Patent Publication Number: US-2004049901-A1

Title: Method for making a multielement acoustic probe using a metallised and ablated polymer as ground plane

Description:
[0001] The invention concerns the field of acoustic probes in particular used for medical imaging and more precisely the field of probes composed of several elements (or channels) excited independently from each other.  
       [0002] Methods of realizing these probes have been described in several documents and in particular in the patent application WO 97/17145. This method consists of first realizing an assembly: printed circuit comprising an interconnection network/layer of piezoelectric material/acoustic matching blades. More precisely, the printed circuit comprises conducting tracks used to contact various acoustic elements. The earth electrode is common to all the elements and is realized by inserting between the acoustic matching blades and the layer of piezoelectric material, a thin metallic or metallized polymer film.  
       [0003] This thin film is then folded over on the sides as illustrated on FIG. 1 which shows the transducer elements T ij  (made from piezoelectric material), and acoustic matching elements A j1  and A j2  whose impedance varies in order to ensure efficient acoustic matching. Each elementary transducer can therefore be controlled between the earth plane P and a metallization Me ij  connected to the interconnection network  1 . To satisfy size restrictions the flexible earth plane is folded over on the sides of the probe, leading due to the radius of curvature of said plane to a dimension generally around 500 μm, thereby increasing the footprint of the probe.  
       [0004] In addition, note that this earth plane located between the piezoelectric elements and the acoustic matching elements causes disturbance in the propagation of the sound waves resulting in loss of acoustic performance of the probe.  
       [0005] The increased size of the probe footprint and/or loss of acoustic performance represent limiting factors for endocardial or cardiological applications which use probes of small footprint and high acoustic performance.  
       [0006] In this context, this invention proposes a new method to manufacture acoustic probes using an original method to manufacture the earth plane.  
       [0007] More precisely, the invention concerns a method to manufacture acoustic probes with unitary piezoelectric transducers, wherein it comprises the following steps:  
       [0008] realization of a connection network comprising primary connections and earth pads, on the surface of a dielectric film;  
       [0009] superposition of a layer of piezoelectric material on the surface of the connection network;  
       [0010] realization of a conducting film on the surface of a flexible film;  
       [0011] ablation over a specific area of the flexible film, exposing the conducting film over the area where the flexible film has been removed;  
       [0012] assembly of the bared conducting film, on the surface of the piezoelectric material and of the flexible film on the surface of the dielectric film;  
       [0013] an operation along a first axis to cut the assembly consisting of the conducting film and the piezoelectric material so as to define the unitary piezoelectric transducers.  
       [0014] Advantageously, the flexible film is a Polyimide film and the conducting film is a metallic film.  
       [0015] Advantageously, the metallic film can be realized in two steps. A first metallization is realized on the surface of the flexible film, then a second metallization is carried out to increase the metallization thickness.  
       [0016] According to a variant of the invention, the flexible film may have a thickness of approximately 10 to 25 μm and the dielectric film may have a thickness of approximately 25 to 50 μm.  
       [0017] According to a variant of the invention, the first metallization step is carried out by spraying or by an electroless process (the electroless process or the metallization using a chemical method consists of dipping the film in a bath saturated in ions of the metal to be deposited. The saturation produces a metallic deposit on the surface of the film). The second step can be realized by electrolytic deposit. During the first metallization step, a thickness of the order of one micron can generally be obtained, whereas the second step produces a thickness which can reach a dozen or so microns.  
       [0018] According to a variant of the invention, the flexible film can be ablated locally by a CO 2  type laser to remove the flexible film locally, leaving only the metallic layer.  
       [0019] Advantageously, the metallic film can be made from copper or nickel.  
       [0020] The method according to the invention may also comprise a third metallization step, with a noble metal, for example gold, to prevent oxidation of the metallic film obtained in the previous steps.  
       [0021] According to a variant of the invention, the assembly of the flexible film on the surface of the dielectric film may also be realized by using an adhesive of type liquid adhesive polymerizable at ambient temperature or when hot.  
       [0022] According to another variant of the invention, the method may comprise the deposit of an intermediate adhesive conducting layer between the piezoelectric material and the dielectric film.  
       [0023] Using the method of the invention, the localized ablation of the flexible film leaves the conducting film only, over a particular area of contact with the layer of piezoelectric material.  
       [0024] Since the conducting film is very thin, there is no significant change in the acoustic properties of the probe. In addition, this thin metallic film is easy to handle since it is supported around the outside by the flexible film. 
     
    
    
     [0025] The invention will be clearer and other advantages will appear on reading the following description given as a non-limiting example, with reference to the attached figures in which:  
     [0026]FIG. 1 illustrates a configuration of acoustic probe according to the known state of the art, comprising an earth plate between the piezoelectric transducers and the matching blades;  
     [0027]FIGS. 2 a  to  2   f  illustrate the main steps of the method according to the invention;  
     [0028]FIG. 3 illustrates a cross-section of a probe manufactured according to the method described in FIGS. 2 a  to  2   f;    
     [0029]FIG. 4 illustrates a step in the method according to the invention comprising the deposit of an intermediate conducting layer between the piezoelectric material and the dielectric film;  
     [0030]FIG. 5 illustrates a cross-section of a probe manufactured using an intermediate conducting layer;  
     [0031]FIG. 6 illustrates a cutting step in order to obtain unitary transducers, included in the method of the invention;  
     [0032]FIG. 7 illustrates a piezoelectric transducer cutting step in a second example of probe manufacturing method according to the invention;  
     [0033]FIG. 8 illustrates a cross-section of a probe manufactured according to the second example illustrated on FIG. 7;  
     [0034]FIG. 9 illustrates a cross-section of a probe manufactured according to FIG. 7 and comprising in addition an intermediate conducting layer. 
    
    
     [0035] We will now describe an example of a unidirectional acoustic probe comprising linear piezoelectric transducers, realized according to the method of the invention.  
     [0036] Generally, the probe comprises a set of unitary piezoelectric transducers, each comprising an earth electrode and a control electrode also called “hot point”, in the field of ultrasound sensors.  
     [0037] Advantageously, to connect these electrodes, a flexible dielectric film is used, on which connections for the hot points and the earth electrodes are realized. FIG. 2 a  illustrates this type of printed circuit.  
     [0038] More precisely, the dielectric film Fd comprises primary connection pads Pc p  which will be opposite the piezoelectric transducers, secondary connection pads Pc s  offset with respect to the transducers and earth pads P M  which will make contact with the earth electrodes.  
     [0039] More precisely, the primary connection pads and the secondary connection pads are connected via conductors and conducting pads realized on the side of the flexible dielectric film opposite that where the piezoelectric transducers are connected. With this type of configuration, contact can be made with all the transducer electrodes from the same side.  
     [0040] We will describe below the steps required to realize an acoustic probe according to the invention.  
     [0041] According to the method of the invention, on the primary connection pads Pc p  (FIG. 2 a ) intended for electrical and mechanical connections a layer of piezoelectric material intended for the manufacture of piezoelectric transducers is deposited (FIG. 2 b ).  
     [0042] Note that the layer C T  does not cover the earth pads PM and the secondary connection pads Pc s  which must be accessible from the side shown on FIG. 2 b . The layer of piezoelectric material is metallized on both sides.  
     [0043] Simultaneously, a conducting film is realized on the surface of a flexible film.  
     [0044] The flexible film may be a polyimide type film of thickness between approximately 10 and 25 μm and metallized on one side as illustrated on FIG. 2 c . A first metallization m 1  can be produced by spraying or electroless process on the flexible film F s . Typically, this metallization m 1  is less than about 1 μm thick.  
     [0045] A second metallization m 2  may then be carried out on the surface of the metallization m 1  by electrolytic deposit of the same metal to reach a thickness of between approximately 5 and 10 μm.  
     [0046] Advantageously, a very thin layer approximately 0.3 μm thick of noble metal which does not oxidize can be flashed (metallization m 3 ) on the surface of the second metallization m 2 .  
     [0047] The metallizations m 1 /m 2 /m 3  therefore form the conducting film F c , of thickness approximately 5 to 10 μm.  
     [0048] In a second step illustrated in FIG. 2 d , the flexible film F s  is locally engraved by CO 2  laser for example, to expose over the area S, the conducting film F c . Advantageously, another flash of gold can be applied over the area S forming the metallization m′ 3 .  
     [0049] The draping can be carried out by pressing under pressure at ambient temperature or at high temperature.  
     [0050] A hot polymerizable liquid adhesive is used to bond the assembly (piezoelectric layer/film F c  interface and film F s /film Fd interface).  
     [0051]FIG. 3 illustrates a cross-section along axis AA′ represented in FIG. 2 f.    
     [0052] When the layer C T  of piezoelectric material metallized on the top and bottom sides is positioned on the primary connection pads Pc p , the film F s  covered by the film F c , intended to drape the entire ceramic layer is positioned on the dielectric film Fd. To do this, the area of the ablated film F s  is greater than that of the piezoelectric material in order to obtain efficient draping, going down onto the dielectric film (FIG. 2 e ).  
     [0053] More precisely, FIG. 3 shows the electrical contact between the lower metallization Me i  of the ceramic and the primary connection pad, and the electrical contact formed between the upper metallization Me s  of the ceramic layer and the earth pad P M  by the conducting film F c  supported by the film F s . The electrical contacts between the various layers are formed by the roughness of the surfaces. On FIG. 3, the very thin layer of adhesive (less than one micron) is not shown, it flows in cavities due to the roughness of the various layers, assembling the surfaces but not however affecting the electrical contacts.  
     [0054] We have just described a variant in which the layer of piezoelectric material is in contact with the dielectric film and the assembly is obtained by a thin layer of liquid adhesive.  
     [0055] According to another variant of the invention, it is also possible to use an intermediate adhesive conducting connection layer C 1 . This intermediate conducting layer C 1  may advantageously be anisotropic conducting material type, i.e. it has the property of being a conductor in a privileged direction and when hot pressed provides electrical contact only, for example, in a direction perpendicular to the plane of the dielectric film Fd, i.e. along a Z axis perpendicular to the (X, Y) plane shown on FIG. 4. This type of resin, whilst providing continuous, uniform adherence over a layer of piezoelectric material deposited on a substrate, only connects along a Z axis, not along X or Y axes, piezoelectric elements to electrical connections located on the dielectric film Fd. Typically, this material may include a binder loaded with conducting particles.  
     [0056] A layer C T  of piezoelectric material is then superposed on the intermediate conducting layer C 1 .  
     [0057] Note that the layers C 1  and C T  do not cover the earth pads PM and the secondary connection pads Pc s  which must be accessible from the side shown on FIGS. 2 b  and  4 .  
     [0058]FIG. 5 illustrates a cross-section of a probe according to the invention comprising the layer C 1 .  
     [0059] Generally, the acoustic impedance of the first layer Ca 1  is relatively high, and the layer Ca 2  represents an acoustic matching layer of lower acoustic impedance.  
     [0060] Typically, the layer Ca 1  may consist of a mixture of thermosetting or thermoplastic resin with metallic loads, type epoxy resin loaded with nickel. The volume resistivity of this type of material may typically be less than 10 −3  Ω.m and its acoustic impedance approximately 9M Rayleigh, the layer Ca 2  advantageously has an impedance of approximately 3 Mega Rayleigh.  
     [0061] The thickness of the film F s  may advantageously lie between approximately 10 and 30 microns to allow correct draping (i.e. follow the shape of the layer of piezoelectric material: most often it is a PZT type ceramic blade of approximate thickness 150-600 μm) and to keep the flexibility of the earth pads. The dimensions of the probe can therefore be reduced by folding and gluing the earth pad on the sides of the absorber.  
     [0062] The assembly operations can be realized under vacuum or under pressure. Typically, either pressure is exerted above the draping film F s , or a vacuum is produced underneath this film. The two effects can also be combined by creating a vacuum on the film F s  and enclosing the assembly in an envelope to which pressure is applied.  
     [0063] When the above-mentioned assembly operation(s) have been carried out, a cutting operation T j  is then carried out to cut the assembly in order to separate the elementary piezoelectric transducers TP i  as shown on FIG. 6. This cutting operation can be carried out with a diamond saw in direction Dy as shown on FIG. 6. Linear transducers are thereby defined, of typical width between 50 and 500 microns. To electrically insulate the linear transducers, the cutting lines stop in the thickness of the dielectric film Fd.  
     [0064] The assembly previously produced can also be cut out by laser.  
     [0065] Lastly, the two types of cutting may be combined. The acoustic matching blades can therefore be cut by laser whereas the piezoelectric material, in this case the ceramic, can be cut using the mechanical saw. The latter cutting method releases the thermal stresses produced when bonding materials which have different thermal expansion coefficients. By cutting the acoustic matching blades first, the thermal stresses in the ceramic are released, so the ceramic does not break during the second cutting.  
     [0066] Once the linear transducers have been produced on the surface of the dielectric film, a traditional conformation operation can then be carried out, in order to realize curved probes, which are extremely useful in the field of ultrasonography.  
     [0067] Through the use of the flexible dielectric film and prior cutting of linear transducers, we obtain a sufficient degree of curvature of said dielectric film to assemble it on the surface of a curved absorber (material absorbing the sound waves). This assembly is carried out traditionally by bonding the flexible film on the surface of said absorber.  
     [0068] We have described the invention in the context of a unidirectional acoustic probe, but the invention may also be applied in the context of a probe with a connection network on the surface of the flexible dielectric film, in order to realize acoustic probes with matrix transducers, covered with linear acoustic matching elements.  
     [0069] In this case, the same method is used as when manufacturing unidirectional probes to deposit a layer of piezoelectric material via a flexible film F s  on a flexible dielectric film Fd (FIG. 2 b ).  
     [0070] A cutting operation is then carried out along an axis Dx to cut the piezoelectric material as illustrated on FIG. 7, which shows the cuts T i  in the layer C T . FIG. 8 illustrates a cross-section along axis BB′, after making the successive deposits of the film F s /F c , the layer C T  and the layers Ca 1  and Ca 2 . Lastly, in the same way as for unidirectional probes with linear transducers, an operation to make cuts T j  is carried out along the Y axis, to cut the assembly Ca 1 /Ca 2 /F c /C T  down into the flexible dielectric film Fd.  
     [0071]FIG. 9 illustrates a cross-section along axis BB′ when using an intermediate conducting layer C 1 . The cuts Tj along the Y axis are then made in the assembly Ca 1 /Ca 2 /F c /C T /C 1 .