JPH11214956A - Surface-acoustic-wave device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface-acoustic-wave device, which is provided with an optimum crystal cutting angle of an LGN that is expected to surpass the quality of convention piezoelectric materials, and the propagation direction of a surface acoustic wave. SOLUTION: In this surface-acoustic-wave device 2, which provides an excitation electrode 2 that generates a surface acoustic wave on a substrate 1 that consists of La3 Ga5.5 Nb0.5 O14 single crystal, and a cutting angle of the substrate and the propagation direction of a surface acoustic wave satisfy ϕ=10 deg.+60 deg.×n1, θ=125 deg.+180 deg.×n2, ψ=70 deg.+180×n3 (where, n1 to n3 are integers) in Eulerian angle representation (ϕ, θand ψ).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a single crystal of lanthanum-gallium-niobium oxide La ₃ Ga _5.5 Nb.
_{The present} invention relates to a surface acoustic wave device using a _0.5 O ₁₄ single crystal as a substrate material.

[0002]

2. Description of the Related Art Conventionally, a surface acoustic wave device in which an excitation electrode for generating a surface acoustic wave is provided on a piezoelectric substrate has been known. Quartz, niobium is used as a substrate material of the surface acoustic wave device. Single crystals such as lithium oxide and lithium tantalate have been used and put to practical use.

[0003] Of these single crystal materials, quartz has a small change in characteristics with respect to temperature change, and thus is suitably used for filters, resonators and the like of mobile phones, etc., but has the drawback of having a small pass bandwidth. I have.

[0004] On the other hand, lithium niobate and lithium tantalate have the advantage of having a large pass band width, and are suitably used for filters of mobile phones, VTRs and the like. It has the disadvantage of being large.

[0005] Lithium tetraborate single crystals are known as a supplement to the above-mentioned disadvantages of the materials, but the requirements have not been sufficiently satisfied due to the deliquescence problem peculiar to lithium tetraborate single crystals.

In recent years, as a material satisfying the above requirements, an electromechanical coupling coefficient (k) used as a performance evaluation of a piezoelectric material has been proposed.
² ) Langasite (La ₃ ) having a large cut surface and propagation direction with a large group delay time temperature coefficient (TCD)
Ga ₅ SiO ₁₄ ) (for example, H.satoh
and A.Mori: Jpn.J.Appl.Phys.Vol.36 (1997) pp.3071-30
63).

[0007] More recently, a single crystal of La ₃ Ga _5.5 Nb _0.5 O ₁₄ (hereinafter abbreviated as LGN) having a crystal structure similar to this langasite has attracted attention as a piezoelectric material, and has a TCD as small as quartz. Moreover, it is highly expected that k ² is larger than the above-mentioned langasite.

However, there has been no report on the optimum crystal cut angle of the above-mentioned LGN and the propagation direction of the surface acoustic wave, and it has been unknown.

Accordingly, the present invention provides a surface acoustic wave device having an optimum crystal cut angle of LGN and a propagation direction of surface acoustic waves, which are expected to have properties superior to conventional piezoelectric materials. With the goal.

[0010]

In order to achieve the above object, the present inventors have developed La ₃ Ga _5.5 Nb which has been actually grown.
By computer simulation performed based on the characteristic values of _0.5 O ₁₄ single crystal (LGN), the cut surface where the basic characteristic values such as the electromechanical coupling coefficient and the group delay time temperature coefficient are optimal, and the propagation direction of the surface acoustic wave are determined. In a surface acoustic wave device in which an excitation electrode for generating a surface acoustic wave is formed on an LGN substrate that has been searched and actually manufactured, the cut angle of the substrate and the direction in which the surface acoustic wave propagates are represented by a right-handed Euler angle display (φ , Θ, ψ), φ = 10 ° + 60 ° × n1, θ =
125 ° + 180 ° × n2, ψ = 70 ° + 180 ° × n
3 (where n1, n2 and n3 are integers). Here, it is assumed that the parameters φ, θ, and さ れ る are allowed as error ranges of about ± 5 °, ± 10 °, and ± 10 °, respectively.

As shown in FIG. 1, the crystal axes are X, Y,
(Φ, θ, ψ) displayed when Z is Z, the propagation direction of the surface acoustic wave is a, the direction perpendicular to the substrate is c, and the direction perpendicular to a and c is b, is referred to as Euler angle display. However, the X axis is particularly defined as the <11.0> direction.

The above-mentioned Euler angle display is based on the point group P32.
As described above, various values can be taken in consideration of the symmetry of one LGN, the periodicity with respect to the surface acoustic wave, and the like.

[0013]

Embodiments of the present invention will be described below in detail with reference to the drawings. La ₃ Ga _5.5 Nb
_{The 0.5} O ₁₄ single crystal (LGN) is mixed with a single crystal raw material (a mixture of La ₂ O ₃ , Ga ₂ O ₃ , and Nb ₂ O ₅ , or La) in a high-frequency heating or resistance heating single crystal growing furnace. ₃ Ga _5.5 Nb _0.5 O ₁₄ ) is heated to a predetermined temperature to melt the raw material.
The seed crystal is immersed in the melt, and the seed crystal is grown at a predetermined rotation speed and pulling speed with respect to the melt.

Next, from the obtained grown single crystal, in terms of Euler angles (φ, θ, ψ), φ = 10 ° (5 ° to 1 °).
5 °), θ = 35 ° (25 ° to 4 °)
Cut out a substrate with a cut surface of 5 °), perform mirror polishing, and propagate the surface acoustic wave on this substrate,
That is, the excitation electrode is formed so as to coincide with ψ = 70 ° (however, allowable in the range of 60 ° to 80 °), and the propagation type surface acoustic wave device S as shown in FIG.

That is, after a metal such as aluminum is formed on the cut surface 1a of the substrate 1 by vacuum evaporation,
Comb-shaped excitation electrodes 2a (input side) and 2b (output side) are formed by a lift-off method and / or an etching method. In the figure, A is a power supply for inputting an AC signal to the excitation electrode on the input side, and B is a detector for detecting an electric signal.

Here, the reason why φ, θ, and ψ are determined to be the above-mentioned angles is that, according to the computer simulation, the crystal orientation and the propagation direction of the substrate surface in the above-mentioned range have the best basic characteristics as a bandpass filter. Because it was expected.

This computer simulation analysis method uses the physical properties of the single crystal grown as described above, and further uses a method such as Cambell (for example, JJCamb).
ellet al: see IEEE.Trans.Sonics.Ultrason.15 (1968) 209). The parameter is c
Constant (N / m ² ), e constant (C / m ² ), relative permittivity, linear expansion coefficient, density (Kg / m ³ ), and their primary temperature coefficient (/ ° C.) and secondary temperature coefficient (/ ° C ² ) was used.

According to the results of the computer simulation, as shown in FIGS. 3 and 4, the electromechanical coupling coefficient (k ² ) is large and the group delay time temperature coefficient (TCD) at room temperature is about 0 ° C./ppm. And the propagation direction of the surface acoustic wave is (φ, θ, ψ) = (10 ° + 6)
0 ° × n1, 125 ° + 180 ° × n2, 70 ° + 18
0 ° × n 3). Note that n1 to n3
Is an integer.

Also, according to the results shown in FIG.
When the TCD is approximately zero (70 ° ± 10 °), φ at which the TCD becomes almost zero is 3 ° to 17 °, θ is 112 ° to 135 °, and according to the results shown in FIG. In the case of about 70 °, φ at which k ² is 0.80% or more is 5 ° to 17 °.
° and θ were 25 to 45 °. The higher the value of k ² , the better, but the reference value is set to 0.80% because the lower limit of k ^{2 in} a practically used LiTaO ₃ single crystal or lithium tetraborate single crystal is approximately 0.80%. %.

Note that the above Euler angles are indicated by the point group P32.
The above numerical values can be taken according to the symmetry of LGN 1 and the periodicity of the surface acoustic wave.

That is, for example, in the first crystal, φ = 10
°, 70 °, 130 °, 190 °, 250 °, 310 °
And θ = 125 ° and 305 ° can be treated as equivalent angles.

With respect to the surface acoustic wave device manufactured as described above, k ² and TCD were measured, and a good measurement of the propagation direction was performed as a band-pass filter. Obtained. Further, the La ₃ Ga _5.5 Nb _0.5 O ₁₄ single crystal has a relatively small surface acoustic wave speed of about 2700 m / sec, and can be miniaturized when used as a low-frequency filter, and is suitably used as a band-pass filter. It is possible.

The surface acoustic wave device can be applied not only to the propagation type but also to various types of filters and vibrators such as a resonator type, and can be appropriately modified and implemented without departing from the gist of the present invention. is there.

[0024]

Next, more specific embodiments of the present invention will be described. [Example 1] La ₃ Ga _5.5 Nb mixed in a harmonic composition ratio
2500 g of a raw material of _0.5 O ₁₄ single crystal, inner diameter φ100 m
m and a height of 90 mm, and charged in a high-frequency single crystal growing furnace while flowing an atmosphere gas adjusted so that Ar: O ₂ = 90: 10.
After melting at ° C, a single crystal was grown by bringing a seed crystal into contact with the melt surface.

Next, the optimum cut plane obtained from the grown crystal by the computer simulation, that is, φ = 10 ° and θ = 12 in Euler angle display as shown in FIG.
A surface of 5 ° and ５ is cut out and mirror-polished, and as schematically shown in FIG. 2, an electrode finger width of about 3 μm, an input electrode 1a having 280 electrode fingers, as schematically shown in FIG. Excitation electrodes and the like consisting of 60 electrode fingers of the output side electrode 1b were formed so as to be in a predetermined direction, and a propagation type surface acoustic wave device S was manufactured. A is an AC power supply, and B is a current detector.

The excitation electrode is formed by depositing aluminum to a thickness of about 3500 ° by a vacuum evaporation method and then forming it in a comb shape by a lift-off method.

Next, the group delay time temperature coefficient (TCD) and the electromechanical coupling coefficient (k ² ) of the surface acoustic wave device manufactured as described above were determined.

Here, the measurement of TCD was calculated from the rate of change of the center frequency (here, about 79 MGHz) with respect to the temperature, and k ² was calculated by the following equation using a network analyzer.

TCD (ppm / ° C.) = Α− (ΔV _O / ΔT) V _OT [ppm / ° C.] (1) k ² = 2 × (V _O −V _m ) / V _O × 100 [%] (2) where α is the coefficient of linear expansion in the propagation direction, V _O is the propagation speed when the substrate surface is electrically open, and V _m is the case when the substrate surface is electrically short-circuited. , And V _OT is the propagation speed at 25 ° C.

As a result of this measurement, the Euler angle display (φ,
θ, ψ), TCD and k ² of the surface acoustic wave device in the propagation direction represented by φ = 10 °, θ = 35 °, ψ = 70 ° are almost 0 (ppm / ° C.) and 0.88, respectively. (%),
Very good results could be obtained.

[0031]

As described above, according to the surface acoustic wave device of the present invention, the group delay time temperature coefficient (TCD) is almost zero because the excitation electrode is formed on the substrate having the optimum cut surface. In addition, it is possible to provide a surface acoustic wave device having an excellent substrate having a very large electromechanical coupling coefficient (k ² ).

The La ₃ Ga _5.5 Nb _0.5 O ₁₄ single crystal has a relatively small surface acoustic wave velocity and can be easily miniaturized when used as a low-frequency filter. An apparatus can be provided.

[Brief description of the drawings]

FIG. 1 is a schematic diagram for explaining Euler angle display.

FIG. 2 is a perspective view schematically showing a surface acoustic wave device according to the present invention.

FIG. 3 is a graph showing a relationship between a cut surface and TCD when ψ = 70 °.

FIG. 4 is a graph showing a relationship between a cut plane and k ² when ψ = 70 °.

[Explanation of symbols]

 1: substrate 1a: cut surface 2: excitation electrode S: surface acoustic wave device

Continuation of front page (72) Inventor Shinji Inoue 3-5 Koikodai, Seika-cho, Soraku-gun, Kyoto Prefecture Kyocera Corporation Central Research Laboratory

Claims (1)

[Claims] 1. A surface acoustic wave device comprising a substrate made of a La ₃ Ga _5.5 Nb _0.5 O ₁₄ single crystal and an excitation electrode for generating a surface acoustic wave, wherein the substrate has a cut angle and a surface acoustic wave. Euler angle display (φ,
A surface acoustic wave device wherein each parameter of θ, ψ) satisfies the following equation. φ = 10 ° + 60 ° × n1, θ = 125 ° + 180 ° × n2, ψ = 70 ° + 180 ° × n3 (where n1, n2 and n3 are integers)