Matched MIC delay line transducer using a series array

Abstract

A transducer for a bulk-mode acoustic delay line comprises a plurality of ries-connected transducers and a series inductance deposited at the end of the delay line. The number and configuration of the series-connected transducers is selected such that the sum of the resistive components of their impedances matches the driving impedance. The value of the inductance is chosen to balance the combined capacitive components of the series-connected transducers. Series contact between the electrodes of adjacent transducers is achieved by configuring the ground layer electrode with a finger-like projection which is overlapped by a similar projection in the top layer electrode of the adjacent transducer. The overlapping portions of the electrodes are permitted to make electrical contact with one another by removing the piezoelectric material from the overlap region.

Government Interests

RIGHTS OF THE GOVERNMENT

The invention described herein may be manufactured, used, and licensed by or for the United States Government for governmental purposes without the payment to me of any royalty thereon.

Description

BACKGROUND OF THE INVENTION

The present invention relates to improvements in bulk-mode delay line transducers and, more particularly, to a transducer which is compact, inexpensive, shock-resistant, and tolerant to manufacturing process variations.

Typical microwave acoustic longitudinal-mode transducers employ a piezoelectric film sandwiched between two metal electrode films. All three films are generally deposited by any suitable technique, in sequence, on the end of a suitable delay line. The equivalent impedance of the transducer includes resistive and capacitive components determined by the dimensions of the piezoelectric film portion clamped between the two electrodes. More particularly, since the transducer is, in effect, a piezoelectric capacitor whose thickness is approximately one quarter of an acoustic wave length, the impedance is inversely proportional to the area and directly proportional to the thickness of the piezoelectric material located between two electrodes. Ideally, the resistive component of the transducer impedance should be matched to the impedance of the driving generator for the delay line (typically 50 ohms). At low frequencies, where the piezoelectric layer is relatively thick, the area of the transducer may be reduced to permit the resistive component of the impedance to be 50 ohms. However, too small a transducer area creates two problems. First, the reduced-size transducer produces an acoustic beam which is so narrow as to render it extremely difficult to locate the beam upon its arrival at the opposite end of the delay line; the proper placement of the receiving transducer is therefore difficult to achieve. Second, it becomes difficult to achieve a reliable bond to an extremely small top electrode. For these reasons, conventional transducers designed to operate in the vicinity of 3.5 GHz should be at least 0.005 inch in diameter. This size results in a value of approximately 1 ohm for the resistive component of impedance and, therefore, some form of impedance matching to the driving generator is required. Conventionally, bonding wires are normally employed to connect this transducer to a matching network.

Ideally, the impedance matching components should be as small and rugged as the delay line itself. In this respect it is desirable to provide an impedance matching system which fits on the end face of the delay line along with the transducer. A conventional transducer, fabricated with a ground layer deposited over the entire end face of the delay line, precludes utilization of the end surface of the insulating delay medium as a substrate for a tuning or matching component. A solution for this problem is described in my prior U.S. Pat. No. 3,688,222 wherein I utilize the concept of overlapping-finger transducer electrodes. More particularly, in may prior patent the ground layer electrode is deposited over only a portion of the end of the delay line; likewise the top layer electrode is deposited over only a portion of the piezoelectric with only finger-like projections of the two electrodes being overlapped. With this configuration both the top layer and the ground layer electrodes are available in the same plane at the end face of the delay line. Moreover some portions of the end face of the delay medium are not coated and therefore are available as a substrate for a matching network. That arrangement achieved its basic goals but was difficult to fabricate because the matching was extremely critical and fabrication controls were not sufficiently refined to permit precise reproduceability.

It is known that many small transducers can be connected in series at the end of a delay line and that each transducer operates independently of the others (reference Weinert et al., "A Thin Film Moasic Transducer For Bulk Waves," I.E.E.E. Transactions on Sonics and Ultrasonics, July 1972, pages 354 through 357). However no attempt has been made to utilize this series transducer approach in a compact rugged package which permits the impedance matching arrangement to fit on the end face of the delay line. Nor has there been any prior art attempt to provide a fabrication technique wherein the series connections between adjacent transducers can be effected simply and reliably.

It is therefore an object of the present invention to provide a multi-transducer array which can be simply fabricated at the end of a delay line utilizing known integrated circuit techniques.

It is another object of the present invention to provide a series array of transducers which can be deposited along with a series tuning inductor at the end of a delay line.

It is another object of the present invention to provide a compact rugged transducer package for a delay line wherein multiple transducers are connected in series using integrated circuit techniques.

SUMMARY OF THE INVENTION

In accordance with the principles of the present invention, each of the series-connected transducers includes a ground layer electrode deposited on the end surface of a delay line and having a finger projecting toward an adjacent electrode. Piezoelectric material is deposited over the electrodes and removed in the region of the projecting fingers. A top layer electrode for each transducer is deposited over the piezoelectric material and extends to contact the exposed portion of the finger projecting from a ground layer electrode of the adjacent transducer. The tuning inductor is a serpentine-shaped inductor connected to the input and output end of the series transducer array and is deposited along with the ground layer electrodes. All connections are thus part of the basic integrated circuit structure. The number and configuration of the transducers is selected such that their combined series resistance matches the impedance of the driving generator. The series-connections between the transducers, on the other hand, reduces the total capacitance accordingly so that this capacitance can be tuned out of the circuit by a relatively large and readily fabricated series inductance.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of specific embodiments thereof, especially when taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a plan view of an end of a delay line on which is fabricated a series-array transducer arrangement according to the present invention;

FIG. 2 is an electrical equivalent circuit of the transducer and inductor of FIG. 1;

FIGS. 3, 4 and 5 are diagrammatic representations of successive stages in the fabrication process of the transducer and inductance of FIG. 1; and

FIG. 6 is a sectional view through a portion of the end of the delay line of FIG. 1 and illustrating the transducer arrangement of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1 of the accompanying drawings, a transducer arrangement according to the present invention is illustrated on the circular end surface of an acoustic delay line. The transducer itself is defined within a diametrically extending strip 11 which is positioned between two spaced terminals 12 and 13. The terminals are in the form of segments of a circle and serve to provide electrical connections to externally of the transducer and delay line. Terminals 12 and 13 are deposited directly on the delay line and the surface. A serpentine inductor 14 is also deposited on the delay line end surface and extends from terminal 12 to the first transducer in an array 15 of series-connected transducers. The last transducer in the array is connected to a further serpentine-shaped inductor 16 which has its other end connected to terminal 13. The transducers in array 15 and the inductor 16 are also deposited on the end surface of the delay line.

The particular transducer array 15 illustrated in FIG. 1 includes 42 transducers arranged in seven rows of six transducers each. The number of transducers utilized in array 15 is by way of example only and not limiting on the scope of the present invention. The number of transducers utilized in any embodiment is selected to permit the total series resistance of all transducers to match the driving impedance of the generator connected to the delay line. The electrical equivalent circuit of the arrangement in FIG. 1 is illustrated schematically in FIG. 2. The impedance of each transducer in array 15 includes a resistive component R and a capacitive component C. These components are illustrated with appropriate subscripts to designate the number of the transducer with which they are associated. Since the transducers are connected in series, the resistances R.sub.1 through R.sub.42 are additive and combine with the resistance R.sub.S to define the total resistive impedance of the equivalent circuit. Resistance R.sub.S represents the series resistance of the thin-film metallic layers and the contact resistance between the various layers and is small relative to the actual resistive component of the transducers. The series-connection of the transducers serves to reduce the total capacitance of the array 15 from the value of a single capacitance in the array. The effect of the capacitance of the array is tuned out at the operating frequency by inductors 14 and 16. The reduced overall capacitance of the array permits inductors 14 and 16 to be larger and therefore more readily fabricated. Inductance 14 is illustrated as being variable in FIG. 2 to point up the fact that a final adjustment in the tuning can be achieved by shorting out various portions of the serpentine inductance after the transducer assembly has been completed.

Referring to FIGS. 3 through 6 of the accompanying drawings, the fabrication steps for the transducer arrangement will now be described. Referring first to FIGS. 3 and 6, the ground layer metalization is first deposited on the substrate 20 defined by the end surface of the delay line. The ground layer metalization includes the ground layer electrode 21 for each transducer in the array as well as the inductances 14 and 16 and terminals 11 and 12. To facilitate the description and understanding herein, inductance 16 and terminals 11 and 12 are not illustrated in FIG. 3; moreover, the ground layer electrodes 21 of only six transducers are illustrated in order to prevent undue complexity in the description. Typically the ground layer metalization is deposited by evaporation directly on the end surface of the substrate 20. For purposes of the specific embodiment being described herein, the ground layer metalization comprises 100 A of chromium and 800 A of gold. The metalization is evaporated over the entire end surface; then, utilizing photoresist protection, the ground pattern around the electrodes, inductors and terminals are selectively etched. It is to be especially noted that each ground layer electrode 21 includes a finger 22 which projects toward the next electrode 21 in the array. As will become clearer from the description of FIG. 5 below, it is this finger 22 at which the top layer electrode of one transducer makes electrical contact with the ground layer electrode of an adjacent transducer.

Referring now to FIGS. 4 and 6 of the accompanying drawings, the piezoelectric layer 23 is deposited over electrodes 21. The piezoelectric material may be zinc oxide, cadmium sulfide, or any known material possessing piezoelectric properties. Typically the piezoelectric layer is sputtered over the region containing electrodes 21 to a thickness of approximately 4000 A. A protective photoresist coating is then applied and the piezoelectric layer 23 is etched at contact portion 24 in each of the fingers 22. The piezoelectric film layer 23 thus covers all of each ground layer electrode 21 except for the small contact portion 24 defined by the selective etching.

Referring now to FIGS. 5 and 6 of the accompanying drawings, the top layer metallization is applied. This is done by first coating the piezoelectric layer 23 with photoresist and then placing a delineation rejection mask configured to permit the top layer electrodes to be positioned as desired. The top layer is then evaporated (typically 100 A chromium, 1000 A gold) and the photoresist and unwanted metal are then removed. The top layer electrode 25 for each transducer overlies a portion of the ground layer electrode 21 of that transducer, this portion being stippled in the drawing for purposes of identification. In addition each top layer electrode 25 extends along the piezoelectric layer 23 and into the contact region 24 of an adjacent transducer. Thus, at one of of its ends the top layer electrode sandwiches piezoelectric layer 23 between it and the ground layer electrode 21. At its other end, top layer electrode 25 makes contact with the ground layer electrode of the next transducer. Series interconnection between the transducers is thus permanent and reliable. Connections to and from the inductors 14 and 16 (not shown) are made in the same way.

In a typical embodiment, the transducers were each configured in a 0.005 inch square, resulting in slightly greater than 1 ohm resistance for each transducer in the array. Multiplied by 42, this resistance is increased to a value of approximately 50 ohms, the driving impedance of a typical driving generator in a microwave system. Naturally the size and number of the transducers can be varied accordingly to effect matching to substantially any impedance value. In an embodiment constructed a 27 nH series inductance (inductances 14 and 16) was sufficient to tune out the overall series capacitance of the transducers.

The structure fabricated as described requires no external matching components and therefore permits the entire delay line-transducer package to be fabricated with minimal length. The package is rugged and the integrated circuit fabrication techniques insure electrical connections which are highly shock-resistant. The arrangement is suitable for mass production using standard techniques. The resulting transducer is insensitive to process variations because a combination of multiple individual transducers tends to average out parameter variations which might occur in a single transducer. Moreover, slight variations in the parameters of individual transducers result in a stagger tuning effect which broadens the passband of the overall transducer. That is, slight differences in the actual frequencies of the transducers combine to provide a broader band width than is possible with a single transducer. Moreover, multiple transducers connected in series can withstand substantially higher applied voltages without electrical breakdown than is possible with a single transducer.

It should also be noted that the arrangement described herein permits multiple transducer arrays to be fabricated in a single manufacturing process.

It will be appreciated that certain specific details described herein are not to be considered limiting features of the present invention. For example, the metal utilized in the electrodes may be varied from that described in the specific example. Moreover the thickness of each layer may differ from that described. It should also be pointed out that the configuration of each transducer can be different to achieve whatever overall effect is desired. Further, the piezoelectric material utilized may differ from transducer to transducer.

I wish it to be understood that I do not desire to be limited to the exact details of construction shown and described, for obvious modifications can be made by a person skilled in the art.

Claims

What is claimed is:

1. A transducer arrangement for a bulk mode acoustic delay line comprising a plurality of series connected, adjacent, closely spaced transducers disposed on an end surface of said delay line, each two adjacent transducers of said plurality of transducers comprising first and second transducers each having,

a thin film metal ground layer electrode, the end of said electrode of said first transducer extending beyond said first transducer and abutting said second transducer,

a layer of electromechanical transducer material disposed on said ground layer electrode and having a transducer material top surface and at least a side surface at right angles thereto and covering all but a specified exposed end portion of the transducer ground layer electrode, said end of said first transducer ground layer electrode being the end of said specified end portion of said first transducer ground layer electrode,

and a thin film metal top layer electrode disposed on said top surface of said transducer material layer and overlying part of said ground layer electrode, the top layer electrode of said second transducer extending beyond said ground layer electrode of said second transducer and into overlying contact with the specified exposed end portion of the ground layer electrode of said first transducer to connect said first and second transducers in series, said top layer electrode of said second transducer being bent at right angles down a said side surface of transducer material and then at right angles again into said overlying contact.

2. The transducer arrangement according to claim 1 wherein said series-connected transducers are arranged in a mosaic pattern of columns and rows, the series connections between transducers defining a zig-zag path.

3. The transducer arrangement according to claim 1 further comprising an inductance connected in series with said transducers and deposited on said end surface of said delay line, the value of said inductance being such that its impedance at the operating frequency of said transducer arrangement is substantially equal to the impedance of the overall capacitance of said series connected transducers.

4. The transducer arrangement according to claim 3 wherein said deposited inductance comprises two deposited inductors, each deposited to one side of said plurality of transducers and each deposited inductor defining a zig-zag path.

5. The transducer arrangement according to claim 2 wherein the metal ground layer electrodes of at least said transducers which are not the end transducers of said columns and rows are T shaped, one part of said T forming a tab which includes said specified exposed end portion.

6. The transducer arrangement according to claim 5 wherein said end surface of said delay line is generally circular and wherein said mosaic pattern resides in a strip extending diametrically across said end surface, said transducer arrangement further comprising two spaced terminals secured to said end surface and electrically-connected to different ends of the series connection of transducers.

7. The transducer arrangement to claim 6 wherein said terminals are configured as segments of a circle disposed on opposite sides of said diametrically-extending strip.

8. The transducer arrangement according to claim 2 wherein said end surface of said delay line is generally circular and wherein said mosaic pattern resides in a strip extending diametrically across said end surface, said transducer arrangement further comprising two spaced terminals secured to said end surface and electrically-connected to different ends of the series connection of transducers.

9. The transducer arrangement according to claim 8 wherein said terminals are configured as segments of a circle disposed on opposite sides of said diametrically-extending strip.