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DESCRIPTION JP2017224969

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DESCRIPTION JP2017224969
Abstract: PROBLEM TO BE SOLVED: To provide a piezoelectric element capable of switching a
resonance frequency in a wide frequency range and capable of downsizing, and a piezoelectric
conversion device using the piezoelectric element. A piezoelectric element (10) comprises a first
piezoelectric layer (111) made of a first piezoelectric material and polarized in the thickness
direction, and a polarization higher than the first piezoelectric material provided on the first
piezoelectric layer (111). A second piezoelectric layer 112 made of a second piezoelectric
material having a small reversal electric field value, and a pair of electrodes sandwiching the first
piezoelectric layer 111 and the second piezoelectric layer 112 in the direction of lamination (first
electrode 121, second And an electrode 122). In the piezoelectric conversion device 20, the
piezoelectric element 10, the first piezoelectric layer 111, and the second piezoelectric layer 112
are smaller than the polarization reversal electric field value of the first piezoelectric material and
more than the polarization reversal electric field value of the second piezoelectric material. A
polarization control voltage application unit (a first DC power supply 211 and a second DC power
supply 212) for applying a polarization control voltage which is a DC voltage between the pair of
electrodes so as to generate a large electric field; And a polarization control voltage inverting unit
(first switch 221, second switch 222) for switching between positive and negative. [Selected
figure] Figure 1
Piezoelectric element and piezoelectric transducer
[0001]
The present invention relates to a piezoelectric element and a piezoelectric conversion device
used for a frequency filter in a communication device, an ultrasonic element that transmits and /
or receives ultrasonic waves, and the like.
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1
[0002]
The piezoelectric body generates mechanical vibration when an alternating electric signal is
input, and generates an alternating electric signal when mechanical vibration is input.
The piezoelectric element converts the electrical signal and the mechanical vibration to each
other by the piezoelectric thin film by utilizing the property of the piezoelectric substance, and is
used as a frequency filter, an ultrasonic element or the like used in wireless communication. In
the frequency filter, only a signal of a predetermined narrow band frequency centered on the
resonance frequency of the piezoelectric thin film is allowed to pass. Usually, a plurality of
piezoelectric elements having different passband frequencies are used in one wireless
communication device.
[0003]
On the other hand, it is known that when a DC bias voltage is applied to a piezoelectric thin film,
the electromechanical coupling coefficient of the piezoelectric thin film changes according to the
magnitude thereof, and thereby the resonance frequency of the piezoelectric thin film also
changes. Here, the electromechanical coupling coefficient is a coefficient that represents a
measure of the conversion efficiency between the energy of electricity and the energy of
mechanical vibration. Non-Patent Document 1 describes that, in 0.67BiFeO3-0.33BaTiO3, which
is a mixed crystal system of BiFeO3 and BaTiO3, the resonance frequency changes by about 4.4%
by changing the DC bias voltage. By making the resonance frequency of the piezoelectric thin
film variable as described above, it is possible to widen the usable frequency band of one
piezoelectric element.
[0004]
However, since it is expected that the number of wireless communication devices will further
increase in the future, each wireless communication device is also required to have a wider
bandwidth. However, when changing the resonance frequency by changing the DC bias voltage
as in the piezoelectric element of Non-Patent Document 1, the range of change of the resonance
frequency is limited, and switching the resonance frequency in a wide frequency range is Have
difficulty.
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2
[0005]
In Non-Patent Document 2, a portion in which two piezoelectric layers made of the same material
are stacked and the polarizations of the two layers are in the same direction (polarization noninverted portions) and the polarizations of the two layers are opposite to each other A
piezoelectric element is described which comprises a laminate having a portion (polarization
inversion portion) which is In this piezoelectric element, the first electrode pair sandwiching the
polarization non-inversion portion and the second electrode pair sandwiching the polarization
inversion portion are provided independently (note that one of the two electrode pairs is
common) Can be used). When an alternating voltage is applied to the first electrode pair, the two
piezoelectric layers vibrate in the same direction, and a resonance having a resonant wavelength
twice the thickness occurs in the entire laminated body, while an alternating voltage is applied to
the second electrode pair. When a voltage is applied, the two piezoelectric layers vibrate in
opposite directions, and a resonance having a resonant wavelength of the same length as the
thickness occurs in the entire stack. Thus, two resonance frequencies can be used by switching
the electrode which applies an alternating voltage in one piezoelectric element. Then, by applying
a DC bias voltage to each of the first electrode pair and the second electrode pair and changing
the values of the DC bias voltages, the resonance frequency can be switched in a wider frequency
range than the configuration of Patent Document 1 .
[0006]
A. Vorobiev and 4 others, "Intrinsically tunable 0.67 BiFeO 3-0.33 BaTiO3 thin film bulk acoustic
wave resonators", APPLIED PHYSICS LETTERS, (USA), published by American Institute of Physics
(American Physical Society Association), December 4, 2012, Vol. 101, pp. 232903-1 to 2329035 Katsuta Katata, Takahiko Yanagiya et al., “Second-order mode polarization inversion resonator
using a PbTiO3 epitaxial thin film”, Technical report of IEICE, electronic information
Communications Society of Japan, October 15, 2014, Vol. 114, No. 263, pp. 29-34
[0007]
The piezoelectric element described in Non-Patent Document 2 operates only one of the
polarization non-inversion part and the polarization inversion part when using one resonance
frequency.
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Therefore, the size of the element must be doubled, which is a disadvantage when incorporated
in a small communication device such as a mobile phone.
[0008]
The problem to be solved by the present invention is to provide a piezoelectric element which
can switch the resonance frequency in a wide frequency range and which can be miniaturized
and a piezoelectric conversion device using the piezoelectric element.
[0009]
The piezoelectric element according to the present invention, which has been made to solve the
above problems, comprises: a) a first piezoelectric layer made of a first piezoelectric material and
having polarization in the thickness direction, and b) laminated on the first piezoelectric layer A
second piezoelectric layer formed of a second piezoelectric material having a polarization
inversion electric field value smaller than that of the first piezoelectric material; c) the first
piezoelectric layer and the second piezoelectric layer; And a pair of electrodes sandwiching in a
direction.
[0010]
The piezoelectric conversion device according to the present invention comprises: a) a first
piezoelectric layer made of a first piezoelectric material and having polarization in the thickness
direction; and b) the first piezoelectric layer laminated on the first piezoelectric layer. A second
piezoelectric layer made of a second piezoelectric material having a polarization inversion
electric field value smaller than that of the material; c) a pair of electrodes sandwiching the first
piezoelectric layer and the second piezoelectric layer in the direction of the lamination; d)
generating an electric field having a value smaller than the polarization inversion electric field
value of the first piezoelectric material and larger than the polarization inversion electric field
value of the second piezoelectric material in the first piezoelectric layer and the second
piezoelectric layer A polarization control voltage application unit that applies a polarization
control voltage that is a direct current voltage between the pair of electrodes, and e) a
polarization control voltage inversion unit that switches between positive and negative of the
polarization control voltage.
[0011]
The polarization inversion electric field value refers to the minimum value of the strength of the
electric field at which the inversion of polarization occurs when an electric field in the reverse
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direction to the polarization is applied to the piezoelectric material.
That is, when an electric field stronger than the polarization inversion electric field value is
applied to the piezoelectric material in the direction opposite to the polarization, the polarization
inversion occurs.
The sum of the absolute value of the polarization inversion electric field value when the
polarization is inverted from positive to negative and the absolute value of the polarization
inversion electric field value when the polarization is inverted from negative to positive is
referred to as a coercive electric field value.
The coercive electric field value is generally used as an index indicating the characteristics of the
piezoelectric body, but in the present invention, the above-mentioned polarization reversal
electric field value is defined in order to define the voltage applied by the polarization control
voltage application unit. Use.
[0012]
The piezoelectric element according to the present invention has a structure in which a first
piezoelectric layer made of a first piezoelectric material and a second piezoelectric layer made of
a second piezoelectric material having a polarization inversion electric field smaller than that of
the first piezoelectric material are laminated. Therefore, by applying a voltage between the pair
of electrodes so as to generate an electric field having a value smaller than the polarization
inversion electric field value of the first piezoelectric material and larger than the polarization
inversion electric field value of the second piezoelectric material, Only the direction of
polarization of the second piezoelectric layer can be controlled without changing the direction of
polarization of the first piezoelectric layer. That is, there are two states, a state in which the
polarization of the first piezoelectric layer and the polarization of the second piezoelectric layer
are in the same direction, and a state in which the polarization of the first piezoelectric layer and
the polarization of the second piezoelectric layer are in the opposite direction. You can switch
between them. By this switching, the piezoelectric element according to the present invention can
use two different resonant frequencies. In order to perform such switching, the piezoelectric
conversion device according to the present invention includes the polarization control voltage
application unit and the polarization control voltage inversion unit.
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[0013]
(1) polarization is maintained even when no electric field is applied, and polarization is reversed
if an electric field stronger than the polarization reversal electric field is applied in the direction
opposite to the polarization, (2) electric field The polarization is maintained even when the
voltage is not applied, and the polarization is not reversed even if the electric field in the
direction opposite to the polarization is applied (the polarization reversal electric field value is
very large), (3) the electric field is applied Polarization is maintained only between the two, and
the polarization also changes in the direction if the direction of the electric field is changed
(polarization reversal electric field value is 0). It is also called "electrostrictive material". )がある
。 In the present invention, (1) or (2) can be used for the first piezoelectric material, and (1) or
(3) can be used for the second piezoelectric material. (1)Examples thereof include PbTiO 3 (lead
titanate), PbZrxTi 1 -xO 3 (lead zirconate titanate) in which a part of Ti (titanium) in PbTiO 3 is
replaced with Zr (zirconium), and the like. (2)Examples of ZnO include zinc oxide (ZnO), AlN
(aluminum nitride), and ScxAl1-xN in which a part of Al in AlN is substituted with Sc (scandium).
(3)Examples of BxSr1-xTiO3 in which a part of Sr (strontium) in SrTiO3 (strontium titanate) is
substituted by Ba (barium) are mentioned.
[0014]
In the piezoelectric element and the piezoelectric conversion device according to the present
invention, since the whole of the first piezoelectric layer and the second piezoelectric layer can
be operated in any of the two states, the piezoelectric element described in Non-Patent Document
2 More compact.
[0015]
The polarization control voltage may be applied at the time of switching between the two states,
and after the polarization of the second piezoelectric layer is reversed, the polarization of the
second piezoelectric layer is maintained even if the application is stopped. The state (inverted
state) is maintained.
Therefore, except at the time of this switching, a DC bias voltage is applied to the first
piezoelectric material and the second piezoelectric material to generate a DC electric field whose
value is smaller than their polarization reversal electric field value, and the magnitude of the DC
bias voltage By changing the amplitude, the resonant frequency can be controlled. Therefore, a
resonant frequency control voltage for applying a resonant frequency control voltage, which is a
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direct current voltage, between the pair of electrodes so as to generate an electric field smaller
than the polarization inversion electric field value of the second piezoelectric material. It is
desirable to include an application unit.
[0016]
The piezoelectric element and the piezoelectric conversion device according to the present
invention only need to have at least the first piezoelectric layer and the second piezoelectric
layer, but the polarization inversion in which the polarization is not reversed by the application
of the polarization control voltage A piezoelectric layer (including a first piezoelectric layer)
formed of a piezoelectric material having an electric field value, and a piezoelectric layer formed
of a piezoelectric material having a polarization reversal electric field value whose polarization is
reversed by application of the polarization control voltage (second piezoelectric The body layers
may be alternately included to have a total of three or more layers. In this case, one of the two
resonance frequencies is larger than the other by the same multiple of the number of
piezoelectric layers.
[0017]
The resonance frequency can be switched in a wide frequency range, and miniaturization can be
achieved by the piezoelectric element and the piezoelectric conversion device according to the
present invention.
[0018]
BRIEF DESCRIPTION OF THE DRAWINGS The schematic block diagram which shows one
Embodiment of the piezoelectric element which concerns on this invention, and a piezoelectric
conversion apparatus.
Schematic which shows the manufacturing method of the piezoelectric element of this
embodiment. Schematic which shows operation | movement of the piezoelectric element of this
embodiment, and a piezoelectric conversion apparatus. Schematic which shows the resonant
wavelength in the piezoelectric element of this embodiment. As a preliminary experiment, a
piezoelectric element in which only the first piezoelectric layer used in the piezoelectric element
of the present embodiment is sandwiched by a pair of electrodes and a piezoelectric element in
which only the second piezoelectric layer is sandwiched by a pair of electrodes are as follows.
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The graph which shows the result of having measured the conversion loss of the resonance of
primary mode, changing the application DC voltage. The graph which shows the result of having
measured the conversion loss of the resonance of primary mode and a secondary mode, changing
the applied DC voltage in the piezoelectric element of this embodiment. The graph which shows
the result of having measured the insertion loss in the piezoelectric element of this embodiment,
changing the frequency of the alternating voltage input from the outside. FIG. 8 is a schematic
configuration view showing an example having four piezoelectric layers, which is another
embodiment of the piezoelectric element according to the present invention. The schematic block
diagram which shows the example which has 3 layers of piezoelectric material layers which
shows other embodiment of the piezoelectric element which concerns on this invention. The
schematic block diagram which shows FBAR which is other embodiment of the piezoelectric
element which concerns on this invention.
[0019]
An embodiment of a piezoelectric element and a piezoelectric conversion device according to the
present invention will be described with reference to FIGS. FIG. 1 is a schematic configuration
diagram of the piezoelectric element 10 and the piezoelectric conversion device 20 of the
present embodiment. In the piezoelectric element 10, a first piezoelectric layer 111 made of
PbTiO3 and a second piezoelectric layer 112 made of PbZrxTi1-xO3 in which a part of Ti in
PbTiO3 is replaced with Zr are stacked. Here, x can take an arbitrary value within the range of
more than 0 and 1 or less, but is 0.53 in this embodiment. The coercive electric field value 2Ec is
470 kV / cm for the first piezoelectric layer 111 (PbTiO3) and 220 kV / cm for the second
piezoelectric layer 112 (PbZrxTi1-xO3), and their polarization reversal electric field value is 2Ec
for the coercive electric field value. It is approximated by Ec which is 1/2. The thicknesses of the
first piezoelectric layer 111 and the second piezoelectric layer 112 are 580 nm in the former
embodiment and 1070 nm in the latter embodiment in the present embodiment, but are not
limited to these thicknesses in the present invention.
[0020]
The piezoelectric element 10 of the present embodiment further includes a first electrode 121
and a second electrode 122. The first electrode 121 and the second electrode 122 are provided
so as to sandwich the first piezoelectric layer 111 and the second piezoelectric layer 112 in the
stacking direction thereof. The first electrode 121 is disposed on the first piezoelectric layer 111
side, and the second piezoelectric layer 112 is disposed on the second piezoelectric layer 112
side. As the first electrode 121, a single crystal in which SrTiO3 is doped with a small amount of
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8
La (3.73 mass%) and polished so that the surface has a (001) plane is used. Here, La is doped to
impart conductivity to SrTiO 3. On the other hand, an electrode made of gold was used for the
second electrode 122. Note that the material of the first electrode 121 and the material of the
second electrode 122 may be interchanged, or another material may be used for the first
electrode 121 and / or the second electrode 122. The second electrode 122 is grounded.
[0021]
The first piezoelectric layer 111 makes the first electrode 121 positive by applying an electric
field having a value higher than the polarization reversal electric field value of the first
piezoelectric layer 111 with the first electrode 121 being positive in advance. Polarization in the
direction is formed. The direction of polarization of the second piezoelectric layer 112 is
controlled by the polarization control voltage application unit 21 and the polarization control
voltage inversion unit 22 as described later, and is not constant.
[0022]
The piezoelectric conversion device 20 according to the present embodiment includes a
polarization control voltage application unit 21 and a polarization control voltage inversion unit
22 in addition to the piezoelectric element 10. The polarization control voltage application unit
21 includes a first DC power supply 211 and a second DC power supply 212, and the
polarization control voltage inversion unit 22 includes a first switch 221 and a second switch
222. The first direct current power supply 211 has a negative electrode connected to the first
electrode 121 and a positive electrode connected to the second electrode 122, and the first
switch 221 is provided between the negative electrode and the first electrode 121. The second
DC power supply 212 has a positive electrode connected to the first electrode 121 and a
negative electrode connected to the second electrode 122, and the second switch 222 is provided
between the positive electrode and the first electrode 121. The magnitude of the polarization
control voltage applied between the first electrode 121 and the second electrode 122 by the first
DC power supply 211 and the second DC power supply 212 is 30 V. As for the magnitude of this
polarization control voltage, an experiment in which a DC voltage is actually applied between the
first electrode 121 and the second electrode 122 to change the value of the voltage is conducted
(see FIG. 6 described later), The polarization of the one piezoelectric layer 111 was not reversed,
and the polarization of the second piezoelectric layer 112 was determined within the range of
reversed voltages.
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[0023]
The piezoelectric conversion device 20 is further provided with a resonance frequency control
voltage application unit 23. The resonance frequency control voltage application unit 23 includes
a variable DC voltage power supply 231 and a third switch 232. The variable DC voltage power
supply 231 is a power supply capable of outputting a DC voltage and changing its output voltage.
The range in which the output voltage is changed by the variable DC voltage power supply 231 is
determined so that the polarization of the second piezoelectric layer 112 is not reversed.
[0024]
The manufacturing method of the piezoelectric element 10 of this embodiment is demonstrated
using FIG. First, a single crystal of (Sr, La) TiO 3 in which SrTiO 3 is doped with La is prepared,
and the first electrode 121 is manufactured by polishing so that the surface becomes (001)
plane. Next, a PbTiO 3 film is formed on the first electrode 121 by an RF magnetron sputtering
method (FIG. 2A) to produce the first piezoelectric layer 111 (FIG. 2B). Here, since (Sr, La) TiO 3
and PbTiO 3 have a small difference in lattice constant, PbTiO 3 of the first piezoelectric layer
111 is epitaxially grown on the first electrode 121. Subsequently, PbZrxTi1-xO3 is deposited on
the first piezoelectric layer 111 by the RF magnetron sputtering method (the same (c)), thereby
producing the second piezoelectric layer 112 (the same (d)). Here, PbZrxTi1-xO3 of the second
piezoelectric layer 112 is also epitaxially grown on PbTiO3 of the first piezoelectric layer 111.
Thus, the first piezoelectric layer 111 and the second piezoelectric layer 112 form an epitaxial
junction structure in which the both are firmly joined. Here, the epitaxial junction structure refers
to a structure in which the crystal orientation in the in-plane direction in one of the two layers is
in agreement with the crystal orientation in the other. For example, if two layers grown in the caxis direction are joined (if the c-axis is perpendicular to the layers), the axis parallel to the two
layers (eg, the a-axis) points in the same direction. And epitaxial junction structure.
[0025]
Subsequently, gold is deposited on the second piezoelectric layer 112 (the same (e)) to produce
the second electrode 122 (the same (f)), thereby completing each layer constituting the
piezoelectric element 10 .
[0026]
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Thereafter, the first electrode 121 and the second electrode are generated so that the first
electrode 121 side is negative and an electric field larger than the polarization inversion electric
field value of the first piezoelectric layer 111 is generated in the first piezoelectric layer 111. By
applying a voltage between 122 (FIG. 2 (g)), an electric field E from the second electrode 122 to
the first electrode 121 is generated, whereby polarization in which the first electrode 121 side is
positive is determined by the first piezoelectric The body layer 111 and the second piezoelectric
layer 112 are formed (the same (h)).
Thus, the piezoelectric element 10 is completed.
[0027]
The operation of the piezoelectric element 10 and the piezoelectric conversion device 20
according to the present embodiment will be described with reference to FIGS. 3 and 4.
[0028]
First, as shown in FIG. 3A, after turning off the third switch 232 of the resonance frequency
control voltage application unit 23, the first switch 221 of the polarization control voltage
inversion unit 22 is turned on and the second switch 222 is turned on. The case of turning off
will be described.
In this case, an electric field E from the second electrode 122 to the first electrode 121 is
generated in the second piezoelectric layer 112, and the polarization P of the second
piezoelectric layer 112 is in the direction of making the first electrode 121 positive. Become. At
this time, an electric field having the same magnitude as that in the second piezoelectric layer
112 is also generated in the first piezoelectric layer 111, but the value of this electric field is the
polarization inversion of PbTiO 3 of the first piezoelectric layer 111. Since the value is smaller
than the electric field value, the direction of the polarization P in the first piezoelectric layer 111
is not affected. Since the polarization P in the first piezoelectric layer 111 and the polarization P
in the second piezoelectric layer 112 are in the same direction at this stage, when electrical or
mechanical vibration is applied from the outside, as shown in FIG. As shown in the right side of
the drawing, a resonance is generated in which the combined thickness d of the thickness d1 of
the first piezoelectric layer 111 and the thickness d2 of the second piezoelectric layer 112 is 1/2
wavelength. Hereinafter, this resonance is referred to as “first-order mode” resonance.
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[0029]
After setting the polarizations P of the first piezoelectric layer 111 and the second piezoelectric
layer 112 in the same direction as described above, the first switch 221 is turned off and the
third switch 232 is turned on (FIG. 3 (b)). . In this state, by adjusting the magnitude of the DC
voltage applied from the variable DC voltage power supply 231, the resonant frequency
corresponding to the resonant wavelength with the thickness d being a half wavelength can be
changed in a range of several%. it can.
[0030]
Next, as shown in FIG. 3C, after turning off the third switch 232 of the resonance frequency
control voltage application unit 23, the first switch 221 of the polarization control voltage
inversion unit 22 is OF, and the second switch 222 The case of turning on will be described. In
this case, an electric field E from the first electrode 121 to the second electrode 122 is generated
in the second piezoelectric layer 112 so that the polarization P of the second piezoelectric layer
112 makes the first electrode 121 side negative. Become. On the other hand, since the value of
this electric field E is smaller than the polarization reversal electric field value of PbTiO 3 of the
first piezoelectric layer 111, the polarization P in the first piezoelectric layer 111 remains in the
direction of making the first electrode 121 positive. is there. Therefore, since the polarization P
in the first piezoelectric layer 111 and the polarization P in the second piezoelectric layer 112
are in opposite directions, when electrical or mechanical vibration is applied from the outside, as
shown in FIG. As shown in the right side of the figure, a resonance having a thickness d of one
wavelength is generated. Hereinafter, this resonance is referred to as “second-order mode”
resonance. The resonance of the second mode is half the resonance wavelength and twice the
resonance frequency as compared to the resonance of the first mode described above.
[0031]
After setting the polarizations P of the first piezoelectric layer 111 and the second piezoelectric
layer 112 in opposite directions as described above, the second switch 222 is turned off and the
third switch 232 is turned on (FIG. 3 (d)). . In this state, by adjusting the magnitude of the direct
current voltage applied from the variable direct current voltage source 231, the resonant
frequency corresponding to the resonant wavelength with the thickness d being one wavelength
can be changed within a range of several percent.
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[0032]
As described above, the resonant frequency corresponding to the resonant wavelength with the
thickness d of 1⁄2 wavelength and the resonant frequency corresponding to the resonant
wavelength with the thickness d of 1 wavelength can be changed in the range of several%. Since
it is possible, the resonant frequency can be switched in a wider frequency range than before.
[0033]
Hereinafter, the experimental result about the piezoelectric element 10 of this embodiment is
shown.
First, as a preliminary experiment, in the piezoelectric element having only the first piezoelectric
layer 111 sandwiched between one pair of electrodes and the piezoelectric element having only
the second piezoelectric layer 112 sandwiched between one pair of electrodes, the resonance of
the first mode Conversion loss was measured. The results are shown in FIG. "PT alone" in the
figure refers to a piezoelectric element in which only the first piezoelectric layer 111 is
sandwiched by a pair of electrodes, and "PZT alone" is a piezoelectric in which only the second
piezoelectric layer 112 is sandwiched by a pair of electrodes Refers to an element. Further, thin
arrows in the same figure indicate the direction of change of the DC voltage when the conversion
loss is measured while changing the DC voltage per unit length applied between the electrodes.
In “PT alone”, conversion loss peaks at two voltages, ie, around −160 kV / cm and around
+310 kV / cm, for the DC voltage per unit length. This means that the directions of polarization
are not aligned at these two voltages, and the polarization is in the process of being reversed. The
coercive electric field value 2Ec of "PT alone" is 470 kV / cm. Similarly, in "PZT only", conversion
loss peaks at two voltages of -100kV / cm and + 120kV / cm, and the coercive field value 2Ec is
220kV / cm. It is. The DC voltage of the region outside the two peaks of conversion loss in these
“PZT only” and inside the two peaks of conversion loss in “PT only” causes the polarization
of the first piezoelectric layer 111 to be reversed. It is considered that the polarization of the
second piezoelectric layer 112 is reversed.
[0034]
The result of having measured the conversion loss, changing the magnitude | size and direction
of the DC voltage applied between the 1st electrode 121 and the 2nd electrode 122 is shown in
FIG. 6 about the piezoelectric element 10 of this embodiment. In this measurement, the
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magnitude of the DC voltage between the first electrode 121 and the second electrode 122 is
such that the DC voltage per unit length does not reverse the polarization of the first
piezoelectric layer 111 as described above. The polarization of the two piezoelectric layers 112
was changed within the range of inversion. Data in ○ in FIG. 6 indicate conversion loss of
resonance in the first mode, and data in Δ indicate conversion loss in the second mode.
According to this figure, when the applied voltage per unit length is gradually changed while
reaching about +100 kV / cm and about -100 kV / cm, a sudden change in conversion efficiency
occurs. And when the applied voltage per unit length is smaller than about -100 kV / cm (larger
in absolute value), the loss of vibration of the primary mode becomes smaller than that of the
secondary mode, and the applied voltage per unit length is When greater than about +100 kV /
cm, the loss of secondary mode vibration is less than that of the primary mode. These results
indicate that if a voltage applied per unit length is smaller than about -100 kV / cm (larger in
absolute value), conversion from the secondary mode to the primary mode takes place, and the
unit voltage per unit length Application of a voltage greater than about +100 kV / cm means that
conversion from the primary mode to the secondary mode takes place.
[0035]
In FIG. 7, for the piezoelectric element 10 of the present embodiment, a DC voltage of −30 V (in
the case of the first mode) or +30 V (in the case of the second mode) as the polarization control
voltage is used for the first electrode 121 and the second electrode 122. The result of having
measured the insertion loss is shown, changing the frequency of the alternating voltage input
from the outside, applying between. Here, the insertion loss refers to the loss of high frequency
power generated when the high frequency power propagates from one terminal to another
terminal in the element inserted in the high frequency circuit. Where the insertion loss is small
(the absolute value of the insertion loss is small), the frequency in the secondary mode is
approximately twice that in the primary mode. This result means that the piezoelectric element
10 according to the present embodiment can be used in both the first mode and the second
mode in which the resonance frequency differs by about twice.
[0036]
FIG. 8 shows another embodiment of the piezoelectric element according to the present
invention. The piezoelectric element 10A shown in the figure includes a first piezoelectric layer
111A made of PbTiO3, a second piezoelectric layer 112A made of PbZrxTi1-xO3, a third
piezoelectric layer 113A made of PbTiO3, and a fourth piezoelectric layer PbZrzTi1-xO3. Four
layers of the piezoelectric layer 114A are stacked in this order, and the four layers are
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sandwiched between the first electrode 121A and the second electrode 122A in the stacking
direction. In this piezoelectric element 10A, the polarization reversal electric field value of the
first piezoelectric layer 111A and the third piezoelectric layer 113A is smaller, and the
polarization reversal electric field value of the second piezoelectric layer 112A and the fourth
piezoelectric layer 114A is larger. Switching between two vibration states is performed by
applying a polarization control voltage between the first electrode 121A and the second
electrode 122A so that an electric field is generated. In the first vibration state, vibration of the
first mode is generated in which the thickness d of the first piezoelectric layer 111A to the fourth
piezoelectric layer 114A is 1/2 wavelength (FIG. 8A), and the second vibration is generated. In
the state, the vibration of the fourth order mode in which the thickness d is two wavelengths
occurs (same (b)).
[0037]
Further, as shown in FIG. 9, the number of piezoelectric layers may be an odd number. In the
piezoelectric element 10B of the figure, a first piezoelectric layer 111B made of PbTiO3, a second
piezoelectric layer 112B made of PbZrxTi1-xO3, and a third piezoelectric layer 113B made of
PbTiO3 are laminated in this order, and these three layers are formed. It has a configuration in
which the first electrode 121B and the second electrode 122B are sandwiched in the stacking
direction. In the piezoelectric element 10B as well, as in the other examples, the polarization
control voltage is applied between the first electrode 121B and the second electrode 122B to
switch between the two vibration states. In the first vibration state, vibration of the primary mode
is generated in which the thickness d of the first piezoelectric layer 111B to the third
piezoelectric layer 113B is a half wavelength (FIG. 9A), and the second vibration is generated. In
the state, the vibration of the third mode in which the thickness d is 3/2 wavelength occurs
(same (b)).
[0038]
The piezoelectric element according to the present invention can have a configuration in which
the piezoelectric layer and the electrode are supported by the support portion 13 as shown in
FIG. The piezoelectric element shown in FIG. 10 is a thin film resonator filter called FBAR (Film
Bulk Acoustic Resonator). The supporting portion 13 of the FBAR 10C is a plate-like member
provided with a cavity 131 by cutting the center thereof. Only a part of the laminate of the first
electrode 121C, the first piezoelectric layer 111C, the second piezoelectric layer 112C, and the
second electrode 122C is supported by the plate-like member of the support portion 13, and the
remaining portion is supported It is on the cavity 131 of the part 13. The portion above the
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cavity 131 can vibrate without being restricted by the support portion 13.
[0039]
The piezoelectric transducer according to the present invention includes a frequency filter used
in communication equipment, an ultrasonic transducer, a diaphragm type aerial ultrasonic
element (pMUT), a piezoelectric transformer, an energy harvester, a piezoelectric actuator, a
piezoelectric motor, a medical device, an ultrasonic microscope, etc. It can be used as an
ultrasonic probe or the like in the measurement apparatus of
[0040]
In an ultrasound probe in an ultrasound microscope, in general, ultrasound generated by a
piezoelectric element is converged by an ultrasound lens and irradiated to a measurement target,
and an ultrasonic wave as a response from the measurement target is collected by the ultrasound
lens. Detection with the piezoelectric element.
Since such an ultrasonic lens is used, the larger the area of the piezoelectric layer of the
piezoelectric element, the larger the sound pressure of the ultrasonic wave converged by the
ultrasonic lens. Therefore, the S / N ratio of the ultrasonic microscope is high. can do. However,
simply increasing the area of the piezoelectric layer reduces the impedance of the piezoelectric
element, which causes an impedance mismatch with the impedance of the measurement system
(usually 50 Ω), resulting in input to the piezoelectric element The high frequency voltage and the
detected ultrasonic waves are converted by the piezoelectric element, and the high frequency
voltage output to the measurement system becomes small. It is possible to match the impedance
value of the piezoelectric element to the impedance of the measurement system by thickening
the piezoelectric layer to increase the area of the piezoelectric layer, in which case the resonant
frequency is small (resonance The wavelength will be long). On the other hand, in the
piezoelectric element according to the present invention, the resonant frequency is higher than
that of the conventional piezoelectric element having the same area and thickness of the
piezoelectric layer in the vibration state of the higher mode among the two states switched by the
polarization control voltage. It can be large (resonant wavelength can be shortened). Therefore, if
the piezoelectric element and the piezoelectric conversion device according to the present
invention are applied to an ultrasonic microscope, the area (and thickness) of the piezoelectric
layer can be increased while matching the impedance of the piezoelectric element with the
impedance of the measurement system. Thereby, the S / N ratio of the acoustic microscope can
be increased.
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[0041]
Further, in the ultrasonic microscope provided with the piezoelectric element and the
piezoelectric conversion device according to the present invention, measurement can be
performed at two different resonance frequencies by switching from the high-order mode to the
first-order mode. For example, when searching for a flaw on a sample, for example, when
ultrasonic waves are applied, harmonics may be generated from the flaw. In that case, ultrasonic
waves are applied to the sample from the piezoelectric element in the first mode, and the
harmonics generated in the sample thereby switch the vibration state of the piezoelectric element
to a higher mode, before the harmonics reach the piezoelectric element. Can be detected by the
piezoelectric element.
[0042]
The present invention is not limited to the above embodiment. For example, although the
piezoelectric layer made of PbTiO3 and the piezoelectric layer made of PbZrxTi1-xO3 are
alternately stacked in the above embodiment, the material of the piezoelectric layer is not limited
to these, and the piezoelectric materials having different polarization reversal electric field values
and being adjacent to each other Any piezoelectric material can be combined as long as it is
possible to bond the body layers. When three or more piezoelectric layers are used, two
piezoelectric layers different in material are alternately stacked in the above embodiment, but the
polarization reversal electric field value is generated by a predetermined electric field (generated
by the polarization control voltage It is also possible to use three or more piezoelectric layers of
different materials, as long as the higher and lower ones are alternately stacked. Furthermore,
although the case where the number of piezoelectric layers is 2 to 4 has been described in the
above embodiment, the number of piezoelectric layers may be five or more.
[0043]
10, 10A, 10B: Piezoelectric element 10C: FBAR 111, 111A, 111B, 111C: first piezoelectric layer
112, 112A, 112B, 112C, second piezoelectric layer 113A, 113B, third piezoelectric layer 114A,
fourth Piezoelectric layers 121, 121A, 121B, 121C ... first electrodes 122, 122A, 122B, 122C ...
second electrodes 13 ... supporting parts 131 ... cavities 20 ... piezoelectric conversion devices 21
... polarization control voltage applying parts 211 ... first DC power supply 212 second direct
current power supply 22 polarization control voltage inverting unit 221 first switch 222 second
switch 23 resonance frequency control voltage applying unit 231 variable direct current voltage
power supply 232 third switch
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