CA1157966A - Method for manufacturing an ultrasonic transducer - Google Patents

Abstract

ABSTRACT
A method for manufacturing an ultrasonic transducer arrangement having groups of jointly controlled transducer elements. One flat side of a strip-shaped piezoelectric body is undetachably connected to a metal coated side of a plastic contact foil. The other side of the plastic contact foil is fastened to a damping body. The piezoelectric body is finely divided to produce individual transducer elements having relatively narrow width. Portions of the plastic contact foil which extend beyond the piezoelectric body are connected as contact tabs to control lines. A plurality of individual transducer elements in the piezoelectric body are electrically coupled to one another to form transducer oscillator groups.

Description

1 157g~6 Back~round of the Invention This invention relates generally to ultrasonic transducers, and more particularly, to a method for manufacturing an arrangement of several ultrasonic transducer elements with electric connecting leads.
In the fields of medical diagnostics and the nondestructive testing of materials, representations in the form of pictures of the interior of a body to be examined are obtained by analysis of ultrasonic pulses which are emitted by ultrasonic transducers arranged at predetermined surface locations of the body.
Controlled high-frequency energy is produced by an electric generator, and con-ducted into the body to be tested by an electromagnetic transducer arrangement, which may be of the piezoelectric or ferromagnetic type. Each transducer element in a transducer arrangement converts high-frequency electrical energy into mechanical ultrasonic energy, of the same frequency. The transducer elements are suitably coupled at their respective vibrating surfaces to the surface of the body to be examined, by the interposition of a coupling means, which may be preselected coupling material.
The ul*rasonic acoustic energy which is generated by the transducers i5 radiated into the body in the form of longitudinal or density waves. Such acoustical waves lead to periodic compression and rarification of the matter in 2Q the direction of propagation. As the waves propagate, echoes thereof are pro-duced at the boundary surfaces between various body parts. Such echo signals are successively received, depending upon their propagation time, by a receiver which may be also used as a transmitter. During reception, the transducers pro-duce output signals corresponding to the echo pulses, the output signals being conducted to electronic circuitry for analysis.
The body to be examined may be parallel-scanned b~ a chain of ultra-sonic transducer oscillators arranged side-by-side, the individual ultrasonic - 1 - t r~

transducers being excited cyclically in accordance with a predetermined program.
An array of such oscillator transducers, which may consist of 50 or more ele-ments, are coupled to an electric connecting lead, illustratively by soldering.
The ultrasonic transducer oscillators of an array may be acoustically subdivided, by fine division, into transducer elements which have substantially greater height than width. Such an arrangement is known as a "transducer comb".
It is desirable that the width of such transducer elements be substantially smaller than one-half of the wave length of the acoustic energy. In arrays which operate at very high frequencies, illustratively 5 MHz, the width of the trans-ducer may be less than 100 microns, thereby rendering the soldering of an elec-trical connecting lead very difficult. Several oscillator transducers may be arranged side-by-side and combined into oscillator groups by connecting them electrically in parallel so as to be jointly controllable. Such a parallel electrical connection is then coupled to a common control line terminal, in the manner described in German Patent 28 29 570.
In a known ultrasonic oscillator arrangement wherein a linear chain of ultrasonic oscillators is produced by arrangeng the ultrasonic oscillators side-by-side, the chain of ultrasonic oscillators can be operated as a sector scanner by appropriate electronic control. The ultrasonic transducer oscillators are formed by soldering a strip-shaped base body of piezoelectric material onto a copper contact foil, approximately 50 microns thick. The opposite flat si~de of the copper foil is fastened to a damping body. The piezoelectric body with the metal foil can be divided by saw cuts into separate ultrasonic oscillators.
Alternatively, the piezoelectric body is connected to a prefabricated metal foil which has been previously cut to form the necessary structure. The portion of metal foil which overhangs the ends of the ultrasonic oscillators can be me-chanically separated and soldered to an electric control conductor, as described 1 157g~

in European Offenlegungschrift 5071.
The step of mechanicall~ separatlng the overhanging metal foil to produce individual contact tabs can cause cracks, and even complete removal, of the contact foil. Moreover, the contact tabs which are produced of copper or other metals tend to become brittle in the vicinity of the soldering joint as a result of reaction with the solders, and can break. If a prefabricated metal foil of the type which is provided with the necessary structure b~ a photoetching technique is used to form the contact tabs, the foil ma~ expand when it is soldered onto the piezoelectric body. Such expansion must be considered during the design of a photoetching mask to determlne the facing between contact tabs.
Of course, such predetermination of the expansion characteristics of the foil requires analysis of the size and shape of the foil, the type of solder to be used, the temperature and duration of heating during soldering, and other material parameters and process steps which are difficult to determine and analyze. The cuts in the foil between the ultrasonic oscillators requires a design of the mask for etching the contact tabs to be performed with high precision. In addi-tion, the metal foils are very thin and easily damaged. Metal foils with rela-tively large thickness of, for example, 50 microns, are not usable because of in-sufficient acoustical coupling caused by interfering acoustical reflections, and excessive stiffness at higher frequencies, illustrativel~ above 2 MHz.
It is, therefore, an object of this invention to simplify the manufac-ture of an ultrasonic transducer arrangement having a large number of ultrasonic oscillators.
It is a further object of this invention to provide finely subdivided ultrasonic oscillators having transducer elements of very small width.
Summary of the Invention The foregoing and other object are achieved by this invention which 1 15796~

provides a methocl ~or manufacturing an ultrasonic transducer arrangement having a plurality of transducer elements which are each provided with contact tabs formed of a contact foil. lhe contact foil is formed of a plastic foil having at least one flat side which is provided with a metal coating. A strip-shaped piezoelectric ~ody which has at least one flat side is undetachably fixed to a surface portion of the metal coating on the contact foil. A further flat side of the contact foil is fastened to a damping body. The thus mounted piezoelec-tric body is then subdivided to form individual transducer elements.
During the fabrication of an ultrasonic transducer comb which has lQ several transducer elements arranged between a matching body and a damping body, the damping body may be used as a support to enhance the mechanical strength during the subsequent fine-division operation which is generally achieved by mechanical cutting. During the subdivision of the piezoelectric body, the piezoelectric body with its metal deposits on both flat sides, as well as the contact foil connected thereto, or at least the metal coating on the contact foil, are cut open. The portion of contact foil which extends beyond the narrow sides of the piezoelectric body can be formed into contact tabs having a width measured along the longitudinal direction of the piezoelectric body which is approximately equal to the width of a transducer element, or to the total width of several jointly controlled transducer elements of an ultrasonic oscillator.
As noted, the contact foil may consist of a continuous plastic foil which is provided with a metal coating. The thickness of the contact foil is substantially smaller than the wave length of the acoustic energy. In one em-bodiment which operates at a frequency of at least 2 MHz, the thickness is at most, for example, 10 microns. The thickness of the metal coating on the flat side of the plastic foil facing the piezoelectric body is not more than 10 mi-crons, and preferably not more than 1 micron. It may be advantageous in some ~ 1579~i embodiments to design the contact foil, or its electrically conducting coating, as a pattern with cutouts, before it is attached to the piezoelectric body.
Such a pattern may be achieved by a photoetching technique or by punching. The connecting tabs are formed after the piezoelectric body is fastened to the con-tact foil.
In a particular embodiment of the invention, the portion of the contact foil which extends beyond the two opposite narrow sides of the piezoelectric body can be designed as contact tabs of which one is connected to an electric control line, and the other is shortened to a predetermined length such that it forms electrically conducting bridges for the transducer elements of each ultrasonic oscillator. In this embodiment, each of the transducer elements is provided with a contact at each of its ends.
All of the transducer elements are provided with a co-mmon connecting lead on the exposed flat side of the piezoelectric body, opposite to the side which contacts the metal foil. This common connecting lead bridges all of the transducer elements by forming a ribbon of electrically conductive material.
Such an electrically conductive ribbon can be formed as an area contact, by fastening the electrically conductive surface of a foil so that the portions of electrically conductive material overhang the narrow sides of the piezoelectric body on which are formed conductor tabs which are connected to the opposite pole of the control voltage. This arrangement forms a cage-like shield for the con-trol leads of the transducer elements. The connection of the contact foil at the lower flat surface of the piezoelectric body, and the shielding foil at the upper flat surface of the piezoelectric body can advantageously be made in a common operation.
Brief Description of the Drawings Comprehension of the invention is facilitated by reading the following I 1 579~i detailed descr;ptlon in conjunction with the annexed drawings J in which:
Figure 1 is a magnified perspective view of an ultrasonic transducer arrangement constructed in accordance with the principles of the invention;
Pigure 2 is a magnified cross-sectlonal representation of a transducer arrangement manufactured in accordance with the inventive process steps; and Figure 3 is a cross-sectional representation of a special embodiment of the ultrasonic transducer arrangement.
Detailed Description Figure 1 shows an ultrasonic tranducer arrangement having a plurality 1~ of linearly arranged transducer elements 2 disposed one behind the other between a damper body 4 and a matching ~ody 5. Several such adjacent transducer elements

2 are electrically connected in parallel with one anotherJ and therefore jointly controlled, to form a plurality of oscillators 6, 7, 8, and 9. In a practical embodiment of the invention, eight transducer elements, of which only four are shown in the Figure, can form an oscillator. Transducer elements 2 can be made by fine-dividing a strip-space or slab-shaped piezoelectric body 10. Damping body 4 is provided with conductor plates 12 and 13 fastened on either side there-of. The surfaces of the conductor plates are provided with electric conductors 14 through 17, onl~ four such electric conductors being shown in this Figure.
2Q. Conductors 14 through 17 serve as control lines for transducer elements 2 and are each connected to a respective one of contact tabs 24 through 28. Contact tabs 24 throug~ 28 are formed from portions of a contact foil 20 which extends outwardly from the transducer arrangement at the end faces of transducer elements 2 A contact foil 22 is similarly attached at the opposite end of transducer elements 2, and is:similarly provided with contact tabs, of which only contact tab 25 is visible in thi~s ~gure.
A common electric return line for providing power to transducer ele-l 1579~

ments 2 is formed by a conductor 30, of which only the end face is visible inthis Figure. Common conductor 3Q is electrically coupled to the opposite pole of a power supply (not shown) from one of conductor plates 12 and 13, by means of a conductor which is not shown in the Figure. During manufacture of the transducer arrangement, piezoelectric body 10 which has been provided on its lower and upper flat sides with metal deposits 18 and 19, respectively, is first connected undetachably to the electrically conducting surface layer of a contact foil which is not more than 10 microns thick. The contact foil consists at least partially of metal and can be advantageously divided so as to cover only the edge zones of piezoelectric body 10. Such division prevents reflections at higher frequencies, particularly 5 MHz and above, which can adversely influence the pulse shape. Contact foils 20 and 22 are preferably formed of plastic, par-ticularly poly-(diphenyloxide pyromellithimide) (polyimide, capton) or also of a polyester foil of an ethylene glycol and terephthalic acid ~mylar), which have great mechanical strength and are also suitable for high temperature stress during the subsequent metallizing and soldering processes. The thickness of the contact foil is preferably substantially less than the wave length of the ultrasonic fre-quency, and may illustratively be 7.5 microns for polyimide, and only approxi-mately 2 microns for mylar.
Figure 2 shows an embodiment of the invention wherein the contact foil is provided with a metal coating 21 and 23 which does not substantially exceed 1 micron, so as to prepare the connection to the piezoelectric body 10. A thin metal coating 21 or 23 of contact foils 20 and 22 can preferably consist of two layers; a first layer 32 and 35, respectively, serves as an adhesion layer and is preferably formed of chromium or titanium. Chromium layer 32 and 35 is at most 1 micron in thickness, and preferably approximately 0.02 microns. Respec-tive layers 33 and 36 which have high electrical conductivity provide good ad-1 157g~

hesion to piezoelectric body 10. Such lligh conductivity layers with good ad-hesion characteristics may be formed of nickel, copper, silver, gold or platinum.
The thickness of this layer is preferably between 0.05 to 0.2 microns.
In some embodiments, it may be advantageous to insert respective middle layers 34 and 37, which are formed of a material which is virtually nonreactive with respective solder layers 42 and 44 which join contact foils 20 and 22 to piezoelectric body 10. In conjunction with layers 32 and 35 of chromium, the solder-stopping layers 34 and 35 may be formed of platinum, and layers 33 and 37 may be formed of gold. In such an embodiment, chromium layers 32 and 35 can be preferably approximately 0.05 microns thick; platinum layers 34 and 37 approxi-mately 0.1 microns thick; and gold layers 33 and 36 approximately 0.2 microns thick; so that the total thickness of the metal coating is not substantially more than 0.35 microns.
Metallization layers 21 and 23 can be preferably applied by sputtering.
This technique permits good adhesion between the metal layers, which is necessary because of the subsequent fine division of piezoelectric body 11. In some embodi-ments, such sputtering can be preferably preceded by a sputter-etc]~l~ng ~roces$, whereby the surface of contact foils 20 and 22 is cleaned to remove adsorption layers and dust, and is additionally activated, to promote adhesion of the metal coating. In other embodiments, metal coatings 21 and 23 can be prepared by ion plating or by vapor deposition. In such cases, contact foils 20 and 22 are first provided with adhesion layers 32 and 35 by ion plating. Adhesion is further enhanced by etching the surface of contact foils 20 and 22 in an oxygen plasma before applying adhesion layers 32 and 35.
The undetachable connection between contact foils 20 and 22 with piezo-electric body 10 must be free of occlusions to prevent separation of transducer elements 2 from contact foils 20 and 22 during and after the fine division of piezoelectric body 10. As noted, such f;ne ~ivision of piezoelectric body 10 produces individual transducer elements 2 which have a small width on the order of 100 microns. Solder layers 4 and 44 are preferably applied as solder alloys or as multl-layer sequences of solder components, as shown in Figure 2. Solder layers 42 and 44 should be formed of a material with a relatively low melting point to prevent complete or partial depolarization of piezoelectric body 10.
The layers within solder layers 42 and 44 may consist, for example, of an indium-tin solder (InSn) in equal parts, which is preferably vapor-deposited as a multi-layer sequence, for instance, in four layers with a total thickness of, for instance, approximately 3 microns. The overall thickness of solder layers 42 and 44 depends upon the roughness of the surface of the piezoelectric body, and may ~e approximately within the range of 0.5 to 6 microns. Solder layers 42 and _ can be connected to metal layer 18 of piezoelectric body 10.
In some embodiments, it may be advantageous to provide metal deposit 18 on piezoelectric body 10 with solder deposits 46 and 48 in the areas where the piezoelectric body couples to contact foils 20 and 22. Solder deposits 46 and 48 can preferably be vapor-deposited. ~ith the aid of a flux, the soldering opera-tion can be performed in air, or, alternatively, as a vacuum soldering joint with-out flux.
The thus formed assembly consisting of piezoelectric body 10 and contact foils 20 and 22, are fastened to a mechanically stable support which may be damping body~4 of the ultrasonic transducer arrangement, to enhance the mechanical strength prior to the subsequent subdivision of the piezoelectric body. Such a joint can be prepared, for example, by a conductive adhesive 52.
As-previously noted, transducer elements 2 are produced by subdividing piezoelectric body lQ. Such subdivision is accomplished, for example, by saw cuts, where the saw cuts are taken from metal deposit 19 and through piezoelectric 1 1579~

bod~ 10 to such a depth that also electrically conducting parts of contact foils 20 and 22 (i.e., metal deposits 21 and 23) are separated. The gaps produced by the saw cuts extend into the overhanging portion of the contact foils, but as shown in Figure 1, not to its outer edge.
Common conductor 30 for controlling transducer elements 2 is fastened to the top side of the transducer elements, for example, by cementing with a layer of conducting adhesive 54, as shown in Figure 2. Matching body 5 can also be fastened to metal layer 19 of piezoelectric body 10 by means of an adhesive 58. As noted above, the layer thicknesses shown in the drawings are magnified for clarification.
Contact tabs 24, 26, 27 and 28, and bridges 25 are formed by cutting open the overhanging portions of contact foils 20 and 22 after the contact foil and piezoelectric body are fastened to damping body 4, and after the fastening matching body 5. In some cases, it may be advantageous to design the contact foil as a profile body with contact tabs 24, 26, 27 and 28, and contact bridges 25 prior to the fastening of its metal deposit and to the piezoelectric body.
After completion of th~ assembly consisting of the contact foil, the piezoelec-tric body, and the damping body, the contact tabs are then connected only to a corresponding control conductor.
The handling of the arrangement during the fabrication of connecting tabs 24 through 28 is facilitated by attaching the contact tabs for adjacent oscillators always alternating on opposite narrow sides o piezoelectric body lQ. Thus, according to Figure 2, oscillator 6 has its corresponding contact tab 24 on the right-hand side of the arrangement, while contact ~oil 22 is disposed on the opposite narrow side, and forms contact bridge 25 for the transducer ele-ments of oscillator 6. The following oscillator 7 has its contac$ tab 29 on the left-hand narrow side of piezoelectric body 10 and is connected to a control 1 157~

conductor of control plate 13, while the contact tab on the right-hand narrow side (not shown in Figure 2) is shortened except for a contact bridge.
Figure 3 shows a particularly advantageous embodiment of the invention.
Transducer elements 2 which are arranged between damping body 4 and matching body 5 are connected on their top side and outer surface region to preferably divided shielding foils 60 and 62, which consists oE electrically conductive material, preferably metal. Shielding foils 60 and 62 are provided with cor-responding metal deposits 61 and 63 which may be formed in the same manner as metal deposits 21 and 23 of contact foils 20 and 22. In addition, metal deposits 61 and 63 may be provided in the area where they are fastened to transducer ele-ments 2 with solder deposits 66 and 68 which are fastened to metal deposit 19 by a soldering process. Metal deposit 19 can preferably be provided with solder deposits 70 and 72 consisting of the same material as solder deposits 46 and 48, and may also be vapor-deposited.
Shielding foils 60 and 62 form a common conducting connection for the other pole of the control voltage of transducer elements 2. The parts of shield-ing conductors 60 and 62 which overhang the lateral end faces of transducer ele-ments 2 are designed as conductor tabs 74 and 75 which are connected to a common conductor of the conductor plates 12 and 13. In a preferred embodiment, the 2Q overhanging portions of the contact foils are not divided into conductor tabs, but are designed as a common bridge-like connection over all transducer elements to connect them at one point to the common counter pole of the control voltage.
Shielding foils 60 and 62 and their contact tabs 74 and 75, extend over the row of contact tabs of contact foils 20 and 22 so as to shield them against high frequency pick-up from the environment. Accordingly, a common connecting con-ductor 30, as shown in Figures 1 and 2, is not necessary for the top side of transducer elements 2.

1 157g~6 In the embodiment of the Figure 3 which is provided with contact foils 20 and 22 as well as shielding foils 60 and 62, piezoelectric body 10 can be connected, after its flat sides are metallized, first to contact foils 20 and 22, and shielding foils 60 and 62, and then fastened to damping body 4. Subsequently, transducer elements 2 can be produced by fine division of piezoelectric body 10 while the outer most extending portions of the upper and lower contact foils are bent at the bottom toward damping body 4. These outer most extending portions are temporarily held by an adhesive plastic strip so that the outer edge of the overhanging portions is not completely sawed through during the fine division operation.
Although the invention has been described in terms of specific embodi-ments and applications, other embodiments and applications, in light of this teaching, would be obvious to persons skilled in the pertinent art. Accordingly, the drawings and descriptions in this disclosure are proffered to facilitate comprehension of the invention and should not be construed to limit the scope thereof.

Claims (25)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for manufacturing an ultrasonic transducer arrangement having a plurality of transducer elements which are provided with contact tabs formed of a contact foil, the method comprising the steps of: coating at least a first flat side of a plastic contact foil with a metal; undetachably connecting a strip-shaped piezoelectric body at a first flat side thereof to a predetermined surface region of said metal coated first flat side of said plastic contact foil;
fastening a second flat side of said plastic contact foil to a damping body; and subdividing said piezoelectric body to form the plurality of transducer elements.

2. The method of claim 1 wherein prior to undetachably connecting said piezoelectric body to said first flat side of said plastic contact foil, there is provided the further step of dividing said plastic contact foil to form a plastic contact foil region for each of the transducer elements, said divided plastic contact foil then being undetachably connected at an edge region thereof to a respective edge region of said first side of said strip-shaped piezoelectric body.

3. The method of claim 1 wherein said plastic contact foil is formed of a selectable one of poly-(diphenyloxide-pyromellithimide) and a polyester foil having an ethylene glycol and terephthalic acid base.

4. The method of claim 3 wherein said plastic contact foil has a maximum thickness of 10 microns.

5. The method of claim 3 wherein said metal coating of said plastic con-tact foil has a maximum thickness of 10 microns.

6. The method of claim 5 wherein said metal coating is deposited on said plastic contact foil in a pattern having cutouts.

7. The method of claim 2 wherein a plurality of the transducer elements are electrically coupled to one another to produce a respective transducer group is electrically controlled jointly, the method having the further step of separating a portion of said plastic contact foil which overhangs said piezo-electric body on one side thereof to form a contact tab for said transducer group.

8. The method of claim 1 wherein there are further provided the steps of:
undetachably connecting a second flat side of said piezoelectric body to a pre-determined portion of a metal coated surface of a shielding foil, said shielding foil having a portion extending beyond said piezoelectric body;
and shaping said extending portion of said shielding foil to form contact tabs.

9. The method of claim 8 wherein said steps of undetachably connecting said piezoelectric body to said plastic contact foil, and said step of undetach-ably connecting piezoelectric body to said shielding foil, are performed in a joint operation.

10. The method of claim 1 wherein said step of coating a first flat side of a plastic contact foil is performed by depositing a plurality of metal layers.

11. The method of claim 10 wherein said step of depositing several metal layers is performed by depositing a first metal adhesion layer of chromium;
and a second metal layer of nickel.

12. The method of claim 10 wherein said step of depositing several metal layers is performed by depositing a first metal layer as an adhesion layer of chromium; and a second metal layer over said first metal layer of a selectable one of gold, copper, silver, and platinum.

13. The method of claim 11 wherein said first layer has a maximum thick-ness of 0.05 microns and said second layer has a maximum thickness of 0.2 microns.

14. The method of claim 10 wherein there is further provided the step of depositing a third metal layer as a solder-stop layer, said third metal layer being interposed between said first and second layers.

15. The method of claim 14 wherein said first, second, and third metal layers are formed by sputtering.

16. The method of claim 15 wherein there is provided the further step of sputter-etching said plastic contact foil prior to sputtering said first, second, and third metal layers.

17. The method of claim 15 wherein said step of depositing said first, second, and third metal layers is performed by a selectable one of ion plating and vapor deposition.

18. The method of claim 16 wherein said step of sputter-etching said plastic contact foil is performed in an ambient oxygen plasma prior to depositing said first metal layer.

19. The method of claim 14 wherein there is further provided the step of depositing a fourth metal layer of solder on said second metal layer, said fourth metal layer being deposited on a predetermined surface area of said second metal layer for connection to said piezoelectric body.

20. The method of claim 19 wherein said step of depositing said fourth metal layer of solder is vapor-deposited.

21. The method of claim 19 wherein there is further provided the further step of depositing a fifth metal layer of solder, said fifth metal layer being deposited on said fourth metal layer.

22. The method of claim 21 wherein there are further provided the steps of depositing sixth and seventh metal layers of solder, so as to produce four layers of solder material.

23. The method of claim 22 wherein said four layers of solder comprise at least one layer of all indium-tin (InSn) solder.

24. The method of claim 22 wherein said four layers of solder have a maximum thickness of 6 microns.

25. The method of claim 1 wherein there is further provided the step of depositing a layer of solder on said first flat side of said strip-shaped piezoelectric body for undetachably connecting said piezoelectric body to said plastic contact foil.