Characteristics on PZT (Pb(ZrxTi1−x)O3) films for piezoelectric angular rate sensor

Sensors and Actuators A 114 (2004) 88–92

Characteristics on PZT (Pb(Zrx Ti1−x )O3) films for piezoelectric angular rate sensor Sung-Ho Lee a,∗ , Masayoshi Esashi b a

b

Institute of Mechanical Systems Engineering, National Institute of Advanced Industrial Science and Technology, 1-2 Namiki, Tsukuba, Ibaraki 305-8562, Japan New Industry Creation Hatchery Center, Tohoku University, 01 Aza Aoba, Aramaki, Aoba-Ku, Sendai, Miyagi 980-8579, Japan Received 30 July 2003; received in revised form 6 March 2004; accepted 6 March 2004 Available online 17 April 2004

Abstract Piezoelectric or magnetostrictive materials, known as smart materials, have been researched widely for sensors or actuators in microsystem technology. In our research, thick sol–gel lead zirconate titanate (Pb(Zr1−x Tix )O3 ) films were fabricated and their characteristics were investigated for angular rate sensor applications. The thickness of the PZT films was 1.5 ␮m, which is required by a vibration angular rate sensor for a good actuation and sensing. The remnant polarization of the PZT films was 12.0 ␮C/cm2 . The electromechanical constants of PZT thin film showed the value of susceptance (B) of 4800 ␮s at capacitance of 790 pF. The PZT films were applied to the vibration angular rate sensor structure and the vibration of 1.78 ␮m in amplitude at the resonant frequency of 35.8 kHz was obtained by the driving voltage of 5 Vp–p of bulk piezoelectric materials with out of phase signal through voltage and inverting amplifier. The oscillating output voltage showed the values of 0.76 and 0.87 V in outer/inner driving electrode at the driving voltage of 5 Vp–p by external actuation using a stacked piezoactuator. © 2004 Elsevier B.V. All rights reserved. Keywords: PZT film angular rate sensor; Sol–gel process; Dry and wet etching method

1. Introduction Piezoelecric materials are attractive for microelectromechanical systems (MEMS) applications, because they have the good properties capable for actuation or sensing in these applications [1–4]. Moreover, the thin film microactuator or sensor of these materials is interesting because of simple design and compatible to mass production processes. Piezoelectric materials, often referred to as “smart materials” are one type of the functional materials utilized in MEMS technology [5]. Development of reliable actuation and high sensitivity to vibration is one of many challenges in thin film MEMS devices. Piezoelectric actuation using bulk ceramic materials is well known, but widespread use in MEMS requires suitable deposition and integration methods, which are compatible with large-scale manufacturing [6–9]. Micromachined vibration inertial sensors are very promising devices for many applications in such areas as automotive safety, consumer products, various industries, such as medicine and space ∗ Corresponding author. Tel.: +81-298-61-7849; fax: +81-298-61-7167. E-mail address: [email protected] (S.-H. Lee).

0924-4247/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2004.03.010

[10–12]. Conventional piezoelectric vibration angular rate sensors use bulk PZT material [13]. These sensors have a non-piezoelectric (usually metal) oscillating body on which PZT resonators are attached with adhesive. This structure, however, needs complicated manufacturing techniques such as the bonding of the PZT resonators, and is not suitable to batch production. Also, these sensors have the severe drawback of the low thermal stability, because the large difference of thermal expansion coefficients between the metal oscillating body and the PZT resonators generates large thermal and mechanical stress. These things affect the property of sensor and actuator using bulk PZT and non-piezoelectric things. Using PZT films as the resonators of vibration angular rate sensors is the promising way to overcome the above mentioned drawbacks. Several methods such as sputtering, chemical vapor deposition (CVD) and sol–gel method have been used to deposit PZT films [14–16]. The sol–gel method has the advantages of easy control of chemical composition and good piezoelectric property compared with other deposition methods. PZT film needs the thickness more than 1 ␮m in thickness for applications of the piezoelectric MEMS, but thick PZT film of more than 1 ␮m in thickness often suffers from cracks and bad crystal orientation.

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In this paper, we describe our process to deposit 1.5 ␮m thick crack-free PZT films with good piezoelectric property by sol–gel method. We also mention the fabrication and basic tests of a vibration angular rate sensor with sol–gel PZT film.

2. Experimental

3. Results and discussion 3.1. Properties of sol–gel PZT film Fig. 1 shows the cross-sectional scanning electron micrograph (SEM) of the PZT film. From the SEM, we confirmed the crack-free PZT film of 1.5 ␮m in thickness. The XRD result of the PZT film was shown in Fig. 2. This result demonstrates that the Pt electrode beneath the PZT film had a good orientation peak of (1 1 1). This result also

Fig. 1. Cross-sectional SEM figure of PZT film.

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Pt(111) Perovskite (100) Perovskite (200)

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demonstrates that the 1.5 ␮m thick PZT film had a preferred tetragonal structure with orientation peaks of (1 0 0) and (2 0 0). The dielectric polarization property of the PZT film was measured with the dielectric analyzer. The remnant polarization of 12.0 ␮C/cm2 , which implies a good ferroelectric property was confirmed as shown in Fig. 3. In Fig. 4, 40

Polarization(µC/cm2)

The properties of PZT material depend upon its composition. Therefore, for high piezoelectric property, its composition should be controlled. Many deposition methods have been proposed for good PZT film. Among these methods, metal organic chemical vapor deposition (MOCVD) or chemical solution deposition (CSD) is easy to control its composition. In this study, the sol–gel method was employed for making the PZT film. Sol–gel PZT film was deposited on 20 mm square silicon substrates with a Pt/Ti/SiO2 layer by multiple spin-coating technique combined with rapid thermal annealing (RTA). A precursor solution of sol–gel PZT was composed of trihydrated lead acetate, titanium iso-propoxide, zirconium iso-propoxide and 2-methoxyethanol for solvent. The composition of the precursor solution for 0.5 M Pb(Zr0.53 Ti0.47 )O3 was considered, which requires to obtain the film with morphotropic phase boundary (MPB). The (1 1 1) oriented 250 nm thick Pt and 150 nm thick Ti film were deposited on a thermally oxidized silicon substrate by sputtering method at room temperature. The process consists of 15 times repeat of the spin-coating and RTA to obtain the 1.5 ␮m thick PZT film. Every five times of spin-coating were followed by one thermal treatment (drying and pyrolysis). The RTA was conducted at 650 ◦ C for 1 min in oxygen atmosphere to change a pyrochlore-structured film to a perovskite-structured crystal film. A 100 nm thick PZT film was realized in each spin coationg. The thick PZT film fabricated in this process was free from cracks. The crystal orientation of the PZT film was measured by X-ray diffraction (XRD) method. To measure dielectric properties of the PZT films, Pt electrode were formed on the PZT films, and a dielectric analyzer (Radiant Technology Inc., RT66A) was used. The electromechanical constants of a PZT film were characterized by the admittance measurement of a transducer with varying frequency using HP LCR meter.

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Driving voltage (V) Fig. 3. Hysteresis loop for thin PZT film.

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Fig. 4. Conductance (G)–susceptance (B) curve of PZT film.

the electromechanical constants of a piezoelectric material with varying frequency were shown. The electrode size was 400 ␮m × 400 ␮m and thickness of film was 1.2 ␮m. As the frequency was increasing, the value of susceptance (B) was increased to 4800 ␮s at capacitance of 790 pF, whereas conductance (G) value was almost constant. In really, the higher susceptance (B) of piezoelectric materials, the more the electromechanical transition value related to sensitivity. It implies the PZT film sensor fabricated by the sol–gel method has the high sensitivity. 3.2. Fabrication of PZT film angular rate sensor We applied our sol–gel PZT film to the fabrication of a vibration angular rate sensor. Fig. 5 illustrates the vibration angular rate sensor developed. A PZT film under driving electrodes oscillates a silicon mass supported by a

Fig. 6. Fabrication process of piezoelectric thin film angular rate sensor.

PZT/Pt/Ti/SiO2 membrane in z-direction. The oscillation of the silicon mass is modulated by the rotation of the sensor around x-axis. The PZT film also detects the modulated oscillation through sensing electrodes. The advantage of the sensor is the simple structure with the PZT film that enables the oscillation and the sensing simultaneously. If we configure the position of electrode properly, it is possible to do both sensing and actuating in the piezoelectric angular rate sensor. Fig. 6 shows the fabrication of the sol–gel PZT film angular rate sensor using bulk micromachining technology. First, the PZT film was deposited on a silicon substrate with a Pt/Ti/SiO2 layer by the above mentioned multiple spin coating processes. Next, an insulation layer of sputtered silicon dioxide and electrodes of sputtering Au/Cr was formed by lift-off and etching respectively. Subsequently, the backside of the silicon substrate was etched to make the mass and the membrane by deep reactive ion etching, and finally a Pyrex glass substrate was bonded with silicon substrate. To confirm the vibration property of the PZT film, the completed sensor was actuated by drive electrode with sine-wave voltage, and the displacement of the mass was measured with a laser Doppler vibrometer (Nihon Kagaku Eng., MLD-102). The output voltage in the vibrating piezoelectric angular rate sensor was measured through the charge amplifier. 3.3. Characteristics of PZT film angular rate sensor

Fig. 5. Outline figure of PZT film angular rate sensor.

Fig. 7 shows the configuration of the upper electrode for driving the PZT film and sensing angular rate simultaneously. The driving electrodes among these electrodes are connected to a voltage follower and inverting amplifier circuit. The electrical signal with the phase difference of 180◦ was applied to the driving electrode. When the driving

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Fig. 7. Electrical circuit diagram for detecting the electric charge from the PZT film angular rate sensor.

Fig. 8. Optical micrograph of fabricated PZT film device with electric circuit.

upper electrode in the PZT film angular rate sensor when the driving voltage was applied to the device. The electrical charge, Q, from the PZT film was measured with charge amplifier in the rotational vacuum chamber. The PZT film was oscillated by the stacked piezoactuator to measure the electrical charge induced in the PZT film without rotating 2 Driving Voltage : 5Vp-p

Amplitude (um)

signal is applied, the PZT film angular rate sensor is oscillated in the vertical direction. The sensing electrodes was connected to the charge amplifier circuit. When the electrical charge is changed, the sensing circuit detects the signal by charge change. When the PZT film angular rate sensor was only vibrated without angular rate from outside, this causes a symmetrical stress distribution. The electrical charge in the sensing electrode is canceled. If the angular rate enters to the PZT film angular rate sensor, the surface charge is changed by coriolis force in the PZT film angular rate sensor. Therefore, we can measure the electrical charge of the sensing electrode from the charge amplifier in the electrical circuit. The fabricated PZT film angular rate sensor with electric circuit is shown in Fig. 8. Fig. 9 shows the angular rate sensor amplitude by stacked piezoactuator using electrical signal at inverting amplifier. The amplitude of 1.78 ␮m in PZT film angular rate sensor was obtained at resonant frequency of 35.8 kHz. The 5 Vp–p driving voltage was applied to stacked piezoactuator through ac power source. The PZT thin film device was settled on the stacked piezoactuator. The amplitude of device was measured with laser Doppler vibrometer. Fig. 10 shows the output voltage through the

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Fig. 9. Vibration property of PZT film angular rate sensor driven by external stacked piezoactuator.

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the stage, because the electrical signal could not be detected from the sensing electrode. The driving voltage was applied to bulk piezoelectric materials with 5 Vp–p with phase difference of 180◦ . The output voltage was translated using the charge amplifier shown in Fig. 8. The output voltage through the C–V converter was 0.87 V at the inner driving electrode and 0.76 V at the outer driving electrode as shown in Fig. 10, respectively.

[8]

[9]

[10] [11] [12] [13] [14]

4. Conclusions We have developed the process to deposit thick sol–gel PZT (Pb(Zrx Ti1−x )O3 ) films combining the multiple spin coating technique and rapid thermal annealing (RTA). Crack-free thick PZT films to apply MEMS in thickness were successfully fabricated by this process. We applied the sol–gel PZT film process to the fabrication of the vibration angular rate sensor. Driving amplitude and output voltage in sol–gel PZT film was evaluated from the electrical circuits. Future works include the characteristics of the angular rate sensor using the piezoelectric film.

Acknowledgements The author wish to thank Dr. T. Iijima at Smart Structure Research Center of National Institute of Advanced Industrial Science and Technology, who gave a lot of valuable advice of sol–gel piezoelectric thin films.

References [1] D. Hauden, MEMS application of piezoelectric materials, in: C. Golassi et al. (Eds.), Piezoelectric materials: Advances in science,

[15] [16]

Technology and Applications, Kluwer Academic Publishers, (2000) 335–346. T. Hattori, Achievements of Japanese micromachine projects, in: Proceedings of the International Conference on Solid-State Sensors and Actuators, Chicago, 16 June 1997, pp. 25–28. T. Ikeda, Fundamentals of Piezoelectricity, Oxford University Press, Oxford, 1990. H. Jaffe, Piezoelectric transducer materials, Proc. IEEE 53 (10) (1965) 1372–1386. E. Holleck, H. Quandt, Materials development for thin film actuators, Microsyst. Technol. 1 (1995) 178–184. P. Muralt, Ferroelectric thin films for microsensors and actuators: a review, J. Micromech. Microeng. 10 (2000) 136. D.B. Hicks, et al., Piezoelectrically activated resonant bridge microaccelerometer, in: Proceedings of Solid State Sensor and Actuator Workshop, Hilton Head, SC, 13–16 June 1994, pp. 225–228. A. Schroth, C. Lee, S. Matsumoto, R. Maeda, Application of sol–gel deposited thin PZT film for actuation of 1D and 2D scanners, Sens. Actuators 73 (1999) 144–152. D.L. Polla, W.P. Robbins, Application of piezoelectric microsystems in structural health monitoring and biomedical engineering, in: Proceedings of the Ninth US–Japan Seminar on Dielectric and Piezoelectric Ceramics, Okinawa, Japan, 2–5 November 1999, pp. 239–242. M. Weinberg, et al., Micromachining inertial instruments, Proc. SPIE 2879 (1996) 26–36. R. Voss, Silicon micromachined vibrating gyroscopes, Proc. SPIE 3224 (1997) 62–73. N. Yazdi, F. Ayazi, K. Najafi, Micromachined inertial sensors, Proc. IEEE 86 (8) (1998) 1640–1659. J.S. Burdess, T. Wren, The theory of a piezoelectric disc gyroscope, IEEE Trans. Aerospace Electron. Syst. AES 22 (4) (1986) 410. S.B. Krupanidhi, N. Maffei, M. Sayer, K. El-Assal, RF planar magnetron sputtering and characterization of ferroelectic PZT films, J. Appl. Phys. 54 (1983) 6601. I.S. Chen, J.F. Roeder, D.J. Kim, J.P. Maria, A.I. Kingon, J. Vac. Sci. Technol. B 19 (5) (2001) 1833. S.K. Dey, K.D. Budd, D.A. Payne, Thin film ferroelectrics of PZT by sol–gel processing, IEEE Trans. Ultrason. Ferroelect. Freq. Contr. 35 (1998) 80.

Biographies Sung-Ho Lee was born in South Korea. He received his PhD degree in mechatronics and precision engineering in 2001 from Tohoku University, Japan, and the BS and MS degrees in materials science and engineering from Pusan National University, South Korea, in 1992, 1995 respectively. From 1994 to 1997, he was a senior researcher at Research Institute of Industrial Science and Technology (RIST) in South Korea. Since 2001, he has been with National Institute of Advanced Industrial Science and Technology (AIST), Japan. His research interests include ceramic microsystem devices, microchemical reactor and microfuel cell system. Masayoshi Esashi was born in Sendai, Japan. He received PhD degree in 1976 from Tohoku University. From 1976 to 1981, he was a research associate in the Department of Electronic Engineering, Tohoku University and an associate professor from 1981 to 1990. Since 1990, he has been a professor at the Department of Mechatronics and Precision Engineering, Tohoku University. He has been a professor at the New Industry Creation Hatchery Center in Tohoku University. He was a director of the Venture Business Laboratory in Tohoku University. His current interest is in micromachining and microtechnology for saving energy and natural resources.