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JP2012222785

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This translation is machine-generated. It cannot be guaranteed that it is intelligible, accurate,
complete, reliable or fit for specific purposes. Critical decisions, such as commercially relevant or
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DESCRIPTION JP2012222785
To provide an electromechanical transducer capable of preventing light from being incident on a
receiving surface without deteriorating mechanical properties of a diaphragm. An
electromechanical transducer has at least one cell structure 2 in which a diaphragm 7 including
one of two electrodes 3 and 8 provided across a gap 5 is movably supported. . In the
electromechanical transducer, the stress relieving layer 9 in which the vibrating membrane 7 and
the acoustic impedance are matched is provided on the vibrating membrane 7, and the light
reflection layer 6 is provided on the stress relieving layer 9. [Selected figure] Figure 1
Electromechanical converter
[0001]
The present invention relates to an electromechanical transducer such as a capacitive
electromechanical transducer used as an ultrasonic transducer or the like.
[0002]
Heretofore, micro mechanical members manufactured by micro machining technology can be
processed on the order of micrometers, and various micro functional devices are realized using
these.
Capacitive micromachined ultrasonic transducers (CMUTs) and other capacitive
electromechanical transducers using such a technology have been studied as alternatives to
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piezoelectric elements. According to such a capacitance type electromechanical transducer, it is
possible to transmit and receive an acoustic wave such as an ultrasonic wave by using the
vibration of the vibrating film, and it is possible to easily obtain excellent broadband
characteristics particularly in liquid. On the other hand, an ultrasonic transducer has been
proposed which receives photoacoustic waves emitted from the inside of a subject by
illuminating illumination light (near infrared rays or the like) onto a measurement object (see
Patent Document 1). In the present transducer, a light reflecting member for reflecting light is
provided, and the light reflecting member is configured to be larger than the receiving surface of
the ultrasonic transducer that receives the photoacoustic wave.
[0003]
JP, 2010-075681, A
[0004]
When a capacitive electromechanical transducer is used as a sensor for receiving a photoacoustic
wave, when light for generating the photoacoustic wave is incident on the device, a photoacoustic
wave is generated on the receiving surface of the device and noise is generated. Become.
If the reflecting member is disposed directly above the receiving surface of the capacitive
electromechanical transducer so as to prevent light from entering in order to prevent such a
situation, then the variation of the spring constant of the vibrating membrane constituting the
device, the vibrating membrane The variation of the deformation amount of As a result, the
sensitivity of the capacitive type electromechanical transducer may be reduced, the variation
thereof, or the bandwidth may be reduced.
[0005]
In view of the above problems, the electromechanical transducer according to the present
invention has at least one cell structure in which a vibrating film including one of two electrodes
provided across a gap is vibratably supported. A stress relieving layer provided on the vibrating
film, and a light reflecting layer provided on the stress relieving layer.
[0006]
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In the electromechanical transducer according to the present invention, the stress relieving layer
is provided on the vibrating film which is the device receiving surface, and the light reflecting
layer is provided thereon. Therefore, since the influence of the stress of the light reflecting layer
does not reach the receiving surface so much, the deformation of the vibrating film hardly
occurs. Thereby, the performance variation of the electro-mechanical transducer in which the
light reflection layer is formed can be reduced, and an elastic wave such as a photoacoustic wave
can be received.
[0007]
It is a figure for demonstrating the embodiment of this invention, and the electro-mechanical
transducer of Example 1. FIG. It is sectional drawing for demonstrating the electromechanical
transducer of Example 2 of this invention. It is a schematic diagram for demonstrating the
photoacoustic apparatus of this invention.
[0008]
The feature of the electromechanical transducer according to the present invention is that a
stress relieving layer is provided on the vibrating film of the cell structure, and a light reflecting
layer is provided on the stress relieving layer. The cell structure has, for example, a gap formed
between the substrate, a first electrode on one surface side of the substrate, a vibrating film
having a second electrode, the first electrode and the vibrating film. And a vibrating membrane
support portion for supporting the vibrating membrane. The cell structure can be manufactured
by a so-called surface type, junction type manufacturing method or the like. The example of FIG.
1 described later has a structure that can be manufactured by a junction-type manufacturing
method, and the example of FIG. 2 described later has a structure that can be manufactured by a
surface-type manufacturing method.
[0009]
Hereinafter, one embodiment of the present invention will be described with reference to FIG.
Fig.1 (a) is a top view of the electrostatic capacitance type electromechanical transducer of this
embodiment, FIG.1 (b) is AB sectional drawing of Fig.1 (a). The electromechanical transducer has
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a plurality of elements 1 having a cell structure 2. Although only four elements 1 are shown in
FIG. 1, the number of elements may be any number. Each element 1 is formed of nine cell
structures 2, but the number of cell structures 2 may be any number.
[0010]
The cell structure 2 according to the present embodiment includes a vibrating membrane 7, a
gap 5 such as a gap, a vibrating membrane supporting portion 4 that vibratably supports the
vibrating membrane 7, and a silicon substrate 3. The vibrating film 7 is single crystal silicon, but
may be a vibrating film (for example, a silicon nitride film) or the like formed by laminating film
formation. The vibrating membrane 7 has a metal (such as an aluminum thin film 8) to be a
second electrode inside or on the outer surface of the vibrating membrane. In the present
invention, a membrane portion made of a silicon nitride film or a single crystal silicon film and a
second electrode portion are collectively expressed as a vibrating film. Further, when the
vibrating film 7 is low-resistance single crystal silicon, single-crystal silicon can be used as the
second electrode, and therefore, the metal serving as the second electrode may not be disposed.
The silicon substrate 3 has low resistance and can be used as a first electrode. When the silicon
substrate is not used as the first electrode, a metal can be formed on the substrate as the first
electrode. In the case where an insulating substrate such as a glass substrate is used as the
substrate, the first electrode is formed on the substrate. The first and second electrodes are
provided across the gap 5.
[0011]
The electromechanical transducer according to the present embodiment has a stress relaxation
layer 9 on the acoustic wave receiving surface. The stress relaxation layer 9 is directly formed on
the vibrating membrane when the first electrode is formed in the vibrating membrane, and is
formed on the first electrode when the first electrode is formed on the vibrating membrane. Is
formed. The stress relieving layer 9 is desirably disposed larger than the entire receiving surface
including the whole cell structure. The stress relieving layer does not increase the amount of
deformation of the vibrating membrane 7 and does not change mechanical characteristics such
as a spring constant. Further, it is preferable that the acoustic impedance is approximately the
same as that of the receiving surface having the vibrating film 7. Specifically, the Young's
modulus is preferably 0 MPa or more and 100 MPa or less, and the acoustic impedance is
preferably 1 MRayls or more and 2 MRayls or less. If the stress relaxation layer has a Young's
modulus of 100 MPa or less, the influence of the stress of the light reflection layer 6 (described
later) on the vibrating film is alleviated, and the rigidity (Young's modulus) is sufficiently small.
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Almost no change in mechanical properties. Further, by having an acoustic impedance of 1
MRayls or more and 2 MRayls or less, the acoustic impedance of the receiving surface of the
acoustic wave becomes substantially the same, and reflection of the acoustic wave at the
interface between the vibrating film 7 and the stress relaxation layer 9 can be suppressed. The
acoustic impedance of the receiving surface is an acoustic impedance that can be converted from
the spring constant of the diaphragm, the mass, the capacitance of the element, etc. For example,
in the case of a CMUT having a center frequency of 1 to 10 MHz, it is 0.01 to 5 MRayls.
However, the acoustic impedance of the receiving surface differs depending on the cell shape and
the like. When the electromechanical transducer of this embodiment is used in a medium having
a low acoustic impedance such as water (the acoustic impedance of water is about 1.5 MRayls), if
the stress relaxation layer is 1 MRayls or more and 2 MRayls or less, The reflection at the
interface between the stress relaxation layer and the medium can be reduced.
[0012]
Furthermore, the electromechanical transducer of the present embodiment has the light
reflection layer 6 on the stress relieving layer 9. The light reflection layer 6 is mainly for
reflecting the light of the wavelength of the light source used to emit light to the subject to
generate the photoacoustic wave, and is a film having a high reflectance to the wavelength of the
light source. I hope there is. As the light reflecting layer 6, Al, Au, a dielectric multilayer film or
the like is used. The light reflecting layer 6 is preferably disposed on the entire surface of the
stress relieving layer 9. More preferably, the light reflecting layer 6 is disposed on all members of
the electro-mechanical conversion device located closer to the subject than the receiving surface.
According to this configuration, it is possible to prevent noise generated when the
electromechanical transducer is irradiated with laser light. The reflectance of the light reflecting
layer 6 is preferably 80% or more, and more preferably 90% or more in light to be used. In
addition, since the light reflection layer 6 is disposed on the receiving surface, it is preferable to
be thin because it is necessary to propagate the acoustic wave with little attenuation. Specifically,
it is preferably 10 μm or less.
[0013]
The driving principle of this embodiment will be described. Here, the element 1 is formed on a
silicon substrate 3 used as a first electrode, and a vibrating film 7 is used as a second electrode.
The element 1 can extract an electrical signal from the first electrode or the second electrode by
providing a lead wiring (not shown) on the substrate or in the through substrate. In the case of
receiving an acoustic wave, a DC voltage is applied to the first electrode or the second electrode
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by voltage application means (not shown). When the acoustic wave is received, the vibrating
membrane 7 is deformed, so that the distance of the gap 5 between the vibrating membrane 7
including the second electrode and the substrate 3 as the first electrode is changed, and the
capacitance is changed. Due to the change in capacitance, a current flows in a lead wire (not
shown). An acoustic wave can be received as a voltage by a current-voltage conversion element
(not shown). Further, a DC voltage and an AC voltage can be applied to the silicon substrate 3
which is the first electrode or the vibrating film 7 which is the second electrode, and the single
crystal silicon vibrating film 7 can be vibrated by electrostatic force. An acoustic wave can also
be transmitted by this.
[0014]
The capacitive electromechanical transducer according to the present embodiment can be used
to receive a photoacoustic wave. A photoacoustic wave is an acoustic wave (typically, an
ultrasonic wave) generated from an object which emits a short pulse laser to the object and
absorbs the light. Therefore, it is necessary to irradiate light such as a laser to a not-shown
object. When scattered light or the like from a light source such as a laser is incident on the
receiving surface of the apparatus, the diaphragm 7 or the like constituting the receiving surface
absorbs the scattered light or the like from the light source and generates an acoustic wave on
the receiving surface. , It becomes noise. In order to prevent this, the light reflection layer is used,
but in the case of a capacitive electromechanical transducer in which the light reflection layer is
provided directly on the receiving surface, the amount of deformation or vibration of the
vibrating film is caused Mechanical properties such as the membrane's spring constant change.
Therefore, as described above, since sensitivity variations and band variations occur between the
cell structures and between the elements, the characteristic deterioration of the device is caused.
On the other hand, in the capacitance type electromechanical transducer of this embodiment, the
light reflection layer 6 is provided on the stress relieving layer 9. Since the stress relaxation layer
9 has a small Young's modulus, even when the stress relaxation layer is hardened and formed, it
is possible to suppress the deformation of the vibrating film and the change of the spring
constant due to the stress at the time of hardening. Further, since the acoustic impedance is
about the same as the receiving surface, it is possible to suppress the reflection of the receiving
acoustic wave at the interface between the stress relieving layer and the receiving surface.
Furthermore, since the light reflecting layer 6 is provided, light does not enter the receiving
surface. Therefore, when using the apparatus of this embodiment as a sensor which receives a
photoacoustic wave, noise can be reduced. Further, since the light reflection layer 6 is disposed
in the vicinity of the receiving surface, it is possible to prevent light such as scattered light
incident from various angles from being incident on the receiving surface. In addition, since the
light reflection layer 6 is integrated with the receiving surface, the capacitive electromechanical
transducer that receives the photoacoustic wave can be miniaturized, and can be easily
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incorporated into another device.
[0015]
In addition, in the capacitance type electromechanical transducer according to the present
embodiment, the light reflection layer supporting layer for supporting the light reflection layer
can be provided between the stress relaxation layer 9 and the light reflection layer 6 (an
embodiment described later) 2). When the light reflecting layer is formed directly on the stress
relieving layer, the light reflecting layer may be bent or deformed due to the stress of the light
reflecting layer because the Young's modulus of the stress relieving layer is low. When the
adhesion between the light reflection layer and the stress relaxation layer is low, the light
reflection layer may be peeled off. In this configuration, since the light reflection layer is formed
on the light reflection layer support layer higher in rigidity than the stress relaxation layer, the
light reflection layer is bent even if the light reflection layer support layer is bonded on the stress
relaxation layer. Or, deformation can be prevented. The Young's modulus of the light reflecting
layer supporting layer that supports the light reflecting layer 6 is preferably 100 MPa or more
and 20 GPa or less. The light reflecting layer is supported by the light reflecting layer supporting
layer, and the light reflecting layer supporting layer and the stress relieving layer can be bonded
by a bonding method or an adhesive having high adhesion. Therefore, as compared with the case
where the light reflection layer is formed directly on the stress relieving layer, the light reflection
layer can be more reliably prevented from being bent or deformed, and the adhesion can be
improved. it can. The light reflecting layer supporting layer supporting the light reflecting layer 6
preferably has an acoustic impedance of about 1 MRayls to about 5 MRayls. By making the
acoustic impedance of the light reflecting layer supporting layer supporting the light reflecting
layer 6 close to the value of the acoustic impedance of the stress relieving layer 9, the light
reflecting layer supporting layer supporting the light reflecting layer 6 and the stress relieving
layer 9 The amount of reflection of the acoustic wave at the interface can be reduced.
[0016]
In the above configuration, the stress relaxation layer 9 is preferably polydimethylsiloxane
(PDMS). What added silica particles etc. to PDMS, fluorosilicone which substituted a part of
hydrogen of PDMS with fluorine, or what added silica particles etc. to fluorosilicone may be used.
The acoustic impedance can be adjusted by adding silica particles or the like. The PDMS has an
acoustic impedance of about 1 MRayls to 2 MRayls, and can suppress reflection of acoustic
waves at the interface between the stress relaxation layer and the receiving surface. Furthermore,
the compatibility with the living body is high. The light reflecting layer supporting layer
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supporting the light reflecting layer 6 desirably has a rigidity higher than that of the stress
relaxation layer 9. When polydimethylsiloxane is used as the stress relaxation layer, for example,
a resin light reflecting layer supporting layer of polymethylpentene, polycarbonate, acryl,
polyimide, polyethylene, polypropylene or the like can be used. However, as long as the rigidity
of the light reflecting layer support layer is higher than that of the stress relaxation layer, it is not
limited thereto. In particular, the acoustic impedance of trimethylpentene is about 1.8 MRayls,
the polycarbonate is about 2.5 MRayls, and the acoustic impedance is 3 MRayls or less, which is
very low. Therefore, the amount of reflection of acoustic waves at the interface between the light
reflecting layer supporting layer supporting the light reflecting layer 6 and the stress relieving
layer 9 can be reduced. Furthermore, when the electromechanical transducer of this embodiment
is used in a medium with low acoustic impedance, the difference in acoustic impedance between
the light-reflecting layer supporting layer supporting the light-reflecting layer 6 and the medium
is small. The amount of reflection of the acoustic wave can be reduced. Furthermore, since the
surface roughness of polycarbonate can be reduced, the surface roughness of the reflective film
can also be reduced, and a decrease in reflectance can be prevented.
[0017]
Hereinafter, the present invention will be described in detail by way of more specific examples.
(Example 1) The structure of the electrostatic capacitance type electromechanical transducer of
Example 1 is demonstrated using FIG. The electromechanical transducer of the present
embodiment has a plurality of elements 1. Although only four elements are shown in FIG. 1, the
number of elements may be any number.
[0018]
The cell structure 2 is composed of a 1 μm thick single crystal silicon vibrating film 7, a gap 5, a
vibrating film support 4 supporting the single crystal silicon vibrating film 7 having a resistivity
of 0.01 Ωcm, and a silicon substrate 3. . The silicon substrate 3 has a thickness of 300 μm and a
resistivity of 0.01 Ωcm. The shape of the vibrating film 7 in this embodiment is a circle having a
diameter of 30 μm, but the shape may be a square, a hexagon or the like. The single crystal
silicon vibration film 7 is mainly made of single crystal silicon, and a layer having a large residual
stress is not formed on the vibration film 7. Therefore, the uniformity between the elements 1 is
high, and variations in transmission / reception performance can be reduced. . In order to
improve the conductive characteristics of the single crystal silicon vibration film 7, an aluminum
thin film 8 of about 200 nm can be formed. In the present configuration, the vibrating membrane
support 4 is silicon oxide, the height of the vibrating membrane support 4 is 300 nm, and the
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gap of the gap 5 is 200 nm.
[0019]
Since the single crystal silicon vibration film 7 and the silicon substrate 3 have low resistance,
they can be used as a first electrode or a second electrode. In the capacitance type
electromechanical transducer of this embodiment, the lead wiring is formed on a silicon
substrate, or by using a silicon substrate having a through wiring, an electric signal can be output
from the first electrode or the second electrode. It can be pulled out. The driving principle of
reception and transmission is as described in the above embodiment.
[0020]
In the capacitive electromechanical transducer of the present embodiment, the stress relieving
layer 9 is disposed on the receiving surface, and the light reflecting layer 6 is disposed on the
stress relieving layer 9. The stress relieving layer 9 is PDMS, and the light reflecting layer 6 is
gold. The acoustic impedance of the stress relaxation layer 9 is 1.8 MRayls, and the thickness is
100 μm. Since the difference in acoustic impedance of the stress relaxation layer 9 and the
silicon vibration film 7 is very small, reflection of the acoustic wave at the interface between the
stress relaxation layer and the receiving surface hardly occurs. When the electromechanical
transducer of the present embodiment is used in a medium having low acoustic impedance such
as water, the difference between the acoustic impedances of the stress relaxation layer 9 and the
medium is very small. The reflection at the interface can be reduced. Therefore, when receiving
the acoustic wave, the strength of the received signal does not deteriorate. The light reflection
layer 6 is for reflecting light of the wavelength of the light source used to generate the
photoacoustic wave, and may be a film having a high reflectance to the wavelength of the light
source. As the light reflection layer 6, Al, a dielectric multilayer film or the like can also be used.
The capacitive electromechanical transducer of this embodiment can be used to receive
photoacoustic waves as described in the above embodiment.
[0021]
(Example 2) The structure of the electrostatic capacitance type electromechanical transducer of
Example 2 is demonstrated using FIG. The configuration of the electromechanical transducer of
the second embodiment is substantially the same as that of the first embodiment. The cell
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structure comprises an upper electrode 37, a vibrating film 36 with a thickness of 1 μm, a gap
34, a vibrating film support 35 supporting the vibrating film 36, an insulating film 33, a lower
electrode 32 and a substrate 30. The substrate 30 is a silicon substrate, the vibrating film 36 and
the vibrating film supporting portion 35 are a silicon nitride film, and the upper electrode 37 and
the lower electrode 32 are aluminum. An oxide film 31 is disposed between the substrate 30 and
the lower electrode 32 to insulate between the two. If the substrate 30 is an insulating substrate
such as a low resistance silicon substrate or glass, the oxide film 31 may be omitted.
[0022]
The substrate 30 has a thickness of 300 μm. In the present configuration, the shape of the
vibrating membrane 36 is a circle having a diameter of 30 μm. The height of the vibrating
membrane support 35 is 300 nm, and the gap of the gap 34 is 200 nm. In addition, the stress
relieving layer 38 is disposed on the receiving surface, and the light reflecting layer 41 is
disposed on the stress relieving layer 38. In order to maintain the rigidity of the light reflection
layer 41, the light reflection layer 41 is formed on the high rigidity light reflection layer
supporting layer 40. The highly rigid light reflecting layer support layer 40 having the light
reflecting layer 41 is bonded to the stress relieving layer 38 by a resin 39.
[0023]
The stress relaxation layer 38 is PDMS. The acoustic impedance of the stress relaxation layer 38
is 1.8 MRayls, and the thickness is 50 μm. The acoustic impedance of the stress relieving layer
38 is desirably 1 MRayls to 2 MRayls, and in this way, reflection of acoustic waves at the
interface between the stress relieving layer and the receiving surface hardly occurs. Therefore,
when receiving the acoustic wave, the strength of the received signal does not deteriorate. The
stress relieving layer 38 can be manufactured by spin coating, dropping, press-fitting using a die,
or bonding a die formed stress relieving layer.
[0024]
The light reflecting layer 41 is gold, and the highly rigid light reflecting layer supporting layer 40
supporting the light reflecting layer 41 is polycarbonate. The Young's modulus of this is 2.5 ×
10 <9> Pa, and the thickness is 100 μm. Since the stress relaxation layer 38 has a low Young's
modulus, the light reflection layer may be bent or deformed due to the stress of the light
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reflection layer 41 or the like. When the adhesion between the light reflecting layer 41 and the
stress relieving layer 38 is low, the light reflecting layer may peel off. In this configuration, since
the light reflecting layer 41 is formed on the light reflecting layer support layer 40 having higher
rigidity than the stress relieving layer 38, even when the light reflecting layer supporting layer
40 is bonded on the stress relieving layer 38, The reflection layer 41 can be prevented from
being bent or deformed. The light reflection layer 41 is supported by the light reflection layer
support layer 40, and the light reflection layer support layer 40 and the stress relaxation layer
38 can be bonded by a bonding method or an adhesive having high adhesion. Therefore, the
adhesion can be improved as compared with the case where the light reflection layer is formed
directly on the stress relaxation layer.
[0025]
The acoustic impedance of the polycarbonate of the light reflecting layer support layer 40 is 2.4
MRayls. The reflection of the acoustic wave at each interface is very small because the difference
in acoustic impedance between the stress relieving layer 38 and the high rigidity light reflecting
layer support layer 40 and the receiving surface is relatively small. Therefore, it can receive
without reducing the intensity of the acoustic wave signal. The high-rigidity light-reflecting layer
supporting layer 40 may have an acoustic impedance substantially equal to that of the stress
relaxation layer 38, and may be acrylic, polyimide, polyethylene, or the like. The acoustic
impedance of the high rigidity light reflecting layer support layer 40 is desirably 1 MRayls to 5
MRayls. A silicone based adhesive can be used as the resin 39 for bonding the high rigidity light
reflecting layer support layer 40 and the stress relieving layer 38. Also, an adhesive such as
epoxy can be used. Also in the present embodiment, the same effects as those of the above
embodiment and the embodiments can be obtained.
[0026]
Example 3 The electromechanical transducer of each of the above examples can be used for a
photoacoustic apparatus that utilizes photoacoustic imaging technology. In photoacoustic
imaging, first, pulsed light is irradiated to a subject, and the acoustic wave generated by the light
absorber absorbing the energy of light propagated and diffused in the subject is received. Then, it
is a technique of imaging information inside the object by using the received signal of the
acoustic wave. According to this technique, optical characteristic distribution information such as
an initial pressure generation distribution or a light absorption coefficient distribution in a
subject can be obtained as image data.
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[0027]
The schematic diagram of the photoacoustic apparatus which can apply this invention is shown
in FIG. The photoacoustic apparatus according to the present invention at least includes a light
source 51, an electromechanical transducer 57 of each of the above-described embodiments as
an acoustic wave receiver, a signal processor 59, and a data processor 50. In the present
embodiment, the light 52 oscillated from the light source 51 is irradiated onto the subject 53 via
the optical member 54 such as a lens, a mirror, or an optical fiber. In the subject, the irradiated
light is absorbed by the light absorber 55 (for example, a tumor or a blood vessel) in the subject
to generate an acoustic wave 56. The acoustic wave receiver 57 receives the acoustic wave 56,
converts it into an electrical signal, and outputs the electrical signal to the signal processing unit
59. The signal processing unit 59 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 50. The data processing unit 50 converts the input signal into image data, and
outputs the image data to the display unit 58. The display unit 58 displays an image based on the
input image data.
[0028]
As described above, according to the photoacoustic apparatus of the present invention, the
electromechanical transducer which is an acoustic wave receiver has a light reflection film, so
that light does not enter the receiving surface, and image data with less noise can be generated.
[0029]
Reference Signs List 1 element, 2 cell structure, 3 substrate (electrode), 4 diaphragm supporting
portion, 5 gap, 6 light reflecting layer, 7 diaphragm, 8 aluminum thin film (electrode), 9 stress
relaxation layer
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