JPH08298347A - Piezoelectric element, its manufacture, and growth method of lithium tetraborate crystal containing twin - Google Patents

Abstract

PURPOSE: To provide a piezoelectric element which can constitute a piezoelectric actuator like a bimorph, by using not a stacked board but a single piezoelectric plate. CONSTITUTION: A single piezoelectric plate 10 is constituted of lithium tetraborate crystal composed of two thin plate type crystal individuals 10b, 10b forming a twin which come into contact with each other on a twin surface 10a. By applying a voltage across the respective crystal individuals 10b, 10b, mechanical displacement is generated in the single piezoelectric plate 10. On the contrary, by applying mechanical displacement to the single piezoelectric plate 10, a voltage is generated between the crystal individuals 10b, 10b.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention includes a piezoelectric element such as a piezoelectric actuator such as a bimorph using a lithium tetraborate (Li ₂ B ₄ O ₇ ) crystal in which a twin crystal is formed, a method for manufacturing the same, and a twin crystal. The present invention relates to a method for growing a lithium tetraborate crystal and a method for producing a seed crystal used for growing a lithium tetraborate crystal containing the twin crystal.

[0002]

2. Description of the Related Art Conventionally, a piezoelectric element called a bimorph has been used, which is formed by laminating two piezoelectric plates, and the applied voltage expands one of the piezoelectric plates and simultaneously compresses the other. Thus, the piezoelectric plate bends in proportion to the applied voltage. It is used as a piezoelectric actuator in the control field. On the contrary, if pressure is applied so that the piezoelectric plate bends, a voltage twice as high as that of one piezoelectric plate is generated. It is used in player pickups and microphones.

Conventionally, in such a piezoelectric element, a ferroelectric ceramic or a single crystal plate thereof is polarized and laminated with an adhesive so as to be inverted with each other. This representative is PZ
T (lead zirconate titanate).

[0004]

However, such a laminated piezoelectric element has a problem in that when it is used for a long period of time, the displacement width varies due to the change with time of the adhesive agent and the reliability is poor. Also, if you use an organic adhesive,
There is also a problem with heat resistance.

The present invention has been made in view of the above circumstances, and includes a piezoelectric element capable of forming a piezoelectric actuator such as a bimorph with a single piezoelectric plate without bonding, a method for manufacturing the same, and a twin crystal. An object of the present invention is to provide a method for growing a lithium tetraborate crystal and a method for producing a seed crystal used in the method for growing a lithium tetraborate crystal containing the twin crystal.

[0006]

In order to achieve the above object, the piezoelectric element according to the present invention has a structure in which each crystal solid of a lithium tetraborate crystal is composed of two crystal solids forming a twin crystal which are in contact with each other at a joint surface. It is characterized by forming a physical contact point.

The piezoelectric element according to the present invention is a single piezoelectric plate composed of a lithium tetraborate crystal body composed of two thin plate-like crystal solids that form twin crystals that are in contact with each other at the bonding surface. It is preferable that the single piezoelectric plate is configured so as to be mechanically displaced by applying a voltage.

The piezoelectric element according to the present invention is a single piezoelectric plate composed of a lithium tetraborate crystal body composed of two thin plate-like crystal solids that form twin crystals that are in contact with each other at the joint surface. A voltage may be generated between the crystal solids by applying mechanical displacement.

The lithium tetraborate crystal body is preferably a crystal body including a bonding surface cut out from a lithium tetraborate crystal in which twin crystals are formed. The method for manufacturing a piezoelectric element according to the present invention, a lithium tetraborate crystal in which twins are generated is cut out in parallel with the growing direction to produce a lithium tetraborate crystal body containing a bonding surface, and the twin crystal of this crystal body is formed. It is characterized in that a conductive film is formed on a part or the entire surface of both crystal solids.

A method for growing a lithium tetraborate crystal containing twins according to the present invention is a method in which a lithium tetraborate crystal containing twins is used as a seed crystal, the seed crystal is immersed in a lithium tetraborate melt, and the Czochralski method is used. Characterized by raising. In the growing method of the present invention, a lithium tetraborate crystal containing twins is used as a seed crystal, and the seed crystal is immersed in a lithium tetraborate melt to obtain EFG (Edge-defined Fi).
It may be configured to pull up in a plate shape by the lm Growth) method.

In the method for producing a lithium tetraborate seed crystal containing twin crystals according to the present invention, a lithium tetraborate single crystal is immersed as a seed crystal in a lithium tetraborate melt, and a temperature gradient directly above the melt is 150 ° C. / It is characterized in that the crystal is made to be cm or more and the lithium tetraborate crystal is pulled up by the Czochralski method and cut out from the lithium tetraborate crystal. In addition,
The upper limit of the temperature gradient is determined so that the crystal does not crack, and is not particularly limited, but is preferably 300 ° C / cm or less.

The piezoelectric element of the present invention focuses on a lithium tetraborate crystal containing twin crystals which have been unappreciated until now, which have been regarded as failures during the manufacturing process during the growth of a single crystal. The present inventors have found that a lithium tetraborate crystal body obtained by cutting out this lithium tetraborate crystal in parallel with the growth direction including the joint surface has a stack of crystal solids reversed at the joint surface, and a twin crystal contacting at the joint surface. It has been found that when a voltage is applied between the crystal solids forming one of the crystal solids, one crystal solid expands, and the other crystal solid compresses to cause mechanical displacement, which can serve as a piezoelectric actuator.

Such a piezoelectric actuator element is
Unlike the conventional bonded piezoelectric element, since it is obtained by cutting out from a single crystal, it does not have a bonded surface, and thus a single piezoelectric plate can constitute a bimorph or the like. This bimorph or the like is extremely reliable because it does not peel off even if it is subjected to vibration for a long period of time.

In the piezoelectric element of the present invention, a lithium tetraborate crystal in which twins are formed is cut out in parallel with the growing direction to prepare a lithium tetraborate crystal body including a bonding surface, and the twin crystal of this crystal body is formed. It can be easily manufactured by forming a conductive film on a part or the entire surface of both crystal solids.

The twin tetraborate crystal in which the twin crystal is generated can be grown by immersing the lithium tetraborate crystal containing the twin crystal in a lithium tetraborate melt as a seed crystal and pulling it by the Czochralski method. Further, the lithium tetraborate crystal in which the twin crystal is generated can be grown by immersing the lithium tetraborate crystal containing the twin crystal as a seed crystal in the lithium tetraborate melt and pulling it up by the EFG method. According to such a method for growing a lithium tetraborate crystal containing twins of the present invention, the position of the bonding surface developed in the obtained crystal can be arbitrarily determined, so that the cutting step can be shortened and cut. It is possible to improve the yield of the crystallites.

A seed crystal of a lithium tetraborate crystal containing twins used at this time is a lithium tetraborate single crystal used as a seed crystal, which is immersed in a lithium tetraborate melt and the temperature gradient immediately above the melt is set to 150 ° C./cm or more. A lithium tetraborate crystal is pulled up by the Czochralski method and cut out from this crystal, whereby it can be easily produced.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic view showing an example in which the piezoelectric element of the present invention is applied to a bimorph, FIG. 2 is a plan view of a wafer in which a bonded surface of a crystal can be visually confirmed by etching, and FIG. 4 is a schematic view showing that the wafer is twinned, and FIG. 4 is a schematic view showing a cut portion of the wafer where twins are generated. FIG. 5 is a perspective view showing a sample cut at the cut portion of FIG. 4. FIG. 5 is a schematic diagram showing a conoscope of a crystal viewed from the direction of the arrow by a polarization microscope, FIG. 7 is a conceptual diagram showing that etching rates of crystal solids contacting each other at a bonding surface are different, and FIG. 10 is a lithium tetraborate crystal containing twin crystals. FIG. 10 (A) is a perspective view showing a seed crystal, and FIG.
Is a perspective view showing a pulled lithium tetraborate crystal,
11A and 11B are views for explaining a method for growing a lithium tetraborate crystal which is another embodiment. FIG. 11A is a perspective view showing a seed crystal, and FIG. 11B is a pulling initial step. The cross-sectional view shown in FIG. 11C is a perspective view showing the pulled lithium tetraborate crystal.

The piezoelectric element of the present invention has a structure in which an electrical contact is formed on each crystal solid of a lithium tetraborate crystal composed of two crystal solids forming a twin crystal which are in contact with each other at the joint surface as described above. Here, the bonded surface is a surface where two crystals forming a twin are in contact with each other. In a lithium tetraborate crystal, for example, when a seed crystal having a (110) orientation is used to obtain a single crystal having the same orientation as the seed crystal, a twin bonding surface has a (112) orientation, and a stack in the C-axis direction is formed. Is reversed to the C ⁺ surface and the C ⁻ surface at the junction surface.

As shown in FIG. 1, a piezoelectric element according to an embodiment of the present invention has an electrode film (electrical contact) of silver or the like on both surfaces of a lithium tetraborate crystal plate (crystal body) 10 including a bonding surface 10a. It has a bimorph 1 in which 11, 11 are laminated. The bimorph 1 is of a cantilever type with one end thereof supported by a support 20. Thin plate-like lithium tetraborate crystal solid 10 forming twins on both sides of the joint surface 10a
b and 10b are in contact with each other with the joint surface 10a as a mirror surface, and these constitute a single piezoelectric plate (crystal body) 10. In addition, the formation of the electrical contact 11 to each crystal solid 10b, 10b,
As described above, the conductive film of metal or the like may be formed on the entire upper surface and lower surface of the crystal plate 10, or may be formed on a part thereof.
You may make it twist.

In such a bimorph 1, the joint surface 10
The polarities of the crystal solids 10b and 10b contacting each other at a are different from each other. As shown in FIG. 1, when a voltage is applied between the crystal solids 10b and 10b, one crystal solid expands and the other crystal solid compresses due to the applied voltage. Therefore, when a voltage is applied, a mechanical displacement occurs as shown by the alternate long and short dash line in FIG. 1, and the displacement actuator can be electrically controlled.

For example, when a voltage of 100 V is applied, a mechanical displacement of about 100 μm can be obtained per cm of length, and electrical energy can be converted into mechanical energy. On the contrary, it is also possible to convert mechanical energy into electric energy. For example, when mechanical displacement is applied to the single piezoelectric plate 10 to compress and contract each crystal solid, a voltage is generated.

The piezoelectric element of the present invention can be constituted by a single piezoelectric plate made of a lithium tetraborate crystal, and unlike the conventional two piezoelectric plates bonded together, it does not require bonding with an adhesive or the like. There is no peeling or other reliability-reducing element at the joint, and it operates stably for a long time. Moreover, it can be used up to a relatively high temperature, specifically up to a melting point of 917 ° C.

The piezoelectric element of the present invention is, for example, a bimorph,
It can be used as a vibrator, a piezoelectric actuator, etc., and is useful as an electro-mechanical energy mutual conversion element. For example, as a piezoelectric actuator, it can be applied to various applications such as a shutter of a camera, a valve that does not use an electromagnetic valve, a vibrator used for generating and receiving ultrasonic waves, a vibrator for twisting, and the like.

The lithium tetraborate crystal body 10 constituting the piezoelectric element of the present invention shown in FIG. 1 can be manufactured, for example, as follows. In order to obtain a lithium tetraborate crystal rod 30 containing twins as shown in FIG. 3, first, a single crystal of lithium tetraborate is used as a seed crystal, for example, by the Czochralski method single crystal growing method. Bonding surface (11
When a seed crystal (single crystal) having an orientation perpendicular to the 2) direction, for example, a (110) orientation, is immersed in a lithium tetraborate melt and then the seed crystal is pulled up while rotating the seed crystal, 1 mm
When the crystal is pulled up to a certain extent, air is blown to the boundary surface between the seed crystal and the melt to give a temperature gradient to the atmosphere directly above the melt, thereby rapidly growing the crystal. As a result, a lithium tetraborate crystal 30 containing twins can be easily obtained. In the crystal growth method by the Czochralski method, the pulling rate is 0.5 to
2.0 mm / hour, the pulling diameter is 2 to 3 inches, and the temperature gradient of the atmosphere directly above the melt is 150 ° C./cm or more.

From the next, a seed crystal 32 containing twins as shown in FIG. 10A is prepared from the crystal rod 30 containing the twins, and the Czochralski method or EFG (Edge-d) is used.
The pulled-up twin crystal bar 30 containing twin crystals in the same orientation as the seed crystal 32 can be easily obtained by a growing method such as the refined film growth method.

FIG. 10 (B) shows a lithium tetraborate crystal rod 30 pulled up by the Czochralski method using the seed crystal 32 shown in FIG. 10 (A). If changed, the generation position of the joint surface 10a in the lithium tetraborate crystal rod 30 can be adjusted accordingly. Therefore, if the joint surface 10a in the seed crystal 32 is determined, the joint surface 10a appearing in the pulled up crystal rod 30 always appears at the same position, and it is not necessary to search for the joint surface 10a each time, and the crystal body 10 The cutting process can be shortened.

Further, since the joint surface 10a of the crystal rod 30 appears corresponding to the position of the joint surface 10a of the seed crystal 32, the center of the crystal rod 30 can be set, for example, by setting the joint surface 10a of the seed crystal 32 as the center. When the bonding surface 10a is developed, the crystal body 10 can be cut out with the highest yield.

The lithium tetraborate crystal 30 can be grown not only by the Czochralski method described above but also by the EFG method, for example. This method uses a seed crystal 32 containing twins as shown in FIG. 11 (A), and this seed crystal 32 is used as a lithium tetraborate melt 3 as shown in FIG. 11 (B).
Soak in 4. In this case, a die 36 is provided on the surface of the crucible, and the die 36 is pulled up directly at a pulling rate of 0.5 to 2 mm / hour. As a result, a plate-shaped lithium tetraborate crystal plate 30 as shown in FIG. 11C can be grown. In particular, when the lithium tetraborate crystal 30 is grown by the EFG method, the yield when cutting out the crystal body 10 is excellent as compared with the Czochralski method.

The joint surface can be confirmed by, for example, acetic acid in water 1: 1.
When the aqueous solution diluted with 0 is applied to the crystal surface with a brush or the like, the bonding surface (112) is etched and can be seen with the naked eye. When a wafer prepared from crystals containing twins is polished, and the above aqueous solution is applied and etched, for example, as shown in FIG.
The boundary line (joint surface) 10a is etched as shown in FIG. The joining surface appearing on the surface occurs parallel to the crystal pulling direction, as shown in FIG. The crystal body 10 shown in FIG. 1 can be obtained by cutting out a (001) face plate as shown by a broken line in FIG. 2 from the lithium tetraborate crystal in which such twinning occurs.

Here, in order to clarify the orientation relationship of the twins, a twin crystal was grown from a seed crystal having a (110) plane orientation as shown in FIG. A (001) face plate was cut out to prepare a thin plate S in which crystal solids A and crystal solids B are alternately arranged as shown in FIG. The thin plate S was polished and observed with a polarization microscope. Since lithium tetraborate is a uniaxial crystal, it has a C-axis, that is, (001)
When a conoscopic image is observed from the direction perpendicular to the plane using a polarization microscope, a cross is visible as shown in the schematic view of FIG. Since the same cross-shaped conoscopic image was observed in both crystal solid A and crystal solid B, it was found that there was no change in the C-axis direction.

Further, the surface of this sample was etched. Since lithium tetraborate has a C axis as a polar axis, there is a distinction between + and − on the C plane, and the etching results are different. As shown in the schematic view of FIG. 7, as a result of etching, when the surface of A is the C ⁺ surface, the surface of B faces the C ⁻ surface, and when the surface of A is the C ⁻ surface, it faces the bonding surface. It was found that the surface of B was the C ⁺ surface. The backside etching result is the opposite. This means that the stacking in the C-axis direction is reversed at the joint surface.

[0032]

[Example] A bimorph was specifically manufactured. A pulling crystal rod containing twins was cut out to prepare a thin plate having a structure in which a joint surface was sandwiched, and the surface was polished to prepare a thin plate having a size shown in FIG. Next, the thickness of about 50μ on the entire surface of both sides of the thin plate.
A silver electrode was coated with m and baked to prepare a bimorph of the present invention as shown in FIG.

For comparison, two thin PZT plates were bonded together with a modified acrylate adhesive to prepare a PZT bonded bimorph having the same dimensions as shown in FIG. In the bimorph of the present invention, the maximum displacement δ when a voltage of 200 V was applied was about 200 μm. On the other hand, in the PZT laminated bimorph, the maximum displacement δ when a voltage of 200 V was applied was about 500 μm. The displacement was measured with a laser displacement meter.

Next, an AC voltage (1 Hz) of 100 V was applied to each of the bimorph of the present invention and the PZT laminated bimorph.
Was applied and vibrated a million times, and the rate of change of the maximum displacement was measured. The result is shown in FIG. In the PZT laminated bimorph, the reduction rate of the maximum displacement was about 5%, but in the bimorph of the present invention, almost no change was observed, and it was confirmed that the reliability was high.

[0035]

Since the piezoelectric element of the present invention can be constituted by a single piezoelectric plate, it can withstand repeated use over a long period of time and has extremely high reliability. Further, according to the method for manufacturing a piezoelectric element of the present invention, the above element can be obtained simply and reliably.

According to the method of growing a lithium tetraborate crystal body containing twins according to the present invention, the position of the joint surface developed in the obtained crystal body can be arbitrarily determined, so that the cutting step can be shortened and The yield of the cut crystal can be improved.

[Brief description of drawings]

FIG. 1 is a schematic view showing an example in which the piezoelectric element of the present invention is applied to a bimorph.

FIG. 2 is a plan view of a wafer in which a crystal bonding surface is made visible by etching.

FIG. 3 is a schematic view showing that a joint surface occurs parallel to a crystal pulling direction.

FIG. 4 is a schematic view showing a cutout portion of a wafer in which twins are generated.

5 is a perspective view showing a sample cut at a cut portion of FIG. 4. FIG.

FIG. 6 is a schematic view showing a conoscope by a polarization microscope of the crystal seen from the arrow direction in FIG.

FIG. 7 is a conceptual diagram showing that the crystal solids contacting each other at the bonding surface have different etching rates.

FIG. 8 is a schematic diagram showing dimensions of piezoelectric plates used in Examples and Comparative Examples.

FIG. 9 is a graph showing changes in displacement of PZT and the bimorph of the present invention when cyclic vibration is applied.

FIG. 10 is a diagram for explaining a method of growing a lithium tetraborate crystal containing twin crystals, which is an embodiment of the present invention, in which (A) is a perspective view showing a seed crystal and (B) is a perspective view. FIG. 3 is a perspective view showing a pulled lithium tetraborate crystal.

11A and 11B are views for explaining a method for growing a lithium tetraborate crystal containing twin crystals according to another embodiment, wherein FIG. 11A is a perspective view showing a seed crystal, and FIG. FIG. 3C is a cross-sectional view showing the initial pulling step, and FIG. 6C is a perspective view showing the pulled lithium tetraborate crystal.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Piezoelectric element (bimorph) 10 Lithium tetraborate crystal body 10a Bonding surface 10b, 10b Twin crystal solid body 11 Electrode 20 Support 30 Lithium tetraborate crystal 32 Seed crystal

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Office reference number FI technical display location H01L 41/22 H01L 41/22 Z

Claims (8)

[Claims] 1. A piezoelectric element, characterized in that an electrical contact is formed on each crystal solid of a lithium tetraborate crystal composed of two crystal solids forming a twin crystal that are in contact with each other at a bonding surface. 2. A single piezoelectric plate composed of a lithium tetraborate crystal body composed of two thin plate-like crystal solids forming a twin crystal contacting at a joint surface, and applying a voltage between the respective crystal solids. A piezoelectric element characterized in that mechanical displacement occurs in the single piezoelectric plate. 3. A single piezoelectric plate composed of a lithium tetraborate crystal body composed of two thin plate-like crystal solids forming a twin crystal which are in contact with each other at a joint surface, wherein mechanical displacement is applied to the single piezoelectric plate A piezoelectric element characterized in that a voltage is generated between the crystal solids due to. 4. The piezoelectric element according to claim 1, wherein the lithium tetraborate crystal is obtained by cutting out a crystal including a bonding surface from a lithium tetraborate crystal in which twinning has occurred. 5. A lithium tetraborate crystal in which twins are formed is cut out in parallel to the growing direction to produce a lithium tetraborate crystal body including a joint surface, and both surfaces of both crystal solids forming a twin crystal of this crystal body are produced. A method of manufacturing a piezoelectric element, which comprises forming a conductive film on a part or the entire surface of the piezoelectric element. 6. A lithium tetraborate crystal containing twins, characterized in that a lithium tetraborate crystal containing twins is used as a seed crystal, and the seed crystal is immersed in a lithium tetraborate melt and pulled up by the Czochralski method. Training method. 7. A tetraborate containing twin crystals, characterized in that a lithium tetraborate crystal containing twin crystals is used as a seed crystal, and the seed crystal is immersed in a lithium tetraborate melt and pulled up into a plate shape from a plate-like gap. Method for growing lithium crystal. 8. A lithium tetraborate single crystal as a seed crystal is immersed in a lithium tetraborate melt, and a temperature gradient immediately above the melt is set to 1
A method for producing a lithium tetraborate seed crystal containing twins, which comprises pulling up a lithium tetraborate crystal by a Czochralski method at 50 ° C./cm or more and cutting out the lithium tetraborate crystal.