Acoustic device and manufacturing method

Abstract

An acoustic device (1000) comprises a sheet (100) having a contact surface (11) for contacting a medium (M) and transceiving an acoustic signal (A) via the contact surface (11). The sheet has a piezoelectric layer (20) comprising a piezoelectric material (20m) configured to convert an acoustic signal (A) via the contact surface (11). The backing layer (30) is acoustically interconnected to a back side (Sb) of the piezoelectric layer (20) opposite the contact surface (11). The backing layer (30) comprises at least two sub-layers (31, 32) of different materials (31m, 32m). The first sublayer (31) includes a first material (31m) having a Young's modulus smaller than that of the piezoelectric material (20m). The second sub-layer (32) includes a second material (32m) having a density higher than that of the first material (31m). At least some of the first material (31m) is arranged between the second material (32m) and the piezoelectric material (20m).

Description

[0001] Technical field and background technology

[0002] The present disclosure relates to acoustic devices including sheets for transducing acoustic signals; and methods of making such acoustic devices.

[0003] Acoustic devices capable of converting relatively low frequencies can be used to achieve relatively large penetration depths, for example, for cardiac / liver imaging or obstetric ultrasound. In addition, acoustic devices with relatively large bandwidths can provide shorter acoustic signals, which can improve (axial) resolution. However, in conventional ultrasonic transducer designs, the options for optimizing frequency response for applications are limited. Lower frequency applications typically require thicker layers of piezoelectric material. This is particularly problematic in acoustic devices that include flexible transducer arrays, where the greater thickness negatively affects the mechanical flexibility of the transducer. Thicker piezoelectric films also require a larger polarization voltage, which can be problematic. Maximization of the transducer bandwidth typically involves a backing material that also affects the flexibility of the transducer.

[0004] It would also be desirable to improve the frequency response of relatively thin and / or flexible acoustic transducer sheets, for example in terms of center frequency and frequency bandwidth, while minimally affecting flexibility and / or peak efficiency. Summary of the invention

[0005] Aspects of the present disclosure relate to an acoustic device, including a sheet, which is preferably relatively thin and / or flexible. The sheet has a contact surface on the front side of the sheet, the contact surface being used to contact a medium and transceive (send and / or receive) an acoustic signal to and / or from the medium via the contact surface. The sheet also has: a piezoelectric layer, which includes a piezoelectric material, the piezoelectric material being configured to convert (generate and / or measure) an acoustic signal via the contact surface (i.e., the front side). The sheet also has: a backing layer, which is acoustically interconnected with the back side of the piezoelectric layer opposite to the contact surface. The backing layer includes at least a first sublayer and a second sublayer. The first sublayer includes a first material having a Young's modulus smaller than that of the piezoelectric material. The second sublayer includes a second material having a density higher than that of the first material. At least some of the first material is arranged between the second material and the piezoelectric material.

[0006] Without being bound by theory, the inventors have found that a backing layer comprising a combination of a relatively dense (heavy) second sublayer connected to a piezoelectric layer via a relatively flexible (soft) first sublayer can act as a mass-spring oscillator. Advantageously, the added mass can allow the resonant frequency of the piezoelectric material to be shifted (from λ / 2 toward λ / 4) without a corresponding increase in the total thickness of the sheet. The 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 set at specific locations where required, which can locally improve the effect and enable further reduction of bending stiffness. Therefore, the present teachings can enable the frequency response of a fully flexible ultrasonic transducer to be optimized in terms of center frequency and frequency bandwidth without negatively affecting the transducer flexibility or peak efficiency. This enables the transducer design to achieve lower frequencies than would otherwise be possible (useful for applications requiring large penetration depths, such as cardiac / liver imaging or obstetric ultrasound) or to achieve higher bandwidths (and therefore higher axial resolution) than would otherwise be possible. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] These and other features, aspects, and advantages of the apparatus, systems, and methods of the present disclosure will become better understood from the following description, appended claims, and accompanying drawings, in which:

[0008] Figure 1A An acoustic device including a sheet is shown;

[0009] Figure 1B A portion of the sheet material that contacts the medium for transmitting and receiving acoustic signals is shown;

[0010] Figure 2A shows a perspective view of a stack of layers forming part of a sheet;

[0011] Figure 2B A cross-sectional view of a stack of layers contacting a medium for transceiving acoustic signals is shown;

[0012] Figure 3A and Figure 3B Portions and stacks of sheets including structured piezoelectric layers are shown;

[0013] Figure 4A and Figure 4B A stack including further structuring of the backing layer is shown;

[0014] Figure 5A and Figure 5B A stack comprising an acoustic matching layer and an additional foil between the piezoelectric layer and a further sublayer of the backing layer is shown;

[0015] Figure 6 and Figure 7 Simulations for various structures and thicknesses of sub-layers in the backing layer are shown. DETAILED DESCRIPTION

[0016] The terms used to describe specific embodiments are not intended to limit the present invention. As used herein, unless the context clearly indicates otherwise, the singular forms "a", "an" and "the" are also intended to include plural forms. The term "and / or" includes any combination and all combinations of one or more items listed in association. It will be understood that the terms "include" and / or "comprise" specify the presence of the features, but do not exclude the presence or addition of one or more other features. It will also be understood that when a particular step of a method is referred to as being after another step, the step may be directly after the other step, or one or more intermediate steps may be performed before performing a particular step, unless otherwise stated. Similarly, it will be understood that when describing a connection between a structure or component, unless otherwise stated, the connection may be established directly or through an intermediate structure or component.

[0017] The present invention is described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. In the accompanying drawings, the absolute and relative sizes of systems, components, layers and regions may be exaggerated for clarity. Embodiments may be described with reference to schematic illustrations and / or cross-sectional illustrations of possible idealized embodiments and intermediate structures of the present invention. In the specification and drawings, the same reference numerals always refer to the same elements. Relevant terms and their derivatives should be interpreted as referring to orientations as described subsequently or as shown in the drawings discussed. Unless otherwise stated, these relative terms are for ease of description and do not require the system to be constructed or operated in a specific orientation.

[0018] Figure 1A An acoustic device 1000 comprising the sheet 100 is shown. Figure 1B A portion of the sheet 100 is shown contacting a medium "M" for transceiving an acoustic signal "A". Typically, an acoustic signal "A" as described herein propagates in the medium "M" as a sound wave. For example, the medium "M" can be a biological medium, such as a tissue, or a non-biological medium, such as a liquid or a solid. In some embodiments, the acoustic signal "A" is an ultrasonic signal, for example having a frequency from 20 kHz to up to several gigahertz. For example, the acoustic device 1000 includes an ultrasonic transducer or an ultrasonic device or forms a part of an ultrasonic transducer or an ultrasonic device.

[0019] 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 the medium "M". More preferably, the front layer 10 is suitable for transceiving an acoustic signal "A" via the contact surface 11, for example, sending an acoustic signal "A" to the medium "M" and / or receiving an acoustic signal "A" from the medium "M". In other or additional embodiments, the sheet 100 includes a piezoelectric layer 20. Typically, the piezoelectric layer 20 includes a piezoelectric material 20m, which is configured to convert (e.g., generate and / or measure) an acoustic signal "A" via the contact surface 11 (i.e., the front side "Sf"). In other or additional embodiments, the sheet 100 includes a backing layer 30. Preferably, the backing layer 30 is acoustically interconnected with the back side "Sb" of the piezoelectric layer 20 opposite to the contact surface 11.

[0020] In some embodiments, the sheet 100 includes a two-dimensional array of acoustic transducers, for example, formed by portions of a piezoelectric layer 20 sandwiched between corresponding electrode layers. In a preferred embodiment, the sheet is a flexible sheet. Most preferably, the sheet 100, including all layers (e.g., 10, 20, 30), is sufficiently flexible so that the sheet can be bent at or below an allowable radius without destroying and / or losing basic functions, such as without destroying any of the acoustic transducers, wires, structural integrity of the layers, etc. For example, the allowable radius can be less than one meter, preferably less than fifty centimeters, more preferably less than twenty centimeters, most preferably less than ten centimeters, or even less than five centimeters. By providing greater flexibility to the sheet, the sheet can be directly applied to various curved surfaces of the medium "M", such as body parts, pipes, etc., and sound waves can be more effectively applied to and / or measured from the medium "M" from all directions.

[0021] In some embodiments, for example, to obtain sufficient flexibility, the thickness of the sheet 100 and / or the combined thickness of the layers 10, 20, 30 can be relatively low, for example less than one centimeter, preferably less than five millimeters, more preferably less than one millimeter, most preferably less than half a millimeter, for example between 50 μm and 500 μm. In other or additional embodiments, various flexible materials and / or bendable layers 10, 20, 30 can be used, such as polymer-based materials. In one embodiment, the piezoelectric layer 20 includes a polymer-based piezoelectric material 20m. For example, the piezoelectric material 20m includes a polarizing film of polyvinylidene fluoride (PVDF), preferably poly(vinylidene fluoride-co-trifluoroethylene) (P(CDF-TrFE) 80 / 20). In another or additional embodiment, the backing layer 30 includes a flexible material and / or a patterned material. In principle, conductive layers such as electrodes can be relatively thin, such as thin metal layers and / or wiring. Preferably, flexible electrodes are provided, for example "MAM electrodes" and / or electrodes comprising metals / oxides made of thermally evaporated materials such as MoCr / Al / MoCr, MoOx / Au / MoOx, MoO3 / Ag / MoO3, etc.

[0022] In some embodiments, the acoustic device 1000 includes a controller 200. In one embodiment, for example, as shown in the figure, the controller 200 is separated from the sheet 100, for example, connected by a wire or can be connected by a wire. It is also conceivable that the controller 200 or the control element is partially or completely integrated as a part of the sheet 100. In one embodiment, the controller 200 is configured to control the generation of an acoustic signal "A" by the sheet 100. For example, the controller 200 is configured to generate an electrical signal E, and one or more transducers including a portion of the piezoelectric layer 20 (in the sheet 100) are configured to convert the electrical signal E into an acoustic signal "A". In another or additional embodiment, the controller 200 is configured to receive and / or process the measurement results of the acoustic signal "A" by the sheet 100. For example, one or more transducers including a portion of the piezoelectric layer 20 (in the sheet 100) are configured to convert the acoustic signal "A" into an electrical signal E, and the controller 200 is configured to measure and / or process the electrical signal E.

[0023] In some embodiments, the acoustic device 1000 is configured to generate, measure and / or process acoustic and / or electrical signals to determine an acoustic image of the medium "M". For example, the acoustic device 1000 includes an ultrasound imaging device or forms part of an ultrasound imaging device. Other applications are also contemplated, such as treatment by sound waves, acoustic streaming, for example, for mixing liquids, inspection of structural elements by sound waves, etc. In one embodiment, the controller 200 includes or accesses a (non-transitory) computer-readable medium storing instructions that, when executed, cause the acoustic device to perform operational actions as described herein.

[0024] Figure 2A The formation (e.g., Figure 1A and Figure 1B 100 is a perspective view of a stack of layers of a portion of a sheet 100 (shown). Figure 2B A cross-sectional view of a stack of layers that contact a medium "M" for transceiving an acoustic signal "A" is shown. As shown 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 side "Sb" of the device 1000 has a second thickness "Tb"; and the front layer 10 between the piezoelectric layer 20 and the front side "Sf" of the device (i.e., the front surface of the contact surface 11) has a third thickness "Tf". As described herein, the backing layer 30 preferably includes at least two different sub-layers 31, 32 and / or sub-structures made of different materials 31m, 32m. Preferably, at least some of the first material 31m is arranged between the second material 32m and the piezoelectric material 20m.

[0025] In some embodiments, the backing layer 30 includes a first sublayer 31, the first sublayer 31 includes a first material 31m or consists essentially of the first material 31m, the Young's modulus of the first material 31m is less than the Young's modulus of the piezoelectric material 20m and / or less than the Young's modulus of the second material 32m. For example, the Young's modulus of the first material 31m is less than the Young's modulus of the piezoelectric material 20m and / or less than the Young's modulus of the second material 32m (at least in the actuation direction indicated by arrow "B", i.e., perpendicular to the layers) by at least 10% (1.1 times), 20% (2.2 times) or 50% (1.5 times), preferably at least two times, three times, up to ten times or more.

[0026] In other or additional embodiments, the backing layer 30 includes a second sublayer 32, the second sublayer 32 includes a second material 32m or consists essentially of a second material 32m, the density of the second material 32m is higher than the density of the first material 31m and / or higher than the density of the piezoelectric material 20m. For example, the density of the second material 32m (the mass per unit volume of the corresponding material) is higher than the density of the piezoelectric material 20m and / or higher than the density of the first material 31m by at least 10% (1.1 times), 20% (2.2 times) or 50% (1.5 times), preferably at least two times, three times, up to ten times or more.

[0027] As described herein, the backing layer 30 including different functional sublayers 31, 32 can allow for providing a sufficient effect of controlling the frequency and / or bandwidth of the transducer even when the backing layer is relatively thin. Therefore, in some embodiments, the second thickness "Tb" of the backing layer 30 is less than the first thickness "Tp" of the piezoelectric layer 20, for example at least two times, three times, five times, up to eight times, ten times or more. For example, the piezoelectric layer 20 has a first thickness "Tp" between 25μm and 250μm, preferably between 50μm and 150μm. For example, the backing layer 30 has a second thickness "Tb" between 5μm and 50μm, preferably between 10μm and 25μm. In other or additional embodiments, the third thickness "Tf" of the front layer 10 is less than the first thickness "Tp" of the piezoelectric layer 20, and preferably also less than the second thickness "Tb" of the backing layer 30. In one embodiment, the third thickness "Tf" is at least two, three, five, ten, up to twenty, fifty, one hundred times or more less than the first thickness "Tp". In another or additional embodiment, the third thickness "Tf" is at least two, three, five, up to eight, ten 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 can be minimally affected and / or acoustic wave transmission can be optimized.

[0028] In some embodiments, for example as shown, the piezoelectric layer 20 comprises or consists essentially of a piezoelectric material 20m, which is arranged between the front electrode layer 15 and the back electrode layer 35. Typically, the electrode layer comprises a conductive material, such as a metal; and / or the electrode layer can be relatively thin, for example having a thickness of less than two microns, preferably less than one micron, for example between 0.1 μm and 0.5 μm. In one embodiment, for example as shown, the back electrode layer 35 can be considered as part of the backing layer 30. However, in view of the negligible thickness / mechanical function, the back electrode layer 35 can also be considered as part of the piezoelectric layer 20. Similarly, the front electrode layer 15 can be considered as part of the front layer 10, as shown, or part of the piezoelectric layer 20. In one embodiment, the front layer 10 includes at least a non-conductive layer in front of the front electrode layer 15 forming the contact surface 11. For example, this can be a relatively thin layer or a larger layer, including an acoustic matching layer or forming an acoustic matching layer.

[0029] In some embodiments, for example, as shown in the figure, the front electrode layer 15 is patterned so that an electrical signal (e.g., voltage) can be selectively applied to and / or measured from a local area of ​​the piezoelectric layer 20, thereby, for example, forming a corresponding transducer. In other or additional embodiments, for example, as shown in the figure, the back electrode layer 35 can be a continuous metal layer or other conductive layer used as a common electrode. In other embodiments (not shown), subdivision can be applied to both the front electrode layer 15 and the back electrode layer 35, or both electrode layers 15, 35 can be continuous layers.

[0030] As described herein, the backing layer 30 preferably includes at least two different sub-layers 31, 32, i.e., includes or essentially consists of two different materials 31m, 32m. In some embodiments, the high-density material 32m for the second sub-layer 32 may include, for example, metal (iron, steel, copper, silver, lead, gold, platinum) or, for example, ceramic. In other or additional embodiments, the flexible material 31m for the first sub-layer 31 may include various polymer materials and organic materials with and without filler additives in the form of solid films and / or as porous foams.

[0031] In some embodiments, two or more sublayers 31, 32 of the backing layer 30 are deposited onto the piezoelectric layer 20 and / or the back electrode layer 35 or the intermediate substrate 33 or are laminated onto the piezoelectric layer 20 and / or the back electrode layer 35 or the intermediate substrate 33, for example Figure 5A and Figure 5B In other or additional embodiments, the stack is constructed starting with a backing layer 30 and depositing or laminating the backside electrode layer 35 and / or the piezoelectric layer 20 onto the backing layer 30 .

[0032] Without being bound by theory, it is observed that acoustic transducers are typically provided with a non-reflective / transmissive backing layer, wherein the primary resonant frequency of the piezoelectric layer may be determined by the thickness "Tp" of the piezoelectric layer 20, which corresponds approximately to half a wavelength (λ / 2), to form a standing wave with a node "N" midway between the antinodes on two opposite ends (at Tp / 2). In contrast, as shown in the present figure and in accordance with some embodiments of the present backing layer 30, the mass-spring effect of the different sub-layers (indicated by the acoustic coupling "B") may cause the node "N" to shift away from the mid-position (at Tp / 2) toward the backing layer 30. This may correspond to a shift of the resonance to lower frequencies (longer wavelengths), for example up to λ / 4. In some embodiments, the coupling "B" may be stronger when the resonant frequency of the mass-spring system formed by the backing layer 30 (e.g., determined by the effective mass and spring constant provided by the materials and structure of the sublayers 31, 32) matches the resonant frequency of the piezoelectric layer 20 (e.g., determined by the thickness "Tp" and the wave speed in the piezoelectric material 20m). If the backing layer completely reflects the wave, a similar effect of shifting the node "N" toward the backing layer can be achieved, but this generally requires the use of a much thicker, high-density layer, which will affect flexibility. For the sake of completeness, it should be noted that this figure shows the amplitude of the standing wave oscillations transverse to the propagation direction, where it will be understood that the wave may actually oscillate primarily in the propagation direction.

[0033] Figure 3A and Figure 3B Portions and stacks of sheets 100 including structured piezoelectric layers 20 and / or backing layers 30 are shown. Figure 4A and Figure 4B A stack including further structuring of the backing layer 30 is shown. In some embodiments, the piezoelectric layer 20 is a structured piezoelectric layer, for example having subdivided elements of piezoelectric material 20m forming corresponding acoustic transducers. For example, the subdivision of the piezoelectric material 20m can correspond to the subdivision of one or both of the electrode layers 15, 35. Preferably, the piezoelectric layer 20 includes piezoelectric material 20m forming an array of pillars 22, which are configured to transceive acoustic signals "A" via the front side "Sf" of the device.

[0034] In some embodiments, the posts 22 or other subdividing elements have a corresponding (maximum) diameter "Dp" (in the plane of the layer), "Dp" being the same as or smaller than the corresponding length "Lp" of the posts (transverse to the plane of the layer, e.g. the direction of propagation of the acoustic signal) or other subdividing elements, e.g. at least 20% (1.2 times), 50% (1.5 times), up to two times, three times or more. The smaller the diameter compared to the length, the higher the transducer density provided at relatively low frequencies. For example, the diameter "Dp" is between 10 μm and 100 μm, preferably between 20 μm and 60 μm. The diameter "Dp" does not need to be constant, e.g. it can be slightly thicker on one side, as shown. In order to provide sufficient separation / decoupling, the (minimum) gap "Gp" can be similar to the diameter "Dp", e.g. preferably smaller. For example, the (minimum) gap "Gp" is between 5 μm and 50 μm, preferably between 10 μm and 30 μm.

[0035] In some embodiments, the piezoelectric layer 20 comprises at least one of a backside piezoelectric layer 23 or a frontside piezoelectric layer 21. Advantageously, a continuous piezoelectric layer 21 and / or 23 may allow for the deposition of additional structures and / or layers. In order to minimize the cross coupling between the subdividing elements 22, the continuous piezoelectric layers 21 and / or 23 preferably have respective thicknesses T1 and / or T2 that are smaller than the length "Lp" of the subdividing elements 22, for example 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 between 1 μm and 30 μm, preferably between 5 μm and 20 μm.

[0036] 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 can form a single piece of material. For example, the layers 21, 23 are essentially composed of the same piezoelectric material 20m as the intermediate structure / pillar; and / or the piezoelectric materials are fused together. In some embodiments, the piezoelectric layer 20 with the pillars 22 can be manufactured according to the method described in the prior disclosure WO 2021 / 167446 A1. In other or additional embodiments, other structuring of the piezoelectric layer 20 is also conceivable. In some embodiments, one or both of the piezoelectric layers 21, 23 can be omitted, for example, the pillars are provided directly between the electrode layers 15, 35. In other or additional embodiments, a structured piezoelectric layer is provided by cutting or otherwise subdividing the piezoelectric layer 20 into separate islands or pillars. For example, the post structure and / or other subdivision of the piezoelectric layer 20 may be advantageous in reducing acoustic and / or mechanical cross coupling between elements in an acoustic device.

[0037] As described herein, the backing layer 30 preferably comprises at least two different sub-layers 31, 32, wherein at least one of the sub-layers is patterned, i.e. subdivided into elements. Preferably, the subdivision of one or more of the at least two different sub-layers 31, 32 corresponds to the division of the piezoelectric layer 20 forming the corresponding transducer. For example, the patterning of one or more sub-layers corresponds to the patterning of at least one of the electrode layers 15, 35 and / or to the patterning of the piezoelectric layer 20. Thus, it will be understood that, for example Figure 3B , Figure 4A , Figure 4B , Figure 5A , Figure 5B The patterning of one or more of the sub-layers shown may also be applied to applications such as Figure 2B The patterning may vary in relative width, alignment relative to the posts, and relative spacing. Other possible variations may include designs in which the heavy material 32m is completely covered by other materials (not shown).

[0038] In some embodiments, for example, Figure 3B , Figure 4A , Figure 4B , Figure 5A , Figure 5B As shown in each of the above, the second sub-layer 32 includes subdivided regions of the second material 32m. In one embodiment, for example, Figure 3B and Figure 5A As shown, the first material 31m of the first sub-layer 31 extends between the subdivided regions of the second material 32m. In another or additional embodiment, for example Figure 4A , Figure 4B , Figure 5B As shown, spaces without material are provided between the subdivided areas of the second material 32m. Combinations are also possible, or a third material (not shown) can be provided between the subdivided areas of the second material 32m and / or the second sublayer 32 can be covered by another layer (not shown). Instead of completely subdividing the second sublayer 32, it is also conceivable to vary the layer thickness of the second material 32m, for example to concentrate more material in some areas than in other areas. In other or additional embodiments, for example as Figure 4B and Figure 5B As shown, the first sub-layer 31 includes subdivided regions of the first material 31m. In one embodiment, for example, as shown, a gap without material is provided between the subdivided regions of the first material 31m. In another or additional embodiment (not shown), another material is provided between the subdivided regions of the first material 31m.

[0039] Preferably, the first sublayer 31 and / or the second sublayer 32 are subdivided according to a pattern, wherein more material 31m, 32m of the respective sublayer 31, 32 is arranged at a position corresponding to the position of the piezoelectric layer 20 where the respective transducer is formed. For example, the material 31m, 32m of the respective sublayer 31, 32 is exclusively or mainly (concentrated) arranged at the position of the pillar 22 (or a similar substructure of the piezoelectric layer 20) and / or at the position of the actuation position determined by the respective (subdivided) electrode layer 15 and / or 35.

[0040] Figure 5A and Figure 5B A stack is shown comprising an acoustic matching layer 12 and an additional foil 33 between the piezoelectric layer 20 and further sub-layers 31 , 32 of the backing layer 30. It will be appreciated that one or both of these features may be combined with any of the preceding embodiments.

[0041] In some embodiments, the transmission of acoustic waves to the medium "M" can be optimized by providing one or more acoustic matching layers 12, which can have a relatively large thickness "Tf", such as up to the second thickness "Tb", or even larger. For example, the one or more matching layers can provide an acoustic impedance gradient so that the acoustic waves from the piezoelectric layer / transducer can smoothly penetrate into the medium "M" and / or so that the acoustic waves can smoothly propagate from the medium "M" to the piezoelectric layer / transducer for detection.

[0042] In other or additional embodiments, the acoustic behavior may be further optimized and / or manufacturing may be facilitated by providing an additional foil 33, which may be considered as part of the backing layer 30. In one embodiment, the acoustic behavior may be further optimized by adjusting the thickness, material and / or distribution of the additional layer 33. In another or additional embodiment, manufacturing may be facilitated by providing a foil 33 for depositing / laminating the backside electrode layer 35 and / or other sub-layers of the backing layer 30.

[0043] Figure 6 and Figure 7 Simulations are shown for various structures and thicknesses of sub-layers in the backing layer 30. Numerical finite element method (FEM) simulations performed in Comsol are used here to illustrate the potential for controlling various parameters of an acoustic device by adjusting the backing layer. Various possible implementations are simulated and compared to a reference monolithic backing design. Frequency responses are simulated, enabling evaluation of parameters including center frequency, bandwidth, and efficiency. Cross-sectional views of various geometries are shown above the corresponding graphs.

[0044] Without being bound by theory, it will be understood that the quality factor or Q factor is a dimensionless parameter that describes the degree of underdamping of an oscillator or resonator. In one definition, the Q factor is the ratio of the initial energy stored in the resonator to the energy lost in one arc of the oscillation period. In another definition, the Q factor is the ratio of the center frequency of the resonator to its bandwidth when subjected to an oscillating driving force. These two definitions give numerically similar but not identical results. In any case, it should be understood that (sinusoidally) driven resonators with higher Q factors resonate with larger amplitudes (at the resonant frequency), but have a smaller frequency range around the frequency at which they resonate, i.e., a smaller bandwidth; and (sinusoidally) driven resonators with lower Q factors resonate with smaller amplitudes (at the resonant frequency), but have a higher frequency range around the frequency at which they resonate, i.e., a larger bandwidth.

[0045] Figure 6 A dual material design is shown in which the relatively high density (heavy) material 32m is steel, and in which the total backing thickness "Tb" is held constant at 14μm. Similar results are obtained using lead as the heavy material 32m (not shown). All quantities are normalized to the performance of the reference design ("Ref"). As shown 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, up to 100% on the right side, where all designs degenerate to the reference design (monolithic PI). The Y-axis indicates: "Q Factor / Q ref ", that is, the relative value of the Q factor, where a lower Q factor corresponds to a higher bandwidth; "f max / f ref ", that is, compared with the reference design, the maximum signal frequency f max ; "M=max|H| compared to M ref ”, i.e., a measure of efficiency calculated as the maximum amplitude of the transfer function H of the acoustic wave in the medium compared to a reference design; and “M / f max Compared to ref", i.e. the same measure but divided by f max and compared with a reference design, which is a measure of efficiency incorporating the resonant frequency shift.

[0046] As shown, with the "S2a" design, when the PI is completely replaced by steel (T33 / Tb=0%), a Q factor reduction of up to 15% can be achieved (top left), with a 22% reduction in resonant frequency (top right). Lower results are achieved using patterned designs "S2b" and "S2c", but these designs provide lower bending stiffness. In all cases, the effect on the radiation efficiency "M" is relatively small, or even increased, compared to the reference. As tested with lead, a stronger effect can be achieved when the material 32m is denser. By optimizing the patterned design, additional performance can be achieved while maintaining low bending stiffness. In a comparative simulation (not shown), the monolithic backing thickness "Tb" was increased from 14μm to 50μm, where a 21% reduction in resonant frequency was observed, while the bandwidth Q was only slightly improved by -2%. Therefore, the use of a dual-material backing can achieve a similar reduction in resonant frequency without increasing the thickness, and with the benefit of increased bandwidth. In further simulations (not shown), it was observed that with a backing layer thickness "Tb" of 50 μm, the dual material design achieved a 40% reduction in resonant frequency and a 20% to 30% reduction in Q factor (tested with lead).

[0047] Figure 7 A three-material backing design is shown. The total thickness remains at its reference value "Tb" = 14 μm, 8 μm is allocated to the material layer 33m (here PI), and the rest is divided between the soft material (disappearing on the left side of the figure) and the relatively high density material (steel, disappearing on the right side). 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; Attenuation α = 0.94 dB / λ. For example, this can correspond to a relatively flexible polymer material, such as polyurethane. On the right side of the figure, the dense material is thin (30% of Tb = 4.2 μm): not much mass is added, resulting in a limited reduction in the resonant frequency (<10%). However, a significant bandwidth can be achieved (Q factor reduction of 10% to 20%). It is associated with a reduction in efficiency, which is more or less significant, depending on the design. A greater reduction in the Q factor can be achieved at a constant backing thickness compared to the dual-material design. It can be noted that there is a significant difference between the designs "S3b", "S3c", "S3d" with different degrees of patterning in the backing layer and the unpatterned backing layer of the design "S3a". Without being bound by theory, it should be noted that in the absence of patterning, the expansion or contraction of the flexible material 31m may be limited. For example, if the Poisson's ratio is close to 0.5, expansion of the material in one direction may require contraction in a lateral direction, which is a limited possibility in an unpatterned interlayer. In summary, the simulations demonstrate the ability of a two-material backing to reduce the resonant frequency and Q factor at constant backing thickness and radiation efficiency; and the ability of a three-material backing to further reduce the Q factor. For both, this can be achieved while reducing the bending stiffness compared to a monolithic backing.

[0048] Among other advantages, the present teachings enable the resonant frequency of the transducer to be reduced 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 a weakly damped resonator. This can reduce the center frequency for a constant thickness, for example, to maintain the mechanical flexibility of the transducer array. Alternatively or in addition, the present teachings can enable increasing / maximizing the bandwidth (and therefore increasing / maximizing 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 a large bandwidth without resorting to the excitation of higher order longitudinal modes. Therefore, transducer efficiency is not sacrificed.

[0049] Some aspects of the present teachings may be embodied as methods of making an acoustic device as described herein. In one embodiment, one or more performance parameters (such as Q factor, center frequency f) of an acoustic transducer in a sheet (100) are calculated as a function of a variable layer thickness and / or a variable pattern of at least one of the plurality of sub-layers 31, 32, 33 forming a backing layer 30. max, efficiency "M"). In another or additional embodiment, the sheet is manufactured according to the layer thickness and / or pattern of at least one of the plurality of sub-layers 31, 32, 33 selected based on the calculated one or more performance parameters. In another or additional embodiment, the calculation is constrained by the maximum thickness of the backing layer and / or the minimum flexibility of the sheet.

[0050] In interpreting the appended claims, it should be understood that the word "comprising" does not exclude the presence of other elements or actions than those listed in a given claim; the word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements; any reference numerals in a claim do not limit its scope; several "means" may be represented by identical or different items or implemented structures or functions; unless otherwise specifically stated, any disclosed device or part thereof may be combined together or separated into further parts. Where one claim refers to another claim, this may indicate synergistic advantages achieved by combining their respective features. However, the fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot also be used to advantage. Therefore, the present embodiment may include all working combinations of the claims, wherein each claim may in principle refer to any preceding claim unless the context clearly excludes it.

Claims

1. An acoustic device (1000), comprising a sheet (100), wherein the sheet (100) has: A contact surface (11) for contacting a medium (M) and transmitting and receiving an acoustic signal (A) via the contact surface (11); a piezoelectric layer (20) comprising a piezoelectric material (20m) configured to convert the acoustic signal (A) via the contact surface (11); and a backing layer (30) acoustically interconnected with a back side (Sb) of the piezoelectric layer (20) opposite to the contact surface (11), in, The backing layer (30) comprises: a first sublayer (31) including a first material (31m) having a Young's modulus smaller than that of the piezoelectric material (20m); and a second sublayer (32) comprising a second material (32m) having a density higher than that of the first material (31m); Wherein, at least some of the first material (31m) is arranged between the second material (32m) and the piezoelectric material (20m).

2. The acoustic device (1000) according to claim 1, wherein: The sheet is a flexible sheet, wherein the thickness and materials of all layers forming the sheet (100) are configured such that the sheet can be bent at or below a predetermined radius of less than ten centimeters without damage and / or loss of basic functions.

3. The acoustic device (1000) according to any one of the preceding claims, wherein: The piezoelectric layer (20) between the front layer (10) and the backing layer (30) has a first thickness (Tp), wherein the backing layer (Tb) between the piezoelectric layer (20) and the back side (Sb) of the device (1000) has a second thickness (Tb), wherein the second thickness (Tb) is at least two times smaller than the first thickness (Tp).

4. The acoustic device (1000) according to any one of the preceding claims, wherein: The sheet (100) comprises: a front layer (10) between the piezoelectric layer (20) and the front side (Sf) of the device including the contact surface (11), wherein the front layer (10) has a third thickness (Tf) smaller than the second thickness (Tb) of the backing layer (30).

5. The acoustic device (1000) according to any one of the preceding claims, wherein: The piezoelectric layer (20) is a structured piezoelectric layer having subdivided elements of piezoelectric material (20m) which form a corresponding acoustic transducer.

6. The acoustic device (1000) according to any one of the preceding claims, wherein: The piezoelectric layer (20) includes: a piezoelectric material (Mp) forming an array of pillars (22), wherein the pillars (22) are configured to transmit and receive acoustic signals (A) via the front side (Sf) of the device; and at least one of a back piezoelectric layer (23) or a front piezoelectric layer (21), wherein the array of pillars (22) is integrally formed with the back piezoelectric layer (23) and / or the front piezoelectric layer (21).

7. The acoustic device (1000) according to any one of the preceding claims, wherein: The second sub-layer (32) comprises subdivided regions of the second material (32m).

8. The acoustic device (1000) according to any one of the preceding claims, wherein: The first sub-layer (31) comprises subdivided areas of the first material (31m).

9. The acoustic device (1000) according to any one of the preceding claims, wherein: At least one of the first sublayer (31) and the second sublayer (32) is subdivided according to a pattern, wherein more material (31m, 32m) of the respective sublayer (31, 32) is arranged at a position corresponding to a position of the piezoelectric layer (20) where a respective transducer is formed.

10. The acoustic device (1000) according to any one of the preceding claims, wherein: The second sublayer (32) includes isolated regions of the second material (32m) with no material between the isolated regions of the second material (32m).

11. The acoustic device (1000) according to any one of the preceding claims, wherein: The first sublayer (31) comprises isolated regions of the first material (31m) with no material between the isolated regions of the first material (31m).

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

13. The acoustic device (1000) according to any one of the preceding claims, 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 of manufacturing an acoustic device (1000) according to any one of the preceding claims, the method comprising: Calculating one or more performance parameters (Q factor, f) of the acoustic transducer in the sheet (100) as a function of the variable layer thickness and / or pattern of at least one of the plurality of sub-layers (31, 32, 33) forming the backing layer (30) max , M); as well as According to one or more performance parameters (Q factor, f max , M), the layer thickness and / or pattern of at least one of the multiple sub-layers (31, 32, 33) is used to manufacture the sheet (100).

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

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