Miniature high frequency focused ultrasonic transducers for minimally invasive imaging procedures

… Systems, 2002. The …, 2002

Spherically focused, high frequency (30-50 MHz) ultrasonic imaging transducers are fabricated by using a piezoelectric polymer film and membrane deflection technique that is compatible with CMOS circuit fabrication. Micromachined ultrasonic transducers with 0.5-2.0 mm-diameter apertures and f-numbers ranging from 1.3-4.0 are fabricated and characterized. The transducers exhibit focused radiation patterns with 50 pm axial resolution and bandwidths of 80-100% around center frequency values. Tissue imaging capabilities of the micromachined ultrasonic transducers are demonstrated through successful imaging of human cadaveric aorta.

IEEE/ASME Journal of Microelectromechanical Systems, 2008

A capacitive micromachined ultrasonic transducer (CMUT) array for minimally invasive medical diagnosis has been developed. Unlike traditional ultrasonic transducers, which generally use a bulky piece of substrate, this transducer array was integrated on a 40-µm-thick micromachined silicon substrate into a probe shape with a typical shank width of 50-80 µm and a shank length of 4-8 mm. For 1-D arrays, 24-96 CMUT devices were integrated on one such silicon probe and formed an accurately configured phase array. In addition to miniaturization, reduction of the substrate thickness also decreases the intertransducer crosstalk due to substrate Lamb waves. Due to its miniature size, this array can be placed or implanted close to the target tissue/organ and can perform high-resolution high-precision diagnosis and stimulation using high-frequency ultrasounds. The issue of conflict between resolution and penetration depth of ultrasonic diagnosis can therefore be resolved. A two-layer polysilicon surface micromachining process was used to fabricate this device. Suspended polysilicon membranes of diameters ranging from 20 to 90 µms and thicknesses from 1.0 to 2.5 µm were used to generate and detect ultrasounds of frequencies ranging from 1 to 10 MHz. B-mode imaging using this transducer array has been demonstrated.

2011

Capacitive micromachined ultrasonic transducers (CMUTs) have been subject to extensive research for the last two decades. Although they were initially developed for air-coupled applications, today their main application space is medical imaging and therapy. This paper first presents a brief description of CMUTs, their basic structure, and operating principles. Our progression of developing several generations of fabrication processes is discussed with an emphasis on the advantages and disadvantages of each process. Monolithic and hybrid approaches for integrating CMUTs with supporting integrated circuits are surveyed. Several prototype transducer arrays with integrated frontend electronic circuits we developed and their use for 2-D and 3-D, anatomical and functional imaging, and ablative therapies are described. The presented results prove the CMUT as a MEMS technology for many medical diagnostic and therapeutic applications.

2002

Capacitive micromachined ultrasonic transducer (cMUT) arrays provide broad bandwidth, high sensitivity, low mechanical impedance, and potential for electronic integration, and thus are promising for medical imaging applications. We have designed and fabricated 1D and 2D cMUT arrays of various sizes using standard integrated circuit fabrication processes. We improved the device parameters for medical imaging applications to achieve fully functional 64-and 128element linear 1D cMUT arrays. We have also built a computer controlled experimental setup for collecting pulse-echo data from the test phantoms using cMUT arrays. In this paper the design and optimization of the immersion cMUTs for medical imaging system are discussed, and the phased array B-scan sector images taken by 1D cMUT arrays are presented.

2014 IEEE 27th International Conference on Micro Electro Mechanical Systems (MEMS), 2014

This paper presents a 1.2 mm diameter high fill-factor array of 1,261 piezoelectric micromachined ultrasonic transducers (PMUTs) operating at 18.6 MHz for intravascular ultrasound (IVUS) imaging and other medical imaging applications. At 1061 transducers/mm 2 , the PMUT array has a 10-20× higher density than the best PMUT arrays realized to date. The PMUTs utilize a piezoelectric material, AlN, which is compatible with CMOS processes. Measurements show a large voltage response of 2.5 nm/V and good frequency matching in air, a high center frequency of 18.6 MHz and wide bandwidth of 4.9 MHz when immersed in fluid. Phased array simulations based on measured PMUT parameters show a tightly focused, high output pressure acoustic beam.

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