JP2000022490A - Surface acoustic wave device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface acoustic wave device where an optimum cut angle is applied to an oxide single crystal of a lanthanum-gallium-niobium group, lanthanum-gallium-tantalum group or lanthanum-gallium-niobium-tantalum group and an optimum propagation direction is given to a surface acoustic wave. SOLUTION: The surface acoustic wave device S is formed by providing exciting electrodes 2a, 2b generating a surface acoustic wave on a piezoelectric substrate made of the oxide single crystal. Each parameter of Euler's angle representation (ϕ, θ, ψ) representing a cut angle of the substrate 1 and a propagation direction satisfies relations of ϕ=a1+60 deg.×b1, θ=a2+180 deg.×b2, ψ=a3+180 deg.×b3 (where, 18 deg.<=a1<=22 deg., 125 deg.<=a2<=130 deg., 148 deg.<=a3<=152 deg., and b1, b2, b3 are integers).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a surface acoustic wave device using a lanthanum-gallium-niobium (and / or tantalum) -based oxide crystal, for example, a single crystal of La ₃ Ga _5.5 Nb _0.5 O ₁₄ as a substrate material. It is about.

[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. Mobile communication devices such as telephones are rapidly spreading. As a substrate material of a surface acoustic wave device used for these communication devices, a single crystal having a large electromechanical coupling coefficient, such as lithium niobate or lithium tantalate, has been put to practical use in a high frequency stage as a bandpass filter. On the other hand, for the intermediate frequency stage, a substrate material such as quartz, lithium tetraborate, etc., whose change in characteristics is small with respect to temperature change, is generally used.

[0003] In recent years, as the demands for downsizing, lower loss, and wider band of the surface acoustic wave device have increased, there has been a demand for a material having a large electromechanical coupling coefficient and having a frequency zero temperature coefficient at room temperature. As a material satisfying the above requirements, a langasite (La) having a cut surface and a propagation direction having a large electromechanical coupling coefficient (k ² ) and a small group delay time temperature coefficient (TCD) used as a performance evaluation of a piezoelectric material. ₃ Ga ₅
SiO ₁₄ ) has attracted attention (for example, H. satoh).
and A. Mori: Jpn. J. Appl. Ph
ys. Vol. 36 (1997) pp. 3071-3
063 etc.).

Further, recently, a La ₃ Ga _5.5 Nb _0.5 O ₁₄ single crystal (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 lanthanum-based material which is expected to have properties superior to conventional piezoelectric materials.
Provided is a surface acoustic wave device having an optimum cut angle of a gallium-niobium-based, lanthanum-gallium-tantalum-based, or lanthanum-gallium-niobium-tantalum-based oxide single crystal, and a propagation direction of a surface acoustic wave. With the goal.

[0007]

Means for Solving the Problems In order to achieve the above object, the present inventors have focused on leaky waves among surface acoustic waves which are considered to have a large electromechanical coupling coefficient among surface acoustic waves, and have actually focused on leaky waves. By computer simulation performed based on the characteristic values of the grown LGN, a cut plane where the electromechanical coupling coefficient of the leaky wave is large and the group delay time temperature coefficient at room temperature becomes zero, and a propagation direction are searched,
Based on this result, a surface acoustic wave device was actually manufactured.

Thus, in a surface acoustic wave device in which an excitation electrode for generating a surface acoustic wave is formed on an LGN substrate, the cut angle of the substrate and the direction in which the surface acoustic wave propagates can be represented by a right-handed Euler angle display (φ, θ). , Ψ), φ = a1 + 60
° × b1, θ = a2 + 180 ° × b2, ψ = a3 + 18
0 ° × b3 (however, 18 ° ≦ a1 ≦ 22 °, 125 ° ≦
a2 ≦ 130 °, 148 ° ≦ a3 ≦ 152 °, b1, b
2, b3 is an integer), it has been found that a surface acoustic wave device having particularly good filter characteristics can be obtained.

Further, since the LGN is good as described above, instead of niobium in the lanthanum-gallium-niobium system, tantalum or a niobium which is a homologous element with niobium is used.
Similarly, good filter characteristics can be obtained by using tantalum.

Incidentally, 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, in particular, the X axis is defined as the <110> direction of the LGN single crystal.

The propagation direction is the LGN of the point group 321.
Considering the symmetry of φ, φ and θ are 60 ° and 180 ° respectively.
Because of the periodicity of the surface acoustic wave of °, φ = 1
0 ° = 70 ° = 130 ° = 190 ° = 250 ° = 310
° and θ = 125 ° = 305 °, which can be treated as equivalent angles.

[0012]

Embodiments of the present invention will be described below in detail with reference to the drawings.

[0013] La ₃ Ga _5.5 Nb _0.5 O ₁₄ single crystal (LG
N) is a single crystal raw material (La ₂ O ₃ , Ga ₂ ) in a single crystal growing furnace of a high frequency heating system or a resistance heating system.
A mixture of O ₃ and Nb ₂ O ₅ , or La ₃ Ga
_The raw material is melted by heating a crucible containing _5.5 Nb _0.5 O ₁₄ ) to a predetermined temperature, a seed crystal is immersed in the melt, and the seed crystal is pulled up by a predetermined number of revolutions with respect to the melt. The crystal grows at a speed.

Next, a substrate having a cut surface with Euler angles of φ = 18 ° to 22 ° and θ = 125 ° to 130 ° is cut out from the obtained grown single crystal, mirror-polished, and the piezoelectric substrate is polished. An excitation electrode is formed so that the direction of propagation of a surface acoustic wave (especially a leaky wave) coincides with １ = 148 ° to 150 ° to produce a propagation type surface acoustic wave device S as shown in FIG. 2, for example. .

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, φ, θ, and ψ were determined to be the above-mentioned angles because the crystal orientation and the propagation direction of the substrate surface in the above-mentioned range were such that the electromechanical coupling coefficient of the leaky wave was large and the room temperature at room temperature was determined by computer simulation. This is because the group delay time temperature coefficient becomes zero at room temperature, and it is expected that the basic characteristics as a bandpass filter will be the best.

The computer simulation analysis method uses the physical property values of the single crystal grown as described above, and further uses a method such as Cambell (for example, JJ.
Cambell et al: IEEE. Trans.
Sonics. Ultrason. 15 (1968)
209). C as a parameter
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.
As a result of the computer simulation, the electromechanical coupling coefficient (k ² ) of the leaky wave is the largest, and the group delay time temperature coefficient (TCD) at room temperature is about 0 ° C./p.
The propagation direction of the cut surface and the surface acoustic wave which is pm is, for example, (φ, θ, ψ) = (18 ° to 222 °, 125 ° to 13 °).
0 °, 148 ° -152 °).

In this propagation direction, k ² is 3% or more, and the group delay time temperature coefficient at room temperature is zero. In addition,
Considering the symmetry of the LGN single crystal of the point group P321, the above propagation direction has periodicity with respect to surface acoustic waves of 60 ° and 180 ° in φ and θ, respectively, so that φ = 10 ° = 70
° = 130 ° = 190 ° = 250 ° = 310 ° and θ
= 125 ° = 305 °, and are treated as equivalent angles. Thereby, each parameter of the optimal Euler angle display (φ, θ, ψ) is (φ = a1 + 60 °)
× b1, θ = a2 + 180 ° × b2, ψ = a3 + 180
° × b3) (18 ° ≦ a1 ≦ 22 °, 12
5 ° ≦ a2 ≦ 130 °, 148 ° ≦ a3 ≦ 152 °, b
1, b2 and b3 are integers). Also, lanthanum-gallium-
Similarly, good filter characteristics can be obtained by using, in place of niobium in the niobium system, tantalum or niobium-tantalum, which is an element analogous to niobium.

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.

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 as described above, and may be appropriately changed without departing from the gist of the present invention. Can be implemented.

[0021]

Next, more specific embodiments of the present invention will be described.

Example 1 La mixed at a harmonic composition ratio
Atmosphere gas prepared by charging 2500 g of ₃ Ga _5.5 Nb _0.5 O ₁₄ single crystal raw material into an Ir crucible having an inner diameter of 100 mm and a height of 90 mm, and adjusting the nitrogen: oxygen = 99: 1 in a high frequency single crystal growing furnace. While feeding
After melting at 500 ° C., a single crystal was grown by bringing a seed crystal into contact with the melt surface.

Next, the optimum cut angle obtained from the grown crystal by the computer simulation, that is, FIG.
In the Euler angle display as shown in FIG.
The surface is cut out so as to obtain a surface of 130 ° to 130 ° and １３０ = 150 °, and is mirror-polished. As schematically shown in FIG. 2, the electrode finger width of about 3 μm on the substrate 1 and the electrode finger of the input side electrode 1a are formed. Excitation electrodes and the like consisting of 280 electrodes and 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 in a comb shape by a lift-off method after being formed to a thickness of about 5000 ° by a vacuum evaporation 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 TCD was measured from the rate of change of the center frequency with respect to the temperature, and k ² was calculated from the following equation using a network analyzer.

TCD (ppm / ° C.) = Α− (ΔVO / ΔT) VOT (ppm / ° C.) (1) k ² = 2 × (VO−Vm) / VO × 100 (%) (2) where α is the coefficient of linear expansion in the propagation direction, VO is the propagation velocity when the substrate surface is in an electrically open state, Vm
Is the propagation speed when the substrate surface is electrically short-circuited (short), and VOT is the speed at 25 ° C.

Euler angle display (φ, θ, ψ) = (20
(°, 128 °, 150 °) are shown in FIG. Here, φ = 20
K ^{2 of the} surface acoustic wave device in the propagation direction represented by °, θ = 125 ° to 130, ψ = 150 ° is 3 from the above formula.
~ 3.1 (%).

The TCD is almost 0 (ppm).
/ ° C), but quartz and lithium tetraborate (LBO) have been put into practical use as materials which are usually single crystal materials used for filters and have a group delay time temperature coefficient of zero at room temperature. Langasite (LGS) has been attracting attention. The shift amount of the center frequency between -20 ° C and + 80 ° C, which is the guaranteed operating temperature range of the SAW filter for mobile communication devices such as mobile phones, is about 100 pp for quartz.
m and LBO are about 700 ppm, and quartz has a small shift in center frequency, but k ² is as small as about 0.1%. Considering miniaturization and stability of the element, the shift in center frequency is large. k ² is an advantage in large LBO and about 0.8%.

In a leaky wave using LGN as a substrate, (φ, θ, ψ) = (18 ° to 22 °, θ = 125 ° to 1
30 °, 148 ° to 152 °) because the shift amount of the center frequency at −20 ° C. to + 80 ° C. is less than 700 ppm of LBO as shown in FIG. 4, and k ² is the LBO of LBO. The leaky wave caused by the LGN is much larger than 0.8% at 3%, which is a great advantage in miniaturization, low loss and stability of the device.

In this embodiment, a lanthanum-gallium-niobium oxide single crystal has been described. However, the same operation and effect can be obtained by replacing tantalum, which is a homologous element with niobium, in place of niobium. That is, for example, La ₃ Ga
_5.5 Ta _0.5 O ₁₄ single crystal or La ₃ Ga _5.5 (Ta, N
b) Even in a _0.5 O ₁₄ single crystal, La ₃ Ga _5.5 Nb
The same operation and effect as the _0.5 O ₁₄ single crystal can be obtained.

[0032]

As described above, according to the surface acoustic wave device of the present invention using a surface acoustic wave, particularly a leaky wave,
Since the excitation electrode is formed on a substrate having an optimal cut surface, a surface acoustic wave provided with an excellent substrate having a very small group delay time temperature coefficient (TCD) and a very large electromechanical coupling coefficient (k ² ). An apparatus can be provided. In particular, a very effective and excellent surface acoustic wave device as a bandpass filter 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 one example of a surface acoustic wave device according to the present invention.

FIG. 3 is a graph showing an insertion loss according to a frequency change of the surface acoustic wave device according to the present invention.

FIG. 4 is a graph showing a frequency change amount according to a temperature change of the surface acoustic wave device according to the present invention.

[Explanation of symbols]

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

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Shinji Inoue 3-5-chome, Seika-cho, Soraku-gun, Kyoto F-term in Kyocera Corporation Central Research Laboratory 5J097 AA06 AA21 BB17 FF01 GG01 GG05 GG07 HA01 KK04 KK07

Claims (2)

[Claims] 1. A lanthanum-gallium-niobium system, a lanthanum-gallium-tantalum system, or a lanthanum-gallium-system.
A surface acoustic wave device comprising a piezoelectric substrate made of a niobium-tantalum-based oxide single crystal and an excitation electrode for generating a surface acoustic wave, wherein a cut angle of the substrate and a propagation direction of the surface acoustic wave are provided. Euler angle display (φ, θ,
A surface acoustic wave device characterized in that each parameter of ψ) satisfies the following equation. φ = a1 + 60 ° × b1, θ = a2 + 180 ° × b2, ψ = a3 + 180 ° × b3 (however, 18 ° ≦ a1 ≦ 22 °, 125 ° ≦ a2 ≦ 13
0 °, 148 ° ≦ a3 ≦ 152 °, b1, b2, and b3 are integers) 2. The surface acoustic wave device according to claim 1, wherein the surface acoustic wave is a leaky wave.