[0001] This invention relates to acoustic transducer arrays, and more Particularly, to a
laminated backing layer for use with such arrays to electrically connect the array
to a circuit element and to substantially eliminate spurious acoustic reflections
caused by the array.
[0002] Ultrasonic imaging systems are widely used to produce images of internal structure
of a specimen or target of interest. A diagnostic ultrasonic imaging system for medical
use forms images of internal tissues of a human body by electrically exciting an acoustic
transducer element or an array of acoustic transducer elements to generate short ultrasonic
pulses that are caused to travel into the body. Echoes from the tissues are received
by the acoustic transducer element or elements and are converted into electrical signals.
circuit element, such as a printed circuit board, flexible cable or semiconductor,
receives the electrical signals. The electrical signals are amplified and used to
form a cross-sectional image of the tissues. These imaging techniques provide a safe,
non-invasive method of obtaining diagnostic images of the human body.
[0003] The acoustic transducer which radiates the ultrasonic pulses is provided with a plurality
of piezoelectric elements arranged in an array with a predetermined pitch. The array
is generally one or two-dimensional. By reducing the pitch of the piezoelectric elements
in the array, and increasing the number of elements, the resolution of the image can
be increased. An operator of the imaging system can control the phase of the electronic
pulses applied to the respective piezoelectric elements in order to vary the direction
of the output ultrasonic wave beam or its focus. In this way, the operator can "steer"
the direction of the ultrasonic wave in order to illuminate desired portions of the
human body without needing to physically manipulate the position of the transducer.
[0004] When one of the piezoelectric elements is energized, acoustic waves are transmitted
both from the front surface facing the imaging target and the rear surface of the
element. It is desirable that the acoustic energy from the rear surface be substantially
attenuated so that the image resolution is not adversely affected. If not attenuated,
the rearward travelling acoustic signals can reflect off the circuit element and return
to the transducer surface, causing a degradation of the desired electrical signal.
[0005] To remedy this situation, a backing layer of an acoustically attenuating material
is disposed between the piezoelectric elements and the circuit element to attenuate
the undesired acoustic energy from the rear surface of the piezoelectric element.
Ideally, this backing layer would have an acoustic impedance matched to the impedance
of the piezoelectric elements so that a substantial portion of the acoustic energy
at the rear surface of the piezoelectric element is coupled into the backing layer.
[0006] A problem with the use of a backing layer between the piezoelectric element and the
circuit element is that of providing electrical interconnection between the particular
piezoelectric elements and the associated circuit elements. The interconnection problem
is more difficult for two-dimensional arrays of more than three rows and columns of
piezoelectric elements, since the internal elements will not have an exposed edge
that accommodates electrical connection. In such two-dimensional arrays, electrical
interconnection between the individual piezoelectric elements and the electric circuit
which receives and processes the electrical signals is generally made in the z-axis
direction perpendicular to the array. However, as the number of elements within the
array increases, and the pitch between the elements decreases, it becomes increasingly
difficult to fabricate this interconnection.
[0007] One approach to provide the interconnection through the backing layer is disclosed
in U.S. Patent No. 4,825,115 by Kawabe et al., entitled ULTRASONIC TRANSDUCER AND
METHOD FOR FABRICATING THEREOF. Kawabe teaches the use of printed wiring boards bonded
directly to the piezoelectric array transducer elements. A backing layer is then molded
onto the array around the boards, which extend outward from the molded backing layer.
While Kawabe provides a reliable interconnection method, th wiring boards provide
a surface for reflection of acoustic wave energy within the backing layer, and worsen
some of the acoustic attenuating properties of the backing layer.
[0008] The deficiencies of the prior art could be overcome if the entire backing layer were
formed from a contiguous block of material. In this way, the overall acoustic attenuating
ability of the backing layer would be improved. However, the use of a solid backing
layer compounds the fabrication problem, in that it is difficult to thread electrical
conductors through the solid backing layer.
[0009] A secondary problem which increases the difficulty of forming the electrical interconnections
is that of undesired cross-talk. Cross-talk is defined as unintended interference
between adjacent signal conductors, which occurs via capacitive or inductive coupling.
In typical applications, cross-talk can be minimized or eliminated through shielding
of the conductive element. While a shield provided around the electrical conductors
which form the interconnections through the backing layer would mitigate the cross-talk
problem, the shield increases the difficulty of providing the conductors through the
solid backing layer.
[0010] Therefore, a critical need exists for an improved method and apparatus for making
electrical interconnection between elements of an acoustic transducer array and corresponding
contacts of an electrical circuit element. Such a technique should provide for the
outputted acoustic energy from the rear surface of the piezoelectric element to be
fully attenuated so that there are substantially no reflections of such energy back
into the transducer element. The technique should also permit relative ease of manufacture
and ready adaptability for large transducer arrays having high numbers of piezoelectric
elements with relatively small pitch.
[0011] In accordance with the above, this invention provides an acoustic transducer for
transmitting acoustic wave energy in response to an electrical signal and for converting
the received acoustic wave energy into an electric signal. The acoustic transducer
includes an array of piezoelectric elements, a backing layer attached at a rearward
face of the piezoelectric elements, a circuit element spaced apart from the piezoelectric
elements by the backing layer, and at least one electrical conductor for each of the
piezoelectric elements to connect the elements to the circuit element. The backing
layer comprises a plurality of layers of acoustic attenuating material integrally
formed into a generally laminate structure. The electrical conductors extend along
a surface of each of the layers and have a predetermined pitch. Each of the layers
has a thickness substantially equivalent to the pitch of the electrical conductors.
[0012] In an embodiment of the invention, each of the electrical signal conductors is separated
by electrical ground conductors which are not in electrical contact with the electrical
signal conductors. The ground conductors would substantially reduce undesired cross-talk
between adjacent electrical conductors of a particular layer. Similarly, selected
layers could be separated by ground layers to substantially reduce undesired cross-talk
between the electrical conductors of adjacent layers.
[0013] The invention further provides a method for fabricating the laminate structure from
a plurality of layers of acoustic attenuating material. Each of the layers is patterned
with a plurality of electrical conductors of a predetermined pitch equivalent to the
thickness of the layers. The patterned layers are then combined into the generally
laminate structure with the electrical conductors extending between a top and bottom
surface of the structure. After combining the individual layers into a laminate structure,
electrical contacts would be disposed on the bottom surface of the laminate structure
for electrical connection to respective ones of the circuit element electrical contacts.
Individual piezoelectric elements of the transducer matrix would be disposed on the
top surface of the laminate structure with each of the piezoelectric elements being
electrically connected to respective ones of the electrical conductors.
[0014] An exemplary embodiment of the invention will now be described with reference to
the drawings, in which:
Fig. 1 is an exploded perspective view of a laminate structure of an acoustic transducer
backing layer of this invention;
Fig. 2 is an enlarged view of a portion of an individual layer of Fig. 1;
Fig. 3 is a perspective view of the backing layer of Figs. 1 and 2 combined into a
single laminate structure;
Fig. 4 is a partial bottom view of the laminate structure illustrating a pattern of
electrical contacts;
Fig. 5 is a partial perspective view of the laminate structure illustrating the individual
piezoelectric elements disposed on a top surface of the laminate structure;
Fig. 6 is a partial perspective view of a single layer of the laminate structure as
in Fig. 2, illustrating an embodiment in which ground conductors alternate with the
electrical conductors;
Fig. 7 is an exploded perspective view as in Fig. 1, in which ground layers are interspersed
with the conductive layers;
Fig. 8a illustrates a partial perspective view of a single layer in which a three-sided
electrical conductor is utilized;
Fig. 8b is a partial top view of the single layer of Fig. 8a; and
Fig. 9 is an exploded perspective view of an acoustic transducer of this invention.
[0015] This invention discloses an improved method and apparatus for making electrical contact
between elements of an acoustic transducer array and corresponding contacts of an
electrical circuit element. The invention provides for the outputted acoustic energy
from the rear surface of the piezoelectric elements to be fully attenuated so that
there are substantially no reflections of such energy back into the transducer element.
In addition, the invention is relatively easy to manufacture and readily adaptable
for large transducer arrays having high numbers of piezoelectric elements with relatively
low pitch.
[0016] Referring first to Figs. 1-5, a method for making an acoustic transducer from a laminate
structure 30 of acoustic attenuating material is illustrated. The laminate structure
30 comprises a plurality of layers 10 (illustrated as layers 10₁ through 10₆) which
are formed from initial sheets of acoustic attenuating material that are cut into
an approximate shape. In the preferred embodiment, the material is an epoxy having
acoustic absorbers and scatterers, such as tungsten, silica, chloroprene particles
or air bubbles. While the material is not limited to epoxy, it must be both electrically
insulating and acoustically absorbing.
[0017] Each of the layers 10 has a front surface 12, a rear surface 14, a top surface 16
and a bottom surface 18. The front surface 12 is provided with an electrode coating
22. The exact thickness of the coating 22 can be selected to exhibit desired acoustic
and structural qualities. If the coating 22 is too thick, it could adversely affect
the acoustic properties of the material by increasing reflectivity of the rearward
traveling acoustic wave. On the other hand, if the coating 22 is too thin, it could
be of low conductivity or prone to discontinuities of conduction. In the preferred
embodiment, the electrode coating 22 comprises a two metal coating having a first
coating of chromium applied to a thickness of approximately 300 Ångströms, and a second
coating of gold applied to a thickness of approximately 3000 Ångströms. The chromium
promotes adhesion to the layer 10 and the gold provides a good quality electrical
conductor which is resistant to oxidation. However, it should be apparent that alternative
materials could be used for the coating 22 as long as the desired electrical conductivity
and material strength requirements are satisfied. The metal coatings can be applied
by sputtering, plating, or other conventional techniques.
[0018] After the conductive coating 22 is applied to each layer 10, the coated layer is
patterned to provide a plurality of conductors 24, as shown in Figs. 1 and 2. Each
conductor 24 extends between the top edge 16 and the bottom edge 18 of the layer 10,
and is separated from adjacent conductors 24 by a gap 26. The patterning could be
produced by a diamond saw, with the saw kerf forming the inter-conductor gap 26. The
saw should cut into the layer 10 to a slight depth to further promote isolation of
the individual conductors 24. Alternatively, the pattern could be produced by conventional
photolithographic techniques.
[0019] The individual conductors 24 are separated by a desired pitch, which is equivalent
to the desired pitch of the piezoelectric elements of the transducer, as will be described
below. In the preferred embodiment, the conductors 24 would have a pitch of 300 microns
with a gap 26 spacing between each conductor of 50 microns. The particular spacing
and pitch shown in the figures is for illustrative purposes only, and is not drawn
to scale.
[0020] Once a plurality of patterned layers 10 is produced, the layers are combined together
into the laminate structure 30 illustrated in Fig. 3. The layers 10 are combined under
temperature and pressure with an electrically insulating epoxy or other adhesive which
fills the gaps between the individual conductors. Each of the patterned layers 10
is oriented so that the conductors 24 extend between an upper surface 32 and a lower
surface 34 of the laminate structure 30. Once the laminate structure 30 is formed,
the external surfaces are lapped with conventional machining processes to produce
substantially flat and/or orthogonal surfaces. An edge portion 28 of each conductor
24 is exposed at the upper and lower lapped surfaces.
[0021] Next, referring to Fig. 4, a pattern of electrical contacts is provided on the lower
surface 34 of the laminate structure 30. An electrode coating 38 is applied to the
lower surface 34, in the same manner as the application of the coating 22 to the individual
layers 10 described above. The bottom conductor 38 is then patterned to provide a
plurality of individual conductive pads 46. Each pad 46 is electrically connected
to an associated end of one of the conductors 24, as illustrated in phantom, and is
patterned to match associated elements of the circuit element.
[0022] The patterning can be produced by a conventional diamond saw, as substantially described
above. The saw would first cut a plurality of vertical kerf lines 42 having dentical
pitch as the gaps 26 between adjacent conductors 24 of the individual layers 10. The
saw would then cut a plurality of horizonal kerf lines 44 spaced at midpoints of the
individual layer 10 thicknesses. However, other patterning techniques such as photolithography
can also be advantageously applied.
[0023] Referring now to Fig. 5, on the top surface 32 of the laminate structure 30, a conductive
coating 36 is applied in the same manner as that described above. However, prior to
patterning the coating 36, a layer of piezoelectric material 48 is bonded onto the
conductive coating. The piezoelectric material 48 may include any material which generates
acoustic waves in response to an electric field applied across the material, such
as but not limited to lead zirconium titanate.
[0024] To bond the piezoelectric material 48 to the conductive coating 36, a low viscosity
electrically insulating adhesive, such as epoxy, is utilized. The adhesive is applied
to a thickness of approximately one micron. The RMS roughness of the piezoelectric
material 48 exceeds the bond thickness, so that when heat and pressure are applied
the peaks of the piezoelectric material penetrate through the epoxy to form electrical
connections between the piezoelectric material and the conductive coating 36.
[0025] Once the piezoelectric material 48 has been bonded onto the conductive coating 36,
a matching layer 68 of graphite or polymer is applied onto the exposed surface of
the piezoelectric material 48. The matching layer 68 increases the forward wave energy
produced by the piezoelectric material 48, as known in the art. The combination of
the piezoelectric material 48, the conductive coating 36 and the matching layer 68
is then patterned with vertical and horizontal kerf lines 42 and 44 in the same manner
as that described above with respect to the bottom surface 34. It should be apparent
that this technique permits self-alignment of the top conductive coating 36 to the
vertical and horizonal kerf lines 42 and 44 with the laminate structure 30.
[0026] It should be apparent that the patterning technique results in a plurality of piezoelectric
elements 52 disposed in a matrix each having an electrical conductor 24 extending
through the acoustic attenuating backing material of the laminate structure 30 to
an electrically conducting pad 46 disposed on a bottom surface 34 of the laminate
structure. The pads 46 at the bottom surface 34 are substantially aligned to the piezoelectric
elements 52 on the upper surface 32. A matrix of piezoelectric elements 52 of any
desired dimension can be produced in this manner by simply increasing the size and
number of the layers 10.
[0027] Referring now to Fig. 6, an alternative embodiment of the laminate structure 30 is
illustrated. The patterning of each individual layer 10 has alternating electrical
signal conductors 62 and ground conductors 64. The pitch of a combined pair of an
electrical conductor 62 and a ground conductor 64 is equivalent to the pitch of a
single electrical conductor 24 described above. The inclusion of interspersed ground
conductors 64 is intended to reduce cross-talk of electrical signals between adjacent
ones of the electrical signal conductors 62. The patterning of each layer 10 is performed
using a diamond saw or a photolithographic process, as is substantially discussed
above. However, at an upper edge of the layer 20, a ground path gap 66 is provided
to prevent electrical connection between the ground conductor 64 and the piezoelectric
element 52 which would be formed by the method discussed above. At a bottom surface
of an assembled laminate structure 30 having the ground conductor 64, the patterning
would match the spacing of the individual layer 10 so that distinct conductive pads
46 are produced for the electrical connection between the circuit element and the
ground conductors 64 and the electrical conductors 62. It is intended that the ground
conductors 64 be electrically connected to a ground potential, so that any cross-talk
signal induced into the ground conductors would be shunted to ground.
[0028] To control cross-talk between electrical conductors 24 of adjacent layers 10, a second
alternative embodiment is illustrated in Fig. 7. In this embodiment, individual layers
10 would be divided in width through a first layer portion 72 and a second layer portion
74. A ground plane 76 is disposed between the first and second layer portions 72 and
74. The ground plane would similarly connect electrically to ground pads 46 disposed
at the lower surface 34 of the assembled laminate structure 30, so as to shunt undesired
cross-talk signals to ground. In a similar manner, thermal conductors could be used
in place of the ground planes 76 to eliminate excess heat from within the laminate
structure 30. The thermal conductors would conduct excess heat within the structure
30 to an external heat sink (not shown).
[0029] Experimentation with acoustic transducers formed in accordance with the teachings
of this invention has demonstrated acceptable levels of cross-talk attenuation for
steering and focusing of a two-dimensional array. In particular, intra-layer cross-talk
levels measured between adjacent conductors 24 demonstrate a cross-talk signal of
-46 Db. Similarly, inter-layer cross-talk measurements made between conductors 24
of adjacent layers 10 demonstrate a cross-talk signal of -45 dB. It is anticipated
that further improvements in cross-talk attenuation can be obtained through use of
the interspersed ground conductors 64 described above.
[0030] It is estimated that the individual electrical conductors 24 would dissipate low
levels of electrical power, and thus would present a minimal thermal problem. The
measured resistance for each individual conductor 24 is less than 1 ohm. Since the
individual piezoelectric elements 52 have resistance of approximately 10,000 ohms
and require approximately 100 volts for the transmit pulse, each conductor 24 can
be expected to carry approximately 10 milliamps. Thus, each electrical conductor 24
could be expected to dissipate approximately 100 microwatts, which is relatively low.
[0031] However, it may be desirable in certain applications to further reduce the resistance
and increase the reliability of each conductor 24. Accordingly, Figs. 8a and 8b illustrate
a third alternative embodiment of the invention. Rather than providing a single planar
electrical conductor 24 which is patterned as described above, the individual layers
10 are patterned prior to applying the conductive coating 22 to form generally rectangular
grooves 82. The rectangular grooves 82 are then coated with an electrically conductive
coating including an electrically conductive backwall 84 and conductive sidewalls
86. The three-sided electrical conductor 80 is then filled with an electrically insulating
epoxy 88, and the individual layers are combined as substantially described above
into the single laminate structure 30. As before, bottom conductive pads 46 are formed
on the lower surface of the laminate structure 30, and individual piezoelectric elements
52 are disposed on an upper surface of the laminate structure. The three-sided conductor
80 would have a resistance roughly one-third of the conductor 24 described above.
[0032] Fig. 9 illustrates an exploded perspective view of an acoustic transducer 50 constructed
in accordance with this invention. The transducer 50 attaches electrically to a circuit
element illustrated as printed circuit board 54. The circuit element can also be a
semiconductor, flexible cable or other device. The circuit board 54 has a plurality
of electrical contact points 56 which match the position of the individual conductive
pads 46 at the lower surface 34 of the transducer 50. In addition, a ground sheet
58 overlays the exposed upper portion of the individual piezoelectric elements 52
and electrically connects to the individual elements.
[0033] In operation, the circuit board 54 provides electrical signals to the individual
conductive pads 46 of the transducer 50. The electrical signals are conducted via
the electrical conductors 24 through the layers 10 of the laminate structure 30 to
the rearward surface of the individual piezoelectric elements 52. The acoustically
transparent ground sheet 58 is disposed on the exposed surface of the transducer 50,
and is placed in contact with the object of interest, such as the patient's skin.
Utilization of the ground sheet 58 at the exposed surface of the piezoelectric elements
52 rather at the rearward surface prevents the patient from receiving an inadvertent
electrical shock.
[0034] The electrical signals which are inputted to the piezoelectric elements 52 are converted
into acoustic wave energy which is transmitted through the ground sheet 58 into the
subject. The wave energy transmitted from the transducer 50 is utilized to achieve
echographic examination. The undesirable transmission of acoustic wave energy from
the rearward faces of the piezoelectric elements 52 is absorbed by the laminate structure
30 formed from the acoustic absorbing material. Reflected wave energy received at
the piezoelectric elements 52 is converted to an electrical signal that is conducted
back through the electrical conductors 24 to the circuit board 54. This received signal
would then be conditioned by known electrical circuitry on the circuit board.
[0035] Using the invention, an array of any desired dimension can be produced by varying
the number and size of the layers 10, and the pitch of the conductors 24 disposed
on each layer.
1. A backing layer for interfacing an acoustic transducer array having a plurality of
transducer elements (52), each of which has a first acoustic impedance and a rear
face, with an electric circuit element (54) having a contact (56) for each transducer
element, the backing layer comprising:
a plurality of layers (10) of acoustic attenuating material integrally formed into
a block (30) having a first face (32) and a second face (34), the block (30) having
an acoustic impedance at the first face (32) which is of a value relative to the first
acoustic impedance such that a selected portion of the element acoustic energy at
the rear face is coupled into the block (30);
at least one electrical conductor (24) for each of the transducer elements (52),
the conductors (24) extending between the first (32) and second (34) faces and having
a predetermined pitch, the conductors (24) for adjacent transducer elements (52) being
electrically isolated from one another;
a plurality of first electrical contacts (36) disposed at the first face (32) for
effecting electrical connection between the rear face of each transducer element and
the corresponding at least one electrical conductor (24); and
a plurality of second electrical contacts (46) disposed at the second face (34)
for effecting electrical connection between the circuit element contact (56) for the
transducer element (52) and the corresponding at least one electrical conductor (24).
2. The backing layer of Claim 1 wherein the electrical conductors (24) are disposed on
a surface of each of the layers (10).
3. The backing layer of Claim 1 or 2 wherein the first electrical contacts (36) are disposed
in a pattern substantially matching the rear face of the transducer array.
4. The backing layer of Claim 1, 2 or 3 wherein the second electrical contacts (46) are
disposed in a pattern substantially matching the electric circuit contacts (56).
5. The backing layer of any one of the preceding claims wherein the electrical conductors
(62) are separated by ground conductors (64), the electrical conductors (62) being
electrically isolated from the ground conductors (64).
6. The backing layer of any one of the preceding claims wherein selected ones of the
layers (10) further include electrical shielding (76) to electrically isolate electrical
conductors (24) of adjacent ones of the layers (10).
7. A method for fabricating an acoustic transducer having a plurality of piezoelectric
elements (52) aligned in a matrix, the method comprising the steps of:
providing a laminate structure (30) comprising a plurality of layers (10) of an
acoustic attenuating material, each of the layers (10) having a plurality of electrical
conductors (24);
disposing electrical contacts (46) on a bottom surface (34) of the laminate structure
(30) electrically connected to respective ones of the electrical conductors (24),
the electrical contacts (46) capable of connection to an external power source; and
disposing the piezoelectric elements (52) on a top surface (32) of the laminate
structure (30), the piezoelectric elements (52) being electrically connected to respective
ones of the electrical conductors (24).
8. The method for fabricating an acoustic transducer of Claim 7 wherein the step of providing
a laminate structure further comprises the steps of:
providing an electrically conductive coating (22) on a surface of each layer (10);
patterning each of the coated surfaces to form the electrical conductors (24);
and
combining the plurality of layers (10) together into the laminate structure (30),
the electrical conductors (24) extending between the top (32) and bottom (34) surfaces.
9. The method for fabricating an acoustic transducer of Claim 7 or 8 wherein the step
of disposing electrical contacts (46) on a bottom surface (34) of the laminate structure
(30) further comprises the steps of:
applying an electrically conductive coating (38) to the bottom surface (34); and
patterning the electrically conductive coating (38) to provide the electrical contacts
(46).
10. The method for fabricating an acoustic transducer of Claim 7, 8 or 9 wherein the step
of disposing the piezoelectric elements (52) on a top surface (32) of the laminate
structure (30) further comprises the steps of:
applying an electrically conductive coating (36) to the top surface (32) and bonding
a piezoelectric layer (48) onto the electrically conductive coating (36); and
patterning both the electrically conductive coating (36) and the piezoelectric
layer (48) to provide the piezoelectric elements (52).