JP2958004B2 - Device using domain-inverted LiNbO 3 substrate - Google Patents

Description

The present invention relates to a piezoelectric device, in particular, LiNbO ₃ having a large electromechanical coupling coefficient and having a domain-inverted region inside a piezoelectric single crystal.
Related to the application of substrates to various devices. (Prior art) It is impossible for a piezoelectric single crystal such as quartz to form a domain-inverted region inside the crystal, and a piezoelectric ceramic or the like is also formed in a substrate as two uniform layers in which the directions of polarization are mutually inverted. Is very difficult to do. For this reason, a device that uses a piezoelectric body in which the poling is aligned in one direction, if excited, the thickness vibrator can excite only odd-order overtone vibrations, and the bending vibrator can bond or split a piezoelectric body with reversed poling. It is impossible to realize with electrodes, and torsional vibration is extremely difficult with single crystal such as quartz. Also, when piezoelectric ceramics is used, it is necessary to bond substrates. , The occurrence of hysteresis in operation as an actuator, and the difficulty in use at high temperatures. (Objects of the Invention) The present invention addresses the defects of various devices which are substantially impossible to realize with the conventional piezoelectric materials as described above, or are designed to perform a number of difficult operations, resulting in unsatisfactory performance. Various devices such as vibrators or actuators that can generate bending, contour, or torsional displacements that do not require sticking or split electrodes and that can excite even-order overtone vibrations. The purpose is to provide. (Summary of the Invention) In order to achieve the above object, in the present invention, LiNbO3
The depth is approximately nλ / 2 (where n = 1,
2, 3,... Where mλ / 2, m =
, M> n, .lambda. Is a desired wavelength) to form a domain-inverted layer or a spontaneously-polarized layer and attach an electrode to the surface of the substrate, thereby providing strong excitation of m-th order overtone thickness vibration. It was made possible. Further, by forming a domain-inverted layer having a depth of about 基板 of the thickness of the substrate on the Z ′ plane of the LiNbO3 single crystal substrate and attaching electrodes to the surface of the substrate, bending displacement or longitudinal displacement in the polarization direction can be prevented. It is intended to occur. In addition, a domain-inverted layer having a predetermined depth is formed on the Y 'plane of a rotating Y plate of a LiNbO3 single crystal substrate, and cut out from this substrate in a direction substantially 45 degrees from the Z' axis in the Y 'plane. The torsion displacement is caused by attaching the electrodes to the torsion. (Examples) Hereinafter, the present invention will be described in detail based on examples shown in the drawings. Prior to the description of the embodiments, a method of manufacturing a LiNbO ₃ piezoelectric substrate having a domain-inverted region used by various devices according to the present invention will be briefly described to facilitate understanding of the present invention. The present inventor has proposed Ti which has been a problem in the field of optical devices.
While studying the phenomenon that the thermal diffusion LiNbO ₃ optical waveguide causes polarization reversal depending on the manufacturing conditions, the LiNbO ₃ substrate +
When Ti is vapor deposited on the C-plane for about 1000 し and heat-treated for several hours at a temperature slightly lower than the Curie temperature of the substrate in air or Ar gas containing water vapor, the spontaneous polarization inversion of the depth of about 12 to 15μm on the + C-plane It was discovered that a domain-inverted layer was formed. Furthermore, even if the TiN diffusion is not performed, the LiNbO ₃ substrate is simply heat-treated for several hours in an Ar gas containing air or water vapor at a temperature slightly lower than its Curie point (about 1,110 ° C.), and depending on the processing time, the thickness of each substrate becomes 1 / It has been found that up to 2 domain-inverted layers are formed. Further, it was confirmed that this phenomenon also occurred in a rotating Y plate in which the + C axis was inclined from the substrates. The device according to the present invention is LiNb manufactured as described above.
It uses an O ₃ substrate, and its application examples are described below for each device type. (1) Thickness vibrator FIG. 1 is a view for explaining the operation of the thickness vibrator according to the present invention.
In this case, the vibration in the mode as shown in the figure, that is, twice the overtone vibration, cannot be generated because the electrodes 2 and 2 have the same sign when the positive and negative signs (arrows) of the generated charges are taken into consideration. On the other hand, in the LiNbO ₃ substrate 3 in which the poling Ps is inverted with respect to the center of the thickness of the substrate as shown in FIG. 4B, the signs of the charges generated at the electrodes 2 and 2 are different from each other. Therefore, a wave as shown by the solid line is generated, and double overtone vibration is excited. When this is generalized and considered, m ·
λ / 2, a wave of n · λ / 2 (where λ is a desired vibration wavelength, m, n = 1, 2, 3,..., m> n) is generated in the domain-inverted layer, and m is an odd number. Choosing an odd-numbered overtone oscillation will cause excitation of an even-numbered overtone, and selecting m even-numbered will cause an even-ordered overtone. Therefore, if the value of m and the depth of the domain-inverted layer are appropriately selected, any overtone oscillation of any order can be obtained. It will be appreciated that it is possible to obtain. Such vibration analysis can be performed using a circuit in which each layer is represented by a Mason equivalent circuit and the mechanical terminals are cascaded and the electrical terminals are connected in series. Then, weak coupling approximation is sufficient. I.e., n order overtone U and effective electromechanical coupling coefficient Ke displacement of the vibration is, U = cosk _n y (where k _n t = nπ, t is the substrate total ^{thickness) ...... (1) Ke 2 =} ( A _n · k _t ) ² (k _t is the electromechanical coupling coefficient of thickness vibration, _An is a coefficient) …… (2) Therefore, _An ² = 2 (1 + cosK _n t− ² cosK _n t _I ) ² / (πn )
² (where t _I is the depth of the domain-inverted layer) FIG. 2 (a) shows the value of the coefficient An calculated for t / t _I = 2 and FIG. 2 (b) shows the value calculated for t / t _I = 40. It can be seen that a high order mode in which a standing wave of about a half wavelength (λ / 2) or an odd multiple thereof stands in the thickness t _I of the domain-inverted layer is strongly excited.
Especially when the boundary of polarization is the center of the thickness of the substrates (t / t _I = 2), only the second, sixth, tenth,... It can be understood that the operation is equivalent to. FIG. 3 shows a LiNbO ₃ Z plate, which has a t / t
_This shows the experimental results of the resonance response when _I = 40. In the low order, the odd-order mode is excited as in the case of a normal oscillator, and when the order increases, a standing wave of about half wavelength stands in the domain-inverted layer. It can be seen that such a higher mode is excited very strongly. Therefore, there is a possibility that it can be used as an ultra-high-order overtone oscillator, which was impossible with a conventional general oscillator. FIG. 4 shows the result of measuring the resonance response of a 128 ° rotated Y plate (t / t _I = 2), which shows a good agreement with the calculation result of FIG. 2 (a). As described above, Li having a domain-inverted layer according to the present invention
The thickness vibrator using the NbO ₃ substrate not only can excite even-order overtone vibration, which has been impossible in the past, but also has the possibility of realizing extremely high-order overtone vibration. (2) Flexural or longitudinal vibrator and actuator Conventional flexural vibrators generally bond every two piezoelectric substrates with an adhesive by reversing the poling, so that the resonance frequency due to the presence of the adhesive layer is reduced. As described above, there is a problem that there is a variation or a difficulty in improving Q, and when this is used as an actuator, there is a defect that not only operation is accompanied by hysteresis but also it is difficult to use at high temperature. However, these problems can be easily solved by using a LiNbO ₃ substrate having a domain-inverted layer. FIG. 5 is a view showing the structure of the bending oscillator according to the present invention, in which a half of the thickness of the LiNbO ₃ substrate 4 is inverted in a positive and negative direction, electrodes 2 and 2 are provided on the front and back sides, and an alternating electric field is applied thereto. There is no need to explain about bending vibration. The best cut angle should be selected for the LiNbO ₃ substrate 4 used as a bending oscillator because its excitation intensity is proportional to the transverse effect electromechanical coupling coefficient. Figure 6 shows that Warner (Warner) such as a rotary angle of 130 ° the dependency that calculated to 150 ° rotation Y plate constant of the electromechanical coupling coefficient K _'23 using the are suitable for rotational Y plate. FIG. 7 is a diagram of experimental results showing the characteristics of a flexural vibrator prototyped using a 128 ° rotation Y plate, wherein the capacity ratio Co / C ₁ is 1;
This is a very small value of about 2 for a bending oscillator, which is convenient when a low-frequency filter is configured. The above-mentioned trial bending oscillator uses a single full-surface electrode. However, as shown in FIG. The capacity ratio could be reduced. FIGS. 9 (a) and 9 (b) show examples of a longitudinal oscillator and a contour oscillator using a LiNbO ₃ substrate having a domain-inverted region, respectively. It will be obvious that electrodes may be applied to any of the surfaces. It goes without saying that the above-described bending vibrator or vertical vibrator becomes an actuator that rotates or moves straight when the electric field applied to its electrode is turned ON / OFF with DC applied. Such an actuator does not need to bond a piezoelectric substrate, so it is not only inexpensive, but also does not generate hysteresis of operation due to the presence of an adhesive layer and can withstand use at high temperatures. It is extremely effective when used for (3) Torsional vibrator and optical deflection device As a LiNbO ₃ substrate having a domain-inverted layer, the in-plane rotation 45
れ ば A torsional vibrator can be obtained by using a rotating plate. For example, as shown in FIG. 10 (a), the entire surface electrodes 7, 7 are formed on the main surface of the substrate 6 which has already been rotated by 128 [deg.] Within its main surface by a 128 [deg.] Rotation.
Is adhered (see FIG. 3B), the main surface becomes a torsional vibrator distorted in the thickness direction. FIGS. 11 (a) and 11 (b) are diagrams of experimental results showing the characteristics. It can be seen that a weak bending vibration is generated in a low frequency portion, but a torsional vibration is strongly excited. Such a torsional vibrator can replace the channel filter transducer, which has conventionally been constructed by bonding piezoelectric ceramics, and can greatly reduce the frequency variation and Q quality caused by bonding. it can. If the electrode 7 is mirror-finished, it can be used as a light deflecting device such as an optical roller. (Effects of the Invention) Since the present invention is configured as described above, an even-order overtone thickness vibrator which cannot be realized with a conventional general piezoelectric resonator is provided, and is conventionally attached. Improves the characteristics of flexural or torsional vibrators that have large frequency variations due to their structure and poor Q, and further removes hysteresis of operation in actuators that utilize flexural or longitudinal displacement, and uses them at high temperatures. Therefore, the present invention exerts a remarkable effect on cost reduction, improvement of characteristics, and expansion of the degree of freedom of use conditions of such a device.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 (a) and (b) are diagrams for explaining the operation of the thickness vibrator according to the present invention, and FIGS. 2 (a) and (b) are calculations showing the results of characteristic analysis thereof FIGS. 3 and 4 are diagrams showing experimental results of different thickness oscillator characteristics according to the present invention, respectively.
FIG. 5 is a view showing the configuration of a bending oscillator according to the present invention, and FIG.
FIG. 7 is a diagram showing a calculation result for obtaining the optimum cutting angle, FIG. 7 is a characteristic diagram showing one experimental result, FIG. 8 is a perspective view showing another embodiment of the bending oscillator according to the present invention, 9 (a) and 9 (b) are perspective views showing the configuration of the substrate for the longitudinal vibrator and the contour vibrator according to the present invention, respectively, and FIG. 10 (a) shows the cutting angle of the torsional vibrator according to the present invention. FIG. 11B is a sectional view taken along the line AA of FIG. 11, and FIGS. 11A and 11B are characteristic diagrams showing the results of the experiment. 3, 4, and 6: LiNbO ₃ substrate having domain-inverted regions, 2, 5, and 7: electrodes.

   ────────────────────────────────────────────────── ─── Continuation of front page    (72) Inventor Haruyasu Ando               Aoba, Aramaki, Sendai City, Miyagi Prefecture (No address)               Department of Communication Engineering, Faculty of Engineering, Tohoku University                (56) References JP-A-60-66483 (JP, A)                 Japanese Patent Publication No. 52-34196 (JP, B2)                 Japanese Patent Publication No. 54-18385 (JP, B2)                 IEICE Technical Report US86-               18L L Bottom 1 NbO Bottom 3 Single crystal               Of domain-inverted layer and its mutual electric value               And its application to children ”(1986), P39-44

Claims (1)

(57) [Claims] The LiNbO3 substrate is heat-treated in an inert gas containing air or water vapor at a temperature slightly lower than the Curie point temperature of the substrate material for a predetermined time, and the LiNbO3 single crystal substrate has a depth of approximately nλ / 2 (where nλ / 2). n = 1, 2, 3,... and mλ / 2, m = 1, 2, 3,.
(n and λ are desired wavelengths) by forming a domain-inverted layer or a spontaneously-polarized layer and attaching an electrode to the surface of the substrate to enable strong excitation of the m-th overtone thickness vibration. Device using a LiNbO3 substrate with a region. 2. 2. The device according to claim 1, wherein the value of m is selected to be an even number, thereby enabling strong excitation of even-order overtone thickness vibration. 3. The LiNbO3 substrate is heat-treated in an inert gas containing air or water vapor at a temperature slightly lower than the Curie point temperature of the substrate material for a predetermined time, and the depth of the LiNbO3 single crystal substrate is approximately 1 A device using a LiNbO3 substrate having a domain-inverted region, wherein a domain-inverted region is formed and a longitudinal displacement in a polarization direction is caused by applying an electrode to the surface of the substrate and forming an electrode on the surface of the substrate. . 4. A LiNbO3 substrate is heat-treated in an inert gas containing air or water vapor at a temperature slightly lower than the Curie point temperature of the substrate material for a predetermined time, and a LiNbO3 single crystal substrate having a predetermined depth is formed on the Y ′ plane of the rotating Y plate. A domain-inverted layer was formed, and a torsion displacement was caused by attaching an electrode to a substrate cut out from the substrate in a direction substantially at 45 degrees to the Z 'axis in the Y' plane thereof. Device using a LiNbO3 substrate with domain-inverted regions.