Patent Publication Number: US-6659954-B2

Title: Micromachined ultrasound transducer and method for fabricating same

Description:
TECHNICAL FIELD 
     This invention relates generally to ultrasound diagnostic systems that use ultrasonic transducers to provide diagnostic information concerning the interior of the body through ultrasound imaging, and more particularly, to micro-machined ultrasonic transducers used in such systems. 
     BACKGROUND OF THE INVENTION 
     Ultrasonic diagnostic imaging systems are in widespread use for performing ultrasonic imaging and measurements. For example, cardiologists, radiologists, and obstetricians use ultrasonic diagnostic imaging systems to examine the heart, various abdominal organs, or a developing fetus, respectively. In general, imaging information is obtained by these systems by placing an ultrasonic probe against the skin of a patient, and actuating an ultrasonic transducer located within the probe to transmit ultrasonic energy through the skin and into the body of the patient. In response to the transmission of ultrasonic energy into the body, ultrasonic echoes emanate from the interior structure of the body. The returning acoustic echoes are converted into electrical signals by the transducer in the probe, which are transferred to the diagnostic system by a cable coupling the diagnostic system to the probe. 
     Acoustic transducers commonly used in ultrasonic diagnostic probes are comprised of an array of individual piezoelectric elements formed from a piezoelectric material by the application of a number of meticulous manufacturing steps. In one common method, a piezoelectric transducer array is formed by bonding a single block of piezoelectric material to a backing member that provides acoustic attenuation. The single block is then laterally subdivided by cutting or dicing the material to form the rectangular elements of the array. Electrical contact pads are formed on the individual elements using various metallization processes to permit electrical conductors to be coupled to the individual elements of the array. The electrical conductors are then coupled to the contact pads by a variety of electrical joining methods, including soldering, spot-welding, or by adhesively bonding the conductor to the contact pad. 
     Although the foregoing method is generally adequate to form acoustic transducer arrays having up to a few hundred elements, larger arrays of transducer elements having smaller element sizes are not easily formed using this method. Consequently, various techniques used in the fabrication of silicon microelectronic devices have been adapted to form ultrasonic transducer elements, since these techniques generally permit the repetitive fabrication of small structures in intricate detail. 
     An example of a device that may be formed using semiconductor fabrication methods is the micro-machined ultrasonic transducer (MUT). The MUT has several significant advantages over conventional piezoelectric ultrasonic transducers. For example, the structure of the MUT generally offers more flexibility in terms of optimization parameters than is typically available in conventional piezoelectric devices. Further, the MUT may be conveniently formed on a semiconductor substrate using various semiconductor fabrication methods, which advantageously permits the formation of relatively large numbers of transducers, which may then be integrated into large transducer arrays. Additionally, interconnections between the MUTs in the array and electronic devices external to the array may also be conveniently formed during the fabrication process. MUTs may be operated capacitively, and are referred to as cMUTs, as shown in U.S. Pat. No. 5,894,452. Alternatively, piezoelectric materials may be used to fabricate the MUT, which are commonly referred to as pMUTs, as shown in U.S. Pat. No. 6,049,158. Accordingly, the MUT has increasingly become an attractive alternative to conventional piezoelectric ultrasonic transducers in ultrasound systems. 
     FIG. 1 is a partial cross sectional view of a MUT  1  according to the prior art. The MUT  1  may have a platform that is rectangular, circular, or may be of other regular shapes. The MUT  1  generally includes an upper surface  2  that is spaced apart from a lower surface  3  that abuts a silicon substrate  5 . Alternatively, a dielectric layer  4  may be formed on the substrate  5  that underlies the MUT  1 . When a time-varying excitation voltage (not shown) is applied to the MUT  1 , a vibrational deflection in the upper surface  2  is developed that stems from the electro-mechanical properties of the MUT  1 . Accordingly, acoustic waves  6  are created that radiate outwardly from the upper surface  2  in response to the applied time-varying voltage. The electro-mechanical properties of the MUT  1  similarly allow the MUT  1  to be responsive to deflections resulting from acoustic waves  7  that impinge on the upper surface  2 . 
     One disadvantage in the foregoing prior art device is that a portion of the ultrasonic energy developed by the MUT  1  may be projected backwardly into the underlying substrate  5 , rather that being radiated outwardly in the acoustic wave  6 , which results in a partial loss of radiated energy from the MUT  1 . Moreover, when ultrasonic energy is coupled into the underlying substrate  5 , various undesirable effects are produced, which are briefly described below. 
     With reference now to FIG. 2, a partial cross sectional view of a MUT array  10  according to the prior art is shown. The array  10  includes a plurality of MUT transducers  1  formed on a silicon substrate  5 . Each transducer  1  is coupled to a time-varying voltage source through a plurality of electrical interconnections formed in the substrate  5 . For clarity of illustration, the voltage source and the electrical interconnections are not shown. An acoustic wave  21  may be conducted into the substrate  5  through a back surface  3 . The wave  21  propagates within the substrate  5  and is internally reflected at a lower surface  18  of the substrate  5  to form a reflected wave  23  that is directed towards an upper surface  19  of the substrate  5 . Consequently, a plurality of reflected waves  23  propagate within the substrate  5  between the upper surface  19  and the lower surface  18 . A portion of the energy present in each reflected wave  23  may also leave the substrate  5  through the surface  18 , to form a plurality of leakage waves  25 . An internal reflection  27  from an end  24  of the array  10  may lead to still further reflected waves  27  and leakage waves  26 . 
     The propagation of acoustic waves  23  and  27  in the substrate  5 , as described above, permits ultrasonic energy to be cross-coupled between the plurality of MUT transducers  1  on the substrate  5  and produce undesirable “cross-talk” signals between the plurality of MUTs  1 , as well as other undesirable interference effects. Still further, the internal reflection of waves in the substrate  5  may adversely affect the acceptance angle, or directivity of the array  10 . 
     Various prior art devices have included elements that impede the propagation of waves in the substrate. For example, one prior art device employs a plurality of trenches between the MUTs  1  that extend downwardly into the substrate  5  to interrupt wave propagation within the substrate  5 . Another prior art device employs a similar downwardly projecting trench, and fills the trench with an acoustic absorbing material in order to at least partially absorb the energy in the reflected waves  23 . Other prior art devices minimize lateral wave propagation by controlling still other geometrical details of the array. Although these prior art devices generally reduce the undesired lateral wave propagation in the substrate, they generally limit the design flexibility inherent in the MUT by reducing the number of design parameters that may be independently varied. Furthermore, the additional manufacturing steps significantly increase the manufacturing cost of arrays that use MUTs. 
     A further disadvantage associated with the prior art devices shown in FIGS. 1 and 2 is that a relatively large parasitic capacitance may be formed between the one or more MUTs  1  and the underlying substrate  5 . Since the MUT  1  is an electro-mechanical device that is generally excited by frequencies in the megahertz range, the formation of parasitic capacitances between the MUTs  1  and the substrate  5  further degrade the performance of the MUTs  1  by producing an additional capacitive load that generally degrades the sensitivity of the MUT. 
     Accordingly, there is a need in the art for micro-machined ultrasonic transducer structures that are capable of producing significant reductions in acoustic wave propagation in the underlying substrate. Further, there is a need in the art for a micro-machined ultrasonic transducer structures that suppress parasitic capacitive coupling between a MUT and an underlying substrate. 
     SUMMARY OF THE INVENTION 
     The invention is directed towards improved structures for use with micro-machined ultrasonic transducers (MUTs), and methods for fabricating the improved structures. In one aspect, a MUT is formed on a substrate and an acoustic cavity is formed within the substrate at a location below the MUT. The acoustic cavity is filled with an acoustic attenuation material to absorb acoustic waves propagated into the substrate, and to reduce the effect of parasitic capacitances on the operation of the MUT. In another aspect, the acoustic cavity is formed below a plurality of MUTs that comprise an array. The acoustic cavity and the acoustic attenuation material substantially reduce cross coupling between the MUTs by preventing wave propagation in the substrate. In still another aspect, a plurality of MUTs abut a dielectric layer with the MUTs being substantially encapsulated by the acoustic attenuation material. In yet another aspect, at least one monolithic semiconductor circuit is formed in the substrate that may be operatively coupled to the MUTs, the circuit being positioned in a non-etched portion of the substrate. In still another aspect, the at least one monolithic semiconductor circuit is formed in the substrate and positioned in a thin substrate layer above the acoustic cavity. In yet another aspect, a plurality of MUTs is attached to one side of a layer of semiconductor material, and a dielectric layer is formed on the opposing side. At least one monolithic semiconductor circuit is formed in the semiconductor material that may be operatively coupled to the MUTs. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partial cross sectional view of a MUT transducer according to the prior art. 
     FIG. 2 is a partial cross sectional view of a MUT transducer array according to the prior art. 
     FIG. 3 is a partial cross sectional view of a MUT transducer assembly according to an embodiment of the invention. 
     FIG. 4 is a partial cross sectional view of a MUT transducer array according another embodiment of the invention. 
     FIG. 5 is a partial cross sectional view of a MUT transducer illustrating a step in a method of fabricating the MUT transducer according to still another embodiment of the invention. 
     FIG. 6 is a partial cross sectional view of a MUT transducer illustrating a step in a method of fabricating the MUT transducer according to still another embodiment of the invention. 
     FIG. 7 is a partial cross sectional view of a MUT transducer illustrating a step in a method of fabricating the MUT transducer according to still another embodiment of the invention. 
     FIG. 8 is a partial cross sectional view of a MUT transducer illustrating a step in a method of fabricating the MUT transducer according to still another embodiment of the invention. 
     FIG. 9 is a partial cross sectional view of a MUT transducer array according still another embodiment of the invention. 
     FIG. 10 is a partial cross sectional view of a MUT transducer illustrating a step in a method of fabricating the MUT transducer according to still yet another embodiment of the invention. 
     FIG. 11 is a partial cross sectional view of a MUT transducer illustrating a step in a method of fabricating the MUT transducer according to still yet another embodiment of the invention. 
     FIG. 12 is a partial cross sectional view of a MUT transducer illustrating a step in a method of fabricating the MUT transducer according to still yet another embodiment of the invention. 
     FIG. 13 is a partial cross sectional view of a MUT transducer array according another embodiment of the invention. 
     FIG. 14 is a partial cross sectional view of a MUT transducer array according yet another embodiment of the invention. 
     FIG. 15 is a partial cross sectional view of a MUT transducer array according still another embodiment of the invention. 
     FIG. 16 is a partial cross sectional view of a MUT transducer array according to yet still another embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is generally directed to ultrasound diagnostic systems that use micro-machined ultrasonic transducers (MUTs) to provide diagnostic information concerning the interior of the body through ultrasound imaging. Many of the specific details of certain embodiments of the invention are set forth in the following description and in FIGS. 3 through 16 to provide a thorough understanding of such embodiments. One skilled in the art will understand, however, that the present invention may be practiced without several of the details described in the following description. Further, it is understood that the MUT described in the embodiments below may include any electro-mechanical device that may be formed on a semiconductor substrate that is capable of emitting acoustic waves when excited by a time-varying voltage, and producing a time-varying electrical signal when stimulated by acoustic waves. Accordingly, the MUT may include a capacitive micro-machined ultrasonic transducer (cMUT), a piezoelectric micro-machined ultrasonic transducer (pMUT), or still other micro-machined ultrasonic devices. Moreover, in the description that follows, it is understood that the figures related to the various embodiments are not to be interpreted as conveying any specific or relative physical dimension, and that specific or relative dimensions related to the various embodiments, if stated, are not to be considered limiting unless the claims expressly state otherwise. 
     FIG. 3 is a partial cross sectional view of a MUT transducer array  30  according to an embodiment of the invention. The MUT transducer array  30  includes a MUT  32  formed on a substrate  34 . The array  30  is capable of receiving ultrasonic waves and generating an output electrical signal, and generating ultrasonic waves in response to input electrical signals. The input and output signals are exchanged with an ultrasound system (not shown) through a plurality of interconnections positioned within the substrate  34 . For clarity of illustration, the interconnecting portions are not shown in FIG.  3 . The MUT  32  may be formed on the substrate  34  through the application of a series of well-known semiconductor fabrication processes. For example, the MUT  32  may be formed by patterning a surface of the substrate using a photolithographic process, and successively adding material layers to the substrate  34  by various material deposition processes. Structural features of the MUT  32  may further be formed by removing selected portions of the deposited material through the application of various etching processes. A dielectric layer may optionally be formed on an upper substrate surface  35  that electrically isolates the MUT  32  from the underlying substrate  34 . Alternatively, the dielectric layer may be incorporated directly into the MUT  32 . 
     Still referring to FIG. 3, the array  30  further includes a cavity  36  that is formed within the substrate  34 . The cavity  36  extends from an upper cavity surface  37  and proceeds downwardly towards a lower substrate surface  39 . The cavity  36  also includes a pair of sidewalls  38  that depend downwardly from the upper cavity surface  37  to the lower substrate surface  39 . The upper cavity surface  37  is separated from the upper surface  35  by a separation layer  31  that is sufficiently thin to prevent the significant propagation of acoustic waves to other portions of the substrate  34 . The cavity  36  may be filled with an acoustic attenuation material  33  having a relatively high acoustic attenuation to provide an acoustically-damped region below the MUT  32 . The dimensions of the cavity  36  and the characteristics of the material  33  cooperatively yield an acoustic impedance that is compatible with the overall acoustic design of the array  30 . For example, the depth “d” of the cavity  36  may be sufficient to allow waves transmitted from the MUT  32  through the surface  35  to be attenuated to a relatively negligible level, since the material  33  is sufficiently lossy to dissipate the acoustic energy present in the waves. Accordingly, the material  33  may include an elastomeric material, such as a room temperature vulcanizing (RTV) elastomer, or various epoxy matrices having dispersed solid metallic, ceramic, or polymeric filler particles of a selected density. Still further, the epoxy matrix may be filled with elastomeric particles or air-filled “micro-balloons” to achieve the desired acoustic properties. The array  30  may be positioned on an acoustic backing member (not shown) to support the array  30  and to provide further acoustic attenuation. 
     FIG. 4 is a partial cross sectional view of a MUT transducer array  40  according to another embodiment of the invention. The MUT transducer array  40  includes a plurality of MUTs  32  formed on a substrate  34  in a predetermined pattern to form the array  40 . A cavity  36  is formed below the plurality of MUTs  32  that extends downwardly from an upper cavity surface  37  towards a lower substrate surface  39 . The cavity  36  is dimensioned to yield a predetermined acoustic impedance when the cavity  36  is filled with a selected acoustic material  33 . 
     FIGS. 5 through 8 are partial cross sectional views that illustrate the steps in a method for fabricating a MUT array according to another embodiment of the invention. Referring to FIG. 5, a MUT  32  is formed on a substrate  34  by a sequence of well-known semiconductor fabrication steps, which may include the formation of a dielectric layer  50  on an upper surface  51  of the substrate  34 . The dielectric layer  50  may include silicon dioxide or silicon nitride, although other dielectric materials including silicon oxynitrides may be used. A layer  53  of silicon dioxide or silicon nitride is deposited on a lower surface  52 . The layer  53  is patterned using standard photolithographic processes to create an opening in the layer  53 , providing access to the back surface  52  of the substrate  34 . 
     Turning now to FIG. 6, the substrate  34  may then be etched to form a cavity  36  that extends from the lower surface  52  to an upper cavity surface  37 , as shown in FIG.  7 . The dielectric layer  50  may also serve as an etch stop layer during the etching process, although other etch stop devices, such as selective doping of the substrate  34 , may also be used. The substrate  34  may be etched using a variety of isotropic or anisotropic solutions in an etching bath to form the cavity  36 . The material properties of the substrate  34  and the composition of the etching bath generally cooperatively determine the shape of the cavity  36 . For example, if the substrate  34  is monocrystalline silicon having a &lt;111&gt; crystalline orientation, then an etching solution comprised of hydrofluoric acid and nitric acid will form a cavity  36  having side walls  38  that extend inwardly at approximately 45 degrees. Alternatively, a &lt;100&gt; monocrystalline material etched with a potassium hydroxide etching solution will yield side walls that extend inwardly at approximately 54.7 degrees. Other internal shapes for the cavity  36  may be obtained using other crystalline configurations in the substrate  34  together with other etching solutions, and are considered to be within the scope of the invention. Similarly, methods other than wet etching may be used to form the cavity  36 . For example, dry etching methods, which include plasma etching, ion beam milling and reactive ion etching may be also used. 
     Referring now to FIG. 8, the cavity  36  may be filled with an acoustic material  33 , which may be comprised of any of the materials identified above. The material  33  may be deposited into the acoustic cavity  36  by direct injection of the material  33  into the cavity  36 , although other methods exist. For example, the material  33  may be sprayed into the cavity  36 . Following the application of the material  33 , the layer  53  may be stripped to expose the surface  52 . The layer  53  may be stripped using various stripping methods, including wet chemical stripping or plasma stripping methods. An acoustic backing member may be positioned below the array to provide further acoustic attenuation. 
     The foregoing embodiments advantageously provide an acoustic cavity below the one or more MUT devices that is filled with an acoustic material to substantially inhibit the propagation of acoustic waves in the substrate. Additionally, the attenuation material generally possesses an acoustic impedance that substantially differs from the substrate material, permitting the MUT to transmit and receive ultrasonic signals more effectively. Still further, by positioning the substantially non-electrically conductive attenuation material below the one or more MUTs, parasitic capacitive coupling effects that may adversely affect the performance of the MUTs are reduced. 
     FIG. 9 is a partial cross sectional view of a MUT transducer array  60  according still another embodiment of the invention. The array  60  includes a plurality of MUTs  32  that are attached to a dielectric layer  50 . The MUTs  32  are further embedded in an acoustic attenuation material  62  that substantially encapsulates the MUTs  32  and abuts the dielectric layer  50  at locations  64 . The material  62  further substantially fills spaces  66  between adjacent MUTs  32  to provide additional resistance to cross-coupling effects. The acoustic attenuation material  62  extends a distance “d” below the layer  50  to ensure that waves propagated into the material  62  are substantially attenuated. 
     Still referring to FIG. 9, the dielectric layer  50  is a thin structure that permits acoustic waves  6  generated by each of the MUTs  32  in the array  60  to be transmitted outwardly, and correspondingly permits reflected acoustic waves  7  to be received by the MNTs  32 . Accordingly, the layer  50  may be comprised of a thin layer of silicon dioxide or silicon nitride, although other alternatives exist. 
     FIGS. 10 through 12 are partial cross sectional views that illustrate the steps in a method for fabricating a MUT array according to another embodiment of the invention. Referring to FIG. 10, a dielectric layer  50  is formed on a substrate  34 . A plurality of MUTs  32  are similarly formed on the substrate  34 , with the dielectric layer  50  interposed between the MUTs  32  and the substrate  34 . Alternatively, the substrate  34  may be comprised of a silicon-on-insulator (SOI) substrate that includes a layer of dielectric material that is spaced apart from the MUTs  32  and positioned within the substrate  32 , so that the MUTs  32  are positioned directly on a silicon surface. An acoustic attenuation material  62  is formed over the plurality of MUTs  32  that substantially encapsulates the MUTs  32 , as shown in FIG.  11 . 
     Turning to FIG. 12, the substrate  34  is substantially removed to expose an upper dielectric surface  64 . If the substrate  34  is an SOI substrate, then the substrate  34  is thinned to expose the insulating layer. In either case, the substrate  34  may be removed by wet etching the substrate  34  in a suitable solution, although other alternative methods exist. For example, the substrate  34  may be removed by employing wet spin etching to remove the substrate  34 . The substrate  34  may also be removed by backgrinding the substrate  34  to expose the surface  64 . 
     In addition to the advantages previously identified in connection with other embodiments, the foregoing embodiments additionally provide an unbounded acoustic cavity that advantageously permits the entire MUT to be encapsulated, so that spaces between adjacent MUTs are filled with the acoustic attenuation material, thus further reducing cross-coupling effects. 
     FIG. 13 is a partial cross sectional view of a MUT transducer array  70  according another embodiment of the invention. The MUT transducer array  70  includes a plurality of MUTs  32  formed on a substrate  34  in a predetermined pattern. A dielectric layer  50  may be interposed between the plurality of MUTs  32  and the substrate  34  to provide electrical isolation. An attenuation cavity  36  is formed below the plurality of MUTs  32  that extends downwardly from an upper cavity surface  37  towards a lower substrate surface  39 . The cavity  36  may be filled with an acoustic attenuation material  33  to yield selected acoustic properties for the array  70 . The array  70  further includes at least one semiconductor circuit  72  that is monolithically formed in the substrate  34  that is positioned proximate to a side of the attenuation cavity  36 . The circuit  72  may include a single semiconductor device, such as a field effect transistor (FET) or a similar device, which is used to drive the MUTs. Alternatively, the circuit  72  may comprise more fully integrated devices. For example, the circuit  72  may include monolithically formed circuits that at least partially perform receiver functions, beamforming processing, or other “front end” processing for the array  70 . Further, the circuit  72  may also include circuits that perform control operations for the array  70 . The semiconductor circuit  72  may be interconnected with the plurality of MUTs  32  and to other circuits external to the array by interconnecting elements formed in the substrate (not shown). The MUT transducer array  70  may be positioned on an acoustic backing member (not shown) to support the array  70  and to provide further acoustic attenuation. 
     FIG. 14 is a partial cross sectional view of a MUT transducer array  80  according yet another embodiment of the invention. The MUT transducer array  80  includes a plurality of MUTs  32  formed on a substrate  34 , which may have a dielectric layer  50  interposed between the plurality of MUTs  32  and the substrate  34 . An attenuation cavity  36  is formed below the plurality of MUTs  32  that extends downwardly from an upper cavity surface  37  towards a lower substrate surface  39 . The cavity  36  may be filled with an acoustic attenuation material  33  to yield selected acoustic properties for the array  80 . The array  80  further includes at least one semiconductor circuit  82  that is monolithically formed in a separation layer  31  at a location above the attenuation cavity  36 , and proximate to the plurality of MUTs  32 . As in the previous embodiment, the circuit  82  may include a single semiconductor device, or the circuit  82  may comprise more fully integrated devices. The semiconductor circuit  82  may be interconnected with the plurality of MUTs  32  and to other circuits external to the array by interconnection elements formed in the substrate (not shown). Alternatively, at least one circuit  82  may be formed in the separation layer  31  at a position approximately below the plurality of MUTs  32  and form interconnections (not shown) with the MUTs  32  through vias (also not shown) that extend from the MUTs  32  to the at least one circuit  82 . The MUT transducer array  80  may be positioned on an acoustic backing member (not shown) to support the array  80  and to provide still further acoustic attenuation. 
     FIG. 15 is a partial cross sectional view of a MUT transducer array  90  according still another embodiment of the invention. The array  90  includes a plurality of MUTs  32  embedded in an acoustic attenuation material  62  that substantially encapsulates the MUTs  32 . A layer  94  comprised of a semiconductor material is interposed between a dielectric layer  96  and the plurality of MUTs  32 . The dielectric layer  96  may be comprised of a thin layer of silicon dioxide or silicon nitride, although other alternatives exist. The array  90  further includes at least one semiconductor circuit  92  that is monolithically formed in the layer  94  at a location proximate to the plurality of MUTs  32 . As described in detail in connection with other embodiments of the invention, the circuit  92  may include a single device, or may comprise more fully integrated devices, including circuits that at least partially perform receiver, beamforming processing, or still other operations. The semiconductor circuit  92  may be interconnected with the plurality of MUTs  32  and to other circuits external to the array by conductive elements formed in the substrate (not shown). Alternatively, at least one circuit  92  may be formed in the layer  94  at a position approximately below the plurality of MUTs  32  and form interconnections (not shown) with the MUTs  32  through vias (also not shown) that extend from the MUTs  32  to the at least one circuit  92 . 
     Fabrication of the array  90  of FIG. 15 may proceed generally as shown in FIGS. 10 through 12. A dielectric layer  96  may be formed on a silicon substrate  34  (as shown in FIG.  10 ). Alternatively, a silicon-on-insulator (SOI) substrate may be used to provide both the substrate  34  and the dielectric layer  96 . In either case, the semiconductor circuits  92  are formed where desired in the layer  94 . The MUTs  32  may then be formed in the layer  94  and a surface of the array  90  that includes the MUTs may be covered with the acoustic attenuation material  62 . The substrate  34  may then be removed by backgrinding, etching, or other similar methods to yield the array  90  shown in FIG.  15 . 
     FIG. 16 is a partial cross sectional view of a MUT transducer array  100  according to yet still another embodiment of the invention. The array  100  is similar to the embodiment shown in FIG. 15 with the dielectric layer  96  removed, and at least a portion of the layer  94  removed, or not formed. Since the layer  96  and  94  are removed, acoustic attenuation due to the layers  96  and  94  are largely eliminated, so that the receiving and transmitting abilities of the MUTs  32  is enhanced. In addition, the layer  94  may be left or formed as islands (not shown) that may be used to form additional circuits  92 , either adjacent to, or between the MUTs  32 . 
     In addition to the advantages present in other embodiments of the invention, the foregoing embodiments include at least one semiconductor circuit that is monolithically formed in the substrate, and positioned in the substrate at a location proximate to the MUTs. The semiconductor circuit advantageously permits at least a portion of the signal processing and/or control circuits for the MUTs to be formed on a common substrate, resulting in significant cost savings through reduced hardware requirements, and savings in fabrication costs. 
     The above description of illustrated embodiments of the invention is not intended to be exhaustive or to limit the invention to the precise form disclosed. While specific embodiments of, and examples of, the invention are described in the foregoing for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled within the relevant art will recognize. For example, the cavity formed behind the MUTs is, as mentioned above, generally filled with an acoustic material, and the filled cavity or the thinned substrate layer are generally backed with acoustic backing material in the form of a layer or backing block having attenuative and impedance characteristics chosen in accordance with the requirements of the particular application. One or the other or both the cavity and backing may alternatively be air-filled, which may be desirable in low frequency applications, or when transmitting acoustic waves into air. The cavity and backing material may have strong attenuative (lossy) properties, or reflective or matching characteristics, depending upon the particular application. Still further, the various embodiments described above can be combined to provide further embodiments. Accordingly, the invention is not limited by the disclosure, but instead the scope of the invention is to be determined entirely by the following claims.