Acoustic devices and methods of manufacture

Abstract

The acoustic device (1000) comprises a sheet (100) having a contact surface (11) for contacting a medium (M), the contact surface (11) being for transmitting and receiving an acoustic signal (A) via the contact surface (11). The sheet has a piezoelectric layer (20) including a piezoelectric material (20m) configured to transduce the acoustic signal (A) via the contact surface (11). A backing layer (30) is acoustically interconnected to a back surface (Sb) of the piezoelectric layer (20) facing the contact surface (11). The backing layer (30) includes at least two sublayers (31, 32) having different materials (31m, 32m). The first sublayer (31) includes a first material (31m) having a Young's modulus lower than that of the piezoelectric material (20m). The second sublayer (32) includes a second material (32m) having a density higher than that of the first material (31m). At least a portion of the first material (31m) is arranged between the second material (32m) and the piezoelectric material (20m).

Description

[Technical Field]

[0001] FIELD OF THE DISCLOSURE The present disclosure relates to an acoustic device comprising a sheet for transducing an acoustic signal, and to a method for manufacturing such an acoustic device. [Background technology]

[0002] Acoustic devices capable of transducing relatively low frequencies can be useful for reaching relatively deep penetration depths, such as cardiac / liver imaging or obstetric ultrasound. Furthermore, acoustic devices with relatively large bandwidths can provide shorter wavelength acoustic signals, which can improve distance resolution. However, typical ultrasound transducer designs offer limited options for optimizing the frequency response for an application. Lower frequency applications typically require thicker layers of piezoelectric material. This is particularly problematic for acoustic devices that include flexible transducer arrays, where thickness adversely affects the transducer's mechanical flexibility. Additionally, thicker piezoelectric films require higher polarization voltages, which can be problematic. Maximizing the transducer's bandwidth typically requires a backing material, which also affects the transducer's flexibility. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. WO2021 / 167446A1 Summary of the Invention [Problem to be solved by the invention]

[0004] There remains a need to improve the frequency response of relatively thin and / or flexible acoustic transducer sheets, e.g., with respect to center frequency and frequency bandwidth, while minimizing impact on flexibility and / or peak efficiency. [Means for solving the problem]

[0005] An embodiment of the present disclosure relates to an acoustic device comprising a sheet, preferably relatively thin and / or flexible. The sheet has a contact surface on a front surface of the sheet for contacting a medium and transmitting and / or receiving (transmitting and / or receiving) acoustic signals to and / or from the medium via the contact surface. The sheet further comprises a piezoelectric layer including a piezoelectric material configured to transduce (generate and / or measure) acoustic signals via the contact surface, i.e., the front surface. The sheet further comprises a backing layer acoustically interconnected to a back surface of the piezoelectric layer opposite the contact surface. The backing layer comprises at least a first sublayer and a second sublayer. The first sublayer comprises a first material having a Young's modulus lower than that of the piezoelectric material. The second sublayer comprises a second material having a density higher than that of the first material. At least a portion of the first material is disposed between the second material and the piezoelectric material.

[0006] Without being bound by theory, the inventors have discovered that a backing layer comprising a combination of a relatively dense (heavy) second sublayer connected to a piezoelectric layer through a relatively flexible (soft) first sublayer can act as a mass-spring oscillator. Advantageously, the added mass allows for a shift in the resonant frequency of the piezoelectric material (from λ / 2 toward λ / 4) without requiring a corresponding increase in the overall thickness of the sheet. An intermediate flexible material can be used to further control the resonant response of the transducer while having only a limited effect on the flexibility of the sheet. Additionally, by patterning one or both layers, mass and / or flexibility can be arranged in specific locations, if needed, which can locally improve effectiveness and further reduce bending stiffness. Thus, the present teachings may enable the frequency response of a fully flexible ultrasound transducer to be optimized in terms of center frequency and frequency bandwidth without adversely affecting the transducer's flexibility or peak efficiency. This allows the transducer design to reach lower frequencies (useful for applications requiring deep penetration depths, such as cardiac / liver imaging or obstetric ultrasound) or to reach higher bandwidths (and therefore axial resolution) than would otherwise be possible.

[0007] These and other features, aspects, and advantages of the devices, systems, and methods of the present disclosure will become better understood with reference to the following description, appended claims, and accompanying drawings. [Brief explanation of the drawings]

[0008] [Figure 1A] 1A and 1B are diagrams illustrating an acoustic device including a sheet. [Figure 1B] 1 illustrates a portion of a sheet in contact with a medium for transmitting and receiving acoustic signals. [Figure 2A] 1 is a perspective view illustrating a stack of layers forming part of a sheet. [Figure 2B] 1 is a cross-sectional view illustrating a stack of layers in contact with a medium for transmitting and receiving acoustic signals. [Figure 3A] 1A-1C illustrate portions of sheets and stacks including structured piezoelectric layers. [Figure 3B] 1A-1C illustrate portions of sheets and stacks including structured piezoelectric layers. [Figure 4A] FIG. 10 illustrates a stack including further structuring of the backing layer. [Figure 4B] FIG. 10 illustrates a stack including further structuring of the backing layer. [Figure 5A] FIG. 10 illustrates a stack including an acoustic matching layer and an additional foil between the piezoelectric layer and a further sublayer of the backing layer. [Figure 5B] FIG. 10 illustrates a stack including an acoustic matching layer and an additional foil between the piezoelectric layer and a further sublayer of the backing layer. [Figure 6] 10A-10C illustrate simulations for various structures and thicknesses of sublayers within a backing layer. [Figure 7] 10A-10C illustrate simulations for various structures and thicknesses of sublayers within a backing layer. DETAILED DESCRIPTION OF THE INVENTION

[0009] The terminology used to describe particular embodiments is not intended to be limiting of the invention. As used herein, the singular forms "a," "an," and "the" are intended to include the plural forms unless the context clearly dictates otherwise. The term "and / or" includes any and all combinations of one or more of the associated listed items. It will be understood that the terms "comprises" and / or "comprising" specify the presence of the stated features but do not exclude the presence or addition of one or more other features. It will be further understood that when a particular step of a method is referenced as following another step, this may follow immediately after the other step, unless otherwise specified, or that one or more intermediate steps may be performed before performing the particular step. Similarly, when a connection between structures or components is described, it will be understood that this connection may be established directly or through intermediate structures or components, unless otherwise specified.

[0010] The present invention is described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. In the drawings, absolute and relative sizes of systems, components, layers, and regions may be exaggerated for clarity. The embodiments are described with reference to schematic and / or cross-sectional illustrations of idealized embodiments and intermediate structures of the invention where possible. In the description and drawings, like numbers refer to like elements throughout. Relative terms, as well as derivatives thereof, should be construed to refer to the orientation as subsequently described or as shown in the drawings in the description. These relative terms are for convenience of description and do not require that the system be fabricated or operated in a particular orientation, unless specified.

[0011] FIG. 1A illustrates an acoustic device 1000 including a sheet 100. FIG. 1B illustrates a portion of the sheet 100 in contact with a medium "M" for transmitting and receiving an acoustic signal "A." Typically, the acoustic signal "A" as described herein propagates through the medium "M" as an acoustic wave. For example, the medium "M" can be a biological medium, such as tissue, or a non-biological medium, such as a liquid or a solid. In some embodiments, the acoustic signal "A" is an ultrasonic signal having a frequency, for example, from 20 kHz to several gigahertz. For example, the acoustic device 1000 includes or is part of an ultrasonic transducer or device.

[0012] In some embodiments, the sheet 100 has a front layer 10. For example, the front layer 10 includes or forms a contact surface 11 on the front side "Sf" of the sheet 100. Preferably, the contact surface 11 is suitable for contacting a medium "M". More preferably, the front layer 10 is suitable for transmitting and receiving, e.g., transmitting and / or receiving, an acoustic signal "A" to / from the medium "M" via the contact surface 11. In other or further embodiments, the sheet 100 includes a piezoelectric layer 20. Typically, the piezoelectric layer 20 includes a piezoelectric material 20m configured to transduce, e.g., generate and / or measure, an acoustic signal "A" via the contact surface 11, i.e., the front side "Sf". In other or further embodiments, the sheet 100 includes a backing layer 30. Preferably, the backing layer 30 is acoustically interconnected to a back side "Sb" of the piezoelectric layer 20, opposite the contact surface 11.

[0013] In some embodiments, sheet 100 includes a two-dimensional array of acoustic transducers, for example, formed by portions of piezoelectric layer 20 sandwiched between respective electrode layers. In a preferred embodiment, the sheet is a flexible sheet. Most preferably, sheet 100, including all layers (e.g., 10, 20, 30), has sufficient flexibility to allow the sheet to bend at or below an acceptable radius without breaking and / or losing essential functionality, e.g., without breaking any of the acoustic transducers, electrical wiring, structural integrity of the layers, etc. For example, the acceptable radius may be less than 1 meter, preferably less than 50 centimeters, more preferably less than 20 centimeters, and most preferably less than 10 centimeters, or even less than 5 centimeters. By providing the sheet with greater flexibility, the sheet can be directly applied to various curved surfaces of medium "M," such as a body part, a pipe, etc., and acoustic waves can be more efficiently applied to and / or measured from various directions into / from medium "M."

[0014] In some embodiments, for example, to achieve sufficient flexibility, the thickness of sheet 100 and / or the combined thickness of layers 10, 20, and 30 can be relatively thin, e.g., less than 1 centimeter, preferably less than 5 millimeters, more preferably less than 1 millimeter, and most preferably less than 1 / 2 millimeter, e.g., 50-500 μm. In other or further embodiments, various flexible materials, such as polymer-based materials, and / or bendable layers 10, 20, and 30 can be used. In one embodiment, piezoelectric layer 20 comprises a polymer-based piezoelectric material 20m. For example, piezoelectric material 20m comprises a polarized film of polyvinylidene fluoride (PVDF), preferably poly(vinylidene fluoride-co-trifluoroethylene) (P(CDF-TrFE)80 / 20). In another or further embodiment, backing layer 30 comprises a flexible and / or patterned material. In principle, conductive layers, such as electrodes, can be relatively thin, e.g., thin metal layers and / or traces. Preferably, flexible electrodes are provided, such as "MAM electrodes" and / or metal / oxide containing electrodes made from thermally evaporated materials such as MoCr / Al / MoCr, MoOx / Au / MoOx, MoO3 / Ag / MoO3.

[0015] In some embodiments, acoustic device 1000 includes controller 200. In one embodiment, for example as shown, controller 200 is separate from sheet 100 and is connected or connectable, for example, by electrical wiring. It is also contemplated that controller 200 or control elements may be partially or fully integrated as part of sheet 100. In one embodiment, controller 200 is configured to control generation of acoustic signal "A" by sheet 100. For example, controller 200 is configured to generate an electrical signal E, and one or more transducers comprising a portion of piezoelectric layer 20 (within sheet 100) are configured to convert electrical signal E into acoustic signal "A." In another or further embodiment, controller 200 is configured to receive and / or process measurements of acoustic signal "A" by sheet 100. For example, one or more transducers comprising a portion of piezoelectric layer 20 (within sheet 100) are configured to convert acoustic signal "A" into electrical signal E, and controller 200 is configured to measure and / or process electrical signal E.

[0016] In some embodiments, acoustic device 1000 is configured to generate, measure, and / or process acoustic and / or electrical signals to determine an acoustic image of medium "M." For example, acoustic device 1000 includes or forms part of an ultrasound imaging device. Other applications may also be contemplated, such as acoustic therapy, acoustic streaming, for example for mixing liquids, and inspection of structural elements with acoustic waves. In one embodiment, controller 200 includes or has access to a (non-transitory) computer-readable medium that stores instructions that, when executed, cause the acoustic device to perform the operations described herein.

[0017] FIG. 2A illustrates a perspective view of a stack of layers forming part of sheet 100, such as that shown in FIGS. 1A and 1B. FIG. 2B illustrates a cross-sectional view of the stack of layers in contact with a medium "M" for transmitting and receiving acoustic signals "A." As illustrated in the figure, the piezoelectric layer 20 between the front layer 10 and the backing layer 30 has a first thickness "Tp," the backing layer 30 between the piezoelectric layer 20 and the back surface "Sb" of device 1000 has a second thickness "Tb," and the front layer 10 between the piezoelectric layer 20 and the front surface "Sf" of the device, i.e., the front surface of contact surface 11, has a third thickness "Tf." As described herein, the backing layer 30 preferably includes at least two different sublayers 31, 32 and / or substructures made of different materials 31m, 32m. Preferably, at least a portion of the first material 31m is disposed between the second material 32m and the piezoelectric material 20m.

[0018] In some embodiments, the backing layer 30 includes a first sublayer 31 that includes or consists essentially of a first material 31m having a Young's modulus that is lower than that of the piezoelectric material 20m and / or lower than that of the second material 32m. For example, the Young's modulus of the first material 31m is at least 10% (1.1 times), 20% (1.2 times), or 50% (1.5 times), preferably at least 2 times, 3 times, up to 10 times, or more, lower than that of the piezoelectric material 20m and / or lower than that of the second material 32m (at least in the actuation direction as indicated by arrow "B", i.e., perpendicular to the layers).

[0019] In other or further embodiments, the backing layer 30 includes a second sublayer 32 that includes or consists essentially of a second material 32m having a density that is higher than that of the first material 31m and / or higher than that of the piezoelectric material 20m. For example, the density of the second material 32m (mass per unit volume of the respective material) is at least 10% (1.1 times), 20% (1.2 times), or 50% (1.5 times), preferably at least 2 times, 3 times, up to 10 times, or more, higher than that of the piezoelectric material 20m and / or higher than that of the first material 31m.

[0020] As described herein, the backing layer 30, including the different functional sublayers 31, 32, can provide sufficient control of the frequency and / or bandwidth of the transducer even when the backing layer is relatively thin. Thus, in some embodiments, the second thickness "Tb" of the backing layer 30 is less than the first thickness "Tp" of the piezoelectric layer 20, e.g., by at least two, three, five, or even up to eight, ten, or more times. For example, the piezoelectric layer 20 has a first thickness "Tp" in the range of 25 to 250 μm, preferably 50 to 150 μm. For example, the backing layer 30 has a second thickness "Tb" in the range of 5 to 50 μm, preferably 10 to 25 μm. In other or further embodiments, the third thickness "Tf" of the front layer 10 is less than the first thickness "Tp" of the piezoelectric layer 20, and preferably less than the second thickness "Tb" of the backing layer 30. In one embodiment, the third thickness "Tf" is at least 2, 3, 5, 10, up to 20, 50, 100, or more times less than the first thickness "Tp." In another or further embodiment, the third thickness "Tf" is at least 2, 3, 5, up to 8, 10, or more times less than the second thickness "Tb." For example, the front layer 10 has a thickness of less than 50 μm, preferably less than 10 μm, or even less than 1 μm. By keeping the front layer 10 relatively thin, flexibility may be minimized and / or acoustic transmission may be optimized.

[0021] In some embodiments, for example, as shown, piezoelectric layer 20 includes or consists essentially of a piezoelectric material 20m disposed between front electrode layer 15 and back electrode layer 35. Typically, the electrode layers include a conductive material such as a metal and / or are relatively thin, e.g., having a thickness of less than 2 micrometers, preferably less than 1 micrometer, e.g., in the range of 0.1 to 0.5 μm. In one embodiment, for example, as shown, back electrode layer 35 can be considered part of backing layer 30. However, given its negligible thickness / mechanical function, back electrode layer 35 could also be considered part of piezoelectric layer 20. Similarly, front electrode layer 15 can be considered part of front layer 10, as illustrated, or can be considered part of piezoelectric layer 20. In one embodiment, front layer 10 includes at least a non-conductive layer in front of front electrode layer 15 that forms contact surface 11. For example, this could be a relatively thin layer or a larger layer that includes or forms an acoustic matching layer.

[0022] In some embodiments, for example, as illustrated, the front electrode layer 15 is patterned so that electrical signals (e.g., voltages) can be selectively applied to and / or measured from localized regions of the piezoelectric layer 20, e.g., forming respective transducers. In other or further embodiments, for example, as illustrated, the back electrode layer 35 can be a continuous metal or other conductive layer that acts as a common electrode. In other embodiments (not shown), subdivisions can be provided in both the front electrode layer 15 and the back electrode layer 35, or both electrode layers 15, 35 can be continuous layers.

[0023] As described herein, the backing layer 30 preferably comprises at least two different sub-layers 31, 32, i.e., comprises or consists essentially of two different materials 31m, 32m. In some embodiments, the high-density material 32m used in the second sub-layer 32 may comprise, for example, a metal (iron, steel, copper, silver, lead, gold, platinum) or, for example, a ceramic. In other or further embodiments, the flexible material 31m for the first sub-layer 31 may comprise various polymeric and organic materials in solid film form and / or cellular foam, with or without filler additives.

[0024] In some embodiments, two or more sublayers 31, 32 of the backing layer 30 are deposited on or laminated against the piezoelectric layer 20 and / or back electrode layer 35, or intermediate substrate 33, as illustrated, for example, in Figures 5A and 5B. In other or further embodiments, the stack is built by starting with the backing layer 30 and depositing or laminating the back electrode layer 35 and / or piezoelectric layer 20 on the backing layer 30.

[0025] Without being bound by theory, it is believed that acoustic transducers typically provide a non-reflective / transmissive backing layer, but the main resonant frequency of the piezoelectric layer is determined by the thickness "Tp" of the piezoelectric layer 20, which corresponds to approximately half a wavelength (λ / 2), and can form a standing wave with one node "N" midway (Tp / 2) between two opposite end antinodes. In contrast, as illustrated in this figure, according to some embodiments of the backing layer 30 of the present invention, the mass-spring effect of different sublayers (indicated by acoustic coupling "B") can shift the node "N" from the midpoint (Tp / 2) toward the backing layer 30. This can correspond to a shift in the resonance to a lower frequency (longer wavelength), for example, up to λ / 4. In some embodiments, coupling "B" may be stronger when the resonant frequency of the mass-spring system formed by backing layer 30 (e.g., determined by the effective mass and spring constant provided by the materials and structure of sublayers 31, 32) matches the resonant frequency of piezoelectric layer 20 (e.g., determined by thickness "Tp" and the wave velocity in piezoelectric material 20m). A similar effect of shifting node "N" toward the backing layer can be achieved if the backing layer perfectly reflects the wave, although this would typically require the use of a significantly thicker, denser layer, which would affect flexibility. For completeness, it is noted that this figure illustrates the amplitude of standing wave oscillations transverse to the direction of propagation; it will be understood that in reality the wave will oscillate primarily in the direction of propagation.

[0026] 3A and 3B illustrate portions of the sheet 100 and stack including a structured piezoelectric layer 20 and / or backing layer 30. FIGS. 4A and 4B illustrate stacks including further structuring of the backing layer 30. In some embodiments, the piezoelectric layer 20 is a structured piezoelectric layer, e.g., having subdivided elements of piezoelectric material 20m that form respective acoustic transducers. For example, the subdivision of the piezoelectric material 20m may correspond to the subdivision of one or both of the electrode layers 15, 35. Preferably, the piezoelectric layer 20 includes piezoelectric material 20m that forms an array of pillars 22 configured to transmit and receive acoustic signals "A" through the front surface "Sf" of the device.

[0027] In some embodiments, the pillars 22, or other subdivided elements, have respective (maximum) diameters "Dp" (within the plane of the layers) that are the same as or smaller than the respective lengths "Lp" of the pillars (transverse to the plane of the layers, e.g., in the direction of propagation of the acoustic signal) or other subdivided elements, e.g., by at least 20% (1.2 times), 50% (1.5 times), or even by factors up to 2 times, 3 times, or more. The smaller the diameter relative to the length, the higher the density of transducers providing relatively low frequencies. For example, the diameter "Dp" is in the range of 10-100 μm, preferably in the range of 20-60 μm. The diameter "Dp" need not be constant, e.g., it may be slightly thicker on one side, as shown. To provide sufficient isolation / decoupling, the (minimum) gap "Gp" may be similar in size to the diameter "Dp," e.g., preferably smaller. For example, the (minimum) gap "Gp" is in the range of 5-100 μm, preferably in the range of 10-30 μm.

[0028] In some embodiments, the piezoelectric layer 20 includes at least one of a back piezoelectric layer 23 or a front piezoelectric layer 21. Advantageously, the continuous piezoelectric layers 21 and / or 23 may allow for the deposition of additional structures and / or layers. To minimize cross-coupling between the subdivided elements 22, the continuous piezoelectric layers 21 and / or 23 preferably have individual thicknesses T1 and / or T2 that are smaller than the length "Lp" of the subdivided elements 22, e.g., by at least two, three, five, up to ten, or more times smaller. For example, the continuous piezoelectric layers 21 and / or 23 have respective thicknesses T1, T2 in the range of 1 to 30 μm, preferably in the range of 5 to 20 μm.

[0029] Preferably, the array of pillars 22 is integrally formed between the back piezoelectric layer 23 and the front piezoelectric layer 21. In other words, the back piezoelectric layer 23, the array of pillars 22, and the front piezoelectric layer 21 may form a single piece of material. For example, the layers 21, 23 may consist essentially of the same piezoelectric material 20m as the intermediate structures / pillars, and / or the piezoelectric materials may be fused together. In some embodiments, the piezoelectric layer 20 with the pillars 22 may be fabricated according to the methods described in the previous International Publication No. WO 2021 / 167446 A1. In other or further embodiments, other structures of the piezoelectric layer 20 may be contemplated. In some embodiments, one or both of the piezoelectric layers 21, 23 may be omitted; for example, the pillars may be provided directly between the electrode layers 15, 35. In other or further embodiments, the structured piezoelectric layer is formed by cutting or otherwise subdividing the piezoelectric layer 20 into individual islands or pillars. For example, pillar structures and / or other subdivision of the piezoelectric layer 20 may be advantageous in reducing acoustic and / or mechanical cross-coupling between elements within the acoustic device.

[0030] As described herein, the backing layer 30 preferably includes at least two distinct sublayers 31, 32, with at least one of the sublayers being patterned, i.e., subdivided into multiple elements. Preferably, the subdivision of one or more of the at least two distinct sublayers 31, 32 corresponds to the division of the piezoelectric layer 20 that forms each transducer. For example, the patterning of one or more sublayers corresponds to the patterning of at least one of the electrode layers 15, 35 and / or corresponds to the patterning of the piezoelectric layer 20. Thus, it will be understood that the patterning of one or more sublayers as shown in FIGS. 3B, 4A, 4B, 5A, and 5B can also be applied to a continuous piezoelectric layer 20, such as that shown in FIG. 2B (not shown). The patterning can vary in terms of relative width, alignment with respect to the pillars, and relative pitch. Other possible variations can include a design (not shown) in which the heavy material 32m is completely covered by another material.

[0031] In some embodiments, the second sublayer 32 includes subdivided regions of the second material 32m, as shown in Figures 3B, 4A, 4B, 5A, and 5B. In one embodiment, the first material 31m of the first sublayer 31 extends between the subdivided regions of the second material 32m, as shown in Figures 3B and 5A. In another or further embodiment, material-free gaps are provided between the subdivided regions of the second material 32m, as shown in Figures 4A, 4B, and 5B. Combinations are also possible, or a third material may be provided between the subdivided regions of the second material 32m (not shown), and / or the second sublayer 32 may be covered by another layer (not shown). Instead of completely subdividing the second sublayer 32, it may be possible to vary the layer thickness of the second material 32m, for example, by concentrating more material in some regions than in others. In other or further embodiments, the first sublayer 31 includes subdivided regions of the first material 31 m, as shown in Figures 4B and 5B for example. In one embodiment, material-free spaces are provided between the subdivided regions of the first material 31 m, as shown for example. In another or further embodiment (not shown), another material is provided between the subdivided regions of the first material 31 m.

[0032] Preferably, the first sublayer 31 and / or the second sublayer 32 are subdivided according to a pattern in which more material 31m, 32m of each sublayer 31, 32 is provided at positions corresponding to the positions of the piezoelectric layer 20 where the respective transducers are formed. For example, the material 31m, 32m of each sublayer 31, 32 is provided (concentrated) exclusively or solely at the positions of the pillars 22 (or similar substructures of the piezoelectric layer 20) and / or at the positions of the actuation positions determined by each (subdivided) electrode layer 15 and / or 35.

[0033] 5A and 5B illustrate a stack including an additional foil 33 between the acoustic matching layer 12 and piezoelectric layer 20 and further sublayers 31, 32 of the backing layer 30. It will be understood that either or both of these features may be combined with any of the previously described embodiments.

[0034] In some embodiments, acoustic wave transmission into medium "M" is optimized by providing one or more acoustic matching layers 12, which may have a relatively large thickness "Tf," such as up to a second thickness "Tb," or even greater. For example, the matching layer(s) may provide an acoustic impedance gradient that allows acoustic waves from the piezoelectric layer / transducer to penetrate smoothly into medium "M" and / or propagate smoothly from medium "M" to the piezoelectric layer / transducer for detection.

[0035] In other or further embodiments, acoustic behavior may be further optimized and / or manufacturing may be facilitated by providing an additional foil 33, which may be considered part of the backing layer 30. In one embodiment, acoustic behavior may be further optimized by adjusting the thickness, material, and / or distribution of the additional layer 33. In another or further embodiment, manufacturing may be facilitated by providing a foil 33 for depositing / laminating the back electrode layer 35 and / or other sublayers of the backing layer 30.

[0036] 6 and 7 illustrate simulations for various structures and thicknesses of sublayers within the backing layer 30. The potential for controlling various parameters of an acoustic device by adapting the backing layer is illustrated here using finite element numerical (FEM) simulations performed in Comsol. Various possible embodiments were simulated and compared to a baseline monolithic backing design. Frequency responses were simulated, allowing for evaluation of parameters including center frequency, bandwidth, and efficiency. Cross-sections of various geometries are illustrated above the corresponding graphs.

[0037] Without being bound by theory, the quality factor, or Q factor, is understood to be a dimensionless parameter that describes how an oscillator or resonator damps. In one definition, Q factor is the ratio of the initial energy stored in the resonator to the energy lost in one radian of a cycle of oscillation. In another definition, Q factor is the ratio of the center frequency of the resonator to its bandwidth when subjected to an oscillating drive force. These two definitions yield numerically similar, but not identical, results. In either case, it is understood that a (sinusoidally) driven resonator with a higher Q factor will resonate at a larger amplitude (resonant frequency) but have a smaller range of frequencies around that frequency at which it resonates, i.e., a smaller bandwidth, and a (sinusoidally) driven resonator with a lower Q factor will resonate at a smaller amplitude (resonant frequency) but have a larger range of frequencies around that frequency at which it resonates, i.e., a larger bandwidth.

[0038] Figure 6 illustrates a dual-material design in which the relatively dense (heavy) material 32m is steel and the total backing thickness "Tb" is held constant at 14 μm. Similar results (not shown) were obtained using lead as the heavy material 32m. All quantities are normalized to the performance of a reference design ("Ref"). As illustrated in the legend, the various lines correspond to different designs "S2a" to "S2c." Along the X-axis, the thickness budget allocated to the polyimide PI sublayer / foil increases, reaching 100% at the right end, with all designs degenerating to the reference design (monolithic PI). The Y-axis represents the "Q-factor / Q ref ”, i.e., a lower Q value corresponds to a higher bandwidth, and the relative Q value, “f max / f ref ”, i.e., the frequency f at which the signal is strongest when compared to the reference design max , "M ref M=max|H| on the reference design, i.e., a measure of efficiency calculated as the maximum amplitude of the transfer function H of the acoustic wave in the medium when compared with the reference design, M / f on the reference design. max ", i.e., the same, but f max, which gives a measure of the efficiency of the combined resonant frequency shift compared to the reference design.

[0039] As illustrated, the "S2a" design achieves a maximum 15% reduction in Q factor (top left) and a 22% reduction in resonant frequency (top right) when PI is completely replaced with steel (T33 / Tb = 0%). Designs "S2b" and "S2c" using patterning are lower, but these designs also result in lower bending stiffness. In all cases, the impact on radiation efficiency "M" is relatively small or even increases compared to the baseline value. A stronger effect can be achieved when the material 32m is denser, as tested with lead. By optimizing the patterning design, further performance can be achieved while maintaining low bending stiffness. In a comparative simulation (not shown), the thickness "Tb" of the monolithic backing was increased from 14 μm to 50 μm. In this case, the resonant frequency decreased by 21%, and only a -2% improvement in bandwidth Q was observed. Therefore, a similar resonant frequency reduction can be achieved using a dual-material backing without increasing thickness, while still benefiting from increased bandwidth. In further simulations (not shown), it was observed that when the backing layer thickness "Tb" was 50 μm, the dual material design allowed for a 40% reduction in resonant frequency and a 20-30% reduction in Q factor (tested with lead).

[0040] Figure 7 illustrates a ternary material backing design. The total thickness is kept at a nominal value "Tb" = 14 μm, with 8 μm allocated to the material layer 31m (here PI) and the remainder split between a soft material (disappearing on the left side of the plot) and a relatively dense material (steel, disappearing on the right side of the plot). The properties of the soft material 31m are density ρ = 1600 kg / m 3, Young's modulus E = 30 MPa, Poisson's ratio ν = 0.495, and damping ratio α = 0.94 dB / λ. For example, this may correspond to a relatively flexible polymer material such as polyurethane. On the right side of the plot, the dense material is thin (30% of Tb = 4.2 μm) and does not add much mass, thus limiting the reduction in resonant frequency (<10%). However, significant bandwidth can be achieved (10-20% reduction in Q). This is accompanied by a more or less significant reduction in efficiency, depending on the design. Compared to the dual-material design, a larger Q reduction can be achieved for a given backing thickness. Note the significant difference between designs "S3b," "S3c," and "S3d," which have varying degrees of patterning in the backing layer, and the unpatterned backing layer of design "S3a." Without being bound by theory, note that in the absence of patterning, the expansion or contraction of the flexible material 31m may be limited. For example, when the Poisson's ratio is close to 0.5, expansion of the material in one direction may require contraction in the transverse direction—a possibility limited by unpatterned interlayers. In conclusion, the simulations demonstrate that a binary backing can reduce the resonant frequency and Q-factor at a constant backing thickness and radiation efficiency, and that a ternary backing can further reduce the Q-factor. In both cases, this can be achieved while reducing bending stiffness when compared to a monolithic backing.

[0041] Among other advantages, the present teachings enable lowering the resonant frequency of a transducer without increasing the thickness of the piezoelectric layer in the transducer device. As described herein, mass can be added to the piezoelectric transducer—essentially making it an underdamped harmonic oscillator. This can lower the center frequency for a given thickness, maintaining, for example, the mechanical flexibility of the transducer array. Alternatively, or in addition, the present teachings can enable increasing / maximizing the bandwidth (and therefore the axial resolution) without increasing the backing thickness. This can also be done while maintaining the mechanical flexibility of the transducer array. Advantageously, the present teachings can obtain large bandwidth without resorting to excitation of higher-order longitudinal modes. Thus, transducer efficiency is not sacrificed.

[0042] Some aspects of the present teachings may be embodied in a method for manufacturing the acoustic devices described herein. In one embodiment, one or more performance parameters (Q-factor, center frequency f) of the acoustic transducers in the sheet (100) may be adjusted. max , efficiency "M", etc.) is calculated as a function of a variable layer thickness and / or variable pattern of at least one of the plurality of sublayers 31, 32, 33 forming the backing layer 30. In another or further embodiment, the sheet is manufactured according to a layer thickness and / or pattern of at least one of the plurality of sublayers 31, 32, 33 selected based on the calculated one or more performance parameters. In another or further embodiment, the calculation is constrained by a maximum thickness of the backing layer and / or a minimum flexibility of the sheet.

[0043] When interpreting the appended claims, it should be understood that the terms "comprises" and "comprises" do not exclude the presence of elements or activities other than those listed in a given claim; the use of the articles "a" or "an" preceding an element does not exclude the presence of a plurality of such elements; reference signs in a claim do not limit its scope; several "means" may be represented by the same or different items or implemented structures or functions; and any of these disclosed devices or portions thereof may be combined together or separated into further parts unless otherwise specified. When one claim refers to another claim, this may indicate synergistic advantages achieved by the combination of the respective features. However, the mere fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot also be advantageously used. Thus, embodiments of the present invention may include all possible combinations of claims, and each claim, in principle, refers to any preceding claim unless the context clearly excludes it. [Explanation of symbols]

[0044] A. Acoustic signal M medium Mp piezoelectric material Sb back side SF front Tb Second thickness Tf Third Thickness Tp First thickness 10 Front layer 11 Contact surfaces 12 Acoustic Matching Layer 15 Front electrode layer 20 Piezoelectric layer 20m piezoelectric material 21 Front piezoelectric layer 22 Pillar 23 Backside piezoelectric layer 30 Backing layer 31, 32 sublayer 31m, 32m Material 33 Foil 35 Back electrode layer 100 sheets 200 Controller 1000 Acoustic Devices

Claims

1. An acoustic device (1000) comprising a sheet (100), the sheet (100) comprising: a contact surface (11) for contacting a medium (M), for transmitting and receiving an acoustic signal (A) via said contact surface (11); a piezoelectric layer (20) comprising a piezoelectric material (20m) configured to transduce the acoustic signal (A) through the contact surface (11); a backing layer (30) acoustically interconnected to a back surface (Sb) of the piezoelectric layer (20) facing the contact surface (11); and The backing layer (30) a first sublayer (31) comprising a first material (31m) having a Young's modulus lower than that of the piezoelectric material (20m); a second sublayer (32) comprising a second material (32m) having a density higher than that of the first material (31m); Including, An acoustic device (1000) wherein at least a portion of the first material (31m) is disposed between the second material (32m) and the piezoelectric material (20m).

2. 2. The acoustic device (1000) of claim 1, wherein the sheet is a flexible sheet and the thickness and materials of all layers forming the sheet (100) are configured to allow the sheet to bend at or below a predetermined radius of less than 10 centimeters without breaking and / or losing essential functionality.

3. 3. The acoustic device (1000) of claim 1 or 2, wherein the piezoelectric layer (20) between the front layer (10) and the backing layer (30) has a first thickness (Tp), and the backing layer (30) between the piezoelectric layer (20) and a back surface (Sb) of the acoustic device (1000) has a second thickness (Tb), the second thickness (Tb) being at least two times smaller than the first thickness (Tp).

4. 4. The acoustic device (1000) of claim 1, wherein the sheet (100) includes a front layer (10) between the piezoelectric layer (20) and the front surface (Sf) of the acoustic device including the contact surface (11), the front layer (10) having a third thickness (Tf) smaller than the second thickness (Tb) of the backing layer (30).

5. 5. An acoustic device (1000) according to any one of claims 1 to 4, wherein the piezoelectric layer (20) is a structured piezoelectric layer having subdivided elements of piezoelectric material (20m) forming respective acoustic transducers.

6. 6. An acoustic device (1000) according to any one of claims 1 to 5, wherein the piezoelectric layer (20) comprises a piezoelectric material (Mp) forming an array of pillars (22) configured to transmit and receive acoustic signals (A) through a front surface (Sf) of the acoustic device, and at least one of a back piezoelectric layer (23) or a front piezoelectric layer (21), the array of pillars (22) being integrally formed with the back piezoelectric layer (23) and / or the front piezoelectric layer (21).

7. 7. An acoustic device (1000) according to any one of claims 1 to 6, wherein the second sublayer (32) comprises subdivided regions of the second material (32m).

8. 8. An acoustic device (1000) according to any one of claims 1 to 7, wherein the first sublayer (31) comprises subdivided regions of the first material (31m).

9. 9. The acoustic device (1000) according to any one of claims 1 to 8, wherein at least one of the first sublayer (31) and the second sublayer (32) is subdivided according to a pattern in which more material (31m, 32m) of each sublayer (31, 32) is provided at positions corresponding to positions of the piezoelectric layer (20) at which respective transducers are formed.

10. 10. An acoustic device (1000) according to any one of claims 1 to 9, wherein the second sublayer (32) comprises isolated regions of the second material (32m) with no material therebetween.

11. 11. An acoustic device (1000) according to any one of claims 1 to 10, wherein the first sublayer (31) comprises isolated regions of the first material (31m) with no material therebetween.

12. 12. An acoustic device (1000) according to any one of claims 1 to 11, wherein the piezoelectric layer (20) comprises a polymer-based piezoelectric material (20m), the first material (31m) comprises a polymer and / or an organic material, and the second material (32m) comprises a metal and / or a ceramic material.

13. 13. An acoustic device (1000) according to any one of claims 1 to 12, wherein the sheet (100) comprises a two-dimensional array of acoustic transducers formed by portions of the piezoelectric layer (20) sandwiched between respective electrode layers (15, 35).

14. A method for manufacturing an acoustic device (1000) according to any one of claims 1 to 13, comprising the steps of: One or more performance parameters (Q value, f ｍａｘ , M) as a function of a variable layer thickness and / or a variable pattern of at least one of the sublayers (31, 32, 33) forming said backing layer (30); The sheet (100) is subjected to the calculation of one or more performance parameters (Q value, f ｍａｘ , M) according to the layer thickness and / or pattern of said at least one sub-layer of the plurality of sub-layers (31, 32, 33); A method comprising:

15. 15. The method of claim 14, wherein the calculation is constrained by a maximum thickness of the backing layer (30) and / or a minimum flexibility of the sheet (100).

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