[0001] The present invention relates to ultrasound transducers. In particular, it relates
to ultrasound transducers of the type which generate and receive longitudinal waves
for use in medical ultrasound imaging.
[0002] In ultrasound transducer technology, various modes of vibration of piezoelectric
material are well known which are useful for generating longitudinal waves. These
include the "plate" mode, in which a relatively flat plate of piezoelectric material
vibrates in a manner such that ultrasound waves are transmitted in a direction normal
to the surface of the plate when electrodes connected to the upper and lower plate
surfaces are energized, and the "bar" mode, in which a long, thin bar of piezoelectric
material having electrodes connected at either end of the bar vibrates to generate
wave transmissions along the longitudinal axis of the bar. There is also a "beam"
mode in which a long, thin bar of piezoelectric material having elongated electrodes
on either side of the bar vibrates to generate wave transmissions which are perpendicular
to the longitudinal axis of the bar, such as in a phased array or linear array transducer.
Also, there are "mixed" modes of vibration, which may include "plate" mode, "bar"
mode, or "beam" mode vibrations, together with lateral vibration modes. These lateral
modes occur to an unacceptable level in piezoelectric material in which the ratio
of the piezoelectric material's height to its width (H/W) is in a ratio of approximately
0.5 to 2 for transducers which utilize the half wavelength resonance mode or in a
ratio of approximately 0.25 to 1 for transducers which utilize the quarter wavelength
resonance mode. As will be understood by those skilled in the art, the lateral modes
of vibration occur in any piezoelectric material to some extent, depending upon the
geometry of each element which comprises the transducer and the properties of the
particular piezoelectric material. It is only a severe problem in half wavelength
transducers when H/W is between about 0.5 and 2 and in quarter wavelength transducers
when H/W is between about 0.25 and 1.
[0003] The particular problem which the present invention is particularly adapted to solve
is readily described by referring to piston type annular transducers of the type used
in annular arrays. Heretofore, a variety of piston type annular array transducers
have been used to provide electronically variable focusing capabilities. In such annular
array transducers, the outer rings of the annular arrays are typically much narrower
than the inner rings or the center piston. This results from the desire to keep the
areas of the various transducer elements substantially equal in order to provide substantially
uniform signals over the depth of penetration of the 'ultrasound. This phenomenon
is well known in the art, and it is common in annular arrays to provide annular elements
which have areas which are substantially equal to each other and to the area of the
central piston.
[0004] The problem, which results from manufacturing annular arrays in the standard manner,
is that the ratio of the height (of the piezoelectric material measured from its substrate)
to the width of the individual elements (measured radially) gets close to 1 in the
outer rings of the annular array. Unfortunately, as noted above, when the ratio of
a transducer's height to its width is in the range of approximately 0.5 to about 2,
the lateral modes of vibration occur at a level which is unacceptable in medical ultrasound
piston transducers. Various attempts have been made heretofore to reduce the lateral
vibration mode of the outer transducer rings. Such methods have included putting dampening
material into the areas between and surrounding the outer rings. Overall, these methods
have yielded little positive results.
[0005] Accordingly, a method for producing an annular array transducer which can be electronically
focused over a large range without having to suffer the problems of lateral mode vibrations
in the outer rings would be desirable. In general, it would be desirable to be able
to select an arbitrary transducer type, e.g. an annular array, without thereby being
forced to accept whatever spurious vibrational modes might occur, e.g. lateral modes
in the outer rings. It would be desirable to have the same vibrational mode, e.g.
plate or bar mode, in all elements of a transducer with any arbitrary geometry, e.g.
the central piston and outer rings of an annular array.
[0006] According to the invention, we provide an improved ultrasonic transducer comprising
a piece of piezoelectric material which has been subdivided into subelements smaller
than the electrodes attached to said subelements, whereby the vibrational mode of
the subelements is determined by their physical shape and dimensions, rather than
by the shape or dimensions of the electrode geometry.
[0007] As hereinafter described with reference to the drawing, the bar vibrational mode
can be combined with any transducer design, because the piezoelectric material is
sawed into a large number of subelements which each vibrate in the bar mode due to
their physical shape and dimensions. These subelements are then electrically connected
to have any desired element geometry. Accordingly, it is possible to design transducers
of arbitrary configuration which have the same vibrational mode in all elements.
[0008] In the Drawing:
FIG. 1 is a top plan view of one embodiment of a transducer utilizing the present
invention: and
FIG. 2 is a side view of a transducer utilizing the present invention:
FIG. 3 is a bottom plan view of the transducer of FIG. 1 illustrating the electrode
pattern of the annular array: and
FIG. 4 is a side view of a transducer utilizing a second embodiment of the present
invention.
[0009] As mentioned above, it is desired to substantially eliminate the lateral mode of
vibration in the outer rings of an annular array. In order to accomplish that result,
a circular piece of piezoelectric material 10, shown in FIG. 1, is sawed, by a semiconductor
dicing saw. for example, into a number of subelements 12. In the preferred embodiment
of the invention, the subelements 12 are substantially square, having an edge length.
W, which is substantially smaller than the height. H, of the piezoelectric material
10. By way of example, if the PZT4 composition of PZT (lead-zirconate-titanate) piezoelectric
material is used, H may be approximately 20 mils (0.5 mm), and W may be approximately
8 mils (0.2 mm) for a 3 MHz medical ultrasound transducer. As shown in FIG. 2, the
saw kerfs 14 extend from a top surface 16 of the piezoelectric material 10 substantially
down to the bottom surface 18. However, in the preferred embodiment of the invention,
the saw kerfs 14 do not extend completely through to the bottom surface 18 of the
piezoelectric material 10. thereby maintaining the structural integrity of the piezoelectric
material 10 and the electrode pattern. However, as will be explained hereinafter,
it is possible to have the saw kerfs 14 extend through the bottom surface 18 with
appropriate changes to the preferred process described below.
[0010] After the saw kerfs 14 are formed through the top surface 16, the subelements 12
of the top surface 16 must be reconnected electrically. While there are a number of
ways in which this can be done, in the preferred method the saw kerfs 14 are filled
with a low viscosity, non-conductive epoxy. Then, in the preferred embodiment, a tri-metal
system is sputtered onto the surface of the epoxy to form the upper electrode 20 which
also functions as an RF shield if electrically connected to ground. As is well known
in the art, a tri-metal system provides a first metal which adheres well to the underlying
material, a second metal which provides coupling between the first metal and a third
metal, and a third metal which is relatively impervious to oxidation and which can
be soldered to easily. In the preferred embodiment of the invention, the first metal
is chrome, the second metal is nickel, and the third metal is copper. One or more
quarter wave acoustic matching layers 22 of, for example, non-conductive, filled epoxy
is then applied over the surface of the top electrode 20 in a manner and for reasons
which are well known in the art.
[0011] Electrodes 24 are formed on the bottom surface 18 in any desired configuration. In
the preferred embodiment of the invention, the electrodes 24 are in the form of an
annular array pattern, as shown in FIG. 3. In the preferred embodiment of the invention,
a layer of conductive material, such as copper, is applied to the bottom of the piezoelectric
material 10. Then, a layer of resist material is printed in the form of the pattern
of the bottom electrodes on the conductive layer, and the exposed portions of the
conductive layer are etched to remove the undesired portions down to the piezoelectric
material 10. An acoustic backing layer 26 is applied to the bottom electrode pattern
24, the purpose of which is well known in the art. As should be obvious, the minimum
interelectrode spacing between the annular rings of the electrode pattern is selected
to insure that no two electrodes can energize the same subelement 12. This can be
accomplished by using an interelectrode spacing which is greater than W times the
square root of 2 (for square subelements 12 having an edge length W).
[0012] Referring now to FIG. 4. an alternative embodiment 28 of the present invention is
shown in cross-section. In this particular embodiment, the piezoelectric material
30 is diced into subelements 32, with the saw kerfs 34 going completely through to
a quarter wavelength mismatching layer 36. The piezoelectric material 30 is itself
approximately a quarter wavelength thick rather than one-half wavelength thick. Again,
one or more quarter wavelength thick matching layers 38 are applied to an electrode
39 on the face 40 of the piezoelectric material 30. The particular material used for
the mismatching layer 36 is selected to have an acoustic impedance of Z
L with a backing layer 37 (on the mismatching layer 36.) having an acoustic impedance
of Z
B, resulting in an input impedance into the mismatching layer 36, as seen from the
piezoelectric material 30, which is (Z
L)
2/Z
B near the frequency for which the layer 36 is approximately one-quarter wavelength
thick. If the subelements 32 are diced completely through the piezoelectric material
30 to the mismatching layer 36, the mismatching layer 36 is preferably conductive
so that the rear electrode pattern 41 may be formed in the mismatching layer 36. In
certain instances, as will be understood by those skilled in the art, optimization
of a particular transducer design may require the mismatching layer 36 to be other
than one-quarter wavelength thick.
[0013] When Z
L is chosen to be relatively large with respect to Z
B, the impedance into the mismatching layer 36 becomes relatively large. Accordingly,
substantially all of the acoustic energy is transmitted through the face 40 of the
piezoelectric material 30, rather than into the mismatching layer 36 and the piezoelectric
material vibrates in a quarter wavelength resonance mode due to the sign change of
the reflection coefficient at the rear boundary 43. as will be obvious to those skilled
in the art. An advantage of manufacturing a transducer 10 in accordance with this
embodiment is that the individual subelements 32 are less fragile since the piezoelectric
material 30 is thinner for a given frequency.
[0014] As this particular embodiment involves a piece of piezoelectric material 30 having
a thickness of about a quarter wavelength rather than a piece of piezoelectric material
30 having a thickness of about one-half wavelength. the height-to-width ratio (H/W)
required to substantially eliminate undesired mixed vibrational modes is governed
by different rules than for a half wavelength thick piece of piezoelectric material
30. Accordingly, the mixed mode of operation will not be experienced in this particular
embodiment unless the height-to-width ratio is substantially in the range of about
0.25 to 1. Accordingly, the individual subelements 32 can have a height-to-width ratio
of approximately 1.25 which makes them structurally stronger than in the embodiment
described with respect to FIG. 2.
[0015] With particular reference to FIG. 2, the height-to-width ratio H/W. is selected to
be substantially greater than 2. In particular, a ratio of 2.5 has been found to be
acceptable.
[0016] While the present invention is particularly adapted for use in annular array type
devices, it could also be used in linear or phased array type devices, in which case
the electrode pattern which is applied at this step would be different. For purposes
of describing the present invention, an annular array electrode pattern is used. Those
skilled in the art will recognize that in appropriate situations the present invention
can be utilized in order to provide a linear array in which the elements operate in
a "bar mode" rather than in the conventional beam mode. Particular advantage can be
taken in that bar mode devices experience greater coupling between electrical energy
and acoustic energy which can provide advantages in linear arrays or phased arrays.
[0017] As hereinbefore described, an annular array ultrasound transducer having individual
annular elements which are well matched to provide substantially equal intensity signals
at various depths and which have excellent frequency match between elements is constructed.
The problems heretofore experienced with annular array transducers have been substantially
eliminated. In addition, using the present invention, medical ultrasound transducers
can be manufactured in any desirable transducer geometry, such as the annular array
described herein, with a uniform vibration mode for all the elements of the transducer.
1. An improved ultrasonic transducer comprising a piece of piezoelectric material
which has been subdivided into subelements smaller than the electrodes attached to
said subelements. whereby the vibrational mode of the subelements is determined by
their physical shape and dimensions. rather than by the shape or dimensions of the
electrode geometry.
2. The improved ultrasonic transducer of Claim 1 wherein the transducer is an annular
array transducer and said subelements are separated by saw kerfs which extend substantially,
but not completely, through said piezoelectric material.
3. The improved ultrasonic transducer of Claim 2 wherein said saw kerfs are filled
with a non-conductive material, and the surface of said piezoelectric material and
said filler material is covered by a conductive electrode.
4. The improved ultrasonic transducer of Claim 3 wherein said conductive electrode
is comprised of a tri-metal system comprising a first metal on the surface of said
piezoelectric material, a second metal on the surface of said first metal, and a third
metal on the surface of said second metal.
5. The improved ultrasonic transducer of Claim 4 wherein said first metal is chrome,
said second metal is nickel, and said third metal is copper.
6. The improved ultrasonic transducer of Claim 1 wherein the transducer is an annular
array transducer and said subelements are separated by saw kerfs which extend completely
through said piezoelectric material.