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JP2014197846

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DESCRIPTION JP2014197846
Abstract: To provide an electromechanical transducer capable of arbitrarily changing the
deflection amount of a vibrating film in an element unit. An electromechanical transducer
includes an element having at least one cell including a first electrode and a second electrode
provided with a gap between the first electrode and the first electrode. A plurality of the first
electrodes 102 and the second electrodes 105 are electrically separated for each element 108,
and a DC voltage is applied to the first electrode 102 to apply a DC voltage independently for
each element 108. Have means. [Selected figure] Figure 1
Capacitance type electromechanical transducer
[0001]
The present invention relates to a capacitive electromechanical transducer such as a capacitive
ultrasonic transducer.
[0002]
As an electromechanical transducer that performs at least one of transmission and reception of
ultrasonic waves, a capacitive micromachined ultrasonic transducer (CMUT), which is a
capacitive ultrasonic transducer, has been proposed (see Patent Document 1).
CMUT is produced using the MEMS process which applied the semiconductor process.
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1
[0003]
FIG. 7 shows a schematic view of a cross section of the CMUT. Here, the first electrode 102 and
the second electrode 105 opposed to the diaphragm 101 across the gap 104 are referred to as a
cell as one set. The vibrating film 101 is supported by a support portion 103 formed on a
substrate 106. The first electrodes 102 are all electrically connected in the CMUT, and a
predetermined DC voltage is applied to the first electrodes 102 so that a desired potential
difference is generated between the first electrodes 102 and the second electrodes 105. It is
uniformly applied. The other second electrode 105 is electrically separated for each element (an
element which is an assembly of cells), and is applied between the first and second electrodes by
applying an AC drive voltage. An alternating electrostatic attraction is generated, and the
vibrating membrane 101 can be vibrated at a certain frequency to transmit an ultrasonic wave.
In addition, when the vibrating membrane 101 receives and vibrates an ultrasonic wave, a
minute current is generated in the second electrode 105 by electrostatic induction, and the
reception signal can be taken out for each element by measuring the current value. .
[0004]
The characteristics at the time of transmitting and receiving these ultrasonic waves are
determined by the amount of deflection of the diaphragm 101 when a DC voltage is applied to
the first electrode 102. Normally, the pressure inside the cell gap 104 is reduced below
atmospheric pressure, and the diaphragm 101 is bent toward the substrate 106 due to the
difference between the atmospheric pressure and the internal pressure of the gap 104. The
amount of deflection of the vibrating membrane 101 is determined by mechanical characteristics
determined by parameters such as the size, shape, thickness, and film quality of the vibrating
membrane. Further, when operating the CMUT, a predetermined potential difference is applied
between the two electrodes in order to enhance the efficiency of transmission and reception of
ultrasonic waves, and the electrostatic attraction generated between the electrodes further
causes the vibrating film 101 to be on the substrate 106 side. It will be in a bent state. When
transmitting ultrasonic waves, electrostatic attraction is inversely proportional to the square of
the distance, so it is more efficient to bring the electrodes closer together. On the other hand, in
the case of receiving ultrasonic waves, the magnitude of the minute current to be detected is
inversely proportional to the distance between the electrodes and proportional to the potential
difference between the electrodes. It becomes.
[0005]
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Special Table 2003-527947
[0006]
However, when the potential difference between the two electrodes is increased, the electrostatic
force acting between the electrodes and the force generated by the pressure difference exceed
the restoring force of the mechanical characteristics of the vibrating membrane.
As a result, the vibrating membrane contacts the electrode on the substrate, resulting in a state
called collapse, and the characteristics of the CMUT are largely changed. Therefore, when the
CMUT normally operates (when not driven in the collapsed state), the potential difference
between the electrodes is set so that the transmission / reception efficiency is high, and the
deflection amount of the vibrating membrane which does not cause the collapse is obtained.
[0007]
Conventionally, all the first electrodes in the CMUT are electrically connected in the CMUT.
Therefore, when operating the CMUT, voltage is uniformly applied to the first electrode. In the
CMUT, variations occur in the above-mentioned vibration film parameters due to various factors
in production. Therefore, even when there is no potential difference between the electrodes, the
deflection of the vibrating membrane varies. Furthermore, the amount of deflection of the
vibrating film during operation also varies.
[0008]
As a result, the efficiency of transmission and reception deviates from an expected value, and in
some cases, collapse occurs in some cells, and the characteristics of transmission and reception
change greatly on an element basis. An object of the present invention is to provide a
capacitance-type electromechanical transducer capable of arbitrarily changing the deflection
amount of a vibrating film in an element unit in order to solve these problems.
[0009]
In the electromechanical transducer according to the present invention, a plurality of elements
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each having one or more cells each including a first electrode and a second electrode provided
with a gap between the first electrode are formed. In the electromechanical transducer, the first
electrode and the second electrode are electrically separated for each element, and a DC voltage
is applied to the first electrode independently for each element. It is characterized in that it
comprises a direct current voltage application means.
[0010]
By using the electromechanical transducer according to the present invention, the amount of
deflection of the vibrating membrane in the electromechanical transducer can be arbitrarily
adjusted in element units.
[0011]
It is a figure explaining the electromechanical transducer concerning a 1st embodiment.
It is a figure explaining the electromechanical transducer concerning a 1st embodiment.
It is a figure explaining the electromechanical transducer concerning a 2nd embodiment. It is a
figure explaining the electromechanical transducer concerning a 3rd embodiment. It is a figure
explaining the electromechanical transducer concerning a 4th embodiment. It is a figure
explaining the electromechanical transducer concerning 5th Embodiment. It is a figure explaining
the conventional electrostatic capacitance type electromechanical transducer.
[0012]
Hereinafter, an embodiment of a capacitive electromechanical transducer according to the
present invention will be described in detail using the drawings.
[0013]
First Embodiment FIG. 1 is a schematic view of a cross section of an electromechanical
transducer according to the present embodiment.
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In this embodiment, the first electrode is described as the upper electrode, and the second
electrode is described as the lower electrode. The vibrating membrane 101 on which the upper
electrode 102 is formed is supported by the support portion 103 formed on the substrate 106,
and vibrates together with the upper electrode 102. A lower electrode 105 is formed on the
substrate 106 at a position facing the upper electrode 102 on the vibrating membrane 101 with
the gap 104 therebetween. In the present invention, a configuration provided with an upper
electrode and a lower electrode facing each other across one gap 104 is called a cell 107. The
element (element) 108 has one or more of the cells 107. Specifically, one cell 107 or a plurality
of (at least two) cells are electrically connected (connected in parallel). Although one cell is
formed by two cells in FIG. 1, the present invention is not limited to this, and a plurality of cells
may be connected in a two-dimensional array. In addition, it is assumed that a plurality of
elements are formed (two or more formed).
[0014]
The upper electrode used in the present invention is selected from metals selected from Al, Cr, Ti,
Au, Pt, Cu, Ag, Mo, Ta, Ni, AlSi, AlCu, AlTi, MoW, AlCr. At least one of the alloys can be selected
and used. In addition, the upper electrode may be provided on at least one of the upper surface,
the back surface, and the inside of the vibrating film, or when the vibrating film is formed of a
conductor or a semiconductor, the vibrating film itself may serve as the upper electrode. is there.
Moreover, as the lower electrode used in the present invention, the same metal as the upper
electrode can be used. When the substrate is a semiconductor substrate such as silicon, the
substrate may double as the lower electrode.
[0015]
FIG. 2 shows a configuration diagram of the electromechanical transducer of the present
embodiment. The electromechanical transducer of this embodiment is characterized in that not
only the lower electrode 105 but also the upper electrode 102 is electrically separated for each
element (each element). In the element, the plurality of upper electrodes 102 and lower
electrodes 105 are electrically connected. In the present embodiment, the upper electrode in the
element (in the element) is individually formed for each cell and electrically connected by the
wiring (not shown) formed on the vibrating film, but it is possible to form one element
Alternatively, one upper electrode may be formed. Further, the lower electrode may be formed
individually for each cell as in this embodiment, or one lower electrode may be formed in an
element unit.
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[0016]
A DC voltage application unit 201 is connected to the upper electrode 102 in an element unit.
The DC voltage application unit 201 applies a desired voltage to the upper electrode
independently for each element, and generates a potential difference between the lower
electrode and the potential between the electrodes. The amount of deflection of the vibrating
membrane 101 is determined by this potential difference. Further, drive detection means 202 are
connected to the lower electrode 105 in units of elements. The drive detection means 202 is
composed of an AC voltage generation means 203, a current detection means 205, and a
protection switch 204.
[0017]
In the configuration of the present embodiment, assuming that the number of elements in the
electromechanical transducer is N, the DC voltage application means 201 also has N of the same
number. The drive detection means 202 also has the same number of elements.
[0018]
Next, the operation of the drive detection unit 202 at the time of ultrasonic wave transmission
and at the time of reception will be described. When transmitting ultrasonic waves, an alternating
voltage is applied by the alternating voltage generation means 203 connected to the lower
electrode 105. As a result, an alternating potential difference is generated between the upper
electrode 102 and the lower electrode 105, and an alternating electrostatic attraction is
generated in the vibrating film 101. The vibrating film 101 vibrates by this electrostatic
attraction, and transmits an ultrasonic wave. At the time of ultrasonic wave reception, the
protection switch 204 connected to the lower electrode 105 is turned off, thereby protecting the
input part of the current detection means 205 from the voltage generated by the AC voltage
generation means 203.
[0019]
When receiving ultrasonic waves, the AC voltage generation means 203 is in a high impedance
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state and does not affect the potential of the lower electrode 105. On the other hand, the
protection switch 204 is turned ON, and the lower electrode 105 and the input part of the
current detection means 203 are connected. When the vibrating membrane 101 vibrates due to
the ultrasonic wave applied from the outside, the capacitance change between the upper and
lower electrodes occurs. Since the upper electrode is fixed at a predetermined potential, a minute
current flows in the wiring of the lower electrode 105 due to the induced charge generated in the
lower electrode 105. By detecting the change of the minute current by the current detection
means 205, it is possible to detect the size of the ultrasonic wave which has caused the change in
capacity. The potential of the lower electrode is fixed at a predetermined potential by the drive
detection means 202 except when transmitting the ultrasonic wave.
[0020]
According to the configuration of the present embodiment, not only the lower electrode but also
the upper electrode is electrically separated in element units, and furthermore, the DC voltage
application means is connected to the upper electrode in element units, so that each element In
addition, DC voltage can be applied independently. Therefore, different electrostatic attractive
forces can be applied to each element, and the deflection amount of the vibrating membrane can
be adjusted. Therefore, the dispersion | variation in the characteristic of ultrasonic transmission /
reception can be reduced.
[0021]
In this embodiment, the DC voltage generating means 203 is connected to the upper electrode
102 and the current detecting means 205 is connected to the lower electrode 105. However, the
lower electrode is connected to the DC voltage generating means, and the upper An electrode
may be connected to the current detection means. In addition, the configuration of the drive
detection means is not limited to the configuration of the present specification, and
configurations other than those described, transmission only, or reception only may be used.
[0022]
Second Embodiment Next, a second embodiment will be described with reference to FIG. The
second embodiment relates to the configuration of wiring from the upper electrode and the lower
electrode. Other than that, it is the same as the first embodiment.
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[0023]
FIG. 3 shows a cross-sectional view of the electromechanical transducer according to the present
embodiment. The through wiring substrate 303 is a lower electrode through wiring 301 (a
through wiring for a second electrode) and an upper electrode through wiring 302 (a through
wiring for a first electrode), which are two types of wirings penetrating the substrate. Have. The
lower electrodes 105 in one element are all connected to one lower electrode through wiring
301. The lower electrode through wiring 301 penetrates from the surface on the lower electrode
side of the through wiring substrate 303 to the surface on the printed circuit board side, and is
connected to the bump electrode 304. The upper electrode 102 is also connected to the upper
electrode through wiring 302 in an element unit, and the upper electrode through wiring 302 is
connected to the bump electrode 304.
[0024]
Bumps 305 are respectively formed on the bump electrodes 304 formed on the side surface of
the printed circuit board of the through wiring substrate 303. The respective wires are connected
to the wires on the printed circuit board 306 via the bumps 305. The electrical signal of the
lower electrode 105 is input to the drive detection means 202 via a wire on the printed circuit
board electrically connected to the lower electrode through wire 302. The electric signal of the
upper electrode 102 is input to the DC voltage application unit 201 via the wiring on the printed
circuit board 306 electrically connected to the upper electrode through wiring 303.
[0025]
The configuration of the present embodiment is characterized by having the same number of
lower electrode through wirings and upper electrode through wirings as the number of elements.
Thus, even when a plurality of elements are provided, the wiring of the upper electrode can be
taken out on the back surface of the substrate while the wiring of the upper electrode 102 is
separated for each element. For this reason, the wiring of the upper electrode can be connected
to the plurality of DC voltage application means 201, respectively, with almost no reduction in
the area of the element for transmitting and receiving the ultrasonic waves (without almost
reducing the transmission and reception efficiency).
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[0026]
Third Embodiment Next, a third embodiment will be described with reference to FIG. The third
embodiment is characterized in that it has control signal generation means for instructing a DC
voltage to be applied to the DC voltage application means 201. Other than that is the same as any
of the first to second embodiments.
[0027]
FIG. 4 shows a configuration diagram of the electromechanical transducer according to the
present embodiment. The vibrating film state detecting means 401 detects the amount of
deflection of the vibrating film 101 (that is, the same as detecting the inter-electrode distance
between the upper electrode and the lower electrode). The magnitude of the current detected by
the lower electrode is inversely proportional to the second power of the distance between the
upper electrode and the lower electrode (hereinafter referred to simply as the distance between
the electrodes) and proportional to the potential difference between the electrodes. In this case,
by connecting a current detection means to the lower electrode 105, it is used as a diaphragm
state detection means.
[0028]
By using the vibrating film state detection means 401 of this embodiment, for example,
ultrasonic waves of a single frequency are transmitted from the outside, and the current
generated at the lower electrode is detected for each element, thereby making it possible to The
difference in deflection can be detected. Alternatively, an AC voltage may be superimposed on a
DC voltage applied to the upper electrode, and a current generated by the superimposed AC
voltage may be detected (described later as a fourth embodiment). Furthermore, as the vibrating
membrane state detecting means, means other than detecting the current may be used. For
example, the deflection amount of the vibrating membrane may be directly measured. As an
example, it is possible to use a method of detecting deflection of a vibrating film using a
piezoresistive effect or a method of optically detecting the amount of deflection.
[0029]
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The signal detected by the vibrating film state detection means is input to the control signal
generation means 402. Based on the detected signal, the control signal generation unit 402
outputs a signal indicating a direct current voltage to be applied to the direct current voltage
application unit 201 so that the deflection amount of the vibrating membrane 101 of the CMUT
becomes a desired state. . The DC voltage application unit 201 generates a DC voltage based on
the signal instructed by the control signal generation unit 402, and applies the DC voltage to the
upper electrode 102. The DC voltage application means 201 performing such an operation can
be easily formed by using a voltage control transmission circuit and a capacitor.
[0030]
According to this embodiment, it is possible to detect the amount of deflection of each vibrating
film 101 for each element. Further, since the DC voltage can be applied to the upper electrode for
each element so as to reduce the deflection amount variation for each element, the deflection
amount of the vibrating membrane can be made more uniform. In addition, even when a
parameter that affects the vibrating membrane 101 changes due to a change with time or a
change in the environment, the state of the vibrating membrane 101 can be adjusted for each
element.
[0031]
Fourth Embodiment Next, a fourth embodiment will be described using FIG. The fourth
embodiment is characterized in that it has an AC voltage superposition means 403 for
superposing an AC voltage on a DC voltage applied to the upper electrode. Other than that is
substantially the same as the third embodiment. FIG. 5 shows a configuration diagram of an
electromechanical transducer according to the present embodiment.
[0032]
The AC voltage superimposing means 403 is connected to the upper electrode 102. An
alternating current voltage is superimposed on the direct current voltage applied by the direct
current voltage application unit 201 with respect to the upper electrode 102 only during a
period in which the deflection amount detection (deflection amount variation measurement) of
the vibrating film is performed. The alternating voltage superimposed on the upper electrode
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102 induces charges in the lower electrode 105 even when the vibrating membrane 101 does
not vibrate, and a current is generated from the lower electrode 105. This current has a value
corresponding to the inter-electrode distance between the upper electrode 102 and the lower
electrode 105, assuming that the AC voltage to be superimposed is constant. Therefore, by
detecting this current by the current detection means 205, it is possible to detect the amount of
deflection of the diaphragm 101 in the form of the distance between the electrodes.
[0033]
It is desirable to avoid the frequency to which the vibrating membrane 101 responds as the
frequency of the AC voltage to be superimposed. Thus, only the inter-electrode distance can be
detected without vibrating the vibrating membrane 101 by the superimposed AC voltage.
[0034]
A signal switching unit 404 is connected to the output of the current detection unit 205, and
outputs an output signal to the control signal generation unit 402 during a period in which the
deflection amount variation measurement is performed. On the other hand, during a period when
deflection amount variation measurement is not performed (when measuring an ultrasonic wave
by vibration of a vibrating film), an output signal is output as a sensor output to the outside of an
image processing apparatus or the like. By having the signal switching means 404, the current
detection means 205 can be used for both of the measurement of deflection amount variation
and the measurement of ultrasonic waves.
[0035]
From the input signal, the control signal generation means 402 outputs a signal for instructing a
DC voltage to the DC voltage application means 201 so as to keep the deflection amount of the
diaphragm 101 in a desired state.
[0036]
According to the configuration of the present embodiment, it is possible to detect the amount of
deflection of the vibrating film for each element by superimposing alternating current voltage
and detecting the current generated in the lower electrode by the current detection means.
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Further, by avoiding the frequency at which the vibrating membrane vibrates as the frequency of
the AC voltage to be superimposed, it is possible to detect the deflection amount of the vibrating
membrane (the distance between the upper and lower electrodes) without vibrating the vibrating
membrane. Therefore, the mechanical vibration characteristics of the vibrating film can be
removed, so that the measurement can be performed with higher accuracy. Further, since the DC
voltage can be applied to the upper electrode for each element so as to reduce the deflection
amount variation for each element, the deflection amount of the vibrating membrane can be
made more uniform. In addition, even when the parameter that affects the vibrating membrane
101 changes due to a change with time or a change in the environment, the amount of deflection
of the vibrating membrane 101 can be adjusted for each element. Further, since the current
detection means 205 can be used for measuring the deflection amount variation and for
measuring the ultrasonic waves, it can be realized with a simple configuration.
[0037]
Fifth Embodiment Next, a fifth embodiment will be described using FIG. The fifth embodiment is
characterized in that the current detection means 205 is configured to switch circuit parameters
at the time of measurement of deflection amount variation and at the time of measurement of
ultrasonic waves. Other than that is the same as the fourth embodiment.
[0038]
The present embodiment will be described using a transimpedance circuit which is a currentvoltage conversion circuit that converts a change in a minute current into a voltage. FIG. 6 shows
a configuration diagram of a transimpedance circuit which is the current detection means 205
according to the present embodiment. 601 is an operational amplifier, 602, 604, 606 are
resistors, 603, 605, 607 are capacitors, and 608 is a circuit element switching means.
[0039]
In FIG. 6, the operational amplifier 601 is connected to the positive and negative power supplies
VDD and VSS. First, the operation at the time of measurement of ultrasonic waves will be
described. The inverting input terminal (−IN) of the operational amplifier 601 is connected to
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the lower electrode 102 via the protection switch 204. The output terminal (OUT) of the
operational amplifier 601 is connected to the inverting input terminal (-IN) by the circuit element
switching means 608 with the resistor 602 and the capacitor 603 connected in parallel, and the
output signal is fed back. ing. The noninverting input terminal (+ IN) of the operational amplifier
601 is connected to the ground terminal (GND) by a resistor 604 and a capacitor 605 connected
in parallel. The voltage of the ground terminal (GND) is an intermediate potential between the
positive power supply VDD and the negative power supply VSS. The values of the resistors 602
and 604 and the values of the capacitors 603 and 605 are respectively the same value, and are
parameters meeting the specifications at the time of ultrasonic wave measurement.
[0040]
Next, at the time of measurement of deflection amount variation, the circuit element switching
means 608 is switched, and the configuration is such that feedback of the operational amplifier is
performed by the resistor 606 and the capacitor 607 connected in parallel (see the circuit
configuration of FIG. ). The resistor 606 and the capacitor 607 are parameters in accordance
with the specifications at the time of measurement of deflection amount variation.
[0041]
According to the present embodiment, it is possible to perform current detection in accordance
with specifications such as the frequency and current size used at the time of deflection amount
variation measurement, and specifications such as the ultrasonic wave frequency and current size
at the time of ultrasonic measurement. In FIGS. 6A and 6B, the circuit element switching means
608 is used only for the feedback portion of the operational amplifier, but the deflection amount
is the same between the non-inverting input (+ IN) of the operational amplifier and the ground
terminal. The element constant may be switched in accordance with switching at the time of
measurement of variation and at the time of ultrasonic measurement.
[0042]
Further, although it has been described in the present specification that the vibrating membrane
101 operates in the conventional mode in which a gap with the lower electrode 105 always
exists at the time of transmission and reception operation of the electromechanical transducer,
the present invention is not limited thereto. The present invention can also be used in operations
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in other states such as the collapse mode in which the vibrating membrane 101 partially
disappears from the lower electrode 105.
[0043]
101 diaphragm 102 upper electrode (first electrode) 103 support portion 104 gap 105 lower
electrode (second electrode) 106 substrate 201 DC voltage application means 202 drive
detection means 203 AC voltage generation means 204 protection switch 205 current detection
means 301 Lower electrode through wiring 302 Upper electrode through wiring 303 Through
wiring board 401 Vibration film state detecting means 402 Control signal generating means 403
AC voltage superimposing means 404 Signal switching means 608 Circuit element switching
means
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