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JP2014140239

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DESCRIPTION JP2014140239
Abstract: The present invention provides an electromechanical transducer suitable for handling
elastic waves of relatively high frequency and capable of increasing the ratio of effective capacity
to parasitic capacity. An electromechanical transducer includes a first electrode (204), a vibrating
portion (203) supported by a supporting portion (206) with respect to the first electrode (204)
via a gap, and a second portion formed in the vibrating portion (203). And a wiring 202 formed
in the vibrating portion 203 and electrically connected to the second electrode 201. In the
vibrating portion 203, the second electrode 201 is disposed closer to the surface of the vibrating
portion 203 facing the first electrode 204 than the wiring 202. [Selected figure] Figure 1
Method of manufacturing capacitive type electromechanical transducer
[0001]
The present invention relates to an electromechanical transducer such as an ultrasonic
transducer and a method of manufacturing the same.
[0002]
An ultrasonic conversion device is a physical quantity conversion device that converts an
electrical signal into an ultrasonic signal or converts an ultrasonic signal into an electrical signal
in order to transmit or receive an ultrasonic wave. For example, the inside of a living body is noninvasive It is used for applications such as ultrasonic examination to be examined in
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In recent years, a capacitive ultrasonic transducer using semiconductor microfabrication
technology has been researched and developed. Since the capacitance type ultrasonic transducer
uses a lightweight diaphragm, it has excellent broadband characteristics in water and air as
compared to the conventional piezoelectric transducer.
[0003]
The basic structure and operation principle of a conventional capacitance type ultrasonic
transducer will be described with reference to the sectional view of FIG. When the ultrasonic
signal (vibration in the ultrasonic band) p (t) transmitted from the object 507 reaches the
vibrating unit 503 supported by the supporting unit 506 in a vibratable manner, the vibrating
unit 503 generates the ultrasonic signal p (t It vibrates according to the waveform of). As a result,
the distance between the upper electrode 501 on the vibrating portion 503 and the lower
electrode 504 disposed on the main surface of the substrate 505 changes by the displacement of
the vibrating portion. The capacitance Ca changes in accordance with the waveform of the
ultrasonic signal p (t). In the present specification, the main surface refers to the surface of the
structure constituting the conversion device that faces the object to which the conversion device
transmits or receives an elastic wave such as ultrasonic waves. Here, by applying an appropriate
DC voltage Vb between the upper electrode 501 and the lower electrode 504 using the DC
voltage source 508, the current generated by the change of the effective capacity Ca using the
current detector 509 The signal i (t) can be detected. However, since the current signal i (t) leaks
through the parasitic capacitance Cp, which is a capacitance between the wiring 502 electrically
connected to the upper electrode 501 and the lower electrode 504, the current signal i (t) The
size of) is reduced according to the size of the parasitic capacitance Cp. When the current signal i
(t) decreases, the intensity ratio (S / N ratio) of the current signal i (t) to noise decreases, and the
accuracy of detecting the received ultrasonic signal p (t) deteriorates. In order to suppress the
reduction of the current signal i (t), it is necessary to increase the ratio Ca / Cp of the effective
capacitance Ca to the parasitic capacitance Cp.
[0004]
As an example of the above-mentioned device, the sound wave conversion device described in
Patent Document 1 can be mentioned. As shown in FIG. 10 (b) of the top view and FIG. 10 (c) of
the cross-sectional view along the broken line G1-G2 of the top view, this sound wave conversion
device comprises a substrate 604 which also functions as a lower electrode, a diaphragm 603,
and a support. A portion 605 is included. The supporting portion 605 is disposed on the main
surface of the substrate 604, and the vibrating portion 603 is vibratably supported by the
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supporting portion 605 to form a gap with the substrate 604. The vibrating portion 603 is made
of a conductive material, and also functions as an upper electrode and a wire. The characteristic
of this sound wave conversion device is that the central portion 601 of the vibrating portion is
shaped so as to be pushed out toward the substrate 604 with respect to the peripheral portion
602 of the vibrating portion. Thus, the distance d4 between the peripheral portion 602 of the
vibrating portion serving as the wiring and the substrate 604 serving as the lower electrode is
longer than the distance d3 between the central portion 601 of the vibrating portion serving as
the upper electrode and the substrate 604 serving as the lower electrode. . Since the capacitance
is inversely proportional to the distance between the electrodes, the ratio Ca / Cp of the effective
capacitance Ca between the central portion 601 and the substrate 604 to the parasitic
capacitance Cp between the peripheral portion 602 and the substrate 604 is The vibration
portion 603 is larger than that in the flat shape.
[0005]
U.S. Patent No. 6,870,937
[0006]
However, in the sound wave conversion device described in Patent Document 1, the entire
vibrating portion 603 is limited to the conductor material, so the range of selection as a material
that can be selected from the viewpoint of density, rigidity, electrical resistance, etc. of the
vibrating film is small (design Low degree of freedom).
In addition, it is necessary to consider an insulation structure separately. As a result, the
structure for increasing the ratio Ca / Cp between the effective capacitance Ca and the parasitic
capacitance Cp is made more lightweight and highly rigid in the vibrating part, and the insulation
between the lower electrode and the upper electrode is improved, and the resistance of the upper
electrode It becomes difficult to reduce the rate.
[0007]
In view of the above problems, the electromechanical transducer according to the present
invention has the following features. An electromechanical transducer includes a first electrode, a
vibrating portion supported by a supporting portion with respect to the first electrode via a gap,
a second electrode formed in the vibrating portion, and the vibrating portion. And a wire
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electrically connected to the second electrode. Then, in the vibrating portion, the second
electrode is disposed closer to a surface of the vibrating portion facing the first electrode than
the wiring.
[0008]
According to the present invention, it is possible to select a material of lower density (light
weight) and higher Young's modulus (high rigidity) than the conductor. As a result, the degree of
freedom in designing the electromechanical transducer is increased, and the vibrating membrane
itself functions as an insulator, so that the insulation between the electrode on the substrate side
and the electrode on the vibrating membrane can be further enhanced. In addition, an optimal
material with low resistivity can be separately used as the electrode. In addition, the second
electrode, the wiring, and the vibrating portion can be made of different materials, and can be an
electromechanical transducer suitable for handling elastic waves such as ultrasonic waves having
a relatively high frequency. The ratio Ca / Cp of the capacitance Ca and the parasitic capacitance
Cp can be increased.
[0009]
The figure which shows the structure of one Embodiment of the electro-mechanical transducer of
this invention. Sectional drawing which shows the structure of other embodiment of the electromechanical transducer of this invention. The figure which shows the structure of Example 1 of
the electro-mechanical transducer of this invention. FIG. 7 is a cross-sectional view showing an
example of a manufacturing process of Example 1; The figure which shows the structure of
Example 2 of the electro-mechanical transducer of this invention. FIG. 7 is a cross-sectional view
showing an example of a manufacturing process of Example 2; FIG. 14 is a cross-sectional view
showing the manufacturing process of the modification of the second embodiment. The figure
which shows the structure of Example 3 of the electro-mechanical transducer of this invention.
FIG. 14 is a cross-sectional view showing an example of a manufacturing process of Example 3;
The figure explaining the ultrasonic transducer of background art.
[0010]
The main feature of the electro-mechanical transducer according to the present invention is that
the second electrode formed on the vibrating portion supported by the supporting portion via the
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air gap with respect to the first electrode is also formed on the vibrating portion. Rather, it is
disposed closer to the surface of the vibrating portion facing the first electrode. That is, in the
vibrating portion of the vibrating portion, the second electrode is arranged to be closer to the
surface of the vibrating portion facing the first electrode than the wiring. The term "vibratable
portion" refers to an inner portion of the vibrating portion surrounded by the support when
viewed in a direction perpendicular to the main surface of the vibrating portion. Based on this
idea, the specific embodiments and examples described below can be configured.
[0011]
The structure of an embodiment of the electro-mechanical transducer according to the present
invention will be described below with reference to FIG. As shown in FIG. 1 (a) of the top view
and FIG. 1 (b) of the cross-sectional view along the broken line A1-A2 of the top view, this
embodiment includes the upper electrode 101 which is the second electrode, the wiring 102, and
the vibrating portion. 103, a lower electrode 104 which is a first electrode, a substrate 105, a
support portion 106, and a connection line 107 are provided. The lower electrode 104 is
disposed on the main surface of the substrate 105, and the support portion 106 is disposed on
the main surface of the lower electrode 104 or the substrate 105 (here, the substrate 105). The
vibrating portion 103 is vibratably supported by the support portion 106 by forming an air gap
110 between the lower electrode 104 and the substrate 105. Here, the air gap 110 has a
cylindrical shape, but is not limited thereto. The upper electrode 101 and the wire 102 are
disposed in the vibrating portion 103, and the connection line 107 is disposed in the vibrating
portion 103 so as to electrically connect the upper electrode 101 and the wire 102. The upper
electrode 101 and the wiring 102 are made of a material whose resistivity is lower than that of
the vibrating portion 103.
[0012]
As a characteristic structure in the present invention, as shown in FIG. 1B, the upper electrode
101 is disposed closer to the surface of the vibrating portion 103 facing the lower electrode 104
than the wiring 102. By arranging the upper electrode 101 and the wiring 102 in this manner,
the distance between the upper electrode 101 and the lower electrode 104 can be made shorter
than the distance between the wiring 102 and the lower electrode 104.
[0013]
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The form in which the upper electrode 101 is disposed can be selected from the following three.
The first one is a mode in which the upper electrode 101 is disposed on the surface of the
vibrating portion 103 facing the lower electrode 104 as shown in FIG. 2A. Thereby, the effect of
increasing the effective capacitance Ca between the upper electrode 101 and the lower electrode
104 can be enhanced. The second one is a mode in which a portion of the vibrating portion 103
is disposed below the upper electrode 101 as shown in FIG. 1B. As a result, even when the
vibrating portion 103 contacts the lower electrode 104, a short circuit between the upper
electrode 101 and the lower electrode 104 can be prevented. The third one is a mode in which
the upper electrode 101 is disposed on the surface of the vibrating portion 103 facing the lower
electrode 104 as shown in FIG. 2B. By doing this, the effect of increasing the effective
capacitance Ca between the upper electrode 101 and the lower electrode 104 can be further
enhanced.
[0014]
Further, as for the position where the wiring 102 is disposed, as shown in FIG. 1B, the surface of
the wiring 102 facing the lower electrode 104 is closer to the lower electrode 104 than the
surface facing the lower electrode 104 of the vibrating portion 101. It should be arranged to be
far away. As shown in FIG. 1B, the wiring 102 can be most easily manufactured by arranging it
on the surface opposite to the surface facing the lower electrode 104 of the vibrating portion
103. Alternatively, as shown in FIG. 2B, by arranging the wiring 102 inside the vibrating portion
103, the effect of electrically insulating the wiring 102 from the outside can be simultaneously
obtained.
[0015]
According to such a configuration, it is possible to have a high rigidity and low density vibration
portion suitable for handling relatively high frequency ultrasonic waves, etc., and compared to
the distance between the upper electrode and the lower electrode. The distance between the
lower electrode and the lower electrode can be increased. Thereby, the effective capacitance Ca
can be increased or the parasitic capacitance Cp can be reduced, so that the ratio Ca / Cp of the
effective capacitance Ca to the parasitic capacitance Cp can be increased to improve the
conversion efficiency of the electromechanical transducer. Can. The operation is basically the
same as that described in the background art with reference to FIG. 10 (a).
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[0016]
Hereinafter, more specific examples will be described. Example 1 The structure of an
electromechanical transducer according to Example 1 of the present invention will be described
with reference to FIG. As shown in FIG. 3A of the top view and FIG. 3B of the cross-sectional view
taken along the broken line B1-B2 of the top view, the present embodiment includes the upper
electrode 201, the wiring 202, the vibrating portion 203, the lower electrode 204, A substrate
205, a support portion 206, connection lines 207, and an insulating layer 208 are included. The
lower electrode 204 is disposed on the main surface of the substrate 205, and the insulating
layer 208 is disposed on the lower electrode 204. The insulating layer 208 has a role of
preventing a short circuit between the upper electrode 201 and the lower electrode 204 if the
vibrating portion 203 and the upper electrode 201 are in contact with the lower electrode 204
side. The support portion 206 is disposed on the main surface of the insulating layer 208 (which
may be the substrate 205 or the lower electrode 204 depending on the structure), and the
vibrating portion 203 forms an air gap between the lower electrode 204 and the substrate 205
by the support portion 206. And is vibratably supported. The upper electrode 201 and the wiring
202 are disposed in the vibrating portion 203, and the upper electrode 201 and the wiring 202
are electrically connected by the connection wire 207 disposed in the vibrating portion 203. The
surface of the upper electrode 201 facing the lower electrode 204 exists in the same plane as the
surface of the vibrating portion 203 facing the lower electrode 204 and is exposed to the air gap.
[0017]
The shape of the lower electrode 204 can be selected as follows. It demonstrates using FIG.3 (c).
Similar to the insulating layer 208, when the lower electrode 204 is formed over the entire main
surface of the substrate 205, flatness is maintained, and a structure formed thereover can be
easily manufactured. Further, by removing the lower electrode 204 present only in the part
facing the wiring 202 which is the blackened part in FIG. 3C, the parasitic capacitance Cp
between the wiring 202 and the lower electrode 204 can be reduced. . Further, as shown by
reference numeral 204 in FIG. 3C, if the lower electrode 204 is formed only in the region directly
under the upper electrode 201, the parasitic capacitance Cp between the wiring 202 and the
lower electrode 204 is further reduced. Can.
[0018]
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An example of a method of manufacturing the electro-mechanical transducer according to this
embodiment will be described below with reference to FIG. As shown in FIG. 4A, in the substrate
205 on which the lower electrode 204, the insulating layer 208, and the sacrificial layer 209
having a shape corresponding to an air gap are formed, the upper electrode 201 is formed on
part of the main surface of the sacrificial layer 209. Form. Next, as shown in FIG. 4B, on the main
surface of the insulating layer 208, the sacrificial layer 209, and the upper electrode 201, an
elastic layer 210 to be a vibrating portion and a supporting portion is formed, and the sacrificial
layer 209 is removed. Next, as shown in FIG. 4C, the connection line 207 is formed on a part of
the elastic layer 210, and the wiring 202 is formed on a part of the main surface of the elastic
layer 210.
[0019]
As the substrate 205, a substrate made of a semiconductor material or insulator material having
high resistivity, for example, single crystal silicon or silicon oxide is preferably used. On the other
hand, when a substrate made of a conductive material is used as the substrate 205, another
insulating layer may be formed between the substrate 205 and the lower electrode 204, or the
substrate 205 may be used as the lower electrode 204. For the lower electrode 204, it is
preferable to use a conductor material with low resistivity, such as gold, aluminum, titanium or
the like. For the insulating layer 208, an insulator material such as silicon nitride is preferably
used. The sacrificial layer 209 is formed at a position where an air gap is desired to be provided
with the vibrating portion 203. The material used for the sacrificial layer 209 needs to use a
material different from the material of the upper electrode 201 and the elastic layer 210 in order
to selectively remove the sacrificial layer 209 later. For example, when aluminum is used for the
upper electrode 201 and silicon nitride is used for the elastic layer 210, polycrystalline silicon is
preferably used for the sacrificial layer 209. For the elastic layer 210, it is preferable to use a
semiconductor material or an insulating material such as silicon nitride, which has a high
Young's modulus, a low density, and a high resistivity.
[0020]
In order to remove the sacrificial layer 209, as shown in FIG. 4B, the etching hole 211 is formed
in the elastic layer 210, and the etching material is supplied from the etching hole 211. As the
etching material, it is necessary to use a material that easily reacts with the material of the
sacrificial layer 209 and hardly reacts with the materials of the upper electrode 201, the
insulating layer 208, and the elastic layer 210. For example, when polycrystalline silicon is used
for the sacrificial layer 209, aluminum is used for the upper electrode 201, and silicon nitride is
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used for the insulating layer 208 and the elastic layer 210, carbon tetrafluoride or xenon
difluoride is used as the etching material. It is suitable. After removing the sacrificial layer 209, a
sealing layer 212 may be formed to close the etching hole 211, if necessary. For example, the
same material as the elastic layer 210 may be used for the sealing layer 212. In the above
example, silicon nitride may be used.
[0021]
It is preferable to use a conductor material with low resistivity, such as gold or aluminum, for the
connection line 207 and the wiring 202. As a method of forming the connection line 207, as
shown in FIG. 4C, for example, the through hole 213 is formed from the main surface side of the
elastic layer 210, and a layer made of the material of the connection line is formed on the side
surface of the through hole 213. A deposition technique can be used. In the above manufacturing
method of this embodiment, a sacrificial layer having a shape corresponding to a void is formed
on the main surface of the substrate, an upper electrode and a vibrating portion are formed on
the sacrificial layer, and a wiring and an upper electrode are formed on the vibrating portion. An
electromechanical transducer is manufactured by forming a connecting wire electrically
connecting to the wire. The sacrificial layer is eventually etched to form an air gap.
[0022]
Example 2 The structure of an electromechanical transducer according to Example 2 of the
present invention will be described with reference to FIG. 5A of a top view and FIG. . The
structure of this embodiment is in common with many parts of the electromechanical transducer
of the first embodiment. The difference in structure from Example 1 is that the insulating layer
208 disposed on the main surface of the lower electrode 204 is not present, and the elastic layer
215 is present on the surface opposite to the main surface of the upper electrode 201. is there.
The elastic layer 215 serves as a part of the vibrating part 203 to increase the rigidity of the
vibrating part, and an insulating layer that prevents the short circuit between the upper electrode
201 and the lower electrode 204 when the vibrating part 203 and the lower electrode 204
contact. It also doubles as a role.
[0023]
An example of a manufacturing method of this embodiment will be described below with
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reference to FIG. As shown in FIG. 6A, in the substrate 205 on which the lower electrode 204 and
the sacrificial layer 209 are formed, the elastic layer 215 is interposed on the surface of the
sacrificial layer 209 to form the upper electrode 201. Next, as shown in FIG. 6B, the elastic layer
210 is formed on the main surfaces of the elastic layer 215 and the upper electrode 201, and the
sacrificial layer 209 is removed. Next, as shown in FIG. 6C, the connection line 207 is formed on
the elastic layer 210, and the wiring 202 is formed on part of the main surface of the elastic
layer 210.
[0024]
Preferred materials used for the substrate 205, the lower electrode 204, the sacrificial layer 209,
the upper electrode 201, the elastic layer 210, the sealing layer 212, the connection line 207,
and the wiring 202 are the same as in the first embodiment. The sacrificial layer 209 is formed
on the main surface of the lower electrode 204 at a position where an air gap is desired to be
provided with the vibrating portion 203. For the elastic layer 215, a semiconductor material or
an insulating material having a high Young's modulus, a low density, and a high resistivity can be
used. In particular, it is preferable to use the same material as the elastic layer 210 to be formed
later. For example, if silicon nitride is used for the elastic layer 210, it is preferable to use silicon
nitride also for the elastic layer 215.
[0025]
As a method of removing the sacrificial layer 209, the same method as that of the first
embodiment can be used. In the present embodiment, the connection line 207 is formed using
the through wiring penetrating the elastic layer 210, but connection is performed without using
the through wiring by performing the following process after the process of FIG. 6B. Line 207
can also be formed. That is, as shown in FIG. 7A, the concave portion 214 is formed by removing
the portion of the elastic layer 210 located immediately above the upper electrode 201. Next, as
shown in FIG. 7B, the connection line 207 is formed on the side surface of the recess 214, and
the wiring 202 is formed on a part of the main surface of the elastic layer 210. The connection
line 207 is formed to electrically connect the upper electrode 201 and the wiring 202. Further, in
order to compensate for the rigidity lowered by forming the concave portion 214, the elastic
layer 216 may be formed on the concave portion 214 by adding the step shown in FIG. 7C after
the step of FIG. 7B. . That is, the elastic layer 216 is formed on the main surface of the upper
electrode 201, the wiring 202, the connection line 207, and the elastic layer 210. As a material
of the elastic layer 216, for example, the same material as the material of the elastic layer 210
can be used.
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[0026]
Example 3 The structure of an electromechanical transducer according to Example 3 of the
present invention will be described with reference to FIG. 8 (a) of a top view and FIG. 8 (b) of a
cross-sectional view taken along dashed line E1-E2 of the top view. . The electromechanical
transducer in this embodiment includes an upper electrode 301, a wire 302, a vibrating portion
303, an insulating portion 304, a substrate 305, a support portion 306, and a connection line
307. The substrate 305 is made of a material with low resistivity, and doubles as the lower
electrode in the above embodiment. The insulating portion 304 and the supporting portion 306
are disposed on the main surface of the substrate 305, and the vibrating portion 303 is
vibratably supported by the supporting portion 306 to form an air gap with the insulating
portion 304 (in some structures, the substrate 305). ing. The upper electrode 301 and the wiring
302 are disposed in the vibrating portion 303, and the upper electrode 301 and the wiring 302
are electrically connected by the connection wire 307 disposed in the vibrating portion 303. The
upper electrode 301 is disposed on the surface of the vibration unit 303 facing the substrate
305, and the wiring 302 is disposed on the main surface of the vibration unit 303.
[0027]
An example of a manufacturing method of this embodiment will be described below with
reference to FIG. As shown in FIG. 9A, the insulating portion 304 and the support portion 306
are formed on the main surface of the substrate 305. Next, as shown in FIG. 9B, the upper
electrode 301 is formed on the functional layer 311 of the multilayer substrate 310. Next, as
shown in FIG. 9C, the support portion 306 and the functional layer 311 of the multilayer
substrate 310 are bonded. Next, as shown in FIG. 9D, the support layer 313 and the insulating
layer 312 of the multilayer substrate are removed, and the through electrode 307 and the wiring
302 are formed in the functional layer 311.
[0028]
As the substrate 305, a substrate made of a material with low resistivity is used. For example, it is
preferable to use a single crystal silicon substrate whose resistivity is lowered by introducing an
impurity. As an example of a method of forming the insulating portion 304 and the supporting
portion 306, there is a method of forming the concave portion 309 in the insulating layer 308
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formed on the main surface of the substrate 305. For the insulating layer 308, an insulator
material such as silicon oxide is preferably used. When the substrate 305 is a substrate made of
single crystal silicon, a method of thermally oxidizing the main surface of the substrate 305 can
be used as a method of forming the insulating layer 308. The depth of the concave portion 309
which becomes the air gap is formed to be shallower than the thickness of the insulating layer
308. The multilayer substrate 310 is a substrate composed of three layers of a functional layer
311, an insulating layer 312 of the multilayer substrate, and a carrier layer 313. The functional
layer 311 is made of a semiconductor material with high resistivity, for example, single crystal
silicon with low density of impurities. The insulating layer 312 of the multilayer substrate is
made of a high resistivity insulator material such as silicon oxide. The carrier layer 313 is made
of, for example, single crystal silicon. As such a multilayer substrate 310, for example, a
commercially available SOI substrate can be used. It is preferable to use a conductor material
with low resistivity, such as gold or aluminum, for the upper electrode 301.
[0029]
As a method of bonding the support portion 306 and the functional layer 311, for example, when
silicon oxide is used for the insulating layer 308 and single crystal silicon is used for the
functional layer 311, anodic bonding or direct bonding can be used. As a method of removing the
support layer 313, for example, when the support layer 313 is made of single crystal silicon,
there is a method of etching using an etching material such as potassium hydroxide or carbon
tetrafluoride. As a method of removing the insulating layer 312 of the multilayer substrate, for
example, when the insulating layer 312 of the multilayer substrate is made of silicon oxide,
etching is performed using an etching material such as hydrofluoric acid or silicon hexafluoride.
There is a way. It is preferable to use a conductor material with low resistivity, such as gold or
aluminum, for the connection line 307 and the wiring 302. As a method of forming the
connection line 307, for example, a method of forming the through hole 314 in the functional
layer 311, and depositing a layer made of the material of the connection line 307 on the side
surface of the through hole 314 can be used. In the manufacturing method of this embodiment,
the upper electrode is formed on the vibrating portion formed on the first substrate, and the
supporting portion and the vibrating portion formed on the main surface of the second substrate
are joined to the vibrating portion. An electromechanical transducer is manufactured by forming
a wire and a connecting wire electrically connecting the upper electrode and the wire.
[0030]
DESCRIPTION OF SYMBOLS 101 ... Upper electrode (2nd electrode), 102 ... Wiring, 103 ...
Vibration part, 104 ... Lower electrode (1st electrode), 105 ... Substrate, 106 ... Support part, 107
... Connection line, 110 ... Air gap
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