JPWO2013065365

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DESCRIPTION JPWO2013065365
The ultrasonic transducer element 20 includes a plurality of capacitive ultrasonic cells 10 each
having a membrane 18 including a lower electrode portion 12A and an upper electrode portion
16A, which are oriented and disposed via a cavity 14 having a circular plan view. The thickness
of the cavity 14 monotonously decreases in a curved manner from the center to the periphery of
the cavity 18.
Ultrasonic transducer element and ultrasonic endoscope
[0001]
The present invention relates to a capacitive ultrasonic transducer element and an ultrasonic
endoscope provided with the ultrasonic transducer element.
[0002]
Ultrasonic diagnostic methods are in widespread use in which ultrasonic waves are applied to the
inside of the body, and an internal state of the body is imaged and diagnosed from echo signals.
An ultrasonic endoscope (hereinafter, referred to as "US endoscope") is one of ultrasonic
diagnostic apparatuses used for ultrasonic diagnostic methods. In the US endoscope, an
ultrasonic transducer is disposed at the distal end rigid portion of the insertion portion
introduced into the body. The ultrasonic transducer has a function of converting an electric
signal into an ultrasonic wave and transmitting it to the body, and also receives an ultrasonic
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wave reflected in the body to convert it into an electric signal.
[0003]
A ceramic piezoelectric material containing lead, which has a large environmental load, such as
PZT (lead zirconate titanate), is mainly used as the ultrasonic transducer. On the other hand, a
Capacitive Micro-machined Ultrasonic Transducer (hereinafter referred to as "c-MUT")
manufactured using MEMS (Micro Electro Mechanical Systems) technology and containing no
lead in its material The development of) is in progress.
[0004]
As disclosed in Japanese Patent Application Publication No. 2005-510264, the c-MUT is an
ultrasonic cell (hereinafter, "US cell") in which the upper electrode portion and the lower
electrode portion are disposed opposite to each other via a cavity portion (cavity). ) As a unit
element. In the US cell, the membrane including the upper electrode part above the cavity
constitutes the vibrating part. Then, an ultrasonic transducer element (hereinafter referred to as
"US element") is configured by arranging a plurality of US cells in which respective electrode
parts are connected by a wiring part.
[0005]
In the US cell, a voltage is applied between the lower electrode portion and the upper electrode
portion to vibrate the membrane including the upper electrode portion by electrostatic force to
generate an ultrasonic wave. Further, when an ultrasonic wave enters the membrane from the
outside, the membrane is deformed and the distance between the two electrode parts changes, so
that the ultrasonic wave is converted into an electric signal from the change in capacitance.
[0006]
Here, since the membrane of the c-MUT is a circular membrane whose entire outer periphery is
fixed (restrained) like the drum membrane, the central portion is easily displaced but the outer
peripheral portion is hardly displaced. For this reason, the outer peripheral portion of the
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membrane has lower transmission / reception efficiency than the central portion. That is, it may
not be said that the ultrasonic endoscope having the conventional capacitive ultrasonic
transducer element and the conventional capacitive ultrasonic transducer element has good
transmission / reception efficiency.
[0007]
An object of the present invention is to provide an ultrasonic transducer element with high
transmission / reception efficiency and an ultrasonic endoscope with high transmission /
reception efficiency.
[0008]
An ultrasonic transducer element according to an embodiment of the present invention
comprises a plurality of capacitive ultrasonic cells each having a membrane including a lower
electrode portion and an upper electrode portion, which are oriented through a cavity having a
circular shape in plan view. The thickness of the cavity monotonously decreases in a curved
manner from the center to the periphery of the cavity.
[0009]
An ultrasonic endoscope according to another embodiment includes a plurality of capacitive
ultrasonic cells each having a lower electrode portion and a membrane including an upper
electrode portion, which are oriented through a circular cavity in plan view. And an ultrasonic
transducer element in which the thickness of the cavity monotonously decreases in a curved
manner from the center to the outer periphery of the cavity.
[0010]
BRIEF DESCRIPTION OF THE DRAWINGS It is an external view for demonstrating the endoscope
system which comprises the ultrasound endoscope of 1st Embodiment.
It is a perspective view for demonstrating the front-end ¦ tip part of the ultrasound endoscope of
1st Embodiment.
It is a perspective view for demonstrating the structure of the ultrasound unit of the ultrasound
endoscope of 1st Embodiment.
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It is a top view for demonstrating the structure of the ultrasound transducer element of 1st
Embodiment. It is a fragmentary sectional view along the VV line of FIG. It is an exploded view of
the ultrasonic cell of the ultrasonic transducer element of 1st Embodiment. It is a fragmentary
sectional view for explaining the manufacturing method of the ultrasonic transducer element of a
1st embodiment. It is a fragmentary sectional view for explaining the manufacturing method of
the ultrasonic transducer element of a 1st embodiment. It is a fragmentary sectional view for
explaining the manufacturing method of the ultrasonic transducer element of a 1st embodiment.
It is a fragmentary sectional view for explaining the manufacturing method of the ultrasonic
transducer element of a 1st embodiment. It is a fragmentary sectional view for explaining the
manufacturing method of the ultrasonic transducer element of a 1st embodiment. It is a crosssectional schematic diagram for demonstrating the drive of the ultrasound transducer element of
1st Embodiment. It is a cross-sectional schematic diagram for demonstrating the drive of the
ultrasound transducer element of 1st Embodiment. It is a fragmentary sectional view of the
ultrasonic transducer element of the modification 1 of a 1st embodiment. It is a fragmentary
sectional view of the ultrasonic transducer element of the modification 2 of a 1st embodiment. It
is a fragmentary sectional view of the ultrasonic transducer element of 2nd Embodiment. It is a
fragmentary sectional view of the ultrasonic transducer element of the modification 1 of a 2nd
embodiment. It is a fragmentary sectional view of the ultrasonic transducer element of the
modification 2 of a 2nd embodiment. It is a fragmentary sectional view of the ultrasonic
transducer element of the modification 3 of a 2nd embodiment.
[0011]
<First Embodiment> An ultrasonic endoscope (hereinafter referred to as "US endoscope") having
an ultrasonic transducer element (US element) 20 and a US element 20 according to the first
embodiment with reference to the drawings. Will be described.
[0012]
<Configuration of Ultrasonic Endoscope> As shown in FIG. 1, the US endoscope 2 constitutes the
ultrasonic endoscope system 1 together with the ultrasonic observation device 3 and the monitor
4.
The US endoscope 2 includes an elongated insertion portion 21 inserted into the body, an
operation portion 22 disposed at a proximal end of the insertion portion 21, and a universal cord
23 extending from the side portion of the operation portion 22. Prepare.
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[0013]
At the proximal end of the universal cord 23, a connector 24A connected to a light source device
(not shown) is disposed. From the connector 24A, a cable 25 detachably connected to the camera
control unit (not shown) via the connector 25A, and a cable 26 detachably connected to the
ultrasonic observation apparatus 3 via the connector 26A It is extended. A monitor 4 is
connected to the ultrasonic observation apparatus 3.
[0014]
The insertion portion 21 is positioned and operated from the distal end side in order from the
distal end side to the distal end rigid portion (hereinafter referred to as "the distal end portion")
37, the curved portion 38 located at the rear end of the distal end 37, and the rear end of the
curved portion 38 A flexible tube portion 39 having a small diameter, a long length and flexibility
up to the portion 22 is continuously provided. An ultrasonic unit (US unit) 30 which is an
ultrasonic transmitting and receiving unit is disposed on the distal end side of the distal end
portion 37.
[0015]
The operation unit 22 includes an angle knob 22A that performs bending control of the bending
unit 38 in a desired direction, an air / water supply button 22B that performs air supply and
water supply operations, a suction button 22C that performs suction operation, and a treatment
that is introduced into the body A treatment tool insertion port 22D or the like which is an
entrance of the tool is disposed.
[0016]
Then, as shown in FIG. 2, the illumination lens cover 31 constituting the illumination optical
system and the observation lens cover 32 of the observation optical system are provided at the
distal end portion 37 provided with the US unit 30 for transmitting and receiving ultrasonic
waves. And, a forceps port 33 which also serves as a suction port, and an air / water feed nozzle
(not shown) are disposed.
[0017]
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As shown in FIG. 3, the ultrasonic array (US array) 40 of the US unit 30 is a group of radial
transducers arranged in a curved shape in a cylindrical shape, in which the long sides of a
plurality of rectangular US elements 20 in plan view are connected. is there.
That is, in the US array 40, for example, 64 US elements 20 each having a short side of 0.1 mm
or less are disposed on the side surface of a cylinder having a diameter of 2 mm.
[0018]
Although the US array 40 is a radial transducer group, the US array may be a convex transducer
group curved in a convex shape.
[0019]
<Configuration of Ultrasonic Transducer Element> As shown in FIG. 4, the ultrasonic transducer
element 20 having a rectangular shape in plan view has a transmitting / receiving unit 60 in
which a plurality of ultrasonic cells 10 are two-dimensionally arranged in a matrix.
An upper electrode terminal 51 and a lower electrode terminal 52 for driving the plurality of
ultrasonic cells 10 are disposed in each US element 20.
The plurality of US elements 20 have their long sides connected to one another.
[0020]
The figures are all schematic diagrams for explanation, and the ratio of the number of patterns,
the shape, the thickness, the size, the size, etc. is different from the actual one.
[0021]
<Basic Structure of Ultrasonic Cell> As shown in FIGS. 5 and 6, in each of the US cells 10, the
lower electrode portion 12A and the upper electrode portion 16A are composed of the lower
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insulating layer 13, the gap adjusting portion 19, the cavity 14 and They are disposed opposite
to each other via the upper insulating layer 15.
Hereinafter, the substrate side in the Z direction is referred to as the lower side, and the
protective layer 17 side is referred to as the upper side.
[0022]
That is, the US cell 10 comprises a lower electrode layer 12, a lower insulating layer 13, a gap
adjusting layer 19 A having a cavity 14, and an upper insulating layer 15 sequentially stacked on
a substrate 11 made of silicon as a base. , Upper electrode layer 16 and protective layer 17. A
portion 15 B of the upper insulating layer 15 immediately above the cavity 14 to the protective
layer 17 constitute the membrane 18. The membrane 18 is illustrated thick in FIG. 6 and the like
for the sake of explanation, but is a thin membrane having a thickness of, for example, 1 μm to
10 μm, which is stretched at the opening of the cavity 14 in a plan view.
[0023]
The lower electrode layer 12 is composed of a lower electrode portion 12A and a lower wiring
portion 12B. The upper electrode layer 16 is composed of an upper electrode portion 16A and an
upper wiring portion 16B. All lower electrode portions 12A of the plurality of US cells 10 of the
US element 20 are connected to the lower electrode terminal 52 via the lower wiring portion
12B. Similarly, all the upper electrode portions 16A of the plurality of US cells 10 of the US
element 20 are connected to the upper electrode terminal 51 via the upper wiring portion 16B.
[0024]
The lower electrode portion 12A, the upper electrode portion 16A, the gap adjusting portion 19
and the cavity 14 are rotationally symmetrical with respect to the central axis O, and are circular
in plan view.
[0025]
Then, the gap adjusting portion 19 made of a high dielectric constant material which adjusts the
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gap which is the thickness (height) of the cavity 14 is on the lower insulating layer 13 inside the
cavity forming portion 14A which is a cylindrical hollow portion. Are located in
In other words, the space without the gap adjusting portion 19 of the cavity forming portion 14A
is the cavity 14. The thickness T of the gap adjusting portion 19 is, for example, 0 (μm)
which is the smallest at the central portion, and monotonously increases in a curved manner
from the central portion toward the outer peripheral portion.
[0026]
That is, the distribution T (r) of the thickness T of the gap adjusting unit 19 having a radius
R is, for example, a function shown in (Expression 1) that monotonously increases in a
curved manner from the central portion Can be expressed by Here, the distance from the central
axis O is r , and the thickness of the outer periphery (r = R) of the gap adjusting unit 19 is
TR . Of course, the distribution T (r) does not have to completely coincide with the function
shown in (Expression 1), and may have some differences, for example, about ± 5%.
[0027]
(Equation 1) T (r) = TR × [1−J0 (2.4 / R × r)] J0 is a zero-order Bessel function, and J0 (0) = 1,
J0 (2.4) = 0 It is.
[0028]
Here, (Expression 1) is an expression using a type 0 Bessel function indicating an envelope of
vibration of a circular flat diaphragm whose outer periphery is constrained.
[0029]
The gap adjusting portion 19 is preferably made of a high dielectric constant material, for
example, SiO 2 (Ks = 3.8) having a relative dielectric constant Ks of 3 or more, for example SiN
having a relative dielectric constant Ks of 5 or more. Is particularly preferred.
Of course, the relative dielectric constant Ks is higher, Al 2 O 3 (Ks = 8.5), Ta 2 O 5 (Ks = 20 to
25), TiO 2 (Ks = 40 to 110), HfOF, or HfO 2 Furthermore, although barium titanate ceramics and
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the like may be used, the upper limit of the relative dielectric constant Ks is about 150 from the
viewpoint of the easiness of the forming method and the like.
[0030]
On the other hand, the thickness (height) t of the cavity 14 monotonously decreases in a
curved manner from the center O of the cavity 14 to the outer periphery corresponding to the
thickness distribution of the gap adjusting portion 19 ( The Bessel function of the 0 type shown
in Equation 2) is represented by t (r).
That is, the cavity 14 has a so-called downward convex shape. Note that t 0 is the thickness
(height) t of the center (r = 0) of the cavity 14 and is equal to the thickness TR of the
outer periphery (r = R) of the gap adjusting unit 19.
[0031]
(Expression 2) t (r) = t0 × J0 (2.4 / R × r) That is, the distribution (changed state) t (r) of the
thickness t of the cavity 14 is the membrane at the time of driving It is the same as 18
theoretical deformation states.
[0032]
Next, a method of manufacturing the US element 20 will be described with reference to FIGS. 7A
to 7E.
[0033]
<Formation of lower electrode layer> A conductive layer made of conductive silicon or metal, for
example, copper, gold, Mo, Ta, or aluminum is sputtered or CVD (chemically deposited) on the
entire surface of the substrate 11 on which the insulating layer (not shown) is formed. The film is
formed by a vapor phase growth method or the like.
Then, after a mask pattern made of resist is formed by photolithography on the conductive layer,
the lower electrode layer 12 is formed by selectively etching away the conductive layer not
covered by the mask pattern.
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[0034]
<Formation of Lower Insulating Layer> The lower insulating layer 13 made of an insulating
material such as SiN is formed, for example, by the CVD method so as to cover the lower
electrode layer 12.
If necessary, the surface of the lower insulating layer 13 is planarized by CMP (Chemical
Mechanical Polish) after film formation. Then, on the lower insulating layer 13, an intermediate
insulating layer 15A made of, for example, SiN is further formed. The thickness t0 (TR) of the
intermediate insulating layer 15A corresponds to the thickness (height) of the cavity forming
portion 14A.
[0035]
As shown in FIG. 7A, the intermediate insulating layer 15A is selectively etched away using a
mask manufactured by photolithography to form a cavity formation portion 14A. When the
lower insulating layer 13 and the intermediate insulating layer 15A are made of different
materials, the intermediate insulating layer 15A is etched with a chemical solution or gas which
selectively etches. On the other hand, when the lower insulating layer 13 and the intermediate
insulating layer 15A are made of the same material, for example, SiN, the surface of the
intermediate insulating layer 15A is planarized by the CMP method if necessary, and then the
intermediate insulating layer 15A is formed. Based on the etching rate, only the cavity forming
portion 14A may be etched to the maximum height (depth) "t0" of the cavity 14.
[0036]
<Gap adjustment portion formation> A gap adjustment layer 19A made of a high dielectric
constant material is formed on the surface of the intermediate insulating layer 15A so as to have
a thickness greater than the maximum thickness (height) "TR (t0)" of the cavity forming portion
14A. . Then, the film thickness equivalent (convex portion) of the gap adjustment layer 19A is
polished by the CMP method to flatten the surface. Then, a structure in which the gap adjusting
layer 19A is embedded only in the cavity forming portion 14A is produced.
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[0037]
As shown in FIG. 7B, by exposing the photoresist through a gray scale photomask or the like, a
resist mask 63 whose thickness monotonously increases from the central portion toward the
outer peripheral portion is manufactured. That is, the thickness of the resist mask 63
monotonously increases in a curved manner from the central portion to the outer peripheral
portion, as shown by (Expression 1).
[0038]
By performing etch back processing under dry etching conditions in which the resist mask 63
and the gap adjustment layer 19A have the same etching rate, as shown in FIG. 7C, it is made of
the high dielectric constant material of the surface profile shown in (Expression 1). The gap
adjusting portion 19 is manufactured inside the cavity forming portion 14A.
[0039]
<Formation of Sacrifice Layer> A sacrificial layer material made of, for example, tungsten is
deposited to a thickness equal to or greater than the maximum thickness (TR) of the cavity
formation portion 14A.
Then, when the polishing by the CMP method is performed, as shown in FIG. 7D, a structure
including the sacrificial layer 61 having a cavity shape is obtained. The maximum thickness t0
(TR) of the sacrificial layer 61 after polishing is, for example, 0.05 μm to 0.3 μm, and
preferably 0.05 μm to 0.15 μm.
[0040]
<Upper Insulating Layer Formation> The upper insulating layer 15 is formed by, for example, the
same method and the same material as the lower insulating layer 13. Then, in order to remove
the sacrificial layer 61, an opening (not shown) into which the etchant flows is formed at a
predetermined position of the upper insulating layer 15.
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[0041]
Cavity Formation The cavity 14 is formed by selectively etching away the sacrificial layer 61. For
example, when tungsten (W) is used as the sacrificial layer 61 and SiN is used as the lower
insulating layer 13 and the upper insulating layer 15, hydrogen peroxide water (H 2 O 2) is used
as the etching agent. When conductive polycrystalline silicon is used as the sacrificial layer 61
and SiN is used as the lower insulating layer 13 and the upper insulating layer 15, xenon fluoride
gas (XeF 2) is used as the etching agent.
[0042]
<Formation of Upper Electrode Layer> The upper electrode layer 16 is formed by the same
method as the lower electrode layer 12.
[0043]
Formation of Protective Layer As shown in FIG. 7E, the upper insulating layer 15 is covered with
the protective layer 17.
The protective layer 17 has not only a protective function but also a function to connect a
plurality of US elements 20. The protective layer 17 is made of a flexible resin such as polyimide,
epoxy, acrylic or polyparaxylene. Among them, polyimide is preferable because it has high
chemical resistance, flexibility, and easy processing. The protective layer 17 may have a twolayer structure in which a biocompatible second insulating layer is further formed on the first
insulating layer.
[0044]
The plan view of the cavity 14 does not have to be a perfect circle, and may be a substantially
circular shape. Furthermore, the plan view of the cavity 14 may be a substantially circular
polygon. That is, the shape of the cavity forming portion 14A is not limited to a cylindrical shape,
and may be a polygonal shape or the like. When the cavity forming portion 14A has a polygonal
prism shape, it is preferable that the shapes of the upper electrode portion 16A and the lower
electrode portion 12A in plan view also be polygonal. And the gap adjustment part 19 should just
have the range of the inscribed circle satisfy ¦ filling the said thickness distribution.
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[0045]
By arranging a plurality of US elements 20 in a radial shape in the connecting direction shown in
FIG. 4, the US array 40 shown in FIGS. 2 and 3 is manufactured. The US array 40 is joined to, for
example, the outer peripheral surface of a cylinder having a predetermined outer diameter.
Further, the coaxial cable bundle 35 is connected to the US array 40, and the US unit 30 is
manufactured.
[0046]
<Operation of US Element / US Cell> Next, the operation of the US element 20 will be described
with reference to FIGS. 8A and 8B. In FIGS. 8A and 8B, the lower insulating layer 13, the upper
insulating layer 15, the protective layer 17 and the like are not displayed, and the description
thereof is also omitted.
[0047]
As shown in FIG. 8A, the lower electrode portion 12A is connected to the voltage signal
generating portion 3A of the ultrasonic observation apparatus 3 through the lower electrode
terminal 52. On the other hand, the upper electrode portion 16A is connected to the capacitance
signal detection portion 3B through the upper electrode terminal 51 and is at the ground
potential. The capacitance signal detection unit 3B detects a capacitance signal (current change)
at the time of ultrasonic wave reception.
[0048]
At the time of ultrasonic wave generation (during driving), the voltage signal generating unit 3A
applies a driving voltage signal including a bias voltage to the lower electrode unit 12A. When a
voltage is applied to the lower electrode portion 12A, as shown in FIG. 8B, the upper electrode
portion 16A of the ground potential is attracted to the lower electrode portion 12A by
electrostatic force, so the membrane 18 including the upper electrode portion 16A has a concave
shape Transform into The outer peripheral portion of the cavity 14 has a small height (thickness)
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t , but the outer peripheral portion of the membrane 18 has a small amount of deformation,
and therefore does not hinder the deformation of the membrane 18.
[0049]
Then, when the voltage application to the lower electrode portion 12A ceases, the membrane 18
recovers to its original shape shown in FIG. 8A by the elastic force. The vibration (deformation /
recovery) of the membrane 18 generates an ultrasonic wave.
[0050]
Here, the cavity 14 of the conventional US element is air (vacuum) whose entire inside is a
dielectric constant "1", and the thickness is the same "t0" from the central portion to the outer
peripheral portion. In other words, the cavity 14 of the conventional US element is the same as
the cavity forming portion 14A of the US element 20.
[0051]
On the other hand, the gap adjusting portion 19 is disposed inside the cavity forming portion
14A of the US element 20. The relative dielectric constant Ks of the gap adjusting portion 19
made of a high dielectric constant material is larger than 1 . For this reason, the dielectric
equivalent distance "Teff" of the gap adjusting portion 19 is represented by (Expression 3) at a
position where the distance from the center of the cavity 14 is "r". Here, the dielectric equivalent
distance "Teff" is an electrostatic distance in which the relative dielectric constant Ks is taken into
consideration with respect to the physical distance "T".
[0052]
(Equation 3) Teff (r) = Tr (r) / Ks Here, when the relative dielectric constant Ks is sufficiently
larger than 1 , for example, when Ks ≧ 3, the dielectric equivalent interval Teff is
substantially 0 . It can be regarded as That is, in the US element 20, the gap adjusting unit 19
can be regarded as not present electrostatically.
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[0053]
For this reason, the dielectric equivalent interval "teff" of the cavity 14 can be approximately
expressed by (Equation 4).
[0054]
(Expression 4) teff = t0−T (r) In other words, the physical distance between the lower electrode
portion 12A and the upper electrode portion 16A, which are disposed opposite to each other via
the cavity 14, is uniform.
However, the dielectric equivalent distance "Teff" which is the effective thickness of the cavity
14, that is, the electrostatic distance (inter-electrode effective distance) has a maximum at the
center (r = 0) and curves toward the outer peripheral portion is decreasing. Since the electrostatic
force is proportional to the effective distance between the electrodes, the US element 20 has an
increased electrostatic force acting between the electrodes than a conventional US element in
which the distance between the electrodes is uniform. For this reason, the US element 20 has a
high generation efficiency of ultrasonic waves.
[0055]
Furthermore, in the US element 20, when driven, the entire lower surface of the membrane 18
drawn to the lower electrode portion 12A is deformed so as to contact and stick to the upper
surface of the gap adjusting portion 19. For this reason, the deformation state of the membrane
18 at the time of driving becomes the same as the upper surface shape of the gap adjusting
portion 19 shown by (Expression 1) or the like.
[0056]
As described above, the shape represented by (equation 1) or the like is a theoretical deformation
shape of the diaphragm.
[0057]
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The ultrasonic waves generated by the US cell 10 that is deformed to the theoretical state
become an ideal waveform with less distortion.
For this reason, the US element 20 having the US cell 10 has a good transmission efficiency of
ultrasonic waves.
[0058]
When the entire lower surface of the membrane 18 is largely deformed so as to stick to the
upper surface of the gap adjusting portion 19, that is, when the gas is present in the cavity 14,
the gas is held in a compressed state. For example, it may be released to the outside through an
opening (not shown) formed to allow the etchant to flow in.
[0059]
Further, in the US element 20, since the gap adjusting portion 19 made of a high dielectric
constant material having higher insulating properties than air is disposed between the electrodes,
the withstand voltage between the electrodes is high.
[0060]
On the other hand, the US element 20 having the US cell 10 also has good ultrasound receiving
efficiency.
As described above, in the US cell 10 in which the inter-electrode distance decreases from the
center to the outer periphery, the capacitance is increased more than in the conventional US cell
in which the inter-electrode distance is uniform.
Therefore, in the US cell 10, even if the membrane 18 is deformed to the same extent as the
conventional US cell, the rate of change of the charge amount is increased. Furthermore,
although the outer peripheral portion of the membrane 18 is restrained by the intermediate
insulating layer 15A, the amount of deformation is small, but since the distance between the
electrodes is short, the change in the amount of charge is large.
[0061]
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As described above, the US element 20 has good ultrasound transmission / reception efficiency.
The US endoscope 2 equipped with the US element 20 has a high efficiency of transmitting and
receiving ultrasonic waves.
[0062]
<Modification of First Embodiment> As shown in FIG. 8A etc., the US element 20 is made of a
high dielectric constant material inside a cavity forming portion 14A which is a cylindrical cavity
formed in the intermediate insulating layer 15A. Since the gap adjusting unit 19 is provided, the
thickness of the cavity is changed. On the other hand, as in the US cell 10A shown in FIG. 9A, a
part of the intermediate insulating layer 15A made of a high dielectric constant material may be
a cavity 14A1 whose thickness changes. That is, the gap adjusting portion of the US element 20A
of the modification 1 of the first embodiment shown in FIG. 9A is a part of the intermediate
insulating layer 15A.
[0063]
Further, as shown in FIG. 9B, the cavity 14B of the US cell 10B of the US element 20B of
Modification 2 of the first embodiment has a thickness curvilinearly from the central portion to
the outer peripheral portion similarly to the US element 20. Although monotonously increasing,
it has a so-called upper concave shape.
[0064]
The US element 20A and the US element 20B have the same effect as the US element 20.
[0065]
Second Embodiment A US element 20C having a US cell 10C according to a second embodiment
will be described below with reference to the drawings.
Since the US cell 10C and the US element 20C are similar to the US cell 10 and the US element
20 of the first embodiment, the same components are denoted by the same reference numerals
and descriptions thereof will be omitted.
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[0066]
As shown in FIG. 10A, in the US cell 10C, the thickness T12 of the lower electrode portion 12C
covered by the lower insulating layer 13 monotonously increases in a curved manner from the
center toward the outer periphery.
For this reason, the physical distance between the lower electrode portion 12C and the upper
electrode portion 16A monotonously decreases in a curved manner from the center toward the
outer peripheral portion. For this reason, the thickness "t" of the cavity 14C monotonously
increases in a curved manner from the central portion toward the outer peripheral portion.
[0067]
The distribution of the thickness T12 of the lower electrode portion 12C of the US element 20C
and the distribution of the thickness "t" of the cavity 14C are the same as the thickness change of
the gap adjusting portion 19 of the US element 20 of the first embodiment. It can be shown by
the type 0 Bessel function of Formula 1) or (Formula 2). That is, also in the US element 20C, the
thickness change state of the cavity 14C is the same as the deformation state of the membrane at
the time of driving.
[0068]
In other words, the upper surface shape of the lower electrode portion 12C covered by the lower
insulating layer 13 is the same as the lower surface shape of the cavity 14C.
[0069]
The lower electrode portion 12C is formed by performing an etch back process using a resist
mask or the like whose thickness monotonously increases from the central portion toward the
outer peripheral portion as in the gap adjustment layer 19A.
[0070]
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<Modification of Second Embodiment> As shown in FIG. 10B, in the US cell 10D of the US
element 20D of the first modification of the second embodiment, the depth is represented by the
type 0 Bessel function in the substrate 11D. A recess monotonously decreasing from the central
portion toward the outer peripheral portion is formed, and the lower electrode portion 12D is
formed thereon.
Therefore, the physical distance between the lower electrode portion 12A and the upper
electrode portion 16D monotonously decreases in a curved manner from the center toward the
outer peripheral portion.
The thickness "t" of the cavity 14D monotonously increases in a curved manner from the central
portion toward the outer peripheral portion.
[0071]
Further, in the US cell 10E of the US element 20E of the modification 2 of the second
embodiment shown in FIG. 10C, the upper electrode portion 16E is deformed into a convex shape
represented by a type 0 Bessel function by the internal stress of the membrane 18 There is. Thus,
the physical distance between the lower electrode portion 12A and the upper electrode portion
16E monotonously decreases in a curved manner from the center toward the outer peripheral
portion. For this reason, the thickness "t" of the cavity 14E monotonously decreases in a curved
manner from the central portion toward the outer peripheral portion.
[0072]
Furthermore, in the US cell 10F of the US element 20F of the third modification of the second
embodiment shown in FIG. 10D, the downward convex cavity represented by the zeroth Bessel
function and the upper cavity represented by the zeroth Bessel function By combining with a
convex cavity, a biconvex cavity 14F is provided. The physical distance between the upper
electrode portion 16F and the lower electrode portion 12F on the substrate 11F monotonously
decreases in a curved manner from the center toward the outer peripheral portion.
[0073]
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The US elements 20C to 20F all have the same effect as the US element 20. Furthermore, unlike
the US element 20, the US elements 20C to 20F do not require the formation of the gap adjusting
portion 19, and are easy to manufacture.
[0074]
As described above, the US elements 20A to 20F have good ultrasound transmission / reception
efficiency as the US element 20 does. The US endoscopes 2A to 2F provided with the US
elements 20A to 20F have high transmission / reception efficiency of ultrasonic waves.
[0075]
The thickness (height) distribution t (r) of the cavities 14 to 14F described in the first
embodiment, the second embodiment, and the modification may be [r] depending on the
constraint condition of the outer periphery of the membrane 18. As long as the thickness t
can be expressed as a function monotonously decreasing in a curvilinear manner as it increases,
it may be an arc function shown in (Expression 5) or a sine function shown in (Expression 6).
[0076]
(Expression 5) t (r) = t0 × [1- (r / R) <2>] (Expression 6) t (r) = t0 × [1-sin (π / 2 × r / R)] The
shape of the cavity is not limited to the shape of the cavities 14 to 14F, and the thickness of the
cavity may be monotonously decreased in a curved manner from the central portion toward the
outer peripheral portion.
For example, the upper surface of the cavity may be greatly curved and the lower surface may be
small, or vice versa, the upper surface of the cavity may be small and the lower surface may be
small and large.
[0077]
The present invention is not limited to the above-described embodiment, and various changes,
modifications, and the like can be made without departing from the scope of the present
invention.
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[0078]
This application is based on Japanese Patent Application No. 2011-240475, filed on Nov. 1,
2011, as a basis for claiming priority, and the above disclosure is made of the present
specification and claims. It shall be quoted in the drawings.
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