FR2810907A1 - Fabrication of multi-element acoustic medical imaging sensor each element is excited independently of the others uses piezo-electric transducers on whose surface a conducting adhesive is applied using heat - Google Patents

Abstract

Heating of the conducting adhesive is done under a vacuum. The conducting adhesive material is a resin polymer loaded with metallic particles. The volume resistance of the adhesive conductor material is less than 10<->3 Ohm centimeters. A deposit of an intermediate conducting thermo-hardened or thermo-plastic layer (C1) is placed between the piezo-electric material and a dielectric film. Method of fabrication of an acoustic sensor incorporating piezo-electric transducers: (a) form a connection network including primary connections (PCp and earth tracks (PM), designed to address the transducers, at the surface of a dielectric (Fd); deposit a layer of piezo-electric material (C(-T)) on the surface of the connection network; deposit a layer of thermo-hardening or thermo-plastic adhesive material (Cd) on the surface of the piezo-electric material surface and the surface of the dielectric film using heat; after heating the assembly comprising the adhesive material layer (C-d) and the piezo-electric material (C1) so as to define unitary piezo-electric transducers (TPi) carry out a cutting operation

Description

The field of the invention is that of acoustic probes, in particular

used for medical imaging and more precisely that of probes composed of several excited elements (or pathways)

independently of each other.

Methods of making these probes have been described in several documents and in particular in patent application WO 97/17145. This method consists in first producing an assembly: printed circuit comprising an interconnection network / layer of piezoelectric material / acoustic adaptation plate. More precisely, the printed circuit comprises conductive tracks making it possible to address various acoustic elements. The ground electrode is common to all the elements and is produced by interposing between the acoustic adaptation blades and the layer of piezoelectric material, a film

thin metallic or metallized polymer.

This thin film is then folded over the sides as shown in Figure 1 which represents transducer elements Tij (made of piezoelectric material), and acoustic adaptation elements Aji and Aj2 including

the impedance varies so as to ensure an efficient acoustic adaptation.

Each elementary transducer can thus be controlled between the plane of

mass P and a Meij metallization connected to the interconnection network 1.

In order to satisfy space constraints, the flexible ground plane is folded over the lateral faces of the probe, leading, due to the radius of curvature of said plane, to a dimension typically of the order of 500 μm,

increasing by the same the footprint of the probe.

It should also be noted that this ground plane located between the piezoelectric elements and the acoustic adaptation elements introduces a disturbing element at the level of the propagation of the acoustic waves.

and leads to a degradation of the acoustic performance of the probe.

The increase in the size of the footprint of the probe and / or the degradation of the acoustic performance constitute limiting factors for endocavitary or cardiological applications which use

Small footprint probes with high acoustic performance.

In this context, the present invention proposes a new method of manufacturing an acoustic probe using an original method of manufacturing the ground plane. More precisely, the subject of the invention is a method for manufacturing an acoustic probe comprising unitary piezoelectric transducers, characterized in that it comprises the following steps: - the production of a connection network comprising primary connections and pads mass, intended to address the piezoelectric transducers, to the surface of a dielectric film; - the superposition of a layer of piezoelectric material on the surface of the connection network; the superposition of a layer of adhesive conductive material of thermosetting or thermoplastic type on the surface of the piezoelectric material and at least partially covering the dielectric film at the level of the ground areas; The assembly of the adhesive conductive material to the surface of the piezoelectric material and to the surface of the dielectric film, by heating; - the cutting after heating of the assembly consisting of the layer of conductive adhesive material and of the piezoelectric material so as to define the transducers

piezoelectric units.

To carry out the assembly by heating, it is in fact possible to take

a thermoplastic or thermosetting adhesive material.

The thermoplastic adhesive changes from a solid state to a liquid (pasty) state upon heating. This change of state is reversible, i.e. at high temperature the thermoplastic adhesive becomes liquid (pasty) and it

becomes solid again at room temperature.

Thermosetting adhesive also changes from liquid state (pasty) to solid state upon heating and unlike thermoplastics

this change of state is not reversible.

The assembly can advantageously be carried out by exerting a pressure at the level of the layer of thermosetting or thermoplastic conductive material, or else by exerting a depression below said layer of thermosetting or thermoplastic conductive material so as to promote adhesion. By virtue of this process, the heated layer of thermosetting or thermoplastic conductive adhesive material makes it possible to obtain a thin conductive layer constituting a ground plane matching the contours of the piezoelectric material and no longer requiring, as in the prior art, the use of an adhesive between a conductive ground plane and the layer of piezoelectric material, a layer of adhesive which introduces

acoustic disturbances and an extra thickness of the order of 2 to 10 microns.

This layer constitutes a draping of the whole of the piezo material.

electric. Advantageously, the thermosetting or thermoplastic conductive material can be a resin loaded with conductive particles, the rate of load being able to be adjusted to obtain the acoustic adaptation, with

the surrounding environment.

The invention will be better understood and other advantages

will appear on reading the description which follows, given as no

limitative and thanks to the appended figures among which: - Figure 1 illustrates an acoustic probe configuration according to the prior art comprising a ground plane between the piezoelectric transducers and the acoustic adaptation blades. - Figures 2a - 2e illustrate the main steps of the method according to the invention; - Figure 3 illustrates a sectional view of a probe manufactured according to the method described in Figures 2a - 2e; - Figure 4 illustrates a cutting step making it possible to obtain unit transducers included in the method of the invention - Figure 5 illustrates a step of cutting piezoelectric transducers in a second example of a method for manufacturing a strain according to FIG. 'invention; - Figure 6 illustrates a sectional view of a probe manufactured according to the second example illustrated in Figure 5. We will describe an example of a unidirectional acoustic probe comprising linear piezoelectric transducers and

carried out according to the method of the invention.

In general, the probe comprises a set of unitary piezoelectric transducers each comprising a ground electrode and a control electrode also called a "hot spot",

in the field of ultrasonic sensors.

To connect all of these electrodes, a flexible dielectric film is advantageously used on which connections are made for the hot spots and for the ground electrodes. The

Figure 2a illustrates such a printed circuit.

More precisely, the flexible film Fd comprises so-called primary Pcp connection areas intended to be opposite the piezoelectric transducers, so-called secondary Pcs connection areas offset with respect to the transducers and PM ground areas intended for

address the ground electrodes.

More precisely, the primary connection areas and the secondary connection areas are connected by means of conductive via and conductive tracks made on the face of the flexible dielectric film opposite to that to which the piezoelectric transducers are connected. With this type of configuration it is possible

address all the transducer electrodes from the same face.

We will describe the necessary stacking of layers below.

to the production of an acoustic probe according to the invention.

According to the method of the invention, a conductive film is then deposited at the level of the primary connection areas Pcp intended to electrically and mechanically connect a layer of material.

piezoelectric for the manufacture of piezoelectric transducers.

This intermediate conductive layer CI may advantageously be of the anisotropic conductive material type, that is to say which has the property of being conductive in a preferred direction and which, when hot pressed, makes it possible to ensure electrical contact only according to, for example, a direction perpendicular to the plane of the flexible film Fd, ie along an axis Z, perpendicular to the plane (X, Y) shown in FIG. 2b. Such a resin thus makes it possible, while ensuring continuous and uniform adhesion at the level of a layer of piezoelectric material deposited on a substrate, to connect only along a Z axis and not on the X or Y axes of the piezoelectric elements to electrical connections located at the level of the flexible dielectric film Fd. Typically this material can include a binder

loaded with conductive particles.

A layer CT of piezoelectric material is then superposed on the conductive intermediate layer C1 as illustrated

in Figure 2c.

It should be noted that the layers C1 and CT do not cover the ground areas PM and the secondary connection areas Pcs which must be accessible from the face shown in the Figures.

2b and 2c.

When the CT layer of piezoelectric material metallized on the lower face and on the upper face is positioned on the layer of conductive material C1, a layer of thermoplastic or thermosetting conductive material Cd is positioned, intended to drape the entire ceramic layer. on the flexible dielectric film. For that

this layer has a surface greater than that of the piezo material

electric so as to achieve effective draping, falling on the film

dielectric (Figure 2d).

Finally, after having positioned the layer of conductive material Cd, one or two acoustic adaptation layers Ca1 and / or

Ca2 to the dimensions of the layer of piezoelectric material (Figure 2c).

According to the invention, the superposition of layers thus produced on the surface of a flexible dielectric film is heated in an autoclave, for example to ensure the assembly of the various layers. More precisely, the intermediate conductive layer C1 and the layer of conductive adhesive material Cd are composed of thermosetting or thermoplastic material. Thus the adhesion of the Cd layer on the flexible film is achieved and in particular at the level of the mass areas PM, and on the surface of the layer of piezoelectric material, the adhesion of the layer of piezoelectric material is also achieved. on the dielectric film via the intermediate layer C1. Figure 3 illustrates a sectional view along the axis AA 'shown in Figure 2c. More precisely, Figure 3 shows the electrical contact between the lower metallization Mei of the ceramic and the primary connection pad via the layer C1, and the placing in electrical contact between the upper metallization Mes of the ceramic layer and the beach

mass PM thanks to the conductive draping layer Cd.

Advantageously, the acoustic matching layer Cal can be made of the same material as the conductive layer Cd, the layer Ca2 representing an acoustic impedance matching layer.

weaker acoustics.

In fact, the conductive layer can typically be produced with a mixture of thermosetting or thermoplastic resin with metallic fillers, such as epoxy resin charged with nickel. The volume resistivity of such a material can typically be less than 10-3 2.m and its acoustic impedance of the order of 9M Rayleigh, an impedance suitable for the Cia layer, the Clb layer rather having an impedance of

the order of 3 Mega Rayleigh.

This constitutes an important advantage from an acoustic point of view since the ground plane thus formed by the conductive layer C1

does not introduce acoustic disturbances.

The thickness of the Cd layer can advantageously be included

between approximately 20 and 60 microns to allow proper draping (i.e.

say to match the shape of the layer of piezoelectric material, it is most often a ceramic blade of the PZT type with a thickness of

the order of 150 - 600 μm and to keep the flexibility of the mass ranges.

In fact, it is thus possible to reduce the size of the probe by folding and

sticking the mass range on the side faces of the absorber.

It should be noted that depending on the nature of the conductive layers Cd and C1 employed, the assembly operation can take place in two stages and in particular if these layers are made from two resins having no

not the same polymerization cycle.

In this case, a first heating step is carried out to

assemble the piezoelectric material on the dielectric film Fd.

Then a second heating step is carried out to assemble the conductive layer Cd to the surface of the dielectric film and to the surface of the piezoelectric material. In all cases, the assembly is obtained by bonding the conductive material, which becomes adhesive at temperature. The operations

assembly can be carried out under vacuum or under pressure.

Typically, it is possible either to exert a pressure above the draping layer Cd, or to exert a depression below this layer. It is also possible to combine the two effects by creating a depression at the level of the Cd layer and by enclosing the whole in an envelope on which pressure is exerted. When the aforementioned assembly operation or operations are performed, a cutting operation Tj of the assembly is then carried out so as to individualize elementary piezoelectric transducers TPi as illustrated in FIG. 4. This cutting operation can be performed. carried out with a diamond saw in the direction Dy illustrated in FIG. 4. Linear transducers are thus defined, the width of which can typically vary between 50 and 500 microns. To electrically isolate linear transducers, the cut lines stop

in the thickness of the dielectric film Fd.

It is also possible to carry out a laser cutting of

the assembly carried out previously.

Finally, it is possible to combine the two types of cutting. Thus the acoustic adaptation blades can be cut with a laser while the piezoelectric material, in this case ceramic, can be cut using a mechanical saw. This latter cutting method makes it possible to release the thermal stresses due to the bonding of materials having different coefficients of thermal expansion. By first cutting the acoustic adaptation blades, the ceramic is freed from thermal stresses and consequently, the ceramic is avoided.

during the second cut.

When the linear transducers are thus produced at the surface of the dielectric film, a shaping operation can be carried out in a conventional manner which makes it possible to produce curved probes,

particularly sought after in the field of ultrasound.

In fact, thanks to the flexible dielectric film used and to the preliminary cutting of linear transducers, a sufficient degree of curvature of said dielectric film is obtained to come and assemble it to the surface of a

absorber (material absorbing acoustic waves) with a curved surface.

This assembly is carried out in a conventional manner by bonding the flexible film to the

surface of said absorber.

We have described the invention in the context of a unidirectional acoustic probe, but the invention can be equally well applied in the context of a probe having a connection network at the surface of the flexible dielectric film, making it possible to produce probes. acoustic comprising matrix transducers, covered with elements

linear acoustic adaptation.

In this case, the procedure is the same as in the method for manufacturing unidirectional probes to deposit a layer of piezoelectric material via a conductive layer.

intermediate Cd on a flexible dielectric film Fd (Figure 2c).

A cutting operation along an X axis of the piezoelectric material is then carried out as illustrated in FIG. 5, showing the cuts Ti in the layer CT and the layer C1 (not shown). FIG. 6 illustrates a sectional view along the axis BB ′, when the successive deposits of the layers Cd, Ca1 and Ca2 have been carried out. Finally, we proceed in a similar manner to the case of unidirectional probes comprising linear transducers, to an operation of Ti cuts along the Y axis, of the entire Cal / Ca2 / Cd / CT / C1 assembly and this up to the flexible dielectric film Fd.

Claims (11)

1. A method of manufacturing an acoustic probe comprising unitary piezoelectric transducers, characterized in that it comprises the following steps - the production of a connection network comprising primary connections (Pcp) and ground pads (PM) , intended to address the piezoelectric transducers, to the surface of a dielectric film (Fd). - the superposition of a layer of piezoelectric material (C-r) on the surface of the connection network; - the superposition of a layer of adhesive material (Cd) of thermosetting or thermoplastic type, conductive on the surface of the piezoelectric material and at least partially covering the dielectric film at the level of the ground pads; Assembling the conductive adhesive material (Cd) to the surface of the piezoelectric material and to the surface of the dielectric film (Fd), by heating; - a cutting operation (Tj) after heating the assembly consisting of the layer of conductive adhesive material (Cd) and of the piezoelectric material (C1) so as to define the Unit piezoelectric transducers (TPj). 2. Acoustic probe manufacturing process according to Claim 1, characterized in that the heating is carried out under vacuum. 3. Acoustic probe manufacturing process according to Claim 1, characterized in that the heating is carried out under pressure. 4. Acoustic probe manufacturing process according to one of claims 1 to 3, characterized in that the conductive adhesive material is a polymer resin loaded with metallic particles. 5. Acoustic probe manufacturing process according to one of claims 1 to 3, characterized in that the volume resistivity of conductive adhesive material is less than about 10-3 ..cm. 6. Acoustic probe manufacturing process according to one of claims 1 to 5, characterized in that it further comprises the filing of a thermosetting or thermoplastic conductive intermediate layer (Ci) between the piezoelectric material and the dielectric film. 7. Acoustic probe manufacturing process according to one of preceding claims, characterized in that it comprises the deposit of at at least one acoustic matching layer on the surface of the layer of conductive adhesive material positioned opposite the layer of material piezoelectric (Cia, C2a). 8. A method of manufacturing an acoustic probe according to claim 7, characterized in that it comprises the deposition of a first acoustic adaptation layer of high impedance on the conductive adhesive material and the deposition of a second adaptation layer. acoustics of low impedance, on the first acoustic matching layer. 9. A method of manufacturing an acoustic probe according to one of claims 7 or 8, characterized in that the adaptation layer impedance at the surface of the conductive adhesive material is constituted by the same said conductive adhesive material. 10. Acoustic probe manufacturing process according to one of claims 6 to 9, characterized in that it comprises: - a first step of assembling the piezoelectric material, the conductive intermediate layer (C1) and the dielectric film by heating; - a second step of assembling the conductive adhesive material on the piezoelectric material / thermosetting conductive intermediate layer / film assembly dielectric, by heating. 11. A method of manufacturing an acoustic probe according to one of claims 1 to 10, characterized in that it further comprises a preliminary cutting operation (Ti) of the layer of piezoelectric material in a direction perpendicular to the cutting axis (Tj) of the assembly formed by the layer of conductive adhesive material / piezoelectric material.