JP4010664B2 - Surface acoustic wave filter - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a SAW filter in which a one-terminal-pair surface acoustic wave (hereinafter referred to as SAW) resonator having electrodes formed on a piezoelectric or ferroelectric substrate is connected in a multi-stage ladder shape.
[0002]
[Prior art]
Conventionally, as a technique related to a SAW filter using a SAW resonator, there is one described in the following literature, for example.
Document 1; IEICE A, J76-A [2] (1993-2), Sato "low loss band filter using SAW resonators" et al P.245-252
Reference 2; IEICE Transactions A, J76-A [2] (1993-2), Tomita et al. “Experiments on SAW filters for mobile radio communications equipment” P.233-244
2A and 2B are diagrams showing a structure and an equivalent circuit of a conventional SAW resonator.
This SAW resonator is formed on a substrate 1 and has an interdigital electrode (hereinafter referred to as IDT) 2 for transmitting and receiving SAW. On both sides of the IDT 2, grating reflectors 3 formed of metal strips are arranged on both sides of the IDT 2 as necessary. The substrate 1 is composed of, for example, a LiTaO ₃ single crystal substrate having a crystal orientation of 36 ° YX, and the IDT ₂ and the reflector 3 are composed of an Al thin film deposited on the substrate 1. An equivalent circuit of the SAW resonator can be represented by an inductor 4 and a capacitor 5 connected in series between a pair of terminals, and a capacitor 6 in parallel thereto, as shown in FIG. 3B, for example.
[0003]
FIGS. 3A and 3B are explanatory views showing the principle of the conventional one-stage constant K-type SAW filter described in Document 1. FIG.
The basic configuration of the multistage ladder-type SAW filter is the one-stage constant K-type SAW filter 10 of FIG. The one-stage constant K-type SAW filter 10 is a filter in which the SAW resonators shown in FIGS. 2A and 2B are connected in a trapezoidal manner as a series arm resonator 11 and a parallel arm resonator 12. According to the theory of the constant K-type filter, as shown in FIG. 3B, by matching the resonance frequency of the series arm resonator 11 with the anti-resonance frequency of the parallel arm resonator 12, the pass band and the attenuation band sandwiching it Thus, a frequency characteristic such as can be obtained, and a bandpass filter can be formed.
Since it is difficult to obtain a sufficient amount of attenuation in the single-stage constant K-type SAW filter 10 as described above, the SAW filter 10 shown in FIG. 2 is usually connected in multiple stages as shown in FIG.
[0004]
FIG. 4 is a connection diagram illustrating an example of a conventional multistage constant K-type SAW filter.
This multi-stage constant K-type SAW filter is obtained by cascading the single-stage constant K-type SAW filter 10 in FIG. 3A in, for example, four stages, and the series-arm resonator and the parallel-arm resonator 12 of each stage. The resonance frequencies are both set to be the same. However, in order to reduce the size, the adjacent series arm resonators and the adjacent parallel arm resonators may be combined in a circuit network and replaced with different resonators having the same resonance frequency but different.
FIGS. 5A to 5E are diagrams showing a manufacturing process of the SAW filter of FIG.
The SAW filter in FIG. 4 is formed, for example, through the steps of FIGS.
[0005]
First, in the process of FIG. 5A, a LiTaO ₃ single crystal substrate 1 having a crystal orientation of 36 ° YX is prepared, and a resist 13 is applied to the pattern formation surface of the substrate 1 by spin coating. In the step of FIG. 5B, an optical mask 14 is set on the substrate 1 coated with the resist 13, and the SAW filter pattern is transferred to the resist 13 by exposing with light 15. In the step of FIG. 5C, the resist 13 unnecessary for development is selectively removed. In the step of FIG. 5D, an Al thin film 16 to be an electrode of the SAW resonator is deposited on the entire upper surface of the substrate 1 from which the unnecessary resist 13 has been removed. 5E, unnecessary portions of the Al thin film 14 are removed together with the resist 13 using an organic solvent.
[0006]
[Problems to be solved by the invention]
However, the conventional SAW filter of FIG. 4 has the following problems.
FIG. 6 is a diagram illustrating the frequency characteristics of the problem of FIG.
In the conventional multi-stage constant K-type SAW filter of FIG. 4, the series arm resonators of each stage all have the same resonance frequency, and the parallel arm resonators of each stage all have the same resonance frequency. As shown in FIG. 6, sufficient attenuation can be obtained in a narrow frequency range, but it is difficult to ensure sufficient attenuation over a wide range. In order to solve this problem and obtain a sufficient amount of attenuation over a wide band, the one-stage ladder-type SAW filter 10 shown in FIGS. 3A and 3B must be cascaded in a larger number of stages. It was. However, increasing the number of cascaded stages not only increases the number of elements, but also increases the loss in the passband, and a technically satisfactory one cannot be obtained.
In addition, in order to obtain sufficient attenuation over a wide frequency without increasing the number of cascaded stages, there is a method of decreasing the resonator capacity by reducing the series arm braking capacity or increasing the parallel arm braking capacity. Although it is considered, the loss in the passband also increases.
[0007]
FIG. 7 is a connection diagram illustrating another example of the multistage constant K-type SAW filter, and FIG. 8 is a diagram illustrating the frequency characteristics of FIG.
This SAW filter is a filter in which a new SAW resonator 17 is connected to the end of the series arm of the SAW filter of FIG. When the SAW resonator 17 having a resonance frequency different from that of each series arm SAW resonator is connected, the attenuation pole having a different frequency is added as shown in FIG. Thus, although the attenuation amount in the attenuation region can be earned, there is a problem that the insertion loss in the pass region increases. Also in this case, the number of elements increases, and the chip size increases.
On the other hand, the document 2 describes a band rejection filter in which a plurality of series SAW resonators are connected as shown in FIG.
[0008]
FIG. 9 is a connection diagram illustrating a conventional band rejection filter, and FIG. 10 is a diagram illustrating the frequency characteristics of FIG.
In the filter of FIG. 9, a plurality of SAW resonators 11 _1, 11 _2, ..., 11 ₅ are connected as the series arm, the resonance frequency f ₀₁ of the respective series-arm SAW resonator 11 ₁ to 11 _5, f _02, f _03, f _04, f ₀₅ are different from each other. In the frequency characteristics of the band-stopping filter to which the SAW resonators 11 _{1 to} 11 _{5 of the} series arm are connected, the attenuation poles of the SAW resonators 11 _{1 to} 11 ₅ are all different and can be in a wide band attenuation range. When the braking capacities of these SAW resonators 11 _{1 to} 11 ₅ are connected in series in the region, the entire braking capacity is reduced. Therefore, unless the logarithm of the SAW resonators 11 _{1 to} 11 ₅ in series arms and the crossing length of each electrode are made very large, the insertion loss in the pass band increases. That is, there is a problem that the chip size is increased or the insertion loss is increased in the low pass band.
[0009]
[Means for Solving the Problems]
In order to solve the above problems, a first invention of the present invention is a one-terminal surface acoustic wave resonance formed by depositing a metal or alloy serving as an electrode on a piezoelectric or ferroelectric single crystal substrate. In a SAW filter in which a plurality of series arm SAW resonators and parallel arm SAW resonators each composed of a plurality of elements are connected in a multi-stage ladder shape and having a pass band and an attenuation band at predetermined positions of frequency, The configuration is as follows. That is, the resonance frequency of each series arm SAW resonator is shifted for each stage of the trapezoidal shape due to the difference in specific gravity of the metal or alloy serving as the electrode.
According to a second invention, in the SAW filter of the first invention, an electrode of one SAW resonator of the plurality of series arm SAW resonators is formed of a first metal, and the plurality of series arm SAW resonances. The electrodes of the other SAW resonators of the child are formed of an alloy in which a second metal having a higher specific gravity than the first metal is blended with the first metal.
[0010]
According to a third aspect of the present invention, in the SAW filter of the first or second aspect, the specific gravity of the metal forming the electrode of the series arm SAW resonator decreases from the first stage of the multistage ladder type to the subsequent stage. It is configured to do.
According to a fourth aspect of the present invention, the SAW filter of the fourth aspect is configured as follows.
That is, the ratio of the second metal in the alloy to be blended with the first metal is set to a coefficient corresponding to the specific gravity of the first metal according to the number of stages in the trapezoid of the SAW resonator. It is set as the ratio calculated | required by the following formula | equation which set it as (beta) and expressed an allowable error as (gamma).
(Α ± γ) / (specific gravity of the second metal / specific gravity of the first metal−1) × (frequency of attenuation region (MHz) × β) / (number of stages−1) [%]
[0011]
A fifth invention is the SAW filter of the second to fourth inventions, wherein the first metal is made of Al, and the second metal is made of a metal selected from metals having a specific gravity of 4-10. ing.
A sixth invention is the SAW filter of the second to fifth inventions, wherein the second metal is made of Ti, Sn, Ni or Cu.
According to a seventh aspect of the present invention, in the SAW filter of the fourth aspect, when the first metal is composed of the Al, the coefficient α is 0.4, the coefficient β is 0.6, and the error is 0. .05.
An eighth invention is the SAW filter of the first to seventh inventions, wherein the piezoelectric or ferroelectric single crystal substrate is composed of a LiNbO ₃ single crystal substrate or a LiTaO ₃ single crystal substrate.
[0012]
A ninth aspect of the SAW filter of the eighth invention, the LiTaO ₃ single crystal substrate, the crystal orientation is the substrate of 36 ° Y-X or X-112 ° Y.
A tenth aspect of the invention is the SAW filter according to the eighth aspect of the invention, wherein the LiNbO ₃ single crystal substrate is a substrate having a crystal orientation of 41 ° YX, 64 ° YX, or 128 ° YX.
According to the first to tenth aspects, the SAW filter is configured as described above.
The resonance frequencies of the plurality of series arm SAW resonators are shifted for each stage due to the difference in specific gravity of the metal serving as an electrode, the sound speed in each SAW resonator is changed, and the attenuation pole is shifted. For this reason, the attenuation range on the high frequency side of the passband is widened, and the above problem can be solved.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a connection diagram of a multistage SAW filter showing an embodiment of the present invention.
This multi-stage ladder-type SAW filter is formed by cascading 1-stage ladder-type SAW filters in four stages of first to fourth stages, and series-arm SAW resonators 21 ₁ , 21 ₂ , which are multi-stage ladder-type series arms. 21 ₃ and 21 _4, and parallel arm SAW resonators 22 ₁ , 22 ₂ , 22 ₃ , and 22 _{4 serving} as ladder-type parallel arms.
As shown in FIG. 3A, each series arm SAW resonance 21 _{1 to} 21 ₄ is formed by vapor-depositing a metal serving as an electrode on the substrate, but the specific gravity of the metal forming these electrodes is changed. It has been. For example, the electrodes of the series arm SAW resonators 21 _{1 to} 21 ₃ are made of an alloy of Al, which is a first metal, and Cu (copper), which is a second metal having a higher specific gravity, and the ratio of Cu Are, in order, 10.18%, 6.78%, and 3,39%. The series-arm SAW resonators 21 _fourth electrode is formed of pure Al. On the other hand, the electrodes of the parallel arm SAW resonators 22 _{1 to} 22 ₄ are all made of an alloy having a Cu content of 10.18%.
[0014]
FIGS. 11A to 11E are cross-sectional views illustrating an outline of the manufacturing process of the SAW filter of FIG.
The SAW filter of FIG. 1 is formed by performing the steps shown in FIGS. 11A to 11C and repeating the steps of FIGS. 11D and 11E.
First, in the process of FIG. 11A, for example, a LiTaO ₃ single crystal substrate 31 having a crystal orientation of 36 ° Y-X is prepared, and a resist 32 is applied to the pattern formation surface of the substrate 31 by spin coating. In the step of FIG. 11B, an optical mask 33 is set on the substrate 31 coated with the resist 32, and the SAW filter pattern is transferred to the resist 32 by exposing with light 34.
[0015]
In the step of FIG. 11C, the resist 32 unnecessary for development is selectively removed. In the step of FIG. 11D, the formation region of the first-stage series arm SAW resonator 21 _{1 and} the formation of the parallel arm SAW resonators 22 _{1 to} 22 ₄ on the substrate 31 from which unnecessary resist 32 has been removed are formed. An alloy thin film 35 to be an electrode is deposited on the predetermined region. In the step of FIG. 11E, an unnecessary portion of the thin film 35 is removed together with the resist 32 using an organic solvent. Through the above steps, the first-stage series arm SAW resonator 21 ₁ and the parallel fourth stage arm SAW resonator 22 _{1-22 4} is formed. Thereafter, the steps of FIGS. 11D and 11E are repeated in order to form the series arm SAW resonators 21 _{2 to} 21 ₄ of the other stages. However, as the thin film 35, the above-described alloys having different Cu contents and pure Al are used.
The multi-stage SAW filter of FIG. 1 having such a configuration has frequency characteristics set by the SAW resonators 21 _{1 to} 21 ₄ and the SAW resonators 22 _{1 to} 22 ₄ , and operates as a bandpass filter.
[0016]
FIG. 12 is a diagram illustrating frequency characteristics of the SAW filter of FIG. The band of FIG. 1 will be described with reference to FIG.
The resonance frequencies of the series arm SAW resonators 21 _{1 to} 21 ₄ are frs _{1 to} frs ₄ and the antiresonance frequencies are fas ₁ to fas ₄ , respectively. The resonance frequencies of the parallel arm SAW resonators 22 _{1 to} 22 ₄ are Where Frp ₁ to frp ₄ and anti-resonance frequencies are fap _{1 to} fap ₄ , respectively, these relationships are as follows.
frs ₁ = fap ₁
frs ₂ > fap ₂
frs ₃ > fap ₃
frs ₄ > fap ₄
frs ₄ > frs ₃ > frs ₂ > frs _{1
frp 4 = frp 3 = frp 2} = frp 1
fap ₄ = fap ₃ = fap ₂ = fap ₁
That is, in the first stage of the ladder type, the resonance frequency of the series arm SAW resonator 21 ₁ is equal to the anti-resonance frequency of the parallel arm SAW resonator 22 ₁ , and the resonance frequency of the series arm SAW resonator becomes the parallel arm in the subsequent stage. It becomes higher than the antiresonance frequency of the SAW resonator. Therefore, the first stage has a constant K-type configuration, but the second and subsequent stages are one-stage ladder filters having a wider band on the high frequency side than the constant K-type configuration.
[0017]
Here, since the entire SAW filter of FIG. 1 is formed by cascading the first-stage to fourth-stage first-stage ladder filters, attenuation determined by different resonance frequencies of the series arm SAW resonators 21 _{1 to} 21 ₄ is provided. The pole will be synthesized. Even if the number of stages is not increased, the high-frequency attenuation range is widened as shown in FIG. Note that frs ₂ ≠ fap ₂ , frs ₃ ≠ fap ₃ , and frs ₄ ≠ fap _4, and the insertion loss in the pass band is slightly increased by deviating from the condition of the constant K-type filter, but the frequency shift of each stage is increased. Since it is small, there is less loss degradation than increasing the number of stages.
That is, if the electrode of the SAW resonator is made of a metal heavier than Al (specific gravity 2.69), the sound speed in the SAW resonator is lower than that of Al, the resonance frequency is shifted to the lower side, and the attenuation region is also lower. Shift to the frequency of. In the SAW filter of FIG. 1, since the specific gravity of the electrode changes for each of the series arm SAW resonators 21 _{1 to} 21 ₄ , the frequency of the attenuation pole set by these SAW resonators 21 _{1 to} 21 ₄ is different. The attenuation range as a whole is expanded.
In the case where a LiTaO ₃ single crystal substrate 22 having a crystal orientation of 36 ° Y-X has a four-stage structure and an attenuation region with a relative bandwidth of 2.84%, the series arm SAW resonators 21 _{2 to} 21 in the _{second to} fourth stages are obtained. _When the relative bandwidths of ₄ are shifted by 0.57%, an attenuation region having a desired bandwidth is obtained.
[0018]
For example, when h / λ (h is the film thickness and λ is the wavelength) is 0.1075, the Cu content in the electrodes of the SAW resonators 21 _{1 to} 21 ₄ is 10.8%, respectively, as described above. When set to 78%, 3,39%, and 0%, an appropriate attenuation range is obtained. This corresponds to changing the ratio of the metal added to Al of the electrodes of the SAW resonators 21 _{1 to} 21 _{3 by} the amount expressed by the following equation (1).
α / ((specific gravity of metal added to Al / specific gravity of Al) −1) × (attenuation band (MHz) × β) / (number of stages−1) [%] (1)
However, α and β are coefficients when the specific gravity of Al is 2.69. Specifically, α is 0.4 and β is 0.6. It is practical to apply equation (1) by adding or subtracting about γ as an allowable error to α.
[0019]
As described above, in the present embodiment, in a multistage ladder type SAW filter which is connected to the SAW resonator 21 ₁ to 21 _4, 22 ₁ to 22 ₄ to ladder type, serial arm SAW resonator other than the series-arm SAW resonators 21 ₄ Since the electrodes of the resonators are made of an alloy of Al and Cu having a specific gravity heavier than that of Al and the composition of the alloys of the SAW resonators 21 _{1 to} 21 ₃ is changed in order, the sound speed of the resonators is different. As a result, the attenuation pole formed by each of the SAW resonators 21 _{1 to} 21 ₄ shifts to a different frequency, and the attenuation region becomes wider. Therefore, the following advantages can be obtained.
(I) Even if the number of cascaded one-stage ladder filters is small, an attenuation region with a desired width can be obtained, and as a result, a low-loss filter can be realized.
(Ii) Since the attenuation region is widened, it is not necessary to provide a SAW resonator for forming an attenuation pole separately at the end of the series arm, and the chip size can be reduced.
(Iii) Since the specific gravity of the electrodes of the series arm SAW resonators 21 _{1 to} 21 ₄ is changed, even if the resonators have the same logarithm and the same cross length, other unnecessary vibration modes or The frequency of the spurious mode such as the vertical or horizontal mode differs for each SAW resonator 21 _{1 to} 21 ₄ , and the influence on the pass band and the attenuation band is small.
[0020]
(Iv) Compared to the case of the band rejection filter as shown in FIG. 9, it is not necessary to increase the logarithm and crossing length of the resonator, so that the chip size can be reduced.
(V) An increase in insertion loss on the low pass band side can be prevented as compared with the case of the band rejection filter as shown in FIG.
(Vi) For each of the series arm SAW resonators 21 _{1 to} 21 ₄ , it can be realized only by changing the metal to be deposited, and no specially complicated process is required. Therefore, the conventional manufacturing equipment can be used as it is.
[0021]
In addition, this invention is not limited to the said embodiment, A various deformation | transformation is possible. Examples of such modifications include the following.
(1) The metal added to Al may be other than Cu, and may be Ti (titanium), Sn (tin), Ni (nickel), or Au (gold) in some cases, and the frequency change by comparing the specific gravity of Cu. If the calculation is performed and the mixing ratio is adjusted based on the calculation result, the same effects as in the above embodiment can be obtained.
(2) In forming the SAW filter, etching may be used instead of lift-off.
(3) The substrate 31 may be a LiNbO ₃ single crystal substrate having a crystal orientation of 41 ° YX, 64 ° YX, or 128 ° YX, or a LiTaO ₃ single crystal substrate having an X-112 ° Y. Good.
(4) The alloy forming the electrodes of the series arm SAW resonators 21 _{1 to} 21 ₃ may not be an alloy in which the same metal is mixed with Al. The selection of Cu, Ti, Sn, Ni, and Au may be changed as appropriate.
[0022]
【The invention's effect】
As described above in detail, the first to tenth inventions are configured such that the resonance frequencies of the plurality of series arm SAW resonators are shifted for each stage depending on the specific gravity of the metal serving as an electrode. The speed of sound in each series arm SAW resonator changes and the attenuation pole shifts. Therefore, it is possible to widen the frequency band of the attenuation band set on the high frequency side of the pass band without sacrificing insertion loss or increasing the number of elements.
[Brief description of the drawings]
FIG. 1 is a connection diagram of a multi-stage ladder type SAW filter showing an embodiment of the present invention.
FIG. 2 is a diagram showing a structure and an equivalent circuit of a conventional SAW resonator.
FIG. 3 is an explanatory diagram showing the principle of a conventional one-stage constant K-type SAW filter.
FIG. 4 is a connection diagram showing a conventional multi-stage constant K-type SAW filter.
5 is a diagram showing a manufacturing process of the SAW filter of FIG. 4;
6 is a diagram showing frequency characteristics of the problem of the SAW filter of FIG. 4;
FIG. 7 is a connection diagram illustrating another example of a multistage constant K-type SAW filter.
FIG. 8 is a diagram illustrating the frequency characteristics of FIG.
FIG. 9 is a connection diagram showing a conventional band rejection filter.
10 is a diagram illustrating the frequency characteristics of FIG. 9. FIG.
11 is a cross-sectional view showing an outline of a manufacturing process of the SAW filter of FIG. 1;
12 is a diagram illustrating frequency characteristics of the SAW filter of FIG. 1;
[Explanation of symbols]
21 _{1 to} 21 ₄ series arm SAW resonator 22 _{1 to} 22 ₄ parallel arm SAW resonator 31 substrate 35 electrode

Claims (10)

A plurality of series-arm surface acoustic wave resonators and parallel arm elasticitys each composed of a one-terminal-pair surface acoustic wave resonator formed by depositing a metal or alloy serving as an electrode on a piezoelectric or ferroelectric single crystal substrate. A surface acoustic wave filter in which surface wave resonators are connected in a multi-stage ladder shape, and has a pass band and an attenuation band at predetermined positions of the frequency,
The surface acoustic wave filter according to claim 1, wherein the resonance frequency of each of the series arm surface acoustic wave resonators is shifted for each stage of the trapezoidal shape due to a difference in specific gravity of the metal or alloy serving as the electrode. The electrode of one surface acoustic wave resonator among the plurality of series arm surface acoustic wave resonators is formed of a first metal, and the other surface acoustic wave among the plurality of series arm surface acoustic wave resonators. 2. The surface acoustic wave filter according to claim 1, wherein the resonator electrode is formed of an alloy in which the first metal is mixed with a second metal having a higher specific gravity than the first metal. The specific gravity of the metal forming the electrode of each series arm surface acoustic wave resonator is configured to decrease from the first stage to the rear stage of the multi-stage ladder type. Surface acoustic wave filter. In the alloy, the proportion of the second metal blended with the first metal depends on the number of steps of the surface acoustic wave resonator in the trapezoidal shape, and the coefficient corresponding to the specific gravity of the first metal is α and 4. The surface acoustic wave filter according to claim 3, wherein the ratio is obtained by the following equation (1) in which β and an allowable error are expressed as γ.
(Α ± γ) / (specific gravity of the second metal / specific gravity of the first metal−1)
× (Frequency of attenuation region (MHz) × β) / (number of stages−1) [%] (1) 5. The surface acoustic wave filter according to claim 2, wherein the first metal is made of Al, and the second metal is made of a metal having a specific gravity of 4 to 10. . 6. The surface acoustic wave filter according to claim 2, wherein the second metal is composed of Ti, Sn, Ni, or Cu. 5. The elasticity according to claim 4, wherein the coefficient α when the first metal is made of Al is 0.4, the coefficient β is 0.6, and the error is 0.05. Surface wave filter. 8. The elastic material according to claim 1, wherein the piezoelectric or ferroelectric single crystal substrate comprises a LiNbO ₃ single crystal substrate or a LiTaO ₃ single crystal substrate. Surface wave filter. 9. The surface acoustic wave filter according to claim 8, wherein the LiTaO ₃ single crystal substrate has a crystal orientation of 36 ° YX or X-112 ° Y. 9. The surface acoustic wave filter according to claim 8, wherein the LiNbO ₃ single crystal substrate has a crystal orientation of 41 ° YX, 64 ° YX, or 128 ° YX.

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