Fabrication Methods for Improved Electromechanical Behavior in Piezoelectric Membranes
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FABRICATION METHODS FOR IMPROVED ELECTROMECHANICAL BEHAVIOR IN PIEZOELECTRIC MEMBRANES M.C. Robinson, P.D. Hayenga, J.H. Cho, C.D. Richards, R.F. Richards, and D.F. Bahr Mechanical and Materials Engineering, Washington State University, Pullman, WA 99164-2920 ABSTRACT Piezoelectric materials convert mechanical to electrical energy under stretching and bending conditions. Optimizing the coupling conversion is imperative to the electromechanical behavior of a micromachined membrane’s performance. This paper discusses analytical calculations that were devised to determine the microscale structure that minimizes residual stress and outlines the implementation of fabrication technique variations including three different electrode configurations, trenching around the membrane, and reducing the total composite residual stress of the support structure using compressive silicon oxide. Lead zirconacte titanate (PZT) films between 1 and 3 µm thick with a ratio of Zr to Ti of 40:60 were deposited onto 3 mm square silicon membranes. The total tensile stress in the composite structure reaches 100 MPa during standard fabrication processing. Utilizing analytical calculations, a structure was determined that lowered the residual stress of the composite to 11 MPa and increased the electromechanical coupling 35 times. Changing the geometry of the electrode coverage decreased the residual stress of the composite by 40%. Trenching around the membrane provided a membrane with boundary conditions that approached simply supported and decreased the composite residual stress by another 16%. A comparison of the electromechanical behavior for these structures will be discussed, showing a route towards increasing electromechanical coupling in PZT MEMS. INTRODUCTION Motivation for this work stems from the study of piezoelectric materials for micro and nanoscale electromechanical devices in a move towards miniaturizing power generation for MEMS. Utilizing piezoelectric materials, which convert mechanical strain energy to electrical energy, is one way in which to achieve micropower. Several research groups have attempted to create membranes that demonstrate this conversion of energy for devices such as actuators and other micropower harvesting techniques.[1,2] Properties of a piezoelectric membrane that influence micropower generation examined in this research include the residual stress of the composite structure, boundary conditions, and the coupling coefficient. Each of these factors is quantified by the resonant frequency, the compliance of the structure, and the electrode configuration that sandwiches the piezoelectric layer. Membranes fabricated at Washington State University were used in several experiments that directly correlate to analytical calculations.[3] Utilization of these analytical expressions that will be further developed with experiment will provide a map to a piezoelectric membrane with optimized electromechanical behavior.
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PROCEDURE The residual stress of a composite piezoelectric membrane structure af
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