WO2025078145A1 - Ultrasonic probe comprising an array of transducer elements - Google Patents

Abstract

The present description relates to an ultrasonic probe comprising an array (200) of transducer elements (201), the probe comprising: - a first piezoelectric layer (210) divided into first piezoelectric sub-elements (211), each comprising a first electrode (215) on a first face (210A) of the first piezoelectric layer, wherein the first dielectric layer comprises a first ground electrode (213) on a second face (210B) of the first piezoelectric layer; - a second piezoelectric layer (220) above the first piezoelectric layer, wherein the second piezoelectric layer comprises a second ground electrode (223) on a first face (220A) of the second piezoelectric layer, and is divided into second piezoelectric sub-elements (221), each comprising a second electrode (225) on a second face (220B) of the second piezoelectric layer; - a ground plane (203) located between the second face of the first piezoelectric layer and the first face of the second piezoelectric layer; - a first collector (230) attached to the first face of the first piezoelectric layer; - a second collector (240) attached to the second face of the second piezoelectric layer.

Description

DESCRIPTION

TITLE: Ultrasonic probe with transducer element array

The present application is based on, and claims priority from, French patent application 2310965 filed on October 12, 2023 and entitled "Ultrasonic probe with matrix of transducer elements", which is considered to be an integral part of the present description within the limits provided by law.

Technical field

[0001] The present description relates generally to ultrasonic probes with a matrix of transducer elements, which may be designated "matrix ultrasonic probes", and in particular to low-frequency matrix ultrasonic probes, i.e. adapted to operate at frequencies typically lower than 2 MHz. The present description relates in particular to curved matrix ultrasonic probes.

Prior art

[0002] Matrix ultrasonic probes generally comprise a plurality of transducer elements arranged next to each other in a matrix array.

[0003] The transducer elements of the matrix network are preferably addressed individually, as opposed to row and column addressing.

[0004] When the transducer elements are piezoelectric transducer elements, each transducer element typically comprises a layer of piezoelectric material, or piezoelectric layer, and front and rear electrodes adapted to apply an electrical excitation signal to the transducer element and/or to recover a signal from the transducer element to be transformed into a signal. electrical. One of the front and rear electrodes can be connected to ground, and the other can be dedicated to the signal.

[0005] Matrix ultrasound probes can be used for 2D or even 3D imaging, particularly in medical imaging applications.

[0006] There is a need for an ultrasonic probe with a matrix of piezoelectric transducer elements, which is suitable for operating at low frequency, i.e. at frequencies typically lower than 2 MHz, at least partially overcoming some of the drawbacks of known matrix ultrasonic probes.

[0007] It would be desirable to have such a matrix ultrasonic probe which can also be curved.

Summary of the invention

[0008] One embodiment overcomes all or part of the drawbacks of known matrix ultrasonic probes.

[0009] One embodiment provides an ultrasonic probe comprising an array of piezoelectric transducer elements, said array comprising:

- a first piezoelectric layer divided into a plurality of first piezoelectric sub-elements each comprising a first electrode at a first face of said first piezoelectric layer, the first dielectric layer further comprising a first ground electrode at a second face opposite the first face of said first piezoelectric layer;

- a second piezoelectric layer stacked above the first piezoelectric layer, the second dielectric layer comprising a second ground electrode at a first face of said second piezoelectric layer and being divided into a plurality of second piezoelectric sub-elements each comprising a second electrode at a second face opposite the first face of said second piezoelectric layer; the first and second piezoelectric layers extending along a main plane, and the second piezoelectric sub-elements being aligned with the first piezoelectric sub-elements in a direction transverse to said main plane;

- a ground plane located between, and in contact with, the second face of the first piezoelectric layer and the first face of the second piezoelectric layer;

- a first collector assembled to the first face of the first piezoelectric layer, and comprising first conductive tracks connected to the first electrodes;

- a second collector assembled to the second face of the second piezoelectric layer, and comprising second conductive tracks connected to the second electrodes; each piezoelectric transducer element comprising one of the first piezoelectric sub-elements aligned with one of the second piezoelectric sub-elements, and a portion of the ground plane between said first and second aligned piezoelectric sub-elements.

[0010] According to one embodiment, the first and second electrodes of the same piezoelectric transducer element are electrically connected to each other, for example the first and second metal tracks connected respectively to said first and second electrodes are electrically connected to each other.

[ 0011 ] According to one embodiment, the first piezoelectric layer and/or the second piezoelectric layer:

- comprises a piezoelectric composite; and/or

- has a thickness between 0.5 and 10 mm.

[ 0012 ] According to one embodiment, the ground plane:

- is a metallic layer, for example a metal plate, for example copper, aluminum or brass; and/or

- extends beyond the stack of the first and second piezoelectric layers by a connection tab adapted to electrically connect the ground plane to a ground outside of said stack; and/or

- has a thickness between 10 and 500 pm.

[0013] According to one embodiment, the matrix comprises:

- a first thickness of piezoelectric material of the first piezoelectric layer between two adjacent first piezoelectric sub-elements on a first face of the ground plane, the first thickness being less than the thickness of the first piezoelectric layer; and/or

- a second thickness of piezoelectric material of the second piezoelectric layer between two second adjacent piezoelectric sub-elements on a second face of the ground plane opposite the first face of said ground plane, the second thickness being less than the thickness of the second piezoelectric layer.

[0014] According to one embodiment, the matrix has a curved shape, the main plane then being curved.

[0015] According to one embodiment, the matrix further comprises a first acoustic impedance matching layer between the first piezoelectric layer and the first collector, said first acoustic impedance matching layer being structured such that each piezoelectric transducer element comprises a portion of said first acoustic impedance matching layer between the first electrode and the first collector, the first acoustic impedance matching layer being for example made of graphite.

[0016] According to one embodiment, the probe further comprises a second acoustic impedance matching layer, the first collector being between the first layer piezoelectric and said second acoustic impedance matching layer, for example between a first acoustic impedance matching layer and said second acoustic impedance matching layer.

[ 0017 ] According to one embodiment:

- the first collector comprises a first insulating substrate, for example a flexible insulating substrate, the first metal tracks being located on or in the first insulating substrate, and being connected to first contact pads connected or connected to the first electrodes; and/or

- the second collector comprises a second insulating substrate, for example a flexible insulating substrate, the second metal tracks being located on or in the second insulating substrate, and being connected to second contact pads connected or connected to the second electrodes.

[0018] According to one embodiment, the first and second piezoelectric sub-elements of each piezoelectric transducer element are divided into several parts, the parts of the same piezoelectric transducer element sharing the same first and second electrodes.

[0019] According to one embodiment, the first piezoelectric sub-elements and the sub-second piezoelectric elements are distributed in two orthogonal directions of the main plane.

[0020] According to one embodiment, the ultrasonic probe further comprises an acoustic attenuation layer on the second collector, a shaper on the acoustic attenuation layer, and a printed circuit on the shaper, the ground plane being electrically connected to a ground via the printed circuit, and the first and second electrodes of a same piezoelectric transducer element being for example also electrically connected to each other by means of the printed circuit.

[0021] One embodiment provides a method of manufacturing an array of piezoelectric transducer elements of an ultrasonic probe, the method comprising: forming a stack of a first piezoelectric layer having a first ground electrode on a second face, a ground plane on the second face of the first piezoelectric layer, and a second piezoelectric layer having a second ground electrode at a first face positioned on the ground plane, the first and second piezoelectric layers extending along a principal plane;

- cutting, in all or part of its thickness, the first piezoelectric layer into several first piezoelectric sub-elements each comprising a first electrode on a first face of the first piezoelectric layer opposite the second face of said first piezoelectric layer;

- cutting, in all or part of its thickness, the second piezoelectric layer into several second piezoelectric sub-elements each comprising a second electrode on a second face of the second piezoelectric layer opposite the first face of said second piezoelectric layer; the second piezoelectric sub-elements being aligned with the first piezoelectric sub-elements in a direction transverse to the main plane; the cuts of the first and second piezoelectric layers stopping at least before the ground plane; the assembly of a first collector comprising first conductive tracks on the first face of the cut first piezoelectric layer, said assembly comprising electrically connecting the first conductive tracks to the first electrodes; assembling a second collector comprising second conductive tracks to the second face of the second cut piezoelectric layer, said assembly comprising electrically connecting the second conductive tracks to the second electrodes; each piezoelectric transducer element comprising one of the first piezoelectric sub-elements aligned with one of the second piezoelectric sub-elements, and a portion of the ground plane between said aligned first and second piezoelectric sub-elements.

[0022] According to one embodiment, the method further comprises forming an electrical connection between the first and second electrodes of the same piezoelectric transducer element, for example by electrically connecting the first and second metal tracks connected respectively to said first and second electrodes.

[0023] According to one embodiment, the first and second piezoelectric layers are not cut over their entire thickness, so as to retain a thickness of piezoelectric material on at least one of the two faces of the ground plane.

[0024] According to one embodiment, the steps of cutting the first and second piezoelectric layers further comprise secondary cuts of the first and second piezoelectric sub-elements so as to divide each piezoelectric transducer element into several parts sharing the same first and second electrodes, the secondary cuts being less deep than the cuts for forming the first and second piezoelectric sub-elements.

[0025] According to one embodiment, the method further comprises assembling a first adaptation layer acoustic impedance to the first face of the first piezoelectric layer before the step of assembling the first collector, so that the first acoustic impedance matching layer is found in the matrix between the first piezoelectric layer and the first collector, the cutting of the first piezoelectric layer including the cutting of said first acoustic impedance matching layer throughout its thickness.

[0026] According to one embodiment, the method further comprises a step of curving the matrix after the steps of cutting the first and second piezoelectric layers, and for example after the steps of assembling the first and second collectors.

Brief description of the drawings

[0027] These characteristics and advantages, as well as others, will be explained in detail in the following description of particular embodiments given without limitation in relation to the attached figures among which:

[0028] Figure 1A is a very schematic perspective view of an example of a matrix of piezoelectric transducer elements of a matrix ultrasonic probe;

[0029] Figure 1B is an exploded perspective view showing details of an array of piezoelectric transducer elements similar to the array of Figure 1A;

[0030] Figure 2A is a very schematic perspective view of a matrix of piezoelectric transducer elements of an ultrasonic probe according to one embodiment;

[0031] Figure 2B is a perspective view showing details of collectors of an array of piezoelectric transducer elements similar to the array of Figure 2A; [0032] Figure 3A, Figure 3B, Figure 3C and Figure 3D are perspective and side views illustrating successive steps of an exemplary method of manufacturing an array of piezoelectric transducer elements of an ultrasonic probe according to one embodiment; and

[0033] Figure 4 is a very schematic perspective view of an ultrasonic probe according to one embodiment.

Description of the embodiments

[0034] The same elements have been designated by the same references in the different figures. In particular, the structural and/or functional elements common to the different embodiments may have the same references and may have identical structural, dimensional and material properties.

[0035] For the sake of clarity, only the steps and elements useful for understanding the described embodiments have been shown and are detailed. In particular, the production of the piezoelectric transducers of the described probes has not been detailed, the described embodiments being compatible with all or most of the known structures of piezoelectric transducers. Furthermore, the production of the control circuits of the piezoelectric transducers has not been detailed, the described embodiments being compatible with the usual control circuits of piezoelectric transducers or the production of the control circuits being within the reach of the person skilled in the art from the indications of the present description.

[ 0036 ] Unless otherwise specified, when referring to two elements connected to each other, this means directly connected without intermediate elements other than conductors, and when we refer to two elements connected (in English "coupled") to each other, this means that these two elements can be connected or be connected by means of one or more other elements.

[0037] In the following description, when reference is made to absolute position qualifiers, such as the terms "front", "back", "top", "bottom", "left", "right", etc., or relative position qualifiers, such as the terms "above", "below", "upper", "lower", etc., or to orientation qualifiers, such as the terms "horizontal", "vertical", etc., reference is made unless otherwise specified to the orientation of the figures.

[0038] Unless otherwise specified, the expressions "about", "approximately", "substantially", and "of the order of" mean to within 10%, preferably to within 5%.

[0039] Unless otherwise specified, when referring to two elements assembled together, this includes another element, or one or more layers, being between the two elements.

[0040] In the following description, when referring to a plane or a principal plane, this plane may be curved. A principal plane may be referred to as a principal surface.

[0041] In the following description, when reference is made to "aligned" or "aligned in the transverse direction" elements, this means that these elements are superimposed one on top of the other and that the edges of the superimposed elements correspond substantially, that is to say that there is little or no overhang between the superimposed elements. It may also be indicated that these elements are facing each other. [0042] In the following description, when reference is made to a probe, an ultrasonic probe or an ultrasonic matrix probe, reference is made, unless otherwise specified, to an ultrasonic probe comprising an array of piezoelectric transducer elements. Furthermore, when reference is made to transducer elements, or to piezoelectric elements, or to transducers, reference is made, unless otherwise specified, to piezoelectric transducer elements.

[0043] In the following description, when reference is made to a collector, unless otherwise specified, reference is made to an electrical collector, which corresponds to a set of conductive tracks insulated from each other in the collector and arranged on an insulating substrate, for example flexible, for example made of a polyimide material.

[0044] Each conductive track is generally terminated at one of its ends by a metal surface intended to be in contact with an electrode of a transducer element, and at its other end by a metal surface making it possible to connect the conductive track to the rest of the ultrasonic probe.

[0045] The following description relates to ultrasonic probes with an array of piezoelectric transducer elements. The piezoelectric transducer elements may be made from a layer of piezoelectric material, or piezoelectric layer, which layer is then divided into several piezoelectric elements.

[0046] The lower the operating frequency of an array ultrasonic probe, the thicker the piezoelectric layer must be. For example, the thickness may be about 6 mm for 400 MHz. However, the thicker the piezoelectric layer, the greater the electrical capacity of the piezoelectric transducer elements of the array is small. In addition, the surface area of each piezoelectric transducer element of the array may be small, further reducing the electrical capacitance.

[0047] To increase the acoustic and electrical performance of the probe, one solution is to superimpose two layers of piezoelectric material, each layer being between two electrodes, one of which can be connected to ground, the other generally being for a signal. This solution aims to reduce the imaginary part of the electrical impedance of the probe. Depending on the arrangement of the electrodes, the polarizations of the two piezoelectric layers are generally normal to the main surfaces of these piezoelectric layers and either in opposite directions or in the same direction.

[0048] For example, one can use a stack of two piezoelectric layers each having a thickness equal to half the thickness of a single piezoelectric layer, and each being between two electrodes. This forms two capacitors stacked on top of each other which can be added. The thickness of the piezoelectric material of each layer is then divided by two, and the capacity of the stack can be multiplied by four for the same total thickness. The imaginary part of the electrical impedance can thus be divided by four.

[0049] Figure 1A is a highly schematic perspective view of an exemplary array 100 of piezoelectric transducer elements of an array ultrasound probe. Figure 1B is an exploded perspective view showing details of an array of piezoelectric transducer elements similar to the array 100 of Figure 1A.

[0050] The matrix 100 comprises two piezoelectric layers superimposed on each other: a first layer piezoelectric 110 surmounted by a second piezoelectric layer 120.

[0051] As illustrated in more detail in Figure 1B, the upper and lower faces of the first piezoelectric layer 110 are metallized, that is to say that the first piezoelectric layer 110 comprises a core of piezoelectric material 111 covered on the upper and lower faces by a metal layer: a first metal layer 112 on the upper face and a second metal layer 113 on the lower face. The first metal layer 112 is structured, that is to say divided in the direction of its thickness, or transverse direction, using several first notches 114 (kerfs), to form several first electrodes 115 separated from each other by these first notches, thus forming several first piezoelectric transducer sub-elements. The first notches 114 therefore correspond to portions of the first piezoelectric layer 110 in which the upper metallization is removed. The second metal layer 113, unstructured, forms a first common ground electrode.

[0052] Similarly, the upper and lower faces of the second piezoelectric layer 120 are metallized, that is to say that the second piezoelectric layer 120 comprises a core of piezoelectric material 121 covered on the upper and lower faces by a metallic layer: a first metallic layer 122 on the lower face and a second metallic layer 123 on the upper face. The first metallic layer 122 is structured, that is to say divided in the direction of its thickness, using several second notches 124 (kerfs), to form several second electrodes 125 separated from each other by these second notches, thus forming several second piezoelectric transducer sub-elements. The second notches 124 therefore correspond to portions of the second piezoelectric layer 120 in which the lower metallization is removed. The second metal layer 123, unstructured, forms a second common ground electrode.

[0053] Preferably, the first notches 114 are arranged opposite the second notches 124. Preferably, the first electrodes 115 are arranged opposite the second electrodes 125.

[0054] The depth of each notch 114, 124 is greater than or equal to the thickness of the metallization of the first metal layer 112, 122, and may extend into the piezoelectric layer, or even into the entire thickness of the piezoelectric layer, or even pass through the second metal layer 113, 123, thus forming several ground electrodes, in which case, it is generally ensured that these ground electrodes can again be electrically connected.

[0055] A collector 130 is disposed between the first and second metallized piezoelectric layers. The collector 130 may comprise an insulating substrate, typically flexible (flex).

[0056] As illustrated in more detail in Figure 1B, the collector 130 comprises:

- an insulating substrate 131, for example flexible, for example made of a polyimide material;

- first metal tracks 132 on a lower face 131A (first face) of the insulating substrate, each first metal track 132 ending with a first metal pad 133, or contact pad;

- second metal tracks 134 on an upper face 131B (second face, opposite the first face) of the substrate insulator, each second metal track 134 terminating in a second metal pad 135, or contact pad.

[0057] Preferably, the second metal tracks 134 are arranged opposite the first metal tracks 132. Preferably, the second metal pads 135 are arranged opposite the first metal pads 133.

[0058] To form the first and second metal tracks and pads respectively, the collector 130 can be locally metallized on each of its first and second faces.

[0059] Each first electrode 115 of the first piezoelectric layer 110 is electrically connected to a first metal pad 133 of the collector, and each second electrode 125 of the second piezoelectric layer 120 is electrically connected to a second metal pad 135 of the collector 130.

[0060] Preferably, the first metal pads 133, the second metal pads 135, the first electrodes 115 and the second electrodes 125 are aligned, an assembly formed by the alignment of a first metal pad 133, a second metal pad 135, a first electrode 115 and a second electrode 125 forming a piezoelectric transducer element 101. The first and second notches 114, 124 also make it possible to facilitate this alignment.

[0061] The surface area of the first metal pads 133, respectively of the second metal pads 135, may be less than the surface area of the first electrodes 125, respectively of the second electrodes 115, in particular in order to compensate for possible alignment errors during the operation of stacking the collector 130 with the two piezoelectric layers 110, 120. Indeed, too large an alignment error may lead to a first metal pad 133, respectively a second metal pad 135, short-circuits a first electrode 115 with another adjacent first electrode, respectively a second electrode 125 with another adjacent second electrode.

[0062] Furthermore, the matrix 100 comprises one or more acoustic impedance matching layers 102, preferably on a first face 100A (front face) of the matrix intended to be oriented on the side of the region of interest. The acoustic impedance matching layer 102 can thus be at least partially in contact with the first piezoelectric layer 110.

[0063] A conductive strip, or sheet 103, for example metallic, is provided at each end and on the outside of the first and second metallized piezoelectric layers 110, 120 for grounding with the rest of the ultrasonic probe. Each conductive grounding strip 103 is in contact with the second metal layers 113, 123 of the first and second piezoelectric layers 110, 120, for example by being folded over the second metal layers 113, 123, and can thus be positioned between the first piezoelectric layer 110, for example the second metal layer 113 of the first piezoelectric layer 110, and the acoustic impedance matching layer 102.

[0064] A disadvantage of the implementation of the matrix 100 of FIGS. 1A and 1B is that the metal tracks 132, 134 and/or the metal pads 133, 135 of the collector 130 can short-circuit the piezoelectric transducer elements 101. In the example of FIGS. 1A and 1B, the first and second electrodes 115, 125, and the first and second metal pads 133, 135 are well aligned, forming piezoelectric transducer elements 101 insulated from each other, and the metal tracks 132, 134 of the collector 130 are not covered by the metallized piezoelectric layers 110, 120, which corresponds to an ideal configuration.

[0065] On the other hand, when the matrix of piezoelectric transducer elements is made up of piezoelectric elements in two directions, for example made up of several rows and columns of piezoelectric elements, short circuits can occur. For example, the collector tracks which connect the piezoelectric elements located most in the center of the matrix can come into contact with the electrodes of the piezoelectric elements located at the periphery of the matrix. This disadvantage can be avoided, for example by covering with an insulating layer, for example made of the same material as the insulating substrate of the collector, the outer faces of the tracks, that is to say the faces which are not in contact with the insulating substrate of the collector, without however covering the metal pads of the collector to maintain electrical contact with the electrodes of the piezoelectric elements. However, the performance of the piezoelectric transducer is degraded when the thickness of the layers located between the piezoelectric layers is increased, since the acoustic impedance of these insulating layers is significantly different from that of the piezoelectric layers. This other drawback can be avoided, for example, by extending the collector tracks so that they extend only opposite the notches 114, 124, or kerfs, of the piezoelectric layers 110, 120. However, in the case of a matrix consisting of a large number of piezoelectric transducer elements, passing all of the tracks requires increasing the width of the opposing notches, and the performance of the probe can then be degraded because this involves either reducing the surface area activates piezoelectric elements (while maintaining the inter-element distance), or increases the inter-element pitch (while maintaining the surface area of each piezoelectric element).

[0066] Furthermore, there may be a problem of alignment of the stack consisting of the two piezoelectric layers 110, 120 and the collector 130, for example when this is carried out visually by an operator. The only visual markers located on the periphery of the matrix are, on the one hand, the ends of the notches 114, 124, or kerfs, which open onto certain lateral faces of the piezoelectric layers 110, 120, and, on the other hand, the patterns formed by the tracks 132, 134 of the collector 130 which extend outside the matrix 100. However, the collector 130 cannot extend from the lateral faces of the piezoelectric layers 110, 120 which are covered by the conductive ground sheet 103, which means that no visual markers can be provided on the collector to guarantee alignment in the direction formed by the plane of the pads 133, 135 and the plane of the conductive sheet 103.

[0067] The inventors propose a matrix ultrasonic probe, which makes it possible to overcome all or part of the drawbacks described above, in particular to respond to the problem of individual electrical connection of the piezoelectric transducer elements, and this, preferably with a simple solution to implement, for example a solution which makes it possible to avoid alignment problems and/or which makes it possible to avoid short circuits between piezoelectric translator elements.

[0068] Embodiments presented also make it possible to have a matrix ultrasound probe which is curved. [0069] Embodiments of matrix ultrasonic probes will be described below. The embodiments described are non-limiting and various variants will become apparent to those skilled in the art from the indications of the present description.

[0070] Figure 2A is a highly schematic perspective view of an array 200 of piezoelectric transducer elements of an ultrasonic probe according to one embodiment. Figure 2B is a perspective view showing details of collectors of an array of piezoelectric transducer elements similar to the array 200 of Figure 2A.

[0071] The array 200 comprises two piezoelectric layers, a first piezoelectric layer 210 and a second piezoelectric layer 220, stacked one above the other. The first and second piezoelectric layers extend along a main plane, which may be curved.

[0072] The material of the piezoelectric layers may be, for example, a ceramic, a composite ceramic, or a crystal. The material of the piezoelectric layers may be, for example, PZT (Lead Zirconate Titanate), a lead-free piezoelectric material, such as BaTiOs (Barium Titanate) or KNN (Potassium Sodium Niobate).

[0073] The thickness of each piezoelectric layer may be between 0.5 and 10 mm, for example equal to approximately 1.8 mm.

[0074] The lower and upper faces of the piezoelectric layers may be metallized, for example by chemical deposition process, by spraying one or more layers, for example of gold, silver or nickel.

[0075] A ground recovery plane 203, or ground plane, is positioned between the two piezoelectric layers. The plane ground can be a conductive layer, for example a conductive plate.

[0076] The ground plane 203 extends laterally at one of its ends beyond the stack of piezoelectric layers by a connection tab 204 adapted to the electrical connection of this ground plane to the outside of the stack of piezoelectric layers.

[0077] The thickness of the ground plane 203 may be between 10 and 500 pm, for example equal to approximately 100 pm.

[0078] The material of the ground plane 203 is for example copper, aluminum or brass. Advantageously, the acoustic impedance of the material of the ground plane 203 is close to that of the piezoelectric layers 210, 220, thus facilitating the propagation of the ultrasonic wave within the ultrasonic probe, in comparison with the example of the ultrasonic probe of FIGS. 1A and 1B.

[0079] The two piezoelectric layers 210, 220 are structured, that is to say divided into several piezoelectric transducer elements 201, this division being carried out in the direction transverse to the main plane of these piezoelectric layers.

[0080] To form the piezoelectric transducer elements 201, each piezoelectric layer is cut over all, or almost all, of its thickness, on either side of the ground plane 203.

[0081] More precisely, the first piezoelectric layer 210 is divided, for example cut, into several first piezoelectric sub-elements 211 and the second piezoelectric layer 220 is divided, for example cut, into several second piezoelectric sub-elements 221, without the ground plane 203 being cut. Thus, the ground plane 203 is common to all the transducer elements. piezoelectric 201. The first and second piezoelectric sub-elements are aligned with each other in the transverse direction, that is to say that each first piezoelectric sub-element 211 is superimposed on a second piezoelectric sub-element 221, with a portion of the ground plane 203 between these two piezoelectric sub-elements, and this, without or with little overhang.

[0082] A piezoelectric transducer element 201 is formed by aligning in the transverse direction a first piezoelectric sub-element 211 and a second piezoelectric sub-element 221, with a portion of the ground plane 203 between the two aligned piezoelectric sub-elements. In other words, a piezoelectric transducer element 201 includes a first piezoelectric sub-element 211 and a second piezoelectric sub-element 221 aligned with the first piezoelectric sub-element, as well as a portion of the ground plane 203 between the two aligned piezoelectric sub-elements.

[0083] This method of cutting the piezoelectric layers 210, 220 on either side of the ground plane 203 makes it possible to achieve good alignment of the two structured piezoelectric layers, i.e. good alignment of the first piezoelectric sub-elements 211 cut from the first piezoelectric layer 210 with the second piezoelectric sub-elements 221 cut from the second piezoelectric layer 220.

[0084] This cutting method also makes it possible to structure the external metallization (the first metal layers) of the first and second piezoelectric layers to form first and second electrodes. Indeed, although not detailed in FIG. 2A, each piezoelectric layer 210, 220 is metallized, in a similar manner to the piezoelectric layers 110, 120 of FIGS. 1A and 1B, except that that the first metal layers are on the outside, and thus the first and second electrodes come into contact with respectively the first and second metal pads 234, 244 of the first and second collectors 230, 240 described later, and that the second metal layers, and thus the common ground electrodes, are on the inside, in contact with the ground plane 203.

[0085] Thus, the upper 210B and lower 210A faces of the first piezoelectric layer 210 are metallized, that is to say each covered by a metal layer:

- a first metal layer on the lower face 210A (first face), this first metal layer being structured, that is to say divided in the direction of its thickness, or transverse direction, using several first notches (kerfs) 214 to form several first electrodes 215 separated from each other by these first notches; and

- a second metal layer on the upper face 210B (second face), this second metal layer being unstructured, forming a first common ground electrode 213 in contact with the ground plane 203; the whole forming the first piezoelectric sub-elements 211.

[0086] Similarly, the upper and lower faces of the second piezoelectric layer 220 are metallized, that is to say each covered by a metal layer:

- a first metal layer on the upper face 220B (second face), this first metal layer being structured, that is to say divided in the direction of its thickness, using several second notches (kerfs) 224, to form several second electrodes 225 separated from each other by these second notches; and

- a second metallic layer on the lower face 220A (first face), this second metallic layer being unstructured, forming a second common ground electrode 223 in contact with the ground plane 203; the whole forming the second piezoelectric sub-elements 221.

[0087] Thus, the ground plane 203 is between the second face 210B of the first piezoelectric layer 210 and the first face 221A of the second piezoelectric layer 220, and is in contact with the first and second common ground electrodes 213, 223.

[0088] Furthermore, this cutting method makes it easier to deform the ultrasonic probe, for example to obtain a curved shape. This cutting method is even more advantageous for bending the stack of piezoelectric layers when the stack also comprises an acoustic impedance matching layer, for example made of graphite, as described later. Indeed, the thickness of the piezoelectric stack, combined with that of the graphite, can make deformation impossible without breaking the materials.

[0089] In the case of curved probes, for example concave or convex, the widths of the cutouts can be adjusted so that, during deformation, the first piezoelectric sub-elements, respectively the second piezoelectric sub-elements, do not touch each other and are not short-circuited.

[0090] As illustrated in the zoomed part of FIG. 2A, the first and second piezoelectric layers 210, 220 are not necessarily cut across their thickness, i.e. a small thickness e1, e2 of piezoelectric material can be maintained on the two faces of the ground plane 203, between two piezoelectric transducer elements 201, forming a heel of piezoelectric material of thickness e1 or e2 on either side of the ground plane 203.

[0091] This heel of thickness el or e2 makes it possible to avoid cutting the ground plane 203 when cutting the first and second piezoelectric layers 210, 220. The thickness el, e2 of each heel is at least equal to the tolerances of the cutting method. This tolerance depends on factors such as the cutting depth, the cutting width, and the dimensions of the transducer elements. This tolerance is for example less than 50 pm, or even less than 10 pm.

[0092] This heel also makes it possible to guarantee an absence of flaking (loss of adhesion) between the ground plane 203 and the piezoelectric material of the first and second piezoelectric layers 210, 220. For this, a heel thickness e1, e2 for example less than 10 μm, or even less than 1 μm, is suitable.

[0093] Each heel can also contribute, with the ground plane 203, to maintaining the rigidity of the assembly during the manufacturing steps of the matrix of transducer elements which follow the cutting of the first and second piezoelectric layers 210, 220. For this, the thickness e1, e2 of the heel is calculated in such a way that the Young's modulus resulting from the stacking of the two heels of piezoelectric material and the ground plane 203 makes it possible to guarantee a vertical deformation of the matrix 200 of transducer elements for example less than 10%, or less than 5% of the total thickness of this matrix.

[0094] By adapting the properties of the ground plane 203, for example its thickness and its material, the heel can promote the formation and maintenance of a curvature of the stack of cut piezoelectric layers, so as to obtain a curved probe. [0095] The uncut thickness e1, e2 of each piezoelectric layer 210, 220 may be defined so as to guarantee rigidity of the assembly before deformation and/or allow deformation without breaking of the piezoelectric layers, even when these piezoelectric layers exceed a limit thickness beyond which they can break, depending on the target radius of curvature, then maintain this deformation. The thickness of this heel may thus be defined as a function of this limit thickness, for example being less than or equal to this limit thickness, while being greater than a value sufficient to ensure rigidity before and after deformation. The thickness e1 and/or e2 is for example between 5 and 100 μm.

[0096] Alternatively, the first and second piezoelectric layers may be cut over their entire thickness, down to the ground plane.

[0097] In addition to its grounding function, the ground plane 203 can form a mechanical support for the two piezoelectric layers 210, 220 once structured.

[0098] For a curved probe, the ground plane 203 can be adapted to follow the curvature given to the two piezoelectric layers 210, 220 structured into piezoelectric transducer elements 201.

[0099] The ground plane 203 can form a neutral fiber between the two piezoelectric layers 210, 220.

[0100] The ground plane 203 can also improve the heat dissipation of the piezoelectric transducer elements 201. Thus, the ground plane can be used as an efficient heat sink, because it is placed directly on a maximized exchange surface with the two piezoelectric layers 210, 220, and within the source of the heating. [0101] A first collector 230 is positioned on the lower face (first face) 210A of the first piezoelectric layer 210, at a first face 200A of the matrix 200, corresponding for example to the front face of the probe, which is the face intended to be oriented on the side of the region of interest. This first face 210A of the first piezoelectric layer 210 is opposite the second face 210B of the first piezoelectric layer in contact with the ground plane 203.

[0102] A second collector 220 is positioned on the upper face (second face) 220B of the second piezoelectric layer 220, at a second face 200B of the matrix 200, corresponding for example to the rear face of the probe, which is the face opposite the front face 200A of the probe. This second face 220B of the second piezoelectric layer 220 is opposite the first face 220A of the second piezoelectric layer in contact with the ground plane 203.

[ 0103 ] As detailed in Figure 2B, the first collector 230 comprises:

- a first insulating substrate 231, for example flexible, for example made of a polyimide material, having a lower face 231A (first face) and an upper face 231B (second face);

- first metal tracks 232 on the lower face 231A of the first insulating substrate 231, each first metal track 232 being intended to connect the piezoelectric transducer elements 201 of the matrix 200 to the rest of the probe and ending with a first metal pad 233, each first metal pad 233 being connected to a contact pad 234 on the upper face 231B of the first insulating substrate 231 by a via 235 passing through the first insulating substrate 231. [0104] As detailed in Figure 2B, the second collector 240 comprises:

- a second insulating substrate 241, for example flexible, for example made of a polyimide material, having a lower face 241A (first face) and an upper face 241B (second face);

- second metal tracks 242 on the upper face 241B of the second insulating substrate 241, each second metal track 242 being intended to connect the piezoelectric transducer elements 201 of the matrix 200 to the rest of the probe and ending with a second metal pad 243, each second metal pad 243 being connected to a second contact pad 244 on the lower face 241A of the second insulating substrate 241 by a via 245 passing through the second insulating substrate 241.

[0105] Preferably, the second contact pads 244 are arranged opposite the first contact pads 234. Preferably, the second metal pads 243 are arranged opposite the first metal pads 233.

[0106] The first electrode 215 of each first piezoelectric element 211 is connected to a first contact pad 234 of the first collector 230, for example through or via a first acoustic impedance matching layer 202 when it is present between the first piezoelectric layer 210 and the first collector 230 (described later). Similarly, the second electrode 225 of each second piezoelectric element 221 is connected to a second contact pad 244 of the second collector 240.

[0107] Preferably, the first contact pads 234, the second contact pads 244, the first electrodes 215 and the second electrodes 225 are aligned, an assembly formed by the alignment of a first contact pad 234, a second contact pad 244, a first electrode 215 and of a second electrode 225 forming, with portions of the ground plane 203 and the first and second common ground electrodes 213, 223, the piezoelectric transducer element 201.

[0108] Thus, each piezoelectric transducer element 201 of the matrix network 200 comprises a first electrode 215 connected to a first track 232 of the first collector 230 and a second electrode 225 connected to a second track 242 of the second collector 240, and comprises a common ground electrode 213, 223 connected to the ground plane 203. The metal tracks 232, 242 of the collectors 230, 240 are adapted to electrically and individually connect the different piezoelectric transducer elements 201. Each transducer element 201 can therefore be controlled individually, unlike addressing by rows and columns.

[0109] The first and second collectors are not necessarily identical. For example, in the case of a curved probe, the collectors may be adapted to the curvature. For example, the pitch between the tracks and pads of the collector closest to the center of the curvature may advantageously be less than the pitch between the tracks and pads of the collector furthest from the center of the curvature.

[0110] The thickness of each collector may be between 10 and 500 μm, for example between 10 and 100 μm, for example equal to approximately 20 μm.

[0111] The first and second electrodes 215, 225 of the same piezoelectric transducer element 201 are preferably electrically connected, for example connected, to each other, via the collectors, preferably outside the stack of piezoelectric layers, the ground plane and the collectors, the two piezoelectric sub-elements 211, 221 of the same piezoelectric transducer element then being driven at the same time.

[0112] For example, the first and second electrodes 215, 225 of the same piezoelectric transducer element 201 are connected to each other by means of a printed circuit board (PCB) assembled to the matrix 200, as illustrated and described later in relation to FIG. 4. The ground plane 203 can also be connected to ground by means of the printed circuit.

[0113] In one embodiment, the piezoelectric layers 210, 220 are oriented so that their respective polarizations are opposite, so that each piezoelectric layer expands or contracts in a similar manner when the same voltage is applied to the first and second electrodes 215, 225 of each same piezoelectric transducer element 201.

[0114] Furthermore, the probe 200 comprises a first acoustic impedance matching layer 202, preferably on the front face 200A of the probe.

[0115] In the example shown, the first acoustic impedance matching layer 202 is arranged between the first piezoelectric layer 210 and the first collector 230. This makes it possible to offset the electrical contact and thus avoid a break in acoustic impedance between the piezoelectric material of the first piezoelectric layer 210 and the first collector 230. The first matching layer 202 is preferably electrically conductive. The acoustic impedance of the first matching layer 202 is preferably between the acoustic impedance of the first collector 230 and that of the first piezoelectric layer 210, for example between approximately 3.10 ⁶ Pa.s/m (Rayl) for a polyimide collector and approximately 30.10 ⁶ Pa.s/m for a piezoelectric ceramic or approximately 20.10 ⁶ Pa.s/m for a piezoelectric composite. The first impedance matching layer 202 may be a matching blade whose acoustic impedance decreases between the first piezoelectric layer 210 and the first collector 230, for example with several layers of different materials.

[0116] The thickness of the first impedance matching layer 202 can be calculated for example to operate as a quarter-wave plate, a thickness for which the interferences promote the passage of the wave around the working frequency.

[0117] The material of the first impedance matching layer 202 may be electrically conductive.

[0118] For example, the first adaptation layer 202 is made of graphite. For example, the acoustic impedance of the first adaptation layer 202 is between 3.10 ⁶ Pa.s/m and 6.10 ⁶ Pa.s/m.

[0119] The piezoelectric transducer elements 201 may each comprise a portion of the first acoustic impedance matching layer 202. The first impedance matching layer 202 may be cut at the same time as the first piezoelectric layer 210 when forming the first piezoelectric sub-elements 211.

[0120] Alternatively or additionally, the probe may comprise a second acoustic impedance matching layer arranged such that the first collector is located between the first piezoelectric layer, or the first acoustic impedance matching layer, and the second acoustic impedance matching layer. The acoustic impedance of the second acoustic impedance matching layer is then preferably between the acoustic impedance of the first collector and that of the medium in which the probe is intended to be installed, for example between about 3.10 ⁶ Pa.s/m (Rayl) for a polyimide collector and about 1.5.10 ⁶ Pa.s/m for water.

[0121] The material of the second impedance matching layer is preferably electrically non-conductive. This material may be, for example, a polymer, for example epoxy, for example a charged polymer.

[0122] The thickness of each acoustic impedance matching layer is for example equal to a quarter of the central wavelength of the acoustic wave. The thickness of each acoustic impedance matching layer may be between 1 and 5 mm, for example equal to approximately 2 mm.

[0123] Each piezoelectric transducer element 201 may also be divided, for example cut, into several parts sharing the same first and second electrodes, for example into four parts, as illustrated in FIG. 3D described later. This may make it possible to reduce the impact of possible lateral modes due to the geometry of the piezoelectric transducer elements.

[0124] Figure 3A, Figure 3B, Figure 3C and Figure 3D are perspective and side views illustrating successive steps of an exemplary method of manufacturing an ultrasonic probe according to one embodiment. The exemplary method is described with reference to the ultrasonic probe 200 of Figures 2A and 2B, but may be adapted to make any other ultrasonic probe according to one embodiment.

[0125] Figure 3A illustrates a structure obtained at the end of the stacking, from top to bottom: of the second piezoelectric layer 220, of the ground plane 203, of the first piezoelectric layer 210, and of the first acoustic impedance matching layer 202. [0126] The ground plane 203 extends beyond the stack of piezoelectric layers by connection tabs 204 adapted to electrically connect the ground plane 203 to ground outside the stack of piezoelectric layers. Four connection tabs 204 have been shown in Figures 3A to 3C, but this is not limiting and there may be fewer, or more.

[0127] Figure 3B illustrates a structure obtained after making cutouts 301 in the thickness of the second piezoelectric layer 220 to form the sub-second piezoelectric elements 221. The cutouts 301 stop at least before the ground plane 203, which is not cut. A thickness e2 of the second piezoelectric layer 220 can be maintained, as described in relation to Figure 2A. The cutouts extend along two orthogonal directions of the main plane. The second piezoelectric sub-elements 221 form, for example, squares in top view, but other shapes are conceivable.

[0128] Figure 3C illustrates a structure obtained after turning over the structure of Figure 3B and making cuts 302 of the first piezoelectric layer 210 and the first impedance matching layer 202 into the first piezoelectric sub-elements 211 by aligning with the second piezoelectric sub-elements 221. The cuts 302 stop at least before the ground plane 203, which is not cut. A thickness el of the first piezoelectric layer 210 can be maintained, as described in relation to Figure 2A. The cuts are made along two orthogonal directions of the main plane. The first piezoelectric sub-elements 211 form, for example, squares in top view, but other shapes are conceivable. [0129] The two piezoelectric layers 210, 220 are not necessarily cut across their thickness, that is to say that a small thickness e1, e2, as shown in FIG. 2A, of piezoelectric material can be maintained on each of the two faces of the ground plane 203, between two adjacent piezoelectric elements, so as to maintain a heel of piezoelectric material, for example of one or several hundred microns, on either side of the ground plane 203. As explained further in relation to FIG. 2A, this makes it possible to stiffen the stack and makes it possible, in the case of a curved probe, to promote deformation without rupture, to maintain the radius obtained after the deformation (curvature) described below.

[0130] Figure 3D illustrates a structure obtained after curving the structure of Figure 3C.

[0131] The curvature can be obtained by heating to deform the piezoelectric material, for example at approximately 80°C. The shape of the curvature can be for example cylindrical, for example spherical, for example parabolic and more generally according to a variable shape in each direction of the matrix.

[0132] The structure of Figure 3D also shows that secondary cutouts 303 have been made, before curvature, to divide each piezoelectric transducer element 201 into several parts, for example four parts. The secondary cutouts are preferably less deep than the cutouts for forming the piezoelectric transducer elements. These secondary cutouts are optional, but can be advantageous, in particular to reduce the impact of possible lateral modes due to the geometry of the elements.

[0133] Then, the first collector 230 is assembled on the first acoustic impedance matching layer 202 of the side of the front face 200A of the matrix 200, and the second collector 240 is assembled on the second piezoelectric layer 220 on the side of the rear face 200B of the matrix 200. The first contact pads of the first collector 230, respectively the second contact pads of the second collector 240, are then electrically connected to the first electrodes, respectively to the second electrodes. The first and second electrodes of the same piezoelectric transducer element 201 can be connected to each other via the first and second collectors. Compared to the example illustrated in FIGS. 1A and 1B, a matrix according to this embodiment advantageously facilitates a visual alignment of the collectors with the piezoelectric layers.

[0134] Then, to form the matrix ultrasonic probe, a second acoustic impedance matching layer 311 can be added to the first collector 230 on the side of the front face 200A of the matrix 200, and an acoustic attenuation layer 312, which can be known by the English term "backing", on the second collector 240 on the side of the rear face 200B of the matrix 200. Other components of the matrix ultrasonic probe are described later in connection with FIG. 4. The second impedance matching layer 311 is preferably electrically non-conductive to avoid short-circuiting the tracks of the first collector 230 which are opposite each other. Otherwise, an insulating layer is interposed between the first collector 230 and the second acoustic impedance matching layer 311.

[0135] Figure 4 is a very schematic perspective view of an ultrasonic probe 400 according to one embodiment. The ultrasonic probe 400 comprises an array of transducer elements, which may be the array 200 of Figure 2, or of Figure 3. [0136] The ultrasonic probe 400 further comprises a second acoustic impedance matching layer 311 on the first collector 230 on the side of the front face 200A of the matrix 200, an acoustic attenuation layer 312, or backing, on the second collector 240 on the side of the rear face 200B of the matrix, a shaper 413 on the acoustic attenuation layer 312, and a printed circuit 414, or PCB, on the shaper 413.

[0137] The ground plane 203 can be connected to ground by means of the printed circuit 414 via the connection tabs 204. The first and second electrodes of the same piezoelectric transducer element can also be connected to each other by means of the printed circuit 414.

[0138] Various embodiments and variations have been described. Those skilled in the art will understand that certain features of these various embodiments and variations could be combined, and other variations will occur to those skilled in the art.

[0139] Finally, the practical implementation of the embodiments and variants described is within the reach of those skilled in the art based on the functional indications given above.

Claims

1. Ultrasonic probe (400) comprising an array (200) of piezoelectric transducer elements (201), said array comprising: - a first piezoelectric layer (210) divided into a plurality of first piezoelectric sub-elements (211) each comprising a first electrode (215) at a first face (210A) of said first piezoelectric layer (210), the first dielectric layer further comprising a first ground electrode (213) at a second face (210B) opposite the first face of said first piezoelectric layer; - a second piezoelectric layer (220) stacked above the first piezoelectric layer, the second dielectric layer comprising a second ground electrode (223) at a first face (220A) of said second piezoelectric layer and being divided into a plurality of second piezoelectric sub-elements (221) each comprising a second electrode (225) at a second face (220B) opposite the first face of said second piezoelectric layer; the first and second piezoelectric layers extending along a main plane, and the second piezoelectric sub-elements (221) being aligned with the first piezoelectric sub-elements (211) in a direction transverse to said main plane; - a ground plane (203) located between, and in contact with, the second face (210B) of the first piezoelectric layer (210) and the first face (220A) of the second piezoelectric layer (220); - a first collector (230) assembled to the first face (210A) of the first piezoelectric layer (210), and comprising first conductive tracks (232) connected to the first electrodes (215); - a second collector (240) assembled to the second face (220B) of the second piezoelectric layer (220), and comprising second conductive tracks (242) connected to the second electrodes (225); each piezoelectric transducer element (201) comprising one of the first piezoelectric sub-elements (211) aligned with one of the second piezoelectric sub-elements (221), and a portion of the ground plane (203) between said aligned first and second piezoelectric sub-elements. 2. Ultrasonic probe according to claim 1, wherein the first and second electrodes (215, 225) of the same piezoelectric transducer element (201) are electrically connected to each other, for example the first and second metal tracks (232, 242) connected respectively to said first and second electrodes are electrically connected to each other. 3. Ultrasonic probe according to claim 1 or 2, wherein the first piezoelectric layer (210) and/or the second piezoelectric layer (220): - comprises a piezoelectric composite; and/or - has a thickness between 0.5 and 10 mm. 4. An ultrasonic probe according to any one of claims 1 to 3, wherein the ground plane (203): is a metal layer, for example a metal plate, for example made of copper, aluminum or brass; and/or extends beyond the stack of first and second piezoelectric layers (210, 220) by a connection tab (204) adapted to electrically connect the ground plane to a ground outside of said stack; and/or - has a thickness between 10 and 500 pm. 5. An ultrasonic probe according to any one of claims 1 to 4, wherein the matrix (200) comprises: - a first thickness (el) of piezoelectric material of the first piezoelectric layer (210) between two first adjacent piezoelectric sub-elements (211) on a first face of the ground plane (203), the first thickness being less than the thickness of the first piezoelectric layer; and/or - a second thickness (e2) of piezoelectric material of the second piezoelectric layer (220) between two second adjacent piezoelectric sub-elements (221) on a second face of the ground plane (203) opposite the first face of said ground plane, the second thickness being less than the thickness of the second piezoelectric layer. 6. Ultrasonic probe according to any one of claims 1 to 5, in which the matrix (200) has a curved shape, the main plane then being curved. 7. An ultrasonic probe (400) according to any one of claims 1 to 6, wherein the matrix (200) further comprises a first acoustic impedance matching layer (202) between the first piezoelectric layer (210) and the first collector (230), said first acoustic impedance matching layer being structured such that each piezoelectric transducer element (201) comprises a portion of said first acoustic impedance matching layer between the first electrode (215) and the first collector, the first acoustic impedance matching layer being for example made of graphite. 8. An ultrasonic probe (400) according to any one of claims 1 to 7, further comprising a second acoustic impedance matching layer (311), the first collector (230) being between the first piezoelectric layer (210) and said second acoustic impedance matching layer, for example between a first acoustic impedance matching layer (202) and said second acoustic impedance matching layer. 9. An ultrasonic probe according to any one of claims 1 to 8, wherein: - the first collector (230) comprises a first insulating substrate (231), for example a flexible insulating substrate, the first metal tracks (232) being located on or in the first insulating substrate, and being connected to first contact pads (234) connected or connected to the first electrodes (215); and/or the second collector (240) comprises a second insulating substrate (241), for example a flexible insulating substrate, the second metal tracks (242) being located on or in the second insulating substrate, and being connected to second contact pads (244) connected or connected to the second electrodes (225). 10. An ultrasonic probe (400) according to any one of claims 1 to 9, wherein the first and second piezoelectric sub-elements of each piezoelectric transducer element are divided into several parts, the parts of the same piezoelectric transducer element sharing the same first and second electrodes. 11. An ultrasonic probe according to any one of claims 1 to 10, wherein the first piezoelectric sub-elements (211) and the second sub-elements piezoelectric elements (221) are distributed in two orthogonal directions of the main plane. 12. An ultrasonic probe (400) according to any one of claims 1 to 11, further comprising an acoustic attenuation layer (312) on the second collector (240), a shaper (413) on the acoustic attenuation layer, and a printed circuit (414) on the shaper, the ground plane (203) being electrically connected to a ground via the printed circuit, and the first and second electrodes of the same piezoelectric transducer element being for example also electrically connected to each other by means of the printed circuit. 13. A method of manufacturing an array (200) of piezoelectric transducer elements (201) of an ultrasonic probe, the method comprising: forming a stack of a first piezoelectric layer (210) provided with a first ground electrode (213) on a second face (210B), a ground plane (203) on the second face (210B) of the first piezoelectric layer, and a second piezoelectric layer (220) provided with a second ground electrode (223) at a first face (220A) positioned on the ground plane, the first and second piezoelectric layers extending along a main plane; - cutting, in all or part of its thickness, the first piezoelectric layer (210) into several first piezoelectric sub-elements (211) each comprising a first electrode (215) on a first face of the first piezoelectric layer (210) opposite the second face of said first piezoelectric layer; - cutting, in all or part of its thickness, the second piezoelectric layer (220) into several second piezoelectric sub-elements (221) comprising each a second electrode (225) at a second face (220B) of the second piezoelectric layer (220) opposite the first face of said second piezoelectric layer; the second piezoelectric sub-elements being aligned with the first piezoelectric sub-elements in a direction transverse to the main plane; the cutouts of the first and second piezoelectric layers stopping at least before the ground plane; - the assembly of a first collector (230) comprising first conductive tracks (232) to the first face (210A) of the first cut piezoelectric layer (210), said assembly comprising the electrical connection of the first conductive tracks (232) to the first electrodes (215); - assembling a second collector (240) comprising second conductive tracks (242) to the second face (220B) of the second cut piezoelectric layer (220), said assembly comprising the electrical connection of the second conductive tracks (242) to the second electrodes (225); each piezoelectric transducer element (201) comprising one of the first piezoelectric sub-elements (211) aligned with one of the second piezoelectric sub-elements (221), and a portion of the ground plane (203) between said aligned first and second piezoelectric sub-elements. 14. The method of claim 13, further comprising forming an electrical connection between the first and second electrodes of a same piezoelectric transducer element (201), for example by electrically connecting the first and second metal tracks. connected respectively to said first and second electrodes. 15. Method according to claim 13 or 14, in which the first and second piezoelectric layers (210, 220) are not cut over their entire thickness, so as to retain a thickness (e1, e2) of piezoelectric material on at least one of the two faces of the ground plane (203). 16. The method of any one of claims 13 to 15, wherein the steps of cutting the first and second piezoelectric layers (210, 220) further comprises secondary cuts of the first and second piezoelectric sub-elements (211, 221) so as to divide each piezoelectric transducer element (201) into several parts sharing the same first and second electrodes (215, 225), the secondary cuts being shallower than the cuts for forming the first and second piezoelectric sub-elements. 17. The method of any one of claims 13 to 16, further comprising assembling a first acoustic impedance matching layer (202) to the first face (210A) of the first piezoelectric layer (210) before the step of assembling the first collector (230), such that the first acoustic impedance matching layer is located in the matrix (200) between the first piezoelectric layer and the first collector, the cutting of the first piezoelectric layer including the cutting of said first acoustic impedance matching layer throughout its thickness. 18. A method according to any one of claims 13 to 17, further comprising a step of bending the matrix (200) after the steps of cutting the first and second piezoelectric layers, and for example after the steps of assembling the first and second collectors.