JPH01311714A - Surface acoustic wave multiple mode filter - Google Patents

Abstract

PURPOSE:To obtain a surface acoustic wave multiplex mode filter with excellent pass band characteristic by forming a means interrupting elastic coupling between resonators. CONSTITUTION:Two resonators comprising an IDT 11 and a reflector 12 arranged at both sides of the IDT 11 are arranged on the surface of a piezoelectric substrate 10 made of crystal or the like. Both ends of each electrode of the reflector are connected. A zinc oxide thin film extended in the propagation direction of a surface acoustic wave is prolonged between the two resonators. The zinc oxide thin film 15 is formed as long as possible between the resonators including a conductor pattern connecting the two IDTs. Moreover, the film 15 may be formed also at the end face of the substrate of the resonator not only between the resonators. Thus, the band pass characteristic as the filter is improved considerably.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to a surface acoustic wave filter constructed by arranging a plurality of resonators, such as a surface acoustic wave multimode filter in which a plurality of resonators are elastically coupled. It is something.

[Prior art]

Two surface acoustic wave (SAW) resonators are placed close to each other,
When they are elastically coupled, two modes of vibration occur. There are two vibration modes, a symmetric mode and an anti-symmetric mode, and a dual mode SAW filter is constructed by matching the resonant frequency of the symmetric mode and the anti-resonant frequency of the anti-symmetric mode.

There are also types that generate two vibration modes within a single resonator, and there are also multimode filters that have a plurality of such resonators.

FIG. 7 is a plan view of an example of the dual mode SAW filter, in which interdigital electrodes are formed on the surface of a substrate 70 such as crystal.
A resonator is configured by the IDT 71 and a reflector 72 sandwiching it, and another resonator is arranged in a direction perpendicular to the SAW propagation direction.

It is also possible to configure a multimode filter by arranging three or more vibrators. Such a multimode resonator type surface acoustic wave filter is described in the Institute of Electronics and Communication Engineers Ultrasound Research Group Material U.
It is shown in S-77-33 etc. There is also a two-boat type in which two interdigital electrodes are arranged in each resonator.

Furthermore, in order to obtain desired characteristics, there are various filters in which a plurality of IDTs are connected in series or in parallel.

〔assignment〕

In the above-mentioned surface acoustic wave multimode filter and other filters, if the resonators are placed close to each other, the eighth
As shown by a solid line 81 in the figure, spurious signals are generated on the second side (lower side depending on the type) of the pass band. This is because a necessary elastic coupling occurs between the resonators, and moat recombination occurs.

In general, the spacing of the resonators is determined to prevent the above-mentioned recombination.
It is said that about 10 times the wavelength of surface acoustic waves is required. However, increasing the spacing between the resonators causes the problem that the chip size also increases.

The present invention aims to solve these problems and obtain a surface acoustic wave multimode filter with good passband characteristics by cutting the elastic coupling.

Furthermore, it is an attempt to efficiently cut bonds using the minimum space.

[Means to solve the problem]

The present invention solves the above problems by forming means for breaking the elastic coupling between the resonators.

That is, in a surface acoustic wave multimode resonator filter in which a plurality of resonators each consisting of an interdigital electrode and a reflector sandwiching the interdigital electrodes are arranged on the surface of a piezoelectric substrate, surface acoustic waves are generated on the substrate surface between the resonators. It is characterized by having a zinc oxide thin film extending in the direction of propagation.

Grooves may be formed in the substrate instead of the zinc oxide thin film. Moreover, the effect can be further improved by using the zinc oxide thin film and the grooves together.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a plan view showing an embodiment of the present invention.

Two resonators each consisting of an IDT II and reflectors 12 placed on both sides thereof are arranged on the surface of a piezoelectric substrate 10 made of crystal or the like. Each electrode of the reflector has a structure in which both ends are connected.

A zinc oxide thin film 15 extending in the direction of propagation of the surface acoustic waves extends between the two resonators. This zinc oxide thin film 15 is preferably formed as long as possible between the resonators, including on the conductor pattern connecting the two IDTs.

Further, it may be formed not only between the resonators but also on the end face side of the substrate of the resonator.

FIG. 2 is a side view of the example shown in FIG.

The thickness of the zinc oxide thin film 15 of about 1 micrometer is sufficiently effective.

By forming a zinc oxide thin film, not only the elastic bond is broken, but also the piezoelectric activity can be reduced.

FIG. 3 is a plan view showing another embodiment of the invention. I
The DT 31 and reflector 32 are the same as in the previous example, but a groove 36 is formed between the resonators.

FIG. 4 is a side view thereof, and the depth of the groove 36 is sufficient to be about the wavelength of the surface acoustic wave.

FIG. 5 shows another embodiment of the invention, in which groove 56 is filled with a zinc oxide thin film 55. In FIG. This allows the groove,
Since both zinc oxide thin films contribute to breaking the elastic bond, the groove depth can also be reduced. The zinc oxide thin film does not have to fill the entire groove.

Alternatively, as shown in FIG. 6, a zinc oxide thin film 65 may be formed on both sides of the groove 66.

Note that the present invention can be used not only for filters having the above structure, but also for general surface acoustic wave filters that use a plurality of other transducers. It can also be used for weighted or divided IDTs.

〔effect〕

According to the present invention, bandpass characteristics as a filter are significantly improved. By reducing the elastic coupling between the transducers, it is possible to prevent the generation of spurious signals, as indicated by the dashed line 82 in FIG.

Further, the means for this purpose can be formed in a small space between the resonators, and the resonators can be arranged close to each other, which is advantageous in terms of miniaturization.

[Brief explanation of the drawing]

FIG. 1 is a plan view showing an embodiment of the present invention, FIG. 2 is a side view thereof, FIG. 3 is a plan view showing another embodiment, FIG. 4 is a side sectional view thereof, and FIG. FIG. 6 is a side sectional view showing another embodiment of the present invention, and FIG. 7 is a plan view of a conventional multimode filter. FIG. 8 is an explanatory diagram comparing the characteristics of a conventional multimode filter and a multimode filter according to the present invention.

Claims (7)

[Claims] (1) In a surface acoustic wave multimode resonator filter in which a plurality of resonators each consisting of an interdigital electrode and a reflector sandwiching the interdigital electrodes are arranged on the surface of a piezoelectric substrate, surface acoustic wave A surface acoustic wave multimode filter comprising a zinc oxide thin film extending in the direction of propagation. (2) The surface acoustic wave multimode filter according to claim 1, wherein the interval between the resonators is 10 times or less the wavelength of the excited surface acoustic wave. (3) In a surface acoustic wave multimode resonator filter in which a plurality of resonators each consisting of an interdigital electrode and a reflector sandwiching the interdigital electrodes are arranged on the surface of a piezoelectric substrate, surface acoustic wave A surface acoustic wave multimode filter characterized by having grooves extending in the propagation direction of the surface acoustic wave. (4) The surface acoustic wave multimode filter according to claim 3, wherein the spacing between the resonators is 10 times or less the wavelength of the excited surface acoustic wave. (5) The surface acoustic wave multimode filter according to claim 3, wherein the depth of the groove is greater than or equal to the wavelength of the surface acoustic wave. (6) In a surface acoustic wave multimode resonator filter in which a plurality of resonators each consisting of interdigital electrodes and reflectors sandwiching the interdigital electrodes are arranged on the surface of a piezoelectric substrate, surface acoustic wave a groove extending in the direction of propagation;
A surface acoustic wave multimode filter characterized in that the grooves are filled with zinc oxide. (7) In a surface acoustic wave multimode resonator filter in which a plurality of resonators each consisting of an interdigital electrode and a reflector sandwiching the interdigital electrodes are arranged on the surface of a piezoelectric substrate, surface acoustic wave a groove extending in the direction of propagation;
A surface acoustic wave multimode filter characterized in that a zinc oxide thin film is formed on both sides of the groove.