Integrated Electronics for Wireless Imaging Microsystems with CMUT Arrays

Integration of transducer arrays with interface electronics in the form of single-chip CMUT-on-CMOS has emerged into the field of medical ultrasound imaging and is transforming this field. It has already been used in several commercial products such as handheld full-body imagers and it is being implemented by commercial and academic groups for Intravascular Ultrasound and Intracardiac Echocardiography. However, large attenuation of ultrasonic waves transmitted through the skull has prevented ultrasound imaging of the brain. This research is a prime step toward implantable wireless microsystems that use ultrasound to image the brain by bypassing the skull. These microsystems offer autonomous scanning (beam steering and focusing) of the brain and transferring data out of the brain for further processing and image reconstruction. The objective of the presented research is to develop building blocks of an integrated electronics architecture for CMUT based wireless ultrasound imaging systems while providing a fundamental study on interfacing CMUT arrays with their associated integrated electronics in terms of electrical power transfer and acoustic reflection which would potentially lead to more efficient and high-performance systems. A fully wireless architecture for ultrasound imaging is demonstrated for the first time. An on-chip programmable transmit (TX) beamformer enables phased array focusing and steering of ultrasound waves in the transmit mode while its on-chip bandpass noise shaping digitizer followed by an ultra-wideband (UWB) uplink transmitter minimizes the effect of path loss on the transmitted image data out of the brain. A single-chip application-specific integrated circuit (ASIC) is de- signed to realize the wireless architecture and interface with array elements, each of which includes a transceiver (TRX) front-end with a high-voltage (HV) pulser, a high-voltage T/R switch, and a low-noise amplifier (LNA). Novel design techniques are implemented in the system to enhance the performance of its building blocks. Apart from imaging capability, the implantable wireless microsystems can include a pressure sensing readout to measure intracranial pressure. To do so, a power-efficient readout for pressure sensing is presented. It uses pseudo-pseudo differential readout topology to cut down the static power consumption of the sensor for further power savings in wireless microsystems. In addition, the effect of matching and electrical termination on CMUT array elements is explored leading to new interface structures to improve bandwidth and sensitivity of CMUT arrays in different operation regions. Comprehensive analysis, modeling, and simulation methodologies are presented for further investigation.Ph.D

Capacitive Micromachined Ultrasound Transducers for Non-Destructive Testing Applications

The need for using ultrasound non-destructive testing (NDT) to characterize, test and detect flaws within metals, led us to utilize Capacitive Micromachined Ultrasound Transducers (CMUTs) in the ultrasound NDT field. This is due to CMUT's large bandwidths and high receive sensitivity, to be a suitable substitute for piezoelectric (PZT) transducers in NDT applications. The basic operational test of CMUTs, conducted in this research, was carried out based on a pulse-echo technique by propagating acoustic pulses into an object and analyzing the reflected signals. Thus, characterizing the tested material, measuring its dimension, and detecting flaws within it can be achieved. Throughout the course of this research, the fundamental parameters of CMUT including pull-in voltage and resonance frequency were initially calculated analytically and using Finite Element Analysis (FEA). Afterward, the CMUT was fabricated out of two mechanically bonded wafers. The device's movable membrane (top electrode) and stationary electrode (bottom electrode) were made out of Boron-doped Silicon. The two electrodes were electrically isolated by an insulation layer containing a sealed gap. The CMUT was then tested and characterized to analyze its performance for NDT applications. In-immersion characterization revealed that the 2.22 MHz CMUT obtained a -6 dB fractional bandwidth of 189%, and a receive sensitivity of 31.15 mV/kPa, compared to 45% and 4.83 mV/kPa of the PZT probe. A pulse-echo test, performed to examine an aluminum block with and without flaws, showed success in distinguishing the surfaces and the flaws of the tested sample

Capacitive micromachined ultrasound transducer (CMUT) design and fabrication for intracardiac echocardiography

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 7MHz center frequency with ~80% fractional bandwidth in both 1-D and 2-D configurations. The 1-D 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 40dB dynamic range from 7 metal wires has been obtained. Several 2-D (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 60V pulse is optimized during transmit operation and 2MPa 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 -25dB. The overall modeling approach is also qualitatively validated by experiments.Ph.D

A BCB Diaphragm Based Adhesive Wafer Bonded CMUT Probe for Biomedical Application

This dissertation presents the design methodology, fabrication procedure, and key experimental characterization results of a linear array of capacitive micromachined ultrasonic transducers (CMUT) for possible ophthalmic anterior segment imaging application. The design methodology involves analytical, 3-D electromechanical finite element analysis, and Verasonics Vantage 128 ultrasonic research platform based diagnostic imaging simulations to develop a technique that minimizes electrical charging and center frequency drift while improving the transduction efficiency. In the design, Bisbenzocyclobutene (BCB), a low K polymer from Dow Chemical Company, has been innovatively used for the first time to fabricate the structural layer of the CMUT diaphragm, realize the interelectrode dielectric spacer, and to act as a low temperature adhesive bonding agent. Additionally, the top CMUT electrode has been placed at the bottom of the diaphragm to affect higher capacitance change that increases sensitivity and provides additional decoupling of the electrical charging effects. Several arrays with element count ranging from 8 to 128 elements and a center frequency range of 5 MHz to 40 MHz have been designed and fabricated. Due to an unforeseen adhesion issue during wirebonding, a 32 channel 40 MHz CMUT array has been packaged manually to validate the fabrication process and CMUT operation. Extensive SEM inspections of the CMUT cross-sections show good agreement with the design specifications. Static and dynamic measurements using a Polytec laser Doppler vibrometer, impedance measurement using an Agilent vector network analyzer, and LCR measurement results are in excellent agreement with analytical and FEA analysis using IntelliSuite. The frequency analysis exhibits high electromechanical coupling coefficient of 0.66 at a low bias voltage of 20 V and high uniformity. A successful measurement of the lower drift of the center frequency 0.32% and higher coupling coefficient verifies the hypothesis that the excellent electrical, structural, and processing characteristics of BCB is a viable option to mitigate the dielectric charging and improve the transduction efficiency of CMUTs

Fem Modelling and Characterization of Ultrasonic Flextensional Transducers

2017 - 2018This work describes the finite element modelling and characterization of ultrasonic flextensional transducer arrays. Flexural acoustic transducers can be piezoelectrically actuated plates or capacitive devices based on the electrostatic attraction between a moving electrode and a substrate. Due to the limited miniaturization allowed by the piezoceramic fabrication process, piezoelectric flexural devices based on bulk ceramics are able to work in the low-frequency ultrasonic range. Capacitive flexural devices, instead, can take advantage of the Silicon micromachining techniques to be fabricated to reach higher frequencies. Capacitive Micromachined Ultrasonic Transducers (CMUTs) are MEMS devices consisting of miniaturized metallized membranes, forced into flexural vibration by an electric signal during transmission, and vice versa generating a voltage signal when actuated by an incident acoustic signal. Due to their low acoustic impedance, CMUT arrays have given excellent results in ultrasound imaging applications. The most recent frontier of ultrasound imaging is real-time volumetric imaging. 3D images have been originally obtained by means of linear phased arrays mechanically titled along the elevation plane. More complex structures like 2D arrays allow electronic beam steering and dynamic focusing in both azimuthal and elevation planes, thus achieving better performance. In order to increase the achievable frame rate, though, part of the front-end transceive and beamforming operations must be performed in probe. Therefore, 2D arrays should be small-sized and easily interfaced with the front end. Nevertheless, 2D arrays with good radiation characteristics require wide apertures with a small pitch between elements, therefore a great number of elements and many channels to wire and control individually. To overcome these issues, much attention is being focused on the design of sparse arrays, which try to achieve comparable performance by counting a lower element number. ... [edited by Author]XVII n.s. (XXXI ciclo

Integrated Circuits for Medical Ultrasound Applications: Imaging and Beyond

Medical ultrasound has become a crucial part of modern society and continues to play a vital role in the diagnosis and treatment of illnesses. Over the past decades, the develop- ment of medical ultrasound has seen extraordinary progress as a result of the tremendous research advances in microelectronics, transducer technology and signal processing algorithms. How- ever, medical ultrasound still faces many challenges including power-efficient driving of transducers, low-noise recording of ultrasound echoes, effective beamforming in a non-linear, high- attenuation medium (human tissues) and reduced overall form factor. This paper provides a comprehensive review of the design of integrated circuits for medical ultrasound applications. The most important and ubiquitous modules in a medical ultrasound system are addressed, i) transducer driving circuit, ii) low- noise amplifier, iii) beamforming circuit and iv) analog-digital converter. Within each ultrasound module, some representative research highlights are described followed by a comparison of the state-of-the-art. This paper concludes with a discussion and recommendations for future research directions

An improved lumped element nonlinear circuit model for a circular CMUT cell

Cataloged from PDF version of article.This paper describes a correction and an extension in the previously published large signal equivalent circuit model for a circular capacitive micromachined ultrasonic transducer (CMUT) cell. The force model is rederived so that the energy and power is preserved in the equivalent circuit model. The model is able to predict the entire behavior of CMUT until the membrane touches the substrate. Many intrinsic properties of the CMUT cell, such as the collapse condition, collapse voltage, the voltage-displacement interrelation and the force equilibrium before and after collapse voltage in the presence of external static force, are obtained as a direct consequence of the model. The small signal equivalent circuit for any bias condition is obtained from the large signal model. The model can be implemented in circuit simulation tools and model predictions are in excellent agreement with finite element method simulations. © 2012 IEEE

CMUT Crosstalk Reduction Using Crosslinked Silica Aerogel

Inter-element acoustic crosstalk is one of the major concerns which restricts the potential deployment of Capacitive Micromachined Ultrasonic Transducers (CMUTs) in Nondestructive Evaluation (NDE) despite its superior transduction capabilities. This thesis investigates the causes of acoustic crosstalk in CMUTs and develops a novel method of CMUT crosstalk reduction by passivating the CMUT top surface by a thin layer of Di-isocyanate enhanced crosslinked silica aerogel. This powerful technique derives its inspiration from the Scholte waves attenuation techniques as used in boreholes at the permeable formations. Analytical and 3D finite element analysis in MATLAB and COMSOL Multiphysics, respectively, show that the developed technique can minimize the crosstalk due to Scholte waves at the fluid-solid interfaces by at least 5 dB more at the nearest neighbor as compared to other published techniques. An added advantage of the developed technique is that the level of Scholte wave attenuation can be controlled by altering the porosity of the aerogel layer. A simple and cost-effective fabrication process employing sol-gel and ambient pressure drying processes for the aerogel layer deposition has been developed that doesn’t interfere with the basic CMUT operation or fabrication techniques