Microelectromechanical systems (MEMS) refer to miniaturized mechanical and electro-mechanical elements that are fabricated through microelectronic processes [...]

Currently, the most advanced micromachined microphones on the market are based on a capacitive coupling principle. Capacitive micro-electromechanical-system-based (MEMS) microphones resemble their millimetric counterparts, both in function and in performance. The most advanced MEMS microphones reached a competitive level compared to commonly used measuring microphones in most of the key performance parameters except the acoustic overload point (AOP). In an effort to find a solution for the measurement of high-level acoustic fields, microphones with the piezoelectric coupling principle have been proposed. These novel microphones exploit the piezoelectric effect of a thin layer of aluminum nitride, which is incorporated in their diaphragm structure. In these microphones fabricated with micromachining technology, no fixed electrode is necessary, in contrast to capacitive microphones. This specificity significantly simplifies both the design and the fabrication and opens the door for the improvement of the acoustic overload point, as well as harsh environmental applications. Several variations of piezoelectric structures together with an idea leading to electrically controlled sensitivity of MEMS piezoelectric microphones are discussed in this paper.

The possibility to obtain aligned clusters of microparticles in a drying water droplet by employing standing flexural plate waves (FPWs) generated by a piezoelectric MEMS transducer has been explored. The MEMS device has a squared cavity etched out in a silicon substrate forming a 6 mm × 6 mm diaphragm composed of a stack of doped silicon (Si) and aluminum nitride (AlN) layers. Metal interdigital transducers (IDTs) placed at the edges of the diaphragm allow to electrically drive the AlN layer to excite flexural plate waves in the diaphragm at its first antisymmetric Lamb mode (A0). The working principle has been validated through finite element analysis and experimentally verified. For experimental testing, a droplet of tap water with an approximate radius of 850 μm has been placed on the diaphragm and let dry at room temperature while applying voltage excitations to two opposed IDTs. After droplet evaporation, dry clusters of microparticles, originally dispersed therein, have been successfully patterned with a regular spacing of half wavelength, as expected, and remained adhered to the diaphragm without the need of additional substances for clot creation. The innovative exploitation of a noncontact and noninvasive flow-field-based approach combined with the evaporation process in a piezoelectric MEMS transducer can enhance the assembly control of microparticles or biological cells in lab-on-chip applications.

A piezoelectric MEMS pressure sensor that exploits the first antisymmetric vibration mode (A0) of Lamb waves is presented. The 6 mm × 6 mm diaphragm used to sense the applied pressure is composed of a stack of doped silicon (Si) and aluminum nitride (AlN) layers with metal interdigital transducers (IDTs) to generate flexural plate waves (FPWs). The working principle has been validated through 2D finite element analysis within the frequency range 10–15 MHz and experimentally verified. A variable pressure has been applied across the diaphragm while measuring the electrical admittance of a single IDT. Experimental data are in good agreement with simulations showing a frequency shift of the admittance peaks when pressure acts on the MEMS diaphragm. For an applied pressure of 170 Pa, a relative frequency variation of 0.25% has been achieved.

Nowadays, the most advanced micromachined microphones on the market are represented with structures using the capacitive coupling principle.Structures and performances of these micro-devices resemble their millimetric counterparts, which are typically used as measuring microphones.In the past decade, thanks to technological progress of the microelectronics industry, microphones using a piezoelectric transduction have been proposed by several teams.Such novel microphones exploit the piezoelectric effect of a thin layer of aluminum nitride, incorporated in their diaphragm structure.In these microphones fabricated with micromachining technology, no fixed electrode is necessary, unlike capacitive microphones.This specificity significantly simplifies both design and fabrication and opens the door for a new improvement of acoustic overload point as well as harsh environmental applications.In this paper, we present and compare two most promising approaches to piezoelectric micromachined microphone design.The first approach is developed by using a flat, circular diaphragm fixed around its perimeter, having the piezoelectric layer with its upper electrode in the vicinity of the clamped region.The other approach involves a square diaphragm cut in a way to create several cantilevers.Such a structure enables a more compliant diaphragm compared to the first approach.

The possibility to steer floating cells dispersed in water by means of flexural plate waves (FPWs) generated by a 9 mm $\times9$ mm piezoelectric MEMS transducer has been explored. The MEMS transducer has a squared cavity etched out in a silicon substrate formed by a 6 mm $\times6$ mm composite diaphragm made of a piezoelectric aluminum nitride (AlN) layer on top of a doped silicon plate. The piezoelectric layer can be electrically actuated by means of metal interdigital transducers (IDTs) placed over the AlN film at the diaphragm edges. Cell alignment has been sought for by inducing standing FPWs in the diaphragm and in the contacting water layer in the cavity by the one-dimensional (1D) acoustic field pattern obtained by exciting two IDTs located symmetrically with respect to the diaphragm centre. The working principle has been validated by means of 2D finite element modelling and simulations. The MEMS transducer has been fabricated using the PiezoMUMPs process and experimentally tested by exploiting a tailored front-end electronic circuit. Inactive fibroblast cells with an approximate diameter of $15~\mu \text{m}$ have been dispersed in demineralized water within the cavity at a concentration in the order of 105 cells/ml. By applying sinusoidal excitation signals to faced IDTs with zero phase shift and peak amplitude of 10 V, lines of cells spaced by half wavelength $\lambda $ /2 = $56~\mu \text{m}$ have been achieved at 12.5 MHz, in good agreement with theoretical predictions and simulation results.

In this work, the possibility to align cells dispersed in water by means of standing flexural plate waves (FPWs) in an underlying substrate has been explored by designing and fabricating a piezoelectric MEMS transducer. The MEMS exhibits a 6x6 mm2 cavity etched out in a silicon substrate forming a volume where cells dispersed in liquid can be steered under electronic control. The diaphragm of the cavity is composed of silicon (Si) and an aluminum nitride (AlN) layer. The generation of FPWs of the A0 mode in the diaphragm is achieved by applying proper excitation voltages to two metal interdigital transducers (IDTs). In turn, acoustic waves are transferred in the liquid, generating a one-dimensional acoustic field pattern thus steering and trapping the dispersed cells in distinct positions. The MEMS device has been fabricated by using the PiezoMUMPs process and experimentally tested by exploiting a tailored front-end circuit. The cavity has been loaded with inert fibroblasts cells with an approximate diameter of 15 μm dispersed in demineralized water with a concentration in the order of 105 cells/ml. By properly driving two IDTs, lines of cells spaced by half wavelength λ/2 = 56 μm have been achieved at 12.5 MHz, in good agreement with theoretical expectations.

The paper presents a technique to obtain an electrically-tunable matching between the series and parallel resonant frequencies of a piezoelectric MEMS acoustic transducer to increase the effectiveness of acoustic emission/detection in voltage-mode driving and sensing. The piezoelectric MEMS transducer has been fabricated using the PiezoMUMPs technology, and it operates in a plate flexural mode exploiting a 6 mm × 6 mm doped silicon diaphragm with an aluminum nitride (AlN) piezoelectric layer deposited on top. The piezoelectric layer can be actuated by means of electrodes placed at the edges of the diaphragm above the AlN film. By applying an adjustable bias voltage Vb between two properly-connected electrodes and the doped silicon, the d31 mode in the AlN film has been exploited to electrically induce a planar static compressive or tensile stress in the diaphragm, depending on the sign of Vb, thus shifting its resonant frequency. The working principle has been first validated through an eigenfrequency analysis with an electrically induced prestress by means of 3D finite element modelling in COMSOL Multiphysics®. The first flexural mode of the unstressed diaphragm results at around 5.1 kHz. Then, the piezoelectric MEMS transducer has been experimentally tested in both receiver and transmitter modes. Experimental results have shown that the resonance can be electrically tuned in the range Vb = ±8 V with estimated tuning sensitivities of 8.7 ± 0.5 Hz/V and 7.8 ± 0.9 Hz/V in transmitter and receiver modes, respectively. A matching of the series and parallel resonant frequencies has been experimentally demonstrated in voltage-mode driving and sensing by applying Vb = 0 in transmission and Vb = −1.9 V in receiving, respectively, thereby obtaining the optimal acoustic emission and detection effectiveness at the same operating frequency.

Ultrasound power delivery can be considered a convenient technique for charging implantable medical devices. In this work, an intra-body system has been modeled to characterize the phenomenon of ultrasound power transmission. The proposed system comprises a Langevin transducer as transmitter and an AlN-based square piezoelectric micro-machined ultrasonic transducer as receiver. The medium layers, in which elastic waves propagate, were made by polydimethylsiloxane to mimic human tissue and stainless steel to replace the case of the implantable device. To characterize the behavior of the transducers, measurements of impedance and phase, velocity and displacement, and acoustic pressure field were carried out in the experimental activity. Then, voltage and power output were measured to analyze the performance of the ultrasound power delivery system. For a root mean square voltage input of approximately 35 V, the power density resulted in 21.6 µW cm−2. Such a result corresponds to the data obtained with simulation through a one-dimensional lumped parameter transmission line model. The methodology proposed to develop the ultrasound power delivery (UPD) system, as well as the use of non-toxic materials for the fabrication of the intra-body elements, are a valid design approach to raise awareness of using wireless power transfer techniques for charging implantable devices.

In this paper, a pioneer partial discharge (PD) loop antenna sensor is presented and examined. It is made of a 70-turn square planar inductor with a side length of 1.8 mm, which is fabricated on top of a silicon substrate in complementary metal oxide semiconductor technology. The microsensor ability to detect corona PD is demonstrated once connected in series with a 60 dB gain amplifier. The behavior is studied at different separation distances from the line through which the PD pulses flow. At 5 cm away, a damped sinusoidal induced voltage with an amplitude of about 100 mV has been measured. The output signal spectrum is highly concentrated around a central resonance frequency of ∼5 MHz. The microsensor response is compared with those of other industrial sensors from Techimp, i.e., horn antennas and high-frequency current transformer sensors. The presented on-chip sensor can be considered a non-intrusive competing solution compared with other heavy and expensive commercial sensors due to its lightweight, compact size, and low cost. In addition, it shows an acceptable signal to noise ratio compared with other commercial electromagnetic wave-based sensors.

We report on a piezoelectric micromachined ultrasonic transducer (PMUT) driven in a nonlinear regime, generating chaotic amplitude modulated ultrasonic waves. At large enough drives, the PMUT enters in the Duffing regime which opens a hysteresis with two available states. By modulating the frequency of the driving signal, the system may switch between both states, and selecting the appropriate modulation frequency enables to enter in the chaotic regime. The chaos is then imprinted as a modulation of the PMUT's amplitude. We characterize this regime in the three accessible domains: electrical, mechanical and acoustic, and demonstrate they are fully correlated. We then focus on the generated acoustic signals and demonstrate that the chaotic modulation propagates according to the PMUT's linear regime. Remarkably, the detected acoustic waves are strongly correlated to the on-chip piezoelectric measurements, regardless of the acoustic beam profile. The frequency spectrum of the chaotic modulation spreads around the ultrasonic carrier, mimicking a noise modulated carrier signal. We exploit this property for jamming applications where the chaotic PMUT is used to mask surrounding acoustic waves. Unlike most jamming applications, our approach does not require driving signals with a broad frequency spectrum, the noisy pattern arising directly from the structure's dynamics. Using two PMUTs, one in the linear and the other in the nonlinear regime, we realize a proof-of-concept where the ultrasound generated by the first PMUT is drowned out by the chaotic PMUT signal. We demonstrate that the carrier frequency of the jamming PMUT does not need to match perfectly the one of the linear PMUT. This chaos generation is generic and could be adapted to any PMUT, and thanks to the rich frequency spectrum of the chaotic modulation, the frequency of the signal to jam does not need to be precisely known.

Chaotic systems, presenting complex and nonreproducible dynamics, may be found in nature, from the interaction between planets to the evolution of weather, but can also be tailored using current technologies for advanced signal processing. However, the realization of chaotic signal generators remains challenging due to the involved dynamics of the underlying physics. In this paper, we experimentally and numerically present a disruptive approach to generate a chaotic signal from a micromechanical resonator. This technique overcomes the long-established complexity of controlling the buckling in micro/nanomechanical structures by modulating either the amplitude or the frequency of the driving force applied to the resonator in the nonlinear regime. The experimental characteristic parameters of the chaotic regime, namely, the Poincaré sections and Lyapunov exponents, are directly comparable to simulations for different configurations. These results confirm that this dynamical approach is transposable to any kind of micro/nanomechanical resonator, from accelerometers to microphones. We demonstrate a direct application exploiting the mixing properties of the chaotic regime by transforming an off-the-shelf microdiaphragm into a true random number generator conforming to the National Institute of Standards and Technology specifications. The versatility of this original method opens new paths to combine the unique properties of chaos with the exceptional sensitivity of microstructures, leading to emergent microsystems.

This paper presents analytic and numerical modelling of a MEMS electrodynamic micro-sensor of dynamic pressure. Two coaxial planar inductors of different diameters are used in the proposed micro-sensor design. Using finite element analysis, the diaphragm resonant frequency and dynamic displacements are evaluated for different diaphragm thicknesses. Then, the total sensitivity is deduced by coupling different physical domains which contribute in the micro-sensor operation. A lumped element model is built in order to study the micro-sensor sensitivity and define the dynamic performance for different resonant frequencies. This model shows that the best sensitivity, within the mV/Pa range, is obtained around the resonant frequency when operation in the audible frequency range, and decreases to the uV/Pa range for ultrasonic frequencies. The obtained sensitivity curves prove that the undamped inductive micro-sensor can offer high pressure sensitivity within a narrow frequency bandwidth.

This article aims at providing a better behavior understanding and performance evaluation of a large-size passive on-chip tapped transformer when used as a transducer in lowfrequency integrated microsystems [30-300 kHz]. Thus, a CMOSbased 1:1 transformer, consisting of 70 turns of 1.6-μm-wide top-level metal spaced by 0.6 μm and with a total length of 1.7 mm, has been investigated and tested. The spatial separation between the two transformer windings was set to 70 μm. An electrical equivalent lumped model has been extracted through an exhaustive specific measurement procedure. The model is useful to simulate and evaluate the voltage transformation ratio (TR) between the two transformer windings. The effects of parasitics and imperfect coupling between transformer windings and through the silicon substrate are outlined from the circuit point of view. We report a magnetic coupling coefficient that does not exceed k = 0.15, with a voltage TR of about 0.39 around the resonance frequency of ~5.3 MHz, when the secondary is unloaded. It has been proven that the inter-windings capacitance introduced by close conductors of each winding, evaluated to 13 pF, assures the most important role in the power transfer. This article has shown that by optimizing properly the transformer realization and limiting some parasitics elements, the inductive and capacitive links could both play a key role in MEMS transducers through tapped transformers operation around 100-kHz frequencies.

This work presents a MEMS device with Aluminum Nitride (AlN) piezoelectric actuators on a silicon suspended plate developed for driving mechanical vortexes in aqueous solution, and hence obtain centrifugation of fluids. Innovatively, the MEMS device has the capability to force ClockWise (CW) or CounterClockWise (CCW) rotations of a fluid drop placed in contact with the MEMS suspended plate. Experimental results show that CW and CCW rotations have been obtained using a 2-mm-diameter drop of water.This capability advantageously could allow the use of the proposed MEMS device for applications where drop centrifugation with different rotation directions can change the chemical or biological properties of the fluids.

A Piezoelectric Micro Electro-Mechanical System (Piezo-M EMS) acoustic transducer able to electrically tune the resonant frequency in receiver and transmitter modes is reported. The proposed 6x6mm squared diaphragm has been simulated by a 2D finite element model, fabricated, and measured. A DC bias voltage applied to the piezoelectric layer produces a controllable stress, thus leading to a matching of the series and parallel resonant frequencies. This allows to increase performances when the transducer is operated as both transmitter and receiver. The resonance can be tuned in a range of +/-70 Hz by a DC voltage of +/-8 V with an estimated sensitivity of 8.87 Hz/V.