JP2000114919A - Surface acoustic wave device - Google Patents

Abstract

(57) [Problem] To have a langasite-type crystal structure expected to have properties superior to conventional piezoelectric materials,
Provided is a surface acoustic wave device having excellent characteristics using a lanthanum-gallium-niobium-based oxide single crystal as a substrate. SOLUTION: As a substrate 11, a lanthanum-gallium-niobium-based oxide single crystal having a langasite-type crystal structure is used.
On the substrate 11, a normalized film thickness h / λ (where h: film thickness of the excitation electrode, λ: wavelength of surface acoustic wave) is 0.003 to 0.003.
Excitation electrodes 12 and 13 having a film thickness satisfying 0.075
To form a surface acoustic wave device S.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a surface acoustic wave using a lanthanum-gallium-niobium oxide single crystal having a langasite-type crystal structure having a large electromechanical coupling coefficient and a small group delay time temperature coefficient as a substrate material. It concerns the device.

[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 or lithium niobate is used as a substrate material of the surface acoustic wave device. And single crystals such as lithium tantalate have been used and put to practical use.

[0003] Among 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 cellular phones and the like, but has the drawback that the pass band width is small. ing.

On the other hand, single crystals of lithium niobate and lithium tantalate have the advantage of a large pass band width, and are suitably used for filters such as mobile phones and VTRs. Has a drawback that the change of is large.

[0005] Lithium tetraborate single crystal is known to compensate for the disadvantages of the above-mentioned materials. However, due to the deliquescence problem peculiar to lithium tetraborate single crystal, etc., it does not sufficiently satisfy the requirements of the above equipment. Did not.

In recent years, a langasite (La ₃ Ga ₅ ) 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. (SiO ₁₄ ) has attracted attention (for example, H. satoh and A. Mor).
i: Jpn. J. Appl. Phys. Vol. 36
(1997) p. 3071-3063).

Furthermore, recently, La ₃ Ga _5.5 Nb _0.5 O ₁₄ single crystal and La ₃ Ga _5.5 Ta _0.5 O ₁₄ single crystal having the same crystal structure as the langasite have been attracting attention as piezoelectric materials. Site-based single crystal is TC
It is highly expected that D is as small as quartz and k ² is larger than quartz.

[0008] However, since the energy of the surface acoustic wave is concentrated on the surface, the propagation characteristics of the surface acoustic wave are greatly affected by the surface condition of the substrate. Actually, in order to configure a surface acoustic wave device, it is essential to form a metal electrode such as an Al vapor-deposited film on the substrate surface.

Therefore, when a surface acoustic wave filter is formed, a metal electrode having an optimum normalized film thickness for the purpose of achieving a large electromechanical coupling coefficient and a group delay time temperature characteristic close to zero is used. However, there has been no report on the optimum and standardized film thickness of the above-mentioned langasite-based single crystal, and it remains unknown.

Therefore, the present invention has a langasite-type crystal structure expected to have properties superior to those of conventional piezoelectric materials, and has excellent characteristics using a lanthanum-gallium-niobium oxide single crystal as a substrate. It is an object of the present invention to provide a surface acoustic wave device.

[0011]

In order to achieve the above object, the present inventors have used a lanthanum-gallium-niobium-based oxide single crystal having a langasite-type crystal structure actually grown as a substrate. An excitation electrode was formed on a substrate to produce a surface acoustic wave device, and an optimum normalized film thickness was determined in consideration of an electromechanical coupling coefficient, a group delay time temperature characteristic, and the like. (D, h: film thickness of excitation electrode,
λ: the wavelength of the surface acoustic wave) was determined to be 0.003 to 0.075. In addition, it has been found that h / λ should be 0.025 to 0.075, particularly when the excitation electrode is mainly composed of aluminum (50% by weight or more).

Further, Euler angles (φ, θ, ψ) indicating the cut angle of the substrate of lanthanum-gallium-niobium oxide single crystal having a langasite-type crystal structure and the direction of propagation of surface acoustic waves (FIG. 3) It was found that the above characteristics were the best when each parameter of the above reference) satisfied the following equation.

Φ = a1 + 60 ° × b1, θ = a2 + 180 ° × b2 ψ = a3 + 180 ° × b3 (where 0 ° ≦ a1 ≦ 60 °, 125 ° ≦ a2 ≦ 16
5 °, 110 ° ≦ a3 ≦ 165 °, b1, b2, and b3 are integers, respectively. Further, even when each parameter of the Euler angle display (φ, θ, ψ) satisfies the following expression, the above characteristics are best. It turned out to be.

Φ = c1 + 60 ° × d1, θ = c2 + 180 ° × d2 ψ = c3 + 180 ° × d3 (where 0 ° ≦ c1 ≦ 60 °, 190 ° ≦ c2 ≦ 21
5 °, 50 ° ≦ a3 ≦ 90 °, d1, d2, and d3 are each integers)

[0015]

Embodiments of the present invention will be described below in detail.

In a high frequency heating type or resistance heating type single crystal growing furnace, a raw material of a lanthanum-gallium-niobium oxide single crystal having a langasite type crystal structure (for example, a grown single crystal is made of La ₃ Ga _5.5 Nb _0.5 O if _{14, La 2 O 3, Ga} 2 O 3, and mixtures Nb ₂ O _5, or by heating the La ₃ Ga _5.5 Nb _0.5 O _14, etc.) crucible containing a predetermined temperature, the raw material The melt is melted, and a 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, the substrate is cut out from the obtained grown single crystal at a cutout angle at which the basic characteristics such as the electromechanical coupling coefficient and the group delay time temperature characteristics are excellent, and mirror polishing is performed. To form the excitation electrode in the direction
For example, a propagation type surface acoustic wave device S as shown in FIGS. 1A and 1B is manufactured.

That is, on the cut surface 11a of the substrate 11, a gold film or an alloy film containing gold as a main component (50% by weight or more), a silver film or an alloy containing silver as a main component (50% by weight or more) Film, copper film or alloy film containing copper as a main component (50 wt% or more), nickel film or alloy film containing nickel as a main component (50 wt% or more), titanium film or alloy containing titanium as a main component Film (50% by weight or more) or an aluminum film or an alloy film containing aluminum as a main component (containing aluminum at 50% by weight or more) (for example, an aluminum-copper alloy film (for example, aluminum-0.5 to 5% by weight copper alloy) Film) or a metal film such as an aluminum-titanium alloy film (for example, an aluminum-0.5-35% titanium alloy film)), and then a lift-off method. Preliminary / or comb-shaped excitation electrode 12 by etching (input side) to form a 13 (output side). In FIG. 1, a power supply for inputting an AC signal to an excitation electrode on the input side, a detector for detecting an electric signal, and the like are not shown.

Here, the electromechanical coupling coefficient is 0.5% or more, and the group delay time temperature characteristic is almost zero (± 0.5 pp).
m / ° C.), the optimum normalized film thickness (h / λ) is 0.003 to 0.015 when the excitation electrode is gold or an alloy containing gold as a main component (50% by weight or more). Or 0.007 to 0.025 for an alloy containing silver as a main component,
0.007 or more for copper or an alloy containing copper as a main component
0.025, 0.007 to 0.025 for nickel or an alloy mainly containing nickel, and 0.01 to 0.04 for titanium or an alloy mainly containing titanium.
It turned out to be 5.

In particular, when the excitation electrode is made of aluminum or an alloy containing aluminum as a main component, the optimum normalized film thickness (h / λ) was determined for the surface acoustic wave device manufactured as described above. 4, 0.025 to 0.075 (most preferably 0.04 to 0.
06), the electromechanical coupling coefficient became maximum, the group delay time temperature characteristic became almost zero, and the result that the basic characteristic as a filter was very excellent was obtained.

Further, when the optimum cut angle and propagation direction at this time were examined, the parameters of the Euler angle display (φ, θ, ψ) indicating the cut angle of the substrate and the propagation direction of the surface acoustic wave were expressed by the following equations. It turned out to be satisfactory.

Φ = a1 + 60 ° × b1, θ = a2 + 180 ° × b2 ψ = a3 + 180 ° × b3 (where 0 ° ≦ a1 ≦ 60 °, 125 ° ≦ a2 ≦ 16
5 °, 110 ° ≦ a3 ≦ 165 °, and b1, b2, and b3 are integers, respectively.

Φ = c1 + 60 ° × d1, θ = c2 + 180 ° × d2 ψ = c3 + 180 ° × d3 (where 0 ° ≦ c1 ≦ 60 °, 190 ° ≦ c2 ≦ 21
5 °, 50 ° ≦ a3 ≦ 90 °, d1, d2, and d3 are integers, respectively. In particular, Euler angle display (90 ° ± 5 °, 155 ° ± 1
0 °, 120 ° ± 10 °), (90 ° ± 5 °, 205 °
± 10 °, 55 ° ± 10 °), (100 ° ± 5 °, 15
0 ± 10 °, 125 ° ± 10 °), (100 ° ± 5 °,
200 ± 10 °, 65 ° ± 10 °), (110 ° ± 5
°, 145 ° ± 10 °, 145 ° ± 10 °), (115
(° ± 7 °, 200 ° ± 10 °, 75 ° ± 10 °) was found to be optimal. Here, ± in parentheses indicates an allowable range.

The surface acoustic wave device is not limited to the propagation type as shown in FIG. 1, but is a resonator type as shown in FIGS. 2 (a) and 2 (b) (in FIG. 2, reference numeral 21 denotes a substrate and 22 denotes an excitation electrode,
23 and 24 are reflector electrodes. ) Can be applied to various types of filters and vibrators, and can be appropriately modified and implemented without departing from the gist of the present invention.

[0025]

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

Example 1 La mixed at a harmonic composition ratio
2500 g of ₃ Ga _5.5 Nb _0.5 O ₁₄ single crystal raw material was charged into an Ir crucible having an inner diameter of 100 mm and a height of 90 mm, and the atmosphere was adjusted in a high frequency single crystal growing furnace so that Ar: O ₂ = 90: 10. The raw material was melted at about 1500 ° C. while flowing gas, and then a single crystal was grown by bringing a seed crystal into contact with the melt surface.

Next, the grown crystal is cut out so as to obtain a plane of φ = 100 °, θ = 145 °, and で in Euler angle notation as shown in FIG. 3 and mirror-polished. As shown, the electrode finger width is about 5 μm (wavelength λ = 20 μm) on the substrate 1 and the number of electrode fingers of the input side electrode 1a is 280.
An excitation electrode having 60 electrode fingers of the output-side electrode 1b was formed so as to be in a predetermined direction, and a propagation type surface acoustic wave device S was manufactured.

The excitation electrode is made of an aluminum-2% by weight copper alloy film having a normalized thickness of 0.05 by a vacuum evaporation method.
It is formed in a comb shape by a lift-off method after being formed to a thickness of about 10,000 ° so that

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 calculated 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− (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 velocity when the substrate surface is electrically open, and V _m is the case when the substrate surface is electrically short-circuited. , V _OT is the speed at 25 ° C.

As a result of this measurement, as shown in FIGS. 6 and 7, in the Euler angle display (φ, θ, ψ), φ = 100 °,
The TCD and k ² of the surface acoustic wave device in the propagation direction represented by θ = 145 ° and ψ = 130 ° are almost 0 (ppm), respectively.
/ ° C), which is the maximum of 0.61 (%), and very good results were obtained.

Next, the relationship between the TCD, k ² of the surface acoustic wave device in the above crystal orientation and propagation direction and the normalized film thickness is described in the fourth.
FIG. 5 and FIG.

As is apparent from the results, the normalized film thickness at which the TCD becomes almost zero and the k ² becomes 0.5% or more is 0.025 to 0.075 (optimally 0.04 to 0.075).
It turned out to be in the range of 6), and very good results were obtained.

When the normalized film thickness is 0.08 or more, the difference in thermal expansion between the crystal substrate and the metal film becomes large, and the frequency temperature characteristic is deteriorated. When the normalized film thickness is 0.02 or less, the resistance of the metal film becomes large and the characteristics are satisfied. The obtained electromechanical coupling coefficient cannot be obtained.

Example 2 La mixed in harmonic composition ratio
2500 g of ₃ Ga _5.5 Nb _0.5 O ₁₄ single crystal raw material was charged into an Ir crucible having an inner diameter of 100 mm and a height of 90 mm, and the atmosphere gas was adjusted in a high frequency single crystal growing furnace so that Ar: O 2 = 90: 10. Was melted at about 1500 ° C. while flowing, and a seed crystal was brought into contact with the melt surface to grow a single crystal.

Next, φ = 100 ° and θ = 200 from this grown crystal in the Euler angle notation as shown in FIG.
2 and 3 are mirror-polished to obtain surfaces of ° and 、, and as shown schematically in FIG.
A resonance type surface acoustic wave device R was fabricated by forming excitation electrodes and the like having a predetermined direction with μm (wavelength λ = 12 μm), 100 electrode finger pairs, and 300 reflector electrode electrodes. .

The excitation electrode is formed by depositing a metal film of the same material as in Example 1 to a thickness of about 3600 ° by a vacuum deposition method so as to have a normalized thickness of 0.03, and then forming a comb shape by a lift-off method. It was done. 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.

As a result of this measurement, the Euler angle display (φ,
θ, ψ), φ = 100 °, θ = 200 °, ψ = 75 °
TCD, k of the surface acoustic wave device in the propagation direction represented by
² is almost 0 (ppm / ° C) and 0.57 (%), respectively.
And very good results were obtained.

[0040]

As described above, according to the surface acoustic wave device of the present invention, a lanthanum-gallium-niobium-based oxide single crystal having a langasite-type crystal structure is obtained after optimizing a normalized film thickness. Since the excitation electrode with the optimum thickness was formed on the substrate consisting of, the group delay time temperature coefficient was almost zero,
In addition, it is possible to provide a surface acoustic wave device having an extremely large electromechanical coupling coefficient (k ² ) and excellent characteristics capable of suppressing generation of ripples (particularly suitable as a bandpass filter).

[Brief description of the drawings]

FIG. 1 is a view schematically showing a propagation type surface acoustic wave device according to the present invention, wherein (a) is a plan view of the surface acoustic wave device;
FIG. 2B is a sectional view taken along line AA ′ of FIG.

FIG. 2 is a diagram schematically showing a resonance type surface acoustic wave device according to the present invention, wherein (a) is a plan view of the surface acoustic wave device,
(B) is a sectional view taken along line BB 'of (a).

FIG. 3 is a schematic diagram for describing Euler angles of a single crystal.

FIG. 4 shows Euler angles (φ, θ, ψ), φ = 100
FIG. 9 is a diagram illustrating a relationship between an electromechanical coupling coefficient of a surface acoustic wave device in a propagation direction represented by °, θ = 145 °, and ψ = 130 ° and a normalized film thickness.

FIG. 5 shows Euler angles (φ, θ, ψ), φ = 100
FIG. 7 is a diagram illustrating a relationship between a group delay time temperature characteristic of a surface acoustic wave device in a propagation direction represented by °, θ = 145 °, and ψ = 130 ° and a normalized film thickness.

FIG. 6 is a diagram showing a relationship between a propagation direction of a surface acoustic wave device represented by Euler angles (100 °, 145 °, ψ) and an electromechanical coupling coefficient.

FIG. 7 is a diagram showing a relationship between a propagation direction of the surface acoustic wave device represented by Euler angles (100 °, 145 °, ψ) and a group delay time temperature characteristic.

[Explanation of symbols]

 11, 21: piezoelectric substrate 12, 13, 22: excitation electrode 23, 24: reflector electrode S: propagation type surface acoustic wave device R: resonance type surface acoustic wave device

Claims (4)

[Claims] 1. An excitation electrode for generating a surface acoustic wave is formed on a substrate made of a lanthanum-gallium-niobium-based oxide single crystal having a langasite-type crystal structure so as to satisfy the following expression. Surface acoustic wave device. 0.003 ≦ h / λ ≦ 0.075 (where h: film thickness of excitation electrode, λ: wavelength of surface acoustic wave) 2. The surface acoustic wave device according to claim 1, wherein the excitation electrode has aluminum as a main component and is formed so as to satisfy the following expression. 0.025 ≦ h / λ ≦ 0.075 (where h: film thickness of excitation electrode, λ: wavelength of surface acoustic wave) 3. The elasticity according to claim 1, wherein each parameter of the Euler angle display (φ, θ, ψ) indicating the cut angle of the substrate and the propagation direction of the surface acoustic wave satisfies the following expression. Surface wave device. φ = a1 + 60 ° × b1, θ = a2 + 180 ° × b2 ψ = a3 + 180 ° × b3 (where 0 ° ≦ a1 ≦ 60 °, 125 ° ≦ a2 ≦ 16
5 °, 110 ° ≦ a3 ≦ 165 °, b1, b2, and b3 are integers, respectively) 4. The elasticity according to claim 1, wherein each parameter of the Euler angle display (φ, θ, ψ) indicating the cut angle of the substrate and the propagation direction of the surface acoustic wave satisfies the following expression. Surface wave device. φ = c1 + 60 ° × d1, θ = c2 + 180 ° × d2 ψ = c3 + 180 ° × d3 (However, 0 ° ≦ c1 ≦ 60 °, 190 ° ≦ c2 ≦ 21
5 °, 50 ° ≦ a3 ≦ 90 °, d1, d2, and d3 are each integers)