Patent Publication Number: US-2018028159-A1

Title: Rearward acoustic diffusion for ultrasound-on-a-chip transducer array

Description:
BACKGROUND 
     The present disclosure relates generally to ultrasound technology. In particular, the present disclosure relates to an apparatus and method for rearward acoustic diffusion for an ultrasound-on-chip transducer array. 
     Ultrasound devices may be used to perform diagnostic imaging and/or treatment, using sound waves with frequencies that are higher with respect to those audible to humans. Ultrasound imaging may be used to see internal soft tissue body structures, for example to find a source of disease or to exclude any pathology. When pulses of ultrasound are transmitted into tissue (e.g., by using a probe), sound waves are reflected off the tissue with different tissues reflecting varying degrees of sound. These reflected sound waves may then be recorded and displayed as an ultrasound image to the operator. The strength (amplitude) of the sound signal and the time it takes for the wave to travel through the body provide information used to produce the ultrasound image. Many different types of images can be formed using ultrasound devices, including real-time images. For example, images can be generated that show two-dimensional cross-sections of tissue, blood flow, motion of tissue over time, the location of blood, the presence of specific molecules, the stiffness of tissue, or the anatomy of a three-dimensional region. 
     SUMMARY 
     In one embodiment, a heat sink device has a non-planar mounting surface and an ultrasound-on-chip device attached to the non-planar mounting surface, the ultrasound-on-chip device including an ultrasonic transducer substrate bonded to an integrated circuit substrate. The non-planar mounting surface of the heat sink device is configured to diffuse acoustic waves that are incident thereupon. 
     In another embodiment, an ultrasound probe includes a housing and an ultrasonic transducer assembly disposed within the housing, the ultrasonic transducer assembly further including a metal heat sink device having a non-planar mounting surface, and an ultrasonic transducer substrate attached to the non-planar mounting surface of the heat sink device. The non-planar mounting surface of the heat sink device may be configured to diffuse acoustic waves that are incident thereupon. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various aspects and embodiments of the disclosed technology will be described with reference to the following Figures. It should be appreciated that the figures are not necessarily drawn to scale. Items appearing in multiple figures are indicated by the same reference number in all the figures in which they appear, and where: 
         FIG. 1  is a perspective view of a handheld ultrasound probe suitable for use with exemplary embodiments; 
         FIG. 2  is an exploded perspective view of the ultrasound probe of  FIG. 1 ; 
         FIG. 3  is a partial cross-sectional view of an exemplary ultrasound-on-chip device suitable for use with exemplary embodiments; 
         FIG. 4  is perspective view of the ultrasonic transducer assembly shown in  FIG. 2 ; 
         FIG. 5  illustrates the ultrasonic transducer assembly shown in  FIG. 2 , with the acoustic lens removed; and 
         FIG. 6  illustrates the ultrasonic transducer assembly shown in  FIG. 5 , with the ultrasound-on-chip device removed to reveal a heat sink having a planar mounting surface; 
         FIG. 7  illustrates a heat sink design in accordance with an exemplary embodiment, including a non-planar mounting surface; 
         FIG. 8  is an enlarged view of the heat sink design shown in  FIG. 7 ; 
         FIG. 9  is a top view illustrating the pattern of the non-planar mounting surface; 
         FIG. 10  is a schematic cross-sectional view illustrating transmission of acoustic energy by an ultrasonic transducer assembly, according to embodiments, in forward and rearward directions; 
         FIG. 11  is an enlarged view of an ultrasound-on-chip/heat sink interface in  FIG. 10 ; 
         FIG. 12  illustrates an alternative embodiment of the pattern of the non-planar mounting surface; 
         FIG. 13  illustrates another alternative embodiment of the pattern of the non-planar mounting surface; 
         FIG. 14  is a perspective view of another type of ultrasound probe suitable for use with exemplary embodiments; 
         FIG. 15  illustrates the ultrasound probe of  FIG. 14  affixed to a patient; 
         FIG. 16  is a top view illustrating an alternative fastening mechanism for the ultrasound probe of  FIG. 14 ; 
         FIG. 17  illustrates the ultrasound probe of  FIG. 14  affixed to the patient; and 
         FIG. 18  is an exploded perspective view of the ultrasound probe of  FIG. 16 . 
     
    
    
     DETAILED DESCRIPTION 
     Medical ultrasound imaging transducers are used to transmit acoustic pulses that are coupled into a patient through one or more acoustic matching layers. After sending each pulse, the transducers then detect incoming body echoes. The echoes are produced by acoustic impedance mismatches of different tissues (or tissue types) within the patient which enable both partial transmission and partial reflection of the acoustic energy. Exemplary types of ultrasonic transducers include those formed from piezoelectric materials or, more recently, capacitive micromachined ultrasonic transducers (CMUTs) that are formed using a semiconductor substrate. 
     In the case of a CMUT device, a flexible membrane is suspended above a conductive electrode by a small gap. When a voltage is applied between the membrane and the electrode, Coulombic forces attract the flexible membrane to the electrode. As the applied voltage varies over time, so does the membrane position, thereby generating acoustic energy that radiates from the face of the transducer as the membrane moves. However, in addition to transmitting acoustic energy in a forward direction toward the body being imaged, the transducers simultaneously transmit acoustic energy in a backward direction away from the patient being imaged. That is, some portion of the acoustic energy is also propagated back through the CMUT support structure(s), such as a silicon wafer for example. 
     When an incident ultrasound pulse encounters a large, smooth interface of two body tissues with different acoustic impedances, the sound energy is reflected back to the transducer. This type of reflection is called specular reflection, and the echo intensity generated is proportional to the acoustic impedance gradient between the two mediums. The same holds true for structures located in a direction away from the patient being imaged, such as a semiconductor chip/metal heat sink interface. If an incident ultrasound beam reaches an acoustic interface at substantially a normal angle (90°), almost all of the generated echo will travel back toward the originating transducer. 
     Typically, for both piezoelectric and capacitive transducer devices, an acoustic backing material is positioned on a back side of an ultrasonic transducer array in order to absorb and/or scatter as much of the backward transmitted acoustic energy as possible and prevent such energy from being reflected by any support structure(s) back toward the transducers and reducing the quality of the acoustic image signals obtained from the patient by creating interference. In general, however, materials that have good acoustic attenuating and scattering properties may also have poor thermal conductivity and/or coefficient of thermal expansion (CTE) mismatches with respect to the transducer substrate material. Accordingly, exemplary embodiments disclosed herein introduce a heat sink device on which an ultrasonic transducer may be attached that provides both acoustic attenuation/scattering capability, as well as thermal dissipation capability. In one embodiment, a metal heat sink device (e.g., copper) may have a non-planar mounting surface and an ultrasound-on-chip substrate attached to the non-planar mounting surface of the heat sink device. 
     As compared to a planar surface, the non-planar mounting surface of the heat sink device can be configured to reduce the amount of acoustic energy reflected off the face of the heat sink device and directed back through the body of the semiconductor substrate toward the transducers. Where the angle of incidence with a specular boundary is less than 90°, the echo will not return to the originating transducer; rather, it is reflected at an angle equal to the angle of incidence (similar to visible light reflecting in a mirror). Moreover, in contrast to an acoustic backing that physically separates the transducer substrate from the heat sink surface, a portion of the exemplary heat sink surface may have direct physical contact with the chip surface. Although heat sinking performance may be optimized using a planar surface with maximum surface area contact between the heat sink and the substrate, this comes at a cost of acoustic performance. Therefore, with such a tradeoff, both the benefits of rearward acoustic diffusion and heat dissipation may be achieved. 
     Embodiments of the present disclosure are described below with reference to the accompanying drawings, in which some, but not all, embodiments of the present disclosure are shown. The present disclosure can be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure clearly satisfies applicable legal requirements. Like numbers refer to like elements throughout. 
     Referring initially to  FIG. 1  and  FIG. 2 , an exemplary ultrasound probe  100  is depicted in a perspective view and an exploded perspective view, respectively. It should be understood, however, that the ultrasound probe  100  depicted in  FIG. 1  and  FIG. 2  represents one exemplary application for the acoustic attenuation features described herein, and other form factors, applications and devices are also contemplated. As shown in  FIG. 1 , the exemplary ultrasound probe  100  is a handheld probe that includes a probe housing  102  having an acoustic lens  104  and shroud  106  disposed at a first end thereof, and a cable assembly  108  disposed at a second end thereof. The shroud  106  prevents direct contact between an ultrasonic transducer assembly  110  ( FIG. 2 ) and a patient (not shown) when the ultrasound probe  100  is used to image the patient. 
     In addition to imaging, the acoustic lens  104  may also be configured to focus acoustic energy to spots having areas of the size required for high-intensity focused ultrasound (HIFU) procedures. Furthermore, the acoustic lens  104  may acoustically couple the ultrasonic transducer assembly  110  to the patient (not shown) to minimize acoustic reflections and attenuation. In some embodiments, the acoustic lens  104  may be fabricated with materials providing impedance matching between ultrasonic transducer assembly  110  and the patient. In still other embodiments, the acoustic lens  104  may provide electric insulation and may include shielding to prevent electromagnetic interference (EMI). Additionally, the shroud  106  and acoustic lens  104  may provide a protective interface to absorb or reject stress between the ultrasonic transducer assembly  110  and the acoustic lens  104 . 
     As also shown in  FIG. 2 , the ultrasonic transducer assembly  110  includes an ultrasound-on-chip device  112  having an ultrasonic transducer array that is covered by the acoustic lens  104  when the ultrasound probe  100  is assembled. An interior region of the ultrasound probe  100 , encapsulated by upper probe housing section  102   a  and lower probe housing section  102   b,  may also include components such as a first circuit board  114 , a second circuit board  116  and a battery  118 . The circuit boards  114  and  116  may include circuitry configured to operate the ultrasonic transducer arrangement  110  in a transmit mode to transmit ultrasound signals, or receive mode, to convert received ultrasound signals into electrical signals. Additionally, such circuitry may provide power to the ultrasonic transducer assembly  110 , generate drive signals for the ultrasonic transducer assembly  110 , process electrical signals produced by the ultrasonic transducer assembly  110 , or perform any combination of such functions. The cable assembly  108  may carry any suitable analog and/or digital signals to and from circuit boards  114  and  116 . 
     An exemplary configuration for the ultrasound-on-chip device  112  is illustrated in the partial cross-sectional view of  FIG. 3 . In the embodiment depicted, the ultrasound-on-chip device  112  includes an ultrasonic transducer substrate  302  bonded to an integrated circuit substrate  304 , such as a complementary metal oxide semiconductor (CMOS) substrate. The ultrasonic transducer substrate  302  may have a plurality of cavities  306  formed therein, and is an example of a CMUT device as described above. The cavities  306  are formed between a first silicon device layer  308  and a second silicon device layer  310 . A silicon oxide layer  312  (e.g., a thermal silicon oxide such as a silicon oxide formed by thermal oxidation of silicon) may be formed between the first and second silicon device layers  308  and  310 , with the cavities  306  being formed therein. In this non-limiting example, the first silicon device layer  308  may be configured as a bottom electrode and the second silicon device layer  310  may be configured as a membrane. Thus, the combination of the first silicon device layer  308 , second silicon device layer  310 , and cavities  306  may form an ultrasonic transducer (e.g., a CMUT), of which six are illustrated in this non-limiting cross-sectional view. To facilitate operation as a bottom electrode or membrane, one or both of the first silicon device layer  308  and second silicon device layer  310  may be doped to act as conductors, and in some cases are highly doped (e.g., having a doping concentration greater than 10 15  dopants/cm 3  or greater). 
     In terms of the aforementioned forward direction toward a subject being imaged, this would be in the upward direction with respect to the view in  FIG. 3 , whereas the backward direction away from the subject being imaged would be in the downward direction with respect to the view in  FIG. 3 . Additional information regarding the fabrication and integration of CMUTs with CMOS wafers may be found, for example, in U.S. Pat. No. 9,067,779, assigned to the assignee of the present application, the contents of which are incorporated by reference herein in their entirety. Again however, it should be appreciated that the ultrasonic transducer substrate  302 /CMOS substrate  304  embodiment represents just one possible configuration for the ultrasound-on-chip device  112 . Other configurations are also possible including, but not limited to, a side-by-side arrangement where transducers and CMOS circuitry are formed on a same substrate, as well as arrays formed from piezoelectric micromachined ultrasonic transducers (PMUTs), or other suitable types of ultrasonic transducers. In still other embodiments, the ultrasound-on-chip device  112  may include an ultrasonic transducer array by itself (i.e., an ultrasonic transducer chip), where CMOS circuitry is located on a different substrate or circuit board altogether. 
     Referring now to  FIG. 4 , a perspective view of the ultrasonic transducer assembly  110  is illustrated in further detail. In the embodiment shown, the ultrasonic transducer assembly  110  includes an interposer circuit board  402  and a heat sink  404  formed from a thermally conductive material such as copper, for example. In the particular view of  FIG. 4 , only side tabs  406  of the heat sink  404  are most clearly depicted, as both the top (mounting) surface of the heat sink  404  and the ultrasound-on-chip device  112  are covered by the acoustic lens  104  for illustration purposes. Subsequent views depict these obscured components in further detail, however. For example,  FIG. 5  depicts the ultrasonic transducer assembly  110  of  FIG. 4 , with the acoustic lens  104  removed to reveal the ultrasound-on-chip device  112 . The interposer circuit board  402  serves as an electrical interface between the ultrasound-on-chip device  112  and the first and second circuit boards  114 ,  116  shown in  FIG. 2 . Connections between a mounting surface of the ultrasound-on-chip device  112  and a first side of the interposer circuit board  402  may be made, for example, using individual wire bonds (not shown) that may in turn be encapsulated by an encapsulant material (not shown). In addition, the interposer circuit board  402  may include one or more connectors  408  configured to mate with corresponding connectors of the first and second circuit boards  114 ,  116 . 
       FIG. 6  illustrates the ultrasonic transducer assembly  110  with the ultrasound-on-chip device  112  removed to reveal a planar mounting surface  410  of a heat sink  404 . Although the heat sink  404  shown in  FIG. 6  provides desired thermal dissipation heat generated by the ultrasound-on-chip device  112 , the properties of the heat sink metal affect how much acoustic energy is reflected and how much is absorbed. In this case, the planar (flat) mounting surface  410  may act as an acoustic reflector that redirects unabsorbed acoustic waves back toward the transducers of the ultrasound-on-chip device  112 . This is undesirable, since such reflected acoustic waves can contribute to false image data. 
     Accordingly,  FIG. 7  illustrates a heat sink design in accordance with an exemplary embodiment, in which a substantial portion of the mounting surface  410  of the heat sink  404  is a non-planar surface  412 . Where geometric features of the mounting surface  410  of the heat sink  404  are made non-planar (as opposed to planar), acoustic sound waves are incident at a non-normal angle with respect to the heat sink surface. Those waves that are not absorbed by the heat sink  406  may be reflected in a direction other than toward the originating transducer and may, in some instances, cancel with one another or at least be scattered in a direction where they may have relatively longer travel times. This in turn may allow for more absorption by the heat sink  406 , reducing interference with acoustic waves being detected from the imaged patient. The non-planar surface  412  may, in one embodiment, encompass an area of the mounting surface  410  corresponding to locations of the ultrasonic transducers of the ultrasound-on-chip device  112  when attached to the heat sink  404 . 
       FIG. 8  is an enlarged view of a portion of the heat sink  404  in  FIG. 7 . The non-planar surface  412  may be defined by forming patterned features in the mounting surface  410 . Exemplary techniques for forming the patterned features of the non-planar surface  412  techniques include, but are not limited to, stamping, molding, etching or other microfabrication techniques. The non-planar pattern may include regular features, such as illustrated in  FIG. 9 , or irregular features as described in further detail hereinafter. The exemplary pattern for the non-planar surface  412  in  FIG. 9  includes a plurality of pyramid structures, each having triangular surfaces  902  converging to a single point  904 . Other patterns are also contemplated, however. 
       FIG. 10  schematically illustrates the propagation of acoustic waves from an ultrasound-on-chip device  112  attached to the heat sink  404 . It should be noted that the device  112  shown in  FIG. 10  is a simplified schematic for illustrative purposes, and does differentiate between the transducer portion and the CMOS integrated circuit portion of the device  112 , other than depicting the CMUT cavities  306 . As will be noted, acoustic waves are transmitted from the transducers in a forward direction into the tissue  1002  of a patient via the acoustic lens  104 , as well as in a backward direction through the substrate material of ultrasound-on-chip device  112  toward the interface with the heat sink  404 . 
     An adhesive material  1004  may be used to securely attach the ultrasound-on-chip device  112  to the interface with the heat sink  404 . The adhesive material  1004  may be any suitable material known in the art, such as an epoxy material, and optionally a tungsten filled epoxy material or epoxy mixture (with tungsten and/or additional elements) selected for acoustic dampening capabilities. In the enlarged view of  FIG. 11 , some of the incident acoustic energy incident upon the surfaces  902  may be transmitted through the interface and into the heat sink  404 , and some of the incident acoustic energy incident upon the surfaces  902  may be reflected (scattered) in directions away from the transducers. In some cases, reflected acoustic waves may cancel with other reflected acoustic waves. 
     As indicated above, other patterns are also possible for a non-planar surface  412  of the heat sink  404 . For example,  FIG. 12  illustrates an alternative embodiment of the pattern of the non-planar mounting surface  412 . Similar to the embodiment of  FIG. 9 , a pattern of prism structures  1202  (also referred to as wedge structures) may be formed (e.g., by stamping) on the mounting surface of the heat sink. The structures  1202  may, for example, be formed in groups of repeating arrangements, where structures in adjacent groups have longitudinal apexes disposed orthogonal to one another. Still another embodiment for a non-planar heat sink surface  412  is illustrated in  FIG. 13 . In this embodiment, the non-planar surface  412  is a sintered surface  1302 , which may be formed using a metallic powder to create an irregular surface. Optionally, a tungsten filled epoxy material may also be applied in bonding an ultrasound-on-chip device  112  to the non-planar surfaces  412  of either  FIG. 12  or  FIG. 13 . 
     In addition to handheld probe embodiments such as depicted in  FIG. 1  and  FIG. 2 , it is further contemplated that other ultrasound probe form factors may incorporate the above described acoustically diffusing heat sink embodiments. For example,  FIG. 14  is a perspective view illustrating an ultrasound probe  1400  that is embodied in a patch configuration, and having an upper housing  1402   a  and a lower housing  1402   b.  The probe  1400  is shown coupled to a patient  1500  in  FIG. 15 , and may be configured to transmit, wirelessly for example, collected ultrasound data to one or more external devices (not shown) for further processing. In the example depicted, the probe  1400  may also be provided with dressing  1502  that provides an adhesive surface for both the probe housing as well as to the skin of the patient. One non-limiting example of such a dressing  1502  is Tegaderm™, a transparent medical dressing available from 3M Corporation. Although not specifically shown in  FIG. 15 , the lower housing  1402   b  may include an opening that aligns with a corresponding opening in the dressing  1502  so that transducer elements of the ultrasound probe  1400  may be acoustically coupled to the patient  1500 . 
     Referring to  FIG. 16 , an alternative fastening mechanism for the ultrasound probe  1400  is illustrated. In the embodiment shown, the ultrasound probe  1400  further includes a buckle  1600  affixed to the upper housing  1402   a  via a post  1602  using, for example, a threaded engagement between the buckle  1600  and the post  1602 . Other attachment configurations are also contemplated, however. As further shown in  FIG. 16 , the buckle  1600  includes a pair of slots  1604  that in turn accommodate a strap  1700  ( FIG. 17 ). In this example, the strap  1700  is wrapped around the patient  1500  and appropriately tightened in order to secure the ultrasound probe  1400  to a desired location on the patient  1500  for acquisition of desired ultrasound data. 
       FIG. 18  illustrates an exploded perspective view of the ultrasound probe  1400  of  FIG. 16 . For ease of illustration and comparison, similar components with respect to the embodiment of  FIG. 1  and  FIG. 2  are designated with similar reference numerals. For example, in addition to the upper housing  1402   a,  lower housing  1402   b  and buckle, the ultrasound probe  1400  further includes an acoustic lens  104  to cover the ultrasound-on-chip device  112 , which in turn is attached to a heat sink device  404 . Although not specifically shown in  FIG. 18 , the mounting surface of the heat sink device  404  may be provided with any of the acoustically diffusing features discussed above, such as for example in any of the embodiments of  FIGS. 7, 12 and 13 . 
     In contrast to the handheld probe embodiment of  FIGS. 1-8  in which the ultrasonic transducer assembly  110  includes an interposer circuit board  402 , the ultrasound-on-chip device  112  and heat sink device  404  are attached directly to a first circuit board  1802 . In addition, the ultrasound probe  1400  further includes, by way of example, a second circuit board  1804  (e.g., for power supply components), an insulator board  1806 , battery  1808  and antenna  1810  (e.g., to enable wireless communication to and from the ultrasound probe  1400 ). In any case, it will be appreciated that the above described thermal and acoustic benefits provided by the heat sink device  404  are applicable to ultrasound probes of various form factors. 
     The techniques described herein are exemplary, and should not be construed as implying any particular limitation on the present disclosure. It should be understood that various alternatives, combinations and modifications could be devised by those skilled in the art from the present disclosure. For example, steps associated with the processes described herein can be performed in any order, unless otherwise specified or dictated by the steps themselves. The present disclosure is intended to embrace all such alternatives, modifications and variances that fall within the scope of the appended claims.