Patent ID: 12220727

DETAILED DESCRIPTION

The present invention relates to a flexible ultrasound transducer1for an ultrasound monitoring system for examining a curved object. The transducer has an integrated circuit structure7and a multi-layered structure2, and an array3of ultrasound transducing elements is arranged in a first layer structure of the multilayer structure. A top view of a 5×5 array3of ultrasound transducing elements3ais schematically shown inFIG.1a. The ultrasound transducing elements are3ain this example pMUT elements. The optimal pitch between elements3ain the array3amay be half of the emittance wavelength of the ultrasound transducer. The array3of ultrasound transducing elements3ais further configured for generating ultrasonic energy propagating along a main transducer axis (Z), which inFIG.1ais perpendicular to the plane in which the array is arranged.

The array3of ultrasound transducing elements3ain the first layer structure4is operated by the use of an array5of control circuits5aand the integrated circuit structure7. As illustrated inFIG.1b, the array5of control circuits5ais arranged in a second layer structure6other than the first layer structure4. The array5of control circuits5ais used for e.g. setting the correct phase and for reading out the phase and/or amplitude of the echo signal from the object being examined.

In this example, the first layer structure4forms a frontplane of the multilayer and the second layer structure6forms a backplane, in which the frontplane is arranged axially above the backplane. The backplane or the second layer structure6is in this example a thin-film transistor (TFT) backplane which is arranged so that each individual transducer element3ais connected to an individual control circuit5a, or TFT circuit.

The control circuits5aare connected to the integrated circuit structure7, which in this example is realized as a plurality of Application Specific Integrated Circuits (ASIC)7a. The ASICs7aare in this case arranged in a separate plane axially below the second layer structure6, and each individual ASIC is configured for supporting a plurality of control circuits5a, such as more than 20, such as more than 100 control circuits5a. The function of the ASICs7amay be multiple, such as generating signals for excitation of the ultrasound transducing elements3a, reading out the phase and/or amplitude of the echo signal from the object being examined and/or for wireless communication with other parts of the ultrasound transducer.

Furthermore, the ultrasound transducer comprises at least one flexible layer arranged so that the bending flexibility of the multi-layered structure2permits the ultrasound transducer) to form a continuous contact with a curved object, such as an arm or a leg, during operation. This will be further discussed in relation toFIGS.2-7below.

FIG.2ashows an embodiment of the layer structure of the multi-layered structure2of the flexible ultrasound transducer1. The multilayer structure2comprises a first layer structure4arranged axially on top of a second layer structure6. The second layer structure6comprises an array5of control circuits5ain the form of an array of TFT circuits, that has been processed axially on top of a backplane flexible layer8. This backplane flexible layer is a polymer layer with desired flexibility, such as a polymer layer comprising or consisting of polyimide.

The ultrasound transducing elements3aare arranged within the first layer structure4and configured for generating ultrasonic energy propagating along a main transducer axis Z. The first layer structure4comprises a frontplane flexible layer9, which function as a membrane in the individual ultrasound transducing elements3a. The frontplane flexible layer9may be the same layer for all individual ultrasound transducing elements3ain the array3of ultrasound transducing elements. The frontplane flexible layer is arranged between a bulk layer10and an array of piezoelements16. The bulk layer10comprises internal walls10aso as to define an array of cavities10bin the bulk layer10and a piezoelement16comprises a piezoelectric material16aarranged between a top metal layer16band a bottom metal layer16c. There is further an electrical connection16darranged between the control circuits5aand the bottom metal layer16c. The electrical connection16dis arranged through the backplane flexible layer9and the bulk layer10and is used by the control circuit5aand the integrated circuit7to apply an AC electric field over the piezoelectric material16a. InFIG.2a, only a bottom connection16dbetween the bottom metal layer16cthe TFT5ais shown, but also the top metal layer16bmay be connected to the TFT5a. As an alternative, either the bottom metal16cor the top metal16bmay be a common contact for the full array of ultrasound transducing elements. In that case, the top metal16bmay be connected to a ground signal.

Consequently, an individual ultrasound transducing element3ain the array3is defined by one of the piezoelements16, a cavity10bof the array of cavities and the portion of the frontplane flexible layer9that is arranged between the piezoelement16and the cavity10b. By applying an AC electric field at the resonance frequency across the piezoelectric material16a, a stress difference between the piezoelectric material and the frontplane flexible layer9is generated, and this will induce a vibration and the emission of an acoustical wave. Typical frequencies are in the range of 50 kHz to 20 MHz. This translates into wavelengths ranging from 1 cm down to <100 um.

The integrated circuit7comprises a plurality of ASICs7a. In this embodiment, the individual ASICs7aare mounted at the opposite side of the backplane flexible layer8than the side of the backplane flexible layer8onto which the second layer structure6is processed using a chip-on-flex technique. Thus, the ASICs7aare arranged axially below the backplane flexible layer9. In one embodiment, the connection between an ASIC7aand the control circuits5ait supports is made through the backplane flexible layer8. In another embodiment, the connection is realised using flexible PCB connectors that go around the backplane flexible layer9. It is also possible to have the second layer structure6arranged axially on top of the first layer structure4, such as on top of the top metal16b. Such a solution may require the second flexible layer6having through holes for the ultrasound emitted by the ultrasound transducing elements.

FIG.2bshows a further schematic illustration of an embodiment of a flexible ultrasound transducer1. The flexible ultrasound transducer1ofFIG.2bhas the same layer structure and function in the same way as the transducer discussed in relation toFIG.2a, but has an additional top flexible layer13arranged as an outermost layer axially above the first layer structure4. This may protect the ultrasound transducing elements3aand the TFTs5aduring bending of the multi-layered structure2, since the outermost layer, in this case the top flexible layer13, may experience the most bending stress during bending. The top flexible layer may have the same thickness and/or consist of the same material as the backplane flexible layer8.

FIG.2cshows a further schematic illustration of an embodiment of a flexible ultrasound transducer1. The flexible ultrasound transducer1ofFIG.2chas the same layer structure and function in the same way as the transducer discussed in relation toFIG.2a, but has an acoustic backing layer11arranged axially below the first4and second6layer structure. The acoustic backing layer11is for reducing the acoustic transmission directed away from the object during operation of the ultrasound transducer1. The acoustic backing layer is in the embodiment shown inFIG.2cin the form of Bragg stack12that comprises multiple layers of alternating high12aand low12bacoustic impedance materials. The Bragg stack12is a “quarter wave mirror”, i.e. designed with the alternating acoustic impedance materials12a,12bhaving a transmissive layer thickness corresponding to one quarter of the wavelength for which the Bragg stack12is designed. Using a Bragg stack12, the acoustic power directed in the negative Z− direction may via constructive interference be reused for emission in the right direction, i.e. in positive Z direction. The Bragg stack12may also be made flexible, i.e. one or both layers of alternating high12aand low12bacoustic impedance may be made of a flexible material.

As an alternative, the acoustic backing layer11may be an acoustic damping layer, in which the power of the emitted ultrasonic wave is reduced.

FIG.2dshows a further schematic illustration of an embodiment of a flexible ultrasound transducer1. The flexible ultrasound transducer1ofFIG.2dhas the same layer structure and function in the same way as the transducer discussed in relation toFIG.2d, but the acoustic backing layer11in the form of a Bragg stack12is instead arranged axially between the first4and second6layer structures. Further, similar to what is shown inFIG.2b, there is a top flexible layer13arranged as the outermost layer, axially above all other layers in the multi-layered stack2. The top flexible layer may be as discussed in relation toFIG.2babove.

The use of at least one flexible layer, such as incorporating the backplane flexible layer8, the frontplane flexible layer9and/or the top flexible layer13in the multilayer structure2, gives the multi layered structure2and the whole flexible ultrasound transducer1a flexibility such that it has a bending flexibility such that the flexible ultrasound transducer1may be bent with a radius of curvature that is less than 5 cm. this allows for use e.g. when examining curved body parts. Thus, the flexible ultrasound transducer1may be wrapped around an arm or a leg, and may further provide for ultrasonic imaging during motion of the object that is being examined, such as during motion of an arm or a leg of a person being examined.

FIGS.3-5show different methods100for producing or manufacturing a flexible ultrasound transducer1according to the present disclosure. The method comprises the below general method steps a)-d), further illustrated inFIG.6:a) arranging101said second layer structure6comprising an array5of control circuits5aaxially above a backplane flexible layer8that is temporarily bonded to a first rigid substrate14;b) arranging102said first layer structure4comprising an array3of ultrasound transducing elements3a;c) forming103the multi-layered structure2of the flexible ultrasound transducer; andd) removing104said first rigid substrate4.

Step a) of arranging101the second layer structure may also comprise arranging an acoustic backing layer11as discussed herein above between the second layer structure6and the backplane flexible layer8, or on top of both the second layer structure6and the backplane flexible layer8.

The first layer structure4may be temporarily bonded to a second rigid substrate15, and the method100may also comprise removing105the second rigid substrate15before forming the multi-layered structure2of the flexible ultrasound transducer1.

FIGS.3a-3fschematically shows an embodiment of the method100for producing a flexible ultrasound transducer1in which the first layer structure4is built up on top of the second layer structure6. First, as seen inFIG.3a, a TFT circuit backplane of the second layer structure6is processed on a backplane flexible layer8that is temporarily bonded to a first rigid substrate14. The backplane flexible layer8may consist of or comprise polyimide. The TFT circuit backplane may comprise IGZO (Indium Gallium Zinc Oxide) and/or LTPS (Low temperature polysilicon) TFTs. Thus, the array5of control circuits (5a) is arranged axially above a flexible layer8.

A bulk layer10in the form of a photo-litho patternable adhesive is deposited on top of the second layer structure6and cavities10bin the bulk layer are formed using photo-lithography, as shown inFIG.3b.

As illustrated inFIG.3c, a frontplane flexible layer9is then laminated axially on top of the bulk layer10, and as discussed herein above, the frontplane flexible layer functions as the membrane in the ultrasound transducing elements.

As illustrated inFIG.3d, the piezoelectric elements16are fabricated axially on top of the frontplane flexible layer9, thereby the forming individual ultrasound transducing elements3ain the form of pMUTs of the flexible ultrasound transducer1.

Further, a connection16dfrom the piezoelectric element16is fabricated through the frontplane flexible layer9and the bulk layer10to the second layer structure6, thereby making a connection between the TFTs and the pMUT. This is illustrated inFIG.3e.

As a last step, the backplane flexible layer8is delaminated from the a first rigid substrate14, thereby providing the flexible ultrasound transducer1, as illustrated inFIG.3f.

FIGS.4a-4fschematically shows an alternative embodiment of the method100for producing a flexible ultrasound transducer1in which the first layer structure4and the second layer structure6are processed independently on different rigid substrates.

The second layer structure6in the form of a TFT control circuit backplane is processed on a backplane flexible layer8, which is temporarily bonded on a first rigid carrier substrate14, seeFIG.4a. Further, the first layer structure4including the ultrasound transducers3ain the form of pMUTs is processed independently from the second layer structure6on a frontplane flexible layer9, which function as the membrane in the ultrasound transducers3a. The frontplane flexible layer9is temporarily bonded to a second rigid carrier substrate15, as seen inFIG.4b.

The bulk layer10with cavities10bis formed on top of the second layer structure6, as illustrated inFIG.4c, and then the frontplane flexible layer9is delaminated from the second rigid substrate15and bonded axially on top of the bulk layer10with cavities10b. Connections16dfrom the piezoelectric element is fabricated through the frontplane flexible layer and the bulk layer to the second layer structure6, thereby making a connection between the TFTs and the pMUT, as illustrated inFIG.4e. Thereafter, the backplane flexible layer8is delaminated from the first rigid carrier substrate14to, thereby forming a flexible ultrasound transducer1.

For both exemplary methods illustrated inFIGS.3and4, an acoustic backing layer11as discussed herein above may be added in the flexible ultrasound transducer1. An example is illustrated inFIGS.5a-5f. This method is similar to the method discussed in relation toFIGS.4a-fabove, but an acoustic backing layer11in the form of a Bragg stack12is processed on top of the backplane flexible layer8before processing the second layer structure6on top of this Bragg stack12, as illustrated inFIG.5a. The Bragg stack comprises multiple layers of alternating high12aand low12bacoustic impedance materials as discussed above. The process then follows the same route, illustrated inFIGS.5b-5f, as the example discussed in relation toFIG.4above.

FIG.7schematically illustrates a flexible ultrasound transducer1during examination of a curved object17, which may for example be an arm or a leg of a patient. Due to the incorporation of at least one flexible layer8, the whole multi-layered structure2is flexible, thereby allowing the ultrasound transducer1to conform to the curved object17. In this example, the whole multilayer structure2conforms to the curved object17, whereas the individual ASICs7aof the integrated circuit structure7, arranged as discrete objects axially furthest away from the curved object17being examined, do not have to be bent.

Experimental Example

A substrate-cavity-flexible substrate-pMUT stack was created and delaminated from a rigid substrate and then again laminated onto a curved plastic substrate. It was found that the pMUT elements still worked after the steps of delamination and lamination onto the curved plastic substrate, with minor changes in the measured peak deflection of the pMUT elements. The results are summarized in Table 1 below:

TABLE 1Measured peak deflection of pMUT elements before and after de- and relaminationPeakPeakFWHMFWHMEstimatedPMUTdeflectiondeflectionBWBWresonanceResonancediameterbeforeafterbeforeafterfreq. in liquidfreq. in air[um][um/s][um/s][kHz][kHz](MHz)(MHz)240574 ± 86181 ± 6039 ± 790 ± 520.6251.25300761 ± 48230 ± 8619 ± 154 ± 340.4250.85400835 ± 61291 ± 23212 ± 134 ± 230.250.5600847 ± 94276 ± 1985.8 ± 0.222 ± 150.1250.25800535 ± 181222 ± 1724.3 ± 0.69.9 ± 100.10.21000376 ± 181321 ± 2724.5 ± 1.415.6 ± 16.10.060.12
In the above the inventive concept has mainly been described with reference to a limited number of examples. However, as is readily appreciated by a person skilled in the art, other examples than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended claims.