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Preparation and properties of PLZT thick films on silicon
Materials Science and Engineering B99 (2003) 195 /198 www.elsevier.com/locate/mseb
Preparation and properties of PLZT thick films on silicon Hong-Jin Zhao a,*, Tian-Ling Ren a, Ning-Xin Zhang b, Ru-Zhong Zuo b, Xiao-Hui Wang b, Li-Tian Liu a, Zhi-Jian Li a, Zhi-Lun Gui b, Long-Tu Li b b
a Institute of Microelectronics, Tsinghua University, Beijing 100084, People’s Republic of China Department of Material Science and Engineering, Tsinghua University, Beijing 100084, People’s Republic of China
Received 14 June 2002; received in revised form 15 September 2002; accepted 21 October 2002
Abstract Lead-lanthanum-zirconate-titanate (PLZT) thick films were prepared on Pt/Ti/SiO2/Si substrates with PZT/PT seeding layer by the screen-printing method. Phase characterization and crystal orientation of the PLZT thick films were investigated by X-ray diffraction analysis (XRD). The ferroelectric hysteresis loop, high-frequency dielectric constant, dielectric loss and piezoelectric constant of the PLZT thick films were measured. The remnant polarization of the silicon-based PLZT thick films was about 32 mC cm 2, the coercive field was about 20 kV cm 1 and the piezoelectric constant d33 was about 630 pC N 1. In the frequency range from 1 to 300 MHz, the dielectric constant was about 3000 and the dielectric loss was less than 0.03, respectively. The PLZT thick films with excellent ferroelectric, high frequency and force /electric coupling properties should be suitable for the ferroelectrics / silicon integrated system, and be a good candidate material for the third-generation (3G) mobile communication and the force / electric coupling microelectromechanical system (MEMS) device applications. # 2002 Elsevier Science B.V. All rights reserved. Keywords: PLZT; Thick films; Screen-printing; High-frequency properties
1. Introduction Lead-lanthanum-zirconate-titanate (PLZT) is one of the most studied perovskite-type ferroelectric materials. PLZT material shows excellent optical /electric, dielectric, piezoelectric, ferroelectric properties. In recent years, with the development of the fabrication technology, PLZT material shows great potential for the microelectromechanical systems (MEMS) [1], and memory [2]. Especially due to its high dielectric permittivity, it is expected to meet the requirements for both isolation and small device size in the front-end radio-frequency (RF) device applications because the device size and the pffiffiffiffi quality factor (Q value) are inverse proportional to o r and the dielectric loss, respectively.
* Corresponding author. Tel.: /86-10-627-85486; fax: /86-10-62771130. E-mail address: [email protected] (H.-J. Zhao).
However, PLZT material, in general, is limited to lowfrequency operation (less than 100 MHz) because of few studies on high-frequency properties. In recent years, with the development of the third-generation (3G) mobile communication, high-frequency and miniature front-end devices for communication system are in great demand. Studies on the PLZT material with excellent electrical properties for the RF front-end device applications (such as filter, antenna and switch, etc) have been one of the hotspots in the fields of communication and microelectronics. The fabrication of PLZT thick films has attracted much attention [3 /5] and various thin-film deposition techniques have been investigated [6 /9]. In this paper, PLZT nano powders were prepared using a sol /gel method. The PLZT ink was formed using PLZT nano powders mixtures and other materials. PLZT thick films were prepared on Pt/Ti/SiO2/Si substrate with PZT/PT seeding layer by the screenprinting method. Ferroelectric hysteresis loop, highfrequency dielectric constant, dielectric loss and piezo-
0921-5107/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-5107(02)00518-4
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H.-J. Zhao et al. / Materials Science and Engineering B99 (2003) 195 /198
electric constant d33 of the PLZT material were measured.
2. Experiments Lead acetate [Pb(CH3COO)2], lanthanum acetate [La(CH3COO)2] and zirconium nitrate [Zr(NO3)4], tetrabutyl titanate [Ti(OC4H9)4] were used as source materials for the preparation of PLZT solution. Lead acetate tri-hydrate and lanthanum acetate were dissolved in 2-methoxyethanol to obtain a Pb /La-precursor solution. After that, tetrabutyl titanate and zirconium nitrate were mixed with the Pb /La-precursor solution to form a PLZT precursor solution. After filtrating, dust and other particulate matter were removed and 0.4 mol l 1 (M) PLZT solution was prepared. PLZT nano powders obtained after vaporizing the solution, were annealed at 750 8C for 30 min to make it crystallized completely. The annealed PLZT powders were mixed again with some boro-silicate glass powders and an organic vehicle to form the ink that was suitable for think-film screenprinting. PLZT thick films were prepared on Pt/Ti/SiO2/ Si substrate with PZT/PT seeding layer by the screenprinting method. The PT thin film was used to debase the annealed temperature of the PZT thin film and the PZT layer to enhance the PLZT thick film adhesion to SiO2 layer. After the screen-printing, the thick films were dried at 150 8C for 10 min in order to remove the solvent. Finally, it was fired in a conveyor belt furnace with a cycle time of 4 h and a peak temperature of 900 8C. Fig. 1 shows the flow diagram for the preparation of the PLZT thick films. Fig. 2 shows the cross-section structure of the PLZT thick films on silicon for the electric properties measurement. Phase characterization and crystal orientation of the PLZT material were investigated by X-ray diffraction analysis (XRD) (D/max-RB). The Pt was sputtered on the surface to introduce electrode for the further electrical measurements. Hysteresis measurement was carried out on a ferroelectric tester (RT6000HVS by Radiant Technology Inc., USA). The dielectric constant and dielectric loss were also measured using an impedance analyzer (HP4291B). Typical poling condition was 30 kV cm1 for 15 min at 1209/2 8C. The piezoelectric constant d33 of the PLZT thick films was measured using the quasi-static d33 measuring instrument (mode ZJ-3A).
Fig. 1. Flow diagram for the preparation of PLZT thick films.
Fig. 2. Measurement structure of PLZT thick films.
3. Results and discussion Fig. 3 is the XRD pattern of the phase characterization and crystal orientation of the PLZT thick films.
From the pattern, the sample exhibits strong peaks at 2u /228 and 2u /388corresponding to the (001) and (111) peaks, respectively. (110), (200) and (211) peaks of
H.-J. Zhao et al. / Materials Science and Engineering B99 (2003) 195 /198
197
Fig. 3. XRD pattern of the PLZT thick films. Fig. 5. Hysteresis loop of the PLZT thick films.
PLZT perovskite structure can also be observed at 2u / 31, 45 and 558. It indicated that the screen-printing derived PLZT thick films were crystallized completely. At the same time, the PLZT thick films with PZT/PT seeding layer showed more highly (001) and (111) preferred orientation. The PLZT thick films with (001) and (111) orientation will have the advantage of high piezoelectric constant due to its sensitivity to the outer stress. Discussing the origin of orientation in sol /gel derived PZT thin films, the mechanism for (001) and (111)orientated perovskite structures growth: the Pt layer deposited on the Si (001) is highly (111)-orientated, thus, the PZT thin films without PT seeding layer would prefer to crystallize in a (111) orientation because of the lattice matching with the Pt layer. At the same time, the lattice constants of PT is similar to that of Pt, so the bottom side of the PT seeding layer starts to crystallize epitaxially from the Pt layer and has a (111) orientation. However, it is interesting to point out that the upper side of the PT film on Pt probably tend to a (001) orientation
Fig. 4. Surface micrograph of the PLZT thick films measurement structure.
[9]. So, the PLZT thin films with the PZT/PT seeding layer prepared using suitable process conditions will be highly (001) and (111)-orientated. Fig. 4 illustrates the surface micrographs of PLZT thick films structure for the measurement of electrical properties. As is known, the spontaneous polarization of ferroelectric materials does not follow the electric field linearly, and thus the polarization /electric field (P /E ) hysteresis loop is resulted. Fig. 5 shows the hysteresis loop of the PLZT. An alternative electric field with sine wave was applied in the experiment. The magnitude of applied electric field was 15/20 kV cm 1 that was higher than the coercive field, and the sine wave frequency was chosen as 20 Hz. As shown in Fig. 5, the PLZT remnant polarization is about 32 mC cm2. The dielectric responses of the silicon-based PLZT thick films, examined in terms of the dielectric constant and dielectric loss as functions of measuring frequency, are shown in Fig. 6(a and b). For the PLZT thick films, the dielectric constant decreases very slowly as the frequency is below 350 MHz. Known from Fig. 6(b), the dielectric loss increases continuously as the frequency increases but is lower than 0.03 as the frequency is below 300 MHz. Fig. 6 shows that the deregulation of dielectric constant and the dielectric loss is occurred when the frequency is about 1 GHz because of the dielectric relaxation. After poling, the dielectric relaxation would be weak. The experimental results show that the PLZT thick films have excellent high-frequency properties: high dielectric constant and low dielectric loss. The measure of the piezoelectric constant d33 which is about 630 pC N 1 shows that the PLZT thick film has excellent piezoelectric property. The experimental results show that the PLZT material is a good candidate material for the force /electric coupling device applications.
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Fig. 6. Dielectric responses of the PLZT thick films (a) dielectric constant; (b) dielectric loss.
4. Conclusions In summary, PLZT nano powders were prepared using a sol /gel method. PLZT thick films were prepared on Pt/Ti/SiO2/Si substrate with PZT/PT seeding layer by the screen-printing method. Ferroelectric hysteresis loop, high-frequency dielectric constant, dielectric loss and piezoelectric constant d33 of the PLZT thick films were measured. The remnant polarization of the PLZT thick film on silicon was about 32 mC cm 2 and the coercive field was about 20 kV cm1. The experimental results showed that the PLZT thick films on silicon have excellent high frequency and force /electric coupling properties. In the spectrum from 1 to 300 MHz, the dielectric constant was about 3000 and the dielectric loss was less than 0.03, respectively. The piezoelectric constant d33 is about 630 pC N 1. The PLZT thick films are suitable for the ferroelectrics-silicon integrated system, and will be a good candidate material for the 3G mobile communication and the force /electric coupling device applications.
Acknowledgements This work is supported by the National ‘973 Project’ of People’s Republic of China (G1999033105) and the
‘985’ Project of People’s Republic of China, and by the Analysis and Testing Foundation of Tsinghua University.
References [1] S. Christopher, Electro-mechanical coupling in 8/65/35 PLZT, Proceedings of the Ninth IEEE International Symposium on Applications of Ferroelectrics, 1994, pp. 357 /360. [2] K. Suu, N. Tani, F. Chu, G. Hickert, T.D. Hadnagy, T. Davenport, Integrated Ferroelectrics 26 (1999) 9 /19. [3] H. Janez, H. Marko, K. Marija, Materials Research Bulletin 34 (1999) 2271 /2278. [4] M. Linnik, O. Wilson Jr, A. Christou, Thick PLZT films grown by sol /gel for optical device applications, Materials Research Society Symposium */Proceedings, vol. 541, 1998, pp. 735 /740 [5] E.B. Araujo, J.A. Eiras, Journal of the European Ceramic Society 19 (1998) 1453 /1456. [6] D.-X. Lu, Z.-Y. Li, L.-B. Huang, Chinese Science Bulletin 39 (1994) 1507 /1510. [7] M. Sedlar, I. Zou, M. Sayer, Integrated Ferroelectrics 6 (1994) 129 /133. [8] K. Tominaga, Y. Sakashita, M. Okada, Integrated Ferroelectrics 5 (1994) 287 /290. [9] K. Ishikawa, K. Sakura, D. Fu, S. Yamada, H. Suzuki, T. Hayashi, Japanese Journal of Applied Physics 37 (1998) 5128 / 5131.
Preparation and properties of PLZT thick films on silicon Hong-Jin Zhao a,*, Tian-Ling Ren a, Ning-Xin Zhang b, Ru-Zhong Zuo b, Xiao-Hui Wang b, Li-Tian Liu a, Zhi-Jian Li a, Zhi-Lun Gui b, Long-Tu Li b b
a Institute of Microelectronics, Tsinghua University, Beijing 100084, People’s Republic of China Department of Material Science and Engineering, Tsinghua University, Beijing 100084, People’s Republic of China
Received 14 June 2002; received in revised form 15 September 2002; accepted 21 October 2002
Abstract Lead-lanthanum-zirconate-titanate (PLZT) thick films were prepared on Pt/Ti/SiO2/Si substrates with PZT/PT seeding layer by the screen-printing method. Phase characterization and crystal orientation of the PLZT thick films were investigated by X-ray diffraction analysis (XRD). The ferroelectric hysteresis loop, high-frequency dielectric constant, dielectric loss and piezoelectric constant of the PLZT thick films were measured. The remnant polarization of the silicon-based PLZT thick films was about 32 mC cm 2, the coercive field was about 20 kV cm 1 and the piezoelectric constant d33 was about 630 pC N 1. In the frequency range from 1 to 300 MHz, the dielectric constant was about 3000 and the dielectric loss was less than 0.03, respectively. The PLZT thick films with excellent ferroelectric, high frequency and force /electric coupling properties should be suitable for the ferroelectrics / silicon integrated system, and be a good candidate material for the third-generation (3G) mobile communication and the force / electric coupling microelectromechanical system (MEMS) device applications. # 2002 Elsevier Science B.V. All rights reserved. Keywords: PLZT; Thick films; Screen-printing; High-frequency properties
1. Introduction Lead-lanthanum-zirconate-titanate (PLZT) is one of the most studied perovskite-type ferroelectric materials. PLZT material shows excellent optical /electric, dielectric, piezoelectric, ferroelectric properties. In recent years, with the development of the fabrication technology, PLZT material shows great potential for the microelectromechanical systems (MEMS) [1], and memory [2]. Especially due to its high dielectric permittivity, it is expected to meet the requirements for both isolation and small device size in the front-end radio-frequency (RF) device applications because the device size and the pffiffiffiffi quality factor (Q value) are inverse proportional to o r and the dielectric loss, respectively.
* Corresponding author. Tel.: /86-10-627-85486; fax: /86-10-62771130. E-mail address: [email protected] (H.-J. Zhao).
However, PLZT material, in general, is limited to lowfrequency operation (less than 100 MHz) because of few studies on high-frequency properties. In recent years, with the development of the third-generation (3G) mobile communication, high-frequency and miniature front-end devices for communication system are in great demand. Studies on the PLZT material with excellent electrical properties for the RF front-end device applications (such as filter, antenna and switch, etc) have been one of the hotspots in the fields of communication and microelectronics. The fabrication of PLZT thick films has attracted much attention [3 /5] and various thin-film deposition techniques have been investigated [6 /9]. In this paper, PLZT nano powders were prepared using a sol /gel method. The PLZT ink was formed using PLZT nano powders mixtures and other materials. PLZT thick films were prepared on Pt/Ti/SiO2/Si substrate with PZT/PT seeding layer by the screenprinting method. Ferroelectric hysteresis loop, highfrequency dielectric constant, dielectric loss and piezo-
0921-5107/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-5107(02)00518-4
196
H.-J. Zhao et al. / Materials Science and Engineering B99 (2003) 195 /198
electric constant d33 of the PLZT material were measured.
2. Experiments Lead acetate [Pb(CH3COO)2], lanthanum acetate [La(CH3COO)2] and zirconium nitrate [Zr(NO3)4], tetrabutyl titanate [Ti(OC4H9)4] were used as source materials for the preparation of PLZT solution. Lead acetate tri-hydrate and lanthanum acetate were dissolved in 2-methoxyethanol to obtain a Pb /La-precursor solution. After that, tetrabutyl titanate and zirconium nitrate were mixed with the Pb /La-precursor solution to form a PLZT precursor solution. After filtrating, dust and other particulate matter were removed and 0.4 mol l 1 (M) PLZT solution was prepared. PLZT nano powders obtained after vaporizing the solution, were annealed at 750 8C for 30 min to make it crystallized completely. The annealed PLZT powders were mixed again with some boro-silicate glass powders and an organic vehicle to form the ink that was suitable for think-film screenprinting. PLZT thick films were prepared on Pt/Ti/SiO2/ Si substrate with PZT/PT seeding layer by the screenprinting method. The PT thin film was used to debase the annealed temperature of the PZT thin film and the PZT layer to enhance the PLZT thick film adhesion to SiO2 layer. After the screen-printing, the thick films were dried at 150 8C for 10 min in order to remove the solvent. Finally, it was fired in a conveyor belt furnace with a cycle time of 4 h and a peak temperature of 900 8C. Fig. 1 shows the flow diagram for the preparation of the PLZT thick films. Fig. 2 shows the cross-section structure of the PLZT thick films on silicon for the electric properties measurement. Phase characterization and crystal orientation of the PLZT material were investigated by X-ray diffraction analysis (XRD) (D/max-RB). The Pt was sputtered on the surface to introduce electrode for the further electrical measurements. Hysteresis measurement was carried out on a ferroelectric tester (RT6000HVS by Radiant Technology Inc., USA). The dielectric constant and dielectric loss were also measured using an impedance analyzer (HP4291B). Typical poling condition was 30 kV cm1 for 15 min at 1209/2 8C. The piezoelectric constant d33 of the PLZT thick films was measured using the quasi-static d33 measuring instrument (mode ZJ-3A).
Fig. 1. Flow diagram for the preparation of PLZT thick films.
Fig. 2. Measurement structure of PLZT thick films.
3. Results and discussion Fig. 3 is the XRD pattern of the phase characterization and crystal orientation of the PLZT thick films.
From the pattern, the sample exhibits strong peaks at 2u /228 and 2u /388corresponding to the (001) and (111) peaks, respectively. (110), (200) and (211) peaks of
H.-J. Zhao et al. / Materials Science and Engineering B99 (2003) 195 /198
197
Fig. 3. XRD pattern of the PLZT thick films. Fig. 5. Hysteresis loop of the PLZT thick films.
PLZT perovskite structure can also be observed at 2u / 31, 45 and 558. It indicated that the screen-printing derived PLZT thick films were crystallized completely. At the same time, the PLZT thick films with PZT/PT seeding layer showed more highly (001) and (111) preferred orientation. The PLZT thick films with (001) and (111) orientation will have the advantage of high piezoelectric constant due to its sensitivity to the outer stress. Discussing the origin of orientation in sol /gel derived PZT thin films, the mechanism for (001) and (111)orientated perovskite structures growth: the Pt layer deposited on the Si (001) is highly (111)-orientated, thus, the PZT thin films without PT seeding layer would prefer to crystallize in a (111) orientation because of the lattice matching with the Pt layer. At the same time, the lattice constants of PT is similar to that of Pt, so the bottom side of the PT seeding layer starts to crystallize epitaxially from the Pt layer and has a (111) orientation. However, it is interesting to point out that the upper side of the PT film on Pt probably tend to a (001) orientation
Fig. 4. Surface micrograph of the PLZT thick films measurement structure.
[9]. So, the PLZT thin films with the PZT/PT seeding layer prepared using suitable process conditions will be highly (001) and (111)-orientated. Fig. 4 illustrates the surface micrographs of PLZT thick films structure for the measurement of electrical properties. As is known, the spontaneous polarization of ferroelectric materials does not follow the electric field linearly, and thus the polarization /electric field (P /E ) hysteresis loop is resulted. Fig. 5 shows the hysteresis loop of the PLZT. An alternative electric field with sine wave was applied in the experiment. The magnitude of applied electric field was 15/20 kV cm 1 that was higher than the coercive field, and the sine wave frequency was chosen as 20 Hz. As shown in Fig. 5, the PLZT remnant polarization is about 32 mC cm2. The dielectric responses of the silicon-based PLZT thick films, examined in terms of the dielectric constant and dielectric loss as functions of measuring frequency, are shown in Fig. 6(a and b). For the PLZT thick films, the dielectric constant decreases very slowly as the frequency is below 350 MHz. Known from Fig. 6(b), the dielectric loss increases continuously as the frequency increases but is lower than 0.03 as the frequency is below 300 MHz. Fig. 6 shows that the deregulation of dielectric constant and the dielectric loss is occurred when the frequency is about 1 GHz because of the dielectric relaxation. After poling, the dielectric relaxation would be weak. The experimental results show that the PLZT thick films have excellent high-frequency properties: high dielectric constant and low dielectric loss. The measure of the piezoelectric constant d33 which is about 630 pC N 1 shows that the PLZT thick film has excellent piezoelectric property. The experimental results show that the PLZT material is a good candidate material for the force /electric coupling device applications.
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H.-J. Zhao et al. / Materials Science and Engineering B99 (2003) 195 /198
Fig. 6. Dielectric responses of the PLZT thick films (a) dielectric constant; (b) dielectric loss.
4. Conclusions In summary, PLZT nano powders were prepared using a sol /gel method. PLZT thick films were prepared on Pt/Ti/SiO2/Si substrate with PZT/PT seeding layer by the screen-printing method. Ferroelectric hysteresis loop, high-frequency dielectric constant, dielectric loss and piezoelectric constant d33 of the PLZT thick films were measured. The remnant polarization of the PLZT thick film on silicon was about 32 mC cm 2 and the coercive field was about 20 kV cm1. The experimental results showed that the PLZT thick films on silicon have excellent high frequency and force /electric coupling properties. In the spectrum from 1 to 300 MHz, the dielectric constant was about 3000 and the dielectric loss was less than 0.03, respectively. The piezoelectric constant d33 is about 630 pC N 1. The PLZT thick films are suitable for the ferroelectrics-silicon integrated system, and will be a good candidate material for the 3G mobile communication and the force /electric coupling device applications.
Acknowledgements This work is supported by the National ‘973 Project’ of People’s Republic of China (G1999033105) and the
‘985’ Project of People’s Republic of China, and by the Analysis and Testing Foundation of Tsinghua University.
References [1] S. Christopher, Electro-mechanical coupling in 8/65/35 PLZT, Proceedings of the Ninth IEEE International Symposium on Applications of Ferroelectrics, 1994, pp. 357 /360. [2] K. Suu, N. Tani, F. Chu, G. Hickert, T.D. Hadnagy, T. Davenport, Integrated Ferroelectrics 26 (1999) 9 /19. [3] H. Janez, H. Marko, K. Marija, Materials Research Bulletin 34 (1999) 2271 /2278. [4] M. Linnik, O. Wilson Jr, A. Christou, Thick PLZT films grown by sol /gel for optical device applications, Materials Research Society Symposium */Proceedings, vol. 541, 1998, pp. 735 /740 [5] E.B. Araujo, J.A. Eiras, Journal of the European Ceramic Society 19 (1998) 1453 /1456. [6] D.-X. Lu, Z.-Y. Li, L.-B. Huang, Chinese Science Bulletin 39 (1994) 1507 /1510. [7] M. Sedlar, I. Zou, M. Sayer, Integrated Ferroelectrics 6 (1994) 129 /133. [8] K. Tominaga, Y. Sakashita, M. Okada, Integrated Ferroelectrics 5 (1994) 287 /290. [9] K. Ishikawa, K. Sakura, D. Fu, S. Yamada, H. Suzuki, T. Hayashi, Japanese Journal of Applied Physics 37 (1998) 5128 / 5131.