Miniature High Frequency Focused Ultrasonic Transducers for Minimally Invasive Imaging Procedures

Proceedings of SPIE, 2005

Capacitive micromachined ultrasonic transducer (cMUT) technology has been recognized as an attractive alternative to the more traditional piezoelectric transducer technology in medical ultrasound imaging for several years now. There are mainly two reasons for the interest in this technology: Micromachining is derived from the integrated circuit technology and therefore shares the well-known advantages and experience of it. Also, capacitive transduction using thin membranes has fundamental superiorities over the piezoelectric transduction mechanism such as wide frequency bandwidth. Capacitive micromachined ultrasonic transducers are essentially capacitor cells where the two plates of the capacitor, the membrane and the substrate, are separated with a vacuum sealed cavity. Typically, a cMUT is made of many microscale capacitor cells operating in parallel. This paper describes a new fabrication technique for building cMUTs which is called the wafer-bonding method. In this method, the cavity and the membrane are defined on separate wafers and brought together by wafer-bonding in vacuum. The wafer-bonding method has several advantages over the traditional sacrificial release method of cMUT fabrication. It allows greater flexibility in the cMUT design which means better device performance. It reduces the number of process steps, device turnaround time, and increases the overall uniformity, reliability. and repeatability. Device examples of one-dimensional and two-dimensional arrays designed to work in the 1 to 50 MHz range with 100 % fractional bandwidth highlight the advantages of this method, and show that cMUT technology is indeed the better candidate for next generation ultrasonic imaging arrays.

Therapeutic ultrasound guided by MRI is a noninvasive treatment that potentially reduces mortality, lowers medical costs, and widens accessibility of treatments for patients. Recent developments in the design and fabrication of capacitive micromachined ultrasonic transducers (CMUTs) have made them competitive with piezoelectric transducers for use in therapeutic ultrasound applications. In this paper, we present the first designs and prototypes of an eight-element, concentric-ring, CMUT array to treat upper abdominal cancers. This array was simulated and designed to focus 30-50 mm into tissue, and ablate a 2-to 3-cmdiameter tumor within 1 h. Assuming a surface acoustic output pressure of 1 MPa peak-to-peak (8.5 W/cm 2 ) at 2.5 MHz, we simulated an array that produced a focal intensity of 680 W/cm 2 when focusing to 35 mm. CMUT cells were then designed to meet these frequency and surface acoustic intensity specifications. These cell designs were fabricated as 2.5 mm × 2.5 mm test transducers and used to verify our models. The test transducers were shown to operate at 2.5 MHz with an output pressure of 1.4 MPa peak-to-peak (16.3 W/cm 2 ). With this CMUT cell design, we fabricated a full eight-element array. Due to yield issues, we only developed electronics to focus the four center elements of the array. The beam profile of the measured array deviated from the simulated one because of the crosstalk effects; the beamwidth matched within 10% and sidelobes increased by two times, which caused the measured gain to be 16.6 compared to 27.4.

Ultrasonics, 2002

We are investigating the use of capacitive micromachined ultrasonic transducers (cMUT's) for use in medical imaging. We propose an ultrasound probe architecture designed to provide volumetric ultrasound imaging from within an endoscope channel. A complete automated experimental system has been implemented for testing the imaging performance of cMUT arrays. This PCbased system includes custom-designed circuit boards, a software interface, and resolution test phantoms. We have already fabricated 1D and 2D cMUT arrays, and tested the pulse-echo imaging characteristics of 1D arrays. Beamforming and image formation algorithms that aim to reduce the complexity of data acquisition hardware are tested via numerical simulations and using real data acquired from our system. Ó

Sensors, 2010

Polyvinilidene fluoride (PVDF) single-element transducers for high-frequency (>30 MHz) ultrasound imaging applications have been developed using MEMS (Micro-electro-Mechanical Systems) compatible techniques. Performance of these transducers has been investigated by analyzing the sources and effects of on-chip parasitic capacitances on the insertion-loss of the transducers. Modeling and experimental studies showed that on-chip parasitic capacitances degraded the performance of the transducers and an improved method of fabrication was suggested and new devices were built. New devices developed with minimal parasitic effects were shown to improve the performance significantly. A 1-mm aperture PVDF device developed with minimal parasitic effects has resulted in a reduction of insertion loss of 21 dB compared with devices fabricated using a previous method.

Microelectronics Journal, 2006

Capacitive micromachined ultrasonic transducers (CMUTs) bring the fabrication technology of standard integrated circuits into the field of ultrasound medical imaging. This unique property, combined with the inherent advantages of CMUTs in terms of increased bandwidth and suitability for new imaging modalities and high frequency applications, have indicated these devices as new generation arrays for acoustic imaging. The advances in microfabrication have made possible to fabricate, in few years, silicon-based electrostatic transducers competing in performance with the piezoelectric transducers. This paper summarizes the fabrication, design, modeling, and characterization of 1D CMUT linear arrays for medical imaging, established in our laboratories during the past 3 years. Although the viability of our CMUT technology for applications in diagnostic echographic imaging is demonstrated, the whole process from silicon die to final probe is not fully mature yet for successful practical applications. q

2008

Ultrasound covers a broad range of applications from underwater exploration and nondestructive evaluation of materials to medical diagnosis and treatment. The ultrasonic transducer plays an important role in determining the resolution, sensitivity, as well as other critical diagnostic capabilities of an ultrasonic detection or imaging system. Currently piezoelectric ultrasonic transducers dominate the market. The device performance of the piezoelectric ultrasonic transducer in medical applications is limited by the material properties and related electrical and acoustic impedance match issues. The fabrication of piezoelectric transducer array requires meticulous handcrafting. It is difficult and expensive to fabricate densely populated piezoelectric array. The Capacitive Micromachined Ultrasonic Technology (CMUT) has emerged as a promising alternative. Compared to piezoelectric technology, the MEMS based CMUT has advantages such as ease of fabrication and the potential to integrate with front-end electronic circuits. CMUT could also provide broader acoustic bandwidth and higher sensitivity over its piezoelectric counterpart, which would improve the image resolution. The main goal of this dissertation work is to develop miniature CMUT devices for minimally invasive biomedical diagnosis and treatment. A two-layer poly-silicon surface ix micromachining process mixed with bulk micromachining process was developed. Based on this process, three prototypes of application were developed in this research:1) a multi-looking imager, 2) a miniaturized invasive ultrasonic probe, and 3) an image-Guided Therapy (IGT) system. Primary testing results including the acoustic/electrical characterization, ultrasonic imaging and flowmetering have been obtained and are discussed. These results indicate that CMUT technology has great potential to become the next-generation transducer technology for the Intravascular Ultrasonic system, invasive blood-flow metering, and therapeutic treatment. x TABLE OF CONTENTS LIST OF FIGURES .

2006

In recent years, medical procedures have become increasingly non-invasive. These include endoscopic procedures and intracardiac interventions (e.g., pulmonary vein isolation for treatment of atrial fibrillation and plaque ablation for treatment of arteriosclerosis). However, current tools suffer from poor visualization and difficult coordination of multiple therapeutic and imaging devices. Dual-mode (imaging and therapeutic) ultrasound arrays provide a solution to these challenges. A dual-mode transducer can provide focused, noncontact ultrasound suitable for therapy and can be used to provide high quality real-time images for navigation and monitoring of the procedure. In the last decade, capacitive micromachined ultrasonic transducers (CMUTs), have become an attractive option for ultrasonic imaging systems due to their fabrication flexibility, improved bandwidth, and integration with electronics. The CMUT's potential in therapeutic applications has also been demonstrated by surface output pressures as high as 1MPa peak to peak and continuous wave (CW) operation. This paper reviews existing interventional CMUT arrays, demonstrates the feasibility of CMUTs for high intensity focused ultrasound (HIFU), and presents a design for the next-generation CMUTs for integrated imaging and HIFU endoscopic catheters.

2006

We present the development of a capacitive micromachined ultrasonic transducer (CMUT) array for noninvasive focused ultrasound ablation of lower abdominal cancers under MR-guidance. While piezoelectric transducers have been traditionally used for high intensity focused ultrasound (HIFU), recent advances in capacitive micromachined ultrasonic transducers (CMUTs) have made them highly competitive with regard to costs, fabrication flexibility, and performance. Even current imaging CMUTs have shown capability of HIFU operation through high power and continuous wave operation. In this paper, we will show our experiments with current imaging CMUTs operated in HIFU mode. In addition, we will show the design and development of CMUT membranes and a transducer array specifically for HIFU ablation lower abdominal cancers.

2007

In the past ten years, high intensity focused ultrasound (HIFU) has become popular for minimally invasive and non-invasive therapies. Traditionally piezoelectric transducers have been used for HIFU applications, but capacitive micromachined ultrasonic transducers (CMUTs) have been shown to have advantages, including ease of fabrication and efficient performance. In this paper, we show the fabrication and testing of CMUTs specifically designed for HIFU. We compare the operation of these designs with finite element models. In addition, we demonstrate that CMUTs can operate under high pressure and continuous wave (CW) conditions, with minimal self-heating, a problem that piezoelectric transducers often face. Finally, we demonstrate MR-temperature monitoring of the heating created by an unfocused HIFU CMUT.

OCEANS Conference, 2002

Capacitive Micromachined Ultrasonic Transducers (CMUTs) were introduced about a decade ago as an alternate method of generating and detecting ultrasound. Since their introduction, considerable research has been done to characterize CMUTs. They have been shown to have broad frequency bandwidth and very good sensitivity. Besides, CMUTs are built on silicon using standard surface micromachining techniques, and therefore have all the

TECCIENCIA, 2013

This paper introduces the reader to a review and analysis of various ultrasound (us) applications in the medical field. First, the transducer is shown, along with a diagram of the basic electronics that it uses to generate and receive signals that allow the reconstruction of images, for medical purposes. Also, us practical uses in medical therapy is shown. This subject is addressed by combining the physical principles involved in the us application, with the implemented technology and the modes of operation for non-invasive studies of pathologies related to brain, heart, and eyes such as injuries, tumors and hematomas. The methodology used in this article also suggests an analysis method for the us scientific and technological research principles combined for their implementation in medical applications.

2005

We have designed, fabricated, and characterized two-dimensional 16x16-element capacitive micromachined ultrasonic transducer (CMUT) arrays. The CMUT array elements have a 250-µm pitch, and when tested in immersion, have a 5-MHz center frequency and 99% fractional bandwidth. The fabrication process is based on standard silicon micromachining techniques and therefore has the advantages of high yield, low cost, and ease of integration. The transducers have a Si 3 N 4 membrane and are fabricated on a 400-µm thick silicon substrate. A low parasitic capacitance through-wafer via connects each CMUT element to a flip-chip bond pad on the back side of the wafer. Each through-wafer via is 20 µ m in diameter and 400 µ m deep. The interconnects form metal-insulator-semiconductor (MIS) junctions with the surrounding high-resistivity silicon substrate to establish isolation and to reduce parasitic capacitance. Each through-wafer via has less than 0.06 pF of parasitic capacitance. We have investigated a Au-In flip-chip bonding process to connect the 2D CMUT array to a custom integrated circuit (IC) with transmit and receive electronics. To develop this process, we fabricated fanout structures on silicon, and flip-chip bonded these test dies to a flat surface coated with gold. The average series resistance per bump is about 3 Ohms, and 100% yield is obtained for a total of 30 bumps.

Piezoelectric and Acoustic Materials for Transducer Applications, 2008

In this chapter the basic principles, the fabrication process, and some modelling approaches of the novel micromachined ultrasonic transducers (MUTs) are described. These transducers utilize the flex-tensional vibration of an array of micro membranes. They are usually called cMUT (capacitive Micromachined Ultrasonic Transducer) or pMUT (piezoelectric Micromachined Ultrasonic Transducer) depending on the actuation principle, electrostatic or piezoelectric. For water coupling applications both these kinds of transducers offer a better matching to the load compared with the typical piezoelectric transducers and therefore they have a larger intrinsic bandwidth. Here emphasis is given to the cMUTs because they have shown good electroacoustic characteristics, which parallel, or even exceed, those of conventional piezoelectric transducers. Good echographic images of internal organs of the human body have been obtained demonstrating the possibilities of this technology to be utilized in commercial 1D and 2D probes for medical applications. At present pMUTs are in a very early stage of development and the potential advantages over the cMUTs are still to be demonstrated.

IEEE Sensors, 2005.

Capacitive micromachined ultrasonic transducers (CMUTs) overcome many limitations of existing ultrasound transducer technologies enabling new applications of ultrasound, especially for medical imaging and treatment. Some of the most important of these advancements are the ability to fabricate transducer arrays with two dimensional geometries and high operating frequencies. Over the past decade, extensive research has been conducted on the fabrication, characterization, and modelling of CMUTs. Current research efforts focus on the integration of CMUTs in systems for new medical imaging tools. This paper briefly reviews CMUT technology and presents imaging results from two CMUT-based imaging systems. The first system is designed for use within a 5-mm endoscopic channel and is based on a two dimensional, 16-element × 16-element, 5-MHz CMUT array. To provide a means of integrating the CMUT array with electronics, each element of the array is connected to a flip-chip bond pad on the back side of the array via a through-wafer interconnect. The array is flip-chip bonded to a custom-designed integrated circuit (IC) that comprises the frontend circuitry for the transducer elements. The array and IC are connected to an FPGA-based data acquisition system that can acquire volumetric imaging data in real time. Volumetric pulseecho and photoacoustic images obtained with this system are presented. The second system is based on a 64-element, 20-MHz, 2-mm diameter CMUT ring array. This array is also designed for use in catheter-based imaging applications. Ring arrays have the advantage of providing space in the center for a guidewire or other catheter-based instrument. Volumetric images obtained with the ring-array system are also presented.

IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control, 2002

In this paper we describe two new types of transducer assemblies fabricated from polyimide films with photolithography that use a polyimide micromachined (MEMS) actuator to mechanically scan an ultrasound beam. Forward viewing transducers pivoted on cantilever hinges and side scanning transducers tilted on torsion hinges were fabricated on polyimide substrates with tables 1.125 mm and 2.25 mm wide. PZT transducers fabricated on these tables operating at 20 MHz and 30 MHz yielded insertion losses of 20-26 dB and fractional bandwidths of 34-49%. The transducer assemblies driven by MEMS actuators produced sector scans of 45-60/spl deg/ in air at resonant frequencies of 32 to 90 Hz and sector scans in fluid of 6-8/spl deg/. Real time images of wire phantoms were obtained using a single channel imaging system based on a personal computer platform with LabVIEW software.