JPS63135010A - Surface acoustic wave resonator - Google Patents

Abstract

PURPOSE:To improve the resonance characteristic by increasing the exciting efficiency of a surface acoustic wave at the midpoint of an interdigital electrode and decreasing the efficiency toward both ends. CONSTITUTION:The exciting efficiency is decreased from the midpoint of the interdigital electrode toward the ends. As a means to decrease the exciting efficiency the midpoint of the interdigital electrode toward both ends, the line width ratio (n=l/L) is decreased from the midpoint of the interdigital electrode toward both ends, where (l) is the width of the electrode finger and L is the pitch of the electrode finger. That is, with the line width ratio (n) given in the vicinity of 0.5, the exciting efficiency is the highest and the exciting efficiency is decreased even when the line width ratio (n) is larger or smaller than the value. On the other hand, the reflection efficiency tends to be higher as the line width ratio (n) gets smaller. Thus, the exciting efficiency is decreased toward both ends by decreasing the line width ratio (n) toward both ends from the midpoint of the interdigital electrode and the reflection efficiency is increased, and the resonance characteristic Q is improved by decreasing the leakage of the excited energy from both ends.

Description

Detailed Description of the Invention [Technical Field] The present invention relates to a surface acoustic wave resonator in which interdigital electrodes are provided on a piezoelectric substrate through which surface acoustic waves propagate. This invention relates to a surface acoustic wave resonator whose characteristics as a resonator are improved by partially changing L.

"Prior art and its problems" Surface acoustic wave elements have traditionally been used for special military purposes, but in recent years they have begun to be used in consumer equipment such as FM tuners and TVs, and are suddenly attracting attention. It has become. Specifically, surface acoustic wave elements include delay elements, oscillators,
It has been commercialized as a filter, etc. The characteristics of these various surface acoustic wave devices are that they are small, lightweight, and highly reliable, and that their manufacturing process is similar to that of integrated circuits, making them suitable for mass production. Nowadays, it is mass-produced as an indispensable electronic component.

Among surface acoustic wave devices with such technical background, some are used as resonators. Conventionally, a surface acoustic wave element that acts as a resonator is shown in FIG. 3, and is constructed as follows.

On the piezoelectric substrate 1 through which surface acoustic waves propagate, there is formed an interdigital electrode 3 consisting of a plurality of electrode fingers 2, 2, . . . 2, which excites surface acoustic waves. As shown in the figure, this interdigital electrode 3 has a line width ratio n=1!, where ρ is the width of the interdigital electrode fingers 2, and L is the pitch, that is, the sum of the electrode finger width and the electrode finger spacing.
/L=0.5.

When a voltage of a specific frequency is applied to the interdigital electrodes 3 of the surface acoustic wave resonator having such a configuration, an electric field is generated between each electrode finger via the piezoelectric substrate 118, and the electric field generated due to the piezoelectricity of the piezoelectric substrate 1 is A strain proportional to the strength of is produced, and this strain propagates on the piezoelectric substrate 1 as a surface wave at a propagation speed determined by the material of the piezoelectric substrate 1. This surface wave is multi-reflected and resonates between the electrode fingers 2, which both excite and reflect the surface acoustic waves.

However, in such a conventional surface acoustic wave resonator, since the line width ratio n is uniform within the interdigital electrode 3, the efficiency of converting electrical energy into mechanical energy, that is, the excitation The efficiency L is constant in each electrode finger 2 constituting this interdigital electrode 3, and the efficiency of reflecting propagated acoustic energy, that is, the reflection efficiency is also constant in each electrode finger 2. Therefore, the mechanical energy converted at both ends of the interdigital electrode 3 escapes to the outside without being reflected inside the interdigital electrode 3, and as a result, leakage loss increases toward both ends. , which caused a reduction in the resonance characteristic Q.

``Object of the Invention'' The present invention has been made in view of the above-mentioned problems. In general, by configuring this interdigital electrode L, the reflection efficiency often increases from the center to the ends of the interdigital electrode, so by configuring this interdigital electrode in this way, L, The purpose is to improve the resonance characteristic Q.

"Structure of the Invention" The present invention provides an interdigital electrode consisting of a plurality of electrode fingers on a piezoelectric substrate through which surface acoustic waves propagate, and multiplexes the surface acoustic waves excited by each electrode finger inside the interdigital electrode. The surface acoustic wave resonator is characterized in that the excitation efficiency of the surface acoustic wave is large at the center of the interdigital electrode and becomes smaller as it approaches both ends of the interdigital electrode.

The resonance characteristic Q of the surface acoustic wave resonator increases as the proportion of the excited surface waves that is confined inside the interdigital electrodes due to multiple reflections increases. Therefore, in the surface acoustic wave resonator of the present invention, the excitation efficiency at both ends of the interdigital electrode is reduced, and conversely, the reflection efficiency at both ends is improved to reduce leakage loss. The resonance characteristic Q of the electrode as a whole is improved, and it is possible to have an instantaneous resonance characteristic.

Various methods can be used to reduce the excitation efficiency as it approaches both ends from the center of the interdigital electrode, but for example, the width of the electrode fingers can be changed! When the pitch of the electrode fingers is set to , and the line width ratio n is set to be above β/, this can be achieved by decreasing the line width ratio n from the center of the interdigital electrode toward both ends. That is, when the line width ratio n is 0.
The excitation efficiency is highest when L is around 5, and the excitation efficiency decreases even if the line width ratio n becomes larger or smaller than this. On the other hand, the smaller the line width ratio n, the higher the reflection efficiency tends to be. Therefore, by decreasing the line width ratio n from the center of the interdigital electrode toward both ends, it is possible to create a state in which the excitation efficiency decreases and the reflection efficiency increases as it approaches both ends. Therefore, the leakage of excited energy from both ends can be reduced, and the resonance characteristic Q can be improved.

By the way, the propagation speed of surface acoustic waves, that is, the sound transport V, is
There is a tendency to change between the surface on which an electrode is formed and the surface on which no electrode is formed, and generally, the surface on which an electrode is formed is different from the surface on which no electrode is formed.

It also slows down the process. Once, when changing the line width ratio n,
Since the proportion of the area where electrodes are formed changes, the sound velocity V partially changes by 1 L. As a result, the frequency expressed by V/2L changes partially, and the resonance characteristic Q changes. This may cause a decrease in the temperature. Therefore, in a preferred embodiment of the present invention, the pitch L is changed so that the frequency becomes constant in response to the partially changing sound speed by changing the line width ratio n. That is, in order to decrease the line width ratio n as it approaches both ends of the interdigital electrode, it is preferable to change the electrode finger width β and also change the pitch L.

"Embodiment of the Invention" FIG. 1 shows an embodiment of a surface acoustic wave resonator according to the present invention.

That is, lithium dioffate with a Y-axis cut rotated by 41 degrees is used as the piezoelectric substrate 1, and a thickness of 100 mm is formed on the piezoelectric substrate 1.
An A1 film of OA was formed, and interdigital electrodes 3 were formed using a normal photolithography technique.

The total number of electrode fingers 2 constituting the interdigital electrode 3 is 10.
One electrode (50,5 pairs) was used, and the pitch at the center of the interdigital electrodes 3 was 5 um. Note that the resonant frequency is approximately 42
It was set to 0MHz.

Then, the line width ratio n (β/L) of the interdigital electrode 3 was set to 0.5 at the center, and was variously changed so that it became smaller toward both ends, and its resonance characteristics were measured. That is, the line width ratio n was changed from 0.5 to 0.4 from the center toward both ends (Example 1), and the line width ratio n was changed from 0.5 to 0.4 from the center toward both ends (Example 1). (Example 2), the line width ratio n was changed from 0.5 to 0.2 from the center toward both ends (Example 3), and the line width ratio n was all 0.3 (Example 3). Four types of surface acoustic wave resonators were created, including one with a diameter of 5 (conventional example). In this case, in the embodiment, since the speed of sound changes in accordance with a change in the line width ratio n, the pitch L is widened toward both ends so that the frequency is kept constant. Therefore, the changes in the line width ratio n in the examples were adjusted by changing the electrode finger width β and also by changing the pitch.

FIG. 2 shows the change in the resonance characteristic Q when the line width ratio n1Fr is changed as described above.

In other words, the line width ratio n decreases to 0 from the center to both ends.
．． In the case where the line width ratio n was changed from 5 to 0.4 (Example 1), the resonance characteristic Q was approximately 1.3 times higher than that in which the line width ratio n was kept constant at 0.5 (conventional example). .

Also, the line width ratio n is 0.5 from the center to both ends.
In the case where the line width ratio n was changed to ~0.3 (Example 2), the line width ratio n
The resonance characteristic Q is approximately 1.4 times higher than that of the conventional example in which Q is kept constant at 0.5.

Roughly speaking, the line width ratio n increases from the center to both ends.
In the case where the line width ratio n was changed from 5 to 0.2 (Example 3), the resonance characteristic Q was approximately 1.4 times higher than that in which the line width ratio n was kept constant at 0.5 (conventional example). .

"Effects of the Invention" As described above, in the surface acoustic wave resonator according to the present invention, the excitation efficiency of the surface acoustic wave is high at the center of the interdigital electrode and becomes lower toward the ends. As a result, leakage loss at the end portion is reduced, and the resonance characteristic Q is improved.

[Brief explanation of the drawing]

Figure 1 is a schematic configuration diagram of the surface acoustic wave resonator according to the present invention, Figure 2 is a chart showing changes in resonance characteristics Q when changing the linewidth ratio n, and Figure 3 is a diagram of the conventional surface acoustic wave resonator. FIG. 2 is a schematic configuration diagram of a surface acoustic wave resonator.

Claims (3)

[Claims] (1) An elastic surface in which an interdigital electrode consisting of a plurality of electrode fingers is provided on a piezoelectric substrate on which surface acoustic waves propagate, and the surface acoustic waves excited by each electrode finger are multiple reflected inside the interdigital electrode. 1. A surface acoustic wave resonator, characterized in that the excitation efficiency of the surface acoustic wave is large at the center of the interdigital electrode and becomes smaller as it approaches both ends of the interdigital electrode. (2) In claim 1, when the width of the electrode fingers is l, the pitch of the electrode fingers, that is, the sum of the electrode finger width and the electrode finger interval is L, and the line width ratio n is l/L. , a surface acoustic wave resonator in which the line width ratio n decreases from the center to both ends of the interdigital electrode. (3) In claim 2, the surface acoustic wave is a surface acoustic wave in which the pitch L is changed so that the frequency becomes constant in response to the sound speed that partially changes by changing the linewidth ratio n. resonator.