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Properties of preferential (Zr0.8,Sn0.2)TiO4 thin films prepared by rf magnetron sputtering for microwave application
Microelectronic Engineering 66 (2003) 648–653 www.elsevier.com / locate / mee
Properties of preferential (Zr 0.8 ,Sn 0.2 )TiO 4 thin films prepared by rf magnetron sputtering for microwave application W.X. Cheng, A.L. Ding*, P.S. Qiu, Y. Zhang, X.Y. He, X.Sh. Zheng State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, China
Abstract Bulk (Zr 0.8 ,Sn 0.2 )TiO 4 ceramics have shown excellent dielectric properties at microwave frequencies; however, the high sintering temperature of the bulk materials is the major obstacle in their use as dielectric resonators to miniaturize microwave circuits. It was possible to obtain highly oriented (111) thin films at substrate temperature of 350 8C and annealing at 650 8C for 10 min by RTA, which is much lower than the bulk sintering temperatures. The electric measurements were conducted on Pt / ZST /Au capacitors. The typical measured small signal dielectric constant and dissipation factor at 100 kHz were 36.5 and 0.0062, respectively. The dielectric constant of the films was comparable to the typical value (38) reported for the bulk ceramics. The temperature coefficient of capacitance was 80.6 ppm / 8C in the measured range of 25–125 8C. The leakage current density was lower than 10 27 A / cm 2 at an applied electric field of 0.4 MV/ cm. The high dielectric constant, which is comparable to the bulk, low dielectric loss, and good temperature stability suggest the suitability of ZST thin films for microwave communications and integrated capacitor applications. 2002 Elsevier Science B.V. All rights reserved. Keywords: (Zr 0.8 ,Sn 0.2 )TiO 4 (ZST) thin film; RF sputtering; Dielectric properties PACS: 77.55. 1 f; 81.15.Cd
1. Introduction Microwave technology has made remarkable progress, along with the recent development of advanced communication systems that include microcellular communications and global positioning systems (GPS). In particular, dielectric resonators have achieved an important position as the key element in microwave integrated circuits and in microwave filters. Dielectric materials which can be used as resonators must exhibit a high dielectric constant ´r (the size of the dielectric resonators is * Corresponding author. Tel.: 186-21-5241-2990; fax: 186-21-5241-3903. E-mail address: [email protected] (A.L. Ding). 0167-9317 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-9317(02)00978-4
W.X. Cheng et al. / Microelectronic Engineering 66 (2003) 648–653
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/2 proportional to ´ 21 ), extremely low dielectric loss (high Q, Q is given by 1 / tan d ), and small r temperature coefficient of resonate frequency. Tin modified zirconium titanate (ZST) meets all these requirements [1]. However, poor sinter-ability and high densification temperature are the major problems to be solved to exploit the excellent properties of bulk ZST ceramics in microwave devices. The bulk ceramics prepared by conventional solid state reaction method with addition of various sintering aids required high sintering temperature ( $ 1200 8C). Attempts have been made to reduce the processing temperature by using liquid phase aids [2], low melting glass addition [3] and chemical processing [4]. So reduction in sintering temperature of ZST is necessary for use as a dielectric resonator in miniaturized communication devices. In the present work, we have fabricated thin films of ZST for application in microwave frequency communications devices. Thin films of dielectric materials offer the advantage of much lower crystallization temperature and larger capacitance than bulk samples and can be integrated in microelectronic devices.
2. Experimental process ZST thin films were prepared by rf magnetron sputtering process. Pt / Ti / SiO 2 / Si(100) were used as substrate. The distance between the sintered ceramic target and the substrate was 30 mm. The following sputter conditions were used in this work: rf power of 100 W, Ar / O 2 (90:10), ambient gas pressure of 6 3 10 23 mbar, the substrate temperature (T s ) was 350 8C. The films were prepared by sputtering on Pt-coated Si(100) substrates, and deposited thin films were annealed at 650 8C for 10 min in oxygen atmosphere by rapid thermal annealing (RTA). The microstructure of ZST / Pt / Ti / SiO 2 / Si(100) was analyzed by a D/ Max2550V (Raguku, Japan) rotating X-ray diffractometer (XRD) with Cu Ka radiation in the angle range from 20 to 608. The surface morphology of the thin film was examined by an SPM-9500J (Shimadzu, Japan) atomic force microscope (AFM). The grain size and surface roughness of the thin film were obtained from AFM images. To investigate the dielectric properties of the prepared ZST thin film, the top electrode of gold (Au) with a diameter of 0.45 mm was prepared on the surface of the ZST film through a shadow mask by evaporation. The capacitance and dielectric loss (tan d ) were measured using a HP4294A LCR impedance analyzer in the frequency range from 1 kHz to 1 MHz. The leakage current of ZST thin films was measured using a 6517A electrometer / high resistance meter (Keithley, USA) by applying dc voltage with a step height of 1 V and a delay time of 30 s.
3. Results and discussion
3.1. Structure and morphology Fig. 1 shows the X-ray diffraction pattern of ZST thin films deposited on Pt-coated Si(100) substrates at substrate temperature of 350 8C and then annealed at 650 8C for 10 min in oxygen atmosphere by RTA. From Fig. 1, we know that the ZST thin film on Pt-coated Si(100) exhibited preferential (111) orientation. The orientation of the ZST film can be explained based on Wulff’s theorem. The reason for (111) orientation is determined by surface energy considerations. Relative
650
W.X. Cheng et al. / Microelectronic Engineering 66 (2003) 648–653
Fig. 1. The XRD pattern of ZST thin film on Pt-coated Si(100) substrate annealed at 650 8C for 10 min.
values of ZST thin film surface energy calculated by Osamu et al. [5] for h111j, h110j, h100j plane are 100, 110, and 103, respectively. So we know that no other plane has a surface energy less than that of the h111j plane. The orientation of ZST thin films was found to be influenced strongly by surface energy. The significantly lower process temperature of ZST thin films compared to bulk sintering temperature show the possibility of exploiting thin films for microwave devices. The surface morphology of the film was characterized by SPM-9500J (Shimadzu) atomic force microscopy (AFM) using tapping mode amplitude modulation. Fig. 2 shows the AFM images of the ZST thin film on Pt-coated Si(100) substrate. It is obvious that the surface is smooth, and the root
Fig. 2. AFM image of ZST thin film on Pt-coated Si(100) substrate annealed at 650 8C for 10 min.
W.X. Cheng et al. / Microelectronic Engineering 66 (2003) 648–653
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mean square (rms) roughness of the surface is 1.86 nm. The quadrilateral column grains grow along with (111) orientation. The average grain size is about 100–200 nm.
3.2. Dielectric properties The temperature coefficient of capacitance (TCC) was determined as follows: TCC 5 h(C125 2 C25 ) /C25 j 3 DT 3 10 6 [ppm / 8C]
(1)
where C125 and C25 are the capacitance at 125 and 25 8C, respectively. The TCC of the ZST thin film was 80.2 ppm / 8C (1 MHz). The value is slightly inferior to ZST ceramic in bulk (0–6 ppm / 8C). In the bulk the TCC of ZST strongly depended on the composition ratio of Zr and Sn ions. It should therefore be possible to improve the TCC of the ZST thin film by controlling the ratio of Zr / Sn. Fig. 3 shows the dielectric constant and dielectric loss as a function of frequency from 1 kHz to 1 MHz. The small signal dielectric constant and dielectric loss of the ZST thin film at the frequency of 100 kHz were 36.5 and 0.0062, respectively. The dielectric constant of the thin film was compared to the value (38) reported for bulk ZST ceramics [1]. The intrinsic dielectric loss value is expected to be even lower than 0.0062 as the effects of series and parallel resistances due to measurement configuration on the measured values of capacitance and loss factor were not modeled. The dielectric properties did not show any appreciable dispersion with frequency up to about 1 MHz indicating good film / electrode interfacial characteristics. However, as the frequency was increased above 1 MHz, the dielectric constant was found to decrease and the loss factor was found to increase with frequency. This behavior was found to be extrinsic in nature as similar behavior was observed at around the same frequency for thin films of other dielectric materials. At frequencies of the order of a few megahertz,
Fig. 3. Dielectric constant and dissipation factor of ZST thin film as a function of frequency.
652
W.X. Cheng et al. / Microelectronic Engineering 66 (2003) 648–653
Fig. 4. Bias dependence of the capacitance and dissipation factor of ZST thin film.
the stray inductance L of the contacts and wires and / or the presence of a finite resistance in series with the film, which may arise due to intrinsic or extrinsic sources, may cause such behavior. The bias stability of MIM capacitors was analyzed in terms of capacitance–voltage (C–V ) and leakage current characteristics to establish their reliability for capacitor applications. The C–V measurement was conducted on MIM capacitors by applying a small ac signal of 10 mV amplitude and 100 kHz frequency across the sample while the dc electric field was swept from a positive bias to negative bias and back again. Fig. 4 shows the C–V characteristics of ZST thin films in MIM configuration. The capacitance and dielectric loss did not show appreciable dependence on the bias voltage. The insulating properties of ZST thin films were analyzed in terms of leakage current density versus electric field (J–E) characteristics, as shown in Fig. 5; the leakage current density of ZST thin films was lower than 10 27 A / cm 2 up to an applied electric field of 0.4 MV/ cm. The low leakage current density indicates good insulating characteristics of ZST thin films and suggests their suitability for integrated capacitor applications.
4. Conclusion Preferential (111) ZST thin films were successfully prepared by rf magnetron sputtering on Pt-coated Si(100) substrate at a temperature much lower than the bulk sintering temperatures. The films were found to be well-crystallized at an annealing temperature of 650 8C. The measured small signal dielectric constant and dissipation factor at 100 kHz were 36.5 and 0.0062, respectively. The MIM capacitors exhibited good temperature and bias stability. The leakage current density was lower than 10 27 A / cm 2 at an applied electric field of 0.4 MV/ cm. All the characteristics of the prepared ZST thin films suggest the suitability of them for microwave and integrated capacitor application.
W.X. Cheng et al. / Microelectronic Engineering 66 (2003) 648–653
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Fig. 5. J–E characteristics of ZST thin film annealed at 650 8C for 10 min.
Acknowledgements The authors would like to thank Dr. Xingui Tang for many useful discussions and are grateful for financial support by the Project of Knowledge Innovation Program of the Chinese Academy of Sciences (no. KGCX2-201-1) and Natural Science Foundation of China (no. 59995520).
References [1] [2] [3] [4] [5]
G. Wolram, E. Gobel, Mater. Res. Bull. 16 (1981) 1455. C.-L. Huang, M.-H. Weng, Mater. Res. Bull. 35 (2000) 881. T. Takada, S.F. Wang, S. Yoshikawa, S.-J. Jang, R.E. Newnham, J. Am. Ceram. Soc. 77 (9) (1994) 2485. S. Hirano, T. Hayashi, A. Hattaori, J. Am. Ceram. Soc. 74 (6) (1991) 1320. O. Nakagawara, Y. Toxota, M. Kobayashi, Y. Yoshino, Y. Katayama, J. Appl. Phys. 80 (1996) 388.
Properties of preferential (Zr 0.8 ,Sn 0.2 )TiO 4 thin films prepared by rf magnetron sputtering for microwave application W.X. Cheng, A.L. Ding*, P.S. Qiu, Y. Zhang, X.Y. He, X.Sh. Zheng State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, China
Abstract Bulk (Zr 0.8 ,Sn 0.2 )TiO 4 ceramics have shown excellent dielectric properties at microwave frequencies; however, the high sintering temperature of the bulk materials is the major obstacle in their use as dielectric resonators to miniaturize microwave circuits. It was possible to obtain highly oriented (111) thin films at substrate temperature of 350 8C and annealing at 650 8C for 10 min by RTA, which is much lower than the bulk sintering temperatures. The electric measurements were conducted on Pt / ZST /Au capacitors. The typical measured small signal dielectric constant and dissipation factor at 100 kHz were 36.5 and 0.0062, respectively. The dielectric constant of the films was comparable to the typical value (38) reported for the bulk ceramics. The temperature coefficient of capacitance was 80.6 ppm / 8C in the measured range of 25–125 8C. The leakage current density was lower than 10 27 A / cm 2 at an applied electric field of 0.4 MV/ cm. The high dielectric constant, which is comparable to the bulk, low dielectric loss, and good temperature stability suggest the suitability of ZST thin films for microwave communications and integrated capacitor applications. 2002 Elsevier Science B.V. All rights reserved. Keywords: (Zr 0.8 ,Sn 0.2 )TiO 4 (ZST) thin film; RF sputtering; Dielectric properties PACS: 77.55. 1 f; 81.15.Cd
1. Introduction Microwave technology has made remarkable progress, along with the recent development of advanced communication systems that include microcellular communications and global positioning systems (GPS). In particular, dielectric resonators have achieved an important position as the key element in microwave integrated circuits and in microwave filters. Dielectric materials which can be used as resonators must exhibit a high dielectric constant ´r (the size of the dielectric resonators is * Corresponding author. Tel.: 186-21-5241-2990; fax: 186-21-5241-3903. E-mail address: [email protected] (A.L. Ding). 0167-9317 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-9317(02)00978-4
W.X. Cheng et al. / Microelectronic Engineering 66 (2003) 648–653
649
/2 proportional to ´ 21 ), extremely low dielectric loss (high Q, Q is given by 1 / tan d ), and small r temperature coefficient of resonate frequency. Tin modified zirconium titanate (ZST) meets all these requirements [1]. However, poor sinter-ability and high densification temperature are the major problems to be solved to exploit the excellent properties of bulk ZST ceramics in microwave devices. The bulk ceramics prepared by conventional solid state reaction method with addition of various sintering aids required high sintering temperature ( $ 1200 8C). Attempts have been made to reduce the processing temperature by using liquid phase aids [2], low melting glass addition [3] and chemical processing [4]. So reduction in sintering temperature of ZST is necessary for use as a dielectric resonator in miniaturized communication devices. In the present work, we have fabricated thin films of ZST for application in microwave frequency communications devices. Thin films of dielectric materials offer the advantage of much lower crystallization temperature and larger capacitance than bulk samples and can be integrated in microelectronic devices.
2. Experimental process ZST thin films were prepared by rf magnetron sputtering process. Pt / Ti / SiO 2 / Si(100) were used as substrate. The distance between the sintered ceramic target and the substrate was 30 mm. The following sputter conditions were used in this work: rf power of 100 W, Ar / O 2 (90:10), ambient gas pressure of 6 3 10 23 mbar, the substrate temperature (T s ) was 350 8C. The films were prepared by sputtering on Pt-coated Si(100) substrates, and deposited thin films were annealed at 650 8C for 10 min in oxygen atmosphere by rapid thermal annealing (RTA). The microstructure of ZST / Pt / Ti / SiO 2 / Si(100) was analyzed by a D/ Max2550V (Raguku, Japan) rotating X-ray diffractometer (XRD) with Cu Ka radiation in the angle range from 20 to 608. The surface morphology of the thin film was examined by an SPM-9500J (Shimadzu, Japan) atomic force microscope (AFM). The grain size and surface roughness of the thin film were obtained from AFM images. To investigate the dielectric properties of the prepared ZST thin film, the top electrode of gold (Au) with a diameter of 0.45 mm was prepared on the surface of the ZST film through a shadow mask by evaporation. The capacitance and dielectric loss (tan d ) were measured using a HP4294A LCR impedance analyzer in the frequency range from 1 kHz to 1 MHz. The leakage current of ZST thin films was measured using a 6517A electrometer / high resistance meter (Keithley, USA) by applying dc voltage with a step height of 1 V and a delay time of 30 s.
3. Results and discussion
3.1. Structure and morphology Fig. 1 shows the X-ray diffraction pattern of ZST thin films deposited on Pt-coated Si(100) substrates at substrate temperature of 350 8C and then annealed at 650 8C for 10 min in oxygen atmosphere by RTA. From Fig. 1, we know that the ZST thin film on Pt-coated Si(100) exhibited preferential (111) orientation. The orientation of the ZST film can be explained based on Wulff’s theorem. The reason for (111) orientation is determined by surface energy considerations. Relative
650
W.X. Cheng et al. / Microelectronic Engineering 66 (2003) 648–653
Fig. 1. The XRD pattern of ZST thin film on Pt-coated Si(100) substrate annealed at 650 8C for 10 min.
values of ZST thin film surface energy calculated by Osamu et al. [5] for h111j, h110j, h100j plane are 100, 110, and 103, respectively. So we know that no other plane has a surface energy less than that of the h111j plane. The orientation of ZST thin films was found to be influenced strongly by surface energy. The significantly lower process temperature of ZST thin films compared to bulk sintering temperature show the possibility of exploiting thin films for microwave devices. The surface morphology of the film was characterized by SPM-9500J (Shimadzu) atomic force microscopy (AFM) using tapping mode amplitude modulation. Fig. 2 shows the AFM images of the ZST thin film on Pt-coated Si(100) substrate. It is obvious that the surface is smooth, and the root
Fig. 2. AFM image of ZST thin film on Pt-coated Si(100) substrate annealed at 650 8C for 10 min.
W.X. Cheng et al. / Microelectronic Engineering 66 (2003) 648–653
651
mean square (rms) roughness of the surface is 1.86 nm. The quadrilateral column grains grow along with (111) orientation. The average grain size is about 100–200 nm.
3.2. Dielectric properties The temperature coefficient of capacitance (TCC) was determined as follows: TCC 5 h(C125 2 C25 ) /C25 j 3 DT 3 10 6 [ppm / 8C]
(1)
where C125 and C25 are the capacitance at 125 and 25 8C, respectively. The TCC of the ZST thin film was 80.2 ppm / 8C (1 MHz). The value is slightly inferior to ZST ceramic in bulk (0–6 ppm / 8C). In the bulk the TCC of ZST strongly depended on the composition ratio of Zr and Sn ions. It should therefore be possible to improve the TCC of the ZST thin film by controlling the ratio of Zr / Sn. Fig. 3 shows the dielectric constant and dielectric loss as a function of frequency from 1 kHz to 1 MHz. The small signal dielectric constant and dielectric loss of the ZST thin film at the frequency of 100 kHz were 36.5 and 0.0062, respectively. The dielectric constant of the thin film was compared to the value (38) reported for bulk ZST ceramics [1]. The intrinsic dielectric loss value is expected to be even lower than 0.0062 as the effects of series and parallel resistances due to measurement configuration on the measured values of capacitance and loss factor were not modeled. The dielectric properties did not show any appreciable dispersion with frequency up to about 1 MHz indicating good film / electrode interfacial characteristics. However, as the frequency was increased above 1 MHz, the dielectric constant was found to decrease and the loss factor was found to increase with frequency. This behavior was found to be extrinsic in nature as similar behavior was observed at around the same frequency for thin films of other dielectric materials. At frequencies of the order of a few megahertz,
Fig. 3. Dielectric constant and dissipation factor of ZST thin film as a function of frequency.
652
W.X. Cheng et al. / Microelectronic Engineering 66 (2003) 648–653
Fig. 4. Bias dependence of the capacitance and dissipation factor of ZST thin film.
the stray inductance L of the contacts and wires and / or the presence of a finite resistance in series with the film, which may arise due to intrinsic or extrinsic sources, may cause such behavior. The bias stability of MIM capacitors was analyzed in terms of capacitance–voltage (C–V ) and leakage current characteristics to establish their reliability for capacitor applications. The C–V measurement was conducted on MIM capacitors by applying a small ac signal of 10 mV amplitude and 100 kHz frequency across the sample while the dc electric field was swept from a positive bias to negative bias and back again. Fig. 4 shows the C–V characteristics of ZST thin films in MIM configuration. The capacitance and dielectric loss did not show appreciable dependence on the bias voltage. The insulating properties of ZST thin films were analyzed in terms of leakage current density versus electric field (J–E) characteristics, as shown in Fig. 5; the leakage current density of ZST thin films was lower than 10 27 A / cm 2 up to an applied electric field of 0.4 MV/ cm. The low leakage current density indicates good insulating characteristics of ZST thin films and suggests their suitability for integrated capacitor applications.
4. Conclusion Preferential (111) ZST thin films were successfully prepared by rf magnetron sputtering on Pt-coated Si(100) substrate at a temperature much lower than the bulk sintering temperatures. The films were found to be well-crystallized at an annealing temperature of 650 8C. The measured small signal dielectric constant and dissipation factor at 100 kHz were 36.5 and 0.0062, respectively. The MIM capacitors exhibited good temperature and bias stability. The leakage current density was lower than 10 27 A / cm 2 at an applied electric field of 0.4 MV/ cm. All the characteristics of the prepared ZST thin films suggest the suitability of them for microwave and integrated capacitor application.
W.X. Cheng et al. / Microelectronic Engineering 66 (2003) 648–653
653
Fig. 5. J–E characteristics of ZST thin film annealed at 650 8C for 10 min.
Acknowledgements The authors would like to thank Dr. Xingui Tang for many useful discussions and are grateful for financial support by the Project of Knowledge Innovation Program of the Chinese Academy of Sciences (no. KGCX2-201-1) and Natural Science Foundation of China (no. 59995520).
References [1] [2] [3] [4] [5]
G. Wolram, E. Gobel, Mater. Res. Bull. 16 (1981) 1455. C.-L. Huang, M.-H. Weng, Mater. Res. Bull. 35 (2000) 881. T. Takada, S.F. Wang, S. Yoshikawa, S.-J. Jang, R.E. Newnham, J. Am. Ceram. Soc. 77 (9) (1994) 2485. S. Hirano, T. Hayashi, A. Hattaori, J. Am. Ceram. Soc. 74 (6) (1991) 1320. O. Nakagawara, Y. Toxota, M. Kobayashi, Y. Yoshino, Y. Katayama, J. Appl. Phys. 80 (1996) 388.