C. Longo

Capacitive micromachined ultrasonic transducer (cMUT) made by a novel "reverse fabrication process"

We report a novel fabrication process of a cMUT array based on the electrostatic effect and realized by silicon micromachining technique. Several fabrication technologies for 1D and 2D cMUT have been presented in the last ten years, differing from each other in the materials used and the process steps involved. They all have in common the presence of micro-holes on the surface of the transducer necessary to evacuate the cavities under the membranes or, in the 2D array, to electrically connect upper to lower pads and to allow the electrical connection to external circuits. The authors of the present work designed and realized a cMUT transducer using a new concept. In the standard process, successive layers are deposited on the silicon wafer up to the silicon nitride structural layer of the micro membranes; our different approach consists in inverting the function of each layer and to build the cMUT capacitive cell starting from the membrane, made of LPCVD silicon nitride coating the silicon wafer, up to the bottom electrode and the backplate. By working on the back of the device, there is no need to use holes in the structural silicon nitride layer to evacuate the cavities and the pads for the electrical connections are on the bottom surface of the device.

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.

Enhanced echographic images obtained improving the membrane structural layer of the cMUT probe

The validity of cMUT technology for medical imag- ing applications has been proved by many authors. The large bandwidth obtainable with capacitive Micromachined Ultrasonic Transducers (cMUT) leads to improved image resolution if compared to their piezoelectric counterpart. However cMUTs actually show lower sensitivity than piezoelectric transducers, mainly due to their non optimized design and fabrication technol- ogy. Recently, we developed in our laboratories a dual-frequency plasma-enhanced chemical-vapour-deposition (DF-PECVD) pro- cess, improving the electrical and mechanical characteristics of the silicon nitride films that we employed to fabricate cMUTs. The DF-PECVD technique makes it possible to control, with extreme precision and over a wide range of values, the stress of the grown silicon nitride film. The result is an increased cohesion and an increased resistivity of the silicon nitride film. As a consequence, the porosity of the obtained DF-PECVD nitride film is extremely reduced thus contributing to effectively sealing the underlying cavities of the cMUT membranes. The improvement of both mechanical and electrical properties of the cMUT structural layer has led to higher transmission and reception sensitivity. Using this silicon nitride deposition technique, we have fabricated 64-elements, 5 MHz echographic probes featuring 100% bandwidth and higher sensitivity characteristics leading to a remarkable improvement in the quality of the echographic images.

Curvilinear capacitive micromachined ultrasonic transducer (CMUT) array fabricated using a reverse process

This paper presents the manufacturing of a flexible 192-elements curvilinear CMUT array developed using our patented reverse fabrication process. In this process the silicon nitride CMUT membranes are surface micromachined over a silicon substrate such that the non-radiating surface of the transducer corresponds to the last layer deposited, while the radiating membranes are uncovered by completely etching the silicon substrate. Before the removal of the silicon substrate a flexible backing material is poured and cured on the non-radiating surface, and after the removal another layer of flexible suited material is applied to cover the membranes. At the end of the process the large 24-mm by 6-mm CMUT die with a total thickness of just 6.5 mum, embedded in the flexible backing and coating layers, was bent to a radius of curvature of less than 10 mm in an azimuth direction, showing great flexibility. The functionality of the device wire-bonded to a flex printed circuit and mounted on a curved backing with a radius of curvature of 25 mm was demonstrated by means of electrical impedance tests and pulse-echo measurements in water. The tested elements showed a pulse-echo center frequency of about 11.0 MHz with a fractional bandwidth of at least 100%, with no apparent performance degradation resulting from curving the array.

P2B-4 Crisscross 2D cMUT Array: Beamforming Strategy and Synthetic 3D Imaging Results

Real-time 3D ultrasound imaging is based on volumetric beam steering and sweeping. Transducer capable to perform volumetric beamforming for medical imaging are typically two dimensional arrays of thousands of active elements. Electronic multiplexing and pre-beamforming is necessary in order to interface such arrays to conventional ultrasound scanners. This paper describes an alternative approach for volumetric beamforming based on a particular two dimensional array of a reduced number of elements obtained by superimposing two linear arrays on the same area. Capacitive Micromachined Ultrasonic Transducer (cMUT) technology is mentioned as a good candidate to approach the problem due to the possibility given by micromachining to fabricate arbitrarily shaped electrode patterns which define the array elements. The beamforming strategy is illustrated and assessed by means of beampattern calculations and synthetic 3D imaging formation. The resulting array is directly connectable to a standard imaging system.

cMUT sensor for applications as a wide-band acoustic receiver in the MHz range

For meticulous calibration of ultrasonic probes, hydrophones are usually used but it is difficult to find hydrophones that can operate up to 30 MHz with a very large bandwidth. Our group has a long experience in fabrication of the new cMUT transducers; these devices can easily operate in the high frequency range due to the silicon micromachining technology used. In order to calibrate large bandwidth high frequency cMUT array probes, we designed and fabricated a single element transducer on the same wafer used for the cMUT array. Since the sensor is fabricated on the same wafer, it has the same characteristics of the probe and, hence, the measurements are possible in the same operative range (3–20 MHz). Our initial results for cMUT single-element sensor are promising.

5F-2 Analysis of Acoustic Interaction Effects and Crosstalk in CMUT Linear Arrays for Medical Imaging

Acoustic interactions occur between the membranes of capacitive micromachined ultrasonic transducers (CMUTs) in liquid-coupled operation, resulting in the excitation of spurious modes with a degradation of the output pressure. In pulse-echo operation of CMUT linear arrays for medical imaging, the acoustic interaction effects manifest as oscillation tails straight after the pulse signal and the echo signal. On the other hand, such oscillations can be observed in the frequency spectra as ripples within the operational bandwidth of the transducer, thus contributing to the cross-talk in CMUT arrays. The cross-talk is detrimental to the quality of the reconstructed image. In this work, the acoustic interaction effects are analyzed through finite element modeling (FEM) and experimental measurements carried out with 128-elements CMUT arrays. An index, the acoustic interaction level (AIL), is introduced to quantify the cross-talk in the actual B-mode pulse-echo imaging operation of a CMUT linear array. The experimental AIL is found approximately -42 dB for the echoes returned from a planar reflector in a single line of view of the reconstructed image

3D Ultrasonic imaging of the human hand for biometric purposes

In this work, 3D ultrasonic images of the internal region of the palm of the human hand are presented and analyzed in order to evaluate the ultrasonic technique for biometric recognition purposes. A commercial ultrasound imaging machine provided with a high frequency (12 MHz) linear array has been employed. The probe is moved in the directional orthogonal to the array and at each step a B-scan is performed and stored to form a 3D matrix. The data from the the 3D matrix are elaborated for achieving 3D ultrasonic palmprints. The results have been compared with corresponding sample obtained with conventional methods and the advantages of the proposed technique are underlined and discussed.

P2P-1 Multilayer cMUT Structure for Improved Sensitivity and Bandwidth

The electrical and mechanical behavior of cMUTs is currently under investigation. The mechanical characteristics of the membranes as well as the air-gap size determine the sensitivity, the maximum pressure level and the operating frequencies of cMUT devices. In this work, we report on a novel multilayer cMUT vibrating structure made of a stack of flexural membrane layers, conveniently interconnected by means of equally spaced pillars. Each membrane layer is metallized in order to apply and detect the electrical signals. A simple analytical lumped parameters model shows that a multilayer cMUT structure can achieve higher sensitivity and maximum pressure level while mantaining broadband behavior. We report finite element modeling (FEM) of possible configurations of multilayer cMUTs for broadband immersion operation in the MHz frequency range. The results obtained show enhanced sensitivity and bandwidth performances in respect of conventional cMUTs

A finite element tool for the analysis and the design of capacitive micromachined ultrasonic transducer (cMUT) arrays for medical imaging

The fabrication technology of capacitive micromachined ultrasonic transducers (cMUTs) is now mature enough to exploit the potential of this new generation of electro‐acoustic transducers in the field of diagnostic medical imaging and non‐destructive evaluation. Converting a demonstrator into a commercial product often requires many design and process refinements, and sometimes significant modifications to the fabrication process. Because each design‐fabrication‐and‐test cycle is time‐consuming and costly, the development of a modeling tool for the computer simulation of cMUTs is an essential aid to reduce the time to market and costs. Much effort has been put by researchers to develop more and more accurate and smart simulation tools, that advanced from the standard equivalent circuit modeling with lumped parameters to more powerful techniques based on the finite element method (FEM). This paper aims to give an overview of the most ordinary modeling approaches used for the simulation of cMUTs. It is shown...