Patent Publication Number: US-2011062824-A1

Title: Ultrasonic transducer, ultrasonic probe and producing method

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an ultrasonic transducer, ultrasonic probe and producing method. More particularly, the present invention relates to an ultrasonic transducer of which sensitivity can be raised with a simple structure, and ultrasonic probe and producing method. 
     2. Description Related to the Prior Art 
     An ultrasonic probe is widely used for medical diagnosis. An ultrasonic transducer is disposed in a distal tip of the ultrasonic probe. The ultrasonic transducer is constituted by backing material, a piezoelectric layer, electrodes, acoustic matching layers, and an acoustic lens. The ultrasonic transducer emits ultrasonic waves to a human body, and receives reflected ultrasonic waves or echo from the human body. An ultrasonic imaging device is supplied with echo information from the ultrasonic transducer, processes the same electrically to produce an ultrasonic image. 
     It is also possible to produce ultrasonic tomographic images by emission of the ultrasonic waves in a scanned manner. Various methods of producing the ultrasonic tomographic images are known. In a method of mechanical scan, the ultrasonic transducer is mechanically rotated, swung or slid. In a method of electronic scan, a plurality of the ultrasonic transducers are arranged in an array. One or more of the ultrasonic transducers to be driven are changed over by an electronic switch or the like. 
     It has been very usual to use one piezoelectric layer to transmit and receive the ultrasonic waves. There is another structure in which a first one of the piezoelectric layers transmits the ultrasonic waves and a second one of the piezoelectric layers receives the reflected ultrasonic waves. For the first one, an inorganic piezoelectric ceramic material is suitably used with a relatively high value of an electromechanical coupling coefficient k33 for high power in transmitting the ultrasonic waves. For a second one, a polymeric piezoelectric compound is suitably used with a small value of a transmission coefficient d33 but with a relatively high value of a reception coefficient g33 for good sensitivity for receiving the reflected ultrasonic waves. 
     In JP-A 6-148154, one of the piezoelectric layers for transmission is disposed on a proximal surface of the acoustic lens, on a side opposite to the human body. The remainder of the piezoelectric layers for reception are disposed in recesses formed in the acoustic lens, on a distal side near to the human body. 
     There is a large difference in acoustic impedance between the human body and the ultrasonic transducers. One or more acoustic matching layers are formed for change the difference in the acoustic impedance stepwise to reduce a loss of the ultrasonic waves in the propagation. However, JP-A 6-148154 does not suggest consideration of the difference in the acoustic impedance between the human body and the ultrasonic transducers. 
     The ultrasonic transducers disclosed in JP-A 6-148154 have a problem of a high manufacturing cost due to a complicated structure in which a recess is formed in the acoustic lens and the ultrasonic transducers for reception are disposed in the recess. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing problems, an object of the present invention is to provide an ultrasonic transducer of which sensitivity can be raised with a simple structure, and ultrasonic probe and producing method. 
     In order to achieve the above and other objects and advantages of this invention, an ultrasonic transducer includes a first piezoelectric layer for transmitting ultrasonic waves to an object in a body. A second piezoelectric layer receives the ultrasonic waves reflected by the object. An acoustic matching layer is disposed between the first and second piezoelectric layers, for constituting an electrode common to the first and second piezoelectric layers. 
     The second piezoelectric layer is positioned nearer to the object than the first piezoelectric layer. 
     The first piezoelectric layer is inorganic, and the second piezoelectric layer is organic. 
     The first piezoelectric layer is inorganic, and the second piezoelectric layer is a complex containing an organic compound and an inorganic compound dispersed in the organic compound. 
     The acoustic matching layer contains metal. 
     The metal is constituted by silver. 
     The acoustic matching layer is a complex containing an organic compound and metal. 
     The organic compound is adhesive. 
     The metal is constituted by metal nano particles. 
     The acoustic matching layer includes a first layer of a material with a relatively low conductivity or a non-conductive material. A conductive material is overlaid on the first layer. 
     Furthermore, a second acoustic matching layer is overlaid on a surface of the second piezoelectric layer opposite to the acoustic matching layer. 
     Each of the first and second piezoelectric layers includes plural element regions arranged in an array in the first direction. 
     Preferably, the second piezoelectric layer includes plural receiving element regions, arranged in a first direction, and having volumes different from one another. 
     The plural element regions have receiving surfaces, which are arranged in the first direction, for receiving the reflected ultrasonic waves, and of which areas are different from one another. 
     The plural element regions have thicknesses different from one another in a direction perpendicular to receiving surfaces thereof. 
     The element regions are arranged two-dimensionally in the first direction and a second direction. 
     Also, an ultrasonic probe having an ultrasonic transducer is provided. The ultrasonic transducer includes a first piezoelectric layer for transmitting ultrasonic waves to an object in a body. A second piezoelectric layer receives the ultrasonic waves reflected by the object. An acoustic matching layer is disposed between the first and second piezoelectric layers, for constituting an electrode common to the first and second piezoelectric layers. 
     Also, an ultrasonic transducer includes a piezoelectric layer. Plural acoustic matching layers are overlaid on the piezoelectric layer. The acoustic matching layers include a first acoustic matching layer, positioned on the piezoelectric layer, formed from a mixture of metal nano particles and adhesive resin, for constituting an electrode for the piezoelectric layer. 
     The metal nano particles are constituted by silver nano particles. 
     The acoustic matching layers are at least three acoustic matching layers of which acoustic impedance is different from one another. 
     Also, an ultrasonic probe having an ultrasonic transducer is provided. The ultrasonic transducer includes a piezoelectric layer. Plural acoustic matching layers are overlaid on the piezoelectric layer. The acoustic matching layers include a first acoustic matching layer, positioned on the piezoelectric layer, formed from a mixture of metal nano particles and adhesive resin, for constituting an electrode for the piezoelectric layer. 
     Also, a method of producing an ultrasonic transducer having a piezoelectric layer is provided, and includes a step of placing a first acoustic matching layer on the piezoelectric layer, the first acoustic matching layer containing a mixture of metal nano particles and adhesive resin, and constituting an electrode for the piezoelectric layer. A second acoustic matching layer different from the first acoustic matching layer is placed on the first acoustic matching layer. The first acoustic matching layer is hardened by heating, so as to attach the first acoustic matching layer to the piezoelectric layer and to attach the second acoustic matching layer to the first acoustic matching layer. 
     Consequently, sensitivity of the ultrasonic probe can be raised with a simple structure, because the acoustic matching layer is disposed between the first and second piezoelectric layers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above objects and advantages of the present invention will become more apparent from the following detailed description when read in connection with the accompanying drawings, in which: 
         FIG. 1  is a perspective view illustrating an ultrasonic diagnostic apparatus; 
         FIG. 2  is an explanatory view illustrating a layered structure of an ultrasonic transducer array; 
         FIG. 3  is a block diagram schematically illustrating circuit elements in the ultrasonic transducer array; 
         FIG. 4  is an explanatory view illustrating one preferred ultrasonic transducer array in which a receiving piezoelectric layer has receiving element regions; 
         FIG. 5  is an explanatory view illustrating another preferred ultrasonic transducer array having three acoustic matching layers; 
         FIG. 6  is a flow chart illustrating a sequence of fabricating the ultrasonic transducer array; 
         FIG. 7  is an explanatory view illustrating a comparative structure of an ultrasonic transducer array having two acoustic matching layers. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) OF THE PRESENT INVENTION 
     In  FIG. 1 , an ultrasonic diagnostic system  2  or apparatus includes a portable ultrasonic imaging apparatus  10  and an ultrasonic probe  11 . The ultrasonic imaging apparatus  10  or device includes a housing  12  and a cover  13  or display unit. An input panel  14  is disposed on an upper surface of the housing  12 , includes plural buttons, trackball and the like, and is operated for inputting various data. A monitor display panel  15  is secured to the inside of the cover  13  for displaying various menus, ultrasonic images, and the like. 
     A hinge  16  keeps the cover  13  rotatable on the housing  12 . The cover  13  is rotatable between an open position and a closed position (not shown), and when in the open position, reveals the input panel  14  and the display panel  15 , and when in the closed position, covers and protects the panels  14  and  15  by opposing an inner surface of the cover  13  to an upper surface of the housing  12 . A grip (not shown) is secured to a lateral face of the housing  12 , and held manually for carrying the ultrasonic imaging apparatus  10  when the cover  13  is in the closed position on the housing  12 . A connecting port  17  is formed in another surface of the housing  12  for connecting the ultrasonic probe  11  removably. 
     The ultrasonic probe  11  includes a scan head  18 , a connector plug  19 , and a cable  20 . The scan head  18  is manually held by a doctor or operator, and caused to touch a patient&#39;s body. The cable  20  extends between the scan head  18  and the connector plug  19 . An ultrasonic transducer array  21  or UT array is incorporated in an end portion of the scan head  18 . 
     In  FIG. 2 , the ultrasonic transducer array  21  includes a base plate  25  and various layers overlaid thereon. The base plate  25  is formed from glass, epoxy resin or the like. The layers include backing material  26 , a lower electrode  27 , a transducer piezoelectric layer  28 , a first acoustic matching layer  29 , a receiving piezoelectric layer  30 , an upper electrode  31 , a second acoustic matching layer  32 , and an acoustic lens  33  in a listed order. 
     The backing material  26  regulates free vibration of the transducer piezoelectric layer  28  in the course of emitting ultrasonic waves, to raise a resolving power of the ultrasonic waves in an emission direction. Various materials can be used for the backing material  26  with property to absorb vibration, and can be organic or inorganic. Preferable examples of the materials include epoxy resin and other resins, chlorinated polyethylene rubber, natural rubber, styrene-butadiene rubber (SBR) and other rubbers, because they have low acoustic impedance and can absorb vibration without dropping the sensitivity. 
     The transducer piezoelectric layer  28  is present in an array of plural element regions of strips, which extend in the EL direction. The element regions of the transducer piezoelectric layer  28  are disposed equidistantly in the array of the AZ direction perpendicular to the EL direction. Filler  34  is charged in gaps or clearances between the element regions of the transducer piezoelectric layer  28  and on their peripheral surface. The array of the element regions are shaped by either surface patterning or cutting (dicing). 
     The transducer piezoelectric layer  28  has a thickness of 0.1-0.5 mm, and is sandwiched by the first acoustic matching layer  29  and the lower electrode  27 . The first acoustic matching layer  29  is electrically conductive, and is used as an upper electrode for the transducer piezoelectric layer  28 . The lower electrode  27  is a thin film of metal. Lead wires (not shown) are connected with respectively the first acoustic matching layer  29  and the lower electrode  27 . The lower electrode  27 , the transducer piezoelectric layer  28  and the first acoustic matching layer  29  are combined to constitute an ultrasonic transducer for transmission and reception. 
     The lead wire connected with the first acoustic matching layer  29  is grounded as illustrated in  FIG. 3 . A lead wire connected with the lower electrode  27  is connected to the ultrasonic imaging apparatus  10 . A transmitter/receiver  41  of  FIG. 3  is incorporated in the ultrasonic imaging apparatus  10 . When a pulse voltage is applied to the transducer piezoelectric layer  28  by the transmitter/receiver  41 , the transducer piezoelectric layer  28  vibrates to generate ultrasonic waves for emission to an object of interest in a body cavity. When the transmitter/receiver  41  receives echo or reflected waves from the object, the transducer piezoelectric layer  28  vibrates to output echo information (reception signal) as a voltage signal. 
     Various inorganic materials with piezoelectric property can be used for the transducer piezoelectric layer  28 . Preferable examples of the materials are PZT and other Pb compounds with piezoelectric property. Among those, specifically preferable examples are PMN-PT and PZN-PT as piezoelectric relaxor single crystal which can have an ultrahigh piezoelectric coefficient. Those are characterized in a high value of the electromechanical coupling coefficient k and in a high value of a ratio of output of ultrasonic waves to applied voltage as efficiency of conversion. 
     The receiving piezoelectric layer  30  operates also as an acoustic matching layer, and cooperates with the first and second acoustic matching layers  29  and  32 . A difference in the acoustic impedance between the transducer piezoelectric layer  28  and a human body is reduced stepwise to increase sensitivity of transmission and reception of ultrasonic waves. The second acoustic matching layer  32  reduces a difference in the acoustic impedance between the receiving piezoelectric layer  30  and the human body stepwise, to increase sensitivity of transmission and reception of ultrasonic waves. 
     Various conductive materials can be used for the first acoustic matching layer  29  with an acoustic impedance lower than that of the transducer piezoelectric layer  28  and higher than that of the receiving piezoelectric layer  30 . A preferable material for the first acoustic matching layer  29  is a baked mixture of metal nano particles (with a diameter of 1-100 nm) and adhesive resin. Preferable metal nano particles are silver nano particles. Specifically, the metal nano particles can exclusively consist of silver nano particles. This is because the silver nano particles have relatively high dispersibility to resin among various metals. When resin with the metal nano particles is baked, particles become bonded in the resin to form a conductive path. Thus, high conductivity can be obtained. Note that the first acoustic matching layer  29  may be a dual layer structure, which can include a first layer of a material without conductivity or a relatively low conductivity, and a conductive layer overlaid on the first layer. 
     In a manner similar to the transducer piezoelectric layer  28 , the receiving piezoelectric layer  30  is constituted by plural element regions of cutting (dicing), or is present with plural element regions of strips of surface patterning for splitting electrically. Filler  35  is charged in gaps of the receiving piezoelectric layer  30  and on its peripheral surface. 
     The receiving piezoelectric layer  30  has a thickness of 0.05-0.3 mm, and sandwiched by the upper electrode  31  and the first acoustic matching layer  29 . The first acoustic matching layer  29  operates as a lower electrode for the receiving piezoelectric layer  30  in addition to operating as upper electrode for the transducer piezoelectric layer  28  as described above. It follows that the first acoustic matching layer  29  functions as a common electrode for the transducer piezoelectric layer  28  and the receiving piezoelectric layer  30 . The upper electrode  31  is a film of metal, to which a lead wire (not shown) is connected. 
     The lead wire connected with the upper electrode  31  is connected to the ultrasonic imaging apparatus  10 . When the receiving piezoelectric layer  30  receives echo or reflected waves, echo information is input to the transmitter/receiver  41  in the ultrasonic imaging apparatus  10  in  FIG. 3 . A combination of the first acoustic matching layer  29 , the receiving piezoelectric layer  30  and the upper electrode  31  constitutes an ultrasonic transducer for reception. 
     Various organic materials with piezoelectric property can be used for the receiving piezoelectric layer  30  with an acoustic impedance lower than that of the first acoustic matching layer  29  and higher than that of the second acoustic matching layer  32 . Among those, preferable materials are PVDF, P(VDF-TrFE) and other fluorocarbon polymers. Those are characterized in a high sensitivity factor g, and relatively high sensitivity to ultrasonic waves. Note that the receiving piezoelectric layer  30  can be formed from complex piezoelectric material formed by dispersing an inorganic compound in an organic compound. 
     Material for the second acoustic matching layer  32  can be a compound of which acoustic impedance is lower than that of the receiving piezoelectric layer  30  and higher than that of a human body. A preferable example of the material is epoxy resin. 
     The acoustic lens  33  converges ultrasonic waves from the transducer piezoelectric layer  28  at an object of interest in a body cavity. A material for the acoustic lens  33  is silicone rubber with a maximum thickness of 1 mm or so. 
     Various adhesive materials can be used for adhesion in overlaying plural layers in the ultrasonic transducer array  21 . Among those, epoxy resin as adhesive agent is specifically preferable for high acoustic transmittance, high strength in attachment, and low cost. 
     In  FIG. 3 , the transmitter/receiver  41  includes a pulser  42 , a receiver  43 , a first A/D converter  44 , an image producing device  45 , a transmitting/receiving switch  46  or TX/RX switch, a receiver  47 , and a second A/D converter  48 . 
     The pulser  42  is connected with the lower electrode  27  by the transmitting/receiving switch  46 . The pulser  42  transmits a pulse voltage or excitation pulse to the lower electrode  27  for the transducer piezoelectric layer  28  to generate ultrasonic waves. 
     The receiver  43  is connected with the lower electrode  27  by the transmitting/receiving switch  46 . The receiver  43  is supplied with echo information (reception signal) from the transducer piezoelectric layer  28  according to reflected waves or echo from the object of interest. 
     The first A/D converter  44  converts the echo information from the receiver  43  into a digital form according to the A/D conversion. The digital echo data from the first A/D converter  44  is input to the image producing device  45 . 
     The pulser  42  and the receiver  43  are connected with the transmitting/receiving switch  46 , which changes over inputs and outputs of those selectively. 
     The receiver  47  is connected with the upper electrode  31 , and amplifies the echo information from the receiving piezoelectric layer  30  in a manner similar to the receiver  43 . 
     The second A/D converter  48  converts the echo information from the receiver  47  into a digital form according to the A/D conversion. The digital echo data from the second A/D converter  48  is input to the image producing device  45 . The image producing device  45  produces an ultrasonic image from the echo information input by the first and second A/D converters  44  and  48 , and outputs the image to the display panel  15 . Various elements included in the transmitter/receiver  41  except for the image producing device  45  are also incorporated for the transducer piezoelectric layer  28  and the receiving piezoelectric layer  30 . 
     Thus, the ultrasonic transducer array can be easily fabricated in a simple structure without high cost, because the first acoustic matching layer  29  with electric conductivity is used as a common electrode for the transducer piezoelectric layer  28  and the receiving piezoelectric layer  30 . The first acoustic matching layer  29  can have the high conductivity owing to the use of resin containing silver nano particles, and can be used sufficiently as common electrode. 
     The resin for the first acoustic matching layer  29  is a thermoset compound and operates as adhesive for the receiving piezoelectric layer  30 . The ultrasonic transducer array  21  can be fabricated easily by reducing the number of steps in the fabrication in comparison with a known method in which adhesive agent is applied to attach the receiving piezoelectric layer  30  after forming the first acoustic matching layer. The number of electrodes of a relatively small thickness can be kept small, to prevent drop of the yield of the fabrication. 
     In the present embodiment, the transducer piezoelectric layer  28  is used for transmission and reception. However, transmission ultrasonic transducer only for transmission without reception can be used instead of the transducer piezoelectric layer  28 . 
     If a transmission line path between the upper electrode  31  and the receiver  47  is very long, a drop of voltage of the echo information between the upper electrode  31  and the receiver  47  will be considerably great according to line resistance of the transmission line path. This will cause degradation of an ultrasonic image according to ultrasonic waves received by the receiving piezoelectric layer  30 , to reduce effect of the substance of the receiving piezoelectric layer  30  with high sensitivity. 
     Thus, the transmission line path for connection between the upper electrode  31  and the receiver  47  should be as short as possible. The receiver  47  is preferably disposed very near to the upper electrode  31 . Specifically, the receiver  47  is contained in the scan head  18  instead of containment in the ultrasonic imaging apparatus  10 . 
     Example 1 
     The ultrasonic transducer array  21  was formed experimentally. The backing material  26  was chlorinated polyethylene rubber and had a thickness of 1 cm. A flexible printed circuit board (FPC) was attached to the backing material  26  by adhesion of epoxy resin of a thermoset type. 
     A film of the transducer piezoelectric layer  28  was formed from C92H (trade name, manufactured by Fuji Ceramics Corporation) as piezoelectric ceramic PZT material. The film of the transducer piezoelectric layer  28  was polished for both its surfaces to have a thickness of 260 microns. A film of metal was formed on one surface of the transducer piezoelectric layer  28  by sputtering Ti, Pt and Au one after another. Then a proximal side of the transducer piezoelectric layer  28  having the metal film was attached to the flexible printed circuit board (FPC) on the backing material  26  by adhesion. To this end, adhesive agent was used, which contained silver nano particles, and was thermally hardened. Acoustic impedance of the transducer piezoelectric layer  28  was approximately 31 Mrayl. Note that the lower electrode  27  was constituted by the flexible printed circuit board (FPC) on the backing material  26  and the metal film of Ti, Pt and Au on the transducer piezoelectric layer  28 . 
     As the first acoustic matching layer  29 , a coating of resin containing silver nano particles (produced by Sumitomo Denki Kogyo Co., Ltd.) was applied to the transducer piezoelectric layer  28  with a thickness of λ/4 (where λ is a wavelength of ultrasonic waves). The first acoustic matching layer  29  was hardened by heating at approximately 180 deg. C. for one (1) hour in the atmosphere. The acoustic impedance of the first acoustic matching layer  29  after heating was approximately 12 Mrayl. 
     The receiving piezoelectric layer  30  was formed from PVDF having acoustic impedance of 4.5 Mrayl. The receiving piezoelectric layer  30  was molded at a thickness of λ/4. An surface electrode or film of metal was formed entirely on one surface of the receiving piezoelectric layer  30 . A second surface of the receiving piezoelectric layer  30  opposite to the surface electrode was attached to the first acoustic matching layer  29 . For the attachment, adhesive agent containing silver nano particles was used. The surface electrode on the receiving piezoelectric layer  30  constituted the upper electrode  31 . 
     A film of epoxy adhesive agent with acoustic impedance of 2 Mrayl was polished to have a thickness of λ/4, and was attached to the upper electrode  31  as the second acoustic matching layer  32  by use of epoxy adhesive agent of a thermoset type. Similarly, the acoustic lens  33  was attached to the second acoustic matching layer  32 . 
     For further examples, elements not included in Example 1 will be hereinafter described. 
     Example 2 
     The transducer piezoelectric layer  28  was formed from PMN-PT (trade name) as a Pb (Mg, Nb) O 3 —PbTiO 3  compound (manufactured by JFE Mineral Corporation) for a piezoelectric relaxor single crystal. The film of the transducer piezoelectric layer  28  was polished for both its surfaces to have a thickness of 240 microns. Acoustic impedance of the transducer piezoelectric layer  28  was approximately 22 Mrayl. 
     In a manner similar to Example 1, the first acoustic matching layer  29  was formed and then hardened by heating at approximately 160 deg. C. for one (1) hour in the atmosphere. The acoustic impedance of the first acoustic matching layer  29  after heating was approximately 11 Mrayl. 
     Comparative Example 1 
     Each of both surfaces of the transducer piezoelectric layer  28  was coated with metals of Ti, Pt and Au successively by sputtering to form a metal film. After this, the transducer piezoelectric layer  28  was attached to the flexible printed circuit board (FPC) positioned on the backing material  26 . Adhesive agent was used for the attachment, was resin containing silver nano particles. The transducer piezoelectric layer  28  on the flexible printed circuit board (FPC) was hardened by heating at approximately 100 deg. C. for one (1) hour in the atmosphere. The transducer piezoelectric layer  28  had acoustic impedance of approximately 31 Mrayl. 
     An upper electrode was disposed instead of the first acoustic matching layer  29 . A film of metal was formed on the transducer piezoelectric layer  28  and was the upper electrode. An acoustic matching layer was formed instead of the receiving piezoelectric layer  30 . Epoxy resin dispersion of zirconia particles was used, had acoustic impedance of 8 Mrayl, was polished to have a thickness of λ/4, and was overlaid on the transducer piezoelectric layer  28  as the acoustic matching layer. Epoxy adhesive agent of a thermoset type was used, and hardened by heating. 
     A film of epoxy adhesive agent with acoustic impedance of 3 Mrayl was polished to have a thickness of λ/4, and was attached as the second acoustic matching layer  32  by use of the epoxy adhesive agent of the thermoset type. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                 Comparative 
               
               
                   
                 Example 1 
                 Example 2 
                 example 1 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Transducer 
                 piezo- 
                 piezo- 
                 piezo- 
               
               
                 piezo- 
                 electric 
                 electric 
                 electric 
               
               
                 electric layer 
                 ceramic PZT, 
                 relaxor single 
                 ceramic PZT, 
               
               
                 28 
                 31 Mrayl 
                 crystal, 
                 31 Mrayl 
               
               
                   
                   
                 22 Mrayl 
               
               
                 First acoustic 
                 resin 
                 resin 
                 x 
               
               
                 matching layer 
                 containing 
                 containing 
               
               
                 29 
                 silver nano 
                 silver nano 
               
               
                   
                 particles, 
                 particles, 
               
               
                   
                 12 Mrayl 
                 11 Mrayl 
               
               
                 Receiving 
                 PVDF, 
                 PVDF, 
                 epoxy resin 
               
               
                 piezo- 
                 4.5 Mrayl 
                 4.5 Mrayl 
                 dispersion of 
               
               
                 electric layer 
                   
                   
                 zirconia 
               
               
                 30 
                   
                   
                 particles, 
               
               
                   
                   
                   
                 8 Mrayl 
               
               
                 Second 
                 epoxy resin, 
                 epoxy resin, 
                 epoxy resin, 
               
               
                 acoustic 
                 2 Mrayl 
                 2 Mrayl 
                 3 Mrayl 
               
               
                 matching layer 
               
               
                 32 
               
               
                   
               
            
           
         
       
     
     Table 1 above indicated a relationship between materials for the layers in Examples and Comparative examples, and values of the acoustic impedance (Mrayl). The transducer piezoelectric layer  28  was formed from the piezoelectric relaxor single crystal and had acoustic impedance of 22 Mrayl only according to Example 2, but was formed from the piezoelectric ceramic PZT and had acoustic impedance of 31 Mrayl according to the remaining examples. 
     The first acoustic matching layer  29  of Examples 1 and 2 was formed from resin containing silver nano particles. The acoustic impedance of the first acoustic matching layer  29  was 12 Mrayl in Example 1, and was 11 Mrayl in Example 2. In contrast, the first acoustic matching layer  29  was not present in Comparative example 1. 
     The receiving piezoelectric layer  30  of Examples 1 and 2 was formed from PVDF with the acoustic impedance of 4.5 Mrayl. In contrast, the resin layer of Comparative example 1 instead of the receiving piezoelectric layer  30  was formed from epoxy resin dispersion of zirconia particles with the acoustic impedance of 8 Mrayl. 
     The second acoustic matching layer  32  in any of the examples was formed from the epoxy resin. The acoustic impedance of the second acoustic matching layer  32  was 3 Mrayl in Example 1, and was 2 Mrayl in all the remaining examples. 
     An experiment was conducted to test sensitivity of reception of the ultrasonic transducer array  21  fabricated according to Examples 1 and 2 and Comparative example 1. An ultrasonic wave comes to have a distorted waveform in the course of propagation, and comes to include higher harmonic components with a frequency an integer times the frequency of the fundamental wave. As a result, higher harmonic components were received with high sensitivity in addition to a fundamental wave in any of Examples 1 and 2 and Comparative example 1. 
     In Comparative Example 1, in contrast, the fundamental wave was received when an output frequency was equal to or more than a predetermined frequency. However, it was impossible to receive second harmonic waves. Note that it was possible to receive second harmonic waves by lowering the output frequency, but the lowered output frequency was insufficient for the ultrasonic imaging. 
     Thus, it was possible with a high sensitivity to receive second harmonic waves in addition to the fundamental wave owing to the receiving piezoelectric layer  30  separately in combination with the transducer piezoelectric layer  28 . 
     In the above embodiments, the ultrasonic probe is a type of convex scan for extracorporeal use. However, a ultrasonic probe of the invention may be a type of a radial scan or a type of a mechanical scan for mechanically rotating, swinging or sliding a single ultrasonic transducer. The feature of the invention can be used in a ultrasonic probe insertable in an instrument channel in an electronic endoscope for in-vivo use, an ultrasonic endoscope structured integrally with an electronic endoscope. 
     In  FIG. 4 , another preferred structure of the ultrasonic transducer array  21  is illustrated. The transducer piezoelectric layer  28  in the ultrasonic transducer array  21  includes transducer element regions  28   a  of the array. The receiving piezoelectric layer  30  includes receiving element regions  30   a - 30   e . The transducer piezoelectric layer  28  and the receiving piezoelectric layer  30  are combined with the base plate  25 , the backing material  26 , the lower electrode  27 , the first acoustic matching layer  29 , the upper electrode  31 , the second acoustic matching layer  32  and the acoustic lens  33 . 
     The receiving element regions  30   a - 30   e  are arranged equidistantly in the AZ direction, and have a two-dimensional multi-element form. The receiving element regions  30   a - 30   e  have receiving surfaces for reflected wave with areas decreasing in the EL direction along the receiving piezoelectric layer  30 , and have volumes different from one another. Each of the receiving element regions  30   a - 30   e  has a specific frequency derived from the volume, shape and the like to receive reflected waves with high sensitivity for specific frequency according to the specific frequency. 
     Waveforms of the ultrasonic waves become distorted by influence of physical property (hardness and the like) of tissue of a body cavity, and will include harmonic components (non linear components) as an integer times the frequency of the fundamental wave. An area, shape and the like of a receiving surface of the receiving element regions  30   a - 30   e  for reflected waves along the receiving piezoelectric layer  30  are predetermined for a specific frequency according to frequencies of the fundamental wave and harmonic waves. This is effective in receiving reflected waves with high sensitivity, especially harmonic components, such as a second harmonic wave, third harmonic wave, fourth harmonic wave and the like. 
     Also, a non-linear parameter (B/A parameter) can be obtained from a ratio between the received fundamental wave and higher harmonic components as information of possibility of a non linear phenomenon on the tissue in a body cavity. It has been expected to utilize the B/A parameter as diagnostic value of next generation because of correspondence to hardness of the tissue. In the present invention, the B/A parameter can be obtained reliably and easily owing to exact examination. 
     In the present embodiment, areas of the receiving element regions  30   a - 30   e  as receiving surfaces for reflected waves arranged along the receiving piezoelectric layer  30  decreases in the EL direction from each of the two ends toward the center. However, these areas of the receiving element regions  30   a - 30   e  can be determined with changes in other manners, for example, can decrease in a direction from the center toward each of the two ends. Also, instead of or in addition to a decrease in the areas of the receiving surfaces of the receiving element regions  30   a - 30   e , it is possible to set the thicknesses different between those in a direction vertical to the receiving surfaces for the reflected waves of the receiving piezoelectric layer  30 . When the thickness is reduced to 1/n time as much as the initial thickness (wherein n is an integer), a specific frequency becomes n times as high as before. For example, when the thickness is reduced to a half as much as the initial thickness, the specific frequency becomes two times as high as before. 
     Example 3 
     An experiment was conducted for the embodiment. In Example 3, the ultrasonic transducer array  21  of Example 1 was repeated with a difference in using the structure of  FIG. 4 . 
     Example 4 
     Example 2 was repeated with a difference in using the structure of  FIG. 4 . 
     Comparative Example 2 
     Comparative example 1 was repeated for comparison. 
     The experiment was conducted to test sensitivity of reception of the ultrasonic transducer array  21  fabricated according to Examples 3 and 4 and Comparative example 2. As a result, higher harmonic components were received with high sensitivity in addition to a fundamental wave in any of Examples 3 and 4. 
     In Comparative Example 2, in contrast, the fundamental wave was received when the output frequency was equal to or more than a predetermined frequency. However, it was impossible to receive a higher harmonic component. Note that it was possible to receive a higher harmonic component by lowering the output frequency, but the lowered output frequency was insufficient for the ultrasonic imaging. 
     Thus, it was possible to receive a higher harmonic component in addition to the fundamental wave owing to the receiving element regions  30   a - 30   e  with the different volumes in combination with the transducer element regions  28   a.    
     In  FIG. 7 , a known ultrasonic transducer array  101  or UT array is illustrated in a manner of JP-A 9-139998. Plural ultrasonic transducers  102  of the array are arranged in the AZ direction. The ultrasonic transducers  102  have a piezoelectric layer  103  and electrodes  104  and  105  for applying voltage to the piezoelectric layer  103 . Lead wires (not shown) are connected with the electrodes  104  and  105 . A base plate  107  and backing material  106  are disposed on a surface of the ultrasonic transducers  102  opposite to an emitting surface for emitting ultrasonic waves. The backing material  106  absorbs unwanted ultrasonic waves from the ultrasonic transducers  102 . 
     There is a considerable difference in the acoustic impedance between the ultrasonic transducers  102  and tissue of a body cavity. That of the tissue is approximately 1.5 Mrayl, in contrast with approximately 25-35 Mrayl as that of piezoelectric ceramic material in the ultrasonic transducers  102 . Due to the difference, ultrasonic waves are reflected by an interface between the ultrasonic transducers  102  and the tissue to cause a loss in the propagation. In view of this, a first acoustic matching layer  108  and a second acoustic matching layer  109  are formed on the ultrasonic transducers  102  on their emission surface. That is changed stepwise from the ultrasonic transducers  102  to the tissue. Note that an acoustic lens  110  operates to converge ultrasonic waves in the EL direction which is perpendicular to the AZ direction of the array. 
     A difference in the acoustic impedance is reduced stepwise by the multi layer structure with plural acoustic matching layers. Theoretically, ultrasonic reflection decreases on an interface of the ultrasonic transducer and tissue of an object of interest, to prevent a loss in the propagation of ultrasonic waves to raise sensitivity in receiving and transmitting ultrasonic waves. However, a total of the interfaces between the acoustic matching layers will considerably large. Adhesive agent present on the interfaces reflects the ultrasonic waves so as to lower the sensitivity of reception and transmission. Another problem arises in that the plural acoustic matching layers must be attached to one another, so that the yield of the fabrication will be small and the process time will be long due to their small thickness. 
     If the acoustic matching layer is in the multi layer structure, a first one of acoustic matching layers on an emitting surface of the ultrasonic transducer should satisfy a condition with an impedance which is lower than that of piezoelectric ceramic material and higher than that of organic substances for forming a second one of the acoustic matching layers on the object side (distal side). However, known materials satisfying the condition are very few. A range of selecting the materials is considerably limited. 
     An embodiment to solve the problem is illustrated in  FIG. 5 . The ultrasonic transducer array  21  includes a first acoustic matching layer  49 , a second acoustic matching layer  50 , a third acoustic matching layer  51  and an acoustic lens  52  in combination with the base plate  25 , the backing material  26 , the lower electrode  27  and the transducer piezoelectric layer  28 . 
     Various conductive materials can be used for the first acoustic matching layer  49  with an acoustic impedance lower than that of the transducer piezoelectric layer  28  and higher than that of the second acoustic matching layer  50 . 
     
       
         
           
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 Baking temperature 
               
               
                   
                 (deg. C.) 
               
            
           
           
               
               
               
               
            
               
                   
                 100 
                 150 
                 200 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Sonic speed 
                 1,560 
                 1,820 
                 2,080 
               
               
                   
                 (m/s) 
               
               
                   
                 Density (kg/m 3 ) 
                 3,910 
                 5,560 
                 7,210 
               
               
                   
                 Acoustic 
                 6.1 
                 10.1 
                 15.0 
               
               
                   
                 impedance 
               
               
                   
                 (Mrayl) 
               
               
                   
                   
               
            
           
         
       
     
     Table 2 indicates the sonic speed, density and acoustic impedance of the acoustic matching layer containing silver nano particles as metal nano particles, for each value of the baking temperature. According to the increase in the baking temperature (deg. C.) from 100 and 150 to 200, the sonic speed (m/s) increases in a sequence of 1,560, 1,820 and 2,080. Similarly, the density (kg/m 3 ) increases in a sequence of 3,910, 5,560 and 7,210. The acoustic impedance (Mrayl) increases in a sequence of 6.1, 10.1 and 15.0. 
     It follows that baking at a high temperature refines the acoustic matching layer to enlarge acoustic impedance. It is possible to determine acoustic impedance at suitable levels by changing the baking temperature in the acoustic matching layer containing silver nano particles. The acoustic matching layer can be used suitably in many types of piezoelectric layers. It is unnecessary to select materials for forming the acoustic matching layer with a desired acoustic impedance, to minimize labor for manual operation. 
     Various materials can be used for the second acoustic matching layer  50  with an acoustic impedance lower than that of the first acoustic matching layer  49  and higher than that of the third acoustic matching layer  51 . Among those, a preferable material is a mixture of zirconia particles and epoxy resin. Various materials can be used for the third acoustic matching layer  51  with an acoustic impedance lower than that of the second acoustic matching layer  50  and higher than that of a human body. Among those, a preferable material is epoxy resin. 
     A sequence of fabricating the ultrasonic transducer array  21  is described by referring to  FIG. 6 . At first, the lower electrode  27  and the transducer piezoelectric layer  28  are overlaid on the backing material  26 , and cut in elements of plural strips by dicing in the step S 10 . The filler  34  is filled in gaps between the elements of the transducer piezoelectric layer  28  and around those. Then an upper surface of the transducer piezoelectric layer  28  is coated with a mixed material as the first acoustic matching layer  49  inclusive of metal nano particles and adhesive resin. See the step S 11 . Then a mixed material is caused to flow and shaped in a film form, inclusive of zirconia particles and epoxy resin, so then the second acoustic matching layer  50  is overlaid on the first acoustic matching layer  49  of the transducer piezoelectric layer  28 . See the step S 12 . The layers are heated at the step S 13 . The first acoustic matching layer  49  is hardened by heat to attach the first and second acoustic matching layers  49  and  50  to the transducer piezoelectric layer  28 . Also, the third acoustic matching layer  51  and the acoustic lens  52  are overlaid in the step S 14  to obtain the ultrasonic transducer array  21 . Note that the step of dicing the lower electrode  27  and the transducer piezoelectric layer  28  into the array can be performed at one time for the acoustic matching layers  49 - 51  after overlaying all the three. In a manner similar to the steps S 10 -S 14 , filler is charged in the gaps after the dicing. 
     As the first acoustic matching layer  49  operates as adhesive to overlay the second acoustic matching layer  50  thereon, the ultrasonic transducer array  21  can be fabricated easily by reducing the number of steps in the fabrication in comparison with a known method in which adhesive agent is applied to form the second acoustic matching layer  50  after forming the first acoustic matching layer. Drop of the yield of the fabrication can be prevented. 
     In the present embodiment, the acoustic matching layers  49 - 51  are present. However, the acoustic matching layers can be two or four or more layers which can include one formed from a mixture of metal nano particles and adhesive resin. An increase in the number of the acoustic matching layers can reduce a difference in the acoustic impedance between those. It is possible to reduce a loss in the propagation owing to a decrease in the reflection in the ultrasonic waves on interfaces between the acoustic matching layers. 
     Example 5 
     An experiment was conducted for the embodiment. In Example 5, the ultrasonic transducer array  21  of Example 1 was repeated with a difference in using the structure of  FIG. 5 . 
     For the first acoustic matching layer  49 , the transducer piezoelectric layer  28  was coated with the resin containing silver nano particles at a thickness of λ/4, where λ is a wavelength of ultrasonic waves. For the second acoustic matching layer  50 , a film of epoxy resin dispersion of zirconia particles with acoustic impedance of 4 Mrayl was polished to have a thickness of λ/4, and was overlaid on the first acoustic matching layer  49 . After this, the second acoustic matching layer  50  was hardened by heating for one (1) hour at approximately 190 deg. C. in the atmosphere. The first acoustic matching layer  49  had acoustic impedance of approximately 14 Mrayl. 
     Example 6 
     Example 2 was repeated with a difference in using the structure of  FIG. 5 . 
     Comparative Example 3 
     Comparative example 1 was repeated with the following differences. 
     The first acoustic matching layer  49  was not present. Instead, a film of metal was formed on the transducer piezoelectric layer  28  and was an upper electrode. 
     For the second acoustic matching layer  50 , epoxy resin dispersion of zirconia particles was used, had acoustic impedance of 8 Mrayl, was polished to have a thickness of λ/4, and was overlaid on the transducer piezoelectric layer  28 . 
     For the third acoustic matching layer  51 , a film of epoxy resin was used, had acoustic impedance of 3 Mrayl, was polished to have a thickness of λ/4, and was overlaid on the second acoustic matching layer  50  by use of epoxy adhesive agent of a thermoset type. 
     Comparative Example 4 
     A film of metal was formed on each of both surfaces of the transducer piezoelectric layer  28  by sputtering Ti, Pt and Au one after another. Then a proximal side of the transducer piezoelectric layer  28  having the metal film was attached to the flexible printed circuit board (FPC) on the backing material  26  by adhesion. To this end, adhesive agent was used, which contained silver nano particles, and was thermally hardened by heating at approximately 100 deg. C. for one (1) hour in the atmosphere. Acoustic impedance of the transducer piezoelectric layer  28  was approximately 31 Mrayl. 
     In Comparative example 4, an upper electrode was disposed instead of the first acoustic matching layer  49 . A film of metal was formed on the transducer piezoelectric layer  28  and was the upper electrode. On the electrode, an acoustic matching layer of glass was overlaid. The acoustic matching layer of glass had acoustic impedance of 14 Mrayl, was polished to have a thickness of λ/4, and was overlaid on the transducer piezoelectric layer  28 . Epoxy adhesive agent of a thermoset type was used, and hardened by heating. In general, the acoustic matching layers are diced for shaping as required for the purpose. It was difficult in the acoustic matching layer of glass to dice, due to low working precision according to slipping of a blade for dicing. Also, polishing was very difficult for the acoustic matching layer of glass, to lower the yield of fabrication. 
     For the second acoustic matching layer  50 , epoxy resin dispersion of zirconia particles was used, had acoustic impedance of 4 Mrayl, was polished to have a thickness of λ/4, and was overlaid on an acoustic matching layer of glass. Epoxy adhesive agent of a thermoset type was used, and hardened by heating. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                   
                   
                 Comparative 
                 Comparative 
               
               
                   
                 Example 5 
                 Example 6 
                 example 3 
                 example 4 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Transducer 
                 piezo- 
                 piezo- 
                 piezo- 
                 piezo- 
               
               
                 piezo- 
                 electric 
                 electric 
                 electric 
                 electric 
               
               
                 electric 
                 ceramic 
                 relaxor 
                 ceramic 
                 ceramic 
               
               
                 layer 28 
                 PZT, 
                 single 
                 PZT, 
                 PZT, 
               
               
                   
                 31 Mrayl 
                 crystal, 
                 31 Mrayl 
                 31 Mrayl 
               
               
                   
                   
                 22 Mrayl 
               
               
                 First 
                 resin 
                 resin 
                 x 
                 glass, 
               
               
                 acoustic 
                 containing 
                 containing 
                   
                 14 Mrayl 
               
               
                 matching 
                 silver nano 
                 silver nano 
               
               
                 layer 49 
                 particles, 
                 particles, 
               
               
                   
                 14 Mrayl 
                 11 Mrayl 
               
               
                 Second 
                 epoxy resin 
                 epoxy resin 
                 epoxy resin 
                 epoxy resin 
               
               
                 acoustic 
                 dispersion 
                 dispersion 
                 dispersion 
                 dispersion 
               
               
                 matching 
                 of zirconia 
                 of zirconia 
                 of zirconia 
                 of zirconia 
               
               
                 layer 50 
                 particles, 
                 particles, 
                 particles, 
                 particles, 
               
               
                   
                 4 Mrayl 
                 4 Mrayl 
                 8 Mrayl 
                 4 Mrayl 
               
               
                 Third 
                 epoxy 
                 epoxy 
                 epoxy 
                 epoxy 
               
               
                 acoustic 
                 resin, 
                 resin, 
                 resin, 
                 resin, 
               
               
                 matching 
                 2 Mrayl 
                 2 Mrayl 
                 3 Mrayl 
                 2 Mrayl 
               
               
                 layer 51 
               
               
                   
               
            
           
         
       
     
     Table 3 above indicated a relationship between materials for layers in Examples 5 and 6 and Comparative examples 3 and 4, and values of acoustic impedances (Mrayl). 
     The first acoustic matching layer  49  of Examples 5 and 6 was formed from resin containing silver nano particles. The acoustic impedance of the first acoustic matching layer  49  was 14 Mrayl in Example 5, and was 11 Mrayl in Example 6. In contrast, the first acoustic matching layer  49  was not present in Comparative example 3. In Comparative example 4, the acoustic matching layer of glass was present instead of the first acoustic matching layer  49 , and had acoustic impedance of 14 Mrayl. 
     The second acoustic matching layer  50  in any of Examples and Comparative examples was formed from epoxy resin dispersion of zirconia particles. The acoustic impedance of the second acoustic matching layer  50  was 8 Mrayl in Comparative example 3, and was 4 Mrayl in the remaining Examples and the remaining Comparative example. The third acoustic matching layer  51  in any of Examples and Comparative examples was formed from epoxy resin. The acoustic impedance of the third acoustic matching layer  51  was 3 Mrayl in Comparative example 3, and was 2 Mrayl in the remaining Examples and the remaining Comparative example. 
     Results of measurement of the sensitivity (dB) and a fractional bandwidth (%) of the ultrasonic transducer array  21  fabricated according to Examples 5 and 6 and Comparative examples 3 and 4 were indicated in Table 4. The sensitivity was a value representing a ratio of the voltage between the input and output, and was higher according to nearness to zero (0). The fractional bandwidth was a value 100 times a ratio obtained by dividing the band width (Hz) by the center frequency for the sensitivity of −0.6 dB (half of the ratio of the voltage between the input and output). The center frequency was approximately 5 MHz for any of Examples 5 and 6 and Comparative examples 3 and 4. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                   
                 Fractional 
               
               
                   
                 Sensitivity (dB) 
                 bandwidth (%) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Example 5 
                 −37.7 
                 69.2 
               
               
                   
                 Example 6 
                 −35.5 
                 70.5 
               
               
                   
                 Comparative example 3 
                 −41.0 
                 68.7 
               
               
                   
                 Comparative example 4 
                 −38.6 
                 65.3 
               
               
                   
                   
               
            
           
         
       
     
     As a result, the sensitivity of Example 5 was still higher than that of Comparative example 3 in spite of nearness in the fractional bandwidth. Note that the number of the acoustic matching layers in Example 5 was two similar to Comparative example 3 on the condition that the first acoustic matching layer  49  was not counted as an acoustic matching layer. Thus, it was possible according to Example 5 to fabricate the ultrasonic transducer array  21  with very high sensitivity even with the same number of steps in the fabrication as Comparative example 3. 
     It was found in Example 5 that the sensitivity was higher by 1 dB than Comparative example 4, under the condition in that the number of the acoustic matching layers in Example 5 was three similar to Comparative example 4 while the first acoustic matching layer  49  was counted as an acoustic matching layer. This was because an adhesive layer was formed to overlay the acoustic matching layer of glass and the second acoustic matching layer  50 , and results in unacceptable effects. In Example 5, however, no adhesive was used to overlay the first and second acoustic matching layers  49  and  50 . 
     It was found in Example 6 that the sensitivity was higher by 2 dB than Example 5. This was because the piezoelectric relaxor single crystal was used with the lower acoustic impedance than the piezoelectric ceramic PZT used in Example 5. 
     It was possible to construct a multi layer structure for an acoustic matching layer easily owing to the first acoustic matching layer  49  operating also as an upper electrode. Sensitivity was higher than a known transducer in view of the same number of acoustic matching layers. 
     Although the present invention has been fully described by way of the preferred embodiments thereof with reference to the accompanying drawings, various changes and modifications will be apparent to those having skill in this field. Therefore, unless otherwise these changes and modifications depart from the scope of the present invention, they should be construed as included therein.