JP6803978B2 - Piezoelectric sensor and its manufacturing method - Google Patents

Description

The present disclosure relates to piezoelectric sensors that detect pressure or force and methods of manufacturing the same.

Sensors that detect pressure or force (load) using a piezoelectric material are known. The pressure sensor of Patent Document 1 has a plurality of laminated piezoelectric bodies and a pair of electrodes located at both ends in the stacking direction of the laminated body composed of the plurality of piezoelectric bodies. In Patent Document 1, a plurality of piezoelectric bodies are bonded by a conductive adhesive interposed between them. Further, the pressure sensor of Patent Document 2 has disk-shaped crystals and disk-shaped electrodes stacked alternately. The electrodes are provided on the crystal by thin film deposition.

Japanese Unexamined Patent Publication No. 5-99775 Japanese Unexamined Patent Publication No. 2008-8873

In the piezoelectric sensor according to one aspect of the present disclosure, the first electrode, the first piezoelectric body, the second electrode, the second piezoelectric body, and the third electrode are laminated with each other in the order in which they are listed in the stacking direction. It is held in a state where the overlapping objects are adhered to each other. The first piezoelectric body and the second piezoelectric body are made of single crystals of the same material, and the angle formed by the stacking direction and the crystal axis is the same. The single crystal has anisotropy with respect to the coefficient of linear expansion in the plane orthogonal to the stacking direction. The thicknesses of the first electrode, the second electrode and the third electrode are t1, t2 and t3, respectively, and the linear expansion coefficients of the second electrode and the third electrode are α2 and α3, respectively, and the single crystal , When the largest coefficient of linear expansion and the smallest coefficient of linear expansion in the plane orthogonal to the stacking direction are α _max and α _min , respectively.
α2> α _max >α3> α _min , and t2>t3> t1.

The method for manufacturing a piezoelectric sensor according to one aspect of the present disclosure includes a laminating step, a heating step, and a cooling step. In the laminating step, the first electrode, the first piezoelectric body, the second electrode, the second piezoelectric body, and the third electrode are laminated in this enumeration order as the order in the laminating direction, and those that overlap each other. A laminated body is formed in which a conductive bonding material is arranged in each of the spaces. In the heating step, the laminate is heated to melt the bonding material. In the cooling step, after the heating step, the laminate is cooled to solidify the bonding material. The first piezoelectric body and the second piezoelectric body are made of single crystals of the same material, and the angle formed by the stacking direction and the crystal axis is the same. The single crystal has anisotropy with respect to the coefficient of linear expansion in the plane orthogonal to the stacking direction. The thicknesses of the first electrode, the second electrode and the third electrode are t1, t2 and t3, respectively, and the linear expansion coefficients of the second electrode and the third electrode are α2 and α3, respectively, and the single crystal , When the largest coefficient of linear expansion and the smallest coefficient of linear expansion in the plane orthogonal to the stacking direction are α _max and α _min , respectively.
α2> α _max >α3> α _min , and t2>t3> t1.

FIG. 1A is a perspective view showing the configuration of the piezoelectric sensor according to the embodiment, and FIG. 1B is an enlarged view of region Ib of FIG. 1A. The flowchart which shows the procedure of the manufacturing method of the piezoelectric sensor of FIG. 3 (a) to 3 (f) are schematic views for explaining the operation of the piezoelectric sensor of FIG. 4 (a) to 4 (e) are schematic views for explaining the operation of the piezoelectric sensor of FIG. It is a figure which shows the structure and thermal stress of the sensor element which concerns on a comparative example and an Example.

Hereinafter, embodiments according to the present disclosure will be described with reference to the drawings. The following drawings are schematic. Therefore, details may be omitted, and the dimensional ratio and the like do not always match the actual ones. Also, the dimensional ratios of the plurality of drawings do not always match.

Dimensions and physical property values (eg, coefficient of linear expansion) depend on temperature. When it is necessary to limit the temperature in determining whether or not the dimensions and physical property values satisfy the requirements according to the present disclosure, for example, the value at room temperature may be used as a reference. The room temperature may be, for example, 20 ± 15 ° C. as defined by the Japanese Industrial Standards (JIS). When it is necessary to define the value of room temperature in more detail, for example, the room temperature may be 20 ° C.

(Overall configuration of sensor)
FIG. 1A is a perspective view showing the configuration of the sensor 1 according to the embodiment.

The sensor 1 includes, for example, a sensor element 3 that converts a pressure (force from another viewpoint) into an electric signal (charge or voltage from another viewpoint), a wiring unit 4 electrically connected to the sensor element 3, and wiring. It has a processing unit 5 that is electrically connected to the sensor element 3 via the unit 4 and performs a predetermined process on a signal from the sensor element 3. The sensor element 3 may be regarded as a sensor (narrowly defined), or the combination of the sensor element 3 and the wiring portion 4 may be regarded as a sensor.

(Overall configuration of sensor element)
The sensor element 3 is, for example, formed in a substantially cylindrical shape, and outputs an electric signal corresponding to the pressure received in the axial direction D1 of the cylinder (for convenience, it may be referred to as the shaft D1). The diameter and height of the cylinder may be set as appropriate. For example, the diameter may be greater than, equal to, or smaller than the height. As an example of the dimensions, the diameter is 3 mm or more and 10 mm or less, and the height is 1 mm or more and 4 mm or less. Further, for example, the diameter is 2 times or more and 5 times or less the height.

The sensor element 3 may be in any direction upward or downward, but in the following description, for convenience, the upper surface is defined as the direction indicated by the arrow in the axial direction D1 (upper surface of the paper). Alternatively, words such as the bottom surface may be used.

The sensor element 3 is composed of, for example, a plurality of layered members laminated in the axial direction D1. The plurality of layered members are, for example, a first electrode 7A, a first piezoelectric body 9A, a second electrode 7B, a second piezoelectric body 9B, and a third electrode 7C in order from the top. Among these layered members, those overlapping each other (7A and 9A, 9A and 7B, 7B and 9B, and 9B and 7C) are adhered to each other on their facing surfaces.

In the following, the first electrode 7A to the third electrode 7C may be simply referred to as “electrode 7” without distinction. Further, the first piezoelectric body 9A and the second piezoelectric body 9B may be simply referred to as "piezoelectric body 9" without distinguishing between them.

When a pressure in the axial direction D1 is applied to the sensor element 3, and thus a pressure in the axial direction D1 is applied to each piezoelectric body 9, each piezoelectric body 9 has an intensity (for example, a voltage value or an amount of electric charge) corresponding to the pressure value. Electrical signal is generated. This signal is taken out by a pair of electrodes 7 overlapping on both sides of each piezoelectric body 9.

The sensor element 3 may be combined with other appropriate members. For example, the sensor element 3 may be supported by a member that contacts substantially the entire lower surface of the sensor element 3 and may be configured to detect the pressure applied to the upper surface. However, the sensor element 3 may be supported on the entire upper surface, supported on the upper surface, the lower surface and / or a part of the side surface, or may be flexibly and deformably supported by the pressure in the axial direction D1.

Further, for example, the sensor element 3 may be used to detect the pressure of a gas or a liquid, or may be used to detect a force (load). The sensor including the sensor element 3 may have an appropriate structure depending on the application. For example, the sensor element 3 may have its upper surface and / or lower surface exposed to the outside (in gas or liquid), may be sandwiched between two rigid members in the axial direction D1, and may have an upper surface and / or lower surface. May be covered with an insulating film having an appropriate thickness. The rigid body member may be in contact with the upper surface and / or the lower surface in its entirety, or may be in contact with a part thereof.

(Shape and dimensions of layered members)
Each of the three electrodes 7 and the two piezoelectric bodies 9 is, for example, a layer having a uniform thickness. The planar shapes of the three electrodes 7 and the two piezoelectric bodies 9 are, for example, identical to each other and are circular in the present embodiment.

The thicknesses of the three electrodes 7 are different from each other. Specifically, assuming that the thickness of the first electrode 7A is t1, the thickness of the second electrode 7B is t2, and the thickness of the third electrode 7C is t3, t1 to t3 satisfy the following equation (1). ..
t2>t3> t1 (1)

The thicknesses tp1 and tp2 of the two piezoelectric bodies 9 (hereinafter, may be referred to as "thickness tp" without distinguishing between them) are, for example, the same as each other. However, these thickness tps may be different from each other. Further, the thickness tp of each piezoelectric body 9 is, for example, thicker than any of the three electrodes 7. In other words, tp> t2. However, the thickness of the piezoelectric body 9 may be thinner than the thickness t2 of the thickest second electrode 7B among the three electrodes 7.

Specific dimensions of the thicknesses t1 to t3 and tp may be appropriately set. An example is as follows.
t1 ≤ 10 μm
50 μm ≤ t2 ≤ 800 μm
20 μm ≤ t3 ≤ 40 μm
400 μm ≤ tp ≤ 1600 μm

Further, from another viewpoint, an example of the dimensions is as follows.
t1 ≦ 0.2 × t2
0.025 × t2 ≦ t3 ≦ 0.8 × t2
t2 ≤ tp ≤ 16 x t2
The range indicated by the above absolute value (μm) and the range indicated by the magnitude relationship with other thicknesses may be appropriately combined.

(Material of layered member (coefficient of linear expansion))
Each electrode 7 is made of a conductor, for example, made of metal. The metal constituting each electrode 7 is generally isotropic with respect to mechanical properties such as linear expansion coefficient (coefficient of thermal expansion) and Young's modulus. The material constituting the first electrode 7A may be appropriately set, and may be the same as or different from the material constituting the second electrode 7B or the third electrode 7C. The materials constituting the second electrode 7B or the third electrode 7C are different from each other, and by extension, their linear expansion coefficients are different from each other.

Each piezoelectric body 9 is made of, for example, a single crystal having piezoelectricity. The single crystal constituting the piezoelectric body 9 has anisotropy with respect to mechanical properties such as the coefficient of linear expansion and Young's modulus. Specifically, for example, the piezoelectric body 9 has anisotropy with respect to the coefficient of linear expansion in the direction parallel to the plane orthogonal to the axial direction D1. The two piezoelectric bodies 9 are made of, for example, single crystals of the same material, and the angles formed by the axial direction D1 and the crystal axis are the same.

The single crystal constituting the piezoelectric body 9 is, for example, crystal (SiO ₂ ). In addition, the single crystal material constituting the piezoelectric body 9 is lithium niobate (LiNbO ₃ ), lithium tantalate (LiTaO ₃ ), lithium tetraborate (Li ₂ B ₄ O ₇ ), potassium niobate (KNbO ₃ ) or It may be a langasite-based compound. However, in the following, the description may be made on the premise that the piezoelectric body 9 is made of quartz.

The materials of the second electrode 7B, the third electrode 7C, and the piezoelectric body 9 may be appropriately selected as long as their linear expansion coefficients satisfy the following equation (2).
α2> α _max >α3> α _min (2)
However, α2 is the coefficient of linear expansion of the second electrode 7B, and α3 is the coefficient of linear expansion of the third electrode 7C. Further, α _max is the largest linear expansion coefficient in the plane orthogonal to the axial direction D1 of the piezoelectric body 9, and α _min is the smallest linear expansion coefficient in the plane orthogonal to the axial direction D1 of the piezoelectric body 9. is there.

By setting the thickness of the layered member (7 and 9) so as to satisfy the above-mentioned (1) and selecting the material of the layered member (7 and 9) so as to satisfy the above equation (2). For example, as will be described in detail later, the risk of damage to the piezoelectric body 9 due to thermal stress can be reduced.

Instead of the above equation (2), the material may be selected so as to satisfy the following equation (2)'.
α2> α _max >α3> α ₉₀ (2) ′
However, α ₉₀ is the coefficient of linear expansion of the piezoelectric body 9 in the direction orthogonal to the direction of α _max in the plane orthogonal to the axial direction D1. Since α _min ≤ α ₉₀ naturally holds in light of the definition of α _min, the equation (2)'is a subordinate concept of the equation (2).

Examples of specific materials are as follows.

For example, it is assumed that the piezoelectric body 9 is made of quartz, and the X-axis of the X-axis (electrical axis), Y-axis (mechanical axis), and Z-axis (optical axis, c-axis) is parallel to the axial direction D1. Here, the coefficient of linear expansion in the X-axis direction and the coefficient of linear expansion in the Y-axis direction are equivalent, and the coefficient of linear expansion in the Z-axis direction is larger than the coefficient of linear expansion in the X-axis direction and the coefficient of linear expansion in the Y-axis direction. large. Therefore, the piezoelectric body 9 has anisotropy with respect to the coefficient of linear expansion in the plane orthogonal to the axial direction D1.

The coefficient of linear expansion in various directions orthogonal to the X-axis direction is not sufficiently disclosed, but in practice, the coefficient of linear expansion in the Z-axis direction is set as the minimum value, and the direction orthogonal to the Z-axis (X-axis or Y-axis). The coefficient of linear expansion in is treated as the maximum value. Following this, when the coefficient of linear expansion in the Z-axis direction is αz and the coefficient of linear expansion in the direction orthogonal to the Z-axis (X-axis or Y-axis) is αxy, α _max = αxy and α _min = αz may be set. ..

In other words, instead of the equation (2) or the equation (2)', the material may be selected so that the following equation (2) ″ is satisfied.
α2>αxy>α3> αz (2) ″
Since α _max ≥ α xy and α _min ≤ α z naturally hold in light of the definitions of α _max and α _min, the equation (2) ″ is a subordinate concept of the equation (2).

αz is about 7.97 ppm / ° C. αxy is about 13.4 ppm / ° C. The Young's modulus of the crystal depends on the direction and the like, but is generally 80 GPa or more and 100 GPa or less.

Therefore, as the material of the second electrode 7B satisfying α2> αxy, for example, the following can be mentioned. In the following parentheses, α indicates the coefficient of linear expansion and E indicates Young's modulus (hereinafter, the same applies). JIS C5210 (phosphor bronze, α = about 18 ppm, E = about 110 GPa), copper (α = about 16.5 ppm, E = about 120 GPa), aluminum (α = about 23 ppm, E = about 70 GPa), silver (α = About 19 ppm, E = about 80 GPa), gold (α = about 14 ppm, E = about 80 GPa), and JIS SUS304 (stainless steel, α = about 18 ppm, E = about 200 GPa).

Further, as a material of the third electrode 7C satisfying αxy> α3> αz, for example, the following can be mentioned. JIS SUS430 (stainless steel, α = about 12 ppm, E = about 200 GPa), iron (α = about 12 ppm, E = about 210 GPa), titanium (α = about 8.5 ppm, E = about 110 GPa), platinum (α = About 9 ppm, E = about 160 GPa), palladium (α = about 11 ppm, E = about 110 GPa), and cobalt (α = about 13 ppm, E = about 210 GPa).

Physical property values such as the coefficient of linear expansion vary depending on the amount of impurities mixed in and the uniformity (non-uniformity) of the crystal. Therefore, whether or not the equation (2) (or the equation (2)'or the equation (2) ") is satisfied is determined only by the value of the coefficient of linear expansion, and is, for example, the material exemplified above. However, it is not determined by whether or not the material is as described above. However, if the material is as described above, the equation (2) or the like can be easily satisfied. Also, based on whether or not the material is as described above, (2) ) It may be estimated whether or not the equation is satisfied.

Various methods have been conventionally proposed for measuring the dimensions or the coefficient of linear expansion, and any of them may be used. For example, a measuring method and instrument whose measurement accuracy is guaranteed to be 0.1 ppm / ° C. (an error of 0.01 ppm / ° C. or less can exist) may be used. The thickness of each layered member (t1 to t3, etc.) may be measured in the sensor element 3 after completion, or may be measured in a state before being bonded. Similarly, the coefficient of linear expansion of each layered member may be measured by disassembling the sensor element 3 after completion, or may be measured in a state before being bonded.

As already mentioned, the material of the first electrode 7A may be appropriately set. That is, the coefficient of linear expansion α1 of the first electrode 7A may be appropriately set. For example, α1 <α _min , α1> α2, or α2 ≧ α1 ≧ α _min may be satisfied.

(Adhesion between layered members)
FIG. 1B is an enlarged view of the region Ib of FIG. 1A. Note that FIG. 1B shows the configuration between the second electrode 7B and the second piezoelectric body 9B, but the configuration between the other electrodes 7 overlapping each other and the piezoelectric body 9 is also the same.

The electrodes 7 and the piezoelectric body 9 that overlap each other are adhered to each other by, for example, a conductive bonding material 11 interposed between them. The material of the bonding material 11 is, for example, solder in a narrow sense (Sn-Pb type) or lead-free solder. Examples of the lead-free solder include Au-Sn series, Au-Si series, Au-Ge series, Sn-Cu series, Sn-Ag series, and Sn-Ag-Cu series.

The thickness tj of the bonding material 11 is thinner than the thickness of the electrode 7 and the thickness of the piezoelectric body 9. For example, the thickness t2 of the second electrode 7B is 50 μm or more, and the thickness t3 of the third electrode 7C is 20 μm or more, whereas the thickness tj of the bonding material 11 is less than 10 μm. In another aspect, for example, the thickness tj of the bonding material 11 is less than 0.2 × t2 and / or less than 0.5 × t3. Therefore, the influence of the bonding material 11 on the thermal stress in the direction orthogonal to the axial direction D1 is small.

(Wiring unit 4 and processing unit 5)
The wiring unit 4 connects, for example, the first electrode 7A and the third electrode 7C. Then, the combination of the first electrode 7A and the third electrode 7C and the second electrode 7B are connected to different terminals (not shown) of the processing unit 5. That is, a sensor element (reference numeral omitted) composed of the first piezoelectric body 9A and electrodes 7 (7A and 7B) on both sides thereof, and a sensor element (reference numeral) composed of the second piezoelectric body 9B and electrodes 7 (7B and 7C) on both sides thereof. (Omitted) is connected in parallel to the processing unit 5.

The wiring portion 4 may be connected to an appropriate position of each electrode 7. For example, the wiring portion 4 is connected to the upper surface of the first electrode 7A, and is connected to the side surface of the second electrode 7B and the third electrode 7C. However, for example, the wiring portion 4 may be connected to the side surface of the first electrode 7A or may be connected to the lower surface of the third electrode 7C.

The structure of the wiring portion 4 may be appropriate. For example, the wiring portion 4 may be composed of a conductor pattern formed on a cable, a bonding wire, and / or a circuit board. The material is also appropriate, and the connection method with the terminal of the electrode 7 or the processing unit 5 is also appropriate.

The processing unit 5 is composed of, for example, an IC (Integrated Circuit). Although not particularly shown, for example, the processing unit 5 includes an amplifier that amplifies the signal from the sensor element 3, a filter that filters the signal from the sensor element 3, and a conversion that converts the signal from the sensor element 3 into another type of signal. It has a unit (for example, an AD converter and / or a modulator) and / or a calculation unit that performs a predetermined calculation on information contained in a signal from the sensor element 3. Further, the processing unit 5 has, for example, an output unit that outputs a signal subjected to the above processing (amplification, filtering, format conversion and / or calculation) to another device.

(Manufacturing method of sensor element)
FIG. 2 is a flowchart showing an example of the procedure of the manufacturing method of the sensor element 3.

First, in step S1, each of the plurality of layered members (7A to 7C, 9A and 9B) constituting the sensor element 3 is formed. For example, each electrode 7 is manufactured by punching or etching a rolled metal plate. Further, for example, the piezoelectric body 9 is manufactured by slicing an artificial quartz at a predetermined cut angle and etching a wafer obtained.

In step S2, the bonding material 11 is arranged on the piezoelectric body 9 and / or the electrode 7. For example, the metal paste to be the bonding material 11 is printed on the upper and lower surfaces of the piezoelectric body 9 by screen printing.

In step S3, all the piezoelectric bodies 9 and electrodes 7 constituting the sensor element 3 are laminated. At this time, the positioning of the piezoelectric body 9 and the electrode 7 in the direction orthogonal to the axial direction D1 is performed, for example, by bringing the positioning member into contact with the side surface (outer edge) of the piezoelectric body 9 and the electrode 7 by a conventionally known laminating machine. Is done by. The alignment around the axis D1 is not performed, for example.

In step S4, the laminate composed of the piezoelectric body 9 and the electrode 7 is heated. As a result, the bonding material 11 is melted. The heating of the laminate may be performed by, for example, a reflow furnace.

In step S5, the laminate composed of the piezoelectric body 9 and the electrode 7 is cooled. As a result, the molten bonding material 11 is solidified, and the piezoelectric bodies 9 and the electrodes 7 that overlap each other are adhered to each other. It should be noted that cooling may be performed at room temperature, for example, and no special device is necessarily required.

In step S5 above, the bonding material 11 solidifies when its temperature reaches the freezing point. Since the freezing point of the bonding material 11 is usually higher than room temperature, the piezoelectric body 9 and the electrode 7 are adhered to each other at a relatively high temperature. For example, in the case of Au-Sn (20 wt% Sn), the freezing point is about 280 ° C. Then, since the sensor element 3 is exposed to room temperature in or after step S5, thermal stress is generated in the sensor element 3 due to the temperature change from the freezing point to the room temperature.

(Thermal stress generated in the piezoelectric body)
The thickness of the layered member (7 and 9) is set so as to satisfy the equation (1), and the layered member (7 and 9) satisfies the above equation (2), (2)'or (2) ". By selecting the material of 9), for example, the risk of damage to the piezoelectric body 9 due to thermal stress can be reduced. This action will be described below.

Here, as described with reference to FIG. 2, the piezoelectric body 9 and the electrode 7 are bonded at a relatively high temperature (for example, a temperature higher than room temperature), and then these members are at a relatively low temperature (for example, a temperature higher than room temperature). For example, it is assumed that the temperature is normal temperature).

(Thermal stress between the electrode and the piezoelectric body)
3A to 3F are schematic views for explaining the influence of the thickness of the electrode 7 and the coefficient of linear expansion on the thermal stress generated between the electrode 7 and the piezoelectric body 9. Here, it is assumed that one piezoelectric body 9 and two electrodes 7 located on both sides thereof are configured. Therefore, for example, the mechanical mutual influence between the two piezoelectric bodies 9 is not considered.

3 (a), 3 (c) and 3 (e) are perspective views showing the piezoelectric body 9 and the electrodes 7 on both sides thereof. These figures show aspects in which the thickness of the electrode 7 and the coefficient of linear expansion differ from each other. Specifically, the electrode 7 is relatively thin in FIG. 3 (a), whereas the electrode 7 is relatively thick in FIGS. 3 (c) and 3 (e). Further, in FIG. 3 (c), the coefficient of linear expansion (αe) of the electrode 7 is relatively large (for example, αe> α _max ), whereas in FIG. 3 (e), the coefficient of linear expansion of the electrode 7 is relatively large. (Αe) is relatively small (eg, αe <α _min (α ₉₀ )).

3 (b), 3 (d), and 3 (f) are schematic views for explaining the thermal stress generated in the piezoelectric body 9 due to the difference in the coefficient of linear expansion between the piezoelectric body 9 and the electrode 7. It corresponds to the aspect of FIG. 3A, FIG. 3C and FIG. 3E, respectively. In these figures, the piezoelectric body 9 is shown, and the region of the thermal stress where the tensile stress is large is hatched. Note that FIGS. 3 (b), 3 (d) and 3 (f) are based on simulation calculation results.

As shown in FIGS. 3A and 3B, when the thickness of the electrode 7 is relatively thin, the thermal stress generated in the piezoelectric body 9 is small. This is the same regardless of whether the coefficient of linear expansion of the electrode 7 is larger or smaller than the coefficient of linear expansion of the piezoelectric body 9.

As shown in FIGS. 3 (c) and 3 (d), the thickness of the electrode 7 is relatively thick, and the coefficient of linear expansion αe of the electrode 7 is larger than the coefficient of linear expansion of the piezoelectric body 9 (for example, α _max ). In some cases, the amount of shrinkage when the temperature drops is larger in the electrode 7 than in the piezoelectric body 9. Therefore, basically, the piezoelectric body 9 receives a compressive force from the electrode 7. That is, basically, the tensile stress is small in the piezoelectric body 9.

However, since the upper and lower surfaces of the piezoelectric body 9 receive a compressive force in the direction along the upper and lower surfaces from the electrode 7, tensile stress is generated on the side surfaces thereof. From another point of view, the Poisson effect occurs. More specifically, since the piezoelectric body 9 has anisotropy with respect to the coefficient of linear expansion in the direction orthogonal to the axial direction D1, both sides of the side surface in the direction in which the coefficient of linear expansion is relatively small. In the region R1, the tensile stress is relatively large as compared with the other regions R2.

As shown in FIGS. 3 (e) and 3 (f), the thickness of the electrode 7 is relatively thick, and the coefficient of linear expansion αe of the electrode 7 is the coefficient of linear expansion of the piezoelectric body 9 (for example, α _min (α ₉₀ )). ), The amount of shrinkage when the temperature drops is larger in the piezoelectric body 9 than in the electrode 7. Therefore, basically, the piezoelectric body 9 receives a tensile force from the electrode 7. More specifically, a relatively high tensile stress is generated on the upper and lower surfaces of the piezoelectric body 9. The relative relationship between the hatching and the magnitude of the stress in FIG. 3 (f) does not appear in the figure, but more specifically, the tensile stress is caused by the anisotropy of the piezoelectric body 9 with respect to the coefficient of linear expansion. Is relatively high on both sides in the direction in which the coefficient of linear expansion is relatively large.

(Thermal stress between piezoelectric bodies)
FIG. 4A is a schematic diagram for explaining the reason why thermal stress is generated between the two piezoelectric bodies 9.

In this figure, the X-axis, Y-axis, and Z-axis attached to each piezoelectric body 9 indicate the X-axis (electrical axis), Y-axis (mechanical axis), and Z-axis (optical axis, c-axis) of quartz. There is. Both of the two piezoelectric bodies 9 are made of quartz and are cut out at the same cut angle (here, at an angle orthogonal to the X axis).

Here, as already described, quartz has anisotropy with respect to the coefficient of linear expansion in the YZ plane. Therefore, when the two piezoelectric bodies 9 are bonded to each other, if the positions around the X axis deviate from each other as shown in the illustrated example, the amount of reduction with respect to the temperature drop in an arbitrary direction in the XZ plane will be different from each other. As a result, even if the two piezoelectric bodies 9 are composed of crystals having the same cut angle (even if the angle formed by the axis D1 and the crystal axis is the same between the two piezoelectric bodies 9), the two piezoelectric bodies 9 are used. Thermal stress is generated between the bodies 9.

4 (b) to 4 (e) are schematic views for explaining the effect of the thickness of the electrode 7 on the thermal stress generated between the two piezoelectric bodies 9. Here, as shown in FIG. 4A, it is assumed that the rotation positions around the X-axis are deviated by 90 ° from each other between the two piezoelectric bodies 9. Further, here, it is assumed that the two piezoelectric bodies 9 and the second electrode 7B between them are configured. Therefore, the influence of the first electrode 7A and the third electrode 7C on the thermal stress is not considered.

4 (b) and 4 (d) are perspective views showing the two piezoelectric bodies 9 and the second electrode 7B between them. These figures show aspects in which the thickness of the second electrode 7B is different from each other. Specifically, in FIG. 4B, the second electrode 7B is relatively thin, whereas in FIG. 4D, the second electrode 7B is relatively thick. In these figures, the coefficient of linear expansion of the second electrode 7B is larger than α _{min and} smaller than α _max .

4 (c) and 4 (e) are schematic views for explaining the thermal stress generated in the piezoelectric body 9 (here, the second piezoelectric body 9B is shown), which are FIG. 4 (b) and FIG. It corresponds to the aspect of 4 (d). In these figures, the piezoelectric body 9 is shown, and the region of the thermal stress where the tensile stress is large is hatched. Note that FIGS. 4 (c) and 4 (e) are based on simulation calculation results.

As shown in FIGS. 4 (b) and 4 (c), when the thickness of the second electrode 7B is relatively thin, a relatively high tensile stress is generated on the surface of the piezoelectric body 9 on the other piezoelectric body 9 side. It occurs over a relatively wide range. The relative relationship between the hatching and the magnitude of the stress in FIG. 4C does not appear in the figure, but more specifically, in each piezoelectric body 9, the tensile stress has a relatively large coefficient of linear expansion. It is relatively high on both sides of the direction. Further, although not particularly shown, the compressive stress increases near the side surface of each piezoelectric body 9 and on both sides in the direction in which the coefficient of linear expansion is relatively small.

As shown in FIGS. 4 (d) and 4 (e), when the thickness of the second electrode 7B is relatively thick, the tensile stress (and compressive stress) generated in the piezoelectric body 9 is relatively small.

From the above, when the second electrode 7B is relatively thin, the tensile stress becomes relatively large due to the thermal stress between the two piezoelectric bodies 9, and when the second electrode 7B is relatively thick, the two piezoelectric bodies are used. It can be seen that the tensile stress between the bodies 9 is relaxed.

(Thermal stress in the embodiment)
In the present embodiment, as shown in the equation (1), the thickness t2 of the second electrode 7B located between the two piezoelectric bodies 9 is thicker than the thicknesses t1 and t3 of the other electrodes 7. Therefore, for example, as described with reference to FIGS. 4 (b) to 4 (d), the anisotropy regarding the coefficient of linear expansion in the plane orthogonal to the axial direction D1 of the piezoelectric body 9 and the two piezoelectric bodies 9 The tensile stress generated in the piezoelectric body 9 due to the misalignment around the axis D1 between them can be relaxed.

The coefficient of linear expansion α2 of the thickened second electrode 7B is orthogonal to the axial direction D1 of the piezoelectric body 9 as shown in the equation (2) (or the equation (2) ′ or (2) ″). It is larger than the maximum coefficient of linear expansion α _max in the plane. Therefore, for example, as can be understood from the explanations in FIGS. 3 (c) and 3 (d), the force received by the piezoelectric body 9 from the second electrode 7B. Is basically a compressive force. As a result, for example, the possibility that the tensile stress generated in the piezoelectric body 9 becomes large due to the thickening of the second electrode 7B is reduced.

Further, as described with reference to FIGS. 3A to 3F, when the electrodes 7 on both sides of the piezoelectric body 9 become thicker, the coefficient of linear expansion αe of the electrode 7 becomes the coefficient of linear expansion of the piezoelectric body 9. The tensile stress increases regardless of whether it is larger or smaller than the comparison. On the other hand, in the present embodiment, as shown in the equation (1), the thickness t1 of the first electrode 7A and the thickness t3 of the third electrode 7C are made thinner than the thickness t2 of the second electrode 7B. , Both of the electrodes 7 on both sides of the piezoelectric body 9 are prevented from being thickened. Further, as shown in (2) (or (2)'or (2) ″ equation), the linear expansion coefficient α2 of the second electrode 7B having the second thickness among the three electrodes 7 is made into a piezoelectric material. The value is set to be between the maximum value and the minimum value of the coefficient of linear expansion of 9. Therefore, the tensile stress generated in the piezoelectric body 9 can be further reduced.

Here, the piezoelectric body 9 is generally more likely to be damaged by a tensile force than by a compressive force. Therefore, by reducing the tensile stress generated in the piezoelectric body 9 as described above, the risk of damage to the piezoelectric body 9 can be reduced.

Since the thickness t1 of the first electrode 7A is the thinnest of the three electrodes 7 (Equation (1)), the thermal stress generated between the first electrode 7A and the piezoelectric body 9 is the smallest. Therefore, the degree of freedom in designing the linear expansion coefficient α1 of the first electrode 7A is higher than that of the linear expansion coefficients α2 and α3 of the second electrode 7B and the third electrode 7C, and may be set as appropriate.

Further, since the third electrode 7C is thicker than the first electrode 7A (Equation (1)), for example, the wiring portion 4 is connected to the upper surface of the first electrode 7A, while the wiring portion 4 is connected to the third electrode 7C. Can be connected to the side. Therefore, for example, it is easy to arrange the sensor element 3 so as to support the lower surface of the sensor element 3.

(Example)
FIG. 5 is a chart showing the configuration and thermal stress of the sensor element according to the comparative example and the embodiment.

In Comparative Examples 1 to 8 and Examples 1 to 18, specific materials and dimensions are set for the sensor element 3. Then, based on the material and dimensions, the thermal stress generated in the piezoelectric body 9 or the like was obtained by simulation calculation.

(Conditions common to Comparative Examples and Examples)
The conditions common to all the comparative examples and the examples are as follows.
Diameter of sensor element 3: 4 mm
First electrode 7A:
Material: Au
Thickness t1: 5 μm
Piezoelectric 9:
Material: Quartz (X-axis and axial direction D1 are parallel)
Thickness tp: 700 μm
Amount of deviation around the X axis between the two piezoelectric bodies: 90 °
Joining material 11:
Material: AuSn
Thickness: 5 μm

The elastic constant of quartz was set as follows, considering the amount of tensor. In the following, the unit is GPa. c11 = 86.72, c12 = 6.89, c13 = 11.88, c14 = -17.92, c15 = 0, c16 = 0, c22 = 86.72, c23 = 11.88, c24 = 17.92 , C25 = 0, c26 = 0, c33 = 107.12, c34 = 0, c35 = 0, c36 = 0, c44 = 57.89, c45 = 0, c46 = 0, c55 = 57.89, c56 =- 17.92, c66 = 39.92.

The coefficient of linear expansion of the crystal was 7.97 ppm / ° C. in the Z-axis direction and 13.4 ppm / ° C. in the direction orthogonal to the Z-axis. The coefficient of linear expansion in the other directions was calculated by interpolation based on the coefficient of linear expansion in the Z-axis direction and the direction orthogonal to the Z-axis direction.

The bonding material 11 was assumed to be solidified at 280 ° C. That is, the thermal stress was calculated on the assumption that the temperature would drop from 280 ° C. to room temperature (20 ° C.).

The thickness t1 of the first electrode 7A and the thickness of the bonding material 11 are sufficiently thinner than the thickness tp of the piezoelectric body 9, and the influence of the first electrode 7A and the bonding material 11 on the thermal stress is sufficient. Small. Further, the diameter of the sensor element 3 is basically independent of the qualitative influence of the thickness of each layer in the sensor element 3 and the coefficient of linear expansion on the thermal stress in the direction orthogonal to the axial direction D1.

(Conditions for Comparative Examples and Examples)
The conditions relating to the material (coefficient of linear expansion α2) and thickness t2 of the second electrode 7B and the material (coefficient of linear expansion α3) and thickness t3 of the third electrode 7C were different from each other between the comparative examples and the examples. ..

Specifically, as a major classification, there are two types, a condition in which the equation (1) is satisfied and a condition in which the equation (1) is not satisfied, and a condition in which the equation (2)'is satisfied and a condition in which the equation is not satisfied. A total of 4 types (2 types x 2 types) of combinations with 2 types were set.

In the column of "(2)'" in FIG. 5, "Y" indicates that the equation (2)'is satisfied, and "N" indicates that the equation (2)' is not satisfied. .. Similarly, in the column of "(1)" in FIG. 5, "Y" indicates that the equation (1) is satisfied, and "N" indicates that the equation (1) is not satisfied. .. Therefore, in the examples, both "(2)'" and "(1)" are "Y", and in the comparative example, at least one of "(2)'" and "(1)" is "(1)". N ".

“Α2” in FIG. 5 indicates the magnitude relationship between the linear expansion coefficient α2 of the second electrode 7B and the linear expansion coefficients α _max and α ₉₀ of the piezoelectric body 9. Similarly, “α3” in FIG. 5 shows the magnitude relationship between the linear expansion coefficient α3 of the third electrode 7C and the linear expansion coefficients α _max and α ₉₀ of the piezoelectric body 9.

As can be understood from these columns, as conditions in which the equation (2)'is not satisfied, more specifically, both α2 and α3 are larger than α _max (Comparative Examples 1 and 5), α2. and that both α3 is smaller than the alpha ₉₀ (Comparative example 3), both the α2 and α3 is set what the size (Comparative examples 2 and 6) between the alpha _max and alpha _90.

In Comparative Examples and Examples, the material of the electrode 7 (7B or 7C) in which α (α2 or α3) is larger than α _max is C5210 (α = 18.2, E = 110 GPa, ν = 0.33). Was assumed. In addition, ν is Poisson's ratio (hereinafter, the same applies). As the material of the electrode 7 in which α is smaller than α _min , JIS 42alloy (42 wt% Ni NiFe alloy, α = 5.5, E = 144 GPa, ν = 0.25) was assumed. As the material of the electrode 7 in which α has a size between α _max and α ₉₀ , SUS430 (α = 11.9, E = 200 GPa, ν = 0.3) was assumed.

The columns "t2" and "t3" in FIG. 5 show the values of the thicknesses t2 and t3. More specifically, t2 = t3 was set as a condition in which the equation (1) was not satisfied. In Comparative Examples and Examples, t2 was set in the range of 30 μm or more and 800 μm or less. t3 was set in the range of 30 μm or more and 200 μm or less. In addition, t2 / t3 is 1 or more and less than 26.7.

(Calculation result of thermal stress)
The column of "σ1" in FIG. 5 shows the maximum value of the tensile stress (thermal stress) generated in the first piezoelectric body 9A obtained by the simulation calculation. Similarly, the column “σ2” in FIG. 5 shows the maximum value of the tensile stress (thermal stress) generated in the second piezoelectric body 9B obtained by the simulation calculation.

As shown in this column, in all the examples, both σ1 and σ2 are 48 MPa or less. On the other hand, in all the comparative examples, at least one of σ1 and σ2 is 49 MPa or more. From this result, it can be confirmed that the maximum value of the tensile stress generated in both of the two piezoelectric bodies 9 can be reduced by satisfying the equations (1) and (2)'.

Further, for example, from the comparison of Comparative Examples 1 to 7 and Example 1, if both the second electrode 7B (thickness t2) and the third electrode 7C (thickness t3) are thickened (Comparative Examples 1 to 4), between them. It can be confirmed that the tensile stress σ2 of the second piezoelectric body 9B sandwiched between the two is increased. Further, when both the second electrode 7B (thickness t2) and the third electrode 7C (thickness t3) are thinned (Comparative Examples 5 to 7), the thermal stress between the two piezoelectric bodies 9 becomes large, and the two piezoelectric bodies 9 become large. It can be confirmed that the tensile stresses (σ1 and σ2) of both bodies 9 become large.

In this simulation calculation, the case where t2 <t3 is not shown. However, in this case, as t2 becomes thinner, the tensile stress of both of the two piezoelectric bodies 9 becomes larger as in Comparative Examples 5 to 7, and as t3 becomes thicker, the tensile stress of the second piezoelectric body 9B becomes larger. Therefore, it is clear that the tensile stress can be reduced in the examples.

Further, for example, when the coefficient of linear expansion (α2 and α3) of both the second electrode 7B and the third electrode 7C is reduced from the comparison of Comparative Examples 1 to 4 (Comparative Example 3), the piezoelectric body 9 has these It can be confirmed that the tensile stress becomes large because the shrinkage amount is larger than that of the electrode 7. It can be confirmed that when the coefficient of linear expansion of both the second electrode 7B and the third electrode 7C is increased (Comparative Example 1), a large tensile stress is generated in the second piezoelectric body 9B sandwiched between them.

Further, for example, when both the second electrode 7B and the third electrode 7C are thick (Comparative Examples 1 to 4), the linear expansion coefficients (α2 and α3) of both the second electrode 7B and the third electrode 7C are obtained. When is set to a magnitude between α _max and α ₉₀ (Comparative Example 2), the maximum value for both σ1 and σ2 is smaller than when the equation (2)'is satisfied (Comparative Example 4) (Comparative Example). While σ2 of 2 is 78 MPa, σ4 of Comparative Example 4 is 81 MPa). From this, it can be confirmed that the action and effect of the equation (2)'is remarkable when combined with the equation (1).

When both the second electrode 7B and the third electrode 7C are thin (Comparative Examples 5 to 7), the thermal stress between the two piezoelectric bodies 9 becomes large, so α2 and α3 are set to α _max and α. Even if the size is between ₉₀ (Comparative Example 6), the maximum value for both σ1 and σ2 is not always smaller than when the equation (2)'is satisfied (Comparative Example 7).

As described above, the sensor 1 (sensor element 3) stacks the first electrode 7A, the first piezoelectric body 9A, the second electrode 7B, the second piezoelectric body 9B, and the third electrode 7C in the stacking direction (axial direction). The order in D1) is such that the objects overlapping each other are laminated and adhered to each other. The first piezoelectric body 9A and the second piezoelectric body 9B are made of a single crystal of the same material, and the angle formed by the axial direction D1 and the crystal axis is the same. The single crystal constituting the piezoelectric body 9 has anisotropy with respect to the coefficient of linear expansion in the plane orthogonal to the axial direction D1. Then, in the sensor 1, the above-mentioned equations (1) and (2) are satisfied.

Therefore, as described with reference to FIGS. 3 and 4, for example, when the electrode 7 and the piezoelectric body 9 are adhered to each other at a relatively high temperature and cooled (steps S4 and S5), the linear expansion coefficients of both are obtained. The possibility that a large tensile stress is applied to the piezoelectric body 9 due to the difference between the two is reduced. As a result, the risk of damage to the piezoelectric body 9 is reduced. Further, for example, even after manufacturing, the possibility that a large tensile stress is repeatedly applied to the piezoelectric body 9 when the temperature change of the sensor element 3 is repeated is reduced, so that the durability is improved.

The setting of the coefficient of linear expansion such as α2 and α3 is actually realized by selecting a specific material. Therefore, for example, the difference in the conditions relating to α2 and α3 is accompanied by the difference in the conditions relating to the Young's modulus of the second electrode 7B and the third electrode 7C. On the other hand, not only the coefficient of linear expansion but also Young's modulus affects the thermal stress. However, the qualitative relationship between the magnitude relationship between the linear expansion coefficient of the electrode 3 and the linear expansion coefficient of the piezoelectric body 9 and the stress distribution in the piezoelectric body 9 described with reference to FIGS. 3 and 4 is the Young's modulus. Does not depend on. Therefore, the tensile stress generated in the piezoelectric body 9 (the maximum thereof) is generated by selecting the material so as to satisfy the equation (2) (or the equation (2)'or the equation (2) ") without considering the Young's modulus. Value) can still be reduced.

Further, as already described with reference to Comparative Examples 1 to 4, when the electrode 7 is relatively thick and the coefficient of thermal expansion of the electrode 7 has a relatively large effect on the thermal stress generated in the piezoelectric body 9, the case where the electrode 7 is relatively thick has a relatively large effect. When the coefficient of linear expansion was between the coefficients of linear expansion α _max and α _{90 of} quartz (piezoelectric body 9), the maximum values for both σ1 and σ2 became smaller (Comparative Example 4). Here, in Comparative Examples 1 to 4, the material of the electrode 7 having a linear expansion coefficient between the linear expansion coefficients α _max and α ₉₀ of the quartz (piezoelectric body 9) is compared with the materials of the other electrodes 7. Therefore, the Young's modulus is the largest, and from the viewpoint of Young's modulus, the thermal stress exerted on the piezoelectric body 9 tends to be large. That is, from the viewpoint of Young's modulus, the tensile stress was small in Comparative Example 3 in which the thermal stress exerted on the piezoelectric body 9 was most likely to be large. Therefore, it may be considered that the influence of Young's modulus is small in the simulation result of FIG. 5, or even if Young's modulus is taken into consideration, Eqs. (1) and (2) (or (2)'or (2) It may be considered that the simulation calculation confirmed that the tensile stress was reduced by selecting the material so as to satisfy the "formula).

Further, in the present embodiment, the first piezoelectric body 9A and the second piezoelectric body 9B are made of quartz whose electric axis (X axis) is parallel to the stacking direction (axial direction D1).

Therefore, for example, as understood from the example (described above) of the materials of the second electrode 7B and the third electrode 7C when the piezoelectric body 9 is quartz, the equation (2) (or the equation (2)' or ( 2) It is easy to select a material generally used for electronic parts as a material satisfying the "formula).

Further, in the present embodiment, when the thicknesses of the first piezoelectric body 9A and the second piezoelectric body 9B are tp1 and tp2,
t2 ≦ tp1 ≦ 16 × t2,
t2 ≦ tp2 ≦ 16 × t2,
50 μm ≦ t2 ≦ 800 μm and 20 μm ≦ t3 ≦ 40 μm.

Within such a range, the effect of the embodiment can be more reliably achieved. In addition, t2 ≦ tp (tp1, tp2) ≦ 16 × t2 and 50 μm ≦ t2 ≦ 800 μm are the ranges of tp and t2 in Examples 1 to 18. Regarding t3, in Examples 1 to 18, it is 30 μm or 40 μm, but unlike t2, it is clear that the smaller t3 is, the smaller the tensile stress of the piezoelectric body 9. However, if t3 is 20 μm or more, it is easy to handle, for example.

Further, in the present embodiment, the first piezoelectric body 9A and the second piezoelectric body 9B have the same shape as each other when viewed in the stacking direction (axial direction D1), and are symmetrical n times when n is 3 or more. (Rotationally symmetric shape that overlaps with itself when rotated at 360 ° / n, circular in this embodiment).

That is, the two piezoelectric bodies 9 have a shape in which misalignment is likely to occur around the axis D1. In such a case, the coefficient of linear expansion tends to be different between the two piezoelectric bodies 9 in the direction orthogonal to the axis D1. Therefore, for example, by making the second electrode 7B thicker, the effect of relaxing the thermal stress between the two piezoelectric bodies 9 can be easily achieved. Even if the two piezoelectric bodies 9 do not have rotationally symmetric shapes, for example, a deviation between the direction and the shape of the crystal axis occurs due to a manufacturing error, so that the effect according to the present disclosure is achieved.

The present invention is not limited to the above embodiments, and may be implemented in various embodiments.

The shape of the sensor element is not limited to a columnar shape. For example, the shape of the cross section orthogonal to the stacking direction of the sensor elements may not be a rotationally symmetric shape, may be a two-fold symmetric shape, or may be an n-fold symmetric shape such as a regular polygon. n may be 2 or more or 3 or more), or may be annular. Further, the shape of the cross section orthogonal to the stacking direction of the sensor elements does not have to be constant in the stacking direction.

The material of the piezoelectric material does not have to be quartz as illustrated in the embodiment. Further, the cut angle may be set as appropriate. For example, the piezoelectric material may be a so-called AT-cut quartz plate.

One electrode (each of the first electrode to the third electrode) may be composed of a plurality of conductive layers (metal layers). Further, the bonding material 11 of the embodiment may be regarded as a part of the electrode. In such a case, the coefficient of linear expansion of the electrode may be determined as to whether or not the requirements of the present disclosure are satisfied by using the value of the entire layer captured as the electrode.

The bonding material 11 is not an essential requirement, and for example, the electrodes (excluding the bonding material 11) may be directly bonded to the piezoelectric body, and the piezoelectric bodies may be bonded to each other by this. Further, for example, a metal layer may be formed on each of the facing surfaces of the two piezoelectric bodies, and the two metal layers may be joined by seam welding or atomic diffusion bonding, and the two metal layers may form an electrode.

The electrode may be provided by forming a metal layer on the piezoelectric body by vapor deposition or the like. In the embodiment, all the layered members constituting the sensor element 3 are bonded together, but they may be bonded step by step. For example, the first electrode and the second electrode may be bonded to the first piezoelectric body, the third electrode may be bonded to the second piezoelectric body, and then both may be bonded to each other.

In the embodiment, two sets of configurations including the piezoelectric body and the electrodes on both sides thereof are connected in parallel, but these may be connected in series.

1 ... Sensor (piezoelectric sensor), 3 ... Sensor element, 7A ... 1st electrode, 7B ... 2nd electrode, 7C ... 3rd electrode, 9A ... 1st piezoelectric body, 9B ... 2nd piezoelectric body.

Claims (5)

The first electrode, the first piezoelectric body, the second electrode, the second piezoelectric body, and the third electrode are held in a laminated state in which the enumeration order is the order in the stacking direction, and are bonded to each other. Ori,
The first piezoelectric body and the second piezoelectric body are made of single crystals of the same material, and the angle formed by the stacking direction and the crystal axis is the same.
The single crystal has anisotropy with respect to the coefficient of linear expansion in the plane orthogonal to the stacking direction.
The thicknesses of the first electrode, the second electrode, and the third electrode are t1, t2, and t3, respectively.
The coefficient of linear expansion of the second electrode and the third electrode was set to α2 and α3, respectively.
When the largest coefficient of linear expansion and the smallest coefficient of linear expansion in the plane orthogonal to the stacking direction of the single crystal are α _max and α _min , respectively.
A piezoelectric sensor in which α2> α _max >α3> α _min and t2>t3> t1. The piezoelectric sensor according to claim 1, wherein the first piezoelectric body and the second piezoelectric body are made of quartz whose electric shaft is parallel to the stacking direction. When the thicknesses of the first piezoelectric body and the second piezoelectric body are tp1 and tp2,
t2 ≦ tp1 ≦ 16 × t2,
t2 ≦ tp2 ≦ 16 × t2,
The piezoelectric sensor according to claim 1 or 2, wherein 50 μm ≦ t2 ≦ 800 μm and 20 μm ≦ t3 ≦ 40 μm. Any of claims 1 to 3, wherein the first piezoelectric body and the second piezoelectric body have the same shape as each other when viewed in the stacking direction, and have a shape n times symmetrical when n is 3 or more. The piezoelectric sensor according to item 1. The first electrode, the first piezoelectric body, the second electrode, the second piezoelectric body, and the third electrode are stacked in this enumeration order as the order in the stacking direction, and are electrically conductive between the overlapping ones. The laminating process that constitutes the laminated body in which the sex bonding material is arranged, and
A heating step of heating the laminate to melt the bonding material, and
After the heating step, a cooling step of cooling the laminate to solidify the bonding material and
Is equipped with
The first piezoelectric body and the second piezoelectric body are made of single crystals of the same material, and the angle formed by the stacking direction and the crystal axis is the same.
The single crystal has anisotropy with respect to the coefficient of linear expansion in the plane orthogonal to the stacking direction.
The thicknesses of the first electrode, the second electrode, and the third electrode are t1, t2, and t3, respectively.
The coefficient of linear expansion of the second electrode and the third electrode was set to α2 and α3, respectively.
When the largest coefficient of linear expansion and the smallest coefficient of linear expansion in the plane orthogonal to the stacking direction of the single crystal are α _max and α _min , respectively.
A method for manufacturing a piezoelectric sensor in which α2> α _max >α3> α _min and t2>t3> t1.

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