Design, Fabrication and Experimental Characterization of PZT Membranes for Passive Low Frequency Vibration Sensing

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Title: Design, Fabrication and Experimental Characterization of PZT Membranes for Passive Low Frequency Vibration Sensing
Author: Zohni, Omar Shariff
Advisors: Andre Mazzoleni, Committee Member
Richard Siergiej, Committee Member
Richard Keltie, Committee Member
Gregory Buckner, Committee Chair
Yong Zhu, Committee Member
Abstract: Low frequency vibration sensing is being used increasingly to monitor the health of machinery and civil structures, enabling “need-based†maintenance scheduling and reduced operating costs. Passive sensors are of particular interest because they don’t require input energy to monitor vibration. Modern vibration sensors are often micro electromechanical systems (MEMS), and are usually very basic in design consisting of a cantilevered beam with some type of deflection sensing circuit. Under the influence of acceleration the beam deflects from its nominal position and its deflection is measured using optical, capacitive or piezoelectric techniques. MEMS sensors tend to exhibit very large stiffness to mass ratios, making them best suited to high frequency vibration sensing. Sensors utilizing the piezoelectric effect can achieve direct energy conversion from the mechanical domain (strain) to the electrical domain (charge) via piezoelectric coupling coefficients. To maximize the electrical output, lead zirconate titanate (PZT) is an excellent piezoelectric material due to its high coupling coefficients. However, the introduction of PZT into standard MEMS processes is problematic because lead is considered a contaminant in most silicon based fabrication facilities. Additional complications with stresses and delamination in thin film stacks have hindered the development of robust fabrication processes for these devices. This dissertation investigates candidate MEMS sensor geometries and fabrication processes for passive low frequency vibration sensing. The addition of silicon nitride (Si3N4) thin films into sol-gel deposited PZT stacks is studied, and the effects of various adhesion layers on delamination and ferroelectric characteristics are quantified. A fabrication process is developed allowing for both front and back side contact for electrical measurements. The effects of thin film stresses on the frequency response of PZT membranes are investigated using experimental, analytical, and computational techniques. Results indicate that thin film stresses in silicon dioxide (SiO2) and Si3N4 can shift the natural frequencies of sensor membranes by as much as 20%. Optimization of sensor membranes is conducted using available numerical methods, particularly finite element analysis (FEA). Coupled electromechanical measurements of fabricated membranes are conducted and experimental results are compared with numerical and analytical solutions. The research outlined in this dissertation represents the first known investigation of passive MEMS vibration sensors specifically targeting such a low frequency range. Also, the integration of PZT into a standard MEMS process requiring low pressure chemical vapor deposition (LPCVD) Si3N4 has not been reported previously. A robust integrated PZT fabrication process is developed which can be used for future work in this field. This process includes a reliable adhesion layer which can be used when deep wet etching of silicon is required. Recommendations for future work and for incorporating these results into packaged sensors are presented.
Date: 2008-12-16
Degree: PhD
Discipline: Mechanical Engineering

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