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DESCRIPTION JP2017085257

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DESCRIPTION JP2017085257
Abstract: To provide a capacitive transducer or the like having a wide band reception band. A
capacitive transducer (1) has a first sub-element each having a cell (2) on which a vibrating
membrane including one of two electrodes formed at intervals is vibratably supported. It has an
element containing a second sub-element. Furthermore, it has a first detection circuit 5, a second
detection circuit 6, and a summing circuit 7 for summing the signals from the first detection
circuit and the second detection circuit. The first subelement is electrically connected to the first
detection circuit 5, the second subelement is electrically connected to the second detection
circuit 6, and the cutoff frequency of the first detection circuit 5 and the The cutoff frequency of
the second detection circuit 6 is different. [Selected figure] Figure 1
Capacitance transducer and information acquisition apparatus including the same
[0001]
The present invention relates to a capacitive transducer, and an information acquisition
apparatus such as a photoacoustic apparatus including the same.
[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.
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1
Capacitive micromachined ultrasonic transducers (hereinafter sometimes abbreviated as CMUT)
using such a technology have been studied as alternatives to piezoelectric elements. According to
such a capacitive transducer, ultrasonic waves and the like can be transmitted and received using
the vibration of the vibrating film, and excellent broadband characteristics can be obtained
particularly in liquid.
[0003]
As such a technology, there is a capacitive transducer that achieves a wide band characteristic by
including a plurality of cells having a vibrating membrane with a high spring constant and a cell
having a vibrating membrane with a low spring constant (see Patent Document 1).
[0004]
U.S. Pat. No. 5,870,351
[0005]
Applying a common voltage from the common electrode and transmitting using a capacitive
transducer that realizes a wide band characteristic by including a plurality of cells having a
vibrating membrane with a high spring constant and a cell having a vibrating membrane with a
low spring constant Reception can be performed.
In that case, a cell having a vibrating membrane with a high spring constant and a cell having a
vibrating membrane with a low spring constant convert efficiency of the vibrating membrane
during reception into an electrical signal, or an electrical signal during transmission. The
conversion efficiency to convert into vibration of the vibrating membrane is different.
Therefore, while wide band characteristics can be realized, the conversion efficiencies of cells
respectively having vibrating membranes having different spring constants are different.
Therefore, the sensitivity may be limited to the smaller conversion efficiency, and the
transmission or reception sensitivity may be reduced.
[0006]
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2
In view of the above problems, according to a capacitive transducer of the present invention, a
first cell on which a first vibrating film including one of two electrodes formed at intervals is
vibratably supported is provided. A second sub-element having a first sub-element having the
second cell, and a second vibrating membrane supported vibratably including a second vibrating
membrane including one of two electrodes formed at a distance from each other; And an element
including In addition, a first detection circuit capable of detecting a signal generated by a change
in capacitance between the two electrodes of the first cell due to the displacement of the first
diaphragm, and a second cell of the second cell due to the displacement of the second
diaphragm. It has the 2nd detection circuit which can detect the signal which arises by the
capacity change between two electrodes, and the summing circuit which adds the signal from the
1st detection circuit and the 2nd detection circuit. The first subelement is electrically connected
to the first detection circuit, and the second subelement is electrically connected to the second
detection circuit. The cutoff frequency of the first detection circuit is different from the cutoff
frequency of the second detection circuit.
[0007]
According to the present invention, an element composed of a plurality of cells is functionally
divided into a plurality of subelements, and different detection circuits are connected to each
subelement to acquire signals, and the acquired signals are summed. Get a signal of one element.
As a result, it is possible to realize a broadband reception band without lowering the reception
sensitivity.
[0008]
A top view showing an example of CMUT concerning an embodiment of the present invention.
The figure which shows an example of the receiving sensitivity of CMUT which concerns on
embodiment of this invention. The figure which shows an example of the receiving sensitivity of
CMUT which concerns on embodiment of this invention. The figure which shows an example of
the output current of CMUT which concerns on embodiment of this invention. The figure which
shows an example of the current voltage gain of CMUT which concerns on embodiment of this
invention. The figure which shows an example of the transimpedance circuit of CMUT which
concerns on embodiment of this invention. The figure which shows an example of the element of
CMUT which concerns on embodiment of this invention. The figure which shows an example of
the element of CMUT which concerns on embodiment of this invention. The figure which shows
an example of the element of CMUT which concerns on embodiment of this invention. The figure
which shows an example of the element of CMUT which concerns on embodiment of this
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3
invention. The figure which shows an example of the receiving sensitivity of CMUT which
concerns on embodiment of this invention. The figure which shows an example of the cell cross
section which CMUT which concerns on embodiment of this invention has. The figure which
shows an example of the photoacoustic apparatus which concerns on embodiment of this
invention. The figure which shows the receiving sensitivity of CMUT which concerns on Example
1 of this invention. The figure which shows the receiving zone | band of CMUT which concerns
on Example 1 of this invention. The figure which shows the minimum received sound pressure of
CMUT which concerns on Example 1 of this invention. The figure which shows the receiving
sensitivity of CMUT which concerns on Example 2 of this invention. The figure which shows the
receiving zone | band of CMUT which concerns on Example 2 of this invention. The figure which
shows the minimum received sound pressure of CMUT which concerns on Example 2 of this
invention. The top view which shows the example of arrangement | positioning of several
subelements.
[0009]
A feature of one aspect according to an embodiment of the present invention is a first subelement and a first sub-element each having a cell of a structure in which a vibrating membrane
including one of two spaced electrodes is vibratably supported. It is to provide an element
including two sub-elements. Then, first and second detection circuits capable of detecting a signal
caused by a change in capacitance between two electrodes of cells of the first and second
subelements, and a summing circuit for summing the signals from these detection circuits And
further comprising Also, the cutoff frequencies of the first and second detection circuits are made
different. Thus, it is intended to obtain a wide band reception band by appropriately combining a
plurality of subelements and a detection circuit having different cutoff frequencies and adding
the signals from these different sets. Capacitance transducers of various configurations may be
included in addition to the embodiments and examples described later as those satisfying the
above-described configuration requirements. For example, as shown in FIG. 20A, there is an
example in which a plurality of subelements are provided by dividing the arrangement area. Also,
as shown in FIG. 20 (b), there is an example where a plurality of sub-elements are provided by
alternately arranging the arrangement areas. Further, as shown in FIG. 20C, there is an example
in which the arrangement area is divided in a checkered pattern and a plurality of sub-elements
are provided. The arrangement region of each subelement may be integrated or divided into a
plurality of regions. Here, the cut-off frequencies (the degree of the magnitude relationship and
the combination do not matter) of the plurality of detection circuits connected to the respective
subelements are different. However, it is preferable to make the overall reception position of the
plurality of sub-elements as far as possible free from deviation when the acoustic wave is
received. Therefore, it is better for the arrangement regions of the plurality of sub-elements to be
overlapped to some extent in an alternating, lattice-like, concentric-like, etc. rather than being
11-04-2019
4
separated separately as shown in FIG.
[0010]
Hereinafter, embodiments of the present invention will be described in detail with reference to
the drawings. In principle, the same components will be denoted by the same reference numerals,
and the description will be omitted or simplified. However, the detailed calculation formulas,
calculation procedures, and the like described below should be appropriately changed depending
on the configuration of the apparatus to which the present invention is applied and various
conditions, and the scope of the present invention is limited to the following description. It is not
intended to
[0011]
A capacitive transducer according to one embodiment (with a CMUT as an example) will be
described using FIGS. 1 and 2. The capacitive transducer 1 of the present embodiment includes
two subelements. In the first sub-element shown in dark gray, the respective second electrodes
17 (see FIG. 12) of the plurality of cells constituting the first sub-element are connected with
each other via the electrode pad 3 Is connected to the detection circuit 5 of FIG. In addition, the
second sub-elements shown in light gray are connected to the second detection circuit 6 through
the electrode pad 4 by connecting the second electrodes 17 of the cell. The cutoff frequencies of
the first detection circuit 5 and the second detection circuit 6 are different. The first detection
circuit 5 and the second detection circuit 6 are connected to the summing circuit 7. The first
electrodes 13 (see FIG. 12) of the cells 2 constituting the two subelements are connected to each
other and connected to the voltage application means 9 through the electrode pads 8. The
number of subelements is not limited to two, and may be three or more, as long as a desired
number of subelements is provided. Further, the number of detection circuits may be increased
according to the number of subelements, and the cutoff frequency of the detection circuit may be
different for each subelement. In the present embodiment, a plurality of sub-elements are
arranged in a concentric or concentric polygon shape. In the present specification, concentric
circles are not limited to true circles, but may be approximately concentric circles.
[0012]
When an acoustic wave such as an ultrasonic wave is received by a capacitive transducer, the
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voltage application means 9 causes the first electrode 13 to generate a potential difference
between the first electrode 13 and the second electrode 17. Apply a DC voltage. Preferably, a
negative voltage is applied to the first electrode 13. When an ultrasonic wave or the like is
received, the vibrating membrane 19 (see FIG. 12) having the second electrode 17 is bent, so that
the distance between the second electrode 17 and the first electrode 13 (the cavity 15 (FIG. The
distance in the depth direction (see) changes, and the capacitance changes. A current flows in the
second electrode 17 due to the change in capacitance. The output current generated from the
cells 2 constituting the first subelement is amplified by the first detection circuit 5 electrically
connected to these cells and converted into a voltage. On the other hand, the output current
generated from the cells 2 constituting the second subelement is amplified by the second
detection circuit 6 electrically connected to these cells and converted into a voltage. The two
signals amplified and converted into voltages by the respective detection circuits are summed up
by the summing circuit 7 to become a voltage signal of one element, and ultrasonic waves and
the like can be extracted as electrical signals.
[0013]
FIG. 2 shows an example of the frequency characteristic of the reception sensitivity of the
capacitive transducer of this embodiment. It is a frequency characteristic of a voltage signal after
amplifying and converting an output current generated by receiving an ultrasonic wave or the
like by a detection circuit. The vertical axis is normalized by the peak value of the reception
sensitivity. The cut-off frequency (about -3 dB frequency) of the first detection circuit (indicated
by a fine broken line) to which the first subelement is connected is 12 MHz, and the second
subelement is connected to the second The cutoff frequency of the detection circuit (indicated by
the coarse dashed line) is 1 MHz. A signal obtained by adding the signal of the first detection
circuit and the signal of the second detection circuit is a signal of the element (indicated by a
solid line). Comparing the -6 dB frequency, the first detection circuit is 2.4 MHz on the low
frequency side (Fmin), 15.6 MHz on the high frequency side (Fmax), and 0 on the low frequency
side (Fmin) in the second detection circuit. .5 MHz, high frequency side (Fmax) is 11.2 MHz.
When these two signals are added, the low frequency side (Fmin) is 0.8 MHz and the high
frequency side (Fmax) is 15.4 MHz, so that a very wide band frequency characteristic can be
obtained.
[0014]
Hereinafter, the details of each component of the present embodiment will be described.
(Transimpedance Circuit) FIG. 6 shows a transimpedance circuit. The transimpedance circuit
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6
includes an operational amplifier 32, feedback resistors 33 and 35, and feedback capacitors 34
and 36. The operational amplifier 32 is connected to the positive and negative power supplies
(VDD, VSS), and the inverting input terminal (−IN) is connected to the second electrode of the
capacitive transducer 1. The output terminal (OUT) is connected to the inverting input terminal (IN) by the feedback resistor 33 and the feedback capacitor 34 connected in parallel, and the
output signal is fed back. The non-inverting input terminal (+ IN) is connected to the ground
terminal (GND) by the feedback resistor 35 and the feedback capacitor 36 connected in parallel.
The voltage of the ground terminal is an intermediate potential between the positive power
supply VDD and the negative power supply VSS. The resistances of the feedback resistors 33 and
35 and the capacitances of the feedback capacitors 34 and 36 are the same. This is preferable
because it eliminates voltage offsets, but this is not a necessary condition. In the present
embodiment, the set value of the feedback resistor 33 and the feedback capacitor 34 is one of
the important factors.
[0015]
(Element Shape, Sub-Element Shape) The shape of the element in the present embodiment is not
particularly limited, and may be, for example, circular or polygonal. In the present specification,
the circular shape is not limited to the shape of a true circle, and may be a substantially circular
shape. As an example of a polygon, shapes, such as a quadrangle, a hexagon, and an octagon, are
mentioned. The shape of the element is preferably substantially circular. Acoustic waves to be
detected often propagate from four sides of the element. Therefore, it is preferable that the
directivity of the element that receives the acoustic wave is wide, and the substantially circular
shape is wider and more preferable than the polygon. Further, as shown in FIG. 7, the abovementioned substantially circular shape is composed of eight or more sides in a polygon
consisting of a line 40 connecting the centers of the cells arranged at the outermost periphery
among the cells constituting the element. It is.
[0016]
Further, the form of the sub-element in the present embodiment is appropriately selected in view
of the above-mentioned problem. Preferably, the subelements are arranged concentrically and
the first subelement is arranged inside the second subelement. For example, as shown in FIG. 8,
the outline of the element is circular, while the line 41 connecting the centers of the cells
arranged at the outermost periphery of the first subelement is octagonal. Further, in the shape of
the second sub-element, a line 40 connecting the centers of the outermost cells is substantially
circular, and a line 42 connecting the centers of the innermost cells is also substantially circular.
11-04-2019
7
The second subelement is hollow in shape. Further, FIG. 9 shows an example of the shape of the
subelement by omitting the cell, the electrode pad and the detection circuit. The elements of FIG.
9 are composed of three subelements. The first subelement 43 has a substantially circular shape
having eight sides. The second sub-element 44 has a hollow shape, the first sub-element side is
substantially circular with eight sides, and the third sub-element side is substantially circular with
12 sides. The third sub-element 45 has a hollow shape, the second sub-element side is
substantially circular with 12 sides, and the outermost periphery is also substantially circular
with 12 sides. The number of subelements may be any desired number, and the number of sides
may be any desired number.
[0017]
As shown in FIG. 10, the shape of the element is a quadrangle, the first sub-element 46 is
substantially circular with eight sides, the second sub-element 47 is a hollow shape, and the first
sub-element side is from eight sides The outermost circumference side may be square. From the
viewpoint of directivity, it is preferable to make the shape of the subelement having a high cutoff
frequency of the detection circuit circular. Because the directivity of high-frequency acoustic
waves is narrow and high in straightness, the intensity of sound increases near the center of the
receiving surface, and the directivity of low-frequency acoustic waves is wide and spreads
radially, so The difference in the intensity of the acoustic wave is reduced. Therefore, in the
example of FIG. 10, detecting the high frequency acoustic wave near the center of the receiving
surface where the first subelement having a high cutoff frequency of the detection circuit has a
higher detection efficiency. That is, by disposing the first subelement connected with the
detection circuit having a high cutoff frequency in a region including the center where high
frequency acoustic waves easily reach, high frequency acoustic waves can be easily detected. In
addition, by disposing a second subelement in which a detection circuit with a low cutoff
frequency is connected to a peripheral area where a low frequency acoustic wave also reaches, it
becomes easy to detect a low frequency acoustic wave. With such a structure, acoustic waves
from low frequency to high frequency can be efficiently detected.
[0018]
(Cut-off frequency) The cut-off frequency of the detection circuit in this embodiment means a
cut-off frequency of a low pass characteristic in which the gain attenuates with an increase in
frequency when the frequency is exceeded. The cutoff frequency of the detection circuit
connected to each subelement is preferably set so as to realize a wide band reception band as a
whole of the CMUT. The assignment of the cutoff frequency to each subelement can be changed
11-04-2019
8
as appropriate depending on the measurement target and the performance of the reception band
to be obtained.
[0019]
In addition, although the cutoff frequency in this embodiment is suitably set according to a
measuring object etc., 0.1 MHz-10 MHz are preferable for the cutoff frequency in the receiving
circuit with a low cutoff frequency, for example. More preferably, it is 0.1 MHz to 5 MHz.
Further, the cutoff frequency in the receiving circuit having a high cutoff frequency is preferably
2 MHz to 20 MHz. More preferably, it is 2 MHz to 15 MHz. That is, what is important in
detection of the photoacoustic wave is that a wide band can be detected from the low frequency
side to the high frequency side, and it is preferable that the low frequency side can be detected
from 0.1 MHz. In the high frequency side (about 20 MHz or more), the attenuation of the
acoustic wave is large in a living body or the like, so in consideration of the attenuation, the
above-mentioned cutoff frequency range is preferable.
[0020]
(Receiving Band of CMUT) The receiving band of a general CMUT will be described with
reference to FIGS. 3, 4 and 5. FIG. FIG. 3 shows reception band characteristics (reception
sensitivity) of the CMUT, FIG. 4 shows output current characteristics of the CMUT, and FIG. 5
shows current-voltage conversion gain characteristics of the detection circuit. The reception band
(reception sensitivity) of the CMUT is determined by the product of the CMUT output current
characteristic and the gain characteristic of the detection circuit. As this detection circuit, a
transimpedance type current voltage amplification circuit is used.
[0021]
The output current I of the CMUT can be expressed as Formula 1 and Formula 2 when the
capacitance change is approximated by parallel plate. I = P / ((Zm + Zr) / (εS * Vb / d ^ 2) +
jωC) (Expression 1) Zm = j * km * ((ω / ω 0 ^ 2) -1 / ω) (Expression 2 Here, P is pressure of
acoustic wave, ε is permittivity of vacuum, S is area of second electrode, Vb is bias voltage
applied between two electrodes, d is gap between electrodes, Zm is vibrating film Mechanical
impedance, Zr is the acoustic impedance of the medium. Also, ω is the angular frequency of the
acoustic wave, C is the total capacitance, km is the spring constant of the diaphragm, and ω 0 is
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9
the resonant frequency. Since total capacitance C is relatively small in Equation 1, it is the
mechanical impedance Zm of the vibrating membrane that is a function of frequency. In addition,
the surface of CMUT is usually used in contact with liquid or gel in many cases. Since the
acoustic impedance Zr of the liquid is larger than the mechanical impedance Zm of the vibrating
membrane, the frequency characteristic of the output current in FIG. 4 is greatly affected. The
frequency at which the mechanical impedance Zm of the vibrating membrane is zero is the
resonant frequency of the vibrating membrane, and at this time, the output current in FIG. 4
takes a maximum value. The peak frequency of the output current of FIG. 4 is 6 MHz.
[0022]
The gain characteristic of the detection circuit shown in FIG. 5 is expressed by Equation 3, and
the cut-off frequency is expressed by Equation 4. G = Rf / (1 + jωRf * Cf) (Expression 3) f ≒ 1 /
(2πRf * Cf) (Expression 4) where G is a circuit gain, Rf is a feedback resistance, Cf is a feedback
capacitance, and ω is an input current Angular frequency, f is the cutoff frequency.
[0023]
Further, in order to stably drive the circuit of FIG. Cf ≧ ((Cin) / (π * GBW * Rf)) ^ 0.5 (Equation
5) where GBW is the gain bandwidth product of the operational amplifier (amplifier gain 0 dB (=
1) × frequency) and Cin is the inverting input of the operational amplifier It is a capacitance
parasitic on the terminal (-IN). In general, when Cin is large, the operation of the operational
amplifier can not catch up, and when Cin is large, the negative feedback circuit becomes unstable
and the circuit itself oscillates and current-voltage conversion can not be performed. , Rf, Cf need
to be selected.
[0024]
For example, in order to change the frequency characteristic of the CMUT of FIG. 3 to the low
frequency side, a method of shifting the resonance frequency of the output current of FIG. 4 to
the low frequency side or shifting the gain characteristic of the detection circuit of FIG. There is.
In order to shift the cutoff frequency of the output current in FIG. 4 to the low frequency side, the
vibrating membrane of the cell may be softened to reduce the spring constant. However, if the
vibrating membrane of the cell is made too soft, the voltage that can be applied to the electrodes
becomes small, and the voltage signal obtained when the acoustic wave is received becomes
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10
small. In addition, since the deflection of the vibrating membrane becomes large and it becomes
difficult to narrow the gap between the electrodes for high sensitivity, the vibrating membrane
needs to have a certain degree of hardness (see Equations 1 and 2). From this, it is preferable to
shift the cutoff frequency of the gain characteristic of the detection circuit of FIG. 5 to the low
frequency side.
[0025]
On the other hand, to change the cutoff frequency of the voltage signal (reception sensitivity) of
FIG. 3 to the high frequency side, the method of shifting the cutoff frequency of the output
current of FIG. 4 to the high frequency side or the gain of the receiving circuit of FIG. There is a
method of shifting the cutoff frequency of the characteristic to the high frequency side. In order
to shift the cutoff frequency of the output current in FIG. 4 to the high frequency side, the
vibrating membrane of the cell may be hardened to increase the spring constant. However, if the
vibrating membrane of the cell is too hard, the large spring constant results in a small voltage
signal obtained when an acoustic wave is received. In addition, since the voltage applied to the
electrodes becomes large, it is necessary to improve the insulation withstand voltage of the
CMUT and to change the device configuration (specifically, change of the power supply, change
for improving the withstand voltage of circuits and devices, etc.). There is a limit to the hardness
of the vibrating film (see Equations 1 and 2). In order to increase the cutoff frequency of the gain
characteristic of the detection circuit of FIG. 5, it is necessary to lower the feedback resistance Rf
to increase the cutoff frequency in order to stably operate the operational amplifier of the
transimpedance circuit. When the feedback resistance Rf is lowered, the gain is lowered, so that
the voltage signal obtained when the acoustic wave is received becomes small. From this, it is
preferable to shift the cut-off frequency of both the output current and the gain characteristic of
the detection circuit in each restriction. As described above, in the case of one output current
(element) and one detection circuit, although it is possible to obtain a wide band frequency
characteristic to some extent, there is a limit.
[0026]
In this embodiment, in order to make the reception sensitivity of FIG. 3 have a wide band
frequency characteristic, detection circuits having different cutoff frequencies are prepared, and
a plurality of cells are connected to each detection circuit. For example, assuming that the
resonance frequency of the output current of the first subelement is 10 MHz and the cutoff
frequency of the first detection circuit is 12 MHz, the frequency of -6 dB of the reception
sensitivity of the first subelement has an Fmin of 2. 4 MHz, Fmax becomes 15.6 MHz. Further,
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assuming that the resonance frequency of the output current of the second subelement is 10
MHz and the cutoff frequency of the second detection circuit is 1 MHz, the frequency of -6 dB of
the reception sensitivity of the second subelement is Fmin of 0. 5 MHz, Fmax becomes 11.2 MHz.
When the reception sensitivities of these two subelements are summed up, the frequency of −6
dB of the summed reception sensitivity becomes Fmin of 0.8 MHz and Fmax of 15.4 MHz. By
summing the output signals obtained from a plurality of subelements connected with detection
circuits having different cutoff frequencies, a very wide band frequency characteristic can be
obtained.
[0027]
The resonant frequencies of the output currents of the first subelement and the second
subelement may be different. If the spring constants of the cells constituting the subelements are
made different, it is possible to change the resonant frequency of the output current of the
subelements. In this case, if the first electrode 13 (see FIG. 12) of the first and second
subelements is common, the DC voltage applied to the first electrode 13 is limited by the (soft)
subelement having a small spring constant. As a result, the output current of the (hard)
subelement having a large spring constant decreases. Therefore, it is preferable to provide the
first electrode 13 for each subelement, and to have a voltage application unit for each
subelement. With this configuration, an optimum DC voltage can be applied to each subelement,
and an acoustic wave can be detected in a state where the output current is high. However, if a
plurality of voltage application means are prepared, the number of wires increases and the load
on the device or circuit increases. From this point of view, the spring constants of the cells
constituting the subelements are substantially the same. Are preferably common.
[0028]
(First electrode (lower electrode) of the cell) DC voltage with the first electrode (lower electrode)
and the second electrode of the cell constituting the subelement being not connected to the
detection circuit as a common electrode Is preferably applied. By applying a common DC voltage
to the common electrode, the number of wirings can be reduced, and the load on the device or
the circuit can be reduced.
[0029]
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12
(Cell Shape) The first cell constituting the first subelement and the second cell constituting the
second subelement can have the same shape and material. Also, the shape and material of each
cell constituting each subelement can be made identical to each other. By making the shape of
the cells constituting the element identical, the sensitivity of the cell having low conversion
efficiency among the cells constituting the element is not limited, so that high band reception
characteristics can be obtained while maintaining high sensitivity. Be The same may not be
completely identical or substantially identical. Here, “substantially the same” indicates that the
size and thickness of the vibrating film of the cell constituting the element, the cavity height, the
insulating film, and the like are within the range of about manufacturing variation. The same
applies to the materials. The range of variation is preferably within a range of 0.5 times to 1.5
times centered on the reference.
[0030]
(Number of Cells Constituting Elements) The number of cells constituting an element may be any
number, and is not limited to the number shown in FIG. It is preferred that the number be such
that the desired resolution and sensitivity are obtained. The number of cells constituting the sub
element is also not particularly limited. However, when the peak frequency of the output current
of the first subelement and the second subelement is between the cutoff frequencies of the first
and second detection circuits, the number of cells of the first subelement is It is preferable to
make it more than the number of cells of a 2nd subelement. More preferably, the number of cells
of the first subelement is in the range of 55% to 95% with respect to the total number of cells
possessed by the element. When the peak frequency of the output current of the first subelement
and the second subelement is larger than the cutoff frequency of the first and second detection
circuits, the number of cells of the first subelement and the number of cells of the first
subelement It is preferable that the number of cells of the two subelements be approximately the
same. More preferably, the number of cells of the first subelement is in the range of 25% to 75%
of the total number of cells.
[0031]
As shown in Equation 3, when the feedback resistance Rf is increased, the gain characteristic of
the detection circuit is increased, and the reception sensitivity is improved. As shown in the
following equation 6 derived from the equations 4 and 5, when the cutoff frequency f is
increased, it is necessary to increase the feedback capacitance Cf necessary to prevent oscillation
of the detection circuit. Therefore, it is only necessary to reduce the feedback resistance Rf to
increase the cutoff frequency, so the circuit gain decreases and the reception sensitivity
11-04-2019
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decreases as the cutoff frequency increases. Cf C ((Cin * 2 * f * Cf) / GBW) (Equation 6)
[0032]
On the other hand, in the detection circuit having a low cutoff frequency, since the feedback
capacitance Cf required to prevent oscillation of the detection circuit is small, the circuit gain can
be increased by increasing the feedback resistance Rf. Further, even if the feedback capacitance
Cf is increased to reduce the feedback resistance Rf, a sufficient circuit gain can be obtained, so
the design freedom is high. In order to add together the reception sensitivities of the two
subelements to widen the reception band, it is preferable that the magnitudes of the two
reception sensitivities be equal to or greater than that shown in FIG. Here, being equal to or
greater than means that the ratio of the peak values of the two reception sensitivities is 0.5 or
more and 0.9 or less. It is preferable that the ratio of peak values of the two reception
sensitivities be 0.7 or more. If either one of the reception sensitivities is extremely large, the
reception band narrows.
[0033]
A method of obtaining a wide band reception sensitivity as shown in FIG. 11 will be described.
The case where the peak frequency of the output current of the first and second subelements is
between the cutoff frequencies of the first and second detection circuits will be described. In
order to maximize the reception sensitivity after addition, it is preferable to adjust the gain of the
second detection circuit to be a wide band by maximizing the gain of the first detection circuit in
the range where the detection circuit does not emit light. In this case, as described above, if the
number of cells constituting the first sub-element is larger than the number of cells constituting
the second sub-element, the reception sensitivity of the first sub-element can be increased. It is
preferable because the later reception sensitivity can be increased.
[0034]
Next, the case where the peak frequency of the output current of the first and second
subelements is larger than the cutoff frequency of the first and second detection circuits will be
described. In order to maximize the reception sensitivity after addition, it is preferable to adjust
the gain of the second detection circuit to be a wide band by maximizing the gain of the first
detection circuit in the range where the detection circuit does not emit light. In this case, as
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14
described above, if the number of cells constituting the first subelement and the number of cells
constituting the second subelement are approximately the same, the reception sensitivity of the
first subelement can be increased. It is preferable because the reception sensitivity after addition
can be increased. Although the number of subelements constituting the element may be any
number, it is preferably about two in consideration of the load of the device and the detection
circuit.
[0035]
(Cell) The cell structure constituting the element according to the present embodiment will be
described with reference to FIGS. 1 and 12. The cell structure 2 includes a substrate 11, a first
insulating film 12 formed on the substrate 11, a first electrode 13 formed on the first insulating
film 12, and a second insulating film on the first electrode 13. It has a membrane 14.
Furthermore, the cell structure 2 has a vibrating film 19 composed of the membrane 16, the
second electrode 17 and the sealing film 18. The vibrating film 19 is disposed apart from the
second insulating film 14 by a cavity 15 which is an interval. When the substrate 11 is an
insulating substrate such as a glass substrate, the first insulating film 12 may be omitted. The
shape of the gap 15 viewed from the top is circular, and the shape of the vibrating part is
circular, but may be square, rectangular or the like. In addition, voltage application means 9 for
applying a voltage between the first electrode 13 and the second electrode 17 of the cell 2 and a
receiving circuit 20 for amplifying the electric signal extracted from the second electrode 17 are
included. doing. The first electrode 13 and the second electrode 17 face each other, and a bias
voltage is applied between the first electrode 13 and the second electrode 17 from the voltage
application means 9. The cell 2 can derive an electrical signal from the second electrode 17.
[0036]
In the present embodiment, the electric signal is drawn from the second electrode 17, but the
electric signal may be drawn from the back side of the substrate 11 using a through wiring or the
like. Moreover, in this embodiment, although the 1st electrode 13 is made into a common
electrode and an electrical signal is drawn from the 2nd electrode 17, you may make it a reverse
structure. That is, the second electrode 17 may be used as a common electrode and the first
electrode 13 may be used as an extraction electrode.
[0037]
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15
(Driving Principle of Capacitance Transducer) In the case of receiving an acoustic wave such as
an ultrasonic wave by the capacitance transducer according to the present embodiment, the
voltage applying means 9 comprises the first electrode 13 and the second electrode 17. A DC
voltage is applied to the first electrode 13 so that a potential difference is generated between
them. When an ultrasonic wave or the like is received, the vibrating film having the second
electrode 17 is bent, so the distance between the second electrode 17 and the first electrode 13
(the distance in the depth direction of the cavity 15) changes and the capacitance Changes. A
current flows in the second electrode 17 due to the change in capacitance. The output current
generated from the cell 2 constituting the first subelement is amplified by the first receiving
circuit 5 and converted into a voltage. On the other hand, the output current generated from the
cell 2 constituting the second subelement is amplified by the second receiving circuit 6 and
converted into a voltage. The two signals amplified and converted into voltages by the respective
receiving circuits are summed up by the summing circuit 7 to become a voltage signal of one
element, and an acoustic wave can be extracted as an electric signal. As described above, the
configuration of the electrodes may be changed to apply a DC voltage with the second electrode
as the common electrode, and the first electrode may be divided into subelements and connected
to the respective receiving circuits. .
[0038]
Further, when transmitting an ultrasonic wave or the like, a DC voltage is applied to the first
electrode (one electrode), an AC voltage is applied to the second electrode (the other electrode),
and the vibrating film 19 is vibrated by electrostatic force. Let Ultrasonic waves and the like can
be transmitted by this vibration. The configuration of the electrodes may be changed, a DC
voltage may be applied with the second electrode as a common electrode, the first electrode may
be divided into subelements, an AC voltage may be applied, and the diaphragm may be vibrated.
[0039]
The electric signal of the one element corresponds to one pixel, and the amplitude and phase
information of the acoustic wave are averaged. In a diagnostic device or the like using the
capacitive transducer 1, an image of an object (measurement object) is formed based on
amplitude and phase information in pixel units.
[0040]
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16
(Photoacoustic Apparatus) FIG. 13 is a block diagram showing a photoacoustic apparatus
according to an embodiment of the present invention. The parts corresponding to FIG. 1 are
assigned the same reference numerals and descriptions thereof will be omitted unless necessary.
The photoacoustic apparatus 500 (hereinafter, abbreviated as “apparatus 500”) of the present
embodiment is characterized in the configuration of the probe (probe) 522. The probe 522 has a
plurality of conversion elements 532 and a holder 534. Each conversion element is a CMUT
according to the above embodiment. The holder 534 is formed in a substantially spherical crown
shape, and holds a plurality of conversion elements 532 along the substantially spherical crown
shape. The conversion elements 532 are held such that the direction of highest reception
sensitivity is concentrated. In the present embodiment, the direction in which the reception
sensitivity of each of the plurality of conversion elements 532 is the highest is directed to a
region including the substantially spherical crown-shaped curvature center of the holder 534.
The output ends of the analog electrical signal of the conversion element 532 are each connected
to the signal wiring. The analog electrical signals output from the conversion elements 532 are
combined by the signal line 536 configured by connecting the signal lines in common, and are
sent to the signal collection unit 240 via the signal line 536. However, the present invention is
not limited to this, and the analog electrical signals output from each of the conversion elements
532 are not combined by the signal line 536 configured by the common connection of signal
lines, but separate signal collecting units 240 as separate signals in parallel. It may be sent out.
[0041]
The irradiation unit 508 is integrated with the probe 522 by being held at the center of the
holder 508. The irradiation unit 508 irradiates the subject 210 with the light 100. In the present
embodiment, the light generated from the light source 200 capable of emitting (oscillating) light
in a pulse shape is guided to the irradiation unit 508 through the mirror 60 and irradiates the
light from the probe 522 side (see FIG. Light in the Z direction). The driving device or position
control unit 538 moves the probe 522. The position control unit 538 moves, for example, the
probe 522 in a spiral shape, and the irradiation unit 508 is an arbitrary position for irradiating
light at a position on a spiral trajectory which is moved by itself by the spiral movement. The
light 100 may be emitted at the position of. In this case, the irradiation unit 508 integrated with
the probe 522 may irradiate the light 100 for each acoustic wave receiving position (light
irradiation position) along with the spiral movement by the position control unit 538. The
conversion element 532 may receive an acoustic wave based on this irradiation, convert it into
an analog electrical signal, and send it to the signal collection unit 240. Thus, when the acoustic
matching liquid is provided between the probe 522 and the subject 210, acoustic wave noise due
to the vibration of the acoustic matching liquid due to the movement of the probe 522 can be
11-04-2019
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reduced. The light source 200 is not limited to one that emits light in a pulse shape, and may be a
light source that emits continuous light, such as a light emitting diode (LED).
[0042]
When light is incident on the subject 210, the absorbers 120 and 140 absorb the light and
thermally expand to generate an acoustic wave 180. The probe 522 receives this acoustic wave,
and the received signal is sent to the signal collecting unit 240 through the signal line 536, and
the image processing unit 260 performs image processing to acquire information inside the
object. it can. The image-processed data can be displayed by the image display unit 280. The
circuit unit may include a circuit unit that transmits and receives signals between the conversion
element, which is a capacitive transducer, and the image processing unit, and a control unit that
controls the image processing unit and the circuit unit. In transmission and reception, when
switching between transmission and reception, beam forming, and the like, the circuit unit is
controlled from the system side.
[0043]
Hereinafter, more specific examples will be described. EXAMPLE 1 This example is a capacitive
transducer composed of two subelements and two detection circuits. Here, reception when the
peak frequency of the output current of the two subelements is between the cutoff frequencies of
the first and second detection circuits and the ratio of the number of cells constituting the two
subelements is changed The band will be described.
[0044]
First, the capacitive transducer of this embodiment will be described. The element of the
capacitive transducer 1 is substantially circular with a diameter of 2 mm, and is composed of two
subelements as shown in FIG. The cells are circular in shape and the diameter of the cavity 15 is
36 μm. Adjacent cells are arranged at an interval of 39 μm. Although the number of cells is
omitted in FIG. 8, the total number of cells actually arranged in the element is 2400. As shown in
FIG. 12, the cell 2 has a 300 μm thick silicon substrate 11, a first insulating film 12 on the
silicon substrate 11, a first electrode 13 on the first insulating film 12, and a first electrode 13.
And the second insulating film 14 of FIG. Furthermore, a vibration film 19 including a second
electrode 17, a membrane 16, and a sealing film 18, and a cavity 15 are provided. The height of
11-04-2019
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the cavity 15 is 150 nm. Furthermore, a voltage application means 9 for applying a bias voltage
between the first electrode and the second electrode and a detection circuit 21 are provided.
[0045]
The first insulating film 12 is a silicon oxide film having a thickness of 1 μm formed by thermal
oxidation. The second insulating film 14 is a 50 nm silicon oxide film formed by plasma
enhanced chemical vapor deposition (PE-CVD). The first electrode 13 is titanium having a
thickness of 50 nm, and the second electrode 17 is an aluminum alloy having a thickness of 100
nm. The membrane 16 and the sealing film 18 are silicon nitride films produced by PE-CVD, and
are formed with a tensile stress of 450 MPa or less. The thickness of the membrane 16 is 400
nm, and the thickness of the sealing film 18 is 850 nm. With such cells, the proportion of the
number of cells constituting the first subelement is changed to produce a capacitive transducer
as shown in FIG.
[0046]
The cutoff frequency of the first detection circuit is 8 MHz, and the cutoff frequency of the
second detection circuit is 1 MHz. The peak frequency of the output current of the two
subelements is 7 MHz. The number of cells constituting the element is 2400, and the shapes of
the cells are substantially the same within the range of manufacturing variation. When the ratio
of the number of cells constituting the first subelement to the total number of cells constituting
the element is 94%, 75%, 57%, 25% and 6%, the maximum value of the reception sensitivity is
shown in FIG. Shown in. Also, FIG. 15 shows the reception band (showing in% the extent to which
it has spread compared to the conventional case), and FIG. From FIG. 14, the reception sensitivity
is maximized when the ratio of the number of cells of the first subelement is around 75%.
Further, as shown in FIG. 15, the reception band is wide by adopting the form of the present
embodiment. Further, as shown in FIG. 16, the minimum received sound pressure is minimum
when the ratio of the number of cells of the first subelement is around 75%. The minimum
received sound pressure is S / N when an acoustic wave is received, and is calculated by the
minimum received sound pressure = 2 ^ 0.5 × integrated noise / maximum sensitivity. The
integration noise is an integration value of circuit noise generated when the CMUT is connected
to the detection circuit, and in the present embodiment, the integration range is 0.5 MHz to 20
MHz. Since the smaller the minimum received sound pressure, the smaller the acoustic wave can
be detected, it is preferable to reduce the minimum received sound pressure.
11-04-2019
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[0047]
From the above, when the peak frequency of the output current of the cell constituting the
element is between the cutoff frequencies of the first and second detection circuits, the number
of cells of the first subelement is set to the second It is preferable to make it more than the
number of cells of a subelement. By setting the number of cells of the first subelement in the
range of 55% to 95% with respect to the total number of cells, acoustic waves can be detected in
a wide band and with high sensitivity, which is preferable.
[0048]
(Embodiment 2) This embodiment is a capacitive transducer comprised of two subelements and
two detection circuits. Here, the peak frequency of the output current of the cells of the two
subelements is larger than the cutoff frequency of the first and second detection circuits, and the
ratio of the number of cells constituting the two subelements is changed. The reception band will
be described. The capacitive transducer of this embodiment can be manufactured in the same
manner as in Embodiment 1. In the present embodiment, the sealing film 18 is 1550 nm.
[0049]
The cutoff frequency of the first detection circuit is 8 MHz, and the cutoff frequency of the
second detection circuit is 1 MHz. The peak frequency of the output current of the capacitive
transducer is 14 MHz. The number of cells constituting the element is 2400, and the shapes of
the cells are substantially the same within the range of manufacturing variation. When the ratio
of the number of cells constituting the first subelement to the total number of cells constituting
the element is 94%, 75%, 57%, 25% and 6%, the maximum value of the reception sensitivity is
shown in FIG. The reception band is shown in FIG. 18 and the minimum reception sound
pressure is shown in FIG. According to FIG. 17, the reception sensitivity is maximized when the
ratio of the number of cells of the first subelement is around 75%. Further, as shown in FIG. 18,
the reception band is wide by adopting the form of this embodiment. Further, as shown in FIG.
19, the minimum received sound pressure is minimized when the ratio of the number of cells of
the first subelement is around 50%. In the present embodiment, the integration range is 0.5 MHz
to 20 MHz and 0.5 MHz to 22 MHz. Since changing the noise integration range changes the
minimum received sound pressure, it is preferable that the minimum received sound pressure is
minimized in the frequency range to be used.
11-04-2019
20
[0050]
From the above, when the peak frequency of the output current of the cell constituting the
element is larger than the cutoff frequency of the first and second detection circuits, the number
of cells of the first sub element and the second sub It is preferable to make the number of cells of
the element approximately the same. By setting the number of cells of the first subelement in the
range of 25% to 75% with respect to the total number of cells, it is preferable because the
acoustic wave can be detected in a wide band and with high sensitivity.
[0051]
The present invention can be applied to a photoacoustic apparatus for obtaining in-vivo
information, a conventional ultrasonic diagnostic apparatus, and the like. That is, it is possible to
realize an object information acquiring apparatus which receives an acoustic wave from an object
and acquires information of the object by using an ultrasonic probe including a plurality of
capacitance type transducers of the present invention. As an example of the information
acquisition apparatus, a light source for irradiating light to an object, an ultrasonic probe for
detecting an acoustic wave from the object excited by the light irradiation, and a signal
processing unit for converting a detection signal into image information And an ultrasonic
diagnostic apparatus comprising: In addition, an ultrasonic probe including a plurality of
capacitance type transducers according to the present invention capable of transmitting an
acoustic wave to a subject and detecting an ultrasonic wave or the like reflected by the subject,
and for converting a detected signal into image information There is an ultrasonic diagnostic
apparatus including a signal processing unit. The signal processing unit processes the signal to
construct an object image. Furthermore, the present invention can be applied to other
applications such as an ultrasonic flaw detector.
[0052]
1: Capacitive transducer 2: cell (first cell, second cell) 5: first detection circuit 6: second detection
circuit 7: summing circuit 13: first electrode 15: distance (cavity 16) Vibrating film (first
vibrating film, second vibrating film) 17: second electrode
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