 min. It is worth noting the ferroelectricity induced by the all low-temperature processing of oxygen-plasma treatment at 200 °C, followed by annealing of the as-coated PZT films at 450 °C. The average remanent polarization as a function of plasma treatment time is shown in Fig. 2. Fig. 3 shows the leakage current density as a function of the applied electric field for the PZT films. It is seen that the leakage current density decreases with the oxygen-plasma treatment time. As the leakage current density is generally attributed to the defect densities, oxygen-plasma treatment is shown to reduce the defect densities perhaps through the oxygen defect passivation [6]. In PZT films, oxygen vacancies appear as natural point defects or arise from switching when electric field is applied, giving rise to the leakage currents.
Fig. 2. The average remanent polarization as a function of the plasma treatment time.
Fig. 3. The leakage current density for the 450 °C-annealed PZT samples with various oxygen-plasma treatment times.
Fig. 1. The P –E hysteresis loop for a 450 °C-annealed PZT samples after the oxygen-plasma treatment for 20 min.
We now discuss a possible mechanism concerning the oxygen-plasma treatment effects leading to crystallization of the PZT film into the perovskite phase. While annealing the as-coated film at 450 °C can give rise to the perovskite nucleation, the growth energy of the perovskite phase, which is about four times the nucleation energy, cannot be supplied by the low-temperature annealing at 450 °C. Then, our results indicate that the oxygen-plasma treatment supplies the growth energy necessary for the perovskite phase growth, which appears to start from the top layers of the film by the oxygen-plasma treatment at 200 °C. In conclusion, by employing oxygen-plasma treatment we have developed a novel method of lowtemperature processing of the PZT ferroelectric thin
E.K. Kang et al. / Current Applied Physics 2 (2002) 407–409
films. Oxygen-plasma treatment at 200 °C, after a 450 °C-annealing of the as-coated films, gave rise to ferroelectric hysteresis loops. The occurrence of the ferroelectricity was explained by the perovskite phase formation starting from the surface layers. In effect, we have demonstrated that the oxygen-plasma treatment can be used as a simple and effective method of lowtemperature processing of the PZT films for practical applications. Higher-temperature plasma treatment is currently being explored for optimal ferroelectric performances. Acknowledgements This work was supported by the Korea Ministry of Science and Technology (National Research Laboratory) and the Korea Research Foundation. A grant from Hynix Semiconductor and measurements at the Korea Basic Science Institute (KBSI)-Seoul Branch are acknowledged.
409
References [1] J.F. Scott, C.A. Arujo, B.M. Melnick, L.D. McMillan, R. Zuleeg, J. Appl. Phys. 70 (1991) 382; H. Tabata, T. Kawai, O. Murata, J. Fujioka, Appl. Phys. Lett. 59 (1991) 2354. [2] T. Mihara, H. Watanabe, C.A. Arujo, Jpn. J. Appl. Phys. 33 (1992) 5281; C.H. Peng, S.B. Desu, Appl. Phys. Lett. 61 (1992) 16. [3] M. Sayer, K. Sreenivas, Science 247 (1990) 1056. [4] Z. Huang, Q. Zhang, R. Whatmore, J. Mater. Sci. Lett. 17 (1998) 1157. [5] Z. Huang, Q. Zhang, R. Whatmore, J. Appl. Phys. 85 (1999) 7355. [6] H.K. Jang, S.K. Lee, C.E. Lee, S.J. Noh, W.I. Lee, Appl. Phys. Lett. 76 (2000) 882. [7] M. Chen, J. Wu, T. Wu, Jpn. J. Appl. Phys. 34 (1995) 4870; K.G. Brooks, I.M. Reaney, R. Klissurska, Y. Huang, L. Bursill, N. Setter, J. Mater. Res. 9 (1994) 2540. [8] M. Klee, R. Eusemann, R. Waser, W. Brandt, H. van Hal, J. Appl. Phys. 72 (1992) 1566; Y. Liu, P. Phule, J. Am. Ceram. Soc. 79 (1996) 495. [9] N. Floquet, J. Hector, P. Gaucher, J. Appl. Phys. 84 (1998) 3815; B.A. Tuttle et al., J. Mater. Res. 7 (1992) 1876.