Patent Publication Number: US-2020289093-A1

Title: Ultrasound transducer assembly having low viscosity kerf fill material

Description:
BACKGROUND 
     Technical Field 
     The present disclosure pertains to ultrasound transducer assemblies and methods, and more particularly to ultrasound transducer assemblies and methods having a kerf fill material in kerfs that extend between adjacent transducer elements. 
     Description of the Related Art 
     Diagnostic ultrasound transducer assemblies typically include a plurality of diced transducer elements arranged along an azimuth axis. The transducer assemblies may be included in a device, such as an ultrasound probe, and are used to transmit and receive ultrasonic energy to produce a meaningful image of a targeted biological structure. The diced transducer elements typically include a piezoelectric material, one or more acoustic matching layers, an acoustic lens, and a backing structure. The spaces or gaps between adjacent transducer elements are generally referred to as kerfs. 
     It is often desirable to provide some mechanical or acoustic isolation between adjacent elements, for example, to reduce crosstalk and improve directivity of the transducer elements in the ultrasound transducer assembly. One method to obtain inter-element isolation (e.g., isolation between adjacent transducer elements) is to leave the kerfs empty, which may be referred to as air kerfs. However, such air kerf architectures generally offer no damping or constraint for lateral vibrations between the adjacent transducer elements, and the impulse response of such ultrasound transducer assemblies will therefore be compromised. 
     An alternative to air kerfs is to fill the kerfs with some type of kerf fill material. For example, a kerf fill material may be utilized that is designed to constrain or attenuate lateral modes; however, such kerf fill materials may contribute to excessive crosstalk due to a stiffness of the kerf fill material. 
     BRIEF SUMMARY 
     The present disclosure, in part, addresses a desire for improved ultrasound transducer assemblies, in which kerfs between adjacent transducer elements may be more completely filled, and in which an ultrasound lens may be more securely attached, than in conventional designs. 
     In at least one embodiment, an ultrasound transducer assembly is provided that includes a plurality of transducer elements, a plurality of kerfs respectively disposed between adjacent transducer elements of the plurality of transducer elements, and a kerf fill material in the plurality of kerfs. The kerf fill material includes a first material having a first viscosity and a solvent that reduces the first viscosity of the kerf fill material to a second viscosity that is less than the first viscosity. 
     In another embodiment, the present disclosure provides a method that includes forming a plurality of kerfs in an ultrasound transducer assembly by dicing through a matching layer and a transducer layer. The kerfs are filled with a kerf fill material, and the kerf fill material includes a mixture of a volatile methylsiloxane (VMS) fluid and at least one of a room temperature vulcanizing (RTV) silicone, an acetoxy, or neutral cure silicone. The method further includes covering a surface of the matching layer with the kerf fill material, and adhesively attaching, by the kerf fill material, an ultrasound lens to the ultrasound transducer assembly. 
     In another embodiment, an ultrasound probe is provided that includes a housing and an ultrasound transducer assembly that is at least partially enclosed within the housing. The ultrasound transducer assembly includes a plurality of transducer elements on the acoustic backing, at least one matching layer on the plurality of transducer elements, a plurality of kerfs extending in a first direction through the at least one matching layer and at least partially into the acoustic backing, and a kerf fill material in the plurality of kerfs. The kerfs in the plurality of kerfs extend in a second direction between adjacent transducer elements of the plurality of transducer elements, and the second direction is transverse to the first direction. The kerf fill material includes a mixture of a volatile methylsiloxane (VMS) fluid and at least one of a room temperature vulcanizing (RTV) silicone, an acetoxy, or neutral cure silicone. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of an ultrasound transducer assembly, which may be a conventional ultrasound transducer assembly. 
         FIG. 2  is a perspective view illustrating an ultrasound probe including an ultrasound transducer assembly, in accordance with one or more embodiments of the present disclosure. 
         FIG. 3  is a cross-sectional view of a transducer assembly taken along the line  3 - 3  shown in  FIG. 2 , in accordance with one or more embodiments of the present disclosure. 
         FIGS. 4 through 7  are cross-sectional views illustrating a method of manufacturing an ultrasound transducer assembly, in accordance with one or more embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     An ultrasound transducer assembly may include an acoustic backing, a plurality of piezoelectric transducer elements on the acoustic backing, and one or more matching layers on the transducer elements. A plurality of kerfs extends through the matching layers and separate adjacent transducer elements from one another. The kerfs are filled with a kerf fill material that includes a first material, such as RTV silicone, and a solvent such as a volatile methylsiloxane (VMS) fluid that reduces the viscosity of the kerf fill material. 
     By reducing the viscosity of the kerf fill material, more complete filling of the kerfs may be achieved. Additionally, the reduced-viscosity kerf fill material facilitates inclusion of one or more additives into the kerf fill material while maintaining a viscosity suitable to completely fill the kerfs. Such additives may be included to alter properties of the kerf fill material, which may be selected depending on a particular application, kerf geometry, or desired operating frequency of the ultrasound transducer. The additives may include powders, microparticles, microspheres or the like, which may alter properties of the kerf fill material such as density, viscosity, thermal conductivity, coefficient of thermal expansion (CTE), acoustic attenuation, or stiffness. 
     In various embodiments, the kerf fill materials provided herein may be provided on an outer surface of a matching layer of an ultrasound transducer assembly, in addition to being provided in the kerfs. In such embodiments, the kerf fill material may be utilized to adhesively attach an ultrasound lens to the outer matching layer. 
       FIG. 1  is a cross-sectional view of an ultrasound transducer assembly  10 , which may be a conventional ultrasound transducer assembly. The x-axis represents the azimuth plane, the y-axis represents the elevational plane, and the z-axis represents depth. 
     The ultrasound transducer assembly  10  includes an acoustic backing  14 , a plurality of transducer elements  13 , a first matching layer  12 , and a second matching layer  11 . A plurality of kerfs  15  physically separates the individual transducer elements  13 , as well as regions of the first and second matching layers  12 ,  11  on the transducer elements  13 . 
     A kerf fill material  16  is provided within the kerfs  15 . However, as shown in  FIG. 1 , the kerf fill material  16  may not completely fill the kerfs  15 . Instead, air voids  17   a,    17   b,    17   c  are present in at least some of the kerfs  15 . The presence of the air voids  17   a,    17   b,    17   c  indicates incomplete filling of the kerfs  15 . This may be caused by various factors, including, for example, the kerfs  15  having a width that is too narrow to be suitably filled by the kerf fill material  16  and/or the kerf fill material  16  having a viscosity that is too high to suitably fill the kerfs  15 . 
     For example, a two-part tin, or platinum curing RTV silicone may be utilized as the kerf fill material  16 . However, typical RTV silicone materials (including, for example, RTV664 and RTV630) generally have a viscosity that is greater than about 100,000 centipoise (cps) and may be greater than about 150,000 cps. This relatively high viscosity can impede the ability of the kerf fill material  16  to consistently and repeatably fill the kerfs  15 , particularly for transducer assemblies having relatively narrow kerf widths and/or relatively long depths. This incomplete kerf filling may contribute to an impulse response with greater variability and reduced performance. 
     Accordingly, for ultrasound transducer assemblies in which the kerf fill material  16  results in partially filled kerfs  15  (e.g., including air voids  17   a,    17   b,    17   c ), excessive variability and unpredictable performance may result. The partially filled kerfs  15  can be a result of very narrow kerfs, viscous kerf fill material (e.g., having a viscosity greater than about 100,000 cps), inability to properly degas the kerf fill material  16 , or any combination of the aforementioned conditions. 
       FIG. 2  is a perspective view illustrating an ultrasound probe  100  including an ultrasound transducer assembly  110 , in accordance with one or more embodiments of the present disclosure. 
     The probe  100  includes a housing  112 , which forms an external portion of the probe  100 . The housing  112  surrounds internal electronic components and/or circuitry of the probe  100 , including, for example, electronics such as driving circuitry, processing circuitry, oscillators, beamforming circuitry, filtering circuitry, and the like. The housing  112  may be formed to surround or at least partially surround externally located portions of the probe  100 , such as a sensor face  120 , and may be a sealed housing, such that moisture, liquid or other fluids are prevented from entering the housing  112 . The housing  112  may be formed of any suitable materials, and in some embodiments, the housing  112  is formed of a plastic material. The housing  112  may be formed of a single piece (e.g., a single material that is molded surrounding the internal components) or may be formed of two or more pieces (e.g., upper and lower halves) which are bonded or otherwise attached to one another. 
     The ultrasound transducer assembly  110  is at least partially enclosed within the housing  112 . The transducer assembly  110  includes a plurality of transducer elements which are electrically coupled to internal circuitry housed within the probe  100 , such as the driving circuitry, processing circuitry, oscillators, beamforming circuitry, filtering circuitry, and the like. 
     The transducer assembly  110  is configured to transmit an ultrasound signal toward a target structure in a region of interest in the patient, and to receive echo signals returning from the target structure in response to transmission of the ultrasound signal. To that end, the transducer elements of the transducer assembly  110  are capable of transmitting an ultrasound signal and receiving subsequent echo signals. In various embodiments, the transducer elements may be arranged as elements of a phased array. Suitable phased array transducers are known in the art. 
     The transducer assembly  110  may include a one-dimensional (1D) array or a two-dimensional (2D) array of transducer elements. The transducer array may include piezoelectric ceramics, such as lead zirconate titanate (PZT), single crystal or may be based on microelectromechanical systems (MEMS). For example, in various embodiments, the transducer assembly  110  may include piezoelectric micromachined ultrasonic transducers (PMUT), which are microelectromechanical systems (MEMS)-based piezoelectric ultrasonic transducers, or the transducer assembly  110  may include capacitive micromachined ultrasound transducers (CMUT) in which the energy transduction is provided due to a change in capacitance. 
     The ultrasound probe  100  may further include an ultrasound lens  114 , which may be included as part of the ultrasound transducer assembly  110 , and which may form a part of the sensor face  120  of the probe  100 . The lens  114  may be any acoustic lens operable to focus a transmitted ultrasound beam from the transducer elements of the ultrasound transducer assembly  110  toward a patient and/or to focus a reflected ultrasound beam from the patient to the transducer elements. The ultrasound lens  114  may have a curved surface shape in some embodiments. The ultrasound lens  114  may have different shapes, depending on a desired application, e.g., a desired operating frequency, or the like. The ultrasound lens  114  may be formed of any suitable material, and in some embodiments, the ultrasound focusing lens  114  is formed of a room-temperature-vulcanizing (RTV) silicone material. 
       FIG. 3  is a cross-sectional view of the transducer assembly  110  taken along the line  3 - 3  shown in  FIG. 2 . The transducer assembly  110  is similar in some respects to the transducer assembly  10  shown and described with respect to  FIG. 1 ; however, kerfs  125  of the transducer assembly  110  are substantially filled by kerf fill material  128 , without formation of voids in the kerfs  125  as will be explained in further detail herein. 
     The transducer assembly  110  includes a plurality of transducer elements  123 , which may be, for example, piezoelectric transducer elements. The transducer elements  123  may be formed of any piezoelectric materials, including, for example, lead zirconate titanate (PZT), polyvinylidene fluoride (PVDF), a combination of lead-magnesium-niobate (PMN) and lead titanate (PT) such as a single crystal PMN-PT, or the like. 
     The transducer elements  123  are formed on an acoustic backing  124 . The acoustic backing  124  may be an attenuation element, which reduces or attenuates undesired acoustic reflections and dissipates thermal energy, such as may be generated by vibrations of the transducer elements  123  during operation of the ultrasound probe  100 . In some embodiments, the acoustic backing  124  is formed of a composite material, such as a composite including metallic particles and, microspheres in a viscoelastic material, a metal/epoxy composite, a tungsten/vinyl composite, or any other suitable materials. 
     As shown in  FIG. 3 , a first matching layer  122  and a second matching layer  121  may be formed on the transducer elements  123 , with each of the transducer elements  123  being covered by respective portions of the first and second matching layers  122 ,  121 . The first and second matching layers  122 ,  121  generally function to increase the transmission of acoustic energy from the high impedance piezoelectric transducer elements  123  to the much lower acoustic impedance of the target to be imaged, such as an organ or other biological structure in a human body. By selecting appropriate matching layer materials and thicknesses, the acoustic impedance can be graded to minimize reflection such that ultrasonic waves emitted by the transducer elements  123  efficiently enter the target for ultrasound imaging. 
     The first and second matching layers  122 ,  121  may be formed of any suitable materials having desired acoustical properties, including, for example, any combination of epoxy or resin materials, fillers, and additives. 
     In some embodiments, the first matching layer  122  may have an acoustic impedance that is greater than an acoustic impedance of the second matching layer  121 . While the ultrasound transducer assembly  110  is shown in  FIG. 3  as including two matching layers, embodiments of the present disclosure are not limited thereto. In various embodiments, more or fewer than two matching layers may be included in the transducer assembly  110 . 
     The acoustic backing  124 , transducer elements  123 , and first and second matching layers  122 ,  121  may be bonded to one another by any suitable technique and/or materials. 
     A plurality of kerfs  125  extend in the depth direction (e.g., along the z-axis) through the first and second matching layers  122 ,  121 , and laterally separate the transducer elements  123  from one another. The kerfs  125  may extend at least partially into the acoustic backing  124 , as shown. In some embodiments, each of the kerfs  125  may extend to a substantially same depth in the transducer assembly  110 . In other embodiments, some of the kerfs  125  may extend to different depths in the transducer assembly  110 . 
     The kerfs  125  may have any suitable width (e.g., along the x-axis), which may depend upon various factors, such as a particular application, operational frequency range, or the like of the transducer assembly  110 . In some embodiments, each of the kerfs  125  may have a substantially same width. In other embodiments, at least one of the kerfs  125  may have a width that is different from at least one other of the kerfs  125 . In some embodiments, the width of the kerfs  125  may be less than about 0.1 mm. In some embodiments, the width of the kerfs  125  may be less than about 50 μm. In some embodiments, the kerfs  125  have a width that is within a range of 20 μm to 40 μm, inclusive. 
     The kerfs  125  are filled with a kerf fill material  128 . In various embodiments, the kerf fill material  128  has attenuation properties to provide suitable isolation between transducer elements  123  while also having a Young&#39;s modulus high enough to adequately constrain lateral modes, but not so high as to inhibit transducer element  123  displacement when generating or receiving ultrasonic waves. 
     In some embodiments, the kerf fill material  128  includes a first material and a solvent which reduces the viscosity of the first material. For example, in some embodiments, the kerf fill material  128  includes a room temperature vulcanizing (RTV) silicone and a solvent which facilitates a viscosity reduction of the RTV silicone. The silicone may be any silicone material, including, for example, a single part (acetoxy or neutral cure) silicone, a two-part (condensation or addition cure) RTV silicone, or an amalgam of a single and two-part silicone system. 
     In some embodiments, the solvent in the kerf fill material  128  includes one or more siloxanes. In some embodiments, the solvent in the kerf fill material  128  includes volatile methylsiloxane (VMS) fluids, which facilitate a viscosity reduction of the kerf fill material  128 , such as RTV silicone. In some embodiments, the kerf fill material  128  has a viscosity less than about 1000 cps (centipoise). In some embodiments, the kerf fill material  128  is an ultra-low viscosity material having a viscosity within a range of about 25 cps to 250 cps, inclusive. 
     By utilizing a solvent, such as a VMS fluid, in the kerf fill material  128 , the kerf fill material  128  may have a viscosity that is significantly reduced as compared to that of a conventional single part or two-part RTV silicone. The solvency of VMS fluids in silicone compounds (e.g., RTV silicone) allows the VMS fluids to serve as a diluent to reduce the silicone compound&#39;s viscosity. VMS fluids are available in a range of different vapor pressures, and thus, in various embodiments, the working life and final material porosity of the composite kerf fill material  128  may be tailored as may be desired, for example, depending on a particular application or desired operational characteristics of the transducer assembly  110 , dimensions of the transducer assembly  110 , dimensions of the kerfs  125 , or the like. 
     As a result of the significant viscosity reduction of the first material (e.g., RTV silicone) in the kerf fill material  128 , the first material can be relatively heavily filled with other materials to further alter the properties of the kerf fill material  128 . For example, the reduced-viscosity RTV silicone can contain a relatively high concentration of additional materials while retaining a desired low viscosity to completely fill the kerfs  125 . In contrast, loading a typical RTV silicone material with a similar concentration of additional materials may increase the viscosity of the RTV silicone material to a point at which it is unsuitable to completely fill the kerfs  125 , and instead may result in formation of voids in the kerfs  125 . 
     As shown in  FIG. 3 , the kerf fill material  128  may include an additive  132 , which may be, for example, any material which is added to the first material (e.g., the RTV silicone), and which alters one or more properties or characteristics of the kerf fill material  128 . In some embodiments, the additive  132  may alter any of density, viscosity, coefficient of thermal expansion (CTE), acoustic attenuation, thermal conductivity, or stiffness of the kerf fill material  128 . The additive  132  includes a material that is different from the first material of the kerf fill material  128 . 
     In various embodiments, the additive  132  may be or include any metallic or metallic oxide powder, polymeric powders, or microparticles such as microspheres. In some embodiments, the additive  132  includes microspheres, which may be any generally spherical microparticles, and which may have a size (e.g., a diameter) between about 1 μm and 1 mm. In some embodiments, the additive  132  includes glass or polymeric microspheres which may decrease density, increase viscosity, and/or reduce CTE of the kerf fill material  128 . The additive  132  may include glass microspheres, which may reduce the CTE of the kerf fill material  128 . In some embodiments, the additive  132  may include hollow microspheres, which may reduce the density of the kerf fill material  128 . Microspheres or finely ground microparticles such as cured silicone can also be included in the kerf fill material  128 , in some embodiments, to increase attenuation and reduce inter-element crosstalk (e.g., crosstalk between the transducer elements  123 ). 
     In some embodiments, the additive  132  includes a powder, such as a powder including one or more of aluminum nitride (AlN), magnesium oxide (MgO), boron nitride (BN), diamond, or copper, which can be added to the kerf fill material  128  to increase thermal conductivity. 
     In some embodiments, the additive  132  is included in at least portions of the kerf fill material  128  that is disposed in the kerfs  125 . In some embodiments, the additive  132  is uniformly distributed throughout the kerf fill material  128 , and the kerf fill material  128  may be a homogenous mixture including the first material (e.g., RTV silicone) and the additive  132 . In other embodiments, the additive  132  is non-uniformly distributed in the kerf fill material  128 . For example, in some embodiments, the additive  132  may be dispersed in the first material with a concentration gradient, for example, with a concentration that increases along the depth (e.g., z-axis) of the kerfs  125 . In some embodiments, the additive  132  may have a concentration that is highest in regions of the kerfs  125  directly between adjacent transducer elements  123 . This may provide the particular altered characteristics of the kerf fill material  128  in a focused region between the transducer elements  123 , while other portions of the kerf fill material  128  may have a lower concentration of the additive  132  or may be substantially free of the additive  132 . 
     In addition to filling the kerfs  125 , the kerf fill material  128  may cover a surface (e.g., the upper surface) of the second matching layer  121 , as shown in  FIG. 3 . The ultrasound lens  114  may be attached to the second matching layer  121  by adhesion provided from the layer of the kerf fill material  128  between the second matching layer  121  and the ultrasound lens  114 . In some embodiments, the kerf fill material  128  has a thickness over the upper surface of the second matching layer  121  that is within a range of 0.01 mm to 5 mm. In some embodiments, the thickness of the kerf fill material  128  over the upper surface of the second matching layer  121  is within a range of 0.1 mm to 0.5 mm, which provides enhanced adhesion between the second matching layer  121  and the ultrasound lens  114 . 
     In some embodiments, the ultrasound lens  114  is formed of RTV silicone material, which may be the same or different from the silicone material which may be included as the first material in the kerf fill material  128 . In some embodiments, the ultrasound lens  114  is formed of a two-part addition cure RTV silicone. The addition of the solvent (e.g., a VMS fluid) in the composite kerf fill material  128  enhances the adhesion of the kerf fill material  128 , thereby improving adhesion of the RTV silicone ultrasound lens  114  to the transducer assembly  110 . 
     Once the transducer assembly  110  has been assembled, e.g., with the ultrasound lens  114  being formed over the outer matching layer (e.g., the second matching layer  121 , as shown), the kerf fill material  128  may be cured. In some embodiments, the solvent in the kerf fill material  128  (e.g., the VMS fluid) is liberated during the curing process, which results in a silicone structure with an increased silicon (Si) chain length and lower final Young&#39;s modulus and shore A hardness (or lower durometer rating). The lower hardness (e.g., as indicated by a lower durometer rating) of the cured kerf fill material  128  may reduce friction and crosstalk between transducer elements  123  of the transducer assembly  110 . 
     The ultrasound lens  114  may form an outer layer of the transducer assembly  110 , and may form an exposed portion of the ultrasound probe  100 . For example, the ultrasound lens  114  may be positioned along the sensor face  120  of the ultrasound probe  100 , as shown in  FIG. 2 . 
       FIGS. 4 through 7  are cross-sectional views illustrating a method of manufacturing an ultrasound transducer assembly, such as the ultrasound transducer assembly  110  shown in  FIG. 3 , in accordance with one or more embodiments. 
     As shown in  FIG. 4 , a method of manufacturing an ultrasound transducer assembly may include forming an ultrasound transducer block  210 . The ultrasound transducer block  210  includes an acoustic backing  224 , an ultrasound transducer layer  223  on the acoustic backing  224 , a first matching layer  222  on the ultrasound transducer layer  223 , and a second matching layer  221  on the first matching layer  222 . 
     The acoustic backing  224 , ultrasound transducer layer  223 , first matching layer  222 , and second matching layer  221  may be laminated or bonded to one another by any suitable material and/or technique. For example, in some embodiments, the layers of the ultrasound transducer block  210  may be bonded to one another by one or more adhesives, such as an epoxy. 
     While the ultrasound transducer block  210  is shown in  FIG. 4  as including two matching layers, embodiments of the present disclosure are not limited thereto. In various embodiments, more or fewer than two matching layers may be included in the ultrasound transducer block  210 . 
     As shown in  FIG. 5 , the method may further include forming a plurality of kerfs  125  which extend through the second matching layer  221 , the first matching layer  222 , and the ultrasound transducer layer  223 . In some embodiments, the plurality of kerfs  125  extend at least partially into the acoustic backing  224 . The kerfs  125  may be formed, for example, by dicing the ultrasound transducer block  210 , thereby forming the acoustic backing  124 , the transducer elements  123 , and separate regions of the first and second matching layers  122 ,  121  as shown in  FIG. 5 . The kerfs  125  thus separate the individual transducer elements  123  from one another, and further separate regions or portions of the first and second matching layers  122 ,  121 . 
     The kerfs  125  may be formed to have any suitable width, e.g., extending between adjacent ones of the transducer elements  123 . In some embodiments, the kerfs  125  may be formed to have a width that is within a range of 20 μm to 40 μm, inclusive. 
     As shown in  FIG. 6 , the method may further include filling the plurality of kerfs  125  with a kerf fill material  128 . The kerf fill material  128  may include a mixture of a first material and a solvent. In some embodiments, the first material of the kerf fill material  128  includes a room temperature vulcanizing (RTV) or single part (acetoxy or neutral cure) silicone, and the solvent includes a volatile methylsiloxane (VMS) fluid. In some embodiments, the kerf fill material  128  may further include one or more additives  132 , which may include at least one of a metallic powder, a metallic oxide powder, microparticles, or microspheres. 
     The kerf fill material  128  may further cover a surface (e.g., an upper surface) of the second matching layer  121 , as shown. The kerf fill material  128  may have a thickness on the surface of the second matching layer  121  that is within a range of 0.1 mm to 0.5 mm, inclusive. 
     As shown in  FIG. 7 , the method may further include attaching an ultrasound lens  114  to the second matching layer  121 . The ultrasound lens  114  may be adhesively attached to the second matching layer  121 , for example, by the kerf fill material  128  that extends between the upper surface of the second matching layer  121  and the ultrasound lens  114 . 
     The ultrasound lens  114  may be formed to have any shape, and in some embodiments, the ultrasound lens  114  is formed to have a curved shape, for example, along an outer surface of the ultrasound lens  114 . The ultrasound lens  114  may be formed of any suitable material, and in some embodiments, the ultrasound focusing lens  114  is formed of a room-temperature-vulcanizing (RTV) silicone material. 
     In various embodiments provided herein, ultrasound transducer assemblies and methods are provided which facilitate improved adhesion of the ultrasound lens  114  to an outer matching layer, such as the second matching layer  121 . The improved adhesion is provided, for example, by the kerf fill material  128  which may include a mixture of a silicone and a volatile methylsiloxane (VMS) fluid. The VMS fluid reduces the viscosity of the silicone, which may facilitate consistent and convenient spreading of the kerf fill material  128  over a surface of the second matching layer  121 . Moreover, the composite kerf fill material  128  may have improved adhesion properties as compared to a conventional RTV silicone. 
     Additionally, embodiments of the present disclosure facilitate tailoring of the kerf fill material  128  to have various properties or characteristics depending on a desired application or design of the ultrasound transducer assembly  110 . For example, by including an additive  132  in the kerf fill material  128 , characteristics such as density, viscosity, coefficient of thermal expansion (CTE), acoustic attenuation, thermal conductivity, or stiffness of the kerf fill material  128  may be altered as may be desired for various applications, kerf geometries, operating frequencies or the like. 
     Moreover, embodiments of the present disclosure facilitate improved filling of the kerfs  125  in the ultrasound transducer assembly  110 . For example, due to the reduced viscosity of the kerf fill material  128 , the kerfs  125  may be completely filled, thereby reducing formation of voids in the kerfs which may otherwise occur when filled with a conventional RTV silicone. Additionally, the kerf fill material  128  provided by the present disclosure more consistently and repeatably fills the kerfs  125 , thereby reducing variations in the kerf filling processes which otherwise may occur when filled with a conventional RTV silicone which generally yields incomplete and inconsistent filling of the kerfs. 
     The various embodiments described herein can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.