Electromechanical Behavior in Micromachined Piezoelectric Membranes
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ELECTROMECHANICAL BEHAVIOR IN MICROMACHINED PIEZOELECTRIC MEMBRANES M.C. Robinson, J.C. Raupp, I. Demir, C.D. Richards, R.F. Richards, and D.F. Bahr Mechanical and Materials Engineering, Washington State University, Pullman, WA 99164-2920 ABSTRACT Piezoelectric materials can convert mechanical and electrical energy, a particularly useful tool in developing micro and nanoscale systems. Characterizing the electromechanical behavior is essential to the design and optimization of the material’s and device’s performance. This paper examines the influence of boundary (clamping) conditions, relative thickness variations between the active one to two micron thick piezoelectric membrane and underlying passive support structure, and the electrode coverage on the electromechanical behavior. Membranes were fabricated with silicon and lead zirconate titanate (PZT) with a ratio of Zr to Ti of 40:60 that provide thickness ratios between 1:2 and 2:1 by depositing the PZT using sequential solution deposition. PZT films contain a tensile stress that accumulates during processing, therefore a compressive stressed layer of tungsten was sputtered on bulk micromachined membranes to produce a near zero net residual stress. A nonlinear finite element numerical simulation technique is utilized for the analysis of the composite thin film. A comparison between the behavioral trends determined by simulation and experimental methods will be discussed. INTRODUCTION There are many factors that guide the optimization of any micro or nanoscale system. Fundamental to the design of a MEMS piezoelectric device is the characterization of the electromechanical behavior. This behavior is described by the coupling coefficient (k2) and the compliance of the structure. Both of these properties can be experimentally determined, however there are several factors that influence them. Several groups have studied PZT and its ferroelectric properties [1-4]. The intention of this research was to determine the materials and structure that optimize these characteristic properties for the micromachined membranes that are fabricated at Washington State University [5]. It is important to conduct numerical simulations for the purpose of reducing the number of experiments required to optimize each property. A finite element code was utilized to determine trends of how specific factors influenced the k2 and the compliance of the structure. In addition to conducting numerical simulations, experiments were conducted that realized trends for both k2 and the compliance based on both mechanical and electrical methods. In order to optimize the membrane performance however, it is necessary to vary the structural components and electrode size. Combining these results with varying the thickness ratio between silicon and PZT should provide an overall peak performance recipe for this micro scale membrane. PROCEDURE The influence of several material properties were examined with both finite element numerical simulation and experiment. Since the ratio of PZT to silicon thickn
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