Composite matching layer

Abstract

Composite matching layer and method of fabricating composite matching layers for ultrasound transducers. A method of fabricating composite matching layers comprises mechanically shaping a high impedance material (120) to yield a trapezoidal-shaped bar or a plurality of parallel pyramidal shaped columns, and fill grooves between said trapezoidal-shaped bars or plurality of parallel pyramidal shaped columns with a low impedance material (130).

Description

FIELD OF THE INVENTION

[0001] The invention relates to ultrasound matching layers, and to a method of manufacturing a composite matching layer.BACKGROUND OF THE INVENTION

[0002] A medical ultrasound probe typically includes many acoustic layers in the configuration, such as a piezoelectric layer, matching layers and a window / or lens and backing. The piezoelectric layer commonly has twenty times or higher acoustic impedance than tissues in a human body. Due to the large impedance mismatch, the ultrasound probe suffers from limited operational bandwidth and poor acoustic energy transmission and reception. To overcome the problems, it is common to employ one or more impedance matching layers between the piezoelectric layer and the window or lens of a probe. Often, a high-performance probe uses two or three matching layers in the design. To achieve even wider bandwidth, it is necessary to increase the number of matching layers to more than three matching layers at the cost of complex fabrication processes, higher manufacturing cost and sensitivity decrease by overall thickness increases.

[0003] Recently, a gradient or continuous matching layer design has been proposed for the sensitivity and bandwidth improvement, using a material with continuously changing impedance. See for example: Haller et al. (1993) "Tapered Acoustic Matching Layers", Sato et al. (2002) "Experimental Investigation of Phased Array Using Tapered Matching Layers", Liz et al. (2017) "Broadband gradient impedance matching using an acoustic metamaterial for ultrasonic transducers", Bian et al. (2021) "Ultra-wideband underwater acoustic transducer with a gradient impedance matching layer", or Zhu et al. (2022) "Enhancement of Ultrasonic Transducer Bandwidth by Acoustic Impedance Gradient Matching Layer". In particular, Liz et al. show a 610 µm long, cone-shape gradient impedance matching layer fabricated with thousands of a 122 µm diameter, silica optical fiber chemically etched in hydrofluoric acid solutions. They demonstrated the benefits of the gradient matching layer in large operational bandwidth improvement. However, it is not easy to control the final cone shape geometry of each fiber in the strong acid solution repeatedly and consistently. In addition, due to the use of the very strong and toxic acid, the fabrication process is not a preferred method to use in a manufacturing facility.

[0004] Zhu et al. show an anisotropic gradient matching material fabricated with nano-tungsten particles and epoxy resin by controlling their volume fractions. The thickness and impedance of each layer was carefully controlled by a multi-step deposition method in which each layer was deposited on top of another layer at a time. This method also shows the benefits of the gradient matching layer in bandwidth improvement. However, its manufacturing processes are very complicated and extremely hard to control both the impedance distribution and layer thickness.

[0005] There is thus a clear need for a less complex, more reliable and more repeatable manufacturing process.SUMMARY OF THE INVENTION

[0006] It is, inter alia, an object of the invention to provide an improved manufacturing process for impedance matching layers for use in ultrasound probes. In particular, to manufacture or fabricate matching layers in a repeatable manner and with good precision. The repeatability and precision are achieved according to this invention by mechanically shaping a slab of high-impedance material to advantageously shaped geometries and filing remaining spaces with a low-impedance material to generate a composite matching layer. By employing mechanical shaping, for example cutting, the manufacturing process can be repeated with near same results. The invention is defined by the independent claims. Advantageous embodiments are defined in the dependent claims.

[0007] According to an aspect of the invention, a method to fabricate a composite matching layer for ultrasound transducers, comprises: mechanically shaping a slab of high-impedance material to form grooves in the slab of high-impedance material in such a way that the remaining high-impedance material forms at least: a trapezoid-shaped bar or a plurality of parallel pyramidal shaped columns; and filling the grooves with a low-impedance material to yield the composite matching layer.

[0008] In an example, mechanically shaping comprises cutting; and each groove is obtained by cutting the high-impedance material with a V-shaped cutting blade with a pre-determined tip angle and a pre-determined tip-shape.

[0009] In an example, mechanically shaping comprises cutting; and each groove is obtained by performing a first cut at a first predetermined angle; and performing a second cut at a second predetermined angle; and wherein the first cut and second cut intersect to form each groove.

[0010] In an example, the first angle and second angle are related such that the first predetermined angle equals the negative of the second predetermined angle.

[0011] In an example, the method further comprises: mechanically processing a first surface of the composite matching layer, wherein the first surface comprises: the tip of the trapezoid-shaped bar, or the tips of the plurality of parallel pyramidal shaped columns.

[0012] In an example, the method further comprises: mechanically processing a second surface of the composite matching layer, wherein the second surface is opposite to the first surface in such a way that the second surface comprises: the base of the trapezoid-shaped bar or the bases of the plurality of parallel pyramidal shaped columns.

[0013] In an example, an impedance of the composite matching layer is controlled by any one or a combination of: a volume fraction of the high-impedance material and low-impedance material; and a geometry of: the trapezoid-shaped bar or the plurality of parallel pyramidal shaped columns.

[0014] According to another aspect of the invention, a method of manufacturing acoustic stacks for ultrasound transducers, comprises manufacturing a composite matching layer as described herein, and bonding a slab of piezoelectric material to the composite matching layer along a surface of the composite matching layer that comprises the base of the trapezoid-shaped bar or the bases of the plurality of parallel pyramidal shaped columns.

[0015] In an example, the method further comprises: mechanically removing material from the acoustic stack to create dice kerfs in such a way that each pillar comprises: a single trapezoid-shaped bar, or a single pyramidal shaped column; and optionally comprising: bonding a de-matching layer to a surface of the piezoelectric layer opposite to the surface of the piezoelectric layer that is bonded to the composite matching layer before creating dice kerfs.

[0016] According to another aspect of the invention, a method of manufacturing ultrasound transducer stacks, comprises manufacturing an acoustic stack as described herein, and coupling an electric circuit to the acoustic stack, wherein the electric circuit is configured to drive individual transducer elements.

[0017] According to another aspect of the invention, a composite matching layer comprises: a first layer comprising a high-impedance material and a low-impedance material, wherein the high impedance material is configured to form: a trapezoidal-shaped bar, or a plurality of parallel pyramidal shaped columns; and wherein the low impedance material is configured to fill the remainder of the first layer.

[0018] According to another aspect of the invention, an acoustic stack comprises as first layer, a composite matching layer as described herein, and a second layer comprising a piezoelectric material bonded to the composite matching layer, along a surface of the composite matching layer that comprises the the base of the trapezoid-shaped bar or the bases of the plurality of parallel pyramidal shaped columns.

[0019] According to another aspect of the invention, an ultrasound transducer stack comprises an acoustic stack as described herein, and an electrical circuit coupled to the piezoelectric material, wherein the electrical circuit is configured to drive individual piezoelectric elements.

[0020] According to another aspect of the invention, an ultrasound probe comprises an ultrasound transducer stack as described herein.

[0021] According to another aspect of the invention, an ultrasound system comprises an ultrasound probe as described herein.

[0022] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Fig. 1A shows an exemplary step in the manufacturing of a composite matching layer. Fig. 1B shows an exemplary step in the manufacturing of a composite matching layer. Fig. 1C shows an exemplary step in the manufacturing of a composite matching layer. Fig. 2A shows an exemplary step in the manufacturing of a composite matching layer. Fig. 2B shows an exemplary step in the manufacturing of a composite matching layer. Fig. 3 shows an exemplary step in the manufacturing of a composite matching layer. Fig. 4A shows an exemplary step in the manufacturing of a composite matching layer. Fig. 4B shows an exemplary step in the manufacturing of a composite matching layer. Fig. 5A shows an exemplary step in the manufacturing of a composite matching layer. Fig. 5B shows an exemplary step in the manufacturing of a composite matching layer. Fig. 5C shows an exemplary step in the manufacturing of a composite matching layer. Fig. 5D shows an exemplary step in the manufacturing of a composite matching layer. Fig. 6A shows an exemplary composite matching layer. Fig. 6B shows an exemplary composite matching layer. Fig. 7A shows an exemplary composite matching layer. Fig. 7B shows an exemplary composite matching layer. Fig. 8 shows an exemplary transducer stack. Fig. 9 shows a frequency response of composite matching layer in accordance with an embodiment. DESCRIPTION OF EMBODIMENTS

[0024] The invention relates to ultrasound imaging, in particular to ultrasound transducers and more particularly to matching layers of ultrasound transducers and methods of manufacturing said matching layers. Matching layers are one or multiple layered materials that are placed on top of transducer elements of an ultrasound probe and which are configured to balance an impedance mismatch between ultrasound transducer elements and a to be imaged tissue. The matching layer combined with a piezoelectric layer (the piezoelectric material comprising ultrasound transducer elements) is typically referred to as an acoustic stack. An acoustic stack further connected to an electric circuit configured to drive individual transducer elements from the piezoelectric layer is typically referred to as an ultrasound transducer stack. An ultrasound transducer stack is further typically integrated into an ultrasound probe. Ultrasound probes may be of any kind, including but not limited to hand-held ultrasound probes such as general imaging probes, transesophageal probes, transthoracic probes, intravenous probes; non-hand-held probes such as ultrasound monitoring patches (referred to broadly as probes in this patent specification); and any other ultrasound probe known to the skilled person.

[0025] According to the invention, a composite matching layer (i.e., a matching layer comprising multiple layers of different material properties) may be manufactured by: i) mechanically shaping a slab of high-impedance material to form grooves in the slab of high-impedance material in such a way that the remaining high-impedance material forms at least: a trapezoid-shaped bar or a plurality of parallel pyramidal-shaped columns; and ii) filling the grooves with a low-impedance material to obtain a desired acoustic impedance and yield the composite matching layer.

[0026] Fig. 1A shows such an exemplary slab of high-impedance material 120.

[0027] Fig. 1B shows the slab of high-impedance material after mechanically shaping the slab of high-impedance material to form grooves 122 in the slab of high-impedance material.

[0028] Fig. 1C shows the slab of high impedance material 120 of Fig. 1B, i.e., with removed material to form grooves 122, wherein the grooves 122 are filled with a low-impedance material 130.

[0029] The grooves 122 as shown in Fig. 1B and 1C are obtained by mechanically shaping material from the slab of high-impedance material. By mechanically shaping this specification refers to processes such as cutting, carving, abrading, molding and / or stamping. However, any process in which a material can be shaped is envisaged. It should be clear to a skilled person in the art what options are available for each of these shaping processes. For example, cutting may comprise cutting with a knife, sawing, laser cutting, waterjet cutting, milling, etc. For example, carving may comprise using tools such as chisels and gouges or rotary carving machines. For example, abrading may comprise grinding with wheels or belts. For example, molding may comprise using dies or compression molding. For example, stamping may comprise punching out. However, it should be clear that this list is not exhaustive.

[0030] An example of a process of shaping according to the invention may comprise molding a material heavily loaded in carbon. For example, carbon particles may be mixed with a high-impedance particles and with a binder (liquid). The mixture may result in a slurry. This slurry may then be compressed into a mold and later exposed to a high temperature to solidify the binder. For composite matching layer the mold will have trapezoidal features on the surface. This will allow the carbon slurry to take a desired shape, e.g., the plurality of pyramidal columns as described above. After solidifying the carbon, the metal mold may be removed by chemical etching. The resulting high impedance material may be in a shape as shown in Fig. 1B.

[0031] In a preferable embodiment, mechanically shaping comprises mechanically removing material. Mechanically removing material may comprise cutting.

[0032] For example, each groove may be obtained by cutting the high-impedance material 120 with a V-shaped cutting blade 220 as shown in Fig. 2A. In Fig. 2A, a first groove 221 is already cut, the V-shaped cutting blade 220 is depicted in a position suitable for cutting a second groove. A third to be cut groove is indicated by dashed lines 223 of where the V-shaped blade would be positioned for cutting said the third groove.

[0033] According to an embodiment of the invention, the V-shaped cutting blade may have a pre-determined tip-angle and a pre-determined tip-shape. An exemplary differing cutting blade 230 is shown in Fig. 2B. Fig. 2B displays the same elements as Fig. 2A (groove 231 of Fig. 2B corresponds to groove 221 in Fig. 2A, future location of V-shaped cutting blade 233 of Fig. 2B corresponds with future location of V-shaped cutting blade 223 in Fig. 2A), with the exception that a V-shaped cutting blade with a different tip-angle and tip-shape is used. It should be understood that further variations in the tip-shape and tip-angle may be employed without deviating from the inventive concept.

[0034] Tip-angles envisaged by the inventors range from 20° to 70°, for example 30°, 40°, 45°, 50 °, 60°. The tip angles are determined by the acoustic impedance of the high-impedance material and the desired lowest acoustic impedance in the composite layer.

[0035] The cutting trajectory followed by the V-shaped cutting blade and any other cutting blade disclosed herein may be as shown by arrows 650 and 750 in Figs. 6A, 6B, 7A and 7B. Note that the direction of cutting may be reversed from the shown arrow and only arrows for some grooves (trajectories followed by a cutting blade) are shown and not for all grooves (trajectories followed by cutting blades).

[0036] In Fig. 2A and 2B the spacing between cuts is shown in regular intervals. It should be understood, that while regular intervals are preferred, in particular as the resulting trapezoid-shaped bar or plurality of parallel pyramidal shaped columns should be aligned with individual transducer elements of a to be added piezoelectric layer, different spacings are possible. In particular, some piezoelectric layers may comprise irregularly spaced transducer elements for optimizing the overall transducer stack performance. In such cases, irregularly spaced trapezoid-shaped bars or plurality of parallel pyramidal shaped columns are to be obtained.

[0037] The number of trapezoid shaped bars may be dependent on the number of transducer elements. As such, a composite matching layer according to the invention may comprise a single trapezoid shaped bar or multiple parallel trapezoid shaped bars, for example 2, 4, 8, 16, 25, 40, 120, 600 or any number therebetween or above parallel trapezoid shaped bars.

[0038] The number of parallel pyramidal shaped columns may depend on the number of transducer elements. As such, a composite matching layer according to the invention may comprise for example 2, 4, 8, 16, 25, 40, 120, 400, 800, 1200, 1600, 2400, 6000, 20000 or any number therebetween or above of parallel pyramidal shaped columns.

[0039] Alternatively to a V-shaped cutting blade, angled cuts may be performed by a single cutting blade as discussed in more detail with relation to Fig. 3. Fig. 3 shows a first groove 321 cut into a slab of high-impedance material 120, a single cutting blade 320, and to be performed cuts 323. In particular, each groove may be obtained by performing a first cut at a first predetermined angle 322; and performing a second cut at a second predetermined angle 323; and the first cut and second cut intersect to form each groove. The first and second angle may further be related such that the first predetermined angle equals the negative of the second predetermined angle. The angle may be determined with respect to any reference frame in which the slab of high-impedance material may be placed. Preferably, the first predetermined angle is defined by 326 and the second predetermined angle is defined by 328 with respect to an imaginary axis 327 as shown in Fig. 3.

[0040] According to the invention, the top and bottom surfaces (named as such in accordance with the drawings) of the high-impedance material may be mechanically processed. Mechanically processing may comprise remove excess material from the top and bottom surfaces. Mechanically processing may comprise polishing or otherwise treating the top and bottom surfaces for achieving a desired finish. For example, some matching layers may comprise conductive layers for aiding in efficient electrical signal transmission or for dissipating heat away.

[0041] The composite matching layer of Fig. 1C, repeated for clarity in Fig. 4A, for example shows excess material 450 and 470, delimited from the edges of the slab of high-impedance material 120 and the low impedance material 130 by a dash-dot-dot line. These excess materials may be cut-off from the composite matching layer resulting in a composite matching layer as shown in Fig. 4B. While Fig. 4A and 4B display both the top and bottom of the composite matching layer to comprise excess material, the inventors also envisage manufacturing methods wherein only one, either the top or the bottom, comprise excess material. Additionally, manufacturing methods wherein this step of cutting excess material is obsolete are also envisaged as discussed in more detail below.

[0042] It should be noted that the tip of the trapezoid shaped bars or plurality of parallel pyramidal columns as shown in Fig. 4B is truncated. In other words, the tip of the trapezoidal-shaped bar and / or the pyramid may be cut off, resulting in a flat surface parallel and opposite to the base surface. This modified structure may be called a truncated pyramid. The tip according to this specification may thus refer to both a single apex tip and a truncated tip. For sake of completeness, a single apex tip comprises for each trapezoid-shaped bar and / or the plurality of parallel pyramidal shaped columns, a single point located at a distance on a geometrical line passing through the centroid of the polygonal base and extending normal to the polygonal base.

[0043] The truncated tip has the benefit that electrical connection can easier be made with an additional layer attached to the composite layer on the surface comprising the truncated tip.

[0044] The base surface of the trapezoid-shaped bar and / or the plurality of parallel pyramidal shaped columns is preferably a polygon. For example, a quadrilateral such as a square, a rectangle or a rhomboid. However, other shapes for the base including but not limited to triangular, pentagonal, hexagonal, etc., are also envisaged.

[0045] The manufacturing of the composite matching layer may thus comprise mechanically processing a first surface of the composite matching layer, wherein the first surface comprises: the tip of the trapezoid-shaped bar or the tips of the plurality of parallel pyramidal shaped.

[0046] The manufacturing of the composite matching layer may thus comprise mechanically processing a second surface of the composite matching layer, wherein the second surface is opposite to the first surface in such a way that the second surface comprises: the base of the trapezoid-shaped bar or the bases of the plurality of parallel pyramidal shaped columns.

[0047] With reference to Figs. 5A, 5B, 5C and 5D, a slightly different manufacturing process for a composite matching layer is described. In particular, this slightly different manufacturing method comprises: i) bringing a slab of high impedance 120 material to the desired dimensions for the composite matching layer, in particular the right height; ii) bonding the slab of high-impedance material with a second slab 140 of a different material. The different material may for example comprise a piezoelectric material. In particular, the second slab of a different material may comprise a piezoelectric layer which contains individual transducer elements. iii) mechanically shaping a slab of high-impedance material to form grooves in the slab of high-impedance material in such a way that the remaining high-impedance material forms at least: a trapezoid-shaped bar or a plurality of parallel pyramidal shaped columns; and iv) filling the grooves with a low-impedance material 130 to yield the composite matching layer.

[0048] According to this manufacturing method it may further be the case that the mechanical removal of material in step iii) extends into the second slab as shown in Fig. 5C.

[0049] Depending on the nature and material of the second slab, it may be possible to already classify the result as shown in Fig. 5D as an acoustic stack comprising a matching layer.

[0050] Through Figs. 2A, 2B, 3, 5B and 5C differing cutting tools for mechanically removing material from the slab of high-impedance material are shown. Based on the variety of tools provided, a skilled person will be able to think of alternatives which are fully envisaged. As stated above, it may for example be possible to stamp out the grooves or shear the material to obtain the grooves. Additionally, the grooves may be obtained by grinding, for example using a rotatory grinder which slowly removes material from the slab of high-impedance material until the grooves are formed.

[0051] The inventors have realized that using mechanical shaping provides a more repeatable and precise fabrication result, particularly when compared to current state of the art methods that fabricate such pyramidal columns using etching as for example described in Li, Z., Yang, D. Q., Liu, S. L., Yu, S. Y., Lu, M. H., Zhu, J., ... & Chen, Y. F. (2017), Broadband gradient impedance matching using an acoustic metamaterial for ultrasonic transducers, Scientific reports, 7(1), 42863. In particular, using mechanical removal approaches, variances between composite matching layers manufactured at different times are smaller than with chemical approaches. Additionally, the trapezoid shaped bar or the plurality of parallel pyramidal shaped columns are to be aligned with individual transducer elements comprised in a piezoelectric layer that may be bonded to the composite matching layer. The pitch mismatch allowable may range between ±0.2 µm and ±20 µm, wherein ±20 µm may be considered a maximum, and ±0.2 µm represents the known smallest allowable mismatches. Nonetheless, the smaller the mismatch the better, and future transducers may require even smaller maximum allowable mismatches. For example, in a transducer with 250 µm pitch the allowed misalignment should be less than 20 µm (0.020 mm). For example, in a transducer with a 145 µm pitch the allowed misalignment should be less than 10 µm (0.010 mm). It may thus be said that the allowable mismatch is roughly between 5 and 20 % for current existing transducers.

[0052] Contrary to mechanical removal of material, chemical removal of material, via processes such as etching, results in composite matching layers which present greater variance between different manufacturing runs. For example, small impurities in the material may inadvertently affect the resultant structures (i.e., the trapezoid shaped bars or plurality of parallel pyramidal columns) by adding imperfections. As a result, the slopes of these structures may present valleys and hills affecting the overall final performance of the ultrasound transducer.

[0053] Figs. 6A and 6B provide a 3D view of a composite matching layer comprising a slab of high-impedance material 120 and a filler low-impedance material 130 comprising trapezoid shaped bars 601 and 602.

[0054] Figs. 6A and 6B provide a 3D view of a composite matching layer comprising a slab of high-impedance material 120 and a filler low-impedance material 130 comprising a plurality of parallel pyramidal shaped columns 601 and 602.

[0055] For clarity not every possible trapezoid shaped bar and pyramidal shaped column is indicated by a reference numeral in Figs. 6A, 6B, 7A and 7B.

[0056] The final impedance of the composite matching layer may be controlled by any one or a combination of: a volume fraction of the high-impedance material and low-impedance material; and a geometry of: the trapezoid-shaped bar or the plurality of parallel pyramidal shaped columns.

[0057] The geometry of: the trapezoid-shaped bar or each of the plurality of parallel pyramidal shaped columns may comprise a polygonal base, preferably a quadrilateral base, with a single apex, wherein the apex is located at a distance on a geometrical line passing through the centroid of the polygonal base and extending normal to the polygonal base, wherein optionally the trapezoid-shaped bar or each of the plurality of pyramidal shaped columns are truncated near the apex yielding a plateau parallel to and opposite to the basis.

[0058] According to an example, the high-impedance material is electrically conductive. An electrically conductive material may be used to make good electrical connections to the piezoelectric material.

[0059] According to an example, the high-impedance material is electrically non-conductive. An electrically non-conductive material may be used when transducer elements in an ultrasound probe make electrical connections directly to the electrode on a piezo layer.

[0060] A high-impedance material within a matching layer, as described herein, may be configured to improve the transmission of ultrasound waves by reducing the acoustic impedance mismatch between the transducer and the tissue. Such a material may comprise ceramics or metals that have high acoustic impedance values. A non-exhaustive list of ceramics may comprise: Lead zirconate titanate (PZT), Barium titanate and Alumina (Aluminum oxide). A non-exhaustive list of metals may comprise: Tungsten, Molybdenum and Stainless steel. It should be understood that also combinations of composites and metals and also other materials not mentioned here are possible. Adhesive bonding agents such as epoxy resins may be utilized to securely attach the piezoelectric layer to the high impedance material.

[0061] A low impedance material within a matching layer, as described herein, may be configured to facilitate the efficient transmission of ultrasound waves by reducing the acoustic impedance mismatch between the transducer and softer tissues. Such a material may comprise polymers, foams or other composites with low acoustic impedance properties. A non-exhaustive list of materials may comprise Polyethylene (PE), Polyurethane foams, and certain epoxy-based composites or resins. It should be understood that combinations of these low impedance materials are also possible to achieve the desired acoustic properties.

[0062] According to the invention, an acoustic stack may be manufactured by bonding a composite matching layer as described herein with a second slab comprising a piezoelectric layer (e.g., bonding layers 120 / 130 with a piezoelectric layer 140 as shown in Fig. 8). A method of manufacturing an acoustic stack for ultrasound transducers thus comprises bonding a slab of piezoelectric material to a composite matching layer manufactured as described above along a surface of the composite matching layer that comprises the base of the trapezoid-shaped bar or the bases of the plurality of parallel pyramidal shaped columns.

[0063] The piezoelectric layer 140 will further comprise individual transducer elements. It is of utmost importance that each trapezoid-shaped bar or each of the plurality of parallel pyramidal shaped columns is aligned within allowable margins with a single transducer element from the piezoelectric layer.

[0064] A piezoelectric layer within the matching layer, as described herein, may be configured to generate ultrasound waves when an electric current is applied and to convert reflected sound waves back into electrical signals. For example, such a piezoelectric layer may comprise piezoelectric ceramics (e.g., PZTs), Relaxor-based piezo material (e.g. PZN-PT, PMN-PT, and PIN-PMN-PT), or piezo composites. It should be understood that also combinations of piezoelectric materials are possible.

[0065] In some examples, the method to manufacture an acoustic stack may further comprise: bonding a de-matching layer to a surface of the piezoelectric layer opposite to the surface of the piezoelectric layer that is bonded to the composite matching layer. E.g., bonding de-matching layer 150 to piezoelectric layer 140.

[0066] A de-matching layer, as described herein, may be configured to enhance the acoustic output. Such a layer may comprise materials with much higher acoustic impedance than the piezoelectric layer. For example, a de-matching layer may comprise tungsten material with about 100 MRayls (100 × 10 6< Pa·s / m = 100 × 10 6< kg / (m 2< s), Rayls are expressed in MKS units). A de-matching layer is further typically of an electrically conductive material.

[0067] In some examples, the method to manufacture an acoustic stack may further comprise: bonding an adhesive layer to a surface of the de-matching layer opposite to the surface of the de-matching layer that is bonded to the de-matching layer. The adhesive layer may further comprise connectors or connections for establishing electric connection between a to-be-connected electric circuit and the piezoelectric layer. E.g., bonding adhesive layer 160 with connections 165 to the de-matching layer 150 to piezoelectric layer 140.

[0068] In some examples, the method to manufacture an acoustic stack may further comprise: mechanically removing material from the acoustic stack to create dice kerfs in such a way that each pillar comprises: a single trapezoid-shaped bar or a single pyramidal shaped column. Creating dice kerfs is typically one of the last steps in manufacturing acoustic stacks. For example, the bonding of a de-matching layer and / or an adhesive layer may be performed before creating dice kerfs. However, bonding the de-matching layer and / or the adhesive layer may also be performed after performing the dice kerfs. The dice kerfs may be generated by any mechanical removal process as described herein, including but not limited to for example cutting, e.g., dicing, sawing or laser cutting. The dice kerf cuts may not extend fully through the backing layer or may not extend into the adhesive layer at all. In some examples, the transducer stack (described below) may be completed before performing the dice kerfs. E.g., holes 800 in Fig. 8 represent such dicing cuts.

[0069] In some examples, the method to manufacture an acoustic stack may further comprise: bonding a ground bond layer to a surface of the composite matching layer opposite to the surface of the composite matching layer that is bonded to the piezoelectric layer. The addition of a ground bond layer is typically performed after the dice kerfs are created. E.g., bonding ground bond layer 190 to the composite matching layer.

[0070] A ground bond may be configured to provide robust electrical grounding and effective insulation, thereby minimizing electrical noise and interference, and enhancing signal quality. This layer may also be configured to maintain the structural integrity of the transducer assembly by providing stable support. Such a ground layer may comprise materials such as a metalized Mylar (polyethylene terephthalate, PET). Adhesives or thermocompression bonding may be employed to bond the conductive layer, to both the composite matching layer.

[0071] In some examples, a conductive layer (not shown) may be disposed between the ground bond layer and the high impedance material. E.g., between the ground bond layer 190 and the composite matching layer 120 / 130.

[0072] A conductive layer within the matching layer, as described herein, may be configured to provide high electrical conductivity, ensuring efficient signal transmission and maintaining reliable electrical contact. Additionally, the conductive layer may be configured to act as an intermediate acoustic matching layer, effectively reducing reflections and improving the transmission of acoustic waves between the high impedance material and the ground layer. Such a conductive layer may comprise materials such as gold (Au), silver (Ag), Copper (Cu), or other highly conductive metals known for their excellent conductive properties and resistance to oxidation. It should be understood that the conductive layer may also comprise combinations of these and other not mentioned materials. Additionally, the conductive layer may be configured to contribute to the thermal management of the ultrasound probe. The A conductive layer may be disposed on the high impedance material 120 or the ground layer 180 by for example sputtering, evaporation, electroplating or adhesion.

[0073] According to the invention, an ultrasound transducer stack may be manufactured by coupling an acoustic stack manufactured as described herein with an electric circuit, wherein the electric circuit is configured to drive individual transducer elements, wherein the piezoelectric layer comprises said individual transducer elements. E.g., by bonding electric circuit 170 to adhesive 160.

[0074] The electric circuit may comprise a flexible printed circuit board (flex PCB or FLEX) or an application specific integrated circuit (ASIC) bonded to a surface of the adhesive layer opposite to the surface of the adhesive layer bonded to the de-matching layer.

[0075] In some examples, the method to manufacture an ultrasound transducer stack may further comprise: bonding a backing layer to a surface of the electric circuit opposite to the surface of the electric circuit that is bonded to the adhesive layer. E.g., bonding backing layer 180 to electric circuit layer 170.

[0076] A backing layer as described herein is typically configured with a high attenuation material to suppress any unwanted scattered reverberations within the acoustic stacks.

[0077] Fig. 8 shows an exemplary transducer stack comprising ground bond layer 190, a composite matching layer 120 / 130 comprising high-impedance material 120 and low impedance material 130, a piezoelectric layer 140, a de-matching layer 150, an adhesive layer 160 comprising connections 165, an electrical circuit layer 170 (e.g., comprising an ASIC or FLEX, alternatively also FPGAs may be used) and a backing layer 180. The connections 165 provide electrical connections between the electrical circuit layer and the piezoelectric layer. Furthermore, Fig. 8 shows the voids 800 that remain when creating the dice kerfs. In this example, the conductive layer and ground bond layer were added after creating the dice kerfs.

[0078] A composite matching layer 120 / 130 according to the invention comprises: a first layer comprising a high-impedance 120 material and a low-impedance material 130, wherein the high impedance material is configured to form: a trapezoidal-shaped bar 601, 602 or a plurality of parallel pyramidal shaped columns 701, 702; and wherein the low impedance material is configured to fill the remainder of the first layer.

[0079] An acoustic stack according to the invention comprises as first layer a composite matching layer as disclosed herein, and a second layer comprising a piezoelectric material 140 bonded to the composite matching layer, along a surface of the composite matching layer that comprises the the base of the trapezoid-shaped bar or the bases of the plurality of parallel pyramidal shaped columns.

[0080] An ultrasound transducer stack according to the invention comprises an acoustic stack as described herein, and an electrical circuit 170 coupled to the piezoelectric material, wherein the electrical circuit is configured to drive individual piezoelectric elements.

[0081] An ultrasound probe according to the invention comprises an ultrasound transducer stack as described herein. In an example, the ultrasound probe may further comprise a communication interface. The communication interface may be configured to provide ultrasound signals to a host.

[0082] An ultrasound system as described herein comprises an ultrasound probe as described herein. In an example, the ultrasound system may further comprise a host. The host may further comprise a communication interface. The communication interface may be configured to receive ultrasound signals from the ultrasound probe. In an example, the host may further comprise a processor for processing the received signals form the ultrasound probe. For example, to generate ultrasound images. The host may also be configured to control the ultrasound probe via the communication interface.

[0083] A skilled person is considered to be familiar with ultrasound probes and ultrasound systems such that these will not be described in further detail.

[0084] A matching layer comprising both high impedance and low impedance materials, as described herein, may be configured to optimize the transmission of ultrasound waves and reduce acoustic impedance mismatches between the transducer and the tissue. Such a layer may include high impedance materials such as ceramics or metals with high acoustic impedance values, including but not limited to Lead zirconate titanate (PZT), Barium titanate, Alumina (Aluminum oxide), Tungsten, Molybdenum, and Stainless steel. In contrast, the low impedance materials may include certain polymers or specialized composites designed to counteract impedance mismatches and attenuate unwanted reflections. Examples of these materials may include Polyvinylidene fluoride (PVDF), Polyurethane, Polystyrene, Epoxy-based composites, Polymer-matrix composites with ceramic fillers, and Carbon fiber-reinforced polymers (CFRP). It should be understood that combinations of these materials, as well as other materials not mentioned here, are possible. The high and low impedance materials together work to ensure efficient transmission of ultrasound waves and improve image clarity. These materials can be finely tuned to achieve specific acoustic properties, significantly enhancing the performance and efficiency of acoustic devices such as ultrasound transducers. The fine tuning may for example be achieved by the particular geometry of the trapezoid-shaped bar or each pyramid as described herein.

[0085] A composite matching layer as described herein has been shown to provide an improved frequency response compared to currently known matching layers. In particular, simulations comparing a three matching layer to the herein described composite matching layer have yielded the frequency response as shown in Fig. 9. In these simulations, the impedance of the Base material is 11 MRayls MRayls (11 × 10 6< Pa·s / m = 11 × 10 6< kg / (m 2< s), Rayls are expressed in MKS units)., which is the same as the first matching layer on the regular design. W 1 and W 2 are 250 µm and 300 µm, respectively. The height of the pyramid structure is 300 µm. "θ 1 " and "θ 2 " are 54 and 48 degrees, respectively. Fig. 9 shows that the frequency response is wider than the regular three matching layer design, which can be even more improved with a higher impedance base material and different geometry.

[0086] It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word "comprising" does not exclude the presence of elements or steps other than those listed in a claim. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. Measures recited in mutually different dependent claims may advantageously be used in combination.

Claims

1. A method to fabricate a composite matching layer for ultrasound transducers, the method comprising: mechanically shaping a slab of high-impedance material (120) to form grooves (122, 221, 231, 321) in the slab of high-impedance material in such a way that the remaining high-impedance material forms at least: - a trapezoid-shaped bar (601, 602) or - a plurality of parallel pyramidal shaped columns (701, 702); and filling the grooves with a low-impedance material (130) to yield the composite matching layer.

2. The method of claim 1, wherein mechanically shaping comprises cutting; and wherein each groove is obtained by cutting the high-impedance material with a V-shaped cutting blade (220, 230) with a pre-determined tip angle and a pre-determined tip-shape.

3. The method of claim 1, wherein mechanically shaping comprises cutting; and wherein each groove is obtained by - performing a first cut at a first predetermined angle (322); and - performing a second cut at a second predetermined angle (323); and wherein the first cut and second cut intersect to form each groove,4. The method of claim 3, wherein the first angle and second angle are related such that the first predetermined angle equals the negative of the second predetermined angle.

5. The method of any one of the preceding claims, further comprising: mechanically processing a first surface (450) of the composite matching layer, wherein the first surface comprises: - the tip of the trapezoid-shaped bar or - the tips of the plurality of parallel pyramidal shaped columns.

6. The method of claim 5, further comprising: mechanically processing a second surface (470) of the composite matching layer, wherein the second surface is opposite to the first surface in such a way that the second surface comprises: - the base of the trapezoid-shaped bar or - the bases of the plurality of parallel pyramidal shaped columns.

7. The method of any of the preceding claims, wherein an impedance of the composite matching layer is controlled by any one or a combination of: a volume fraction of the high-impedance material and low-impedance material; and a geometry of: - the trapezoid-shaped bar or - the plurality of parallel pyramidal shaped columns.

8. A method of manufacturing acoustic stacks for ultrasound transducers, the method comprising: manufacturing a composite matching layer according to any one of the preceding claims, and bonding a slab of piezoelectric material (140) to the composite matching layer along a surface of the composite matching layer that comprises the - the base of the trapezoid-shaped bar or - the bases of the plurality of parallel pyramidal shaped columns.

9. The method of claim 8, further comprising mechanically removing material from the acoustic stack to create dice kerfs in such a way that each pillar comprises: - a single trapezoid-shaped bar or - a single pyramidal shaped column; and optionally comprising: bonding a de-matching layer (150) to a surface of the piezoelectric layer opposite to the surface of the piezoelectric layer that is bonded to the composite matching layer before creating dice kerfs.

10. A method of manufacturing ultrasound transducer stacks, the method comprising manufacturing an acoustic stack according to claim 9, and coupling an electric circuit (170) to the acoustic stack, wherein the electric circuit is configured to drive individual transducer elements.

11. A composite matching layer, the composite matching layer comprising: a first layer comprising a high-impedance material (120) and a low-impedance material (130), wherein the high impedance material is configured to form: a trapezoidal-shaped bar (601, 602) or a plurality of parallel pyramidal shaped columns (701, 702); and wherein the low impedance material is configured to fill the remainder of the first layer.

12. An acoustic stack comprising as first layer, a composite matching layer according to claim 11, and a second layer comprising a piezoelectric material (140) bonded to the composite matching layer, along a surface of the composite matching layer that comprises the - the base of the trapezoid-shaped bar or - the bases of the plurality of parallel pyramidal shaped columns.

13. An ultrasound transducer stack comprising an acoustic stack according to claim 12, and an electrical circuit (170) coupled to the piezoelectric material, wherein the electrical circuit is configured to drive individual piezoelectric elements.

14. An ultrasound probe comprising an ultrasound transducer stack according to claim 13.

15. An ultrasound system comprising an ultrasound probe according to claim 14.

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