Ultrasonic transducer and manufacturing method therefor

This invention relates to a method of manufacturing an ultrasonic transducer. First, a plurality of printed boards in each of which a plurality of leads are formed in a line are stacked. The end portions of the leads protrude from each printed board. These lead end portions are inserted into a plurality of lead holes of an alignment jig. The plurality of printed boards are buried in the back surface of this alignment jig, and a backing layer is formed by resin molding. After that, the alignment jig is removed from the surface of the backing layer, and the surface of the backing layer is flattened. Since the end portions of the leads are exposed to this flattened surface of the backing layer, discrete electrodes formed on the back surfaces of transducer elements are electrically connected to these lead end portions. The accuracy of lead arrangement is thus improved by the use of the alignment jig. This reduces alignment errors of leads with respect to the discrete electrodes of the transducer elements.

BACKGROUND OF THE INVENTION
 The present invention relates to an ultrasonic transducer as a main
 component of an ultrasonic probe of an ultrasonic apparatus such as an
 ultrasonic diagnostic apparatus and a method of manufacturing the same
 and, more particularly, to a two-dimensional array type ultrasonic
 transducer in which transducer elements for reversibly converting an
 electrical signal and an ultrasonic signal are arrayed in a matrix
 (two-dimensionally) and a method of manufacturing the same.
 Various apparatuses using ultrasonic waves are widely used in the field of
 medicine. Of these apparatuses using ultrasonic waves, the most
 extensively used is an ultrasonic diagnostic apparatus which obtains
 tomographic images of soft tissues of living bodies by using the
 ultrasonic pulse reflection method. This ultrasonic diagnostic apparatus
 is known as a noninvasive method and displays a tomographic image of a
 tissue. Compared to other diagnostic apparatuses such as an X-ray
 diagnostic apparatus, an X-ray computer tomographic apparatus, a magnetic
 resonance imaging apparatus, and a nuclear medicine diagnostic apparatus,
 an ultrasonic diagnostic apparatus has the advantages that it can display
 images in real time, is small and inexpensive, has high safety with no
 exposure to X-rays, and is capable of blood flow imaging by using the
 ultrasonic Doppler method.
 For these reasons, ultrasonic diagnostic apparatuses are extensively used
 in examinations of hearts, abdomens, mammary glands, and urinary organs,
 and in obstetrics and gynecology. Also, an ultrasonic diagnostic apparatus
 can display, e.g., heart beats and the motions of an unborn child in real
 time with a simple operation of bringing an ultrasonic probe into contact
 with the body surface, and can allow repetitive examinations because of
 its high safety. Additionally, examinations can be readily performed even
 on the bedside.
 To generate a tomographic image of the interior of an object to be
 examined, an ultrasonic diagnostic apparatus scans an internal section of
 the object via an ultrasonic probe. This scan is classified into two types
 in accordance with the principle of scanning: one is mechanical scan by
 which an ultrasonic transducer is mechanically moved, and the other is
 electronic scan which uses electronic switching and delay control of
 arrayed transducer elements. The scan is also classified into two other
 types in accordance with the range of scanning: one is two-dimensional
 scan which scans a section with an ultrasonic beam, and the other is
 three-dimensional scan which scans an internal three-dimensional region of
 an object with an ultrasonic beam. The currently most frequently used
 three-dimensional scan is to translate or axially rotate, a
 two-dimensional scan type ultrasonic probe either manually or
 mechanically.
 Since this three-dimensional scan requires a long scanning time, the time
 resolution of the scan is low, so the scan is impractical. To improve the
 time resolution, it is essential to use a two-dimensional array type
 ultrasonic probe in which a plurality of transducer elements are arrayed
 in a matrix and to scan a three-dimensional region electronically by using
 electronic switching and delay control of these transducer elements.
 In the manufacture of a two-dimensional array type ultrasonic transducer to
 be incorporated into this two-dimensional array type ultrasonic probe, one
 of the most difficult problems to solve is a method of extracting leads
 from transducer elements arrayed at fine pitches.
 One example is a method by which a conductor pad array formed in a matrix
 on the surface of a printed board is adhered to a discrete electrode array
 of transducer elements. Methods of electrically connecting the conductor
 pads on a printed board to the discrete electrodes of the transducer
 elements are: (1) a method using a conductive adhesive; (2) a method of
 heating, under pressure, an anisotropic conductive film sandwiched between
 the conductor pad array and the discrete electrode array of the transducer
 elements; and (3) a method of contact-bonding metal bumps formed on the
 conductor pads and on the discrete electrodes.
 Unfortunately, in the method (1) the interval between the transducer
 elements must be increased in order to avoid conduction between adjacent
 discrete electrodes. Also, the methods (2) and (3) have the problems of,
 e.g., breakage of the transducer elements, depolarization (polarization
 shift) of the transducer elements heated in the connecting operation, and
 thermal deformation of the printed board, since these methods require
 pressing and/or heating. Further, these methods (1) to (3) have the common
 problem that the acoustic characteristics (wavelength, frequency band) of
 the ultrasonic transducer itself degradate in accordance with the acoustic
 characteristics of the printed board directly adhered to the transducer
 elements.
 Furthermore, in the case the transducer plate is divided to obtain the
 transducer element array after the transducer plate is sticked on the
 printed circuit board, wiring on the printed circuit board are breaked.
 Therefore the element array after is defective in division.
 To solve these problems, a method of burying a plurality of leads in a
 backing layer is being studied. In this method, elements certainly are
 separated to each other by gaps attained to the backing layer. However,
 the yield is low because the alignment of the leads with the discrete
 electrodes is not easy.
 BRIEF SUMMARY OF THE INVENTION
 It is an object of the present invention to reduce alignment errors of
 leads with respect to discrete electrodes of transducer elements in a
 two-dimensional array type ultrasonic transducer and in a method of
 manufacturing the same.
 The present invention relates to a method of manufacturing an ultrasonic
 transducer. First, a plurality of printed boards in each of which a
 plurality of leads are formed in a line are stacked. The end portions of
 the leads protrude from each printed board. These lead end portions are
 inserted into a plurality of lead holes or lead slits of an alignment jig.
 The plurality of printed boards are buried in the back surface of this
 alignment jig, and a backing layer is formed by resin molding. After that,
 the alignment jig is removed from the surface of the backing layer, and
 the surface of the backing layer is flattened. Since the end portions of
 the leads are exposed to this flattened surface of the backing layer,
 discrete electrodes formed on the back surfaces of transducer elements are
 electrically connected to these lead end portions. The accuracy of lead
 arrangement is thus improved by the use of the alignment jig. Therefore,
 alignment errors of the leads with respect to the discrete electrodes of
 the transducer elements can be reduced.
 Additional objects and advantages of the invention will be set forth in the
 description which follows, and in part will be obvious from the
 description, or may be learned by practice of the invention. The objects
 and advantages of the invention may be realized and obtained by means of
 the instrumentalities and combinations particularly pointed out
 hereinafter.

DETAILED DESCRIPTION OF THE INVENTION
 Practical embodiments of an ultrasonic transducer and a method of
 manufacturing the same according to the present invention will be
 described in detail below with reference to the accompanying drawings.
 Note that an ultrasonic transducer is a main component for transmitting
 and receiving ultrasonic waves in an ultrasonic probe of an ultrasonic
 diagnostic apparatus.
 First Embodiment
 As shown in FIGS. 1 and 2, an ultrasonic transducer according to the first
 embodiment has a backing layer 1 for damping ultrasonic waves. Stacked
 flexible printed boards 10 are buried in this backing layer 1. Leads 6 are
 formed in a line in each printed board 10. Note that these leads 6 are
 buried in the substrate material 9, or formed on the substrate material 9
 and covered with an insulating layer. Electronic circuits such as
 amplifier circuits may be formed on each printed board 10.
 The front ends of the leads 6 are exposed to the front surface of the
 backing layer 1 and fixed by conductive resins 40. The rear ends of the
 leads 6 protrude from the back surface of the backing layer 1. Transducer
 elements 3 are arrayed in a matrix on the front surface of the backing
 layer 1. Discrete electrodes 51 are formed on the back surfaces of the
 transducer elements 3. Common electrodes 53 for grounding are formed on
 the front surfaces of the transducer elements 3. The front ends of the
 leads 6 are electrically connected to the discrete electrodes 51 via
 contacts 11'. Acoustic matching layers 4 having conductivity are mounted
 on the transducer elements 3. A conductive film 16 for grounding is formed
 on these acoustic matching layers 4.
 A method of manufacturing the ultrasonic transducer according to this
 embodiment will be described below.
 FIG. 3A shows a plate-like alignment jig 7 for aligning the leads 6 with
 the discrete electrodes 51 of the transducer elements 3. Lead holes 8 are
 formed in a matrix in this alignment jig 7 in accordance with the
 arrangement of the transducer elements 3. These lead holes 8 are formed to
 be slightly larger than the diameter of the leads 6 so as to allow
 insertion of the leads 6. The lead holes 8 need not be substantially
 rectangular holes as shown in FIG. 3A but can be circular lead holes 8' as
 shown in FIG. 3B. This alignment jig 7 is manufactured by forming lead
 holes 8, by mechanical processing such as drilling, laser processing, or
 etching processing, in a plate made of a material, such as ceramics,
 metal, or resin, which allows easy drilling. That is, the alignment jig 7
 is made of a material, such as ceramics, metal, or resin, which is easy to
 process, and holes are formed by mechanical processing such as drilling,
 laser processing, or etching processing, each having high processing
 accuracy. Consequently, the lead holes 8 can be arranged with high
 accuracy.
 The alignment jig need not be a single member, but may have a multilayered
 structure made of workable materials such as a ceramic, metal, and resin.
 The lead hole formed in the alignment jig need not be a through hole
 extending through the alignment jig, but may be a blind groove formed
 midway along the thickness of the alignment jig.
 First, as shown in FIG. 4, the printed boards 10 are stacked. In these
 printed boards 10, the leads 6 are buried in a line in substrate materials
 9. The end portions of these leads 6 protrude, into the form of a comb,
 from the printed boards 10. The printed boards 10 are so stacked that the
 leads 6 are arranged in a matrix.
 Next, as shown in FIGS. 5 and 6, the end portions of the leads 6 are
 inserted into the lead holes 8 of the alignment jig 7. Since the lead
 holes 8 are arranged in accordance with the arrangement of the transducer
 elements 3, the leads 6 are also arranged in accordance with the
 arrangement of the transducer elements 3. As shown in FIG. 6A, the gaps
 between the lead holes 8 and the leads 6 are filled with the conductive
 resins 40. Consequently, the printed boards 10 are fixed to the alignment
 jig 7. Note that the material to be charged into the lead holes 8 need not
 be a conductive resin and can be an insulating resin.
 In the above step, the leads 6 of one printed board 10 are inserted into
 one line of the lead holes 8. However, the leads 6 of a plurality of
 printed boards 10 can also be inserted into one line of the lead holes 8.
 Also, in the above structure the leads 6 are formed in a line in one
 printed board 10. However, the leads 6 can also be formed in a plurality
 of lines in one printed board 10.
 As shown in FIGS. 7 and 8, a relatively soft resin S for backing having
 high ultrasonic damping performance is formed by molding in a rectangular
 parallelepiped region R on the back surface of the alignment jig 7
 including the stacked printed boards 10. As a consequence, the printed
 boards 10 are buried in the resin S. As this resin S, a material having
 appropriate acoustic impedance and appropriate acoustic attenuation by
 which it functions as an acoustic damper is chosen. The resin S changes
 into the backing layer 1 by hardening.
 Another method of forming the backing layer is to insert a resin sheet in a
 gap between printed boards and then inject a resin in the gap.
 As shown in FIG. 9, the alignment jig 7 is removed from the surface of the
 backing layer 1 by, e.g., mechanical processing or etching. After that,
 the surface of the alignment jig 7 is flattened by cutting it together
 with the leads 6. This exposes the ends of the leads 6 to the surface of
 the backing layer 1. The alignment jig 7 can deteriorate the acoustic
 characteristics (e.g., the wavelength and frequency band) of the
 transducer elements 3. This method can avoid this inconvenience by the
 removal of the alignment jig 7.
 As shown in FIG. 10, a contact layer 11 as a thin metal film is formed on
 the flattened surface of the backing layer 1 by a technique such as vapor
 deposition or sputtering. The function of this contact layer 11 is to
 increase the contact area between the leads 6 and the discrete electrodes
 of the transducer elements 3 and thereby improve the reliability of the
 electrical connection between the leads 6 and the discrete electrodes of
 the transducer elements 3. More specifically, the contact layer 11 is a
 stacked structure of chromium and gold. A conductive resin may be formed
 thin by a technique such as printing. The thickness of this contact layer
 11 is much shorter than the wavelength of ultrasonic waves generated by
 the transducer elements 3. So, the contact layer 11 does not change the
 acoustic characteristics.
 As shown in FIG. 11, the contact layer 11 is separated by grooves 12 in
 accordance with the arrangement of the leads 6, thereby forming the
 contacts 11, isolated from each other.
 Next, as shown in FIG. 12, a piezoelectric plate 2 such as piezoelectric
 ceramics is adhered by a conductive adhesive onto the contacts 11' on the
 surface of the backing layer 1. A thin metal film is formed on each of the
 front and back surfaces of this piezoelectric plate 2. A baked silver
 electrode is a typical example of this thin metal film. However, some
 other material or some other method is also usable.
 As shown in FIG. 13, an acoustic matching layer 14 made of a conductive
 resin having a predetermined acoustic impedance is formed to have a
 predetermined thickness on the surface of the piezoelectric plate 2. This
 conductive resin is, e.g., an epoxy resin filled with a silver frit. Since
 this acoustic matching layer 14 has conductivity, it is electrically
 connected to the thin metal film on the front surface of the piezoelectric
 plate 2.
 As shown in FIG. 14, grooves 15 are formed crosswise to extend from the
 surface of the acoustic matching layer 14 to the outermost portion of the
 backing layer 1 through the piezoelectric plate 2 in accordance with the
 arrangement of the leads 6. With these grooves 15, the transducer elements
 3 are arrayed in a matrix. Therefore elements electrically are separated
 to each other.
 Subsequently, the surfaces of the acoustic matching layers 14 are covered
 with a conductive film 16 formed by stacking a thin metal film and a
 resin. This conductive film 16 is, for example, formed by laminating
 thin-film silver on the surface of a polyethylene film. The conductive
 film 16 is connected to the common electrodes 53 of the transducer
 elements 3 via the acoustic matching layers 14. The transducer elements 3
 can be grounded from this resin film 16. In this embodiment, conductive
 film 16 may be single layer film (e.g. metal foil).
 Through the above steps, the ultrasonic transducer shown in FIG. 1 is
 completed. As described above, in this method the leads 6 are arranged by
 the alignment jig 7. This increases the arrangement accuracy of the leads
 6 and reduces alignment errors with respect to the arrangement of the
 transducer elements 3.
 Also, in this method the electrodes of the piezoelectric plate 2 are
 electrically connected to the leads 6 via the contacts 11'. This improves
 the reliability of this electrical connection. Additionally, since a
 conductive adhesive is used in this connection. The low pressure and heat
 generated by this adhesive connection causes only an acoustic
 characteristic deterioration. Furthermore, after the backing layer 1 and
 the piezoelectric plate 2 are adhered to each other, the individual
 transducer elements 3 are separated by the grooves 15. Accordingly, even
 the conductive adhesive used to adhere the backing layer 1 and the
 piezoelectric plate 2 can also be separated by the grooves 15. In this
 invention, the leads 6 are expanded from elements 3, therefore elements 3
 electrically can be separated to each other by grooving after connecting
 between leads 6 and elements 3 using conductive adhesive. Therefore, this
 method does not cause breakage or polarization shift of the piezoelectric
 plate 2. Also, the conductive resin used as an adhesive does not generate
 any electrical leak between adjacent transducer elements 3. This further
 improves the productivity of the method.
 Furthermore, the backing layer 1 interposed between the transducer elements
 3 and the printed boards 10 reduces the influence of the printed boards 10
 on the acoustic characteristics.
 In the aforementioned method, as shown in FIG. 11, the grooves 12 are
 formed after the thin-film electrode 11 is formed on one principal surface
 of the backing layer 1, thereby dividing this thin-film electrode 11 in
 accordance with the wiring patterns 6. However, in this embodiment the
 step of forming the grooves 12 is not always necessary. That is, these
 grooves 12 need not be formed because the thin-film electrode 11 can be
 divided by forming the grooves 15 after the formation of the conductive
 resin layer 14 serving as the acoustic matching layers 4.
 The grounding circuit can also be realized by forming grounding conductor
 patterns in the printed board 10 between a plurality of leads arrayed as
 described above or can be set by forming grounding conductor layer in the
 printed board 10. It is possible by forming a grounding circuit like this
 to reduce crosstalk between the wiring patterns.
 Moreover, electronic circuits such as amplifiers and correction circuits
 can also be formed on the printed boards 10. This reduces the number of
 manufacturing steps and achieves high-density packaging.
 In the above method, in the step shown in FIG. 9 the alignment jig 7 is
 removed from the backing layer 1. As shown in FIGS. 15 and 16, this
 alignment jig 7 can also be cut, instead of being removed, to leave a thin
 alignment jig 7' behind. If this is the case, the thickness of this thin
 alignment jig 7' is substantially adjusted to be an odd integer multiple
 of .lambda./4 where .lambda. is the wavelength of ultrasonic waves (center
 frequency) generated by the transducer elements 3. Hence, the ultrasonic
 waves generated by the transducer elements 3 are transmitted through the
 alignment jig 7', without being reflected by it, and propagate to the
 backing layer 1. Consequently, the acoustic characteristics remain
 unaffected.
 Second Embodiment
 The second embodiment of the present invention will be described below. In
 the explanation of this second embodiment, the same reference numerals as
 in the first embodiment denote the same parts, and a detailed description
 thereof will be omitted. In this second embodiment, as shown in FIG. 17,
 leads 20 are not formed in printed boards but buried in a backing layer 1,
 as isolated bar-like conductive wire materials. The front and rear ends of
 each lead 20 are exposed, and the remaining portion is covered with an
 insulating material 22.
 Steps of manufacturing an ultrasonic transducer according to the second
 embodiment will be described below.
 First, as shown in FIG. 18, a pair of alignment jigs 7a and 7b having lead
 holes 8 formed in a matrix in accordance with the arrangement of
 transducer elements 3 are opposed to each other. Next, as shown in FIG.
 19, the leads 20 are inserted between the lead holes 8 of the alignment
 jig 7a and the lead holes 8 of the alignment jig 7b. The gaps between the
 lead holes 8 and the leads 20 are filled with a conductive resin. The
 uncovered front and rear ends of the leads 20 project from the alignment
 jigs 7a and 7b. Therefore, adjacent ones of the leads 20 do not contact
 each other, and these leads 20 are accurately arranged in accordance with
 the arrangement of the transducer elements 3. Since the lead holes 8 of
 the alignment jigs 7a and 7b are accurately arranged in accordance with
 the arrangement of the transducer elements 3, it is readily possible to
 accurately arrange the leads 20 in accordance with the arrangement of the
 transducer elements 3. This obviates cumbersome adjustment of the
 arrangement of the leads 20.
 Note that the leads 20 can also be arranged by a single alignment jig 7a,
 not by the pair of alignment jigs 7a and 7b. If this is the case, a
 surface formed by using the alignment jig 7a is the surface to be brought
 into contact with the transducer elements 3.
 Next, as shown in FIGS. 20 and 21, a backing resin S is formed by molding
 in a rectangular parallelepiped region R between the alignment jigs 7a and
 7b including the leads 20. Consequently, the leads 20 arranged as above
 are buried in the resin S. As this resin S, it is preferable to select a
 material having appropriate acoustic impedance and appropriate acoustic
 attenuation by which it functions as an acoustic damper. The resin S
 changes into the backing layer 1 by hardening.
 As shown in FIG. 22, the alignment jig 7a is removed from the surface of
 the backing layer 1 by using a technique such as mechanical processing or
 etching. After that, the surface of the backing layer 1 is flattened by
 cutting it together with the leads 20. This exposes the ends of the leads
 20 to the surface of the backing layer 1. The alignment jig 7b on the back
 surface of the backing layer 1 can be either removed or left behind. When
 this backing layer 7b is removed, the back surface of the backing layer 1
 need not be flattened by cutting. The rear ends of the leads 20 projecting
 from the lead holes 8 are used as electrical connecting terminals for
 external circuits.
 As shown in FIGS. 23 and 24, a contact layer 11 as a thin metal film having
 a stacked structure of chromium and gold is formed on the flattened
 surface of the backing layer 1 by a technique such as vapor deposition or
 sputtering. This contact layer 11 is divided crosswise by grooves 12 to
 form contacts 11' for increasing the electrical contact area between the
 end portions of the leads 20 and discrete electrodes of the transducer
 elements 3. Since the contacts 11' are very thin, they do not deteriorate
 the acoustic characteristics.
 After that, following the same procedure as in the first embodiment, the
 transducer elements 3 are formed in a matrix on the contacts 11' on the
 surface of the backing layer 1.
 As described above, this embodiment can achieve an effect similar to that
 of the first embodiment, e.g., the bar-like leads 20 can be arranged in
 accordance with the arrangement of the transducer elements 3 by using the
 pair of alignment jigs 7a and 7b.
 This second embodiment employs the leads 20 whose central portions are
 covered with an insulating material. However, coaxial wires manufactured
 by forming conductive layers on the coatings of the leads 20 can also be
 used. Since the leads 20 are shielded by the outer conductive layers,
 crosstalk between these leads can be reduced.
 Third Embodiment
 The third embodiment of the present invention will be described below. In
 the explanation of this third embodiment, the same reference numerals as
 in the first and second embodiments denote the same parts, and a detailed
 description thereof will be omitted.
 As shown in FIG. 25, transducer elements 3 are arrayed on the surface of a
 backing layer 1 in which printed boards 10 are buried. Common electrodes
 53 of the transducer elements 3 are connected together to a conductive
 layer 25 for grounding. Acoustic matching layers 26 are mounted on the
 transducer elements 3 via the conductive layer 25. In the backing layer 1,
 the printed boards 10 of the first embodiment or leads 20 of the second
 embodiment are buried.
 Steps of manufacturing an ultrasonic transducer will be described below. In
 this embodiment, the steps up to the formation of the backing layer 1 are
 the same as in the first and second embodiments described above, so a
 detailed description of these steps will be omitted.
 First, as shown in FIG. 26, a piezoelectric plate 2 such as piezoelectric
 ceramics is adhered on contacts 11' of the backing layer 1. The common
 electrodes 53 are formed on the surface of this piezoelectric plate 2, and
 discrete electrodes 51 are formed on the back surface. Next, as shown in
 FIG. 27, grooves 27 are formed crosswise to extend from the surface of the
 piezoelectric plate 2 to the outermost portion of the backing layer 1.
 With these grooves 27, the piezoelectric plate 2 is divided, forming a
 plurality of transducer elements 3 in a matrix.
 As shown in FIG. 28, a conductive film 28 formed by stacking a thin metal
 film and a resin is adhered to the surfaces of the common electrodes 53 of
 the transducer elements 3 by a conductive adhesive. This conductive film
 28 is, e.g., a film formed by laminating a copper foil on the surface of a
 polyimide film, a film formed by laminating a metal foil on the two
 surfaces of a resin film, or a metal foil.
 As shown in FIGS. 29 and 30, an epoxy resin layer 29 is formed on the
 surface of the conductive film 28 so as to have a thickness suited to the
 acoustic matching characteristics. This epoxy resin layer 29 is divided
 crosswise with grooves 30 not to reach the conductive film 28 by using a
 dicing machine or a laser, thereby forming acoustic matching layers 29'.
 The material of the acoustic matching layers 29' is not restricted to an
 epoxy resin but can be some other type of resin. Also, each acoustic
 matching layer 29' need not be a single layer but can have a multilayered
 structure.
 Note that the type of laser is selected and the intensity of the laser is
 adjusted so that the conductive film 28 is not divided and only the epoxy
 resin layer 29 on this conductive film 28 is selectively divided.
 The intensity of the laser is adjusted so that a conductive layer (metal
 layer) is divided and a resin layer is not divided in the case conductive
 film 28 has a conductive layer and a resin layer.
 Through the above steps, the ultrasonic transducer shown in FIG. 25 is
 manufactured. This embodiment can achieve an effect similar to those of
 the first and second embodiments. In addition, the conductive film 28
 below the acoustic matching layers 29' is not divided but connected to the
 common electrodes 53 of the transducer elements 3. Therefore, no
 conductivity need be given to the acoustic matching layers, unlike in the
 first and second embodiments. This extends the range of selecting the
 material of the acoustic matching layers and facilitates the use of a
 multilayered structure. Consequently, optimum acoustic matching
 characteristics can be easily attained.
 In the above embodiment, the acoustic matching layers 26 are formed by
 forming the epoxy resin layer 29 on the conductive film 28 and dividing
 this epoxy resin layer 29. However, the present invention is not limited
 to this arrangement. That is, a resin layer laminated on a metal foil,
 which is a material having superior acoustic characteristics and has a
 predetermined thickness, can also be used as the conductive film 28,
 without forming the epoxy resin layer 29. In this case, the acoustic
 matching layers 26 are formed by forming grooves, reaching the metal foil,
 in the resin layer formed on the metal foil.
 Fourth Embodiment
 The fourth embodiment of the present invention will be described below. In
 the explanation of this fourth embodiment, the same reference numerals as
 in the first, second, and third embodiments described above denote the
 same parts, and a detailed description thereof will be omitted.
 In this embodiment, as shown in FIG. 31, leads 35 are connected in an end
 portion 42 projecting from a printed board 36 of an insulating material
 37. A plurality of such printed boards 36 are overlapped. The connecting
 portions 42 are inserted into lead slits 39 of an alignment jig 38 as
 shown in FIG. 32, and gaps are fixed with an insulating resin. The lead
 slits 39 are formed parallel at the same pitch as the Y-direction pitch of
 transducer elements 3 in the alignment jig 38.
 The alignment jig need not be a single member, but may have a multilayered
 structure made of workable materials such as a ceramic, metal, and resin.
 The lead slit formed in the alignment jig need not be a through hole slit
 extending through the alignment jig, but may be a blind groove formed
 midway along the thickness of the alignment jig.
 In this embodiment, the leads 35 of one printed board 36 are inserted into
 on lead slit 39. However, the leads 35 of a plurality of printed boards 36
 can also be inserted into one lead slit 39. Furthermore, in this
 embodiment one line of leads 35 are formed in one printed board 36.
 However, a plurality of lines of leads 35 can also be formed in one
 printed board 36. If this is the case, a plurality of lines of leads 35 of
 one printed board 36 are inserted into a plurality of adjacent lead slits
 39.
 Next, as shown in FIG. 33, a backing layer 1 is formed on the back surface
 of the alignment jig 38. The printed boards 36 are buried in this backing
 layer 1. The alignment jig 38 is removed from the surface of the backing
 layer 1 by a technique such as mechanical processing or etching. After
 that, the surface of the backing layer 1 is flattened by cutting it
 together with the connecting portions 42 of the leads 35. The isolated end
 portions of the leads 35 are exposed to the cut surface of the backing
 layer 1. The alignment jig 38 does not deteriorate the acoustic
 characteristics because it is removed.
 Instead of removing the alignment jig, the alignment jig may be cut thin to
 leave the thin alignment jig. In this case, the thickness of the thin
 alignment jig is adjusted to an odd number multiple of .lambda./4 where
 .lambda. is the wavelength of the center frequency of an ultrasonic wave
 generated by the transducer element. The ultrasonic wave generated by the
 transducer element is not reflected by the alignment jig but passes
 through the alignment jig. The ultrasonic wave then reaches the backing
 layer. The thin alignment jig does not deteriorate the acoustic
 characteristics.
 Finally, as in the first embodiment, the transducer elements 3 are arrayed
 on the surface of this backing layer 1, thereby completing an ultrasonic
 transducer.
 In this embodiment, the connected leads are isolated in the step of cutting
 the alignment jig thin. However, the leads can be isolated by forming
 grooves deep enough to divide the connected leads in the step of dividing
 the piezoelectric plate by vertical and horizontal grooves.
 This embodiment can achieve an effect similar to those of the above
 embodiments. Additionally, the connecting portions of the leads are
 inserted into the lead slits of the alignment jig. This by far improves
 the workability compared to the aforementioned embodiments in which
 isolated leads are individually inserted into isolated holes.
 Additional advantages and modifications will readily occur to those skilled
 in the art. Therefore, the invention in its broader aspects is not limited
 to the specific details and representative embodiments shown and described
 herein. Accordingly, various modifications may be made without departing
 from the spirit or scope of the general inventive concept as defined by
 the appended claims and their equivalents.