Title: Capacitive Micromachined Ultrasound Transducer (CMUT) Design and Fabrication for Intracardiac Echocardiography
Dr. Levent Degertekin, ECE, Chair , Advisor
Dr. Fatih Sarioglu, ECE
Dr. Muhannad Bakir, ECE
Dr. Omer Inan, ECE
Dr. Peter Hesketh, ME
The objective of this research is to develop capacitive micromachined ultrasonic transducer (CMUT) arrays with novel geometry for intracardiac echocardiography (ICE) imaging along with a novel reliable CMUT fabrication process to improve the system performance. We used custom CMOS electronics and monolithically integrated our CMUT arrays to CMOS chips. The arrays are designed for 9-Fr (<3mm) ICE catheters over a total area of about 2.6x11-mm2 at around 7-MHz center frequency with ~80% fractional bandwidth in both 1D and 2D configurations. The 1D array transducer includes 64 channels with beam-steering capabilities for cross sectional ICE imaging application at distance range of about 5-cm. The ICE image with 40-dB dynamic range from 7 metal wires has been obtained. Several 2D (sparse) arrays are designed based on signal-to-noise ratio (SNR) optimization capable of generating volumetric images. The CMUT-on-CMOS technique is used for arrays integration with our ASICs using vias for top and bottom electrode connections to the related electronics pads. A 60-V pulse is optimized during transmit operation and 2-MPa surface pressure has been achieved that is in agreement with our simulation results. We also developed an improved CMOS compatible low temperature sacrificial layer fabrication process for CMUTs. The process adds the fabrication step of silicon oxide evaporation which is followed by a lift-off step to define the membrane support area without a need for an extra mask. The parasitic capacitance is reduced about 15% and device long-term test demonstrates 72-hours stable output pressure showing no significant degradation on performance. We have also developed a new energy-based calculation method for CMUT performance evaluation that is valid during both small and large signal operation since well-known frequency and capacitance based coupling coefficients definitions are not valid for large signal and nonlinear operation regimes. The quantitative modeling results show that CMUTs do not need DC bias to achieve high efficiency large signal transduction: AC only signals at half the operation frequency with amplitudes beyond the collapse voltage can provide energy conversion ratio (ECR) above 0.9 with harmonic content below -25-dB. The overall modeling approach is also qualitatively validated by experiments.