JP2000252788A - Surface acoustic wave device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a surface acoustic wave device with a small size and low power consumption that has a characteristic of a natural single phase unidirectional transducer. SOLUTION: Stimulation electrodes 2a, 2b to generate a surface acoustic wave are provided onto a substrate 1 having a Langasite structure and made of an oxide single crystal containing at least lanthanum, gallium and VA group elements. The surface acoustic wave device S satisfies any of conditions (1); 90 deg.<=ϕ<100 deg., 145 deg.<=θ<=170 deg., 110<=ψ<=140 deg., (2); 100 deg.<=ϕ<110 deg., 140 deg.<=θ<=165 deg., 120 deg.<=ψ<=155 deg., and (3); 110 deg.<=≃<=120 deg., 120 deg.<=θ<=160 deg., 135 deg.<=ψ<=170 deg., where ϕ, θ, ψare parameters denoting Euler's angle representation that indicate a cut angle of the substrate 1 and a propagation direction or a surface acoustic wave.

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 langasite structure and, in particular, using a single crystal oxide containing at least lanthanum, gallium, niobium and / or tantalum as a substrate material. Yes, especially natural single-phase unidirectional converters (Natural
Single-Phase Unidirection
The present invention relates to a surface acoustic wave device capable of realizing a low-loss SAW filter using an al-Transducer (NSPUDT) characteristic.

[0002]

2. Description of the Related Art In recent years, mobile communication terminals such as portable telephones have rapidly become widespread. It is essential for the terminal to be small and light and to have low power consumption, and there is a strong demand for such components in the terminal. As an example of a surface acoustic wave device which is one of the components of the portable terminal, a single-phase signal source having a phase of 18
A transmitting and receiving surface acoustic wave device that selectively connects only a signal of a specific frequency band by arranging a comb-shaped transmitting and receiving converter connected to two terminals different from each other by 0 ° has been widely put into practical use. I have.

In order to realize a surface acoustic wave device, the electromechanical coupling coefficient (k ² ) is large and the power flow angle (PF)
A) is required to have characteristics such as a small temperature characteristic (TCD) and a small temperature characteristic (TCD), and a crystal having a good temperature characteristic has been most widely used. However, quartz has good temperature characteristics, but has a high sound speed and a small electromechanical coupling coefficient.

Recently, a langasite-based single crystal has been attracting attention as a new piezoelectric single crystal, and this single crystal has a lower sound speed than a quartz crystal, has an electromechanical coupling coefficient of two to three times, and has a temperature characteristic comparable to that of a quartz crystal. Therefore, it is expected to be a great piezoelectric material in place of quartz. Among these langasite-based single crystals, La ₃ Ta _0.5 Ga _5.5 O ₁₄ (LGT) has attracted particular attention (see, for example, JP-A-10-284983).

Conventionally, in such a surface acoustic wave device, it is required to suppress the insertion loss and the ripple inside the band and the spurious outside the band as well. An ordinary comb-shaped electrode uses a structure in which the width of the electrode finger is λ / 4 with respect to the wavelength λ of the surface acoustic wave. There is a disadvantage that the theoretical minimum loss is 6 dB and the insertion loss cannot be suppressed low.

On the other hand, in order to eliminate such disadvantages,
Using floating electrodes, changing the electrode thickness periodically,
By using a group-type IDT, the traveling direction of the elastic wave can be biased in one direction, and the insertion loss can be reduced.

However, these electrodes have disadvantages in that the electrode finger width with respect to the wavelength is smaller than usual, so that it is very difficult to manufacture them, and it is very difficult to design such as weighting, so that it is difficult to obtain necessary frequency characteristics. In addition to low insertion loss, the performance of the surface acoustic wave device requires improvement in frequency characteristics such as flattening of phase characteristics, ripples in a pass band, and suppression of spurious components outside a band.

As one of means for solving such a drawback, a one-way characteristic can be obtained despite the use of a normal electrode having an electrode finger width and electrode finger spacing of λ / 4 due to the anisotropy of the piezoelectric substrate itself. There has been proposed a natural single-phase one-way converter having the following. Conventionally, a quartz substrate, a LiNbO ₃ single crystal, a LiTaO
_Three single crystals and Li ₂ B ₄ O ₇ single crystals are known.

However, NSPUDTs using these conventional materials are limited in practical use because of a small electromechanical coupling coefficient k ² , no zero temperature coefficient, and a non-zero power flow angle. There are drawbacks. Further, as described above, the surface acoustic wave velocity on the substrate is a problem in reducing the size and weight of the surface acoustic wave device, and the lower the better, the faster the sound speed of the conventional material is 3000 m.
/ S or higher, there is a problem in terms of size and weight reduction.

An object of the present invention is to provide a surface acoustic wave device having excellent characteristics by eliminating or reducing such conventional disadvantages.

[0011]

In order to achieve the above object, the present inventors have focused on an oxide single crystal having a langasite structure and containing at least lanthanum, gallium, and a group VA element. La ₃ Ga _5.5 Nb _0.5
The NSPUDT operation, the electromechanical coupling coefficient, the group delay time temperature coefficient, the sound velocity, and the power flow angle were obtained by computer simulation and actual trial production based on the characteristic values of O ₁₄ single crystal and La ₃ Ga _5.5 Ta _0.5 O ₁₄ single crystal. In a surface acoustic wave device having an excitation electrode for generating a surface acoustic wave on a substrate of an actually manufactured LGN by searching for a cut surface and a propagation direction of the surface acoustic wave at which the basic characteristic values such as Is 90 degrees in the right-handed Euler angle display (φ, θ, ψ).
When ≦ φ <100 °, 145 ° ≦ θ ≦ 170 °, 110
1 when ° ≦ ψ ≦ 140 °, 100 ° ≦ φ <110 °
40 ° ≦ θ ≦ 165 °, 120 ° ≦ ψ ≦ 155 °, 1
When 10 ° ≦ φ ≦ 120 °, 120 ° ≦ θ ≦ 160 °,
It was found that 135 ° ≦ ψ ≦ 170 °. In addition,
FIG. 1 shows the case where the crystal axes are X, Y, and Z, the propagation direction of the surface acoustic wave is a, the direction perpendicular to the substrate is c, and the directions perpendicular to a and c are b (φ, θ). , Ψ) are called Euler angles.

Further, in a transmission / reception type surface acoustic wave device, when a bidirectional electrode or a unidirectional inversion electrode is used for one of a transmission electrode and a reception electrode, an extremely small loss surface acoustic wave device can be obtained. .

[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 is fed into a single crystal growing furnace of a high-frequency heating method or a resistance heating method in a single crystal raw material (La ₂ O
₃ , a mixture of Ga ₂ O ₃ and Nb ₂ O ₅ , or La ₃
The raw material is melted by heating the crucible containing Ga _5.5 Nb _0.5 O ₁₄ ) to a predetermined temperature, the 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, from the obtained grown single crystal, in terms of Euler angles (φ, θ, ψ), when 90 ° ≦ φ <100 °, 145 ° ≦ θ ≦ 170 ° and 100 ° ≦ φ <110 ° When 140 ° ≦ θ ≦ 165 °, 110 ° ≦ φ ≦ 120
At the time of °, cut out the substrate of the cut surface of 120 ° ≦ θ ≦ 160 °, perform mirror polishing, and the direction in which the surface acoustic wave propagates on this substrate, that is, 110 ° on each cut surface.
≤ψ≤140 °, 120 ° ≤ψ≤155 °, 135 ° ≤
Excitation electrodes are formed so as to be equal to ψ ≦ 170 °, and, for example, a propagation type surface acoustic wave device S as shown in FIG. 2 is manufactured.

That is, after a metal such as aluminum is formed on the cut surface 1a of the substrate 1 by vacuum evaporation,
A normal excitation electrode 2a (transmitting electrode) having an electrode finger-to-electrode spacing of と of the wavelength λ by a lift-off method and / or an etching method, for example, is adjacent to the electrode finger at an electrode finger-to-electrode spacing of λ / 8. The two electrode fingers form a double electrode 2b (receiving electrode) having the same electric field period. In addition,
In the drawing, A is a power supply for inputting an AC signal to the excitation electrode on the transmission 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 propagation direction obtained by the computer simulation was confirmed by trial measurement, and the crystal direction and the propagation direction of the substrate surface in the above range were determined. , NSPUDT
This is because it was the most basic characteristic as a surface acoustic wave device utilizing the operation.

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) 2
09) and the first order perturbation theory. The parameters include c constant (N / m ² ), e constant (C / m ² ), relative permittivity, linear expansion coefficient, and density (Kg / m ² ).
³ ) and their primary temperature coefficient (/ ° C) and secondary temperature coefficient (/ ° ^C2 ) were used.

According to the results of the above computer simulation, for example, in the case of La ₃ Ga _5.5 Nb _0.5 O ₁₄ , FIGS. 3 to 6 show that the electromechanical coupling coefficient (k ² ) is large and the group delay time at room temperature is large. Temperature coefficient (TCD)
Is about 0 ppm / ° C., the power flow angle is almost 0 °, and the propagation direction of the cut surface and the surface acoustic wave that efficiently generates NSPUDT operation is (φ, θ, ψ) 90 ° ≦
When φ <100 °, 145 ° ≦ θ ≦ 170 °, 110 °
≦ と き ≦ 140 °, 100 ° ≦ φ <110 ° 14
0 ° ≦ θ ≦ 165 °, 120 ° ≦ ψ ≦ 155 °, 11
When 0 ° ≦ φ ≦ 120 °, 120 ° ≦ θ ≦ 160 °, 1
It was found that 35 ° ≦ ψ ≦ 170 °.

Here, the NSPUDT operation will be described. Intermodal coupling coefficient kappa ₁₂ obtained by normalizing from perturbation theory in electrode period λ is represented by the number 1. h is the thickness of the electrode, KE in the first term on the right side indicates electrical perturbation, and the second term indicates mechanical perturbation. Phase angle 2φo optimum NSPUDT phase condition operation is obtained conditions negligible electrical perturbation terms are as Equation 2 (for example, if the electrode film thickness is large), the kappa ₁₂ in the phase angle 2φM mechanical perturbation terms As a result, the strongest NSPUDT operation is obtained when φM = 45 °. FIG.
10 °, 145 °, ψ) shows φM, but ψ = 14
It can be seen that φM = 45 ° around 5 °. NSPU
The DT operation is maximum at φM = 45 °, but φM = 20 °
A practical operation can be obtained at 60 °.

[0020]

(Equation 1)

[0021]

(Equation 2)

The above Euler angle display is based on the space group P3
The above numerical values can be taken according to the symmetry of the LGN 21 and the periodicity with respect to the surface acoustic wave. That is, for example, (φ,
θ, ψ) is (φ + 120 °, θ, ψ), (φ + 60 °,
360 ° -θ, ψ), (120 ° -φ, θ, 180 °-
ψ), (60 ° -φ, 360 ° -θ, ψ) and have the same characteristics. Further, (0 °, θ, ψ) is equivalently symmetric with (0 °, θ, 180 ° -ψ) and has the same characteristics.

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. In addition, La ₃ Ga _5.5 Nb _0.5 O ₁₄ single crystal (LG
N), as shown in FIG. 6, the surface wave velocity is 2700 m /
In seconds, it is smaller than the surface wave velocity of quartz of 3150 m / sec, and when used as a low frequency filter, it can be miniaturized and can be suitably used as a bandpass filter. Here, when a filter having the same frequency is manufactured from LGN and quartz, the electrode finger width is proportional to the surface wave velocity, so that the size of the IDT portion can be reduced by about 15%. Furthermore, the fact that k ² and the dielectric constant are larger than those of quartz also contributes to miniaturization, so that further miniaturization can be expected. Also,
Since the NSPUDT operation is used, the insertion loss is reduced, which is optimal for power saving.

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

The composition is mainly lanthanum-gallium-niobium, lanthanum-gallium-tantalum,
In the case of a lanthanum-gallium-niobium-tantalum single crystal, a similar effect can be expected since niobium and tantalum are VA group elements.

[0026]

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 raw material of _0.5 O ₁₄ single crystal (LGN)
The above-mentioned raw material was charged at about 1500 ° C. in an Ir crucible having a height of 100 mm and a height of 90 mm while flowing an atmosphere gas in a high-frequency single crystal growing furnace whose flow rate was adjusted so that Ar: O ₂ = 90: 10. After melting, 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 above computer simulation, ie, FIG.
Φ = 110 °, θ = 14 in Euler angle display as shown in
Cut out to obtain a 5 ° surface, perform mirror polishing,
As schematically shown in FIG. 2, an electrode finger width of about 6 μm
m, a transmission type surface acoustic wave device in which excitation electrodes and the like having 20 pairs of electrode fingers of the transmission-side electrode 2a and 20 pairs of electrode fingers of the double-side reception electrode 2b are formed in a predetermined direction. S was produced.

The excitation electrode is formed by depositing aluminum to a thickness of about 6000 ° by a vacuum evaporation method, and then forming it in a comb shape by a lift-off method.

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

Here, the sound speed is determined by the center frequency and λ (24 μm).
m), the TCD was calculated from the rate of change of the center frequency (here, about 112 MHz) with respect to the temperature, and k ² was calculated from the admittance circle by the network analyzer. As a result of this measurement, in the Euler angle display (φ, θ, ψ), φ = 110 °, θ = 145 °, ψ =
Sound velocity of the surface acoustic wave device in the propagation direction represented by 145 °,
TCD, k ² , and PFA are about 2700 (m /
s), 0 (ppm / ° C), 0.4 (%), 0.15
(°), and very good results could be obtained.
However, since PFA cannot be measured, an analysis value was used.

For comparison in the same direction, NSPUDT
Propagation type surface acoustic wave devices having 20 pairs of transmission / reception double electrodes, each of which does not operate, were manufactured. Since the excitation wavelength and the wavelength of the reflected wave are different from each other in the double electrode, there is no reflection effect of the excitation wavelength, and the double electrode is a bidirectional converter, so that a natural one-way converter operation cannot be obtained.

As a result, as shown in FIG.
At 2.5 MHz, the insertion loss of the NSPUDT operation waveform 3 was about 5 dB lower than that of the bidirectional waveform 4.
Also, it can be seen that the spurious level of the third harmonic in the NSPUDT operation waveform shown in FIG. 9A is greatly improved as compared with the comparison characteristic shown in FIG. 9B. Since FIGS. 8 and 9 are intended to compare the insertion loss by the NSPUDT operation, the reflection loss of the measurement system is not matched with the measurement system. It is apparent from the above that the propagation characteristics required for the surface acoustic wave device are excellent.

[Example 2] 2500 g of a single crystal raw material mixed in a composition ratio represented by La ₃ Ga _5.5 Ta _0.5 O ₁₄ was prepared, and Ir having an inner diameter of 100 mm and a height of 90 mm was prepared.
Crucible and Ar: O in high frequency single crystal growing furnace
The raw material was melted at about 1550 ° C. above the melting point while flowing an atmosphere gas whose flow rate was adjusted so that ₂ = 90: 10, and then a seed crystal was brought into contact with the melt surface to grow a single crystal.

Next, φ = 110 ° and θ = 145 ° in the Euler angle display as shown in FIG. 1 which are the optimal cut planes obtained by the computer simulation from the grown crystal.
Then, the same surface acoustic wave device S as in Example 1 was manufactured. The conditions such as the electrode material, structure, and thickness were all the same as in Example 1.

Next, regarding the manufactured surface acoustic wave device,
Group delay time temperature coefficient (TCD), electromechanical coupling coefficient (k
² ) was determined using the same method as in Example 1.

As a result of this measurement, the Euler angle display (φ,
θ, ψ), φ = 110 °, θ = 145 °, ψ = 145
Sound velocity of surface acoustic wave device in the propagation direction expressed in °, TC
D, k ² and PFA are approximately 2700 (m /
s), 0.5 (ppm / ° C), 0.31 (%), 0.2
3 (°), and very good results were obtained. However, since PFA cannot be measured, an analysis value was used.

The same substrate is used for comparison with NSP.
As a result of fabricating a transmission type surface acoustic wave device having a double electrode structure of 20 pairs of transmission and reception in which no UDT operation occurs, FIG.
As shown in the figure, NSPU at a frequency of 112.5 MHz
In the DT operation waveform 5, the insertion loss was reduced by about 2 dB as compared with the bidirectional waveform 6. This proved that the propagation characteristics required for the surface acoustic wave device were excellent. Since the purpose of FIG. 10 is to compare the insertion loss by the NSPUDT operation, the reflection loss of the measurement system is not matched with the measurement system.

The present results show that the effect of substituting Nb for Ta does not change much,
a is added to the entire single crystal at a low ratio,
It is considered that b and Ta have similar characteristics in terms of valence and chemical characteristics.

Also, from the results, La ₃ Ga _5.5 Nb
The value of x in _0.5-x Ta _x O ₁₄ is the same effect can be expected in the range of 0 to 0.5.

[0040]

As described above, according to the surface acoustic wave device of the present invention, since the excitation electrode is formed on the substrate having the optimum cut surface with good NSPUDT operation, the electromechanical coupling coefficient is large and the temperature is high. The characteristics are good, the power flow angle is small, the sound speed is low, and in addition to the NSPUDT operation, characteristics with low loss and small spurious and ripple are obtained.
A small surface acoustic wave device with low power consumption and excellent filter characteristics can be realized.

[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 the relationship between k ² and the propagation direction when φ = 110 ° in LGN.

FIG. 4 is a graph showing the relationship between TCD and propagation azimuth when φ = 110 ° in LGN.

FIG. 5 is a graph showing the relationship between PFA and propagation azimuth when φ = 110 ° in LGN.

FIG. 6 is a graph showing the relationship between the surface acoustic wave velocity and the propagation direction when φ = 110 ° in LGN.

FIG. 7 is a graph showing the relationship between the reflection coefficient and φM in the case of LGN: (110 °, 145 °, ψ).

FIG. 8 is an actual measurement result of a filter characteristic indicating an NSPUDT operation in the case of LGN: (110 °, 145 °, 145 °).

FIG. 9 (a) shows LGN: (110 °, 145 °, 14
5 °) in the NSPUDT operation. (B) is LGN: (110 °, 145 °, 145)
°) Wide-band frequency characteristics in bidirectional operation.

FIG. 10 shows a graph of (110) having a composition of La ₃ Ga _5.5 Ta _0.5 O _14.
(°, 145 °, 145 °) is an actual measurement result of the filter characteristic indicating the NSPUDT operation.

[Explanation of symbols]

 1: substrate 1a: cut surface 2a: transmission side electrode 2b: reception side electrode 3: NSPUDT operation waveform 4: bidirectional waveform 5: NSPUDT waveform 6: bidirectional waveform S: surface acoustic wave device

Claims (1)

[Claims] 1. A surface acoustic wave device having an excitation electrode for generating a surface acoustic wave on a substrate having a langasite structure and made of an oxide single crystal containing at least a lanthanum, gallium, and VA element. The Euler angle display (φ, φ) indicating the cut angle of the substrate and the propagation direction of the surface acoustic wave.
θ, ψ), wherein each parameter satisfies one of the following conditions (1) to (3). (1) 90 ° ≦ φ <100 °, 145 ° ≦ θ ≦ 170
°, 110 ° ≦ ψ ≦ 140 ° (2) 100 ° ≦ φ <110 °, 140 ° ≦ θ ≦ 165
°, 120 ° ≦ ψ ≦ 155 ° (3) 110 ° ≦ φ ≦ 120 °, 120 ° ≦ θ ≦ 160
°, 135 ° ≦ ψ ≦ 170 °