Abstract:
A transducer having a ceramic element in which the ceramic is elevated above a polymer and a method of manufacturing the transducer. The transducer comprises a piezo-composite element comprising a ceramic element embedded in epoxy. In an array, the ceramic elements may be in the form of posts. The plurality of ceramic elements is slightly elevated above the polymer and in staggered arrangement with the polymer. The element is manufactured by first grinding the face of the composite and removing damaged ceramic by acid etching the ceramic. The epoxy is removed by plasma etching so that the ceramic is above the epoxy. The composite is sputter plated so that a maximum temperature that could damage the plating is not exceeded. The ceramic is then poled so that a maximum temperature that could damage the plating is not exceeded. Contacts are then attached to the plating adjacent the ceramic.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention is directed to a piezoelectric composite to which plating is applied and a method for applying such a plating to provide improved adhesion and reliability, and specifically to a ceramic piezoelectric composite ultrasonic transducer having an array of ceramic posts and a method for manufacturing such transducers.  
       BACKGROUND OF THE INVENTION  
       [0002]     Ultrasonic transducers incorporate one or more piezoelectric vibrators which are electrically connected to a pulsing-receiving unit in the form of an ultrasonic test unit. The piezoelectric transducer converts an electrical pulse from the pulsing unit and converts the electrical signal to a mechanical vibration which is transmitted through a material such as a metal to which it is coupled. The piezoelectric material has the ability to receive a mechanical vibration from the material to which it is coupled and convert it to an electrical pulse which is sent to the receiving unit. By tracking the time difference between the transmission of the electrical pulse and the receipt of the electrical signal and measuring the amplitude of the received wave, various characteristics of the material can be determined. The mechanical pulse is generally in the frequency range of about 0.5 MHz to about 25 MHz, so it is referred to as an ultrasonic wave from which the equipment derives its name. Thus, for example ultrasonic testing can be used to determine material thickness or the presence and size of imperfections within a material.  
         [0003]     Ultrasonic transducers can be used in pairs of send only and receive only units. However, frequently the transducers are transceivers that both send and receive the pulses. The transducer may be a single element or a single transducer may be comprised of a plurality of ceramic elements. This invention is broadly directed to the transducer or probe that is comprised of a ceramic element or elements and improved methods of manufacturing such transducers. Such transducers that utilize the composite ceramic include single element transducers, dual element transducers and arrayed (phased array) transducers.  
         [0004]     These transducers currently are produced by providing a single-piece ceramic material, such as a lead-zirconate-titanate (PZT) ceramic. This ceramic is processed to produce a plurality of spaced columns/posts or planes of preselected side projecting from a solid piece of the ceramic which is unaffected by the processing. This unaffected solid piece of ceramic is referred to as the ceramic backbone. This invention is more narrowly directed to improvements in processing of the ceramic element that is used in an ultrasonic probe, the probe including not only the ceramic, but also the matching layers, the backing, the case and the connectors.  
         [0005]     After the plurality of spaced columns or spaced planes, also referred to as a diced ceramic, has been formed, the spacing between the columns or planes is filled with an epoxy polymer. Sufficient epoxy polymer is applied to form a continuous layer of epoxy overlying the diced ceramic and opposite the ceramic backbone.  
         [0006]     The ceramic backbone is then removed by grinding. To assure complete removal of the ceramic backbone, the ceramic removal operation extends below the backbone and slightly into the diced ceramic, removing a small portion of each post or plane. This is not significant, as it is important to maintain a flat surface with a smooth surface finish. The surface standard for parallelism is 0.0002″ after grinding, and the surface finish is about 35,000 Angstrom units or smoother, typically between about 15,000 to about 35,000 Angstrom units. After the backbone has been removed, the workpiece is flipped over and the epoxy polymer is removed by grinding. Again, some small portion of each post may be removed, but it is important to maintain a flat surface with a smooth surface finish. It is not important whether the epoxy polymer or the ceramic backbone is removed first, although the processing is somewhat easier if the ceramic backbone is removed first. At this point in the processing, the workpiece comprises a plurality of ceramic posts embedded in an epoxy polymer. Both sides of the workpiece are then finish ground. After finish grinding, the ceramic posts are depressed typically from about 15,000 to about 30,000 Angstroms below the epoxy polymer. The depression of the posts can be reduced to 2000 Angstroms below the surface of the epoxy by an optional polishing step. A cross-section of a prior art multi-arrayed transducer after an optional polishing operation is shown in  FIG. 1  with a layer of plating  32  applied over its surface, depicting the ceramic posts  12  lying below the surface of the epoxy  24 .  
         [0007]     The ceramic is cleaned in an ultrasonic cleaner to remove any damaged ceramic. The power setting of the cleaners are adjustable, and the power setting is adjusted to a level at which plating on posts is not removed is not removed from the posts during cleaning. After cleaning, the ceramic is rinsed followed by plasma cleaning, the ceramic workpiece is sputter plated, and the plating is tested for adhesion. The ceramic workpiece is then dice-deactivated and poled to activate the ceramic.  
         [0008]     While this process can produce an effective transducer, there are problems associated with such transducers. These problems are associated with ceramic posts or planes being depressed below the surface of the epoxy. The sputter plating process provides a very thin plating over the surface. Total plating thickness is about 15000 Angstroms, which is applied by a line of sight process. Because the ceramic posts are depressed below the surface of the epoxy polymer, it is possible that the sputtering process may not provide a uniform coating of the surface, particularly along the perpendicular surfaces extending between the parallel planes of epoxy polymer and ceramic material. In addition, since the sputter plating operation is performed at temperatures of about 120° C. (about 250° F.), the epoxy is free to expand unrestrained above the ceramic posts or planes. Even though this expansion is small, because of the thinness of the plating deposited by the sputter plating process, it can be sufficient to damage the thin plating extending in the vertical direction along the epoxy polymer between the ceramic posts and the horizontal surface of the epoxy, causing poor performance of the ceramic, such as low capacitance. After sputtering, the ceramic is dice-deactivated and poled to activate the ceramic. The temperatures for poling can be in the range as high as about 100-110° C. After poling, contacts are soldered to the plating.  
         [0009]     Another problem with this configuration is that the depressed ceramic is difficult to solder. As a result, the solder heat is borne by the epoxy during the soldering process, causing it to expand, and further increasing the possibility that the thin plating may fracture, thereby causing bad solder connections.  
         [0010]     A transducer with a plurality of elements formed from a ceramic, which elements are not depressed below the polymer, would overcome many of the difficulties associated with the prior art transducers described above, but such a transducer and a method for fabricating such a transducer is heretofore unknown to the art.  
       SUMMARY OF THE INVENTION  
       [0011]     The present invention provides a transducer having a ceramic element in which the ceramic is elevated above a polymer. This favorable configuration of the transducer is produced as a result of the method of manufacturing the transducer. The transducer may comprise a piezo-composite element comprising a ceramic element embedded in epoxy, such as is found in a single element transducer. The transducer may comprise a piezo-composite element having a plurality of ceramic elements embedded in epoxy and, the ceramic elements separated from one another by a polymer. The plurality of piezo-ceramic elements may be as simple as a dual element transducer in which two ceramic elements are separated by a non-conductive polymer, a four element array of ceramic elements or it may comprise an array having a larger number of piezo-ceramic elements, each element similarly separated. For example a 50×50 array, having about 2500 elements, with each element being rectangular and having an edge of about 0.002 inches can readily be manufactured. In an array, the ceramic elements may be in the form of posts, each post being an emitter acting as a plurality of wave point sources when excited, or the ceramic elements may be in the form of strips, forming a planar interface, each strip being an emitter of a planar wave when excited. The plurality of ceramic elements is slightly elevated above the polymer and in staggered arrangement with the polymer. As a result, the face of the composite comprising the ceramic and epoxy does not comprise a truly planar arrangement. The face includes a conductive layer such as a noble metal applied over it. Each of the ceramic elements forming emitters is individually connected to the drive signal and to ground in parallel by contacts soldered to the conductive layer. By exciting the individual ceramic members simultaneously, acoustic performance is significantly improved as the present invention provides improved reliability in the electrical connection between the ceramic element(s) and the plating. In this configuration, a transducer incorporating the ceramic made in accordance with the present invention also provides a lower acoustic impedence and better power transfer, particularly to rough surfaces. When in post configuration, a transducer incorporating a ceramic made in accordance with the present invention will also provide improved resolution.  
         [0012]     The transducer of the present invention is formed by first providing a single-piece ceramic material having a first side and a second side. This ceramic is processed from the first side to produce a plurality of spaced columns/posts or planes of preselected size projecting from the second side, which is a solid piece of the ceramic forming a backbone unaffected by the processing. The spacing between the columns or planes is then filled with a nonconductive material such as a polymer. Sufficient polymer is applied to form a continuous layer overlying the diced ceramic and opposite the ceramic backbone. The ceramic backbone is then removed by grinding, the workpiece is flipped over, and the polymer is removed by grinding. A small portion of each post may be removed, as long as a flat surface with a smooth surface finish is provided. The workpiece comprising a plurality of ceramic posts embedded in a polymer is then finish ground. At this point the ceramic posts are depressed typically from about 15,000 to about 30,000 Angstroms below the polymer. The preparation techniques are virtually identical to the prior art techniques.  
         [0013]     Now, the workpiece is first etched in an acid solution. The acid solution is selected to preferentially attack the ceramic. After the workpiece has been etched for a sufficient time, the workpiece is then removed from the acid and cleaned with deionized water solution for a period of time sufficient to neutralize the acid. The workpiece is then dried with a non-reactive gas and the part is then plasma etched. The plasma etching etches the polymer so that it is depressed below the surface of the ceramic elements, exposing the ceramic posts. After plasma etching, the surface of the transducer is sputter-plated at temperatures below about 75° C. The ceramic elements are then dice de-activated, and then poled at temperatures below about 60° C. After poling, the contacts can be soldered to the metallized transducer elements in the conventional manner.  
         [0014]     A significant advantage of the present invention is that each of the plurality of ceramic elements forming the transducer of the present invention extends above the polymer matrix by as much as about 25,000 Angstroms, which provides a different surface than provided by the prior art transducers. If too much of the ceramic is exposed, it becomes susceptible to cracking.  
         [0015]     The above-mentioned advantage overcomes a series of problems associated with the prior art transducers, thereby producing additional advantages. Because the ceramic elements extend above the polymer matrix, the ceramic will constrain the expansion of the polymer. However because the sputter plating is performed at a much lower temperature than previous sputter plating operations, the expansion will not as severe, leading to less expansion and less stress applied to the thin, fragile plating.  
         [0016]     In a similar fashion, the dice de-activation and poling are at temperatures well below the prior art temperatures, so that these operations provide less expansion to the transducer materials, again resulting in less stress applied to the thin, fragile plating.  
         [0017]     Another advantage that results from the ceramic elements extending above the polymer matrix is that the heat resulting from the soldering of the contacts to the elements will result in the ceramic elements being subject to the heat of soldering. This is desirable, as ceramic materials can more readily react to heat than the polymer matrix. Ceramic materials typically have a low coefficient of thermal expansion, so this heat will have less of an effect on the fragile plating, which will experience lower stresses with lower expansion.  
         [0018]     Another advantage of the process of the present invention is that the acoustic layers are more readily adhered to ceramic posts projecting above the polymer. Acoustic layers are applied over the face of the ceramic. When the ceramic posts in an array are depressed below the surface of the polymer, it is difficult for the material adhering the acoustic layers to the face of the transducer to wet the top of the posts. Proper wetting of the posts is required in order to obtain efficient coupling between the very small posts that generate the ultrasound and the acoustic layer(s). Failure to achieve a good contact between the posts and the acoustic layer can result in a transducer having low gain. Such failures typically occur when the adhesive used to adhere an acoustic layer to the face of the ceramic failed to contact the top of the ceramic posts. Additionally, by providing a ceramic face having the ceramic posts projecting slightly above the polymer, it is possible to provide an electrical connection to the ceramic post by wedge-bonding rather than by soldering. Wedge bonding is not possible when the polymer projects above the ceramic posts, as the polymer typically is not sufficiently hard or strong to support the wedge bonding application.  
         [0019]     Another advantage of the present invention is that the acid etching of the ceramic material in an ultrasonic bath after the grinding operation attacks the grain boundaries of the ceramic posts. The acid is formulated to accomplish this, and combined with the ultrasonic vibrations, assists in removing damage areas from the ceramic posts, thereby providing a more structurally sound surface on which to adhere the subsequently applied sputter plating.  
         [0020]     Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]      FIG. 1  is a cross-sectional view of a prior art multi-arrayed transducer with a plating applied over its surface, depicting the ceramic posts lying below the surface of the epoxy.  
         [0022]      FIG. 2  is a cross-sectional view of a multi-arrayed transducer.  
         [0023]      FIG. 3  is a perspective view of ceramic posts after dicing.  
         [0024]      FIG. 4  is a cross-sectional view of a ceramic block filled with a non-conductive polymer.  
         [0025]      FIG. 5  is a front view of the transducer after finish grinding operations have been completed.  
         [0026]      FIG. 6  is a cross-sectional view of the workpiece of the present invention after plasma etching, depicting the ceramic posts lying above the surface of the epoxy.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]      FIG. 2  depicts a multi-arrayed transducer  10  that incorporates the piezo-electric composite of the present invention. The transducer is comprised of a plurality of piezo-composite ceramic posts  12 . Each ceramic post  12  is spaced from an adjacent ceramic post  12  by a nonconductive material  14 , such as a polymer. In a preferred embodiment, the polymer is an epoxy. The piezo-ceramic composite posts are characterized by their ability to vibrate when exposed to an electrical excitation, thereby generating a mechanical or sound wave, hereinafter referred to as an acoustic wave, and to generate an electrical pulse when excited by an incident acoustic wave. A preferred ceramic material is lead-zirconate-titanate (PZT), although other equivalent materials such as polycrystalline relaxor materials (PZN-PT materials) and biased electrorestrictor materials (PMN-PT ferroelastic relaxor materials) may be used as is known in the art. Each ceramic post  12  or element includes an electrical connection  16  to allow the elements to be connected in parallel with a device that includes a power source. The device typically is an ultrasonic test unit that, in addition to including a power source, includes an ability to condition signals received by the test unit and to allow the signals to be displayed. The ultrasonic test units are well known in the art and, while used in connection with the present invention, are not part of the present invention. The transducer  10  typically includes a facing material  18  that facilitates coupling the multi-arrayed transducer to a test piece.  
         [0028]     The piezo-composite ceramic posts  12  of the present invention are initially provided as a block of ceramic material of preselected size. The preselected size is chosen on the basis of the transducer size. The ceramic block is typically mechanically cut into a plurality of posts, yielding a two dimensional array of posts. The methods of performing such cuts are well-known and any acceptable method for cutting the block may be used. It is preferred that the ceramic block be cut using a dicing saw. The diced ceramic block  20  having the ceramic posts projecting upward form a ceramic backbone  22  is shown in  FIG. 3 , the spacing between the posts being the kerfs remaining after the block has been diced.  
         [0029]     The kerfs of the diced ceramic block  20  are then filled with a nonconductive material  14 , as depicted in  FIG. 4 , which is a cross-section of a block  20  filled with an epoxy  24  to provide structural support, particularly in the transverse direction under a shear load, for the thin and brittle ceramic posts  12 . As can be seen in  FIG. 4 , the epoxy  24  forms an epoxy backbone  26  that overlies the ceramic posts  12 , and fills the interstitial areas  28  between the posts.  
         [0030]     Both the ceramic backbone  22  and the epoxy backbone  26  must be removed. The ceramic backbone  22  and the epoxy backbone  26  typically initially are rough ground to remove the bulk of the material. It is desirable to rough grind both the ceramic backbone  22  and the epoxy backbone as close as possible to the ceramic posts without exposing the ceramic posts. However, sometimes the grinding may extend slightly below the backbones  22 ,  26 . The next mechanical operation is a finish grinding operation to produce a smooth, uniform surface. Each side of the epoxy-filled ceramic is subjected to the grinding operations. The finish grinding operation can be accomplished by any acceptable methods that produce a smooth uniform surface. Linear grinding, lapping and back grinding all are acceptable finish grinding steps.  FIG. 5  depicts the face of the finish ground transducer  30  after finish grinding.  FIG. 5  depicts the ceramic posts  12  surrounded by the epoxy  24 , the ceramic posts  12  being depressed about 15,000-30,000 Angstrom units below the surface of the epoxy  24 .  
         [0031]     In order to place the ceramic posts  12  in the same plane as the epoxy  24 , the finish ground workpiece is placed in an acid solution. The acid solution is selected to etch the piezo-ceramic composite posts as a pre-plating step. While the acid selected will depend upon the specific ceramic material used, a solution of HBF 4  and HNO 3  has been effective in etching PZT ceramic. More specifically, a preferred 2000 milliliter solution having about 200 milliliters of HNO 3  (50% concentrated acid by volume) and about 4 milliliters of HBF 4  (50% concentrated acid by volume) was mixed by adding the concentrated acids to about 1796 milliliters of water to yield a solution of about 0.1% HBF 4  by volume and about 5% HNO 3  by volume. This solution is exemplary of the solution utilized in the best mode for practicing the invention. It will be understood that other concentrations of the disclosed acids and other acids may be used, as long as the acid etches the grain boundaries of the ceramic grains. The workpiece is vibrated in this acid solution in an ultrasonic cleaner for a time sufficient to etch the ceramic posts. The preferred time for etching is about 30 seconds, although the etching time will depend on the concentration of the acid and the acids used. The time must be sufficient to etch the grain boundaries of the posts to facilitate removal of ceramic material which may have been damaged during the grinding operations. All ultrasonic cleaners described herein operate at 80 Khz, although the ultrasonic cleaners may be operated at different frequencies, so long as the required process step is successfully accomplished.  
         [0032]     After etching, the workpiece is removed from the cleaner and the acid is neutralized. The preferred method for neutralizing the acid is a multi-step wash with deionized water. The workpiece is first rinsed with deionized water for about two minutes. Then, the workpiece is then placed into a second ultrasonic cleaner having deionized water for a preselected period of time, about 3-4 minutes, after which it is spray dried, with a non-reactive gas such as nitrogen, although an inert gas may be used. In the preferred embodiment, dry filtered air is used to dry the workpiece.  
         [0033]     Next, the epoxy  24  is preferentially removed so that the ceramic posts  12  do not remain depressed below the epoxy  24 . The epoxy  12  is preferentially removed from the surface of the workpiece by plasma etching. Plasma etching of the epoxy  24  is accomplished with a high energy gas stream that removes the epoxy  24  from the surface of the workpiece, but does not adversely affect the ceramic posts  12 . Oxygen is the preferred plasma etching gas. During the plasma etching operation, the plasma stream may reach a temperature in the range of about 250° F.-290° F. The plasma etching was accomplished at a rate of about 1500 Angstroms per minute and continued for a sufficient length to time, to produce ceramic posts  12  having the required elevation over the epoxy. The amount of time required for the plasma etch will vary, depending upon the amount of epoxy on the workpiece after either finish grinding or polishing, a longer time being required for a greater amount of epoxy. Since etching the surface to achieve a plane including both the epoxy  24  and the ceramic posts  12  of a few atomic layers in thickness is extremely difficult to accomplish, the preferred embodiment etches the epoxy  24  sufficiently so that the ceramic posts  12  are slightly above the adjacent epoxy  24 , but no more than about 25000 Angstroms above the epoxy. This preferred configuration with the ceramic posts projecting above the epoxy is shown in  FIG. 6 . Both sides of the workpiece are plasma etched in this fashion.  
         [0034]     Immediately after the plasma etching process is completed, the workpiece is placed into the sputtering chamber, and the opposed faces of the etched workpiece are sputter plated at a maximum temperature of about 75° C. (167° F.) and more preferably at a maximum temperature of about 62° C. (144° C.). The selected temperature is related to the expansion of the polymer. If the temperature is too high, the expansion of the polymer is too great and the sputtered plating is adversely affected. For epoxy, the temperature is about 10° C. to about 15° C. above the t g  (glass transition temperature) of the epoxy. As is clear, the maximum sputtering temperature will vary from polymer composition to polymer composition, as the expansion/contraction of the polymer is the determining factor affecting the plating. The combination of the lower sputtering temperature of the atoms, the etching of the ceramic posts, and the ceramic posts  12  being above or about at the same level as the epoxy provides better adhesion of the plating to the surface of the workpiece. Because of the differences in thermal expansion between the ceramic posts, the epoxy and the metallic materials comprising the plating, avoiding the elevated temperatures of the prior art processing and maintaining the plating process at 75° C. and below reduces thermally induced stresses in the very thin plating as the workpiece cools, thereby providing a workpiece having a higher reliability, since the probability of failure due to plating failure is reduced. The plating is preferably applied as a trilayer of titanium, palladium and silver. The titanium is applied as a first layer to a thickness of about 300 to about 600 Angstrom units under a vacuum of about 1.5 mTorr. The palladium layer is applied over the titanium to a thickness of about 2000-3000 Angstroms under a vacuum of about 4 mTorr. The silver layer is applied over the palladium layer to a thickness of about 9,000-12,000 Angstroms under a vacuum of about 4 mTorr. In order to maintain the temperature at 60° C. or below, it is necessary to sputter at a current of about 500 milliamps, which is carefully controlled. The initial voltage was 408 volts, but the potential in not carefully controlled and will vary depending upon the target material. However, to avoid exceeding the maximum temperature, it is necessary to sputter the silver in a plurality of stages. At the above-noted amperage and voltage, four separate plating stages of 10 minutes with an intervening period of time for cooling is required.  
         [0035]     After sputter plating, a plated side of the workpiece is poled to activate the piezo-ceramic material at a temperature of up to about 60° C. (140° F.). Preferably the plating temperature is maintained at a temperature below about 60° C. Poling entails inducing a high voltage field across the ceramic. The ceramic is immersed in a dielectric fluid to prevent arcing. The present invention accomplishes poling at a temperature below about 60° C., which is a lower temperature than the prior art poling temperature of 110° C. recommended by the manufacturer of PZT. The present invention also accomplishes poling at a higher voltage/unit thickness, up to about 170 volts (V) per 0.001″, which is also higher than the voltage of p to a bout 150 volts per 0.001″ recommended by the manufacturer of PZT. The poling provides an electrical potential to each of the individual ceramic posts. The advantage of the lower poling temperature is that the epoxy expansion is reduced so as not to adversely affect the plating. The poling temperature that may be used will vary from polymer to polymer being dependent on the thermal expansion of the polymer and the temperature required to accomplish poling. The posts are electrically connected in parallel, so that a short electrical pulse from the power supply will cause each of them to vibrate simultaneously and generate an acoustic pulse. A reflected pulse causes the ceramic posts to vibrate and generate an electrical signal. The slight differences in timing receipt and amplitude of the reflected pulse from the different ceramic posts and the corresponding electrical signal can be resolved and conditioned by the ultrasonic equipment to which the transducer is attached to provide meaningful information to a trained technician.  
         [0036]     Subsequent finishing operations may be performed on the workpiece in order to produce a finished transducer, such as providing acoustic layers to the transducer face. However, these processes are well known to the art, and this invention does not make further contribution to these well-known practices.  
         [0037]     While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.