EP3670004A1 - Ultrasonic transducer with vibrating membrane with capacitive effect at high bandwidth - Google Patents

Abstract

The invention relates to an ultrasonic transducer with a vibrating membrane with capacitive effect (1), comprising: - a support (13) with a cavity (14) is provided; - a vibrating membrane (11) fixed to the support (13) and covering the cavity (14); -a conductive element (101) separated from the membrane (11) by the cavity (14); -characterized in that: -the vibrating membrane (11) has a resonance frequency in membrane mode fm and a resonance frequency in plate mode verifying the relationship fm> fp; -an excitation circuit (2) has terminals connected between the vibrating membrane (11) and the conductive element (101), and configured to apply between its terminals an electrical signal whose maximum frequency fo verifies the relationship fm> 1.5 ^∗ fo; or a measurement circuit (3) is connected between the vibrating membrane (11) and the conductive element and configured to measure the variations in capacity up to a frequency fo.

Description

The invention relates to acoustic transducers in the ultrasound range, and in particular such transducers with a vibrating membrane with capacitive effect.

Ultrasound is pressure waves whose frequency range starts at 20 KHz and extends up to a few tens of MHz. The ultrasound propagates at a speed depending on the propagation medium, around 343m / s in air, and 1500m / s in water. The waves which propagate undergo an absorption, which is all the more important as their frequency is high. Furthermore, when the wave encounters a discontinuity in the propagation medium, only part of the wave is transmitted, and the other part is reflected.

• créer une absorption d'ondes ultrasonores, par exemple pour un échauffement localisé de matière ;
• pour l'émission et la réception d'ondes par exemple pour une application de transmission d'informations;
• pour l'analyse de la réflexion d'ondes sur des obstacles par exemple pour une application de télémétrie.

Different applications use ultrasonic transducers to:

• create an absorption of ultrasonic waves, for example for a localized heating of material;
• for transmitting and receiving waves, for example for an information transmission application;
• for the analysis of wave reflection on obstacles, for example for a telemetry application.

For such applications, there is a growing need for miniaturized electroacoustic transducers for ultrasonic propagation in a fluid medium. In a fluid, an acoustic wave is generated by the movement of a moving surface. At the level of the moving surface, the acoustic intensity of the source is equal to the impedance of the medium multiplied by the square of the speed of the moving surface. For a given frequency, the greater the amplitude of the movements of the movable surface of the transducer, the more intense the source.

For miniaturized applications, vibrating membrane transducers are developed. Such transducers include membranes, suspended above cavities, formed in a support or open. The diameter of the circular cavities is generally between a few tens and a few hundreds of microns. The thickness of such membranes is generally greater than 50-100nm and up to several microns. The resonance frequencies of the complete device depend on the geometry of the assembly and the materials.

A special case of vibrating membrane transducer is the vibrating membrane transducer with capacitive effect. The membrane of an emitter is for example subjected to an electrostatic force by application of a difference in alternating potential between this membrane and a conductive electrode housed in the bottom of the cavity. For a detector, the value of the capacity formed between the membrane and the electrode housed at the bottom of the cavity is determined at all times by the deformation of the membrane, and therefore by the instantaneous incident pressure on the membrane. Detection is done by measuring variations in this capacity.

The displacement of the membrane is maximum at the resonant frequency of this membrane. The emission intensity is therefore maximum at the resonant frequency of its membrane. It is the same for the detection of a wave: the sensitivity of a sensor is maximum at the resonance frequency of its membrane.

Ultrasonic transducers are therefore generally associated with an optimal operating frequency which is determined by the resonance of their membrane. The quality coefficient of the mechanical resonator including the membrane determines the bandwidth of the transducer. A bandwidth is conventionally defined by the frequencies corresponding to a halving of the acoustic intensity with respect to the resonance, on either side of this resonance frequency. The widest bandwidths can be of the same order of magnitude as the resonance frequency: for example, a 1MHz resonant frequency transducer with a bandwidth of 600kHz, i.e. a bandwidth of 700kHz to 1300kHz, is considered to be a broadband transducer. Outside the bandwidth, the amplitudes of vibration can be several orders of magnitude lower than the amplitudes at resonance.

This resonant operating mode implies that each application requires a specific transducer, due to very different ultrasonic frequencies which cannot be covered by the same transducer: obstacle detection is typically carried out at 40 kHz with a range of a few meters in the air , gestures are captured between 100 kHz and 400 kHz with a range of a few tens of centimeters in the air, fingerprint detection is performed between 1 MHz and 10 MHz with a millimeter range in an inhomogeneous medium, and medical imaging ultrasound uses frequencies between 5MHz and 50MHz in an aqueous medium.

The document WO2012010786 offers an ultrasonic transducer with a vibrating membrane with a capacitive effect. In this transducer, a cavity of a support is maintained under vacuum under a membrane. The document proposes to operate the transducer at a frequency lower than the resonant frequency and to consider a wider range of operable frequencies, with performance varying according to the frequency operated.

There is therefore a need for designs of transducers having wider operating frequency bands. Furthermore, for telemetry applications, there is a need to minimize the blind area near them.

The invention aims to solve one or more of these drawbacks. The invention thus relates to an ultrasonic transducer with a vibrating membrane with capacitive effect, as defined in the appended claims.

The invention also relates to the variants of the dependent claims. Those skilled in the art will understand that each of the features of the dependent claims and the description can be combined independently with the features of an independent claim, without constituting an intermediate generalization.

• [Fig.1] est une vue en coupe schématique selon un plan vertical d'un exemple de transducteur ultrasonore à membrane vibrante à effet capacitif selon un mode de réalisation de l'invention ;
• [Fig.2] est une vue en coupe schématique selon un plan vertical d'un exemple de transducteur ultrasonore à membrane vibrante à effet capacitif selon un autre mode de réalisation de l'invention ;
• [Fig.3] est une vue en coupe schématique selon un plan vertical d'un exemple de transducteur ultrasonore à membrane vibrante à effet capacitif selon un autre mode de réalisation de l'invention ;
• [Fig.4] est une vue en coupe schématique selon un plan vertical d'un exemple de transducteur ultrasonore à membrane vibrante à effet capacitif selon un autre mode de réalisation de l'invention ;
• [Fig.5] est une vue en coupe schématique selon un plan horizontal d'une matrice de transducteurs ultrasonores selon le mode de réalisation de la figure 4;
• [Fig.6] est une vue en coupe schématique selon un plan vertical d'un exemple de transducteur ultrasonore à membrane vibrante à effet capacitif selon un autre mode de réalisation de l'invention ;
• [Fig.7] est un diagramme illustrant des résultats de mesure d'amplitudes de vibration dans l'air d'une membrane pour différentes tensions d'excitation.

Other characteristics and advantages of the invention will emerge clearly from the description given below, by way of indication and in no way limitative, with reference to the appended drawings, in which:

• [ Fig. 1 ] is a schematic sectional view along a vertical plane of an example of ultrasonic transducer with vibrating membrane with capacitive effect according to an embodiment of the invention;
• [ Fig. 2 ] is a schematic sectional view along a vertical plane of an example of an ultrasonic transducer with a vibrating membrane with capacitive effect according to another embodiment of the invention;
• [ Fig. 3 ] is a schematic sectional view along a vertical plane of an example of an ultrasonic transducer with a vibrating membrane with capacitive effect according to another embodiment of the invention;
• [ Fig. 4 ] is a schematic sectional view along a vertical plane of an example of an ultrasonic transducer with a vibrating membrane with capacitive effect according to another embodiment of the invention;
• [ Fig. 5 ] is a schematic sectional view along a horizontal plane of a matrix of ultrasonic transducers according to the embodiment of the figure 4 ;
• [ Fig. 6 ] is a schematic sectional view along a vertical plane of an example of an ultrasonic transducer with a vibrating membrane with capacitive effect according to another embodiment of the invention;
• [ Fig. 7 ] is a diagram illustrating the measurement results of vibration amplitudes in the air of a membrane for different excitation voltages.

The figure 1 is a schematic sectional view along a vertical plane of an exemplary transducer 1 of the ultrasonic type with vibrating membrane with capacitive effect according to a first embodiment of the invention. The transducer 1 comprises a support 13 in which a cavity 14 is formed. The cavity 14 is for example cylindrical. In the example illustrated, the support 13 notably includes a substrate 131 in the form of a plate, and a dielectric layer 132 also in the form of a plate. In the example illustrated, the support 13 also includes a conductive element 101. The substrate 131 and the dielectric layer 132 are here fixed to the conductive element 101. The dielectric layer 132 has a bore defining the side walls of the cavity 14. The bore depth of the cavity 14 is less than the height of the layer 132 or of the assembly of the two layers 132 and 102. The bottom 141 of the cavity 14. is thus advantageously delimited by a layer of dielectric 15 Part of the conductive element 101 is thus housed under the cavity 14, under the dielectric layer 15

A vibrating membrane 11 is fixed to the support 13 and covers the cavity 14. The membrane 11 has an external upper face 113 and an internal lower face 114. The membrane 11 is arranged opposite the conductive element 101. The membrane 11 and conductive element 101 are separated by cavity 14 and dielectric layer 15.

In the example illustrated, the membrane 11 is fixed to the dielectric layer 132 of the support 13 by means of an electrode 102. As detailed below, the electrode 102 is only an optional component for exciting the membrane 11. The electrode 102 is here in the form of a plate. The electrode 102 is here fixed on an upper face of the dielectric layer 132 and has a shape similar to this by being traversed by the same bore. The electrode 102 is in electrical contact with the membrane 11 at the periphery of the cavity 14.

The conductive element 101 forms an electrode of the transducer 1. An excitation circuit 2 has its terminals connected on the one hand to the electrode 102 and on the other hand to the conductive element 101. By applying a alternating potential between its terminals, the excitation circuit 2 creates an electric field between the membrane 11 and the conductive element 101, which subjects the membrane 11 to an electrostatic force and causes it to flex. The transducer 1 therefore has a capacitive effect.

In linear regime, the displacement d of the center of the membrane 11 in a direction normal to its plane at rest is proportional to the applied force F and to the stiffness modulus of the membrane: F = D ^∗ d

avec E le module de Young et η le coefficient de Poisson du matériau de la membrane 11 et h son épaisseur.For a membrane 11 forming a plate, in the absence of tension: $$\mathrm{D}=\mathrm{E}*{\mathrm{<figure-callout\; id="3"\; label="h"\; filenames="imgf0001.png,imgf0003.png"\; state="\{\{state\}\}">h</figure-callout>}}^{\mathrm{3}}/\mathrm{12}*\left(\mathrm{1}-{\mathrm{<figure-callout\; id="2"\; label="\eta "\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">\eta </figure-callout>}}^{\mathrm{2}}\right)$$

with E the Young's modulus and η the Poisson's ratio of the material of the membrane 11 and h its thickness.

In the mechanics of vibrations, the theory makes it possible to distinguish different vibratory behaviors according to the geometry and the realization of the vibrating membrane 11.

To simplify, we can first analyze different one-dimensional objects, such as a beam and a rope. A beam will have a behavior and a resonant frequency mainly determined by its geometry (its length and its section) and the Young's modulus of the material which composes it. The behavior of a rope will be essentially defined by its tension. The tighter the string, the higher its resonant frequency.

• un comportement de type plaque, déterminé par la géométrie de l'objet et son matériau. Une fréquence de résonance fp est associée à ce comportement ;
• un comportement de type membrane, principalement défini par la tension dans l'objet. Une autre fréquence de résonance fm est associée à ce comportement.

Similarly, for two-dimensional objects, we meet both:

• a plate-like behavior, determined by the geometry of the object and its material. A resonance frequency fp is associated with this behavior;
• a membrane type behavior, mainly defined by the tension in the object. Another resonance frequency fm is associated with this behavior.

The object's resonant frequency is the quadratic sum of the resonant frequencies governed by each of these two behaviors.

For a circular object of radius R embedded on its periphery, the resonance frequency fr is defined by the relation: $$\mathrm{Fr}\left(\mathrm{R}\right)=\mathrm{\surd }\left({\mathrm{fm}}^{\mathrm{2}}\left(\mathrm{R}\right)+{\mathrm{fp}}^{\mathrm{2}}\left(\mathrm{R}\right)\right)$$

The resonance frequency fm in membrane mode can in particular be defined in this case by the following relationship, with T the tension of the object (in N / m) and s its surface density (in kg / m ² ): $$\mathrm{fm}=\frac{2.405}{2\pi \ast R}\ast \surd \left(\mathrm{T}/\mathrm{S}\right)$$

The resonance frequency fp in plate mode can in particular be defined in this case by the following relationship, with p the density of the circular object (in kg / m ³ ): $$\mathrm{fp}=\frac{11.84}{{\mathrm{<figure-callout\; id="2"\; label="R"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">R</figure-callout>}}^2}\ast \surd \left(\mathrm{E}\ast {\mathrm{<figure-callout\; id="2"\; label="h"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">h</figure-callout>}}^2/\mathrm{\rho }\right)$$

Those skilled in the art will be able to define empirically or analytically the resonance frequencies fp and fm for other geometries of vibrating membranes.

According to a preferred aspect of the invention, the vibrating membrane 11 of the transducer 1 verifies the following relationship: fm> fp. Preferably, the vibrating membrane 11 of the transducer 1 verifies the following relationship: fm> 1.5 ^∗ fp, and even more preferably fm> 2.5 ^∗ fp. The vibrating membrane 11 of the transducer 1 therefore has a preponderant membrane mode compared to its plate mode. Advantageously, by satisfying this inequality, one can place oneself in a mode where the membrane presents a significant displacement far from the resonance, that is to say in linear mode.

According to the invention, the excitation circuit 2 is configured to apply between its terminals a signal whose frequency components are included in the frequency interval [0-fo], fo verifying the relation f0 <fr, and preferably fo <0.66 ^∗ fr (i.e. fr> 1.5 ^∗ fo). So we deduce f0 <fr <fm. Thus, the membrane 11 is excited at a frequency significantly lower than its resonance frequency in membrane mode, the displacements of the membrane are not caused by a resonance phenomenon, but by a mechanism of forced oscillations, which makes it possible to have a wide range of operable excitation frequencies, from very low frequencies (a few Hz) up to 0.66 ^∗ fr, over which the performances are constant. The same transducer 1 can thus be used for many different applications. The use of forced oscillations also makes it possible to generate short pulses, unlike resonant excitations, and therefore, for telemetry applications for example, to minimize the blind zone. The use of forced oscillations also makes it possible to increase the excitation power at constant frequency. Advantageously, the excitation circuit 2 is configured to apply between its terminals an excitation signal with a maximum frequency fo verifying the relation fr> f0, and advantageously fr> 1.5 ^∗ fo

Advantageously, the excitation circuit 2 is configured to apply between its terminals a signal such as the ratio between the total electric power applied between these terminals and the electric power applied over a frequency range between 0.9 ^∗ fr and 1.1 ^∗ fr is at least equal to 10. Thus, most of the excitation power is applied outside the resonance range.

The diagram of the figure 7 illustrates results of measurement of vibration amplitudes in the air of a membrane for different excitation voltages. A membrane above a 2µm cavity was excited at fo = 5Hz for different excitation voltages. The fundamental resonance of this type of membrane is located at frequencies between 20MHz and 30MHz. The curve in solid line corresponds to an excitation voltage of 16 V, the curve in broken line corresponds to an excitation voltage of 12V, the curve in dotted lines corresponds to an excitation voltage of 8V, and the curve in dash -point corresponds to an excitation voltage of 4V. The amplitudes of deformation at 5 Hz are therefore in forced oscillation mode, and these amplitudes increase with the excitation voltage, as shown in this diagram. Amplitudes of a few tens of nanometers have been obtained for a cavity diameter of 2 µm. These very large deformations without resonance make it possible to generate ultrasound.

Ultrasound was emitted experimentally between 20 kHz and 140 kHz by membranes 15 nm thick suspended above circular cavities of 10 µm in diameter.

• l'élément conducteur 101 peut par exemple présenter une épaisseur comprise entre 150 et 250nm, par exemple 200nm. L'élément conducteur 101 peut par exemple être réalisé en tungstène, en aluminium, en titane, en Cuivre, en or, ou en une combinaison de ces matériaux ;
• la couche de diélectrique 132 peut par exemple présenter une épaisseur comprise entre 0,8 et 1,25 µm, par exemple 1 µm. La couche de diélectrique 132 peut par exemple être réalisée en SiO₂ ;
• le substrat 131 peut par exemple être réalisé en verre, en quartz, en alumine, en silicium recouvert d'une couche diélectrique ou encore en SiN ;
• la cavité 14 peut présenter un diamètre compris entre 5 et 50µm, par exemple 10 µm (définissant la longueur suspendue de la membrane 11) ;
• l'électrode 102 peut par exemple présenter une épaisseur comprise entre 80 et 150nm, par exemple 100nm. L'électrode 102 peut par exemple être réalisée en tungstène, en aluminium, en titane, en Cuivre, en or, ou tout autre matériau ou alliage conducteur. L'électrode 102 peut être formée par un dépôt d'un matériau conducteur sur un support isolant ;
• la membrane 11 peut par exemple présenter une épaisseur comprise entre 5 et 25nm, par exemple 10nm. La membrane 11 peut par exemple présenter une couche de carbone amorphe. La membrane 11 peut être fixée au support 13 sans précontrainte en tension.

The components of transducer 1 can have the following dimensions and compositions:

• the conductive element 101 may for example have a thickness of between 150 and 250 nm, for example 200 nm. The conductive element 101 can for example be made of tungsten, aluminum, titanium, copper, gold, or a combination of these materials;
• the dielectric layer 132 may for example have a thickness of between 0.8 and 1.25 μm, for example 1 μm. The dielectric layer 132 can for example be made of SiO ₂ ;
• the substrate 131 can for example be made of glass, quartz, alumina, silicon covered with a dielectric layer or even SiN;
• the cavity 14 may have a diameter between 5 and 50 μm, for example 10 μm (defining the suspended length of the membrane 11);
• the electrode 102 can for example have a thickness of between 80 and 150nm, for example 100nm. The electrode 102 can for example be made of tungsten, aluminum, titanium, copper, gold, or any other conductive material or alloy. The electrode 102 can be formed by depositing a conductive material on an insulating support;
• the membrane 11 may for example have a thickness of between 5 and 25 nm, for example 10 nm. The membrane 11 may for example have an amorphous carbon layer. The membrane 11 can be fixed to the support 13 without prestressing in tension.

Advantageously, the membrane 11 has a thickness at most equal to 100 nm. The membrane 11 can advantageously be provided to vibrate in the cavity 14 over an amplitude of at least 5% of the suspended length and less than the depth of the cavity.

The diameter of the cavity 14 can be reduced in order to increase the resonant frequency of the membrane 11.

A continuous or very low frequency electrostatic force can be applied by the excitation circuit 2 to impose an initial mechanical voltage on the vibrating membrane 11. The excitation circuit 2 will then apply a potential difference between the electrode 102 and the element 101 with a DC or very low frequency component (for example at most equal to 50 Hz), so as among other things to be able to modulate the sensitivity and the dynamics of the transducer 1.

The figure 2 is a schematic sectional view along a vertical plane of an exemplary transducer 1 of the ultrasonic type with vibrating membrane with capacitive effect according to a second embodiment of the invention. The transducer 1 comprises a support 13, a conductive element 101, an electrode 102 and a membrane 11 identical to those of the first embodiment. In this example, a measurement circuit 3 is electrically connected between the membrane 11 and the conductive element 101 (via the connection via 103).

The measurement circuit 3 measures the charge displacements linked to the instantaneous variation in capacitance between the electrode 102 and element 101, induced by the vibrations of the membrane 11.

The measurement circuit 3 may also apply a potential difference between the electrode 102 and the element 101 with a DC or very low frequency component (for example at most equal to 50 Hz), so as to be able to modulate the sensitivity and the dynamics. transducer 1.

One can also consider connecting an excitation circuit 2 and a measurement circuit 3 as described above between the membrane 11 and the conductive element 101. The connection of the excitation circuit 2 and the measurement circuit 3 can be set implemented selectively and independently by respective switches. It is then possible to independently process the emission and reception of an acoustic signal, for example to implement a telemetry mode.

The figure 3 is a schematic sectional view along a vertical plane of an example of ultrasonic transducer with vibrating membrane with capacitive effect according to a third embodiment of the invention. The transducer 1 comprises an excitation circuit 2 (and may include a measurement circuit 3 in addition or as a replacement), a conductive element 101, an electrode 102 and a membrane 11 identical to those of the first embodiment. In this example, the support 13 comprises at least one conduit 104 placing the interior of the cavity 14 in communication with the exterior of the transducer. The conduits 104 allow the cavity 14 of the transducer 1 (and therefore the internal face 114 of the membrane 11) to be placed in communication with the external face 113 of the membrane 11. It is thus possible to balance the pressures between the faces 113 and 114 of the membrane 11 and ensure that all the cavities are at the same pressure, in this case atmospheric pressure. The conduits 104 can also be replaced by grooves on the upper surface of the dielectric layer 132.

The figure 4 is a schematic sectional view along a vertical plane of an example of ultrasonic transducer with vibrating membrane with capacitive effect according to a fourth embodiment of the invention. In this example, the membrane 11 includes several superimposed layers of different materials. The membrane 11 thus includes a superposition of a layer 111 and a layer 112 of different materials. The layer 111 is for example formed from a conductive material such as Titanium. The layer 111 can for example have a thickness between 3 and 7 nm, typically 5 nm. The layer 112 is for example formed from amorphous carbon. The layer 112 may for example have a thickness of between 8 and 12 nm, typically 10 nm. Such a configuration turns out to be optimal to favor on the one hand the capacitive effect by the use of the conductive layer 111, and on the other hand to favor the flexibility and the tenacity of the membrane 11 by the use of the layer 112.

In the various examples, if the membrane 11 includes a conductive layer, one can dispense with interposing an electrode 102 between this membrane 11 and the circuits to which it is connected. In particular, the embodiment which follows describes a membrane 11 including a combination of a conductive layer and a layer chosen for its mechanical properties: the electrode 102 can be dispensed with by direct connection of a circuit to the conductive layer. This corresponds to the example of the figure 6 , using the membrane 11 described with reference to the figure 5 .

The figure 5 is a schematic sectional view along a horizontal plane of a matrix of ultrasonic transducers 1 according to the embodiment of the figure 4 . The transducers can be arranged in a matrix of 500 by 500 transducers 1 along two perpendicular axes. A corresponding matrix of cavities 14 is thus formed in a common layer of dielectric 132. The cavities 14 of the same column of transducers 1 are placed in communication via conduits 109. At the ends of the matrix, the transducers 1 are placed in communication with the outside by conduits 104. Thus, each transducer 1 has its cavity in communication with the external face of its membrane via conduits 104 and 109 if necessary. The same membrane 11 can be used to cover all of the cavities 14.

The pressure in the cavities 14 can also be different from the surrounding pressure. Peripheral sealing can thus be implemented, if the different cavities 14 of the matrix are placed in communication with one another.

In order to be able to orient the transmission or reception beam, a network of transducers 1 may comprise a plurality of conductive elements 101 (for example arranged in parallel) and / or a plurality of electrodes 102 (for example in parallel. One can for example form a plurality of channels. Parallel conductive elements 101 can be positioned perpendicular to the parallel electrodes 102. Network elements are thus delimited by the superposition of an electrode 102 and a conductive element 101 and individually addressable. The pitch between the conducting elements of 101 or 102 can be reduced to the pitch of the network of elementary transducers. An elementary transducer diameter reduced to 10 μm with a pitch of 15 μm makes it possible for example to perform beam forming (beam forming in English) up to more than 10 MHz in the air.

Claims (12)

[Ultrasonic transducer with vibrating membrane with capacitive effect (1), comprising: - a support (13) in which at least one cavity (14) is provided; - a vibrating membrane (11) fixed to the support (13) and covering the cavity (14); - a conductive element (101) separated from the membrane (11) by the cavity (14); - characterized in that : - the vibrating membrane (11) has a resonance frequency in membrane mode fm and a resonance frequency in plate mode fp verifying the relationship fm>fp; - an excitation circuit (2) has terminals connected between the vibrating membrane (11) and the conductive element (101), and configured to apply between its terminals an electrical signal whose maximum frequency fo verifies the relationship fr> fo , fr being the resonance frequency of the membrane; and or - a measurement circuit (3) is connected between the vibrating membrane (11) and the conductive element and configured to measure the variations in capacity up to a frequency fr> fo. Ultrasonic transducer (1) according to claim 1, wherein the vibrating membrane (11) is configured to check the relationship fm> 1.5 ^∗ fp. Ultrasonic transducer according to claim 1 or 2, in which an excitation circuit (2) has terminals connected between the vibrating membrane (11) and the conductive element (101), and configured to apply between its terminals an electrical signal of so that the ratio between the total electric power applied between these terminals and the electric power applied over a frequency range between 0.9 ^∗ fr and 1.1 ^∗ fr is at least equal to 10. Ultrasonic transducer (1) according to any one of the preceding claims, in which an excitation circuit (2) has terminals connected between the vibrating membrane (11) and the conductive element (101), and configured to apply an electrical signal between its terminals with the maximum frequency fo verifying the relation fr> 5 ^∗ fo. Ultrasonic transducer (1) according to any one of the preceding claims, in which said support (13) places an external face (113) of the membrane in communication with the cavity (14) delimited by an internal face (114) of the membrane. . Ultrasonic transducer (1) according to claim 5, in which a matrix of cavities (14), including said cavity, is formed in the support (13), several of said cavities being in communication (109), a respective conductive element (101) being housed under each of said cavities (14), the ultrasonic transducer comprising a matrix of vibrating membranes (11), including said vibrating membrane, and as defined in claim 1 and covering respective cavities. Ultrasonic transducer (1) according to any one of the preceding claims, in which said membrane (11) has a thickness at most equal to 100 nm. Ultrasonic transducer (1) according to any one of the preceding claims, in which said membrane (11) is a combination of several layers of different materials (111, 112). An ultrasonic transducer (1) according to any one of the preceding claims, wherein said membrane includes a layer of amorphous carbon (112). An ultrasonic transducer (1) according to any one of the preceding claims, wherein said membrane includes a layer of titanium (111). Ultrasonic transducer (1) according to any one of the preceding claims, further comprising an electrode (102) in electrical contact with the membrane and with the excitation circuit (2) or the measurement circuit (3) if necessary. Ultrasonic transducer (1) according to any one of the preceding claims, in which an excitation circuit (2) or a measurement circuit (3) has terminals connected between the vibrating membrane (11) and the conductive element (101 ), the excitation circuit (2) or the measure (3) being further configured to apply a potential difference with a DC component or a component at a frequency less than 50 Hz.

Images