Composite ultrasonic transducer

In a composite ultrasonic transducer, each of a plurality of piezoelectric ceramic columns included in a resin plate has a given circular cross-section and passes through the resin plate in a direction of a thickness of the resin plate, and central axes of the piezoelectric ceramic columns are regularly arranged at positions corresponding to nodes of a regular triangle network.

BACKGROUND OF THE INVENTION 
1. 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. 
2. 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 MRayl which is much higher than an 
acoustic impedance of approximately 1.5 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 initially was fabricated by arranging 
piezoelectric ceramic columns each having a circular shape in a cross 
section plane 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 
.mu.m. 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 .mu.m or more is not employed. 
In the field of the 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 
.mu.m using the dicing technique. Consequently, a plurality of 
piezoelectric ceramic columns each having a square cross-section of 
approximately 150 .mu.m.times.150 .mu.m 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 the lateral spreading of ultrasonic waves generated 
by the vertical mode resonance, leading to a 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 a reduction of the frequency of the employed 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. Pat. No. 5,164,920) proposes a 
composite ultrasonic transducer as shown in FIG. 6 fabricated using 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 
by a longitudinal cross-section plane 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 a first arbitrary ceramic column 2a having the 
hexagonal cross-section to face with an angle twice the taper angle toward 
one side of a second ceramic column located closest to the first ceramic 
column without being arranged parallel thereto. In other words, those 
sides of adjacent ones of the ceramic columns 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 the taper angle of the 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 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. 
SUMMARY OF THE INVENTION 
One object of the present invention is to provide a composite ultrasonic 
transducer which can be fabricated relatively easily and which achieves 
sufficiently suppressed undesired lateral mode resonance. 
A composite ultrasonic transducer according to the present invention 
includes a resin plate, and a plurality of fine piezoelectric ceramic 
columns regularly arranged therein, and is characterized in that 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 that the central axes of the plurality of 
piezoelectric ceramic columns are arranged at one major surface 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.

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 in a longitudinal cross-section plane 
including a longitudinal central axis of the column, and has a circular 
shape in a cross-section plane perpendicular to the central axis. In other 
words, each of piezoelectric ceramic columns 2b is not tapered and has a 
constant cross-sectional diameter and is particularly shaped as a right 
circular cylinder as shown in FIGS 1A AND 1B. The central axes of these 
piezoelectric ceramic columns 2b are arranged at positions corresponding 
to substantially all nodes of a regular triangle network at one major 
surface of resin plate 3b (hereinafter referred to as "regular triangle 
network array"), wherein the nodes are especially located at the vertices 
of equilateral traingles making up the array as shown in FIG. 1A. 
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 onto 
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 .mu.m, and an 
x-ray absorber pattern 12b formed of a tungsten film with a thickness of 5 
.mu.m. 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 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 .mu.m, and each cylinder may have a 
cross-sectional diameter of 30 .mu.m and a height of 300 .mu.m. 
Referring to FIG. 2D, resin molding using nickel mold 13 generates a resin 
mold 14. Resin mold 14, after being 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 .mu.m, and each hole may have a cross-sectional diameter of 30 .mu.m 
and a depth of 300 .mu.m. 
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.degree. C. to remove binder therefrom, and thereafter sintered at 
1200.degree. 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 .mu.m, and each ceramic column has a cross-sectional 
diameter of approximately 25 .mu.m and a height of approximately 250 
.mu.m, giving an aspect ratio of approximately 10. 
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 embedded in the 
remaining resin 17 in the form of a plate 3b. Consequently, composite 
ultrasonic transducer 1b including where a plurality of fine piezoelectric 
ceramic columns 2b 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 into the composite ultrasonic 
transducer 1b or to 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 .mu.m and a gold layer having a thickness 
of 0.4 .mu.m 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 a resin 
material. In composite transducer 1b of the first example, spacing of 
central axes of a plurality of fine piezoelectric ceramic columns 2b was 
38 .mu.m, and each ceramic column 2b had a cross-sectional diameter of 25 
.mu.m and a height of 110 .mu.m giving an aspect ratio of 4.4. 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 .mu.m, and each ceramic column 2 
had a square cross-section of 25 .mu.m.times.25 .mu.m and a height of 110 
.mu.m. 
The first example and the example for comparison were tested and 
consequently an 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 a 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 
that 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 was 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 had a frequency of approximately 12 MHz 
which is higher than half of the frequency, namely 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 .mu.m, and each 
ceramic column 2b had a cross-sectional diameter of 46 .mu.m and a height 
of 230 .mu.m giving an aspect ratio of 5. The composite ultrasonic 
transducer of the second example was tested and consequently ultrasonic 
waves caused by the vertical mode resonance of 5.8 MHz were observed. 
However, a 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. On 
the other hand 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 does not run or extend continuously. As also seen in FIG. 3, 
each respective ceramic column 2b is surrounded at equal spacing distances 
by six neighboring columns 2b arranged in an equilateral hexagonal 
pattern. 
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. 
Pat. No. 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 thus 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. 
Although the present invention has been described and illustrated in 
detail, it is clearly understood that the same is by way of illustration 
and example only and is not to be taken by way of limitation, the spirit 
and scope of the present invention being limited only by the terms of the 
appended claims.