JP3155639B2 - Surface acoustic wave resonator and method of manufacturing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface acoustic wave resonator suitable for a filter device and a filter device comprising the surface acoustic wave resonator.

[0002]

2. Description of the Related Art A surface acoustic wave device can be manufactured by using photolithography technology, as in the case of LSIs which are produced in extremely large quantities.
In the HF band etc., as a suitable high-frequency signal processing device,
For example, an intermediate frequency filter of a television receiver, V
Widely used in the field of TR local oscillators and the like.

Recently, due to the small size and light weight of the surface acoustic wave device, it has been applied as a high frequency filter in the field of mobile communications such as pagers and mobile phones. For example, in a mobile phone, since transmission and reception are performed by a single antenna, a filter device including a surface acoustic wave device is often used as an antenna duplexer that separates a transmission signal and a reception signal.

The characteristics of such a filter device require low loss and a large degree of out-of-band suppression. As a surface acoustic wave device for such a demand, for example, IDT
There is a filter device configured using a type filter or a surface acoustic wave resonator.

[0005] For example, as shown in FIG.
This is a configuration in which the interdigital electrode 4 provided with the bonding pad 6 is disposed thereon.

[0006] These surface acoustic wave devices are much more versatile than transversal type filter devices comprising a pair of input and output interdigital electrodes as described in, for example, Japanese Patent Application Laid-Open No. 62-116008. Has the advantage that the loss can be reduced.

An object of the present invention is a surface acoustic wave resonator and a filter device using the same.

The surface acoustic wave resonator can constitute a UHF band or quasi-microwave band filter device by using a reactance element, similarly to a conventional crystal resonator.

With respect to the above surface acoustic wave device,
For example, JP-A-58-131810, JP-A-58-131810
There is a technique disclosed in, for example, US Pat.

In addition, "The Institute of Electrical and Electronics Engineers, 1975, Ultrasonics Symposium, Proceedings, 381-384 (IEEE, 1975, ULT)
RASONICS SYMPOSIUM PROCEE
DINGS, pp. 381-384) ", and" Electronic Letter 1985, 21
Vol. 25/26, pp. 1211-1212 (ELEC
TRONICS LETTERS 5th Decem
1985, Vol. 21, No. 25/26,
pp. 1211-1212) ".

[0011]

When a filter device is composed of a surface acoustic wave resonator and a reactance element, not only the resonance characteristics of the resonator but also the damping capacitance and conductance of the resonator become problems. .

When a so-called ladder-type filter device is composed of a series-connected inductance element and a surface acoustic resonator connected to each connection point as a low-pass filter,
In order to achieve matching with an external circuit, the damping capacity of the surface acoustic wave resonator is limited.

Further, in order to reduce the loss of the filter device itself, it is necessary that the conductance and the braking capacity of the surface acoustic wave device are small.

On the other hand, the cut-off frequency of the filter in the high frequency range is determined by the damping capacitance of the surface acoustic wave resonator and the element value of the inductance element. It is necessary to be.

Further, in the above-described filter device, a stop characteristic can be obtained at the resonance frequency of the surface acoustic wave resonator used, and in order to sufficiently increase the degree of suppression in the stop characteristic, the surface acoustic wave resonance is required. It is necessary to increase the conductance of the child.

As described above, when configuring the above-described filter device, the electrical characteristic values of the surface acoustic wave resonator are subject to various restrictions.

When the above-described filter device is configured using a conventional surface acoustic wave resonator, it is difficult to satisfy all the conditions, and there is a limit in reducing the loss of the filter device.

Accordingly, the present invention is to solve the above-mentioned problems of the conventional device, and to provide a surface acoustic wave resonator suitable for the configuration of a filter device, a filter device using the same, and a method of manufacturing the surface acoustic wave resonator. With the goal.

[0019]

In order to solve the above-mentioned problems, the present invention comprises a substrate and at least two IDTs.
An electrode group connected in series above, and connected in parallel to the electrode group
Surface acoustic wave resonator having a capacitive element and a series
A plurality of inductance elements connected to the
Surface acoustic wave resonators connected in series with multiple inductors
It is characterized in that it is connected to a connection point of a sensing element.

[0020]

[0021]

[0022]

[0023]

According to the present invention, the following surface acoustic wave
A pendulum is provided. That is, in a surface acoustic wave resonator including a substrate, an electrode group in which at least two or more interdigital electrodes are connected in series, and a capacitive element connected in parallel to the electrode group, at least two of the interdigital electrodes are connected. The capacitance value of the electrode group connected in series is C ₀ , the capacitance value of the capacitance element connected in parallel to the electrode group is C ₁ , and the capacitance value of the elastic surface resonator is C, and the following expression is satisfied. Subject to

[0025]

(Equation 5)

[0026]

(Equation 6)

[0027]

(Equation 7)

[0028]

(Equation 8)

(Where fr is the resonance frequency of the surface acoustic wave resonator, ft is the center frequency of the pass band of the filter device, and fc is the value of the filter device determined by the capacitance value and the inductance element value of the surface acoustic wave resonator. Cutoff frequency, k
² is the electromechanical coupling coefficient, Cs is the capacitance per pair of interdigital electrodes, ls is the loss in the passband of the filter device, u is
A constant determined by the piezoelectric material, N is the number of electrode pairs, and W is the electrode opening length) The surface acoustic wave resonator provided with the electrode group and the parallel capacitance.

[0030]

Further, according to the present invention , the following method is provided as a method of manufacturing the surface acoustic wave resonator .

That is, when manufacturing a surface acoustic wave resonator in which an electrode and a capacitor are provided on a piezoelectric substrate, a first electrode is formed of a first conductive thin film on the piezoelectric substrate, and then a second electrode is formed. Forming a conductive thin film on a first electrode, and forming a second electrode different from the first electrode by the second conductive thin film, the method comprising: And forming the second electrode,
After covering the conductive thin film on the piezoelectric substrate on which the capacitive element is to be formed with the protective pattern and forming the first and second electrodes,
A method of manufacturing a surface acoustic wave resonator for manufacturing a capacitive element can be considered.

[0033]

As described above, in the present invention, a plurality of interdigital transducers formed of a conductive thin film on a piezoelectric substrate are connected in series to provide a one-opening surface acoustic wave resonator, and the interdigital transducers connected in series are provided. , A capacitive element connected in parallel was provided.

By configuring the surface acoustic wave resonator in this manner, the surface acoustic wave resonator is configured by changing the capacitance of a capacitance element connected in parallel separately from the braking capacitance of the surface acoustic wave resonator. Since the element value can be set to the capacitance value required in such a case, the manufacture of the filter device is facilitated and the performance of the filter device can be improved.

[0035]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

As shown in FIG. 1, a case is considered in which a ladder-type filter is constituted by including a one-opening surface acoustic wave resonator 1 and an inductance element 2.

Although not in agreement with FIG. 1, for ease of analysis, a ladder type comprising two inductance elements connected in series and a single-aperture surface acoustic wave resonator connected in parallel to the connection point. Consider one filter stage (a so-called T-type filter).

For simplicity, the resistance is assumed to be sufficiently small to be negligible, the susceptance of the surface acoustic wave resonator is B, and the reactance of the inductance is X / 2.
Then, the image impedance Zi can be expressed as follows.

[0039] Zi = √ ((X / B ) · (1-B · X / 4)) here, is X = ωL _1, further, ω is the angular frequency,
L ₁ is the value of the series inductance.

When the equivalent circuit constants of the surface acoustic wave resonator are L ₂ and C ₂ and the braking capacity is C, B = ωC− (1 / (ωL ₂ −1 / (ωC ₂ ))). .

Assuming that the resonance frequency of the surface acoustic wave resonator is fr (this value corresponds to the stop frequency of the filter), f
Assuming that r = 1 / (2π · ₂ (L ₂ C ₂ )) and the characteristic impedance of the circuit is Rs, the following equation is obtained from the above equation as a matching condition at the passband frequency ft.

[0042] _{C = L 1 / ((Rs} 2 + π 2 ft 2 L 1 2) · (1-fr 2 ·
P / (ft ² −fr ² ))) where P = 8 k ² (2 k ²
+ Π ² ) / π ⁴ , k ² is an electromechanical coupling coefficient.

The cut-off frequency fc of the attenuation region is
fc ≒ (1 / 2π) √ (4 / (L ₁ C) − (1 / L ₂ C
₂ )) can be approximated.

Using the above relational expression for the above-mentioned equation of the matching condition, the following equation is obtained as the matching condition when rearranged.

[0045]

(Equation 9)

Here, P = 8 k ² (2 k ² + π ² ) / π ⁴ C: braking capacity of surface acoustic wave resonator k ² : electromechanical coupling coefficient fc: cut-off frequency of attenuation band ft: center frequency of pass band fr: center frequency of stop band Rs: impedance of circuit For example, 128 ° Y-XL which is a typical substrate material
Using iNbO ₃ , ft is 950 MHz, fr is 8
Consider that a 20 MHz, fc constitutes a 1.3 GHz filter device.

Since Rs is usually 50 Ω, C becomes 3.6 pF according to the above equation.

On the other hand, assuming that the capacitance per unit length of a pair of interdigital electrodes is Cs, the number of electrode pairs of the interdigital electrode is N, and the electrode opening length is W, C is expressed as NWCs, and if the opening length is 100 μm. Then, in order to obtain the above capacitance value, the number of electrode pairs is 75.

However, in this case, the resonance impedance of the surface acoustic wave resonator becomes a large value of about 10Ω, and the resonance characteristics required for the filter configuration cannot be obtained. Conversely, if the number of electrode pairs is increased so that the resonance impedance becomes sufficiently small, there may be inconveniences such as a large braking capacity C and an extremely short aperture length.

In order to solve such a problem, a configuration in which a plurality of surface acoustic wave resonators are connected in series is used.

By connecting a plurality of resonators of which the number of electrode pairs and the aperture length are appropriately selected in series, the overall capacitance of the resonator becomes smaller than that of a single resonator, and the resonance impedance is greatly increased. Can be avoided.

However, in order to realize a low-loss filter device, the following problem occurs.

The braking capacity of the surface acoustic wave resonator required for matching is expressed by the above equation, but when an approximate expression of the braking capacity required to realize a desired loss is obtained, it is expressed as follows. .

That is, the loss ls in the pass band of one stage of the ladder-type filter is given by a rough approximation: susceptance B and conductance G of the surface acoustic wave resonator, and reactance X of the series inductance, ls ≒ 10Log (1 + GX (1 + √BX) )) (Log
Is the common logarithm). As a result of an experimental investigation of the conductance in the pass band of the surface acoustic wave resonator, it was newly found that there is a relationship proportional to the number N of electrode pairs and the electrode opening length W.

That is, G in the above-mentioned passband is u
Can be expressed by the following equation as a proportional constant.

G = uNW The above equation and the above-mentioned relational expression of the cut-off frequency fc are substituted into the above-mentioned relational expression of the loss, and the following equation relating to the braking capacity C ₀ is obtained by rearranging.

[0057]

(Equation 10)

Here, ls: loss of one stage of the ladder-type filter u: constant determined by the substrate material N is the number of electrode pairs, W is the electrode opening length When the loss of one stage of the filter device is about 0.5 dB, Can be approximately the same, but 0.5d
When an attempt is made to obtain a loss of B or less, the value of the braking capacity determined by the loss becomes even smaller, so that it does not match the value of the braking capacity determined by the matching conditions, and a conventional surface acoustic wave resonator is used. Then, it is not possible to realize a filter device with a lower loss.

In order to solve the above-mentioned problems of the conventional device, in the present invention, an inductance element connected in series as shown in FIG. 1 and a single-opening surface acoustic wave resonator connected in parallel at each connection point are provided. In the ladder-type filter device having the structure shown in FIG. 2, three single-opening surface acoustic wave resonators are connected in series as shown in FIG.
A configuration was adopted in which a capacitance element was connected and provided in parallel with the surface acoustic wave resonator connected in series. In this case, assuming that the capacitance value of the capacitance element connected in parallel is C ₁ and the sum of the damping capacitance of the surface acoustic wave resonator and C ₁ is C, the condition of the capacitance value determined from the matching condition and the loss is , C is expressed as in Equation 9, C ₀ is expressed in Equation 10, and C ₁ is expressed as the difference between the capacities determined from Equations 9 and 10.

As described above, according to the present invention, the capacitance value required from the matching condition is obtained as the sum of the damping capacitance of the surface acoustic wave resonator and the capacitance value of the capacitive element, and is required from the loss. The value of the braking capacity is obtained as the braking capacity value of the surface acoustic wave resonator alone, and is smaller than that of the conventional surface acoustic wave resonator.
It became easy to satisfy each condition, and the degree of freedom in design increased.

In this embodiment, 128 ° Y−X L
A conductive thin film of aluminum, an aluminum alloy, or the like is formed on the surface of the iNbO ₃ piezoelectric substrate 3, and the IDT 4 and the capacitive element 5, which are surface acoustic wave resonators, and these electrode patterns and the terminals of the package are connected. A bonding pad 6 for connection is formed.

Of course, the substrate is not limited to a piezoelectric substrate.
For example, a magnetostrictive element may be used.

Note that the package is omitted in FIG.

FIG. 4 shows an example of the frequency characteristic of the filter device of the present invention, showing the characteristic of a three-stage ladder filter. As shown in the figure, the loss in the pass band could be improved by about 0.2 dB as compared with the conventional case.

Next, a second embodiment of the present invention is shown in FIG.

This embodiment has a configuration in which the capacitive element is provided in a direction perpendicular to the propagation direction of the surface acoustic wave. In the configuration in which the capacitive element is provided in the surface acoustic wave propagation direction as in the first embodiment, when the number of serially connected stages of the surface acoustic wave resonator is large,
Although the second embodiment is advantageous for effectively utilizing the substrate surface, the arrangement of the second embodiment is advantageous for effectively utilizing the substrate surface when the number of series connection stages is small.

Next, a third embodiment of the present invention is shown in FIG.

In this embodiment, a capacitive element is formed on a substrate (for example, a glass substrate or the like) which is different from the surface acoustic wave resonator, and is mounted on a package, and then electrically connected in parallel by wires 7. Configuration.

The structure of this embodiment has the advantage that the degree of freedom in layout is further increased because each substrate can be arranged in a separate package.

FIG. 7 shows a method of manufacturing a surface acoustic wave resonator used in the first and second embodiments of the present invention.

In the method, when a surface acoustic wave resonator having an electrode and a capacitive element provided on a piezoelectric substrate is manufactured, the first electrode is formed from a first conductive thin film on the piezoelectric substrate. A method of manufacturing a surface acoustic wave resonator comprising: forming a second conductive thin film on a first electrode; and forming a second electrode different from the first electrode by the second conductive thin film, In the step of forming a first electrode and the second electrode,
After covering the conductive thin film on the piezoelectric substrate on which the capacitive element is to be formed with the protective pattern and forming the first and second electrodes,
A method for manufacturing a surface acoustic wave resonator for manufacturing a capacitive element.

In this case, the second conductive thin film is preferably thinner than the first conductive thin film. Here, it is preferable that the capacitance element includes not only the capacitance part provided in parallel with the surface acoustic wave resonator but also the IDT. Now, in a surface acoustic wave device used at a high frequency, an electrode having a relatively thin film thickness of about 100 nm is used in order to avoid unnecessary reflection, loss, and the like of the surface acoustic wave generated by the IDT.

However, in order to connect wires for wiring, the above-mentioned film thickness is too thin and may cause a contact failure. Therefore, usually, a bonding pad having a large film thickness is first formed, and then the bonding pad is formed. A method of forming and patterning a thin film of an interdigital electrode is performed.

When the capacitive element is formed on the same substrate, it is possible to form the same thin film as the interdigital electrode. However, particularly when the resistance of the thin film is a problem, the method of the present embodiment described below is used. Resistance can be reduced.

Hereinafter, description will be made with reference to FIG.

First, (a) an aluminum thin film as a first conductive thin film having a thickness of, for example, about 500 nm is formed on the surface of the piezoelectric substrate 3 by a method such as evaporation or sputtering.

(B) Next, a bonding pad is formed from the aluminum thin film by using a photolithography technique.

At this time, the portion of the aluminum thin film where the capacitive element is to be formed is covered with a photoresist and left without being etched.

(C) Next, the film thickness 1 for the surface acoustic wave resonator
An aluminum thin film (thinner than the first conductive thin film) as a second conductive thin film having a thickness of about 00 nm is formed.

(D) Further, similarly to the aluminum thin film as the first conductive thin film, a surface acoustic wave resonator is formed by photolithography.

(E) Finally, in order to prevent the electrode pattern formed up to the previous step from being etched, the electrode pattern of the capacitive element is formed by using the photolithography technique as described above.
To complete the manufacturing.

According to the present method, a thick film is required for a portion that needs to be manufactured with a thick conductive thin film such as a bonding pad, and a portion that needs to be manufactured with a thin conductive thin film such as a surface acoustic wave resonator. Can be manufactured with a thin film, and a surface acoustic wave device suitable for the purpose and application can be constructed.

[0083]

According to the present invention, in order to construct a low-loss filter device, the opening length of the interdigital transducer of the IDT type resonator, the number of electrode pairs, and the like are increased, and the resonator at a frequency deviated from the resonance point is formed. Even if the resistance value is reduced, the capacitance value does not increase, so that the configuration of the filter device is simplified and the performance of the device can be improved. Further, according to the method for manufacturing a surface acoustic wave resonator of the present invention, efficient manufacturing can be performed.

[Brief description of the drawings]

FIG. 1 is an example of a configuration diagram of a filter device according to the present invention.

FIG. 2 is a configuration diagram of a surface acoustic wave resonator according to a first embodiment of the present invention.

FIG. 3 is a configuration diagram of a conventional surface acoustic wave resonator.

FIG. 4 is an example of a frequency characteristic of the filter device according to the present invention.

FIG. 5 is a configuration diagram of a surface acoustic wave resonator according to a second embodiment of the present invention.

FIG. 6 is a configuration diagram of a surface acoustic wave resonator according to a third embodiment of the present invention.

FIG. 7 is an explanatory diagram of a method for manufacturing a surface acoustic wave resonator according to the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Surface acoustic wave resonator, 2 ... Inductance, 3 ... Piezoelectric substrate, 4 ... Interdigital electrode, 5 ... Capacitance element, 6 ... Bonding pad, 7 ... Wire, 8 ... First aluminum thin film, 9 ... Second Aluminum thin film.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Kazushi Watanabe 292 Yoshida-cho, Totsuka-ku, Yokohama-shi, Kanagawa Prefecture Inside Hitachi, Ltd. Video Media Research Laboratories (72) Akinori Yuhara 292 Yoshida-cho, Totsuka-ku, Yokohama-shi, Kanagawa (56) References JP-A-53-112643 (JP, A) JP-A-58-47311 (JP, A) JP-A-4-288720 (JP, A) JP-A-4 −249906 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H03H 9/145 H03H 3/08 H03H 9/25

Claims (4)

(57) [Claims] 1. A substrate, a group of electrodes and at least two series-connected interdigital electrodes, surface acoustic wave resonator and a capacitive element connected in parallel with said electrodes, and, in series
A plurality of inductance elements connected to each other, and the surface acoustic wave resonator is connected to a plurality of series-connected inductors.
Characterized by being connected to a connection point of a conductance element.
Filter device. 2. A surface acoustic wave resonator comprising a substrate, an electrode group in which at least two or more interdigital electrodes are connected in series, a capacitance element connected in parallel to the electrode, and an inductance element. A filter device comprising a ladder-type filter. 3. A surface acoustic wave resonator comprising a substrate, an electrode group in which at least two or more interdigital electrodes are connected in series, and a capacitive element connected in parallel to the electrode group.
The capacitance value of an electrode group in which at least two IDTs are connected in series is C ₀ , the capacitance value of a capacitance element connected in parallel to the electrode group is C ₁ , and the capacitance value of the elastic surface resonator is C. , Provided that the following equation is satisfied: (Equation 2) (Equation 3) (Equation 4) (Where fr is the resonance frequency of the surface acoustic wave resonator, ft
The center frequency of the passband of the filter device, fc is the cutoff frequency, k ² of the filter device is determined by the capacitance value and the inductance element value of the surface acoustic wave resonator, the electromechanical coupling coefficient, Cs is interdigital electrodes Capacity per pair, ls
Is the loss in the pass band of the filter device, u is a constant determined by the piezoelectric material, N is the number of electrode pairs, and W is the electrode opening length) The electrode group and the parallel capacitance are provided. Child. 4. When manufacturing a surface acoustic wave resonator provided with an electrode and a capacitor on a piezoelectric substrate, a first electrode is formed of a first conductive thin film on the piezoelectric substrate, and then a second electrode is formed on the piezoelectric substrate. Forming a conductive thin film on a first electrode, and forming a second electrode different from the first electrode by the second conductive thin film, the method comprising: Electrodes, and
In the step of forming the second electrode, the conductive thin film on the piezoelectric substrate on which the capacitive element is to be formed is covered with a protection pattern, and after forming the first and second electrodes, the elastic element for manufacturing the capacitive element is formed. A method for manufacturing a surface acoustic wave resonator.