Abstract:
An ultrasound transducer array ( 250 ) in which the piezoelectric layer ( 256 ) and the matching layer(s) ( 258 ) have different sub-dicing. In one embodiment, the piezoelectric layer ( 256 ) is diced only once and the matching layer(s) ( 258 ) is diced more than once. A resulting transducer shows improved bandwidth, crosstalk and noise performance.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of Provisional Patent Application Ser. No. 60/084,506, entitled “A Method to Build High Bandwidth, Low Crosstalk, Low EM Noise Transducer,” filed May 6, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to transducers, and particularly, to improved ultrasound transducer arrays. 
     2. Description of the Related Art 
     Ultrasound machines are used to non-invasively obtain image information about the structure of an object which is hidden from view and has become widely known as a medical diagnostic tool. As shown in FIG. 1, medical ultrasonic transducer arrays conventionally are fabricated from a block of ceramic piezoelectric material within which individual elements are defined and isolated from each other by sawing at least partially through the block of piezoelectric material making a number of cuts with a dicing saw. In particular, as shown in FIG. 1 a transducer array  100  includes a layer  104  of transducer elements  104   a - 104   f . The transducer elements  104   a - 104   f  are laid down on a backing medium  102 . The backing medium  102  serves to support the transducer structure. One or more acoustical matching layers  106  (including elements  106   a - 106   f ) and  107  (including elements  107   a - 107   f ) may be laid down on top of the transducer layer  104 . The piezoelectric layer  104  may be formed of any piezoelectric ceramic material such as lead zirconate titanate (PZT). The matching layers  106 ,  107  and the transducer layer  104  may be glued to one another using an epoxy, such as Der332. The layers are then diced by forming kerfs  110   a - 110   e  with a standard dicing machine. Typically the kerfs  110   a - 110   e  are made both in the direction parallel to the paper and perpendicular to the paper. 
     Typically, the ratio of the width to the thickness of the piezoelectric elements  104   a - 104   f  is optimized to about 0.5. The ratio of the width to thickness of the matching layers is typically ignored. As is generally known, the basic requirement for the transducers is high bandwidth, low pulse length, low crosstalk to the neighboring elements. However, if the width and thickness ratio of the matching layer is close to 1, lateral and thickness vibration mode will have a much stronger coupling, which in turn, will provide higher crosstalk, and unpredictable spectrum and pulse, which can degrade image quality. 
     Conventionally, the elements are sub-diced in order to change the width and thickness ratio of the ceramic piezoelectric material. For example, for the Siemens 5L40 transducer, the element is sub-diced once so that each sub-element width is around 116 micrometers and the thickness of the PZT element is about 256 micrometers. The resulting ratio of about 0.46 for the piezo-active layer results in a very good value for K T  (electromechanical coupling coefficient) and low coupling. However, for the matching layer the thickness is typically about 130 micrometers resulting in a ratio of about 0.91, leading to a relatively strong coupling between the thickness mode and the unwanted lateral mode. 
     SUMMARY OF THE INVENTION 
     These drawbacks of the prior art are overcome in large part by an ultrasound transducer array according to the present invention. According to one embodiment of the invention, the piezoelectric elements and the matching layer(s) are diced with different sub-dicing. In particular, according to one embodiment the PZT is sub-diced once but at the same time the first matching layer is sub-diced twice to obtain a more optimum ratio for the matching layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A better understanding of the present invention is obtained when the following detailed description is considered in conjunction with the following drawings in which: 
     FIG. 1 is a diagram of an exemplary ultrasound transducer array according to the prior art; 
     FIG. 2 is a diagram of an exemplary ultrasound transducer array according to an embodiment of the invention; 
     FIGS. 3A-3E illustrate formation of an ultrasound transducer array according to an embodiment of the invention; 
     FIGS. 4A-4D illustrate formation of an ultrasound transducer array according to another embodiment of the invention; 
     FIG. 5 illustrates an exemplary ultrasound transducer element according to an embodiment of the invention; and 
     FIGS. 6A-6D,  7 A- 7 D,  8 A- 8 D and  9 A- 9 D illustrate test results comparing performance of an exemplary ultrasound transducer array according to an embodiment of the invention with prior transducer arrays. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 2, a diagram illustrating a transducer array  250  in accordance with an embodiment of the present invention is shown. In particular, the array includes an interconnecting circuit or flexible circuit  254  disposed upon a support structure or backing block  252 . As is known, the flexible circuit  254  serves to provide the respective signal electrodes and corresponding traces or leads. The flexible circuit  254  generally has a plurality of adjacent traces or leads (not shown) extending from opposite sides of the block. The flexible circuit  254  may be made of a copper layer bonded to a polyimid material, typically a KAPTON-Flexible circuit, manufactured by Sheldahl of Northfield, Minn. The material forming the backing block  252  may be acoustically matched to the flexible circuit  254 . Further, the acoustic impedance of the flexible circuit  254  is approximately equal to that of the epoxy material for gluing the flexible circuit to the backing block. 
     A plurality of piezoelectric elements  256   a - 256   c  are disposed upon the flexible circuit  254 . Kerfs  260 ,  262  separate the piezoelectric elements  256   a - 256   c . According to an embodiment of the present invention, a matching layer of elements  258   a - 258   f  is provided on top of the piezoelectric elements  256   a - 256   c . As shown, the matching layer elements  258   a - 258   f  are diced smaller than the underlying piezoelectric elements  256   a - 256   c . Thus, additional kerfs  264   a-c  separate the elements  258   a ,  258   b , the elements  258   c ,  258   d  and the elements  258   e ,  258   f , respectively. 
     FIG.  3 A-FIG. 3E illustrate a method for producing an ultrasound transducer array according to an embodiment of the present invention. In particular, as shown in FIG. 3A, an interconnecting circuit or flexible circuit  204  is provided on a support structure or backing block  202 . As shown in FIG. 3B, a piezoelectric layer  206  is disposed on the flexible circuit  204 . The backing block  202 , the flexible circuit  204  and the piezoelectric layer  206  may be glued to one another by use of a known epoxy adhesive. The epoxy adhesive is placed between the backing block between the flexible circuit and the piezoelectric layer. The layers are secured to one another by affixing all layers together and applying pressure to the layers. 
     As shown in FIG. 3C, the piezoelectric layer  206  is diced by forming kerfs  208   a - 208   c  therein with a standard dicing machine. As a result of the dicing operation, a plurality of transducer elements  212   a   14   212   d  are formed. 
     Next, as shown in FIG. 3D, one or more matching layers  214  may be laminated in a known manner on top of the diced piezoelectric layer  206 . Finally, as shown in FIG. 3E, the matching layer or layers  214  are diced by introducing kerfs  218   a - 218   d . Further, cuts coincident with kerfs  208   a - 208   c  may be introduced. 
     An alternate method for producing a low crosstalk, low EM noise ultrasound transducer, according to the present invention, is shown in FIGS. 4A-4D. A substrate  4000  as shown in FIG. 4A includes a thin matching layer  4002  bonded to a PZT layer  4004 , a flexible circuit layer  4006  and a thin backing layer  4008 . The thin backing layer  4008  may be about 0.15 mm. 
     In FIG. 4B, a series of kerfs  4010   a ,  4010   b , and  4010   c  are cut into the substrate from the thin backing layer  4008  side. The kerfs  4010   a - 4010   c  are extended to the top of the PZT layer  4004 . In the next step, as illustrated in FIG. 4C, the array substrate may be flipped over to expose the front surface for the matching sub-dicing cut. That is, the matching layer  4002  may be sub-diced to result in kerfs  4012   a ,  4012   b ,  4012   c  and  4012   d . As will be discussed in greater detail below, standard kerf filling material (not shown) or other known methods may be employed to hold the elements together during this process. In addition, cuts coincident with kerfs  4010   a - 4010   c  may be made. Finally, as shown in FIG. 4D, a thick backing layer  4014  is applied to the thin backing layer  4008 . 
     As noted above, kerf filling may be desirable between the dicing steps described above with regard to FIG.  4 . In particular, the standard DC734RTV filling material could be used for kerf filling as well as to line the thick backing  4014 . Alternatively, a thin (3 micron) barrier material may be used between the DC734RTV and epoxy used to bond the thick backing. If air kerfs are desired, they may be obtained by bonding the barrier material with thin sheets to the diced surfaces and a thick backing bonded to the barrier material. Alternatively, the thick backing  4014  may be bonded to the thin backing using a thin adhesive. Furthermore, if the PZT layer or the first conductive matching layer were not diced completely through, a fully covered grounded plane for the array which would reduce the EM noise level compared to a conventional transducer array would result. 
     A closeup of an exemplary element of a transducer array, in accordance with the present invention, is shown in FIG.  5 . In particular, the element  500  includes a backing material  502  which is cut for a 200 μm backing layer portion  504 . A 25 μm flexible circuit  505  is then provided. A PZT layer  506 , about 175 μm wide and 370 μm thick, is then added. Next are first and second matching layers  508 ,  510 , respectively. According to one embodiment, the first matching layer  508  is about 190 μm thick, and the second matching layer is about 78 μm thick. A kerf  512  separates the matching layer elements. Finally, an ultrasound transducer lens  514  is applied to the top of all of the elements in the array. 
     The efficacy of the use of matching layers having different sub-dicing than the PZT layer has been experimentally demonstrated. In particular, a transducer array according to the present invention (e.g., as shown in FIG. 5) was tested for “acceptance angle” in comparison with the Siemens 3.5 MHz phase array and another manufacturer&#39;s 3.5 MHz phase array. The acceptance angle is the −6 dB relative amplitude frequency for a two-way pin target angularly displaced from the transducer. 
     FIGS. 6A-6D illustrate the results for the test low crosstalk transducer. In particular, FIG. 6A illustrates the detected amplitude as a function of frequency. As can be seen, the angle at which the relative amplitude is −6 dB is 52°. A similar diagram (FIG. 8A) is shown for the non-modified case. As shown, the acceptance angle there is ˜28°. Finally, the result for the other manufacturer&#39;s array is shown in FIG.  9 A. There, the −6 dB acceptance angle is 48.84°. 
     FIGS. 6B,  8 B and  9 B illustrate the −12 dB and peak frequency curves for each angle for each of the tested transducers. FIGS. 6C,  8 C and  9 D illustrate the acceptance angle for several frequencies in 1 MHz steps. FIGS. 6D,  8 D and  9 D illustrate the frequency spectra from 0-60 in 10 degree steps. As can be seen, the spectrum for the test device remains the same over a range of frequencies. 
     The results are summarized in Table 1, below: 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 ACCEPTANCE ANGLE 
               
             
          
           
               
                   
                   
                 −12 dB Center 
                 −12 dB Center 
                 −12 dB Center 
               
               
                   
                 ACCEPT 
                 Frequency at 
                 Frequency at 
                 Frequency at 
               
               
                 TYPE 
                 ANGLE 
                 0° (MHz) 
                 20° (MHz) 
                 40° (MHz) 
               
               
                   
               
             
          
           
               
                 Low 
                 52.09 
                 4.0 
                 3.8 
                 3.7 
               
               
                 crosstalk 
               
               
                 #1 
               
               
                 Standard 
                 27.97 
                 4.0 
                 3.8 
                 3.5 
               
               
                 3.5 MHz 
               
               
                 array 
               
               
                 Other 
                 44.24 
                 3.9 
                 3.85 
                 3.75 
               
               
                 manuf. 
               
               
                 Array 
               
               
                   
               
             
          
         
       
     
     Finally, FIG. 7 illustrates the waveforms at various angles for the test element. Thus, FIG. 7A illustrates the pulse at 0°; FIG. 7B illustrates the pulse at 10°; FIG. 7C illustrates the pulse at 20°; and FIG. 7D illustrates the pulse at 30°. As can be seen, the pulse remains substantially the same over the entire range of frequencies.