JP2017224969A - Piezoelectric element and piezoelectric converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a piezoelectric element which can switch resonant frequency in a wide frequency range and can be made compact, and to provide a piezoelectric converter using the piezoelectric element.SOLUTION: A piezoelectric element 10 includes: a first piezoelectric layer 111 composed of a first piezoelectric material and having polarization in the thickness direction; a second piezoelectric layer 112 laminated on the first piezoelectric layer 111, and composed of a second piezoelectric material having a polarization inversion electric field value smaller than that of the first piezoelectric material; and a pair of electrodes (first electrode 121, second electrode 122) sandwiching the first piezoelectric layer 111 and the second piezoelectric layer 112 in the lamination direction. A piezoelectric converter 20 includes: the piezoelectric element 10; a polarization control voltage application unit (first DC power supply 211, second DC power supply 212) for applying a polarization control voltage of DC 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 but larger than the polarization inversion electric field value of the second piezoelectric material, in the first piezoelectric layer 111 and the second piezoelectric layer 112; and a polarization control voltage inversion unit (first switch 221, second switch 222) for switching positive/negative of the polarization control voltage.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a frequency filter in a communication device, a piezoelectric element used for an ultrasonic element that transmits and / or receives ultrasonic waves, and a piezoelectric transducer.

  The piezoelectric body generates mechanical vibration when an alternating electrical signal is input, and generates an alternating electrical signal when the mechanical vibration is input. Piezoelectric elements convert electrical signals and mechanical vibrations with a piezoelectric thin film by utilizing the properties of the piezoelectric body, and are used as frequency filters, ultrasonic elements, etc. used in wireless communication. In the frequency filter, only a signal having a predetermined narrow band frequency centering on the resonance frequency of the piezoelectric thin film is passed. Usually, a plurality of piezoelectric elements having different passband frequencies are used in one wireless communication device.

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 depending on the magnitude thereof, and the resonance frequency of the piezoelectric thin film also changes accordingly. Here, the electromechanical coupling coefficient is a coefficient representing a standard of the conversion efficiency between the energy of electricity and the energy of mechanical vibration. Non-Patent Document 1 describes that in 0.67 BiFeO ₃ -0.33BaTiO ₃ , which is a mixed crystal system of BiFeO ₃ and BaTiO ₃ , the resonance frequency changes by about 4.4% by changing the DC bias voltage. Thus, by making the resonance frequency of the piezoelectric thin film variable, the frequency band that can be used by one piezoelectric element can be widened.

  However, since the number of wireless communication devices is expected to increase further in the future, each wireless communication device is required to have a wider bandwidth. However, when the resonance frequency is changed by changing the DC bias voltage as in the piezoelectric element of Non-Patent Document 1, the range of change in the resonance frequency is limited, and switching the resonance frequency in a wide frequency range is not possible. Have difficulty.

  Non-Patent Document 2 includes a laminate of two piezoelectric layers made of the same material, where the two layers have the same polarization (polarization non-inversion portion) and the two layers have opposite polarizations. A piezoelectric element including a laminated body having a portion (a polarization inversion portion) is described. In this piezoelectric element, a first electrode pair sandwiching the polarization non-inversion part and a second electrode pair sandwiching the polarization inversion part are provided independently (note that one of the two electrode pairs is common) Can be used). When an AC voltage is applied to the first electrode pair, the two piezoelectric layers vibrate in the same direction, and a resonance having a resonance wavelength twice the thickness of the entire laminate occurs, whereas an AC 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 resonance wavelength having the same length as the thickness occurs in the entire laminate. In this way, two resonance frequencies can be used by switching the electrode to which the AC voltage is applied 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 these DC bias voltages, the resonance frequency can be switched in a wider frequency range than the configuration of Patent Document 1. .

A. Vorobiev and 4 others, “Intrinsically tunable 0.67BiFeO3-0.33BaTiO3 thin film bulk acoustic wave resonators”, APPLIED PHYSICS LETTERS, (USA), American Institute of Physics, December 4, 2012, Volume 101 Pages 239033-1 to 232903-5 Katsukichi Katada, Takahiko Yanagiya and two others, "Second-order mode polarization inversion resonator using PbTiO3 epitaxial thin film", IEICE technical report, published by the Institute of Electronics, Information and Communication Engineers, October 15, 2014, No. 1 Volume 114 Number 263 Pages 29-34

  The piezoelectric element described in Non-Patent Document 2 operates only one of the polarization non-inversion part and the polarization inversion part when one resonance frequency is used. For this reason, the size of the element has to be doubled compared to the conventional size, which is disadvantageous when incorporated into a small communication device such as a mobile phone.

  The problem to be solved by the present invention is to provide a piezoelectric element that can switch the resonance frequency in a wide frequency range and can be miniaturized, and a piezoelectric transducer using the piezoelectric element.

The piezoelectric element according to the present invention, which has been made to solve the above problems,
a) a first piezoelectric layer made of a first piezoelectric material and having polarization in the thickness direction;
b) a second piezoelectric layer made of a second piezoelectric material provided on the first piezoelectric layer and having a polarization reversal electric field value smaller than that of the first piezoelectric material;
c) A pair of electrodes sandwiching the first piezoelectric layer and the second piezoelectric layer in the stacking direction.

The piezoelectric transducer according to the present invention is
a) a first piezoelectric layer made of a first piezoelectric material and having polarization in the thickness direction;
b) a second piezoelectric layer made of a second piezoelectric material provided on the first piezoelectric layer and having a polarization reversal electric field value smaller than that of the first piezoelectric material;
c) a pair of electrodes sandwiching the first piezoelectric layer and the second piezoelectric layer in the stacking direction;
d) An electric field having a value smaller than a polarization reversal electric field value of the first piezoelectric material and larger than a polarization reversal electric field value of the second piezoelectric material is generated 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 DC voltage between the pair of electrodes,
e) A polarization control voltage inverting unit that switches between positive and negative of the polarization control voltage.

  The polarization reversal electric field value refers to the minimum value of the electric field strength at which polarization reversal occurs when an electric field in the opposite direction to the polarization is applied to the piezoelectric material. That is, if an electric field stronger than the polarization reversal electric field value is applied to the piezoelectric material in the direction opposite to the polarization, the reversal of polarization 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. However, in the present invention, the polarization inversion electric field value is defined in order to define the voltage applied by the polarization control voltage application unit. And use.

  The piezoelectric element according to the present invention has a configuration 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 value 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 a polarization reversal electric field value of the first piezoelectric material and larger than a polarization reversal electric field value of the second piezoelectric material, Only the direction of polarization of the second piezoelectric layer can be controlled while maintaining 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 opposite directions. Can be switched between. By this switching, the piezoelectric element according to the present invention can use two different resonance frequencies. In order to perform such switching, the piezoelectric transducer according to the present invention includes the polarization control voltage application unit and the polarization control voltage inversion unit.

In piezoelectric materials, (1) polarization is maintained even when no electric field is applied, and polarization is reversed if an electric field stronger than the polarization inversion electric field value is applied in the opposite direction to the polarization, (2) electric field Polarization is maintained even when no electric field is applied, and polarization does not reverse even when an electric field in the opposite direction to the polarization is applied (the polarization inversion electric field value is very large), (3) An electric field is applied There is a material in which polarization is maintained only during the period, and if the direction of the electric field is changed, the polarization also changes in that direction (the polarization inversion electric field value is 0. Also referred to as “electrostrictive material”). 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. Examples of (1) include PbTiO ₃ (lead titanate), PbZr _x Ti _1-x O ₃ (lead zirconate titanate) in which part of Ti (titanium) in PbTiO ₃ is replaced with Zr (zirconium), etc. Is mentioned. Examples of (2) include ZnO (zinc oxide), AlN (aluminum nitride), and Sc _x Al _1-x N in which a part of Al in AlN is replaced by Sc (scandium). Examples of (3) include Ba _x Sr _1-x TiO ₃ in which a part of Sr (strontium) in SrTiO ₃ (strontium titanate) is replaced with Ba (barium).

  In the piezoelectric element and the piezoelectric conversion device according to the present invention, the whole of the first piezoelectric layer and the second piezoelectric layer can be operated in any of the two states. Can be made smaller.

  The polarization control voltage may be applied at the time of switching between the two states. Even if the application is stopped after the polarization of the second piezoelectric layer is reversed, the polarization of the second piezoelectric layer is not changed. The state (inverted state) is maintained. Accordingly, except for this switching time, a DC bias voltage that generates a DC electric field having a value smaller than the polarization inversion electric field value is applied to the first piezoelectric material and the second piezoelectric material, and the magnitude of the DC bias voltage is increased. By changing the height, the resonance frequency can be controlled. Therefore, the piezoelectric conversion device applies a resonance frequency control voltage, which is a DC voltage, between the pair of electrodes so as to generate an electric field smaller than the polarization reversal electric field value of the second piezoelectric material. It is desirable to provide an application unit.

  The piezoelectric element and the piezoelectric conversion device according to the present invention may have at least the first piezoelectric layer and the second piezoelectric layer, but the polarization inversion does not invert the polarization by the application of the polarization control voltage. A piezoelectric layer (including the first piezoelectric layer) made of a piezoelectric material having an electric field value, and a piezoelectric layer (second piezoelectric layer) made of a piezoelectric material having a polarization reversal electric field value whose polarization is reversed by application of the polarization control voltage 3 layers or more in total, including body layers). In this case, one of the two resonance frequencies is larger than the other by a multiple of the same number as the number of piezoelectric layers.

  With the piezoelectric element and the piezoelectric transducer according to the present invention, the resonance frequency can be switched over a wide frequency range, and the size can be reduced.

1 is a schematic configuration diagram showing an embodiment of a piezoelectric element and a piezoelectric conversion device according to the present invention. Schematic which shows the manufacturing method of the piezoelectric element of this embodiment. Schematic which shows operation | movement of the piezoelectric element and piezoelectric conversion apparatus of this embodiment. 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 between a pair of electrodes and a piezoelectric element in which only the second piezoelectric layer is sandwiched between a pair of electrodes, The graph which shows the result of having measured the conversion loss of the resonance of the primary mode, changing the applied DC voltage. The graph which shows the result of having measured the conversion loss of the resonance of the primary mode and the 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, changing the frequency of the alternating voltage input from the outside in the piezoelectric element of this embodiment. The schematic block diagram which shows the example which has four piezoelectric material layers which are other embodiment of the piezoelectric element which concerns on this invention. The schematic block diagram which shows the other embodiment of the piezoelectric element which concerns on this invention, and shows the example which has three piezoelectric material layers. The schematic block diagram which shows FBAR which is other embodiment of the piezoelectric element which concerns on this invention.

Embodiments of a piezoelectric element and a piezoelectric transducer 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 transducer 20 of the present embodiment. In the piezoelectric element 10, the first piezoelectric layer 111 made of PbTiO _3, the second piezoelectric layer 112 made of PbZr _x Ti _1-x O ₃ where part of Ti in PbTiO ₃ has been replaced by Zr is laminated Yes. Here, x can take any value within the range of greater than 0 and less than or equal to 1, but is set to 0.53 in the present embodiment. The coercive electric field value 2Ec is 470 kV / cm for the first piezoelectric layer 111 (PbTiO ₃ ) and 220 kV / cm for the second piezoelectric layer 112 (PbZr _x Ti _1-x O ₃ ). Is approximated by Ec which is 1/2 of the coercive electric field value 2Ec. The thickness of the first piezoelectric layer 111 and the second piezoelectric layer 112 is 580 nm for the former and 1070 nm for the latter in this embodiment, but is not limited to these thicknesses in the present invention.

The piezoelectric element 10 of this 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 obtained by doping SrTiO ₃ with a small amount (3.73 mass%) of La and polished so that the surface becomes a (001) plane was 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, and another material may be used for the first electrode 121 and / or the second electrode 122. The second electrode 122 is grounded.

  The first piezoelectric layer 111 is positive on the first electrode 121 side by applying an electric field having a value higher than the polarization reversal electric field value of the first piezoelectric layer 111 in advance. Directional polarization is formed. The polarization direction 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.

  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 DC power supply 211 has a negative electrode connected to the first electrode 121 and a positive electrode connected to the second electrode 122, and a 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 a 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 30V. The magnitude of this polarization control voltage is measured by actually applying a DC voltage between the first electrode 121 and the second electrode 122 to change the value of the voltage (see FIG. 6 to be described later) The polarization of the first piezoelectric layer 111 was determined within the voltage range in which the polarization of the second piezoelectric layer 112 was not reversed.

  The piezoelectric transducer 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 source 231 and a third switch 232. The variable DC voltage power source 231 is a power source that can output a DC voltage and change the output voltage. The range in which the output voltage is changed by the variable DC voltage power source 231 is determined so that the polarization of the second piezoelectric layer 112 is not reversed.

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 a (001) plane. Next, a PbTiO ₃ film is formed on the first electrode 121 by RF magnetron sputtering (FIG. 2A), thereby producing the first piezoelectric layer 111 (FIG. 2B). Here, since the difference in lattice constant between (Sr, La) TiO ₃ and PbTiO ₃ is small, PbTiO _{3 in} the first piezoelectric layer 111 is epitaxially grown on the first electrode 121. Subsequently, PbZr _x Ti _1-x O ₃ is formed on the first piezoelectric layer 111 by RF magnetron sputtering ((c)), thereby producing the second piezoelectric layer 112 (same as above). (d)). Here, PbZr _x Ti _1-x O ₃ of the second piezoelectric layer 112 is also epitaxially grown on the PbTiO _{3 of} the first piezoelectric layer 111. Thus, the first piezoelectric layer 111 and the second piezoelectric layer 112 are formed with an epitaxial junction structure in which 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 matched with the crystal orientation in the other. For example, when two layers grown in the c-axis direction are joined (when the c-axis is perpendicular to the layer), the axis parallel to the two layers (for example, the a-axis) points in the same direction It can be said that this is an epitaxial junction structure.

  Subsequently, gold is vapor-deposited on the second piezoelectric layer 112 (same (e)) to produce the second electrode 122 (same (f)), whereby each layer constituting the piezoelectric element 10 is completed. .

  Thereafter, the first electrode 121 and the second electrode are generated so that an electric field that is negative on the first electrode 121 side and is larger than the polarization reversal electric field value of the first piezoelectric layer 111 is generated in the first piezoelectric layer 111. By applying a voltage between the two electrodes 122 (FIG. 2 (g)), an electric field E directed from the second electrode 122 to the first electrode 121 is generated, whereby the first electrode 121 side has a positive polarization. Formed on the body layer 111 and the second piezoelectric layer 112 ((h)). Thus, the piezoelectric element 10 is completed.

  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.

First, as shown in FIG. 3 (a), the third switch 232 of the resonance frequency control voltage applying unit 23 is turned OFF, the first switch 221 of the polarization control voltage inverting unit 22 is turned ON, and the second switch 222 is turned ON. The case where it is turned off will be described. In this case, an electric field E from the second electrode 122 toward the first electrode 121 is generated in the second piezoelectric layer 112, and the polarization P of the second piezoelectric layer 112 is in a direction in which the first electrode 121 side is 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, and the value of this electric field is the polarization of PbTiO _{3 in} the first piezoelectric layer 111. Since it is smaller than the reversal electric field value, the direction of the polarization P in the first piezoelectric layer 111 is not affected. At this stage, the polarization P in the first piezoelectric layer 111 and the polarization P in the second piezoelectric layer 112 are in the same direction. Therefore, when electrical or mechanical vibration is applied from the outside, FIG. ) as shown on the right side of FIG in resonance to the thickness d ₁ of the first piezoelectric layer 111 1/2 wavelength thickness d of the thickness d ₂ combined in the second piezoelectric layer 112 is produced. Hereinafter, this resonance is referred to as “first-order mode” resonance.

  Thus, after setting the polarization P of the first piezoelectric layer 111 and the second piezoelectric layer 112 in the same direction, the first switch 221 is turned off and the third switch 232 is turned on (FIG. 3B). . In this state, by adjusting the magnitude of the DC voltage applied from the variable DC voltage power source 231, the resonance frequency corresponding to the resonance wavelength with the thickness d being ½ wavelength can be changed within a range of several percent. it can.

Next, as shown in FIG. 3C, the third switch 232 of the resonance frequency control voltage applying unit 23 is turned OFF, the first switch 221 of the polarization control voltage inverting unit 22 is set to OF, and the second switch 222 is set. The case where is turned on will be described. In this case, an electric field E from the first electrode 121 toward the second electrode 122 is generated in the second piezoelectric layer 112, and the polarization P of the second piezoelectric layer 112 is in a direction in which the first electrode 121 side is negative. Become. On the other hand, since the value of the electric field E is smaller than the polarization inversion electric field value of PbTiO ₃ in the first piezoelectric layer 111, the polarization P in the first piezoelectric layer 111 remains in a direction in which the first electrode 121 side is positive. It is. Accordingly, since the polarization P in the first piezoelectric layer 111 and the polarization P in the second piezoelectric layer 112 are opposite to each other, when electrical or mechanical vibration is applied from the outside, FIG. As shown in the diagram on the right side, a resonance having a thickness d of one wavelength is generated. Hereinafter, this resonance is referred to as “second-order mode” resonance. In the secondary mode resonance, the resonance wavelength is ½ and the resonance frequency is twice that of the above-described primary mode resonance.

  After setting the polarization P of the first piezoelectric layer 111 and the second piezoelectric layer 112 in the 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 DC voltage applied from the variable DC voltage power source 231, the resonance frequency corresponding to the resonance wavelength with the thickness d being one wavelength can be changed within a range of several percent.

  As described above, each of the resonance frequency corresponding to the resonance wavelength having the thickness d of 1/2 wavelength and the resonance frequency corresponding to the resonance wavelength having the thickness d of 1 wavelength can be changed within a range of several percent. Therefore, it is possible to switch the resonance frequency in a wider frequency range than before.

  Hereinafter, the experimental result about the piezoelectric element 10 of this embodiment is shown. First, as a preliminary experiment, resonance in the first mode is performed for a piezoelectric element in which only the first piezoelectric layer 111 is sandwiched between a pair of electrodes and a piezoelectric element in which only the second piezoelectric layer 112 is sandwiched between a pair of electrodes. The conversion loss of was measured. The result is shown in FIG. In the figure, “PT alone” indicates a piezoelectric element in which only the first piezoelectric layer 111 is sandwiched between a pair of electrodes, and “PZT alone” indicates a piezoelectric in which only the second piezoelectric layer 112 is sandwiched between a pair of electrodes. Refers to an element. The thin arrows in the figure indicate the direction of change in the DC voltage when the conversion loss is measured while changing the DC voltage per unit length applied between the electrodes. In “PT alone”, the conversion loss peaks at two voltages with a DC voltage per unit length of around −160 kV / cm and around +310 kV / cm. This means that in these two voltages, the polarization directions are not aligned and the polarization is in the process of reversing. The coercive electric field value 2Ec of “PT alone” is 470 kV / cm. Similarly, with “PZT alone”, the conversion loss peaks at two voltages, the DC voltage per unit length is around -100 kV / cm and +120 kV / cm, and the coercive field value 2Ec is 220 kV / cm. It is. The polarization of the first piezoelectric layer 111 is reversed by a DC voltage outside the two peaks of conversion loss in “PZT alone” and inside the two peaks of conversion loss in “PT alone”. The polarization of the second piezoelectric layer 112 is considered to be reversed.

  FIG. 6 shows the result of measuring the conversion loss while changing the magnitude and direction of the DC voltage applied between the first electrode 121 and the second electrode 122 for the piezoelectric element 10 of the present embodiment. In this measurement, the 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. In FIG. 6, the data with ◯ indicates the conversion loss of resonance in the primary mode, and the data with △ indicates the conversion loss in the secondary mode. According to this figure, when the applied voltage per unit length is gradually changed and reaches about +100 kV / cm and about -100 kV / cm, a sudden change in conversion efficiency occurs. When the applied voltage per unit length is smaller than about -100 kV / cm (large in absolute value), the loss of vibration in the primary mode is smaller than in the secondary mode, and the applied voltage per unit length is When it is larger than about +100 kV / cm, the vibration loss of the secondary mode is smaller than that of the primary mode. These results show that when a voltage applied per unit length is smaller than about -100kV / cm (large in absolute value), the conversion from the secondary mode to the primary mode occurs and the unit length per unit length This means that if a voltage greater than about +100 kV / cm is applied, conversion from the primary mode to the secondary mode occurs.

  In FIG. 7, for the piezoelectric element 10 of this embodiment, a direct current voltage of −30 V (in the primary mode) or +30 V (in the secondary mode) is applied to the first electrode 121 and the second electrode 122 as the polarization control voltage. The result of having measured insertion loss, changing the frequency of the alternating voltage input from the outside, applying between them. Here, the insertion loss refers to a loss of high-frequency power that occurs when high-frequency power propagates from one terminal to another terminal in an element inserted in the high-frequency circuit. Where the insertion loss is small (its absolute value is small), the frequency in the secondary mode is about twice that in the primary mode. This result means that the piezoelectric element 10 of the present embodiment can be used in both the primary mode and the secondary mode whose resonance frequencies are different by about twice.

FIG. 8 shows another embodiment of the piezoelectric element according to the present invention. The piezoelectric element 10A shown in the figure, the first piezoelectric layer 111A, the second piezoelectric layer 112A composed of _{PbZr x Ti 1-x O 3} , the third piezoelectric layer 113A made of PbTiO ₃ consisting of PbTiO _3, and, Four layers of the fourth piezoelectric layer 114A made of PbZr _x Ti _1-x O ₃ are stacked in this order, and the four layers are sandwiched between the first electrode 121A and the second electrode 122A in the stacking direction. In this piezoelectric element 10A, the polarization inversion electric field value of the first piezoelectric layer 111A and the third piezoelectric layer 113A is smaller than that of the second piezoelectric layer 112A and the fourth piezoelectric layer 114A. Switching between the 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, a primary mode vibration is generated in which the total thickness d from the first piezoelectric layer 111A to the fourth piezoelectric layer 114A is ½ wavelength (FIG. 8A), and the second vibration is generated. In the state, a fourth-order mode vibration having a thickness d of 2 wavelengths occurs ((b)).

Further, as shown in FIG. 9, the number of piezoelectric layers may be an odd number. The piezoelectric element 10B in the figure, the first piezoelectric layer 111B made of PbTiO _3, the second piezoelectric layer 112B composed of _{PbZr x Ti 1-x O 3} , and a third piezoelectric layer 113B made of PbTiO ₃ in this order The three electrodes are stacked and sandwiched between the first electrode 121B and the second electrode 122B in the stacking direction. In the piezoelectric element 10B as well, switching between the two vibration states is performed by applying a polarization control voltage between the first electrode 121B and the second electrode 122B as in the other examples. In the first vibration state, primary mode vibrations having a thickness d that is a total of 1/2 wavelength from the first piezoelectric layer 111B to the third piezoelectric layer 113B are generated (FIG. 9 (a)), and the second vibration is generated. In the state, the vibration of the third-order mode with the thickness d of 3/2 wavelength is generated ((b)).

  As shown in FIG. 10, the piezoelectric element according to the present invention can take a configuration in which a piezoelectric layer and an electrode are supported by a support portion 13. The piezoelectric element shown in FIG. 10 is a thin film resonator filter called FBAR (Film Bulk Acoustic Resonator). The support portion 13 of the FBAR 10C is a plate-like member provided with a cavity 131 by hollowing out the center. In the laminated body of the first electrode 121C, the first piezoelectric layer 111C, the second piezoelectric layer 112C, and the second electrode 122C, only a part thereof is supported by the plate-like member of the support part 13, and the remaining part is supported. It is on the cavity 131 of the part 13. The portion on the cavity 131 can vibrate without being restricted by the support portion 13.

  The piezoelectric transducer according to the present invention includes a frequency filter used in communication equipment, an ultrasonic transducer, a diaphragm-type airborne ultrasonic element (pMUT), a piezoelectric transformer, an energy harvester, a piezoelectric actuator, a piezoelectric motor, a medical device, an ultrasonic microscope, and the like. It can be used for an ultrasonic probe or the like in the measuring apparatus.

  In an ultrasonic probe in an ultrasonic microscope, in general, ultrasonic waves generated by a piezoelectric element are converged by an ultrasonic lens and irradiated to a measurement target, and ultrasonic waves that are responses from the measurement target are collected by the ultrasonic lens. Detecting with the piezoelectric element. Since such an ultrasonic lens is used, the larger the area of the piezoelectric layer of the piezoelectric element, the greater the sound pressure of the ultrasonic wave converged by the ultrasonic lens, so the S / N ratio of the ultrasonic microscope is increased. can do. However, simply increasing the area of the piezoelectric layer reduces the impedance of the piezoelectric element, thereby creating an impedance mismatch with the impedance of the measurement system (usually 50Ω), which is then input to the piezoelectric element. The high-frequency voltage or the detected ultrasonic wave is converted by the piezoelectric element and the high-frequency voltage output to the measurement system becomes small. If the piezoelectric layer is made thicker as the area of the piezoelectric layer is increased, the impedance value of the piezoelectric element can be adjusted to the impedance of the measurement system. The wavelength becomes long). On the other hand, in the piezoelectric element according to the present invention, the resonance frequency is higher than that of a conventional piezoelectric element having the same area and thickness of the piezoelectric layer in the high-order mode vibration state among the two states switched by the polarization control voltage. It can be increased (resonance wavelength is shortened). Therefore, if the piezoelectric element and the piezoelectric transducer 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 ultrasonic microscope can be increased.

  Further, in an ultrasonic microscope including the piezoelectric element and the piezoelectric transducer according to the present invention, it is possible to perform measurement at two different resonance frequencies by switching from the higher order mode to the primary mode. For example, when searching for scratches on a sample, for example, if an ultrasonic wave is irradiated, harmonics may be generated from the scratches. In that case, the piezoelectric element is irradiated with ultrasonic waves in the first-order mode, thereby switching the vibration state of the piezoelectric element to a higher-order mode before the harmonics generated in the sample reach the piezoelectric element. Can be detected by the piezoelectric element.

The present invention is not limited to the above embodiment. For example, in the above embodiment, piezoelectric layers made of PbTiO ₃ and piezoelectric layers made of PbZr _x Ti _1-x O ₃ are alternately laminated, but the material of the piezoelectric layers is not limited to these, and the polarization inversion electric field value Any piezoelectric body can be combined as long as the piezoelectric layers are different and can adjoin adjacent piezoelectric layers. In addition, when three or more piezoelectric layers are used, in the above embodiment, two types of piezoelectric layers of different materials are alternately stacked, but the polarization inversion electric field value is generated by a predetermined electric field value (generated by the polarization control voltage). 3 or more types of piezoelectric layers of different materials can be used as long as the higher and lower electric field values are alternately stacked. Furthermore, although the case where the piezoelectric layer has 2 to 4 layers has been described in the above embodiment, the number of piezoelectric layers may be 5 or more.

10, 10A, 10B ... Piezoelectric element 10C ... FBAR
111, 111A, 111B, 111C ... first piezoelectric layers 112, 112A, 112B, 112C ... second piezoelectric layers 113A, 113B ... third piezoelectric layers 114A ... fourth piezoelectric layers 121, 121A, 121B, 121C ... First electrode 122, 122A, 122B, 122C ... second electrode 13 ... support 131 ... cavity 20 ... piezoelectric transducer 21 ... polarization control voltage application unit 211 ... first DC power supply 212 ... second DC power supply 22 ... polarization control voltage Inversion unit 221 ... first switch 222 ... second switch 23 ... resonance frequency control voltage application unit 231 ... variable DC voltage power source 232 ... third switch

Claims (5)

a) a first piezoelectric layer made of a first piezoelectric material and having polarization in the thickness direction;
b) a second piezoelectric layer made of a second piezoelectric material provided on the first piezoelectric layer and having a polarization reversal electric field value smaller than that of the first piezoelectric material;
c) A piezoelectric element comprising: a pair of electrodes sandwiching the first piezoelectric layer and the second piezoelectric layer in the direction of the lamination. The piezoelectric element according to claim 1,
The piezoelectric element, wherein the first piezoelectric layer and the second piezoelectric layer have an epitaxial junction structure. a) a first piezoelectric layer made of a first piezoelectric material and having polarization in the thickness direction;
b) a second piezoelectric layer made of a second piezoelectric material provided on the first piezoelectric layer and having a polarization reversal electric field value smaller than that of the first piezoelectric material;
c) a pair of electrodes sandwiching the first piezoelectric layer and the second piezoelectric layer in the stacking direction;
d) An electric field having a value smaller than a polarization reversal electric field value of the first piezoelectric material and larger than a polarization reversal electric field value of the second piezoelectric material is generated 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 DC voltage between the pair of electrodes,
e) A piezoelectric conversion device comprising a polarization control voltage reversing unit that switches between positive and negative of the polarization control voltage. The piezoelectric conversion device according to claim 3,
A resonance frequency control voltage applying unit that applies a resonance frequency control voltage, which is a DC voltage, between the pair of electrodes so as to generate an electric field having a value smaller than a polarization reversal electric field value of the second piezoelectric material. A piezoelectric transducer. The piezoelectric conversion device according to claim 3 or 4,
The piezoelectric conversion device, wherein the first piezoelectric layer and the second piezoelectric layer have an epitaxial junction structure.

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