Method of forming a multilayer acoustic impedance converter for ultrasonic transducers

Abstract

A method of forming an ultrasonic transducer comprises coupling a front polymer layer of uniform thickness to a piezoelectric element. A front metal layer is coupled to the polymer layer on a side of the front polymer layer opposite the piezoelectric element for transmitting acoustic energy between the front polymer layer and a propagation medium. The front polymer layer and the front metal layer define a front acoustic impedance converter, wherein the front polymer layer completely isolates the piezoelectric element from the front metal layer.

Claims

1. A method of forming an ultrasonic transducer comprising the steps of: providing a piezoelectric element having a high characteristic acoustic impedance; coupling a front polymer layer to said piezoelectric element, said front polymer layer having a continuous, uniform thickness along a length of the front polymer layer; and coupling a front metal layer to said front polymer layer on a side of said front polymer layer opposite said piezoelectric element for transmitting acoustic energy between said front polymer layer and a propagation medium having a low characteristic acoustic impedance lower than the high characteristic acoustic impedance of the piezoelectric element, wherein said front polymer layer and said front metal layer define a front acoustic impedance converter effective to convert between a characteristic acoustic impedance of the piezoelectric element and a characteristic acoustic impedance of the propagation medium, wherein said front acoustic impedance converter has an effective characteristic acoustic impedance between said piezoelectric element and said propagation medium characteristic acoustic impedances, and wherein said front polymer layer completely isolates said piezoelectric element from said front metal layer.

2. The method of claim 1, wherein said front polymer layer and said front metal layer comprise discrete continuous layers.

3. The method of claim 1, further comprising the step of coupling a backing absorber to said piezoelectric element, said backing absorber having a characteristic acoustic impedance.

4. The method of claim 1, further comprising the step of coupling a quarter wavelength matching layer to said front metal layer, said quarter wavelength matching layer configured to be in contact with said propagation medium.

5. The method of claim 1, wherein a thickness of said front metal layer (t.sub.m1) and a thickness of said front polymer layer (t.sub.p1) are substantially:
t.sub.m1=Z.sub.C/(.sub.m2f.sub.0); and
t.sub.p1=V.sub.p.sup.2.sub.p/(2f.sub.0Z.sub.C), wherein, Z.sub.C is said effective characteristic acoustic impedance of said front acoustic impedance converter, .sub.m is a density of said front metal layer, f.sub.0 is a predetermined center resonant frequency, V.sub.p is a velocity of sound in said front polymer layer, and .sub.p is a density of said front polymer layer.

6. The method of claim 1, wherein thicknesses of said front metal layer and said front polymer layer are selected based on densities of said front metal layer and said front polymer layer, said effective characteristic acoustic impedance of said front acoustic impedance converter, a predetermined center resonant frequency of said ultrasonic transducer, and a velocity of sound in said front polymer layer.

7. A method of forming an ultrasonic transducer comprising the steps of: providing a piezoelectric element having a high characteristic acoustic impedance; coupling a front polymer layer to said piezoelectric element, said front polymer layer having a continuous, uniform thickness along a length of the front polymer layer; and coupling a front metal layer to said front polymer layer on a side of said front polymer layer opposite said piezoelectric element for transmitting acoustic energy between said front polymer layer and a propagation medium having a low characteristic acoustic impedance lower than the high characteristic acoustic impedance of the piezoelectric element, wherein said front polymer layer and said front metal layer define a front acoustic impedance converter effective to convert between an impedance of the piezoelectric element and an impedance of the propagation medium, wherein said front polymer layer completely isolates said piezoelectric element from said front metal layer, and wherein said thickness of said front polymer layer is less than one quarter of the wavelength of a predetermined center resonant frequency of said ultrasonic transducer.

8. A method of forming an ultrasonic transducer comprising the steps of: providing a piezoelectric element; coupling a front polymer layer to said piezoelectric element, said front polymer layer having a continuous, uniform thickness along a length of the front polymer layer; coupling a front metal layer to said front polymer layer on a side of said front polymer layer opposite said piezoelectric element for transmitting acoustic energy between said front polymer layer and a propagation medium; wherein said front polymer layer and said front metal layer define a front acoustic impedance converter, and wherein said front polymer layer completely isolates said piezoelectric element from said front metal layer; coupling a low characteristic acoustic impedance layer having a first characteristic acoustic impedance to said front metal layer; and coupling a high characteristic acoustic impedance layer having a second characteristic acoustic impedance higher than said first characteristic acoustic impedance to said low characteristic acoustic impedance layer for being in contact with said propagation medium.

9. The method of claim 8, wherein said high characteristic acoustic impedance layer comprises at least one of a metal and a high characteristic acoustic impedance polymer.

10. The method of claim 9, wherein said high characteristic acoustic impedance polymer comprises at least one of a polyimide and polyester.

11. The method of claim 10, wherein said low characteristic acoustic impedance layer comprises at least one of rubber and latex.

12. A method of forming an ultrasonic transducer comprising the steps of: providing a piezoelectric element; coupling a front polymer layer to said piezoelectric element, said front polymer layer having a continuous, uniform thickness along a length of said front polymer layer; and coupling a front metal layer to said front polymer layer on a side of said front polymer layer opposite said piezoelectric element for transmitting acoustic energy between said front polymer layer and a propagation medium, wherein said front polymer layer and said front metal layer define a front acoustic impedance converter, and wherein said front polymer layer completely separates said front metal layer from said piezoelectric element; and wherein said thickness of said front polymer layer is less than one quarter of a wavelength of a predetermined center resonant frequency of said ultrasonic transducer.

13. The method of claim 12, wherein said front polymer layer and said front metal layer comprise discrete continuous layers.

14. The method of claim 12, further comprising the step of coupling a backing absorber to said piezoelectric element, said backing absorber having a characteristic acoustic impedance.

15. The method of claim 12, further comprising the step of coupling a quarter wavelength matching layer to said front metal layer, said quarter wavelength matching layer configured to be in contact with said propagation medium.

16. The method of claim 12, further comprising the steps of: coupling a low characteristic acoustic impedance layer to said front metal layer; and coupling a high characteristic acoustic impedance layer to said low characteristic acoustic impedance layer for being in contact with said propagation medium.

17. The method of claim 12, wherein a characteristic acoustic impedance of said propagation medium is lower than a characteristic acoustic impedance of said piezoelectric element, and wherein said front acoustic impedance converter has an effective characteristic acoustic impedance between characteristic acoustic impedances of piezoelectric element and said propagation medium.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) Understanding of the present invention will be facilitated by consideration of the following detailed description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings, in which like numerals refer to like parts and in which:

(2) FIG. 1A is a metal polymer multilayer impedance converter for ultrasonic transducers, according to an embodiment of the invention;

(3) FIG. 1B is a mass and spring model for the metal polymer multilayer impedance converter of FIG. 1A;

(4) FIG. 2 illustrates the impedance performance of a prior art quarter wavelength matching layer with Z.sub.R=1.5 MRayl, loaded with water;

(5) FIG. 3 illustrates the impedance performance of a polyimide/copper multilayer impedance converter designed for 6.8 MHz, according to an embodiment of the invention;

(6) FIG. 4 illustrates an ultrasonic transducer with a polymer-metal multilayer impedance converter, according to an embodiment of the invention;

(7) FIG. 5 illustrates an embodiment of an ultrasonic transducer with a multilayer impedance converter on a front face of a piezoelectric layer and a backing absorber layer, according to an embodiment of the invention;

(8) FIG. 6 illustrates a simulated performance of the ultrasonic transducer of FIG. 4, according to an embodiment of the invention;

(9) FIG. 7A is a sectional view of an ultrasonic transducer having an impedance converter of the invention, according to an embodiment of the invention;

(10) FIG. 7B is a sectional view of an ultrasonic transducer having double matching or impedance converter layers, according to another embodiment of the invention;

(11) FIG. 8 illustrates an experimental observation of an output waveform as a function of time after a short pulse excitation of an ultrasonic transducer of FIG. 7A; and

(12) FIG. 9 illustrates a frequency response curve resulting from a Fourier transform of the waveform of FIG. 8.

DETAILED DESCRIPTION OF THE DRAWINGS

(13) Reference will now be made to various embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. It is to be understood that the figures and descriptions of the present invention have been simplified to illustrate elements that are relevant for a clear understanding of the present invention, while eliminating, for purposes of clarity, many other elements found in typical ultrasonic transducers. Because such elements are well known in the art, and because they do not facilitate a better understanding of the present invention, a discussion of such elements is not provided herein. The disclosure herein is directed to all such variations and modifications known to those skilled in the art.

(14) In prior art ultrasonic transducers, a layer of low characteristic impedance material, with a thickness of one quarter of the wavelength of the center frequency of the transducer, is provided between the piezoelectric element and a propagation medium. Exemplary low characteristic impedance materials for air ultrasonic transducers include aerogels and plastic foams. For imaging transducers such as those useful in medical applications, such low characteristic impedance materials include substantially pure polymer and/or polymer loaded with powder and/or fibers. The use of a layer of low characteristic impedance material, referred to as a matching layer or an impedance converter, improves the conversion ratio of electric to acoustic energy in transmitting of acoustic signals, as well as preventing or reducing undesirable phase shift, compared to ultrasonic transducers with no matching layer. However, such matching layers have disadvantages, including an undesirably large thickness for some applications. In addition, if the thickness of the matching layer varies from one quarter of a wavelength of the acoustic wave, the conversion ratio decreases, resulting in observable phase shifts. A matching layer thus is generally undesirable in transducers where broadband or wideband performance (e.g., consistent performance over a wide band of acoustic wavelengths) is required. For example, pulse excitation and reception, often employed in medical ultrasonic imaging, requires good broadband performance. Moreover, as suitable materials are not single phase, scattering of acoustic energy occurs, resulting in undesirable propagation loss. Still further, it is difficult to manufacture suitable materials to obtain specific desired characteristic acoustic impedances, resulting in problems relating to mass production of such transducers for specific applications.

(15) At least some of the drawbacks associated with prior art transducers are addressed in an embodiment wherein a transducer includes a piezoelectric element and a polymer layer disposed on the piezoelectric element. A metal layer is disposed on the polymer layer. The polymer layer and the metal layer together constitute an impedance converter. The thicknesses of the polymer layer and the metal layer are so selected as to provide the impedance converter with an effective characteristic acoustic impedance intermediate the characteristic acoustic impedances of the piezoelectric element and of the propagation medium. Advantageously, by selecting the thicknesses of the metal and polymer layer, a range of effective characteristic acoustic impedances is available. The thickness of this impedance converter may be configured to be much less than one quarter of the wavelength of the target frequency of the acoustic signals.

(16) According to an aspect of the invention, the condition wherein the impedance converter is substantially thinner than one quarter wavelength is compensated for by means of an additional material layer having a relatively higher impedance (or higher density material) positioned on the lower impedance side (propagation medium side) of the converter. In this manner, the additional layer compensates for otherwise degraded converter performance and operates to provide (i.e. recovers) substantially the original impedance conversion function. The higher impedance material layer (e.g. metal layer) is positioned between the thickness reduced converter layer (e.g. polymer layer) and the lower impedance region adapted to be converted to a higher impedance.

(17) The ratio of thicknesses of the metal layer and the polymer layer may be determined based on a predetermined center resonant frequency of the transducer and material parameters. By adapting the thicknesses of both layers to be thinner than one quarter wavelength in each material, in conjunction with determining the thickness ratio among the layers, the method and apparatus of the present invention provides the function of a quarter wavelength impedance converter. Further, the present invention enables the design of arbitrary conversion ratios according to the selection of thickness ratios for each of the layers and the corresponding layer component materials.

(18) Further, by combining two conventionally available material layers with high and low impedances (e.g. a metal layer and a polymer layer), an arbitrary effective acoustic impedance Z.sub.m is synthesized as described herein, having the same function as that of a quarter wavelength impedance converter with acoustic impedance Z.sub.m. The method and apparatus of the present invention thus mitigates the problems associated with obtaining specific values of acoustic impedance Z.sub.m according to conventional quarter wavelength designs.

(19) Advantageously, an impedance converter having a desired characteristic acoustic impedance can readily be fabricated from commercially available metal and polymer materials, thereby facilitating mass production of impedance converters and reducing costs of production compared to prior art matching layers. Good performance over a broadband range around the center resonant frequency may be obtained, so that a transducer with an impedance converter according to the invention is suitable for applications, such as medical imaging, requiring good broadband performance.

(20) Referring now to FIGS. 1A-1B, an ultrasonic transducer 100 with a multilayer acoustic impedance converter 120 is schematically illustrated. Transducer 100 includes a piezoelectric body or element 110 having a high characteristic acoustic impedance Z.sub.P (e.g., 20-3010.sup.6 kg/m.sup.2s or MRayl). Multilayer acoustic impedance converter 120 includes a polymer layer 130 having a predetermined thickness t.sub.p and a metal layer 140 having a predetermined thickness t.sub.m. In an exemplary embodiment, the layers 130, 140 each may be of substantially uniform thickness t.sub.p and t.sub.m respectively. In other configurations, layers 130, 140 may have varying thicknesses t.sub.p and t.sub.m respectively. The effective or equivalent characteristic acoustic impedance Z.sub.C of the combined polymer layer 130 and metal layer 140 is adapted to provide appropriate functionality as an acoustic impedance converter 120. In an exemplary embodiment, polymer layer 130 has parallel opposing planar first surface 132 and second surface 134. Metal layer 140 has parallel opposing planar first surface 142 and second surface 144. In an exemplary configuration, metal layer 140 may be of copper, brass, aluminum, steel or other suitable metal. First surface 132 of polymer layer 130 is bonded or otherwise coupled to piezoelectric body or element 110 of ultrasonic transducer 100. First surface 142 of metal layer 140 is bonded or otherwise coupled (e.g. via electroplating metal onto polymer or via electroless plating) to second surface 134 of polymer layer 130. Second surface 144 of metal layer 140 may be disposed in contact with a propagation medium 150 (for example, water or a biological tissue of an animal, such as a human) having a low characteristic acoustic impedance Z.sub.R (e.g., 1.5 MRayl). It is understood that metal layer 140 may include a thin material coating, for example, a paint coating, having negligible effect on the effective characteristic acoustic impedance of the converter 120, and interposed between metal layer 140 and propagation medium 150. The presence of such a thin layer is also intended to be interpreted as metal layer 140 being in contact with propagation medium 150. Metal layer 140 serves to transmit ultrasonic energy between propagation medium 150 and piezoelectric element 110 through polymer layer 130. The layers 130, 140 may be directly in contact with one another at respective surfaces 134, 142, or may be coupled via a thin adhesive layer disposed therebetween. The thin adhesive layer has no significant effect (i.e. negligible effect) on the effective acoustic impedance of acoustic impedance converter 120.

(21) Still referring to FIG. 1A, an ultrasonic wave 160 originates from piezoelectric element 110 and propagates normal to the surface of polymer layer 130 from piezoelectric element 110 towards metal layer 140 and into propagation medium 150.

(22) The thicknesses t.sub.m and t.sub.p of the metal and polymer layers 140, 130 respectively, may be selected so that the effective characteristic acoustic impedance Z.sub.C of impedance converter 120 is between that of propagation medium 150 (i.e., Z.sub.R) and the active piezoelectric element 110 (i.e., Z.sub.P).

(23) FIG. 1B illustrates a mass and spring model used to calculate the wave impedance or specific acoustic impedance Z.sub.2 of acoustic impedance converter 120 as seen from the surface 132 of polymer layer 130 to metal layer 140. A mass 170 represents metal layer 140 and a spring 180 represents polymer layer 130. Spring 180 has an end point 190. The following calculations and conditions are provided by way of example to illustrate how impedance conversion takes place by means of the polymer and metal layers. Actual values are not limited to the precise values described herein but based on practical requirements for the specific application. For example, certain applications may not require a wide bandwidth, while in other cases, design of bandwidth and sensitivity are achieved as the cumulative effect of each section (e.g. front and back) without satisfying the below criteria. In any event, for calculations of specific acoustic impedance Z.sub.2, the mass M of mass 170 is calculated as:
M=.sub.mt.sub.m per unit area,(1)

(24) wherein .sub.m is the density of metal layer 140; and t.sub.m is the thickness of metal layer 140.
Likewise, the spring constant K of spring 180 is calculated as:
K=c.sub.33/t.sub.p=.sub.pV.sub.p.sup.2/t.sub.p(2)

(25) wherein c.sub.33 is the stiffness constant in the direction of thickness t.sub.p; t.sub.p is the thickness of polymer layer 130; .sub.p is the density of polymer layer 130; V.sub.p is the acoustic velocity in polymer layer 130.
Equation (2) uses the known relationship V.sub.p=(c.sub.33/.sub.p).
In the model above, the mass of polymer layer 130 was neglected. However, a part of polymer layer 130 proximate to metal layer 140 moves with metal layer 140 such that at least a portion of the mass of polymer layer 130 influences the mass of metal layer 140. When metal layer 140 is thinner than polymer layer 130, the mass M of mass 170 may, therefore, be approximated as:
M=.sub.mt.sub.m+0.4(.sub.pt.sub.p)(3)
As is known in the art, the resonant frequency f.sub.o of impedance converter 120 may then be calculated as:

(26) $\begin{matrix} f_{0} = \frac{1}{2 p} \sqrt{\frac{K}{M}} & (4) \end{matrix}$

(27) As described below, the specific acoustic impedance at end point 190 of spring 180 is the highest at a resonance condition. When subjected to an ultrasonic wave 160 (see FIG. 1A), end point 190 displaces to the left in FIG. 1B, i.e., towards mass 170, and spring 180 generates a force pushing mass 170 to the left, at a given start time. Consequently, mass 170 starts to move and is displaced to the left. The displacement of mass 170 is a maximum after one quarter cycle of the resonant frequency, since the displacement of mass 170 lags by one quarter cycle the displacement of the end point 190 of spring 180 and the amplitude of the spring oscillation is greatest at a resonant frequency. At one quarter cycle after the start time, spring 180 elongates to a maximum length and the force from spring 180 is the largest. At this time, end point 190 returns to its original position, and the vibration velocity of end point 190 reaches its maximum in one cycle period as the mass 170 is now moved to the left and exerts force on spring 180.

(28) As is known in the art, the specific acoustic impedance Z.sub.2 of impedance converter 120 is given by the force at end point 190 divided by the velocity at that time. Since at a resonant frequency, the force on end point 190 is at a maximum, the specific acoustic impedance Z.sub.2 becomes a maximum at the resonant frequency. In an ideal model, without spring losses, the specific acoustic impedance Z.sub.2 approaches infinity at the resonant frequency and the resonance is sharp. However, the radiation or propagation medium impedance Z.sub.1 is attached to mass 170 and its effect is equivalent to a resistive load. The propagation medium impedance Z.sub.1 thereby serves to damp the resonance. As a result, the resonance is broadened. An analysis of mass 170 and spring 180 with radiation or propagation impedance Z.sub.1 at mass M provides the specific acoustic or wave impedance Z.sub.2, at resonant frequency f.sub.o as seen from end point 190 as:
Z.sub.2=MK/Z.sub.1(5)

(29) wherein, Z.sub.1=Z.sub.R=.sub.1V.sub.1 is the characteristic acoustic impedance of propagation medium 150 (e.g., about 1.5 MRayl); .sub.1 is the density of propagation medium 150; and V.sub.1 is the acoustic velocity of propagation medium 150.
Thus, using the practical parameters of Z.sub.1=Z.sub.R and M (i.e., mass per unit area of metal layer 140) and K (i.e., spring force per unit area divided by displacement for polymer layer 130), the specific acoustic impedance Z.sub.2 of converter 120 has a much higher value than the radiation or propagation impedance Z.sub.R. This acoustic impedance converter 120 having thinned polymer and metal layers, has the same function as the well known quarter wavelength relatively thick matching layer in contact with propagation medium 150. In the conventional quarter wavelength matching layer case, the specific wave impedance of the quarter wavelength layer as seen from the back side is converted to
Z.sub.2=Z.sub.m.sup.2/Z.sub.1(6)

(30) wherein, Z.sub.m is the characteristic acoustic impedance of the quarter wavelength matching layer. In the prior art, this quarter wavelength matching layer is bonded to the front surface of a piezoelectric layer (having characteristic acoustic impedance Z.sub.PZT=30 MRayl) in an ultrasonic transducer. Impedance Z.sub.2 is the wave or specific acoustic impedance seen from the piezoelectric layer. Thus, the propagation medium acoustic impedance Z.sub.1 is up-converted to Z.sub.2, which is close to Z.sub.PZT.

(31) As is known in the art, for a piezoelectric material having a high characteristic acoustic impedance Z.sub.PZT, the specific acoustic impedance Z.sub.2 of converter 120 has to be close to Z.sub.PZT for an efficient energy transfer between the piezoelectric material and impedance converter 120 and Z.sub.1=Z.sub.R (i.e., acoustic impedance of propagation medium 150). In an ideal matching condition, if Z.sub.2=Z.sub.PZT, Z.sub.m has to be equal to (Z.sub.PZTZ.sub.R). However, as a practical matter, the value of the specific acoustic impedance Z.sub.2 of converter 120 need not be identical to the value of the characteristic acoustic impedance Z.sub.PZT of the active piezoelectric material. In exemplary embodiments, the specific acoustic impedance Z.sub.2 of converter 120 is not significantly different from the value of the characteristic acoustic impedance Z.sub.PZT of piezoelectric element 110 and the condition Z.sub.1<Z.sub.m<Z.sub.PZT generally holds true. The value of Z.sub.m is conventionally chosen to be between Z.sub.1 and Z.sub.PZT depending on the design requirements for the particular application.

(32) Still referring to FIG. 1A in conjunction with FIG. 1B, the thickness t.sub.m of metal layer 140 and the thickness t.sub.p of polymer layer 130 may be calculated as follows. Equation (4) above states:

(33) $f_{0} = \frac{1}{2 p} \sqrt{\frac{K}{M}}$
Inserting the values for K and M from Equations (2) and (1) respectively in Equation (4),

(34) $\begin{matrix} f_{0} = \frac{Vp}{2 p} \sqrt{\frac{r_{p}}{r_{m} t_{m} t_{p}}} & (7) \end{matrix}$
Further, from Equation (6) above,
Z.sub.C.sup.2=Z.sub.1.Math.Z.sub.2(8)
and from Equation (5) above,
Z.sub.1.Math.Z.sub.2=MK(9)
Thus, from Equations (8) and (9),
Z.sub.C=(MK)(10)
This equation means the value Z.sub.C may be chosen by selecting materials with thicknesses that yield suitable values of M and K. The value Z.sub.C may be called an effective characteristic acoustic impedance of acoustic impedance converter 120 and provides for selection of an effective characteristic acoustic impedance for a multilayer impedance converter. While the structures associated with the aforementioned cases are distinct, the effect of the impedance conversion is the same. The impedance Z.sub.1 is converted to Z.sub.2 and the multilayer converter structure has its effective acoustic impedance Z.sub.C as Z.sub.m. If the conversion ratio Z.sub.2/Z.sub.1 is the same for both cases, then Z.sub.C corresponds to Z.sub.m, thereby being equivalent in function.
Inserting values of M and K from Equations (1) and (2) respectively into Equation (10), there is obtained
Z.sub.C=V.sub.p.Math.(.sub.m.Math..sub.p.Math.t.sub.m/t.sub.p)(11)
Equations (7) and (11) can be solved for t.sub.m and t.sub.p as follows:
Equation (7) is first solved for V.sub.p and the value of V.sub.p is substituted into Equation (11). t.sub.m may then be determined as below:
t.sub.m=Z.sub.C/(.sub.m2pf.sub.o)(12)
The thickness t.sub.m of metal layer 140 is linearly dependent on the desired effective characteristic acoustic impedance Z.sub.C of impedance converter 120, and is inversely dependent on the density of the metal of metal layer 140 and the center resonant frequency of transducer 100.
Equations (7) and (11) are solved by eliminating the term .sub.mt.sub.m by making a product of terms of left side of Equations (7) and (11) to get f.sub.0Z.sub.C, and by making a product of terms of right side of these two equations to get Vp.sup.2.sub.p/t.sub.p. From equality of the left and right products, we get t.sub.p as below:
t.sub.p=V.sub.p.sup.2.sub.p/(2pf.sub.oZ.sub.C)(13)
The thickness t.sub.p of polymer layer 130 is inversely dependent on the center resonant frequency f.sub.o of transducer 100 and the desired effective characteristic acoustic impedance Z.sub.C of impedance converter 120. The thickness t.sub.p of polymer layer 130 is directly linearly dependent on the density of the polymer of polymer layer 130. The thickness t.sub.p of polymer layer 130 is further proportional to the square of the acoustic velocity in the polymer layer 130. Thus, for a given or required Z.sub.C for a given application and a given center resonant frequency f.sub.o, thickness t.sub.m of metal layer 140 and thickness t.sub.p of polymer layer 130 may be calculated using Equations (12) and (13). Both thicknesses t.sub.m and t.sub.p are linearly related to the center resonant frequency f.sub.o of transducer 100. The ratio of the thickness t.sub.m of metal layer 140 to the thickness t.sub.p of polymer layer 130 may be expressed as
t.sub.m/t.sub.p=Z.sub.C.sup.2/(.sub.mV.sub.p.sup.2.Math..sub.p)(14)
The thickness ratio is accordingly independent of the center resonant frequency f.sub.o of transducer 100. The ratio of the metal thickness t.sub.m to the polymer thickness t.sub.p increases with the square of the desired effective characteristic acoustic impedance Z.sub.C of impedance converter 120. It will be understood that the values of thicknesses t.sub.m and t.sub.p calculated using Equations (12) and (13) may serve as starting points for the design of acoustic impedance converter 120 and may be varied therefrom without departing from the scope of the invention. The thicknesses t.sub.m and t.sub.p may be varied depending on the commercial availability of the chosen materials of standard thicknesses. These variations in the thicknesses of t.sub.m and t.sub.p from those determined through Equations (12) and (13) are intended to be within the scope of the present invention.

(35) It will be further understood that an acoustic impedance converter may perform satisfactorily even though the thicknesses t.sub.m and t.sub.p may not satisfy Equations (12) and (13). A desired overall performance for an ultrasonic transducer may be achieved with a non-ideal front acoustic impedance converter and a non-ideal back impedance converter, both of which may deviate from the values determined using the method described herein. However, the phase shift resulting from the front and back matching layer(s) may be cancelled by using a higher resonant frequency for the front matching layer(s) and a lower resonant frequency for the back matching layer(s) relative to the center resonant frequency. Yet another example is an ultrasonic transducer with no back matching layer (i.e., with air backing), which may use double front acoustic impedance converters in order to provide a structure with sufficiently wide bandwidth for a given application. In such a transducer each individual acoustic converter may deviate from the ideal values. However, the effective combined characteristic acoustic impedance may provide satisfactory overall performance because of the cancelling effect of the two acoustic impedance converter structures, wherein one of the acoustic impedance converter may be configured for a higher resonant frequency and the other for a lower resonant frequency relative to the center resonant frequency of ultrasonic transducer 100.

(36) Referring now to FIG. 2, there is illustrated a chart 200 depicting the results of a specific acoustic impedance Z.sub.2 calculation using a one dimensional wave propagation model (i.e., not using the mass and spring model), as known in the art, for wave or specific acoustic impedance Z.sub.2 of a quarter wavelength matching layer for which the characteristic acoustic impedance has an ideal value of Z.sub.m=6.710.sup.6 Rayl and Z.sub.R=1.510.sup.6 Rayl for the propagation medium loaded at the front side of the quarter wavelength matching impedance layer of an ultrasonic transducer. The wave impedance 210 seen from the back of the quarter wavelength matching layer shows a peak at 6.8 MHz with a value of specific acoustic impedance Z.sub.2=3010.sup.6 Rayl which is equal to the characteristic acoustic impedance of the piezoelectric material, Z.sub.PZT. However, as set forth above, it is difficult to obtain a material having the desired ideal value of characteristic acoustic impedance Z.sub.m=6.710.sup.6 Rayl for fabricating the quarter wavelength matching layer of an ultrasonic transducer.

(37) Referring now to FIG. 3, a chart 300 illustrates the specific acoustic impedance Z.sub.2 seen from polymer layer 130 to propagation medium 150 for a multilayer impedance converter 120 of FIG. 1A calculated using the same rigorous one dimensional wave propagation model as for FIG. 2. In an exemplary configuration, polymer layer 130 takes the form of a polyimide layer with density r.sub.p=1454 kg/m.sup.3, sound velocity V.sub.p=2175 m/s and thickness t.sub.p=20 mm (about 1/16 of the wavelength). In this embodiment, metal layer 140 takes the form of a copper layer 12 with density r.sub.m=8960 kg/m.sup.3, sound velocity V.sub.m=5010 m/s and thickness t.sub.m=19.7 mm (about 1/37 of the wavelength). This exemplary configuration of impedance converter 120 yields specific acoustic impedance Z.sub.2=30 MRayl at 6.8 MHz peak which ideally matches to characteristic acoustic impedance Z.sub.PZT of bulk PZT which typically has Z.sub.PZT=30 MRayl. It is further noted here that the quarter wavelength in polyimide layer 130 is 80 mm and the copper layer is 184 m at 6.8 MHz and, therefore, 20 mm as per the present design is much thinner than a quarter of the wavelength at the center resonant frequency of the piezoelectric active material. It is understood that the design of the same impedance conversion ratio at a different frequency can be accomplished using the same ratio of thickness to wavelength for the same materials.

(38) The impedance curve 310 of impedance converter 120 (of FIG. 1A) shown in FIG. 3 is very close to that of the ideal impedance performance depicted by curve 210 shown in FIG. 2. Thus, selection of suitable thicknesses and material parameters of the polymer 130 and metal 140 layers provides for an arbitrary value of an equivalent characteristic acoustic impedance. It is noted that the thicknesses of each of polymer layer 130 and metal layer 140 are much thinner than the quarter wavelength matching layers of the same materials.

(39) Referring now to FIG. 4, there is schematically illustrated an ultrasonic transducer 400 configured to operate at high frequency (i.e., in the MHz region) according to an embodiment of the invention. Transducer 400 includes a piezoelectric element 110 having a front acoustic impedance converter 120 bonded to piezoelectric element 110 at a boundary plane 112. A backing absorber 410 is bonded to the piezoelectric element 110 at a boundary plane 114. Impedance converter 120 may be bonded to piezoelectric element 110 using an adhesive such as an epoxy. Boundary plane 114 is opposite to boundary plane 112. Thus, backing absorber 410 is attached to piezoelectric element 110 on a face opposite the face of piezoelectric element 110 on which is attached front impedance converter 120. Front acoustic impedance converter 120 contacts propagation medium 150. It will be understood that front acoustic impedance converter 120 may contact propagation medium 150 through a thin intervening layer, such as a paint coating, having a negligible effect on the effective characteristic acoustic impedance of converter 120.

(40) Generally, piezoelectric element 110 (for example, a piezoelectric ceramic layer) has a high characteristic acoustic impedance Z.sub.1 (about 20-30 MRayl depending on the configuration and the material, e.g., Z.sub.PZT approximately equal to 30 MRayl)). Propagation medium 150 generally has a relatively low characteristic acoustic impedance Z.sub.R (for example, about 1.5 MRayl). Acoustic impedance converter 120 includes a polymer layer 130 of thickness t.sub.p and a metal layer 140 of thickness t.sub.m bonded to polymer layer 130. The thicknesses t.sub.m and t.sub.p for the metal layer 140 and the polymer layer 130 have been selected based on the desired or predetermined equivalent or effective characteristic acoustic impedance Z.sub.C of acoustic impedance converter 120. An ideal value of specific acoustic impedance Z.sub.2, (which, as noted above is close to characteristic acoustic impedance Z.sub.PZT) determined by the effective characteristic acoustic impedance Z.sub.C can be obtained as shown in FIG. 3. Thus, a wideband transducer can be configured more easily and economically than the prior art systems and methods.

(41) Generally, the vibration of piezoelectric element 110 excites acoustic waves in a forward direction (to the left in FIG. 4) toward propagation medium 150 and also in a rearward direction (to the right in FIG. 4) toward backing absorber 410. Some of the energy of acoustic waves propagating in a rearward direction is transmitted to backing absorber 410. The remainder is reflected at back boundary 414 between piezoelectric element 110 and backing absorber 410. A certain amount of reflection is desirable to enhance the sensitivity of transducer 400 to drive and to receive ultrasonic waves. This reflection at back boundary 414 is a function of the characteristic acoustic impedance of backing absorber 410. If the acoustic absorption in backing absorber 410 is not sufficiently high, ultrasonic waves reflected from an end surface 412 of backing absorber 410 may be reflected back to piezoelectric element 110 and may overlap with a front wave propagating towards propagation medium 150. Disadvantageously, the ultrasonic waves may be destructively or constructively added depending on the frequency, and multiple resonance peaks may be formed on the frequency response curve of transducer 400. Another problem caused by excessively low acoustic absorption by the absorption material of backing absorber 410 is that the thickness of backing absorber 410 has to be increased so as to absorb substantially all backward waves and prevent reflection from end surface 412. If the thickness of backing absorber 410 is excessive, then the transducer 400, with its backing absorber, may not fit within the limitations of the transducer holder or housing, the size of which may be constrained depending on the application of transducer 400.

(42) FIG. 5 depicts an ultrasonic transducer 500 having a back impedance converter 560 for backing absorber 410 according to another embodiment of the invention. Generally, characteristic acoustic impedance (MRayl) and acoustic absorption (decibel/centimeter or dB/cm) are closely related and cannot be controlled independently. A material having a high acoustic absorption often has a low characteristic acoustic impedance. The desired characteristic acoustic impedance of a backing absorber 410 may vary depending on the material and structure of the active piezoelectric layer. Examples of the active piezoelectric layer having various desirable characteristic acoustic impedances for backing absorber 410 include bulk PZT, 2-2 composite, 1-3 composite and single crystals.

(43) As shown in the exploded view of FIG. 5, transducer 500 includes a PZT/polymer 2-2 connectivity composite array 520, a front acoustic impedance converter 120 attached (e.g. bonded) to a front surface 526 of array 520 via a grounding layer 510, a flexible printed circuit board 540 with conductor traces 530 (e.g., copper), a shielding conductive layer 550 (e.g. copper) and a backing absorber 410. Backing absorber 410 is attached (e.g. bonded) to a back surface 528 of array 520 through conductive shielding layer 550, flexible circuit board 540 and conductor traces 530. Flexible printed circuit board 540 serves to provide a route for electrical signals and also functions as a back acoustic impedance converter 560 including the polymer of the flexible printed circuit board 540 and the metal of shielding conductive layer 550 to up-convert the low characteristic acoustic impedance of backing absorber 410. Composite array 520 includes multiple narrow elongated elements 524 (for example, about 10 millimeters (mm)0.1 mm) of PZT with kerfs or channels 522 (for example, of about 50 micrometers (m) width) therebetween filled with a polymer, such as epoxy. Each piezoelectric element 524 of composite array 520 may be driven with different signals having different phases to steer beam direction. Backside electrodes (not shown) of composite array 520 are connected to conductive traces 530 of flexible printed circuit board 540, along a first surface 542 of flexible printed circuit board 540. The flexible printed circuit board 540 is coupled along a second surface 544 thereof, opposite to first surface 542, to back acoustic impedance converter 560 which, in turn, is coupled to a backing absorber 410. Because of the narrow width geometry of piezoelectric elements 524, the characteristic acoustic impedance Z.sub.PZT of piezoelectric elements 524 may be as low as 15 MRayl. In this embodiment, acoustic impedance converter 120 has metal layer 140 of thickness t.sub.m=15 mm as a front layer and polymer layer 130 of thickness t.sub.p=40 mm on metal layer 140. This exemplary metal-polymer multilayer acoustic impedance converter 120 yields specific acoustic impedance Z.sub.2=15 MRayl at 5.2 MHz, which Z.sub.2 matches the Z.sub.PZT of 2-2 PZT/polymer composite array 520.

(44) In the illustrated embodiment of FIG. 5, polymer layer 130 of acoustic impedance converter 120 may be of polyimide and metal layer 140 may be of copper. The thickness of copper layer 140 may be so selected as to provide an appropriate acoustic impedance conversion from the low characteristic acoustic impedance of propagation medium 150 (Z.sub.R) to the high characteristic acoustic impedance Z.sub.PZT of PZT/polymer composite array 520. A second copper layer 510 is interposed between acoustic impedance converter 120 and PZT/polymer array 520 as a connection to ground. Copper layer 510, however, does not function as an acoustic impedance converter because layer 510 has a characteristic acoustic impedance similar to that of piezoelectric elements 524 and because layer 510 is directly bonded to piezoelectric elements 524 as a grounded electrode. Therefore, the presence of copper layer 510 does not influence the design of composite or multilayer acoustic impedance converter 120. It will be understood that PZT composite array 520, front matching or acoustic impedance converter 120 and back acoustic impedance converter 560 are shown separately (i.e., not bonded or otherwise coupled) for illustrative purposes only.

(45) To provide backing absorber 410 with an appropriate acoustic impedance conversion, back acoustic impedance converter 560 in FIG. 5 may be provided between piezoelectric array 520 and backing absorber 410. Generally, the characteristic acoustic impedance of backing absorber 410 is about 4-10 MRayl, which is higher than the characteristic acoustic impedance of front propagation medium 150, which may be about 1.5 MRayl, for example. The desired effective acoustic impedance Z.sub.C of back acoustic impedance converter 560 may be selected to be consistent with the desired bandwidth and sensitivity of transducer 500. For example, when the characteristic acoustic impedance of backing absorber 410 is 5 MRayl, and a required specific backing acoustic impedance is 10 MRayl, the desired effective characteristic impedance of the back acoustic impedance converter Z.sub.C(=(MK)=(Z.sub.1.Math.Z.sub.2)) is 7.07 MRayl. This value of the effective characteristic acoustic impedance of back acoustic impedance converter 560 is obtained by using a metal (for example, copper) layer 550 with thickness t.sub.m=24 mm and a polymer (for example, polyimide) layer 540 with thickness t.sub.p=29 mm, for a transducer having a center resonant frequency of 5.2 MHz. The selection of appropriate materials and thicknesses t.sub.m, t.sub.p for metal layer 550 and polymer layer 540 for back acoustic impedance converter 560 interposed between an active piezoelectric element 524 and a backing absorber 410 is made in substantially the same manner as for front acoustic impedance converter 120 between active piezoelectric array 520 and front propagation medium 150. Back acoustic impedance converter 560 converts the low characteristic acoustic impedance Z.sub.1 of backing absorber 410 to a higher specific acoustic impedance Z.sub.2 which is the wave impedance or specific impedance as seen from active piezoelectric array 520 to the interior of backing absorber 410. An appropriate value for specific acoustic impedance Z.sub.2 is determined from the desired bandwidth and sensitivity of transducer 500. The desired value of the effective characteristic acoustic impedance, Z.sub.C, of back acoustic impedance converter 560 is calculated using the equation Z.sub.C=square root of the product of Z.sub.2 and Z.sub.1 (i.e. Z.sub.C=(Z.sub.2Z.sub.1)). The thickness t.sub.m of metal layer 550 is determined based on the desired effective characteristic acoustic impedance Z.sub.C of back acoustic impedance converter 560, the density of the metal of metal layer 140, and the center resonant frequency f.sub.o of transducer 500. The thickness t.sub.p of polymer layer 540 is calculated based on the desired effective characteristic acoustic impedance Z.sub.C of back acoustic impedance converter 560, the density of the polymer of polymer layer 540, the acoustic velocity in the polymer of polymer layer 540, and the center resonant frequency f.sub.0 of transducer 500.

(46) Table I below lists the material parameters for an exemplary propagation medium (water), an exemplary piezoelectric active material (PZT), an exemplary metal (copper), a polyimide and Polyvinylidene fluoride (PVDF). As is known in the art, the characteristic acoustic impedance of a material is given by the product of the density of the material and the velocity of sound in the material.

(47) TABLE-US-00001 TABLE I Material parameters used for design of various examples in Table II Propagation PZT- PVDF (for medium 5H Copper Polyimide matching) Density 1000 7500 8960 1454 1780 (kg/m.sup.3) Velocity 1500 4800 5010 2175 2100 (m/s)

(48) The following Table II compares the two calculated values of specific acoustic impedances Z.sub.2, one calculated by a mass and spring model and the other calculated by a rigorous one dimensional model for low and high values (15 and 30 MRayl) of Z.sub.P, where the value for the characteristic acoustic impedance of the propagation medium Z.sub.1=1.5 MRayl was used. Table II shows that specific acoustic impedance Z.sub.2 calculated using the mass and spring model is close enough for actual use.

(49) TABLE-US-00002 TABLE II Examples of designed acoustic impedance converters to match low (~15 MRayl) and high (~30 MRayl) characteristic acoustic impedances of piezoelectric layers, with a propagation medium, such as water or human muscle, having a characteristic acoustic impedance of Z.sub.R = 1.5 MRayl. Copper Polymer Z.sub.2, M-K Z.sub.2 1-D fo = p thickness: t.sub.m thickness: t.sub.p model model Materials (K/M) (MHz) (mm, t.sub.m/l) (mm, t.sub.p/l) (MRayl) (MRayl) Cu-polyimide 2.6 32.8, 0.017 82, 0.098 14.1 15.8 Cu-polyimide 2.6 54.4, 0.028 54, 0.065 28.8 30 Cu-polyimide 5.2 16.4, 0.017 41, 0.098 14.1 15.8 Cu-polyimide 5.2 27.2, 0.028 27.0, 0.065 28.8 30 Cu-polyimide 10.4 8.4, 0.017 20.5, 0.098 14.1 15.8 Cu-polyimide 10.4 13.6, 0.028 13.5, 0.065 28.8 30 Cu-PVDF 5.2 14.3, 0.015 49, 0.12 13.0 15.5 Cu-PVDF 5.2 26.3, 0.027 31, 0.078 28.9 30.1

(50) As seen in Table II, when the materials and desired specific acoustic impedance are selected, the thickness ratio of polymer layer 130 to metal layer 140 is generally constant for any given frequency. For example, for copper and polyimide, when Z.sub.2=30 MRayl is selected, a series of f.sub.0=2.6 MHz, 5.2 MHz, and 10.4 MHz gives the same thickness ratio of copper/polyimide @1, consistent with Equation (14) above. If a polymer other than polyimide is used, the density and acoustic velocity may differ from that of polyimide, such that the thickness ratio will differ, as may be appreciated from Equation (14) above.

(51) Referring now to FIG. 6, there is shown a plot 600 of signal strength, represented as pressure as a function of frequency (Hz) for a simulated ultrasonic transducer in transmitter mode, using a one dimensional model (either the Mason model or the KLM model yield effectively the same result). Backing absorber 410 (see FIG. 4 or 5) has a low characteristic acoustic impedance of 4.5 MRayl. Piezoelectric element 110 (see FIG. 4) has a high characteristic acoustic impedance of 30 MRayl; acoustic impedance converter 120 is composed of a polymer layer 130 of 33 m polyimide and a metal layer 140 of copper with various thicknesses as shown in FIG. 6. The resonant frequency is f.sub.0@5.2 MHz and the thickness of PZT is 350 mm. Curves 610, 620, 630 and 640 represent the signal strength for thicknesses of 0 m, 10 m, 20 m and 30 m for metal layer 140 of copper. A separate calculation, according to the method described hereinabove, shows that the combination of a 33 m polyimide polymer layer 130 and a 20 m copper metal layer 140 yields an effective characteristic acoustic impedance Z.sub.C of 6.1 MRayl, and 1.5 MRayl of the front propagation medium and is converted to a specific acoustic impedance of 25 MRayl at the front face of PZT layer. This specific acoustic impedance is somewhat lower than the characteristic impedance of PZT. However, for both high sensitivity and wide bandwidth purposes, such a slightly lower impedance at the front face of the PZT layer may be advantageous. For example, FIG. 6 shows curve 630 for 20 mm copper having high sensitivity and wideband and symmetric frequency response, with a center frequency of 5.2 MHz. The structure of 20 mm metal layer 140 of copper and 33 mm polymer layer 130 of polyimide corresponds to a hypothetic material with characteristic acoustic impedance Z.sub.m=6.1 MRayl with one quarter wavelength thickness. If the design is altered to obtain a different designed center frequency f.sub.o, the PZT thickness is also different and is inversely proportional to the frequency f.sub.o. It will be understood that the thicknesses t.sub.m and t.sub.p are also different for a different center resonant frequency f.sub.o. For such a different frequency design, the values of t.sub.m/l and t.sub.p/l may still be kept constant as can be seen in Table II.

(52) FIG. 6 illustrates a simulation of an acoustic output of an ultrasonic transducer 400 (of FIG. 4) acting as a transmitter. The same multilayer acoustic impedance converter structure 120 functions also for an ultrasonic transducer acting as a receiver. The reciprocity principle for an acoustic device generally states that a given structure of a transducer should have the same bandwidth as a receiver as that as a transmitter. Therefore, multilayer acoustic impedance converter 120 described herein may be advantageously used in medical ultrasonic imaging systems. For example, transducer 400 (of FIG. 4) transmits acoustic beams in a short pulse into a human body which beams are then reflected from a target material such as a human organ. The acoustic beams are scanned and the reflections are received by the same transducer 400 (of FIG. 4) and analyzed to reconstruct the image of the human organ, for example, for display on a display device. For such purposes, transducer 400 (FIG. 4) is used as both transmitter and receiver; use of the acoustic impedance converter disclosed herein provides good wideband performance. Metal layer 140 (FIG. 4) serves to transmit ultrasonic energy generated by piezoelectric element 110 (FIG. 4) to propagation medium 150 (FIG. 4) through polymer layer 130 (FIG. 4). Metal layer 140 (FIG. 4) also serves to transmit/receive ultrasonic energy reflected from the target material in propagation medium 150 (FIG. 4) to piezoelectric element 110 (FIG. 4) through polymer layer 130 (FIG. 4).

(53) Referring now to FIG. 7A, another exemplary embodiment of an ultrasonic transducer 700 with acoustic impedance converter 120 according to the invention is illustrated. Ultrasonic transducer 700 has an active piezoelectric element 110 of suitable material, such as PZT. Active piezoelectric element 110 has front and back planar parallel faces. Backing absorber 747 may be formed from any low characteristic acoustic impedance material, such as air, water or an absorber material. Good wideband frequency response may be achieved if acoustic impedance converter 120 is appropriately designed as described herein. Front propagation medium 150 may be water or a human body. Acoustic impedance converter 120 has a polymer layer 130 bonded to the front face of active piezoelectric layer 110, and a metal layer 140 bonded to polymer layer 130. In an exemplary embodiment, both layers 130, 140 are of substantially uniform thicknesses t.sub.p, t.sub.m respectively, and these layers function as an impedance converter as already described. The polymer and metal layers are configured so as to have an effective acoustic impedance between that of the piezoelectric material and the propagation medium. The equivalent or effective characteristic acoustic impedance of the multilayer impedance converter 120 is Z.sub.C2 and the intermediate characteristic acoustic impedance is Z.sub.C2<Z.sub.PZT. Matching layer 710 may be a polymer layer having with a one quarter wavelength thickness and an acoustic impedance Z.sub.m1 bonded to a front face of acoustic impedance converter 120, for facing propagation medium 150. The characteristic acoustic impedances of the quarter wavelength matching layer 710 and acoustic impedance converter 120 are chosen to satisfy the relationship Z.sub.1<Z.sub.m1<Z.sub.C2<Z.sub.PZT.

(54) Referring now to FIG. 7B, another embodiment of the present invention is illustrated wherein first and second sets of multilayer acoustic impedance converters 120 and 743, respectively, are configured as double matching layers for a transducer 801. Outer acoustic impedance converter 743 is configured according to the structure of multilayer acoustic impedance converter 120 as described herein. A lower characteristic acoustic impedance layer 744 (such as a rubber or latex layer) is bonded to metal layer 140 of inner acoustic impedance converter 120 and a higher characteristic acoustic impedance layer 745 (e.g. a metal, polymer or plastic layer) is adapted to be in contact with propagation medium 150. Outer multilayer acoustic impedance converter 743 functions as a quarter wavelength matching layer 710 as shown in FIG. 7A, and the equivalent characteristic acoustic impedance Z.sub.C1 and Z.sub.C2 are so chosen as to satisfy the relationship Z.sub.1<Z.sub.C1<Z.sub.C2<Z.sub.PZT. Inner acoustic impedance converter 120 has a polymer inner layer 130 bonded to piezoelectric element 110, and an outer metal layer 140 bonded to polymer inner layer 130.

(55) Double matching layers have been utilized in prior art ultrasonic transducers using air as a backing absorber. The first layer disposed directly on a PZT element is a quarter wavelength matching layer of material having a high characteristic acoustic impedance, which characteristic acoustic impedance is lower than that of PZT. The second layer is disposed between the first layer and the propagation medium (e.g., water). The high characteristic acoustic impedance material of the first layer in prior art air backing transducers consists of a thin layer of glass (or composite material). The required thickness of such a layer is, for example, 0.52 mm for a transducer having a 2.6 MHz center operating frequency, or 0.26 mm thickness for a transducer having a 5.4 MHz center operating frequency. However, such thin layers of material have proven difficult to manufacture in large scale production systems. The double matching layer apparatus and method as described herein makes it possible to obtain a desired wideband performance that is capable of such large scale manufacturing.

(56) Experimental testing of the embodiment shown in FIG. 7A was accomplished wherein a uniform PZT plate of 0.85 millimeter (mm) thickness with a center operating frequency of 2.6 MHz was used as piezoelectric element 110. Polymer layer 130 comprised a 33 mm PVDF layer. Metal layer 140 comprised a 77 mm layer of brass. It is understood that for such applications the acoustic properties of brass are very similar to those of copper. Matching layer 710 was a 220 mm polyimide layer. The layers were bonded by low viscosity, negligibly thin epoxy bonding layer.

(57) FIG. 8 shows operational results associated with transducer 700 of FIG. 7A. When the transducer 700 of FIG. 7A is driven by a sharp pulse, the observed waveform 810 is shown in chart 800 of FIG. 8. The frequency performance of the Fourier transformed spectrum 910 of the waveform 810 of FIG. 8 is illustrated in chart 900 of FIG. 9. As can be appreciated from FIG. 8, a sharp pulse waveform 810 is obtained. As can be seen by curve 910 in FIG. 9, wide frequency performance (about 56% relative bandwidth) was achieved.

(58) As previously described, the thicknesses t.sub.p and t.sub.m of polymer layer 130 and metal 140 of acoustic impedance converter 120 may be varied from the values determined using Equations (12) and (13) without departing from the scope of the invention. At a constant center resonant frequency, as the thickness t.sub.p of polymer layer 130 may be increased from the theoretical value obtained using Equation (13), and the thickness t.sub.m of metal layer 140 may be correspondingly decreased from the theoretical value obtained using Equation (12). As the thickness t.sub.m of metal layer 140 approaches zero, the thickness t.sub.p of the polymer layer 130 approaches the thickness of a conventional quarter wavelength matching layer. Thus, as the thickness t.sub.p of polymer layer 130 is decreased from the thickness of a quarter wavelength matching layer, the deviation in the resonance response due to a thinner polymer layer 130 may be compensated by adding metal layer 140 of a given thickness t.sub.m. The thickness t.sub.p of polymer layer 130 may be decreased to one-tenth of the theoretical value determined using Equation (13) and still provide adequate transducer performance depending on the requirements of a given application.

(59) It is to be understood that when thicknesses deviate from their original values, the impedances Z.sub.C are different and the function of impedance conversion of the layer pair is likewise different. However, other layers, such as the backing absorber converter and the outermost converter layer (in the case of double layer matching) similarly influence the performance and design these layers to compensate the difference to satisfy overall performance. Thus, in one embodiment, the thickness t.sub.p of polymer layer 130 may range from between about one-tenth of the theoretical value determined using Equation (13) and less than the thickness of a conventional quarter wavelength matching layer for a given center resonant frequency of the transducer. It will be appreciated that alternative embodiments may have thickness t.sub.p of polymer layer 130 about two-tenth, three-tenth, four-tenth, and so on, of the theoretical value determined using Equation (13) and be advantageously employed in different applications requiring different transducer performances. It will further be appreciated that alternative embodiments may also have thicknesses t.sub.p of polymer layer 130 of 1.1, 1.2, 1.3 (etc.) times the theoretical value determined using Equation (13) for different applications.

(60) In other embodiments, only one of the thicknesses t.sub.p and t.sub.m may be varied from the values determined using Equations (12) and (13). For example, the thickness t.sub.p of polymer layer 130 may be half of the theoretical value obtained using Equation (13), without changing the corresponding theoretical thickness t.sub.m of metal layer 140. As will be understood by one skilled in the art, such a combination would result in the resonant frequency of acoustic impedance converter 120 being increased by a factor of approximately the square root of two (2) (i.e., 1.414) from the predetermined center resonant frequency of the transducer. The resulting deformation in the response curve may be useful in other applications of a special frequency response, including but not limited to nondestructive evaluation using ultrasound energy and Doppler flow speed detection.

(61) Variations and modifications to the disclosed embodiments are within the scope of the invention. For example, while the piezoelectric units are generally shown as relatively thin and flat layers, other shapes and forms may be employed. Surfaces that are disclosed as being on and in contact with one another may have interposed therebetween thin layers of materials such as adhesives having little or no effect on the acoustic impedance of the structure.

(62) While the foregoing invention has been described with reference to the above embodiments, various modifications and changes can be made without departing from the spirit of the invention. Accordingly, all such modifications and changes are considered to be within the scope of the appended claims.

Method of forming a multilayer acoustic impedance converter for ultrasonic transducers

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