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JP2005236868

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DESCRIPTION JP2005236868
PROBLEM TO BE SOLVED: To provide an ultrasonic transducer capable of increasing the
amplitude of a vibrating film at the time of vibration and reducing a DC bias voltage and an AC
signal voltage. SOLUTION: An upper electrode composed of a vibrating film 1 formed of an
insulator and a conductive film 2 formed on the vibrating film, and a plurality of irregularities are
formed on the surface of the upper electrode facing the vibrating film. An ultrasonic transducer
for generating an ultrasonic wave by bringing a lower electrode 12 into close contact with each
other and applying an alternating current signal between the upper electrode and the lower
electrode. The electrode has a laminated structure. [Selected figure] Figure 5
Ultrasonic transducer
[0001]
The present invention relates to an electrostatic ultrasonic transducer that generates constant
high sound pressure over a wide frequency band.
[0002]
Conventional ultrasonic transducers are mostly resonant type using piezoelectric ceramic.
Here, the configuration of a conventional ultrasonic transducer is shown in FIG. Conventional
ultrasonic transducers are mostly resonant type using piezoelectric ceramic as a vibrating
element. The ultrasonic transducer shown in FIG. 1 uses a piezoelectric ceramic as a vibrating
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element to perform both conversion from an electrical signal to ultrasonic waves and conversion
from ultrasonic waves to electrical signals (transmission and reception of ultrasonic waves). The
bimorph type ultrasonic transducer shown in FIG. 1 is composed of two piezoelectric ceramics
61 and 62, a cone 63, a case 64, leads 65 and 66, and a screen 67.
[0003]
The piezoelectric ceramics 61 and 62 are bonded to each other, and the lead 65 and the lead 66
are connected to the surface opposite to the bonding surface, respectively. Since the resonance
type ultrasonic transducer utilizes the resonance phenomenon of piezoelectric ceramic, the
transmission and reception characteristics of the ultrasonic wave become good in a relatively
narrow frequency band around the resonance frequency.
[0004]
Unlike the resonant ultrasonic transducer shown in FIG. 1 described above, the electrostatic
ultrasonic transducer is conventionally known as a broadband oscillation type ultrasonic
transducer capable of generating high sound pressure over a high frequency band. FIG. 2 shows
a specific configuration of the broadband oscillation type ultrasonic transducer. In the
electrostatic ultrasonic transducer shown in FIG. 2, a dielectric 131 (insulator) such as PET
(polyethylene terephthalate resin) having a thickness of about 3 to 10 μm is used as a vibrator.
For the dielectric 131, the upper electrode 132 formed as a metal foil such as aluminum is
integrally formed on the upper surface thereof by a process such as evaporation, and the lower
electrode 133 formed of brass is the lower surface of the dielectric 131 It is provided to contact
the part. The lower electrode 133 is connected to the lead 152 and fixed to a base plate 135
made of Bakelite or the like.
[0005]
Further, a lead 153 is connected to the upper electrode 132, and the lead 153 is connected to a
DC bias power supply 150. A DC bias voltage for attracting the upper electrode of about 50 to
150 V is constantly applied to the upper electrode 32 by the DC bias power supply 150 so that
the upper electrode 132 is attracted to the lower electrode 133 side. 151 is a signal source. The
dielectric 131 and the upper electrode 132 and the base plate 135 are crimped by the case 130
together with the metal rings 136, 137 and 138 and the mesh 139.
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[0006]
On the surface of the lower electrode 133 on the side of the dielectric 131, a plurality of
microgrooves of approximately several tens to several hundreds of μm having an uneven shape
are formed. Since this minute groove serves as an air gap between the lower electrode 133 and
the dielectric 131, the distribution of capacitance between the upper electrode 132 and the
lower electrode 133 changes minutely. The random minute grooves are formed by manually
roughening the surface of the lower electrode 133 with a file. In the electrostatic ultrasonic
transducer, the frequency characteristics of the ultrasonic transducer shown in FIG. 2 are as
shown by curve Q1 in FIG. 6 by thus forming an infinite number of capacitors having different
sizes and depths of the air gaps. It is broadband.
[0007]
In the ultrasonic transducer configured as described above, a rectangular wave signal (50 to 150
Vp-p) is applied between the upper electrode 12 and the lower electrode 133 in a state where a
DC bias voltage is applied to the upper electrode 132. There is. Incidentally, as shown by a curve
Q2 in FIG. 6, the frequency characteristic of the resonance type ultrasonic transducer has a
center frequency (resonance frequency of the piezoelectric ceramic) of, for example, 40 kHz, ± 5
kHz with respect to the center frequency which is the maximum sound pressure. At a frequency
of -30 dB relative to the maximum sound pressure. On the other hand, the frequency
characteristic of the broadband oscillation type ultrasonic transducer of the above configuration
is flat from 40 kHz to 100 kHz, and is about ± 6 dB at 100 kHz as compared to the maximum
sound pressure (see Patent Documents 1 and 2) .
[0008]
Here, in the broadband oscillation type ultrasonic transducer, it is necessary to increase the
amplitude of the vibrating membrane of the upper electrode in order to obtain a high sound
pressure level for the following reasons. That is, the sound pressure level (SPL) of the ultrasonic
wave emitted from the ultrasonic transducer is the device area S, the vibration velocity of the
vibrating film v, and the applied voltage V applied between the upper electrode and the lower
electrode Then, there is a relationship of SPL∝S ・ v ・ V. In order to increase the sound
pressure level, any one or all of the device area S, the vibration velocity v of the vibrating film,
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and the applied voltage V may be increased. However, in consideration of miniaturization of the
ultrasonic transducer, the device area S, We do not want to increase the applied voltage V.
[0009]
Therefore, in order to satisfy the demand for increasing the sound pressure level, it is necessary
to increase the vibration velocity v of the vibrating membrane. Here, the vibration velocity v of
the vibrating film depends on the vibration frequency and amplitude, but in order to increase the
vibration velocity of the vibrating film, and hence the sound pressure level, from the necessity of
obtaining a high sound pressure level It can be seen that it is necessary to increase the amplitude
of the vibrating membrane. Japanese Patent Laid-Open No. 2000-50387 Japanese Patent LaidOpen No. 2000-50392
[0010]
However, in the conventional broadband oscillation type ultrasonic transducer, since the thin film
of the upper electrode and the bulk material of the lower electrode are adsorbed using a DC bias
voltage of several tens to several hundreds of volts, There is a problem that a sufficient amplitude
of the vibrating membrane can not be obtained. Furthermore, the diaphragm of the upper
electrode is driven by an alternating current signal of several hundred volts. In addition, the DC
bias voltage is a high voltage, and the AC signal as a signal applied to the thin film of the upper
electrode and the lower electrode is also a high voltage. It has led to increased power and cost.
[0011]
The present invention has been made in view of such circumstances, and can increase the
amplitude (film displacement) of the vibrating film at the time of vibration to obtain a large
sound pressure, and at the same time, the DC bias voltage and the AC signal voltage. An object of
the present invention is to provide an ultrasonic transducer with a reduced effect.
[0012]
In order to achieve the above object, the invention according to claim 1 is characterized in that
an upper electrode composed of a vibrating film formed of an insulator and a conductive film
formed on the vibrating film is opposed to the vibrating film of the upper electrode. Surface has a
plurality of concavities and convexities, the lower electrode being in contact with the upper
electrode and the lower electrode, and an ultrasonic signal is generated by applying an
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alternating current signal between the upper electrode and the lower electrode An ultrasonic
transducer, wherein the upper electrode has a laminated structure.
[0013]
The invention according to claim 2 is the ultrasonic transducer according to claim 1, wherein
each conductive film of the stacked upper electrode is formed on the power supply side through
a wiring portion formed at an end of each layer of the upper electrode. At the time of connection,
in order not to short-circuit each conductive film, a margin portion in which the conductive film
is not formed is alternately provided at both ends of the upper surface of the vibrating film, for
each layer of the stacked upper electrodes. .
[0014]
The invention according to claim 3 is characterized in that, in the ultrasonic transducer according
to claim 1 or 2, the number of stacked upper electrodes is an odd number.
[0015]
The invention according to claim 4 is characterized in that, in the ultrasonic transducer according
to any one of claims 1 to 3, the thickness of each vibrating film in the stacked upper electrodes is
different.
[0016]
The invention according to claim 5 is an ultrasonic speaker, characterized in that the ultrasonic
transducer according to any one of claims 1 to 4 is included.
[0017]
As described above, according to the ultrasonic transducer in accordance with the present
invention, it is possible to use the upper electrode consisting of the vibrating film formed of an
insulator and the conductive film formed on the vibrating film, and the vibrating film of the
upper electrode. Ultrasonic waves are generated by bringing the upper electrode and the lower
electrode into close contact with each other and applying an alternating current signal between
the upper electrode and the lower electrode. Since the upper electrode has a laminated structure,
the amplitude of the diaphragm can be increased during vibration, and the DC bias voltage and
the AC signal voltage can be reduced.
[0018]
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Therefore, by configuring an ultrasonic speaker using this ultrasonic transducer, the amplitude of
the vibrating film can be increased at the time of vibration, and an ultrasonic speaker in which
the DC bias voltage and the AC signal voltage are reduced can be realized. .
[0019]
Hereinafter, embodiments of the present invention will be described in detail with reference to
the drawings.
The configuration of the ultrasonic transducer according to the embodiment of the present
invention is the same as that of the wide frequency band oscillation type ultrasonic transducer
shown in FIG. 2 except for the configuration of the electrode portion. Will be described, and
duplicate explanations will be omitted.
FIG. 3 shows the configuration of the electrode part of the ultrasonic transducer according to the
embodiment of the present invention, and FIG. 4 shows the electrode structure of the
conventional electrostatic type wide frequency band oscillation type ultrasonic transducer.
FIG. 3 shows the structure and specific configuration of the film (vibration thin film) used in the
present invention.
This is in line with the film and electrode configuration method of a general multilayer capacitor.
FIG. 3A is a developed view of the electrode portion of the ultrasonic transducer according to the
present embodiment, and FIG. 3B is a cross-sectional view thereof.
[0020]
As shown in FIG. 4, the electrode structure of a conventional wide frequency band oscillation
type ultrasonic transducer comprises a lower electrode 12, a vibrating film 1 formed of an
insulator, and a conductive film 2 deposited on the upper surface of the vibrating film 1. And the
lower electrode 12 is in contact with the vibrating film 1 side of the upper electrode 10.
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That is, the upper electrode is configured to be a single layer.
[0021]
On the other hand, in the electrode structure of the ultrasonic transducer according to the
embodiment of the present invention, the upper electrode of the ultrasonic transducer has a
multilayer structure as shown in FIG. 3 (a).
That is, the upper electrode portion 10-2 having the same structure is stacked on the upper
electrode portion 10-1 in which the conductive film 2 (vapor deposition portion) is formed on the
upper surface of the vibrating film 1.
Here, a margin 2A where the conductive film is not deposited is formed at one end of the upper
electrode 10-1 and the upper electrode 10-2 stacked on the upper electrode 10-1 is A margin 2A
is formed at an end of the electrode 10-1 opposite to the margin 2A. In FIG. 3A, for convenience
of explanation, a state in which the upper electrode portion is laminated in two layers is shown,
but in fact, as shown in FIG. 3B, a plurality of upper electrode portions having two or more layers
The upper electrode 10 is configured by laminating the
[0022]
As shown in FIGS. 3A and 3B, margin portions (non-electrode vapor deposited portions) 2A are
alternately provided at both ends of the vibrating film portion of the upper electrode portion, and
overlapping electrodes (conductive films) It is configured not to short circuit. Also, this
configuration makes it easy to take out the electrode. In FIG. 3B, the upper electrode formed by
laminating the upper electrode portion is expressed relatively thickly as compared to the lower
electrode, but this is for the purpose of explaining the electrode structure in an easy-tounderstand manner. The upper electrode is formed considerably thinner than the lower
electrode.
[0023]
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Also, in the real case, in order to press the upper electrode to the lower electrode side, tension is
applied in the shape of the case or the like. Since the upper electrode is not bonded unless
tension is applied, the vibration behavior of the upper electrode becomes unstable, which is not
preferable. When there is tension, the upper electrode (vibrating film) stacked by the force
vibrates almost integrally. The upper electrode portions 10-1 and 10-2 form the wiring portion
10 by spraying metallikon metal, and lead wires are connected to the wiring portion to wire to an
external power source or the like.
[0024]
Metallicon is a method of spraying and covering a molten metal of a metal such as tin, zinc,
aluminum, copper, brass, gold, silver, nickel silver, nickel, iron, etc., and an electric spray method
using electric arc heat There are gas thermal spraying methods that use oxygen, acetylene
flames. Metallikon can also be applied to materials other than metals, such as porcelain, glass,
and wood. In the electrode structure of the ultrasonic transducer according to the present
embodiment configured as described above, a capacitor is formed by each conductive film of the
upper electrode and the lower electrode. At this time, the force acting between the electrodes is
expressed by the following equation.
[0025]
F = (ε / 2) · (V / d) <2> · S (1) where F is a suction force acting on the electrode, ε is a dielectric
constant of the vibrating film, V is a voltage applied between the electrodes, d Is the distance
between electrodes, and S is the electrode area. As understood from the above equation (1), the
suction force F is proportional to the electrode area. This is because the charge stored in the
surface electrode increases in proportion to the area.
[0026]
Therefore, as in the electrode structure of the ultrasonic transducer according to the embodiment
of the present invention, accumulation is achieved by making the electrode area of the upper
electrode, that is, the conductive film substantially larger, by forming the upper electrode in a
laminated structure. The amount of charge can be increased, and as a result, the suction force
acting on the upper electrode can be increased, so that the displacement of the vibrating film can
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be increased, and the sound pressure can be increased. The frequency characteristic of the sound
pressure level of the ultrasonic transducer in this case is as shown by a curve Q3 in FIG.
[0027]
Next, FIG. 5 shows a specific example of the electrode structure of the ultrasonic transducer
according to the embodiment of the present invention. In the figure, the thickness of the first
layer and the third layer of the upper electrode portion is d / 4, the thickness of the second layer
is 2 d / 4, and the total thickness of the upper electrode is d. When the upper electrode is formed
of a film having a thickness d, the electrostatic force F acting on the upper electrode is expressed
by the above equation (1). However, in the example shown in FIG. 5, since the conductive film 2
of the second layer is short-circuited to the lower electrode 12, the electrostatic force (attractive
force) F1, F2 acting on the conductive film 2 of the first layer F3, F3,. The sum of the electrostatic
force (attracting force) F3 acting on the conductive film 2 of the layer becomes the electrostatic
force acting on the conductive film 2 of the upper electrode with respect to the lower electrode
12. Essentially, the electrostatic force is basically only the attraction force, so the present
embodiment shown in FIG. 5 is shown by an arrow as a force acting on the charge accumulated
in the conductive film 2.
[0028]
In this case, F1 = F3 = 16F, F2 = -4F when the electrostatic force F of the above equation (1) is
considered, and the electrostatic force acting on the conductive film of the upper electrode is 16F
+ 16F-4F = 28F in total. Compared to the case where the upper electrode is formed of a film
having a thickness d, one layer is significantly increased. What is important here is that the first
and third layers of the upper electrode should be as thin as possible, as long as they can maintain
electrical durability (pressure resistance) and mechanical durability (vibration failure). . Also, the
total thickness of the conductive film of the upper electrode is preferably as thin as possible
without being stuck to 10 μm.
[0029]
Further, in the example shown in FIG. 5, it is a necessary condition that the thickness of the
vibrating film 1 of the first layer and the third layer be thinner than the thickness of the vibrating
film 1 of the second layer. Further, in the case of adopting a five-layer structure as a whole, the
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vibrating films 1 of the first layer, the third layer and the fifth layer (conductive films 2 formed in
each layer have the same potential. The second and fourth vibration films 1 (the conductive film
2 formed on each layer have the same potential as the lower electrode). ) It is important to be
thinner. This is to increase the attraction force acting on the conductive film of the upper
electrode, and the relationship of the electrostatic force shown in FIG. 5 derived from the above
equation (1), that is, acting on the conductive film 2 of each layer. It is clear from the direction of
force.
[0030]
According to the ultrasonic transducer according to the embodiment of the present invention, the
upper electrode composed of the vibrating film formed of an insulator and the conductive film
formed on the vibrating film, and the surface of the upper electrode facing the vibrating film
Ultrasonic waves that generate ultrasonic waves by bringing the upper electrode and the lower
electrode into close contact with each other and applying an alternating current signal between
the upper electrode and the lower electrode. Since the upper electrode has a laminated structure,
the suction force acting on the upper electrode can be increased, so that the amplitude of the
vibrating film can be increased at the time of vibration, and the DC bias voltage and The AC
signal voltage can be reduced.
[0031]
The figure which shows the structure of the conventional resonance type ultrasonic transducer.
The figure which shows the structure of the conventional wideband oscillation type | mold
ultrasonic transducer. The figure which shows the electrode structure of the ultrasonic
transducer which concerns on embodiment of this invention. The figure which shows the
electrode structure of the conventional ultrasonic transducer. The figure which shows the
example of the electrode structure of the ultrasonic transducer which concerns on embodiment
of this invention. The figure which shows the frequency characteristic of the sound pressure level
of an ultrasonic transducer.
Explanation of sign
[0032]
DESCRIPTION OF SYMBOLS 1 ... Vibrating film, 2 ... Conducting film (vapor deposition part), 2A ...
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Margin part, 10, 10-1, 10-2 ... Upper electrode, 11 ... Wiring part, 12 ... Lower electrode, 20 ...
Lead wire
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