CLOSED-LOOP FINITE ELEMENT DESIGN OF ARRAY ULTRASONIC TRANSDUCERS FOR HIGH FREQUENCY APPLICATIONS

open_access
Open Access
Author:
Kim, Jeong Nyeon
Graduate Program:
Engineering Science and Mechanics
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 10, 2019
Committee Members:
Judith Todd Copley, Dissertation Advisor/Co-Advisor
Judith Todd Copley, Committee Chair/Co-Chair
Richard L. Tutwiler, Committee Member
Bernhard R. Tittmann, Committee Member
Susan E Trolier-Mckinstry, Outside Member
Keywords:
Thin film PZT
PMUT
Array
Finite element modeling
PZFlex
Ultrasound transducer
Abstract:
Since Paul Langevin’s discovery of active sonar in 1917, ultrasound transducers have evolved in multiple forms that include single element, single element on a wedge, single element with cylindrical lens, single element with spherical lens, linear arrays, annular arrays, two- dimensional (2D) arrays, and phased arrays, among others. They have been applied in sound navigation and ranging (SONAR), structural health monitoring (SHM), nondestructive testing (NDT), nondestructive evaluation (NDE), medical/biomedical sensing/imaging, and biometric sensing/imaging. This dissertation focuses on the development of high frequency phased array transducers for two specific applications – scanning acoustic microscopy, and biometric imaging for small electronics. Closed-loop finite element studies were conducted in three dimensions using PZFlex, a commercial finite-element method software. A 5 MHz, thickness-mode, linear array for an acoustic microscope, and a flexible 10 MHz, bending-mode, piezoelectric, micromachined ultrasonic transducer (PMUT) 2D array, plus a flexible 38 MHz bending-mode, PMUT 2D array for finger-print and finger-vein imaging, were virtually prototyped and their respective performances were predicted. The scanning acoustic microscope (SAM) has been a well-recognized tool for both visualization and quantitative evaluation of materials at the microscale, since its invention in 1974. While there have been multiple advances in SAM over the past four decades, some issues still remain to be addressed. First, the measurement speed is limited by the mechanical movement of the acoustic lens. Second, a single element transducer acoustic lens only delivers a predetermined beam pattern for a fixed focal length and incident angle, thereby limiting control of the inspection beam. Here, a development of a phased array probe as an alternative is proposed to overcome these issues. Preliminary studies to design a practical, high frequency, phased array, acoustic microscope probe were explored. A linear phased array, comprising 32 elements and operating at 5 MHz, was modeled using PZFlex. This phased array system was characterized in terms of electrical input impedance response, pulse-echo and impulse response, surface displacement profiles, mode shapes, and beam profiles. PMUT using lead-zirconate-titanate, PbZr0.52Ti0.48O3 (PZT), thin films are currently being investigated for miniaturized, high frequency, ultrasound systems, and their microfabrication processes explored. For example, Liu et al. developed a process to remove the PZT from an underlying rigid Si substrate, creating the potential for developing curved arrays [138, 139]. This dissertation aims to improve the design of flexible PMUT arrays by developing 3D models using PZFlex. A 10 MHz 2D array PMUT device, working in 3-1 bending mode, was designed. A circular unit-cell was structured from the top, comprising a platinum (Pt) electrode, a PZT active layer, a bottom Pt electrode and a titanium (Ti) passive layer, all placed concentrically on a polyimide (PI) substrate. The active PZT layer had a diameter of 46 µm and a thickness of 1 µm. The passive Ti layer was 59.8 µm diameter and 1 µm in thickness. The PI substrate was 20 µm thick. Below the passive Ti layer, another 7 µm thick PI passive layer and 13 µm deep cavity with 46 µm diameter was added concentric to the PZT layer. The dimensions were selected to have a resonance frequency at 10 MHz under water load and air backing. The pulse-echo and spectral response analysis of the unit-cell predicted its bandwidth to be 87%. Mode shapes of the unit-cell were modeled to discover undesirable cross coupling to higher modes. A 2D array, consisting of 256 (16×16) unit-cells, was created and characterized in terms of pulse-echo response, spectral response, surface displacement profile, cross-talk, and beam profiles. Iterations to find a robust design of the flexible PMUT array with increased resonance frequency and low operating voltage were continued. A PMUT array has to be operated at very low voltage to be embedded and run in small electronic devices, such as smart-phones, and smart-watches. A 38 MHz, flexible, PMUT array operating at 3 Volt peak-to-peak (Vpp) driving voltage was designed. To achieve these goals, a unit-cell, consisting of four 3-1 bending mode diaphragms, were devised. The quad diaphragm unit-cell was structured with 40 µm × 40 µm × 500 nm PZT layer on top of 40 µm × 40 µm × 1 µm Ti elastic layer which had four (2×2)10 µm × 10 µm × 5 µm cavities beneath it. The cavities had 11 µm of interspacing to next cavities. Four pairs of 10 µm × 10 µm top and bottom Pt electrodes were placed concentrically with the cavities by sandwiching the PZT layer. The top and bottom Pt electrodes had thicknesses of 50 nm and 100 nm, respectively. A PI substrate was placed beneath the Ti layer, surrounding the cavities, with 8 µm thick, including the 5 µm deep cavities. The pulse-echo and spectral response analysis of the quad diaphragm unit-cell revealed its bandwidth to be 32.2 %. A 2D array was constructed with 16×16 unit-cells, consisting of 1024 (32×32) diaphragms. This array was evaluated in terms of pulse-echo response, spectral response, surface displacement profile, cross-talk, and beam profiles.

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