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JP2017029759

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DESCRIPTION JP2017029759
Abstract: To provide a capacitive transducer drive method, a drive device and the like capable of
reducing variations in transmission sound pressure caused by characteristic variations of
capacitive transducers used in ultrasonic transducers and the like. Kind Code: A1 An electrostatic
capacitance type transducer 1 having an element 3 having a cell 2 of a structure in which a
vibrating membrane 11 including an electrode 10 of one of a pair of electrodes formed across a
gap 8 is vibratably supported. It is a driving method. In the mode in which the acoustic wave is
received, a voltage smaller than the pull-in voltage is applied as the reception bias voltage. In the
mode for transmitting the elementary acoustic wave, a voltage smaller than the reception bias
voltage is applied as a transmission bias voltage. [Selected figure] Figure 1
Method and device for driving capacitive transducer
[0001]
The present invention relates to a driving method, a driving device and the like of a capacitive
transducer used as an ultrasonic transducer or the like.
[0002]
Heretofore, micromachine components manufactured by micromachining technology can be
processed on the order of micrometers, and various microfunctional devices are realized using
this.
04-05-2019
1
Capacitance transducers using such technology are being investigated as alternatives to
piezoelectric elements. According to such a capacitive transducer, the vibration of the vibrating
film can be used to transmit and receive an acoustic wave such as an ultrasonic wave (hereinafter
sometimes represented by an ultrasonic wave), and in particular, it is excellent in liquid Wide
band characteristics can be easily obtained. In the present specification, acoustic waves include
those called sound waves and ultrasonic waves.
[0003]
The ultrasonic diagnostic apparatus transmits an ultrasonic wave from a capacitive transducer to
a subject, receives a reflection signal from the subject with a capacitive transducer, and captures
an ultrasonic image based on the received signal. It is. Patent Document 1 proposes a method for
suppressing the reduction in sensitivity due to the collapse state of a capacitive transducer.
Further, Patent Document 2 proposes a method of driving a capacitive transducer with respect to
the increase of the sound pressure of the transmitted ultrasonic wave and the improvement of
the reception efficiency of the reflected signal.
[0004]
International Publication 2009/075280 Japanese Patent Application Laid-Open No. 2006122344
[0005]
The capacitive transducer is configured by collecting a plurality of elements having cells of a
structure in which a vibrating membrane including one of a pair of electrodes provided with a
gap is vibratably supported. Ru.
The elements may have variations in the characteristics of the respective elements due to
variations in film thickness at the time of manufacture. When the acoustic wave is transmitted by
applying the same bias voltage to a plurality of elements and applying the same transmission
drive voltage to a plurality of elements, the intensity of the acoustic wave transmitted from the
elements in one capacitive transducer May vary. When the intensity of the transmitted acoustic
wave is dispersed, the reflected wave from the object is dispersed, which may cause distortion in
the ultrasonic image based on the received signal or a reduction in resolution.
04-05-2019
2
[0006]
When transmission and reception are performed in a state where a high bias voltage is applied as
in the technique described in Patent Document 1, the intensity variation of the transmitted
acoustic wave becomes large due to the non-linear acoustic wave intensity characteristic of the
capacitive transducer. There is a thing. Further, in the technology of Patent Document 2,
although the reception sensitivity of the reflected acoustic wave is adjusted by switching the bias
voltage at the time of transmission and reception in a stepwise manner, the drive is performed in
consideration of variations in the characteristics of a plurality of elements. It can not be said that
[0007]
In view of the above problems, a driving method of a capacitive transducer according to the
present invention includes a cell of a structure in which a vibrating film including one of a pair of
electrodes formed with a gap is vibratably supported A driving method of a transducer provided
with a plurality of elements, has the following features. That is, in a mode in which at least a part
of the plurality of elements receives an acoustic wave, a voltage smaller than the minimum
voltage of the pull-in voltages of the elements is applied to the elements as a reception bias. In
the mode in which the element group transmits an acoustic wave, a voltage smaller than the
reception bias is applied to the element group as a transmission bias.
[0008]
In view of the above problems, a drive device of a capacitive transducer according to the present
invention has a cell of a structure in which a vibrating film including one of a pair of electrodes
formed with a gap is vibratably supported A driving device for a transducer provided with a
plurality of elements, which has the following features. That is, a voltage control unit is provided
which controls the voltage applied between the pair of electrodes. The voltage control unit
applies a voltage smaller than the minimum voltage of the pull-in voltages of the element group
to the element group as a reception bias in a mode in which at least a part of the plurality of
elements receives an acoustic wave. Do. Then, in a mode in which the element group transmits an
acoustic wave, a voltage smaller than the reception bias is applied to the element group as a
transmission bias.
04-05-2019
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[0009]
According to the present invention, by setting the bias voltage for reception smaller than the
pull-in voltage, it becomes possible to drive the element without pulling-in at the time of
reception driving. Further, by making the transmission bias voltage smaller than the reception
bias voltage, it is possible to transmit a sound pressure larger than the sound pressure when
driving with the transmission and reception bias voltages the same.
[0010]
The top view and AB sectional drawing explaining an example of the electrostatic capacitance
type transducer of this invention. The figure which shows an example of a transmission drive
voltage and an intensity | strength (transmission sound pressure) characteristic of an acoustic
wave, and an example of the time waveform of a transmission drive voltage. The figure which
shows an example of the time waveform of the sound pressure of the upper surface of an
element. FIG. 6 is a diagram for explaining a drive device and a transmission / reception circuit
which drive a capacitive transducer. The perspective view of an ultrasonic probe. Sectional
drawing explaining an example of the manufacturing method of the electrostatic capacitance
type transducer of this invention. FIG. 2 is a top view illustrating the capacitive transducer of
Example 1; FIG. 7 is a view showing an example of a time waveform of a transmission drive
voltage and an example of transmission drive voltage and an intensity (transmission sound
pressure) characteristic of an acoustic wave for explaining the first embodiment. FIG. 7 is a view
showing an example of a time waveform of a transmission drive voltage and an example of
transmission drive voltage and an intensity (transmission sound pressure) characteristic of an
acoustic wave for explaining the first embodiment.
[0011]
In the present invention, in the mode for receiving acoustic waves, a voltage smaller than the
pull-in voltage is applied to the element as a reception bias. Then, in a mode in which the element
transmits an acoustic wave, a voltage smaller than the reception bias is applied to the element as
a transmission bias.
[0012]
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4
Hereinafter, embodiments of the present invention will be described with reference to the
drawings. FIG. 1 is a view showing an example of a capacitive transducer according to the
present invention, FIG. 1 (a) is a top view, and FIG. 1 (b) is a cross-sectional view taken along line
A-B of FIG. 1 (a). In this embodiment, the capacitive transducer 1 includes a plurality of elements
3 (elements) each having the cell 2 of a structure in which a vibrating membrane including one
of the pair of electrodes formed with a gap therebetween is vibratably supported. Equipped with
Although only three elements 3 are illustrated in FIG. 1A, the number of elements may be any
number. In addition, although each element 3 is configured of 44 cells 2, the number of cells may
be any number. Further, the arrangement of the cells 2 may be a lattice arrangement, a zigzag
arrangement, or any arrangement. Furthermore, the outline of the element 3 may be a rectangle
as shown in FIG. 1A, a square, a hexagon or the like.
[0013]
As shown in FIG. 1 (b), the cell 2 includes a substrate 4, a first insulating film 5 formed on the
substrate 4, a first electrode 6 formed on the first insulating film 5, and a first electrode 6. And
the second insulating film 7 on the electrode 6 of FIG. Furthermore, a vibrating membrane 11
including the second electrode 10 and the membrane 9, a vibrating membrane support 12 for
supporting the vibrating membrane 11, and a gap (cavity) 8 are provided. When the substrate 4
is an insulating substrate such as a glass substrate, the first insulating film 5 may be omitted. The
shape of the gap 8 viewed from the top is circular, and the shape of the vibrating portion is
circular, but may be square, rectangular or the like. In addition, voltage application means 13 for
applying a bias voltage between the first electrode 6 and the second electrode 10 of the cell 2
and voltage application means 14 for applying a transmission drive voltage to the second
electrode 10 are provided. ing.
[0014]
The membrane 9 of the vibrating film 11 is an insulating film. In particular, since a silicon nitride
film can be formed with a low tensile stress, for example, a tensile stress of 300 MPa or less,
residual stress in the silicon nitride film can prevent large deformation of the vibrating film,
which is desirable. The membrane 9 of the vibrating film 11 may not be an insulating film. For
example, a low resistance silicon single crystal of 1 Ωcm or less can be used as the membrane 9.
In that case, the membrane can also be used as a second electrode.
04-05-2019
5
[0015]
In the capacitive transducer of the present embodiment, the first voltage application means 13
can apply a bias voltage to the first electrode 6. Note that the second electrode 10 is fixed to the
ground potential. In the present invention, the ground potential does not necessarily have to be 0
V but represents a reference potential which the transmission / reception circuit has. When a
bias voltage is applied to the first electrode 6, a potential difference is generated between the
first electrode 6 and the second electrode 10. Due to this potential difference, the vibrating
membrane 11 is displaced to the point where the restoring force of the vibrating membrane and
the electrostatic attractive force are balanced. When the acoustic wave reaches the vibrating
membrane 11 in this state, the vibrating membrane 11 vibrates, whereby the capacitance
between the first electrode 6 and the second electrode 10 changes, and a current flows in the
second electrode 10. Flow. This current is an electrical signal corresponding to the intensity of
the acoustic wave, and this current is output through the second electrode pad 41 connected to
the second electrode 10. Also, in a state where the first voltage application means 13 applies a
bias voltage to the first electrode 6, the second voltage application means 14 applies a
transmission drive voltage to the second electrode 10 (that is, transmission to the bias voltage)
An acoustic wave is transmitted by superimposing a drive voltage. The transmission drive voltage
may be any waveform as long as it can transmit a desired acoustic wave. A desired waveform
may be used, such as a unipolar pulse, a bipolar pulse, a burst wave or a continuous wave.
[0016]
Here, “pull-in” will be described. For example, when focusing on one cell, when the voltage
applied to the first electrode 6 increases, the restoring force of the vibrating film 11 and the
electrostatic attractive force are balanced, and the vibrating film 11 contacts the insulating film 7
on the lower surface of the gap 8 Lead to Thus, the contact of the vibrating membrane 11 to the
lower side is called pull-in, and the voltage at the time of pull-in is called pull-in voltage. Since the
distance between the first electrode 6 and the second electrode 10 is closer as the bias voltage is
higher, the conversion efficiency of converting the received acoustic wave into an electrical
signal or converting the electrical signal into an acoustic wave is higher. However, when a bias
voltage equal to or higher than the pull-in voltage is applied between the electrodes, and the
diaphragm contacts the lower surface of the gap, the frequency characteristics of the cell largely
change, and the reception sensitivity of detectable acoustic waves also greatly changes. In
addition, the intensity and frequency characteristics of the acoustic wave that can be transmitted
also change significantly. That is, in the case of the element unit, when the element 3 to which
the voltage higher than the pull-in voltage is applied and the element 3 not to be mixed are
mixed in the element group to be driven, the dispersion of the reception sensitivity becomes
04-05-2019
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large.
[0017]
In the present embodiment, in a mode in which at least a part of the plurality of elements
receives an acoustic wave, a voltage smaller than the minimum pull-in voltage of the pull-in
voltages of the elements in the element group is received The voltage is applied to the element
group. In addition, when focusing on one element, in the present specification, “element pulls
in” indicates that all cells in the element are pulled in. That is, the voltage at which all cells in
the element pull in is taken as the pull in voltage of the element. When there are a plurality of
elements, the pull-in voltage may differ from element to element. Therefore, in the present
embodiment, the reception bias voltage is made smaller than the pull-in voltage which is the
minimum among the pull-in voltages for each element. As a result, since all elements in the
element group are driven in a state where they do not pull in during reception driving, variation
in reception sensitivity for each element can be reduced, which is preferable. In the mode in
which the element group transmits an acoustic wave, a voltage smaller than the reception bias
voltage is applied to the element group as a transmission bias voltage. Furthermore, at the time
of transmission drive, the transmission drive voltage is superimposed on the transmission bias
voltage for the element group of transmission drive target. The elements to be driven for
transmission can perform linear electronic scanning of ultrasonic waves by switching in time
series. In the present embodiment, it is preferable to make the sum of the transmission bias
voltage and the transmission drive voltage smaller than the minimum pull-in voltage. The
transmission drive voltage is, for example, the maximum value of the amplitude of the waveform
shown in FIG. 2B, and indicates the amplitude in the direction of increasing the bias voltage.
When the element 3 to which the voltage equal to or higher than the pull-in voltage is applied is
mixed with the element 3 which is not the pull-in voltage, the variation of the transmission sound
pressure becomes large. In the present embodiment, as described above, in the element group, it
is preferable to drive the element group in a non-pull-in state at the time of transmission driving,
because the variation in transmission sound pressure for each element can be reduced.
[0018]
The capacitance type transducer of this embodiment can be manufactured by a semiconductor
microfabrication process. The thickness of the insulating film 7 and the membrane 9 and the
height of the gap 8 may vary due to film formation variations during manufacturing. Due to
manufacturing variations, the distance between the first electrode 6 and the second electrode 10
will vary. Further, since the thickness of the membrane 9 constituting the vibrating membrane
04-05-2019
7
11 and the thickness of the second electrode 10 also vary, the spring constant of the vibrating
membrane 11 also varies. When the distance between the first electrode 6 and the second
electrode 10 is dispersed or the spring constant of the diaphragm 11 is dispersed, the pull-in
voltage of the cell 2 is dispersed. The pull-in voltage fluctuates. In the capacitive transducer 1
including a plurality of elements 3 having variations in pull-in voltage caused by the
manufacturing variations, when the same bias voltage is applied to the element group, all
elements 3 of the element group are not pulled in To drive. This makes it possible to reduce the
variation in reception sensitivity. Further, with respect to the sum of the bias voltage and the
transmission voltage, the variation in the transmission sound pressure can be reduced by driving
with all the elements 3 of the element group not being pulled in. In order to obtain a clear
ultrasonic image by increasing the reception sensitivity of the acoustic wave, it is preferable to
set the reception bias voltage as high as possible within the above conditions.
[0019]
When driving with the reception bias voltage and the transmission bias voltage the same, the
transmission bias voltage is also high, so the transmission drive voltage is limited. Therefore, the
intensity of acoustic waves that can be transmitted is also limited. For example, when the
minimum pull-in voltage is 100 V and the reception bias voltage is 80 V, the transmission drive
voltage (that is, the amplitude of the absolute value) is less than 20 V when the transmission bias
voltage is also 80 V. Unlike this, when the bias voltage for transmission is made smaller than the
bias voltage for reception as in the present embodiment, as shown in FIG. 2A, the strength of the
acoustic wave that can be transmitted by the capacitive transducer is improved. Can do. This is
discussed in more detail below.
[0020]
FIG. 2A is an example of transmission drive voltage and intensity (transmission sound pressure)
characteristics of an acoustic wave. The horizontal axis is the ratio of the transmission drive
voltage to the pull-in voltage, and the vertical axis is the intensity ratio of the transmission sound
pressure. The sequence is the ratio of the bias voltage of the transmission to the pull-in voltage.
The transmission sound pressure intensity ratio on the vertical axis is the series Vdc0.5 (a series
(indicated by x) in which the ratio Vdc of the transmission bias voltage to the pull-in voltage is
0.5) when the transmission drive voltage / pull-in voltage is 0.49. The transmission sound
pressure of is normalized to 1. The curve of each series is a quadratic approximate curve
obtained by least squares approximation of the plotted points. The transmission sound pressure
on the vertical axis is directly above the element 3 when the acoustic wave is transmitted by
04-05-2019
8
applying a bias voltage to the first electrode 6 of the capacitive transducer and a transmission
drive voltage to the second electrode 10. Acquiring a time waveform of an acoustic wave, it is the
maximum value of one-sided amplitude of the acquired time waveform.
[0021]
FIG. 2B shows the time waveform of the transmission drive voltage, and FIG. 3 shows the time
waveform of the sound pressure on the surface of the element 3. The horizontal axis in FIG. 2B
and FIG. 3 is time (μsec). The vertical axis in FIG. 2B is a voltage ratio normalized by the
maximum value, and the transmission drive voltage is a rectangular bipolar pulse wave. The
rectangular pulse width is 50 nsec. The vertical axis in FIG. 3 is the sound pressure ratio
normalized by the maximum value. The transmission drive voltage is not limited to the
rectangular bipolar pulse wave as shown in FIG. 2 (b), but may be a unipolar pulse wave on one
side or a burst wave, and it may be a waveform that provides desired frequency characteristics
and transmission sound pressure. Just do it. FIG. 2A shows the result of calculation of the sum of
the transmission bias voltage and the transmission drive voltage up to 99% or less of the
minimum pull-in voltage. For example, assuming that the ratio Vdc of transmission bias voltage
to pull-in voltage is 0.8 (indicated by ♦) and transmission drive voltage / pull-in voltage is 0.19,
the intensity ratio of transmission sound pressure is 0.7. If it is intended to further increase the
transmission sound pressure by increasing the transmission drive voltage, it is necessary to lower
the transmission bias voltage by an amount corresponding to the increase in the transmission
drive voltage. For example, assuming that the ratio of transmission bias voltage to pull-in voltage
is 0.7 (indicated by ▪) and transmission drive voltage / pull-in voltage is 0.29, the transmission
sound pressure intensity ratio is 0.86. In order to obtain the highest transmission sound
pressure, for example, if the ratio of the transmission bias voltage to the pull-in voltage is 0.5
(indicated by x) and the transmission drive voltage / pull-in voltage is driven at 0.49,
transmission is performed. The sound pressure intensity ratio is 1. When driving with the sum of
the transmission bias voltage and the transmission drive voltage at 99% or less of the minimum
pull-in voltage, the transmission drive voltage / pull-in voltage is within the range of less than 0.5
to obtain the maximum transmission sound pressure. By increasing and driving at the same time,
it is possible to obtain even greater intensity of the transmission sound pressure.
[0022]
For example, the ratio of the bias voltage for reception to the pull-in voltage is 0.8, and the bias
voltage for transmission and the bias voltage for reception are equalized to drive transmission
(indicated by ◆). In this case, the ratio of the maximum transmission drive voltage is 0.19, and
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the intensity ratio of transmission sound pressure obtained under this condition reaches up to
only 0.7. On the other hand, as in the present embodiment, the ratio of the reception bias voltage
to the pull-in voltage is 0.8, and the ratio of the transmission bias voltage is 0.5 (that is, smaller
than the reception bias voltage). Then, the ratio of the maximum transmission drive voltage is
0.49 and the intensity ratio of the obtained transmission sound pressure is 1.0, as indicated by a
line indicated by x. Assuming that the ratio of the transmission bias voltage is 0.7 (in this case
also smaller than the reception bias voltage), the ratio of the maximum transmission drive
voltage is 0.29 as shown by the line indicated by ■, and the obtained transmission sound
pressure The intensity ratio of is 0.86. A transmission sound pressure larger than the
transmission sound pressure when driving with the transmission and reception bias voltages the
same can be obtained.
[0023]
That is, by making the transmission bias voltage smaller than the reception bias voltage and
making the transmission drive voltage smaller than the difference between the pull-in voltage
and the transmission bias voltage, the sound when driving with the transmission and reception
bias voltages the same Sound pressure equal to or higher than pressure can be transmitted.
Although the ratio of the reception bias voltage to the pull-in voltage has been described above
as 0.8, it may be a value other than 0.8 smaller than 1.
[0024]
Further, in the case of driving the transmission bias voltage in a range smaller than the reception
bias voltage, it is preferable that the transmission bias voltage be low in order to obtain the same
transmission sound pressure intensity. For example, to obtain a transmission sound pressure
intensity ratio of 0.6, the transmission bias voltage is set to 0.8 as the minimum pull-in voltage
(indicated by ♦), and the transmission drive voltage / pull-in voltage is set to 0.165. An intensity
ratio of 0.6 is obtained. Similarly, an intensity ratio of 0.6 can be obtained by setting the
transmission bias voltage to 0.5 of the minimum pull-in voltage (indicated by x) and setting the
transmission drive voltage / pull-in voltage to 0.345. However, comparing the slopes of tangents
at the point where the same intensity ratio is obtained, the slope of the tangent is smaller when
the transmission bias voltage is lower. The smaller the inclination, the smaller the dispersion of
the intensity ratio of the transmission sound pressure that occurs when the applied voltage is
dispersed. It is preferable to drive under the driving condition with a small inclination.
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[0025]
The variation in applied voltage means that the effective electric field strength of the bias voltage
applied to the first electrode 6 is dispersed. That is, it is assumed that the pull-in voltage is
different for each of the elements 3 constituting the capacitive transducer 1. In this case, since
the first electrode 6 is common to the elements 3, when a common bias voltage is applied to the
first electrodes 6, the effectively applied bias voltage differs for each element 3. Furthermore,
when a common transmission drive voltage is applied to the second electrode 10, the
transmission drive voltage that is effectively applied differs for each element 3. As the
transmission bias voltage is higher, the influence on the intensity dispersion of the transmission
sound pressure due to the difference between the effectively applied bias voltage and the
transmission drive voltage is larger, so that the transmission sound pressure intensity dispersion
is different as in this embodiment. It is preferable to drive under the smaller drive condition. As
apparent from FIG. 2 (a), it is preferable to drive the transmission bias voltage in a region which
is equal to or less than 1/2 of the minimum pull-in voltage.
[0026]
The vibrating film 11 can be vibrated normally by making the transmission drive voltage smaller
than the transmission bias voltage. In the capacitive transducer, when the amplitude of the
transmission drive voltage is made larger than the transmission bias voltage, the vibrating
membrane 11 may not vibrate normally. The normal vibration of the vibrating membrane 11
does not exceed the position of the vibrating membrane 11 in the initial state where the bias
voltage is not applied, in the opposite direction to the direction in which the position of the
vibrating membrane 11 changes when the bias voltage is applied. It means to do. When the
transmission drive voltage is equal to or higher than the transmission bias voltage, the frequency
characteristic of the element 3 changes significantly. Therefore, since the intensity variation of
the transmission sound pressure becomes large and the desired intensity ratio of the
transmission sound pressure can not be obtained, it is preferable to make the transmission drive
voltage smaller than the transmission bias voltage.
[0027]
For example, in order to obtain a transmission sound pressure intensity ratio of 0.2, the
transmission bias voltage is set to 0.3 times the minimum pull-in voltage (indicated by ● in FIG.
2A), and the transmission drive voltage / pull-in voltage is It is preferable to set it as 0.24. The
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combination of the transmission bias voltage and the transmission drive voltage is the slope of
the approximate curve including the point at which the desired transmission sound pressure
intensity ratio is obtained, the diaphragm 11 vibrates normally, and the desired transmission
sound pressure strength is obtained. It is preferable to set to the conditions with small.
[0028]
Next, an example of a drive device is shown in FIG. An apparatus such as an ultrasonic diagnostic
apparatus includes a system control unit 16, a bias voltage control unit 17, a transmission drive
voltage control unit 18, a transmission / reception circuit 19, an ultrasonic probe 20, an image
processing unit 21, a display unit 22, and the like. The drive device includes a bias voltage
control unit 17, a transmission drive voltage control unit 18, and the like. The ultrasonic probe
20 is a transmitting and receiving probe including a capacitive transducer 1 that transmits an
acoustic wave to a subject and receives the acoustic wave reflected from the subject. The
transmission and reception circuit 19 is a circuit that supplies a bias voltage and a drive voltage
supplied from the outside to the ultrasonic probe 20, and processes an acoustic wave received by
the ultrasonic probe 20 and outputs the processed acoustic wave to the image processing unit
21. . The bias voltage control unit 17 supplies a bias voltage to the transmission / reception
circuit 19 in order to supply the bias voltage to the ultrasonic probe 20. The bias voltage control
unit 17 includes a power supply and a switch (not shown), switches the transmission bias voltage
and the reception bias voltage with a switch at a timing instructed by the system control unit 16
and supplies the transmission / reception circuit 19 with the switch. The transmission drive
voltage control unit 18 supplies the transmission drive voltage to the transmission / reception
circuit 19 in order to supply the transmission drive voltage to the ultrasonic probe 20. At a
timing instructed by the system control unit 16, a waveform capable of obtaining a desired
frequency characteristic and the intensity of the transmission sound pressure is supplied to the
transmission / reception circuit 19. The image processing unit 21 performs image conversion
(for example, a B-mode image, an M-mode image, and the like) using the signal output from the
transmission / reception circuit 19, and outputs the image to the display unit 22. The display
unit 22 is a display device that displays the image signal output from the image processing unit
21. The image display unit 22 can be configured separately from the drive device and the like.
The system control unit 16 is a circuit that controls the bias voltage control unit 17, the
transmission drive voltage 18, the image processing unit 21, and the like.
[0029]
FIG. 4B shows an example of the transmission / reception circuit. The transmission / reception
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circuit 26 includes a transmission unit 23, a reception preamplifier 24, and a switch unit 25. At
the time of transmission driving, the bias voltage applied from the bias voltage control unit 17 is
applied to the ultrasonic probe 20 in accordance with the transmission bias voltage instructed
from the system control unit 16 of FIG. 4A. Similarly, according to the transmission drive voltage
instructed from the system control unit 16, the voltage applied from the transmission drive
voltage control unit 18 is applied to the ultrasonic probe 20 via the transmission unit 23. When
the transmission drive voltage is applied, the switch unit 25 is in the open state, and the signal
does not flow to the reception preamplifier 24. When the transmission drive voltage is not
applied, the switch unit 25 is in the closed state and is in the reception state. The switch unit 25
is formed of a diode or the like (not shown) and serves as a protection circuit to prevent the
reception preamplifier 24 from being broken. When an acoustic wave is transmitted from the
ultrasonic probe 20 and the acoustic wave reflected by the object returns to the ultrasonic probe
20, the ultrasonic probe 20 receives the acoustic wave. At the time of reception driving, the bias
voltage applied from the bias voltage control unit 17 is applied to the ultrasonic probe 20
according to the reception bias voltage instructed from the system control unit 16 of FIG. 4A.
Since the switch unit 25 is closed, the reception signal is amplified by the reception preamplifier
24 and sent to the image processing unit 21.
[0030]
FIG. 5 shows an example of an ultrasonic probe which is an object information acquiring
apparatus. FIG. 5 is a perspective view of the ultrasonic probe. The ultrasonic probe 27 is
composed of a capacitive transducer 1, an acoustic matching layer 28, an acoustic lens 29, and a
circuit board 30. The capacitance type transducer 1 of FIG. 8 has the same configuration as the
capacitance type transducer 1 of FIG. 1, and as shown in FIG. 5, a large number of elements 3 are
arranged in the X direction like a one dimensional array. Although FIG. 5 shows a onedimensional array, the elements 3 may be arranged in a two-dimensional array, or may be
another shape such as a convex type. The capacitive transducer 1 is mounted on the circuit
board 30 and electrically connected. The circuit board 30 may be a board integrated with the
transmission / reception circuit 19 shown in FIG. 4A, or may be connected to the transmission /
reception circuit 19 as shown in FIG. 4A via the circuit board 30. An acoustic matching layer 28
is provided on the surface side to which the capacitive transducer 1 transmits the acoustic wave
in order to match the acoustic impedance with the object. The acoustic matching layer 28 may be
provided as a protective film for preventing electrical leakage to the subject. An acoustic lens 29
is disposed via the acoustic matching layer 28. It is preferable that the acoustic lens 29 be one
that can match the acoustic impedance between the subject and the acoustic matching layer 28.
If an acoustic lens 29 having a curvature in the Y direction as shown in FIG. 5 is provided,
acoustic waves spreading in the Y direction can be narrowed at the focal position of the acoustic
lens. Since the acoustic wave spreading in the X direction can not be narrowed as it is, the
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acoustic wave is narrowed at the focal position by shifting the timing of transmitting the acoustic
wave for each element 3 (element group) and performing transmission driving by beam forming.
Can do. The shape of the acoustic lens 29 is preferably a shape that can obtain the desired
distribution characteristic of acoustic waves. Further, the type and shape of the acoustic
matching layer 28 and the acoustic lens 29 may be selected according to the type of the subject
to be used, or may not be provided. The reception signal including the information of the subject
obtained by supplying the bias voltage and the transmission drive voltage to the ultrasonic probe
27 and the acoustic wave reflected from the subject is transmitted or received via the cable (not
shown) It is transmitted to the processing unit 21.
[0031]
An example of the manufacturing method of the capacitive transducer of this embodiment is
shown using FIG. FIG. 6 is a cross-sectional view taken along the line A-B of FIG. As shown in FIG.
6A, the first insulating film 32 is formed on the substrate 31. The substrate 31 is a silicon
substrate, and the first insulating film 32 is for forming insulation with the first electrode. When
the substrate 31 is an insulating substrate such as a glass substrate, the first insulating film 32
may not be formed. The substrate 31 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 It will be scattered. Since
this variation is a variation in conversion efficiency, it is a variation in sensitivity and band.
Therefore, the substrate 31 is preferably a substrate having a small surface roughness.
Furthermore, the first electrode 33 is formed. The first electrode 33 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.
[0032]
Next, a second insulating film 34 is formed. The second insulating film 34 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 34 may not be formed because a
membrane described later is an insulator. Furthermore, it is formed in order to prevent the first
04-05-2019
14
electrode from being etched at the time of removing the sacrificial layer performed in a step
subsequent to this step. If the first electrode is not etched by the etching solution or etching gas
at the time of removing the sacrificial layer, the second insulating film 34 may not be formed.
Similarly to the substrate, when the surface roughness of the second insulating film 34 is large,
the distance between the first electrode and the second electrode due to the surface roughness
varies among the cells, so the surface roughness is small. Small dielectrics are desirable. For
example, a silicon nitride film, a silicon oxide film or the like.
[0033]
Next, as shown in FIG. 6 (b), a sacrificial layer 35 is formed. The sacrificial layer 35 is desirably
made of a material having a small surface roughness. As with 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 among the cells, so a sacrificial layer with a small
surface roughness is desirable . Also, in order to shorten the etching time for removing the
sacrificial layer, a material having a high etching rate is desirable. In addition, there is a need for
a sacrificial layer material in which the insulating film and the membrane are not substantially
etched with respect to the etchant or etching gas for removing the sacrificial layer. When the
insulating film or the membrane is etched with respect to the etchant or etching gas for
removing the sacrificial layer, the thickness variation of the vibrating film and the distance
variation between the first electrode and the second electrode occur. 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 and bandwidth among the cells. In the case where the
insulating film or the membrane is a silicon nitride film or a silicon oxide film, it is preferable to
use a sacrificial layer material which has a small surface roughness and is difficult to etch the
insulating film or the membrane. For example, amorphous silicon, polyimide, chromium or the
like. In particular, since a chromium etching solution hardly etches a silicon nitride film or a
silicon oxide film, it is desirable when the insulating film or the membrane is a silicon nitride film
or a silicon oxide film.
[0034]
Next, as shown in FIG. 6C, a membrane 36 is formed. The membrane 36 desirably has low tensile
stress. For example, a tensile stress of 500 MPa or less is good. The silicon nitride film can be
stress controlled and can be made to have a low tensile stress of 500 MPa or less. When the
membrane has a compressive stress, the membrane causes sticking or buckling and is greatly
deformed. Also, in the case of high tensile stress, the membrane may be broken. Thus, the
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membrane 36 desirably has low tensile stress. For example, it is a silicon nitride film capable of
stress control and low tensile stress.
[0035]
Next, an etching hole (not shown) is formed, and the sacrificial layer 35 is removed through the
etching hole to seal the etching hole. For example, sealing can be performed using a silicon
nitride film or a silicon oxide film. The sacrificial layer removal step or the sealing step can also
be performed after the formation of the second electrode.
[0036]
Next, as shown in FIG. 6D, the second electrode 37 is formed. The second electrode 37 is
desirably made of a material having a small residual stress, such as aluminum. When the
sacrificial layer removing step or the sealing step is performed after the formation of the second
electrode, the second electrode is desirably a material having etching resistance to the sacrificial
layer etching and heat resistance. For example, titanium or the like. Although the second
electrodes 37 are electrically separated in FIG. 6D, they may be electrically connected. FIG. 6E
shows a state in which the voltage application means 13 and the voltage application means 14
are connected to the first electrode 33 and the second electrode 37, respectively. Although the
sacrificial layer 35 is shown in FIG. 6 (e), it is finally removed to form a gap here.
[0037]
In the present embodiment, the minimum pull-in voltage of the element 3 constituting the
capacitive transducer 1 may be obtained by measuring the pull-in voltage of the element 3 for
actually transmitting and receiving the acoustic wave, or the capacitance The pull-in voltage of a
pull-in voltage measurement element (TEG) placed around the mold transducer may be
measured. However, when the pull-in voltage is measured by an element for transmitting and
receiving an acoustic wave, the insulating film of the element is charged to cause variations in
characteristics. Therefore, it is preferable to measure the pull-in voltage of TEG. The pull-in
voltage can be measured by measuring the capacitance when the bias voltage is changed. As the
bias voltage is increased, the capacitance also increases, and the capacitance does not change at
a certain voltage. This voltage is a pull-in voltage. The pull-in voltage can also be measured by
measuring the change in resonance frequency when the bias voltage is changed. As the bias
04-05-2019
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voltage is increased, the resonant frequency is lowered, and the resonant frequency is shifted to a
higher frequency at a certain voltage. This voltage is a pull-in voltage. The pull-in voltage may be
measured by a method capable of obtaining a desired accuracy, and any measurement method
may be used.
[0038]
Further, it is also possible to estimate the pull-in voltage by calculation by measuring the film
thickness and dielectric constant of each film formed and the diameter of the cell in the
manufacturing process described above. The pull-in voltage can be calculated by calculating the
relationship between the capacitance and the diaphragm displacement using the finite element
method or the like, taking the capacitance as a polynomial approximation of displacement, and
solving its first and second partial derivatives. It can. The film thickness can be measured using
an optical interference method or a stylus type surface shape measuring device. The dielectric
constant can be determined from the capacitance and electrode area, the distance between the
upper and lower electrodes, and the dielectric constant of vacuum, by forming a film between the
upper and lower electrodes and measuring the capacitance between the electrodes. The diameter
of the cell can be optically measured using a microscope or the like. A film thickness measuring
element for measuring the film thickness is preferably arranged in the vicinity of the element in
order to predict the characteristics of the element for transmitting and receiving the acoustic
wave by calculation. In order to grasp the film formation variation of the film formation
apparatus used in the semiconductor microfabrication process using a silicon substrate or the
like, it is sufficient to provide only the necessary number at a desired position.
[0039]
The minimum pull-in voltage of the element 3 can be predicted by using the measurement and
calculation shown above. For example, the case where 50 elements 3 constituting the capacitive
transducer 1 are arranged in a one-dimensional array will be described. A film thickness
measurement element is provided for each of the devices 3, and the film thickness and dielectric
constant of each film and the cell diameter are measured in the manufacturing process. Based on
the measured data, the pull-in voltage of each element 3 is calculated by the finite element
method. Also, the pull-in voltage of the elements 3 arranged at both ends of the one-dimensional
array is measured. The pull-in voltage can be accurately predicted by considering the deviation
between the measured value and the calculated value of the elements 3 at both ends in the
correction of the calculated value of the remaining elements. The number of elements to be
measured may be one, or a plurality of measurements may be performed to correct the
04-05-2019
17
calculated value using the average value of the deviation from the calculated value as the
correction coefficient. Alternatively, a plurality of TEGs may be provided in the vicinity of the
elements 3 constituting the capacitive transducer 1, and a value obtained by measuring the pullin voltage of TEG may be used as a measurement value. A pull-in voltage of 3 may be measured.
Although the minimum pull-in voltage of the pull-in voltages of the respective elements 3 in the
element group can be known by the method as described above, the pull-in voltage may be
predicted or measured by another method. Such minimum pull-in voltage is acquired, for
example, at the time of manufacture, and based thereon, the reception bias voltage, transmission
bias voltage and the like of the elements of the drive device are set.
[0040]
The capacitance type transducer according to the present embodiment draws out an electrical
signal from the second electrode 37 by using a lead wire (not shown) electrically connected to
the second electrode pad 41 of FIG. 1A. Can do. When ultrasonic waves are received by the
capacitive transducer, a DC voltage is applied to the first electrode 33. When ultrasonic waves
are received, the vibrating membrane 38 having the second electrode 37 is deformed, so the
distance of the gap between the second electrode 37 and the first electrode 33 changes, and the
capacitance changes. The change in capacitance causes a current to flow in the lead-out line. This
current is subjected to current-voltage conversion by the transmission / reception circuit 26
shown in FIG. 4B, and ultrasonic waves can be received as a voltage. Further, a direct current
voltage can be applied to the first electrode 33, a transmission drive voltage can be applied to the
second electrode 37, and the vibrating film 38 can be vibrated by electrostatic force. By this,
ultrasonic waves can be transmitted.
[0041]
By driving the capacitive transducer manufactured as described above according to the driving
method of the present embodiment, the following effects can be obtained. That is, it is possible to
reduce variations in the intensity of acoustic waves transmitted from the elements to be driven in
one capacitive transducer, which are generated when driving with the same bias voltage for
transmission and reception. As a result, the variation in the reflected wave from the object is
reduced, distortion of the ultrasonic image based on the received signal is reduced, and the
resolution is improved.
[0042]
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18
Embodiment 1 An embodiment of the present invention will be described below with reference to
FIGS. 7 and 8. FIG. FIGS. 7 (a) and 7 (b) are top views of the capacitive transducer of this
embodiment, and FIG. 7 (b) is an enlarged schematic view of FIG. 7 (a). FIG. 8A shows the time
waveform of the transmission drive voltage applied to the elements of the capacitive transducer
in this embodiment.
[0043]
The external dimensions of the capacitive transducer 1 shown in FIG. 7A are 7.5 mm in the Y
direction and 44 mm in the X direction. The external shape of the element 3 is 0.2 mm in the X
direction and 4 mm in the Y direction, and is arranged in 196 one-dimensional arrays. The
schematic diagram which expanded a part of Fig.7 (a) is FIG.7 (b), and the AB sectional drawing
of FIG.7 (b) is FIG.6 (d). The cells 2 constituting the element 3 have a circular shape, and the
diameter of the gap 8 is 31 μm. The cells 2 are densely arranged as shown in FIG. 7B, and the
cells 2 constituting one element 3 are arranged at an interval of 34 μm from the adjacent cells.
That is, the shortest distance between the adjacent cells 2 is 3 μm. Although the number of cells
is omitted in FIG. 7B, in actuality, 702 cells 2 are arranged in one element 3.
[0044]
The cell 2 includes a silicon substrate 4 with a thickness of 300 μm, a first insulating film 5
formed on the silicon substrate 4, a first electrode 6 formed on the first insulating film 5, and a
first electrode 6. And the second insulating film 7 of FIG. In addition, a vibrating membrane 11
including the second electrode 10 and the membrane 9, a vibrating membrane support 12 for
supporting the vibrating membrane 11, and a gap 8 are provided. The height of the gap 8 is 240
nm. Furthermore, voltage application means 13 for applying a bias voltage between the first
electrode and the second electrode, and voltage application means 14 for applying a
transmission drive voltage to the second electrode are included. The first insulating film 5 is a
silicon oxide film having a thickness of 1 μm formed by thermal oxidation. The second
insulating film 7 is a 100 nm silicon oxide film formed by PE-CVD. The first electrode 6 is
titanium having a thickness of 50 nm, and the second electrode 10 is aluminum having a
thickness of 100 nm. The membrane 9 is a silicon nitride film produced by PE-CVD, is formed
with a tensile stress of 450 MPa or less, and has a thickness of 1400 nm.
04-05-2019
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[0045]
A pull-in voltage measuring element (TEG) is placed around the capacitive transducer as
described above. The pull-in voltages of the TEG disposed close to the elements at both ends of
the capacitive transducer and the central element are 203 V, 207 V, and 216 V sequentially from
the left end. In addition, in order to measure the film thickness of each film formed in the abovedescribed manufacturing process, when the element 3 at the left end is the first element and the
element 3 at the right end is the 196th element, The film thickness measurement elements are
arranged at five intervals of 8 mm from the vicinity of the element of (1) to the vicinity of the
196th element. The diameter of the gap 8 is measured at 10 points in the element in which the
film thickness measuring element is disposed in the vicinity. The average of ten points of each
element is the diameter of the gap 8. There is no difference in the diameter of the gap 8 and is
31 μm. When the pull-in voltage is calculated by the finite element method based on the
measurement results of the diameter of the gap 8 and the film thickness of each film, 226 V, 224
V, 230 V, 236 V in order from the film thickness measurement element in the vicinity of the first
element. , 240V. The difference between the calculated value and the measured value is about
10%, and the minimum pull-in voltage is predicted based on the tendency of the pull-in voltage
calculated from the film thickness measurement result. Then, it can be predicted that the element
having a pull-in voltage of 224 V calculated from the film thickness measurement result is the
element having the minimum pull-in voltage, and the actual pull-in voltage of the element is
about 201 V. The pull-in voltage of the element 3 constituting the capacitive transducer 1 of this
embodiment can be predicted to vary from 201V to 216V. It is 201V when the difference
between the calculated value and the measured value of -10% is taken into consideration as the
minimum value 224V of the pull-in voltage calculated from the film thickness measurement
result (201.6 V, so the decimal part is rounded down). Moreover, it is 216V when the calculated
value and the deviation -10% of a measured value are considered to 240V of the maximum value
of the pull-in voltage calculated from the film thickness measurement result.
[0046]
Next, an acoustic matching layer 25 μm thick is formed on the capacitive transducer 1. In this
embodiment, a silicone adhesive having an acoustic impedance of 1.082 MRayls and an
attenuation coefficient of 1.47 × F <1.44 dB / cm / MHz (F is a frequency) is used. Furthermore,
an acoustic lens with an acoustic impedance of 1.22 M Rayls, an attenuation coefficient of 3.1 ×
F <1.4> dB / cm / MHz, and an average thickness of 530 μm is formed on the acoustic matching
layer.
04-05-2019
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[0047]
Next, transmission driving is performed using the manufactured capacitive transducer. First, the
case where the reception bias voltage and the transmission bias voltage are the same will be
described as a comparative example. A bias voltage of 160 V is applied to the first electrode 6
with the reception bias voltage and the transmission bias voltage being 80% of the minimum
pull-in voltage. The transmission drive voltage is applied to the second electrode 10 with the
ratio of the transmission drive voltage to the minimum pull-in voltage being 0.05, 0.1, 0.19. The
time waveform of the transmission drive voltage is shown in FIG. The horizontal axis of FIG. 8A
indicates time (μs), and the vertical axis indicates transmission drive voltage (V). The series
shows the ratio of the transmission drive voltage to the minimum pull-in voltage 0.05-0.19. The
waveform of the transmission drive voltage is a dipole wave with a pulse width of 45 ns as shown
in FIG. 8A, and the absolute value of the amplitude on the plus side and the minus side of the
waveform is the transmission drive voltage. The characteristic of the transmission sound
pressure at that time is shown in FIG. The curves in FIG. 8B are second-order polynomial
approximation curves of the plots when transmitted under each condition. The horizontal axis in
FIG. 8 (b) is the ratio of the transmission drive voltage to the minimum pull-in voltage. The
ordinate represents the transmission sound pressure transmitted by one element of the element
3 constituting the capacitive transducer 1 when the transmission drive voltage is applied, and
indicates the transmission sound pressure after passing through the acoustic matching layer and
the acoustic lens. There is. FIG. 8 (b) shows the transmission sound pressure of an element
having a minimum pull-in voltage of 201 V and an element having a maximum pull-in voltage of
216 V. The element having the largest pull-in voltage effectively reduces the potential difference
between the first electrode and the second electrode, so that the conversion efficiency decreases
and the transmission sound pressure decreases.
[0048]
Assuming that the transmission drive voltage is 14% of the minimum pull-in voltage, the element
having the minimum pull-in voltage transmits an acoustic wave of 340 kPa, and the element
having the maximum pull-in voltage transmits an acoustic wave of 260 kPa. The difference in
transmission sound pressure at this time is 80 kPa, and the transmission sound pressure varies
by ± 13% with respect to the average value 300 kPa of the transmission sound pressure. Since
the transmission sound pressure varies, the intensity of the acoustic wave reflected from the
subject also varies. When transmitting the acoustic wave dispersed from the capacitive
transducer and receiving the acoustic wave reflected from the subject, a bias voltage for
reception is applied to the first electrode 6. In this comparative example, since the transmission
bias voltage and the reception bias voltage are the same, a voltage of 160 V is applied. As in the
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case of applying the transmission bias voltage, the receiving sensitivity is also different because
the potential difference applied between the first electrode and the second electrode is effectively
different for each element, and the same acoustic wave is received ± 13% It fluctuates. In the
case of transmission and reception, the finally obtained received signal varies by ± 26%.
Generally, it is preferable that the variation of the ultrasonic probe that performs transmission
and reception is ± 25% or less as the finally obtained reception signal. Therefore, when the
transmission bias voltage and the reception bias voltage are the same, the variation is It is
difficult to make it smaller.
[0049]
Next, as the present embodiment, transmission driving in the case where the bias voltage of
transmission is made smaller than the bias voltage of reception using the capacitance type
transducer manufactured above will be described. In this embodiment, the reception bias voltage
is 80% of the minimum pull-in voltage, and the transmission bias voltage is 50% of the minimum
pull-in voltage. A bias voltage of 100 V transmission is applied to the first electrode 6. The
transmission drive voltage is applied to the second electrode 10 as the ratio of the transmission
drive voltage to the minimum pull-in voltage is 0.05, 0.1, 0.2, 0.3, 0.4, 0.49.
[0050]
The time waveform of the transmission drive voltage is shown in FIG. 9 (a). The horizontal axis of
FIG. 9A indicates time (μs), and the vertical axis indicates transmission drive voltage (V). The
series shows the ratio 0.2-0.49 of the transmission drive voltage to the minimum pull-in voltage.
The waveform of the transmission drive voltage is a dipole wave having a pulse width of 45 ns
similar to that of FIG. 8A, and the absolute value of the amplitude on the plus side and the minus
side of the waveform is the transmission drive voltage. The characteristic of the transmission
sound pressure at that time is shown in FIG. The curves in FIG. 9 (b) are second-order polynomial
approximation curves of the plots when transmitted under each condition. The vertical and
horizontal axes in FIG. 9 (b) are the same as in FIG. 8 (b). The sequence of FIG. 9 (b) is also similar
to that of FIG. 8 (b).
[0051]
In order to obtain the transmission sound pressure 340 kPa similar to that of the comparative
04-05-2019
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example, the transmission drive voltage is applied 60 V to the second electrode 10 as 30% of the
minimum pull-in voltage. The element with the lowest pull-in voltage transmits an acoustic wave
of 340 kPa, and the element with the highest pull-in voltage transmits an acoustic wave of 280
kPa. The difference in transmission sound pressure at this time is 60 kPa, and the transmission
sound pressure varies by ± 22.7% with respect to the average value of the transmission sound
pressure. As compared with the case where the reception bias voltage and the transmission bias
voltage are the same voltage as in the comparative example, the transmission sound pressure can
be obtained by making the transmission bias voltage smaller than the reception bias voltage as in
this embodiment. Can reduce the variation of Further, when performing the reception operation,
160 V is applied to the first electrode 6 with the reception bias voltage being 80% of the
minimum pull-in voltage as in the comparative example. As in the comparative example, since the
variation of the reception sensitivity when receiving the same acoustic wave is ± 13%, the sound
pressure is transmitted as in the present embodiment, and the final time when the acoustic wave
reflected from the object is received The dispersion of the received signal obtained as a result is
± 19.7%. As described above, by driving the transmission bias voltage to be smaller than the
reception bias voltage, it is possible to reduce the variation of the ultrasonic probe that performs
transmission and reception.
[0052]
Although the preferred embodiments and examples of the present invention have been described,
the present invention is not limited to these embodiments and examples, and various
modifications and changes can be made within the scope of the present invention.
[0053]
1 · · · Capacitance type transducer, 2 · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 2 · · · · · · · · · · film
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