JPH0243364B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a piezoelectric vibrator, and particularly to a piezoelectric vibrator that uses shear surface waves (Love surface waves) and has good frequency-temperature characteristics.

(Prior Art) In general, a piezoelectric vibrator used at high frequencies is made of a thin piezoelectric material plate with parallel plate surfaces, as shown in Figure 1a.
Electrodes 2, 2' are attached to both sides of the electrode. The vibration state of such a piezoelectric vibrator utilizes standing waves, that is, thickness vibration, which are formed by complete reflection of a plane wave with a wavefront parallel to the plate surface at the plate surface. Therefore, its resonant frequency is inversely proportional to the thickness of the plate.

Now, when trying to use this piezoelectric vibrator to obtain a resonant frequency above the VHF band from the fundamental vibration with the half-wavelength displacement distribution shown in Figure 1b, it is difficult to support it because the required thickness is too thin. becomes. In order to avoid this, odd-numbered higher-order vibrations (third-order in the figure) shown in Figure 1c are used, but with this, the electrodes attached to the plate surface can effectively collect piezoelectrically induced charges. As a result, the equivalent series capacitance decreases in inverse proportion to the square of the high frequency order, and the frequency adjustment range of an oscillator using such a vibrator or the bandwidth of a filter narrows accordingly, which poses a problem in practical use.

In order to avoid these drawbacks, surface wave transducers have appeared. FIG. 2 is a structural diagram of such a surface wave vibrator, in which interdigital electrodes 11, 11' are attached to a piezoelectric substrate 10 having a thickness that can be considered as a semi-infinite body compared to the wavelength. Since the energy of the surface waves excited by the electrodes literally concentrates on the surface, the thickness of the piezoelectric substrate 10 has little to do with it, and since it can be firmly supported with adhesive etc., the manufacturing and support problems seen in the thickness vibrator described above are The problem is alleviated.

However, the commonly used surface waves are so-called Rayleigh waves, which have components of longitudinal waves and shear waves (SV waves) perpendicular to the plate surface, which have the following four drawbacks. ing.

The first drawback is that there are few cutting directions with good frequency temperature coefficients.

The second drawback is that since there is a longitudinal wave component, there is energy loss due to acoustic radiation into the air, and therefore, in order to increase the resonance sharpness Q, it is necessary to encapsulate the device in a vacuum.

The third drawback is that, unlike plane waves in thickness vibration, Rayleigh surface waves cannot be sufficiently reflected at end faces.

Therefore, at present, interdigital reflectors 1 as shown in FIG. 2 are provided on both sides of the interdigital electrodes 11 and 11'.
2, 12' are provided. This reflector 12,1
The electrodes 2' are formed by attaching them by vapor deposition in the same way as the interdigital electrodes 11 and 11', or by directly cutting grooves in the piezoelectric substrate 10 by etching or the like.
However, in order to obtain good reflection, the elements 12, 12' of the reflectors 12, 12' are formed at equal intervals.
A, 12a', 12b, 12b'... are required in number of several hundred or more, and it is difficult to manufacture them with high precision. Further, since the reflectors 12, 12' occupy several times the length on the left and right sides of the interdigital electrodes 11, 11', the piezoelectric substrate 10 becomes large, which is disadvantageous in terms of material costs and miniaturization.

A fourth drawback is that the interdigital reflectors 12, 12' have sharp frequency characteristics, and the frequency of the interdigital reflectors 12, 12'
It is inversely proportional to the spacing between a, 12a', 12b, 12b'... However, once the reflectors 12 and 12' are formed, they cannot be easily changed, making it difficult to adjust the frequency.

On the other hand, vibrators that utilize shear surface waves are also being considered. FIG. 3 is a block diagram of a piezoelectric vibrator using such shear surface waves, in which interdigital electrodes 21 and 21' are attached on a piezoelectric substrate 20. Note that this point is the same as in the case of FIG. 2, but in FIG. ' fingers 21a, 21
It is possible to excite shear waves (SH waves) surface waves that are parallel to a', . . . 21c, 21c'.
This type of parallel surface wave is called a Love surface wave, and it does not exist in a homogeneous, non-piezoelectric substrate. However, it is possible to create a two-layer structure by attaching a layer on the surface where the sound velocity of the shear wave is slower than the sound velocity of the base, or by attaching a surface film to take advantage of its mass effect, or by attaching a metal film to utilize its mass effect and piezoelectric reaction. It is possible to create surface waves with concentrated energy on the surface by using methods such as At this time, the surface layer or film does not necessarily need to cover the entire surface of the plate, and there may be a gap smaller than the wavelength, so there is sufficient room for forming interdigital electrodes, etc.

Now, since this Love surface wave has little acoustic radiation, the second drawback mentioned above is eliminated. Furthermore, although it cannot be said to be perfect with right-angled end faces, considerable reflection can be obtained, so if the end faces 23 and 24 (Fig. 3) are provided parallel to the displacement direction and perpendicular to the plate surface, the standing wave is formed and resonance is obtained. Therefore, the interdigital reflector is not necessary within a certain Q range.

(Problems to be Solved by the Invention) As described above, the piezoelectric vibrator that generates Love surface waves can eliminate the second drawback of Rayleigh waves, and can also be used within a certain range of Q. There is no need for a blind reflector, and the third and fourth drawbacks can also be eliminated, which is effective. However, this piezoelectric vibrator cannot improve the first drawback, that is, the frequency temperature characteristics, it is difficult to adjust the frequency, and furthermore, it cannot be applied when it is desired to improve the Q.

Therefore, by forming an inclination on the end face, the present invention achieves complete reflection in a simple manner without providing an interdigital reflector, and further improves Q. In addition, frequency adjustment is easy, and the frequency It is an object of the present invention to provide a piezoelectric vibrator that has good temperature characteristics and can eliminate the drawbacks of the conventional ones.

(Means for Solving the Problems) The present invention includes forming an excitation electrode on the surface of a piezoelectric material substrate, and a surface layer on a part of the surface of the substrate.
Alternatively, in a piezoelectric vibrator that is provided with a surface film and generates Love surface waves by the excitation electrode, at least one end face of the substrate parallel to the displacement direction of the Love surface waves has a predetermined inclination angle; This piezoelectric vibrator is characterized by reduced energy loss of Love surface waves.

(Function) The piezoelectric vibrator of the present invention completely reflects parallel surface waves by forming an inclination on at least one end face, eliminates attenuation of standing waves, and further improves Q.

(Example) Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

Now, according to analysis, if the substrate is an anisotropic material such as a piezoelectric material, perfect reflection cannot be obtained from the right-angled end faces as shown in Figures 2 and 3, and mode conversion occurs during reflection. It has been found that the Q decreases due to the energy loss, or that spurious waves increase due to coupling with other vibration modes. This is because the elastic coefficient C' ₅₆ , which is zero in the case of isotropy, has a finite value in the case of anisotropy.

According to theoretical calculations, perfect reflection can be achieved by giving the end face an inclination angle α determined by equation (1).
The above inconvenience will be eliminated.

α=tan ^-1 (C' ₅₆ /C' ₆₆ )...(1) (However, C' ₅₆ and C' ₆₆ are stiffness elastic constants.) Figure 4 shows an embodiment of the piezoelectric vibrator according to the present invention. In the figure, 30 is a piezoelectric substrate, 31 and 31' are interdigital electrodes, and fingers 31a, 31b, 31
c, 31a', 31b', and 31'c. Each interdigital electrode 31, 31' is formed on the surface of the piezoelectric substrate 30, and excites the piezoelectric substrate 30 to generate Love surface waves. Although not shown, a surface layer or film is applied to the surface portions other than the interdigital electrodes 31 and 31' so that the condition for the existence of Love surface waves described above is satisfied. Here, the interdigital electrode 3
The X' axis is parallel to the fingers 31a, 31a'... of 1, 31',
If you select the Y' axis perpendicular to the plate surface and the Z' axis perpendicular to both axes and set the coordinate system as shown,
C′ ₅₆ and C′ ₆₆ in equation (1) above are stiffness elasticities for this coordinate system. Both end surfaces 3 of the piezoelectric substrate 30
2 and 33 are parallel to the X' axis as shown, and are formed so that their surfaces form an inclination angle α given by equation (1) with the Y' axis. In this way, the end face 32,
The reflection at 33 is perfect, resulting in a piezoelectric vibrator with high Q and little spurious. In the example shown here, the tilt angle α is formed on each end face in consideration of love surface waves on both end faces of the substrate, but even if only one end face is tilted, a certain degree of effect can be achieved. Can be done.

Incidentally, the frequency adjustment can be easily performed by mechanically or chemically polishing the end surfaces 32 and 33, or by forming a film of metal, dielectric, etc. by vapor deposition, sputtering, or the like.

Furthermore, in the piezoelectric vibrator using Love surface waves according to the present invention, a vibrator having good temperature characteristics can be obtained by appropriately selecting the cutting angle of the piezoelectric substrate.

Next, regarding the cutting angle, temperature characteristics, etc., a case where quartz crystal is used as the piezoelectric material will be specifically explained.

FIGS. 5a and 5b are explanatory diagrams illustrating a method of cutting a crystal in order to make the temperature coefficient of the resonance frequency zero, and numeral 41 in the figure indicates the crystal.

As shown in Figure 5a, the X' axis is +X of the crystal 41.
axis (electrical axis perpendicular to the plane of the paper), and rotate the Y' axis by β° (33° to 39°) from the +Y axis (mechanical axis) of the crystal to the +Z axis (optical axis). If the end face is cut and formed by tilting the end face by an angle α (1° to 11°), the temperature characteristic of the resonant frequency becomes zero. Note that the frequency temperature characteristic has an upwardly convex quadratic curve shape.

Figure 6 shows the relationship between the cutting angle, the apex temperature of the frequency-temperature characteristic, and the necessary inclination of the end face. By adjusting the cutting angle β and the inclination angle α, the required temperature range (- It can be seen that a vibrator suitable for use at temperatures between 60°C and 100°C can be obtained.

Also, as shown in Figure 5b, the X' axis is set to +X of the crystal.
Good frequency-temperature characteristics similar to the above can be obtained when the cutting angle β is further increased to 127° to 133° and the inclination angle is −2° to −8° while aligning the cutting angle with the axis (perpendicular to the plane of the paper). It will be done.

FIG. 7 shows the cutting angle corresponding to FIG. 5b, the apex temperature of the frequency-temperature characteristic, and the angle of inclination α of the end face required at that time.

Incidentally, the inclination angle α in FIGS. 6 and 7 shows a calculated value, but experiments have shown that there is a fairly permissible range, and there is no problem even if the angle deviates from this value by several degrees. or,
The width of the vibrator in the X direction is the length of the fingers of the interdigital electrodes.
It is sufficient to have at least 10 wavelengths, and the width of the piezoelectric substrate should be at most 20% of the width.

(Effects of the Invention) As described above in detail, according to the present invention,
At least one end face of the substrate parallel to the displacement direction of the Love surface wave has a predetermined inclination angle, and by reducing the energy loss of the Love surface wave at the end face of the substrate, it is possible to create a vibrator using Rayleigh waves. It is possible to provide a piezoelectric vibrator with a good frequency temperature coefficient by eliminating the disadvantages of the present invention.

Further, according to the piezoelectric vibrator of the present invention, the Q can be particularly high, and the frequency can be easily adjusted.

[Brief explanation of the drawing]

Figure 1 is an explanatory diagram illustrating a conventional piezoelectric vibrator with electrodes attached to both sides of a plate of piezoelectric material, Figure 2 is a configuration diagram of a conventional surface wave vibrator using Rayleigh waves, and Figure 3 is a diagram illustrating the structure of a conventional surface wave vibrator that uses Rayleigh waves. Configuration diagram of piezoelectric vibrator using waves,
FIG. 4 is a configuration diagram of a piezoelectric vibrator according to the present invention, and FIG.
The figure is an explanatory diagram explaining the method of cutting a crystal to make the temperature coefficient of the resonant frequency zero, and Figures 6 and 7 show the relationship between the cutting angle and the peak temperature of the frequency temperature characteristic, and the required end face shape at that time. It is a figure showing the relationship between an inclination angle and a cutting angle. 1... Plate of piezoelectric material, 2, 2'... Electrode, 10, 2
0, 30...Piezoelectric base, 11, 11', 21, 2
1', 31, 31'... interdigital electrode, 12, 12'
...Bing-shaped reflector, 23, 24, 32, 33... End face, 41... Crystal.

Claims (1)

[Scope of Claims] 1. An excitation electrode is formed on the surface of a piezoelectric material substrate, and a surface layer or surface film for confining vibration energy is provided on the surface of the substrate excluding the excitation electrode, and the excitation electrode is used to increase the love surface. In a piezoelectric vibrator that generates waves, at least one end surface of the substrate parallel to the displacement direction of the Love surface wave is α=tan ^-1 (C′ ₅₆ /C′ ₆₆ ), where α is the end surface of the substrate. A piezoelectric vibrator characterized in that the inclination angles C′ ₅₆ and C′ ₆₆ have inclination angles indicated by the stiffness elastic coefficient of the substrate, and energy loss of Love surface waves at the end face of the substrate is reduced. 2 A surface layer or a surface film is provided on a part of the surface of the piezoelectric material substrate on which the excitation electrode is formed, excluding the excitation electrode, to generate a Love surface wave, and the substrate parallel to the displacement direction of the Love surface wave. In the piezoelectric vibrator having a predetermined inclination angle on at least one end face of the piezoelectric material, a crystal is used as the piezoelectric material, and the normal line (Y' axis) thereof is 33° to 39° from the Y axis of the crystal in the Z axis direction,
Or, when the rotation angle is 126° to 134°, and the Y′ axis has a rotation angle of 33° to 39°, the end face is 1° to 11° in the Z-axis direction from the normal to the surface, and 126° to 134°. A piezoelectric vibrator characterized in that it is tilted by -2° to -8° when the piezoelectric vibrator has a rotation angle of -2° to -8°.