Patent Description:
As known, ultrasonic transducers are devices capable of emitting and receiving acoustic waves (in particular, ultrasounds at a frequency comprised between <NUM> and <NUM>) in fluid (liquid or gaseous) and/or solid propagation media, by converting electro-mechanical, acoustic or light energy.

In particular, MUT ("Micromachined Ultrasonic Transducers") semiconductor ultrasonic transducers are known that are manufactured using silicon bulk micromachining and/or surface micromachining processes. The MUT transducers comprise membranes capable of vibrating in both acoustic waves transmission condition and acoustic waves reception condition; currently, the membrane vibrational operation is based on piezoelectric effects (piezoelectric MUTs, PMUTs) or electrostatic effects (capacitive MUTs, CMUTs).

The efficiency of the emitted/received energy electroacoustic conversion, the frequency response gain and the bandwidth are identification parameters of the MUTs. They depend both on factors typical of the MUTs (such as geometric structure and materials of the transducers, which determine a mechanical impedance of the MUT) and on factors typical of the acoustic wave propagation media (such as density of the propagation media and speed of conveyed sound, which determine an acoustic impedance thereof).

Different air ultrasonic applications are known, such as distance measure and object and environment imaging, based on the detection of the pulse echo, i.e. on the transmission of acoustic waves (e.g., of an ultrasonic pulse) and on the reception of ultrasonic echoes generated by the reflection and diffusion of the acoustic waves in the environment. The spatial distribution and harmonic content of ultrasonic echoes are caused by density variations in the propagation medium, and are indicative of objects and/or inhomogeneities present therein.

In this application, the usable frequencies also depend on the dimensions of the objects or on their characteristics; in particular, for the detection of objects having very small dimensions (e.g. up to <NUM>), very high-frequency acoustic waves are to be used, e.g. of the order of MHz.

Another example of air ultrasonic application is ultrasonic communication, which involves transmitting and receiving a modulated signal through an acoustic channel. In this application, the bandwidth directly affects the measurement resolution (pulse echo detection) or data transmission/reception (ultrasonic communication).

Another possible ultrasonic application is the minimally invasive treatment of patient tissues, where focused ultrasonic waves are used for the ablation of tissues, which are destroyed due to high heat. In this case, high-power ultrasonic waves are focused towards a target point, so as to obtain a rise in temperature up to <NUM>-<NUM>.

In the article "<NPL> it is emphasized that wireless power transfer (WPT) through acoustic waves may achieve higher efficiencies than inductive coupling when the distance exceeds the dimension of the transducer by several times. This document teaches that arrays of ultrasonic phased elements have high power concentration on the receivers and allow the efficiency of power transfer to be increased.

The article "<NPL>, describes a PMUT device formed by an array of piezoelectric micromachined ultrasonic transducers (PMUTs) having the shape of concentric rings and demonstrates how this configuration allows manufacturing of miniaturized devices capable of generating very high acoustic and high focused intensities.

<NPL>, discloses a transducer having two cavities and coupling two PMUT designs to obtain different coupling behaviours.

<NPL>, teaches an array of PMUT elements arranged side-by-side, wherein, to increase the membrane displacement or output pressure, without a significant change in the resonant frequency, insulation trenches are formed between the cell.

<NPL>, discloses an ultrasonic transducer array, with a central membrane array and a single concentric ring and focuses on a single resonant frequency.

<CIT> discloses a PMUT element, with a single membrane working at a single frequency; the membrane is suspended through Si arms to reduce mechanical cross-talk with the outside world.

<CIT> discloses an ultrasonic transducer having a single membrane working at a single frequency where the anchor part is weakened through a buried cavity.

<CIT> discloses a pressure sensor formed by a single membrane and including a piezo-resistor.

Starting therefrom, it is desirable to manufacture piezoelectric micromachined ultrasonic devices that may work both in air and in aqueous environment, with high detection accuracy, even at much greater distances than the wavelengths used, in various fields of application, in a flexible manner.

The aim of the present invention is to provide a piezoelectric micromachined ultrasonic device that meets the indicated requirements.

According to the present invention, a PMUT ultrasonic transducer and the manufacturing process thereof are provided, as defined in the attached claims.

For a better understanding of the present invention, some embodiments thereof are now described, purely by way of non-limiting example, with reference to the attached drawings, wherein:.

The following description refers to the arrangement shown; consequently, expressions such as "above", "below", "top", "bottom", "right", "left" relate to the attached figures and are not to be intended in a limiting manner.

The present invention originates from inventors' research performed on PMUT ultrasonic transducers with concentric ring structure.

To this end, reference is made to <FIG> showing a test structure <NUM> formed in a body <NUM> of semiconductor material, here monocrystalline silicon.

The body <NUM> has a face 2A having a piezoelectric element <NUM>, of annular-shape, deposited thereon, and accommodates a cavity <NUM>, also of annular-shape and underlying the piezoelectric element <NUM>.

The cavity <NUM> has an internal diameter D_int and a width w_c; the piezoelectric element <NUM> has a width w_r, smaller than the width w_c of the cavity <NUM>.

In practice, the body <NUM> defines a membrane <NUM> overlying the cavity <NUM> and carrying the piezoelectric element <NUM>.

In the inventors' studies, it has been noted that the resonance frequency of the test structure <NUM> of <FIG>, in a first approximation, is substantially independent of the internal diameter D_int of the cavity <NUM> and mainly depends on the width w_c of the cavity <NUM>.

In particular, <FIG> shows the substantial invariability of the resonance frequency from the internal diameter D_int of the cavity <NUM> in test structures having a constant width w_c of the cavity <NUM>, here equal to <NUM>, and different internal diameter D_int of the cavity <NUM>.

<FIG> instead shows that the resonance frequency strongly depends on the width w_c of the cavity <NUM>, in test structures having a constant internal diameter D_int (here equal to <NUM>) and different widths w_c of the cavity <NUM>.

The inventors' studies have also highlighted that the acoustic pressure measured on the surface (in proximity to the face 2A of the body <NUM>) significantly depends on the width w_c of the cavity <NUM> and, to a lesser extent, on the internal diameter D_int.

Taking into account the above, the inventors have invented an acoustic transducer capable of emitting and detecting multi-frequency acoustic waves, as discussed hereinbelow with reference to <FIG>.

<FIG> show an acoustic transducer <NUM> of PMUT type having concentric rings.

In detail, the acoustic transducer <NUM> comprises a body <NUM> of semiconductor material, such as silicon, for example with a monocrystalline structure, having a face 12A.

The body <NUM> accommodates a plurality of concentric buried cavities 12A-<NUM>; of these, the concentric buried cavity 12A arranged in a central position (hereinafter also referred to as central buried cavity 12A) has a circular shape and the other concentric buried cavities 12B-<NUM> have an annular shape. In practice, the concentric buried cavities 12B-<NUM> are spaced outward in the radial direction from the central buried cavity 12A.

The concentric buried cavities 12A-<NUM> (generically referred to as the concentric buried cavities <NUM> when it is not desired to distinguish them) have widths w1-w8 increasing in the radial direction, here from the central buried cavity 12A, where w1 represents the radius of the central buried cavity 12A.

According to an embodiment, the widths w1-w8 of the concentric buried cavities 12A-<NUM> are chosen so that they have a linearly decreasing resonance frequency (see <FIG>).

For example, w1 = <NUM>; w2 = <NUM>; w3 = <NUM>; w4 = <NUM>; w5 = <NUM>; w6 = <NUM>; w7 = <NUM> and w8 = <NUM>.

In general terms, the widths wi of the first cavities may vary between a minimum value of <NUM> and a maximum value of <NUM>.

The concentric buried cavities <NUM> are mutually spaced in a uniform manner, for example by <NUM>-<NUM>.

The concentric buried cavities <NUM> also have a depth e.g. of <NUM>-<NUM>, in particular of <NUM>.

The concentric buried cavities <NUM> are coplanar with each other and extend at a short distance from the face 11A of the body <NUM>. For example, the distance d1 between the top side of the concentric buried cavities <NUM> and the face 11A of the body <NUM> (<FIG>) is comprised between <NUM> and <NUM>, in particular <NUM>.

The concentric buried cavities 12A-<NUM> delimit from below, inside the body <NUM>, respective thin membranes 13A-<NUM>; therefore each thin membrane 13A-<NUM> extends between a respective concentric buried cavity 12A-<NUM> and the face 11A of the body <NUM>.

A plurality of piezoelectric elements 15A-<NUM> extends on the face 11A of the body <NUM>. The piezoelectric elements 15A-<NUM> (generically referred to as the piezoelectric elements <NUM> when it is not desired to distinguish them) each extend on a respective thin membrane 13A-<NUM> (see in particular the enlarged detail of <FIG>, showing the concentric buried cavities 12A-12C, the respective thin membranes 13A-13C and the respective piezoelectric elements 15A-15C).

The piezoelectric elements 15A-<NUM> have a circular / annular shape similar to that of the respective concentric buried cavities 12A-<NUM> and therefore comprise, in the embodiment shown, a piezoelectric element 15A, of circular shape and central position, also referred to as the central piezoelectric element 15A, and a plurality of piezoelectric elements 15B-<NUM>, of annular shape and arranged concentrically with each other and with the central piezoelectric element 15A.

In practice, each piezoelectric element 15A-<NUM> forms, with the respective concentric buried cavity 12A-<NUM>, a transducer unit, indicated by 40A-<NUM>.

As visible in particular in <FIG>, each piezoelectric element <NUM> comprises a layer stack formed by a bottom electrode <NUM>, a piezoelectric layer <NUM> and a top electrode <NUM> and is electrically insulated from the body <NUM> by an insulating layer <NUM>.

A passivation layer <NUM> may cover, upwardly and laterally, the layer stacks <NUM>-<NUM>. For example, as shown in <FIG>, showing a detail of the electrical connection of the bottom electrode <NUM> and of the top electrode <NUM>, the protection layer <NUM> may be formed by a double layer (first and second protection layers 21A and 21B), mutually superimposed.

The piezoelectric layer <NUM> may be formed, for example, by PZT (Pb, Zr, TiO3) or by other piezoelectric material, i.e. capable of transforming electrical energy into vibrational mechanical energy or vice versa.

The insulating layer <NUM> may be of TEOS (tetraethylorthosilicate); the bottom electrode <NUM> is of electrically conductive material, e.g. of titanium (Ti) or platinum (Pt); the top electrode is of electrically conductive material, e.g. of TiW; the protection layers 21A and 21B are of insulating material. For example, the first protection layer 21A may be of silicon oxide and the second protection layer 21B may be of silicon nitride deposited by CVD (Chemical Vapour Deposition).

A deep buried cavity <NUM> extends below some of the concentric buried cavities <NUM> and, precisely, of the concentric buried cavities <NUM> having smaller width, for the reasons explained hereinbelow. In the embodiment shown, the deep buried cavity <NUM> extends below the central buried cavity 12A and the two concentric buried cavities 12B, 12C adjacent thereto.

In the embodiment shown, therefore, the deep buried cavity <NUM> is centrally arranged and has a circular shape; for example, it may have a radius R of <NUM> (diameter of <NUM> pm).

In general, the second buried cavity <NUM> has a diameter comprised between <NUM> and <NUM> and a depth comprised between <NUM> and <NUM>.

Furthermore, the top surface of the deep buried cavity <NUM> extends at a distance d2 from the face 11A of the body <NUM> which is comprised between <NUM> and <NUM>, for example of <NUM>.

The deep buried cavity <NUM> has a depth e.g. of <NUM>-<NUM>, in particular of <NUM>, and delimits on the bottom a thick membrane <NUM> having a thickness d2, in the example considered, of <NUM>.

As shown in <FIG>, the bottom electrode <NUM> and the top electrode <NUM> of each piezoelectric element <NUM> are connected to respective conductive tracks <NUM>, <NUM> (also referred to as the bottom conductive track <NUM> and the top conductive track <NUM>), for example formed in the same material layer as the respective electrodes <NUM>, <NUM>.

The conductive tracks <NUM>, <NUM> may be separated, for example as shown in <FIG>. Here, in the first part of the conductive tracks <NUM>, <NUM> (from the respective electrodes <NUM>, <NUM>) of each piezoelectric element <NUM>, the conductive tracks <NUM>, <NUM> are superimposed, so that only the top conductive track <NUM> is visible.

In particular, in the first part of the conductive tracks <NUM>, <NUM>, near the electrodes <NUM>, <NUM>, the conductive tracks <NUM>, <NUM> of the piezoelectric elements 15A-15C (as well as of the piezoelectric elements 15D-<NUM>, not visible) extend through interruptions in the piezoelectric elements <NUM> that are respectively more external.

Alternatively, <FIG>, the bottom electrodes <NUM> of all the piezoelectric elements <NUM> may be connected to each other (for example to ground) and the top electrodes <NUM> of all the piezoelectric elements <NUM> may be connected to each other. In this case, the rings of the piezoelectric elements 15B-<NUM> may be complete, without interruptions, and may be biased at a common biasing voltage V, for example provided by a voltage generator <NUM>, for example external and coupled to the top conductive tracks <NUM> through a voltage application pad <NUM>.

In both cases, the conductive tracks <NUM>, <NUM> are connected to electronics <NUM> (<FIG>).

The electronics <NUM> comprises, in a known manner and not shown, a power supply unit, configured to generate the biasing voltages, and a control unit, configured to control the transduction steps (acoustic wave emission and/or acoustic wave detection).

The electronics <NUM> may be integrated into the body <NUM> together with the acoustic transducer <NUM> or may be integrated in a separate die and coupled to the conductive tracks <NUM>, <NUM> through pads, as shown schematically in <FIG>.

In use, in a known manner, the application of a variable voltage, for example an alternating voltage, to the piezoelectric elements 15A-15B, causes generation of a strain and vibration of the respective thin membranes 13A-<NUM> and emission of acoustics waves in the medium where the acoustic transducer <NUM> is located (acoustic wave generator operating mode).

Conversely, in the rest condition of the thin membranes 13A-<NUM> (in presence of a DC bias or in absence of applied voltage, depending on the material of the piezoelectric layer <NUM>), an acoustic wave hitting the thin membranes 13A-<NUM> causes vibration of the latter and therefore the onset of a variable stress in the relative piezoelectric elements 15A-<NUM>; these elements therefore generate corresponding electrical signals usable by the electronics <NUM> for detecting the parameters of the received acoustic wave (acoustic wave receiver operating mode).

The acoustic transducer <NUM>, due to the different widths of the concentric buried cavities 12A-<NUM>, is capable of rapidly switching from the acoustic wave emitter operating mode to the acoustic wave detector operating mode.

In fact, as discussed above, due to the different widths of the thin membranes 13A-<NUM>, each transducer unit 40A-<NUM> has a different resonance frequency.

For example, with the widths w1-w8 and the dimensions of the thin membranes 13A-<NUM> indicated above, for silicon membranes, the resonance frequency values indicated in the following table I have been experimentally detected:.

Due to the different resonance frequencies of the thin membranes 13A-<NUM> and to their adjacent arrangement, the acoustic transducer <NUM> has a resonance frequency comprised between the minimum frequency value (resonance frequency of the maximum width cavity, here the concentric buried cavity <NUM>) and the maximum frequency value (resonance frequency of the minimum width cavity, here the first cavity 12A) and therefore is broadband.

By neglecting for the moment the effect of the thick membrane <NUM>, as a result, the acoustic transducer <NUM> has a reduced quality factor Q with respect to the single transducer units 40A-<NUM> and therefore tends to rapidly damp oscillations.

This is advantageous, for example, when the acoustic transducer <NUM> is used as a distance meter.

In fact, in this case, the acoustic transducer <NUM> may be controlled to operate, in a first step, in the generator operating mode and generates a high-frequency acoustic wave and, in a second step, in the receiver operating mode, and detects the echo of the emitted acoustic wave.

To this end, in the first step, the thin membranes 13A-<NUM> are caused to vibrate by the respective piezoelectric elements 15A-<NUM>, but they are rapidly damped, due to the high band of the system, and are rapidly caused to rest, to be able to operate in a detection condition.

In this manner, the acoustic transducer <NUM> may perform distance measurement operations very rapidly, without having to wait long times between the two operating steps. As a result, it is capable of operating even at a short distance.

According to another embodiment, the transducer units 40A-<NUM> may be controlled individually and/or in selected groups, applying control voltages only to the selected transducer unit(s) 40A-<NUM>. This may be useful in some applications, such as for example for focusing an acoustic beam for tumor ablation in the brain.

As mentioned above, in the acoustic transducer <NUM>, the deep buried cavity <NUM> allows the acoustic waves emitted at high frequency to be amplified.

In fact, the acoustic power emitted by a PMUT acoustic transducer is subject to a damping dependent on the wave frequency and on the propagation medium, according to the equation: <MAT> wherein:.

For example, at the frequency of <NUM>, the coefficient α has a value of <NUM> in water and <NUM> in air.

In general, therefore, the waves emitted at higher frequencies are subject to greater attenuation than those at lower frequency. Furthermore, if the acoustic transducer <NUM> works at frequencies of the order of MHz and the propagation medium is air, the attenuation is rather high.

The deep buried cavity <NUM> obviates this problem. In fact, the acoustic pressure of a wave emitted by a membrane depends on the square of the ratio between the thickness and the radius of the membrane, as well as on its stiffness (Young's Modulus and on the density of the material of the membranes <NUM>, <NUM>, here monocrystalline silicon). By designing the thick membrane <NUM> so that it has a resonance frequency in the operational field of the narrower membranes 13A-13D (and where the attenuation in air is maximum), the acoustic wave emitted by the narrower membranes 13A -13D may be amplified.

For example, for a thickness of the thick membrane <NUM> of <NUM> (distance d2 in <FIG>) and diameter 2R of <NUM>, the thick membrane <NUM> has a resonance frequency of <NUM> and therefore acts so as to amplify the oscillations of the membranes 13A-13C/D that operate at similar frequencies.

In practice, the thick membrane <NUM> resonates with the overlying thin membranes 13A-<NUM> which are actuated. Due to the large diameter of the thick membrane <NUM>, the latter is subject to a high deformation which originates a very high acoustic pressure at a near resonance frequency, in the example considered, at <NUM>. Since this frequency is inside the frequency band where the acoustic propagation is strongly attenuated by the air (or other medium which the acoustic transducer immersed in and which has strong attenuation at high-frequency), the membrane <NUM> contributes to making the acoustic transducer <NUM> operative even in such operating conditions.

The acoustic transducer <NUM> may be formed as shown in <FIG>.

In <FIG>, a wafer <NUM> of semiconductor material, such as monocrystalline silicon, has been subject to first processing steps for forming the buried deep cavity <NUM>.

For example, the deep buried cavity <NUM> may be formed with the process described in <CIT> (corresponding to <CIT>), according to which, starting from an initial silicon wafer, a plurality of adjacent and communicating trenches are formed, separated by silicon pillars; then an epitaxial growth is performed in an oxidizing environment, which leads to the top closure of the trenches and to a migration of the silicon atoms of the pillars, with formation of the buried cavity <NUM>.

Then, <FIG>, the concentric buried cavities <NUM> (only some shown in <FIG>) are formed in the epitaxial layer <NUM> grown on the buried cavity <NUM>, by using the same process described above.

Thus, the thin membranes 13A-<NUM> (only some shown in <FIG>) and the thick membrane <NUM> are also formed.

The wafer thus obtained, indicated by <NUM>, is formed by a substrate <NUM> (intended, after dicing the wafer <NUM>, to form the body <NUM>) having for example a depth of <NUM> and a face 51A.

Subsequently, <FIG>, the piezoelectric elements 15A-<NUM> (only some shown) are formed on the face 52A of the substrate <NUM>, by depositing and defining the respective layers.

After dicing the wafer <NUM>, a plurality of acoustic transducers <NUM> are obtained (<FIG>).

According to another embodiment, shown in <FIG>, the membranes 13A-<NUM> and <NUM> are of porous silicon.

In detail, with reference to <FIG>, a wafer <NUM> comprises a substrate <NUM> of semiconductor material, here silicon, for example monocrystalline, and a deep sacrificial region <NUM>, thermally grown or deposited on the substrate <NUM>. The deep sacrificial region <NUM> may be, for example, of silicon oxide, BPSG (Boron Phosphorous Silicon Glass) or silicon nitride.

In <FIG>, a first structural layer <NUM> of porous silicon is deposited on the substrate <NUM> and covers the deep sacrificial region <NUM>. For example, the first structural layer <NUM> may be deposited by an LPCVD (Low Pressure Chemical Vapour Deposition) process from pure silane gas, at a pressure of <NUM> mtorr at <NUM>. The first porous silicon layer <NUM> may have a thickness comprised between <NUM> and <NUM>.

In a manner known to the person skilled in the art, the porous silicon of the first structural layer <NUM> is characterized by the presence of micropores which make it permeable to liquids, in particular to etchants, such as, in case of a deep sacrificial region <NUM> of silicon oxide, HF (hydrofluoric acid).

Then, <FIG>, the deep sacrificial region <NUM> is removed by HF etching through the first structural layer <NUM>. Since this type of etching allows the removal of silicon oxide, but does not remove silicon, at the end of the etching the deep cavity <NUM> is formed in the wafer <NUM>.

In <FIG>, a second structural layer <NUM>, again of porous silicon, is deposited on the first structural layer <NUM>. The second structural layer <NUM> may be deposited using the same parameters indicated above for the first structural layer <NUM>.

The first structural layer <NUM> and the second structural layer <NUM> together form an intermediate structural layer <NUM>, having a thickness equal to about the distance between the deep buried cavity <NUM> and the concentric buried cavities 12A-<NUM>.

In <FIG>, a plurality of superficial sacrificial regions <NUM> (only some shown) corresponding to the concentric buried cavities 12A-<NUM> to be formed are created on the surface of the intermediate structural layer <NUM>. The superficial sacrificial regions <NUM> may be, for example, of thermally grown or deposited silicon oxide.

In <FIG>, a third structural layer <NUM> of porous silicon is deposited on the intermediate structural layer <NUM> and covers the superficial sacrificial regions <NUM>. The third structural layer <NUM> may be deposited using the same parameters indicated above for the first structural layer <NUM> and may have thickness of about <NUM>, and in any case corresponding to the desired thickness of the thin membranes 13A-<NUM>.

The third structural layer <NUM> and the intermediate structural layer <NUM> together form a final structural layer <NUM>, having a thickness equal to the distance d2 (<FIG>).

Then, <FIG>, the superficial sacrificial regions <NUM> are removed by HF etching through the third structural layer <NUM>, forming the concentric buried cavities 12A-<NUM>.

In <FIG>, the piezoelectric elements 15A-<NUM> (only some of which shown) are formed, by depositing and defining the respective layers.

The acoustic transducer <NUM> and the manufacturing processes described herein have numerous advantages.

In fact, forming transducer units 40A-<NUM> with thin membranes 13A-<NUM>, concentric with each other and having variable dimensions, as described above, allows a multifrequency acoustic transducer, with high detection accuracy and quick response, to be obtained.

Thanks to the presence of the deep buried cavity <NUM>, the acoustic transducer <NUM> is capable of emitting high-power acoustic waves, detectable at a long distance, and has high sensitivity even when operating in media having high attenuation, such as air, and high frequencies. Advantageously, this amplification occurs by exploiting the portion of the body <NUM> underlying the thin membranes 13A-<NUM>, and therefore without requiring further space for forming the thick membrane <NUM>. This contributes to maintaining the dimensions of the acoustic transducer <NUM> small.

By operating several transducer units 40A-<NUM> simultaneously, wide band acoustic signals, with rapid vibration damping, may be emitted and detected.

The acoustic transducer <NUM> may also be manufactured using usual semiconductor technique process steps and therefore at reduced costs.

It also has small overall dimensions which allow it to be used in portable devices.

Finally, it is clear that modifications and variations may be made to the acoustic transducer and to the manufacturing process described and illustrated herein without thereby departing from the scope of the present invention, as defined in the attached claims.

For example, the concentric buried cavities 12A-<NUM> may be in different number.

They may also have dimensions variable in a different manner than shown. In particular, according to a different embodiment, the cavities might have a decreasing dimension starting from the center. In this case, the deep buried cavity <NUM> might have an annular shape and extend below the first outermost cavities.

Or, the concentric buried cavities <NUM> might have variable width in groups, so that the buried cavities of each group have the same width, and each group is arranged between a first group of larger dimension cavities and a second group of smaller dimension cavities.

Alternatively, the dimension sequence of the first cavities <NUM> might not be monotonous, and therefore not always increasing (or decreasing, depending on the direction of observation).

The number of concentric buried cavities <NUM> having the deep buried cavity <NUM> extending therebelow may vary, in particular depending on the desired resonance frequencies.

The central buried cavity <NUM> might also have an annular shape.

Claim 1:
A PMUT acoustic transducer (<NUM>), comprising:
a body (<NUM>) of semiconductor material having a face (11A) ;
a plurality of first buried cavities (12A-<NUM>) arranged concentrically to each other and extending at a distance from the face (11A) of the body (<NUM>);
a plurality of first membranes (13A-<NUM>) formed by the body (<NUM>) and including a central membrane (13A) and a plurality of peripheral membranes (13B-<NUM>) surrounding the central membrane (13A) and spaced outward in the radial direction, each first membrane (13A-<NUM>) extending between a respective first buried cavity (12A-<NUM>) of the plurality of first buried cavities and the face (11A) of the body (<NUM>);
a plurality of piezoelectric elements (15A-<NUM>) on the face of the body, each piezoelectric element extending above a respective first membrane (13A-<NUM>) of the plurality of first membranes;
wherein the first membranes (13A-<NUM>) have different widths (w1-w8), defining respective different resonance frequencies.