JPH09153766A - Saw(surface accoustic wave) element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent electrodes from being discharged and destroyed by pyroelectricity. SOLUTION: Concerning a surface acoustic wave(SAW) element having interdigital electrode transducers(IDT) 2 and 3 and a sound absorbing member 51 outside these IDT 2 and 3 so as to absorb unwanted waves in SAW excited by the IDT 2 and 3 on a piezoelectric board 1, the sound absorbing member 51 is respectively electrically connected with paired electrodes at the IDT 2 and 3. Then, a resistance value between these paired electrodes through the sound absorbing member 51 is set from 1×10<3> Ω to 1×10<8> Ω.

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 element such as a surface acoustic wave filter for high frequencies, which is provided with a mechanism for preventing discharge breakdown due to pyroelectricity.

[0002]

2. Description of the Related Art A surface acoustic wave is a wave whose energy is concentrated near the surface of the elastic body among the waves propagating in the elastic body. The surface acoustic wave element, which is a device utilizing the surface acoustic wave, has a slow wave propagation speed of about 100,000 times slower than the electromagnetic wave propagation speed, so that it can be miniaturized and reduced in weight, and the wave propagates on the surface. Because it can be driven and detected at any place,
Since it has the advantage that it can be used over a wide frequency range from 10 MHz to several GHz, it is currently widely used for filters, delay lines, resonators, oscillators and the like. The surface acoustic wave element comprises a piezoelectric substrate (1) and an electrode transducer. A typical surface acoustic wave filter is
As shown in FIG. 2, on the piezoelectric substrate (1), an apodized interdigital transducer (IDT) (2), which is weighted so that the crossing length changes on the input side, has a crossing length on the output side. A fixed normal form IDT (3) is provided opposite to each other. The pitch of the electrode fingers of each IDT (2) (3) is constant and the same. Further, in order to prevent the surface acoustic wave excited by the input side IDT (2) from being reflected or propagating to other than the output side IDT (3), that is, the unnecessary wave of the surface acoustic wave is eliminated. To absorb, IDT (2) (3)
A sound absorbing material (5) is arranged outside the.

Many of the piezoelectric materials used for the piezoelectric substrate (1) have a pyroelectric property. Pyroelectricity is the property of producing polarized charges on the surface by heating. Therefore, not only the heating and frictional heat in the manufacturing process, but also the heat generated by the circuit board or the device on which the surface acoustic wave element is installed in the product accumulates charges on each electrode finger of the IDT (2) (3). However, a phenomenon occurs in which the charge accumulated between the electrode fingers is discharged. A surface acoustic wave filter for signals of a frequency of about several tens of MHz used in conventional TVs, VTRs, etc. can be used even if discharge occurs due to electric charges generated by the pyroelectricity of the piezoelectric substrate (1). I couldn't get my finger to break. However, in the surface acoustic wave filter, when the operating frequency of the filter becomes high and the wavelength corresponding to the operating frequency becomes short, the pitch of the electrode fingers becomes narrower accordingly,
As a result, the width of the electrode finger also becomes narrow. For example TV and VT
In the case of a high frequency surface acoustic wave filter used for a second intermediate frequency (usually 400 MHz or 600 MHz) such as a BS tuner having a frequency higher than R, if the IDT is a double electrode type, the width of the electrode finger is 1 μm. It will be about. At this time, if the discharge occurs between the electrode fingers, the electrode fingers are damaged,
It gives a fatal defect to the filter performance.

In order to solve such a problem, the one described in Japanese Patent Publication No. 59609/1993 is known. As shown in FIG. 3 (a), in each IDT (2) (3), a resistance link path (90) for connecting a pair of electrodes is provided with a conductive material to form a pair of electrodes. The occurrence of discharge is prevented by eliminating the bias of the electric charge between the two. In addition, the one described in JP-A-6-29770 is known. As shown in FIG. 3 (b), a pair of discharge terminals (91) facing each other are provided on the piezoelectric substrate (1) for each IDT (2) (3) to form a pair of discharge terminals. (91) is connected to a pair of electrodes at each IDT. Then, the discharge terminals (9
1) One side is formed with a protruding end toward the other side. As a result, a discharge is generated between the pair of discharge terminals (91), so that no discharge occurs at the electrodes and damage is prevented.

[0005]

However, the resistance link path (90) described in Japanese Patent Publication No. 59609/1993 needs to have a high resistance value so that no signal is lost and no discharge occurs. Therefore, in order to form the resistance link path (90) with the same conductive material as the electrodes, it is necessary to make the cross-sectional area of the resistance link path (90) extremely small or to make the length extremely long. Have difficulty. For example, aluminum (specific resistance is 2.5 × 10 ^-8 Ωm) is used as a conductive material, and the resistance link path (90) has a resistance value of 1 kΩ and a length of 2 mm.
In order to form the, the cross-sectional area needs to be 0.05 μm ² . In addition, if the resistance link path (90) is made of a material different from that of the electrodes, the number of manufacturing steps increases and the production cost increases. The discharge terminal (9
In the case of 1), damage to the terminal (91) due to discharge, especially terminal (9)
The protruding end of 1) is inevitable. As a result, the characteristics of the device may be changed, and the protruding end is damaged, so that there is a problem in reliability as to whether or not the next discharge will occur between the discharge terminals (91).

[0006]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a surface acoustic wave device capable of preventing discharge due to pyroelectricity without increasing the number of manufacturing steps.

[0007]

Means for Solving the Problems To solve the above problems,
The present invention is configured as follows. ID on the piezoelectric substrate
In a surface acoustic wave element comprising T and a sound absorbing material outside the IDT, which absorbs unnecessary waves of surface acoustic waves excited by the IDT, the sound absorbing material is electrically connected to a pair of electrodes in the IDT. The electrically conductive sound absorbing material that is electrically connected and has a resistance value of 1 × 10 ^{3 to} 1 × 10 ⁸ Ω between the pair of electrodes through the sound absorbing material.

[0008]

[Operation and effect] Even if electric charge is biased to the pair of electrodes of the IDT due to pyroelectricity, the electric conduction is conducted through the conductive sound absorbing material before the electric discharge occurs between the electrodes, so that the electric discharge is prevented from occurring. it can. Further, the electrodes forming a pair of IDTs are electrically connected via a conductive sound absorbing material, but the signal of the electrodes is not affected by the resistance of the sound absorbing material. Furthermore, since the above-described actions and effects can be realized by using a conductive sound absorbing material as the sound absorbing material, the number of manufacturing steps is not increased.

[0009]

Embodiments of the present invention will be described below in detail with reference to the drawings. As shown in FIG. 1, the surface acoustic wave filter of the present invention has an input side IDT on a piezoelectric substrate (1).
(2) and the output side IDT (3) are arranged so as to face each other. The input-side IDT (2) is an apodized shape that is weighted so that the crossing length changes, and is formed so that the pitch of the electrode fingers is constant. On the outside of the electrode fingers, the input pads (21) (21)
And each electrode finger and input pad (21)
Connected by bonding. An input signal is applied to one side (upper side in the figure) of the input pad (21) and the other side (lower side in the figure) is grounded. A dummy electrode (7) is provided outside the input side IDT (2) and between the input pads (21) and (21) to the output side tip of the input side IDT (2). It is formed in parallel with the same pitch. This is because the input side IDT (2) changes due to the change in the crossing length of the input side IDT (2).
Is provided in order to prevent the phase of the surface acoustic wave excited by the laser light from shifting in the longitudinal direction of the electrode fingers. Output side IDT
(3) is a normal form with a constant intersection length and the input side IDT
It is formed at the same interval as the pitch of the electrode fingers of (2). Output pads (31) and (31) are formed on the outside of each electrode finger by wire bonding. One (upper side in the figure) of the output pad 31 transmits an output signal, and the other (lower side in the figure) is grounded. Between the input-side IDT (2) and the output-side IDT (3), the radio wave generated at the input-side IDT (2) is transmitted to the output-side IDT (2).
A shield electrode (4) is provided so that T (3) does not receive directly. The shield electrode (4) is grounded. The above structure is the same as that of the conventional surface acoustic wave filter (FIG. 2).
Further, the same material as the conventional one is used. That is, as the material of the piezoelectric substrate (1), LiNbO ₃ that propagates in a Y-cut X direction in 128 degrees rotation, LiTaO _{3 that} propagates in 112 degrees from the X-cut Y axis, etc. are used, and various electrodes (2) (3) (4) Aluminum is used as the material of (7). Also, various electrodes (2) (3) (4)
The thickness of (7) is about 0.1 to about 0.14 μm. Since LiNbO ₃ and LiTaO ₃ are trigonal crystals, they have not only piezoelectricity but also pyroelectricity. Further, the propagation velocity of the surface acoustic wave in the piezoelectric substrate (1) is 3000 to 4000 m / s.

The dummy electrode (7) is connected to each input pad (21) (21).
Is electrically connected to From each input pad (21) (21) to the position of the sound absorbing material provided outside the input side IDT (2),
A pattern wiring (60) is formed in each. Similarly, the pattern wiring (61) is formed from each output pad (31) (31) to the position of the sound absorbing material provided outside the output side IDT (3). The material and thickness of the pattern wirings (60) (61) are the same as those of the various electrodes (2) (3) (4) (7). After the above various electrodes and wirings are formed, the conductive sound absorbing material (51) is provided outside the IDTs (2) and (3). Conductive sound absorbing material (51)
Is the input side pattern wiring (60) through the conductive sound absorbing material (51)
To prevent the resistance between (60) and the output side pattern wiring (61) (61) from becoming 1 × 10 ^{3 to} 1 × 10 ⁸ Ω, use a conventional sound absorbing material such as epoxy resin with metal. It is formed by mixing a conductive material such as. The thickness of the conductive sound absorbing material (51) is about 50 μm
This is no different from the thickness of the conventional sound absorbing material (5).
The minimum resistance value of 1 × 10 ³ Ω is derived from the input impedance and the output impedance when the surface acoustic wave filter is replaced with an equivalent circuit. If the resistance is lower than this value, the input signal will be attenuated and lost. The maximum resistance value of 1 × 10 ⁸ Ω is derived from the insulation resistance between the electrode fingers. If the resistance value is higher than this value, the discharge cannot be prevented.

The surface acoustic wave filter having the above-described structure has a structure in which the IDTs (2) and (3) and the dummy electrodes (7) have a space between the electrode fingers even if charge is accumulated on the electrode fingers due to pyroelectricity. Before the electric discharge occurs, the pattern wirings (60) (61) and the conductive sound absorbing material (51) are brought into conduction, and the bias of the electric charge between the electrode fingers can be eliminated, and as a result, the electric discharge can be prevented. Furthermore, through the input pad (21) and the output pad (31) which are grounded,
Since electric charges due to pyroelectricity can be removed, it is possible to prevent electric discharge between the electrode fingers and other members. Also, pattern wiring (60) (61)
Can be formed in the same step as the step of forming the various electrodes (2) (3) (4) (7). The conductive sound absorbing material (51) is a conventional sound absorbing material.
Since it can be formed in the step of forming (5), there is no increase in manufacturing cost and manufacturing time due to the increase in manufacturing steps.

The radio wave generated at the input side IDT (2) is reflected by the shield electrode (4) and returned to prevent an influence on the input signal. (4) is arranged obliquely as compared with FIG. 2 so as not to be parallel to the longitudinal direction of the electrode fingers.

The above description of the embodiments is for explaining the present invention, and should not be construed as limiting the invention described in the claims or reducing the scope. or,
The configuration of each part of the present invention is not limited to the above-described embodiment, and it is needless to say that various modifications can be made within the technical scope described in the claims. For example, although the present embodiment has described the surface acoustic wave filter, other surface acoustic wave elements such as a resonator and a delay line may have the piezoelectric substrate (1) having pyroelectricity and I
The present invention can be applied to any surface acoustic wave device utilizing DT because it has the same problem.

[Brief description of the drawings]

FIG. 1 is a plan view of a surface acoustic wave filter of the present invention.

FIG. 2 is a plan view of a conventional surface acoustic wave filter.

FIG. 3 is a plan view of a surface acoustic wave filter having a conventional means for preventing electrode destruction due to discharge.

[Explanation of symbols]

 (1) Piezoelectric substrate (2) Input side IDT (3) Output side IDT (51) Conductive sound absorbing material

Claims (1)

[Claims] 1. A comb-shaped electrode transducer is provided on a piezoelectric substrate (1), and a sound absorbing material (5) is provided outside the electrode transducer to absorb unnecessary waves of surface acoustic waves excited by the electrode transducer. In the surface acoustic wave element, the sound absorbing material (5) is electrically connected to each pair of electrodes of the electrode transducer, and the resistance value between the pair of electrodes is 1 through the sound absorbing material. × 10 ^{3 to} 1
A surface acoustic wave element which is a conductive sound absorbing material (51) having a resistance of × 10 ⁸ Ω.