Optimization of PZT-based MEMS
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Optimization of PZT-based MEMS Firas Akasheh, Todd Myers, Susmita Bose and Amit Bandyopadhyay School of Mechanical and Materials Engineering Washington State University, Pullman WA 99164-2920, U.S.A ABSTRACT Due to their excellent piezoelectric properties, PZT ceramics are attractive materials for many MEMS applications. By depositing a PZT film on a micro-machined platinized silicon substrate, membranes can be actuated in the flexural mode. In this work, 2D arrays of PZT actuated membranes, which can be used as ultrasonic transducers, have been designed, fabricated, and tested for their ferroelectric response. The device behavior was also modeled using FEA. Data from FEA runs were used to evaluate the device performance based on its effective coupling coefficient (efficiency), acoustic impedance, and resonance frequency. Results show that the device-coupling factor is significantly affected by the PZT and silicon layers’ thickness and the top electrode configuration. Results also indicate a considerable flexibility in optimizing the device performance under a wide range of operational requirements, which makes such devices attractive for different applications. INTRODUCTION Piezoelectrically actuated membranes are one of the most common structures employed in various MEMS sensing and actuation applications. By including a piezo-active film in the multilayered membranes, the electromechanical coupling effect exhibited by this class of materials can be used to generate and/or sense flexural motion of the membrane. Applications of membrane based MEMS include micro pumps, accelerometers, pressure sensors, and ultrasonic transducers, to name a few. The piezoelectric material of choice in MEMS applications has been lead zirconate titanate (PZT) due to their high dielectric and electromechanical constants. In this work, we consider a special type of MEMS devices, piezoelectric micromachined ultrasonic transducers (pMUTs). MEMS based ultrasonic transducers can open several novel applications including 3D volumetric imaging and high frequency imaging. 3D imaging systems require 2D arrays of a large number of closely spaced, micro-sized transducer elements [1]. A conventional transducer consists of a 1D array of separately wired bulk pieces (dimensions on the order of millimeters) of PZT. The increase in complexity and size refinement needed to realize the required 2D arrays is beyond the capabilities of the current dicing and cabling technology. In pMUTs, a novel concept in ultrasonic transducers, the sound radiating elements are PZT-driven micro-machined membranes, while the connection and electrodes are deposited films, making the sought 2D arrays easy to realize and manufacture in a commercial set-up [2-4]. In this paper, we report on our investigation on the design, modeling and fabrication of pMUT devices. First, a description of the intended device and fabrication details are presented. Then the computer model developed to simulate and predict the device behavior and performance are presented along with results on the
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