JP2015146972

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DESCRIPTION JP2015146972
Abstract: To provide a capacitive transducer that reduces side lobes. A capacitive transducer is
provided with an element (14) having a plurality of cells (12) on which a vibrating membrane (9)
including one of a pair of electrodes formed with a gap is vibratably supported. The distance
between the pair of electrodes at the end of the element is larger than the distance between the
pair of electrodes at the center of the element. [Selected figure] Figure 1
Capacitance transducer and method of manufacturing the same
[0001]
The present invention relates to a capacitive transducer and a method of manufacturing the
same.
[0002]
Heretofore, micromachine components manufactured by micromachining technology can be
processed on the order of micrometers, and various microfunctional devices are realized using
these.
Capacitance transducers using such technology are being investigated as alternatives to
piezoelectric elements. According to such a capacitive transducer, it is possible to transmit and
receive an ultrasonic wave using the vibration of the vibrating film, and in particular, it is
possible to easily obtain excellent broadband characteristics in liquid.
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[0003]
There is a capacitive transducer including an element in which cells are arranged in a square
shape or a rectangular shape and the distance between adjacent cells is uniform (see Patent
Document 1). Further, there is a capacitive transducer in which the transmission efficiency or
reception sensitivity of the cell at the element end is lower than the transmission efficiency or
reception sensitivity of the cell at the center of the element (see Patent Document 2).
[0004]
JP, 2008-98697, A U.S. Patent 8456985
[0005]
When ultrasonic waves are transmitted by a capacitive transducer including cells in which cells
are arranged in a square or rectangular shape and the distance between adjacent cells is uniform,
uniform sound pressure is emitted at the end and center of the elements. Therefore, side lobes
are easily generated in the ultrasonic beam.
The image quality of an ultrasound image using this ultrasound beam may be degraded by side
lobes. Also in the case of reception, the image quality may be degraded as well.
[0006]
In addition, in capacitive type transducers where the transmission efficiency or reception
sensitivity of the cell at the element end of the capacitive transducer is lower than that of the cell
at the center, the shape of the cell at the end of the element is different from the shape of the cell
at the center ing. While this configuration enables apodization to reduce side lobes, the
frequency characteristics of transmission efficiency and reception sensitivity of each cell are
different, so that signals of unnecessary frequency bands are also acquired, particularly when
ultrasonic waves are received. The SN ratio may be degraded.
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[0007]
The present invention has been made based on such problem recognition. An object of the
present invention is to reduce side lobes in capacitive transducers.
[0008]
The present invention adopts the following configuration. That is, a capacitive transducer
comprising an element having a plurality of cells vibratably supported by a vibrating membrane
including one of a pair of electrodes formed with a gap therebetween, which is a capacitive
transducer The capacitance type transducer is characterized in that a distance between a pair of
electrodes of an end cell is wider than a distance between a pair of electrodes of a cell at a
central portion of the element.
[0009]
The present invention also adopts the following configuration. A method of manufacturing a
capacitive transducer comprising an element having a plurality of cells, comprising: forming a
plurality of first electrodes; a plurality of second electrodes respectively corresponding to the
plurality of first electrodes Forming a plurality of cells comprising a pair of the first electrode and
the second electrode by vibratably forming a vibrating film including the electrodes of The
distance between the first electrode and the second electrode in the cell at the end of the device
is larger than the distance between the first electrode and the second electrode in the cell at the
central portion of the device. It is a manufacturing method of a capacitance type transducer.
[0010]
According to the present invention, side lobes in a capacitive transducer can be reduced.
[0011]
FIGS. 2A and 2B are a top view and a cross-sectional view of the capacitive transducer of
Example 1; FIGS.
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The top view and AB sectional drawing of the capacitive transducer of Example 2. FIG. 7A to 7C
are cross-sectional views illustrating a method of manufacturing a capacitive transducer. FIG. 2 is
a block diagram illustrating the configuration of a subject information acquisition apparatus.
[0012]
Hereinafter, preferred embodiments of the present invention will be described with reference to
the drawings. However, the dimensions, materials, shapes and relative positions of components
described below should be appropriately changed according to the configuration of the
apparatus to which the invention is applied and various conditions, and the scope of the present
invention is not limited. It is not the thing of the meaning limited to the following description.
[0013]
The present invention is made for an ultrasonic capacitive transducer, and can be applied to an
apparatus and method for transmitting or receiving ultrasonic waves using the transducer. The
subject of the present invention further includes an apparatus using ultrasonic echo technology,
which transmits ultrasonic waves to a subject such as a living body and receives echo waves
reflected and propagated inside the subject. By generating data based on an echo wave, it is
possible to acquire characteristic information reflecting the difference in acoustic impedance
inside the object.
[0014]
Further, in the capacitive transducer according to the present invention, in addition to the echo
wave, when the light from the light source is irradiated to the subject, photoacoustic effect
generates and propagates in the light absorber inside the subject. It can be used to receive waves.
By analyzing the photoacoustic wave, it is possible to acquire functional information and optical
characteristic information inside the object. Since these devices obtain characteristic information
by performing processing by the signal processing unit on the received echo waves and
photoacoustic waves and then analyzing them by the information processing device, they can be
called an object information acquisition device. . By converting the characteristic information into
image data and displaying it on the display unit, internal inspection such as diagnosis can be
performed.
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[0015]
The present invention can also be grasped as a control method of a subject information
acquisition apparatus, or a subject information acquisition method, or an acoustic wave
measurement method. Furthermore, these methods can also be grasped as a program that
realizes an information processing unit such as a CPU or a circuit. Moreover, it can be grasped
also as a manufacturing method of a capacitance type transducer characteristic to the present
invention, and a manufacturing method of a probe using the same.
[0016]
When using a capacitive transducer for acquiring characteristic information, it is preferable to
use a probe in which one or more elements are arranged. Also, if the subject is held and the
probe is scanned, a wide range of measurements can be made. If the subject is a breast, for
example, holding using a plate-like member or a bowl-like member is preferable. The ultrasonic
wave in the present invention is a typical example of an acoustic wave also referred to as a sound
wave or an elastic wave, and does not limit the wavelength or the like.
[0017]
Hereinafter, an embodiment of the present invention will be described with reference to FIG. FIG.
1 (a) is a top view of a capacitive transducer according to the present invention, and FIG. 1 (b) is
a cross-sectional view taken along line A-B of FIG. 1 (a). A plurality of cells 12 are formed in the
element 14 of the capacitive transducer of the present invention. Although the number of
elements included in the capacitive transducer is one in FIG. 1, it may be any number. Here, the
element is one element of a capacitive transducer in which the signal extraction electrodes of all
the cells constituting the element are common. That is, the output of the electric signal is
performed in the unit of this element. Further, although the number of cells included in the
element 14 is 15 in FIG. 1, it may be any number.
[0018]
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The vibrating membrane 9 is vibratably supported by the cell. The vibrating membrane 9
includes the second electrode 1, and the second electrode 1 is provided so as to sandwich the
gap 3 (cavity) with the first electrode 2. In FIG. 1, the vibrating membrane is configured such that
the second electrode 1 is sandwiched between the first membrane 7 and the second membrane
8, but the vibrating membrane is vibratable and has the second electrode. It suffices to use only
the second electrode or only the first membrane and the second electrode. Although described
later, reference numeral 4 denotes an etching path, 6 denotes a sealing portion, 10 denotes a
substrate, and 11 and 15 denote first and second insulating films.
[0019]
The first electrode or the second electrode is used as an electrode for applying a bias voltage or
an electrode for applying an electric signal or extracting an electric signal. In FIG. 1, the first
electrode is used as an electrode to which a bias voltage is applied, and the second electrode is
used as a signal extraction electrode. Electrodes to which a bias voltage is applied are also
common in the device. The bias voltage may be common to the elements. On the other hand, the
signal extraction electrode must be electrically separated for each element.
[0020]
In the element of the capacitive transducer of FIG. 1, the distance between the pair of electrodes
of the cell at the end of the element is wider than the distance between the pair of electrodes of
the cell at the center of the element. The transmission efficiency or the reception sensitivity
becomes lower as the distance between the pair of electrodes of the cell becomes wider, so that
the transmission efficiency or the reception sensitivity of the cell at the element end can be
lowered. Therefore, it is possible to reduce the side lobes generated again in the ultrasonic beam
as compared to a capacitive transducer having the same transmission efficiency or reception
sensitivity from the center to the end of the element. Therefore, it is possible to reduce ultrasonic
signals from targets that are not in the direction of the ultrasonic beam, and to form high-quality
ultrasonic images.
[0021]
Furthermore, the spring constant of the vibrating membrane of the cell at the end of the element
may be smaller than the spring constant of the vibrating membrane of the cell at the center of
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the element. The frequency characteristic of the transmission efficiency or reception sensitivity is
determined by the effective spring constant of the cell. The effective spring constant depends on
the force obtained by subtracting the electrostatic force from the restoring force of the vibrating
membrane. In the capacitive transducer of this configuration, the distance between the pair of
electrodes of the cell at the end of the element is larger than the distance between the pair of
electrodes of the cell at the center of the element. The electrostatic force of the cell is smaller.
[0022]
As in this configuration, by making the spring constant of the vibrating membrane of the cell at
the end of the element smaller than the spring constant of the vibrating membrane of the cell at
the central part of the element, the effective spring constants of all the cells in the element can be
obtained. It can be the same. Then, the frequency characteristics of transmission efficiency and
reception sensitivity of all cells constituting the element become almost the same. Therefore,
since the signal of an unnecessary frequency band is not acquired at the time of receiving an
ultrasonic wave, it is possible to prevent the deterioration of the image quality without
deteriorating the SN ratio. Therefore, the capacitance type transducer of this configuration
enables apodization to reduce side lobes and can reduce deterioration of the S / N ratio, so that
high-quality ultrasonic images can be formed.
[0023]
Further, the sum of the restoring force and the electrostatic attractive force of the vibrating
membrane of the cell at the end of the element may be equal to the sum of the restoring force of
the vibrating membrane of the central portion of the element and the electrostatic attractive
force. In the capacitive transducer of this configuration, the effective spring constants of all the
cells in the element can be made the same, and the frequency characteristics of transmission
efficiency and reception sensitivity of all the cells constituting the element will be the same. .
Therefore, since the signal of an unnecessary frequency band is not acquired at the time of
receiving an ultrasonic wave, it is possible to prevent the deterioration of the image quality
without deteriorating the SN ratio. Therefore, the capacitance type transducer of the present
configuration enables apodization to reduce side lobes and can reduce deterioration of the SN
ratio, so that high-quality ultrasonic images can be formed.
[0024]
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For example, when the vibrating membrane shape is circular, the restoring force FM of the spring
can be described as equation (1). FM = kM · x (1) where KM is the spring constant of the
vibrating membrane and x is the displacement of the vibrating membrane. Also, the electrostatic
force FE can be described as in equation (2). Here, ε 0 is the permittivity of vacuum, S is the
area of the vibrating film, V is the bias voltage, and d is the effective distance before applying the
bias voltage.
[0025]
The sum of the restoring force and the electrostatic attractive force of the vibrating membrane of
the cell can be described as equation (3). Therefore, the effective spring constant of the cell can
be described as equation (4).
[0026]
When the sum of the restoring force and the electrostatic attractive force of the vibrating
membrane of the cell at the end of the element and the sum of the restoring force of the
vibrating membrane of the central part of the element and the electrostatic attractive force are
the same Not only that each effective spring constant includes an equal range. The effective
spring constants of the cell at the central portion and at the end of the element can be described
as equation (5) when c and e are respectively added to the lower right subscripts.
[0027]
ここで、de>dcであるので、kMe<kMcとなる。 If the spring constant of the end of the
element and the spring constant of the central part are determined so as to satisfy the equation
(5), the frequency characteristics of transmission efficiency and reception sensitivity of all the
cells constituting the element can be made the same. Therefore, since the signal of an
unnecessary frequency band is not acquired at the time of receiving an ultrasonic wave, it is
possible to prevent the deterioration of the image quality without deteriorating the SN ratio.
Therefore, the capacitance type transducer of the present configuration enables apodization to
reduce side lobes and can reduce deterioration of the SN ratio, so that high-quality ultrasonic
images can be formed.
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[0028]
The driving principle of the present invention will be described. The capacitive transducer can
extract an electrical signal from the second electrode by using the signal extraction wiring. In the
present embodiment, the electrical signal is drawn by the lead wiring, but a through wiring or the
like may be used. Further, in the present embodiment, an electrical signal is extracted from the
second electrode, but may be extracted from the first electrode.
[0029]
When an ultrasonic wave is received by a capacitive transducer, a direct current voltage is
applied to the first electrode 2 by a voltage application unit (not shown) to generate a potential
difference between the electrodes. In this case, the second electrode 1 may be fixed to the ground
voltage. The ground voltage indicates a direct current reference potential which a current-voltage
conversion circuit (not shown) (not shown) has. When ultrasonic waves are incident, the
vibrating membrane 9 having the second electrode 1 is deformed, so that the distance of the gap
3 between the second electrode 1 and the first electrode 2 is changed, resulting in the
capacitance Changes. Due to the change in capacitance, a current is output from the second
electrode 1 and a current flows in the lead-out wiring. This current can be converted into a
voltage by a current-voltage conversion circuit (not shown) to receive ultrasonic waves. As
described above, the DC voltage may be applied to the second electrode and the electrical signal
may be extracted from the first electrode by changing the configuration of the lead wiring. Also,
the current-voltage conversion circuit is preferably provided in the probe 402 of FIG.
[0030]
Moreover, in the case of generating an electric potential difference between the first electrode 2
and the second electrode 1 when transmitting ultrasonic waves, an alternating voltage (including
pulse voltage) as a transmission signal to the second electrode 1 And the vibrating membrane 9
can be vibrated by electrostatic force. By this, ultrasonic waves can be transmitted. Also in the
case of transmission, an alternating voltage may be applied to the first electrode to vibrate the
vibrating film by changing the configuration of the lead wiring.
[0031]
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The manufacturing method of the present invention will be described with reference to FIG. FIG.
3 is a cross-sectional view of the capacitive transducer of the present invention, which has
substantially the same configuration as FIG. FIG. 3 is a cross-sectional view taken along a line AB
in FIG.
[0032]
As shown in FIG. 3A, the first insulating film 61 is formed on the substrate 60. The substrate 60
is a silicon substrate, and the first insulating film 61 is provided to form insulation with the first
electrode. When the substrate 60 is an insulating substrate such as a glass substrate, the first
insulating film 61 may not be formed. The substrate 60 is preferably a substrate having a small
surface roughness. When the surface roughness is large, the surface roughness is transferred
also in the film forming step after the present step, and the distance between the first electrode
and the second electrode due to the surface roughness is determined between the cells , Each
element will vary. This variation is a variation in transmission and reception sensitivity.
Therefore, the substrate 60 is preferably a substrate having a small surface roughness.
[0033]
Next, the first electrode 51 is formed. The first electrode 51 is desirably a conductive material
having a small surface roughness, such as titanium or aluminum. Similar to the substrate, when
the surface roughness of the first electrode is large, the distance between the first electrode and
the second electrode due to the surface roughness varies among the cells and between the
elements, so the surface roughness Small conductive materials are desirable.
[0034]
Next, a second insulating film 65 is formed. The second insulating film 65 is desirably an
insulating material having a small surface roughness, and when a voltage is applied between the
first electrode and the second electrode, electricity between the first electrode and the second
electrode is generated. It is formed to prevent a short circuit or a dielectric breakdown. In the
case of driving at a low voltage, the second insulating film 65 may not be formed because the
first membrane layer described later is an insulator. As in the case of the substrate, when the
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surface roughness of the second insulating film is large, the distance between the first electrode
and the second electrode due to the surface roughness varies between cells and between
elements, so the surface A second insulating film having a small roughness is desirable. For
example, a silicon nitride film, a silicon oxide film or the like.
[0035]
Next, as shown in FIG. 3B, a sacrificial layer 53 is formed. The sacrificial layer 53 is desirably
made of a material having a small surface roughness. As in the case of the substrate, when the
surface roughness of the sacrificial layer is large, the distance between the first electrode and the
second electrode due to the surface roughness varies between cells and between elements, so the
surface roughness Small sacrificial layers are desirable. Also, in order to shorten the etching time
of etching for removing the sacrificial layer, a material having a high etching rate is desirable.
[0036]
In addition, a sacrificial layer material is required in which the second insulating film, the first
membrane layer, and the second electrode are not substantially etched with respect to the
etchant or etching gas for removing the sacrificial layer. When the second insulating film, the
first membrane layer, and the second electrode are substantially etched with respect to the
etchant or etching gas for removing the sacrificial layer, the thickness variation of the vibrating
film, the first electrode and the first electrode A variation in distance between the two electrodes
occurs. Variations in the thickness of the vibrating film and variations in the distance between
the first electrode and the second electrode result in variations in sensitivity among the cells and
among the elements. When the second insulating film, the first membrane layer is a silicon
nitride film or a silicon oxide film, the surface roughness is small, and the etching solution in
which the second insulating film, the first membrane layer, and the second electrode are not
etched Chromium is preferred.
[0037]
Next, as shown in FIG. 3C, a sacrificial layer 83 and a sacrificial layer 93 are formed by repeating
the process of FIG. 3B. The sacrificial layer 53 is for forming the cell gap in the central part of the
device, and the sacrificial layers 83 and 93 are for forming the cell gap toward the end of the
device. Therefore, the sacrificial layer 53 is thinner than the sacrificial layer 83, and the
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sacrificial layer 83 is thinner than the sacrificial layer 93.
[0038]
From FIG. 3 (d) to FIG. 3 (f), the steps of forming the vibrating film including the second electrode
and forming the gap by removing the sacrificial layer will be described. In FIG. 3, the vibrating
membrane is composed of the first membrane, the second electrode and the second membrane,
but may be composed of any number of layers including the second electrode.
[0039]
As shown in FIG. 3D, a first membrane layer 57 including a first membrane is formed. The first
membrane layer 57 desirably has low tensile stress. For example, a tensile stress of 300 MPa or
less is good. The silicon nitride film can be stress controlled and can have a low tensile stress of
300 MPa or less. When the first membrane has a compressive stress, the first membrane causes
sticking or buckling and is largely deformed. Also, in the case of high tensile stress, the first
membrane may be broken. Therefore, the first membrane layer 57 desirably has low tensile
stress.
[0040]
Next, as shown in FIG. 3E, a second electrode 52 is formed, and an etching hole (not shown) is
further formed. Thereafter, the sacrificial layer 53, the sacrificial layer 83, and the sacrificial
layer 93 are removed from the etching holes through an etching path (not shown). The second
electrode 52 is desirably made of a material having low residual stress and heat resistance. When
the residual stress of the second electrode is large, a second electrode with a small residual stress
is desirable because it causes a large deformation of the vibrating membrane. In addition, it is
preferable to use a material that does not cause deterioration or increase in stress due to the
temperature at the time of forming the sealing layer for forming the second membrane layer or
the sealing portion.
[0041]
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In the case of removing the sacrificial layer in a state where the second electrode is exposed, it is
necessary to etch the sacrificial layer while applying a photoresist or the like for protecting the
second electrode. It is desirable to have a second electrode with etch resistance that allows the
sacrificial layer to be etched without the photoresist and with the second electrode exposed, as
the stress on the photoresist or the like makes the first membrane easier to stick. Sticking means
that the vibrating film which is a structure adheres after the sacrificial layer is removed. For
example, titanium, aluminum silicon alloy, etc. are desirable.
[0042]
Next, as shown in FIG. 3 (f), a second membrane layer 58 including a second membrane is
formed. In this process, a second membrane is formed, and a sealing portion that seals an etching
hole (not shown) is formed. By forming the second membrane layer 58, the second membrane
can be formed to form a vibrating membrane having a desired spring constant, and the etching
hole can be sealed.
[0043]
When the sealing process of the etching hole and the process of forming the second membrane
are the same as in this process, the vibrating film can be formed only by the film forming
process. Therefore, the thickness of the vibrating film can be easily controlled, and variations in
the spring constant or deflection of the vibrating film due to the thickness variations can be
suppressed, thereby reducing variations in the reception or transmission sensitivity between cells
or elements. can do. The step of sealing the etching hole and the step of forming the second
membrane may be separate steps. It is also possible to form a second membrane and then form a
seal, or to form a seal and then form a second membrane.
[0044]
Also, the second membrane layer is desirably a material having low tensile stress. As with the
first membrane, when the second membrane has compressive stress, the first membrane causes
sticking or buckling and is greatly deformed. Also, in the case of high tensile stress, the second
membrane may be broken. Therefore, low tensile stress is desirable for the second membrane
layer. The silicon nitride film can be stress controlled and can have a low tensile stress of 300
MPa or less.
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[0045]
The sealing portion may be formed so that liquid and outside air do not enter the gap. In
particular, when sealing is performed under reduced pressure, the vibrating film is deformed by
atmospheric pressure, and the distance between the first electrode and the second electrode is
shortened. Since the sensitivity of transmission or reception is proportional to the 1.5th power of
the effective distance between the first electrode and the second electrode, sealing under reduced
pressure to keep the gap at a pressure lower than atmospheric pressure, Transmission or
reception sensitivity can be improved. The effective distance is the sum of the value obtained by
dividing the insulating film between the first electrode and the second electrode by the relative
permittivity and the gap. After this process, wiring not connected to the first electrode and the
second electrode is formed by a process not shown. The wiring material may be aluminum or the
like.
[0046]
According to such a manufacturing method, it is possible to manufacture a capacitive transducer
having the configuration necessary to achieve the object of the present invention. Hereinafter,
the present invention will be described in detail by way of more specific examples.
[0047]
First Embodiment An embodiment of the present invention will be described below with
reference to FIG. FIG. 1 (a) is a top view of a capacitive transducer according to the present
invention, and FIG. 1 (b) is a cross-sectional view taken along line A-B of FIG. 1 (a). The element
14 of the capacitive transducer of the present invention is composed of 15 cells 12. Although the
number of elements included in the capacitive transducer is one in FIG. 1, it may be any number.
[0048]
In the cell 12, a vibrating film 9 including the first electrode 2 and the second electrode 1
provided with the gap 3 interposed therebetween is vibratably supported. The vibrating
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membrane 9 has a configuration in which the second electrode 1 is sandwiched between the first
membrane 7 and the second membrane 8. The first electrode 2 is an electrode for applying a bias
voltage, and the second electrode 1 is a signal extraction electrode. The shape of the vibrating
membrane in this embodiment is circular, but the shape may be square, hexagonal or the like. In
the case of a circular shape, since the vibration mode is axisymmetric, it is possible to suppress
the vibration of the diaphragm due to the unnecessary vibration mode.
[0049]
The first insulating film 11 on the silicon substrate 10 is a silicon oxide film with a thickness of 1
μm formed by thermal oxidation. The second insulating film 15 is a silicon oxide film with a
thickness of 0.1 μm formed by Prasma Enhanced Chemical Vapor Deposition (PE-CVD). The first
electrode is aluminum with a thickness of 50 nm, and the second electrode 4 is aluminum with a
thickness of 100 nm. The first membrane 7 and the second membrane 8 are silicon nitride films
manufactured by PE-CVD, and are formed with a tensile stress of 200 MPa or less. The diameters
of the first membrane 7 and the second membrane 8 are 25 μm, and the thicknesses thereof are
0.4 μm and 0.7 μm, respectively.
[0050]
In the element of the capacitive transducer of FIG. 1, the distance between the pair of electrodes
of the cell at the end of the element is wider than the distance between the pair of electrodes of
the cell at the center of the element. The gap depth of the cell at the end of the device is 0.25
μm, and the gap depth of each of the cells at the center of the device is 0.2 μm and 0.15 μm.
According to this configuration, the transmission efficiency or the reception sensitivity decreases
as the distance between the pair of electrodes of the cell increases, so that the transmission
efficiency or the reception sensitivity of the cell at the element end can be lowered. Therefore, it
is possible to reduce the side lobes generated again in the ultrasonic beam as compared to a
capacitive transducer having the same transmission efficiency or reception sensitivity from the
center to the end of the element. Therefore, it is possible to reduce ultrasonic signals from targets
that are not in the direction of the ultrasonic beam, and to form high-quality ultrasonic images.
[0051]
The distance between the pair of electrodes of the cell at the end of the element may be larger
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than the distance between the pair of electrodes of the cell at the center of the element. The gap
depth of the cell at the end of the element, the first insulating film thickness, and the vibration
thickness However, it is sufficient if the gap depth of the cell at the central portion of the element
is larger than the first insulating film thickness and the vibration film thickness. When the
vibration film thickness of the cell at the end of the element is larger than the vibration film
thickness of the cell at the center of the element, the vibrating membrane size of the cell at the
element end is larger than the vibrating membrane size of the cell at the central part of the
element. Thereby, the spring constant of the diaphragm of the cell at the end of the element can
be made the same as the spring constant of the diaphragm of the cell at the center of the
element, and the frequency characteristics of transmission efficiency or reception sensitivity of
all cells in the element are You can do the same.
[0052]
It is desirable that the gap depth of the cell at the end of the element and the first insulating film
thickness be larger than the gap depth of the cell at the center of the element and the first
insulating film thickness. In this configuration, the size of the vibrating membrane of all the cells
in the element can be made the same, which facilitates design. Also, the radiation impedances of
all the cells in the device can be made the same, and the etching times of the sacrificial layers of
all the cells in the device can be made the same. The distance between the pair of electrodes at
the end of the element and the distance between the pair of electrodes at the center of the
element may be designed depending on the shape of the ultrasonic beam to be formed. For
example, if a Gaussian beam is used, it may be designed according to the distribution.
[0053]
In addition, since the diaphragm shape of the cell is the same in the element, the frequency
characteristics of the transmission efficiency and the reception sensitivity of all the cells
constituting the element are substantially the same. Therefore, since the signal of an unnecessary
frequency band is not acquired at the time of receiving an ultrasonic wave, it is possible to
prevent the deterioration of the image quality without deteriorating the SN ratio. Therefore, the
capacitance type transducer of the present configuration enables apodization to reduce side
lobes and can reduce deterioration of the SN ratio, so that high-quality ultrasonic images can be
formed. In addition, the cell shape same includes not only the case where the shape is
strictly the same but also an error such as an error in the manufacturing process that the
frequency characteristics of the conversion efficiency of the cell can be regarded as the same.
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[0054]
Second Embodiment The configuration of a capacitive transducer according to a second
embodiment will be described with reference to FIG. FIG. 2A is a top view of the capacitive
transducer of the present invention, and the configuration of the capacitive transducer of the
second embodiment is substantially the same as that of the first embodiment. Therefore, the
explanation will focus on the differences.
[0055]
The capacitive transducer of FIG. 2 includes the second electrode 31, the first electrode 32, the
gap 33, the etching path 34, the sealing portion 36, the first membrane 36, the second
membrane 37, the vibrating membrane 39, A substrate 40, a first insulating film 41, a cell 42, an
element 44, and a second insulating film 45 are included.
[0056]
In the element of the capacitive transducer of FIG. 2, the spring constant of the diaphragm of the
cell at the end of the element is smaller than the spring constant of the diaphragm of the cell at
the center of the element.
The frequency characteristic of the transmission efficiency or reception sensitivity is determined
by the effective spring constant of the cell. The effective spring constant depends on the force
obtained by subtracting the electrostatic force from the restoring force of the vibrating
membrane. In the capacitance type transducer of this configuration, since the distance between
the pair of electrodes of the cell at the end of the element is wider than the distance between the
pair of electrodes of the cell at the center of the element, the electrostatic force of the cell at the
end of the element is It is smaller than the electrostatic force of the cell at the center of the
element.
[0057]
As in this configuration, the spring constant of the vibrating membrane of the cell at the end of
the element is smaller than the spring constant of the vibrating membrane of the cell at the
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center of the element. The constant can be the same. Therefore, the frequency characteristics of
the transmission efficiency and the reception sensitivity of all the cells constituting the element
are substantially the same. Therefore, since the signal of an unnecessary frequency band is not
acquired at the time of receiving an ultrasonic wave, it is possible to prevent the deterioration of
the image quality without deteriorating the SN ratio. Therefore, the capacitance type transducer
of the present configuration enables apodization to reduce side lobes and can reduce
deterioration of the SN ratio, so that high-quality ultrasonic images can be formed.
[0058]
As described above, in the capacitive transducer according to the present invention, the distance
between the pair of electrodes of the cell at the end of the element is wider than the distance
between the pair of electrodes of the cell at the center of the element. The transmission efficiency
or the reception sensitivity becomes lower as the distance between the pair of electrodes of the
cell becomes wider, so that the transmission efficiency or the reception sensitivity of the cell at
the end of the element can be reduced. Therefore, it is possible to reduce the side lobes
generated again in the ultrasonic beam as compared to a capacitive transducer having the same
transmission efficiency or reception sensitivity from the center to the end of the element.
Therefore, it is possible to reduce ultrasonic signals from targets that are not in the direction of
the ultrasonic beam, and to form high-quality ultrasonic images.
[0059]
<Example of application> The above-mentioned capacitive type transducer can be applied to a
probe which receives or transmits an acoustic wave using it. For example, in FIG. 4, the probe
402 comprises a plurality of elements 403. When the transmission unit 405 controls the
transmission acoustic wave according to an instruction of the information processing unit 406,
an acoustic wave is generated from each element. On the other hand, at the time of reception, the
electric signal output from each element is subjected to processing (for example, amplification or
AD conversion) by the signal processing unit 404.
[0060]
FIG. 4 shows a state in which the above probe is used as a component of the object information
acquisition apparatus. First, as the characteristic information, a case where a light absorber
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inside the object 401 absorbs light from a light source (not shown) to generate a photoacoustic
wave will be described. At this time, the photoacoustic wave propagates inside the object and is
received by the element. The electrical signal output from the element is input to the signal
processing unit and subjected to signal processing. The information processing unit generates an
initial sound pressure distribution, an absorption coefficient distribution, and the like in the
subject by known image reconstruction processing based on the signal input from the signal
processing unit. At the time of diagnosis, such information may be converted into image data and
displayed on the display unit 407 as necessary. In the present specification, a configuration
including a signal processing unit and an information processing unit may be referred to as a
processing unit.
[0061]
Next, the case of acquiring echo information inside the subject will be described. At this time, the
acoustic wave is transmitted from each element by the control signal transmitted by the
transmission unit. The acoustic wave reflected at the acoustic impedance boundary inside the
subject is again received by the element. The received signal output from the element is
subjected to known signal processing, reconstruction processing, and image data conversion as
in the case of the photoacoustic wave. In the case of an apparatus using this reflected wave, the
probe for transmitting the acoustic wave may be provided separately from the probe for
receiving. Furthermore, the capacitive transducer according to the present invention can be
applied to a device having the function of a device using photoacoustic waves and a device using
echo waves.
[0062]
The probe may be a mechanical scan, or may be a handheld (hand-held) type in which a user
such as a doctor or a technician holds the probe and moves it relative to the subject. In
particular, when mechanically scanning a subject that is a living body, stable measurement can
be performed by holding the subject by the holding unit. If the subject is a breast, a plate-like or
bowl-like holding means is preferred.
[0063]
1: second electrode, 2: first electrode, 3: gap, 9: vibrating membrane, 12: cell, 14: element
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