SUBJECT: Ph.D. Dissertation Defense
BY: Toby Xu
TIME: Monday, January 12, 2015, 3:00 p.m.
PLACE: Love Building, 109
TITLE: Material And Array Design For CMUT Based Volumetric Intravascular And Intracardic Ultrasound Imaging
COMMITTEE: Dr. F. Levent Degertekin, Chair (ME)
Dr. Oliver Brand (ECE)
Dr. Peter J. Hesketh (ME)
Dr. Karim Sabra (ME)
Dr. Todd Sulchek (ME)


This research aims to improve capacitive micromachined ultrasonic transducer (CMUT) based imaging catheters for intravascular ultrasound (IVUS) and intra-cardiac echocardiography (ICE) for 3-D volumetric imaging through integration of high-k thin film material into the CMUT fabrication and array design. CMUT-on-CMOS integration has been recently achieved and initial imaging of ex-vivo samples with adequate dynamic range for IVUS at 20MHz has been demonstrated; however, for imaging in the heart, higher sensitivities are needed for imaging up to 4-5 cm depth at 20MHz and deeper at 10MHz. Consequently, one research goal is to design 10-20MHz CMUT arrays using integrated circuit (IC) compatible micro fabrication techniques and optimizing transducer performance through high-k dielectrics such as hafnium oxide (HfO2). This thin film material is electrically characterized for its dielectric properties and thermal mechanical stress is measured. Experiments on test CMUTs show a +6dB improvement in receive (Rx) sensitivity, and +6dB improvement in transmit sensitivity in (Pa/V) as compared to a CMUT using silicon nitride isolation (SixNy) layer. CMUT-on-CMOS with HfO2 insulation is successfully integrated and images of a pig artery were successfully obtained with a 40dB dynamic range for 1x1cm2 planes. For array design, a novel dual gap, dual frequency 2D array was designed, fabricated and verified against a large signal model for CMUTs. Three different CMUT element geometries (2Rx 1Tx) were designed to cover a large bandwidth from 20 to 40MHz. A system level framework for designing CMUT arrays was described including effects from imaging design requirements, acoustical cross-talk, bandwidth, signal-to-noise (SNR) optimization and considerations from IC limitations for pulse voltage. Electrical impedance measurements and hydrophone measurements comparisons between design and experiment show differences due to variations in fabrication parameters and model assumptions.