JP3390537B2 - Surface acoustic wave filter - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface acoustic wave filter using a surface acoustic wave resonator. [0002] In recent years, portable telephones and cordless telephones have been reduced in size and weight and have been rapidly spreading. A surface acoustic wave filter using a small and lightweight surface acoustic wave resonator is employed as a high-frequency circuit of these portable telephones and cordless telephones. The characteristics of a filter used in a radio unit such as a mobile phone are required to have a low loss in a band and a large amount of attenuation outside a band. As a filter satisfying such requirements, a surface acoustic wave filter shown in FIG. 14 has been proposed. This surface acoustic wave filter is shown in FIG.
As shown in the figure, using the surface acoustic wave resonator A as a series element,
The surface acoustic wave resonators B are connected in parallel in a ladder type using the parallel elements as parallel elements. FIG. 14B shows a frequency characteristic of the impedance of the surface acoustic wave filter.
FIG. 14C shows the frequency characteristics of the insertion loss. A surface acoustic wave resonator which is a series element A of a surface acoustic wave filter has a frequency characteristic of an anti-resonance frequency f2 as shown by a solid line in FIG. When the resonator has the frequency characteristic of the resonance frequency f1 as shown by a broken line in FIG. 14B, the surface acoustic wave filter operates at the frequency f1 as shown in FIG.
4 shows passband characteristics having a passband between f2 and f2. [0005] The specifications of the surface acoustic wave filter in a cellular phone or the like are different since the center frequency and the pass band differ depending on the system. Therefore, the filter design value is changed for each system, the crystal orientation of the piezoelectric substrate is changed, Or to change the material of the conductive substrate itself. Generally, the characteristics of a surface acoustic wave filter tend to be such that a steep filter characteristic cannot be obtained when an attempt is made to widen a pass band. This is inconvenient when is close to each other. Major factor in determining the bandwidth of the surface acoustic wave filter is an electro-mechanical coupling coefficient K ² of the substrate material of the piezoelectric substrate. Therefore, if the substrate material of the piezoelectric substrate is determined, the upper limit of the bandwidth is also substantially determined, and the lower limit of the bandwidth and the steepness of the filter characteristics are also substantially determined. It was difficult to freely control the characteristics of the wave filter. An object of the present invention is to provide a surface acoustic wave filter capable of controlling a filter characteristic such as obtaining a steep filter characteristic regardless of the bandwidth or narrowing the bandwidth. An object of the present invention is to provide a surface acoustic wave filter having a piezoelectric substrate and a plurality of surface acoustic wave resonators formed on the piezoelectric substrate and connected in a ladder form. The surface acoustic wave filter is formed on the piezoelectric substrate, and has an additional capacitance connected in series or parallel to the surface acoustic wave resonator. In the above-described surface acoustic wave filter, the additional capacitance includes a first electrode layer formed on the piezoelectric substrate, a dielectric layer formed on the first electrode layer,
It is desirable to have a second electrode layer formed on the dielectric layer. In the surface acoustic wave filter described above,
The first electrode layer of the additional capacitance is connected to an input / output electrode or ground electrode of the surface acoustic wave resonator, and the second electrode layer is connected to a ground electrode or input / output electrode of the surface acoustic wave resonator. And the additional capacitance is desirably provided in parallel with the surface acoustic wave resonator. In the above-described surface acoustic wave filter, the first electrode layer or the second electrode layer of the additional capacitance is connected to an input / output electrode of the surface acoustic wave resonator, and the additional capacitance is It is desirable to be provided in series with the surface acoustic wave resonator. In the above-described surface acoustic wave filter, the surface acoustic wave resonator has a pair of comb-shaped electrodes facing each other on the piezoelectric substrate, and the pair of comb-shaped electrodes of the surface acoustic wave resonator Wherein the additional capacitance is provided in parallel with the surface acoustic wave resonator, with the pair of comb electrodes serving as electrodes, the dielectric layer serving as a dielectric, and Is desirable. In the above-described surface acoustic wave filter, the dielectric layer is preferably a silicon oxide layer or a silicon nitride layer. In the above-described surface acoustic wave filter, the additional capacitance is formed on the piezoelectric substrate, and a first electrode connected to an input / output electrode of the surface acoustic wave resonator; A second electrode opposed to the first electrode at a predetermined interval, and a dielectric made of the piezoelectric substrate between the first electrode and the second electrode; It is desirable to be provided in series with the surface acoustic wave resonator. According to the present invention, there is provided a surface acoustic wave filter having a piezoelectric substrate and a plurality of surface acoustic wave resonators formed on the piezoelectric substrate and connected in a ladder form. And has an additional capacitance connected in series or parallel to the surface acoustic wave resonator, so that a steep filter characteristic can be obtained regardless of the bandwidth, or the bandwidth can be narrowed to control the filter characteristic. be able to. [0013] In the above-described surface acoustic wave filter, the additional capacitance is formed by the first electrode layer formed on the piezoelectric substrate;
Since it is composed of the dielectric layer formed on the first electrode layer and the second electrode layer formed on the dielectric layer, it can be easily provided on the piezoelectric substrate. In the above-described surface acoustic wave filter, the first electrode layer of the additional capacitance is connected to the input / output electrode or the ground electrode of the surface acoustic wave resonator, and the second electrode layer is connected to the ground electrode or the input electrode of the surface acoustic wave resonator. By connecting to the output electrode, the additional capacitance may be provided in parallel with the surface acoustic wave resonator, or the first electrode layer or the second
The additional capacitance may be provided in series with the surface acoustic wave resonator by connecting the electrode layer to the input / output electrodes of the surface acoustic wave resonator. In the above-described surface acoustic wave filter, the surface acoustic wave resonator has a pair of comb electrodes facing each other on the piezoelectric substrate, and the pair of comb electrodes is provided on the pair of surface electrodes. The device area can be increased by providing the formed dielectric layer, providing an additional capacitance as a pair of comb-shaped electrodes as electrodes, using the dielectric layer as a dielectric, and providing the dielectric layer in parallel with the surface acoustic wave resonator. In the above-described surface acoustic wave filter in which an additional capacitance can be provided without forming the dielectric layer, if the dielectric layer is formed of a silicon oxide layer or a silicon nitride layer, excellent characteristics such as insulation resistance and excellent characteristics can be obtained. In the above-described surface acoustic wave filter, the additional capacitance is formed on the piezoelectric substrate, the first electrode connected to the input / output electrode of the surface acoustic wave resonator, and the first capacitance on the piezoelectric substrate. A second electrode facing the first electrode at a predetermined interval, and a dielectric composed of a piezoelectric substrate between the first electrode and the second electrode to form a new dielectric layer or the like. Without adding, the additional capacitance can be provided in the surface acoustic wave resonator. A surface acoustic wave filter according to a first embodiment of the present invention will be described with reference to FIGS. The surface acoustic wave filter according to the present embodiment includes a plurality of surface acoustic wave resonators S1,
S2, S3, and S4 are connected in a ladder configuration. By connecting an additional capacitor in parallel to a part of the series element and the parallel element, the steepness of the filter characteristic is increased. As shown in FIG. 1A, a first-stage series element A 'has an additional capacitance Cd1' connected in parallel to a surface acoustic wave resonator S1, and a first-stage parallel element B 'has an elastic The additional capacitance Cd2 'is connected in parallel to the surface acoustic wave resonator S2, and the second-stage series element A is constituted only by the surface acoustic wave resonator S3.
The second-stage parallel element B includes only the surface acoustic wave resonator S4. FIG. 1B shows an equivalent circuit of the surface acoustic wave filter according to this embodiment. The equivalent circuit of the first-stage series element A 'includes an equivalent capacitance C1, an equivalent inductance L1, an inter-terminal capacitance Cd1 of the surface acoustic wave resonator S1, and an additional capacitance Cd1' connected in parallel. The equivalent circuit of the element B 'includes an equivalent capacitance C2, an equivalent inductance L2, an inter-terminal capacitance Cd2 of the surface acoustic wave resonator S2, and an additional capacitance Cd2' connected in parallel. Is composed of an equivalent capacitance C3, an equivalent inductance L3, and an inter-terminal capacitance Cd3 of the surface acoustic wave resonator S3. The equivalent circuit of the second-stage parallel element B is equivalent to the equivalent capacitance C4, equivalent inductance L4 of the surface acoustic wave resonator S4. Consists of FIG. 2 shows the frequency characteristics of the surface acoustic wave filter according to this embodiment. FIG. 2A shows the frequency characteristics of the impedance, and FIG. 2B shows the frequency characteristics of the insertion loss. 2 (a) and 2 (b), the solid line indicates the characteristic when no capacitance is added, and the broken line indicates the additional capacitance Cd.
This is a characteristic when 1 'and the additional capacitance Cd2' are connected in parallel. In general, assuming that the equivalent capacitance of the surface acoustic wave resonator S is Co, the equivalent inductance is Lo, and the capacitance between terminals is Cd, the resonance frequency fr and the antiresonance frequency fa are expressed by the following equations. fr = (1 / 2π) ｛1 / (Co · Lo) ^1/2 ｝ fa = (1 / 2π) ｛(1 / Lo) (1 / Co + 1 / C
d)｝ ^1/2 where an additional capacitance Cd ′ is provided between the terminals of the surface acoustic wave resonator S.
Are connected in parallel, the anti-resonance frequency fa 'shifts to the resonance frequency side as shown in the following equation. Fa '= (1 / 2π) × ｛(1 / Lo)
(1 / Co + 1 / (Cd + Cd ′))｝ ^{1/2 In} the case of the present embodiment, the additional capacitance Cd is added to the surface acoustic wave resonator S1.
1 'are connected in parallel, the additional capacitance Cd2' is connected in parallel to the surface acoustic wave resonator S2, and the anti-resonance frequency of the surface acoustic wave resonators S1 and S2 is reduced as shown by the broken line in FIG. It changes to the resonance frequency side, and the apparent electromechanical coupling coefficient K ²
Becomes smaller. Therefore, as shown by the broken line in FIG. 2B, the steepness of the insertion loss in the pass band increases, and a steep filter characteristic can be obtained. FIG. 2 (a)
, The surface acoustic wave resonators S1 and S2 have a resonance frequency f
It is designed so that 1 = f2. As described above, according to this embodiment, a steep filter characteristic can be obtained by adding a capacitance to a part of the surface acoustic wave resonator connected in a ladder configuration. A surface acoustic wave filter according to a second embodiment of the present invention will be described with reference to FIGS. The surface acoustic wave filter according to the present embodiment is configured by connecting two surface acoustic wave resonators S1 and S2 in a ladder type. The bandwidth is narrowed by connecting additional capacitors in parallel to all the series elements and parallel elements. As shown in FIG. 3A, the first-stage series element A 'has an additional capacitance Cd1' connected in parallel to the surface acoustic wave resonator S1, and the first-stage parallel element B 'has an elastic An additional capacitance Cd2 'is connected in parallel to the surface acoustic wave resonator S2. FIG. 3B shows an equivalent circuit of the surface acoustic wave filter according to the present embodiment. The equivalent circuit of the series element A 'is a surface acoustic wave resonator S
1 comprises an equivalent capacitance C1, an equivalent inductance L1, an inter-terminal capacitance Cd1 and an additional capacitance Cd1 'connected in parallel. The equivalent circuit of the parallel element B' is equivalent to the equivalent capacitance C2 of the surface acoustic wave resonator S2 and the equivalent inductance L2. , Terminal capacitance Cd2
And an additional capacitor Cd2 'connected in parallel. FIG. 4 shows the frequency characteristics of the surface acoustic wave filter according to this embodiment. FIG. 4A shows the frequency characteristic of the impedance, and FIG. 4B shows the frequency characteristic of the insertion loss. 4 (a) and 4 (b), the solid line shows the characteristics when no capacitance is added, and the broken line shows the additional capacitance Cd.
This is a characteristic when 1 'and the additional capacitance Cd2' are connected in parallel. In FIG. 4A, the surface acoustic wave resonator S
1, S2 is designed so that the resonance frequency f1 = f2. As described above, generally, when an additional capacitor is connected in parallel between the terminals of a surface acoustic wave resonator, the anti-resonance frequency shifts toward the resonance frequency. In the case of this embodiment, the additional capacitance Cd1 'is connected in parallel to the surface acoustic wave resonator S1, and the additional capacitance Cd2' is connected in parallel to the surface acoustic wave resonator S2, as shown by the broken line in FIG. Then, the anti-resonance frequencies of the surface acoustic wave resonators S1 and S2 greatly change to the resonance frequency side. Therefore, as shown by the broken line in FIG. 4B, it is possible to obtain a filter characteristic with a narrow pass band width. As described above, according to the present embodiment, the pass band width can be controlled without changing the piezoelectric substrate by adding a capacitance to the ladder-type surface acoustic wave resonator. A surface acoustic wave filter according to a third embodiment of the present invention will be described with reference to FIGS. The surface acoustic wave filter according to the present embodiment has a 36 ° rotation Y-plate LiTaO ₃ substrate 1 as the piezoelectric substrate 10.
A plurality of surface acoustic wave resonators S <b> 1 to S <b> 10 are connected in a ladder-type configuration. By connecting an additional capacitor in parallel to a part of the series element and the parallel element, the steepness of the filter characteristic is increased. As shown in FIG. 5, the first-stage series element A ′ includes a surface acoustic wave resonator S1.
, An additional capacitor Cd1 'is connected in parallel, a first-stage parallel element B' is connected in parallel with a surface acoustic wave resonator S2, and an additional capacitor Cd2 'is connected in parallel. The parallel elements B of the second to fourth stages are constituted only by the surface acoustic wave resonators S3, S5, and S7, and the surface acoustic wave resonators S4, S
6, the fifth-stage series element A '
Is an additional capacitor Cd9 'connected in parallel to the surface acoustic wave resonator S9, and the fifth parallel element B' is connected to the surface acoustic wave resonator S1.
0 is connected in parallel with an additional capacitor Cd10 '. As shown in FIG. 6, the surface acoustic wave resonator S is composed of about 100 pairs of comb-shaped excitation electrodes 12 (FIG. 6).
(C)) is replaced with about 50 pairs of reflectors 14 and 16 (FIG. 6).
(B)). FIG. 6 shows an example of a structure in which an additional capacitance Cd is added to a surface acoustic wave resonator S which is a series element. As shown in FIG. 6A, from the input electrode 12A and the output electrode 12B of the excitation electrode 12,
Each of the electrode layers 18 and 20 is extended rightward in FIG. 6A, and is superposed on the right side of the reflector 16. That is, as shown in FIG. 6D, the electrode layers 18 and 20 formed on the piezoelectric substrate 10 are overlapped with the silicon oxide film 22 interposed therebetween. Thereby, the additional capacitance C is provided between the input electrode 12A and the output electrode 12B of the surface acoustic wave resonator S.
d are connected in parallel. FIG. 7 shows an example of a structure in which an additional capacitance Cd is added to a surface acoustic wave resonator S which is a parallel element. In the case of a parallel element, the output side of the surface acoustic wave resonator S is grounded. Therefore, as shown in FIG. 7A, the output electrode 12B of the excitation electrode 12 is commonly connected to the reflectors 14 and 16 and grounded. Is done. On the input side, the reflectors 14 and 16 pass through the electrode layer 2.
4 is extended in the center direction, and as shown in FIG. 7B, an input electrode 12A is formed thereon via a silicon oxide film 22. Thereby, the additional capacitance Cd is connected in parallel between the input electrode 12A of the surface acoustic wave resonator S and the ground (output electrode 12B). The capacitance value of the additional capacitance Cd is such that when the silicon oxide film 22 is formed by P-CVD, its dielectric constant is about 4.3. Is 0.2 × 0.
When the distance is about 2 mm, the additional capacitance Cd is about 2 pF. When the silicon oxide film 22 is used as the dielectric film, excellent characteristics such as insulation resistance and excellent characteristics can be obtained. Further, when P-CVD is used, the silicon oxide film can be manufactured at a low temperature, so that damage to the element can be avoided and a dense and good-quality dielectric film can be obtained. it can. FIG. 8 shows the frequency characteristics of the surface acoustic wave filter according to this embodiment. FIG. 8A shows the frequency characteristics of impedance, and FIG. 8B shows the frequency characteristics of insertion loss. In the case of a surface acoustic wave resonator with no added capacitance,
As shown by the solid line in FIG. 8A, the difference between the resonance frequency and the anti-resonance frequency is about 3%. On the other hand, in the case of the surface acoustic wave resonator in which additional capacitors are connected in parallel, FIG. As shown by the broken line, the difference between the resonance frequency and the anti-resonance frequency is about 1.7%. In this embodiment, the electrode pitch of the surface acoustic wave resonator is changed so that the resonance frequency when no capacitance is added and the resonance frequency when the capacitance is added match the desired frequency. . FIG. 8B shows the frequency characteristics of the surface acoustic wave filter configured as shown in FIG. 5 by connecting the additional capacitance in parallel to some of the surface acoustic wave resonators and matching the resonance frequencies. . The solid line shows the characteristics of the surface acoustic wave filter configured without adding a capacitance, and the broken line shows the characteristics of the surface acoustic wave filter of this embodiment with the added capacitance. As is clear from FIG. 8B, the filter characteristics become steep by adding the capacitance. As described above, according to the present embodiment, a steep filter characteristic can be obtained by adding a capacitance to a part of the surface acoustic wave resonator connected in a ladder configuration. In the above embodiment, the additional capacitance is connected in parallel to the surface acoustic wave resonator. However, by connecting the additional capacitance in series to the surface acoustic wave resonator, a sharp filter characteristic can be obtained regardless of the bandwidth, , The filter characteristics can be controlled. FIG. 9 shows an example of a structure in which the additional capacitance Cd is connected in series to the surface acoustic wave resonator S. As shown in FIG. 9A, the input electrode 12A of the excitation electrode 12 is divided into two electrode layers 12Aa and 12Ab, and as shown in FIGS. The electrode layers 12Ab are overlapped with the silicon oxide film 22 therebetween. Thereby, the additional capacitance Cd is connected in series to the input electrode 12A of the surface acoustic wave resonator S. FIG. 10 shows another example of the structure in which the additional capacitance Cd is connected in series to the surface acoustic wave resonator S. As shown in FIG. 10A, the input electrode 12A of the excitation electrode 12 is divided into two electrode layers 12Aa and 12Ab, and the piezoelectric substrate 10
It is made to oppose at a predetermined distance above. Although no new dielectric such as a silicon oxide film is provided between the electrode layers 12Aa and 12Ab, as shown in FIG. 10B, a dielectric is provided between the electrode layers 12Aa and 12Ab. It has a capacitor structure in which the piezoelectric substrate 10 is sandwiched. Thereby, the additional capacitance C is applied to the input electrode 12A of the surface acoustic wave resonator S.
d are connected in series. The 36 ° rotated Y plate Li which is the piezoelectric substrate 10
The relative dielectric constant of the TaO ₃ substrate is ε ₂₂ (depth direction) = 42, ε
_{Since 33} (propagation direction) = 42, the electrode layers 12Aa, 1
Assuming that the length of 2Ab is 1 mm and opposing each other with an interval of 0.1 mm, the additional capacitance is 0.02 pF.
In order to increase the capacitance value of the additional capacitance, FIG.
As shown in FIG. 0 (c), the comb electrodes of the electrode layers 12Aa and 12Ab may be used, and the effective length of the opposing electrodes may be increased. A surface acoustic wave filter according to a fourth embodiment of the present invention will be described with reference to FIGS. The surface acoustic wave filter according to the present embodiment has
A plurality of surface acoustic wave resonators S <b> 1 to S <b> 6 are connected in ladder form on a 6 ° rotating Y-plate LiTaO ₃ substrate 10. The bandwidth is narrowed by connecting additional capacitors in parallel to all the series elements and parallel elements. As shown in FIG. 11, a first-stage series element A 'has an additional capacitance Cd1' connected in parallel to a surface acoustic wave resonator S1, and a first-stage parallel element B 'has a surface acoustic wave resonance element. An additional capacitor Cd2 'is connected in parallel to the resonator S2, a second-stage series element A' is connected in parallel to the surface acoustic wave resonator S3, and an additional capacitor Cd3 'is connected to the surface acoustic wave resonator S3. The additional capacitance Cd4 'is connected in parallel to the wave resonator S4, the third series element A' is connected to the surface acoustic wave resonator S5 in additional capacitance Cd5 ', and the third parallel element B' is An additional capacitance Cd6 'is connected in parallel to the surface acoustic wave resonator S6. In this embodiment, an additional capacitance may be provided in each surface acoustic wave resonator by the above-described structure, but a structural example taking into account the feature of adding capacitance to all surface acoustic wave resonators is shown in FIG. FIG. FIG. 12A is a cross-sectional view of the surface acoustic wave resonator, FIG. 12B is an enlarged plan view of a part of the excitation electrode 12, and FIG. 12C is FF ′.
It is a line sectional view. In this structure example, as shown in FIG. 12A, a silicon oxide film 26 is formed on the entire surface of the surface acoustic wave resonators S1 to S6. Excitation electrode 12, reflectors 14, 16
When silicon oxide film 26 is formed so as to cover the entire area, an additional capacitance using silicon oxide film 26 as a dielectric is formed. As shown in FIG. 12B, one set of comb-shaped electrodes 1
2 and 12 'are formed so as to face each other, so that when the silicon oxide film 26 is formed on the entire surface, FIG.
As shown in (c), a silicon oxide film 26 is present on the piezoelectric substrate 10 between the pair of comb electrodes 12 and 12 '. Therefore, a pair of comb electrodes 12, 12 '
Are connected in parallel to the surface acoustic wave resonators S1 to S6. The comb electrodes 12, 12 'are 100 pairs, the opening length A is 0.3 mm, and the electrode finger pitch L is 1
When the thickness H of the electrode layer is 0.7 μm, the metallization ratio is 1: 1, and the dielectric constant of the silicon oxide film 26 is about 4.3, the capacitance value of the additional capacitance Cd is about 0.1 pF.
FIG. 1 shows a frequency characteristic of the surface acoustic wave filter according to the present embodiment.
3 is shown. The solid line shows the characteristics when no capacitance is added, and the broken line shows the characteristics when the additional capacitance is connected in parallel. When the additional capacitors are connected in parallel as compared with the case where no capacitors are added, it is possible to obtain a filter characteristic with a narrower pass band as shown by a broken line. As described above, according to the present embodiment, the pass bandwidth can be controlled without changing the piezoelectric substrate by adding a capacitance to the ladder-type surface acoustic wave resonator. The present invention is not limited to the above embodiment, and various modifications are possible. For example, in the above embodiment, a silicon oxide film is used as the dielectric film, but another dielectric film such as a silicon nitride film may be used. Further, as a method of forming the dielectric film, other than the P-CVD method, the dielectric film may be formed by another CVD method or a sputtering method. As described above, according to the present invention, according to the present invention, a piezoelectric substrate and a plurality of surface acoustic wave resonators formed on the piezoelectric substrate and connected in a ladder form are provided. A surface acoustic wave filter having an additional capacitance formed on a piezoelectric substrate and connected in series or in parallel to the surface acoustic wave resonator, so that a steep filter characteristic can be obtained regardless of the bandwidth, , The filter characteristics can be controlled. In the above-described surface acoustic wave filter, the additional capacitance is formed by the first electrode layer formed on the piezoelectric substrate,
Since it is composed of the dielectric layer formed on the first electrode layer and the second electrode layer formed on the dielectric layer, it can be easily provided on the piezoelectric substrate. In the above-described surface acoustic wave filter, the first electrode layer of the additional capacitance is connected to the input / output electrode or the ground electrode of the surface acoustic wave resonator, and the second electrode layer is connected to the ground electrode or the input electrode of the surface acoustic wave resonator. By connecting to the output electrode, the additional capacitance may be provided in parallel with the surface acoustic wave resonator, or the first electrode layer or the second
The additional capacitance may be provided in series with the surface acoustic wave resonator by connecting the electrode layer to the input / output electrodes of the surface acoustic wave resonator. In the above-described surface acoustic wave filter, the surface acoustic wave resonator has a pair of comb electrodes facing each other on the piezoelectric substrate, and the pair of comb electrodes is provided on the pair of surface electrodes. The device area can be increased by providing the formed dielectric layer, providing an additional capacitance as a pair of comb-shaped electrodes as electrodes, using the dielectric layer as a dielectric, and providing the dielectric layer in parallel with the surface acoustic wave resonator. In the above-described surface acoustic wave filter in which an additional capacitance can be provided without forming the dielectric layer, if the dielectric layer is formed of a silicon oxide layer or a silicon nitride layer, excellent characteristics such as insulation resistance and excellent characteristics can be obtained. In the above-described surface acoustic wave filter, the additional capacitance is formed on the piezoelectric substrate, the first electrode connected to the input / output electrode of the surface acoustic wave resonator, and the first capacitance on the piezoelectric substrate. A second electrode facing the first electrode at a predetermined interval, and a dielectric composed of a piezoelectric substrate between the first electrode and the second electrode to form a new dielectric layer or the like. Without adding, the additional capacitance can be provided in the surface acoustic wave resonator.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing a surface acoustic wave filter according to a first embodiment of the present invention. FIG. 2 is a graph showing a frequency characteristic of the surface acoustic wave filter according to the first embodiment of the present invention. FIG. 3 is a diagram illustrating a surface acoustic wave filter according to a second embodiment of the present invention. FIG. 4 is a graph showing frequency characteristics of a surface acoustic wave filter according to a second embodiment of the present invention. FIG. 5 is a diagram showing a surface acoustic wave filter according to a third embodiment of the present invention. FIG. 6 is a diagram illustrating an example of a structure of a surface acoustic wave resonator that is a series element in a surface acoustic wave filter according to a third embodiment of the present invention. FIG. 7 is a diagram illustrating an example of a structure of a surface acoustic wave resonator that is a parallel element in a surface acoustic wave filter according to a third embodiment of the present invention. FIG. 8 is a graph showing a frequency characteristic of a surface acoustic wave filter according to a third embodiment of the present invention. FIG. 9 is a diagram showing an example of a structure in which an additional capacitance is connected in series to a surface acoustic wave resonator. FIG. 10 is a diagram showing another example of a structure in which an additional capacitance is connected in series to a surface acoustic wave resonator. FIG. 11 is a diagram illustrating a surface acoustic wave filter according to a fourth embodiment of the present invention. FIG. 12 is a diagram illustrating an example of a structure of a surface acoustic wave resonator in a surface acoustic wave filter according to a fourth embodiment of the present invention. FIG. 13 is a graph showing frequency characteristics of a surface acoustic wave filter according to a fourth embodiment of the present invention. FIG. 14 is a diagram showing a conventional surface acoustic wave filter. DESCRIPTION OF SYMBOLS 10: Piezoelectric substrate 12 ... Exciting electrode 12A ... Input electrode 12B ... Output electrodes 14, 16 ... Reflectors 18, 20, 24 ... Electrode layers 22, 26 ... Silicon oxide films A, A '... Series Element B, B '... Parallel elements S1 to S10 ... Surface acoustic wave resonators C1 to C4 ... Equivalent capacitance L1 to L4 ... Equivalent inductance Cd1 to Cd4 ... Terminal capacitance Cd1' to Cd10 '... Additional capacitance

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-5-22074 (JP, A) JP-A-6-167384 (JP, A) JP-A-6-152317 (JP, A) JP-A 64-64 68114 (JP, A) JP-A-7-273597 (JP, A) JP-A-5-235684 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H03H 9/145 H03H 9 / 64

Claims (1)

(57) Patent Claims 1. A piezoelectric substrate, formed on said piezoelectric substrate, the surface acoustic wave filter having a plurality of surface acoustic wave resonators connected in a ladder, wherein formed on the piezoelectric substrate, have a connected additional capacitance to said surface acoustic wave resonator, the additional capacity, the piezoelectric
A first electrode layer formed on a conductive substrate, and the first electrode
A dielectric layer formed on the layer; and a dielectric layer formed on the dielectric layer.
A second electrode layer, and the first capacity of the additional capacitance is provided.
The electrode layer is an input / output electrode of the surface acoustic wave resonator or a ground.
Connected to an electrode, wherein the second electrode layer is provided with the surface acoustic wave.
Connected to the ground electrode or input / output electrode of the resonator,
The surface acoustic wave filter, wherein a capacitor is provided in parallel with the surface acoustic wave resonator .