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Fabrication of ultrasonic arrays with 7 μm PZT thick films as ultrasonic emitter for object detection in air
Sensors and Actuators A 123–124 (2005) 614–619
Fabrication of ultrasonic arrays with 7 m PZT thick films as ultrasonic emitter for object detection in air Hong Zhu a,∗ , Jianmin Miao a , Zhihong Wang b , Changlei Zhao b , Weiguang Zhu b a b
Micromachines Centre, School of Mechanical and Production Engineering, Nanyang Technological University, Singapore 639798, Singapore Microelectronics Centre, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore Received 13 September 2004; received in revised form 24 January 2005; accepted 8 March 2005 Available online 28 April 2005
Abstract Lead zirconate titanate (PZT) thick films with thickness up to 20 m with relatively good property have been deposited on silicon wafers using a modified sol–gel slurry process. Submicrosized or nanosized PZT powder was dispersed into regular PZT sol–gel solutions to prepare the composite film. Ultrasonic arrays have been fabricated with 7 m PZT thick film on the silicon membrane diagrams. The property of the PZT thick film, the fabrication process, the vibration characteristics and the ultrasonic emission property of the fabricated arrays are reported. The ultrasonic transducer can generate ultrasound with pressures up to 3.6 Pa. © 2005 Elsevier B.V. All rights reserved. Keywords: Ultrasonic array; PZT thick film; Micromachining
1. Introduction Nowadays, the medical industry requires ultrasonic transducers to operate at high frequencies to improve the resolution of the image of small structures or even microstructures of the human body. There is a need to fabricate very small ultrasonic micro transducers with high directivity and high sensitivity for ultrasonic biomicroscopy. It is impossible to use traditional PZT ceramics to fabricate high frequency ultrasonic transducer, because such high frequency requires the thickness of the PZT ceramics plate to be as thin as 50 m, which is difficult to machine due to its brittleness and poor mechanical strength [1]. Many efforts have already been spent on micromachined ultrasonic transducer for in-air use such as ultrasonic imaging or object detection [2–8]. Generally, the thickness of the PZT film on a micromachined ultrasonic sensor can be less than 1 m, but the micromachined ultrasonic emitter will require the thickness ∗ Corresponding author. Present address: Data Storage Institute, DSI Building, 5 Engineering Drive 1 (off Kent Ridge Crescent, NUS), Singapore 117608, Singapore. Tel.: +65 6874 7825; fax: +65 6776 6527. E-mail address: zhu [email protected] (H. Zhu).
0924-4247/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2005.03.037
of the PZT film to be several micrometers or greater as the generative force of the actuator increases as the film thickness increases. Up to now, the best way to fabricate PZT thin film is by sol–gel method, but generally the thickness of the PZT films prepared by this method can not be more than 1 m due to the high intrinsic stress of the film. Some results have been achieved on deposition of thick PZT film above 10 m [9–14], and now more efforts should be spent on the integration of the thick PZT film with microelectrical mechanical systems (MEMS) structure. In this paper, we report the modified sol–gel method by mixing the PZT powder into regular sol–gel solution to prepare the thick PZT composite film, the fabrication process of the ultrasonic array with this PZT thick film and the performance of this device. 2. Preparation of thick PZT film by modified sol–gel slurry method The PZT slurry was prepared by dispersing submicrosized (0.1–0.3 m) or nanosized (30–80 nm) PZT powder into regular pure PZT sol–gel solution [9,10]. Changing the mass
H. Zhu et al. / Sensors and Actuators A 123–124 (2005) 614–619
Fig. 1. SEM cross-section of a 14 m PZT thick film.
ratio between the powder and the pure sol–gel solution in the slurry will change the thickness of each PZT layer. The more powder inside the slurry, the thicker each PZT layer after spin coating. The mass ratio between the PZT powder and the pure sol–gel solution in the slurry used in the fabrication of the device is 1:2 in weight. The concentration of the pure sol–gel solution is 30 wt.%. The preparation of the PZT film by the slurry method is just the same as the regular sol–gel deposition method: spin coating (2000 rpm, 30 s), prebaking (250 ◦ C, 2 min), sintering (500 ◦ C, 2 min) of each layer and final annealing of the film (650 ◦ C, 30 min). During the deposition, the first layer of PZT film should be deposited with pure sol–gel solution to lay a good foundation for the final film and also to improve the adhesion between the successive PZT layers and the bottom electrode. The next two or three layers of the film are deposited with the slurry, after which a pure sol–gel solution is deposited so that the solution can penetrate into the microcracks and holes in the film to improve the film density so as to achieve a better electrical properties of the film. After rounds of alternating deposition between the slurry and the pure sol–gel solution when the desired thickness of the film is reached, the final layer should be deposited with pure sol–gel solution in order to get a good flat surface for the final film. The temperature and time of the final annealing is crucial to the property of the PZT thick film. Generally, higher temperature and longer time annealing is required to achieve a better property of the film. Since high temperature may have bad effects on the bottom electrode, and also the composition of the PZT may change during long time annealing, we choose the final annealing temperature and time to be 650 ◦ C and 30 min, respectively. Fig. 1 shows the cross-section of a PZT thick film under SEM. The thickness of the fabricated film is about 14 m. There are some small cracks and holes inside the film, which indicate that the density of the film is not so high. It is only because of these small cracks and holes, which greatly re-
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Fig. 2. XRD pattern of the 14 m PZT thick film annealed at 650 ◦ C for 30 min.
duced the intrinsic stress in the films, that PZT films up to 20 m in thickness can be deposited. Fig. 2 shows the XRD pattern of the thick film (14 m in thickness) annealed at 650 ◦ C for 30 min. The X-ray diffraction (XRD) pattern showed only perovskite PZT phase present in the film and there is no pyrochlore phase. The XRD graph shows that the film is with (1 1 0) orientation preferential texture, which has better piezoelectric properties. The hysteresis loops of two kinds of thick PZT films shown in Fig. 3 are almost the same, even though the film thickness is quite different. The remnant polarization (Pr) of the film about 7.5 c/cm2 is not so high compared with that of PZT bulk ceramics about 23 c/cm2 . This is because that there are many small cracks and holes inside the film, as voids in the PZT film deteriorate its electrical property. Nevertheless, the new modified PZT slurry deposition method is a good way to fabricate PZT thick film, which have relatively good ferroelectric property, and can be used in practical PZT sensor and actuator fabrication.
Fig. 3. Ferroelectric hysteresis loops of two kinds of PZT films with thickness 3.5 and 7 m fabricated on the ultrasonic arrays.
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Since the major part of the deposited thick film is made of PZT powder in the slurry, the property of the powder is crucial to the property of the final deposited film. During the final annealing at 650 ◦ C, only the sol–gel PZT phase is crystallized and the crystallinity of the PZT powder phase does not change. Nanosized PZT powder and submicrosized powder have been used in the preparation of the thick PZT film. The SEM cross-section view and ferroelectric property measurement results show that films deposited with submicrosized PZT powder in the slurry are less dense, have more cracks and holes in the film but have better ferroelectric properties than those deposited with nanosized PZT powder in the slurry. Therefore, we preferred to use the PZT slurry with submicrosized PZT powder inside.
3. Structure and fabrication process of the ultrasonic array The resonance frequencies of the transmitters can be adjusted for a specific application by changing the thickness and width of the diaphragm. In this work, the PZT layers were 3.5 and 7 m in thickness and the widths of transmitters were designed as 0.25, 0.5, 1, 1.5 and 2 mm, respectively. Fig. 4 shows the schematic structure of a single cell in the ultrasonic arrays with PZT thick film. Si3 N4 is used as the etching mask during the backside silicon KOH etching and also used to compensate the compressive stress in SiO2 layer. Another layer of non-doped silicon glass (NSG) is deposited onto Si3 N4 layer. The function of this NSG layer is to act as a buffer layer between PZT film and Si3 N4 layer, as PZT cannot be well prepared directly on Si3 N4 layer. A polyimide layer has been deposited to insulate the top electrode from the bottom electrode and also to reduce the parasitic capacitance induced by the conducting wires. Etching of PZT thick film is a challenging step in the fabrication process. There are several ways to etch PZT thin film, such as RIE using Cl2 gas, ion milling using Ar ions, or wet etching by chemical solution containing HCl + HF or HNO3 + HF. The wet etching is not a preferred method, as the chemical solution may penetrate into the films under the etching mask through the microcracks and holes present in the film. This will cause the chemical to further attach the underlying PZT layer protected by the mask. This is also
Fig. 4. Schematic cross-section structure of a single transmitter in the ultrasonic arrays with PZT thick film.
the main reason why many MEMS devices with PZT thin film patterned by wet etching process often have electrical short circuit in the device. To fabricate MEMS device with thin PZT film, the best etching method is ion milling, during which there is no poisonous gas like Cl2 used in the RIE etching process. But for the pattering of PZT thick film, all the above three processes are not good enough. As shown in Fig. 1, there are some cracks and holes in the fabricated PZT thick film. During wet etching process, the etching solution will penetrate through these cracks and holes into the active part of the PZT film above the silicon diagram in the device. The etching rate of PZT by ion milling or RIE is low, so it may take very long time to pattern PZT thick film using ion milling or RIE method. It seemed that new methods should be developed to pattern PZT thick film. In order to solve the etching problem in the patterning of thick PZT film, the polyimide insulator layer is added in the device. The polyimide layer is used to separate the active PZT above the diagram (used to actuate the structure to emit ultrasonic sound) from the other part of PZT film. By adding this polyimide layer, there is no need to etch the active part of the PZT thick film. In the fabrication process, wet etching of PZT is used to open the contacting hole on the bottom electrode, which is far way from the active PZT part. The distance between the contacting hole on the bottom electrode and the nearest PZT cell is above 2 mm in the design. Even though during the etching process, some etching solution penetrates into the PZT under the etching mask, this area of PZT will be covered by polyimide during the fabrication. The detailed fabrication processes of the ultrasonic array shown in Fig. 5 are: (a) thermal wet and dry oxidation on both sides of the 4 in. wafer until the thickness of SiO2 layer reaching 1.8 m; LPCVD deposition of Si3 N4 about 250 nm and NSG about 500 nm on both sides of the wafer; (b) backside dry etching of NSG, Si3 N4 and SiO2 by RIE using CF4 + O2 to open the etching window for the silicon diagram; (c) backside silicon diagram anisotropic wet etching by KOH until the remained thickness of silicon about 50 m; (d) bottom electrode Pt (180 nm)/Ti (20 nm) deposition on the front side by sputtering using the lift-off method; (e) deposition and sintering of thick PZT by modified sol–gel slurry method; PZT patterning by wet etching using HCl + HF + H2 O to open the contacting hole for the bottom electrode; (f) spin coating, patterning and curing of polyimide insulator layer to prevent the electrical short circuit between the top and bottom electrode; (g) top electrode Pt (180 nm)/Ti (20 nm) deposition on the front side by sputtering using the lift-off method; and finally etching the backside silicon by RIE using CF4 + O2 until the desired thickness left about 20 m in this device. Fig. 6 is the image of the ultrasonic arrays with 7 m thickness PZT film fabricated on a 4 in. wafer. Each white point is a single PZT emitter cell; the dark square around the white array points is the contacting hole of the bottom electrode. In this device, each cell has its own bottom electrode, while the entire cells in the array using a common upper electrode, which is above the polyimide layer. Unlike the fabrication of
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ferroelectric mems device with thin PZT film, this ultrasonic array with thick PZT film have obtained 100% yield of the device due to the good thick PZT layer. It is quite often to get bad points in the MEMS device with PZT thin film due to the electrical short circuit between the top and bottom electrode due to the particle and defects in the film.
4. Vibration characteristic and ultrasonic sound emission property Fig. 7 is a typical vibration response of a transmitter with a size of 1.5 mm × 1.5 mm in the fabricated array after input of an ac driving voltage of 500 Hz, 40 V (p–p) with PZT film under 30 V dc bias. The thickness of the PZT layer in the device is 7 m, and the thickness of bottom silicon in the diagram structure is about 20 m. The peak-to-peak displacement of
Fig. 7. The displacement and the driving voltage of a transmitter in the ultrasonic array.
Fig. 5. Schematic fabrication process of the ultrasonic array device.
Fig. 6. Fabricated ultrasonic array devices with 7 m PZT thick film in 4 in. wafer.
Fig. 8. Frequency dependence of the peak-to-peak displacement of a 1.5 mm × 1.5 mm transmitter in the ultrasonic array at bias of 30 V and Vp–p of 20 V.
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The output sound pressure level (SPL) of the transmitter versus the ac driving voltage at a fixed dc bias applied onto the transmitter was also measured as shown in Fig. 10, which shows that the SPL increases with the increase of the ac driving voltage. The SPL within the range of 102–110 dB is very good for such a small transmitter. The ultrasound emission measurement results show that the device could have practical applications.
5. Conclusion
Fig. 9. Frequency dependence of output sound pressure of a 1.5 mm × 1.5 mm transmitter in the ultrasonic array at bias of 30 V and Vp–p of 20 V.
the cell is about 0.2 m measured by scanning laser doppler vibrometer (SLDV) made by Polytec GmbH, Germany. In Fig. 8 the frequency dependence of the peak-to-peak displacement of a single cell in the ultrasonic array is shown. The first order of the resonance frequency is 57.75 kHz, while the second order of the resonance frequency is 202 kHz. At the first order of resonance frequency, which the ultrasonic emitter will operate at, the peak-to-peak displacement of the single cell is about 4.2 m, which is really large enough. The sound pressure measurement versus frequency of a single cell in the device has also been carried out as shown in Fig. 9, with an output of 3.6 Pa at the resonance frequency of 57.6 kHz. The two resonance frequencies measured by vibration displacement test and sound pressure test are almost the same, which shows that the results of the two different measurements are accurate.
Fig. 10. Output sound pressure level of a transmitter vs. diving voltage at resonance frequency.
Ultrasonic arrays with 7 m PZT thick film on the silicon membrane diagrams as ultrasonic emitter have been successfully fabricated by silicon micromachining with 100% yield. The modified sol–gel slurry method is a good way to fabricate PZT thick film up to 20 m with relatively good ferroelectric property with Pr as high as 7.5 c/cm2 . The polyimide layer is introduced to avoid etching of PZT thick film, which still provides challenges for future work. The vibration characteristics and the ultrasonic emission property of the fabricated array show that ultrasonic transducer can generate the ultrasound with pressure as high as 3.6 Pa, which could have practical application in object detection in air. The preparation method of thick PZT film and the micromachining process can be used in other ferroelectric device fabrication using thick PZT films.
References [1] M. Lucacs, M. Sayer, S. Fostser, Single element high frequency PZT sol–gel composite ultrasound transducers, IEEE Trans. Ultrasonics Ferroelectronics Frequency Control 47 (2000) 148–159. [2] K. Yamashita, H. Katata, M. Okuyama, H. Miyoshi, G. Kato, S. Aoyagi, Y. Suzuki, Arrayed ultrasonic microsensors with high directivity for in-air use using PZT thin film on silicon diagrams, Sens. Actuators A 97–98 (2002) 302–307. [3] P. Muralt, Ferroelectric thin films for micro-sensors and actuators: a review, J. Micromech. Microeng. 10 (2000) 136–146. [4] P. Muralt, PZT thin films for microsensors and actuators: where do we stand, IEEE Trans. Ultrasonics Ferroelectronics Frequency Control 47 (2000) 903–915. [5] M. Lebedev, J. Akedo, Y. Akiyama, Actuation properties of lead zirconate titanate thick films structured on Si membrane by the aerosol deposition method, Jpn. J. Appl. Phys. 39 (2000) 5600–5603. [6] J. Baborowski, P. Muralt, N. Ledermann, S. Petitgrand, A. Bosseboeuf, N. Setter, Ph. Gaucher, PZT coated membrane structures for micromachined ultrasonic transducers, in: Proceedings of the 13th IEEE International Symposium on Applications of Ferroelectrics, Nara, Japan, May 28–June 1, 2002, pp. 483–486. [7] T. Iijima, K. Kunii, Preparation of texture-controlled lead zirconate titanate diaphragm-type film actuator using a chemical solution method, Jpn. J. Appl. Phys. 40 (2001) 5740–5742. [8] E. Defay, C. Millon, C. Malhaire, D. Barbier, PZT thin films integration for the realization of a high sensitivity pressure microsensor based on a vibrating membrane, Sens. Actuators A 99 (2002) 64–67. [9] Z. Wang, W. Zhu, C. Zhao, O.K. Tan, Dense PZT thick films derived from sol–gel based nanocomposite Process, Mater. Sci. Eng. B 99 (2003) 56–62.
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Biographies Hong Zhu received his B.Eng. from the Department of Materials Science and Engineering, University of Science and Technology Beijing, China in 1990, and his Ph.D. from Shanghai Institute of Metallurgy, Chinese Academy of Sciences in 1995. He worked as a visiting scholar in Graduate School of Engineering Science, Osaka University, Japan from 1998 to 2000. In October 2000, he joined School of Mechanical and Production Engineering, Nanyang Technological University, Singapore as a research fellow in Micromachines (MEMS) Center. In August 2004, he joined Data Storage Institute, Agency for Science, Technology and Research (A*STAR), Singapore as a senior research engineer. His research interests include fabrication and characterization on ferroelectric sensor and actuator, new MEMS fabrication technology and device development. Jianmin Miao received his Dipl.-Ing. and Dr.-Ing. from the Department of Electrical Engineering and Information Technology at the Darmstadt University of Technology, Germany in 1991, 1996 respectively. He was the head of R&D with Lectret S.A. (Switzerland and Singapore) with prime responsibility for the joint project of integrated silicon microphones with the Institute of Microelectronics in Singapore. In September 1998, he joined the Nanyang Technological University, Singapore as a faculty. He is currently an associate professor and the director of the Micromachines (MEMS) Center. His research interests include MEMS design, technology development and characterization. He is the member of the Institute of Electrical and Electronics Engineers. Zhihong Wang received the B.S. degree in magnetics and the M.S. degree in solid-state physics from Lanzhou University, Gansu and the Ph.D. degree in microelectronics from Xi’an Jiaotong University, Shaanxi, China in 1984, 1989 and 1999, respectively. In 1999, he joined the Department
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of Advanced Component Development, SAE Magnetics (H.K.) Ltd., as principal engineer and manager. In 2000 he joined Nanyang Technological University, Singapore, where he is currently a research fellow with Sensors and Actuators Laboratory, School of Electrical and Electronic Engineering. His early research involved preparation and characterization of various electronic ceramics and films, design, fabrication and reliability assessment of piezoelectric devices. His current research interests include thin film piezoelectric measurement, ultrasonic transducers and thick film ferroelectric sensors and actuators for micro electromechanical systems and biomedical applications. Changlei Zhao received his B.Eng. degree in Foundry Engineering at the Northwestern Polytech University in 1996 and M.Eng. degree in Materials at Shanghai Institute of Ceramics, Chinese Academy of Science in 1999, respectively. It was during his undergraduate years that he became interested in designing and developing functional materials. This interest led him turn to ferroelectric and piezoelectric studies within the Functional Ceramics Research and Design Centre at SICCAS immediately after his bachelors degree. During his graduate research, his interest in piezoelectric materials for sensors and actuators was established. He is currently studying on the ferroelectric film actuators and electronic devices as a doctoral candidate. Weiguang Zhu received his B.Sc. and M.Sc. from Shanghai Jiao Tong University, China, and Ph.D. from Purdue University, USA. Currently, he is a professor at Nanyang Technological University, Singapore. Dr. Zhu is a member of American Physical Society, American Ceramic Society, IEEE, Materials Research Society, and a fellow of IES. He has presented many invited talks and served as the session chairman various committees in international conferences. Dr. Zhu is a member and ChairElect of Executive Board of Asian Ferroelectric Association; a member of Academic Committee of the Chinese National Open Laboratory on Inorganic Functional Materials, of the Chinese Academy of Sciences; member of Academic Committee of Electronic Materials Research Laboratory of Xi’an Jiaotong University, a Key Laboratory of the Chinese Ministry of Education. He has published more than 150 referred papers in international journals, and has been invited as the guest editor for Journal of Ferroelectrics and Related Materials, Journal of Ceramics International and Ceramic Transactions, member of editorial board of Journal of Ferroelectrics, visiting professor at EPFL of Switzerland, RWTH at Aachen of Germany Xi’an Jiaotong University of China, and Xi’dian University of China, and invited professor at the French National Institute on Condensed Matters of Chemistry at Bordeaux (ICMCB-CNRS). Dr. Zhu has received the Outstanding Achievement Award from 16th International Symposium of Integrated Ferroelectrics. His research interests includes electronic materials, thin films, ferroelectrics, high-k dielectrics, microwave tunable dielectrics, gas sensors, high-Tc superconductors, superionic conductors, shape memory alloys, atomic diffusion, optical and electron holography, crystallography, and ferroelectric composites.
Fabrication of ultrasonic arrays with 7 m PZT thick films as ultrasonic emitter for object detection in air Hong Zhu a,∗ , Jianmin Miao a , Zhihong Wang b , Changlei Zhao b , Weiguang Zhu b a b
Micromachines Centre, School of Mechanical and Production Engineering, Nanyang Technological University, Singapore 639798, Singapore Microelectronics Centre, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore Received 13 September 2004; received in revised form 24 January 2005; accepted 8 March 2005 Available online 28 April 2005
Abstract Lead zirconate titanate (PZT) thick films with thickness up to 20 m with relatively good property have been deposited on silicon wafers using a modified sol–gel slurry process. Submicrosized or nanosized PZT powder was dispersed into regular PZT sol–gel solutions to prepare the composite film. Ultrasonic arrays have been fabricated with 7 m PZT thick film on the silicon membrane diagrams. The property of the PZT thick film, the fabrication process, the vibration characteristics and the ultrasonic emission property of the fabricated arrays are reported. The ultrasonic transducer can generate ultrasound with pressures up to 3.6 Pa. © 2005 Elsevier B.V. All rights reserved. Keywords: Ultrasonic array; PZT thick film; Micromachining
1. Introduction Nowadays, the medical industry requires ultrasonic transducers to operate at high frequencies to improve the resolution of the image of small structures or even microstructures of the human body. There is a need to fabricate very small ultrasonic micro transducers with high directivity and high sensitivity for ultrasonic biomicroscopy. It is impossible to use traditional PZT ceramics to fabricate high frequency ultrasonic transducer, because such high frequency requires the thickness of the PZT ceramics plate to be as thin as 50 m, which is difficult to machine due to its brittleness and poor mechanical strength [1]. Many efforts have already been spent on micromachined ultrasonic transducer for in-air use such as ultrasonic imaging or object detection [2–8]. Generally, the thickness of the PZT film on a micromachined ultrasonic sensor can be less than 1 m, but the micromachined ultrasonic emitter will require the thickness ∗ Corresponding author. Present address: Data Storage Institute, DSI Building, 5 Engineering Drive 1 (off Kent Ridge Crescent, NUS), Singapore 117608, Singapore. Tel.: +65 6874 7825; fax: +65 6776 6527. E-mail address: zhu [email protected] (H. Zhu).
0924-4247/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2005.03.037
of the PZT film to be several micrometers or greater as the generative force of the actuator increases as the film thickness increases. Up to now, the best way to fabricate PZT thin film is by sol–gel method, but generally the thickness of the PZT films prepared by this method can not be more than 1 m due to the high intrinsic stress of the film. Some results have been achieved on deposition of thick PZT film above 10 m [9–14], and now more efforts should be spent on the integration of the thick PZT film with microelectrical mechanical systems (MEMS) structure. In this paper, we report the modified sol–gel method by mixing the PZT powder into regular sol–gel solution to prepare the thick PZT composite film, the fabrication process of the ultrasonic array with this PZT thick film and the performance of this device. 2. Preparation of thick PZT film by modified sol–gel slurry method The PZT slurry was prepared by dispersing submicrosized (0.1–0.3 m) or nanosized (30–80 nm) PZT powder into regular pure PZT sol–gel solution [9,10]. Changing the mass
H. Zhu et al. / Sensors and Actuators A 123–124 (2005) 614–619
Fig. 1. SEM cross-section of a 14 m PZT thick film.
ratio between the powder and the pure sol–gel solution in the slurry will change the thickness of each PZT layer. The more powder inside the slurry, the thicker each PZT layer after spin coating. The mass ratio between the PZT powder and the pure sol–gel solution in the slurry used in the fabrication of the device is 1:2 in weight. The concentration of the pure sol–gel solution is 30 wt.%. The preparation of the PZT film by the slurry method is just the same as the regular sol–gel deposition method: spin coating (2000 rpm, 30 s), prebaking (250 ◦ C, 2 min), sintering (500 ◦ C, 2 min) of each layer and final annealing of the film (650 ◦ C, 30 min). During the deposition, the first layer of PZT film should be deposited with pure sol–gel solution to lay a good foundation for the final film and also to improve the adhesion between the successive PZT layers and the bottom electrode. The next two or three layers of the film are deposited with the slurry, after which a pure sol–gel solution is deposited so that the solution can penetrate into the microcracks and holes in the film to improve the film density so as to achieve a better electrical properties of the film. After rounds of alternating deposition between the slurry and the pure sol–gel solution when the desired thickness of the film is reached, the final layer should be deposited with pure sol–gel solution in order to get a good flat surface for the final film. The temperature and time of the final annealing is crucial to the property of the PZT thick film. Generally, higher temperature and longer time annealing is required to achieve a better property of the film. Since high temperature may have bad effects on the bottom electrode, and also the composition of the PZT may change during long time annealing, we choose the final annealing temperature and time to be 650 ◦ C and 30 min, respectively. Fig. 1 shows the cross-section of a PZT thick film under SEM. The thickness of the fabricated film is about 14 m. There are some small cracks and holes inside the film, which indicate that the density of the film is not so high. It is only because of these small cracks and holes, which greatly re-
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Fig. 2. XRD pattern of the 14 m PZT thick film annealed at 650 ◦ C for 30 min.
duced the intrinsic stress in the films, that PZT films up to 20 m in thickness can be deposited. Fig. 2 shows the XRD pattern of the thick film (14 m in thickness) annealed at 650 ◦ C for 30 min. The X-ray diffraction (XRD) pattern showed only perovskite PZT phase present in the film and there is no pyrochlore phase. The XRD graph shows that the film is with (1 1 0) orientation preferential texture, which has better piezoelectric properties. The hysteresis loops of two kinds of thick PZT films shown in Fig. 3 are almost the same, even though the film thickness is quite different. The remnant polarization (Pr) of the film about 7.5 c/cm2 is not so high compared with that of PZT bulk ceramics about 23 c/cm2 . This is because that there are many small cracks and holes inside the film, as voids in the PZT film deteriorate its electrical property. Nevertheless, the new modified PZT slurry deposition method is a good way to fabricate PZT thick film, which have relatively good ferroelectric property, and can be used in practical PZT sensor and actuator fabrication.
Fig. 3. Ferroelectric hysteresis loops of two kinds of PZT films with thickness 3.5 and 7 m fabricated on the ultrasonic arrays.
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Since the major part of the deposited thick film is made of PZT powder in the slurry, the property of the powder is crucial to the property of the final deposited film. During the final annealing at 650 ◦ C, only the sol–gel PZT phase is crystallized and the crystallinity of the PZT powder phase does not change. Nanosized PZT powder and submicrosized powder have been used in the preparation of the thick PZT film. The SEM cross-section view and ferroelectric property measurement results show that films deposited with submicrosized PZT powder in the slurry are less dense, have more cracks and holes in the film but have better ferroelectric properties than those deposited with nanosized PZT powder in the slurry. Therefore, we preferred to use the PZT slurry with submicrosized PZT powder inside.
3. Structure and fabrication process of the ultrasonic array The resonance frequencies of the transmitters can be adjusted for a specific application by changing the thickness and width of the diaphragm. In this work, the PZT layers were 3.5 and 7 m in thickness and the widths of transmitters were designed as 0.25, 0.5, 1, 1.5 and 2 mm, respectively. Fig. 4 shows the schematic structure of a single cell in the ultrasonic arrays with PZT thick film. Si3 N4 is used as the etching mask during the backside silicon KOH etching and also used to compensate the compressive stress in SiO2 layer. Another layer of non-doped silicon glass (NSG) is deposited onto Si3 N4 layer. The function of this NSG layer is to act as a buffer layer between PZT film and Si3 N4 layer, as PZT cannot be well prepared directly on Si3 N4 layer. A polyimide layer has been deposited to insulate the top electrode from the bottom electrode and also to reduce the parasitic capacitance induced by the conducting wires. Etching of PZT thick film is a challenging step in the fabrication process. There are several ways to etch PZT thin film, such as RIE using Cl2 gas, ion milling using Ar ions, or wet etching by chemical solution containing HCl + HF or HNO3 + HF. The wet etching is not a preferred method, as the chemical solution may penetrate into the films under the etching mask through the microcracks and holes present in the film. This will cause the chemical to further attach the underlying PZT layer protected by the mask. This is also
Fig. 4. Schematic cross-section structure of a single transmitter in the ultrasonic arrays with PZT thick film.
the main reason why many MEMS devices with PZT thin film patterned by wet etching process often have electrical short circuit in the device. To fabricate MEMS device with thin PZT film, the best etching method is ion milling, during which there is no poisonous gas like Cl2 used in the RIE etching process. But for the pattering of PZT thick film, all the above three processes are not good enough. As shown in Fig. 1, there are some cracks and holes in the fabricated PZT thick film. During wet etching process, the etching solution will penetrate through these cracks and holes into the active part of the PZT film above the silicon diagram in the device. The etching rate of PZT by ion milling or RIE is low, so it may take very long time to pattern PZT thick film using ion milling or RIE method. It seemed that new methods should be developed to pattern PZT thick film. In order to solve the etching problem in the patterning of thick PZT film, the polyimide insulator layer is added in the device. The polyimide layer is used to separate the active PZT above the diagram (used to actuate the structure to emit ultrasonic sound) from the other part of PZT film. By adding this polyimide layer, there is no need to etch the active part of the PZT thick film. In the fabrication process, wet etching of PZT is used to open the contacting hole on the bottom electrode, which is far way from the active PZT part. The distance between the contacting hole on the bottom electrode and the nearest PZT cell is above 2 mm in the design. Even though during the etching process, some etching solution penetrates into the PZT under the etching mask, this area of PZT will be covered by polyimide during the fabrication. The detailed fabrication processes of the ultrasonic array shown in Fig. 5 are: (a) thermal wet and dry oxidation on both sides of the 4 in. wafer until the thickness of SiO2 layer reaching 1.8 m; LPCVD deposition of Si3 N4 about 250 nm and NSG about 500 nm on both sides of the wafer; (b) backside dry etching of NSG, Si3 N4 and SiO2 by RIE using CF4 + O2 to open the etching window for the silicon diagram; (c) backside silicon diagram anisotropic wet etching by KOH until the remained thickness of silicon about 50 m; (d) bottom electrode Pt (180 nm)/Ti (20 nm) deposition on the front side by sputtering using the lift-off method; (e) deposition and sintering of thick PZT by modified sol–gel slurry method; PZT patterning by wet etching using HCl + HF + H2 O to open the contacting hole for the bottom electrode; (f) spin coating, patterning and curing of polyimide insulator layer to prevent the electrical short circuit between the top and bottom electrode; (g) top electrode Pt (180 nm)/Ti (20 nm) deposition on the front side by sputtering using the lift-off method; and finally etching the backside silicon by RIE using CF4 + O2 until the desired thickness left about 20 m in this device. Fig. 6 is the image of the ultrasonic arrays with 7 m thickness PZT film fabricated on a 4 in. wafer. Each white point is a single PZT emitter cell; the dark square around the white array points is the contacting hole of the bottom electrode. In this device, each cell has its own bottom electrode, while the entire cells in the array using a common upper electrode, which is above the polyimide layer. Unlike the fabrication of
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ferroelectric mems device with thin PZT film, this ultrasonic array with thick PZT film have obtained 100% yield of the device due to the good thick PZT layer. It is quite often to get bad points in the MEMS device with PZT thin film due to the electrical short circuit between the top and bottom electrode due to the particle and defects in the film.
4. Vibration characteristic and ultrasonic sound emission property Fig. 7 is a typical vibration response of a transmitter with a size of 1.5 mm × 1.5 mm in the fabricated array after input of an ac driving voltage of 500 Hz, 40 V (p–p) with PZT film under 30 V dc bias. The thickness of the PZT layer in the device is 7 m, and the thickness of bottom silicon in the diagram structure is about 20 m. The peak-to-peak displacement of
Fig. 7. The displacement and the driving voltage of a transmitter in the ultrasonic array.
Fig. 5. Schematic fabrication process of the ultrasonic array device.
Fig. 6. Fabricated ultrasonic array devices with 7 m PZT thick film in 4 in. wafer.
Fig. 8. Frequency dependence of the peak-to-peak displacement of a 1.5 mm × 1.5 mm transmitter in the ultrasonic array at bias of 30 V and Vp–p of 20 V.
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The output sound pressure level (SPL) of the transmitter versus the ac driving voltage at a fixed dc bias applied onto the transmitter was also measured as shown in Fig. 10, which shows that the SPL increases with the increase of the ac driving voltage. The SPL within the range of 102–110 dB is very good for such a small transmitter. The ultrasound emission measurement results show that the device could have practical applications.
5. Conclusion
Fig. 9. Frequency dependence of output sound pressure of a 1.5 mm × 1.5 mm transmitter in the ultrasonic array at bias of 30 V and Vp–p of 20 V.
the cell is about 0.2 m measured by scanning laser doppler vibrometer (SLDV) made by Polytec GmbH, Germany. In Fig. 8 the frequency dependence of the peak-to-peak displacement of a single cell in the ultrasonic array is shown. The first order of the resonance frequency is 57.75 kHz, while the second order of the resonance frequency is 202 kHz. At the first order of resonance frequency, which the ultrasonic emitter will operate at, the peak-to-peak displacement of the single cell is about 4.2 m, which is really large enough. The sound pressure measurement versus frequency of a single cell in the device has also been carried out as shown in Fig. 9, with an output of 3.6 Pa at the resonance frequency of 57.6 kHz. The two resonance frequencies measured by vibration displacement test and sound pressure test are almost the same, which shows that the results of the two different measurements are accurate.
Fig. 10. Output sound pressure level of a transmitter vs. diving voltage at resonance frequency.
Ultrasonic arrays with 7 m PZT thick film on the silicon membrane diagrams as ultrasonic emitter have been successfully fabricated by silicon micromachining with 100% yield. The modified sol–gel slurry method is a good way to fabricate PZT thick film up to 20 m with relatively good ferroelectric property with Pr as high as 7.5 c/cm2 . The polyimide layer is introduced to avoid etching of PZT thick film, which still provides challenges for future work. The vibration characteristics and the ultrasonic emission property of the fabricated array show that ultrasonic transducer can generate the ultrasound with pressure as high as 3.6 Pa, which could have practical application in object detection in air. The preparation method of thick PZT film and the micromachining process can be used in other ferroelectric device fabrication using thick PZT films.
References [1] M. Lucacs, M. Sayer, S. Fostser, Single element high frequency PZT sol–gel composite ultrasound transducers, IEEE Trans. Ultrasonics Ferroelectronics Frequency Control 47 (2000) 148–159. [2] K. Yamashita, H. Katata, M. Okuyama, H. Miyoshi, G. Kato, S. Aoyagi, Y. Suzuki, Arrayed ultrasonic microsensors with high directivity for in-air use using PZT thin film on silicon diagrams, Sens. Actuators A 97–98 (2002) 302–307. [3] P. Muralt, Ferroelectric thin films for micro-sensors and actuators: a review, J. Micromech. Microeng. 10 (2000) 136–146. [4] P. Muralt, PZT thin films for microsensors and actuators: where do we stand, IEEE Trans. Ultrasonics Ferroelectronics Frequency Control 47 (2000) 903–915. [5] M. Lebedev, J. Akedo, Y. Akiyama, Actuation properties of lead zirconate titanate thick films structured on Si membrane by the aerosol deposition method, Jpn. J. Appl. Phys. 39 (2000) 5600–5603. [6] J. Baborowski, P. Muralt, N. Ledermann, S. Petitgrand, A. Bosseboeuf, N. Setter, Ph. Gaucher, PZT coated membrane structures for micromachined ultrasonic transducers, in: Proceedings of the 13th IEEE International Symposium on Applications of Ferroelectrics, Nara, Japan, May 28–June 1, 2002, pp. 483–486. [7] T. Iijima, K. Kunii, Preparation of texture-controlled lead zirconate titanate diaphragm-type film actuator using a chemical solution method, Jpn. J. Appl. Phys. 40 (2001) 5740–5742. [8] E. Defay, C. Millon, C. Malhaire, D. Barbier, PZT thin films integration for the realization of a high sensitivity pressure microsensor based on a vibrating membrane, Sens. Actuators A 99 (2002) 64–67. [9] Z. Wang, W. Zhu, C. Zhao, O.K. Tan, Dense PZT thick films derived from sol–gel based nanocomposite Process, Mater. Sci. Eng. B 99 (2003) 56–62.
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Biographies Hong Zhu received his B.Eng. from the Department of Materials Science and Engineering, University of Science and Technology Beijing, China in 1990, and his Ph.D. from Shanghai Institute of Metallurgy, Chinese Academy of Sciences in 1995. He worked as a visiting scholar in Graduate School of Engineering Science, Osaka University, Japan from 1998 to 2000. In October 2000, he joined School of Mechanical and Production Engineering, Nanyang Technological University, Singapore as a research fellow in Micromachines (MEMS) Center. In August 2004, he joined Data Storage Institute, Agency for Science, Technology and Research (A*STAR), Singapore as a senior research engineer. His research interests include fabrication and characterization on ferroelectric sensor and actuator, new MEMS fabrication technology and device development. Jianmin Miao received his Dipl.-Ing. and Dr.-Ing. from the Department of Electrical Engineering and Information Technology at the Darmstadt University of Technology, Germany in 1991, 1996 respectively. He was the head of R&D with Lectret S.A. (Switzerland and Singapore) with prime responsibility for the joint project of integrated silicon microphones with the Institute of Microelectronics in Singapore. In September 1998, he joined the Nanyang Technological University, Singapore as a faculty. He is currently an associate professor and the director of the Micromachines (MEMS) Center. His research interests include MEMS design, technology development and characterization. He is the member of the Institute of Electrical and Electronics Engineers. Zhihong Wang received the B.S. degree in magnetics and the M.S. degree in solid-state physics from Lanzhou University, Gansu and the Ph.D. degree in microelectronics from Xi’an Jiaotong University, Shaanxi, China in 1984, 1989 and 1999, respectively. In 1999, he joined the Department
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of Advanced Component Development, SAE Magnetics (H.K.) Ltd., as principal engineer and manager. In 2000 he joined Nanyang Technological University, Singapore, where he is currently a research fellow with Sensors and Actuators Laboratory, School of Electrical and Electronic Engineering. His early research involved preparation and characterization of various electronic ceramics and films, design, fabrication and reliability assessment of piezoelectric devices. His current research interests include thin film piezoelectric measurement, ultrasonic transducers and thick film ferroelectric sensors and actuators for micro electromechanical systems and biomedical applications. Changlei Zhao received his B.Eng. degree in Foundry Engineering at the Northwestern Polytech University in 1996 and M.Eng. degree in Materials at Shanghai Institute of Ceramics, Chinese Academy of Science in 1999, respectively. It was during his undergraduate years that he became interested in designing and developing functional materials. This interest led him turn to ferroelectric and piezoelectric studies within the Functional Ceramics Research and Design Centre at SICCAS immediately after his bachelors degree. During his graduate research, his interest in piezoelectric materials for sensors and actuators was established. He is currently studying on the ferroelectric film actuators and electronic devices as a doctoral candidate. Weiguang Zhu received his B.Sc. and M.Sc. from Shanghai Jiao Tong University, China, and Ph.D. from Purdue University, USA. Currently, he is a professor at Nanyang Technological University, Singapore. Dr. Zhu is a member of American Physical Society, American Ceramic Society, IEEE, Materials Research Society, and a fellow of IES. He has presented many invited talks and served as the session chairman various committees in international conferences. Dr. Zhu is a member and ChairElect of Executive Board of Asian Ferroelectric Association; a member of Academic Committee of the Chinese National Open Laboratory on Inorganic Functional Materials, of the Chinese Academy of Sciences; member of Academic Committee of Electronic Materials Research Laboratory of Xi’an Jiaotong University, a Key Laboratory of the Chinese Ministry of Education. He has published more than 150 referred papers in international journals, and has been invited as the guest editor for Journal of Ferroelectrics and Related Materials, Journal of Ceramics International and Ceramic Transactions, member of editorial board of Journal of Ferroelectrics, visiting professor at EPFL of Switzerland, RWTH at Aachen of Germany Xi’an Jiaotong University of China, and Xi’dian University of China, and invited professor at the French National Institute on Condensed Matters of Chemistry at Bordeaux (ICMCB-CNRS). Dr. Zhu has received the Outstanding Achievement Award from 16th International Symposium of Integrated Ferroelectrics. His research interests includes electronic materials, thin films, ferroelectrics, high-k dielectrics, microwave tunable dielectrics, gas sensors, high-Tc superconductors, superionic conductors, shape memory alloys, atomic diffusion, optical and electron holography, crystallography, and ferroelectric composites.