EP2349593A1

EP2349593A1 - Cmut cell consisting of a membrane of nanotubes or nanowires or nanobeams

Info

Publication number: EP2349593A1
Application number: EP09752873A
Authority: EP
Inventors: Bérengère LEBENTAL; Anne Ghis
Original assignee: Laboratoire Central des Ponts et Chaussees; Commissariat a lEnergie Atomique CEA; Ecole Nationale des Ponts et Chaussess ENPC; Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Current assignee: Laboratoire Central des Ponts et Chaussees; Ecole Nationale des Ponts et Chaussess ENPC; Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 2008-11-21
Filing date: 2009-11-20
Publication date: 2011-08-03
Also published as: US20110242932A1; JP2012509631A; FR2939003A1; FR2939003B1; US8873341B2; WO2010057992A1

Abstract

The invention relates to a capacitive electroacoustic CMUT transducer that includes at least one membrane (105, 205) capable of oscillation under the effect of an electric field and/or an acoustic wave, the membrane consisting of one or more layers (301-306) of juxtaposed nanotubes or nanowires or nanobeams, and to an acoustic imaging device of UHF sonar including such transducers.

Description

CMUT CELL FORMED OF A MEMBRANE OF NANO-TUBES OR NANO-THREADS OR NANO-BEAMS

DESCRIPTION

TECHNICAL AREA

The invention relates to the field of electroacoustic transducers micro-machined capacitive type also called "cMUT", and provides a device with one or more cMUT transducers having an improved membrane.

The invention provides improvements in particular in terms of stiffness and thickness of the membrane, range of the operating frequency range of the device, bandwidth.

It applies in particular to the field of high frequency acoustic imaging.

STATE OF THE PRIOR ART

Devices for generating and detecting ultrasonic acoustic waves may include micromachined ultrasonic transducers, that is, devices capable of converting acoustic energy into electrical energy and vice versa, and which are manufactured using thin film production of microsystems or micro-devices.

There are several types of transducers: - pMUT transducers (pMUT for "piezoelectric MUT") which are formed of piezoelectric elements.

- The mMUT transducers (mMUT for "Magnetostrictive" MUT) whose operating principle is based on magnetostriction.

- cMUT (capacitive MUT) transducers that use electrostatic force.

To generate acoustic waves in solid media, ultrasonic transducers made from piezoelectric ceramics (pMUTs) are particularly well suited because their acoustic impedance is generally of the same order of magnitude as that of the solid materials in which the waves are generated. On the other hand, the use of pMUTs transducers in fluid media poses problems of impedance mismatch.

To overcome this problem, transducers with piezoelectric composites can be implemented. In these transducers, impedance matching layers, such as quarter-wave layers, are added to the piezoelectric ceramic material of the transducer. This has the disadvantage of making the manufacture of such transducers more complex.

An alternative to pMUTs transducers is the use of cMUT transducers whose operation is based on the electrostatic effect. Such transducers may be formed of a metallized membrane disposed above a metal plate acting as an electrode, and may be provided for emitting ultrasound for example in air or in water.

The operation of these transducers is based on an electrostatic attraction between a membrane and a metal plate when a voltage is applied to this plate. In wave reception mode, the vibrations of the membrane caused by the acoustic waves can be measured by capacity variation, or by strain gauge.

One of the main interests of cMUTs is their low acoustic impedance, which makes them well suited for use in a fluid environment.

The ultrasonic transducers may be arranged in a matrix to form an acoustic imaging device, as has been described for example in EP 1 414 739 B1.

The cMUTs transducers are generally formed of a gold-coated silicon nitride membrane disposed on the backside of a doped silicon plate.

In the document: Surface Micromachined

Capacitive Ultrasonic Transducers, I. Ladabaum, IEEE

Trans. On Ultrasonics, Ferroelectrics and Frequency Control, Vol 45 No. 3, 1998, an example of such a cMUT transducer is given.

In the document: A novel method for fabricating capacitive micromachined ultrasonic transducers with ultrathin membranes, L. L. Liu, in 2004 IEEE International Ultrasonics, Ferroelectrics, and

Frequency Control Joint 50 ^th Anniversary Conference, p. 497, an exemplary embodiment of a cMUT transducer with an alumina membrane, is meanwhile given.

The production of cMUT transducers by micromachining generally comprises a step of releasing a membrane.

A method consisting of structuring a sacrificial layer before depositing the membrane over, creating holes in the membrane to be able to remove the sacrificial layer, then sealing the holes after release of the membrane, is given for example in document US 2007/0161896 A1. .

Another method is to deposit a membrane made independently on a structured substrate.

The realization of cMUTs of small size is therefore difficult to implement insofar as the existing deposition processes do not allow to achieve sufficiently high form factor membranes, the membranes are generally formed too thick for a too small surface.

The release methods also tend to weaken the membranes.

Furthermore, the cMUT transducers obtained using such methods have a limited accessible frequency range, for example from 20 kHz to 10 MHz.

A cMUT transducer having layers of stress reducing materials has been disclosed in FR 2,880,232 (A1). Document US 2007/0215964 A1 discloses a cMUT transducer with a membrane comprising secondary structures which make it possible, inter alia, to vary the mass and the rigidity of the membrane independently, which leads to a better sensitivity of the device.

Such a device raises problems as regards the complexity of its implementation, its cost of implementation, its size, the operating frequencies to which it provides access.

In US Patent 2006/0116585 (A1) is provided a device formed of a stack of layers in which the transmission and reception are decoupled.

Bandwidth and transducer efficiency are increased by this method. Such a device poses problems in particular in terms of complexity of implementation, and cost of implementation.

There is the problem of finding a new cMUT transducer device that provides improvements over the disadvantages mentioned above.

STATEMENT OF THE INVENTION

The invention uses a cMUT type capacitive electroacoustic transducer cell comprising one or more membranes formed respectively of one or more layers of nanotubes and / or nano-wires and / or nano-beams.

By "nano-son" or "nano-beams", or

"Nanotubes" means bar-shaped elements having a critical dimension or a diameter (for nanowires and nanotubes) which can be between 0.5 nm and 5 μm.

The nanowires may have a cylindrical or substantially cylindrical shape, with a rounded profile.

The nano-beams may have a parallelepipedal or substantially parallelepipedal shape.

The term "nanotubes" means bars having a hollow central portion and having a diameter which may be between 0.5 nm and 5 μm.

The length of the nanowires or nano-beams or nanotubes may be for example between 50 nm and 1 mm.

The invention thus provides a capacitive electro-acoustic transducer of the cMUT type comprising: at least one membrane designed to oscillate under the effect of an electric field and / or an acoustic wave, the membrane being formed of one or more layers formed of juxtaposed nanotubes, and / or nano-son and / or nano-beams juxtaposed (e) s.

The membrane may be formed of at least one layer of nanotubes and / or nano-wires and / or nano-beams parallel to each other.

The assembly of the nanotubes and / or nano-wires and / or nano-beams in the membrane enables the transducer of the invention to benefit from the performances of the NEMS resonators to perform the actuation function of an ultrasonic transducer while being miniature and rigid. In other words, the transducer according to the invention has small dimensions and high working frequencies. According to one possible implementation, the device may comprise one or more membranes each formed of one or more layers of nanotubes and / or nano-wires and / or nano-parallel beams separated by a distance from the order of the equilibrium distance of the van der Waals interaction.

This membrane may be disposed above at least one cavity and may be provided to oscillate under the effect of an electric field when the device is in emission mode.

The membrane may be provided to oscillate under the effect of an acoustic wave, when the device is in reception mode.

Such a device makes it possible to generate and detect high frequency waves, in particular in a fluid medium with possibly one or more gradients of physical properties, and optionally comprising solid inclusions, and possibly irregular boundaries. According to one possibility, the membrane may comprise one or more so-called "bonding" layers, contiguous with nanotubes and / or nanowires and / or nano-beams.

According to another possibility, the membrane may be further formed: of one or more layers called "connecting", contiguous to nanotubes and / or nano-son and / or nano-beams.

Such layers may be provided to allow the nano-son, nanotubes and / or nano-beams to be bonded and / or to ensure a tightness of the membrane. The bonding layer or the bonding layers may be based on a material chosen to be more deformable than the layer (s) of nanotubes and / or nanowires and / or nano-beams of the membrane. The membrane may include:

one or more layers of nanotubes or nano-wires or nano-beams of a first size, in particular of a first critical dimension,

one or more layers of nanotubes or nano-wires or nano-beams of a second size, different from the first size, in particular a second critical dimension different from the first critical dimension.

This can for example increase the bandwidth, modulate the operating frequency, decouple the transmission and reception frequencies.

According to one possible implementation, the membrane may be formed of at least a first layer of nanotubes and / or nano-wires and / or nano-beams oriented in a first direction and at least a second layer of nanotubes and / or nano-wires and / or nano-beams oriented in a second direction, orthogonal to the first direction. According to a particular embodiment, the membrane may be formed of one or more layers of nanotubes, and / or nanowires and / or nano-beams.

In this case, according to one possibility, the membrane may comprise at least one graphene-based bonding layer contiguous to at least one layer of nanotubes, and / or nano-beams and / or nano-wires. Covering the membrane formed by the layer (s) of nanotubes, nano-wires, nano-beams makes it possible to guarantee impermeability of the membrane and efficiency of actuation without disturbing its mechanical properties.

According to another possibility, the membrane may be formed of one or more layers of nanotubes, and / or a nano-beam layer and / or a nano-son layer, interposed between at least a first layer of graphene and at least one second layer of graphene.

According to a particular embodiment, the membrane may be formed of a self-supporting core consisting solely of nanowires and / or nano-beams and / or nanotubes.

The capacitive electroacoustic transducer may also comprise: at least one actuation and / or detection electrode, and at least one cavity above which the membrane is intended to oscillate, the membrane being located facing said cavity.

The electroacoustic capacitive transducer may further comprise:

means for applying a fixed potential to said electrode,

means for applying a variable potential to said electrode.

In this case, the transducer may be provided to operate as a transmitter or transmitter / receiver. The capacitive electroacoustic transducer may include:

means for applying a fixed potential to said electrode; means forming a capacimeter.

In this case, the transducer may be provided to operate as a receiver or transmitter / receiver.

According to one possible implementation, the electroacoustic capacitive transducer may furthermore comprise:

at least one first electrode, and at least one first cavity above which a membrane is intended to oscillate, the membrane being located opposite said first cavity,

at least one second electrode, and at least one second cavity in which a membrane opposite said second electrode and said second cavity is intended to oscillate. According to one possibility of implementation, the first cavity may be for example intended to form an emitting part of the device, while the second cavity may for example be intended to form the receiving part of the device. According to a possible implementation of the transducer, the same membrane may be located opposite the first cavity and the second cavity.

According to one possible implementation, the transducer may comprise a plurality of membranes located above the same cavity. According to one possible implementation, the transducer may comprise a membrane located above several cavities or a matrix of cavities. The transducer may comprise several membranes superimposed above or opposite one and the same cavity.

The invention also relates to an acoustic imaging device or UHF sonar comprising a matrix of capacitive electroacoustic transducers of cMUT type as defined above.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood on reading the description of exemplary embodiments given, purely by way of indication and in no way limiting, with reference to the appended drawings in which:

FIGS. 1A and 1B illustrate an example of a cMUT transducer according to the invention, comprising a membrane formed of nanotubes or nano-wires or nano-beams, according to a view from above and in a cross-sectional view,

FIGS. 2A, 2B, 2C, 2D, 2E illustrate examples of cMUT cells operating respectively as transmitters or as receivers or transceivers according to the invention,

FIG. 3 illustrates an exemplary pulse operation of a cMUT transducer device according to the invention, FIG. 4 illustrates an example of transient operation of a cMUT transducer device according to the invention,

FIG. 5 illustrates an example of steady-state operation of a cMUT transducer device according to the invention,

FIGS. 6A and 6B illustrate a membrane formed of a network of nanotubes in an exemplary cMUT transducer according to the invention, the membrane being respectively represented in a view from above and in a cross-sectional view,

FIGS. 7A and 7B illustrate a membrane of a cMUT transducer according to the invention, the membrane being formed of a network of nanotubes of high density, and respectively represented in a view from above and in a cross-sectional view,

FIGS. 8A to 8D illustrate various network arrangements (x) of nanotubes forming or included in a membrane of cMUT transducer devices according to the invention,

FIGS. 9A to 9F illustrate various network arrangements (x) of nano-wires, or nano-beams or nanotubes in a membrane of a cMUT transducer according to the invention,

FIGS. 10A and 10B illustrate a membrane of a cMUT transducer formed of several layers of nanotubes of different orientations, according to a view from above and in a cross-sectional view, FIGS. HA and HB give examples of resonance frequency curves of devices cMUTs according to the invention comprising a membrane formed of nanotubes of 1 μm in length and a membrane formed of nanotubes and 1 nm in radius, and obtained respectively at using numerical simulations for a membrane in air and in water,

FIGS. 12A and 12B give examples of resonance frequency curves of devices cMUTs according to the invention respectively comprising a membrane formed of nanotubes with a radius of 1 nm and a membrane of 1 TPa of Young's modulus, and obtained respectively using measurements made in the air and in the water,

FIG. 13 illustrates an example of a cMUT transducer device according to the invention, comprising a hermetic cavity,

FIG. 14 illustrates an example of a cavity of oblong shape in a cMUT transducer device according to the invention,

FIG. 15 illustrates an example of a cMUT transducer device according to the invention, comprising several membranes facing the same cavity,

FIG. 16 illustrates an example of a cMUT transducer device according to the invention, comprising a membrane facing several cavities; FIG. 17 illustrates an example of a cMUT transducer device according to the invention; having several superimposed membranes and having different orientations.

Identical, similar or equivalent parts of the different figures bear the same numerical references so as to facilitate the passage from one figure to another.

The different parts shown in the figures are not necessarily in a uniform scale, to make the figures more readable.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

An example of a device according to the invention, comprising at least one micro-machined ultrasonic transducer cell cMUT is given in FIGS. 1A and 1B (FIG. 1A showing a cross-sectional view X 'X of the device while in FIG. a top view of the device is given).

The cell comprises at least one electrode 107 which may be metallic, for example gold-based, and which rests on a substrate 100 which may for example be based on silicon or on silicon oxide and on which conductive zones rest. 103a, 103b, which can play the role of electric mass. The cell also comprises at least one membrane 105 (shown in oscillation in FIG. 1A), the ends of which are connected to the zones 103a, 103b, in order to put the membrane 105 to ground.

The membrane 105 may be supported by blocks 104, for example based on insulating material such as than SiO2 on which it rests. The blocks 104 make it possible to maintain the membrane 105 above a cavity 110 unveiling the electrode 107 and in which the membrane 105 is intended to vibrate. The depth (defined in a direction parallel to that of the vector k of the orthogonal coordinate system [O; /; j; k] given in FIGS. 1A and 1B) of the cavity 110 may be for example of the order of 100 nm or included for example between 100 nm and 1000 nm. The cavity 110 may for example have a rectangular or square shape, the lateral dimensions (defined in a direction parallel to the plane [O; /; j] given in Figures IA and IB) may be of the order of 1 micron, or for example between 300 nm and 3 μm, and adjusted to optimize certain operating parameters of the device such as the operating frequency, the vibration amplitude of the membrane 105.

The membrane 105 may be formed of nanotubes, for example carbon nanotubes, or nano-wires or nano-beams, for example based on semiconductor material such as Si, or for example based on a material dielectric such as Si3N ₄ . The nanowires, or nano-wires, or nano-beams, are arranged and can be arranged in a single layer, or several superimposed layers.

The membrane 105 may be formed of juxtaposed rows of nanotubes or nano-son or aligned nano-beams. The nanotubes or nano-son or nano-beams can be parallel to each other, which allows to obtain very strong Van der Waals lateral interactions.

A membrane formed of aligned parallel nano-wires or nanotubes may have a much higher Young's modulus than a conventional membrane formed of a crystalline material-based layer. In addition, thanks to the small diameter of these nanotubes, nanowires and nano-beams the membrane thus formed has a thickness, of the order of a nanometer, smaller than that of a conventional membrane.

Nano-wires or nanotubes or nano-beams can be distributed over several layers or thicknesses.

By adapting the dispersion of the nanotubes around their average properties, for example their average radius, it is possible to adapt the frequency range at which the membrane is intended to vibrate, and by the same the bandwidth of the device.

Such a membrane also has the advantage, besides its increased rigidity, that its thickness is controllable to a fraction of a nanometer.

A hydrophilic or hydrophobic character can also be imparted to the membrane. This is facilitated especially when the membrane is based on carbon nanotubes.

By adapting the type of nanotubes or nano-wires or nano-beams used, it is possible to control certain physical parameters of the membrane such as its density, its stiffness and its thickness. The membrane 105 may comprise one or more sheets or layers referred to as "connecting" to make it possible to bind the nanotubes or nanowires or nano-beams and possibly to ensure a seal and / or to guarantee efficiency of actuation of the membrane without disturbing its mechanical properties. In the case for example where the membrane 105 is formed of one or more layers of juxtaposed nanotubes, the layer (s) "connection" may (for example) be in the form of one or more layers of graphene. The addition of one or more layers of graphene to the nanotubes, can make it possible to reinforce the cohesion and the tightness of the membrane, without disturbing the displacements of the membrane, because of the small number of layers and the module of Young of these layers much lower than that of nanotubes.

A bonding layer allowing a sealing and cohesive reinforcement of the membrane, based on a material other than graphene may optionally be provided. The bonding layer (or bonding layers) may be provided based on a more deformable material than said nanotubes and / or nanowires and / or nano-beams of the membrane.

For example with Young's modulus equal or substantially equal, the bonding layer may be provided to have a moment of inertia at least five times lower than all the nanotubes and / or said nano-son and / or or said nano-beams of the membrane. According to another possibility, with a moment of equal or substantially equal inertia, the bonding layer may be provided so as to have a module of Young at least five times lower than the assembly formed by the nanotubes and / or said nanowires and / or said nano-beams.

The membrane 105 may be covered, in regions surrounding the cavity 110, with metal zones 112a, 112b, for example based on aluminum, resting on parts of the latter. The contact of the metallic material of the zones 112a, 112b with the membrane can be improved for example by oxidation, for example by RIE (RIE) treatment or by functionalization of the nanotubes, for example using groups COOH.

Such metal zones 112a, 112b may make it possible to secure the membrane 105 to the edges of the cavity 110 in order to limit the energy losses and the decrease in operating frequencies due to poor contact between the membrane and the support.

To generate a wave, a DC bias voltage is applied, as is an AC voltage on the electrode 107.

To perform a detection of an incident wave, a DC bias voltage is applied to the membrane 105. It is the movements of a surrounding fluid that then vibrate the membrane 105. These vibrations are detected by a measurement of variable capacitance between the electrode 107 and the membrane 105.

For example, a method as described in the following article can be used as a detection method: Measurement of Nano-displacement based on in-plane suspended MOSFET detection compatible with a front-end CMOS process "by E. Colinet et al. in 2008 IEEE Int. solid-states circuits conf, session 18, MOS MEDLEY, 18.2.

For an imposed alternating voltage, it is possible to detect variations in capacitance at the level of the emitting membrane 105 for determining the interferences between an emitted wave and a wave reflected by a target medium.

According to a variant of the exemplary cell cMUT which has just been described, it is also possible to implement a device with two very close cavities above which a single membrane 105 is arranged, or having a first cavity above which there is a first membrane, and a second cavity of which is a second membrane, distinct from the first membrane. In this case, a first cavity may be provided to make the transmitting part of the device, while a second cavity may be provided for the receiving part.

When the two cavities are sufficiently close, for example separated by a distance d such that d << λ, the incident wave at the receiving cavity is identical to the incident wave at the emitting cavity, and more easily measurable.

In Figures 2A-2E different examples of cMUTs cell arrangements and modes of operation of these cells are given.

In FIG. 2A, a cMUT transducer cell of the type of that previously described in connection with FIGS. 1A and 1B is given. This cell operates as an acoustic wave transmitter 200 and comprises a membrane 105 intended to vibrate in a cavity 110 facing an electrode 107. The device is also provided with means 210 for applying a variable potential Va to the electrode 107, and means 212 for applying a fixed potential Vs on the membrane 105.

In FIG. 2B, a cMUT cell of the type of that described previously with reference to FIGS. 2A and 2B, is also shown. The cell operates as an acoustic wave receiver 202 and comprises a membrane 105, as well as means 212 for applying a fixed potential Vs on the electrode 107, and means 220 forming a capacimeter, for measuring a capacitance variation δCm representative of waves received by the membrane 105.

Another example of a cMUT cell is given in Figure 2C. The cell operates as a transceiver and comprises a membrane 105 suspended above a first cavity 110 and a second cavity 170. The first cavity 110 belongs to the emitting part of the cell. The cell also comprises means 210 for applying a variable potential Va to the membrane 105, and means 212 for applying a fixed potential Vs to the electrode 107. The second cavity 170 belongs, in turn, to the receiving part of the cell. The cell also comprises means 212 for applying a fixed potential Vs to a second electrode 207, and means 220 for measuring a capacitance variation δCm of the membrane 105, connected to the second electrode 207. Another example of a cMUT cell is given in FIG. 2D. The cell operates as a transceiver and has a membrane 105 and a single cavity 110 to act as both transmitter and receiver. The cell also comprises means 212 for applying a fixed potential Vs to an electrode 107, and means 220 for measuring a capacitance variation δCm of the membrane 105, connected to the electrode 107. Following the emission of acoustic waves 200, a measurement of the echo return time at the transmitter makes it possible to locate a reflective zone of the waves and the frequency analysis of these echoes makes it possible to identify the nature of the zones traversed by these waves. Another example of a cMUT cell is given in Figure 2E. The cell operates as a transceiver and comprises two membranes 105, 205 and two cavities 110, 270, a first membrane 105 being intended to vibrate in the first cavity 110, a second membrane 205 being intended to vibrate in the second cavity 170, the second membrane 205 being distinct from the first membrane. In this case, the first cavity may be provided to make the transmitting part of the device, while a second cavity may be provided for the receiving part.

In either of the examples which have just been given, the membrane or membranes are formed of one or more layers of nanotubes and / or nano-threads and / or nano-beams, parallel. The cells can each be associated with one or more modules integrated in the same medium: for example at least one amplification module, for example at least one memory module, for example at least one energy recovery module, for example information transmission and reception modules, for example at least one module for signal processing adapted to perform operations such as, for example, quadruple decompositions, comparisons.

These modules can possibly be placed at a distance from the cell, the connection of the cell to this remote electronics can be made by technologies such as wire-bonding or a TSV (TSV) integration with through vias. A matrix of emitters or receivers or emitters / receivers as described above and independently controlled by an active stamping based on transistors can be realized.

One can thus form a micrometric dimension imaging device operating at ultra-high frequency for example between 10 MHz and 10 GHz, in particular between 100 MHz and 200 MHz when the device is intended to operate in a liquid medium such as water , or for example between 500 MHz and 5 GHz when the device is intended to operate in the air.

In Figures 3, 4, 5, different modes of operation of a cell cMUT are given.

A cMUT cell may have a transient mode, a pulse mode of operation (Figure 3) or a sinusoidal operating mode (Figure 4).

In pulse mode (FIG. 3, using a voltage pulse (signal Sl) applied to the transmitter, the transmitter can in turn transmit an acoustic wave in the form of a pulse (Signal S2) whose signal time spreading depends on the bandwidth of the transmitter and the surrounding environment.The pulse S2 propagates in the medium and reaches the receiver at the distance d after a time Ti with _{Cfi and} the speed of sound in the middle). Frequency analysis of a received signal S3 and its comparison with the transmitted signal S2 makes it possible in one go to determine the transfer function of the medium for the frequencies present in the spectrum of the transmitter and the receiver. The pulse S2 can be partially reflected by the medium, which can be translated at the receiver by successive echoes. The echo return time at the transmitter makes it possible to locate the reflective zone and their frequency analysis to analyze the nature of the interfaces traversed by the pulse S2.

In sinusoidal mode (FIG. 4), the emitted wave can be in the form of a transient sine wave (signal S20), generated using a sinusoidal voltage (signal SlO). A signal transmitted between t = 0 and t = T2 is received (signal S30) by the receiver at the distance d between t = T1 and t = T2 + T1. It is a sinusoid whose amplitude depends on ω, the pulsation, in rad / s. The transfer function of the medium at the frequency w is determined. Between t = 0 and t = T3, with T3 equal to several periods, the transmitted wave depends on the region of the space located within a few wavelengths of the transmitter. Its emission amplitude between t = 0 and t = T3 as a function of the frequency w makes it possible to define a near-field acoustic impedance variable according to ω. At the transmitter, sinusoids echoes interfere with the transmitted wave. The amplitude and the phase shift of these echoes makes it possible to deduce the position and the nature of the interfaces traversed by the wave.

In steady state mode (FIG. 5), the transmitter emits a wave (signal S200) of amplitude A at the frequency w, for example around 1 GHz in the air and for example of the order of 100 MHz in the water. The wave generated using a voltage SlOO applied to the transmitter propagates in the medium and interferes with its echoes, causing transient behavior. After a certain time depending on the medium and the speed of propagation of the waves in the medium, the wave reaches a steady state. At the transmitter and receiver located at a distance d from the transmitter, two sinusoids (respectively S200 and S300) are measured. At the transmitter (signal S200), the amplitude and the phase shift of the reflected wave (which depend on the frequency) correspond to the overall acoustic impedance of the surrounding medium. At the receiver (signal S300), this is the transfer function of the medium traversed.

Various embodiments of a membrane of a cMUT transducer according to the invention will now be given. The membrane 105 may be formed of an array of parallel and disjoint nanotubes (the network being shown in FIGS. 6A and 6B, respectively in a cross-sectional view and in a view from above).

The membrane 105 may be formed of a dense network of nanotubes juxtaposed and parallel to each other (the network being shown in FIGS.

7A and 7B, respectively according to a cross-sectional view and in a view from above).

A dense membrane can be formed. The membrane is said to be "dense" when it is impermeable to the molecules of a fluid surrounding the transducer.

The density limit depends on the fluid considered and its interaction with the carbon atoms forming the membrane.

The density limit may be for example such that a space of the order of 0.6 nm between two parallel nanotubes can be obtained for a membrane 105 of carbon nanotubes of a transducer device intended to operate in water .

The density limit may also depend on physical characteristics such as the dimensions of nanotubes, their hydrophilic or hydrophobic character, the lateral interaction energy of the nanotubes forming the membrane.

Nanotubes distributed over several layers, for example over 3 layers, can make it possible to form the membrane 105 (FIG. 8A). According to one possible implementation, the transducer membrane is formed of a first set of nanotubes of a first size, in particular of a first diameter, arranged in one or more superimposed layers 301, 302, 303, as well as a second set of nanotubes of a second size, in particular of a second diameter, arranged in one or more superimposed layers 306, 307, 308, covering the first set (Figure 8B).

In FIG. 8C, the membrane 105 is formed of a stack of layers 301, 302, 303, of nanotubes contiguous to a so-called "bonding" layer 320, for example based on graphene, intended to bind the nanotubes and possibly to ensure a tightness of the membrane. More particularly, the graphene bonding layer can cover the gaps between neighboring nanotubes of the nanotube layer which carries it to form a planar structure.

Another example of implementation of the membrane 105 is given in FIG. 8D. In this other example, the membrane 105 is formed of a stack of layers 301, 302, 303 of nanotubes located between a first stack 322 of several bonding layers, for example based on graphene, and a second stack 332 of several layers. binding, for example based on graphene. In a layer of aligned but non-contiguous nanotubes, the density of the membrane depends on the spacing between the nanotubes. If this spacing is greater than the diameter of a nanotube, a deposit of one or more additional thickness (s) of nanotubes may be provided to fill any possible gaps between the nanotubes, and to increase the density of the membrane. The additional nanotubes are likely to be deposited naturally in the interstices between the nanotubes.

The thickness of the membrane depends on the number of layers of nanotubes or bonding layers formed and can be controlled very precisely, regardless of the density and the Young's modulus.

The dispersion of the dimensions of the nanotubes around a mean value is controllable and makes it possible to adjust the frequency bandwidth at which the membrane is able to vibrate.

In the case where the membrane is formed of a network of nanotubes, this network can be made for example using a method of Langmuir Blodgett, as described in the document: Langmuir Blodgett films of Single-Wall Carbon Nanotubes : Layer-by-layer deposition and in-plane orientation of nanotubes Jpn. J. Appl. Phys. Flight. 42 (2003) pp. 7629-7634, or by dielectrophoresis as in the document entitled Frequency Dependence of the structure and electrical behavior of carbon nanotubes assembled by dielectrophoresis Nanotechnology 16 (2005) 759-763.

The graphene sheets can be made according to the method described in Synthesis of graphene based nanosheets via chemical reduction of exfoliated graphite oxide Carbon 45 (2007) 1558-1565.

In this case, the network of nanotubes can be manufactured by a method such as that described in document WO 2007/126412 A2. The leaflets of graphene can be deposited for example by capillary dielectrophoresis on the network of nanotubes. The natural affinity between nanotubes and graphene sheets makes it possible to fix the graphene sheet on the nanotubes. It is also possible to envisage a deposition by dielectrophoresis or a surface functionalization of a first sheet.

After removal of the solution of graphene sheets, a network of nanotubes can be deposited by dielectrophoresis on graphene. After rinsing the graphene solution, we start again with a solution of graphene mono-sheet.

An impermeable triple layer formed of a dense network of nanotubes interposed between two sheets of graphene can thus be obtained.

A structure comprising several layers of nanotubes can be produced using several solutions of different compositions.

The density limit therefore depends on the fluid considered and its interaction with the carbon atoms forming the membrane. The lower limit of the distance between the nanotubes may be for example of the order of 0.6 nm for a membrane 105 of nanotubes of a transducer device intended to operate in water.

The equilibrium distance of nanotubes between them being of the order of 0.35 nm, a membrane of nanotubes linked together by interaction van der Waal is impermeable to water molecules.

An example of a method for producing a cMUT transducer according to the invention will now be given.

On a substrate 100, the electrode is made

107, as well as the zones 103a, 103b and the insulating blocks 104 forming the cavity 110 for example by means of lithography steps, for example using an e-beam type electron beam. .

The layer on which the membrane 105 is formed may be functionalized by specific chemical groups, for example hydrophobic groups or amino chemical groups, in order to increase the affinity of this layer for nanotubes.

The membrane can be made directly suspended above the cavity.

The membrane may be made on a substrate having no cavity, the cavity being made, for example by HF etching, for example after formation of the membrane.

The membrane 105 may also be disposed over a recessed cavity, for example by means of a process commonly known as "nano-imprint" on the functionalized substrate or not.

It is possible to make contact over the membrane by depositing a metal layer on top of the membrane 105, which is etched to form metal zones 112a, 112b serving as zones for holding the membrane, and allowing to increase the strength of the bonding between the membrane 105 and the support of the membrane.

For the production of a matrix of cMUT cells, functionalization of the deposition zone can be performed for each of the cells. This method corresponds to self-assembly methods for example such as those commonly called "patterned affinity templates".

The membrane 105 may be formed with nanotubes parallel to each other oriented in the same direction. The alignment of the nanotubes to form the membrane has several advantages over a non-aligned nanotube membrane. The alignment makes it possible to obtain a high density of nanotubes for a small thickness of the membrane 105. Indeed, a monolayer of non-aligned nanotubes would contain a vacuum, which should be filled by adding additional layers of nanotubes, the number additional layers necessary being all the more important as the disorder of the nanotubes is important.

A configuration with parallel nanotubes makes it possible to obtain a membrane with better mechanical properties, in particular greater rigidity and a small thickness. The interaction energy between two parallel nanotubes separated by a distance of the order of the equilibrium distance of the van der Waals interaction between nanotubes is much greater than the interaction energy between nanotubes. crossing, resulting in improved membrane cohesion. In the case where the membrane 105 is formed only with parallel nanotubes and in the same direction, the movement of the nanotubes is more homogeneous. An organization of nanotubes in parallel nanotube network makes it possible to obtain a transducer having improved detection efficiency.

A membrane formed of aligned nanotubes makes it possible to increase the lateral cohesion of the membrane and its sealing without reducing the rigidity of the membrane and therefore its operating frequency.

Thanks to the high Young's modulus of the nanotubes, the nanotube transducer membrane has a rigidity greater than that of the usual membranes.

A wider range of usage frequencies, and in particular higher frequencies of use, can thus be obtained. The controllable dispersion of the properties of the nanotubes around an average value allows a broadening of the bandwidth of the device favorable to its implementation in an electroacoustic transducer. The variable number of nanotube walls makes it possible to increase the density of the membrane for a constant nanotube radius and a quasi constant Young's modulus.

The thickness of the membrane depends on the number of layers of nanotubes or layers of graphene used and is therefore very precisely controllable, independently of density and Young's modulus.

The realization of successive layers of carbon nanotubes of different sizes can be obtained by changing only the solution used during the deposition.

The implementation of a transducer device according to the invention, with a membrane of carbon nanotubes, can be obtained without having to perform a membrane release step, which also reduces the number of technological steps and allows a reduction scale compared to the embodiments of transducers according to the prior art.

Because of its design that can be called "bottum-up" that is to say from assembly of nanoscale elements to form a functional final device of larger dimensions, the miniaturization of the membrane is simplified the constituent elements of the membrane are structurally of micron or nanometric dimensions. As a result, the accessible resonant frequencies are higher than those of prior art devices due to the reduced cavity size and properties of the materials.

In one or other of the embodiments that have just been given, the membrane 105 has a small thickness, for example between 1 nm and 30 nm depending on the number of layers of nanotubes or nano-son employed. The examples of membranes described above in connection with FIGS. 7A-7B and 8A-8D comprise carbon nanotubes arranged on one or more layers. The membrane may be formed based on nanotubes of material (x) other than carbon, for example electrically conductive nanotubes or electrically conductive nanotubes such as boron nitride nanotubes, possibly made conductive for example by doping or by functionalization.

Non-metallic nanotubes may optionally be rendered conductive by doping or metallization or functionalization. According to other examples, the membrane (s) of the cMUT transducer according to the invention may optionally be formed of parallel nano-beams or nano-wires of cylindrical or parallelepipedal shape, possibly bound by a layer of link.

The membrane of a cMUT transducer according to the invention may optionally be formed of a mixture of different nanotubes, or a mixture of different nanowires or a mixture of nanotubes and nanowires.

In FIG. 9A, a transducer membrane 105 cMUT comprises a core made up of parallel nano-wires 401, of cylindrical shape and covered by a bonding layer 420 intended to bind the nanowires. The nanowires may have been made conductive and may for example be semi-conductor nano-son, for example based on silicon, possibly doped. The bonding layer 420 provided for bonding the nanowires may for example be based on graphene.

In FIG. 9B, a cMUT transducer membrane 105 is in turn formed of nano-beams 501 parallel to one another and of parallelepipedal shape, the nano-beams being covered by a bonding layer 520 which can make it possible to bind the nano-wires. or strengthen the connection between nano-wires. The nano-beams may have been made for example by a technique called "nano-imprint lithography".

In FIG. 9C, a cMUT transducer membrane 105 is formed of a layer of nanotubes

301 parallel covered by a bonding layer 420 provided to allow to bind the nanowires or strengthen the connection between the nanowires.

In FIG. 9D, the membrane 105 comprises parallel rows 402 of aligned nano-wires 401 covered by a bonding layer 420.

In FIG. 9E, the membrane 105 comprises parallel rows 502 of aligned nano-beams 501 covered by a bonding layer 520.

In FIG. 9F, the membrane 105 comprises parallel rows 302 of aligned nanotubes 301 covered by a connecting layer 320. The membrane 105 may be provided with a self-supporting core consisting solely of nano-wires and / or nano-beams and / or nano-tubes.

The nano-wires or nano-beams or nanotubes are, in the examples which have just been given, bars of critical dimension which can be between 0.5 nm and 5 μm (the critical dimension being the smallest dimension of these dimensions). "Nano-wires" or "nano-beams", or "nano-beams" except for their thickness and which is defined in a direction parallel to that of the vector / orthogonal reference [O; /; j; k] given in the figures 9A to 9F).

In the examples which have just been given, the nano-wires or nano-beams, or nanotubes, have a length L (the length L being defined in a direction parallel to that of the vector j of the orthogonal reference [O; k] given in Figures 9A to 9F), which may be between 50 nm and 1 mm.

Exemplary embodiments of a cMUT transducer cell having one or more graphene-based layers for bonding and sealing nanotube arrays have been provided. Other materials may be used to form such a layer or such layers, for example boron nitride.

The bonding layer of the nanotubes or nano-wires that can be used to form a cMUT transducer membrane may be based on a crystalline material chosen to be more deformable than said nanotubes and / or nanowires. and / or nano-beams forming the membrane, having for example a modulus of Young equal or substantially equal but a moment of inertia ten times weaker than that of the nanotubes and / or nano - son and / or nano - beams, or having a Young 's module ten times weaker and a moment of inertia equal to or substantially equal to that of the nanotubes and / or nanowires and / or nano-beams.

Such a bonding layer can be made for example by an ALD (ALD for "atomic layer deposition") method, which makes it possible to form layers of small thickness, for example with a thickness of between 5 Angstroms and 5 nanometers.

A hydrophilic or hydrophobic character may be imparted to the membrane, depending on the intended application for the cMUT transducer. A membrane of carbon nanotubes may for example be rendered hydrophobic or hydrophilic with functionalized nanotubes.

The membrane can be treated in order to render it hydrophilic, for example using functionalized COOH groups on the nanotubes.

According to another possibility, the membrane can be treated in order to render it hydrophobic, for example by electropolymerization.

It is also possible to directly produce a hydrophilic or hydrophobic membrane, from nanotubes or nano-wires themselves previously conditioned prior to their assembly, for example by functionalization of the nanotubes or by means of a suitable coating ("coating" according to the Anglo-Saxon terminology) formed on nanotubes. The size of a cMUT transducer elementary cell according to the invention is favorable for its integration into a matrix to form a miniaturized ultra-high frequency acoustic imaging device or a very high resolution micro-sonar.

In the examples of cMUT transducers that have been given, the cavity has a rectangular shape.

Cavities having other shapes: circular, or oval, or square, may also be contemplated.

A membrane formed of several superimposed layers of nanotubes or nano-son of different orientation between the layers, can also be realized, particularly in the case where the cavity has a square shape.

In FIGS. 10A and 10B an example of membrane 105 formed of layers 610, 620, 630, 640 superimposed on nanotubes of different orientations between the layers, is given (FIG. 10A represents a view from above of the membrane 105, while FIG. FIG. 10B gives a cross-sectional view of this membrane 105).

The membrane comprises an alternation of layers 610, 630 whose nanotubes 601a are oriented in a first direction, parallel to that of the vector / orthogonal reference [O; /; j; k] given in FIGS. 10A and 10B, and of layers 620, 640 whose nanotubes 601b are oriented in a second direction, parallel to that of the reference vector j orthogonal [0; /; j; k], orthogonal to the first direction.

These different orientations can be obtained for example by dielectrophoresis from potentials successively applied to several sets of pairs of opposite electrodes.

In the figures HA and HB are given, examples of curves C1 and C2 of resonance frequency as a function of the Young's modulus of a transducer membrane according to the invention formed of carbon nanotubes of 1 μm in length, and of 1 nanometer radius. The curve C1 is representative of measurements made in the air while the curve C2 is representative of measurements made in the water. FIGS. 12A and 12B are given, examples of C3 and C4 resonance frequency versus modulus curves of a transducer membrane according to the invention formed of carbon nanotubes 1 nanometer in radius and 1 TPa of Young's modulus. Curve C3 is representative of measurements made in air while curve C4 is representative of measurements made in water.

Different variants may be envisaged for the manufacture of a cMUT transducer according to the invention.

According to a first possibility, it is possible to form the cavity 110 before forming the membrane 105. In this case, the membrane 105 can then be made directly on the support 100 or else made outside the support 100 can be carried on the support 100. According to a second possibility, it is possible to form the membrane 105 outside the support 100 and then to transfer the membrane 105 to the support 100, and then to form the cavity 110. According to a third possibility, the membrane 105 can be formed on the support 100, then to form the cavity 110.

An example of a cMUT cell arrangement comprising a membrane 105, formed above a hermetic cavity 770, is given in FIG. 13. In this example, the cavity 770 is closed and delimited by walls 710a, 710b, 710c, 71Od lateral which can be formed at least partially by support blocks of the membrane 150, and by the membrane 150 itself and a support.

Such an arrangement can make it possible to couple resonance modes, which depend on the height of the cavity, with the modes of the membrane.

Such an arrangement may make it possible to increase the quality factor and / or the amplitude of vibration, in particular in the bandwidth of the device.

The realization of a cMUT cell with one or more cavities that are not completely closed or sealed, and in particular that have lateral openings, can allow fluid to be evacuated through these openings, and to avoid damping the displacements of the membrane, in particular for frequencies different from the resonance modes and which depend on the height of the cavity. A cMUT cell with one or more oblong shaped cavities may be provided. A cavity 710 of oblong rectangular shape, with a length D, of the order of 2 to 1000 times the width d is for example represented in FIG. 14. This makes it possible to avoid transverse parasitic modes which can be related to effects. on board.

An exemplary embodiment of cMUT cell provided with several electrodes, including an electrode for actuating the membrane and a reading electrode, has already been given in connection with FIG. 2C. According to other embodiments, cells having a plurality of actuation electrodes and / or a plurality of independent reading electrodes and each facing one or more membranes may be provided. A matrix arrangement of the electrodes, comprising several rows of electrodes, can also be provided. With an arrangement comprising a plurality of electrodes arranged in the same cavity, it is possible to obtain an improved spatial sensitivity in reception, possibly to know displacements at different points of a membrane, possibly to control the displacement of certain portions of the membrane independently. Another example of a cMUT cell is given in FIG. 15. This device comprises several membranes 205a, 205b, 205c disposed above one or more cavities and facing one or more electrodes for actuation and / or one or more measuring electrodes. This may make it possible to improve the evacuation of the fluid in which the membranes are intended to vibrate and to limit the damping due to certain parasitic modes. Such an arrangement may make it possible to improve the evacuation of the fluid, in particular when the cavity or the cavities are closed laterally by lateral walls which prevent a lateral evacuation of the fluid.

In one case, for example, where the membranes 205a, 205b, 205c are respectively disposed above a first electrode, a second electrode, and a third electrode, emission and reception by neighboring membranes can be implemented. An emission of out-of-phase waves by neighboring membranes may also be implemented, which makes it possible to obtain a directivity and / or an emitted power.

In one case, where the membranes 205a, 205b, 205c, are respectively disposed above a first detection electrode, a second detection electrode, and a third detection electrode, an improvement of the sensitivity spatial reception can be obtained. Another example of a cell cMUT is given in FIG. 16. This device comprises several cavities HO ₁₁ , HO ₁₂ , 710, ₃ , 710 ₂ , 71O ₂₂ , 71O ₂₃ , 71O ₃ i, 71O ₃₂ , 71O ₃₃ , arranged according to a matrix of several rows of cavities above which a membrane 150 is intended to vibrate. Cavities of very small size, for example of critical dimension or of width d between 50 nm and 500 nm can be implemented.

The implementation of small cavities can in particular make it possible to obtain high operating frequencies, as well as to make the production of a suspended membrane based on parallel nanotubes easier.

According to one variant, a cell with cavities of different sizes may be implemented and allow the same device to transmit and / or receive at different frequencies.

Examples of cells with rectangular shaped cavities have been given previously. Other shapes such as polygonal shapes, or hemisphere or sphere, can also be provided for the cavities, in particular as a function of the power and / or directivity and / or amplitude and / or the transmission frequency, and / or the sensitivity and / or frequency of the reception frequency and / or reception bandwidth, which it is desired to obtain.

Another example of a cMUT cell is given in FIG. 17, and comprises several membranes 705a, 705b, 705c, 705d of nanotubes, and electrodes 703i, 7032, 7033, 703 ₄ , 703 ₅ , 703e, 703 ₇ arranged on a support and oriented in different orientations, in pairs of electrodes facing each other.

A first membrane 705a is in contact with a first pair of electrodes 703i, 703 ₅ arranged opposite to each other, while a second membrane 705b is in contact with a second pair of electrodes 7032, 703e arranged face to face, a third membrane 705c is in contact with a third pair of electrodes 7033, 703 ₇ arranged face to face, a fourth 705d membrane is in contact with a fourth pair of electrodes 703 ₄ , 703s. The membranes 705a, 705b, 705c, 705d thus have different orientations to each other.

In this example, the set of electrodes has a polygon-forming arrangement, with one or more cavities within the polygon, each nanotube membrane being disposed at an angle different from that of the other membranes.

Such electrode arrangement, in pairs, and having different orientations between it, can be used to form membranes

By applying a suitable voltage on a first pair of electrodes 703i and 703 ₅ , facing each other, it is possible to promote the deposition of a layer of nanotubes aligned and oriented according to a first orientation determined by the field between the electrodes to form the first membrane 703a.

Then, by applying a suitable voltage on a second pair of electrodes 7032 and 703e, located facing one another, it is possible to promote the deposition of a layer of nanotubes aligned and oriented in a second orientation determined by the electric field between the electrodes 7032 and 703e, to form the first membrane 703a. This is done immediately by applying a voltage between the electrodes 7033 and 703 ₇ , to form the third membrane 703c, then applying a voltage between the electrodes 703 ₄ and 703s, to form the fourth membrane 703d.

Claims

1. capacitive electroacoustic transducer cMUT comprising: at least one membrane (105, 205a, 205b, 705a, 705b, 705c, 705d) designed to oscillate under the effect of an electric field and / or an acoustic wave, the membrane being formed of one or more layers (301-306) of nanotubes and / or nanowires and / or nano-beams.

The capacitive electroacoustic transducer cMUT according to claim 1, wherein the membrane (105, 205, 305, 405, 505, 605) is formed of at least one layer, parallel nanotubes and / or parallel nano-wires. and / or parallel nano-beams.

3. capacitive electroacoustic transducer cMUT according to claim 2, wherein the membrane (105, 205, 305, 405, 505, 605) is formed of parallel rows of aligned nanotubes and / or aligned nano-wires and / or nano-aligned beams.

4. Capacitive electroacoustic transducer cMUT according to one of claims 1 to 3, wherein the membrane is formed of one or more layers (301-306) of nanotubes and / or nanowires and / or nanobeams, parallel separated (e). ) s a distance of the order of the equilibrium distance of the van der Waals interaction.

5. capacitive electroacoustic transducer cMUT according to one of claims 1 to 4, the membrane being further formed: at least one layer (322, 332, 420, 520) called "connection", contiguous to the nanotubes and / or nano-wires and / or nano-beams.

6. capacitive electroacoustic transducer cMUT according to claim 5, the bonding layer being more deformable than all of said nanotubes and / or said nanowires and / or said nano-beams of the membrane.

7. Capacitive electroacoustic transducer cMUT according to one of claims 1 to 6, the membrane having a core consisting solely of nanotubes and / or nano-son and / or nano-beams.

8. Capacitive electroacoustic transducer cMUT according to one of claims 1 to 7, the membrane (305) comprising:

one or more layers (301, 302, 303) of nanotubes and / or nano-wires and / or nano-beams of a first critical dimension,

one or more layers (306, 307, 308) of nanotubes and / or nano-wires and / or nano-beams of a second critical dimension, different from said first critical dimension.

9. capacitive electroacoustic transducer cMUT according to one of claims 1 to 8, wherein the membrane is formed of at least a first layer (610) nanotubes (601a) and / or nano-wires and / or nano-beams oriented in a first direction and at least one second layer of nanotubes and / or nano-wires and / or nano-beams oriented in a second direction, orthogonal to the first direction.

10. capacitive electroacoustic transducer cMUT according to one of claims 1 to 9, further comprising: at least one cavity (110, 170) above which at least one membrane is intended to oscillate.

11. capacitive electroacoustic transducer cMUT according to claim 10, comprising at least one membrane, formed of one or more layers of nanotubes and / or nanowires and / or nano-beams, and located above several cavities. .

12. capacitive electroacoustic transducer cMUT according to one of claims 10 or 11, comprising a plurality of membranes (205a, 205b, 205c) each formed of one or more layers of nanotubes and / or nano-son and / or nano-beams, the membranes being located above the same cavity (710).

13. capacitive electroacoustic transducer cMUT according to one of claims 9 to 12, wherein a membrane (150) is located above several cavities or a matrix of cavities (71On, 710i2, 710i3 71O ₂ I, 71o _22, 71o _23, 71o ₃ I, 71o _32, 71o _33).

14. capacitive electroacoustic transducer cMUT according to one of claims 1 to 13, comprising several superposed membranes (705a, 705b, 705c, 705d), each formed of one or more layers of nanotubes and / or nanowires and or nano-beams, the membranes having different orientations.

15. capacitive electroacoustic transducer cMUT according to one of claims 10 to 14, one or more of said cavities being closed.

16. capacitive electroacoustic transducer cMUT according to one of claims 1 to 15, further comprising: at least one electrode (107, 207) for actuating the membrane and / or at least one electrode (107, 207) of detection.

17. The capacitive electroacoustic transducer cMUT according to claim 16, further comprising: - means (210) for applying a fixed potential to said electrode, and / or means (212) for applying a variable potential to said electrode, and / or means (220) for forming a capacimeter.

18. Acoustic imaging device or UHF sonar comprising a matrix of capacitive electro-acoustic transducers cMUT according to one of claims 1 to 17.