Abstract:
A system for attaching an acoustic element to an integrated circuit includes various ways in which to connect piezoelectric ceramic or micro-machined ultrasonic transducer (MUT) elements to an integrated circuit (IC), thus reducing the number of conductors required to connect the acoustic element to the IC by combining the signals in the IC. In another aspect of the invention, the transducer elements include an electrically conductive acoustic layer comprising a backing layer and/or a de-matching layer that is connected to an IC.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates generally to ultrasonic transducers, and, more particularly, to a system for attaching the acoustic element of an ultrasonic transducer to an integrated circuit.  
         BACKGROUND OF THE INVENTION  
         [0002]    Ultrasonic transducers have been available for quite some time and are particularly useful for non-invasive medical diagnostic imaging. Ultrasonic transducers are typically formed of either piezoelectric elements or micro-machined ultrasonic transducer (MUT) elements. The piezoelectric elements typically are made of a piezoelectric ceramic such as lead zirconate titanate (commonly referred to as PZT), with a plurality of elements being arranged to form a transducer assembly. A MUT is formed using known semiconductor manufacturing techniques resulting in a capacitive ultrasonic transducer cell that comprises, in essence, a flexible membrane supported around its edges over a silicon substrate. By applying contact material, in the form of electrodes, to the membrane, or a portion of the membrane, and to the base of the cavity in the silicon substrate, and then by applying appropriate voltage signals to the electrodes, the MUT may be energized such that an appropriate ultrasonic wave is produced. Similarly, when electrically biased, the membrane of the MUT may be used to receive ultrasonic signals by capturing reflected ultrasonic energy and transforming that energy into movement of the electrically biased membrane, which then generates a receive signal  
           [0003]    The transducer elements can be combined with control circuitry forming a transducer assembly, which is then further assembled into a housing possibly including additional control electronics, in the form of electronic circuit boards, the combination of which forms an ultrasonic probe. This ultrasonic probe, which may include various acoustic matching layers, backing layers, and dematching layers, may then be used to send and receive ultrasonic signals through body tissue.  
           [0004]    In the past, joining an acoustic sensor, such as a piezoelectric ceramic transducer element or a MUT element, to the electrical control circuitry required the use of many individual wires to connect each element of the transducer array to the control circuitry. In the case of large transducer arrays having many hundreds or thousands of elements, large wiring harnesses were required. Unfortunately, a large wiring harness increases the bulk and cost of the ultrasonic probe. For ultrasonic probes designed to be used inside the human body, it is desirable to reduce the overall size of the ultrasonic probe and cable. One manner of reducing the size of the probe is to provide the transducer element control electronics on an integrated circuit (IC) assembly. An IC in proximity to the transducer array may be used to transmit and receive from many small transducer elements and may also be used to combine the signals, thereby reducing or eliminating the bulky and expensive cables that typically connect the ultrasonic probe elements to the control electronics.  
           [0005]    Placing the transducer array over the IC results in greater packaging efficiency. Unfortunately, there is no convenient way to connect the piezoelectric ceramic or MUT transducer elements to the control electronics.  
           [0006]    Therefore, it would be desirable to have a way in which to connect both the piezoelectric ceramic and MUT elements of an ultrasonic transducer array directly to an IC.  
         SUMMARY  
         [0007]    The invention is a system for attaching an acoustic element to an integrated circuit (IC). The system provides various ways to connect piezoelectric ceramic or MUT transducer elements to an IC, thus reducing the number of conductors required to connect each element of the transducer array to the IC by combining the signals in the IC. In another aspect of the invention, the transducer elements include an electrically conductive acoustic layer having a backing layer and/or a dematching layer that is connected to an IC.  
           [0008]    Other systems, methods, features, and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present invention, and be protected by the accompanying claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The present invention, as defined in the claims, can be better understood with reference to the following drawings. The components within the drawings are not necessarily to scale relative to each other, emphasis instead being placed upon clearly illustrating the principles of the present invention.  
         [0010]    [0010]FIG. 1 is a cross-sectional schematic view of a transesophageal (TEE) ultrasonic probe.  
         [0011]    [0011]FIG. 2 is a cross-sectional schematic view illustrating a portion of the ultrasonic transducer of FIG. 1.  
         [0012]    [0012]FIG. 3 is a cross-sectional schematic view of an alternative embodiment of the ultrasonic transducer of FIG. 2.  
         [0013]    [0013]FIG. 4 is a cross-sectional schematic view illustrating another alternative embodiment of the ultrasonic transducer of FIG. 2.  
         [0014]    [0014]FIG. 5A is a plan view illustrating the IC in the ultrasonic transducer of FIG. 2.  
         [0015]    [0015]FIG. 5B is a plan view illustrating a footprint of a two-dimensional (2D) acoustic sensor.  
         [0016]    [0016]FIG. 5C is a plan view illustrating the redistribution layer of FIG. 2 including the transducer array footprint of FIG. 5B.  
         [0017]    [0017]FIG. 6A is a plan view illustrating an alternative embodiment of the IC of FIG. 5A.  
         [0018]    [0018]FIG. 6B is a plan view illustrating a footprint of a one-dimensional (1D) acoustic sensor.  
         [0019]    [0019]FIG. 6C is a plan view illustrating a redistribution layer including the transducer array footprint of FIG. 6B.  
         [0020]    [0020]FIG. 7 is a cross-sectional schematic view illustrating an alternative embodiment of the ultrasonic transducer of FIG. 2.  
         [0021]    [0021]FIG. 8A is a cross-sectional schematic view illustrating one of the piezoelectric ceramic transducer elements of FIG. 2.  
         [0022]    [0022]FIG. 8B is a cross-sectional schematic view illustrating an alternative embodiment of the piezoelectric ceramic transducer element of FIG. 8A.  
         [0023]    [0023]FIG. 9 is a cross-sectional schematic view illustrating another alternative embodiment of the ultrasonic transducer of FIG. 2.  
         [0024]    [0024]FIG. 10 is a cross-sectional schematic view illustrating an alternative embodiment of the ultrasonic transducer of FIG. 4.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]    The invention to be described hereafter is applicable to piezoelectric ceramic and micro-machined ultrasonic transducer (MUT) elements connected to an integrated circuit (IC).  
         [0026]    [0026]FIG. 1 is a cross-sectional schematic view of a transesophageal (TEE) ultrasonic probe  100 . The ultrasonic probe  100  includes a probe housing  110  that contains an ultrasonic transducer. The ultrasonic transducer includes an acoustic sensor  120  commonly comprising a number of transducer array elements (to be described in further detail below). The transducer elements may be piezoelectric ceramic or micro-machined ultrasonic transducer (MUT) elements. An acoustic window  112 , through which ultrasonic energy is both transmitted from and received by the ultrasonic probe  100 , is located along a surface of the probe housing  110  and in front of the acoustic sensor  120 .  
         [0027]    The acoustic sensor  120  is joined to an integrated circuit (IC)  140  through an interface  130 . The interface  130  includes a redistribution layer  145  (shown and described in FIG. 2) applied over the active circuitry of the integrated circuit  140 . The active circuitry of the IC  140  can be formed on the surface of a suitable substrate ( 150  of FIG. 2) and is typically fabricated over a silicon (Si) substrate. However, other semiconductor substrate materials may be used to fabricate the IC  140 . The IC substrate is bonded to a circuit board  155  having acoustic impedance that matches the acoustic impedance of the IC substrate. The circuit board  155  is bonded to a backing material  160  by thin bonds to prevent acoustic reflections from the bonded surfaces.  
         [0028]    The acoustic sensor  120  typically includes many hundreds or thousands of transducer elements, and preferably includes 2500 ultrasonic elements. Each of the elements requires an electrical connection to the electronic control circuitry (not shown) associated with the ultrasonic probe  100 . The IC  140  allows many such electrical connections to be combined, thereby reducing the number of individual connections within the ultrasonic probe  100 .  
         [0029]    Typically, an acoustic backing  160  is applied behind the circuit board  155  in order to absorb any ultrasonic energy that migrates through the circuit board  155 . A heat sink  170  is applied behind the acoustic backing  160  in order to remove heat from the acoustic sensor  120  and the IC  140 . The acoustic sensor  120 , interface  130 , IC  140 , circuit board  155 , acoustic backing  160  and heat sink  170  comprise an ultrasonic transducer  200 .  
         [0030]    [0030]FIG. 2 is a cross-sectional schematic view illustrating a portion of the ultrasonic transducer  200  of FIG. 1. Although omitted from the ultrasonic transducer  200  of FIG. 2, there is commonly a circuit board ( 155  of FIG. 1), an acoustic backing  160  and heat sink  170  (as shown in FIG. 1) associated with the ultrasonic transducer  200 . Furthermore, matching and dematching layers are omitted for clarity.  
         [0031]    The ultrasonic transducer  200  includes an acoustic sensor  220  that, in this embodiment, comprises a plurality of piezoelectric ceramic transducer elements, an exemplar one of which is illustrated using reference numeral  210 . A plurality of piezoelectric ceramic transducer elements  210  are arranged in an array, which typically includes many hundreds or thousands of individual transducer elements, and in a preferred embodiment, includes 2500 elements. Each piezoelectric ceramic transducer element  210  includes an element metalization layer  212   a  applied to the upper surface as shown. The element metalization layer  212   a  provides an electrical ground connection for each element  210 . The ground connection typically includes a conductor (not shown) connecting each element  210  to a suitable electrical ground. In accordance with an aspect of the invention, each piezoelectric ceramic transducer element  210  is joined to the active circuitry  218  associated with IC  140  through a redistribution layer  145 . The redistribution layer  145  can be applied over the active circuitry  218 , the IC pads, an exemplar one of which is illustrated using reference numeral  224 , and the die passivation layer  214  located on the IC  140 . The IC substrate  150  and the active circuitry  218  comprise the IC  140 . The die passivation layer  214  is applied over the active circuitry  218  and the IC pads  224 , leaving the IC pads  224  exposed.  
         [0032]    The redistribution layer  145  includes a number of redistribution conductors, an exemplar one of which is shown using reference numeral  222 , in contact with portions of the active circuitry  218  through the IC pads  224 . The redistribution layer  145  also includes a secondary passivation layer  216  applied over the redistribution conductors  222  and the die passivation layer  214 . The redistribution conductors  222  redistribute the connections of the active circuitry  218 , through the IC pads  224 , to appropriate locations corresponding to each piezoelectric ceramic transducer element  210 . The redistribution conductor  222  is a conductive material that connects each IC pad  224  to a respective metal contact  232 , and can be formed using fine trace IC technology. Each metal contact  232  corresponds to a piezoelectric ceramic transducer element  210 .  
         [0033]    The die passivation layer  214  and the secondary passivation layer  216  can be formed of, for example, but not limited to, silicon dioxide or polymer. The redistribution layer  145  aids in reducing capacitive coupling between the active circuitry  218  and the transducer elements  210 . An additional passivation layer (not shown) can be applied between the die passivation layer  214  and the redistribution conductors  222  to further reduce capacitive coupling between the active circuitry  218  and the transducer elements  210 , and is applicable to the other embodiments discussed. Furthermore, the secondary passivation layer  216  aids in leveling the uneven surface resulting from the formation of the active circuitry  218  over the IC substrate  150 .  
         [0034]    In one embodiment, each piezoelectric ceramic transducer element  210  is joined to a respective metal contact  232  using a conductive element  228 . The conductive element  228  can be, for example, a solder ball, or bump, that forms an electrical contact between the metal contact  232  and an element metalization layer  212   b  applied to the underside of each piezoelectric ceramic transducer element  210 . In this manner, electrical contact between the piezoelectric ceramic transducer element  210  and the active circuitry  218  is achieved. Although illustrated in FIG. 2 using solder bumps as the conductive element  228 , a variety of other techniques are available for electrically connecting the metal contact  232  to the element metalization layer  212   b  of each piezoelectric ceramic transducer element  210 . For example, instead of solder bumps, gold bumps can be used. Further, conductive adhesive or conductive polymer bumps can be used to connect the metal contact  232  to the element metalization layer  212   b.  Further still, as will be described below with respect to FIG. 4, a technique known as “thin-line bonding” can be used to connect the element metalization layer  212   b  directly to the metal contact  232  resulting in a direct ohmic connection. In such an embodiment, the surface of the secondary passivation layer  216  and the metal contact  232  can optionally be lapped flat, or planarized, to level the surface of the redistribution layer  145  and of the integrated circuit  140 , prior to thin-line bonding the element metalization layer  212   b  to the metal contact  232 .  
         [0035]    Adhesive material  226  fills the gaps between each conductive element  228  and the space between each piezoelectric ceramic transducer element  210  and the secondary passivation layer  216 . The adhesive  226  is typically non-conductive and can be a variety of adhesives such as, for example but not limited to, epoxy. The adhesive  226  can also function as a dematching material, which acts as an acoustic reflector.  
         [0036]    The ultrasonic transducer  200  is typically constructed by forming the secondary passivation layer  216  and the metal contacts  232  over the active circuitry  218  of the IC  140 . The conductive elements  228  are then deposited over the metal contacts  232 . The adhesive  226  is then deposited and the material from which the transducer elements  210  are formed is bonded to the secondary passivation layer  216 , resulting in an electrical connection between the element metalization layer  212   b  and the conductive elements  228 . The transducer elements  210  are then formed by removing a portion of the material that forms the transducer elements  210  and the element metalization layers  212   a  and  212   b.  For example, the transducer elements  210  can be cut using a dicing saw stopping before the saw contacts the metalization layer  212   b.  The remaining material of the transducer elements  210  and the metalization layer  212   b  can then be removed by, for example, burning with a laser. The resulting saw kerf  215  creates the independent transducer elements  210  and removes the electrical connection between transducer elements  210 .  
         [0037]    [0037]FIG. 3 is a cross-sectional schematic view of an alternative embodiment of the ultrasonic transducer  200  of FIG. 2. The ultrasonic transducer  300  of FIG. 3 includes an acoustic sensor  320  comprising a plurality of MUT elements  310 . The MUT elements  310  are formed on a MUT substrate  330 . Each MUT element  310  includes one or more MUT cells (not shown). If more than one MUT cell is used to form the MUT element  310 , the MUT cells forming the MUT element typically are commonly connected. As known to those having ordinary skill in the art, MUT elements  310  can be fabricated on a substrate  330 , such as silicon, using semiconductor device processing technology. Each MUT element  310  includes an electrical ground connection (not shown) typically extending over the surface of each MUT element  310 . The ground connection typically includes a conductor (not shown) connecting each MUT element  310  to a suitable electrical ground.  
         [0038]    In accordance with an aspect of the invention, each MUT element  310  includes one or more small diameter holes, referred to as vias, an exemplar one of which is illustrated using reference numeral  325 . The via  325  extends through the MUT substrate  330  and makes contact with each MUT element  310 . Each via  325  is doped so as to be electrically conductive, thereby providing an electrical connection to each MUT element  310 . In accordance with this aspect of the invention, each MUT element  310  is located adjacent to a via  325 . Each via  325  extends through the MUT substrate  330  and contacts one of the conductive elements  328 . In this manner, the conductive via  325  electrically couples the MUT element  310  to the conductive element  328 . It should be noted that all the alternative conductive elements mentioned above in FIG. 2 apply to FIG. 3. Each conductive element  328  makes contact with a respective metal contact  332 . Each metal contact  332  makes electrical contact with a respective redistribution conductor  322 , which in turn makes electrical contact with a respective IC pad  324 . Each IC pad  324  contacts a portion of the active circuitry  318 . In this manner, the active circuitry  318  located on the IC  340  makes electrical contact, through the redistribution conductors  322 , to the conductive elements  328  and to each MUT element  310 . Adhesive material  326  fills the gaps between each conductive element  328  and the space between the MUT substrate  330  and the secondary passivation layer  316 . The adhesive  326  is typically non-conductive and similar to the adhesive  226  described above.  
         [0039]    [0039]FIG. 4 is a cross-sectional schematic view illustrating another alternative embodiment of the ultrasonic transducer  200  of FIG. 2. The ultrasonic transducer  400  of FIG. 4 includes acoustic sensor  420 , which in this example includes a plurality of piezoelectric ceramic transducer elements, an exemplar one of which is illustrated using reference numeral  410 . Each piezoelectric ceramic transducer element  410  includes an element metalization layer  212   a  applied to the upper surface as shown. The element metalization layer  212   a  provides an electrical ground connection for each element  410 . The ground connection typically includes a conductor (not shown) connecting each element  410  to a suitable electrical ground. Each piezoelectric ceramic transducer element  410  also includes an element metalization layer  412   b  applied to the underside as shown. The element metalization layer  412   b  allows direct electrical contact to be made between the piezoelectric ceramic transducer element  410  and the metal contact  432 . As mentioned above, such a connection is typically referred to as a “thin-line bond.” The thin-line bond uses an adhesive polymer material to mechanically connect the metal contact  432  directly to the exposed surface of the element metalization layer  412   b  of the piezoelectric ceramic transducer element  410 . The thin-line bond is achieved because of the microscopic surface roughness that exists on the exposed surface of the element metalization layer  412   b  and the metal contact  432 . This microscopic surface roughness provides a direct ohmic connection between the metal contact  432  and the element metalization layer  412   b.  The metal contact  432  is connected through the redistribution conductor  422  to the IC pad  424 . The IC pad  424  connects to the active circuitry  418  located on the IC  440 . The redistribution layer  445  includes the redistribution conductors  422  and a secondary passivation layer  416 , which is similar to the secondary passivation layer  216 , described above.  
         [0040]    Similar to the ultrasonic transducer  200  of FIG. 2, the ultrasonic transducer  400  is typically constructed by forming the secondary passivation layer  416  and the metal contacts  432  over the active circuitry  418  and IC pads  424  of the IC  440 . The exposed surface of the secondary passivation layer  416  and portions of the metal contacts  432  can then be lapped, or planarized, flat.  
         [0041]    The adhesive  426  is then deposited and the material from which the transducer elements  410  are formed is bonded to the secondary passivation layer  416 , resulting in a thin-line bond electrical connection between the element metalization layer  412   b  and the metal contacts  432 . The transducer elements  410  are then formed by removing a portion of the material that forms the transducer elements  410  and the element metalization layers  412   a  and  412   b.  As described above, the transducer elements  410  can be cut using a dicing saw stopping before the saw contacts the metalization layer  412   b.  The remaining material of the transducer elements  410  and the metalization layer  412   b  can then be removed using, for example, a laser. The resulting saw kerf  415  creates the independent transducer elements  410  and removes the electrical connection between transducer elements  410 .  
         [0042]    [0042]FIG. 5A is a plan view illustrating the IC  140  of FIG. 2. The IC  140  includes a plurality of IC pads, an exemplar one of which is illustrated using reference numeral  224 . The IC pad  224  connects to the active circuitry  218  and corresponds to the IC pad  224  of FIG. 2.  
         [0043]    [0043]FIG. 5B is a plan view illustrating a footprint of a two-dimensional (2D) acoustic sensor  500 . For simplicity, the transducer array footprint  500  illustrates 16 transducer elements, an exemplar one of which is illustrated using reference numeral  510 , arranged in a 4×4 array. However, a typical transducer array includes many hundreds or thousands of transducer elements.  
         [0044]    [0044]FIG. 5C is a plan view illustrating the redistribution layer  145  of FIG. 2 including the transducer array footprint  510  of FIG. 5B shown using a dashed line. Each redistribution conductor  222  connects one of the IC pads  224  to one of the pads  510  of the transducer array footprint  500 . The redistribution conductors  222  in the redistribution layer  145  make electrical connections between the IC pads  224  and each respective transducer array element  510 . Accordingly, both the transducer array footprint  500  and the design of the active circuitry  218  can be independently optimized and each transducer array element  510  connected to the appropriate portion of the active circuitry  218 .  
         [0045]    [0045]FIG. 6A is a plan view illustrating an alternative embodiment of the IC of FIG. 5A. The IC  640  includes a plurality of IC pads, an exemplar one of which is illustrated using reference numeral  624 . The IC pad  624  connects to the active circuitry  618 .  
         [0046]    [0046]FIG. 6B is a plan view illustrating a footprint of a one-dimensional (1D) acoustic sensor  605 . The transducer array footprint  605  includes a plurality of transducer elements, an exemplar one of which is illustrated using reference numeral  610 , arranged in a 1×8 array.  
         [0047]    [0047]FIG. 6C is a plan view illustrating an alternative embodiment of the redistribution layer  145  of FIG. 5A. The redistribution layer  645  includes the transducer array footprint  605  of FIG. 6B. Each redistribution conductor  622  connects one of the IC pads  624  to one of the pads  610  of the transducer array footprint  605 . Further, in FIG. 6C, the dotted lines illustrate the outline of the transducer array footprint  605  shown in FIG. 6B. In this manner, the redistribution conductors  622  in the redistribution layer  645  make electrical connections between the IC pads  624  and each respective transducer array element  610 . Accordingly, both the transducer array footprint  600  and the design of the active circuitry  618  can be independently optimized and each transducer array element  610  connected to the appropriate portion of the active circuitry  618 .  
         [0048]    [0048]FIG. 7 is a cross-sectional schematic view illustrating an alternative embodiment of the ultrasonic transducer  200  of FIG. 2. The ultrasonic transducer  700  includes an acoustic sensor  720 , which comprises a plurality of piezoelectric ceramic transducer elements, an exemplar one of which is illustrated using reference numeral  710 . Each piezoelectric ceramic transducer element  710  includes an element metalization layer  712   a  applied to the upper surface as shown. The element metalization layer  712   a  provides an electrical ground connection for each element  710 . The ground connection typically includes a conductor (not shown) connecting each element  710  to a suitable electrical ground. Each piezoelectric ceramic transducer element  710  also includes an element metalization layer  712   b  applied to the underside of each piezoelectric ceramic transducer element  710  as shown. In the embodiment shown in FIG. 7, each conductive element  728  is a conductive polymer bump that is lapped flat, or planarized, as illustrated, and then metalized with a metalization layer  715 . The adhesive layer  726  is also planarized and metalized. Each conductive element  728  is located over a respective metal contact  732 . The element metalization layer  712   b  on the underside of each piezoelectric ceramic transducer element  710  is thin-line bonded to the metalization layer  715  applied over each conductive element  728 .  
         [0049]    The redistribution layer  745  includes the redistribution conductors  722  and a secondary passivation layer  716 , which is similar to the secondary passivation layer  216  described above.  
         [0050]    Each metal contact  732  is connected through a respective redistribution conductor  722  to a respective IC pad  724 . The IC pad  724  connects to the active circuitry  718  located on the IC  740 . The gap between the secondary passivation layer  716  and the exposed surface of the element metalization layer  712  on the piezoelectric ceramic transducer element  710  is filled with a layer of adhesive  726 . The adhesive  726  is similar to the adhesive  226  described above. As described above, the transducer elements  710  can be cut using a dicing saw stopping before the saw contacts the metalization layer  712   b.  The remaining material of the transducer elements  710  and the metalization layer  712   b  can then be removed using, for example, a laser. The resulting saw kerf  715  creates the independent transducer elements  710  and removes the electrical connection between transducer elements  710 .  
         [0051]    [0051]FIG. 8A is a cross-sectional schematic view illustrating one of the piezoelectric ceramic transducer elements of FIG. 2. The piezoelectric ceramic transducer element  800  includes a first matching layer  802  and a second matching layer  804  located over piezoelectric ceramic element  806 . The matching layers  802  and  804  are electrically conductive and generally include a metalization layer  811  applied over the matching layer  802 . The matching layers  802  and  804  help to match the acoustic impedance of the piezoelectric ceramic element  806  (approximately 30 megarayls (MRayls)) to the acoustic impedance of the patient (approximately 1.5 MRayls). The MRayl is a unit of measure of acoustic impedance.  
         [0052]    For example, by using the matching layers  802  and  804 , in a ¼ wave arrangement, the 1.5 MRayl acoustic impedance of the patient can be closely matched to the 30 MRayl acoustic impedance of the piezoelectric ceramic element  806 . Alternatively, instead of using a pure piezoelectric ceramic element, the acoustic impedance of the piezoelectric ceramic element  806  can be altered by fabricating the element  806  using a composite piezoelectric ceramic material.  
         [0053]    In accordance with another aspect of the invention, a layer of an electrically conductive acoustic material is bonded to the surface of the piezoelectric ceramic element  806  opposite that of the matching layer  804 . In one embodiment, the electrically conductive acoustic material is a dematching layer  808  bonded to the piezoelectric ceramic element  806  as shown. The dematching layer  808  acts as an acoustic reflector and may be a high impedance dematching layer, constructed of, for example, tungsten-carbide, having a cobalt or nickel binder and having an acoustic impedance of approximately 80-100 MRayls. Alternatively, a low impedance dematching layer, constructed of, for example a polymer or polymer mixtures, such as, for example, epoxy having an acoustic impedance of approximately 3 MRayls and epoxy-metal mixtures such as epoxy-silver having an acoustic impedance of approximately 4.5 MRayls. For example, the electrically conductive dematching layer  808  might be tungsten-carbide, having acoustic impedance on the order of 80-100 MRayls. In this manner, the high impedance dematching layer  808  reflects acoustic energy back towards the piezoelectric ceramic element  806 , which has an acoustic impedance of approximately 33 MRayls. The dematching layer  808  is bonded to the IC  840 , which has acoustic impedance on the order of 19 MRayls. Both surfaces of the dematching layer reflect waves, as known to those having ordinary skill in the art. The small amount of energy that passes through the dematching layer  808  is transmitted into the IC substrate  850 , circuit board  855  and into the backing  860  and absorbed. Both the circuit board  855  and the backing  860  match the acoustic impedance of the IC substrate  850 .  
         [0054]    As known to those having ordinary skill in the art, other than the interface  830 , which is similar to the interface  130  described above, all elements in FIG. 8A are attached using acoustic bonds. The IC  840  is similar to the IC  140  described above and can be joined to the dematching layer  808  through interface  830 , which is similar to interface  130  described above.  
         [0055]    Alternatively, the electrically conductive dematching layer  808  might be a layer of epoxy-silver having acoustic impedance on the order of 4.5 MRayls. In this embodiment, the low impedance dematching layer  808  reflects acoustic energy back towards the piezoelectric ceramic element  806  that has an acoustic impedance of approximately 33 MRayls. The dematching layer  808  is bonded to the IC  840 , which has acoustic impedance on the order of 19 MRayls. Both surfaces of the dematching layer reflect waves as known to those skilled in the art. The small amount of energy that passes through the dematching layer  808  is transmitted into the IC substrate  850 , circuit board  855  and into the backing  860  and absorbed. Both the circuit board  855  and the backing  860  match the acoustic impedance of the IC substrate  850 . As known to those having ordinary skill in the art, additional dematching layers can be added in alternating acoustic impedances to decrease the acoustic energy transmitted through the IC  840 , circuit board  855  and into the backing  860 .  
         [0056]    [0056]FIG. 8B is a cross-sectional schematic view illustrating an alternative embodiment of the piezoelectric ceramic transducer element  800  of FIG. 8A. The transducer element  810  includes one matching layer  812 , over which is applied metalization layer  811 . The matching layer  812  is applied over the piezoelectric ceramic element  814 , which has acoustic impedance of approximately 33 Mrayls. The acoustic impedance of the piezoelectric ceramic element  814  is closely matched to the acoustic impedance of the IC substrate  850 . This configuration may be desirable in some cases because it can provide a larger operating bandwidth. The transducer element  810  could also be constructed using a composite polymer and piezoelectric ceramic element  814 , which has acoustic impedance matched to the acoustic impedance of the IC substrate  850 .  
         [0057]    Another configuration is to place the transducer element on a backing having conductors over the redistribution layer on the IC. Such a backing including conductors is described in commonly assigned U.S. Pat. No. 5,267,221, entitled “BACKING FOR ACOUSTIC TRANSDUCER ARRAY” to Miller et al.  
         [0058]    As known to those having ordinary skill in the art, other than the interface  830 , which is similar to the interface  130  described above, all elements in FIG. 8B are attached using acoustic bonds.  
         [0059]    In this embodiment, the electrically conductive acoustic layer applied to the circuit board  855  is a backing layer  816 . The backing layer  816  acts as an acoustic absorption material, thereby absorbing any acoustic energy that travels through the IC  840  and the circuit board  855 . Furthermore, the dematching layer  808  of FIG. 8A may be combined with the backing layer  816  of FIG. 8B to achieve the desired acoustic performance of the piezoelectric ceramic transducer element  800 .  
         [0060]    [0060]FIG. 9 is a cross-sectional schematic view illustrating another alternative embodiment  900  of the ultrasonic transducer  200  of FIG. 2. The ultrasonic transducer  900  includes an acoustic sensor  920  that, in this embodiment, comprises a plurality of piezoelectric ceramic transducer elements, an exemplar one of which is illustrated using reference numeral  910 . A plurality of piezoelectric ceramic transducer elements  910  are arranged in an array, which typically includes many hundreds or thousands of individual transducer elements, and in a preferred embodiment, includes 2500 elements. Each piezoelectric ceramic transducer element  910  includes an element metalization layer  912   a  applied to the upper surface as shown. The element metalization layer  912   a  provides an electrical ground connection for each element  910 . The ground connection typically includes a conductor (not shown) connecting each element  910  to a suitable electrical ground. In accordance with an aspect of the invention, each piezoelectric ceramic transducer element  910  is joined to the active circuitry  918  associated with IC  940  through a die passivation layer  914  in which the metal contacts  932  are connected directly to the IC pads  924 , thereby eliminating the redistribution conductors referred to above. The die passivation layer  914  can be applied over the active circuitry  918  and IC pads, an exemplar one of which is illustrated using reference numeral  924 . The die passivation later  914 , can be formed using, for example, silicon dioxide or polymer.  
         [0061]    Each IC pad  924  is in electrical contact with a corresponding portion of the active circuitry  918  and the underside of metal contact  932 . As illustrated in FIG. 9, the redistribution conductors and the secondary passivation layer referred to above are omitted because the transducer elements  910  are each aligned over corresponding IC pads  924 .  
         [0062]    In one embodiment, each piezoelectric ceramic transducer element  910  is joined to a respective metal contact  932  using a conductive element  928 . The conductive element  928  can be, for example, a solder bump that forms an electrical contact between the metal contact  932  and an element metalization layer  912   b  applied to the underside of each piezoelectric ceramic transducer element  910 . In this manner, electrical contact between the piezoelectric ceramic transducer element  910  and the active circuitry  918  is achieved. Although illustrated in FIG. 9 using solder bumps as the conductive element  928 , the variety of other techniques mentioned above with respect to FIG. 2 are available for electrically connecting the metal contact  932  to the element metalization layer  912   b  of each piezoelectric ceramic transducer element  910 .  
         [0063]    Adhesive material  926  fills the gaps between each conductive element  928  and the space between each piezoelectric ceramic transducer element  910  and the die passivation layer  914 . The adhesive  926  is typically non-conductive and can be a variety of adhesives such as, for example but not limited to, epoxy.  
         [0064]    The ultrasonic transducer  900  is typically constructed by forming the die passivation layer  914  and the metal contacts  932  over the active circuitry  918  of the IC  940 . The conductive elements  928  are then deposited over the metal contacts  932 . The adhesive  926  is then deposited and the material from which the transducer elements  910  are formed is bonded to the die passivation layer  914 , resulting in an electrical connection between the element metalization layer  912   b  and the conductive elements  928 . The transducer elements  910  are then formed by removing a portion of the material that forms the transducer elements  910  and the element metalization layers  912   a  and  912   b  by, for example, cutting and lasing as described above.  
         [0065]    [0065]FIG. 10 is a cross-sectional schematic view illustrating an alternative embodiment  1000  of the ultrasonic transducer  400  of FIG. 4. The ultrasonic transducer  1000  of FIG. 10 includes acoustic sensor  1020 , which in this example includes a plurality of piezoelectric ceramic transducer elements, an exemplar one of which is illustrated using reference numeral  1010 . Each piezoelectric ceramic transducer element  1010  includes an element metalization layer  1012   a  applied to the upper surface as shown. The element metalization layer  1012   a  provides an electrical ground connection for each element  1010 . The ground connection typically includes a conductor (not shown) connecting each element  1010  to a suitable electrical ground. Each piezoelectric ceramic transducer element  1010  also includes an element metalization layer  1012   b  applied to the underside as shown. The element metalization layer  1012   b  allows direct electrical contact to be made between the piezoelectric ceramic transducer element  1010  and the metal contact  1032 . As mentioned above, such a connection is typically referred to as a “thin-line bond.” The thin-line bond electrically connects the metal contact  1032  directly to the exposed surface of the element metalization layer  1012   b  of the piezoelectric ceramic transducer element  1010 , typically using an adhesive polymer. The thin-line bond is achieved because of the microscopic surface roughness that exists on the exposed surface of the element metalization layer  1012   b  and the metal contact  1032 . This microscopic surface roughness provides a direct ohmic connection between the metal contact  1032  and the element metalization layer  1012   b.    
         [0066]    The metal contact  1032  connects directly to the IC bond pad  1024  without the use of the redistribution conductor described above. The IC pad  1024  connects to the active circuitry  1018  located on the IC  1040 . The surface of the secondary passivation layer  1016  and a portion of the metal contact  1032  is lapped, or planarized, and metalized to level the surface of the IC  1040  and to provide an even surface over which to attach the transducer elements  1010 .  
         [0067]    The ultrasonic transducer  1000  is typically constructed by forming the IC bond pads  1024  directly over portions of the active circuitry  1018 . The metal contacts  1032  are then formed over the IC bond pads  1024 , and the die passivation layer  1014  and the secondary passivation layer  1016  are then formed over the active circuitry  1018  and the IC pads  1024  of the IC  1040 . The exposed surface of the secondary passivation layer  1016  and portions of the metal contacts  1032  are then lapped, or planarized, flat and metalized. The element metalization layer  1012   b  is then thin-line bonded to the planarized surface of the secondary passivation layer  1016 , resulting in a thin-line bond electrical connection between the element metalization layer  1012   b  and the metal contacts  1032 . The transducer elements  1010  are then formed by removing a portion of the material that forms the transducer elements  1010  and the element metalization layers  1012   a  and  1012   b  by, for example, cutting and lasing as described above.  
         [0068]    It will be apparent to those skilled in the art that many modifications and variations may be made to the present invention, as set forth above, without departing substantially from the principles of the present invention. For example, the present invention can be used with piezoelectric ceramic and MUT transducer elements. All such modifications and variations are intended to be included herein.