Composite ultrasonic transducer

Abstract: In a composite ultrasonic transducer (1b), each of a plurality of piezoelectric ceramic columns (2b) included in a resin plate (3b) has a circular cross-section and passes through the resin plate (3b) in a direction of a thickness of the resin plate (3b), and central axes of the piezoelectric ceramic columns (2b) are arranged at positions corresponding to nodes of a triangle network.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a composite ultrasonic transducer formed by regularly arranging a plurality of piezoelectric ceramic columns in a resin plate. Such a composite ultrasonic transducer is applicable to a medical ultrasonic diagnostic apparatus and an industrial nondestructive inspection apparatus. Description of the Background Art A piezoelectric ceramic plate has been utilized for a long time as an ultrasonic transducer. However, the piezoelectric ceramic plate has an acoustic impedance of approximately 30 10 6 Kg/m 2 S (MRayl) which is much higher than an acoustic impedance of approximatelyl 1.5 10 6 Kg/m 2 S (MRayl) of any biological object, and therefore has a low efficiency of transmitting ultrasonic waves from the piezoelectric ceramic plate to the biological object. In addition, compared with piezoelectric resin such as polyvinyliden fluoride, the piezoelectric ceramic plate has a low efficiency in receiving an ultrasonic signal to convert it to an electric signal while having a high efficiency of converting an electric signal to an ultrasonic signal. In view of these problems, a composite ultrasonic transducer formed of a resin plate including an array of a plurality of small piezoelectric ceramic columns has been proposed and studied (see IEEE Trans. Sonics Ultrasonics, Vol. SU-32, 1985, pp. 481-497). A composite ultrasonic transducer at the initial stage was fabricated by arranging piezoelectric ceramic columns each having a circular shape at a cross section perpendicular to a longitudinal axis and filling the space between those ceramic columns with resin. The piezoelectric ceramic columns each had a cross-sectional diameter of at least approximately 300 µm. It is known that various characteristics of the composite ultrasonic transducer depend on the dimension of the piezoelectric ceramic column and the frequency of the ultrasonic wave. For example, if the composite ultrasonic transducer is used in a higher frequency range, piezoelectric ceramic columns each having a smaller cross-sectional area should be used in view of the sensitivity characteristic. Owing to such circumstances, in the field of the medical ultrasonic diagnostic art using ultrasonic waves in the frequency range of at least 2.5 MHz, the composite ultrasonic transducer including the array of piezoelectric ceramic columns each having the cross-sectional area of 300 µm or more is not employed. In the field of semiconductor art around 1980, a dicing technique using a diamond saw to cut a silicon substrate began to be employed. The dicing technique was also utilized for fabricating a composite ultrasonic transducer which can be used in the frequency range of 2.5 MHz or more. For example, according to Japanese Patent Laying-Open No. 58-22046, a piezoelectric ceramic plate is first adhered onto a ferrite substrate, and the ceramic plate is laterally and vertically cut with a pitch of 300 µm using the dicing technique. Consequently, a plurality of piezoelectric ceramic columns each having a square cross-section of approximately 150 µm × 150 µm are arrayed on the ferrite substrate at positions corresponding to nodes of a square network (hereinafter referred to as "square network array"). Cut grooves between the piezoelectric ceramic columns are filled with a resin layer and thereafter the resin layer and the plurality of piezoelectric ceramic columns are separated from the ferrite substrate to form a plate-like composite ultrasonic transducer as schematically illustrated in the plan view of Fig. 4A and the side view of Fig. 4B. Specifically, a plurality of fine piezoelectric ceramic columns 2 each having the square cross section are arrayed in the square network in a resin plate 3 in a composite ultrasonic transducer 1. A problem of composite ultrasonic transducer 1 is that an undesirable lateral mode of high-frequency resonance occurs in a direction parallel to a major surface of plate-like transducer 1, while a desired vertical mode of ultrasonic oscillation in a direction of the thickness of transducer 1 is generated. If the lateral mode resonance occurs in a frequency range close to a frequency band of the vertical mode ultrasonic oscillation used, for example, for the ultrasonic diagnosis, the lateral mode resonance accelerates lateral spread of ultrasonic waves caused by the vertical mode resonance, leading to reduction of the resolution of an ultrasonic image. In order to avoid the reduction of the resolution, a central frequency used for the diagnosis is limited to half the lateral mode resonance frequency or less. The resolution of the ultrasonic image is also reduced by reduction of the frequency of used ultrasonic waves. Generally, the frequency of the lateral mode resonance of the composite ultrasonic transducer is inversely proportional to the pitch of the array of the piezoelectric ceramic columns. Therefore, the array pitch may be made finer in order to increase the frequency of the lateral mode resonance. In composite ultrasonic transducer 1 as illustrated in Figs. 4A and 4B, one arbitrary side of one arbitrary piezoelectric ceramic column 2 having the square cross-section faces parallel to one side of another ceramic column located closest to the one arbitrary ceramic column. It is considered that the lateral mode resonance is likely to occur due to the interaction between the sides facing in parallel to each other and close to each other. With such circumstances and progress in the x-ray lithography art, Japanese Patent Laying-Open No. 4-232425 (U.S. Patent 5,164,920) proposes a composite ultrasonic transducer as shown in Fig. 6 fabricated using the x-ray lithography. Referring to the perspective view of Fig. 6, a composite ultrasonic transducer 1a includes a plurality of tapered piezoelectric ceramic columns 2a regularly arranged in a resin plate 3a. Specifically, each of tapered piezoelectric ceramic columns 2a has a trapezoidal shape at a longitudinal cross-section including a longitudinal central axis, and has a hexagonal shape at a cross-section perpendicular to the central axis. Each of the piezoelectric ceramic columns 2a is formed to have the hexagonal cross-section in order to densely arrange ceramic columns 2a in resin plate 3a. Each of the piezoelectric ceramic columns 2a is tapered in order to allow one side of one arbitrary ceramic column 2a having the hexagonal cross-section to face with an angle twice the taper angle to one side of another one ceramic column located closest to the one ceramic column without facing in parallel thereto. In other words, those sides facing closest to each other are not parallel to each other so that the interaction between those sides decreases and thus the undesirable lateral mode resonance is considered to be suppressed. It is considered that, as taper angle of piezoelectric ceramic columns 2a is made larger, the undesirable lateral mode resonance could be suppressed more effectively. However, if the taper angle is made too large, the desired vertical oscillation mode in the longitudinal direction of piezoelectric ceramic columns 2a could become non-uniform. Further, even if the x-ray lithography is used, it would be difficult to form fine piezoelectric ceramic column 2a having precisely controlled taper angle and hexagonal cross-section. Ultrafine scale piezoelectric composite materials for high frequency ultrasonic imaging arrays are known from B.G. Pazol et al. IEEE Ultrasonics Symposium, November 1995. SUMMARY OF THE INVENTION One object of the present invention is to provide a composite ultrasonic transducer which can be fabricated relatively easily with a sufficiently suppressed undesired lateral mode resonance. A composite ultrasonic transducer according to the present invention as set forth in claim 1 includes a resin plate, and a plurality of fine piezoelectric ceramic columns regularly arranged therein, each of the piezoelectric ceramic columns has a substantially circular shape in a cross-section perpendicular to a longitudinal central axis of each column and substantially passes through the resin plate in the direction of the thickness of the plate, and the central axes of the plurality of piezoelectric ceramic columns are arranged at one major surface at the whole of the resin plate at positions substantially corresponding to nodes of a regular triangle network. The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1A schematically shows a plan view of composite ultrasonic tr ansducer according to one embodiment of the present invention, and Fig. 1B shows a side view thereof. Figs. 2A-2J are cross-sectional views schematically showing a process of manufacturing the composite ultrasonic transducer shown in Fig. 1. Fig. 3 is a schematic plan view showing a location where a loop of a stranding wave is to be generated if lateral mode resonance occurs in the composite ultrasonic transducer of the present invention. Fig. 4A schematically shows a plan view of a composite ultrasonic transducer according to the prior art, and Fig. 4B shows a side view thereof. Fig. 5 is a schematic plan view showing a location of a loop of a standing wave in the lateral mode oscillation generated in the composite ultrasonic transducer shown in Fig. 4A. Fig. 6 is a schematic perspective view showing another example of a composite ultrasonic transducer according to the prior art. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the plan view of Fig. 1A and the side view of Fig. 1B, one example of a composite ultrasonic transducer according to one embodiment of the present invention is schematically shown. A plurality of piezoelectric ceramic columns 2b are regularly arranged in a resin plate 3b in a composite ultrasonic transducer 1b. Each of piezoelectric ceramic columns 2b has a rectangular shape at a longitudinal cross-section including a longitudinal central axis of the column, and has a circular shape at a cross-section perpendicular to the central axis. In other words, each of piezoelectric ceramic columns 2b is not tapered and has a constant cross-sectional diameter. The central axes of these piezoelectric ceramic columns 2b are arranged at positions corresponding to nodes of a regular triangle network at the whole of one major surface of resin plate 3b (hereinafter referred to as "regular triangle network array"). The schematic cross-sections of Figs. 2A-2J show one example of a manufacturing process of the composite ultrasonic transducer illustrated in Figs. 1A and 1B. Referring to Fig. 2A, an x-ray sensitive resist layer 11 is formed on a conductive substrate 10. Synchrotron radiation (SR) is directed to resist layer 11 through an x-ray mask 12. X-ray mask 12 includes a membrane 12a formed of silicon nitride with a thickness of 2 µm, and an x-ray absorber pattern 12b formed of a tungsten film with a thickness of 5 µm. X-ray absorber pattern 12b includes a plurality of circular openings arrayed to form the regular triangle network. A stencil mask (metal mesh without the membrane) fabricated by the photolithography and plating may be used as the x-ray mask. Referring to Fig. 2B, resist layer 11 subjected to the SR radiation is developed, and a resist structure 11a is formed. Referring to Fig. 2C, a nickel mold 13 is formed by plating with nickel using conductive substrate 10 as an electrode for plating. Nickel mold 13 includes a plurality of fine cylinders arranged according to the regular triangle network array. For example, the central axes of the cylinders are arranged with a spacing of 46 µm, and each cylinder may have a cross-sectional diameter of 30 µm and a height of 300 µm. Referring to Fig. 2D, resin molding using nickel mold 13 generates a resin mold 14. Resin mold 14 separated from mold 13 has a negative structure generated by the structure of mold 13, and includes a plurality of fine holes arranged according to the regular triangle network array. For example, the central axes of the holes are arranged with a spacing of 46 µm, and each hole may have a cross-sectional diameter of 30 µm and a depth of 300 µm. Referring to Fig. 2E, slurry of piezoelectric ceramic is applied onto resin mold 14, and the slurry is dried to form a dry cake 15 of the piezoelectric ceramic. Referring to Fig. 2F, resin mold 14 is removed from ceramic cake 15 using oxygen plasma 16. Referring to Fig. 2G, piezoelectric ceramic cake 15 is heated to 500°C to remove binder therefrom, and thereafter sintered at 1200°C to produce a slightly contracted sintered piezoelectric ceramic structure 15a. The spacing of axes of fine ceramic columns included in sintered piezoelectric ceramic structure 15a is, for example, approximately 38 µm, and each ceramic column has a cross-sectional diameter of approximately 25 µm and a height of approximately 250 µm. Referring to Fig. 2H, piezoelectric ceramic structure 15a is covered with, for example, epoxy resin 17, and accordingly the space between the fine ceramic columns is filled with resin 17. Referring to Fig. 2I, the base of ceramic structure 15a and the base of filling resin 17 are removed by polishing to leave a plurality of fine piezoelectric ceramic columns 2b with a desired height. Consequently, composite ultrasonic transducer 1b where a plurality of fine piezoelectric ceramic columns 2b are regularly arranged in resin plate 3b is obtained. Generally, if the length of each piezoelectric ceramic column is reduced, or the composite ultrasonic transducer is made thinner, the frequency of ultrasonic waves generated by the vertical mode resonance tends to become higher. Referring to Fig. 2J, an upper electrode 18a and a lower electrode 18b are formed in order to input an electric signal to composite ultrasonic transducer 1b or output an electric signal therefrom. Each of electrodes 18a and 18b is formed, for example, by depositing a chromium layer having a thickness of 0.1 µm and a gold layer having a thickness of 0.4 µm by sputtering. As the first example of the present invention, composite ultrasonic transducer 1b as shown in Figs. 1A and 1B was actually fabricated according to the process steps shown in Figs. 2A-2I using lead zirconate titanate (PZT) as a piezoelectric material and epoxy resin as an epoxy material. In composite transducer 1b of the first example, spacing of central axes of a plurality of fine piezoelectric ceramic columns 2b was 38 µm, and each ceramic column 2b had a cross-sectional diameter of 25 µm and a height of 110 µm. Similarly to the composite ultrasonic transducer of the first example, a composite ultrasonic transducer 1 as shown in Figs. 4A and 4B was actually fabricated as an example for comparison, according to the process steps shown in Figs. 2A-2B using PZT and epoxy resin. In this example for comparison, the spacing of central axes of a plurality of fine piezoelectric ceramic columns 2 was 38 µm, and each ceramic column 2 had a square cross-section of 25 µm × 25 µm and a height of 110 µm. The first example and the example for comparison were tested and consequently ultrasonic frequency of approximately 12 MHz generated by the vertical mode resonance was observed in both of the first example and the example for comparison. Although the undesirable lateral mode resonance was not observed in the first example of the present invention, the lateral mode resonance was observed with frequency of approximately 20 MHz and an electromechanical coupling coefficient of approximately 20% in the example for comparison. In order to avoid the influence of the undesirable lateral mode resonance occurred in the composite ultrasonic transducer, the vertical mode resonance frequency should be at most half the lateral mode resonance frequency. However, in the case of the composite ultrasonic transducer of the example for comparison, it is impossible to prevent the ultrasonic waves generated by the vertical mode resonance from being influenced by the undesirable lateral mode resonance, since the ultrasonic waves caused by the vertical mode resonance have the frequency of approximately 12 MHz which is higher than half of the frequency, about 20 MHz, caused by the undesirable lateral mode resonance. As the second example of the present invention, a composite ultrasonic transducer having only its dimensions changed relative to the composite ultrasonic transducer of the first example was actually fabricated. Specifically, according to the second example, the spacing of central axes of a plurality of piezoelectric ceramic columns 2b was 69 µm, and each ceramic column 2b had a cross-sectional diameter of 46 µm and a height of 230 µm. The composite ultrasonic transducer of the second example was tested and consequently ultrasonic waves caused by the vertical mode resonance of 5.8 MHz was observed. However, the lateral mode resonance was not observed in the range of 2-18 MHz. As heretofore described, the undesirable lateral mode resonance is generated in composite ultrasonic transducer 1 where piezoelectric ceramic columns 2 each having a square cross-section are arranged according to the square network array, while the undesirable lateral mode resonance is not observed in composite ultrasonic transducer 1b where piezoelectric ceramic columns 2b each having a circular cross-section are arranged according to the triangle network array. There could be two reasons for it as below. The first reason is that if piezoelectric ceramic column 2b has a circular cross-section as shown in Fig. 3, the side of ceramic column 2b is formed of a curved surface instead of a flat surface. More specifically, when the undesirable lateral mode resonance propagates from one piezoelectric ceramic column to an adjacent ceramic column through the interaction of the sidewalls thereof, the thickness of a resin layer 3b between the sidewalls locally varies. Therefore, development and propagation of the lateral resonance mode having a specific frequency would be suppressed by non-uniformity of the thickness of the resin layer intervening between the sidewalls of ceramic columns 2b adjacent to each other. The second reason is as follows. If piezoelectric ceramic columns 2 are arranged according to the square network array as shown in Fig. 5, the location of the loop of the standing wave generated by the undesirable lateral mode resonance forms a straight line as shown by broken line 4. If the piezoelectric ceramic columns 2b are arranged according to the triangle network array, the location of the loop of the standing wave in the undesirable lateral mode resonance forms the hexagonal network as shown by broken line 4b of Fig. 3. Accordingly, if piezoelectric ceramic columns 2 are arranged according to the square network array as shown in Fig. 5, the lateral mode resonance is likely to occur since the location of the loop of the standing wave linearly continues. On the other hand, if the piezoelectric ceramic columns 2b are arranged according to the triangle network array as shown in Fig. 3, the lateral mode resonance is suppressed since the location of the loop of the standing wave couldn't run continuously. In composite ultrasonic transducer 1b of the present invention, the effect of the circular cross-section of each piezoelectric ceramic column 2b and the effect of the regular triangle network array of ceramic columns 2b are combined to suppress the undesirable lateral mode resonance, and consequently the undesirable lateral mode frequency is not observed. Piezoelectric ceramic columns 2b each having the circular cross-section could be disadvantageous for densely arranging them in the resin plate compared with piezoelectric ceramic columns 2a each having the hexagonal cross-section as shown in Fig. 6. However, the composite ultrasonic transducer preferably includes piezoelectric ceramic columns with a volume fraction of approximately 40% in the resin plate considering the sensitivity as described in Japanese Patent Laying-Open No. 60-97800 (U.S. Patent 4,683,396). The actual volume fraction of the piezoelectric ceramic columns in the resin plate of the composite ultrasonic transducer of the first example according to the present invention is 39%. Accordingly, the volume fraction of approximately 40% is easily achieved even if piezoelectric ceramic columns 2b each has the circular cross-section. Use of piezoelectric ceramic columns 2b each having the circular cross-section instead of the hexagonal cross-section is not disadvantageous. According to the present invention, a composite ultrasonic transducer that can be relatively easily fabricated with a sufficiently suppressed undesirable lateral mode resonance as described above can be provided.

Claims: A composite ultrasonic transducer comprising: a resin plate (3b); and a plurality of fine piezoelectric ceramic columns (2b) regularly arranged in said resin plate (3b), wherein each of said ceramic columns (2b) has a substantially circular shape in a cross-section perpendicular to a longitudinal central axis of each column, and substantially passes through said resin plate (3b) in a direction of a thickness of the resin plate (3b), characterized in that said central axes of said plurality of piezoelectric ceramic columns (2b) are arranged at positions substantially corresponding to nodes of a regular triangle network at the whole of one major surface of said resin plate (3b). The composite ultrasonic transducer according to claim 1, wherein each one of said ceramic columns (2b) has a constant cross-sectional diameter in a direction of said longitudinal central axis. The composite ultrasonic transducer according to claims 1 or 2, wherein each one of said ceramic columns (2b) is a respective right circular cylinder with said longitudinal central axis perpendicular to said major surface of said resin plate (3b). The composite ultrasonic transducer according to claims 2 or 3, wherein said arrangement of said plurality of columns is defined by three dimensional parameters [height of said columns; diameter of said columns; spacing between two columns] which have the following valves in µm: [230; 46; 69] or [110; 25; 38] or [250; 25; 38] or [300; 30; 46]. The composite ultrasonic transducer according to any of claims 1 to 4, wherein at least a group of said ceramic columns (2b) in a central region of said composite ultrasonic transducer are each respectively surrounded at equal spacings by six neighbouring ones of said ceramic columns (2b) arranged in a hexagonal pattern as seen in a plane parallel to said major surface of said resin plate. The composite ultrasonic transducer according to any of claims 1 to 5, wherein said regular triangle network consists of said nodes arranged at vertices of equilateral triangles.

Inventors: HIRATA YOSHIHIRO, HIRATA, YOSHIHIRO, Hirata, Yoshihiro Sumitomo Electric Indust., Ltd.
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1998
Main classification: B06B1
IPC codes: H04R17, G01N29, B06B1, A61B8