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JP2012095112

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DESCRIPTION JP2012095112
An ultrasonic wave generation unit capable of emitting an ultrasonic wave having a flat wave
front is provided. A bottom portion (120) having a disk-shaped lower electrode (122) is formed
on a substrate (110) in a cMUT (100), and a hollow cylindrical support portion (130) is formed
thereon. An upper surface portion 140 including an annular upper electrode 142 and a
nonconductive vibration mass 144 formed on the central axis C side of the upper electrode 142
is formed on the support 130. When an alternating voltage is applied between the lower
electrode 122 and the upper electrode 142, an electrostatic attraction force acts on the portion
where the electrodes face each other. On the other hand, the vibrating mass portion 144 is
higher in rigidity and less likely to be deformed than the other portions. As a result, in the upper
surface portion 140, the peripheral portion including the upper electrode 142 is curved, and the
central portion including the vibrating mass portion 144 vibrates while maintaining the flat
shape. Therefore, the cMUT 100 has a flat wavefront and can emit an ultrasonic wave with good
directivity. [Selected figure] Figure 1
Ultrasonic generator unit
[0001]
The present invention relates to an ultrasound generation unit, in particular to a capacitive
ultrasound generation unit.
[0002]
In recent years, capacitive micromachined ultrasonic transducers (cMUTs) have attracted
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attention as ultrasonic elements.
In general, a cMUT has a lower electrode disposed on a substrate, an upper electrode disposed in
a thin film facing the lower electrode, and a cavity located between the lower electrode and the
upper electrode. When a voltage is applied between the lower electrode and the upper electrode,
an electrostatic attraction force acts between the electrodes. Therefore, when an alternating
voltage is applied between the lower electrode and the upper electrode, the electrostatic
attraction force acting between the electrodes changes, and the thin film vibrates. Due to this
vibration, the cMUT emits an ultrasonic wave.
[0003]
For example, Patent Document 1 discloses a technology related to a cMUT capable of efficiently
transmitting and receiving ultrasonic energy, in which at least one of the upper electrode and the
lower electrode includes a plurality of electrode elements. In the cMUT according to Patent
Document 1, at least one of the upper electrode and the lower electrode is divided into a plurality
of elements, thereby preventing the cMUT from being damaged by the contact between the
upper electrode and the lower electrode.
[0004]
JP, 2007-527285, A
[0005]
In general, in cMUT, the thin film is vibrated in the primary vibration mode.
In the primary vibration mode, the displacement of the central portion of the vibrating thin film
is generally largest. Therefore, the thin film vibrates by being deformed from a flat plate shape in
a stationary state to a curved surface shape in a convex shape or a concave shape. For this
reason, when the thin film deforms from a flat plate into a convex shape, an acoustic field of
ultrasonic waves diffused around the cMUT is formed, and when the thin film deforms from a flat
into a concave shape, the ultrasonic waves focused from the cMUT Sound field is formed. As a
result, in the cMUT in which the thin film is curved and vibrates in a convex shape or a concave
shape as compared to the case where a parallel sound field in which neither diffusion nor
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focusing is formed in the traveling direction of the ultrasonic wave, the emitted ultrasonic wave
The directivity of the sound is reduced and the sound pressure is also reduced.
[0006]
Then, an object of this invention is to provide the ultrasonic wave generation unit which can
inject ¦ emit an ultrasonic wave with high directivity.
[0007]
In order to achieve the above object, one aspect of the ultrasonic wave generation unit of the
present invention comprises a bottom portion positioned on a substrate, a top portion opposite to
the bottom portion with a gap, and a support portion, The bottom portion has a first electrode,
and the top portion has a second electrode, and includes a central region and a peripheral region
located around the central region, and the support portion includes the bottom surface and the
upper surface. A gap is formed in a part of the gap, and when the lower surface is seen from the
upper surface, at least a part of the second electrode overlaps with a part of the gap It is
characterized in that the surrounding area is less rigid than the central area.
[0008]
In addition, in order to achieve the above object, another aspect of the ultrasonic wave
generation unit of the present invention includes a bottom portion positioned on a substrate, a
top portion opposed to the bottom portion with a gap, and a support portion. The bottom surface
has a first electrode, the top surface has a second electrode, and a gap is opposed to the bottom
surface, and the supporting portion is the bottom surface and the top surface. A gap is formed in
a part of the gap, and at least one of the first electrode and the second electrode has an inner end
and an outer end, and the lower surface portion from the upper surface portion side. When
viewed, the inner end is positioned to overlap the gap, and all of the outer peripheral ends of the
second electrode are positioned to overlap the support.
[0009]
According to the present invention, it is possible to provide an ultrasonic wave generation unit
capable of emitting an ultrasonic wave with high directivity, since the central portion of the
vibration surface is unlikely to be curved.
[0010]
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The figure which shows the outline of an example of cMUT which concerns on the 1st
Embodiment of this invention.
FIG. 6 is a diagram for explaining the transient phenomenon of the operation of the cMUT
according to the first embodiment of the present invention.
The figure for demonstrating the deformation ¦ transformation of the upper surface part of
cMUT which concerns on the 1st Embodiment of this invention.
The figure for demonstrating the deformation ¦ transformation of the upper surface part of
cMUT which concerns on a 1st comparative example.
The figure for demonstrating the deformation ¦ transformation of the upper surface part of
cMUT which concerns on a 2nd comparative example. The figure which shows the outline of an
example of cMUT which concerns on the 1st modification of the 1st Embodiment of this
invention. The figure which shows the outline of an example of cMUT which concerns on the 2nd
modification of the 1st Embodiment of this invention. The figure which shows the outline of
another example of cMUT which concerns on the 2nd modification of the 1st Embodiment of this
invention. The figure which shows the outline of an example of cMUT which concerns on the 2nd
Embodiment of this invention. The figure which shows the outline of an example of cMUT which
concerns on the 3rd Embodiment of this invention. The figure for demonstrating a mode that the
upper surface part of cMUT which concerns on the 3rd Embodiment of this invention deform ¦
transforms. The figure which shows the outline of an example of cMUT which concerns on the
4th Embodiment of this invention. The figure which shows the outline of an example of cMUT
which concerns on the 5th Embodiment of this invention.
[0011]
First Embodiment First, a first embodiment of the present invention will be described with
reference to the drawings. The outline of the configuration of the capacitive Micromachined
Ultrasonic Transducers (cMUT) 100, which is a capacitive ultrasonic generation unit according to
the present embodiment, is shown in FIG. FIG. 1A schematically shows a top view of the cMUT
100 viewed from the traveling direction of the ultrasonic wave emitted from the cMUT 100, and
FIG. 1B schematically shows a sectional view viewed from the side thereof.
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[0012]
As shown in this figure, in the cMUT 100 according to the present embodiment, a disk-shaped
bottom portion 120 is located on a substrate 110, for example. The substrate 110 and the
bottom portion 120 may be formed integrally or separately. An axis passing through the center
of the disc of the bottom portion 120 and perpendicular to the top surface of the substrate 110
is defined as a central axis C. Furthermore, the central axis C side is defined as the inner side, and
the side away from the central axis C is defined as the outer side. The bottom portion 120 has,
for example, a disk-shaped lower electrode 122. The circular center of the lower electrode 122
coincides with the central axis C. On the bottom surface portion 120, a support portion 130
having a hollow cylindrical shape is formed. The central axis of the cylinder formed by the inner
side surface of the support portion 130 and the central axis of the cylinder formed by the outer
side surface both coincide with the central axis C.
[0013]
A disc-shaped upper surface portion 140 is formed on the support portion 130. The circular
center of the upper surface portion 140 also coincides with the central axis C. The upper surface
portion 140 includes an upper electrode 142 and a vibrating mass portion 144. The upper
electrode 142 has an annular shape. The central edge of the annular upper electrode 142 is
referred to as the inner end, and the outer circumferential edge is referred to as the outer end.
The inner end and the outer end of the upper electrode 142 according to the present
embodiment are both circular. The center of the circle at the inner end and the center of the
circle at the outer end coincide with the central axis C. The vibrating mass portion 144 is formed
inside the inner end of the upper electrode 142. The vibrating mass portion 144 also has a disk
shape, and the center thereof coincides with the central axis C. Here, the vibrating mass portion
144 is nonconductive. According to the above configuration, the cMUT 100 has a cavity 150
which is a cylindrical space surrounded by the bottom surface portion 120, the support portion
130, and the top surface portion 140. The cavity 150 is a vacuum.
[0014]
In the cMUT 100 according to the present embodiment, the diameter of the lower electrode 122
(D1 in FIG. 1A) is larger than the diameter of the cavity 150 (D2 in FIG. 1A). Also, the diameter of
the outer end of the upper electrode 142 is larger than the diameter of the cavity 150. Also, the
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diameter of the inner end of the upper electrode 142 is smaller than the diameter of the cavity
150. Furthermore, when the line connecting the middle point between the inner end and the
outer end of the upper electrode 142 is referred to as a width center Cw1, the diameter of the
circle drawn by the width center Cw1 is smaller than the diameter of the cavity 150.
[0015]
That is, when viewed from the normal direction of the substrate 110 (when the lower surface
120 is viewed from the upper surface 140), the upper electrode 142 is positioned so as to
overlap with a part of the cavity 150. Also, when viewed in the normal direction of the substrate
110, the outer end of the upper electrode 142 is positioned to overlap the support portion 130,
and the inner end is positioned to overlap the cavity 150. Therefore, the upper electrode 142 has
an outer end positioned to overlap the support 130 as viewed in the normal direction of the
substrate 110 and an inner end positioned to overlap the support 130 as viewed in the normal
direction to the substrate 110. As an end. Further, a width center Cw1, which is a line connecting
middle points of the inner end and the outer end of the upper electrode 142, is positioned to
overlap the cavity 150 when viewed from the normal direction of the substrate 110.
[0016]
Further, in the cMUT 100 according to the present embodiment, the Young's modulus of the
vibrating mass portion 144 is larger than the Young's modulus of the other portion of the upper
surface portion 140. Furthermore, the specific gravity of the vibrating mass portion 144 is larger
than the specific gravity of the other portion of the upper surface portion 140. Here, the upper
electrode 142 is formed of, for example, aluminum, the vibrating mass portion 144 is formed of,
for example, SiO 2, and the other portion of the upper surface portion 140 is formed of, for
example, SiN.
[0017]
In the present embodiment, although the vibrating mass portion 144 is formed inside the inner
end of the upper electrode 142, the outer peripheral edge of the vibrating mass portion is formed
outer than the inner end of the upper electrode 142. It may be done. That is, the upper electrode
142 and the vibrating mass portion 144 may be disposed at an overlapping position as viewed
from the direction of emission of the ultrasonic waves.
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[0018]
Thus, for example, the lower electrode 122 functions as a first electrode, for example, the bottom
surface 120 functions as a bottom surface, for example, the upper electrode 142 functions as a
second electrode, for example, the upper surface 140 The region of the vibrating mass 144
functions as the central region, for example, the region of the upper surface 140 including the
upper electrode 142 other than the vibrating mass 144 functions as the peripheral region, for
example, the support 130 Functions as a support, and for example, the cavity 150 functions as
an air gap.
[0019]
Next, the operation of the cMUT 100 according to the present embodiment will be described.
When a voltage is applied between the lower electrode 122 and the upper electrode 142, an
electrostatic attraction force is exerted between the lower electrode 122 and the upper electrode
142 which are opposed to each other. Here, the substrate 110 is sufficiently hard, and the lower
electrode 122 and the bottom portion 120 including the same hardly deform. On the other hand,
the upper surface portion 140 including the upper electrode 142 is easily deformed. Therefore,
when the electrostatic attraction is applied, the upper surface portion 140 including the upper
electrode 142 is deformed. When the voltage applied between the lower electrode 122 and the
upper electrode 142 is alternating current, the electrostatic attraction force acting between the
lower electrode 122 and the upper electrode 142 changes with the change in voltage, and the
upper surface portion 140 vibrates. Do. The cMUT 100 generates an ultrasonic wave by the
vibration of the upper surface portion 140. In general, the vibrating upper surface portion 140 is
also referred to as a "membrane" or a "vibrating membrane" or the like. Moreover, the part
containing the bottom face part 120, the support part 130, the top face part 140, the cavity 150
grade ¦ etc., May be collectively called a "cell."
[0020]
The polarity of the voltage applied to the lower electrode 122 and the upper electrode 142 does
not matter. Between the lower electrode 122 and the upper electrode 142, if there is a potential
difference between the opposing lower electrode 122 and the upper electrode 142, regardless of
whether a positive potential or a negative potential is applied to any of the electrodes. There is an
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electrostatic attraction that is attractive. The electrostatic attraction force F (N) of charges of
different polarities is defined by the following equation (1). Here, A is the area of the electrode (m
<2>), V is the applied voltage (V), l is the distance between the electrodes (m), ε0 is the vacuum
dielectric constant, and εs is the dielectric ratio of the medium.
[0021]
In order to vibrate the upper surface portion 140 of the cMUT 100 efficiently, a DC bias voltage
is applied between the lower electrode 122 and the upper electrode 142. When a DC bias voltage
is applied, the top surface portion 140 is displaced to the bottom surface portion 120 side.
Energy is stored in the cMUT 100 by the displacement of the upper surface portion 140. Here,
when a pulse voltage is superimposed on the DC bias voltage between the lower electrode 122
and the upper electrode 142, the upper surface portion 140 largely vibrates so as to release the
energy stored by the DC bias voltage.
[0022]
Here, as an example, the situation in the case where the primary vibration mode is excited in the
upper surface portion 140 of the cMUT 100 according to the present embodiment will be
described with reference to FIG. In this embodiment, as shown at the top of FIG. 2, a +50 V DC
bias voltage is applied between the lower electrode 122 and the upper electrode 142, and a sine
wave AC pulse wave (having an amplitude of 50 V) Drive voltage is superimposed on one wave.
The displacement of the upper surface part 140 at this time is described in the middle of FIG. 2,
and the generated sound pressure is described in the lower part of FIG.
[0023]
At time t1 immediately before applying the drive voltage, the upper surface portion 140 is
stationary in a state where the DC bias voltage is applied, that is, the upper surface portion 140
is displaced toward the bottom surface portion 120 as described above. At this time, the sound
pressure is zero. When an AC pulse wave is applied in this state, the top surface portion 140 is
further displaced to the bottom surface portion 120 side. At time t2 at which the drive voltage
reaches a maximum value, the displacement speed of the upper surface portion 140 is
maximized. At time t2 at which the displacement speed of the upper surface portion 140 is
maximized, the sound pressure once exhibits a positive maximum value. The maximum negative
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pressure is obtained at time t4, which is slightly delayed from time t3 at which the membrane is
displaced most to the negative side. At time t5 at which the drive voltage takes a minimum value
of 0 V, the upper surface portion 140 is displaced to a position lifted above the initial position. At
time t6 slightly behind t5, the maximum positive pressure is obtained.
[0024]
Next, the deformation of the upper surface portion 140 will be further described with reference
to FIG. When a voltage is applied between the lower electrode 122 and the upper electrode 142,
the strongest electrostatic attractive force acts on the width center Cw1 of the annular upper
electrode 142. On the other hand, since the portion where the electrode is not formed inside the
inner end of the upper electrode 142 does not have conductivity, no electrostatic attraction force
acts on that portion. In addition, the vibrating mass portion 144 has a Young's modulus larger
than that of the other portion of the upper surface portion 140. Therefore, the portion of the
vibrating mass portion 144 is higher in rigidity and less likely to be deformed than the other
portions of the top surface portion 140.
[0025]
As described above, when a DC bias voltage is applied between the lower electrode 122 and the
upper electrode 142, and when a voltage is further superimposed on the DC bias voltage, the
upper surface portion 140 of the cMUT 100 is as shown in FIG. Transform into That is, the
portion including the upper electrode 142 is curved in the direction of the bottom surface
portion 120, and the central portion including the vibrating mass portion 144 which is difficult
to deform such as a curve is displaced in the direction of the bottom surface portion 120 while
maintaining the flat shape. Do.
[0026]
Thereafter, when the voltage applied between the lower electrode 122 and the upper electrode
142 is reduced, the upper surface portion 140 tries to return to the original shape, and the
inertial force is opposite to that of the bottom surface portion 120 as shown in FIG. Displace in
the direction. Also at this time, the central portion including the vibrating mass portion 144
maintains a flat plate shape. Therefore, ultrasonic waves generated by deformation as shown in
FIGS. 3A and 3B have a flat wave front parallel to the flat plate formed by the vibrating mass
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portion 144, and the normal direction of the flat plate Proceed to
[0027]
For comparison, consider the case of the first comparative example in which the upper surface
portion 140 does not temporarily have the vibrating mass portion 144, the upper surface portion
140 has uniform rigidity, and includes the disk-shaped upper electrode 142. In such a first
comparative example, as shown in FIG. 4A, the upper electrode 142 receives the strongest
electrostatic attraction at the central axis C portion, and the upper surface portion 140 has the
central axis C portion. Are most displaced in the direction of the bottom 120 and curve as a
whole. In addition, when the voltage applied between the lower electrode 122 and the upper
electrode 142 is reduced, the upper surface portion 140 tries to return to the original shape and
is opposite to the bottom surface portion 120 by inertia force as shown in FIG. Displace in the
direction. At this time, the top surface portion 140 is displaced to a position farthest from the
bottom surface portion 120 at the portion of the central axis C, and is curved as a whole. The
ultrasonic waves generated by changing the shapes shown in FIGS. 4A and 4B travel in the
normal direction of the upper surface portion 140. Therefore, the ultrasonic waves emitted by
the cMUT according to the first comparative example have their wave fronts made convex or
concave, and diffuse or converge. For a cMUT having no vibrating mass portion 144 and the
upper electrode 142 formed on one surface of the upper surface portion 140, see, for example,
Japanese Patent No. 4370120.
[0028]
As described above, in the cMUT 100 according to the present embodiment, the presence of the
vibrating mass portion 144 can emit an ultrasonic wave having a flat (or substantially flat)
wavefront. The flat wavefront travels along the normal direction of the upper surface of the
vibrating mass 144 without diffusing or converging (the wavefront is orthogonal to the normal
direction). Therefore, the cMUT 100 according to this embodiment can generate an ultrasonic
wave with high directivity. That is, this configuration improves the directivity of the generated
ultrasonic waves and suppresses the reduction in the generated sound pressure.
[0029]
It is preferable that the diameter of the vibrating mass portion 144 be sufficiently large so that
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the emitted ultrasonic wave has a sufficiently wide flat wave front. In addition, when the specific
gravity of the vibrating mass portion 144 that vibrates to generate ultrasonic waves is larger
than the specific gravity of the other portion of the upper surface portion 140, it has the function
of increasing the mass of the vibrating portion. As a result, the kinetic energy of the vibrating
portion increases, and the output of the ultrasonic wave generated by the cMUT 100 increases.
[0030]
In the present embodiment, the diameter of the outer end of the upper electrode 142 is larger
than the diameter of the cavity 150. For comparison, the case of the second comparative example
in which the outer end of the upper electrode 142 is closer to the central axis C than the outer
periphery of the cavity 150 will be considered. In such a second comparative example, since the
strongest electrostatic attraction force acts on the width center Cw1 of the annular upper
electrode 142, the portion of the width center Cw1 of the upper electrode 142 is displaced the
most. As a result, as shown in FIG. 5, the top surface portion 140 of the second comparative
example has a wavy shape. However, even in this case, as described above, since the vibrating
mass portion 144 maintains the flat shape and is displaced in parallel, the generated ultrasonic
waves have their wave fronts made flat and parallel to the normal direction of the flat plate.
Progress and directionality is high. That is, the problem of the present invention is solved even
with the configuration of the second comparative example.
[0031]
In the second comparative example, the displacement of the vibrating mass portion 144 is
smaller as compared to the case of the present embodiment described with reference to FIG.
Therefore, the output of the ultrasonic wave emitted by the cMUT according to the second
comparative example is smaller than the output of the ultrasonic wave emitted by the cMUT 100
according to the present embodiment. The cMUT according to the second comparative example
is more suitable than the cMUT 100 according to the present embodiment in the case where the
output of ultrasonic waves may be smaller.
[0032]
In the present embodiment, the diameter of the outer end of the upper electrode 142 is larger
than the diameter of the cavity 150. As can be seen from the comparison with the second
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comparative example described above, in the cMUT 100 according to the present embodiment,
since the upper electrode 142 is deformed with the boundary between the support 130 and the
cavity 150 as a fulcrum, The amplitude is larger than that of the second comparative example.
That is, the configuration of the cMUT 100 according to the present embodiment, in particular,
the positional relationship between the cavity 150 and the upper electrode 142 contributes to
increasing the displacement of the vibrating mass portion 144 and emitting a strong ultrasonic
wave. Therefore, the diameter of the outer end of the upper electrode 142 is preferably larger
than the diameter of the cavity 150.
[0033]
First Modification of First Embodiment Next, a first modification of the first embodiment of the
present invention will be described. Here, in the description of the present modification, only
differences from the first embodiment will be described, and the same parts will be denoted by
the same reference numerals and descriptions thereof will be omitted. In this modification, in
order to increase the rigidity of the central portion of the upper surface portion 140, the central
portion is made thicker than the other portions.
[0034]
FIG. 6 (a) is a top view of the cMUT 100 according to this modification as viewed from the
traveling direction of the applied ultrasonic waves, and FIG. 6 (b) is a cross-sectional view as
viewed from the side thereof. Show. The cMUT according to the present modification is obtained
by replacing the vibrating mass portion 144 in the first embodiment with a vibrating mass
portion 146. Here, the vibrating mass portion 146 is made of a material having the same Young's
modulus as the other portions of the upper surface portion 140, and is thicker than the other
portions of the upper surface portion 140. The portion of the vibrating mass portion 146 is
thicker than the other portions of the top surface portion 140, and therefore has high rigidity
and is less likely to be deformed. The configuration other than the vibrating mass unit 146 of the
cMUT 100 according to this modification is the same as that of the first embodiment.
[0035]
As described above, the vibrating mass portion 146 is less likely to be deformed than the other
portions of the upper surface portion 140. For this reason, when an AC voltage is applied
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between the lower electrode 122 and the upper electrode 142, the vibrating mass portion 146
vibrates while maintaining a flat plate shape. Therefore, in the cMUT 100 according to the
present modification, as in the cMUT 100 according to the first embodiment, the wave front is
flat, travels parallel to the normal direction of the flat, and emits ultrasonic waves with high
directivity. can do.
[0036]
Second Modified Example of First Embodiment Next, a second modified example of the first
embodiment of the present invention will be described. Here, in the description of the present
modification, only differences from the first embodiment will be described, and the same parts
will be denoted by the same reference numerals and descriptions thereof will be omitted. The
cMUT 100 according to the present modification differs from the cMUT 100 according to the
first embodiment in the outer shape, and also the shapes of the upper electrode 142, the
vibrating mass portion 144, and the like.
[0037]
FIG. 7 (a) schematically shows a top view of the cMUT 100 according to the present modification
as viewed from the traveling direction of the applied ultrasonic waves, and FIG. 7 (b)
schematically shows a cross-sectional view as viewed from the side thereof. Show. The cMUT
according to this modification has a rectangular shape as viewed from the direction of travel of
the ultrasonic waves. In the present modification, the vibrating mass portion 144 has a
rectangular plate shape. The upper electrode 142 has a configuration in which four trapezoidal
electrodes are gathered as shown in FIG. 7 (a). The shorter side of the parallel sides of each
trapezoidal electrode is located on the upper electrode 142 side, and the longer side is located on
the support 130 side. The non-parallel sides of the trapezoidal electrodes face the adjacent sides
with a gap. Therefore, as shown in FIG. 7A, the entire upper electrode 142 has a substantially
rectangular shape having an inner end and an outer end, and the end portion of the vibration
mass 144 is formed on the inner end side. It is configured to support. A slit is inserted in a
portion corresponding to the diagonal of the substantially rectangular upper electrode 142. The
portion corresponding to the slit contributes to increasing the displacement of the upper surface
portion 140. In addition, the shapes of the lower electrode 122 and the cavity 150 viewed from
the traveling direction of the ultrasonic wave irradiated from the cMUT are also quadrangle.
[0038]
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Also according to this modification having such a configuration, the vibrating mass portion 144
is less likely to be deformed than the other portions of the upper surface portion 140 as in the
first embodiment. For this reason, the cMUT 100 can emit an ultrasonic wave whose wavefront is
flat and travels parallel to the normal direction of the flat plate and has high directivity.
[0039]
Note that, in the present modification described with reference to FIG. 7, the upper electrode 142
and the vibrating mass portion 144 are disposed at an overlapping position when viewed from
the emission direction of ultrasonic waves, that is, the normal direction of the substrate 110. But
it does not have to overlap. For example, as in the first embodiment, the vibrating mass portion
144 may be disposed inside the inner end of the upper electrode 142. Also in this case, the same
function as in the first embodiment or the present modification is provided.
[0040]
Furthermore, the top view of an example of cMUT 100 which concerns on another modification
is shown in FIG. In this example, as shown in FIG. 8, the shapes of the cMUT and each part are
hexagonal as viewed from the traveling direction of the emitted ultrasonic wave. Also in this case,
the same operation and function as the cMUT 100 according to the above-described modified
example and the first embodiment are performed. Further, the shape is not limited to a
quadrangle or a hexagon, and may be a triangle, an octagon, or any other shape, and the upper
electrode 142 may be a shape having a slit in a part thereof. In any of the above cases, the same
effects as in the first embodiment can be obtained.
[0041]
Second Embodiment Next, a second embodiment of the present invention will be described. Here,
in the description of the present embodiment, only differences from the first embodiment will be
described, and the same parts will be denoted by the same reference numerals and descriptions
thereof will be omitted. The outline of the configuration of the cMUT 100 according to the
present embodiment is shown in FIG. FIG. 9A schematically shows a top view of the cMUT 100 as
viewed from the traveling direction of the ultrasonic wave emitted from the cMUT 100, and FIG.
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9B schematically shows a cross-sectional view as viewed from the side thereof.
[0042]
The cMUT 100 according to the present embodiment is obtained by replacing the ring-shaped
upper electrode 142 in the first embodiment with a disk-shaped upper electrode 242. Here, the
upper electrode 242 is located at the central portion of the upper surface portion 140, and is
disposed at a position overlapping the vibrating mass portion 144 when viewed from the
traveling direction of the ultrasonic waves, ie, the normal direction of the substrate 110. Also in
the present embodiment, the upper electrode 242 is positioned so as to overlap with a portion of
the cavity 150 when viewed in the normal direction of the substrate 110.
[0043]
When a voltage is applied between the lower electrode 122 and the upper electrode 242, an
electrostatic attractive force is generated at a portion where the lower electrode 122 and the
upper electrode 242 face each other. The vibrating mass portion 144 has a Young's modulus
larger than that of the other portion of the upper surface portion 140, and thus is less likely to
be deformed than the other portion. As a result, the top surface portion 140 of the cMUT 100 is
deformed in the same manner as the cMUT 100 according to the first embodiment described
with reference to FIG. That is, the central portion including the vibrating mass portion 144
having a high Young's modulus and difficult to deform such as a curve maintains a flat plate
shape, a peripheral portion having a low Young's modulus and easy to deform is curved, and the
upper surface portion 140 vibrates.
[0044]
Therefore, when an alternating voltage is applied between the lower electrode 122 and the upper
electrode 242, the cMUT 100 according to the present embodiment has a flat wave front, and
travels parallel to the normal direction of the flat plate Is high, can emit ultrasound.
[0045]
In FIG. 9, the size of the upper electrode 242 matches the size of the vibrating mass portion 144,
and the position of the upper electrode 242 also matches the position of the vibrating mass
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portion 144.
However, the upper electrode 242 may be larger or smaller than the vibrating mass portion 144.
Further, the positions of the upper electrode 242 and the vibrating mass portion 144 may not be
identical when viewed from the normal direction of the substrate.
[0046]
Third Embodiment Next, a third embodiment of the present invention will be described. Here, in
the description of the present embodiment, only differences from the first embodiment will be
described. An outline of the configuration of the cMUT 300 according to the present embodiment
is shown in FIG. FIG. 10 (a) schematically shows a top view of the cMUT 300 as viewed from the
traveling direction of the ultrasonic wave irradiated from the cMUT 300, and FIG. 10 (b)
schematically shows a cross-sectional view as viewed from the side thereof.
[0047]
As shown to this figure, in cMUT300 which concerns on this embodiment, the disk-shaped
bottom face part 320 is formed on the board ¦ substrate 310. As shown in FIG. An axis passing
through the center of the disc of the bottom portion 320 and perpendicular to the disc is defined
as a central axis C. The bottom portion 320 has, for example, a ring-shaped lower electrode 322.
The center side edge of the ring-shaped lower electrode 322 is referred to as the inner end, and
the outer side edge is referred to as the outer end. The inner end and the outer end of the lower
electrode 322 according to the present embodiment are circular. The center of the circle at the
inner end and the center of the circle at the outer end coincide with the central axis C.
[0048]
On the bottom surface portion 320, a support portion 330 having a hollow cylindrical shape is
formed. Here, the central axis of the inner side surface of the support portion 330 and the central
axis of the outer side surface coincide with the central axis C. A disk-shaped upper surface
portion 340 is formed on the support portion 330. The upper surface portion 340 has a diskshaped upper electrode 342. The circular centers of the upper surface portion 340 and the
upper electrode 342 also coincide with the central axis C. Thus, the cMUT 300 has a cavity 350
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which is a cylindrical space surrounded by the bottom surface 320, the support 330, and the top
surface 340. The cavity 350 is a vacuum.
[0049]
In the cMUT 300 according to the present embodiment, the diameter of the outer end of the
lower electrode 322 is larger than the diameter of the cavity 350. Also, the diameter of the inner
end of the lower electrode 322 is smaller than the diameter of the cavity 350. Furthermore, when
the line connecting the middle point between the inner end and the outer end of the lower
electrode 322 is referred to as a width center Cw3, the diameter of the circle drawn by the width
center Cw3 is smaller than the diameter of the cavity 350. Also, the diameter of the upper
electrode 342 is larger than the diameter of the cavity 350.
[0050]
That is, when viewed from the normal direction of the substrate 310, the inner end of the lower
electrode 322 is positioned to overlap the cavity 350, and the outer end of the lower electrode
322 is positioned to overlap the support 330, and the width center Cw3 The circle drawn is
positioned to overlap the cavity 350. Further, the outer peripheral portion of the upper electrode
342 is positioned so as to overlap the support portion 330 when viewed in the normal direction
of the substrate 310.
[0051]
Thus, for example, the lower electrode 322 functions as a first electrode, for example the bottom
surface 320 functions as a bottom surface, for example the upper electrode 342 functions as a
second electrode, for example the upper surface 340 It functions as an upper surface part, for
example, the support part 330 functions as a support part, for example, the cavity 350 functions
as a gap part. The lower electrode 322 has an inner end located so as to overlap the cavity 350
when viewed from the normal direction of the substrate 310 and an outer end located so as to
overlap the support 330 as an end portion. All of the outer periphery 342 is positioned to
overlap the support 330 as viewed in the normal direction of the substrate 310.
[0052]
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Next, the operation of the cMUT 300 according to the present embodiment will be described.
When a voltage is applied between the lower electrode 322 and the upper electrode 342, the
strongest electrostatic attractive force acts on the width center Cw3 of the lower electrode 322.
On the other hand, the portion inside the inner end of the lower electrode 322, that is, the central
portion surrounded by the annular lower electrode 322 has no conductivity, and thus no
electrostatic attraction force acts on the central portion. Therefore, as shown in FIG. 11, a portion
of the upper surface portion 340 facing the lower electrode 322 is curved. On the other hand, a
portion of the upper surface portion 340 facing the central portion surrounded by the annular
lower electrode 322 is not curved. Thus, the central portion is displaced in parallel while
maintaining the flat shape.
[0053]
Therefore, when an alternating voltage is applied between the lower electrode 322 and the upper
electrode 342, the cMUT 300 according to the present embodiment has a flat wave front, and
travels parallel to the normal direction of the flat plate Is high, can emit ultrasound.
[0054]
In the present embodiment, the diameter of the outer end of the upper electrode 342 is larger
than the diameter of the cavity 350.
As in the description of the first embodiment, in the cMUT 300 according to the present
embodiment as well, the upper electrode 342 is deformed with the boundary between the
support portion 330 and the cavity 350 as a fulcrum, so the amplitude of the upper surface
portion 340 is growing. That is, the positional relationship between the support portion 330 and
the cavity 350 and the upper electrode 342 in the configuration of the cMUT 300 according to
the present embodiment contributes to the emission of strong ultrasonic waves by increasing the
amplitude of the upper surface portion 340. Further, it is preferable that the diameter of the nonconductive central portion surrounded by the lower electrode 322 be sufficiently large so that
the emitted ultrasonic wave has a sufficiently wide flat wave front.
[0055]
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Fourth Embodiment Next, a fourth embodiment of the present invention will be described. Here,
in the description of the present embodiment, only differences from the third embodiment will be
described, and the same parts will be denoted by the same reference numerals and descriptions
thereof will be omitted. An outline of the configuration of the cMUT 300 according to the present
embodiment is shown in FIG. FIG. 12 schematically shows a cross-sectional view of the cMUT
300 as viewed from the side. The cMUT 300 according to the present embodiment is obtained by
replacing the circular upper electrode 342 in the third embodiment with an annular upper
electrode 442.
[0056]
The inner edge of the annular upper electrode 442 is referred to as the inner end, and the outer
edge is referred to as the outer end. The inner end and the outer end of the upper electrode 442
according to the present embodiment are circular. The center of the circle at the inner end and
the center of the circle at the outer end coincide with the central axis C. Also, the diameter of the
outer end of the upper electrode 442 is larger than the diameter of the cavity 350. Also, the
diameter of the inner end of the upper electrode 442 is smaller than the diameter of the cavity
350. Furthermore, when the line connecting the middle point between the inner end and the
outer end of the upper electrode 442 is referred to as a width center Cw4, the diameter of the
circle drawn by the width center Cw4 is smaller than the diameter of the cavity 350.
Furthermore, the diameter of the inner end of the upper electrode 442 is larger than the
diameter of the inner end of the lower electrode 322.
[0057]
That is, when viewed from the normal direction of the substrate 310, the upper electrode 442
has an inner end positioned to overlap with the cavity 350 and an outer end positioned to
overlap with the support portion 330 as an end. A circle drawn by the center Cw 4 is positioned
to overlap the cavity 350. Further, the inner end of the upper electrode 442 is positioned to
overlap the lower electrode 322 when viewed in the normal direction of the substrate 310.
[0058]
When a voltage is applied between the lower electrode 322 and the upper electrode 442, an
electrostatic attraction force acts on a portion where the lower electrode 322 and the upper
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electrode 442 are opposed to each other. On the other hand, the portion inside the inner end of
the upper electrode 442, that is, the central portion surrounded by the ring-shaped upper
electrode 442, has no conductivity, so no electrostatic attraction force acts on the central portion.
Therefore, when an alternating voltage is applied between the lower electrode 322 and the upper
electrode 442, the lower electrode 322 and the upper electrode 442 in the upper surface portion
340 face each other, and between the lower electrode 322 and the upper electrode 442 Where
the cavity 350 is present, the top portion 340 is curved and vibrates. On the other hand, the
central portion of the upper surface portion 340 where the lower electrode 322 and the upper
electrode 442 do not face each other is not curved. As described above, when an alternating
voltage is applied between the lower electrode 322 and the upper electrode 442, the upper
surface portion 340 vibrates such that the central portion maintains a flat shape and is displaced
in parallel.
[0059]
Therefore, when an AC voltage is applied between the lower electrode 322 and the upper
electrode 442, the cMUT 300 according to the present embodiment has a flat wave front, which
travels parallel to the normal direction of the flat plate, and has directivity Is high, can emit
ultrasound.
[0060]
Here, the fact that the inner end of the lower electrode 322 is inside the inner end of the upper
electrode 442 has the following meaning.
Increasing the size of the lower electrode 322 that does not contribute to the mechanical
characteristics of the upper surface portion 340 corresponds to increasing the area on which the
electrostatic attractive force acts, that is, increasing the electrode area A in the equation (1). The
electrostatic attraction force F can be increased by increasing the electrode area A. Therefore, the
inner end of the lower electrode 322 is inside the inner end of the upper electrode 442, and
increasing the electrode area A contributes to increasing the electrostatic attraction force F.
[0061]
Fifth Embodiment Next, a fifth embodiment of the present invention will be described. Here, in
the description of the present embodiment, only differences from the fourth embodiment will be
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described, and the same parts will be denoted by the same reference numerals and descriptions
thereof will be omitted. The outline of the cross section which looked at cMUT300 which
concerns on this embodiment from the side is shown in FIG. The cMUT 300 according to the
present embodiment is obtained by replacing the ring-shaped lower electrode 322 in the third
embodiment with a disk-shaped lower electrode 522.
[0062]
As shown in FIG. 13, the center of the disk-shaped lower electrode 522 coincides with the central
axis C. In addition, the diameter of the disk-shaped lower electrode 522 is larger than the
diameter of the cavity 350. That is, the circumferential portion of the lower electrode 522 is
positioned to overlap the support portion 330 when viewed in the normal direction of the
substrate 310. In addition, the upper electrode 442 has an inner end located so as to overlap the
cavity 350 when viewed in the normal direction of the substrate 310 and an outer end located so
as to overlap the support portion 330 as an end. Furthermore, the diameter of the circle drawn
by the width center Cw4, which is a line connecting the middle points of the inner end and the
outer end of the upper electrode 442, is smaller than the diameter of the cavity 350. That is, the
circle drawn by the width center Cw4 is positioned to overlap the cavity 350.
[0063]
When a voltage is applied between the lower electrode 522 and the upper electrode 442, an
electrostatic attraction force acts on a portion where the lower electrode 522 and the upper
electrode 442 are opposed to each other. On the other hand, since the portion inside the inner
end of the upper electrode 442, ie, the central portion surrounded by the ring-shaped upper
electrode 442, does not have conductivity, no electrostatic attraction force acts on the central
portion.
[0064]
Accordingly, in the upper surface portion 340, the lower electrode 522 and the upper electrode
442 face each other, and the upper surface portion 340 is curved where the cavity 350 exists
between the lower electrode 522 and the upper electrode 442. On the other hand, the central
portion of the upper surface portion 340 where the lower electrode 522 and the upper electrode
442 do not face each other is not curved. Thus, the central portion is displaced in parallel while
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maintaining the flat shape. When an AC voltage is applied between the lower electrode 522 and
the upper electrode 442, the upper surface portion 340 vibrates such that the central portion
maintains a flat shape and is displaced in parallel.
[0065]
Therefore, when an AC voltage is applied between the lower electrode 522 and the upper
electrode 442, the cMUT 300 according to the present embodiment has a flat wave surface, and
the cMUT 300 travels parallel to the normal direction of the flat plate. Is high, can emit
ultrasound.
[0066]
[Combination of Each Embodiment] Also in the third embodiment, the fourth embodiment, or the
fifth embodiment, as in the first embodiment, in the central portion of the upper surface portion,
the other portion of the upper surface portion A vibrating mass portion having a larger Young's
modulus may be provided to increase the rigidity of the central portion.
Also in the second embodiment, the third embodiment, the fourth embodiment, or the fifth
embodiment, as in the first modification of the first embodiment, the central portion of the upper
surface portion The thickness may be increased to increase the stiffness of the central portion. In
addition, the Young's modulus of the central portion may be increased, and the thickness may
also be increased. With any of these, the central portion is more difficult to bend, and the
directivity of the generated ultrasonic waves is further improved.
[0067]
Further, also in the second embodiment, the third embodiment, the fourth embodiment, or the
fifth embodiment, the configuration of each portion is not limited to a circular shape, and the
second modification of the first embodiment As in the example, it may be a quadrangle or a
hexagon, or a polygon such as a triangle, an octagon or the like, or any other shape. Also in the
first embodiment, the second embodiment, the third embodiment, the fourth embodiment, or the
fifth embodiment, as in the second modification of the first embodiment, The upper electrode
may have a shape provided with a slit. In any of these cases, the same effects as in each
embodiment can be obtained.
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[0068]
The present invention is not limited to the above embodiment as it is, and at the implementation
stage, the constituent elements can be modified and embodied without departing from the scope
of the invention. In addition, various inventions can be formed by appropriate combinations of a
plurality of constituent elements disclosed in the above embodiment. For example, even if some
components are removed from all the components shown in the embodiment, the problems
described in the section of the problem to be solved by the invention can be solved and the
effects of the invention can be obtained. The configuration from which this component is deleted
can also be extracted as an invention. Furthermore, components in different embodiments may
be combined as appropriate.
[0069]
DESCRIPTION OF SYMBOLS 100 ... cMUT, 110 ... board ¦ substrate, 120 ... bottom part, 122 ...
lower electrode, 130 ... support part, 140 ... upper surface part, 142 ... upper electrode, 144 ...
vibration mass part, 146 ... vibration mass part, 150 ... cavity, 240 ... upper surface portion 242
upper electrode 300 cMUT 310 substrate base 320 bottom electrode 322 lower electrode 330
support portion 340 upper surface portion 342 upper electrode 350 cavity 442 upper electrode
, 522 ... lower electrode.
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