U.S. patent application number 12/156616 was filed with the patent office on 2009-02-12 for high frequency ultrasound transducers based on ceramic films.
Invention is credited to Bjorn A. J. Angelsen, Tonni F. Johansen, Helge Kristiansen.
Application Number | 20090039738 12/156616 |
Document ID | / |
Family ID | 34915227 |
Filed Date | 2009-02-12 |
United States Patent
Application |
20090039738 |
Kind Code |
A1 |
Angelsen; Bjorn A. J. ; et
al. |
February 12, 2009 |
High frequency ultrasound transducers based on ceramic films
Abstract
A design and a manufacturing method of ultrasound transducers
based on films of ferro-electric ceramic material is presented, the
transducers being particularly useful for operating at frequencies
above 10 MHz. The manufacturing technique can involve tape-casting
of the ceramic films, deposition of the ceramic films onto a
substrate with thick film printing, sol-gel, or other deposition
techniques, where manufacturing methods for load matching layers
and composite ceramic layers are described. The designs also
involve acoustic load matching layers that provide particularly
wide bandwidth of the transducers, and also multi-band operation of
the transducers. The basic designs can be used for elements in a
transducer array, that provides the frequency characteristics of
the single element transducers, for array steering of the focus and
possibly also direction of a pulsed ultrasound beam at high
frequencies and multi-band frequencies.
Inventors: |
Angelsen; Bjorn A. J.;
(Trondheim, NO) ; Johansen; Tonni F.; (Trondheim,
NO) ; Kristiansen; Helge; (Oslo, NO) |
Correspondence
Address: |
Lance J. Lieberman, Esq.;Cohen, Pontani, Lieberman & Pavane
Suite 1210, 551 Fifth Avenue
New York
NY
10176
US
|
Family ID: |
34915227 |
Appl. No.: |
12/156616 |
Filed: |
June 3, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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11074990 |
Mar 8, 2005 |
|
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12156616 |
|
|
|
|
60551155 |
Mar 8, 2004 |
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Current U.S.
Class: |
310/364 |
Current CPC
Class: |
H01L 2224/73253
20130101; H01L 2224/16225 20130101; H01L 2224/48091 20130101; H01L
2224/48091 20130101; H01L 2224/73265 20130101; B06B 1/0692
20130101; H01L 2924/00014 20130101 |
Class at
Publication: |
310/364 |
International
Class: |
H01L 41/047 20060101
H01L041/047 |
Claims
1. A method of electrical connection to the electrodes of an
ultrasound transducer or transducer array that radiates and
receives ultrasound waves through a front face into a load
material, with a back face on the opposite side, where the hot
element electrodes and a common ground electrode are accessible
from said back face at conducting electrode pads, and electrical
connections between said electrode pads and other circuits are made
at conductor pads on said other circuits with a glue between said
electrode pads and said conductor pads, said glue containing
conducting particles, and the electrical connection between said
electrode pads and said conductor pads is furthered by conducting
humps on one of said electrode pads and conductor pads, and by
applying pressure to the connection, the distance between said
humps and the opposite pad is made so small that said conducting
particles in said glue provides via said humps electrical
connection between said electrode pads and said conductor pads,
while outside said conducting humps the conductivity of the glue is
so low that no electrical connection is made.
2. An ultrasound transceiver system comprising an ultrasound
transducer or transducer array and at least one electronic
integrated circuit chip, where the electrical connections for said
integrated circuit chip are accessible at conductor pads at one
side of said chip, and electrical connections between said
conductor pads and said transducer array is made according to claim
1.
3. An ultrasound transceiver system according to claim 2, where the
transceiver is connected to an ultrasound imaging system through a
cable, the electrical wires of the cable being connected to the
transceiver at cable connection pads in said conducting film
structure, said cable wires being connected to said cable
connection pads through one of soldering, and gluing with glue
containing conducting particles.
4. An ultrasound transceiver system according to claim 3, where
stress relieve between the cable and the transducer is obtained by
gluing the jacket of the cable to said backing material.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a division of U.S. patent application
Ser. No. 11/074,990 filed Mar. 8, 2005 and claims priority from
U.S. Provisional Patent Application Ser. No. 60/551,155 which was
filed on Mar. 8, 2004.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention is directed to technology and design
of efficient ultrasound transducers for high frequencies. The
invention has special advantages where the highest frequencies are
above 10 MHz, but has also applications for transducers at lower
frequencies.
[0004] 2. Description of the Related Art
[0005] Medical ultrasound imaging at frequencies above .about.10
MHz, has a wide range of applications for studying microstructures
in soft tissues, such as the composition of small tumors or a
vessel wall. Ultrasound transducers for medical applications are
currently based on ferro-electric, ceramic plates as the active
material, that vibrates in thickness mode. When polarized, the
materials show piezoelectric properties with efficient
electromechanical coupling. However, the characteristic impedance
of the ceramic material (Z.sub.x.about.33 MRayl) is much higher
than that of the tissue load material (Z.sub.L.about.1.6 MRayl). In
order to get adequate thickness vibration amplitude of the plate
for efficient power coupling into the tissue load material, one
must operate the plates at thickness resonance, typically
L.sub.x=.lamda./2 resonance. Here L.sub.x is the plate thickness,
.lamda.=c.sub.l/f is the wavelength of longitudinal waves normal to
the plate with wave velocity c.sub.l and frequency f. The resonance
makes the transducer efficient in a band of frequencies around a
center frequency f.sub.0=c.sub.l/.lamda..sub.0=c.sub.l/2L.sub.x.
Acoustic matching plates between the ceramic plate and the load are
used to improve the power coupling to the load, a technique that
increases the bandwidth of the transducer resonance.
[0006] With the well known composite technique, where the ceramic
plate is diced into small posts, and the interpost space is filled
with polymer, the efficient characteristic impedance is reduced to
.about.15 MRayl, which is still around 10 times higher than the
characteristic impedance of the load, such as soft tissue or water.
Transducers of composite material must therefore also operate in
thickness resonant mode, albeit one can obtain some wider bandwidth
than with the transducers of whole ceramic.
[0007] Hence, both with whole and composite ceramic, the resonant
operation requires that the plate thickness is inversely
proportional to the center frequency of the operating transducer
band. This requires thicknesses in the range of 200-20 .mu.m for
center frequencies in the range of 10-100 MHz. Today, lapping of
the ceramic plate is the common technology to manufacture plates
with correct thickness, which becomes difficult and expensive at
thicknesses in ranges below 50-60 .mu.m, corresponding to
frequencies above 30-40 MHz. Composite ceramic/epoxy material is
also difficult to make for frequencies above 15 MHz, and it is
hence a general need for efficient methods to manufacture
transducers with a functioning high frequency band above .about.10
MHz.
[0008] The invention devices a solution to these problems as
described below.
SUMMARY OF THE INVENTION
[0009] The invention presents a new design of ultrasound
transducers where the active electromechanical coupling material is
ferroelectric, ceramic films that are made piezoelectric through
electric polarization. The piezoelectric layer is made either by
tape casting techniques and subsequently adhered to a substrate
after sintering, or by direct printing of the film onto a
substrate, or growing of the films onto the substrate with
sintering of the ceramic film after adhesion to the substrate. The
acoustic contact between the piezoelectric plate and the load
material is through the substrate, where the substrate after the
adhesion of the piezoelectric films is modified into layers with
descending characteristic acoustic impedance from the piezoelectric
plate to the load material, so that it performs an acoustic
impedance matching between the load material and the piezoelectric
plate, to increase the transducer bandwidth.
[0010] A preferred substrate material is the semiconductor silicon
(Si), since micro-machining manufacturing techniques for this
material are well developed. The characteristic acoustic impedance
of Si is 19.6 MRayl, a convenient impedance to be the acoustic
matching layer closest to the transducer with Chebychef matching
response. With 3 layer Chebychef matching, a second load matching
layer can then be made as a composite of Si and polymer to a
characteristic impedance of 7.3 MRayl, while the final matching
layer close to the load is a polymer with characteristic impedance
of .about.2.7 MRayl. With a 2 layer Chebychef matching, the Si
layer is coated with polymer with characteristic impedance
.about.2.7 MRayl.
[0011] For maximally flat type of matching with 3 layers to the
load, a substrate based on glass or fused quartz may be used with a
characteristic impedance of .about.14 MRayl, which is the layer
closest to the transducer. The 2.sup.nd layer then has a
characteristic impedance .about.4 MRayl, which is obtained by a
composite of glass and polymer with characteristic impedance of
.about.2 MRayl, where the 3.sup.rd layer is polymer with
characteristic impedance of .about.2 MRayl.
[0012] For maximally flat type of matching with 2 layers to the
load, a substrate based on Mg may be used with a characteristic
impedance of .about.10 MRayl, which is the layer closest to the
transducer. The 2.sup.nd layer can then be a polymer with
characteristic impedance .about.2.5 MRayl. A 2-layer Chebychef type
of matching can be obtained with a substrate layer closest to the
transducer .about.13 MRayl with a matching layer closest to the
load of polymer with characteristic impedance .about.3 MRayl.
[0013] With thick film printing of the front electrode and
ferroelectric ceramic film onto a Si-substrate, the Si substrate
can first be covered with a film layer that provides sufficient
diffusion isolation between the Si-substrate and the ceramic to
avoid contamination of the ceramic during the sintering process
with temperatures .about.1000.degree. C. Useful materials for this
isolating film layer is for example a film of ceramic material or
Al.sub.2O.sub.3. This isolating film layer could also be the
electrode film itself. A typical electrode material for this
isolation is Pt, and the electrode can typically be used as a
ground electrode for the finished transducer. The hot transducer
electrode (also Pt typically) is printed on the top side of the
ceramic film, and can be printed into divided areas to form
individual, hot electrodes of elements in a transducer array.
[0014] Electrical connection between the transducer electrodes and
other circuitry, can be obtained through gluing with conducting
glues. A specially convenient low cost connection technique, is to
use a conducting glue composed of conducting particles with
diameter .about.5-10 .mu.m dissolved with .about.10 vol % in an
isolating glue. Conducting humps at the connection spots provide so
low distance to the opposite connector, that the conducting
particles of the glue forms a connection at the humps, with low
conductivity of the glue outside these connecting humps.
[0015] For dual frequency band operation of the transducer
elements, a conducting paste with similar characteristic impedance
as the ceramic film, can be printed on top of the Pt-electrodes,
with thickness .about..lamda./4 for the lowest frequency band
operation. In this lowest resonance band, the ceramic film is also
.lamda./4 thick, so that the total thickness of the ceramic film
and the electrode is .about..lamda./2 in the low resonance band. In
the higher frequency resonance band, the ceramic and the electrodes
are .lamda./2 each, so that the total thickness of the ceramic and
the electrode provides .lamda. resonance of the transducer. Typical
material for these electrodes can be a Ag/Pd mixture.
[0016] As the element electrodes of a transducer array according to
this principle participate in the resonance definition of the
transducer element, the separation of the electrodes provides
improved acoustic isolation between the array elements, for
improved array performance.
[0017] Other objects and features of the present invention will
become apparent from the following detailed description considered
in conjunction with the accompanying drawings. It is to be
understood, however, that the drawings are designed solely for
purposes of illustration and not as a definition of the limits of
the invention, for which reference should be made to the appended
claims. It should be further understood that the drawings are not
necessarily drawn to scale and that, unless otherwise indicated,
they are merely intended to conceptually illustrate the structures
and procedures described herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] In the drawings:
[0019] FIG. 1, show an example of a film transducer structure
according to the invention with a 2-layer acoustic matching to the
load material, together with example electro/acoustic transfer
functions and pulse envelopes,
[0020] FIG. 2, show an example of a film transducer structure
according to the invention with a 3-layer acoustic matching to the
load material, together with example electro/acoustic transfer
functions and pulse envelopes, and
[0021] FIG. 3, shows an example of a film transducer according to
the invention with a multi-band operation, and
[0022] FIG. 4, shows examples of array structures with multiple
transducer elements based on ceramic film as the active material,
where also structures for multi-band operation is shown, and
[0023] FIG. 5, shows an example according to the invention how
electrical connection to the array and cooperating electronic
circuits can be made with a conducting glue, and
[0024] FIG. 6, shows another example according to the invention of
connecting the array electrodes to an integrated circuit with a
glue containing conducting particles.
DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS
[0025] A cross section of an example transducer embodiment with two
acoustic matching layers according to the invention, is shown in
FIG. 1a. In this FIG. 100 shows a cross section of a complete
transducer assembly, where 101 is a substrate, and 102 shows the
ferroelectric ceramic film. The film is covered with a front
electrode 103 and a back electrode 104, where the electrodes
together function as an electric port to couple electric energy to
the transducer vibrations. The electrodes are also used to polarize
the ceramic film in the direction normal to its thickness, where
the polarization direction is indicated by the arrow P1, 105. With
this polarization, the ceramic film becomes piezoelectric. For best
possible function of the transducer, the electrodes should be as
thin as possible, with thickness preferably less than .lamda./10,
where .lamda. is the wavelength of bulk waves in the thickness
direction in the electrodes.
[0026] Free ceramic films with electrodes can be manufactured with
tape casting techniques, sintered at .about.1200.degree. C., and
adhered onto the substrate after the sintering. Adhesion of the
tape-casted transducers onto the substrate can be done by gluing,
ultrasound or anodic bonding techniques, etc. The ceramic films can
also be thick film printed onto the front electrode on the
substrate, or deposited through sol-gel techniques, and then
sintered. These film deposition techniques put stronger
requirements on the substrate, as the substrate must withstand the
sintering temperatures, and leaves a limited group of substrates to
be used.
[0027] Silicon (Si) is highly interesting for this purpose, as the
characteristic acoustic impedance has an interesting value for Si
to participate as an acoustic matching layer to the load impedance.
There also exist well developed micro machining techniques for Si,
which is very valuable for manufacturing of the acoustic impedance
matching layers to the load material. Si can withstand sintering
temperatures of 850-900.degree. C., which provides adequate
piezoelectric properties of the ceramic film, although not as good
as when sintering at 1200.degree. C. However, there is some
problems of contamination of the ceramic film from the Si
substrate, which requires that the Si substrate is covered with a
diffusion blocking layer, before the printing. This phenomenon was
addressed in U.S. patent application Ser. No. 10/180,990, where a
solution was given by using a thick isolation film of a material
with close to the same characteristic acoustic impedance as the
ceramic film, so that the isolation film thickness participates in
the definition of the transducer resonance frequencies. The
isolation film could be made of the ceramic material itself, or
other materials with close to the same characteristic acoustic
impedance.
[0028] In this patent, the isolation film is made so thin that the
resonance frequency is mainly defined by the piezoceramic film
itself, with negligible effect of the isolation film on the
transducer resonance frequencies. As the isolation film is thin,
one can choose a larger group of materials for this film, where
still the ferroelectric ceramic material itself is a candidate, but
also alumina (Al.sub.2O.sub.3), or the conducting front electrode
which then must be made sufficiently dense and thick to function as
a diffusion blockage. A practical requirement is that the thickness
of the front electrode and possibly added isolation film is less
than .lamda./10, where .lamda. is the wave length for bulk waves in
the thickness direction of the films.
[0029] In FIG. 1a is shown a separate diffusion isolation film 106
in direct contact with the substrate 101, and the front electrode
103 is printed on top of the isolation film. The front electrode is
typically based on Pt, but Ag/Pd combinations are also interesting.
The ceramic film 102 is then printed onto the front electrode 103
and finally the back electrode 104 is printed onto the ceramic
film.
[0030] The tape casting technique allows higher sintering
temperatures of the ceramic than sintering on Si, which gives
better piezoelectric properties of the tape casted film. However,
the lower electromechanical coupling of the film sintered at
850-900.degree. C. is acceptable in many applications. When a
tape-casted ceramic transducer is adhered to the substrate, one do
not need the sintering diffusion isolation film 106. With
ultrasound or anodic bonding techniques, this film layer would be
missing, and with gluing it could illustrate the glue film. Since
the high temperature sintering is not done on the substrate
material, this technique allows the use of a larger group of
substrates than when the substrate is sintered together with the
printed film. Examples of such substrates are glass or fused
silica, aluminum, and magnesium.
[0031] According to the invention, the substrate is part of the
acoustic impedance coupling between the transducer and the load.
For Si, glass and other substrates one can etch the substrate down
to adequate thickness to function as a first acoustic matching
layer 107 between the ceramic transducer and the load material 109.
A second acoustic matching layer 108 can be made of polymer, for
example adhered to the glass layer by spin coating techniques.
Another method is to adhere a Mg film of correct thickness onto the
tape-casted ceramic, before adhering the final polymer film. Both
these methods provide a 2-layer acoustic impedance matching between
the transducer and the load material 109.
[0032] FIG. 1b shows as the solid line (110) the electro-acoustic
transfer function for a 70 MHz transducer with a glass substrate
matching layer. The transfer function is from voltage over the
electrodes to pressure in the transmitted wave. The ceramic film
has a characteristic acoustic impedance of 33 MRayl with thickness
22 .mu.m and acoustic propagation velocity of 4330 m/s. The ceramic
film is covered with Pt electrodes of thickness 1.5 .mu.m (104) and
2 .mu.m (103). Pt has a characteristic acoustic impedance of 85
MRayl. The glass matching layer has a characteristic acoustic
impedance of 13 MRayl and a thickness of 19 .mu.m with an acoustic
velocity of 5200 m/s, and the polymer matching layer has a
characteristic acoustic impedance of 3 MRayl and a thickness of 11
.mu.m with acoustic velocity of 3000 m/s.
[0033] The dashed line (111) in FIG. 1b shows the electro-acoustic
transfer function for the same film transducer with Mg as the first
matching layer (characteristic impedance of 10 MRayl, thickness 21
.mu.m, propagation velocity of 5760 m/s) and polymer with
characteristic impedance 2.5 MRayl and thickness 11 .mu.m.
[0034] We see that the transducer has an efficient 3-dB bandwidth
from 44 MHz to 97 MHz for glass substrate and 47 MHz to 93 MHz for
Mg substrate, giving a center frequency of 70 MHz with 74% (glass)
and 66% (Mg) relative bandwidth. The envelope of a typical pulse is
shown in FIG. 1c where the solid line 112 represents glass matching
and the dashed line 113 represents Mg matching. The pulses have a
.about.15 dB duration of 38 nsec (glass) and 40 nsec (Mg) with a
1.sup.st temporal tail lobe that is 19 dB (glass) and 21 dB (Mg)
below the main lobe, giving a .about.25 dB pulse duration of 62
nsec (glass) and 67 nsec (Mg).
[0035] Wider bandwidths can be obtained with a 3-layer acoustic
matching as illustrated in FIG. 2a where 200 indicates the complete
transducer assembly including acoustic impedance matching layers to
the load. As in FIG. 1a, 102 shows the ceramic film with the
electrodes 103 and 104 adhered to the substrate 101 with the
isolation film 106. The transducer has three acoustic matching
layers 201, 202, and 203 between the ceramic film and the load,
where 201 is made of bulk substrate material, 202 is made as a
composite of the substrate material and polymer, and 203 is made of
bulk polymer.
[0036] The matching layers are made in a first step by plane
etching, or grinding, or other, of the substrate down to the total
thickness of layers 201 and 202. In a second step, the substrate is
further etched to form voids 205 in located regions so that one in
these regions are left with the thickness of layer 201 while the
remnant substrate material 204 sticks out with the thickness of the
layer 202. Polymer is then adhered to fill the voids 205 in layer
202 and to build layer 203 to correct thickness, for example using
spin coating techniques. Hence, the 2.sup.nd layer (202) is build
as a composite of polymer and the substrate material, where the
relative volume fill of these materials defines the characteristic
impedance of this layer, according to known principles.
[0037] With this method, both a 3-1 composite and a 2-2 composite
can be built, as illustrated in FIGS. 2d and 2e respectively. In
FIG. 2d, the voids 205 in layer 202 are etched in a continuous
pattern, leaving the remnant substrate material as posts 204, so
that a 3-1 composite is formed when the voids are filled with
polymer. In FIG. 2e, the voids 205 are etched in a two-dimensional
pattern (depth and length) leaving the remnant substrate material
as 2-dimensional stripes 204 to form a 2-2composite. The Figure
shows the pattern as linear stripes, but a 2-2composite could also
be formed by etching the voids as circles, for improved performance
with an annular array.
[0038] The 3-layer matching is conveniently used with a
Si-substrate, where the ceramic films are either thick film printed
onto the substrate before sintering as described above, or by
adhering a tape casted ceramic film onto the Si substrate. Anodic
bonding of the ceramic transducer to the substrate is then
interesting. An Si substrate is particularly interesting because
one have well defined etching methods to manufacture the
voids/posts 205/204 for the 2.sup.nd matching layer 202 in FIG.
2a.
[0039] An example electro-acoustic transfer function from voltage
across the electrodes to the pressure in the wave in the load, is
shown as the solid line (206) in FIG. 2b. The ceramic layer and
electrodes are the same as for the transfer functions in FIG. 1b,
while the 1.sup.st .lamda./4 matching layer (201) made of
crystalline Si has a characteristic impedance of 19.6 MRayl and a
thickness of 31 .mu.m and acoustic propagation velocity of 8440
m/s. The 2.sup.nd, composite .lamda./4 matching layer (202) has a
characteristic impedance of 7.3 MRayl and a thickness of 20 .mu.m
with a propagation velocity of 5500 m/s, while the 3.sup.rd
.lamda./4 matching layer made of polymer has a characteristic
impedance of 2.7 MRayl and thickness of 9 .mu.m with a propagation
velocity of 2400 m/s.
[0040] We note that the transfer function has an efficient band
width from 40 MHz to 104 MHz providing a center frequency of 72 MHz
with a relative 3-dB bandwidth of 89%. The envelope of the impulse
response for this structure is shown as the solid line (208) in
FIG. 2c, where we note the -15 dB duration of the main lobe is 35
nsec, the 1.sup.st temporal sidelobe pulse tail is -30 dB below the
main lobe, and the -35 dB duration of the pulse is 86 nsec.
[0041] With sputter etching, or a combination of chemical and
sputter etching, it is possible to create voids/posts 205 and 204
also in a glass substrate. The dotted line 207 in FIG. 2b shows the
electro-acoustic transfer function with the same ceramic film and
electrodes for a 3-layer load matching build on glass, where the
first glass .lamda./4 layer has a characteristic acoustic impedance
of 14 MRayl and thickness of 21 .mu.m with an acoustic propagation
velocity of 5610 m/s, the 2.sup.nd composite .lamda./4 layer has a
characteristic impedance of 4 MRayl and thickness of 12 .mu.m with
an acoustic propagation velocity of 3300 m/s, and the 3.sup.rd
polymer .lamda./4 layer has a characteristic impedance of 2 MRayl
and a thickness of 11 .mu.m and an acoustic propagation velocity of
3000 m/s.
[0042] We note that the transfer function has an efficient
bandwidth from 47 MHz to 95 MHz providing a center frequency of 71
MHz with a relative 3-dB bandwidth of 68%. The envelope of the
impulse response for the glass substrate is shown as the dotted
line 209 in FIG. 2c, where we note the .about.15 dB duration of the
main lobe is 40 nsec, the pulse tail is -27 dB below the main lobe,
and the -35 dB duration of the pulse is 70 nsec. We note that even
if the relative bandwidth of the 3-layer glass matching has not
increased from the 2-layer glass matching, the pulse length is
reduced.
[0043] We note that the glass substrate produces less ripple in the
electro-acoustic transfer function and lower tail in the impulse
response. However, the difference is so low that the Si substrate
is preferred in many situations due to more developed micro
machining techniques.
[0044] A multiband operation of the transducer can be obtained by
adding another layer at the back of the transducer, with close to
the same characteristic impedance as the ceramic, as shown in FIG.
3a, where 300 indicates the complete transducer assembly. This
Figure shows the same transducer and matching structure as in FIG.
2a, with the added layer 301 at the back-side of the structure. A
typical electro-acoustic transfer function from the voltage between
the electrodes to the vibration velocity in the load material, is
shown in FIG. 3b. The transfer function has a lower efficient band
302 from 20 MHz to 35 MHz, and a higher efficient band 303 from 58
MHz to 76 MHz. The total efficient band is 117% around a center
frequency of 48 MHz. However, the whole band cannot be accessed
with one single wide-band pulse, rather one can use the transducer
for pulses with center frequencies .about.25 MHz and .about.70
MHz.
[0045] With the low frequency pulse one can produce a less
resolution image with deeper penetration, and switching to the high
frequency band one can obtain a higher resolution image for objects
at closer range. The dual band is also interesting for harmonic
imaging where one transmits a pulse with center frequency .about.30
MHz, and receives a 2.sup.nd harmonic band of this pulse around
.about.60 MHz. One can even transmit a pulse with center frequency
.about.22 MHz and receive a 3.sup.rd harmonic band with center
frequencies .about.66 MHz, for example to detect small gas
bubbles.
[0046] The back layer can be made with conducting material, for
example a mixture of Ag and Pd which can be deposited with
tape-casting or thick film printing techniques. The back layer will
then function as an added electrode, which is convenient for some
techniques of electrical connection to the transducer, as described
below.
[0047] The middle of the low frequency band is found where the
total thickness of the ceramic and the back layer is
.about..lamda./2, referred to as .lamda./2 resonance. At the middle
of the high frequency band, the total thickness of the ceramic and
the back layer is .about..lamda., referred to as .lamda. resonance.
In standard transducers with electrodes at the surface of the high
impedance region, the electromechanical coupling at the mechanical
.lamda. resonance is low, because the integral of the electric
field across the whole wavelength of the plate thickness is close
to zero. However, with the structure in FIG. 3a, the back electrode
is found in the middle of the resonant structure, with .lamda./2
distance to the front electrode, which provides good electric
coupling to the mechanical .lamda. resonance.
[0048] An ultrasound transducer array can be made as in FIG. 4a,
which shows a complete array assembly 400 with similar thickness
structure as in FIG. 2a, with the difference that the back
electrode is physically split into hot electrodes 401 of the
individual array elements 402 defined by the hot electrodes 401.
The front electrode 103 is a common ground electrode. An element
definition with the electrodes only, without cutting the ceramic
material between the elements, provides adequately low acoustic
coupling between neighbor array elements, provided the array
elements are relatively wide compared to their thickness, and the
signal delay between neighboring elements is relatively low. This
situation found with switched arrays, and especially with annular
arrays.
[0049] A lower acoustic coupling between the elements is obtained
by separated, added back layers as illustrated in FIG. 4b, with a
similar acoustic function as 301 in FIG. 3a. The back layers 403
are deposited only at the hot element electrodes with separation
between, and are composed of a material with characteristic
impedance close to that of the ceramic. As the back layers have
strong influence on the vibration pattern of the structure, the
separation of the back layer between the elements reduces the
acoustic coupling between the elements. The structure is also
interesting for a phased array operation with so large delays
between the element signals that one obtains angular direction
steering of the beam. The back layer is conveniently deposited onto
the ceramic film with thick film printing techniques, followed by
sintering. A conducting back layer, for example a mixture of Ag and
Pd, then simplifies electrical connection to the hot
electrodes.
[0050] Electrical connection to single element and array
transducers, can conveniently be done with a conducting glue
technique, where the glue contains small conducting particles (e.g.
diam .about.10 .mu.m) in an isolating glue substance (particle vol
% .about.10%). FIG. 5 shows an example embodiment of this technique
for an annular array, where FIG. 5a shows a side view and FIG. 5b
shows a top view of an array and integrated circuit assembly. Both
the array and the integrated circuit are mounted on a polymer
based, acoustically absorbing transducer backing material 500 that
is coated with a metal conducting film pattern 501 for electrical
connection to the element electrodes 502 of the annular array 520.
In this particular embodiment the array is indicated with a 3-layer
matching as presented in FIG. 2, where the array radiates and
receives ultrasound waves through the front surface indicated by
the arrows 521. FIG. 5a indicates how the connection points between
the film conductors and the element electrodes are obtained with
conducting humps 503 (e.g. dimension .about.50 .mu.m) on the metal
pattern, shown both in FIGS. 5a and 5b. The distance between the
element electrodes 502 and the humps 503 is so low that single
conducting particles 504 provide electrical connection between the
humps and the element electrodes. The front ground electrode 103 is
connected to the pattern through an especially tall metal hump 505.
Outside the humps, the distance between the electrodes and the
metal film pattern is so large that the glue with the particles
provide low electrical conductivity between the electrodes and the
metal film below. Hence, each film conductor on the surface of the
backing connects to a defined electrode at the humps.
[0051] The same method is conveniently used to connect the
integrated circuit 506 to connecting film pattern 501 that further
connects to the transducer array electrodes 103 and 502. Humps 507
on the metal pattern are here made under metal connection pads 508
on the integrated circuit. The integrated circuits can typically be
made along the lines described in U.S. Pat. No. 6,540,677 B1, which
allows connection to an imaging instrument through a reduced set of
connecting wires, down to a single coaxial cable 509. The cable can
be connected to the integrated circuit using the same gluing
technique through the pads 510 on the circuit on the metal film
pattern 501 on the backing. For stress relieve of the cable wires
and the circuit, the cable jacket is glued by the block 511 onto
the backing material 500.
[0052] Yet another embodiment according to the invention of
connecting the transducer and the integrated circuit, is shown in
FIG. 6, where FIG. 6a shows a side view and FIG. 6b shows a top
view of the embodiment. The electronic circuit 601 is mounted
directly on the substrate 600 where the ultrasound transducer array
602 is adhered. The substrate is under the transducer array
modified to participate in the load matching network, as described
above, so that the transmission and reception of ultrasound waves
is through the substrate indicated by the arrows 608. In this
particular example is illustrated a 3-layer matching as described
in relation to FIG. 2, where the polymer 609 of the acoustic
matching layer is spun to coat the whole front face of the
substrate. The back side of the assembly is coated with a polymer
607 with especially high acoustic absorption.
[0053] The electric connections to the integrated circuit is done
at connection pads 603, where connection to a conduction pattern
are obtained at conducting humps 604 on a conducting film connector
pattern 605, 606, using the conducting glue technique as with the
embodiment in FIG. 5. The bottom electrode of the array 612 is used
as a common ground electrode and connects via the film conductor
606, while the top electrodes of the individual elements connect to
the circuit via film conductors 605. Isolation between these
element connection conductors and the array element electrodes can
for example be obtained by an isolation film 610 on top of the
array element electrodes, and conducting humps 611 on the element
electrodes 602 connect the element electrodes through the isolating
film 610 to the film conductors 605. Connection to the external
cable 509 is in this embodiment similar to that in FIG. 5. The
electronic circuit and the array are conveniently coated with an
acoustically and electrically isolating polymer 607.
[0054] Further variations to the embodiments of FIGS. 5 and 6 can
clearly be done within the general scope of the invention. For
example is it in some applications pertinent to glue the electronic
circuit directly to the back side of the array, making electric
contact between the transducer electrodes and the integrated
circuit through humps and a conducting glue as described.
[0055] Hence, while there have shown and described and pointed out
fundamental novel features of the invention as applied to preferred
embodiments thereof, it will be understood that various omissions
and substitutions and changes in the form and details of the
devices illustrated, and in their operation, may be made by those
skilled in the art without departing from the spirit of the
invention. For example, the presented transfer functions are
calculated with a selected set of material parameters and layer
thicknesses, and adjustments and improvements in the transfer
function characteristics can be obtained by adjustments of the
parameters. One should note that according to the principle of the
invention, the piezoelectric layers in these Figures could be given
different thicknesses for tuning of the active frequency bands to
desired requirements. One should also note that the transducer
plates could be curved to provide focusing of the ultrasound beam,
with negligible modifications in the transfer functions, or lenses
could be placed in front of the plates to participate in the
focusing of the ultrasound beam.
[0056] It is also expressly intended that all combinations of those
elements and/or method steps which perform substantially the same
function in substantially the same way to achieve the same results
are within the scope of the invention. For example, one can group
together many element transducers of the type shown into a
transducer array according to well known principles. An array of
such element transducers can also be obtained by cutting through
the film layers with a laser or similar techniques, to divide the
transducer into smaller elements grouped side by side to form an
array radiating surface.
[0057] Moreover, it should be recognized that structures and/or
elements and/or method steps shown and/or described in connection
with any disclosed form or embodiment of the invention may be
incorporated in any other disclosed or described or suggested form
or embodiment as a general matter of design choice. It is the
intention, therefore, to be limited only as indicated by the scope
of the claims appended hereto.
[0058] Thus, while there have shown and described and pointed out
fundamental novel features of the invention as applied to a
preferred embodiment thereof, it will be understood that various
omissions and substitutions and changes in the form and details of
the devices illustrated, and in their operation, may be made by
those skilled in the art without departing from the spirit of the
invention. For example, it is expressly intended that all
combinations of those elements and/or method steps which perform
substantially the same function in substantially the same way to
achieve the same results are within the scope of the invention.
Moreover, it should be recognized that structures and/or elements
and/or method steps shown and/or described in connection with any
disclosed form or embodiment of the invention may be incorporated
in any other disclosed or described or suggested form or embodiment
as a general matter of design choice. It is the intention,
therefore, to be limited only as indicated by the scope of the
claims appended hereto.
* * * * *