Experimental study of in-plane poled piezoelectric diaphragm-based MEMS devices

• Main Author: Shen, Zhiyuan.
• Other Authors: Miao Jianmin
• Format: Theses and Dissertations
• Language: English
• Published: 2013
• Subjects: DRNTU::Engineering::Electrical and electronic engineering::Microelectromechanical systems
• Online Access: http://hdl.handle.net/10356/54062
• Institution: Nanyang Technological University

In recent years, piezoelectric diaphragms with an in-planed poling scheme have been studied and have demonstrated improved actuation over traditional diaphragms. In this study, the design, fabrication and characterization of an in-plane poled MEMS piezoelectric diaphragm will be investigated, with the aim of demonstrating the diaphragm’s effectiveness in sensitivity improvement and its applications as an acoustic transducer and energy harvester. The in-plane poling of the diaphragm was realized by adopting a double-sided aligned double spiral electrode. The in-plane poling induces the in-plane elongation of the material via the d33 piezoelectric effect. The elongation is converted to an out-of-plane displacement due to the confined boundary condition. The patterned electrode induces a polarization distribution in the device. The existing analytical results of piezoelectric diaphragms are mainly based on uniform material properties assumption. These results must be modified by considering the polarization distribution before they can be used to predict the performance of the diaphragm in this design. Two modeling strategies—a uniform field modeling strategy and a distributed material modeling strategy—were used to cope with the material distribution and to simulate the mechanical behavior of the diaphragm. In the uniform field modeling strategy, the device was divided into several regions. The polarization in each region is assumed to be uniform. In the distributed material modeling strategy, the modeling process imitates the poling process of the diaphragm. A finite element code was developed to reorient the material polarization distribution according to the poling field calculated. The influence of different drive configurations and diaphragm and electrode geometry parameters on deflection has been calculated by the uniform field modeling. The resonance frequencies and mode shapes have been analyzed by the distributed material modeling. The diaphragm was fabricated by the micro electromechanical system (MEMS) process. The piezoelectric material is a lead zirconate titanate (PZT) ceramic. Au/Cr electrodes were deposited on bulk PZT wafers by sputtering and patterned by the lift-off process. SU-8 thick-film and acrylate plates were used to form the structural layer. The impedance and quasi-static displacement spectra of the diaphragm were measured after poling. A diaphragm bearing double-sided patterned electrodes can be actuated by more drive configurations than a diaphragm bearing only a single-sided electrode. Compared to the single-sided electrode drive, a double-sided, out-of-phase drive configuration has been proven to be able to increase the coupling coefficient of the fundamental resonance from 7.6% to 11.8%. The displacement response of the diaphragm can be increased from 2.6 nm/V to 8.6 nm/V. The diaphragm working in the vibration frequency can generate and receive acoustic waves in the out-of-plane direction. The acoustic transmission performance of the diaphragm has been characterized using a setup built in an anechoic chamber. The first four resonance modes of the diaphragm simulated by the distributed material modeling have been verified by impedance and velocity spectra. The sensitivity and sound pressure level of a single element and the directivity of an array have been measured. The 1 kHz sensitivity of such a diaphragm has the magnitude of 126.21 V/Pa, which is one order of magnitude larger than the sensitivity of reported thick-film sandwich structure. This research also demonstrates the application of the in-plane poled diaphragm as an energy harvester. The energy harvester was mounted on a mechanical shaker which simulates a vibration source. A resistor was connected to the harvester to form a closed-circuit loop. The obtained load power with respect to the load resistance and the input vibration frequency was measured. The load power reaches maximum when the resistor matches the output impedance of the diaphragm and when the input frequency coincides with the anti-resonance of the diaphragm. The influence of a proof mass on the resonance and energy harvesting has also been investigated. A proof mass weighted two times heavier than the original diaphragm is found to be able to greatly enhance the energy harvesting ability of the diaphragm. A power density of 22.59 μW/cm2 was realized.