Multilayer ultrasound transducer and the method of manufacture thereof

A three crystal ultrasound transducer in which each crystal has a thickness that is measured in the z-range direction that varies as a function of its position along the x-elevation direction. The impedance of the transducer is reduced compared with uniform thickness transducers thereby providing a better electrical match between the ultrasound transducer and the ultrasound system to which it is coupled.

FIELD OF THE INVENTION 
This invention relates to a multilayered ultrasound transducer and the 
method of manufacture thereof, and, more particularly, to a multilayered 
ultrasound transducer that has a plurality of piezoelectric layers that 
are each non-uniform in thickness. 
BACKGROUND OF THE INVENTION 
Some ultrasound transducers utilize a single-layer of piezoelectric 
material to form the transducer elements. Single-layer transducers have 
the disadvantage that when operated at higher frequencies, the layer's 
impedance increases greatly so that a mismatch in impedances occurs 
between the transducer and the ultrasound system to which it is coupled. 
Due to this mismatch of impedances, the transfer of energy to the 
transducer is decreased due to reflection of energy by the transducer. 
Ultrasound transducer having multiple layers of piezoelectric material are 
also known. In some ultrasound transducers the layers of piezoelectric 
material are uniform in thickness. These transducers with uniform 
thickness piezoelectric layers suffer from limited bandwidth and poor 
signal-to-noise ratio due to higher side lobes, especially in in-depth 
imaging. In addition, they are limited by the lack of control of the slice 
thickness in the elevation direction. 
U.S. Pat. Nos. 5,415,175 ("the '175 patent")and 5,438,998 ("the '998 
patent"), both of which are assigned to the present assignee and are 
specifically incorporated herein by reference, disclose an ultrasound 
transducer that has two layers of piezoelectric material stacked one on 
top of the other in the z-range direction as shown in FIGS. 12 and 13 of 
the '175 and '998 patents. Each layer has a thickness in the z-range 
direction and a width in the x-elevation direction extending from a first 
end to a second end. The thickness of each layer is non-uniform, and more 
particularly, the thickness is at a maximum at the first and second ends 
and the thickness is at a minimum at a point about midway therebetween. As 
shown in FIG. 12, the top layer of piezoelectric material has a concave 
surface which will face the region of examination when the transducer is 
in use. The bottom layer also has a concave surface which faces a backing 
block on which the bottom layer is disposed. In the embodiment shown in 
FIG. 13 the concave surface of the bottom layer faces the top layer of 
piezoelectric material. 
It is thus desirable to provide an ultrasound transducer that has a reduced 
impedance and an improved electrical match to the ultrasound system to 
which it is coupled. It is also desirable to provide an interconnect 
circuit that is simple in construction, maintains the same number of 
traces as a single layer design and has all of the traces extending from 
one side of the transducer. 
SUMMARY OF THE INVENTION 
According to a first aspect of the invention there is provided a three 
crystal ultrasound transducer including a first piezoelectric layer having 
a thickness in a range direction and a width in an elevation direction 
wherein the width extends from a first end to a second end and the 
thickness of the first piezoelectric layer is at a maximum at the first 
and second ends and the thickness is at a minimum at a point about midway 
between the first and second ends, a second piezoelectric layer disposed 
on the first piezoelectric layer, the second piezoelectric layer having a 
thickness in the range direction and a width in the elevation direction, 
wherein the width extends from a first end to a second end and the 
thickness of the second piezoelectric layer is at a maximum at the first 
and second ends and the thickness is at a minimum at a point about midway 
between the first and second ends, a third piezoelectric layer disposed on 
the second piezoelectric layer, the third piezoelectric layer having a 
thickness in the range direction and a width in the elevation direction 
wherein the width extends from a first end to a second end and the 
thickness of the third piezoelectric layer is at a maximum at the first 
and second ends and the thickness is at a minimum at a point about midway 
between the first and second ends, and an interconnect circuit disposed 
between the first, second and third piezoelectric layers wherein the 
interconnect circuit can deliver an excitation signal to the first, second 
and third piezoelectric layers thereby causing each piezoelectric layer to 
generate an ultrasound signal. 
According to a second aspect of the invention there is provided a three 
crystal ultrasound transducer including a first piezoelectric layer having 
a thickness in the range direction and a width in the elevation direction 
wherein the thickness of the first piezoelectric layer is non-uniform 
along its width, a second piezoelectric layer disposed on the first 
piezoelectric layer, the second piezoelectric layer having a thickness in 
the range direction and a width in the elevation direction wherein the 
thickness of the second piezoelectric layer is non-uniform along its 
width, a third piezoelectric layer disposed on the second piezoelectric 
layer, the third piezoelectric layer having a thickness in the range 
direction and a width in the elevation direction wherein the thickness of 
the third piezoelectric layer is non-uniform along its width, and an 
interconnect circuit disposed between the first, second and third 
piezoelectric layers wherein the interconnect can deliver an excitation 
signal to the first, second and third piezoelectric layers thereby causing 
each piezoelectric layer to generate an ultrasound signal. 
According to a third aspect of the invention there is provided a three 
crystal ultrasound transducer including a first piezoelectric layer, a 
second piezoelectric layer disposed on the first piezoelectric layer, a 
third piezoelectric layer disposed on the second piezoelectric layer and 
an interconnect circuit having a first center pad, a second center pad, 
and a third center pad on which are disposed the first, second and third 
piezoelectric layers respectively and a plurality of traces coupled to the 
first, second and third center pads wherein the plurality of traces extend 
from the same side of each of the piezoelectric layers.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS 
FIG. 1 is a schematic view of an ultrasound system 10 for transmitting and 
receiving ultrasound signals. The system 10 is used to generate an image 
of an object 12 or body that is located in a region of examination. The 
ultrasound system 10 has transmit circuitry 14 for transmitting electrical 
signals to a transducer 16, receive circuitry 18 for processing signals 
received by the transducer 16 and a display 20 for displaying the image of 
the object 12 in the region of examination when the transducer is in use. 
FIG. 2 shows a partial perspective view of a portion of a linear transducer 
array according to a preferred embodiment of the present invention. Not 
all of the elements that would make up transducer 16 have been illustrated 
in order to clarify the description of the invention. The linear array 
includes a backing block 22, a first layer of piezoelectric material 24 
disposed on top of the backing block 22, a second layer of piezoelectric 
material 26 disposed on top of the first layer of piezoelectric material 
24, and a third layer of piezoelectric material 28 disposed on top of the 
second layer of piezoelectric material 26. An interconnect circuit (not 
shown) is disposed between the backing block 22 and the first layer of 
piezoelectric material 24, between the first and second layers of 
piezoelectric material 24 and 26, respectively, and between the third 
layer of piezoelectric material 28 and an acoustic matching layer (not 
shown). Kerfs 30 extending in the x-elevation direction separate the 
transducer elements from one another in the y-azimuth direction so that 
the transducer elements are sequentially arranged in the y-azimuth 
direction. In a preferred embodiment the kerfs 30 extend partially into 
the backing block 22 to electrically and acoustically isolate the 
transducer elements from one another. 
Each of the three-layers of piezoelectric material is identical in 
dimension. Each layer has a width extending in the x-elevation direction 
from a first end 32 to a second end 34 and a thickness t(x) extending in 
the z-range direction. The thickness of each transducer element varies as 
a function of its position along the x-elevation direction. 
FIG. 3 is a cross-sectional view of a three crystal design taken along the 
x-elevation direction according to a preferred embodiment of the present 
invention. In a preferred embodiment, the backing block 22 has a top 
surface 40 that is convex in shape. The first layer piezoelectric material 
24 is positioned so that its poling direction faces towards the backing 
block 22 as indicated by the arrow. The second layer of piezoelectric 
material 26 is disposed so that its poling direction faces away from the 
backing block 22 as indicated by the arrow and the third layer of 
piezoelectric material 28 is disposed so that its poling direction faces 
towards the backing block 22 as indicated by the arrow. Each of the 
three-layers of piezoelectric material have a width w extending in the 
x-elevation direction from the first end 32 to the second end 34 of the 
layers and a thickness t(x) extending in the z-range direction. The 
thickness t(x) varies as a function of its position along the x-elevation 
direction and, in a preferred embodiment, the thickness t(x) of each layer 
is at a maximum at the first and second ends 32 and 34 and the thickness 
is at a minimum at a point about midway between the first and second ends 
32 and 34. 
In a preferred embodiment, a first acoustic matching layer 42 and static 
shield (not shown) are disposed on top of the third layer of piezoelectric 
material 28. In another preferred embodiment, a second acoustic matching 
layer 43 is disposed on top of the first acoustic matching layer 42. 
Preferably, the acoustic matching layer, like the three-layers of 
piezoelectric material, has a non-uniform thickness. If a second acoustic 
matching layer is provided, the static shield is disposed over the second 
acoustic matching layer. In a preferred embodiment, the static shield is a 
gold-coated mylar layer that is coupled to the transducer chassis ground 
to prevent radio frequency interference. Such a static shield is 
commercially available from Sheldahl of Northfield, Minn. 
In a preferred embodiment, each of the three-layers of piezoelectric 
material has a width w of about 14 mm. The maximum thickness of each of 
the three-layers is about 0.006 inches and the minimum thickness of each 
layer is about 0.003 inches. In a preferred embodiment, two acoustic 
matching layers are disposed on top of the third layer of piezoelectric 
material. Preferably, a high impedance acoustic matching layer is disposed 
directly on the third layer of piezoelectric material and a low impedance 
matching layer is disposed on the high impedance matching layer. In a 
preferred embodiment, the low and high impedance matching layers have a 
thickness that varies as a function of its position along the x-elevation 
direction and preferably has a maximum thickness at its outer ends and a 
minimum thickness at a point about midway between the outer ends. In a 
preferred embodiment, the minimum thickness of the low impedance matching 
layer is about 0.0054 inches and its maximum thickness is about 0.0086 
inches. The minimum thickness of the high impedance layer is about 0.0048 
inches and its maximum thickness is about 0.008 inches. The first, second 
and third layers of piezoelectric material have a radius of curvature of 
about 6.420 inches. The low and high impedance acoustic matching layers 
have a radius of curvature of about 11.123 inches. None of the figures 
have been drawn to scale. 
In a preferred embodiment each transducer element is composed of the 
following elements. The first, second and third layers are composed of 
piezoelectric material lead zirconate titanate (PZT), however, they may be 
composed of other materials such as a composite like polyvinylidene 
fluoride (PVDF), an electro-restrictive material such as lead magnesium 
niobate (PMN) or a composite ceramic material or other suitable material. 
The high impedance matching layer is formed of Dow Corning's epoxy resin 
DER 332 with Dow Corning's hardener DEH 24 filled with 9 micron alumina 
oxide particles from Microabrasive of Westfield, Mass., and 1 micron 
tungsten carbide particles available from Cerac Incorporated of Milwaukee, 
Wis. The low impedance matching layer is formed of Dow Corning's epoxy 
resin DER 332 with Dow Corning's hardener DEH 24. 
Interposed between the backing block, the first layer 24, the second layer 
26, the third layer 28 and the acoustic matching layer 42 is an 
interconnect circuit 50 (illustrated by the dark lines) which couples the 
transducer to the transmit and receive circuits 14 and 18 when the 
transducer is in use. The interconnect circuit 50 is preferably divided 
into two parts, a signal flex circuit 52 and a ground flex circuit 70 with 
the common part between the signal and ground flex circuits designated as 
46. 
FIG. 4 is a view of the signal flex circuit 52 in its unwrapped state. The 
signal flex circuit 52 has an area 54 that is formed solely by a layer of 
copper. In a preferred embodiment the layer of copper 54 has a thickness 
ranging from about 0.0002 inches to about 0.0005 inches, and more 
preferably has a thickness of about 0.0003 inches, extending from one side 
of area 54 is a plurality of traces 56. 
The individual traces 56 are preferably copper which has been disposed on a 
polyimide film 48 such as KAPTON.TM. which is commercially available from 
the E. I. DuPont Company. The individual traces 56 are electrically 
isolated from one another by the layer of polyimide 48 as is well known. 
With reference to both FIGS. 3 and 4, the area 54 has a first center pad 
area 58 that, when the transducer is constructed, will be disposed between 
the backing block 22 and the first layer of piezoelectric material 24. The 
area 54 has a second center pad area 60 that, when the transducer is 
constructed, will be disposed between the second layer 26 and the third 
layer 28. An area 62 connects the first and second center pads 58 and 60 
and simply wraps around a side of the first and second layers 24 and 26 
when the transducer is constructed as shown in FIG. 3. Because no traces 
are formed in area 54 the construction of the transducer is simplified. 
Alignment is only required between the kerfs 30 that define the transducer 
elements and the traces 56 in the signal flex circuit 52. 
Referring to FIG. 3 the ground flex circuit 70 has a first and second 
branch 72 and 74, respectively, that are formed by a layer of copper 
having a thickness ranging from about 0.0002 inches to about 0.0005 inches 
and, more preferably, has a thickness of about 0.0003 inches. When the 
transducer is constructed both the ground and the signal traces extend 
from the same side of the transducer and are joined at area 46 which has 
the signal traces 56 on the underside thereof separated from the layer of 
copper that forms the ground plane by the layer of polyimide 48. 
When the transducer is coupled to the ultrasound system and an excitation 
signal is output by the transmit circuit 14, the signal flex circuit 52 
delivers the excitation signal to the first, second and third layers 24, 
26 and 28. Upon receipt of the excitation signal, the first, second and 
third layers 24, 26 and 28 convert the excitation signal to a pressure 
wave which is emitted from the transducer as an ultrasound beam. The 
ultrasound beam is directed into a region of examination to which the 
transducer is pointed. As the ultrasound beam encounters various 
structures in the region of examination, ultrasound waves are reflected 
back to the transducer. The reflected ultrasound waves are converted to 
electrical signals by the first, second and third layers and delivered to 
the receive circuitry 18 where they are processed and displayed on display 
20. 
To construct the transducer shown in FIG. 2, the first, second, third 
layers 24, 26 and 28 and signal and ground flex circuits are assembled as 
shown in FIG. 3. Kerfs 30 (see FIG. 2) are diced in the x-elevation 
direction through the acoustic matching layers, the ground and signal flex 
circuits and through the first, second and third layers and preferably 
partially into the backing block as is well known. The kerfs 30 are 
located to cut between the ground and signal traces of the ground and 
signal flex circuits so that each trace leads to an individual transducer 
element. Because the signal and ground traces extend from the same side of 
the transducer and the area 54 of the signal flex circuit does not have 
any traces, the process of correctly positioning the kerfs 30 is 
simplified. 
It has been found that a multilayered transducer constructed of 
piezoelectric layers having non-uniform thickness in the x-elevation 
direction provides better matching of the transducer to the ultrasound 
system which results in increased bandwidth and improved signal-to-noise 
ratio. In particular, because the layers of piezoelectric material are 
assembled based upon their poling direction which are acoustically in 
series and electrically in parallel, the following relationships apply 
based upon the KIM or Mason models: 
EQU .xi.(N)=.xi.(1)N.sup.2 
EQU Z(N)=Z(1)N.sup.2 
EQU V(N)=V(1)N, 
where .xi.(N) and .xi.(1) are the dielectric constants for N layers and for 
a single-layer respectively, Z(N) and Z(1) are acoustic impedance for N 
layers and for a single-layer respectively, and V(N) and V(1) are applied 
voltage for N layers and for a single-layer respectively. It can be seen 
that the impedance decreases significantly with a multilayered 
construction. 
FIG. 5 is a table listing the parameters measured for two, single layer 
non-uniform thickness transducers, two, two-layered non-uniform thickness 
transducers and a three-layered non-uniform thickness transducer. Listed 
on the right hand side of the table are three parameters namely; (a) the 
acoustic impedance Z at antiresonance for a single transducer element, (b) 
the clamping capacitance .xi. at 100 KHz for a single transducer element 
and (c) the round trip impulse response to flat target at center frequency 
for a single transducer element. Across the top line of the table is an 
indication of the array type and serial number of the array tested. The 
first two columns are for a single layer non-uniform thickness transducer 
having a design according to the '175 patent and the '998 patent. The next 
two columns are for a two-layered non-uniform thickness transducer having 
a design according to the '175 patent and the '998 patent. The last column 
is for a three-layered non-uniform thickness according to the present 
invention. 
As an example, for a two-layer design an improvement of about 51/2 db or 
better in signal-to-noise ratio has been measured and confirmed end for a 
three-layer design an improvement of about 8 db or better in 
signal-to-noise ratio has been measured and confirmed. 
FIG. 6 is an example of a typical one-layer ultrasound transducer acoustic 
impedance frequency response plot resulting from operation of such 
transducer. 
FIG. 7 is an example of a two crystal design ultrasound transducer acoustic 
impedance frequency response plot resulting from the operation of the two 
crystal transducer. Comparing the graphs shown in FIGS. 6 and 7 it can be 
seen that a reduction in anti-resonant frequency bulk impedance was 
reduced from 11.638 .OMEGA. to 2.896 .OMEGA., a ratio of 4.018=N.sup.2 
=2.sup.2. 
While this invention has been shown and described in connection with the 
preferred embodiments, it is apparent that certain changes and 
modifications, in addition to those mentioned above, may be made from the 
basic features of the present invention. Accordingly, it is the intention 
of the Applicant to protect all variations and modifications within the 
true spirit and valid scope of the present invention.