Liquid Density Sensing Using Resonant Flexural Plate Wave Device with Sol-Gel PZT Thin Films

LIQUID DENSITY SENSING USING RESONANT FLEXURAL PLATE WAVE DEVICE WITH SOL-GEL PZT THIN FILMS

Jyh-Cheng Yu* and Huang-Yao Lin

Department of Mechanical and Automation Engineering National Kaohsiung First University of Science and Technology 2, Juoyue Rd., Nantz District, Kaohsiung 811, Taiwan, R.O.C.

ABSTRACT This paper presents the design, fabrication and preliminary experimental results of a flexure plate wave (FPW) resonator using sol-gel derived lead zirconate titanates (PZT) thin films. The resonator adopts a two-port structure with reflecting grates on the composite membrane of PZT and SiNx. The design of the reflecting grating is derived from a SAW resonator model using COM theory to produce a sharp resonant peak. The comparison between the mass and the viscosity effects from the theoretical expression illustrates the applications and the constraints of the proposed device in liquid sensing. Multiple coatings of sol-gel derived PZT films are adopted because of the cost advantage and the high electromechanical coupling effect over other piezoelectric films. The fabrication issues of the proposed material structure are addressed. Theoretical estimations of the mass and the viscosity effects are compared with the experimental results. The resonant frequency has a good linear correlation with the density of low viscosity liquids, which demonstrate the feasibility of the proposed device.

INTRODUCTION

Acoustic wave devices have attracted enormous attention for sensor application because that the wave velocity and damping are sensitive to external disturbances such as temperature, pressure, additive mass, and viscosity [1]. Among them, the phase velocity of lamb waves, unlike the velocity of surface acoustic wave (SAW), depends on the thickness of the propagating plate. They can be considered as two Rayleigh waves that are strongly coupled and propagating on both sides of the plate. Two vibrating modes can propagate through the plate independently, namely the symmetric and the anti- symmetric lamb modes. The anti-symmetric zero mode A0, also called “flexural plate wave” (FPW), propagating on a thin membrane with thickness 5% or less of the acoustic wavelength, has a phase velocity lower than the sound velocity of loading liquid. FPW sensors are suitable for liquid sensing because a slow mode of propagation, such as A0 mode, will minimize the radiation energy loss. The excitation and the detection of the acoustic waves are most readily accomplished by the use of interdigital transducers (IDTs) [2] on thin piezoelectric plate that is used to be realized from the etching processes on a bulk piezoelectric substrate. Piezoelectric thin films have the cost advantage over crystalline materials. Many literatures addressed FPW sensors using ZnO[3] and AlN[5]. The electromechanical coupling effect and the dielectric constant of PZT are much higher than AlN and ZnO, which makes PZT films potentially suitable for sensor application. However, the polycrystalline structure of PZT and the required high temperature of heat treatment during the coating process complicate the realization. The potential applications of FPW sensors to chemical and liquid sensing have attracted a lot of research interests. Costello et al.[3] proposed a simple theory for the mass sensitivity of a delay-line oscillator with ZnO on silicon nitride membrane, and modeled the attenuation of plate waves in contact with viscous liquids. Laurent et al.[4] addressed the configuration design of the FPW devices using AlN and ZnO on silicon membrane, and showed that the FPW device has a large mass sensitivity compared to other acoustic devices. Weinberg et al.[5] derived the fluid-damping model for resonant FPW devices. To increase the differentiability of the resonant frequency shift, reflecting gratings are added to the FPW devices that is first reported by Joshi[6] using a Y-X lithium niobate plate. Nakagwa[7] also adopted the same configuration but on an AT-cut quartz substrate. Mass, tensile stress, and viscosity effects might couple when in contact with liquids, which will complicate the differentiablity in sensor applications. Very few literatures address the practical issues and strategy in liquid density sensing.
This study will discuss the design and application issues for the FPW resonator using the sol-gel derived PZT on silicon nitride membrane as shown in Figure 1. We will derive the design of the reflecting grating using the Coupling of Modes (COM) theory, and apply to the FPW devices. The mass loading when the device is in contact with liquid will introduce the deviation of resonant frequency that is determined by the liquid density and viscosity. The constraints of the proposed device in liquid density sensing will be discussed. Finally, ©EDA Publishing/DTIP 2007 ISBN: 978-2-35500-000-3

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