JPH0998055A - Surface acoustic wave device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the noises which are caused by the bulk waves by fixing a surface acoustic wave device onto the inner surface of a package by means of an adhesive having its acoustic impedance equal or extremely equal to that of a substrate. SOLUTION: A surface acoustic wave element which can excite the vertical leaky waves is put into a package consisting of a stem 1 and a cover 2. A piezoelectric substance 4 is fixed on the surface of the stem 1 via an adhesive layer 3 different from the conventional one. That is, the layer 3 uses adhesive which has its acoustic impedance that is higher than the conventional adhesive consisting primarily of the epoxy resin containing the silver powder and equal or extremely equal to the acoustic impedance of the substance 4. In other words, the layer 3 uses an adhesive that consists primarily of the epoxy resin containing gold powder. As a result, the noises due to the reflected waves can be effectively reduced.

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 device having a surface acoustic wave element housed in a package.

[0002]

2. Description of the Related Art Conventionally, a surface acoustic wave element has been used as a circuit element such as a resonator filter and a signal processing delay line in a communication device such as a mobile phone. Along with this, there is an increasing need for surface acoustic wave devices that can be used in the gigahertz band.

As a method of realizing a surface acoustic wave element that can be used in such a high frequency band, a method of using a wave having a high acoustic velocity and a method of narrowing the electrode pitch by fine electrode processing are considered. The former method includes a method using a high acoustic velocity substrate typified by diamond and a method using so-called leaky surface acoustic waves. A leaky surface acoustic wave is a wave that propagates on the surface while radiating energy in the depth direction of the elastic body. By appropriately selecting the cut surface or the surface acoustic wave propagation direction, the propagation loss is reduced and the Rayleigh surface wave is reduced. It is possible to obtain higher propagation velocities than waves.

[0004] As a practical substrate which is able to excite the typical LSAW, quartz LST cut, lithium tantalate _{(LiTaO 3) 36 ° Y-} X cut of lithium niobate (LiNbO ₃₎ 41 ° Y-X cut, 6
A 4 ° Y-X cut is known. Recently, a leaky surface acoustic wave, which is called a longitudinal wave type leaky wave, which exceeds a high shear wave velocity has been found on a lithium tantalate substrate, a lithium niobate substrate, and a lithium tetraborate (Li ₂ B ₄ O ₇ ) substrate. Has been done. For example, regarding the lithium tantalate substrate, there is a report in the "15th Symposium on Fundamentals and Applications of Ultrasonic Electronics" (Proceedings of the lecture distributed on November 28, 1994), and regarding lithium niobate, "LiNbO ₃ Substrate Leakage Surface Acoustic Wave Characteristics and New Cut "(The Institute of Electrical Communication Engineers, Vol.J69-C, 10, pp1309-1318,19
86), and regarding the lithium tetraborate substrate,
"Longitudinal leaky wave propagating in lithium tetraborate substrate"
(Scholarly Society 150 Committee 39th Workshop Material, June 23, 1994)
There is a report.

In the surface acoustic wave device using such a longitudinal wave type leaky wave, research has been conducted on a cut surface and a propagation direction in which an electro-mechanical coupling coefficient, a propagation efficiency, a temperature characteristic and the like are optimal. .

When the surface acoustic wave device is mounted on a circuit board, the surface acoustic wave device is housed in a package consisting of a stem (1) and a cover (2) as shown in FIG. In FIG. 4, the surface acoustic wave device is formed by forming a plurality of comb-shaped electrode pieces (not shown) on the surface of a piezoelectric substrate (4), and an adhesive layer on the stem (1).
It is fixed by (9). On the stem (1), a plurality of terminal pins (6) for input and output are provided so as to project downward while keeping insulation from the stem (1), and a single ground pin (7).
Are electrically connected to the stem (1) and project downward. Then, the above-mentioned electrodes and the corresponding terminal pins (6)
And the base end of each of them are connected to each other by a bonding wire (8).

The adhesive layer (9) is mainly composed of a thermosetting resin such as an epoxy resin or a silicone resin, and is used as a metal powder, generally a fine silver powder, to obtain conductivity. Has been added. As the piezoelectric substrate (4), if the piezoelectric substrate capable of exciting the above-mentioned longitudinal wave type leaky wave is adopted and the cut surface and the surface acoustic wave propagation direction are appropriately selected, it is possible to use the high frequency usable in the gigahertz band. A high-performance surface acoustic wave device is obtained.

[0008]

However, the leaky surface acoustic wave is a surface acoustic wave which propagates while radiating a bulk wave into the inside of the substrate although the energy of the wave is concentrated on the surface of the substrate. Therefore, in FIG. As indicated by the wavy line, the bulk wave W emitted in the depth direction is reflected at the interface between the piezoelectric substrate (4) and the adhesive layer (9), and the reflected wave W'is received by the receiving electrode. As a result, there is a problem that noise is generated in the output signal. In particular, the longitudinal leaky wave has a large amount of bulk waves radiated into the inside of the substrate, and the above problem is serious.

An object of the present invention is to provide a surface acoustic wave device capable of reducing the noise caused by the bulk wave radiated in the depth direction of the substrate.

[0010]

SUMMARY OF THE INVENTION A surface acoustic wave device according to the present invention includes a surface acoustic wave element having a substrate having at least a surface layer portion having piezoelectricity and an electrode for exciting a surface acoustic wave, and a surface acoustic wave element inside the package. In the surface acoustic wave device housed in
The surface acoustic wave element is fixed to the inner surface of the package by an adhesive layer having the same acoustic impedance as that of the substrate or the same acoustic impedance as possible. Specifically, the surface acoustic wave element has a substrate and an electrode arrangement capable of exciting a leaky surface acoustic wave (longitudinal wave type leaky wave) having longitudinal waves as a main component.

In the surface acoustic wave device of the present invention,
Since the acoustic impedances of the substrate and the adhesive layer are substantially the same, the sound pressure transmittance and the energy transmittance at the interface between the substrate and the adhesive layer are approximately 1. Therefore, most or all of the bulk wave radiated to the inside of the substrate is not reflected at the interface, propagates inside the adhesive layer, and is attenuated inside the bonding layer.

[0012]

In the surface acoustic wave device according to the present invention, all or most of the bulk wave radiated to the inside of the substrate is not reflected at the interface between the substrate and the adhesive layer, but is transmitted and attenuated inside the bonding layer. Therefore, the generation of noise due to the bulk wave is effectively prevented.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention applied to a surface acoustic wave filter will be described in detail with reference to the drawings. As shown in FIGS. 1 and 2, a surface acoustic wave filter according to the present invention includes a surface acoustic wave element capable of exciting a longitudinal leaky wave in a package including a stem (1) and a cover (2). It is configured to house. The surface acoustic wave device of this example uses a piezoelectric substrate (4) made of a lithium tantalate single crystal and having a cut surface in Euler angle display (90 °, 90 °, 31 °). A plurality of comb-shaped electrodes (5) are formed on the surface of (4). The thickness of the piezoelectric substrate (4) is, for example, 300 to 400 μm. A DC sputtering device is used to form the electrode (5), and the thickness of the Al-
After forming a thin film of Cu alloy, the thin film was formed into a predetermined pattern by an ordinary photolithography method.

The piezoelectric substrate (4) is formed on the surface of the stem (1) by
It is fixed by an adhesive layer (3) having a property different from the conventional one. That is, the adhesive layer (3) has a higher acoustic impedance than that of an adhesive containing silver powder-added epoxy resin as a main component, which has been generally used in the past, and has a piezoelectric substrate.
It is formed by using an adhesive having the same or as much as possible acoustic impedance as that of (4), specifically, an adhesive whose main component is an epoxy resin to which fine gold powder is added. The thickness of the adhesive layer (3) is, for example, 100 to 200 μm.

A plurality of input / output terminal pins (6) are provided on the stem (1) so as to project downward while maintaining insulation from the stem (1), and a single ground pin (7). Stem (1)
It is electrically connected to and protrudes downward. Each of the electrodes and the corresponding base end of the terminal pin (6) are connected to each other by a bonding wire (8).

By the way, generally, the acoustic impedance of a certain medium is defined by the product of the density of the medium and the speed of sound. Here, the acoustic impedances of the media A and B are respectively Z
_A and Z _B , the sound pressure transmittance τ _P from medium A to medium B
And the energy transmittance τ _E are represented by the following equations 1 and 2, respectively.

[Equation 1] τ _P = 2Z _B / (Z _A + Z _B )

[Equation 2] τ _E = 4Z _A Z _B / (Z _A + Z _B ) ²

Therefore, if the acoustic impedances of the medium A and the medium B are set to be equal, both the sound pressure transmittance τ _P and the energy transmittance τ _E become 1 and the reflection of sound waves at the interface between both media is prevented. Can be done.

In the surface acoustic wave filter, since the acoustic impedances of the piezoelectric substrate (4) and the adhesive layer (3) are substantially the same, the sound pressure at the interface between the piezoelectric substrate (4) and the adhesive layer (3). The transmittance and the energy transmittance are approximately 1,
As indicated by the wavy line in FIG. 2, the bulk wave W radiated in the depth direction is hardly reflected on the interface and propagates inside the adhesive layer (3). Therefore, the noise caused by the reflected wave, which has been a problem in the conventional device, can be effectively reduced.

FIG. 3 shows a surface acoustic wave filter of the present invention having an adhesive layer containing fine gold powder and a conventional surface acoustic wave filter having an adhesive layer containing fine silver powder. The measured values of the frequency characteristics are graphed and the two are compared. The solid line represents the frequency characteristic of the surface acoustic wave filter of the present invention, and the broken line represents the frequency characteristic of the conventional device.

As is apparent from the figure, in the frequency characteristics of the surface acoustic wave filter of the present invention, the amount of attenuation outside the pass band is several dB to 10 dB higher than in the conventional case.
This is because, in the conventional device, the noise due to the reflected wave is superimposed on the frequency component outside the pass band, whereas in the device of the present invention, the amount of this noise superimposed is significantly reduced. Further, even in the pass band, according to the device of the present invention, it is possible to suppress the ripple caused by the reflected wave.

The description of the above embodiments is for the purpose of describing the present invention, and should not be construed as limiting the invention described in the claims or reducing the scope thereof.
In addition, the configuration of each part of the present invention is not limited to the above-described embodiment, and it goes without saying that various modifications can be made within the technical scope described in the claims. For example, in the above embodiment,
The acoustic impedance was adjusted by using high-density fine gold powder as the metal powder to be added to the adhesive, but the present invention is not limited to this, and other metal powder having a density according to the acoustic impedance to be realized is used. A method or a method of using an adhesive containing a resin other than an epoxy resin as a main component can also be adopted. It is also possible to adjust the acoustic impedance by changing the hardness of the adhesive.

In the above embodiment, the piezoelectric substrate (4)
As the substrate, a lithium tantalate substrate with Euler angles (90 °, 90 °, 31 °) is used, but other cut surfaces and surface acoustic wave propagation directions can be used, and other piezoelectric materials such as niobate can be used. It is also possible to use lithium or lithium tetraborate. Further, although the can type package is adopted in the above-mentioned embodiment, it is also possible to adopt another package, for example, a surface mount type package. Further, the present invention is not limited to the surface acoustic wave filter and can be implemented in various surface acoustic wave devices such as a resonator, a delay line, a convolver, and the like.

[Brief description of drawings]

FIG. 1 is a perspective view showing a configuration of a surface acoustic wave device according to the present invention.

FIG. 2 is a cross-sectional view of the surface acoustic wave device.

FIG. 3 is a graph comparing the frequency characteristics of the surface acoustic wave device of the present invention and the conventional device.

FIG. 4 is a sectional view of a conventional device.

[Explanation of symbols]

 (1) Stem (2) Cover (3) Adhesive layer (4) Piezoelectric substrate (5) Electrode (6) Terminal pin (7) Earth pin (8) Bonding wire

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tatsuro Usuki 2-5-5 Keihan Hondori, Moriguchi-shi, Osaka Sanyo Electric Co., Ltd. (72) Kenichi Shibata 2-5 Keihan-hondori, Moriguchi-shi, Osaka No. 5 Sanyo Electric Co., Ltd.

Claims (2)

[Claims] 1. A surface acoustic wave device in which a surface acoustic wave element having a substrate having at least a surface layer portion having piezoelectricity and an electrode for exciting a surface acoustic wave is housed inside a package. The surface acoustic wave device is characterized in that the surface acoustic wave element is fixed to the inner surface of the package by an adhesive layer having the same acoustic impedance or the same acoustic impedance as that of the substrate. 2. The surface acoustic wave device according to claim 1, wherein the surface acoustic wave element has a substrate and an electrode arrangement capable of exciting a leaky surface acoustic wave having a longitudinal wave as a main component.