Predictive Modeling of Piezoelectric Micromachined Ultrasonic Transducers

  • Shadi Khazaaleh

Student thesis: Master's Thesis

Abstract

Over the past few decades, industrial applications of ultrasound have been expanding remarkably, mainly in the field of biomedical imaging. Current conventional ultrasonic transducers are mainly based on bulk piezo-ceramics operating in thickness mode, which requires large driving voltage and suffers from several limitations. Alternative MEMS-based ultrasonic transducers have been receiving researchers' interest to overcome the drawbacks of conventional transducers by utilizing micromachining techniques from the integrated circuit (IC) industry. Research has been focusing mainly on two types of MEMS ultrasound transducers, capacitive and piezoelectric micromachined ultrasonic transducers (cMUT and pMUT). PMUTs have several advantages over cMUTs that broadens their field of application, namely in portable devices; therefore, pMUTs seem to be an attractive alternative that could substitute current bulk ultrasonic transducers. In this thesis, pMUTs with frequencies ranging from 250 kHz up to 10 MHz were designed, modeled and simulated using several simulation tools for predicting key performance parameters. Investigated designs included devices consisting of fully-clamped membranes, and pMUTs with partially-clamped membranes by introducing trenches along the circumference. The fabricated devices were then characterized using three testing setups: Twyman-Green interferometry (TGI), digital holographic microscopy (DHM), and needle hydrophone mounted on scanning tank. DHM measurements indicated that the fundamental resonance mode of trenched pMUTs had comparable displacements with their fully-clamped counterpart of similar frequency while having a flatter mode shape, which is believed to enhance the output acoustic pressure. The experimental results were found to be generally in good agreement with the simulation model. Furthermore, the effect of packaging the devices was studied for two applications; fingerprint identification and gesture recognition. In fingerprint identification packaging, the device suffered from low contrast ratio due to heavy reflections along the propagation path. Contrast ratio was enhanced by introducing multiple matching layers across the packaging stack. In gesture recognition packaging, excessive ringing was observed due to reflections from the packaging enclosure. This issue was resolved by adjusting the gap between the pMUT and the enclosure to be equal to half or quarter wavelength in air.
Date of AwardDec 2016
Original languageAmerican English
SupervisorJaime Viegas (Supervisor)

Keywords

  • Ultrasonic Transducers
  • Piezoelectricity
  • Microelectromechanical systems
  • Ultrasonic imaging systems
  • Resonance Frequency
  • IntelliSuite
  • COMSOL Multiphysics.

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