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DESCRIPTION JP2004080193

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DESCRIPTION JP2004080193
An object of the present invention is to provide a fine, high-performance ultrasonic transducer
with less deterioration of piezoelectric and dielectric properties. A backing material (14), a
plurality of laminated piezoelectric bodies (11) arranged in an array on the backing material (14),
and a plurality of acoustic matching layers (12, 13) respectively arranged on each laminated
piezoelectric body It is an ultrasonic transducer which it has. The stacked piezoelectric body is a
stacked body including first and second piezoelectric bodies stacked alternately and completely
polarized in the opposite directions, and first and second stacked bodies respectively disposed on
opposite side surfaces of the stacked body. Arranged in direct contact with the two external
electrodes, the upper surface of the first piezoelectric body and the lower surface of the second
piezoelectric body thereon, and being electrically connected to the first external electrode and Ushaped insulation Disposed in direct contact with the first internal electrode in contact with the
second external electrode through the second part, the upper surface of the second piezoelectric
member and the lower surface of the first piezoelectric member thereon, and the second external
electrode And a second inner electrode which is electrically connected and in contact with the
first outer electrode through a U-shaped insulating portion. [Selected figure] Figure 4
Ultrasonic transducer and method of manufacturing the same
The present invention relates to an ultrasonic transducer mainly used for a medical ultrasonic
diagnostic apparatus and a nondestructive inspection apparatus, and more particularly to an
ultrasonic transducer using a laminated piezoelectric material and a method of manufacturing
the same. . [0002] In the field of medical ultrasonic diagnostic equipment and nondestructive
testing equipment, ultrasonic transducers for transmitting and receiving ultrasonic waves include
PZT (zircon-lead titanate) based piezoelectric ceramics, relaxor and lead titanate. Based
piezoelectric single crystals are used. Conventionally, as an ultrasonic transducer of a medical
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ultrasonic diagnostic apparatus, a one-dimensional array probe in which a plurality of PZT
piezoelectric ceramic vibrators processed in a strip shape are arrayed is mainly used. Recently, in
order to enhance the sensitivity of this one-dimensional array probe or to enable low voltage
driving, an ultrasonic transducer using a laminated piezoelectric material has been developed.
[0004] Further, in response to a demand from a conventional two-dimensional (tomographic)
image to a three-dimensional image, research has been conducted on a two-dimensional array
probe in which minute rod-like vibrators are two-dimensionally arranged. In the two-dimensional
array probe, since one element is small, the impedance becomes high and the transmission /
reception sensitivity is lowered when the conventional piezoelectric ceramic is used. For this
reason, also in a two-dimensional array probe, one using a laminated piezoelectric material has
been studied. The laminated piezoelectric material has a structure in which a plurality of
piezoelectric materials and internal electrodes are alternately laminated, and co-firing is
performed using PZT ceramics as the piezoelectric materials and Pt, Ag / Pd as the internal
electrodes (1100 to 1250). C). These techniques are described in US Pat. L. Goldberg, IEEE
Transaction on Ultrasonics, Ferroelectrics and Frequency Control, vol. 41, no. 5 September 1994,
pp. 761-771 and the like. The internal electrodes in such a laminated piezoelectric body are
coated, for example, with an insulating material such as glass every other layer of the end of the
internal electrode, and commonly connected internal electrode ends exposed every other layer
with a conductive material Conduction is taken by that. Although it is necessary to carry out the
polarization treatment of the piezoelectric body after connecting the wiring of the internal
electrodes, it becomes structurally difficult to apply an ideal electric field as the vibrator becomes
finer.
As a result, sufficient polarization processing can not be performed. Furthermore, the mechanical
load generated by the material connecting the wiring of the internal electrode on the side surface
of the vibrator also increases, making it difficult to sufficiently bring out the characteristics
originally possessed by the piezoelectric body. In fact, it has been observed that the vibrator
produced by co-firing has a tendency that the electromechanical coupling coefficient is lowered
although the effective capacity is increased. On the other hand, since the co-fired laminated
piezoelectric material is expensive and difficult to manufacture, a piezoelectric plate having a
plurality of gap grooves is laminated in advance with an adhesive, and then divided by dicing to
obtain A method of producing an acoustic probe has been proposed (Japanese Patent Laid-Open
No. 2001-29346). However, this method can not easily align the gap grooves in the upper and
lower layers. In addition, since the adhesive laminate is used, there is a problem that mechanical
strength is insufficient and division by dicing is difficult. Therefore, the present invention
provides a fine, high-performance ultrasonic transducer having sufficient mechanical strength
with less deterioration of piezoelectric and dielectric properties such as an electromechanical
coupling coefficient. To be an issue. Further, according to the present invention, a fine highperformance ultrasonic transducer having sufficient mechanical strength with less deterioration
of piezoelectric / dielectric characteristics such as an electromechanical coupling coefficient is
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used by using a co-fired laminated piezoelectric material. The object is to provide a method of
easy manufacture. According to one aspect of the present invention, there are provided a backing
material, a plurality of laminated piezoelectric bodies arranged in an array on the backing
material, and each of the laminated piezoelectric bodies. A laminate including a plurality of
acoustic matching layers disposed on top of each other, wherein the laminated piezoelectric
material is alternately laminated, and includes first and second piezoelectric materials completely
polarized in opposite directions to each other; First and second external electrodes respectively
disposed on opposing first and second side surfaces of the laminate, an upper surface of the first
piezoelectric body, and the first laminated on the first piezoelectric body A first inner portion
disposed in direct contact with the lower surface of the second piezoelectric body, electrically
connected to the first external electrode, and in contact with the second external electrode
through a U-shaped insulating portion An electrode, an upper surface of the second piezoelectric
body, and the second piezoelectric body The first external body is disposed in direct contact with
the lower surface of the first piezoelectric body stacked on top of the other, electrically
connected to the second external electrode, and via the U-shaped insulating portion. An
ultrasonic transducer is provided, characterized in that it comprises a second inner electrode in
contact with the electrode.
According to another aspect of the present invention, a plurality of piezoelectric materials and
internal electrodes are alternately stacked and fired at the same time, and each piezoelectric
material is subjected to polarization treatment and cut into a predetermined shape, and then the
piezoelectric material is removed. Forming a U-shaped insulating portion every other layer at one
end of the internal electrode while forming the external electrode connected to the exposed end
of the internal electrode while maintaining the temperature below the Curie temperature of
Providing a multilayer piezoelectric body, arranging a plurality of the multilayer piezoelectric
bodies in an array on a backing material, and arranging an acoustic matching layer on the
multilayer piezoelectric bodies arranged in the array. A method of manufacturing an ultrasonic
transducer is provided. DETAILED DESCRIPTION OF THE INVENTION Embodiments of the
present invention will be described below with reference to the drawings. FIG. 1 is a crosssectional view showing the structure of an example of a laminated piezoelectric material used for
an ultrasonic transducer according to an embodiment of the present invention. The laminated
piezoelectric body shown in FIG. 1 has a plurality of piezoelectric bodies 1a and 1b laminated
only via the internal electrodes 2a and 2b, and one end of each of the internal electrodes 2a and
2b is provided on one side. It includes an insulating material 4 to be coated every other layer,
and external electrodes 3a and 3b electrically connected to the ends of internal electrodes 2a and
2b exposed on the other side. As piezoelectrics 1a and 1b, piezoelectric ceramics such as PZT or
barium titanate or piezoelectrics such as PZNT (Pb (Zn1 / 3Nb2 / 3) O3-PbTiO3) or PMNT (Pb
(Mg1 / 3Nb2 / 3) O3-PbTiO3) Single crystals are used and are completely polarized in the
directions shown by the arrows in the figure. The two types of piezoelectric materials stacked
alternately and polarized in opposite directions can be referred to as first and second
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piezoelectric materials. Specifically, the first piezoelectric body 1a and the second piezoelectric
body 1b disposed thereon are joined only through the first internal electrode 2a. The first inner
electrode 2 a is electrically connected to the first outer electrode 3 a and is in contact with the
second outer electrode 3 b via the insulating material 4. In addition, the second piezoelectric
body 1b and the first piezoelectric body 1a disposed thereon are bonded only via the second
internal electrode 2b. The second inner electrode 2 b is electrically connected to the second outer
electrode 3 b and is in contact with the first outer electrode 3 a via the insulating material 4. Pt,
Ag / Pd or the like can be used as the internal electrodes 2a, 2b, and an Au / Cr sputter film or
the like is used as the external electrodes 3a, 3b.
Further, as the insulating material 4, a material which is cured at a temperature equal to or lower
than the Curie temperature of the piezoelectric body 1, for example, an epoxy resin is used. The
laminated piezoelectric material used in the embodiment of the present invention can be
manufactured, for example, by the following method. First, the piezoelectric body 1 and the
internal electrode 2 are alternately laminated, and the laminated piezoelectric body is
manufactured by simultaneous firing. The thickness per one layer of the piezoelectric body 1 is
about 10 to 300 μm, the thickness of the internal electrode 2 is about 0.5 to 10 μm, and two to
ten layers are stacked. At this time, it is desirable that the width and length of the laminated
piezoelectric material be sufficiently large in order to remove the non-polarized portion or the
non-uniformly polarized portion by polarization after processing. For example, in the case of
obtaining a plate-like vibrator having an area of 12 mm × 24 mm, the width and the length are
respectively set to about 22 mm and 34 mm or more. After lamination, the internal electrodes 2
are temporarily connected every other layer, and the piezoelectric body 1 is subjected to
polarization processing by applying an electric field of 1 to 3 kV / mm for several minutes.
Thereafter, it is cut into a plate to be used as an ultrasonic transducer, for example, an area of
about 12 × 24 mm. After being subjected to the polarization treatment, it is cut into a
predetermined shape, and after cutting, in all piezoelectric layers, a completely polarized state to
the end, that is, a state in which the polarization direction is uniform in all planes is obtained .
Next, in order to remove the end of the internal electrode 2 exposed on one side surface of this
laminated piezoelectric body, concave grooves are formed on every other layer using a dicing
saw or the like, and the side surfaces of the laminated dielectric To form. The groove depth (in
the lateral direction in FIG. 1) is preferably about 10 to 100 μm. If the thickness is less than 10
μm, it is difficult to sufficiently insulate. On the other hand, if the thickness exceeds 100 μm,
the portion to which voltage can not be applied becomes a non-negligible size as an invalid
portion, and the piezoelectric characteristics may be degraded. In particular, when the depth of
the groove is determined so as to satisfy the condition represented by the following formula (1),
the deterioration of the piezoelectric characteristics is hardly caused and the characteristics of
the ultrasonic transducer are improved, which is preferable. W2 / w1 ≦ 0.15 (1) where w1 is the
width of the piezoelectric body and w2 is the depth of the groove. The width of the groove
(vertical direction in FIG. 1) is preferably about 10 to 30 μm. If it is less than 10 μm, it is
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difficult to sufficiently insulate, while if it exceeds 30 μm, the effect of mechanical load due to
the insulating material to be filled appears, and the piezoelectric characteristics may be degraded.
In particular, when the width of the groove is determined so as to satisfy the condition
represented by the following formula (1), the deterioration of the piezoelectric characteristics is
hardly caused and the performance of the ultrasonic transducer is improved, which is preferable.
T2 / t1 ≦ 0.5 (2) (where, t1 is the thickness of the piezoelectric body, and t2 is (groove width) /
2. The grooved side is immersed in an etching solution such as ammonium fluoride or nitric acid.
By this etching process, the concave grooves are isotropically etched, so that the U-shaped shape
is obtained with the corners removed. The most preferable shape is a semi-circle because the
cross-sectional area is small and the stress is not concentrated. Similarly, U-shaped grooves are
also formed on the opposite side surfaces so that the grooves on both side surfaces are
alternated as shown in the drawing. Since the groove formed on the side surface of the laminated
piezoelectric material has a U-like shape and there are no corners, it is advantageous in terms of
mechanical strength without fear of cracking from the corners. In addition, since the crosssectional area of the groove is smaller than that of the concave rectangular groove, the
mechanical load is small, which leads to the improvement of the piezoelectric characteristics. In
the U-shaped groove thus formed, the insulating material 4 such as epoxy resin is applied / cured
or deposited by sputtering to be filled. Although the value of acoustic impedance is about 3.2
Mrayls in a normal epoxy resin, the smaller the value is, the more the characteristics such as the
electrical coupling coefficient improve. For example, the acoustic impedance of a partially special
epoxy resin such as EPO-TEK301 (manufactured by epoxy technology) is about 3 Mrayls, and the
acoustic impedance of liquid or air is about 2 Mrayls. Therefore, it is most preferable that the Ushaped insulating portion be composed of a groove and an insulating material such as liquid or
air filled in the inside. Since the piezoelectric layer is subjected to polarization processing in the
previous step, subsequent processes such as coating and curing of the insulating material must
be performed at a temperature equal to or lower than the Curie temperature of the piezoelectric.
The Curie temperature is determined according to the type of piezoelectric material, and is, for
example, about 200 to 300 ° C. for PZT piezoelectric ceramics and about 175 ° C. for PZNT
piezoelectric single crystals. After the U-shaped insulating portion is formed, an electrode film to
be the external electrodes 3a and 3b is formed on both side surfaces by sputtering. The
formation of the external electrodes 3a and 3b is also performed at a temperature equal to or
lower than the Curie temperature of the piezoelectric body as described above.
By the method as described above, 10 layers of piezoelectric material 1 having an area of 12 ×
24 mm and a thickness of 60 μm are stacked via the internal electrode 2 and processed into a
strip-shaped vibrator having a width of 0.200 mm. A laminated piezoelectric body as shown in
FIG. When the electromechanical coupling coefficient of this laminated piezoelectric material was
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measured, about 66% was obtained as the value of the electromechanical coupling coefficient
k'33. This value is almost the same as the value 69% of the unstacked bulk piezoelectric material.
Therefore, it was confirmed that the characteristic deterioration resulting from lamination hardly
occurs by using the embodiment of the present invention. The impedance characteristics of the
laminated piezoelectric transducer obtained here are shown in the graph of FIG. In the graph of
FIG. 2, the size of the arrow corresponds to the size of the coupling coefficient. For comparison,
FIG. 3 shows the impedance characteristics of a laminated piezoelectric material produced by
trial manufacture using the prior art. Specifically, a laminated piezoelectric body was
manufactured by adopting the method described in JP-A-2001-102647. In the laminated
piezoelectric body thus manufactured, the portion of the insulating material is concave, and the
cross section thereof is larger than that of the U-shaped case. Moreover, since the adhesive
curing process at 200 ° C. or higher is included, the polarization process is performed after
laminating a plurality of piezoelectric layers. For this reason, an ineffective portion occurs at the
end of the piezoelectric layer, and in some cases, as shown in the graph of FIG. 3, unnecessary
vibration may occur, and the electromechanical coupling coefficient may also decrease. As shown
in the graph of FIG. 2, the laminated piezoelectric vibrator manufactured by the above-described
method has no unnecessary vibration, and the electromechanical coupling coefficient maintains
the characteristics of the bulk piezoelectric which is not laminated. Be done. In such a laminated
piezoelectric material, the piezoelectric material layer is laminated only via the internal electrode
and produced by co-firing, so there is no adhesive. For this reason, the decrease in mechanical
strength due to the adhesive is avoided. In addition, since the laminate is produced and cut into a
desired shape, it can be produced without a complicated process such as alignment performed by
the grooves provided in the piezoelectric body. In addition, since the apparent dielectric constant
is increased by a factor of the number of layers, the ultrasonic transducer according to the
embodiment of the present invention using this laminated piezoelectric body can be expected to
improve characteristics such as a significant improvement in sensitivity. FIG. 4 is a schematic
view showing the configuration of an ultrasonic transducer according to an embodiment of the
present invention. The illustrated ultrasonic transducer is a one-dimensional array probe in
which a plurality of laminated piezoelectric bodies 11 in which a first acoustic matching layer 12
and a second acoustic matching layer 13 are sequentially laminated are one-dimensionally
arranged on a backing material 14 It is.
Although not clearly shown in FIG. 4, the laminated piezoelectric body 11 has a structure
laminated in the same direction as the case of FIG. 1, and is manufactured by the method
described above. Such a one-dimensional array probe can be manufactured, for example, by the
following method. First, the above-described laminated piezoelectric body 11 is adhered to the
backing material 14, and then the acoustic matching layers 12 and 13 are formed. Finally, array
processing is performed in the direction orthogonal to the side on which the external electrodes
(not shown) are formed, and signal lines (not shown) are wired to the external electrodes on one
side of each array, and the other side The external electrodes of are commonly connected to the
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GND line (not shown). In the ultrasonic transducer of the present embodiment, the apparent
dielectric constant of each transducer is improved, and the electromechanical coupling
coefficient can maintain the characteristics of the bulk piezoelectric material which is not
stacked. Therefore, it is possible to significantly improve the sensitivity and reduce the drive
voltage. FIG. 5 is a schematic diagram showing the configuration of an ultrasonic transducer
according to another embodiment of the present invention. The illustrated ultrasonic transducer
is a two-dimensional array probe in which a plurality of stacked piezoelectric bodies 11 in which
a first acoustic matching layer 12 and a second acoustic matching layer 13 are sequentially
stacked are two-dimensionally arranged on a backing material 14. It is. Such a two-dimensional
array probe basically prepares one row, performs array processing, connects the signal line and
the GND line, and then basically bonds the plurality of transducer groups for one row, except for
the above-mentioned first-order It can be manufactured by the same method as the original array
probe. An example of a method of manufacturing the two-dimensional array probe shown in FIG.
5 is shown in FIG. First, as shown in FIG. 6A, the laminated piezoelectric members 11 for one row
are bonded to a flexible printed circuit (FPC) 15 having a predetermined wiring pattern. The
laminated piezoelectric body 11 can have, for example, the structure shown in FIG. Next, the
ground side electrode 16 and the signal side electrode 17 of the FPC 15 are connected to the
electrodes of the laminated piezoelectric body 11 as shown in FIG. 6B using a thin film electrode
18 such as an Au sputter film. Further, as shown in FIG. 6C, the acoustic matching layers 12 and
13 are formed in the ultrasonic transmission / reception direction, and array division processing
is performed as shown in FIG. 6D. Finally, as shown in FIG. 6E, a filling resin is filled between the
divided elements, and the backing material 14 is adhered. By the above process, one row of a
two-dimensional array probe is produced.
A two-dimensional array probe can be obtained by stacking and joining a plurality of vibrators
for one row as shown in FIG. 6 (f). FIG. 7 is a cross-sectional view showing the structure of
another example of the laminated piezoelectric material used for the ultrasonic transducer
according to the embodiment of the present invention. The laminated piezoelectric body shown is
basically the same as that shown in FIG. 1, and can be manufactured by the same process as
described above. However, as the insulating material 4, a resin having a low adhesive strength
with the piezoelectric body 1 such as an epoxy resin is used. By using such a resin, an air gap as
illustrated is formed between the concave groove on the side surface of the piezoelectric body 1
and the insulating material 4. Even when no air gap is formed, a state in which the piezoelectric
body 1 and the insulating material 4 are mechanically loosely bonded can be obtained. With such
a structure, the mechanical load applied to the side surface of the piezoelectric body 1 is reduced,
so that the deterioration of the piezoelectric / dielectric characteristics such as the
electromechanical coupling coefficient can be reduced. An epoxy resin was formed as the
insulating material 4 by sputtering, and a laminated piezoelectric body as shown in FIG. 7 was
produced, as shown in FIG. 7, having a partial gap between the concave groove and the insulating
material (epoxy resin). As a result of evaluating the obtained laminated piezoelectric body, the
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electromechanical coupling coefficient k'33 value is about 67%, and better characteristics are
obtained compared with the case of using an epoxy adhesive having high adhesive strength with
the piezoelectric body. was gotten. Furthermore, the laminated piezoelectric body used in the
ultrasonic transducer according to the embodiment of the present invention may have a crosssectional structure as shown in FIG. In the laminated piezoelectric material shown in FIG. 8, the
U-shaped concave grooves 5 are all voided. This void can be formed by the following method
after producing the U-shaped groove by the same process as that described with reference to FIG.
The U-shaped groove is filled with an etchable material such as a resist material and hardened.
The material filled here is dissolved and removed using an etching solution after forming external
electrodes on both sides. Alternatively, a void can be formed by the same process even if a Ushaped groove is filled with a material which is melted by heat or the like such as wax. As
described above, since the acoustic impedance of air is 2 Mrayls or less, by making the U-shaped
groove hollow inside, it is possible to remarkably improve the piezoelectric characteristics such
as the mechanical coupling coefficient. This is because the inner surface of the groove receives
no mechanical load because it is hollow.
As a result of trial manufacture and evaluation of the laminated piezoelectric body having such a
structure, an electromechanical coupling coefficient almost equal to that of the bulk piezoelectric
body was obtained. As described in detail above, according to one aspect of the present
invention, fine high performance with sufficient mechanical strength with less deterioration of
piezoelectric / dielectric characteristics such as electromechanical coupling coefficient An
ultrasonic transducer is provided. In addition, according to another aspect of the present
invention, a multilayer high-performance ultrasonic transducer which has a sufficient mechanical
strength and which has a sufficient reduction in piezoelectric / dielectric characteristics, such as
an electromechanical coupling coefficient, and which is sintered simultaneously is used. Methods
are provided that are easily manufactured using the body. The present invention can be suitably
used for a medical ultrasonic diagnostic apparatus, a nondestructive inspection apparatus and
the like, and its industrial value is enormous. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a
cross-sectional view showing the structure of a laminated piezoelectric material in an ultrasonic
transducer according to an embodiment of the present invention. FIG. 2 is an impedance
characteristic diagram of a laminated piezoelectric material in an ultrasonic transducer according
to an embodiment of the present invention. FIG. 3 is an impedance characteristic diagram of a
conventional laminated piezoelectric body. FIG. 4 is a schematic diagram illustrating the
configuration of a one-dimensional array ultrasonic transducer according to an embodiment of
the present invention. FIG. 5 is a schematic diagram showing the configuration of a twodimensional array ultrasonic transducer according to another embodiment of the present
invention. FIG. 6 is a process diagram for explaining a manufacturing process of a twodimensional array ultrasonic transducer according to an embodiment of the present invention.
FIG. 7 is a cross-sectional view showing the structure of a laminated piezoelectric body in an
ultrasonic transducer according to an embodiment of the present invention. FIG. 8 is a cross-
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sectional view showing the structure of a laminated piezoelectric body in an ultrasonic
transducer according to an embodiment of the present invention. Explanation of the code 1a, 1b
... Piezoelectric 2a, 2b ... Internal electrode 3a, 3b ... Conductive film (external electrode) 4 ...
Insulator 5 ... U-shaped groove 11 ... Laminated piezoelectric body 12: first acoustic matching
layer 13: second acoustic matching layer 14: backing material 15: flexible printed circuit board
16: ground side electrode 17: signal line side electrode 18: thin film electrode
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