Supervisor Ph.D. student Prof. Nicola A. Lamberti Monica La Mura Scientific Referees Prof. Alessandro S. Savoia Prof. Giosuè Caliano Ph.D. Course Coordinator Prof. Ernesto Reverchon This 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. For the design of optimized CMUT devices, accurate modeling is mandatory: the propagation of the acoustic wave produced by a source made of multiple vibrating membranes radiating into a fluid-like medium cannot be fully described by analytical models, thus CMUT devices must be simulated by Finite Elements Models (FEM). In this work, a simplified model of a wide aperture multicell CMUT device was used to develop a tool to support the design process. The model was used to investigate the design parameters variation effect on the static and dynamic performance of CMUT arrays composed of circular cells. The collapse voltage, the membrane deflection profile and the static capacitance of Reverse-Fabricated CMUT devices (RF-CMUTs) were computed by varying the membrane radius and thickness and the cavity height. Since devices fabricated by the Reverse Fabrication Process are built from top to bottom, the Silicon Nitride base can be made very thin, and the devices can be backed by arbitrarily designed backing layers. The backing material effect on the pulse-echo behavior of immersed CMUT devices was investigated, and the reverberation phenomenon reduction was observed by matching the backing material acoustic impedance to the acoustic impedance of the propagating medium. In order to investigate the performance of a sparse CMUT array, a 3D FEM model is needed, since there is no axial symmetry in a sparse array layout. For this reason, a full model of a reverse CMUT cell was implemented, tested and experimentally validated. Due to the long simulation time required by the FE analysis of a finite transducer element, a mesh optimization study was carried out on a simpler structure, i.e. an infinite transducer. The results of this optimization process were used to mesh a CAD-imported model of a sparse array element. The studied array element is part of a multi-chip module (MCM) comprising a CMUT array based on a density tapered Fermat’s spiral and an analog front end (AFE) ASIC wafer-bonded to the transducer array by means of a Benzocyclobutene (BCB) layer. The 10-mm array is designed for broadband operation around 7 MHz in immersion operation, and is made by 256 1.0λ-wide elements consisting of 19 hexagonally tiled circular cells. The proposed model was used to perform a harmonic analysis in ANSYS, in order to compute the element factor that modulates the array radiated pressure field. The study of the array element directivity is important to assess the beam steering capabilities of the device. The beam pattern was computed by varying the mechanical boundary conditions applied to the array element, in order to investigate the effects of an acoustic isolation of the array element, achievable by performing trenches in the BCB layer or in both the BCB and the structural nitride. The results obtained for the device included in an infinitely extended structure are in good agreement with the measurement performed on the probe head prototype featuring the MCM, though some differences in the main lobe and side lobe level exist, probably due to the incorrect compensation for the hydrophone directivity applied. The FEM model of the wide aperture multicell CMUT was used as a basis to model a flextensional array of circular membranes actuated by piezoelectric disks, housed inside the cavity and glued to the rear of the membrane. The transducer array was designed for broadband reception operation in concrete-coupled condition, in order to perform efficient acoustic emission measurements for the monitoring of concrete structures. The piezoelectric flextensional array design process was based on the computation of the device reception transfer function, obtained in concrete-coupled conditions by varying the geometrical parameters of the elastic plate, of the piezoelectric disks, of the structural layer and of the backing. The resulting device has a 200 kHz wide -6 dB reception sensitivity bandwidth around the center frequency of 112 kHz, thus is suitable for acoustic emission techniques applied to concrete structures.

Among the various non-destructive techniques for health monitoring in structures, the Acoustic Emission (AE) is well known in scientific literature. Ultrasonic waves emitted by the creation and propagation of cracks in concrete or Reinforced Concrete specimens are usually collected by means of ultrasonic sensors. The signals must be treated in front-end readout process with preamplifiers and filters, to be able to set a proper trigger level and to cut the background noise (belonging to different frequency ranges). In addition, the post processing of the data is important to “clean up” the dataset, removing fake events, and to extract the proper information, useful for structure damage assessment. In this paper, the authors present the experimental set up and the transducers used to acquire the AE signals recorded during a four-point bending test on a RC beam. The ad hoc realized amplifier and filtering circuit used in the test are also described. Then, an example of an AE signal is also reported, in terms of frequency spectrum analysis and noise filtering.

In this paper, first experimental results obtained on a new resonant humidity sensor are presented. The resonant sensor is made of a piezoelectric material coated with a hygroscopic material, therefore able to adsorb the water molecules contained in the surrounding air. The adsorbed water increases the sensitive layer mass, thus varying the sensor resonance frequency. The sensor is included in an electronic oscillator that tunes its oscillating frequency with the sensor resonance frequency. The output voltage signal is sampled and processed by a microcontroller in order to measure the resonance frequency. By relating the device resonance frequency to the amount of water adsorbed by the polymeric layer, an accurate air humidity measurement can be obtained.

The monitoring of concrete buildings is important to assess their solidity and promptly intervene in case of damage. Concrete structures suffer the formation of micro-cracks and the propagation of macro-cracks, due to ageing or to mechanical stresses. These cracks are sources of acoustic emission, and can be distinguished according to the frequency and intensity of the acoustic wave they generate; thus, transducers with high sensitivity and wide bandwidth are needed. Flextensional piezoelectric transducers are known for these characteristics, which make them suitable for acoustic emission applications. For this reason, we designed a flextensional transducer optimized for acoustic emission techniques applied to concrete structures. The performance of the device in receive mode in concrete-coupled operation was investigated by running harmonic analyses on a finite element model. The geometrical parameters of the device were designed in order to obtain a broadband receive sensitivity.

Due to the growing demand for humidity control systems, there is a need for small, cheap, accurate and reliable sensors for air relative humidity (RH) measurements. In this paper, a resonant air relative humidity sensor is presented. The sensor is based on a quartz crystal coated on both surfaces with a hygroscopic polymer. Due to the adsorption of water molecules, the polymer mass increases accordingly with the surrounding air humidity, causing the decrease of the device resonance frequency. By measuring the sensor resonance frequency, an accurate RH measurement can be obtained. Experimental results are shown, comparing the fabricated sensor behavior with a commercially available RH sensor.

Assessment of existing structures by means of non-destructive techniques is an important research topic in modern engineering, because of its potential in estimating structural health status without introducing undesired damage. Among several recognized techniques, Acoustic Emission (AE) analysis is growing interest in the scientific community, since it can be used to detect the initiation of cracking processes, to determine cracking typology and to extract other information from the specimen. In this paper, the authors present the results of a four-point bending test on a Reinforce Concrete (RC) beam, performed at the University of Salerno. The test has been conducted according to a loading protocol characterised by several loading-unloading cycles, and both mechanical and acoustic data have been recorded. The results will be presented in terms of standard AE parameters, commonly adopted in literature, such as Felicity ratio, signal strength and cumulative signal strength, historic index and b value. It will be underlined that these parameters allow to detect interesting moments of the phenomenon (cracking onset and development) and to assess the damage of the structure.

Density is one of the characteristic properties of a substance, that can be used to understand other physical and chemical properties. On-line density sensors are of interest in many applications: the radioactive monitoring in waste storage tanks, in pipelines for process control in the petrochemical industry, in the production of chemical reagents, in food processing, in the production of paper and textiles and so on. In this paper, a piezoelectric density sensor is proposed and analyzed with FE methods. The idea at the base of the present work relies in the well-known property of piezoelectric structures to vary their resonant behavior depending on load conditions. The active element of the proposed density sensor is a cheap piezoceramic bimorph, widely used in buzzers and telephone receivers. The bimorph is clamped all around by tightening its border to the edge of a rigid open chamber in which the liquid is poured; the flexural resonance frequency of the membrane is modified by the liquid mass. The liquid volume is the volume of the chamber and therefore the membrane resonance frequency can be related to the liquid mass density. We analyze the proposed sensor by a FEM commercial code (ANSYS®) to test the device capability and to design the chamber in which the liquid is poured, in order to maximize the device sensitivity; we computed the sensor resonance frequency by varying the chamber height and for three different values of the chamber radius. The obtained results demonstrate that best sensitivity is obtained with smaller values of both height and radius.

Conclusions Ultrasound (US) imaging is the preferred technique for the investigation of human body tissues, such as organs, muscles, tendons and vessels. In fact, US imaging offers several advantages over other available imaging techniques: it is completely harmless and non-invasive, provides real-time images and is cost-effective. Images of anatomic structures are obtained by transmitting and receiving ultrasonic acoustic waves propagating through the human body, by means of an electroacoustic transducer device. The fabrication technology of transducer arrays for ultrasound imaging based on piezoelectric materials is nowadays very mature and therefore hardly improvable. During the last two decades, very promising results were obtained by new devices, fabricated by silicon micromachining technology and based on the electrostatic cell working principle. Capacitive Micromachined Ultrasonic Transducers (CMUTs) are MEMS devices consisting of miniaturized metallized membranes [1]; the membranes are forced into flexural vibration by an electric signal during transmission, and vice versa generate a voltage signal when actuated by an incident acoustic signal. The performance of CMUT arrays for US imaging applications are excellent [2,3]. Moreover, the CMOS-compatible fabrication technology enables the integration of the transducers with a dedicated front-end electronic circuit, Fig. 1. (a) The cell layout of a CMUT device; (b) a fully-packed CMUT probe head; (c) a fully assembled CMUT probe for ultrasound imaging.

Liquids density measurements are highly required in both research and industrial applications, from process control in petrochemical industry to product monitoring in food processing. Commercial devices are often expensive or unwieldy; therefore, in a previous work, a cheap and handy resonant sensor, based on a piezoelectric bimorph, was introduced. The operating principle is based on the variation of the device electrical impedance, due to the liquid mass. By fixing the liquid volume, the frequency variation can therefore be related to the liquid density. The maximum sensitivity of the device was obtained when the resonance frequency was higher. In this paper, two piezoelectric bimorphs with smaller diameter, and therefore resonating at higher frequencies, were considered as active elements and analyzed by means of a FEM commercial code, in order to maximize the device sensitivity, varying the liquid height. Finally, the sensitivity was computed, by using the height with the best sensitivity, and varying the liquid density. Best results were obtained by using the smallest diameter bimorph. First experimental measurements confirmed simulation results.

When Capacitive Micromachined Ultrasonic Transducers (CMUTs) are coupled with water, they show high front-face acoustic reflectivity, due to the impedance mismatch between the transducer substrate material, typically based on silicon, and the propagation medium. During pulse-echo operation, surface reflectivity is responsible for multiple reflections of the received acoustic signals, which result in a set of unwanted echoes. In ultrasound imaging applications, this signal reverberation creates artifacts and reduces the image contrast. In this paper, a method to reduce front-face reflectivity is proposed, and a Reverberation Level (RL) index is introduced in order to quantify the unwanted reverberation of the signal returned to the transducer surface. The proposed method combines the increase of the bias voltage, the application of an optimized resistive load and the addition of a low-impedance acoustic backing to CMUTs realized by Reverse Fabrication Process (RFP). In this way, the mechanical energy conversion and transmission to the backing, as well as the electrical energy dissipation, are improved, thus reducing the energy reflection into the medium. The proposed method is analyzed by means of Finite Element simulations and is experimentally validated by characterizing single-element RFP-CMUTs, provided with different backing materials and electrical loads. In the analyzed prototypes, a RL reduction of 8.6dB is obtained.

Density is one of the characteristic properties of a substance and on-line density sensors are of interest in many industrial fields. In this paper, a piezoelectric density sensor is proposed and analyzed with FE methods and experimentally. The active element of the proposed density sensor is a cheap piezoceramic bimorph, widely used in buzzers and telephone receivers; the bimorph is clamped all around tightening its border to the edge of a rigid open chamber in which the liquid is poured. The resonance flexural frequency of the membrane is modified by the liquid mass, the liquid volume is the volume of the chamber and therefore the liquid mass density can be obtained by the membrane resonance frequency variation. First measurements on an ad hoc realized prototype confirm the device capability.

Due to the widespread use of portable devices, there is a strong focus on possible methods of interaction between man and the device, in order to exchange information. Capacitive touchscreens are commonly used on the front of smartphones or tablets; nevertheless, for some applications it can be useful to have a pointing solution on the back of the device, so that fingers do not cover the visible area. The solution proposed in this paper is to turn the plastic back cover of the device into a position encoder, by gluing some piezoelectric small plates, electrically connected in parallel, on its inner surface. Moving the finger on the back cover causes a variation of the electrical impedance of the structure, thus the desired position can be revealed and reported on the front screen. In this work, the performances of a trackpad based on piezoelectric elements are investigated by Finite Elements Analysis, and an automatic method for the recognition of the finger position is proposed. Experimental tests were also conducted on a prototype, in order to confirm the capabilities of the proposed device.

Two technological options to achieve a high deposition rate, low stress plasma-enhanced chemical vapor deposition (PECVD) silicon nitride to be used in capacitive micromachined ultrasonic transducers (CMUT) fabrication are investigated and presented. Both options are developed and implemented on standard production line PECVD equipment in the framework of a CMUT technology transfer from R & D to production. A tradeoff between deposition rate, residual stress and electrical properties is showed. The first option consists in a double layer of silicon nitride with a relatively high deposition rate of similar to 100 nm min(-1) and low compressive residual stress, which is suitable for the fabrication of the thick nitride layer used as a mechanical support of the CMUTs. The second option involves the use of a mixed frequency low-stress silicon nitride with outstanding electrical insulation capability, providing improved mechanical and electrical integrity of the CMUT active layers. The behavior of the nitride is analyzed as a function of deposition parameters and subsequent annealing. The nitride layer characterization is reported in terms of interfaces density influence on residual stress, refractive index, deposition rate, and thickness variation both as deposited and after thermal treatment. A sweet spot for stress stability is identified at an interfaces density of 0.1 nm(-1), yielding 87 MPa residual stress after annealing. A complete CMUT device fabrication is reported using the optimized nitrides. The CMUT performance is tested, demonstrating full functionality in ultrasound imaging applications and an overall performance improvement with respect to previous devices fabricated with non-optimized silicon nitride.

Ultrasonic transducers for broadband watercoupled operation in the low ultrasonic 30-100 kHz frequency range find their application in different industrial and biomedical areas, such as non-destructive testing and therapy. In this paper we propose the design and the fabrication process of a multi-cell, piezoelectrically-actuated, flexural-mode transducer structure, characterized by a low mechanical impedance leading to wideband and high sensitivity immersion operation and by a high flexibility in the definition of the active area shape. After investigating the use of different materials for the different components, we report on the design of a fabrication process flow for the precise assembly of the transducer, based respectively on epoxy resin and wire-bonding for mechanical and electrical interconnection of the individual parts. A reduced-thickness (<10 mm) and weight (350 g) transducer prototype, featuring 121 circular cells placed in a hexagonal tiling arrangement covering a quasi-circular area of 80-mm diameter, is designed, fabricated, and characterized. A transmit sensitivity of 5 kPa/V at 100 kHz resulted from hydrophone measurements performed at 150 mm while the pulse-echo response from a planar surface placed at the same distance showed a 125% -6dB fractional bandwidth centered at 53 kHz.

In phased array systems, a partial beamforming can be carried out inside the probe to reduce the number of signals to be processed by the main system. Such technique, known as micro-beamforming, involves the application of a delay-and-sum beamforming to small groups of adjacent array elements using dedicated electronics. In this paper we show that, due to the electrostatic spring-softening effect, the phase response of Capacitive Micromachined Ultrasonic Transducers (CMUTs) can be controlled by varying the bias voltage. Based on this phenomenon, we propose a phase shift micro-beamforming method of CMUT arrays. We analyze the performance obtainable by applying this method to a typical phased array configuration for medical imaging. We show that, with relatively high quality factor CMUTs that still allow broadband immersion operation, it is possible to obtain up to 90° phase shifts by varying the bias voltage. Further, by inverting the sign of the bias voltage, we show that it is possible to obtain an additional 180° phase shift. Simulation results show that, with a 40% -3 dB one-way fractional bandwidth CMUT, the grating lobe level of the transmission radiation patterns, obtained with the proposed micro-beamforming technique, may be kept below -23 dB using broadband excitation signals for maximum steering angles of ±45°.

In recent years, the increasing usage of tablet PCs and smartphones has made obsolete the traditional keyboard and mouse as input devices, replaced by multi-touch or stylus-based input solutions that are perceived as being more user-friendly. Over the past 30 years, has been shown in literature and by patents, several solutions based on different types of interaction, from optics to acoustics. Our objective is to make surfaces, also wide, which have not been designed for use as a touch-screen, in particular with the new operating systems that widely use the intuitive features of the touch-methodology, usable. The goal of the work is to realize an autonomous stylus, at low cost, capable of generating flexural waves in any surface, which can be detected with a sensors system, also simple and low-cost, with an appropriate accuracy, in order to detect the position of the stylus, both to be used as pointer, either to be used as an input device for natural writing or drawing.

In this work an experimental evaluation of the possiblity to influence and control the fluid film forces in the gap of a lubricated journal bearing by means of a rotating travelling wave is carried out. The travellig wave is generated by two power actuators opportunely positioned on the outer surface of the bearing and electrically driven with a phase shift of 90 degrees. Each transducer is designed to work at the natural frequency of the radial nonaxisymmetrical mode 0-5 (23.6 kHz). Experimental results show that the travelling wave is capable to control the motion of an oil drop on the inner surface of the bearing and that it is capable to put in rotation a rotor layed on the drop oil via the viscous forces in the oil drop itself.

In dermatological aesthetic piezoelectric actuators are widely used: the skin treatment is obtained by the bending vibration, along the length, of a steel foil with a thickness of about 0.5 mm; the vibration frequency is in the ultrasonic range, to avoid annoying noise. In this paper a piezoelectric actuator able to excite a bending motion in the steel foil is described; the active part of the actuator is a piezoelectric Langevin-type transducer soliciting the foil at one edge. The actuator was designed by using ANSYS with the objective to obtain a system with high efficiency, low losses, high mechanical stiffness and low encumbrance. Best results were obtained by means of a Langevin actuator with a stepped horn displacement amplifier, whose total length is lambda/2 at the resonance frequency; the Langevin is connected to the foil by an ad hoc support. The ANSYS results computed in operating conditions show a well sustained bending vibration of the foil with stress values, in all the actuator components, far from the limit value in the material.