JP3264074B2 - Laminated ferroelectric and bonding method thereof - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention is a single crystal ferroelectrics, e.g. lithium niobate (LiNbO ₃₎ substrate or lithium tantalate (LiTaO ₃₎ substrate and a piezoelectric ceramic, e.g., lead zirconate titanate (PZT) or lanthanum-added The present invention relates to a laminated ferroelectric material in which a material mainly composed of lead zirconate titanate (PLZT) is firmly joined without using an adhesive and integrated, and a joining method thereof.

[0002]

Some of the prior art ferroelectric often show a piezoelectricity, a piezoelectric ceramic or a single crystal ferroelectric by utilizing the properties have been widely used for various filters and oscillators.

However, ferroelectric single crystal ingots obtained by the single crystal pulling method and piezoelectric ceramics after sintering do not exhibit piezoelectricity as they are, and polarization treatment can be performed by applying a DC electric field at a high temperature. Required. By this polarization processing, spontaneous polarization in the ferroelectric substance is oriented in the direction of the applied electric field. The polarization once oriented is preserved even when the electric field is removed, and can function as a piezoelectric body.

A typical single crystal ferroelectric is LiN
There are bO ₃ and LiTaO ₃ , and single crystals with high uniformity are commercially available. Further, piezoelectric ceramics represented by PZT and PLZT are widely used because of their ease of shape processing and low cost.

[0005] These materials have been practically used for a long time because of their piezoelectricity and their electromechanical coupling constant.
Optimum ones are selected according to conditions such as the fractional bandwidth and the center frequency, and many devices have been devised.

However, the polarization of the ferroelectric that has been subjected to the polarization treatment is a single polarization state in which all directions are in the same direction in the single crystal. In the piezoelectric ceramics, the directions of polarization of individual crystal grains are different, but they are in the same direction in the crystal grains, and the macroscopic polarization direction of the entire ceramic is in a fixed direction.

In recent years, it has become possible to form a domain-inverted layer using the pyroelectricity of LiNbO ₃ or LiTaO ₃ , and the range of application is expanding.

FIG. 6 is a schematic view of a conventional bending oscillator utilizing the reversal of polarization. In FIG. 6, reference numeral 11 denotes LiNbO.
₃ is a substrate, 61 is an adhesive, 62 is the excitation electrode.

[0009] As shown, the two LiNbO ₃ substrate 11, the polarization direction P _s which is shown in the substrate are bonded via an adhesive 61 so that the opposite. When a voltage is applied to this substrate by the excitation electrode 62, if the plate expands on one substrate, the other contracts, and the bending vibration is excited .

[0010] However, the conventional bending vibrator, the presence of the adhesive 61 between the substrates, has been a major cause of variations in the reduction and frequency of Q.

Therefore, as described above, by utilizing the phenomenon that a domain-inverted layer is formed by applying pretreatment and heat treatment to one substrate, a domain-inverted ferroelectric material without an adhesive layer by an adhesive is used. Substrates can now be manufactured and are attracting attention.

[0012]

However, LiNb
The formation of the domain-inverted layer in the O ₃ or LiTaO ₃ substrate requires a high temperature near the Curie point, and in order to perform the process at a lower temperature, Ti diffusion or proton exchange is performed. It was necessary to perform heat treatment after changing the composition of the crystals in the surface layer. Further, this polarization inversion layer has
In iNbO ₃ , there is a restriction that it is formed on the + z plane, and in LiTaO ₃ , there is a restriction that it is formed on the −z plane .

[0013] In addition, the polarization of the orientation by the electric field applied 18
It was impossible to intentionally form a domain having an arbitrary direction in the same crystal only by being able to reverse 0 #.

This point is more difficult because the polarization in the ferroelectric is not a single domain structure in the piezoelectric ceramics, and it has been very difficult to produce layers that are mutually inverted.

[0015]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention provides a method for mirror-polishing the surfaces of two or more single-domain ferroelectrics to be joined, and After the surfaces to be cleaned are made hydrophilic, the mirror surfaces are brought into close contact with each other, and then heated, they are firmly and directly joined to each other at the atomic level.

Here, the direct bonding means that the substrates are brought into close contact with each other by van der Waals force acting between OH groups generated by performing hydrophilic treatment between clean substrates, and these substrates are subjected to a heat treatment. By adding, van der Waals bonds between OH groups are replaced with atomic level bonds such as covalent bonds and ionic bonds between constituent atoms of the ferroelectric,
A chemically and physically stable bond is obtained.

[0017]

By the above means, the single crystal ferroelectric and the piezoelectric ceramic can be firmly joined without using an adhesive.

In particular, in a ferroelectric substance exhibiting pyroelectricity, by joining the polarization directions of the polarization axes together, an electrostatic attraction is exerted by the surface charges appearing during the heat treatment, whereby the adhesion is further increased and the polarization directions are adjusted. Strong bonding can be obtained at a lower temperature than when no bonding is performed.

Further, if the ferroelectrics in a single domain are joined with their polarization directions being opposite to each other, a domain-inverted structure can be easily formed. It becomes possible to easily obtain a domain wall structure having good reproducibility.

Further, if the ferroelectric material is cut at a certain angle with respect to the polarization axis, a laminated ferroelectric material having a polarization structure in an arbitrary direction can be obtained by appropriately selecting the cut angle. it can.

In the case of a ferroelectric material having the above structure and exhibiting piezoelectricity, the relationship between the polarization directions of the ferroelectric materials directly joined and the direction of the voltage applied to the joined material are appropriately selected. You can control the vibration mode.

In addition, since the bonding surface is formed by bonding at a lattice level which is physically and chemically very stable, the respective domains are formed by mechanical polishing or chemical etching. It can be made thinner. Therefore, it is possible to accurately control the thickness of each of the laminated structures and to obtain a substrate with little change in characteristics over time.

Hereinafter, embodiments of the present invention will be described.

[0024]

【Example】

(Embodiment 1) FIGS. 1A and 1B are perspective views showing an embodiment of a joined body of the present invention and a joining method thereof. The cut angle of the substrate is 128 ° rotation ycut, and the substrate is cut perpendicular to the axis rotated 128 ° in the z-axis direction from the y-axis of the crystal. These single crystals are divided into single domains, and the polarization is parallel to the z-axis.

In the following, the direction of polarization is indicated by an arrow in the drawing, and the crystal orientation is also indicated. As shown in FIG. 1A, two LiNbO ₃ substrates 11, which have been polished to a mirror surface and cleaned so that no dust or dirt is present on the bonding surface, are aligned in crystal orientation, and the polarization directions are reversed. As shown in FIG.
As shown in FIG. Washing for obtaining a clean surface necessary for bonding can be obtained, for example, by immersing a mixed solution of sulfuric acid and hydrogen peroxide in a heated solution for several minutes and then washing with pure water. By this treatment, the surface of the substrate is cleaned and hydrophilized, and hydroxyl groups (OH groups) are adsorbed on the surface by hydrogen bonding. In this state, the substrates are joined by overlapping. When the two substrates are brought into close contact with each other, care should be taken so that foreign matter does not enter the bonding interface.

LiNbO ₃ substrate 1 adhered at room temperature
Even at this point, the pieces 1 are strongly bonded to each other, and a value of several tens of kg / cm ² can be obtained in terms of tensile strength. However, it is considered that the bonding at this time is due to an atomic force and a hydrogen bond between OH groups existing on the substrate surface.
Therefore, when moisture or a chemical penetrates into the joint interface, it is easily separated. Therefore, heat treatment is performed to obtain a stronger bonding state. The heat treatment is performed at a temperature not exceeding the Curie point of LiNbO ₃ . By this heat treatment, a dehydration reaction occurs, and the bonds between the atoms at the bonding interface become atomic-level bonds such as covalent bonds and ionic bonds, so that a strong bonding is achieved between the two substrates. If the heat treatment temperature exceeds the Curie point, the orientation of the polarization in the dielectric material will vary, so the temperature must always be lower than this temperature.

According to the above-described procedure, a joined body having a reversed polarization direction can be obtained without using an adhesive.

Further, since the polarization direction of the obtained joined body can be freely selected by selecting the cut angle of the crystal, it is possible to design an optimum combination according to the desired vibration mode. Furthermore, if the thicknesses of the substrates to be joined are the same, the boundary of polarization is at the center of the substrate, so that when used as a vibrator, in-plane vibration is hardly excited, and spurious vibration is suppressed. Furthermore, when a DC voltage is applied to the vibrator structure as a piezoelectric actuator, good linearity and large amplitude can be obtained because there is no adhesive which is one of the causes of hysteresis.

When the substrate is a LiTaO ₃ substrate,
The same can be said. The following is an example.

(Embodiment 2) FIG. 2 is a side view showing a joined body according to a second embodiment of the present invention. In FIG. 2, reference numeral 21 denotes a LiTaO ₃ substrate. The cut angle of the substrate is zc
ut.

This embodiment is almost the same as the first embodiment,
The difference is that LiTaO ₃ has a lower Curie point than LiNbO ₃ .

Therefore, when the zcutLiTaO ₃ substrates are joined together, the heat treatment temperature must not exceed the Curie point of LiTaO _{3 for} the same reason as described above.

When the substrate of this embodiment is applied to a thickness vibrator, resonance at one wavelength becomes possible with respect to the conventional half-wavelength resonance, so that it works equivalently to a vibrator having a half plate thickness. For this reason, the upper limit of the frequency, which has been limited by the processing accuracy when thinning the sheet, can be doubled, so that a higher-frequency thickness oscillator can be obtained by the conventional technique.

Next, an embodiment in which the directions of polarization of the substrates to be joined are the same will be described. Embodiment 3 FIG. 3 is a side view showing a joined body according to a third embodiment of the present invention. What is characteristic in this embodiment is that electrostatic attraction acts between the substrates during the heat treatment. This is LiNbO ₃
Is caused by pyroelectricity. Pyroelectric materials generate heat on their surface by applying heat and changing the temperature. The polarity of this charge is determined by the direction of polarization. In the case of the present embodiment, since the polarization directions are matched between the two substrates, charges of opposite polarities are generated between the substrates and the surfaces of the respective substrates are brought closer together electrostatically. Even if the temperature is raised quickly during heating, there is no particular problem since the thermal expansion coefficients of the two substrates are completely equal, and a greater electrostatic attraction acts, which is convenient. However, when the polarities of both substrates are not matched as in the second embodiment, charges of the same polarity are generated on the surfaces of the respective substrates. If the electric repulsion exceeds the bonding strength between the substrates, the two substrates are peeled off from each other and cannot be bonded firmly. It is necessary to pay attention to this point when joining a pyroelectric ferroelectric substrate cut perpendicularly to the polarization direction. In the case of joining in the reverse direction of the polarization, it is advisable to gently heat the device or to apply a pressure between the substrates. The polarization direction is cut at an angle.
When the polarization direction is reversed even if the substrate is
Higher joint strength when treated at the same temperature
I know that .

Next, an example of joining piezoelectric ceramics will be described. (Embodiment 4) FIGS. 4 (a) to 4 (e) are side views showing a joined body according to a fourth embodiment of the present invention. In the figure, reference numeral 41 denotes a PZT or PLZT piezoelectric ceramic mainly composed of lead zirconate titanate or lanthanum-added lead zirconate titanate.

PZT and PLZT piezoelectric ceramics are the most widely used piezoelectric materials at present because of their high electromechanical coupling coefficient and easy molding. In particular, PLZ
T-based piezoelectric ceramics are densely sintered and transparent ones can be obtained. Therefore, the surface condition of the mirror surface required for bonding by the present method can be relatively easily obtained.

As shown in the figure, several combinations of piezoelectric ceramics having different polarization directions are joined. The method is as described above.

Since the piezoelectric ceramics have almost no anisotropy in the coefficient of thermal expansion, it is possible to easily realize the joining by selecting the polarization direction almost freely. Therefore, it is possible to obtain an unprecedented polarization structure, and it is possible to freely design according to the vibration mode when used in an actual device. This is merely an example, and the polarization direction can be changed according to the purpose, and almost infinite combinations are possible. In this embodiment, the case of piezoelectric ceramics is shown, but a similar structure can be achieved by using a single crystal ferroelectric. Further, there is no problem even if the materials to be joined are of different types, and the degree of freedom of design increases even in the combination of materials.

However, in the case of joining different materials, it is necessary to pay attention to the coefficient of thermal expansion of both materials. The following is an example.

As shown in FIG. 5, zcutLiNbO ₃
The substrate 11 and the zcutLiTaO ₃ substrate 21 are joined. One zcutLiNbO ₃ substrate has a planar direction (x
(Axial direction) is 154 × 10 ⁻⁷ / ° C., and the other zcutLiTaO ₃ substrate has a value of 161.
× 10 ^-7 / ° C. Unlike Examples 1 and 2, there is a difference in the coefficient of thermal expansion between the substrates to be joined. However, since the crystal morphologies are almost the same, they have good compatibility, and even if heated up to the Curie point of LiTaO ₃ , the difference in the coefficient of thermal expansion is small. There are no cracks due to this, and joining is possible even with dissimilar materials .

Although all of the embodiments have been described with respect to only an example in which two substrates are joined, it is naturally possible to join three or more substrates, and the polarization direction thereof can be freely selected. Further, different materials can be used. The gist of the present invention resides in a laminated ferroelectric substance whose polarization direction can be arbitrarily selected at the time of bonding substrates and a bonding method thereof, and includes all bonded bodies including the above conditions.

Here, LiNbO ₃ , LiTaO ₃ , P
Although examples of ZT and PLZT have been described, similar processing is possible in principle with other ferroelectrics.

[0043]

According to the present invention, as described in the embodiments above, the single crystal ferroelectric and the piezoelectric ceramic can be firmly joined by the above means without using an adhesive. At this time, since the polarization directions of the two layers can be set arbitrarily, a laminated ferroelectric having a structure that has not been conventionally obtained can be easily obtained.

Further, since all the bonding surfaces are chemically and physically stable, the bonding substrate of the present invention can be processed. Further, the piezoelectric device made of the laminated ferroelectric material of the present invention can be used as an adhesive layer. Because of the absence of the above, the characteristics are good and the change with time is small. In particular, in ferroelectrics that exhibit pyroelectricity, by joining together with the polarization direction of the polarization axis, electrostatic attraction due to the surface charge works during heat treatment, further increasing the adhesion and comparing with the case where the polarization direction is not matched. Thus, a strong bond can be obtained at a lower temperature.

[Brief description of the drawings]

FIG. 1 is a perspective view showing a joined body and a joining method thereof according to a first embodiment of the present invention.

FIG. 2 is a side view showing a joined body according to a second embodiment of the present invention.

FIG. 3 is a side view showing a joined body according to a third embodiment of the present invention.

FIG. 4 is a side view showing a joined body of a fourth embodiment.

FIG. 5 is a side view showing a joined body according to a fifth embodiment of the present invention.

FIG. 6 is a side view schematically showing a conventional bending oscillator.

[Explanation of symbols]

11 LiNbO ₃ substrate 21 LiTaO ₃ substrate 41 Piezoelectric ceramics 61 Adhesive 62 Excitation electrode

────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kazuo Eda 1006 Kazuma Kadoma, Kadoma-shi, Osaka Matsushita Electric Industrial Co., Ltd. (56) References JP-A-7-45801 (JP, A) JP-A-5 -178695 (JP, A) JP-A-5-178697 (JP, A) JP-A-60-51700 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C30B 1/00- 35/00 H03H 9/00-9/76 CA (STN) JICST file (JOIS) WPI (DIALOG)

Claims (14)

(57) [Claims] 1. A single solid-state ferroelectric material comprising two or more single-domain single-crystal ferroelectrics cut out at a certain angle with respect to the polarization direction of a crystal, and having any given polarization direction in a single solid. A laminated ferroelectric material characterized by the presence of a depressed domain. 2. A laminated ferroelectric, wherein two or more piezoelectric ceramics subjected to a polarization treatment are directly bonded, and domains having arbitrary polarization directions are present in a single solid. 3. The method according to claim 2, wherein two or more single crystal ferroelectrics are divided into single domains.
The surface to be joined is mirror-polished, the surface to be joined is cleaned and hydrophilized, and the mirror surfaces are brought into close contact with each other, and then heated below the Curie point of the single crystal ferroelectric to join the joined surfaces. A method of joining laminated ferroelectrics, comprising a step of firmly directly joining each other by substituting for joining at the atomic level. 4. A method for subjecting two or more piezoelectric ceramics to polarization treatment, polishing the surfaces to be joined to a mirror surface, cleaning and hydrophilizing the surfaces to be joined, and bringing the mirror surfaces into close contact with each other. A method of heating at a temperature below the Curie point to replace the bonding at the bonding surface with bonding at the atomic level so as to make a direct and strong bonding to each other. 5. The method according to claim 1, wherein the single crystal ferroelectric is lithium niobate (Li).
2. The laminated ferroelectric according to claim 1, wherein the ferroelectric is selected from the group consisting of NbO ₃ ) and lithium tantalate (LiTaO ₃ ). 6. A piezoelectric ceramic comprising lead zirconate titanate (PZT) and lanthanum-added lead zirconate titanate (PL).
3. The laminated ferroelectric according to claim 2, wherein the ferroelectric is any one selected from the group consisting mainly of ZT). 7. A ferroelectric single crystal comprising lithium niobate (Li)
4. The method according to claim 3, wherein the method is selected from NbO ₃ ) and lithium tantalate (LiTaO ₃ ). 8. A piezoelectric ceramic comprising lead zirconate titanate (PZT) and lanthanum-added lead zirconate titanate (PL).
5. The method according to claim 4, wherein the method is any one selected from the group consisting mainly of ZT). 9. A mirror-polished surface of two or more single-domain single-crystal ferroelectrics to be joined, and the surface to be joined is cleaned and hydrophilized to change the crystal orientation of the ferroelectric. In addition, after the mirror surfaces are brought into close contact with each other so that the polarization directions are opposite to each other, heating is performed below the Curie point of the single crystal ferroelectric, and the bonding at the bonding surface is replaced with a bonding at an atomic level so that the bonding is firmly performed. A method of joining a domain-inverted single crystal ferroelectric, comprising a step of directly joining. 10. The surface to be joined of two or more single-domain single-crystal ferroelectrics is mirror-polished, the surface to be joined is cleaned and hydrophilized, and the polarization direction of the ferroelectric and the crystal After the mirror surfaces are brought into close contact with each other in all directions, heating is performed below the Curie point of the single crystal ferroelectric, and the bonding at the bonding surface is replaced with bonding at the atomic level to directly bond each other firmly. Method of bonding single crystal ferroelectrics. 11. A surface to be bonded of two or more piezoelectric ceramics subjected to a polarization treatment is mirror-polished to clean and hydrophilize the surface to be bonded so that the polarization direction of the piezoelectric ceramics is reversed. After the mirror surfaces are brought into close contact with each other, the piezoelectric ceramics are heated below the Curie point of the piezoelectric ceramics, and the bonding of the bonding surfaces is replaced with an atomic-level bonding to firmly directly bond each other. Joining method. 12. A mirror-polished surface of two or more piezoelectric ceramics which have been subjected to a polarization treatment is mirror-polished, and the surface to be bonded is cleaned and hydrophilized. A method for bonding piezoelectric ceramics, comprising the steps of: bonding the piezoelectric ceramics to each other, heating the same to a temperature below the Curie point of the piezoelectric ceramics, replacing the bonding at the bonding surface with atomic-level bonding, and firmly directly bonding each other. 13. The method for bonding a single crystal ferroelectric according to claim 9, wherein the single crystal ferroelectric is selected from lithium niobate and lithium tantalate. 14. A piezoelectric ceramic comprising lead zirconate titanate (PZT) and lanthanum-added lead zirconate titanate (PZT).
13. The method for bonding piezoelectric ceramics according to claim 11, wherein the method is any one selected from the group consisting mainly of (LZT).