U.S. patent application number 11/058282 was filed with the patent office on 2005-08-18 for piezoelectric element and method of manufacturing the same.
This patent application is currently assigned to FUJI PHOTO FILM CO., LTD.. Invention is credited to Miyoshi, Tetsu.
Application Number | 20050179345 11/058282 |
Document ID | / |
Family ID | 34836395 |
Filed Date | 2005-08-18 |
United States Patent
Application |
20050179345 |
Kind Code |
A1 |
Miyoshi, Tetsu |
August 18, 2005 |
Piezoelectric element and method of manufacturing the same
Abstract
An array of piezoelectric elements is easily manufactured by
making insulating portions at side surfaces smaller. The
piezoelectric element includes: a multilayered structure in which
piezoelectric material layers and internal electrode layers are
alternately stacked; first insulating films formed by using an AD
method, for covering a first group of internal electrode layers at
a first surface of the multilayered structure; second insulating
films formed by using the AD method, for covering a second group of
internal electrode layers at a second surface of the multilayered
structure; a first external electrode connected to the second group
of internal electrode layers and insulated from the first group of
internal electrode layers at the first surface; and a second
external electrode connected to the first group of internal
electrode layers and insulated from the second group of internal
electrode layers at the second surface.
Inventors: |
Miyoshi, Tetsu;
(Kaisei-machi, JP) |
Correspondence
Address: |
SUGHRUE MION, PLLC
2100 PENNSYLVANIA AVENUE, N.W.
SUITE 800
WASHINGTON
DC
20037
US
|
Assignee: |
FUJI PHOTO FILM CO., LTD.
|
Family ID: |
34836395 |
Appl. No.: |
11/058282 |
Filed: |
February 16, 2005 |
Current U.S.
Class: |
310/328 |
Current CPC
Class: |
H01L 41/0472 20130101;
Y10T 29/49128 20150115; H01L 41/083 20130101; Y10T 29/435 20150115;
Y10T 29/42 20150115; B06B 1/064 20130101; H01L 41/293 20130101;
H01L 41/27 20130101; H01L 41/273 20130101 |
Class at
Publication: |
310/328 |
International
Class: |
H01L 041/083 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 18, 2004 |
JP |
2004-040829 |
Claims
1. A piezoelectric element comprising: a multilayered structure in
which a plurality of piezoelectric material layers and a plurality
of internal electrode layers are alternately stacked, said
plurality of internal electrode layers including a first group of
internal electrode layers and a second group of internal electrode
layers; first insulating films formed by using an aerosol
deposition method, for covering said first group of internal
electrode layers at a first surface of said multilayered structure;
second insulating films formed by using the aerosol deposition
method, for covering said second group of internal electrode layers
at a second surface of said multilayered structure; a first
external electrode electrically connected to said second group of
internal electrode layers and insulated from said first group of
internal electrode layers by said first insulating films at the
first surface of said multilayered structure; and a second external
electrode electrically connected to said first group of internal
electrode layers and insulated from said second group of internal
electrode layers by said second insulating films at the second
surface of said multilayered structure.
2. The piezoelectric element according to claim 1, wherein said
first group of internal electrode layers and said second group of
internal electrode layers are alternately disposed.
3. The piezoelectric element according to claim 1, wherein said
plurality of piezoelectric material layers and said plurality of
internal electrode layers are formed by stacking plural sheets,
each including a sheet of a piezoelectric material mixed with a
binder and a solvent and at least an electrode material formed
thereon, and baking the stacked plural sheets.
4. The piezoelectric element according to claim 1, wherein said
plurality of piezoelectric material layers are formed by using the
aerosol deposition method.
5. A method of manufacturing a piezoelectric element having a
multilayered structure, said method comprising the steps of: (a)
fabricating a multilayered structure in which a plurality of
piezoelectric material layers and a plurality of internal electrode
layers are alternately stacked, said plurality of internal
electrode layers including a first group of internal electrode
layers and a second group of internal electrode layers; (b) forming
first insulating films for covering said first group of internal
electrode layers at a first surface of said multilayered structure
by using an aerosol deposition method; (c) forming second
insulating films for covering said second group of internal
electrode layers at a second surface of said multilayered structure
by using the aerosol deposition method; (d) forming a first
external electrode electrically connected to said second group of
internal electrode layers and insulated from said first group of
internal electrode layers by said first insulating films at the
first surface of said multilayered structure; and (e) forming a
second external electrode electrically connected to said first
group of internal electrode layers and insulated from said second
group of internal electrode layers by said second insulating films
at the second surface of said multilayered structure.
6. The method of manufacturing a piezoelectric element according to
claim 5, wherein said first group of internal electrode layers and
said second group of internal electrode layers are alternately
disposed.
7. The method of manufacturing a piezoelectric element according to
claim 5, wherein step (a) includes stacking plural sheets, each
including a sheet of a piezoelectric material mixed with a binder
and a solvent and at least an electrode material formed thereon,
and baking the stacked plural sheets.
8. The method of manufacturing a piezoelectric element according to
claim 5, wherein step (a) includes forming said plurality of
piezoelectric material layers by using the aerosol deposition
method.
9. The method of manufacturing a piezoelectric element according to
claim 5, wherein: step (b) includes forming a mask to be used for
forming said first insulating films; and step (c) includes forming
a mask to be used for forming said second insulating films.
10. The method of manufacturing a piezoelectric element according
to claim 9, wherein the mask formed at at least one of step (b) and
(c) includes one of a resist mask and a metal mask.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a piezoelectric element in
which piezoelectric material layers and electrode layers are
alternately formed, and the present invention further relates to a
method of manufacturing the piezoelectric element.
[0003] 2. Description of a Related Art
[0004] Multilayered structures in each of which insulating
(dielectric) layers and electrode layers are alternately formed are
utilized for various uses such as multilayered capacitors,
piezoelectric pumps, piezoelectric actuators and ultrasonic
transducers. In recent years, with the developments of MEMS (micro
electro-mechanical systems) related devices, elements each having
such a multilayered structure have been microfabricated still
further and packaged more densely.
[0005] In microfabrication of an element having opposed electrodes,
the smaller the area of the element is made, the smaller the
capacity between the electrodes becomes, and therefore, a problem
occurs that the electrical impedance of the element rises. For
example, when the electrical impedance rises in a piezoelectric
actuator, the impedance matching can not be taken with a signal
circuit for driving the piezoelectric actuator and power becomes
difficult to be supplied, and thereby, the performance as the
piezoelectric actuator is degraded. Alternatively, in an ultrasonic
transducer using a piezoelectric element, detection sensitivity of
ultrasonic wave is dropped. Accordingly, in order to enlarge the
capacity between electrodes while microfabricating the element,
plural piezoelectric material layers and plural electrode layers
are alternatively stacked. That is, the capacity between electrodes
of the entire element can be made larger by connecting the stacked
plural layers in parallel.
[0006] As a method of manufacturing a piezoelectric element having
a multilayered structure, conventionally, a bulk method employing
bulk piezoelectric materials has been known. In the bulk method, a
technique of alternately stacking the bulk piezoelectric materials
that have been cut in desired thicknesses and electrode layers, and
securing them with an adhesive or bolts is used. However, this
technique is generally used for manufacturing a relatively large
piezoelectric element and not suitable for manufacturing a minute
piezoelectric element for the following reason. That is, because
piezoelectric materials that have been cut into thin pieces are
brittle and easy to break, and the handling of them is difficult.
Especially, the piezoelectric material having a thickness of 100
.mu.m or less is easy to break. Further, because the
microfabrication is difficult by the technique, the manufacturing
process becomes complicated. Furthermore, with respect to finished
products, there is a problem about attachment performance due to
the adhesive, and a problem occurs that stress is produced in the
bonded part. Accordingly, the manufacturing yield is reduced and
the cost of manufacturing is increased by the technique, and
therefore, the method is not suitable in view of productivity.
[0007] Then, manufacturing the piezoelectric element by using a
film forming technology such as a green sheet method has been under
study.
[0008] In the green sheet method, a green sheet (piezoelectric
material sheet) is used, which is formed by a mixture of a powder
of a piezoelectric material having no plasticity with a binder of
an organic material or the like. In this technique, plural
piezoelectric material sheets, onto which paste of an electrode
material is applied by screen printing or the like, are stacked and
baked at high temperature of about 1000.degree. C. so as to allow
the binder to fly from the piezoelectric material sheets, and
thereby, the piezoelectric materials are made into strong
films.
[0009] In a piezoelectric element manufactured using such a
technique, in order to connect the plural internal electrode layers
to each other, interconnection is performed on the side surfaces of
the piezoelectric element. FIG. 9 is a sectional view for
explanation of a general interconnecting method of a piezoelectric
element having a multilayered structure. The piezoelectric element
100 includes plural piezoelectric material layers 101, plural
internal electrode layers 102 and 103, and side electrodes 104 and
105.
[0010] The internal electrode layers 102 are formed so that one
ends thereof extend to one wall surface of the piezoelectric
element, and internal electrode layers 103 are formed so that one
ends thereof extend to the other wall surface of the piezoelectric
element. Thereby, the internal electrode layers 102 are connected
to the side electrode 104 and insulated from the side electrode 105
by insulating regions 106. Contrary, the internal electrode layers
103 are connected to the side electrode 105 and insulated from the
side electrode 104 by insulating regions 106. By applying a
potential difference between the side electrode 104 and the side
electrode 105, a voltage is applied between the internal electrode
layers 102 and the internal electrode layers 103, and each
piezoelectric material layers 101 disposed therebetween expands and
contracts by the piezoelectric effect.
[0011] However, in the internal electrode layers 102 and 103, the
insulating regions 106, in which no electrode is formed, are
provided for insulating the electrode layers from either of the
side electrodes. The insulating regions 106 do not expand nor
contract even when a voltage is applied between the internal
electrode layers 102 and 103. On this account, first regions that
expand and contract, and second regions that do not expand and
contract exist within the piezoelectric material layers 101, and
therefore, there is a problem that stress is concentrated between
these regions and they are easy to break.
[0012] As a related technology, Japanese Patent Application
Publication JP-A-60-128683 discloses a method of manufacturing a
long-life multilayered piezoelectric actuator (the second page,
FIG. 2). According to JP-A-60-128683, internal electrodes are
formed on the entire surface of green sheets mainly composed of a
piezoelectric ceramic material, the green sheets are stacked in
layers and sintered, machining is performed on both side surfaces
of the sintered material, ends of the both side surfaces are
insulated with a resin every other layer, and conducting layers are
formed on the respective surfaces over the processed material.
Thereby, durability can be improved compared to the conventional
ones in which electrodes can not be formed all over the surfaces,
and further, microfabrication can be performed because alignment is
not required for forming internal electrodes partially.
[0013] However, the epoxy resin used for insulating the side
surfaces of the element has a low withstand voltage, and therefore,
the thickness of the resin must be made larger. For example, when
the voltage for driving the piezoelectric element is 200V to 300V,
the resin is required to have a thickness of 20 .mu.m to 30 .mu.m.
In the case where the thickness of the resin is large, a problem
occurs when an ultrasonic probe is manufactured by arranging plural
piezoelectric elements in a one-dimensional or two-dimensional
array form. That is, a filling material is poured between the
plural piezoelectric elements when the ultrasonic probe is
manufactured. However, in the case where gaps between the plural
piezoelectric elements arranged are as small as 50 .mu.m, it
becomes difficult to pour the filling material into these
piezoelectric elements due to the thickness of the resin.
SUMMARY OF THE INVENTION
[0014] The present invention is achieved in view of the
above-mentioned problems. An object of the present invention is to
easily manufacture an array of piezoelectric elements having
multilayered structures by making insulating portions on side
surfaces small.
[0015] In order to solve the above described problems, a
piezoelectric element according to the present invention comprises:
a multilayered structure in which a plurality of piezoelectric
material layers and a plurality of internal electrode layers are
alternately stacked, the plurality of internal electrode layers
including a first group of internal electrode layers and a second
group of internal electrode layers; first insulating films formed
by using an aerosol deposition method, for covering the first group
of internal electrode layers at a first surface of the multilayered
structure; second insulating films formed by using the aerosol
deposition method, for covering the second group of internal
electrode layers at a second surface of the multilayered structure;
a first external electrode electrically connected to the second
group of internal electrode layers and insulated from the first
group of internal electrode layers by the first insulating films at
the first surface of the multilayered structure; and a second
external electrode electrically connected to the first group of
internal electrode layers and insulated from the second group of
internal electrode layers by the second insulating films at the
second surface of the multilayered structure.
[0016] Further, a method of manufacturing a piezoelectric element
having a multilayered structure according to the present invention
comprises the steps of: (a) fabricating a multilayered structure in
which a plurality of piezoelectric material layers and a plurality
of internal electrode layers are alternately stacked, the plurality
of internal electrode layers including a first group of internal
electrode layers and a second group of internal electrode layers;
(b) forming first insulating films for covering the first group of
internal electrode layers at a first surface of the multilayered
structure by using an aerosol deposition method; (c) forming second
insulating films for covering the second group of internal
electrode layers at a second surface of the multilayered structure
by using the aerosol deposition method; (d) forming a first
external electrode electrically connected to the second group of
internal electrode layers and insulated from the first group of
internal electrode layers by the first insulating films at the
first surface of the multilayered structure; and (e) forming a
second external electrode electrically connected to the first group
of internal electrode layers and insulated from the second group of
internal electrode layers by the second insulating films at the
second surface of the multilayered structure.
[0017] According to the present invention, the insulating portions
on the side surfaces can be made small by forming the first
insulating films for covering the first group of internal electrode
layers at the first side surface of the multilayered structure and
second insulating films for covering the second group of internal
electrode layers at the second side surface of the multilayered
structure by using the aerosol deposition method. As a result, an
array of piezoelectric elements having multilayered structures can
be easily manufactured.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a perspective view showing a structure of a
piezoelectric element according to one embodiment of the present
invention;
[0019] FIG. 2 is a flowchart showing a method of manufacturing the
piezoelectric element according to one embodiment of the present
invention;
[0020] FIGS. 3A and 3B are diagrams for explanation of steps of
forming PZT layers and internal electrode layers in the method of
manufacturing the piezoelectric element as shown in FIG. 2;
[0021] FIGS. 4A to 4D are diagrams for explanation of steps of
forming insulating films in the method of manufacturing the
piezoelectric element as shown in FIG. 2;
[0022] FIGS. 5A and 5B are diagrams for explanation of steps of
forming external electrodes in the method of manufacturing the
piezoelectric element as shown in FIG. 2;
[0023] FIG. 6 shows a device to be used for forming films in
accordance with the AD method;
[0024] FIG. 7 is a perspective view showing a piezoelectric element
array including the piezoelectric element as shown in FIG. 1;
[0025] FIG. 8 is a partially sectional perspective view showing an
example in which the piezoelectric element array as shown in FIG. 7
is applied to an ultrasonic probe; and
[0026] FIG. 9 is a sectional view for explanation of a general
interconnecting method of a piezoelectric element having a
multilayered structure.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0027] Hereinafter, embodiments of the present invention will be
described in detail by referring to the drawings. The same
reference numerals are assigned to the same component elements and
the description thereof will be omitted.
[0028] FIG. 1 shows a structure of a piezoelectric element
according to one embodiment of the present invention. A
piezoelectric element 10 as shown in FIG. 1 is a micro columnar
structure having a bottom surface with sides having a length of
about 0.3 mm to about 1.0 mm, and a height of about 2.1 mm. The
piezoelectric element 10 includes plural PZT (Pb (lead) zirconate
titanate) layers 1, plural first internal electrode layers 2,
plural second internal electrode layers 3, plural first insulating
films 4, plural second insulating films 5, plural third insulating
films 6, a first external electrode 7, and a second external
electrode 8.
[0029] In the embodiment, the PZT layers 1 are formed by the green
sheet method. By the way, the PZT layers 1 may be formed by using
the aerosol deposition (AD) method as a film forming method. The
internal electrode layers 2 and the internal electrode layers 3 are
alternately disposed between the plural PZT layers 1. The external
electrode 7 covers one side surface and a bottom surface of the
piezoelectric element 10, the external electrode 8 covers the other
side surface and a top surface of the piezoelectric element 10, and
the external electrodes 7 and 8 are insulated by the insulating
films 6.
[0030] The internal electrode layers 2 are electrically connected
to the external electrode 7 at the one side surface and insulated
from the external electrode 8 by the insulating films 4 located on
the other side surface. The internal electrode layers 3 are
insulated from the external electrode 7 by the insulating films 5
located on the one side surface and electrically connected to the
external electrode 8 at the other side surface. These plural
insulating films 4 and 5 and plural insulating films 6 are formed
by using the AD method as a film forming method.
[0031] By applying voltages between the internal electrode layers 2
and the internal electrode layers 3 via the external electrodes 7
and 8, the PZT layers 1 expand and contract by the piezoelectric
effect. Such a piezoelectric element is used for piezoelectric
pumps, piezoelectric actuators, ultrasonic transducers for
transmitting and receiving ultrasonic waves in an ultrasonic probe,
and so on. Further, in the piezoelectric element having the
multilayered structure as shown in FIG. 1, areas of the opposed
electrodes are larger than that in a single layer piezoelectric
element, and thereby, electric impedance can be made lower.
Therefore, compared to the single layer piezoelectric element, the
piezoelectric element having the multilayered structure operates
more efficiently for the applied voltage.
[0032] Next, a method of manufacturing the piezoelectric element
according to the embodiment will be described by referring to FIGS.
2 to 5B. FIG. 2 is a flowchart showing the method of manufacturing
the piezoelectric element according to the embodiment. Further,
FIGS. 3A to 5B are diagrams for explanation of the method of
manufacturing the piezoelectric element according to the
embodiment.
[0033] First, at steps S1 to S3 as shown in FIG. 2, a multilayered
structure including the PZT layers 1, the internal electrode layers
2 and the internal electrode layers 3 is formed. At step S1, as
shown in FIG. 3A, plural sheets formed by arranging conductor paste
on the piezoelectric material sheets (green sheet) are stacked. The
piezoelectric material sheet is formed by a mixture of a powder of
a piezoelectric material having no plasticity with a binder of an
organic material or the like and a solvent. Further, the conductor
paste is a paste of electrode material obtained by mixing a metal
powder with a binder of an organic material or the like and a
solvent, and applied onto the piezoelectric material sheet by
screen printing or the like.
[0034] At step S2, the stacked plural sheets are applied with heat
and pressure to be pressure bonded, and then the piezoelectric
material and the electrode material are simultaneously baked.
Thereby, the binders contained in the green sheets and conductor
paste fly so that a multilayered structure is formed in which the
PZT layers and the internal electrode layers are integrated. At the
time of baking, the organic components and moisture are removed
from the piezoelectric material sheets and the paste of the
electrode material, and thereby, the volumes of the piezoelectric
material sheets and the electrodes are reduced.
[0035] At step S3, the multilayered structure formed at step S2 is
cut into desired sizes. Thereby, as shown in FIG. 3B, a
multilayered structure 11 including the PZT layers 1, the internal
electrode layers 2 and the internal electrode layers 3 and having a
desired size is formed.
[0036] Then, at steps S4 to S7, the insulating films 4 to 6 are
formed on the multilayered structure 11. At step S4, as shown in
FIG. 4A, a resist mask having a desired pattern is formed on a
first surface of the multilayered structure 11 by using the
photolithography process. By the way, in place of the resist mask,
a metal mask may be formed. At step S5, as shown in FIG. 4B, the
insulating films 4 and the insulating films 6 are formed on the
first surface of the multilayered structure 11 by using the resist
mask, and then, the resist mask is removed.
[0037] At step S6, the multilayered structure 11 is turned upside
down, and, as shown in FIG. 4C, a resist mask having a desired
pattern is formed on a second surface opposite to the first surface
of the multilayered structure 11 by using the photolithography
process. At step S7, as shown in FIG. 4D, the insulating films 5
and the insulating films 6 are formed on the second surface of the
multilayered structure 11 by using the resist mask, and then, the
resist mask is removed.
[0038] When the insulating films 4 to 6 are formed at these steps
S5 and S7, the AD method is used as a film forming method, which
will be described later. As a material of the insulating films 4 to
6, for example, PZT, alumina, zirconic (Zr) oxide such as zirconia
(ZrO.sub.2), titanium oxide (TiO.sub.2), or the like can be used.
That is, any material may be used that can form an insulating film
by using the AD method as the film forming method.
[0039] Then, at steps S8 and S9, the external electrodes 7 and 8
are formed. At step S8, as shown in FIG. 5A, conductor paste such
as silver paste or palladium silver paste is applied to the
portions except for the front surface and rear surface of the
multilayered structure 11 and the insulating films 6. Then, at step
S9, drying and baking of the conductor paste is performed. Thereby,
as shown in FIG. 5B, the external electrodes 7 and 8 are formed. At
the time of baking, the organic components and moisture are removed
from the conductor paste, and thereby, the volume of the conductor
paste is reduced.
[0040] Here, the AD method used for forming the insulating films in
the embodiment will be described. FIG. 6 shows a device to be used
for forming films in accordance with the AD method. This device has
an aerosol generating container 20 and a film forming chamber 30.
An aerosol refers to fine particles of a solid or liquid floating
in a gas. In the embodiment, PZT fine particles having an average
particle diameter of 0.2 .mu.m to 1 .mu.m are used for forming the
insulating films.
[0041] The aerosol generating container 20 is a container in which
a powder of the film forming raw material is located and the
aerosol is generated. In the aerosol generating container 20, a
carrier gas lead-in part 21, an aerosol lead-out part 22, and a
pressure regulating nozzle 23 are provided. The carrier gas lead-in
part 21 introduces a carrier gas as a gas to be used for carrying
the raw material powder into the aerosol generating container 20.
That is, the raw material powder located in the aerosol generating
container 20 is blown up by the carrier gas so as to generate the
aerosol.
[0042] As the carrier gas, dry air, nitrogen gas, argon gas, oxygen
gas, helium gas, or the like is used. Further, the aerosol lead-out
part 22 draws the aerosol generated in the aerosol generating
container 20 and guides it to the film forming chamber 30. The
pressure regulating nozzle 33 is used when the pressure difference
between the aerosol generating container 20 and the film forming
chamber 30 is adjusted.
[0043] Such an aerosol generating container 20 is mounted on a
vibrating base 24. The vibrating base 24 provides vibration to the
aerosol generating container 20 to agitate the raw material powder
located inside thereof, and thereby, increases the generation
efficiency of the aerosol.
[0044] In the film forming chamber 30, an exhaust pipe 31, an
aerosol lead-in part 32, a nozzle 33 and a movable stage 34 are
provided. The exhaust pipe 31 is connected to a vacuum pump and
exhausts air from inside of the film forming chamber 30. The
aerosol lead-in part 32 is connected to the aerosol lead-out part
22 of the aerosol generating container 20 and introduces the
aerosol generated in the aerosol generating container 20 into the
film forming chamber 30. The nozzle 33 sprays the aerosol
introduced via the aerosol lead-in part 32 toward the multilayered
structure 11 placed on the movable stage 34.
[0045] In the embodiment, the insulating films 4 to 6 are formed by
using the AD method. According to the AD method, the material
powders of the insulating films 4 to 6 are sprayed on a foundation
layer such as the PZT layers 1 and the internal electrode layers 2
and 3 at a high speed and impinged thereon to form films, and
thereby, so-called "anchoring" phenomenon occurs in which the
material powders cut into the foundation layers. As a result, an
anchor layer (a layer into which the material powders cut) is
formed on each of the PZT layers 1 and the internal electrode
layers 2 and 3 as the foundation layers.
[0046] The depth of the anchor layer is different depending on the
quality of material of the foundation layer and the powder speed,
and normally, on the order from 10 nm to 100 nm. Therefore, by
observing the interfaces of the respective layers contained in the
piezoelectric element 10, it can be discriminated that the method
of manufacturing the piezoelectric element according to the present
invention has been used.
[0047] According to the embodiment, an insulating film having a
high withstand voltage can be formed by using the AD method, and
thus, the insulating film can be made thinner. For example, when
the voltage for driving the piezoelectric element is 200V to 300V,
the insulating film can be made to have a small thickness of 2
.mu.m to 3 .mu.m. Further, since the insulating film formed by
using the AD method is denser and stronger than the insulting film
formed by an epoxy resin or the like, the mechanical strength can
be improved. Furthermore, since the internal electrode layers 2 and
the internal electrode layers 3 are located on the entire area of a
surface of each PZT layer 1, stress is not concentrated on a
partial region of the PZT layer 1. As a result, the durability of
the piezoelectric element can be made better compared to that of
the conventional piezoelectric element. By the way, in the
embodiment, the external electrodes 7 and 8 are formed by employing
the conductor paste. However, the external electrodes 7 and 8 may
be formed by sputtering platinum. In this case, titanium (Ti) is
desirably used as an adhesion layer.
[0048] By employing the piezoelectric elements according to one
embodiment of the present invention described above, a
piezoelectric element array can be fabricated. For example, a
piezoelectric element array as shown in FIG. 7 is fabricated by
arranging the plural piezoelectric elements 10 in a two-dimensional
array form.
[0049] FIG. 8 is a partial sectional perspective view showing an
example in which the piezoelectric element array as shown in FIG. 7
is applied to an ultrasonic probe. This ultrasonic probe includes
an ultrasonic transducer array 40 having plural ultrasonic
transducers for transmitting and receiving ultrasonic waves, an
acoustic matching layer 41, an acoustic lens 42, and a backing
layer 43. These parts 40 to 43 are accommodated in a casing 44.
Further, interconnections drawn from the plural ultrasonic
transducers of the ultrasonic transducer array 40 are connected via
a cable 45 to an ultrasonic imaging apparatus main body.
[0050] The ultrasonic transducer array 40 includes plural
ultrasonic transducers 40a arranged in a two-dimensional matrix
form and filling materials 40b such as an epoxy resin located
between those ultrasonic transducers 40a.
[0051] The acoustic matching layer 41 is formed by glass, ceramic,
epoxy resin mixed with metal powder, or the like that can easily
transmit ultrasonic waves, and provided for eliminating a mismatch
of the acoustic impedance between an object to be inspected such as
a living body and the ultrasonic transducer. Thereby, the
ultrasonic wave transmitted from the ultrasonic transducer
propagates efficiently within the object.
[0052] The acoustic lens 42 is formed by silicon rubber, for
example, and provided for focusing an ultrasonic beam, which has
been transmitted from the ultrasonic transducer array 40 and passed
the acoustic matching layer 41, at a predetermined depth.
[0053] The backing layer 43 is formed by a material providing large
acoustic attenuation such as epoxy resin or rubber mixed with
powder of a metal, ferrite or PZT, and provided for rapidly
attenuating unwanted ultrasonic wave generated by the ultrasonic
transducer array 40.
[0054] Such an ultrasonic probe is fabricated in the following
manner, for example. A common electrode is formed on a substrate
such as ceramic such as Macor (registered trademark) or glass that
can be used as the acoustic matching layer, and the plural
piezoelectric elements 10 are arranged thereon in a two-dimensional
array form as shown in FIG. 7. Then, filling materials are located
between those piezoelectric elements 10. Thereby, the ultrasonic
transducer array 40 and the acoustic matching layer 41 are
fabricated. By the way, the acoustic matching layer having plural
layers may be provided by further bonding another acoustic matching
layer to the substrate. Furthermore, separate interconnections are
drawn from the respective ultrasonic transducers (piezoelectric
elements 10), the backing layer 43 is located, and the acoustic
lens 42 is located at the acoustic matching layer 41 side.
[0055] In the case where the piezoelectric element according to the
embodiment is applied to the ultrasonic probe, the withstand
voltage of the insulating films can be made higher by forming the
insulating films by using the AD method, and even when the voltage
for driving the ultrasonic transducer is 200V to 300V, the
insulating film can be made to have a small thickness of 2 .mu.m to
3 .mu.m. Therefore, interferences between the plural ultrasonic
transducers can be suppressed. Further, when a gap length between
the plural ultrasonic transducers is on the order of 50 .mu.m, the
insulating portions on the side surfaces never cause a barrier when
the filing material such as an epoxy resin is poured between the
ultrasonic transducers.
[0056] Thereby, a large number of ultrasonic transducers can be
packaged densely, and therefore, an ultrasonic probe can be
obtained that is capable of obtaining an ultrasonic image with high
resolving power. Further, an ultrasonic probe including the
two-dimensional ultrasonic transducer array, to which the
piezoelectric elements according to the present invention are
applied, can be utilized in an ultrasonic diagnostic apparatus for
acquiring three-dimensional ultrasonic images in real time because
the plural ultrasonic transducers are packaged densely.
* * * * *