Riccardo Carotenuto

Capacitive micromachined ultrasonic transducer (CMUT) arrays for medical imaging

Abstract 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.

Design, fabrication and characterization of a capacitive micromachined ultrasonic probe for medical imaging

In this paper we report the design, fabrication process, and characterization of a 64-elements capacitive micromachined ultrasonic transducer (cMUT), 3 MHz center frequency, 100% fractional bandwidth. Using this transducer, we developed a linear probe for application in medical echographic imaging. The probe was fully characterized and tested with a commercial echographic scanner to obtain first images from phantoms and in vivo human body. The results, which quickly follow similar results obtained by other researchers, clearly show the great potentiality of this new emerging technology. The cMUT probe works better than the standard piezoelectric probe as far as the axial resolution is concerned, but it suffers from low sensitivity. At present this can be a limit, especially for in depth operation. But we are strongly confident that significant improvements can be obtained in the very near future to overcome this limitation, with a better transducer design, the use of an acoustic lens, and using well matched, front-end electronics between the transducer and the echographic system.

A new low voltage piezoelectric micromotor based on stator precessional motion

In this work a piezoelectric motor is described whose stator is composed of a cylindrical steel axle fitted at the center of a thin piezoelectric membrane. The rotor consists of a cylindrical permanent magnet, pressed in contact with the top surface of the axle, by means of magnetic forces. A travelling wave, at the natural flexural vibration of the thin piezoelectric membrane, is excited via the piezoelectric effect. The vertical displacement of the membrane is geometrically amplified by the central axle, obtaining a wide precessional motion of the axle. On this motion is based the transmission mechanism of the proposed motor. The motor is able to give a relatively high speed (/spl ap/3500 rpm) and torque (1.8/spl middot/10/sup -5/ N/spl middot/m) by using a commercial piezoelectric membrane (diameter 32 mm, thickness 0.2 mm), driven at relatively low voltage (18 V P-P). The very small thickness of the stator makes this motor suitable for microsystem applications. A simple analytical model of the transmission mechanism is discussed, and the predicted results are compared with experimental measurements, with a satisfactory agreement.

Micromachined ultrasonic transducers using silicon nitride membrane fabricated in PECVD technology

Capacitive ultrasonic transducers, consisting of thin membranes scratched over a conducting backplate, offer many advantages compared to piezoelectric transducers, such as low impedance mismatch, low energy density and low cost. Recent developments in microfabrication technology have spurred novel design for transducers in the ultrasonic range both for air and water applications. In this paper we report the fabrication process of transducers using PECVD deposition technology. With this process it is possible to change the stress from compressive to tensile, varying the temperature and time parameters. The resulting film is of very good quality and experiences irreversible modification after annealing process which prevents changes at lower temperature. Using this technology we succeeded in fabricating transducers with 3.8 MHz resonant frequency in air.

Electromechanical coupling factor of capacitive micromachined ultrasonic transducers

Recently, a linear, analytical distributed model for capacitive micromachined ultrasonic transducers (CMUTs) was presented, and an electromechanical equivalent circuit based on the theory reported was used to describe the behavior of the transducer [IEEE Trans. Ultrason. Ferroelectr. Freq. Control 49, 159-168 (2002)]. The distributed model is applied here to calculate the dynamic coupling factor k(w) of a lossless CMUT, based on a definition that involves the energies stored in a dynamic vibration cycle, and the results are compared with those obtained with a lumped model. A strong discrepancy is found between the two models as the bias voltage increases. The lumped model predicts an increasing dynamic k factor up to unity, whereas the distributed model predicts a more realistic saturation of this parameter to values substantially lower. It is demonstrated that the maximum value of k(w), corresponding to an operating point close to the diaphragm collapse, is 0.4 for a CMUT single cell with a circular membrane diaphragm and no parasitic capacitance (0.36 for a cell with a circular plate diaphragm). This means that the dynamic coupling factor of a CMUT is comparable to that of a piezoceramic plate oscillating in the thickness mode. Parasitic capacitance decreases the value of k(w), because it does not contribute to the energy conversion. The effective coupling factor k(eff) is also investigated, showing that this parameter coincides with k(w) within the lumped model approximation, but a quite different result is obtained if a computation is made with the more accurate distributed model. As a consequence, k(eff), which can be measured from the transducer electrical impedance, does not give a reliable value of the actual dynamic coupling factor.

An approximated 3-D model of the Langevin transducer and its experimental validation

In this work, an approximated 3-D analytical model of the Langevin transducer is proposed. The model, improving the classical 1-D approach describing the thickness extensional mode, allows us to predict also the radial modes of both the piezoelectric ceramic disk and the loading masses; furthermore, it is able to describe the coupling between radial and thickness extensional modes. In order to validate the model, the computed frequency spectrum is compared with that obtained by measurements carried out on 13 manufactured samples of different thicknesses to diameter ratios. The comparison shows that the model predicts with quite good accuracy the resonance frequencies of the two lowest frequency modes, i.e., those of practical interest, all over the explored range. Finally, the coupling effect between thickness and radial modes on the frontal displacement is measured and discussed.

Vibration maps of capacitive micromachined ultrasonic transducers by laser interferometry

A 1.8-mm /spl times/ 1.8-mm capacitive micromachined ultrasonic transducer (CMUT) element is experimentally characterized by means of optical measurements. Optical displacement measurements provide information on the resonant behavior of the single membranes and also allow us to investigate the dispersion in the frequency spectrum of adjacent membranes. In addition, higher order mode shapes are observed, showing that either symmetrical or asymmetrical modes are excited in CMUT membranes. Laser interferometry vibration maps, combined with quantitative displacement measurements, provide information about the quality and repeatability of the fabrication process, which is a basic requirement for 2D array fabrication for ultrasound imaging.

Design and fabrication of a cMUT probe for ultrasound imaging of fingerprints

Optical fingerprint scanners suffer from limited depth of penetration and are particularly sensitive to the surface conditions of the skin. Fingerprint scanners based on ultrasounds offer the possibility to explore the surface and the underlying tissues of the finger and to detect blood flow, leading to enhanced robustness and reliability in biometric applications. Capacitive Micromachined Ultrasonic Transducers (cMUTs) have shown to have great potential for use in medical imaging applications. The ease of fabricating broadband high-frequency ultrasound transducers makes the cMUT technology a good candidate for ultrasound based biometrics. This paper presents the design, fabrication and characterization of a cMUT linear array probe optimized for near-field ultrasound imaging. Ultrasound images of fingerprints are obtained using a customized 3D ultrasound scanning system.

cMUT echographic probes: design and fabrication process

The electrostatic capacitive, silicon micro fabricated, ultrasonic transducer (cMUT), approached in the last years, is a new promising alternative to the piezoelectric transducer for echographic probes. The cMUT transducer inherently has a larger bandwidth for immersion application and, because it takes advantage of the well established microelectronic technology it is, potentially, less expensive and gives much more flexibility in the design of complex 1D and 2D arrays than piezoelectric transducers. In perspective, a further advantage of the cMUT is the possibility to be integrated with the front-end electronics on the same silicon wafer. In this paper the design and the fabrication process of a 64-elements cMUT probe is described. We are fabricating an array with a pitch of 0.245 mm, kerf 27 Am, elevation 14 mm, glued on a commercial backing and soldered using a typical connection-comb to permit the electrical connection to the printing circuits, as used in commercial probes. With the addition of the biasing voltage, the probe is ready to be connected to a commercial echographic system, like Technos/spl reg/ (ESAOTE). Due to the inherently large bandwidth, the probe can be used as a linear array at about 7 MHz, and as a phased array at about MHZ..

The effects of membrane metallization in capacitive microfabricated ultrasonic transducers.

The mechanical effects of the metal layer on the membranes of capacitive micromachined ultrasonic transducers (CMUTs) are analyzed in this paper by means of finite element simulations. The influence of electrode size and thickness on the electrostatic behavior of the single CMUT cell, including diaphragm displacement, cell capacitance, and collapse voltage, is explored. The effect on device sensitivity is investigated through the transformation factor of the cell, that is computed by FEM and compared with the parallel plate model prediction. It is found that for a non-negligible electrode thickness, as in the majority of fabricated devices, both the static and dynamic performance of the cell can be affected in a significant way. Thus, the effects of membrane metallization must be taken into account in CMUT design and optimization.