JPH0322703A - Saw resonator - Google Patents

Abstract

PURPOSE:To obtain a single spectrum by setting only one state as the reflection state of two elastic surface waves that a reflector has and unifying only main resonance as the resonance state. CONSTITUTION:An electrode pattern consists of a piezoelectric material flat plate 100, an inter-digital electrode(IDT) 101, the reflector 102, the positive and negative electrodes 103 and 104 of the IDT 101, conductor strips 105 and 106 which perform the reflecting operation of the reflector 102, and a connecting conductor 107 for connecting the conductor strips 105 and 106. Here, the IDT 101 consists of the inter-digital electrode which has a period lambda/4, where lambda is the wavelength of the elastic surface wave and the reflector 102 consists of the conductor strip groups 105 and 106 having a period lambda/2. The reflecting structure having the period lambda/2 is used, so only a stress wave of main resonance is reflected. Consequently, the single-spectrum resonator is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a SAW resonator using surface acoustic waves.

[Conventional Technique Wl] FIG. 4 shows an example of the electrode structure of a conventional SAW resonator. The names of the parts in the diagram are: 40o is a piezoelectric flat plate;
1 is an IDT, 402 is a reflector, 405 and 404 are each an IDT.
The positive and negative electrodes of the DT, 405 and 406, etc. are conductor strips that act as a reflector, and 407 is a connecting conductor of the conductor strips. Figure 4 is an enlarged view of a part of the electrode pattern on one side of a conventional SAW resonator, an IDT (abbreviation for interdigital transducer) that excites and receives surface acoustic waves, and a pair of reflectors. This is what is illustrated. IDT (101 ) and a reflector (102). The IDT is made by arranging interdigital electrodes each having a conductor width of λ/4 at a pitch of λ/4. However, λ
is the wavelength λ of the surface acoustic wave that generates vibrational displacement when the SAW resonator is in operation. λ is approximately equal to four times the width of the IDT electrode. FIG. 4 408 shows λ-th. Furthermore, the length Lg between points P and S is determined by the IDT and the reflector. Resonance occurs and SA
When constructing an oscillator and a filter using a W resonator, there are problems such as abnormal oscillation and the inability to obtain an amount of operational attenuation outside the band of the filter.

In FIG. 10, the resonance characteristics of the SAW resonator are illustrated using the frequency on the horizontal axis and the current passing through the resonator on the vertical axis. In the figure, the large resonance at frequency @f1 is the main resonance (100
0), and a small resonance of frequency f2 (1 001'
I is a sub-resonance, and the sub-resonance is unnecessary. SAW
Depending on the design conditions of the resonator, the main resonance and the sub-resonance may have the same magnitude. The causes of these two resonance states will be explained with reference to FIGS. 8 and 9. Figure 8 is S
Distance between the IDT of the AW resonator and the reflector GM I' g
Figure 9 shows the case of Lg=λ/2.Both figures show the SAW resonance in the series resonance frequency f>-ft in the main resonance and sub-resonance states. The vibrational displacement state of the child is expressed as the stress wave F of the surface acoustic wave.First, the 8th
In the figure, 800 is a piezoelectric flat plate, 801 and 802 are IDT interdigital electrodes with a width of approximately λ/4, and 803 and 80
4,805 is a conductor strip having a conductor width of approximately λ/4 of the reflector. Solid line 806 is the stress wave state of the main resonance, broken line 8
07 is a stress wave state of sub-resonance. Next, in Figure 9, 90
0 is a piezoelectric flat plate, 901 and 902 are IDs similar to those in Fig. 8.
Similarly, interdigital electrodes 903, 904, and 905 are conductor strips of the reflector. A solid line 906 indicates a state of stress waves corresponding to main resonance, and a broken line 907 indicates a state of stress waves corresponding to sub-resonance. Subresonance occurs in both cases of Fig. 8 and Fig. 9. occurs at a size that cannot be ignored. Therefore, the present invention is intended to solve these problems, and the purpose is to manufacture a SAW resonator that has no sub-resonance but is negligibly small, and to create a stable SAW oscillator with a single spectrum and no spurious. The purpose of the present invention is to provide the market with a SAW filter that can have a large amount of operational attenuation outside the passband.

[Means to solve the problem] The SAW resonator of the present invention is characterized by the following items.

(11) In a cavity type SAW resonator formed on the surface of a piezoelectric flat plate, an IDT that transmits and receives surface acoustic waves, and a pair of reflectors that reflect the surface acoustic waves on both sides of the IDT. , the IDT is comprised of interdigital electrodes with a period of λ/4, where λ is the wavelength of surface acoustic waves, and the reflector is comprised of a reflective structure with a period of λ/2 or between λ/2 and λ/4; 2) Also, the reflecting structure of the reflector with a period of λ/2 is composed of a group of conductor strips with a width of approximately λ/2; (3)
In addition, the reflection structure of the reflector with a period of λ/2 is approximately λ/2
(4) The distance Lg between the IDT and the reflector is approximately Lg = λ/4, where λ is the wavelength of the surface acoustic wave; 5) Also, the distance Lg between the IDT and the reflector is approximately Lg=λ/2, where λ is the wavelength of the surface acoustic wave.

[Function] According to the above configuration of the present invention, the reflection state of the two surface acoustic waves of the conventional reflector can be reduced to only one state, and as a result, SAW#:. The resonance state of the pendulum can be unified to only the main resonance.

The mechanism by which a conventional SAT resonator has two resonance states can be explained with reference to FIGS. 8 and 9. The state of reflection of the surface acoustic wave in the reflector is such that the end of the conductor strip (IJ) that performs each reflection operation becomes a node of the stress wave. Therefore, two resonant conditions occur corresponding to the two ends of the conductor strip. In the present invention, the width of the conductor strip of the reflector is set to 17 times the wavelength λ of the surface acoustic wave.
2, it is possible to limit the state in which the node of the stress wave of the surface acoustic wave comes to the end of the conductor strip that performs the reflection operation to be unique. As a result, only a single resonant state can be achieved.

[Example] FIG. 1 is a detailed diagram of an electrode pattern in a SAW resonator of the present invention. The names of each part in the figure are: 100 is a piezoelectric flat plate, 10101 is an IDT within a dotted line, 10201 is a reflector within a dotted line, 105 and 104 are the positive and negative electrodes of the interdigital electrodes of the IDT, and 105 and 106 are reflectors. 107 is a connecting conductor for connecting 105, 106, etc. The electrode width Wt of the interdigital electrodes of the IDT 101 is set to approximately 1/4 of the wavelength λ of the surface acoustic wave excited in the IDT, and the pitch between the electrode fingers is also approximately λ/4. In this case, the period of the interdigital electrodes of the IDT is 108 λ in the figure. Usually, λT is set to a value very close to the above-mentioned λ. Next, a reflector is a structure for reflecting the surface acoustic waves excited in the INT and propagating from side to side perpendicular to the interdigital electrodes to form a resonator. The conductor strip I that has a reflective action
The electrode width WR of Jp is configured to be λ/2 compared to the conventional λ/4. Also, the picture between conductor strips is approximately λ/2.
shall be. Further, the distance dLg between the IDT and the reflector is set to 174 of λ. (Length between points P and S in Figure 1).

Next, a method for forming the above-mentioned IDT and reflector will be described. First, 100 piezoelectric flat plates are made of quartz and LiTaO.
, , LimbO, etc. are sliced into flat plates and their surfaces are mirror-polished to finish. Next, a thin film of a conductive metal such as At, Au, or O is formed by vapor deposition or sputtering on the surface of the piezoelectric flat plate, and then an electrode pattern is formed by photo-etching. Typically, the number of conductor strips that a reflector has is in a large array of ioo to 1000 conductor strips. The principle of reflection of surface acoustic waves in a reflector is that a reflection structure (such as a conductor strip) is placed periodically on the propagation path of the surface acoustic wave, and the reflection occurs at the end of the reflection structure different from the piezoelectric surface. A mismatch in acoustic impedance causes small wave reflections, and when a large number of them gather together, total reflection of the surface acoustic wave occurs. Due to the nature of surface acoustic waves, the wave energy is localized within about one wavelength from the surface, so the acoustic impedance mismatch mentioned above can be solved by slightly etching the surface of the piezoelectric flat plate to create long and narrow grooves. It can also be configured by arranging them.

FIG. 11 is a cross-sectional view of a reflector in which this method is implemented in accordance with an example of the present invention. In the figure, 1100 is a piezoelectric flat plate, 110
1 and 1102 are IDT interdigital electrodes, 1103 and 11
04 etc. is an array of grooves with a radiation of λ/2 configured as a reflector. Further, the length Lg is the distance between the IDT and the reflector and takes a value of approximately λT/4 or λt/2.

Next, FIG. 2 shows another embodiment of the present invention. Since FIG. 2 is the same as FIG. 1 of the present invention except that the distance Lg between the IDT (201) and the reflector (202) is L=λT/2, the explanation will be omitted.

Next, FIG. 3 shows another embodiment of the reflector used in the SAW resonator of the present invention. In the figure, 3oO is a piezoelectric flat plate, 50
1 and 302 are both reflectors, but 301 is a reflector with an electrode width and an inter-electrode pitch of λ/2, while 502 is a reflector with an electrode width and an inter-electrode pitch of λ/4. Also 304 and 3
05, 507, 308, etc. are conductor strips that perform a reflective operation, and 305, 306, etc. are connection conductors for connecting the conductor strips. The distance L between both reflectors is set to Ls=λ/2 in order to achieve phase matching of the vibrational displacement that exists between both reflectors. The reason why pitches λ/2 and λ/4 are used together as shown in FIG. 3 is that the reflection operation of the reflector 3010 with pitch λ/2 becomes a second-order mode reflection for the wavelength λ, which slightly degrades the reflection performance. Picture λ/4
The purpose of this reflector is to compensate for this drawback. In this way, the Q value of the main resonance of the SAW resonator can be maintained large, and the magnitude of the sub-resonance can be suppressed to a small value.

The sub-resonance 1002 indicated by the broken line in FIG. 10 described above showed improved resonance characteristics.

Finally, the embodiments of the present invention are shown in FIGS. 1, 2, and 3.
The state of vibrational displacement when the resonator is in a resonant state is shown as a stress wave r of a surface acoustic wave. FIGS. 5, 6, and 7 show states of stress waves subjected to vibrational displacement corresponding to FIGS. 1, 2, and 3, respectively. In the figure, 500, 600 and 7
00 etc. are piezoelectric flat plates, 501, 502, 601, 6
02 is the interdigital electrode of IDT, 503, 504, 605,
604, 701 to 704, etc. are conductor strips of the reflector. The stress wave corresponding to the vibration of the main resonance is indicated by the solid line 505
, 605, 705, and the stress waves corresponding to the vibrations of the sub-resonance are indicated by broken lines 506, 606, 707. In both figures, in the reflector with a pitch of λ/2, the subresonant stress wave exhibits a displacement state that becomes a node at the center of the conductor strip for reflection, and does not satisfy the reflection phase condition of the reflector. Therefore, no wave reflection occurs. Therefore, waves are transmitted and no resonance phenomenon occurs. In FIG. 7, the sub-resonant stress waves are reflected only by λ/4 reflector sections such as 705 and 704.

[Effects of the Invention] As described above, according to the present invention, by using a reflection structure with a period of λ/2, only the stress wave of the main resonance of the SAW resonator is reflected, and a resonator with a single spectrum is generated. realizable.

Furthermore, if reflection structures with periods λ/2 and λ/4 are used together, the Q value of the main resonance can be maintained high and the size of the sub-resonance can be suppressed to a small level, so oscillation at the sub-resonance in the SAW oscillator can be suppressed. In addition to preventing this, the degree of freedom in designing a SAW filter formed by connecting a large number of SAW resonators is increased, and the pass characteristics can be improved, which will bring many benefits in the future.

103, 104... Interdigital electrodes 105, 106
・・・・・・Conductor strip

[Brief explanation of drawings]

FIG. 1 is a detailed view of the electrode pattern in one embodiment of the SAW resonator of the present invention, FIG. 2 is a diagram similar to FIG. 1 in another embodiment of the present invention, and FIG. 3 is a diagram of the present invention. A detailed view of the electrode nozzle of the reflector included in the SAW resonator of the invention, FIG.
Detailed diagrams of electrode nozzles of the AW resonator, and FIGS. 5, 6, and 7 are displacement state diagrams shown by stress waves during resonance shown in FIGS. 1, 2, and 3 of the present invention. , Fig. 8 and Fig. 9 are displacement state diagrams shown by stress waves during resonance of a conventional SAW resonator, Fig. 10 is a resonance characteristic diagram shown by a SAT resonator, and Fig. 11
The figure is a sectional view of a reflector structure included in the SAW resonator of the present invention. 1ro0・・・・・・・・・・・・Piezoelectric flat plate 1
0 1 ・・・・・・・・・・・・・・・ Engineering DT or higher

Claims (5)

[Claims] (1) I that transmits and receives surface acoustic waves on the surface of a piezoelectric flat plate
DT, and 1 that reflects the surface acoustic wave on both sides of the IDT.
In a cavity type SAW resonator formed of a pair of reflectors, the IDT is composed of interdigital electrodes with a period of λ/4, where the wavelength of the surface acoustic wave is λ, and the reflector has a period of λ/2. Alternatively, a SAW resonator comprising reflective structures of λ/2 and λ/4. (2) The reflection structure of the reflector with a period of λ/2 is approximately λ/2
2. The SAW resonator according to claim 1, wherein the SAW resonator comprises a group of conductor strips having a width. (3) The reflective structure of the reflector with a period of λ/2 is approximately λ/2
2. The SAW resonator according to claim 1, wherein the SAW resonator comprises an array of grooves formed on a surface of a piezoelectric material having a width. (4) The SAW resonator according to claim 1, wherein the distance Lg between the IDT and the reflector is approximately Lg=λ/4, where λ is the wavelength of the surface acoustic wave. (5) The SAW resonator according to claim 1, wherein the distance Lg between the IDT and the reflector is approximately Lg=λ/2, where λ is the wavelength of the surface acoustic wave.