Patent Description:
As known, proximity sensors comprise time-of-flight sensors. In particular, the time-of-flight sensors may be ultrasonic transducers, for example made using MEMS ("Micro Electro-Mechanical Systems") technology.

In use, ultrasonic transducers are immersed in a fluid (generally having low density, e.g. air) and are controlled to generate and detect ultrasonic acoustic waves (e.g., with a frequency comprised between <NUM> and <NUM>, for example equal to <NUM> or <NUM>). In detail, as exemplarily shown in <FIG>, the ultrasonic transducer T generates an ultrasonic wave (or emitted wave, indicated in <FIG> with the reference We) which, in the presence of objects O arranged along the acoustic wave propagation direction, is reflected forming an ultrasonic echo (or reflected wave, indicated in <FIG> with the reference Wr) detectable through the same ultrasonic transducer T; the time distance between the emission of the emitted wave We and the reception of the reflected wave Wr is indicative of a relative distance D between the ultrasonic transducer T and the detected object O. As a result, by measuring the time-of-flight, it is possible to have information about the object O to be detected. In greater detail, the ultrasonic transducer T comprises a membrane which, for example piezoelectrically or capacitively, is controllable to oscillate in order to generate the emitted wave We. This membrane is also configured to oscillate when the reflected wave Wr impinges on the ultrasonic transducer T; therefore this allows the reflected wave Wr to be detected, for example piezoelectrically or capacitively.

<FIG> shows, as a function of time, a displacement (indicated in <FIG> with the reference Xm) of the membrane of the ultrasonic transducer T with respect to a rest position of the membrane (i.e. corresponding to the position that the membrane assumes when it is not oscillating, and therefore when the ultrasonic transducer T is neither emitting nor detecting). As it may be noted, the graph of <FIG> shows a first waveform Xm,e indicative of the displacement of the membrane that generates the emission of the emitted wave We, and a second waveform Xm,r indicative of the displacement of the membrane generated by the reception of the reflected wave Wr in succession to each other. The first waveform Xm,e has a first portion with an oscillatory trend along a ring-up interval Tup, where the respective peaks of the first waveform Xm,e have a maximum amplitude increasing as a function of time, and a second portion consecutive to the first portion and with an oscillatory trend along a ring-down interval Tdown, where the respective peaks of the first waveform Xm,e have a maximum amplitude decreasing as a function of time (e.g., in an exponential manner). The ring-up interval Tup corresponds to the time interval necessary to cause the membrane to oscillate at the desired emission frequency of the emitted wave We, while the ring-down interval Tdown corresponds to the time interval necessary to interrupt the membrane oscillation (e.g., the ring-down interval Tdown ends when the membrane oscillation has a maximum amplitude smaller than a maximum threshold amplitude, for example comparable to measurement noise). The second waveform Xm,r has instead an oscillatory trend along an echo interval Techo, where the respective peaks of the second waveform Xm,r have a first maximum amplitude increasing and then decreasing as a function of time. Generally, between the emission of the emitted wave We and the reception of the reflected wave Wr (in other words, between the end of the ring-down interval Tdown and the beginning of the echo interval Techo) there is a blind-zone interval Tblind which, during the correct operation of the ultrasonic transducer T, is not null. In detail, the sum of the ring-up interval Tup, the ring-down interval Tdown and the blind-zone interval Tblind defines the time-of-flight TTOF of the ultrasonic transducer T. As a result, the blind-zone interval Tblind is correlated to the relative distance D of the object O with respect to the ultrasonic transducer T.

However, when the relative distance D is less than a threshold relative distance, the emitted wave We and the reflected wave Wr end up partially superimposing (i.e. the blind-zone interval Tblind is zero) making it difficult to discriminate the reflected wave Wr from the emitted wave We, and therefore complicating or making it impossible to measure the relative distance D. In other words, the threshold relative distance (also known as the blind region) is the minimum detectable relative distance D between the object O and the ultrasonic transducer T, without any loss of information due to the superposition of the emitted wave We and the reflected wave Wr.

The known ultrasonic transducers T are optimized during design to have a high figure of merit (e.g., greater than <NUM>), so as to reduce their energy loss during operation. However, a smaller bandwidth of the energy spectrum of the membrane oscillation and a lower damping of this oscillation correspond to an increasing figure of merit. In other words, a greater ring-down interval Tdown corresponds to a greater figure of merit, and therefore the blind-zone interval Tblind is reduced. Since the blind-zone interval Tblind decreases, the risk of superimposition of the emitted wave We and the reflected wave Wr and therefore the risk of not being able to detect the relative distance D increases. As a result, as the figure of merit increases, the minimum detectable threshold relative distances increase. This is mainly due to the fact that the ultrasonic transducer T usually operates immersed in a low-density propagation medium (e.g., air) which does not allow effective damping of the membrane oscillation especially at the working oscillation frequencies typical of the membrane (e.g., about <NUM>), and has as a result the fact that the known ultrasonic transducers T cannot detect objects O that are too proximate thereto.

Known solutions to increase the damping of the membrane oscillation comprise the use of passive dampers (e.g., layers of suitable polymeric material arranged on the membrane) or active dampers (e.g., supplying the membrane with counter-phase excitations following active pulses which cause the membrane to oscillate) or of algorithms to detect the decay variation, caused by the reflected wave Wr, of the envelope of the second portion of the first waveform Xm,e in order to compensate for the effects thereof. However, these solutions have numerous criticalities such as identifying suitable dampers which allow, even as the environmental conditions of the fluid having the ultrasonic transducer T immersed therein vary, a correct coupling between the impedance of the ultrasonic transducer T and the power dissipation of the fluid, or coupling problems between the impedance of the ultrasonic transducer T and the power dissipation of the fluid, or an excessive dependence on the shape of the second waveform Xm,r and on the object O to be detected.

Document <CIT> relates to an ultrasonic microintegrated acoustic transducer of a MEMS (MicroElectroMechanical System) type with reduced stress sensitivity and to the manufacturing process thereof.

Document <CIT> relates to a micropump MEMS device for moving or ejecting a fluid, in particular a microblower or flowmeter.

The aim of the present invention is to provide a MEMS ultrasonic transducer device and a manufacturing process of the same that overcome the drawbacks of the prior art.

According to the present invention, a MEMS ultrasonic transducer device and a manufacturing process of the same are provided, as defined in the annexed claims.

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

<FIG> schematically shows an ultrasonic transducer device <NUM> made using MEMS technology, and therefore hereinafter also referred to as the MUT ("Micromachined Ultrasonic Transducer") device. For example, the MUT device <NUM> is integrated into a die <NUM>.

The MUT device <NUM> comprises one or more MEMS ultrasonic transducer elements (or MUT elements) <NUM>. In the embodiment exemplarily shown in <FIG>, the MUT device <NUM> comprises a plurality of MUT elements <NUM> mutually arranged side by side, for example aligned on rows and columns. In the embodiment of <FIG>, the MUT elements <NUM> have a circular shape (see also <FIG>).

Each MUT element <NUM> is connected independently, through electrical connections <NUM> and pads <NUM>, shown schematically, to a control unit <NUM>, generally formed in a different die <NUM>, for example formed as an ASIC (Application Specific Integrated Circuit). Alternatively, the MUT elements <NUM> may be connected to groups, wherein the MUT elements <NUM> of a group are controlled separately and the groups are controllable separately, to reduce the number and simplify the electrical connections.

With reference to <FIG>, the die <NUM> comprises a semiconductor body <NUM> of semiconductor material, such as silicon, for example monolithic, having a first and a second main face 3A, 3B and forming the plurality of MUT elements <NUM>.

Each MUT element <NUM> comprises a central fluidic passage <NUM> and one or more lateral fluidic passages <NUM>. For each MUT element <NUM>, the central fluidic passage <NUM> and the one or more lateral fluidic passages <NUM> open on the second main face 3B by a central opening <NUM> and, respectively, one or more lateral openings <NUM> (one for each lateral fluidic passage <NUM>). Optionally, the lateral openings <NUM> may be connected to an external fluidic circuit not shown, for example to allow the suction of a liquid or a gas contained in a tank, or directly with the external environment, for example for the suction of air from the environment; similarly, the central openings <NUM> may be connected to an external fluidic circuit not shown or to the outside, according to the intended application.

As shown in detail in <FIG>, each MUT element <NUM> further comprises a first chamber <NUM>, arranged in proximity to the first main face 3A of the semiconductor body <NUM>, and a second chamber <NUM>, extending between the first chamber <NUM> and the second main face 3B of the semiconductor body <NUM>.

Each MUT element <NUM> further comprises lateral trenches <NUM> (one for each lateral opening <NUM>) extending between the lateral openings <NUM> and the second chamber <NUM>, and a central trench <NUM> extending between the first chamber <NUM> and the central opening <NUM>, through the second chamber <NUM>. In case each MUT element <NUM> comprises a single lateral trench <NUM>, the central trench <NUM> and the lateral trench <NUM> are arranged side by side to, at a distance from, each other; otherwise, in case each MUT element <NUM> comprises two or more lateral trenches <NUM>, the central trench <NUM> is interposed between the lateral trenches <NUM> (in particular, it is central with respect to the latter which are arranged, in bottom view, around the central trench <NUM>, i.e. they are radially external with respect to the central trench <NUM>).

Each lateral trench <NUM> defines a respective lateral fluidic passage <NUM>, and the central trench <NUM> defines the central fluidic passage <NUM>.

The lateral trenches <NUM>, the central trench <NUM> and the second chamber <NUM> define a fluidic recirculation path (not shown) that fluidically connects the first chamber <NUM> with the outside of the MUT device <NUM>. The fluidic recirculation path is configured to allow the recirculation of the fluid or gas present in the first chamber <NUM>, as better described below.

The portion of the semiconductor body <NUM> between each first chamber <NUM> and the first main face 3A of the die <NUM> forms a respective membrane <NUM> and on each membrane <NUM>, above the first main face 3A, a respective piezoelectric element <NUM> is arranged.

The first and second chambers <NUM>, <NUM> (here exemplarily having a circular shape) have centers aligned with each other along a central axis <NUM>. The membrane <NUM> is therefore concentric with the chambers <NUM>, <NUM> and, in the example considered, it also has a circular shape. According to the embodiment shown in <FIG>, the first chamber <NUM> has an area (in top view in <FIG>, or in bottom view in <FIG>) that is greater than the second chamber <NUM>, so that one of its peripheral zones, here circular ring shaped, protrudes laterally with respect to the second chamber <NUM>. Nevertheless and in a manner not shown, the first chamber <NUM> may also have an area that is smaller than or equal to the second chamber <NUM>.

The lateral trenches <NUM> extend vertically (perpendicularly to the first and second main faces 3A, 3B of the semiconductor body <NUM>) between the lateral openings <NUM> and the second chamber <NUM>. In the embodiment shown in <FIG>, the lateral openings <NUM> are four for each MUT element <NUM> and have a curved shape, with a circular crown sector, circumferentially aligned with each other. The lateral trenches <NUM> here have the same shape and area as the lateral openings <NUM> and have a smaller internal diameter than the second chamber <NUM> so that the latter surrounds them at a distance.

The central trench <NUM> of each MUT element <NUM> here has a cylindrical shape parallel and concentric to the central axis <NUM> of the respective MUT element <NUM>, has the same area (in bottom view) as the respective central opening <NUM>, and traverses the second chamber <NUM> to reach the first chamber <NUM>.

The piezoelectric element <NUM> may have the structure shown in the section of <FIG>.

In detail, the piezoelectric element <NUM> is formed above an insulating layer <NUM>, for example formed by the superposition of a thermally grown silicon oxide layer and a dielectric layer, as hereinafter discussed in detail with reference to <FIG>, and covers the entire first main face 3A of the semiconductor body <NUM>. Above the insulating layer <NUM>, there extends a stack <NUM> comprising: a bottom electrode <NUM>, of electrically conductive material, for example of titanium (Ti) or platinum (Pt); a thin-film piezoelectric region <NUM>; and an top electrode <NUM>, for example of TiW. The bottom electrode <NUM> is in electrical contact with a first contact line <NUM> (for example, it is formed in the same layer and patterned through known photolithographic steps). A first and a second dielectric layer <NUM>, <NUM>, for example of silicon oxide and silicon nitride deposited by CVD (Chemical Vapour Deposition) extend on the stack <NUM>. A second contact line <NUM> of conductive material, for example of aluminum and/or copper, extends above the dielectric layers <NUM>, <NUM> and into an opening <NUM> thereof, to electrically contact the top electrode <NUM>. Optionally, a passivation layer <NUM>, for example of silicon oxide and/or nitride deposited by CVD, covers the entire top surface of the die <NUM>, except for the electrical connection openings (above the pads <NUM>). In practice, the contact lines <NUM>, <NUM> form the electrical connections <NUM> of <FIG> and allow: in order to generate the emitted wave We by the MUT element <NUM>, the electrical connection of one of the electrodes <NUM>, <NUM> (e.g. the bottom electrode <NUM> of all the actuator elements <NUM>) to a reference potential, typically to ground, and the bias of the other of the electrodes <NUM>, <NUM> (e.g. of the top electrode <NUM>) to an alternating actuation voltage; or, in order to detect the reflected wave Wr, the acquisition of a detection potential difference between electrodes <NUM> and <NUM> induced by the impingement of the reflected wave Wr on the membrane <NUM>.

The MUT device <NUM> operates similarly to the known devices (e.g., to the ultrasonic transducer T of <FIG>). In particular, in use the MUT device <NUM> is surrounded by a propagation medium (a fluid such as liquid or gas, in particular air) wherein acoustic waves (in detail, ultrasonic waves), generated or detected by the MUT device <NUM>, propagate.

When the MUT elements <NUM> are operated in own transmission modes (i.e. they work as actuators), the membranes <NUM> are caused to vibrate by the piezoelectric elements <NUM> and the vibrations of the membranes <NUM> cause the generation and propagation in the acoustic wave propagation medium. In particular, the alternating actuation voltage (e.g., at a frequency comprised between about <NUM> and about <NUM> and with a voltage equal to about <NUM> V) is applied across the electrodes <NUM> and <NUM>. The application of the alternating actuation voltage between the electrodes <NUM> and <NUM> causes the contraction and expansion of the thin-film piezoelectric region <NUM> and the consequent deflection of the membrane <NUM> in the vertical direction, alternately moving away from and towards the chambers <NUM> and <NUM>, causing corresponding increases and decreases in the volume of the chambers <NUM> and <NUM>. These volume variations cause the propagation medium (hereinafter, air) present in the first chamber <NUM> to be moved pneumatically and to undergo recirculation with respect to the environment external to the MUT device <NUM>. In fact, such volume variations allow, in alternate succession to each other, partial depletion and filling steps of the first chamber <NUM>. Each partial depletion step of the first chamber <NUM> comprises the suction, towards the second chamber <NUM> and through the central trench <NUM>, of the air present in the first chamber <NUM> and subsequently its expulsion into the external environment through the central trench <NUM> and the lateral trenches <NUM>. Each partial filling step of the first chamber <NUM> instead comprises the suction, through the central trench <NUM> and the lateral trenches <NUM> and towards the second chamber <NUM>, of the air present in the external environment and subsequently its introduction into the first chamber <NUM> through the central trench <NUM>.

When the MUT elements <NUM> are operated in own reception modes (i.e. they work as sensors), the acoustic waves coming from the propagation medium (i.e. the reflected waves Wr) impinge on the membranes <NUM> and induce the vibration thereof. This induced vibration of each membrane <NUM> causes both the air recirculation in the first cavity <NUM>, similarly to what has been previously described, and a stress in the piezoelectric element <NUM> which in turn generates the detection potential difference between the electrodes <NUM> and <NUM>, indicative of the vibration of the membrane <NUM> induced by the impinging acoustic waves.

The reception mode and the transmission mode are alternative to each other: the MUT device <NUM> may therefore operate only in reception, only in transmission, or both in reception and transmission, but in time periods alternated to each other.

The MUT device <NUM> may be provided, according to an embodiment, as described hereinafter with reference to <FIG>, exemplarily showing the manufacturing of a single MUT element <NUM> (the other MUT elements <NUM> of the MUT device <NUM> are manufactured simultaneously, laterally arranged side by side, in a manner not shown).

Initially, <FIG>, the second chamber <NUM> of each MUT element <NUM> is formed in a wafer <NUM> of semiconductor material, for example monocrystalline silicon. For example, the manufacturing process described in the <CIT> (corresponding to <CIT>) and briefly summarized below may be used for the purpose.

In detail, above the wafer <NUM>, a mask <NUM> of resist is formed having honeycomb lattice openings. Using the mask <NUM>, an anisotropic etch of the wafer <NUM> is performed, so as to form a plurality of trenches <NUM>, having a depth of for example <NUM>, communicating with each other and delimiting a plurality of columns <NUM> of silicon.

Subsequently, <FIG>, the mask <NUM> is removed and an epitaxial growth is performed in a reducing environment. As a result, an epitaxial layer, for example N-type and having thickness <NUM>, grows above the columns <NUM>, closing the trenches <NUM> upwardly.

An annealing step is then performed, for example for <NUM> minutes at <NUM>, preferably in a hydrogen, or, alternatively, nitrogen atmosphere.

As discussed in the aforementioned patents, the annealing step causes a migration of the silicon atoms which tend to move to a lower energy position. As a result, also owing to the close distance between the pillars <NUM>, the silicon atoms of these migrate completely and the second chambers <NUM> are formed. Above the second chambers <NUM> a silicon layer remains, partially formed by epitaxially grown silicon atoms and partially by migrated silicon atoms and forming a closing layer <NUM> of monocrystalline silicon.

Then, <FIG>, another epitaxial growth is performed, with a thickness of a few tens of micrometers, for example equal to <NUM>, from the closing layer <NUM>. In this manner the wafer <NUM> comprises a first thick region <NUM> of monocrystalline silicon overlaying the second chambers <NUM>.

Subsequently, <FIG>, the first chambers <NUM> are formed in the first thick region <NUM>, for example by repeating the manufacturing process described in the European patent <CIT> and previously described with reference to <FIG>. In this manner, the wafer <NUM> has a first and a second face corresponding to the first and second main faces 3A, 3B of the semiconductor body <NUM> and accommodates, above the second chambers <NUM>, the first chambers <NUM> and the membranes <NUM>.

Then, <FIG>, using a masking layer not shown, holes <NUM> are formed, one for each first chamber <NUM>, which extend each from the first face 3A of the wafer <NUM> to the respective first chamber <NUM>. The holes <NUM>, having diameter of, for example, <NUM>, are preferably formed in proximity to an outer edge of the respective membranes <NUM>, so as not to alter the elastic features thereof.

Then, <FIG>, a thermal oxidation is performed, forming an oxide layer with a thickness, for example, of <NUM>. In particular, there are formed a coating oxide portion 116A on the sides of each of the first chambers <NUM>, a first and a second superficial oxide portion 116B and 116C on the faces 3A and, respectively, 3B of the wafer <NUM> and a closing oxide portion 116D within each of the holes <NUM>. The first chambers <NUM> are then completely covered by the coating oxide portions 116A and the holes <NUM> are closed by the closing oxide portions 116D.

Subsequently, <FIG>, on the first superficial oxide portion 116B, a dielectric material layer <NUM>, for example of TEOS (tetraethylorthosilicate), is deposited which, together with the first superficial portion 116B, forms the insulating layer <NUM> of <FIG>. The dielectric material layer <NUM> may have, for example, a thickness of <NUM>.

Then, the piezoelectric elements <NUM> are formed on the dielectric material layer <NUM>. For example, with reference to <FIG>, first the electrodes <NUM> and the first contact lines <NUM> are formed, using known deposition and masking techniques. Then, a thin-film piezoelectric layer (e.g. of PZT - Pb, Zr, TiO3) and an electrode layer are deposited in succession, which are defined by known masking and definition techniques so as to form the thin-film piezoelectric regions <NUM> and the top electrodes <NUM>. Then, the first and second dielectric layers <NUM>, <NUM> are deposited and defined, forming the openings <NUM>; the second contact lines <NUM> are formed and the passivation layer <NUM>, which is thus opened on the pads (not shown), is deposited and defined.

Then, <FIG>, a first deep etch of the silicon is performed from the back, through the second superficial oxide portion 116C, up to reaching the second chamber <NUM>, forming the lateral trenches <NUM> and the lateral openings <NUM>. Furthermore, this etch also forms part of the central trench <NUM> (up to reaching the second chamber <NUM>) and the central opening <NUM>. In this step, the coating oxide portions 116A operate as an etch stop.

Furthermore, in <FIG>, a second deep etch of the silicon is performed from the back, through the central opening <NUM> up to reaching the first chamber <NUM> (i.e. through the coating oxide portion 116A), ending the formation of the central trench <NUM> and putting it in fluid communication with the first chamber <NUM>.

After carrying out the final manufacturing steps, including opening the contacts and dicing the wafer <NUM>, the MUT device <NUM> of <FIG> is obtained.

According to another embodiment shown in <FIG>, the MUT device <NUM> is provided by using porous silicon and sacrificial regions.

In detail, with reference to <FIG>, a wafer <NUM> comprises a substrate <NUM> of semiconductor material, here silicon, for example monocrystalline, and a first sacrificial region <NUM>, thermally grown or deposited on the substrate <NUM> (e.g., on a first face 201A of the substrate <NUM>). The first sacrificial region <NUM> is intended to form the second cavity <NUM> and 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 first face 201A of the substrate <NUM> and covers the first 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 structural layer <NUM> may for example 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 identified by the presence of micropores which make it permeable to liquids, in particular to etchants, such as HF (hydrofluoric acid).

Furthermore, with reference to <FIG>, a second sacrificial region <NUM> is formed in the first structural layer <NUM>, for example through thermal growth. The second sacrificial region <NUM> is in contact with the first sacrificial region <NUM>, is intended to form the portion of the central trench <NUM> comprised between the first and second cavities <NUM>, <NUM> and may be, for example, of silicon oxide, BPSG (Boron Phosphorous Silicon Glass) or silicon nitride.

Then, <FIG>, a second structural layer <NUM> of porous silicon is deposited on the first structural layer <NUM> and on the second sacrificial region <NUM>. Furthermore, a third sacrificial region <NUM> is formed in the second structural layer <NUM>, for example through thermal growth. The third sacrificial region <NUM> is in contact with the second sacrificial region <NUM>, extends on the latter and on the second sacrificial region <NUM>, is intended to form the first cavity <NUM> and may be, for example, of silicon oxide, BPSG or silicon nitride. The first, the second and the third sacrificial regions <NUM>, <NUM>, <NUM> together form a sacrificial region <NUM> of silicon oxide, BPSG or silicon nitride.

In <FIG>, a third structural layer <NUM> of porous silicon is deposited on the second structural layer <NUM> and on the third sacrificial region <NUM>. The part of the third structural layer <NUM> superimposed on the third sacrificial region <NUM> is intended to form the membrane <NUM>. The first, the second and the third structural layers <NUM>, <NUM> and <NUM> together form a structural region <NUM> of porous silicon. The structural region <NUM> forms, with the substrate <NUM>, the semiconductor body <NUM> (where the first main surface 3A is at the structural region <NUM> and the second main surface 3B is at the substrate <NUM>).

Furthermore, <FIG>, the piezoelectric element <NUM> is formed on the second structural layer <NUM>, similarly to what has been previously described with reference to <FIG>.

Then, <FIG>, a first deep etch of the silicon is performed from the second main surface 3B up to reaching the sacrificial region <NUM>, forming the lateral trenches <NUM> and the lateral openings <NUM>. Furthermore, this etch also forms part of the central trench <NUM> (up to reaching the position of the second chamber <NUM>) and the central opening <NUM>.

In <FIG>, a second etch (a chemical etch, e.g. through HF) is performed to remove the sacrificial region <NUM>, for example from the first main surface 3A and through the structural region <NUM>. Since this type of etch allows the removal of silicon oxide (i.e. of the sacrificial region <NUM>), but does not remove the silicon, at the end of the etch the first and second cavities <NUM> and <NUM> and the part of the central trench <NUM> comprised therebetween are formed in the wafer <NUM>, thus obtaining the MUT device <NUM>.

From an examination of the characteristics of the invention made according to the present invention, the advantages that it affords are evident.

In particular, the MUT device <NUM> allows, owing to the second chamber <NUM> and the trenches <NUM> and <NUM> of each MUT element <NUM>, the recirculation of the air present in the first cavities <NUM>. It has been verified that this increases the damping of the oscillations of the membranes <NUM>, thus reducing the ring-down interval Tdown. This reduces the probability of superimposition between the emitted wave We and the reflected wave Wr and increases the minimum detectable relative distance between the object to be detected and the MUT device <NUM>. In detail, this is due to the fact that the air recirculation generates an energy loss in the oscillations of the membranes <NUM> and avoids the heating of the air present in the first cavities <NUM>, due to the oscillations of the membranes <NUM> which periodically compress the volumes of the first cavities <NUM>. Since the ring-down oscillations of the membranes <NUM> are a function of the air temperature in the first cavities <NUM>, preventing a temperature increase in the first cavities <NUM> avoids the amplification of the oscillations of the membranes <NUM>, thus improving the measurement accuracy of the MUT device <NUM>. In other words, the bandwidth of the energy spectrum of the oscillations of the membranes <NUM> grows and, for example, may exceed <NUM> (unlike the known ultrasonic transducers wherein it is generally less than <NUM>%).

Furthermore, the structure of each MUT element <NUM> is specifically designed to improve its frequency response. In particular, although the air recirculation in the first cavity <NUM> may introduce spurious peaks in the frequency spectrum of the acoustic pressure present on the membrane <NUM> (i.e. additional peaks with respect to the desired peak which is indicative of the oscillations of the membrane <NUM> caused by the emitted wave We and by the reflected wave Wr), having the membranes <NUM> at the first main surface 3A of the semiconductor body <NUM> and the central and lateral trenches <NUM>, <NUM> facing the second main surface 3B of the semiconductor body <NUM> minimizes the number and the amplitude of these spurious peaks and increases the frequency distance thereof with respect to the peak indicative of the emitted wave We and of the reflected wave Wr; this prevents such possible spurious peaks from superimposing on the peak indicative of the emitted wave We and the reflected wave Wr, and therefore from affecting the measurement of the distance of the object to be detected (as it would happen instead if at least part of the trenches <NUM> and <NUM> were facing the first main surface 3A also having the membranes <NUM> facing thereto).

The structure of the MUT elements <NUM> allows the detection of the distance of the objects to be detected without requiring the use of post-processing algorithms of the detected signals, and minimizes the dependence of the measurement on external variable factors such as the properties of the propagation medium (unlike the known solutions).

The MUT device <NUM> has small external dimensions and high flexibility and versatility.

For example, with the solution described, it is possible to form a die <NUM> having side S = <NUM> comprising <NUM> (40x40) MUT elements <NUM> each having a diameter K (dimension of the second chamber <NUM>, see <FIG>) of <NUM> and arranged at a distance L of <NUM>. The thickness of the die <NUM> (thickness of the semiconductor body <NUM>) may be between <NUM> and <NUM>.

Furthermore, in each MUT element <NUM>, the first chamber <NUM> may have a diameter of <NUM> and thickness <NUM>, the second chamber <NUM> may have a diameter of <NUM> and thickness <NUM>, the distance dcc (<FIG>) between the first chamber <NUM> and the second chamber <NUM> may be variable between about <NUM> and about <NUM> and the membrane <NUM> may have, for example, a thickness of about <NUM>. The central trench <NUM> may have a diameter dt comprised between about <NUM> and about <NUM> and the lateral trenches <NUM> have a smaller diameter than the central trench <NUM> and for example may have a diameter of <NUM>. The thin-film piezoelectric region <NUM> may have a thickness of <NUM> and the piezoelectric element <NUM> may have an overall thickness comprised between <NUM> and <NUM>.

In particular, <FIG> shows the dependence of the bandwidth BW (e.g., at -<NUM> dB) of the energy spectrum of the oscillations of the membranes <NUM> as a function of the distance dcc and the diameter dt. As the distance dcc increases, the maximum value of bandwidth BW increases, and as the diameter dt increases, the bandwidth BW has an increasing and then decreasing trend, with the maximum value of bandwidth BW which is at diameters dt that increase the more the distance dcc decreases. In particular, the bandwidth BW is maximized when the diameter dt is comprised between about <NUM> and about <NUM> and the distance dcc is comprised between about <NUM> and about <NUM>.

The values indicated above are however only indicative and in particular the shape and dimensions of the chambers <NUM>, <NUM> and of the trenches <NUM>, <NUM> may vary widely, according to the application and the desired flow volumes.

For example, according to a different embodiment (<FIG>) the MUT device <NUM> comprises a plurality of MUT elements <NUM> of quadrangular shape, for example squared, having side K = <NUM> (side of the second chamber <NUM>), arranged at a distance L = <NUM>. The piezoelectric element <NUM> is also squared herein. Other shapes (e.g. oval, hexagonal, octagonal, etc.) and other dimensions are however possible.

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

Claim 1:
A MEMS ultrasonic transducer, MUT, device (<NUM>), comprising a semiconductor body (<NUM>) of semiconductor material having a first (3A) and a second (3B) main surface and integrating a first MUT element (<NUM>) which includes:
a first chamber (<NUM>) extending into the semiconductor body (<NUM>) at a distance from the first main surface (3A);
a membrane (<NUM>) formed by the semiconductor body (<NUM>) between the first main surface (3A) and the first chamber (<NUM>) ;
a piezoelectric element (<NUM>) extending on the first main surface (3A) of the semiconductor body (<NUM>) above the membrane (<NUM>); and characterized by further comprising
a second chamber (<NUM>) extending into the semiconductor body (<NUM>) between the first chamber (<NUM>) and the second main surface (3B);
a central fluidic passage (<NUM>) extending into the semiconductor body (<NUM>) from the second main surface (3B) to the first chamber (<NUM>) and traversing the second chamber (<NUM>); and
one or more lateral fluidic passages (<NUM>) extending into the semiconductor body (<NUM>) from the second main surface (3B) to the second chamber (<NUM>),
wherein the one or more lateral fluidic passages (<NUM>), the central fluidic passage (<NUM>) and the second chamber (<NUM>) define a fluidic recirculation path that fluidically connects the first chamber (<NUM>) with the outside of the semiconductor body (<NUM>).