JPH06164306A - Surface acoustic wave resonator - Google Patents

Abstract

PURPOSE:To provide a small-sized small-capacitance surface acoustic wave resonator by arranging an interdigital transducer with a metal whose specific gravity is large on the surface of a rotation Y cut LiTaO3 piezoelectric substrate and exciting Love wave type surface acoustic waves in the direction of an X axis. CONSTITUTION:The interdigital transducer 2 provided with terminal 4 and 4' is provided on the surface of the rotation Y cut LiTaO3 piezoelectric substrate 1 cut by a prescribed angle within a range where a rotation angle from a Y axis is -10 deg. to +50% on a Y-Z plane with the Y axis as a normal line. The electrode of the interdigital transducer 2 is not an electrode having a uniform thin film and is sufficiently thickened by the metal whose specific gravity is large such as gold, platinum and silver, so that an effect equivalent to the uniform film can be obtained and pseudo surface acoustic waves can be transformed to the Love wave type surface acoustic waves. Thus, a Love wave type surface acoustic wave resonator much smaller than the surface acoustic wave resonator utilizing a LiTaO3 piezoelectric substrate for not largely depending on the accuracy of cut rotation angle whose capacitance ratio is small can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an energy trap type surface acoustic wave resonator used as a resonance element in a voltage controlled oscillator (VCO) of electronic equipment, especially communication equipment.

[0002]

2. Description of the Related Art When a frequency synthesizer is used in a communication device, a voltage controlled oscillator (VCO) is required to have a wide frequency variable range. Therefore, when a surface acoustic wave resonator is used as the resonance element of the voltage controlled oscillator, the surface acoustic wave resonator has a small capacitance ratio (a value proportional to the reciprocal of the difference between the resonance frequency and the anti-resonance frequency) and the electromechanical coupling is small. A large coefficient (k ² ) is required. An example of a surface acoustic wave resonator that meets such requirements is an energy trap type surface acoustic wave resonator (hereinafter abbreviated as SAW resonator) using a lithium tantalate (LiTaO ₃ ) piezoelectric substrate. Conventional lithium tantalate (LiTa
There are the following two types of SAW resonators using a piezoelectric substrate of O ₃ ). One is a Rayleigh wave type surface wave existing on a piezoelectric substrate propagating in an X-cut-112 ° rotation Y direction, and the other is a 36 ° angle.
It utilizes a pseudo surface acoustic wave (leaky wave type surface acoustic wave) existing on a piezoelectric substrate propagating in the Y-cut-X direction.
Since the SAW resonator using the Rayleigh wave type surface wave has a relatively small electromechanical coupling coefficient k ² of about 0.7%, when the SAW resonator is constructed, the capacitance ratio is about 250 and cannot be reduced so much. . Since the coupling coefficient k ² is small, it is necessary to increase the number of electrodes of the grating reflector and the number of electrode pairs of the IDT, which is disadvantageous for downsizing. On the other hand, the SAW resonator using the pseudo surface acoustic wave is a leaky wave propagating while radiating a bulk wave in the piezoelectric substrate, and thus generally has a large propagation attenuation amount, but has a cutting rotation angle of 36.
In the case of a 36 ° Y-cut X-propagating LiTaO ₃ piezoelectric substrate at 0 °, the propagation attenuation is almost 0, and the coupling coefficient k ² is also 4.
Since it is relatively large at 7%, it has been put to practical use.
However, since this type of wave is essentially a leaky wave, there is a drawback that attenuation occurs when the cut rotation angle deviates from 36 °.

FIG. 3A is a characteristic diagram of the surface wave velocity with respect to the cutting rotation angle of the LiTaO ₃ substrate. As shown in FIG. 3B, the horizontal axis represents the cutting rotation angle θ from the Y axis in the YZ plane.
And the surface wave propagates in the X-axis direction. As shown in FIG. 3, it is known that a Rayleigh wave indicated by a broken line having a slow surface wave velocity and a pseudo surface acoustic wave (leaky wave) having a high surface wave velocity are present on the rotating Y-cut LiTaO ₃ piezoelectric substrate. ing. Further, the electromechanical coupling coefficient k ² is defined by the following equation.

[Equation 1] However, V _f : Surface wave velocity of free surface V _m : Surface wave velocity of surface short circuit (Metalized Surface)

[0004]

From the above equation, the electromechanical coupling coefficient k ² becomes larger as the difference between the surface free surface wave velocity V _f and the surface short circuit surface velocity V _m becomes larger. Therefore, in the case of Rayleigh waves, the coupling coefficient k ² is extremely small because they almost overlap. On the other hand, the pseudo surface acoustic wave has the largest difference in surface wave velocity when the cutting rotation angle is 0 ° (also called Y plate), and the cutting rotation angle is 36 ° (commonly called 3 plate).
This is about 1.3 times that of a 6 ° Y plate). Ie 36 °
It can be seen that the 0 ° Y plate has a larger k ² and a smaller capacity ratio than the Y plate. However, since the pseudo surface acoustic wave is a leaky wave, the propagation attenuation amount of the surface wave becomes 0 near 36 ° Y, but the accuracy of the cutting rotation angle is a problem, and the propagation attenuation amount becomes 0 ° when the rotation angle is 0 °. There is a problem that it becomes large and cannot be used as it is. Also, Li
A Love-wave type surface acoustic wave resonator using TaO ₃ as a piezoelectric substrate has not yet been put into practical use. An object of the present invention is to provide a Love wave type surface acoustic wave resonator that is smaller than the conventional surface acoustic wave resonator using a LiTaO ₃ piezoelectric substrate, has a small capacitance ratio and does not largely depend on the accuracy of the cutting rotation angle. To provide.

[0005]

The surface acoustic wave resonator of the present invention has a predetermined angle in the range of −10 ° to + 50 ° from the Y axis on the YZ plane with the Y axis as the normal line. On the surface of the rotary Y-cut LiTaO ₃ piezoelectric substrate cut by the above, a interdigital transducer formed of a metal having a large specific gravity is arranged.
The piezoelectric substrate has a basic structure configured to excite a Love wave type surface acoustic wave in the X-axis direction, and is the same as the interdigital transducer on the surface wave propagation paths on both sides of the interdigital transducer. A grating reflector formed of a metal having a large specific gravity is provided so that a Love wave type surface acoustic wave is excited in the X-axis direction of the piezoelectric substrate.

That is, in order to put into practical use a Love wave type surface acoustic wave resonator which has not been realized so far, LiTaO
₃ By attaching a heavy substance with a slow sound velocity on the piezoelectric substrate to lower the surface acoustic wave velocity and making it slower than the slow transverse wave shown in Fig. 3, the coupling coefficient k ² is almost the same or higher, and the pseudo surface acoustic wave is generated. This is a change to a Love wave type surface wave with no propagation attenuation. At this time, the interdigital transducer (ID
Even if the electrode of T: Interdigital Transducer is not a uniform thin film electrode, gold (Au), platinum (Pt), silver (Ag)
An equivalent effect to a uniform film (though the film thickness is considered to be equivalent to approximately 1/2) can be obtained by thickening a metal with a large specific gravity such as It can be converted into surface waves. Furthermore, the range of the cutting angle of the rotary Y-cut is -10 ° to + 50 ° (170 ° to 180 ° in Fig. 3).
° and 0 ° to 50 °), as is clear from FIG. 3, it is clear that a coupling coefficient k ² equal to or higher than that of the case of 36 ° Y-cut-X propagation LiTaO ₃ can be obtained, and rotation There is an advantage that the influence of the precision of the cut angle on the propagation attenuation amount is eliminated.

[0007]

1 is a basic configuration diagram showing a first embodiment of the present invention, and FIG. 2 is a configuration diagram showing a second embodiment. In the first embodiment of FIG. 1, the range of the cutting angle of the rotary Y-cut is −10 ° to + 50 ° (170 ° to 180 ° and 0 ° in FIG. 3).
1 to 50 °) shows a basic configuration in which a interdigital transducer (IDT) 2 having terminals 4 and 4 ′ is arranged on the surface of a piezoelectric substrate 1 having an IDT 2 having a relatively large number of logarithms.
This is a surface acoustic wave resonator configured by only 2. Also,
The second embodiment of FIG. 2 is a Love wave type surface acoustic wave resonator having a structure in which a grating reflector 3 made of the same heavy metal electrode material as the IDT 2 is arranged on both sides of the IDT 2 of the basic configuration of FIG. 36 ° Y cut-X
As compared with a conventional quasi surface acoustic wave resonator in which an electrode structure similar to that shown in FIG. 2 is formed of a light metal such as aluminum on a directional propagation LiTaO ₃ piezoelectric substrate, the reflector 3 has a larger electromechanical coupling coefficient k ² . Since the number of electrode fingers can be reduced (to 1/2 or less), the size can be reduced, and at the same time, a surface acoustic wave resonator having a small capacitance ratio can be realized. As the measured value of the electromechanical coupling coefficient k ^2, a value of about 11% or more was obtained as compared with the conventional value of 4.7%. In the above embodiment, the basic configuration of only the IDT 2 shown in FIG.
The grating reflectors 3 are provided on both sides of the IDT 2 shown in FIG.
In the above description, the present invention is also applied to a surface acoustic wave resonator in which the electrode fingers of the IDT 2 are provided with break points and weighted so as to form a rhombus in order to improve the spurious response. It goes without saying that you can do it.

[0008]

By implementing the present invention, the chip size can be made smaller and the size can be reduced as compared with the conventional surface acoustic wave resonator using the pseudo surface acoustic wave. Furthermore, since the capacity ratio can be reduced,
When it is used for a voltage controlled oscillator or the like, the frequency variable range can be broadened, and the practical effect is extremely large.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing a first embodiment of the present invention.

FIG. 2 is a configuration diagram showing a second embodiment of the present invention.

FIG. 3 is a relationship diagram of a rotation cut angle and a surface wave velocity in a rotation Y-cut LiTaO ₃ substrate.

[Explanation of symbols]

 1 Piezoelectric substrate 2 Interdigital transducer (IDT) 3 Grating reflector 4 and 4'terminal

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tsutomu Aida 2-3-13 Toranomon, Minato-ku, Tokyo Kokusai Electric Co., Ltd. (72) Inventor Hiroshi Shimizu 1-22, Yagiyamahonmachi, Taihaku-ku, Sendai-shi, Miyagi Number 12

Claims (2)

[Claims] 1. A surface of a rotary Y-cut LiTaO ₃ piezoelectric substrate, which is cut at a predetermined angle in the range of −10 ° to + 50 ° from the Y-axis on the YZ plane with the Y-axis as a normal line. A surface acoustic wave resonator in which a comb-shaped transducer formed of a metal having a large specific gravity is provided in the piezoelectric substrate and a Love wave type surface acoustic wave is excited in the X-axis direction of the piezoelectric substrate. 2. A surface of a rotary Y-cut LiTaO ₃ piezoelectric substrate, which is cut at a predetermined angle in the range of −10 ° to + 50 ° from the Y-axis on the Y-Z plane with the Y-axis as a normal line. In addition, the interdigital transducer formed of a metal having a large specific gravity and the grating reflector formed of a heavy metal having the same specific gravity as the interdigital transducer are disposed on the surface wave propagation paths on both sides of the interdigital transducer. And a surface acoustic wave resonator configured to excite a Love wave type surface acoustic wave in the X-axis direction of the piezoelectric substrate.