ACTUATION OF PIEZOELECTRIC STRUCTURES WITH FERROELECTRIC THIN FILMS HAVING MULTIPLE ELEMENTS

Abstract

 A method for driving piezoelectric elements of a micro-system. The piezoelectric elements comprising a ferroelectric thin film, and being configured to be part of any one or a combination of items of a list comprising: a cantilever, a bridge, a diaphragm, a manifold of complex patterns of plates that may include trenches, slits, and include plate elements having different thickness; and being further configured to comprise at least 2 types of parallel plate electrode capacitors, the first type of capacitor having on a first side of the ferroelectric thin film a first type of electrode and the second type of capacitor having a second type of electrode electrically disjoined from the first type of electrode, forming a patterned surface of electrodes of the first side, and the first and second type of capacitors having a common electrode on a second side of the ferroelectric thin film opposite to the first side, and configured to face both the first type of electrode and the second type of electrode. The ferroelectric thin film is fixed on an elastic layer, forming together an elastic structure. The method comprises exciting in at least an exciting burst of an alternating electrical driving signal distributed to a first and a second signal, each active in different halves of the vibration period, and of one polarity only, thus substantially zero, in the other half period when it is not active, whereby the first and the second signal may have either one of the same polarity in their active half period, or opposite polarity, the first signal driving the first type of capacitor during a first half of a vibration period, and the second signal driving the second type of capacitor during a second half of the vibration period, thereby enabling an excitation of a flexural elastic structure, therewith enabling a deformation of the elastic structure, whereby further the vibration period is configured to correspond to a basic resonance of the elastic structure, the exciting burst comprising a determined number of resonance cycles.

Description

Technical field

[0001] The present invention relates to piezoelectric micro-machined ultrasonic transducers (pMUT), particularly for the case of using ferroelectric thin films as piezoelectric elements.

Background

[0002] Piezoelectric laminated plates, bridges and cantilevers are useful for a number of applications in micro sensors, micro actuators, and micro transducers ^1,2. Figure 1 shows a generic type in the form of a cantilever (cross section) containing a piezoelectric thin film 103 sandwiched between a top electrode 101 and a bottom electrode 104, thus forming a parallel plate capacitor. This functional unit is attached to an elastic layer 106, frequently and optionally through a buffer or adhesion layer 105, the elastic layer 106 being attached to a micromachined substrate 107. The elastic layer 106 is bent when an electric field is applied across the piezoelectric thin film 103. The elastic layer 106 is normally at least as thick as the piezoelectric thin film 103 in order to place the neutral plane of the bent structure outside the piezoelectric layer, i.e. inside the elastic layer 106.³ The working principle is based on the in-plane mechanical stress (T₁, T₂) generated by the piezoelectric effect upon application of an electric field. In case of parallel plate capacitor structures, as depicted in figure 1, this electric field E is perpendicular to the plane (indicated by arrows between the top and bottom electrodes 101 and 104 in figure 1), thus having only component 3, i.e., in direction 3 of the coordinate axes drawn in figure 1 and represented as E₃ in figure 2. The in-plane stress is governed by an effective piezoelectric coefficient e_31,f, whose derivation^3,4, and measurement^5,6 are found in the literature. The generated in-plane stresses are obtained as:

[0003] In polar ionic crystals, or even in all inorganic piezoelectric materials with polar order, the coefficient e_31,f is negative, and the generated in-plane stress thus tensile (positive) when the electric field is parallel to the internal polarization of the material (arrow 103a). This is the situation depicted in figure 1. The tensile stress generated in the piezoelectric film will force the cantilever to bend upwards with a constant curvature under the top electrode (see, e.g. ⁷). This bending effect is sketched in figure 2 in an exaggerated manner. Figure 2 shows a bending of a flexural plate by a piezoelectric thin film, wherein:

t_p is a thickness of piezoelectric thin film 203, between electrodes 201 and 204;

R is a bending radius or inverse curvature valid at a neutral plane 208;

z is a radial coordinate of the curved structure having its origin at the neutral plane 208; and

d_m is a length of the lever for the bending moment by the line forces created by the piezoelectric effect, which corresponds to about the distance between the center plane of the piezoelectric film and the neutral plane of the complete elastic stack.

The piezoelectric stress forms a line force F₁=T₁t_p acting on the neutral plane with a moment M₁=F₁d_m. Ideally, the neutral plane is inside the passive elastic layer 206. The moment increases with the lever length. The opposing moment by the rigidity of the elastic layer 206, however, increases much stronger with increase of its thickness. For the total electromechanical coupling, there is thus an optimum of the thickness relation t_p/t_e, which depends on the relative rigidities, but is typically found around ½ (see, e.g. ^3,8,9).
Further to cantilevers, the principle can be extended (see, e.g. ref¹⁰) to diaphragm-like clamped plates, bridges, and combinations of these, like for instance perforated plates, ^{11 12} or cantilever arrays.

[0004] When dealing with excursions of plates and bridges, it is easily seen that the plate must have regions with positive curvature, and regions with negative curvature. For instance, a circular plate deflecting upwards in the center has there a negative curvature, and consequently a positive curvature at the border where it is clamped (i.e., the first derivative of the deflection function is zero at the border). Theoretically, one could soften this border by etching trenches or by partial liberation, but in practice, one has to avoid building too fragile structures, and often an air-tight membrane is required. The changes of curvature may be preselected by the electric field, which can be chosen parallel and antiparallel to the internal polarization of the material, leading to regions with positive and negative stress, and positive and negative curvatures. An example configuration of a piezoelectric laminated plate is sketched as a cross section of a round clamped plate in figure 3. The schematic illustration of the cross section shows that this has two types of top electrodes mounted on a piezoelectric thin film 303, i.e., a first type of top electrode 301 and a second type of top electrode 302, and a bottom electrode 304 of floating or grounded type. It can be realized with an arrangement with the second type of top electrode 302 being for example a central electrode and the first type of electrode 301 being for example an external, ring-shaped electrode, and the bottom electrode 304 being for example a common bottom electrode that is floating, or at ground. Giving a negative voltage to the central electrode and a positive voltage to the outer electrode leads to compressive stress below the former, and to tensile stress below the latter if the polarization is fixed as indicated by an arrow 303a. Condition is that the internal polarization is stable and does not change with the electric field. This is the case in polar materials that are not ferroelectric. Examples are the wurtzite structures aluminum nitride (AlN), alloys of aluminum nitride and scandium nitride (AlScN) and zinc oxide (ZnO). In case of ferroelectric thin films, the local polarization switches to become parallel to the external electric field as soon as the electric field is larger than the coercive field (E_c). Figure 3 further shows at least a buffer layer 305, an elastic layer 306 and a micromachined substrate 307.

[0005] The present invention relates to devices with ferroelectric thin films. Ferroelectric films of PZT (Pb(Ti,Zr)O₃) are known to exhibit the strongest piezoelectric coefficients, and are thus attractive for a number of applications where high strokes and piezoelectric coupling coefficients are important, such as for ink-jet printing, ¹³ and linear actuators as needed for instance in autofocus lenses. The disadvantage of ferroelectric thin films is the ferroelectric switching occurring at a so-called coercive field E_c. Figure 4 shows the generated in-plane stress as a function of the electric field. Although the ferroelectric stress can be much higher (limited to about 600 MPa), and generated at lower electric fields, the advantage of the linear AIScN materials is the fact that one can drive the stress also to compressive values. The bipolar operation of ferroelectric films has in addition the disadvantage of accelerated fatigue due to polarization fatigue. This was very much studied for the memory applications (see, e.g. ref ¹⁴). Note that the piezoelectric coefficients are proportional to the internal polarization. In summary, figure 4 shows a comparison of piezoelectric performance of ferroelectric thin film 401 and 402, with non-ferroelectric, polar piezoelectric thin films 403, 404, 405, and 406. The latter show a linear behavior across the complete range from strongly negative to strongly positive electric field. The ferroelectric thin films switch as soon as the electric field is larger than about 20 to 50 kV/cm. For larger electric fields, the polarization is always parallel to the applied electric field, and thus the in-plane stress becomes positive (branches 401, 402) for both signs of the electric field.
PZT is not the only possible ferroelectric thin film material. Suitable ferroelectric materials are oxides of the general formula ABO₃, with the three categories of di-valent Atom A (Pb²⁺, Ba²⁺) combined with a 4-valent cation B (Ti⁴⁺, Zr⁴⁺), tri-valent Atom A (Bi³⁺) combined with a tri-valent cation B (Fe³⁺) (BiFeO₃), and mono-valent A (Li⁺, K⁺¹, Na⁺¹), combined with a 5-valent B (Nb⁺⁵, Ta⁺⁵, W⁺⁵), as in LiNbO₃ or KNbO₃. Many of these compounds form solid solutions among each other and are optimized for highest properties often along so-called morphotropic phase boundaries. The best known examples are [PbZrO₃]_0.52[PbTiO₃]_0.48 (PZT), and [Pb(Mg_1/3Nb_2/3O₃]_0.66[PbTiO₃]_0.33 (PMN-PT), and (K_0.5,Na_0.5)NbO₃ (KNN). In the so-called leadfree materials, Pb²⁺ is either replaced by a suitable combination with Bi³⁺: (K_(1-x), Na_x)_0.5Bi_0.5TiO₃, which can be combined in a solid solution with BaTiO₃, or is from the family of KNN with additional dopants (Li, As, Ta, ...). Furthermore, there is a large number of potential dopants that may improve a material concerning ferroelectric domain motion, electrical leakage, thermal behavior, etc.: Mn, Nb, Ca, Fe, Sc, La, Y, Sm, Gd, Ce, etc.

[0006] For integration, also the buffer or adhesion layers play a role (105, 305, 1505). They may play additional roles as chemical barriers, for texturing the electrode material, and for promoting the growth of the ferroelectric thin film outside the electrode area. Suitable buffer layers can be TiO₂, ZrO₂, Ta₂O₅, ZnO, Al₂O₃, and MgO. Suitable electrodes deposited before the ferroelectric thin film are Pt with (111)-texture, LaNiO₃ and SrRuO₃ with (100) texture, or other, similar conducting perovskite oxide thin films. The top electrodes are from the same materials, or additionally made of aluminum, or gold. In the latter case, chromium or TiW adhesion layers might be applied.

[0007] In many cases, the piezoelectric element may be driven by a unipolar voltage (see figure 5, representing a unipolar operation of ferroelectric thin film of PZT, in which the ferroelectric material is never depolarized by a field in opposite direction of the internal polarization of the ferroelectric material). The advantage of such operation is a better stability of the internal polarization and thus of the piezoelectric coefficients. The stability can even be improved by a so-called poling step including the application of a high field at higher temperature (e.g. 150 °C) (see, e.g. ref. ¹⁵). This mode is a good solution for linear actuators. There is also hardly any fatigue, as shown in ref. ¹⁶ for the high stress / low strain case. However, there are actuation modes with alternating currents where tensile and compressive stress generation would be an advantage. This is the case for ultrasonic wave generation as used for ultrasound imaging for instance. The use to 2 different electrodes on the same diaphragm was probably first described in ref. 8 ⁸ for the stator of an ultrasonic micro motor, see figure 6, representing a piezoelectric laminated diaphragm with inner and outer electrodes to excite positive and negative stress for more optimal excitation of the desired vibration mode. The photo in figure 6 is from a work on an ultrasonic stator for a micromotor. ⁸ It shows an outer electrode 601 and its contacts, an inner electrode 602 and its contacts, and the access to the bottom electrode (604). In the same paper, it was shown by analytical modeling that the use of two types of electrodes, situated at regions with opposite curvature, increase drastically the coupling coefficient, i.e., the amount of mechanical work done by the electrical power. The point is, however, that a compressive stress cannot by created as soon as the electric field increases beyond the coercive field, at which the stress switches to a tensile value.
Micro-machined Ultrasonics Transducers (MUT) are investigated since a number of years with the idea to replace the "classical" piezoelectric ultrasound probes used for medical ultrasonic imaging. The latter are made of piezoelectric bulk ceramics of PZT, or PNM-PT, and offer certainly the best performance/cost ratio for most of the cases. The MUTs find their justification in large arrays. Indeed, micro fabrication is more suited for miniaturization and for the fabrication of probe arrays with many elements. The classical probes reach very high coupling factors (ratio of produced mechanical energy per invested electrical energy, or vice versa) of up to K²=0.8 (80 %). The only known MUT principles able to reach such high values are those based on electrostatic attraction in the capacitance formed by a membrane and a counter electrode separated by a vacuum cavity of about 10 nanometers. This MUT is called c-MUT. ¹⁷ The MUT principle employing a piezoelectric thin film - a so called pMUT - was also investigated for a while by the interested community, but was unable to reach high enough coupling factors.^{9 18 19} A K² of 5 % is already very good among the published results. Figure 7 shows one of the best results ever achieved to the date of the present invention (from the research group of the inventor). The coupling coefficient increased up to 11 % because in this case a dc bias was superimposed to the ac signal, in order to avoid polarization switching (unpublished result). The element had only one top electrode covering the central part up to 0.7 times the radius. In other words, figure 7 shows a pMUT resonator with the basic resonance at 2.5 MHz, and a k² of 11 % (k=0.33). Part (a) on the left side contains a measured susceptance curve (lm(Y)), wherein the large negative value is to be noted. Part (b) on the right side shows a schematic cross section through the device. The diameter of the diaphragm is 200 µm, the device layer of the SOI (silicon on insulator) has a thickness of 10 µm, and the PZT film is 4 µm thick. This curve is published for the first time in the present patent application, but the device is from the same batch as those published in refs ^{15 16}. The electrode version was of type CE.
Echo experiments with pMUT's based on PZT thin films were made earlier by the research group of the present inventor, and showed the feasibility of such devices.⁹ Figure 8 shows an experiment carried out in air. The acoustic wave was generated with one transducer (called emitter in figure 8) and received with a second one (called receiver in figure 8). It can be seen that the received signal starts to rise about 100 µs after the emitted signal started to be generated. This is the signal of the wave that travelled the distance once (indeed, the sound velocity in air corresponds to about 3 cm per 100 µs). The weaker signals received afterwards repeat themselves with a period of about 180 µs, corresponding to travel times of about 6 cm, i.e., twice the distance between the devices. One can conclude that the sound wave is reflected at the receiving pMUT, again reflected at the passive emitter pMUT surface, and detected again by the receiver. Figure 8 shows 3 echos, corresponding to a total path of 21 cm. It is therefore quite plausible that the wave can be detected at distances of over 30 cm, as claimed in the article. In other words, figure 8 shows an echo experiment with 2 pMUTs. In figure 8(a) the schematic of the experimental set-up is shown; and figure 8(b) is a screen shot of signals. A first pMUT is emitting a short pressure wave burst (5 cycles in this case). The upper curve in figure 8a shows the drive signal given to the emitter (actuator). The lower curve shows the signal captured by the receiver (sensor) after amplification by a charge amplifier. Both pMUTs were fabricated in the same batch and work at identical frequency of 98 kHz. The received signal shows some ringing, i.e., it is longer than the original signal supplied to the emitter. It is also seen that several echoes are received. The wave goes back and forth between emitter and receiver.
The aim of this invention is to enhance the performance of pMUT structures with ferroelectric thin films, to create operation conditions at which ferroelectric films may be used at their superior value as material for high piezoelectric stress development, and overall to boost pMUT properties.
In summary of the prior art references as far as included in the section describing the background of the invention background, it is found that

• working pMUT's have been demonstrated by the research group of the present inventor,

and that:

(1) in these prior art references, the piezoelectric materials are PZT thin films;

(2) the designs of the device included specific measures to avoid parasitic capacities for achieving highest possible coupling coefficients;

(3) the designs included the use of two types of electrodes, depending on the curvature of the membranes at a given phase of the excitation;

(4) the designs included the use of SOI wafers for the definition of the diaphragm (or plate, or membrane); and

(5) the designs did not include any specific features of the driving and read-out electronics in case of ferroelectric thin films as piezoelectric elements.

Further prior art references include patent publications discussed below.
International publication WO 2016/054448 A1, entitled "Piezoelectric Micro-machined Ultrasonic Transducers Having Differential Transmit and Receive circuitry", to Chirp Microsystems, describes an application for driving the signals to the two electrodes of a piezoelectric element. These elements need 2 voltages to define an electric field (bottom and top electrode). The drawings of WO 2016/054448 A1 show an electrode version with a single ring element, similar to the one tested in version RSE of figure 7b discussed herein above. The central issue of WO 2016/054448 A1 seems to be that two separate circuits are needed for transmission (producing an outgoing wave) and reception (measuring intensity of incoming wave). WO 2016/054448 A1 concentrates on only 2 electrical connections.
In US publication US 2016/0262725 A1, to University of California, an application related to a manifold of pMUT elements is described. The description of the pMUT electrodes is the simple one with a piezoelectric film sandwiched between 2 electrodes (normal).
US 2016/0262725 A1 focuses on how to control the element array.

Summary of invention

[0008] In a first aspect, the invention provides a method for driving piezoelectric elements of a micro-system, the piezoelectric elements comprising a ferroelectric thin film, the piezoelectric elements being configured to be part of any one or a combination of items of a list comprising: a cantilever, a bridge, a diaphragm, a manifold of complex patterns of plates that may include trenches, slits, and include plate elements having different thickness; and the piezoelectric elements being further configured to comprise at least 2 types of parallel plate electrode capacitors, the first type of capacitor having on a first side of the ferroelectric thin film a first type of electrode and the second type of capacitor having a second type of electrode electrically disjoined from the first type of electrode, forming a patterned surface of electrodes of the first side, and the first and second type of capacitors having a common electrode on a second side of the ferroelectric thin film opposite to the first side, and configured to face both the first type of electrode and the second type of electrode. The ferroelectric thin film is fixed on an elastic layer, forming together an elastic structure. The method comprises exciting in at least an exciting burst of an alternating electrical driving signal distributed to a first and a second signal, each active in different halves of the vibration period, and of one polarity only, thus substantially zero, in the other half period when it is not active, whereby the first and the second signal may have either one of the same polarity in their active half period, or opposite polarity, the first signal driving the first type of capacitor during a first half of a vibration period, and the second signal driving the second type of capacitor during a second half of the vibration period, thereby enabling an excitation of a flexural elastic structure, therewith enabling a deformation of the elastic structure, whereby further the vibration period is configured to correspond to a basic resonance of the elastic structure, the exciting burst comprising a determined number of resonance cycles.

[0009] In a preferred embodiment, the step of exciting comprises excitations' duration shorter than a half period, the duration being defined as period/n, where n is an integer.

[0010] In a further preferred embodiment, the alternating electrical driving signal is derived from an AC or RF source through a simple device containing diodes for splitting the signal into a branch with negative voltages, and a branch with positive voltages.

[0011] In a further preferred embodiment, a thickness of the piezoelectric thin film ranges from 10 nm to 10 µm.

[0012] In a further preferred embodiment, the elastic layer of the flexural structure is a thin film single or multilayer comprising Si₃N₄/SiO₂, SiC, or machined from a silicon substrate using silicon micromachining in techniques with dry etchers and silicon on insulator (SOI) substrates.

[0013] In a second aspect, the invention provides an acoustic device, comprising an elastic layer supporting at least 2 types of parallel plate electrode capacitors formed with a piezoelectric thin film on one side of the elastic layer, the elastic layer being anchored with anchors in a frame structure consisting mainly of a thicker substrate material, the piezoelectric thin film comprising ferroelectric materials; wherein the first type of parallel plate electrode capacitor has on a first side a first type of electrode and the second type of parallel plate electrode capacitor has a second type of electrode electrically disjoined from the first type of electrode, and the first and second type of parallel plate electrode capacitors have a common electrode on a second side opposite to the first side, and configured to face both the first type of electrode and the second type of electrode; wherein the common electrode carries the piezoelectric film, which comprises on its surface opposite to the common electrode the first type and the second type of electrodes that are separated on the surface, and wherein a shape of each of the first and the second of the electrodes follows a required distribution of signs of curvature radii to accomplish an optimal excitation of a desired mechanical vibration of the elastic layer; and wherein the first type and the second type of electrodes are situated at the interface between elastic and piezoelectric layer, and the common electrode on top of the piezoelectric layer.

[0014] In a further preferred embodiment, the anchors are a determined number of narrow bridges.

[0015] In a further preferred embodiment, each of the anchors comprises a complete clamping around a border of the elastic layer.

[0016] In a further preferred embodiment, the piezoelectric film is either one of polar or ferroelectric nature.

[0017] In a further preferred embodiment, a thickness of the piezoelectric thin film ranges from 10 nm to 10 µm.

[0018] In a further preferred embodiment, the elastic layer of the flexural structure is a thin film, a single or multilayer comprising Si₃N₄/SiO₂, SiC, or machined from a silicon substrate using silicon micromachining in techniques with dry etchers and silicon on insulator (SOI) substrates.

[0019] In a further preferred embodiment, the acoustic device is embedded in a gaseous, liquid, or solid environment for the emission of acoustic waves; and the reception of acoustic waves.

[0020] In a third aspect, the invention provides an acoustic device comprising a flexural elastic structure which is excited to resonance by electric fields across a piezoelectric layer sub-divided into a set of parallel plate capacitors, leading to an optimal deformation of an elastic structure by the piezoelectric stress for the emission of emitting acoustic waves with the elastic structure towards an object in an adjacent medium on one side; the elastic structure comprising a piezoelectric layer sub-divided into a set of parallel plate capacitors in which the application of an AC electric field causes a pattern of induced piezoelectric stress; the piezoelectric parallel plate capacitors being grouped into 2 types, such that at maximum amplitude within a half-period, one type covers the area of compressive stress induced by the electric field in the piezoelectric layer, the other type covers the area of tensile stress in the piezoelectric layer; the elements of the same group are poled in the same way; the elements of different groups either in the opposite direction; the two excitation signals have opposite polarity combined with a phase difference of 180 ° or they are poled in the same direction, and the two excitation signals have same polarity combined but a phase difference of 180 °; the acoustic waves are reflected from the object in front of the elastic structure and received on the elastic structure, whereby the reflected acoustic waves deform the elastic structure, which is enabled for being excited to resonance by an incoming acoustic wave arriving through an adjacent medium from one side and experiencing a resonant deformation; the elastic structure comprising a piezoelectric layer sub-devided into a set of parallel plate capacitors in which the resonant deformation causes a pattern of induced stress that creates piezoelectric charges and voltages; the piezoelectric parallel plate capacitors being grouped into 2 types, such that at maximum amplitude within a half-period, one type covers the area of compressive stress induced by the wave in the piezoelectric layer, the other type covers the area of tensile stress in the piezoelectric layer; the elements of the same group are poled in the same way; the elements of different groups either in the opposite direction; and a sum of the two signal is combined for the output signal, or they are poled in the same direction, and the difference of the signal is used as output signal.

[0021] In a further preferred embodiment, the acoustic device further comprises extracting means configured for extracting from the signals and storing digitally for signal treatment, any one of the list comprising signal peaks, signal delays, and complete wave form; and whereby a plurality of piezoelectric elements of a micro-system is arranged in an array in order to increase the information density on the objects to be detected.

[0022] In a fourth aspect, the invention provides a use of the acoustic devices, comprising reflecting the acoustic waves from the object in front of the elastic structure and receiving the acoustic waves on the elastic structure, whereby the reflected acoustic waves deform the elastic structure, creating electrical currents and voltages signals by a direct piezoelectric effect in the piezoelectric layer, and opening switches after each of the excitation bursts, to allow the electrical currents and voltages signals to pass to an amplifying circuit, where the signals are added or subtracted, depending on a polarization given to the two types of parallel plate electrode elements.

[0023] In a fifth aspect, the invention provides an electronic circuit configured to drive at least a first type and a second type of piezoelectric elements of a microsystem, the first type and the second type of piezoelectric elements being configured to be part of any one or a combination of items of a list comprising a cantilever, a bridge, a diaphragm, and the piezoelectric elements being configured in a form of parallel plate electrode capacitors with an electrode system of bottom and top electrodes, whereby the first type and the second type of piezoelectric elements comprise at least a first type and a second type of electrodes, and that are adapted for a desired deformation of the at least first type and second type of piezoelectric elements, meaning that the first type of electrodes extends over a surface that exhibits a first curvature of a first sign during a maximal deflection in a given mode, and that the second type of electrodes exhibits a second curvature for a second sign during the maximal deflection at the same moment as the first one, the electronic circuit being further enabled to produce a unipolar pulse signal for driving, that gives the voltage input to the piezoelectric elements only in a half-period when the element needs to produce tensile stress, and in a half period requiring compressive stress, gives a voltage supply of substantially zero voltage.

[0024] In a further preferred embodiment, the electronic circuit further comprises a first supply line for the half-period when the first or the second piezoelectric element needs to produce tensile stress, and a second supply line for the half-period requiring compressive stress in the first or the second piezoelectric element, the first supply line being connected to the first type of electrodes, the second supply line to the second type of electrodes, in each case to one of the 2 types of electrodes called the active ones, a third supply line which is common for both the first type and the second type of piezoelectric elements, usually called "common" or ground (GND), the electric field defined between the first signal and the common in type 1 elements, and between the second signal and the common between in type 2 elements, the electronic circuit further configured such that an electric field in the second type of piezoelectric element points in opposite direction, as compared to an electric field in the first type of piezoelectric elements.

[0025] In a further preferred embodiment, the electronic circuit further comprises a multitude of the described electronic circuits to drive an array of pMUT single transducers (pMUT cell), allowing also for addressing parts of the array cells, and with different phases with respect to the time in order to allow for beam steering of the emitted wave.

[0026] In a further preferred embodiment, the electronic circuit is further enabled for a sensing mode in which ultrasonic waves are intended to be detected by type 1 and type 2 elements of opposite poling directions leading to the generation of the same sign of voltage and current, wherein the electronic circuit is configured to sum up the signals from the two types for the signal production, by any one of the item in the following list: a connection of the lines by a switch that occurs when opening the receive line, a summing up of the signals after a preamplifier.

[0027] In a further preferred embodiment, the electronic circuit further comprises a first supply line for the first half-period (V0), and a second supply line for the second half-period (V180°), whereby the first supply line and the second supply line are of the same polarity, being either always positive, or always negative, the first supply line is connected to the first type of piezoelectric elements, the second supply line is connected to the second type of piezoelectric elements, a third supply line which is common for the first and the second piezoelectric elements (GND), and defines an electric field between V0 to GND and V180° to GND, the signals detected by type 1 and type 2 elements having the same poling directions leading to the generation of opposite sign of voltage and current, wherein the electronic circuit is configured to measure the difference between the signals from the two types for the signal generation.

[0028] In a further preferred embodiment, the electronic circuit is further enabled for a sensing mode, in which ultrasonic waves are intended to be detected, wherein the electronic circuit is configured such that the generated voltage and current of the first type of electrodes have the opposite sign of the generated voltage and current if the second type of electrodes, a difference of the signals from the first type of electrode and the second type of electrode is used for signal production, after the switch opening the receive lines, whereby the signals are given to two ports of an operational amplifier, thus amplifying the difference between the two signals.

[0029] In a sixth aspect, the invention provides a use of the method as described herein above in any one of applications of the following list: finger print detectors, flow sensors, bio-medical sensors, micropumps for fluidic elements in biomedical applications, non-destructive testing.

Brief description of the figures

[0030] The invention will be better understood through the detailed description of preferred embodiments of the invention and in reference to the figures, wherein

figure 1 shows a basic structure for applying a piezoelectric thin film in a parallel plate configuration to bend a flexural plate, cantilever, bridge or membrane, according to prior art;

figure 2 shows a bending of a flexural plate by a piezoelectric thin film in an exaggerated view, according to prior art;

figure 3 illustrates a schematic pMUT cross-section having two types of top electrodes, and a floating or grounded bottom electrode, according to prior art;

figure 4 shows a comparison of piezoelectric performance of ferroelectric thin film with non-ferroelectric, polar piezoelectric thin films, according to prior art;

figure 5 contain a graph illustrating a unipolar operation of ferroelectric thin film of PZT, according to prior art;

figure 6 shows a piezoelectric laminated diaphragm with inner and outer electrodes to excite positive and negative stress for more optimal excitation of the desired vibration mode, according to prior art;

figure 7 shows an example of a pMUT resonator, according to prior art;

figure 8 shows an echo experiment with 2 pMUTs, according to prior art;

figure 9 illustrates a pair of unipolar RF input signals to electrodes as a function of time, the two signals having opposite polarity;

figure 10 illustrates a version with a pair of unipolar RF input signals that is working with one driving signal polarity only;

figure 11 illustrates an example of a circuit for generating sinus curves with truncation of one polarity, generating a pair of unipolar RF input signals of opposite polarity (compatible with fig. 9);

figure 12 illustrates an example schematic of wiring and signal treatment;

figure 13 illustrates the use of the pMUTs according to the invention in the case where the same element is used for emission and reception (in contrast to fig. 8);

figure 14 illustrates the use of an array of pMUTs, the pMUTs being part an acoustic wafer, attached to a connectic wafer distributing the signals of type 1 and type 2;

figure 15 schematically depicts a cross section through one pMUT element of the array, the top electrode differentiates between type 1 and type 2 elements, signals Y2 and Y3 are used to drive the elements;

figure 16 schematically depicts a cross section through one pMUT element of the array, the top electrode differentiates between type 1 and type 2 elements; signal Y2 and the inverted Y3 are used to drive the elements;

figure 17 depicts a possible geometrical arrangement of the via contacts as seen from the top; and

figure 18 depicts an example configuration for exciting a plurality of elements.

Detailed description of preferred embodiments of the invention

[0031] The present invention relates to piezoelectric micro-machined ultrasonic transducers (pMUT), particularly for the case of using ferroelectric thin films as piezoelectric elements. It addresses the issue of applying several elements that can be poled differently according to the local sign of curvature of the plate generating ultrasonic waves.
The present invention is also related to a driving circuit for supplying the elements with the necessary, time dependent voltages, and how to collect the signals generated by the same piezoelectric elements upon reception of the backscattered ultrasonic waves.
One difference in the present invention compared to prior art reference ⁹, is that in principle only one transducer is used for both functions of generating and receiving an acoustic wave.
One important idea of the present invention is to drive a multi-electrode device with unipolar signals distributed in time so as to create tensile stress at the correct time interval for a given vibration mode and electrode. Unipolar operation allows for a higher polarization, and thus higher piezoelectric activity, and in addition, leads to a longer life time in as much as ferroelectric fatigue is excluded. On the issue of figure 7, it was described that the electromechanical response is enhanced by the application of a DC bias. For the case shown there, only one electrode was active (case CE of fig. 7b). The ideal bias amounts to about the AC signal height, giving a signal of A(sin(ωt) + 1), where ω means the angular frequency and t the time. So if we want to have more effective excitation of the wave, we need to have an activation in each interval of a period, i.e., with 2 types of electrodes, one activated in the first half, the second in the second half of the period, we come to the first step of the invention, proposing that we apply two different bias voltages, one of the size A for the positive signal in, say, the first half of the vibration period to electrode type 1 (signal V+= A(sin(ωt) + 1)), and a bias of -A given to the second half of the period to electrode type 2 s(signal V-=A(sin(ωt) - 1)). The AC signal is the same for both. We then get a signal shape as shown in fig. 9a. In each half period, the optimal polarity is then applied to the appropriate electrodes working in segments requiring tensile stress. We see in figure 9a, that this simple method indeed avoids the switching of the ferroelectric material as it us unipolar, however, the signal is not entirely restricted to the half-period (as indicated by the dotted area), which would be ideal, but reaches somewhat into the other half-period. This situation is avoided by using a circuit as proposed in figure 11: The signal may be derived from a pure sinus from which the negative part is truncated to generate V+, and the positive part is truncated to achieve V-. These signals V+ and V- can be used to drive multielectrode resonators as shown in figs 9b and 9c. Fig. 9b shows the situation in one of the half periods (say at phase 0°), and Fig. 9c in the other one (phase 180°).

[0032] Alternatively, one may work with one polarity only (the ferroelectric elements will be poled accordingly) but with two signals having a phase shift of 180 ° to each other. This is shown in fig. 10.

[0033] Figs. 9b and 9c show an example of a device vibrating in the fundamental mode of a plate with patterned top electrodes, excited with the signals as described above. The hardware of such a device is as shown in fig. 3: figs. 9b and 9c show the deformations. The device contains a plate comprising a ferroelectric thin film 903, with a first type of top electrode 901 mounted on top of the ferroelectric thin film 903, a second type of top electrode 902 also mounted on top of the ferroelectric thin film 903, and a bottom electrode 904 mounted on an opposite side of the ferroelectric thin film 903. The ferroelectric thin film 903 with its electrodes is further mounted to an elastic layer 906, by means of a buffer layer 905. The whole plate structure is further attached on to an outer periphery on a micromachined substrate as shown in fig. 3. The polarization directions 903a and 903b are opposite to each other.
The principle of the excitation of the plate is shown in fig. 9b and fig. 9c. These figures illustrate RF input signals (on the left on figures 9b and 9c) to electrodes of the plate (on the right of figures 9b and 9c), as a function of time, referred to as solution 1 herein. The plate in figures 9b and 9c is shown in a simplifier manner, i.e., without a number of the structural features represented in figure 3, for an easier reading. The 2 RF signals V₊ and V_- have only one polarity with respect to the GND. V₊ is given to the second type of top electrode 902, and V_- to the first type of top electrode 901. V₊ is only positive with respect to the ground. The field created in the piezoelectric ferroelectric thin layer 903 has thus always the same sign for a given electrode type: for example, the polarization below the second type of electrode 902 as shown at location 903b points always "down", i.e., from the second type of top electrode to the bottom electrode 904. This downward directed polarization may be prepared by an initial poling process. V_- is always negative with respect to the bottom electrode 904 connected to ground potential. Its halfwave appears half a period later (180 degrees shift in the amplitude). It always creates a field pointing up, the polarization is therefore also always pointing up. In other words, the polarization below the first type of electrodes 901 as shown at locations 903a points always "up", i.e., from the bottom electrode 904 towards the first type of top electrode 901. Hence, the polarization in this example is pointing up below the first type of electrodes 901, i.e., also called outer electrodes herein, and pointing down below the second type of electrode 902, i.e, also called inner electrode herein. The electric fields are only applied in half waves, so as the field direction is parallel to the polarization in every capacitor. Of course, in a preferred embodiment, one could exchange the polarization directions of inner and outer electrodes, when exchanging the driving signals as well. One could as well exchange the roles of bottom and top electrodes.

[0034] Figure 10 shows an alternative (referred to as solution 2 herein) using only one polarity of driving signals, e.g. only positive voltages. A second signal V₁₈₀ corresponds then to a first signal V₀ that is phase shifted by 180° but else the same. The film is then poled in same way at both electrode types. There is no change in the wiring of the device as compared to the example of figures 9b and c. The signals to and from inner electrodes 1002 (second type of electrode) and outer electrodes 1001 (first type of electrode) must be isolated from each other.
Figure 11 shows an example circuit for the realization of the positive and negative input signals, phase shifted in addition by 180°, based on a circuit of operational amplifiers simulating a threshold free diode. The circuit generates sinus curves (not shown in figure 11) with truncation of one polarity:

• Y1 = input of sinus signal;
• Y2 = positive half-wave output; and
• Y3 = negative half-wave output.

The frequency limit of operational amplifiers may go up to 200 MHz (LM7171 of Texas Instruments). In a preferred embodiment, the circuit targets at 10 to 20 MHz only, for which LM7171 is thus a good choice. Alternatively, one may also produce pulses with simpler circuits, and not care about a perfect sinusoidal signal shape. By inverting the sign of one signal (Y2 or Y3), the required signals for the version for the generation of V₀ and V₁₈₀ is realized.
One advantage of the principle according to the invention is that we do not depolarize the ferroelectric thin film. At each electrode we stay in the unipolar mode. After the excitation, the complete transducer is well poled and ready for receiving the reflected acoustic waves in an optimal situation.
In general, one uses the same acoustic device for the generation of the primary wave, and for detecting the response of the environment in the form of reflected waves. This measurement is made by an amplifier in a receiver circuit (1205). This one needs to be protected from the power input used to generated the primary waves. For this purpose, there are gates 1203 opening only after the wave generation is finished.
An example interface electronic circuit which is schematically illustrated at figure 12 contains a supply part 1201 delivering the positive and negative voltage pulses, i.e., the signals V₊ and V_-, and the ground (V=0), or the only 180° shifted signal. In addition, there is a receiver circuit 1205 collecting the response signals from the piezoelectric elements, once the voltages pulses are stopped. In order to protect the amplifiers, the receiving part is opened with a switch after the decay of the last voltage pulse. In other words, figure 12 contains a schematic of wiring and signal treatment. The supply part 1201 is configured as a signal generator to excite a pMUT 1202 for wave emission. There are three lines connected to the latter for signals Y2, Y3 and ground (GND). If the signal Y3 shifted by 180 ° with respect to signal Y2, is in addition of opposite polarity, the 3 signals are V₊, V_-, and GND (Option 1). If Y2 and Y3 have the same polarity, it is V₀, V₁₈₀ and GND (Option 2). At the end of the pulse sequence, a signal from the pulse generator opens the switches 1203 to allow the response signal to the amplifier. Depending on the chosen option, node 1204 includes the addition (option 1) or the subtraction (option 2) of the signals.
One advantage about detecting waves is that we deal now with a low-voltage situation, meaning that the response to incoming waves does not lead to a switching of the ferroelectric. For the reception we deal with the direct piezoelectric effect, i.e., the creation of charges (electric displacement field D₃) as a function of deformation (in plane strain S₁ and S₂ are caused by a deformation of the plate due to the incoming pressure wave):

The two different types of elements will undergo strains with opposite signs at a given time. In solution 1 (figure 9), they have also opposite polarizations. For this reason, the created charges will have the same sign, and one can simply add them in a circuit (this leaves the voltage free to adapt for each element), or simply to add the two lines after the protection switches. This simplifies the reception circuit. In solution 2, the two types of elements exhibit the same polarity, and the difference of the currents of voltages is the signal to be collected.
The burst of supply signals may contain for instance 6 periods, lasting 300 ns at a frequency of 20 MHz. If the wave is reflected at a distance of 1 mm, the acoustic path length amounts to 2 mm, and such an echo is then received after 1350 ns, assuming a sound velocity of water (1484 m/s). This is well possible if the ringing of the acoustic device is damped sufficiently (In medical ultrasound, the overall Q-value of the device is usually kept at about 3).
The above presented innovation is based on considering a combination of simple elements as mechanical, vibrating structures. However, given the todays means of finite element modeling, more complex shapes could be applied, that can be described as a manifold of complex patterns of plates that may include trenches, slits, ridges, and plate segments of different thickness.
So far we have considered that the time period is split into two halves. A generalization is obtained if we include also excitation times of fractions of the period T: i.e., T/n, for cases including moving waves inside the acoustic device.

Packaging and electrical circuits towards applications:

[0035] The electronics presented in fig. 12 is thought for using the same element for emission and detection by implementing switches to separate the emission period (connection to signal generator) from the detection period (connection to amplifier). This situation is schematically drawn in fig, 13, showing the physical situation of the two periods.

[0036] A considerable enhancement of the performance of ultrasonic transducers for acoustic imaging is obtained by the use of linear of even two-dimensional arrays. The diameter of the pMUT elements must be chosen near half of the wavelength in the medium through which the waves have to propagate (e.g., 80 µm in water at 10 MHz). Arrays allow for a better spatial resolution (as illustrated in fig. 14), and also to access better to high resolution in real time. The price to pay is mainly on the level of electrical contacts and electronics. For larger arrays, it becomes impossible to have the wiring on the same level as the pMUT structures. The wiring problem can be overcome by attaching a connectic wafer (1402) to the so-called acoustic wafer (1401) containing the pMUT structures. The signal lines are guided from the front side to the backside of the acoustic wafer through vias (1404), and then inside the connectic wafer to contacts at the periphery of the device. The vias are metallized through-holes. The assembly process of connectic and acoustic wafer must assure the connection of vias to the counter contacts on the connectic wafer.

[0037] A possible design of the acoustic wafer is shown in fig. 15. The top electrodes are patterned for forming the piezoelectric elements of type 1 and type 2. They are connected to the vias (1508) leading across the through holes with insulation (1509) to the backside of the substrate (1507), from where the contact is made to the connectic wafer. The bottom electrode is common to both elements and extends over the complete element except for the via regions, and regions outside of the active membrane for avoiding parasitic capacities. In the version shown, emission signal bursts derived from signals Y3 and Y2 are in use (bipolar). The ferroelectric layer (1503) is then poled as given in the drawing (1503a, 1503b), where also the poled regions are highlighted.

[0038] An alternative is shown in fig. 16. The role of bottom and top electrodes are inverted. In this version, the top electrode is the common one, and covers most of the element. This version should be better for avoiding parasitic capacities with objects in the medium in front of the device, or pick-up of RF signals by objects in the medium (antenna effect). A further variant is implemented, as an inverted Y3 signal is used, leading to a different polarization direction in one type of the piezoelectric elements (1603a) as compared to the other one (1603b).

[0039] In fig. 17, a top view is shown corresponding to the acoustic wafer version of fig. 15. The active regions are below top electrodes 1703 and 1704. The regions with uncovered ferroelectric layer are white, i.e., represented in non/textured surfaces. The ferroelectric layer is limited to the contour 1702. The grey zone 1701 corresponds to the situation of uncovered bottom electrode. The black features are the contact and via lines (1705, 1706, 1707, 1708), below which there is no bottom electrode for avoiding shorts, and parasitic capacities. The complete structure can be covered in addition with an insulating layer protecting from the medium (e.g. a polymer layer of parylene).

[0040] In case of larger arrays, the most reasonable way to proceed is a line by line scan. For emission, all elements of a line can be excited at the same time from one source in a parallel way, as illustrated in fig. 18. The lines 1801 and 1802 supply the RF bursts to excite the elements of line n. There is a linear array of switches outside the array for choosing the line. For detection, however, all elements must be read-out separately in order to profit from the high resolution of the array. One possibility is to dispose of one amplifier circuitry (as shown in fig. 12) per column. Conductor lines orthogonal to the excitation lines serve to collect the signals (1804, 1805 from the two types of piezoelectric elements per cell). The FET structures provide the switches to let only one cell (n,m) to the column read out lines 1804 and 1805. The control line 1803 opens the switches. It is for the same line n as for the excitation signal if the back-reflected echo signal is measured.

Industrial applicability

[0041] The first interest for pMUT's in the years 2000 to 2008 was motivated by high resolution ultrasonic imaging, mainly for medical applications. However, the achievable coupling factor was too small for competing with standard bulk versions using very high performing piezoelectric ceramics and mono-crystals in the longitudinal resonance mode. Recently, however, it was recognized that pMUT's may be used for finger print detection. Currently they are made of AIN thin films and exhibit quite a low coupling K² of about 1 %. PZT versions, as presented in this invention, are stronger, and even more strong with the proposed invention. The need for higher emission power arises from the idea to have the finger print detector below the screen of a smart phone. But there will be many other applications where finger print detectors are added. Hence, it is thought that there will be a large market for such devices. Other applications can also be foreseen for echo-type sensors as the experiment described in figure 8. It is possible to detect the position or presence of objects or persons on a relatively short distance of the order of meters.

Citations

[0042] 

1. Muralt, P. Ferroelectric thin films for microsensors and actuators: a review. Micromech.Microeng. 10, 136-146 (2000).

2. Muralt, P. Recent progress in materials issues for piezoelectric MEMS. J.Am.Ceram.Soc.. 91, 1385-96 (2008).

3. Muralt, P. Piezoelectric thin films for MEMS. Integr. Ferroelectr. 17, 297-307 (1997).

4. Muralt, P., Kholkin, A., Kohli, M. & Maeder, T. Piezoelectric actuation of PZT thin film diaphragms at static and resonant conditions. Sens. Actuators A 53, 397-403 (1996).

5. Dubois, M.-A. & Muralt, P. Measurement of the effective transverse piezoelectric coefficient e31,f of AIN and PZT thin films. Sens. Actuators A 77, 106-112 (1999).

6. Prume, K., Muralt, P., F., C., Schmitz-Kempen, T. & Tiedke, S. Piezoelectric thin films: evaluation of electrical and electromechanical characteristics for MEMS devices. IEEE Trans UFFC 54, 8-14 (2007).

7. Meng, Q., Mehregany, M. & Deng, K. Modeling of the electromechanical performance of piezoelectric laminated microactuators. J.Micromech.Microeng. 3, 18-23 (1993).

8. Dubois, M.-A. & Muralt, P. PZT thin film actuated elastic fin micromotor. IEEE Trans Ultrason. Ferroelectr. Freq. Control 45, 1169-1177 (1998).

9. Muralt, P. et al. Piezoelectric micromachined ultrasonic transducers based on PZT thin films. IEEE Trans UFFC 52, 2276-88 (2005).

10. Ledermann, N., Muralt, P., Baborowski, J., Forster, M. & Pellaux, J.-P. Piezoelectric PZT thin film cantilver and bridge acoustic sensors for miniaturized photoacoustic gas detector. J.Microelectromech.Systems 14, 1650-1658 (2004).

11. Schächtele, J., Goll, E., Muralt, P. & Kaltenbacher, D. Admittance Matrix of a Trapezoidal Piezoelectric Heterogeneous Bimorph. IEEE Trans UFFC 59, 2765-2775 (2012).

12. Kaltenbacher, K. et al. Sound Transducer for insertion in an ear. US Patent 2012/0053393 A1 (2011).

13. Fujii, E. et al. Preparation of (001)-oriented PZT thin films and their piezoelectric applications. IEEE Trans UFFC 54, 2431-2438 (2007).

14. Tagantsev, A. K., Stolichnov, I., Colla, E. & Setter, N. Polarization fatigue in ferroelectric films: Basic experimental findings, phenomenological scenarios, and microscopic features. J.Appl.Phys. 90, 1387-1402 (2001).

15. Kohli, M. & Muralt, P. Poling of ferroelectric films. Ferroelectrics 225, 155-162 (1998).

16. Mazzalai, A., Balma, D., Chidambaram, N., Matloub, R. & Muralt, P. Characterization and fatigue of the converse piezoelectric effect in PZT films for MEMS applications. J.MEMS 24, 831-838 (2015).

17. Haller, M. I. & Khuri-Jakub, B. T. A surface micromachined ultrasonic air transducer. IEEE Trans UFFC 43, 1-6 (1996).

18. Belgacem, B., Calame, F. & Muralt, P. Thick PZT sol-gel films for pMUT transducers performances improvement. in IEEE Ultrasonics Symposium 922-925 (IEEE, 2006).

19. Belgacem, B., Calame, F. & Muralt, P. Piezoielectric micromachined ultrasonic transducers with thick PZT sol-gel films. J Electroceram 19, 369-373 (2007).

Claims

1. A method for driving piezoelectric elements of a micro-system,
the piezoelectric elements comprising a ferroelectric thin film,
the piezoelectric elements being configured to be part of any one or a combination of items of a list comprising: a cantilever, a bridge, a diaphragm, a manifold of complex patterns of plates that may include trenches, slits, and include plate elements having different thickness; and
the piezoelectric elements being further configured to comprise at least 2 types of parallel plate electrode capacitors, the first type of capacitor having on a first side of the ferroelectric thin film a first type of electrode and the second type of capacitor having a second type of electrode electrically disjoined from the first type of electrode, forming a patterned surface of electrodes of the first side, and the first and second type of capacitors having a common electrode on a second side of the ferroelectric thin film opposite to the first side, and configured to face both the first type of electrode and the second type of electrode;
the ferroelectric thin film being fixed on an elastic layer, forming together an elastic structure;
the method comprising:
exciting in at least an exciting burst of an alternating electrical driving signal distributed to a first and a second signal, each active in different halves of the vibration period, and of one polarity only, thus substantially zero, in the other half period when it is not active, whereby the first and the second signal may have either one of the same polarity in their active half period, or opposite polarity, the first signal driving the first type of capacitor during a first half of a vibration period, and the second signal driving the second type of capacitor during a second half of the vibration period, thereby enabling an excitation of a flexural elastic structure, therewith enabling a deformation of the elastic structure, whereby further the vibration period is configured to correspond to a basic resonance of the elastic structure, the exciting burst comprising a determined number of resonance cycles.

2. The method of claim 1, wherein the step of exciting comprises excitations' duration shorter than a half period, the duration being defined as period/n, where n is an integer.

3. The method of claim 1, wherein the alternating electrical driving signal is derived from an AC or RF source through a simple device containing diodes for splitting the signal into a branch with negative voltages, and a branch with positive voltages.

4. The method of claim 1, wherein a thickness of the piezoelectric thin film ranges from 10 nm to 10 µm.

5. The method of claim 1, wherein the elastic layer of the flexural structure is a thin film single or multilayer comprising Si₃N₄/SiO₂, SiC, or machined from a silicon substrate using silicon micromachining in techniques with dry etchers and silicon on insulator (SOI) substrates.

6. An acoustic device, comprising
an elastic layer supporting at least 2 types of parallel plate electrode capacitors formed with a piezoelectric thin film on one side of the elastic layer, the elastic layer being anchored with anchors in a frame structure consisting mainly of a thicker substrate material, the piezoelectric thin film comprising ferroelectric materials;
wherein the first type of parallel plate electrode capacitor has on a first side a first type of electrode and the second type of parallel plate electrode capacitor has a second type of electrode electrically disjoined from the first type of electrode, and the first and second type of parallel plate electrode capacitors have a common electrode on a second side opposite to the first side, and configured to face both the first type of electrode and the second type of electrode;
wherein the common electrode carries the piezoelectric film, which comprises on its surface opposite to the common electrode the first type and the second type of electrodes that are separated on the surface, and wherein a shape of each of the first and the second of the electrodes follows a required distribution of signs of curvature radii to accomplish an optimal excitation of a desired mechanical vibration of the elastic layer;
wherein the first type and the second type of electrodes are situated at the interface between elastic and piezoelectric layer, and the common electrode on top of the piezoelectric layer.

7. The acoustic device of claim 6, wherein the anchors are a determined number of narrow bridges.

8. The acoustic device of claim 6, wherein each of the anchors comprises a complete clamping around a border of the elastic layer.

9. The acoustic device of claim 6, wherein the piezoelectric film is either one of polar or ferroelectric nature.

10. The acoustic device of claim 6, wherein a thickness of the piezoelectric thin film ranges from 10 nm to 10 µm.

11. The acoustic device of claim 6, wherein the elastic layer of the flexural structure is a thin film, a single or multilayer comprising Si₃N₄/SiO₂, SiC, or machined from a silicon substrate using silicon micromachining in techniques with dry etchers and silicon on insulator (SOI) substrates.

12. The acoustic device of claim 6, being embedded in a gaseous, liquid, or solid environment for the emission of acoustic waves; and the reception of acoustic waves.

13. An acoustic device comprising a flexural elastic structure which is excited to resonance by electric fields across a piezoelectric layer sub-divided into a set of parallel plate capacitors, leading to an optimal deformation of an elastic structure by the piezoelectric stress for the emission of emitting acoustic waves with the elastic structure towards an object in an adjacent medium on one side; the elastic structure comprising a piezoelectric layer sub-divided into a set of parallel plate capacitors in which the application of an AC electric field causes a pattern of induced piezoelectric stress; the piezoelectric parallel plate capacitors being grouped into 2 types, such that
at maximum amplitude within a half-period, one type covers the area of compressive stress induced by the electric field in the piezoelectric layer, the other type covers the area of tensile stress in the piezoelectric layer; the elements of the same group are poled in the same way; the elements of different groups either in the opposite direction; the two excitation signals have opposite polarity combined with a phase difference of 180 ° or they are poled in the same direction, and the two excitation signals have same polarity combined but a phase difference of 180 °;
the acoustic waves are reflected from the object in front of the elastic structure and received on the elastic structure, whereby the reflected acoustic waves deform the elastic structure, which is
enabled for being excited to resonance by an incoming acoustic wave arriving through an adjacent medium from one side and experiencing a resonant deformation; the elastic structure comprising a piezoelectric layer sub-devided into a set of parallel plate capacitors in which the resonant deformation causes a pattern of induced stress that creates piezoelectric charges and voltages; the piezoelectric parallel plate capacitors being grouped into 2 types, such that
at maximum amplitude within a half-period, one type covers the area of compressive stress induced by the wave in the piezoelectric layer, the other type covers the area of tensile stress in the piezoelectric layer; the elements of the same group are poled in the same way; the elements of different groups either in the opposite direction; and a sum of the two signal is combined for the output signal, or they are poled in the same direction, and the difference of the signal is used as output signal.

14. The acoustic device of claim 13, further comprising
extracting means configured for extracting from the signals and storing digitally for signal treatment, any one of the list comprising signal peaks, signal delays, and complete wave form; and
whereby a plurality of piezoelectric elements of a micro-system is arranged in an array in order to increase the information density on the objects to be detected.

15. A use of the devices of any one of claims 13 and 14, comprising refl
ecting the acoustic waves from the object in front of the elastic structure and receiving the acoustic waves on the elastic structure, whereby the reflected acoustic waves deform the elastic structure,
creating electrical currents and voltages signals by a direct piezoelectric effect in the piezoelectric layer, and
opening switches after each of the excitation bursts, to allow the electrical currents and voltages signals to pass to an amplifying circuit, where the signals are added or subtracted, depending on a polarization given to the two types of parallel plate electrode elements.

16. An electronic circuit configured to drive at least a first type and a second type of piezoelectric elements of a microsystem, the first type and the second type of piezoelectric elements being configured to be part of any one or a combination of items of a list comprising a cantilever, a bridge, a diaphragm, and the piezoelectric elements being configured in a form of parallel plate electrode capacitors with an electrode system of bottom and top electrodes, whereby the first type and the second type of piezoelectric elements comprise at least a first type and a second type of electrodes, and that are adapted for a desired deformation of the at least first type and second type of piezoelectric elements, meaning that

the first type of electrodes extends over a surface that exhibits a first curvature of a first sign during a maximal deflection in a given mode, and that

the second type of electrodes exhibits a second curvature for a second sign during the maximal deflection at the same moment as the first one,

the electronic circuit being further enabled to produce a unipolar pulse signal for driving, that gives the voltage input to the piezoelectric elements only in a half-period when the element needs to produce tensile stress, and in a half period requiring compressive stress, gives a voltage supply of substantially zero voltage.

17. The electronic circuit of claim 16, further comprising
a first supply line for the half-period when the first or the second piezoelectric element needs to produce tensile stress, and a second supply line for the half-period requiring compressive stress in the first or the second piezoelectric element, the first supply line being connected to the first type of electrodes, the second supply line to the second type of electrodes, in each case to one of the 2 types of electrodes called the active ones,
a third supply line which is common for both the first type and the second type of piezoelectric elements, usually called "common" or ground (GND), the electric field defined between the first signal and the common in type 1 elements, and between the second signal and the common between in type 2 elements,
the electronic circuit further configured such that an electric field in the second type of piezoelectric element points in opposite direction, as compared to an electric field in the first type of piezoelectric elements.
The electronic circuit of claim 20, further comprising a multitude of the described electronic circuits to drive an array of pMUT single transducers (pMUT cell), allowing also for addressing parts of the array cells, and with different phases with respect to the time in order to allow for beam steering of the emitted wave.

18. The electronic circuit of claim 17, further enabled for a sensing mode in which ultrasonic waves are intended to be detected by type 1 and type 2 elements of opposite poling directions leading to the generation of the same sign of voltage and current, wherein the electronic circuit is configured to sum up the signals from the two types for the signal production, by any one of the item in the following list: a connection of the lines by a switch that occurs when opening the receive line, a summing up of the signals after a preamplifier.

19. The electronic circuit of claim 16, further comprising
a first supply line for the first half-period (V0), and a second supply line for the second half-period (V180°), whereby the first supply line and the second supply line are of the same polarity, being either always positive, or always negative, the first supply line is connected to the first type of piezoelectric elements, the second supply line is connected to the second type of piezoelectric elements, a third supply line which is common for the first and the second piezoelectric elements (GND), and defines an electric field between V0 to GND and V180° to GND,
the signals detected by type 1 and type 2 elements having the same poling directions leading to the generation of opposite sign of voltage and current, wherein the electronic circuit is configured to measure the difference between the signals from the two types for the signal generation.

20. The electronic circuit of claim 19, further enabled for a sensing mode, in which ultrasonic waves are intended to be detected, wherein the electronic circuit is configured such that
the generated voltage and current of the first type of electrodes have the opposite sign of the generated voltage and current if the second type of electrodes,
a difference of the signals from the first type of electrode and the second type of electrode is used for signal production, after the switch opening the receive lines, whereby the signals are given to two ports of an operational amplifier, thus amplifying the difference between the two signals.

21. A use of the method according to claim 1 in any one of applications of the following list: finger print detectors, flow sensors, bio-medical sensors, micropumps for fluidic elements in biomedical applications, non-destructive testing.

Drawing