JPH10284983A - Surface acoustic wave device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a small-sized surface wave device with a pass band by providing an interdigital electrode on a substrage surface, constituting a substrate of LTG single crystal and selecting a segment angle from LTG crystal and the surface acoustic wave propagating direction of the substrate. SOLUTION: The surface acoustic wave device is provided with a pair of interdigital electrodes 2 on a substrate 1 surface. LTG single crystal is used for the substrate 1. LTG signal crystal is a crystal type which belongs to a dot group 32. When the segment angle and the surface acoustic wave propagating direction in the surface acoustic wave device are expressed, patterning is executed by combinating ≃=-5 to 5 deg., 5-15 deg., 15-25 deg. and 25-35 deg. (35 deg. is not included) with the ranges of θ and ψ so as to permit the surface acoustic wave propagating direction to be a prescribed direction. The device is generally suitable for a filter in a band where the frequency is 10-500 MHz, especially 10-300 MHz and also SAW speed is large so that it is useful for the minituarization of a surface acoustic wave delay element.

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 interdigital electrodes on a single crystal substrate.

[0002]

2. Description of the Related Art In recent years, mobile communication terminals such as mobile phones have rapidly become widespread. It is desired that the terminal be particularly small and lightweight because of its portability. In order to reduce the size and weight of the terminal, it is essential that the electronic components used in the terminal are also small and lightweight. A surface acoustic wave device, that is, a surface acoustic wave filter is frequently used. The surface acoustic wave device has a structure in which interdigital electrodes for exciting, receiving, reflecting, and propagating surface acoustic waves are formed on the surface of a piezoelectric substrate.

[0003] Important characteristics of a piezoelectric substrate used in a surface acoustic wave device include a surface acoustic wave velocity (SAW velocity) and an electromechanical coupling coefficient (k2). FIG. 4 shows the characteristics of a conventionally known representative substrate for a surface acoustic wave device. These characteristics are described in Yasutaka Shimizu, "Propagation Characteristics of Surface Acoustic Wave Materials and Present Situation of Utilization," IEICE Transactions A, Vol. J76-A, No. 2, pp. 129-1
37 (1993). Hereinafter, the piezoelectric substrates for various surface acoustic wave devices are distinguished by the symbols in FIG.

[0004] As can be seen from FIG.
6LT has a SAW speed of more than 4000m / s,
It is suitable for constructing a filter for a high-frequency part of a terminal. The reason will be described below. For mobile communication represented by mobile phones, various systems have been put into practical use in various countries around the world, and any system has a frequency of 1 GHz.
Front and rear frequencies are used. Therefore, the center frequency of the filter used in the terminal high frequency unit is about 1 GHz. In the case of a surface acoustic wave filter, the center frequency is substantially proportional to the SAW speed of the piezoelectric substrate used, and substantially inversely proportional to the width of the electrode finger formed on the substrate. Therefore, in order to increase the frequency, a substrate having a high SAW speed, for example, 6
4LN and 36LT are preferred. The high-frequency filter is required to have a wide band having a pass bandwidth of 20 MHz or more. In order to achieve a wide band, the electromechanical coupling coefficient k2 of the piezoelectric substrate must be large. For this purpose, 64LN and 36LT are frequently used.

On the other hand, as an intermediate frequency of a mobile terminal, a frequency band of 70 to 300 MHz is used. When a filter having this frequency band as a center frequency is configured using a surface acoustic wave, the 64LN, 3
When 6LT is used, the width of the electrode finger formed on the substrate is
The filter needs to be much larger than the filter used in the high frequency section.

The above will be described by estimating specific numerical values. The electrode finger width d of the surface acoustic wave converter constituting the surface acoustic wave filter and the center frequency f0 of the surface acoustic wave filter
F0 = V / (4d) (1) is approximately satisfied between the above and the SAW speed V of the piezoelectric substrate to be used. When a SAW speed is 4000 m / s and a surface acoustic wave filter having a center frequency of 1 GHz is constructed, the electrode finger width is obtained from the above equation (1) by d = 4000 (m / s) / (4 × 1000 (MHz)) = 1
μm. On the other hand, when an intermediate frequency filter having a center frequency of 100 MHz is formed using a piezoelectric substrate having a SAW speed of 4000 m / s, the electrode finger width required for this is d = 4000 (m / s) / (4 × 100 ( MHz)) = 10
μm, and the required electrode finger width is ten times as large as that of the filter in the high frequency part. Increasing the electrode finger width means that the surface acoustic wave device itself also increases. Therefore, in order to reduce the size of the surface acoustic wave intermediate frequency filter, it is necessary to use a piezoelectric substrate having a small SAW speed V as apparent from the above equation (1).

An ST crystal shown in FIG. 4 is known as a piezoelectric substrate having a relatively low SAW speed. Since the effective SAW speed of the piezoelectric substrate is affected by the electrode finger structure formed on the substrate, a simple statement cannot be made. However, in the case of ST quartz, it is known that the SAW speed is 3130 to 3155 m / s. Have been. This value is 64LN, 36LT
About ／ of that of the above, which is advantageous for miniaturization.

For the above-mentioned reason, the surface acoustic wave filter for the intermediate frequency of the mobile communication terminal has heretofore mostly been constituted by the ST quartz piezoelectric substrate. As is clear from FIG. 4, the electromechanical coupling coefficient k2 of the ST quartz substrate is 0.1
7%, which is particularly small among various piezoelectric substrates. The fact that k2 is small means that only a filter having a very narrow passband can be constructed.

[0009] Analog systems have been mainly used as mobile communication systems, that is, portable telephone systems, and their channel width is 1 in the domestic NTT specification.
Since the band was very narrow, 2.5 kHz, 30 kHz in the AMPS specification in the United States, and 25 kHz in the TACS specification in Europe, the electromechanical coupling coefficient k2 of the ST quartz substrate was not a problem. However, in recent years, digital mobile communication systems have been developed from the viewpoints of effective use of frequency resources, compatibility with digital data communication, and the like.
It has been put into practical use and is rapidly spreading. The channel width of this digital system is very wide, for example, 200 kHz in the European cellular phone GSM system and 1.7 MHz in the cordless telephone DECT system. When such a broadband intermediate frequency filter is formed of a surface acoustic wave filter, it is difficult to realize the ST crystal substrate.

[0010]

As described above,
Conventionally, in a surface acoustic wave device, when a piezoelectric substrate having a large electromechanical coupling coefficient is used, the pass band can be widened,
There is a problem that the element size is increased due to the high SAW speed of the substrate. On the other hand, when a substrate having a relatively low SAW speed is used to reduce the size of the element, the electromechanical coupling coefficient is too small. There is a problem that the pass band cannot be widened, and in any case, sufficient characteristics as a surface acoustic wave filter for an intermediate frequency cannot be obtained.

An object of the present invention is to provide a small surface acoustic wave device having a wide pass band.

[0012]

The above object is achieved by the following constitution. An interdigital electrode is provided on the surface of the substrate, and the substrate is a single crystal represented by the chemical formula La3Ta0.5Ga5.5O14 (LTG). A surface acoustic wave device in which φ, θ, and 存在 are present in any of the following regions when expressed as (φ, θ, ψ) in Euler angles. Area φ0 group {φ = −5 to 5 °} (excluding 5 °) area φ0-1 θ = 95 to 130 ° ψ = 0 to 30 ° area φ0-2 θ = 45 to 85 ° ψ = 0 to 15 ° region φ0-3 θ = 0 to 85 ° ψ = 15 to 45 ° region φ0-4 θ = 0 to 30 ° ψ = 75 to 90 ° region φ0-5 θ = 140 to 180 ° ψ = 75 to 90 ° region φ10 group {φ = 5 to 15 °} (excluding 15 °) area φ10-1 θ = 55 to 125 ° ψ = -40 to 40 ° area φ10-2 θ = 125 to 180 ° ψ = -35 to − 10 ° region φ10-3 θ = 125 to 180 ° ψ = 20 to 55 ° region φ10-4 θ = 0 to 55 ° ψ = -55 to -25 ° region φ10-5 θ = 0 to 55 ° ψ = 5 35 ° region φ10-6 θ = 0 to 25 ° ψ = 65 to 90 ° region φ10-7 θ = 130 to 180 ° ψ = -75 to −65 ° region φ20 group ｛ = 15 to 25 ° (excluding 25 °) area φ20-1 θ = 50 to 120 ° ψ = -35 to 30 ° area φ20-2 θ = 120 to 180 ° ψ = -30 to 0 ° area φ20- 3 θ = 140 to 180 ° ψ = 35 to 65 ° region φ20-4 θ = 0 to 85 ° ψ = −60 to −35 ° region φ20-5 θ = 0 to 50 ° ψ = -5 to 30 ° region φ20 -6 θ = 0 to 25 ° ψ = 55 to 90 ° region φ20-7 θ = 110 to 180 ° ψ = -65 to -55 ° region φ30 group φ = 25 to 35 ° (excluding 35 °) Area φ30-1 θ = 0 to 75 ° ψ = −60 to −40 ° area φ30-2 θ = 0 to 30 ° ψ = 45 to 75 ° area φ30-3 θ = 50 to 85 ° ψ = -30 to − 10 ° region φ30-4 θ = 50-85 ° ψ = 20-30 °

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A specific example of the present invention will be described in detail with reference to the drawings. FIG. 1 shows a configuration example of a surface acoustic wave device according to the present invention. This surface acoustic wave device has a pair of interdigital electrodes 2 on the surface of a substrate 1. The substrate 1 has LTG
Use a single crystal. The LTG single crystal is a crystal type belonging to the point group 32.

The x-axis, y-axis and z-axis in the figure are orthogonal to each other. The x-axis and the y-axis are in the in-plane direction of the substrate 1,
The x-axis defines the propagation direction of the surface acoustic wave. The z axis perpendicular to the substrate surface is the cutout angle (cut plane) of the single crystal substrate.
Is specified. The relationship between the x-axis, y-axis, and z-axis and the X-axis, Y-axis, and Z-axis of the langasite single crystal can be expressed by Euler angles (φ, θ, ψ).

When the cutout angle and the propagation direction in the surface acoustic wave device of the present invention are represented by (φ, θ, ψ), φ, θ
And ψ are present in each of the above regions.

In the φ0 to φ30 groups, there are combinations of φ, θ and ψ in which the SAW speed is as low as 2700 m / s or less and the coupling coefficient is 0.4% or more. In the φ0 group-1 to φ0 group-3, φ10 group-1 to φ10 group-6, φ20 group-1 to φ20 group-7, φ30 group-1 to φ30 group-4, the coupling coefficient is 0.8. %, There are combinations of φ, θ, and ψ. Further, in the φ0 group-3, the φ10 group-1 to the φ10 group-6, the φ20 group-1 to the φ20 group-7, and the φ30 group-2, combinations of φ, θ, and ψ at which the coupling coefficient is 1% or more are Exists.

Since the LTG single crystal is trigonal,
Due to the symmetry of the crystal, combinations of Euler angles equivalent to each other exist. For trigonal substrates, φ = 120-240 °
And φ = 240-360 ° (-120 ° -0 °) is φ
= 0-120 °, and θ = 360-18
0 ° (0 to −180 °) is equivalent to θ = 0 to 180 °, and ψ = 270 to 90 ° is equivalent to ψ = −90 to 90 °. For example, φ = 130 ° and φ = 250 ° are equivalent to φ = 10 °, θ = 330 ° is equivalent to θ = 30 °, and ψ = 240 ° is equivalent to ψ = 60 °.

In the case of a trigonal crystal substrate, by examining the range of φ = 0 to 30 °, it is possible to know the characteristics of all cut angles and propagation directions.

Therefore, in order to know the characteristics of all the cut angles and propagation directions in the LTG single crystal substrate, it is necessary to consider the range of φ0 = 0 to 30 °, θ0 = 0 to 180 °, and ψ0 = −90 to 90 °. It is sufficient to check only this (φ0, θ0, ψ
From the combination of (0), an equivalent combination of (φ, θ, ψ) showing the same characteristics at φ = 30 to 120 ° can be seen. Specifically, in the range of 30 ° ≦ φ ≦ 60 °, φ = 60 ° −φ0, θ = 180 ° −θ0, ψ = ψ0, and in the range of 60 ° ≦ φ ≦ 90 °, φ = 60 ° + φ0, θ = 180 ° −θ0, ψ = −ψ0, and in the range of 90 ° ≦ φ ≦ 120 °, φ = 120 ° −φ0, θ = θ0, ψ = −ψ0, (φ0, (φ, θ, ψ) equivalent to (θ0, ψ0) can be obtained. Then, based on the symmetry described above, the characteristics at all (φ, θ, ψ) can be known.

The following are specific examples of equivalent combinations. The equivalent to (0 °, 120 °, 30 °) is (60 °, 60 °, 30 °), (60 °, 60 °, −30 °), (120 °, 120 °, −30 °) Since φ = 120 ° and φ = 0 ° are equivalent, (0 °, 120 °, −30 °) is also equivalent. The equivalent to (10 °, 70 °, -20 °) is (50 °, 110 °, -20 °), (70 °, 110 °, 20 °), (110 °, 70 °, 20 °) It is.

Each of the above-mentioned regions defined in the present invention includes the equivalent combinations of (φ, θ, ψ) thus obtained.

The LTG single crystal used in the present invention is generally represented by the chemical formula La3Ta0.5Ga5.5O14,
The conventionally known langasite (La3Ga5SiO
14) (Preliminary proceedings of the 44th Federation of Applied Physics-related lectures NO.1 p213
Lecture NO.28p-N-1). In the present invention, when the LTG single crystal is applied to a substrate of a surface acoustic wave device, the above-described surface acoustic wave device with high characteristics is realized by selecting a crystal cutting direction and a surface acoustic wave propagation direction. LTG single crystal is
Inevitable impurities such as Al, Zr, Fe, Ce, N
d, Pt, Ca and the like may be contained. The method for producing the LTG single crystal is not particularly limited, and it may be produced by a normal single crystal growing method, for example, a CZ method.

The dimensions of the substrate are not particularly limited, but generally the surface wave propagation direction is about 4 to 10 mm, the direction orthogonal thereto is about 2 to 4 mm, and the thickness is about 0.2 to 0.4 mm.

Interdigital electrodes 2 and 2 formed on a substrate 1
Is a thin film electrode for exciting, receiving, reflecting, and propagating a surface acoustic wave, and is formed in a periodic stripe shape. The interdigital electrodes are patterned so that the surface acoustic wave propagation direction is the above-described predetermined direction. The interdigital electrodes may be formed by vapor deposition or sputtering using Au, Al, or the like. The electrode finger width of the interdigital electrode may be appropriately determined according to the frequency to which the surface acoustic wave device is applied, and is generally about 2 to 10 μm in a preferable frequency band to which the present invention is applied.

The surface acoustic wave device of the present invention is generally suitable for a filter in a frequency band of 10 to 500 MHz, particularly 10 to 300 MHz. Further, since the surface acoustic wave device of the present invention has a low SAW speed, it is also useful for downsizing a surface acoustic wave delay element.

[0026]

The present invention will be described below with reference to examples. C
A crystal was grown by the Z method to produce an LTG single crystal, which was cut out to obtain an LTG substrate. A surface acoustic wave converter consisting of a pair of interdigital electrodes was formed on the surface of the substrate to obtain a surface acoustic wave device. The interdigital electrode is formed on both the input side and the output side by vapor deposition of Al, and has a thickness of 0.3 μm.
The electrode finger width d is 15 μm, and the electrode pitch (4d) is 60 μm.
The number of electrode finger pairs was set to 20. A plurality of devices were manufactured by changing the cutout angle of the substrate and the propagation direction of the surface acoustic wave.

The cutout angle of the substrate and the propagation direction of the surface acoustic wave in these devices are represented by Euler angles (φ,
θ and ψ) are shown in FIGS. 2 and 3.

The SAW speed and the electromechanical coupling coefficient k2 of these devices were measured. The SAW speed is obtained from the center frequency of the filter. For the electromechanical coupling coefficient k2, the two-terminal admittance of surface acoustic wave conversion is measured.
From this, it was obtained by a well-known Smith equivalent circuit model. These results are shown in FIG. 2 and FIG.

As is apparent from FIGS. 2 and 3, the SAW speed is 2700 in the range from the group of φ0 to the group of φ30.
m / s or less, and electromechanical coupling coefficient is 0.4%
Since there is a combination of φ, θ, and な る which is twice as large as that of the conventional ST crystal and more than that of the conventional ST crystal, it is advantageous for downsizing and broadening of the surface acoustic wave device. Further, the SAW speed is 2700 in the range from the group φ0 to the group φ30.
m / s or less, and a combination of φ, θ, and な る whose electromechanical coupling coefficient is 1% or more and 5.5 times or more as large as that of the conventional ST crystal exists. This is advantageous due to miniaturization and broadband.

The effects of the present invention are apparent from the results of the above examples.

[0031]

According to the present invention, φ, θ and ψ are defined in the region φ.
When it is within the 0 group to the φ30 group, the SAW speed is small and the electromechanical coupling coefficient is large, so that the surface acoustic wave device can be miniaturized and the pass band width as a filter can be widened. Good characteristics can be obtained as an intermediate frequency surface acoustic wave filter of a terminal.

[Brief description of the drawings]

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

FIG. 2 is a table showing characteristics of the surface acoustic wave device according to the present invention.

FIG. 3 is a table showing characteristics of the surface acoustic wave device according to the present invention.

FIG. 4 is a table showing characteristics of a typical substrate for a conventional surface acoustic wave device.

[Explanation of symbols]

 1 substrate 2 interdigital electrode

Claims (1)

[Claims] An interdigital electrode is provided on a surface of a substrate, wherein the substrate is a single crystal represented by a chemical formula of La3 Ta0.5 Ga5.5 O14 (LTG), and a cut-out angle of the substrate from an LTG crystal and a cut-off angle of the substrate. When the surface acoustic wave propagation direction is expressed as (φ, θ, ψ) in Euler angles, φ, θ, and ψ
Is a surface acoustic wave device existing in any of the following regions. Area φ0 group {φ = −5 to 5 °} (excluding 5 °) area φ0-1 θ = 95 to 130 ° ψ = 0 to 30 ° area φ0-2 θ = 45 to 85 ° ψ = 0 to 15 ° region φ0-3 θ = 0 to 85 ° ψ = 15 to 45 ° region φ0-4 θ = 0 to 30 ° ψ = 75 to 90 ° region φ0-5 θ = 140 to 180 ° ψ = 75 to 90 ° region φ10 group {φ = 5 to 15 °} (excluding 15 °) area φ10-1 θ = 55 to 125 ° ψ = -40 to 40 ° area φ10-2 θ = 125 to 180 ° ψ = -35 to − 10 ° region φ10-3 θ = 125 to 180 ° ψ = 20 to 55 ° region φ10-4 θ = 0 to 55 ° ψ = -55 to -25 ° region φ10-5 θ = 0 to 55 ° ψ = 5 35 ° region φ10-6 θ = 0 to 25 ° ψ = 65 to 90 ° region φ10-7 θ = 130 to 180 ° ψ = -75 to −65 ° region φ20 group ｛ = 15 to 25 ° (excluding 25 °) area φ20-1 θ = 50 to 120 ° ψ = -35 to 30 ° area φ20-2 θ = 120 to 180 ° ψ = -30 to 0 ° area φ20- 3 θ = 140 to 180 ° ψ = 35 to 65 ° region φ20-4 θ = 0 to 85 ° ψ = −60 to −35 ° region φ20-5 θ = 0 to 50 ° ψ = -5 to 30 ° region φ20 -6 θ = 0 to 25 ° ψ = 55 to 90 ° region φ20-7 θ = 110 to 180 ° ψ = -65 to -55 ° region φ30 group φ = 25 to 35 ° (excluding 35 °) Area φ30-1 θ = 0 to 75 ° ψ = −60 to −40 ° Area φ30-2 θ = 0 to 30 ° ψ = 45 to 75 ° Area φ30-3 θ = 50 to 85 ° ψ = -30 to − 10 ° region φ30-4 θ = 50-85 ° ψ = 20-30 °