Abstract:
Techniques for fabricating high frequency ultrasound transducers are provided herein. In one embodiment, the fabrication includes depositing a copperclad polyimide film, a layer of epoxy on the copperclad polyimide film, and a polyvinylidene fluoride film on the epoxy. The assembly of materials are then pressed to bond the polyvinylidene fluoride film to the copperclad polyimide film and to form an assembly. The polyvinylidene fluoride film being one surface and the copperclad polyimide film being the other surface. The area behind the copperclad polyimide film surface is filled with a second epoxy, and then cured to form an epoxy plug.

Description:
PRIORITY AND RELATED APPLICATION 
   This application claims priority to U.S. Provisional Patent Application Ser. No. 60/574,094, filed on May 25, 2004, entitled “Design and Fabrication of a 40-MHZ Annular Array Transducer,” which is hereby incorporated by reference in its entirety. 

   FIELD OF THE INVENTION 
   The present invention is directed to design and fabrication of high frequency ultrasound annular array transducers. 
   BACKGROUND OF THE INVENTION 
   The field of high-frequency ultrasound (“HFU”) imaging, using frequencies above 20 MHz, is growing rapidly as transducer technologies improve and the cost of high bandwidth electronic instrumentation decreases. Single element focused transducers, however, are currently used for most HFU applications. These single element transducers are limited in their application due to their inherent small depth of field, which limits the best image resolution to a small axial range close to the geometric focus of the transducer. 
   HFU transducers primarily utilize single element focused transducers fabricated with polyvinylidene fluoride (“PVDF”) membranes as their active acoustic layer. These transducers are relatively simple to fabricate but suffer from a fairly high two-way insertion loss (≈40 dB) because of the material properties of PVDF. As a result, methods have focused on improving the insertion loss by optimizing the drive electronics and electrical matching. Single element PVDF transducers continue to be the primary transducer choice for HFU applications and have been fabricated using a ball-bearing compression method. 
   Similarly, methods of fabricating single element HFU transducers using ceramic material have been refined. A number of ceramic devices have been fabricated successfully to operate in the HFU regime. Ceramic devices have an inherent advantage over PVDF based transducers because of their low insertion loss. Ceramic materials, however, are typically used for flat arrays because they are difficult to grow or to press into curved shapes. Fabricating HFU ceramic transducers into concave shapes is known in the art through the use machining, coating, lapping, laminating and/or heat forming techniques for bonding and shaping curved transducers. These known fabrication techniques are used to construct single element transducers, and are not used to construct an array transducer. 
   Both PVDF and ceramic transducers have been used to great success for ophthalmic, dermatological, and small animal imaging. Current methods aim to fabricate individual array elements on the order of λ/2; these small dimensions necessitate advances in interconnects and electronics to fully implement the technologies. Accordingly, there exists a need for a technique for the feasible design and fabrication of a high frequency annular array transducer. 
   SUMMARY OF INVENTION 
   It is an object of the present invention to provide a HFU transducer with large bandwidth, providing fine scale axial resolution, and small lateral beamwidth, which permits imaging with resolution on the order of a wavelength. An array transducer permits electronic focusing that both improves the depth of field of the device and permits a two-dimensional image to be constructed, and with a relatively limited number of elements. 
   It is a further object of the present invention to construct, bond, and form a concave annular array transducer out of an active piezoelectric material, polyimide film, and epoxy using a ball-bearing compression method. 
   It is yet another object of the present invention that the active piezoelectric material of the transducer can be polyvinylidene fluoride (“PVDF”). PVDF is an advantageous material for fabricating high frequency transducers because the material can be press fit into a curved shape. PVDF also provides a better acoustic impedance match to water and biological tissue. 
   It is a further object of the present invention to demonstrate the feasibility of a new method to construct PVDF based annular arrays. 
   In order to meet these objects and others that will become apparent with respect to the disclosure herein, the present invention provides techniques for fabricating high frequency ultrasound multiple ring focused annular array transducers. In one embodiment, the fabrication includes depositing a copperclad polyimide film, a layer of epoxy on the copperclad polyimide film, and a PVDF film on the epoxy. The assembly of materials are then pressed to bond the polyvinylidene fluoride film to the copperclad polyimide film, and to form an assembly. The PVDF film being one surface and the copperclad polyimide film being the other surface. The area behind the copperclad polyimide film surface is filled with a second epoxy, and then cured to form an epoxy plug. 
   Advantageously, the active acoustic element of the transducer is a PVDF film with one side coated in gold and acting as the ground plane. A positive array pattern of the transducer is formed on a copper clad polyimide film (“flex circuit”). The flex circuit and PVDF are bonded together, press fit into a spherical shape, and then back filled with epoxy. Transducer performance can be characterized by measuring pulse/echo response, two-way insertion loss, electrical cross talk, and the complex electrical impedance of each array element before and after complex impedance matching. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram which illustrates a positive array pattern of a high frequency annular array transducer. 
       FIG. 2  is an assembly view which illustrates a press fit device used to assemble a high frequency annular array transducer. 
       FIG. 3  is a plan view which illustrates the electrical traces and contact pads of the positive array pattern portion of the high frequency annular array transducer. 
       FIG. 4  is a plan view which illustrates electronic access to the transducer annuli through a customized printed circuit board connected to the array pattern of the transducer. 
       FIG. 5  is an assembly view which illustrates a high frequency transducer. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1 , an exemplary positive array pattern of a transducer is shown. The circuit patterns are designed as positive images with a computer-aided design (“CAD”) software package. QuickCAD is used in a preferred embodiment, which is commercially available from Autodesk Inc. The transducer has an aperture  110  with a number of equal area rings, known as annuli  140 , and separated by a designated annuli spacing  150  between the annuli  140 . In a preferred embodiment, the transducer has a total aperture of 9 mm with five equal area rings separated by 100 μm spacings. Transducer electrical traces  155  permit access to each annulus, and can have the same designated spacing as the annuli spacing  150  between the annuli  140 . In a preferred embodiment, the electrical traces that permit access to each annulus and the spacing between the traces are 100 μm. 
   From the CAD file, a transparent film with a positive array image is generated by a commercial offset print shop. This method of creating the positive image permits line widths and spacings of smaller than 100 μm. 
   The array pattern  100  is formed on a material commonly used to fabricate flex circuits, such as for example, single sided copper clad polyimide film. In a preferred embodiment, the single sided copper clad polyimide film is RFlex 1000L810, which is commercially available from Rogers Corp. located in Chandler, Ariz. or any equivalent supplier. In the preferred embodiment, the polyimide film is 25-μm thick, the copper is 18-μm thick, and an adhesive layer bonding the copper to the polyimide is 20-μm thick. Before creating the array pattern  100 , the polyimide is coated with a uniform thickness of positive photoresist, which is commercially available from Injectorall located in Bohemia, N.Y. or any equivalent supplier. 
   The copper array pattern  100  is fabricated onto the flex circuit using standard copper etching techniques. In a preferred embodiment, the positive array image is placed on top of the photoresist coated polyimide and exposed to ultraviolet (“UV”) light for 2-3 minutes in a UV fluorescent exposure unit, which is commercially available from AmerGraph located in Sparta, N.J. or any equivalent supplier. The polyimide is then transferred to a liquid developer, which removes the photoresist that is exposed to UV light. The developed film is agitated in a ferric chloride bath until all the copper in the areas lacking photoresist are etched away. 
   Once the array pattern  100  is fabricated, a microscope can be used to view the finished array pattern  100  to ensure that the line widths and spacings between the transducer electrical traces  155  are uniform and of the correct size. After removing the remaining photoresist, which can be done with steel wool or with acetone, the array pattern  100  should be tested for electrical continuity between the annuli  140  and copper contact pads  170 . Test patterns are used to ensure correct line width spacing for both annuli spacing  150  and transducer electrical traces  155 . And in a preferred embodiment, test patterns are utilized to ensure 100 μm spacing for both the ring separations and line widths. 
   Referring to  FIG. 2 , an annular array transducer is assembled using a press fit device and layers of material using compression to bond and form the assembly into a concave shape. In a preferred embodiment, the press fit device is constructed of aluminum. The press fit device shown in  FIG. 2  uses a base plate  210 , a pressure plate  260 , and a ball bearing  270  to apply uniform pressure to a polyvinylidene fluoride (“PVDF”) film  230 , epoxy  240 , and copperclad polyimide film  250 . A top plate  275  presses the ball bearing  270  into the PVDF  230 , epoxy  240 , and copperclad polyimide  250  assembly. The base plate  210  has a central hole  220  in which a tube  215  is inserted. In an preferred embodiment, the tube  215  is made of Teflon and the ball bearing  270  is made of stainless steel. 
   Assembly of the transducer begins by inserting a tube  215  into a baseplate  210 . A polyimide film  250 , on which an array pattern  100  is fabricated, is centered over the tube  215  with the copper side facing in a direction opposite to that of the base plate  210 , shown facing in the upward direction. An epoxy layer  240  is deposited onto the copperclad polyimide film  250  and array pattern. As used herein, “epoxy” is understood as including any resinous bonding agent. In a preferred embodiment, a single drop of Hysol RE2039 or HD3561 epoxy, which is commercially available from Loctite Corp. located in Olean, N.Y., is placed onto the array pattern. A PVDF film  230  is then deposited on the epoxy  240 . In a preferred embodiment, a 4 cm by 4 cm section of PVDF membrane, such as that commercially available from Ktech Corp. located in Albuquerque, N.Mex. or any equivalent supplier, is placed over the epoxy. The PVDF can be 9 μm thick and have one side metallized with gold, where the metallized side forms a ground plane of the transducer and should face in a direction opposite to that of the epoxy  240 . A ring  265  is placed over or on top of the PVDF film  230 , and clamped with a pressure plate  260 . The pressure plate permits the layers of material to move slightly while also stretching during the press fit, thus avoiding crinkling of the films at the edge of the transducer. In a preferred embodiment, the ring  265  can be made of Teflon. 
   A ball bearing  270  is pressed into the PVDF film  230  by applying pressure to a top plate  275  that is in contact with the ball bearing  270 . In a preferred embodiment, the ball bearing  270  is made of stainless steel and has an outside diameter of 18 mm. The PVDF film  230  and the copperclad polyimide film  250  are bonded together with the epoxy  240 , and formed to have a spherically curved shape comprising a concave surface  290  and a convex surface  285 . After compression, epoxy is deposited in the tube  215 , such that a plug of epoxy  225  fills the area behind the convex surface  285  of the copperclad polyimide film  250 . The assembly can then be placed into a vacuum chamber to ensure bubbles are not present on the backside of the copperclad polyimide film  250 . In a preferred embodiment, the press fit device is turned over and the Teflon tube is filled with epoxy. The whole press fit device is then placed into a vacuum chamber at approximately 8 Torr. The degassing lasts as long as necessary to ensure that no bubbles are present on the backside of the polyimide, which is approximately 40 minutes. 
   In an exemplary embodiment, the epoxy plug has an outside diameter of 13 mm, while the active array has an outside diameter of 6 mm. The wider epoxy plug ensures a more spherically curved transducer face and avoids crinkles at the edge of the transducer. 
   After degassing, cure time of the epoxy plug  225  can be reduced by placing the assembled transducer into an oven. In a preferred embodiment, after the degassing process the press fit device is moved into a 40 degree Celsius oven to reduce the epoxy cure time. When the epoxy cures, the transducer is separated from the tube  215 . The resultant transducer assembly includes an epoxy plug  225  bonded to the convex surface  285  of the copperclad polyimide film  250 . Referring to  FIG. 3 , the electrical traces and their contact pads remain exposed by trimming away any excess material. 
     FIG. 5  illustrates an exemplary embodiment, where an epoxy  510 , such as silver epoxy EE129-4 which is commercially available from Epoxy Technology located in Billerica, Mass. or any equivalent supplier, is used to join the conductive side of the PVDF film  230  to a ground connection via the metal cap  530  and metal connector  520 . The metal cap  530  and metal connector can comprise two separate units, or be constructed as a single unit. In an alternative embodiment, the ground connection can also be made by joining the conductive side of the PVDF film to ground traces on the polyimide. 
   Referring to  FIG. 4 , in order to electronically access the annuli  140 , a customized printed circuit board (“PCB”)  410  can be fabricated to enable electronic access to the annuli  140  through the printed circuit board traces  470 . The PCB  410  has a connector  420  on one side and a series of smaller connectors  430  on the opposing side. Cables  440  are connected to each of the smaller connectors  430 . An additional advantage of the PCB  410  is that surface mount inductors  480  can be soldered directly onto the PCB  410  for impedance matching. The inductors shown in  FIG. 4  are connected in series to the printed circuit board traces  470 , but can also be in parallel to the printed circuit board traces  470 . A mounting bracket made from aluminum rod can hold the transducer  460  and PCB  410 . The polyimide film  450  is then wrapped around and inserted into the connector  420 . Thus, the PCB  410  enables electronic access from the cables  440  to the PCB traces  470  through a series of connectors  430 . The PCB traces  470  are electronically connected to the transducer electrical traces  155  through a connector  420 . The transducer electrical traces  155  are electronically connected to the annuli  140 . 
   In a preferred embodiment, the first connector  420  is a 20-pin zero insertion force (“ZIF”) connector, which is commercially available from Hirose Electric located in Simi Valley, Calif. or any equivalent supplier. The smaller connectors  430  are miniature MMCX-BNC connectors, which are commercially available from Amphenol or any equivalent supplier. The Cables  440  are BNC cables, such as RG-174 50 Ohms of 0.87 meters length. 
   In an exemplary embodiment, prior to applying the press fit technique described above, an adhesive material such as tape can be applied to the electrical traces located on the polyimide film. This prevents the epoxy from adhering to the polyimide films, allowing the polyimide film to flex after the fabrication process without breaking the electrical traces. Similarly, an adhesive material such as tape can be placed on the polyimide traces leading out to the ZIF connector&#39;s contact pads, and removed subsequent to fabrication. The polyimide film is held in position with an adhesive material such as tape and centered over the Teflon ring. The adhesive material is removed after the pressure plate is secured but before the press fit is applied. Once the top plate is secured and the ball bearing has been pressed into the assembly, the screws holding the pressure plate can be loosened. A copper conductive adhesive material such as copper conductive tape is positioned on the backside of the PCB in order to form a ground plane and reduce electrical noise. 
   In a preferred embodiment, the results from a piezoelectric transducer modeling software package, such as PiezoCAD that is commercially available from Sonic Concepts located in Woodinville, Wash. or any equivalent supplier, is used to determine the best impedance matching for maximizing the two-way pulse/echo response. Based on the model results, an appropriate surface mount inductor is selected and soldered directly onto the PCB board. The complex impedance can again be measured to ensure that the reactance at the center frequency is in fact zero. Impedance matching eliminates the complex component at a desired frequency for better transducer efficiency. 
   In an exemplary embodiment, a 5-ring annular array transducer is fabricated with equal area elements and 100 μm spacing between the annuli. The total transducer aperture is 9 mm and the radius of curvature is also 9 mm. The inner and outer radii of the annuli when projected onto a plane are 0, 1.95, 2.05, 2.81, 2.90, 3.47, 3.56, 4.02, 4.11 and 4.50 mm. The projected spacings between elements can sometimes be slightly less than 100 μm because the initial pattern is designed as a planar layout and then press fit into a spherical curvature. 
   In an exemplary embodiment, impedance measurements are made of each annulus in order to determine the most efficient electrical matching. Based on piezoelectric transducer modeling, the transducer capacitance is matched with an inductor connected in parallel and located on the PCB. Parallel inductance is selected because it results in a larger improvement for the two-way insertion loss but with a decrease in bandwidth. All of the array elements can utilize the same matching inductance. When using a single matching inductance, however, the frequency at which the matched reactance occurs can vary somewhat for each ring. In a preferred embodiment, a value of 0.33 μH is calculated as the best matching at 40 MHz. In the ideal case the reactive component for each ring should be zero at 40 MHz. 
   In an exemplary embodiment, the total transducer aperture can be 6 mm with a geometric focus of 12 mm. In this embodiment, the inner and outer radii of the annuli when projected onto a plane are 0, 1.22, 1.32, 1.8, 1.9, 2.26, 2.36, 2.65, 2.75 and 3.0 mm. In this arrangement, the transducer capacitance is matched with an inductor connected in series and located on the PCB. The inductor value of 0.82 μH is calculated as the best matching at 40 MHz. 
   Impedance matching may also increase the pulse/echo response for the same excitation signal. An increase in pulse/echo sensitivity can be achieved at the cost of reduced bandwidth. Impedance matching also improves the two-way insertion loss over the unmatched case. 
   PVDF based annular arrays can be constructed using a copper clad polyimide film to form the array electrode pattern. After impedance matching, the performance of the array elements should be similar to what has been reported for single element PVDF transducers. 
   Those of ordinary skill in the art will appreciate that the foregoing discussion of certain embodiments and preferred embodiments are illustrative only, and does not limit the spirit and scope of the present invention, which is limited only by the claims set forth below.