Flexible Ultrasound Array

Abstract

The present invention provides a flexible ultrasonic transducer (1) for use in an ultrasonic monitoring system for inspecting curved objects. The ultrasonic transducer (1) includes an integrated circuit structure (7) and a multi-layer structure (2), the multi-layer structure (2) including an array (3) of ultrasonic transducer elements (3a) disposed in a first layer structure (4) and configured to generate ultrasonic energy propagating along a primary transducer axis Z, and an array (5) of control circuitry (5a) disposed in a second layer structure (6). The array (5) of control circuitry and the integrated circuit structure (7) are configured to operate the array (3) of ultrasonic transducer elements in the first layer structure (4). Furthermore, the multi-layer structure (2) includes at least one flexible layer (8, 9) arranged such that bending flexibility of the multi-layer structure (2) enables the ultrasonic transducer (1) to form continuous contact with the curved object during operation.

Description

[Technical field]

[0001] The present concepts relate to the field of ultrasound imaging, and more particularly to flexible ultrasound transducers comprising an array of ultrasound transducer elements. [Background technology]

[0002] Large, two-dimensional ultrasound arrays have several applications in the medical market and consumer electronics, including medical imaging, activity recognition, directional sound, fingerprint detection, and mid-air haptics.

[0003] The standard construction of a micromachined ultrasonic transducer (MUT) is known in the art. A small drum is constructed on which a suspended membrane is placed to create a small cavity. The dimensions of this cavity combined with the stiffness of the membrane determine the resonant frequency of a particular MUT. As an example, a MUT can be driven by the piezoelectric effect (pMUT). Applying an alternating electric field at the piezoelectric material's resonant frequency creates a stress difference between the piezoelectric material and the membrane, which induces vibration and emission of acoustic waves. Typical frequencies are 50 kHz to 20 MHz. This corresponds to wavelengths of 1 cm to less than 100 μm for waves in air. Applications that use beamforming to focus on transmission or image small spots on reception require larger arrays of ultrasonic transducer elements.

[0004] To achieve ideal beamforming technology, the optimum pitch between subsequent elements in the array is half the wavelength. Depending on the stack used, this means that the pitch between elements is close to the dimensions of the individual transducer elements. As a result, for high density pMUT elements, there is very little space between elements.

[0005] Additionally, each pMUT may require small circuits next to it to set the correct phase, read the phase / amplitude, etc. This circuitry may be relatively complex and may not have space on the same plane as the pMUT.

[0006] Thus, there is a need in the art for improved designs of ultrasonic transducer elements that can be manufactured in large arrays. Furthermore, ultrasonic transducer elements manufactured on flat, rigid substrates may not be suitable for scanning curved objects. An operator must move or press the flat transducer element against the curved object to be inspected, making image reconstruction, etc. difficult. Thus, there is a need in the art for ultrasonic transducer elements that allow for improved inspection of curved objects. Summary of the Invention

[0007] It is an object of the present invention to at least partially overcome one or more limitations of the prior art, in particular to provide a flexible ultrasonic transducer that can be used to inspect curved objects.

[0008] According to a first aspect of the present invention there is provided a flexible ultrasonic transducer for use in an ultrasonic monitoring system for inspecting a curved object, comprising: The ultrasonic transducer includes an integrated circuit structure and a multi-layer structure, the multi-layer structure comprising: an array of ultrasonic transducer elements disposed in a first layer structure and configured to generate ultrasonic energy propagating along a primary transducer axis (Z); an array of control circuits arranged in a second layer structure; the array of control circuits and the integrated circuit structure are configured to operate the array of ultrasonic transducer elements of the first layer structure; The multi-layer structure further includes at least one flexible layer arranged to enable bending flexibility of the multi-layer structure to enable said ultrasonic transducer to form continuous contact with a curved object during operation.

[0009] The flexible ultrasonic transducer of the present invention is for transmitting ultrasonic waves to a target and can be used in an ultrasonic monitoring system.

[0010] Such a system may also include separate processing means for processing the resultant echo signals obtained from the object under examination, display means for displaying an image obtained by the ultrasound transducer, and separate communication means for transferring information from the processing means to the display means.

[0011] The multi-layer structure of the ultrasonic transducer is a stack of different layers or layer structures. Each layer structure may include different sub-layers. The integrated circuit structure may be part of the multi-layer structure, i.e. it may form one of the layers of the multi-layer structure, or it may be disposed as a separate element attached to the multi-layer structure, such as the bottom layer. The bottom layer may be a backplane flexible layer.

[0012] The functionality of the integrated circuit structure may be multiple, such as providing signals for exciting the ultrasonic transducer elements, reading out the signals of the ultrasonic transducer elements, etc. The integrated circuit structure may thus be configured to process the resulting echo signals obtained from the object under test, and further, the integrated circuit structure may provide wireless communication with other units, such as a display system of the ultrasonic monitoring system.

[0013] An array of ultrasonic transducer elements is disposed on the first layer structure. The array may be a two-dimensional array. The surface area of ​​the array is at least 100 cm. 2 , for example at least 400 cm 2 That's fine too.

[0014] The flexible ultrasonic transducer may be in the form of a sheet. The sheet may have a first outer surface and a second outer surface arranged opposite each other, i.e. having normal vectors pointing in two different parallel directions. The outer layers of the multi-layer structure may form the first and second outer sheets, respectively. The surface area of ​​the sheet, for example the surface area of ​​the first or second outer surface, may be at least 100 cm 2 , for example, at least 400 cm 2 may be also possible.

[0015] The first layer structure may form part of or constitute a "front plane" of the flexible ultrasound transducer.

[0016] One ultrasonic transducer may be a piezoelectric micromachined ultrasonic transducer (pMUT). A pMUT is a flexible membrane placed over a small cavity. The dimensions of the cavity in combination with the stiffness of the membrane may determine the resonant frequency of the pMUT. In one example, a pMUT is driven by the piezoelectric effect; that is, a stress difference between the piezoelectric material and the membrane is generated by applying an alternating electric field at the resonant frequency of the piezoelectric material.

[0017] Alternatively, one ultrasonic transducer element may be a capacitive MUT (cMUT), in which case actuation is performed by an air gap within the MUT and electrodes on either side of the air gap.

[0018] The resonant frequency of the ultrasonic transducer may be in the range of 50 kHz to 20 MHz, which corresponds to wavelengths from 1 cm to less than 100 μm.

[0019] The array of ultrasonic transducer elements is further configured to generate ultrasonic energy that propagates along a primary direction, i.e., the transduction axis Z. The locations of different layers and layer structures of the multilayer may be defined in this disclosure as being "axially above" or "axially below" other layers and layer structures. "Axial" refers to the transduction axis Z, and layers and layer structures that are axially above other layers and layer structures are therefore located further along the positive Z direction. Thus, when the transduction axis Z is pointing vertically upward, "axially above" corresponds to "vertically above" and "axially below" corresponds to "vertically below".

[0020] An array of control circuits is disposed in the second layer structure, which may be a single layer, such that the control circuits and integrated circuit structure are used to operate the ultrasonic transducer elements.

[0021] The second layer structure may be disposed axially below the first layer structure, and thus the second layer structure may form or be part of a "backplane" of the flexible ultrasonic transducer.

[0022] The second layer structure may be made of a thin-film semiconductor material such as IGZO (Indium Gallium Zinc Oxide) and / or LTPS (Low Temperature Poly Silicon). Therefore, the second layer structure may be made of a control circuit using IGZO and / or LTPS.

[0023] Further, the multi-layer structure includes at least one flexible layer disposed within the structure such that the entire multi-layer structure and the ultrasonic transducer element are flexible and allow the ultrasonic transducer to continuously contact the curved object during inspection. Thus, the flexible layer allows the ultrasonic transducer element to conform to the curved object during inspection.

[0024] The at least one flexible layer may be at least one of a backplane flexible layer and a frontplane flexible layer.

[0025] A flexible membrane in a pMUT may form one of the flexible layers, but an ultrasonic transducer may also include at least one flexible layer other than a flexible layer associated with or part of an ultrasonic transducer for generating ultrasonic waves, such as at least one flexible layer other than a flexible layer or membrane that is part of a pMUT as described above.

[0026] The flexible ultrasonic transducer is for inspecting curved objects. A suitable curved object may be an object having a curvature with a bending radius of less than 20 cm, for example less than 10 cm. Having at least one flexible layer in the multi-layer structure allows the flexible ultrasonic transducer to bend and conform to such curved objects, and the outer layers of the multi-layer structure can therefore form a continuous contact with the curved object. As an example, all layers of the multi-layer structure may bend upon contact with the curved object.

[0027] The first aspect of the invention is based on the insight that flexible ultrasonic transducers can be made by incorporating flexible layers in a multi-layer structure in which an array of ultrasonic transducer elements and an array of control circuits are arranged in different planes. The inventors have found that it may be disadvantageous if the control circuits vibrate together with the ultrasonic transducer elements. Therefore, the array of ultrasonic transducer elements is arranged on the front plane, while the array of control circuits is arranged on the back plane of the flexible ultrasonic transducer. This has the further advantage that the front plane and the back plane can be processed separately. That is, different layers or layer structures of the multi-layer structure may come from different suppliers.

[0028] Manufacturing constraints can be avoided when the frontplane and backplane are processed independently of each other, and thus the temperature constraints for processing the backplane are not limited by the thermal properties of the frontplane.

[0029] In addition, because the array of ultrasonic transducers and the array of control circuitry are located on different planes, it is possible to densely array ultrasonic transducers such as piezoelectric micromachined ultrasonic transducers (pMUTs) and array complex control circuitry underneath the ultrasonic transducers.

[0030] In addition, the flexible ultrasonic transducer design allows for a 100cm 2 More than, for example, 400cm 2 It is possible to manufacture large-area flexible ultrasonic transducers.

[0031] The entire ultrasonic transducer is flexible, so it can be adapted to fit around curved objects during ultrasonic imaging. Therefore, the flexible ultrasonic transducer of the present disclosure can be used for medical imaging, motion recognition on curved substrates, and non-destructive testing around curved substrates. The curved object or substrate to be inspected can be a part of the human body such as an arm or a leg, or other object such as a door handle or a car dashboard.

[0032] Flexible ultrasonic transducers can be used with moving objects, such as moving parts of the human body, and therefore may be glued or fixed to the body.

[0033] In an embodiment of the first aspect, each ultrasonic transducer element of the array of the first layer structure is connected to an individual control circuit of the array of the second layer structure.

[0034] Thus, each control circuit may be configured to control one ultrasonic transducer element.

[0035] Alternatively, an individual control circuit may be arranged to control a plurality of ultrasonic transducer elements, such as at least two, e.g., at least four ultrasonic transducer elements. As an example, an individual control circuit may be arranged to control a 2x2 subarray of ultrasonic transducer elements.

[0036] In an embodiment of the first aspect, the second layer structure is disposed axially above the backplane flexible layer of the at least one flexible layer.

[0037] Thus, the control circuitry may be processed directly on the backplane compliant layer, for example a flexible polymer layer.

[0038] The integrated structure or circuitry may include any standard or custom devices, such as complementary metal oxide semiconductor (CMOS) devices, to support a particular functionality, for example a wireless ASIC or a digital signal processor (DSP).

[0039] In an embodiment of the first aspect, the integrated circuit structure is comprised of a number of Application Specific Integrated Circuits (ASICs). As an example, at least one ASIC may support a number of individual control circuits.

[0040] Thus, the control circuits may be driven by multiple dedicated ASICs, and thus one ASIC may be configured to support or drive multiple separate control circuits.

[0041] The ASICs may be specially constructed to perform a particular function or task and may be configured to excite the ultrasonic transducer elements as well as read output signals from the array of ultrasonic transducer elements, and the ASICs may be configured to wirelessly communicate with other portions of the ultrasonic monitoring system.

[0042] As an example, multiple ASICs may be implemented as individual devices axially below the backplane flexible layer.

[0043] As a result, the ASIC may be mounted directly onto the backplane flexible layer, for example using chip-on-flex technology.

[0044] Alternatively, however, the ASIC may be incorporated into any layer of the multi-layer structure.

[0045] In an embodiment of the first aspect, the first layer structure includes an array of piezoelectric elements disposed on a front plane flexible layer, and a bulk layer including an interior wall to define an array of cavities in the bulk layer, wherein an ultrasonic transducer element in the array is defined by one of the piezoelectric elements, a cavity of the array of cavities, and a portion of the front plane flexible layer disposed between the piezoelectric element and the cavity.

[0046] The front flexible layer may be one of the flexible layers arranged such that the bending flexibility of the multi-layer structure enables the ultrasonic transducer to form continuous contact with a curved object during operation.

[0047] Alternatively, the front plane flexible layer may be disposed directly on the bulk layer. Additionally, the array of cavities and the array of piezoelectric elements may be arranged such that there is one single cavity associated with each piezoelectric element. As an example, the cavities may be disposed below the piezoelectric elements.

[0048] The ultrasound transducer elements may be or include MUTs configured to be electronically activated.

[0049] In this way, the ultrasonic transducer element can be driven by the piezoelectric effect. Applying an alternating magnetic field near or at the resonant frequency of the piezoelectric element causes the flexible layer of the front plane to vibrate, thereby emitting acoustic waves. The dimensions of the cavity can be adjusted to achieve different resonant frequencies. In this way, the front flexible layer may function as the membrane of the ultrasonic transducer element.

[0050] The ultrasonic transduction element may be or may include a piezoelectric micromachined ultrasonic transducer (pMUT).

[0051] By way of further example, the ultrasonic transduction element may be or may include a capacitive multi-mode ultrasonic transducer (cMUT).

[0052] In an embodiment of the first aspect, the array of control circuits includes an array of Thin Film Transistors (TFTs).

[0053] TFTs may include semiconductors such as Indium Gallium Zinc Oxide (IGZO) and / or Low Temperature Poly Silicon (LTPS), dielectric layers and metal contacts, which may be deposited on a substrate, such as a polymer on a glass wafer.

[0054] Also, one TFT may be mounted on one ultrasonic transducer element. In this manner, the flexible ultrasonic transducer may include a pMUT frontplane separated from a TFT backplane.

[0055] In an embodiment of the first aspect, the multi-layer structure further comprises an acoustic backing layer that reduces acoustic transmission disposed axially below the first layer structure or directed away from the curved object during operation.

[0056] In this manner, an acoustic backing layer can be added to the stack to manage waves that are directed away from the test object, thus mitigating problems caused by misdirected acoustic radiation.

[0057] As an example, the acoustic backing layer may be disposed axially below the second layer structure. Thus, the acoustic backing layer may be disposed axially below the array of control circuits.

[0058] Additionally, the acoustic backing layer may be an acoustic damping or reflecting layer in the form of a Bragg stack, which may include multiple layers of alternating high and low acoustic impedance materials.

[0059] As a result, the acoustic backing layer may have an absorptive nature, whereby acoustic waves are substantially attenuated or absorbed, or it may have a reflective nature, whereby acoustic waves are reflected.

[0060] The acoustic damping layer may be obtained by using a material such that the emitted ultrasonic waves lose their power.

[0061] The acoustic reflection layer may be obtained by using an acoustic Bragg stack (Bragg reflector) that includes multiple layers of alternating high and low acoustic impedance materials, which may help reflect ultrasonic waves. The Bragg stack may be designed using the quarter-wave rule. Thus, each of the multiple layers of the Bragg stack may have a thickness corresponding to a quarter of the wavelength for which the Bragg stack is designed.

[0062] Multiple alternating layers of high and low acoustic impedance materials can be used to allow flexibility while providing bandgap reflection at the target acoustic application frequencies. Bandgap engineering can be used to enable even broadband functionality.

[0063] In an embodiment of the first aspect, the flexible ultrasound transducer element further comprises a top flexible layer disposed as an axially outermost layer of the first layer structure.

[0064] The top flexible layer may be arranged such that the bending flexibility of the multi-layer structure allows the ultrasonic transducer to form continuous contact with a curved object during operation.

[0065] Having the flexible layer be the axially outermost layer, such as axially above other layers in a multi-layer structure, may be advantageous since such materials may bend more easily during operation of the ultrasonic transducer. The outermost layers will experience the highest bending or flexural stresses, and it may be useful to avoid having the ultrasonic transducer elements and control circuitry as the outermost layers, since they are more sensitive to bending.

[0066] One or all of the at least one flexible layer may comprise a polymer, such as a polyimide.

[0067] In an embodiment of the first aspect, the multi-layer structure has a bending flexibility that allows the flexible ultrasonic transducer to be bent with a radius of curvature of less than 5 cm.

[0068] The radius of curvature may be defined as bending in the positive or negative Z direction, i.e., in the direction along the principal translation axis. The minimum radius of curvature may be less than 5 cm, such as 1-4 cm.

[0069] In another aspect of the inventive concept, there is provided a method for manufacturing a flexible ultrasonic transducer as claimed in any preceding claim, comprising the steps of: a) axially disposing a second layer structure including an array of control circuitry over a backplane flexible layer that is temporarily attached to a first rigid substrate. b) disposing a first layer structure including an array of ultrasonic transducer elements; c) forming a multi-layer structure of a flexible ultrasonic transducer; d) removing the first rigid substrate. This aspect may generally offer the same or corresponding advantages as the previous aspect.

[0070] Step a) may include depositing a second layer structure containing the array of control circuits directly onto the backplane flexible layer.

[0071] Step b) may include providing a bulk layer, forming a cavity in the bulk layer, for example using a photolithographically patternable adhesive, laminating a flexible layer onto the bulk layer, and disposing an array of piezoelectric elements on the front flexible layer.

[0072] Step d) may be peeling the backplane flexible layer from the first rigid substrate.

[0073] In an embodiment of the second aspect, the first layer structure is formed on the second layer structure, so that step b) of disposing the first layer structure may be performed on the second layer structure disposed in step a), and step c) may thus be part of step b).

[0074] Thus, sublayers of a first layer structure, such as a bulk layer, a front flexible layer, and a piezoelectric element, may be disposed sublayer-by-sublayer on top of a second layer structure.

[0075] In an embodiment of the second aspect, the first layer structure is temporarily adhered to a second rigid substrate, and the method includes removing the second rigid substrate prior to forming the flexible ultrasonic transducer multi-layer structure.

[0076] In this way, the first layer structure may be formed on another rigid substrate. This is advantageous in that the first layer structure and the second layer structure may be manufactured in different manufacturing processes optimized for the first layer structure and the second layer structure, respectively. Thus, the first and second layer structures may be provided by different suppliers, and the method may then comprise forming the multi-layer structure by placing the pre-manufactured first layer structure on the pre-manufactured second layer structure after peeling from the rigid substrate. [Brief description of the drawings]

[0077] The above and additional objects, features and advantages of the inventive concept will be better understood from the following illustrative and non-limiting detailed description taken in conjunction with the accompanying drawings, in which like reference numerals are used for like elements unless otherwise noted, and in which:

[0078] [Figure 1a] FIG. 2 is an illustration of an array of ultrasonic transducer elements. [Figure 1b] FIG. 2 is an explanatory diagram showing first and second layer structures in an ultrasonic transducer. [Figure 2a] 1 illustrates an exemplary embodiment of a layer structure in a flexible ultrasound transducer. [Figure 2b] 1 illustrates an exemplary embodiment of a layer structure in a flexible ultrasound transducer. [Figure 2c] 1 illustrates an exemplary embodiment of a layer structure in a flexible ultrasound transducer. [Figure 2d] 1 illustrates an exemplary embodiment of a layer structure in a flexible ultrasound transducer. [Figure 3a] 1 illustrates an exemplary embodiment of a method for manufacturing a flexible ultrasound transducer. [Figure 3b] 1 illustrates an exemplary embodiment of a method for manufacturing a flexible ultrasound transducer. [Figure 3c] 1 illustrates an exemplary embodiment of a method for manufacturing a flexible ultrasound transducer. [Figure 3d] 1 illustrates an exemplary embodiment of a method for manufacturing a flexible ultrasound transducer. [Figure 3e] 1 illustrates an exemplary embodiment of a method for manufacturing a flexible ultrasound transducer. [Figure 3f] 1 illustrates an exemplary embodiment of a method for manufacturing a flexible ultrasound transducer. [Figure 4a] 1 illustrates an exemplary embodiment of a method for manufacturing a flexible ultrasound transducer. [Figure 4b] 1 illustrates an exemplary embodiment of a method for manufacturing a flexible ultrasound transducer. [Figure 4c] 1 illustrates an exemplary embodiment of a method for manufacturing a flexible ultrasound transducer. [Figure 4d] 1 illustrates an exemplary embodiment of a method for manufacturing a flexible ultrasound transducer. [Figure 4e] 1 illustrates an exemplary embodiment of a method for manufacturing a flexible ultrasound transducer. [Figure 4f] 1 illustrates an exemplary embodiment of a method for manufacturing a flexible ultrasound transducer. [Figure 5a] 1 illustrates an exemplary embodiment of a method for manufacturing a flexible ultrasound transducer. [Figure 5b] 1 illustrates an exemplary embodiment of a method for manufacturing a flexible ultrasound transducer. [Figure 5c] 1 illustrates an exemplary embodiment of a method for manufacturing a flexible ultrasound transducer. [Figure 5d] 1 illustrates an exemplary embodiment of a method for manufacturing a flexible ultrasound transducer. [Figure 5e] 1 illustrates an exemplary embodiment of a method for manufacturing a flexible ultrasound transducer. [Figure 5f] 1 illustrates an exemplary embodiment of a method for manufacturing a flexible ultrasound transducer. [Figure 6] 1 shows a general overview of process steps in a method for manufacturing a flexible ultrasonic transducer. [Figure 7] 1 illustrates a schematic of a flexible ultrasonic transducer during inspection of a curved object. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0079] The present invention relates to a flexible ultrasonic transducer 1 for an ultrasonic monitoring system for inspecting curved objects. The transducer comprises an integrated circuit structure 7 and a multi-layer structure 2, in which an array 3 of ultrasonic transducer elements is arranged in a first layer of the multi-layer structure. In Fig. 1a, a top view of a 5x5 array 3 of ultrasonic transducer elements 3a is shown diagrammatically. The ultrasonic transducer elements 3a are pMUT elements in this example. The optimal pitch between the elements 3a of the array 3 may be half the emittance wavelength of the ultrasonic transducer. The array 3 of ultrasonic transducer elements 3a is further configured to generate ultrasonic energy propagating along a main transducer axis (Z), which in Fig. 1a is perpendicular to the plane in which the array is arranged.

[0080] The array 3 of ultrasonic transducer elements 3a in the first layer structure 4 is operated by means of an array 5 of control circuits 5a and an integrated circuit structure 7. As shown in FIG. 1b, the array 5 of control circuits 5a is arranged in the second layer structure 6 rather than in the first layer structure 4. The array 5 of control circuits 5a is used, for example, to set the correct phase and / or read out the phase and / or amplitude of the echo signals from the examined object.

[0081] In this example, a first layer structure 4 forms a multi-layer frontplane and a second layer structure 6 forms a backplane, the frontplane being arranged axially above the backplane. The backplane or second layer structure 6 is in this example a thin film transistor (TFT) backplane, arranged such that the individual conversion elements 3a are connected to individual control circuits 5a, or TFT circuits.

[0082] The control circuit 5a is connected to an integrated circuit structure 7, which in this example is realized as a number of ASICs (Application Specific Integrated Circuits) 7a. The ASICs 7a are in this case arranged in another plane axially below the second layer structure 6, each ASIC being configured to support a number of control circuits 5a, for example more than 20, for example more than 100 control circuits 5a. The functions of the ASICs 7a may be multiple, such as generating signals for exciting the ultrasonic transducer elements 3a, reading out the phase and / or amplitude of echo signals from the object to be examined, and / or generating signals for wireless communication with other parts of the ultrasonic transducer.

[0083] Additionally, the ultrasonic transducer includes at least one flexible layer arranged such that the bending flexibility of the multi-layer structure 2 allows the ultrasonic transducer 1 to be in continuous contact with a curved object, such as an arm or leg, during operation, as will be further described in relation to Figures 2-7 below.

[0084] Figure 2a shows an embodiment of a layer structure of a multi-layer structure 2 of a flexible ultrasonic transducer 1. The multi-layer structure 2 comprises a first layer structure 4 arranged axially on a second layer structure 6. The second layer structure 6 comprises an array 5 of control circuits 5a in the form of an array of TFT circuits axially processed on a backplane flexible layer 8. This backplane flexible layer is a polymer layer having the desired flexibility, for example a polymer layer comprising or consisting of polyimide.

[0085] The ultrasonic transducer elements 3a are arranged in a first layer structure 4 and are configured to generate ultrasonic energy propagating along a main transducer axis Z. The first layer structure 4 includes a front plane flexible layer 9, which functions as a membrane for each ultrasonic transducer element 3a. The front plane flexible layer 9 may be the same layer for all individual ultrasonic transducer elements 3a in the array of ultrasonic transducer elements 3. The front plane flexible layer is arranged between a bulk layer 10 and an array of piezoelectric elements 16. The bulk layer 10 includes an inner wall 10a to define an array of cavities 10b in the bulk layer 10, and the piezoelectric elements 16 include a piezoelectric material 16a arranged between an upper metal layer 16b and a lower metal layer 16c. Furthermore, an electrical connection 16d is provided between the control circuit 5a and the lower metal layer 16c. The electrical connection 16d is arranged through the back plane flexible layer 9 and the bulk layer 10 and is used by the control circuit 5a and the integrated circuit 7 to apply an alternating electric field on the piezoelectric material 16a. 2a, only the bottom connection 16d between the bottom metal layer 16c and the TFT 5a is shown, but the top metal layer 16b may also be connected to the TFT 5a. Alternatively, either the bottom metal layer 16c or the top metal layer 16b may be a common contact for the full array of ultrasonic transducer elements. In that case, the top metal layer 16b may be connected to a ground signal.

[0086] As a result, each individual ultrasonic transducer element 3a in the array 3 is defined by one of the piezoelectric elements 16, a cavity 10b of the array of cavities, and a portion of the front plane flexible layer 9 disposed between the piezoelectric element 16 and the cavity 10b. Application of an alternating electric field at the resonant frequency across the piezoelectric material 16a creates a stress difference between the piezoelectric and the front plane flexible layer 9, which induces vibration and emits acoustic waves. Typical frequencies are in the range of 50 kHz to 20 MHz. This corresponds to wavelengths from 1 cm to less than 100 μm.

[0087] The integrated circuit 7 includes a number of ASICs 7a. In this embodiment, the individual ASICs 7a are mounted on the side of the backplane flexible layer 8 opposite to the side on which the second layer structure 6 is fabricated using chip-on-flex technology. The ASICs 7a are thus located axially below the backplane flexible layer 9. In one embodiment, the connection between the ASICs 7a and the supporting control circuitry 5a is made through the backplane flexible layer 8. In another embodiment, the connection is realized using a flexible PCB connector that runs around the backplane flexible layer 9.

[0088] It is also possible that the second layer structure 6 is arranged axially above the first layer structure 4, such as above the upper metallic layer 16b. Such a solution requires that the second flexible layer 6 has through holes for the ultrasound waves emitted by the ultrasound transducer elements.

[0089] Fig. 2b is a further schematic diagram of an embodiment of a flexible ultrasonic transducer 1. The flexible ultrasonic transducer 1 of Fig. 2b has a similar layer structure and function as the transducer described in relation to Fig. 2a, but has an additional top flexible layer 13 arranged as the outermost layer in the axial direction on top of the first layer structure 4. This allows the ultrasonic transducer elements 3a and the TFTs 5a to be protected when the multi-layer structure 2 is bent, since the outermost layer, in this case the top flexible layer 13, may be subjected to the greatest bending stress when bent. The top flexible layer may have the same thickness and / or be made of the same material as the backplane flexible layer 8.

[0090] FIG. 2c is a further schematic diagram of an embodiment of a flexible ultrasonic transducer 1. The flexible ultrasonic transducer 1 of FIG. 2c has the same layer structure and function in the same way as the transducer described in relation to FIG. 2a, but has an acoustic backing layer 11 arranged axially below the first layer structure 4 and the second layer structure 6. The acoustic backing layer 11 is intended to reduce acoustic transmission directed away from the object during operation of the ultrasonic transducer 1. The acoustic backing layer is in the form of a Bragg stack 12, which in the embodiment shown in FIG. 2c includes multiple layers of alternating high 12a and low 12b acoustic impedance materials. The Bragg stack 12 is designed as a "quarter wave mirror", i.e. with alternating acoustic impedance materials 12a, 12b with a thickness of the transmission layer corresponding to ¼ of the wavelength for which the Bragg stack 12 is designed. With the Bragg stack 12, acoustic power directed in the negative Z direction can be reused by constructive interference to be emitted in the correct direction, i.e. in the positive Z direction. The Bragg stack 12 can also be flexible, i.e., one or both of the alternating layers of high acoustic impedance 12a and low acoustic impedance 12b can be made of a flexible material.

[0091] Alternatively, the acoustic backing layer 11 may be an acoustic damping layer, whereby the power of the emitted ultrasonic waves is reduced.

[0092] Fig. 2d further illustrates a schematic representation of an embodiment of a flexible ultrasonic transducer 1. The flexible ultrasonic transducer 1 of Fig. 2d has a similar layer structure and function as the transducer described in relation to Fig. 2d, but an acoustic backing layer 11 in the form of a Bragg stack 12 is instead arranged axially between the first layer structure 4 and the second layer structure 6. Furthermore, there is a top flexible layer 13 arranged as the outermost layer, axially above all other layers in the multilayer stack 2, similar to that shown in Fig. 2b. The top flexible layer may be as described in relation to Fig. 2b above.

[0093] The use of at least one flexible layer, such as the incorporation of a backplane flexible layer 8, a frontplane flexible layer 9 and / or a top flexible layer 13 in the multi-layer structure 2, provides bending flexibility to the multi-layer structure 2 and the flexible ultrasound transducer 1 as a whole, such that the flexible ultrasound transducer 1 can be bent with a radius of curvature of less than 5 cm, allowing it to be used, for example, in examining curved body parts. In this manner, the flexible ultrasound transducer 1 can be wrapped around an arm or leg, and can perform ultrasound imaging in accordance with the movement of the object being examined (e.g., the movement of the arm or leg of the examinee).

[0094] 3 to 5 show a different method 100 for manufacturing a flexible ultrasonic transducer 1 according to the present disclosure. The method comprises the following general method steps a) to d), which are further illustrated in FIG. a) placing 101 a second layer structure 6 including an array 5 of control circuits 5a axially above a backplane flexible layer 8 which is temporarily adhered to a first rigid substrate 14; b) Depositing 102 a first layer structure 4 including an array 3 of ultrasonic transducer elements 3a. c) forming 103 a multi-layer flexible ultrasonic transducer structure 2; d) Removing the first rigid substrate 4, step 104.

[0095] Step a), step 101 of arranging the second layer structure, may also include a step of arranging an acoustic backing layer 11 as described herein between the second layer structure 6 and the backplane flexible layer 8, or on both the second layer structure 6 and the backplane flexible layer 8.

[0096] The first layer structure 4 may be temporarily adhered to a second rigid substrate 15, and the method 100 may include a step of removing the second rigid substrate 15 prior to forming the multi-layer structure 2 of the flexible ultrasonic transducer 1.

[0097] 3a-3f show diagrammatically an embodiment of a method 100 for manufacturing a flexible ultrasonic transducer 1, in which a first layer structure 4 is built on a second layer structure 6. First, as seen in FIG. 3a, the TFT circuit backplane of the second layer structure 6 is processed on a backplane flexible layer 8 temporarily bonded to a first rigid substrate 14. The backplane flexible layer 8 may be made of or may include polyimide. The TFT circuit backplane may be made of Indium Gallium Zinc Oxide (IGZO) and / or Low temperature polysilicon (LTPS) TFTs. Thus, an array 5 of control circuits (5a) is arranged axially above the flexible layer 8.

[0098] As shown in FIG. 3b, a bulk layer 10 in the form of a photolithopatternable adhesive is deposited on top of the second layer structure 6, and a cavity 10b in the bulk layer is formed using photolithography.

[0099] Thereafter, as shown in FIG. 3c, a front plane flexible layer 9 is axially laminated on top of the bulk layer 10, which acts as a membrane for the ultrasonic transducer element as described herein above.

[0100] As shown in FIG. 3 d , piezoelectric elements 16 are fabricated axially on the front plane flexible layer 9 , thereby forming individual ultrasonic transducing elements 3 a in the form of a pMUT of a flexible ultrasonic transducer 1 .

[0101] Additionally, a connection 16d from the piezoelectric element 16 is made through the front plane flexible layer 9 and the bulk layer 10 to the second layer structure 6, thereby providing a connection between the TFT and the pMUT. This is shown in Figure 3e.

[0102] As a final step, the backplane flexible layer 8 is peeled off from the first rigid substrate 14, thereby providing the flexible ultrasound transducer 1, as shown in Figure 3f.

[0103] 4a-4f show diagrammatically another embodiment of a method 100 for manufacturing a flexible ultrasonic transducer 1, in which the first layer structure 4 and the second layer structure 6 are processed independently on different rigid substrates.

[0104] A second layer structure 6 in the form of a TFT control circuit backplane is processed on a backplane flexible layer 8 which is temporarily glued on a first rigid carrier substrate 14, as in Fig. 4a. Furthermore, the first layer structure 4 containing the ultrasonic transducer elements 3a in the form of pMUTs is processed, independently of the second layer structure 6, on a frontplane flexible layer 9 which serves as a membrane for the ultrasonic transducer elements 3a. As can be seen in Fig. 4b, the frontplane flexible layer 9 is temporarily glued to a second rigid carrier substrate 15.

[0105] As shown in Fig. 4c, a bulk layer 10 with a cavity 10b is formed on the second layer structure 6, and then the front plane flexible layer 9 is peeled off from the second rigid substrate 15 and axially glued on top of the bulk layer 10 with the cavity 10b. A connection 16d from the piezoelectric element is made through the front plane flexible layer and the bulk layer to the second layer structure 6, thereby forming a connection between the TFT and the pMUT, as shown in Fig. 4e. The back plane flexible layer 8 is then peeled off from the first rigid carrier substrate 14, thereby forming the flexible ultrasonic transducer 1.

[0106] For both exemplary methods illustrated in Figures 3 and 4, an acoustic backing layer 11 as discussed herein may be added in the flexible ultrasonic transducer 1. An example is shown in Figures 5a-5f. This method is similar to the method described in connection with Figures 4a-4f above, but as illustrated in Figure 5a, an acoustic backing layer 11 in the form of a Bragg stack 12 is processed on top of the backplane flexible layer 8 before processing a second layer structure 6 on top of this Bragg stack 12. This Bragg stack consists of multiple layers of alternating high 12a and low 12b acoustic impedance materials as described above. The process then follows the same route as the example described in connection with Figure 4 above, as illustrated in Figures 5b-5f.

[0107] 7 shows a schematic representation of the flexible ultrasound transducer 1 when inspecting a curved object 17, such as for example an arm or leg of a patient. Due to the incorporation of at least one flexible layer 8, the entire multi-layer structure 2 is flexible, which allows the ultrasound transducer 1 to conform to the curved object 17. In this example, the entire multi-layer structure 2 conforms to the curved object 17, whereas the individual ASICs 7a of the integrated circuit structure 7, which are positioned as the axially furthest individual objects from the curved object 17 to be inspected, do not have to bend. EXAMPLES

[0108] The substrate-cavity-flexible substrate-pMUT stack was fabricated, peeled from the rigid substrate, and then re-laminated onto a curved plastic substrate. The results showed that the pMUT element was still functional after the peeling and lamination steps, although there was a slight change in the measured peak deflection of the pMUT element. The results are summarized in Table 1 below.

[0109] Table 1: Measured peak deflections of pMUT elements before and after peeling and relamination. TIFF0007655855000001.tif78151

[0110] Thus, the inventive concept has been described with reference to a limited number of examples. However, as one of ordinary skill in the art will readily appreciate, other examples than those disclosed above are equally possible within the scope of the inventive concept as defined by the appended claims.

Claims

1. A flexible ultrasonic transducer (1) for an ultrasonic monitoring system for inspecting curved objects, the ultrasonic transducer (1) comprising an integrated circuit structure (7) and a multi-layer structure (2), the multi-layer structure (2) comprising: a first layer structure (4) constituting a front plane of a flexible ultrasonic transducer, the first layer structure (4) including an array (3) of ultrasonic transducer elements (3 a) arranged in the first layer structure (4) and configured to generate ultrasonic energy propagating along a positive direction of a main transducer axis Z, and a front plane flexible layer (9); a second layer structure (6) constituting a backplane of a flexible ultrasonic transducer and including an array (5) of control circuits (5a) arranged on the second layer structure (6), the array (5) of control circuits (5a) and an integrated circuit structure (7) configured to operate the array (3) of ultrasonic transducer elements of the first layer structure (4); and a backplane flexible layer (8) disposed at a position farther along the negative direction of the main transducer axis Z than the second layer structure (6); Including, the integrated circuit structure (7) includes a plurality of ASICs, at least one ASIC supporting a plurality of individual control circuits (5a); The plurality of ASICs are mounted as separate elements (7a) at positions farther along the negative direction of the main vibration axis Z than the backplane flexible layer (8); the entire ultrasonic transducer (1), including the integrated circuit structure (7) and the multi-layer structure (2), is flexible, thereby enabling the ultrasonic transducer (1) to form a continuous contact with the curved object during operation; The first layer structure (4) is disposed at a position farther along the positive direction of the main vibrator axis Z than the second layer structure. Flexible ultrasonic transducer (1).

2. A flexible ultrasonic transducer (1) as described in claim 1, characterized in that each ultrasonic conversion element (3a) of the array (3) of the first layer structure (4) is connected to an individual control circuit (5a) of the array (5) of the second layer structure (6).

3. a front plane flexible layer (9) of the first layer structure (4) disposed between the array of piezoelectric elements (16) and the bulk layer; the bulk layer (10) includes interior walls (10a) so as to define an array of cavities (10b) within the bulk layer (10); 2. A flexible ultrasonic transducer (1) as described in claim 1, characterized in that an ultrasonic conversion element (3a) in the array (3) is defined by one of the piezoelectric elements (16), a cavity (10b) of the array of cavities, and a portion of the front plane flexible layer (9) disposed between the piezoelectric element (16) and the cavity (10b).

4. 2. A flexible ultrasonic transducer (1) according to claim 1, characterized in that the array (5) of control circuits (5a) comprises an array of thin film transistors (TFTs).

5. The flexible ultrasonic transducer (1) of claim 1, characterized in that the multi-layer structure (2) further comprises an acoustic backing layer (11) disposed axially below the first layer structure (4) to reduce acoustic transmission directed away from the curved object during operation.

6. The flexible ultrasonic transducer (1) according to claim 5, characterized in that the acoustic backing layer (11) is arranged at a position farther along the negative direction of the main vibration axis Z than the second layer structure (6).

7. The acoustic backing layer (11) is an acoustic damping or reflecting layer in the form of a Bragg stack (12), 6. A flexible ultrasonic transducer (1) according to claim 5, characterized in that the Bragg stack (12) comprises multiple layers of alternating high (12a) and low (12b) acoustic impedance materials.

8. The flexible ultrasonic transducer (1) as described in claim 1, further comprising an upper flexible layer (13) arranged as the outermost layer at a position farther along the positive direction of the main vibration axis Z than the first layer structure (4).

9. 2. The flexible ultrasonic transducer (1) according to claim 1, characterized in that the multi-layer structure (2) has a bending flexibility such that the flexible ultrasonic transducer (1) can be bent with a radius of curvature of less than 5 cm.

10. A method (100) for manufacturing a flexible ultrasonic transducer (1) according to any one of claims 1 to 9, comprising: a) positioning (101) said second layer structure (6) including an array (5) of control circuits (5a) at a position further along the positive direction of the primary vibration axis Z than a backplane flexible layer (8) temporarily attached to a first rigid substrate (14); A step (102) of disposing the first layer structure (4) including an array (3) of ultrasonic transducer elements (3 a) at a position farther along the positive direction of the main vibration axis Z than the second layer structure (6); c) forming (103) a multi-layer structure (2) of a flexible ultrasonic transducer; d) removing (104) the first rigid substrate (14); e) placing a plurality of ASICs, at least one of which supports a plurality of individual control circuits (5a) as separate elements (7a) at a location further along the negative direction of the primary vibration axis Z than said backplane flexible layer (8); The method includes:

11. 11. The method according to claim 10, characterized in that the first layer structure (4) is built on top of the second layer structure (6).

12. The first layer structure (4) is temporarily adhered to a second rigid substrate (15); 11. The method according to claim 10, characterized in that the method comprises a step (105) of removing the second rigid substrate (15) before forming the multi-layer structure (2) of the flexible ultrasonic transducer (1).

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