JPH02198211A - Surface acoustic wave resonator - Google Patents

Abstract

PURPOSE:To improve the design accuracy of a surface acoustic wave resonator by providing an interdigital transducer in which interdigital electrodes having plural parallel electrode fingers mesh with each other to one side of a piezoelectric substrate and a surface acoustic wave reflector located aside thereto and connecting the electrode finger closest to the reflector to the reflector. CONSTITUTION:Interdigital electrodes 2, 3 having plural parallel electrode fingers 2a, 3a are formed interdigitally to one face of a piezoelectric substrate 1 made of a piezoelectric single crystal such as LiTaO3, LiNbO3, Li2B4O7 or crystal to form an interdigital electrode 4. Then grating reflectors 5, 5' comprising interdigital electrodes connecting both ends of plural parallel metallic strips are arranged to clip the electrode 4. Then the electrode 4 and the reflector 5 are formed by evaporating aluminum on the substrate 1 with selective etching. Then the electrode finger closest to the reflector 5 in the electrode fingers 2a, 3b is connected to the reflector 5 via connection metallic patterns 7, 7'. Thus, the parasitic capacitor is reduced and the insertion loss is reduced.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to surface acoustic wave technology and also to the electrode structure of a surface acoustic wave device, and is most effective when applied to, for example, a grating reflector of a surface acoustic wave resonator. Regarding technology.

[Prior art] LiTaO3, LiNb0. , Li2B4O7, piezoelectric single crystals such as quartz, and piezoelectric thin films such as Zn○ are used as one of the applications of surface acoustic wave devices that utilize the piezoelectricity of
There is a surface acoustic wave resonator.

As shown in FIG. 6, the surface acoustic wave resonator includes two comb-shaped electrodes 2 and 3 having a plurality of parallel electrode fingers 2a and 3a on one main surface of a piezoelectric substrate 1 made of a piezoelectric single crystal. Grating reflectors (hereinafter simply referred to as reflectors) 5 made of metal film strips such as aluminum, groups (grooves) of piezoelectric substrates, etc. are arranged on both sides of IDT (interdigital transducer) TLLs 12 which are interlocked with each other. However, there is a resonator with high resonance sharpness (Q) by generating a standing wave of surface acoustic waves between the reflectors 5 ("Nikkei Electronics J, February 15, 1982 issue, No. 185"). Pages 189 to 189). [Problems to be Solved by the Invention] In the surface acoustic wave resonator as described above, the reflector 5 is disposed adjacent to the DT electrode. If it consists of the surface of a piezoelectric substrate or an embedded metal strip (usually, each metal strip is electrically connected to each other), the leakage electric field from the IDT electrode 4 causes a stray capacitance between it and the reflector 5. There is a problem that occurs.

That is, in the surface acoustic wave resonator, as shown in FIG.
Since V is excited by applying the ground potential GND to the other comb-shaped electrode 3, it is in an electrically floating state with the outermost electrode finger 2a of the IDT electrode 4. A stray capacitance C□ is generated between the reflector 5 and the reflector 5. Furthermore, a metal stem 6 connected to a ground point is provided on the back surface of the piezoelectric substrate 1 as shown in FIG. 7, and the stray capacitance C2 between this metal stem 6 and the reflector 5 is also
The stray capacitance with the ground point is increased via the stray capacitance C□.

As described above, when the parasitic capacitance of the IDTtlt pole 4 increases, the insertion loss between the input and output transducers increases when two or more sets of IDT electrodes are used. When used, there is a disadvantage that the variable range of the oscillation frequency becomes narrow. Moreover, the stray capacitance C□ between the IDT electrode 4 and the reflector 5 changes depending on the aperture length and the distance between the electrode finger 2a or 3a and the reflector 5, and the stray capacitance C□ between the reflector 5 and the metal stem 6 The capacitance C2 also changes depending on the number of reflector strips, aperture length, line width ratio, substrate distortion, etc.

Therefore, when designing a surface acoustic wave resonator, the stray capacitances C1 and C2 must also be calculated as one of the parameters, but the estimation of the stray capacitances is very complicated and has poor accuracy. As a result, conventional surface acoustic wave resonators have the problem that it is difficult to achieve desired characteristics with high precision, and that when the number of reflector strips is increased or decreased, the characteristics fluctuate greatly.

This invention was made in view of the above-mentioned problems, and its purpose is to improve the design accuracy of surface acoustic wave resonators and to further reduce fluctuations in characteristics due to design changes. .

[Means for Solving the Problems] In order to achieve the above object, the present invention forms an interdigital transducer formed by interlocking comb-shaped electrodes having a plurality of parallel electrode fingers on one main surface of a piezoelectric substrate, and In a surface acoustic wave resonator in which a surface acoustic wave reflector is arranged adjacent to an interdigital transducer,
Among the plurality of parallel electrode fingers, the parallel electrode finger closest to the surface acoustic wave reflector and the electrode of the reflector are connected by a connection pattern.

[Operation] According to the above-described means, the potential of the outermost electrode finger of the IDT fl pole (on the side closer to the reflector) and the potential of the electrode of the reflector are the same, so that the potential between the IDT electrode and the reflector becomes the same. The stray capacitance of the surface acoustic wave resonator is reduced, and the parasitic capacitance of the IDT electrode can be reduced, thereby increasing the design accuracy of the surface acoustic wave resonator and further reducing fluctuations in characteristics due to design changes. In addition, the insertion loss of the surface acoustic wave resonator can be reduced, and the resonant frequency can be increased to a higher band.

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

In the embodiment of FIG. 1, LiTa0. , LiNbo
3. A plurality of parallel electrode fingers 2a, 3a are arranged on one main surface of the piezoelectric substrate 1 made of piezoelectric single crystal such as Li2B4O or quartz.
The comb-shaped electrodes 2 and 3 are formed so as to mesh with each other, and the IDT? It constitutes the u-pole 4. A grading reflector 5.5' is arranged on one main surface of the piezoelectric substrate 1 so as to sandwich the IDT electrode 4. The grading reflector 5.5' is made of an interdigital electrode formed by connecting both ends of a plurality of parallel metal strips. The IDT electrode 4 and reflector 5 are formed by depositing aluminum on the substrate 1 and patterning it by selective etching.

In this embodiment, among the electrode pattern constituting the reflector 5 and the plurality of electrode fingers 2a and 3b of the comb-shaped electrodes 2 and 3 forming the IDT electrode 4 facing the reflector, this reflector is 5 and the electrode finger closest to the IDT electrode 4 are connected by a connecting metal pattern 7.7''. With this structure, the potential of the reflector 5 and the electrode finger of the IDT electrode 4 that is closest to the reflector are connected by the connecting metal pattern 7.7''. Since the potentials of the electrode fingers on the nearby side match, the stray capacitance becomes invisible and the parasitic capacitance of the IDT electrode 4 is reduced.

The connection metal pattern 7 is connected to the IDT electrode 4 and the reflector 5.
It can be formed at the same time as the aluminum electrode that is formed.

Furthermore, in the surface acoustic wave resonator of the embodiment shown in FIG.
By connecting the comb-shaped electrode 3, which is connected to the electrode constituting the reflector 5 through the connection metal pattern 7, to the ground point as shown by the broken line in the figure, the IDT electrode 4
parasitic capacitance can be reduced. That is, since the metal stem 6 (see FIG. 7) provided on the back surface of the piezoelectric substrate 1 is at ground potential, by fixing one reflector 5 to the ground potential, the reflector 5 and stem 6 in FIG. The capacitance C2 between them can be made zero. As a result, the ID
The parasitic capacitance of the T electrode 4 can be further reduced.

FIG. 2 shows an embodiment in which a pair of comb-shaped electrodes 2 and 3 constituting an IDTffi pole 4 have the same number of electrode fingers 2a and 3a. When the comb-shaped electrodes 2 and 3 have the same number of electrode fingers, the electrode fingers closest to the reflector 5 are different from each other. That is, in FIG. 2, the comb-shaped electrode 3 has its electrode fingers closest to the reflector 5 on the left, and the comb-shaped electrode 2 has its electrode fingers closest to the reflector 5' on the right. Even in such a case, if the electrode constituting the reflector 5' or 5 and the electrode finger 2a or 3a closest to it are
By connecting each with the connecting metal patterns 7a and 7b,
The parasitic capacitance of the IDT electrode 4 can be reduced.

Further, FIGS. 3 and 4 show an embodiment in which the present invention is applied to a surface acoustic wave resonator in which two or more IDT electrodes 4 are provided.

In these embodiments, the electrode constituting the left reflector 5 is connected to the ID closest to it by the connecting metal pattern 7.
The electrodes constituting the reflector 5' on the right side are connected to the electrode fingers of the comb-shaped electrode 3 of the T-electrode 4 by the connection metal pattern 7', and the electrodes of the comb-shaped electrode 3'' of the T-electrode 4' are connected to the electrode fingers of the comb-shaped electrode 3 of the T-electrode 4'. Furthermore, in the embodiment of FIG. 4, the electrodes constituting the central reflector 5'' are connected to the electrode fingers of the IDT electrodes 4' and 4'' by connecting metal patterns 7'' and 7'', respectively. The embodiments shown in FIGS. 3 and 4 are also connected in the same manner as in the first embodiment.

By connecting the comb-shaped electrode 3 on the side to which the reflector 5 is connected to the ground point, the stray capacitance Jft between the reflector and the stem is reduced.
In addition to being able to eliminate C2, a modification similar to that shown in FIG. 2 can be considered.

Furthermore, conventionally, in order to eliminate the stray capacitance that occurs between the IDT electrodes in a surface acoustic wave resonator that has multiple IDT electrodes, a shield electrode (at ground potential) is provided between the IDT electrodes, and the stray capacitance that occurs between the IDTffi electrodes is A technique has been proposed to release stray capacitance through a shield electrode, but in the embodiments shown in FIGS. Since it performs the same function as a conventional shield electrode, there is no need to provide a shield electrode, and the surface acoustic wave resonator can be made smaller accordingly.

FIG. 5 shows still another embodiment of the present invention.

These embodiments are applied to a surface acoustic wave resonator in which the IDT electrode 4 is a so-called avoidized electrode in order to reduce transverse mode spurious.

Of these, the embodiment shown in FIG. 5 is similar to the embodiments shown in FIGS. are connected by a connecting metal pattern 7.

In the above embodiment, it has been explained that the electrodes constituting the reflector 5 are formed of a vapor-deposited aluminum layer, but the present invention is not limited thereto. It can be applied not only to the case where the electrode is formed, but also to the case where it is a conductive buried electrode formed by diffusing metal into the surface of the piezoelectric substrate by ion implantation or the like.

When the reflector 5 is a buried electrode, the connection metal pattern 7 in the above embodiment may be a buried electrode or a vapor-deposited metal film.

[Effects of the Invention] As explained above, the present invention forms an interdigital transducer in which comb-shaped electrodes having a plurality of parallel electrode fingers are interlocked on one main surface of a piezoelectric substrate, and In a surface acoustic wave resonator in which a surface acoustic wave reflector is arranged, the parallel electrode finger on the side closest to the surface acoustic wave reflector among the plurality of parallel electrode fingers is connected to the electrode of the reflector. Since the connection is made using the pattern, the design accuracy of the surface acoustic wave resonator can be improved, and the fluctuation in characteristics due to design changes can be reduced. In addition, it has the effect of reducing insertion loss and increasing the resonant frequency band. Additionally, since the connection pattern is connected to the ground point, stray capacitance between the reflector and stem can be eliminated, which further reduces parasitic capacitance of the IDT electrode and insertion loss. can.

[Brief explanation of the drawing]

FIG. 1 is a plan view showing a first embodiment of a surface acoustic wave resonator according to the present invention, and FIG. 2 is a plan view showing a modification thereof. 3 is a plan view showing a second embodiment of the surface acoustic wave resonator according to the present invention; FIG. 4 is a plan view showing a third embodiment of the surface acoustic wave resonator according to the present invention; FIG. 6 is a plan view showing a fourth embodiment of a surface acoustic wave resonator according to the present invention, and FIG. 6 is a plan view showing an example of a conventional surface acoustic wave resonator. FIG. 7 is an explanatory cross-sectional view thereof. 1...Piezoelectric substrate, 2a, 3a...electrode fingers, 2゜3...comb-shaped electrode, 4...IDT electrode, 5...
... Grating reflector, 7... Metal pattern for connection. Figure 4 Figure 5 111)-11 Figure Figure 5

Claims (2)

[Claims] (1) An interdigital transducer is formed by interlocking interdigitated electrodes having a plurality of parallel electrode fingers on one main surface of a piezoelectric substrate, and a surface acoustic wave reflector is disposed adjacent to the interdigital transducer. In the surface acoustic wave resonator, the parallel electrode finger on the side closest to the surface acoustic wave reflector among the plurality of parallel electrode fingers and the electrode of the reflector are connected by a connection pattern. surface acoustic wave resonator. (2) The surface acoustic wave resonator according to claim 1, wherein the connection pattern is connected to a ground point.