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JP2015213224

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DESCRIPTION JP2015213224
The present invention provides a technique for suppressing damage to a through electrode even
in a manufacturing method using sacrificial layer etching in a capacitive transducer fabricated on
the through electrode substrate. SOLUTION: A capacitance type transducer comprising a cell 20
having a vibrating film 10 including a second electrode 8 provided on both sides of a gap 6 with
respect to a first electrode 4 on a substrate 1 having a through electrode 3 is manufactured. It is
a method. In the manufacturing method, a first electrode is formed on a substrate having a
through electrode, an electrode protection layer 5 for protecting the through electrode and the
first electrode is formed, and a material having an etching rate different from that of the through
electrode is formed on the electrode protection layer. The sacrificial layer 6a is formed, and a
layer constituting a part of the vibrating film is formed on the sacrificial layer. Furthermore, the
etching hole 11 is formed in the layer which constitutes a part of the vibrating film, and the
sacrificial layer is etched using an etchant having a higher etching rate for the sacrificial layer
than the etching rate for the through electrode to seal the etching hole. Forming the sealing layer
12. [Selected figure] Figure 1
Method of manufacturing capacitive transducer, and capacitive transducer
[0001]
The present invention relates to a method of manufacturing a capacitive transducer
manufactured using a semiconductor process or the like, a capacitive transducer, and the like.
[0002]
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1
Although an ultrasonic transducer using a piezoelectric material has been used, in recent years, a
capacitive ultrasonic transducer (CMUT: Capacitive Micromachined Ultrasonic Transducer)
manufactured using a semiconductor process has been developed.
The capacitive ultrasonic transducer, for example, can vibratably provide a vibrating film
including one of two parallel parallel plate electrodes opposed to the cavity structure, and can
transmit or receive ultrasonic waves by applying an electric field between the electrodes It is a
device that Since such capacitive ultrasonic transducers have excellent characteristics such as a
wide frequency band, they can be used for medical diagnosis by ultrasonic waves with higher
accuracy than before, and they are attracting attention as promising technologies. ing.
[0003]
In the capacitive transducer, ultrasonic vibration is generated by electrostatic force obtained by
applying an electric field between the electrodes, and when ultrasonic waves are received, the
vibrating film is minutely displaced by the ultrasonic waves, and thus, Capacitance change occurs
and the signal is detected. A capacitive transducer is thus a device capable of converting
mechanical energy and electrical energy. As this manufacturing method, there is a sacrificial
layer etching method in which a sacrificial layer is formed in a portion to be a cavity and then
this sacrificial layer is etched to form a cavity structure having a gap between electrodes. Patent
Document 1 discloses, as a sacrificial layer etching method, a method in which a sacrificial layer
residue is to be removed by simple immersion etching after electric field etching of the sacrificial
layer.
[0004]
JP, 2011-259371, A
[0005]
On the other hand, a through electrode substrate is often used as a substrate for forming the
cavity structure.
However, when forming a cell on a substrate having a through electrode by using the abovedescribed sacrificial layer etching method, the influence on the through electrode may be a
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problem. An object of the present invention is to provide a technique capable of reducing the
influence on such a through electrode.
[0006]
A cell having a first electrode, and a vibrating film including a second electrode provided with the
gap between the first electrode and the first electrode on the first surface side of the substrate
having the through electrode of the present invention and vibratably supported. A method of
manufacturing a capacitive transducer comprising the steps of: Forming the first electrode on the
first surface side of the substrate having the through electrode; Forming an electrode protection
layer for protecting the through electrode and the first electrode; Forming a sacrificial layer made
of a material having an etching rate different from that of the through electrode on the electrode
protective layer; Forming a layer for forming a part of the vibrating film on the sacrificial layer;
Forming an etching hole in a layer for forming a part of the vibrating film, and etching the
sacrificial layer using an etchant having a higher etching rate to the sacrificial layer than an
etching rate to the through electrode; Forming a sealing layer for sealing the etching hole;
[0007]
In addition, the capacitive transducer according to the present invention can be vibrated
including a first electrode and a second electrode provided on the first surface side of the
substrate having the through electrode with the first electrode and the first electrode interposed
between the first electrode and the first electrode. It is a capacitive transducer, comprising a cell
having a supported vibrating membrane, wherein the gap is formed by a sacrificial layer etching
method. Then, an electrode protection layer for protecting the through electrode and the first
electrode is formed, and the through electrode is formed of copper, or an alloy containing
copper, or gold, or an alloy containing gold.
[0008]
According to the present invention, a capacitance type transducer can be suitably manufactured
by forming a cell on a penetration electrode substrate by a sacrificial layer etching method which
suppresses damage to the penetration electrode.
[0009]
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FIG. 1 is a cross-sectional view showing a basic configuration example of a capacitive transducer
of the present invention.
FIG. 6 is a diagram for explaining a method of manufacturing the capacitive transducer of
Example 1. FIG. 6 is a diagram for explaining a method of manufacturing the capacitive
transducer of Example 1. The figure explaining the defect of the penetration electrode in
preparation process of a penetration electrode substrate. FIG. 2 is a top view of an example of a
capacitive transducer. Explanatory drawing of the to-be-examined object information acquisition
apparatus using the electrostatic capacitance type ¦ mold transducer of this invention.
[0010]
In the present invention, the vibration supported by the first electrode on the first surface side
(upper surface side) of the substrate having the through electrode includes the first electrode and
the second electrode provided with the gap between the first electrode and the first electrode. In
the method of making a capacitive transducer comprising a cell having a membrane, the
following steps are carried out. A sacrificial layer made of a material having an etching rate
different from that of the through electrode is formed on an electrode protection layer formed to
protect the through electrode and the first electrode. Then, the sacrificial layer is etched through
the etching hole using an etching solution in which the etching rate to the sacrificial layer is
larger than the etching rate to the through electrode. In order to suppress damage to the through
electrode, materials for the through electrode, the sacrificial layer, the etching solution, and the
like are appropriately selected. The etching solution may further etch the sacrificial layer at a
higher etching rate than the etching rate of the electrode protection film. Further, the etching
solution may have a higher etching rate to the sacrificial layer than an etching rate to a layer for
forming a part of the vibrating film. Thus, in the capacitive transducer fabricated on the through
electrode substrate, the damage to the through electrode is suppressed even in the
manufacturing method using the sacrificial layer etching.
[0011]
A capacitive ultrasonic transducer according to an embodiment of the present invention and a
method of manufacturing the same will be described using a cross-sectional view showing a basic
configuration. Example 1 FIG. 1 is a cross-sectional view of a capacitive transducer according to
Example 1 of the present invention, and only one cell is shown to simplify the description. FIG. 1
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shows a part of a capacitive transducer formed on a through electrode substrate, and shows a
cross sectional structure of one cell including a vibrating membrane.
[0012]
In the capacitance type transducer of this embodiment, a through hole is formed in the substrate
1, and an insulating film 2 for electrically separating the through electrode 3 described below is
formed on the surface of the substrate 1 and inside the through hole. There is. The through
electrode 3 is formed by filling the through hole between the opposing first surface (main
surface) and the second surface (back surface) of the substrate 1. A first electrode 4 is formed on
the first surface side (main surface side) of the substrate 1, and an electrode protection film 5 for
insulating or protecting the first electrode 4 is formed. The electrode protective film 5 also
protects the through electrode 3 in the manufacturing process. A gap 6 for forming a cavity
structure, a first membrane 7, a second electrode 8 and a second membrane 9 are sequentially
formed on the electrode protective film 5, and the first membrane 7, the second electrode 8 and
the second membrane 9 are formed. The diaphragm 9 is configured with the membrane 9.
[0013]
These constitute one cell, and as shown in FIG. 4 which is a top view, a large number of cells 20
are actually arranged. FIG. 1 shows one cell in the AB cross section of FIG. As shown in FIG. 4, an
element 30 is configured by a plurality of cells 20, and in one element 30, at least one of the first
electrode 4 and the second electrode 8 is electrically connected. In the manufacturing process,
the etching hole 11 is opened in a part of the vibrating film 10, and the sacrificial layer etching is
performed through the etching hole 11. The etching holes 11 are closed by forming the sealing
layer 12. Although the sealing layer 12 is formed on the entire surface as a part of the membrane
in the configuration of FIG. 1, the sealing layer 12 may be formed limited to the area around the
etching hole 11.
[0014]
The operating principle of the capacitive transducer as described above will be described. An
ultrasonic wave is generated by vibrating the vibrating film 10 by the electrostatic force obtained
by applying an electric field between the opposing first and second electrodes. When ultrasonic
waves are received, the vibration film 10 is slightly displaced by the ultrasonic waves, causing a
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change in electrostatic capacitance between the electrodes, and a signal is detected. The
sensitivity of a capacitive transducer comprising opposing first and second electrodes is inversely
proportional to the square of the gap between the electrodes. In order to fabricate a device with
higher sensitivity, it is desirable to control the gap with high precision, and it is required to
precisely fabricate a cavity structure including the vibrating film and the gap. A sacrificial layer
etching technique is often used to form such a narrow gap cavity structure. In addition, in order
to produce a device with high sensitivity or high efficiency and miniaturization, it is necessary to
shorten the wiring length or to densify the cell arrangement, so the substrate which forms the
capacitive transducer is penetrated. Electrode substrates are often used.
[0015]
FIGS. 2A and 2B show manufacturing methods of the capacitive transducer of this embodiment.
First, the insulating film 2 is formed on the substrate 1 in which the through holes of FIG. The
thickness of the substrate 1 is about 100 μm to 1000 μm, and the size of the through hole is a
hole having a diameter of about 10 μm to 300 μm. As a material of the substrate 1, a material
having excellent smoothness and heat resistance such as silicon and glass is selected. For
example, in the case of a silicon substrate, a thermal oxide film or the like is generally used as the
insulating film 2, but as a manufacturing method or material, any film forming method or
material capable of forming an insulating film having excellent insulating properties and
smoothness. I do not mind. In the case of using a dielectric substrate such as glass having high
insulating properties, it is not necessary to form the insulating film 2.
[0016]
Next, the through electrode 3 is formed as shown in FIG. The through electrode 3 is desired to be
an electrode which has low resistance and can sufficiently fill the through hole. In general,
copper or the like having a low electric resistance and capable of easily forming an electrode by
an electrolytic plating method or the like is used. The surface of the substrate including the end
face of through electrode 3 is smoothed using a CMP (Chemical Mechanical Polishing) process or
the like. In particular, it is desirable to remove the through electrode and foreign matter that has
overflowed from the substrate surface. Capacitive transducers require precise gap control.
Therefore, it is desirable that the surface roughness of the substrate surface on the main surface
side is sufficiently smaller than the size (thickness) of the gap 6. If the thickness of the gap 6 is
about 200 nm, the maximum value of the surface roughness ( It is desirable to smooth Rmax) to
20 nm or less. However, when the through electrode 3 is subjected to a smoothing process, as
shown in FIG. 3, irregularities 17 called "dishing" in which the wiring peculiar to CMP is
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excessively polished or "erosion" in which the insulating film is excessively polished often occur.
Do. It also occurs when the through electrode is damaged by an internal defect in the film
forming condition of the through electrode 3 or a foreign material involved in the CMP process.
The size of the unevenness such as the dishing is on the order of several μm, and it is not easy
to control. This will be described in detail.
[0017]
In FIG. 3, the gap between the substrate and the through electrode or the wiring becomes
unevenness having a controlled film thickness or more. That is, even if a protective film is formed
thereon, since it is difficult to sufficiently cover a recessed portion such as a pinhole, it is not easy
to prevent the etchant from entering from the pinhole portion. The through electrode substrate
often uses a material having a low resistance as a main component of the through electrode
material (in the present specification, meaning to occupy the majority of the composition). In the
case of a substrate in which a through hole is formed in a silicon substrate or the like and a
copper electrode is embedded therein, asperity is likely to occur between the substrate and the
copper through electrode as described above. This is because dishing is likely to occur in the
CMP step for smoothing the surface after the copper is embedded, and unevenness is likely to
occur due to the difference in thermal expansion coefficient with the substrate and the like. In
such a structure, formation of an insulating film or the like by plasma CVD having high coverage
is performed to insulate and protect the through electrode. However, because the insulating film
has a thickness on the order of several hundreds of nm while the dishing size has irregularities
on the order of several micrometers, and high temperature processing is required, cracks easily
occur on the insulating film due to the difference in thermal expansion coefficient. Etc, it is
difficult to completely protect the electrode. Therefore, when the sacrificial layer is etched in the
process of manufacturing the capacitive transducer on the through electrode substrate, the
through electrode is likely to be damaged due to the pinhole as described above. In the present
embodiment, such problems are overcome by appropriately selecting the through electrode, the
first electrode, the electrode protective film, the sacrificial layer, the material for etching, and the
like as described later.
[0018]
Next, as shown in FIG. 2C, the first electrode 4 is formed on the main surface side of the
substrate 1. At this time, the first electrode 4 is disposed in contact with the through electrode 31, and the electrode protection film 5 is formed on the first electrode 4. Here, the electrode
protection film 5 is also formed on the through electrode 3. When the substrate 1 is insulating,
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the first electrode 4 may be formed directly on the surface of the substrate 1. Thus, the electrode
protection film 5 simultaneously covers the through electrode 3-2 and is formed to serve as
insulation and protection. The first electrode 4 is connected to the electrode pad 15 on the
second surface side (rear surface side) on the opposite side to the main surface side. As a
material used for the first electrode 4, titanium, tungsten, an aluminum alloy, or the like is used
as a metal having excellent smoothness and high heat resistance. The film thickness of the first
electrode 4 is about 50 nm to 1000 nm. The electrode protective film 5 is formed of a silicon
oxide film, a silicon nitride film, or the like by plasma CVD using a dielectric having high
coverage and capable of being formed at a relatively low temperature.
[0019]
A plurality of electrode pads 15 and 16 are formed on the back surface side for electrode
extraction with the outside and for protection of the through electrodes. As these electrode pads,
a metal material for connection with an external electric field circuit board is used, and an
electrode in which titanium, nickel, gold or the like is combined is used. For the purpose of only
protecting the through electrode, the dielectric using plasma CVD described above may be
formed.
[0020]
Next, as shown in FIG. 2D, a sacrificial layer 6a is formed to form a cavity. It is desirable that the
sacrificial layer 6a formed in consideration of the thickness of the gap be a material which is
excellent in smoothness and heat resistance and which is easy to etch the sacrificial layer. When
a material mainly containing copper is used as a material used for the through electrode 3,
titanium, tungsten, or an alloy material of titanium and / or tungsten is used as a material of the
sacrificial layer 6 a. An etching solution containing hydrogen peroxide water as a main solution is
used for etching titanium and tungsten. Therefore, the difference in etching rate with copper
which is the through electrode material is large (the etching rate for the sacrificial layer is higher
than the etching rate for copper), and the sacrificial layer is a sacrificial layer even if the
protective film 5 covering the through electrode 3 The through electrode is not easily damaged
by etching. The preferred etching selectivity is 100 or more. Furthermore, if the etching
selectivity is 1000 or more, it is more preferable because damage to the through electrode due to
the sacrificial layer etching is hardly caused.
[0021]
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Next, as shown in FIG. 2E, the first membrane film 7 and the second electrode 8 which are
insulator layers are formed. The second electrode 8 is preferably a metal excellent in conductivity
and heat resistance. Further, a metal film capable of controlling stress is desirable, and titanium,
molybdenum, an aluminum alloy or the like is used. As the first membrane film 7, a silicon nitride
film or the like using plasma CVD excellent in coverage and stress control is formed. As the
second electrode 8, titanium, an aluminum alloy, copper, molybdenum or the like is formed by
using a sputtering method or an electron evaporation method. Furthermore, the second
membrane film 9 is formed as shown in FIG. Similar to the first membrane film 7, the second
membrane film 9 is desirably formed of a silicon nitride film or the like. Thus, the vibrating film
10 formed of the first membrane 7, the second electrode 8 and the second membrane 9 is
formed, and the second membrane film 9 has an effect of stress relaxation and a function as a
protective film of the second electrode 8. Will be fulfilled.
[0022]
Next, as shown in FIG. 2G, an etching hole 11 is formed in a part of the vibrating film 10. The
etching holes 11 are formed on the sacrificial layer 6a using a dry etching method or the like.
After that, when an alloy of titanium or tungsten is used as the sacrificial layer 6a, the sacrificial
layer 6a is removed by performing immersion etching using a solvent mainly containing
hydrogen peroxide water heated to about 40 degrees Celsius or more. Ru. Then, after sufficiently
rinsing with pure water, the cavity or gap 6 having a narrow gap is dried by performing IPA
(isopropyl alcohol) and HFE (hydrofluoroether) treatment. By protecting the first electrode 4 and
the second electrode 8 in the sacrificial layer etching step as in the steps of FIGS. 2-1 (c) to (f),
even if the etching is performed for a long time using immersion etching, the electrodes are
damaged. Little to give.
[0023]
Next, the etching hole 11 can be closed by forming the sealing layer 12 as shown in FIG. The
sealing layer 12 may be formed with a limited area at the sealing portion, or may be formed
entirely as a part of the membrane.
[0024]
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Next, as shown in FIG. 2I, contact holes (connection holes) 13 are formed on the main surface
side of the through electrode 3-2 and a part of the second electrode 8. Similar to the formation of
the etching holes 11, it is formed using a dry etching method. Next, as shown in FIG. 2J, the
second electrode 8 and the through electrode 3-2 are connected by the electrode wiring 14 and
connected to the electrode pad 16 on the back surface side.
[0025]
When an electrode material mainly composed of copper is used for the through electrode 3, the
etching speed difference between the sacrificial layer and the through electrode is large as
described above by using titanium, tungsten or an alloy thereof for the sacrificial layer 6a.
Therefore, even if it is a through electrode which can not be protected by the protective layer by
dishing, it is possible to manufacture a capacitive transducer having a through electrode which is
hardly damaged at the time of etching the sacrificial layer.
[0026]
As described above, according to the method of manufacturing the capacitive transducer of this
embodiment, a sacrificial layer etching method capable of suppressing damage to the through
electrode on the through electrode substrate using the low resistance wiring material is used. It
can be used to form capacitive transducers. Therefore, RC delay and parasitic capacitance can be
reduced or high density mounting can be performed, and a device with high yield can be
manufactured.
[0027]
Example 2 Example 2 will be described. In the second embodiment, copper or an alloy material
mainly made of copper is used as the through electrode in the process of FIG. The formation
method is preferably electrolytic plating. As the sacrificial layer 6a in FIG. 2D, a metal excellent in
smoothness and heat resistance, for example, titanium or aluminum, or an alloy mainly
containing at least one of titanium and aluminum is formed. Further, in the step of FIG. 2G, after
the etching holes 11 are formed, the diluted layer of hydrofluoric acid is used as a sacrificial
layer etching solution to remove the sacrificial layer 6a. It is desirable that the concentration of
dilute hydrofluoric acid in the etching solution be 5% or less. By setting the etching solution to
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dilute hydrofluoric acid, damage to the through electrode 3 can be further reduced, etching can
be performed at normal temperature, and the sacrificial layer can be easily removed. After
etching the sacrificial layer, the drying step is performed in the same manner as in Example 1,
and then the sealing layer 12 is formed. Thereafter, contact formation and wiring can be
performed to manufacture a capacitive transducer having a through electrode.
[0028]
EXAMPLE 3 Example 3 will be described. In the third embodiment, gold or a gold-based alloy
material is used as the through electrode in the step of FIG. The formation method is preferably
electrolytic plating. As the sacrificial layer shown in FIG. 2D, one having excellent smoothness
and heat resistance is desirable, and further, a metal which can be easily removed by etching at
normal temperature is desirable. For example, chromium, an aluminum alloy, titanium, or
molybdenum is formed. When chromium is used for the sacrificial layer 6a, the sacrificial layer is
etched using a mixed solution of ceric ammonium nitrate and nitric acid and / or perchloric acid
as an etchant. When molybdenum is selected as the sacrificial layer 6a, the sacrificial layer can
be easily removed by using hydrochloric acid as a sacrificial layer etching solution. The formation
method of the sacrificial layer 6a can use vapor deposition, a sputtering method, or the like.
[0029]
Since the gold used for the through electrode has excellent corrosion resistance, it is possible to
widen the choice of the sacrificial layer material at the time of sacrificial layer etching and to
select various sacrificial layer materials. In addition, since the Young's modulus of gold is small,
stress generated between the substrate and the gold through electrode in the through hole can
be relaxed. As in Example 1, after the sacrificial layer is etched, a drying step is performed, and
then the sealing layer 12 is formed. Then, by forming the contact and wiring, it becomes possible
to manufacture a capacitive transducer having a through electrode.
[0030]
(Embodiment 4) The apparatus described in the above embodiment can be applied to an object
information acquiring apparatus such as an ultrasonic diagnostic apparatus using an acoustic
wave and an ultrasonic image forming apparatus. An acoustic wave from a subject is received by
a capacitive transducer, and an output electrical signal is used to reflect the subject's information
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reflecting the optical characteristic value of the subject such as the light absorption coefficient,
and the subject reflecting the difference in acoustic impedance. Sample information etc. can be
acquired.
[0031]
FIG. 5A shows an object information acquisition apparatus using a photoacoustic effect. The
pulsed light generated from the light source 2010 is irradiated to the subject 2014 via the
optical member 2012 such as a lens, a mirror, and an optical fiber. The light absorber 2016
inside the object 2014 absorbs the energy of the pulsed light and generates a photoacoustic
wave 2018 which is an acoustic wave. The capacitive transducer 2020 in the probe 2022
receives the photoacoustic wave 2018, converts it into an electrical signal, and outputs the signal
to the signal processing unit 2024. The signal processing unit 2024 performs signal processing
such as A / D conversion and amplification on the input electric signal, and outputs the signal
processing to the data processing unit 2026. The data processing unit 2026 acquires object
information (characteristic information reflecting the optical characteristic value of the object
such as a light absorption coefficient) as image data using the input signal. Here, the signal
processing unit 2024 and the data processing unit 2026 are collectively referred to as a
processing unit. The image display unit 2028 displays an image based on the image information
input from the data processing unit 2026 including the image information generation unit. In the
present embodiment, the capacitive transducer receives the photoacoustic wave generated by the
light from the front light source being irradiated to the object and converts it into an electrical
signal.
[0032]
FIG. 5B shows a subject information acquiring apparatus such as an ultrasonic echo diagnostic
apparatus using reflection of acoustic waves. The acoustic wave transmitted from the capacitive
transducer 2120 in the probe 2122 to the subject 2114 is reflected by the reflector 2116. The
capacitive transducer 2120 receives the reflected acoustic wave 2118 (reflected wave), converts
it into an electrical signal, and outputs the electrical signal to the signal processing unit 2124.
The signal processing unit 2124 performs signal processing such as A / D conversion and
amplification on the input electric signal, and outputs the signal processing to the data
processing unit 2126. The data processing unit 2126 acquires object information (characteristic
information reflecting a difference in acoustic impedance) as image data using the input signal.
Here, the signal processing unit 2124 and the data processing unit 2126 are also referred to as a
processing unit. The display unit 2128 displays an image based on the image data input from the
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data processing unit 2126.
[0033]
The probe may be one that scans mechanically or one that is moved by a user such as a doctor or
an engineer relative to the subject (handheld type). Moreover, in the case of the apparatus using
a reflected wave like FIG.5 (b), you may provide the probe which transmits an acoustic wave
separately from the probe which receives. Furthermore, the apparatus has both the functions of
the apparatus shown in FIGS. 5A and 5B, object information reflecting the optical characteristic
value of the object, and object information reflecting the difference in acoustic impedance. Both
may be acquired. In this case, the capacitive transducer 2020 in FIG. 5A may transmit not only
the photoacoustic wave but also the transmission of the acoustic wave and the reception of the
reflected wave.
[0034]
1 · · · · · · · · · penetration electrode, 4 · · first electrode, 5 · · electrode protective film, 6 · · · gap
(cavity) 6a · · · sacrificial layer Layer for forming a portion), 8 · · · second electrode, 10 · · ·
vibrating membrane, 11 · etching hole, 12 · · sealing layer, 20 · · ·
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