JPH037162B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (1) Technical Field of the Invention The present invention relates to a piezoelectric vibrating element, and more particularly to a piezoelectric vibrating element that uses lithium niobate (LiNbO ₃ ) single crystal and has improved resonance characteristics.

(2) Background of the technology When a suitable electrode is formed on a piezoelectric material such as quartz crystal or lithium tantalate (LiTaO ₃ ) and an alternating current electric field is applied to this electrode, the piezoelectric material generates stress with a frequency equal to the applied electric field, and When the frequency of the applied electric field matches the natural frequency of the piezoelectric material, resonance occurs and strong vibrations are obtained. Since such resonators have high performance, they are widely used as oscillation circuits, filters, delay lines, etc. in communication devices.

(3) Prior art and problems Figure 1 is a perspective view showing the main structure of a conventional piezoelectric vibrating element using a single crystal of LiTaO _{3 .} It has a structure in which opposing electrodes 3 and 4 are patterned on opposing main surfaces 2a and 2b of a single crystal blank plate (hereinafter simply referred to as blank plate) 2, respectively.

In this configuration, when an alternating current electric field is applied between the opposing electrodes 3 and 4, thickness shear mode vibration occurs,
As schematically shown by curve a, this thickness shear vibration occurs over a wide circular range including the center of the blank where the circular electrode 3 is located, and this vibration is strong in the center and becomes weaker toward the periphery. .

(4) Object of the Invention In view of the above-mentioned conventional problems, an object of the present invention is to provide a piezoelectric vibrating element that is small in size, has a large mechanical coupling coefficient, and has improved vibration characteristics.

(5) Structure of the Invention According to the present invention, this object is made of a piezoelectric body made of lithium niobate single crystal, and the opposing principal surfaces of the piezoelectric body are perpendicular to the Y axis and have an X-Z plane as the principal plane. Move the Y plate around the X axis from the Y axis to the Z axis direction.
It is formed by the cut-out surface of a rotating Y plate rotated within a range of 165±5 degrees, and the longitudinal direction of the opposing main surface makes an angle within a range of 90±5 degrees with respect to the X axis of the rotating Y plate. This is achieved by providing a piezoelectric vibrating element characterized by having a rectangular piezoelectric material plate cut out in a direction.

(6) Examples of the invention Embodiments of the present invention will be described below with reference to the drawings.

The inventor of this application uses a lithium niobate (LiNbO ₃ ) single crystal as a piezoelectric material, and when cutting a wafer from this LiNbO ₃ single crystal, the mechanical coupling coefficient of the thickness shear fast mode is large and the mechanical coupling coefficient of the thickness shear slow mode is large. A surface with a small mechanical coupling coefficient was selected, and when forming a vibrating element on a lithium niobate wafer, the vibrating element was, for example, a strip type, and its rectangular cross section was selected to be perpendicular to the vibration change.

FIG. 3 is a diagram showing the dependence of the mechanical coupling coefficient k (hereinafter simply referred to as coupling coefficient) on the rotation angle θ (deg) around the X axis in the LiNbO ₃ single crystal 11 shown in FIG. , the vertical axis represents the coupling coefficient k, the horizontal axis represents the rotation angle θ, the curve represents the coupling coefficient in the thickness longitudinal mode, the curve represents the coupling coefficient in the thickness shear slow mode, and the curve represents the coupling coefficient in the thickness shear fast mode. Shows angle dependence. From the changes in the coupling coefficients shown by these curves, the angles θ at which the coupling coefficient for the thickness-shear fast mode is large and that for the thickness-shear slow mode are small are around 35 degrees and 165 degrees as indicated by arrows b and c.

Among the above two angles, the wafer cutting angle has a large coupling coefficient k, and a small displacement direction Φ (deg) as shown by the arrow d in FIG. 4, so that the vibration wave advances parallel to the wafer cutting surface. The angle shall be 165°.

By such wafer cutting, a wafer in which vibrations in the thickness longitudinal mode and the thickness shear slow mode are suppressed can be obtained. Also, the wafer cutting angle θ is
Considering the angle dependence of the displacement direction near θ=165°, the effect of the present invention will not be impaired even if it is selected within the range of ±5 degrees.

Note that when θ=35°, the displacement direction θ at this angle is approximately −1.5 degrees, so the vibration wave does not proceed parallel to the cutting surface, so it is not determined as the cutting angle.

Figure 5 shows the X of the series resistance Rs (Ω) obtained by the inventor of the present application in order to consider the traveling direction of vibration waves in a wafer cut at a cutting angle of 165 degrees.
This is a diagram showing degree dependence with respect to the axis, in which the vertical axis represents the series resistance Rs, the horizontal axis represents the rotation angle φ (deg) with respect to the X axis, and the series resistance Rs is minimum at φ=90°. Therefore, as shown in FIG. 6, the vibration wave propagates in the direction perpendicular to the X-axis (φ=90°) indicated by arrow e in the wafer 12 cut out at a cutting angle of 165°, and the piezoelectric wave in the piezoelectric element 13 The body blank is formed by cutting out a rectangular blank so that its longitudinal direction is the same as the direction of arrow e.

FIG. 7 is a perspective view showing the main structure of, for example, the strip type piezoelectric vibrating element 13 formed in this way.
In the figure, reference numeral 14 indicates a piezoelectric material plate made of LiNbO ₃ single crystal, and reference numeral 15 indicates a counter electrode.

As schematically shown by the curve f in the figure, the vibration in the piezoelectric element 13 becomes a strong and equal vibration mode throughout the center width direction of the element 13 (direction perpendicular to the arrow e), and becomes stronger as it moves toward both ends of the blank plate 14. become weak. In this way, the blank plate 14 can be made smaller.

FIG. 8 is a diagram showing the resonance characteristics (admittance) of the strip type piezoelectric vibrating element when the longitudinal direction of the vibrating element 13 in FIG. The frequency (MHz) and the vertical axis represent the resonance strength (dB). Referring to the same figure, only one strong resonance peak appears at frequency
It is found around 4.4MHz and provides a high-characteristic resonance in which spurious waves other than the thickness-shear first mode do not appear.

Figure 9 shows the longitudinal direction of the strip type vibrating element
This is a diagram showing the resonance characteristics of an element cut out from a wafer in a direction that makes 85 degrees (φ = 85 degrees) with the axis. Although a small spurious signal indicated by the symbol h is observed in the diagram, it has the same high characteristics as in Figure 8. resonance can be obtained.
Also, in Fig. 10, the angle φ is
With the resonance characteristics of the vibrating element cut out by increasing 5 degrees from 90 degrees and setting φ=95 degrees, high characteristic resonance can be obtained as in the above case.

In Figure 11, φ is 80 degrees, and in Figure 12, φ is 100 degrees.
This is a diagram showing the resonance characteristics of a vibrating element cut out from a wafer, with the code i or j in each case.
The spurious waves caused by the unnecessary waves shown in are strong and the resonance characteristics are getting worse.

Thus, when cutting out a vibrating element from a wafer, a vibrating element with high resonance characteristics can be obtained by setting the angle φ between the longitudinal direction of the element and the X axis to φ=90±5°.

(7) Effects of the Invention As described in detail above, according to the present invention, a piezoelectric vibrating element with improved resonance characteristics using a lithium niobate single crystal as a piezoelectric material is provided, and this vibrating element may be of a strip type. Since it is possible to downsize the piezoelectric vibrating element, it is highly effective in improving the reliability and functionality of the piezoelectric vibrating element.

[Brief explanation of the drawing]

Figure 1 is a perspective view of a conventional piezoelectric vibrating element, Figure 2 is a diagram showing a wafer cutting surface for explaining an embodiment of the present invention, and Figure 3 is a diagram showing the angular dependence of the mechanical coupling coefficient of LiNbO ₃ single crystal. , FIG. 4 is a diagram showing the angular dependence of the displacement direction, FIG. 5 is a diagram showing the rotation angle dependence of the series resistance in a wafer cut out at the cutting angle of the present invention, and FIG. FIG. 7 is a perspective view of a piezoelectric element according to the present invention, and FIGS. 8 to 12 are diagrams showing resonance characteristics of the piezoelectric vibrating element according to the present invention. 2...LiTaO ₃ single crystal piezoelectric plate, 3, 4, 15
...Counter electrode, 11...LiNbO ₃ single crystal, 12
... Wafer, 13 ... Strip type piezoelectric vibrating element, 14 ... LiNbO ₃ single crystal piezoelectric element plate.

Claims (1)

[Scope of Claims] 1. A piezoelectric material made of a single crystal of lithium niobate, the opposing principal surfaces of which are perpendicular to the Y-axis and arranged in the X-Z direction.
Formed by the cut surface of a rotated Y plate whose main surface is a flat plane, rotated around the X axis within a range of 165 ± 5 degrees in the direction from the Y axis to the Z axis, and the longitudinal direction of the opposing main surface A piezoelectric vibrating element comprising a rectangular piezoelectric material plate cut out in a direction having an angle within a range of 90±5 degrees with respect to the X axis of the rotating Y plate. 2. The piezoelectric vibrating element according to claim 1, wherein the piezoelectric vibrating element is of a strip type.